The Geological Society of America Field Guide 9 2007 Tectonic evolution of the San Juan Islands thrust system, Washington E.H. Brown B.A. Housen E.R. Schermer Department of Geology, Western Washington University, Bellingham, Washington 98225, USA ABSTRACT The mid-Cretaceous San Juan Islands–northwest Cascades thrust system is made up of six or more nappes that are a few kilometers or less thick, up to one hundred kilometers in breadth, and that were derived from previously accreted Paleozoic and Mesozoic terranes. This fi eld trip addresses many questions regard- ing the tectonic evolution of this structural complex, including the homeland of the terranes and the process of post-accretionary dispersal that brought them together, how thrusting in the San Juan Islands might have been related to coeval orogenic activity in the neighboring Coast Plutonic Complex, and the origin of blueschist metamorphism in the thrust system relative to subduction and nappe emplacement. The geology of this trip has many counterparts in other outboard regions of the Cordillera, but some aspects of the tectonic processes, as we understand them to date, seem to be unique. Keywords: San Juan Islands, thrust faults, terranes, blueschist metamorphism, kine- matic analysis, paleomagnetism. INTRODUCTION San Juan Islands–northwest Cascades thrust system are poorly known and have been the focus of our recent work. Rocks and structures of the San Juan Islands of northwest Many aspects of the lithology, structure, and metamorphism Washington record a long and complex history related to Cor- are similar to the Mesozoic evolution of other parts of the Cordil- dilleran convergent margin tectonism. The area is underlain by lera; other aspects may be unique to the San Juan Islands. The east- the San Juan Islands–northwest Cascades thrust system, made west transect across the San Juan Islands during this fi eld trip will up of nappes a few kilometers or less thick and up to 100 km in highlight the different terranes juxtaposed by the thrust system, breadth (Figs. 1, 2), thrust onto the continental margin during and structures formed before, during and after high-pressure – mid-Cretaceous time (e.g., Misch, 1966; Brown, 1987; Bran- low-temperature (HP-LT) metamorphism. The trip builds on ear- don et al., 1988). The nappes have an oceanic history, indicat- lier work that identifi ed the main terranes and structures in the ing accretion to the edge of the North American continent, but San Juan thrust system (e.g., McClellan, 1927; Danner, 1966; they also bear clear evidence of interaction with the continen- Vance, 1975; Whetten et al., 1978; Brandon et al., 1988). Our tal margin long preceding their emplacement in Washington. recent results on structure, metamorphism, geochronology, and Their mid-Cretaceous arrival in Washington as thrust sheets paleomagnetism will provide a forum for discussions that bear was likely the consequence of some type of post-accretionary on the tectonic history and correlation with other Cordilleran fragmentation and dispersal. The timing and mechanisms of the terranes. We will compare and contrast units from the external, accretion, dispersal and fi nal emplacement of terranes of the unmetamorphosed parts of the thrust system to the more internal Brown, E.H., Housen, B.A., and Schermer, E.R., 2007, Tectonic evolution of the San Juan Islands thrust system, Washington, in Stelling, P., and Tucker, D.S., eds., Floods, Faults, and Fire: Geological Field Trips in Washington State and Southwest British Columbia: Geological Society of America Field Guide 9, p. 143–177, doi: 10.1130/2007.fl d009(08). For permission to copy, contact [email protected]. ©2007 The Geological Society of America. All rights reserved. 143 144 Brown et al. Figure 1. Regional setting of the San Juan Islands—northwest Cas- cades area in the northwest Cordillera. AX—Alexandria; BA—Baker CH terrane; CC—Cache Creek terrane of Miller (1987); CH—Chugach AX terrane; EK—Eastern Klamath terrane; FR—Franciscan complex; GV—Gravina belt; GVS—Great Valley sequence; H—Huntington 56 terrane; IZ—Izee terrane; MT—Methow basin; QS—Quesnellia; 135 SC–FR—Straight Creek–Fraser River fault; SF—Shoo Fly complex; ST—Stikinia; WA—Wallowa terrane; WJ—Western Jurassic belt; GV ST WR—Wrangellia ; WTrPz—Western Triassic and Paleozoic belt; AX YT YT—Yukon-Tanana terrane. Sources: Burchfi el et al. (1992a); Gehrels 128 and Kapp (1998); Wheeler and McFeely (1991). B.C.—British 56 AK Columbia; CA—Cali fornia; cc—Cache Creek belt; Cz—Cenozoic B.C. rocks and surfi cial deposits; ID—Idaho; mc—McCloud belt of Miller WR (1987); OR—Oregon; NV—Nevada; Wash.