Paradoxes in Geology

Paradoxes in Geology

The illustration on the front cover represents the Indus Gorges, half way between Patan and Dasu (N-Pakistan). The river incises the metaplutonic rocks of the Cretaceous Kohistan Arc. Photo by Jean-Pierre Burg.

Attendants of the Symposium on "Paradoxes in Modern Geology", held in Beijing on 13 and 14 September 1999, to honour the 70th birthday of Professor Kenneth Jingwah Hsfi.

Paradoxes in Geology Edited by

Ueli Briegel ETH-Z Swiss Federal Institute of Technology Department of Earth Sciences Z(Jrich, Switzerland

Wenjiao Xiao Laboratory of Lithosphere Tectonic Evolution Institute of Geology and Geophysics Chinese Academy of Sciences Beijing, China

2001 ELSEVIER Amsterdam

- London

- New

York-

Oxford

- Paris - Shannon

- Tokyo

ELSEVIER SCIENCE B.V. Sara Burgerhartstraat 25 P.O. Box 211, 1000 AE Amsterdam, The Netherlands 92001 Elsevier Science B.V. All rights reserved This work is protected under copyright by Elsevier Science, and the following terms and conditions apply to its use: Photocopying Single Photocopies of single chapters may be made for personal use as allowed by national copyright laws. Permission of the Publisher and payment of a fee is required for all other photocopying, including multiple or systematic copying, copying for advertising or promotional purposes, resale, and all forms of document delivery. Special rates are available for educational institutions that wish to make photocopies for non-profit educational classroom use. Permissions may be sought directly from Elsevier Science Global Rights Department, PO Box 800, Oxford OX51DX, UK; phone: (+44) 1865 853333, e-mail: [email protected]. You may also contact Global Rights directly through Elsevier's home page (http://www.elsevier.com), by selecting 'Obtaining Permissions'. In the USA, users may clear permissions and make payments through the Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01923, USA; (+1)(978) 7508400, fax: (+1)(978) 7504744, and in the UK through the Copyright Licensing Agency Rapid Clearance Service (CLARCS), 90 Tottenham Court Road, London WlP 0LP, UK; phone: (+44) 207 631 5555; fax: (+44) 207 631 5500. Other countries may have a local reprographic rights agency for payments. Derivative Works Tables of contents may be reproduced for internal circulation, but permission of Elsevier Science is required for external resale or distribution of such material. Permission of the Publisher is required for all other derivative works, including compilations and translations. Electronic Storage or Usage Permission of the Publisher is required to store or use electronically any material contained in this work, including any chapter or part of a chapter. Except as outlined above, no part of this work may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without prior written permission of the Publisher. Address permissions requests to: Elsevier Science Global Rights Department, at the mail, fax and e-mail addresses noted above. Notice No responsibility is assumed by the Publisher for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or form any use or operation of any methods, products, instructions or ideas contained in the material herein. Because of rapid advances in the medical sciences, in particular, independent verification of diagnoses and drug dosages should be made. First edition 2001 Library of Congress Cataloging in Publication Data A catalog record from the Library of Congress has been applied for. British Library Cataloguing in Publication Data Paradoxes in Geology 1. Geology I. Briegel, Ueli I1. Xiao, Wenjiao 551 ISBN 0-444-50560-1 @ The paper used in this publication meets the requirements of ANSI/NISO Z39.48-1992 (Permanence of Paper). Printed in The Netherlands.

FOREWORD This volume is dedicated to Kenneth Jingwah Hsfi, Professor emeritus of the Swiss Federal Institute of Technology (ETH-Zfirich). It collects the proceedings of the Symposium on "Paradoxes in Modern Geology", held in Beijing on the 13th and 14th September 1999, to honour his 70th birthday. Paradox, as stated by the Oxford Advanced Learner's Dictionary, is a "statement that seems to say the opposite of common sense or the truth, but which may contain a truth". It seems to me very typical of Ken Hsfi, a "scholar" of Popper, not to search for the truth, but to try to find fault in generally accepted views. As a result, controversies eventually end up as paradoxes. Unfortunately the scientific community has not always been open to Ken's paradoxical ideas and the degree of frustration was varying through time, as he himself reports in the last chapter of this volume. Ten years ago at the occasion of Ken Hsti's 60th birthday, a Symposium on "Controversies in Modern Geology" was held in Zfirich and in 1991, a book with the same title was printed by Academic Press (ISBN 0-12-510340-9). It is redundant and would be an act of piracy to rewrite Ken Hsfi's curriculum, which was so thoughtfully reported in that foreword by the late Prof. Seymour Schlanger, one of the best friends of Ken. Knowing that not everyone has access to the 1991 volume, I would like to cite two paragraphs here: " . . . . I n order to formulate.., an appreciation of the first 60 years of Professor Hsfi (or Ken, as he prefers) it is necessary, according to further definition, to "grasp the nature, worth, quality and significance" of his complex and productive path across the geological landscape and, equally important in Ken's life, his path through the universe of cultures. It is beyond the experience of any one person to completely "grasp" all of the aspects of Ken's life and work to date .... This extraordinary range of interest and competence may well be due to a rather unique set of interacting and reinforcing aspects of his professional evolution and personal history. One of these is, of course, his innate intense intellectual curiosity about "how the world works" combined with his ability to master the fundamentals of the various sciences that converge in the field of geology. Another, perhaps equally important, has been his successive immersion in three widely disparate cultures: Chinese, American and European each with its own philosophical worldview, educational system, framework of personal and professional relationships and hierarchies, all separated by linguistic gulfs. Further, the necessity of having to face the complex set of challenges that mark such a global career, coupled with exposure to geologists of diverse backgrounds, surely has also been a powerful factor in the development of his universalist approach to geology .... " The wide variety of paradoxes, as listed hereafter by Sun Shu, which Ken has worked on or at least initiated discussions on within the scientific community is just one indication of the manifold fields of interests he has been involved in, often simultaneously, not only over the last 10 years but throughout his scientific career. The broad discussion of paradoxes surely is fruitful for the advancement of science in general and especially in the Geosciences as demonstrated by Ken several times. I would like to mention only two examples, which illustrate this statement

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Foreword

Kenneth Jingwah Hsfi, Professor emeritus of the Swiss Federal Institute of Technology (ETH-Zfirich).

Foreword

vii

very clearly: the Flysch problem and the desiccation of the Mediterranean. Both have been disputed for many years and Ken was never refuted. Not only Ken himself propelled the geoscience community forward with his controversial statements, quite a few of his former students, who are today in key positions at universities and in industry, are influencing in a similar open-minded way the present-day thinking. This was greatly shown during the Symposium, where 28 papers were presented, most of which are included in this volume. The authors originating from China, Taiwan, Russia, U.S.A., Italy, France, Australia, Great Britain and Switzerland have been delineating paradoxes and problems in the fields of tectonics, basic and applied geosciences, petrology, paleoceanography, paleoclimatology and paleogeography, kinematics and modelling. Three of the few scientists that missed the meeting contributed an article to this volume. I would like to express my gratitude to Jerry Winterer, He Qixiang and Celal Seng6r; their contributions to the volume are greatly appreciated. Ueli Briegel Ziirich, April 2001 It was in the late 1980s when I was a college student that I came to know the big name of Prof. K. J. Hsti. In 1992, I started a Ph.D. program concerning the early Mesozoic orogeny in South China, which probably was the biggest tornado Prof. Hsti unleashed several years ago in geological community. I went to NW Zhejiang, SE China, to investigate the tectono-sedimentological evolution of the area that is a key to testifying Prof. Hsti's tectonic views about South China. To my surprise, some aspects have been proven correct and many other lines of evidence still kept coming out. Therefore, although I heard his lecture several times, I was longing to meet him. The 1998 summer, for me, was a pleasant time, for I had the chance to meet Prof. Hsfi. Invited by Prof. H.H. Chen, he visited our laboratory. Afterward, we kept close contact and I learned from him not only geology, but also environmental protection. In May-July, 1999, when I was in Ztirich as a visiting scholar to the "Institut ftir Geophysik", ETH-Zurich, Prof. Hsti kindly invited me several times to his home and we exchanged views including that on the South China tectonics. Gradually, I realized that Prof. Hsti has achieved many others in his academic career besides geology, as you can see in Prof. S. Sun's introduction in this volume. On September 13-14, 1999, in order to honor his great contribution to science, in particular that to the geological progresses in China, under the sponsorship of the Laboratory of Lithosphere Tectonic Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, and also supported in various ways by many other different institutions and individuals, the "Festschrift Symposium on Paradox in Modern Geology on the Occasion of Celebrating Ken Hsfi's 70th Birthday" was held in Beijing and I was very honored to be in the organization staff. Speakers invited from around the world came to Beijing, attended the 2-day meeting and many of them also went to the field trip afterward to Xinjiang, China and to the Pamirs, Pakistan. Most of the contributors presented their manuscripts, which form the main body of the monograph. In the mean time, Elsevier, who already published many of Prof. Hsfi's works, sincerely showed their interest in publishing the

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Foreword

monograph. After consulting with Prof. Hsfi and other contributors, Dr. Ueli Briegel and I signed the contract as the editors with Elsevier to publish a monograph entitled " Paradoxes in Geology" and later Ueli kindly took the main job of editing. As readers will see in the m o n o g r a p h , the topics include T E C T O N I C S & & STRUCTURAL GEOLOGY, PETROLOGY, PALEOGEOGRAPHY P A L E O C L I M A T O L O G Y , and OTHERS. In this wide range of research area, Prof. Hsfi, cooperating with his former colleagues and students, has achieved greatness and everything indicates, to a certain extent, that he is a man outside of the mainstream. He does not take for granted "received opinion" or "common sense." He has no patience to engage in "mopping up actions." Instead, he always sees the opportunity for breakthroughs to establish a new paradigm. Sometimes he only had an idea which inspired others. Sometimes he himself would bull his way through like a "tornado" despite united opposition. Some of his ideas were simple and ingenious. That is the way he understands "Paradox", a tenet contrary to received opinion, or a statement that is seemingly contradictory or opposed to common sense and yet is perhaps true. The purpose of holding this symposium is a reassessment of some classical problems in geology. Some are no longer controversial such as the turbidite problem, the Franciscan problem, etc. Others are still to be resolved such as the nature of radioactivity, the significance of archipelago orogenesis, or whether global change has been a blessing to mankind. As Prof. B. F. Windley states in his recent book review on Prof. Hsfi's newly published book with his Chinese colleague "Geologic Atlas of China: an Application of the Tectonic Facies Concept to the Geology of China", "We cannot ignore, and should take careful account of " the "new and challenging conceptual overview" of Prof. Hsfi. Prof. Hsfi always emphasizes that "Scientific truth cannot be verified, only falsified." We hope more false notions in science will be falsified, which will inspire great progress in future. Ueli and I sincerely would like to dedicate this monograph to Prof. K.J. Hsfi and those who are involved in the long-term geological cooperation between China and other countries. Meetings like this symposium will certainly lead to further cooperation between the Chinese scientists and international counterparts and greatly contribute to our science. During the meeting, Prof. S. Sun, Director of the Academic Committee of the Laboratory of Lithosphere Tectonic Evolution, extended enormous effort to get all the things well arranged. Prof. X.H. Liu, Director of the Laboratory of Lithosphere Tectonic Evolution, hardly recovered from the 4-month expedition to Antarctica, led all the staff in the laboratory to arrange the festschrift issue. I am sincerely grateful to my colleagues and friends for their various kinds of help, the number of which is too enormous to mention here. All contributors are thanked and I am greatly indebted to Prof. E. Moores of University of California at Davis for his time and advice in the monograph issue. I am sincerely similarly indebted to Prof. F. Heller for his generosity with his time, ideas, and support while I was in Zfirich. Wen-Jiao Xiao

Revised December 2000, Qijiahuozi, Beijing June, 2000, Adlisberg, Zfirich

CONTENTS

Foreword ........................................................................................ Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

v 1

Shu Sun Part I. Tectonics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.

The p a r a d o x of geosyncline hypothesis and orogenic analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

5 7

Jiliang Li and Wenjiao Xiao 2.

Early Mesozoic collapse of the Late Paleozoic archipelago in South China . . . . . . . . . . . . . . . . . . .

15

Wenjiao Xiao, Shu Sun, Jiliang Li and Haihong Chen 3.

Mesozoic tectonics of Fujian, Eastern China . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

39

Zuyi Zhou and Zongting Liao 4.

S o n g p a n - G a r z e belt: fore-arc accretion or back-arc collapsing? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

55

Pan Guitang, Xu Qiang and Jiang Xinsheng 5.

R o t a t i o n a l collision and the T a n - L u transform fault . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

65

Wang Qingchen 6.

The p a r a d o x of an ocean-dipping Benioff zone: example from T a i w a n . . . . . . . . . . . . . . . . . . . . . . .

77

Chia- Yu Lu 7.

Thin-skinned plate tectonics: the re-invention of a crazy idea . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

101

Peter Finckh and Kenneth J. Hsu" 8.

Oil in 'holes-in-the-continent' (relict oceanic basins) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

113

Sergey Aplonov 9.

Structural rocks: stratigraphic implications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

131

A. M. C. ~eng6r and Mehmet Sakm,c Part II. Mechanics of Overthrusting and Landsliding . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

229

10.

231

Rock mechanics and the p a r a d o x of overthrusting tectonics . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Ueli Briegel 11.

The flow of giant rock landslides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

245

Christopher R. J. Kilburn Part III. Petrology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

267

12.

269

Concepts that led to mechanisms of movement of mass and energy in earth . . . . . . . . . . . . . . . . .

F. W. Dickson 13.

Crystallization histories of alkaline intrusions in West Texas: some paradoxes and answers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

277

Max F. Carman, Jr. and Stanley Indest 14.

A paradox: the ultramafic rock belt ( U M R B ) as a m e t a m o r p h o s e d tectonic m6lange in northern Dabie M o u n t a i n s , eastern central China . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

323

Xu Shutong, Liu Yican and Wu Weiping 15.

N d isotopes vs. Ken Hsfi's tectonic facies: a case study of crust evolution in South China . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

343

Xin-Hua Zhou Part IV. Paleogeography, Paleoceanography and Paleoclimatology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

351

16.

353

The Messinian salinity crisis in the M e d i t e r r a n e a n . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Maria Bianca Cita

Contents

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17.

A review of the catastrophic extinction at the end of the Cretaceous and its scientific implication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

361

He Qixiang 18.

The oldest biogenous pelagic sediments above Mesozoic oceanic basement: a review . . . . . . . . .

373

Edward L. Winterer 19.

Unveiling secrets locked in G r e a t G h o s t Lake in T a i w a n . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

387

Chen-Tung Arthur Chen Part V. Applied and Basic Sciences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

411

20.

413

At a turning point in petroleum geology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Stefan M. Luthi 21.

Exploitation of lithium in brines by Hsfi's m e t h o d . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

421

Chen Yancheng and Kenneth J. Hsu" 22.

Preliminary report on experiments to m o n i t o r C-14 beta-decay . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

429

Kenneth J. HsfL Min-Pen Chen and Chao-Kai Huang Part VI. Postscript . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

437

23.

439

The career of a frustrated conformist . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

Kenneth J. Hsu" A u t h o r index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

453

Subject index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Paradoxes in Geology Ueli Briegel and Wenjiao Xiao, eds. 9 2001 Elsevier Science B.V. All rights reserved.

INTRODUCTION Shu Sun Laboratory of Lithospheric Tectonic Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, P.R. China

Good morning, Professor and Mrs. Hsti, Ladies and Gentlemen, friends and colleagues: It is a great privilege for me to be here to deliver a speech to congratulate Professor Kenneth Jinghwa Hsti on his 70th birthday. Prof. Hsfi (or Ken, as he prefers) is one of the greatest theorists of our time in geoscience. The late Prof. S. O. Schlanger of the Northwestern University of the United States, said in 1989, on the occasion of his 60th birthday: "We all will agree that the world of geology would have been a much duller one without the enlivening presence of Ken on the science." Ken has not been sitting during the last decade. As a Chinese geologist and a friend of Ken, I am proud of Ken's outstanding scientific achievements. Ken was born in Nanking, China on July 1, 1929. He had his B.Sc. from the National Central University, Nanking, China in 1948, M.A. from the Ohio State University, Columbus, Ohio in 1950, and Ph.D. from U.C.L.A. in 1953. Ken was a research geologist of the Shell Development Company, 1954-63. He taught then in US universities until 1967 when he was called to the Chair of Experimental Geology at the Swiss Federal Institute of Technology (ETH), Ztirich. He stayed there until his retirement in October 1994. At ETH he established several laboratories: of rock mechanics, stable-isotope geochemistry, physical sedimentology, and quaternary geology. He has trained several dozens of doctoral candidates, and numerous students of his are now professors in well known international institutions. Ken has been active in professional societies and international cooperative research programs. He was the Editor-in-Chief (1973-78) and the President (197882) of the International Association of Sedimentologists. He served in various committees and panels of the JOIDES (Joint Oceanographical Institutions of Deep Earth Sampling), and participated in five deep-sea drilling cruises to the South Atlantic Ocean and to the Mediterranean Sea. He was the Chairman of the International Commission on Marine Geology (1980-1988), and he has represented the International Union of Geological Sciences at the ICSU's (International Council of Scientific Unions) International Geosphere and Biosphere Program since 1988. Ken has contributed greatly, during the last four decades of his professional career, to the revolution in earth science, and his work has established new fundamental principles of geology and geological oceanography. Ken's work during the 1960s on the tectonic m61anges of the California Coast Ranges has provided a key for the understanding of the origin of mountains, and an

2

S. Sun

independent verification of the theory of seafloor spreading which was to revolutionize the earth science. The theory by Ken and his colleagues during the 1970s on the desiccation of the Mediterranean has led to the understanding of the genesis of giant salt deposits of the world. The discovery also provided the key to interpret the evolution and dispersal of animals and of plants in Europe, western Asia and Africa during the last several million years. The theory has furthermore led to a new working philosophy in geology called actualistic catastrophism. Ken's research during the 1980s on the catastrophic biologic extinction at the end of the Mesozoic Era 65 million years ago has inspired him to formulate a radically new interpretation of the history of life on Earth. The implication of the discovery is a slogan survival of the luckiest, instead of the Darwinian survival of the fittest. Ken has been working during the 1990s on the physics of time. Finally, I might add that Ken has been considered a modern man of Renaissance. He has been searching for a common language between science and arts, and published several articles on the fractal geometry of music. Unlike those productive scientists who devote all their energy to scientific research, Ken has never lost sight of the fact that his aim in doing science is for the betterment of mankind. Ever since 1977, he has devoted a large share of his research efforts to promote the geology of the land of his birth, China. He has invited numerous Chinese scientists to his institution for advanced training, and he has worked in the field in China almost every summer, even under very difficult rugged mountainous conditions, during the last 20 years. His new tectonic facies map of China was published recently and represents a completely original interpretation of the geology of China. Furthermore, he has been developing a new process of potash, magnesium and lithium exploitation from lake brines to provide much needed chemicals for the reconstruction of China and other developing countries. Ken served in various scientific organizations to promote the advancement of science, and he has devoted much effort toward international cooperation and world peace. While serving on a United Nations expert panel, he presented key arguments to support a policy to suspend the dumping of nuclear wastes onto seabeds, and he has been active in calling public attention to the immorality of governmental policies to export nuclear wastes from industrial to developing countries. His interpretation of biologic evolution has led to a conclusion that the history of life is exemplified by co-evolution and symbiosis, there is little scientific evidence to support the assumption by social Darwinism that the essence of life is ruthless competition among hostile groups. Recognizing his scientific contributions, Ken has been honored by numerous international organizations and academies of sciences. The most notable one is the Wollaston Medal of the Geological Society of London, the oldest and the most prestigious prize in geology. He is an honorary fellow of numerous professional societies and a foreign associate of several academies, among those the U.S. National Academy of Sciences, the Third World Academy of Sciences, and Academia Sinica located at Taipei, China. Our meeting today will be aimed at the discussion on paradoxes in modern geology. Paradox, as defined in a dictionary, is statement contrary to received opinion. As Sir Karl Popper taught us, the progress of science consists of

Introduction

3

falsification of received opinions. Paradox is thus a start towards progress. Ken is one of the most creative scientists. Thanks to his experiences in three different countries, China, US, and Switzerland, he learned that there are no universally received opinions. The experiences gave him the opportunity to often make statements contrary to received opinions. When those statements were verified by research, he could publish his opinions. His some 400 articles and 20 books represent the fruits of those statements contrary to received opinions. The very first paradox was his origin of geosyncline. The received opinions gave a special significance to a mythical concept called geosyncline. He stated, contrary to those opinions, that geosynclines are simple regions underlain by thin Earth's crust. The second paradox in his career was his work at Ventura. The received opinion at the time, based upon the misconception of the nature of the Grand Banks Event, was that turbidite was a blanket of sheet sand. The Ventura data gave him the indication that turbidite was shoe-string sand. It was contrary to received opinions, his report was given to the Shell consultant Ph. Kuenen for an expertise. Kuenen recognized the merit and published immediately his famous paper on Longitudinal Transport of Turbidity Currents, Spring 1956. The third paradox was his statement on the origin of dolomite. The received opinion at that time was dolomitization by seepage reflux. Brines circulate downward through the sabkha. His contrary opinion was evaporative pumping. Dolomitizing brines do not seep down, they are pumped up through the sabkha sediments. The fourth paradox was his work on thermal isostasy. The received opinion was Airy hypothesis, based upon an erroneous concept that the upper mantle has no strength and the Earth's crust floats on the mantle like icebergs on water. He stated, contrary to received opinion, that upper mantle has finite strength, and the isostatic equilibrium related to basin-subsidence is caused by mantle-density changes. The fifth paradox was his work on Franciscan. The five principles of m61anges are all paradoxical, contrary to received opinion based upon the three laws of stratigraphy. The sixth paradox was his work on overthrusting tectonics. The received opinion at that time, thanks to Hubbert and Rubey, 1959, was gravity sliding. He pointed out that they were wrong because they ignored the role of cohesive strength. The door was open for plate-tectonic interpretations. The seventh paradox was his work on landsliding. The received opinion, thanks to Ron Shreve, was that land slides. His paradox was that landslide debris flows like a thixotropic liquid. The eighth paradox was his work on saline giants. The received opinion, thanks to Lyell and Bischopf, was that giant evaporites were formed by accretion under u n i f o r m i t a r i a n conditions. His statement, based upon the discovery of Mediterranean evaporite, is the creation of saline giants by catastrophic event. The ninth paradox was his theory of Messinian desiccation. The received opinion, based upon the extensive work by the French in the Med, postulated evaporitedeposition on shallow sea-bottom, and subsequent subsidence. He came up with the outrageous idea of deep desiccated basin.

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S. Sun

The tenth paradox was his work on thin-skinned plate tectonics. The received opinion, based upon earthquake observation of Circum-Pacific Benioff zones, was that orogenesis results from plate displacement. His statement, based upon geophysical data in the Alps, postulates thin-skinned plate-tectonics, with a detachment horizon at Moho. The eleventh paradox was his work on the tectonics of South China. He came out with the outrageous hypothesis of "Huanan Alps, not South China Platforms". The twelfth paradox was his K/T boundary. The received opinion was catastrophism and gradual evolutionary change, even across the K/T boundary, because of natural selection. The K/T data indicate survival of the luckiest in a catastrophe. The thirteenth paradox was the Strangelove Ocean. The received opinion was the Darkness at Noon or Impact Winter scenario of Alvarez et al. He stated instead that extinction did not result from killing, but from failure to reproduce under stressed environment. The fourteenth paradox was his archipelago model of orogenesis. The received opinion was mountain-building because of plate-collision. He stated instead that mountain-building took place in plate-interior because of backarc basin collapses. The fifteenth paradox was to extract lithium from Qaidam brines. The received opinion was that traces of lithium in the brines could not be extracted by evaporation. He stated instead that lithium could be extracted after enrichment through the concurrent precipitation of magnesium chloride and evaporation of water. The sixteenth paradox was that global warming may be a blessing to mankind. The received opinion emphasizes the story of greenhouse catastrophe. He pointed out the historical evidence that little ice ages were the time of climatic catastrophe. Ken has made other paradoxical statements, including his patented process for EOR, his question on natural radioactivity, and so on. Due to the limitation of time, I cannot say more about them. You can see, what listed above gives a general idea that he contributed to the progress of science through the falsification of established opinions. He has made major contributions in almost every specialized discipline of geology. A number of his famous friends and his students will give papers in this symposium to define Ken's role in advancing the earth science during the late half century. In 1985, Prof. Huang Ji-qing, an outstanding pioneer of earth science and a famous academician in China, said: "Prof. Hsfi is well known in the international geological circle for his erudition, nimble thinking, original ideas, and deep understanding." Prof. Huang emphasized: "As a Chinese geoscientist of the old generation, I am very proud of his great achievements." Today, we see again his shining footprints in the above-mentioned paradoxes, when we come here, the great China where Ken was born, to celebrate Ken's 70th birthday. Prof. Hsti and Mrs. Hsfi, please receive my sincere blessing with a famous Chinese proverb, "Happiness as plentiful as water in the East Sea and lives as long as pine in the South Mountain. That's all for my talk. Thank you all very much.

Part I

TECTONICS

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Paradoxes in Geology Ueli Briegel and Wenjiao Xiao, eds. 9 2001 Elsevier Science B.V. All rights reserved. Chapter 1

THE P A R A D O X OF G E O S Y N C L I N E H Y P O T H E S I S AND OROGENIC ANALYSIS

Jiliang Li* and Wenjiao Xiao Laboratory of Lithospheric Tectonic Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, P.R. China

ABSTRACT The main hypothesis that dominated the geological domain before plate tectonics was geosyncline. The geosyncline concept, however, put forward a hundred-year puzzle to geologists. The hypothesis of global cratonization is a misconception both in philosophy and science, and the geosyncline hypothesis left a legacy of history to us. The plate tectonics theory replaced the geosyncline hypothesis, impelled and is impelling geoscience. If geologists had given a correct definition to geosynclines, this term would have been useful in describing rock associations of folded mountain ranges. The method showing tectonic environments with rock associations is useful. The method became the cornerstone of tectonic facies analysis (Hsfi, 1991, 1995; Hsfi et al., 1999; Li, 1992a, 1992b; Robertson, 1992). The tectonic facies analysis can be used to classify tectonic units in orogenic belts and to compile tectonic maps (Hsfi et al., 1999). INTRODUCTION Plate tectonics has given us a new framework in which orogenies can be investigated. Orogenic belts have been widely accepted as an integral part of the results of convergent plate motion (e.g. Seng6r, 1990). However, the main hypothesis that dominated geological thinking before plate tectonics was geosyncline. About 30 years ago, Ken Hsti wrote a paper entitled "The Odyssey of Geosyncline" (1973) for the Pettijohn Symposium, in which the concept, activity, cycle, subsidence and magmatism of geosynclines were reviewed. At the end of the paper, Hsfi concluded that if geologists gave a correct definition to geosynclines, this term would be useful in describing rock associations of folded mountain ranges. Thirty years passed. My teacher Ken Hsfi tells me to write a paper " The paradox of Geosyncline". I (Wenjiao Xiao) simply could not figure out his intention. Today, the term geosyncline has been eliminated from geological textbooks. Many students may not know what the real meaning of geosyncline is. Then, what is the purpose of

* Corresponding author: fax: +86-10-62010846; E-mail: [email protected]

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my paper? I think the following reasons should be the starting point of the paper. The geosyncline hypothesis is the first approach to orogens, in terms of which many geologists thought all their life (Hsfi, 1972; Seng6r, 1990). Although the hypothesis has become historical already, we must give it fair appraisal. Secondly, it is necessary to let students understand why a tectonic hypothesis that ruled geological thinking for a hundred years died away so quickly. Thirdly, there are still a number of geologists using terms such as geosynclinal area, orogenic movement phases, folded mountain belts, geanticlines, etc. We must tell them why the plate tectonic theory is better than the geosyncline hypothesis in explaining mountain building and why the former replaced the latter. I hope this paper can answer these questions.

AMBIGUITY AND CONFUSION OF CONCEPTS In James Hall's paper (1859), there are three main points. Firstly, there are great thick Paleozoic strata along the Atlantic border of North America. Secondly, thick sedimentary strata caused deep subsidence. Thirdly, vertical uplift formed folded mountain ranges. James Dana (1866, 1873) complemented and developed Hall's thought, and advocated that geosynclinal subsidence and folded mountain were produced by compression induced by crustal contraction. According to the studies in recent years (Hatcher, 1972, 1989), neither Hall's loading-subsidence idea nor Dana's compression-induced depression can explain the origin of a series of sedimentary sequences on a passive continental margin. At present, it is common knowledge that a great thickness of sedimentary sequence is deposited on a passive continental margin, which originated in an extensional tectonic environment. The extensional subsidence is related to the upwelling of abnormal mantle and thinning of the crusts. In the nineteenth century, the mechanism of subsidence and sedimentation of passive continental margin was unknown to the geological community. So Hall and Dana's misconceptions were hard to avoid. Dana (1873) had known that Hall's vertical uplift of mountains was incorrect. He put forward a new mechanism of horizontal compression produced by a contracting Earth. Such a mechanism was undoubtedly an improvement. The compression produced by contraction of the Earth, however, would be symmetrical, which contradicts the observations of the interior structure of actual mountain belts. Griggs (1939) conducted a series of experiments to simulate mountain-building processes. The experimental results were similar to actual asymmetrical mountain chains, suggesting that the convection mechanism is more reasonable than Dana's contraction. Hall's (1859) concept of geosyncline puts forward a hundred-year puzzle to geologists. Many geologists tried to find out the answer to what a geosyncline is and where an actual one would be found. Wang (1972) summarized the genetic series and physiographic equivalents of geosynclines (Table 1). It is clear that geosynclines could form in various stress fields and multifarious physiographic environments. In logic, if a concept has unlimited extensions, it would lose the necessary condition of independent existence. Such an ambiguous concept is eventually replaced by a new, clear and definite concept.

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Table 1 Genetic series and physiographic equivalents of geosynclines (after Wang, 1972) Geotectonic units

Physiographic equivalents

Tensile open series Hedreocraton and parageosynclines Platform Extensional miogeosynclinal ridge and trough Extensional eugeosynclinal ridge and trough Tiefcraton

Continent Continent Terrace Continental rise Ocean basin (abyssal plain and seamount complex)

Compressional Tiefcraton Compressional Compressional Compressional

Ocean basin and trench Sedimentary outer arc Inter deep Volcanic inner arc

closed series (distal to craton) miogeosynclinal ridge eugeosynclinal furrow eugeosynclinal ridge

Compressional closed series (Proximal to craton) Miogeosynclinal furrows Exogeosyncline Molasse basin Platform Hedreocraton

Idiogeosyncline Idiogeosyncline Shelf or epicontinental seas Continent Continent

ROCK ASSOCIATION AND GEOSYNCLINE During the development of the geosyncline hypothesis, a number of geologists, such as Hall (1859), Dana (1866, 1873), Haug (1900), Kay (1951), Bertrand (1887), Arbenz (1919), Krynine (1941), Pettijohn (1957) and Wang (1972) elucidated activities of geosynclines in different space and time settings. They put forward a series of sedimentary associations to reflect these activities (tectonic cycle). Hsti (1972) summarized the evolution of the thought of geosynclinal cycles (Fig. 1). Wang (1972) further developed the concept of geotectonic cycles and included igneous activities in these cycles (Table 2). The method of showing tectonic environments with rock associations is useful. This became the cornerstone of the tectonic facies analysis (Hsfi, 1990, 1995; Hsti et al., 1999; Li, 1992a, 1992b). The tectonic facies analysis can be used to classify the tectonic units in orogenic belts and to compile tectonic maps (Hsfi et al., 1999). Therefore, the geosyncline hypothesis has left a legacy to us. A PHILOSOPHIC QUESTION OF THE GEOSYNCLINE HYPOTHESIS The geosyncline hypothesis suggested that a mobile belt such as geosyncline tends to stabilize through orogeny, the so-called cratonization. Mobile is the starting point of a geosyncline and craton is its terminal point. Kober (1923) called craton of continents hochkraton (high craton) and that of oceans tiefkraton. Obviously, he thought the whole world's ocean was a craton. The geosyncline hypothesis considered many mobile belts in the beginning and with the accretion of continents,

J. Li and W. Xiao

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Fig. 1. Compilation of different views of geosynclinal cycles from Hsfi (1972).

cratons gradually grew up until the whole globe became a craton. This is compatible to the hypothesis of heat death that the entropy gradually increases till the earth becomes a cold planet. This hypothesis is erroneous. Firstly, oceans in the Earth are newborn basins with the newest basaltic floor (Hess, 1962; Vine and Mathews, 1963) and neotectonic activities (Hess, 1962). In continents, ancient orogenic belts may be active again. For example, Tienshan is a Permian orogen and Kunlun is a Triassic one, however, in Cenozoic there are strong neotectonic activities including high uplift, folding and thrusting with earthquakes. Even in the North China Craton, there are also neotectonic activities including transpressional and transextensional faulting, uplifts and subsidence, in connection with earthquakes. The facts mentioned above indicate that the hypothesis of global cratonization is a misconception both in philosophy and science. GEOSYNCLINE AND PLATE TECTONICS Most founders of plate tectonics were influenced by the geosyncline hypothesis. They tended to safeguard and prolong the hypothesis. In 1972, the Society of Economic Paleontologists and Mineralogists convened a symposium on Modern and Ancient Geosynclinal Sedimentation (see Datt and Shawer, 1974), aimed at searching where the actual geosyncline is and what its sedimentary characteristics are.

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Table 2 Sedimentation and igneous activities in a complete tectonic circle Tectonic units Tensile open series Hedreocraton Platform Miogeosyncline Eugeosyncline Tiefcraton

Sedimentation (oceanward prograding sedimentation)

Igneous activities

Thin quartzite-limestone suite

None

Quartzite-limestone and feldspathic sandstone-subgraywacke Quartzwacke turbidite and pelagic sediments (apron) Pelagic sediments

Generally none

Compressional closed series Sedimentation (continentward Tectonic units prograding flysch sedimentation) Tiefcraton and trench Pelagic and cannibalized turbidites Eugeosynclinal ridge Erosion or neritic carbonate sedimentation (phosphate etc.) Eugeosynclinal furrow Cannibalized flysch sedimentation upon apron Miogeosynclinal ridge Erosion or neritic carbonate sedimentation Miogeosynclinal furrow Flysch emplacement upon quartzitelimestone and subgraywacke sequence Exogeosyncline "Clastic wedge" to molasse type sediments on quartzite-limestone suite Molasse basin Molasse sedimentation Platform and craton

With or without oceanic tholeiitic lava Ophiolite Igneous activities Ophiolite Ophiolite Ophiolite and other volcanics Andesitic volcanics Generally none Plutonism in orthogeosyncline area Plateau basalt flow

At that time, several years after the plate tectonic model was formulated, there were a b u n d a n t data about various tectonic environments such as mid-ocean ridge, passive continental margin, rift valley, island arc, back arc basin, fore arc basin, subduction complex (m61ange zone), trench, oceanic basin, oceanic island arc, etc. Most participants argued that sedimentary prisms correspond to miogeosyncline, but nobody could find an actualistic setting that neared or paralleled the eugeosyncline. In the geosyncline hypothesis there was also no place for a m61ange zone. These deficiencies suggested that the geosyncline hypothesis could not explain reasonably the orogeny and orogenic belts. Therefore, Datt (1974) asked: "what is likely to be the fate of the geosyncline in the next 20 years?" He guessed there would be two probable answers: "Some would argue that the entire concept is dead or dying and that the term itself should be put quietly to rest in the archives of science history. Others, however, have sought to rationalize every detail of the old geosynclinal concepts with the new global tectonics." Although most geologists have sought to rationalize geosynclinal concepts, these concepts died in 1980s. Science is inexorable, and newly emerging ideas eventually replace the old ones. The plate tectonics theory replaced the geosyncline hypothesis, which has impelled and is continuously impelling geoscience to develop led by mobilism.

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H o w e v e r , the h u n d r e d - y e a r history o f the geosyncline was the p e r i o d in which all the v a r i o u s b r a n c h e s of geology d e v e l o p e d a n d r e a c h e d m a t u r i t y . T h e a b u n d a n t scientific legacy left by the d e a d geosyncline h y p o t h e s i s will be a s t r o n g s u p p o r t to the d e v e l o p m e n t of the new global tectonics. Therefore, geologists s h o u l d explore this scientific treasure to use to f u r t h e r the a c h i e v e m e n t o f plate tectonics. ACKNOWLEDGEMENTS T h e a u t h o r s w o u l d like to t h a n k Profs. K. J. Hsfi a n d S. Sun for s u p p o r t o f this w o r k . This p a p e r has benefited significantly f r o m Dr. Ueli Briegel's insightful c o m m e n t s on, a n d suggestions for, an early version o f the m a n u s c r i p t . M u c h of the w o r k p r e s e n t e d here has been financially s u p p o r t e d by v a r i o u s grants of the N a t i o n a l N a t u r a l Science F o u n d a t i o n of C h i n a (49672148) a n d the K e y P r o g r a m s of M i n i s t r y of Science a n d T e c h n o l o g y o f C h i n a (915-96-06-03, 915-96-07). REFERENCES Arbenz, P., 1919. Probleme der Sedimentation und iher Bezichungen zur Gebirgsbidung in der Alpen. Naturf. Gesell. Zurich, 64: 246-275. Bertrand, M., 1887. la Chaine des Alpeset la formation du continent european. Soc. Geol. France Bull., 15: 423-447. Dana, J.D., 1866. Observations on the origin of some of the earth's features. Am. J.S.i., Ser. 2, 42: 252-253. Dana, J.D., 1873. On the origin of mountains. Am. J.S.i., Ser. 3, 5: 347-350. Datt, R.H. Jr., 1974. The geosyncline concept. Soc. Econ. Paleontol. Mineral. Spec. Pub., 19: 1-13. Datt, R.H. Jr. and Shawer, R.H. (Eds.), 1974, Modern and Ancient Geosynclinal Sedimentation. Soc. Econ. Paleontol. Mineral. Spec. Pub., 19: 380pp. Griggs, D., 1939. A theory of mountain-buildiing. Am. J. Sci., 237: 641-646. Hall, J., 1859. Description and figures of the Organic Remains of the Lower Heldberg Group and the Oriskany Sandstone, Natural History of New york, Vol. 3, Van Benthsen, New York, 532pp. Hatcher, R.D. Jr., 1972. Development model for the southern Appalachians. Geol. Soc. Am. Bull., 83: 2735-2760. Hatcher, R.D. Jr., 1989. Tectonic synthesis of the US Appalachians. Chapter 14, In: Hatcher, R.D. Jr., Thomas, W.A., and Viele, G.W. (Eds.), The Appalachian-Quachita Orogen in the United States. The Geological Society of America, F-2: 511-535. Haug, E., 1900. Les Geosynclinascx et les aires continentales. Soc. Geol. France Bull., 28: 617-711. Hess, H.H., 1962. History of oceanic basins. Petrological studies: A volume in honor of A.F. Buddington. The Geological Society of America, 599-620. Hsfi, K.J., 1972. The concept of geosyncline, yesterday and today. Trans. Leicester Lit. Phil. Soc., 66: 26-48. Hsfi, K.J., 1973. The odyssey of geosyncline. In: Ginsburg, R.N. (Ed.), Evolving Concepts in Sedimentology, Johns Hopkins University Press, Baltimore, 66-92. Hsfi, K.J., 1991. The concept of tectonic facies. Bull. Tech. Univ. Istanbul, Istanbul, 44: 25-42. Hsfi, K.J., 1995. The Geology of Switzerland and an Introduction to Tectonic Facies. Princeton University Press, Princeton, 250pp. Hsfi, K.J., Sun Shu, Wang Qinchen, Chen Haihong and Li Jiliang, 1999. Tectonic Facies Map of China, Science Press, Beijing, 155pp. Kay, G.M., 1951. North American geosynclines. Geol. Soc. Am. Mem., 12: 143pp. Kober, L., 1923. Lehrbuch der Geologie, Holder-Ichler-Tempsky, Vienna, 425pp. Krynine, P.D., 1941. Paleogeographic and tectonic significance of sedimentary quartzites. Geol. Soc. Am. Bull., 52: 1915-1916.

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Krynine, P.D., 1941. Paleogeographic and tectonic significance of sedimentary graywackes. Geol. Soc. Am. Bull., 52:1917. Krynine, P.D., 1941. Paleogeographic and tectonic significance of arkoses. Geol. Soc. Am. Bull., 52:19181919.

Li, J.L., 1992a, On the tectonic facies in collision orogenic belts. In: Li, Q.B. et al. (Eds.), Modern Geological Science Monograph (I), Nanjing University Press, Nanjing, 9-22. Li, J.L., 1992b, The fundamental problems of the tectonics in southeastern China (in Chinese). In: Li, J.L. (Ed.), Structure and Tectonic Evolution of Lithosphere in Southeastern China, China Science and Technology Press, Beijing, 1-11. Li, J.L., Sun, S., Hao, J., Chen, H.H., Hou, Q.L. and Xiao, W.J., 1999. On the classification of collision orogenic belts. Scientia Geologica Sinica, 34(2): 129-138. Pettijohn, F.J., 1957. Sedimentary Rocks, New York, Harper, 718pp. Seng6r, A.M.C., 1990. Plate tectonics and orogenic research after 25 years: a Tethyan perspective. EarthSci. Rev., 27: 1-201. Vine, F.J. and Mathews, D.H., 1963. Magnetic anomalies over oceanic ridges. Nature, 199: 947-949. Wang, C.S., 1972. Geosynclines in the new global tectonics. Geol. Soc. Am. Bull., 83: 2105-2110.

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Paradoxes in Geology Ueli Briegel and Wenjiao Xiao, eds. 9 2001 Elsevier Science B.V. All rights reserved. Chapter 2

EARLY M E S O Z O I C COLLAPSE OF THE LATE P A L E O Z O I C ARCHIPELAGO IN S O U T H CHINA

Wenjiao Xiao*, Shu Sun, Jiliang Li and Haihong Chen Laboratory of Lithospheric Tectonic Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, P.R. China

ABSTRACT The significance of the Permian-Early Triassic tectono-paleogeography of South China is reviewed in the light of data concerning the amalgamation of multiple blocks and the inter-block basins. The Yangtze microcontinent, Simao, Lincang, Jiangnan, and Cathaysian remnant arcs, together with the various kinds of geological terranes, were sparsely distributed along the southern active margin of the Eurasian continent in Pz-T1, which is very similar to the present SE Asia archipelago. The front arc of the South China archipelago has been preferred to the southern parts of the Lincang and Cathaysian arcs. South to the front arc are the Central Burma, Shan-Thai-Malay and Hainan blocks. After the collapse of the inter-block basins and final amalgamation of these South China blocks in the Middle Triassic, these blocks were linked by m61ange zones and fold-thrust belts (FTBs) with the main deformation time duration of Tz-J. The Changning-Menglian m~lange and the Coastal Fujian m61ange marked the collisional orogeny between Gondwana and Eurasia in the western and eastern parts of South China, respectively. The early Mesozoic multi-block collage process of South China gave rise to the Mesozoic continental growth of southern Eurasia. INTRODUCTION The expression of South China is used in a broad sense, referring to the tectonic realm bordered on the northwest by the Songpan-Gantze deformed belt; on the north by the Sino-Korean block; on the west by the Tibetan Plateau; on the south by the Dongnanya block (Fig. 1). South China has emerged as an ideal place to study the processes of continental collision and evolution of continental collages, offering numerous continental and oceanic fragments stitched together by continental collisions that are located in the huge orogenic collages between the Gondwana and Eurasian continents (Seng6r, 1985, 1990; Sun et al., 1991; Hsfi and Chen, 1999).

*Corresponding author. Fax: + 86-10-62010846; E-mail: [email protected]

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Fig. 1. Tectonic map of China showing the location of Figures 2, 8 and 12 (Modified after Hsfi et al., 1990; Sun et al., 1991).

This paper discusses some of the interaction between the various kinds of fragments by temporal-spatial analysis of the foreland fold-and-thrust belts (FTBs), together with some other geological and geophysical data. The geology of South China is to be reinterpreted in the light of data concerning the multiple forearc accretion/backarc basin collapse processes along the southern active margin of the Eurasian continent. The aim is to clarify the roles of forearc growing and backarc collapsing in Mesozoic orogeny, with some examples, rather than considering in detail the tectonics of South China. To achieve the goal, attempts of structural/ sedimentologic analyses of the Permian to the Early Mesozoic tectono-sedimentary processes in this area have been made, including the orogenic analyses of the DabieTanlu-Sulu orogenic collage, the Western Yunnan orogenic collage, and the SE China orogenic collage. D A B I E - T A N L U - S U L U OROGENIC COLLAGE The Dabie-Tanlu-Sulu orogenic collage (Fig. 2) marks the northern margin of South China, separating the North China block to the north from the South China

Early Mesozoic collapse of the Late Paleozoic archipelago in South China

17

Fig. 2. Tectonic map of Dabie-Tanlu-Sulu orogenic collage showing the location of Figures 3, 4, 5 and 6 (modified after Xiao, 1997).

block to the south, forming one of the world's most accessible transpressional boundaries (e.g. Mattauer et al., 1985; Peltzer et al., 1985; Hsfi et al., 1987; Wang et al., 1996; Zhang, 1997). The geology of the Dabie Mountains has been controversial since the last century (Seng6r, 1985) and the establishment of the age of the main deformation has been no easy matter. Based on the division of five longitudinal zones characterized by distinct rock assemblages and separated by major steep faults, Mattauer et al. (1985) interpreted the Dabie Mountains as a "Caledonian" collision belt with "Hercynian" and Triassic intracontinental deformation, whilst the extensive investigations on the Ultra-High-Pressure (UHP) rocks in the Dabie Mountains and the paleomagnetic studies have suggested a orogenic belt of Early Mesozoic age. Recently, Meng and Zhang (1999) tried to reconcile the controversy about the time of collision of the North and South blocks by proposing a Paleozoic collision belt and a Triassic one which were located in the northern and southern parts of the Dabie mountains, respectively. From a geodynamic point of view, however, during the last two decades, opinions on the collage style of the South China block to the North China block have changed from the rotation to the indentation model. Paleomagnetic and geological data have demonstrated a scissor suturing between the North China block and the Yangtze block in T3-J (Zhao and Coe, 1987), forming a continent-island arc-microcontinent

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collisional collage (Zhai et al., 1994; Xue et al., 1997). Yin and Nie (1993) proposed that the collision as accomplished by indentation of the South China block into the North China block involved no significant rotation (Yin and Nie, 1996). While we agree with Yin and Nie (1996) about the role of indentation process in the genesis of the tectonic scenario of the North and South China amalgamation, we are at the same time aware of the structural and paleomagnetic data which are highly in agreement with the rotation model (Zhao and Coe, 1987; Enkin et al., 1992; Xiao, 1997; Zhu et al., 1998; Meng and Zhang, 1999). Instead of the non-rotation model, the evidence both in geology and geophysics indicates a rotation-indentation model (Gilder et al., 1999; Meng and Zhang, 1999). Intracontinental deformation was widespread both in the North China block and the South China block during the collision between these two major blocks in China (Fig. 2). The Huangshi-Wuhan foreland FTB is located to the south of the Dabie orogenic collage (Figs. 3 and 4). Paleomagnetic data suggest that the North China block and the South China block collided first in the Permian near the eastern part of the North China block, then they progressively scissored together as the South China block rotated clockwise with respect to the North China block (Zhao and Coe, 1987; Enkin et al., 1992; Xiao, 1997; Zhu et al., 1998; Gilder et al., 1999). Tanlu fault and its relationship to the D a b i e - T a n l u - S u l u orogenic collage

The UHP metamorphic rocks outcropping in Dabie and Sulu regions suggest that the Tanlu fault has had transcurrent movement (Chapter 5, this volume). Recent multidisciplinary investigations have revealed that the Tanlu fault zone serves as a special continental transform fault (Okay and Seng6r, 1992; Chapter 5, this volume). Also, the

Fig. 3. The N-S cross-sectionalfeatures of the Dabie Shan orogenicbelts (modifiedafter Wang and Cong, 1998). See text for discussion.

Early Mesozoic collapse of the Late Paleozoic archipelago in South China

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Fig. 4. Structural cross-section of Wuhan with zonation of structural styles (after Xiao, 1997).

Fig. 5. Structural cross-section of Caohu (after Xiao, 1997).

paleomagnetic data accessible for Tanlu fault (e.g. Xing et al., 1995; Gilder et al., 1999) show that the displacement is less than 700 km as stated in the previous studies (Xu et al., 1987). The sedimentary facies analysis suggested that the Tanlu is a special part of the Dabie-Sulu m61ange trending N E - S W (Zhang, 1997; Gilder et al., 1999). There was much debate about that also (Zhang, 1997; Xiao, 1997). Recent structural style analysis demonstrates that the foreland (FTBs) in the vicinities of the Tanlu fault, such as the Susong FTB (Figs. 3 and 5), are highly in accordance with the collisional event between the South China and the North China blocks. The Xu-Huai foreland FTB is located to the N W of the Tanlu fault, as shown in the following paragraphs. Thus, the Tanlu fault was a phenomenon of multiple structural mosaic in which the Early Mesozoic orogenic collage structures had been cut or overprinted by the later Mesozoic to Cenozoic strike-slip faulting.

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Xu-Huai foreland FTB According to our field work and the previous work in Xu-Huai (Xia and Huang, 1984; Xu et al., 1993; Xiao, 1997), this region is characterized by zonation from SEE to NWW: (1) the root zone which is composed of Precambrian metamorphic basement rocks; (2) the duplex and nappe zone which is composed of large-scale nappes and duplex structures; (3) the linear fold-thrust zone; and (4) foreland fold zone (Fig. 6). These observations have demonstrated that the tectonic vergence is NW-directed. The latest marine stratum involved in the thrusting is the Upper Permian (Fig. 7), implying that the foreland fold-thrust deformation took place after the Late Permian, according to the analysis of tectonic facies (Xiao, 1997). Although the Xu-Huai thrust belt was recently postulated as a lateral ramp of the main thrust of the Early Mesozoic orogeny (Xu et al., 1995) along which the strikeslip "Tanlu fault" occurred, its structural zonation and the latest marine stratum

Fig. 6. Zonation of the Xu-Huai foreland FTB (modified after Xia and Huang, 1984; Xu et al., 1995; Xiao, 1997).

Fig. 7. Cross-section of the Xu-Huai foreland FTB (modified after Xia and Huang, 1984; Zhang, 1997; Xu et al., 1995; Xiao, 1997).

Early Mesozoic collapse of the Late Paleozoic archipelago in South China

21

involved in the thrusting have demonstrated that the South China block collided with the North China block in the late stage of the Late Permian (Xiao, 1997). The tectonic zonation of the Xu-Huai foreland FTB recorded the collage history and was characterized by a thrusting roughly perpendicular to the original trend of the Dabie-Tanlu-Sulu orogen in the late stage of the Late Permian (Xiao, 1997). The reason for the present N E - S W trend is probably due to the large-scale rotation during the Mesozoic collage of the South China block to the North China block. WESTERN YUNNAN OROGENIC COLLAGE In the vast expanse of the mountainous Western Yunnan orogenic collage in the southwestern part of South China, the western Yunnan region, two major mountain belts are prominent from west to east: the Gaoligong Mountains and the Ailaoshan Mountains. These mountain belts have signatures of collision of continental blocks or remnant arcs such as the Yangtze microcontinental block, Simao, Baoshan (Shan-Thai-Malay), and the Tengchong block (Central Burma) as well, mostly during the early Mesozoic (Fig. 8). There are tectonic m61ange zones and foreland FTBs between these blocks. Investigations of these FTBs would be of significant importance in placing temporal-spatial constraints on the detailed scenario of South China and East Asia as well (Zhong, 1998).

Fig. 8. Simplified geological map of the Sijiang region in southwestern China (modified from Chen et al., 1996).

22

W. X i a o et al.

The Central Burma block west of the Gaoligong Mountains is composed of a metamorphosed basement, the "Gaoligong Group" of amphibolite facies metamorphism with a Middle Proterozoic age (Yunnan Bureau of Geology, 1990; Wopfner, 1996), and a late Paleozoic marine sedimentary cover with fossils, although these cover rocks were metamorphosed in most places and are scarcely cropping out. Most of the areas of the northern region are covered by Quaternary Tengchong volcanic rocks (Fig. 9). The Shan-Thai-Malay Block is bounded by a major thrust fault with the Gaoligong basement on the west and by the ChangningMenglian m61ange zone on the east (Chen and Xiao, 1999). The block is composed of thick successive marine sediments from the Sinian to Triassic. Upper Paleozoic glaciomarine deposits combined with the presence of cold-water faunas and Glossopteris have been reported in the region, suggesting a Gondwana origin (Liu et al., 1993; Wopfner, 1996). The Changning-Menglian m61ange zone, a main Paleotethys suture, is a metamorphic complex with chaotic blocks of dismembered ophiolite and carbonates of seamount origin in a sheared flysch matrix of greenschist or amphibolite facies metamorphism. The Lincang Arc terrane, east of the m61ange zone, is dominated by late Paleozoic and early Mesozoic granites intruding into the metamorphic basement of the arc (Liu et al., 1993; Chen and Xiao, 1999). East of the arc is the late Paleozoic Jinghong metamorphic complex, a m61ange zone between the arc and Simao block where the Paleozoic shallow marine carbonates are extensively covered by Mesozoic continental red beds (Fig. 10) (Chen and Xiao, 1999). Further east is the Mojiang metamorphic belt, a m61ange with dismembered ophiolite and chaotic blocks in extensively sheared matrix, overthrust by the Ailaoshan Precambrian basement of the Yangtze block (Fig. 11) (Chen and Xiao, 1999). GAOLIGONG SHAN

W

/r j ..t

"

E

~

e

l

t

Fig. 9. Structural cross-section of Gaoligongshan (after Chen and Xiao, 1999). LINCANG E _

Fold-thrust belt

M61ange

"~

Fold-thrust belt M61ange

Fig. 10. Structural cross-section of Lincang showing folding-thrusting deformation features (after Chen and Xiao, 1999).

Early Mesozoic collapse of the Late Paleozoic archipelago in South China

23

T E C T O N I C F R A M E W O R K OF SE C H I N A The southeastern part of the archipelago was an active margin since the Late Precambrian. There were the Yangtze microcontinent, Jiangnan arc and Cathaysian arc (Fig. 12). The Coastal Fujian m61ange and the Shilu m61ange in between the Cathaysian arc and the Hainan block to the southeast, a probable Gondwana block, marked the collisional orogeny between Gondwana and Eurasia. In Central South China east of the Yangtze folded belt lies a long mountain range, the Xuefeng Mountains. A Precambrian basement, the Banxi and Lengjiaxi Groups, or Sibao Group in southern extension of the mountain, is overthrust over the Yangtze deformed belt (Fig. 13) (Chen and Xiao, 1999). Between the two distinct tectonic provinces is a tectonic m61ange zone made of chaotic blocks and matrix (Chen et al., 1993b). Although many geochemists insist that SE China is characterized by a Precambrian orogenesis (Li et al., 1994; Li and McCulloch, 1996; Chen and Jahn, 1998), Jurassic red beds have been involved in the m61ange in

Fig. 11. Structural cross-section of Ailaoshan showing folding-thrusting deformation features (after Chen and Xiao, 1999).

Fig. 12. Tectonic map of SE China showing the major block pattern showing the location of Figures 13, 14 and 15 (modified after Chen et al., 1993a).

24

W. Xiao et al.

Fig. 13. Cross-section in the Xuefengshan area showing thrust and imbrication tectonics. some localities of southwestern Hunan suggesting overthrusting persisted at least into the late Jurassic. Paleozoic fossils have been found in sheared matrix formerly named Precambrian Banxi Group in the northeastern extension of the m61ange belt in Anhui and Jiangxi. South of the mountains are Permo-Triassic deep-water turbidites with radiolarites and basalts. Further east in Jinggangshan region and southern Jiangxi is a Paleozoic forearc accretionary complex with flysch broken formations and ophiolite blocks (Hsfi and Chen, 1999). Zhao et al. (1995) discovered the late Paleozoic radiolarian silicalite in the "Shuangqiao Shan Group", which is a block within the ophiolitic m61ange (He et al., 1999). He et al. (1996) also reported late Paleozoic fossils in rocks of the "Shuangqiao Shan Group" in the ophiolitic m61ange. He et al. (1999) proposed that the "Banxi Group" is not a Pre-Sinian metamorphic stratigraphic unit but a complex tectonic mosaic though the possibility of Precambrian components in Banxi complex exists (Hsfi and Chen, 1999). Zhang et al. (1997) reported island arc basalts they had found in South China. This suggests that the forearc accretionary mechanism played an important role in the tectonic evolution of South China (Seng6r and Okurogullari, 1991; Seng6r et al., 1993; Li, 1993; Li et al., 1993; Ma, 1998). East of the m61ange country is a Paleozoic arc terrane, the Wuyi-Yunkai Arc covered by late Paleozoic marine sediment as a passive margin cover of the Jiangnan Block. In the Nanpanjiang area of the western Guangxi Province, a thick and widely distributed sequence of Permo-Triassic pyroclastic turbidites is well exposed. Within the deformed flysch country, dismembered ophiolite blocks as well as giant pieces of carbonate platforms from the Devonian to Triassic have also been recognized. Lying northeast to Jiangxi and Zhejiang, is the NW Zhejiang FTB that recorded the temporal-spatial characteristics of the collage of the Jiangnan arc terrane versus the Yangtze block (Xiao et al., 1997). Another wide metamorphosed m61ange zone has been recognized east of the Wuyi Mountains with both early Paleozoic and early Mesozoic ages, demarcating the Jiangnan and Cathaysian blocks. The SW Fujian foreland FTB characterizes the inter-collage process between the Jiangnan and Cathaysian blocks (Hou et al., 1995; Li et al., 1996). NW Zhejiang FTB

Structures associated with this episode are represented in the Southern Margin of the Yangtze block by a system of northwest vergent contractional faults that root southeastward beneath the metamorphic and related plutonic rocks of the Chencai

Early Mesozoic collapse of the Late Paleozoic archipelago in South China

25

Complex. Structural styles can be zoned as follows based on assemblages of duplexes, imbricate fans and fold styles (Fig. 14). (1) Northwest Zone. Broad synclines separated by narrow anticlines dominate the structure in this zone. They are Jura Mountain-type folds (Xiao et al., 1997). (2) Central Zone. Large-scale imbricate fan. The major fold structures within this zone are large-amplitude folds, in which large-scale fault-bent folds can be observed. (3) Southeast Zone. Complex duplex and tight fold structures (Xiao et al., 1997). The closely spaced tight folds and stacked thrusts juxtapose the basement overlying the Paleozoics. The total shortening is estimated to be 40%-50%, and some multiduplexes show 60% shortening (Xiao et al., 1997). Intimately associated with these thrusts are synorogenic terrestrial foreland basins, such as the Upper Triassic, Jurassic and Lower Cretaceous basins. Within these basins, small-scale folding and reverse faulting can be observed, but their total shortening is less than 10% (Xiao, 1995). (4) Root Zone. The Root Zone lies structurally above the sedimentary sequences of the passive continental margin of the southern margin of the Yangtze block and encompasses a wide variety of fault-bounded tectonic assemblages ranging in age from Late Proterozoic or Paleozoic to Early Mesozoic (Kong et al., 1995). The metamorphic rocks of the Chencai Complex, including metavolcanic and metasedimentary rocks, experienced multiple stages of overturn folding and northwestward ductile thrusting. As defined by Kong et al. (1995), the Chencai Group represents totally a complex antiformal structure. Actually, this antiformal structure includes a vast number of multi-stacks with imbrication or juxtaposing of the various rocks in the Chencai Complex (Xiao, 1995). Both the meta-volcanic and meta-sedimentary rocks are tectonically interleaved with ultramafics and associated rocks, and with overlying syn-orogenic clastic rocks of Molasse basins. This tectonic zone including various ultramafic and mafic blocks has been considered the main component of the tectonic m61ange whose emplacement age is still controversial (Shui, 1984; Kong et al., 1995). Our preliminary zoning on the basis of the analysis of the structural styles is quite similar to the standard model for FTBs of Lowell (1985), thus suggesting a decreasing deformational degree from southeast to northwest. This vergence is the result of the continuous northwestward shortening of the Jiangnan Arc.

S W Fujian FTB and Cathaysian frontal arc

The tectonics of the Southern margin of the Jiangnan arc is characterized by the foreland FTB where the youngest stratum involved in the FTB is Early Triassic turbidites and contourites (Hou et al., 1994; Zhou, 1996), and the latest molasse is Late Triassic in age (Li et al., 1996). For the tectono-sedimentary analysis, interested readers should refer to Zhou et al. (1996). The Cathaysian arc terrain is extensively covered by the Jurassic and Cretaceous pyroclastic rocks and red beds, and only in a few localities are the late Paleozoic marine beds overlying an early Paleozoic metamorphosed basement seen. This Mesozoic volcanic region may be the signature of a frontal arc terrain of the South China archipelago. In coastal Fujian, a Late Mesozoic m61ange zone has been

i

A

bO

i
65%. The REE pattern of the basalts interbedded with these volcaniclastic rocks are similar to those formed in a continental rift setting

Mesozoic tectonics of Fujian, Eastern China

43

(Wang et al., 1982). The lower part of the Permian sequence in the basin consists of coalbeds interbedded with sandstones and sandy siltstones, while the upper part comprise siltstones, calcareous siltstones and radiolarian-bearing siliceous sedimentary rocks. Lower Triassic. The lower Triassic Xikou Formation consists of sandstones, calcipelites, mud-siltstones and siltstones. Deep-water turbidite sedimentary structures are developed in the Xikou Formation, among which are Bouma sequences, convolute laminations, ball-and-pillow structures, scour marks and flute marks. The Palaecurrent direction determined from flute casts and climbing-ripple beddings is around 300 ~ in the central area of the basin. In the same area, contourites and carbonate gravity flow deposits were identified (Yang et al., 1993). The basin was asymmetric with water depth decreasing towards the northwestern part of the basin. Syndepositional slumps and brecciation of strata are also well developed. The grain size frequency diagram and C-M diagram of the 35 samples indicate suspension and saltation transport (Zhou, 1992a). The rate at which sediments of the Xikou Formation were accumulated (0.136-0.460 m per 100 years) exceeds that of the Paleozoic deposits (0.01-0.17 m per 100 years) in the region and is within the range of that of the flysch deposits in other foreland basins in the world (0.151.927 m per 100 years, Schwab, 1986). Discriminant function analysis of sandstones from the Xikou Formation using 11 major element oxides as variables and adopting the values of the unstandardized discriminant coefficients for sandstones suites of eastern Australia (Bhatia, 1983) suggest that the provenance of the Xikou Formation was an active continental margin (Zhou, 1992a). Another prominent feature of the Xikou Formation is its well-developed structures. While some of these structures are of the syndepositional origin, the majority are caused by the post-depositional compression evident from the closely spaced mesoscopic scale isoclinal folds, chevron folds, duplexes and associated cleavages (Zhou, 1992b; Yang et al., 1993). Middle Triassic-Upper Jurassic. The Middle Triassic calcareous sandstones were deposited in small scale shallow marine depressions. They are overlain unconformably by the thick Upper Triassic conglomerates, pebble-bearing sandstones (Fig. 2). The Lower to Middle Jurassic quartz conglomerate, coarse and fine sandstone and siltstones interbedded with marine deposits are in turn overlain unconformably by thick tufaceous sandy conglomerates, sandstones and siltstones of the Jurassic. The accumulation rates of these sediment range between 0.078 m and 0.490 m per 100 years, close to those of the molasse deposits in other foreland basins (0.10-0.40 m per 100 years, Schwab, 1986). In summary, the Mesozoic successions of the west Fujian evolved from Early Triassic flysch deposits to the coal-bearing upper Triassic to Upper Jurassic molasse deposits, forming a typical foreland basin sedimentary sequence (Fig. 2). This foreland basin trended ENE and increased in size during its late evolution. In the early Triassic, the foredeep was in Da'tian, central Fujian, where deep-water turbidites were deposited. Subsequently, as the size of the basin increased, the water depth decreased. This process culminated in Late Triassic and Late Jurassic when the extremely thick (with a maximum thickness of 3400 m in Da'tian) molasse sequence was rapidly deposited and the width of the basin reached its maximum.

Z . Z h o u and Z. Liao

44

Age & Rock Units 156 Ma

Lithology

t

,.V...V

V-.-i

,.-V.-..V

....

Tectonstratigraphy

Changlin Fm, . . ... .. ... ... ... ... . . . . . . ! J3c I

' - "

"::"

-

" "::"

-

"

"~'i'

~.~:~.o..:~.~:~

:

,

163 Ma Zhangping ~ ~ ~ ~ " .o Fm J2z • ;-.; ; - . - : ~ .~ 187Ma

...........

Molasse

99

'"~.: ~-:~.i ~-:~.i ~:" Deposits

in Foreland Basin

.:."~i:."~~:."~~

Ushan Fm J1

. .......... ..:! ..............

208 Ma .............

i

' Wenbingshan . . . . . . . . . . . . . . Fm ............. ~->. 9 C~. 9 .'~" T3w ,...

230 Ma

9

,:,: . . . ~-,: : 9 ~ ; :

.,~:~..,~:~..~'.~'.

Anren Fm T2n 240Ma

..........

9. - . . . . .

~ : : ~ - : ~ _:~. ...............

Flysch Deposits

in

:-.: . . . = . . . . . .

Xikou

Fm Tlx 245 Ma 250 Ma

...... = .".,, = ;, ' , ' , " , _. -=._.. -=. _.. = ,,

Foreland Basin

~ ............ ............

Shelf Deposits

'

'

~-.--.•

C 360Ma

"'.--=."--.".i--.

-~i-~l

i

;>. - ,::.. -- ,-:r- -!

Tianwadong Fm :>~;=" ~'-'" D3t ................ : '_~," " L ' . ' . , ' " ,:'.:~:" 9 , . . . .

376 Ma

- 9. . . .

"7 . . . . .

Fluvial Deposits

~.,

..-~.. ,,,~.. ,.-~.. ,.

Tuffaceous

Sandstone

Coal-bearing

sandstone

,s. B~o, ~995'

Conglomerate Fine Sandstone Calcpelite Mudstone Sandstone Mud-Siltstone Siliciclastic

Carbonate Carbonate

F i g . 2. G e n e r a l i z e d

Upper

Sandy

Conglomerate

Paleozoic-Mesozoic

stratigraphic

~=~

column

of west Fujian

foreland

basin.

Mesozoic tectonics of Fujian, Eastern China

45

Foreland fold-and-thrust belt

Widely developed folds and low-angle thrusts in the west Fujian have been recognized by regional geological survey and drill cores that penetrated into both the sedimentary cover and the basement. The foreland fold and thrust belt of the west Fujian is characterized by multistage and multilevel thrusting of the cover and by basement decoupling. The most prominent folds were developed in the Lower Permian coal-bearing sediments, and the folding in those rocks is intense and complex. This coal-bearing Lower Permian sequence also acted as the detachment zone which enabled the development of imbricate thrusts and folds in the overlying sedimentary cover. Other detachment zones in the basin are Lower and Upper Carboniferous strata. Low angle thrusts in the basin usually occur in groups with very small dips (15 ~ 20 ~ and strike ENE-WSW (Tao, 1987). They occur not only as cover thrusts related to the different layers of the detachment, but also as steeper thrusts in the basement of the basin. Kinematic studies of the thrusts reveal two main stages of thrusting. The early thrusting episode has been related to early Mesozoic north-south convergence between two continental blocks (Fig. 3) and was overprinted by early Cretaceous thrusting from east to west (Fig. 4). In extreme cases, the northwest limbs of some anticlines are overturned. Microfabric studies show that the direction of the early stages of the thrusting was from SSE to NNW, with the root of the thrust belt, therefore, located to the south of the basin. Permian- Triassic granites

Permian to Triassic S-type granites in western Fujian foreland basin generally cut across the fractures and the anticlinal axes. They are alkaline granites with high 87Sr/86Sr ratios and low Fe3+/(Fe 2+ + Fe 3+) ratios. In addition, they are also LREE enriched (Wang and Liu, 1986). A discriminant diagram of the Rb against Y + Nb suggests a syncollisional and volcanic arc origin (Zhou et al., 1996). Most of these granites postdate the closure of the previous ocean basin and may be related to the partial melting of the partly subducted oceanic crust or to voluminous sediments accumulated between the continents before the final closure of the intervening ocean basin. EARLY MESOZOIC C O N T I N E N T A L COLLISION IN SE CHINA Studies of the geology of southeast Asia and of South China in the last decade or so have culminated in the recognition of several continental blocks that assembled mainly during the Later Paleozoic-Early Mesozoic (e.g., Seng6r, 1990). The MiddleLate Triassic collision between the South China block and the Indochina block along the Black River (or Song Da) suture and the Red River (or Song Ma) suture is well documented, which is in marked contrast to the little attention that has been paid to the possible suture zones further east in the northern shelf of the South China Sea and the southern margin. The similarity in Cambrian sedimentary successions, ore deposits, trilobites and brachiopods between Hainan Island and Australia, together

46

Z. Z h o u and Z. Liao

Fig. 3. Schematic cross-section to show the evolution of west Fujian foreland basin. Symbols are the same as those in Figure 2.

Fig. 4. Structural profile from Longyan to show the thrusting in western Fujian.

with the discovery o f U p p e r Paleozoic glacio-marine deposits of similar origin to those in Tibet, west Y u n a n and SE Asia (Yu, 1989) suggest that H a i n a n Island is a f r a g m e n t that rifted away f r o m G o n d w a n a at or after the end of the Paleozoic. The location o f the suture zone on H a i n a n Island a n d its vicinity remains a p r o b l e m to be

Mesozoic tectonics of Fujian, Eastern China

47

solved. Wang (1986) proposed that the suture zone extends along the Reiqiong Strait that separates Hainan Island from mainland China. Others have argued that it is located in northern Hainan (Zhen, 1989) or is cross-cutting the island in S W - N E direction (Metcalfe, 1996) Further east, Hsfi et al., (1990) postulated the existence of a Mesozoic (Triassic or Jurassic) collision between Hainan and the Donnanya (Southeast Asian) Blocks. However, it is worth mentioning here that the suture zone to the south and to the east of Fujian in SE China are of different origins and ages (e.g., Zhou, 1992b). The deep water flysch deposits of the Lower Triassic Xikou Formation and the intense deformation of these deposits, the formation of the S-type granites and the first episode of thrusting in the western Fujian fold-and-thrust belt have all been interpreted to be genetically related to a continent-continent collision which took place to the south of the west Fujian foreland basin in the Early Triassic. This collision marked the beginning of the Early Mesozoic orogeny in the region. However, deformation did not stop as collision ceased. Continuing post-collisional convergence made the orogeny a long process less and led to the uplift of colliding continental blocks, resulting in the deposition of the thick molasse in the adjacent western Fujian foreland basin (Fig. 2). The difficulty in identifying the suture zone has been due mainly to the massive magmatic and deformation activity related to the subduction process to the east of Fujian in the Taiwan Strait in the Cretaceous (Fig. 5), and to the submergence of the area beneath the South China sea. However,

Fig. 5. Schematic cross-section to show the evolution of east Fujian-Taiwan area. A: western Fujian Allemanide; C: eastern Fujian Celtide.

48

z . Z h o u and Z. Liao

geophysical data from the region record some evidences of the existence of an ENE trending tectonic zone. The Bouguer anomaly trend in east Fujian swings from NE to ENE at the southern border of the west Fujian foreland basin. Folding and thrusting have resulted in the thickening of the crust in the basin. Along the coast from Nan'ao to Hong Kong is an ENE tending tectonic zone characterized by a relatively low aeromagnetic anomaly and gravity gradient, in support of the proposed ultrabasic igneous rocks at a depth of 8-12 km (unpublished report of No.909 Aeromagnetic Survey Brigade, Ministry of Geology and Mineral Resources of China). In the Pearl River Mouth Basin, three ENE trending aeromagnetic lows are also attributed to the deep basic intrusive rocks (Guong et al., 1989). In order to unravel the basement structure beneath the Tertiary basins of the northern South China Sea shelf, an integrated geological-geophysical approach has been used to interpret several S-N profiles for which there are gravitational, magnetic, seismic and borehole data supplemented by onshore data. Forward and inversion iterative modeling was applied to the profiles. These data enabled the postulation of the Paleozoic and metamorphic rocks and Upper Mesozoic sedimentary and metamorphic rocks in the basement. Fault mapping revealed an ENE fault system crosscut by younger NW trending faults. All these phenomena are consistent with a collisional event. A schematic evolution for west Fujian is proposed in Fig. 3. The two regional unconformities within the synorogenic molasse and separating molasse from the underlying flysch (Fig. 2) are reflections of two major stages of tectonic relaxation in the collision zone. The coarse sediment above the unconformities actually represents flexural rebound of the thrust belt as the uplifted area was eroded, and the finer sediment above the two coarse units might represent renewed uplifts that were related to the continuing N-S post-collisional compression. The traditional practice of relating these two unconformities to "Indosinian" and "Yanshanian" orogenies, respectively, should therefore be reassessed. The sedimentary sequence and evolution of the west Fujian foreland basin are comparable to those of typical foreland basins. Thrusting and decollement took place along ramps at different stratigraphic levels, and the detached pieces could not displace freely on a perfect "sole" of the southern Appalachian type (Zhu, 1989). Finally, the Lower Triassic deep marine fine turbidites in the foredeep of west Fujian foreland basin conformably overlie the Upper Permian deposits (Fig. 2), showing that the transition from passive margin to active margin was gradual. The fact that neither mud nor shale was deposited over fine turbidites also reveals that the compression was a slow and continuous process. In addition, the lack of evidence for metamorphism shows that the related continental collision was a mild one. MESOZOIC TECTONIC EVOLUTION OF EASTERN FUJIAN East of the NNE-trending " F u ' a n - N a n ' a n " fault (Fig. 1), from Mazu island to Nanao island in east Guangdong, there exists a tectonic zone that is more than 400 km long and 50 km wide (on land). Metamorphic rocks, migmatites, granites, and volcanic rocks crop out in this tectonic zone. From northwest to southeast, the rock sequence grades from lower greenschist facies to upper greenschist facies to

Mesozoic tectonics of Fujian, Eastern China

49

lower amphibolite facies (Xie et al., 1985). This shows that the intensity of the metamorphism increases seawards. The occurrence of andalusite and sillimanite in the lower amphibolite facies, and the intensive migmatization and magmatism show that this is a metamorphic belt of high temperature and low pressure. The study of metamorphic rocks also shows that metamorphism of the zone has a close genetic relation to shear-related deformation. The structural deformation features, deformation field, microstructure, and structural geochemistry of the zone show that this was a sinistral transcurrent fault during the early Mesozoic (Li, 1988; Zhou, 1992b). Three types of granites are found in the zone. One is formed during shearing processes as a result of heating and replacement. It exhibits the same metamorphic and deformation features as the surrounding rocks. Another is a I-type granite in the area with Rb-Sr radiometric ages of these granites ranging exclusively between 90 and 120 Ma. Studies on Nd-Sr isotopes of the Cretaceous magmatic rocks indicate that magmatic sources from the mantle increase towards the sea (Huang and Sun, 1986). The third type of granite is the A-type granites with their ages range from 90 to 95 Ma. These A-type granites are formed during the tectonic relaxation stage following the intensive compression regime. There are more than 40 basic and ultrabasic rock bodies along the tectonic zone. Changji ultrabasic rock bodies are typical in the central area of the zone. Magnetic and gravity data suggest that these rocks are not present at depth (Fig. 6). AFM, Mg-CaO-AI203, F e / ( F e O + MgO)-SiO2 diagrams for 18 samples from Changji ultrabasic rocks show an ophiolitic affinity. The gabbro of Taohuashan (with a Rb-Sr whole rock age of 104.9-+-3.9 Ma, 87Sr/86Sr = 0.7062 + 0.0001) in Quanzhou is also surrounded by migmatites indicating some form of tectonic emplacement. The distribution patterns of both REE and trace elements of these rocks exhibit four main types of rocks, i.e., metamorphic peridotite, ultramafic accumulate complex, accumulate gabbro and massive gabbro (Zhou, 1992; Lu et al., 1994). Radiometric ages of the rocks obtained from the tectonic zone can be divided into three major groups within ranges of 62-84 Ma, 90-120 Ma and 158-198 Ma. Each age group coincides with a tectonic event, i.e., subduction along the Central

Fig. 6. A profile crossing the Changji ultrabasic rock body, east Fujian. 1 - Late Jurassic rhyolite, 2 - ultrabasic rock, 3 - fault, 4 - drill hole.

50

z . Z h o u and Z. Liao

Range in Taiwan (62-84 Ma), subduction (90-120 Ma) and sinistral shearing (158-198 Ma) along the eastern Fujian tectonic zone (Zhou, 1992). According to Hilde et al., (1977), there was an oceanic ridge system connecting the Pacific and the Tethys during the Mesozoic. This E-W trending ridge system with its northward subduction zones was dissected by several N-S trending transform faults. As no Early Cretaceous rocks show signs of shear deformation, it is postulated that the east Fujian tectonic zone had undergone shearing deformation related to the NNE trending sinistral transform fault during Late Triassic to Early Jurassic. This event not only resulted in remelting of rocks in the area to generate the basic-acid magmatic activity, but also formed the linear dynamo-thermal metamorphic zone along the transform fault. This transform fault has, like the other two transform faults (the Central Range, Taiwan, Palau-Kyushu Ridge) to the east, undergone a transition from shearing to compression as the margin evolved into a subduction zone. The east and west sides of the tectonic zone had different geological histories during and before the Mesozoic. The presence of high temperature metamorphic belt, the simultaneous tectonic emplacement of oceanic rocks and submarine volcanics, the occurrence of I-type granite, volcanism on the land, and the regular evolution pattern of Nd-Sr isotopes for Cretaceous magmatic rocks clearly indicate that they are Celtides of a late Mesozoic collision between the SE China block and the Dongnanya (Southeast Asian) block, and the collision took place in the Early Cretaceous after the elimination of Gunanhai ocean (Paleo-South China Sea) between the two blocks. The widespread thrusting involving Paleozoic and Lower Mesozoic strata in central and western Fujian represents the related Allemanide. Thus, the evolution history of the Mesozoic east Fujian can be divided into two major stages (Fig. 5). During Late Triassic to Early Jurassic, sinistral shearing along the east Fujian tectonic zone is manifested not only by the ductile deformation, but also by metamorphism, migmatization and volcanism that was related to the shearing deformation. During Early Cretaceous, the area turned into a compression regime related to subduction along the west side of the Taiwan Strait. The emplacement of ophiolites and the deep buried basic rocks interpreted from geophysical data (Zhou, 1992b) in the eastern Fujian Celtide as well as the westward thrusting in the western Fujian Allemanide were consequences of this subduction process. This transition from a transform to a subduction system in eastern Fujian, however, is not the only one in the west Pacific region. Studies on Hokkaido in northern Japan (Maruyama and Seno, 1986), the Median Tectonic Line (MTL) in Japan, the Central Range in Taiwan, and the Palau-Kyushu Ridge (Karig, 1971) all indicate that these areas too underwent a similar process of transition younging eastwards. Therefore, the NNE-trending transform fault related to the Tethyan ocean ridge underwent, in sequence, the transition from transform fault to subduction system. This transition process should enlighten our understanding of the accretion history of continents in east Asia. CONCLUSIONS 1. The Cathysia paradox and its related controversies were caused to a large extent by the malpractice of relating pre-Cambrian rocks to Cathysia as well as relating

Mesozoic tectonics of Fujian, Eastern China

2.

3.

4.

5.

6.

51

Lower and Upper Paleozoic rocks to Caledonian and Hercynian fold belts, respectively. Plate tectonic interpretation of the geology and geochronology of the pre-Cambrian rocks has brought forward different evolution histories. Each continent has its own evolution history. The Mesozoic sedimentary successions of the west Fujian evolved from Early Triassic flysch deposits to the coal-bearing Upper Triassic to Upper Jurassic molasse deposits, forming a typical foreland basin sedimentary sequence. The formation of this foreland basin was related to a continent-continent collision which took place to the south of the west Fujian foreland basin in the Early Triassic. This collision marked the beginning of the Early Mesozoic orogeny in the region. The first stage of thrusting in the west Fujian foreland fold and thrust belt and the S-type synorogenic granites were results of this collision. The two regional unconformities within the synorogenic molasse and separating molasse from the underlying flysch are results of two major stages of tectonic relaxation in the collision zone. The traditional practice of relating these two unconformities to "Indosinian" and "Yanshanian" orogenies, respectively, should therefore be reassessed. The evolution of Mesozoic eastern Fujian can be divided into two major stages, i.e., the Late Triassic-Early Jurassic sinistral shearing stage and Early Cretaceous subduction stage. Both stage exhibited their own characteristics in metamorphism, migmatization, magmatism and deformation. The overprinting of these processes complicated the geology of the east Fujian tectonic zone. The transition from a transform to subduction system presents a basic dynamic scheme for the continental accretion in east Asia. This, together with the Early Mesozoic collision to the south of the west Fujian foreland basin, shows that, the oceanic subduction and continent-continent collision afterwards is the major accretion scheme in SE China. During the northward movement of Tethyan continents, the disappearing of the intervening oceans and the resultant continental collision may be the major form of continental accretion. The NNE trending transform faults that originated from the Tethyan ridges are important constraints not only in that they caused the shear deformation on land, but also in that they laid the foundation for later compression and related magmatism due to the change of movement direction of Pacific plates. The tectonics of Fujian therefore provides a window to understand the interaction of Tethyan and Pacific plates. The tectonic facies model of Hsfi (1995) can be applied to provide a basic framework for the tectonic evolution and continental accretion history of the region. For the Late Mesozoic collision event, the Celtide is the east Fujian tectonic zone and the Allemanide is the western Fujian foreland and thrust belt. This scheme predicts that the related Raetide is located to the southeast of the studied area. The western Fujian Allemanide is, however, actually the location where two allemanides overprinted, i.e., the early Mesozoic Allemanide and the Late Mesozoic Allemanide, which resulted in the complicated pattern of thinskinned sedimentary cover thrusting in the area.

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ACKNOWLEDGEMENT I h a d the u n i q u e o p p o r t u n i t y to j o i n in a field excursion led by K.J. Hsfi in F u j i a n in 1987 w h e n I was d o i n g m y Ph. D research in the region. T h e excursion t o u c h e d a l m o s t all the m a j o r geological p r o b l e m s t h a t h a d puzzled local geologists for years. I was deeply impressed by the insight a n d vision K.J. Hsfi s h o w e d d u r i n g the excursion. M u c h of m y w o r k in F u j i a n a n d later w o r k in the geology of S o u t h C h i n a h a v e been inspired by K.J. Hsfi's p a p e r s a n d b o o k s which will r e m a i n a m a j o r source o f i n s p i r a t i o n a n d i n n o v a t i o n . K.J. Hsfi also p r o v i d e d useful c o m m e n t s for the m a n u s c r i p t . T h e late Profs. X. Z h u a n d X.Z. Bian are also r e m e m b e r e d for their e n c o u r a g e m e n t a n d help d u r i n g the research. R. Hall helped in the d r a w i n g o f figures. This is a N S F C s u p p o r t e d research project ( G r a n t N o . 49202034, 49876013).

REFERENCES Ai, C.X., Chen, B.W. and Huang, H., 1985. Preliminary study of iron ore-control tectonics of south Fujian and east Guangdong. Bulletin of the Institute of Geology of the Chinese Academy of Geological Sciences, 13: 53-66. (in Chinese, with English abstr.) Bhatia, M.R., 1983. Plate tectonics and geochemical composition of sandstones. Journal of Geology, 91: 611-627. Bian, X.Z., Chu, Z.X. and Zhou, W.D., 1993. Framework of Paleozoic-Mesozoic tectonic evolution of Fujian Province. Geology of Fujian, 12(4): 280-291. ( in Chinese, with English abstr.) Grabau, A.W., 1924. Stratigraphy of China. Part I, Paleozoic and older. The Geological Survey of Agriculture and Commerce, Peking, No.528. Guo, L.Z., Shi, Y.S. and Ma, R.S., 1983. Formations and evolutions of Meso-Cenozoic active continental margin and island arc of west Pacific. Acta Geologica, 57(1): 11-18. (in Chinese, with English abstr.) Guo, L.Z., Shi, Y.S., Ma, R.S., Ye, S.F. and Lu, H.F., 1983. Tectonostratigraphic terranes of Southeast China. Journal of Nanjing University (Natural Sciences Edition), 20(4): 732-749. Guong, Z.S., Jin, Q.J.,Wang, S.S. and Meng, J.M., 1989. Geology, tectonics and evolution of the Pearl River Mouth Basin, In: Zhu, X. (Ed.), Chinese Sedimentary Basins, Elsevier, Amsterdam, pp. 181-196. Hilde, T.W.C., 1977. Evolution of the western Pacific and its margin. Tectonophysics, Vol. 38. Hou, Q.L. and Li, J.L., 1993. Preliminary study on the foreland thrust and fold belt, southwestern Fujian. In: J.L. Li (Ed.), Lithospheric Structure and Evolution of Southeast Chinese Lands and Seas. China Science and Technology Press, Beijing, pp. 27-33. (in Chinese) Hsfi, K.J., 1995. The Geology of Switzerland: An Introduction to Tectonic Facies. Princeton University Press, 250. Hsfi, K.J., Sun, S. and Li, J.L., 1987. Huanan Alps, not South China Platform. Scientia Sinica, B series: 1107-1115. Hsfi, K.J., Li, J.L., Chen, H., Wang, Q.C., Sun, S. and Sengor, A.M.C., 1990. Tectonics of South China: Key to understanding the West Pacific Geology. Tectonophysics, 183: 9-39. Hsfi, K.J., Sun, S.,Wang, Q.C., Chen, H.H. and Li, J.L., 1998. Tectonic Facies Map of China, Science Press, 155. Huang, X. and Sun, S.H., 1986. Studies on Nd, Sr isotopes of Cretaceous magmatic rocks from Fujian. Acta Petrologica, 2(2): 50-63. Karig, D.E., 1971. Origin and development of marginal basins in the western Pacific. J. Geophys. Res., 76:2542-2561. Li, S.G., 1988. The microstructure and petrofabric features and stress field of Changle-Nanao metamorphic belt. Bulletin of the Nanjing Institute of Geology and Mineral Resources, 9(2): 117-131. (in Chinese, with English abstr.) Lu, H., Jia, D., Wang, Z., Guo, L., Shi, Y. and Zhang, Q., 1994. Tectonic evolution of the Dongshan terrane, Fujian province, China. Journal of South American Earth Sciences, 3/4: 349-365.

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Maruyama, S. and Seno, T., 1986. Orogeny and related plate motions: example of Japanese islands. Tectonophysics, 137: 305-329. Metcalfe, I., 1996. Gondwana dispersion, Asian accretion and evolution of eastern Tethys. Australian Journal of Earth Sciences, 43: 605-623. Ren, J.S., 1986. Some problems about the geotectonics of South China. Scientia Bulletin, 49-51. (in Chinese) Ren, J.S., Jiang, C.F., Zhang, Z.K. and Qin, D.Y., 1980. Geotectonics of China and its Evolution, Science Press, Beijing. (in Chinese) Schwab, F.L., 1986. Sedimentary "signatures" of foreland basin assemblages: real or counterfeit? In: Allen, P. and Homewood, P. (Ed.), Foreland Basins, Blackwell Scientific Publications, Oxford, pp. 359-410. Seng6r, A.M.C., 1990. Plate tectonics and orogeny research after 25 years: A Tethyan perspective. Earth Science Review, 27: 1-201. Shui, T. and Xu, B., 1988. Geology of the metamorphosed basement in Zhejiang-Fujian region, Science Press, Beijing. Tao, J.H., 1987. Thrust tectonics and its formation mechanism in the east part of southwest Fujian. Geology of Fujian, 6(4): 249-270. (in Chinese, with English abstr.) Wang, D.P., Liu, Z.J. and Wang, D.Q., 1982. Paleogeography and structures of Makeng-type iron ore deposits, Fujian. Journal of Changchun College of Geology, 3: 43-58. (in Chinese, with English abstr.) Wang, D.Z. and Liu, C.S., 1986. Distribution patterns and genetic series of granites of HercynianIndosinian cycle in Southeast China. Acta Petrologica Sinica, 2(4): 1-13. (in Chinese, with English abstr.) Wang, E.K. and Liu, C., 1992. Is Cathysia a unified continent? In: Li, J.L. (Ed.), Lithospheric Structure and Evolution of Southeast Chinese Lands and Seas. China Science and Technology Press, Beijing, pp. 96-105. (in Chinese, with English abstr.) Wang, H.Z., 1986. Geotectonic development of China. In: Yang, Z., Chen, Y. and Wang, H. (Eds.), The Geology of China. Clarendon Press, Oxford, pp. 235-276. Wang, X. and Yang, S.F., 1992. Structure characteristics of the Early Paleozoic collisional orogenic belt in South China. In: Li, J.L. (Ed.), Lithospheric Structure and Evolution of Southeast Chinese Lands and Seas. China Science and Technology Press, Beijing, pp. 111-118. (in Chinese) Xie, D.K., Xu, M., Zhou, Y. and Shi, H., 1985. Metamorphism of the Mesozoic metamorphic belt in coastal Fujian. Bulletin of the Nanjing Institute of Geology and Mineral Resources, 6(1): 53-63. (in Chinese, with English abstr.) Yang, M.F., Li, J.L., Hao, J., Hou, Q.L. and He, H.Q., 1993. The discovery of Lower Triassic contourites and carbonate gravity flow deposits in southwest Fujian. In: Li, J.L. (Ed.), Lithosphere Structure and Geological Evolution of Southeast less China, Metallurgy Industry Press, Beijing, pp. 213-218. (in Chinese) Yu, Z.Y., 1989. The determination of the early Permian glacialmarine deposit in Hainan island and its tectonic significance. Journal of Nanjing University (Natural Science Edition), 25(1): 108-119. (in Chinese, with English abstr.) Zhen, J.Z., 1989. Tectonics of western Hainan. MSc thesis, Tongji University, Shanghai. (in Chinese) Zhou, Z.Y., 1989. The study on the basement of SE China: A review. Geology of Fujian, 8(1): 46-53 (in Chinese, with English abstr.) Zhou, Z.Y., 1992a. The depositional environment and tectonic setting of Xikou Formation in western Fujian. Experimental Petroleum Geology, 14(2): 135-142. (in Chinese, with English abstr.) Zhou, Z.Y., 1992b. Eastern Fujian tectonic zone. In: Liu, G.D. (Ed.), Geological-Geophysical Features of China Seas and Adjacent Regions, Science Press, Beijing, pp. 320-327. (in Chinese) Zhou, Z.Y., Lao, Q.Y., Chen, H.J., Ding, S.J. and Liao, Z.T., 1996. Early Mesozoic orogeny in Fujian, southeast China. In: Hall, R. and Blundell, D. (Eds.), Tectonic Evolution of Southeast Asia. Geological Society Special Publication No. 106, pp. 549-556. Zhu, X., 1989. Remarks on Chinese Meso-Cenozoic sedimentary basins. In: Zhu, X. (Ed.), Chinese Sedimentary Basins, Elsevier, Amsterdam, pp. 1-5.

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Paradoxes in Geology Ueli Briegel and Wenjiao Xiao, eds. 9 2001 Elsevier Science B.V. All rights reserved. Chapter 4

S O N G P A N - G A R Z E BELT: FORE-ARC ACCRETION OR BACK-ARC COLLAPSING?

Pan Guitang, Xu Qiang* and Jiang Xinsheng Chengdu Institute of Geology and Mineral Resources, 610082 Chengdu, Sichuan, P.R. China

ABSTRACT The Bayan Har Basin thought to be the preexistence of the Songpan-Garze belt, is a back-arc basin among a series of Polyarc-basin systems behind the QiangtangTaniantawong front, the arc beginning in the Late Paleozoic. The three margins surrounding this "triangular" basin have their own tectonic dynamic properties. The eastern margin started as the western passive continental margin of the Yangtze block (D-T2), then changed into a foreland basin related to the North China block and the Qiangtang block during the middle to late Triassic (Ladinian). Because of the westward oblique subduction of the Yangtze block, two subduction m61anges, the South Kunlun m61ange in the north and the Hoh XilGarze-Litang m61ange in the south, were formed on both north and south sides of this basin at almost the same period. The stratigraphic sequences and sedimentological characteristics indicate that the main body of the Bayan Har Basin is composed of Triassic flysch sediments deposited in a foreland basin during the Triassic.

INTRODUCTION Previously, the Garze-Litang belt was thought to be the Songpan-Garze Indian-china geosyncline or folded belt by most geologists (Zhenqian et al., 1993, 1984; Biwie et al., 1987; Zhen et al., 1995). Some other geologists studied the evolution of the Bayan Har Basin applying to plate tectonics model and concluded that it was a paleo-oceanic basin opened during the Late Paleozoic after the consumption of the Qinling-Qilian-Kunlun oceanic plate and closed in the Late Triassic. The Hoh Xil-Jingshajiang suture is just the trace of the already disappeared paleo-oceanic floor (Hsfi, K.J., 1977, 1991; Chen Zhuliang, 1994). The Songpan-Garze belt represents a big 500-km-wide subduction complex, filling the remnant Paleotethys trapped between the Laurasia, Yangtze and Qiangtang *E-mail: [email protected]

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great blocks in the front of the Kunlun Shan compression arc system. Seng6r (1987), suggested that it is one of the largest unparallel sutures in the world, and inferred an oceanic lithosphere under (Seng6r et al., 1988). Based on the thin skin tectonics of Hsfi's (1981), Guitang and Zhenqian (1991) divided the eastern Tibet plateau collision orogenic belt (including the Garze-Songpan area) into three units: the eastern Tibet subduction block, the Yidun collision belt (including the Jinshajiang suture and the Garze-Litang suture) and the Yangtze subduction block. Now we have realized that the Songpan-Garze belt is not the remnant of the Tethys but a back-arc basin from the Late Paleozoic to Early Mesozoic. This belt is not a giant forarc accretional subduction complex but an abyssal fill in a back-arc basin. This basin started to be of existence by rifting on the western continental margin of the Pan Cathaysia during the Devonian and became a back-arc basin by seafloor spreading (Hsfi, K.J. et al., 1995). Therefore, we have to deal with the question: forearc accretion or backarc subduction? This is a "crucial"problem to be solved while studying the tectonics in this region.

THE TECTONIC F R A M E W O R K AND EVOLUTION OF THE S O N G P A N - G A R Z E BELT The following tectonic units have been recognized in the Songpan-Garze and surrounding area (Fig. 1). (1) the Kunlun forearc overthrust belt (the Burhan Budai-Ngola Shan volcanic arc, Datong shan-Huan Yuan uplift and the m61ange in the south of the Kunlun Shan) (2) the Gonghe backarc basin flysch (3) the A'nemaqin Shan m61ange and the Xiqin Shan uplift (4) the Bayan Har foreland flysch basin, the Longmen Shan foreland overthrust belt and the Sichuan foreland basin (s) the Garze-Litang arc-continent collision suture zone (6) the Dege-Yidun island arc (Hou Liwei et al., 1994) (7) the Hoh Xil-Jinshajiang arc-continent collision suture zone (8) the Qamdo backarc foreland and the Jiangda-Deqen volcanic arc The predecessor of the Songpan-Garze Orogenic belt, i.el, the Bayan Har Basin is one of largest backarc basins within the poly-arc-basin system behind the Late Paleozoic Taniantawen frontal arc. The tectonic dynamics of the three boundaries surrounding this "triangular" basin are different from each other. In the east, the western margin of the Yangtze block was a passive continental margin from the Devonian to the Middle Triassic, then it began to wedge westward during the Late Middle Triassic (Ladinian). The basin floor oblique subducted along both the northern and southern margins to form the southern Kunlun subduction complex in the north and the Hoh Xil-Garze-Litang subduction complex in the south. However, the Triassic flysch, the main body of the Bayan Har Basin, is the infill of a peripheral foreland basin.

Songpan-Garze belt." fore-arc accretion or back-arc collapsing?

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Fig. 1. Tectonic map of the Tibet plateau and adjacent districts.

CHARACTERISTICS OF THE B U R H A N B U D A I - N G O L A SHAN CONTINENTAL M A R G I N VOLCANIC ARC AND F O R E A R C BASIN A Proterozoic-Paleozoic composite magmatic arc developed along the north side of the middle Kunlun faults and a complete Indian-china magmatic arc along the eastern margin of the Qaidam block appeared which bends northwestwards turning along the Burhan Budai-Ngola Shan. During the Early Indian-china magmatism most occurred along the south slope of the eastern segment of the eastern Kunlun Shan, the eastern middle Kunlun and the southern Kunlun faults, forming a 350 km long and 35 km wide magma belt, composed mainly of quartz diorite, granhodiorite, monzonitic granite, Karlsteinite and granite. During the Late Indian-china, the magmatism belt shifted to the Ngola Shan area. Through the amount of rock bodies intruded in Paleozoic-Triassic strata and even into the middle Triassic Junzihe formation, the country rock is remarkably altered with strong silicification, hornfels, local hybridization and generative many-wall rock Xenoliths of various shapes and sizes. The rock type of the

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Fig. 2. The Burhan Budai-Ngola Shan Triassic volcanic arc and forearc accretion wedge.

magmatic arc is granodiorite with local concentration of dark minerals along the margin. It is worth mentioning that the Caledonian and Hercynian magmatic arc belt and its basement has been overthrusted southward onto the southern Kunlun m61ange while the Indian-China magmatic arc may be an island arc-front arc formed by westward wedging of the Yangtze block. The Indian-China granite and the inner arc of the Burhan Budai-Ngola Shan was formed along the eastern Kunlun and the upper Triassic Ngola Group, i.e., calcalkaline and alkaline volcanics unconformably overlies the lower and Middle Triassic and older strata. In the front of this epicontinental volcanic arc, a remarkable forearc basin accretion wedge occurred. From the Qinghai Nanshan to the Xinghai area, the lower Triassic Longwuhe Group is interbedded with basin volcanics, volcanoclastics, silicate, siliceous slate etc. The Hongshuichuan Formation is composed of coarse clastics and polymictic volcanoclastics, indicating sediment resources related to the island arc (Fig. 2). ABOUT THE A ' N E M A Q I N OPHIOLITE Ml~LANGE The A'nemaqin ophiolite m61ange is 500 km long, beginning in the east at Maqen, going westward through Majixueshan, Huashixia, to the Buqing Shan.

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The ophiolite is composed of ultramafic rocks, gabbro cumulates, homogeneous gabbro, diabase gabbro dike, basic lava (pillow-lava and block shaped), radiolarian chert etc., mostly appearing as tectonic blocks together with some other kinds of blocks mixed in sandy slaty flysch. The geochemical composition indicates that the basin volcanics around the Buqenshan area is seafloor tholeiitic with SiO2 content of 49%, TiO2 + 1.3%, K20_0.5% and P205 about 0.1%, which is quite similar to the typical seafloor tholeiite, and the REE Partitioning chart is of the LREE deficiency flat type. The tholeiite around the Majixueshan area is of the alkaline inclined, the REE chart is LREE slight enriched type, showing oceanic island feature while the basin volcanics in Xia Dawo are both oceanic ridge and oceanic island type (Qinghai Geological bureau, 1991; Jiang Chunfa et al., 1992). Based on the Early to Middle Triassic radiolarian from chert (Jiang Chunfa et al., 1992) and the 260 Ma Rb-Sr isochronal dating of the Xia Dawo volcanics and regional tectonic analysis, the ophiolite was formed during the Late Permian through to the Middle Triassic. The purple red muddy cherts sampled (1994) at 14th kilometre of the ~ a q a n - G a n g d a highway yielded Devonian-Carboniferous radiolarian fossil id~ t!fied to be Eatactior sp. and susceptible Tenticulite (D) by Fengqinglai et al. (1996). The chert in the Bayan Har Group on the north of the Zogqen area between the Bayan Har folded belt and Garze-Litang belt also yielded Early Carboniferous radiolarian fossils. Therefore, it is implied that the ophiolites of the Bayan Har Ocean and south of the eastern Kunlun was formed as early as the Early Carboniferous or even earlier. Besides, the massive Permian carbonate rocks on the A'nemaqin Shan in Toson Hu-Huashixia-Maqen are rich in shallow marine fossils such as fusulinids, brachiopods and crinoids. Some exposures indicate a certain amount of bioclastic limestones. Even in the Maqen area, the carbonate blocks were overthrust southward on the Jurassic coal strata. In the north of Huashixia and Toson Hu valley, many limestone thrust blocks can be seen seated on the mountain. Consequently, it means that the A'nemaqin Shan ophiolite belt may be the product of a backarc basin, which is quite similar to the tectonics of the South China Sea. The main subduction belt of the real Bayan Har Ocean has been buried under thick flysch in the Triassic foreland depression, i.e., the Bayan Har Group or southward overthrusted volcanic arc zone.

