Structural geology and tectonic evolution of the Sognefjord Transect, Caledonian orogen, southern Norway: a field trip guide (GSA Field Guide 19)

Field Guide 19

Structural Geology and Tectonic Evolution of the Sognefjord Transect, Caledonian Orogen, Southern Norway—A Field Trip Guide

by Alan Geoffrey Milnes GEA Consulting Grand-Rue 7C CH-2035 Corcelles NE Switzerland Fernando Corfu University of Oslo Department of Geosciences Postboks 1047 Blindern N-0316 Oslo Norway

Field Guide 19 3300 Penrose Place, P.O. Box 9140

Boulder, Colorado 80301-9140, USA

2011

Copyright © 2011, The Geological Society of America (GSA), Inc. All rights reserved. GSA grants permission to individual scientists to make unlimited photocopies of one or more items from this volume for noncommercial purposes advancing science or education, including classroom use. For permission to make photocopies of any item in this volume for other noncommercial, nonprofit purposes, contact The Geological Society of America. Written permission is required from GSA for all other forms of capture or reproduction of any item in the volume including, but not limited to, all types of electronic or digital scanning or other digital or manual transformation of articles or any portion thereof, such as abstracts, into computer-readable and/or transmittable form for personal or corporate use, either noncommercial or commercial, for-profit or otherwise. Send permission requests to GSA Copyright Permissions, 3300 Penrose Place, P.O. Box 9140, Boulder, Colorado 80301-9140, USA. GSA provides this and other forums for the presentation of diverse opinions and positions by scientists worldwide, regardless of their race, citizenship, gender, religion, or political viewpoint. Opinions presented in this publication do not reflect official positions of the Society. Copyright is not claimed on any material prepared wholly by government employees within the scope of their employment. Published by The Geological Society of America, Inc. 3300 Penrose Place, P.O. Box 9140, Boulder, Colorado 80301-9140, USA www.geosociety.org Printed in U.S.A. GSA Books Science Editors: Marion E. Bickford and Donald I. Siegel Library of Congress Cataloging-in-Publication Data Milnes, A. G. Structural geology and tectonic evolution of the Sognefjord Transect, Caledonian orogen, southern Norway : a field trip guide / by Alan Geoffrey Milnes, Fernando Corfu. p. cm. — (Field guide ; 019) Includes bibliographical references. ISBN 978-0-8137-0019-9 (pbk.) 1. Geology, Structural—Norway. 2. Orogeny—Norway. 3. Plate tectonics—Norway. 4. Geology— Fieldwork—Norway. I. Corfu, Fernando, 1949– II. Title. QE633.N8M55 2011 551.809483ʹ8—dc22 2011002497 Cover: Sognefjorden, ~50 km from the mouth, looking NE into a small side arm. Mountains in the background are ~900 m above sea level; water depth to the right of the photo is ~1300 m (locality Austrheim, Stop 3.3). Photo by Geoffrey Milnes.

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Contents

Preface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . v Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .vii Abstract . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1 Chapter 1. An introductory outline . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Autochthon-Parautochthon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 2. Caledonian Allochthon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 3. Late- to Post-Collisional Extensional Shear Zones . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Lærdal-Gjende Fault . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Nordfjord-Sogn Detachment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 4. Plate Tectonic Cartoon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 Chapter 2. The Sognefjord transect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 1. Crustal Structure. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9 2. Structural Synthesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Eastern Segment of the Sognefjord Transect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12 Central Segment of the Sognefjord Transect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13 Western Segment of the Sognefjord Transect . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15 3. Retro-Deformation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Balancing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Depth Control . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16 Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 4. Eclogite Exhumation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18 5. An Orogenic Timetable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20 Chapter 3. The field trip itinerary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21 Day 1. Valdres-Jotunheimen. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 Stop 1.1. Søndrol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 Stop 1.2. Vangsmjøsi. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22 Stop 1.3. Øye . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 Stop 1.4. Tyin Road Profile. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 Stop 1.5. Tyedalen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28 Stop 1.6. Lorteviki–Eidsbugarden . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 Day 2. Inner Sognefjorden . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 31 Stop 2.1. Årdal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34 Stop 2.2. Lærdal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 Stop 2.3. Eide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36 Stop 2.4. Sogndal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 Stop 2.5. Slinde. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39 Stop 2.6. Hermansverk . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

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Contents Day 3. Outer Sognefjorden . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 Stop 3.1. Hella . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 43 Stop 3.2. Sæle . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46 Stop 3.3. Austrheim . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46 Stop 3.4. Kyrkjebø . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46 Stop 3.5. Råsholm. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 Stop 3.6. Hellebø . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 Stop 3.7. Bekkeneset . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50 Day 4. Solund . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53 Stop 4.1. Losna . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53 Stop 4.2. Hersvik . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57 Stop 4.3. Hyllestad . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57 Day 5. Askvoll-Atløy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59 Stop 5.1. Vårdalsneset . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59 Stop 5.2. Gjervik. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63 Stop 5.3. Kviteneset . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65 Stop 5.4. Brurestakken . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65 Day 6. Fensfjorden-Lindås . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65 Stop 6.1. Kjekallevågen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68 Stop 6.2. Osterfjorden . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68 Stop 6.3. Holsnøy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71 Stop 6.4. Stalheim. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75 References Cited . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77

Preface

The field trip described in this guide starts in the “Norwegian Alps,” the high mountain massif called Jotunheimen, and runs out along Sognefjorden, the world’s longest fjord, to the islands along the west coast of Norway. Geologically, the Sognefjord transect provides a complete cross section through the Caledonian orogenic belt, of Paleozoic age, which in Norway stretches along the west coast from Stavanger to North Cape, a distance of ~2000 km. This transect in southern Norway provides a superb and exceptionally well-documented example of late collisional tectonics in an Alpine-type orogen. It provides a continuous, 250-km-long cross section from the cratonic foreland, in the east, through the heavily deformed continental margin, to the remains of the Caledonian ocean complex, in the west. Detailed structural data are available along the whole transect, together with good stratigraphic, radiometric, petrological, and geophysical control. These data have been analyzed in terms of the kinematics and relative ages of the different deformation phases, and correlated along the whole transect. Logistically, the whole route is ideal for field trips, both individually and in groups. It is easily accessible and well exposed, with good communications and different types of accommodations and services. The itinerary described in this guide is based on Excursion 28 of the 33rd International Geological Congress held at Oslo in August 2008. Many other itineraries are possible and practical, however, and it is hoped that this guide will also prove useful for planning trips on a do-ityourself basis. Some parts of the trip are mainly “stop and look,” because of the large distances to be covered, but there are some stops with short crossings of rough (possibly wet) terrain, requiring good footgear (leather walking boots or rubber boots). Also, the weather is unpredictable, and the worst has to be expected (rain, wind, temperatures down to 10 °C in summer), requiring good wind and rain protection (wind-proof, water-tight anorak and rain trousers, umbrella). Most of the trip can be carried out anytime between March and October. May–July are the best months, both weatherwise and because of the long daylight hours. The high mountain parts of the trip (Stop 1.4 to Stop 1.6) may be hindered because of snow conditions, in which case the itinerary can be completed by driving directly along the main Oslo-Bergen road E16 (kept open year round) from Stop 1.3 to Stop 2.1. For weeklong weather forecasts, consult the web site of the Norwegian Meteorological Institute and the Norwegian Broadcasting Corporation; www.yr.com/English/. A useful piece of geological equipment, apart from field book, compass-clinometer, etc., is a pair of binoculars. For the overnight accommodation at some of the localities, participants should bring along a sheet sleeping bag, although there is also the possibility of renting one when required. Each participant should have a high-visibility vest, easily accessible throughout the trip, because some road sections are on main roads. In addition to the present Field Guide, the basic data for the trip are to be found in two papers: Milnes et al. (1997) and Wennberg et al. (1998). For an overall view of the Sognefjord transect and a comparison with the Alps, see Milnes (1998). The whole area of the trip is covered by excellent 1:250,000-scale geological maps of the Norwegian Geological Survey (NGU), map sheets Årdal, Florø, and Bergen. These should be purchased before the trip (order through the NGU web site: www.ngu.no). A recent publication of the Geological Society of Norway (Ramberg et al., eds., 2008) gives an easily read and beautifully illustrated overview of the geology of Norway and its continental shelf, including several chapters relevant to the present field trip (Chapters 3–7) and an overview map of Norway at a scale of 1:2,000,000. A good road map for the whole trip is the Cappelens 1:335,000-scale map CK2 (“Sør-Norge nord”—ISBN 978-82-02-27260-9); see Cappelens web site: www.cappelenkart.no. For planning the trip it is essential to obtain up-to-date ferry v

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Preface timetables. These can be obtained in English from the ferry company’s web site: www.fjord1.no/en/. The timetables needed for the trip described here are numbers 14-107, 14-173, 14-251, 14-321, 14-415, and 14-431. Google provides web access to almost all types of accommodation and services along the route if you search by town name. The web address of the Norwegian telephone database is www.telefonkatalogen. no. In Norway, the emergency telephone numbers are 110 (Fire), 112 (Police), and 113 (Medical emergency).

Acknowledgments

Many people have knowingly or unknowingly contributed to the production of this guide. We think particularly of the many graduate students, researchers, and university teachers who have taken part in field trips along part or all of this transect since we started our research program in the mid-1970s. We thank them all for the many discussions and arguments, in fair weather and foul, during which the ideas set out here crystallized and, hopefully, matured. We would like to express special thanks to Michael Heim, Urs Schärer, Andreas Koestler, Thomas Dietler, Mattias Lundmark, and Ole Petter Wennberg, for their fine research and their companionship in various phases of this work. The itinerary of the field trip as described here is based on Excursion 28 of the 33rd International Geological Congress, which was held in Oslo in August 2008. We wish to thank the participants on that trip (photo below) for their interest and inspiring evening discussions over expensive beers, and for their encouragement for writing this guide. We would also like to thank Torgeir Andersen and Håkon Austrheim for providing maps and diagrams from the guide that they prepared for the 33IGC Excursion 29, which we have used to illustrate the Atløy part of our excursion (Figs. 38–40 in this guide).

The itinerary in this Field Guide was followed on Excursion 28 of the International Geological Congress (IGC) in Norway, in August 2008. Here, “on top of the world,” are the IGC participants, with lake Tyin behind and the high mountain area of Jotunheimen in the background. In the photo, from left to right: Stefano Mazzoli, Michael Szpunar, Marion (“Pat”) Bickford, Miles Osmaston, Mattias Lundmark, George DeVries Klein, Allah Bakhsh Kausar, Fernando Corfu (co-leader), Egle Sinkune, Greg Dunning, Petras Sinkunas, Florencia Bechis, and Rômulo Machado.

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The Geological Society of America Field Guide 19 2011

Structural Geology and Tectonic Evolution of the Sognefjord Transect, Caledonian Orogen, Southern Norway—A Field Trip Guide Alan Geoffrey Milnes GEA Consulting, Grand-Rue 7C, CH-2035 Corcelles NE, Switzerland Fernando Corfu University of Oslo, Department of Geosciences, Postboks 1047 Blindern, N-0316, Oslo, Norway

ABSTRACT The Sognefjord transect described in this field guide starts in the “Norwegian Alps,” the high mountain massif called Jotunheimen, and runs out along Sognefjorden, the world’s longest fjord, to the islands along the Norwegian west coast. Geologically, it provides a complete cross section through the Caledonian mountain belt, and represents an exceptionally well-documented example of late collisional tectonics in an Alpine-type orogen. It is comparable with the Alps as a natural laboratory for orogenic studies, being both easily accessible and well exposed, with a long history of geological research and excellent geological map coverage. The transect exposes several major tectonic structures, including the Jotun thrust complex (Days 1–2), with a demonstrable displacement of 200–300 km, the extensional Nordfjord-Sogn Shear Zone (Days 4–6), with up to 50 km of normal displacement, and the eclogitic orogenic root, with evidence for ductile rebound under predominantly gravitational forces (Day 3). The field trip starts on the cratonic foreland of the Caledonian orogen, in the east (Day 1), continues through the heavily deformed continental margin (Days 1–3), and ends in the remains of the Caledonian ocean complex, in the west (Days 4–6). Detailed structural data are available along the whole transect, together with good stratigraphic, radiometric, petrological, and geophysical control. These data have been analyzed in terms of the kinematics and relative ages of the different deformation phases, and used to reconstruct the crustal geometry at different stages backward in time (kinematic modelling) and to imitate the process of orogenic root collapse (dynamic modelling). The itinerary is based on Excursion 28 of the International Geological Congress, which was held at Oslo in August 2008. MANUSCRIPT ACCEPTED BY THE SOCIETY 3 AUGUST 2010

Milnes, A.G., and Corfu, F., 2011, Structural Geology and Tectonic Evolution of the Sognefjord Transect, Caledonian Orogen, Southern Norway—A Field Trip Guide: Geological Society of America Field Guide 19, 80 p., doi:10.1130/2011.0019. For permission to copy, contact [email protected]. © 2011 The Geological Society of America. All rights reserved.

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 CHAPTER 1  An introductory outline

The Sognefjord transect in southern Norway cuts through the Caledonian orogenic belt, whose collisional phase culminated in Late Silurian and Early Devonian times. It provides a superb and exceptionally well-documented example of late collisional tectonics in an Alpine-type orogen (Milnes, 1998). One of the world’s longest and deepest fjords (Sognefjorden), together with its head valleys and the rugged mountains of Jotunheimen, provides a continuous, 250-km-long cross section from the cratonic foreland, in the east, through the heavily deformed continental margin to the remains of the Caledonian oceanic complex, in the west. The location of the transect is shown in Figure 1. Detailed structural data are available along the whole transect, together with good stratigraphic, radiometric, petrological, and geophysical controls. These data have been analyzed in terms of the kinematics and relative ages of the different deformation phases, and correlated along the whole cross

section. The resulting synthesis has been used, in conjunction with the other data, to carry out a retro-deformation, reconstructing the crustal geometry at different stages backward in time (Milnes et al., 1997), and as a basis for a dynamic model of orogenic root collapse (Milnes and Koyi, 2000). These papers form the basis of the present field trip, which has been run for students and visiting international groups innumerable times in the past two decades and was included as Excursion 28 of the program of the 33rd International Geological Congress (33IGC), held in Oslo in summer 2008. The latter part of the field trip diverges northward and southward from Sognefjorden, briefly studying the footwall and hanging wall of the spectacular Nordfjord-Sogn Detachment (the main theme of Excursion 29 of the 33IGC; see Andersen and Austrheim, 2008), and particularly its southward continuation as the Bergen Arc Shear Zone (Wennberg et al., 1998).

Figure 1. Sognefjord transect through the Caledonides of southern Norway. Rectangle shows the location of the tectonic map, Figure 2. D—Denmark.

