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. 2) is characterized by mid to late Paleozoic PACIFIC NAPPES MN OC terranes overlain by Mesozoic terranes. The structurally lowest EAN MELANGE component of the nappe complex is the East Sound Group in the N San Juan Islands and correlative Chilliwack Group in the Cas- CZ BELTS CZ cades. These are island arc derived sedimentary and volcanic rocks of Devonian–Permian age (Danner, 1966; Vance, 1975; Misch, SC-FR fault restored 1966; Tabor et al., 2003). Calc-alkaline Devonian plutonic rocks 100 KM presumed to be related to this arc are the Turtleback and Yellow Aster Complexes of the San Juan Islands and Cascades, respec- Figure 3. Regional geology shown with hypothetical restoration of the tively (Mattinson, 1972; Whetten, et al., 1978; Brandon et al., Straight Creek–Fraser River fault. (SC-FR fault) based on ~170 km of 1988; Tabor et al., 2003). This assemblage is likely related to arc displacement (e.g., Umhoefer and Schiarizza, 1996). Abbreviations: HB—Hicks Butte inlier, HL—Harrison Lake stratigraphic sequence, rocks that extend from California to northern British Columbia MN—Manastash Ridge inlier, PRC—Pacifi c Rim Complex, SE— and mark mid-late Paleozoic convergence along the continental Settler Schist. See Table 1 for other unit abbreviations. margin (McCloud belt of Miller, 1987). Higher in the nappe pile, in both the San Juan Islands and Cascades, is a disrupted section including Permian to Juras- The highest nappes in the San Juan Islands–northwest Cas- sic ribbon chert, Permian HP-LT schist, ocean island basalt, cades thrust system are Late Jurassic rocks that include ophio- Permian limestone bearing Tethyan fusulinids (exotic to North litic plutonic rocks, mid-oceanic-ridge basalt, ribbon chert, and America), and other materials (Fig. 2, Table 1). In the San Juan arc-derived mudstone-sandstone. Units of these upper nappes Islands, units are Orcas Chert, Deadman Bay Volcanics, and Gar- that we will examine include rocks in the Lopez fault zone, the rison Schist (Brandon et al., 1988), observed on this fi eld trip. In Constitution Formation, Fidalgo Ophiolite and Easton Meta- the Cascades, this zone is referred to as the Bell Pass Mélange morphic Suite. These units are closely similar to terranes in the and in addition to the above mentioned rock types includes the western Jurassic belt, Franciscan Complex and Coast Range 10 × 4 km Twin Sisters dunite slab (Tabor et al., 2003). Rocks Ophiolite of the Klamath Mountains and California Coast and structures of this zone are similar to the “Cache Creek belt” Range (e.g., Brown and Blake, 1987; Garver, 1988; Blake and of Miller (1987) that extends sporadically from northern British Engebretson, 1994; J.S. Miller et al., 2003). Columbia to California and apparently represents accretionary The nappe geometry portrayed in Figure 2B and described mélange of mainly oceanic rocks. above interprets the Cascades and San Juan Islands nappe piles TABLE 1. KEY TO UNITS Thrust system units EASTON METAMORPHIC SUITE (EA)—Late Jurassic ocean floor and trench deposits, well-recrystallized Early Cretaceous blueschist. FIDALGO COMPLEX (FC)—Late Jurassic arc-related ophiolite, minimal fabric, incipient prehnite-pumpellyite metamorphism. CONSTITUTION FORMATION (CO)—Late Jurassic trench deposits, minimal fabric, incipient blueschist metamorphism. LUMMI FORMATION (LM)—Late Jurassic ocean floor and trench deposits, penetrative fabric, incipient blueschist metamorphism. LOPEZ STRUCTURAL COMPLEX (LS)—Jurassic to Early Cretaceous ocean floor and trench deposits, incipient blueschist metamorphism. TWIN SISTERS DUNITE (TS)—Mantle-derived ultramafic tectonite. TURTLEBACK COMPLEX (TB) and correlative YELLOW ASTER COMPLEX (YA)—early to middle Paleozoic gabbro/tonalite, and paragneiss in YA, minimal fabric, amphibolite, greenschist and prehnite-pumpellyite facies metamorphism. GARRISON SCHIST (GA) and correlative VEDDER COMPLEX (VC)—ocean floor deposits, Permian epidote-amphibolite and blueschist metamorphism. ORCAS CHERT including DEADMAN BAY VOLCANICS (OC) and correlative BELL PASS MELANGE (BP)—Triassic-Jurassic chert, lesser oceanic-island basalt in OC and BP; exotic blocks of Early Cretaceous sandstone-argillite, Twin Sisters Dunite, Yellow Aster Complex, and Vedder Complex in BP; Garrison Schist and limestone with Permian Tethyan fusulinids in OC. EAST SOUND GROUP (ES) and correlative CHILLIWACK GROUP including Cultus Formation (CH)—Silurian to Jurassic island arc, McCloud fauna, minimal fabric, incipient blueschist metamorphism. NOOKSACK FORMATION (NK)—Jurassic to Early Cretaceous island arc possibly formed contiguous with Wrangellia. Slaty fabric, incipient prehnite- pumpellyite metamorphism. INGALLS TECTONIC COMPLEX (ING)—Early to Late Jurassic ocean floor and forearc or backarc–related ophiolite, prehnite-pumpellyite metamorphism and thermal aureole. Occurs east of the Straight Creek–Fraser River fault, but is correlative with the higher nappes in the thrust system.

Mélange belts HELENA-HAYSTACK MELANGE (HH)—serpentinite matrix, blocks of graywacke, mudstone, chert, basalt-rhyolite and 150–170 Ma gabbro-tonalite. WESTERN MELANGE BELT (WM)—scaly argillite matrix, blocks are mostly Late Jurassic–earliest Cretaceous lithic sandstone/siltstone, some 150–160 Ma gabbro-tonalite blocks. EASTERN MELANGE BELT (EM)—mostly meta-chert and greenstone, Devonian-Jurassic fossils, 165–190 Ma tonalite-gabbro, Permian Tethyan fusulinids.

Footwall units to the San Juan Island thrust system HARO FORMATION and SPIEDEN GROUP (HS)—Triassic to Early Cretaceous arc-derived sedimentary rocks, zeolite facies metamorphism. WRANGELLIA (WR)—Paleozoic arc and Triassic ocean plateau complex, microcontinent, zeolite facies metamorphism.

Cascade crystalline core, part of the Coast Plutonic Complex TONGA FORMATION (TG)—Early Cretaceous trench deposits and arc volcaniclastic rock, greenschist and amphibolite facies. CHIWAUKUM SCHIST (CW)—Early Cretaceous accretionary complex, Barrovian amphibolite facies metamorphism.