THE TRIASSIC IN THE BAYAN HAR AND ADJACENT B A S I N - THE F O R E L A N D BASIN The Bayan Har basin was filled with Triassic flysch sediments of great thickness, mostly clastic turbidite. Influenced by strong deformation, the basin is shortened and thickened, the sediment strata were intensely folded to form upright, recumbent, and even overturned folds. The paleogeography reconstruction of this area indicates that the original dimension of the Triassic basin was at least 5-10 times larger than the present one, comparing with the baggiest modern submarine fan on seafloor in the Bengal bay (Garganti and Van Haver, 1988; Loreng, 1988).

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Sediments of the active continental margin on the southern side of the eastern Kunlun arc The Burhan Budai-Ngola Shan forearc accretional wedge. The wedge sediments like mostly the middle Triassic Hongshuichuan formation, and the Naochanjiangou formation consist of coarse clastics, tuffaceous rocks and volcanic clastics. The enormous thickness, the quick changing facies and the graded bedding show that the sediments belong to a submarine turbidity sequence. The paleocurrent direction data show a complicated and variable paleocurrent system, possibly related to the island configuration (Lou Jianning, 1995). The Gonghe backarc basin and its continental margin sediments. It is mainly distributed along the north of the A'nemaqin Shan volcanic island arc zone and the Xiqin Shan relic arc. The northwestern boundary is the Kunlun arc bend. The Lower Triassic is the Longwuhe Group, the Middle Triassic is the Gulangti Formation (Jiang Xinsheng et al., 1996). The Longwuhe Group mainly occurs in the area between the Longwuhe bayou and the Jianzha Shinaohe and is 2300-6450 m thick. The lower part is made up of coarse clastics, and the massive conglomerate is more than 100 m thick. The lateral change is very quick. The sediments are fining upward and consist of several cycles. The upper part is dominated with silt and slate intercalated by limestone, bearing ammonites and some plant fossils. The graded bedding, convolute bedding and sole mark show that it is turbidite sediment. The paleocurrent direction reflects a general southward tendency. The Dickinson chart analysis shows a tectonic of passive margin-island arc type. The Middle Triassic Gulangti Formation has a far wider extension than the Lower Triassic. The outstanding feature of the Gulangti formation is the carbonate debris flow sediment in the clastics, reflecting process as on the continental margin related to backarc seafloor spreading during the Early-Middle Triassic. Most sections have at least four suites of carbonate debris flow sediments, all occurring in the lower part of the section, indicating a strong regional tectonic activity in the Early and Middle Triassic. The field observation and mapping and the laboratory work indicate a southward dipping continental margin. The development of grain-flow sediment and the REE geochemical feature of terringenous constituents in Tongde area indicate an island-continental margin.

The Songpan-Garze Triassic passive continental margin and foreland basin sediments Early Triassic flysch-Early Bayan Har Group. It is mainly distributed to the north of the Mado-Xikequ-Changmahe zones, in the northeast of the Bayan Har Shan. It is composed of medium-coarse grained arkose, intercalated with silt slates. The upper part containing more silty slates, and also exotic limestone blocks of different sizes which bear a lot of Carbo-Permian fossils. In the Changmahe area, 40-200 m thick purple conglomerates, unconformable on Permian and are covered conformable by the Middle Triassic (Qinghai Geology bureau, 1991). The Bochiguo

Songpan-Garze belt: fore-arc accretion or back-arc collapsing?

61

Formation slope turbidite in the back Longmenshan belt lies comformable on the Permien basalt. The turbidite indicators, for example load casts, flute cast, ripple marks, graded bedding, mostly show A-B beds of turbidites mixed with a large amount of olistostromes. Paleocurrent direction data show northeastward in the north district and south or southwestward divergent tendency in the southern area (Fig. 3). The REE features indicate that the lower part is formed in an island tectonic setting, and the upper part in a passive continental margin. Middle Triassic flysch-Middle Bayan Har Group. In the Bayan Har Shan area there is a succession of medium-fine arkoses, arkosite, silt and silty slate. This is the most typical Bayan Har Shan deposit, with some a siliceous sliding on continental rise southeastward, deposited in the medium-inner fan. The large amount of paleocurrent data reflects a southeast-directed flow, and the sediment basin gradually shifts to the south. The REE indicates that the source area is an islandarc-oceanic island arc.

Fig. 3. Sedimentary feature of the Bayan Har foreland Basin in the Triassic.

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Late Triassic flysch-Late Bayan Har Group. It is located in middle to southern Bayan Har Shan, stretching southward to the north of Hoh Xil-Zhidoi-ShiquGarze. The most distinct feature in this area is the occurrence of a large amount of dark muddy-siliceons rocks (Barnes, 1988).

Qiangtang-Qamdo block and the Triassic forearc accretional wedge in its northeast margin A lot of detailed information and analytical results have been documented on defining the Yidun island arc. Some authors suggest it to be formed oceanic crust (Hou Liwei et al., 1994). Important mention is that the volcanic island arc and the southwestward oblique subduction accretion, related to Kenan Group, extended from Xijin Ulan Hu, Goulushankeco and Dengeryoungqu to the south of ZhrdoXiewo. To the north this group is in contact with the Bayan Han Group. The Kenan Group is generally composed of metaphomosed, deeply deformed clastics and volcanics intercalated with carbonate rocks. The regional dynamic metamorphism has reached greenschist facies, and no original sedimentary structures are preserved. Pillow-lava and ultrabasic blocks in it show that the lower part of the Kenan Group consists of seafloor sediments. The sedimentary sequence changes upward from oceanic island to forearc-subduction accretional wedge deposits. The trace elements of graywacke indicate an environment of active continental margin and oceanic island. Many studies indicate that (1) during the Early Paleozoic the Qiongtang-Qamdo block was formed, the Prototethys frontal arc zone on the northward subducting along Qinnidong, then during the early late Paleozoic, the oceanfloor subduction and collision assembled the forearc accretion wedge to be deformed and folded, and later a Devonian-Carboniferous carbonate platform is formed on it., (2) the backarc seafloor spreading began in the Carboniferous-Permian (possibly in the Early Permian), and the ORB around Xijin Ulan Huanea was its relics (Zhang Yifu et al., 1994), (3) during the Late Permian to Early-Middle Triassic, the Jinshajiang oceanfloor subducting westwards formed the Jianda-Weixi continental margin volcanic arc and its forearc accretional wedge. In this area Late Triassic flysch is the largest sedimentary body, and the typical main features are as follows: It is a huge pile of turbidite, contourite and slope olistostostrome of terrigenous clastics on the continental slope-rise-abyssal basin. Based on the distribution of sediments in this basin, the early sedimentation was filled from northern part of the basin with coarse deposits, and then the depositional center shifted southward during the middle and late periods. The late sediments are mainly in the south and consist of fine deposits. The paleocurrent data show a general southeastward-directed (Miall, 1984). Based on regional depositional features, the distribution and shapes of the sedimentary bodies, supply styles, the mineral composition, the formation and evolution of the Bayan Har basin is shown in Fig. 3. It generally reflects that the Triassic flysch in the western part of the basin was formed on top of the continent oceanic transitional lithosphere during the backarc spreading, controlled by the

Songpan-Garze belt." fore-arc accretion or back-arc collapsing?

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Yangtze block's westward wedging and the backarc subduction. The Triassic flysch in the eastern part shows an evolutionary process from passive continental margin to foreland basin (Pan Guitang, 1994, 1996). POSTSCRIPT I (Pan Guitang) got acquainted with Mr. Hsfi 20 years ago in 1979, when he was giving lectures on sedimentology in Chengdu. In 1980, I got a chance to join his expedition to the Yarlu Zangbo suture zone. His immense geological knowledge, capable sensibility to field geological phenomenon, and profound but easily understood original train of thought have left a deep impression on me. Since the 80s, through reading his works and papers, I understand his brilliant ideas and remarkable contributions to the basic problems of earth science. His academic ideas are simple, and clear but full of wisdom and have inspired several generations of geologists. Informed that in 1991 he was giving lectures in Qingdao, I made a special trip to learn again from him for two weeks. Between 1993 and 1995, Mr. Hsfi lead international expeditions to Tibet three times. Knowing that I have been engaged in field work on the plateau for many years, he invited me to lead and acompany the expedition teams. We made extensive geological surveys on Yarlun Zangbo and Xianquanhe along the China-Nepal highway and the Qinghai-Tibet highway respectively. After that we have published "Tectonic Evolution of the Tibetan Plateau: A working hypothesis based on the archipelago model of orogenesis" in the International Geology Review. In this paper, we not only refined Tibet plateau tectonic framework but also thoroughly analyzed the geological history of the plateau, and further promoted the archipelago orogenesis model. I believe that this is one of the most valuable works on the Tibet plateau up to now over the world, and explains the very complicated problem very concisely. On the occasion of celebrating Hsfi's 70th birthday, he invited me again to answer the question "Songpan-Graze belt: forearc accretion or backarc collapsing?" I know he already has his own ideas on it. As this question is quite attractive and my colleagues and I have done some works on it, we are pleased to join this discussion. We thank Mr. Ueli Briegel who made many comments on our manuscript. REFERENCES Barnes, P.M., 1988. Submarine Fan Sedimentation at a Convergent Margin: the Cretaceous Mangapokia Formation. New Zealand, Sedimentary Geology, 59: 155-178. Chen Zhuliang, 1994. 100 year of Tethys geology. Tethys geology. No. 18, pp. 1-22. (in Chinese) Chen Biwie et al., 1987. Geological structure of the three river district. Geological Publish House. (in Chinese) Fengqinglai, Liu Penpei and Ye Mei, 1996. Tectono-Palaeographic pattern of Palaeotethyan Stage in South China. Geological Science and Technology Information, 15, No. 3, 1-6. Garganti, E. and Van Haver, 1988. The Indus Clastics, Forearc Basin Sedimentation in the Ladakh Himalayan (India). Sedimentary Geology, 59: 237-249. Hsfi, K.J., 1977. Tectonic evolution of the Mediterranean Basin. In: Narin, A.E.M. et al. (Eds.), The Ocean Basin and Margins. Vol. 4, pp. 29-75. Hsfi, K.J., 1981. Thin-skinned plate-tectonic model for collision-type orogenesis. Scientia Sinica, 24, pp. 44-48.

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Hsfi, K.J., 1991. The concept of Tectonic facies. Bull. Tech. Univ. Istanbul, 44, No. 1-2, 25-42. Hsfi, K.J., Pan Guitang, Seng6r, A.M.C., et al., 1995. Tectonic evolution of the Tibetan plateau: A working hypothesis based on the archipelago model of orogenesis. IGR, 37: 473-508. Hou Liwei et al., 1994. Yidun arc orogeny and mainly mineral system. Geological Publish House. (in Chinese) Jiang Chunfa et al., 1992. Open and close structure in Kunlun belt. Geological Publish House. (in Chinese) Jiang Xinsheng, Pan Guitang, Yan Yangji, et al., 1996. Triassic Sedimentary Framework and TectonoPaleogeographic Evolution of Qinling-Qilian-Kunlun Junction Region, Progress in Geology of China (1993-1996) Papers to 30th IGC. Loreng, J.C., 1988. Triassic-Jurassic rift-Basin Sedimentology History and Methods. Van Nostrand Keinhold Company, New York. Lou Jianning, 1995. Study on the basic? About the formation and evolution of Tethys. Tethys Geology, No. 19, pp. 1-8. (in Chinese) Liu Zhenqian, 1984. Re-recognize about Tethys--Himalayas tectonic province. Geological Publish House. (in Chinese) Liu Zhenqian et al., 1993. Classification of tectonic-magmatic zone and distribution law of mineralization in the three river district. Geological Publish House. (in Chinese) Miall, A.D., 1984. Principles of Sedimentary Basin Analysis. Springer, New York, 490pp. Pan Guitang 1994. The evolution of Tethys in globe ocean-continent transition, Tethys geology, No. 18, pp. 23-40. (in Chinese) Pan Guitang, and Liu Zhenqian, 1991. Basic feature of Tethys tectonic. Diangzi S&T University press. (in Chinese) Pan Guitang, 1996. Cenozoic Deformation and Stress Patterns in Eastern Tibet and Western Sichuan, Geowissenschaften 14 (1996) 7-8. Qinghai Geological bureau, 1991. Qinghai Regional Geology. Geological Publish House. (in Chinese) Seng6r, A.M.C., 1987. Tectonics of the Tethys sides: Orogenic collage development in a collision setting. Ann. Rev. Earth Planet Sci., 15: 213-224. Seng6r, A.M.C., Altiner, D., Cin, A., Ustaomer, T. and Hsfi, K.J., 1988. Origin and assembly of the Tethy-side orogenic at the expense of Gondwana-land. Geo. Soc. London Spec. Publ., Vol. 37, pp. 119-181. Wang Zhen et al., 1995. Granite and its mineralization in eastern Tibet. Southwestern Jiaotong University Press. (in Chinese) Zhang Yifu et al., 1994. Formation and evolution of the Hohxil-Bayan Har Triassic sedimentary basin. Qinghai People's Press. (in Chinese)

Paradoxes in Geology Ueli Briegel and Wenjiao Xiao, eds. 9 2001 Elsevier Science B.V. All rights reserved.

Chapter 5

R O T A T I O N A L COLLISION AND THE T A N - L U T R A N S F O R M FAULT

Wang Qingchen Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, P.R. China

ABSTRACT Two aspects of the Tancheng-Lujiang (Tan-Lu) Fault Zone impede a clear understanding of its kinematics, one being the potential significance of the structure orientations, and the other the strike-slip motion along the Fault. Tectonic facies distribution, as well as recent paleomagnetic and seismic tomography data, imply that the Tan-Lu Fault Zone was initiated at the turn of the Paleozoic-Mesozoic, when the Yangtze and Sino-Korean Cratons collided. Rotational and northward translational motion of the Yangtze Craton caused a sinistral shear along the eastern margin of the Sino-Korean Craton. As a result, the Tan-Lu Fault Zone was acting as a transform fault. Indentation of the Sino-Korean Craton into the Yangtze Craton is estimated to amount to 450 km, that resulted in a 520-km-displacement in the shallow crust along the Tan-Lu Fault Zone during the Late Triassic to Early Cretaceous, as well as a 200-km-offset of the subducted sliver of the Yangtze Craton at mantle depth. Some portion of these amounts of shearing have transformed into compression and thrust in the Dabie-Sulu orogenic belt that terminates the Tan-Lu Fault Zone.

INTRODUCTION The NNE-trending "deep fracture zone" within eastern China (Fig. 1) was first recognized and named Tancheng-Lujiang (Tan-Lu) Fault Zone in the 1950s (see Xu, 1993), and has been considered as one of the nine famous structures in the world (Uemura and Mizutani, 1984). The Tan-Lu Fault Zone is an active fault zone, along which several earthquakes with magnitude above seven occurred during the past 300 years. However, although the Tan-Lu Fault Zone has been studied for more than 40 years, especially in the 1980s (for example, Xu, 1980; Xu et al., 1987; Klimetz, 1983; Zhang et al., 1984), several important questions remain to be answered (Xu, 1993), such as: When was the Tan-Lu Fault Zone initiated? What is the nature of the Tan-Lu Fault Zone? What is the maximum horizontal displacement along the Tan-Lu Fault Zone? There are still many controversies in answering these basic and deeply studied questions. Why? The key to the solutions of these questions lies in recognizing two

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Fig. 1. (A) Sketch map of the Tan-Lu Fault Zone: LA- Luxi arc structure; XHA- Xu-Huai arc structure; | and | indicating wedged Jurassic-Cretaceous basins; | Ning-Wu pull-apart basin; | Lu-Zong pullapart basin. A-A' and B-B' are positions of seismic tomography lines in Fig. 3. (B) Opposite vergence along of the Tan-Lu Fault Zone shown by tectonic facies. Abbreviations in the index maps: SK, Sino-Korean Craton; YZ, Yangtze Craton; CZ, Collision zone.

entangling problems in understanding the T a n - L u Fault Zone. Only by exposing and keeping off these pitfalls, can one see the T a n - L u Fault Zone in its true light. The author will discuss these aspects first, and then try to answer the abovementioned questions, based on tectonic facies analyses, as well as paleomagnetic and seismic-tomography data published recently. TWO KEYS FOR UNDERSTANDING

THE TAN-LU FAULT ZONE

Key 1: potential significance of the structure orientation. It can be seen clearly from geological maps that the Dabie orogenic belt extends W N W and the T a n - L u Fault Zone runs N N E (Fig. 1). These orientations have mislead people who considered all faults with W N W trend as collision-related structures and all faults with N N E trend as members of the T a n - L u Fault Zone family. As a result, some Precambrian structures that happen to run N N E have been misunderstood as part of an "ancient T a n - L u Fault Zone" (Fang et al., 1986). To avoid the impartation, one should recognize that the N N E strike is by no means unique to the T a n - L u Fault Zone and the orientation of the collision-related structures are never restricted to W N W . Key 2."potential meaning of the sinistral strike-slip motion. The T a n - L u Fault Zone has been inferred to as a sinistral strike-slip fault zone for many years (Xu, 1980). No agreement on the m a x i m u m horizontal displacement along the T a n - L u Fault Zone has been reached. The displacements were reported as 740 km (Xu, 1980, 1993), or 510 km (Sun et al., 1993), or 120 km (Li, 1994). One of the reasons might be that different measuring marks have been used. However, the real mistake is

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considering strike-slip motion as a unique feature of wrench faults, which always offset geological objects they cut through. To avoid this mistake, one should realize that strike-slip motion can develop not only along wrench fault, but also along other kinds of faults, such as transform faults. To distinguish the various faults, one should investigate carefully the nature and kinematics, especially the termination, of each fault. TECTONIC FACIES ON EACH SIDE OF THE T A N - L U FAULT ZONE A tectonic facies analyses method was established by a Sino-Swiss group led by Profs. K. J. Hsfi and Sun Shu who studied Phanerozoic geology of China (Hsfi et al., 1998). A set of names for the tectonic facies, namely alemanides, celtides and rhaetides were proposed by Hsfi (1995) to describe the composition and deformation of the tectonic units. The alemanides are characterized by folding and thrusting of the sedimentary covers, the celtides by ductile deformation under various metamorphic conditions, and the rhaetides by the rigid basement nappes. Among these tectonic facies, the rhaetides play the role of a motor during mountain building. On the western side of the Tan-Lu Fault Zone (Fig. 1), the thin-skinned deformation of the foreland-thrust belt has been categorized as the alemanide of the Dabie Mountains. The ultrahigh-pressure (UHP) metamorphic rocks and their country rocks of amphibolite facies constitute the celtides. The hanging wall of the UHP rocks bearing unit, the northern Dabie gneisses that have been reheated by the Cretaceous intrusions, play the role of an overthrust rigid basement. On the eastern side of the Tan-Lu Fault Zone (Fig. 2), the celtide is composed of UHP rocks and their country rocks, the alemanide composed of Paleozoic shallow marine strata with thin-skinned deformation in the northeastern Shandong, while the rhaetide is composed of rigid Precambrian basement under the northern Jiangsu. Tectonic vergence could be recognized by the spatial distribution of rhaetideceltide-alemanide, because that the rhaetides always overthrust on top of the celtides and those in turn, on top of the alemanides. Therefore, a southern vergence is indicated in the Dabie Mountains on the western side of the Tan-Lu Fault Zone, while a northern vergence is manifested in Shandong and Jiangsu on the eastern side. The opposite vergence implies a left-lateral displacement at the shallow level along the Tan-Lu Fault Zone. On the other hand, while the overthrust wedges moved to south and north respectively in a sinistral movement, the underthrust wedges move in the opposite direction with a dextral sense of shear. This are indeed the results of seismic studies of the earthquakes along the Tan-Lu Fault Zone. ROTATIONAL COLLISION BETWEEN THE SINO-KOREAN CRATON AND THE YANGTZE CRATON Recent paleomagnetic data (see Table 1 of Zhu et al., 1998) showed that the Yangtze Craton collided first at the southeastern margin of the Sino-Korean Craton during the Late Permian. The clockwise rotation of the Yangtze Craton caused a

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Fig. 2. (A) Scenario of rotational collision between the Sino-Korean (S-K) and Yangtze (Yz) Cratons during Pz-J3. (B) Net motion of the combined rotation (R) and translation (T) of the Yangtze Craton resulted in an indentation of the Sino-Korean Craton. During the rotational collision, the indented triangular area was enlarged from AAEF to AAE"F", while the Tan-Lu Fault zone was elongated from AF to AF".

gradually westward-migrating collision. By the Late Triassic, the Yangtze Craton had rotated about 60 ~ and moved translationally northward at the same time (Zhu et al., 1998). Such motion continued until the Middle Jurassic when the two Cratons became one kinetically united continent. The rotational collision scenario has brought several important implications concerning the nature of the T a n - L u Fault Zone.

,

The T a n - L u Fault Zone did not exist before the initial collision between the Sino-Korean and Yangtze Cratons in the Late Permian. It is likely that the S i n o - K o r e a n C r a t o n had a " J " - s h a p e d southeast corner and the Yangtze Craton had a more or less straight northern margin before the Permian (Fig. 2). The clockwise rotation of the Yangtze Craton during the Late Permian to the Late Triassic resulted in a collision zone, whose trend was first parallel to the eastern margin of the Sino-Korean Craton, say, N N E in terms of today's orientation, then rotated to NE, afterward to W N W (Fig. 2). At the same time, a northward movement of the Yangtze Craton caused a sinistral shear along the eastern margin of the Sino-Korean Craton. It is the sinistral shear that initiated the T a n - L u Fault Zone.

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DEEP STRUCTURE ALONG THE TAN-LU FAULT ZONE Gravity and magnetic anomalies, as well as seismic data, indicated that while a narrow mantle upwelling zone has resulted in crustal thinning along the T a n - L u Fault Zone, there is no obvious vertical displacement at the depth of the M o h o (Wei and Teng, 1993). Furthermore, a recent seismic t o m o g r a p h y study showed that the narrow mantle upwelling beneath the T a n - L u Fault Zone extends down to a depth of 71 km and merges into the asthenosphere low-velocity layer at a depth of 120 km (Xu et al., 2000). Another important p h e n o m e n o n displayed by seismic t o m o g r a p h y studies (Xu et al., 2000) is that the Yangtze Craton subducted northward beneath the SinoKorean Craton on both sides of the T a n - L u Fault Zone (Xu et al., 2000). The subduct slab of the Yangtze Craton could be traced down to 240 km and was broken off by a tongue-shaped mantle upwelling at a depth of 120-170 km (Fig. 3). It could

Fig. 3. Seismic tomography lines across the Tan-Lu Fault Zone (see their positions in Figure 1). Yz - Yangtze Craton; SK - Sino-Korean Craton; UHP - ultrahigh-pressure metamorphic zone; T-L F . - Tan-Lu Fault Zone; T-M F . - Taihu-Mamiao fault; W-S F. - Wuhe-Shuihou fault; J-X F . Jiashan-Xiangshui fault.

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be measured that the upper intersections (at a depth of 120 km) of the subducted slab and ascended mantle have been offset in the range of 200 km by the T a n - L u Fault Zone. However, the lower intersections and the slab below (at depths of 150-240 km) seem to show no obvious displacement along the Tan-Lu Fault Zone. This indicates that the subduct slab was not offset by the T a n - L u Fault Zone before the slabbreakoff event, and that the maximum sinistral displacement along the T a n - L u Fault Zone is not more than 200 km. DISCUSSION The above-mentioned new data provide an opportunity to re-examine the following questions. 1. When was the Tan-Lu Fault Zone initiated?. The hypothesis of Precambrian origin should be discarded first. Obviously, if a fault has offset the collision zone between the two cratons, no offset could happen before the collision event. Paleomagnetic (Zhu et al., 1998) and geochronological data (Li et al., 1993) indicated that the Sino-Korean and Yangtze Cratons were collided at the turn of the Late Permian to the Early Triassic. Therefore, the T a n - L u Fault Zone should be born in the same or a later period. The mylonite in the T a n - L u Fault Zone was dated as 95-144 Ma (Xu and Zhu, 1994). However, these data refer to the ductile deformation in the last stage, if the hypothesis of multi-stage activity of the Tan-Lu Fault Zone is accepted. The initial shear must be earlier than these records. The tomography images showed that the subducted sliver of the Yangtze Craton has been offset about 200 km at a depth o f 120 km, while the break-off portion remains in place below 150 km. This implies that the offset started later than the break-off event. The subducted slab started to break off at about 180 Ma, as is manifested by the magmatism that originated when the mantle upwelling squeezed up from the narrow rift of the broken slab (Xu, 1998). The paleomagnetic data indicated that the translational motion of the Yangtze Craton started at the very beginning of its rotational collision onto the Sino-Korean Craton. This is supported by two northwest verging arc structures in the cover of the Sino-Korean Craton, one being the Xu-Huai arc (Xu et al., 1994) and the other the Luxi arc (XHA and LA in Fig. 1). The youngest strata folded in these arc structures are Upper Permian to Lower Triassic. This implies that the ductile deformation was produced by a northwestward thrust along the T a n - L u Fault Zone in the period of the Middle Triassic to Jurassic. The Luxi arc is cut by wedge-shaped narrow basins near Tancheng (Fig. 1). These NW-trending narrow basins, as well as NE-NNE-trending pull-apart basins (Fig. 1) near Nanjing and Lujiang (Jiang and Xu, 1993) were bounded by brittle normal faults and filled by sediments of the Late Jurassic to Early Cretaceous. The overprint of brittle deformation on originally plastic deformation indicates that strata on both east and west sides of the T a n - L u Fault Zone have uplifted for a couple of kilometers since its sinistral shearing. Therefore, the Tan-Lu Fault Zone was initiated in the Early Triassic and continues its sinistral shear during the Jurassic to the Early Cretaceous. The

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subducted sliver of the Yangtze Craton was not offset until the Middle Jurassic, when mantle upwelling squeezed it up. 2. What is the nature of the Tan-Lu Fault Zone?. Several hypotheses have been suggested (see review in Xu and Zhu, 1994) and they can be summarized as three tectonic models, i.e., suture line between the Sino-Korean and Yangtze Cratons (Jia, et al., 1990), wrench fault (Xu et al., 1993), and intracontinental transform fault (Li, 1975; Zhang et al., 1984; Okay and Seng6r, 1992; Yin and Nie, 1993). The suture line model is based on a hypothesis that the T a n - L u Fault Zone separated the Sino-Korean Craton from either the "Jiaodong terrane" (Jia et al., 1990) or the "lower Yangtze terrane" (Chen et al., 1993). However, the so-called "Jiaodong terrane" is composed of two parts, with its northern part belonging to the Sino-Korean Craton and southern part to the U H P - H P belt, while the "lower Yangtze terrane" is an appendix of the Yangtze Craton. In fact, the T a n - L u Fault Zone could be subdivided in three sections. To the north of Tancheng, the north section cuts into the Sino-Korean Carton; between Tancheng and Lujiang, the middle section separates the Sino-Korean in the west from the Sulu U H P - H P complex in the east; on the south of Lujiang, the south section separates the WNWtrending Dabie U H P - H P complex from the folded cover of the Yangtze Craton. Both the Sulu and Dabie U H P - H P metamorphic complexes are products of continental collision and represent the suture zone between the Sino-Korean and Yangtze Cratons (Wang et al., 1996). Therefore, far from being the suture zone itself, the T a n - L u Fault Zone cuts the suture zone that is represented by the Dabie-Sulu U H P - H P complex. The wrench fault model (Xu et al., 1993) is based on the supposed large-scale horizontal sinistral displacement. A series weakness of the model is that the largescale strike-slip displacement of the T a n - L u Fault Zone stops abruptly at its south end near Guangji (Fig. 1). On the south of Guangji, the sedimentary cover of the Yangtze Craton has never been cut and offset by the T a n - L u Fault Zone. Instead, the folds in the sedimentary cover turn their axes from NNE-trending on the east of Guangji to E - W and WNW-trending on the west. In fact, on the western side of the southern section of the T a n - L u Fault Zone sinistral horizontal displacement, if any, was transformed into complex deformation in the Dabie U H P - H P belt. Similarly, the sinistral horizontal displacement on the east side of the middle section of the T a n - L u Fault Zone was transformed into complex deformation in the Sulu U H P - H P belt. Therefore, the T a n - L u Fault Zone is by no means a wrench fault. The transform fault model, therefore, is favored by the author in the sense that the horizontal displacement has been transformed into the overthrust of the rhaetides facies on each side of the T a n - L u Fault Zone. The transform fault model could also explain why the fault terminates exactly where the Dabie overthrust on top of the Yangtze foreland. 3. What is the maximum horizontal displacement along the Tan-Lu Fault Zone? As mentioned above, the maximum horizontal displacements have been inferred to be from 120 to 740 km by different authors, one author even used different offset markers. On the other hand, paleomagnetic data have indicated that the Yangtze Craton has experienced both translational and rotational motions during its collision

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with the Sino-Korean Craton. Therefore, to estimate the displacement, it is necessary not only to choose the right geological markers, but also to take into account the style of motion. The most often employed markers have included lithofacies belts, intrusive bodies, tectonic belts, fold axes, fault lines, geophysical anomalies, metamorphic belts, volcanic activity belts, paleontological zones, and mineral deposit belts (Xu, 1993). However, have these markers indeed been offset? Is it possible that these markers developed at different localities before motion along the Tan-Lu Fault Zone studied? For example, Jurassic volcano-sedimentary basins cannot be considered as offset measurement, because they might have developed independently in both the Dabie and Sulu regions. The Proterozoic basins as well forming separately in south Liaoning and Xu-Huai area (Ji et al., 1980) can hardly be used as offset markers, either. In fact, the estimation of the displacement along the Tan-Lu Fault Zone is far from an easy job, if its transform nature and rotational collision between the Yangtze and Sino-Korean Cratons are taken into account. For example, taking the now metamorphosed phosphorites at Haizhou (A in Fig. 4), Susong (B in Fig. 4), and Qichun (C in Fig. 4), they all scatter from east to west along the northern margin of the Yangtze Craton. However, the total distance between these phosphorite deposits estimated as 600 km by Sun et al. (1993) does not represent the real horizontal displacement along the southern section of the Tan-Lu Fault Zone. If rotation is taken into account, the horizontal displacement (D) of geological markers at the northern margin of the Yangtze Craton should be calculated by D2= 2L2(1-cos0). The L in the equation represents the original distance between the reference markers and the Euler pole (O in Fig. 4), which is assumed to be normal to, and 4~ of the Tan-Lu Fault Zone (Gilder et al., 1999). 0 represents the rotation angle by which these phosphorite deposits have been rotated during the collision event. Geochronological data indicated that these phosphorites were metamorphosed during 220 4-10 Ma (Sang et al., 1987). Since then, the Yangtze Craton had rotated about another 10~ (Zhu et al., 1998), which gives a reasonable estimation of 0. If the Euler pole was located 500 km from the Tan-Lu Fault Zone (Gilder et al., 1999), the L for the Haizhou phosphorite (O-A), is about 500 km, while the L for the Susong phosphorite (O-B), is about 650 km. The Haizhou phosphorite has been displaced for about 87 km (A-A ~) since the Late Triassic. The Susong phosphorite should have been rotated to a locality about 113 km (B-B') further (Fig. 4). However, a southward thrust in the Dabie orogenic belt has displaced the Susong phosphorite to its present location (B"). Therefore, the distance between the Haizhou and Susong phosphorites (A'-B") is not the real offset by the Tan-Lu Fault. On the other hand, the combined rotation and translation of the Yangtze Craton resulted in a net motion that equals the indentation of the Sino-Korean Craton into the Yangtze Craton (Figs. 2 and 4). The indenter is represented by a triangle AEFB", with E at the Hanzhong massif of the Qinling Mountains, F at Haizhou, and B" at Susong. In the context, the Tan-Lu Fault Zone is but one side of the triangular indenter. The maximum indentation (I in Fig. 4) could be estimated as I = 2 S/EF, where S being the area of the triangle AEFB",

R o t a t i o n a l collision a n d the T a n - L u

73

transform fault

(A)

D 2 = 2L2(I-cos O) D: A-A ' B-B' L." O-A, O-B .... ~

j...

(B) I=2S/EF

~

B"

Fig. 4. Diagram showing how to calculate displacement along the Tan-Lu Fault Zone when taking into count the rotation (A) and indentation (B). See text for detailed explanation.

and E F the side normal to the height (I). The measured distance of E F is about 960 km, that of EB" about 840 km, and FB" about 520 km, while the estimated I is about 450 km. Therefore, the southeastern corner of the Sino-Korean Craton has indented into the Yangtze Craton by about 450 km since collision started. Obviously, the indentation distance decreases towards both sides away from the starting point (A in Fig. 2). During the same period, the net motion of the Yangtze Craton along the Tan-Lu Fault Zone was about 520 km. However, this motion has nothing to do with the discussed offset, that has a significant meaning only when a definite reference marker is cut and body pairs move apart. A direct measurement from the seismic-tomography image showing the slab break-off gives an estimation of about 200 km offset at mantle depth (see foregoing pages). Another estimation is 120 km offset at crustal depth based on magnetic anomalies (Li, 1994). If these estimations could be accepted, it seems that the strikeslip motion along the Tan-Lu Fault Zone varies with depth. Significant motion at shallow structural level might have resulted from thin-skin deformation, while offset at depth was caused by mantle upwelling.

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CONCLUSION Based on the above discussion, the a u t h o r arrives at the following conclusion. The T a n - L u Fault Z o n e was b o r n in the Early Triassic, which is later than the start of the collision between the Yangtze and S i n o - K o r e a n Cratons. R o t a t i o n and n o r t h w a r d m o t i o n of the Yangtze C r a t o n cause a net result of indentation by the S i n o - K o r e a n Craton. Sinistral shear along the east m a r g i n of the S i n o - K o r e a n C r a t o n initiated the T a n - L u Fault Z o n e as a t r a n s f o r m fault. I n d e n t a t i o n of the S i n o - K o r e a n C r a t o n into the Yangtze C r a t o n is estimated as 450 km, that resulted in an a p p a r e n t displacement of 520 k m in the shallow crust along the T a n - L u Fault Z o n e during the Late Triassic to Early Cretaceous, as well as a 200-km offset of the subducted sliver of the Yangtze C r a t o n at mantle depth. A p o r t i o n of these a m o u n t s of shearing have resulted in compression and thrusting in the D a b i e - S u l u orogenic belt that terminates the T a n - L u Fault Zone. ACKNOWLEDGEMENT The study has been financially supported by the N a t i o n a l N a t u r a l Science F o u n d a t i o n of China (No. 49772150) and Chinese A c a d e m y of Sciences (No. KZ951-A1-401). The a u t h o r is grateful to Prof. K. J. Hsfi who helped a lot in learning his creative idea of the tectonic facies analyses and its application in Chinese geology. T h a n k s are due to Dr. Ueli Briegel for his constructive c o m m e n t s on an original draft of the manuscript. REFERENCES Chen, H. Liu, Y. and Hu, Y., 1993. Research on tectonics and long-range prospective areas of petroleum, and gas in Sichuan, Hubei, and Anhui Provinces. In: Special paper of Tongji University. Tongji University Press, Shanghai, pp. 8-26. Fang, Z., Ding, M., Xiang, H., Ji, F. and Li, R., 1986. Basic characteristics of the Tancheng-Lujiang fault zone: Kexue Tongbao, 31: 1405-1411. Gilder, S., Leloup, P.H., Courtillot, V., Chen, Y., Coe, R., Zhao, X., Xiao, W., Halim, N., Cogne, J.-P. and Zhu, R., 1999. Tectonic evolution of the Tancheng-Lujiang (Tan-Lu) Fault via Middle Triassic to Early Cenozoic Paleomagnetic data. J. Geophy. Res., 104(B7): 15365-15390. Hsfi, K.J., 1995. The geology of Switzerland and an introduction to tectonic facies. Princeton University Press, 250pp. Hsfi, K.J., Sun, S., Wang, Q., Chen, H. and Li, J., 1998. Tectonic facies map of China. Science Press, Beijing, 155 pp. Ji, F., Fang, Z. and Li, R., 1980. Basic characteristics of the Tan-Lu fault zone. In: Fang Z. (Ed.), 1980, Tan-Lu Fault, Seismological Press, Beijing, pp. 27-82. (in Chinese) Jia, D., He, Y., Shi, Y. and Lu, H., 1990. Decking history of the East Shandong composite terrane, East China. In: Wiley, T.J., Howell, G.P. and Wong, F.L., (Eds.), Terrane Analysis of China and the Pacific Rim: Circum-Pacific Council for Energy and Mineral Resource, Houston, pp. 345-346. Jiang, B. and Xu, J., 1993. Formation and evolution of the Ning-Wu pull-apart basin, Lower Yangtze River. In: Xu, J. (Ed.), The Tancheng-Lujiang Wrench Fault System, John Wiley and Sons, Chichester, pp. 193-200. Klimetz, M.P., 1983. Speculation on the Mesozoic plate tectonic evolution of eastern China. Tectonics, 2: 139-166. Li, C.Y., 1975. Legend for Asian geological map (1:5000000). Chiense Academy of Geological Sciences, Beijing.

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Li, S., Chen, Y., Cong, B., Zhang, Z., Zhang, R., Liou, D., Hart, S.R. and Ge, N., 1993. Collision of the North China and Yangtze Blocks and formation of coesite-bearing eclogites: Timing and processes. Chemical Geology, 109: 70-89. Li, Z., 1994. Collision between the North and South China blocks: a crustal-detachment model for suturing in the region east of the Tanlu fault. Geology, 22: 739-742. Okay, O.I. and Seng6r, A.M.C., 1992. Evidence for intracontinental thrust-related exhumation of the ultra-high-pressure rocks in China. Geology, 20:411-414. Sang, B., Chen, Y. and Shao, G., 1987. The Rb-Sr ages of metamorphic series of the Susong Group at southeastern foot of the Dabie Mountains, Anhui Province, and their tectonic significance. Reg. Geol. China, (4): 364-370. (in Chinese) Sun, J., Chen, F., Wang, G. and Jin, Y., 1993. Correlation between the Subei-Jiaonan and Dabie blocks and horizontal displacement of the Tancheng-Lujiang fault zone. In: Xu, J. (Ed.), The TanchengLujiang Wrench Fault System, John Wiley and Sons, Chichester, pp. 97-104. Uemura, T. and Mizutani, S., 1984. Geological Structures, Wiley, Chichester, 309pp. Wang Q., Zhai M., and Cong B., 1996. Regional Geology. In: Cong B. (Ed.), Ultrahigh-pressure metamorphic rocks in the Dabieshan-Sulu region of China, Science Press, Beijing; Kluwer Academic Publishers, Dordrecht, pp. 8-26. Wei, S. and Teng, J., 1993. Lithosperic structure and geophysical field, Tancheng-Lujiang Fault Zone, Eastern China. In: Xu, J. (Ed.), The Tancheng-Lujiang Wrench Fault System, John Wiley and Sons, Chichester, pp. 149-168. Xu, J., 1980. The horizontal displacement of the Tancheng-Lujiang fault zone and its geological significance. In: Scientific Papers on Geology for International Exchange, No.l, Geol. Publ. House, pp. 129-142. (in Chinese) Xu, J., 1993. Historical review and present setting. In: Xu, J. (Ed.), The Tancheng-Lujiang Wrench Fault System, John Wiley and Sons, Chichester, pp. 3-16. Xu, J., Ma, G., Tong, W., Zhu, G. and Lin, S., 1993. Displacement of the Tancheng-Lujiang Wrench Fault System and its geodynamic setting in the Northwestern Circum-Pacific. In: Xu J. (Ed.), The Tancheng-Lujiang Wrench Fault System, John Wiley and Sons, Chichester, pp. 51-76. Xu, J. and Zhu, G., 1994. Tectonic models of the Tan-Lu Fault Zone, Eastern China. International Geology Review, 36: 771-784. Xu, J., Zhu, G., Tong, W., Cui, K. and Liu, Q., 1987. Formation and evolution of the Tancheng-Lujiang wrench fault system: a major shear system to the Northwest of the Pacific Ocean. Tectonophysics, 134: 273-310. Xu, P., Sun, R., Liu, F., Wang, Q. and Cong, B., 2000. Seismic tomography showing subduction and slab breakoff of the Yangtze block beneath the Dabie-Sulu orogenic belt. Chinese Science Bulletin, 45: 70-73. Xu, S., Liu, Y., Jiang, L., Su, W. and Ji. S., 1994. Tectonic Regime and Evolution of Dabie Mountains. Science Press, Beijing, 175pp. Yin, A. and Nie, S., 1993. An indentation model for the North and South China collision and the development of the Tan-Lu and Honam fault systems, Eastern Asia. Tectonics, 12: 801-813. Zhu. R., Yang. Z., Wu. H., Ma. X., Huang B., Meng Z. and Fang D., 1998. Paleomagnetic constraints on the tectonic history of the major blocks of China during the Phanerozoic. Science in China (series D), 41 (Supp.): 1-19. Zhang, Zh.-M., Liou, J.G. and Coleman, R.G., 1984. An outline of the plate tectonics of China. Geol. Soc. Amer. Bull., 95: 295-321.

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Paradoxes in Geology Ueli Briegel and Wenjiao Xiao, eds. 9 2001 Elsevier Science B.V. All rights reserved.

Chapter 6

THE P A R A D O X OF AN O C E A N - D I P P I N G BENIOFF ZONE: EXAMPLE F R O M TAIWAN

Chia-Yu Lu Geological Institute, National Taiwan University, 245 Choushan Road, Taipei, Taiwan

ABSTRACT In Taiwan, the Paleozoic/Mesozoic basement is regarded as a rifted continental margin by back-arc spreading from the southeastern coast of the China mainland. The Backbone Range Slate Belt is interpreted as a part of the Miocene accretionary wedge. The Lishan Fault is referred to as the suture between the Philippine Sea plate and the Eurasia. Following the concept of tectonic facies in an archipelago model of intraplate orogenesis, a back-arc basin collapse orogenesis model is proposed to interpret the ocean-dipping Benioff zone of Taiwan. The back-arc basin collapse orogenesis model explains the mechanism, which transforms continental-dipping into oceanic-dipping Benioff zone as well as oceanic subduction into continental subduction.

INTRODUCTION Structural geologists have always known that lithospheric plates are not rigid. The geology of the west Pacific is an excellent demonstration that the zone of orogenic strain is not limited to a plate margin. Intraplate deformation causes the orogenesis that eliminates back-arc basins, thereby causing arc-continent or arc-arc collision (Hsfi, 1994). The theory of back-arc basins collapsing proposed by Dalziel (1981) postulates that back-arc orogenesis starts when the ocean lithosphere of a back-arc basin is consumed and subducted behind an arc, which could either be the frontal arc or a remnant arc. In the case of latter, the remnant arc would become once again an apparent frontal arc, or the frontal arc of a "micro-plate," and the arc-magnetism is active again because of partial melting of the subducted ocean lithosphere and/or of the subducted sediments of the former back-arc basin. Hsfi (1994) carried the idea of back-arc basins collapse a step further and has formulated the archipelago model of orogenesis to explain the origin of all mountains. Considering back-arc basin intra-plate features, the southern continental margin of Asia is not the plate margin. Hsfi (1994) places the plate boundary of Asia south of the Banda-Sunda arc of Indonesia and east of the Mariana arc of the west Pacific. Between the frontal arcs and the mainland are numerous remnant arcs and basins.

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The archipelago model (Hsfi, 1994) challenges the current concept that the analogues for geologic formations in the mountains are to be found on the ocean margins. Hsfi suggested that the rocks in the mountains are mostly sediments and volcanics in, or on the margin of back-arc basins and those on island arcs or remnant island arcs. A few back-arc basins have been drilled, but there are no systematic studies yet to verify or falsify the postulates of the archipelago model with information from ocean drilling. The Indonesian Archipelago is the actualistic analogue to study the geologic evolution of mountains, the South China Sea of the Archipelago is the best actualistic analogue to study the opening and closing of a back-arc basin, and Taiwan is an actualistic analogue of a collision mountain belt. The recognition of the tectonic evolution of the Taiwan Mountain Belt may help to understand the geological processes leading to the origin of mountains. PROBLEMS IN TECTONIC INTERPRETATION Taiwan lies about 150 km off the southeastern coast of the China mainland (Fig. l a). It is separated from the latter by the Taiwan Strait with an average waterdepth of 100 m. East of Taiwan, a submarine slope plunges down into the Pacific Ocean at a grade of 1 : 10, reaching a depth of more than 4000 meters at a distance of some 50 km from the coast. The earliest general geological studies began in 1895 when the Japanese occupied the Taiwan Island. Three years later, a general geologic map (1:800,000) of Taiwan was compiled by Y. Ishii and published (Chang, 1966). After the outbreak of the war between Japan and Russia in 1904, an island-wide investigation of fossil fuel resources was undertaken and extensive geologic surveys in various fields were carried out. From the Restitution of Taiwan at the end of World War II, the Chinese geologists took over these jobs; much work has been done, mostly concerning the basic geological mapping and stratigraphic subdivisions (Ho, 1988). Since the advent of the plate tectonic theory, many geologists have considered Taiwan as an active mountain belt formed by the collision between the Luzon Arc and the Eurasian continent. Their studies have contributed much to the understanding of the Taiwan orogenic edifice. Most authors assumed a simple Benioff zone in the Cenozoic plate-tectonic framework of Taiwan, whereas Biq (1972, 1977) proposed a dual-trench structure in the Taiwan-Luzon region. He suggested that the North Luzon Trough marks an active east-dipping subduction zone and is the boundary between the Philippine Sea Plate and Eurasia, and that the Manila Trench was extending into Taiwan as an intra Asian plate subduction zone during the early Pleistocene continent-arc collision that strongly tectonized the island. If the simple Benioff zone tectonic framework and the notion that the Tertiary rocks of the Central Range are Eurasian continental-margin deposits are accepted, there are still many questions, which remain to be answered: l) Where is the suture between the Philippine Sea Plate and Eurasia in Taiwan? 2) How did the arc and the continent collide? The collision has been variously interpreted as a consequence of the impingement by the Luzon Arc moving

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Fig. 1. Location map and main stratigraphy unit of Taiwan (Modified after Ho, 1988). (a) General tectonic map (upper left corner): Isobaths in meters, large open arrow showing the direction of convergence (Philippine Sea Plate relative to Eurasia). (b) Major thrust faults as heavy lines with triangles on the upthrust side (CC - Chfichih Fault; LS - Lishan Fault; LAF - Laonungchi Fault; ZF - Central Range Fault; CF - Coast Range Fault). A, B, C, D locations of geological transects in Fig. 2. L - Liuguai, W - Wanda reservoir, Y - Yakou. Circled A, B, C, and D, major tectonic lines, described also in Fig. 4.

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4)

5)

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in a northwesterly or north northeasterly direction, with a clockwise or counterclockwise rotation. When was the onset of the collision between the Asian continental margin and the Luzon Arc? The beginning of the collision has been suggested by different authors to be in the Middle Miocene, Late Miocene, Early Pliocene, Late Pliocene or PlioPleistocene. Why is the Luzon Arc so rigid? The collision zone of the Luzon Arc in Taiwan is only the length of the Coastal Range (about half time length of the Central Range); there has no collision been found yet to the south of the Coastal Range. How has the Central Range been formed by the collision of the Luzon Arc? Where have the sediments of this part of the Manila Trench accretionary wedge gone? Only small portions of them are exposed on the Hengchun Peninsula (Kenting M61ange at southernmost tip of Taiwan) and at the south end of the Coastal Range (Lichi M61ange). Furthermore, the Kenting M61ange seems more likely to consist of deformed continental margin deposits of Eurasia. Why is the Taiwan Island in a unique position at the present intersection of two convergent plate junctions of opposite polarity? This article is an attempt to answer these questions.

THE TECTONIC F R A M E W O R K OF THE TAIWAN REGION The island of Taiwan is on the boundary between the Eurasian and the Philippine Sea Plate and, more exactly, on the junction of the Luzon Arc and the Ryukyu Arc (Fig. 1). The lithosphere of Eurasia around Taiwan includes the crust of the Chinese continental margin and the oceanic crust of the South China Sea. The Philippine Sea Plate is subducting beneath the continental part of Eurasia at the Ryukyu Trench, and overriding the crust of the South China Sea along the Manila Trench. The west facing Luzon Arc, associated with the North Luzon Trough, extends to the north from southern Luzon of the Philippine Arc and finally becomes the Coastal Range and Longitudinal Valley of Taiwan (Biq, 1964; Bowin et al., 1978; Chai, 1972; Defant et al., 1989). The Manila Trench is a deep-sea furrow lying west of, and parallel to, the North Luzon Trough (Biq, 1972; Ludwig et al., 1967). A submarine ridge separates the two, but the Manila Trench gradually disappears near the southern coast of Taiwan. The Ryukyu Trench extends from southern Japan to eastern Taiwan. It is associated with the southeast facing Ryukyu Arc and Okinawa Trough (Fig. la) (Bowin et al., 1978; Tsai, 1986; Tsai et al., 1977). The Ryukyu arc-trench system was built up on the edge of the Eurasian continent from the Early Miocene time (Kizaki, 1986; Letouzey and Kimura, 1986), whereas the Okinawa Trough was opened up in the Late Pliocene (Lee et al., 1980; Letouzey and Kimura, 1986; Viallon et al., 1986). The Okinawa Trough is an incipient intercontinental back arc basin that has developed behind the Ryukyu arc-trench system (Shinjo et al., 1999). Early Cretaceous oceanic crust has been recognized to the east of the Huatung Basin (oceanic crust west of the Gagua Ridge, Yeh, K. Y. and Lallemand, S. personal communication, 1999). As a consequence of the opening of the South China

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Sea from the Late Oligocene onwards, many parts of the Philippine-Taiwan archipelago are interpreted as the rifting fragments of the southeastern China continental margin (Holloway, 1982; Hsfi et al., 1990; Taylor and Hayes, 1980; Taylor and Hayes, 1983). The back arc basins of south China were formed behind a frontal arc at the China-Pacific plate-boundary. By the end of the Cretaceous, the western boundary of the Pacific plate was located east of the Philippines. Subduction of the Pacific plate during the Paleogene caused the separation of the Mariana system (or more exactly a Paleo-Mariana Arc, underlain by the continental crust of Palau/Ryukyu, Mariana Ridge, and Mariana Arc) from the Philippines, which was then still attached to Mainland Asia, because the South China Sea basin was not yet there. This process changed the climate of south China from a Sahara desert to a maritime climate (Hsti et al., 1995). The Luzon Arc developed along the Philippine Sea Plate in Early Miocene or Late Oligocene time (Bachman et al., 1983; Karig, 1983; Richard et al., 1986). Paleogeographic data from DSDP cores indicate that the Philippine Sea was formed by a series of back arc spreading centers since the Paleogene (Hayes, 1982; Hilde and Lee, 1984; Lewis and Hayes, 1983; Matsubara and Seno, 1980; Seno and Maruyama, 1984). The Philippine Sea Plate is presently rotating clockwise and moving toward the west-northwest at a speed of 7-9 cm/yr, in the Taiwan-Luzon region (Katsumata and Sykes, 1969; Minster and Jordan, 1979; Seno, 1977; Seno et al., 1987). Paleomagnetic results from ocean drilling and from land on the Philippine Sea Plate indicate progressive northward movement of the entire plate during the Tertiary. Using these paleomagnetic results it is possible to reconstruct the geodynamics of the Philippine Sea Plate since 50 Ma. The southern part of the Philippine Sea PlateEurasia plate boundary has been largely a strike-slip boundary from 50 Ma to 15 Ma (Hall et al., 1995). Geological information from Japan, the Philippines and Taiwan indicates a change of motion from north-northwest to west-northwest around 5 Ma ago (Matsubara and Seno, 1980; Sarewitz and Karig, 1986; Seno and Maruyama, 1984; Teng, 1987). The east-dipping subduction of the Manila Trench is inferred to begin in late Oligocene to middle Miocene time, based on geological and geophysical data (Hayes and Lewis, 1984; Lewis and Hayes, 1984; Taylor and Hayes, 1980, 1983). Many authors have been trying to reconstruct the paleo-tectonics of Taiwan by following these Cenozoic tectonic events (Barrier and Angelier, 1986; Chai, 1972; Chi et al., 1981; Hsfi et al., 1995; Huang et al., 1995; Lu and Hsfi, 1992; Suppe, 1984; Suppe, 1988; Suppe et al., 1981; Teng, 1990). Their contributions serve as building blocks in the construction of the tectonic history in this paper. Thus the tectonic evolution of Taiwan depends on the development of all those features mentioned above. AN OUTLINE OF THE GEOLOGY OF TAIWAN Taiwan can be divided into five major tectono-stratigraphic units including, from the west to the east: the Coastal Plain and the Western Foothills; the Hsfiehshan Range; the Backbone Range; the Paleozoic/Mesozoic basement; and the Coastal Range (Fig. 1). The Central Range of Taiwan, in this paper, includes the Hsfiehshan Range, the Backbone Range and the Paleozoic/Mesozoic basement.

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The stratigraphy of the Coastal Plain and the Western Foothills are known as a passive margin shallow marine clastic sequence of Oligocene-Miocene-Quaternary age. The strata are fossiliferous and little metamorphosed, and folding and faulting have deformed them. However, their stratal continuity has not been severely disrupted. The stratigraphy is divided naturally into two sequences by a regional lower Oligocene unconformity (Huang, 1982). The units near the unconformity function widely as the basal d6collement in the Western Foothills fold-and-thrust belt (Suppe, 1987). They are succeeded by a Pliocene and Pleistocene molassic sequence of foreland basin sediments, four kilometers thick, derived from the eastern mountain chains. No clastic sediment was found encroaching from the east before 3 Ma (NN15) (Huang, 1976). A gradually transition from northern to southern Taiwan into deep-water facies of Neogene sedimentary rocks in the Western Foothills was described by Ho (1975). The Hsfiehshan Range is composed of a Eocene to Miocene passive margin shallow marine sequence. It appears on the surface to be quartzites and slates of prehnite-pumpellyite to lower greenschists facies. Most of the quartzite layers in this unit retain stratal continuity and can be traced for tens of kilometers. Slaty cleavage is well developed in the shale layers. This sequence is characterized by transgressive facies and rift facies (Lu et al., 1991; Sun, 1982; Wang, 1987) and is deposited in a system of major normal-fault basins that strike roughly parallel to the trend of the South China Sea (Sun, 1982). The normal faulting was still active, though less strongly, in the Miocene. Some of these faults have been reactivated during the Pleistocene compression in the Foothills as reverse faults or tear faults (Bonilla, 1975; Suppe, 1986). The Backbone Range, which we shall discuss in detail later, is generally composed of two rock units: the Eocene "slate", composed mainly of dark gray flysch and phyllite, containing interbeds of whitish to dark colored metasandstones, limestone lenses and irregular conglomerate bands (Ho, 1975); and the Miocene "slate", composed of dark gray argillites, phyllites and flysch with occasional interbeds of dark gray compact sandstone and disseminated marly nodules. Both units were metamorphosed under prehnite-pumpellyite facies to lower greenschist facies conditions (Chen et al., 1983b) and are characterized by the intense layer parallel shearing with or without slaty cleavage. The stratal continuity is very poor in both units; most layers cannot be traced for 100 m. Features of the broken-formation and M61ange are common. The Paleozoic/Mesozoic basement is also called the Gutaiwan Arc in this paper. This basement is composed of three lithologic units and all of these were metamorphosed under greenschist to lower amphibolite facies conditions (Yen, 1962). The first rock unit consists of quartz-mica schists, phyllite, metasandstone, and metaconglomerate, in association with exotic blocks, ranging in size from centimeters to kilometers, of marble, quartzite as well as dismembered ophiolitic blocks of metabasite, amphibolite and ultramafic rocks. This unit has been interpreted to be a m61ange complex in an ancient accretionary wedge or suture zone (Hsfi, 1988; Lu, 1986; Yui et al., 1988; Yui et al., 1990). The scarce fusulinids and corals found within the marble layers suggest that at least a part of the unit is late Permian in age (Yen, 1953). Dinoflagellate fossils found in the quartz-mica

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schist indicate its middle Jurassic (?) to lower Cretaceous age (Chen, 1989). Recent 40Ar/39Ar dating of high-pressure rocks in this unit reveals a high-pressure metamorphic age of ~94 Ma (Lo and Yui, 1992). This means that this unit was subjected to a Mesozoic orogen. The second unit consists exclusively of massive marble. No fossil has been found within this unit. Radiometric dating suggests a Paleozoic age (Jahn, 1988; Jahn et al., 1986). The third unit consists of granitoid rocks. U-Pb, Rb-Sr, K - A r and 40Ar/39Ar dating of these rocks has yielded three groups of ages: 85-90 Ma, 30-40 Ma and later than 10 Ma (Jahn et al., 1986; Jahn et al., 1973; Lan et al., 1990; Lo and Onstott, 1995; Yui et al., 1990). The general lithologic distribution and structures of the Paleozoic/Mesozoic basement of the Backbone Range and the Hsfiehshan Range are shown in four transects in Fig. 2, whose locations are shown in Fig. 1. A mylonitic fault zone, associated with a kilometer- scale overturned synform structure in the Backbone belt, always marks the contact between the Paleozoic/Mesozoic basement and the

Fig. 2. Cross sections showing the general structures and lithology of the Paleozoic/Mesozoic basement and the "Slate" Belt. (A) The Suao Hualien Highway (modified after (Yen, 1960); (B) The Hoping-bei river (modified after Chiao, 1991). (C) The Central cross-Island Highway (modified after (Lu, 1991b; Wang Lee and Lu, 1981; Yang and Lo, 1986; Yen, 1960); (D) The eastern part of the Southern Cross Island Highway (modified after (Yen, 1960).

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Backbone Range. The boundary between the Hsfiehshan Range and the Backbone Range on the Central Cross Island Highway is interpreted as a back-thrust, based on the shear sense of the pressure shadows in the slate (Clark et al., 1991). The Coastal Range is composed of deformed rocks of the Miocene volcanic arc and fore-arc basin of the Luzon Arc, together with an overlying Pliocene and Pleistocene molasse sequence of four to five kilometers in thickness and the Lichi M61ange (Ho, 1982). THE LOCATION OF THE "PLATE" BOUNDARY Sutures (Gansser, 1964) mark zones along which oceanic lithosphere has been totally consumed. A subduction complex, whose being often wrecked into a m~lange reveals the closure of a preexisting oceanic trench on top of the subduction zone, and therefore commonly marks a suture zone boundary. According to this principle, Biq (1969; 1971a; 1972; 1973; 1976b; 1977)placed the present-day suture boundary along the zone which is marked by the outcrops of the Lichi M61ange, that means the region to the west of the Longitudinal Valley including the Paleozoic/Mesozoic basement, the Backbone Range, the Hsfiehshan Range, the Foothills and the Coastal Plain belongs to Eurasia. Tsai (Tsai et al., 1977; Tsai, 1978; 1986), based on seismic data, agrees with Biq's observation. On the other hand, Bowin et al. (1978), Suppe (1981; 1984; 1987), Ernst (1983), Ernst et al. (1985) and Angelier et al. (1986), based on bathymetric and geophysics data, proposed a single subduction zone on the western margin of the Central Range. This model suggests that most part of Taiwan rode over oceanic crust. Yen (1968) used available geophysical, geochemical and geological data to show that the crust under most parts of Taiwan is of continental type, but the crust under the Coastal Range is transitional to the oceanic type. Ho (1982, 1988) from a geological point of view also indicated that the Taiwan Island is not underlain by oceanic crust. Chi et al. (1981), Sun (1982, 1985), Suppe (1988) concluded that foothills and thrust belt belong to Eurasia. Suppe (1988) recognized that the possible "fragments" formally belong to Eurasia, but are splitted off from the Oligocene-Miocene South China Sea. The sea floor spreading thus carries possibly Paleogene sediments of the Central Range. The stratigraphy in Hengchun records collision with rifted fragments of China and the Oligocene and early Miocene oceanic crust on the south side of the South China Sea (Suppe, 1988). The northern extension of the Hengchun collision zone is the Central Range. The Central Range is the main source of the rocks of the collision stratigraphy in the Coastal Range. Therefore, the suture must be somewhere on the west margin of the Central Range or in the Central Range itself. DISCUSSION ON THE ORIGIN OF MI~LANGE M61ange are heterogeneous tectonic mixtures of blocks and/or lens-shaped components derived from different lithologies including continent/oceanic

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basement, flysch and other sediments entombed in a pervasively sheared matrix (Greenly, 1919; Hsfi, 1974). In m61ange units, an original stratigraphic suite cannot be deduced from the observed superposition of subunits and elements included (Hsfi, 1976). Lichi MOlange

The Lichi M61ange (Fig. 1) consists of clasts, blocks and slabs of igneous and sedimentary rocks (Biq, 1969, 1971a; Hsfi, 1976; Hsfi, 1988). The igneous rocks belong exclusively to the ophiolite suite (Liou et al., 1977). The fragmented sedimentary strata in the Lichi unit are mainly interbedded turbidite sandstone and hemipelagic shale. The style of their deformation is, in some instances, similar to that of broken formation in the Franciscan M61ange. The basal mafic breccia displays the effects of partial ocean ridge type metamorphism under greenschist to rare amphibolite-facies conditions (Liou and Ernst, 1979). Fossils of mixed origin and ages, Oligocene to the middle Pliocene, have been found in both the matrix and the exotic blocks of the Lichi M61ange (Chang, 1967; 1968; 1969; Chang and Chen, 1970; Chi, 1982; Huang et al., 1979). Biq (1969; 1971a; 1972; 1976ab) suggested that Lichi M61ange was a tectonic m61ange. However, Wang (1976), from a stratigraphic viewpoint, stated that the Lichi "Formation" is the olistostromal facies of the underlying sequences (Takangkou Formation and Chimei Formation). Among the exotic blocks that slid into the Lichi Formation are turbidite sandstones derived from the Takangkou Formation and mafic and ultramafic rock-fragments rewashed from an ophiolite slab obducted on the continent during an arc-continent collision. Thus the Lichi Formation cannot be considered as a tectonic m61ange. Suppe et al. (1981) suggested that the ophiolite unit in the Lichi M61ange (the East Taiwan Ophiolite) appears to have formed along a major left-lateral transform fault of the China Basin Range system about 700 to 1000 km southeast of its present position in the Coastal Range of Eastern Taiwan; Page and Suppe (1981) suggested the olistostromal origin of the Lichi M61ange due to gravity sliding. They proposed that Lichi M61ange was not formed in a trench or subduction zone because the m61ange is contained within the stratigraphic sequence of the volcanic island arc. The Lichi M61ange was interpreted to be a fault-scarp olistostromal facies of the underlying (Takangkou) formation deposited on the west flank of the Luzon Trough during the collapse of the forearc basin when arc-continental collision began. However, because there is a consensus that the m61ange marks the suture of the arc-continent collision (Biq, 1972; Chen, 1991b; Hsfi, 1988; Huang et al., 1995; Lu and Hsfi, 1992; Teng, 1980; Teng, 1987), the observations of olistostromes were largely ignored. Barrier and Angelier (1986) postulated that the origin of the blocks in Lichi M61ange is probably linked to a high angle slope of oceanic origin. Their data indicate that the Lichi M61ange was deposited rapidly, during a period of time that marked a change in the regional tectonics and relate this event to a change in direction of plate motion that probably occurred around 4 Ma ago. The Lichi M61ange thereby may have been strongly sheared, which resulted in a change of the dominant strike-slip movement into dominant thrust movement. We shall discuss this event later.

Chia- Yu Lu

86

Kenting M~lange The Kenting M61ange (Figs. 1 and 3) is a 2000-meter thick chaotic argillaceous sediment-mass widely exposed on the Hengchun Peninsula at the southern extremity of Taiwan (Biq, 1977; Tsan, 1974a, b). The intensely sheared fragments in the argillaceous matrix were described by Pelletier and Stephan (1986). The Kenting M61ange includes exotic blocks of ophiolites and Neogene conglomerates, reef limestone, deep-water turbidite sandstone, slump deposits and hemipelagic shale. The blocks of layered rocks have shown little disruption of stratal continuity. Yet the close proximity of rocks of different origins indicated the m61ange nature of the Kenting rock-mass. This type of slabs and blocks, mixed together in a collision zone, has been called thrust-sheet type of m61ange by Taira and Tashiro (1987) and has been observed in the collision zone between Eurasia and the Bonin Arc in the FujiHakune region of Japan (Hsfi, 1988). A variety of fossils have been found in the Kenting M61ange. Chi (1982) shows that the oldest assemblages are Eocene in age, and some others are Oligocene, but most are late Miocene. The youngest species indicate a late Miocene age. However, Huang et al. (1983) have discovered early Pliocene nannofossils in the Kenting blocks. Huang (1984; Huang et al., 1985) claimed that the blocks in the Kenting

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The paradox of an ocean-dipping Benioff zone: example from Taiwan

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M61ange contain the characteristic late Miocene (N14-N17) and early Pleistocene (N22) planktonic formaminifers. The late Miocene blocks have no reworked fauna, but the Pliocene and Pleistocene horizons bear abundant reworked early, middle and late Miocene planktonic formaminifers. The investigation of Pelletier (1985) gave a Middle to Late Miocene (NN 11) age that confirmed the result given by Chi (1982). Biq (1972, 1977) proposed that the Kenting M61ange is a tectonic m61ange. Pelletier and Stephan (1986) interpreted the Kenting "Formation" as a tectosedimentary m61ange. Further ideas by Tsan (1974a) and Page and Lan (1983) suggested that large-scale debris flows were the origin of the Kenting M61ange, considering that the ophiolitic exotics probably came from the East Taiwan Ophiolite (Liou et al., 1977). However, the exotic conglomerate has a sandy matrix and it does not have the same bimodal distribution of clasts as muddy conglomerates formed by submarine slides (Hsfi, 1988). Moreover, the reconstruction of the Miocene paleogeography of eastern Taiwan (Suppe, 1988; Teng, 1990) shows no evidence of the presence of a high source area from which the Kenting mass might have slid down. Besides, the penetrative shear phenomena and the large variation in composition of the exotic block also indicate subduction as the forming mechanism for the Kenting M61ange. The recognition of the Pingtung Valley (Fig. 1) as the northernmost onshore part of the Manila Trench is based not merely upon the morphological aspect of the valley, but also upon certain geological and geophysical features strongly indicative of the active subsidence of the Pingtung area (Biq, 1977; muang et al., 1985). The trench is diagnostically marked on land by its large amount of m61ange. In summary, the Kenting M61ange is a subduction m61ange because it is a mixture of rocks from different tectonic environments. These rocks were shear-deformed in a trench environment. The features in the Kenting M61ange suggest that these rocks have experienced breaking, mixing and shearing which support the view that this m61ange was processed in a subduction zone. The Kenting M61ange may have formed in late Miocene and re-deformed later (Plio-Pleistocene) due to the oblique convergence that we shall describe later. THE BACKBONE RANGE SLATE BELT Recognizing that there is a subduction zone east of the Pingtung valley, we might predict an accretionary wedge extending northward from the southernmost part of Taiwan. The first possible candidate is the Backbone Range Slate Belt (BRSB). The BRSB was first mapped as the outcrop zone of a Tertiary slate series in the Geology and Mineral Resource Map of Taiwan compiled by Y. Ichikawa (Chang, 1966). During the last five decades, normal stratigraphic methodology was practiced and few questioned the applicability of the law of superposition, lateral continuity, and paleontological dating. Despite lines of evidence that suggest the internal complexity of this mass of strata, generations of researchers portrayed it as an orderly succession of simply deformed formations ranging in age from Eocene to Middle Miocene. The local geologic maps of the BRSB often give the reader the impression that, on a small scale, the "formations" or their enveloping surfaces are more or less

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Chia- Yu Lu

horizontal in attitude. In the reports associated with these geologic maps, it was always told that rocks bearing Eocene fossils and those bearing Miocene species were alternately found in cross sections and the boundary between the two fossiliferous zones was an unconformity, weakly folded. This has led many observers to believe that each of the cross-island highways runs across this "unconformity" several times. The BRSB is a huge mass of slate, phyllite, greywacke and metagraywacke, constituting a sedimentary sequence comparable to the Alpine Flysch. "Immature terrigeneous detritus from basement and sediment, and sedimentary cover rocks suggest strong orogenic uplift and erosion as a source of flysch sedimentation" (Tercier, 1947). This is the classical definition of the term flysch, introduced by Studer (1827), (Hsfi, 1960). Depending on the subsequent tectonic evolution of the sedimentary basins, the flysch sequences seem to be preserved in stratigraphic continuity with their basement, or sheared off forming rootless tectonic units. Increasing deformation and dismembering of these rocks leads to the formation of broken flysch formation (Hsfi, 1968) and finally of tectonic m~langes. The rocks of the BRSB have been severely sheared, so that layer-parallel cleavage is well developed. The "pinch and swell" structure is common in the sandstone units. The lateral continuity of the sedimentary formation is limited, and few beds can be traced for more than a hundred meters. Igneous rocks such as pillow lavas, diorite, and gabbro are not uncommon. There are also minor andesite, dolerite, and other pyroclastic products of less-mafic composition demonstrating that there was some synsedimentary arc volcanic activity (Chen, 1991a). The ophiolites are apparently derived from the disruption of former ocean crust, and occur as exotic blocks in the matrix of slate and greywacke. Also included in the group are chlorite-sericite-albite rocks. The so-called tuffs are, in many instances, slightly metamorphosed pelitic rocks or basaltic rocks. Yui et al. (1994), based on the available tectonodiscrimination diagrams and spidergrams of the chemical composition of the metabasites from the slate formations of Taiwan, described that these metabasites can be geochemically split into two groups: one of the volcanic-arc (calc-alkaline) affinity and the other of intra-plate (alkali) affinity. Despite other arguments (Chung, 1995; Yui et al., 1995), this result is similar to that of metabasite in the Paleozoic/Mesozoic Basement (Yui et al., 1990) and could be fitted into the "accretionary wedge" model of Lu and Hsfi (1992). The fossils found are distributed in a number of scattered exposures and their ages range widely from the Cretaceous (Yen et al., 1956), through the Eocene (Chang, 1972), the Oligocene (Hashimoto and Matsumaru, 1975) the late Oligocene (Huang, 1980; Lee, 1977) to the Miocene (Chang, 1973, 1974, 1975). Often fossils of different ages were found in the same "Formation". The difficulties in practicing the normal stratigraphic methodology are well described by Ho (1988), as follows: "Complete faunal zonation is, however, hardly possible because no continuous fossiliferous section is observed and barren stratigraphic sections are quite common . . . . However, the abundant fossil evidence gives only an improved knowledge of the distribution of rocks of different ages in the argillaceous sequence, it provides little help in clarifying different stratigraphic units that could be used in field mapping." The traditional stratigraphic practice is

The paradox of an ocean-dipping Benioff zone." example from Taiwan

89

such that older fossiliferous rocks are automatically included as reworked strata and as being a part of the younger strata. Ho (1986) divided the BRSB into two stratigraphic units: the Miocene Lushan "Formation" and the Eocene Pilushan "Formation" based on the fossils found in these rocks. He added that, however, "no distinct lithologic break has been found between the Lushan Formation and the Pilushan Formation. The essentially similar argillaceous lithology of these two rock units makes the boundary arbitrarily defined at most places. Distinction between these two formations has to be made largely on fossil evidence and this faunal boundary cannot be applied in field mapping. The distribution of these two formations on the geologic maps is, therefore, quite uncertain. However, no good evidence of an angular discordance has ever been discovered". Biq (1962) compared the rocks in the BRSB to the Bfindnerschiefer or Schistes Lustr6 of the Pennine Alps, now well known as metamorphosed hemipelagic mud composed of deep-water argillaceous sediment, containing ophiolites, and representing a part of the Alpine Paleogene accretionary wedge. In many localities of the eastern flank of the Central Range, the Lushan "Formation", like the Pilushan "Formation", is in direct contact with the Paleozoic/ Mesozoic basement, and thus their contact relationship must be an unconformity or a fault zone. This subject is widely discussed in Suppe et al. (1976), Tsan (1977), and Chen et al. (1983a). The nappe stacking structures of the BRSB were studied by Biq (1976a,b, 1985) and Pelletier and Hu (1984). Field work showed the general structure of the BRSB to be a synform or synclinorium (Lu, 1991b; Yang and Lo, 1986) with penetrative flat-lying thrusts and layer parallel foliation indicating that shear deformation is dominated in the BRSB. Dismembered ophiolites and shallow-water limestone, deep-water turbidite sandstone, slump deposits and hemipelagic shale occur as exotic blocks and are very common. The deep-sea depositional setting of the northern part of BRSB is studied by Teng et al. (1991). Many ophiolitic rocks occurring as exotic blocks, for example at Liukuai, the Wanda reservoir and Yakou (Fig. 1), are little affected by metamorphism, and the pillow structures and igneous textures of basalt, and gabbro and ultramafic rocks are commonly recognizable. We note that the flysch is not everywhere associated with ophiolites. We have, therefore, established four map units in making our tectonic map: two broken formations (designated Central Range Flysch Nappes and Coastal Range Flysch Nappes; and two m61anges (Kenting M61ange and Lichi M61ange). These units are shown in Fig. 1. The South China Sea is subducted under the Philippine Sea Plate along the Manila Trench. As the northern extension of the accretionary wedge of the Manila Trench, the boundary between the BRSB and the Hsfiehshan Range, we suggest, is the suture between Eurasian and Philippine Sea plate. Partly in accordance with the statement above is Biq's view. He has suggested that there is a virtual arc-and-trench relationship between the Backbone Range and the adjacent Hsfiehshan Range, the latter range being actually a subduction zone, albeit Ampferer in type (Biq, 1989).

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Chia- Yu Lu

From the above arguments, we conclude that the BRSB is a Miocene accretionary wedge occurring in the form of flysch nappes, as a complex of broken formations and a m61ange. G U T A I W A N ARC AS A D R I F T E D BLOCK The rifting characters of the Paleogene depositional basin of Taiwan has been well studied by many workers, (e.g. Chou, 1973; Lu et al., 1991; Sun, 1982, 1985; Teng et al., 1991; Wang, 1987). Based on the results of their studies of northern Taiwan stratigraphy, Teng et al. (1991) concluded that the Paleogene basin west of the Paleozoic/Mesozoic basement (Gutaiwan) was a west-dipping half-graben, which had experienced a rifting stage from the Late Cretaceous to the Early Miocene and an immediate drifting stage from the Early Miocene to the Middle Miocene. The counterpart of this half-graben on the west side of the Taiwan Strait has been found along the southeastern coast of the Chinese continental margin (Ru and Piggot, 1986; Sun, 1982, 1985). The equivalent of the Mesozoic M61ange in Gutaiwan has also been found near Quanzhou on the southern coast of Fujian (Hsfi et al., 1990). Therefore, the Gutaiwan Arc is interpreted as a rifted block of Mesozoic collision zone of southeastern China. COLLISION-TYPE O R O G E N Y Accepting the above arguments and following the tectonic facies concept (Hsfi, 1994; Hsfi et al., 1995), we have recognized three tectonic units in the Taiwan orogenic belt: (I) A l e m a n i d e the foreland thrust belt (cover thrusts) represented by the Hsuehshan Range and the foothills, together with a foreland molasse basin. (II) C e l t i d e - the Miocene Accretionary Wedge identified by flysch nappes, broken formations of the Backbone Range Slate Belt and the Kenting M61ange. The western part of this region turned into Alemanide, and the eastern part transformed into Celtide, during the Plio-Pleistocene arccontinent collision. (III) Rhaetide - the overriding plate consisting of the Paleozoic/Mesozoic basement, (1R and 2C) Neogene Volcanic Arc (2R), intraplate Lichi M61ange (2C) and forearc basin filled with flysch and molasse (2C). The distribution of these three tectonic units is shown in Figs. 1 and 3. As listed below, these tectonic units can be divided further into subunits in certain areas by the tectonic lines shown in Figs. 3, and 4. Eurasia (A) Foothills t h r u s t - Changhua thrust (Kaohsiung- Tanshui line, (A) line) (i) Foothills Folded Belt (B) Front Range Thrust (ii) Front Range Thrust Belt (Chishan- Chinkuashih line, (B) line)

The paradox of an ocean-dipping Benioff zone: example from Taiwan

91

Fig. 4. Diagram showing the distribution in time and space of rock groups of Taiwan. (Modified after (Biq, 1974; Biq et al., 1985). Key 1. Gutaiwan Arc; 2. Shelf sediments, partly converted into low-grade metamorphic rocks (hatched); 3. flysch and flyshoid sediments; 4. marble; 5. ophiolitic m61ange; 6. molasse; 7. arc andesite, N: northern Taiwan, S: southern Taiwan.

(iia) Mainly Miocene rocks (iib) Mainly Oligocene rocks Philippine Sea Plate (C) Plate Boundary Thrust (Hengchun- Ilan line, (C) line) (iii) Central Range Arc (iiia) Miocene Accretionary Wedge (iiib) Paleozoic/Mesozoic Basement (D) East Rift Valley strike slip Fault (Taitung- Hualien Line, (D) line) (iv) Coast Range Volcanic Arc The distribution in time and space of rock groups of Taiwan is shown in Figure 4. The upper limit level shows the time of the emplacement of each unit. THE TECTONIC EVOLUTION OF TAIWAN Based on the foregoing descriptions, major conclusions are put forward as follows: 1) The suture between the Philippine Sea Plate and Eurasia in Taiwan is located along the Lishan Fault (Hengchun-Ilan line).

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Ch&- Yu Lu

Fig. 5. The tectonic evolution of Taiwan. (See text) (Modified after Lu and Hsti, 1992).

2) The Taiwan Mountain Belt consists of two collision belts, the first collision resulted from the oblique convergence between the Gutaiwan Arc and Eurasia (Fig. 5 A, B, C) and the second one (Fig. 5 D, E) might be the result of the changing plate convergence direction from N N W to WNW, that converted the Longitudinal fault (D fault in Fig. 4) from a dominated strike-slip fault into a dominated thrust fault.

The paradox of an ocean-dipping Benioff zone." example from Taiwan

93

3) The first collision began at Late Miocene time. The second collision (between the Coastal Range and the Gutaiwan Arc) occurred during the Plio-Pleistocene. 4) The rocks of the Central Range of Taiwan were severely deformed and metamorphosed during the Miocene collision. After the second collision, the Taiwan area has been transversely shortened and uplifted into a mountain belt. 5) The BRSB represents the accreted materials filling the gap between the Chinese continental margin and the west-advancing Gutaiwan Arc. 6) Hsfi (1994) places the eastern plate boundary of Asia east of the Mariana arc of the west Pacific. Between the frontal arcs and mainland are numerous remnant arcs and basins (Fig. 6). Many Early Cretaceous oceanic basins are created at the westernmost part of the Philippine basin. This old oceanic crust with higher density causes the later oceanic dipping subduction because of the transform fault which join two opposite subduction polarities together. The following sums up the tectonic development of the Taiwan Mountain Belt: F r o m the Late Oligocene to the Early Miocene, the continental margin near southeastern China was rifting and numerous back arc basins formed at the eastern side of South China Sea (Ru and Piggot, 1986). A part of the Gunanhai

Fig. 6. Back-arc basins and sutures in Southeast-Asia. M.T.- Manila Trench, K.M.- Kenting M~lange, P.T.- Palawan Trough, S - Sulu Ridge, R - Reed Bank, M - Malay Penisula. Hatched areas represent oceanic crust.

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Chia- Yu Lu

M61ange and the Dongnanya Block (Gutaiwan Arc) (Hsti et al., 1990) were rifted to the southeast. The back-arc collapse of the South China Sea began approximately at the middle Miocene (~15 Ma) (Fig. 5B). At the same time the deep-sea and hemipelagic sediments were deposited in the South China Sea. Synsedimentary volcanic activity is demonstrated by the tuffecious layers and volcanic rocks found in many places within the Oligocene- Miocene sequence of Taiwan (Biq, 1997). The northern tip of the Gutaiwan Arc collided with the Chinese continental margin around 9 Ma ago (Fig. 5 C, D) and the Kenting M61ange was formed. Because the Chinese continental margin is extended in a N 60~ direction, whereas the trend of the Gutaiwan Arc and the concomitant Manila Trench is N 16~ there was an oblique transgressive collision between the advancing arc and the passive continent. The collision zone propagated from the north to the south, which resulted in the younger deformation in the Kenting M61ange and the gradual transition from northern to southern Taiwan into deep-water facies of Neogene sedimentary rocks in the Western Foothills. During this period, the Coastal Range Arc began to grow after a strike-slip fault zone was developed within the Philippine Arc on the Philippine Sea Plate (Richard et al., 1986). The ophiolites in the latter formed Lichi M61ange which might have slumped into the pull-apart basin within this strike-slip fault zone (Guhuadong Trough). A change of motion of the Philippine Sea Plate from north-northwest to westnorthwest took place around 5 Ma ago (Matsubara and Seno, 1980; Sarewitz and Karig, 1986; Seno and Maruyama, 1984; Teng, 1987) (Fig. 5 D). This movement has drastically changed the fault styles around the Philippine Sea Plate. The faults around the Gutaiwan Arc and the Luzon Arc have changed from a dominated strike-slip fault style into a dominated thrust fault style. The Coastal Range Arc started to collide with the Gutaiwan Arc. In the same period, the post-collision deformation was penetratively developed within the collision zone between the Gutaiwan Arc and the Chinese continental margin. Around 3 Ma ago, the Lichi M61ange was in its active formation between the Coastal Range Island and the Gutaiwan Arc. Synkinematically, the Coastal Range arc began to indent into the Gutaiwan Arc, and back thrusting was initiated (Lu, 1991a; Stanley et al., 1981). To the extent of this collision having been an oblique collision, strong strike-slip movements remobilized many zones of earlier structures (Angelier et al., 1986, 1990; Biq, 1971b, 1981, 1989; Lu et al., 1991). Finally the Taiwan Island was rapidly uplifted and eroded (Fig. 5 E). ACKNOWLEDGMENT I am grateful to Professor Ken Hsfi for advice and encouragement during this work. This paper could not have been finished without his help. REFERENCES Angelier, J., 1990. Geodynamic evolution of the eastern Eurasian margin-Foreword. Tectonophysics, 183(1-4), vii-x.

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Angelier, J., Barrier, E. and Chu, H.T., 1986. Paleostress trajectories related to plate collision in the Foothills fold-thrust belt of Taiwan. Mem. Geol. Soc. China, 7: 201-217. Angelier, J., Bergerat, F., Chu, H.T. and Lee, T.Q., 1990. Tectonic analysis and the evolution of a curved collision belt: the Hsuehshan Range, northern Taiwan. Tecton-physics, 183(1-4): 77-96. Bachman, S.B., Lewis, S.D. and Schweller, W.J., 1983. Evolution of a forearc basin, Luzon Central Valley, Philippines. Amer. Assoc. Pet. Geol. Bull., 67:1143-1162. Barrier, E. and Angelier, J., 1986. Active collision in Eastern Taiwan: the Coastal Range. Tectonophysics, 125: 39-72. Biq, Chingchang, 1962. Schistes-lustres-type sediments in Taiwan. Mem. Geol. Soc. China: 103-106. Biq, Chingchang, 1964. Taiwan and the Alps: an attempt at comparison of island and mountain arcs. Rept. XXII Int. Geol. Congr., 22(11), 220-238. Biq, Chingchang, 1969. Role of gravitational gliding in Taiwan tectogenesis. Bull. Geol. Surv. Taiwan: 42p. Biq, Chingchang, 1971a. Comparison of m61ange tectonics in Taiwan and in some other mountain belts. Petrol. Geol. Taiwan: 79-106. Biq, Chingchang, 1971b. A fossil subduction-zone in Taiwan. Proc. Geol. Soc. China: 146-154. Biq, Chingchang, 1972. Duel-trench structure in the Taiwan-Luzon region. Proc. Geol. Soc. China: 65-75. Biq, Chingchang, 1973. Kinematic pattern of Taiwan as an example of actual continent-arc collision. Report of the Seminar on Seismology, US-ROC Cooperative Science Program: 21-26. Biq, Chingchang, 1974. Taiwan. In: Spencer, A.M. (Ed.), Mesozoic and Cenozoic orogenic belts. Geol. Soc. London, pp. 501-511. Biq, Chingchang, 1976a. Alpinotype Taiwan revisited. Petrol. Geol. Taiwan: 1-14. Biq, Chingchang, 1976b. Orogen and metallogeny in T a i w a n - plate-tectonically examined. Miscellaneous Publication of the Geological Survey of India (Seminar on Tectonics and Metallogeny of South and East Asia), 34 (Part 1), 1-16. Biq, Chingchang, 1977. The Kenting M61ange and the Manila Trench. Proc. Geol. Soc. China: 119-122. Biq, Chingchang, 1981. Collision Taiwan-style. Mem. Geol. Soc. China: 91-102. Biq, Chingchang, 1989. The Yushan-Hsuehshan megashear zone in Taiwan. Proc. Geol. Soc. China, 32(1), 7-20. Biq, Chingchang., 1997. Taiwan. In: Moores, E.M. and Fairbridge, R.W. (Eds.), Encyclopedia of European and Asian regional geology, pp. 711-717. Biq, Chingchang, Shyu, C.T., Chen, J.C. and Boggs, S., 1985. Taiwan: geology, geophysics and marine sediments. In: A.E.M. Nairn, F.G. Stehli and S. Uyeda (Eds.), The Ocean Basins and Margins, Vol. 7A, The Pacific Ocean. Plenum Press, New York and London, pp. 530-550. Bonilla, M.G., 1975. A review of recently active faults in Taiwan. U.S. Geol. Survey Open-File Report, 72: 75-41. Bowin, C., Lu, R.S., Lee, C.S. and Schouten, H., 1978. Plate convergence and accretion in Taiwan-Luzon region. Am. Ass. Petrol. Geol. Bull., 62: 1645-1672. Chai, B.H.T., 1972. Structural and tectonic evolution of Taiwan. Am. J. Sci., 272: 389-422. Chang, L.S., 1966. Outline of Geology of Taiwan (in Chinese). Mining History of Taiwan:Taipei,Taiwan, 1: 35-66. Chang, L.S., 1967. A biostratigraphic study of the Tertiary in the Coastal Range, eastern Taiwan, based on smaller foraminifera (I. Southern part). Proc. Geol. Soc. China: 64-76. Chang, L.S., 1968. A biostratigraphic study of the Tertiary in the Coastal Range, eastern Taiwan, based on smaller foraminifera (II. Northern part). Proc. Geol. Soc. China: 19-33. Chang, L.S., 1969. A biostratigraphic study of the Tertiary in the Coastal Range, eastern Taiwan, based on smaller foraminifera (IV. Middle part). Proc. Geol. Soc. China: 89-101. Chang, L.S., 1972. Eocene/Miocene hiatus and N conglomerate in the Central Range of Taiwan. Proc. Geol. Soc. China: 93-98. Chang, L.S., 1973. A biostratigraphic study of the so-called Slate Formation in Taiwan based on smaller foraminifera: III. Sankuan Hsuiluan area along the upper courses of the Tanshuiho and the Yulochi. Proc Geol Soc China: 69-84. Chang, L.S., 1974. A biostratigraphic study of the so-called slate formation in Taiwan based on smaller foraminifera: IV. northernmost part of the central Taiwan. Proc. Geol. Soc. China: 85-94. Chang, L.S., 1975. Biostratigraphy of Taiwan. Geology and Paleontology of Southeast Asia: 337-361.

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Chang, L.S. and Chen, T.H., 1970. A biostratigraphic study of the Tertiary along the Hsiukuluanchi in the Coastal Range, eastern Taiwan, based on smaller Foraminifera. Proc. Geol. Soc. China: 115-128. Chen, C.H., 1989. A preliminary study of the fossil Dinoflagellates from the Tananao Schist, Taiwan (in Chinese). Master Thesis, National Taiwan University. Chen, C.H., Chu, H.T. and Chuang, T.Y., 1983a. Some structural problems of the Central Range of Taiwan(in Chinese). Bull. Centr. Geol. Sur., 2: 1-16. Chen, C.H., Chu, H.T., Liou, J.G. and Ernst, W.G., 1983b. Explanatory notes for the metamorphic facies map of Taiwan. Spec. Publ. Cent. Geol. Surv.: 1-3. Chen, P.Y., 1991a. Basaltic-andesitic volcanic rocks from the areas of Changshihchiao and Hsiangyang, Southern E-W Cross Island Highway, Taiwan. Spec. Publi. Cent. Geol. Survey: 127-159. Chen, W.S., 1991b. Translated title: Origin of the Lichi Melange in the Coastal Range, eastern Taiwan. Special Publication of the Central Geological Survey, Papers for symposium on Rocks of Taiwan, 5. Chi, W.R., 1982. The calcareous nannofossils of the Lichi m~lange and the K6nting melange and their significance in the interpretation of plate-tectonics of Taiwan region. Ti-Chih: 99-114. Chi, W.R., Namson, J. and Suppe, J., 1981. Stratigraphic record of plate interactions in the Coastal Range of eastern Taiwan. Mem. Geol. Soc. China: 155-194. Chiao, V.C., 1991. The structural evolution of the Hoping area, eastern Taiwan (in Chinese). Master Thesis, National Taiwan University. Chou, J.T., 1973. Sedimentology and paleogeography of the Upper Cenozoic system of western Taiwan. Proc. Geol. Soc. China: 111-144. Chung, S.L., 1995. The geochemical characteristics of the metabasites from the Slate Formation complex: Discussion. Journal of the Geological Society of China, 38(2): 173-178. Clark, B., Fisher, D. and Lu, C.-Y., 1991. The Hsuehshan Range: a large scale crustal pop-up structure. Taiwan International Collaborative Research for Understanding Subduction-collision System in Taiwan, Abstract: 249-258. Dalziel, I.W.D., 1981. Back-arc extension in the Southern Andes; a review and critical reappraisal. In: Vine, F.J. and Smith, A.G. (Eds.), Extensional tectonics associated with convergent plate boundaries, pp. 319-335. Defant, M., Dario, J., Maury, R.C., De Boer, J. and Joron, J.L., 1989. Geochemistry and tectonic setting of the Luzon arc, Philippines. Geol. Soc. Am. Bull., 101: 663-672. Ernst, W.G., 1983. Mountain building and metamorphism: a case history from Taiwan. In: Hsfi, K.J. (Ed.), Mountain Building Process. Academic Press, London, pp. 247-256. Ernst, W.G., Ho, C.S. and Liou, J.G., 1985. Rifting, Drifting, and Crustal Accretion in the Taiwan sector of the Asiatic Continental Margin Transect, Central Mountain Range, Taiwan. In: Howell, D.G. (Ed.), Tectonostratigraphic Terranes of the Circum-Pacific Region. Circum-Pacific Counc. Energy Miner. Resour., Houston, Tex., pp. 375-389. Gansser, A., 1964. The geology of the Himalayas. Wiley-Interscience, New York. Greenly, E., 1919. The geology of Angelsey, Great Britain. Mem. Geol. Surv.: 980p. Hall, R., Fuller, M., Ali, J.R. and Anderson, C.D., 1995. The Philippine Sea plate: magmatism and reconstruction. In: Taylor, B. and Natland, J. (Eds.), Active margin and marginal basins of the western Pacific continental. AGU, Washington DC, United States, pp. 371-404. Hashimoto, W. and Matsumaru, K., 1975. On the Lepidoczclina bearing limestone exposed at the southern cross mountain highway, Taiwan. Contr. Geol. Paleon, Southeast Asia, 16:103-116. Hayes, D.E., 1982. Magnetic lineations in the marginal seas of southeast Asia and their tectonic implication: Geology and tectonics of the Luzon-Marianas region, Philippine SEATAR Committee, Spec. Pub. Philippine Bureau of mines and geosciences, pp. 21-30. Hayes, D.E. and Lewis, S.D., 1984. A geophysical study of the Manila Trench, Luzon, Philippines: Crustal structure, gravity, and regional tectonic evolution. J. Geophys. Res., 89: 9171-9195. Hilde, T.W.C. and Lee, C.S., 1984. Origin and evolution of the West Philippine basin: a new interpretation. Tectonophysics, 102: 85-104. Ho, C.S., 1975. An introduction to the geology of Taiwan: explanatory text of the geologic map of Taiwan. Ministry of Economic Affairs, R.O.C., 153pp. Ho, C.S., 1982. Tectonic Evolution of Taiwan: Explanatory Text of the Tectonic Map of Taiwan. Ministry of Economic Affairs, R.O.C., 1-126pp. Ho, C.S., 1986. A synthesis of the geologic evolution of Taiwan. Tectonophysics, 125: 1-16.

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Seno, T., 1977. The instantaneous rotation vector of the Philippine Sea plate relative to the Eurasian plate. Tectonophysics, 42: 209-226. Seno, T. and Maruyama, S., 1984. Paleogeographic reconstruction and origin of the Philippine Sea. Tectonophysics, 102: 53-84. Seno, T. et al., 1987. Redetermination of the Philippine Sea plate relative to the Eurasian plate motion. Eos, Trans. Am. Geophys. Union, 68: 1474. Shinjo, R., Chung, S.L., Kato, Y. and Kimura, M., 1999. Geochemical and Sr-Nd isotropic characteristics of volcanic rocks from the Okinawa Trough and Ryukyu Arc: implication for the evolution of a young, intracontinental back arc basin. Journal of Geophysical Research, 104: 10591-10608. Stanley, R.S., Hill, L.B., Chang, H.C. and Hu, H.N., 1981. A Transect Through the Metamorphic Core of the Central Mountains, Southern Taiwan. Mem. Geol. Soc. China, 4: 443-473. Studer, B., 1827. Remarques geognostiques sur quelques parties de la chaine septentrionale des Alpes. Ann. Sci. Nat. Paris, 11: 1-47. Sun, S.C., 1982. The Tertiary basins of offshore Taiwan. 2nd ASEAN Council on Petroleum (ASCOPE) Conference, Manila, 1981, Proc.: 125-135. Sun, S.C., 1985. The Cenozoic tectonic evolution of offshore Taiwan. Energy, 10: 421-432. Suppe, J., 1981. Mechanics of mountain building in Taiwan. Mem. Geol. Soc. China, 4: 67-89. Suppe, J., 1984. Kinematics of arc-continent collision, flipping of subduction, and back-arc spreading near Taiwan. Mem. Geol. Soc. China, 6: 21-33. Suppe, J., 1986. Reactivated normal faults in the western Taiwan fold and thrust belt. Mem. Geol. Soc. China, 7: 187-200. Suppe, J., 1987. The active Taiwan mountain belt. In: Schaer, J.P. and Rodgers, J. (Eds.), The Anatomy of mountain Ranges. Princeton Univ. Press, pp. 277-293. Suppe, J., 1988. Tectonics of arc-continent collision on both sides of the South China Sea: Taiwan and Mindoro. Acta Geologica Taiwanica (26): 1-18. Suppe, J., Liou, J.G. and Ernst, W.G., 1981. Paleogeographic origins of the Miocene east Taiwan ophilolite. Am. Jour. Sci., 281: 228-246. Suppe, J., Wang, Y., Liou, J.G. and Ernst, W.G., 1976. Observations of some contacts between basement and Cenozoic cover in the Central Mountain, Taiwan. Proc. Geol. Soc. China, 19: 59-70. Taira, A. and Tashiro, M., 1987. Historical Biogeography and Plate tectonic Evolution of Japan and Eastern Asia. Tokyo, Terra Sc. Publ., 221p. Taylor, B. and Hayes, D.E., 1980. The tectonic evolution of the South China Basin. In: Hayes, D.E. (Ed.), The tectonic and geologic evolution of the Southeast Asian seas and islands. Geophysics. Monogr. Ser. AGU, Washington D. C., pp. 89-104. Taylor, B. and Hayes, D.E., 1983. Origin and history of the South China Basin. In: Hayes, D.E. (Ed.), The tectonic and geologic evolution of the Southeast Asian seas and islands: Part 2. Geophysics. Monogr. Ser. AGU, Washington D. C., pp. 23-56. Teng, L. S., 1980. On the origin and tectonic significance of the Lichi Formation, Coastal Range, eastern Taiwan (in Chinese). Ti-Chih, 2: 51-62. Teng, L.S., 1987. Stratigraphic records of the late cenozoic Penglai Orogeny of Taiwan. Acta Geol. Taiwanica, NTU, 25: 205-224. Teng, L.S., 1990. Geotectonic evolution of late Cenozoic arc-continent collision in Taiwan. Tectonophysics, 183: 57-77. Teng, L.S. et al., 1991. Tectonic aspects of the Paleogene depositional basin of northern Taiwan. Proc. Geol. Soc. China, 34: 313-336. Tercier, J., 1947. Le flysch dans la s6dimentation alpine. Eclogae geol. Helv., 40: 164-198. Tsai, Y.B., 1978. Plate subduction and the Plio-Pleistocene orogeny in Taiwan. Pet. Geol. Taiwan, 15: 1-10. Tsai, Y.B., 1986. Seismotectonics of Taiwan. Tectonophysics, 125: 17-38. Tsai, Y.B., Teng, T.L., Chiu, J.M. and Liu, H.L., 1977. Tectonic implication of the seismicity in the Taiwan region. Mem. Geol. Soc. China, 2: 13-42. Tsan, S.F., 1974a. The Kenting Formation: a note on Hengchun Penisula stratigraphy. Proc. Geol. Soc. China, 17: 131-133. Tsan, S.F., 1974b. Stratigraphy and structure of the Hengchun Penisula , with special reference to a Miocene olistrome. Bull. Geol. Surv. Taiwan, 24: 99-108.

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Tsan, S.F., 1977. Remarks on the Suao section of the Central Range of Taiwan. Mem. Geol. Soc. China, 2: 141-145. Viallon, C., Huchon, P. and Barrier, E., 1986. Opening of the Okinawa basin and collision in Taiwan: a retreating trench model with lateral anchoring. Earth Planet. Sci. Lett., 80: 145-155. Wang, C.S., 1976. The Lichi Formation of the Coastal Range and arc-continent collision in eastern Taiwan. Bull. Geol. Sur. Taiwan: 73-86. Wang Lee, C. and Lu, C.Y., 1981. Structure of the Tananao Schist between Lo-shao and Tzemuchiao, eastern Taiwan (in Chinese). Ti-Chih, 3: 121-134. Wang, Y., 1987. Continental margin rifting and Cenozoic tectonics around Taiwan. Mem. Geol. Soc. China, 9: 227-240. Yang, C.N. and Lo, W., 1986. Geologic structures of the Tananao Schist and its cover in the Tayuling area, Central Range of Taiwan. Ti-Chi, 7:11-33. Yen, T.P., 1953. On the occurrence of late Paleozoic fossils in the metamorphic complex of Taiwan. Bull. Geol. Surv. Taiwan, 4: 23-29. Yen, T.P., 1960. A stratigraphical study on the Tananao Schist in northern Taiwan. Bull. Geol. Surv. Taiwan, 12: 53-66. Yen, T.P., 1962. The grade of metamorphism of the Tananao Schist. Bull. Geol. Soc. Taiwan, 5: 101-108. Yen, T.P., 1968. Crustal features of Taiwan region. Proc. Geol. Soc. China: 130-136. Yen, T.P., Sheng, C.C., Keng, W.P. and Yang, Y.T., 1956. Some problems on the Mesozoic formation of Taiwan. Bull. Geol. Surv. Taiwan, 8: 1-14. Yui, T.F., Lu, C.Y. and Lo, C.H., 1988. A speculative tectonic history of the Tananao Schist of Taiwan. Proc. Geol. Soc. China, 31 (2): 7-18. Yui, T.F., Lu, C.Y. and Lo, C.H., 1990. Tectonic evolution of the Tananao Schist Complex of Taiwan. In: Auboun, J. and Bougois, J. (Eds.), Tectonics of Circum-Pacific Continental Margins, pp. 193-209. Yui, T.F., Wu, T.-w. and Lu, C.Y., 1994. Geochemical characteristics of metabasites from the Slate Formation complex of Taiwan. Journal of the Geological Society of China, 37: 53-67. Yui, T.F., Wu, T.-w. and Lu, C.Y., 1995. Geochemical characteristics of metabasites from the Slate Formation of Taiwan: Reply. Journal of the Geological Society of China, 38(2): 179-182.

