Mélanges: Processes of Formation and Societal Significance (GSA Special Paper 480)

Special Paper 480 . . THE GEOLOGICAL SOCIETY • OF AMERICA® Mélanges: Processes of Formation and Societal Significance ...

Author: John Wakabayashi | Yildirim Dilek

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Special Paper 480 . . THE GEOLOGICAL SOCIETY • OF AMERICA®

Mélanges: Processes of Formation and Societal Significance

edited by

John Wakabayashi Department of Earth and Environmental Sciences California State University, Fresno Fresno, California 93740 USA Yildirim Dilek Department of Geology Miami University Oxford, Ohio 45056 USA

Special Paper 480 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. In addition, an author has the right to use his or her article or a portion of the article in a thesis or dissertation without requesting permission from GSA, provided the bibliographic citation and the GSA copyright credit line are given on the appropriate pages. 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, sexual orientation, 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 Mélanges : processes of formation and societal significance / edited by John Wakabayashi, Yildirim Dilek. p. cm. — (Special paper ; 480) Includes bibliographical references. ISBN 978-0-8137-2480-5 (pbk.) 1. Melanges (Petrology) 2. Orogenic belts. I. Wakabayashi, John. II. Dilek, Yildirim, 1957–. QE471.15.M44M446 2011 551.8′2—dc23 2011016085 Cover: Shale matrix mélange of the Franciscan Complex exposed in a sea cliff in the San Simeon area, central California coast. Vertical dimension of view is approximately 2 meters. This photo shows dark gray shale enclosing blocks of graywacke (gray), basalt (greenish), and graphitic schist (dark gray-black). The photo also shows spatial variability of deformation in a mélange from domains with a strong matrix fabric and deformed blocks to those with minimal deformation of block and matrix. Photo, cropped differently for front and back cover, by Cathy Busby.

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Contents Introduction: Characteristics and tectonic settings of mélanges, and their significance for societal and engineering problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . v John Wakabayashi and Yildirim Dilek Part I. Mélange Generation in Oceanic Fracture Zones in Abyssal Settings 1. Serpentinite matrix mélange: Implications of mixed provenance for mélange formation . . . . . . . 1 John W. Shervais, Sung Hi Choi, Warren D. Sharp, Jeffrey Ross, Marchell Zoglman-Schuman, and Samuel B. Mukasa 2. Geochemical mapping of the Kings-Kaweah ophiolite belt, California—Evidence for progressive mélange formation in a large offset transform-subduction initiation environment . . . . . . . . . . . 31 J. Saleeby Part II. Mélange Formation Associated with Subduction Initiation 3. Constraints on the evolution of the Mesohellenic Ophiolite from subophiolitic metamorphic rocks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75 R. Myhill 4. Role of plutonic and metamorphic block exhumation in a forearc ophiolite mélange belt: An example from the Mineoka belt, Japan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95 Ryota Mori, Yujiro Ogawa, Naoto Hirano, Toshiaki Tsunogae, Masanori Kurosawa, and Tae Chiba Part III. Mélange Development in Subduction-Accretion Complexes and in Collisional Settings 5. Mélanges of the Franciscan Complex, California: Diverse structural settings, evidence for sedimentary mixing, and their connection to subduction processes . . . . . . . . . . . . . . . . . . . . . . . 117 John Wakabayashi 6. Tectonic evolution of the Ankara Mélange and associated Eldivan ophiolite near Hançili, central Turkey . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143 Anne Dangerfield, Ron Harris, Ender Sarıfakıoğlu, andYildirim Dilek 7. Petrology of a Franciscan olistostrome with a massive sandstone matrix: The King Ridge Road mélange at Cazadero, California . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171 Rolfe Erickson 8. Sedimentary block-in-matrix fabric affected by tectonic shear, Miocene Nabae complex, Japan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189 Soichi Osozawa, Terry Pavlis, and Martin F.J. Flower

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Contents 9. Numerical estimation of duplex thickening in a deep-level accretionary prism: A proposal for network duplex . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 207 Hikaru Ueno, Ken-ichiro Hisada, and Yujiro Ogawa 10. Tectonic, sedimentary, and diapiric formation of the Messinian mélange: Tertiary Piedmont Basin (northwestern Italy) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 215 Andrea Festa 11. Recognition of a trench-fill type accretionary prism: Thrust-anticlines, duplexes, and chaotic deposits of the Pliocene-Pleistocene Chikura Group, Boso Peninsula, Japan . . . . . . . . 233 Satoru Muraoka and Yujiro Ogawa 12. Implication of dark bands in Miocene–Pliocene accretionary prism, Boso Peninsula, central Japan . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 247 Yoko Michiguchi and Yujiro Ogawa Part IV. Significance of Mélanges for Engineering and Applied Geology 13. Geopractitioner approaches to working with antisocial mélanges . . . . . . . . . . . . . . . . . . . . . . . . 261 Edmund W. Medley and Dimitrios Zekkos

The Geological Society of America Special Paper 480 2011

Introduction: Characteristics and tectonic settings of mélanges, and their significance for societal and engineering problems John Wakabayashi Department of Earth and Environmental Sciences, California State University at Fresno, Fresno, California 93740, USA Yildirim Dilek Department of Geology, Miami University, Oxford, Ohio 45056, USA

Mélanges occur widely in collisional and accretionary orogenic belts around the world and represent mappable geological units consisting of blocks of different ages and origin, commonly embedded in an argillitic, sandy, or serpentinite matrix showing high stratal disruption and a chaotic internal structure. Understanding the mélange-forming processes and the significance of mélanges and related units in the geological record is of first-order significance in documenting the tectonic evolution of mountain belts; therefore, these chaotic rock units have attracted much attention in field-based structural studies since the nineteenth century. The term mélange has evolved to cover tectonic, sedimentary, and/or diapiric processes (Silver and Beutner, 1980) and tectonic settings of mélange formation, since its first use in 1919 by the British geologist Edward Greenly for the “Gwna Group” of the Mona Complex in Anglesey, north Wales (Greenly, 1919). In his classic work in the Franciscan Complex, Hsü (1968) proposed to use “mélange” only for tectonic mélanges and therein started a long-lived debate on the definition of the mélange term as well as on the processes involved in mélange formation. This controversy on the definition and formation of mélanges is livelier than ever in present time and requires a more systematic approach in mélange studies and better communication among the mélange researchers. To this end, we organized and convened a topical session on mélanges during the Geological Society of America (GSA) Annual Meeting in Denver in 2007. The session was well attended by many international scientists from North America, Europe, and the Pacific Rim countries. This session was sponsored by the GSA International and the Structural Geology and Tectonics Divisions, and it brought together earth scien-

tists in research communities from around the world, who do not ordinarily interact at the same meetings, in order to add an interdisciplinary dimension to our discussions on mélanges. It provided an excellent forum to discuss the new advances on the mélange concept as well as the diverse mélange types and mélange-forming processes based on some case studies. This Special Paper emanated from this successful GSA topical session. The benchmark GSA Special Paper 198 on mélanges, published more than 25 years ago (Raymond, 1984a), continues to influence research on mélanges, as testified by the frequent citation of the papers in it. The papers in the current volume build upon the solid foundation provided by the papers of Special Paper 198, as well as those published before and since, and include applications of new methodologies, exploration of new subjects, and a more international focus. The geographic spread of mélange localities around the world is also broader in parallel with the larger international authorship in the current volume. Given the three-dimensional (3-D) complexity of mélanges, it is of little surprise that field studies form the foundation of all of the research presented in this volume. Beyond this common linkage, the papers here span a broad spectrum of features and focus. We streamlined the chapters according to a relative conceptual chronology related to mélange development (or impact) and related processes that begin with formation on the abyssal ocean floor (Part I), then proceed to subduction initiation (Part II), and accretionary wedge development and orogenic belt formation (Part III). The final section concentrates on the impact of mélanges on societies by way of their engineering properties (Part IV). We provide below first a synoptic summary of the

Wakabayashi, J., and Dilek, Y., 2011, Introduction: Characteristics and tectonic settings of mélanges, and their significance for societal and engineering problems, in Wakabayashi, J., and Dilek, Y., eds., Mélanges: Processes of Formation and Societal Significance: Geological Society of America Special Paper 480, p. v–x, doi:10.1130/2011.2480(00). For permission to copy, contact [email protected]. © 2011 The Geological Society of America. All rights reserved.

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chapters in this volume, and then we conclude with some general statements about mélanges and their significance. PART I. MÉLANGE GENERATION IN OCEANIC FRACTURE ZONES IN ABYSSAL SETTINGS The papers by Shervais et al. (Chapter 1) and Saleeby (Chapter 2) present integrated field, petrological, and geochemical data on serpentinite-matrix mélanges of different ages in California. Both studies provide field and geochemical evidence that blockin-matrix fabrics developed in serpentinite within an abyssal fracture zone. Shervais et al. present field evidence for subduction initiation along a fracture zone based on the presence of highpressure, high-temperature (HP-HT) garnet-amphibolite blocks from a serpentinite shear zone structurally beneath unmetamorphosed ophiolitic rock of the Jurassic Coast Range ophiolite. The serpentinite matrix mélange may represent the original subduction interface. The authors suggest that exhumation was aided not only by the lower density of the serpentinite matrix compared to the top of the lower plate (oceanic crust) and the upper plate (suboceanic mantle), but was also driven by the positive volume change associated with serpentinite in a confined zone that might have served to force the serpentinite upward. Saleeby presents evidence for subduction initiation along an abyssal fracture zone, based on his work in the Kings-Kaweah ophiolite belt of the southern Sierra Nevada. He suggests a long duration of time (~190 m.y.) between the early Ordovician abyssal ocean crust formation and the Permo-Carboniferous mélange development in an abyssal fracture zone environment. He argues that initiation of subduction along this fracture zone was followed by the development of supra-subduction zone igneous rocks. The evidence for subduction initiation comes from a ca. 255 Ma Sm/Nd age and (HP-HT) metamorphism of garnet-amphibolite blocks in a serpentinite matrix mélange. Saleeby argues that the emplacement of garnet-amphibolite blocks in the mélange was related to serpentinite diapirism through the upper plate of a subduction system, instead of back along the subduction interface through channel flow. PART II. MÉLANGE FORMATION ASSOCIATED WITH SUBDUCTION INITIATION The rock record of subduction initiation is evaluated in Myhill’s paper (Chapter 3) on the metamorphic sole of the Vourinos and Pindos ophiolites in the western Hellenides (Greece). This topic of subduction initiation is also covered in the papers by Shervais et al. and Saleeby (Part I), and to a lesser extent, in the papers by Mori et al. (Mineoka belt, Japan, Chapter 4), and Wakabayashi (Franciscan Complex, California, Chapter 5) in Part III. Myhill presents detailed metamorphic evidence and argues that metamorphic soles, which are the thin high-grade metamorphic sheets commonly found beneath Tethyan ophiolites, were formed at lower pressures than commonly thought, and that they

were therefore not necessarily associated with subduction initiation as has been widely assumed. He demonstrates that the hightemperature metamorphism of the metamorphic sole beneath the Mesohellenic ophiolites (Pindos and Vourinos) occurred during intra-oceanic thrusting (but not subduction) near a ridge crest and soon after subduction initiation, and that slices and blocks of the sole were incorporated into a subophiolitic mélange during further thrusting associated with ophiolite emplacement. Mori et al. (Chapter 4) present a model for the complex tectonic evolution of the Mineoka ophiolitic mélange belt in the Boso Peninsula of central Japan, based on field relations, geochronology, and petrology. The evolution of this mélange includes early HP-HT, possibly associated with a subduction initiation event, followed by considerable deformation and mixing involving triple junction interaction and evolution. An early stage of ductile deformation at deep crustal levels was associated with syn-subduction exhumation of metamorphic rocks, including HP-HT rocks, followed by a brittle phase of deformation developed at much shallower levels, as rocks were incorporated into the mélange zone. Their geochronological data suggest initiation of subduction at ca. 33–39 Ma, followed by development of the ophiolitic mélange between 15 and 18 Ma. Mori et al. propose that the present Izu arc may be an analog of the Mineoka mélange belt. PART III. MÉLANGE DEVELOPMENT IN SUBDUCTION-ACCRETION COMPLEXES AND IN COLLISIONAL SETTINGS In his process-oriented approach to delineating the tectonic settings of mélange formation, Wakabayashi (Chapter 5) divides and examines the classic Jurassic-Eocene Franciscan Complex mélanges of coastal California into distinct structural groups. The structurally highest mélanges in the Franciscan Complex may have formed at or shortly after subduction initiation, marking the initial subduction interface, whereas the mélanges separating coherent nappe sheets may represent later-developed paleo megathrust horizons within the accretionary prism. He argues that the large displacements associated with these nappes may have been largely accommodated along the borders of the mélanges rather than within them. He presents field and petrographic evidence supporting pre-tectonic sedimentary mixing of mélanges (development of block-in-matrix structure and introduction of exotic blocks), including those most likely to be classified as entirely tectonic mélanges (internappe mélanges). He shows the presence of “two cycle” high-P rocks, which were subducted to blueschist-facies depths, then exhumed and re-worked as sedimentary deposits, and then resubducted again to blueschist depths and exhumed. Dangerfield et al. (Chapter 6) present structural, geochronological, and geochemical data from the Eldivan ophiolite, which occurs as a coherent block in the Ankara Mélange in north-central Turkey. The Ankara Mélange is part of the İzmir-Ankara-Erzincan

Introduction: Characteristics of mélanges, and their significance for societal and engineering problems suture zone and represents a classic Tethyan colored mélange. Dangerfield and her co-authors show development of the Eldivan ophiolite in a suprasubduction zone setting, followed by its integration into the Ankara Mélange as an oceanic block. Detrital zircon U/Pb analyses from the mélange and the overlapping epiclastic sandstones show that mélange development occurred between ~143 Ma and 105 Ma, consistent with the regional geochronological data. The authors argue that although the development of the İzmir-Ankara-Erzincan suture zone involved continental collision tectonics, its overall evolution resembles the formation of mélange terrains in the southwest Pacific rather than that of a Himalayan-type continental collision. Erickson (Chapter 7) presents field, petrographic, and geochronological data from a Cretaceous sandstone-matrix olistostrome in the Franciscan Complex in northern California that collectively provide critical constrains on the exhumation age and patterns of various blocks and the depositional age of the olistostrome. The majority of these blocks themselves are Franciscan-derived. The evolution of the olistostrome includes initial subduction burial of the blocks; their subsequent exhumation and exposure as blueschist-, eclogite-, and amphibolite-facies blocks; their deposition some time after 83 Ma; and partial re-subduction to prehnite-pumpellyite facies conditions subsequently. As shown in the paper by Wakabayashi, Erickson’s work also demonstrates the sedimentary reworking of previously metamorphosed Franciscan rocks, including “high-grade” blocks formed during the earliest stages of Franciscan subduction at ca. 165 Ma. Osozawa et al. (Chapter 8) use map and outcrop relationships from excellent coastal exposures of the Miocene Nabae complex of Japan and petrofabric studies to show that the blockin-matrix fabric observed in this mélange was a result of early sedimentary sliding rather than tectonism. They demonstrate that the amount of shear strain associated with foliation development in the mélange matrix was minimal, and that this deformation was vastly inadequate to account for the introduction of exotic blocks of chert and basalt into the shale matrix. Their data also show evidence for reworking of clasts that include penetrative fabrics developed in an older subduction complex. Ueno et al. (Chapter 9) document complex duplex structures from accretionary complex rocks of the Jurassic-Cretaceous Chichibu Belt of Japan, and show how some of these structures have been previously (and mistakenly) interpreted as block-in-matrix features in the absence of good exposures. The superb coastal exposures allow detailed characterization of the structures and estimation of the amount of structural thickening associated with tectonic duplexing. They describe “network duplexes,” which are themselves composed of duplexes of smaller orders, and calculate thickening of a factor of ~6–13 at the greenschist facies level of this subduction-accretionary complex. Festa (Chapter 10) presents detailed field relationships from the Piedmont Basin in northwest Italy that developed as an episutural basin after the main stage Alpine collision, and he documents structures that formed at burial depths of 2–3 km. Utilizing sedimentary structures and sedimentary contact relationships (for

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establishing sedimentary origins), shear-sense indicators, and progressive strain and rotation of rocks toward faults, he distinguishes between mélanges that were formed by tectonic strain, sedimentary sliding, and diapiric emplacement. He then links the development of different types of mélanges to the regional tectonics, during which faulting (along with the formation of tectonic mélanges) may have triggered gas hydrate disassociation and rise of overpressured fluids (diapiric emplacement, preferentially following fault zones), triggering gravitational collapse and development of sedimentary mélanges. Muraoka and Ogawa (Chapter 11) present observations on mélanges, duplexes, and folds that they interpret to have formed in a trench-fill environment, the shallowest level of preservation of an accretionary prism. The evidence comes from fine coastal exposures of the Plio-Pleistocene Chikura Group on the Boso Peninsula of Japan. The Lower Chikura Group units are interpreted to have been deposited in the trench in advance of the thrust front and later incorporated into the accretionary wedge by seaward propagation of the thrust front; the Upper Chikura Group units, on the other hand, were originally deposited in a trench slope basin setting. The lower Chikura Group deposits include evidence for interaction of methane-rich fluids from associated chemosynthetic biocommunities that suggest a trenchfill environment similar to the modern Sagami Trough. Chaotic deposits or mélanges include those with diapiric (intrusive) field relationships as well as those that appear to represent submarine slides, whereas the duplexes and thrust anticlines record significant tectonic shortening in the coherent units. Michiguchi and Ogawa (Chapter 12) examine the internal structure of the Miocene-Pliocene accretionary prism complex exposed in the Boso Peninsula, Japan. They show that dark bands found in siltstones are the products of different deformation mechanisms in an accretionary prism toe and the frontal thrust region. The host rocks include both coherent stratal and chaotic units (as in mélanges). Their map, outcrop, and microscopic analysis suggests that some of these features formed as a result of high pore fluid pressure as shear fractures, whereas others formed as tensional fractures associated with different states of stress and deformation modes. One of their dark band types represents flexural-slip faults associated with folding, another type represents sliding planes formed during submarine landslides, whereas yet another type consists of thrust faults formed during accretion. PART IV. SIGNIFICANCE OF MÉLANGES FOR ENGINEERING AND APPLIED GEOLOGY The paper by Medley and Zekkos (Chapter 13) focuses on the geological engineering aspects of mélanges, bringing a societal relevance and significance to mélange studies and research. This topic has been largely overlooked in purely academic studies of mélanges, although it has been a subject of many detailed investigations in engineering geology, whose results have been published in the past 16 years. Up to now, many engineers and geologists in engineering and environmental geology force-fit

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block-in-matrix geology into a layer-cake stratigraphic interpretation, commonly with disastrous consequences, because this sector of applied geology has not kept pace with the most recent advances in recognition of mélanges. Medley and Zekkos fully describe the engineering issues of dealing with mélanges, including both theory and case studies. SUMMARY COMMENTS AND CONCLUSIONS Mélange Classification: Descriptive Rather than Genetic Schemes Recommended Most of the previous mélange studies, including those in GSA Special Paper 198, offered detailed classification or definitions of mélanges and their sub-types. It is clear that a uniform classification scheme has merit, given that the term mélange is used differently by many authors. However, caution is urged, especially when genetic significance is attached to a definition, given how difficult it may be to ascertain mélange origins from first-order field observations, particularly for mélanges that appear strongly deformed, such as those described by Osozawa et al. (Chapter 8) and Wakabayashi (Chapter 5). The classification schemes proposed by Cowan (1985) are largely descriptive and are therefore more useful than a genetic definition presented, for example, by Sengör (2003), wherein a purely tectonic origin is a requirement for the term mélange. Raymond (1984b) proposed a detailed classification scheme, but its ultimate application required some knowledge of the genesis of the mélange. Although this may seem a regressive definition, we recommend a broad definition of mélange as a bedrock unit with a matrix and variety of blocks included in it, similar to the recommendation of Silver and Beutner (1980). In fact Silver and Beutner (1980) noted that in addition to the more common block-in-matrix fabric, some mélanges have a block-on-block fabric, a structural style that appears to best fit the Mineoka ophiolite belt of Japan (Takahashi et al., 2003; also Mori et al., Chapter 4). Festa (Chapter 10) makes a similar recommendation for a descriptive, rather than genetic definition of the term mélange. Chapter 5 by Wakabayashi proposes mélange categories based on structural-tectonic settings that are derived from 3-D field relationships. This scheme has the primary goal of connecting the mélanges to large-scale processes during evolution of active plate margins, but it does not directly aid evaluation of strain and sedimentary processes in mélange formation in the way that a scheme such as Cowan’s (1985) does. Accordingly, we think that there is no single unifying classification or nomenclature scheme for mélanges, nor should there be, because different schemes serve different purposes. We recommend that authors writing about block-in-matrix units be as specific as possible about the descriptive aspects of these units, so that readers are not misled into applying their own definition of “mélange” that may differ markedly from that intended by the author. In many ways, the problem of mélange classification and nomenclature parallels that of the term ophiolite, for

which numerous definitions also exist (e.g., Dilek, 2003; Dilek and Furnes, 2009). Sedimentary versus Tectonic Mixing in Mélanges An increasing amount of field evidence has been presented in the past few decades, illustrating the significant contributions of sedimentary mixing to even some of the most (apparently) sheared mélanges (e.g., Aalto, 1989; Osozawa et al., 2009, Chapter 8; Wakabayashi, Chapter 5). These studies support the conclusions of earlier research (Cowan and Page, 1975; Cowan, 1978). Some of the most extreme examples include the sedimentary introduction of exotic blocks into nappe-bounding mélanges in the Franciscan Complex, which may have accommodated tens of km or more of displacement (Wakabayashi, Chapter 5). The studies of Osozawa et al. (2009, Chapter 8) also show that most or all exotic blocks in the Nabae Complex of the Shimanto Belt of Shikoku, Japan, and the Yuwan accretionary complex of the Ryukyu Islands, respectively, were integrated into the mélange by pre-tectonic phases of submarine sliding. Osozawa et al. (Chapter 8) argue that the deformation that produced the matrix foliation in the mélanges that they have examined records relatively minimal shear strain, which cannot account for introduction of exotic blocks or development of block-in-matrix fabrics. Aalto (1989) and Wakabayashi (Chapter 5) document a range of textures from undeformed sedimentary breccias to strongly foliated shale mélange matrix. Although evidence points to submarine sedimentary (gravity) sliding as a main contributor to the development of blockin-matrix fabrics in many of the most tectonized mélanges, sedimentary sliding was not a major process in the formation of block-in-matrix fabrics in all mélanges. Some mélanges clearly have a tectonic or diapiric origin. Festa (Chapter 10) summarizes effectively the criteria for distinguishing diapiric versus tectonic mélanges, and provides field examples of both. For diapiric mélanges, the diagnostic feature is opposing shear sense on opposite mélange contacts, a criterion that was first applied by Orange (1990) and subsequently used by Dela Pierre et al. (2007), as well as Muraoka and Ogawa (Chapter 11) and Festa (Chapter 10). For tectonic mélanges, an important field characteristic is an increasing degree of deformation and rotation of fabric elements as a fault or shear zone is approached (Festa, Chapter 10). Significance of Mélanges and Mélange Types in Orogenic Belt Development Mélanges are characteristic features of modern and ancient convergent plate boundaries, and rank with ophiolites and HP– low-temperature metamorphic rocks as critical recorders of convergent plate margin processes. Mélanges provide critical insights into sedimentary and structural evolution in the accretionary prism and forearc basin environments, including evidence for large-scale material movement (particularly in cross-sectional view) in accretionary wedges. Mélanges form as subduction of

Introduction: Characteristics of mélanges, and their significance for societal and engineering problems oceanic lithosphere is punctuated by a collisional process (see discussion in Dangerfield et al., Chapter 6, although they argue for a noncollisional origin for the particular mélange of their study), and/or terminated by the final stages of continental collision (Festa, Chapter 10). In addition to recording subduction- and collision-related sedimentary and tectonic processes, mélange formation may also include pre-subduction tectonics, including deformation along abyssal fracture zones (Shervais et al., Chapter 1; Saleeby, Chapter 2) and at oceanic core complexes (Saleeby, Chapter 2), as well as supra-subduction zone oceanic crust evolution (Dangerfield et al., Chapter 6; Shervais et al., Chapter 1). Mélanges also offer major insights into the most extreme vertical movements along convergent plate margins: the exhumation of high-pressure metamorphic rocks (Shervais et al., Chapter 1; Saleeby, Chapter 2; Mori et al., Chapter 4; Wakabayashi, Chapter 5; Erickson, Chapter 7). Societal Significance Mélanges, by the very nature of their chaotic block-inmatrix structure, pervasive and strong internal deformation and clay-rich soil contents, are prone to landsliding as well as creating problems because of the great contrast in ease of excavation of block and matrix (Medley and Zekkos, Chapter 13). Hence, they pose major challenges for engineering projects developed on them as well as for water supplies and infrastructure. Therefore, mélange terrains cause first-order societal problems for the people in California, Japan, Italy, Scotland, Greece, Cyprus, Turkey, the Philippines, and many other countries, where ophiolites and mélanges occur abundantly. Furthermore, most engineers and engineering geologists continue to treat mélangecontaining bedrock by using the basic principles of stratigraphy and by assuming a layered structure for their formation, and fail to account for the 3-D variation of many key parameters such as rock strength and ease of excavation. This ill-informed approach results in disastrous engineering problems, leading to significant property damage and casualties. It is thus highly important for the academic community and the practicing geological and civil engineers to convey their learned experience and knowledge on mélanges and mélange structures to each other through publishing in common literature and in conference proceedings in order to maximize the dissemination of their scientific and applied findings. The academic community in particular should continue to strive to remedy the knowledge gaps through interaction with the applied community as well as through implementing contemporary reforms in undergraduate education that would revive field instruction and field-based, observation-oriented earth science education. We hope that this GSA Special Paper presents an important step in this mission of closing the knowledge gap in the purely scientific and engineering aspects of mélanges and mélange-forming processes and their significance for engineering and societal issues. August 2010

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REFERENCES CITED Aalto, K.R., 1989, Franciscan Complex olistostrome at Crescent City, northern California: Sedimentology, v. 36, p. 471–495, doi:10 .1111/j.1365-3091.1989.tb00620.x. Cowan, D.S., 1978, Origin of blueschist-bearing chaotic rocks in the Franciscan Complex, San Simeon, California: Geological Society of America Bulletin, v. 89, p. 1415–1423. Cowan, D.S., 1985, Structural styles in Mesozoic and Cenozoic mélanges in the Western Cordillera of North America: Geological Society of America Bulletin, v. 96, p. 451–462, doi:10.1130/0016-7606(1985)962.0.CO;2. Cowan, D.S., and Page, B.M., 1975, Recycled Franciscan material in Franciscan mélange west of Paso Robles, California: Geological Society of America Bulletin, v. 86, p. 1089–1095, doi:10.1130/0016 -7606(1975)862.0.CO;2. Dangerfield, A., Harris, R., Sarıfakıoğlu, E., and Dilek, Y., 2011, this volume, Tectonic evolution of the Ankara Mélange and associated Eldivan ophiolite near Hançili, central Turkey, in Wakabayashi, J., and Dilek, Y., eds., Mélanges: Processes of Formation and Societal Significance: Geological Society of America Special Paper 480, doi:10.1130/2011.2480(06). Dela Pierre, F., Festa, A., and Irace, A., 2007, Interaction of tectonic, sedimentary and diapiric processes in the origin of chaotic sediments: An example from the Messinian of Torino Hill (Tertiary Piedmont basin, northwestern Italy): Geological Society of America Bulletin, v. 119, p. 1107–1119, doi:10.1130/B26072.1. Dilek, Y., 2003, Ophiolite concept and its evolution, in Dilek, Y., and Newcomb, S., eds., Ophiolite Concept and the Evolution of Geologic Thought: Geological Society of America Special Paper 373, p. 1–16. Dilek, Y., and Furnes, H., 2009, Structure and geochemistry of Tethyan ophiolites and their petrogenesis in subduction rollback systems: Lithos, v. 113, p. 1–20, doi:10.1016/j.lithos.2009.04.022. Erickson, R., 2011, this volume, Petrology of a Franciscan olistostrome with a massive sandstone matrix: The King Ridge Road mélange at Cazadero, California, in Wakabayashi, J., and Dilek, Y., eds., Mélanges: Processes of Formation and Societal Significance: Geological Society of America Special Paper 480, doi:10.1130/2011.2480(07). Festa, A., 2011, this volume, Tectonic, sedimentary, and diapiric formation of the Messinian mélange: Tertiary Piedmont Basin (northwestern Italy), in Wakabayashi, J., and Dilek, Y., eds., Mélanges: Processes of Formation and Societal Significance: Geological Society of America Special Paper 480, doi:10.1130/2011.2480(10). Greenly, E., 1919, The Geology of Anglesey: Great Britain Geological Survey Memoir, v. 1, 980 p. Hsü, K.J., 1968, The principles of mélanges and their bearing on the FranciscanKnoxville paradox: Geological Society of America Bulletin, v. 79, p. 1063– 1074, doi:10.1130/0016-7606(1968)79[1063:POMATB]2.0.CO;2. Medley, E.W., and Zekkos, D., 2011, this volume, Geopractitioner approaches to working with antisocial mélanges, in Wakabayashi, J., and Dilek, Y., eds., Mélanges: Processes of Formation and Societal Significance: Geological Society of America Special Paper 480, doi:10.1130/2011.2480(13). Michiguchi, Y., and Ogawa, Y., 2011, this volume, Implication of dark bands in Miocene–Pliocene accretionary prism, Boso Peninsula, central Japan, in Wakabayashi, J., and Dilek, Y., eds., Mélanges: Processes of Formation and Societal Significance: Geological Society of America Special Paper 480, doi:10.1130/2011.2480(12). Mori, R., Ogawa, Y., Hirano, N., Tsunogae, T., Kurosawa, M., and Chiba, T., 2011, this volume, Role of plutonic and metamorphic block exhumation in a forearc ophiolite mélange belt: An example from the Mineoka belt, Japan, in Wakabayashi, J., and Dilek, Y., eds., Mélanges: Processes of Formation and Societal Significance: Geological Society of America Special Paper 480, doi:10.1130/2011.2480(04). Muraoka, S., and Ogawa, Y., 2011, this volume, Recognition of trench-fill type accretionary prism: Thrust anticlines, duplexes and chaotic deposits of Pliocene-Pleistocene Chikura Group, Boso Peninsula, Japan, in Wakabayashi, J., and Dilek, Y., eds., Mélanges: Processes of Formation and Societal Significance: Geological Society of America Special Paper 480, doi:10.1130/2011.2480(11). Myhill, R., 2011, this volume, Constraints on the evolution of the Mesohellenic Ophiolite from subophiolitic metamorphic rocks, in Wakabayashi, J., and Dilek, Y., eds., Mélanges: Processes of Formation and Societal