Overlap units NANAIMO GP. (NA)—Late Cretaceous epicontinental marine sedimentary rock, zeolite facies. CHUCKANUT FORMATION and related units (CN)—Eocene fluviatile sedimentary rock, virtually unmetamorphosed. 148 Brown et al. to be approximately at the same level and laterally contiguous. AGE OF THRUSTING This is based in part on correlations of terranes between the two regions (as shown in Fig. 2 and Table 1). The structural model The age of assembly of the nappes is uncertain because also assumes a simple in-sequence assembly of the nappe pile. observed structures could potentially have formed during one Because the stratigraphy is not exposed under the broad nappe of of many tectonic events, including initial accretion going back Easton Suite between the Cascades and San Juan Islands, out-of- to the Paleozoic for the older terranes, post-accretionary ter- sequence thrust models relating nappes in these two areas could be rane translation of at least hundreds of kilometers, emplace- viable. Cowan and Bruhn (1992) proposed that Cascades nappes ment of nappes into the regional geologic setting of north- lie at a higher structural level than those in the San Juan Islands. west Washington, and deformation related to the Eocene and McGroder (1991) favors a break in continuity of nappes in the younger fold and thrust belt affecting the Nanaimo Group hidden zone between the Cascades and San Juan Islands caused and Chuckanut Formation (England and Calon, 1991). Cer- by out-of-sequence thrusting and folding of the nappe pile. tainly some metamorphic fabric and possibly some fault Peripheral to the San Juan Islands nappe pile along its boundaries are inherited from events pre-dating assembly of northwest fl ank are the arc-derived Late Triassic Haro Forma- nappes in their present setting (Brown et al., 2005). However, tion, the Late Jurassic–Early Cretaceous Spieden Group, and the there is good evidence for major mid-Cretaceous assembly. Late Cretaceous Nanaimo Group bearing detritus from the thrust This deformation is referred here to as the thrusting event. system. These units lack evidence of HP-LT metamorphism and Nappes of the thrust system were emplaced and unroofed in penetrative tectonite fabric, and thus are considered to be “exter- the San Juan Islands vicinity by the time of deposition of the nal” to the thrust system (Brandon et al., 1988). Owing to the dif- Nanaimo Group (Vance, 1975); the oldest part of the Nanaimo ferent tectonic and metamorphic histories, a fault is assumed to known to bear detritus from the thrust system is ca. 85 Ma separate the nappe pile from the external units. This fault, named (latest Campanian-earliest Santonian; Brandon et al., 1988). A the Haro fault, cannot be directly observed, but is inferred to dip maximum age for thrusting in the San Juan Islands is given by under the nappe pile based on regional dips and a gravity survey fault juxtaposition of Late Aptian (112–115 Ma) fossiliferous (Johnson et al., 1986; Palumbo and Brandon, 1990). The Haro rock with 124 Ma HP-LT metamorphic rock on Lopez Island fault may have been reactivated during south-vergent thrusting in (one of our fi eld trip stops). In the Cascades, a population of the Cowichan fold and thrust belt (England and Calon, 1991). detrital zircons in the Nooksack Formation (footwall to the The ultimate footwall to nappes of the San Juan Islands– nappes) gives a maximum depositional age of 114 Ma, and a northwest Cascades thrust system is problematic in the San Juan large sandstone raft in the Bell Pass Mélange bears 119 Ma Islands, but clearer in the Cascades. Based on arguments given detrital zircons (Brown and Gehrels, 2007). above that external units underlie the San Juan Island nappes and More precise ages of thrusting are known for two localities: observation that Wrangellia underlies Nanaimo Group units on K-Ar whole rock ages of 87 and 93 ± 3 Ma were obtained for two Vancouver Island, one could infer that Wrangellia is basement mylonite samples from the west fl ank of the Twin Sisters Dunite to the San Juan Island nappes (e.g., Cowan and Bruhn, 1992). (Armstrong in Brown, 1987). Movement on the Windy Pass In the Cascades, evidence indicates that nappes are thrust over thrust is dated at ca. 94 Ma by relationships with U-Pb zircon- the southern end of the Coast Plutonic Complex. In the central dated plutons that predate, postdate and are involved in thrusting Cascades, the Ingalls Complex, a component of the San Juan (R.B. Miller et al., 2003). Thus, major displacement is broadly Islands–northwest Cascades thrust system, is thrust over Chi- bracketed between ca. 115 and 85 Ma based on youngest terranes waukum Schist and Mount Stuart batholith along the Windy involved and the age of rocks bearing detritus of the nappes, and Pass Thrust (Figs. 2, 3; Miller, 1985). In the northwest Cascades, a more limited time frame is suggested to be ca. 90–95 Ma from the relatively undeformed Jurassic-Cretaceous Nooksack Group dated rocks in two fault zones. which underlies the nappe pile (e.g., Misch, 1966) appears to be a southern extension of the Harrison Lake stratigraphic sequence in METAMORPHISM the southern British Columbia Coast Plutonic Complex (Fig. 3; Monger and Journeay, 1994). Along its western fl ank, the Coast Most units in the San Juan Islands–northwest Cascades Plutonic Complex is intrusive into Wrangellia. Thus, one inter- nappe pile show effects of Cretaceous HP-LT metamorphism. pretation for the regional structure is that Wrangellia and the The degree of recrystallization and metamorphic fabric devel- Coast Plutonic Complex constituted a structural block in mid- opment varies greatly, even within the same units. In the Cas- Cretaceous time that served as footwall to the San Juan Islands– cades, evidence of HP-LT metamorphism is found virtually northwest Cascades thrust system in both the San Juan Islands in all thrust system units of Jurassic or older age. The blue- and Cascades (e.g., Brown, 1987; McGroder, 1991; Monger and schist facies Easton Metamorphic Suite in the Cascades bears Brown, 2008). Other interpretations place nappes of the San Juan synkinematic metamorphic minerals dated at 120–130 Ma by Islands–northwest Cascades thrust system within, and as part of, K-Ar and Rb-Sr (Brown et al., 1982; Armstrong and Misch, the country rock of the Coast Plutonic Complex (Monger and 1987). Rock units younger than 120 Ma (Nooksack Forma- Journeay, 1994; Cowan and Brandon, 1994). tion and sandstone in the Bell Pass Mélange) lack defi nitive Tectonic evolution of the San Juan Islands thrust system, Washington 149