Paradoxes in Geology Ueli Briegel and Wenjiao Xiao, eds. 9 2001 Elsevier Science B.V. All rights reserved.

Chapter 7

T H I N - S K I N N E D P L A T E T E C T O N I C S : T H E R E - I N V E N T I O N OF A CRAZY IDEA

Peter Finckh a and Kenneth J.

Hsfi b

aHolzwiesstr. 13, CH-8704 Herrliberg, Switzerland bFrohburgstr. 96, CH-8006 Zfirich, Switzerland

ABSTRACT The plate-tectonic theory postulates that mountain ranges are formed as a result of plate-to-plate collisions, where the lithosphere plates remain intact and have been subducted. Geophysical investigations in the Alps revealed the velocity structure in the Earth's crust, with low velocity channels at or near the Moho. There is also an absence of deep-seated earthquakes, implying the absence of a Benioff Zone. These facts led Ken Hsti to the conclusion that the crustal layers were detached from the underlying mantle of lithospheric plates during orogenic deformations. His theory of thin-skinned plate-tectonics was dismissed as a crazy idea in the late 1970s. The Swiss National Research Programme 20 (NRP 20) carried out a series of seismic traverses across the Swiss Alps, using the reflection and refraction methods, dynamite as well as Vibroseis | as sources. All major traverses in the east, west and south show prominent Moho reflections and strong intracrustal energy returns. Interpretations by N R P 20 clearly indicate Moho detachment, as well as imbrication of crustal slices. The postulate of thin-skinned plate tectonics is thus no longer a crazy idea. Old north-dipping reflections are truncated by the more recent south-dipping reflectors. The seismic evidence thus confirms another of Hsfi's interpretations of Late Cretaceous Eo-Alpine deformation. His tectonic-facies model suggested that the Schams sedimentary cover above the "Briangonnais Swell" was thrust southward, when the ocean crust of the adjacent north Penninic basin was subducted down a north-dipping zone.

INTRODUCTION The plate-tectonic theory postulates that mountain ranges are formed as a result of plate-to-plate collisions, where the lithosphere plates remain intact and have been subducted (Fig. 1). Gravity measurements indicate clearly crustal thickening under the Alps, and in a tentative geological interpretation ductile yielding of the crust during deformation was postulated (Kissling et al., 1983). This over-simplified theory is incorrect.

102

P. Finckh and K. J. Hs~t"

Fig. 1. S-wave-velocitycross section from Basel to Milano as derived from the simultaneous inversion of all available dispersion data of seismic surface waves. Hatched area indicates the range of uncertainty of crustal lithospheric and asthenospheric boundaries. (From Panza and Mueller, 1978).

Reviewing the geological and geophysical evidence, Hsfi (1979) pointed out that both the crust and mantle are characterised by a layered structure. Seismic refraction studies indicated the presence of one or more intracrustal low-velocity (seismic) layers, the lowest of which lies just above the Moho (Fig. 2). He also noted that the crustal thickening is not simply the result of thickening of individual crustal layers. There are more crustal layers in regions of thicker crusts than in regions of thinner crust, and the piling of crustal slices detached from Moho is an expression of the Alpine deformation. Continental lithospheric plates of the Alps have not been subducted into the mantle as ocean lithosphere of the Pacific. This fact explains the absence of a seismic active Benioff Zone under the Alps. The detachment of crust above Moho in plate-tectonic deformations is analogous to the decollement deformation of the detached sedimentary cover above the Jura basement. Hsfi used the expression thin-skinned plate-tectonics to designate his theory of deformation. It was dismissed as a crazy idea by his colleagues and the manuscript was refused for publication by recognised European geological journals but was

103

Thin-skinned plate tectonics." the re-invention o f a crazy idea

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Fig. 2. Schematic velocity-depth structure for the continental crust of the Hercynian close to the Rhinegraben together with petrological interpretation. 1 - Cenozoic sediments (near-surface low-velocity layer); 2 - Mesozoic (and Paleozoic) sediments; 3 - upper crystalline basement consisting of metamorphic rocks, such as gneisses and schists (zone of positive velocity gradient); 4 - lacolithic zone of granitic intrusions (sialic low-velocity zone); 5 - migmatites (middle crustal layer); 6 - amphibolites (high velocity tooth); 7 - granulites (lower crustal layer); and 8 - ultramafic (uppermost mantle). (From Mfiller, 1977)

published elsewhere (Hsfi, 1979). Panza and Mueller (1978) published in the same idea the conventional postulate that lithosphere plates have been subducted down to depth of more than 200 km below the Alps (see Fig. 1). Stephan Mueller and colleagues initiated the Swiss National Research Programme 20 to investigate the crustal structure under the Alps. Several seismic profiles were shot across the Alps (Lehner et al., 1997; see also Fig. 3). The velocitystructuring of the crust and mantle of the traverses are shown by Fig. 4. The seismic investigations have thus permitted the re-invention of a crazy idea (cf. Fig. 4 and 5). A Southern Traverse seismic section is shown by Fig. 6. The seismic continuity, interpreted as the Moho between lower crust and upper mantle, is repeated. A lower Moho, marked by a lower-crust/mantle contact A, is southerly dipping, at a depth ranging from about 45 to 50 kin. An upper Moho, also marked by a lower-crust/ mantle contact C, dips gently to the north, from about 37 to 42 km depth. The mantle below the upper Moho has been thrust along the surface B over the lower crust, i.e., the present-day Moho. Three very prominent zones of north-dipping intracrustal reflectors are present. The lower (D) dips gently to the north, D, from 25 to 35 km depth. The intermediate (E) marks the contact between the upper and lower crust, and also dips gently to the north, from 22 to 28 km depth. The highest northdipping reflector F, from 15 to 20 km depth, is more steeply dipping than E and D. The F-reflectors are cut by a flat thrust G in the upper crust at about 20 km depth. Higher reflectors, representing crustal thrusts in the Penninic Alps, are nearly flat surfaced. Hsfi (1995) interpreted the two sets of reflectors as the manifestations of

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106

P. Finckh and K. J. Hs~"

Fig. 5. Recent crustal structure under the Alps. Note the thickening of the crust under the Alps caused by a postulated crustal underthrusting. The basement underthrust led to marginal deformation and shortening of sedimentary cover in the Jura Mountains and also led to isostatic uplift of the Helvetic and Pennine Alps. (From Hsfi, 1995) two phases of orogenic deformations: the north-dipping reflectors F are Eo-Alpine, and the flat thrusts G are Neo-Alpine. The present Moho A is a detachment surface, separating the more brittle deformation of the crust from the ductile deformation of the mantle. The seismic section of the Western Traverse is shown by Fig. 7. There is only one prominent Moho (A'), dipping gently southward from about 35 to more than 50 km depth. There are two or more zones of south-dipping intracrustal reflections. The lower G', from the surface down to 20 km depth, marks a zone of flat thrusts, correlative to G in the Southern Traverse. Sandwiched between the south-dipping discontinuities A' and G' are two prominent zones of north-dipping reflectors. The more northerly zone under the Val d'Annivers F2' ranges from 15 to 30 km depth. The more southerly zone under Zermatt F I', correlative to F in the Southern Traverse, ranges from 20 to 35 km depth. The north-dipping reflectors are truncated at the bottom by the zone of deformation above the Moho A', and they are truncated at the top by the flat thrusts G'. The structural relation clearly indicates that the north-dipping structures have been formed by earlier deformations. The results of the Western Traverse confirm thus Hsfi's postulate of two phases of crustal faulting and/or subduction in the Alps. The Eastern Traverse is shown by Fig. 8. The Moho A" dips gently southward from about 45 to more than 60 km depth. A flat thrust-zone G" gently southerly dipping, is identified at a depth of less than 10 km. Sandwiched between are north-dipping reflectors E" and F", correlative to the reflectors E and F of the Southern Traverse and by the flat thrusts G" reflector. The evidence confirms thus again the earlier origin of north-dipping structures. The discovery of north-dipping reflectors is a surprising result. The scientists of the N R P 20 team have minimised their importance. The north-dipping structures are commonly considered the subsurface continuation of the Neo-Alpine backthrusting

Thin-skinned plate tectonics." the re-invention of a crazy idea

107

Fig. 6. Composite of seismic sections along the Southern Traverse utilising dynamite sources. For location see Fig. 3. Vertical scale is Two-Way-Traveltime,for approximate depth conversionmultiply with 3 km/s. (Fig. 9). Applying the Archipelago Model of Orogenesis, Hsfi (1995) identified those reflectors as the evidence for very important Eo-Alpine deformations during the Mesozoic. Underthrusting along north-dipping subductions should be marked at the surface by steep escarpments and in the surface by a Benioff Zone. The coarse breccias of the Breggia Nappe are evidence of the existence of such an escarpment on the south side of a so-called Brian~onnais Swell. The presence of a north-dipping Benioff Zone is now verified by the seismic evidence. Paleogeographic reconstructions by Hsti revealed that the southern margin of Europe was an archipelago of back-arc basins and remnant arcs prior to the NeoAlpine deformation. Subduction of oceanic crust under the basins is evidenced by the presence of ophiolite m61anges and of high-T/high P metamorphics in the Penninic Alps. Radiometric dates and stratigraphic data indicate that the

Fig. 7. Composite of seismic sections along the Western Traverse utilising dynamite sources. For location see Fig. 3.

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110

P. Finckh and K. J. Hsfi

Fig. 9. Schematic cross sections to illustrate the present day structure of the Adriatic microplate in relations to the Alps. (From Giese et al., 1992) deformation took place during the late Mesozoic, and the orogenic deformations have been referred to as Eo-Alpine Orogenesis (Trfimpy,1973). Hsfi (1994) adopted the hypothesis that back-arc collapses were the cause of the Eo-Alpine deformations. Well known to Alpine geologists, the Briangonnais Swell of the French Alps extended northward to western Switzerland and eastward to the Schams region. Hsti (1994) considered the Swell a relic island-arc. The sediments of the Brianqonnais/ SubBrianqonnais facies, including those in the Schams area, were the cover of the arc. The Briangonnais Swell was separated from Europe by the "Valais Trough." Hsfi considered the trough as one or more back-arc basins between the relic island-arc and the European margin. The margin was an active margin south of the Helvetic Passive Margin. The rocks of the Adula Nappe constituted such an active margin, and the margin was bounded on the south side by a north-dipping subduction zone (Hsti, 1995). The ocean floor in the back-arc basin between the "Swell" and the Adula margin led eventually to its "collapse," leading ultimately to an arc-continent collision. The collision should have produced a north-dipping suture zone and southerly vergent structures in the Eo-Alpine foreland thrust belt. The NRP results revealed the presence of such a suture zone. It is well known to Alpine geologists that southerly vergent structures are present in the Schams area; their contrast to the northerly vergence of the Neo-Alpine structures has in fact been a puzzle for a century (Hsti, 1995, 117-125). The site of deep sedimentation south of the "Briangonnais Swell" has been designated Piemont Trough. Interpreting on the basis of the Archipelago Model, the swell was a relic arc and the trough was a Mesozoic back-arc basin, bounded by a north-dipping subduction zone in Switzerland and a west-dipping subduction zone in France and in Italy. Submarine sliding of the sediments on the steep arc-escarpment of the Brianqonnais relic arc caused the deposition of thick Jurassic breccias. The back-arc was subducted down the north- and west-dipping subduction zones, and the subducted rocks are metamorphosed. A belt of Cretaceous high-T/high-P rocks

Thin-skinned plate tectonics." the re-invention o f a crazy idea

111

extend from the Schams area to western Switzerland, the French Alps, Corsica (Hsti, 1997; Caby, 1997; Cadoppi, et al., 1997) and from there to the Tyrrhenian Sea between Sardinia and Calabria (Kastens, et al., 1997). The Piemont back-arc basin collapsed through the collision of the Brianqonnais and Italy. The oceanic crust was subducted and the Upper Cretaceous Flysch were deposited in a foredeep when subduction along a west- and east-dipping subduction started the neo-Alpine deformations. The Piemont Backarc Basin was, however, completely consumed in early Tertiary. The ultimate arc (Brianqonnais)-continent(Europe) collision are manifested by the Neo-Alpine deformations. Crustal slices above the Moho were detached and transported northward during the Tertiary to produce the northerly vergent structures is crustal and foreland deformations. SUMMARY Many of the mistaken concepts of the classic geosyncline theory are incorporated in the new paradigm of global tectonics. The basic idea of Suess/Wegener/Argand that the Tethyan Mountains were formed by collision of continents was incorporated in the concept of plate collision of the plate-tectonics theory. Hsti recognised in 1979 that the Alps were not formed by plate-collision: crustal slices detached from the Moho have been piled on top of one another, produced both crustal and cover deformations. His theory was dismissed as a crazy idea. A thorough investigation of the crustal structures verified his postulates. The Alps are underlain by piles of crustal slices, pushed together during two stages of Alpine deformation. The crazy idea was reinvented and developed into the archipelago model of orogenesis. The new theory, based upon interpretations of actualistic deformations, provides an explanation of the north-dipping reflectors discovered by the scientists of N R P 20. The first author was a member of the N R P 20 team, but became self-employed before he could complete geological interpretations of the geophysical results. He is indebted to Professor Kenneth Hsti for his invitation to present this paper during the Beijing Symposium. Without his co-authorship the paper would not have been written. REFERENCES Caby, R., 1997. The External Brianngonnais/Piemont-Ligurian boundary in the western Alps: Tertiary exhumation of a west-dipping subduction complex (abstract). Third Workshop on Alpine Geological Studies, Biella, Earth Science Department, Univ. Milan. Cadoppi, P., Riccio, I. and Tallone, S., 1997. Revised tectonostratigraphic setting of the outer envelope of the Dora Maira nappe (abstract). Third Workshop on Alpine Geological Studies, Biella, Earth Science Department, Univ. Milan. Frei, W., Heitzmann, P., Lehner, P. and Valasek, P., 1989. Die drei Alpentraversen von NFP 20. Bull. Ver. Schweiz. Pet. Geol. Ing., 55: 13-3. Giese, P., Roeder, D. and Scandone, P., 1992. The fragmented Adriatic microplate: evolution of the Southern Alps, the Po basin and the northern Apennines. In: Blundell, D., Freeman, R., and Mueller, St. (Eds.), 1979. A continent revealed: The European Geotraverse, Cambridge University Press, 275 pp. Hsti, K.J., 1979. Thin-skinned plate tectonics during Neo-Alpine Orogenesis. Am. Jour. Sci., 279: 353-366. Hsti, K.J., 1994. Archipelago model of orogenesis. Geology Today (December),107-110.

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Hsfi, K.J., 1995. Geology of Switzerland, an Introduction to Tectonic Facies. Princeton University Press, Princeton, NJ. 250pp. Hsfi, K.J., 1997. Archipelago model for Alpine orogenesis (abstract). Third Workshop on Alpine Geological Studies, Biella, Earth Science Department, Univ. Milan. Kastens, K.A., Mascles, J. et al., 1997. Proc. ODP, Scientific Results, 107. College Station, TX (Ocean Drilling Program). 772pp. Kissling, E., Mueller, St., and Werner, D.,1983. Gravity anomalies, seismic structure and geothermal history of the Central Alps. Annales Geophysicae 1, 37-46. Kissling, E., Ansorge, J., and Baumann, M., 1997. Methodological considerations of 3-D crustal structure modeling by 2-D seismic methods. In: Pfiffner, O. A., Lehner, P., Heitzmann, P., Mueller, St., and Steck, A., (Eds.), Deep structure of Switzerland. BirkhS,user Basel. 380pp. Lehner, P., Mueller, St. and Trfimpy, R, 1997. Deep structure of the Swiss Alps: an introduction. In: Pfiffner, O. A., Lehner, P., Heitzmann, P., Mueller, St., and Steck, A., (Eds.), Deep structure of Switzerland. BirkhS,user Basel. 380pp. Panza, G.F. and Mueller, St., 1978. The plate boundary between Eurasia and Africa in the Alpine area. Mem. Ist. Geol. Univ. Padova, 33: 43-50. Mfiller, St., 1977. A new model of the continental crust. Am. Geophys. Union Mon., 20: 289-317. Trfimpy, R.T., 1973. The timing of orogenic events in the central Alps. In: Dejong, K.A. and Schjolton, R. (Eds.), Gravity and Tectonics: 229-252, Wiley, New York.

Paradoxes in Geology Ueli Briegel and Wenjiao Xiao, eds. 9 2001 Elsevier Science B.V. All rights reserved.

Chapter 8

OIL IN ' H O L E S - I N - T H E - C O N T I N E N T ' (RELICT OCEANIC BASINS) Sergey Aplonov Oceanology Institute, St. Petersburg State University, Russia, and TETHYS Geodynamical Research Center

ABSTRACT In 1988, Ken Hsfi suggested a paradoxical idea that sedimentary basins of North-West China Junggar, Tarim and Qaidam, regarded earlier as cratonic blocks, in reality are relict back-arc basins, and that they are underlain by Palaeozoic oceanic and not by thinned continental crust. At the same time, relict Triassic oceanic crust in the basement of Northern West Siberia (named Obsky Paleocean) was discovered. Today, more than ten years later, we have got new evidence confirming the oceanic type of the crust underlaying deep sedimentary basins, not only in West Siberia, but also in the North Caspian Basin and the basins of Russian Arctic Shelf. The deep sedimentary basins floored by ancient oceanic crust are characterized by an anomalously thin (at about 10 km) crust covered with anomalously thick (12-15 km, and somewhere more than 20 km) sedimentary cover, an anomalously high crustal density (more than 2.9 g/cm 3) and the presence of hidden linear magnetic anomalies over them, which are very much alike the linear magnetic anomalies over mid-oceanic ridges. Using special methods of adaptive band-pass filtering of the total magnetic anomalies, we revealed linear magnetic anomalies over all of the investigated deep sedimentary basins. Then we tried to date the linear magnetic anomalies in terms of Vine-Matthews concept, like it is done with present-day marine magnetic anomalies. As a result, it was established that in the basins of the Eastern Barents and Northern Kara Shelf, and in the North Caspian Basin, the best spreading intervals correspond to Devonian, in the South Kara Basin to Carboniferrous, and in the Obsky Paleocean in West Siberia i to Triassic, and this last interval was confirmed by the direct measurement of paleoceanic floor age. Present-day "holes-in-the-continent" may be the result of two types of final stages of ancient ocean evolution. In the first case (West Siberia, Eastern Barents Shelf) a shorttime (15-20 Ma) spreading formed a narrow band of oceanic crust which later was not subducted, but was imprinted into surrounding continent and avalanched. Such a structure is classified as an aborted ocean. In the other case (North Caspian Basin, Kara Shelf), the oceanic block is partly subducted and only after that, a

114

S. Aplonov

"passive" isostatic subsidence starts. It is better to classify such a structure as a "trapped ocean". So, present day continents have holes filled with crust of the oceans that escaped hundred million years ago. We will never manage to touch this crust, but our idea on the Earth's evolution will not be complete without knowledge about this crust. Fortunately even through thick sedimentary cover we can see geophysical evidence of the ancient oceanic crust preserved in the "holes-in-the-continent". And today we keep learning to find and interpret them.

INTRODUCTION Everyone knows that by the end of the seventies, it was clear that most of the largest sedimentary basins are underlain by stretched and thinned continental crust, and the received opinion was that it is the result of continental rifting. It is the wellknown McKenzie model (McKenzie, 1978). But still, there were some doubts. First of all, sedimentary basins' basement is not like continental crust, even one transformed by rifting. Secondly, it is physically impossible that rifted continental crust could subside to great depths (sometimes more than 20 km). To hesitate is not the same as to declare, that the deepest depressions of sedimentary basins are floored, at least in part, by oceanic rocks. And Ken Hsti did it. In 1988, he suggested a paradoxical idea that sedimentary basins of North West C h i n a - Junggar, Tarim and Qaidam, regarded earlier as cratonic blocks, are in reality relict back-arc basins, and that they are underlain by Paleozoic oceanic and not by thinned continental crust (Hsfi, 1988). In the middle of the eighties, I discovered relict Triassic oceanic crust in the basement of Northern West Siberia. In 1988, a paper on this paleocean named Obsky (after the river of Ob, the largest one in West Siberia) was published (Aplonov, 1988). That was the starting point of our correspondence with Ken Hsti, which resulted in a joint paper (Aplonov et al., 1992). In this paper I'll try to demonstrate, what we know about deep sedimentary basins today, ten years later, and it is up to you to decide if it is a resolved paradox or still controversial. I will show you some examples from sedimentary basins of Russia. It is not only West Siberia, but also the North Caspian Basin and the Arctic Shelf.

T H E C R U S T U N D E R N E A T H DEEP S E D I M E N T A R Y BASINS Today we have more and more evidence confirming the oceanic type of the crust underlaying deep sedimentary basins. This type of crust is the result of a more advanced process than usual continental rifting, In this case, rifting is followed by sea-floor spreading, and "aborted oceans" are formed with no following subduction; they remain to be "holes-in-the-continents" with sedimentary basins over them.

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115

In Fig. 1, we can see the map of the basement depth over the Barents and Kara Seas. The basement here is depressed to the depth of 16-18 km. On the crustal sections we can see that the consolidated crust the crust between basement and Moho surfaces without sedimentary s e q u e n c e - is only about 10 km thick. The next example is from the North Caspian Basin (Fig. 2): here the basement has a depth of 22-23 km. It is the deepest sedimentary basin in the world. The consolidated crust here is only 8-10 km thick. So, the first characteristic feature of deep sedimentary basins is anomalously thin crust covered with anomalously thick sedimentary cover. 10 km is a little more than a standard oceanic crust, but it is too little for a continental crust (even a rifted one). The second characteristic feature is the anomalously high crustal density. It can be supposed from the first glance at this gravity map from North West Siberia (Fig. 3) that the area of the deepest basement corresponds to a vast positive anomaly. We have analyzed both gravity and refraction data and it was confirmed that regional positive gravity anomalies are caused by anomalously high basement density and not by Moho uplift. This allowed us to predict the basement density over the deep sedimentary basins. In Fig. 3, we can see the example from North West Siberia (the Obsky Paleocean Depression). Earlier it was obvious that the crustal density over sedimentary basins is higher than that over surrounding areas, but to what extent? And if the average basement density over the deepest parts of sedimentary basins reaches 3 g/cc, it can be hardly regarded as a continental crust, even a rifted one.

TWO TYPES OF B O U N D A R I E S B E T W E E N BLOCKS What are the borders between paleoceanic and continental blocks in the basement of sedimentary basins? We have analyzed gravity, magnetic, reflection and refraction data along the transects across sedimentary basins. There are borders of two types there (Fig. 4). The first one is over North West Siberia. The Obsky Paleocean block which is 300 km wide is limited with normal faults. Another one (the North Caspian Basin) is more complicated. The high density paleoceanic block is limited with normal faults in the north (like in the Obsky Paleocean), but the southern border of the paleoceanic block is a thrust with a horizontal displacement of 250 km. I think that two types of borders are the result of two types of final stages of ocean evolution. In the first case (West Siberia), a short-time spreading formed a narrow band of oceanic crust which later was not subducted, but was imprinted into the surrounding continent and accreted. This structure is a real aborted ocean (Burke, 1976). In the other case (North Caspian), the oceanic block is partly subducted and only after that does a "passive" isostatic subsidence start. It is better to call such a structure a "trapped ocean" (Caroll et al., 1990). This is probably the reason for different present-day depths of the sedimentary basins. The West Siberian aborted ocean was formed only by isostatic forces (with present-day depth of 14-15 kin), and the North Caspian trapped ocean was initially subducted and then added with isostatic subsidence. As a result, its present-day depth is eight km more than that of the West Siberian aborted ocean.

116

S. Aplonov

Fig. 1. Relief of the basement surface and consolidated crustal thickness over the Barents and K a r a Shelves. N B - N o r t h Barents basin, S B - South Barents basin, N N Z - N o r t h N o v a y a Zemlya basin, U - Uyedineniya basin, S K - South K a r a basin, N K V N o r d k a p p - V a r a n g e r basin, O B - Obsky Paleocean, Y K H - Yenisey-Khatanga Trough, O L - Olga basin, BJ - Bjornoja basin, A I D - Arctic Institute Depression, SM - Smidt Depression, P R - Perseya High, CB - Central Bank High, F D F e d y n s k y High, C B R - Central Barents High, LP - L o p p a High, U S H - U s h a k o v High, VZ - Vize High, N N Z S - N o r t h N o v a y a Zemlya Saddle, L U D - Ludlow Saddle, NSS - N o r t h Siberian Swell, N Y N o r t h Yamal Uplift.

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Fig. 3. Regional gravity map and density of the consolidated crust over North West Siberia.

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Fig. 4. Two types of paleoceanic blocks' borders in the basement of the deep sedimentary basins.

S. Aplonov

120

MAGNETIC ANOMALIES And now, one more and the most wonderful evidence of the paleoceanic nature of the deep sedimentary basins are hidden linear magnetic anomalies over them, which are very much alike the linear magnetic anomalies over mid-oceanic ridges. The idea to look for spreading anomalies over sedimentary basins was quite natural, as many scientists noted linearity of the magnetic field there. I formulated the hypothesis as follows: Paleozoic spreading, like the modern one went simultaneously with the Earth magnetic field reversals. So, Paleozoic and Early Mesozoic oceanic crust like Jurassic, Cretaceuos and Cenozoic was marked with linear magnetic anomalies. After the spreading had stopped, some ancient oceanic blocks were not subducted, and hence, were destroyed. In any case, the magnetic linearity there could be preserved. This idea was encouraged by paleomagnetic investigations over the Uralian ophiolites. In the 80s, normally and reverse magnetized blocks were detected in the Devonian pillow lavas (Pechersky et al., 1983). It means that although remnant magnetization of the former oceanic crust is decreasing, its direction could be preserved for at least 400 My! But another question is what happens to striped magnetization while oceanic crust turns into the basement of the deep sedimentary basin, subsiding to great depths and being covered with sediments? During this transformation, if the temperature of oceanic basalts exceeds the Courie point even once, their remnant magnetization will be lost, and then it would be useless to look for spreading anomalies over sedimentary basins. In the southern part of the Obsky Paleocean, where it is practically degenerated into continental rift (only 50 km wide), and after that, where its floor can be drilled, we have investigated basement basalts (Tables 1 and 2). First of all, it was confirmed that basalts there differ from basalts in the other widespread continental West Siberian Triassic rifts. Their chemical composition is similar to that of MORB. Secondly, maximum paleotemperatures of the boundary between

Table 1 Chemical composition of the Obsky Paleocean basalts

Main oxides SiO2 TiO2 A1203 Fe203 FeO MnO MgO CaO Na20 K20

Obsky paleocean basalts

Continental rifts of West Siberia

MORB basalts

East Siberian traps flood basalts

Aplonov, 1 9 8 6

Zhuravljov, 1 9 8 6

Ringwood, 1977

Kutolin, 1977

48.26 1.77 15.20 2.87 7.79 0.15 7.68 10.72 2.52 0.38

50.51 1.04 17.38 4.29 6.52 0.18 5.68 9.02 2.83 1.51

49.92 1.48 15.67 2.48 7.84 0.14 7.75 11.32 2.63 0.23

50.22 1.27 16.21 4.72 6.91 0.20 5.43 9.28 2.91 1.32

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Table 2 Magnetic characteristics of the Obsky Paleocean basalts No. 1 2 3 4 5 6 7 8

Borehole area

No. of samples

Isotopic age, Ma

Q ~-Ir/Ii

Saymovskaya Aleksandrovskaya Strezhevskaya Medvedevskaya Zvezdnaya Pokachevskaya Konitlorskaya Bolsheretchenskaya

5 5 5 3 2 2 4 4

235 230 232 217 215 220 223 238

5.31 5.29 10.51 6.27 8.34 11.21 7.65 5.45

Tc,~ 333 342 305 385 330 310 295 325

sedimentary cover and basement are not more than 150-170~ while the Courie point of basalts is higher than 300~ Thirdly, the Obsky Paleocean basalts have high remnant magnetization that is 5-10 times higher than induced magnetization. So, the transformation of an ancient oceanic block into the basement of sedimentary basin does not erase striped remnant magnetization. But it is very difficult to find out spreading signals, because (1) their source is very deep (today there is no ocean floor at the depth of 15-20 km), (2) during hundreds of million years, magnetic sources appear in the ocean lithosphere, which are not connected with spreading, and which conceal low-amplitude high-frequency spreading signals by intensive regional anomalies. Using special methods of adaptive band-pass filtering we revealed linear magnetic anomalies above all of the investigated deep sedimentary basins. In Fig. 5, we can see an example from the Arctic Shelf and Northern West Siberia. D A T I N G THE M A G N E T I C S I G N A T U R E S But even this is not all. Of course, the presence of linear magnetic anomalies over the deepest parts of sedimentary basins is a strong evidence for their oceanic nature. But we went further and tried to date the linear magnetic anomalies in terms of the Vine-Matthews concept (Vine and Matthews, 1963), like it is done with present-day marine magnetic anomalies. This is a rather more complicated problem. First of all, magnetic polarity scales for Paleozoic are not precise enough. Besides, in an ocean, we always have "a point of reference" - - zero age crust in the mid-oceanic ridge crest. But we are not even sure that the extinct spreading center was in the ancient oceanic block which is the basement of the sedimentary basin now. The following method for age determination of the oceanic crust was created (Fig. 6). The thing is that we tried filtered spreading signal with different intervals of the Palaeozoic magnetic polarity scale. The best fit was the most probable spreading interval. The procedure included about 100,000 successive approximations for each basin. When we started this part of the work, we were not sure whether we would get positive results. But the result has exceeded all expectations. In Fig. 7 we can see the

Fig. 5. Linear magnetic anomalies over the deep sedimentary basins of the Arctic Shelf and North West Siberia.

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Stage I Adaptive band-pass filtering of the total magnetic anomalies

Separation of 'spreading' anomalies

Stage 2

Stage 3

Iterative Comparison search of filtered for probable spreading ___> ('spreading') and calculated interval magnetic on the Paleozoic anomalies magnetic polarity scale Generation of magnetically active layer model

Stage 4 Determination of the ancient sea-floor spreading parameters

Interval Ma

Half-rate Latitude cm/year degrees

Fig. 6. Method for determination of the age of paleoceanic crust in the deep sedimentary basins.

ce ~elation between calculated and filtered magnetic anomalies for deep sedimentary basins. In the basins of the Eastern Barents and Northern Kara Shelf, and in the North Caspian Basin, the best spreading intervals correspond to Devonian. The best fit of calculated and filtered magnetic anomalies over the South Kara Basin was obtained for Carboniferous, and in the Obsky Paleocean for Triassic, and only this last interval was confirmed by the direct measurement of paleoceanic floor age. As a result, we managed to create the age scheme for the paleoceanic basement of deep sedimentary basins. In Fig. 8 we can see the example from the East Barents Shelf. This was our starting point for the following basin modeling, that was not available in any other method. Now, let us discuss how these speculations refer to petroleum exploration.

EVIDENCE FOR P E T R O L E U M Nearly all simple anticline pools within onshore Russia are already explored today. Further prospects are connected with unconventional accumulations in the lower part of the sedimentary cover of the largest onshore basins. It is very demanding to explore unconventional accumulations because they do not depend on present day anticlines. To discover these objects, very complicated and expensive seismic methods are traditionally used by the western geophysicists, and during the last decade, in our country too. Although this approach allows us to discover oil and gas by "wild cat" method and brings results, it is still ineffective and frankly speaking, needs no scientific support. We have chosen another way. Our way is not so simple and short, but we hope it will allow us to significantly raise the effectiveness of petroleum exploration. It is as follows: We try to restore the origin of the sedimentary basin, the early stages of its development, the history of its folded structure formation and reformation, and the history of generation, migration and accumulation of hydrocarbons. Having done all

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Fig. 7. Correlation between calculated and filtered linear magnetic anomalies for deep sedimentary basins.

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Fig. 8. Linear magnetic anomalies and paleoceanic crust age over the East Barents Shelf.

these operations, we managed to detect hydrocarbon accumulation zones in the lower sequence of the sedimentary cover; we were able to predict the type of the zone and accumulations comprised, and to point areas of maximum density of predicted hydrocarbon resources. This step-by-step succession is based on two basic hypotheses: The first one is that high petroleum potential of the sedimentary basin is determined by correlation of its evolution with global cyclicity of sedimentation. If the sedimentation in some basin during some stage of evolution correlates with the global cyclicity, this basin is called harmonic, and if it does not correlate, it is called disharmonic. Only harmonic sedimentary basins have high petroleum potential, because harmonic formation contains (1) major source rocks and (2) what is more important, major regional seals that preserve oil and gas.

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Fig. 9. Vendian and Phanerozoic global cycles and sedimentation cyclicity curves for main Russian platforms. Let us examine this in three of the Russian platform regions. In the upper part of Fig. 9, we can see the Vendian and Phanerozoic global cycles. There are three of them: Vendian-Early Palaeozoic, Middle-Late Palaeozoic, and Mesozoic-Cenozoic. Each of them starts from a land stage, then follows transgression, then epicontinental seas, and finally, regression.

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The next in Fig. 9 is the sedimentation cyclicity curve for East Siberia. Harmonic sedimentation can be observed only in the Late Precambrian and Cambrian. Later you can see only disharmonic development. As a result, two-thirds of the East Siberian hydrocarbons belong to these ancient deep parts of the sedimentary section. Later sediments, Palaeozoic and Mesozoic, are of great thickness in the East Siberian sedimentary basins, but there is nearly no petroleum there, because of the absence of source rocks and regional seals. The next curve is for the Timan Pechora basin. Let's remember that it is the onshore continuation of the Devonian East Barents spreading center. The main stage of harmonic sedimentation is in the Silurian, Devonian and Carboniferous, and this is the main oil and gas bearing sequence of Timan Pechora. The third curve is for West Siberia. This is the most beautiful sedimentary basin of Russia and at the same time the most rich in hydrocarbons. It originated in the Triassic, as we remember, in the place of the Obsky aborted ocean, and developed harmonically all the way up. In order to see the beautiful structure of the West Siberian sedimentary cover, let us have a look at the simplified lithologic column of the Urengoy borehole area (Fig. 10). All formations are drilled here from land stage to regression. Sedimentation rates vary by a factor of 5-7 for different formations, but their duration is practically the same about 10 My (total accumulation time is about 30 My). The second basic hypothesis is that only those sedimentary basins are harmonic and rich in oil and gas, which originated during the land stages by intensive rifting followed by a short time of spreading, that means they originated in the "holes-in-thecontinent". We have already seen this in the Timan Pechora Basin (the Devonian rifting and spreading within the Barents Sea) and in West Siberia (the Triassic rifting and spreading within the Obsky Paleocean). Looking at the global view (Fig. 11), we can see much more amazing facts. It is obvious that land stages in Phanerozoic global cycles correspond to supercontinent formation. In these palinspastic maps, all harmonic basins are shown, which originated in Devonian on the margins of the relatively small Euroamerica, including the East Barents, Timan Pechora and North Caspian. These harmonic basins originated in the Triassic on the margins of huge Pangea. All in all, 95% of all petroleum reserves of the world belong to these basins.

CONCLUSIONS This is what we think today of the problem that was formulated by Ken Hsfi as a paradox many years ago. We should be neither astonished nor offended with some reviewers from geological magazines who can not believe in this theory. Science seems to be conservative enough, and to some extent it is good. New ideas are often supressed with "received opinions" and that is not good. Only 30 years ago some reviewers, who were full of "common sense", rejected the idea on the origin of marine magnetic anomalies, that was later called "Vine-Matthews hypothesis" and that can now be regarded as one of the best discoveries of the century.

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Fig. 10. Simplified lithologic column of Urengoy borehole area (North West Siberia).

But nature itself is always more complicated than the schemes we make to describe it. We got used to thinking that Paleozoic oceanic crust can be preserved only in ophiolites. But this is not true. Present-day continents have holes filled with crust of the oceans that vanished hundred million years ago. We will never manage to touch this crust, but our ideas on the Earth's evolution will not be complete without knowledge about this crust. Fortunately, even through the thick sedimentary cover, we can see geophysical evidence of the ancient oceanic crust

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Fig. 11. Palinspastic maps for Devonian and Triassic and harmonic sedimentary basins, which originated in the 'holes-in-the-continents' on the margins of Euroamerica and Pangea.

p r e s e r v e d in the " h o l e s - i n - t h e - c o n t i n e n t " . A n d t o d a y we k e e p l e a r n i n g to find a n d interpret them.

REFERENCES Aplonov, S.V., 1986. Geodinamika rannemezozoiskogo Obskogo paleookeana. IO AN SSSR, Moscow (in Russian). Aplonov, S., 1988. An aborted Triassic ocean in West Siberia. Tectonics, 7, pp. 1103-1122. Aplonov, S.V., Hsfi, K.J. and Ustritsky, V.I., 1992. Relict back-arc basins of Eurasia and their hydrocarbon potentials. The Island Arc, pp. 70-76. Burke, K., 1976. Development of graben associated with the initial rupture of the Atlantic Ocean. Tectonophys., 36: pp. 93-112. Caroll, A.R., Liang, Y. and Graham, S.A., 1990. Junggar Basin, northwest China: Trapped Late Paleozoic Ocean. Tectonophys., 181(1/4): 1-14. Hsfi, K.J., 1988. Relict back-arc basins: Principles of recognition and possible new examples from China. In: Kleinpell, K.L. and Paola, C. (Eds.), New perspectives in Basin Analysis, Springer-Verlag, New York, pp. 245-263. Kutolin, V.A., 1977. Problemy petrokhimii v petrologii bazal'tov. Nauka, Moscow (in Russian). McKenzie, D.P., 1978. Some remarks on the development of the sedimentary basins. Earth Planet. Sci. Lett., 40: 25-32.

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Pechersky, D.M., Didenko, A.N. and Kurenkov, S.A., 1983. Geologo-petromagnitnye issledovanija komplexa parallel'nykh daek Mugodzhar. Izvestiya Akademii Nauk SSSR, Fizika Zemli, 5:31-40 (in Russian). Ringwood, A.E., 1977. Composition of the core and implications for the origin of the Earth. Geochem. J., 11: 111-135. Vine, F.J., Matthews, D.H., 1963. Magnetic anomalies over ocean ridges. Nature, 199: 947-949. Zhuravl'ov, E.G., 1986. Trappovaja formatsija Zapadno-Sibirskoy plity. Izvestiya Akademii Nauk SSSR, Geologija, (7), 26-32 (in Russian).

Paradoxes in Geology Ueli Briegel and Wenjiao Xiao, eds. 9 2001 Elsevier Science B.V. All rights reserved.

Chapter 9

S T R U C T U R A L ROCKS: STRATIGRAPHIC I M P L I C A T I O N S

A. M. C. Seng6r and Mehmet Sakln9 ]TO Maden Fakiiltesi, Jeoloji B6liimii ve Avrasya Yerbilimleri Enstitiisii, Ayaza~a 80626, ]stanbul, Turkey

ABSTRACT The three great classes of rock types, namely sedimentary, igneous and metamorphic, are defined strictly on a genetic basis and represent records of the sedimentary, magmatic and metamorphic processes taking place in and, on the surfaces of, rocky planets. The genetic basis of the three great classes is best shown by a review of the history of understanding of their origin. The tri-partite subdivision of the major rock types omits, however, one major class, namely those rocks formed dominantly through structural processes, herein called structural rocks. Omission of this major class of rock type has resulted from the late recognition of the incidence frequency, and the large scale, of the structural processes generating rock types distinct from those included in the other three classes. Another reason for the late recognition of the structural rocks is the still prevalent confusion of structural rocks with metamorphic rocks. Structural rocks form entirely along fault zones, whose sizes may range from those barely mappable at usual mapping scales to transcontinental shear zones of thousands of km of offset and subduction zones, along which displacement magnitudes may reach tens of thousands of km. Structural rocks are conveniently divided into fault rocks and m61anges. Broken and disrupted formations and chaos structure constitute intermediary types between the two. A different classification, used in this paper, divides them into coherent and incoherent structural rocks. The coherent ones are further subdivided into brittle and ductile structural rocks. Structural rocks may become metamorphosed after formation or they may originate in metamorphic conditions, which makes their recognition difficult. They constitute rock bodies of various sizes, herein called keiroliths (Greek, Ke;pco-to shear and k;Oos- rock), ranging from insignificant volumes along small faults, that are usually not mapped separately from the fault trace, to immense complexes forming considerable portions of entire continents. Structural rocks commonly young into the generating fault zone when the fault has only just individualised as a throughgoing shear plane, but young outwards into fault walls in mature shear zones of considerable displacement, or in belts consisting of many such shear zones (called keirogens). M61ange wedges and other thrustrelated fault rocks commonly young towards the footwall. This generalisation is generally (but not always) valid and what it expresses probably results from the vicissitudes of developing shear zones, the circulation of fluids in them and the

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changing local stress fields owing to developing asperities resulting from diverse causes during the history of individual fault zones and whole keirogens. Structural rocks should be classifed stratigraphically following the rules of lithostratigraphic classification as all other major rock types. Thus, the lithosphere should be divided comprehensively, using a single classification scheme, into contiguous lithozones, with which only biostratigraphic and magnetostratigraphic zones overlap (and chemostratigraphic zones for marine sedimentary rocks). The spatial relationships of lithozones with one another and with biostratigraphic and magnetostratigraphic (and chemostratigraphic) zones are further records of geological processes. Interpretation of such spatial relationships in terms of processes enable the stratigrapher to establish temporal relationships of rock bodies forming the lithozones of the lithosphere and reconstruct its geological history. In this process, structural rocks have critically important messages to give. Their omission from the reconstruction has resulted in the fabrication of seriously deficient geological histories in the past. INTRODUCTION Earth history, which is nothing more than a concatenation of dominantly diachronous earth processes, can only be written if the earth processes can be read from their products, which are their only record that we can observe. Life leaves its traces in fossils (which in most instances are petrified images of organisms) and other earth processes in rocks. It is therefore of importance to regard rocks as records of earth processes. We study those records to learn about the processes. Rocks, and spatial relationships among rocks, tell us about the processes that formed them and the relationships among the processes. Physical geology deals with the processes as such; historical geology with their significance in the life story of our planet (and other rocky space objects, such as meteorites, asteroids and rocky planets). Accordingly, the first task of the geologist interested in earth history is to order rocks in space and time. This he or she can only do by knowing what process (or processes) each rock signifies. Only by knowing the processes that created a group of rocks can he or she make inferences about the significance of telling rock a from rock b and about their present spatial relationships. The purpose of this paper is to define structural rocks as a fourth great class of rock type formed by structural processes--in addition to sedimentary, igneous and metamorphic rocks, formed, respectively, by sedimentary, magmatic and metamorphic processes--and to discuss some of their stratigraphic implications. Various types of structural rocks have long been known and studied in widely different contexts, such as fault gauges, mylonites (collectively known as 'fault rocks': Sibson, 1977; Wise et al., 1984; Schmid and Handy, 1991; Passchier and Trouw, 1996, p. 98), m61anges (Greenly, 1919a, pp. 65-66, 193-195; Bailey and McCallien, 1950a, b, 1953, 1961, 1963; Gansser, 1955, 1959, 1964, 1974; Hsti, 1968, 1971; cf. Erol, 1981) and chaos (Noble, 1941; Wright and Troxel, 1969, 1984; Wernicke and Burchfiel, 1982). Because the fact that structural rocks form a fourth major coherent rock group of equal standing to the other three major rock groups has not been appreciated, they have seldom been separately identified, mapped and named

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(m61anges have become an exception after their r61e has been recognised in identifying plate tectonic processes by Hamilton, 1969, Dewey and Bird, 1970; Dickinson, 1970 and Hsfi, 1971), the identification of the processes that create them has been slower (e.g., Hsfi, 1971; Hanmer and Passchier, 1991, pp. 3-4). That is why some major earth structures, such as the giant transcontinental shear zones in Asia ($eng6r et al., 1993; ~eng6r and Natal'in, 1996) have long been overlooked, whose only widespread record they make up. As Higgins (1971, p. 1) wrote 'In many cases, overlooking the presence of cataclastic rocks has led to erroneous interpretations of structure, stratigraphy, and geologic history.' This paper is not a review of structural rocks. An enormous literature has grown about them in the last three decades and there are good reviews one could consult to learn about their various kinds and properties (e.g., Gansser, 1974; Hsfi, 1974; Nicolas and Poirier, 1976; White et al., 1980; Wernicke and Burchfiel, 1982; McCall, 1983; Raymond, 1984; Wise et al., 1984; Moore, 1986; Hanmer and Passchier, 1991; Schmid and Handy, 1991; Lallement, 1992; Passchier and Trouw, 1996). This paper instead aims at pointing to their unity in forming an independent major rock type class in equal standing to sedimentary, igneous and metamorphic rock type classes and to discuss some of their hitherto neglected stratigraphic implications. To that end, we first present what we understand under stratigraphy in the next section by presenting a history of the discovery of biostratigraphy and argue that even this most abstract of all stratigraphic methods is based on an entirely geometrical concept. The interpretation of the geometrical aspects of biostratigraphy in terms of time is based on how biological and geological processes are thought to be reflected in its geometrical features. In the section following that, we summarise the history of thought on understanding the origin of sedimentary, igneous and metamorphic rocks ernphasising the fact that their temporal aspects are derived entirely also from their geometrical properties interpreted in terms of their formative processes. Finally, we define and briefly describe the structural rocks, again emphasising the significance of their spatial relationships in terms of their temporal relationships. In the light of this discussion, we recommend that in their stratigraphic classification, the rules of lithostratigraphy should be employed. We end up by emphasising the importance of this stratigraphic approach to structural rocks for understanding the geological history of the lithosphere. ON THE N A T U R E OF STRATIGRAPHY: HISTORY OF THE INVENTION OF BIOSTRATIGRAPHY AS AN ILLUSTRATION

What is stratigraphy? The branch of geology that infers temporal relationships from spatial relationships is called stratigraphy. It may be thought of as a science of sequence rather than of strata. The term stratigraphy seems to have been first used by William 'Strata' Smith to define the science of strata description (Smith, 1817). William Smith has long been wrongly credited with the invention of biostratigraphy. We discuss this miscredit briefly below to underline the importance of process in understanding the temporal relationships of not only rock bodies with one another, but of any piece of the

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geological record, which consists entirely of solid or conceptual geometrical entities. In the historical review below, we show that biostratigraphy became a powerful tool in establishing temporal interrelationships of rock bodies, once it was appreciated that fossils can be grouped into conceptual geometrical objects called biozones, whose spatial relationships with one another and with the rock bodies, into which they penetrate, can be interpreted in terms of temporal relationships in the framework of biological and geological processes.

History of the invention of biostratigraphy as illustrative of the nature of stratigraphy

The recognition that fossils could be used to order in time, and to correlate with one another, the rocks in which they are contained, was probably the single greatest discovery in the history of geology after Steno's enunciation of the principles of lithostratigraphy in the seventeenth century and before the rise of plate tectonics in the twentieth. It made the construction of a detailed earth history possible, and the honour of having recognised that this was so has most commonly been accorded to the English engineer William Smith, 'the father of English geology,' by geologists, historians of geology, and general science historians (e.g., out of many, see Woodward, 1902; Cox, 1948; Fenton and Fenton, 1952, p. 70ft.; Kummel, 1970, p. 8; Bruno, 1987, p. 216). Some historians of geology (e.g., Laudan, 1987; Gohau, 1990) and recently some geologists such as Hallam (1989) lately have given Georges Cuvier and his friend and co-worker Alexandre Brongniart equal credit, on the basis of their recognition, in the Paris Basin, that certain fossil assemblages seemed to occur only in (or between) certain horizons and that they could be used to order these horizons in time (Cuvier and Brogniart, 1808, 1811, 182211969]). But they were sharply criticised by Sarjeant (1988, 1991, 1992), who rightly pointed out that Cuvier's and Brongniart's work well post-dated Smith's results that had become common knowledge in England and elsewhere through his unpublished table which he had so generously entrusted to almost anybody who would ask (Smith, 1815; for photographs of the original manuscript table and its transcription, see Sheppard, 1917, plate X and Kummel, 1970, frontispiece) and that it was unlikely that this had escaped Cuvier's wide-ranging notice. Despite this, we believe that Cuvier (and not Cuvier and Brongniart together!) deserves the entire credit for having invented biostratigraphy (see also Rudwick, 1997a, b; ~;eng6r, 1998, p. 21, note 25). First, let us look at the comparative chronology of the evolution of the ideas of Smith and Cuvier: Smith wrote down his ideas on the occurrence of certain fossils in certain beds for the first time on 5th January, 1796 in his 'book' which was nothing more than a private notebook (Phillips, 1844, p. 17f.1) Smith's 'friend and pupil' John Farey mentioned that Smith had already explained his 'discovery' to his friends in 1795 (Geikie, 1905, p. 394, footnote). De Launay wrote that Smith's 'stratigraphic' studies had already begun in 1794, but does not say on what source this early date was based (de Launay, 1905, p. 77). It was presumably the beginning of the construction of the Somersetshire Coal Canal, in the construction of which

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William Smith had been employed as resident engineer for six years (Woodward, 1902, p. 5). Woodward (1911, p. 33), wrote that Smith had made his fossil collection first in 1795. The earliest date is provided by Smith himself: 'After twenty-four years of intense application to such an abstruse subject as the discovery and delineation of the British Strata, the reader may easily conceive the great satisfaction I feel in bringing it to its present state of perfection' (Smith, 1815, p. 1). But we know that what had impressed him twenty-four years earlier had been only the regular lie of the Red Marl with respect to the Lias (Woodward, 1902). Even in his notes of 1792 and 1793 there is as yet no mention of fossils as index to strata. What made him aware of this relationship was his work on the Somersetshire Coal Canal (see esp. Eyles, 1969, pp. 151ft.). Smith himself says that the first widely disseminated news of his discovery was a manuscript table showing Order of Strata and their imbedded organic remains, in the vicinity of Bath; examined and proved prior to 1799 (published in Smith, 1815, as [Table] No. 1; Smith had dictated it in 1799 to his two clerical friends Joseph Townsend and Benjamin Richardson: Eyles, 1969, p. 144), 'a copy of which being shown to me by the late W. Reynolds of Colebrook-dale, about twelve years since, and being assured by him and others of its multiplication and general distribution, even to the East and West Indies, I am induced to publish it in the original form.' (Smith, 1815, p. 9). Smith continues to relate the history of his 'early discoveries' as follows: 'The thickness of several of the strata were underrated, and some few mistakes were made in other parts of the paper 2, which, in the progress of the work, will be corrected; but this short account of my early discoveries of regularity in the succession and courses of the strata within twenty miles of Bath, with the peculiar extraneous fossils by which they are characterized, became a key to the further courses of these and all other varieties in this island, which was easily transferred to the same kind of strata on the continent.' (Smith, 1815, p. 9). In 1817 Smith pointed out again that the first time his 'original method of tracing the Strata by the organized Fossils imbedded therein' was circulated in written form had been in 1799 (Smith, 1817, p. v). It seems thus clear that his method was developed sometime between 1795 and 1799 and his first ideas probably began crystallising in late 1795 so that he jotted them down in January, 1796. In fact, Eyles (1969, p. 153) thinks that Smith's discovery was made during the last three months of the year 1795. It is however, remarkable that he made no mention of fossils in an earlier table of strata he had prepared in 1797, but instead explicitly mentioned the simultaneous origin of the 'Freestone Ironstone Clay Ironstone or any other Mineral Body as well as C o a l . . . in heaps but regularly dispersed throughout such parts of the Globe as the Creator of all things thought fit to place them in' (Douglas and Cox, 1949, p. 185) in the best Stenonian/Wernerian lithostratigraphic tradition (which, in those days, was common knowledge)! Smith's first list of fossils contains also inorganic inclusions such as 'pyrites and ochre' under 'fossils, petrifactions, &c. &c.' (Smith, 1815, [Table] No. I; also see his manuscript table of 1799 in Kummel, 1970, where these words are legible). In his Strata Identified by Organised Fossils, Containing Prints on Colored Paper of the Most Characteristic Specimes in Each Stratum, Smith wrote, while describing the fossils of the Clunch Clay and Shale (=Oxford Clay: Arkell, 1933, table I: Oxfordian)

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that 'The upper part of this thick Stratum contains large incurved oysters or Gryphea, so much resembling others I have collected from remote parts, of a clay which now appears to be Oak-tree clay [= Kimeridge Clay." Arkell, 1933, table I." Kimmeridgian] as to be distinguished with difficulty; but this is only one of the many instances of the general resemblance of organized Fossils, where the Strata are Similar.' (Smith, 1816, p. 22, our italics). One of the many instances referred to was that 'Cockscomb Oysters are also common both to that rock, and to the Septaria above; in fact those large Clay-balls found plentifully in the deep cutting of the north Wilts Canal seemed to partake both of the inhabitants of the rock above, and of that below the Clay; the Trigonia of the Clay-balls being the same species as those large ones which compose the chief part of some beds of stone, about four feet thick, near the bottom of the Swindon rock. It may at first appear that the identification of Strata, by the organized fossils they contain, would in such cases be somewhat doubtful; but in the course of the work I shall make further remarks on such apparent repetitions, which will rather show the great utility of them,...' (Smith, 1816, p. 18). Smith thus had stumbled upon what we now know to be a thoroughly uncharacteristic oyster for the rocks he was working in, with a range from the Jurassic to Eocene, but he had no idea what to do with it except to fall back, almost certainly unknowingly, onto the old assumption by Baldassari (1751, quoted after Edwards, 1967, p. 36) that similar fossils occurred in similar rocks (which he thought would be useful and welcome news for well-sinkers and others 'in these thick Strata of Clay, abounding with alternations of stony matter and organized Fossils:' Smith, 1816, p. 18), an assumption that negates the whole essence of biostratigraphy.t All these show, we think clearly, that Smith did not, at least not originally, consider fossils as being independent of the rocks in which they were embedded. In sharp contrast, that is what Cuvier did explicitly, for the first time. In the same month, only 16 days after Smith jotted down his first notes on the index r61e of fossils in strata in private, Cuvier made public his views on the extinction of some elephant species in his seminal lecture on living and fossil elephants, which was published in the same year (Cuvier, 1796; see Rudwick, 1976, p. 157, note 1, on the confusion about the exact date). This work, however, was a follow-up on his initial studies published earlier in a short abstract in co-authorship with E. Geoffroy Saint-Hilaire (Cuvier and Geoffroy [Saint-Hilaire], 17953), in which it was clearly stated that "Finally, the citizens [i.e. Cuvier and Geoffroy] have discovered that the Mammoth, the animal whose bones are found in Siberia and elsewhere, and which has been regarded always as an elephant, is indeed of the same genus, but, though very closely allied to the Asian elephant, is distinct enough to be considered a separate species" (Cuvier and Geoffroy [Saint-Hilaire], 1795, p. 90). Cuvier later stated in his partial autobiography that the 1796 paper was the first place in which he mentioned his views on extinct animals. It is indeed true that the great anatomist first made his views public about the existence of a world anterior to ours that had

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v a n i s h e d t h r o u g h some c a t a s t r o p h y in his f a m o u s 1796 lecture. It was with the following clear w o r d s t h a t C u v i e r h a d a n n o u n c e d his theory: 'But one science that at first seems to have not much of a close relation with anatomy, that which deals with the structure of the earth, which collects the monuments of the physical history of the globe, and seeks to trace in a daring way the table of the revolutions which it has proven; in one word, geology, could establish in a sure manner the various facts serving as its basis only by the aid of anatomy. Everybody knows that in Siberia, in Germany, in France, in Canada, and even in Peru, bones of enormous animals are found in the earth which cannot belong to any of the species that now live in these climates. For example, one has found them in all the northern parts of Europe, Asia, and America, which resemble the bones of elephants and the texture of their ivory that of their defenses to such a degree that all scholars have taken them to be as such to this day. Others seem to be bones of rhinoceros and indeed approach them much. Now there are neither elephants nor rhinoceros in the torrid zone of the Old World. How come are their cadavers found in such great numbers in the northern parts of the two continents? On this issue the conjectures are dime-a-dozen: some suppose that vast inundations had transported them there; others that the peoples of the south had taken them there during large military expeditions. The inhabitants of Siberia believe in good faith that these bones belong to a subterranean animal similar to our mole, which never allows itself to be taken alive; they call it mammoth, and the horns of the mammoth, which resemble ivory, form an extremely profitable branch of commerce. None of these could satisfy an enlightened mind. The hypothesis of Buffon would have been the most plausible, had it not been combatted by reasons of a different type. To him, the earth, detached as an incandescent mass from the sun, had begun to cool by the poles. It was there the organic nature began. The first-formed species, those that most needed the heat, were chased towards the equator as the refrigeration progressed. Having thus crossed all the latitudes, there is nothing surprising in the fact that one finds their remains all over the place. A close examination made by anatomy of these bones has taught us that they are never so similar to those of the elephants as to be regarded as belonging to the same species and we can thus dispense with all these explanations. The teeth and the jaws of the mammoth do not at all resemble those of the elephant. A glance at the same parts of the animal of Ohio will suffice to show that it is still farther removed. These animals would have differed in the same way and more from the elephant as the dog does from the jackal and the hyena; considering that the dog tolerates the cold of the north, while the other two live only in the south, it could have been the same with these animals, of which we only know the remains. But in dispensing with the necessity of admitting a gradual refrigeration of the earth, in distancing the pathetic ideas presented to our imagination by the ice and the frost of the north invading the presently so cheerful countries, into which novel difficulties would we be thrown now by these new discoveries? What has become of these two enormous animals of which one finds only the vestiges, and of others of which the earth offers us all over the place their remains, and of which none probably exists today? The rhinoceros of Siberia is entirely different from all the known rhinoceros; the same applies to the alleged bears of Ansbach 4, to the fossil crocodile of Maastricht 5, to the species of deer 6 of the same place, to the twelve-footlong animal without incisors and with digits armed with claws 7, of which a skeleton was

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discovered in Paraguay: none with a living analogue. Why, finally, is there no petrified human bone? All these facts, analogous to each other, and against which no established fact can be brought, seem to me to prove the existence of a world anterior to ours, destroyed by some catastrophe. But what was that primitive earth? What was this nature not subordinated to the empire of man? And what revolution could have annihilated it to the point of leaving only the semi-decomposed bones as traces? It is not for us to enter into the vast field of conjectures these questions present. The more audaceous philosophers have entertained them. The modest anatomist, called to detailed investigations, to scrupulous comparison of the objects submitted to his eyes and to his scalpel, is content with the honour of having opened this new path to the genius who will dare to tread it.' (Cuvier, 1796, pp. 442-445; italics Cuvier's; see Rudwick, 1997a, pp. 21-24 for an independent translation). The idea that fossils delineate time horizons is thus expressed in a language whose clarity leaves nothing to be desired. The great anatomist not only pointed out that the extinct animals provide an upper time limit for the world in which they lived, but by mentioning (as it later turned out, incorrectly) the absence of fossil human bones 8, he stressed that the present fauna provides a lower time limit for the present world. Both these limits he thought coincided and provided a time-horizon that demarcated, in the rock record, the present world from a former one (Fig. 1). Cuvier's particular examples turned out to be unfortunate and he has been later much abused by historians of geology for having got his facts wrong, but those historians thus only have betrayed their misunderstanding of the nature of scientific progress and of the momentous importance of Cuvier's idea of fossils providing time-lines for our investigations of the past (see Stephen Jay Gould's elegant words in defense of Cuvier in his Foreword in Smith, 1993). In 1799 Cuvier published on the fossil elephants and their geological implications more extensively, now incorporating the Siberian geological observations as to the strata in which these fossils occurred and Pallas' speculations to explain them, which we know he had learned through his Hamburger friend J. A. H. Reimarus already in 1797 (Coleman, 1964, p. 113). Cuvier repeatedly stressed the confinement of these fossils to barely indurated strata--clearly the deposits of the last convulsion of the earth as quoted extensively above. Therefore Cuvier knew, at the latest by the date of his initial joint abstract in 1795, that some animals had vanished from the face of the earth and, by 1796, that this was related to some world-wide event (process.t) that also affected the rocks, in which the fossils represented embedded time-lines. By the time Smith's unpublished table began circulating, Cuvier had thus already k n o w n - - a n d made k n o w n - - t h a t some fossil animal species had temporal ranges 9 confined by definite limits reflected in their record entombed in rocks and that this had had physical causes that had been nearly universal in effect. When he joined Brongniart in producing the mineralogical geography of Paris, Cuvier did not think he was doing anything novel in principle. That it was novel for the Paris basin geology, he and his co-author stressed with the following words: 'The region in which this capital is situated is perhaps one of the most remarkable that has been

Structural rocks." stratigraphic implications

139 Highest occurrence of taxon a in sequence

/-

Range of man i

Biohorizon

Geographical (horizontal) extremes of occurrence of taxon a

itt

"- Lowest occurrence of taxon a in sequence

Time surface (='Cuvier horizon') marking the disappearance of one taxon and the appearance of another

A

Range zone of taxon a

Range of mammoth

'

9

[ ~

B

Range zone of taxon b

Highest occurrence of taxon b in sequence

Geographical (horizontal) extremes of occurrence of taxon b Lowest occurrence of taxon b in sequence

Fig. 1. A. Schematic depiction of the 'Cuvier horizon' defined herein to be that time surface separating the time of extinction of one species and the time of appearance of another. This concept is implicit in the writings of Cuvier. Cuvier thought that the fossil species he was dealing with ranged far on the surface of the earth (even globally?) and did not concern himself with their spacial limitations. Hence a onedimensional representation suffices to represent a Cuvier horizon. B. Alexander von Humboldt, the founder of modern physical geography, criticised Cuvier's neglect of the geographical range of species and argued that, as in the case of the present-day animals and plants, the distribution of fossil organisms must also have been limited by geographical factors. This, yon Humboldt thought, would make the employment of biostratigraphy much more difficult than implied in Cuvier's writings. These ideas of von Humboldt were the forerunners of the concept of biostratigraphic zone.

observed yet, by the succession of diverse terranes that make it up and by the extraordinary remains of ancient organisms it harbours. Thousands of marine shells with which shells of fresh water alternate regularly make up the principal mass; bones of terrestrial animals entirely unknown, even by genus, fill certain parts; other bones of species noticeable by their large size, of which we find some relatives only in very distant countries, are distributed in the most superficial beds; a very marked character of a past irruption of the sea from the south-east is marked in the forms of the capes and in the directions of the principal hillocks; in brief, there is no other place that can instruct us better on the last revolutions that terminated the formation of our continents. However, this terrain has been little studied from this viewpoint, and although for a long time it has been inhabited by considerably educated men, what has been written on it is confined to a few fragmentary essays and always of a purely mineralogical character, without any regard to the organised fossils, or of purely zoological, without any attention to the position of these fossils." (Cuvier and Brongniart, 1811, pp. lf.; for an independent translation, see Rudwick, 1997a, p. 133).

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All this, some might argue, would entitle Cuvier to an equal share in the erection of biostratigraphy. We believe that in the field methodology of biostratigraphy he might deserve equal credit with Smith, or even yield to Smith, for, as he himself so generously pointed out once, the field geology of their joint Paris papers was mainly the work of his friend Brongniart. But in the theory of biostratigraphy, Cuvier takes most, if not all the credit. It is our conviction here that if it were not for that theory, the field method would not have been used so widely and diffused so rapidly, nor probably would it have survived, as the writings of William Smith himself betray! Cuvier's theory of universal catastrophes annihilating whole faunas and their replacement by entirely newer ones in immense areas, if not globally, as wrong as it was, gave a robust reasoning to biostratigraphy that neither Lyell's uniformitarianism nor Darwin's theory of gradual evolution later was able to shake (see also Neumayr, 1887, p. 26; Suess, 1901, p. 1). Cuvier had shown that abrupt changes in the terrestrial (or even local) biota had taken place and he saw immediately that these could provide reference points, on the basis of which a temporal ordering might be possible. Apart from his specific theories of catastrophism, that reasoning underlying biostratigraphy has remained unchanged since he first uttered it. This principle, however, could not have been erected had he not imagined devastating, large-scale events, inspired by the incredibly wide distribution of mammoth and rhinoceros carcasses in Siberia reported mainly by Pallas, and by Pallas' own hypotheses. Armed with that principle, Cuvier would not have succumbed to any apparent exceptions in the biostratigraphic order as clearly William Smith did and Lyell, as mocked by De La Beche in his sketch of the Professor Ichthyosaurus, was more than ready to do and would have been overjoyed if one such exception could have been demonstrated (see the delightful and informative Chapter 4 in Gould, 1987, and his Fig. 4.1 reproducing De La Beche's caricature). Also the very idiom 'organised fossils' implying an order, i.e. a model, was Cuvier's (see Cuvier and Brongniart, 1808, p. 422; 1811, p. 2). It appears in Smith's writings first in his 1816 book; Smith's own earlier description had been 'imbedded organic remains' (so in his 1799 table: see Smith, 1815), stressing the empirical aspect (i.e. where the fossils occur) in sharp contrast to Cuvier's theoretical emphasis (i.e. that they are 'ordered in time'). The fact that Smith adopted it we think is ample evidence that he fell under Cuvier's influence as soon as the latter's views became known to him. Without in any way wishing to belittle it, we emphasise here that William Smith's great service was more to the ordering of strata in the field by a rule-of-thumb methodology (as shown by his reference to the utility of his method to well-sinkers) than to subdividing the past of the planet on the basis of its rock and fossil record and in the framework of two theories of earth-behaviour and development of life, i.e. processes (as shown by Cuvier's emphasis on the revolutions of the past than on the succession of strata recording them). The distinction between these, not always appreciated by geologists, is immense. No activity, not ordered within the framework of a theory about process(es), can be called science (even simple historiography: see Seng6r, in press) and thus it was Cuvier alone who invented biostratigraphy and then raised it to the status o f a science.

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N o t h i n g shows this m o r e clearly than the report he read to the A c a d e m y at the Louvre on 1 l t h August, 1806, on F a t h e r Andr6 Chrysologue de Gy's ThOorie de la Surface A ctuelle de la Terre (Chrysologue de Gy, 1806): '... some of the principal objects which appear to us necessary to be profoundly studied, in order to make geology a science of facts, and before attempting, with any hope of success, to answer the grand problem of the causes which have reduced our globe to its actual state. To this end we ought, 1st, To search if the division of great chains in one middle crest, and two orders of lateral crests, observed by Pallas, and developed by de Luc, is invariable, and examine, as Mr. Ramond has done in the Pyrenees, the causes which sometimes mask them. 2ndly, To examine if there is also any thing certain or uniform in the succession of secondary strata, if such a kind of stone is always below such another, and vice versa. 3rdly, To operate in a similar manner on the fossils, determine the species which appear first, and those which come only later; discover if these two sorts never accompany each other, if there are any alternations in their appearance; that is, if the first found appear a second time, and if the second have then disappeared. 4th, To compare the fossil with the living species more rigorously than has hitherto been done, and determine if there is any relation between the antiquity of the beds, and the similarity or dissimilarity of fossils with the living beings. 5th, To determine if there is any uniform relation of climate between fossils and those living beings which most resemble them; as, for example, if they migrated from the north to the south, the east to the west, or if there have been mixtures and irradiations. 6th, To determine what fossils have lived where they are now found, what others have been transported there, and if there are, in this respect, uniform rules with regard to the strata, species, or climates. 7th, To follow in detail their different strata throughout their whole extent, whatever may be their doublings, inclinations, ruptures, and slopings; and also to determine what counties belong to one and the same formation, and what others have been formed separately. 8th, To follow the horizontal beds and those which are inclined in one or different ways, to determine if there is any relation between the greater or less constancy in their horizontal position, antiquity, or nature. 9th, To determine the valleys in which the re-entering and salient angles correspond, and those in which they do not; also those in which the strata are the same on both sides, and those in which they differ, in order to discover if there is any relation betwen these two circumstances, and if each of them taken apart has any analogy with the nature and antiquity of the strata composing the heights which limit the valleys' (Cuvier et al. l~ in Chrysologue de Gy, 1806, pp. 326ff.) This is a research p r o g r a m m e that the palaeontologist and geologist 11 Cuvier himself was following as the reader of his Discours pr~liminaire (Cuvier, 1812, v.1) would at once recognise, but one which William Smith himself p r o b a b l y could not have outlined, m u c h less actually carried out. W h e n Cuvier presented his results to the public in his palaeontological magnum opus, the Recherches sur les Ossemens Fossils de Quadrupeds, witness his enthusiastic exclamation on w h a t he believed had been the results of his (and his colleagues') efforts: 'The i m p o r t a n c e of these researches [on the relations of fossils with the strata in which they are contained] is

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quite obvious. It is to them alone that we owe the commencement even of a Theory of the Earth; as, but for them, we could never have even suspected that there had existed any successive epochs in the formation of our earth, and a series of different and consecutive operations in reducing it to its present state. By them alone we are enabled to ascertain, with the utmost certainty, that our earth has not always been covered over by the same envelope; because we are thoroughly assured that the organised bodies to which fossil remains belong must have lived upon the surface, before they came to be buried, as they now are, at a great depth. It is only by means of analogy, that we have been enabled to extend to the primitive formations, the same conclusions which are furnished directly for the secondary formations by the fossils; and if there had only existed formations of strata in which there were no extraneous fossils, it could never have been asserted that these several formations had not been formed all together.' (Cuvier, 1812, v. 1, p. 3512). The contemporaries of Smith and Cuvier also conceived the relative merits of the two men's contributions much along the lines that we have outlined. Rupke (1983) has assembled the relevant evidence from the British side and we use his book to underline a few points. First, when Smith was given the inaugural Wollaston Medal, the council of the Geological Society was conscious of his 'being a great original discoverer in English Geology; and especially for his having been the first, in this country, to discover and to teach the identification of strata, and to determine their succession by means of their embedded fossils' (Extract from the minutes of the Council, 1 l th January, 1831, quoted from the Proceedings of the Geological Society, i, p. 271 by Rupke, 1983, p. 191; italics ours). The Council minutes thus emphasise Smith's originality in England and no global credit was claimed. The reason for this is provided by Fitton's implication made in his Notes on the History of English Geology published in 1833 that the dissemination of Smith's results had been of secondary importance in establishing biostratigraphy as compared with the formal influence from abroad, especially from Cuvier's Essai sur la G~ographie MinOralogique (Rupke, 1983, p. 193): 'that publication has given a greater impulse to geological science; bringing into view distinctly, and for the first time, that great class of deposits which connects the secondary strata with the products of still subsisting operations, establishing on impregnable ground the importance of zoological inquiries to the history of the earth, and affording some of the most masterly examples of the investigation of local details.' Rupke (1983, p. 192) identifies the glorification of Smith not as an attempt to set the historical record straight, but as an attempt by the English school of geology at self-definition. Smith, the self-educated country boy making a great discovery in the best Baconian tradition with no prejudice or theory, suited uniquely well the image of the juvenile Geological Society founded on a Baconian philosophy of science (Woodward, 1907, p. 14). Those who might think that we read modern thought into Cuvier's ideas would perhaps be admonished by von Humboldt's insightful critique of Cuvier's biostratigraphy in 1823, which reflects the spirit of some of our modern concerns: 'Another objection [against using 'organised fossils' uncritically in identifying formations], drawn from the influence which climates exert even on pelagic animals, appears to me still more important. Although the seas, from well-known physical

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causes, have, at immense depths, the same temperature at the equator and within the temperate zone, yet we see, in the present state of our planet, the shells of the tropics (among which the univalves predominate, as they do among the testaceous fossils) differ much from the shells of northern climates. The greatest number of those animals adhere to reefs and shallows; whence it follows, that the specific differences are often very sensible in the same parallel on opposite coasts. Now if the same formations are repeated and extended to immense distances, from east to west, from north to south, and from one hemisphere to the other, is it not probable, whatever may have been the complicated causes of the ancient temperature of our globe, that variations of climate must have modified, heretofore as now, the types of organisation; and that the same formation (that is, the same rock placed in the two hemispheres between two homonymous formations) would have enveloped different species? It no doubt often happens, that superposed beds present a striking difference in their fossil organic remains. But can we thence conclude, that after a deposit was formed, the beings which then inhabited the surface of the globe were all destroyed? It is incontestable that generations of different types have succeeded to one another. The ammonites, which are scarcely to be found among transition rocks, attain their maximum in the beds that represent, on different points of the globe, the muschelkalk and Jura limestone; they disappear in the upper beds of the chalk, and above that formation. The echinites, extremely rare in alpine limestone, and even in muschelkalk, become on the contrary very common in the Jura limestone, chalk, and tertiary formations. But nothing proves that this succession of different organic types, this gradual destruction of genera and species, coincides necessarily with the periods at which each formation took place.' (von Humboldt, 1823a, pp. 52f.; in the French original: von Humboldt, 1823b, pp. 41f.). This is the earliest plea we are aware of to keep lithostratigraphy and biostratigraphy distinct and the first firm statement that the two do not necessarily coincide and neither represents a real chronostratigraphy. Von Humboldt's points about the influence of geography are not terribly different from those which Berry pointed out when discussing limitations of zones in 1987 (see his pp. 157ff.). Von Humboldt sounds so modern, because he was criticising a very modern view of the r61e of fossils in stratigraphy. Fig. 2A shows a Venn diagram illustrating the concept of a taxon space. This is a two dimensional representation of a four-dimensional conceptual object embodying all individuals of a particular taxon that have ever lived. We map this taxon space into a three-dimensional set of all preserved fossils of its members and thus obtain a biozone known as taxon range zone (Fig. 2B). Taxon range zones are the most fundamental units of biostratigraphy and von Humboldt (1823a) was the first to sense their existence (Fig. 1B). They are three-dimensional interpenetrable geometric entites whose relationships can best be defined in the most complete fashion by using the concepts and terminology of the set theory and the Venn diagrams to graphically depict sets. Fig. 2C shows the definition using Venn diagrams of the most commonly used biozones in stratigraphy. In a similar vein one can think of every rock unit as a distinct lithozone. Lithozones are called lithostratigraphic units and, unlike biozones, they are not interpenetrable. They are defined by rock characteristics. These characteristics range from such simple ones as colour through mineralogical composition,

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grain size, structural fabric on a variety of scales to any combination of these. The requirement is that they be recognisable in the field at most by means of a hand-lens. The geologist defines the lithozones in terms of formations (plus various subdivisions and groupings of formations into smaller and larger units, respectively) and maps

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Fig. 2A. A Venn diagram showing the concept of a taxon space. Taxon space is a four-dimensional set of all individuals of a taxon that have ever existed. B. A Venn diagram showing a taxon range zone, which is the most fundamental of all biostratigraphic zones. A taxon range zone is a three-dimensonal conceptual entity representing the record of a four dimensional conceptual entity formed by the taxon space (see Fig. 2A). C. Venn diagrams illustrating the definition of the most commonly used biozones in stratigraphy as set relations among a number of sets representing various taxon range zones (for discussion of these biostratigraphic zones, see Salvador, 1994).

them out with a view to obtaining as complete a picture as possible of the 3-dimensional mosaic of the earth's lithosphere. The first thing he or she usually does in the field is to separate the three great classes of rock types, namely the sedimentary, igneous and metamorphic rocks in his or her area. Below we present a critical review of the history of recognition of these three classes to underline that this recognition was, exactly as in the case of the recognition of biozones,

process-based. THE THREE GREAT CLASSES OF ROCK TYPES Of the three great classes of rock types, the igneous and the sedimentary rocks had long been known to mankind. The neolithic man living in the central Anatolian settlement of (~atal H6yfik in 6500 BC extensively used and traded obsidian produced by the local volcanoes and depicted one of them in frightful eruption on the wall of his shrine (Mellaart, 1967; Seng6r, 1997). It could not have escaped him that the precious stone he shaped into sharp tools was once an incandascent lava. Similarly, the silting activity of rivers was a phenomenon commonly noted in antiquity.

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Sedimentary rocks We begin our history of the g r o w t h of u n d e r s t a n d i n g of the s e d i m e n t a r y rocks by q u o t i n g H e r o d o t u s ' i n t e r p r e t a t i o n of the geological history of Egypt. The father of history gave an elaborate description of h o w he t h o u g h t the river Nile m u s t have created E g y p t in the past ages: 'The greater portion, then, of this country whereof I have spoken was (as the priests told me, and I myself formed the same judgement) land acquired by the Egyptians; all that lies between the ranges of mountains above Memphis to which I have referred seemed to me to have been once a gulf of the sea, just as the country about Ilion and Teuthrania and Ephesus and the plain of the Maeander, to compare these small things with the great [note that Herodotus was not describing what he thought a unique event, but the consequences of a well-known process, namely sedimentation]. For of the rivers that brought down the stuff to make these lands there is none worthy to be compared for greatness with one of the mouths of the Nile; and the Nile has five mouths. There are also other rivers, not so great as the Nile, that have wrought great effects;... Now in Arabia, not far from Egypt, there is a gulf of the sea entering in from the sea called Red 13, of which the length and narrowness is such as I shall show: for length, it is forty days' voyage for a ship rowed by oars from its inner end to the wide sea; and for breadth, it is half a day's voyage at the widest. Every day the tide ebbs and flows therein. I hold that where now is Egypt, there was once another such gulf; one entered from the northern sea towards Aethiopia, and the other, the Arabian gulf of which I will speak, bore from the south towards Syria; the ends of these gulfs pierced into the country near each other, and but a little space of land divided them. Now if the Nile choose to turn his waters into this Arabian gulf, what hinders that it be not silted up by this stream in twenty thousand years? Nay, I think that ten thousand would suffice for it. Is it then to be believed that in the ages before my birth a gulf even much greater than this could not be silted up by a river so great and so busy?' (Herodotus, II. 10-11). It is h a r d to believe, were it n o t for his u n d e r e s t i m a t i o n of time necessary to silt up the R e d Sea, that this I o n i a n of Halicarnassus is addressing us f r o m twenty five centuries ago! The w o r d sediment in the sense we use it t o d a y appears for the first time in Steno's Canis Carchariae Dissectum Caput (The Dissection of the H e a d of a C a r c h a r i a s Shark: Stenonis, 1667), in which Steno develops in five consecutive conjectures a t h e o r y of h o w s e d i m e n t a r y rocks form: 'Conjecture 1: Soil [= terra] from which bodies resembling parts of animals does not seem to produce these bodies today . . . . Conjecture 2: The said soil does not seem to have been firm when the bodies to were produced in it . . . . Conjecture 3: Nor can there be strong opposition to the belief that the said once covered with water . . . . Conjecture 4: There seems also to be no objection to the belief that the said at some time in the past mixed with water . . . .

are dug referred soil was soil was

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Conjecture 5: I cannot see anything to prevent us from regarding the said soil as sediment gradually accumulated from water. [Nee qvicqvam obstare video, qvo minus

eandem terram pro aqvae sedimento sensim congesto habeamus].' (Stenonis, 1667, pp. 93-99; in Pollock's translation in Scherz, 1969, pp. 97-105). Flfigel (2000) has recently pointed out that here, by employing the w o r d sediment, the a n a t o m i s t Steno was m a k i n g use of a medical concept that he was already familiar with. In following the implications of sedimentation in an aqueous environment, Steno drew the following stratigraphic consequences in his m o r e f a m o u s w o r k of 1669. H a v i n g declared in his dedication to his patron, the Medici Duke, that ' . . . regions where shells and other similar deposits of the sea are dug up . . . were sediments of the turbid sea' ((Stenonis, 1669, p. 2; in Winter's translation, 1916, p. 206), Steno observes the following on p. 29ff.: 'Concerning the position of the strata the following can be considered as certain: 1. At the time when a given stratum was being formed, there was beneath it another substance which prevented the further descent of the comminuted matter; and so at the time when the lowest stratum was being formed either another solid substance was beneath it, or if some fluid existed there, then it was not only of a different character from the upper fluid, but also heavier than the solid sediment of the upper fluid. 2. At the time when one of the upper strata was being formed, the lower stratum had already gained the consistency of a solid . . . . 4. At the time when any given stratum was being formed, all the matter resting upon it was fluid, and, therefore, at the time when the lowest stratum was being formed, none of the upper strata existed.' (Stenonis, 1669, pp. 29-30; Winter, 1916, 229-230). The above three items enunciate what we today k n o w under the designation of the

principle of superposition (cf. Schenck, 1960). H a v i n g established the principle of superposition of strata, Steno proceeds to establish their individual geometry: 'As regards form, it is certain that at the time when any given stratum is being produced, its lower surface, as also its lateral surfaces, correspond with the surfaces of of the lower substance and lateral substances, but that the upper surface was parallel with the horizon, so far as possible; and that all strata, therefore, except the lowest, were bounded by two planes parallel with the horizon. Hence it follows that strata either perpendicular to the horizon or inclined toward it, were at one time parallel with the horizon.' (Stenonis, 1669, pp.30-31; White, 1916, p. 230, with minor modification). This is the principle of original horizontality of beds (Schenck, 1960). Steno finally pointed out that the beds deposited in a basin m u s t continue u n i n t e r r u p t e d as far as the edges of that basin: 'At the time when any given stratum was being formed it was either encompassed on its sides by another solid substance, or it covered the entire spherical surface of the

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earth. Hence it follows that in whatever place the bared sides of the strata are seen, either a continuation of the same strata must be sought, or another solid substance must be found which kept the material of the strata from dispersion.' (Stenonis, 1669, p. 30; Winter, 1916, p. 239). This last item is today known as the principle of original continuity (Schenck, 1960). Note that the three principles that Steno erected concerning the spatial and temporal characteristics of strata were all derived from the assumption that they are deposited in a fluid medium, i.e., from the assumption of a process. Both the recognition of sedimentary rocks as such and their stratigraphic characteristics were based on the process that was assumed to have created them. This process Steno had observed along the present day river mouths in northern Europe and he deductively reconstructed it from the preserved shark tooth fossils in Tuscany on the basis of the carcharodon shark head that he had examined (Stenonis, 1667).

IgHeous rocks The recognition of the origin and the deduction of the stratigraphic characteristics of igneous rocks proceeded along a very similar path. Only lava had ever been observed to harden into rock. The origin of the subvolcanic and the plutonic rocks had to be inferred from their hypothesised mode of formation and the various characteristics observed in the field. Although the presence of molten rock in subterranean 'channels' had already been conjectured (DK31B52; Bollack, 1969a, pp. 88-89; DK31B51; Bollack, 1969a, pp. 62-63; also see the commentary on Timaeus by Proclus of Byzantium cited by Bollack, 1969b, p. 228; in addition, Seneca, in his Quaestiones Naturales, III; Bollack, 1969a, pp. 88-89 and 1969b, pp. 227-228) and some of the changes observable on the surface of the earth had been ascribed to it (Helmbold in Cherniss and Helmbold, 1957, p. 275) already in the fifth century BC by Empedocles and metallic veins have been described both by Diodorus Siculus and Pliny the Elder. The first reference to rocks later to be called 'plutonic' by Richard Kirwan that we are aware of occurs in the Abbe Lazzaro Moro's famous book De Crostacei e de gli Altri Marini Corpi Che si Truovano su' Monti (1740) and in an entirely hypothetical context. The main point of Moro's book is that all mountains are made by vertical uplift under the influence of subterranean heat. In Book II, ch. XV, Moro counters the arguments of those who point out that uplifts must create void spaces in the bowels of the earth. He says: 'I must.., say that in those places where mountains rise, empty spaces may be avoided in two ways: For the first, the space, that is made by the subterranean fire by throwing out so much material remains full of burning fire-matter. This fills it up to such an extent that no empty spaces remain. When the fire abates, the space becomes full of air or other fluid materials.' (Moro, 1740, p. 300).

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Only thirteen years later the famous German geologist Johann Gottlob Lehmann wrote in his Abhandlung von den Metalmfittern und der Erzeugung der Metalle (1753) that 'the dykes which we find in mines, appear to be only the branches and shoots of an immense trunk [= Stokk; Stock in Lehmann's original], which is placed at a prodigious depth in the bowels of the earth; but in consequence of its great depth, we have not yet been able to reach the trunk [Lehmann says that it is so deep that it cannot be reached at all]. The large dykes are its principal branches, and the slender ones its inferior twigs.' (Lehmann, 1753, p. 178, quoted from Werner, 1791, p. 3014). Filling of empty spaces created within pre-existing rock has been a concept wellknown to miners centuries before Moro. But almost all miners thought that these 'veins' and 'dykes' (= vena profunda in Agricola, 1556, Book Ill), 'sills' (= vena dilatata, ibid.) and even stocks (= vena cumulata, ibid.) were filled either from above or by 'infiltration.' Agricola (1544[ 1956]) was of the opinion that these bodies formed in two ways: one was by filling fissures opened simultaneously with the origin of the 'country rock,' the other by filling those that post-dated the origin of the country rock by some time. He thought that the infills were created by appropriate mixtures of water and earth (in the sense of the four-element theory) and the mixture was determined by the degree of the prevailing heat or cold (an idea Agricola took from Albertus Magnus and a Muslim scientist called Gilgil by Albertus: Agricola, 154411956], book V, passim). In all writers who later treated the origin of crosscutting rock bodies we see similar ideas developed as summarised by Werner (1791, pp. 7-50). What is common to all is the idea that these bodies originated by disrupting a pre-existing rock mass and the space thus crated was filled by material younger than the host rock body. This cross-cutting relationship is the essence of the stratigraphic significance of the igneous rocks, even in case of sills, which may be thought of as cross-cutting an infinitely thin plane separating two concordant tabular bodies of rock. James Hutton, in his Theory of the Earth (1795), has made full use of the power of this relationship to show that the cross-cutting bodies of igneous rock could not have been filled from above in a sedimentary manner. In the second volume of the Theory of the Earth he quotes de Luc at length (Hutton, 1795, pp. 81-85), describing what the central European miners call a Bergkeil (= Mountain wedge: see Fig. 3). De Luc called this a 'most embarrassing question' (1779, p. 362) for the theory that the dykes represented open fissures filled from above by sedimentation, because if that theory were true it would be impossible to explain the support of the Berggkeil. In it Hutton saw 'the clearest evidence that an enormous mass of the mountain had been raised by a subterranean force; that this force had acted upon an enormous column of melted mineral, the specific gravity of which is great; and that this fluid mass had suspended a great wedge of this mountain, or raised it up.' (Hutton, 1795, p. 85).

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'Bergkeil' (=Mountainwedge) Dyke

Fig. 3. Schematic representation of the idea of Bergkeil (= Mountain wedge) used in the eighteenth century to argue against the possibility of the filling of dykes from above (i.e. in the neptunian way). If the spaces containing the dykes were originally gaping fissures, it becomes impossible to support the Bergkeil.

H u t t o n does not, however, refer to Werner's defense (possibly against de Luc), which is that crossing dykes do not need to be of the same age and, in by far the most cases, they are not. In cases where they are, irregularities along the dyke walls may still support the mountain wedge above the open fissures that later will be filled to become dykes (Werner, 1791, pp. 100-101; Werner, 1809, p. 96). All other inferences of H u t t o n concerning plutonic bodies of rock were based on his deduction that since erosion seemed capable of levelling the land in a relatively short time, an uplifting agency was required to restore it. This he thought was heat, expressed in the introduction of large amounts of molten rock into the earth's stratified crust. He had earlier claimed that strata at the bottom of the sea was consolidated by heat introduced by layer-parallel veins. By contrast, cross-cutting mineral veins displayed 'Every species of fracture, and every degree of dislocation and contortion...and there is no other general principle to be observed in examining their form.' (Hutton, 1788, p. 269). N o t only their external form, but their internal structure betrayed violence. H u t t o n begins this argument with an analogy: '... for example, a tree or a rock shall be found simply split asunder, although there be no doubt with regard to some power having been applied in order to produce the effect, yet we are left merely to conjecture at the power. But when wedges of wood or iron, or frozen water, should be found lodged in the cleft, we might be enabled, from this appearance, to form a certain judgement with regard to the nature of the power which had been applied. This is the case with mineral veins. We find them containing matter, which indicates a cause; and every formation in this case is interesting to the theory.' (Hutton, 1788, pp. 269-270).

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The sentences just quoted betray the hypothetical nature of Hutton's ideas concentrating on processes. He assumed that the mineral veins and basaltic dykes tore the rocks asunder to make space for themselves and in the process deformed and uplifted them. He considered the volcanoes as showing clearly the nature and the magnitude of the requisite force '... when fire bursts forth from the bottom of the sea, and when the land is heaved up and down, so as to demolish cities in an instant, and split asunder rocks and solid mountains, there is nobody but must see in this power, which may be sufficient to accomplish every view of nature in erecting land, as it is situated in the place most advantageous for that purpose.' (Hutton, 1788, p. 271). He then questioned whether those operations of 'extreme heat' (ibid. p. 272) were exceptional events or whether 'are operations natural to the globe.' The answer, he thought was 'plain: These operations of the globe, remain at present with undiminished activity, or in the fullness of their power.' But if these operations were as natural as H u t t o n assumed, he reasoned further that they must also take place within the crust, that molten rock must solidify in rents and fissures it creates at depths beyond our gaze: 'Here, therefore, is an important point to be determined; for, if it shall appear, that much of this melted matter, analogous to lava, has been forced to flow among the strata which had been formed at the botom of the sea, and now are found forming dry land above its surface, it will be allowed, that we have discovered the secret operations of nature concocting future land, as well as those by which the present habitable earth had been from the bottom of the abyss. Here, therefore, we shall at present rest the argument, with endeavouring to show that such is actually the case.' (Hutton, 1788, pp. 276-277). It was these theoretical views that led H u t t o n to assume that molten rock must have invaded the strata deposited in the sea in wide parts of the world and that igneous rock is a c o m m o n constituent of the earth's crust 15. His observations in Scotland amply furnished him with evidence to support his case. Cross-cutting relationships, now commonplace in first-year geology texts, were the only temporal inferences H u t t o n made on igneous rocks and these remained the only ones until Darwin observed cumulate layering in a solidifed lava lake on James Island (now San Cristobal) in the Galapagos Archipelago in 1835 and inferred that this must be a c o m m o n process also in plutonic rocks (Darwin, 1844, pp. 111-114). The 'stratigraphy' Darwin's observation revealed was nothing more than sedimentation in a liquid environment and therefore obeyed the same three stratigraphic principles set out by Steno for sedimentary rocks: since the process was the same, so were the stratigraphic principles applied to its products.

Metamorphic rocks The last of the three great classes of rocks to be recognised and named was the metamorphic rocks (see also Daubr6e, 1860; Daly, 1917 and Miyashiro, 1973,

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Appendix, for reviews of the history of ideas on metamorphism). We are unaware of any recognition of metamorphosis of rocks in antiquity. When the middle Ages opened, mankind seems to have been aware only of igneous (but most commonly interpreted as a result of combustion of coal and sulphur) and sedimentary processes and their products, igneous and sedimentary rocks. Of the two last questions of Jean Buridan's (1300-1358) Questiones super tres primos libros metheororum et super majorem partem quarti a magistro, question 20 inquires 'Has the dry land been at another time where the sea is today, and, conversely, has the sea been once where the dry land is today and will it return there?' (Duhem, 1958, p. 293; Gohau, 1987, p. 32). The Parisian master answers this in the affirmative by pointing out that erosion causes uplift by making lands 'lighter' and sedimentation depresses ocean floors by making them 'heavier.' He envisages a bimodal earth where one hemisphere is occupied by the ocean and the other by land. He thus thinks that by continuous subsidence, the ocean bottom eventually reaches the centre of the earth and then gradually emerges in the other hemisphere as land (but turned inside out). In this process, sediment is turned into rock at the bottom of the ocean: 'metals, rock crystal, and other stones form by coagulation of a terrestrial or aqueous mixture and that this coagulation would be brought about by a very intense refrigeration or by the failure of heat for a very long time.' (quoted from Duhem, 1958, p. 305). This may be viewed as a process akin to diagenesis. He notes, however, that such processes can only take place near the surface, yet minerals and metals are also found at great dephts, 'although at such depths there are hardly bodies capable of furnishing either the gases or the vapours. Other minerals, it seems, were transported to these places' (quoted from Duhem, 1958, p. 305). This is the first mention we know, in the literature of geology, of a whole region of earth being transported bodily and gradually from one physicochemical environment into another one. Master Buridan did not make any further use of his inference, because he did not think that the interior of the earth was particularly hot (or cold) and thus he saw no reason for the rocks that had formed at near-surface conditions to undergo any further changes. Just a little more than four centuries later, James Hutton used an argument similar to Buridan's (but using heat instead of cold) to show how sediment petrifies into rock and how one type of rock may become metamorphosed into a different type rock (cf. Daubr6e, 1860, pp. 10-13). The first inference of change of one type of rock into another may have been made by Arduino of the Tertiary fame. In his Osservazioni chimiche sopra alcuni fossili ('Chemical observations on some fossils:'Venice, 1779) he hypothesised that the dolomites of Lavin in Vicenza formed from Secondary limestones. He was led to this inference by the brecciated state of the dolomites. He thought that the limestone had been first brecciated and then the modifying igneous agent rose from the deep along fissures. As we shall see below, von Buch followed him in this interpretation nearly half a century later. Hutton is usually cited as the first person who had a sophisticated idea of what we today call metamorphism. His first example of metamorphosis under the effect of heat was the origin of anthracite (what he called caput mortuum and

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'perfect coal,' 1788, p. 241; we now know it to be the coal of the highest metamorphic rank): 'In the process of vegetation, there are produced oily and resinous substances; and from the collection of these substances at the bottom of the ocean, there are formed strata, which have afterwards undergone various degrees of heat, and have been variously changed, in consequence of the effects of that heat, according to the distillation of the more volatile parts of those bodies has been suffered to proceed .... and it is only in proportion as certain chemical separations take place that these inflammable bodies are changed in their substance by the application of heat.' (Hutton, 1788, p. 239). Through these chemical reactions, Hutton assumed, under the effects of heat, that the oily substances became bituminous and the bituminous substances became coaly. He observed also that 'The strata of fossil coal is found in almost every intermediate state as well as in those of bitumen and charcoal.' (Hutton, 1788, p. 241). Hutton then mentions the change of chalk into a 'solid marble' by the effect of heat (Hutton, 1788, p. 254). This, he thought, happened by 'fusion' and all strata that are consolidated he believed were consolidated by this means. It is clear from Hutton's writings that this 'fusion' did not happen everywhere with the same intensity. In some places it led to simple diagenesis, in others to metamorphism and in still others to complete melting, to use anachronistic labels to describe his inferences. He inferred that this heating was by means of raising the isotherms at sea bottom: 'there has been exerted an extreme degree of heat below the strata formed at the bottom of the sea' (Hutton, 1788, p. 263). In 1798, Charles D. William ('Guglielmo') Thomson (see Gunther, 1939; Waterston, 1965) noticed that altered and mineralised pieces of limestones were embedded in the lavas of the Somma in the Vesuvius and conjectured that they were probably Apennine limestone so altered by the effects of heat supplied by the Vesuvian volcanism. Thomson further asked whether the famous Carrara marble favoured by Michelangelo may not have been similarly generated (Daubr6e, 1860, p. 14). Hutton's younger friend Sir James Hall was also impressed by the presence of crystalline limestone xenoliths in basalts. In his famous experiments, he noticed that above a certain pressure the carbonate of lime retained its carbonic acid and, upon heating and pressuring, it agglutinated in the form of a solid, commonly crystalline substance (Hall, 1806, 1812). On the continent, A.-J.-M. Brochant de Villiers discovered a passage of sedimentary rocks into crystalline rocks in the Tarentaise Alps in France (Brochant de Villiers, 1808). Using arguments similar to Hutton's, he put forward the hypothesis that the crystalline, micaceous, talciferous limestones, micaceous, talcifreous and amphibolitic schists in this region must have been originally sedimentary and probably a part of the so-called Transition Rocks (and not of the Primitive rocks as until then supposed) owing to their content of plant fossils and similarity with their alleged correlatives in Germany (Daubr6e, 1860, p. 19).

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In his Description of the Western Islands of Scotland (1819), Macculloch interpreted amphibolitic schists as argillites modified by the effect of heat. Daubr6e (1860, p. 16) points out that it was only after Napol6on's final defeat at Waterloo (18th June 1815) that Hutton's ideas could spread rapidly on the continent. Contrary to the claims of a number of revisionist historians, Hutton's views were fairly well-known on the continent even before that: Von Buch, for example, was initially on the side of the neptunists (von Buch, 180011867]). After some years, he reluctantly confessed (von Buch, 180911867], p. 518): So we stand dismayed and crushed over the results, to which the appearance of Montdor has led us. If the porphyry of the Puy de D6me, of Sarcou, of Puy de Nug6re originated from granite, so could also the beds of the Montdor owe their origin to the metamorphosis of the granite (not melting) and the basalt could be a fluid product of these rocks. But even the most enthusiastic vulcanists would not dare to view this result as a general one and to apply it to the German basalts. Finally with complete surrender (von Buch, 1818-1819[1877], p. 17, he 'gladly agree[d] with Dr. Hutton'. It is curious that the 1809 passage has escaped Daubr6e's (1860) attention while writing his history of ideas on metamorphism, because it is here, for the first time, that there is talk of metamorphosis (in the G e r m a n original: Veriinderung) of rocks with emphasis that it is not meant to mean melting! In a published letter to the Imperial and Royal Domain-Inspector Alois von Pfaundler, von Buch corrects an interpretation of Count Joseph Marzari-Pencati concerning the contact relations of granite and limestone west of Predazzo (Fig. 4),

Fig. 4. Sketch map by Leopold von Buch (in von Buch, 182311877]. p. 90) showing the granite/limestone contact west of Predazzo, where Count Joseph Marzari-Pencati had thought that the granite had metamorphosed the limestone much in the manner encounterd in Sir James Hall's experiments, an idea with which von Buch enthusiastically agreed.