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4

Chapter 1

The overall structure displayed by the Sognefjord transect can be summarized in terms of two main collisional complexes and two major, post-collisional, extensional shear zones (Fig. 2). The collisional complexes are designated AutochthonParautochthon and Caledonian Allochthon in the legend to Figure 2. These were actively deforming in Late Silurian and Early Devonian times. The extensional shear zones postdate the collision and were active in the Middle and Late Devonian. The more easterly zone is called the Lærdal-Gjende Fault where it crosses the Sognefjord transect. The more westerly zone, which approximately follows the Norwegian Atlantic coastline where it crosses the transect, is called the Nordfjord-Sogn Detachment in Figure 2. These extensional shear zones can be used to subdivide the transect into three segments: an eastern segment (footwall and hanging wall of the Lærdal-Gjende Fault), a central segment

(footwall of the Nordfjord-Sogn Detachment), and a western segment (hanging wall of the Nordfjord-Sogn Detachment). On the field trip the Caledonian orogen (Autochthon-Parautochthon below, Allochthon above) will be studied separately in each segment: Days 1–2, eastern segment; Day 3 (with one Stop on each of Days 4 and 5), central segment; Days 4–5, western segment. The post-collisional shear zones will be studied where the transect crosses them: Days 1–2, Lærdal-Gjende Fault; Days 4–5, Nordfjord-Sogn Detachment. On Day 6 the trip follows a branch of the Nordfjord-Sogn Detachment, the Bergen Arc Shear Zone, southward from the Sognefjord transect, to study the Caledonian structures in the hanging wall. A brief introduction to each of the major tectonic elements is given below, followed by a cartoon outline of the supposed plate tectonic development of the Caledonian orogen, in comparison with the Alps, as general background.

Figure 2. Tectonic map of the Sognefjord region, southern Norway, showing the location of the Sognefjord transect and the location of the detailed structural profile, Figure 4 (center line of the transect strip). BASZ—Bergen Arc Shear Zone; BSB—Baltic Shield Basement; HFSZ— Hardangerfjord Shear Zone; LGF—Lærdal-Gjende Fault; NSD—Nordfjord-Sogn Detachment; WGC—Western Gneiss Complex. Adapted from Milnes et al. (1997).

An introductory outline

5

1. AUTOCHTHON-PARAUTOCHTHON

2. CALEDONIAN ALLOCHTHON

The lower of the two collisional complexes, known as the Autochthon-Parautochthon in the literature (cf. Roberts and Gee, 1985), is represented along the Sognefjord transect by the Baltic Shield Basement (BSB in Fig. 2) and its northwestward continuation as the Western Gneiss Complex (WGC in Fig. 2). The Baltic Shield Basement forms the whole of southern Norway. It is now part of the Baltic Shield, and it was part of a shield area, Baltica, also in late Precambrian and early Paleozoic times, forming the rigid, cratonic foreland of the Caledonian orogen. In the eastern segment of the transect it is seen in a series of windows through the allochthonous units. The Western Gneiss Complex makes up most of the central segment of the Sognefjord transect and consists of two zones that are clearly defined on a regional scale: a southeastern zone (forming the mountains below and surrounding Jøstedalsbreen, Norway’s largest ice cap), which consists of Precambrian basement similar to the Baltic Shield Basement, and a western zone in which this basement has been “Caledonized” (deformationally and metamorphically overprinted during the Caledonian orogeny—hence the term Parautochthon). The Caledonized zone of the Western Gneiss Complex contains the well-known eclogite bodies of western Norway (“e” localities in Fig. 2).

The upper collisional complex is known as the Caledonian Allochthon and contains a number of more or less far-traveled nappe units derived from the cover of the Western Gneiss Complex, the rifted margin of the ancient craton, Baltica, and parts of the oceanic area that lay outboard of this margin, known as Iapetus (cf. Stephens, 1988). The Allochthon was overthrust from the northwest onto the Autochthon-Parautochthon in Late Silurian and Early Devonian times (ca. 425–395 Ma, sometimes referred to as the Scandian orogenic phase). Thrusting took place on a basal décollement zone, sometimes referred to as the Main Caledonian Detachment Zone, with an overall displacement of 200–300 km (Hossack et al., 1985; Fossen, 1992). The term detachment is used in this guide to designate any low-angle shear zone on which the displacement is so large that it can only be deduced from regional considerations. A detachment may be extensional or contractional and is usually accompanied by a zone of intense shearing or mylonitization that may be several kilometers thick. The shear zone at the base of the Caledonian Allochthon is a contractional detachment, related to crustal shortening (and thickening), whereas the Nordfjord-Sogn Detachment (see below) is an extensional detachment, related to crustal extension (and thinning). The Caledonian Allochthon has been subdivided into Lower, Middle, and Upper nappe units on the basis of structural position and/or rock association, which reflect different paleogeographic localities within the pre-Caledonian plate tectonic configuration (Roberts and Gee, 1985). The Lower Allochthon mainly represents the sheared-off, late Precambrian–early Paleozoic cover of the Autochthon-Parautochthon, although it locally contains slices derived from both underlying and overlying units. In the eastern segment of the Sognefjord transect it is represented by strongly sheared and disrupted early Paleozoic metasediments, but southeastward it grades into a more or less coherent foreland fold-and-thrust belt, dying out and becoming autochthonous south of Oslo (see Fig. 1: Caledonian front). The Middle Allochthon in the Sognefjord transect is represented by a large mass of Precambrian basement (called the Jotun Complex) and its late Precambrian–early Paleozoic sedimentary cover (Valdres “sparagmites” overlain by early Paleozoic platform sediments up to Wenlock in age). This unit is sometimes referred to as the Jotun Nappe or Jotun-Valdres Nappe Complex. It is generally interpreted as part of the rifted continental margin of Baltica (cf. Emmett, 1996) on the basis of stratigraphic affinity with the Lower Allochthon. Locally, it contains felsic intrusions and associated dike complexes, formed in the Silurian during the initial stages of nappe development (Lundmark and Corfu, 2007). The Upper Allochthon in southern Norway is characterized by Ordovician–Early Silurian ophiolite complexes with island arc and marginal basin affinities, and associated sedimentary and intrusive rocks (e.g., Pedersen et al., 1988; Andersen and Andresen, 1994). These units are mixed with scattered continental fragments, some in a mélange-like association reminiscent of the

Figure 2. (continued).

6

Chapter 1

Pennine Zone of the Central Alps (cf. Milnes, 1978, 1998; Froitzheim et al., 1996). The Upper Allochthon represents the remains of the Iapetus oceanic complex. On the Sognefjord transect, the Upper Allochthon is only seen in the western segment, in the hanging wall of the Nordfjord-Sogn Detachment. Higher units or units derived from the Laurentian side of Iapetus have not been identified along the Sognefjord transect. Apart from the huge Jotun Nappe, the Middle and Upper Allochthons in southern Norway are commonly fragmentary and dismembered, with different tectonic units bearing different local names or their assignment being controversial. In order to clarify their affinities in this Field Guide, local names will be followed where appropriate (in parentheses), if their affinity is established, or with question marks and/or alternatives when this is not the case.

two major, late to post-collisional, extensional shear zones. The more easterly of these is known as the Lærdal-Gjende Fault where it crosses the Sognefjord transect (LGF in Fig. 2; cf. Milnes and Koestler, 1985), and the Hardangerfjord Shear Zone farther to the southwest (HFSZ in Fig. 2; cf. Fossen, 1992; Fossen and Hurich, 2005). The low-angle, NW-dipping Lærdal-Gjende Fault marks the southeastern border of what has traditionally been known as the “Faltungsgraben” (Goldschmidt, 1912). This initially enigmatic structure is now known to be a large-scale half graben, with the Lærdal-Gjende Fault marking its southeastern margin. The downthrow on this fault is estimated at ~8 km, bringing down the Middle Allochthon, which along this transect is composed of the Precambrian Jotun Complex, making up most of the high mountain area of Jotunheimen. Nordfjord-Sogn Detachment

3. LATE- TO POST-COLLISIONAL EXTENSIONAL SHEAR ZONES Lærdal-Gjende Fault Along the Sognefjord transect the Caledonian Allochthon and the underlying Autochthon-Parautochthon are cut through by

The western end of the Sognefjord transect is intersected by a much larger low-angle, extensional shear zone known as the Nordfjord-Sogn Detachment (Fig. 2; see, e.g., Andersen, 1998; Braathen et al., 2004), represented at the present erosion level by a 1–2-km-thick zone of mylonitized rocks, topped by a brittle fault (Solund Fault or Dalsfjord Fault of probable Mesozoic age;

Figure 3. Schematic cartoon of the orogenic evolution of collisional orogens, with approximate Caledonide and Alpine time limits (adapted from Milnes, 1998). Key: cf—cratonic foreland; rm—rifted continental margins and microcontinents; oc—oceanic complex; ll—spreading centers. In southern Norway the cratonic foreland of the Caledonides is represented by the Baltic Shield Basement and the Western Gneiss Complex (Autochthon-Parautochthon; see Fig. 2). The Lower Allochthon consists of the stripped-off cover of the craton, with some basement slices; the Middle Allochthon represents basement and cover units from the rifted continental margins; and the Upper Allochthon contains remnants of the oceanic complex with fragments of oceanic crust, microcontinents, subducted flakes, island arcs, etc., often in mélange-like associations.

An introductory outline see Torsvik et al., 1992). The hanging wall of the Nordfjord-Sogn Detachment contains the erosional remnants of a large Devonian basin or group of basins, with coarse clastic sediments lying on the eroded stumps of the Caledonian Allochthon (mainly the Upper Allochthon, consisting of the Solund-Stavfjord Ophiolite Complex). The footwall is built of gneisses belonging to the Western Gneiss Complex (Fig. 2), showing collision-related high-grade metamorphism and polyphase deformation, and enclosing eclogite bodies and high-pressure schists. The displacement on the Nordfjord-Sogn Detachment is estimated to be in the region of 50 km. South of the Sognefjord section this detachment extends into the North Sea, but in the Bergen area it is at least partly represented by the Bergen Arc Shear Zone (BASZ in Fig. 2), an oblique-lateral ramp in the Devonian extensional detachment system in western Norway (Wennberg et al., 1998). 4. PLATE TECTONIC CARTOON The Scandinavian Caledonides and the Alpine System are generally described, since the advent of plate tectonics, in terms of the opening and closing of an ocean within a previously consolidated super-continent, the late Precambrian continent Rodinia in the Caledonides, and the Permo-Triassic continent Pangea in the Alps. This model—or better, “comic strip”—is little more than the traditional “Wilson cycle” and is sketched with some refinements in Figure 3. For each orogen, one envisages the opening and closing of an oceanic area, with the rifting and thinning of the continental margins and the isolation or partial isolation of microcontinents during the extensional phase (“pre-orogenic

7

extension,” Milnes, 1998) and the development of subduction zones, island arcs, backarc basins, and all their corollaries during the subsequent closing phase (“pre-collisional contraction”). During the closing phase, oceanic crust is mainly subducted or obducted, but locally continues to be formed in backarc basins. The end of this phase and the start of the collisional phase is often placed shortly after the age of the youngest backarc ophiolites (Fig. 3: in the Alps, ca. 60 Ma; in southern Norway, ca. 440 Ma; Dunning and Pedersen, 1988). The phase of collisional contraction can be subdivided into early and late collisional phases, of which the early phase comprises the reconstitution of the original crustal thickness out of the faulted continental margins and microcontinents and their obducted ophiolite fragments (Fig. 3: compare the original and final frames of the “comic strip”). The structures (faults, basins, titled blocks, etc.) formed during pre-orogenic extension largely determine the geometry and dimensions of the tectonic units in the reconstituted crust. However, none of the processes in the cartoons shown in Figure 3 necessarily leads to any change in overall plate motion; any changes in this respect are mainly caused by forces outside the system. In the collisional phase, and particularly during “late collisional contraction” (Fig. 3), the approach of the cratonic forelands is eventually prevented, and a fundamental rearrangement of plate motions on a global scale is initiated (cf. Cloos, 1993). The next step in the process (not shown in Fig. 3) is the one that is the main focus in the present Field Guide and the one that is summarized for the Sognefjord transect in Chapter 2, in preparation for the description of the field localities themselves in Chapter 3.

 CHAPTER 2  The Sognefjord transect

Although the general outline of early Caledonian and Scandian orogenesis and late–post-orogenic extensional tectonics is well established, there have been few compilations of detailed structural data across the whole belt that could aid in advancing past the conceptual stage. An exception is the ValdresJotunheim-Sognefjord–West Coast cross section, here called the Sognefjord transect, which has been studied in detail along its entire length, with numerous Ph.D. and Masters’ theses at various institutions in the 1970s, 1980s, and 1990s. These data were compiled and synthesized for the first time in the mid-1990s and were accompanied by a composite profile as geometrically correct as possible (Fig. 1 in Milnes et al., 1997, a foldout cross section with horizontal and vertical scales of 1:600,000). The line of section is plotted in Figure 2, and a reduced version of the cross section is shown in Figure 4. The latter will be used as the connecting link in the present Field Guide, with the different Stops plotted on the section at their correct structural positions. In the original version (Milnes et al., 1997) the main sources of data were summarized with reference to a series of “panels.” These are also shown in Figure 4, but the large number of detailed references that were cited are not repeated in the present guide. Within each panel, the geometry, kinematics, and history of the Scandian deformation were described in detail, and are sufficiently well documented to be correlated with neighboring panels to underpin the reconstructions. In addition, two other types of data provide important constraints. First, although there is as yet no deep reflection seismic profile along the transect, other types of geophysical data provide invaluable information on crustal structure. These will be discussed first (Section 1). Second, the absolute dating of some of the events, by stratigraphic or radiometric means, provides an orogenic timetable that is sufficiently reliable for checking the consistency and plausibility of the movement picture. This type of information will be discussed in connection with the structural synthesis (Section 2). Based on the structural synthesis, an attempt is made to retro-deform the Sognefjord crustal segment—i.e., to reconstruct crustal conditions along the transect by successively removing each deformation phase backward in time, taking into account different lines of argument related to depth of burial at different stages (Section 3). The possibilities of retro-deformation are, however, limited, only reaching back as far as the formation of the eclogites. In the final section of this introductory overview, the kinematic modeling (retro-deformation) is carried further to show how it throws light on the processes of eclogite exhumation and how it has been used for developing a dynamic model of orogenic root collapse (Section 4). This chapter ends with an overall summary

of the tectonic evolution of the South Norwegian Caledonides in the form of an “orogenic timetable” for easy reference and as a basis for discussion (Section 5). 1. CRUSTAL STRUCTURE A recent compilation of geophysical data on Moho depth under Scandinavia (Kinck et al., 1993) shows a maximum Moho depth of almost 40 km under the eastern part of the Sognefjord transect, decreasing slowly westward to 20 km at position B (Fig. 6), where the Baltic Shield Basement is overlain by phyllites that reached the greenschist-amphibolite facies boundary. The depth of the main Caledonian detachment (marked LA, Lower Allochthon, in Fig. 6) is a major control on the whole retro-deformation, as it never exceeded 20 km. As noted earlier, the Lower Allochthon at the eastern end of the transect probably never even sank below 10 km; because it contains the cover of the Western Gneiss Complex, it was obviously stripped off before this complex was eclogitized (present exposures representing depths of 60–70 km). Further depth control on the Middle Allochthon is provided by

The Sognefjord transect

Figure 6. Retro-deformation of the Sognefjord transect (from Milnes et al., 1997). The “present cross section” at the top is a true-scale simplification of the detailed structural cross section shown in Figure 4. NSD—Nordfjord-Sogn Detachment; LGF—Lærdal-Gjende Fault; UA—Upper Allochthon; MA—Middle Allochthon; LA—Lower Allochthon; WGC—Western Gneiss Complex.