evidence of high-pressure metamorphism. In the San Juan TECTONIC EVOLUTION Islands, aragonite (Fig. 4) and lawsonite are widely devel- oped in Jurassic and older rocks that are otherwise relatively A number of features and arguments point to primary accre- unaltered (Vance, 1968; Glassley et al., 1976). This incipient tion and residence of terranes of the San Juan Islands–northwest HP-LT metamorphism has been considered to be related to Cascades thrust system along the continental margin prior to mid-Cretaceous thrusting (Brandon et al., 1988; Maekawa mid-Cretaceous assembly in the present nappe pile. As Brandon and Brown, 1991) but so far the only isotopic ages available, et al. (1988) note, the presence of detritus in sandstones from Ar-Ar muscovite, indicate metamorphism at 124 Ma (Brown diverse sources, including metamorphic rock, chert, and silicic et al., 2005) and ca. 137–154 Ma (Lamb, 2000), older than the arc volcanic rock (e.g., Constitution Formation) suggests prox- emplacement phase of thrusting. imity to a “continent-like” landmass. They also note that else- The age of blueschist metamorphism relative to thrusting where in the Cordillera correlatives of Paleozoic terranes of the is critical to understanding the tectonics of the thrust system. San Juan Islands–northwest Cascades thrust system (e.g., East If aragonite was formed during thrusting, burial on the order Sound Group) accreted long before the mid-Cretaceous. Addi- of 20 km is required at the ~200 °C temperature estimated for tional arguments and evidence are provided by the: (1) the Yellow metamorphism (Brandon et al., 1988), indicating a great thick- Aster Complex (Figs. 2 and 6; Table 1), a pre-Devonian terrane ness of overlying nappes. An alternative concept that blueschist with links to the continent indicated by beds of quartz arenite metamorphism in the thrust system is inherited from an event and a suite of detrital zircons that match those of the miogeo- predating nappe emplacement may be possible for the older cline (Brown and Gehrels, 2007); and (2) Permian blueschist terranes. However, Schermer et al. (2007) showed that HP-LT metamorphism in some units (Garrison Schist, Vedder Complex; metamorphism lasted during several phases of brittle deforma- Armstrong et al., 1983), indicating that these rocks were involved tion that followed juxtaposition of the internal San Juan Island in convergent margin tectonics long before thrusting in the San nappes, including the late Aptian Richardson rocks. If all of Juan Islands–northwest Cascades system. the HP-LT metamorphism in the San Juan Islands is related to Although terranes of the thrust system are similar to the same subduction zone, the time span of deformation and other outboard units of the Cordillera, especially those in the metamorphism in that subduction zone could be several tens Klamath Mountains with which they have been correlated of millions of years (at least from 124 Ma to some time after (see below), some aspects of the thrust system are unique. 112 Ma, but likely beginning earlier). The subduction zone The stacking sequence of the San Juan Islands–northwest model requires emplacement in the San Juan Islands vicinity Cascades thrust system is older on the bottom, younger on after HP-LT conditions ended, and on structures that are not top, approximately reversed from that generally understood exposed in the internal nappe pile (Schermer et al., 2007). Fig- for primary accretion, as in the Klamath Mountains where the ure 5 summarizes various interpretations of the age of meta- oldest rocks are on top (Irwin, 1981). The duration of assem- morphism relative to deformation. bly of the terranes is a few tens of millions of years at most,

1.0 mm

A B

Figure 4. Aragonite in the San Juan Islands. (A) Coarse aragonite from marble in the Orcas Chert unit, McGraw-Kittinger quarry, Orcas Island (Vance, 1977, p. 194). The sample shown is a single crystal exhibiting twin lamellae on a cleavage surface that extends across the entire speci- men. (B) Aragonite veins crossing foliation in the Constitution Formation, South Beach, San Juan Island. 150 Brown et al.

SW directed thrusting NW directed Brown, 1987 thrusting 87-90 Ma NW Cascades HP-LT 120-130 Ma

San Juan Is. NW directed thrusting

Maekawa & HP-LT Brown, 1991 penetrative cleavage

SW directed thrusting HP-LT Cowan & Brandon, 1994 local penetrative cataclasis cleavage

D1 D2 SW-NE NW contraction thrusting Bergh 2002 NW-SE strike-slip penetrative cleavage HP-LT

penetrative Brown et al., cleavage thrusting 2005 HP-LT 124 Ma

SW-NE contraction NW-SE extension emplacement Schermer in SJI et al, 2007 penetrative cleavage veins & brittle faulting HP-LT

remagnetization Burmester et al. 2000 remagnetization in the eastern SJI sometime during K normal chron rotation of SJI rocks after remagnetization

140 130 120 110 100 90 Ma 80

Figure 5. Interpreted sequence of deformational and metamorphic events in the San Juan Islands (SJI) thrust system presented in different reports. Absolute time of events is for the most part only loosely constrained in the reports referenced here. HP-LT—high-pressure–low-temperature.

much briefer than the ~300 m.y. period of accretion that built all the Paleozoic rocks. We are not aware of anywhere else the Klamath complex (Irwin, 1981). Cretaceous blueschist along the Cordillera that Paleozoic rocks are affected by Cre- metamorphism in the San Juan Islands–northwest Cascades taceous blueschist metamorphism. Thus, the building process thrust system affects not only Jurassic-Cretaceous Franciscan of the San Juan Island nappe pile is different than that under- type rocks as in the Klamath Mountains, but also apparently stood for other parts of the Cordilleran margin. Cretaceous Rocks Jurassic Rocks 30 177 Ma SPIEDEN GROUP 25 155 Ma Sentinal Island Fm.

70 er

20 fossil age

b 60

um r EASTON SUITE 15

N

be 50

40 10

Num 30 5 224 Ma

20 0 80 100 120 140160 180 200220 240 260 280 300 10 238 Ma 0 119 80 120 160 200 240 280 25 sandstone in 148 Ma 60 165 BELL PASS 20 MELANGE 50 FIDALGO COMPLEX er 15 40

ber Numb 143 um 30 10

N 233 20 5

10 237Ma 0 80 100 120 140 160 180 200220 240 260 280 300 0 80 120 160 200 240 280 3 ss in BPM 70 148 Ma 1 60 200 600 1000 1400 1800 2200 2600

50 LUMMI FORMATION 152 Ma

ber 40 30 125 um

N 30 TONGA FORMATION

20 ber 20 10 um

N 0 80 120 160 200 240 280 10 149 Ma

40 0 CONSTITUTION 80 100 120 140 160 180 200220 240 260 280 300

er 30 FORMATION 80 153 Ma 70 Numb 20 60 NOOKSACK

r GROUP 10 50 be

um 40

0 N 80 120 160 200 240 280 30 Ma 20 114 Ma

10 0 80 100 120 140160 180 200220 240 260 280 300 Early Paleozoic Rock Ma 25 1825 YELLOW ASTER COMPLEX 20

15

2069 2321 number 10 2528 1404 5 960 3316

0 800 1200 1600 2000 2400 2800 3200 Ma Figure 6. Detrital zircon age distributions in terranes of the San Juan Islands–northwest Cascades thrust system (Spieden Group from Housen and Fanning, unpublished; other units from Brown and Gehrels, 2007). 152 Brown et al.