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where the Count had claimed to have found granite overlying the limestone. Von Buch points out that this had resulted from confusing apparent dip with true dip, but enthusiastically supports the Count's opinion that the granite here had metamorphosed the limestone much in the manner encountered in Sir James Hall's experiments (von Buch, 182111877]). This is a topic to which von Buch devoted much attention in the years that followed in connexion with the origin of Dolomite as a rock type and the Alps as an uplifted mountains range. Daubr6e (1860, pp. 17-19) has rightly pointed out that these studies of yon Buch has generated much enthusiasm for studies on rock metamorphism. In a series of three papers, von Buch (1824a[1877], 1825a[1877], 1825b[1877]) developed the hypothesis that dolomite was metamorphosed limestone, in which the magnesium content was greatly increased by augite porhyry intrusions into the limestone, which uplifted and heavily fractured it. He held the heat responsible for the metamorphism, but emphasised the chemical transformation, the homogeneity of which he found puzzling. In another paper, he pointed out that the same augite porphyry was also responsible for uplifting the Alps, an idea, which he had hinted at already in the 1821 paper cited above (von Buch, 1824b[1877]). Von Buch meticulously described how he thought dolomites passed laterally into limestones and how as soon as one left the area where augite porhyry was seen in contact with the dolomites, dolomites disappeared and were replaced by limestones. In the dolomites, most of the fractures contain vugs with drusy structure. 'How much would one not wish to see in all these the effect of high temperature, with which the augite porphyry breaks through the lower beds, pushes up the dolomite high into columns, towers and pyramids, metamorphoses the dense rocks such as basalt into more coarsely crystalline ones and thus annihilates any sign of bedding, destroys fossils, causes shrinkage, joints and burstings, in which the drusy structure is seen. How clearly does it not appear that it is always the dense limestone that lies under the dolomite but above the red sandstone and that gets leached, metamorphosed and handled in this way!' (von Buch, 1824a[1877], pp. 87-88). Von Buch then describes how he found limestone debris everywhere consisting of pieces not yet fully converted to dolomite. He gives the impression of dolomitisation disappearing as one wanders away from the thermal and chemical influence of the augite porphyry. From von Buch's writings we get a clear impression of a front of metamorphism moving across the rock, metamorphosing it as it goes. This is an impression we have already obtained from Hutton's writings and von Buch gives a vivid description of how he thinks metamorphism spreads ahead of the invading hot mass of the augite porphyry. The first occurrence in the geological literature of the verb 'being metamorphosed' seems to be by Bou6 (1823, p. 200), when he described the alteration of bituminous slates into burnt clay as a result of spontaneous ignition of coal beds near Dyssart in county Fife in Scotland (cf. Bonin et al., 1997, p. 28). Brochant's student and later associate l~lie de Beaumont returned to his master's old mapping area in the late twenties to take a closer look at the crystalline rocks of

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the area of the Belledonne Massif and its sedimentary frame. He mapped the region and investigated in detail the partly metasedimentary terrain southeast of the Belledonne Massif now known to be occupied by a series of slices belonging to the Ultradauphinois tectonic unit (e.g. Gwinner, 1978, pp. 181,297, 351,352; Debelmas, 1982, Fig. 7). l~lie de Beaumont first studied those around Petit C~eur (l~lie de Beaumont, 1828a) and then those exposed at the Chardonet Pass farther to the southwest (l~lie de Beaumont, 1828b). He confirmed Brochant's observations and disagreed with Bakewell (1823a, esp. ch. VIII), ascribing the intercalation of dark schists, crystalline limestones and quartzites to the Lias on the strength of the belemnites found in the limestones and argued that the plant fossils of 'coal age' (i.e. Carboniferous as correctly identified by Bakewell, 1823a, ch. VIII, 1823b, pp. 410411) also belonged to the same formation (this was the beginning of the controversy known as the 'affair of Petit-Cceur,' between those who denied the plant fossils their age-diagnostic characteristic, i.e. l~lie de Beaumont and his party, and those who denied that there was at Petit-Coeur an undisturbed sequence, i.e. Bakewell and his supporters. For a history of this affair, see Favre, 1867, pp. 358-382; Ellenberger, 1958, pp. 19-20). In this region l~lie de Beaumont noticed an increase in the 'modification' of rocks eastward expressed in the progressive coalification of the plant material, passing of black slates into green and wine-coloured schists, and the limestones becoming [sic] gypsum. On the first geological map of France (1/500,000" Dufr6noy and l~lie de Beaumont, 1840, sheets Lyon and Marseill6e), this transition is marked by a sharp line passing through the Chardonet Pass (4~ and 44~ on the Marseill6e sheet) and following the arc of the Western Alps. It separates the 'unmodified' rocks (to the west) from the 'modified' rocks. The transition l~lie de Beaumont likened to a piece of wood half coalified: in one half the plant tissue would be recognisable, in the other all plant tissue would have disappeared and coalification would be complete and even graphite could be generated (l~lie de Beaumont, 1828b, p. 362). The implication of a front is clear, as it is in von Buch's description. But l~lie de Beaumont also thought that the sections he studied in the Tarentaise indicated that they were deposited 'at the depths of a very deep sea, when the most intensively studied parts of the Jurassic deposits were laid down along the shores, crowned at intervals by great coral reefs. The central part of the Alps seem to offer to our regards pelagic deposition; [whereas] hills in the vicinity of Bath and Oxford present to us littoral deposition.' (Elie de Beaumont, 1828b, p. 377, italics l~lie de Beaumont's). The 'modification' (i.e., metamorphism), then, affected these very deep sea deposits. How the intrusions came to affect these, we can only understand if we understand the distinction l~lie de Beaumont made between normal metamorphism and accidental metamorphism16. He thought that at a time when there was already life on earth, sufficient temperatures may have obtained at depths of 1000 m or so from the surface of the earth to keep most rocks in a liquid state. In other words, the thickness of the earth's crust at those times may have been only about one kin. Now, l~lie de Beaumont

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further believed that the accumulation of coal beds, corals and mussel-banks showed that most Palaeozoic seas had had a small depth. Yet the entire thickness of the Palaeozoic strata reach several thousand meters. The weight of even a small basin would thus have been enough to 'fold in' its basement. Every newly laid down bed would push the basin bottom closer to the red-hot interior, l~lie de Beaumont thought that this would heat up the lower parts of the basin sufficiently to change the texture, even the structure, of the original sediments. The thicker the sediment package in a basin, the greater would be the effects of metamorphism (Vogt, 1846, pp. 247-248). But such conditions would only obtain, if there is an extraordinarily high geotherm; l~lie de Beaumont believed that this must have been the case in the Palaeozoic. In the later eras, however, the earth would have cooled down sufficiently to allow the presence of a much thicker crust (its present thickness l~lie de Beaumont later estimated to be less than 50 km thick: l~lie de Beaumont, 1852, p. 1237) and the 'normal metamorphism' would not take place as easily and, as generally, as it did during the Palaeozoic. This was one reason, he further thought, why metamorphism was more widespread in Cambrian rocks than in those of later times (Vogt, 1846, p. 165). In later times, metamorphism became confined to the vicinity of large intrusions. Until 1875, the external massifs of the Alps were believed to be intrusive bodies and l~lie de Beaumont thought their intrusion had been responsible for the metamorphism of the Jurassic rocks not only in the Tarentaise, but in all regions of the Alps, where the sedimentary rocks came close to these massifs (Vogt, 1846, p. 293). Metamorphism as a consequence of heating owing to the proximity of an intrusive body, l~lie de Beaumont called 'accidental metamorphism' and thought that accidental metamorphism became relatively more widespread with respect to normal metamorphism in the younger periods of earth history. The idea that the grade of metamorphism is an indication of the antiquity of a rock body, which remained prevalent well into the second half of the twentieth century, is a left-over of this anti-uniformitarian interpretation of l~lie de Beaumont. In 1860, his student Auguste Daubr6e was to call his accidental metamorphism, 'metamorphism of juxtaposition' (corresponding to the currently used term 'contact metamorphism': Daubr6e, 1860, pp. 54-59) and his 'normal metamorphism' 'regional metamorphism' (ibid., pp. 59-65), a term still in use. At about the same time as l~lie de Beaumont was formulating these ideas, Lyell reached similar conclusions. In the third volume of his Principles of Geology, he devoted chapters XXV and XXVI to a discussion of granite and gneiss. After having shown that granite is an igneous rock, he concludes, in the XXVIth chapter that 'when voluminous masses of melted rock have been for ages in an incandescent state, in contact with sedimentary deposits, they must produce some alteration in their texture, and this alteration may admit of every intermediate gradation between that resulting from perfect fusion, and the slightest modification which heat can produce.' (Lyell, 1833, p. 367). He says that 'The geologist is conducted, step by step, to this theory by direct experiments on the fusion of rocks in the laboratory, and by observation of the changes in the composition

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and texture of stratified masses, as they approach or come in contact with igneous veins and dikes. In studying the latter class of phenomena, we have the advantage of examining the condition of the rock at some distance from the dike where it has escaped the influence of heat, and its state where it has been near to, or in contact with, the fused mass. The changes thus exhibited may be regarded as the results of a series of experiments, made on a great scale by nature under every variety of condition, both as relates to the mineral ingredients of the rocks, the intensity of heat or pressure, the celerity or slowness of the cooling process, and other circumstances.' (ibid., pp. 367-368).

Lyell was familiar with l~lie de Beaumont's ideas on metamorphism in the Alps and cites his 1829 paper on the mountains of Oisans in the Belledonne Massif in support of his own views (Lyell, 1833, p. 372). He also remarks that 'we must suppose the heat which alters the strata to proceed, in almost all cases, from below upwards, and to act with greatest intensity on the inferior strata. If, therefore, several sets of argillaceous strata or shales be superimposed upon each other in a vertical series of beds in the same district, the lowest of these will be converted into hornblendeschist, while the uppermost may continue in the condition of clay-slate.' (Lyell, 1833, pp. 373-374). There is certainly a great resemblance between the ideas of Elie de Beaumont and Lyell concerning metamorphism, with the exception that Lyell perceived no change in time with respect to either the extent or the intensity of metamorphic phenomena on earth. It was this insistence on the uniformity of metamorphic phenomena through time that led him to invent a new term for these rocks. He first reminded his readers that what was in those days commonly called 'primary' rocks such as granite and gneiss were nothing more than rocks made by heat in the bowels of the earth. Thus, Lyell (1833, p. 374) called them 'hypogene', from the Greek brc3- sub, below and y ; v o g ~ t - b i r t h , origin. The granite, as H u t t o n had earlier shown, was a magmatic rock. Lyell now proposed to call all the 'stratified primary rocks' metamorphic (from ~te~b,- trans and gopqb~- form), i.e. transformed rocks (Lyell, 1833, pp. 374-375). The important thing is that both l~lie de Beaumont and Lyell perceived metamorphism, as did H u t t o n and Leopold von Buch earlier, as the invasion of a pre-existing rock body by a set of temperature and/or pressure conditions sufficiently different from those that obtained during the formation of that body as to alter its mineralogical composition and texture. We can think of this as the intersection of two or three sets: one representing the rock body, i.e. a set of points in rock-space, another a penetrable space, of which every point corresponds with a temperature value (temperature-space) and yet another set, every point of which corresponds with a pressure value (pressure-space). Intersections of these sets will be the rock volumes that will undergo metamorphism as shown in Fig. 517. The problem with metamorphism is to know which conditions ('which sets') a metamorphic rock represents: maximum conditions of metamorphism or the last set of conditions it underwent? If an extremist view is adopted of the last possibilty, no metamorphic rocks should be encountered on the earth's surface at all, which is

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Fig. 5. Illustration of the concept of metamorphic front with Venn diagrams. R is a set of rock-space points, T is a set of temperature-space points and P is a set of pressure-space points. Metamorphism occurs in the rock-space where more than one of these sets intersect.

clearly not the case. The kinetics of metamorphic reactions, however, is such that they usually represent the highest temperature conditions encountered, as this drives out most of the volatiles which are the most important metamorphic catalysts as Daubr6e (1860) showed already in the middle of the last century (cf. Yardley, 1989, pp. 177-186). Unlike sedimentation and igneous activity that create new masses of rock, metamorphism thus converts a pre-existing rock mass into another type of rock by the passage through the rock of temperature and pressure conditions, plus volatiles, behind 'fronts' of metamorphism. Provided we can ascertain the metamorphic path, isograds can be taken to approximate, within a few million years at the very most (cf. Yardley 1989, pp. 177-186), isochronous surfaces, as Lyell (1838, pp. 521-522) and Read (1949) pointed out (also see Lyell's excellent discussion on determining the age of metamorphic rocks as opposed to the age of rocks later affected by metmorphism in Lyell, 1838, ch. XXV). Younging directions of metamorphism within a metamorphic terrain, then, would be perpendicular to the metamorphic front at any point on the front as clearly implied by Lyell (1838, pp. 521-522) and illustrated in Fig. 6 and 7. The same applies to the high-pressure metamorphic rocks. In other words, the

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A. M. C. ,,~eng6"r and M. Sakm, c

Fig. 6. A. Younging direction in metamorphic rocks and movement direction of metamorphic fronts. Temperature space T invades rock space R along the x direction. Metamorphic front is parallel with the edge of the temperature space (which is an isochronous surface) within the permission of the kinetics of metamorphic reactions. Dashed arrows show direction of the movement of the metamorphic front. Note that the younging direction of metamorphic rocks (defined to be perpendicular to the metamorphic front at any point on the front) are not parallel with the advance of the metamorphic front. B. Metamorphic front expands from the saggital point c with coordinates (x,y) into the rock space R. Metamorphic front is parallel with the edge of the temperature space (which is an isochronous surface) within the permission of the kinetics of metamorphic reactions. Dashed arrows show direction of movement of the metamorphic front, which in this case is parallel with the younging direction in the metamorphic rocks.

y o u n g i n g d i r e c t i o n p o i n t s f r o m the inside o f the f r o n t to the o u t s i d e as s h o w n in Fig. 7. N o t e t h a t m o v e m e n t d i r e c t i o n o f the m e t a m o r p h i c f r o n t a n d the d i r e c t i o n o f y o u n g i n g in m e t a m o r p h i c r o c k s n e e d not be p a r a l l e l (e.g. as in Figs. 6A a n d 7B). A p a r t f r o m the t h r e e m a j o r r o c k g r o u p s we h a v e discussed in this section, t h e r e is yet a n o t h e r t y p e o f rock, f o r m e d by s t r u c t u r a l p r o c e s s e s o n l y . T h e s e are the r o c k s we t e r m in this p a p e r ' s t r u c t u r a l r o c k s ' , w h i c h we discuss next.

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Fig. 7. Typical younging patterns of prograde metamorphism in various types of metamorphic environments: A - Contact metamorphism around a batholith. B - Regional barrovian metamorphism consequent upon crustal thickening and lithospheric detachment. B 1 - Pre-crustal thickening state with normal lithospheric thickness. T~-T3 are normal geothermal gradients. P is lithostatic pressure. B2 - After crustal thickening, but before geothermal gradient has attained equilibrium. Arrows show instantaneous younging direction in metamorphic rocks. In this case motion of the metamorphic front (which is vertically upward) and the younging direction are not parallel. C - High P/low T metamorphism. P1-P4 isobaric lines. Heavy arrow is subduction direction, lighter arrow younging direction of metamorphism (and the resulting metamorphic rocks at this scale).

STRUCTURAL

ROCKS

I f a rock has been f o r m e d by structural processes only, or if such processes were the determining factors in its origin, it is a structural rock. A folded sandstone, for example is not a structural rock, because, a l t h o u g h structurally affected, it remains a sandstone. In other words, the structural process in question, namely folding, has not generated a new rock, but altered the configuration of a pre-existing one that remains readily recognisable for what it had been before the influence of the mechanical process that affected it. But if the same sandstone is milled along a fault zone to produce a cataclasite, it has become what is k n o w n in the literature a 'fault rock' and thus falls under our designation 'structural rock.' A l t h o u g h its m a i n c o m p o n e n t s are derived from the pre-existing sandstone, the new rock no longer has the texture of the old one, nor do its original grains keep their relative positions with respect to one a n o t h e r as they did in the original sandstone. Thus the structural rock is readily distinguishable from the p a r e n t sandstone in the field as a distinct rock. M a n y fault rocks have added veining owing to circulating fluids along the fault and c o n t i n u o u s faulting c o m m o n l y deforms, brecciates, and mills down the solidifed fills of such veins. Thus the fault rock m a y even acquire a composition different from that of the p a r e n t rock. All cataclasites, mylonites, chaos structure

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and m61anges are structural rocks. They define independent bodies within the earth's lithosphere and should be mapped separately. As we have seen above, sedimentary, igneous and metamorphic rocks are identified by the processes that created them and are mapped separately thus giving us information about the spatial/temporal characteristics of the sedimentation, igneous and metamorphic processes in the area being mapped. Structural rocks also give us information about the structural processes that formed them. As we did with the other three classes of rock types, we first present below a critical history of the development of our understanding of structural rocks as a background for our discussion of their stratigraphic significance. Brief h&torical review History of ideas on the development of structural rocks to the 20th century:

Structural rocks must have been seen and mapped by miners and geologists from the beginning of mining activity. Agricola, for instance, describes 'dykes' (= vena profunda) that clearly offset other dykes (Fig. 8A) and recognises those dykes that are 'massive,' those that have 'drusy structure' and those that are 'dumb' (Fig. 8B). The massive dykes 'have no water and little air', the drusy ones, 'rarely water, but commonly air' and the dumb ones 'commonly water' (Agricola, 1556[1953], p. 56). In one case, he shows extensive fracturing around a dyke, which gives the impression of being emplaced along a shear zone (Fig. 8C). Given the geology of Saxony and the northern Czhech Republic (e.g. Dorn and Lotze, 1971, fig. 49; Prescher, 1987, pp. 27, 56, 60), especially in the vicinity of Chemnitz and J~chymov (formerly Joachimsthal,

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Fig. 8. A. Two 'dykes' designated A, apparently offsetting 'dykes' BC and BD. B. Various types of 'dyke' (or 'vein') filling: A and B have massive fills, C and D fills with drusy structure and E and F have 'dumb' fills. C. 'Dyke' with extensive jointing around it. A are 'fissures that come from the east' and B are those that 'come from the west.' 'Dykes' (or 'veins') in Figs. 8A and 8C may represent fault rock fills. The same may be true for 'dykes' C and D in Fig. 8B. All are from Agricola's De Re Metallica, Book III (Agricola, 155611953]).

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Fig. 9. John Whitehurst's (1778, plate II) section at Matlock High-Tor in Derbyshire, across the valley of the River Derwent clearly showing a body of fault rock along a normal fault. See text for discussion.

where Agricola was the physician of the mining community), there is little question that some of the 'dykes' he describes were fault rock-filled faults. The earliest clear recognition of a fault rock that we are aware of is that by Whitehurst (1778), in which the author describes and illustrates (Fig. 9 herein) a section at Matlock High-Tor in Derbyshire. In this section, the numbers 1, 2, 3, 4, 5, etc. show the corresponding strata, '...whence it appears that they have been burst, dislocated, and thrown into confusion, by some violent convulsion of Nature.' (Whitehurst, 1778, p. 154; italics Whitehurst's). Whitehurst continues his description thus: 'The strata which compose the top of Masson Mountain, are elevated about one hundred fathoms above the summit of Matlock High-Tor, N~ and the same beds are depressed about fifty fathoms below the foot of it, at the river, as shewn in the plate [herein Fig. 9]. A, represents a great fissure or chasm, filled up with the fragments of the upper and adjacent strata'

(ibid., italics Whitehurst's) Whitehurst believes that his great fissure was responsible for excavating the valley of the Derwent River and that the strata 1 and 2, missing to the right of the picture were believed to have been 'swallowed up into that enormous cleft; and if in this instance, the same thing may have happened in many others; if not universally, in all mountainous countries, wherein the upper strata have disappeared.' (Whitehurst, 1778, p. 155).

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A b r a h a m Gottlob Werner was much influenced by these ideas of Whitehurst and his New Theory of the Origin of Dykes and Veins is ascribes explicitly the origin of many dykes and veins to faulting: 'When a vein [or dyke] traverses the strata of a mountain, it happens almost always that the corresponding strata on its hangingwall side are found much lower down than on its footwall side; this difference in level in the parts of the same stratum or bed, bears a proportion to the width of the vein [or dyke]. (Werner, 1791, pp. 84-85; 1809, p. 81). Most of the faulting Werner describes is normal faulting as the description above shows. He thought that these normal faulting events were similar to landslides, taking place especially in wet years (Werner, 1791, p. 64; 1809, pp. 61-62), and happening because the mountains had dried up, shrunk and also become unable to carry their own weight after the last depression of world-wide sea-level according to his ~eognostic theory (Werner, 1791, p. 51; 1809, pp. 49-50). But he also thought th" ~arthquakes generated veins or dykes and gave those that formed during the 17~3 earthquake in Calabria as examples (Werner, 1791, p. 64; 1809, p. 62). Werner observed that some of the veins and dykes contained conglomerates and breccias. The rounded clasts Werner used as evidence of water transport and employed them as support for his theory that veins were filled from above (e.g. Werner, 1791, pp. 68-69; 1809, 65-66). But some vein filling showed evidence that they were derived from the walls of the vein: 'When they are of considerable size, their structure is parallel with that of the walls of the country rock; which incontrovertibly shows them to have been only slid down, without having been rolled. But when these fragments are of a smaller size, they assume all kinds of directions, which proves that they had been tumbled confusedly into an empty space. It may be remarked, that it is only in rocks possessing a slaty or foliated structure, as gneiss, micaschist and clay slate, that we find this peculiarity in arrangement and position of the fragments and debris of rocks found in veins.' (Werner, 1791, p. 70; in this case the English translation in 1809 is not sufficiently faithful to Werner's meaning for us to be able to refer to it). Some of Werner's descriptions of rounded clasts filling 'veins' probably correspond with true sedimentary breccias, such as the Ger611gneis ('pebble-gneiss') at Markersbach in the county of Schwarzenberg (Prescher, 1987, p. 52) amidst micaschists (Dorn and Lotze, 1971, fig. 49: in the micaschists beween Aue and Annaberg). Werner also noted that the vein filling itself is in places fragmented forming a true breccia (=Triimerstein [sic], Werner, 1791, p. 71-72; 1809, p. 68; also 1791, p. 127; 1809, p. 121). In others, the vein and the country rock appeared to have been 'melted together' (Werner, 1791, p. 94; 1809, p. 90) 19. 'In places where this peculiarity occurs, the rock has had a strong attraction for the substance of the vein introduced into the rent, and has become so intimately mixed with it, that they now appear to be one and the same substance, at least it is not easy to mark a line of separation between the rock and the vein.' (Werner, 1791, p. 94; 1809, p. 90).

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Such 'veins' or 'dykes' passed into the country rock by gradational contacts, Werner noted, most frequently occurred in quartz- and hornstein-rich veins and in quartzrich gneisses. He also noted that such veins and dykes were rare. Werner further remarked that some of the vein or dyke walls were decomposed, especially those that occurred in granites, gneisses, micaschists, slates and porphyries. In such instances it is only a part of the vein or dyke wall that had been altered. Werner emphasised that it was never the quartz, but commonly feldspars (into white kaolinite: Werner, 1791, p. 130; 1809, p. 124), more frequently the hornblendes and micas that had become altered. He observed that this change extended in places well into the country rock, 'even to a fathom' (Werner, 1791, p. 128; 1809, p. 122). In other places, another sort of alteration was seen and attacked principally feldspars, micas and hornblendes turning them into a greenish clay and mottled stone. This, Werner remarked, used to be called a gneiss in the Freiberg mining region (Werner, 1791, p. 131, 1809, p. 125). To geologists familiar with the geology of the Bohemian Massif in the region of the Erzgebirge, the descriptions by Werner quoted in the previous paragraph clearly answer to the mylonitised shear zones such as the ones known from the granulites and gabbros in which potassium feldspars and biotites retrograded from garnets are seen (Fig. 10; county D6beln, west of Rol3wein: Prescher, 1987, p. 27) and gabbros show beautifully developed shear zones (Flasergabbro: Prescher, 1987, p. 56); or to the highly sheared and recrystallised granites near Rochlitz in the northwestern part of the Granulitgebirge (Prescher, 1987, p. 60).

Fig. 10. Deformed granulites cut by mylonitised shear zones at Troischaufelsen in the Mulden Valley, to the west of Rol3wein in the county D6beln (from Prescher, 1987, p. 27). Werner very likely misinterpreted such mylonitic shear zones as dykes or veins.

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The reason why we have given such detailed quotations and descriptions from Werner was to emphasise that unless the process that created the rocks he observed was recognised, no amount of careful description could make him understand their message. He never realised that many of his veins or dykes were no veins at all, but just cataclasised or mylonitised shear zones 2~ see Fig. 11. He even believed that within his veins depostion was of a more tranquil nature than at the free surface of the earth (Werner, 1791, p. 117; 1809, p. 112). But soon at least some structural rocks were recognised for what they are in the geological circles. Lyell, in the glossary he appended to the third volume of his Principles of Geology provides a description and illustration of a fault (Fig. 12), emphasising the fact that the fault is commonly 'accompanied by a crack or fissure varying in width from a mere line to several feet, which is generally filled with broken stone, clay, &c . . . . ' (Lyell, 1833, p. 68 of the Appendices). Only a year later, the great Swiss geologist Arnold Escher von der Linth depicted structural rocks in a water colour painting of the Sardona G r o u p southeast of the village of Matt in the Sernf Valley, at the frontier between the cantons of Glarus and Graubfinden in Switzerland. In this water colour (Fig. 13) he clearly showed the Eocene Flysch below and the Permian Verrucano (Sernifit) above, with the mylonitic Jurassic Lochseitenkalk in between. The abnormality of this contact had been observed and correctly interpreted by his father Hans Conrad Escher von der Linth (1809; see

Fig. 11. Metasomatised mylonite on the Aurela Ridge group in the sheared and uralitized granulites of the San Gabriel Mountains in California, looking like a 'dyke' ending abruptly against the granulites. Note other 'pod-like' bodies of metasomatised mylonites surrounded by sheared granulites (from Hsti, 1955, plate 33).

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Fig. 12. Section across a fault given by Lyell in the Glossary he appended to the third volume of his Principles of Geology (1833, p. 68 of the Appendices). It is essentally a vertical fault filled with fault rock.

Staub, 1954, pp. 131-132), but Arnold Escher did not yet, in 1834, publish anything on this issue. The next year, Rudolf von Carnall described in some detail a body of fault rock formed by shear along a fault plane, calling it a product of 'shearing off' (=Abreibung: von Carnall, 1835, p. 145). He had puzzled over the origin of what he calls 'fissure-mass' (= Kluftmasse" ibid.). He first pointed out that fissures along faults could not have opened by one asperity on one wall climbing on another at the opposite wall thereby forcing the walls open, because, he argued, the anthracite forming the fault walls was not nearly strong enough to be able to support the asperities in the way suggested. He thought it was the shearing off of the wall material that was filling the fissures along the faults: 'Thus does it [i.e. what he calls 'fissure filling'] differ from its surroundings: it is softer, from slates plastic clay had formed, from sandstone strips hardly coherent sands, in which larger clasts are randomly embedded. Thus originated also the coal rubble [= Trfimmer], milled down to the finest size, and not infrequently kneaded together like plastic clay. Thus originated also those films, as did the planes of parting of the rest of the fissure mass running parallel with both walls of the fissure, and that is why also we see cleats in the surrounding cross-rocks corresponding with the position of the fissure .... To me, there is no other way to explain these occurrences.' (von Carnall, 1835, p. 146). In 1846, Arnold Escher von der Linth's geological description of the Canton of Glarus was published in a geographical series published by Oswald Heer and J. J. Blumer-Heer. It is in this publication that Escher for the first time speaks of the Lochseitenkalk as an 'altered limestone' (= ver(inderter Kalkstein: Escher, 1846, p. 68). He describes it as follows (ibid., p. 69): '... that it consists, at Ringgenkopf, partly of dark blue, dense, brittle limestone containing belemnites and one ammonite similar to Ammonites polyplocus REIN. But in other parts it is made up of pure crystalline marble, whose white matrix is cut by numerous greyish patches and veins, which mostly very gradually pass into the

c~

c~

~.,~~

Fig. 13. Arnold Escher von der Linth's water colour of the Sardona Group as seen in 1834 from the eastern peak of the Camperdunes Ridge, southeast of the village of Matt in the canton of Glarus, Switzerland. Note the thin band of the Jurassic Lochseitenkalk (limestone mylonite) separating the Permian Verrucano above from the Eocene flysch below. > From Staub (1954, plate X).

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martix and melt into it . . . . In other parts of its mentioned areas of distribution, it exhibits almost everywhere this crystalline aspect and in such places contains no distinct fossils.' The 300 m thickness near Vorab, Escher thought, was a result of tectonic thickening and in many places observed that the thickness reduced down to almost nothing. As we know from his own publications (Escher von der Linth, 1841, 1846), and from the publications of geologists who knew both Escher and his outcrops in Glarus (e.g. Murchison, 1849, pp. 246-253, esp. p. 248), he considered the anomalous position of the Permian Verrucano and the Jurassic Lochseitenkalk atop the Eocene flysch t o h a v e resulted from a 'colossal overthrusting or a bending of the beds,' despite the difficulties that he thought such an interpretation involved (Escher von der Linth, 1841, p. 61). In a letter he wrote to Murchison on 5th December, 1848, Escher referred to the Lochseitenkalk as having undergone 'more or less metamorphic modifications' (cf. Trtimpy, 1991, p. 389, italics mine). What these metamorphic modifications were, he elucidated by likening the Lochseitenkalk to the St. Leonhard Limestone in the canton of Valais, which was used for building high ovens and that were thus exposed to the high temperatures for a long time (Escher von der linth, 1846, p. 69). A year after Escher's death in 1872, Armin Baltzer, another perceptive Alpine geologist, published his study of the Gl~rnisch, the isolated mountain massif southwest of Glarus, on the left bank of the Linth (Baltzer, 1873). In this book, he gave an account of the Lochseitenkalk, the process of the modification of the texture of which he called 'mechanical metamorphism.' Baltzer certainly knew Escher well and admired him (Heim and Hugi, 1914). It is almost certain that Escher spoke to him of the 'metamorphism' of the Lochseitenkalk, as he had done to Murchison, and its possible temperature dependence. This, we think is reflected in Baltzer's interpretation of the modifications that the Lochseitenkalk had undergone. In the following we quote Baltzer in full, first his description and then his interpretation of the Lochseitenkalk" 'At Gl~irnisch, I observed it [i.e. the Lochseitenkalk] in a strongly modified form. Here, the hinge of the northern fold [of the Glarus double fold." cf. Trfimpy, 1991] is very close and therefore the mechanical contact effect was very strong. The few metre-thick limestone is dense, light-coloured and striped in a flamy way. Clearly recognisable fossils do not exist. Above Luchsingen, the mechanical compression was so violent, that on the spot it splits into a rubble of rhomboedric platelets. Away from the Eocene it is more compact, but cut by many small fissures .... Apparently it was the high temperature generated by friction that altered the dark high mountain limestone, rich in organic humus-rich materials,.., into the lighter Lochseitenkalk. I regard the contorted layers of dense, hard substance with the wavey beds...as the mechanically modified crystalline or oolitic high mountain limestone. The view of the Lochseiten pieces unwittingly brought before my eyes the analogy with the grey, granular cast iron and the wiry wrought iron. The latter corresponds with the denser, lighter, wiry bands. According to Heim, there are here and there slickensides at the contacts between the Eocene and the limestone.' (Baltzer, 1873, p. 58).

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A l b e r t H e i m , in his great b o o k t h a t laid the f o u n d a t i o n s of m o d e r n s t r u c t u r a l geology, Untersuchungen fiber den Mechanismus der Gebirgsbildung im Anschluss an die Geologische Monographie der T6di-Windgdllen-Gruppe ( = I n v e s t i g a t i o n s on the M e c h a n i s m of M o u n t a i n - B u i l d i n g a p p e n d e d to the G e o l o g i c a l M o n o g r a p h on the T 6 d i - W i n d g / i l l e n - G r o u p : cf. Milnes, 1979), describes the L o c h s e i t e n k a l k in the following terms: 'Lochseitenkalk is often seen from a great distance as a conspicuous straight line extending along the mountain slopes. [see Fig. 13 herein] In the north it forms nearly a flat plate that dips very gently north; in the south one that dips south. In longitudinal ridges this plate appears as a horizontal line, which surrounds the mountain peaks at a certain elevation and delimits the grey-green and violette Verrucano downwards. On some peaks, the Verrucano has been eroded away, while the Lochseitenkalk remained . . . . In general, the Lochseitenkalk is not thick, but exhibits very strong thickness changes. In places it measures 1 to 3 m, at the Lochseite only 1/2 to 2/3 m. Above the Bischofalp at the southern slope of the K/irpfstock it is also only 2/3 m, at the Niederenalp upper echelon 7 m. At very few points, as above the Alp Walenbfitz, at a few places of the Weisstannenthal, it is completely squeezed out. More usually it is 10 to 20m thick . . . . at the Mannen 100-150m thick. Eastward and westward it decreases down to 10-20m, but towards the Vorab swells to 600m . . . . In general, the Lochseitenkalk is thicker in the southern fold, where we are closer to its hinge, than in the northern fold. The Lochseitenkalk was described by Escher as "modified limestone." Baltzer used the expression "mechanical metamorphism" for it .... which, as far as I know, makes its first appearance in the literature there, although similar products themselves had been often described before. The Lochseitenkalk is almost always light grey. Very commonly it is penetrated by a flaser texture of slickensides, so that, when hit, it disintegrates into small more or less lense-shaped or nodular pieces. The slickensides are in places lined by thin clayeytalcose skins. In places, the lower contact of the Lochseitenkalk is thoroughly worked by slickensides, whereas a t - - n o t uncommonly very nearby--places nothing of the sort is observed. In most places it has become crystalline, fine-grained, marbly and cut by numerous whitish, more rarely yellowish and reddish patches, veins and streaks . . . . In this state it was compared by Escher with the limestone of St. Leonhard in Valais, after the latter had long been employed as high oven stone, but Escher did not further express himself on the origin of this structure. Very commonly lighter, harder, contorted layers and patches alternate with more grey, softer, crystalline layers. This appearance rightly reminded Baltzer of the difference between the wiry wrought iron and the granular cast iron. Above Luchsingen below the Glfirnisch, Baltzer found the Lochseitenkalk "on the spot it split into a rubble of rhomboedric platelets" or at least cut by many small joints. In some places, the Lochseitenkalk is almost pulverized and partly re-cemented by calcite, or it becomes a breccia by internal disruption and displacement of the disrupted pieces . . . . Talcose-shiny schistose parts or epidote veins cut the limestone bank. The surfaces of the disrupted pieces are rusty where they have been dissolved into a breccia. At Hahnenst6ckli, Escher found the Lochseitenkalk disrupted in part into a breccia, which was embedded in a brown calcareous slate. The latter, probably originating from the Lias or the Dogger, here forms the matrix of the breccia. I have had prepared a fairly large number of thin sections of the Lochseitenkalk. The same features repeat at a microscopic scale as those observed macroscopically. Finely crystalline layers alternate with microdense layers--all strongly

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squashed, bent, and displaced along countless little faults, so that in some samples hardly a cubic millimetre has remained without being displaced with respect to its neighbour. Real veins with secretions are very rare--there was not enough space for gaping fissures. Fossils are mostly missing where the limestone is 100m thick. The mechanical metamorphism may have destroyed them, if they were originally present.' (Heim, 1778a, pp. 139-141). This was the time, when, on the other side of the Atlantic, fault rocks were also beginning to be recognised. But the immense, previously essentially totally unknown areas the American geologists had to map and understand, infested with all sorts of danger both from natural and human causes, did not allow them as yet to make detailed studies of them. Nevertheless Powell noted in the scientific part of his historic Colorado Canyon report that along faults with separated walls, 'in many places we find no definite line of separation between the strata in place and the fallen strata, and there is a space of greater or lesser extent, sometimes several hundred feet wide, between the two series, composed of fragments of the same rocks, in some cases thrown down promisciously, and found much mixed...' (Powell, 1875, p. 183; see also his figs. 65 and 66). He was describing structural rocks of diverse grain sizes along faults of varying scales. Perhaps at this point we should pause and take stock of what had been learned of structural rocks by the end of the third quarter of the nineteenth century: First, it became clear that they were products of shearing deformation within a tabular body parallel with bounding faults. As Heim says, by 1873, their description was commonplace (for one rather nice example, see Daubr6e, 1852, pp. 122-123, and esp. plate 2, fig. 30). Secondly, their origin was seen not only in the mechanical milling down of wallrocks of faults, but also in ductile stretching and recrystallisation. Escher (1846) sensed that temperature had to do something with their origin and Baltzer (1873) perceptively suggested that it might have been the frictional sliding that elevated the temperature within the generation space of fault rocks 21. Heim's meticulous description of the Lochseitenkalk and his marvellous outcrop (Heim, 1878b, Plates VII, VIII, esp. figs. 5 and 6, XVI, figs. 4-7) and thin-section (Heim, 1878b, Plate XV, fig. 10) sketches have greatly popularised the idea of a new rock type created as a consequence of the mostly mechanical deformation of a pre-existing rock type. However, the employment by both Escher and Baltzer of the term 'metamorphism' to describe the mode of formation of the structural rock of the Lochseitenkalk proved most unfortunate. In his review of metamorphism Daubr6e also used the term metamorphisme de structure, by which he meant the formation of foliation in rocks (Daubr6e, 1860, pp. 65-67). He thought that this kind of metamorphism originated 'by pressure and sliding' (ibid. p. 66). The ill-effect of these regrettable appellations was that metamorphic and structural rocks and the processes that generate them became hopelessly mixed up in the geological literature in the next century to come.

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While structural studies were making giant strides in the Alps, another place of development of structural c o n c e p t s - - c o m m o n l y in the light of the Alpine discoveries--was the Scottish Northwest Highlands. The whole thing there had started with a controversy between Murchison and James Nicol with regard to gneisses appearing higher than fossiliferous rocks--Murchison maintaining there was an original sequence, with Nicol arguing that the outcrop faulted and the higher gneisses was/were elevated along steep faults above the younger Palaeozoic rocks (see Oldroyd, 1990, for a superb account of this controversy and the resulting progress in our understanding of the structure of mountain ranges). It turned out that both contestants were wrong--Murchison more so than Nicol--and that the gneisses were thrust n o r t h w e s t w a r d over the L a u r e n t i a n foreland along gently inclined (about 25 ~ planes (cf. Peach and Horne, 1884, Peach et. al., 1888, 1907). Charles Lapworth, of Ordovician fame, was the first one to recognise the importance of considerable thrusting in the Northwest Highlands (Lapworth, 1883). With the thrusting, he also recognised, along the thrust-planes, the presence of structural rocks. In his 1883 paper, we read only a foreboding of what was to come: 'The final results of my investigations in the Durness-Eriboll region...seem to me to indicate most distinctly the probable truth of the theory which has long appeared to myself to be the only plausible solution of the Highlands difficulty. I believe that we have in the so-called metamorphic Silurian region of the Highlands of Scotland a portion of an old mountain system, formed of a complex of rock formations of very different geological ages. These have been crushed and crumpled together by excessive lateral pressure, locally inverted, profoundly dislocated, and partially metamorphosed.' (Lapworth, 1883, p. 121; italics mine). During his Highland work Lapworth familiarised himself with the observations and ideas on Alpine thrusts and studied Heim's 1878 book (Lapworth, 1885, p. 558). In 1885, when he summarised his work on the structural rocks of the Eastern Gneisses, he did so in terms of Baltzer's and Heim's words and also introduced one of his own: 'The old planes of schistosity become obliterated, and new ones are developed; the original crystals are crushed and spread out, and new secondary minerals, mica and quartz, are developed. The most intense mechanical metamorphism [note Baltzer's and Heim's influence.t] occurs along the grand dislocation (thrust) planes, where the gneisses and pegmatites resting on those planes are crushed, dragged; and ground out into a finely-laminated schist (Mylonite, Gr. mylon, a mill) composed of shattered fragments of the original crystals of the rock set in a cement of secondary quartz, the lamination being defined by minute inosculating lines (fluxion lines) of kaolin or chloritic material and secondary crystals of mica. Whatever rock rests immediately upon the thrust-plane, whether Arch~ean, igneous, or Palaeozoic, &c., is similarly treated, the resulting mylonite varying in colour and composition according to the material from which it is formed. The variegated schists which form the transitional zones between the Arnaboll gneiss and Sutherland mica-schists are all essentially mylonites in origin and structure, and appear to have been formed along many dislocation planes, some of which still show

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between them patches of recognisable Archa~an and Palaeozoic rocks. These variegated schists (Phyllites or Mylonites) differ locally in composition according to the material from which they have been derived, and in petrological character according to the special physical accidents to which they have been subjected since their date of origin forming frilled schists, veined schists, glazed schists, &c., &c. The more highly crystalline flaggy mica schists, &c., which lie generally to the east of the zones of the variegated schists, appear to have been made out of similar materials to those of the variegated schists, but to have been formed under somewhat different conditions. They show the fluxion structure of the mylonites; but the differential motion of the particles seems to have been less, while the chemical changes much greater. In some of these crystalline schists (the augen schists) the larger crystals of the original rock from which the schist was formed, are still individually recognisable, while the new matrix containing them is a secondary matrix of quartz and mica arranged in the fluxion planes. While the mylonites may be described as microscopic pressure-breccias with fluxion-structure, in which the interstitial dusty, siliceous, and kaolinitic paste has only crystallsised in part; the augen-schists are pressure breccias, with fluxion structure, in which the whole of the interstitial paste has crystallised out. The mylonites were formed along the thrust planes, where the two superposed rock systems moved over each other as solid masses; the augen-schists were probably formed in the more central parts of the moving system, where the all-surrounding weight and pressure forced the rock to yield somewhat like a plastic body. Between these augen-schists there appears to be every gradation, on the one hand to the mylonites, and on the other to the typical mica-schists composed of quartz and mica.' (Lapworth, 1885, pp. 558-559, italics Lapworth's). In the above discussion of mylonites and related rocks, Lapworth emphasised not only mechanical milling of the constituent grains of the mylonitised rocks, but also new mineral growth and fluxion, i.e. flow. His contemporaries certainly understood Lapworth to include plastic deformation of individual grains in his definition of mylonite. Lewis (1885) described 'optically distorted' quartz from rocks, which he believed were identical in character to Lapworth's mylonites--showing, as we would now say, crystal plasticity (Schmid and Handy, 1991). In their great 1907 memoir, the Highlands team led by Peach and Horne, frequently wrote of plastic deformation within mylonites. Lapworth was writing at a time, when detailed mechanisms of plastic deformation of rock materials were just beginning to be studied and understood (see, for example, the brief historical introduction in Hirth and Lothe, 1968, esp. pp. 3-8). That process is still continuing and constitutes one of the most exciting fields in structural geology (for reviews, see Schmid, 1982; Nicolas and Poirier, 1976; Baumann, 1986; Karato and Toriumi, 1989). Moreover, study of the brittle deformations within shear zones also has experienced a great acceleration after the widespread appreciation of the importance of large strike-slip fault zones (e.g., Wilson, 1960; Tchalenko, 1970; Wilcox et al., 1973; Bartlett et al., 1981; Sylvester, 1988; also see Hancock, 1985; Scholz, 1989; King and Sammis, 1992). Work since Lapworth has concentrated mainly on distinguishing different kinds of fault rocks, finding criteria of doing so and, in the process, understanding the various deformation mechanisms that create and further evolve them. For overviews, we refer the interested reader to Waters and

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Campbell (1935), Hsfi (1955), Higgins (1971), White et al. (1985), Sibson (1977), Schmid and Handy (1991) and Passchier and Trouw (1996). None of this work has changed the basic idea that developed since Lyell's days that fault rocks are basically formed dominantly through shear-controlled mechanical reconstitution of rock materials and that they are formed as a deformation front migrates into a previously undeformed rock. In the following section we present a brief review of the types of structural rocks now recognised and their stratigraphic implications. But before doing so, we need to review the development of ideas on two kinds of structural rock, which, although only varieties of fault rocks, have hitherto not been considered under that heading: namely m61anges and chaos structures. History of ideas on the development of the concept of mdlange." M61anges were recognised later than other kinds of fault rocks, because their recognition was built upon the recognition of what in the Scottish highlands was called a crush breccia. The concept of m61ange was invented by Edward Greenly, when he was working on the geology of Anglesey (Greenly, 1919a, b). He had come to Anglesey after resigning his post with the Geological Survey of Great Britain, where he had been a member of the Scottish Highlands team (Strahan, 1919). He was thus closely familiar with the concept of fault rocks associated with thrusting. The high state of disruption of the rocks, which are now collected under the designation Mona Complex upon Greenly's suggestion (1919a, p. 39) had been known already in the last quarter of the nineteenth century (see the review in Greenly, 1919a, pp. 1-13). They had been called 'crush-conglomerates' and 'crush breccias' by Matley (1913) and Greenly compared them with what he believed were similar crush-conglomerates from the Isle of Man (1919a, p. 65). He knew that he was looking at fault rocks, of the kind that had become so familiar to the workers of the Survey in the Scottish Northwest Highlands, but it was the much larger scale that induced Greenly to give them a new name: 'Autoclastic phyllite-and-grit m61ange. This is really a gigantic autoclast, extending over many miles of country in several districts, as does the "crust conglomerate" of the Isle of Man, but on a still greater scale. It consists essentially of lenticular strips and lumps of grit (Figs. 4 and 5 [here reproduced as Fig. 15A&B respectively]) floating in a schistose matrix that sweeps curving around them (Plates VII, XXIII [VII is reproduced herein as Fig. 14 ]). Rounded fragments are rare, but in the Cemaes district the m61ange is more of a pseudo-conglomerate than elsewhere, having less parallel structure, both in the forms of the lumps and the nature of the matrix. Almost everywhere the lumps are phacoidal, their boundary curves meeting at sharp angles in the direction of the strike. The matrix is now a fine greenish or pale straw-coloured schist, composed of minutely flaky sericite...in curving sheets and seams, always with a considerable quantity of chlorite and some finely granular quartz. In the matrix of the northern breccias these minerals are but feebly developed, and the colour may be a dull grey. The matrix not only sweeps round the phacoids, but often invades them, so that they are traversed by films of it along which they tend to part. And where the larger augen are not close together, it is full of smaller and smaller augen, as well as detached clastic grains that have been torn from them. In the cores of the augen the clastic grains retain often their original rounded outlines quite uninjured, but along the margins and the fissile films they are deformed and flattened. This may take place without "crushing," but it is

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Fig. 14. A view of the 'autoclastic m+lange' of Greenly (1919a, plate VII), which is essentially a broken formation. Coast, near Porth Cadwaladr, Bodorgan.

possible that matter has been stolen from the sides of the grain, thus thinning it. Undulose extinction is general, but far from universal. The matrix of the grit has become a mosaic, with which almost all the grains, felspar [sic] as well as quartz, interlock, especially at their "ends," in the direction of the strike; so that the old clastic outlines are better seen in ordinary light than between the nicols. Many of the grains have broken up, optically, into a mosaic. In the unsheared areas there is a tendency to marginal enlargement of the grain at the expense of the matrix. In the shear-zones the mosaic of the grain becomes gradually incorporated with that of the matrix, into which it merges and finally disappears. The crystalline elements of such mosaic tend to be fusiform and nemablastic [i.e. columnar or prismatic minerals growing in a schist fabric], especially towards the end of the augen. These grit-augen are often excessively quartz-veined . . . . There is usually a system parallel to the minor axis, doubtless a "stretching system," but more and more come in, sometimes to such an extent that in the meshes of their intricate net-work the original structure of the grit can hardly be made out.' (Greenly, 1919a, pp. 65-66). Greenly's description just given, portrays a tectonically disrupted and internally strained phyllite-sandstone sequence, to the extent that the original sedimentary

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A

B .

"'"~

.

.

.

"1"""

,

.

,;

"

\

C

Fig. 15. Greenly's sketches to illustrate the formation of his autoclastic m61ange. A. Disruption of a bed by shearing and the inital formation of phacoids: Breaking down of Tyfry Beds (Greenly, 1919a, Fig. 68). B. As the deformation front moves into the body of a coherent rock body, the original coherence is destroyed and numerous phacoids develop along shear planes: Passage from Fydlyn (not yet m61anged) to Gwna Beds (m61anged) (Greenly, 1919a, fig. 32). C. Fully developed autoclastic m61ange. Greenly's 'diagram of autoclastic m~lange' (Greenly, 1919a, fig. 69).

geometries can no longer be recognised. This is essentially a broken formation. How this happened, he describes as follows: 'Principles of Autoclastic MOlange.--The essential characters of an autoclastic m61ange may be said to be the general destruction of original junctions, whether igneous or sedimentary, especially of bedding, and the shearing-down of the more tractable material until it functions as a schistose matrix in which the fragments of the more obdurate rocks float as isolated lenticles or phacoids. Now this condition is general throughout the higher tectonic horizons of the Mona Complex, which, at that time, were in the zone of fracture, so that all attempts at minor folding resulted only in thrusts and

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other ruptures. By the accidents of recumbent folding and erosion, most of the Gwna beds that are known to us are on those horizons, and are consequently in the condition of m61ange, to which they lent themselves readily by reason of their very heterogeneous character. The early stages of development may be seen in Tyfry Beds of the Pentraeth Inliers (Fig. 68 [herein Fig. 15A], on Twyny Parc in the Bodorgan headlands, and especially well at the top of the Fydlyn Group (Fig. 32 [herein Fig. 15B]). At the top of that massive deposit the thin alternating beds come on, these are soon stripped into phacoids transversely to the bedding; which rapidly becomes imperceptible, and they thus pass into typical autoclastic m61ange .... In the eastern Middle Region, for example, nothing else is to be seen [but mOlange] along 11 miles of strike, and in the eastern Aethwy Region it occupies 20 square miles of country. In a country of this kind, the larger masses are found to be arranged as trains of lenticles overlapping en ~chelon. (see the diagram, Fig. 69 [herein Fig. 15C] as well as Figs. 4, 5, 165-6, 169, Plates VII [herein Fig. 14], XXII, and the parts of the one-inch map about Pentraeth, Llaniestyn, and elsewhere.) The major axes of the lenticular cores are slightly curved, the form of the line being highly characteristic,--a sigmoidal curve approximating to a straight line. Upon a map, the masses that can be separated out appear as if embedded in a homogeneous and structureless matrix. But this country-rock is itself built up of interdigitating lenticular bodies; and, could we take in the region at a glance, it would present itself to us as a m61ange of torn and sheared lenticular masses of all sizes, from such as are two or three miles in length to the smallest that the eye can see, of spilite lava, diabase, quartzite, limestone, jasper, and grit, floating in an undifferentiated but schistose body that is a weft of all the more easily deformable elements, itself pervaded throughout by the same lenticular structure.' (Greenly, 1919a, pp. 193-195, italics Greenly's). Greenly emphasised that although several members within a m61ange could be separated out on a map, where this is not possible, the m61ange should be mapped as a unit (Greenly, 1919a, p. 66, note 1). Greenly's inferences have stood the test of time almost intact. Shackleton (1954) has shown that the sequence he had inferred in the M o n a Complex was upside down, but his inferences concerning the m61ange and the mechanism of its formation remain correct. Shackleton (1969) tried to show that Greenly's m61ange was a submarine slide breccia, but his reasons all pertain to his inability, in pre-plate tectonic days, of conceiving how such a jumble of rocks could possibly form tectonically and then get overlain by sedimentary rocks (Wood's, 1974, arguments for a non-tectonic origin of the Gwna m61ange we find equally unconvincing). Shackelton correctly saw that all sorts of environments were represented in Greenly's 'general m61ange' and all were brecciated (Shackleton, 1969, pp. 9-10). Where extreme tectonism did not efface original boundaries, the breccia contours could be seen to be angular. The size range of blocks went from microscopic to several kin. Shackleton could not relate the disruption to any set of recognised structures affecting the m61ange body as a whole. (ibid., p. 10; also see Wood, 1974). F r o m all this, he concluded that only a submarine slump could explain the origin of the m61ange. This was the common opinion before plate tectonics 22 (with the remarkable exception of Bailey and McCallien, 1950, 1953, 1961, 1963: see below; also see Sir Edward Bailey's tectonic opinion on the Anlgsey m61anges in Wood, 1974, p. 335), but once 'faults' could be imagined along which displacements

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could be measured by thousands, nay tens of thousands of kilometres (i.e. the subduction zones), all the characteriscics that had earlier baffled the students of m61anges could be readily explained. The word m61ange did not become popular after Greenly. Even he himself did not use it when summarising his Anglesey work later (e.g. Greenly, 1922). The word and the concept was revived, when James McCallien and a doctoral student of his were surprised by the jumble of limestones and pillow lavas occurring in a matrix of sheared schists, serpentinites and, in places, even mafic volcanics near Ankara in Central Turkey in the late forties. Because the full story of this 'second discovery' of the m61ange concept is essentially unknown even in Turkey, we give it here in some detail. W. J. McCallien had been invited by the Turkish government to teach geology to the students of the Geographical Institute, in which capacity he had served between 1944 and 1949. McCallien had not brought his family to Turkey so he had ample time on weekends to roam around Ankara to do some fieldwork in the compay of a young and enthusiastic undergraduate student: O~uz Erol. Their initial wanderings bafO-d McCallien: Erol recalls the frustation of his teacher that it did not seem st~ ~ghtforward to erect a stratigraphy, let alone to get a handle on the structure. The only fossiliferous rocks they initially could find were the well-bedded fusulinaceous Permo-Carboniferous black limestones already reported by Chaput (1936, pp. 25f.). Chaput had pointed out that the bituminous and dark-coloured brecciated limestones bearing the fusulinid Staffella occurred nowhere in outcrop, but had been encountered only in the surficial float 23. He thus did not indicate them in his "Old Elmadagl Series" that appears on his map (Chaput et al., 1930). There is only the faintest suggestion on Chaput's map of the complications that his Elmadagl Series in reality exhibited, for he had mapped the light-coloured, recrystallised limestone units as large lenses and thin layers tectonically interleaved within the Elmada(gl Series. When McCallien and Erol wandered through this series, they could never find the Permo-Carboniferous, well-bedded, black limestones to continue more than about 10 metres before abruptly ending against slates or sheared mafic igneous rocks. Moreover, Chaput's 'calcareous marbles' also looked embedded in the same slate/ mafic rock 'matrix 24'. They further often talked about the matrix being 'sheared' forming a 'plastic medium' in which the blocks were 'churned.' But McCallien seemed to despair, for, owing to war conditions, no topographical base maps were available to him to begin a mapping campaign; he kept searching for an area to study where he could find a simpler, regularly bedded sequence. In 1946, by a lucky coincidence, the Army Topographical Survey published in a local journal 25 an example of a 1/25,000-scale map of a small area to the south of Imrahor in the south-eastern periphery of Ankara (for locations see Fig. 16) as an example of the fruits of its then new aerophotogrammetric work. Erol copied this 'sample map' and used it as a base to complete an honour's thesis in this region. It was here that they found the first pillow-lava outcrop which McCallien identified for Erol as such. This was the first pillow-lava occurrence ever reported from Turkey (McCallien, 1950). This exercise gave Erol his first taste of detailed mapping and in 1947, after graduation, he commenced a Ph.D. thesis under McCallien. Topographic maps continued to be a problem, but Erol obtained, not entirely legally--because

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A. M. C. ~eng6r and M. Sakm, c

civilians were still not authorised to use 1/25,000 m a p s - - a set of maps from a military friend that covered a 625 km 2 area to the southeast of Ankara, and began mapping in its northern part. He had to cut his maps into small enough pieces to be able to hide them quickly in his pocket in case he was 'caught.' Under these extraordinary circumstances, he mapped a large number of blocks embedded in sedimentary and igneous matrices in the part of his area nearest to Ankara, namely in the ~alda~, imrahor, and the Kaya~ regions (Fig. 17A). McCallien kept asking Erol to establish a stratigraphy and Erol kept coming back to him with the plea that it did not seem possible. Finally both agreed that they were looking at a jumbled mixture of blocks floating in the two matrices (Fig. 17A&B). It was in the ~alda~ region that Erol also noted that the blocks and the foliation defined planes dipping to the north. McCallien hypothesised that this probably indicated a north to south tectonic transport. Erol further thought that the non-metamorphic clastic rocks, wedged into the jumbled rocks he was mapping in the Klbnsyayla district in the Elmada(~l region (Fig. 16), resembled the regulary bedded Liassic clastics in the Yakaclk area (first found by Vadfisz in 1913" also see Chaput, 1936, p. 46) and correlated them. All this indicated that the area had been highly tectonised and apparently during the Alpine orogeny. Erol finished his thesis in 1949, but had to defend it in McCallien's absence, for the latter had already left Turkey by that time. His conclusions about the 'mixed series' he formulated as follows in his thesis: 'The first characteristic that catches the eye in this blocky series, of which we have tried above to outline the rock types, is the presence of large limestone blocks and intermediary material that surrounds them. The sedimentary parts of this intermediary material were sheared, mixed up and the igneous parts were pushed in between various rock types. The "pillow lavas" within the igneous parts are clear evidence for submarine eruptions (Bailey, 1936, p. 1721). The green tufts must also be of volcanic origin. Thus we found it appropriate to call this mixed belt containing: blocks of possible Permo-Carboniferus (perhaps also Triassic) age, a flysch series whose age is provisionally assigned to the Mesozoic, greywackes thought to be still older, pillow lavas, and other eruptive material, the Elmada~l Blocky Series (= Boulder Bed Series). We do not wish to say much about the origin of these mixed series, which is still a topic of debate. But, it is clear that the blocks within the Elmada~gl series are not a result of the "northerly tilting, like fish scales, of the Elmada~l Massif and its Mesozoic cover during the Cainozoic" as believed by Chaput (1931, p. 8326). The generation of the blocks must have occurred in an environment in which the pillow lavas, indications of older (probably Mesozoic) submarine eruptions, could spread, because the blocks and the intermediary material only rarely are in contact with fracture zones. Some of the normal faults in the area cut the Tertiary and are thus younger. In brief, I wish to stress especially that this series is a "mixed series" consisting of the blocks and of the intermediate material. Although we do not have clear evidence for its age, we provisionally treat it as Mesozoic. The real age can only be established after finding evidence for the age of the intermediary material. This "Boulder Bed Series," in which other rock types predominate over the green eruptives has been distinguished from the usual "Mesozoic Mixed Series," in which the serpentinites are dominant and which occurs in the southern part of our area.' (Erol, 1949, pp. 21f.; 1956, p. 18).

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A. M. C. ~eng6r and M. Sakln, c

r,.,~

r..~~

r..,~

Fig. 17. Two examples of Erol's field slips. A. Region southeast of Bursal village. B. Region northeast of K6miircfi. Note the blocks floating in a matrix. (For locations, see Fig. 16).

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A. M. C. ~eng6"r and M. Saktn,c

Erol wrote all these in 1949. During the examination, he remembers with amusement now, how the old professor of geology in Ankara, Ahmet Sevket Birand, expressed surprise that all this could happen in a lake. Erol, horrified, exclaimed: 'A lake! This was an ocean, at the bottom of which all these eruption events and submarine slidings were taking place to create this mixed series!' (Prof. O~uz Erol, personal communication, 15th December, 1994). Birand was baffled, and Erol thought from the expression on his face that he would probably fail the examination. To his great relief, he passed with flying colours. In the meantime, McCallien was communicating with his old friend, the renowned Scottish tectonician Sir Edward Bailey, about this extraordinary area. He asked Bailey whether he would want to go and tour it to see whether a reasonable interpretation could be formulated. Bailey accepted, funds were raised through the generosity of the Carnegie Trust, the University College of the Gold Coast (where McCallien was now employed), and the Mineral Research and Exploration Institute of Turkey in Ankara (the Turkish Geological Survey), and the two went to Ankara in the summer of 1950. Bailey and McCallien (1953) relate the background of their work with the following words: 'In the five years preceding our joint visit it had been impossible for W. J. McCallien to undertake detailed mapping, since, quite properly under existing conditions, a foreigner attempting such a task is subjected to endless questioning. On the other hand, one of his research students [in reality the only one he had], Oguz Erol, starting in 1947 and working under close supervision, produced a valuable thesis, the title of which when translated reads: "A Study of the Geology and Geomorphology of the Region S.E. of Ankara in Elma Dagi and its surroundings." This account is illustrated with a careful geological map (not as yet published 27) on the scale of 1:100,00028. For our present purpose it is only necessary to say that Erol firmly established a broken block structure for the greywacke-limestone complex of the Ankara outcrop, and in many cases separated individual blocks of limestone on this map. He also found numerous micro-fossils in limestones, some of which have been determined by pal~eontologists as Middle Permian. We had the great advantage of Erol's company and guidance in the area he knows so well during one week of our visit.' (Bailey and McCallien, 1953, p. 406). At the time Erol was doing his military service and had to use a vacation leave to lead his former professor and Sir Edward in his area. During the one-week field excursion Erol led for the two British geologists, not one word was spoken about m61ange. Erol says that he first saw the word m61ange in the published papers of Bailey and McCallien. Bailey and McCallien then moved to the Alaca district some 160 km east of Ankara where the ancient Hittite sites were being excavated by the Turkish Historical Society (Tfirk Tarih Kurumu) and they lodged in its guest house. There too they found rock associations similar to those mapped by Erol to the southeast of Ankara. Their month's travels around Ankara and Alaca districts showed Bailey what his colleague had seen earlier and it was presumably Bailey who remembered that a

Structural rocks." stratigraphic implications

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similar association had been described f r o m Anglesey in 1919 by Greenly. They thus decided to use Greenly's terminology: 'Our facilities, as provided by the Geological Survey [of Turkey], included jeep transport, which took us "over hill, over dale" to "wander everywhere", like Puck in Shakespeare's fairy tale. We saw great tracts of broken rocks, consisting mainly of greywackes, limestones and the Steinmann trinity. They rivalled in abandoned disarray Puck's most mischiveous attempts to puzzle and confuse mere mortal man. Yet, with the help of fossils, mostly of the micro order and found quite commonly both in the limestones and the cherts, the M.T.A. map (which forms the basis of our plate VIII) has diagrammatically divided the area into Palaeozoic and Mesozoic, with local refinements such as Permo-Carboniferous, Lias, Oolite and Jurassic-Cretaceous. We established undoubted important recurrences of rock types at more than one horizon; but, broadly speaking, we are satisfied that the main greywackes are Devonian, the main limestones Permo-Carboniferous, in an inclusive sense, and the main developments of the Steinmann trinity Mesozoic . . . . All the above formations are broken to give what we may call the Ankara M61ange ... out of respect for descriptions that Greenly and Matley have furnished of a similar tectonic m61ange in Anglesey and the adjoining Lleyn peninsula of Wales (cf. Greenly, 1919). We definitely know from our own experience of some 6000 sq. km. of m61ange exposed along the Ankara outcrop, and it is obvious that this is only a small fraction of an immense whole." (Bailey and McCallien, 1953, p. 405; also see Bailey and McCallien, 1950a, pp. 3f.; 1950b, pp. 18-19). Erol's recollection is that the hypothesis of how this immense m61ange had f o r m e d had come f r o m Sir E d w a r d Bailey. It was f o r m u l a t e d as follows: "(12) We have mentioned under heading (4) how the Ankara m61ange, exposed in the Ankara anticlinal, reappears eastwards in the Alaca H6yfik group of anticlinals. The width of separating Early Tertiary outcrop occupying the intervening synclinal is, in this traverse, 60 km. The synclinal outcrop diminishes greatly northwards past (~ankiri; but it increases correspondingly southwards and eastwards past Kir~ehir, after which the synclinal may be named. (13) A remarkable feature of the Kir~ehir synclinal is that its Early Tertiary outcrop is in very many places interrupted by great spreads of marble and crystalline schists intruded by granite or diorite. These rocks are clearly pre-Tertiary and have yielded debris to the same. Similar rocks are found occasionally in direct contact with the assemblage characteristic of the Ankara m61ange. We shall speak of these crystallines as the Kir~ehir crystallines. (14) The highly accentuated synclinal disposition of the Kir~ehir crystallines with reference to the Ankara m61ange shows that the former constitute part of a great nappe de recouvrement covering the latter. In the anticlinal areas the early Tertiaries rest preferentially on the m~lange, because the anticlines originated with concomitant erosion before the early Tertiaries started to accumulate. In the synclinal areas the Early Tertiaries rest preferentially on the Kir~ehir crystallines, because here erosion has spared extensive masses of the nappe de recouvrement. (15) We believe that the Kir~ehir nappe is most probably a Pontide nappe, and that the thrust at its base, which we suggest be called the 'Anatolian thrust', is essentially a boundary between Pontic (Black Sea) and Taurus (Mediterranean) pre-Tertiary rocks, which differ in facies. The Ankara m61ange we interpret as Taurus rocks beneath the

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A. M. C. ~eng6"r and M. Saktn,c

Anatolian thrust; the Taurus Mountains as a continuation of similar rocks bulldozed towards the south. (16) The horizontal displacement postulated along the Anatolian thrust amounts to about 350 km. This is greater than the minimum displacement measured along other thrusts yet described. The T6rnebohm thrust in Scandinacia has a minimum displacement of about 130 km." (Bailey and McCallien, 1950a, pp. 5-6; also see 1950b, pp. 20-21; 1953, pp. 437-438). The timing of this monstrous thrusting was given by the youngest rocks bulldozed by the hypothesised Klr~ehir Nappe and the oldest rocks lying unconformably on both the m61ange and the Klr~ehir crystallines, after some of the microfossil material Bailey and McCallien had collected had been studied. These rocks were Upper Jurassic and Eocene respectively (see Bailey and McCallien, 1953, pp. 407 and 480): We suggest as an explanation the following sequence of events. In Late Cretaceous times Kirsehir crystallines (Pontic) were thrust long distances over Ankara and Alaca H6yfik Mesozoics and Pala~ozoics (Taurus). The thrust we call the Anatolian Thrust, and the thrust-mass the Anatolian Nappe. Continuance of movement started to fold the Anatolian Nappe together with its m61ange foundation, thereby initiating the Ankara and Alaca H6yfik anticlinals, and the Ayas and Kirsehir synclinals. Concurrent erosion removed the Anatolian Nappe in anticlinal areas, leaving enormous klippes in synclinal areas. Before the end of Late Cretaceous times general submergence introduced the sea into some of the erosional hollows floored by Ankara M61ange between the klippes. The process of submergence was accentuated during the Eocene. Eventually Oligocene seas and lagoons spread over previously upstanding remnants of the Kirsehir crystallines. Succeeding post-Oligocene elevation was accompanied by accentuation of the anticlinal and synclinal deformation. Thus Mid-Tertiary and later erosion has revealed the Mesozoic and Palaeozoic materials of the Ankara M61ange beneath Eocene (and Late Cretaceous) in anticlinal areas, and the Kirsehir crystallines beneath Oligocene in synclinal areas. (Bailey and McCallien, 1953, p. 438). Thus Bailey and McCallien thought they could sketch a complete MesozoicCainozoic tectonic history for Asia Minor. Obviously the m61ange was the most important part of this story and it not only has remained so until today, but has become much amplified in significance. The story we reviewed above in some detail shows that the m61ange concept was rediscovered by Erol and McCallien almost exactly three decades after Greenly had first proposed it in 1919. It was Erol's utter frustration that no sensible stratigraphy could be erected in his area and McCallien's insistence that he should try anyway that finally led the former to tune his observations to a scale fine enough so that they finally discovered that they were looking at a jumbled sequence of rocks. Erol remembers how often the two went off to check yet another area where Erol tried to show McCallien why a normal Stenonian/Cuvierian stratigraphy was not possible. In the process they developed their terminology of blocks, lenses, lenticules, matrix, intermediate material etc. Their premature interpretations included mixing during submarine eruptions and submarine sliding. Finally, the arrival of the better-informed Sir Edward showed

Structural rocks." stratigraphic implications

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them that the problem they were grappling with was a very real one, but that it had been solved some three decades earlier by Greenly: this gave them their terminology. The presence of the Steinmann trinity in the m61ange drew Sir Edward's attention to Steinmann's own area, the Arose Schuppenzone (Steinmann's Aufbruchszone) between the higher-lying Austroalpine and lower-lying Penninic elements, and thus a giant thrust similar to the Austroalpine basal thrust above the Aroser Schuppenzone was conceived: this inspired the interpretation. Although later shown to be wrong in significant details, the interpretation that the m61ange had been generated in a giant shear zone, essentially a huge fault zone, remained victorious, at least to the present day! At the time, too, this giant thrust fault was the most exciting part of their discovery. The mammoth overthrusting this hypothesis suggested, exceeded by almost 200% the largest thrust displacement then known! This implied a great mobility of Asia Minor and that it had a structure very similar to Suess', Argand's and Staub's Alps: A giant, south-facing sandwich (as opposed to the north-facing All "x" with the Pontic ranges on top, the geosynclinal fill in the middle plus some bt ~ Jozed Taurus material, and the Taurus itself on the bottom. All this made a mockery of the then prevalent Kober-Stillean model of symmetric orogens and vast, rigid Zwischengebirge in between: '(17) We are conscious that our conclusions differ considerably from Kober's conception of a Zwischengebirge between the Pontic and Taurus Mountains, and also from Par6jas's interpretation of transversal anticlinals and synclinals in the same region' (Bailey and McCallien, 1950a, p. 4; 1950b, p. 21; also see 1953, p. 438). Ihsan Ketin, by that time a fairly young 'senior' member of the Turkish geological community, put the monstrous 'Anatolian Thrust' to test. Were the 'Klr~ehir crystallines' really a massive, rootless nappe, a gigantic klippe of the Pontides floating on the Taurus, whose rocks it had crushed and sheared into what eventually had become the Ankara M61ange? Was Turkey indeed an immense sandwich consisting of the Pontides on top, the Ankara M61ange in the middle, and the Taurides at the base, similar to the 'schema of the Alpine cross-section' of Staub (1924, fig. 2), which Bailey probably had in mind? With the express purpose of testing these ideas, Ketin mapped an area to the east of Ankara in the summer months of 1953 and 1955 within the grand curve of Klzlhrmak, the classical Halys, about the city of Yozgat, as a part of the national 1/100,000-scale geological mapping program of the Mineral Research and Exploration Institute of Turkey in Ankara (Ketin, 1956a). Ketin indeed confirmed the presence of the Klr~ehir crystallines in the area, consisting of calcschists, marbles and amphibole schists, cut by both mafic and felsic plutons with compositions ranging from olivine gabbro to granite. Above the crystallines, however, was the serpentinite-radiolarite association, in places sitting directly on the metamorphics (as in west of Kaman, Ketin noted: Ketin, 1956b, p. 169), in others being thrust onto a Lower Eocene Flysch. Nowhere could Ketin find a contact that showed that the metamorphics were above the m61ange. Moreover, in the area he mapped, the

188

A. M. C. Seng6r and M. Sakm,c

serpentinite-radiolarite association did not look terribly chaotic. He described the following 'beds' in one section: "50-60m. grey-green, thinly-bedded sandy limestone 25-30 m. calcitised, amygdaloidal spilite 15-20 m. red limestone and marl 8-10 m. pillow lavas 15-20 m. red limestone, marl and schist 25 m. grey, light coloured platy-limestones and marl 20 m. radiolaria-bearing limestone 25 m. grey marl and limestone 6 m. breccia of basic lava 10 m. radiolarite, limestone, marl 30-32 m. grey, schistose marl 15-18 m. red limestone and marl 8 m. grey schist and marl 18 m. red limestone and marl 10-12 m. spilite 6 m. red limestone 22 m. grey schist and marl 3 m. lava 20 m. grey schist and marl 50 m. serpentinite 5 m. radiolarian limestone 8 m. amygdaloidal spilite 20 m. grey limestone and marl 5m. diabase 80 m. grey-bluish schist and marl 20m. calcitised and silicified basalt 80-90 m. grey marl, schist and in part sandstone 8-9 m. red radiolarite and limestone 120-130m. sandstone and sandy limestone, marl 8-10 m. volcanic tuff and spilite 20m. semi-crystalline limestone block 45-50m. serpentinite and lava 120-130m. red limestone, silica- and manganese-rich beds, serpentinite, etc." (Ketin, 1956a, pp. 8-9; see also his sections in his fig. 3). Ketin found Turonian through Maastrichtian fossils in the sedimentary layers and assumed a fairly regular succession of deep-sea sedimentary and volcanic rocks 29 at the b o t t o m of a geosyncline. Although locally he mapped thrusts, he further assumed that what he had mapped was the original stratigraphic disposition and the crystalline massif of Klr~ehir had been the floor of that geosyncline at least in part. The metamorphic rocks, he thought were indeed older than the serpentiniteradiolarite series, but not much older. He estimated they were probably Mesozoic in age (Ketin, 1956b, p. 169). Thus, not much remained of Bailey and McCallien's grand hypothesis of the structure of Asia Minor, which failed Ketin's test 3~ With it, the m61ange concept was once more relegated to the dusty archives of the history of geology, except in the

Structural rocks." stratigraphic implications

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later writings of Bailey and McCallien (1961, 1963) and Gansser (1955, 1959, 1964, pp. 7-8). However, Gansser, a Swiss, was a child of the Alps, where jumbled r o c k s known at least since Bachmann ( 1 8 6 3 ) - were more the rule than the exception, so while describing what he called the coloured m61ange of Iran, he did not feel he was revealing a new world. In the light of all this it is very remarkable that the m61ange concept had to be reinvented for the second time, two decades after Erol and McCallien's reinvention and one decade after the rejection of Bailey and McCallien's tectonic interpretation of the structure of Asia Minor by Ketin. Hsfi relates in detail how he himself had reinvented it in his history of the evolution of the models on the Franciscan Complex in California (Hsfi, 1985, esp. p. 57f.). Interestingly, it was another Swiss geologist, H61i Badoux, who told Hsfi that what he thought he had invented had been invented already by Bailey and McCallien in 1950 and that it had been called m61ange (Hsfi, 1985, p. 57). Hsfi had agonised over the Franciscan rocks much like Erol and McCallien had done over the Ankara rocks. He finally came up, independenly of either Greenly or Bailey and McCallien, with the idea that he was looking at a tectonic jumble, essentially at a mega-fault breccia, but initially mixed in a submarine slide environment. Once he conceived the process that had created the rocks he was looking at, he was able to map them in an intelligible way and developed a methodology of mapping (Hsfi, 1968, 1990), which was essentially the same as Greenly's. Let us remember that Greenly was able to understand the Mona m61anges owing to his experience in the Northwest Highlands of Scotland, where fault rocks had been mapped in great detail after the recognition of large-scale thrusting and Matley's earlier work, also inspired by the Highlands discoveries. In other words, Greenly first recognised (read: hypothesised!) a process and then sorted out the structure. Bailey and McCallien simply used Greenly's hypothesis and experience. Hsfi, having been educated in the more parochial (and more devoutly Baconian) U.S. environment of the fifties, had to reinvent all that through much agonising only to be told at the end by an Alpine geologist that he had reinvented the wheel (see Hsfi, 1985 and 1990). But the great importance of the Franciscan m61anges came when it was realised that they represented the sweep of an ocean floor for at least 1000 km (Hamilton, 1969; Dewey and Bird, 1970; Dickinson, 1970; Hsfi, 1971). Such fault displacements had not been hitherto recognised. Nor fault rocks, of the volume of the Franciscan Complex, had been recognised before. With the recognition of this new process, namely subduction, m61anges acquired great significance as indicators of past subduction and they began to be recognised the world over. History of Ideas on the Development of the Concept of Chaos Structure." Chaos structure is a term introduced by the United States Geological Survey geologist Levi Noble in 1941 to describe the great confusion of rock types encountered in the Black Mountains, in the southern part of the Amargosa Range to the northeast of the Death Valley in eastern California. Below we give in Noble's words his recognition of this structure. It is nothing more than the re-invention of the concept of m61ange in complete ignorance of Greenly's work: 'In 1932 a set of vertically taken air photographs was made for the Geological Survey to aid in mapping the geology of Death Valley and the Black Mountains. One feature in

190

A. M. C. ~engO'r and M. S a k m l

the photographs, a crazy-quilt pattern--or more literally a mosaic--of large areas of rock in the Black Mountains, attracted immediate attention as bizzare and without obvious explanation. One of the photographs (P1. 6) gives a general idea of the great number of the blocks, their irregular shape, and their chaotic arrangement; and from the scale of the photograph, about 2500 feet to the inch [about 1/200,000], one may get a rough idea of their size. The rocks of which the blocks consist represent a stratigraphic range of many thousands of feet, but all of them are younger than the pre-Cambrian gneiss upon which they rest and from which they are separated by a great thrust fault. This fault is the Amargosa overthrust, and the mosaic of fault blocks constitutes a composite thrust plate which will be called the Amargosa chaos. The characteristic features of the chaos shown in Plate 6 are: (1) The arrangement of the blocks is confused and disordered---chaotic. (2) The blocks, though mostly too small to map, are vastly larger than those in anything that could be called a breccia; most of them are more than 200 feet in length, some are as much as a quarter of a mile, and a few are more than half a mile in length. (3) They are tightly packed together, not separated by much finer-grained material. (4) Each block is bounded by surfaces of movement--in other words, each is a fault block. (5) Each block is minutely fractured throughout, yet the original bedding in each block of sedimentary rock is clearly discernible and is sharply truncated at the boundary of the block. Commonly the bedding, even of incompetent beds, is not greatly distorted. None of the geologic terms in common use appear exactly to fit this mosaic of large tightly packed individual blocks of different ages occupying a definite zone above a major thrust fault. The feature suggests a fault breccia on a cyclopean scale, yet it is not a fault breccia in the orthodox sense. Although it is a thrust plate, it is shattered over large areas to a degree that appears to be unique. But more important than these considerations is the fact that the great areal extent o f the feature makes it impossible to map separately the geologic units o f the mosaic and makes it necessary therefore to treat the assemblage, despite its heterogeneity, much as a geological formation be treated. Like a formation, then, it requires a name that will indicate both its type locality and its character.

"Amargosa does the one, for the Black Mountains, in which Virgin Spring lies, form part of the Amargosa Range; and "chaos" as the preceding paragraphs have tried to show, does the other. The Amargosa chaos or features resembling it are widespread in the southern Death Valley region, and if they occur in other regions the term chaos, as a common noun, may prove to be a useful geological term.' (Noble, 1941, pp. 963-965, italics ours). N o b l e a s s u m e d that the A m a r g o s a chaos h a d originated along thrust faults. Later W r i g h t and Troxel (1969, 1984) showed that this interpretation was w r o n g a n d the faults creating the chaos were n o r m a l dip-slip in nature, bringing y o u n g e r rocks over older. They n o t e d that m a p p a b l e b o u n d i n g faults in the chaos always o m i t t e d section and assumed that the chaos was a p r o d u c t of coalescing listric n o r m a l faults. Wernicke and Burchfiel (1982, p. 109) p o i n t e d out, h o w e v e r , that the same effect can be p r o d u c e d along planar n o r m a l faults of large displacement. W r i g h t a n d Troxel (1984, plate I, cross-section A-A') a p p e a r to have a d o p t e d this suggestion.

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Seng6r was shown the chaos structure in the field by Brian Wernicke and he was greatly impressed with its similarity to m61ange. Especially in the Emigrant Canyon south of the Death Valley, where, at the Emigrant Spring locality, Wernicke pointed out the chaos formed from the variegated rock types of the Johnnie and Noonday formations, largely embedded in a shattered argillite matrix, in places ground down to rock flour, ~;eng6r remembers exclaiming 'coloured m61ange!' The two main differences between the m61anges that form along subduction zones and the chaos structure forming along large normal faults are the amount of displacement along the generative fault zones and the evolution of the ambient pressure/temperature regime as the rocks evolve. Along subduction zones entire paleogeographic realms disappear and their representatives are now only encountered as exotic blocks within the m61ange. Erosion is the only agent that can destroy record of former environments in an extensional environment and therefore, in principle, chaos has no 'exotic' blocks. Chaos structure evolves in a progressively unroofing environment where ambient pressures and temperatures continuously decrease. M61anges, especially those along subduction zones, have more complicated paths of evolution because of the vicissitudes of the tectonic regimes reigning in accretionary wedges and subduction channels (e.g. Cloos, 1982, 1984; Dobretsov and Kirdyashkin, 1992; Le Pichon et al., 1993).

The lesson o f this history

We have reviewed, in summary fashion (except for the history of discovery of the Ankara m61ange) in the foregoing paragraphs the ideas on structural rocks beginning with the sixteenth century. What this history shows us is that structural rocks could only be recognised once the processes that created them were understood. These processes had to be hypothesised, commonly on very flimsy data, and then the deductions made from such hypotheses had to be tested by extensive, careful observations. It was during these tests that the concept of structural rocks evolved. However, because structural rocks are very commonly found in metamorphic states, having generally formed in metamorphic environments, they were widely confused with metamorphic rocks. Debates about what constitutes metamorphism were raging while the first detailed maps of structural rocks were being made (cf. Daly, 1917). Daly thought that generation of mylonites was a 'very rare' phenomenon. This is understandable, because the geologists' appreciation of how very mobile the lithosphere is has developed only very gradually and, only after the invention of plate tectonics, the geologist really had to come to grips with very large fault displacements all over the place as the history of the m61ange concept so nicely illustrates. It had to be reinvented twice (three times if one includes the recognition of chaos structure in the history of the m61ange concept) before it came to stay and its eventual success was due to the discovery of subduction and, with it, immense fault displacements. The realisation that normal faults can also have very large displacements (Wernicke and Burchfiel, 1982; Wernicke, 1985; see esp. Snow and Wernicke, 2000) followed the recognition of the great mobility of the

192

A. M. C. ~eng6r and M. Sakm,c

lithosphere within the framework of plate tectonics. Thus, structural rocks were commonly confused with metamorphic rocks and their messages were seldom heard distinctly, as emphasised by Higgins (1971) for fault rocks and by Hsfi (1968) for m61anges.

M A I N TYPES OF S T R U C T U R A L R O C K S A N D T H E I R STRATIGRAPHIC IMPLICATIONS What is a structural rock?

Before we review the main types of structural rocks, this is perhaps the place to define them precisely and contrast them to the metamorphic rocks. Figure 18 shows that structural rocks originate, at the intersection of a velocity space with a rock space. The nature of the velocity space to create sructural rocks is an extremely complicated one and cannot be discussed adequately in this paper. However, to illustrate its nature crudely, consider some of its properties exhibited in Fig. 19. Figure 19A shows a cross-section of an undeformed cube that will undergo simple shear as shown in Figs. 19B and C. To generate a structural rock the strain must contain at least some incompatibilities in it, because compatible strain cannot break the r o c k - it would just generate tectonite (cf. Ramsay and Huber, 1983, session 3, esp. fig. 3.1; only in some ultramylonites the rock will be essentially all 'matrix' with no grains; such a structural rock may be described in terms of compatible strain). For simplicity, we assume (I) that elongation due to shear strain will take place along a 'ridge-transform fault' system within the rock and (II) that all the ridges formed in any one 'row' will share, at any one time, the total 'spreading' (i.e. elongation) equally. We further assume that, (III) within the 'ridges,' the 'spreading' will generate antitaxial veins (Ramsay, 1980; Ramsay and Huber, 1983, pp. 243-245). For a

( Generation of structural rocks Fig. 18. Origin of structural rocks illustrated as an intersection of a velocity space portrayed as the set V fixed with respect to a rock volume represented by set R. The characteristics of the velocity space is given by equations 1-3. Structural rocks form at the intersection of the two sets. If other sets representing T and P spaces are added, rocks forming at the intersections of all the sets will be metamorphic structural rocks reflecting the conditions of the T and P sets.

Structural rocks." stratigraphic implications

193

Fig. 19. A simple model for the origin of structural rocks. A. Cross-section of an undeformed rock cube with sides a. Diagonal lines represent direction of motion trajectories when the 'ridge-transform fault' system depicted begins 'spreading.' Ridges are those at 90 ~ to the diagonal 'transform fault' orientations. B. The same after a strain (~,) of 0.58. Note the rotation of the original 'ridge-transform fault' system and the amount of 'spreading' here assumed to form antitaxial veins (shown in detail in Fig. 19D). See Fig. 20 for an illustration of this 'row-wise' movement in structural rocks. C. The same after a strain of 1.43. Note further rotation and the opening of new 'ridges' in the form of new antitaxial veins. D. Enlarged view of the rotated ridge-transform system depicted in B (~ is the shear strain corresponding with the rotation angle). E. Sketch of the 'ridge-transform fault' system with the distribution of strains if ductile homogeneous deformation is assumed along the shear surfaces and within the new material accreted by the 'ridges,' which here takes the form of antitaxial fibre growth. F. Theoretical foliations drawn on the basis of the geometry shown in C and E. Note the 'S' and the 'C' surfaces, in addition to the presence of foliations exhibiting 'reversed shear sense.' See Fig. 21 illustrating an example of this from the structural rocks of the Maggia Nappe in Ticino, Switzerland. s t r u c t u r a l r o c k to f o r m , the ' s p r e a d i n g v e l o c i t y ' m u s t be > 0 ( o t h e r w i s e r o c k c a n n o t be g r a n u l a t e d a n d t h e t o t a l s t r a i n will be c o m p a t i b l e ) . U n d e r t h e s e c o n d i t i o n s , e l o n g a t i o n (e) a l o n g a n y o n e ' r o w ' o f ' r i d g e s ' is g i v e n b y (for t h e p a r a m e t e r s fr, a a n d b, see Fig. 19): e = [cos 90 ~ - {ft x + [tan -1

(a/(b + a x t a n 7r))} x a~ cos fr]

{a/(b + a x t a n ft)}] x b

(1)

N a t u r a l l y , the e v a l u e is d i f f e r e n t for a n y o n e r o w , w h i c h c a n be c a l c u l a t e d o n c e t h e r o w w i d t h s are fixed. T h e d i s p l a c e m e n t dn a l o n g a n y ' r o w ' i o f a n y o n e p a r t i c l e n w i t h r e s p e c t to its initial p o s i t i o n a l o n g the r o w is g i v e n by:

d n - e/pi x ( n - 1),

(2)

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where Pi is the total number of 'ridges' in that row and n is the nth rigid block that carries the particle n. The relative displacement A n m - - d n - d m of two particles n and m on two different blocks in different rows i and j is then: Anm = [ei/Pi x ( n -

1 ) ] - [ej/pj x ( m -

1)]

(3)

The velocity 12nm of n with respect to m is then 12nm = d(nm)/dt. The equations given above describe the relative motion of blocks (i.e. grains) in a shear zone, which move with respect to each other within the dictates of the shear and in the framework of our assumptions I-III given above. For the purposes of this discussion we ignore the flattening component in the definition of the velocity space given above, because its elongation component along the rows is taken into account by the equation 2, which is our primary interest here. However, we note its importance for the behaviour of our 'blocks' (i.e., grains) in cataclastic rocks by leading to internal deformation and/or further disruption of the grains. The intersection of a velocity field possessing the properties governed by equations 1-3 and the assumptions I-III, and fixed with respect to a rock body, with the rock body itself, will generate a structural rock. Its kind, on the other hand, will depend on rate of shear and the ambient P and T and volatile kind and availability conditions where the shear is taking place. Figure 20 shows that the 'row-wise deformation' does indeed take place in sheared rocks (Gregg, 1978, figs. 2 and 3). It does so at all scales (for the megascopic end of the scale see esp. Dickinson, 1996). Our model also predicts reverse shear sense along the 'C'-planes of our model in a mylonite. Fig. 21 shows an example of such a case from the mylonitised orthogneisses of the Maggia Nappe in Ticino, Switzerland (Simpson, 1981, fig. 3. 39 C). Naturally, the model we present is unrealistically simple. It ignores rotations and considerable internal (both brittle and ductile) strain of grains that commonly characterise structural rocks. However, these features can be easily added to our model they would only greatly complicate the mathematical definition. For instance, individal 'grains' produced in our model may undergo rotations and induce strain onto their surroundings. The 'ridge-transform fault' system we describe may look as a result of such rotations and generate further strains within the grains or between the grains and/or an entirely new ridge-transform system may nucleate and evolve. But the important feature of our model is that it allows strain incompatibilities to develop in rocks so as to lead to grain-size reduction and grainsize growth by means of the postulated 'spreading' process. The spreading need not be expressed always by antitaxial fibre growth as we assumed. It may be syntaxial and would lead to the growth of the walls of the 'spreading centres.' The 'spreading' may also be accomplished by crystal plasticity and grain boundary sliding in mylonites, where, instead of brittle, well-defined ridge-transform systems, bands of strain of various widths may develop essentially playing the same r61e. If a velocity space gives rise to pure shear structures in a rock body, the resulting rock products are not structural rocks according to the definition we here adopt, because pure shear can only be maintained without also leading to considerable

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195

Fig. 20. A. Details of garnet grain shape from the Moretown member of the Missisquoi Formation in central Vermont, USA (Gregg, 1978, fig. 2, with permission). B. Gregg's (1978) reconstruction of the shear history of the granet grains shown in Fig. 20A (Gregg, 1978, fig. 3, with permission). Note the 'row-wise' movement of the individual pieces along shear ('C') planes. See Gregg (1978) for discussion and more examples.

simple shear in appreciable areas if there is concomitant volume loss. Some shales, for instance, may be compacted down to 38 % of their original thickness (Potter et al., 1980, p. 59), meaning that, if tectonically shortened, they would have been able to sustain a shortening of 62% by pure shear alone. This is considerable, but the shale will remain a shale after such a shortening unless metamorphosed. Cleavage generation by mechanical rotation of platy minerals or pressure solution alone do not change the rock type affected (e.g., a limestone with numerous stylolite horizons and correlative veins will be recognised as the parent limestone). Therefore,

A. M. C. ~eng6r and M. Sakm, c

196

5m

'

' '

Fig. 21. Sheared granitic gneisses exhibiting foliations showing 'reversed sense of shear' from the Maggia Nappe, Ticino, Switzerland. (From Simpson, 1981, fig. 3.39 C, with permission).

products of pure shear, unless accompanied by metamorphism, are not new rock types, but their old selves, only strained. That is why tectonites, defined by Sander (1930, p. 62), as those rocks, the relative motion of whose parts in their fabric can be summed, are not necessarily structural rocks (also see Turner and Weiss, 1963, p. 38). By contrast simple shear can entirely disrupt a preexisting rock and reconstitute it into a completely new rock type without any help from metamorphism. Perhaps one can draw an analogy here between the origin of sedimentary rocks and structural rocks. A rock is disintegrated by erosion, its products are transported elsewhere, where they are deposited to become sedimentary rocks by diagenesis. Similarly, a rock is disrupted by structural processes, its products are transported elsewhere, where they are 'deposited' to become structural rocks. As the limit between diagenesis and metamorphism is not always easy to draw, so it is also between the induration processes of structural rocks and metamorphism.

Types of structural rocks Conventionally one can divide structural rocks into fault rocks and m61anges. Chaos structure (Noble, 1941, esp. p. 964; Wright and Troxel, 1984, esp. figs. 7, 8, 9 and the geological map; for the interpretation here preferred, see Wernicke and Burchfiel, 1982, p. 109), broken formations (Hsif, 1968) and disrupted formations (Raymond, 1984) are intermediate between fault rocks and m61anges. Table I summarises the kinds and characteristics of the main structural rock types in terms of a classification adopted mainly from Higgins (1971) and Passchier and Trouw

Table 1 Classification of the primary structural rock types. Modified mainly from Higgins (1971), Raymond, (1984b) and Passchier and Trouw (1996). References indicated by superscript are as follows: 1. Higgins (1971), 2. Passchier and Trouw (1996), 3. Lundberg and Moore (1986) and Lucas and Moore (1986), 4. Hsti (1968, p. 1066), 5. Raymond (1984b), 6. Cowan and Miller (1981), 7. Noble (1941), Wright and Troxel (1969, 1984), Wernicke and Burchfiel (1982), 8. Cloos (1982, 1984), Cloos and Shreve (1988a, b) primary cohesion 1 (Incohesive rocks) 2 ~= Incohesive 9- breccia (>30% of angular grams) (D

R o c k s with primary c o h e s i o n 1 ( C o h e s i v e rocks) 2

E

Rocks without fluxionl structure

E, ,.o ~D

Fault gouge (dominantly fine-grained matrix; may be foliated, commonly phacoidal, fragments may be augen-shaped)

,

Micro- and macro-scale

Macro- and mega-scale

Rocks with fluxionl structure Micro- and macro-scale

.=, 9 ~D

C O M P L E X

Macro- and mega-scale

without

Broken Proto-mylonite formation (10-50% (original matrix) 1, 2 macrofabric, e.g. ~-~ Mylonite 1, 2 beds, in .~ (Foliated & "~ part usually > intact)4, 5 lineated; ~D elemets ,~ mostly w/monoclinic ",~ Cataclasite 1 .~ DisE symmetry; 50-90% membered (Cohesive =~ formation "E matrix) cataclasite) 2 (90% bedding, matrix) 1,2 destroyed) 5 Micro-fault breccia (>30% of angular grains)

Rocks with fluxion structure Exotic blocks without

with

Exotic blocks

Exotic blocks without with

9

Neomineralisation-recrystallistion dominant over cataclasis 1

Cataclasis dominant over neomineralisation-recrystallistion 1

&

-- Incohesive = cataclasite (50m

Fig. 28. Idealised columnar section of the condensed sequences preserved in karstic caves in Jurassic limestons of west Sicily (from Ager, 1981, fig. 3.1, with permission). If the reader would turn the picture upside down, the Toarcian to Kimmeridgian condensed sequence in older Sinemurian-Pliensbachian rocks would be like an igneous intrusion from below. As it is its stratigraphic features are identical to a would-be layered mafic intrusion injected from the top.

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remarked, 'Logic requires that a single system of rules be applied to all bodies of rock so as to partition the earth's crust comprehensively into conterminously defined lithic units.' Figure 29 gives an example how this m e t h o d o l o g y may be appiled to structural rocks. It shows Higgins' 'hypothetical fault zone' and our stratigraphic classification of it (we deliberately chose Higgins' hypothetical example to show a large range of rock types in distinct units). It is a 1/75,000 scale m a p and all rock bodies visible at this scale can be m a p p e d as formations. There are seven rock types, but X X I V different rock bodies. It is clear that rock bodies I-VI, I X - X , X V I - X V I I , X X - X X I V must be m a p p e d as separate formations. By contrast, VII and VIII may be m a p p e d as members of the formation d, X I - X I V as members of g and X V I I I X I X as members of i, because they seem to be genetically more closely related to their associated 'formation' with which they are in contact than to other rocks. If m a p p i n g reveals clear field evidence that any of the similar rock types were once contiguous bodies, then those bodies may be grouped into one formation. Any m a p p a b l e variations in the rock types or other visible characteristics such as texture within the identified formations shown in Fig. 29 may be further separated

Fig. 29. Higgins' (1971, fig. 30) geological map of a hypothetical fault zone showing typical relations of various types of structural rock. Roman numerals indicate the separate rock bodies into which the outcrop of the shear zone is divided whereas lower case letters indicate the formations into which we have divided the rock bodies. Lower case letters with superscripts are members of the associated formations.

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out as members. The entire fault rock zone illustrated in Fig. 29, on the other hand, may be designated as a Group. These rocks obey all the usual stratigraphic sequence criteria, provided the processes responsible for those criteria are kept in mind: Any sedimentary rock unconformably covering the fault rocks are younger than them, so are any igneous or neptunian dykes cutting them. Any rocks cut by the fault zone in which the fault rocks formed are older than the fault rocks. Careful structural work may reveal the younging direction of the fault rocks and these may be checked by such methods as fission track dating on epidotes (Bar et al., 1974) or K - A r dating of illite crystals within fault rocks (e.g., Shafiqullah et al., 1988). The thing that needs to be kept in mind is that stratigraphic relationships are expressions of geological processes and without reference to them no stratigraphy can be done. An empirical stratigraphy such as that Werner and Cuvier dreamed of and most of our current stratigraphic guides and text-books still recommend that makes no reference to hypothetical reconstructions of processes, is an empty dream that cannot be realised. CONCLUSIONS The purpose of this paper was to underline that rocks forming in shear zones within the lithosphere by disrupting and reconstituting pre-existing rocks through shear processes make up a considerable portion of the earth's rocky rind and they should be recognised and mapped as such. We find such recommendations as 'Mylonite should therefore not be used as a rock name in a stratigraphic sequence' (Passchier and Trouw, 1996, p. 102) as fundamentally mistaken and misleading, because, if followed, it would result in eliminating from the geological maps of not inconsiderable areas whole bodies of rock, whose message concerning the geological history of the area are critically important. Rock types are determined according to the processes that f o r m them. Therefore, rock classification is not empirical but entirely genetic, as the names of the major rock groups, namely sedimentary, igneous and metamorphic, clearly imply. This is further underlined by the late recognition of metamorphic rocks and, later still, of structural rocks. In the foregoing paragraphs we have presented a historical discussion of the characteristics of the major rock groups within the context of the growth of understanding concerning their origin. That discussion has shown that all rock groups form discrete rock bodies within the lithosphere with spacial features resulting from their peculiar modes of formation that allow inference of their temporal relations with one another. That is why we chose to define stratigraphy as a science that infers temporal relations from spacial ones. All the temporal aspects of sedimentary rocks are functions of deposition in a fluid environment, whereas all temporal aspects of igneous rocks result both from the process of intrusion into, or extrusion onto, pre-existing rock bodies and sedimentation within the intruded magma pockets or extruded lava lakes. In case of sedimentary rocks (and very rarely in igneous, metamorphic and structural rocks) biozones represent geometric bodies that can penetrate into rock bodies and thus provide an independent temporal control. Temporal aspects of metamorphic rocks are ultimately tied to the movement of temperature and pressure fronts within rock bodies. That generally temperature is a more efficient factor in metamorphism and

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that p r o g r a d e m e t a m o r p h i s m tends to leave a m o r e widespread signature t h a n r e t r o g r a d e m e t a m o r p h i s m are accidents of the kinetic characteristics of m e t a m o r p h i c reactions a n d do n o t affect the principle of the t e m p o r a l significance of m e t a m o r p h i c fronts (but they do affect the o p e r a t i o n of the geologist in the field) as R e a d (1949) emphasised m o r e t h a n half a century ago. T e m p o r a l aspects of structural rocks d e p e n d mostly on cross-cutting relationships a n d principles of structure superposition (which, by the way, is a stratigraphic principle). Simple rules of t h u m b , such as the n a r r o w i n g of newly-developing shear zones versus the widening of old, e s t a b l i s h e d shear belts of c o n s i d e r a b l e d i s p l a c e m e n t are useful as w o r k i n g hypotheses, but need to be checked against stratigraphic and isotopic age data. As H a r l a n d (1992) e m p h a s i s e d , the e a r t h ' s l i t h o s p h e r e m u s t be divided c o m p r e h e n s i v e l y into c o n t e r m i n o u s l y defined lithic units. Each one of these units is the result of one or m o r e geological process. The i n t e r p r e t a t i o n of the spacial relationships of these c o n t e r m i n o u s l y defined units in the light of their f o r m a t i v e processes to infer their t e m p o r a l relationships is the c o m p r e h e n s i v e m e t h o d of s t r a t i g r a p h y a n d the ultimate goal of geology. Figure 30 shows c o n t e r m i n o u s rock

Fig. 30. A portrayal of the earth's crust in terms of sets of rock-, taxon-, temperature-, pressure-, volatileand velocity-space points. RI-R8 are rock-stratigraphic units (of any rank); BI-B7 are taxon-range zones (of any taxon level) and PZ I-III are polarity zones of the magnetic field (no chemostratigraphic zones are shown so as not to render the picture undecipherable). These are the permanently-fixed components of the earth's crust unless later destroyed by erosion, metamorphism or cataclasis. T is a set of temperature-value points, P, a set of pressure-value points, H, a set of volatile points, V a set of velocity values forming a velocity-space. These latter are evanescent and continuously change in time and affect the rocks and their fossil contents, i.e. the biozones. The velocity-space here depicted need not have the restrictions we put on it in our Fig. 18 above, and may thus represent erosion and deposition as well as structural rock genesis. This diagram shows why the earth's crust (and the lithosphere) should be divided comprehensively into contiguous rock units using a single rock-stratigraphic classification system. All sets shown in this figure (representing the full range of the geological processes) other than the rock sets are modifiers on the rock sets.

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bodies as r e p r e s e n t e d by sets with square outlines for g r a p h i c effect. Elliptical sets of t a x o n r a n g e zones p e n e t r a t e t h e m p e r m a n e n t l y , giving rise, in their m u l t i f a r i o u s relations o f intersection, inclusion a n d exclusion, to diverse o t h e r kinds of b i o z o n e s (Fig. 2C). Sets of t e m p e r a t u r e - (T), pressure- (P), volatile- (H) a n d velocity-space (V) p o i n t s go in a n d o u t o f them, creating, at their areas of influence and, at their intersections, a c o m p l e x a r r a y o f m e t a m o r p h i c , igneous, a n d structural r o c k types a n d influencing the r e c o r d of the b i o s t r a t i g r a p h i c a n d m a g n e t i c p o l a r i t y zones. T h u s , the geological time unrolls to c o m p o s e the w o n d e r f u l epic o f this beautiful p l a n e t o f ours, which is the great g o o d f o r t u n e of the geologist to h e a r a n d to a d m i r e . . . a n d to find solace in the u t t e r evanescence o f all things, except the intelligence t h a t desires to c o m p r e h e n d them.

ACKNOWLEDGEMENTS This p a p e r is an o u t g r o w t h o f the s t r a t i g r a p h y a n d historical geology courses we h a v e j o i n t l y t a u g h t over the years a n d d u r i n g which h a d to p r o n o u n c e the n a m e K e n Hsfi to o u r students m a n y times over. It is a small t o k e n o f o u r g r a t i t u d e to h i m for the things he t a u g h t us. A p a p e r of this scope leaves us u n d e r so m a n y o b l i g a t i o n s t h a t to a c k n o w l e d g e t h e m all w o u l d necessitate a list t h a t w o u l d rival its bulk. But we wish to t h a n k p a r t i c u l a r l y o u r s t u d e n t s at the i s t a n b u l T e c h n i c a l U n i v e r s i t y w h o f u n c t i o n e d as g u i n e a pigs as we tried o u r ideas a n d a p p r o a c h e s o n t h e m , B r i a n W e r n i c k e , to w h o m we owe o u r k n o w l e d g e o f the c h a o s structure, a n d X a v i e r Le P i c h o n w h o kindly read a n d c o m m e n t e d on o u r s t r u c t u r a l rock model.

