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View Entire Issue Vol. 6, No. 2 February 1996 INSIDE • Penrose Conference Reports, p. 14, 16 GSA TODAY • Rocky Mountain Section Meeting, p. 24 A Publication of the Geological Society of America • North-Central Section Meeting, p. 30 Alternate Origins of the Coast Range Ophiolite (California): Introduction and Implications William R. Dickinson, Department of Geosciences, University of Arizona, Tucson, AZ 85721 Clifford A. Hopson, Department of Geological Sciences, University of California, Santa Barbara, CA 93106 Jason B. Saleeby, Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125 ABSTRACT Correctly interpreting the tectonic evolution of the California continental margin requires understanding the ori- gin of the Jurassic Coast Range Ophio- lite, which represents a fragment of mafic-to-ultramafic crust of oceanic character lying depositionally beneath the western flank of the Great Valley forearc basin in fault contact with the Franciscan subduction complex of the California Coast Ranges. Three con- trasting hypotheses for genesis of the ophiolite as seafloor are each based on internally consistent logic within the framework of plate tectonics, but are mutually exclusive and lead to strikingly different interpretations of regional tectonic relations, even though each assumes that the Sierra Nevada batholith to the east represents the eroded roots of a magmatic arc linked to subduction along the Mesozoic continental margin. To encourage the further work or analy- Multiple basaltic sills of the sheeted dike and sill complex, Point Sal remnant of the Middle Jurassic Coast sis needed to develop a definitive inter- Range ophiolite. The ridge in background exposes sheeted sills and (to left of tree on the skyline) base of pretation, summary arguments for each the overlying pillow lavas. hypothesis of Coast Range Ophiolite genesis in mid- to late Jurassic time are presented in parallel: (1) backarc now incorporated within the continental larger than our area of disagreement. We spreading behind an east-facing intra- block (Bailey et al., 1970). The overall span each interpret the Coast Range Ophiolite oceanic island arc that then collided and of Middle to Late Jurassic radiometric ages layered assemblage as a profile of mafic amalgamated with the Sierran continen- for igneous components of ophiolite and crust and lithosphere of oceanic character, tal-margin arc; (2) paleoequatorial mid- postophiolite hypabyssal intrusions is and we infer that this profile was formed ocean spreading to form oceanic litho- ~170 to 155-150 Ma (Hopson et al., 1981, through magmatism induced by mantle sphere that was then drawn northward 1991; Saleeby et al., 1984; Mattinson and upwelling linked to lithospheric extension toward a subduction zone in front of Hopson, 1992). Understanding correctly or “spreading.” We each also argue for the Sierran continental-margin arc; and the origin and emplacement of the Coast emplacement of the ophiolite within the (3) forearc spreading within the forearc Range Ophiolite is essential for under- conceptual framework of plate tectonics, region of the Sierran continental-margin standing the Mesozoic evolution of the taking the Sierra Nevada composite batho- arc in response to transtensional defor- Cordilleran continental margin (Saleeby, lith to the east to be the deeply eroded mation during slab rollback. 1992). The time is long past when geosci- roots of Jurassic-Cretaceous magmatic arc entists could assume that all ophiolites belts, and regarding Franciscan rocks of INTRODUCTION formed in the same way or have the same the California Coast Ranges farther west Widely distributed exposures of the tectonic significance. as part of the subduction complex Jurassic Coast Range Ophiolite in the Cali- With the help of co-authors, we out- accreted near the trench that was paired fornia Coast Ranges represent deformed line here three divergent views on the ori- with the Sierran-Klamath arc assemblage and structurally dismembered segments gin of the Coast Range Ophiolite. We of oceanic crust and uppermost mantle emphasize that our areas of agreement are Ophiolite continued on p. 2 IN THIS ISSUE GSA TODAY February Vol. 6, No. 2 1996 Alternate Origins of the Coast SAGE Remarks ......................... 20 Range Ophiolite (California): GSA TODAY (ISSN 1052-5173) is published Introduction and Implications .... 1 Environment Matters ................... 22 monthly by The Geological Society of America, Inc., Calendar .............................. 23 with offices at 3300 Penrose Place, Boulder, Colorado. Washington Report ..................... 10 Mailing address: P.O. Box 9140, Boulder, CO 80301- Rocky Mountain Section Meeting ....... 24 9140, U.S.A. Second-class postage paid at Boulder, Book Nook ............................ 