Notwithstanding the important contributions of many previ- Islands, respectively, that they interpreted to indicate northwest- ous studies of the San Juan Islands–northwest Cascades thrust directed thrusting. Brandon et al. (1993) disputed this conclu- system, the homeland of the nappes and the tectonic process of sion for the San Juan Islands, suggesting that lineations mapped their transport and emplacement remain unresolved issues. Three by Maekawa and Brown (1991) are the product of differential published interpretations (Fig. 7) are: solution-mass-transfer, not thrusting. Cowan and Brandon (1994) (1) An orogen-normal contractional model in which the nappes described folds and Riedel shears in the Lopez and Rosario fault formed as continental borderland terranes that were caught zones that they interpret to indicate southwest transport of the in a collision zone between the offshore Wrangellian micro- nappes (orogen-normal). In the eastern San Juan Islands, Lamb continent and North America (Brandon and Cowan, 1985; (2000) reported northeast vergent (orogen-normal) isoclinal folds Brandon et al., 1988; Rubin et al., 1990; McGroder, 1991; dated by synkinematic mica at ca. 137–154 Ma (see above) in Burchfi el et al., 1992b; Cowan and Brandon, 1994; Monger rocks inferred to be related to the Easton Suite. Bergh (2002) and Journeay, 1994). observed folds, stretching lineations, and shear zones in the (2) A transcurrent-transpressional model in which the nappe ter- Lopez and Rosario fault zones supporting both orogen-normal ranes originally accreted or were deposited south (or north?) and orogen-parallel displacement and conceived the two-stage along the margin from their present location and then moved model described above and shown in Figures 5 and 7. Burmester coastwise, fi nally stacking up in a reentrant of the continen- et al. (2000) found that many of the rocks in question have been tal margin formed by the south end of Wrangellia (Brown, reoriented after acquiring their magnetization, which developed 1987; Maekawa and Brown, 1991; Brown and Dragovich, during or after the fabric was formed; therefore they suggested 2003; Monger and Brown, 2008). that the orientation of the fabrics cannot be used to determine (3) A two-phase model in which terranes were fi rst juxtaposed direction of transport in the present frame of reference. Brown by orogen-normal thrusting along the continental margin et al. (2005) determined that fabric in blueschist tectonite of the south of Wrangellia, and then underwent orogen-parallel Lopez fault zone predates thrusting and they suggested that much thrusting and strike-slip faulting (Bergh, 2002). of the kinematic analysis in the San Juan Islands has been carried Resolution of the emplacement history of the San Juan out on similar pre-thrust fabric and therefore may not be use- Islands–northwest Cascades thrust system is central to our ful in understanding emplacement of the nappes. Gillaspy (2004) understanding of mid-Cretaceous orogeny in the Pacifi c North- and Schermer et al. (2007) found that faults and extension veins west, including: the cause of crustal thickening and Barrovian indicate a protracted period of orogen-normal shortening coupled metamorphism in the crystalline core, the origin of the Nanaimo with orogen-parallel extension during aragonite metamorphism basin, and the confi guration of terranes along the North American that postdates thrusting, juxtaposition of the terranes, and pene- margin in the Early Cretaceous. On a broader regional scale, the trative fabric formation. The different interpretations are summa- San Juan Islands–northwest Cascades thrust system is relevant rized in Figure 5. To more effectively make use of these structural to understanding evolution of the 1500-km-long Coast Plutonic observations, the challenge for future workers is to understand Complex which extends from northwest Washington to Alaska. the age of outcrop-scale structures relative to the age of emplace- Based on their interpretation as orogen-normal contractional ment of the nappes. features, thrusts of the San Juan Islands and northwest Cascades have been correlated with thrusts in northern British Columbia Regional Considerations and Alaska and cited as evidence for a west-vergent thrust sys- tem that extends virtually the entire length of the Coast Plutonic Another strategy for establishing nappe displacements is Complex and has accommodated many hundreds of kilometers consideration of regional geology. Because units of the San of mid-Cretaceous shortening between the Insular superterrane Juan Islands–northwest Cascades thrust system bear evidence and North America (Rubin et al., 1990). of residence along the continental margin prior to emplacement in the present day setting, direct accretion of these rocks from Kinematics of Outcrop Scale Structures the west, the Pacifi c basin, seems improbable. Derivation of the nappes from the northeast is envisaged in the contractional One approach to understanding displacement of nappes in model of Brandon and Cowan (1985) and McGroder (1991) the San Juan Islands–northwest Cascades thrust system is kine- which invokes a root zone for the nappes along the northeastern matic analysis of outcrop scale structures. Such studies to date edge of the Cascade core in the approximate area of the Ross yield somewhat disparate results (Fig. 5). Brown (1987), working Lake fault zone (Figs. 2 and 3). In this view, during Wrangellia in the Cascades, reported a set of orogen-normal stretching linea- collision the nappes were driven to the southwest, riding over tions in the Easton Suite coeval with 120–130 Ma blueschist min- the Cascade core and the northeastern fl ank of Wrangellia. erals (see above). Younger orogen-parallel lineations were found Regional geologic features cited as supportive of this model are: in mylonite zones separating Cascades nappes (ca. 90 Ma, see coeval crustal thickening in the Cascade core suggesting thrust above). Smith (1988), and Maekawa and Brown (1991) mapped loading, contractional structures in the Cascade core, and inter- orogen-parallel stretching lineations in the Cascades and San Juan pretation that the Nanaimo Group was deposited in a foreland McGroder, 1991 Late Jurassic 98 Ma ST WR CPC MT QS WR CC SK NWCS CORE QS terranes 94 Ma A NWCS CH MT NK MT SK QS CPC NWCS terranes

Brown, 1987 90-95 Ma Early Cretaceous MT Plutons CORE WR CPC QS WR NK

NWCS B ? F

NWCS

F 100 km

E. to mid-Cretaceous Late Cretaceous CPC Bergh, 2002

WR CPC WR

F C area of EA San Juan EA Islands F

Lopez fault Lopez zone Rosario fault Rosario fault zone fault zone 100 km zone

Figure 7. Schematic drawings of three published models for tectonic evolution of the San Juan Islands–northwest Cascades thrust system (NWCS). (A) Contractional model of McGroder (1991). Terranes of the thrust system were formed in a basin between Wrangellia and the con- tinental margin. Convergence between these masses thrust the intervening terranes as nappes over the Cascade crystalline core (including the Skagit migmatite complex) and onto the eastern edge of Wrangellia, achieving orogen-normal shortening of some 400-500 km. (B) Transcurrent model of Brown (1987). Terranes of the San Juan Islands–northwest Cascades thrust system are interpreted to have accreted 100s of km south of their present site and south of Wrangellia. Blueschist metamorphism and orogen-normal fabrics were recorded in the Easton Suite. Post- accretionary displacement moved the terranes northward along the coast as a fore arc sliver, driven by dextral-oblique Farallon–North America convergence, until they collided with a reentrant in the continental margin formed by the south end of Wrangellia. (C) Two-phase model of Bergh (2002). Terranes of the San Juan Islands–northwest Cascades thrust system lay south of Wrangellia and developed orogen-normal contractional structures during the D1 phase in response to high-angle Farallon–North America convergence. D2 structures include NW and SE coastwise displacements as low-angle wedge extrusions caused by sinistral-oblique Farallon convergence. CPC— Coast Plutonic Complex; F—Farallon plate. Other abbreviations as in Fig. 1 and Table 1. 154 Brown et al.