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Significance: Geological Society of America Special Paper 480, doi:10.1130/2011.2480(03). Orange, D.L., 1990, Criteria helpful in recognizing shear-zone and diapiric mélanges: Examples from the Hoh accretionary complex, Olympic Peninsula, Washington: Geological Society of America Bulletin, v. 102, p. 935– 951, doi:10.1130/0016-7606(1990)1022.3.CO;2. Osozawa, S., Morimoto, J., and Flower, F.J., 2009, ‘Block-in-matrix’ fabrics that lack shearing but possess composite cleavage planes: A sedimentary mélange origin for the Yuwan accretionary complex in the Ryukyu island arc, Japan: Geological Society of America Bulletin, v. 121, p. 1190–1203, doi:10.1130/B26038.1. Osozawa, S., Pavlis, T., and Flowers, M.F.J., 2011, this volume, Sedimentary block-in-matrix fabric affected by tectonic shear, Miocene Nabae complex, Japan, in Wakabayashi, J., and Dilek, Y., eds., Mélanges: Processes of Formation and Societal Significance: Geological Society of America Special Paper 480, doi:10.1130/2011.2480(08). Raymond, L.A., 1984a, editor, Mélanges: Their Nature, Origin and Significance: Geological Society of America Special Paper 198, 170 p. Raymond, L.A., 1984b, Classification of mélanges, in Raymond, L.A., ed., Mélanges: Their Nature, Origin and Significance: Geological Society of America Special Paper 198, p. 7–20. Saleeby, J., 2011, this volume, Geochemical mapping of the Kings-Kaweah ophiolite belt, California—Evidence for progressive mélange formation in a large offset transform-subduction initiation environment, in Wakabayashi, J., and Dilek, Y., eds., Mélanges: Processes of Formation and Societal Significance: Geological Society of America Special Paper 480, doi:10.1130/2011.2480(02). Sengör, A.M.C., 2003, The repeated discovery of mélanges and its implications for the possibility and the role of objective evidence in the scientific enterprise, in Dilek, Y., and Newcomb, S., eds., Ophiolite Concept and the

Evolution of Geologic Thought: Geological Society of America Special Paper 373, p. 385–446. Shervais, J.W., Choi, S.H., Sharp, W.D., Ross, J., Zoglman-Schuman, M., and Mukasa, S.B., 2011, this volume, Serpentinite matrix mélange: Implications of mixed provenance for mélange formation, in Wakabayashi, J., and Dilek, Y., eds., Mélanges: Processes of Formation and Societal Significance: Geological Society of America Special Paper 480, doi:10.1130/2011.2480(01). Silver, E.A., and Beutner, E.C., 1980, Melanges: Geology, v. 8, p. 32–34, doi:10 .1130/0091-7613(1980)82.0.CO;2. Takahashi, A., Ogawa, Y., Ohata, Y., and Hirano, N., 2003, The mode and nature of faulting and deformation and the emplacement history of the Mineoka Ophiolite, NW Pacific Rim, in Dilek, Y., and Robinson, P.T., eds., Ophiolites in Earth History: Geological Society of London Special Publication 218, p. 299–314. Ueno, H., Hisada, K.-I., and Ogawa, Y., 2011, this volume, Numerical estimation of duplex thickening in a deep-level accretionary prism: A proposal for network duplex, in Wakabayashi, J., and Dilek, Y., eds., Mélanges: Processes of Formation and Societal Significance: Geological Society of America Special Paper 480, doi:10.1130/2011.2480(09). Wakabayashi, J., 2011, this volume, Mélanges of the Franciscan Complex, California: Diverse structural settings, evidence for sedimentary mixing, and their connection to subduction processes, in Wakabayashi, J., and Dilek, Y., eds., Mélanges: Processes of Formation and Societal Significance: Geological Society of America Special Paper 480, doi:10.1130/2011.2480(05).

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Serpentinite matrix mélange: Implications of mixed provenance for mélange formation John W. Shervais Department of Geology, Utah State University, Logan, Utah 84322-4505, USA Sung Hi Choi Department of Geology and Earth Environmental Sciences, Chungnam National University, Daejeon 305-764, South Korea Warren D. Sharp Berkeley Geochronology Center, 2455 Ridge Road, Berkeley, California 94709, USA Jeffrey Ross* Department of Geosciences, Stony Brook University, Stony Brook, New York 11794-2100, USA Marchell Zoglman-Schuman† University of South Carolina, Columbia, South Carolina 29208, USA Samuel B. Mukasa Department of Geological Sciences, University of Michigan, Ann Arbor, Michigan 48109-1005, USA

ABSTRACT Serpentinite matrix mélange represents a significant, if less common, component of many accretionary complexes. There are two principal hypotheses for the origin of serpentinite mélange: (1) formation on the seafloor in a fracture zone–transform fault setting, and (2) formation within a subduction zone with mixing of rocks derived from both the upper and lower plates. The first hypothesis requires that the sheared serpentinite matrix be derived from hydrated abyssal peridotites and that the block assemblage consist exclusively of oceanic rocks (abyssal peridotites, oceanic basalts, and pelagic sediments). The second hypothesis implies that the sheared serpentinite matrix is derived from hydrated refractory peridotites with supra-subduction zone affinities, and that the block assemblage includes rocks derived from both the upper plate (forearc peridotites, arc volcanics, sediments) and the lower plate (abyssal peridotites, oceanic basalts, pelagic sediments). In either case, serpentinite mélange may include true mélange, with exotic blocks derived from other sources, and serpentinite broken formation, where the blocks are massive peridotite.

*Current address: Bechtel Savannah River, Inc., Aiken, South Carolina 29801, USA. † Current address: 2623 Withington Heights Peak Drive NE, Rio Rancho, New Mexico 87144, USA. Shervais, J.W., Choi, S.H., Sharp, W.D., Ross, J., Zoglman-Schuman, M., and Mukasa, S.B., 2011, Serpentinite matrix mélange: Implications of mixed provenance for mélange formation, in Wakabayashi, J., and Dilek, Y., eds., Mélanges: Processes of Formation and Societal Significance: Geological Society of America Special Paper 480, p. 1–30, doi:10.1130/2011.2480(01). For permission to copy, contact [email protected]. © 2011 The Geological Society of America. All rights reserved.

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Shervais et al. The Tehama-Colusa serpentinite mélange underlies the Coast Range ophiolite in northern California and separates it from high-pressure/temperature (P/T) metamorphic rocks of the Franciscan complex. It has been interpreted both as an accreted fracture zone terrane and as a subduction-derived mélange belt. Our data show that the mélange matrix represents hydrated refractory peridotites with forearc affinities, and that blocks within the mélange consist largely of upper plate lithologies (refractory forearc harzburgite, arc volcanics, arc-derived sediments, and chert with Coast Range ophiolite biostratigraphy). Lower plate blocks within the mélange include oceanic basalts and chert with rare blueschist and amphibolite. Hornblendes from three amphibolite blocks that crop out in serpentinite mélange and sedimentary serpentinite yield 40Ar/39Ar plateau ages of 165.6–167.5 Ma, similar to published ages of highgrade blocks within the Franciscan complex and to crystallization ages in the Coast Range ophiolite. Other blocks have uncertain provenance. It has been shown that peridotite blocks within the mélange have low pyroxene equilibration temperatures that are consistent with formation in a fracture zone setting. However, the current mélange reflects largely upper-plate lithologies in both its matrix and its constituent blocks. We propose that the proto-Franciscan subduction zone nucleated on a large offset transform fault–fracture zone that evolved into a subduction zone mélange complex. Mélange matrix was formed by the hydration and volume expansion of refractory forearc peridotite, followed by subsequent shear deformation. Mélange blocks were formed largely by the breakup of upper plate crust and lithosphere, with minor offscraping and incorporation of lower plate crust. We propose that the methods discussed here can be applied to serpentinite matrix mélange worldwide in order to understand better the tectonic evolution of the orogens in which they occur.

INTRODUCTION Accretionary Complexes and Mélange Formation Accretionary complexes formed during plate convergence, subduction, and collision represent a fundamental component of the plate tectonics paradigm, along with ophiolites, island arcs, and paired metamorphic belts. Accretionary complexes typically include a large fraction of material derived from the upper plate as sediments and as tectonic slices eroded from the overlying crust and lithosphere, as well as material derived from the lower, subducting plate by faulting and subduction accretion (Bailey et al., 1964; Moore and Karig, 1980; Moore et al., 1980; Hamilton, 1988; von Huene and Scholl, 1991). Deformation is dominantly compressional, but thickening of the accretionary wedge typically results in extensional deformation of the thickest part of the accretionary wedge even as compressional deformation continues in the thin leading edge (Platt, 1986). Accretionary complexes may also be deformed by large-scale, trench-parallel shear that both erodes and accretes new material (Moore et al., 1980; Howell, 1989; Wright and Wyld, 2007). Understanding the origin and significance of mélange assemblages is critical to our understanding of convergent plate boundaries, and places important constraints on the tectonic history of ancient orogens. Accretionary mélange complexes are conceptually and formatively tied to plate tectonics, and their

presence in any orogen reveals its plate tectonic origin. This is especially important for the Archean and early Paleoproterozoic, where mélange formation may represent our best evidence for the onset of Phanerozoic-style plate tectonics (Shervais, 2006). In their classic configuration, accretionary complexes consist largely of clastic sediments deposited within the trench as turbidites or submarine fan deposits, which are subsequently deformed by thrust faulting to form coherent sheets of metasediment or disrupted to form shale-matrix mélange—the classic argille scagliose of the Ligurian Alps (Hsü, 1968; Page, 1978; Raymond, 1984). Formation of shale-matrix mélange may involve either or both tectonic disruption and olistostromes (debris flows), which commonly form broken formations that consist of formerly intercalated wackes and shales, with rare exotic blocks (Abbate et al., 1970; Cowan, 1978, 1982; Aalto, 1981; Raymond, 1984). Broken formations may also form by layer parallel extension of intercalated wackes and shales in response to extensional deformation of the accretionary wedge (Byrne, 1984). In contrast, true mélange requires some component of exotic material, whose provenance is unrelated to the matrix material. These exotic blocks may include igneous or sedimentary rocks of oceanic affinity (e.g., pillow lavas, chert) or metamorphic rocks formed from previously subducted material, mixed by either tectonic or debris flow processes (e.g., Bailey et al., 1964; Hsü, 1972; Cowan, 1978, 1982; Hall, 1980; Cloos, 1984). Regardless of their mode of formation, these complexes represent

Serpentinite matrix mélange: Implications of mixed provenance for mélange formation subduction mélanges formed in response to the continuous and ongoing subduction of material deposited into the trench on an active plate margin. Other mélange formations represent passive margin sediments, seamounts, and fringing coral-reef limestones that formed during or after rifting along a passive continental margin, only to be dismembered during collision and emplacement of an overriding ophiolite (i.e., obduction) onto the passive margin (Yilmaz and Maxwell, 1984; Shervais, 2001; Metcalf and Shervais, 2008). Examples of these obduction mélanges (and their associated thrust complexes) include the Hawasina nappes and Haybi mélange, which underlie the Semail ophiolite of Oman (Robertson, 1986; Bechennec et al., 1988, 1990), and the Mamonia complex of Troodos (Lapierre et al., 2007). The primary distinction between an obduction mélange and a true subduction mélange is that the former is thrust over the passive margin from which it was derived (in the lower plate of a collision zone), whereas the latter consists largely of sediments derived from an active volcanic arc or its underlying plutons (in the upper plate of the subduction zone) and may be unassociated with a collisional event. Igneous elements of the passive margin obduction mélange typically date to the initiation of rifting and are unrelated to the later convergence.

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may also form as a result of volume expansion during the serpentinization process. Serpentinization involves ~50% volume expansion, depending on the primary mineralogy. Formation of block texture during serpentinization results in fractures where serpentinization is concentrated (O’Hanley, 1991, 1992). As the serpentinite expands, it may be forced out along these fractures, and blocks of partially serpentinized massive peridotite may be forced upward between adjacent fractures, forming the classic block-in-matrix texture of a broken formation without imposition of an external stress field; subsequent tectonic shear will exploit these preexisting “shear zones” to cause further deformation (e.g., Shervais et al., 2005a) (Fig. 1). Serpentinite mélange, like argille scagliose, may also form by sedimentary processes as debris flows or olistostromes. Sedimentary serpentinites are common in the Mesozoic of California (e.g., Moiseyev, 1970; Lockwood, 1971; Phipps, 1984) and have even been observed to have formed Quaternary debris flows (Cowan and Mansfield, 1970). In the Marianas forearc,

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Serpentinite Matrix Mélange Serpentinite matrix mélange represents a relatively minor fraction of most accretionary complexes, but their significance far outweighs their relative volume (e.g., Saleeby, 1979, 1984; Carpenter and Walker, 1992; Wright and Wyld, 1994; Malpas et al., 1994; Blake et al., 1995; Lennykh et al., 1995; Tankut et al., 1998; Coleman, 2000; Wakabayashi, 2004; Guillot et al., 2004; Choi et al., 2008a, 2008b). Serpentinite mélange exposes samples of the upper mantle and may be a prime carrier that brings high-grade metamorphic blocks (blueschist, eclogite, amphibolite, garnet amphibolite) back to the surface from depth (e.g., Bailey et al., 1964; Moore, 1984; Ross and Sharp, 1988; Oh and Liou, 1990; Baldwin and Harrison, 1992; Harlow et al., 2004; Beane and Liou, 2005; Tsujimori et al., 2006). The occurrence of chlorite-actinolite rinds on many blueschist and eclogite “knockers” implies transport within a serpentinite matrix, even when these blocks are found within a shale matrix mélange. A serpentinite mélange typically consists of a sheared serpentinite matrix with blocks of unsheared, partially serpentinized peridotite, volcanic rocks, chert, and high-grade metamorphic rocks. The sheared serpentinite matrix commonly comprises lizardite and chrysotile, with minor brucite, magnetite, and carbonate; antigorite is less common but may be the dominant serpentine mineral in some mélange belts. Serpentinite may also form a broken formation, in which blocks of massive, unsheared serpentinite float in a matrix of highly sheared or foliated serpentinite (e.g., Shervais et al., 2005a) (Fig. 1). Serpentinite broken formations may form tectonically in response to an external shear stress; alternatively, they

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Figure 1. Outcrop scale view of serpentinite broken formation, Black Diamond Ridge, Stonyford, California. (A) View to NE of massive lherzolite or harzburgite blocks in matrix of sheared serpentinite; crest of ridge is massive lherzolite. (B) Serpentinite broken formation with phacoidal lherzolite or harzburgite blocks (dark green, reddish brown) in sheared serpentinite matrix (pale blue green).

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serpentinite mud volcanoes are common and feed debris flows within the trench (e.g., Ishii et al., 1992; Fryer et al., 1995, 1999, 2000). These deposits may contain a variety of exotic clasts derived from their parent serpentinite body, such as blueschists, as well as intermingled clay, sand, and fossils. Their introduction into a trench setting may result in the tectonic recycling of this sedimentary mélange, forming many of the features we currently identify with tectonic mélange. Nonetheless, ultimately the source of these sedimentary serpentinites must itself have been mixed prior to their formation, especially where they carry high-pressure metamorphic rocks. A major question about the origin of serpentinite matrix mélange hinges on the provenance of the serpentinite itself: Does it represent oceanic mantle, hydrated on the seafloor long before accretion to the subduction complex, or does it represent the hanging wall of a subduction zone, hydrated during plate convergence? Is the mixed assemblage the result of tectonic processes, or does it form as a serpentinite diapir within forearc sediments (e.g., Ishii et al., 1992; Fryer et al., 2000)? These disparate origins require distinctly different histories, and each implies different assemblages of tectonic inclusions (e.g., Blake and Jayko, 1990; Blake et al., 1995; Wakabayashi, 2004; Choi et al., 2008a, 2008b). In this contribution we summarize evidence for the origin of the Tehama-Colusa serpentinite mélange in northern California, which is juxtaposed tectonically against both the Coast Range ophiolite and the Franciscan complex (Hopson and Pessagno, 2004, 2005; Shervais et al., 2004a, 2005a). This evidence includes mapping and petrologic studies by Shervais and co-workers (Shervais and Kimbrough, 1985, 1987; Shervais and Hanan, 1989; Shervais et al., 2004a, 2004b, 2005a, 2005b, 2005c), mineral chemistry and isotopic studies by Choi et al. (2008a, 2008b), and published reports by other workers (Jayko and Blake, 1986; Blake et al., 1987, 1992; Jayko et al., 1987; Huot and Maury, 2002; Hopson and Pessagno, 2004, 2005; Hopson et al., 2008). Previous studies going back to Saleeby (1983) have proposed a fracture zone origin for this mélange belt (Saleeby, 1983; Shervais and Kimbrough, 1987; Coleman, 2000; Hopson and Pessagno, 2004, 2005). Our data imply a more complex interpretation, in which a proto-Franciscan subduction zone nucleated on a largeoffset transform (oceanic fracture zone); subsequent extension of the upper plate, and magmatism associated with this extension, created the refractory forearc mantle and the overlying ophiolitic crust (Choi et al., 2008a, 2008b). Convergence during and after this process resulted in a variety of included blocks derived from both the upper and lower plates (Jayko and Blake, 1986; Shervais and Kimbrough, 1987; Blake et al., 1987, 1992; Huot and Maury, 2002; Hopson and Pessagno, 2004, 2005; McLaughlin et al., 1990; McLaughlin and Ohlin, 1984). METHODS The study of mélange formation typically involves two components: field observation and investigation of the prov-

enance of the mélange matrix and constituent blocks within the mélange. In this contribution we summarize the results of our field mapping projects in two locations, and the results of other field investigations in these locations. Field studies document the lithologies that occur within the mélange, their distribution, and how they relate to the surrounding rocks. Some lithologies are distinct and require an exotic source, e.g., amphibolite blocks and metasedimentary blocks. Sizes of the blocks range from several kilometers to less than a meter, so many blocks cannot be mapped even at larger scales. However, their location can be noted, along with their composition and distribution. We also review the provenance of knockers within the mélange, with particular focus on the petrology, mineral chemistry, and whole-rock geochemistry of metavolcanic and peridotite blocks within the mélange. For the mafic volcanic rocks, major and trace element concentrations can be used for comparison with volcanic rocks from known tectonic settings, using Harker diagrams and trace element tectonic discrimination diagrams. For the peridotite blocks and even the sheared serpentinite matrix, we can use the major element composition of relict mineral phases. Cr-spinel is especially useful because it is typically unaffected by the serpentinization process, with the exception of forming magnetite or ferrichromite rims. Abyssal peridotites are characterized by spinels with Cr#s [= 100Cr/(Cr+Al)] of ~10 to ~59; whereas supra-subduction zone–forearc peridotites are characterized by spinels with Cr#s ~35 to ~84 (Dick and Bullen, 1984; Ishii et al., 1992; Metcalf and Shervais, 2008). Although these groups overlap at Cr#s ~40–60, there are commonly samples that lie outside the overlap and provide diagnostic compositions (e.g., Choi et al., 2008a). The application of powerful new tools for trace element microanalysis (e.g., laser ablation ICP-MS [inductively coupled plasma–mass spectrometry]) will allow even more detailed studies in the future (e.g., Jean et al., 2010). Chert blocks of different provenance may also be distinguished if their radiolarian faunal assemblages are documented (e.g., Hopson and Pessagno, 2004, 2005). Cherts associated with the Coast Range ophiolite are characterized by a trend upsection from polytaxic Tethyan faunas to oligotaxic Boreal faunas (Pessagno and Blome, 1990; Pessagno et al., 2000; Murchey in Shervais et al., 2005c). Many of these cherts are also alumina rich, reflecting high volcanic ash contents (Hopson et al., 1981), but this is not true for all locales. In contrast, Franciscan abyssal cherts are characterized by higher silica contents, a wide range in ages, and classic ribbon chert intercalated with thin mudstone layers (Karl, 1984; Murchey, 1984). The mélange matrix is more difficult to characterize because of its variability and structural incoherence. However, X-raydiffraction studies can establish the phases present and distinguish among the serpentine phases, and microprobe studies of relict spinel compositions can establish the provenance of their protolith. The sheared matrix may preserve fabric elements indicative of shear sense and relative strain, but more commonly they simply reflect a poorly defined foliation.

Serpentinite matrix mélange: Implications of mixed provenance for mélange formation GEOLOGIC SETTING Coast Range Ophiolite The Coast Range ophiolite of California separates rocks of the Franciscan complex, a classic subduction zone accretionary complex in central and northern California, from forearc basin sedimentary rocks of the Great Valley Group (Bailey et al., 1970; McLaughlin et al., 1988; Shervais et al., 2004b; Hopson et al., 2008) (Fig. 2). Although the contact between the ophiolite and the Great Valley Group is commonly faulted, the original sedimentary contact is preserved locally (Blake et al., 1987). Igneous rocks of the ophiolite have been dated at 172 Ma to 161 Ma, although in the Diablo Range ages range down to 148 Ma (Hopson et al., 1981, 2008; Shervais et al., 2005c; Mattinson and Hopson, 2008). The ophiolite is commonly interpreted as a suprasubduction zone ophiolite that forms the basement of the Great Valley forearc basin (Evarts, 1977; Shervais and Kimbrough, 1985; Lagabrielle et al., 1986), but recent data show that more complex scenarios involving ridge collisions and late mid-oceanridge basalt (MORB) overprints are required (Giaramita et al., 1998; Evarts et al., 1999; Shervais et al., 2004b, 2005b, 2005c; and others). However, other investigators argue for more traditional models of formation at a mid-ocean-ridge spreading center (Hopson et al., 1981, 2008; Ingersoll, 2000; Dickinson, 2008). Serpentinized peridotite that underlies mafic igneous rocks of the Coast Range ophiolite has long been considered to represent the basement of mantle lithosphere upon which the ophiolite was constructed (e.g., Bailey et al., 1970; Hopson et al., 1981; Jayko et al., 1987; Shervais, 1990; Huot and Maury, 2002; Shervais et al., 2005a, 2005b). In contrast, Hopson and Pessagno (2004, 2005) proposed that the Tehama-Colusa serpentinite mélange in northern California represents an oceanic fracture zone unrelated to the ophiolite, and juxtaposed against it during subsequent accretion within the Franciscan subduction zone. This proposal echoes earlier suggestions by Saleeby (1983), Jayko and Blake (1986), and Shervais and Kimbrough (1987), but develops this hypothesis more fully based on their detailed synthesis (Hopson and Pessagno, 2004, 2005; Dickinson, 2008). Franciscan Complex The Franciscan complex of California is the classic example of an accretionary complex formed in response to the subduction of oceanic lithosphere (e.g., Bailey et al., 1964; Hsü, 1968; McLaughlin and Ohlin, 1984; McLaughlin et al., 1982, 1990; Blake et al., 1985, 1987, 1988; Ernst, 1993; Wakabayashi, 1999). The Franciscan complex in northern California comprises three NNW-trending belts that decrease in age and metamorphic grade from east to west: the Eastern Belt, Central Belt, and Coastal Belt (Bailey et al., 1964; McLaughlin et al., 1982). The Central Belt represents the classic shale-matrix mélange that is commonly associated with the Franciscan accretionary complex. It contains knockers of graywacke, greenstone, serpentinite,

5

chert, limestone, blueschist, eclogite, amphibolite, and garnet amphibolite (Bailey et al., 1964; Blake and Jones, 1974; Blake et al., 1988) as well as arkosic wackes interpreted as slope basin deposits (Becker and Cloos, 1985). The Coastal Belt, which lies outboard of the Central Belt, is a Schüppenzone that consists of east-tapered thrust wedges that generally young structurally downward toward the west and include rocks as young as Miocene in age, some of which are obducted over the older part of the Coastal Belt (McLaughlin et al., 1982; Aalto et al., 1995; Blake et al., 1985; McLaughlin et al., 2000). The Eastern Belt consists of coherent blueschist-facies metagraywacke and metabasalt, as well as exotic blocks in metamorphosed mélange with a metagraywacke matrix (Bailey et al., 1964; Blake and Jones, 1974; McLaughlin and Ohlin, 1984; Blake et al., 1988; Ernst, 1993). The metasediments have Early to mid-Cretaceous depositional ages and mid- to Late Cretaceous metamorphic ages (Blake et al., 1982). The easternmost unit in this belt is the Southfork Mountain schist, a penetratively deformed quartz-albite-lawsonite metagraywacke of Early Cretaceous age (120.5 Ma: Wakabayashi and Dumitru, 2007), and its blueschist-facies mafic member, the Chinquapin metabasalt. The Southfork Mountain schist crops out along the western margin of the Tehama mélange in the northern part of the study area, but it is replaced by other units of the Eastern Belt (Valentine Springs Formation, Yolla Bolly terrane) and in places, Central Belt rocks, farther south. TEHAMA-COLUSA SERPENTINITE MÉLANGE The Tehama-Colusa serpentinite mélange extends from Elder Creek in the north to Wilbur Springs in the south (Fig. 2). The northern portion of this belt was mapped by Blake et al. (1992), and it was described in some detail by Huot and Maury (2002) and by Hopson and Pessagno (2004, 2005); the reader is referred to their work for a complete survey of this belt. The southern end of this belt was mapped by McLaughlin et al. (1990). We present here an overview of the mélange between Elder Creek and Stonyford, California, where we have mapped parts of this mélange in detail. Jayko and Blake (1986) discuss the distribution and significance of foliated metasedimentary rocks in the Tehama mélange belt. The Tehama-Colusa mélange includes two main segments connected by a thin selvage of sheared serpentine that separates schists of the Franciscan complex on the west from the Coast Range ophiolite or wackes and mudstones of the Great Valley Group to the east: (1) the Tehama mélange segment, which structurally underlies the Elder Creek massif of the Coast Range ophiolite, and surrounds the Chrome peridotite block, a huge isolated knocker of harzburgite-dunite at the southern end of the Tehama mélange belt; and (2) the Colusa mélange, which underlies and partially surrounds the Stonyford volcanic complex and continues south to the area around Wilbur Springs (Fig. 2). The mélange is separated from adjacent rocks of the Great Valley Group in the south by the Stony Creek fault (Brown, 1964), and from the Elder

6

122° 44' W

40'

35'

Shervais et al. 122° 30' 25.5' W 40° N

Geologic map of Tehama-Colusa mélange Adapted from Jennings and Strand (1960) Hopson and Pessagno (2005) Shervais et al. (2005a, 2005b, 2005c)

Round Mtn

v

Crowfoot Point

Thomes Creek v

Tehama-Colusa mélange Elder Creek ophiolite Franciscan complex

45'

Hz

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Chrome

Stonyford volcanic complex

Grindstone Creek Alder Springs

Snow Mtn. volcanic complex Jurassic (?) Great Valley Series Cretaceous Great Valley Series Tertiary sediments

30'

30'

Quaternary alluvium Black Diamond Ridge

Places

ms v Lz

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v Little Stony Creek Hz

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10 0

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15'

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Wilbur Springs

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122° 30' W

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Figure 2. Geologic map of the Tehama-Colusa mélange. Sheared serpentinite dominates the thinner outcrop belts of the mélange, whereas kilometer-scale and smaller blocks make up much of the outcrop belt where it is wider. Kilometerscale blocks of harzburgite (Hz) or lherzolite (Lz) form much of the mélange around Stonyford and Chrome, and volcanic blocks (v) are common in the north. After Jennings and Strand (1960), Blake et al. (1992), Hopson and Pessagno (2005), and Shervais et al. (2005b). Dashed-line boxes show outline of detailed maps in Figure 3.

Serpentinite matrix mélange: Implications of mixed provenance for mélange formation Creek ophiolite in the north by the Beehive Flat fault (Jayko and Blake, 1986; Jayko et al., 1987). The Stony Creek fault trends N-S and forms a high-angle range front fault along this segment of the Coast Range. The Beehive Flat fault is a younger, NWtrending, low-angle detachment fault that cuts out section from the lower part of the Elder Creek ophiolite (Jayko et al., 1987). The serpentinite mélange belt is separated from the Franciscan complex to the west by the Coast Range fault, a high-angle struc-

ture that truncates an older tectonic contact between these units (Jayko and Blake, 1986) (Fig. 2). Tehama Mélange Exposures of sheared serpentinite north of Grindstone Creek (~39° 30′ N, Fig. 2 and Fig. 3A) constitute the Tehama mélange belt; the northern part of this belt has also been referred 122° 30' W 40° 00' N

122° 45'

Fault

Elder Creek Middle Fork

Tehama-Colusa mélange Massive peridotite blocks

Paskenta Fault Zone

Volcanic ± chert blocks

lt

Round Mountain

N

Elder Creek South Fork

Flat Fau

Coast Range

Elder Creek ophiolite Be eh ive Lz

Foliated metasediments

Digger Creek ssp

Elder Creek ophiolite

Paskenta Crowfoot Point

Franciscan complex Lo

Great Valley Group

Thomes Creek

0 St yC on lt

u Fa

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u Fa

he

rC

re

Chrome Harzburgite block

Hz

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e Fa Coa

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ang

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ru

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ge

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ek

1

re

an

st

tR

hru

as

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ng

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gS Su

7

Grindstone Creek

39° 40' 122° 30'

Figure 3 (Continued on following page). Geologic maps of the Tehama and Colusa mélange segments. (A) Tehama mélange segment, showing distribution of largest tectonic blocks (after Blake et al., 1992; Huot and Maury, 2002; and Shervais, unpublished). (B) Colusa mélange segment around Stonyford, showing distribution of massive peridotite blocks and the Stonyford volcanic complex (after Brown, 1964; Shervais et al., 2005a, 2005b, 2005c). Hz—harzburgite; Lz— lherzolite; ssp—sheared serpentinite; SFVC—Stonyford volcanic complex; Qal—Quaternary alluvium.