—Washington. CC QS cc C O A units that experienced subduction and HP-LT metamorphism. In S T particular, we would like to consider how the geology of the area ST mc P relates to various hypotheses regarding the origin and paleo geog- L U Yalacom raphy of the terranes, and the evolution of deformation before, 130 T O during, and after emplacement in their current location. 50 N MT fa u I lt lt C au C f TECTONIC SETTING O WR M FR P SC- L E QS The San Juan Islands–northwest Cascades thrust system lies X mc at the south end of the 1500 km long Coast Plutonic Complex, a belt of continental arc plutons and metamorphic country rock 50 116 that formed from Late Jurassic to Early Cenozoic (Figs. 1, 2). MT B.C. Fig. 2 QS Wash. Outboard of the Coast Plutonic Complex and intruded by it is CPC the Insular superterrane composed of the co-joined Wrangellia and Alexander terranes. Inboard of the Coast Plutonic Complex ID are rocks of the Early Cretaceous continental margin, including Cz the Methow stratigraphic sequence in Washington. Detritus, cur- rent indicators and stratigraphy in the Methow sequence indicate Columbia absence of an outboard sediment source until ca. 110 Ma (Ten- 46 Embayment WA nyson and Cole, 1978; Haugerud et al., 2002), thus we view the Cz BA locale of the Washington Cascades and San Juan Islands as an cc OR ocean basin until that time. Major orogenic activity characterizes IZ H the region from ca. 110–80 Ma, during which nappes of the San WJ Blue Mtns. 42 Juan Islands–northwest Cascades thrust system were emplaced, 125 cc the Coast Plutonic Complex was intruded by voluminous arc 200 kilometers TrPz plutons, and country rock of the complex was locally buried to W cc Klamath depths of up to 35 km (in the “Cascade crystalline core”; Figs. 2 EK Mountains 42 mc 116 and 3) and was deformed by orogen-normal and orogen-parallel FR CA displacements. Overlapping the waning stages of this orogenic NV No. pulse was development of the Nanaimo stratigraphic sequence, Sierra bearing detritus from the San Juan Islands–northwest Cascades cc SF thrust system as well as from the Coast Plutonic Complex, in GVS mc an elongate basin extending north from the San Juan Islands. In Eocene time the orogen was cut obliquely and displaced ~170 km (estimates range from 90 to 190 km; e.g., Vance, 1985; Misch, 1977) by the N-S dextral, strike-slip Straight Creek–Fraser River fault system. Restoration of the fault shows the San Juan Islands– northwest Cascades thrust system to have lain along the southern margin of Wrangellia and the Coast Plutonic Complex (Fig. 3) CPC 123 Q B.C. VC CH WA HZ NA NORTHWEST WR YA BP T ES 121 HS TB 49 A CZ NK LM Mt Baker OC R CO EA o s CH TS A' s GA YA L FC EA a NK k e EA LS f CH a u l SAN JUAN t LS CASCADES ISLANDS HH HH EM M D e D M l a F n g e EM Q WM B CRYSTALLINE e l t N s T PUGET SOUND WM CORE 88-96 Ma plutons 48 30 kilometers T t CW l u a f . R r TG e s CN a r F - . Windy k CN Pass C t Thrust h g i a MS r T t ING S EA A Figure 2 (on this and following page). San Juan Islands–northwest Cascades thrust system and surroundings. Based on compilation by Brown and Dragovich (2003) and references therein. Abbreviations given in Table 1. (A) Map. B.C.—British Columbia; WA—Washington. 146 Brown et al. A A' Mt Baker Twin Sisters Lummi window Shuksan Chilliwack Orcas Island Range Island YA thrust batholith GA VC NA OC FC YA FC TS CO LM CN CH EA SL EA EA BP CH HS TB ES TB NK ES OC BP CH BP CH CC NK WRANGELLIA depositional or intrusive contact 10 km Straight B fault contact no vertical exaggeration Creek fault Figure 2 (continued). (B) Cross section. Q in Late Cretaceous time. South of this orogenic complex is the U E S Columbia Embayment, an area covered primarily by Cenozoic Mid-Late NE LLIA volcanic rocks, thought to be underlain by primitive crust, and Cretaceous MT Plutons B.C. considered in some models to be a possible homeland for the WA Ros thrust system nappes (e.g., Davis et al., 1978; Vance et al., 1980). Jur. - s Lake East and south of the Columbia Embayment are accreted terranes HZ SESE Fault Zo of the Blue Mountains, and Klamath Mountains respectively E. Cret. N ne (Fig. 1), the latter especially bearing similarities to units of the WRANGE Georgia S Plutons San Juan Islands–northwest Cascades thrust system. CW HL Cascade N LLIA trai Crystalline STRUCTURAL STRATIGRAPHY NA t B.C. ING Core WA PRC NK EA EA WPT N The nappe pile of the San Juan Islands–northwest Cascades NWCS HB thrust system (Fig.