basin caused by emplacement of San Juan Islands–northwest rocks had all been remagnetized during or after folding, and that Cascades nappes. However, several aspects of regional geology the predominantly normal polarity of the remagnetized directions pose problems for this interpretation. indicated to them that this remagnetization occurred during the (1) The contractional model invokes transit of nappes of the Cretaceous Long-Normal Chron (116–83.5 Ma). The remagne- San Juan Islands–northwest Cascades thrust system over tized directions from the San Juan Islands are scattered, how- the Cascade crystalline core (Fig. 3) at precisely the time ever, indicating that a signifi cant amount of rotation and/or tilt of great magmatic arc activity in that region. No rocks occurred after this remagnetization event. related to this arc activity are found in the San Juan Islands Paleomagnetic studies of the unmetamorphosed “external” or Cascades, except where nappes lap onto the southern units of the San Juan Islands have more promising results. The edge of the Cascade core in the vicinity of the Windy Pass exception is the Haro Formation; Hults and Housen (2000) have thrust (Figs. 2 and 3). found that these rocks were also remagnetized prior to folding, (2) Nappes of the San Juan Islands–northwest Cascades thrust despite their lack of any signifi cant metamorphism. system carry metamorphic aragonite acquired prior to (as The rocks of the Spieden Group have complex magnetizations, well as after) thrusting. Aragonite has been shown experi- with the majority of these clastic rocks having poorly resolved mentally to invert quickly to calcite outside its stability fi eld magnetizations. Dean (2002) found three magnetic components in at elevated temperature except under conditions of abnor- most of the Late Jurassic Spieden Bluff Formation samples, which mally low T/P, less than 10 °C/km (Carlson and Rosenfeld, yielded an inconclusive paleomagnetic fold test. The Early Cre- 1981). Transit of the thrust system nappes over the active taceous Sentinel Island Formation has a simpler, two-component arc would place them in a region of abnormally high T/P, magnetization in some of the rocks. Dean (2002) found that the precluding preservation of aragonite. second-removed component from the Sentinel Island Forma- (3) The elongate, orogen-parallel Nanaimo basin is fl anked not tion passes the inclination-only paleomagnetic fold test, with the by terranes of the San Juan Islands–northwest Cascades best-clustered inclinations occurring at 100% untilting. The mean α thrust system, but by plutonic rocks of the Coast Mountains. inclination of 64°, 95 = 7.8°, suggests an Early Cretaceous paleo- Thrust system terranes occur south along strike from the latitude of 46° N. Comparing this direction with that expected for Nanaimo (Figs. 2 and 3), and thus the basin is not likely a the present-day location of Spieden Island calculated from a stable consequence of nappe loading. North America reference pole (Housen et al., 2003), a latitudinal Many workers have envisaged a southerly origin of some translation of 1500 ± 1000 km is estimated for these rocks. or all of the terranes of the San Juan Islands–northwest Cas- The Nanaimo Group has been the subject of extensive paleo- cades thrust system, in the Columbia embayment, Klamath magnetic study, primarily from outcrops in the Canadian Gulf Mountains, or California Coast Range (e.g., Davis et al., 1978; Islands (Ward et al., 1997; Enkin et al., 2001; Kim and Kodama, Vance et al., 1980; Brown and Blake, 1987; Garver, 1988; Burch- 2004), with limited work from Orcas Island (Housen et al., fi el et al., 1992b). Davis et al. (1978) and Vance et al. (1980) 1998). All of these studies have found that most Nanaimo Group proposed that the Mesozoic ophiolitic terranes of the San Juan rocks have poorly defi ned magnetizations (~60% “failure rate” Islands–northwest Cascades system formed in a “pull-apart gap” reported for most sample collections). However, a signifi cant in southeastern Oregon and subsequently moved northward and number of samples in all of these studies (a few 100 out of ~1000 were obducted onto the continent. Geologic features cited in sup- samples collected) have well-defi ned magnetizations that pass a port of the model are: (1) thrust emplacement of the Ingalls ophio- reversals or fold test. Studies of inclination error, notably Kim lite over the south edge of the Cascade core, (2) absence from and Kodama (2004), suggest that inclination error in these sedi- eastern Oregon and western Idaho of some continental margin ments is moderate (8–10°), and that when corrected for the paleo- terranes that are part of the Mesozoic assemblage to the north magnetic inclinations in these rocks place the Nanaimo Basin at and south along the Cordillera, and (3) Sr isotope ratios and seis- a paleolatitude of 41° N during Campanian-Maastrichtian time. mic velocities indicating primitive crust underlying the Columbia Using a Late Cretaceous North American reference pole for embayment. More recent geophysical evidence for a deep crustal comparison, a translation of 1600 ± 900 km is indicated for these rift in the Columbia embayment is a linear break in the gravity rocks since ca. 75 Ma. fi eld running along the southern margin of the embayment (Riddi- Related constraints on the Late Cretaceous paleogeography hough et al., 1986). of the San Juan Islands also come from paleofaunal data from the Nanaimo Group rocks. Kodama and Ward (2001) argued Paleomagnetic and Other Constraints of Paleogeography that the lack of rudistid bivalves in the otherwise well-preserved paleofauna of the Nanaimo Group can be used to constrain the Paleomagnetic studies of the rocks in the San Juan Islands paleolatitude of these rocks. Rudistids are tropical to subtropi- have had mixed success in constraining their tectonic history, cal reef forming bivalves, and are common in a number of Late with the main complication being an extensive remagnetization Cretaceous marginal basin rocks from Baja California to Central that has affected all of the “internal” units that have experienced California. Using estimated locations of rudist-bearing basins, high P-T metamorphism. Burmester et al. (2000) found that these and the locations of anoxic black shales (Marca Shale) that mark Tectonic evolution of the San Juan Islands thrust system, Washington 155 the presence of a cold-water upwelling zone along the ancient California margin, Kodama and Ward (2001) suggested that the Nanaimo Group rocks were located at or north of the location 140 of the Moreno Basin (central California, 42° N reconstructed paleolatitude) at 75 Ma. Some additional support for this con- Aleut D F ian - KA straint comes from the recognition of a marine reptile fauna from W ra n Nanaimo Group rocks on Vancouver Island, which share some g ALAS provinciality with the marine reptile fauna of the Moreno Forma- e ll tion from central California (Nicholls and Meckert, 2002). Another set of data, detrital zircon age distributions, has also been used to test paleogeographic constraints on the location of the Nanaimo Group rocks. Mahoney et al. (1999) used the pres- ence of several Archean-aged zircons to indicate that the Nanaimo Group rocks had been located no more than 500 km south of its P-MF present-day location, during Late Cretaceous time. Using the same set of data, Housen and Beck (1999) compared variations in the 50 mm/yr detrital zircon age distributions as a function of stratigraphic posi- tion within the Nanaimo Group. They argued that variations in Yakutat Protero zoic-aged zircons support a source of detritial zircons from terrane, the Mazatzal and Yavapai orogens in southwest North America, and transform that northward migration of the Nanaimo Basin during its deposi- displacement tion was consistent with other paleomagnetic evidence, and plate North 60 motion estimates. The analyses of Kodama and Ward (2001), and Kim and Kodama (2004) also supported the conclusion of Housen America and Beck (1999), that the Nanaimo Group reached the “moder- plate