17

18

Chapter 2

the Ordovician-Silurian cover of the Dalsfjord Complex (Herland Group, Fig. 4), which also underwent only low-grade metamorphic conditions. The other type of depth control, which has to be applied to rock masses that do not undergo internal deformation, is that the erosion level at each stage in the retro-deformation must lie above the previous one; i.e., once-eroded parts are not allowed to reappear in the cross section at some later time. This is particularly important for the Jotun Complex, as parts must have been eroding away throughout much of the later stages of the orogeny (Fig. 6). Results The results of the exercise are shown in Figure 6 as a sequence of frames (Milnes et al., 1997). At the top, the constructed present-day cross section (from Fig. 4) is presented in a simplified form. The retro-deformation starts with a Late Devonian reconstruction (Fig. 6, frame 4, ca. 365 Ma), after the end of movement on the Nordfjord-Sogn Detachment and the LærdalGjende Fault. Because there is little information on the erosion level at this time, the erosion surface was drawn just slightly above the present level (Fig. 6, A–B and C–D). From this starting point, the first step is the removal of the effects of the Mode II extension in the upper crust (the extensional shear zones, i.e., the Lærdal-Gjende Fault and the Nordfjord-Sogn Detachment), including the removal of the coarse clastic sequences in the Middle Devonian basins. This reconstructs conditions at the end of the Mode I extension phase (Fig. 6, frame 3, ca. 385 Ma). Focusing on the Western Gneiss Complex, this removes the Sognefjord-D4 deformation and places the eclogites at a depth of 30 km by movement in the footwall of the Nordfjord-Sogn Detachment, a crustal-scale extensional detachment with a displacement of ~50 km (Fig. 6). Strong footwall uplift must have taken place and must have resulted in a high mountain chain, because it was true uplift. In the hanging wall, in contrast, the basal Devonian is at the Earth’s surface today, it was at the Earth’s surface during deposition, and it lies at the Earth’s surface at this stage of orogenic evolution, i.e., strong uplift and erosion did not take place. In comparison, the footwall loses 30 km of crust by erosion (or rather, gains it, during the retrodeformation step from frame 4 to frame 3). The next step is to remove the Mode I extension and to reconstruct conditions at the end of contraction (Fig. 6, frame 2, ca. 395 Ma). This pushes the Middle Allochthon (Jotun Complex) to its farthest outreach over the foreland. For this step we have used an estimated 20 km of reversed movement on the main Caledonian detachment (the Jotunheimen-D4 phase). At deeper levels, this step removes the asymmetrical, top-to-W or NW folds (Sognefjord-D3) in the Western Gneiss Complex. This step in the retro-deformation has relatively little effect on eclogite depth. It shows that at the end of the contractional phase, the presently exposed eclogites lay at ~40 km and were almost completely retrograded to amphibolite facies because the all-pervasive Sognefjord-D2 deformation was nearing completion.

The last step is the reconstruction of conditions at the time of eclogite formation (Fig. 6, frame 1, ca. 410 Ma). This is more speculative but is still subject to important constraints. The metasediments in the Jotunheimen Detachment, which is in its later phase of development (Jotunheimen-D3), must remain at shallow depths, as must the top of the rigid wedge of the Baltic Shield (Fig. 6, point B). But the eclogites must move downward to 60–70 km at the same time as the main deformation in the surrounding gneisses (Sognefjord-D2) is removed. This leads to a deep root, which can only be achieved by a process of downfolding of the thrust zone separating the Baltic Shield basement from the Upper Allochthon, after the Middle Allochthon has passed and after the Lower Allochthon has been stripped off. It is the destruction of this root that lifts the eclogites from a 60 to 70 km depth to a 40 km depth, which produces the Sognefjord-D2 deformation, with bulk subvertical shortening and bulk horizontal E-W extension. This process started to take place while the horizontal contraction of the rheological upper crust was continuing. 4. ECLOGITE EXHUMATION The retro-deformation of the Sognefjord transect (Fig. 6) is a typical example of kinematic modeling. It is not concerned with the forces involved, only with the probable geometry of the crustal segment at different points of time, based on the structural data and their interpretation in terms of strain and movement. No hypotheses concerning material properties or stress distributions are involved, no conceptual models related to plate tectonics are used, and no preconceived ideas about the end result of the reconstruction are entertained. The end result is determined by how far back in time the available data can be reasonably interpreted using the accepted techniques of modern structural geology. One of the most interesting results of the Sognefjord retro-deformation was that it provided new insights into the exhumation of the well-known Norwegian eclogite bodies (Milnes et al., 1997). Up to that time the exhumation and preservation of the eclogites had been explained in terms of the extensional collapse of the Caledonian orogen and by rapid uplift of the Western Gneiss Complex in the footwall of the Nordfjord-Sogn Detachment. Exhumation was conceived to be a direct result of the lateto post-orogenic extensional tectonics. With regard to eclogite formation, the process envisaged by most workers is that of in situ eclogitization. The eclogite parageneses in the mafic pods are considered to be the remnants of a pervasive eclogite facies metamorphism that affected the whole crustal root at its deepest extent, toward the end of orogenic contraction. With regard to eclogite exhumation, the Western Gneiss Complex has served as one of the main examples of the process of uplift and unroofing that results from crustal extension, because it lies now in the footwall of a spectacular extensional shear zone, the NordfjordSogn Detachment. Although the large-scale situation is clearly more complicated, the Western Gneiss Complex has even been regarded as a huge metamorphic core complex (Krabbendam and Dewey, 1998).

The Sognefjord transect However, already in the early 1990s it had been recognized that a major phase of pervasive deformation and amphibolite facies metamorphism in the Western Gneiss Complex postdated eclogite formation and predated the process of eclogite exhumation caused by crustal extension (the latter process is often referred to as “extensional collapse,” cf. Dewey, 1988). At an early stage it was recognized that the amphibolite facies overprinting showed no evidence of major non-coaxial deformation, whether contractional or extensional (e.g., Andersen and Jamtveit, 1990). Along Sognefjord, this phase of deformation was later labeled Sognefjord-D2, characterized by the absence of consistent kinematic indicators and suggesting a general “pure shear” strain regime (Milnes et al., 1997). This led directly to the idea that the root that had been reconstructed by retrodeformation (Fig. 6, frame 1) was gravitationally unstable and “collapsed upwards,” carrying the enclosed eclogites upward and decompressing them isothermally from ~20 kbar to 10 kbar pressure corresponding to a depth decrease of 20–30 km. Hence, in Milnes et al. (1997), eclogite exhumation was interpreted in terms of three successive phases, each dominated by a different process: (1) ductile rebound of the orogenic root (Fig. 6, frames 1 and 2, exhumation rate 2–3 mm a–1), (2) buoyant flexure at the transition from contractional to extensional tectonics in the upper crust (Fig. 6, frames 2 and 3, exhumation rate ~1 mm a–1), and (3) crustal extension along major detachments, or “extensional collapse” (Fig. 6, frames 3 and 4, exhumation rate 1–2 mm a–1). In later articles the first two exhumation processes were grouped together, as they could not be clearly distinguished (Milnes, 1998; Milnes and Koyi, 2000). Of the total of ~60 km of exhumation, which is estimated for the eclogites in outer Sognefjorden, about half was accomplished by buoyancy-driven processes. The field relations along the Sognefjord transect indicate that the component of eclogite exhumation from ductile rebound (1) took place before any movement on major extensional detachments, (2) proceeded more rapidly than the later extension-related exhumation, and (3) contributed an amount of exhumation of the same order of magnitude as the later extension. The structural analysis of the Sognefjord transect showed that an orogenic root formed at the end of collision and suggested the idea that the root was gravitationally unstable and collapsed by a mechanism of ductile rebound as contractional deformation ceased (Milnes et al., 1997). Subsequently, dynamic modeling was carried out using the geometry produced by the retro-deformation and inserting realistic material properties and time limits (Koyi et al., 1999; Milnes and Koyi, 2000). The models showed that the process of ductile rebound of an orogenic root, which had been earlier suggested as a possibly important orogenic process, but never demonstrated on a field example (e.g., Platt, 1993; Avouac and Burov, 1996), was indeed a plausible explanation of the relations observed along Sognefjorden. In the model (Fig. 7) the outcrops now exposed along the shores of Sognefjorden are indicated as they were toward the end of the process. In this Field Guide, the profile indicated in the model is the object of study during Day 3.

19

Figure 7. Dynamic model, based on the structural analysis and retrodeformation of the Sognefjord transect, showing the strain distribution resulting from the ductile rebound of an orogenic root (from Milnes and Koyi, 2000). The deformation in this numerical model is caused solely by gravitational forces derived from the unstable low-density, low-viscosity root. (A) Starting point of the model, dimensioned according to the retro-deformation of the Sognefjord transect (cf. Fig. 6, frame 1). (B) Strain distribution when the gravity-driven collapse (ductile rebound) of the orogenic root was almost complete (cf. Fig. 6, between frames 2 and 3). (C) Enlargment of part of B, showing the Sognefjord profile through the Western Gneiss Complex and the three structural regimes studied on Day 3 of the field trip.

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Chapter 2

Figure 8. An orogenic timetable for the South Norwegian Caledonides (after Milnes et al., 1997). NSD— Nordfjord-Sogn Detachment; LGF—Lærdal-Gjende Fault; KGOC—Karmoy-Gullfjellet Ophiolite Complex; SSOC—Solund-Stavfjord Ophiolite Complex; WGC—Western Gneiss Complex; OSLO—continental sedimentation in the Oslo graben; defm—deformation; metm—metamorphism; sedm—sedimentation.

5. AN OROGENIC TIMETABLE In order to visualize the complicated temporal relationships discussed in this chapter, and to provide a basis for discussion during the field trip, an orogenic timetable is included in which the events discussed are plotted against a geological time scale (Fig. 8). The amount of radiometric and strati-

graphic data from southern Norway is relatively small and subject to relatively large uncertainties, and the early Paleozoic time scale itself is not well established. The present compilation, therefore, is only a rough outline and should not be taken too literally. We estimate that a margin of error of ca. 5–10 Ma must be assumed before significant age differences can be identified.

 CHAPTER 3  The field trip itinerary

The field trip is subdivided into six “Days,” each containing a number of “Stops,” based on the itinerary of Excursion 28 of the 33rd International Geological Congress (33IGC) held at Oslo in August 2008. The approximate location of the overnight stops and the route of the excursion are shown in Figure 9. Clearly, the days and stops described here can be combined in different ways, and, correspondingly, overnight stops can be chosen at different locations. There are various types of accommodations available along the route: camping sites, youth hostels, pensions, hotels; and these can now be researched on the Internet. As the latter part of the itinerary is dependent on ferry times, planning needs to

be based on current ferry timetables, which can be downloaded from web site www.fjord1.no. The geological part of the 33IGC trip started at Fagernes and ended at Gudvangen (Fig. 9), but with small adjustments the trip could conveniently be started in Oslo (a 3 h drive to Fagernes) and ended in Bergen (omitting the last 33IGC locality, Stop 6.4 in this Field Guide). The sequence of days and stops can, of course, be arranged in different ways. However, the 33IGC sequence is used as a basis, as it proved to be practical, allowing sufficient time for discussion on the outcrops, stops for buying provisions, etc., with a comfortable margin. In the following, the precise location of each Stop,

Dale Leirvik

Sogndal

Eidsbugarden

Brekkestranda Fagernes Gudvangen

Figure 9. Geological map of part of southern Norway, showing the route taken on Excursion 28 of the 33IGC in August 2008, which formed the basis for the present Field Guide. The overnight stops are indicated with open rings. The itineraries for Days 1–6 are detailed in this chapter of the guide. “Day 0” and “Day 7” were the 33IGC travel days, from Oslo to the first overnight stop (Fagernes), and from the last overnight stop (Gudvangen) back to Oslo, respectively.

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22

Chapter 3

with instructions on how to find the best outcrops, is given before the geological descriptions. Figures 4A and 4B together form a complete geological cross section along the Sognefjord transect (from Milnes et al., 1997). The structural positions of the excursion stops have been plotted on the appropriate parts of this cross section, together with a route map, at the beginning of the itinerary for each day. DAY 1. VALDRES-JOTUNHEIMEN The stops on Day 1 (Fig. 10) show different aspects of the thrust zone at the base of the Caledonian Allochthon, in the eastern segment of the Sognefjord transect (called the Jotunheimen Detachment Zone in Fig. 4). The overall structure can be described as a “tectonic sandwich.” The “slices of bread” above and below (the Jotun Complex, and the Baltic Shield Basement, respectively) behaved in a rigid manner and show practically no signs of Caledonian deformation, whereas the “butter” between represents the main Caledonian thrust zone and consists of intensely sheared phyllites and other metasediments, mainly of Paleozoic age, belonging to the Lower Allochthon. Southeast of the Sognefjord transect the main Caledonian thrust zone grades into a foreland fold-and-thrust belt with structural relations similar to the Jura Mountains, the foothills of the Canadian Rockies, and the Valley and Ridge Province of the Southern Appalachians. In fact, it has been suggested that the tectonic sandwich of this area is an exhumed equivalent of the Appalachian deep detachment, which was postulated on the basis of reflection seismic profiling (Milnes, 1987; cf. Iverson and Smithson, 1982; Hatcher et al., 1987). The mechanics of the thin-skinned, foreland fold-and-thrust belt in southern Norway, which extends southeastward to the south of Oslo, can thus be understood in terms of “bulldozer tectonics,” i.e., the critical tectonic taper model (e.g., Chapple, 1978; Dahlen, 1990; Malavieille, 2010), with the rigid Jotun Complex of the Middle Allochthon as the upper plate, causing the push from behind (the “bulldozer”). The displacement on the thrust zone is estimated at 200–300 km, with overthrusting from NW to SE, but there is clear evidence of late-stage, reversed movement on the same zone, with a top-to-NW sense of movement, possibly with a displacement of 10–20 km. At an even later stage the LærdalGjende Fault developed, also with top-to-NW kinematics, cutting discordantly through the main Caledonian thrust zone (Heim et al., 1977; Koestler, 1983; Lutro and Tveten, 1996). The complicated structural history of the main Caledonian thrust zone and its later extensional overprinting will be the theme of Day 1, together with some aspects of the Precambrian history of the Jotun Complex above and the Baltic Shield Basement below. Stop 1.1. Søndrol Location Stop 1.1 lies on the main E16 road, ~40 km in direction Bergen from Fagernes. From the road intersection and bridge at the extreme eastern end of lake Vangsmjøsa (Hemsingbru), continue