NOTES 1. See Cox (1942) for a facsimile of this note expressing admiration for the 'wonderful order and regularity with which Nature has disposed of these singular productions and assigned to each class its peculiar Stratum.' Note the absence of any time connotation here. We know from Smith's later writings that, according to him, repetition of similar strata might bring about repetition of the same fossils 5. la Baldassari! (see below). 2. Presumably the MS shown to him by W. Reynolds. 3. Not mentioned by Coleman (1964), nor by Rudwick (1976). Cuvier's discovery 3f the distinctness of the Mammoth and thus implicitly of its extinct status was thus earlier than even the first private notes of Smith. 4. In cave deposits in Bavaria. See Rudwick (1997a, p. 22, note 17). 5. Cuvier soon identified this animal as being a sea lizard, related to monitors. It was in fact the first Mosasaurus fossil ever found (Rudwick, 1997a, p. 24, note 18, wrongly attributes the name Mosasaurus to Cuvier; it was in fact coined by William Conybeare, in Parkinson's Outlines of Oryctology, 1922: see de Graaf and Rompen, 1995 and Bardet and Jagt, 1996). 6. Rudwick (1997a, p. 24, note 18) points out that in reality the 'deer antlers' were remains of Cretaceous marine turtles, which Cuvier correctly identified once he saw the fossils himself. 7. This is the animal Cuvier was soon to call Megatherium, the giant sloth (cf. Rudwick, 1997a, pp. 25-32). 8. The absence of human bones in strata containing remains of extinct animals was hotly defended by Cuvier and after his death the debate lasted until the great discoveries in the Milhill cave, Brixham, in 1858! Here William Pengelly, a former champion of John MacEnery who had dug Kent's Hole to find

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human implements mixed with fossils of extinct mammals, had supervised the digs. Pengelly had led Sir Charles Lyell through the excavations by the time the latter read his presidential address to the Geological Section of the British Association at Aberdeen. At this lecture, Lyell reversed his earlier opinion on the absence of human fossils mixed together with those of extinct mammals expressed earlier in his presidential address to the Geological Society in London in 1851 (cf. Bailey, 1962, pp. 181-183). Lyell's reluctance to accept the existence of human bones mixed with those of extinct animals stemmed from his religious desire to keep man a special creation, despite the fact that the Bible expressly mentions ante-diluval humans, Noah being the best-known. Cuvier has often been similarly accused. We contend here that Cuvier's motives to keep man an entirely post-diluvial animal stemmed not from his religious convictions that God had specially created man after the last great catastrophe that had wiped out the ante-diluvial world, but from his desire to make biostratigraphy a neat tool to subdivide earth history. Fossil man's appearance in the rock record provided for Cuvier a lower limit to the post-diluvial deposits, while the disappearance of the ante-diluvial fauna an upper limit for the diluvial deposits. The heat of the debate in the scientific circles was fuelled because of the stratigraphical, and thus historical geological, and thus geodynamical considerations and the debate raged significantly between those whom Seng6r elsewhere identified as the predecessors of the KoberStillean catastrophist/regularist school ($eng6r, 1982a, b; 1991, 1996, 2000), who believed no humans could have co-existed with antediluvial animals, and those who were the predecessors of the WegenerArgandian actualist/irregularists, who believed in the possibility of an antediluvial human fauna. This is clearly shown by the conversion of Lyell, at the time of the debate the leader of the latter school, despite his religious convictions. Jules Verne provided possibly the best outline of the character of the fundamental nature of the debate as conceived by the enlightened public in the mid-nineteenth century, from the mouth of the hero of his Journey to the Centre of the Earth, Professor Otto Lidenbrock, though regrettably introducing it with what later turned out to be forgeries of stone axes and, in one case, a human jaw bone, introduced by workmen into Boucher's de Perthes pits in the quarries of Moulin-Quignon, near Abbeville, in the department of the Somme (cf. Bailey, 1962, p. 183, about the forgery). Professor Lidenbrock appears in Verne's novel not as a paleontologist, but a mineralogist/geologist interested in the evolution of the earth, an enthusiastic opponent of the views of Elie de Beaumont, and it is from that angle that he rejoices upon stumbling upon a human skeleton, later a specimen alive, contemporaneous with mastodons! (see Verne, [1864], ch. XXXVIII). Verne's grouping of l~lie de Beaumont with Cuvier underlines the issue at stake. Verne (himself a religious man, indeed an 'intelligent, but orthodox Roman Catholic,' especially before 1870; later his religious views broadened to embrace a sort of pan-Christian deism: Verne, 1976, p. 9; also see pp. 36, 206) puts not one word about the religious implications of antediluvial man into the mouth of his hero during his enthusiastic subterranean lecture about the significance and the history of discovery of antediluvial man, but heavily underlines his significance in the geological history of the globe. 9. At the time Cuvier mentioned only the upper limits. If anybody is inclined to think that he might have assumed all animals to have the same lower limit, namely the Creation (e.g., Bourdier, 1969, esp. p. 41), we may perhaps remind that his insistence that no fossil human bones had been found shows that he was indeed aware of different lower limits exhibited by different animal groups. 10. This text was signed jointly by Leli6vre, Hatiy, and Cuvier, but Cuvier had written it alone and read it (Coleman, 1964, p. 113; also see Smith, 1993, items 209 and 272, and p. 234, where Smith pointed out that 'Cuvier was presumably the author of those reports for which he served as reporter' as was the case here). The text of our quotation is from an anonymous English translation published in 1809 in the Annals of Philosophy, v. 33, pp. 315-316, which we altered in a few places to make it correspond better with the original. See Rudwick (1997a, pp. 106-108), for an independent translation; Rudwick does not refer to the earlier anonymous translation that we had been using in improved form as reading material for our classes in Istanbul since the early nineties. The report was also published in 1807 in Journal des Mines, v. 21, pp. 413-430 and in the ProcOs verbaux of the AcadOmie des Sciences, 1913, 3(1804-1807), pp. 408-4 13. 11. We disagree with Coleman's verdict that Cuvier was not a geologist, but only a palaeonotlogist, and that his most unoriginal contributions were those he made to geology (Coleman, 1964, pp. 114 and 115), which we think is unjust on a man who founded biostratigraphy as a scientific discipline on the joint pillars of palaeontology and geology. Here, we find ourselves in agreement with the opinion that

214

12.

13. 14.

15. 16.

17. 18.

19. 20.

21.

22. 23.

A. M. C. ,,~eng6r and M. S a k m l

Karl Marx once expressed that Cuvier was not just a geologist, but that he was a 'great geologist' (Marx, 1866[1965], p. 257). In the separate edition of the Discours under the title Discours sur les Rbvolutions de la Surface du Globe, et sur les Changemens qu'elles ont Produits dans le R@ne Animal (Cuvier, 1825), pp. 58-59. The passage above we have quoted from the first English edition of the Discours under the title Essay on the Theory of the Earth, translated by Robert Kerr with Robert Jameson's notes (Cuvier, 1813), pp. 54-55 with slight alterations to improve the conformity to the French original. See Rudwick (1997a, p. 205) for an independent translation. Herodotus' 'Red Sea' is our Gulf of Aden and Indian Ocean; his 'Arabian gulf' is our Red Sea. The English text we give here is Anderson's translation (Werner, 1809, p. 29). We only changed Anderson's vein, which he used to translate Gang, to dyke, despite the popular knowledge of Werner's theory in the English-speaking world as 'theory of veins.' For the history of the word 'magma,' see Bonin et al. (1997, pp. 31-33) and Gohau (1998). Bonin et al. (1997, p. 28) write that l~lie de Beaumont distinguished a normal metamorphism from an abnormal one and quote Frapolli (1846[sic], pp. 615[sic] and 625) as their authority. The Frapolli reference should be 1847 instead of 1846 and p. 615, 616. Moreover, Frapolli refers to Vogt's textbook (1846) as his source, which Vogt says is based on l~lie de Beaumont's lectures in the l~cole des Mines (Paris). Frapolli indicates that l~lie de Beaumont had spoken of these ideas already in 1833 in the very year Lyell coined the term 'metamorphic rocks' (Lyell, 1833, pp. 374-375). On p. 247 Vogt says expressly that l~lie de Beaumont spoke of an accidental metamorphism and this could be contrasted with a normal metamorphism. On his p. 625 Frapolli's (1847) text reads as if the term abnormal metamorphism is not due to l~lie de Beaumont. In this paper we use accidental metamorphism as l~lie de Beaumont's term. One can add more sets to this picture to represent, for instance, pore water or CO2 as metamorphic catalysts. This is our translation of the main title of Werner's famous book Neue Theorie vonder Entstehung der Gginge (1791). In the English translation by Charles Anderson, published in 1809 in Edinburgh, the title was translated as New Theory of the Formation of Veins. In his book Werner discusses both veins and dykes as being the result of the same phenomena. '...das Ganggestein mit dem Gebirgsgesteine verbunden, und, so zu sagen in solches verflossen...' But one should remember that even in our own day, shear zones containing cataclasites and mylonites are at times likened to dykes and indeed in many instances, structural rocks behave like intrusions. Hst~ (1955, p. 252) followed, for instance, the example of Hammer (1914) in likening mylonites to dykes. He wrote that 'More commonly, mylonitic rocks superficially resemble dikes about a foot thick. They are bounded on both sides by thrust planes, and the direction of movement is indicated by the drag or bending of the layered granulites near the "dike." In places, the larger "cataclastic dikes" send offshoots into granulites along their layering planes; the offshoots end abruptly against granulite (pl. 33 [herein Fig. 11]. Local, podlike bodies of metasomatized mylonites, in the neighborhood of, yet apparently not connected with, the main part of the "dike," have also been observed.' M61ange bodies also form intrusions, including those in dyke form (e.g., Cloos, 1984). To see how reasonable Baltzer's guess was, consider that the lithostatic overburden over the Glarus thrust at the Gl~irnisch was at least 3 km (i.e. about 1 kb or more and 100~ to 200~ at most: see Trfimpy, 1980). If we assume that the thrust was moving between Anomaly 6 time (19.4 Ma ago) and Anomaly 5 time (8.9 Ma ago) and that it probably accommodated at least some 30 km shortening (cf. Trfimpy, 1980; Dewey et al., 1989) we get something like a rate of 3 mm/a. This is an absolute minimum, because the time interval of the movement is chosen to be a maximum (which easily accommodates the time constant of 107a), whereas the shortening was taken to be a minimum. The rate of motion could easily have been of the order of 1 cm/a. In that case, considerable shear heating becomes inescapable along the thrust plane (cf. Scholz, 1980). Observations indeed suggest that the thrust plane was heated up to 400~ (Breigel and Goetze, 1978), despite the extremely fine-grained original texture of the Lochseitenkalk (1 ~tm). Note that Shackleton had earlier used the term m~lange to describe a series of subaerial volcanic slump and flow deposits in the Kathwanga area of the Kavirondo Rift Valley (Shackleton, 1951, p. 360). Chaput likened these rocks to the black limestones he had found in Dikilita~ to the west of Ankara that contained both Staffella and members of the group Craticulifera, thus giving a latest

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24.

25. 26.

27.

28.

29. 30.

31.

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Carboniferous ('Ouralien' of Chaput, 1936, p. 35) to Permian age. Prof. O. Erol (personal communication, 15th December, 1994) tells us that this Dikilita~ could not be identified any more and suspects that Chaput had consulted local peasants and jotted down a possibly traditional, but no longer current name he was told. Professor Erol (personal communication, 15th December, 1994) remembers that even then (19451946) they used the terms matrix and block to describe what they were seeing. Eventually they even distinguished in the field a sedimentary matrix consisting mainly of slates with subordinate sandstones from an igneous matrix formed dominantly from oxidised mafic volcanics and mafic tufts. The Harita Dergisi (Cartographic Journal) Erol's critique of Chaput is here somewhat unjust, but it is not his fault. He was reading only a poor Turkish translation by Prof. Hfimit Nfifiz [Pamir], of Chaput's article, which had been published bilingually, for Erol could not read French. What Chaput actually had written was that the ancient massif of Elma Dafgl and its Mesozoic cover had been in places strongly broken and imbricated indicating a push towards the north-west (Chaput, 1931, p. 28). What Erol had found was the opposite vergence. This map is still unpublished. Petty professional jealosies between geographers and geologists in Turkey were the main cause why the Mineral Research and Explortion Institute did not publish Erol's map. Erol recalls with gratitude that the text of his thesis and a generalised 1/100,000-scale map only could be published by that institute owing to the beneficial intervention of Professor Hamit Nafiz Pamir. As a result, Erol's careful work on the m61ange was not followed up for years and only in the 70's did we begin to understand its tectonic significance in Turkey. This is not entirely correct. Erol's main mapping was carried out on a base with scale 1/25,000, but his eventual thesis map had to be of 1/100,000 for not the entire thesis area (some 1500 km 2) was covered by the larger scale maps. His main area of concentration, however, was in the smaller region mentioned above covered by the 1/25,000-scale maps. We should recall that both Ketin and Sir Edward Bailey and W. J. McCallien thought, following the conventional wisdom of the day, that serpentines had been originally submarine volcanic rocks. We now know that Ketin had grossly underestimated the structural complications in his area (see his marvellous recantation in the great synthesis on the distribution of thrusting in time and space in the geological history of Turkey, which he wrote for the symposium organised by the Turkish Geological Society upon the occasion of his retirement from active teaching in 1983: Ketin, 1985). But in essence he was right: The profile we quote above is now known to belong to a thrust-stacked sequence within an accretionary complex of late Cretaceous age that was thrust on top of the Klr~ehir Massif in the earliest Maastrichtian to earliest Eocene interval (see G6rfir et al., 1984, esp. fig. 9). The Klr~ehir metamorphics are pre-medial Cretaceous in age and Dr. Necdet Ozgiil (pers. comm. 1985) correlated them with the Paleozoic-Mesozoic Taurus sequences, i.e. a Gondwana-Land sequence south of the Neo-Tethys. Their major metamorphism was thus possibly sometime in the medial Mesozoic, probably early to medial Cretaceous. Ketin made the same sorts of mistakes Argand had in the Alps, with similar fruitful results. Seng6r recalls, with very nostalgic feelings, how his friends and later colleagues Professor ihsan Seymen (a former doctoral student of Ketin) and Professor Fazh Y. Oktay showed him, in a terribly hot summer day in 1977, Ketin's locality west of Kaman, where he had observed the 'concordant' position of the serpentinite-radiolarite series on the metamorphic rocks of the Klr~ehir Massif. 'If Ketin had just looked another kilometre or so around the corner,' Seymen said, pointing to the slope, 'he would have seen the shallow thrust contact truncating the metamorphics and he would have understood that he was just looking at an accordance.' Seng6r and Yllmaz (1981) followed Ketin's assessment of the age of the Klr~ehir metamorphics and considered the Klr~ehir as a simple continuation of the Apulian platform eastward. This mistake was corrected by G6rfir et al. (1984), who showed that the Klr~ehir, indeed a part of the northern margin of Gondwana-Land until sometime in the Triassic, parted company with Apulia later (this recognition became possible when Naci G6rtir documented the subsurface connexion between the suture south of the Klr~ehir massif, discovered in the early seventies by F. Y. Oktay, with the izmir-Ankara suture of Seng6r and Yllmaz, 1981). For the present state of the art on the story of the Ankara M61ange and the Klr~ehir Massif see esp. two studies: G6rtir et al. (1984) and Seymen (1985). The English version here is identical to that published in Nature with the exception of a few words and the figure added to the paper in Nature.

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Trfimpy, R., 1980. Geology of Switzerland--A Guide-Book, Part A: An Outline of the Geology of Switzerland. Wepf & Co., Basel, 104 pp. + lfoldout plate. Triimpy, R., 1991. The Glarus nappes: A controversy of a century ago. In: Miiller, D.W., McKenzie, J.A. and Weissert, H. (Eds.), Controversies in Modern Geology (Hsii-Festschrift), Academic Press, London, pp. 385-404. Turner, F.J. and Weeiss, L.E., 1963. Structural Analysis of Metamorphic Tectonites. McGraw-Hill Book Co., New York, ix + 545pp. Vad~sz, M.E., 1913. Liasfossilien aus Kleinasien. Mitt. Jb. Geol. Reichsanst. (Budapest), 21" 57-82. Verne, J.-J., 1976. Jules Verne: a Biography (translated and adapted by Roger Greaves). An Arnold Lent Book, Taplinger Publishing Company, New York, xii + 245pp. Vogt, C., 1846. Lehrbuch der Geologie und Petrefaktenkunde Zum Gebrauche bei Vorlesungen und zum Selbstunterrichte Theilweise nach L. l~lie de Beaumont's vorlesungen an der Ecole des Mines, Erster Band. Friedrich Vieweg und Sohn, Braunschweig, XIX + 436pp. Waters, A.C. and Campbell, C.D., 1935. Mylonites from the San Andreas Fault Zone. Am. Jour. Sci., 5th Ser., 29: 473-501. Waterston, C.D. (1965). William Thomson (1761-1806): a forgotten benefactor. University of Edinburgh Journal, 22: 122-134. Werner, A.G., 1791. Neue Theorie von der Entstehung der G~inge mit Anwendung auf den Bergbau besonders den freibergischen. Gerlachische Buchdruckerei, Freiberg, 256pp. Werner, A.G., 1809. New Theory of the Formation of Veins; with its application to the art of working mines, translated from the German, to which is added an appendix, containing notes illustrative of the subject; by Charles Anderson. Archibald Constable and Company, Edinburgh and Constable, Hunter, Park and Hunter, London, 259pp. Wernicke, B., 1985. Uniform-sense normal simple shear of the continental lithosphere. Canadian Journal of the Earth Sicences, 22: 108-125. Wernicke, B. and Burchfiel, B.C., 1982. Modes of extensional tectonics. Journal of Structural Geology, 4: 105-115. White, S. H., Drury, M. R., Ion, S. E. and Humphreys, F. J., 1985. Large strain deformation studies using polycrystalline magnesium as a rock analogue: Part I. Grain size palaeopiezometry in mylonite zones. Physics of the Earth and Planetary Interiors, 40: 201-207. Whitehurst, J., 1778. An Inquiry into the Original State and Formation of the Earth; Deduced From Facts and the Laws of Nature to Which is Added an Appendix Containing Some General Observations of The Strata in Derbyshire. Printed for the Author by J. Cooper, London, ii + [ii] + 199 pp. + VI plates. Wilcox, R.E., Harding, T.P. and Seely, D.R., 1973. Basic wrench tectonics. American Association of Petroleum Geologists Bulletin, 57: 74-96. Wilson, G., 1960. The tectonics of the 'Great Ice Chasm', Filchner Ice Shelf, Antarctica. Proceedings of the Geologists' Association, 71(2): 130-138. Winter, J.G. translator and commentator, 1916. The Prodromus of Nicolaus Steno's Dissertation Concerning a Solid Body Enclosed By Process of Nature Within A Solid. University of Michigan Studies, Humanistic Series, Vol. XI--Contributions to the History of Science--Part II, The Macmillan Company, New York, pp. 166-283. Wise, D.U., Dunn, D.E., Engelder, J.T., Geiser, P.A., Hatcher, R.D., Kish, S.A., Odom, A.L. and Shamel, S., 1984. Fault-related rocks: Suggestions for terminology. Geology, 12: 391-394. Wood, D.S., 1974. Ophiolites, m61anges, blueschists, and ignimbrites: early Caledonian subduction in Wales. In: Dott, R.H., Jr. and Shaver, R.H. (Eds.), Modern and Ancient Geosynclinal Sedimentation, SEPM Spec. Pub., 19: 334-344. Woodward, H., 1902. Life of William Smith the "Father of English Geology". Proceedings of the Bath Natural History and Antiquarian Field Club, 10: 1-14. Woodward, H.B., 1907. The History of the Geological Society of London. Geological Society, London, xix + [i] + 336pp. Woodward, H.B., [1911]. History of Geology. Watts & Co., London, 154pp. Wright, L.A. and Troxel, B.W., 1969. Chaos structure and Basin and Range normal faults: evidence for a genetic relationship. Geol. Soc. Am. Spec. Pap., 121: 580-581. Wright, L.A. and Troxel, B.W., 1984. Geology of the Northern Half of the Confidence Hills 15-Minute Quadrangle Death Valley Region, Eastern California: the Area of the Amorgose Chaos. State of

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California, The Resources Agency, Department of Conservation, California Division of Mines and Geology, Map Sheet 34, Plate I, explanatory text, vi + 31pp. Yardley, B.W.D., 1989. An Introduction to Metamorphic Petrology, Longman Scientific & Technical, Essex, xii + [i] + 248pp. Zinovyev, S.V., 1992. Stress-metamorficheskie Kompleksy Buchtarminskogo zvena Irtyshskoi Zoni Smyatiya. Rossiskaya Akademiya Nauk, Sibiskoe Otdelenie, Obedinennyi Institut Geologi, Geofiziki i Mineralogii, Novosibirskii Gosudarstvennii Universitet, Novosibirsk, 130pp. Zoback, M.L., Jachens, R.C. and Olson, J.A., 1999. Abrupt along-strike change in teconic style: San Andreas fault zone, San Francisco Peninsula. Jour. Geophys. Res., 104: 10,719-10,742.

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Part II

MECHANICS OF OVERTHRUSTING AND LANDSLIDING

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Paradoxes in Geology Ueli Briegel and Wenjiao Xiao, eds. 9 2001 Elsevier Science B.V. All rights reserved. Chap ter 10

ROCK M E C H A N I C S AND THE P A R A D O X OF O V E R T H R U S T I N G TECTONICS

Ueli Briegel* Swiss Federal Institute of Technology, D-ERD W, ETHZ, CH-8092 Ziirich, Switzerland

ABSTRACT The phenomenon of overthrusting was recognised by geologists some 150 years ago. Since the beginning of this century, geologists and engineers have been trying to understand the mechanics and kinematics of these large-scale structures. Different approaches to the generation of the thrustplane and the movement along the existing decollement-zone have been reported. During the last 40 years, experimental rock deformation has developed into the key discipline to produce numbers for the physical properties of the different rocks involved. Ken Hsti set up a Rockdeformation Lab at ETH, and his students have contributed substantially to the understanding of the behaviour of different rocks in overthrust-settings such as Calcite, Anhydrite, Gypsum, Clay and synthetic mono- and bimineralic rocks. Such characteristic values can be used to formulate mathematical models in order to evaluate the possibility and credibility of the kinematic models proposed. Although we have identified today a large amount of physical properties for minerals and rocks, the extremely complex matter of overthrust tectonics is far from a final solution. Too many parameters are to be considered simultaneously whereas the present experimental techniques do not allow to handle more than just a few at a time. The paradox persists. INTRODUCTION At the turn of the eighteenth century, Elie de Beaumont's geological view of the world was still prevalent: all sediments were laid down on a preexisting relief of crystalline and volcanic rocks. Today's inclined portions were accumulated on the flanks of such early basins (Fig. 1). So displacing, folding and thrusting had to be "invented" yet. Probably the first overthrust was the one reported by Weiss in 1826, the "Lausitzer Hauptverwerfung" in Sachsen Germany, as quoted by Rothpletz in 1894, where a granite sits on top of Turonian marls. However, at that time, relative dating

*E-mail. [email protected]

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Fig. 1. Section of Elie de Beaumont's "Versuch einer Zusammenstellung der relativen Alter gewisser Lagen, und gewisser Systemevon Gebirgen".

was only possible with fossils and the relationship between Granite and Marl was not at all clear. One of the first reported overthrusts of sediment over sediment is the Glarus overthrust. In 1840, the Swiss scientist Arnold Escher recognized the inverse strata in that area. However, he did not dare to publish his finding. Years later, he visited the Sernftal with the honourable Scotsman Sir Roderick Impey Murchison and wanted to be confirmed by that famous geologist. Murchison published the section of the Tschingelh6rner in 1849. Two sentences of that publication should illustrate how new this idea still was (p. 248) (Fig. 2): I was convinced that M.Escher was correct in his delineation and mapping of the ground, although he ingenously urged me to try in every way to detect some error in his views, so fully was he aware of the monstrosity of the apparent inversion. And Murchison writes further: I dare not pretend to offer an explanation of the "modus operandi" by which such a marvellous mutation of order has been produced over so vast an area. However he states clearly that no folding seems to be involved (Fig. 3). Contrary to that statement, Albert Heim in 1878 postulated his "Glarus Doublefold" and kept up this paradox for another 20 years, although Marcel Bertrand reinterpreted Heim's sketch correctly in 1884 without ever having been in the area. The famous Mointhrust in Scotland was recognised only later (1861) because the age of the overlaying Gneiss had been debated for a long time.

F I R S T A P P R O A C H E S TO T H E O V E R T H R U S T P R O B L E M At the beginning of this century, many got interested in the mechanics of overthrusts. In 1909, Smoluchovsky showed that large overthrusts are not possible. He imagined a granite block 100 miles, long and calculated the force to be applied for a movement at its base (Fig. 4).

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Fig. 2. Murchison's section from Elm to Flims.

Fig. 3. The Glarus "Double-Fold" by Heim, 1882.

Fig. 4. Smoluchovsky's consideration. The same force (pressure) would occur at the base of a column of rock with height b . e . Assuming the coefficient of friction for iron on iron as 0.15 and b = 100 miles, the pillar would measure 15 miles, which is obviously impossible. He speculated that either one would need a slope of 1:6.5 or the coefficient of friction was much lower. At that time and in the following decades, not much was known about the physical properties of rocks. This changed in the fifties, when the first experimental data on rocks deformed under triaxial conditions became available from a few Labs in the United States. With sophisticated experimental techniques, the scientists try to simulate burial conditions. Inside high-strength pressure vessels, they apply a confining pressure on the jacketed sample, a rock cylinder of may be 12 m m diameter and 24 mm length. They may also apply a pore pressure while pumping water into the sample, a furnace will set the chosen temperature and a piston driven into the vessel at a certain speed will deform the sample at a specific strain-rate. They record the stress on the sample with a load-cell and the deformation with a strain-gage. The records of each experiment can be plotted in a stress-strain diagram. The mechanical behaviour of Solnhofen limestone is obviously changing around 100 mPa confining pressure at the stated conditions (Fig. 5). This change in

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Fig. 5. Four samples of Solnhofen limestone deformed at different confining pressures at room temperature and the respective stress-strain-diagram (Rock-def. lab ETH). deformation mechanism is also visible by looking at the deformed s a m p l e s - loss of cohesion through fracturing below and ductile deformation without visible damage above 100 m P a - as well as at the stress-strain c u r v e s - distinct stress drops below and stress increase above 100 mPa after reaching the ultimate elastic strength. The brittle-ductile transition is commonly demonstrated by the Mohr-Coulomb envelope (Fig. 6), a purely geometrical representation of the measured stresses in the Normal (x) and Shear-Stress (y) coordinate system. Several experimental results are plotted, e.g. the stress-value at the yield point preceding brittle failure is taken as the centre of a circle on the x-axes with radius corresponding to the confining pressure of the respective experiment. The envelope of these circles represents the boundary between the brittle and the ductile field. For a detailed explanation, please refer to Hubbert and Rubey, 1959 pp. 123-125. E F F E C T I V E PRESSURE CONCEPT In 1959, the famous and much-discussed papers by Hubbert and Rubey were published. They introduced the principle of effective pressure to the overthrust problem. From oil wells, they knew the occurrence of very high pore-pressures in certain formations and made two important assumptions to evaluate once more, the possible size of overthrust blocks. The fracture at the base, the detachment horizon, is developed continuously and propagates like a dislocation. Therefore, the force to produce it is quite small at any given time and can be neglected. Therefore r0 can also be neglected and azz equals pgz. This assumption was much debated later, also by Ken Hsfi (e.g 1969).

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Fig. 6. Mohr-Coulomb diagram for Volterra-Gypsum (Ko, 1993, p. 45). With the introduction of a high pore-fluid pressure ()~ =0.9), they calculate an 8 km thick block to be possibly as much as 137 km long. For gravitational gliding with the same )~, they need a slope of 3.3 ~ or 1.6 ~ with )~ =0.95 (Fig. 7). Finally, they admitted that the task was extremely difficult and more work had to be done. We agree to this statement at once, let us only look at the boundary conditions they used. The block does not encounter any resistance on the front plane! The ramp and the toe wedge were invented and again a debate started on the toe effect, once more alimented by Ken Hsfi (1970). Raleigh and Griggs (1963) suggested that an eroding toe will double the needed force and a "riser" would quadruple the force suggested by Hubbert and Rubey. R O C K M E C H A N I C A P P R O A C H E S TO THE P R O B L E M A N D THE ETH LAB F O R E X P E R I M E N T A L R O C K D E F O R M A T I O N More and more experiments in different Labs contributed to the knowledge of the physical properties of rocks. By 1966, all the available data had been collected in the GSA Memoir 97: " H a n d b o o k of Physical Constants" which is still a standard reference volume today. In 1969, Ken Hsti published "A Preliminary Analysis of the Statics and Kinetics of the Glarus Overthrust", delineating the size, temperature, velocity and morphology already quite close to later findings. He recognised the Lochsiten limestone as the plastically deformable lubrication material and suggested performing creep-tests on this material. Then came the time of improved calculators and many people started to model overthrust-mechanics. Again one of the crucial problems was the development of the decollement plane or zone. Ken Hsti initiated two very important projects at ETH: he started one of the first Rock-deformation Labs on mainland Europe (it is still the only one in Switzerland) and had one of his Ph.D. students model overthrusts with the finite element method. After a few years of development, we were able to join the scientific community with several major contributions.

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Fig. 7. Hubbert and Rubeys overthrust-block (1959 Fig. 23) compared to Raleigh and Griggs model of the Pine Mountain overthrust (1963 Fig. 6). The eroding toe is to the right, the riser in the middle of the sketch.

Briegel and G6tze on Lochsitenkalk We tried to estimate the differential stress recorded in the dislocation structure of Lochsiten limestone, comparing naturally and experimentally deformed rocks. The found 300~ and 200 mPa differential stress indicate probably the last increment and do not mirror the main overthrust conditions (probably 400~ and much lower differential stress below 100 mPa) (Fig. 8).

Schmid on mylonite and superplastic flow Using experimental results from London, Zfirich and Canberra, Schmid worked out the flow laws for Calcite rocks, plotted new Deformation Mechanism Maps and came up with extremly low shear stresses at the base of the Glarus overthrust (Fig. 9). From his Fig. 62 (400~ he extrapolated a shear stress of less than 1 mPa for a grain-size of 1 ~ and strainrate of 10-1~ The conditions assumed, however, are still quite vague. An additional 100~ e.g., would cause the strainrate to increase by 2 orders of magnitude for the same result, or at a grain-size of 100 ~t, the stress would reach 100 mPa. We do not know the grain size during the overthrusting phase, as the present 1 ~t of Lochsiten limestone indicates again most probably the very last increment of at least partially brittle deformation, nor do we know the exact temperature.

Rock mechanics and the paradox of overthrusting tectonics

237

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U. Briegel

238

Briegel and Mfiller on anhydrite F r o m a large number of experiments, we deduced the first flow law for anhydrite. This was a prerequisite for the attempt to model the Jura overthrust as outlined below.

MODELLING Mfiller and Hsti published in 1980, their results on the finite element analysis of thrust-block generation and the application to the Jura decollement. The boundary conditions have been improved a lot compared to Hubbert and Rubey (Fig. 7). In a first attempt, they found that it is impossible to generate a large overthrustblock within a homogenous rock body, as the strength in most elements near the moving end exceeds stability before a decollement plane is propagated. Therefore, they introduced a layered initial body with different rock-properties and indeed, the results became promising. A decollement zone is generated within a low-strength layer near the base of the body (Fig. 3 and 4, in Mfiller and Hsfi, 1980). With this background, with the experimental results for anhydrite (Fig. 10) and the assumption of a widespread evaporite zone below the Molasse basin, as confirmed in different bore holes, they tackled the paradox of the Jura decollement (Fig. 11). The "Fernschubhypothese", a push from the Alps (Laubscher 1961) in conjunction with some gravity effect across the Molasse basin could be negated

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240

U. Briegel

quite clearly. On the other hand, a mechanism of underthrusting seems feasible and the introduced instabilities showed up exactly where large faults are present today, at the boundary of the folded Jura against the plateau-Jura. M a n y more experimental investigations were made in the E T H Rock-deformation Lab on gypsum, anhydrite, clay, serpentinite and on a variety of synthetic rocks. ( H o s h i n o , Briegel, Mtiller, Schmid, B a u m a n n , J o r d a n , Nfiesch, O l g a a r d , Del'Angelo, Ko) F r o m this myriad of projects I may mention two very important results for the understanding of overthrust-mechanics. 1. Peter Jordan (1987) performed a series of experiments with two-phase synthetic aggregates of Calcite and Halite and came up with some astonishing results: The mechanical strength of a highly sheared polyphase rock is controlled by the weakest phase which is rather quickly forming continuous layers parallel to the bulk shear plane (c-surfaces) for simple shear. A starting material with a random distribution of the phases will reach this ideal low strength configuration by a mechanical redistribution of the phases in the initial stage of the deformation. (Fig. 12) During this initial period, the strength is higher than the strength of the final foliated rock. The mechanical characteristics and, in particular, the ultimate bulk strength of the material is predominantly a function of the mineral composition of the rock and the strength of each phase, and is strongly influenced by the deformation mechanisms of each mineral, and the shape, orientation and distribution of the various mineral particles or domains. As a general rule, the weaker minerals have a much greater influence on the ultimate bulk strength than would be expected from their volumetric contribution. 2. Suzchung Ko (1993) investigated thoroughly the influence of pore pressure due to d e h y d r a t i o n of h y d r o u s minerals. Large volume changes and water productions accompany several different mineral reactions, most of them known to play a role in tectonic and metamorphic active areas. The behaviour

Fig. 12. Deformed synthetic rock of Halite (black, 24% vol.) and Calcite (gray). The initially random distribution of the two phases is transformed in a shear-banding with thin bands of Halite. Between neighbouring Halite grains, tiny cataclastic shear-zones develop (white in picture b) and get filled with Halite. Such weak zones tend to propagate and finally build continous Halite bands as indicated by the arrows. Maximum principle stress is vertical (Jordan, 1987).

Rock mechanics and the paradox of overthrusting tectonics

241

of the affected rock depends much on the residence time of the fluid. If the porosity is low, pressure builds up (Fluid Volume always exceeds the solid Volume reduction) and the deformation tends towards the brittle field. In parallel, the friction along existing shearzones would decrease. For an evaporite decollement, as temperature increases with burial or just depth, gypsum will dehydrate releasing a considerable amount of water. Since gypsum flows plastically at very shallow depths at geologic strain rates, and because it is often associated with salt, it is likely to have a very low porosity and permeability prior to dehydration. Thus, fluid pressures can potentially build up to equal the overburden, reducing the effective pressure to zero (Fig. 13). After Ken Hsii's retirement, the Lab continued to develop. A torsion-apparatus (developed by Mervin Paterson of ANU, Canberra) was installed and Dave Olgaard, Lisa Dell'Angelo, Iona Stretton and others continued to contribute to the overthrust-problem. All Lab work (on: anhydrite, gypsum, shale and torsion of Solnhofen limestone and Carrara marble) concerned the strength and evolution of the decollement zone, in particular, mechanisms that would make it weaker. Dave Olgaard (pers.communication) summarizes: The dehydration of gypsum results in a large solid volume decrease, meaning a potentially large increase in porosity and permeability. Thus with time, the dehydrating matrix will not be able to retain the fluid, fluid pressure will decrease and the rock will again strengthen, both because of the increase in effective pressure and because the product solid phase, anhydrite, is stronger than the reactant gypsum. Below the gypsum dehydration depth, anhydrite deforming by dislocation creep can still form a decollement because it is weaker than dolomite and probably weaker than limestone, although this last point is debateable (Dell'Angelo and Olgaard, 1995).

Fig. 13. Volume change during typical single-phase dehydration reactions. The volume of water production is adjusted for the equilibrium temperature and pressure conditions of each reaction. (Ko, 1993 Fig. 1-1)

242

u. Briegel

However, if anhydrite undergoes a significant reduction in grain size by dynamic recrystallization, it can weaken significantly with strain and there will be a mechanism change from dislocation creep to "superplastic" or diffusion-controlled creep (torsion experiments by Stretton and Olgaard, 1997). Now anhydrite deformation is very grain size sensitive with fine-grained zones forming weak decollement. Shale can make a good decollement because it contains a lot of water and has a very low permeability even though the porosity is still quite high. Therefore fluid pressures are often very high and effective stresses very low. Unless the shales are strongly lithified, they will probably densify with deformation by shear-induced compaction. Thus shale decollemont zones may retain water. Dehydrating gypsum encased in shale is an ideal weak d e c o l l e m o n t - a large fluid source and no place for the fluid to go because the surrounding rocks have very low permeability. These ideas are based on the experiments on shales. The torsion experiments provide considerable insight into the development of shear zones and they have only begun to reveal the mechanisms of localisation. Solnhofen limestone maintains its fine grain size and the strength remains constant with strain at high temperature even though the texture changes significantly (Casey, Kunze and Olgaard, 1998). Presumably, there are enough impurities to prevent grain boundary migration and the deformation mechanism is grain size sensitive, but not texture sensitive. Carrara marble deforming at the same conditions does show some strain softening, and this time the mechanism appears to be rotation of the crystals into orientations for easy slip. However, what is interesting about all the Carrara work is that there was no evidence for localisation even though many conditions were ripe: (1) a very high n (=10) which means that a small local weakening will lead to a drastic increase in strain rate. (2) a drastic reduction in grain size by dynamic recrystallization and the conditions of deformation that are believed by some (e.g. Rutter) to be within a "grain-size sensitive" regime. (3) an observed strain softening in stress-strain curves without localisation (4) the argument that the experiment design prevents localization doesn't hold because if one does the calculations, more than 90% of the load (torque) is taken up by the outer half of the solid cylinder, over which the strain rate varies by only a factor of 2 (Paterson and Olgaard, 2000), (5) we often did observe localization in Carrara samples and once in Solnhofen, but in each case it was likely caused by sample inhomogeneity. OUTLOOK The paradox of overthrust-mechanism persists (Fig. 14). As we have seen, there are so many parameters involved, such as material, stratigraphy, grain-size, temperature, stress, porosity, strain-rate, fluid-chemistry, fluid-pressure, and all are somehow dependent on and interfering with each other. Look at Trtimpy's 1980 cartoon and you understand, that the problem is not so simple as the geometrically probably perfect models (e.g. for Pine Mountain, Fig. 6) may suggest. Even for a good experimentalist and/or modelist, only two or three of these parameters can be investigated at the same time. There are many more pieces to be

Rock mechanics and the paradox of overthrusting tectonics

243

Fig. 14. Geological (top) and palinspastic section through the central Glarus alps. (From Trfimpy, 1980 Fig. 24).

found or developed to see even an outline of the puzzle which Ken Hsfi and others started to hook together and whereof we have just a few boundary- or cornerpieces so far. REFERENCES De Beaumont Elie, 1838. "Elie de Beaumont's Versuch einer Zusammenstellung der relativen Alter gewisser Lagen und gewisser Systeme von Gebirgen." In: Geologische Elementarkarte l stes Blatt. Im Verlag des Landes-Industrie-Comptoirs, Weimar. Bertrand, M. 1884. Rapports de Structure des Alpes de Glaris et du bassin houiller du Nord. Bull. Soc.g~ol.de France, III e s~rie, t. 12, Paris. Briegel, U. and Goetze, C. 1978. Estimates of stress recorded in the dislocation structure of Lochseiten limestone (Switzerland). Tectonophysics, 48:61-76 Casey, M., Kunze, K., Olgaard, D.L. 1998. Texture of Solnhofen limestone deformed to high strains in torsion, J. Struct. Geol., 20[2/3]: 255-267. Dell'Angelo, L.N., Olgaard, D.L. 1995. Experimental deformation of fine-grained anhydrite: evidence for superplasticity, J. Geophys. Res., 100, [B8], 15,425-15,440. Handbook of Physical Constants, 1966. Sydney P. Clark, Jr., Editor. Geol. Soc. Amer. Memoir 97. Heim, Albert, 1882. Die Glarner Doppelfalte. Vierteljahrsschrift Natf. Ges. ZUrich, 27.Jg., II.Heft. Hsfi, K.J. 1969-1. Role of cohesive strength in the mechanics of overthrust faulting and of landsliding. Geol. Soc. America Bull., 80: 927-952. Hsfi, K.J. 1969-2. Role of cohesive strength in the mechanics of overthrust faulting and of landsliding: Reply. Geol. Soc. America Bull., 80: 955-960. Hsfi, K.J. 1969-3. A preliminary analysis of the statics and kinetics of the Glarus overthrust. Eclogae geol. Helv., 62(1): 143-154 Hsfi, K.J. 1970. Cohesive Strength, Toe Effect, and the Mechanics Imbricated Overthrusts. Proc. 2nd Congr. Int. Soc. Rock Mechanics, Beograd 1970, II, 3-36: 247-250. Hubbert, M.K. and Rubey, W.W. 1959. Role of pore pressure in the mechanics of overthrust faulting. Geol. Soc. America Bull., 70:115-166.

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U. Briegel

1961. Role of Fluid pressure in the mechanics of overthrust faulting: reply to Discussion by Francis Birch. Geol. Soc. America Bull., 72: 1445-1452. 1969. Role of cohesive strength in the mechanics of overthrust faulting and of landsliding: Discussion. Geol. Soc. America Bull., 80: 953-954. Jordan, Peter (1987). The deformational behaviour of bimineralic limestone halite aggregates. Tectonophysics, 135:185-197 Ko, S.C. 1993. Dehydration-induced pore pressure anomalies and mechanical weakening in rocks, Ph.D. thesis, ETH-Zfirich, Switzerland. Laubscher, H.P. 1961. Die Fernschubhypothese der Jurafaltung. Eclogae Geol. Helv., 54:221-282 Murchison, R.I. 1849. On the Geological Structure of the Alps, Quarterly Journ. Geol. Soc. London Vol., Pt.1, pp. 157 ff. Mfiller, W.H. und Briegel, U. 1977. Experimentelle Untersuchungen an Anhydrit aus der Schweiz. Eclogae Geol. Helv. 70(3) Mfiller, W.H. und Briegel, U. 1978. The rheological behaviour of polycrystalline Anhydrite. Eclogae Geol. Helv. 71(2) Mfiller, W.H. and Briegel, U. 1980. Mechanical aspects of the Jura overthrust. Eclogae Geol. Helv. 73(1) Mfiller, W.H. and HsiJ, K.J., 1980. Stress Distribution in Overthrusting Slabs and Mechanics of Jura Deformation. Roch Mech. Suppl., 9: 219-232. Paterson M.S. and Olgaard, D.L. 2000. Rock deformation tests to large shear strains in torsion. J. Struct. Geology, 22: 1341-1358. Raleigh, C.B. and Griggs, D.T. 1963. Effect of the toe in the mechanics of overthrust faulting. Geol. Soc. America Bull., 74: 813-830. Rothpletz, A. 1894. Geotektonische Probleme. Schweizerbarth, Stuttgart. Schmid, S.M. 1975. The Glarus overthrust: field evidence and mechanical model. Eclogae Geol. Helv. 68(2): 247-280. Schmid, S.M. 1982. Laboratory experiments on rheology and deformation mechanisms in calcite rocks and their applications to studies in the field. Mitt. Geol. Inst. ETH u. UNIZ (NF)241. 106p. Stretton, I., Olgaard, D.L. 1997. A transition in deformation mechanism through dynamic recrystallization evidence from high strain, high temperature torsion experiments, Eos, Trans. Am. Geophys. Union, 78(46): F723. Smoluchovski, M.S. 1909. Some remarks on the mechanics of overthrusts. Geol. Mag. New ser. Dec.V, Vol. VI, p.204 TriJmpy, R. 1980. Geology of Switzerland, a guide book. Wepf & Co, Basel. ISBN 3-85977-062-4. -

Paradoxes in Geology Ueli Briegel and Wenjiao Xiao, eds. 9 2001 Elsevier Science B.V. All rights reserved.

Chapter 11

THE F L O W OF GIANT ROCK L A N D S L I D E S

Christopher R. J. Kilburn Benfield Greig Hazard Research Centre, Department of Geological Sciences, University College London, Gower Street, London WC1E 6BT, U.K.

ABSTRACT Giant, rapid landslides, or sturzstroms, are among the most powerful natural hazards on Earth. They occur as whole mountainsides are displaced kilometres within minutes. Thirty years ago, popular explanations attributed high sturzstrom velocities to exotic mechanisms for reducing friction, notably the entrapment of cushions of air. Ken Hsfi challenged this view in the 1970s, arguing that sturzstrom behaviour is a natural result of collections of rock fragments. Hsti's view today prevails, supported by advances in granular-flow dynamics. The latest modifications of Hsfi's model assume that sturzstroms lose most of their energy by deformation in a basal boundary layer and, possibly, in narrow horizons within the body of the landslide. The favoured model views material in the boundary layer as a Bingham plastic which, for most of a sturzstrom's emplacement at high deformation rate, behaves approximately as a Newtonian fluid. This model provides a physical basis for forecasting the runout lengths of sturzstroms and for improving methods for assessing their hazard.

INTRODUCTION Sturzstroms are giant landslides of shattered rock that can travel kilometres within minutes. They occur when unstable volumes exceed ~106-10 v m 3, and their runout lengths tend to increase with initial volume (Fig. 1). Their speed, size and travel distance are often taken to imply that sturzstroms have an unusually high mobility compared to smaller landslides, and this has driven numerous investigations into how volume controls sturzstrom friction (Hsfi, 1975; Erismann, 1979; Melosh, 1987). Such studies have fallen into two classes: (1) those which invoke external mechanisms (e.g., trapped air or rock melting) to enhance mobility, and (2) those which relate mobility to properties inherent to colliding rock masses. Hsti (1975) championed the latter approach which, although unfashionable at the time, has since come to prevail among sturzstrom models. Building on Hsti's approach, this paper argues that volume is important to sturzstrom behaviour because it controls the energy available for transport rather than the inherent mobility of a landslide.

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STURZSTROM CHARACTERISTICS Sturzstrom volumes on Earth range from --~106 m 3 to about 30 km 3 (Fig. 1). They occur in virtually all major rock types, from carbonates, through metamorphic rocks, to volcanic material. Their deposits consist of collections of fragments, from fine grains to blocks the size of a house. The fine material appears normally to be concentrated either within the lower layers of a deposit or along narrow horizons at different levels throughout its thickness (Fig. 2). The simplest interpretation is that

The flow o f giant rock landslides

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Fig. 1. (A) The runout length of sturzstroms tends to increase with sturzstrom volume. Simple boundarylayer models (Eq. (20)) expect L to increase with the square root of volume. The dashed lines show the limits L= BV 1/2, where the upper and lower values of B are 40 km -1/2 and 5.5 km -1/2. (B) The ratio H/L of vertical drop height to runout length decreases as sturzstrom volume increases. Boundary-layer models (Eq. (15)) yield H/L=bV -1/6, shown by the dashed lines with upper and lower values for b of 0.22 km -1/3 and 0.075 km-1/3. (C) The ratio H/V 1/3 for sturzstroms lies within two preferred ranges of values. Most data suggest H/V ~/3~ 1 (lower box; for which V > 0.02 km3); these may correspond to when collapse occurs almost directly onto the runout surface (z/y 1 in Fig. 3).

d e f o r m a t i o n and the c o m m i n u t i o n of rock tends to be focused along a small n u m b e r of horizons ( H e n d r o n and Patton, 1985; Hewitt, 1988; Kilburn and Sorensen, 1998). Also consistent with focussed d e f o r m a t i o n is the tendency for sturzstrom deposits to preserve their large-scale, pre-failure stratigraphy, such that a pre-failure sequence of, say, c a r b o n a t e s over gneiss, would yield a sturzstrom deposit with disrupted c a r b o n a t e levels sitting on b r o k e n gneiss. Indeed, the exposed upper portions of some giant sturzstrom deposits, such as at K6fels and Flims in the Alps, preserve fine structures at scales of 0.1-1.0 m (Erismann, 1979). These latter deposits are r e m a r k a b l e when seen in outcrop, in that they a p p e a r f r o m a distance to consist of massive rock faces ( ~ 1 0 - 1 0 0 m across), when they in fact consist of rock masses which have been shattered at the centimetre scale, but which have experienced no relative d e f o r m a t i o n during emplacement. They are the equivalent of a threedimensional jigsaw puzzle that has travelled several kilometres without any of the pieces becoming dislodged. At the very largest scale, s t u r z s t r o m deposits a p p e a r as sheets, m u c h wider than thick, whose periphery is often lobate or even divided into tongues following t o p o g r a p h i c depressions (Hsfi, 1975, 1978). Such g e o m e t r y is typically associated with fluid behaviour which, in the context of rock emplacement, can only be

248

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Fig. 2. All sturzstrom deposits consist of broken rock. Near the source (top) heavily fractured hard rock (white) overlies a thinner base of gravel and small debris, possibly mixed with mud or clay (stipple), along which deformation is initially concentrated; secondary clay-rich layers within the body of the landslide may later develop as preferred horizons for deformation (Fig. 5). Away from the source (bottom), the whole sturzstrom has become much thinner and a larger basal fraction consists of a small debris supported by fine-grained matrix (stipple with grey and black debris). This layer is overlain by giant angular blocks (white). Notice the poor mixing between different stratigraphic horizons (white, grey and black levels). The black box in the top diagram shows the relative thickness of the lower section. Although idealised, these sections are based on observations at Vaiont, Italy (Hendron and Patton, 1985; top) and in the Karakoram Himalayas (Hewitt, 1988; bottom).

The flow of giant rock landslides

249

explained if the bulk shape of the rock mass is controlled by individual fragments (again from sand to the size of a house), each of which can move in response to local conditions, just as a fluid spreads according to the movement of its molecules. Together, the basic features of sturzstrom deposits suggest that sturzstroms are emplaced as collections of fragments through which deformation is concentrated along their base and, possibly, also along narrow bands at higher levels. These essential elements were certainly recognised at least forty years ago. Uncertainty occurred when trying to explain why deformation was concentrated and why such deformation could allow sturzstroms to travel great distances. STURZSTROM VOLUME AND MOBILITY Albert Heim pioneered the modern study of giant rock landslides following a series of landslide disasters in the Alps (Heim, 1882, 1932; Hsti, 1978). He noticed that, for a given vertical drop height H, a sturzstrom tended to travel a greater horizontal distance L as the pre-failure volume V became larger (Figs. 1, 3 and 6). Applied to a sliding rigid block, the ratio H/L is simply related to the friction coefficient g along the sliding plane as (Hayashi and Self, 1992):

H / L = ~t

(1)

Equation (1) is important because it relates geometric features of sturzstrom travel (H/L) to an effective roughness (g) of the sliding surface, most of which happens to be the sturzstrom-ground contact. The effective roughness, in turn, depends on the interaction between uneven ground and the irregular base of the sturzstrom. The more frequently the irregularities in the ground and the sturzstrom interact, the greater the value of ~t and hence of H/L. Applying Eq. (1), therefore, the inverse relation between sturzstrom volume and H/L (Fig. 1) suggests that larger volumes can trigger mechanisms for reducing the effective interlocking between ground and sturzstrom. An early lubricating model of enduring popularity proposed that sturzstroms are able to glide over cushions of trapped air (Shreve, 1968). Essential conditions for air entrapment are that: (1) topography must allow a sturzstrom to detach itself from the ground and fly for a short distance before crashing back down and compressing the air caught underneath; and (2) the compressed air must remain trapped for a time long enough to allow a significant runout for the landslide. Although flying sturzstroms may seem unlikely, one such was observed near the Swiss village of Elm in 1881, when a block undercut by quarrying collapsed onto a ledge, was deflected into the air and returned to the ground a few hundred metres downslope (Hsti, 1975, 1978). Such circumstances are rare, however, and so cannot be considered essential for the emplacement of sturzstroms. Even when air-launching does occur, air entrapment requires poor leakage from a landslide. For very wide landslides, it is plausible that lateral gas loss affects only the margins of the moving body (provided the velocity is not large enough to tear the slide apart (Erismann, 1979)) and, hence, that gas entrapment depends primarily on the upper levels of the landslide acting as an effective cap. Effective capping, in turn, is only feasible if the upper layers of a sturzstrom are virtually impermeable

250

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Fig. 3. Definition sketch of key sturzstrom dimensions. The top figure shows a geometry commonly assumed in models, for which the source (stipple) is viewed as a block whose dimensions are independent of the drop height H. A more realistic situation (bottom) shows that slope failure often occurs near the runout slope and suggests that, by virtue of the stress field required to form a new failure surface, the drop height H may vary approximately with the cube root of sturzstrom volume (Fig. 1).

(Erismann, 1979), a condition that is unlikely to be satisfied by the shattered, fragmented nature of sturzstrom deposits. The air-cushion hypothesis was further weakened by its inability to explain why larger sturzstroms have smaller apparent coefficients of sliding friction (Erismann, 1979), and by the discovery on the airless lunar surface of possible sturzstrom deposits (Howard, 1973). Alternative models therefore returned to processes operating within a landslide itself, since these would not rely on environmental controls (the entrapment of air) and might naturally explain the influence of landslide size on mobility. Given the fragmental nature of sturzstrom deposits, the interaction between fragments is an obvious starting point to investigate sturzstrom dynamics. Hsfi (1975) pursued this approach while air-cushioning was still the popular explanation for long sturzstrom runout. He reintroduced Albert Heim's studies of Alpine sturzstroms (Heim, 1882, 1932), whose results had laid dormant for over 40 years.

The flow of giant rock landslides

251

Heim (1932) had postulated that collisional energy losses were important to the behaviour of sturzstroms, but had lacked a theoretical support for the idea. Two decades later, Bagnold (1954) introduced the idea of grain-dispersive pressure among collections of moving fragments. He argued that, by colliding with each other, the grains in a granular mass can force themselves apart (the collisions producing the socalled grain-dispersive pressure), so helping faster grains to jump beyond their slower neighbours and reducing their frictional contact with respect to the static state (Bagnold, 1954). Making a crucial connection between Heim's hypothesis and Bagnold's granular-flow model, Hsfi (1975, 1978) conjectured that grain-dispersive pressures could explain why the apparent friction coefficients of sturzstroms are smaller than those typical of sliding rigid blocks. He formally proposed the name sturzstrom to avoid implying a sliding emplacement for giant landslides and cited the comminuted rock matrix as a principal source of dispersive pressures (Hsfi, 1975). Although also proposed independently (Howard, 1973) and applied in the field (e.g., McSaveney, 1978), the simple granular-flow model was challenged on the grounds that, while bulk friction may be reduced, the total rate of energy loss throughout a landslide would be too rapid to allow sturzstroms to travel the distances observed (Melosh, 1979, 1986, 1987). Modifications of Hsfi's sturzstrom model have thus sought conditions that favour lower rates of collisional energy loss by some combination of (1) increasing the effective size of fragments, (2) introducing an interstitial fluid, and (3) reducing the proportion of the sturzstrom volume in which the most energetic collisions occur. The key ideas can be summarized as follows.

(1) Increasing the effective size of fragments. Fragments increase their effective size when, instead of bouncing as individuals, they move as groups of fragments, so that collisions within groups are minimised and most energy is lost by contact among the outer fragments of adjacent groups. Melosh (1979, 1986, 1987) suggested that such behaviour could be induced by the passage of sound (acoustic) waves through a landslide. Triggered by shaking during collapse, the acoustic waves could also temporarily reduce overpressures on fragments, especially along the lower layers of a sturzstrom, thereby further decreasing the effect of frictional contacts. (2) Introducing interstitial fluids. Although mud may sometimes be available (Hsfi, 1975), the granular nature of most sturzstrom matrices has been used to support models invoking a gaseous and pressurized interstitial fluid that gradually escapes during transport. Thus Kent (1966) proposed fluidisation by air entrapment, while others have appealed to frictional heating for vaporising trapped groundwater (Habib, 1967; Goguel, 1978) or for dissociating carbonate rocks to yield CO2 (Erismann, 1979). Erismann (1979) further considered frictional rock melting as a means of increasing landslide mobility. (3) Reducing the volume of energetic collisions. As collisional energies increase, fragments not only move faster but also attempt to increase their mean separation. Using computer simulations (Campbell and Brennan, 1985; Straub, 1996), Campbell (1989) and Straub (1997) have argued that, under suitable

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C. R. J. Kilburn

conditions over rough ground, bouncing might be strong enough to carry a sturzstrom over a rarified layer containing a few, but rapidly colliding fragments. Although the collisions are energetic, the volume of the basal layer is small enough to ensure that the total rate of energy dissipation is much less than that expected from collisions throughout the landslide mass, thereby reducing the sturzstrom's bulk resistance. All the above mechanisms are remarkable in their imagination and in their appeal to extreme conditions. Being extreme is not by itself a motive for exclusion, but it does imply unusual conditions that should be evaluated with caution. Thus, as for Shreve's air-cushion hypothesis (Shreve, 1968), the pressurized-gas models all suffer from the need for the upper layers of sturzstroms to be virtually impermeable, an unlikely condition for fragmental flow. The extreme of rock melting does occur (Erismann, 1979), but has been observed in only a handful of larger deposits, so that its general application is questionable (Melosh, 1987). For rarified basal layers to support a giant landslide, the impact velocity of basal fragments must exceed ~10 (gh) 1/2 (Campbell, 1989), where h is landslide thickness and g is gravitational acceleration. For typical collapse thicknesses of sturzstroms, the minimum implied fragment velocities and impact pressures (pu 2, where p is fragment density) are 140-300 m s-1 and ~108 Pa. Even for the ideal (frictionless) conversion of potential to kinetic energy, such velocities imply (a) collapse heights of ~2-10 km (greater if friction is important), similar to and greater than the maximum values observed on Earth (Fig. 6), and (b) impact pressures similar to the compressional strength of intact rock (Lockner, 1995), so it is unclear whether or not such fragments (which probably have lower strengths due to internal flaws) could sustain such collisions for any significant length of time before disintegrating. Compared with the alternatives, only the acoustic model of Melosh (1979) appears as a plausible general mechanism for reducing sturzstrom friction. However, as with all the previous models, the acoustic fluidisation hypothesis cannot quantify the observed decrease in H/L with increasing sturzstrom volume. Indeed, to address this problem in particular, it is convenient to return to Hsti's starting point of using the established Bagnold model for concentrated granular flow (Bagnold, 1954; Hsfi, 1975). STURZSTROMS AS G R A N U L A R FLOWS Energy is dissipated in granular flows by inelastic collisions between fragments, by friction (rolling and sliding) between fragments, and by the deformation of any fluid between the fragments. The relative importance of each factor depends on the volumetric concentration of fragments, the size distribution of fragments, the rheology of any interstitial fluid, and the velocity of deformation (Fig. 4). Deformation rate is a crude measure of the energy available for motion. At low rates of deformation, fragments collide only weakly and most energy is lost either through frictional contact or by deforming an interstitial fluid. At high deformation rates, collisions occur so frequently that inelastic energy losses dominate granularflow resistance. The values of transitional deformation rates between these extremes

253

The f l o w o f giant rock landslides

0

~ ,,,~ .l...a

\\\\\\\-,."-.

.4...0

r 9

9

Deformation Rate Fig. 4. Landslide behaviour varies with the volume concentration of solid fragments and the rate of deformation. At small deformation rate or solid concentration (Region 1), most energy is dissipated by

deforming material (typically mud) between larger fragments. At large deformation rate and solid concentration (Region 3), most energy is dissipated by collisions among large fragments. Between these extremes (Region 2) both collisions and fluid deformation are important. M, D, and S, refer to notional fields for mudflows, debris flows and sturzstroms. The relative positions of M and D illustrate the increasing importance of fragment collisions among debris flows. Two sturzstrom fields are shown: that in Region 3 assumes all solid material loses most energy by collisions; that in Region 1 considers fine solids to behave as an effective fluid, so that the effective solid concentration is that only of large fragments. The shaded border on the left corresponds to long term creep. The curves show the shapes of field boundaries expected from Bagnold's relation (Eq. (21)). For constant physical properties, the dashed and solid boundaries correspond to Ba ~ 40 and Ba ~ 400; when physical properties vary, the boundaries will move towards the top right as the matrix viscosity increases and as the mean size and density of colliding fragments decrease (Eq. (21)).

increase as the c o n c e n t r a t i o n of fragments becomes larger and the resistance of any interstitial fluid increases (Fig. 4). Applied to landslides, a range of dynamic b e h a v i o u r can be expected according to the p r o p o r t i o n of interstitial fluid and the rate of deformation. Since the typical interstitial fluid is mud, an i m m e d i a t e possibility is that sturzstroms f o r m one end of a continuous spectrum of landslide types that grade, with increasing m u d content, t h r o u g h debris flows into mudflows (Fig. 4; Pierson and Costa, 1987; Iverson 1997). However, if the fragments in dry landslides consist of larger pieces within a matrix of very- title material, ambiguity arises as to what constitute~ an interstitial fluid. Titus, while collisions control energy dissipation f r o m the large fragments, frictional effects m a y determine energy losses from the matrix, so that the matrix behaves effectively as an interstitial fluid (Fig. 4). Initial a r g u m e n t s that Bagnold flow dissipates energy too quickly to explain s t u r z s t r o m r u n o u t (Melosh, 1986, 1987) a s s u m e d that energy dissipation is d o m i n a t e d by collisions between fragments t h r o u g h o u t the landslide. A complete analysis, however, m u s t consider the potential influence (a) of b o u n d a r y - l a y e r flow and (b) of energy loss being controlled by fine material behaving as an interstitial fluid.

C. R. J. Kilburn

254 BOUNDARY-LAYER FLOW IN STURZSTROMS

To explain the preservation of prefailure stratigraphy, d e f o r m a t i o n during transport must be restricted to narrow horizons within a sturzstrom; otherwise material from the prefailure ground level could be expected somewhere within the body of the final deposit (Erismann, 1979). Since deformation must occur between the moving sturzstrom and stationary ground, the simplest inference is that significant deformation, and hence loss of energy and momentum, occurs at least within a narrow boundary layer along the base of the landslide. As a sturzstrom advances, its basal material is the first to be slowed by stationary ground. Owing to collisions between neighbouring fragments, retardation of the basal layer induces a deceleration in the layer of fragments immediately above. At a rate controlled by the ability of slow fragments to decelerate their overlying neighbours, the retarding effect of the ground is gradually felt through an increasing amount of the granular mass. If te is the emplacement time of a sturzstrom and th is the time for the retarding effect to involve the whole landslide thickness h, then boundary-layerflow and wholebody flow can be distinguished according to whether th/te is much greater than or much smaller than unity. Thus (a) for th/te >> 1, emplacement is complete before the retarding effect of the ground has penetrated deeply into the landslide, so that only collisions in a basal boundary layer actively contribute to energy dissipation (Fig. 5c), but (b) for th/te ~

o,

o

% _._._,,

,,.,o"-'f" G~ "

S"

] "

(

~

oJ

o-,

Fig. 2. Location of Wildhorse Mountain Plug and mafic sills in and around Big Bend National Park, West Texas. 1 - Rattlesnake Mountain (RMS); 2 - Pefia Mountain; 3 - Slickrock Mountain; 4 - A1 Reed Trail; 5 - Lonsdale Locality 175; 6 - Lonsdale Locality 354; 7 - Cigar Mountain; 8 - Leon Mountain; 9 - Adobe Walls Draw; 1 0 - Bone Spring Hills.

Table 1 Chemical Analyses Analyst: Sample

Unknown

Voight W H 76-2

SiO2 TiO2 A1203 Fe203 FeO MnO MgO CaO Na20 K20 P205 H20

68.13 0.30 16.00 1.92 1.12 0.08 0.51 1.31 5.46 4.73 0.18 0.24

67.13 0.27 15.14 2.09 1.22 0.08 0.48 1.32 5.13 4.74 0.18 2.01"*

Total

99.98

99.79

P.B.

Ellested *

*Reported in Lonsdale (1940) **Loss on ignition, H20, C02, organic, etc.

K.C.

280

M. F. Carman, Jr. and S. Indest

Petrology

The plug is composed of glomerocrysts and single phenocrysts of multiphase feldspars, ferroaugite, biotite and very sparse quartz in a groundmass of mostly alkali feldspar and quartz. Overall, the rock contains about 85% feldspar, 6% quartz and 9% minor accessories (Table 2) and we can treat it as essentially a quartz feldspar rock. Its texture is seriate, with coarse biotite and feldspar phenocrysts grading down to very fine groundmass grains. General mineralogy

The large feldspar phenocrysts show the relationship of the phases very well. They are epitaxially grown, each phase mantling the previously crystallized one (Fig. 3). A plagioclase core formed first, this was overlain by anorthoclase and that was covered with sanidine; none of these phases crystallized simultaneously. Plagioclase is commonly albite twinned, anorthoclase is pericline or albite-pericline twinned in all sections and sanidine in phenocrysts is not twinned, so they are easily distinguished with the microscope. All phases are distinctly zoned. Euhedral feldspar microphenocrysts (0.25-2.0 mm) typically have small plagioclase cores with anorthoclase mantles and sanidine rims. Plagioclase does not have a sharp optical boundary with anorthoclase and albite lamellae often project from plagioclase for a short distance into anorthoclase. Microprobe data demonstrate a continuous zoning trend from plagioclase to anorthoclase across the optical boundary zone in all crystals probed. On the other hand, the boundary between anorthoclase and sanidine is very sharp; the outer edge of anorthoclase is generally a distinct clean boundary and there is an abrupt drop in the An content in sanidine relative to anorthoclase across the boundary. This suggests that there may have been a period when anorthoclase stopped growing and before sanidine started to nucleate. Detailed textural relations

Table 2 Average Modal Analysis (From Indest, 1978) Quartz Groundmass feldspar Plagioclase in phenocrysts Anorthoclase in phenocrysts Sanidine in phenocrysts Biotite Ferroaugite Apatite Other* Total Total feldspar phenocrysts Total phenocrysts

6.5 54.0 3.5 5.7 21.4 4.0 1.4 0.7 2.8 100.0 30.6 36.0

*Other includes magnetite, hydrous iron oxides, deuteric phyllosilicates and zircon.

Crystallization histories o f alkaline intrusions in West Texas

281

Fig. 3a. Large (4 ram) phenocryst, showing plagioclase core (P); diamond shaped pericline twinned anorthoclase inner mantle (A), with typical anorthoclase habit dominated by {-201} and { 100} pinacoids; and outer mantle of sanidine (S). View approximately down the b crystallographic axis. 25X UXN.

Fig. 3b. Zoning trend in phenocryst of 3 a. Dot (.) traverse is from tip to tip of the long axis of the crystal, cross (+) traverse is across short axis of crystal. Showing essentially continuous zoning from plagioclase (P) through anorthoclase (A), a gap from about Or29 to Or40 followed by sanidine (S).

282

M. F. Carman, Jr. and S. Indest

demonstrate that the three phases of the ternary feldspar, plagioclase, anorthoclase and sanidine, grew sequentially, no two phases crystallizing at the same time (Indest, 1978). Measurements of optical axial angle and microprobed compositions, 2Vz = 123 ~ (An20) to 2Vz - 100~ (An40) show plagioclase to be in the high structural state; while 2V• and optic axial plane orientation of sanidine, 38 ~ to 52~ and AP perpendicular to (010), show it to be low sanidine. Much of the sanidine is optically homogeneous, but the glomerocrysts, phenocryst mantles and the smaller euhedral grains show local microperthitic patches. Judging from this, and from the condition of alkali feldspars from neighboring intrusions (Altman, 1970; Patel, 1973; and Carman et al., 1975) it is most likely that the optically homogeneous alkali feldspars are cryptoperthitic. Pyroxene (0.2-0.8 mm) is compositionally homogeneous ferroaugite, typical of pyroxenes from other syenites. Biotite (0.1-8 mm) shows reaction rims of magnetite and pyroxene, and this, together with its large phenocrystic form, indicate that it crystallized early when the magma was deeper than its present level, still in a subjacent magma chamber in which PH20 was sufficient for the biotite to be stable. Apatite occurs both in the groundmass and as inclusions in the other minerals; magnetite also is found as minute inclusions in pyroxene, biotite and feldspar, but it also occurs as separate microphenocrysts up to 0.8 mm in size. Quartz appears mostly as small anhedral groundmass grains, but there are rare small subhedral microphenocrysts up to 0.65 mm. Zircon occurs as minute < 0.1mm euhedral grains included in, or closely associated with, the mafic minerals. Details o f feldpar zoning

The crystallization history of Wildhorse Mountain Plug, insofar as we can decipher it, is largely preserved in the zonation of its ternary feldspars and this was the subject of detailed electron microprobe study in probe traverses through selected grains (Indest, 1978). As noted earlier, the phenocrysts show a compositional trend from plagioclase (andesine-oligoclase) to anorthoclase and then to sanidine on the rim. Each separate phase is also zoned: plagioclase An content decreases from core to rim, anorthoclase shows decreasing An and Ab percent, together with increasing Or percent from plagioclase boundary to sanidine boundary, and in sanidine the Ab/ Or ratio decreases in many crystals from interior outward. This zoning trend in sanidine is counter to that predicted from laboratory studies of phase relations (Bowen and Tuttle, 1950; Brown, 1993 and Nekvasil, 1994). Microphenocrysts show similar trends in these phases. Zoning trends of groundmass grains, which contain anorthoclase and sanidine or just sanidine, are the same as those of the phenocrysts, but there is more variation in geometry and direction of zoning; whereas zoning of the phenocrysts and microphenocrysts is generally symmetrical in traverses across grains, this is seldom the case for the groundmass grains. There are several factors that control the geometry and chemical nature of zoning in these feldspar crystals. These factors, at least in part a direct function of the degree of super-cooling the driving force of crystallization, are: 1) Nucleation rate 2) Crystallization rate

Crystallization h&tories of alkaline intrusions in West Texas

283

3) Diffusion rates of the crystallization components toward and away from the crystal-liquid interface 4) How the spatial relationship of growing crystals affects the diffusion of the crystallization components These factors cannot be evaluated quantitatively in studying naturally occurring rocks; rather, they require crystallization experiments under controlled conditions. However, they must be kept in mind when trying to interpret the results of measured zoning in the rocks. The problem of getting the full extent of zoning from a geometrically restricted slice through a given crystal was minimized by analyzing a number of grains that show greatest optical zoning. The geometry of the slice and the size of area of analysis, which is a function of the electron beam size, can also inhibit the ability to analyze the outermost edge of a crystal. In the crystallization sequence another geometric problem arises. Some feldspar phases exhibit a tendency for crystal growth in a preferred direction, causing compositional zones to be much broader in the preferred direction. When probing such crystals, with analyzed points equally spaced, one usually obtains a better picture of the actual zoning trend from a traverse parallel to the preferred direction, where zones are broader. In the non-preferred directions, with narrower compositional zones, there are fewer analyzed points, and, depending on the relationship of probe spot size to width of each zone, each analysis could be an average of a wider compositional range, resulting in misrepresentation of the extreme compositions of each zone and the inner and outer limits of each phase. With these considerations in mind we have chosen two phenocrysts, three microphenocrysts and five groundmass grains as representative of the zoning found in the feldspars of Wildhorse mountain. In the zonation of the phenocrysts from plagioclase to anorthoclase and then sanidine, there is a distinct gap between anorthoclase and sanidine compositions between about Or30 and Or35, across which An content decreases from 5-15% in anorthoclase to less than 5% in sanidine. In Fig. 3 is a typically zoned phenocryst that illustrates these features, except that the gap between anorthoclase and sanidine is An30-42, which may be due to missing a narrow zone in the probing. Figure 4 depicts zoning in the anorthoclase-sanidine mantle of a phenocryst in detail. Anorthoclase shows oscillatory zoning between Or13 and Or28; while sanidine has complexly variable zoning, first increasing in Or and then oscillating. The trend, however, is broadly toward an increase in Or outward through the inner part of the mantle, contrary to that expected from experimental data, and then dropping back to lower Or in the outer part. Other phenocrysts show quite similar zoning patterns. Grains between 2.00 and 0.25 mm longest dimension are arbitrarily called microphenocrysts. They range from euhedral to subhedral, and typically contain all three ternary phases. Figure 5 illustrates such a subhedral grain in which a tiny oligoclase core has a thin anorthoclase mantle and a large sanidine outer mantle that is quite thick in the [100] direction and very thin in the [001] direction; so thin, in fact, that it was only possible to get a single analysis in the sanidine in that direction. Through the thick sanidine, however, there is a general drop in Or from inner to outer mantle, but in the outer half the slight oscillatory zoning trends toward increase in Or. The very small plagioclase core has a low An content, as would be

284

M. F. Carman, Jr. and S. Indest

Fig. 4a. Medium sized (2.7 mm) phenocryst, showing euhedral plagioclase core (P), large anhedral pericline twinned anorthoclase inner mantle (A) and conformable sanidine outer mantle (S). Section perpendicular to the a crystallographic axis. 45X UXN. expected for a grain nucleating late in the period during which plagioclase was forming. A euhedral microphenocryst in which only anorthoclase and sanidine were detected is depicted in Fig. 6 which shows a general increase in Or from the inner to the outer mantle. The lack of plagioclase may be the result of not having cut the grain through its center, but the important feature is the trend in Or content in the sanidine. Rare, nearly perfectly euhedral grains have a relatively large core of plagioclase and thin mantle of anorthoclase (Fig. 7), but no sanidine outer mantle. The lack of a sanidine mantle might be explained by their being xenocrysts, but they are otherwise compositionally in perfect accord with the rest of the rock. Another explanation may be that perfect crystal faces made nucleation of sanidine difficult on such surfaces. In summary, for microphenocrysts there appears to be little or no gap between plagioclase and anorthoclase compositions, but that between anorthoclase and sanidine persists. There is a distinct tendency for Or content to increase outward in sanidine mantles. Figure 8 shows seriate groundmass with euhedral microphenocrysts. Analyses of all groundmass grains probed are shown in Fig. 9, representing one hundred and

Crystallization histories of alkaline intrusions in West Texas

285

Fig. 4b. Zoning trend in phenocryst of 4 a. Dot (') traverse from center of plagioclase core through anorthoclase and sanidine mantles along the long axis of crystal, cross (+) traverse across the short axis of crystal from right to left from sanidine rim to sanidine rim at the widest point. The anorthoclase-sanidine gap is from about 0r28 to about 0r37.

Fig. 4c. Sketch of phenocryst of 4 a, showing location of probed spots in anorthoclase and sanidine mantles and illustrating the oscillatory zoning in them. Dashed lines connect zones of like composition.

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M. F. Carman, Jr. and S. Indest

Fig. 5. Zoning trend and sketch of small (0.41 mm) microphenocryst with small euhedral plagioclase core and strongly oscillatory zoned anorthoclase mantle and large subhedral sanidine mantle, all dominated by {100} and {001} forms. View along b crystallographic axis. Traverse across long axis of crystal illustrates slight oscillatory zoning in sanidine, with small general trend toward decreasing Or content.

Fig. 6. Zoning trend in small microphenocryst (0.27 mm) with anorthoclase core and sanidine mantle, showing general increase in Or outward.

Crystallization histories of alkaline intrusions in West Texas

287

Fig. 7a. Large (1.2 ram) euhedral microphenocryst, with large plagioclase core (P), anorthoclase mantle (A) and no sanidine, dominated by {001} and {010} forms. View along a crystallographic axis. 80X UXN.

Fig. 7b. Zoning trend in microphenocryst of 7 a. Dot (.) traverse parallel to long axis and cross (+) traverse parallel to short axis of crystal. Both traverses completely across crystal.

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M. F. Carman, Jr. and S. Indest

Fig. 8. Groundmass of interior of plug, showing seriate, mostly euhedral microphenocrysts. 180 UXN.

Fig. 9. Illustrates the composition range of one hundred and eleven spot analyses on thirteen groundmass grains less than 0.25 mm in size.

Crystallization histories o f alkaline intrusions in West Texas

289

eleven points analyzed in thirteen crystals < 0.25 mm in size. This diagram shows that the groundmass consists of both anorthoclase and sanidine and that the gradation from one to the other is continuous, no gap. Figures 10 and 11 illustrate five representative groundmass grains in which the three larger ones, > 0.10 mm long, have zoning with outward increase in Or; two of these have slight oscillatory

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290

M. F. Carman, Jr. and S. Indest

zoning. The two smaller grains, < 0.10 mm long, show no symmetrical pattern, and probably represent a size range at which the factors mentioned at the beginning of the discussion of zoning play a controlling role in determining the geometry of compositional distribution.

Summarizing the zoning of feldspar phases." 1) Plagioclase is simply and normally zoned from core to rim, An content ranging from 40 % in the cores of crystals to just below 20% on the plagioclase rims. This result would be expected in crystals forming in a stable, but relatively rapidly cooling magma in a rather shallow setting. Phenocrysts are present in rocks at the contact of the intrusion, so that plagioclase must have been forming prior to emplacement at its present position, probably in a differentiating magma chamber at not too great a depth below the existing surface. 2) The oscillataory zoning shown in the anorthoclase in Fig. 4c is similar to that found in the other anorthoclase phenocryst mantles. This suggests that they were growing as the magma was moving up to its present position, with possible connection to the surface, and accompanying volcanism causing periodic release in pressure and boiling off of water content. 3) The zoning in sanidine is of two general kinds: (a) continuous, though sometimes slightly oscillatory, zoning from the anorthoclase-sanidine boundary, with increasing Or and a small decrease in An outward in the crystal; and (b) an increase in Or outward from the anorthoclase-sanidine boundary, with a reversal to increase in Ab at the outermost rim. It is believed that the second trend represents the true picture, and that the lack of detection of increase in Ab in some crystals was the result of inability to probe the outermost rim of some grains, owing to such things as inclusions in the rim, the rim being thinner than the area covered by the probe beam, and badly oriented sections.

Crystallization history of feldspars The phase relations of the systems A b - O r - A n - H 2 0 and A b - O r - A n - Q - H 2 0 have been studied in considerable detail in the laboratory and theoretically, using thermodynamic calculations. The topology of the relationships is quite complex, especially near the temperature minimum in the Ab-Or (alkali feldspar) part of the systems, and there is an enormous literature on the subject. This was fairly extensively reviewed in the 1993 NATO Feldspar Conference publication "Feldspars and Their Reactions" (Parson, 1994). In the critical factors that control crystallizing phases, temperature, pressure and composition (liquid and solid), water content of the liquid phase (expressed as water activity [aw], or the partial pressure of water [PH20]) and SiO2 have the greatest effect on the compositions and temperatures at which the feldspar phases form. The presence of quartz in the Wildhorse Mountain Plug indicates silica saturation, and the biotite phenocrysts show the presence of water. The probable range of pressure is from about 0.5 kbar, based on overburden

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291

at time o f intrusion, and a b o u t 4 kbar, based on the differentiated n a t u r e of the intrusion, which suggests a relatively shallow m a g m a c h a m b e r at the beginning of crystallization. A m a g m a with such a high salic content is very unlikely to exist separated f r o m m o r e mafic constituents at a depth of 15 km, c o r r e s p o n d i n g to a pressure of 5 kbars (Tuttle a n d Bowen, 1958). U n d e r such conditions metastable hypersolvus crystallization can occur as discussed below. The zoning patterns in Wildhorse crystals indicate changing conditions during crystallization that almost certainly reflect the rise of the plug to its final position and, we postulate, the gradual or periodic d r o p in pressure a n d water content. Nekvasil recently calculated the phase relations of the feldspars in the granite system, and Fig. 12 shows the crystallization paths u n d e r conditions of 2 k b a r and

Fig. 12. Ternary feldspar crystallization paths, as calculated from thermodynamic considerations by H. Nekvasil. (Adapted from Nekvasil, 1990, figures 4, 7 and 9) (a) Shows polythermal solvus sections at aw = 1.0 (water saturated), and at 2 kbar Qz saturated (solid line), 2 kbar with no Qz (dashed line) and at 5 kbar with no Qz (dotted line). The heavy solid lines represent composition trends of plagioclase, anorthoclase and sanidine in Wildhorse Mountain rocks. (b) Shows polythermal solvus sections at aw = 0.1, and 2 kbar Qz saturated (solid line), 2 kbar with no Qz (dashed line) and at 5 kbar with no Qz (dotted line).

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M. F. Carman, Jr. and S. Indest

5 kbar, water saturated and nearly dry, and silica saturated and silica absent in her calculations. Also shown in the figure is the crystallization path of phenocrysts and microphenocrysts of Wildhorse Mountain, as determined by electron microprobe analysis. First, it is to be noted that the Wildhorse Mountain crystals most closely match the 2 kbar water and silica saturated curves. Secondly, we see that both water content and silica have significant effects in moving the paths toward the Ab corner and toward the Ab-An and Ab-Or sides of the diagram; thus decreasing the Or content of plagioclase and the An content of alkali feldspar. Simply raising pressure in the drier conditions has relatively little effect, whereas in the water-saturated case the pressure effect is quite significant, moving the paths in the same sense as does silica. Data for silica saturation at pressures higher than 2 kbar are not available, but presumably they would move the plagioclase part of the curve farther toward the Ab-An side and closer to the Wildhorse curve. We therefore conclude that the first part of crystallization of the Wildhorse Mountain Plug took place at something like 2 kbar or higher and water saturation, and that crystallization was following a normal fractionation path. At the Ab end of the plagioclase curve, and in the range of anorthoclase compositions, the sharp bend in the curve marks a striking departure from the calculated curve for 2 kbar with silica, which we will attempt to explain and use in deciphering the crystallization history. Aided by Nekvasil's calculations, W. L. Brown (1993) developed liquid fractionation lines and feldspar paths for the ternary feldspars under a variety of conditions, each one at constant pressure, including those at "intermediate PH20" ( < 5 kbar) or aw (Fig. 13) and at 2 kbar and aw = 0.1 (Fig. 14). These come closest to matching conditions believed to have been obtained during the latter part of the crystallization of the plug. It should be noted how sensitive these curves are to differences in aw, and, indirectly, pressure as it affects water content. Line C-E in is the boundary line between alkali feldspar (sanidine) and plagioclase (here including anorthoclase), and point C is the critical end point of that boundary. In figure 13a, at "intermediate" aw above 2 kbar pressure, Wildhorse bulk composition lies between liquid fractionation lines Q and R and it should crystallize plagioclase, fractionating to the boundary line CE, where it would be joined by sanidine which becomes more Ab rich as it forms. Figure 13b shows where the crystallization path would lie, between rcl and ~3, approximating the Wildhorse curve for anorthoclase compositions. Figure 14 shows what the liquid fractionation lines and feldspar paths should be in the latest stages of crystallization of the magma if it is moving into its final position with low aw = 0.1 and 2 kbar pressure. Wildhorse bulk composition, lying just above C and to the left of liquid fractionation line U (Fig. 14a), should miss the boundary curve and move toward the minimum point M. The feldspar path (Fig. 14b) should move from re4 to ~0K and then toward qgM with decreasing Or content in anorthoclase. As noted in describing the zoning, sanidine epitaxially mantles anorthoclase, however its Or content generally increases outward, instead of decreasing as all the phase diagrams indicate it should; although, in some instances the outermost portion of the sanidine of Wildhorse Mountain a swing back toward Ab was detected. Clearly, some sort of metastable hypersolvus crystallization has occurred.

Crystallization histories o f alkaline intrusions in West Texas

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Fig. 13. Liquid fractionation lines and feldspar paths at PH20, or aw at constant P, intermediate between 5 kbar and 2 kbar, but closer to 2 kbar. (From Brown, 1993, Fig. 10) (a) The boundary line is even down to C, the special line CM corresponding to the coalescence of the plagioclase and sanidine fields. Liquidus lines from Or and An either meet the boundary line or converge on M. (b) Fractionation paths for plagioclase (re), sanidine (or) and (re + c~) are given, that for (re + or) stopping at rcK=~K (= KE). Solidus paths for plagioclase may bend back with a minimum in Ab {e.g. (R) and (U)}. Paths for plagioclase and sanidine which meet the solvus stop at KE, both sanidine and plagioclase ceasing to fractionate, being replaced by feldspar which follows the unique solidus path ~PK-~PM; crystallization again changes from subsolvus-simultaneous to hypersolvus. The Wildhorse Mountain bulk composition, lying between lines (R) and (Q) should, under equilibrium conditions, crystallize plagioclase down to the solvus between re2 and r~K at which point it would be joined by sanidine on the line from rcK to ~2, they crystallize together until the liquid reaches rcK= crK (= KE). At that point sanidine ceases crystallization and a single feldspar of anorthoclase composition (here designated as "plagioclase") would form down toward ~PM,becoming more Ab-rich. These last t w o figures display only the p a t h o f the liquid in the t e r n a r y system A b - O r - A n a n d do n o t t a k e into a c c o u n t the crystallization o f quartz. F i g u r e 15 depicts the e q u i l i b r i u m p h a s e relations o f the q u a t e r n a r y system A b - O r - A n - Q at a b o u t 1 k b a r , in w h i c h the W i l d h o r s e M o u n t a i n Plug lies c o m p o s i t i o n a l l y . E F G H is the two f e l d s p a r liquidus surface, w h i c h stops s h o r t o f the base o f the t e t r a h e d r o n . L i q u i d s b e l o w it will crystallize sanidine first, those a b o v e it will f o r m plagioclase first, a n d those on it will crystallize the two feldspars t o g e t h e r . G H W X is the p l a g i o c l a s e - q u a r t z b o u n d a r y surface, liquids b e l o w it will crystallize q u a r t z first a n d those a b o v e it will crystallize plagioclase first. H G S is the s a n i d i n e - q u a r t z b o u n d a r y surface on w h i c h sanidine a n d q u a r t z crystallize t o g e t h e r . A line f r o m M to M ' traces the locus o f a t e m p e r a t u r e m i n i m u m b e t w e e n N a - r i c h a n d K - r i c h alkali feldspars. This, h o w e v e r , does n o t exist in the system A b - O r - A n - H 2 0 at 2 k b a r a n d aw = 0.1 (Nekvasil a n d Lindsley, 1990). Lines lettered with P, A a n d S s c h e m a t i c a l l y illustrate

294

M. F. Carman, Jr. and S. Indest

Fig. 14. Liquid fractionation lines and feldspar paths at P -- 2 kbar and aw = 0.1.(From Brown, 1993, Fig. 11) (a) The boundary line is even ("eutectic") from E to N and odd ("peritectic") from N to C, the special line NM lying in the sanidine field. Liquidus lines from An meet either the boundary line EC or converge on M; liquidus lines from Or meet either the boundary line from E to N or converge on M. (b) Fractionation paths for plagioclase (rt). sanidine (~r) and (rt + or) are given, that for (rt + ~r) stopping at crN. Solidus paths for plagioclase, here including anorthoclase, may bend back with a minimum in Ab {e.g. (U)}. Paths for plagioclase and sanidine, which meet the solvus, are of two kinds. Simultaneous twofeldspar fractionation occurs on solvus paths poor in Ab but it stops at rtN and aN, plagioclase ceasing to fractionate whereas sanidine continues and follows the unique solidus path aN - qgM; crystallization thus changes from subsolvus-simultaneous to hypersolvus. Paths corresponding to the line (R) involve twofeldspar sequential fractionation, plagioclase at first (rt5- rt6) followed by sanidine alone (cr6 - q~); the path thus jumps across the solvus between 71;6 and or6. Liquids between lines (U) and (V) do not reach C, instead, they curve into the minimum at M and, as a result crystallize only "plagioclase", including anorthoclase. The bulk composition of Wildhorse Mountain rocks lies to the left of line (U) and should fractionate along a line to the left of 7T 4 - - q9 K - - qgM,forming first Na-rich plagioclase and then anorthoclase under equilibrium conditions.

the plagioclase, anorthoclase and sanidine crystallization paths on the front face of the tetrahedron. Curve L1-L6 schematically shows the postulated liquid fractionation path in the third dimension. In the system shown in Fig. 15, initial plagioclase (P1) is andesine that is zoned outward to oligoclase (P2). The liquid, meanwhile, will

295

Crystallization histories of alkaline intrusions in West Texas

An

X

P1

Cl W

Ab-

H

G

L6L5 Or

Fig. 15. Equilbrium phase relations in the system A b - O r - A n - Q at about 1 kbar. E F G H is the twofeldspar liquidus surface which stops short of the base of the tetrahedron. Liquids below it will crystallize sanidine first, those above it will form plagioclase first, and those on it will crystallize the two feldspars together. G H W X is the plagioclase-quartz boundary surface, liquids below it will crystallize quartz first, while those above it will crystallize plagioclase first. HGS is the sanidine-quartz boundary surface on which sanidine and quartz will crystallize together. A line from M to MO traces the locus of a temperature

minimum between Na-rich and K-rich alkali feldspars. It does not exist in the system Ab-Or-An-H20 at 2 kbar and aw = 0.1 (Nekvasil and Lindsley, 1990). Lines lettered with P, A and S schematically show the plagioclase (P), anorthoclase (A) and sanidine (S) crystallization paths on the front face of the tetrahedron. Curve L1-L6 schematically shows the postulated liquid fractionation path in the third dimension. Details are in the text.

move downward on a curved path (L l - L 2 ) toward the two feldspar boundary and in general toward the feldspar-quartz b o u n d a r y surface. The crystallization of anorthoclase initiates a greater removal of Or from the liquid as the solid phase (anorthoclase) moves from A2 to A3, still lower in the tetrahedron. This causes the liquid path to move less strongly toward the Or corner (L2-L3) and to be convex toward that corner. As the liquid (L3-L4) reaches the region of the "critical end line" of the two feldspar surface anorthoclase ceases formation and is followed by sanidine which becomes more Or-rich as crystallization proceeds ($4-$5). In the final stage of crystallization the liquid (L5-L6) meets the quartz-sanidine surface and moves toward the minimum on the quartz-feldspar boundary surface, in a direction of Ab enrichment; while sanidine becomes more Ab-rich ($5-$6), following the more expected crystallization path, and crystallizes with quartz. If crystallization had followed the theoretical path (Fig. 14 b) anorthoclase would have continued crystallizing, becoming more Ab-rich, and the liquid would have moved away from the two feldspar surface toward the A b - Q line.

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M. F. Carman, Jr. and S. Indest

Brown (1993), in discussing ideal water-buffered liquid fractionation lines and feldspar zoning paths, describes two sequential feldspar paths in which the crystallization of "plagioclase" (which includes anorthoclase in the sense used in this paper) ceases and the crystallizing phase jumps a gap to sanidine. Brown states "If the two phases are close in composition and thus in lattice parameters, the second phase may overgrow the first epitaxially; . . . . " This fits the case for Wildhorse feldspars. The gap in composition from Or30 to Or35 between anorthoclase and sanidine in Wildhorse rocks is suggestive of such a jump; and if the bulk composition of the rocks were a bit more potassium-rich, placing it near R in figure 14 a, we would postulate such a jump between L3 and L4. Indeed, such a jump may be the case under natural conditions different from those of the theoretical diagrams, for example, with silica included and at lower pressure and aw in the last stages of crystallization. In either case, we are left with the paradox that the sanidine is zoned in the wrong direction, toward increasing Or rather than Ab, only going toward Ab at the latest stages of its formation. We conclude, as previously stated, that metastable hypersolvus crystallization occurred. It must be remembered when discussing these crystallization trends that the exact compositions of the liquid cannot be precisely related to measured crystal compositions because of the postulated metastable conditions and the lack of knowledge of amounts of solid at any given composition. During nonequilibrium subsolidus crystallization of anorthoclase and sanidine, all compositions, liquid and solid, may be anomalous for the temperature at which crystallization is occurring. This metastable crystallization might be caused by the magma starting to move to a position of final shallow emplacement, the temperature dropping relatively quickly, turbulence promoting rapid nucleation, and the change in temperature promoting rapid crystal growth. The occurrence of strong oscillatory zoning in anorthoclase and the presence of anorthoclase in the groundmass suggests that these events may have started while anorthoclase was still crystallizing. In summary, it appears that during early crystallization, when plagioclase was forming, conditions were approaching equilibrium and fractionating crystals were following the pattern of experimental determination, with relatively regular continuous compositional zoning. With the beginning of anorthoclase formation, the pattern of zoning changed, becoming strongly oscillatory over a fairly wide range of compositions, and, in general diverging from the curves that matched conditions for plagioclase formation. The crystallization of sanidine, also, does not match the expected trends, except in the very latest part of its formation.

Conclusions

The Wildhorse Mountain intrusion was formed from a quartz syenitic magma which probably was derived by differentiation of a more mafic parent in a subjacent magma chamber (Carman, et al., 1975), intruded at a shallow depth as a somewhat complexly shaped plug, and cooled relatively rapidly under confining pressures of a few kilobars or less. The rock is 85% ternary feldspar, and as such can be interpreted to a first approximation by ternary feldspar phase relations. From the compositonal

Crystallization histories of alkaline intrusions in West Texas

297

zoning patterns observed in the rocks, we postulate the following sequence of events in the formation or this intrusion: 1. Crystallization of ferroaugite, biotite and plagioclase, forming the basis of phenocrysts, in a relatively shallow magma chamber at confining pressure between 2 kbar and 4 kbar. Relatively quiet conditions, with cooling and crystallization rapid enough to cause continuous compositional zoning in plagioclase from andesine to oligclase. 2. Magma starts to rise toward final shallow emplacement at about 700 m or less, probably connected to volcanic eruption above, with periodic release of pressure and variation in volatile content in residual liquid. Anorthoclase nucleates and grows rapidly, owing to turbulence associated with irruption and relatively rapid drop in temperature. Supercooling starts, and the variations in temperature and drop in volatile content account for the departure from phase relations determined by thermodynamic calculations and laboratory experiments. 3. Final emplacement, with gradual cessation of volcanism, relatively quiet conditions returning and strong supercooling. Liquid has reached the composition where sanidine is the feldspar phase, and very rapid overgrowth on anorthoclase occurs under metastable hypersolvus conditions. Sanidine is zoned toward more Or-rich compositions, counter to theoretical expectations. 4. In a final quiet stage cooling rate has slowed, and, as the residual liquid reaches the quartz-sanidine boundary, an approach to equilibrium conditions occurs, and sanidine and liquid move toward the temperature minimum at more Ab-rich compositions. Ultimately, after complete solidification, the system is below the solvus and some exsolution occurs in the sanidine mantle of some phenocrysts, as evidenced in sparse patches of microperthite. The irregular and patchy nature of the microperthite may be due to spotty distribution of interstitial water-rich residual liquid which facilitates exsolution in the alkali feldspars (Smith and MacKenzie, 1961). F O R M A T I O N OF SYENITIC BODIES IN SHALLOW MAFIC SILLS In the second intrusion to be discussed, we wish to illustrate the use of the compositional zoning of ternary feldspars, among other criteria, in deducing the crystallization history and magmatic evolution of the Rattlesnake Mountain Sill (RMS, Fig. 2); and to show how this led to the formation of syenite bodies in this sill. The Rattlesnake Mountain Sill (RMS) is one of several Na-rich mafic sills that were intruded at depths of several hundred to a few thousand meters. They range from 30 to 80 m thick, and from one kilometer to several kilometers in lateral extent. All are aphyric, with aphanitic chilled margins, and all contain syenite bodies of various shapes and sizes enclosed within them. We propose several mechanisms for the formation of these syenite bodies, which occur mostly as relatively thin subhorizontal sheets distributed through the sills; but vertically elongated cylindroidal forms and rounded ocelli are also found. The different forms show a systematic distribution in the sills that leads to an understanding of how they were

298

M . F. Carman, Jr. and S. Indest

emplaced. At least eleven sills have such features in the Big Bend area, and at least four have them all. These sills do not have a single thick layer like those found in many other mafic alkaline intrusions, which have been explained by large-scale crystal settling. The near absence of phenocrysts when the Big Bend magmas were emplaced explains the lack of large-scale settling. But this raises major questions as to how the syenite bodies formed. The main rocks of the sills are syenodiorites and syenogabbros, composed of olivine, Fe-Ti oxides, plagioclase, apatite, augite, biotite, alkali feldspars and analcite, approximately in that order of crystallization. In the coarser interior of the sills these mafic rocks have a strongly ophitic texture, an important feature in explaining the formation of plagioclase-rich syenites. The RMS will serve as an example of the others. It is a tongue-shaped body, 80 m thick, with a roughly circular outcrop 3000 m in diameter, in which syenite sheets are most abundant in the central 25 m (Figs. 16 and 17). Modal and chemical analyses of sections through the sill SECTION OF RATTLESNAKE MOUNTAIN SILL(RMS) 80 --r_/_ / / / / / , / / / / / J Aguja Formation OCELLI

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Fig. 16. Schematic columnar section through the Rattlesnake Mountain Sill (RMS), illustrating the occurrence of the different types of syenite bodies in the intrusion.

Crystallization histories of alkaline intrusions in West Texas

299

Fig. 17. Plagioclase-rich (PR,1) and plagioclase-poor(PP, 2) syenite sheets, illustrating PR and PP sheets combined. Central Sheet Zone, RMS. Hammer, lower right, gives scale. show no evidence of large-scale crystal settling in syenodiorite (Figs. 18 and 19). Note the slight lowering of K-feldspar and increase of plagioclase in the Central Zone relative to the Border Zone, which is reflected in the lower K20 -k- Na20 and higher CaO in the Central Zone, suggesting that the Central Zone syenodiorite may represent Border Zone syenodiorite with the subtraction of alkali feldspars (syenite). Occurrence of syenite bodies

The syenites show a range of composition, from a plagioclase-rich type (PR), closer to the syenodiorite, but with fewer mafic minerals and which contains essential plagioclase; to a plagioclase-poor type (PP) consisting largely of alkali feldspar; but these two types have a strong bimodal distribution. Except for their proportions, the mineralogy of these rocks is similar to that of the syenodiorite. Chemical analyses published by Carman, et al. (1975, Table 1) show that combining Central Zone syenodiorite, PR syenite and PP syenite in the proportions 84.5%, 8% and 7.5%, respectively, gives the composition of the Border Zone syenodiorite. The proportions used for that calculation are those of the two types of sheets measured in detail in the field in the Central Zone, and this also suggests that the Central Zone syenodiorite was formed from the Border Zone type of magma by the subtraction of the two types of syenite. PR sheets range from about 5 to 30 cm thick (Fig. 20) and they are confined to the central 20 m of the sill (Fig. 16). Textures are coarse-grained to pegmatoid and simplectic (Fig. 21). PP sheets occur throughout the sill, but they are also concentrated in the center (Fig. 16). They range from 2 cm to 1 89 thick in the

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central part of the sill (Figs. 17 and 20), and become thinner and more fine-grained toward the margin. They are frequently bordered on one or both sides by PR syenite (Fig. 21). They range from aphanitic in chilled border rock to fine-grained to medium-grained in the Central Zone, and locally coarse-grained and diktytaxitic around miarolitic cavities in thicker sheets (Fig. 22).

Crystallization histories of alkaline intrusions in West Texas

301

Fig. 20. PR (1) and PP (2) syenite sheets. Central Sheet Zone, RMS. Length of scale, left center, 0.5 m.

Cylindroidal bodies occur in a 10 m zone immediately below the central sheet-rich zone (Fig. 16) They are vertically elongated and range from several centimeters up to nearly 2 m in length, and from 5 to 40 cm in diameter (Fig. 23), and a few are quite irregular in shape (Fig. 24). They often stand up in differential erosion. Most are composed of PP syenite, though a few PR ones and some transitional between the two are found (Fig. 25). Ocelli appear in a zone 1 m thick about 5 m below the top of the sill (Fig. 26). These consist of fine-grained PP syenite, and most have a large central filling of analcite.

In situ formation o f syenites The occurrences described suggest in situ formation for the syenite bodies. Mineralogy and textures give strong evidence for fractional crystallization.

302

M. F. Carman, Jr. and S. Indest

Fig. 21. PP syenite sheet bordered on both sides by PR syenite. Largest dark patches in PR syenite are simplectic intergrowths of vermicular augite enclosed in large plates of plagioclase.

Fig. 22. Miarolitic cavities with locally coarse-grained and diktytaxitic texture in PP syenite sheet. Central Sheet Zone, RMS. Scale is in centimeters.