ate” paleolatitude of ~43° N at 75 Ma, consistent with the so-called F-QCF “Baja-BC” (Baja–British Columbia) hypothesis. Pacific Taken together, these paleogeographic data would be most plate consistent with the “Klamath origin” models discussed above. Complicating this correlation, however, are the proposed ties between the San Juan Islands rocks and Wrangellian or North Cascades basement, as abundant paleomagnetic data from strati- 120

fi ed rocks of Wrangellia/Insular affi nity (Wynne et al., 1995,

Enkin et al., 2003), or barometrically corrected plutonic rocks C (Housen et al., 2003) both indicate more southerly paleolatitudes a s (36 N, and 3000 ± 700 km of translation) for these units during 300 KM c

a

mid-Cretaceous time (93–88 Ma). LRF d

e

5

Modern Analogues? Juan a CAN. 0

r de Fuca c U.S.

plate Modern tectonic regimes along the western North American 8 mm/yr margin (Fig. 8) that serve as possible analogues for emplacement of the San Juan Islands–northwest Cascades thrust system via coastwise movement are collision zones formed by northward displacement of: (1) Siletzia against the south end of Wrangellia Siletzia, (e.g., Wells et al., 1998), and (2) the Yakutat terrane against the fore-arc displacement southeast corner of Alaska in the Saint Elias orogen (Plafker et al., 1994). Siletzia lies in the Cascade forearc, driven by a combina- Figure 8. Modern-day analogues of orogen-parallel thrusting in the tion of oblique plate convergence and Basin and Range extension Pacifi c Northwest. F-QCF—Fairweather-Queen Charlotte fault, (Wells et al., 1998). Seismic refl ection allows identifi cation of DF—Denali fault, LRF—Leech River fault. References: Plafker et al. (1994), Wells et al. (1998); Bruhn et al. (2004). Siletzian rocks under Wrangellia to depths of 15–20 km along shallow to moderately north-dipping faults (Clowes et al., 1987). Total northward displacement is not known, but Beck (1984) suggested paleomagnetic discordance indicates as much as 300– 156 Brown et al.

400 km. The current rate of arc-parallel transport is 6–8 mm/yr at ever, caution must be exercised to avoid a nasty fall on the slick the northern end of the terrane (Wells and Simpson, 2001). seaweed-covered rocks that may be present. Please pay attention to The Yakutat terrane is moving north along the Fairweather– the fi eld trip guides as the departure time draws near, to ensure you Queen Charlotte transform fault at 45–50 mm/yr relative to are on the vessel, and the trip can run in a safe and timely fashion. North America (Plafker et al., 1994; Bruhn et al., 2004). At the After we have fi nished the Stuart Island stop, participants will re- corner area in southern Alaska where plate interaction changes embark for a ~45 min trip to Spieden Island. from transform to convergent, the Yakutat terrane is colliding with the continent (Fig. 8). A north-dipping Benioff zone and the Stop 1-1. Fossil Cove, Stuart Island, Nanaimo Group Wrangell magmatic arc in this region both testify to signifi cant (Fig. 10) subduction of the Yakutat terrane (and probably other materials). The convergent zone is marked by a thin-skinned accretionary The Nanaimo Group comprises a set of 11 formations, ranging complex of Cretaceous and younger rocks displaced northward from Turonian to Maastrichtian in age, composed of clastic marine on gently to moderately dipping thrust faults (Bruhn et al., 2004). and deltaic sedimentary deposits (Fig. 10). The ages of these rocks Displacements are strongly partitioned between strike-slip faults are constrained by biostratigraphy (e.g., Haggart, 1994), and mag- and thrusts. Both analogues are characterized by low-dip thrusts netostratigraphy (Enkin et al., 2001). These rocks were deposited accommodating margin-parallel displacement indicating that in a large marginal basin, extending ~175 km from its southern- such structure, as possibly fi ts the San Juan Islands–northwest most extent in the San Juan Islands to its northernmost extent on Cascades thrust system, is not a tectonic anomaly. Vancouver Island. The Nanaimo Group contains several elements that are of tectonic interest. Structurally, the Nanaimo Group FIELD TRIP GUIDE rocks (along with the Paleocene-Eocene Chuckanut Formation) are folded as part of the Cowichan fold and thrust belt (England The fi eld trip guide begins at Friday Harbor, San Juan Island and Calon, 1991; see also Mustoe et al., this volume, and Blake (Fig. 9). Before departing, be certain that you have brought along and Engebretson, this volume). One of the primary constraints on warm clothes, raingear, and good fi eld boots. the age of uplift and thrusting of the metamorphosed “interior” Please do not use rock hammers or collect specimens any- domain of the San Juan Islands is the presence of metamorphosed where on this trip unless specifi cally advised. sandstone clasts interpreted as being derived from the Constitution Forma tion that are found in conglomerates of the Extension Forma- DAY 1 tion of the Nanaimo Group on Orcas and Stuart Islands (Brandon et al., 1988). On a larger scale, age distributions of detrital zircons Day 1 is spent primarily on the terranes “external” to the San (Housen and Beck, 1999; Mahoney et al., 1999), paleomagnetism Juan Islands thrust system. These units are the Haro Formation, (Ward et al., 1997; Housen et al., 1998; Enkin et al., 2001; Kim and Spieden Group, and Nanaimo Group. They broadly overlap in age Kodama, 2004), and fossil assemblages (Kodama and Ward, 2001) with rocks in the nappe pile but are distinguished by their absence have been used to evaluate possible large-scale displacements of of, or very low-grade (zeolite facies), metamorphism, and, in the the Nanaimo Group rocks. case of the Spieden and Nanaimo Groups, an absence of penetra- On Stuart Island, the turbidites and sandstones of the Haslam tive tectonite fabric. These units are important to understanding the Formation, the conglomerates of the Extension Formation, and younger portion of the tectonic history of the San Juan Islands. the sandstones and siltstones of the Pender Formation can be The fi eld trip will begin with a drive from Friday Harbor across found (Fig. 10). A stop at Fossil Cove, on the NW end of Stuart San Juan Island to picturesque Roche Harbor, on the northern end Island (a boat trip of ~45 minutes), allows for examination of the of San Juan Island. We will depart from the boat ramp at Roche bedding and sedimentary structures in these rocks, as well as the Harbor, taking a chartered craft to Stuart and Spieden Islands. many fossils (primarily Inoceramus). Time permitting, we may We will be landing on public access beach areas, but please note stop at a beach where the Extension Formation crops out, in order that only the intertidal zone in these areas is considered to be pub- to examine the conglomerate clasts of this interesting unit. lic property, and that the uplands are privately owned. Access to Spieden Island in particular is restricted by its owner. Stop 1-2. North Shore Spieden Island, Spieden Group (Fig. 11) Directions and Other Instructions Before departing on the Humpback Hauling vessel, be certain Spieden Island is one of the largest (perhaps the largest) pri- that you have brought warm clothes and your lunch. Even if the vately owned island in the San Juan archipelago. It has a color- weather appears to be sunny, raingear is recommended. A lifejacket ful history, most notably as “Safari Island,” when in the 1970s (provided on the vessel) is required at all times, and please do not a group of investors purchased the island with the bright idea forget yours on the beach. If you are prone to seasickness, please of transforming it into a private exotic game hunting reserve. take appropriate precautions. The vessel has a landing-craft type The island was stocked with many species of exotic game ani- ramp, so we will be able to disembark on relatively dry land. How- mals (mostly Asian and African deer, goat, sheep, and antelope Kn 75 ORCAS Kn Kn ISLAND 70 70 65 Kn 1-1 Kn 55 84 Kn STUART 30 ISLAND