on E16 along the steep southern shore of the lake (tunnels, cliff sections) until the landscape opens out. After 3 km, there is a sharp curve in the road with a parking area on the right (677809N, 47892E) and a wide view of the lake and the surrounding mountains. High-visibility vests must be worn! Description This is a viewpoint stop, serving as an introduction to the sandwich tectonics, which will be studied on Day 1. In the mountains south of the lake (Figs. 11A and 11B) a prominent break of slope can be seen (actually, it can be followed across country to the south for several tens of kilometers), with Precambrian crystalline rocks of the Jotun Complex in the cliffs above (Middle Allochthon), and argillaceous metasediments (Vang phyllites, Lower Allochthon) forming the gentle slopes below (Fig. 11C). This line marks the top of the main Caledonian thrust zone in this area. Most of the movement (estimated at 200–300 km), however, must have taken place within the phyllites, as there is commonly very little sign of mylonitization in the Jotun crystallines at the contact (e.g., Fig. 11D, 1-m-thick mylonite). The contact between the Vang phyllites and the underlying Precambrian basement of the Baltic Shield (outcropping here in the Vang window) is not well exposed but has a similar character: heavily deformed phyllites and quartzites lying on undeformed Precambrian granites and migmatites (see Stop 1.3). North of lake Vangsmjøsi the top of the main Caledonian thrust zone is less easy to identify because an isoclinally folded sequence of Precambrian arkose (local name “Valdres sparagmites”) and other metasedimentary units occur below the Jotun Complex crystalline rocks, which, however, still form the upper parts of the mountain slopes. These relationships will be studied in more detail at Stops 1.4 and 1.5 (see particularly Fig. 14B for a schematic representation of relations in the thrust zone north of the lake and its head valley). Stop 1.2. Vangsmjøsi Location Return along E16 to the road intersection at Hemsingbru (~3 km) and turn left across the bridge. Continue up the winding road to the next intersection (Hen) and turn left again. The road now rises to a fine viewpoint above Vangsmjøsi (view similar to Stop 1.1), and then descends again to lake level, following the northern shore of the lake. Stop 1.2 consists of road sections just before (east of) a prominent, very deep road cut (678061N, 47373E). Description From the viewpoint above Vangsmjøsi down to the lakeshore, the road follows the contact between the Precambrian basement of the Vang window and the overlying phyllites (better seen at Stop 1.3). Afterward, the contact lies beneath the lake, and the road outcrops show typical phyllite relationships: highly heterogeneous and irregularly foliated-folded at hand-specimen scale, but very monotonous and uniform at the outcrop scale and

The field trip itinerary

23

Eidsbugarden

1.5

1.6 1.4 1.3

1.2 1.1 1

Fagernes

1.5

1.4

1.6

1.2 1.3

1.1

Figure 10. Route map for Day 1, with the structural positions of the excursion stops plotted on a reduced copy of the geological cross section shown in Figure 4.

24

Chapter 3

A

B

Fig. 11A

Fig. 11D

Fig. 11C

C

D

Figure 11. (Stop 1.1.) (A) View of the mountain Grindane from Stop 1.1, showing the basal thrust of the Jotun Nappe (Middle Allochthon) and the buildings of Søndrol farm (base camp for mapping in the 1970s). (B) Field book sketch of the mountains Grindane and Bergsfjellet made at Stop 1.1, showing the basal thrust (see Fig. 11A) and the main tectonic units: Precambrian Jotun crystalline complex (brown), Precambrian basement of the Vang window (red), and Paleozoic phyllites of the main Caledonian thrust zone (light green). The main movement direction of the Jotun Nappe relative to the basement was from the NW. (C) Contact of Jotun crystallines (forming the overhang) on Paleozoic phyllite on the mountain Grindane (see Fig. 11B), marked by ~1-m-thick mylonite (geologist Michael Heim). (D) The same contact ~10 km to the SW, on the mountain Rankonøsi (geologist Ondrej Voborny).

The field trip itinerary larger. Except for some prominent masses of typical dark quartzite (lenses, boudins, isoclinal fold hinges), mappable horizons are lacking, and the only constant structure is the phyllite foliation, which, although irregular, seems to dip consistently to the SE, i.e., toward the foreland, in the direction of the main movement. This is the wrong direction for development in a top-to-SE thrust zone. Also, the angle between the shear zone margins and the phyllite foliation, a regional feature in this area, would indicate a much lower shear strain than expected in a thrust zone of this magnitude. In the road outcrops to be studied here (Figs. 12B and 12C), the general SE dip of the main foliation is well seen, together with well-developed shear bands indicating top-to-NW sense of shear. This strong evidence of reverse movement in the main Caledonian thrust zone, which at this Stop practically obliterates any structures related to the main thrusting (top-to-SE), is a typical feature of the Lower-Middle Allochthon in southern Norway, as illustrated by the diagrams taken from Fossen (1992) (Figs. 12D and 12E). The summit of the mountain directly above these outcrops, Skutshorn (see Fig. 12A), consists of Jotun crystallines, and the prominent light-colored cliffs below consist of Valdres sparagmites (coarse, current-bedded arkoses seen in blocks to the west of the road cut) involved in a series of recumbent, isoclinal folds (Heim, 1979), together forming the Middle Allochthon. These folds are truncated at the basal contact, along which a mélange-like zone of heterogeneous lithology separates the sparagmites (Middle Allochthon) from the phyllites (Lower Allochthon) below. The mélange-like zone will be studied in detail at Stop 1.4, together with the basal contact of the zone, known as the Valdres thrust, inside the main Caledonian thrust zone. Since the Valdres thrust truncates the overlying folds, it is thought that the structure above the thrust represents an early stage in the thrust movement, which was later cut through and transported passively on top of the phyllites. The early part of the top-to-SE (contractional) thrusting belongs to the Jotunheimen-D2 phase, and the later part to the Jotunheimen-D3 phase (see Fig. 4). The reversed movement in the thrust zone belongs to Jotunheimen-D4 (Fossen’s “mode I extension”), and development of the Lærdal-Gjende Fault (see Stop 1.5) represents Jotunheimen-D5 (Fossen’s “mode II extension”), i.e., both developing under the post-collisional extension. Stop 1.3. Øye Location Continue westward along the road north of lake Vangsmjøsi to the intersection with E16 at Eidsbru and park near the store (678309N, 46721E). There are outcrops on both sides of E16 and along the local roads on each side of the main road; see locality sketch (Fig. 13A). High-visibility vests must be worn!

25

phyllite is exposed in the road cut along the secondary road just east of the E16 (Fig. 13A). The contact to the basement runs along (but is hidden below) the road. Several outcrops on the western side of E16, south of the store and along the small river, display a complex contact zone between migmatitic gneisses and the “Øye granite.” Migmatitic gneiss, exposed along the river (Figs. 13C and 13D), was formed during the Gothian orogeny at ca. 1520 Ma, as indicated by a U-Pb age for zircon and monazite determined on an associated gneiss near the location of Stop 1.1 (Corfu, 1980a, 1980b). The river outcrop also shows a crosscutting pegmatite dike, which is likely related to the Øye granite. Another set of exposures south of the intersection with E16 displays the contact zone between relatively fine-grained gray, massive granite and country rock xenoliths (Fig. 13E). More homogeneous, porphyritic granite is visible farther south in fresh road cuts at the right-hand turn of the road. The granite is part of a large pluton some 20 km long and 5 km wide, which extends to the northwest and occupies a major window through phyllite on Fillefjell up to ~1300 m. The relatively fine-grained border facies seen at this locality gives way abruptly to the more common coarse-grained appearance with up to 3-cm K-feldspar megacrysts in a 0.5–1 cm matrix of blue quartz, biotite, and plagioclase (Fig. 13C). One such outcrop is exposed a few hundred meters farther east (but for practical reasons cannot be visited during the field trip). The granite has been dated by U-Pb zircon and titanite to 930 Ma (Corfu, 1980a) and belongs to a suite that is widespread in the basement of southern Norway and southwestern Sweden. The late- to post-tectonic suite is also coeval with the Rogaland anorthosite and appears to be the result of melting following lithospheric delamination and/or crustal underthrusting (Andersson et al., 1996; Duchesne et al., 1999; Lundmark and Corfu, 2008a). The Caledonian effects on the granite are minimal. Titanite U-Pb ages have not been affected, and neither have the Rb-Sr ages on muscovite found on post-magmatic joints. Deformation is not evident even in the outcrops closest to the phyllite. The rapid gain in elevation of the contact between basement and phyllite, from 1300 m on Fillefjell, can be viewed as local basement uplift in the footwall of the Hardangervidda Shear Zone (Fossen and Hurich, 2005). Other authors have speculated that the basement in these windows along the southeast side of the “Faltungsgraben” may not be fully autochthonous, and may even be allochthonous (the “Windows allochthon” of Rice, 2005). There is no positive evidence that the basement in these windows could be allochthonous, although antiformal stacks due to underplating below décollements are common in analogue models (e.g., Malavieille, 2010, Fig. 2), and we prefer to assume that it represents more or less autochthonous Baltic Shield basement. Stop 1.4. Tyin Road Profile

Description The outcrops at this location exemplify the lithological diversity of the Baltic Shield Basement (Fig. 13B) and its lack of deformation near the contact with the overlying phyllite. The

Location From Stop 1.3, follow E16 up to Tyinkrysset (convenient stop for fuel and provisions), and from there take national road 53

26

Chapter 3

WNW

A

ESE

B

Stop 1.2

WNW

ESE

C E

N

D

Figs. 12B and 12C

Figure 12. (Stop 1.2.) (A) Field party on the opposite side of lake Vangsmjøsi from Stop 1.2, which lies at the base of the mountain Skutshorn. (B) Part of the road section at Stop 1.2: typical aspect of the Paleozoic phyllites of the main Caledonian thrust zone (geologist Ole Petter Wennberg). (C) Close-up of shear bands in Figure 12B, looking NE, indicating top-to-NW sense of shear (cf. Fig. 12D). These belong to the Jotunheimen-D4 deformation phase of Milnes et al. (1997), and correspond with the D2 phase of Fossen (1992) (see Figs. 12D and 12E). (D) Schematic view of the two principal phases of deformation in the main Caledonian detachment zone (Fossen, 1992). The D1 phase of Fossen corresponds with Jotunheimen-D3, and the D2 phase of Fossen corresponds with Jotunheimen-D4 (cf. Fig. 5). (E) Regional overview of top-to-NW structures (D2 = Jotunheimen-D4) in the main Caledonian detachment zone SW of the present area (from Fossen, 1992). WGR—Western Gneiss Region; UBN—Upper Bergsdalen Nappes; LBN—Lower Bergsdalen Nappes; HRNC—Hardangervidda-Ryfylke Nappe Complex.

The field trip itinerary

Fig. 13D

A

27

C

Fig. 13D

Fig. 13E

Fig. 13E from stop 1.2

Øye granite

E16

B Metasediment Amphibolite

Fig. 13C

Early crust Gabbro

Granite

Granitic gneiss

Late mafic dike

Sheared anorthosite

Early mafic dikes

E D

F

Figure 13. (Stop 1.3.) (A) Topographic map of the area around Stop 1.3. The contact between the basement and the overlying graphitic and/or quartzitic schists of the Vang Formation is covered by the road from Stop 1.2, where it intersects with E16. The black dashed line marks the contact zone between the Øye granite (to the south) and the country rocks (to the north). (B) Pictorial representation of relationships between the different lithological elements and structures in the autochthonous Precambrian basement, including the Øye granite (marked Granite; from Milnes and Koestler, 1985). (C) Schematic representation of contact relationships of Øye granite. The normally coarse-grained and K-feldspar porphyritic granite (a) gradually becomes finer grained and equigranular (b, c) as the contact is approached. The country rock (d) consists of amphibolite (A) invaded by multiple sets of granite and pegmatite veins (G) (from Corfu, 1980b). (D) Amphibolitic country rock cut by granitic veins and pegmatite. (E) Details of the contact zone: composite xenoliths with country rock and early granitic veins. (F) The stave church from Øye, just south of Stop 1.3, was built about A.D. 1200. It was dismantled in 1747, but many pieces were found later in the foundations of the new church and used to rebuild the original church at a slightly different location in the 1950s (looking ENE, toward lake Vangsmjøsi and Stop 1.2).

28

Chapter 3

toward Tyin and Årdal. The road profile starts 2.3 km after leaving the intersection with E16, just before a major left curve in the road (678740N, 45895E), and heads up the road from there to the shore of the lake Tyin. High-visibility vests must be worn! Description The road to Tyin provides a good section from the phyllite (Vang Formation, Lower Allochthon) to the Jotun crystallines passing through the sedimentary formations interpreted to be the cover of the crystalline rocks of the Jotun Complex (Valdres Zone, Figs. 14A and 14B) and composed of the lithologically mixed Tya Series and the more monotonous Valdres sparagmite (all Middle Allochthon). The underlying Vang Formation consists of black-gray, graphitic quartz-biotite-sericite schists, quartzose phyllites with abundant quartz veins, and rare marble, all displaying the main E- to SE-dipping schistosity with the crenulation cleavage (Fig. 14C) described at Stop 1.2. They are followed by rocks of the Tya Series, a zone resembling a tectonic mélange composed of thin black quartzites and greenish chlorite schists and phyllites with local carbonate layers and pods (Figs. 14D and 14E). The Tya Series has been correlated with the Mellsenn Formation farther to the east, a sequence of slates and quartzites containing Early Ordovician fossils (Loeschke and Nickelsen, 1968). The Valdres sparagmite (Fig. 14F) consists of light-gray to green meta-arkoses, characteristically with purple to pink feldspar clasts, black bands of heavy minerals, and local cross-bedding. The sequence is isoclinally folded. Dating of biotite, muscovite, and feldspar from sparagmite and the Tya Series by Rb-Sr yields ages of ca. 390 Ma (Schärer, 1980a, 1980b). Long stretches of the contact between sparagmite and the overlying Jotun crystallines are characterized by a prominent quartz-pebble conglomerate, which supports a depositional relationship of sparagmite on the Jotun crystallines before inversion during the thrusting process (see also Figs. 15D and 23D). Unfortunately, the conglomerate is missing at the present locality, but it will be seen on the long alternative route at Stop 1.5. In this region the Jotun crystallines consist mainly of monzonitic hornblende gneiss with local metagabbroic

units. Just at the top of the ascent, directly west of the southern tip of the lake, the crystalline consists of fine-grained greenish rocks from a late fault, probably a subsidiary of the Devonian fault system related to late Caledonian extension. The Jotun crystallines will be examined in greater detail at Stop 1.6. Stop 1.5. Tyedalen Location The program in Tyedalen (the valley that extends from lake Tyin down to Årdal, at the head of Sognefjord, along national road 53) depends on time and weather conditions. From Stop 1.4 continue along the main road toward Årdal for ~25 km to the small tarn, Holsbruvatn, and stop at a large parking area at the NW end of the tarn (679648N, 44152E). At Holsbruvatn, highvisibility vests must be worn! Description The geological map and profile shown in Figures 15A–15B are taken from the geological map at a scale of 1:50,000, which was published for this area (Koestler, 1989). The parking area lies exactly where the Lærdal-Gjende Fault crosses the road, and large in situ outcrops of the fault rock (cataclasites, Fig. 15E), as well as innumerable fresh boulders of the same material around the parking area, can be studied. The structural relations, however, are not well seen here and will be studied in detail at Stop 2.2. At a large scale, the Lærdal-Gjende Fault cuts discordantly through the basal units of the Jotun-Valdres Nappe, the Vang Formation, and the upper part of the Baltic Shield at different localities along its trace (here, the base of the nappe). It therefore postdates all the deformational structures seen earlier in Day 1 and belongs to the regional deformation phase Jotunheimen-D5 (see Fig. 5). The fine-weather, plenty-of-time program (3 h away from the vehicles) involves a short but strenuous ascent of ~300 m from the tunnel ~2 km back along the road from Holsbruvatn (679378N, 44411E), marked by the red line in Figure 15A. The