Crystallization histories of alkaline intrusions in West Texas

303

Fig. 23. PP cylinder in Upper Cylinder Zone in RMS, exposed in three dimensions. Dark band in syenodiorite around cylinder is wetting to make cylinder show better. For example, strongly zoned plagioclase in syenodiorite is often mantled with alkali feldspar ranging from anorthoclase to outer rims of sanidine (Fig. 27). In fact, throughout all rocks of the sill, the minerals show epitaxial compositional zoning, as seen in microprobe analyses of feldspars in the different rock types (Fig. 28a). These show the ranges of zoned compositions, which form overlapping series going from syenodiorite to PR syenite to PP Syenite. Note that plagioclase in syenodiorite is more anorthite-rich than that of PR syenite, although they overlap. Mantles on plagioclase in syenodiorite range from anorthoclase to thin rims of sanidine; while those in PR syenite are divided, with an inner mantle of anorthoclase and an outer mantle of sanidine. In PP syenites anorthoclase is mantled with sanidine. Tracing the centers of the composition fields (Fig. 28b), sanidine is most K-rich in syenodiorite, becoming progressively more Na-rich through PR syenite to PP syenite, the expected progression in a fractionating ternary feldspar system. Also,

304

M. F. Carman, Jr. and S. Indest

Fig. 24. Irregular "cylinder" of transitional syenite. Upper Cylinder Zone, RMS. Hammer, upper right end of cylinder gives scale.

note the successive movement outward and toward the Ab corner of the diagram going from syenodiorite to PR syenite and then to PP syenite. This is diagnostic of increased water in the PR and then PP syenite liquids, an important point when considering their viscosities. The clinopyroxenes show similar overlapping series between syenodiorite and PR and PP syenites (Fig. 29), with increasing Fe and then Na content as fractionation proceeds. The overall picture is that of a coherent transitional system ranging from syenodiorite to PP syenite. This evidence of fractional crystallization illustrates how a syenodiorite parent magma evolved to form syenitic liquids. The rare earth elements pattern supports the magmatic differentiation of the syenodiorite (Fig. 30). The Border Zone (RM 178), which has only a few syenite sheets, is taken as closest to the intruding magma. The Central

Crystallization histories of alkaline intrusions in West Texas

305

Fig. 25. Overlapping cylinders with radiating veinlets: PR (1) cut by transitional type (2) and PP (3). Upper Cylinder Zone, RMS. Length of scale, right center, 10 cm.

Fig. 26. Syenite ocelli in syenodiorite. Arrow shows the "up" direction in sample. Scale is in centimeters.

306

M. F. Carman, Jr. and S. Indest

Fig. 27. Photomicrograph of Central Zone syenodiorite, showing plagioclase with anorthoclase mantles with thin sanidine rims. X43 UXN. Zone syenodiorite (RM 162) is depleted in light and heavy elements, with PR (RM 185A) and PP (RM 173C) syenite successivey enriched in them. The europium anomaly in the syenites reflects the separation of plagioclase from the differentiating liquid. Moreover, mass balance calculations support the formation of the syenites from the syenodiorite (Fig. 31). Such calculations, using chilled border rock as a parent and averages of multiple PR and PP syenite compositions as daughter end products, and assuming simple Rayleigh fractionation, show that with crystallization of reasonable proportions of early forming minerals, liquids of the compositions of PR and PP syenites can be produced. Crystallization of about 25 to 30% will produce PR syenite liquid and about 45 to 50 % will produce PP syenite liquid. Two different programs were used to make mass balance calculations for the purpose of cross checking the results ( G P P - - Baker, et al, 1985, and XLFRAC Stormer and Nicholls, 1978), and the results were esentially identical. R 2 is the residual difference between calculated chemical compositions and those measured in the daughter rocks. For GPP R 2 less than 1.00 is considered acceptable, and for X L F R A C the acceptable limit is 0.20.

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The problem of mechanisms for the formation of syenite sheets The demonstration of in situ magmatic differentiation for the formation of syenitic liquids does not solve the question of how the liquids were aggregated in the places and the forms in which they occur in the sill. The modal analyses demonstrate there was no large-scale settling of early-forming heavy minerals, such as has been

308

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Fig. 29. Microprobe analyses of RMS clinopyroxenes, from Cameron et al. (1986) a) Salitic augiteferroaugite data points. (b) Sodic pyroxenes.

most widely called upon to explain the single thick syenite layers in other alkaline mafic bodies. Nevertheless, some degree of gravity settling was operating in the sill. This is shown by small festoons of graded layers, on a scale of 3-5 cm thick and 10 cm long, which give an appearance almost of cross bedding (Fig. 32). They occur in the upper part of the Central Zone. The grading is seen in the color; the upper part of the festoon is lighter, essentially PR syenite, grading downward into a darker band. Thin section shows the color difference to be due to a slightly higher concentration of ophitic patches in the lower part of the festoon and their paucity in the leucocratic band. What process, or processes, could account for the occurrence of relatively thin sheets of syenite of two distinctly different compositions interspersed through the sill, one formed when the magma was only 25-30 % crystallized and the other only when

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_ 1.8-2.0 Gpa (Tsai et al., 2000) are unlikely to be produced in both island arc or overriding plate environment which are of "low pressure terrane". The metamorphosed lava with the meta-conglomerate at the base mentioned in the section "plagioclase-amphibolite" should not be the island arc products. As for the occurrence of ultramafic bodies in UMRB, all the harzburgite lherzolite, serpentinite, pyrigarnite and eclogite tectonically emplaced into gneissic rocks as was proved by drilling from 1958 to 1960 and recent field mapping (1/50,000) in the area covering the UMRB. Indeed, there are some ultramafic bodies such as Jiaoziyan, Shacun and Daoshichong pyroxenites are intrusives. However the Zhujiapu and Xiaohekou bodies (mainly pyroxenite) are unlikely to be the post collisional intrusives (see the "rock blocks") as suggested by Zhang et al. (1996) and Li et a1.(1999).

Some important features related to the tectonic setting of UMRB or north Dabie Complex Major geologic discontinuity across the UMRB. Not only stratigraphic differences as mentioned in the, section "geologic outline", but also the marked changes of lithology, geologic history and fauna assemblage are found across the UMRB. This is among the most important characteristics that point to the UMRB itself as the suture (Moores/Twiss, 1995) between Sino-Korea and Yangtze plate. Petrochemical features of the meta-flysch (including Luzhenguan and Foziling Group). The Luzhenguan Group at the base of Foziling Group consists of mozonitic gneiss, plagioclase amphibolite, mica-quartz schist, mica-schist, thin-bedded quartzite and marble, and their protoliths are greywacke, lithic sandstone, arkose, pelite and intermediate volcanics. The Foziling Group is composed of metasandstone of lower maturity, slate, phyllite and thin-bedded quartzite intercalated with schists. Their protoliths are sandstone, silt and pelite. Petrochemical features of sandstone indicate that it was the deposit close to continental-island arc or active continent margin (Fig. 10a,b) and is probably at the base of an accretionary prism. The REE pattern and trace element analyses give the same tectonic setting (Fig. 1l c,d, and e). It is unlikely that the products formed at the passive continental margin as suggested by Okay et al. (1993). Occurrence of eclogite and eclogitic rocks. The recently discovered eclogite and eclogitic rocks in different country rocks in UMRB are the most important discriminators in precluding the "island arc" and "hanging wall slab" environment, both of which are of low pressure terrane. The geochemical features show some relic slices of oceanic crust preserved, though they are of minor amount.

338

S. Xu, Y. Liu and W. Wu

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The ultramafi'c rock belt (UMRB)

339

Chaotic rock blocks of different environments and &tensively sheared matrix expressed as mylonitization (thrust) and folding. It is also the diagnostic feature of the UMRB (Xu et al. 1994) and characterizes the Franciscan m61ange (Hsfi 1968). It is known that belts of thrusts and folds can be the distinct tectonic units. Two families of such units can be recognized in an orogen: the foreland belts on continental lithosphere and the accretionary prisms at zones where oceanic lithosphere subducts (Seeber, 1983). By deformation and metamorphism, the UMRB is no doubt at the zone of accretionary prism but most of the oceanic rock blocks are presumed to be changed in some geochemical features by high temperature overprinting, or remain to exhumation. Multiple isotopic ages of both eclogite and gneissic country rocks. This is also the pronounced phenomenon of the UMRB (cf Figs. 6 and 8). Theoretically, plate motion or subduction is a successive process. The basaltic rocks in the leading edge of a descending slab would arrive at mantle depth earlier and would form the older eclogite, while the basaltic rocks in the middle portion or at the trailing edge of a descending slab would arrive at mantle depth later and form the younger eclogite. If the active subduction of an ancient oceanic plate in the Dabie Mountains started from late Sinian to early Paleozoic; and the following collision between Sino-Korea and Yangtze plate operated during early Carboniferous to late Triassic as is commonly believed, there should be the eclogites of different ages rather than the ages concentrated in a limited range (ca 220-230 Ma.). The most important fact is that the concordance of the isotopic ages with the metamorphic evolution of eclogite: 457 Ma, 264 Ma, 220-230 Ma and 210Ma seem to reflect eclogite metamorphism. The 190 Ma age is thought to reflect the high temperature overprinting, causing intensive deformation of eclogite at the early stage of exhumation of eclogite and eclogitic rocks.The important result of isotopic dating of tonalitic gneiss in UMRB is the zircon U-Pb age around 225-230 Ma (CF Fig. 8) which are absent from the gneisses beyond the UMRB (Fig. 9). It implies that the monzonitic and granodioritic gneisses or the Dabie complex have never subducted to great depth (autochthonous), and are likely to be the underthrust basement of Yangtze plate underlying the UMRB. CONCLUSION Combination of lines stated above leads us to the conclusion that the UMRB is a suture between Sino-Korea and Yangtze plate. As for the nature of the suture, we prefer the "meta-tectonic m61ange" instead of the early suggestion "meta-ophiolite m61ange" because only a few relic slices of oceanic crust have been found. Geologists should never term the geologic unit as "island arc complex" based only on the presence of some amount of island arc materials, because it is possible that both continental and island materials can be involved in a tectonic m61ange during subduction and exhumation. With the meta-tectonic m61ange, the architecture of the Dabie Mountains can be summarized in a sketched section from Huoshan to Huangshi (insert in Fig. 1). The areal distribution of UMRB in the west portion may be formed by thin-skin tectonics with an underlying master detachment, by which the UMRB or northern Dabie complex may have been transported far from its root.

340

S. Xu, Y. Liu and W. Wu

Stripped of the thin-skin component, the boundary (suture) between Sino-Korea and Yangzte plate in Hubei Province may appear much narrower as it does in the east portion in Anhui Province. The eclogite belt (or UHPMB) is also a tectonic m61ange coherent with the UMRB, but it will not be discussed in the present paper. ACKNOWLEDGEMENT This project is financially supported by NSFC (No. 49794041), Ministry of Territory and Resources of China and Anhui Bureau of Geology and Mineral Resources. The zircon U-Pb analyses were performed by Prof. Li Huiming. Prof. Borming Jahn of the Rennes University of France checked the sample 98702 of SmNd isotope analyse. The zircon samples are carefully prepared and selected by Prof. Fang Zhong of Nanjing University. We give many thanks to them. We are grateful to Prof. Ken Hsfi for his review and improving the English of our manuscript. We also give thanks to our colleague engineer Lu Yiqun for completing the illustrations. REFERENCES Ames, L., Tilton, G.R. and Zhou, G.Z., 1993. Timing of collision of the Sino-Korea and Yangtze cratons: U-Pb zircon dating of Coesite-bearing eclogites. Geology, 21: 339-342. Bhatia, M.R., 1983. Plate tectonics and geochemical composition of sandstone. J. Geology, 91: 611-621. Bhatia, M.R. and Crook, K.A.W., 1986. Trace element characteristics of graywackes and tectonic setting discrimination of sedimentary basins. Contrib. Mineral. Petrol., 192(2): 181-193. Bureau of Geology and Mineral Resources of Anhui Province, 1997. Stratigraphy of Anhui Province. China University of Geosciences Press, pp. 31-198. Chavagnac, V. and Jahn, B., 1996. Coesite-bearing eclogite from the Bixiling complex, Dabie Mountains. Chemical Geology, 133: 29-51. Coleman, R.G., 1977. Ophiolites. Springer-Verlag, Berlin, Heidelberg, New York. 229pp. Condie, K.C., 1989. Plate Tectonics and Crustal Evolution (third edition). Pergamon Press, 165-207. Cuthbert, S.J. and Carswell, D.A., 1990. Formation and exhumation of medium-temperature eclogite in the Scandinavian Caledonides. In: Carswell (Ed.), Eclogite Facies Rocks. Blackie, Glasgow and London, pp. 180-203. Defant, M.J. and Drummond, M.S., 1990. Derivation of some modern arc magmas by melting of young subducted lithosphere. Nature, 347: 662-665. Eide, E., 1995. A model for the tectonic history of the HP and Ultrahigh Pressure Metamorphism Coleman, R.G. and Wang, X. (Eds.), Cambridge University Press, Cambridge, pp. 391-426. Ge Ninjie, Li Shuguang, Peng Zhonghua, et al., 1993. Mineral composition and metamorphic P-T conditions of the eclogite in the eastern part of the Dabie Mountains and their genetic implication. Acta Geologica Sinica (in Chinese with English abstract), 67(2): 109-122. Hacker, B.R. and Wang, Q., 1995. Ar/Ar geochronology of ultrahigh-pressure metamorphism in central China. Tectonics, 14: 994-1006. Hacker, B.R., Raschbacher, L., Webb, L., Ireland, T., Walker, D. and Dong, S., 1998. U/Pb zircon ages constrain the architecture of the ultrahigh-pressure Qinling-Dabie Orogen, China. Earth and Planetary Science Letters, 161: 215-230. Hsfi, K.J., 1968. Principles of m~langes and their bearing on the Franciscan-Noxville paradox. Geol. Soc. Amer. Bull., 79: 1063-1074. Ji. S. and Martignole, J., 1994. Ductility of Garnet as an indicator of extremely high temperature deformation. J. Structural Geology, 16(7): 985-996. Jian P., Yang W., Li, Z. and You H., 1997. Isotopic chronological evidence for the Caledonia Xongdian eclogites in the western Dabie Mountains (in Chinese with English abstract). Acta Geologica Sinica, 71: 133-141.

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Lin, Qianlong, Shi Quanzheng and Fu Guanghong, 1985. A preliminary research on the outline of plate tectonics and distribution of mineral deposits of eastern Qinling, Henan Prov. Regional Geology of China, 17(2): 77-84 (in Chinese with English abstract). Li Caiyi, Ma Jianguo and Chen Ruibao, 1988. New information of the sequence and age of Erlangping Group. (in Chinese with English abstract). Henan Geology, 6(4): 33-39. Li Shuguang, Jagoutz, E., Xiao Yilin, Ge Ningjie and Chen Yizhi, 1996. Chronology of ultrahigh-pressure metamorphism in the Dabie Mountains and Su-Lu terrane: I. Sm-Nd isotope system. Science in China (series D), 39(6): 597-609. Li Shuguang, Hong Ji'an, Li Huiming and Jiang Laili, 1999. U-Pb zircon ages of the pyroxenite-gabbro intrusions in Dabie Mountains and their geological implications. Geological Journal of China Universities, 5(3): 351-355. Liu Yican, Xu Shutong, Jiamng Laili, Wu Weiping, Su Wen and Chen Guanbao, 1999. Geochemical constrains to the meta-ophiolite from the metamorphosed mafic-ultramafic belt in the northern part of the Dabieshan orogen. Journal of Mineralogy and Petrology, 19(1): 68-73 (in Chinese with English abstract). Moores, E.M. and Twiss, R.J., 1995. Tectonics, W.H. Freeman and Company, New York, pp. 212-241. Okay, A.I., Xu Shutong and Seng6r, A.M.C., 1989. Coesite from the Dabieshan eclogites, central China. Eur. J. Mineral., 1: 595-598. Okay, A.I., 1993. Petrology of diamond and coesite-bearing metamorphic terrane: Dabie Shan, China. Eur. J. Mineral., 5: 659-673. Okay, A.I. and Seng6r, A.M.C., 1993. Tectonics of an ultrahigh-pressure metamorphic terrane: the Dabie Shan/Tongbai Shan orogen, China. Tectonics, 12(6): 1320-1334. Roser, B.P. and Korsch, R.J., 1986. Determination of tectonic setting of sandstone-mudstone suites using SiO2 content and KzO/Na20 ratio. J. Geology, 94: 635-650. Seeber, L., 1983. Large scale thin-skin tectonics. Reviews of Geophysics and Space Physics, 21(6): 1528-1538. Seyfert, C.K. (Ed.), 1987. The Encyclopedia of Structural Geology and Plate Tectonics, Van Nostrand Reinhold Company. New York, 561-634. Tsai Chin-Ho, Liou J.G. and Ernst, W.G., 1998. Eclogite facies relics and retrogressed garnet-peridotite in the North Dabie Complex, central-eastern China, and suggested implications for regional tectonics (abstract). In: Collection of International Workshop on UHP Metamorphism and Exhumation. Stanford University, pp. A-153. Tsai chin-Ho and Liou, J.G., 2000. Eclogite-facies relics and inferred ultrahigh-pressure metamorphism in the North Dabie Complex, central-eastern China. American Mineralogist, 85: 1-8. Wang, Q., Cong, B., Zhai, M. and Spark, J.W., 1994. A possible paleozoic island arc: petrochemical evidences from North Dabie gneiss. In: Annual Peport (1993-1994) of The Laboratory of Lithosphere Tectonic Evolution of Institute of Geology. Chinese Academy of Sciences, Seismology Press, Beijing. pp. 31-37. Wang, Q., Liu, X., Maruyama, S. and Cong, B., 1995. Top boundary of the Dabie UHPM rocks, central China. J. SE Asia Geosci., 11: 295-300. Wang, X., Liou, J.G. and Mao H.K., 1989. Coesite-bearing eclogite from Dabie Mountains in central China. Geology, 17: 1085-1088. Wang, Q., Xu, J., Wang, J., Zhao, Z., Wang, R., Qiu, J., Xong, X., Sang, L. and Ben H., 2000. Confirmation of the adakite in northern Dabie Mountains and its relationships with ultrahigh pressure metamorphism (in Chinese). Chinese Science Bulletin, 45(10): 1017-1024. Wei Chunjing, Shan Zhenggang, Zhang Lifei, Wang Shiguang,Chang Zongguang, 1997. Confirmation of North Dabie eclogite and its geologic implication. Chinese Science Bulletin (in Chinese), 17: 1832. Xu Zhiqin, 1987. Etude tectonique et microtectonique de la Chaine Paleozoique et Triasique des Quilins (Chine). Thesis de Doctorat. Univ. Sci. Tech. du Languedoc. Xu Shutong, Su Wen, Liu Yican and Jiang Laili, 1991. Diamonds from the high pressure metamorphic rocks, eastern Dabie Mountains. Chinese Science Bulletin (in Chinese), 36(17): 1318-1321. Xu Shutong, Okay, A.A., Ji Shouyuan, Seng6r, A.M.C., Su Wen, Liu Yican and Jiang Laili., 1992a. Diamonds from Dabie Shan metamorphic rocks and its implication for tectonic setting. Science, 256: 80-82.

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Xu Shutong, Jiang Laili, Liu Yican and Zhang Yong, 1992b. Tectonic framework and evolution of the Dabie Mountains, eastern China. Acta Geologica Sinica, 5(3): 221-238. Xu Shutong, Liu Yican, Jiang Laili, Su Wen and Ji Shouyuan, 1994. Tectonic Regime and Evolution of Dabie Mountains ( in Chinese with English abstract), Science Press, Beijing, 1-86. Xu Shutong, Jiang Laili, Liu Yican and Su Wen, 1996. Structural Geology and Ultrahigh pressure Metamorphic Belt of the Dabie Mountains in Anhui Province. Field Guide Book of 30th IGC(T328). Geological Publishing House, Beijing, China. Xu Shutong, Wu Weiping, Su Wen, Jiang Laili and Liu Yican, 1998a. Meta-granitoid from the highultrahigh pressure metamorphic belt in the Dabie Mountains and its tectonic significance, (in Chinese with English abstract). Acta Petrological Sinica, 14(1): 42-59. Xu Shutong, Liu Yican,Su Wen, Jiang Laili and Wu Weiping, 1998b. Eclogite in northern Dabie Mountains and its tectonic implication (Abstract). In: Collection of International Workshop on UHP Metamorphism and Exhumation, Stanford University, pp. A-151. Xu Shutong, Liu Yican, Jiang Laili, Su Wen and Wu Weiping, 1999a. Confirmation of the ultrahighpressure metamorphic belt in the Dabie Mountains and related problems. Progress in Natural Science, 9(1): 15-23. Xu Shutong, Liu Yican, Su Wen, Wu Weiping, Jiang Laili and Wang Rucheng, 1999b. Geometry, kinematics and tectonic implication of the deformed garnets in the foliated eclogite from the ultrahigh pressure metamorphic belt in the Dabie Mountains, eastern China (in Chinese with English abstract) Acta Petrologica Sinica, 15(3): 321-337. Xu Shutong, Liu Yican, Su Wen, Wang Rucheng and Jiang Laili, 1999c.Discovery of the eclogite and its oetrography in the northern Dabie Mountains (in Chinese). Chinese Science Bulletin, 44(13): 14521456. Xu Shutong, Liu Yican, Su Wen, Wang Rucheng, Jiang Laili and Wu Weiping, 2000. Discovery of the eclogite and its petrography in the northern Dabie Mountains. Chinese Science Bulletin, 45(3): 273-278. You Zhendong, Han Yujing, Yang Weiran, Zhang Zeming, Wei Bize and Liu Rong, 1996. The Highpressure and Ultrahigh Pressure Metamorphic Belt in the East Qinling and Dabie Mountains, China, China University of Geosciences Press, Wuhan, 25-26. Zhang, Q., Ma Baolin, Liu Ruoxin, Zhao Dasheng, Fan Qicheng, Li Qi and Li Xiuyun, 1995. A continental lithosphere slice obove a subduction zone - - Geochemistry of Raobazhai ultramafic rock, Anhui Province (in Chinese). Science in China, 25(8): 865-873. Zhang, R.Y., Liu, J.G. and Tsai, C.H., 1996. Petrogenesis of a high-temperature metamorphic terrane: a new tectonic interpretation for the northern Dabieshan, central China. Journal of Metamorphic Geology, 14: 319-333.

Paradoxes in Geology Ueli Briegel and Wenjiao Xiao, eds. 9 2001 Elsevier Science B.V. All rights reserved.

Chapter 15

Nd ISOTOPES VS. KEN HSU'S TECTONIC FACIES: A CASE STUDY OF CRUST EVOLUTION IN SOUTH CHINA

Xin-Hua Zhou Institute of Geology and Geophysics, Chinese Academy of Sciences, P.O. Box 9825, Beijing 100029, P.R. China

ABSTRACT The Nd isotope approach, both initial ratio and model age, has been employed to constrain the source characteristics of both igneous and sedimentary systems in two case studies in South China, which would provide timing and source constraints on the granitoids in South China Block and the source province of sedimentary rocks in Yangtze Block, which is compatible and supplementary to the study of tectonic facies proposed by Ken Hsfi in the same region. It would be a great challenge to test this combined approach in regions with a distinct tectonic background.

INTRODUCTION The tectonic framework and crustal evolution of South China have been a longstanding hot debate in the geological community of China. A variety of tectonic models have been proposed, in which South China has been described geologically as the well-known Cathaysia landmass, the Caledonia geosyncline, an arc-trench-basin system and the collision tectonics (e.g. Huang et al., 1977; Hsti et al., 1987) either in the Precambrian or in the Paleozoic. However, none of the above tectonic models has been well acknowledged as a successful and dominant theory in delineating the geological evolution of South China. One of the major controversies in this field is related to the origin of granitoids in the region, which are widely spread over eleven administrative provinces, with ages ranging from Neoproterozoic to late Mesozoic, generally younging towards the coast. It has puzzled the Chinese geologists for a long time (e.g. Jahn et al., 1990, Fig. 1). Whether this distribution pattern is related to any tectonic event and indicative of coeval tectonism or is required to be interpreted in different ways in different geological times remains unsolved and is quite debatable. The introduction of the concept of the tectonic facies (Hst~, 1995, Hsfi and Chen, 1999) has challenged all existing models, and provides unique insights into tectonic origins of lithostratigraphic units observed and mapped by geologists. It has been applied to a typical and well-known orogeny in Europe, the Alps, hence, to lead the author to revisionism in writing the Geology of Switzerland (Hsfi, 1995). It also

344

Xin-Hua Z h o u

-

Yangtze Cover

B

r~

6t ]sx 4f ]sx f isxLsxl SX

Metasediments or m61ange

(b)

SX

2

1.4

1.6

1.8

I

I

2.0

I

I

2.2

taG Fig. 1. Comparison of TGc histogram of (a) post Sinian sediment cover from Yangtze Craton, and (b) metasediments or m61ange from western Zhejiang and southern Anhui. SX ShangxiGroup, S Q Shaunxiwu Group, Z -- Sinian, the others stand for the abbreviations of relevant periods of Paleozoic Era, as usually used in geological time scale. Modified from Zhou (1998).

impacts on interdisciplinary studies, and it greatly helps to recognize and to understand relevant information in different subjects. The Chinese geochemists have been inspired to adopt this concept to justify a variety of models regarding the origin of granitoids in South China. We will present a case study in this paper on a new insight into the origin of granitoids in South China. T H E Nd ISOTOPE A P P R O A C H , D A T A C O L L E C T I O N AND NOTATION Nd isotope has provided enormous information on the evolutionary history of continental crust since the mid 80s. Nd isotopic compositions of clastic sediments have yielded much information on growth history of continental crust as they reflect average crust residence ages. This approach is based on the Nd isotopes, as an accumulated radiogenic tracer of the natural decay of its parent isotope 147Sm,which is one of the rare earth elements and broadly used in geochemical studies.

345

N d isotopes vs. Ken Hsu"s tectonic facies

The relevant parameter is often expressed as initial ratio (after decay correction) or model age. Several authors (McColluch and Wasserburg, 1978; Depaolo, 1981; O'Nions et al., 1983) have proposed different forms of model ages, such as the chondritic uniform reservoir model age, T c H u a , the depleted model age, TDM and the crustal residence age, TCR, to delineate the evolution of terrestrial material. The crustal residence age has been widely and successfully applied in a variety of studies (Allegre et al., 1984; Goldstein et al., 1984; Michard et al., 1985; Zhou and Goldstein, 1990; Zhou et al., 1997). However, by its definition, the application of Tca is restricted to the sedimentary mass of the earth. It is required to extend this method beyond the sedimentary rocks if one wants to solve any problem related to igneous systems. Based on the author's experience in studying the source province of metamorphic rocks in southwestern Zhejiang (Zhou et al., 1992), it has been postulated that to extend the application of the crustal residence age, Tca to most rocks of crustal origin, such as igneous and metamorphic rocks. Taking the form and calculation of two-stage model, one can define this model age as the generalized crustal residence age, Tcc. If the first stage of the evolving parameter, Sm/Nd, in the calculation of two-stage model age, can be reasonably assumed or calculated by some available data, thus, one is able to calculate the generalized crustal residence age TGc. Actually, for a same system with identical parameter (Sm/Nd), we could have TDM--- TCR = T~c, but, with distinct or broader geologic and geochemical meaning of the generalized crustal residence age, T~c. This postulated and modified Nd model age, T~c, can be applied to characterize the source material for the given rocks in a given region, i.e. to geochemically characterize the lithostratigraphic unit sampled and analyzed. Logically, these modified model ages must be related to the model of tectonic facies, as proposed by Hsfi (1995), which constrains the source material characteristics. In this study, as a case, we will test the above hypothesis by correlating any relationship between the geochemical characters of the source material of granitoids and tectonic facies in the region studied in South China. Following tradition, we take a standard epsilon notation and model age expression as most of the international authors, such as Depaolo and Wasserburg (1976), do for the data of Sm-Nd isotopic system cited in this paper. That is, for the epsilon notation:

8Nd(/)- {[(143Nd/144Nd)sample/(143Nd/144Nd)mantle]-

1}, 104

where, (143Nd/144Nd)sample and (143Nd/144Nd)mantle are the Nd isotopic ratios of the measured sample and the mantle of relevant models (Chondritic uniform reservoir for TCHUR, and depleted mantle for TI~M and T~c) at time t, respectively. To avoid possible discrepancy in parameter used for the model age calculation, the initial ratio of the Nd isotopes is emphasized to construct the pattern diagram in this study. All the age corrections are based on published quality geochronological data. Major reference sources for Nd isotope data are from Huang et al. (1986, 1989), Jahn et al. (1990), Li et al. (1992), and author's unpublished data.

346

Xin-Hua Zhou

CALEDONIA GRANITOIDS OF SOUTH CHINA The Nd isotope approach has been applied to investigate the source characteristics of Caledonia granitoids as a first step to test the hypothesis. 24 granitoids in the region within the age range from 340 Ma to 470 Ma are selected for this purpose and three major types of pre-Caledonia crustal rocks in the region are also selected as potential candidates of sources. They are (1) the sedimentary cover, such as Carboniferous, Devonian, Silurian, Ordovician, Cambrian and Neoproterozoic (Sinian) sediments; (2) the Precambrian basement exposed on the surface in the Zhejiang and Fujian provinces with well-accepted geochronological evidence and (3) the M61ange and metasediments. All the Nd isotopic ratios, 143Nd/144Nd, of whole rock samples have been corrected back to 430 Ma, a typical average granitoid age in Caledonia, as the initial ratio for comparison. The results are as follows: granitoids (-4 t o - 1 4 ) , sediment cover (-9 to -15), m~lange and/or metasediments (-3 t o - 1 2 , with one exception,-1) and basement (-13 t o - 2 0 ) respectively. Clearly, the initial ratios of both the sediments and the basement are too small to match the range of the granitoids while it seems that only the m61ange and/or metasediments show quite similar range as the granotoids. Based on their source signature, as expressed by the Nd isotope initial ratio, this comparison strongly suggests that there is a remarkable affinity of source characteristics between a part of the m61anges and metasediments, such as the Shangxi Group in Anhui, and the Shuanqiaoshan Group in Jiangxi and the granitoids. This observation implies that these m61ange and/or metasediments are possible candidates for the granitoids source material, at least based on their Nd isotopic signatures. Secondly, the initial Nd isotope ratios and the model ages are plotted onto the regional geological map (Fig. 1), and the results even surprisingly show a regular pattern, forming a series of contours of this isotopic parameter. The pattern is trending in a NE-SW direction, with less enriched isotopic parameter in the center, such a s - 4 t o - 6 in end values, and more enriched for the outer parts, such a s - 1 0 to -12 in end values. The above observation could be used to argue for a hypothesis of a m61ange and/or metasediment origin for the granitoids, in which the less enriched isotopic parameter is interpreted as a mixed character dominated by juvenile crustal mixture (its geological expression being as young orogenic material, such as those of volcanic origin), whereas the more enriched isotopic parameter is interpreted as that dominated by recycled and old crust. Thus, the accretionary wedge, or flysch facies, should be the first candidate for the tectonic expression of source material. This implication has been supported by a geochemical study (Xing, 1990), in which a local Neoproterozoic low-grade metasedimentary rock was taken as a source material to model the granitoids. Furthermore, the Nd isotopic plot pattern would bear a clue on the distribution of tectonic facies during the pre-Caledonia age in the region, which would place constrains on the distribution of the tectonic facies in the region and is compatible with both proposed tectonic models of collision and archipelago. This observation has been further confirmed by another independent study based only on the Sr isotope data, but showing a fairly similar pattern (Sun and Xu, 1990).

347

N d isotopes vs. Ken Hsfi's tectonic facies

I M P L I C A T I O N TO C L A S S I F I C A T I O N OF G R A N I T O I D S Similar observations have been made for the granitoids with different geological ages, such as Neoproterozoic (>850 Ma), Early (200-260 Ma) and Late Mesozoic (65-190 Ma) granitoids. In all the cases, the source characteristics of granitoids require a major component with strong affinity to that of metasediments in the region although other components could be involved in the source processes in different degrees. For example, in the early Mesozoic, the older, recycled components could play a significant role. However, it would be in less degree for the late Mesozoic ones. In short, it can be deduced that the origin of granitoids is constrained by both the nature of possible source material and the petrogenesis environment, which is usually expressed as the tectonic facies. On the basis of this study and combined with previous work, a granitoid classification is proposed, in which the genesis, occurrence and type of granitoids would be mainly constrained by the nature of source material and tectonic facies (Table 1).

S O U R C E P R O V I N C E : A CASE S T U D Y OF W E S T E R N Z H E J I A N G A N D SOUTHERN ANHUI The Nd isotope approach is also applied to study the source province for a given region in the Yangtze Block, South China. A case study is taken in southern A n h u i western Zhejiang, where the Sinian and Paleozoic sediments are well exposed, as well as a debatable m61ange and/or metasediments (traditionally, in the Neo- to Meso-Proterozoic, such as the Shanxi G r o u p in Anhui, the Shuanqiaoshan G r o u p in

Table 1 A proposed classification of granitoids Source

Subduction

Collision

Case Type S China Celtide A Oceanic-sediments, No magmatic Arc, water saturated Celtide B Oceanic-sediments, magmatic Arc, water saturated Celtide A Oceanic crust, dry-water unsaturated Rhaetide, Continental crust

Case

G l o b a l Type S China

Global

TDM 8Nd(T) esr(T)

SaS

Pz

Andino-S SaC

Mz

Hercyno-S M

M-L

M-H

SbS

Pz

Andino-I SbC

Pz

Hercyno-S

M-H

M-L

NeoPt Andino- I, OcC M-

Pz

Hercyno-I LL

H

L

Mz

Hercyno-S H

LL

H

OcS

CcS

Pz

S-

LL - extreme low, L - low, M - median, H - high

CcC

L

348

Xin-Hua Zhou

HANCeHOU

CHANGSHA o -9.0fi.8.2 ~-11.0/2.1

GUIYANG -7.1/1.72

-7.8/1.83

12.7/221 [03/2.01

o

GUANGZHOU

-6.2/1.71 ~Nd/(t)/tGC ~. Contoursof ENd/(t)

Fig. 2. Sketchmap showingthe regular distribution of eNd (t) and TGc of early Paleozoicgranitoids (340460Ma). Modified from Zhou (1998).

Jiangxi and the Shaunxiwu Group in Zhejiang). The existing crustal sedimentary cover, including those of m61ange and/or metasediments is sampled for Nd isotope investigation. All isotopic parameters are calculated to the crustal residence age, TCR, or TGc, and then plotted in a histogram (Fig. 2). From this diagram, four remarkable features can be seen. (1) Both sediments and metasediments fall closely in their respective narrow range, which implies a highly uniform homogeneity in the source region with respect to rare-earth elements and precludes any disturbances caused by tectonic processes. (2) Data from metasediments, geologically older, show a depleted isotopic signature and younger TGo which indicates a dominant role of juvenile materials in their source. (3) In strong contrast, data from the sediments, geologically younger, show an enriched isotopic signature and older TGc, which suggests a dominant role of recycled ancient materials in their source. (4) The above and observed pattern does apply to both the southern Anhui and western Zhejiang regions. Assuming the majority of metasediments are of pre-Sinian age, as assumed by most geologists, the study of Nd general crust residence age, Toc, reveals that they must have been derived from a juvenile source region, such as erosion of island-arc volcanism, even meta-volcanic rocks, with minor input of recycled materials, which in turn requires an active tectonic environment, such as one being compatible with a paleo-island arc, continental active margin, back-arc basin or marginal basin.

N d isotopes vs. Ken Hsii's tectonic facies

349

The major characteristics are that the source materials are juvenile in composition, and are from an active tectonic background. For the cover sediments (the Sinian plus Paleozoic), the results demonstrate that the source must have been derived from an ancient, recycled crust, of apparent Paleoproterozoic age. The time boundary of two formations is constrained just at the end of the Sinian, ca. 800-900 Ma, when local tectonic framework would have changed dramatically. This Neoproterozoic collision event, as evidenced by a series of studies (Zhou and Zhu, 1992; Li, 1992; Hsfi and Chen, 1999), did change the regional tectonic settings, which could have provided a vast and stable erosion provenance for post-Sinia strata, and thus caused the older apparent crustal residence age. On the other hand, the pre- and post-collisional tectonic processes did involve in island-arc, back-arc basin, active margin and accretionary wedge, which would have been the reservoir of juvenile materials as recorded in the metasediments with younger TGc. Nevertheless, this result could also be compatible with a part of the metasediments originating from the m61ange components of Paleozoic age. However, if this is the case, the juvenile materials must be balanced by a large proportion of older recycled component, to yield a relatively younger apparent crustal residence age as observed. In addition, the results show a sharp contrast between the sedimentary cover, aging from the Sinian to Carboniferous, i.e. pre-Permian, and the metasediments and a few Permian rocks. This observation remarkably indicates that there were episodic and dramatic changes of the tectonic regime in the Late Neoproterozoic and Permian, which is compatible with the collision tectonics models proposed by some authors (Hsfi et al., 1990; Li, 1992; Zhou and Zhu, 1992). CONCLUDING REMARKS The Nd isotope approach has been successfully employed to constrain the source characteristics of both sedimentary and igneous systems in the case study of granitoids in South China and the source province in the Yangtze Block,which is compatible and supplementary to the study of tectonic facies in the same region. It would be a great challenge to test this combined approach in regions with a distinct tectonic background. ACKNOWLEDGMENTS: This study was initiated, encouraged and supported by Professor Ken Hsfi when the author made an academic visit at ETH, conducting a joint project on lithosphere evolution of SE China. Ken Hsfi actually worked as a coauthor all the time. The project is also partially supported by the NSF of China. This paper is dedicated to the special symposium on the occasion of Professor Ken Hsfi's seventieth birthday. REFERENCES Allegre, C.J. and Rousseau D., 1984. The growth of the continent through geological time studied by Nd isotope analysis of shales. Earth Planet. Sci. Lett., 67: 19-34.

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Depaolo, D.J., 1981. Neodymium isotopes in the Colorado Front Range and crust-mantle evolution in the Proterozoic. Nature, 291: 193-196. Depaolo, D.J. and Wasserburg, G.J., 1976. Nd isotopic variations and petrogenetic model. Geophys. Res. Lett., 3: 249-252. Dia, A., Allegre, C.J. and Erlank, A.J., 1990. The development of continental crust through geological time: the South African case. Earth Planet. Sci. Lett., 98: 74-89. Goldstein, S.L., O'Nions, R.K. and Hamilton, P.J., 1984. A Sm-Nd isotopic study of atmospheric dust and particulates from major river systems. Earth Planet. Sci. Lett., 70: 221-236. Hsii, K.J., 1995. The geology of Switzerland and an introduction to tectonic facies. Princeton University Press, Princeton, New Jersey. Hsii, K.J., and Chen H.H., 1999. Geologic Atlas of China. Elsevier, Amsterdam. Hsfi, K.J., Li, J.L., Chen, H.H., Wang, Q.C., Sun, S. and Seng6r, A.M.C., 1990. Tectonics of South China: key to understanding West Pacific geology. Tectonophysics, 183: 9-39. Hsii, K.J., Sun, S., Li, J.L., and Chen, H.H., 1987. Huanan Alps, not South China Platform. Science in China (Series B), 31:1107-1115. Hsii, K.J., Sun, S., Li, J.L., Chen, H.H., Pen, H., and Seng6r, A.M.C., 1988. Mesozoic overthrust tectonics in South China. Geology, 16:418-421. Huang, J.-Q., Ren, J.S., Jiang, C.-F., Zhang, Z.M. and Xu, Z.-Q., 1977. An outline of the tectonic characteristics of China. Acta Geologica Sinica, 51:117-135 (in Chinese). Huang, X., Sun, S.H., Depaolo, D.J. and Wu, K.L., 1986. Nd-Sr isotope study of Paleozoic granitoids and the basement of southern China. Acta Petrologica Sinica, 5:28-36 (in Chinese). Huang, X., Sun, S.H., Depaolo, D.J. and Wu, K.L., 1989. Nd-Sr isotope study of Cretaceous magmatic rocks from Fujian Province. Acta Petrologica Sinica, 2:50-63 (in Chinese). Jahn, B.-M., Zhou, X.-H. and Li, J.-L., 1990. Formation and tectonic evolution of southeastern China and Taiwan: isotopic and geochemical constraints. Tectonophysics, 183: 145-160. Li, J. L., 1992. On the tectonics of Southeast China. In: Li, J.L. (Ed.), Structure and evolution of Southeast China Lithosphere. Publisher of Science and Technology, Beijing, pp. 1-24 (in Chinese). Li, X. H., Zhao, Z.H., Gui, X.T. and Yu, J. S., 1992. Samarium-neodymium and zircon uranium-lead isotopic constraints on the age of formation of the Precambrian crust in southeast China. China Journal of Geochemistry, 11:111-120. McColluch, M.T. and Wasserburg, G.J., 1978. Sm-Nd and Rb-Sr chronology of continental crust formation. Science, 200: 1003-1011. Michard, A., Gurrier, P., Soudant, M., and Albarede, F., 1985. Nd isotopes in French Phanerozoic shales: external vs. internal aspects of crustal evolution. Geochim. Cosmochim. Acta, 49: 601-610. O'Nions, R.K., Hamilton, P.J. and Hooker, P.J., 1983. A Nd-isotope investigation of sediments related to crustal development in the British Isles. Earth Planet. Sci. Lett., 63: 229-240. Sun, M.Z. and Xu, K.Q., 1990. On the origin of Caledonian granitoids in southern China. Journal of Nanjing University (Earth Sciences), 4:10-22 (in Chinese). Xing, F.M., 1990. Late Precambrian Jingning granitoids in southern Anhui. Science in China (B series), 11: 1185-1195. Zhou, X.H. and Goldstein, S.L., 1990. Continental growth and sediment evolution: Nd isotope profile of the Sinian Formation, North China. Contribution to ICOG, Australia Geological Society, Abstracts, 27: 116. Zhou, X.H., 1998. Isotope geochemical constraints on the tectonic evolution of Southeastern China. In: Tu Guangzhi and Chow, T.J. (Eds.), Isotope geochemistry research in China. Science Press, Beijing, pp. 63-73. Zhou, X.H., Cheng, H. and Zhong, Z.H., 1992. The source province of metamorphic rocks from Southwest Zhejiang a case study. In: Memoir of Lithosphere Tectonic Evolution Research (1). Seismological Press, Beijing, pp. 114-120. Zhou, X.H., Hu, S.L., Goldstein, S.L., Li, J.L. and He, J., 1997. Episodic growth of the continental crust in SE China: Nd isotopic approach. In: Wang, H.Z., Jahn, B.-M., and Mei, S.L. (Eds.), Origin and history of the earth. Proceedings of the 30th International Geological Congress, Vol. 1, VSP BV, Utrecht, The Netherlands, pp. 143-153. Zhou, X.M. and Zhu, Y.H., 1992. Magma mixing and Precambrian geology of Jiangshan-Shaoxing Fault zone. Science in China (series B), pp. 296-303.

Part IV

PALEOGEOGRAPHY, PALEOCEANOGRAPHY AND P A L E O C L I M A T O L O G Y

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Paradoxes in Geology Ueli Briegel and Wenjiao Xiao, eds. 9 2001 Elsevier Science B.V. All rights reserved.

Chapter 16

THE MESSINIAN SALINITY CRISIS IN THE MEDITERRANEAN

Maria Bianca Cita Department of Earth Sciences, University of Milano, Milano, Italy

ABSTRACT A dramatic change in paleogeography and paleoceanography affected the Mediterranean area in the terminal part of the Miocene Epoch, when the connections with the Atlantic were restricted and then interrupted altogether as a result of plate motions combined with lowering of sea-level. Consequences of this disconnection include the deposition of over one million cubic kilometers of salts in the deepest parts of the Mediterranean basins, subaerial erosion and canyon cutting along the passive continental margins, the destruction of all marine biota. The big picture came out from the first drilling campaign of the Glomar Challenger in the Mediterranean in 1970, in the early days of the DSDP, under the illuminated leadership of Bill Ryan and Ken Hsti, originating a lot of interest and controversial issues. Four more drilling campaigns were accomplished in the Mediterranean in 1975, 1986, 1995, each of them providing new data supportive of the deep basin desiccation model originally proposed. Researches on Messinian peripheral basins on land have been very active, and in the late nineties an unprecedented stratigraphic resolution has been obtained combining multiple biostratigraphies, magnetostratigraphy and astrocyclostratigraphy. So, we now know that the initiation of the salinity crisis proper (= first deposition of evaporitic limestones following a period of restricted circulation) was strictly synchronous in the western (Sorbas, Spain), central (Caltanissetta, Sicily) and eastern Mediterranean (Gavdos, Greece) and occurred 5.9 My ago. We also know that the termination of the salinity crisis (= sudden return of open marine conditions in the Mediterranean) was strictly synchronous. In this latter case, the record includes not only sections exposed on land, but also the deep-sea record: indeed, the last two Mediterranean cruises (ODP Legs 160 and 161) proved beyond any doubt that the flooding of the Mediterranean was synchronous, and occurred 5.33 My ago. The base of the Trubi Formation at Eraclea Minoa (Sicily) that materializes the event, has been recently validated by IUGS as the GSSP for the Zanclean Stage and for the Pliocene Epoch. What remains controversial and may originate paradoxical interpretations is the 3D reconstruction of the Messinian events with special reference to the extension and significance of the intramessinian unconformity, and erosional surfaces.

354

M. B. Cita

FOREWORD The Mediterranean is a mosaic of old (Jurassic-Cretaceous) and new (MiocenePliocene) small ocean basins along a major plate boundary (the Eurasia/Africa Plates). A dramatic change in paleogeography and paleoceanography affected the Mediterranean area in the terminal part of the Miocene Epoch, as a result of plate motions combined with lowering of sea-level. The present day Mediterranean is strongly elongated longitudinally (3860 km versus an average of approximately 600 km N-S). The surface covers an area of circa 2.5 millions square kin, and the volume is 3.7 cubic kin. Water depth ranges from 350m (Gibraltar Strait, Sicily Channel) to 5020 m (Matapan Trench), with the exception of the shallower Adriatic Sea. Mean water depth is slightly less than 1500m. The Mediterranean is a silled basin with three major sills: from W to E they are the Strait of Gibraltar, the Sicily Channel and Bosphorous- Dardanelles. Surficial temperature increases from W to E, but is stable at the bottom (around 13~ Also, salinity increases eastward, reaching 39%0 in the Levantine Basin, which is one of the saltiest seas of the world, second only to the Red Sea. Climate is hot and dry on the southern shorelines; temperate and more or less humid to the north. Circulation is antiestuarine. The present day water budget of the Mediterranean is strongly negative, with an annual loss by excess evaporation over precipitation / runoff estimated at 3310 cubic km (Hsfi, Cita, Ryan, 1973).

DATA AND IDEAS The disconnection of the Mediterranean from the Atlantic in the Messinian resulted in the deposition of thick evaporites in the deepest part of the various basins (i.e., Balearic, Tyrrhenian, Ionian, Levantine, Aegean). The big picture came out from the first drilling campaign in the Mediterranean in 1970, under the inspiring leadership of Bill Ryan and Ken Hsfi (Ryan, Hsfi et al., 1973). A deep basin desiccation model was proposed (Hsfi, Ryan and Cita, 1973; Hsti, Cita, Ryan, 1973) whose basic arguments may be summarized as follows: 1. Messinian evaporites recovered from the deep sea include intertidal, subtidal and supratidal facies that suggest complete desiccation (Ryan, Hsfi et al.; 1973; Hsfi, 1973). 2. Messinian evaporites buried in the subbottom of the Mediterranean, as interpreted from seismic reflection profiles, have a volume of one million cubic km, which corresponds to one-sixth of the salinity of the world Ocean (Ryan, 1973). 3. Erosional surfaces were created subaerially during evaporitic draw-down on the passive continental margins: they are detectable on seismic reflection profiles and are calibrated by numerous commercial and scientific wells (Ryan, 1976; Ryan and Cita, 1978; Montadert et al., 1978). 4. Subaerial canyon cutting in deep-sea fans was documented in at least three major rivers debouching in the Mediterranean: the Nile, the Rhone, and the Ebro (Choumakov, 1973; Ryan, 1978; Barber, 1981; Clauzon, 1982).

The Messinian salinity crisis in the Mediterranean

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5. The marine biota that populated the Mediterranean in pre-salinity crisis times disappeared altogether. The Pliocene fauna re-immigrated from the Atlantic (Cita, 1973; Wright, 1979; Sgarrella et al., 1997). 6. Sediments deposited in pre-Messinian and post-Messinian times recovered from the deep sea, are of open marine facies, in contrast with sediments associated with the evaporites which are either barren, or essentially non marine in nature (Cita, 1973; Cita et al., 1978, and later works). Substantial evidence supportive of the deep basin desiccation model, which was originally proposed on just a few, although very strong data, was provided by four additional drilling campaigns carried out by the Deep Sea Drilling Project / Ocean Drilling Program in 1975 (LEG 42, see Hsfi, Montadert et al., 1978); in 1986 (Leg 107, see Kastens, Mascle et al., 1987); in 1995 (Leg 160, see Emeis et al, 1996; Leg 161, see Comas et al., 1996). Unfortunately, the Messinian has never been a major scientific target in any of these expeditions, or even a minor one. Important findings include: 1. Tidal facies of Messinian evaporites recovered in the deepest abyssal plain of the entire Mediterranean inclusive of bird's eye structures in dolostones, entherolitic folding in anhydrites, potash salts (DSDP Site 374, see Fig. 1). 2. Non marine subaqueous facies of thick Messinian lacustrine sediments in the Tyrrhenian (ODP Site 652). 3. Open marine facies of pre-Messinian sediments underlying evaporites on the Sardinia Margin (ODP Site 654), Menorca (DSDP Site 372), Florence Rise (DSDP Site 375). 4. Record of Messinian Gap, dissolution facies and erosional surface, in the Eratosthenes Seamount (ODP sites 965-968, see Major and Ryan, 1999). 5. Record of Messinian conglomerates and erosional surface in the Alboran Basin (ODP Site 976, see Iaccarino et al., 1999). TIME F R A M E W O R K Stratigraphically oriented investigations on the Messinian (latest Miocene) and Zanclean (earliest Pliocene) stages have been very active in the last twenty years. In the late nineties, an unprecedented stratigraphic resolution has been obtained combining multiple biostratigraphies (based on planktic and benthic foraminifera, and on calcareous nannofossils), magnetostratigraphy, isotopic stratigraphy and astrocyclostratigraphy (Hilgen et al., 1995; Hilgen and Langereis, 1988, 1993; Krijgsman et al., 1999a; Cita and McKenzie, 1999). We know now that the initiation of the "Salinity Crisis" proper (coinciding with the first deposition of evaporitic limestone following a period of restricted circulation) was strictly synchronous in the western Mediterranean (Sorbas Basin, Spain) in the Central Mediterranean (Caltanissetta Basin, Sicily, Italy) and in the Eastern Mediterranean (Gavdos, south of Crete, Greece) and occurred 5.9 My ago. This datum could not be identified in the deep-sea record, because the transition from the evaporites to the underlying succession could be recovered only on basin margins associated with unconformities.

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Fig. 1. Location of all Mediterranean drillsites. DSDP 121-134, Leg 13 (Ryan, Hsfi et al., 1973); DSDP 371-377 Leg 42A su, Montadert et al., 1978); ODP 650657, Leg 107 (Kastens, Mascle et al., 1987); ODP 963-973, Leg 160 (Emeis, Robertson et al., 1996); ODP 974-979, leg 161 (Comas, Zahn, et al., 1996). After Cita et al., 1999.

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Fig. 2. Correlation of the different units and formation of the Vena del Gesso Basin to the geomagnetic polarity time scale of Hilgen et al., 1995; with new astronomical ages for the late Messinian polarity reversals from Krijgsman et al., 1999, and to the Mediterranean Messinian Formations of the Caltanissetta Basin of Sicily and the Sorbas Basin of Spain. The right side of the figure shows the major Messinian events of the Mediterranean salinity crisis with their astronomical ages. After Krijgsman et al., 1999b, modified.

The Messinian stage, now validated internationally with the lower boundary defined by a GSSP (Global Stratotype Section and Point) in Morocco with an age of 7.24 My and an upper boundary coinciding with the Zanclean GSSP, with an age of 5.33 My (see Fig. 2), has a duration of 1,900,000 y. The duration of the "Salinity Crisis" is much shorter, only 630,000 y based on multiple stratigraphic criteria, with astronomical cycles being the most powerful tool (precession cycles). The Pliocene flooding (sensu Cita, 1975; 1982) proved to be isochronous throughout the Mediterranean, from the Alboran Sea to the Balearic, T.yrrhenian, Ionian and Levantine Basin (Spezzaferri et al., 1998; Iaccarino et al., 1999; Cita et al., 1999) as well as on land (Hilgen e Langereis, 1993; Sgarrella et al., 1997). MULTI-STEP MODELS The model for evaporite sedimentation was initially based on the stratigraphic record of the Caltanissetta Basin in Sicily (Decima and Wezel, 1973), with the distinction of the "lower evaporates" and "upper evaporates" separated by an erosional surface. The lower evaporites are known only from land: they are barren or associated with marine sediments. They may contain a salt layer, which in turn may contain exposure surfaces (Lugli, 1999). Erosional surfaces evidenced in seismic reflection profiles across the Balearic Margin (Ryan and Cita, 1978;

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Montadert et al., 1978) extend underneath the salt layer, suggesting that evaporitic drawdown occurred since the very beginning of the "Salinity Crisis" s.s. The upper evaporites are known both from land and from the deep-sea record. According to the latest publications by Krjigsman et al. (1999a, 1999b) their duration is of approximately 280,000 y. A brackish water fauna that includes Paratethyan immigrants ("lago-mare" sensu Ruggieri, 1967; see also Bonaduce and Sgarrella, 1999) is often recorded in association with the upper evaporites, with special reference to their topmost part. The termination of the Messinian "Salinity Crisis" coincides with the re-establishment of open marine conditions in the entire Mediterranean. Notwithstanding the wealth of deep-sea drilling data (46 drillsites, 23 of them penetrating Messinian sediments), and notwithstanding the high resolution, integrated stratigraphy now available, worked out in several peripheral, piggyback or thrust-top basins from orogenic belts surrounding the Mediterranean as the Betic Cordillera, the Apennines, the Central Sicilian basin and some Greek Islands, some major controversies still exist in the literature, as shown by the two cartoons recently proposed by Clauzon et al. in 1996 (left side of Fig. 3) and by Martin et al., in 1999 (right side of Fig. 3), that want to offer alternate interpretations of the Messinian "Salinity Crisis". Both models assume that the evaporites in the deep sea are diachronous versus those accumulated in the peripheral perched basins. However, according to Clauzon et al., the peripheral basins underwent evaporitic sedimentation prior to the deep-basin desiccation, whereas according to Martin et al. the peripheral basins are younger. Paradoxically, at 5.55 My the Mediterranean was

Fig. 3. Comparison of contradictory depositional models recently proposed as alternate to the deep basin desiccation model, using the same time-scale. Modified after Clauzon et al., 1996 (left side) and Martin et al., 1999 (right side).

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empty according to one model, whereas according to the other, it was filled by normal salinity waters. CONCLUDING REMARKS

A lack of a 3D perspective is noticed in most land-based scientists, including those responsible for the magnificent high resolution stratigraphy available. Also noticed is a lack of interest of marine geologists and geophysicists, in this "big science" theme. A lack of adequate technology prevents a final word. Indeed, in order to solve forever the remaining controversial issues, one should drill, and continuously core the depocenter of at least one of the major basins, and possibly two (Balearic and Ionian). Is the Intramessinian Gap so well calibrated by Krjigsman et al. (1999a, 1999b) and lasting some 90,000 y or four precession cycles also expressed in the deepsea record, or has deposition been continuous in the deepest salt ponds? Numerical modelling only may help to provide boundary conditions for the reconstruction of a realistic intra-Messinian and post-Messinian scenario. REFERENCES Barber, P.M., 1981. Messinian subaerial erosion of the Proto-Nile delta. Mar. Geol., 44: 253-272. Bonaduce, G. and Sgarrella, F., 1999. Paleoecological interpretation of the latest Messinian sediments from southern Sicily (Italy). Mem. Soc. Geol. Ital., 54: 83-92. Choumakov, I.S., 1973. Pliocene and Pleistocene deposits of the Nile Valley in Nubia and Upper Egypt (abstract in English). In: Ryan, W.B.F., Hsfi, K.J., et al., Initial Reports of the Deep Sea Drilling Program, Vol. 13, Part 2, U. S. Government Printing Office, Washington D.C., pp. 1241-1242. Cita, M.B., 1973. Mediterranean Evaporite: paleontological arguments for a deep basin desiccation model. In: Messinian Events in the Mediterranean. Kon. Ned. Akad. Weten., Amsterdam, pp. 203-223. Cita, M.B., 1975. The Miocene-Pliocene boundary. History and definition. In: Saito, T., Burckle L. (Eds.), Late Neogene Epoch Boundaries. Sp. Publ. Micropal., pp. 1-30. Cita, M.B., 1982. The Messinian salinity crisis in the Mediterranean: a review, Alpine-Mediterranean geodynamics. Geodyn. Ser. 7, AGU, Washington D.C., pp. 113-140. Cita, M.B. and McKenzie, J.A. (Eds.), 1999. Cycles, events, sea levels in Messinian times. Mem. Soc. Geol. Ital. 54, Roma. Cita, M.B., Racchetti, S., Brambilla, R., Negri, M., Colombaroli, D., Morelli, L., Ritter, M., Rovira, E., Sala, P., Bertarini, L. and Sanvito, S., 1999. Changes in sedimentation rates in all Mediterranean drillsites document basin evolution and support starved basin conditions after early Zanclean flood. Mem. Soc. Geol. Ital., 54: 143-159. Cita, M.B., Wright, R.C., Ryan, W.B.F. and Longinelli, A., 1978. Messinian paleoenvironments. In: Hsfi, K.J., Montadert., L., et al., Initial Reports of the Deep Sea Drilling Program, Vol. 42, Part 1, U.S. Government Printing Office, Washington D.C., pp. 1003-1035. Clauzon, G., 1982. Le Canyon Messinien du Rh6ne" une preuve decisive du "Desiccated Deep-Basin model" (Hsfi, Cita and Ryan, 1973). Soc. G6ol. France Bull., 24: 597-610. Clauzon, G., Suc, J.-P., Gautier, F., Berger, A. and Loutre, M.-F., 1996. Alternate interpretation of the Messinian salinity crisis: controversy resolved? Geology, 24: 363-366. Comas, M.C., Zahn, R., Klaus, A., et al., 1996. Proc. ODP, Init. Repts. 161. College Station, TX (Ocean Drilling Program). Decima, A. and Wezel, F.C., 1973. Late Miocene evaporites of the Central Sicilian Basin, Italy. In: Ryan, W.B.F., Hsfi, K.J., et al., Initial Reports of the Deep Sea Drilling Program, Vol. 13, Part 2, U. S. Government Printing Office, Washington, D.C. pp. 1234-1239. Emeis, K.C., Robertson, A.H.F., Richter, C., et al., 1996. Proc. ODP Init. Repts. 160. College Station, TX (Ocean Drilling Program).

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Hilgen, F.J., Krijgsman, W., Langereis, G.C., Lourens, L.J., Santarelli, A. and Zachariasse, W.J., 1995. Extending the astronomical (polarity) time scale into the Miocene. Earth Plan. Sci. Lett., 136:495-510. Hilgen, F.J. and Langereis, G.C., 1988. The age of the Miocene-Pliocene boundary in the Capo Rossello area (Sicily). Earth Plan. Sci. Lett., 91: 214-222. Hilgen, F.J. and Langereis, G.C., 1993. A critical re-evaluation of the Miocene-Pliocene boundary as defined in the Mediterranean. Earth Plan. Sci. Lett., 118:167-179. Hsfi, K.J., 1973. The desiccated deep basin model for the Messinian events. In: Drooger, C.W. (Ed.), Messinian Events in the Mediterranean, pp. 60-70. Hsfi, K.J. and Cita, M.B., Ryan, W.B.F., 1973. The origin of the Mediterranean evaporites. In: Ryan, W.B.F., Hsfi, K.J., et al., Initial Reports of the Deep Sea Drilling Program, Vol. 13, Part 2, U. S. Government Printing Office, Washington D.C., pp. 1203-1231. Hsfi, K.J., Montadert., L., et al., 1978. Initial Reports of the Deep Sea Drilling Program, Vol. 42, Part 1, U.S. Government Printing Office, Washington D.C. Hsfi, K.J., Ryan, W.B.F. and Cita, M.B., 1973. Late Miocene desiccation of the Mediterranean. Nature, 242: 240-244. Iaccarino, S., Castradori, D., Cita, M.B., Di Stefano, E., Gaboardi, S., Mc Kenzie, J.A., Spezzaferri, S. and Sprovieri, R., 1999. The Miocene-Pliocene boundary and the significance of the earliest Pliocene flooding in the Mediterranean. Mem. Soc. Geol. Ital., 54: 109-131. Kastens, K.A., Mascle, J., Auroux, C., et al., 1987. Proc. ODP Init. Rep. 107. College Station, TX (Ocean Drilling Program). Krijgsman, W., Hilgen, F.J., Raffi, I., Sierro, F.J. and Wilson, D.S., 1999a. Chronology, causes and progression of the Messinian salinity crisis. Nature, 400: 652-655. Krijgsman, W., Hilgen, F.J., Marabini, S. and Vai, G.B., 1999b. New paleomagnetic and cyclostratigraphic age constraints on the Messinian of the Northern Apennines (Vena del Gesso Basin, Italy). Mem. Soc. Geol. Ital., 54: 25-33. Lugli, S., 1999. Geology of the Realmonte salt deposit, a desiccated Messinian basin (Agrigento, Sicily). Mem. Soc. Geol. Ital., 54: 75-81. Major, C. and Ryan, W.B.F., 1999. Eratosthenes Seamount: record of late Miocene sea-level changes and facies related to the Messinian salinity crisis. Mem. Soc. Geol. Ital., 54: 47-59. Martin, J.M., Braga, J.C. and Sanchez Almazo, I., 1999. The Messinian record of the outcropping marginal Alboran Basin deposits: significance and implications. In: Zahn, R., Comas, M.C. and Klaus, A., (Eds.), Proc. ODP, Sci. Results, 161, College Station, TX (Ocean Drilling Program), pp. 543-551. Montadert, L., Letouzey, J. and Mauffret, A., 1978. Messinian event: seismic evidence. In: Hsfi, K.J., Montadert., L., et al., 1978. Initial Reports of the Deep Sea Drilling Program, Vol. 42, Part 1, U.S. Government Printing Office, Washington D.C., pp. 1037-1050. Ruggieri, G., 1967. The Miocene and later evolution of the Mediterranean Sea. In: Adams and Ager (Eds.), Aspects of Tethyan biogeography. Syst. Assoc. Publ., 7, pp. 283-290. Ryan, W.B.F., 1973. Geodynamic implication of the Messinian crisis of salinity. In: Messinian Events in the Mediterranean. Kon. Ned. Akad. Weten., Amsterdam, pp. 26-28. Ryan, W.B.F., 1976. Quantitative evaluation of the depth of the western Mediterranean before, during and after the late Miocene salinity crisis. Sedimentology, 23: 791-813. Ryan, W.B.F., 1978. Messinian badlands on the southeastern margin of the Mediterranean Sea. Mar. Geol., 27: 349-363. Ryan, W.B.F. and Cita, M.B., 1978. The nature and distribution of Messinian erosional surfaces. Indicators of a several kilometers deep Mediterranean in the Miocene. Mar. Geol., 27: 193-230. Ryan, W.B.F., Hsfi, K.J., et al., 1973. Initial Reports of the Deep Sea Drilling Program, Vol. 13. U.S. Government Printing Office, Washington D.C. Sgarrella, F., Sprovieri, R., Di Stefano, E. and Caruso, A., 1997. Paleoceanographic conditions at the base of the Pliocene in the Southern Mediterranean basins. Riv. Ital. Paleont. Strat., 103: 207-220. Spezzaferri, S., Cita, M.B. and Mc Kenzie, J.A., 1998. The Miocene/Pliocene boundary in the eastern Mediterranean: results from ODP Leg 160, Sites 967 and 969. In: Robertson, A.H.F., Emeis, K.C., Richter, C. and Camerlenghi, A. (Eds.), Proc. ODP, Sci. Results, 160, College Station, TX, (Ocean Drilling Program), pp. 9-28. Wright, R., 1979. Benthic foraminiferal repopulation of the Mediterranean after the Messinian (late Miocene) event. Palaeogeogr., Palaeoclimatol., Palaeoecol., 29:189-214.

Paradoxes in Geology Ueli Briegel and Wenjiao Xiao, eds. 9 2001 Elsevier Science B.V. All rights reserved.

Chapter 17

A REVIEW OF THE C A T A S T R O P H I C E X T I N C T I O N AT THE E N D OF THE C R E T A C E O U S AND ITS SCIENTIFIC I M P L I C A T I O N

He Qixiang CCOP Technical Secretariat, 2nd Floor, OMO Bldg., 110/2 Sathorn Nua Road, Bangrak, Bangkok 10500, Thailand

ABSTRACT The faunal mass extinction at the end of the Cretaceous remained a puzzle until the late 70s when the extraterrestrial impact was proposed as the cause of the extinction. Sedimentary and geochemical evidence strongly suggests that a Strangelove ocean existed as a consequence of the impact. The catastrophic extinction is an integrated result from the drastic environmental devastation. The old dogma which excluded catastrophic evolution on the earth has to be abandoned. The extraterrestrial process has doubtlessly played a significant role in the evolution of the earth as well as in the evolution of life on the earth. It has opened a new era for geoscientists to find out solutions for some geological puzzles for which the old dogma has failed to give an explanation.

INTRODUCTION Sudden changes of the fossil fauna in geological records were discovered in the first half of the nineteenth century. Since then they have been used as criteria to subdivide the strata into different ages. Controversies, however, have occurred in explanation of such changes since the very beginning of the discovery. Some scholars, such as the French anatomist George Cuvier, postulated environmental catastrophes to explain the mass extinction. Old forms of life, as they believed, were killed by the events happened in a very short time. Based upon his religious believing, Cuvier attributed the mass mortality to a catastrophe of the Noachian deluge-alike (Hsfi, 1986b). Charles Lyell, a younger contemporary of Cuviers, preferred an explanation to the contrary. He believed in the immutability of the physical laws. Natural processes, in his opinion, were the ones taking place at a uniform rate. There was no such catastrophe that could change the way of the evolution of life on the earth. It led to the theory of uniformitarianism that ruled earth science for almost a century. As a loyal follower of Lyells', Charles Darwin proposed natural selection as the driving force of evolution. As to the mass extinction observed around the stratigraphic boundaries, he complained about the stratigraphic gaps or the imperfection of the geological records, an excuse no one could prove or deny then (Hsti, 1986b).

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The most distinctive mass extinction occurred at the end of the Cretaceous. Among the organisms that were wiped out, none was so peculiar as the dinosaurs. Since the first discovery of the giants in 1770, various assumptions had been made to explain their sudden disappearance, but none was satisfactory. A theory, which owed the dinosaur extinction to an extraterrestrial impact, was proposed in the late 70s, based on the evidence of unusual enrichment of cosmic materials in the boundary clay. The theory has sparked off a dispute not only on the event itself, but also related to a new philosophy of science, in which catastrophic extraterrestrial events are respected as one of the driving forces of the evolution of life on the planet earth. Although the debate continues on the characteristics of the extraterrestrial body, there seems a common understanding among the people on the catastrophic mass extinction. Based on the studies of various evidences, the integrated consequence of the drastic environmental devastation caused by a catastrophic extraterrestrial impact is the killer of the organisms at the end of the Cretaceous (Hsfi, 1986a, Hsfi et al., 1982). EVIDENCE OF A G R E A T D Y I N G In 1770, the first skeleton of a dinosaur was discovered at Maastricht, the Netherlands. It was studied by Cuvier. Cuvier named it Mosasaurus as an extinct giant reptile. Since then, a variety of skeletons of the same kind have been found in many localities in the world. Richard Owen proposed the name of Dinosauria for the group of the reptiles that had been flourishing during the Mesozoic and disappeared all of a sudden by the end of the Cretaceous (Hsfi, 1986). There has been a long debate over the extinction of the dinosaurs. Though there were arguments against it, the main stream of explanation had followed the dogma of Darwinism which attributed the sudden change of a fossil assemblage around the Cretaceous/Tertiary boundary to artifacts of the imperfect records, and excluded any possibility of catastrophic event. Further evidence came up in the 70s while studies were contributed to the K/T boundary at Gubbio, Italy and other localities over the world as well as in deep sea drilling cores. (Alvarez et al., 1979, Hsfi, 1980) The radiometric dating was then well developed. Paleomagnetic stratigraphy was also available for dating deep sea sediments. It was found that the Cretaceous/Tertiary boundary clay which is only a few centimeters in thickness was exactly within the magnetostratigraphic epoch of C-29-R which covers less than 500,000 years of time span. Taking into account the low sedimentation rate of the clay, there is no stratigraphic gap at all. Mass extinction and catastrophic changes did take place at the end of the Cretaceous. Not only the dinosaurs, but also other organisms on land or in the ocean sustained great loss. Russell (1979) made a count of the number of genera of fossil organisms which lived just before and after the end of the Cretaceous. The diversity of floating marine microorganisms was reduced by 42%, of bottom-dwelling marine organisms by 49% and of swimming marine organisms by 70% across the K/T boundary. At a brief glimpse, it seems that the shallow marine benthic organisms were little affected by the terminal Cretaceous event in view of the relatively small diversity

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reduction of several groups. However, a closer look reveals that of the total of 115 species in the boundary sequence at Denmark, 60 species have a range restricted to the Maastrichtian, 44 species are restricted to the Danian, and only 11 species or about 10% occur in both the Maastrichtian and Danian (Birkelund and Hakansson, 1982). Ninety percent of the species became extinct within a very short time interval. Surlyk and Johansen (1984) studied the brachiopods of the same sequence, and found that the Maastrichtian fauna became extinct by 80%, whereas the diversity reduction was practically nil. Ocean swimmers as a group suffered the greatest loss, having been reduced from 332 to 99 genera, or some 70%, despite the Danian replacement. All the ammonites and belemnites and almost all the marine reptiles became extinct at the end of the Cretaceous. Almost all the calcareous nanoplankton became extinct in a very brief transitional interval of less than 50,000 years, whereas the damage to the siliceous and other noncalcareous plankton was considerably less. Concerning the deep marine benthic organisms, the Upper Cretaceous taxa differ only slightly from those in the Paleocene. Numerous species evolved across the K/T boundary with little or no changes. However, there is no bioturbation in the boundary clay as indicated by the well-preserved lamination, that may suggest mass mortality of burrowing organisms. On land, other giant reptiles with body weight more than 25 kg also became extinct in addition to the extinction of the dinosaurs. No great changes took place within the plant kingdom except for the extinction of the squilapollenites flora at the high northern latitudes (Hickey, 1981). The lack of tree pollen in the boundary, which is abundant in both Cretaceous and Paleocene floras, indicates that forests were catastrophically destroyed. The biotic record shows that a catastrophic event did come suddenly and without forewarning. Many taxa were eliminated. Particularly hard hit were the floaters and the swimmers in the oceans, as well as the giant reptiles on land. Other groups suffered drastic population reduction, but many taxa recovered quickly and did not become extinct. EVIDENCE OF E X T R A T E R R E S T R I A L MATERIALS A possibility of an extraterrestrial cause for extinction of dinosaurs was postulated in 1956 by de Laubenfels who attributed the mass death of dinosaurs to the hot air released from a large body impact. No evidence, however, was provided. H. Urey (1973), a Nobel Laureate, reaffirmed the possibility of a comet impact, as temperature changes around the K/T boundary were confirmed in the late seventies when oxygen stable isotopes were used as a paleotemperature-meter (Boersma et al., 1979, Emiliani et al., 1981). Direct evidence was needed then to testify the postulation. W. Alvarez (1979) was one of the pioneers to study the paleomagnetic stratigraphy of the Gubbio section. He later cooperated with the others at the Lawrence Berkeley Laboratory to study the geochemistry of the K/T boundary clay and to determine the abundance of some very rare elements, such as iridium (Ir), osmium (Os) and other so-called siderophile

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elements which are abundant in meteorites, but rare in the earth's crust. The results were surprising. W.Alvarez found that the concentration of iridium in the boundary clay at Gubbio reaches 6.35 ppb, almost 30 times higher than the background. A similar anomaly was also found in the so-called Fish Clay at the Stevens Klint, Denmark, where the abundance of iridium reaches 6.5 ppb upon a backgroup of 0.3 ppb. Unusual enrichments of iridium and other siderophiles and a corresponding depletion of the rare earth elements were discovered in the marine sections across the K/T transition in Spain, France, Germany, Austria, Tunisia, Israel, Pakistan, Russia, North America and New Zealand as well as in deep-sea cores. The K/T boundary clay occurs as well in some continental sequences on land. In the continental facies of the Raton Basin, New Mexico and Colorado, the K/T boundary clay, similar to tonstein, consists of nearly pure well-crystallized kaolinite, and lies at the top of a carbonaceous shale unit below a coal bed. Iridium and other siderophile elements are enriched in a 10-cm interval in the clay and in the overlying coal and shale. The maximum iridium concentration is several orders of magnitude higher than that of the background (Orth et al., 1982), Similar boundary sections with iridium anomalies were found in other localities of terrestrial sedimentation in other parts of the world (Pillmore et al., 1984, Smit and van der Kaars, 1984). In addition to the geochemical evidence, spherules resembling microtektite and shock quartz have been identified in K/T boundary clays, either in the sections on land, or in deep-sea cores, which further indicates a collision with an extraterrestrial body.

EVIDENCE OF E N V I R O N M E N T A L DEVASTATIONS Nobody exactly knows if dinosaurs died of hunger or of cooling. Evidence is favourable to an explanation of the devastation of the environmental system on the earth, which triggered a chained process to cause the terminal Cretaceous extinction. As observed in the sections all over the world, the K/T boundary is marked with a thin layer of laminated clay, which, comparing to the overlying and underlying sequences, is devoid of carbonate and enriched in iridium and other siderophile elements. The boundary clay at the Stevns Klint, Denmark, is intercalated between the latest Cretaceous Maastrichtian Chalk and the early Paleocene Cerithium Limestone. Whereas the Maastrichtian Chalk and Cerithium Limestone are intensively bioturbated, the Fish Clay shows little evidence of bioturbation. It contains a high content of organic carbon, and is devoid of carbonate. The boundary clay at Gubbio, Italy occurs between carbonate strata which are reddish brown in colour and contain about 5% insoluble residue. The boundary clay is about l cm thick and contains more than 50% insoluble residue. Alvarez et al. (1980), analyzed the concentrations of 28 trace elements in the insoluble residue fraction of the boundary sequence and found that 27 out of the 28 elements including numerous rare earth elements show very similar patterns of random abundance variation with time, but iridium increases by a factor of 30 above the background values.

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The boundary clay at Carravaca, Spain, is ten times thicker than that at Gubbio. The basal part which is a 0.5 cm thick red lamina, has an insoluble residue of about 70% and is characterized by an unusual enrichment of iridium and other siderophiles, and by a corresponding depletion of the rare earth elements (Smit, 1982, Smit and ten Kate, 1982, Smit and Romein, 1985). The dark gray marl overlying the red lamina has an insoluble residue varying between 40 and 60 % in weight. The siderophiles are somewhat enriched, but the rare earth elements are not much depleted. The sediment marking the boundary of a K/T transition at deep-sea sites is not invariably a clay, but a marl or a calcareous ooze at numerous sites. The boundary clay at Site 524 in the South Atlantic is about 2 cm thick. The late Cretaceous marls contain usually 30 to 40 % CaCO3, but 5 cm above the boundary clay we find a pelagic sediment with only 10%, returning to normal (40% or more) in sediments some 5m above the contact. However, at other DSDP sites in the Atlantic and the Pacific, the boundary sediments deposited in open oceans at paleodepth of a few thousand meters are marls or calcareous ooze. A colour change from red or white to gray, green or black is observed at many sites across the K/T boundary. At the outcrop at Stevens Klint, for example, the Cretaceous chalk is white and Tertiary chalk is white too, but the boundary clay is black. Both the colour and the laminated structure indicate that the bottom water was devoid of oxygen at the time of deposition of the boundary clay. The black colour owes its origin to very finely disseminated organic carbon which is 4-5 times higher than the adjacent sediments in abundance. Oxygen-isotope anomalies around the K/T boundary suggest a rapid temperature fluctuation at the beginning of the Tertiary Period. Taking the oxygen isotope record of the DSDP Site 524 as an example (Fig. 2), the 0180 is relatively constant during the latest Maastrichtian and early Paleocene with a background value of about 1.0%o. A perturbation occurs at around 3m above the boundary with a change to-2.0%o. This represents an increase of 5~ of the bottom temperature of the ocean water at that site. The temperature change did not take place immediately at the end of the Cretaceous. It was built up over a period of some 30,000 or 40,000 years. The 0180 values of the fine fraction which consists practically only of nanofossils indicates that the surface temperature at this site during the late Maastrichtian and early Paleocene times was about the same as the bottom temperature. A perturbation of 0180 is observed at the boundary, indicating a cooling phase right after the boundary event which was marked by the horizon with the first appearance of the iridium anomaly. 01SO perturbations or temperature changes are also observed at other places. Unfortunately, not all studies came to the same conclusion. Boersma and Shackleton (1981) found no systematic change of bottom temperature, but perhaps a slight drop of surface temperature across the boundary at the Pacific DSDP site 465. Carbon stable isotope records provide a critical clue to reveal the drastic environmental changes of the ocean waters at the end of the Cretaceous. In the marine environments, fractionation of the carbon isotopes between the surface and bottom waters is accomplished via a photosynthesis-respiration mechanism (Broecker and Peng, 1982). In the photic zone, plants preferentially incorporate carbon 12 from the dissolved inorganic carbon reservoir into the organic

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matter produced during photosynthesis.With the sinking of the organic matter down to the bottom, carbon 12 is transferred from the surface to the bottom waters, establishing a carbon isotope gradient of around 2%0 to 3%0. In a profile from a midlatitude Pacific Ocean site (Fig. 1), the 013C value ranges from about 2%0 in the nearsurface water to a low of about 0.2%0, which corresponds to the oxygen-minimum zone. If for some reason the plankton production in the ocean was completely suppressed, a mixing of ocean water would eliminate the carbon isotope gradient in a very short time, of the order of decades. Such a sterile ocean is called by Hsfi a "Strangelove" ocean. A sudden destruction of oceanic life or the "Strangelove" effect, has been suggested as a mechanism to produce the negative carbon shift of about 3%o in the planktonic skeletons of the earliest Tertiary, whereas the benthic skeletons do not show a change in their isotopic composition (Broecker and Peng, 1982, Hsfi and McKenzie, 1985). Such an anomaly, first reported by Brennecke and Anderson (1977), has been detected at many sites where the sedimentation was continuous across the K/T boundary. At DSDP Site 524, a negative anomaly of about 3%~ occurred at a horizon where the maximum iridium concentration was detected (Fig. 2). This anomaly was registered in the bulk sample, which consisted mainly of nanoplankton. A second depletion occurs in both the bulk and the fine fractions at a horizon 1.2 m above the "iridium spike" after the first appearance of Globigirina pseudobulloides in the sediments 50,000 years younger as estimated from sedimentation rates. Fossil tests of the benthic species Gavelinella beccariformis were analyzed above and below the boundary sediments. The ol3c values of the

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Fig. 1. Profiles of carbon 13 content of dissolved inorganic c a r b o n (ol3c), dissolved oxygen ( 0 2 ) , and total dissolved inorganic carbon (ECO2) from a typical mid-latitude Pacific Ocean station (17~ 172~ This figure, redrawn from Kroopnick et al. (1977), depicts a typical surface-to-bottom carbon isotope gradient in the marine environment.

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benthic skeletons from the Cretaceous are 1 or 2%~ less positive than those of the bulk or those of a fine fraction, which consists almost exclusively of nanoplankton. This is in fact the normal oceanic pattern of fractionation of carbon isotopes between the surface and bottom waters. However, the 013C value of the benthic skeletons from the earliest Tertiary are similar or more positive than those of the coexisting planktonic fossils. This is an abnormal pattern. An earlier investigation of carbon isotope values across the K/T boundary at DSDP Site 465A revealed the same pattern (Boersma and Shackleton, 1981). The 013C of the planktonic species indicates a negative shift of about -1.5%o across the boundary, whereas the carbon isotope value of the benthic species shows no systematic variation across the boundary. As a consequence, the value for the earliest Tertiary planktonics was slightly more negative than that of the benthic from the same sample, even though the latest Cretaceous gradient was normal. Planktonic foraminifera of that age have 013C values more than 1%o more positive than that of the contemporaneous benthic species. The 013C records across the K/T boundary have provided critical evidence to a Strangelove ocean. Supported by the considerable evidence presented in the previous context, a meteor impact was responsible for mass extinction at the end of the Cretaceous. Computer modelling has revealed the possible damage of terrestrial environments by the fall of a solid giant with a mass of 1017 or 1018g (Silver and Schultz, 1982). One scenario suggested that the high temperature (105 ~ of the mushroom cloud rising from the impact site should produce considerable NOx

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Fig. 2. Geochemical anomalies of the boundary section at site 524, South Atlantic (after Hsfi and McKenzie, 1985). The boundary sediment at site 524 is a marl except for the boundary clay that has 4% calcite. Note that considerable Cretaceous nanofossils, with Tertiary chemical signals, are found in sediments deposited during the first 30,000 yr of Tertiary time. Note also the Strangelove perturbance at this site.

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through the combination of atmospheric nitrogen and oxygen. It would in turn (1) destroy the ozone layer of the stratosphere, (2) cause large-scale defoliation of terrestrial plants, (3) be oxidized and fall eventually as acid rain to change the pH of the oceans. At the same time, the ejecta envelope around the earth may have reduced the transmission of sunlight sufficiently to suppress the photosynthesis of the phytoplankton for at least several months. The pollen record across the K/T boundary has provided evidence of forest destruction. The data from the San Juan Basin, New Mexico, shows that the percentage of tree pollen ranges from 70 to 85% in the late Cretaceous and in the early Tertiary assemblages of the region. However, in the boundary sediment, the tree pollen constitutes less than 1% of the assemblage, which consists almost entirely of fern spores. The temporary suppression of photosynthesis and catastrophic environmental pollution could cause a drastic change in ocean chemistry which could have resulted in an ocean almost devoid of plankton. As calculated by Lewis et al. (1982), an impact of a 10~8g asteroid could produce 2x1037 NO molecules and a cometary event of equal-iridium magnitude (1019g, 40 km/s entry velocity) could produce a NO yield of 2x104~ molecules. Adding all the oxidized molecules (2x104~ to a 75 m-deep surface layer of the ocean could reduce the pH of the sea water in this layer by half a unit. The pH gradient of the oceans, like the carbon isotope gradient is also a product of plankton photosynthesis. When dissolved CO2 in surface waters is no longer being extracted by the plankton, the pH fractionation of ocean waters stops. Adding this additional acid (HNO3) to the ocean could drastically suppress plankton fertility until the acid-rich surface layer was destroyed by the ocean mixing process. A corollary of the postulate of a Strangelove ocean is the release of CO2 from the oceans to the atmosphere. Equilibration with the Strangelove ocean could increase the partial pressure of CO2 in the atmosphere by a factor of 3. A greenhouse effect resulting from increased atmospheric CO2 may have caused a global warming. A second corollary of the postulate is the accumulation of nutrients in ocean waters as a result of mass mortality and decreased productivities. The plentitude of nutrients could be the cause of monospecific blooms of the nannoplankton of the earliest Tertiary oceans. In conclusion, the mass extinction at the end of the Cretaceous was most probably caused by a catastrophic environmental disaster as a result of an extraterrestrial impact. The event stopped the photosynthesis-respiration processes in the ocean waters and resulted in a Strangelove ocean. CO2 not utilized during photosynthesis could be transferred from the ocean to the atmosphere leading to the marked increase in the atmospheric CO2 content and consequently major climatic changes, which killed the dinosaurs.

COMET VERSUS ASTEROID IMPACT The biological and geochemical evidence supports a large body impact by the end of the Cretaceous, whereas the hypothesis of catastrophic volcanism fails to explain the siderophile enrichment and the occurrence of the shock quartz in the boundary

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clay, even though the consequences of environmental devastation derived from those two alternative causes are very similar. Finding an anomalous concentration of iridium in the boundary clay at Gubbio, Italy, Alvarez et al. (1979, 1980) first interpreted the fact as the evidence of a supernova explosion. They later concluded that the event was a large-body impact, after they found that the trace element chemistry of the boundary clay is different from what they expected from a supernova. Alvarez et al. (1980) then postulated that an asteroid, with a composition of Type I carbonaceous chondrite and a mass of 10~Vg, was involved in the terminal Cretaceous impact. Kyte et al. (1980), noting the high concentration of platinumgroup metals in the Danish boundary and the worldwide distribution of the cosmic fallouts, suggested that the impact body may have been cometary and that the event might be a much magnified version of the Tunguska event of 1908. Hsfi (1980, 1984) postulated a cometary impact because the chemical pollution of cyanides and heavy metals might explain the selectivity of the terminal Cretaceous extinction. The cometary hypothesis received further support after Smit and Klaver (1981) discovered sanidine spherules, as well as anomalous concentrations of volatile metals in the K/T boundary clay of Spain, which resembled the potassium-rich spherules recovered in the Tunguska region where the bolide was most probably a comet. The identification of the extraterrestrial body, however, needs more evidence. Even though the cometary impact is still the most probable option, the cyanide poisoning is not very likely the cause for the terminal Cretaceous extinction. The cyanides in comets, if there had been any, would have been dissociated and thus detoxified in the mushroom cloud after the impact. It in fact, does not make much sense in arguing whether the impact was caused by a comet or an asteroid. The important fact is that an extraterrestrial event triggered an environmental catastrophe and caused the long-term destruction of the habitat of many animals and plant species. Mass extinction was a consequence of environmental devastation (Hsfi, 1992). CONCLUSIONS The evidence of sedimentology and geochemistry suggests an extraterrestrial impact at the end of the Cretaceous, which caused a mass extinction. The geological records of the marine sequences across the K/T boundary show continuous sedimentation. The stratigraphic gap postulated by Darwin in order to explain the sudden change of fauna assemblage does not exist at all. The extraterrestrial impact is strongly supported by the unusual enrichment of iridium and other sederophile elements which are enriched in meteorites and rare in the earth. The concentration of iridium in the boundary clay which usually is a few centimeters thick, could be tens of times higher than the background value. No other explanation is satisfactory besides an extraterrestrial input. A catastrophic event on the earth may look strange to geologists, but is not surprising to the astronomers. Regular visiting of extraterrestrial bodies to the earth is a common phenomenon as far as the universe is considered as an interrelated system.

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Such catastrophic extraterrestrial impacts did happen in the history cyclically. At other era boundaries where mass extinction occur, there is also evidence of extraterrestrial impacts. Even though no geochemical anomalies have been found across the normally defined Precambrian/Cambrian boundary, an iridium anomaly has been found in a clay at a horizon called the China-C marker which defines the base of a formation containing the first trilobites. A Strangelove perturbation was also found at the predicted horizon (Xing and Luo, 1984). A sharp perturbation of 013C values o f - 2 to 5%o has been detected across the Permo/Triassic boundary in a clay characterized by an iridium anomaly. The pattern of geochemical changes is thus similar to that observed across the K/T boundary (Chen, 1982). It doesn't make too much sense to argue if the extraterrestrial body was an asteroid or a comet. Geological evidence is not sufficient enough to identify the outsider. All evidence supports an environmental devastation caused by a chained process triggered by the extraterrestrial impact. The temporary black out caused by ejecta dust, green house effect and temperature changes, acid rain, chemical pollution, and severe damage of the ocean environment and ecosystem might play roles to different extents in different places and times. The drastic environmental change is the reason that killed the biota in a geologically short time. It was a catastrophe indeed. The catastrophic extinction at the end of the Cretaceous requires a change of scientific views towards the evolution of the earth. Uniformitarianism or Darwinism which excluded catastrophic evolution as a possibility, is no longer satisfactory to interpret nature. People use the term 'revolution' to describe a great change of the scientific philosophy. The concept of Plate tectonics has been regarded as a revolution of earth sciences, which liberated geoscientists from fixism and made them adopt the philosophy of activism. The discovery of catastrophic events and their great impacts on the evolution of life, therefore, should be also accepted as a revolution in geoscience, because it has liberated the people from uniformitarianism. The catastrophic evolution has provided another tool to interpret the puzzles we have ever met in the history. ACKNOWLEDGEMENT I heard of the Cretaceous terminal extinction for the first time when I was in Zfirich working as a visiting scholar under Ken Hsfi. I was then shocked to breathe the fresh air of free discussion in the field of science. I adopted the philosophy of catastrophic evolution only when I finished the stable isotope measurements for the D S D P Site 524. I am in debt to Ken Hsfi for the great help he has ever given to me in my endeavour to geoscientific studies. This paper, therefore, is contributed to his seventieth birthday. REFERENCES Alvarez, L., 1987. Mass extinction caused by large bolide impacts. Lawrence Berkeley Laboratory (Berkeley, California) Report No. 22786.

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Alvarez, L., Alvarez, W., Asaro, F. and Michel, H., 1979. Extraterrestrial cause for the CretaceousTertiary extinction: experiment and theory. Lawrence Berkeley Laboratory (Berkeley, California) Report No. 9666. Alvarez, L., Alvarez, W., Asaro, F. and Michel, H., 1980. Extraterrestrial cause for the CretaceousTertiary extinction. Science, 208:1095-1108. Alvarez, W., Alvarez. L., Asaro, F. and Michel, H., 1979. Experimental evidence in support of an extraterrestrial trigger for the Cretaceous-Tertiary extinction (abstract). EOS (American Geophysical Union Transactions), 60:734 Birkelund, T., 1981, Extinction at the end of the Cretaceous in boreal shelf seas-a multicausal event [abs.]: Conference on Large Body Impacts and Terrestrial Evolution, Huston, Texas, Lunar and Planetary Institute, Abstracts. p. 5. Birkelund, T. and Hakansson, E., 1982. The terminal Cretaceous extinction in boreal shelf seas a multicausal event. In: Silver, L.T. and Schultz, P.H. (Eds.), Geological implications of impacts of large asteroids and comets on Earth. Geol. Soc. Am., Spec. Paper, 190: 373-384. Boersma, A. and Shackleton, N., 1981. Oxygen and carbon-isotope variations and planktonic-foraminifer depth habitats, late Cretaceous to Paleocene, central Pacific, DSDP sites 463 and 465, in Initial Reports of the Deep Sea Drilling Project: U.S. Government Printing Office, Washington D.C., Vol. 62: 495-512. Boersma, A., Shackleton, N., Hall, M. and Given, Q., 1979. Carbon-and oxygen-isotope records at DSDP 384 (North Atlantic) and some Paleocene paleotemperatures and carbon-isotope variations in the South Atlantic, in Initial Reports of the Deep Sea Drilling Project: U.S. Government Printing Office, Washington D.C., Vol. 43: 695-718. Brennecke, J.C. and Anderson, T.F., 1977. Carbon isotope variation in pelagic carbonates (abstract), EOS Trans. AGU, 58:415. Broecker, W.S. and Peng, T.-H. 1982. Tracers in the Sea. Eldigo, New York. 690p. Chen, J., 1982. Carbon isotopic variation in carbonates at the boundary between Permian and Triassic in China. In: Developments in Geoscience. Science Press, Beijing, pp. 247-254. Emiliani, C., Kraus, E.B. and Shoemaker, E.M., 1981. Sudden death at the end of the Mesozoic. Earth and Planetary Science Letters, 55: 318-334. Hickey, L., 1981. Land plant evidence compatible with gradual, not catastrophic change at the end of the Cretaceous. Nature, 292: 529-531. Hsfi, K.J., 1980. Terrestrial catastrophe caused by cometary impact at the end of Cretaceous. Nature, 285: 201-203. Hsfi, K.J., 1984. A scenario for the terminal Cretaceous event. Initial Report DSDP 73:755-763 Hsti, K.J., 1984. Geochemical markers of impacts and of their effects on environments. In: Holland, I.I.D., Trendall, A.E. (Eds.), Patterns of Change in Earth Evolution, Dahlem Konferenzen. Springer-Verlag, Berlin, Heidelberg, New York, Tokyo: pp. 63-76. Hsfi, K.J., 1986a. Environmental changes in times of biotic crisis. In: Raup, D.M. and Jablonski, D. (Eds.), Patterns and Processes in the History of Life. Springer, Berlin, 297-312. Hsii, K.J., 1986b. The Great Dying. Harcourt, Brace and Jovanovich, San Diego. Hsfi, K.J., 1992. Challenger at Sea. Princeton University Press, New Jersey, 408pp. Hsfi, K.J. and McKenzie, J.A., 1985. A "Strangelove ocean" in the earliest Tertiary, in Sunddquist, E.T., and Broecker, W.S., (Eds.), The Carbon Cycle and Atmospheric CO2: Natural Variations Archean to Present. American Geophysical Union Geophysical Monograph 32: 487-492. Hsfi, K.J., He, Q., McKenzie, J.A., Weissert, H., Perch-Nielsen, K., Oberhansli, H., Kelts, K., LaBreccque, J., Tauxe, L., Krahenbuhl, U., Percival, S.F., Wright, R., Karpoff, A.M., Petrersen, N., Tucker, P., Poore, R.Z., Gombos, A.M., Pisciotto K., Carman, M.F. and Schriber, E., 1982. Mass mortality and its environmental and evolutionary consequences. Science, 216: 249-256. Hsfi, K.J., LaBrecque, J.L., and others 1984. The initial reports of the Deep Sea Drilling Project: U.S. Government Printing Office, Washington, D.C., v.73: 798p. Kroopnick, P.M., Margolis, S.V. and Wong, C.S., 1977. 13C variations in marine carbonate sediments as indicators of the CO2 balance between atmosphere and oceans. In: Andersen, N.R. and Malahoff, A. (Eds.), The Fate of Fossil Fuel CO2 in the Oceans. Plenum Press, New York, pp. 295-321. Kyte, F., Zhou, Z. and Wasson, J.T., 1980. Siderophile-enriched sediments from the Cretaceous-Tertiary boundary. Nature, 288: 651-656. Laubenfels, M.W., 1956. Dinosaur extinction: one more hypothesis Jour. Paleontology, 30: 207-212.

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Lewis J.S., Watkins, G.H., Hartman, H., Prinn, R.G., et.al., 1982. Chemical consequences of major impact events on Earth. In: Silver, L.T. and Schultz, P.H., (Eds.), 1982. Geological implications of impacts of large asteroids and comets on Earth. Geol. Soc. Am., Spec. Paper, 190: 215-221. Orth, C.J., Gilmore, J.S., Knight, J.D., Pillmore, C.L., Tschudy, R.H. and Fassett, J.E., 1982. Iridium abundance measurements across the Cretaceous/Tertiary boundary in the San Juan and Raton Basins of northern New Mexico. Geol. Soc. Am. Spec. Paper, 190: 423-433. Pillmore, C.L., Tschudy, R.H., Orth, C., Gilmore, J.S. and Knight, D.J., 1984. Geologic framework of nonmarine Cretaceous-Tertiary boundary sites, Raton Basin, New Mexico and Colorado. Science, 223: 1180-1183. Russell, D.A., 1979. The enigma of the extinction of dinosaurs. Ann. Review Earth and Planet. Sci., 7: 163-182. Silver, L.T. and Schultz, P.H. (Eds.), 1982. Geological implications of impacts of large asteroids and comets on Earth. Geol. Soc. Am., Spec. Paper, 190: 329-352. Smit, J. and Klaver, G., 1981. Sanidien spherules at the Cretaceous-Tertiary boundary indicates a large impact event. Nature., 292: 47-49. Smit, J. and van der Kaars, S., 1984. Terminal Cretaceous extinction in Hall Creek area, Montana. Science, 223: 1171-1179. Smit, J. and ten Kate, W.G.H.Z., 1982. Trace element patterns at the Cretaceous-Tertiary boundary: consequences of a large impact. Cretaceous Research, 3: 307-332. Smit, J., 1982. Extinction and evolution of planktonic foraminifera at the Cretaceous/Tertiary boundary after a major impact. In: Silver, L.T. and Schultz, P.H. (Eds.), Geological implications of impacts of large asteroids and comets on Earth. Geol. Soc. Am., Spec. Paper, 190: 329-352. Surlyk, F. and Johansen, M.B., 1984. End-Cretaceous brachiopod extinction in the chalk of Denmark. Science, 223: 1174-1177. Urey, Harold, 1973. Cometary collisions and geological periods. Nature, 242: 32-33. Xing, Y. and Luo, H. 1984. Precambrian-Cambrian boundary candidate, Meishucun, Jinning, Yunnan, China. In: Special issue on the Precambrian-Cambrian boundary. Geological Magazine, 121(3): 143-154.

Paradoxes in Geology Ueli Briegel and Wenjiao Xiao, eds. 9 2001 Elsevier Science B.V. All rights reserved. Chapter 18

THE O L D E S T B I O G E N O U S PELAGIC S E D I M E N T S A B O V E M E S O Z O I C O C E A N I C BASEMENT: A REVIEW

Edward L. Winterer* Scripps Institution of Oceanography, La Jolla, California 92093-0220, USA

ABSTRACT Review of ocean-drilling data on the nature of biogenous sediments overlying Mesozoic oceanic crust, supplemented by data from sediments depositionally above ophiolites, shows that pre-Oxfordian basal sediments are generally radiolarites, whereas younger oceanic crust generally is overlain by calcareous sediments. The middle Cretaceous is exceptional in having less carbonate-rich basal pelagic sediments. The dominant controls on the nature of basal pelagic biogenous sediments are the level of the carbonate compensation depth (CCD) relative to the crest of active ocean spreading ridges, where new oceanic crust is formed. This depth fluctuates in both space and time, but has irregularly deepened since Middle Jurassic times, when calcareous coccolithophorids increased markedly in abundance. Selective preservation, especially in the early-burial diagenetic environment, accentuates the contrasts between radiolarian-rich sediments below the CCD and calcareous sediments above.

INTRODUCTION In addition to the still-disputed question of the significance of ophiolites in the continents (are they tectonically emplaced slices of normal oceanic lithosphere, or do they mainly represent lithosphere generated in "supra-subduction zone" environments), there is a related, long-standing question: why are the oldest biogenous sediments overlying most pre-Cenozoic ophiolite sequences siliceous--generally radiolarites, while younger ophiolites are commonly overlain by pelagic limestone? This compositional shift has been attributed to the evolution and the increase in relative abundance over siliceous radiolarians of the most common calcareous planktonic organisms, i.e., foraminifers and coccolithophorids. In principle, increasing abundance of the calcareous plankton would have deepened the carbonate compensation depth (CCD) from a level shallower to a level deeper than the typical mid-ocean spreading-ridge crests.