Pe Pt 1-2 JKs SPIEDEN ISLAND o fault ar Trh H O 1-3 r ca s t TrJo hrust Pt TrJo Pt TrJo

Ro sa JKc rio TrJo th Jc ru st Jc SAN JUAN R ISLAND o ch e o H rb r r a d averton V . e all B ey rd. SHAW Friday Harbor ISLAND

PTrd

San Juan Vall le Ave ey rd. y

Arg Lime Kiln Point Pg Jc

W es 1-4 t Si t d l e r d. Bailer Hill rd. u . a f

TrJo rd int y Kn = Nanaimo Group a

JKs = Spieden Group Po le B

tt k JKl = Lopez Structural Complex Ca c Jc = Constitution Formation u B TrJo = Orcas Chert Trh = Haro Formation N American

Camp . JKl PTrd = Deadman Bay Volcanics n Cattle Pg = Garrison Schist L t's ket Point 2-1 ic Pe = East Sound Group P 2.0 km Pt = Turtleback Complex 2-2

Figure 9. Map of San Juan Island and vicinity. Solid circles locate fi eld trip stops. Sources are Brandon et al. (1988) and Burmester et al. (2000). 158 Brown et al.

74 Turn Point 70 50

65 Kne

78 50 48 34 65 Kne 85 Knh 62 Prevost 28 64 76 80 Harbor Satellite 83 Knh 48 Fossil Kne 83 76 55 Island 78 Cove 72 72 81 63 50 58 71 83 Stop 1-1 65 Knp 64 62 71 Knp 56 Knh A 53 65 70

Nanaimo Group: 56 70 54

75 75 Knp: Pender Fm 55 Kne: Extension Fm 61 50 Reid Harbor 61 Knh: Haslam Fm Knp Knp 45 Kne Kne 63 50 Stuart Island 32

14 N 22 55 26 scale 1 km Figure 10 (on this and following page). Geology of Stuart Island, from Mercier (1977). (A) Geologic map. 24 22

species). Needless to say, the concept of hunting exotic game in trip back to the Roche Harbor boat ramp, where the seaborne por- the midst of an ecological paradise did not work out; the island tion of this trip will end. reverted to Spieden Island, and the descendants of the surviving After leaving the Roche Harbor boat ramp, we will drive to creatures can be seen cavorting around the island today. Davidson Head, parking on the shoulder of the road at the “neck” Geologically, Spieden Island, and nearby Sentinel Island, are of the head. We will then walk northwest along the beach, exam- the only known occurrences of the late Jurassic–early Cretaceous ining the exposures of the Haro Formation in the intertidal zone. Spieden Group. The Spieden Group is composed of two forma- Fans of fresh oysters will be certain to notice the abundant (likely tions, the Oxfordian-Kimmeridgian Spieden Bluff Formation, and seeded) oysters present on the Haro Formation outcrops. the uppermost Valanginian Sentinal Island Formation (Fig. 11). The ages of these units are constrained by biostratigraphy Directions to Stop 1-3 ( McClellan, 1927, Haggart, 2000), primarily via fossils of From Roche Harbor waterfront, drive southwest on Reuben Buchia. The rocks of both formations are clastic sediments, with Memorial Drive. fi ner-grained turbidite deposits characterizing the Spieden Bluff 0.2 mi Go left on Roche Harbor Road. Formation, and volcaniclastic-rich sandstone, mudstone, and con- 0.9 Go left (NW) on Afterglow Drive. glomerates characterizing the Sentinel Island Formation. The rocks 1.8 Neck of Davidson Head; park on gravel shoulder on also display some soft-sediment deformation features; some have a right side of road. very weak anastomosing scaly cleavage, and have been folded. Our fi eld trip stop will be located on a wave-cut bench, Stop 1-3. Davidson Head, San Juan Island, Haro Formation exposed at low tide, on the north shore of Spieden Island. Here we will see outcrops of both formations, and localities that dis- The north shore of San Juan Island is home to one of the play the locally abundant macrofossils. We will have ~30 min most geographically restricted units in the San Juan Islands—the at this location; please follow the instructions of the trip leaders Late Triassic (Norian) Haro Formation. This unit crops out on closely. After we re-embark, the vessel will take us on a ~40 min Davidson Head, and is a 700-m-thick mixed volcaniclastic unit. Tectonic evolution of the San Juan Islands thrust system, Washington 159

B

Figure 10 (continued). (B) Composite stratigraphic section of Nanaimo Group units exposed on Stuart Island. In these fi gures, the Pender Formation is referred to as the Ganges Formation, a now superceded formation name.