8

Shervais et al.

to as the Round Mountain mélange (Jayko and Blake, 1986; Shervais and Kimbrough, 1987; Blake et al., 1992; Huot and Maury, 2002). The Tehama mélange lies southwest of the Elder Creek massif of the Coast Range ophiolite, separated from the ophiolite and its Crowfoot Point breccia unit by the Beehive Flat fault (Jayko and Blake, 1986; Blake et al., 1992; Huot and Maury, 2002). A high-angle fault along the western boundary of the mélange separates it from the Franciscan complex (South Fork Mountain schist: Blake et al., 1988, 1992); this fault is generally regarded as the Coast Range fault. Mélange blocks here consist largely of basalt and chert, commonly forming composite knockers of chert sitting depositionally on pillow lava. Small blocks of massive peridotite and diabase are less common (Huot and Maury, 2002). A second major exposure of the Tehama mélange occurs near the town of Chrome, ~20 km south of Elder Creek (Fig. 3A).

A large kilometer-scale block of harzburgite (~3 km across and ~8 km long) forms a bulge in the mélange belt, which is only a few tens or hundreds of meters wide on either side of this block. This block of partially serpentinized but massive harzburgitedunite underlies Red Mountain and contains several chromite mines, including the Grey Eagle mine, an open pit excavation ~500 m across. Several smaller pits are found upslope from the main pit. The podiform chromite deposits within dunite are now largely mined out, but remnants are still visible. Colusa Mélange The southern segment of the Tehama-Colusa mélange extends from Grindstone Creek in the north to Wilbur Springs in the south (Fig. 2). In the north it surrounds the Stonyford volcanic complex (Shervais and Hanan, 1989; Shervais et al.,

n Fra

39° 30'

nC

ca

cis

Great Valley Group

x ple

om

Tehama-Colusa mélange Massive peridotite blocks Stonyford volcanic complex

Black Diamond Ridge Volcanic sandstone blocks ssp

Franciscan complex

Lz Qal

Great Valley Group Gravelly Ridge Conglomerate

SFVC

Stonyford ssp

N

Hyphus Creek

Little Stony Creek Franciscan Complex

B 122° 40'

5 km

Basaltic sandstone Great Valley Group

Hz

Hz ssp

Hz 39° 15' N 122° 30' W Figure 3 (Continued).

Quaternary alluvium

Serpentinite matrix mélange: Implications of mixed provenance for mélange formation 2005a, 2005b, 2005c); farther south the mélange is almost entirely peridotite, both massive and sheared, forming a serpentinite broken formation. The mélange near Stonyford appears to largely underlie the volcanic complex itself (as documented by deep exposures and fault offsets), but it also wraps around its eastern margin, at least in part (Fig. 3B). Blocks of cumulate gabbro, isotropic gabbro, quartz diorite, and pyroxenite underlie the volcanic complex and are also exposed to the northeast. Massive, unsheared harzburgite and lherzolite form large kilometer-scale blocks both north and south of the volcanic complex (Shervais et al., 2005a; Choi et al., 2008a, 2008b), whereas additional volcanic blocks and metamorphic blocks lie to the west. North of the Stonyford complex the crest of Black Diamond Ridge is underlain by massive lherzolite, and the lower elevations are underlain by sheared serpentinite with smaller (dekameter-scale to meter-scale blocks) of massive serpentinite (Fig. 1). South of the Stonyford complex, massive unsheared harzburgite blocks are exposed along Hyphus Creek and Little Stony Creek. Franciscan metasediments and metavolcanics— equivalent to the Valentine Springs Formation—crop out along the western boundary of the mélange, and Tithonian mudstones of the Great Valley Group are faulted against the eastern boundary of the mélange (Fig. 3B). Serpentinite mélange mapped in the Wilbur Springs area by McLaughlin et al. (1990)—their Grizzly Creek mélange unit— contains a similar assemblage of Coast Range ophiolite–derived blocks and is likely correlative with the Colusa mélange. South of Wilbur Springs (Fig. 2) sedimentary serpentinites are common, interbedded with Lower Cretaceous Great Valley Group mudstones (Moiseyev, 1970; Phipps, 1984; Carlson, 1981; McLaughlin et al., 1990; Campbell et al., 1993). These detrital serpentinites were derived from protrusions of sheared serpentinite mélange on the seafloor; they contain small amounts of clay and macrofossils that confirm their detrital origin and a Hauterivian age (McLaughlin et al., 1990). Similar deposits are found in the Mariana forearc (Fryer et al., 1995, 2000). SERPENTINITE MATRIX OF THE TEHAMACOLUSA MÉLANGE This mélange matrix consists of phacoidal lithons a few centimeters to tens of centimeters in size, encased by serpentinite microlithons and sheared scaly serpentinite that may be broken into continually smaller scaly fragments (Fig. 4A). The matrix is commonly homogeneous in appearance, but in places it displays distinct variations in color that reflect changes in serpentine mineralogy, possibly containing clasts of one serpentinite in a matrix of another (Figs. 4B, 4C). The scaly matrix defines a subvertical foliation that generally trends ~N-S (subparallel to the trend of the mélange outcrop belt), but in detail it wraps around the margins of mélange blocks, and in places it dips parallel to local fault contacts (Fig. 5). Huot and Maury (2002) also note zones of nonfoliated granular serpentinite that they interpret as late diapirs formed within the mélange.

9

S-C fabrics are observed near both Stonyford and Elder Creek (Dennis and Shervais, 1991; Huot and Maury, 2002). Dennis and Shervais (1991) described serpentinite mylonite in a fault block of mélange within the Franciscan complex near Stonyford (Hyphus Creek area) that has well-developed S-C fabrics, which indicate tops down-to-the-east–dextral shear along a low-angle

A

B

C

Figure 4. Outcrop close-ups of sheared serpentinite matrix, Tehama mélange. (A) Intercalated green lizardite and blue antigorite-bearing sheared serpentinite; S-C shear bands in blue serpentinite indicate dextral shear sense (foliation trends N-S). (B) Contact between green lizardite and blue antigorite-bearing serpentinite. (C) Centimeter-scale clast of green lizardite in sheared blue serpentinite; millimeter-scale clasts are common. All photos are north of Toomes Camp Road, Paskenta.

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detachment surface. Foliated serpentinite exposed east of Black Diamond Ridge (Stonyford) displays a crude S-C fabric that dips to the east and also indicates top down-to-the-east–dextral movement within the mélange. In the northern Tehama mélange, welldeveloped S-C fabrics indicate dextral shear along dominantly N-S–trending shear planes (Fig. 4A). An outcrop in Digger Creek displays both dextral shear S-C-C′ fabric and an asymmetric fold in foliated serpentinite adjacent to a N-S–trending fault contact with an ophiolite dike complex that indicates west-side-up offset (Fig. 6). The dextral shear S-C-C′ fabric is consistent with other indicators of dextral shear in the Colusa mélange. More commonly, sheared and foliated serpentinites do not display consistent shear sense indicators. X-ray-diffraction studies show that the sheared serpentinite matrix consists largely of lizardite, with minor chrysotile, brucite, Cr-spinel, and magnetite. Magnesite veins are reported locally. In parts of the Tehama mélange, the sheared matrix includes zones of indigo blue antigorite-bearing serpentine intercalated with more common green lizardite plus chrysotile. The foliated blue antigorite-bearing serpentinite commonly contains millimeter- to centimeter-scale clasts of pale-yellow-green lizardite-chrysotile (Fig. 4). The presence of antigorite as a relict phase (with dextral S-C fabrics) within the dominantly lizardite-chrysotile matrix implies original formation at temperatures of 300–640 °C, followed by retrogression to temperatures 55) consistent with derivation from highly refractory peridotite of supra-subductionzone affinity (Fig. 7A; Huot and Maury, 2002). The enrichment of these highly refractory spinels in TiO2 (Fig. 7B) implies reaction with a refractory magma similar in composition to boninite. The occurrence of high-Cr, high-Ti spinel is characteristic of a highly refractory harzburgite or dunite protolith, formed in a

supra-subduction setting by hydrous melt extraction (e.g., Choi et al., 2008a). LITHOLOGY OF BLOCKS IN TEHAMACOLUSA MÉLANGE In this section we review the variety of tectonic blocks (knockers) found within the mélange, grouped according to lithology. Blocks with inferred upper plate provenance include those derived from the Coast Range ophiolite or its underlying mantle lithosphere. Blocks with inferred lower plate provenance are those with clear oceanic affinities or those that have been subducted and metamorphosed along high-pressure/temperature (P/T) trajectories to blueschist or higher grade. A few blocks of low-grade metavolcanics and foliated metasediments cannot be firmly assigned to either of these groups (e.g., Jayko and Blake, 1986) and are considered separately.

A

East Dike complex

West Serpentinite

B

West C′

ssp C

Hz

Figure 5. Sheared serpentinite matrix (ssp) wraps around harzburgite block (Hz), Tehama-Colusa mélange. Edge of small, meter-scale harzburgite block (west) in sheared serpentinite matrix, which wraps around margin of boulder. Toomes Camp Road, Paskenta.

S

East Figure 6. Sheared serpentinite, Digger Creek, Tehama mélange segment. (A) Outcrop view of sheared serpentinite with dextral S-C-C′ fabric (west) in contact with dike complex (east); foliated serpentinite at contact deformed into asymmetric fold by down-to-east throw on fault; person is sitting on contact, which trends ~N-S. (B) Close-up of sheared serpentinite, with dextral S-C-C′ shear fabric. Scale aligned parallel to contact, which trends ~N-S.

Serpentinite matrix mélange: Implications of mixed provenance for mélange formation Mantle Peridotites

11

the sheared serpentine matrix in preserving their primary mantle textures and mineral compositions, despite partial to extensive serpentinization. The massive blocks near Chrome and Hyphus– Little Stony Creeks are harzburgite, with less common dunite and chromite. Data presented by Shervais et al. (2005a) and Choi et al. (2008a, 2008b) show that the mineral compositions in these unsheared massive peridotites are highly refractory, with Cr-rich spinels (Cr#s > 35: Fig. 7A) and low minor-element contents in pyroxene. Spinels with Cr#s of 35–55 have low TiO2, but high Cr# spinels commonly have high TiO2, consistent with boninite melt reaction (Fig. 7B). Pyroxene compositions are refractory, with low

Mantle peridotites form the largest and most common blocks within the mélange (Fig. 1). These blocks range in shape from rectangular to phacoidal lozenges and in size from 4%–13%. The dispersed stratigraphic positions of the mafic units, lying within primarily argillaceous strata, raise the question as to the time interval over which the units were erupted. This raises further the question as to whether coherent differentiation trends should be expected for the suite as a whole. Major element variation (Fig. 12) shows some coherency, however. There is a general trend of Al2O3, CaO, and TiO2 enrichment, and MgO depletion with SiO2. There is no coherency to alkalis, and with the elevated K2O value of 1.32% for sample C6, at least some alkali mobility is suggested. An iron enrichment trend is suggested for the entire suite on the basis of the FeO/MgO versus SiO2 plot. On this plot the data straddle the boundary between island arc tholeiite and calc-alkaline–boninite lava series, as defined by Reagan and Meijer (1984) for the Island of Guam in the Mariana forearc. The three lowest stratigraphic level samples (C2, C3a, and C4) have some major element similarities to boninites, with MgO ranging from 9% to 13%, and with SiO2 at ~52%. These samples also show elevated Ni and Cr contents, which is consistent with a boninitic affinity. In general, however, the suite more closely resembles an arc tholeiite association. Trace element data for the cover-strata volcanic rocks are consistent with an arc tholeiite–transitional boninite lava series. Select trace element variation diagrams for the mafic volcanic members, as well as basalts from the Kings-Kaweah ophiolite belt, are presented in Figure 11 (after Pearce, 1982; Shervais, 1982). On the V versus Ti plot the cover strata suite spreads along the arc tholeiite–calc-alkaline–boninite field. For Cr versus Y the three high-Mg rocks plot in the boninite field, and the rest in the boninite-arc tholeiite transition. For Zr versus Ti the three high-Mg rocks again plot in the boninite field, and the rest in the arc tholeiite field. All Kings-Kaweah ophiolite belt basalts plot within the MORB field. The two cover-strata volcanic rocks with the lowest SiO2 (samples C3a and C8) were analyzed for Nd and Sr isotopes (Table DR1). Initial isotopic composition corrections were made for a nominal age of ca. 215 Ma, which is justified below in the discussion of the age of the cover sequence. Initial εNd for these samples is +8.2 (±0.4) and 7.9 (±0.4), respectively. The Sri values are 0.7036 and 0.7040, respectively. The Nd data point to a strong depleted mantle component, whereas the Sr data suggest a seawater component added to the depleted mantle component. The initial εNd values fall within the upper range of values typically measured for arc tholeiite-boninite series rocks, whereas the Sri values lie at the lower range (Taylor et al., 1994;

56

J. Saleeby 16

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Figure 12. Major element variation diagrams for cover-strata volcanic rocks. Island arc tholeiite–calc-alkaline–boninite boundary on FeO/MgO versus SiO2 diagram from Reagan and Meijer (1984). Data in Table DR8 (see footnote 1).

Stern et al., 1991). Thus the mantle source for these lavas appears to have been at the more highly depleted end of the spectrum for typical sources of such lavas. The association of early-stage boninitic-affinity lavas, followed by arc tholeiites, is typical of the initial stages of suprasubduction zone magmatism following subduction initiation (Stern and Bloomer, 1992; Bloomer et al., 1995; Stern, 2004). Relatively shallow-level partial melting of partially serpentinized harzburgite, like that of the Kings River ophiolite, is a likely source for the earlier boninitic-affinity lavas with deeper melting of a similar depleted source that gave rise to the tholeiitic lavas. The relatively advanced state of time integrated LILE source depletion, as indicated by the Nd and Sr isotopic data, is consistent with abyssal mantle lithosphere akin to that of the Kings River ophiolite, forming the principal source rock in the mantle wedge that rendered the mafic lavas. Stratigraphic Setting and Age Constraints of Proximal Supra-Subduction Zone Volcanic Rocks The eruption of proximal volcanic rocks of the cover strata began at the cessation of radiolarian chert deposition in the Cala-

veras complex, and ceased by the time of siliciclastic turbidite deposition of the western Kings sequence. These relationships are exhibited for the sample C2 basaltic unit that lies along the Calaveras–Kings sequence contact, and the sample C8 basaltic unit that is mainly faulted against, but appears to be in local depositional contact with, the turbidites (Fig. 3). The local depositional contact is likely an angular unconformity, now transposed, representing an unknown hiatus. Age constraints for the cover strata are only quite broad, and thus the time interval over which proximal volcanism occurred is not well constrained. Accumulation of Calaveras chert-argillite began after a putative ~30 m.y. of radiolarian ooze deposition above ca. 295 Ma Kaweah serpentinite mélange pillow basalt. This suggests that Calaveras chert-argillite sedimentation began at ca. 265 Ma, or in the Middle Permian. The inclusion of Permian shallow-water-limestone slide blocks within chaotically deformed Calaveras chert-argillite (Saleeby, 1979) only constrains hosting chert-argillite deposition to the Permian or younger. Ammonoid remains from several localities within Kings sequence argillites indicate an early Mesozoic age (Saleeby et al., 1978). U/Pb zircon ages of 170 ± 1 Ma on a hornblende andesite dike that crosscuts the sample C3 volcanic unit (Saleeby and Sharp, 1980), and of 152 ± 1 Ma on

Geochemical mapping of the Kings-Kaweah ophiolite belt, California a trondhjemitic dike that cuts the siliciclastic turbidites, further indicate an early Mesozoic age for much of the section (Saleeby and Dunne, 2011). The 255 ± 20 Ma Sm-Nd isochron age for the sample M9 garnet amphibolite putatively further constrains proximal supra-subduction zone volcanism to have initiated in early Mesozoic time. Maximum age constraints for the sample C3 volcanic unit are provided by U/Pb zircon age data on sample C3b, an angular dacite clast of ~30 dm diameter that lies in a basaltic tuff breccia (sample C3a). The size and textural immaturity of the clast indicate that it probably underwent only one cycle of transport within the hosting submarine pyroclastic flow. This further suggests that its source was proximal to the vent complex for the mafic flow. In terms of bulk composition (Table DR7 and Fig. 12) and the presence of plagioclase microphenocrysts, completely recrystallized, the dacite clast is similar to silicic members of the boninite-arc tholeiite suites of Chichijima in the Bonin Islands forearc (Taylor et al., 1994). Possible consanguinity between the dacite clast and its mafic host suggests that the age of the clast could approximate the age of the sample C3 volcanic unit and other similar units proximal to the sample C3 site (Fig. 3). Sample 3b yielded a meager population of fine euhedral zircon. Figure 13A is an age frequency plot for 206Pb/238U ages determined by laser ablation ICP-MS (inductively coupled-plasma mass spectroscopy) techniques on 23 fine euhedral grains extracted from the clast. Analytical data are presented in Table DR9a. The data define a mean age of 219.1 ± 2.9 Ma. Ages for a number of grains range to as young as earliest Cretaceous, similar to the highly discordant zircon from the Kaweah serpentinite mélange plagiogranites (Saleeby and Sharp, 1980). The zircon grains of sample C3b, exhibiting the younger ages, are considered disturbed by Sierra Nevada batholith contact metamorphism and have been omitted from the Figure 13A analysis. The distribution of 207Pb/206Pb ages for those grains included in the age analysis (Table DR9a) suggests that most of these ages are concordant, and with the small scatter of the 206Pb/238U ages (MSWD = 1.4), 219 Ma is taken as the eruption age for the dacite clast source. U/Pb zircon data indicating an age of ca. 200 Ma was presented for the sample C1 dioritic “feeder” dike (Saleeby and Sharp, 1980). These age constraints suggest a Late Triassic to earliest Jurassic age for the principal phase of supra-subduction zone volcanism in the cover strata. The onset of supra-subduction zone volcanism in the Late Triassic in conjunction with stratigraphic relations (Fig. 3) places a Middle Triassic bound on the termination of hemipelagic sedimentation for the Calaveras complex. Approximate age and sediment provenance constraints for the siliciclastic turbidite of sample C9 are provided by U/Pb ages of its detrital zircon population (Table DR9b and Figs. 13B and 13C). Sample C9 consists of a 10-cm-thick sandstone bed graded to medium sand size along its base, where it consists of ~60% quartz, ~20% feldspar, plagioclase dominant, and ~20% lithic fragments, both volcanic and metamorphic. Recrystallization inhibits the precision of the determined modes and may have skewed the analysis away from a higher lithic content. Fig-

57

ure 13B shows an age probability plot for back to 500 Ma on the basis of 37 206Pb/238U ages. In terms of age constraints on the turbidite, the strong peak at 175–160 Ma suggests a late Middle Jurassic maximum depositional age. The minimum age of the turbidite section is constrained by it and its first cleavage being cut by a 152 ± 1 Ma trondhjemite dike swarm (Saleeby and Dunne, 2011). The 175–160 Ma peak corresponds to one of two major early Mesozoic magmatic flux events in the SW Cordilleran arc, the other being during the Triassic (Stern et al., 1981; Chen and Moore, 1982; Fiske and Tobisch, 1978; Busby-Spera, 1984; Barth et al., 1997; Ducea, 2001; Saleeby and Dunne, 2011). The sample C9 detrital zircon clearly reflects detrital inputs from both early Mesozoic arc belts. The full spectrum of 206Pb/238U detrital zircon ages for sample C9 is given in Figure 13C, with comparative plots from regional Paleozoic siliciclastic units that constituted proximal parts of the SW Cordilleran passive margin and overlying lower Mesozoic eolianites, which were possible sources for the ancient detrital zircon populations. Of particular interest are the spectra from continental slope-rise strata that occur in southern Sierra Nevada pendants as basement for the eastern facies of the Kings sequence (Saleeby and Busby, 1993; and unpub. data), and similar pendant rocks from the northern Mojave Desert region (A. Chapman and J.B. Saleeby, unpub. data). The spread of ages between ca. 0.95–2.1 Ga and 2.45–2.8 Ga corresponds to a series of peaks in the C9 turbidite. In contrast, lower Paleozoic passive-margin shelf rocks of the Snow Lake pendant of the eastcentral Sierra Nevada, the central Mojave Desert, and Death Valley (Grasse et al., 2001; Barbeau et al., 2005) do not yield spectra similar to the Late Archean-Proterozoic population of sample C9. Sample C9 peaks in the 390–640 Ma range are not readily explained with the southern Sierra–northern Mojave source spectra. Ages in the 390–400 Ma range could reflect fringing arc rocks emplaced into and erupted over lower Paleozoic (Shoo Fly) slope-rise strata of the northern Sierra Nevada (Saleeby et al., 1987), whereas detrital zircon with ages ranging from 390 to 640 Ma are abundant in lower Mesozoic eolianites that spread across the western reaches of the SW Cordilleran passive margin (Dickinson and Gehrels, 2003; Fig. 13C). The distribution of Paleozoic SW Cordilleran passive margin strata along the California region is pursued further below. In summary, proximal supra-subduction zone submarine mafic volcanic rocks of the Kaweah serpentinite mélange cover strata were erupted in Late Triassic to earliest Jurassic time. Upper Middle to lower Upper Jurassic siliciclastic turbidites appear to rest unconformably on the youngest of these mafic flows. A vent complex has not been identified for the Late Triassic–earliest Jurassic flows. Intercalation of the flows with argillite-rich strata suggests episodic volcanism off the flanks of a vent complex within a basinal setting. Such a vent complex has been identified for Late Triassic–earliest Jurassic arc tholeiitic to transitional calc-alkaline submarine mafic volcanics of the Jasper Point–Penon Blanco Formation ~100 km north of the Kings-Kaweah ophiolite belt along the Sierra Foothills belt (Saleeby, 1982; Snow, 2007). This volcano-plutonic complex

58

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N = 25

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Figure 13. U/Pb zircon age data for cover strata samples. (A) Frequency plot of 206Pb/238Pb ages on single igneous zircon grains from sample C3b dacite block. (B) Probability plot of younger spectrum of 206Pb/238Pb ages for detrital zircon from sample C9 siliciclastic turbidite. (C) Probability plots of 206Pb/238Pb ages for detrital zircon of C9 siliciclastic turbidite in comparison to 206 Pb/238Pb ages for detrital zircon populations from Paleozoic siliciclastic units of the southern Sierra Nevada region and for lower Mesozoic eolianites of the SW Cordillera, which were potential source components for the turbidites (after Grasse et al., 2001; Dickinson and Gehrels, 2003; Barbeau et al., 2005; Chapman et al., 2011). Data in Tables DR9a and DR9b (see footnote 1). MSWD—mean square of weighted deviates; Pz—Paleozoic.

Geochemical mapping of the Kings-Kaweah ophiolite belt, California was constructed on the Permo-Carboniferous Tuolumne ophiolitic mélange. The Jasper Point–Penon Blanco volcanic construct appears to have constituted a broad pillowed shield with hemipelagic sediment intervals followed by submarine volcaniclastic cone construction. The upper volcaniclastics interfinger northward along the Foothills belt with argillite-rich strata, producing a section similar to the principal stratigraphic section in which the Kaweah serpentinite mélange cover strata volcanics lie. It seems likely that the mafic flows in this serpentinite mélange cover strata likewise formed off the flanks of a major constructional center, presently not exposed. Thus the pattern of early Mesozoic supra-subduction zone mafic submarine volcanism constructed on a disrupted and accreted late Paleozoic ophiolitic substrate is a regional pattern along the Foothills belt. Furthermore, the Jasper Point–Penon Blanco construct, and part of its intrusive roots, rest unconformably beneath Upper Jurassic siliciclastic turbidites (Saleeby, 1982; Ernst et al., 2009), further attesting to a common early Mesozoic tectonic history along the Foothills belt. TECTONICS OF MÉLANGE FORMATION IN A LARGE OFFSET TRANSFORM–SUBDUCTION INITIATION ENVIRONMENT In this section a tectonic model is developed that accounts for the polygenetic abyssal magmatic history of the KingsKaweah ophiolite belt, its progressive disruption to form ocean floor mélange, and its accretion into the SW Cordilleran active margin and ensuing residence in a supra-subduction zone environment. A number of initial questions must be addressed in order for such a model to be seriously considered: (1) Was the Kings-Kaweah ophiolite belt “obducted” onto the Cordilleran margin? (2) Given up to an ~190 m.y. hiatus in abyssal magmatism, is the implied residence time within the abyssal realm reasonable within the physical constraints of seafloor spreading rates and characteristic sizes of major ocean basins? And (3) Given a major abyssal transform phase for the Kings-Kaweah ophiolite belt, did the plate kinematics of this plate juncture circuit directly into the plate kinematics of the Cordilleran margin? Or alternatively, was the abyssal transform phase decoupled from Cordilleran tectonics, and the transform assemblage merely accreted en masse, independent of its transform history? This question may also be posed as: Is there evidence within the SW Cordillera, independent of the transform history recorded in the Kings-Kaweah ophiolite belt, for late Paleozoic transform tectonics having affected the region? Gross Emplacement Geometry The Kings-Kaweah ophiolite belt was not obducted onto the SW Cordilleran margin. Regionally pervasive deformation structures and fabrics associated with the ophiolite belt are steep to vertical. West-dipping mid-crustal reflectors along the western Sierra Nevada–Great Valley transition directly north of the Kings River region (Miller and Mooney, 1994) are commonly cited as

59

evidence for eastward obduction of the Foothills belt, including its ophiolitic basement, in the Late Jurassic. However, basement core, as well as surface map data, shows that such reflectors underlie voluminous Early Cretaceous Sierra Nevada batholith rocks and subordinate pendants of Foothills belt rocks (May and Hewett, 1948; Saleeby, 2007). The referenced reflectors are continuous and highly coherent, and would not survive in such a state after the intrusion of copious batholithic intrusions. Furthermore, these reflectors coincide with a patch of recent seismicity (Gilbert et al., 2007), and thus it is much more likely that the reflectors are young and potentially active structures related to ongoing mantle-lithosphere removal processes and related lower crustal flow beneath the region (Zandt et al., 2004; Le Pourhiet et al., 2006; Saleeby et al., 2003). The emplacement geometry of the Kings-Kaweah ophiolite belt appears to have constituted a lithosphere-scale wedge that was accreted to the hanging wall of a newly established subduction zone. Serpentinite diapirs were sourced from deep enough below the accreted wedge to entrain high-pressure metamorphic blocks from underplated abyssal lithosphere, and the diapirs penetrated up through the accreted ophiolite belt without entrainment of hypothetical continental structural basement for the belt, assuming an obduction geometry. Early Mesozoic boninitic to arc tholeiitic volcanics that were erupted through the Kings-Kaweah ophiolite belt were likewise sourced within a mantle wedge composed of hydrous depleted peridotites with strong time integrated LILE depletions. Finally, Early Cretaceous members of the Sierra Nevada batholith that were intruded through the Kings-Kaweah ophiolite belt lack any evidence of having interacted with hypothetical continental structural basement for the ophiolite belt. The above is displayed in the Figure 14 plot of initial εNd verses Sri for principal igneous suites of the southern Sierra Nevada (DePaolo, 1981; Pickett and Saleeby, 1994; Clemens-Knott et al., 2011; and data reported here). The plot shows data fields for the KingsKaweah ophiolite belt, cover strata mafic volcanics, shallow-level Early Cretaceous Sierra Nevada batholith rocks emplaced into the Kings-Kaweah ophiolite belt, deep-level Early Cretaceous Sierra Nevada batholith rocks emplaced south of, but along strike of, the Kings-Kaweah ophiolite belt, and Late Cretaceous Sierra Nevada batholith rocks emplaced into North American crustal rocks of the axial to eastern Sierra Nevada batholith. The plot also shows a representative field for Proterozoic sialic basement of the SW Cordillera region, an important component for the axial to eastern Sierra Nevada batholith suite. The western Foothills suites display a geochemical maturation of the underlying mantle wedge with time, starting with its proto-composition as depleted mantle of Kings-Kaweah ophiolite belt affinity. Progressive addition of slab-derived fluids and minor terrigenous sediment components to the mantle wedge with time progressively shifted the εNd and Sri values progressing from the early Mesozoic suprasubduction zone mafic volcanics to the cross-cutting batholithic units (cf. DePaolo, 1981; Lackey et al., 2005; Stern et al., 1991; Clemens-Knott et al., 2011). In contrast, the entire axial to eastern Sierra Nevada batholith suite shows a strong Proterozoic North

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ancy would have worked in favor of a subduction initiation event, given the correct tectonic circumstances. It appears that the SW Cordilleran margin and the adjacent Panthalassa basin presented such circumstances at the end of the Paleozoic.

A B C D

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KKOB (Pz) KSM cover strata volcanics (Lt. Triassic - E. Jurassic) Shallow w. SNB (125-115 Ma) Deep w. SNB (115-100 Ma)

E F

e. SNB (100-85 Ma) SW Proterozoic basement

Figure 14. Plot of initial εNd verses Sri for samples from the KingsKaweah ophiolite belt (KKOB), its cover strata mafic volcanics, crosscutting plutons of the Early Cretaceous Sierra Nevada batholith (SNB), axial to eastern batholithic rocks of the southern Sierra Nevada, and SW Cordilleran Proterozoic basement (DePaolo, 1981; Pickett and Saleeby, 1994; Clemens-Knott et al., 2011; data presented in Table DR1 [see footnote 1]). Also delineated is the geochemical expression of the Foothills suture. Pz—Paleozoic.