*E-mail: [email protected]

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Coccoliths first appear in Upper Triassic strata, becoming abundant in the late Jurassic. Other locally abundant calcareous-shelled organisms in the pelagic realm were Halobia (Triassic) and Bositra (Jurassic), probably as bivalves attached to floating seaweed, and the pelagic crinoid Saccocoma (Upper Jurassic). Nektonic belemnites and the calcitic aptychi of aragonitic ammonites are scattered sparsely in moderately deep-water Mesozoic pelagic sediments, but not directly on ophiolite. A clarification in our thinking about both ophiolites and the earliest biogenous sediment overlying them has come about from more than 30 years of drilling in the ocean basins of the world. At all drill sites that reached Cenozoic oceanic crust, the oldest biogenous sediments are at least somewhat calcareous, and at most of these sites the sediments are nannofossil or foram-nannofossil limestone, commonly admixed with proportions of clay or volcanogenic minerals. These drill results match the observations from Cenozoic ophiolite sections on the continents. For Mesozoic oceanic crust, we have about four dozen drill sites where the coring captured samples of not only the upper surface of Jurassic and Cretaceous oceanic crust, but also the immediately overlying sediments, sediments that could be dated paleontologically. This body of data not only greatly augments the observations of Mesozoic ophiolite sequences on the land, but also puts constraints on water depths in which the sediments accumulated, to the extent of applicability of the well-known empirical relation between depth to ocean crust and age of crust, which predicts a starting depth at the normal, oceanic spreading ridge crest of about 2700 m (Sclater et al., 1971; Sclater and Detrick, 1973). It is the aim of this brief paper to review these data and then to review the alternative hypotheses to explain the temporal shift from siliceous to carbonate sediments as the first biogenous deposits on oceanic crust. BASAL SEDIMENTS ON N E W L Y - F O R M E D CRUST IN THE EXTANT OCEANS In recent decades, an extensive campaign of sampling along and near the active spreading ridge axes has yielded valuable data on the nature of the early sediments above and within newly-formed oceanic crust. Much of the near-surface sampling effort has been directed at hydrothermal vents and little effort has been given to nonvent areas along the spreading axes or to areas beyond the immediate influence of axial processes. Some of what we know is gleaned from bottom photographs showing coatings of new sediment on lava flows or clouds of sediment stirred by submersibles. Efforts to drill very young crust have been largely unsuccessful, owing to the extensive fracturing in the brittle lavas. Most of what we know about early sediments on ocean crust is derived from cores taken during the Deep Sea Drilling Project (DSDP) and Ocean Drilling Program (ODP). Oceanic basement has been reached at 355 of the 1151 drill sites (to September, 1999). Cores of near-basment sediments were recovered at most sites that reached basement. Because biogenous sediments overlying Cenozoic oceanic basement are almost everywhere calcareous, Mesozoic sediments are herein emphasized. Mesozoic basement has been drilled at only 45 sites (about 13% of all basement sites), about equally divided among the three main ocean basins. Thirteen sites are in Upper Cretaceous crust, 23 in Lower Cretaceous (Lower

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Cretaceous oceanic plateaus are favorite targets), 7 in Upper Jurassic, and 2 in Middle Jurassic. Hydrothermal sediments

The very first sediments on oceanic crust are commonly deposits from hydrothermal spring emanations close to the active spreading center, accumulated in irregular mounds, pipes and chimneys that commonly contain sulfides or layers and wads of Fe and Mn oxides (reviewed in Jenkyns, 1986). In other places, normal biogenous basal sediments are but lightly impregnated with Fe or Mn oxides. Where crustal extension is minimal, and fracturing and fluid circulation are confined to shallow levels, hydrothermal deposits may lack sulfides and be characterized by low P-T metamorphic facies, e.g. zeolite-prehnite-pumpellyite and greenschist, in contrast to more extensional, open systems, with massive sulfides, low-temperature zeolites such as phillipsite and smectite clays (Schiffman et al., 1990). Veins, slope breccias and neptunian dikes

In some places, especially where serpentinites are at the seafloor, the serpentinite is riven by hydrothermal calcite veins and cut by open fractures into which fine and coarse sediments from the seafloor trickled and tumbled, forming neptunian dikes and sills (Bernoulli and Weissert, 1983, Bernoulli et al., 1978). In such places, as well as on normal fast-spreading ocean floor where fault scarps develop as the newformed crust moves away from the spreading center, the oldest sediments may include slope breccias derived from the fault scarps (Milkert et al., 1996). Even between scarps, where the virgin seafloor is relatively flat, bottom photographs show significant roughness in the form of collapsed lava-lake roofs and pillow breccias. Turbidites

Where early seafloor relief is strong, sediments deposited on highs may be carried by turbidity currents to adjacent lows, for example, from an adjacent continental margin, as along segments of active spreading ridges in the Gulf of California (Lonsdale, 1989) Biogenous calcareous sediments originally deposited on oceanic topograpic highs may be redeposited as turbidites on oceanic crust at depths below the regional CCD, but be preserved against dissolution by rapid burial beneath the undersaturated waters at the seafloor. Pelagic sediments

It may thus be some time until the new crust has been transported by spreading away from the spreading axis and its active tectonism before "normal" pelagic sediments begin to dominate deposition. Because of the almost ubiquitous presence of bottom currents in the ocean, currents moving typically at a few cm/sec, and because of the ceaseless activity of benthic organisms, stirring up newly deposited sediment or

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inhibiting its settling onto the bottom, fine-grained sediments may be tardy in settling permanently on the bare rocky crust, except in sheltered locales, in pits and behind obstacles to currents. Only gradually are the initial irregularities smoothed by sedimentation. Commonly the earliest biogenous sediments contain admixtures of hydrothermal components, and are stained reddish or dark gray by Fe or Mn oxides. Non-biogenous pelagic sediments

Besides sediments of biologic origin, sediments that result from settling out of the water column onto newly-formed crust at spreading ridges include wind-borne sediment from the continents, volcanic ash and fine sediment brought to the sea by streams and carried in suspension by ocean currents. Where a spreading ridge is close to a continent, e.g., in the Gulf of California, terrigenous sediments, in this example interlayered with diatomaceous sediments, blanket and are sandwiched between lava flows of the active ridge crest. At greater distances, e.g., on segments of the Juan de Fuca ridge, both turbidites and suspended clays from the continent rest directly on very young ridge basalt (Johnson and Holmes, 1989). In regions close to glaciers, ice rafted sediments can be deposited directly onto zero-age ocean crust, e.g., in the Greenland Sea or on spreading ridges in the Southern Ocean. Biogenous pelagic sediments and the radiolarite vs. limestone problem

In much of the world ocean, the sediments on oceanic crust (save for spotty hydrothermal deposits) are biogenous composed of the skeletal remains of planktonic one-celled organisms. These are siliceous radiolarians and diatoms and calcareous algae (coccolithophorids, commonly termed nannofossils or simply nannos) and foraminifers. The particular composition of the biogenous sediment depends on the relative abundance of each type of organism living in the water column and the degree which their remains are preserved as they fall to the seafloor and are buried. The relative abundance of the living populations is largely a function of the fertility of the surface waters, where most of these organisms live. The waters of highest fertility are generally dominated by radiolarians and diatoms, while coccolithsophorids and foraminifera tend to dominate in waters of lower fertility. Soil nutrients are brought to the sea by streams, and hence near-continent waters are commonly more fertile than mid-ocean waters. The sinking of organic matter removes it from being available to organisms in the surface ocean, but where oceanographic conditions enforce upwelling of deeper, nutrient-enriched waters, as in certain coastal zones where winds cause near-coast surface waters offshore, where they are replaced by upwelled deeper, nutrient-rich waters. Regions of marked divergence of surface currents, e.g., along the equator, are also the sites of upwelling and thus of enhanced fertility, as are some high-latitude regions where density profiles allow recirculation of deeper waters back to the surface. In the present-day ocean, the CCD varies in depth from less than 3 km in some coastal regions to more than 5 km in part of the Atlantic (Berger and Winterer, 1974). Virtually everywhere, the crests of actively spreading ocean ridges are above

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the CCD and thus the biogenous pelagic sediment on new ocean crust is dominated by calcareous planktonic foraminifers and coccolithophorids. Special circumstances of high fertility, such as on the ridge crests in the Gulf of California, result in diatomrich sediment, but with some calcareous remains as well. With few exceptions, the basal pelagic biogenous sediments on older ocean crust are also calcareous all through the Cenozoic and back as far as the middle Cretaceous, suggesting that the CCD was deeper than about 2700 m during all that time. Prior to middle Cretaceous time, some parts of the spreading ridge system were above the CCD (calcareous), some parts were below (siliceous) and some parts were close to it (cherty and clayey carbonate). Baumgartner (1987) advanced the hypothesis that it was mainly winnowing of radiolarian tests by currents that removed them from shallow areas ("swells") in the Middle Jurassic, leaving only calcareous sediments behind. Winnowing by currents on highs is effective only to the extent that the particles are very close to the sediment-water interface. Swell sediments are generally not marked by evidences of significant currents, such as ripple marks, cross stratification or size sorting. In fact, in most swell-facies strata there are diagenetically calcified tests of radiolarians, as witness to their former presence. Siliceous tests tend to dissolve during sinking from the surface waters to the seafloor, and only a small proportion survive the journey, resulting in slow accumulation rates compared to surface production rates. Silica dissolution at swell depths is faster than at adjacent basinal depths, and is the opposite for carbonate. Swell sediments generally are replete with evidences of thorough burrowing, a process that quickly buries newly deposited particles. Pore waters in the burial environment on the swells are nearly saturated for carbonate but markedly undersaturated for opaline silica. The result is selective preservation of carbonate on swells and selective preservation of silica in adjacent basins. It thus appears that selective dissolution is the more logical explanation for the depth differentiation of siliceous from calcareous pelagic sediments. As shown in Table 1, which lists all drill sites that sampled basal sediments on Mesozoic oceanic crust, there was also a significant difference between oceans: except for the middle Cretaceous, when the CCD was nearly everywhere not much deeper than about 3000 m, the Atlantic remained calcite-preserving from the Callovian onward, while the Pacific waters were more dissolving of calcite at risecrest depths. Approximate curves for the time variations in the depth of the CCD in the oceans is given in van Andel, 1975, There was also a regional difference in habitat for certain calcareous organisms, even at paleodepths above the CCD, as for example for the calpionellids, which are broadly distributed in Tithonian-Aptian sediments in the Atlantic and Indian Oceans and their margins, as far east as New Guinea (Colom, 1965), while they are as yet unreported from the Pacific. NORMAL OCEAN CRUST Composition

For normal oceanic spreading centers, the typical basalt is "Normal" Mid-OceanRidge Basalt (N-MORB), of a fairly narrow range of composition, inferred to be

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Table 1 DSDP and ODP data for Mesozoic basal sediments Ocean

Age of basal sediments

Lithology

Site

Approx. Paleolat.

Atlantic tMaestrichtian

nanno chalk

2

35S

tCampanian

zeolitic clay

9

25N

tCampanian tCampanian late Albian late Albian late Albian Albian Aptian late Aptian Berriasian-Valanginian Tithonian Oxfordian-Kimmeridgian Oxfordian-Kimmeridgian Oxfordian-Kimmeridgian Callovian

nanno chalk nanno ooze nanno marl ooze nanno-bearing claystone marly chalk black nanno claystone black nanno claystone nanno chalk nanno limestone calc. claystone& limestone clayey limestone Limestone clayey limestone nanno claystone

10 354 137 530 550 386 361 417/18 387 391 367 100 105 534

25N 0 20N 35S 40N 25N 50S 15N 25N 20N 15N 15N 25N 20N

Pacific Maestrichtian

nanno limestone

192

30N

Maestrichtian

zeolitic claystone

323

60S

Campanian

cherty limestone

163

10S

Cenomanian-Turonian *Cenomanian Barremian-Albian

radiolarian clay claystone cherty claystone

66 169 164

30S 25S 20S

tAptian tAptian tAptian

cherty limestone Limestone cherty limestone

289 807 464

30S 30S 5S

tAptian

volc. ss, clystn.

317

30S

*Barremian-Aptian Hauterivian late Hauterivian early Hauterivian Berriasian-Valanginian tTithonian

radiolarian claystn nanno marlstone cherty nanno limestone cherty nanno limestone nanno claystone nanno limestone

462 166 303 304 307 167

35S 25S 10S 10S 25S 30S

E. Cretaceous]L. Jurassic Bathonian-Callovian

cherty claystone chert& radiolarite

595/96 801

25S 10S

Campanian Campanian

marly nanno ooze nanno claystone

211 239

35S 25S

Coniacian

detrital clay

250

40S

Cenomanian

nanno limestone and clay

212

40S

middle Albian

detrital clay

257

50S

*middle Albian

nanno ls& rad. clay

260

40S

early Alhian Aptian tValanginian Berriasian Oxfordian

detrital clay claystone nanno-bearing claystn nanno-bearing claystn nanno-bearing claystn

256 259 249 765 261

45S 50S 50S 30S 35S

Indian

*= post-basement flow or sills t -- original depth shallower than normal, e.g. on oceanic plateau.

(continued)

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derived from relatively undepleted, fertile mantle. We have an empirical model for the original starting depth, based on a mass of empirical data from bathymetry, seismic-reflection, magnetic-reversal and drilling data. The newly-formed basalts at the spreading ridge crest, where the first sediments are deposited, typically have a depth of about 2700 4- 300 m. Cooling causes contraction of the lithosphere and deepening of the seafloor as the lithosphere moves away from the spreading axis, generally following the empirical relation depth to basement - 2700 m + K(Age~/2), where Age is in m.y. and K is commonly about 360 (Berger and Winterer, 1974). The empirical relations are effectively independent of spreading rates. We can thus estimate the water depth at which any layer of sediment was deposited and infer paleoceanographic conditions, e.g., the depth of the calcite compensation depth, from the lithologic character of the sediments. Similarly, oceanic lithosphere created at a spreading center in a backarc basin is also generally of N-MORB composition, and the spreading ridge axes are at normal oceanic spreading-ridge depths, e.g., about 2700 m. Crust forming in some young backarc basins, may be influenced by additions of melting-point-lowering water ascending from the subducting slab beneath, and thus have magma compositions different from N-MORB.

Structure

Normal oceanic lithosphere, from the limited data at hand, shows a great range of structure, depending in part on the rates of magma supply from partial melting in the mantle. At relatively fast-spreading ridges, magma supply is copious and a thick (i.e., > 5 km) crust develops, comprising, atop ultramafic rocks of the mantle, layered and massive gabbro connected by a system of diabasic dikes and sills to overlying pillow basalt flows. Where spreading rates are slow, and especially during the early history of speading in a newly-formed ocean basin, serpentinized ultramafic mantle rocks are commonly present at the ocean floor, overlain directly by sediments. The implication is that magma generation was feeble at these places, and that space created by extensional rifting allowed diapiric rise of serpentinite. In some places, strong variability in depth and thickness of oceanic crust occurs along the spreading axis, as for example at Iceland and the Reykjanes Ridge, or along fracture-zone bounded segments of ridge. In short, there is no "standard" crustal structure by which to judge if an ophiolite meets the standard.

Table footnote continued Mesozoic crust, from data in the published volumes of DSDP and ODP (up to Leg 177), grouped by ocean, and listed in order of increasing age of basal sediments, is shown. Only sites where sediments rest on basalt are listed; other types of oceanic crust, e.g., gabbro, serpentinite, signal possible abnormal initial depths. Sites with non-calcareous sediments on basement are shown in bold-face type. The paleolatitudes, approximate to about +5 ~ are from maps in Firstbrook et al., 1979.

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OPHIOLITE Data from ophiolite sections on land

Any general understanding of the nature of the early sediments deposited on ocean crust should try to take advantage of the sediments atop ophiolites, because these can extend our knowledge into conditions during times prior to that represented by the oldest crust preserved in the modern ocean basins. The oldest ocean lithosphere known from drilling and magnetic-anomaly data to be preserved in the ocean basins is about 170 Ma (Bathonian-Callovian, Middle Jurassic), and the only knowledge we have of older oceanic lithosphere is from study of ophiolites tectonically emplaced onto the continents. Ophiolites range in age from Cenozoic to Precambrian, but great caution must be exercised in interpreting these as remnants of oceanic lithosphere generated at oceanic spreading ridges, or regarding their sedimentary cover as representative of contemporaneous oceanic sediments for which the depth rules of normal ocean crust necessarily apply. A serious impediment to interpreting sediments deposited on ophiolite--and by no means are sediments preserved atop all ophiolites-- is that neither the paleodepth nor the tectonic/paleogeographic setting of most ophiolites is as well fixed as for oceanic crust and overlying basal sediments from the actual oceans. Furthermore, many ophiolites are extensively disrupted by tectonism associated with their emplacement and by subsequent deformation, with the result that original pseudostratigraphy is disturbed, and fragmented. Data from drilling, dredging, deep-sea cameras, submersibles and seismic studies in the actual oceans have provided us with but a sketchy picture of the structure of oceanic lithosphere to compare with the structure of ophiolites on the land. Compositional data about the ocean crust, from analyses of rocks recovered from drill holes and by dredging, is abundant but come mainly from very few localities, and for the most part come from the upper, pillow-basalt layers. Gabbros have been cored extensively at one site in the Indian Ocean, and still deeper layers are represented mainly by highly deformed serpentinite in diapirs that crop out at the seafloor. How are we to determine that a particular ophiolite was created at an oceanic spreading center? And if not there, then where?

Composition

Some ophiolites have the N-MORB chemistry and petrology of typical mid-ocean ridge basalt and for these, e.g., the Upper Cretaceous Oman ophiolite (Murton, 1989) and the 2 Ga Purtuniiq ophiolite in northern Quebec (Scott et al., 1990), an ocean spreading-ridge environment can be fairly safely assumed. On the other hand, the basaltic members in many other ophiolites have the chemistry and petrology of rocks typically associated with island arcs or with other supra subduction-zone environments, e.g., boninites. Boninitic lavas suggest derivation from depleted mantle sources, e.g., sub-oceanic lithosphere, that has been partially remelted by addition of water derived from the subducting slab.

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They are generally among the oldest lavas in oceanic arcs, but also occur in transtensional transform zones, as on Cyprus (Murton, 1989).

Structure

The supra-subduction-zone ophiolites, on the other hand, commonly do have some of the features of the pseudostratigraphy of oceanic lithosphere, i.e., ultramafics (commonly serpentinized), overlain by layered and isotropic gabbro, diabasic dikes and sills, and pillow basalt and are thus logically inferred to be the result of lithospheric accretion at a spreading axis. The dikes and sills in some of these suggest only small amounts of extension, as in the California Coast Range ophiolite. This well-studied ophiolite appears to have been emplaced in a forearc setting, and shows both island-arc and boninitic petrology (Robertson, 1989, 1990). The problem with supra-subduction zone attribution lies in identifying any such spreading axes in modern oceanic-arc settings, above a subduction zone. A number of competing models for supra-subduction zone ophiolites have been proposed. A preferred model is for a functioning spreading axis in a forearc above a functioning subduction zone, but no actualistic example has been identified in any modern forearc region. Many forearcs show evidence of extensional faulting, but without traces of a spreading center. Because modern forearcs are typically at shallower depths than normal ocean floor, this model presents the further difficulty of estimating the paleodepth of the spreading axis, that is, the depth at which sediment accumulation begins. We have no empirical basis for estimating paleodepths in forearcs unless we have independent knowledge of the depth of the paleo-CCD. A variant of this model is to generate boninites in a transtensional transform fault setting above a subduction zone, as proposed for Cyprus boninitic rocks (Murton, 1989). S e d i m e n t s on ophiolite

Leaving aside basal hydrothermal deposits, some of which include molds of calcareous fossils, and excluding allochthonous sediments, examples of basal sediments resting conformably on ophiolites of increasing ages are: 9 Upper Cretaceous: Cyprus: Campanian radiolarian claystone (Robertson and Hudson, 1974) 9 Middle Cretaceous: Tibet: Xigaze ophiolite, overlain conformably by radiolarite (Marcoux et al., 1982), Oman: Samail ophiolite, intralava Cenomanian radiolarites (Tippet et al., 1981) 9 Lower Cretaceous: variable: in some places they are calcareous, in some siliceous. On the Costa Rica (Nicoya Peninsula): radiolarite (Baumgartner, 1987) 9 Upper Jurassic (Kimmeridgian-Tithonian): No examples certainly known of this age. By contrast, virtually all ophiolites covered by sediments Oxfordian or older have radiolarite or siliceous claystone as first sediments (except for hydrothermal,

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volcanoclastic or turbidite clastics). Examples from the Jurassic and Triassic include (* designates possible supra-subduction zone bodies): 9 Middle Jurassic and Oxfordian Apenninnes: (Marcucci and Conti, 1995) *Coast Ranges, Calif. (Baumgartner et al., 1987) Costa Rica, Nicoya Peninsula (Baumgartner, 1987) *N. Calif.: Josephine ophiolite (Harper, 1984) 9 Lower Jurassic California, Marin Headlands (Murchey, 1984) Costa Rica, Nicoya Peninsula (Baumgartner, 1987) 9 Upper Triassic: Baja California., San Hippolito: radiolarite on pillows, with interpillow Halobia, which might be the result of confined environments where waters could reach saturation for calcite). (Pessagno et al., 1979) Greece: radiolarite (w/Halobia) on pillow basalts (Smith et al., 1975) in a "marginal" ocean basin (Smith and Spray, 1984)

TRENDS IN MESOZOIC BASAL SEDIMENTS FROM DRILL AND OPHIOLITE DATA A few broad generalizations about basal sediments can be drawn from the data from the actual oceans and from ophiolites: A. Calcium carbonate, by ages 9 Most Campanian (exception: Cyprus) and Maestrichtian non-hydrothermal basal sediments are calcareous, deposited above the CCD. 9 Aptian-Coniacian basal sediments are carbonate-poor, deposited close to the CCD, but some original carbonate may have been removed by diagenesis (see Oxygenation) 9 Kimmeridgian-Barremian sediments are calcareous, deposited above the CCD. 9 Oxfordian sediments are slightly calcareous, deposited close to CCD. 9 Callovian basal sediments are marly in the Atlantic (above CCD) but noncalcareous elsewhere (below CCD). 9 Pre-Callovian basal sediments are non-calcareous everywhere, deposited below CCD. B. Calcium carbonate, by regions 9 The least dissolved basal sediments, age by age, are in the Atlantic. The Indian and Pacific are more or less alike in terms of dissolution. C. Oxygenation, i.e, black shale 9 Black shales deposited at spreading-center depths are a feature of the middle Cretaceous of the Atlantic, while in the Indian (Neo-Tethyan) and Pacific Oceans, basal sediments were deposited in oxygenated waters. Diagenesis of

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organic-rich sediments may include reactions leading to sulfide phases, e.g., pyrite and hydrotroilite, and acidic waters that can dissolve carbonate fossils. The evolving outlines of oceanic basins and their connections during the Mesozoic provide a relatively simple explanation for the space and time variations in Mesozoic basal sediments. In the Central Atlantic, following a Late Triassic-Earliest Jurassic episode of rifting and hypersaline lagoonal conditions, ocean waters first appeared in the narrow Central Atlantic basin in Early Jurassic time, derived in part from boreal sources via the shallow northern seaways between Europe and North America/ Greenland. Slow seafloor spreading gradually widened the Central Atlantic, which terminated southwestward in the Gulf of Mexico, a saline near-cul-de-sac during most of the Jurassic. It is uncertain whether there was an oceanic connection via Central America into the Pacific in the Jurassic. The South Atlantic remained closed until late in the Early Cretaceous. The Central Atlantic connected in deep water via the Mediterranean Tethys to the Neo-Tethys Indian Ocean, and thence across wide openings north of Australia-New Guinea (then locked against Antarctica) and south of Indo-China. The Pacific was hemmed in on the north, east and south by continental margins, and formed one huge ocean with the Tethys a kind of triangular bay narrowing gradually westward toward the Mediterranean Tethys. Triassic and Jurassic seafloor spreading in the "Indian" sector of the Tethyan ocean led to subduction beneath southern Asia, commonly with the formation of back-arc basins such as those preserved in Tibet, and commonly with ophiolitic m61anges in the suture zones between island arcs and the adjacent backarc basins (Hsfi et al., 1995). Fragments of the southern, Gondwana continent, some with their characteristic southern faunas and floras, from time to time rifted away and drifted northward to collide with Asia, leaving suture zones peppered with dismembered ophiolites. By the end of Jurassic time, a connection was established between Atlantic and Pacific across Central America, making the Tethyan zone a circum-global seaway in low latitudes. The Atlantic remained relatively "lagoonal" and carbonate-preserving with respect to the "estuarine" Pacific, with a shallower CCD up to the present time. The CCD has irregularly deepened in all ocean to levels below the average depth of spreading ridges since the increases in abundance of coccolithophorids in the Late Jurassic and of planktonic foraminifers in the mid-Cretaceous. The further deepening of the CCD in the early Oligocene, associated with extensive Antarctic glaciation, did not affect the composition of basal sediments but did shift the composition of biogenous sediments deposited in deeper waters toward the more calcareous.

SUMMARY Based largely on data from ocean drilling, but partly on data from ophiolites, a review of the types of sediments deposited on oceanic crust soon after the formation of the crust, shows that prior to Oxfordian time, basal biogenous sediments were mainly siliceous, radiolarian-rich and essentially carbonate-free. Relatively low production rates of calcareous plankton kept the CCD mainly shallower than the crest of spreading ridges. During Late Jurassic and Early Cretaceous time, following

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increases in a b u n d a n c e of calcareous c o c c o l i t h o p h o r i d s , the C C D d e e p e n e d a little a n d limey sediments a d m i x e d with clay a n d silica were d e p o s i t e d on s p r e a d i n g ridges in the Pacific a n d its w e s t w a r d extension b e t w e e n Asia a n d G o n d w a n a l a n d , at least at low paleolatitudes. By contrast, n e w - f o r m e d ocean crust in the A t l a n t i c a n d its eastern, " M e d i t e r r a n e a n " arm, where the C C D h a d d r o p p e d to as m u c h as 4500 m (Winterer, 1998), was generally overlain by calcareous sediments. D u r i n g midC r e t a c e o u s times ( A p t i a n - T u r o n i a n ) , in spite of a great increase in a b u n d a n c e of p l a n k t o n i c foraminifers, the C C D generally shallowed to levels close to a b o u t 3000 m, with the c o n s e q u e n c e t h a t sediments of this age on new ocean crust are c o m m o n l y c a r b o n a t e - p o o r . D u r i n g L a t e C r e t a c e o u s time, the C C D d e e p e n e d again, so t h a t in m o s t places b i o g e n o u s sediments d e p o s i t e d on s p r e a d i n g ridges were c a r b o n a t e - r i c h once m o r e . This c o n d i t i o n has persisted t h r o u g h C e n o z o i c times. REFERENCES Baumgartner, P.O., 1987. Age and genesis of Tethyan Jurssic radiolarite, Eclog. geol. Helv.,80: 831-879. Baumgartner, P.O., De Wever, P. and Murchey, B., 1987. Correlation of Tethyan and W-North American Jurassic zonations and age of Franciscan terranes, Terra Cognita, 7: 210. Berger, W.H. and Winterer, E.L., 1974. Plate stratigraphy and the fluctuating carbonate line. In: Jenkyns, H.C. and Hsfi, K.J. (Eds.), Pelagic sediments: on land and under the sea. Int. Assoc. Sedimentologists, Spec. Pub. No. 1, 11-48. Bernoulli, D., Garrison, R.E. and McKenzie, J., 1978. Petrology, isotope geochemistry, and origin of dolomite and limestone associated with basaltic breccia, Hole 373A, Tyrrhenian Basin. In: Hsfi, K.J. and Montadert, L. (Eds.), Init. Repts., Deep Sea Drill. Proj., Washington D.C., U.S. Gov't Print. Off. 42, Pt. 1,541-558. Bernoulli, D. and Weissert, H., 1983. Sedimentary fabrics in serpentinite breccias (ophicalcites) and their significances, South-Pennine Arosa Zone, Switzerland. In: Proc. Conf. on Ophiolites: oceanic tectonics and metamorphism. Florence, Italy. Ptiagora, Bologna, 8: 7p. Colom, G., 1965. Essais sur la biologie, la distribution g6ographique et stratigraphique des Tintinno~diens fossiles, Eclog. geol. Helv., 58: 319-334. Firstbrook, P.L., Funnell, B.M., Hurley, A.M. and Smith, A.G., 1979. Paleoceanographic Reconstructions., 160-0 Ma. Deep Sea Drilling Project, La Jolla, 41p. Harper, G.D., 1984. The Josephine ophiolite, northwestern California, Geol. Soc. Amer. Bull.,95: 10091026. Hsfi, K.J., Pan, G., Seng6r, A.M.C., Briegel, U., Chen, H., Chen, C., Harris, N., Hsfi, P., Li, J., Lee, T., Li, Z.X., Li, C.L., Powell, C., Wang, C., and Winterer, E.L., 1995. Tectonic evolution of the Tibetan Plateau: a working hypothesis based on the archipelago model of orogenesis.Internat. Geol. Rev., 37: 473-508. Irwin, W.P. and Coleman, R.G., 1972. Preliminary map showing global distribution of alpine-type ultramafic rocks and blueschists. U. S. Geol. Surv,.Misc. Field Studies Map; MF- 0340. Jenkyns, H.C., 1986. Pelagic environments, In: Reading, H.G. (Ed.), Sedimentary Environments and Facies, Oxford, Blackwell, 2nd Edit., 349-354. Johnson, H.P. and Holmes, M.L., 1989. Evolution in plate tectonics; The Juan de Fuca Ridge. In: Winterer, E.L., Hussong, D.M. and Decker, R.W. (Eds.). The Eastern Pacific Ocean and Hawaii, Geol. Soc. Amer., The Geology of North America, N, 73-91. Lonsdale, P., 1989. Geology and tectonic history of the gulf of California. In: Winterer, E.L., Hussong, D.M. and Decker, R.W. (Eds.), The Eastern Pacific Ocean and Hawaii, Geol. Soc. Amer., The Geology of North America, N, 499-521. Marcoux, J., De Wver, P., Nicholas, A., Girardeau, J., Xiao Xuchang, Chang Chengfa, Wang Naiwen, Zao Yougong, Bassoulet, J.P., Colchen, M. and Mascle, G., 1982. Preliminary report on depositional sediments on top of volcanic member: the Xigatze ophiolite (Yarlung- Zangbo suture zone, Zizang, China). Ofioliti,6: 31-32.

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Marcucci, M. and Conti, M., 1995. Radiolarian biostratigraphy of the cherts in the sedimentary cover of the Apenninic ophiolites (Italy). In: Baumgartner, P.O., et al. (Eds.) Middle Jurassic to Lower Cretaceous Radiolaria of Tethys: Occurrences, Systematics, Biochronology, Mem. Geol. (Lausanne), 23: 799-812. Milkert, D., Alonso, B., Liu, L., Zhao, X., Comas, M. and de Kaenel, E., 1996. Sedimentary facies and depositional history of the Iberia Abyssal Plain. In: Whitmarsh, R.B. et al., (Eds.), Proc. Ocean Drill. Prog., 149: 685-704. Murchey, B., 1984. Biostratigraphy and lithostratigraphy of cherts in the Franciscan Complex, Marin Headlands, California. In: Blake, M. (Ed.), Franciscan Geology of Northern California. Pac. Sect., SEPM, Los Angeles,43: 51-70. Murton, B.J., 1989. Tectonic controls on boninite genesis. In: Sanders, A.D. and Norry, M.J. (Eds.), Magmatism in the Ocean Basins. Geological Society Spec. Pub. 42: 347-377. Pessagno, E.A., Jr., Finch, W. and Abbott, P.L., 1979. Upper Triassic Radiolaria from the San Hipolito Formation, Baja California. Micropaleontology,25: 160-197. Robertson, A.H.F., 1989. Palaeoceanography and tectonic setting of the Jurassic Coast Range ophiolite, central California: evidence from the extrusive rocks and the volcaniclastic cover. Mar. Petrol. Geol.,6: 194-220. Robertson, A.H.F., 1990. Sedimentology and tectonic implications of ophiolite-derived clastics overlying the Jurassic Coast Tange ophiolite, Northern California. Amer. Jour. Sci., 290: 109-163. Robertson, A.H.F. and Hudson, J.D. 1974. Pelagic sediments in the Cretaceous and Tertiary history of the Troodos Massif, Cyprus. In: Jenkyns, H.C. and Hsfi, K.J. (Eds.), Pelagic sediments: on land and under the sea. Int. Assoc. Sedimentologists, Spec. Pub. No. 1,403-436. Schiffmann, P., Evarts, R.C., Williams, A.E. and Pickthorn, W.J., 1990. Hydrothermal metamorphism in oceanic crust from the Coast Range ophiolite of California: fluid-rock interaction in a rifted island arc. In: Peter, Tj., Nicolas, A. and Coleman (Eds.), Ophiolite Genesis and Evolution of the Oceanic Lithosphere. Proc. Ophiolite Conference, Muscat, Oman, 1990., Kluwer, Dordrect, 399-425. Sclater, J.G., Anderson, R.N. and Bell, M.L., 1971. Elevation of ridges and evolution of the central eastern Pacific. Jour. Geophys. Res., 76: 7888-7915. Sclater, J.G. and Detrick, R., 1973. Elevation of mid-ocean ridges and the basement age of JOIDES drilling sites. Geol. Soc. Amer. Bull., 85: 1547-1554. Scott, J.D., St.-Onge, M.R., Lucas, S.B. and Helmstaedt, H., 1990. Geology and chemistry of the Early Proterozoic Purtuniq ophiolote, Cape Smith Belt, Northern Quebec, Canada. In: Peter, Tj., Nicolas, A. and Coleman (Eds.), Ophiolite Genesis and Evolution of the Oceanic Lithosphere. Proc. Ophiolite Conference, Muscat, Oman, 1990., Kluwer, Dordrect, 1990, 817-849. Smith, A.G., Hynes, A.J., Menzies, M., Nisbet, E.G., Price, I., Welland, M.J. and Ferri6re, J., 1975. The stratigraphy of the Othris Mountains, eastern central Greece: a deformed Mesozoic continental margin sequence, Eclog. geol. Helv.,68: 463-481. Smith, A.G. and Spray, J.G., 1984. A half-ridge transform model for the Hellenic-Dinaric ophiolites. In: Dixon, J.E. and Robertson, A.H.F. (Eds.) The Geological Evolution of the Eastern Mediterranean, Oxford, Blackwell Scientific, 629-644. Tippet, P.R., Pessagno, E.A. and Smewing, J.D., 1981. The biostratigraphy of sediments in the volcanic unit of the Samail ophiolite. Jour. Geophys. Res.,86: 2756-2762. van Andel, Tj., 1975. Mesozoic/Cenozoic calcite compensation depth and the global distribution of calcareous sediments, Earth Planet. Sci. Lett.,26: 187-195. Winterer, E.L., 1998. Paleobathymetry of Mediterranean Tethyan Jurassic pelagic sediments, Mem. Soc. Geol. Ital., 53: 97-131.

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Paradoxes in Geology Ueli Briegel and Wenjiao Xiao, eds. 9 2001 Elsevier Science B.V. All rights reserved.

Chapter 19

U N V E I L I N G SECRETS L O C K E D IN GREAT G H O S T LAKE IN TAIWAN

Chen-Tung Arthur Chen Institute of Marine Geology and Chemistry, National Sun Yat-Sen University, Kaohsiung 804, Taiwan

ABSTRACT Distinctive white laminations appear in the generally black sediments of the anoxic Great Ghost Lake in Taiwan. These white laminations have lower porosity, organic matter content and C/N ratio, but higher C/S, N/S ratios and magnetic susceptibility, and contain organic matter most probably derived from phytoplankton. This suggests that they have been deposited in cooler and drier periods. The organic matter of the black sediments is mainly from land plants, suggesting deposition under warmer and wetter climates. These high resolution data, the first not only in Taiwan but also for southeast Asia, are in good agreement with the local climatological records deduced from documentary sources since 1681 and direct instrumental measurements since 1900. Major white laminations appear in BC 250, BC 80, 90 AD, 420-520 AD, 620-700 AD, 850-930 AD, 1350-1430 AD, 1550 AD, 1730-1800 AD and 1960 AD, and correspond to not only the periods of cold/dry and frequent dust storms, but also population declines based on the historical and natural records of China. These seem to reflect large scale climatic changes.

INTRODUCTION Why does Great Ghost Lake in Taiwan have such a name? There are lots of legends. One is that it is a forbidden land, where, if someone were foolish enough to enter, he would never return. Then, there's another story. About twenty years ago, the local governments of Taiwan tried to stock many lakes in Taiwan with various kinds of fish. They had great success but not with Great Ghost Lake. The fish in Great Ghost Lake could never survive even after repeated attempts. Back in 1984, realizing that acid rain had become such a serious problem at home, I started to look at its various effects on the lakes in Taiwan. I started out with only a general survey. That was the first time a large survey had ever been attempted, so we only sampled surface water. Very quickly we (Chen and Wang, 1997) found that many high mountain lakes are acidic (Fig. 1). This is partly due to the fact that these lakes have very low buffering capacity (Fig. 2). This means the lake water in those

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Fig. 1. The pH of lake waters Zones I through III are described in the text. (Shaded area represents where elevations > 2000m)

places becomes acidic with even only a small amount of acid added. Then, we turned our attention to alpine and subalpine lakes and did a more detailed study of these lakes. Thereby we found out that Great Ghost Lake was not only the deepest lake in Taiwan, but also the only one where the subsurface waters were anoxic almost all year-round (Chen and Wang, 1990). This explains why those fish couldn't have survived in Great Ghost Lake because there is no dissolved oxygen in the water. With the lack of oxygen, the organic matter in the mud which accumulated on the bottom of the lake was also well preserved. And without any fish or other large organisms living in the lake, the mud was never disturbed. There was more to

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Fig. 2. The alkalinity of lake waters Zones I through III are described in the text. (Shaded area represents where elevations> 2000 m)

the story. Because the lake is deep with steep slopes, the wind and waves could not disturb the mud either; in other words, there is always older mud lying beneath the younger mud. The mud with a well preserved sequence (Fig. 3) had become an archive of past climatic events (Lou and Chen, 1997). GREAT GHOST LAKE Great Ghost Lake is located north of the Yao-Bye Mt. at 22 ~ 52'15"N, 120 ~ 51'15"E in Kaohsiung County in southern Taiwan (Fig. 4). The length and width of

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Fig. 3. Dark sediments with white laminations

this 2150-m high lake are 650 m and 260 m, respectively. In total, the lake area is about 10.87 ha, and the drainage area is 90.3 ha. The maximum depth is 40 m in the rainy season, and about 2 m less in the dry season. The average water depth is 15.4 m. The local strata belong to the Miocene Lushan Formation which is composed mostly of argillite and slate. This area is one of the wettest in Taiwan, and the annual rainfall, mostly concentrated from May to September, is about 4200 mm. The annual mean temperature is about 13~ Except during the brief winter turnover, the deep water is anoxic most of the year. Important though is that the sediments are well protected from oxidization or bioturbation after deposition, so, of course, the environmental signals are well recorded (Chen et al., 1993).

PAST C L I M A T I C SIGNALS Most of the sediments are dark brown (or black). These are interlaced with white laminations (Fig. 3). White laminations are finer in grain size, and they have less sorting and their organic matter content is lower. There are some coarse plant fragments, but the major grains are silt and fine sand. The rest is clay, medium sand and coarse sand. No gravel could be found in the sediments. The depths with the obvious white laminations are at 3, 14, 23, 30, 51, 58, 63, 75, 79 and 84 cm

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Fig. 4. Location of Great Ghost Lake from the top. Their thickness ranges from 0.1 to 2 cm. In fact, some of the thick white laminations (such as those at 51, 58 and 75 cm) are composed of several thin laminations. Note the twigs and small fragments of leaves frequently found beneath the white laminations. This may suggest rapid or catastrophic changes in the environment. Figures 5 and 6 show the distributions of the Relative Brightness Index (RBI), the contents of water, total organic carbon (TOC), total nitrogen (TN), total sulfur (TS), and the atomic ratios of TOC/TN (C/N), TOC/TS (C/S) and TN/TS (N/S). The lower RBI segments are more black, and have a higher water content or porosity. They also have a higher C, N, S contents and C/N ratio, but lower C/S and N/S ratios. But on the other hand, the higher RBI segments are lighter in color, have lower water and organic matter contents, lower C/N ratio, but higher C/S and N/S ratios. Generally, more than 90% of the nitrogen in lake sediments is organic. Carbon and sulfur are mostly from the organic matters in the sediments of the

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Fig. 5. Distributions of (a) RBI (relative brightness index); (b) WC (water content); (c) TOC (total organic carbon); (d) TN (total nitrogen) and (e) TS (total sulfur) in the sediments of Great Ghost Lake. The dashed lines show the mean values. Arrows 1, p and s show the components of land plants, phytoplanktons and shore soils, respectively.

oligotrophic lake. Shore soil is the major source of the sediments, but it contains very little organic matter, in fact, less than one-third of that in the white laminations (Figs. 5c and 5e). It can be said, therefore, that the organic matter in the sediments is mainly from the phytoplankton and plants on land. Phytoplankton has lower C/N, but higher C/S and N/S ratios than the plants (Huang and Chen, 1994). It seems that the organic matter in the black sediments are mostly from plants, but the one in the white laminations is largely from phytoplankton. The older sediments in the lower part of this core have lower C/S and N/S ratios than the plants (Figs.6c and 6d). This could have come from either the decomposition of the organic matter that reduces organic carbon and nitrogen contents, or it could be from the sulfate reduction in the porewater that increases the inorganic sulfur content in the sediments. The Pb-210 method was used to calculate the modern sedimentation rate. The sediments containing high organic carbon were used for radioactive C-14 dating by the Institute of Geological and Nuclear Sciences in New Zealand.

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Fig. 6. Distributions of (a) RBI (relative brightness index); (b) C/N (atomic TOC/TN ratio); (c) C/S (atomic TOC/TS ratio); (d) N/S (atomic TN/TS ratio) in the sediments of Great Ghost Lake. The dashed lines show the mean values. Arrows 1, p and s show the components of land plants, phytoplanktons and shore soils, respectively.

They used an Accelerator Mass Spectroscope. The results of these datings are given in Fig. 7a. A uniform stratum and nearly closed terrain surrounds Great Ghost Lake, but note the considerable variations in the colors and organic matter compositions of the sediments. Also look at the sharp boundaries between the black and white sediments. These may once again imply abrupt environmental changes. Why were there such changes? Perhaps they were due to changes in the redox potential of lake water, abrupt landslides, mud flows or climatic changes. Remember that the C/N ratio is identical for organic matter decomposed under either aerobic or anaerobic conditions, so such variations in the organic matter components of the sediments could not have been induced by the changes in the redox states. On the other hand, there is the possibility that the steep slopes of the lake and drainage areas may have caused landslides or mud flows. This would have disrupted normal deposition. But this can't be true because of the flourishing vegetation around the lake which protects the soil from strong erosion.

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Fig. 7. Comparison between the RBI curve and the historical/natural records: (a) C-14 dating and sedimentation rate; (b) RBI; LIA: Little Ice Age, MWP: Medieval Warm Period, C: cold, W: warm, D: dry and M: moist; (c) MI (moisture index) in China. Dashed lines I& II represent the mean values during 1000 1900 AD and 0 ~ 1000AD, respectively; and (d) PI (precipitation index, moving average of sixty years) in Central North China (modified from Lou et al., 1997). Additional evidence which follows suggests that landslides or mud flows seldom affect depositions in this lake. First of all, the sediments are all composed of fine grains without coarse sand or gravel which normally accompanies strong flows. And the ages of the sediments are linearly related to depth (Fig. 7a). Secondly, the organic matter content of the white lamination is several times greater than that of the soil and its composition is typical of that of phytoplankton. In contrast, we would expect that the deposits of the mud flows should contain much soil with low organic matter content but with plant detritus imbedded. Thus, the major factor affecting the characteristics of the sediments should be the climate. Since land plants are more sensitive to temperature and moisture than aquatic organisms, they are much more influenced by any climatic changes. During the warm, wet periods, the drainage area is covered with flourishing terrestrial plants and these contribute more organic matter with higher C/N ratios to the lake sediments. But during the cool/dry periods, there is an increase in the ratio

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of phytoplankton to land plants so sediments contain less organic matter with lower C/N ratios. The white laminations have similar features to those of the phytoplankton. This is why we postulate that they were formed under the cool/dry climate (Meyers et al., 1993; Nakai, 1986).

COMPARISONS WITH CHINESE HISTORICAL AND N A T U R A L RECORDS According to the 2500 year-long record the high RBI segments appeared in BC 250, BC 80, 90 AD, 420-520 AD, 620-700 AD, 850-930 AD, 1350-1430 AD, 1550 AD, 1730-1800 AD and 1960 AD. During the 1350-1950 AD period, sediments had higher RBI than those before 1350 AD. The cold "Little Ice Age" (LIA) occurred from 1550-1850 AD (Lamb, 1977), but some scientists have proposed that the LIA began in 1250 AD after the period of 900-1300 AD which was known as the "Medieval Warm Period" (MWP). Fig. 7b shows that the low and high RBI segments correlate well with the global MWP and LIA. The LIA is a global phenomenon, but there is a time lag depending upon the region. The climate in China became cold in the twelfth century, and this must have been the beginning of the LIA. The RBI of the lake sediments increased after 1200 AD, but were obviously higher after 1350 AD. This makes us conclude that the beginning of the LIA in Taiwan was about 1350 AD. Chinese climate fluctuations could also be extracted from the historical records of floods and droughts (Fig. 7c, Chu, 1973). The climate was drier and more variable during the last millennium than during the period of 1000-2000 BP. The drier periods are well correlated with the white laminations. From these historical drought/flood and precipitation records, central North China could be divided into several dry/wet periods including 11 major dry periods (Fig. 7d). Note that the dry periods mostly coincided with the high RBI segments, but there are some inconsistencies in 300-500 AD, 700 AD and 900 AD, etc. (Fig. 7d). Perhaps this is due to differences in sampling and in the interpretation of these historical documents. The high RBI segments are mostly correlated with the cool/dry periods of the historical records, yet there are slight differences in time or magnitude depending on the area. Most of the white laminations appear in the historical dry periods, but not all dry periods occur with the presence of white laminations. We have to remember that China is a vast extent of land with diverse geography and climate. The point is that the average effect of the climate is reflected in the sediments of Great Ghost Lake and this means that there must have been large scale climate fluctuations. The high RBI segments also coincide with the cold periods recorded in the Dunde Ice Core of the Qinghai-Tibetan Plateau in China. Not only the three highest RBI segments (500 AD, 650 AD, 900 AD) correspond to the three coldest periods recorded in the Ice Core, but also some small white laminations (in 90 AD, 280 AD and 1250 AD, etc.) correspond with the colder periods. Also note that the black ones correlate with the warmer periods (Lou et al., 1997).

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Fig. 8. Comparison between the RBI curve and the historical/natural records: (a) RBI; (b) 3180 in the Dunde Ice core; (c) DF (dustfall frequency, events/decade) in North China; (d) LF (141 Chinese lake-area fluctuation frequency, %) in China; and (e) CLF (49 Chinese coastal lake-area fluctuation frequency, %) (modified from Lou et al., 1997).

During the cold periods, the amplified Siberian High makes the weather in the area of 30-45~ in northern China drier. The cold, dry periods are contemporaneous. As far as dust fall frequency is concerned, notice in Fig. 8c that the southward moving Siberian High enhances the front activities and the frequency of dust storms. To sum up, the white laminations correspond well with the periods of the frequent dust storms (500 AD, 700 AD, 900 AD, 1100 AD and the LIA, etc.). Fig. 8d shows the lake-area fluctuations of 141 lakes during the last 2500 years in China. In terms of the whole of China, there were three periods of lake expansion. This is indicative of more rainfall and more frequent floods. In contrast, lake regression occurred in the dry periods. The relevance of the RBI profile to the lake-area fluctuation records is not as obvious as, say, that of the RBI to the moisture index (Fig. 7c). It's also not as important as the precipitation fluctuation in central North China (Fig. 7d). This is because of different time resolution. Only near

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500 AD and 1650-1980 AD did the low water levels of the lakes correspond to the high RBI segments. Human activities might have greatly affected lake evolution (e.g. through land reclamation). The records of the lake-area fluctuations were hampered by complex sampling problems. Nevertheless, the lake-evolution records exhibit similar trends to the moisture index or the precipitation fluctuation in central North China (Shi, 1993). For example, they all show the major dry periods of 300-700 AD and 1000-1300 AD, but the lake-area records do not show details for these two periods. In Fig. 8e it's obvious that the high RBI segments correspond better to the periods of Chinese coastal lake-area fluctuation frequency. The dry periods of BC 90, 90 AD, 500 AD, 900 AD, 1400 AD and 1750 AD all match.

SPECULATION ON F O R M A T I O N MECHANISMS Most of the white laminations in Great Ghost Lake correlate with the cool/dry periods according to the Chinese historical and natural records, while the black sediments correspond to the warm/wet periods. A few time and magnitude deviations within these records exist because of the limitations of the dating methods, sensitivity, resolution and differences in the local geography (Chen et al., 1993). At present Western China is the major source of aerosols lingering over the North Pacific. Frequent outbursts of dust storms occur in western China especially during the dry and cold periods. In fact, a large dust veil can indeed extend thousands of kilometers eastward. Most coarse black or fine white sediments reveal a single mode in size distribution. But note the sediments in the LIA are bimodal being a mixture of black and white sediments. It seems probable that these sediments originate from two different sources. The white sediments sometimes appear as individual 1~2-mm thin laminations or as a 30-mm thick layer composed of several thin laminations. They sometimes form sharp, but sometimes smeared boundaries with the black sediments. The white sediments have a grain size distribution like loess and they are present in periods corresponding to the high dustfall and/or frequent dust storm periods. This probably tells us that part of the inorganic matter in the white laminations could be aeolian dust. As vegetation flourishes around Great Ghost Lake during the warm/wet periods, plants on the land supply plenty of organic matter to the lake. At the same time, less dust is blown into the atmosphere in the arid regions, and most of this freight in the air is washed down by rains before it arrives at the lake (Fig. 9). In other words, little dust reaches the lake, and that which does could be coated with organic matter. That would make these dust particles indistinguishable from the black sediments. This may explain why the lake deposits are seen to contain black sediments only. As the climate gets colder and drier, phytoplankton instead of land plants become the major source of organic matter due to the inhibited growth of the plants on land. Because of the low rainfall, less material is transported from the drainage area into the lake. Aeolian dust or the soil from the shore with low organic matter content are

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Fig. 9. Pathway of aelion dusts to the Great Ghost Lake.

then deposited together with phytoplankton to form white laminations. The presence of the twig and leaf fragments immediately beneath the white laminations may suggest that defoliation was induced by some abrupt cooling.

FLUX OF METALS IN THE PAST 2600 YEARS With an increasing population and industrialization, human activities have become the dominant factor affecting the biogeochemical cycling of many trace metals in Taiwan. In fact, globally the amount of material removed from the Earth's crust each year by mining is now of the same order of magnitude as new material generated by such fundamental geological processes as mountain building, erosion

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and oceanic crust formation. As a matter of fact, anthropogenic fluxes of many potentially toxic trace metals to the atmosphere have even exceeded the natural fluxes. In the same way, the current ensemble of changes in Taiwan population growth, urbanization and i n d u s t r i a l i z a t i o n - have had more profound ramifications than any previous changes. Probably of greatest importance is the rapid industrialization in Taiwan over the past 40 years. This has made air pollution and acid rain two major environmental problems. There is the concern that acidified water from acid rain could mobilize metals, which of course would generate a threat to public health. The high mountain lakes are clearly the most threatened by the effect of acid rain. Of course this means that the distribution of metals in the near-surface sediments of these lakes is likely to be affected. There is a strong interest in metal fluxes, so it is necessary to understand the various pathways. The Earth's atmosphere has been known to transport both natural and anthropogenically mobilized trace elements. Of these, lead is one of the easiest to study. As far as atmospheric input is concerned, lead dominates. Aeolian anthropogenic lead was being produced from the development of the cupellation technology since about 4500 BP, and then its production increased markedly in the late eighteenth century at the beginning of the Industrial Revolution. Furthermore, around 1940, there was a sharp increase in atmospheric lead concentrations due to lead emissions from automobiles. In Taiwan the auto industry started to take off about ten years after that. The anthropogenic source of lead is relatively easy to identify. This is because, in most ores, lead has Pb-206/Pb-207 ratios which are lower than the natural lead of soil and soil-related components. There are two widely used methods of reducing the grain-size effects to evaluate the natural or anthropogenic sources of metals in the sediments. First we can compare the relative abundance with respect to aluminum. Secondly, we can leach the sample with dilute acid. The iron and manganese oxides, the surface of clay minerals, organic matter and carbonates are the major phases of metal association in sediments (Wann et al., 1997). Acid-leached metals represent most of the metals in these phases, but they do not include those in the lattice of clay minerals and rock debris. Those ores are of natural origin and we know they are not changed by anthropogenic inputs (Chen et al., 2001). In order to identify the anthropogenic sources, it is also useful to compare recent data with historical data. Unfortunately, these are lacking in Taiwan. Sediments deposited over a time span of 2600 yrs in Great Ghost Lake became useful. Not only could we establish a baseline, but we could also identify natural variations. In Fig. 10 are plotted the vertical distributions of 16 total metal concentrations (A1, As, Cd, Ce, Cr, Cs, Cu, Fe, Mg, Mn, Ni, Pb, Rb, Sr, V and Zn). The values vary a great deal, in fact, far more than can be explained through the possible analytical errors. In order to be certain that these are not caused by sampling errors, another core was examined. Similar peaks were found. It seems that the peaks in Fig. 10 are related to the RBI peaks. The white laminations at 3, 23, 30, 51, 58, 63 and 75 cm have higher concentrations of aluminum, caesium, iron, magnesium, nickel, rubidium and zinc, but lower concentrations of arsenic, cadmium, cerium, manganese and strontium. The first group of metals

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Fig. 10. Vertical distributions of total metal concentrations.

are mostly from the crust while the second group are more associated with organic matter (Wann et al., 1995; Wann and Chen, 1996). The vertical distribution of acid-leached metals are plotted in Fig. 11. These peaks also correlate with the RBI. Look at the near surface enrichment. Aluminium, cadmium, cerium, copper, lead and zinc are apparent. This suggests a possible anthropogenic input. With the help of the Pb-210 and C-14 datings, the depth scale

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Fig. l l. Vertical distributions of acid leached metal concentrations.

was then changed to the age of the sediments. Also the metal concentrations were converted to fluxes, (Figs. 12 and 13). It can be clearly seen that since about 1350 AD, or the onset of the Little Ice Age, most metal concentrations have increased. The highest rate of increase obviously occurred after about 1950 A D when the industrialisation age started to gain its hold on Taiwan.

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Fig. 12. Fluxes of total metals (~tg/cm2/yr) in different years. The total aeolian deposition rates of Wu (1996) are marked at the bottom. Recall from Fig. 7c the aeolian particle fluxes also started to increase after 1350 AD. The higher RBI and, the higher metal fluxes are most likely due to the higher rate of the long-range air transport of crust material. The crust material can best be represented by aluminium which is a relatively conservative element with no

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Fig. 13. Fluxes of acid-leached metals (~tg/cm2/yr) in different years. The total aeolian deposition rates of Wu (1996) are marked at the bottom.

significant anthropogenic source. In order to look at other effects, the total metal and acid-leached metal fluxes relative to the aluminium fluxes are plotted in Figs. 14 and 15. The increasing trend since 1350 A D has now largely disappeared, which confirms the major contribution of Asian dusts. Here the increasing trends after 1950 are clear. Let's look at lead in Figure 16. In order to see if this is due to pollution,

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Fig. 14. Fluxes of total metal/A1 ratios in different years.

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Fig. 15. Fluxes of acid-leached metal/A1 ratios in different years.

we measured the acid-leached lead in a shorter core collected near the longer core. Both cores show higher fluxes since 1950. Further, the Pb-206/Pb-207 ratios are also plotted. The ratios decrease after 1950 AD. This reinforces our conclusion of an anthropogenic origin of lead near surface (Wann et al., 1995).

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Fig. 16. Vertical distributions of acid-leached lead and Pb-206/Pb-207 ratios.

With the exception of winter time, the hypolimnion of Great Ghost Lake is anoxic with active redox cycles of iron and manganese. Upward migration and diffusion of reduced species could also alter their concentrations, but the effects are probably smaller than variations due to the differences in the source material. Keep in mind that the onset of the anthropogenic lead increase in Great Ghost Lake occurred about 1950. Other studies show similar results. The worldwide lead emissions from automobiles since 1940 have probably played an important role in these sharp increases. Some recent records, however, indicate that the lead concentration has started to decrease in recent years due to the phasing out of leaded gasoline. The record in Great Ghost Lake does not show a decrease in lead concentration near the top yet. This could be because leaded gasoline in still widely used in Taiwan. Since Great Ghost Lake is located in a remote subalpine region, the anthropogenic metals there must have come from atmospheric fallouts. We've already seen the high solubility of trace metals in atmospheric particles. It follows that most of the anthropogenic lead in the sediment of Great Ghost Lake was dissolved from aeolian particles and subsequently sequestered by iron oxides. To conclude, the aeolian Asian dust particles seem to have played a significant role in the flux of metals in Great Ghost Lake. The fluxes have generally been higher during dry periods, especially since 1350 AD. On the other hand, local pollution from certain trace metals seem to have gained importance since 1950 AD (Wann and Chen, 1996). CAUSES OF CATASTROPHIC POPULATION DECREASES IN CHINA IN THE PAST According to Thomas Robert Malthus the human population increases exponentially with time. By and large, the population in China over the last 2,500

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years has followed the Malthusianism theory. It is, however, important to point out that the exponential curve has been punctuated quasiperiodically. There are some obvious explanations for several population declines, and these are mostly related to wars, rebellions, plagues and crop failures. Wars or rebellions had different causes, some started because of aggression or suppression, some due to infighting and some may have even been brought on by climatic changes. For instance, bad harvests and famines due to a cluster of cold years in northern Europe forced a mass migration of people southward. In March BC 58, 263,000 of the Helvetii, and some 100,000 of the Boii and other Celts crossed the Jura Mountains to settle in France. This movement resulted in direct conflict with the Roman Empire as Julius Caesar refused to let these people enter the Roman province. Only 130,000 of the invaders survived (Hsfi, 1997). A similar story occurred in China as well when hungry people had nothing to lose and were willing to risk their lives in order to fill their empty stomachs. Take the northern nomads as an example. They started to invade China when the cluster of cold years came about the time of the birth of Christ. Widespread drought resulted in widespread famines, and that fuelled rebellions. The Yellow Turban Rebels, who rose in 184 AD, were in fact accused of killing 800,000 people. In the total chaos, the mortality rate must have been even higher for the old, the young and the weak, thus further increasing the death toll. Also farms had been left untilled, waterworks unattended and forests cut or burnt. This would have directly or indirectly led to ever poorer harvests the following years, and even more deaths (Yoshino, 1996; Hsfi, 1997). Continued droughts must have been worse than wars. When Wang Mong toppled the Western Han dynasty in 9 AD, the population was close to 60 million. Continued droughts led people to cannibalism, and the population was quickly reduced to 15 million within about 25 years. The number gradually returned to 60 million 130 years later. However, it didn't stop there. The later years of the Eastern Han, Three Kingdoms and Western Jin dynasties still experienced one drought after another. In 309 AD, even the Yellow River and the Yangtze River desiccated. Such events corresponded to a population decrease from 60 million in 188 AD to 14 million in 221 AD, a reduction of 77% in a mere 33 years. The population would not climb back to 25 million until 348 AD. In Fig. 17 is plotted the history of the Chinese population on the right with the periods of population decrease indicated (Chao and Xie, 1988; Yoshino, 1996). On the left are the RBI signals from Great Ghost Lake. Recall that these white layers with higher RBI values correspond to drier periods. I speculate that severe droughts in China killed people off. In the meantime, enhanced dust production left its mark in the sediments of Great Ghost Lake. I should point out that even without droughts, wars and large-scale civil unrests could have also induced higher dust production because farms would have been more exposed and forest coverage reduced during war, leading to more wind-blown particles. If the above is proven true, it would indicate that climatic variations play a pivotal role not only in the civilization of mankind, but indeed, may also be a matter of life and death. In this highly advanced world with gimmicks invented every day, young people tend to think that mankind is able to control natural hazards, and even

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Fig. 17. RBI in the sediments of Great Ghost Lake and the population in China. Numbers 1 through 14 indicate periods when the population showed a sharp decrease. dominate nature. Such notions are dangerous, and I do hope my research will provide a lesson that people must respect nature for this is a spirit frequently forgotten by our young generation.

ACKNOWLEDGMENTS I must acknowledge the funding agencies in Taiwan that have supported my research. These are the National Science Council, the Environmental Protection Administration and the Kenting National Park. C o m m e n t s by academician K. Hsti were most useful. Dr. J.Y. Lou, Ms. B.J W a n g and Ms. H.I. H u a n g provided assistance.

REFERENCES Chao, W.L. and Xie, H.Q., 1988. Population History of China, Peoples Press, Beijing, 645pp (in Chinese). Chen, C.T.A. and Wang, B.J., 1990. Winter overturning of the anoxic Great Ghost Lake, Proceedings, National Science Council, 14:11-15. Chen, C.T.A., Lou, J.Y. and Wann, J.K., 1993. Preliminary paleoclimatological records from high mountain lakes in Taiwan, Terrestrial, Atmospheric and Oceanic Sciences, 4: 321-329.

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Chen, C.T.A. and Wang, B.J., 1997. Acidification of lakes and reservoirs in Taiwan, Geochemical Journal, 31: 345-355. Chen, C.T.A., Lou, J.Y., Wann, J.K. and Lan, H.C., 2001. Eolian dust flux recorded in the sediments of Great Ghost Lake, Taiwan, China. Quaternary Sciences, 21:18-28 (in Chinese, English abstract). Chu, K.C., 1973. A preliminary study on the climatic fluctuations during the last 5,000 years in China, Scientia Sinica, 16(2): 226-256. Hsfi, K.J., 1997. Sun, climate, hunger, and mass migrations. Science in China, 41: 449-472. Huang, B.T. and Chen, C.T.A., 1994. Stoichiometry of particulate organic carbon, hydrogen, oxygen, nitrogen and sulfur in lakes and reservoirs in Taiwan, Oceanologia et Limnologia Sinica, 25(1): 29-37 (in Chinese with English abstract). Lamb, H.H., 1977. Climate, present, past and future, Methuen, London, 835pp. Lou, J.Y., Chen, C.T.A. and Wann, J.K., 1997. Paleoclimatological records of Great Ghost Lake in Taiwan, Science in China, 40: 284-292. Lou, J.Y. and Chen, C.T.A., 1997. Paleoclimatological and paleoenvironmental records since 4000 BP in the sediments of alpine lakes in Taiwan, Science in China, 40(4): 424-431. Meyers, P.A., Takemura, K. and Horie, S., 1993. Reinterpretation of Late Quaternary sediment chronology of Lake Biwa, Japan, from correlation with marine glacial-interglacial cycles, Quaternary Research, 39:154-162. Nakai, N., 1986. Paleoenvironmental features of Lake Biwa deduced from carbon isotope compositions and organic C/N ratios of the upper 800-m sample of 1400-m cored column, Proceedings of the Japanese Academy 62 B: 279-282. Shi, S.H., 1993. The periodic analyses of climatic change during historical times in central north China, Mem. of Nanjing Inst. of Geog. and Limnol., Acad. Sinica, 10:81-88 (in Chinese with English abstract). Wann, J.K., Chen, C.T.A., Lin, Z.M. and Jiang, S.J., 1995. Sources and sinks of lead and other trace metals enriched in the surface sediments of remote subalpine lakes in Taiwan, Terrestrial, Atmospheric and Oceanic Sciences, 6: 379-392. Wann, J.K. and Chen, C.T.A., 1996. Distribution of trace metals in the surface sediments of two pristine subalpine lakes in Taiwan, Chemistry and Ecology, 12: 67-89. Wann, J.K., Chen, C.T.A. and Wang, B.J., 1997. A seasonally anoxic mountain lake with an active Fe cycle in tropical Taiwan, Aquatic Geochemistry, 3: 21-42. Wu, Y.L., 1996. Investigation of total acid deposition and analysis of various mediums, Environment Protection Administration Report, Taipei, pp. 52-55. Yoshino, M., 1996. Population variations in China and environmental changes. Proceedings of Aichi University: History, Asian History and Geography. 5:1-26 (in Japanese).

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Part V

A P P L I E D AND BASIC SCIENCES

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Paradoxes in Geology Ueli Briegel and Wenjiao Xiao, eds. 9 2001 Elsevier Science B.V. All rights reserved. Chapter 20

AT A T U R N I N G POINT IN P E T R O L E U M GEOLOGY*

Stefan M. Luthi Delft University of Technology, Delft, The Netherlands

ABSTRACT In this chapter, I discuss recent technological and scientific developments in petroleum geology, how they affect the future of the profession, and how Ken Hsfi has been and continues to be - - involved in this evolution.

IS G E O L O G Y D R I V E N BY T E C H N O L O G Y ? Have all great insights in geology been triggered by breakthroughs in technology and physical sciences? The question may be provocative, but perhaps not entirely unjustified. For years, the squabble regarding the earth's age that opposed Lord Kelvin to some of his contemporary geologists was at a dead end. Only the discovery of radioactivity and its causes put an end to the impasse. As it turned out, the geologists were "right", and Kelvin was "wrong", but the insights came from the physicists and chemists, not the geologists: the earth is a slow nuclear reactor that generates heat through fission of unstable isotopes. Lord Kelvin based his estimates of the earth's age on the assumption that our planet steadily cooled down from an initially hot state to its present temperature, and from the internal heat flow he back-calculated its age. He did not account for the additional heat source provided by nuclear reactions and, therefore, ended up short with his estimate. Today, geologists commonly use isotope geochemistry for determining the age of rock samples. In a similar way, many other important forward strides in geology have been made possible, or at the least been greatly aided, by developments in other sciences and in technology. Plate tectonics got their major boost once the magnetic stripes on the ocean floor were mapped out with a magnetometer. Paleoclimatology is unthinkable without mass spectrometers. Sequence stratigraphy could not have been developed without high-resolution seismic and sophisticated processing algorithms.

* Talk given at the Symposiumon Paradoxes in Modern Geology, in honor of Prof. Kenneth Hsti's 70th birthday. Chinese Academy of Sciences, Beijing, China.

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A D V A N C E S IN P E T R O L E U M G E O L O G Y Perhaps no field, however, has benefited as much from technology as has petroleum geology. The methods employed by petroleum geologists some fifty years ago bear no resemblance with what is used nowadays. The modern petroleum geologist works with high-resolution seismic lines, often in three-dimensions, which allow him to make a good initial assessment of the hydrocarbon potential in a basin. He is able to have an exploration well drilled in up to 3000 m water depth into a desired target located well over 10,000 m below the rig floor. He can direct the well into a geological target using the noise of the drill bit as a seismic source to explore the target ahead of the bit. He can lower a sonde into an exploration well that images the layering on the borehole wall with a resolution of one cm. Or he can deploy another sonde capable of pumping a fluid sample out of a geological layer of interest with a depth accuracy of better than 50 cm. However, of all the tools available to the petroleum geologists, the logging-whiledrilling (LWD) tools are the most powerful ones. They allow him to make measurements of the layer penetrated by the drill bit during the drilling process and to have them sent up the mud column via mud pulses in real time. If the drill bit hits a hydrocarbon-bearing zone, the resistivity measurement will indicate this immediately through an increase in its reading. The geologist can thus make immediate decisions: set casing, start coring, or retrieve a sample of the potential reservoir rock. L W D measurements are particularly valuable in deviated drilling and combined with a geological model of the target: the actual measurement is continuously compared with the expected response, and any difference between the two is either attributed to a change in rock composition, or to a different geometry between reality and the model. In either case, the geologist can adjust the direction of the drill bit towards another layer. The art of directional drilling is, in fact, so advanced that today a layer three meters thick can be followed for several kilometers without the drill bit ever leaving the layer, even if the layer dip changes. The longest horizontal well drilled to date is over ten kilometers long; it corresponds to a rig located on the university campus in Delft drilling into a l a y e r - several kilometers d e e p - under the center of Rotterdam. In the very near future, another major development will be available to the geologist: a real-time borehole imaging tool will allow him to monitor the layering traversed by the drill bit from the comfort of his control room on the rig. He can thus adjust the direction of drilling without having to use a geometric model. The analogy with doctors making endoscopic examinations using videocameras on fiberoptic cables is perhaps not very appealing, but quite accurate. Such technology is particularly useful in development drilling where producing wells have to be accurately placed and completed in order to efficiently drain the reservoir. In the field of reservoir development and management, permanent monitoring is about to revolutionize the conventional procedures. It consists of periodic surface or downhole measurements that capture the progress of reservoir depletion. Repeated seismic surveys, so-called 4-D seismic monitoring, have been successfully used for this, particularly when gas or steam is involved because of their good seismic contrasts. Yet newer and better methods of monitoring are being developed. They

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consist of deploying permanent sensors in the boreholes whose responses are recorded at a surface facility. Reservoir pressure, temperature, and resistivity will be measured at various depths in each well. The geologist or engineer will be able to call up the data each morning upon reaching his office, just like he checks how the stock market is doing. Data display versus time, or geographic location, allows him to identify problems as soon as they arise. The impact of technology on petroleum geology and engineering can best be demonstrated by two figures: 9 Exploration success rates have steadily risen throughout the last decades. Thirty years ago, one well out of ten struck oil or gas. Today, two to three out of ten are successful. 9 Recovery rates have increased from an average of 25% in the 1960s to about 40% today. The underlying reason for this vastly increased efficiency is, however, not technology, but knowledge. Technology merely provided the tools to undertake previously impossible operations, which in turn enhanced our knowledge. This was supplemented by new research in the generously funded research and development centers of oil companies and national energy agencies. It all resulted in a much better understanding of the geology of hydrocarbon-bearing formations, of fluid flow in porous media, of evaluating the deposits for their recoverable reserves, and of identifying by-passed zones in fields that had been on stream for a long time. Thus, technology and science went hand in hand, taking turns in leading each other, both advancing at a rapid pace. K E N HSU, P E T R O L E U M G E O L O G I S T Ken Hsfi started his professional life in the oil industry, quite some time before the new technology mentioned above was available. He joined the Shell Development Company in the 1950s, first working in research, then in operations. During a field trip to Galveston Island in the 1980s, I met his former mentor, Rufus LeBlanc. When I introduced myself as a former student of Ken, he immediately answered in his exuberant Cajun style, "Oh God, good old Ken, he worked so hard. You know, all this sand here, from Houston to New Orleans, I don't think he left a single grain unturned in his studies." LeBlanc was referring to the painstaking work Ken did for Shell by analyzing the textural and mineralogical changes of the sands along the Gulf Coast. His objective was to investigate what controls the maturity of sediments, i.e. the relative amount of quartz over other minerals, but also the degree of rounding of the sand grains. He found that the observed changes in maturity along the beach sands Gulf Coast could not be attributed to abrasion, or the mechanical instability of feldspar to the action of waves and currents (Hsti, 1960). Rather, he noted that the coarse fraction had the same maturity as the Mississippi sediments, and only the finer grains originating from the local streams were richer in quartz. These local rivers come from regions where chemical weathering is very strong. Ken therefore attributed the changes in maturity to the chemical weathering and the climate in general of the source terrane rather than the length of sediment transport.

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The paradigm stands and it is generally accepted now that sediment maturity reflects the climate. When Ken moved to operations, he was assigned to work on the sedimentology of the Ventura field in California. He started off with what he knew best by then: grain size analysis and maturity studies. He then continued with well correlation, compaction studies, structural interpretations and finally synthesized the results in what was to become a landmark paper in the field (Hsfi, 1977). The reservoir sands, he convincingly demonstrated, were deposited as elongated bodies by laterally restricted currents in the axial part of the Ventura basin. Doing his part to improve exploration success, he challenged conventional views which were based on the passive-margin type model and unrestricted flow of the turbidity currents by postulating that the exploration strategy in tectonically active areas should be changed: " . . . the search for sand reservoirs in tectonically active regions might be directed toward the central parts of the ancient depositional basins.., parallelism of structural axis and trend of sand bodies in folded deep sea basins suggests an influence on structural trends.., anticlinal traps might be expected to coincide with maximum sand development, and stratigraphic traps might be expected on the flanks of such folds." (Hsfi, 1977) Exploration has since moved to more distant, deeper and more complicated targets. Today's frontier areas lie in "ultradeep" waters on both sides of the Atlantic Ocean, where turbidite deposits in water depths over 2500 m hold great promise, but the drilling and particularly the completion technology still have to be developed for them to become commercially viable. Other frontier areas include very deep Paleozoic carbonates on the Asian continent, intracratonic fluvial systems in Africa, fractured basement blocks in Southeast Asia and overthrusted rocks in the foothills of the Andes. "In the 1970s and 1980s... innovative geophysical methods were indispensable for exploration .... We have found more than we need for the next half century. What we need now are better methods to exploit, i.e. to produce the natural resources which we already have found." (Hsfi, 1997) Petroleum geologists have explored better than expected and almost ran themselves out of business. But the task is not done. World consumption continues to increase, and many giant fields are aging. The biggest frontier now might well be the development of known reserves, which are neither recoverable with today's technology nor economically viable with the present price range of crude oil. These are not only the gigantic tar sand deposits, the oil shales and the gas hydrates, all of which still await technological breakthroughs. They are, above all, those reserves left behind in "depleted" fields. In fact, for the total amount of oil produced so far, which is estimated to be somewhere around 800 billion barrels (8-1011 barrels), an amount twice as high has been left behind. To put this into perspective: a new field discovered in the deep waters of the Atlantic typically holds a few (1-5) billion barrels of recoverable oil, which is able to cover a few (,-~2-8) weeks of the world's demand for oil. If only 10% of the oil left behind in "depleted" fields is recovered, the world's oil consumption can be satisfied for an additional six years. This may not

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seem much, but it is considerably more than what a single discovery of a giant field can contribute. I have faced four areas of great interest to the industries: 1. Exploitation of metal from brines in seawater 2. In-situ exploitation of metals from subterranean formations 3. Improved recovery methods of hydrocarbons from depleted oil fields, coal, lignite, oil shale and tar sands 4. Direct recovery of in-situ burning of thermal energy from subterranean formations (Hsfi, 1997)

ON, ON* Since his retirement from the Swiss Federal Institute of Technology (ETH) in Zfirich, Ken Hsfi has spent much time thinking about the problem of enhanced recovery of oil from aging fields. He has developed a concept termed "in-situ reactor" for some mining applications points 1 and 2 in the above list and secured a US patent for it (Hsfi et al., 1997). Subsequently, he adapted this idea to the recovery of hydrocarbons in a concept he terms "hydrologic cells". Its purpose is to increase the recovery of hydrocarbons dramatically through an effective sweep of a block within the reservoir. This is achieved with massive hydraulic fracturing in one or more injection wells, and similarly massive fracturing in one or more producing wells. The volume contained between these hydraulic fractures will be swept evenly and efficiently by a steadily advancing injection front. Ken works currently on demonstrating the feasibility of this concept in a field experiment on a large scale (Fig. 1). M. King Hubbert was a physics student.., yet he made his greatest contributions in... hydrogeology with his theory of groundwater motion, in petroleum geology with his research on hydraulic fracturing, and in tectonics with his analysis of the pore pressure in overthrust mechanics. (Hsfi, 1989) Ken does not point out that in the oil industry and in the energy sector in g e n e r a l - Hubbert is known above all for the "Hubbert Peak". After analyzing data from the United States, Hubbert (1956) concluded that US oil production will peak in the early 1970s, and world production in 1995, with productions following roughly a Gaussian curve. Hubbert's prediction regarding the US turned out to be correct. The overwhelming importance and unpredictability of oil production from the Middle East, with huge swings in its output in the 1970s and 1980, make it difficult to verify his prediction on world production. More recently, however, Campbell and Laherr6re (1998) fitted multiple Hubbert curves to production data

*A call used by runners of the Hash House Harrier clubs ("drinkers with a running problem") to signal to their fellow runners that they have found the right trail.

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Fig. 1. Ken Hsfi's vision for enhanced oil recovery includes massive fracturing. from all major oil provinces and concluded that by the year 2010 the end of "cheap oil" will have arrived. By this they imply that conventional oil produced primary and secondary recovery methods at prices below about 10 US dollars will become very scarce by that time. Therefore, they conclude, increasingly sophisticated and capital-intensive enhanced oil recovery methods will have to be employed to meet world demand, making the commodity more expensive. Based on the Hubbert peak model, Campbell and Laherr6re (1998) predict that by the year 2075 total conventional oil production will have fallen below one billion barrels per year, down from a current rate of about 25 billion barrels. If, however, non-conventional production methods are taken into account, total production at that time will still be around five billion barrels per year, falling under one billion only by about the year 2100. Thus non-conventional production can add another 25 years one human generation to the life span of oil. Ken Hsfi's ideas are very much along this line of thinking. His creativity combines innovative geology and new technology to propose new methods for producing nonconventional fossil fuels. He is determined to go ahead with large-scale field experiments in order to test his ideas, just like Lord Kelvin, M. King Hubbert, and many other great minds have spent considerable time putting their creativity to practical use. If the geological departments of our universities teach their students to apply theoretical geology to geological engineering, one might effect a revolution, and.., a renaissance of geological education. A consequence of ignoring the importance of geological engineering is that the best minds are preempted in the irrelevance of ivory-tower activities, while the society cries in vain for help to solve urgent problems. (Hsfi, 1997)

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ACKNOWLEDGEMENTS I wish to t h a n k several friends f r o m large oil c o m p a n i e s for p r o v i d i n g e s t i m a t e s o n e x p l o r a t i o n success a n d r e c o v e r y rates. S c h l u m b e r g e r s u p p o r t e d m e d u r i n g m y c h a n g e f r o m i n d u s t r y to a c a d e m i a . K e n Hsfi, as always, was a s t i m u l a t i n g s o u r c e o f ideas t h a t h e l p e d c o n c e i v e this p a p e r , w h e t h e r he i n t e n d e d it o r not. A n d finally, m y son Louis, art s t u d e n t at the R i e t v e l d a c a d e m y in A m s t e r d a m , g r a t e f u l l y c o n t r i b u t e d Fig. 1 b u t declines all responsibility. REFERENCES Campbell, C.J. and Laherr6re, J.H., 1998. The end of cheap oil. Scientific American, March 1998, pp. 78-83. Hsfi, K.J., 1960. Texture and mineralogy of the recent Gulf Coast. Jour. Sed. Petrology, 30: 380-403. Hsfi, K.J., 1977. Studies of the Ventura field, California. In: Facies geometry and genesis of Pliocene turbidites. Bull. American Assoc. Petrol. Geol., 61(2): 137-168. Hsfi, K.J., 1989. Physical Principles of Sedimentology. Springer Verlag. 233p. Hsfi, K.J., 1997. A reform in earth science education after a revolution in earth science. Episodes, 20(3): 151-157. Hsfi, K.J., Hsu, P. and Dickson, F.W., 1997. In-situ reactor for recovery of metals and salts. United States Patent 5,645,322, granted July 8, 1997. Hubbert, M.K., 1956. Nuclear energy and the fossil fuels. American Petroleum Institute, Drilling and Production Practices, pp. 7-25.

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Paradoxes in Geology Ueli Briegel and Wenjiao Xiao, eds. 9 2001 Elsevier Science B.V. All rights reserved.

Chap ter 21

EXPLOITATION OF LITHIUM IN BRINES BY HSU'S M E T H O D

Chen Yancheng ~, and Kenneth J. Hsfi 2 1Geological Institute for Chemical Minerals Product, Ministry of Chemical Industry, P.R. China; 2Tarim Associates AG, Frohburgstrasse 96, 8006 Zfirich, Switzerland

ABSTRACT Lithium compounds have many applications in industry, but the use is limited by the rarity of lithium in the earth's crust. Lithium is abundant as a trace element in seawater and in brines of salt lakes, but the exploitation is prevented by the high cost of extraction. Numerous international institutions have tried in vain to find a method in exploiting lithium from sea water or from very concentrated brines. Traditional methods of salt exploitation by fractional crystallization are not applicable in exploiting lithium in Dead Sea type of brines. Experimental methods have been tried since 1970 in China. Those are (1) precipitation of lithium as a carbonate, (2) extraction by ion-exchange resin, and (3) extraction by other organic compounds. Small amounts of lithium have been obtained by such methods, but the costs are so high that the lithium so obtained cannot be marketed for profits. Hsfi recognized that the problem of exploiting lithium from brines lay in the difficulty of separating a magnesium precipitate from the brine progressively enriched in lithium. The traditional "batch" methods do not remove the precipitate in a pond until the crystallization of that salt is completed. After that, the residual brine can be drained from one pond and transferred to another where the precipitation of another salt starts. Hsfi invented an "assembly line" method. The residual brine of seawater or salt lakes after potassium exploitation is commonly supersaturated with magnesium chloride. Such a brine can be induced to flow boustrophendonically in an evaporating pond, alias Helminthoid evaporator, which consists of a series of parallel troughs. Bischofite is precipitated while the brine is concentrated by solar evaporation. The precipitate is removed mechanically by vehicles driven to the partitions separating the boustrophenonically arranged troughs. Brines containing about 100 ppm LiC1 can be evaporated to produce an enriched concentrate containing 1-2% LiC1. The concentrate can then be processed in a factory to produce LiC1 or LizCO3. We have carried out experiments in the laboratory and in the field to verify the applicability of Hsfi's method, and found that the lithium compounds thus produced can be marketed for profit.

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INTRODUCTION Lithium is a silvery white light metal with a specific gravity of 0.534 and its melting point is 179~ It is an important resource metal to propel the world forward. Lithium batteries, for example, have many applications. Their light weight gives them a preference over lead batteries: lithium batteries are thus used in electric automobiles, torpedoes, or other battery-powered gadgets or machines. An alloy of Mg, A1 and Li has high strength and light weight. The alloy is used by automakers to make combustion-engine vehicles. The light lithium alloy is also a useful reducing agent to process rare-metal compounds in the metallurgical industry. A high energy fuel with the high temperature and fast-combustion character is made of lithium compounds. The fuel is used in rockets and manned spaced vehicles. Lithium is also used as a coolant and a conduction medium of heat in a nuclear reactor, or as a solvent for uranium and thorium in a liquid fuel reactor. Finally, lithium is an additive in making glass, ceramics, cement, fireworks, explosives, etc. LiBr can be used as refrigerant to replace freon in refrigerators. Lithium can be refined from lithium minerals, but the world's production comes largely from the extraction from brines in salt lakes of U.S.A. and Chile. Because of the high production cost, lithium is at present not widely used. CHINESE EFFORTS TO EXTRACT LITHIUM FROM BRINES Lithium bearing minerals are rare and lithium is extracted from pegmatite minerals in Xinjian. Lithium is present as dissolved ions in the brines of salt lakes of China. One rich occurrence is the Qarhan Lake of Qinghai. The lake brine is derived from evaporation of groundwater during transport from its source in the mountains into the hydrologically enclosed Qaidam Basin. The less soluble major ions Ca 2+, HCO3_ and SO 2- have been precipitated as cement in sediments, and the residual brine consists mainly of potassium and magnesium chloride. Lithium is present as a trace element in a concentration of between 50 and 100 ppm. The Qinghai Potash Works Ltd. has built evaporating ponds for potash exploitation. After the least soluble salt sodium chloride is precipitated in the first pond, the residue enriched in potassium and magnesium is transferred to a harvest pond. The salt precipitated from that supersaturated solution is polyhalite KC1.MgC1.6H20. The precipitate is then processed in a factory to produce KC1 fertilizer. The last residual solution, called laolu, is a supersaturated solution of magnesium chloride. This laolu has a LiC1 concentration of 100-300 ppm. Continued evaporation causes the precipitation of bischofite MgC1.6H20. The residual brine is still saturated with respect to magnesium chloride, but it is more enriched in LiC1. After much of the bischofite is precipitated, the last laolu may contain several percent of LiC1. However, the volume of the residual brine would be far too small to be separated from the bischofite precipitate. Eventually the laolu is evaporated completely, and LiC1, together with other salts of trace elements, is crystallized as impurities in the bischofite, which is dumped as waste in the desert.

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Lithium in Qarhan brines, laolu and in waste dumps has a very large volume, and they represent the most important potential resources. Different methods have been tried out during the last four decades to extract such lithium. Of these, the three most successful are described in the following sections. Carbonate precipitation method Dissolved magnesium and lithium in brines can be precipitated as carbonates. A NazCO3 reagent injected into a lithium-bearing magnesium-chloride solution causes the following chemical reactions: Na2CO3 + 2Mg 2+ + 6H20 = MgC03 96H20 (bischofite) + 2Na + Na2CO3 + 2Li + = LizCO3 + 2Na + Lithium carbonate can then be separated from bischofite in the factory. Lithium is produced by the carbonate-precipitation method from the brines of the Mohave Desert (California, USA), in Chile and in Sichuan, China. The method is, however, not suitable for extracting lithium from Qarhan, or from other brines saturated with magnesium chloride, such as that of the Dead Sea, Israel. The difficulty lies in the fact that the mixed carbonate precipitates induced by sodium carbonate have a very large Mg/Li ratio. Much of the reagent is thus wasted to precipitate bischofite which cannot be economically marketed. For example, some 1000 tons of Na2CO3, costing some 200,000 US$ have to be used as agent to produce 1 ton of Li2CO3 worth about 4000 US$. The process is thus obviously not economical. Ion-exchange resin method The method is to place artifical resin in a brine to extract dissolved Li + by absorption. The research team of the Qinghai Potash Works made use of a resin from the Fifth Institute of the Ministry of the Second Machinery Industry of China. Only small amounts of Li can be extracted from the waste brine. The resin is, however, very expensive, and it can only be used for two or three times before its adsorbing energy is lost. The method has proved to be uneconomical. Organic extraction method A research team of the Qinghai Institute of Salt Lakes has developed an organicextraction reagent. They tried since the middle 1970s to use the reagent to extract lithium from the brine of a salt lake in Qinghai. Again small amounts of Li can be extracted, but the production cost is very high. Furthermore, the environmental contamination by the organic solvent is such a serious problem that the test had to be suspended. The use of organic solvents to extract lithium is not an option today. HSI~I'S M E T H O D OF L I T H I U M E X T R A C T I O N F R O M BRINES The economy dictates that the carbonate precipitation method is applicable if the lithium in a residual solution is enriched to about 1 or 2%, when sodium carbonate

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Y. Chen and K. J. Hsii

can be injected into this Li-concentrate to precipitate an enriched lithium mixture. Lithium in the laolu of Qinghai can reach such a concentration after a 100-fold reduction of volume by evaporation of water and similarly in the crystalline water in the bischofite. Hsfi proposed to the Geological Institute for Chemical Minerals Product, Ministry of Chemical Industry, P.R.C. to carry out laboratory and field experiments to verify the practicability and profitability of his invented process. The results of the experiments are given in the following sections. The laboratory experiment, January-June, 1996

Chen experimented in 1995 with various carbonates. He found that sodium carbonate is the only chemically reactive reagent to precipitate magnesium and lithium chlorides from the laolu. The carbonate precipitation method is commercially applicable only if the Mg/Li ratio in the brine is reduced to 20/1 or less. Chen noted, however, that the volume of the residual laolu became far too small to be separated from the bischofite precipitate long before the lithium chloride concentration reached 1%. Hsfi joined Chen in April, 1996. He found a simple solution: instead of the "batch process" to remove the bischopfite from an evaporating vessel after the lithium in the laolu is concentrated, an "assembly-line process" could be substituted to remove the solid precipitate while lithium enrichment is taking place. Hsfi designed an experiment, carried out from April 14 to 22. The starting material was a laolu from Qarhan. The brine was initially placed in the first series in 16 evaporting dishes. After half of the brine of each dish was evaporated, the residual brine was separated from the bishofite precipitate and placed in a second series of 8 evaporating dishes. Again after half of the brine of each dish was evaporated, the residual brine was separated from the bischofite precipitate and placed in a third series of 4 evaporating dishes. Again after half of the brine of each dish was evaporated, the residual brine was separated from the bishofite precipitate and placed in the last evaporating dish.The last residual laolu is named Lithium Brine 1. We repeated the experiment again during the last week of April, and the last residual laolu is named Lithium Brine II. The two series of experiments verified the following: 1) An enriched lithium concentrate of 3.2 or 4.5% could be produced by successive fractional crystallization when the bischofite precipitate is steadily removed from the evaporating dishes. 2) The Li loss as impurity in bischofite is small; it is only about 1-2% during the last stage of bischofite precipitation. Field test." July, 1995-November, 1996

For a field test to enrich lithium in laolu, a Helminthoid-type evaporation-pool was constructed according to the patented design of Hsfi (2001). The evaporation energy is solar.The facility occupies an area of 325 m 2, and the evaporation pool consists of the ten parallel evaporation troughs. Each trough is 24.5 m long, 1 m wide

Exploitation of lithium in brines by Hsii's method

425

Table 1 Chemical analyses of Brines during the first and second series of laboratory tests Item Spec. gravity M g 2+ ( g / L ) M g C I 2 (g/L)

Lake brine

Laolu

1.2350 57.89

1.3520 124.42

1.3795 113.64

1.3569 111.90

487.39

438.35

403.35

227.03

Li + ( g / L )

0.01245

0.11808

LiC1 (g/L)

0.0752

0.7212

Lithium Brine I

Lithium Brine II

8.15

5.21

45.18

31.82

K + (g/L)

14.91

0.2931

1.26

0.44

K C I (g/L)

28.43

0.5589

2.4025

0.839

Na +

29.05

1.22

1.81

1.22

NaC1

73.88

3.10

4.60

3.10

Li impurity in bishofite

-

-

0.30%

0.16%

LiC1 i m p u r i t y

-

-

1.84%

0.98%

K impurity

-

-

0.073%

0.08 %

KC1 i m p u r i t y

-

-

0.14%

0.15%

Na impurity

-

-

0.15%

0.19%

NaC1 impurity M g 2 + / L i + (brine)

4649.8

1053.69

0.38% 12.9

0.48% 21.81

MgC12/LiC1 (brine)

3019.0

675.80

9.7

0 0

6 0.32

-

0.03

0. 1

13.99

B20 3 Cs + ( m g / L )

0 0

Br (g/L)

-

I (g/L)

-

0.0009

0.0054

0.197

Srl + ( g / L )

-

0. 00094

0. 0021

-

Total (g/L)

-

331.12

486.86

-

and 0.5 m deep. The troughs are separated by partitions which are open at alternate ends. The brine pumped into the head of the first trough flows down the length of the trough to the end, where the partition is breached so that the brine can now flow into the next parallel trough. The brine then flows down to the end of that trough, where the partition is breached so that the brine can now flow into the third parallel trough. With a configuration like the Helminthoid type of worm-trails, the brine entering the pool at the beginning of the first trough could flow boustrophendonically through the next troughs and the lithium concentrate can be harvested at the end of the 10th trough. The brine is steadily evaporated by exposure to dry air and sun, and the water loss causes the precipitation of bischofite. The solid precipitate is steadily removed so that the Mg/Li ratio in the brine is diminishing. When the brine at the end of the 10th trough has been enriched to have a LiC1 concentration of 1% or more, it could be harvested for processing in a factory. The experiment was carried out from July, 1995 to November, 1996 on the side of the Qarhan Lake in Qinghai. The Helminthoid-pool site was constructed 200 m north of the waste-brine pumping station of the First Ore Dressing Plant of the Qinghai Potash Work. The construction began on July 28, 1995. Troughs were excavated and were paved with a magnesium cement and the floor and walls of each trough were lined with a black PVC plastic foil to prevent brine leakage.

Y. Chen and K. J. Hsfi

426

Table 2 Chemical analyses of Brines during first and second series of laboratory tests Item Specific gravity Li + (g/L) LiC1 ( % ) Mg 2+ (g/L) MgC12 (%) B (g/L) Br (mg/L) Sr (mg/L) I (mg/L) Rb (mg/L) Cs (rag/L) K (g/L) KC1 Na (g/L) NaC1

Lake brine

Waste brine

Lithium brine

1.2350 0.01245 0.00616 57.89 18.36 0 30 _ 0.9 _ 0 14.91 2.30 29.05 5.98

1.3379 0.08036 0.036 126 36.9 0.162 27.1 65.5 0.461 < 0.0512 < 0.0512 0.594 0.085 1.49 0.283

1.3700 4.026 1.79 96.2 27.5 1.08 122.7 2056 2.60 0.102 < 0.051 0.758 0.106 1.50 0.278

On May 28, 1996 a laolu from the Waste-Brine Pool of the Orient Refining Potash Plant of the Lianyungang Chemical Industrial Designing Institute was pumped into the Helminthoid evaporator. The pool had a capacity of 122.5 m 3. After the pool was filled up, an additional volume of 55 m 3 was pumped into the pool to replace the material loss during the course of the experiment. The solar evaporation test was completed on October 20, 1996, after 169 days and nights of evaporation. The average daily material-loss was 22 mm/day. It should be recalled that the water lost by evaporation led to supersaturation of the brine and the precipitation of bischofite with crystalline water. The precipitate was removed manually. The total material loss from the pool was thus about twice the average of the 10 mm/day water-evaporation. The specific gravity and composition of the waste brine are shown in Table 2. The results show that the LiC1 content of the waste brine was enriched 64 times from 0.036 to 1.79%, to a concentration sufficiently enriched for profitable chemical processing in factories. The last enrichment was, however, achieved only through an additional process of accelerated evaporation. The most concentrated brines from the last trough of the Helminthoid pool were pumped into a smaller pool of 5 m 2 in area and 0.1 m in depth. In such a shallower device, the efficiency of water loss was greater. ECONOMICS OF HSU'S M E T H O D OF L I T H I U M E X T R A C T I O N We have used as starting material an unusually, enriched laolu containing 360 ppm LiC1. The average laolu may have a concentration of about 120-150 ppm. The volume of waste brine available from the Potash Work is about 10,000,000 cubic meters, containing more than 1500 tons LiC1. Assuming a 50% efficiency, the recoverable LiC1 of 750 tons is worth 3 million US$.

Exploitation of lithium in brines by Hsfi's method

427

To concentrate l0 million cubic meters of waste brine of 150 ppm LiC1 to a concentration of 1.5%, the material loss should be 99%. At an experimental rate of about 22 mm per day or about 8m per year, this material loss could be achieved by evaporating laolu in helminthoid evaporators spread over an area of evaporation of 1,250,000 m 2. Taking into consideration the partitions, etc.,the facilities can be constructed over an area of about two square kilometers. We have been able to estimate the labor and material cost on the basis of constructing the experimental pool. We believe that the projected facility to process 10 million cubic meters of waste brine could be constructed with an investment of 10 million US$. Another 5 million US$ for constructing a factory to process the brine concentrate should be added. The annual income from the 750 tons of LiC1 would be about 20% of the investment. Subtracting the labour and material cost of the production at about 2 million US$ per year, the annual earnings should be about 1 million US$, or about 7% of the investment. The profit from exploitation of lithium is thus marginal at best, if no by-product is marketed. The profit from the by-product bischofite can be considerably more than that from the lithium exploitation. We analysed the bischofite precipitates and found that those formed during the earlier stages of lithium production are not enriched in LiC1. CaSO4 is the only unacceptable impurity that prevents the bischofite from being an adequate raw material for electrolysis to produce metallic Mg. We have developed a patented process to purify the salt (Hsfi, 2001), and we found that the purified bischofite could be marketed at more than 100 US$ per ton. The amount of bischofite produced as a byproduct during lithium exploitation from laolu are in the megaton-range. If one or two megatons are purified and sold, the income would be 100-200 million US$. Subtract the production, processing and transport costs from the earnings, we believe that the profit of marketing bischofite could be 10-20 millions US$ per year from the processing of the annual waste brine of the Qinghai Potash Work. There are additional profits to be earned through the extractions of bromine, iodine, strontium, caesium, boron, and other trace elements in the lithium concentrate. In conclusion, the Hsfi method is the only method so far that can be applied to extract lithium economically from brines in salt lakes of the Dead Sea Type.

PARADOX OF L I T H I U M EXPLOITATION Several Chinese institutions failed in their attempts during 30 years to develop a process of extracting lithium from the Qarhan brines. Chen and his colleagues tried in vain for a year. Consulting the experts, Chen was told that it would be physically impossible to extract lithium from brines through fractional crystallization: the volume of residual brine enriched in lithium would be too small to be separated from the mass of solid precipitate. Hsfi arrived at Zhuozhou in the afternoon of April 13, 1995. Analysing the problem, he recognized that the traditional wisdom of "batch production" had led to the dead end. One does not have to wait until the lithium is enriched to remove the

428

Y. Chen and K. J. Hsfi

bischofite precipitate. Why should the "batch production" mentality be the orthodox one? The mentality has been conditioned by traditional practice. Salt precipitate is commonly harvested only after the evaporation is completed, because removal of solids during evaporation requires the acquisition of a vessel to dredge up the precipitate from the bottom of an evaporating pond. Such a procedure is rarely practised, because of the high cost of aquiring and transporting a dredging vessel inland to where brine lakes exist. While the "assembly-line" procedure of removing bischofite was practised during the laboratory test, the Helminthoid construction was not invented until the early summer of 1995 shortly before the field test. In order to remove the bischofite precipitate, the workers should have some place to stand on. Partitions for troughs are ideal for that purpose. The next step was to breach the partitions at alternate ends so that the brine could steadily evaporate. While the brine is continuously fed at one end, the lithium concentrate is continuously harvested at the other end. The basic ideas are so simple that Hs~i met strong opposition from the examiners of the U.S. Patent Office, who examined the patentability of his invention. It was argued that a modification of an old patented process is legally not patentable, if the modification is obvious. Tens of thousands of words and many thousands of dollars were spent to convince the bureaucrats of the patent office that what he considered obvious was not obvious to hundreds of scientists doing research on lithium exploitation from brines. Chen had to submit an affidavit to the court to explain why the invention of a Helminthoid Evaporator was not an obvious modification of traditional facilities of salt exploitation by solar evaporation. Finally the patent was approved after an hour-long conversation between Hsfi's patent lawyer and the patent-examiner, after the lawyer threatened a law-suit. Chen used a Chinese proverb to describe the invention: the idea is so simple that a child can understand, but so paradoxical that even a devil fails to think of it. The Occidental equivalent is the Columbus' Egg. The lesson learned from the experience is clear: analyse your problem before you spend the rest of your life working on it. REFERENCE Hsfi, K.J., 2001. Process and Apparatus for Recovery of Lithium in a Helminthoid Evaporator. (US Patent Appln. 08/844092). United States Patent 2001, in press.

Paradoxes in Geology Ueli Briegel and Wenjiao Xiao, eds. 9 2001 Elsevier Science B.V. All rights reserved.

