The Late Cretaceous San Juan thrust system, San Juan Islands, Washington Mark T. Brandon* Geological Survey of Canada Pacific Geoscience Centre P.O. Box 6000 Sidney, British Columbia V8L 4B2 Canada Darrel S. Cowan and Joseph A. Vance Department of Geological Sciences University of Washington Seattle, Washington 98195 *Present address: Department of Geology and Geophysics, Yale University, P.O. Box 6666, New Haven, Connecticut 06511-8130. 221 © 1988 The Geological Society of America, Inc. All rights reserved. Copyright is not claimed on any material prepared by government employees within the scope of their employment. All materials subject to this copyright and included in this volume may be photocopied for the noncommercial purpose of scientific or educational advancement. Published by The Geological Society of America, Inc. 3300 Penrose Place, P.O. Box 9140, Boulder, Colorado 80301 GSA Books Science Editor Campbell Craddock Printed in U.S.A. Library of Congress Cataloging-in-Pub6cation Data Brandon, M. T. (Mark Thomas) The late Cretaceous San Juan thrust system, San Juan Islands, Washington I Mark T. Brandon, Darrel S. Cowan, and Joseph A. Vance. p. cm.-(Special paper I Geological Society of America ; 221) Bibliography: p. ISBN 0-8137-2221-7 I. Thrust faults (Geology)-Washington (State)-San Juan Islands. 2. Geology, Stratigraphic-Cretaceous I. Cowan, Darrel S., 1945- 11. Vance, Joseph A., 1930- . III. Title. IV. Series: Special papers (Geological Society of America) ; 221. QE606.5.U6B73 1988 55 l.TT0979774-dcl9 88-4003 CIP Cover Pboto: Satellite image of the San Juan Islands, provided by Advanced Satellite Productions, Inc., Vancouver, B.C., Canada. Late Cretaceous thrust faults of the San Juan-Cascades system are shown in yellow. Early Tertiary faults, related to accretion of a more outboard set of terranes, are shown in red. The northwest-striking islands visible in the west central part of the image correspond to broad, northwest-trending fo lds in the Naniamo Group, a syn-orogenic basin that formed peripheral to the San Juan-Cascades thrust system. The metropolitan areas are Vancouver (northeast), Victoria (west central), Bellingham (east central), and Port Angeles (south central). 10 9 876 5432 11 Contents Acknowledgments . v Abstract ............................................................. I Introduction . .. .. .. .. .. .. .. .. 2 Previous work . .. .. .. .. .. .. .. 2 Use of stratigraphic and tectonostratigraphic terms . .. .. .. 5 Regional setting . .. .. .. .. .. .. .. 5 Overview of the San Juan Islands . 7 Paleozoic and lower Mesozoic rocks of the thrust system . II Turtleback terrane: Paleozoic plutonic and volcanic rocks . II Composition and age of the Turtleback Complex . II Stratigraphy and age of the East Sound Group . .. .. .. .. 13 Origin of the Turtleback and East Sound units .. .. .. .. 14 Regional correlation of the Turtleback and East Sound units . .. .. 15 Deadman Bay terrane: Permian-Lower Jurassic chert. basalt, and limestone . .. .. .. .. .. .. .. 15 Stratigraphy and age of the Deadman Bay Volcanics . .. .. 15 Stratigraphy and age of the Orcas Chert . .. .. .. .. 18 Origin and tectonic setting . .. .. .. .. .. .. 19 Regional correlation . .. .. .. .. .. .. 20 Garrison terrane: Permo-Triassic mafic schist . .. .. .. .. 21 Composition . .. .. .. .. .. .. .. 21 Age ............................................................. 21 Structural relationships . .. .. .. .. .. .. .. 21 Origin and tectonic setting . .. .. .. .. .. .. 24 Regional correlation . .. .. .. .. .. .. .. 24 Upper Mesozoic rocks of the thrust system . .. .. .. .. 25 Constitution Formation: Upper Mesozoic clastic sequence . .. .. 25 Stratigraphy . .. .. .. .. .. .. .. 26 � ............................................................. n Provenance of clastic rocks . 27 iii IV Contents Origin and tectonic setting . .. .. .. .. .. .. 27 Regional correlation . .. .. .. .. .. .. .. 29 LOpez Structural Complex: Late Cretaceous imbricate fault zone . .. 29 Rock units and ages . .. .. .. .. 29 Internal structural relations . .. .. .. .. .. .. 31 Decatur terrane: Upper Mesozoic ophiolite, arc, and clastic sequence .. .. 31 Composition and age of the Fidalgo Complex .. ... .. .. 31 Geochemistry and tectonic setting of the Fidalgo Complex . .. .. 33 Stratigraphic and age of the Lummi Formation . .. .. .. 34 Metamorphism of Decatur rocks .. .. .. ... .. .. 34 Regional correlation . .. .. .. .. .. .. ... 35 External units: Mesozoicuni tsfo rward of, and below, the San Juan thrust system . .. .. .. .. .. .. .. 36 Haro terrane: Upper Triassic and Jura-Cretaceous volcaniclastic units . .. .. .. .. .. .. .. 36 Haro Formation (Upper Triassic) . 36 Spieden Group (Upper Jurassic and Lower Cretaceous) . .. .. 