Figure 14. (Stop 1.4.) (A) Geological map of the area around Stop 1.4, showing the location of the photos along the Tyin road profile (Figs. 14C– 14F). From bottom to top: green, Vang phyllites; light yellow, Tya Series; dark yellow, Valdres sparagmites; red, brown, Jotun crystallines. (B) Pictorial summary of relationships in the deformed zone (the main Caledonian thrust zone) between the autochthonous Baltic Shield Basement and allochthonous Jotun crystalline complex (from Milnes and Koestler, 1985). The position of the Tyin road profile is shown in red. The main top-to-SE movement on the Valdres thrust took place after the formation of the Valdres-S2 foliation (belonging to what is now called the Jotunheimen-D2 deformation phase; see Fig. 5) and before the top-to-NW reversed movement in the Vang phyllites seen at Stop 1.2 (earlier called Vang-D2; correlated now with the regional phase Jotunheimen-D4; see Fig. 5). (C) Start of the Tyin road profile: a prominent cliff on the left of the road, driving up, showing the contact between the Vang Formation (below) and the base of the Tya Series (above). The contact itself is known as the Valdres Thrust (geologist Geoff Milnes standing on the thrust). At this locality the Vang Formation consists of black graphitic schists (with typical folded quartz veins). The Tya Series begins with a succession of light- to dark-gray, generally rusty quartzites, also with typical quartz veining. Figures 14C–14E are a series of photos of the road section above this locality (see Fig. 14A). (D) Tya Series: gray quartzite and quartzitic schist, followed by black rusty schists (possibly a tectonic intercalation of the Vang Formation); above these, gray rusty quartzites and rusty dark schists. (E) Near the top of the Tya Series: rusty marble in sericite-chlorite schists. (F) Valdres sparagmite lying above the Tya Series. Sparagmites are light-gray meta-arkoses, commonly banded, with typical purple to pink feldspar clasts and feldspathic veins. There is good evidence that the Tya Series is in inverted stratigraphic position and represents the cover of the sparagmites and Jotun basement before thrusting. The words base, top, below, and above in these captions (Figs. 14C–14F) refer to present-day positions. This is not obvious at the present locality, but local examples of inverted current bedding have been observed (and heftily discussed!) in the sparagmites along this section.

The field trip itinerary

B A

29

Jotun Complex (Precambrian)

Valdres zone Valdres S2

Fig. 14F Fig. 14E Figs. 14C and D

Vang S2

Sparagmite

Tya Series Valdres Thrust Vang Fm. Phyllite/ schist Quartzite

Vang Thrust Basement Complex (Precambrian)

D

C

Vang F m

. Tya Seri e

E

s

F

A Stop 1.5

cro ss se ig.

,F

on

cti B

15

22 21 20 Jotun (NW of LGF)

7 LGF

15 14 13 Jotun (SE of LGF)

6 sparagmite

5

4 3 2 Tya Series

23 Vang Fm.

B

NW

SE

D C

Lær

dal-

G

eF jend

ault

E

The field trip itinerary upper part of this traverse is seen in Figure 15C (below the word Lærdal on the marked trace of the Lærdal-Gjende Fault). The profile starts in mylonitic Valdres sparagmite, containing greenish schist zones (mylonitized basement?) and enters heterogeneous, deformed Jotun crystalline toward the top. At this position, on a prominent rock terrace, deformed conglomerates similar to the characteristic quartz conglomerates of the Bygdin area (Hossack, 1968, see Fig. 15D) are encountered, interleaved with heterogeneously sheared Jotun crystallines. In the one area where relatively undeformed relations can be observed (Grønsennknipa, Hossack, 1972) the Bygdin conglomerate lies in stratigraphic contact with Jotun Complex rocks and is overlain by the Valdres sparagmites. Along the deformed base of the Middle Allochthon, this sequence is often reversed, suggesting widespread tectonic inversion of the stratigraphy, admittedly now dismembered and in places isoclinally folded. In bad weather but with plenty of time, a series of roadside stops can be made along the almost continuous road section as one drives back along national road 53 toward Tyin and Stop 1.6, using Figure 15A as a guide. Different stops show Valdres sparagmites with inverted(?) current bedding, Tya Series comparable with Stop 1.4, and Vang phyllites with dominant top-to-NW structures. The distance from Stop 1.1 to Stop 1.5 is 40 km as the crow flies, across strike, and the relations observed in Day 1 can be extrapolated at least 100 km along strike, in spite of all the local heterogeneities and variations. In the area of the Vang window, it is estimated that the phyllites lay at a depth of ~10 km at the end of thrusting, in Tyedalen perhaps 10–15 km, and on the NW side of the Faltungsgraben (e.g., Stop 2.6) ~15–20 km (new biotite, small garnets), suggesting a gently NW-dipping midcrustal detachment at the time of the main top-to-SE thrusting (Jotunheimen-D3 phase, Fig. 5). Stop 1.6. Lorteviki–Eidsbugarden Location Drive back on national road 53 to the southern end of lake Tyin (the top of the profile of Stop 1.4) and then take to the left (minor road 252). Stop 1.6 is on the shore by the hut Lorteviki (679040N, 45923E). Description The Lorteviki outcrops (Fig. 16A) display crystalline rocks of the Jotun Complex in various stages of shearing (Figs. 16B

31

and 16C). Gneiss samples collected in the vicinity have yielded a U-Pb zircon age, indicating magmatic crystallization at ca. 1690 Ma. These data also yield evidence for a severe disturbance at ca. 910 Ma, coinciding with the age of titanite, and interpreted as indicating the time of amphibolite facies metamorphism and deformation (Schärer, 1980a). The syenitic to monzonitic gneisses are locally intruded by younger gabbros, giving ages of 1250 Ma (Schärer, 1980a). Most of the deformation seen in these gneisses is probably Sveconorwegian. In this region the Caledonian overprint was weak and localized to discrete shear zones and faults (Fig. 16B). It is recorded by the growth of green biotite, but it did not manage to completely reset the Rb-Sr system of the original brown biotite or of feldspar (Schärer, 1980a), and it did not affect U-Pb in zircon or titanite. Both Lorteviki and Eidsbugarden (tourist hotels at the end of road 252) offer good views of the majestic Jotunheimen mountains (Figs. 16D and 16E), which consist largely of twopyroxene, monzonitic to dioritic gneisses (mangerite to jotunite) with local peridotite lenses and gabbros (see geological map of Koestler, 1989). Lundmark et al. (2007) determined protolith ages of 1660–1630 Ma for variably retrograded high-grade gneisses, also documenting a complex Sveconorwegian history with high-grade metamorphism, at least two episodes of anatectic melting at 954 and 933 Ma, and the generation of granitic pegmatites repeatedly between 950 and 925 Ma. DAY 2. INNER SOGNEFJORDEN The main emphasis of Day 2 (Fig. 17) is on the structure and content of the large half-graben that is traditionally known as the “Faltungsgraben” (after Goldschmidt, 1912): the Lærdal-Gjende Fault (the southeastern bounding fault of the half-graben), the Jotun Complex (mainly Precambrian crystalline rocks that have been little affected by the Caledonian orogeny but contain some Caledonian intrusions) occupying the core of the half-graben, and the main Caledonian thrust zone, which emerges from under the Jotun Complex on the northwestern side of the half-graben (the continuation of the thrust zone studied on Day 1). The Jotun Complex in this region can be subdivided into three main parts. The Lower Jotun Nappe, which we traversed on Day 1, consists of plutonic rocks variously deformed and metamorphosed (at amphibolite facies) during the Sveconorwegian orogeny, but without Caledonian intrusives. The Upper Jotun Nappe reached granulite facies conditions and anatexis

Figure 15. (Stop 1.5.) (A) Part of the geological map, 1:50,000-scale sheet 1517 IV, Hurrungane (Koestler, 1989), showing the area of Stop 1.5 (the exact program depends on time and weather conditions). Legend: 20–22—lithological units in the Jotun Complex to NW of the fault zone; 7—cataclasites of the Lærdal-Gjende Fault (LGF) zone (cf. Fig. 15E); 13–15—lithological units in the Jotun Complex to SE of the fault zone; 6—Valdres sparagmites underlying the Jotun Complex (Bygdin-type conglomerates indicated with ring symbols, cf. Fig. 15D); 2–5—lithological units in the Tya Series; 23—phyllites of the Vang Formation (Paleozoic) underlying the Valdres Thrust (black toothed line). (B) Geological cross section along the line marked in Figure 15A (legend same as in Fig. 15A). (C) Distant view across Tyedalen, looking NNE, showing the trace of the Lærdal-Gjende Fault. In the background are the mountains of the main Jotunheimen range (photo by Michael Heim). (D) Photo of typical Bygdin-type quartz conglomerate (from the type area at Bygdin to SE of Tyedalen); see unit 6 in Figures 15A and 15B. (E) Close-up of typical hard and coherent cataclasite from the Lærdal-Gjende Fault at the Holsbruvatn locality (unit 7 in Figs. 15A and 15B).

32

Chapter 3

A Læ

rda

l-G

de jen

Fau

lt

Eidsbugarden

Stop 1.5

Stop 1.6 Stop 1.4

B

Mafic dike

Early mylonite Syenite

Gabbro

Fig. 16C Cataclas

ite

Syenite

Late mylonites and shear zones

Gabbro Thrust or modified erosional surface

C

D

E Figure 16. (Stop 1.6.) (A) Geological map of the Tyin area, showing the location of Stop 1.6 and the tourist hotels at Eidsbugarden (convenient overnight accommodations). The location of Stop 1.5 and the end of the Tyin road profile (Stop 1.4) are also shown. Color legend as in Figure 14A. (B) Pictorial summary of Precambrian relations in the Jotun Complex in the Tyin area (from Milnes and Koestler, 1985). (C) Mylonitic gneiss of the Jotun Complex, presumably resulting from Sveconorwegian deformation. (D) Participants on an excursion following the Uppsala Caledonide Symposium in 1981 stand on mylonitized amphibolite facies, monzosyenitic gneiss at Lorteviki, while admiring the parent pyroxene gneisses of the Jotunheimen in the background. (E) Cluster of buildings at Eidsbugarden, at the end of lake Bygdin, with the high mountains of Jotunheimen in the background (mainly of pyroxene granulites, with some peridotite).

The field trip itinerary

33

Eidsbugarden Sogndal Leikanger

2.6

2.4 2.1a

2.3

2.5

2.1b 2.2

Faltungsgraben

2.5 2.4 2.6

2.3

2.2

2.1

Figure 17. Route map for Day 2, and the structural position of the excursion stops plotted on a reduced copy of the geological cross section shown in Figure 4. UA—Upper Allochthon; MA—Middle Allochthon.

34

Chapter 3

and was deformed during the Sveconorwegian (late Precambrian) orogeny, and was later intruded by Silurian granite. The Upper Jotun Nappe can be subdivided into two parts with different compositions and histories. The segment northeast of Årdal (forming the Jotunheimen mountains proper, see Day 1) comprises 1.66–1.63 Ga mangerites and jotunites, with gabbro and peridotite, metamorphosed up to granulite facies, with local anatexis and deformation during the Sveconorwegian orogeny (950–900 Ma). This segment contrasts with the segment centered in inner Sognefjorden that is dominated by 965 Ma anorthosites, which underwent high-grade metamorphism between 950 and 890 Ma, and which were much more strongly affected by Caledonian events than the peripheral parts. The distinction is seen in the density of Silurian granitic dikes, the degree of Caledonian hydration and retrogression, and the degree of resetting of U-Pb systems (Lundmark and Corfu, 2007, 2008a, 2008b; Lundmark et al., 2007). From the point of view of regional tectonics, controversial questions are: What lies at depth below the Jotun Complex? Does the main Caledonian thrust zone studied on Day 1 really correlate with the more complex zone of phyllites, sparagmites, mylonitized basement, mafic schists, and garnet schists that emerges on the other side (e.g., Stop 2.6)? Is the Precambrian of the Baltic Shield Basement really continuous with the Precambrian basement of the Western Gneiss Complex of the Jøstedalsbreen Massif, without any intervening disturbance or, as some have postulated, suture? Our mapping led us to answer both the latter questions in the affirmative, but in spite of the deep incision of Sognefjorden, the base of the half-graben is not exhumed, and there is an “exposure gap” of at least 25 km along the Sognefjord transect (between Stop 2.2 to a short distance past Stop 2.4). However, in inner Hardangerfjorden (Fig. 2), far to the southwest of the present area, there is continuity of exposure at least between the Lower and Middle Allochthon on both sides of the Faltungsgraben. In Figure 4 the reconstruction of the base of the Jotun Complex (approximately the base of the Middle Allochthon) as descending to a maximum of ~5 km below sea level, where it is truncated by the Lærdal-Gjende Fault, is based on a recent reassessment of the gravity data (Skilbrei, 1990) and the “mid-crustal discontinuity” on an interpretation of the Sognefjord refraction seismic profile (Iwasaki et al., 1994). Based on rather clear stratigraphic and lithological indications, the continuity of the Jotunheimen detachment and the Baltic Shield Basement beneath the area of inner Sognefjorden prior to the development of the Lædal-Gjende Fault is the simplest hypothesis, and there are few, if any, indications that a more complicated situation (such as that postulated by Rice, 2005) may have prevailed. Stop 2.1. Årdal Location From Eidsbugarden, drive back to the intersection with national road 53, and turn right toward Årdal (passing on the way