American lithosphere or crustal component that is consistent with the North American crustal host rocks for the inner Sierra Nevada batholith zones. The boundary between the distinctive eastern continental affinity and western oceanic affinity Sierra Nevada batholith coincides with a profound tectonic break in metamorphic framework rocks wherein Paleozoic ophiolitic rocks lie to the west, and early Paleozoic Cordilleran passive-margin-affinity rocks lie to the east. The pre-batholithic boundary is named the Foothills suture, shown by its expression in the Sierra Nevada batholith εNd versus Sri in Figure 14, and discussed further below. Aging of Oceanic Plates As noted earlier, an ~190 m.y. hiatus in abyssal magmatism may not seem reasonable within the context of modern plate tectonics. The oldest known abyssal crust is ca. 168 Ma in the western Pacific, and it is nearing subduction into the Mariana trench (Ogg and Smith, 2004). However, the geometry of the continental masses and ocean basins was very different throughout the Paleozoic, as compared with the Cenozoic. During the Pangean supercontinent era the Panthalassa ocean basin occupied nearly two-thirds of Earth’s surface (Murphy and Nance, 2008). Thus such an extended abyssal-realm residence for the Kings-Kaweah ophiolite belt does not appear to pose any problems that cannot be accounted for in our current understanding of plate tectonics through geologic time. An interesting consideration regarding the abyssal magmatic hiatus is the implication of the residence of such old oceanic lithosphere in the Panthalassa basin. Based on simple principles of conductive cooling of oceanic lithosphere as it ages off its respective spreading axis, such old oceanic lithosphere should have carried strong negative buoyancy forces. Such strong negative buoy-

Transform Tectonics and Subduction Initiation along the SW Cordilleran Margin One of the definitive features of the SW Cordilleran margin is a regionally extensive zone of Permo-Carboniferous transform truncation that coincides with the pre-batholithic metamorphic framework of the Sierra Nevada (Davis et al., 1978; Walker, 1988; Kistler, 1990). Evidence supporting this event includes the high-angle truncation of NE-SW–trending facies boundaries and fold-thrust structures in the Neoproterozoic-Paleozoic passive margin sequence along the eastern Sierra Nevada region, and the truncation of the regional Paleozoic “McCloud” fringing arcmarginal basin system that ran outboard of the central to northern Cordilleran passive margin (Rubin et al., 1990). In conjunction with this truncation event was the transpressive accretion of NW-trending strike-slip ribbons along the truncation zone, which constitute the principal Paleozoic metamorphic framework units for the Sierra Nevada batholith. Figure 15 shows the distribution of Paleozoic metamorphic framework ribbons in the Sierra Nevada as well as the southern termination of the McCloud arc in the eastern Klamath Mountains. The Sierra Nevada batholith framework ribbons occur along the western Foothills, where they are variably covered by lower Mesozoic active margin strata as poly-metamorphosed pendants within this batholith, and as the eastern wall of the batholith along the Owens Valley region. The general distribution of the displaced ribbons relative to the zone of Permo-Carboniferous continental truncation is shown in the inset map of Figure 15. Essential features of Figure 15 are the delineation of regional tectonic domains based on facies relations and petrotectonic assemblages of Paleozoic rock exposures. The domains include areas once occupied by Paleozoic rocks that have been intruded out by Mesozoic plutons or covered by Mesozoic or Cenozoic strata. The principal domains are the western North America craton margin, the passive margin shelf, its slope and rise, the McCloud fringing arc with remnants of its subduction complex, and Panthalassa lithosphere that was accreted to the SW Cordilleran margin. The principal Paleozoic rock exposures for the Sierra Foothills belt and the southeastern Klamath Mountains are differentiated within the context of the regional domains. Of critical importance in the western Sierra are exposures of Paleozoic ophiolitic mélange and metamorphic tectonites of the Feather River, Bear Mountain, and Tuolumne complexes, all of which have igneous and metamorphic age, and a number of structural relations that are similar to those of the ophiolitic ductile shear zones and serpentinite mélange of the KingsKaweah ophiolite belt (Saleeby, 1990). Co-extensive with these Paleozoic oceanic basement exposures are belts of Calaveras complex chert-argillite, all broadly constrained in age from

Eastern Klamath Mtns.

TR

a uil ah Co

Paleozoic Passive Margin domains

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sf or m

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Cratonic Shelf Slope/rise Shoo Fly slope/ rise outcrop belt

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ial Ax

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t ea Gr

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y lle Va

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e ing h nic ato r C

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ath De

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Calaveras hemipelagic belts Upper mantle + mafic crustal metamorphics Bear Mountain Central metamorphic belt Feather River Kings-Kaweah Trinity Tuolumne Regional domain

on

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ma rgi ato n nh in g e

thru Golco st b nda elt

Geochemical mapping of the Kings-Kaweah ophiolite belt, California

0

100 km

olf W e t hi W

Mojave Desert

Figure 15. Maps showing principal Paleozoic tectonic elements and facies systems of the SW Cordillera in relation to the Permo-Carboniferous continental truncation zone, Sierra Foothills Paleozoic ophiolite and related (Calaveras) chert-argillite belts, and key Paleozoic tectonic elements of the eastern Klamath Mountains. Some of the larger isolated plutons and basal contacts of the lower Mesozoic overlap units of the Sierra Foothills and the Klamath Mountains are delineated for location purposes. No attempts were made to restore Basin and Range extension nor Middle Jurassic dextral drag along the Sierra Nevada and its proximal backarc region. The strikeslip ribbons of the Sierra Nevada are reconstructed from stratigraphic relations of metamorphic pendant belts. (Sources: D’Allura et al., 1977; Irwin, 1977; Schweickert et al., 1977; Saleeby et al., 1987; Kistler, 1990; Saleeby, 1990; Saleeby and Busby, 1993; Metcalf et al., 2000; Dickinson and Lawton, 2001; Glazner et al., 2005; Stevens and Green, 2000; Stevens et al., 2005; Stevens and Stone, 2005; Nadin and Saleeby, 2008; Saleeby and Dunne, 2011.) Pz—Paleozoic; Cz—Cenozoic.

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Permo-Carboniferous to Permo-Triassic (Cox and Pratt, 1973; Schweickert et al., 1977; Behrman and Parkison, 1978; Watkins et al., 1987; Saleeby, 1979). As with the Calaveras complex that is spatially related to the Kings-Kaweah ophiolite belt, these belts are interpreted as mainly “Permian” hemipelagic accumulations that rested above, and were accreted along with, Panthalassa lithosphere fragments. The locus of accretion is the Foothills suture, the join between North American and Panthalassan lithosphere. The suture is hypothesized to have had a polyphase history, first, of Permo-Carboniferous sinistral transform motion, followed by latest Permian sinistral oblique thrust imbrication during subduction initiation, and then re-deformation within the axis of superposed early Mesozoic arc magmatism. The eastern wall of the suture in the central to northern Sierra consists of lower Paleozoic slope-rise strata of the Shoo Fly complex, and plutons and overlap strata of the southernmost McCloud arc (D’Allura et al., 1977; Schweickert et al., 1977; Saleeby et al., 1987; Rubin et al., 1990; Harding et al., 2000). Similar lower Paleozoic sloperise strata lie in southern Sierra pendants east of the intruded-out Foothills suture, and form basement for the eastern facies of the lower Mesozoic Kings sequence (Saleeby and Busby, 1993; and unpub. data). The northward continuation of the Foothills suture is suggested below to have extended into the southern Klamath Mountains, but it has since been severely overprinted by early Mesozoic subduction-related thrusting (Davis et al., 1978). The inner margin of the truncation zone along the Sierra Nevada and northern Mojave Desert has been completely intruded out by the Mesozoic batholithic belt. Nevertheless, the inner truncation boundary can be regionally delineated by stratigraphic and sedimentary facies discontinuities between NW-trending pendant belts and by regional geochemical discontinuities in the batholithic belt (Moore and Foster, 1980; Walker, 1988; Kistler, 1990; Dunne and Suczek, 1991; Saleeby and Busby, 1993; Greene et al., 1997; Stevens et al., 2005; Lackey et al., 2005; Saleeby and Dunne, 2011). The inner truncation structure is shown as the Axial Sierra fault in Figure 15. The trace of the Permo-Carboniferous Axial Sierra fault has been disrupted by Mesozoic dextral faults (Lahren and Schweickert, 1989; Kistler, 1990; Saleeby and Busby, 1993; Nadin and Saleeby, 2008). The distribution of passive-margin shelf facies rocks along and adjacent to the truncation zone has been further shuffled by Permo-Triassic and mid-Cretaceous thrust faults, and conceivably by early Mesozoic extensional faults (Stevens and Greene, 2000; Stevens and Stone, 2005; Nadin and Saleeby, 2008; Saleeby and Dunne, 2011). The principal superposed Mesozoic structures are shown in Figure 15 in green, some preserved in batholithic and pendant rocks, and some largely cut out by batholithic rocks. In the extreme southwestern Sierra Nevada and adjacent Mojave Desert, large-magnitude extensional structures of Late Cretaceous age that exhumed the Sierra Nevada batholith to lower crustal levels have obscured pre-batholithic pendant relationships (Nadin and Saleeby, 2008; Saleeby et al., 2007). Elsewhere in the Sierra Nevada, exhumation of the batholith has been limited to mid- to upper crustal levels, leaving pendant stratigraphy and batholith petrochemical patterns

intact to the extent that the complex structural relations along the truncation zone can be reasonably constrained (Fig. 15). The accreted Panthalassa lithosphere domain extends across the projected trace of the Foothills suture in the eastern Klamath Mountains region as the Trinity peridotite, which forms depositional basement for the southernmost McCloud arc, and the Klamath Central Metamorphic Belt that consists of Paleozoic MORB metabasites that were partly subducted eastward beneath the Trinity peridotite in mid-Paleozoic time (Wallin and Metcalf, 1998; Metcalf et al., 2000; Barrow and Metcalf, 2006). The Trinity peridotite consists of serpentinized harzburgite, dunite, and lherzolite that are similar to the long-term LILEdepleted mantle that rendered the crustal section for the Kings River ophiolite. Plagioclase lherzolite that equilibrated under shallow upper-mantle, diapiric conditions (Quick, 1981) yields an Sm-Nd mineral–bulk rock isochron and an initial εNd that are indistinguishable from those determined for the Kings River ophiolite mafic crustal section (Jacobsen et al., 1984; Shaw et al., 1987). The high εNd value indicates that the Trinity peridotite was derived from the Panthalassa-depleted MORB mantle. The Trinity peridotite was ascending and undergoing partial melting beneath a Panthalassa spreading center at the same time that the Kings River ophiolite crustal section was generated along a Panthalassa spreading center by the partial melting of a Trinity-like peridotite. Unfortunately, vestigial lenses of peridotites retaining the remnants of high-temperature mantle-flow fabrics in the Hog and Red Mountain slabs of the Kings River ophiolite are serpentinized and contact metamorphosed, so they are not well suited for mineral isotopic studies that could more directly test a Trinity peridotite affinity. The Trinity peridotite was incorporated into a supra-subduction zone environment during the Early Silurian initiation of the southern segment of the McCloud arc system (Metcalf et al., 2000), whereas the Kings River ophiolite remained in the Panthalassa abyssal realm until the Late Permian. This is consistent with the Kings River ophiolite being generated at a fast spreading ridge, as concluded above, which potentially generated large expanses of abyssal lithosphere, including that of the Trinity peridotite, capable of rendering highly divergent tectonic evolutionary paths of potentially derivative ophiolitic fragments. The inset map of Figure 15 shows the southwest Cordillera truncation zone in a regional context. The truncation structure has been equated with the Mojave-Sonora megashear (Anderson and Silver, 1979), a zone of Late Jurassic sinistral shear that has produced little translation that can be documented. The term California-Coahuila transform has been adopted from Dickinson and Lawton (2001) as the Permo-Carboniferous truncation and translation zone, so as not to confuse this plate juncture for the superposed intraplate supra-subduction zone strain of the “megashear.” The Caborca block is shown in the northwestern Mexico region astride the transform. This block corresponds to a fragment of the passive margin shelf that was translated 500–800 km along the transform from the truncated shelf at Sierra Nevada latitudes (Stevens et al., 2005). Stratigraphic relations along the truncation locus indicate that the principal phase of sinistral

Geochemical mapping of the Kings-Kaweah ophiolite belt, California translation was during the Pennsylvanian (Stevens et al., 2005). Isolated exposures of slope-rise strata along the northwest margin of the Caborca block could be facies changes across the margin of the block and/or additional strike-slip ribbons displaced within the transform zone. The inset map also shows the locus of Triassic arc magmatism, which runs inboard and along the trace of the transform zone (Stern et al., 1981; Chen and Moore, 1982; Busby-Spera, 1984; Barth et al., 1997; Saleeby and Dunne, 2011). The actual age range of this “Triassic” arc belt is from the latest Permian (ca. 248 Ma) into the Early Jurassic (ca. 200 Ma), with the highest flux of plutons occurring in Early to Middle Triassic time. The principal phase of transform truncation and displacement along the California-Coahuila system, and the onset of “Triassic” arc magmatism, are in accord with the transform phase of development, and the subduction initiation emplacement of the Kings-Kaweah ophiolite belt. Translation and Emplacement of the Kings-Kaweah Ophiolite Belt The favored hypothesis for the generation, displacement, and emplacement of the Kings-Kaweah ophiolite belt is shown in Figure 16, and an overview of the critical phases of geologic history recorded in this ophiolite belt are summarized in Table 2. Lacking definitive constraints, a simple approach is adopted for the initial plate configuration whereby the genesis of the Kings River ophiolite along a fast-spreading ridge is depicted to have been proximal to a large offset transform roughly aligned with the future California-Coahuila transform (Fig. 16A). The fracture zone is shown extending into the Cordilleran passive margin as a marginal offset in the Neoproterozoic rifted margin. The marginal offset is shown as the outer edge of what was to become the Caborca block in the Permo-Carboniferous. The configuration of the passive margin to the south of the Caborca block native site (present geographic coordinates) is poorly constrained owing to subsequent tectonic overprints (Dickinson and Lawton, 2001; Nance and Linnemann, 2008). The possible existence of additional continental ribbons displaced from the outer edge of the Caborca block is also poorly constrained, with one possibility being the older continental basement elements within the Chortis block of the Central America isthmus (Rogers et al., 2007). Vast tracts of new abyssal lithosphere are shown to have been generated in Early Ordovician time (Fig. 16A), capable of rendering highly divergent evolutionary paths of its potential ophiolitic fragments, as indicated by the contrasting histories of the Kings River ophiolite and the Trinity peridotite. Considering the distribution of the continents and major ocean basins in the early Paleozoic (Murphy and Nance, 2008), the seafloor spreading kinematics of Figure 16A could have circuited directly into Iapetus plate motions. A hypothetical site for the Early Ordovician ascent of the Trinity peridotite beneath a Panthalassa ridge segment is shown adjacent to the “Kings River ophiolite” ridge segment, across a major transform fracture zone. The “Trinity” ridge-transform segments are considered likely nucleation sites

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for the initiation of east-dipping intra-oceanic subduction that rendered the southern segment of the McCloud arc, which was formed by Early Silurian time as recorded in the Klamath Mountains (Wallin and Metcalf, 1998). Once subduction initiated for the McCloud arc, the plate configuration depicted requires that the oceanic transform zone separating the Kings River ophiolite and Trinity ridge segments became active as a boundary transform, possibly analogous to the modern South Sandwich transform that bounds the southern Scotia Arc (British Antarctic Survey, 1985). Devonian–Early Mississippian fold and thrust belt structures developed along the southern McCloud backarc and adjacent Cordilleran shelf region suggest an early phase of McCloud arc impingement along the Cordilleran margin (cf. Smith and Miller, 1990) the details of which are not treated here. Figure 16B shows the plate geometry in the Late Pennsylvanian (295–290 Ma), entailing the initiation of the CaliforniaCoahuila transform, and intra-oceanic rifting along the KingsKaweah ophiolite belt–hosting transform. The initiation of a transform-spreading geometry similar to the Cayman Trough is adopted (CAYTROUGH, 1978; Rosencrantz et al., 1988) whereby ephemeral short spreading-center segments lie nested between transform walls with metamorphic core complex segments. The b–b′ cross section diagrammatically shows Ordovician MORB crust and mantle lithosphere (Kings River ophiolite) along the edge of the transform deforming into a metamorphic core complex and being entrained into the reactivated leaky transform zone. The geometry and kinematics of the putative core complex differ from oceanic core complexes that have been described in the literature (cf. Baines et al., 2003; Schroeder and John, 2004; Tucholke et al., 2008) in that the driving transtensional deformation is affecting the edge of an aged oceanic plate along an intra-oceanic rift, as opposed to transform plate edges developed proximal to more or less steady-state abyssal spreading centers. In this cross section a detachment fault similar to that imaged at the mid-Atlantic Ridge–Atlantis fracture zone intersection (Canales et al., 2004) is shown disrupting the Kings River ophiolite crustal section and rooting into a neovolcanic rift along which new oceanic lithosphere is generated. The Kings River ophiolite Moho ductile shear zone is shown forming along the asthenospheric corner flow zone off the shoulder of the neo-rift zone (after Baines et al., 2008). Entrainment of the Kings River ophiolite into the rift system requires the nucleation of a transform segment between this ophiolite and its native abyssal plate, as shown in the Figure 16B inset. Figure 16C shows the plate geometry in the Early Permian (285–260 Ma), with the growth of an oceanic microplate within the dilational transform system. The microplate is named the Foothills ophiolite belt microplate for its current expression as the principal ophiolitic exposures of the Sierra Foothills, including the Feather River, Bear Mountains, Tuolumne and KingsKaweah sub-belts and the co-extensive Calaveras hemipelagic belts. The Foothills ophiolite belt microplate was composed primarily of “abnormal” oceanic crust with an abundance of surfaced serpentinized upper mantle rocks, mafic metamorphic

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Figure 16. Generalized plate tectonic model for the generation of the Kings River ophiolite along a fast-spreading ridge in the Early Ordovician Panthalassa ocean basin, followed by the generation of the Kaweah serpentinite mélange in a Permo-Carboniferous leaky transform–slow-spreading-ridge system like that of the Cayman Trough, and the emplacement of the Kings-Kaweah ophiolite belt along the Foothills suture during subduction initiation. Insets show diagrammatic cross-sectional relationships at key time intervals. The Foothills ophiolite belt (FOB) consists of the Feather River, Bear Mountains, and Tuolumne sub-belts, in addition to the KingsKaweah ophiolite belt and the overlying and co-extensive Calaveras chert-argillite belts. Color-coding for details of the Kings-Kaweah ophiolite belt is the same as in Figures 2 and 3, and for other regional features (Fig. 15). Paleolatitude and orientation of the southwest North American craton are after Cocks and Torsvik (2002) and Nance and Linneman (2008). See text for details. KRO—Kings River ophiolite; Pz—Paleozoic.

Geochemical mapping of the Kings-Kaweah ophiolite belt, California

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TABLE 2. OUTLINE OF PETROGENETIC AND TECTONIC EVENTS RECOGNIZED IN THE KINGS-KAWEAH OPHIOLITE BELT (KKOB) Geologic time* (Ma) 125–100

Tectonic regime Intrusion of gabbroic to tonalitic plutons of western zone Sierra Nevada batholith and resulting contact metamorphism.

152–125

Second slaty cleavage formed in cover strata turbidites.

157–148

Intrusion of tonalitic intrusive sheets and basalt-trondhjemite solitary and sheeted dike sets with sinistral shear, and local dynamothermal contact metamorphism.

160–152

Nonconformable deposition of siliciclastic turbidites, serpentinite diapiric dike emplacement, and formation of first cleavage in turbidites.

170–157

Intrusion of gabbroic to tonalitic sheets with longitudinal dextral shear and local dynamothermal contact metamorphism, and local basalt to andesite solitary dike emplacement.

190–175

Early cover strata erosion, possible surfacing of serpentinite diapirs.

205–195

Intrusion of dioritic and trondhjemitic solitary dikes.

225–190

Submarine eruption of boninitic to arc tholeiitic pillowed and volcaniclastic flows, local submarine dacite dome-tuff eruption, deposition of tuffaceous and siliceous argillite, and sandstones-conglomerates derived from chert-rich source.

265–225

Deposition of Calaveras chert-argillite, near pervasive soft sediment deformation and inclusion of Permian shallow-water-limestone blocks.

ca. 255

Emplacement of KKOB to hanging wall of neo-subduction zone, and underlying high-pressure † metamorphism with subsequent emplacement of derivative metamorphic blocks into KSM by serpentinite diapirism.

295–255

Oceanic transform displacement of KKOB to SW Cordilleran transform margin.

295–265

Pelagic sedimentation of radiolarian oozes.

ca. 295

Diffuse oceanic spreading, transform generation of ophiolitic ductile shear zones, core complex § deformation, and transform capture of KRO , and initiation of submarine serpentinite diapirism and ocean floor mélange formation.

484–295

Residence of KRO in Panthalassa ocean basin with sparse pelagic sedimentation of metalliferous radiolarian ooze.

ca. 484

Seafloor spreading generation of KRO at fast spreading center in Panthalassa ocean basin.

*Time intervals not shown lack resolvable geologic record. Italics denote approximate upper and lower bounds on time constraints. † KSM—Kaweah serpentinite mélange. § KRO—Kings River ophiolite.

tectonites deformed along transform shear zones and core complex segments, dispersed basaltic-hypabyssal carapaces, mafic and ultramafic rubble piles, and variably deformed pelagic oozes. The c–c′ cross section shows a diagrammatic profile across the transform zone with tectonic slabs of the Kings River ophiolite forming a median ridge that faced into an axial deep along which pelagic and hemipelagic oozes of the Calaveras complex accumulated. During this time interval the Kings-Kaweah ophiolite belt was undergoing progressive deformation into ophiolitic mélange by transform shearing and large-magnitude displacements, and progressive serpentinite diapirism. The Calaveras complex is also being progressively deformed from soft sediment to lithification states by transform shearing as well as by slumping driven by vertical tectonism and conceivably transform seismicity. Viewing Figure 16C in a regional context, displacements related to the growth of the Foothills ophiolite belt microplate

are shown circuiting into the California-Coahuila transform and displacing the Caborca block. Such transform motion conceivably continued farther into the Rheic suture zone that transected the interior of Mexico, and which accommodated the impingement of Gondwana with the southern margin of Laurentia (Nance and Linnemann, 2008). Ribbons of slope-rise facies strata of the Cordilleran passive margin and superimposed southernmost McCloud arc rocks were entrained in the transform zone and accreted along the continental truncation zone inboard of the encroaching abyssal lithosphere. The autochthonous sloperise facies system lying inboard of the truncation zone, which also constituted part of a marginal basin behind the McCloud arc, underwent an unknown amount of shortening along the Golconda thrust belt during the later Permian phases of transform activity. This in part may account for the disproportionate along-strike dimensions of the strike-slip ribbons as compared

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with the current across-strike width of the slope-rise facies belt, compounded by additional dispersal of the ribbons along Mesozoic dextral faults (Fig. 15). Golconda thrust belt convergence is interpreted to have been driven by a reversal in the McCloud arc, which, based on the disruption of eastern Klamath arc activity and depositional overlap by the shallow-water reefal McCloud limestone, occurred in Early to Middle Permian time (Irwin, 1977; Stevens, 2009). A possible mechanism shown for driving the reversal was the collision of a major ocean island chain that formed in Panthalassa low latitudes, the remnants of which are dispersed along the northern Cordillera as the Cache Creek terrane, and from which shallow-water limestone blocks and clasts were derived and regionally reworked into early Mesozoic hemipelagic and volcaniclastic deposits (Davis et al., 1978; Ross and Ross, 1983). A number of Permo-Carboniferous events along the inboard wall of the transform system reflect extensional and/or transtensional deformation interpreted to be kinematically linked to the California-Coahuila transform regime. These events include extensional exhumation of the southern end of the McCloud arc subduction complex (Barrow and Metcalf, 2006), the termination of McCloud arc–related volcanism in the northernmost Sierra Nevada with subsidence that promoted overlapping hemipelagic sedimentation (D’Allura et al., 1977), the development of a borderland facies belt oriented along the truncated edge of the passive margin shelf (Stone and Stevens, 1988), and widespread extensional tectonism in the passive margin east of the truncation zone (Smith and Miller, 1990). This extensional-transtensional regime was abruptly overprinted by Golconda thrust belt deformation, and then later by Permo-Triassic sinistral transpression, interpreted below to mark initiation of subduction along the transform zone. The simplistic plate geometry adopted in Figure 16 offers an explanation for an enigma regarding the Calaveras complex belts of the Sierra Foothills. Permo-Carboniferous reefal limestones that formed on the Cache Creek ocean island chain are biogeographically distinct from the Permian McCloud arc capping reefal limestones, and both are distinct from coeval shelf limestones of the Cordilleran passive margin (cf. Ross and Ross, 1983). Blocks and slabs of the Cache Creek–type limestones are common in Permo-Triassic chert-argillite belts accreted to the margin of the McCloud arc, stretching from the Sierra Foothills to the southern Yukon (Davis et al., 1978). However, in the Calaveras complex belts of the Sierra Foothills blocks and clasts of both Cache Creek and McCloud-type limestones are present. This is the only region where fragments of both limestone types are present in the same rock assemblages, which is in line with the Figure 16C reconstruction whereby blocks of each were readily sourced in submarine landslides from the inboard wall of the transform system and delivered into the Calaveras depositional trough(s). The Foothills ophiolite belt microplate appears to have been juxtaposed with the truncated SW Cordilleran margin in the latest Permian (Fig. 16D). As final juxtaposition progressed, Panthalassa

lithosphere began converging with the truncated Cordilleran margin, and a new subduction zone nucleated. Permo-Carboniferous “abnormal” oceanic lithosphere of the Foothills ophiolite belt microplate was susceptible to accretion to the hanging wall of the new subduction zone owing to its buoyancy that resulted from widespread serpentinization. According to the Figure 16 model the majority of the impinging oceanic lithosphere was old (≥200 Ma) and consisted of “normal” ridge crest–generated lithosphere of Kings River ophiolite affinity. Juxtaposition of aged cold oceanic lithosphere with the buoyant “abnormal” oceanic lithosphere ribbon presented ideal circumstances for subduction initiation (cf. Stern, 2004). The ca. 255 Ma Sm/Nd age for the Kaweah serpentinite mélange garnet amphibolite block is taken as the approximate time of subduction initiation. High-pressure metamorphism is envisaged to have occurred along the neosubduction megathrust with the high temperature (~800 °C) of metamorphism recorded in the garnet amphibolite block possibly reflecting the subduction of a warm mid-Permian segment of the Foothills ophiolite belt microplate outer edge. Fragments of the high-P metamorphic selvage that formed along the neo-subduction megathrust were entrained in serpentinite diapirs and transported up to crustal levels in the proto-forearc wedge, possibly erupting in mud volcanoes. The d–d′ cross section diagrammatically shows the en masse accretion of the Kings-Kaweah ophiolite belt to the hanging wall of a neo-subduction zone and its juxtaposition with the passive margin, para-autochthonous strike-slip ribbons along the Foothills suture. The Kings River ophiolite transform median ridge is shown hypothetically persisting as a bathymetric high forming a proto-forearc ridge, facing inward to the Calaveras axial deep sediment wedge that was likewise accreted en masse along the Foothills suture. Serpentinite diapirs are shown, sourced from the subduction megathrust zone where they entrained fragments of the high-P metamorphic selvage that formed along the neo-megathrust. The upper plate of the neo-subduction zone responded to oblique convergence by sinistral transpression as recorded by Permo-Triassic east-directed thrusting of the Sierra Nevada– Death Valley thrust system (Stevens and Stone, 2005) and similarage thrust structures of the Mojave Desert region (Miller and Sutter, 1982; Walker, 1988), now highly dispersed in the northwest Mojave region by large-magnitude Late Cretaceous extension (Chapman et al., 2011). Partly coincident with Permo-Triassic transpression, relatively low-volume arc plutonism initiated along the trace of the truncation zone (Stern et al., 1981; Chen and Moore, 1982; Barth et al., 1997; Saleeby and Dunne, 2011). Such arc plutonism is recorded for end of Permian–Early Triassic time, whereas Late Triassic and earliest Jurassic arc activity is recorded by widespread voluminous silicic ignimbrites, some of which were ponded in large submarine calderas that were nested in a regional arc graben system (Fiske and Tobisch, 1978; BusbySpera, 1984, 1988; Schweickert and Lahren, 1989). The temporal and spatial relations of supra-subduction zone magmatism, as presented in the Figure 16 model, seem at odds with the generalized subduction initiation model of Stern (2004).