37 Nanaimo Group: Upper Cretaceous syn-orogenic clastic sequence . .. .. .. .. .. .. .. .. .. 37 Post-orogenic unit: The Eocene Chuckanut Formation . .. .. .. 38 High-pressure metamorphism and its relation to thrusting . .. .. 38 Terrane accretion and Late Cretaceous thrusting . .. .. .. .. 42 Concluding remarks . .. .. .. .. .. .. .. 45 Appendices A. Summary of fossil ages and isotopic dates . .. .. .. .. 47 B. Summary of geochemical analyses . .. ... .. .. 64 References cited .. .. .. .. .. .. .. .. 78 Acknowledgments We are grateful to E. H. Brown, S. Y. Johnson, D. L. Jones, J.W.H. Monger, and R. W. Tabor for critical and helpful reviews of this paper and an earlier version; to R. L. Armstrong, R. B. Forbes, C. W. Naeser, R. A. Zimmerman, and R. E. Zartman for providing unpublished isotopic dates; and to W. R. Danner, H. !go, D. L. Jones, M. J. Orchard, E. A. Pessagno, Jr., N. M. Savage, and W. V. Sliter for providing and clarifying various paleontological ages. We thank J. T. Whetten for giving us access to unpublished geochemical data, and D. Mogk and M. A. Dungan for microprobe analyses of Garrison amphiboles. Our study has profitedfrom discussions with: P. R. Carroll, A. Drinkhouse Dailey, S. Y. Johnson, J. I. Garver, P. Misch, S. A. Monsen, J. E. Muller, and J. T. Whetten. Cowan and Brandon acknowledge support from the National Science Foundation (grants EAR 76-13127 and EAR 79-10827), the Society of Sigma Xi, and the Corporation Fund of the Department of Geological Sciences, University of Washington. Field accommodations were kindly provided by the University of Washington Friday Harbor Laboratories. Brandon is grateful to the National Science and Engineering Research Council of Canada for a post-doctoral fellowship at Pacific Geoscience Centre during which this manuscript was completed. We also thank the Department of Geological Sciences at the University of Washington and the Pacific Geoscience Centre for clerical and drafting support. Geological Society of America Special Paper221 1988 ABSTRACf The San Juan Islands expose a thick and regionally extensive sequence of Late Cretaceous thrust faults and nappes, referred to as the San Juan thrust system. This thrust system, which straddles the southeastern edge of the Wrangellia terrane of Van­ couver Island, contains important information on the accretionary history of Wrangellia and other, related, far-traveled terranes. Nappes of the thrust system contain a diverse group of rocks ranging from early Paleozoic to middle Cretaceous in age. Based on contrasts in stratigraphy, metamorphism, and geochemistry, we identify five terranes within, and peripheral to, the thrust system. These terranes were widely separated from each other, and also from Wrangellia, until the Late Jurassic: (1) the Haro terrane, an Upper Triassic arc-volcanic sequence; (2) the Turtleback terrane, a Paleozoic arc­ plutonic and volcanic unit; (3) the Deadman Bay terrane, a Permian to Lower Jurassic oceanic-island sequence containing Tethyan-fusulinid timestones; (4) the Garrison ter­ rane, a Permo-Triassic, high-pressure metamorphic unit; and (5) the Decatur terrane, a Middle to Upper Jurassic ophiolite and superimposed arc-volcanic sequence. Thick Jura-Cretaceous clastic units are linked to these older San Juan terranes and to Wran­ gellia, either as directly overlapping units or by the presence of clastic material derived from the terranes. The voluminous amount of clastic material in these Jura-Cretaceous units requires a large, subaerially exposed source region. We infer that this source region was a continent-like landmass, presumably part of continental America (North or Central?). Late Cretaceous thrusting juxtaposed these older terranes and disrupted the Jura­ Cretaceous clastic units. Very low-temperature high-pressure metamorphic assem­ blages, including lawsonite and aragonite, were developed during this event, and formed as a direct result of thrust-related burial to depths of about 20 km. Stratigraphic evi­ dence indicates that structural burial, metamorphism, and subsequent uplift back to the surface all occurred during a very short time interval, between 100 and 84 Ma, with average vertical transport rates of about 2 km/m.y. The Upper Cretaceous Nanaimo Group, a syn-orogenic basin to the north of the San Juan system, contains cobbles of metamorphosed rocks from the San Juan nappes, and therefore records the erosional unroofing of the thrust system. We envision the San Juan system to be a short-lived collision-like orogen, rather than a long-lived subduction complex. This conclusion is based primarily on the diver­ sity of rock units involved and the punctuated nature of the deformation. What remains unclear is the cause of this orogenic deformation. The clastic-rich Jura-Cretaceous units imply