Stop 1.5). For Stop 2.1 there are two alternatives (see Fig. 18A). The 2.1a site (679762N, 43784E) is a parking area on the side of the road, high over the fjord, ~4 km after driving past Stop 1.5. From the parking area, walk up the old mountain road that starts directly on the southeast side of national road 53. It offers numerous exposures along the road and an incomparable view of the fjord, but it is not recommended in rainy periods because of the potential for rockfall. The alternative (or additional) site 2.1b is a large picnic area beside the main road along Årdalsfjorden (678610N, 42686E). The road between sites 2.1a and 2.1b provides excellent views of the geological relationships in the 1000-m-high rock walls on the northwest side of the fjord and lake Årdalsvatnet (Figs. 18B and 18C). Details can also be examined at the ferry quay at Fodnes, where the ferry to Mannheller will be taken later in Day 2. Description The main focus at this stop is the Årdal intrusion and the related dike complex, a swarm of predominantly medium- to fine-grained leucocratic dikes intruding the partly retrograded granulite facies rocks of the Upper Jotun Nappe. The dikes are mainly granitic in composition, and they commonly contain xenoliths and range in size from centimeters to several tens of meters across, locally >200 m. They tend to be subparallel, north-south striking, and eastward dipping. Dikes in the central Upper Jotun Nappe generally exhibit a foliation defined by biotite, and show boudinage and pinch-and-swell structures as well as shear-induced folding (Figs. 18C and 18D; cf. Lundmark and Corfu, 2007). The dikes had historically been considered to be Caledonian intrusives (e.g., Goldschmidt, 1916; Schärer, 1980b), but an Rb-Sr whole rock age of ca. 900 Ma (Koestler, 1982) changed that perception. Recent U-Pb work, however, has demonstrated that the dikes are indeed Silurian in age (427 Ma), showing that the earlier Rb-Sr isochron was a result of extensive crustal contamination and incomplete mixing, also evident in the large amount of inherited zircon xenocrysts present in the rocks (Lundmark and Corfu, 2007, 2008b). The latter paper concludes that “the geometry of the dyke complex was primarily controlled by Caledonian pre- to syn-magmatic faults reflecting a top-to-southeast, non-coaxial, strain field” (Lundmark and Corfu, 2008b, p. 987). The strain field is interpreted to reflect nappe translation, and the age of the Årdal dike complex is therefore a minimum age for the Caledonian thrusting of the Jotun Complex in western Norway. Rheological changes induced by the magmatism permitted further hydration and deformation of the country rocks in the vicinity of the dikes in a continued top-to-SE, non-coaxial strain field, possibly reflecting continued translation of the Upper Jotun Nappe and its emplacement on top of the Lower Jotun Nappe. The final major modification of the architecture of the nappe complex was the development of top-to-NW normal faults (Lundmark and Corfu, 2008b), such as the Lærdal-Gjende Fault to be studied at Stop 2.2.

The field trip itinerary

A

35

Stop 2.1a Fig. 18B

Stop 2.1b

Stop 2.2

B

C

D

Figure 18. (Stop 2.1.) (A) Geological map of parts of inner Sognefjorden (from Lutro and Tveten, 1996), showing the location of Stop 2.1 (alternative sites 2.1a and 2.1b) and Stop 2.2. The red area approximately marks the core of the Årdal intrusion, but the intense veining extends far outside the core into the surrounding granulites (marked in brown). (B, C) Views of the steep rock walls on the northwest side of lake Årdalsvatnet, from the road between sites 2.1a and 2.1b. (D) Typical appearance of the granitic dikes in the vein network surrounding the Årdal intrusion, which is of Silurian age. Various generations can be recognized, showing variable amounts of deformation. Note also the relationship between fold size and vein thickness (geologist Fernando Corfu).

36

Chapter 3

Stop 2.2. Lærdal Location The best continuous profile through the Lærdal-Gjende Fault zone, including the footwall and hanging wall, occurs along the old main road from the village of Lærdalsøyri toward the now abandoned ferry quay at Revsnes. This runs along the southern shore of Lærdalsfjorden, whereas the present main road, national road 5, enters a tunnel on the north side of the fjord. Drive to the new ferry quay at Lærdalsøyri, along the old road, and park in the large parking area. The road profile of Stop 2.2 starts at the first outcrops on the old road, near the quay, and runs for ~500 m northwestward around the bay to where the road disappears into the first tunnel (677530N, 41678E; Fig. 19A). Note the ferry crossing between Stops 2.2 and 2.3; allow 20 min to drive from Stop 2.2 through the tunnel to the Fodnes ferry quay, and consult the current ferry timetable in order to judge how much time can be spent at Stop 2.2 (ferry route 14-107 Fodnes-Mannheller; see web site www.fjord1.no). If the ferry timetables for the trip have not been downloaded previously, a current ferry timetable pamphlet for the whole region should be obtained on board this first ferry in the itinerary. Description The profile starts in veined migmatitic gneisses of the Baltic Shield basement, with no sign of deformation postdating the veining (no Caledonian overprinting). It then passes through sequences of metasediments, increasingly mylonitized, grading into mylonites and ultramylonites derived from crystalline protoliths, and ends at the tunnel entrance in post-mylonitic cataclasites and in a 20-cm-wide zone of fault gouge (Figs. 19B– 19D). Cataclasites and country rock of the Jotun Complex can be viewed by traversing along the track that extends from the tunnel entrance along the fjord shore. Across the fjord, the topographic trace of the fault can be clearly seen (Fig. 19E). As in Tyedalen (Stop 1.5), part of the cataclasite in the fault zone is more resistant than many high-grade gneisses and stands out as a cliff or ridge across the mountainside (see also Fig. 15E). It is estimated that the cataclasite, up to 200 m thick, marks the main fault core, representing most of the hanging wall–down, top-toNW movement, probably with 20–30 km of displacement (see Fig. 4). The narrow zone of brittle reactivation and gouge formation is probably much younger (Mesozoic, coeval with tectonic events responsible for the development of the North Sea Basin; cf. Andersen et al., 1999) and of much less importance. The kine-

matics and significance of the mylonites and ultramylonites are problematic and will be a main point of discussion: Are they all related to the Lærdal-Gjende Fault, i.e., top-to-NW, or are they earlier, thrust related, i.e., top-to-SE? Stop 2.3. Eide Location From Lærdal, drive back through the Fodnes tunnel to the ferry crossing between Fodnes and Mannheller. The ferry crossing and the new highway between Mannheller and Kaupanger (national road 5) provide many opportunities to view (but not to stop and scrutinize!) various expressions of the Årdal dyke complex. After passing the industrial area north of Kaupanger, take the side road to Vestrheim, and then turn immediately to the right on a short dirt road that leads to a quarry (and horse training facility). Stop 2.3 (678679N, 40366E) is a partially active quarry; the extent and range of the visit may have to be reduced if quarrying is in progress. Description The walls of the quarry at Stop 2.3 (for location, see Fig. 20A), and especially the numerous blocks lying around, give a very good overview of the dominant features of the high-grade metamorphosed, 965 Ma Jotun anorthosite-gabbro-troctolite suite (Lundmark and Corfu, 2008a). The youngest major phase of the suite, a meta-troctolite, commonly exhibits primary compositional layering, and decimeter-scale orbicular coronas made up of shells of spinel, pyroxene, garnet, and amphibole surrounding olivine nodules in a plagioclase matrix (Griffin, 1971; Figs. 20B–20D). Griffin (1971) and Griffin et al. (1985a) suggest that the coronas formed by two stages of subsolidus reaction during magmatic cooling in the mid- to lower crust (S tectonite

B

C

D

E

The field trip itinerary

63

really run in one day (as was the case of the 33IGC excursion), a close watch has to be kept on timing, as four different ferries have to be caught. These are Dale-Eikenes (ferry route 14-415, at the start of the day), Askvoll-Gjervik (ferry route 14-431, between Stops 5.1 and 5.2), Gjervik-Fure (ferry route 14-431, after Stop 5.4), and Lavik-Oppedal (ferry route 14-251, at the end of the day). Download current timetables from web site www.fjord1 .no/en before the planned field trip.

5. Amphibolite facies mylonites mainly formed under noncoaxial top-to-W movement are related to large-scale movement on the extensional detachments active during the late orogenic extension of the Caledonides. These structural relationships are taken to indicate overall coaxial deformation in the lower crust, partly coeval with extensional detachment in the upper crust during exhumation (Engvik and Andersen, 2000).

Description The Vårdalsnes eclogite is in the upper part of the Western Gneiss Complex, structurally ~3 km below the Dalsfjord Fault at the top of the Nordfjord-Sogn Detachment. The body was mapped in great detail by A.K. Engvik, and the description given here is taken from her paper (Engvik and Andersen, 2000). The eclogite occurs as layers and lenses, variably retrograded to amphibolite. It is composed of garnet and omphacite with varying amounts of barroisite, actinolite, clinozoisite, kyanite, quartz, paragonite, phengite, and rutile, cut through by quartz veins and high-grade shear zones (Figs. 37B and 37C). The rocks record a five-stage evolution connected to Caledonian burial and subsequent exhumation (Fig. 37D): 1. A prograde evolution through amphibolite facies; 2. Formation of L>S-tectonite eclogite (T = 680 ± 20 °C, P = 16 ± 2 kbar) related to the subduction of continental crust; a lack of asymmetrical fabrics and the orientation of eclogite facies extensional veins indicate that the deformation regime during formation of the L>S fabric was coaxial (with vertical stretching, see Fig 37D); 3. Formation of subhorizontal eclogite facies foliation, with a horizontal stretching direction (see Fig. 33D); disruption of eclogite lenses and layers between symmetrical shear zones characterizes the dominantly coaxial deformation regime of stage 3. Locally occurring mylonitic eclogites (T = 690 ± 20 °C, P = 15 ± 1.5 kbar) with topto-W kinematics may, however, indicate that non-coaxial deformation was also active at eclogite facies conditions; 4. Development of a widespread regional amphibolite facies foliation (T = 564 ± 44 °C, P S stretching lineation in the boudins (adapted from Fig. 2 of Engvik and Andersen, 2000). (C) Detailed structural map of a part of the area with eclogite L>S tectonite. Notice the eclogite facies extensional veins oriented normal to the stretching lineation, and the N-S–oriented quartz veins associated with amphibolitization (adapted from Fig. 2 of Engvik and Andersen, 2000). (D) Pressure (P), temperature (T), and structural evolution for the Vårdalsnes eclogite. Boxes numbered 1–5 indicate P-T conditions for the five tectono-metamorphic stages described in the text. Boxes A, B, and C indicate the bulk strain regime and characteristic structures developed (adapted from Fig. 7 of Engvik and Andersen, 2000). Mg—margarite; Qtz—quartz; Zo—zoisite; Ky—kyanite; V—vapor. (E) Participants on the 33IGC excursion examine the Vårdalsnes eclogite at the site shown in Figure 37C and enjoy the splendid view across the fjord.

A

N 2 km

Fig. 38C Fig. 38B

Solund-Stavfjord Ophiolite Complex

Sunnfjord Melange

Fig. 40A Herland Group

Høyvik Group

Dalsfjord Suite top of NSD WGC

B

C

Figure 38. (Stop 5.2.) (A) Simplified geological map of Atløy with excursion localities in the hanging wall of the Nordfjord-Sogn Detachment (NSD). The accompanying cross section illustrates the structural relationships. Axial traces of large scale folds (overturned to recumbent, in blue) predate the Herland Group unconformity. In northern Atløy the Sunnfjord Mélange lies directly on the Høyvik Group. This contact is a low-angle angular unconformity. Thus, three major unconformities are preserved on Atløy: the Dalsfjord Suite–Høyvik Group contact, the Høyvik Group–Herland Group contact, and the Herland Group–Sunnfjord Mélange contact. (Figure courtesy of T.B. Andersen and H. Austrheim.) WGC—Western Gneiss Complex. (B) Low-grade mylonitic schists with top-to-W kinematic indicators exposed in the shore sections below the road at Stop 5.2 (coin is ~1.5 cm in diameter). (C) Dalsfjord Fault being inspected by participants on the 33IGC excursion.

The field trip itinerary Stop 5.3. Kviteneset Location Drive through the tunnel and continue north along road 608 for ~2.5 km, to a cattle grid. Stop near the small creek with prominent road sections, at a lefthand curve (Fig. 39B: 681140N, 28679E), a few hundred meters before reaching the northern end of the peninsula. Description This stop shows the contact between the Precambrian Dalsfjord Suite and the sedimentary Høyvik Group (Fig. 39A). The nature of the contact exposed by the road is not clear owing to the strong foliation developed during the Middle Ordovician (ca. 450 Ma) pre-Scandian deformation, but near the top of the hill to the west the depositional contact, including a basal conglomerate, is remarkably well preserved. The Dalsfjord Suite comprises banded felsic gneisses, less deformed gabbro, monzonites, and alkaline mangeritic rocks. Monzonitic rocks of the Dalsfjord Suite contain abundant mesoperthite, which is commonly found as typical clastic grains in the psammites of the Høyvik Group. Regionally the Dalsfjord Suite rocks are correlated with similar rocks in the Jotun and Lindås Nappes, an interpretation supported by U-Pb data (Corfu and Andersen, 2002). Locally, an up to 10-m-thick zone, highly enriched in Fe oxides and muscovite, possibly representing a paleo-lateritic weathering zone in the Dalsfjord gneisses, is present along the unconformity. The basal deposits consist of deformed pebbly conglomerates and a massive bluish subarkosic metasandstone (Granesund Formation). This is succeeded by quartz-rich mica schists, locally with layers of dolomitic marble (Kvitanes Formation) and metapsammites and schists (Atløy Formation). The Høyvik Group was deformed and metamorphosed in a pre-Scandian, Ordovician orogenic event, as indicated by the Ar-Ar plateau age of ca. 449 Ma for muscovite (ferri-phengite) in the main greenschist facies foliation at this locality (Andersen et al., 1998). Stop 5.4. Brurestakken Location Drive back along road 608 and continue past Gjervik, rounding the southern coast of Atløy. Stop and park at a small abandoned quarry on the right of the road, with the hill Brurestakken to the left (near the S in Sjøralden in Fig. 40A), ~1 km before Herland. The profile begins on the shores of lake Sjøralden at 680761N, 28100E (Fig. 40C) and ends on the summit of the hill Brurestakken at 6807615N, 280327E (Fig. 40D). Description This stop features the unconformity between the Høyvik Group and the overlying Herland Group, the stratigraphy of the Silurian Herland Group and the Sunnfjord Mélange, and the structures related to Scandian thrusting (Fig. 40B).