Geochemical mapping of the Kings-Kaweah ophiolite belt, California In the Figure 16 model a relatively low-volume calc-alkaline to locally shoshonitic arc was established along the eastern Sierra region before Late Triassic–earliest Jurassic boninitic-arc tholeiitic volcanism occurred along the Foothills belt. In the Stern (2004) model, which is based on the Izu-Bonin–Mariana arc system, such proto-forearc volcanism develops in response to profuse slab rollback as negatively buoyant lithosphere founders during subduction initiation (cf. Hall et al., 2003) and the upper plate of the new subduction zone undergoes regional extension. This form of subduction initiation is termed spontaneous and in theory arises primarily from buoyancy contrasts across transform faults in abyssal lithosphere. Stern (2004) also presents an end member alternative to spontaneous initiation that is termed induced, which results from far-field plate forces rendering the necessary component of convergence for the initiation and sustenance of subduction. The Macquarie Ridge–Puysgur Trench– Fiordland plate juncture system is cited as an example of ongoing induced subduction initiation, and emphasis is placed on the importance of upper plate compressive deformation, opposed to extension, as the sign of induced initiation. The Permo-Triassic subduction initiation event of the SW Cordillera resembles the induced subduction initiation regime that is occurring today along the Macquarie Ridge–Puysgur Trench– Fiordland system. There the transform system that is evolving into a new subduction zone extends from abyssal lithosphere into passive margin–type lithosphere of the Campbell-Challenger plateau, where transpressive deformation is in large part producing the New Zealand landmass. As of yet, however, this system has not produced any known boninitic-arc tholeiitic volcanic associations in an extensional proto-forearc setting. Returning to the Permo-Triassic event of the SW Cordillera, subduction initiation appears to have been in response to far-field oblique compressive forces, as evidenced by regional transpressive deformation of the upper plate. Perhaps as subduction progressed, the convergence trajectory changed to a stronger normal component, whereupon negative buoyancy forces in the aged downgoing slab began to dominate the dynamics of the system, and a phase of profuse slab rollback ensued. The result of this rollback was the inflow of asthenosphere into the forearc mantle wedge, promoting boninitic magma genesis at shallow levels and arc tholeiitic magma genesis at deeper levels. The already heated axial region of the arc responded to regional extension by changing the mode of arc magmatism to a series of dispersed large-volume submarine calderas, as opposed to a chain of calc-alkaline to shoshonitic plutons, which were conceivably overlain by andesitic strata-cones. The common occurrence of silicic ash components within the siliceous argillites that the Kaweah serpentinite mélange coverstrata mafic volcanics were erupted into is consistent with distal large-volume silicic eruptions occurring intermittently during the forearc region mafic volcanism. The relationships outlined above suggest that the Permo-Triassic subduction initiation regime along the SW Cordillera has elements of both induced and spontaneous initiation, as defined by Stern (2004), and thus appears to be a hybridization of the process as defined.

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Initiation of Franciscan Subduction The regional geology of California is distinguished by the late Mesozoic–early Cenozoic Franciscan subduction complex, which produced the California Coast Range basement regime. Regional tectonic relations, as well as the history of the Coast Range ophiolite and the initial high-pressure metamorphism recorded in the Franciscan complex, call for a subduction initiation event in the Middle Jurassic (Stern and Bloomer, 1992; Anczkiewicz et al., 2004; Shervais et al., 2005). The timing of this event corresponds well with the onset of a high–magmatic flux event of late Middle to Late Jurassic age that is recorded within the early-stage Sierra Nevada batholith and its metavolcanic pendants (Stern et al., 1981; Chen and Moore, 1982; Snoke et al., 1982; Ducea, 2001). Such a subduction initiation event calls for a major change in the plate tectonic regime of the SW Cordillera in the time interval between the production of the Triassic arc belt and the late Middle to Late Jurassic belt. This time interval corresponds to a distinct lull in early Mesozoic arc magmatism of the SW Cordillera (Stern et al., 1981; Chen and Moore, 1982; Barth et al., 1997; Saleeby and Dunne, 2011), dextralsense transpression in the region (Saleeby et al., 1992; Saleeby and Dunne, 2011), and to unconformities in the forearc region as recorded in the Kaweah serpentinite mélange cover strata and the Jasper Point–Penon Blanco sequence. In absolute age this lull occurred between 200 and 175 Ma. The explanation favored here for these ill-defined events is that they record the dextral-sense oblique collision of the Insular Superterrane, a major composite arc terrane of peri-Gondwana affinity that impinged northward and accreted into the northern Cordillera region in the Middle to Late Jurassic, and which was progressively compressed and accreted into the northern Cordillera in the Early Cretaceous (Davis et al., 1978; McClelland et al., 1992; Getty et al., 1993). As the Insular Superterrane slid northward along the SW Cordilleran margin toward its northern Cordillera accretion site it left a leaky transform–spreading ridge system in its wake along which the Coast Range and other southern Cordillera Middle Jurassic ophiolites formed (Saleeby et al., 1992). Nucleation of Franciscan subduction followed in the wake of Insular Superterrane northward migration along the leaky transform system. CONCLUSIONS The Kings-Kaweah ophiolite belt underwent two distinct phases of abyssal MORB magma genesis. The first phase was at 484 ± 18 Ma (Early Ordovician), which generated a complete abyssal crust and depleted upper mantle sequence probably along a fast-spreading ridge in the Panthalassa ocean basin. The second phase is constrained by 295 ± 15 U/Pb zircon ages on rare felsic intrusives, and by a 299 ± 32 Ma Sm-Nd isochron age on a wide range of mafic to rare felsic rocks. This Permo-Carboniferous phase of abyssal magmatism occurred along a large offset transform fracture zone. During this phase of magmatism and tectonism a part of the Early Ordovician abyssal lithosphere that

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was proximal to the fracture zone was disrupted into an oceanic metamorphic core complex and was cut into a series of elongate slabs by transform-related ductile shear zones. Deformation progressed within the transform zone to the state of producing mélange. Mélange mixing was facilitated by copious serpentinization and related diapiric mobilization, some of which surfaced onto the seafloor. Pillowed abyssal tholeiite eruptions, ophicalcites, and radiolarian oozes mingled with serpentinite debris along fault scarps and debris aprons. As deformation and pelagic sedimentation progressed, biogenic oozes were mixed and ultimately replaced by siliceous argillite derived from terrigenous material and distally derived volcanic ashes. The hemipelagic deposits were mobilized by submarine mass wasting and accumulated along axial troughs within the transform system. Regional tectonic relations of the SW Cordillera indicating a late Paleozoic transform truncation of the NeoproterozoicPaleozoic passive margin may be directly linked to the transform history of the Kings-Kaweah ophiolite belt. Direct kinematic linkage between the oceanic transform system and the passive margin truncation zone led to the juxtaposition of the ophiolite belt with a veneer of para-autochthonous strike-slip ribbons that together were accreted along the truncation locus. Emplacement of the ophiolite belt occurred by its en masse accretion to the hanging wall of a new subduction zone along the preexisting transform juncture starting at ca. 255 Ma. Related arc magmatism began in the eastern Sierra by ca. 248 Ma. Fragments of high-pressure oceanic metamorphic rocks that formed along the new subduction zone were locally entrained by serpentinite diapirs that intruded up into the overlying ophiolite belt, which introduced the exotic blocks into the mélange assemblage. In Late Triassic to earliest Jurassic time, fluid-assisted melting of depleted peridotites accreted to the hanging wall of the subduction zone rendered boninitic to arc-tholeiitic mafic magmas that were erupted within and above the ophiolite belts’ hemipelagic cover strata. Later in the Jurassic, siliciclastic turbidites spread across the KingsKaweah ophiolite belt as the forearc region matured. Detritus for the turbidites was supplied mainly by exhumed early Paleozoic passive margin–related strata and by early Mesozoic arc rocks of the eastern Sierra Nevada region. By late Middle to Late Jurassic time (170–148 Ma) small calc-alkaline plutons and dike swarms were emplaced into the Kings-Kaweah ophiolite belt within a transtensional deformation field, and by ca. 125 Ma copious gabbroic to tonalitic plutons of the Sierra Nevada batholith intruded and pervasively contact metamorphosed the ophiolite belt. Regional relations of the SW Cordilleran truncation and subduction initiation event, as well as the geologic history of the Kings-Kaweah ophiolite belt, suggest that subduction initiation was driven primarily by far-field plate tectonic forces and involved, first, highly oblique subduction whose tangential components were directly inherited from the prior transform phase of motion. Once subduction had been ongoing, and as the subduction trajectory evolved to a stronger normal component, aged cold, early Paleozoic abyssal lithosphere that was entering the trench began to founder, leading to a distinct slab rollback phase. This

resulted in regional extension across the arc and forearc environment, leading to a reorganization of arc magmatism to scattered large-volume submarine calderas and to dispersed boninitic-arc tholeiitic volcanism in the forearc region. In this context the early Mesozoic SW Cordilleran subduction initiation event resembles a hybridization between far-field-controlled “induced subduction initiation” and a more local buoyancy force–controlled “spontaneous subduction initiation,” as defined by Stern (2004). ACKNOWLEDGMENTS The author is indebted to C.A. Hopson for decades of inspiration in pursuit of addressing the ophiolite problem. Conversations with P.D. Asimow, J.M. Eiler, M.C. Gurnis, J.W. Hawkins, C. Şengor, J.W. Shervais, and R.J. Stern were of great value for the data interpretation presented. Direct assistance, acquisition of spikes and standards, and helpful tips in radiogenic isotopic geochemistry from J.G. Wasserburg, D.A. Papanastassiou, H. Ngo, and J.C. Chen are kindly acknowledged. Additional assistance in various aspects of the geochemical analytical work presented here by T. Bunch, A. Chapman, and M. Ducea are kindly acknowledged. Assistance in database compilation of Great Valley basement cores and technical drafting by Z.A. Saleeby is acknowledged. Assistance in mineral separation procedures by M. Chaudhry and I. Saleeby was essential for this study. The author thanks Harvey and Bobby Ruth for their hospitality while doing fieldwork, and their assistance in fieldwork logistics. Critical reviews by E.A. Miranda, K.D. Putirka, and J. Wakabayashi were a great asset. A grant from the Gordon and Betty Moore Foundation helped bring this study to completion. (Caltech Tectonics Observatory Contribution no. 119.) REFERENCES CITED Ague, J.J., and Brimhall, G.H., 1988, Magmatic arc asymmetry and distribution of anomalous plutonic belts in the batholiths of California: Effects of assimilation, crustal thickness, and depth of crystallization: Geological Society of America Bulletin, v. 100, p. 912–927, doi:10.1130/0016 -7606(1988)1002.3.CO;2. Anczkiewicz, R., Platt, J.P., Thirlwall, M.F., and Wakabayashi, J., 2004, Franciscan subduction off to a slow start: Evidence from high precision Lu-Hf garnet ages on high grade blocks: Earth and Planetary Science Letters, v. 225, p. 147–161, doi:10.1016/j.epsl.2004.06.003. Anderson, T.H., and Silver, L.T., 1979, The role of the Mojave-Sonora megashear in the tectonic evolution of northern Sonora, in Anderson, T.H., and RoldanQuintana, J., eds., Geology of Northern Sonora: Hermosillo, Sonora, Sonora Instituto de Geologia, Universidad Autonoma de Mexico, p. 59–68. Aumento, F., Loncarevic, B.D., and Ross, D.I., 1971, Hudson geotraverse: Geology of the Mid-Atlantic Ridge at 45°N: Royal Society of London Philosophical Transactions, ser. A, v. 268, p. 623–650. Babcock, J.M., Harding, A.J., Kent, G.M., and Orcutt, J.A., 1998, An examination of along-axis variation of magma chamber width and crustal structure on the East Pacific Rise between 13°30′N and 12°20′N: Journal of Geophysical Research, v. 103, p. 30,451–30,467, doi:10.1029/98JB01979. Baines, A.G., Cheadle, M.J., Dick, H.J.B., Scheirer, A.H., John, B.E., Kusznir, N.J., and Matsumoto, T., 2003, Mechanism for generating the anomalous uplift of oceanic core complexes: Atlantis Bank, Southwest Indian Ridge: Geology, v. 31, p. 1105–1108, doi:10.1130/G19829.1. Baines, A.G., Cheadle, M.J., John, B.E., and Schwartz, J.J., 2008, The rate of oceanic detachment faulting at Atlantis Bank, SW Indian Ridge: Earth and Planetary Science Letters, v. 273, p. 105–114.

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Constraints on the evolution of the Mesohellenic Ophiolite from subophiolitic metamorphic rocks R. Myhill* Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge, Cambridgeshire, CB2 3EQ, UK

ABSTRACT Narrow, discontinuous bands of high-grade subophiolitic metamorphic rocks, comprising predominantly amphibolite facies metabasites with rare metasediments, are observed at the contact between the complexes and subjacent mélanges of the Mesohellenic Ophiolite exposed in northwestern Greece. Both conventional and pseudosection thermobarometry have been used to yield estimated peak pressure-temperature (P-T) conditions of these tectonic sheets. Toward the leading edge of the ophiolite, subophiolitic rocks of the Vourinos Complex record peak metamorphic temperatures of 770 ± 100 °C. Pressures of 4 ± 1 kbar beneath the Vourinos are estimated on the basis of hornblende composition and are similar to the expected pressures from ophiolitic overburden. Beneath the exposed Dramala Complex, at the trailing edge of the ophiolitic body southwest of the Vourinos, estimated temperatures reached 800 ± 40 °C and 12.00 ± 1.27 kbar at the top of an apparent inverted metamorphic gradient imposed by discrete phases of accretion. High pressure assemblages beneath ophiolitic bodies imply exhumation relative to the overlying ophiolite. Estimated homologous temperatures in the upper plate are similar to those inferred for channeled exhumation during continental collision. Mineral assemblages lower in the Dramala sole indicate reduced temperatures and peak pressures. Similar pressures obtained within lower temperature sole rocks beneath Vourinos and Pindos suggest that a shallowly dipping thrust may have been responsible for obduction. Peak temperatures and pressures are in agreement with those estimated for secondary thrust propagation beneath a proto-arc after subduction in an intra-oceanic setting.

*[email protected] Myhill, R., 2011, Constraints on the evolution of the Mesohellenic Ophiolite from subophiolitic metamorphic rocks, in Wakabayashi, J., and Dilek, Y., eds., Mélanges: Processes of Formation and Societal Significance: Geological Society of America Special Paper 480, p. 75–94, doi:10.1130/2011.2480(03). For permission to copy, contact [email protected]. © 2011 The Geological Society of America. All rights reserved.

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INTRODUCTION The emplacement of ophiolites onto relatively buoyant continental margins, and the style of deformation beneath these complexes of dense oceanic crust and upper mantle during obduction, are controversial topics. In addition to work on the ophiolitic bodies themselves, constraints can also be derived from subophiolitic rocks. Autochthonous sequences are commonly observed between rocks deformed little by ophiolitic emplacement and the ophiolitic complex itself. These can take the form of tectonically disrupted mélanges composed of a mixture of lithologies. Although many mélanges are described as having chaotic structure (Şengör, 2003), discernible sequences are commonly present. The origin of lithologies contained within the mélange, and the degree of deformation and lithologic mixing imposed by the emplacement of the ophiolite, are often difficult to resolve. Nevertheless, the presence of a mélange is extremely important, as its low strength provides an ideal substrate for ophiolitic movement, and lithologies within the mélange provide constraints on transport of the overlying thrust sheets. Directly beneath the ophiolite, higher grade metamorphic rocks can be observed, separated from the underlying mélange by an abrupt tectonic and metamorphic contact. These rocks are commonly referred to as metamorphic soles, and constitute highly deformed sheets or sequences of sheets (700 °C (Green, 1982) and in some cases a loss of hornblende within these zones (Liagkouna samples aw0 and aw4), suggestive of temperatures approaching 900 °C (Green, 1982; Rushmer, 1991). The high proportion of mafic minerals within the sample suggests that several percent of the rock mass may have been lost as a felsic melt. Using the modeled pseudosection for the locality (Fig. 6), the low plagioclase content of the sample (12%) is suggestive

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of pressures of ~12 kbar, greatly in excess of the ~5–7 kbar estimated by the semiquantitative thermobarometer of Ernst and Liu (1998), despite the assumed increased reliability owing to the lack of titanite. High pressures are also modeled using the average P-T mode of THERMOCALC (not bulk composition dependent) for a garnet-bearing pyroxenite boudin present within a quartzofeldspathic shear zone. This anhydrous assemblage yields an optimum P-T estimate of 650 °C and 9.70 ± 0.78 kbar for a water activity of 0.1. For a more reasonable temperature of 800 °C based on the hornblende-plagioclase thermometry, pressures of 12.00 ± 1.27 kbar are attained. Fits are worsened by including hornblende, as expected from the lack of prograde hornblende within the boudin. Increasing water activity also worsens the fit, while increasing the optimum temperature and pressure. A sample recovered from 80 m below the contact (aw4) is indicative of lower pressures than those undergone by the uppermost sole. The general lack of epidote within the pargasiteplagioclase-titanite-ilmenite peak assemblage, and the relative abundance of plagioclase, indicate that peak pressures did not exceed 8 kbar (Fig. 6). A thin metabasic unit at the base of the section (aw7) unusually contains remnants of unaltered plagioclase, enabling hornblende-plagioclase thermometry based on the equilibrium equation edenite + albite = richterite + anorthite (Holland and Blundy, 1994). For a nominal pressure of 5 kbar, this formulation yields a temperature of 625 ± 40 °C. The presence of rutile within the sample is also extremely uncommon, being commonly viewed as a high-pressure indicator within metabasic rocks. The modeled phase diagram provided for this locality (Fig. 6) shows a joint presence of rutile and titanite with plagioclase between 4 and 10 kbar. If the modeled bulk composition can be viewed as an accurate representation of this sample, the lack of significant epidote and ilmenite in the sample suggests that pressures remained ~5.5 kbar. The lack of clinopyroxene suggests that peak temperatures were below ~600 °C. Combining the temperature estimates for the high- and low-grade Liagkouna soles yields an apparent inverse metamorphic gradient of 175 °C over 120 m. Observations of albite and prehnite formation from initial plagioclase in the low- and hightemperature samples suggest that retrogressive reactions continued to temperatures 10 kbar from this neargranulite-facies sample require an explanation other than that of “static” overburden of the ophiolitic complex. DISCUSSION Possible Setting of Sole Protolith Formation The dominance of metabasaltic rocks within the high-grade metamorphic units is in marked contrast to the Zavordhas and Avdella mélanges, where sediments are significantly more common, and so determining the setting of these rocks will help constrain large-scale geotectonic reconstructions. Vourinos and Pindos subophiolitic amphibolitic rocks can be divided into two distinct geochemical groups, which can also be distinguished on the basis of metamorphic grade (Jones and Robertson, 1991; Spray, 1980). Rocks of alkalic, within-plate-basalt (WPB) affinity are generally of lower metamorphic facies than those of MORB character. Evidence for a strong supra-subduction zone signature is lacking. Trace element compositions of Vourinos metamorphic sole rocks have WPB signatures, similar to greenschist facies metabasites within the Zavordhas mélange (Engwell, 2008). The implied setting of the high-grade subophiolitic metamorphic rocks is within an oceanic basin, away from significant terrigenous input, with lower grade metabasites derived from closer to the continental margin. The high-grade metasedimentary subophiolitic rocks are commonly quartz rich and may represent metamorphosed semi-pelites and cherts also derived from a sediment-starved basin environment. Similar affinities beneath the Vourinos and Pindos suggest the presence of a single fault above which the two complexes were initially obducted, prior to later disruption. P-T Conditions and Exhumation Temperature estimates from the high-grade metamorphic rocks studied here greatly exceed half the temperature of oceanic mantle above a subduction zone, so overriding of the mantle wedge during stable subduction cannot be the only cause of heating. To create these high temperatures invoking significant shear heating, recent upwelling is required, coupled with dynamothermal metamorphism from the moving of a hot ultramafic body over the tip of a newly initiated thrust (Spray, 1984), in agreement with the short time gap between ophiolite formation and cooling of the sole (Liati et al., 2004; Roddick et al., 1979). The low pressures recorded by most of the metamorphic sole are similar to those estimated by reconstructing Vourinos Ophiolite thickness. A folded and sheared Vourinos section (Rassios and Dilek, 2009) can be reconstructed, and, coupled with reasonable density assumptions can be used to obtain a pressure of

Constraints on the Mesohellenic Ophiolite evolution from subophiolitic metamorphic rocks ~4–5 kbar. The present-day thickness of the Pindos is only ~4 km (Rassios and Dilek, 2009), but tectonic disruption makes reconstruction of original thickness difficult. Nevertheless, it may be that all of the Liagkouna sole has undergone some exhumation to higher levels. The rarely observed higher pressure assemblages could not have been attained by simple overthrusting, and do appear to have required exhumation. High pressures within ophiolitic soles are not uncommon, and indeed it has been suggested that pressures exceeding those of ophiolitic overburden are ubiquitous in the geological record (Wakabayashi and Dilek, 2000, 2003). The mechanism for this exhumation is still poorly understood. Detailed analysis is beyond the scope of this paper, but observations from the Mesohellenic Ophiolite are suggestive of a regime similar to that of Himalayan exhumation between ca. 24 and ca. 18 Ma (White et al., 2002). The intense ductile deformation observed and related to early emplacement within the Vourinos Complex (e.g., Rassios et al., 1994) has been related to processes relevant at ca. 30 km depths (Ross et al., 1980), suggestive of lower mantle section exhumation. High homologous mantle temperatures indicated by peak temperatures within the sole, and the propagation of a thrust juxtaposing cold against overlying hot material, are both reminiscent of rocks exhumed beneath the South Tibetan Detachment System (e.g., Beaumont et al., 2001; Caddick et al., 2007; Jamieson et al., 2002). The tectonic geometry and scale of both systems are also similar. Inverse pressure gradients inferred from Liagkouna samples, and previously from the Semail Ophiolitic Sole (Gnos, 1998), suggest that exhumation was focused in the upper plate as for Himalayan exhumation, and high temperatures would place the uppermost sole in a similar deformational regime to the overlying peridotite. If exhumation at the base of ophiolitic complexes is indeed similar to channel flow, like that proposed for the Himalaya, erosion is necessary. There is ophiolitic material beneath the Cretaceous limestone on the Pelagonian margin, but no detailed studies have so far been conducted to determine its origins. Possible Tectonic Scenarios The presence of hot, shallow mantle above the lower plate of the thrust sampled by the metamorphic rocks is possible if the thrust propagated within the hanging wall of a preexisting subduction zone, where sub-arc upwelling resulted in temperatures sufficient (Kelemen et al., 2003) to induce near-granulite-facies metamorphism. In this case, burial to 20–40 km is not required to obtain amphibolite-facies metamorphism in the mantle wedge, as would be the case directly above an initiating subduction zone (Wakabayashi and Dilek, 2003). The alternative explanation of ridge subduction (Brown, 1998; Sisson and Pavlis, 1993) has also been proposed as a general model of sole formation (Shervais, 2001) and could similarly explain the high temperatures recorded. However, this explanation fails to account for the high pressures observed within many subophiolitic metamorphic rocks worldwide, or the presence of supra-subduction zone signatures within

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some soles (Encarnacion, 2004; Parlak et al., 2006; Pomonis et al., 2002; Saha et al., 2005; Wakabayashi et al., 2010). In the specific case of the Mesohellenic Ophiolite, it also fails to account for a crustal-level ophiolitic complex with supra-subduction zone signatures represented by the Aspropotamos Complex. Furthermore, pressures indicated by much of the Vourinos and Pindos soles suggest that mantle flow within the wedge above the thrust would have been strongly inhibited on time scales relevant to ophiolite obduction. Consequently, the observed volumes of supra-subduction zone-nature crust by 10%–20% melting of peridotite could not have been created. On this basis, a single scenario is favored, where a nascent arc developed above a subduction zone until reaching the buoyant continental margin, after which subduction stalled and a secondary thrust propagated through the overlying crust and upper mantle, facilitating obduction. The temperatures recorded by the sole suggest that this thrust propagated down through the crust and mantle before significant cooling had occurred. The high T, low P metamorphism common beneath the Mesohellenic Ophiolite is a good fit for subduction near a rift, and this is reflected in the favored model, with the emergent thrust located ~40 km from a proto-arc rifting environment. IMPLICATIONS Development of the Mesohellenic Ophiolite Processes occurring during the initiation of intra-oceanic subduction have recently been studied with numerical models. Although these models are rheology dependent, and as such prone to uncertainties, their application is of use in complex multivariable systems. Hall et al. (2003) showed that the initiation of subduction may result in rapid rollback of the lower plate after a few million years. This rollback may be favored further when initiated in warm regions, such as close to mid-oceanic ridges (Patel et al., 2007). The implications of this are that soon after the initiation of subduction, mantle melting can create a proto-arc and a marginal proto-arc above the subducting slab. Further combined metamorphic, thermodynamical, and rheological numerical modeling (Nikolaeva et al., 2008) showed that growth of new crust of a volume and areal extent comparable to that observed within the Mesohellenic Ophiolite could be formed within 5 m.y. of thrust inception and after ~500 km of subduction. Importantly, these models do not suggest significant mantle flow where the thickness of the mantle wedge is 30 km depth (farther west than shown) to beneath Dramala. MORB— mid-oceanic-ridge basalt; HT—high temperature. (3) The complexes are thrust over the oceanic crust and onto Pelagonia. The Aspropotamos is overridden by a Tertiary thrust after significant cooling of the ophiolite. Further compression leads to polarity reversal of subduction, after which closure of the oceanic basin continues until the trailing end of the Mesohellenic Ophiolite is back-thrust onto the passive margin of Apulia.

Constraints on the Mesohellenic Ophiolite evolution from subophiolitic metamorphic rocks The cessation of sea floor spreading was probably related to the collision of Africa with Europe (Smith, 2006). Convergence resulted in intra-oceanic thrusting and westward-dipping subduction, possibly initiating close to the center of the basin. The usual arguments against subduction initiation by ridge collapse—that the sole is of a different composition from the overlying ophiolite (Searle and Cox, 1999; Shervais, 2001)—cannot be invoked here, as the subduction zone developed is not the source of the metamorphic sole. Extension in the upper plate during subduction resulted in the formation of thick crustal cumulates in a nascent-arc setting, with flanking regions developing MORB-island arc transitional crustal sequences. Vourinos is proposed to be the site of the proto-arc, with Dramala further to the west. Within the lava sequences, supra-subduction zone signatures increase stratigraphically upward and toward the developing arc. This evolution is observed both in the Mesohellenic Ophiolite and farther north into the Albanian Mirdita Complexes (Beccaluva et al., 2005; Dilek et al., 2007). Subduction continued until proto-subduction of Pelagonia transferred convergence to a secondary thrust in the upper plate, largely responsible for ophiolitic emplacement. Underthrust material is a potential source of fluid responsible for late-stage igneous activity. For example, boninitic dikes are thought to be produced by fluid sourced from a high-temperature slab, causing melting of already depleted mantle (Ishikawa et al., 2005). The metamorphic rocks studied exhibit abundant evidence for fluid flow within the uppermost underthrust slab, and this may have imparted a prominent chemical signature on the overlying ophiolitic rocks. A later emergent thrust resulted in underthrusting of the Aspropotamos, as proposed by Jones et al. (1991). The original location is constrained by the presence of only a crustal section. The supra-subduction zone chemistry of the upper lavas and low Ti dikes (Jones et al., 1991; Rassios and Moores, 2006) are more akin to the Vourinos Complex than the Dramala. This later thrust is specific to the Mesohellenic Ophiolite, and is not required to explain features of other Tethyan ophiolites. Formation of the Sole and Mélange The secondary thrusting in this model results in basaltic material being overridden by hot mantle sequences, causing mylonitization in the lowermost upper plate and hightemperature metamorphism in the uppermost lower plate where it is juxtaposed against mantle material. This material becomes the metamorphic sole and represents the earliest obduction of the overlying ophiolitic bodies. The broad division of “sole” compositions into two groups, the higher grade amphibolites of MORB and greenschist facies basic rocks of within-plate basalt (WPB) composition (Jones, 1990) may reflect their environments of formation and times of accretion. While the MORB rocks were accreted before the ophiolite overrode the subduction zone, WPB rocks may have been sourced from rift-related volcanics of

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the Pelagonian continental margin. The underlying greenschist facies mélange contains sandstone pebbles within a matrix absent in sandstones (Ghikas, 2007), implying that even the uppermost mélange may not have been transported from deep within the Pindos Basin. As such, the variations in metamorphic grade with composition record the transport and cooling of the overlying ophiolitic body. Kinematic indicators within the lowermost mantle sequence are in good agreement with those within the sole and the underlying mélange. Beneath the Vourinos ophiolite, ductile structures within the metamorphic “sole” are consistent with emplacement to the northeast. For the Liagkouna outcrop of the Pindos, previous workers undertook detailed structural analysis (Jones, 1990; Jones and Robertson, 1991). Ductile deformation in the form of strong isoclinal folding here indicates early motion of the ophiolite toward the northeast, along a vector of 040°, much the same as the Vourinos and in agreement with ductile-brittle fabrics within the Dramala Complex itself (Rassios, 1991). Thus, assuming that relative rotations about a vertical axis have not resulted in exactly opposite apparent motions, it can be providing the metamorphic rocks beneath both the Dramala and Vourinos Complexes were formed beneath the same fault system. Similar fabrics to those observed in the sole are also present within the underlying mélange. In the subophiolitic exposures beneath Vourinos, and within the ophiolite itself, folds verge away from the ophiolite exposures to the southeast near the Zavordhas Monastery, and northeast to the north of Skoumtsa, apparently related to footwall compression and emplacement of the ophiolitic body onto the margin of Pelagonia. Emplacement of the Ophiolitic Bodies Given that kinematic indicators within the lowermost mantle sequence are in good agreement with those within the sole and mélange, it is likely that the continued motion of the ophiolitic bodies was facilitated by mylonitic zones in peridotite and shearing and faulting within overridden lavas that formed the hightemperature sole. After sole formation, further emplacement onto the Pelagonian Platform was enabled by deformation within deposits on the passive continental margin. These deposits, which would have been increasingly dominated by unconsolidated sediments, became the sub-ophiolitic mélanges. Faulting would have been facilitated within these sediments by dewatering, which would have led to high pore-fluid pressures and encouraged brittle failure. This may explain how such narrow bands of high-grade metabasics could be preserved beneath the ophiolite during emplacement onto the continental margin. CONCLUSIONS Important constraints on the emplacement of ophiolitic complexes can be provided by field work coupled with modern metamorphic studies. The Mesohellenic Ophiolite provides a useful