11 Colorado, and at additional mailing offices. Postmas- North-Central Section Meeting ......... 30 ter: Send address changes to GSA Today, Membership GSAF Update .......................... 12 Services, P.O. Box 9140, Boulder, CO 80301-9140. 1995 Annual Meeting Revisited ......... 36 Copyright © 1996, The Geological Society of America, Penrose Conference Reports Inc. (GSA). All rights reserved. Copyright not claimed Fault-Related Folding ................ 14 GSA Meetings ......................... 38 on content prepared wholly by U.S. Government Argentine Precordillera .............. 16 employees within the scope of their employment. Per- Classifieds ............................. 39 mission is granted to individuals to photocopy freely all 1996 GeoVentures ..................... 19 Northeastern Section Meeting Update . 39 items other than the science articles to further science and education. Individual scientists are hereby granted GSA on the Web ....................... 19 permission, without royalties or further requests, to make unlimited photocopies of the science articles for use in classrooms to further education and science, and to make up to five copies for distribution to associates in the furtherance of science; permission is granted to Ophiolite continued from p. 1 We thank conveners R. G. Anderson, make more than five photocopies for other noncom- mercial, nonprofit purposes furthering science and edu- D. M. Miller, and R. M. Tosdal for arrang- cation upon payment of the appropriate fee ($0.25 per (Fig. 1). We concur that the east flank of ing the 1993 Penrose Conference on page) directly to the Copyright Clearance Center, 27 the Franciscan subduction complex was Jurassic Cordilleran magmatism at which Congress Street, Salem, Massachusetts 01970, phone (508) 744-3350 (include title and ISSN when paying). thrust beneath and otherwise faulted our opposing thoughts were pointedly Written permission is required from GSA for all other against the Coast Range Ophiolite , which juxtaposed, and we dedicate the following forms of capture, reproduction, and/or distribution of formed the westernmost segment of the discussions to the memory of E. H. Bailey any item in this journal by any means. GSA provides this and other forums for the presentation of diverse floor of the Great Valley forearc basin (who started it all). opinions and positions by scientists worldwide, regard- lying between Sierran arc and Franciscan less of their race, citizenship, gender, religion, or politi- trench. 1. COAST RANGE OPHIOLITE cal viewpoint. Opinions presented in this publication do not reflect official positions of the Society. We nevertheless ascribe generation of AS BACK-ARC–INTER-ARC the Coast Range Ophiolite to three differ- BASIN LITHOSPHERE ent tectonic settings: (1) Dickinson infers SUBSCRIPTIONS for 1996 calendar year: William R. Dickinson, Department of Society Members: GSA Today is provided as part of “backarc” seafloor spreading behind a Geosciences, University of Arizona, membership dues. Contact Membership Services at migratory east-facing intraoceanic island (800) 472-1988 or (303) 447-2020 for membership Tucson, AZ 85721 arc, which collided with the west-facing information. Nonmembers & Institutions: Free with Richard A. Schweickert, Department of paid subscription to both GSA Bulletin and Geology, Sierran arc along the continental margin Geological Sciences, University of Nevada, otherwise $45 for U.S., Canada, and Mexico; $55 else- (as intervening oceanic lithosphere was where. Contact Subscription Services. Single copies Reno, NV 89557 consumed), to lodge the migratory arc and may be ordered from Publication Sales. Claims: For Raymond V. Ingersoll, Department of Earth nonreceipt or for damaged copies, members contact its backarc seafloor against the continental and Space Sciences, University of California, Membership Services; all others contact Subscription margin; (2) Hopson infers “midocean” Services. Claims are honored for one year; please allow Los Angeles, CA 90024-1567 sufficient delivery time for overseas copies. seafloor spreading along an intraoceanic ridge crest, followed by tectonic transport The concept that the Coast Range STAFF: Prepared from contributions from the of the resulting seafloor to the continental Ophiolite was formed by backarc-interarc GSA staff and membership. margin (as Sierran subduction drew it ever spreading behind an east-facing intra- Executive Director: Donald M. Davidson, Jr. Science Editor: Suzanne M. Kay closer), until the ophiolite docked against oceanic island arc that was accreted to the Department of Geological Sciences, Cornell University, the continental margin prior to the onset continent in Jurassic time by arc collision Ithaca,
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