65

The Herland Group (Brekke and Solberg, 1987) consists of two formations. The base of the Sjøralden Formation is defined by fluvial conglomerates that overlie the Høyvik Group unconformably. They are followed upward by the Brurestakken Formation, a succession of shallow-marine sandstones, fine-grained wackes, mud-rich sandstones, a black mud-shale sequence, and a calcareous unit, locally with a mid-Silurian (Wenlock) shelly fauna, the diagnostic fossils being Pentamerus sp. The Brurestakken Formation comprises three coarse clastic units sandwiched between sandstone-shale and fossiliferous calcareous zones. The deformation of the Herland Group is related to layerparallel shortening associated with the obduction and emplacement of the 443 Ma (Dunning and Pedersen, 1988) SolundStavfjord Ophiolite, with a fold-and-thrust belt geometry related to the SE-directed tectonic transport during the Scandian orogeny, and late W-vergent, asymmetrical back-folds formed during the extension of the orogen (Fig. 40B). The Sunnfjord Mélange, formed during obduction and emplacement of the ophiolite, links the oceanic and continental rocks in this region. The contact between the mélange and the Herland Group is highly sheared, although locally an unconformable depositional contact is preserved (Fig. 38A). The mélange contains a variety of clasts, those in the conglomerates reflecting a bimodal source of both continental and oceanic affinity and formation in a rapidly deepening foreland basin. The rapid subsidence was probably a result of the loading on the continental margin by the advancing ophiolite nappe and its cover. DAY 6. FENSFJORDEN-LINDÅS During Days 4 and 5 we looked at the Nordfjord-Sogn Detachment and the geology of the footwall and hanging wall of this major extensional detachment. Losna (Stop 4.1) is practically the southernmost exposure of the detachment, which then descends below sea level into Sognesjøen and never reappears (except speculatively on some offshore seismic profiles, e.g., Færseth et al., 1995). However, it was recently discovered that a major extensional shear zone does come onshore to the south, in the Bergen area. This is known as the Bergen Arc Shear Zone, and at its northern end Devonian conglomerates lie unconformably on deeply eroded Caledonian Allochthons in the hanging wall (Wennberg, 1998; Wennberg et al., 1998). Although the geometry of the Bergen Arc Shear Zone is quite different from that of the Nordfjord-Sogn Detachment, the shear vector is remarkably similar: The mean orientation of the shear direction in the north (area nearest to the detachment) is found to be 20° to 276° top-to-W, as compared to 12° to 289° on Losna (Stop 4.1). However, the shear zone orientation is much steeper (mean dip 55° to 208°). This means that it is an oblique-slip shear zone, and it is interpreted as a lateral ramp, branching off from the main Nordfjord-Sogn Detachment. The minimum displacement in the shear direction is estimated to be ~16 km. A prominent set of NE-SW–striking normal faults of Devonian age (Larsen et al., 2003), which are developed in the hanging wall of the

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Chapter 3

Høyvik

A Høyvik Group

Kvitanes

Atløy Fm. (meta-psammite + schist) Kvitanes Fm. (mica-schist + marble) Granesund Fm. (cgl., arkose) Nonconformity

Dalsfjord Suite Monzonitic gneiss + gabbro

Stop 5.3

500 m

Granesund

B

N

Stop 5.3 Figure 39. (Stop 5.3.) (A) Geological map of the Kviteneset area, Atløy, showing the nonconformity between the Dalsfjord Suite and the Høyvik Group (unpublished map, T.B. Andersen, 1985). The best preserved localities are along the top and just over on the SW side of the hill between Granesund and Høyvik (symbolized with black dots). (B) Panoramic view of Stop 5.3, looking west (from http://kart.sesam.no/3d/).

Figure 40. (Stop 5.4.) (A) Detailed geological map of the area around Brurestakken. (Unpublished map by T.B. Andersen, 1985; figure courtesy of T.B. Andersen and H. Austrheim.) (B) Restored, pre-D3 vertical section of the Herland area (from Andersen et al., 1990), showing the structural and stratigraphic relationships between the continental units (Dalsfjord Suite, Høyvik Group, and Herland Group cover) and the Sunnfjord Mélange and Solund-Stavfjord Ophiolite Complex. (C) Participants on the 33IGC excursion on the western shore of lake Sjøralden, inspecting fluvial conglomerates of the Sjøralden Formation (Herland Group) and the well-exposed unconformity above the Høyvik Group. (D) View from Brurestakken toward the islands Alden and Tviberg in the west (see also Fig. 34E). The islands are composed of units of the Solund-Stavfjord Ophiolite Complex. A white granitic pluton, related to the syntectonic Sogneskollen pluton discussed on Day 4, is visible near the shore of the first island, Tviberg (geologist Petras Sinkunas).

The field trip itinerary

67

A

Fig. 40C Fig. 40D

Fig. 40D

Fig. 40C

B C

D

68

Chapter 3

Bergen Arc Shear Zone, are thought to be related to this extensional shearing. Unfortunately, many of the most interesting outcrops of the Bergen Arc Shear Zone are difficult to access. Day 6 is conceived as a stop-and-look day following the main road toward Bergen (Fig. 41). It starts in the Western Gneiss Complex in the footwall of the Bergen Arc Shear Zone, continues through remnants of the Lower, Middle, and Upper Allochthons within the shear zone itself, partially overprinted by brittle faulting (represented by the Fensfjord Fault), and ends, in the hanging wall, in a large nappe complex, the Lindås Nappe, which is dominated by granulite facies rocks (anorthosite, gabbro, charnockite, mangerite) and whose tectonostratigraphic position is controversial (Middle or Upper Allochthon?). One subunit in the Lindås nappe, the Holsnøy subunit, contains the famous “eclogitized” Caledonian shear zones (Austrheim and Griffin, 1985; Austrheim, 1987, 1990; Bingen et al., 2004; Glodny et al., 2008), which will be studied as the final stop in this cross section. The day ends with a long drive along the main Bergen-Oslo road (E16) to Gudvangen (Sognefjorden again), with some buswindow geology and a short stop in the anorthosites of the Jotun Complex along the way. Stop 6.1. Kjekallevågen Location South of Sognefjorden, the route from the Day 5 localities follows the main E39 road toward Bergen, and takes off along road 570 following the northern shore of inner Fensfjorden for ~7 km. At this point, road 570 crosses a bridge over a small arm of the fjord (Kjekallevågen). Just after the bridge, stop at a parking place and picnic area on the left (673551N, 30449E). Description From this parking place a good view of the Bergen Arc Shear Zone is obtained. It runs from the low islands visible on the horizon to the northwest along the whole length of Fensfjorden to the deep inlet visible toward the southeast (Fig. 42A), a distance of ~50 km—all under water (except for its marginal parts). It comes to land to the southeast, and a cross section through the whole zone will be studied at Stop 6.2. The islands at its northern end, however, have been mapped in detail, to produce the structural profile a–aʹ shown in Figure 42B. One of the islands exposes a fine eclogite in the footwall of the Bergen Arc Shear Zone (Winswold, 1996) embedded in the gneisses of the Western Gneiss Complex (cf. Stop 3.6), as well as part of the transition zone into the shear zone proper (mylonitic gneisses, Figs. 42C and 42D), although most of the actual shear zone is submerged here. Although there are few data from the shear zone itself in this northern area, the structural data from the footwall and hanging wall show some interesting features (Fig. 42E). First, the prominent stretching lineation in the Western Gneiss Complex, parallel to the axes of a set of tight upright folds and developed under amphibolite facies conditions (with superficial similarities to parts of the Sognefjord profile, cf. Stops 3.4 and 3.5, although

structural mapping of this complex between Fensfjorden and Sognefjorden has not yet been carried out), has a similar orientation to the shear vector in the Bergen Arc Shear Zone (Fig. 42Ea). Nevertheless, detailed mapping along the northern shore of Fensfjorden has allowed the transition from the Western Gneiss Complex to the overprinted, lower grade mylonitic gneisses of the Bergen Arc Shear Zone proper to be described in detail (Wennberg et al., 1998). Second, the bedding data from the Devonian islands (Byrknesøyene, see Figs. 42A and 42B) show irregular orientations, with no obvious folding but rather with a constant dip of 10°–30° to the northeast (Fig. 42E, c). Third, the internal structure of the Lindås nappe in this area is dominated by a major, almost isoclinal, upright synform (Børillen synform), affecting the main amphibolite-facies foliation in the gneisses of the unit (Fig. 42E, b). The axial trace of this synform runs parallel to the trace of the Bergen Arc Shear Zone but disappears under water and the Devonian islands in the north (Fig. 42A). Based on a detailed analysis of the metamorphic and structural history of the whole area (Borthen, 1995), the deformation leading to the dominant foliation, and the later isoclinal folding, are thought to be of Caledonian age, and the lack of similar folding in the Devonian conglomerates indicates that the folding predates their deposition. Hence, on Figure 41 the cross section through Stops 6.1–6.3 shows a stratigraphic unconformity at the base of the Devonian conglomerates, truncating the Børillen synform in the Lindås Nappe. From the Stop 6.1 parking place, a spectacular geomorphological phenomenon can, however, not be seen! The very low topography of the islands on the opposite side of Fensfjorden is typical of the Norwegian strandflat, the rock plain just above present sea level, which is found all along the Norwegian west coast and which is thought to be an erosional surface of late Pliocene– Pleistocene age (the product of a combination of glacial erosion, marine erosion, and subaerial weathering; see Holtedahl, 1998; Aarseth and Fossen, 2004). In this region the strandflat is ~30 km wide and undulates between 0 and 50 m above sea level, with a mean around 20 m. What is not seen is the depth of the Fensfjorden. At the level of the Mongstad oil refinery (seen from this stop), the detailed subsurface topographic maps (made for laying the oil pipelines) show extremely steep fjord walls descending to flat fjord bottoms at depths exceeding 500 m. The mode of formation of this spectacular “hidden topography,” with steep-sided U-shaped valleys separated by large flat-topped mountains and plateaus, is still problematic. Stop 6.2. Osterfjorden Location From Stop 6.1, return along road 570 to the intersection with E39, and turn right toward Bergen. The localities follow in sequence along the main road, which follows the shore of Osterfjorden toward Knarvik and the floating bridge across Osterfjorden to Bergen: 6.2a, Bjørsvik (UTM 30795E, 67276N); 6.2b, Ostereidet, petrol station (UTM 30704E, 67263N); 6.2c,

The field trip itinerary

69

Brekkestranda

Gudvangen

6.4 6.1 6.3 6.2

6.4

6.3 Main thrust

Øygarden Gneiss Complex Lindås Nappe BASZ = Bergen Arc Shear Zone BS = Bjørillen Synform FF = Fensfjord Fault SFC = Sotra-Fedje Culmination

6.2

6.1

Major Bergen Arc

Figure 41. Route map for Day 6, with the structural position of the excursion stops plotted on a generalized cross section through the Bergen area (approximately along the line of Stops 6.1–6.3, from Milnes and Wennberg, 1997), and a reduced copy of the geological cross section shown in Figure 4 for Stop 6.4. The colors used on the Bergen cross section are as follows, from right to left: pink—Western Gneiss Complex; dark olive—Kvalsida Gneiss (possible Jotun equivalent); dark green—Major Bergen Arc Zone (ophiolites and related rocks); middle olive—Lindås Nappe (possible Jotun equivalent); violet—Ulriken subunit of Lindås Nappe: light green—Minor Bergen Arc Zone (sediments with Late Ordovician–Early Silurian fossils); pink—Øygarden Gneiss Complex (continuation of Western Gneiss Complex). On the cross section for Stop 6.4: UA—Upper Allochthon; MA—Middle Allochthon.

A

a´

Fig. 42D

a

n tio ec 41 s s . os Fig Cr in

Stop 6.1

b´ Stop 6.2

b

Figure 42. (Stop 6.1.) (A) Overview structural map of the Bergen Arc Shear Zone (BASZ) in the area south of Sognefjorden (see Fig. 41). (B) Structural profile through the Bergen Arc Shear Zone at the northern end of Fensfjorden, marked a–aʹ in Figure 42A (after Wennberg et al., 1998). (C) Field party studying mylonitic gneisses within the Bergen Arc Shear Zone at the south end of the structural profile, Figure 42B, looking SE along Fensfjordenen. The island in the background consists of Devonian conglomerates in the hanging wall of the shear zone (geologists Ole Petter Wennberg, Inger Winsvold, Sigrid Borthen). (D) Sense of shear indicators in mylonitic gneisses toward the footwall margin of the Bergen Arc Shear Zone (photo by Ole Petter Wennberg). (E) Structural data from the footwall and hanging wall of the Bergen Arc Shear Zone (from Wennberg et al., 1998).

B Fig. 42C

a

a´

D

C

E

The field trip itinerary Totland picnic area (UTM 30612E, 67245N); 6.2d, Sauvåg picnic area (UTM 30479E, 67247N). High-visibility vests must be worn at all these stops, and care must be taken to keep off the road, which is at times quite busy. Description This sequence of stops provides a brief introduction to the different tectonic units in the Bergen area, both inside and outside the Bergen Arc Shear Zone (Wennberg, 1998). These units are summarized in the general cross section shown in Figure 41, which is drawn along the line indicated in Figure 42A. The sequence of stops along the main road is marked on the structural profile, Figure 43A (line of section b–bʹ is marked in Fig. 42A), and on the geological map of the area, Figure 43B (based on the detailed structural map, scale 1:13,000, in Wennberg, 1998). Stop 6.2a is situated within the Bergen Arc Shear Zone, near its footwall margin. The late brittle part of the movement (Fensfjord Fault) is concentrated in the deep gully just before the main road to Bergen disappears into the tunnel, opposite the road intersection to Bjørsvik, but brittle deformation effects overprint the whole road section east of the gully. On each side of the gully, packets of black garnet-mica schist occur, probably remnants of the onetime cover of the Western Gneiss Complex or possibly the Lower Allochthon. The tunnel entrance marks the contact with the Kvalsida Gneiss unit, containing mylonitized rocks of Jotun affinity lying within the Bergen Arc Shear Zone. Stop 6.2b lies in the central part of the Bergen Arc Shear Zone, and the main road sections show heterogeneously banded and mylonitized Kvalsida Gneiss units (Fig. 43C). Stop 6.2c lies in the hanging wall, just outside the margin of the Bergen Arc Shear Zone. The contact between the Lindås Nappe (Fosnøy subunit) and the underlying Major Bergen Arc zone is exposed in the road section that starts at the picnic area and runs northwestward on the inside curve of the road (DANGER!). The hanging-wall margin of the Bergen Arc Shear Zone is not seen here, but it has been studied in detail along the shore of Osterfjorden and subjected to detailed structural analysis (Fig. 43D; see Wennberg, 1996). Stop 6.2d lies within the Lindås Nappe (Fosnøy subunit) approximately on the hinge of the Børillen synform (see Stop 6.1), hence the subhorizontal foliation (as opposed to the steeply dipping foliation and contact at Stop 6.2c). Crosscutting but strongly folded felsic veins of the Ostereidet dike swarm (Wennberg et al., 2001) are well exposed (Figs. 43E and 43F). One controversial point in the regional geology has been the relationship between the Lindås Nappe and the Upper Jotun Nappe that we crossed on Day 2. The two are similar in terms of lithology, metamorphism, and age, and they have both been intruded by Silurian syntectonic leucocratic granitic (to trondhjemitic?) dikes (Fig. 43F). The only apparent difference is the presence of eclogitized shear zones in parts of the Lindås Nappe (Holsnøy subunit, Stop 6.3) and their absence in the Upper Jotun Nappe. Tectonically, however, they differ in that the Lindås