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starting point for these studies, given its large spatial extent and the breadth of work previously carried out. The supra-subduction zone affinity Mesohellenic Ophiolite requires formation above a westward-dipping slab (Smith and Rassios, 2003) to form the proto-arc Vourinos and transitional Dramala Complex crust and upper mantle sequences. Although the slab subducting beneath these complexes would appear to be an ideal source for metamorphic soles, both the presence of a supra-subduction zone affinity crustal section complex and high peak temperatures recorded within lower pressure metamorphic rocks are strong evidence of sole formation in the uppermost lower plate of a secondary thrust propagating within the subduction mantle wedge. This thrust may have formed as a result of upper plate stresses developed during attempted subduction of the passive margin of Pelagonia. Initiation marginward of the Aspropotamos Complex facilitated much of the initial emplacement of the Mesohellenic Ophiolite from the Pindos Basin northeast onto the Pelagonian Platform. Later thrusting then resulted in the Zygosti-Rodiani, Vourinos, and then the Dramala Complexes overriding the Aspropotamos (Jones, 1990). The previously observed MORB-WPB and amphibolite-greenschist division of rocks beneath the complex (Engwell, 2008) can be explained by continued sole accretion during oceanic obduction and earliest margin emplacement. Some metamorphic samples collected are indicative of high pressures similar to those previously documented from beneath many other ophiolitic complexes (Wakabayashi and Dilek, 2000, 2003). High homologous mantle temperatures indicated by peak temperatures within the sole and the propagation of a thrust juxtaposing cold against overlying hot material are both reminiscent of rocks exhumed beneath the South Tibetan Detachment System (e.g., Beaumont et al., 2001; Caddick et al., 2007; Jamieson et al., 2002). It is therefore possible that the processes of exhumation may be similar in both tectonic environments. It is hoped that future studies will be able to further test these hypotheses beneath ophiolites with better preserved soles. Future advances in metamorphic study, and in particular the incorporation of titanium into hornblende models and development of a metabasic melting model (Powell and Holland, 2008), will greatly enhance our ability to obtain detailed pressure-temperature-time (P-T-t) histories from subophiolitic metamorphic rocks. ACKNOWLEDGMENTS This project would not have been possible without the help and encouragement of T. Holland of the University of Cambridge, who supervised this master’s project. The author is also indebted to A. Rassios for her invaluable assistance and many enjoyable discussions on the evolution of the Hellenic ophiolites. D. Ghikas, R. Sparkes, and the other participants in the Aliakmon River Project provided much appreciated support during fieldwork. K. Gray and C. Haywood gave valuable assistance in sample preparation and obtaining microprobe data. I thank J. Wakabayashi and E. Moores for detailed and thoughtful

reviews, which substantially improved this contribution. Further thanks are due to J. Wakabayashi for follow-up discussions. This research was supported by the Aliakmon River Project under the auspices of the Greek Institute of Geology and Mineral Exploration (IGME). Fieldwork was financially supported by Peterhouse, Cambridge, and the Public Power Corporation of Greece; and electron probe microanalysis was made possible by the Department of Earth Sciences, University of Cambridge. REFERENCES CITED Beaumont, C., Jamieson, R.A., Nguyen, M.H., and Lee, B., 2001, Himalayan tectonics explained by extrusion of a low-viscosity crustal channel coupled to focused surface denudation: Nature, v. 414, p. 738–741, doi:10.1038/414738a. Beccaluva, L., Coltorti, M., Saccani, E., and Siena, F., 2005, Magma generation and crustal accretion as evidenced by supra-subduction ophiolites of the Albanide-Hellenide Subpelagonian zone: The Island Arc, v. 14, p. 551– 563, doi:10.1111/j.1440-1738.2005.00483.x. Brown, E.H., 1977, The crossite content of Ca-amphibole as a guide to pressure of metamorphism: Journal of Petrology, v. 18, p. 53–72. Brown, M., 1998, Ridge-trench interactions and high-T low-P metamorphism, with particular reference to the Cretaceous evolution of the Japanese Islands: Geological Society [London] Special Publication 138, p. 137–170. Brunn, J.H., 1956, Etude géologique du Pinde septentrional et de la Macédoine occidentale: Annales Géologiques des Pays Helléniques, v. 7, 358 p. Caddick, M.J., Bickle, M.J., Harris, N.B., Holland, T.J., Horstwood, M.S., Parrish, R.R., and Ahmad, T., 2007, Burial and exhumation history of a Lesser Himalayan schist: Recording the formation of an inverted metamorphic sequence in NW India: Earth and Planetary Science Letters, v. 264, p. 375–390, doi:10.1016/j.epsl.2007.09.011. Dale, J., Holland, T., and Powell, R., 2000, Hornblende-garnet-plagioclase thermobarometry: A natural assemblage calibration of the thermodynamics of hornblende: Contributions to Mineralogy and Petrology, v. 140, p. 353– 362, doi:10.1007/s004100000187. Diener, J.F., Powell, R., White, R.W., and Holland, T.J., 2007, A new thermodynamic model for clino- and orthoamphiboles in the system Na2O-CaOFeO-MgO-Al2O3-SiO2-H2O-O: Journal of Metamorphic Geology, v. 25, p. 631–656, doi:10.1111/j.1525-1314.2007.00720.x. Dilek, Y., Furnes, H., and Shallo, M., 2007, Suprasubduction zone ophiolite formation along the periphery of Mesozoic Gondwana: Gondwana Research, v. 11, p. 453–475, doi:10.1016/j.gr.2007.01.005. Dimo-Lahitte, A., Monie, P., and Vergely, P., 2001, Metamorphic soles from the Albanian ophiolites: Petrology, 40Ar/39Ar geochronology, and geodynamic evolution: Tectonics, v. 20, p. 78–96, doi:10.1029/2000TC900024. Elitok, M., and Drüppel, K., 2008, Geochemistry and tectonic significance of metamorphic sole rocks beneath the Beyşehir-Hoyran ophiolite (SWTurkey): Lithos, v. 100, p. 322–353, doi:10.1016/j.lithos.2007.06.022. Encarnacion, J., 2004, Multiple ophiolite generation preserved in the northern Philippines and the growth of an island arc complex: Tectonophysics, v. 392, p. 103–130, doi:10.1016/j.tecto.2004.04.010. Engwell, S., 2008, The geology of the Aliakmon Valley, Greece [B.Sc. thesis]: Edinburgh, Edinburgh University, 32 p. Ernst, W.G., and Liu, J., 1998, Experimental phase-equilibrium study of Al- and Ti-contents of calcic amphibole in MORB—A semiquantitative thermobarometer: American Mineralogist, v. 83, p. 952–969. Gartzos, E., Dietrich, V.J., Migiros, G., Serelis, K., and Lymperopoulou, T., 2009, The origin of amphibolites from metamorphic soles beneath the ultramafic ophiolites in Evia and Lesvos (Greece) and their geotectonic implication: Lithos, v. 108, p. 224–242. Ghikas, C., 2007, Structural and tectonics of a subophiolitic mélange (Zavordhas Mélange) of the Vourinos Ophiolite (Greece) and kinematics of ophiolite emplacement [M.S. thesis]: Miami, Ohio, University of Miami, 35 p. Ghikas, C., Dilek, Y., and Rassios, A.E., 2010, Structure and tectonics of subophiolitic melanges in the western Hellenides (Greece): Implications for ophiolite emplacement tectonics: International Geology Review, v. 52, p. 423–453, doi:10.1080/00206810902951106.

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Vamvaka, A., Kilias, A., Mountrakis, D., and Papaoikonomou, J., 2006, Geometry and Structural Evolution of the Mesohellenic Trough (Greece): A New Approach: Geological Society [London] Special Publication 260, p. 521–538. van Hinsbergen, D.J.J., Hafkenscheid, E., Spakman, W., Meulenkamp, J.E., and Wortel, R., 2005, Nappe stacking resulting from subduction of oceanic and continental lithosphere below Greece: Geology, v. 33, p. 325–328, doi:10.1130/G20878.1. Wakabayashi, J., and Dilek, Y., 2000, Spatial and temporal relationships between ophiolites and their metamorphic soles: A test of models of forearc ophiolite genesis, in Dilek, Y., Moores, E., Elthon, D., and Nicolas, A., eds., Ophiolites and Oceanic Crust: New Insights from Field Studies and the Ocean Drilling Program: Geological Society of America Special Paper 349, p. 53–64. Wakabayashi, J., and Dilek, Y., 2003, What constitutes ‘emplacement’ of an ophiolite?: Mechanisms and relationship to subduction initiation and formation of metamorphic soles: Geological Society [London] Special Publication 218, p. 427–448. Wakabayashi, J., Ghatak, A., and Basu, A.R., 2010, Suprasubduction-zone ophiolite generation, emplacement, and initiation of subduction: A perspective from geochemistry, metamorphism, geochronology, and regional geology: Geological Society of America Bulletin, v. 122, p. 1548–1568, doi:10.1130/B30017.1. White, N.M., Pringle, M., Garzanti, E., Bickle, M., Najman, Y., Chapman, H., and Friend, P., 2002, Constraints on the exhumation and erosion of the High Himalayan Slab, NW India, from foreland basin deposits: Earth and Planetary Science Letters, v. 195, p. 29–44, doi:10.1016/S0012 -821X(01)00565-9. Williams, H., and Smyth, W.R., 1973, Metamorphic aureoles beneath ophiolite suites and alpine peridotites; tectonic implications with west Newfoundland examples: American Journal of Science, v. 273, p. 594–621, doi:10.2475/ajs.273.7.594. Zimmerman, J., 1972, Emplacement of the Vourinos ophiolitic complex, northern Greece, in Shagam, R., Hargreaves, R.B., et al., eds., Studies in Earth and Space Sciences: A Memoir in Honor of Harry Hammond Hess: Geological Society of America Memoir 132, p. 225–239.

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Role of plutonic and metamorphic block exhumation in a forearc ophiolite mélange belt: An example from the Mineoka belt, Japan Ryota Mori* Master’s Program of Science and Technology, University of Tsukuba, Tsukuba 305-8572, Japan Yujiro Ogawa† Doctoral Program in Earth Evolution Sciences, University of Tsukuba, Tsukuba 305-8572, Japan Naoto Hirano§ Laboratory for Earthquake Chemistry, University of Tokyo, 7-3-1 Hongo, Bunkyo, Tokyo 113-0033, Japan Toshiaki Tsunogae Masanori Kurosawa Tae Chiba Doctoral Program in Earth Evolution Sciences, University of Tsukuba, Tsukuba 305-8572, Japan

ABSTRACT We investigated the field relations, metamorphic and deformation conditions, age, and chemistry of basaltic, plutonic, and metamorphic blocks in the Mineoka ophiolite mélange belt, Boso Peninsula, central Japan, to clarify their emplacement mechanisms. We considered internal and external deformation of the blocks in the context of the complicated processes by which the ophiolite mélange belt was formed in a forearc setting. A two-stage history leading to the present-day forearc sliver fault zone was revealed: an early stage of deep ductile deformation followed by an episode of brittle deformation at shallower levels. Both stages were the result of transpressional stress conditions. The first stage produced subduction-related schistosity with microfolding and mylonitization and then brecciation during exhumation in the intraoceanic subduction zone, from a maximum depth of garnet-amphibolite facies or eclogitic facies. The second stage was characterized by strong, brittle shear deformation as the rocks were incorporated into the present-day fault zone. The first incorporation of the oceanic plate to the side of the Honshu arc might have occurred during the Miocene, and was followed by right-lateral oblique subduction that has continued ever since the Boso triple junction arrived at its present-day position, thus forming the paleo-Sagami trough plate boundary.

*Current address: Masuo 3-8-25, Kashiwa 277-0033, Japan; [email protected]. † Corresponding author, current address: Yokodai 1-127-2 C-740, Tsukubamirai 300-2358, Japan; [email protected]. § Current address: Center for Northeast Asian Studies, Tohoku University, Sendai 980-8576, Japan. Mori, R., Ogawa, Y., Hirano, N., Tsunogae, T., Kurosawa, M., and Chiba, T., 2011, Role of plutonic and metamorphic block exhumation in a forearc ophiolite mélange belt: An example from the Mineoka belt, Japan, in Wakabayashi, J., and Dilek, Y., eds., Mélanges: Processes of Formation and Societal Significance: Geological Society of America Special Paper 480, p. 95–115, doi:10.1130/2011.2480(04). For permission to copy, contact [email protected]. © 2011 The Geological Society of America. All rights reserved.

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INTRODUCTION Places where ophiolitic mélanges are associated with plutonic and metamorphic blocks provide a key to understanding the emplacement mechanism of such blocks during mélange formation. However, little information on these matters has been published, except for part of the Franciscan (e.g., Cowan and Page, 1975; Cloos, 1984; Wakabayashi, 2004, and others) and Ankara mélanges (Dilek and Thy, 2006). Karig (1980) presented the idea that high-pressure metamorphic blocks (“knockers,” blocks in a mélange belt) are brought to the shallow part of the continental margin along a forearc sliver fault. Mann and Gordon (1990) discussed examples of such strike-slip faults in relation to the emplacement of the Guatemalan, Franciscan, and Sambagawa metamorphic rocks. Although these concepts demonstrate that exhumation of deep metamorphic rocks as a consequence of strike-slip faulting is possible, they do not explain the deformation history of those rocks. Cloos (1984) presented an interesting model of exhumation of high-pressure metamorphic blocks via a subduction channel as a return flow, but not in relation to an ophiolite mélange belt. This return flow model was followed by digital experiments by Gerya et al. (2002), but a detailed connection between such a model and field relationships was not attempted. Cowan and Silling (1978), Pavlis and Bruhn (1983), and Iwamori (2003), on the other hand, used an analogue and digital simulation to propose a corner-flow model for exhumation of subduction-related metamorphic rocks, and noted that a rapid uplift occurs along a backstop in the forearc area, especially in the case of oblique subduction. The models proposed by Cloos (1984), Cowan and Silling (1978), Pavlis and Bruhn (1983), and Iwamori (2003) are particularly plausible for a forearc in an oblique subduction setting in which deep-level rocks are selectively exhumed to shallow levels. The examination of relatively young forearc mélange belts, with associated metamorphic and plutonic rocks, may provide an important test to these models of mélange exhumation. However, the deformation of such units has received little study. Previous studies of the rock associations of the Mineoka belt, which is the chief concern in this paper (Figs. 1 and 2), suggested that the emplacement mechanism of the ophiolite sequence can be elucidated by consideration of the field relations and conditions at the time of emplacement, particularly with reference to the mechanism by which the associated plutonic and metamorphic rocks were exhumed (Ogawa and Taniguchi, 1988; Sato et al., 1999; Sato and Ogawa, 2000; Hirano et al., 2003; Takahashi et al., 2003; Ogawa and Takahashi, 2004; Mori and Ogawa, 2005). The general field relations of such blocks, as well as the basaltic and sedimentary rock blocks, are given in Ogawa et al. (2009) with chemical data of major, trace, and rare earth elements without giving new radiometric age data and any tectonic interpretation. This chapter provides new radiometric ages and chemical data from basaltic and other igneous rocks and evaluates these

data in the context of chemical data and field relations of previously described rocks to propose a tectonic model for exhumation of the plutonic and metamorphic blocks of the Mineoka belt. TECTONIC SETTING OF THE MINEOKA BELT The Mineoka belt lies in the Boso triple junction area. The Boso triple junction is the only known Trench-Trench-Trench– type (TTT-type) triple junction in the NW Pacific (Fig. 1), and formed when the Izu island arc moved to its present position in the Miocene (Seno and Maruyama, 1984; Ogawa et al., 1989; Takahashi and Saito, 1997). The Izu arc (the easternmost boundary of the Philippine Sea plate) then collided with the Honshu arc, and the NE boundary of the Philippine Sea plate has since been subducting obliquely under the Eurasia or Northeast Japan plate, forming successive accretionary prisms in the early Miocene and late Miocene to Pliocene, and it is still doing so today (Ogawa and Taniguchi, 1988; Ogawa et al., 1989; Saito, 1992; Hanamura and Ogawa, 1993; Yamamoto and Kawakami, 2005; Ogawa et al., 2008). During the middle Miocene, clastic rocks were deposited in the area of the present-day Mineoka belt. They are composed of three types of rock fragments: ophiolitic rocks, continental rocks, and volcanic island arc rocks (Ogawa, 1983; Ogawa et al., 2008, 2009), which shows that those three distinctive types of crustal material were locally available at that time. In addition, this area includes part of the Cretaceous to Paleogene Shimanto accretionary complex, which is widely developed in SW Japan as the shallow expression of the Sambagawa metamorphic rocks (Isozaki and Maruyama, 1991). The mechanism of emplacement of the ophiolitic rocks in this area during the middle Miocene is thought to have been a kind of incorporation that is an extrusion of ophiolitic rocks into the forearc area, bringing several different kinds of rocks together (Ogawa and Taniguchi, 1988; Sato et al., 1999). Thus, during the middle Miocene, the plate boundary was an ophiolite-bearing fault zone (the Mineoka ophiolite). Since then, the Mineoka belt has been an active fault system in a right-lateral (dextral) fault belt (Nakajima et al., 1981; Ogawa, 1983; Mori and Ogawa, 2005), as well as providing the backstop between the accretionary prism and the forearc basin (Fig. 2). The total strike-slip displacement along the fault belt may be more than 100 km (Lallemant et al., 1996). SUMMARY OF FIELD RELATIONS OF OPHIOLITIC AND OTHER ROCKS IN THE MINEOKA BELT The lithologies of the ophiolitic, plutonic, and metamorphic blocks and the surrounding rocks of the Mineoka belt are distributed in different ways in the three sub-belts (northern, central, and southern) of the Mineoka belt, which, in total, is 5 km in width (Fig. 2) (Taniguchi, 1991; Chiba, 2008). During the early stages of research, the ophiolitic rocks of the Mineoka belt were formerly thought to form an ophiolite mélange, as blocks of various

Role of exhumation in a forearc ophiolite mélange belt, Mineoka belt, Japan

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Figure 1. Index map of the localities in this paper. Shown are the Mineoka ophiolite belt, the Sofugan Tectonic Line (S.T.L., arrow) in the Izu arc (Shichito Ridge) collision to the Honshu arc, and the Ohmachi Seamount and Hahajima Seamount (arrow). Adopted and modified from Yuasa (1985).

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Figure 2. General lithologic map of the middle Boso Peninsula, showing the northern, central, and southern sub-belts of the Mineoka belt. The forearc basin (Miura Group on the north) and accretionary prism (Emi Group on the south and farther southward) of Miocene and Pliocene time (adopted and modified from Takahashi et al., 2003). Cross-section lines in Figures 3 and 4 are shown as A, B, and C. T-T-T—trench-trench-trench.

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Role of exhumation in a forearc ophiolite mélange belt, Mineoka belt, Japan rock types were identified within sheared serpentinite matrix (Fig. 3) (Ogawa, 1983; Ogawa and Taniguchi, 1987, 1988). However, recent studies show that the structural style is not that of block-in-matrix, but rather one of a collage of unsheared blocks of various lithologies bounded by discrete fault zones (Takahashi et al., 2003; Ogawa and Takahashi, 2004) (Fig. 4). The serpentinite, mostly composed of lizardite and chrysotile from harzburgite tectonite, does not form a matrix but is one of the main block lithologies. Most of the lithologies can be found in fault contact with each of the other lithologies present. It is only within rela-

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tively narrow (~1 m wide) fault zones that the serpentinite and clastic rocks are sheared. Some such faults cut plutonic and metamorphic blocks (Takahashi et al., 2003; Chiba, 2008). The field relationships of these blocks and their major, trace, and rare earth element chemistry are described in Ogawa et al. (2009). In the northern sub-belt, alkalic basalt blocks are dominant (Ogawa et al., 2009), and they are in fault contact with clastic rocks of either the Eocene Shimanto Supergroup or the Miocene Hota Group (Kawakami, 2004). Alkali basalt blocks are Miocene in age (Hirano and Okuzawa, 2002; Hirano et al., 2003).

pillow lava & dolerite sheet complex Kojima Formation diorite

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Figure 3. Previous model of profiles of the eastern part of the Mineoka belt. (A) Central and southern sub-belts. (B) Mineoka-Sengen area (central sub-belt). (C) Heguri-Naka area (central sub-belt). Note that these sections are drawn as if all of the ophiolitic blocks are surrounded by sheared serpentinite, but the present model is not the case (see details in text). Adopted and modified from Ogawa (1983).

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Figure 4. New model of the profile of the central and southern sub-belts of the Mineoka belt (eastern part along line C of Fig. 2). Adopted and modified from Takahashi et al. (2003). Note that most of the ophiolitic blocks are shown as faulted, with massive serpentinite in the north of the profile, but with sandstone-mudstone in the south.

In the central sub-belt of the Mineoka Hills area, both large and small blocks of tholeiitic basalt are in fault contact with massive serpentinite (mostly completely serpentinized harzburgite tectonite) (Takahashi et al., 2003). These blocks are mostly Paleogene in age, from basalt, chert-limestone, and clastic rocks (Mohiuddin and Ogawa, 1998). In the southern sub-belt, the blocks are almost aligned on the southern boundary of the Mineoka belt in fault contact with the Hota Group, and the basaltic rocks are of Cretaceous age as far as measured and as mentioned below. Every block of the Mineoka belt is bounded by a right-lateral (dextral) fault with a thrust component, or by serpentinite and/or sedimentary rocks. The large-scale fault pattern in the belt is that of a typical dextral Riedel shear zone (Ogawa, 1983; Mori and Ogawa, 2005), suggesting that dextral shears prevail within the 5-km-wide Mineoka belt. The size of most blocks is in the order of tens of meters, but blocks or slabs of hundreds of meters in scale are not uncommon (Ogawa et al., 2009). Blocks are generally oblate or prolate in shape, but spherical blocks are also common. If elongated, the long axis of blocks would tend to be parallel to, or slightly oblique to, the trend of the Mineoka belt (WNW). Therefore, the Mineoka belt might represent an intermediate model between the previous mélange models depicted by Figure 3 and the fault belt model by Figure 4. This intermediate model differs from that of Figure 3 in which only some of the serpentinite boundaries are sheared, and from that of Figure 4 in which individual blocks have rounded ends. It is important to note that some plutonic and metamorphic blocks have an oblate or prolate shape and lie within narrow sheared serpentinite fault belts surrounded by massive serpentinite or sedimentary rocks.

AGE OF OPHIOLITIC AND OTHER IGNEOUS ROCKS AND THEIR COVER OF PELAGIC SEDIMENTARY ROCKS Igneous blocks include tholeiite, which is variably altered but retains its igenous textures, relatively fresh alkali basalts, and unaltered high-Mg andesite and diorite (tonalite). Alkali basalt is relatively fresh, and the high-Mg andesite and diorite (tonalite) are fresh. One of the basaltic rocks in the middle sub-belt is of late Eocene age (47 ± 10 Ma, Ar-Ar whole rock isochron; Hirano et al., 2003). Such rocks constitute the most common igneous rock type in the Mineoka belt. They are older than the associated plutonic and metamorphic rocks, which are Oligocene, as shown below (ca. 40 Ma or younger). However, considering that these radiometric ages were obtained previously, they are more or less uncertain, without any good plateau ages except for one alkali basalt of ca. 20 Ma Ar-Ar age (Hirano and Okuzawa, 2002). In contrast to the above-reviewed ages, another study showed that a block of radiolarian chert is of Albian age (ca. 100 Ma) (Ogawa and Sashida, 2005). We conducted 40Ar/39Ar incremental heating analyses of two tholeiitic basalts (NH980303-03 from Hashimoto and NH011117-01 from Shinyashiki), a high-Mg andesite (RM050929-05 from Hinata) and a diorite (BM31YD from Yamada) from the southern sub-belt, and an alkali basalt (RM050425-04 from Isomura) from the central sub-belt (details of these localities are given in Ogawa et al., 2009). The spectra of some samples show young ages on the lower temperature gas fractions (Figs. 5A, 5B, 5C) that may be a consequence of Ar loss from weathering and/or alteration of the

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Figure 5. Ar-Ar data plot based on new data of basaltic and andesitic rocks of Cretaceous age: A for Hashimoto, B for Shinyashiki, C for high-Mg andesite at Hinata, D for alkalic basalt at Isomura, and E for diorite at Yamada (for local names of occurrence, see Ogawa et al., 2009). Plateau ages are shown for A through D, and an isochron age for E (Naoto Hirano, original data). Inverse isochrons and age spectra are in the upper part of each figure. MSWD—mean square of weighted deviates. MSWD = SUMS/(n-2) (York, 1968). All errors are 2σ. Fresh and separated groundmass from basaltic samples were crushed to 100–300 μm grains and wrapped in aluminum foil (70 mm in length, 10 mm in diameter) with flux monitors for biotite (EB-1, Iwata, 1998), K2SO4, and CaF2. The samples were irradiated for 24 h in the Japan Material Testing Reactor (JMTR), Tohoku University. During the irradiation, samples were shielded by Cd foil to reduce thermal neutron-induced 40Ar from 40K (Saito, 1994). The Ar extraction and Ar isotopic analyses were done at the Radioisotope Center, University of Tokyo. During incremental heating, gases were extracted in 10 steps between 600 and 1500 °C. The analytical methods are described by Ebisawa et al. (2004).

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samples. Most samples show a well-developed plateau spanning more than 50% of the gas released and including 3–10 heating steps. The 30.6 ± 1.6 Ma plateau age of RM050425-04 (Fig. 5D), may be affected by excess 40Ar as indicated by the 40Ar/36Ar ratio of 304.5 ± 5.8, which is higher than the atmospheric ratio (295.5) in the inverse isochron (Fig. 5D). The 25.5 ± 2.3 Ma age from the inverse isochron diagram thus appears the best age estimate for sample RM050425-04 (Fig. 5D). On the other hand, the plateau ages (80.6 ± 1.7 Ma for NH980303-03; 85.0 ± 3.9 Ma for NH011117-01; 28.6 ± 5.1 Ma for RM050929-05; 37.7 ± 1.2 Ma for BM31YD) should be accepted as the best age estimates because the initial 40Ar/36Ar ratio of each sample corresponds to the atmospheric ratio in the inverse isochrons, respectively (Figs. 5A, 5B, 5C, and 5E). These findings confirm that although one tholeiitic basalt in the central sub-belt is of Paleogene age, those in the southern sub-belt are Cretaceous, and the alkali basalt, andesitic volcanic rocks, and diorite in any of the sub-belts are either Eocene or Oligocene, younger than 40 Ma. These ages demonstrate that rocks of the ophiolite belt formed long before they arrived at their present positions during the Miocene. Another important component of the pelagic cover on some of the basaltic blocks is the Paleogene to Miocene limestonechert sequence, the Kamogawa Group (Mohiuddin and Ogawa, 1998). This sequence is composed of bedded limestone and chert with sporadic ash intercalations and has been dated from Paleocene (ca. 60 Ma) to early Miocene (ca. 18 Ma) on the basis of foraminifers, which means that part of the oceanic plate had existed in pelagic realms. The alkali basalts are much younger, yielding Ar-Ar ages of ca. 20 Ma (Hirano and Okuzawa, 2002; Hirano et al., 2003). In addition to the above-reviewed ages, we obtained K/Ar wholerock dates on three volcanic samples. These analyses were con-

Sample number Kj060921 Hinata51 BM04HG2

ducted by Geochronology and Isotope Chemistry of Ontario, Canada. The host andesitic tuff breccia for the high-Mg andesite fragment we dated (sample RM050929-05; Ar/Ar age of 28.6 ± 5.1 Ma) yielded a 15.6 ± 0.5 Ma age, and andesitic pumice-fall deposits at Kojima at Kamogawa Harbor (Kj-5) yielded an age of 5.8 ± 0.3 Ma. Previously published K/Ar dates from plutonic blocks are as follows: the Yamada knocker (a dioritic body in sheared serpentinite) yielded 40.9 ± 2.1 Ma (whole rock), and a similar diorite block in the Shingan-ji area yielded 27.9 ± 1.8 Ma (hornblende) and 24.1 ± 1.4 Ma (plagioclase) (Hirano et al., 2003). Metamorphic blocks (hornblende schists mentioned above) have K-Ar ages as follows: a hornblende from Byobu-jima gave 33.1 ± 2.3 Ma (Hiroi, 1995a), and that from a block from Heguri-Naka, 39.5 ± 2.2 Ma (Hiroi, 1995b). Those data are summarized in Table 1. The new and published ages indicate that the tholeiitic volcanic rocks formed first, followed by formation of the plutonic and metamorphic rocks, which were in turn followed by alkalic basalts and andesites. Therefore, as Hirano et al. (2003) noted, within the Tertiary rocks, the tholeiitic basalt-bearing ophiolitic suite (ophiolite proper) is the oldest, followed by younger island-arc plutonic and metamorphic rocks, and then still younger alkalic basalts. The youngest rocks are andesitic tuff breccia derived from the Izu island arc during the Miocene. These rocks were probably extruded in the Izu frontal area, and may overlie the older (28.6 ± 5.1 Ma) high-Mg andesite. These relationships suggest that at ca. 40 Ma, after incorporation of Cretaceous chert and Cretaceous to Eocene tholeiitic basalt, the area became an island arc environment with arc plutonism and formation of crust with high Mg andesite. Alkalic basalt was erupted, with no obvious connection to arc activity, and pelagic limestone and chert deposition continued throughout the various stages of development of the Mineoka belt rocks.

TABLE 1. RADIOMETRIC AGES OF IGNEOUS AND METAMORPHIC ROCKS OF THE MINEOKA BELT Location Rock Method, whole rock or wh in. Age (Ma) Reference ht.* Kojima Andesite tuff K-Ar wh 5.8 ± 0.3 This study Hinata

Andesite tuff

K-Ar wh

15.6 ± 0.5

This study

Toge

Alkali basalt

Ar-Ar wh in. ht.

19.62 ± 0.9

RM050425-04

Isomura

Alkali basalt

Ar-Ar wh isochron

25.5 ± 2.3

Hirano and Okuzawa (2002) This study

RM050929-05

Hinata

High-Mg andesite

Ar-Ar plateau

28.6 ± 5.1

This study

Yamada

Diorite

Ar-Ar plateau

37.7 ± 1.2

This study

Kamogawa Harbor Heguri-Naka

Hornblende schist

K-Ar hornblende

33.1 ± 2.3

Hiroi (1995a)

Garnet amphibolite

K-Ar hornblende

39.5 ± 2.2

Hiroi (1995b)

BM31YD Byobu-jima Heguri-Naka BM591(Bentenjima) NH980303-03

Kamogawa Harbor Hashimoto

Tholeiite

Ar-Ar isochron

47 ± 10

Hirano et al. (2003)

Tholeiite

Ar-Ar plateau

80.6 ± 1.7

This study

NH011117-01

Shinyashiki

Tholeiite

Ar-Ar plateau

85.0 ± 3.9

This study

* wh in. ht.—whole rock incremental heating for plateau.