Chapter 22

P R E L I M I N A R Y R E P O R T O N E X P E R I M E N T S TO M O N I T O R C-14 BETA-DECAY

Kenneth J. Hsfi l, Min-Pen Chen 2 and Chao-Kai Huang 2 I Tarim Associates AG, Frohburgstrasse 96, CH 8006 Zfirich, Switzerland, 2Institute of Oceanography, National Taiwan University, Taipei 10617, Taiwan

ABSTRACT Natural radioactive decays have been assumed spontaneous. The decays are assumed to have a constant half-life, despite contrary experimental and empirical evidence. Postulating that beta-decay is a neutrino-neutron interaction, an experiment has been carried out in Taiwan to monitor the decay rate of C-14. The rate has varied within the range of 1% during the past year, and the variation is positively correlative to the hourly cosmic counting rate at Beijing. The range is about 800 times greater than the random background-variation. The experimental results are consistent with the postulate that beta-decay is activated by neutrinos from cosmic rays. INTRODUCTION In search for a physical theory of time, we noted that one physical phenomenon giving a measure of time is the radioactive decay. Going back to the Descartian argument that all forces are transmitted by particle interaction, we started with a postulate that radioactive decay is caused by particle interaction, and that the particle causing the decay is, let us say, an unknown particle called chronon. The beta-decay can thus be expressed by an expression (Hsfi, 1992):

(A,Z) + c =~ ( A , Z + 1) + e -

(1)

In order to bring the idea from the realm of metaphysics into physics, we searched then for a natural particle, such as one in the cosmic rays that can be the chronon. The only candidate is the neutrino, because radioactive decays can take place in the center of the earth, and the neutrino is the only particle that can penetrate into the earth's interior to cause a particle interaction. Since the chronon does not have a measurable mass or momentum or charge, the experimental expression of Eq. (1) is

(A,Z) =~ ( A , Z + 1) + e -

(2)

This phenomenon has been experimentally observed and is called beta-decay. When this phenomenon was first discovered, a serious crisis threatened to undermine the backbone of physics. The charges are conserved during the decay, the mass is

430

K. J. Hsfi, M.-P. Chen and C.-K. Huang

conserved within measurable accuracy, but the momentum is apparently not conserved. Faced with the observation, physicists had to make a choice. Either momentum conservation for particle interactions had to be abandoned, or something was being emitted in the reaction that could not be observed, but which carried off just the right magnitude of momentum to make everything work out right. One of the "czars" of theoretical physics in the 1930s, Wolfgang Pauli, declared that the second alternative was the only acceptable one. Later, Fermi coined the name neutrino for the unobserved particle that must have been emitted in the reaction, so that the beta-decay is expressed in the standard model by (Krauss, 1992): (A,Z)=~(A,Z+

1)+e-+n

(3)

Either-or propositions overlook alternative solutions. Instead of an antineutrino being emitted in a spontaneous decay, the neutron may have acquired energy and momentum through the capture or collision with a neutrino, so that the experimental relation Eq. (2) is expressed by (Hsfi, 1994): (A, Z) + n =~ (A, Z + 1) + e -

(4)

A comparison of Eqs. (1) and (4) suggests that the chronon activating the betadecay is a neutrino. Physicists objected the formulation of Eq. (4) because the existence of n in Eq.(3) has been observed; its measured energy-spectrum from the reactor (reaction chains consisting of beta-decays) is consistent with the theory to within a few percent of that postulated by Pauli when he first "invented" the reaction. We could get around that problem, however, by writing the equation (Wang, 1998): (A, Z) + v ==~ (A, Z + 1) + e - +2n

(5)

The physical implications of the postulate (5) are: l) Beta-decays are neutrino-captures, or more precisely neutrino-neutron interactions. 2) Beta-decays can theoretically trigger chain reactions of neutrino captures, or neutrino-neutron interactions, similar to chain reactions after fission induced by neutron-captures. Eq. (5) implies that the rate of beta-decay is a function of neutrino flux. This postulate may explain the phenomenon of the relatively rapid rate of free-neutron decay, which takes about 11 minutes, whereas neutrons in an atomic nucleus, probably shielded from neutrinos by an energy field, decay more slowly or not at all (Hsti, 1996). C O U N T I N G OF DPM OF C- 14 DECAY Brookhaven scientists have done work on beta-decays of Si-37, Ti-44, Bi-207 and of other radioactive elements (Alburger and Harbottle, 1990). They found that the decay rates varied systematically up to a few percent during a period of up to four years of continuous observation. They could not find, however, any correlation of the varying decay rates with measurable physical parameters. Another element that may have very large variations in decay-rate is carbon-14. Assuming a constant

Preliminary report on experiments to monitor C-14 beta-decay

431

half-life, the carbon-14 ages calibrated against historical dates or tree-rings have discrepancies as large as 10%. The orthodox interpretation is to assume a high C-14 production-rate in the past so that the apparent C-14 age is younger than the real age (NOAA 1999). This kind of correction has now been applied to all experimentally determined ages, but such a correction does not necessarily bring more agreement between C-14 and historic dates. Equation (5) postulates that the discrepancies of C-14 ages from historical dates could be a result of a variation of the beta-decay rate as a function of neutrino flux, with or without the additional complicating factor of a varying production rate. To test this hypothesis, we have been monitoring in Taiwan the apparent decay rate by counting the electrons from the C-14 decay since the beginning of 1998. A liquid scintillation-counter, Model 1215-111, with unquenched LSC standards has been used. The standard set consists of two activity standards and one blank standard. The activity standards are precisely calibrated, sealed unquenched samples of carbon-14 doped (1.14C) stearic acid and of tritium doped (7(n)-3H)-cholesterol in 10 ml of xylene-based scintillation solution. The two labeled compounds are produced by DuPont NEA, Belgium and Amersham International, UK. The unquenched blank standard is a sample of 10 ml of xylene-based scintillation solution. The standards are supplied in 20 ml capacity low-potassium content Wheaton vials. These are flame-sealed and secured with white silicon mastic and an aluminum cap. Standards are deoxygenated with pure nitrogen before sealing. The Carbon-14 standards are calibrated against reference standards of n-hexadecane-114C supplied by the National Institute of Standards and Technology, Standard Reference Material (SRM) No. 422C, the estimated accuracy of which was 0.81%. The absolute activity of the standards is within 0.5% of the reference standards of NIST SRMs. The scintillation counter was installed in a "normally clean laboratory," providing a satisfactory operational environment. Lead-shielding was installed according to the maker's instruction. Additional shielding was installed around the vial sensor and under the instrument. The selector-voltage was set at 240V. Fuses fitted in the fuse-carriers on the back-panel were of the correct rating for the local electric supply and according to the label. In carrying out our experiments, C-14 and blank standards provided by the Wallace Co., Taipei, Taiwan are installed in the liquid Scintillation Counter. Disintegrations per minute (DPM) are accounted alternately every 3600s. The counting results are recorded by automatically recording magnetic discs, and the daily averages are automatically computed and recorded. The DPM results are compared to the data published by NOAA on sunspot number and hourly cosmic ray counting-rate/256 at Beijing (Olsson, 1975). DISCUSSION OF E X P E R I M E N T A L RESULTS The decay rate of the C-14 standard at the start of the experiment on April 1, 1998 was 98,400 counts per minute (CPM), but the CPM was almost down to 98,000 in September (Fig. 1). Carbon 14 has a half-life of 5730 years. The ratio N/N0 is about 10 -4, so that the theoretical decrease of DPM count should be less than 10 during the first year. The measured CPM decrease was thus about 30 times the theoretical

K. J. Hsff, M.-P. Chen and C.-K. Huang

432 98400

I

'

I

'

I

'

I

'

I

'

I

'

I

I

,

I

,

I

,

I

,

I

,

I

,

I

'

98300 98200 98100 98000 97900 o O

97800

m

a

97700 97600 97500 97400

m

4/I/1998

5/2/1998

6/2/1998

7/3/I998

8/3/1998

9/3/1998

1 0/4/1998'

Date

Fig. 1. Variation of C-14 CPM April-October, 1998.

value. The possibility was suggested that the experimental results could reflect a decline of the counting-efficiency of the instrument. The makers came in early September, 1998 to check the counter. They could find no decline in the counting efficiency, and they found actual decreases in CPM values for both the C-14 and the tritium standards. We now believe that the decrease of CPM reflected a true decline of beta-decay rate, correlative to a decrease in cosmic-ray intensity during the same time-interval (see Fig. 2). The maker returned again in mid-October to make new calibrations. The record of the background noises has been registered by measuring the CPM of the blank sample, and the values ranged from 13.5 to 14.5 during the last 6 months (Fig. 3). The C-14 decay has since then been registered in disintegrations per minute (DPM). The value decreased from a high of about 100,000 in early November to a low of 99,200 in mid-May. The signals are thus 4 orders of magnitude greater than the background noise. The range of the DPM variations is about 800 or 0.8%, corresponding to a range determined by the maker in 1997 prior to our purchase of the counter. The range of signal-variations is thus also 4 orders of magnitude greater than the background variations. Our results verified thus the earlier work by Alburgh and Harbottle that the beta decay rate of radioactive elements can vary significantly if the rate is monitored over a period of months or years. Having established that the measurements are not artifacts of defective instruments, but are indicative variations of the C-14 decay-rate, we searched for parameters correlative to the signals. Like Alburgh and Harbottle, we found no obvious correlations to temperature, pressure or other environmental parameters. Our hypothesis relating beta-decay to neutrino flux suggested a possible correlation

Preliminary report on experiments to monitor C-14 beta-decay

433

Fig. 2. Variation of C-14 DPM, October 1998-May 1999, compared to varying hourly cosmic-ray counting-rate.

Fig. 3. Variation of C-14 DPM, October 1998-May 1999, compared to random background variation.

of the disintegration rate to solar-neutrino flux, which may have an inverse correlation to sunspot activity (Boehm and Vogel, 1992). We could find, however, little correlation of the D P M and sunspot-number signals. The latter has a somewhat irregular cyclicity, quasi-monthly or semi-monthly. The average sunspot-number

434

K. J. Hsii, M.-P. Chen and C.-K. Huang

Fig. 4. Variation of C-14 DPM, October 1998-May 1999, compared to varying sunspot number.

showed, however, little decrease during the 6-month period that could be correlative to the reduction of the counts of the disintegrations per minute of the C-14 standard (Fig. 4). We were able to compare our signals with the measurements of the intensity of the cosmic rays. There is a temporal trend of the 1% reduction of C-14 DPM parallel to the 5% reduction of the hourly cosmic-ray counting-rate measured at Beijing from early November, 1998 to early February, 1999. There are also parallels in the interruptions of the decline-trend when both DPM and cosmic ray counts rose. There seems, however, a time-lag, of a few days to about a week, of the C-14 DPM signals. It seems that the decay-rate is not directly affected by cosmic-ray particles themselves that are measurable, but by particles which are disintegration products of measurable cosmic-ray particles. The major components of the sea-level cosmic radiations such as those counted in Beijing are m-mesons and neutrinos; three-quarters of the charged particles are m-mesons. Those are the decay-products of positive and negative p-mesons in cosmic rays, which are copiously created in cosmic rays by collisions of primary and secondary nucleons (i.e., protons and neutrons) and other p-mesons with nuclei of air molecules. The m-mesons may penetrate more than a few meters into the ground. The small values of background CPM and their lack of correlation to cosmic ray intensity indicate, however, that few such mesons have been able to penetrate the shielding of the scintillation counter. Our experimental results suggest a working hypothesis that neutrinos were the only particles from the cosmic rays that did penetrate the shielding to influence the rate of carbon-14 beta-decay. The preliminary experimental results are thus consistent with our postulate that beta-decays are activated by neutrinos (alias chronons) from cosmic rays. The betadecay rate of radioactive elements seems to be a function of the neutrino fluxes: the

Preliminary report on experiments to monitor C-14 beta-decay

435

rate is faster at times of higher neutrino fluxes. A final verification of this conclusion will be to measure the beta-decay rate in an underground mine, into which only neutrinos can penetrate. We are planning this experiment in Taiwan. A statistical correlation relating the C-14 beta-decay rate and neutrino flux can be obtained through a monitoring of the neutrino flux. If the relation is established, a C-14 standard could be an inexpensive monitoring device for neutrinos. PHILOSOPHICAL IMPLICATIONS The question whether beta-decay is a neutrino-neutron interaction or not, has practical implications. If neutrino beams can activate beta decays, one could theoretically accelerate decay-rates for detoxification of radioactive waste while generating energy. A more intriguing question is the reality of the neutrino-chain reaction. Neutrons may trigger fission chain reactions when more neutrons are released after fission through neutron-capture. If Eq. (5) is correct that 2 neutrinos are released for each neutrino-capture, a chain reaction of beta-decays is theoretically possible. We have, in fact, invoked this possibility to interpret the extremely rapid rate of propagation of earthquake-fractures (Hsfi, 1994). We might go one step further and ask whether the so-called cold fusion could be a manifestation of the neutrino chain-reaction. The most profound implication of the experiment is not the practical, but the philosophical implication. Physicists championing the theory of relativity question the physical reality of time (Davies, 1994). It was stated: the mistake of pre-relativity physics was to identify time too closely with human experience. Relativity physics has shifted the moving present out of the superstructure of the universe, into the minds of human beings, where it belongs. In the absence of an acceptable theory of the mind in physics, any discussion of physical time must necessarily exclude the consideration of the now, and the apparent forward flow of time, because these are meaningless concepts within the concept of ordinary space-time. The experimental verification that time is a physical phenomenon measurable by the particle-interactions, and the rate of the interactions is a function of timedependent flux of chronons, alias neutrinos. The considerations of the now and of the apparent forward of time are not meaningless concepts. On the contrary, one wonders if the concept of ordinary space-time is only a mathematical concept, and not a valid physical concept. REFERENCES Alburger, D.E. and Harbottle, G., 1990. Physical Review C, 41, 2320-2324; also D.E. Alburger, personal communication, 1994. Boehm, F. and Vogel, P. 1992. Physics of Massive Neutrinos. Cambridge Univ. Press, Cambridge, 249p. Davies, P.C.W, 1994. The Physics of Time Assymetry. Univ. Calif. Press, Berkeley, 214p. Hsti, K.H., 1996. Terrestrial, Atmospheric and Ocean Sciences, Taipei, Taiwan, 7: 239-255. Hsti, K.J., 1992. Proc. Nat. Acad. Sci., 89: 10222-10226. Hsti, K.J., 1994. Why Newton was not a Chinese. Abschiedsvorlesung. Tarim Associates, Zurich, 17p. Hsfi, K.J., 1994. Proc. Int. Workshop on Science Frontiers and Priority Setting of NSFC, Beijing 1994. 54-60.

436

K. J. Hsii, M.-P. Chen and C.-K. Huang

Krauss, L.M., 1992. The Fifth Essence: The Search of Dark Matter in the Universe. New York: Basic Books Publishers, 341p. NOAA, 1999. (ftp://ftp.ngdc.noaa.gov/STP/SOLAR DATA). Olsson, E. (Ed.), 1975. Radiocarbon Variations and Absolute Chronology. Wiley New York, 652p. Wang Henry, 1998. Institute of Physics, Academia Sinica, Taipei, personal communication.

Part VI

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Paradoxes in Geology Ueli Briegel and Wenjiao Xiao, eds. 9 2001 Elsevier Science B.V. All rights reserved.

Chapter 23

T H E CAREER OF A F R U S T R A T E D C O N F O R M I S T

Kenneth J. Hsti* Frohburgstr. 96, CH-8006 Z~irich, Switzerland

My Swiss students organized a symposium on the occasion of my 60th birthday. I have been considered a controversial scientist, and the Festschrift was organized on the theme of Controversies in Modern Geology (Mtiller et al., 1991). When I was approaching 70, my Chinese students wanted to do something again. I was flattered, and at the same time moved; I hoped something good could come out of it. When they came to consult me about a theme, I blurted out: "Why don't you invite my friends to Beijing to evaluate Ken Hsfi's crazy ideas?" They decided to do exactly that, and the symposium was entitled Paradoxes in

Modern Geology. Paradoxes contradict received opinions and paradox can be considered a euphemism for crazy ideas. People with crazy ideas are non-conformists. I, like many others, actually prefer to be a conformist: it is more comfortable to voice "me-tooism". Yet we beg to differ when our intellectual honesty is to prevail. Science is a rational expression in human languages. Rationalities differ, however, in different countries, where people speak different languages and see things differently. I have been a citizen of four countries, and I did not have to try very hard when I ventured to differ from the rational norm of one particular establishment. A crazy idea of a Chinaman may have been derived from a rational norm in China. Science progresses through the falsification of the falsehoods that are a part of an orthodox. Paradoxes could thus lead to advancement of science. My friend Sun Shu was very kind to enumerate Ken Hsti's 18 paradoxical opinions, and those were evaluated by the speakers who came to the Beijing Symposium. Now the editors of this opus offered me a rostrum at the forum to reminisce. How and why did I think differently? Where or from whom did I get my crazy ideas? After 70, one can do what his heart desires, he can do no wrong. Confucious said that, and I learned it in school. Now, after repeated rejections of my manuscripts, I am not so certain about the Confucian wisdom. People do not seem to like to hear what I have to say. I originally planned to write a manifesto of a crusade against religious mentality in science. On second thought, I remember my friend Bob Ginsburg's advice: be gracious. He is right. I can post other opinions, but I don't have to be querulous. I shall thus attempt in this essay to recall modestly the

* E-mail: [email protected]

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circumstances leading to the formulation of the "18 paradoxes". I felt unconformable when I had to differ; I wanted to conform, but I have been a frustrated conformist. Geology is a science founded by the intellectual giants of Great Britain, James Hutton, William Smith, Charles Lyell, Charles Darwin, among others, and they carried on the tradition of Francis Bacon. The Baconian induction is well known. Facts are to be observed and collected, multiple working hypotheses formulated, and a process of exclusion should lead infallibly to the detection of the true cause. As promised by Karl Popper's philosophy of falsification, the truth may be revealed after all the falsehoods are unmasked. Science in its progress has, however, not always followed the Baconian method. Facts and phenomena are infinite in number, and they cannot all be tabulated and arranged. The process of exclusion is not infallible, because the truth that is not tabulated remains elusive. What is not included cannot be excluded, and we may be no closer to the truth after all the known working hypotheses are proven wrong. Geologists endowed with Baconian wisdom may encounter little difficulty in making break-throughs: new facts are revealed by new methods, new tools. Driven by the pressure to make even more discoveries, career scientists tend to disdain the apparent mopping-up operations of verifications or falsifications. Orthodoxes are established on the basis of induction. Facts that cannot be explained are dismissed as irrelevant. Those facts are, however, often the stuff that makes scientific revolutions, as T.S. Kuhn reminded us. When deductive conclusions do not make sense, something must be wrong, and often the fault lies with the presumptions of the orthodoxy. I am writing this short essay to illustrate on the basis of my experiences that paradoxes contradicting received opinions are often needed to rescue science from being trapped at dead ends. I was a conformist when I was young. After I completed my undergraduate studies in China, I came to the Ohio State. Ed Spieker was my first mentor. He taught not only geology, but also the history of science at Columbus. I should have learnt from Spieker the philosopher, but I did not: I was too young. A favorite topic in geology of the post-war decades, as one might recall, was orogenic chronology. Recognizing my interests in reading, Spieker assigned me a library-research: I was to collect evidence to date the deformation of the Appalachian mountains. I was a 19year-old bookworm devoid of field experience, and I depended exclusively upon secondary information, often misunderstood or misinterpreted. There was also that eagerness to please the person who had been kind to me. My master's thesis was a typical beginner's exercise (Hsfi, 1950). I was a conformist, and I relied upon authorities to champion a cause. I did my Ph.D research on metamorphic petrology at U.C.L.A., a Baconian stronghold. There was a T.V. series on a police detective Sargeant Friday. His favorite expression was that of my teachers: "Facts, m'am, I want facts." There was a naive faith that facts would infallibly lead to the right conclusion. In my first two years of field work, I collected all too many facts. I measured the dip and strike of every layering. It was a time-consuming work-therapy, but the facts were confusing. There are N-S trending and there are E - W trending structures. Assuming

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that they have all been formed during one single period of deformation, the geology did not make sense. The eventual enlightenment came one day when a simple idea hit: why should I assume that there was only one episode of deformation and metamorphism? Unburdened from the preconceived notion, the rest was simple. The jumble of polymetamorphic structures and mineral-assemblages could be sorted out. The dissertation was nevertheless a work of orthodoxy: the young neophyte deferred to the establishment elders, citing their opinions as if those were expressions of natural laws (Hsfi, 1955). I was unemployed after I finished my studies in 1953. There was no job market for metamorphic petrologists. I was eventually hired as a trainee in sedimentology by Shell. The theory of turbidity-currents sedimentation was an innovative idea. The young turks at the Shell Research had no difficulty in being receptive. The relevant question to petroleum geologists then was not so much the depositional mechanism, as the geometry of oil-bearing reservoirs. I worked with Bob Nanz then. He had just completed his study of alluvial reservoirs of the Seeligson Field in south Texas. He taught me the methodology of identifying sedimentary units formed by a certain process of sedimentation, i.e., to identify a "genetic unit" on the basis of stratigraphic correlation. It was not a difficult task, as I found out after I went to Ventura to study the Pliocene turbidites there. Those sediments were deposited during rare events and are sandwiched between sediments accumulated under normal conditions. A sand unit, marked by the electric log markers AO and AP, is present between two shale units. I could define its distribution in the Ventura Field as an elongate sand-body deposited by turbidity currents flowing down the long axis of the Ventura Basin. The explanation by deductive reasoning was simple: a turbidity current originating from a point source should have laid down a shoe-string sand (Hsfi, 1977). My interests in turbidites continued after I came to Zfirich. Nature has performed an experiment: the subaqueous slide at Horgen on the side of Lake Zfirich triggered turbidity-current sedimentation. My first student at Zurich, Kerry Kelts, helped me to start lake research, and our first achievement was to demonstrate the longitudinal transport of turbidity currents (Kelts and Hsfi, 1980). The conclusion was later verified experimentally in the laboratory (Hsfi et al., 1984). Still my paradoxical interpretation of longitudinal transport is not generally accepted, and the orthodox continues to portray turbidites as fan-shaped submarine delta deposits. John Shelton, my colleague of the Shell days, was to discuss the question at Beijing. Unfortunately, he cancelled his trip at the last minute. Even when I was a student, we argued about the origin of mountains. The geosynclinal theory was the paradigm. Geosyclines, areas submerged under deeper water or underlain by thick sediments, should be the precursor of mountains, we were told. There were many theories, and all of us were confused: everyone talked about geosynclines, but nobody seemed to have ever seen one. In 1956, I went to the annual meeting of the American Geophysical Union. George Wollard exhibited his crustal section across North America. Gravity data were processed on the basis of Airy's model of isostasy, crustal thickness was deduced and compared to seismic results. A glance at the profile enlightened me,

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I could see that geosynclines are simply regions underlain by thin Earth's crust (Hsfi, 1958). For the first time in my life, I learned to escape the tyranny of semantic traps. The aim of science is to comprehend, not to find words or slogans. When my friend Frank Dickson resigned in 1959 his position as the project-leader to study the chemistry of dolomitization, he recommended me as a substitute to the Shell management. I changed my specialization from clastic-sedimentation to experimental geochemistry. I soon realized, however, that the key to understanding is not chemistry but hydrodynamics: dissolved magnesium ions have to be brought in by moving water to induce dolomitization. Shortly after I received my assignment, I went to visit a Shell office at Midland, Texas. Stratigraphical studies defined the spatial distribution of Permian carbonates in the West Texas Basin. Dolomite predominates in the shelf regions, where groundwater movement must have been active, and limestones are mainly basinal, where there was little circulation. This significance of the pattern did not escape other geologists, but the path of circulation remained a puzzle. My colleagues in Shell preferred the seepage-reflux hypothesis; that was the orthodox opinion. They postulated that magnesium-rich waters from supratidal lagoons descended through sabkha sediments to induce dolomitization there. I did not like the idea, because the hypothesis had been contradicted by actual observations: the sediments under a Recent lagoon are lime muds, not dolomite. My knowledge in hydrodynamics was rudimentary. Groundwater moves through aquifers, I was taught. Chemical analyses of groundwater seemed to argue against my own idea. Where a groundwater is magnesium-rich, it did not seem to move fast enough to dolomitize. Where groundwater moves rapidly, the water does not contain enough magnesium to change a lime sediment into a dolomite (Hsfi, 1966). I was puzzled, but I left Shell before I could solve the riddle. Dave Kinsman came to Ztirich in 1968 to talk about the Recent dolomite on the sabkhas of Abu Dhabi. He did a good job collecting data, but he continued to adopt the seepage-reflux hypothesis. I made a simple hydrologic calculation, and I could tell that he was wrong: the magnesium-rich water could not have moved down fast enough through the sabhka sediments. What was the alternative? The alternative was that groundwater in sabkha sediments does not descend, but ascends. Come to think of it, this is indeed possible: a hydrodynamic potential is established when the groundwater table is lowered because of the loss of vadose water by evaporation. I offerred the idea to Kinsman, but he was too obstinate to consider any paradox. I had no choice but to tackle the problem myself. Thanks to the help of Chris Siegenthaler, we demonstrated evaporative pumping in the laboratory (Hsfi and Siegenthaler, 1969). Thanks to the help of Jean Schneider and Judy McKenzie, we demonstrated the applicability of the theory to interpret the genesis of sabkha dolomites (Hsfi et al., 1980). I was hoping that Schneider or McKenzie would contribute a paper to the symposium, but they could not come. The Airy model of isostasy formed the physical basis of the theory of continental drift, and Wollard data seemed to have verified the Airy model. There was, however, a nagging exception: the seismically determined thickness of the crust of western

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North America is thinner than that predicted by gravity data. The discrepancy did not go away when geophysical measurements became increasingly more precise. The inspiration came suddenly. Airy was wrong, because his model is physically incorrect. Airy postulated more than 100 years ago the floatation of lighter solid crust on a liquid substratum. We now know that the substratum or the upper mantle is not a liquid, but a solid of considerable strength. Earth's crust cannot float on the mantle. The zone of isostastic compensation lies at a level where the mantle strength is negligible. The strength decreases with increasing temperature, and becomes negligible only at a depth of 100 km or more. The classic concepts of lithosphere and asthenosphere by Joseph Barrell were exhumed: the lithospheric shell of finite strength includes not only the crust but also the uppermost mantle. Continents cannot drift, and they did not. Continents and their underlying uppermost mantle constitute lithospheric plates, and the plates move. The new understanding was to form the geophysical basis of plate-tectonics. When I first proposed that the lithospheric segments are isostatically adjusted, not according to the Airy model, but according to the Pratt model, the manuscript was advancing an idea contradictory the received opinion. It was rejected. Only John Rodgers appreciated the rationale and published the paper in the American Journal of Science (Hsti,1965). Dozens of years have gone by, and this concept of relating subsidence to mantle-density change, alias thermal isostasy, has become the new orthodoxy. Numerous computations on the origin of sedimentary basins are made on the basis of this paradigm. Li Jiliang reviewed in this volume the changing concepts of geosynclines. I wanted to work on the Franciscan rocks, because my late wife Ruth fell in love with the California Coast Range. I persuaded the Shell management, even though little oil had been produced from such basement rocks. The Franciscan are so deformed that the lateral continuity of sedimentary layering is broken. I recognized the Franciscan M61ange, because I saw slabs, blocks or fragments of different rocks, all sheared and/or fractured, and mixed together as a m61ange. A m61ange is called a mOlange because it is a m61ange (Hsfi, 1967) I found no echos from the establishment. I was dismissed as a Don Quixote waving his lance at a windmill which was the United States Geological Survey. The idea was also disliked by my colleagues in Switzerland. A student's manuscript on the ophiolite m61anges of the Arosa Zone in the Alps was publishable only after the word mOlange was expurgated and substituted by the word unit. The truth prevailed eventually, and I have received much appreciation from my peers for an achievement which is not much more than calling a spade a spade. My greatest satisfaction came, however, from field geologists of the Third World: Cuba, Equador, China, Turkey, Argentine, etc., where they applied the m61ange concept to solve the riddles of their geology, like Xiao, Pan, Wang, and Xu did in this volume. When I joined ETH in 1967, I had to teach structural geology, in which I had to be concerned about the mechanics of overthrusting. Hubbert and Rubey had postulated in 1959 that overthrusting was induced by gravity under conditions of high-pore pressure in sediments. Their theory was immediately accepted as the paradigm. The two were the most prominent geologists of their time, and there

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seemed to be no alternative. Another decade was to elapse before the theory of platetectonics was innovated. I had been a great admirer of King Hubbert, and I was certain that gravity sliding was the answer. I was neverthless troubled by their conclusion because I did not quite understand.When I had to teach, however, I had to. I had to be answerable to students; I had to understand the basic equation: Tcrit. - - TO @

(1 - ~.)tan 4)

The formula is so simple that we do not really need the symbols, we could write critical strength = cohesion + (pore - pressure factor) x friction In other words, strength consists of cohesion and friction (or internal friction) The term r0 is, under normal circumstances, much smaller than the term (l-k) tan 4). Hubbert and Rubey rightly assumed that the cohesion can be neglected under normal circumstances, for the sake of easy calculations. In fact, a rock always has cohesion before it fractures, and the cohesion term, however small, is far from negligible when the friction term approaches to null. Computers were coming to use in universities in the late 1960s, and there was no need to neglect a meaningful term. Thanks to the help of Chris Siegenthaler, the computational results clearly showed that Hubbert and Rubey were mistaken (Hsti, 1969). Gravity-sliding cannot be the answer, and we have to adopt an alternative overthrusting mechanism compatible to the theory of plate-tectonics. In search for an alternative, we discovered superplasticity. The possibility of superplastic flow of rocks was first demonstrated by Ueli Briegel in our rockmechanics laboratory. He and Walter Mfiller, using the finite-element method, verified that superplastically deformed gypsum or anhydrite forms the detachment layer of decollement overthrusting in Jura Mountains (Mtiller and Hsti, 1980). With his first-hand experience, Briegel gave at Beijing, a review of the paradox of overthrusting mechanics. Mylonites are rocks deformed by superplastic flows, and they are commonly present under large overthrust masses. Briegel found that the mylonite under the Glarus Thrust in the Alps have flowed superplastically under relatively high temperature and slow strain rate. Using experimental data, I estimated the rate of overthrusting and found that the Glarus overthrusts were moving at a rate of a few centimeters per year (Hsti, 1969b). I was pleased when I learned that Le Pichon had come to the same conclusion on the basis of his plate-tectonic theory. The fascination with the cohesion factor in the overthrusting tectonics led to the next inquiry: what would happen if cohesion is indeed negligible as Hubbert and Rubey assumed? Yes, there will be gravity sliding, but the phenomenon is a landslide not an overthrust. Ron Shreve invoked the mechanism to interpret the Blackhawk Landslide of California: the detached slab was assumed to have slid on an aircushion. It was a clever idea. Few understood the mathematical formulation, and the air-cushion hypothesis became the orthodox for a few decades. A closer look at the phenomenon led me, however, to conclude that Shreve was mistaken. The Blackhawk Slab did not slide. It was broken up and the debris flowed like a thixotropic liquid (Hsti, 1975). I have not had time since then to work on the

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landslide problem, and I was pleased to learn from Chris Kilburn during this symposium that giant landslides did indeed flow. Through a twist of fate, I was asked in 1968 to serve as a sedimentologist on Glomar Challenger. That was the beginning of my 20-year involvement with the deepsea drilling program. The project has always been dominated by big-ocean scientists, but I was sufficiently persuasive to bring the drillship to an inland sea. I proposed the drilling without any knowledge of the regional geology. The Mediterranean had to be drilled, I told my colleagues on the JOIDES Planning Commission, because it is there; it is there between the two continents Europe and Africa. Continuous seismic profiling had discovered salt domes under the Mediterranean seabed, but the salt was considered Triassic: it was the conventional wisdom then that the Mediterranean owed its origin to an oceanization of a continental crust. The discovery of an Upper Miocene evaporite formation was a great surprise to everybody. Even more surprising was the size of the deposit: the Messinian is a "saline giant." It had been thought that giant evaporite formations were formed by accretion of small lagoonal deposits. The deep-sea drilling results have falsified that hypothesis: the Mediterranean evaporite was formed when this inland sea was changed into a giant salt-pond or a giant salt pan (Hsfi, 1972). French marine geophysicists held on to their prejudice that the Mediterranean was once a marine shelf; the Miocene evaporite was deposited on shallow bottom when the shallow sea was desiccated. Italian sedimentologists found evidence of deep-water sedimentation, and they argued that the Mediterranean was never desiccated. Salt crystals settled out of saline brine when the Mediterranian was isolated from the Atlantic. How do we harmonize the evidence of shallow-water sedimentation in a deepbasin? The answer is simple: shallow water sediments would be deposited in a deep basin if the basin is desiccated. This is the model of a dessicated deep basin by Hsfi, Ryan, and Cita (Hsfi, et al., 1973). The geological profession was incredulous because the postulate of catastrophic desiccation contradicts the Lyellian principle of uniformitarianism. The passion cooled down, and the evidence is clear. Eventually the deep-basin desiccation model was adopted by elementary textbooks. Maria Cita came to Beijing to present the results of the latest studies of the Messinian desiccation. After the Professor of Geophysics retired at ETH, I was asked by the administration to find a successor. I made friends with geophysicists and broadened my interests to include crustal geophysics. Stephan Mfiller came to join us from Karlsruhe and he initiated systematic gravity and seismic surveys across the Alps. He found a zone of high attenuation of seismic energy under the Jura mountains. His discovery led to my postulate of thin-skinned plate-tectonics: Earth's continental crust of an overriding plate can be detached from its underlying mantle at the Moho and be overthrust on top of an underthrust continental crust during orogenic deformations (Hsfi, 1979). The overthrust crust in the Alps is cut into numerous horizontal slabs called the Austro-alpine Nappes, and the underthrust crust, now deformed and metamorphosed under high temperatures, are the Penninic Nappes. The idea was to lead the formulation of the concept of the tectonic facies (Hsfi, 1994).

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I offerred Mfiller a co-authorship of the1979 opus. He accepted the offer at first, but then declined when he was told: "It is just another of Ken Hsfi's crazy ideas." Mfiller told me that he was a serious scientist and he had his reputation to protect. He spent the next 10 years and 20 million swiss francs so that the "crazy idea" was re-invented by orthodox geologists. Peter Finckh, who worked on Mfiller's project, was kind enough to come to Beijing and tell us the efforts leading to the re-invention. The thin-skinned plate-tectonics identified the Moho as the detachment surface in continental collisions. The continental crust above the Moho in one lithospheric plate is stripped off and overthrust onto the crust of another, while the small ocean basin in between was consumed by plate-subduction. Ian Dalziel called the orogenic phenomenon "backarc-basin collapse". Is the theory applicable to China? Using the approach of comparative anatomy of mountains, I proposed that the Precambrian rocks south of the Yangtze are not the basement of the so-called Jiangnan Landmass, but that they represent rigid-basement thrust sheets. The idea was an outrage, only the late Zhu Xia had an open mind to recommend the publication of the manuscript (Hsti, 1981). After working for several years in south China, the hypothesis is verified (Hsti, et al. 1988). When the idea of Huanan Alps, not South China Platform was first published, it was considered another outrage. With the progress of science, however, paradox becomes orthodox and the mechanism of backarc-basin collapse has become a standard model to interpret the geology of south China (Zhou Zuyi, this volume) and of Taiwan (Lu, this volume) The postulate of thin-skinned plate-tectonics also explains the puzzling geochemistry of the South China granites. The orthodoxy adopted the model of Andean orogenesis: the deformation and the magmatism of southeast China had been considered a consequence of subduction of the Pacific Plate under the Asian continent. The geochemistry of Chinese granites is, however, definitely not the Andean type. The isotope signatures indicate that the granites were formed when the flysch sediments of "collapsed" backarc basins were "partially melted" (Zhou Xinhua, this Volume). I learned isotope geochemistry from Mike Lloyd at Shell, and we were pioneers in isotope sedimentology when we worked on the first deep-drilling samples (Hsfi and Lloyd, 1972). Later Judy McKenzie wrote me from Scripps that she would like to come to Ztirich to do a dissertation on stable isotopes. As I became more involved in deep-sea drilling projects, we began to apply isotope geochemistry to paleoceanography. We needed help, and I invited Nick Shackleton to teach in my place when I took a sabbatical to China in 1979. He brought with him the surprising discovery that the ocean was heated up for about 5~ from top to bottom at the end of the Cretaceous, when the dinosaurs became extinct. It had to be a catastrophic event, I thought. There could not have been an explosion of terrestrial thermal energy. The excess energy supply must have been triggered by an extraterrestrial event (Hsfi, 1980). I sent a summary of my speculative ideas in a manuscript to Nature, even before I knew of the discovery by Alvarez et al. of an iridium anomaly at the K/T boundary clay. I took the galley proof of my 1980 article to Glomar Challenger, as we were scheduled to drill Tertiary objectives in the South Atlantic. On the long cruise to the

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first drillsite, I was able to convince my shipmates to forego a previously scheduled objective so that we could drill the K/T boundary in the Cape Basin. It was a success. Although the isotope analyses of our cores could not verify Shackleton's paleotemperature change across the boundary, we did find a remarkable carbonisotope anomaly. I did not quite understand the results until Wally Broecker came to Zfirich for a visit. "It is the signature of a Strangelove Ocean, " he told us. The unusually high carbon-12 values in the earliest Tertiary samples indicate that the ocean was almost sterile after a meteorite hit (Hsfi and McKenzie, 1984). He Qixiang played an active role in developing the concept of Strangelove Ocean, and he contributes a paper on this theme to the symposium. I left China in 1948. After I first returned in 1977, I was invited by Sun Shu and his colleagues at the Chinese Academy of Science to join their ambitious project on the tectconic evolution of China. The standard model had postulated that the suture of plate-collision is marked by a zone of ophilite m61anges, ophiolites being the relics of the ocean crust between two colliding continents. From the outset, we had to recognize that there would be too many "sutures" if we adopted the orthodoxy. Huang Jiqing, the Grand Old Man of Chinese geology, came up with his Theory of Polycyclic Deformation. There were only two continents and one ocean at any one time, he thought, but the continents came together and were pulled apart again so that there were several sutures marking the collisions (at different times) of the same two continents. Huang's theory became the orthodox. After 20 summers of field work in China, we came up with a paradox that there were no colliding continents, nor was there an intervening ocean. China was the site of an archipelago on the southeast side of the Paleozoic and Mesozoic Asia. The paleogeographic position was similar to that of the Indonesian Archipelago on the southeast of the Cenozoic Asia. There were numerous arcs and backarc basins in the region. The ophiolites are, however, not the sutures of plate-collisions. They are the relics of the ocean floor of back-arc basins. They mark the position of subduction zones of backarc basins (Hsfi, Chen, et al. 1999). Pan Guitang went with me on several expeditions to Tibet, and he was asked to evaluate our archipelago model on the basis of his study of the Songpang-Garze Flysch. Not all the backarc basins have collapsed. Where an ancient backarc basin survived arc-arc collisions, they became "holes in continent"; Tarim, Junggar, and Qaidam are such holes (Hsfi, 1988). "Holes in continent" are also common on the Russian Platform. After the publication of my paper on relic backarc basins, Sergey Aplonov contacted me and suggested the applicability of the idea to explain the origin of the Western Siberian, the Caspian and other large petroliferous basins in the territory of the former Soviet Union. He came to Beijing to present this conclusion. Shortly before my retirment, a Polish geologist asked me to give him a review of the history of my publications. I thought about it for the first time, and realized that the degree of acceptability of my ideas has been inversely proportional to my professional standing. I had little difficulty in getting my thesis published; a student's exercises had to have many facts and few paradoxes. Later, I met perceptive editors like

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John Rodgers and Guy Davies; they understood, and published the unconventional ideas of my youth. After I joined the JOIDES research, our work was not burdened by censorship; our manuscripts were published as cruise reports. I ran into opposition, when I became less diffident and spoke my mind. Thanks to the insight of Eldredge Moores, my voluntary contributions were published in Geology. I was to receive "flak", however, and my articles were repeatedly singled out for discussion. My luck finally ran out after I became a senior citizen. Younger editors took over and research activities became a business of cut-throat competition. Editors acted like postal clerks, and preferred to use no judgment of their own. Manuscripts were sent for reviewing by impertinent post-docs in "timbuctoo colleges". Ignorant remarks by specialists were forwarded verbatim to add insult to injury. Censorship in science was practised with the excuse of peer reviewing, forgetting that peers are supposed to be persons of equal quality or equal standing. Being a victim of the system, I talked about that at the dinner table with my family. My son Peter, who was a law student, advised me that I should no longer try to publish in scientific journals. "If your idea is any good," he said, "it should be useful. You can write it up and file a patent application." He was right. I submitted, during the last decade, more than a dozen manuscripts to scientific journals, and almost all of those, including a few invited contributions, were rejected. Meanwhile I filed 13 patents, and all but one were granted. One patent teaches a process to extract lithium from brines of salt lakes (Hsti, 2001). The received opinion was that the traces of lithium in the brines of the Qaidam Basin could not be extracted by solar evaporation. I came up with a simple idea that "the impossible" could be done. Chen Yancheng presented at this symposium the saga of our cooperation. While studying the origin of potassium-rich brines of Qaidam Basin, I again became interested in the hydrodynamics of evaporative pumping. One underestimates the vertical movement across relatively impermeable aquicludes, not recognizing the role of a very large cross-sectional area A. Instead of the specific flow rate q, Darcy's law states that the total flow rate Q is Q = k(AH/AL)A

This simple relation is the basis of my numerous patents on mining, oil, and water technologies. Stefan Lfithi pointed out at the Beijing Symposium that we are at a turning point in petroleum geology. An application of the new E O R (enhanced oil recovery) technology may increase the world's recoverable oil-reserves by a factor of 50% or more. My first Ph. D. student at ETH, Kerry Kelts helped me to start our lake research, and my last student Andreas Lehmann concluded the program. After thirty years of data-collection, I finally realized that the earth's climate changed from an optimum to a little ice age around 4000 BP. I called up Kelts in Minnesota, and he told me that he had been telling me that all those years, but I was too absent-minded to pay any attention to his discovery. After my retirement in 1994, I was invited to Taiwan. I was told by Arthur Chen of his discovery of two white bands in the

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Great Ghost Lake. Being a geochemist, Chen was not sure of their origin, but it was clear to me that they consist of eolian dust from Northwest China. Radiometric dates suggested that they were deposited during two little ice ages in historical times. Chen presented in this opus a detailed report of the "unveiled secrets", while I went on to polemize received opinions again. Environmental activists, politicians, and the press blamed anthropogenic activities as responsible for the current global warming. I gave an alternative intepretation (Hsfi, 2000): instead of a "greenhouse catastrophe", global warming could actually be a blessing to mankind. Yuk Yung did not rule out this possibility in his talk given at the Beijing Symposium. We were all taught the Darwinian theory in school, and few of us bothered to read The Origin of Species. My preoccupation with mass extinctions propelled me to study the classic. I wanted to know the scientific basis for this revolutionary theory. I was surprised that Darwin made a fundametal mistake in failing to recognize that evolution is the history of life on earth. He did not realize that a theory of evolution should tell us what has happened, not what could have happened. Darwin considered mass extinctions as artifacts of the imperfection of the geological record. He is wrong. A century after Darwin, science has established that an extraordinary environmental change triggered the catastrophic evolutionary changes at the end of the Cretaceous Period. Darwin's theory of natural selection is wrong: when a catastrophe hit, the survivors may not have been the fittest but the luckiest (Hsti, 1986). Darwin came to his conclusions because he was misled by the uniformitarian philosophy of geology. Hutton saw neither a beginning nor an end in Earth history. Lyell was convinced of uniform rates in Earth processes. They did not perceive the arrow of time. My hero in science is William Smith. He gave us stratigraphy and he made us aware of the geologic time. Stratigraphic paleontology is the documentary basis for a history of life on earth, and evolution is unthinkable if there has not been the flow of time. I spent the last years at ETH searching for a physical theory of time (Hsfi, 1992). My daughter Elisabeth was an inspiration. She was doing a master's thesis at Cambridge on the significance of the tick-tick time. I may have been a doctor of philosophy, but I had never worried about such philosophical questions. When I did, I was horrifield by what I learned. A prominent physicist told us The mistake of pre-relativity physics was to identify time too closely with human experience. Relativity physics has shifted the moving present out of the superstructure of the universe, into the minds of human beings, where it belongs. In the absence of an acceptable theory of the mind in physics, any discussion of physical time must necessarily exclude the consideration of the now, and the apparent forward flow of time, because these are meaningless concepts within the content of ordinary space-time. I never did understand the ordinary space-time, but I always deferred to the authority. Einstein is after all the great scientist of the 20th century. If his theory of relativity tells us that time is a human illusion, who am I to argue? Who am I indeed? I am a frustrated conformist, driven by an intellectual honesty not to accept any authoritarian dogmas. Theoretical physicists want us to believe that they have found the truth, when they express their understanding in

450

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mathematics. Their arrogance dictates that truth cannot be expressed in a daily life language. In my anti-establishmentarian frame of mind, I see mathematics as just another language. Truth is perceived by painters, musicians, writers, philosophers, or any curious mind; physicists have no monopoly (Hsti, 1996a). The apparent flow of time is common sense. If common sense does not make sense in the space-time concept of the theory of relativity, we are facing a choice whether the theory or common sense is nonsensical. As a scientist, I have no choice but to believe in common sense, we rely on common sense to intepret scientific observations. I believed in the flow of time and posed the question whether the elementary unit of time, alias chronon, is the atom, the fundamental particle, in space and time (Hsti, 1996b). This question introduces the talk presented by Chen Min-pen at the Beijing Symposium, he and I started the inquiry on the nature of chronon with an experiment to monitor the beta-decay rate of carbon-14. The unit measurement of geological time has been years or m.y. (million years) which are multiples of a second t. To measure geologic time, we presume spontaneous radioactive decay and constant half-life of naturally occurring radioactive minerals. When I started to look into the meaning of time, I found that both orthodox opinions are untested assumptions. Is natural radioactivity spontaneous? Since radioactivity can be induced in the laboratory by particle interactions, why should the natural radioactivity be different? Is the half-life of radioactive decay a constant? Why should the C-14 dating be unreliable when constant half-life is assumed? Have there been thorough experimental investigations of decay-half-lives? Chen Min-pen, Chao Kai-huang and I set up an experiment to explore the validity of the received opinion. We came to the paradoxical conclusion that natural radioactivity is not spontanous, and that the halflife of C-14 decay is not a constant. Beta decay, in our interpretation, involves a particle-particle interaction between a neutron and a W I M P (weakly interactive massive particle), and the W I M P is probably some kind of a cosmic neutrino. The experimental interpretation gives physical meaning to time. It takes time for a naturally radioctive element to decay. This flow of time is measured in terms of collisions per astronomic unit of time. The decay rate depends thus upon the flux of the W I M P causing the neutron decay. Time is not an illusion, it is an absolute entity, but the entity is only measurable in conjunction with events. We do not live in an arbitrary space-time world. We live in a real world where actions involving material things and flow of time. Mass and time are analogues. Mass is measured with reference to velocity or acceleration. Mass is absolute, but is only measurable within the framework of an action. I have a mass, you have a mass, Neil Armstrong has a mass. We stay always in the earth's gravitational field, and our mass at any given moment is a constant, expressible by our weight or by the momentum of our movement. Weight is not absolute, but relative to the gravitational attraction. Time is measured with reference to events. Time is absolute, but is only measurable within the framework of an action. The action of radiometric decay is measured by the rate of particle interactions, and the rate depends on the density or

The career of a frustrated conformist

451

flux of the interacting particles. One hundred million years determined radiometrically for the Cambrian Period may not have the same absolute duration as 100 million years for the Cretaceous, if the fluxes of the W I M P were signifcantly different. With this perception, I shall come to conclude this outbursts of a frustrated conformist. What is life? What is my life? Life is a continuous chain of actions of a self-organizing system. The system has its inception when an egg is fertilized. Actions synthesizing carbon dioxide and water to make sugar and to propel the continuity of the self-organizing system. The system ceases to be self-organizing when living organisms die, when dead bodies decay and are to become carbon dioxide and water following physical laws. What is my life? I thought about the question when I was asked to write an autobiography by the Commonwealth Publishers of Taipeh. I planned a trilogy: Search, Vanity, and Nirvana. The first of the trilogy is published: Search is a story of my youth (Hsfi, 1996c) The second of the trilogy is an expression of my taoistic conviction. Everything that I did as a frustrated conformist is Vanity. Who cares, after another century, for the paradoxes of " m o d e r n geology"? I have only been a self-organizing system, propelled by a "software p r o g r a m " to act in response to external actions. I have been fortunate. I have done many things, written many publications, filed more than a dozen patents. I have had successes, friends, and family. At the twilight of my life, I have suffered the loss of persons dear to me: friends, parents, and my first wife, among others. It is just a matter of time before I shall join them. The traditional Chinese philosophy seeps deeply into my being. Perhaps, Einstein is right. Time is an illusion. The world is an illusion. Life is an illusion and vanity. Nevertheless, I cannot have the privilege of achieving the Taoistic Wu-Wei (doing nothing). I thought I was to find peace after retirement and to write the third trilogypart of my life. But, no, I have no control over the self-organizing system that is I. I still go to my office every week day, try to market my patented technologies. I still have the compulsion to write. I still travel. More and more, I understand the wisdom of St. Paul: Life is a service to God, and death is the reward.

REFERENCES Hsfi, K.J., 1950. Evidence for the dating of Paleozoic orogenies of eastern North America. M.A. Thesis, Ohio State University, Columbus, Ohio, 259p. Hsfi, K.J., 1955. Granulites and Mylonites of the Region about Cucamonga and S. Antonio Canyons, San Gabriel Mountains, California. Univ. of Calif. Publ. in Geology, Vol. 30: Berkeley, CA, 129p. Hsfi, K.J., 1958. Isostasy and a theory for the origin of geosynclines. American Journal of Science, 256: 305-327. Hsfi, 1965. Isostasy, crustal thinning, mantle changes, and the disappearance of ancient land masses. American Journal of Science, 263: 97-109. Hsfi, 1966. Origin of dolomite in sedimentary sequence: a critical analysis. Mineralium Deposits, 2: 133-138.

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Hsfi, K.J., 1967. Mesozoic Geology of the California Coast Ranges- a new working hypothesis. In: J.P. Schaer, (Ed.), Etages Tectoniques, Baconni6re, Neuch~tel, pp. 279-296. HsiJ, K.J., 1969a. Role of cohesive strength in the mechanics of overthrust faulting and of land-sliding. Geological Soc. America Bulletin, 80: 927-952. Hsii, K.J., 1969b. A preliminary analysis of the statics and kinetics of the Glarus Overthrust. Ecologae Geologicae Helvetiae, 62: 143-154. Hs~i, K.J., 1972. Origin of saline giants: a critical review after the discovery of the mediterranean evaporite. Earth Science Rev., 8: 371-396. Hsfi, K.J., 1975. Catastrophic debris streams generated by rock falls. Geol. Soc. Am. Bull., 86: 129-140. Hsii, K.J., 1977. Studies of Ventura Field, California, I: facies geometry and genesis of Lower Pliocene turbidites. Amer. Assoc. Petrol. Geologists, 61: 137-168. Hsi.i, K.J., 1979. Thin-skin plate tectonics during Neo-alpine orogenesis. American Journal of Science, 279: 353-366. Hsfi, K.J., 1980. Terrestrial catastrophe caused by cometary impact at the end of Cretaceous. Nature, 285: 201-203. Hsfi K.J., 1981. Thin-skinned tectonic model for collision-type orogenesis. Sci. Sin., 24:100-110. Hsii, K.J., 1986. Darwin's three mistakes, Geology, 14: 532-534. HsiJ, K.J., 1988. Relic back-arc basins. In: Kleinspehn, K.L. and Paola, C. (Eds.), New Perspectives in Basin Analysis, Springer Verlag, N.Y., pp. 246-263. Hsfi, K.J., 1992. In search of physical theory of time. Proc. Nat. Acad. Sci., 89, 1022-1026. Hsfi, K.J., 1994. The Geology of Switzerland: an Introduction to Tectonic Facies. Princeton Univ. Press, Princeton, N.J., 250p. Hsfi, K.J., 1996a. In search of a common language. Institute For Advanced Study, Berlin, Jahrbuch, 87-95. Hsfi, K.J., 1996b. Are chronons the elementary particles in space and time? Terrest.Atm. and Oceanic Sci. (Taipeh), 7: 239-255. Hsfi, K.J., 1996c. Search and Aloneness (in Chinese). Commonwealth Publishers, Taipeh, 474p. Hsfi, K.J., 2000. Klima macht Geschicte. Orell Fiissli, Ziirich, 334p. HsiJ, K.J., 2001. Process and Apparatus for Recovery of Lithium in a Helminthoid Evaporator. (US Patent Appln. 08/844092). United States Patent 2001, in press. Hsii, K.J., Chen Haihong, et al., 1999. Geologic Atlas of China. Elsevier, Amsterdam, 24 plates and 362p. Hsfi, K.J. and Lloyd, R.M. 1972. Stable-Isotope Investigations of Sediments from the DSDP III Cruise to the South Atlantic. Sedimentology, 19: 45-58. HsiJ, K.J. and McKenzie, J.A., 1984. A "Strangelove" Ocean in the Earliest Tertiary. In: Sundquist, E.T. and Broecker, W.S. (Eds.) The Carbon Cycle and Atmospheric CO2, Natural Variations Archaen to Present. Am. Geoph. Union, Geoph. Mono. 32: pp. 487-492. Hsii, K.J., McKenzie, J.A. and Schneider, J., 1980. Movement of subsurface water under the sabkha, Abu Dhabi, UAE and its relation to dolomite genesis, SEPM Spec. Publ., no. 28, 11-30. Hsii, K.J., Ryan, W.B.F. and Cita, M.B., 1973. Late Miocene Desiccation of the Mediterranean, Nature, 242: 240-244. Hsfi, K.J. and Siegenthaler, C., 1969. Preliminary experiments on hydrodynamic movement induced by evaporation and their bearing on the dolomite problem. Sedimentology, 12:11-25. Hsii, K.J., Siegenthaler, C. and Kleboth, P., 1984. Longitudinal Transport of Turbidity C u r r e n t s - a Model Study of Horgen events. Sedimentology, 38: 187-194. Hsti, K.J., Sun Shu and Li Jiliang, 1988. Huanan Alps, not South China Platform. Academia Sinica Proc. Ser. B for 1988, 31: 109-119. Kelts, K. and Hsii, K.J., 1980. Resedimented facies of 1875 Horgen slumps in Lake Zfirich and a process model of longitudinal transport of turbidity currents. Eclogae geol. Helv., 73: 271-281. Mfiller, D.W., McKenzie, J.A. and Weissert, H. (Eds.), 1991. Controversies in Modern Geology. Academic Press, London, 490p. Miiller, W.H. and Hsii, K.J., 1980. Stress Distribution in overthrusting slabs and mechanics of Jura deformation. Rock Mechanics, Suppl. 9: pp. 219-232.

AUTHOR INDEX Aplonov, S., 113 Briegel, U., 231 Carman, Jr., M.F., 277 Chen Yancheng, 421 Chen, C.-T. A., 387 Chen, Haihong, 15 Chen, Min-Pen, 429 Cita, M.B., 353 Dickson, F.W., 269 Finckh, P., 101 He Qixiang, 361 Hsfi, K.J., 101,421,429, 439 Huang, Chao-Kai, 429 Indest, S., 277 Jiang Xinsheng, 55 Kilburn, C.R.J., 245

Li, Jiliang, 7, 15 Liao, Zongting, 39 Liu Yican, 323 Lu, Chia-Yu, 77 Luthi, S.M., 413 Pan Guitang, 55 Saklnq, M., 131 Seng6r, A.M.C., 131 Sun, Shu, l, 15 Wang Qingchen, 65 Winterer, E.L., 373 Wu Weiping, 323 Xiao, Wenjiao, 7, 15 Xu Qiang, 55 Xu Shutong, 323 Zhou, Xin-Hua, 343 Zhou, Zuyi, 39

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SUBJECT

INDEX Ankara M61ange, 185-186, 191, 198 Anorthoclase, 282-287, 289-290, 292-297, 303, 318 Antarctic glaciation, 383 Anthropogenic fluxes, 399 Anthropogenic input, 400 Anthropogenic lead, 406 Anticlinal traps, 416 Antineutrino, 430 Apatite, 282 Apennine limestone, 153 Apennines, 358, 382 Aplonov, Sergey, 447 Arc-trench-basin system, 343 Archipelago, 30 Archipelago model, 77-78, 107 Archipelago tectonics, 31 Arctic Shelf, 114, 121 Arduino, 152 Argand's Alps, 187 Argillite, 390 Arnaboll gneiss, 173 Arosa Schuppenzone, 187 Asia, 383-384, 416 Asia Minor, 186-189 Astrocyclostratigraphy, 353, 355 Atlantic, 353-354, 377, 382-383, 416 Augite, 312, 319 Australia, 30 Austroalpine Nappes, 445 Austroalpine basal thrust, 187

Aborted ocean, 113-115 Absorption, 423 Abu Dhabi, 442 Abyssal plain, 355 Accelerated evaporation, 426 Accelerator Mass Spectroscope, 393 Accretion of continents, 9 Accretionary complex, 24 Accretionary prisms, 200, 205, 335, 337, 339, 349 Acid-leached metals, 400 Acid rain, 368, 387, 399 Acoustic fluidisation, 260 hypothesis, 252 Active continental margin, 335, 337, 348-349 Active spreading ridge, 374-376 Adriatic Sea, 354 Adula Nappe, 110 Aegean Basin, 354 Aeolian dust, 397-398 Aeolian particle fluxes, 402 Africa, 416 Africa Plate, 354 Agricola, G., 149, 162-163 Ailaoshan, 21-22 Air-cushion hypothesis, 250 Air entrapment, 249, 251 Air pollution, 399 Airy model, 442-443 Alakol-Junggar-Turfan, 205 Albite, 280 Alboran Basin, 355 Alboran Sea, 357 Alemanide facies, 50-51, 67, 90 Alkali feldspars, 282 Alkaline igneous rocks, 278 Alkaline mafics, 277, 308 Alps, 102, 156-158, 173, 238-239, 247, 249 crustal structure under, 106 Altaids, 200, 205-207 Amargosa overthrust, 190 Amargosa Range, 189 Amphibolite facies, 323, 330, 334 Anatolian thrust, 185-187 Andean-Cordilleran belt, 335 Andes, 416 A'nemaqin Shan m61ange, 56, 58-60 Anglesey, 175, 179 Anglesey m~langes, 178 Anhedral pericline twinned anorthoclase, 284 Anhui, 24, 327, 340, 344, 346-349 Ankara, 179, 181, 185, 189

Ba-zoned orthoclase, 270-271 Bachmann, J., 189, 198 Backarc basin collapse, 16, 77, 446 Backarc basins, 11, 27-31, 55-56, 326, 348-349, 377, 379, 383 Backarc spreading, 77 Backbone Range, 77, 81-84, 87-90 Background variations, 432 Bacon, Francis, 440 Badoux, Heli, 189 Bagnold flow, 253 Bagnold Number, 259 Bailey, Edward, 184, 187-189 Baizhangyan, 329 Baizhangyan eclogite, 330 Bakewell, R., 156 Baldassari, 136 Balearic Basin, 354, 357 Baltzer, R. A., 170-173 455

456 Banda-Sunda arc, 77 Banded gneiss, 334 Banxi Group, 23-24 Baoshan, 21 Barents, 113, 115, 123, 127 Barrell, Joseph, 443 Basal layer, 256 Basal shear stress, 254-260 Basalt sediments, 382-383 Basalts, 24, 377, 379 Basement, 346 Basement basalt, 120 Basement density, 115 Bath, 135 Batholith, 335 Bathymetry, 379 Bayan Har foreland flysch basin, 55-56 Bayan Har Group, 59-62 Bayan Har Shan, 62 Baykalide orogenic system, 206 Belledonne Massif, 156, 158 Benioff Zone, 101-102, 107 Bergkeil, 149-150 Bertrand, Marcel, 232 Beta-decay, 430, 434, 450 rate, 431-432, 435 Betic Cordillera, 358 Big Bend, Texas, 277-278, 298, 315-319 Bingham fluid, 256, 259 Bingham interpretation, 261 Bingham plastic, 245 Biogenous sediments, 373, 376, 383 Biostratigraphy, 133, 134-145, 353 Biotite, 282, 297 Biozones, 143 Bipyramidal quartz, 270-271 Bird's eye structures, 355 Bischofite precipitates, 427 Black Mountains, 190 Black sediments, 392, 397 Black shale, 382 Blackhawk Landslide, California, 444 Bochiguo Formation, 60-61 Bohemian Massif, 166 Bonin Arc, 86 Borehole imaging tool, 414 Bosphorus-Dardanelles, 354 Bottom currents, 375 Bou6, 155 Boundary clay, 361-365, 369 Boundary layer rheology, 253-255, 263 Boundary layers, 256-264 Breccia, 165, 171, 178, 189-190 Briangonnais Swell, 101, 107, 110 Briegel, Ueli, 444 Brines, 421 in seawater, 417 British Strata, 135

Subject Index Brittle deformation, 236 Brittle-ductile transition, 233, 241 Brittle failure, 233 Brittle m61ange, 200 Brochant de Villiers, A.-J.-M., 153, 155-156 Broecker, Wally, 447 Broken formation, 176-177, 196, 198 Brongniart, Alexandre, 134, 138-140 Bulk deformation, 255-256 Bulk friction, 251 Bulk rheological properties, 261 Bulk strength, 240 Bfindnerschiefer, 89 Buqing Shan, 58 Burchfiel, B.S., 190 Burhan Budai-Ngola Shan, 56-58 Burhan Budai-Ngola Shan forearc, 60 Buridan, 152 Burma, 15, 21-22, 29 C-14 dating, 392, 394 C- 14 production rate, 431 Caesar, Julius, 407 Calabria, 165 Calcareous algae, 376 Calcareous nannofossils, 355 Calcareous plankton, 373 Calcareous planktonic foraminifers, 377 Calcite compensation depth, 379 Caledonia geosyncline, 343 "Caledonian" collision belt, 17 Caledonian fold belt, 39-41, 51 Caledonian magmatic arc belt, 58 California Coast Range, 381-382, 443 Callander Bay, Canada, 317 Caltanissetta Basin, Sicily, 353, 355, 357 Canyon cutting, 354 Cape Basin, 447 Capture Front, 314 Carbon-14 standards, 431 Carbon isotope gradient, 366 Carbon isotopes, fractionation, 367 Carbonate compensation depth, 373, 377, 382-384 Carbonate precipitation method, 423 Carbonate turbidite, 27 Carbonates, 399 Carrara marble, 153, 242 Carravaca, Spain, 365 Caspian Basin, 113-115, 123, 127 Cataclasis, 211 Cataclasites, 161, 194, 199, 205 ~atal H6yfik, 145 Cathaysia, 39, 41-42, 50, 56, 343 Cathaysian remnant arc, 15, 23-25, 29, 32 CCD, 373, 377, 382-384 Celtide facies, 42, 50-51, 67, 90 Chain reactions, 430

Subject Index Changhua thrust, 90 Changmahe, 60 Changning-Menglian m61ange, 15, 22, 27, 32 Chao Kai-huang, 450 Chaos structures, 161, 175, 189-191, 196, 198 Chaput, E., 179 Chemical potentials, 271 Chemical weathering, 415 Chen, Arthur, 448 Chen Min-pen, 450 Chen Yancheng, 447-448 Chencai Complex, 24-25 Chile, 423 Chilled margins, 297 Chimei Formation, 85 China, 446 tectonic evolution, 447 Chronons, 429-430, 434 Chrysologue de Gy, A., 141 Cimmerian continent, 33 Circum-global seaway, 383 Cita, Maria, 445 Clastic sediments, 344 Clay minerals, 399 Climate, 395 Climatic changes, 393 Climatic fluctuations, 395 Clinopyroxenes, 304 Coal beds, 157 Coastal Plain, 82 Coastal Range, 80-81, 84, 94 Coccolithophorids, 373, 376-377, 383-384 Cold fusion, 435 Collapsed backarc basin, 28 Colliding fragments, 252 Collision, 39 inelastic, 252 Collision tectonics, 343 Collisional model, 260 Collisional orogens, 205, 326 Compatible strain, 192 Compositional zoning, 297, 303, 318 Concentration gradients, 271 Concordia age, 330 Confining pressure, 233, 297 Confucius, 439 Conglomerates, 355 Continental collision, 15 Continental margins, 383 Continental rifting, 114, 120 Continents, accretion of, 9 Contraction cracks, 315 Convective overturn, 272 Cooling behaviour, modelling, 310 Core-mantle boundary, 273 Corundum, 330 Cosmic counting rate, 429 Cosmic rays, 429, 434

457 intensity, 432 Costa Rica, 381-382 Coulomb relation, for sliding, 256 Cratonic blocks, 113-114 Cratonization, 9 Crude oil, 416 Crush breccia, 175 Crust, 400, 402 thinning, 8 Crustal density, 115 Crustal residence age, 345, 348-349 Crustal structure, under the Alps, 106 Crystal plasticity, 174, 194 Crystal settling, 312 rates, 311 Crystallization experiments, 283 history, 277, 290-297 paths, 291-292 rate, 282, 312 sequence, 283 Crystallizing magma, 310 Cuvier, Georges, 134, 136-141, 210 Cycled stress energy, 269-270 Cycling liquefaction energies, 272 Cylindroidal forms, 315-316 Cyprus, 381 Czech Republic, 162 Dabie Mountains, 17, 67, 323-324 Dabie orogenic belt, 66 Dabie orogenic collage, 18 Dabie-Sulu orogenic belt, 65, 74 Dabie-Tanlu-Sulu orogenic collage, 16, 21, 31 Dabie UHP-HP complex, 71 Dalziel, Ian, 446 Dana, James, 8 Daoshichong pyroxenite, 327, 337 Darwin, Charles, 140, 151,440, 449 Darwinism, 362 Da'tian, 43 Datong shan-Huan Yuan uplift, 56 Daubr6e, Auguste, 153-155, 157, 159, 172 Davies, Guy, 448 De La Beche, 140 De Luc, J.A., 149-150 Dead Sea, 421,423 Death Valley, 189, 191 Debris-flow, 198, 253 Decay rates, 430-431, 434435 Decollement, 241-242 Decollement zone, 231,235, 238, 241 Deep-sea fans, 354 Deformation, 240-242, 247, 249, 252, 339, 380 of Bingham fluid, 259 rate, 245, 253, 256, 259, 263 velocity, 252 Deformation mechanisms, 236-237, 240

458 Dege-Yidun island arc, 56 Dehydrating gypsum, 242 Dehydration, 240-241 Dengeryoungqu, 62 Depleted oil fields, 417 Derbyshire, 164 Descending slabs, 339 Desiccation, 354 model, 353, 355 Detachment horizon, 234 Deviated drilling, 414 Diabasic dikes, 379, 381 Diagenesis, 196 Diapirs, 380 Diatomaceous sediments, 376 Diatoms, 376 Dickson, Frank, 442 Differential diffusion, 271 Differential stress, 236 Differentiating liquid, 306 Differentiation, 277 Diffusion, 283 rate, 283 Diffusion-controlled creep, 242 Diffusion-controlled growth, 271 Dinosaurs, 362 extinction, 363 Disequilibria, 269, 272-273 Disequilibrium textures, 271 Dislocation creep, 241-242 Dislocation density, 237 Displacement, 200-202, 205 Disrupted formations, 196 Dissolution, 382 Dissolution facies, 355 Dissolution-precipitation reactions, 269-270 Dolomite, 442 Dolomitisation, 155 Dongnanya block, 15, 47, 50, 94 Dry periods, 395 DSDP cores, 81 Ductile deformation, of feldspar, 334 Dufr6noy, P.-A.-O., 156 Dunde Ice Core, of the Qinghai-Tibetan Plateau, 395 Durness-Eriboll, 173 Dust, production, 407 Dust fall frequency, 396 Dust storms, 396-397 Dykes, 150-151, 162, 163, 165, 166-167, 210 Dynamic recrystallization, 242 Earthquakes, 165, 199 East Asia, 51 Eastern Fujian Celtide, 47 Eastern Tibet subduction block, 56 Easy slip, 242 Ebro, 354

Subject Index Eclogite, 323, 326-327, 329, 333-334, 337, 339 Eclogite belt, 324, 340 Eclogitic facies, 323, 326, 329 metamorphism, 330, 333, 336 Effective capping, 249 Effective pressure, 241 concept of, 234-235 Effective rheology, 260-261 Effective size, of fragments, 251 Effective stresses, 242 Egypt, 146 Elastic strength, 233 l~lie de Beaumont, L., 155-157, 158, 231 Elm, 249, 259 Elongation component, 194 Emigrant Canyon, 191 Emplacement time, 261 Endothermic dissolution, 269, 271-273 Energetic collisions, 251 Energy dissipation, 252-254, 259-260 Enhanced oil recovery, 418, 448 Entherolitic folding, 355 Entropy increases, 272 Entropy production, 269 Environmental catastrophe, 369 Environmental changes, 365 Eo-Alpine, 106 Epitaxial mineral growth, 271,277 Eratosthenes Seamount, 355 Erlangping Group, 326 Erol, O., 181, 184, 189 Erosion, 398 Erosional surface, 355 Erzgebirge, 166 Escher von der Linth, Arnold, 167-168, 170-172, 232 Eugeosyncline, 11 Euhedral feldspar, 280, 284 Eurasia, 15-16, 23, 30, 32-33, 77-78, 80, 91 Eurasia plate, 354 Eurasian margin, 39 Euroamerica, 129 Europe, 383 Europium anomaly, 306 Evaporation, 422, 424, 426 Evaporative pumping, 448 Evaporites, 354-355, 357 Evaporitic drawdown, 358 Exhumation, 323, 330, 333, 337, 339 Exothermic crystallization energies, 271-273 Exothermic precipitation, 269 Experimental equilibria, 273 Experimental rock deformation, 235-238 Experimental studies, 277 Exploration, success rates, 415 Exploration wells, 414 Extension, 202 Extensional rifting, 379

Subject Index Extensional subsidence, 8 Extinction, 361 of dinosaurs, 363 Extraterrestrial bodies, 362 Extraterrestrial impact, 361,368-369 Farey, John, 134 Fault buckling, 200 Fault gouges, 132, 200, 202 Fault plane, 199 Fault rocks, 161, 175, 200-210 Fault zones, 131, 198, 200-201,205 Faunal mass extinction, 361 Feldspar phases, 277, 283, 290 zoning, 290 zoning paths, 293-294, 296 Feldspars, 307 crystallization history, 290-296 ductile deformation, 334 Fengqinglai, 59 Fermi, 430 Fernschubhypothese, 238 Ferroaugite, 297 Finckh, Peter, 446 Finite element analysis, of thrust-block generation, 238-239 Finite element method, 235 Fish Clay, 364 Fission chain reactions, 435 Flims, 247 Florence Rise, 355 Flow laws, 236, 238 Flow m61ange, 200 Fluid deformation, 253 Fluid injection, 200 Fluid pressures, 241-242 Fluidisation, 260 Flux, of metals, 398-406 Flysch, 22, 24, 27, 29, 39, 43, 48, 51, 55, 59-60, 63, 85, 88-89, 170, 181, 187 Flysch nappes, 90 Foothills thrust, 90 Foram-nannofossil limestone, 374 Foraminifera, 376 Forearc accretion, 16, 24 Forearc basin, 11,326 Forearc regions, 200 Forearc setting, 381 Forearcs, 381 Foreland basins, 43, 51, 55 Foreland belts, 339 Foreland fold-and-thrust belts, 16 Foziling Group, 337 Fractal patterns, 270 Fractional crystallization, 301,304, 315, 319, 424 Fractionating crystals, 296 Fractionation, of carbon isotopes, 367 Fractionation path, 277

459 Fragmental flow, 252 Fragments effective size, 251 velocities, 252 Franciscan Complex, 189 Franciscan M61ange, 85, 339 Franciscan rocks, 443 Free-neutron decay, 430 French Alps, 110 Friction coefficients, 251,263 Frictional effects, 251,253 Front Range Thrust, 90 Frontal arc, 31 Fu'an-Nan'an fault, 48 Fu'an-Nan'jin fault, 40 Fuji-Hakune, 86 Fujian, 25, 29, 31, 39, 40-43, 45-46, 48, 50-51, 346 Fujian foreland basin, 42, 44-45, 48 Fujian foreland FTB, 24-25, 47 Fujian m61ange, 15, 23, 32 Gabbros, 379-381 Galapagos Archipelago, 151 Gansser, A., 189 Gaoligong Mountains, 21-22 Garnets, 326 Garze-Litang belt, 55, 59 Garze-Litang suture, 56 Gas hydrates, 416 Gavdos Basin, Greece, 353, 355 Geological engineering, 419 Geophysical basis, of plate tectonics, 443 Geosynclines, 7, 9-12, 41, 55, 188, 441 Gibraltar Strait, 354 Ginsburg, Bob, 439 Glarus, 170 Glarus Alps, 243 Glarus overthrust, 232, 235-236 Global warming, 368 Glomar Challenger, 446 Glossopteris, 22 Gneiss, 335-336, 339 Gondwana, 15, 22-23, 29, 32, 46, 383-384 Gonghe backarc basin, 56, 60 Gongjialing harzburgite, 327 Gornostaev shear belt, 205 Goulushankeco, 62 Grain boundary migration, 242 Grain boundary sliding, 194 Grain-dispersive pressure, 251 Grain size, 236-237 Granite genesis, 272 Granite problem, 269, 272-273 Granite system, 291 Granites, 45, 335 Granitoid classification, 347 Granitoids, 343-344, 346-347

Subject Index

460 Granodiorites, 335 Granular flows, 252-253, 259 model, 251 Granulite facies, 323,326, 329-330, 333-334, 336 Granulitgebirge, 166 Gravitational acceleration, 254 Gravitational fields, 269-270, 272-273 Gravity, 115 Gravity sliding, 444 Great dying, 362-363 Great Ghost Lake, 388 Greenhouse effect, 368 Greenland, 383 Greenland Sea, 376 Greenly, Edward, 175-179, 185, 189 Groundmass grains, 284, 288-289 Guangdong, 48 Guangji, 71 Guangxi, 24, 27-29 Gubbio, 362-365, 369 Guhuadong Trough, 94 Guizhou, 27 Gulangti Formation, 60 Gulf of California, 376-377 Gulf of Mexico, 383 Gunanhai M61ange, 93-94 Gutaiwan Arc, 82, 90, 94 Hainan block, 15, 23, 30, 32 Hainan Island, 29, 46-47 Haizhou, 72 Half-life, 429 Hall, James, 8, 153, 155 Harmonic sedimentary basins, 125, 129 Harmonic sedimentation, 127 Harzburgite, 329 Heat flow, 413 Hefei basin, 324, 326 Heim, Albert, 170, 171-172, 173, 232, 249, 250-251 Helminthoid evaporator, 421,428 Helminthoid pool, 425 Helvetic Passive Margin, 110 Hengchun Peninsula, 80, 84 Hercynian fold belt, 41-42, 51 Hercynian-Indosinian fold belt, 42 "Hercynian" intracontinental deformation, 17 Hercynian magmatic arc belt, 58 "Hercynian" movement, 40 High-resolution seismic lines, 414 Hob Xil-Jingshajiang suture, 55-56 Hob Xil-Zhidoi-Shiqu-Garze, 62 Hokkaido, 50 Hong Kong, 48 Hongshuichuan Formation, 58, 60 Hot spots, 273 Howford Bridge, Scotland, 318 Hsfi, Ken J., 189, 23!, 415-419,424, 428,439-451

Hsfiehshan Range, 81-84, 89 Huanan Alps, 39, 446 Huang Jiqing, 447 Huangshi, 339 Huangshi-Wuhan foreland FTB, 18 Huangweihe, Yuexi County, 329 Huangweihe eclogite, 329-330 Huashixia, 58 Huatung Basin, 80 Huazhuang, Shucheng County, 329 Huazhuang eclogite, 330 Hubbert, M. King, 417-419, 443-444 Hubei Province, 339 Hunan, 24, 29 Huoshan, 339 Hutton, James, 149, 150-155, 158, 440, 449 Hydraulic fracturing, 417 Hydrocarbon accumulation zones, 125 Hydrocarbon-bearing zone, 414 Hydrocarbon potential, 414 Hydrologic cells, 417 Hydrothermal activity, 271 Hydrothermal deposits, 375, 381 Hypersolvus crystallization, 291 Hypolimnion, 406 Iceland, 379 Igneous minerals, 269 Igneous rocks, 148-151,210, 345 Impact, extraterrestrial, 361 Impact pressures, 252 Indian Ocean, 377, 380, 382 Indonesia, 30 Indonesian Archipelago, 78, 447 Indosinian fold belt, 42 "Indosinian" movement, 39-40 "Indosinian" orogeny, 48, 51 Industrialisation age, 401 Initial fragmentation, 260 In-situ reactor, 417 Intergranular reactions, 270

International Stratigraphic Guide, 208 Interstitial fluid, 251,253, 259, 314, 317, 319 Intracontinental foldbelts, 205 Intraplate orogenesis, 77 Intrusions, 278, 282, 291,296, 298, 312, 315-316, 318-319, 334 Intrusive bodies, 198 Intrusive concepts, 270 Intrusive mechanisms, 269 Inverse mountains, 273 Inyo Mountains, 270 Ion-exchange resin method, 423 Ionian Basin, 354, 357 Iridium anomaly, 446 Iridium spike, 366 Irtysh shear belt, 205 Island-arc volcanism, 348

Subject Index Island arcs, 11, 17, 24, 42, 335-337, 349, 380-381, 383 Isle of Man, 175 Isostasy, 442 Isostatic subsidence, 114-115 Isotopic ages, 339 Isotopic signature, 348 Isotopic stratigraphy, 355 James Island, 151 Japan, 31, 50 Jianda-Weixi, 62 Jiangda-Deqen volcanic arc, 56 Jiangnan block, 24 Jiangnan remnant arc, 15, 23-25, 28-29, 32 Jiangxi, 24, 28, 346, 348 Jiaodong terrane, 71 Jiaoziyan pyroxenite, 327, 337 Jingangshan Mountains, 28 Jinggangshan, 24 Jinghong m61ange zone, 27 Jinghong metamorphic complex, 22 Jinshajiang, 62 Jinshajiang suture, 56 JOIDES Planning Commission, 445 Josephine ophiolite, 382 Juan de Fuca ridge, 376 Junggar, 113-114, 207, 447 Jura, 240 Jura decollement, 238 Jura Mountains, 407 Juvenile crustal mixture, 346 Juvenile materials, 348-349 K/T boundary, 363-365, 367, 446-447 Kara Basin, 113 Kara Sea, 115 Kara Shelf, 113, 123 Karakoram, 248 K/irpfstock, 171 Katili, 200 Keiroliths, 131,201-202, 205 Kelts, Kerry, 441,448 Kelvin, Lord, 413 Kenan Group, 62 Kenting M61ange, 80, 86-87, 89, 94 Ketin, Ihsan, 187-189 Kilauea Lava Lake, 314 Kilburn, Chris, 445 Kinsman, Dave, 442 Kir~ehir crystallines, 186-188 Kir~ehir synclinal, 185 K6fels, 247 Kuhn, T.S., 440 Kunlun, 10, 57 Kunlun arc, 56, 60 Lacustrine sediments, 355

461 Lake-area fluctuations, 396-397 Lake sediments, 391 Lake Zfirich, 441 Laminations, 390-392, 394-395, 397-398 Landslides, 165, 245, 251-252, 254, 261,393-394 behaviour, 253 size, 250 volume, 259 Laolu, 422, 424, 426-427 Lapworth, Charles, 173 Laurasia, 55 Lava flows, 376 Lava-lake roofs, 375 Leading edge, of descending slabs, 339 Leadville, Colorado, 270 Lehmann, Johann Gottlob, 149 Lengjiaxi Group, 23 Levantine Basin, 354, 357 Li Jiliang, 443 Liaoning, 72 Lichi M61ange, 84-85, 89, 94 Life, 451 Lincang remnant arc, 15, 22, 27, 31-32 Lincoln quartz monzonite porphyry, 270 Liquid fractionation lines, 292-294, 296 Liquid fuel reactors, 422 Lishan Fault, 77, 91 Lithium, 422, 424, 448 Lithodemic units, 205 Lithosphere, 379 Lithospheric plates, 443 Lithozone, 143 Little Ice Age, 401 Lloyd, Mike, 446 Localisation, 242 Lochseitenkalk, 170-172, 235-236 Loess, 397 Logging-while-drilling tools, 414 Longitudinal Valley, 80, 84 Longjingguan plagioclase amphibolite, 333 Longmen Shan foreland overthrust belt, 56 Longmenshan belt, 61 Longwuhe Group, 58, 60 Longyan, 46 Lower Yangtze terrane, 71 Lujiang, 70 Luotian, 327 Lushan "Formation", 89 L~thi, Stefan, 448 Luxi arc, 70 Luzhenguan Group, 337 Luzon Arc, 78, 80-81, 84 Lyell, C., 140, 157-159, 167-168, 175, 440, 449 M-mesons, 434 Maastricht, 362 Maastrichtian fauna, 363 McCallien, W. James, 179, 181, 184, 188-189

462 MacCulloch, J., 154 McKenzie, Judy, 442, 446 Macroviscous flow, 260 Mafic magmas, 277, 316 Mafic sills, 297-318 Maggia Nappe, 194 Magma, 311,313-314, 316, 379 intruding, 318 rapidly cooling, 290 Magma chamber, 282, 291,296-297, 318 Magma viscosities, 311 Magmatic arc, 28 Magmatic crystallization, 271 Magmatic differentiation, 304, 307, 312, 319 Magmatic evolution, 297 Magmatic rock, 158 Magnetic anomalies, 113, 120-121, 123, 380 Magnetic reversal, 379 Magnetite, 282 Magnetometer, 413 Magnetostratigraphy, 353, 355 Magnitude deviations, 397 Majixueshan, 58-59 Manila Trench, 78, 80, 87, 89, 94 Manshuihe, 335 Mantle abnormal, upwelling, 8 partial melting in, 379 Mantle convection cells, 273 Mantle-core interactions, 273 Mantle plumes, 273 Mantle strength, 443 Maohuayan, Tongcheng County, 329 Maohuayan eclogite, 330 Maqen, 58 Marginal basin, 348 Mariana arc, 77, 81, 93 Mariana Ridge, 81 Marin Headlands, California, 382 Marzari-Pencati, Joseph, 154 Mass extinction, 361-362, 367 Mass spectrometers, 413 Matapan Trench, 354 Matley, 175 Matrix, 334, 339 Mature fault zone, 200 Mazu, 48 Mechanism change, 242 Mechanisms of rearrangement, 271 Median Tectonic Line, 50 Mediterranean, 30, 353-355, 357-358, 384, 445 Mediterranean evaporite, 445 Mediterranean Tethys, 383 Meishan, 326 M61ange, 11, 22-25, 27-29, 31, 56, 84-85, 131-132, 162, 175, 177-179, 184-185, 187-189, 191-192, 196, 198, 200, 205, 326, 339-340, 344, 346-349, 383, 443

Subject Index M61ange diapirism, 200, 202 Melts, 273 Menorca, 355 Mesozoic orogeny, 16 Meta-ophiolite m61ange, 337 Meta-volcanic rocks, 348 Metal association, 399 Metals concentrations of, 401 distribution of, 399 flux of, 398-406 Metamorphic front, 160 Metamorphic m~lange, 200 Metamorphism, 152, 155-159, 161, 170, 172, 173, 191, 196, 200, 211,323, 336, 339, 345 Metamorphosed limestone, 155 Metamorphosis, 154 Metasediments, 346-349 Meteorites, 364 Microcontinent collisional collage, 17-18 Microsyenite, 278 Microtektite, 364 Mid-ocean ridge, 11, 120 Mid-ocean ridge basalt, 380 Middle East, 417 Miocene accretionary wedge, 90 Miogeosyncline, 11 Model ages, 345 Mohave Desert, California, 423 Moho, 101-102, 115 Moho detachment, 101 Mohr-Coulomb envelopes, 233, 235 Mointhrust, 232 Moisture index, 397 M ojiang metamorphic belt, 22 Molasse, 25, 39, 43, 48, 51 Molasse basin, 238 Momentum, 430 Mona Complex, 178 Monteregian rocks, 317 Moores, Eldredge, 448 MORB, 120 Moro, A. Lazzaro, 148 Morocco, 357 Morotu District, Sakhalin, 318 Mountain building, 398 Mountain lakes, 387 Mozitan-Xiaotian fault zone, 324, 327 Mud flows, 253, 393, 394 Mud pulses, 414 MUller, Stephan, 445-446 MUller, Walter, 444 Multi-component liquid, 315 Multiphase feldspars, 280 Murchison, Roderick I., 170, 173, 232 Mush Zone, 314 Mylonite bands, 334

Subject Index Mylonites, 132, 161, 173-174, 194, 200, 205, 210, 236, 323, 327, 329, 339, 444 Mylonitic gneisses, 200 Mylonitized amphibolite, 333 Nan'ao, 48 Nanjing, 70 Nanofossils, 365 Nanoplankton, 367 Nanpanjiang, 24, 27, 29 Nanwan Formation, 324 Nanz, Bob, 441 Naochanjiangou Formation, 60 Natural hazards, 407 Natural selection, 361 Nd isotope, 344, 348 Nd isotope ratios, 346 Near-basement sediments, 374 Neo-Alpine, 106-107 Neutrino captures, 430 Neutrino flux, 430-432, 435 Neutrino-neutron interactions, 429-430, 435 Neutrinos, 429-430, 434 Neutron capture, 435 Neutrons, 434-435 New Guinea, 377 New Mexico, 364 New ocean crust, 377 Nicol, James, 173 Nicoya Peninsula, Costa Rica, 382 Nile, 354 Noble, Levi, 189-190 Nomads, northern, 407 Normal faulting, 165 North America, 383 North China block, 16-19, 21, 31, 55 North China Craton, 10 North Dabie Complex, 330 North Luzon trough, 78 NRP 20 seismic reflection lines, 104, 111 Nucleation rate, 282, 296 Oblique subduction, 200 Obsky Paleocean, 113-115, 120-121, 123, 127 Ocean-dipping Benioff zone, 77 Oceanic arcs, 381 Oceanic basement, 374 Oceanic basin, 11 Oceanic crust, 333, 373-376 formation, 399 Oceanic island arc, 11 Oceanic lithosphere, 373, 379, 381 Oceanic ridge, 202 Oil fields, depleted, 417 Oil recovery, enhanced, 418, 448 Oil shales, 416 Oisans, 158 Okinawa Trough, 80

463 Oligoclase, 318 Oligotrophic lakes, 392 Olistostromes, 198 Olivine, 312 Oman ophiolite, 380-381 Ophicalcites, 198 Ophiolite, 22, 27, 59, 86, 335, 373-374, 380-383 Ophiolitic m61ange, 24 Ophitic patches, 314, 318-319 Ordinary space-time, concept of, 435 Organic extraction method, 423 Organic matter, 388, 390-395, 397, 399-400 Orogenesis, archipelago model, 107 Ortolevan reaction cells, 270 Oscillatory zoning, 271-272, 283, 290, 296 Otago, New Zealand, 317 Overriding plate, 335, 337 Overthrust blocks, 234 Overthrust conditions, 236 Oxygenation, 382 Ozone layer, 368 p-mesons, 434 Pacific, 77, 365, 377, 382-383 Pacific plate, 39, 326 Palau-Kyushu Ridge, 50 Palau/Ryukyu, 81 Paleo-carbonate compensation depth, 381 Paleo-island arc, 348 Paleo-Mariana Arc, 81 Paleoceanography, 354 Paleogeography, 354 Paleomagnetic investigations, 29, 31, 33, 67 Paleomagnetic stratigraphy, 363 Paleotemperature-meter, 363 Paleotethys, 22, 27, 55 Pangea, 127, 129 Papoose Flat, 270-271 Paris Basin, 134, 138 Partial melting, in the mantle, 379 Particle interactions, 429-430, 435 Passive continental margin, 8, 11,337, 354 Passive-margin model, 416 Pauli, Wolfgang, 430 Pearl River Mouth Basin, 48 Pelagic sediments, 373-374, 376-377 Pennine Alps, 89, 107 Penninic basin, 101 Penninic Nappes, 445 Pentraeth Inliers, 178 Pericline, 280 Permeability, 241-242 Petroleum exploration, 123 Phase chemistry, 278 Philippine Sea plate, 77-78, 80, 89, 91, 94 Photosynthesis, 368 Photosynthesis-respiration mechanism, 365 Physical theory of time, 449

Subject Index

464 Phytoplankton, 392, 394-395, 397-398 Pillow basalts, 27, 379-381 Pillow breccias, 375 Pillow lava, 179, 181 Pilushan "Formation", 89 Pine Mountains, 242 Pingtung Valley, 87 Plagioclase, 282-287, 290, 292-297, 299, 302, 306, 312, 319 Plagioclase amphibolites, 333, 336-337 Plankton production, 366 Plastic deformation, 174, 333 Plate Boundary Thrust, 91 Plate motion, 7 Plate tectonics, geophysical basis, 443 Pliocene turbidites, 441 Plutonic rocks, 151 Pollution, local, 406 Polycomponent silicate liquids, 272 Polycyclic deformation, theory of, 447 Pontic ranges, 187 Popper, Karl, 440 Population declines, 407 Pore pressure, 240 Porosity, 242 Powell, J.W., 172 Pratt model, 443 Precipitation fluctuation, 397 Precipitation index, 394 Precipitation reactions, 269-270 Predazzo, 154 Pressure solution, 195 Protolith, 334-335, 337 Protons, 434 Pseudoplastic behaviour, 260 Pull-apart basins, 70 Pure shear, 194-196 Pyroxene, 282 Qaidam, 113-114, 447 Qaidam Basin, 422, 448 Qaidam block, 57 Qamdo backarc foreland, 56 Qarhan brines, 423, 427 Qarhan Lake, Qinghai, 422, 425 Qiangtang block, 55 Qiangtang-Taniantawong front, 55 Qichun, 72 Qin-Fang, 28 Qinghai, 422-425 Qinghai Nanshan, 58 Qinghai-Tibetan Plateau, 395 Qingshan, 327 Qinling Group, 327 Qinling-Qilian-Kunlun oceanic plate, 55 Qinnidong, 62 Qishici harzburgite, 327 Quanzhou, 49

Quartz, 295 Quartz microsyenite plugs, 277 Quartz-sanidine boundary, 297 Quartz-sanidine surface, 295 Quartz syenitic magma, 318 Quartzite, 327 Quebec, 380 Radioactive decays, 429 Radiolarians, 373, 376-377 Radiolarites, 24 on pillows, 382 Radiometric dating, 362 Rainfall, low, 397 Raobazhai, Huoshan County, 329 Raobazhai eclogite, 330 Raobazhai harzburgite, 327 Raton Basin, 364 Rattlesnake Mountain Sill, 297-298, 303, 307-308, 310-316, 319 Rayleigh fractionation, 306 Reaction cells, 273 Read, H.H., 159, 211 Rearrangement mechanisms, 271 Recovery rates, 415 Recycled materials, 348 Reef, 27 Reimarus, J.A.H., 138 Reiqiong Strait, 47 Relative Brightness Index, 391 Relativity, theory of, 435 Relict back-arc basins, 113-114 Remnant arcs, 21 Remnant magnetization, 121 Reservoir depletion, 414 Reservoir development, 414 Residual liquid, 314-316 movement, 312 Resisting stress, 255 Resistivity measurement, 414 Retrogressive metamorphism, 323, 326 Reverse shear sense, 194 Reykjanes Ridge, 379 Reynolds Number, 259 Rhaetide facies, 42, 51, 67, 71, 90 Rheology, 252, 256-257, 261,264 effective, 260-261 Rhone, 354 Richardson, Benjamin, 135 Ridge axes, 379 Ridge basalt, 376 Ridge-transform fault, 194 Rift valley, 11 Rifted continental crust, 114 Rock deformation, 231 experimental, 235-238 Rock melting, 252 Rocks, physical properties, 231,235

Subject Index Rodgers, John, 443, 448 Rootless rock blocks, 323 Row-wise deformation, 194 Rubey, W.W., 443-444 Runout lengths, 258, 260-263 Russian Arctic Shelf, 113 Ryan, William, 445 Ryukyu Arc, 80 Ryukyu Trench, 80 Sabkha dolomites, 442 Saint-Hilaire, E. Geoffroy, 136 Salinity Crisis, 355, 358 Salt lakes, 422 San Andreas Fault, 198-199 San Francisco Bay, 199 San Hippolito, Baja California, 382 San Juan Basin, New Mexico, 368 Sanidine, 280,282-287,289-290,292-297,303, 318 Sardinia Margin, 355 Saxony, 162, 231 Schams, 101, 110 Schwarzenberg, 165 Scintillation counter, 431,434 Scotland, 154 Scottish Highlands, 173, 175, 189 Seafloor spreading, 383 Sedimentary rocks, 146-148, 151, 157, 196, 210 Sedimentation cyclicity curve, 127 Sedimentation rate, 127, 394 Sediments, maturity of, 415-4 16 Sediments atop ophiolites, 380 Segregation, 314, 316 4-D seismic monitoring, 414 Seismic reflection, 354, 379 Selective dissolution, 377 Selective preservation, 377 Self-organizing system, of life, 451 Seneca, 148 Seng6r, Celal, 191 Sernf Valley, 167 Serpentine m61ange, 42 Serpentinites, 375, 379-380 Shackleton, Nick, 446-447 Shacun pyroxenite, 327, 337 Shan-Thai-Malay block, 15, 21-22, 27, 29-30, 32 Shandong, 67 Shangxi Group, 346-347 Shaunxiwu Group, 348 Shear planes, 131,240 Shear strains, 192, 205 Shear stresses, 236, 315 Shear zones, 131, 133, 166-167, 172, 176, 194, 200, 205, 210, 241-242, 334 Shelton, John, 441 Shilu m~lange, 23, 29, 32 Shock quartz, 364, 368

465 Shonkin Sag, 318 Shreve, Ron, 444 Shuanghe, 333 Shuangqiaoshan Group, 24, 346-347 Shuihou-Wuhe shear zone, 324 Sibao Group, 23 Siberia, 113-115, 120-121, 127, 136, 140 Siberian High, 396 Sichuan, 423 Sichuan foreland basin, 56 Sicilian Basin, Central, 358 Sicily, 205 Sicily Channel, 354 Siegenthaler, Chris, 442, 444 Silica saturation, 292 Silicate liquids, 269 Simao, 15 Simao block, 22, 32 Simao remnant arc, 21, 27, 30 Simple shear, 192, 195-196, 240 Sino-Korean block, 15 Sino-Korean Craton, 65, 67-68, 70-74 Sino-Korean plate, 324, 326, 330, 336-337, 339 Slate, 390 Slope breccias, 375 Smith, William, 133-136, 140-141,440, 449 Smoluchovsky, M.S., 232 Solar-neutrino flux, 433 Solidification rate, 315 of the sill, 310, 315 Solidification Front, 311, 313-314, 319 Solnhofen limestone, 233, 242 Solutions, 273 Somersetshire Coal Canal, 134-135 Song Da, 45 Song Ma, 45 Songpan-Gartze deformed belt, 15, 55-56, 60 Songpan-Gartze Flysch, 447 Sorbas Basin, Spain, 353, 355 Source material, 345-347, 349, 406 South America, 30 South Atlantic, 365 South China, 15-16, 23-24, 27, 30, 32, 45, 52, 81, 343-344, 346-347 South China archipelago, 23, 25, 29-30 South China block, 18-19, 21, 33, 39 South China granites, 446 South China Plate, 29 South China Sea, 45, 48, 80-82, 89, 93 South China Sea block, 39 South East China, 40-42 South East China block, 50 South East China orogenic collage, 16 Southeast Asia, 30, 45-46 Space-time, concept of, 435 Spieker, Ed, 440

466 Spinel, 330 Spontaneous decay, 430 Spreading axes, 381 Spreading centres, 121,201,377, 380, 382 Spreading ocean ridges, 376-377, 379, 384 Spreading rates, 379 Square Butte, Montana, 318 Standard epsilon notation, 345 Staub's Alps, 187 Steinmann trinity, 187 Stenonis, N., 134, 147-148 Stevns Klint, 364 Stick-slip, 200 Strain, 242 Strain rates, 232, 236-237, 241-242 Strain softening, 242 Strangelove ocean, 361,366-368, 447 Stratigraphic nomenclature, 205 Stratigraphic principles, 151 Stratigraphic traps, 416 Stratigraphy, 212 Strength, 240, 242 Stress, 237, 242, 271 Stress drops, 233 Stress energies, 271 Strike-slip faulting, 174, 200 Structural rocks, 161-192 types, 192-210 Sturzstroms, 245-247, 250-252, 254, 259, 261 Subduction, 11, 39, 189, 200, 327, 330, 339, 383 Subduction zones, 191,205, 324, 380-381 Suess' Alps, 187 Sulu, 18 Sulu UHP-HP complex, 71 Sun Shu, 439, 447 Sunspot activity, 433 Supercooling, 282, 297 Superplastic creep, 242 Superplastic flow, 236, 444 Supersaturation, 426 Supra-subduction zones, 373, 381 Susong, 72 Susong FTB, 19 Susong Group, 326 Suspended clays, 376 Suspension Zone, 314 Sutherland mica-schists, 173 Sutures, 324, 335, 337, 339 Swiss Alps, 101 Syenite, 282, 301-302, 304, 306-315, 318-319 Syenitic bodies, 277, 297-318 Syenitic liquids, 307 Syenitic ocelli, 305, 310, 316-317 Syenodiorites, 298-300, 303-304, 306, 309, 311, 314-315, 318-319 Syenograbbros, 298 Symplectite, 330 Synthetic rocks, 240

Subject Index Ta'erhe, 335 Taiwan, 47, 50, 77-80 Central Range, 50, 81, 84, 89 tectonic evolution, 91-94 Taiwan Strait, 47 Takangkou Formation, 85 Tan-Lu fault, 19, 65-74, 326 Tancheng, 70 Taniantawen frontal arc, 56 Tar sand, 416 Tarentaise, 156 Tarim, 113-114, 447 Taurus, 187 Taurus Mountains, 186 Taxon space, 145 Tectonic erosion, 335 Tectonic evolution of China, 447 of Taiwan, 91-94 Tectonic facies, 7, 9, 20, 39, 51, 66, 74, 90, 343, 345, 347, 445 Tectonic m61ange, 324 Tengchong block, 21 Tengchong volcanic rocks, 22 Ternary feldspars, 278, 282, 292, 296, 303, 318 compositional zoning, 297 crystallization paths, 291 Ternary systems, 293 Tethyan plate, 39 Texas, 277, 315, 318 Theory of relativity, 435 Thermal plumes, 269 Thin-skin tectonics, 339 Thinned continental crust, 113-114 Thinning of the crusts, 8 Thomson, Charles D. William, 153 Thrust-block generation, finite element analysis, 238-239 Thrustplanes, 173, 231 Tibet, 46, 381,383, 447 Tibetan Plateau, 15 Tidal facies, 355 Tienshan, 10 Timan Pechora basin, 127 Time, physical theory, 449-450 Timor, 30 Toe effect, 235 Tonalitic gneiss, 323, 334-335, 339 Tongbai Mountains, 326-327 Torsion experiments, 242 Toson Hu-Huashixia-Maqen, 59 Total nitrogen, 391 Total organic carbon, 391 Total sulfur, 391 Townsend, Joseph, 135 Toxic trace metals, 399 TP conditions, 271 TP path, 271

Subject Index Trace elements, 399 Trace metals, 406 Trailing edge, of descending slabs, 339 Transform faults, 67, 71, 74, 201-202, 205 Trapped ocean, 114-115 Tree rings, 431 Trench, 11 Troxel, B.W., 190 Tunguska event, 369 Turbidites, 375-376, 416, 441 Turbidity currents, 416 Tyrrhenian Basin, 354-355, 357 Ultra-High-Pressure (UPH) rocks, 17-18, 67 Ultrabasic rock bodies, 49 Ultramafic rock belt, 327-335 Ultramafic rocks, 379 Underthrusting, 240 United States, 200, 232 Unroofing, 200 Upwelling, 376 Upwelling of abnormal mantle, 8 Uralian ophiolites, 120 Urengoy borehole, 127 Vadfisz, 181 Vaiont, 248 Valais Trough, 110 Veins, 165-167 Velocity of deformation, 252 Ventura Field, California, 416 Ventura Pliocene turbidites, 441 Verrucano, 171 Vesuvius, 153 Viscous behaviour, 257, 259 Viscous stress, 259 Volcanic eruption, 297 Volcanism, 290, 368 Volcanoclastic rocks, 198 Von Buch, Leopold, 152, 154-156, 158 Von Carnall, 168 Von Humboldt, A., 143 Wangyang eclogite, 330 Wangyang village, Tongcheng County, 329 Waste brine, 427 Weiss, 231 Weisstannenthal, 171 Werner, A.G., 149-150, 165-166, 210 Wernicke, Brian, 190-191 Western Foothills, 82 Western Fujian Alemanide, 47 Western Yunnan orogenic collage, 16 Whitehurst, John, 164 Whole-body deformation, 260 Whole-body flow, 254 "Wild cat" methods, 123

467 Wildhorse Mountain Plug, 278-297, 318 Wollard, George, 441,442 Wright, L.A., 190 Wuyi Mountains, 24 Wuyi-Yunkai Arc, 24, 28 Xia Dawo, 59 Xiaohekou bodies, 337 Xiaohekou pyroxenite, 328 Xijin Ulan Hu, 62 Xikou Formation, 43, 47 Xinghai, 58 Xinjian, 422 Xiqin Shan, 60 Xiqin Shan uplift, 56 Xongdian eclogite, 330 Xu-Huai, 72 Xu-Huai foreland FTB, 19-21 Xuefeng Mountains, 23, 28-29 Yangshan, 326 Yangtze, 27-28,407, 446 Yangtze block, 17, 21-22, 24-25, 29-31, 55, 63,347 Yangtze Craton, 65, 67-74, 344 Yangtze folded belt, 23 Yangtze microcontinent, 15, 23, 32 Yangtze para-platform, 39 Yangtze plate, 324, 326, 330, 337, 339 Yangtze subduction block, 56 "Yanshanian" movement, 39-40 "Yanshanian" orogeny, 48, 51 Yao-Bye Mountain, 389 Yarlu Zangbo suture zone, 63 Yellow River, 407 Yellow Turban Rebels, 407 Yidun collision belt, 56 Yidun island arc, 62 Yield point, 233 Yield strength, 256-257, 259 Yuk Yung, 449 Yunan, 46 Yunnan, 21, 30-31 Yuwanshui Formation, 27 Zhangbaling Group, 326 Zhejiang, 24, 26-27, 41-42, 344-349 "Zhen'he-Da'pu" fault, 42 Zhrdo-Xiewo, 62 Zhu Xia, 446 Zhujiapu, 327 Zhujiapu bodies, 337 Zhujiapu pyroxenite, 328 Zircon, 282 Zogqen, 59 Zone-melting, 272 Zoned sanidines, 277 Zoning patterns, 283

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