71

Nappe is structurally juxtaposed on Paleozoic ophiolitic units of the Upper Allochthon (Major Bergen Arc), whereas no such (proven) units appear to be present underneath the Jotun Nappe Complex. The geochemical and isotopic composition of the Årdal dike complex, however, is best explained by the melting of sedimentary rocks similar to those invoked for the genesis of granitic rocks at this stop and for the Sogneskollen intrusion (cf. Stop 4.3) (Skjerlie et al., 2000; Lundmark and Corfu, 2007). If the Lindås and Upper Jotun have a common origin, and both were derived from the margin of Baltica, then more complex tectonic solutions are needed to bring the Lindås on top of the ophiolites (Middle Allochthon on top of Upper Allochthon!). The alternative is that they have distinct origins, as implied in Figure 41 (from Milnes and Wennberg, 1997). Stop 6.3. Holsnøy Location At Knarvik, take road 564 for Holsnøy, reached over two bridges and one intervening island. Follow the road west, and then northwestward to Rossland; continue north and then westward to Sætrevik, and from there walk to the east side of the bay (672400N, 27884E). Description The island of Holsnøy is geologically a part of the Lindås Nappe (the Holsnøy subunit, see Milnes and Wennberg, 1997) and is composed of a granulite facies AMCG (anorthosite, mangerite, charnockite, and granite) complex, locally overprinted by eclogite facies metamorphism (Figs. 44A and 44B). Ages reported so far for the magmatic emplacement of the complex range from ca. 1240 to 951 Ma with high-grade metamorphism at 929 Ma (Bingen et al., 2001; Glodny et al., 2008). The complex has generally been considered to be related to the high-grade rocks of the Upper Jotun Nappe seen on Days 1 and 2, an inference supported by the geochronological work (but see discussion for Stop 6.2). Subduction of the Baltic margin during the Caledonian orogeny led to the partial eclogitization of the granulites at 430 Ma, followed by an amphibolite facies overprint at 414 Ma. The mode of development of the various paragenetic assemblages and related structures in these rocks has become a prime example for the importance of fluids in enabling the progress of mineral reactions and, conversely, their complete inhibition under dry conditions (e.g., Austrheim and Griffin, 1985; Austrheim, 1987, 1990; Jamtveit et al., 1990, 2000; Austrheim and Boundy, 1994; Jolivet et al., 2005; Bjørnerud and Austrheim, 2006). The region has also become an important area for studying the behavior of isotopic systems and the interpretation of radiometric ages (e.g., Bingen et al., 2001, 2004; Kühn et al., 2000, 2002; Camacho et al., 2005; Glodny et al., 2008; Andersen and Austrheim, 2008). The general interpretation is that the granulites were subducted in the Silurian, reaching eclogite facies conditions (650– 750 °C and 15–17 kbar) at which they underwent brittle fracturing. The fractures were infiltrated by fluids, which promoted further

b

6.2d

B

6.2d

A

C

6.2c

Fig. 43D Askvik

6.2b

6.2a

6.2c

E

D

F

Figure 43. (Stop 6.2.) (A) Structural profile through the Bergen Arc Shear Zone (BASZ) along the Osterfjord cross section, marked b–bʹ in Figure 42A (after Wennberg et al., 1998). LC—Lindås Complex; MaBA—Major Bergen Arc; KG—Kvalside Gneiss; WGC—Western Gneiss Complex. (B) Geological map of the Osterfjorden cross section, showing the location of the short stops included in Stop 6.2 (simplified after Wennberg, 1998). (C) Heterogeneous mylonite gneisses of the Kvalside unit, in the central part of the Bergen Arc Shear Zone, at short stop 6.2b (geologist Fernando Corfu). (D) Structural summary of the superimposed fabrics owing to reversal of shear sense, across the hanging-wall margin of the Bergen Arc Shear Zone (from Wennberg, 1996). The upper zone of sinistral kinematic indicators represents the nappe movements that predate the development of the Bergen Arc Shear Zone; the lower zone of dextral kinematic indicators shows the structures developed during movement in this shear zone. The transition zone marks the hanging-wall margin of this shear zone where the two sets of shear indicators are superimposed. (E, F) Two examples of crosscutting but strongly folded felsic veins in the Lindås nappe at short stop 6.2d.

6.2b 6.2a

b´

The field trip itinerary

A

73

C

Stop 6.3

D

B

Stop 6.3

E

Figure 44. (Stop 6.3.) (A) Geological map of Holsnøy, showing the distribution of the main lithologies: red and orange for mangeritecharnockite, and dotted-brown for anorthosite to gabbro. (B) Map showing the distribution and intensity of eclogitized shear zones and the degree of eclogitization (from Jamtveit et al., 1990). (C) Classical appearance of eclogitization in the Lindås Nappe. Eclogite develops symmetrically around a central vein filling a fracture in the original granulite (from Jamtveit et al., 1990). (D) Eclogite fingers penetrate into anorthosite, starting from an eclogite shear zone at the bottom of the picture (from Jamtveit et al., 1990). (E) Participants of the 33IGC excursion study the eclogitized outcrops at Stop 6.3 (photo by George DeVries Klein).

74

Chapter 3

A

B

Stops 2.3 + 2.4

Stop 2.1

Stop 6.4

Stop 2.2

Stop 6.4

Fig. 45B

C D

Figure 45. (Stop 6.4.) (A) Regional distribution of anorthositic-gabbroic phases within the Jotun Nappe Complex (from Wanvik, 2000). (B) Geological map of the Gudvangen-Mjolfjell massif, focused especially on the industrial quality of the anorthosite in terms of purity and solubility in consideration of its potential for extracting aluminum (from Wanvik, 2000). (C) View from Stahlheim toward Gudvangen and Nærøyfjorden in the NNE. The rounded mountain top to the left, Jordalsnuten, consists of pure anorthosite and therefore has very little vegetation. (D) Anorthosite is extracted for various industrial purposes, not the least as an ornamental stone, as seen in this picture.

The field trip itinerary shearing, enabling crystallization of the eclogite facies paragenesis (Figs. 44C–44E). Subsequent deformation and/or fluid intrusion was controlled largely by eclogite, which is rheologically weaker than granulite. The eclogites occur in anastomosing shear zones, mostly trending NW, with the mineralogy of eclogitized anorthosite consisting of omphacite, garnet, kyanite, zoisite, phengite, rutile, quartz, and amphibole. Stop 6.4. Stalheim Location On the drive back to Oslo from Bergen (route E16, through Voss), leave the main highway at Stalheim and stop near the hotel (67470N, 37390E) for a brief overview of the zone. From there, descend to the valley and drive ~2 km northward, cross a small bridge, and stop on the side road to the anorthosite quarry to the left (67476N, 376398E). Description Stalheim offers an excellent view of Nærøydalen, with Gudvangen and Nærøyfjord in the background. The spectacular Nærøyfjord is ranked as a UNESCO World Heritage Site. The rounded mountaintop Jordalsnuten to the left is essentially bare of vegetation owing to its nutrient-poor anorthositic composition (Fig. 45C). The anorthosite can be examined in blocks near two quarries at the foot of Jordalsnuten. The variety present at this

75

location is highly leucocratic (70) is easily soluble in mineral acids, and the bytownite plagioclase of the Inner Sogn anorthosite makes it well suited for industrial processes based on acid leaching. The high aluminum content, ca. 31% Al2O3, has made these occurrences interesting for various industrial applications, especially as an alternative raw material for the Norwegian aluminum industry” (Wanvik, 2000, p. 103). A process currently under development and testing would co-produce aluminum, silica, and calcium and would consume CO2, which represents an interesting modern environmental application. Current mining activity produces anorthosite as a reflective material for mixing with asphalt or concrete in pavement construction, as a refractory material in industrial applications, for the manufacture of mineral wool, and for other applications.

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Bingen, B., Skår, Ø., Marker, M., Sigmond, E.M.O., Nordgulen, Ø., Ragnhildstveit, J., Mansfeld, J., Tucker, R.D., and Liégeois, J.-P., 2005, Timing of continental building in the Sveconorwegian orogen, SW Scandinavia: Norwegian Journal of Geology, v. 85, p. 87–116. Bjørnerud, M.G., and Austrheim, H., 2006, Geophysics: Hot fluids or rock in eclogite metamorphism?: Nature, v. 440, E4, doi:10.1038/nature04714. Bøe, O., 1997, Structural evolution of the Nordfjord-Sogn Shear Zone in Solund, Losna, outer Sognefjord, Western Norway [unpublished thesis]: University of Bergen, Norway, 116 p. Borthen, S., 1995, Børillen synform in the Lindås Complex, Western Norway— Petrography, structural geology and regional significance [unpublished thesis]: University of Bergen, Norway, 143 p. Braathen, A., Osmundsen, P.T., and Gabrielsen, R.H., 2004, Dynamic development of fault rocks in a crustal scale detachment: An example from western Norway: Tectonics, v. 23, TC4010, doi:10.1029/2003TC001558. Brekke, H., and Solberg, P.O., 1987, The geology of Atløy, Sunnfjord, western Norway: Geological Survey of Norway (NGU) Bulletin, v. 410, p. 73–94. Camacho, A., Lee, J.K.W., Hensen, B.J., and Braun, J., 2005, Short-lived orogenic cycles and the eclogitization of cold crust by spasmodic hot fluids: Nature, v. 435, p. 1191–1196, doi:10.1038/nature03643. Chapple, W.M., 1978, Mechanics of thin-skinned fold-and-thrust belts: Geological Society of America Bulletin, v. 89, p. 1189–1198, doi:10.1130/0016 -7606(1978)892.0.CO;2. Chauvet, A., and Dallmeyer, R.D., 1992, 40Ar/39Ar mineral dates related to Devonian extension in the southwestern Scandinavian Caledonides: Tectonophysics, v. 210, p. 155–177, doi:10.1016/0040-1951(92)90133-Q. Chauvet, A., and Séranne, M., 1994, Extension-parallel folding in the Scandinavian Caledonides: Implications for late-orogenic processes: Tectonophysics, v. 238, p. 31–54, doi:10.1016/0040-1951(94)90048-5. Cloos, M., 1993, Lithospheric buoyancy and collisional orogenesis: Subduction of oceanic plateaus, continental margins, island arcs, spreading ridges, and seamounts: Geological Society of America Bulletin, v. 105, p. 715– 737, doi:10.1130/0016-7606(1993)1052.3.CO;2. Corfu, F., 1980a, Geologie der Jotun Decke sowie U-Pb und Rb-Sr Geochronologie des darunterliegenden präkambrischen Schildes, Fillefjell, Südnorwegen [Ph.D. thesis]: ETH Zürich, Switzerland, 138 p. Corfu, F., 1980b, U-Pb and Rb-Sr systematics in a poly-orogenic segment of the Precambrian shield, central southern Norway: Lithos, v. 13, p. 305–323, doi:10.1016/0024-4937(80)90051-1. Corfu, F., and Andersen, T.B., 2002, U-Pb ages of the Dalsfjord Complex, SWNorway, and their bearing on the correlation of allochthonous crystalline segments of the Scandinavian Caledonides: International Journal of Earth Sciences, v. 91, p. 955–963, doi:10.1007/s00531-002-0298-3. Cuthbert, S.J., Carswell, D.A., Krogh-Ravna, E.J., and Wain, A., 2000, Eclogites and eclogites in the Western Gneiss Region, Norwegian Caledonides: Lithos, v. 52, p. 165–195, doi:10.1016/S0024-4937(99)00090-0. Dahlen, F.A., 1990, Critical taper model of fold-and-thrust belts and accretionary wedges: Annual Review of Earth and Planetary Sciences, v. 18, p. 55–99, doi:10.1146/annurev.ea.18.050190.000415. Dewey, J.F., 1988, Extensional collapse of orogens: Tectonics, v. 7, p. 1123– 1139, doi:10.1029/TC007i006p01123. Dewey, J.F., Ryan, P.D., and Andersen, T.B., 1993, Orogenic uplift and collapse, crustal thickness, fabrics and facies changes: The role of eclogites, in Prichard, H.M., Alabaster, T., Harris, N.B.W., and Neary, C.R.,eds., Magmatic Processes and Plate Tectonics: Geological Society [London] Special Publication 76, p. 325–343. Dietler, T.N., 1987, Struckturgeologische und tektonische Entwicklung der Western Gneiss Complexes im Sognefjord-Querschnitt, westliches Norwegen [Ph.D. thesis]: ETH Zürich, Switzerland, 233 p. Duchesne, J.C., Liégeois, J.P., Vander Auwera, J., and Longhi, J., 1999, The crustal tongue melting model and the origin of massive anorthosites: Terra Nova, v. 11, p. 100–105, doi:10.1046/j.1365-3121.1999.00232.x. Dunning, G.R., and Pedersen, R.B., 1988, U/Pb ages of ophiolites and arcrelated plutons of the Norwegian Caledonides: Implications for the development of Iapetus: Contributions to Mineralogy and Petrology, v. 98, p. 13–23, doi:10.1007/BF00371904.

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Contents Preface

Day 2. Inner Sognefjorden

Acknowledgments Abstract Chapter 1. An introductory outline I Autochthon-Parautochthon

2 Caledonian Allochthon 3 I ate- to Post-Collisional Extensional Shear Zones

Lrerdal-Gjende Fault Nord{jord-Sogn Detachment 4 Plate Tectoni c Cartoon

Chapter 2. The Sognefiord transect I Crustal Stru cture

2. Structural Synthesis

Eastern Segment of the Sogne{jord Transect Central Segment o(the Sogne{jord Transect Western Segment of the Sognefjord Transect 3 Retro-Deformation

Balancing Depth Control R.esJJiJs. 4. Eclogite Exhumation 5. An Orogenic Timeta ble

Chapter 3. The field trip itinerary Day I. Valdres-Jotunheimen

Stop Stop Stop Stop Stop Stop

1.1. S¢ndrol 1.2. Vangsmj¢si 1.3. @ye 1.4. Tyin Road Profile 1.5. Tyedalen 1.6. Lorteviki-Eidsbugarden

. . THE GEOLOGICAL SOCIETY • OF AMERICA®

3300 Penrose Place • P.O. Box 9140 Boulder, CO 80301 -9140, USA

Stop Stop Stop Stop Stop Stop

2.1. 2.2. 2.3. 2.4. 2.5. 2.6.

A.rdal Lrerdal Eide Sogndal Slinde Hermansverk

Day 3. Outer Sognefjorden

Stop 3.1. Stop 3.2. Stop 3.3. Stop 3.4. Stop 3.5. Stop 3.6. Stop 3.7.

Hella Srele Austrheim Kyrkjeb¢ Rasholm Helleb¢ Bekkeneset

Day 4 . So lund

Stop 4.1. Losna Stop 4.2. Hersvik Stop 4.3. Hyllestad Day 5. Askvoll-Atl¢y

Stop 5.1. Stop 5.2. Stop 5.3. Stop 5.4.

Vardalsneset Gjervik Kviteneset Brurestakken

Day 6 . Fensfjorden-Lindas

Stop Stop Stop Stop

6.1. 6.2. 6.3. 6.4.

Kjekallevagen Osterfiorden Holsn¢y Stalheim

References Cited

ISBN 978-0-81 37-0019-9