Role of exhumation in a forearc ophiolite mélange belt, Mineoka belt, Japan PETROCHEMISTRY OF BASALTIC, PLUTONIC, AND METAMORPHIC ROCKS Because most of the basaltic rocks are altered without any fresh phenocrysts, we cannot ascertain their tectonic origin by major element chemistry alone. Abundances and ratios of trace and rare earth elements are more useful for elucidating the tectonic setting of magmatism, because they are comparatively immobile during alteration. Basaltic rocks in the Mineoka belt have been divided into tholeiitic and alkali (Ogawa and Taniguchi, 1989a, 1989b; Taniguchi and Ogawa, 1990; Hirano et al., 2003). The tectonic setting of tholeiitic basalts of the Mineoka belt is disputed, although most of these basalts exhibit typical normal mid-oceanic-ridge basalt (N-MORB), or ocean floor basalt chemistry with minor amounts of island arc tholeiite (IAT) (Hirano et al., 2003). New trace- and rare-earth-element data were obtained by inductively coupled-plasma mass spectrometry (Ogawa et al., 2009). The rocks thought to have IAT chemistry by Hirano et al. (2003) show a negative anomaly of Nb and Ta in spider diagrams with a large ion lithophile element–enrichment pattern (Fig. 6), suggesting a kind of island arc setting (Ogawa and Takahashi, 2004). The now-subducted plate, to which these basalts and associated pelagic sedimentary rocks belong, has been called the Mineoka plate (Ogawa and Taniguchi, 1988; Sato et al., 1999). The younger plutonic rocks show a well-developed island arc signature (Figs. 6 and 7). The alkali basalts are within plate basalts (WPB) (Fig. 6), and their younger ages indicate that they erupted independently of arc activity within the Mineoka plate. Besides these basaltic and plutonic rocks, a new andesitic rock was found at Hinata: an andesitic tuff breccia including a high-magnesian andesite clast, as already described (Fig. 7). Considering that most diorite bodies are dated ca. 40 Ma or younger, the area probably became part of an island arc after ca. 40 Ma, which may overlap the previous MORB-type lithology. Metamorphic rocks show mostly island arc affinity. The tholeiitic basalts of the Mineoka plate may be as old as Cretaceous to Paleocene; after 40 Ma most of the igneous rocks show island arc affinities, with some alkali basalt. The implications will be discussed further in the following chapters. DEFORMATION OF PLUTONIC AND METAMORPHIC ROCKS Plutonic Blocks As noted by Mori and Ogawa (2005), the plutonic and metamorphic blocks have undergone several stages of deformation, initially more ductile and later more brittle. The Yamada knocker (a dioritic body in fault contact with serpentinite) in the southern sub-belt of the Mineoka belt provides a typical example of the two stages of deformation (Fig. 8). The initial, more ductile deformation is characterized by mylonitic formation along centimeter to millimeter–wide ductile zones within faults, which are

103

developed inside the blocks. Quartz grains are deformed with wavy extinction, whereas plagioclase is cataclastically broken. Internal shears have various orientations and are characterized by metamorphic minerals such as prehnite and hornblende. The later, more brittle deformation is characterized by brecciation restricted along faults of Riedel shears, associated with extensive cataclastic deformation only in the surface boundaries of the blocks (Fig. 8). Analcime veins within shear planes are near the boundaries with the serpentinite, providing evidence of uplift to relatively shallow levels. However, the shearing does not penetrate the serpentinite body, which is almost massive and contains no shear zones more than tens of centimeters wide. The late shearing in the dioritic blocks was strong enough to create systematic Riedel shears. The internal shear planes are arranged in a small circle associated with strike-slip faults with a thrust component, most of which are dextral, although some are sinistral. This style of deformation suggests that the plutonic and metamorphic rocks underwent several episodes of shearing (conjugate sets) at depth with conical symmetry induced by a transpressional regime (Mori and Ogawa, 2005). Similar two-stage deformation is observed in basaltic rocks from the eastern end of the Mineoka belt (Takahashi et al., 2003; Ogawa and Takahashi, 2004). The first stage is penetrative deformation by shearing and cataclasis within the blocks, together with zeolite, prehnite, or calcite veins that fill near-vertical conjugate shears. This stage of deformation has been suggested to have occurred near a spreading center, coeval with hydrothermal activity that formed the vein fillings (Ogawa and Takahashi, 2004). In the second stage of deformation the basaltic blocks are strongly deformed mostly along the boundaries of the different kinds of blocks apparently by dextral transpressional shearing with flower structures. These shear zones are characterized by Riedel shear systems, as described in the bodies at Shinyashiki (Takahashi et al., 2003) and at Benten-jima (Ogawa and Takahashi, 2004). Metamorphic Blocks Four metamorphic blocks in the Mineoka belt are composed mostly of schistose metabasite, ranging from meters to tens of meters in size with oblate to prolate shapes (Fig. 9). These blocks have all been metamorphosed to amphibolite facies (Ogo and Hiroi, 1991) but show retrograde facies of epidote-amphibolite or greenschist facies. Their chemistry has a characteristic island arc signature, except for the possible MORB-type Sumoba-ishi Island block. The quartz and psammitic (quartz, feldspar, muscovite, garnet, and monazite are the components) schists indicate deposition of the original sedimentary rocks as chert and terrigenous sandstone (Fig. 10). This combination of pelagic sediments and terrigenous sands indicates sedimentation near a trench, close to a continent (Ogo and Hiroi, 1991). Three examples of metamorphic blocks in Kamogawa Harbor and one on land at Heguri-Naka exhibit similar deformation: the Kana-shima block off Byobu-jima (Mori and Ogawa, 2005), the Sumoba-ishi Island block, and the Heguri-Naka block

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U

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WPB type

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Zr

Figure 6. Representative spider diagrams of basaltic rocks, first described by Hirano et al. (2003), and later, some trace and rare earth element (REE) data were added by Ogawa et al. (2009). Trace elements are normalized by normal mid-oceanic-ridge basalt (N-MORB) after Saunders and Tarney (1984) and REE by Primordial Mantle after Sun and McDonough (1989). Analyzed by N. Hirano and M. Kurosawa. WPB—within plate basalt; IAB—island arc basalt.

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(Fig. 9). The Kana-shima block contains epidote veins, the Sumoba-ishi block contains chlorite veins, and the Heguri-Naka block contains prehnite veins. All of these are products of retrograde metamorphism. Thus the initial schistosity was followed by the development of epidote or chlorite veins, followed by isoclinal microfolding, and then brecciation that preserved the essential traces of the foliation, and finally the formation of cataclasite along faults (Fig. 10) (Mori and Ogawa, 2005). Penetrative schistosity is observed in both metabasite and quartz and psammitic schist layers, but all the lithological boundaries are faults (Fig. 10). The schistosity of these metamorphic rocks (at least three of the four) appears to be a product of compression during subduction,

Er

Tm

Yb

Lu

which is supported by the IAT chemistry of the metabasite as well as by the occurrence of quartz feldspar–bearing psammitic schist. The metamorphic conditions for these blocks were calculated from the chemistry of some of their minerals in Table 2, and are also summarized in Table 2. The average temperature and pressure of metamorphism were estimated for garnet-bearing psammitic schist at Byobu-jma to be 500–550 °C and 500 MPa, respectively, on the basis of the stability of chalcopyrite and the quantity of jadeite components in aegirineaugite within the schist (Ogo and Hiroi, 1991). The pressure-temperature (P-T) conditions are attributed to the intermediate series of regional metamorphism in subduction zones (Ogo and Hiroi, 1991). Our own temperature estimates based on plagioclase and hornblende compositions (Table 1) and the Holland and

Role of exhumation in a forearc ophiolite mélange belt, Mineoka belt, Japan 10

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Andesite tuff breccia (Hinata) 10

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Diorite (Hinata) Hinata rocks -2

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-3

10

Rb Ba

Th

U

K

Nb Ta

La

Ce

Sr

Nd P

Hf

Zr

Sm Ti

Tb

Y

Figure 7. Representative spider diagrams of andesitic, high-Mg andesite, and plutonic rocks at Hinata. Trace and rare earth elements are normalized by N-MORB after Saunders and Tarney (1984) and Sun and McDonough (1989), respectively. Sample lithologies from Hinata and Kobata are shown. For local names of occurrence, see Ogawa et al. (2009). Analyzed by S. Haraguchi, T. Ishii, and M. Kurosawa.

Late shear R1 R2 R1 (S2)

S1

Figure 8. Polished thin section photo (left), and enlargement of the boundary to serpentinite (upper right) of diorite from Yamada, west of Heguri-Naka. Note in the sketch (lower right) that the block boundary has early foliation (S1) that is dislocated by later shear (S2), interpreted as R1 with additional R2. Note that the early foliation is mylonitic of a more ductile deformation (wavy extinction of quartz but cataclasis of plagioclase), whereas the late foliation is more brittle. Strong late shear is mostly dextral in the horizontal frame.

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Blundy (1994) thermometer, suggest metamorphic temperatures of 560–600 °C (Table 2), which is nearly consistent with the results of Ogo and Hiroi (1991). In addition, we applied a semiquantitative geothermobarometer of Ernst and Liu (1998) for a Byobu-jima sample and obtained P-T conditions of 620 °C and 1.5 GPa (Table 3), which is the highest pressure condition so far reported from metamorphic

A

C

rocks in this area. We confirmed that plagioclase, rutile, and titanite are present in the sample, as this method is applicable only under the saturation of Al2O3 and TiO2. As shown in Figure 11A, no exsolution lamella of the Ti-bearing phase has been identified in hornblende, suggesting that the mineral probably preserved the primary composition. This figure also demonstrates that rutile grains are enclosed within

B

Benten-jima

D

Kana-shima

Figure 9. Outcrop photos of four metamorphic blocks. Note that all are of either oblate or prolate shape. (A) Byobu-jima Island (central bar ~ 3 m long); (B) Sumoba-ishi Island (lower circular island); (C) Kana-shima Island (the island is ~10 m in diameter), all in the Kamogawa Harbor area; (D) Heguri-Naka block entrapped within sheared serpentinite.

Role of exhumation in a forearc ophiolite mélange belt, Mineoka belt, Japan texturally retrograde titanite. Because of the higher stability field of rutile (P >1.3 GPa at 600 °C) than that of titanite (P 0.6 (type 3) are categorized as arc peridotites and show the most depletion, whereas type 2 are transitional between arc and MORB mantle. Eldivan mantle Cr-spinels overlap arc and MORB fields in Dick and Bullen’s diagram for Cr-spinel Cr# versus Mg# (Fig. 4A). They also plot in the supra-subduction zone mantle field defined by Kamenetsky et al. (2001) in a TiO2 wt% versus Al2O3 diagram, with some points in the overlap between supra-subduction zone and MORB mantle fields (Fig. 4B).

147

5

4

3

2

1

0 Figure 3. Whole rock Al2O3 weight percentages for mantle rocks from continental, ocean-floor, and subduction-trench settings compared with the Eldivan ophiolite mantle. The range in Al2O3 weight percentages in the Eldivan ophiolite spans the range of ocean-floor and subduction-trench mantle. It is also similar to that seen in the Oman ophiolite and somewhat higher than in the Orhaneli ophiolite in the western İzmir-Ankara-Erzincan suture zone and the Brooks Range ophiolite in Alaska, interpreted to have formed in supra-subduction zone settings. Data sources for other mantle compositions are as follows: continental (Carter, 1970; Frey and Prinz, 1978); ocean floor (Shibata and Thompson, 1986; Paulick et al., 2006; Seifert and Brunotte, 1996); subduction trench (Fisher and Engel, 1969; Ishii, 1985; Ishii et al., 1992); Oman (Takazawa et al., 2003); Brooks Range (Harris, 1995); Orhaneli ophiolite (Sarıfakıoğlu et al., 2009).

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N-MORB, except for sample 126a (Fig. 6A2, bold line), which has a negative Nb anomaly, again suggesting a subduction influence. Diabase Dikes Diabase dikes are found mostly in parallel or sheeted arrays that cut screens of massive gabbro. Many dikes show chilled margins, and some preserve flow fabrics. Shear fractures within the dikes have gouge zones filled with epidote and chlorite, which are common seafloor hydrothermal alteration minerals, indicating that the fractures probably formed during seafloor metamorphism. Dikes show two primary orientations, NNW-SSE and E-W, with dips from 40° to 90°. In one locality, horizontal sheeted dikes feed vertical pillow basalts and sheet flows, indicating a 90° rotation of the units about a horizontal axis. Dikes are primarily basaltic andesite to andesite with an SiO2 range of ~56–58 wt% (Fig. 5). Element concentrations are most similar to N-MORB (2 times above and below N-MORB concentrations) compared with other units in the Eldivan ophiolite, with

REE patterns also similar to N-MORB (Fig. 6B1). Low ratios of Th/Ta = 0.82–4.40, La/Nb = 1.22–1.93, and La/Yb are similar to massive gabbro values and again near N-MORB. One exception is sample 203 (Fig. 6B1, bold line), which shows a slight elevation in LREE with no depletion in the high field strength elements (HFSE), similar to enriched MORB (E-MORB) (Sun and McDonough, 1989). Th/Ta and La/Nb ratios in this sample are similar to those in the other dikes, although La/Yb = 2.42 is higher, reflecting the higher LREE concentrations. Trace element patterns (Fig. 6B2) show characteristic N-MORB and low LILE abundances compared with the HREE, with hydrothermal alteration reflected in varying concentrations of mobile Rb, Ba, Th, U, K, and Sr. As seen in the REE, sample 203 (Fig. 6B2, bold line), shows higher concentrations of LILE, beginning with Nb and sloping downward to HREE, with a slight increase in Hf and Zr. The absence of a negative Nb-Ta anomaly suggests that higher values of LILE are not due to a subduction component but perhaps to a less depleted mantle source.

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0.2

Troodos Ocean basin

A

0.1 1.0

0.9

0.8 0.7 0.6 0.5 Mg# (Mg/Mg+Fe2+)

0.4

0.3

Figure 4. (A) Cr# (Cr/Cr+Al)/Mg# (Mg/Mg+Fe2+) of Cr-spinel, the Eldivan serpentinized peridotite. Fields of abyssal (dashed line) and arc peridotites (solid line) are taken from Dick and Bullen (1984). (B) TiO2 wt% vs. Al2O3 wt% in Cr-spinel of the Eldivan ophiolite. Fields of supra-subduction zone (SSZ; solid line) and MORB (dashed line) are from Kamenetsky et al. (2001). Data are plotted with Cr-spinel data from the Orhaneli ophiolite (Sarıfakıoğlu, 2009), Brooks Range ophiolite (Harris, 1995), ocean basin peridotites (Shibata and Thompson, 1986; Morishita et al., 2007), Troodos and Oman ophiolites (Augé and Johan, 1988; Takazawa et al., 2003; Tamura and Arai, 2006), and Mariana peridotites (Ishii et al., 1992) for comparison. The Eldivan ophiolite plots mostly within the field of arc peridotites (A) and supra-subduction zone peridotites (B), indicating its subduction influenced character.

Tectonic evolution of the Ankara Mélange and associated Eldivan ophiolite near Hançili In tectonic discriminant diagrams, dike rocks plot in fields of supra-subduction zones (Figs. 7A, 7F), island-arc tholeiites (IAT) (Figs. 7B, 7C, 7G) and backarc basin basalts (BABB) (Figs. 7D, 7E), and with some values plotting in the overlap of subduction zone influenced and MORB fields (Figs. 7B, 7D, 7G). Dikes plot consistently in the supra-subduction zone and backarc basin field in diagrams E and F (Fig. 7), which use the most immobile trace elements of La, Nb, and Yb to infer a tectono-magmatic setting. Additional ternary discriminant diagrams (Fig. 8) also plot dikes in subduction related fields. In diagrams B and C, dikes plot almost entirely as island arc basalt and island arc tholeiites, respectively. Diagram D has scatter between the BABB and MORB fields, whereas A and E do not discriminate between subduction and MORB basaltic rocks. Volcanic Rocks Volcanic rocks include both basaltic and rhyolitic units as blocks and broken thrust sheets within a serpentinized matrix. Basalt occurs as pillows, sheet flows, and brecciated units up to tens of square meters in area. Large blocks of basalt hundreds of meters in diameter protrude up through serpentinite to form a hummocky landscape typical of eroded mélange units. Silicic volcanic units are found only as small blocks on the meter to decimeter scale.

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Major element chemistry reveals a compositional range of volcanic rocks from basalt and andesite to dacite and rhyolite. Basaltic rocks contain 45–53 wt% SiO2 and plot mainly in the basalt field in the total-alkali silica diagram with some overlap into the basaltic andesite field (Fig. 5). Rhyolitic rocks contain 63–74 wt% SiO2 and fall into rhyolite and dacite fields (Fig. 5). Basalts show three distinct geochemical signatures: 1. The most dominant pattern shows LREE depletion characteristic of N-MORB (Fig. 6C1), with low ratios of Th/La = 0.33–2.05, La/Nb = 0.56–2.10, and La/Yb = 0.49–1.29. Element concentrations are equal to and up to 3.5 times more than N-MORB concentrations, giving these basaltic rocks the highest elemental concentrations when compared to the massive gabbros and sheeted dikes. The trace element diagrams for these basaltic volcanics (Fig. 6C2) show a large amount of variation, particularly with mobile elements of Rb, Ba, K, and Sr, which reflect their alteration. However, patterns still show the low LILE abundance (compared with HREE) that is typical of N-MORB. Discriminant diagrams for N-MORB-like basaltic rocks show more scatter than dike rocks but plot in fields of MORB more often than subduction-influenced fields (Figs. 7A–7G). In diagrams B, C, and E, basaltic rocks plot almost entirely within the MORB field, whereas some samples plot close but somewhat outside MORB fields in B, D, and G. In diagrams E and F, which use the

Rhyolite

72 Rhyodacite Dacite

68

Basaltic volcanics

Comendite Pantellerite

Silicic volcanics Gabbro

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Trachyte

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Trachyandesite

Phonolite

Alkaline basalt

Basanite Nephelite

52 Sub-alkaline basalt 48

44

40 0

0.1

1.0

10

Nb/Y

Figure 5. International Union of Geological Sciences (IUGS) total alkali silica classification diagram, illustrating the distribution of rock types in the Eldivan ophiolite. Volcanic rocks are basaltic and rhyolitic, whereas dike rocks plot between the two in the basaltic andesite and andesite fields.

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Sample 274, 280, 282 Sample 272 N-MORB

0.1 La

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Yb Lu


Figure 6. REE (1) and incompatible element (2) diagrams for (A) gabbro, (B) diabase dikes, and (C) volcanic rocks. REE elements were normalized to chondritic meteorite compositions (McDonough and Sun, 1995). Normal mid-oceanic-ridge basalt (N-MORB) reference line (Sun and McDonough, 1989) is marked by a thin dotted line on all diagrams. For comparison of the different rock types within the Eldivan ophiolite, white and gray fields are plotted in each trace element diagram: volcanic rocks in white (A2, B2), gabbro and diabase dikes in gray (A2, B2), and alkaline rocks in gray (C2). Special samples mentioned in the text are distinguished by bold solid or dashed lines.

Tectonic evolution of the Ankara Mélange and associated Eldivan ophiolite near Hançili most immobile elements, most basalt samples plot in the MORB field. Ternary discriminant diagrams have similar results (Fig. 8), with basalts falling clearly into the MORB fields in diagrams B, C, and D. Diagrams A and E do not distinguish between MORB and subduction-related basaltic rocks. 2. Contrastingly, the geochemistry of samples 16, 19, 20, 21, and 142 (Fig. 6C1, light gray lines) shows highly elevated immobile LREE up to 17 times that of N-MORB and HREE below N-MORB concentrations, giving steeply sloping patterns typical of alkaline ocean island basalts (OIB) (Fig. 6C), but reflect alteration in high K abundance. Element ratios of La/Yb are correspondingly high (La/Yb = 16.21–22.47). Sample 272 (Fig. 6C1, bold line) is subparallel to these samples, showing low abundances in HREE but only a moderate LREE elevation. Trace element patterns for samples 16, 19, 20, 21, and 142 (Fig. 6C2, gray field) show the same elevation in the LILE as seen in the LREE, with some variation in mobile elements, most notably Ba and K. Sample 272 (Fig. 6C2, bold line) has LILE abundances more elevated than the N-MORBtype basalts but less than the alkaline samples. The absence of a Nb anomaly, combined with the low HREE concentrations, suggests that this sample source was not modified by subduction but perhaps was derived from a more heterogeneous source transitional between those that produce N-MORB and OIB basalts. These rocks consistently plot in fields for OIB, WPB, and alkaline basalt on discriminant diagrams (Figs. 7A, 7B, 7D, 7F). In diagrams where alkaline fields are not present, these rocks plot outside all fields (G) or overlap both the IAT and MORB fields (C), and cannot be discriminated. In diagram E, these samples plot in the E-MORB field. Similar results are seen with additional ternary discriminant diagrams (Fig. 8). In all diagrams, these rocks plot as within plate alkali (A, B, D), ocean island tholeiite (C), and WPB (E). 3. The third geochemical signature is seen in basaltic sample 274 (Fig. 6C1 and 2, bold dashed line), which shows LILE enrichment and HFSE (Nb, Ta) depletion relative to N-MORB, suggesting a subduction-influenced source (Fig. 6C2). A subduction-influenced source is also reflected in a slight LREE enrichment and high ratios of Th/La (6.48), La/Nb (2.85), and La/Yb (3.66) (Fig. 6C1). Rhyolitic samples have two separate trace and REE signatures. Most samples are similar to N-MORB in their lower LREE concentrations compared with HREE (Fig. 6C1) but have overall flatter patterns that could reflect higher degrees of differentiation in the magma chamber. This same pattern is also seen in the trace elements (Fig. 6C2), with LILE slightly lower in concentration than HREE, with the exception of Zr and Hf, which show higher abundances. The effects of secondary alteration are seen in the scatter of mobile elements, especially Ba, U, Th, K, and Sr. Alternatively, rhyolitic samples 280 and 282 (Fig. 6C2, bold dashed line) closely match basaltic sample 274. LREE abundances are elevated, reflected in Th/Ta, La/Nb, and La/Yb ratios similar to sample 274 (Fig. 6C1). LILE (Fig. 6C2) are also more abundant except for negative concentrations of Nb, Ta, and Ti,

151

characteristic of subduction zones. Some scatter is still seen in the mobile elements, particularly Rb, K, and Sr. Despite the two signatures seen in REE and trace elements, all of the rhyolitic samples plot in fields for volcanic arc granite (Fig. 7, H, I) in discriminant diagrams of Pearce et al. (1984a) that are based on immobile elements of Nb, Ta, and Yb. Interpretation of Whole-Rock and Mineral Chemistry Three different magma affinities are present in the Eldivan ophiolite: N-MORB, alkaline (OIB), and supra-subduction zone. The occurrence of three distinct geochemical signatures in this small area of exposure (~20 km2) implies a high degree of mixing of either (1) upper and lower plate blocks during tectonic emplacement or (2) magma sources during seafloor formation. Mixing of upper and lower plate units is plausible, considering the current imbricated structure of the ophiolite in the mélange. This has been suggested to account for alkaline rocks within the mélange that are interpreted as seamounts accreted into the serpentine mélange from the downgoing plate (Floyd, 1993; Tüysüz et al., 1995; Tankut et al., 1998). Accretionary mixing of an N-MORB downgoing plate, which included seamounts, with a supra-subduction zone upper plate could explain the geochemical variation in the Eldivan ophiolite, although few modern analogues of this process exist. Similar chemical variations to those seen in the Eldivan ophiolite are found in modern backarc supra-subduction zone basins due to mixing different magma sources rather than upper and lower plate components. Such supra-subduction zones or backarc ocean basins are extensional upper plate basins that form above subduction zones owing to lower plate movement away from the upper plate through slab rollback. The combination of extension and subduction in supra-subduction zone backarc settings creates conditions of both mantle depletion and enrichment, which result in basalts of different compositions (Sinton and Fryer, 1987; Price et al., 1990; Stern et al., 1990; Eissen et al., 1994; Hawkins and Melchior, 1985; Dril et al., 1997; Fretzdorff et al., 2002; Sinton et al., 2003). Basalts in the North Fiji, Lau, Mariana, Manus, and East Scotia backarc basins show an overprint of LILE enrichment on N-MORB geochemical patterns, which increase with proximity to the subducting slab. Compositional zoning in the Lau basin, with LILE enriched basalt on the west edge near the arc and N-MORB types in the young central spreading center, show that LILE enrichment decreases as rifting continues, owing to decreased subduction influence from slab rollback (Hawkins and Melchior, 1985; Pearce et al., 1984b). Likewise, initial rifts in the Mariana trough erupt basalts similar to those of the Mariana arc, where older rift zones erupt N-MORB (Stern et al., 1990). North Fiji and East Scotia spreading ridges erupt basalt transitional between N-MORB and alkaline basalt owing to influence from hotspot volcanism (Price et al., 1990; Eissen et al., 1994; Fretzdorff et al., 2002). This chemical array is similar to that seen in the Eldivan ophiolite and occurs entirely in the upper plate, caused by mixing of variably depleted

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Tectonic evolution of the Ankara Mélange and associated Eldivan ophiolite near Hançili and enriched mantle sources or melts (Sinton and Fryer, 1987; Price et al., 1990; Stern et al., 1990; Dril et al., 1997). In this study the backarc basin or supra-subduction zone mixing is favored for the Eldivan ophiolite. Although incompatible and REE diagrams are dominated by N-MORB patterns, there are noticeable subduction and alkaline influences (samples 126a, 203, 280, and 272), as seen in modern backarc and intraarc settings. Additionally, basalt mostly plots as N-MORB, with some scatter into other fields (Figs. 7 and 8), but dike rocks plot consistently within subduction-influenced fields, including IAT, backarc basin, and supra-subduction zone, with minor overlap into N-MORB fields (Figs. 7 and 8). These dike compositional patterns provide direct evidence that the Eldivan ophiolite was at one time in a supra-subduction setting, as the sheeted dike complex represents ocean floor construction. Additionally, rhyolitic volcanics also plot in volcanic arc fields (Fig. 7), giving more evidence for a significant subduction influence. Evidence for a supra-subduction zone setting is also found in the mantle sequence of the Eldivan ophiolite. Cr-spinel (Cr#s 0.47–0.70) plots within fields for mantle more depleted than ocean rift mantle and closer to transitional supra-subduction settings (Fig. 4) similar to Oman-type ophiolites as defined by Harris (1992). This is also supported in whole-rock Al2O3 wt% of the Eldivan ophiolite, which indicates the degree of partial melt extraction, and could be expected for ocean crust in the complex melting regime of a backarc basin. It also closely matches Al2O3 wt% concentrations from the Oman ophiolite but is slightly higher than the Brooks Range ophiolite and Orhaneli ophiolite (western İzmir-Ankara-Erzincan suture zone), all thought to have formed in supra-subduction zone settings (Fig. 3). Finally, a supra-subduction zone interpretation is consistent with other studies along the İzmir-Ankara-Erzincan suture zone in the Ankara Mélange, Dağkuplu Mélange, and Kırşehir block ophiolitic massifs. Other areas of the Ankara Mélange contain alkaline basalts (Çapan and Floyd, 1985, 1993; Tankut et al., 1998), N-MORBs (Tankut, 1984; Tankut et al., 1998), and IAT basalts (Tankut, 1984; Tankut et al., 1998; Tüysüz et al., 1995) similar to the Eldivan ophiolite. Our discovery of suprasubduction zone dikes is consistent with the discovery of supra-

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subduction zone plagiogranite in the Ankara Mélange (Dilek and Thy, 2006). Similarly, the Dağkuplu Mélange in the western İzmirAnkara-Erzincan suture zone shows the same variety of alkaline, N-MORB, and supra-subduction zone geochemistry (Göncüoğlu et al., 2006; Sarıfakıoğlu, 2006; Sarıfakıoğlu et al., 2009). Suprasubduction geochemistry also characterizes Cretaceous ophiolites from the Kırşehir block (Çiçekdağ and Sarıkarıman massifs) (Yalınız et al., 1996, 2000a; Floyd et al., 2000). Most of these previous studies use geochemistry of crustal volcanic rocks, cumulate sequences, dike complexes, and massive gabbros for their interpretation without data from associated mantle peridotite. Including analyses of the peridotite provides additional evidence for high degrees of melt extraction inconsistent with a MORB tectonic model for the Eldivan ophiolite. Sarıfakıoğlu et al. (2009) used both mineral and whole-rock geochemistry of crust and mantle rocks to interpret the Orhaneli ophiolite in the western İzmir-Ankara-Erzincan suture zone as a supra-subduction zone ophiolite. Cr-spinel data from lherzolite and harzburgite of the Orhaneli mantle sequence closely match the Eldivan ophiolite (Fig. 4). Whole-rock Al2O3 from the Orhaneli ophiolite is generally lower but is still within the range of the Eldivan ophiolite (Fig. 3). These results show the same continuity in mantle composition as seen in the crustal-sequence geochemical data that argue for some supra-subduction influence from the western to central İzmir-Ankara-Erzincan Ocean. EPI-OPHIOLITIC SEDIMENTARY COVER UNITS Epi-ophiolitic sediment occurs as blocks, intercalated sediment in pillow lobes, and layered sediments depositionally overlying pillow basalts. Blocks are generally meter to decimeter sized blocks of pelagic, radiolarian-bearing limestone and minor chert within the serpentinized matrix of the mélange but not in direct contact with ophiolitic units. Some radiolarian-bearing red chert is intercalated within pillow basalt lobes. Layered sediments overlying the ophiolite consist of interbedded chert and limestone, and chert interbedded with pillow basalt and volcanic breccia. Individual beds are centimeters thick, but sediment sequences can reach 10 m. In one locality, shale and minor sandstone turbidites were found within the epi-ophiolitic cover. Karadağ Formation

Figure 7. Nine discriminant diagrams for basaltic rocks of the Eldivan ophiolite. In diagrams A–G, basaltic rocks generally plot in MORB fields, although some show too much scatter to be conclusive. Dikes mostly fall into island arc tholeiite (IAT) fields with some overlap in MORB fields. Alkaline basalts plot consistently in ocean island basalt (OIB) or within plate basalt (WPB) fields. Diagrams H and I are for granitic rocks. Samples from the Eldivan ophiolite all plot in volcanic-arc granite fields. These diagrams are from (A) Shervais (1982); (B) Pearce and Norry (1979); (C) Pearce (1982); (D) Woodhead et al. (1993) and Floyd et al. (2000); (E) Floyd et al. (1991); (F) Pearce et al. (1981); (G) Pearce and Cann (1973); (H) Pearce et al. (1984a); (I) Pearce et al. (1984a). BABB—backarc basin basalt; E-MORB—enriched MORB; N-MORB—normal MORB; SSZ— supra-subduction zone; CAB—continental arc basalt.

The Karadağ Formation overlies the Ankara Mélange along an angular unconformity and, because of multiple deformation phases, also overlies the mélange tectonically as a result of later thrust faulting. It is made up of intercalated volcanic and coarse siliciclastics at its base that grade upward into finer sandstones and mudstones and finally clay-rich limestone (Akyürek et al., 1980; Hakyemez et al., 1986). It is interpreted as flysch deposited in a foredeep setting near a continental margin during the onset of collision. The Kursunluduz Member of the Karadağ Formation contains chert bands alternating with red pelagic limestone (Akyürek et al., 1980; Hakyemez et al., 1986). The presence of

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Figure 8. Discriminant ternary diagrams for basaltic rocks of the Eldivan ophiolite. Basaltic rocks are black diamonds, dikes are gray squares, and alkaline basalts are light gray diamonds. In diagrams A–E, basaltic rocks fall mostly into the field of MORB with little overlap into subduction-influenced fields. In diagrams that do not distinguish between MORB and subduction fields, basalts plot in both fields. Dikes from the Eldivan ophiolite fall into subduction-related fields in diagrams that distinguish between MORB and arc-related rocks. In those that do not, dikes plot in the MORB-arc fields. Alkaline basalts consistently plot in enriched ocean island fields or within plate basalt (WPB) fields. Fields are from (A) Meschede and Casey (1986); (B) Wood (1980); (C) Mullen (1983); (D) Cabanis and Lecolle (1989); (E) Pearce and Cann (1971). WPA—within plate alkaline basalt; WPT—within plate tholeiitic basalt; OIT—ocean island tholeiite; OIA—ocean island alkalic basalt; CAB—continental arc basalt; PMORB—plume MORB; VAB—volcanic arc basalt; IAB—island arc basalt; VAT—volcanic arc tholeiite; OFB—ocean floor basalt; LKT— low-K tholeiite. See Figure 7 caption for additional abbreviations.

Tectonic evolution of the Ankara Mélange and associated Eldivan ophiolite near Hançili

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Praegglobotruncana stephani, Rotaliapora apenninica, Hedbergella sp., Ticinella sp., Globigerina sp., Textulariella sp., Cuneolina sp., and Valvulammina sp. radiolarians suggests the age of the Karadağ is Cenomanian to Campanian (Akyürek et al., 1980). Near the Eldivan ophiolite, the Karadağ Formation consists mostly of pelagic limestone and chert, and some clastic material, including sandstone lenses with graded and cross-beds. The angular unconformity between the Karadağ Formation and the underlying imbricated ophiolitic material of the Ankara Mélange suggests that the Karadağ Formation was deposited after or during imbrication of the ophiolite, and is in part correlative with the overlying Maastrichtian flysch of Norman (1984).

al. (2006). Single point analyses were taken with a 35 µm and a 25 µm diameter beam according to grain size. Common Pb corrections are for 204Pb, using an initial Pb composition from Stacey and Kramers (1975). Uncertainties are 1.0 for 206Pb/204Pb, 0.3 for 207 Pb/204Pb, and 2.0 for 208Pb/204Pb. Detrital zircon age extractor and ISOPLOT 3.00 (Ludwig, 2003) were used to determine and sort reliable age data. This detrital zircon age extractor extracts significant peak ages based on at least three grain analyses and the number of grains constituting each peak age. Results are listed in Table A3.

U/Pb Age Analysis of Detrital Zircon in Sandstone Units

Detrital zircon age populations from sandstone in the mélange and the Karadağ Formation have different minimum, maximum, and peak ages, suggesting that they were sourced from different terranes (Fig. 9). Detailed analysis of the mélange sandstone shows an age distribution from 143.2 ±2 Ma to 164.1 ±1 Ma with a peak age of 153 Ma. The Karadağ sandstone shows an age distribution from 105.2 ±5 Ma to 166 ±3 Ma, with a peak age of 130 Ma. The youngest peak age is used here as a proxy for the maximum age of deposition, which is consistent with the stratigraphic positions of the sandstones. The maximum age of the mélange sandstone, and Eldivan ophiolite, is 143.2 ±2 Ma, whereas the maximum age of the Karadağ sandstone is 105.2 ±5 Ma. Peak ages here are interpreted to represent the average age of the terrane dominantly being eroded at the time of deposition. An inherited fraction of zircon from the Neoproterozoic to Paleoproterozoic is present in both the mélange and the Karadağ sandstones (Fig. 9). Detrital zircons of similar age from the Tauride block in southwestern Turkey were documented by Kröner and Şengör (1990), who attributed them to the southern Angara

Sandstone samples were collected from a block within the Ankara Mélange directly adjacent to basalt and from the Karadağ Formation, which unconformably overlies the mélange. The age and tectonic source region for sandstone samples from the mélange and overlying Karadağ Formation were investigated through detrital zircon and sandstone petrography. Siliciclastic material was scarce, and these two samples represent the only sandstones found within the study area. The entire sample collected was processed for detrital zircons. The error introduced by the limited sample size and small number of zircons found within each sample is recognized, however the results are consistent with age data obtained by other methods throughout the suture zone. Zircon U-Pb age analyses were conducted by laser-ablation multicollector inductively coupled-plasma mass spectrometry (LA-MC-ICPMS) at the Arizona LaserChron Center. Analytical methods follow those described in Gehrels (2000) and Gehrels et

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Figure 9. Detrital zircon age populations for sandstones within the mélange (A) and the Karadağ Formation (B), indicating different source terranes for the two units. The larger graphs show the entire zircon population, including those that have Precambrian ages. The inset graph shows the detail of the peak ages. For the ophiolitic sandstone, the distribution of ages is narrow, with a peak age of ca. 153 Ma. The Karadağ sandstone has a larger range in ages, and a peak age of ca. 130 Ma.

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craton of Siberia. Dilek and Thy (2006) also found Proterozoic zircon in plagiogranite from the ophiolite near Ankara and interpreted them as a subduction recycled component from the Rhodope-Strandja Massif in northwestern Turkey and southeastern Bulgaria. These terranes may also have supplied the Paleoproterozoic to Neoproterozoic zircon grains in the mélange and Karadağ sandstones.

No deep water fauna are recorded. These data suggest that the Karadağ sandstone was derived from the carbonate system of a continental margin and was deposited in a nearby marginal basin. This is consistent with the interpretation of the Karadağ Formation as flysch deposited on the imbricated ophiolite, most likely in a foredeep setting near the continental margin, created as continuing subduction brought the Kırşehir and SakaryaPontide terranes together.

Sandstone Petrography STRUCTURE OF THE ELDIVAN OPHIOLITE Sandstone from both formations is compositionally and texturally immature, with low percentages of quartz and angular to subangular clasts. Despite alteration and secondary authigenic growth, the sandstone samples yielded two very different petrographic provenance results. The mélange sandstone is dominated by volcanic lithic fragments (52.33%) and plagioclase (25.33%), with minor quartz (8.66%), K-feldspar (2.00%), and clay minerals (11.66%). In contrast, the overlying Karadağ sandstone is made up of carbonate mud clasts (with some authigenic clay) (45.33%), plagioclase (28.00%), bioclastic grains (15.00%), and quartz (11.33%), with minor volcanic lithic material (0.33%). The composition of the mélange sandstone with its high percentage of volcanic lithic fragments implies that it was sourced from a nearby volcanic terrane. This idea agrees with the tectonic discriminant diagrams of Dickinson et al. (1983) (not shown). There are a number of sources for lithic fragments in the İzmir-Ankara-Erzincan Ocean, including seamounts, island arcs (Tankut, 1984; Tankut et al., 1998; Tüysüz et al., 1995), and the Pontide continental arc to the north. However, the Pontide arc is younger (Turonian) than detrital zircon grains found in the mélange sandstone, suggesting that it is not the source for volcanic lithics. The other plausible volcanic sources are oceanic, giving more evidence for intra-oceanic subduction away from the continental margin. Sandstone from the Karadağ Formation contains virtually no volcanic lithics, bioclastic grains, carbonate mud, or plagioclase grains. The bioclastic material in this sample is a mix of echinoderm, bryozoan, brachiopod, bivalve, and foraminiferan grains, a compositional variation that suggests a well-developed but relatively shallow carbonate system.

The angular unconformity between the Karadağ Formation and the underlying imbricated mélange, and the subsequent shortening of both, imply multiple phases of deformation (Fig. 10). The first phase involved dismemberment of the Eldivan ophiolite and serpentinite mélange development (Fig. 10A). Where paleohorizontal indicators exist, they show predominantly steep dips. Elongated blocks are also commonly vertical, with gaps between them filled with serpentine. If the mélange is associated with an accretionary wedge, it is likely the sections investigated formed near the backstop region, the area where accreted thrust sheets are progressively rotated to steeper dips by accretion of new material beneath them. Accretionary wedge development commonly produces isoclinally folded units with mostly sub-horizontal fold hinge lines. Although horizontal fold hinge lines are found within the serpentinite matrix and in overlying Karadağ Formation units, no hinge lines were found in the several blocks we investigated. Another way to explain the mostly steep dips in mélange blocks is by strike-slip deformation, which would produce steeply plunging hinge lines. There is no evidence of these in any part of the field area. The second phase of deformation occurred after the Karadağ Formation was deposited above the Ankara Mélange. During this second phase, the Karadağ Formation and underlying ophiolite and serpentine were thrust along southwardverging thrust faults (Fig. 10B). Some age constraints for these events are provided by detrital zircon populations from the mélange and Karadağ sandstones.

Epi-ophiolitic cover Karadag Formation

Pillow basalt Sheeted dikes

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Gabbro Serpentinized peridotite Figure 10. Multiple deformational phases are found in the Ankara Mélange. The first phase imbricated and rotated the oceanic units into mostly vertical units, encased in serpentine mélange (A). Subsequent phases involved shortening of the Ankara Mélange and the unconformably overlying Karadağ Formation (B). Subsequent events included serpentine diapirism and thrusting of mélange over late Miocene deposits of the Hançili Formation.

Tectonic evolution of the Ankara Mélange and associated Eldivan ophiolite near Hançili Timing of Ophiolite Imbrication Detrital zircon ages from sandstone within and above the Ankara Mélange provide limits on the timing of mélange formation and dismemberment of the Eldivan ophiolite. Zircon grains as young as 143 ±2 Ma within a sandstone of the mélange indicate that it was incorporated into the mélange after this time and before the youngest age of zircon grains within the unconformably overlying Karadağ sandstone, which yields ages of 105 ±5 Ma. Further imbrication of the ophiolite and the Karadağ Formation occurred after 105 ±5 Ma. Imbrication of the Eldivan ophiolite between 143 ±2 and 105 ±5 Ma is consistent with data from other parts of the suture zone that suggest collapse of the ocean basin had begun about this time. For example, radiolarians in limestone deposits from the Kirazbaşı foredeep complex are as old as ca. 135 Ma (late Valanginian) (Tüysüz and Tekin, 2007). Intra-oceanic thrusting began prior to 90 Ma near the Kırşehir block (Yalınız et al., 2000b) and 93 ±2 Ma in the western İzmir-Ankara-Erzincan Ocean (Önen, 2003). Granitoids of 94.9 ±3.4 Ma, in the Kırşehir block, interpreted as the result of supra-subduction zone ophiolite obduction, also suggest that subduction must have been active before 95 Ma (Boztuğ et al., 2007). Restoration of the Eldivan Ophiolite The orientation of sheeted dikes in ocean crust is commonly used as a proxy for spreading ridge orientation. For the Eldivan ophiolite, it would represent the orientation of a supra-subduction zone spreading ridge. Commonly, sheeted dikes are perpendicular to overlying basaltic flows that they feed. Most dikes in the Eldivan Ophiolite strike NNW-SSE and are steeply dipping (Fig. 11A). A minor component of E-W dikes is also found, but most are horizontal. In one locality, which represents the largest single mélange block of basaltic rock in the Hançili region, a series of horizontal sheeted dikes is in contact with vertical pil-

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low basalts and sheet flows that strike E-W, indicating that the entire igneous section has rotated 90° about a horizontal axis. Sedimentary blocks are also steeply dipping, indicating a similar amount of rotation about a horizontal E-W axis. Assuming that other sheeted dikes in the region underwent a similar horizontal axis rotation, applying this rotation to the NNE-SSW dikes provides the original orientation of the suprasubduction zone spreading ridge that produced the Eldivan ophiolite. The rotation maintains the steep dip of the NNW-SSE dikes but changes the average strike to near N-S (Fig. 11B), which is subparallel to the İzmir-Ankara-Erzincan suture zone. However, oroclinal bending of this suture zone from indentation of the Kırşehir block indicates an additional 90° of counterclockwise vertical axis rotation. Paleomagnetic studies used to test the oroclinal bend hypothesis found that the Ankara Mélange near the Eldivan ophiolite has undergone at least 30° of counterclockwise vertical axis rotation since the Eocene, and that it may have already been rotated counterclockwise by even more before this time (Kaymakci et al., 2003). According to Kaymakci et al. (2003), the Çankırı basin underwent rotation during the Eocene Epoch through the midMiocene, and perhaps as early as the Paleocene Epoch. Near the Eldivan ophiolite the Çankırı basin margin rotated 33° counterclockwise during Oligocene time. A clockwise vertical axis rotation of 33° was applied to correct for these rotations, which moves the average strike direction of most sheeted dikes to 041, which was most likely the orientation in the Eocene (Fig. 11C). To completely restore the İzmir-Ankara-Erzincan suture zone back to its pre-collisional indentation trend, an additional 52° of clockwise rotation is needed. Correcting for the horizontal and vertical axis rotations of the sheeted dikes shows that they are subparallel to the İzmirAnkara-Erzincan suture zone, which implies mostly orthogonal motion of the spreading ocean with respect to the subduction boundary represented by the suture zone, indicating little strike slip motion in the creation of the suture zone. These results are

Figure 11. Stereographs of poles to dike attitudes in sheeted dike units of the Eldivan ophiolite. Solid black lines are average strike of sheeted dikes and most likely the spreading ridge: (A) Prior to any restoration. (B) Restored about a horizontal axis according to paleo-horizontal controls. (C) Partially restored to original orientation by 30° of post-Eocene clockwise vertical axis rotation documented from paleomagnetic data. Oroclinal bending of the İzmir-Ankara-Erzincan suture zone indicates that at least another 60° of clockwise rotation is needed to restore the dikes back to their original orientation, which would be near E-W.

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consistent with those found by Fayon et al. (2001) and Whitney et al. (2001), who concluded that the northern part of the Kırşehir block was deformed and exhumed by orthogonal collision. However, whereas the İzmir-Ankara-Erzincan suture zone may have formed through orthogonal motion, Fayon et al. (2001) and Whitney et al. (2001) give evidence for a later oblique collision of the Tauride platforms with the southern Kırşehir block, exhuming the southern Kırşehir block through left-lateral wrench faulting. TECTONIC EVOLUTION Review of the İzmir-Ankara-Erzincan Ocean Understanding the overall tectonic evolution of this ocean is crucial to reconstructing the role played by the Eldivan ophiolite. Age constraints of various events throughout the İzmir-AnkaraErzincan suture zone indicate three main phases of İzmir-AnkaraErzincan Ocean evolution: a constructional phase, a destructional phase, and a suturing phase (Fig. 12A). The constructional phase began with rifting at least as old as the late Carnian–early Norian Stages (ca. 215 Ma), based on radiolarians associated with MORB in the central and western parts of the suture zone (Bragin and Tekin, 1996; Tekin et al., 2002). Other radiolarians suggest that it developed into an ocean basin by late Bajocian time (Tüysüz and Tekin, 2007), and seamounts formed on the ocean floor during the Jurassic and Cretaceous Periods (Rojay et al., 2001, 2004; Tankut et al., 1998). Destruction of the ocean basin by intra-oceanic subduction is documented by supra-subduction zone ophiolites, the oldest in the Ankara Mélange, yielding a U/Pb zircon age of 179 ±15 Ma (Dilek and Thy, 2006). Intra-oceanic subduction continued through the Late Cretaceous Period, creating supra-subduction zone ocean crust in the central İzmir-Ankara-Erzincan Ocean, now part of the Kırşehir block (Yalınız et al., 1996; Yalınız et al., 2000b), and the Dağkuplu mélange (Göncüoğlu et al., 2006; Sarıfakıoğlu, 2006; Sarıfakıoğlu et al., 2009). Late Valanginian (ca. 135 Ma) (Tüysüz and Tekin, 2007) to Paleocene (Koyçiğit, 1991) radiolarians are found in foredeep deposits along the Sakarya-Pontide margin, suggesting that active subduction against the continent began in the Early Cretaceous Period. Other events documenting subduction at this time are the occurrence of accretion complexes along the Pontide margin, which were metamorphosed at ca. 100 Ma (Okay et al., 2006), and Turonian Epoch (ca. 90 Ma) through Paleocene Epoch magmatism in the Ponticles (Yılmaz et al., 1997). It is important to note that the oldest age of supra-subduction zone ophiolites, 179 ± 15 Ma, predate the oldest foredeep deposits (ca. 135 Ma) against the continent, suggesting that intra-oceanic extension occurred prior to subduction against the continental margin. Thrusting and imbrication of the Eldivan supra-subduction zone basin in the central İzmir-Ankara-Erzincan suture zone occurred between 105 and 143 Ma, as shown by detrital zircon ages from this study. In the western İzmir-Ankara-Erzincan Ocean, thrusting began at least by 94 Ma, as recorded by the age of a metamorphic sole

(Önen, 2003). This constrains a destructive phase of subduction that began ca. 179 ±15 Ma and ended as early as ca. 60 Ma. Final closure of the İzmir-Ankara-Erzincan Ocean occurred through continental block collision, collisional indentation, and suturing of the Kırşehir block and the larger Anatolide-Tauride platform with the Sakarya-Pontide terranes during the Late Cretaceous Period to the Miocene Epoch. The first evidence of continental collision comes from post-collisional granitoids of the Kırşehir block, which yielded Rb-Sr whole-rock and 207Pb-206Pb zircon ages from 110 ±14 Ma (Güleç, 1994) to 74.9 ±3.8 Ma (Boztuğ et al., 2007). Exhumation of the collision zone in the Central Pontides, based on stratigraphic constraints, and granitoids of the Kırşehir block, based on apatite fission-track ages, documents collision between 86 and 93 Ma and 57 and 62 Ma, respectively (Okay et al., 2006; Boztuğ and Jonckheere, 2007). Collisional indentation of the Kırşehir block caused at least 90° of counterclockwise rotation, 33° of which is well constrained since the Eocene. Ages of continental block collision young away from the central part of the suture zone, where the Kırşehir block is present. In the western part of the suture, where the Kırşehir block is absent, and collision of the Sakayra-Pontide terrane occurred only within the Anatolide-Tauride block,

Figure 12. (A) Age constraints for the evolution of the İzmir-AnkaraErzincan suture zone through time. Three main phases are identified: (1) an initial construction phase in which the ocean basin was forming through ridge spreading with hotspot volcanism creating seamounts on the ocean floor, (2) destruction of the ocean basin through intraoceanic subduction that resulted in intra-oceanic seafloor spreading above a subduction zone and arc magmatism, and (3) collision and suturing of the Kırşehir and Anatolide-Tauride continental blocks with the Sakarya-Pontide terranes. Numbers in the time line and map correspond with the source of age data (below) and sample locations, respectively. The sample location for detrital zircons of this study is represented with a black star. Sources for data are as follows: (1) Tekin et al. (2002); (2) Bragin and Tekin (1996); (3) Dilek and Thy (2006); (4) Rojay et al. (2004); (5) Rojay et al. (2001); (6) Göncüoğlu et al. (2006); (7) Tüysüz and Tekin (2007); (8) Önen (2003); (9) Yalınız et al. (2000); (10) Koçyiğit (1991); (11) Yalınız et al. (1999); (12) Boztuğ and Jonckheere (2007); (13) Yılmaz et al. (1997); (14) Okay et al. (2006); (15) Kaymakci et al. (2003); (16) Boztuğ et al. (2007) and references therein; (17) Fayon et al. (2001). (B) Schematic cartoon model for the evolution of the Eldivan ophiolite during the Cretaceous, using the Philippine Sea plate and Mariana trough as an analogue. Early Cretaceous time documents the beginning of subduction and upper plate extension, as evidenced by supra-subduction zone (SSZ) basalt, foredeep complexes along the continental margin, and ophiolitic metamorphic soles. In the Late Cretaceous Period supra-subduction zone upper plate subduction began along the Sakarya-Pontide margin, causing active volcanism in the Pontide continental arc. The latest Cretaceous Period through the Oligo-Miocene Epochs was characterized by collision and suturing of the Kırşehir block (KB) and AnatolideTauride platform with the Sakarya-Pontide terrane, as evidenced by post-collisional granitoids and fission-track (FT) exhumation ages of the Kırşehir block. Fission-track ages also indicate that wrench faulting exhumed the southern Kırşehir block through left-lateral, strikeslip motion owing to later oblique collision of the Tauride platform with the Kırşehir block. OIB—ocean island basalt.

Tectonic evolution of the Ankara Mélange and associated Eldivan ophiolite near Hançili

13

6 1

e ya Zo8 n S akar

7 4

4

2 4 10 4 4 12 5

12

Kirsehir 9 block 11

Izm i

Erzincan suture arank A r-

12

Anatolide-Tauride block

40Ar/39Ar regional 40Ar/39Ar age of metamorphic sole (8) metamorphic age (8) Active Pontide magmatic arc (13)

Zircon U-Pb SSZ ocean crust crystallization age (3)

Radiolaria from blocks of pelagic sediment assoc. w/ OIB and SSZ basalt (2,6,9)

Oldest radiolaria from limestone and chert blocks (1,2)

Radiolaria from intercalated sediment on OIB pillow basalts (4,5,6) Crystallization ages (various methods) of post-collisional Central Anatolian granitoids (11, 16)

Rotation in the Cankiri basin (15)

Radiolaria from forearc basin deposits (10)

200

Radiolaria from blocks of pelagic sediment (2)

Radiolaria from the Kirazbasi foredeep complex (7,14) FT 1st phase exhumation age of post-collisional granitoids (12) Metamorphism of accretionary complex (14) FT 2nd phase exhumation age of post-collisional granitoids (12,17)

180

160

Exhumation of accretionary complex (14)

140

Ma

120

80

100

Constructional phase Construction of the ocean basin through ocean ridge spreading and seamount volcanism

60

Destructional phase Destruction of the ocean basin through intra-oceanic subduction, creating SSZ ocean crust, forearc basins, and intra-oceanic thrusting

40

Suturing phase Collision of the Kirsehir and Anatolide-Tauride blocks with the Sakarya-Pontide terrane

20

0

Iz

uture can s -Erzin a r a k n mir-A

220

de onti a-P Intr

Central Pontides

14

sut ure

Ist

on e ul z b ure n a S ut

Inter-Tauride

RhodopeRhodope Strandja Massif

159

Detrital zircon U-Pb age, Karadag Fm, this study

220

200

180

160

140

Ma

120

100

80

60

40

20

0

Detrital zircon U-Pb age, epi-ophiolitic sandstone, Eldivan ophiolite, this study

A N

S

25-100 Ma

65 to 135 Ma

Ankara Mélange

65?-179 Ma

Subduction of SSZ upper plate

S-P

B

KB

S-P

Upper plate SSZ extension

KB

S-P

KB

160

Dangerfield et al.

exhumation is documented by 40Ar/39Ar metamorphic cooling ages of 48 ±12 Ma (Önen, 2003). These age constraints suggest that the collision of the Kırşehir block with the Central Pontides may have occurred significantly earlier than collision between the Anatolide-Tauride block with the rest of the continental margin. The age of final suturing (no more deformation) between the Kırşehir block and Anatolide-Tauride platform with the Sakarya-Pontide terrane is not constrained. Boztuğ and Jonckheere (2007) attribute a second phase of granitoid exhumation in the Kırşehir block at 28–30 Ma to collision of the Arabian-African platform in the east, where Fayon et al. (2001) interpret exhumation of granitoids at 35 Ma to be from collision of the Anatolide-Tauride platform. Shortening continued into the late Miocene and Pliocene, as indicated by thrusting of the Ankara Mélange over the edge of the Hançili basin. Plate Tectonic Setting The Eldivan ophiolite was created in the upper plate of the İzmir-Ankara-Erzincan Ocean during oblique intra-oceanic subduction as part of a backarc basin. This created a suite of geochemical signatures, as supra-subduction zone melting modified an N-MORB mantle, which was mixed with an enriched OIB mantle that had previously created seamounts on the ocean floor (Fig. 12B). The current Philippine Sea plate and Mariana trough supra-subduction zone basins are suggested as modern analogues for the tectonic setting of the Eldivan ophiolite, İzmir-AnkaraErzincan Ocean, and Ankara Mélange. The Philippine Sea plate formed as an upper-plate suprasubduction zone basin caused by intra-oceanic subduction (Harris, 2003). Later, the Philippine Sea supra-subduction zone ocean basin began to subduct to the west, creating the Japan, Ryukyu, and Luzon arcs. In a similar way, formation of the Eldivan intra-oceanic basin began subduction beneath the Pontides, creating the Pontide magmatic arc. Subduction of the Eldivan oceanic basin beneath the Pontides allowed several large fragments of mostly supra-subduction zone oceanic crust and some mantle to accrete to the margin, serpentinize, and produce mélange in the forearc between 105 and 143 Ma (Fig. 12B). Eventually the subduction zone was choked by collision of the Kırşehir block with the Sakarya-Pontide terrane, which further imbricated the ophiolite with the overlying Karadağ Formation. Collision continued to indent the continental margin and rotate the Eldivan ophiolite from its original E-W orientation to its current position on the western edge of the large omega-shaped İzmir-Ankara-Erzincan suture zone. How far the Kırşehir block has traveled is not yet constrained. However, according to the Philippine Sea plate model, as supra-subduction zone basins open along a continental margin, fragments of the margin are rifted off and travel away as the basin opens (Harris, 2003). Closure of the supra-subduction zone basin eventually brings many of these fragments back into collision with parts of the original continental margin from which

they were rifted. These processes are illustrated in many parts of the equatorial Pacific and Indonesian regions (Harris, 2003). CONCLUSIONS 1. The dismembered Eldivan ophiolite is a remnant of the İzmir-Ankara-Erzincan Ocean branch of the northern NeoTethys that evolved as a supra-subduction zone basin between the Gondwana-derived Kırşehir and Anatolide-Tauride blocks and the Sakarya-Pontide margin. 2. Parts of the İzmir-Ankara-Erzincan Ocean were accreted to the southern Asian margin as it subducted beneath it. These fragments were incorporated into the serpentine-rich Ankara Mélange. 3. During accretion, most of the units scraped from the İzmir-Ankara-Erzincan Ocean were imbricated, steeply inclined, and later broken into blocks surrounded by serpentinite. These include fragments of mostly oceanic crustal material, limestone, chert, and rare sandstone. 4. The ages of some blocks in the Ankara Mélange are younger than 143 ±2 Ma, with imbrication and initial destruction of the ocean basin having occurred between 143 ±2 Ma and 105 ±5 Ma. These ages are older than those of imbrication of the İzmir-Ankara-Erzincan Ocean in the west, which is documented at ca. 94 Ma. 5. Intra-oceanic volcanic arcs or seamounts are likely source terranes for sandstone units associated with the Eldivan ophiolite, suggesting that the ophiolite formed in an intra-oceanic subduction zone away from significant continental influence. 6. Studies of sheeted dike orientations indicate that the spreading ridge of intra-oceanic supra-subduction zone basins was most likely subparallel to the southern margin of Asia before indentation of the Kırşehir block, and has since been rotated nearly 90° counterclockwise. 7. The tectonic setting and evolutionary history of the Eldivan ophiolite can be characterized as a Western Pacific–type suture system in contrast to the more classic Himalayan-type suture that involves subduction of large tracts of MORB-like oceanic lithosphere and juxtaposition far-traveled of continental blocks of different affinities. This interpretation may also apply to many other Cretaceous ophiolite-bearing suture zones of the Eastern Mediterranean. ACKNOWLEDGMENTS Funding for this project was provided by the American Association of Petroleum Geologists and U.S. National Science Foundation grant EAR 0337221. We would especially like to acknowledge the Maden Tetkik ve Arama Genel Müdürlüğü (MTA) in Turkey for supporting the fieldwork for this study, and we thank Mustafa Sevin, Esra Esirtgen, and Serdal Alemdar for their tremendous help in the field. Also, we thank Victor Valencia at the University of Arizona, who dated our detrital zircon samples, and Steve Nelson and Mike Dorais for critical reviews of our manuscript.

0.06

0.02

1.79

8.46

0.12

42.20

0.18

0.00

0.04

0.01

97.82

Al2O3

Fe2O3

MnO

MgO

CaO

Na2O

K2O

P2O5

Total

0.11

8.75

0.78

0.01

0.00

0.00

0.00

0.10

110

126a

0.75

0.25

0.02

1.22

2.11

9.06

9.24

0.18

0.04

1.03

4.61

5.41

5.73

0.13

11.19 10.32 0.14 5.95

8.48

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