The Haro Formation has only experienced zeolite facies meta- Directions to Stop 1-4 morphism, and thus the contact between the Haro Formation and 0.0 Return on Afterglow drive to Roche Harbor Road; the high-P, low-T metamorphic rocks immediately to the south reset odometer and go left (east-southeast). of Davidson Head represents a fundamental structural boundary 1.3 mi Go right (south) on West Valley Road. in the San Juan Islands. This contact is nowhere exposed, but is 2.8 Go right (west) on Mitchell Bay Road. inferred to be a thrust fault (the Haro Thrust), based primarily 5.6 Go left (south) on Westside Road. on the large-scale structural architecture of the San Juan–north 9.7 Turn in at entrance to Lime Kiln Point Park and follow Cascades nappes (e.g., Brandon et al., 1988). trail to coast. Spieden Island A 65 80 63 42

U 10 U D 5 D Sentinel Island 45 33 15 22 24 Lower Cretaceous Sentinel Island Formation U upper member D lower member N Upper Jurassic Spieden Bluff Formation upper member 2000 200 400 lower member faults meters anticline strike/dip contour interval = 10 meters

B AGE GROUP FORMATION MEMBER THICKNESS LITHOLOGY DESCRIPTION

not exposed 600 m + sandstone Haurterivian Upper member Spieden Group Early Cretaceous conglomerate and minor Sentinel Island Formation Crudely stratified volcanic Crudely stratified

Unconformity MassiveMassive and thinly and bedded thinly fossiliferousbedded sandstone fossiliferous and siltstone sandstone and siltstone 140140 m m

Lower Unconformity member Massive and thinly Valanginian

Lower member Lower bedded fossiliferous 20 m sandstone and siltstone Massive and crudely Upper member stratified volcanic breccia and conglomerate, Late 80 m Jurassic minor sandstone, Formation Lower Oxfordian or not exposed siltstone and tuff member Spieden Bluff Kimmeridgian

Figure 11. Geology of Spieden and Sentinel Islands, after Johnson (1981) and Dean (2002). (A) Geologic map. (B) Schematic section of the Spieden Group. Tectonic evolution of the San Juan Islands thrust system, Washington 161

Stop 1-4. Lime Kiln Point State Park, San Juan Island on fusulinids, conodonts and radiolaria. The Tethyan fusulinids (Fig. 12) link this unit to the “Cache Creek belt” of mélanged oceanic rock extending along the Cordilleran margin from California to north- Lime Kiln Point is a famous venue for orca whale spotting. ern British Columbia (Miller, 1987). The limestone is largely For geologists, the locality is important for its exposures of lime- recrystallized to aragonite marble (Vance, 1968). stone that bears Permian Tethyan Fusulinids, known to have The Deadman Bay Volcanics are separated from the over- grown at a tropical latitude and suggesting large displacements lying Orcas Chert unit by an east-dipping thrust fault; however of the terrane (Danner, 1966, 1976; Monger and Ross, 1971). these two units are regarded by Brandon et al. (1988) as parts of The limestone occurs as layers and irregular masses within a a single terrane based on their mutual similarity of age, lithology, sequence of ocean island pillow basalt fl ows and breccias, named and chemical signature of ocean island basalts. the Deadman Bay Volcanics (Brandon et al., 1988). The age of the Return to Friday Harbor via West Side Road and Bailer Hill unit as a whole ranges from Early Permian to Late Triassic based Road (Fig. 9).

Figure 12. Geology of Lime Kiln Point, reproduced from part of Fig. 9 in Brandon et al. (1988). 162 Brown et al.

DAY 2 Brandon (1994) applied a “symmetry based statistical analy- sis” of asymmetric folds and Riedel shears, concluding that the On day 2, we will examine the Rosario and Lopez fault structures formed by southwest thrusting. Bergh (2002) divided zones on San Juan and Lopez Islands, two of the major structures structures into an early set of folds, foliation and lineations in the San Juan Islands thrust system. related to southwest contraction (D1), and a later set of lineations and shears (D2) formed by northwest displacement as exhibited Directions to Stop 2-1 at this locality (Fig. 14B). 0.0 Intersection of Argyle Ave and Spring Street in Friday The Rosario thrust at this locality dips northeast. Footwall to Harbor; head south on Argyle Ave. the thrust is the Triassic-Jurassic Orcas Chert which is dominantly 1.0 mi Beginning of Cattle Point Road. composed of ribbon chert with lesser pillow basalt, mudstone, 7.1 Turn right (south) on Pickett’s Lane in American and limestone (Vance, 1975). In the hanging wall is the Late Camp Park. Jurassic Constitution Formation, mostly composed of volcanic- 7.6 Go right (west) on Salmon Banks Road (dirt road). rich graywacke sandstone. The fault at this locality (mapped in 7.9 End of road; park. detail by Brandon et al., 1988) is marked by an imbricate zone ~100 m wide bearing lenses and rafts of ribbon chert, sandstone, Stop 2-1. South Beach, American Camp National Park, mudstone, greenstone, and most signifi cantly HP-LT greenschist- San Juan Island (Fig. 13) amphibolite of the Permian Garrison Schist unit. Amount and timing of displacement on the Rosario thrust are Americans and British disputed the boundary between their diffi cult questions. Vance (1975) noted that the overlying Consti- respective territories in the early 1800s and set up military camps tution Formation bears detritus that appears to be derived from on San Juan Island, which both sides claimed. War nearly broke the underlying Orcas Chert, Garrison Schist, and Deadman Bay out in 1859 when an American settler shot a pig belonging to Volcanics and proposed that the contact is an unconformity. The the British Camp. The international boundary dispute was fi nally imbricate structure and inclusion of the Garrison Schist in the resolved by arbitration in 1872, in favor of the Americans. deformation zone, however, suggested a fault of large displacement This part of the Rosario thrust, well exposed at the water’s to Maekawa and Brown (1993) and Cowan and Brandon (1994). edge, has been a key locality for interpretations of San Juan Islands structural evolution (summarized in Figs. 5 and 7). Directions to Stop 2-2 Maekawa and Brown (1991) observed shear zones with fault 8.7 mi Retrace route to Cattle Point Road. Turn right (east) drag and northwest trending lineations at this locality and sug- and drive to Cattle Point. gested dominantly northwest thrusting (Fig. 14A). Cowan and 10.8 Parking for Cattle Point.

Figure 13. Bedrock geology of the South Beach area, American Camp, reproduced from Brandon et al. (1988). Legend as in Fig.12. Figure 14. Structural analysis of fabrics in the Rosario Thrust at South Beach, San Juan Island by (A) Maekawa and Brown (1991), and (B) Bergh (2002). These interpretations are in mutual agreement, indicating northwest thrusting. Cowan and Brandon (1994) interpret these structures to be part of a pattern of Riedel shears that together with fold orien- tations statistically indicate southwest-vergent thrusting (i.e., toward the viewer with respect to Fig. 14B). 164 Brown et al.

Lopez Structural Complex 1991; Cowan and Brandon 1994; Bergh, 2002) (Fig. 15). These structures are subparallel to the northern boundary of the Lopez At Cattle Point and subsequent stops on Lopez Island, we Structural Complex, the Lopez fault, where most of the offset is will see rocks and structures of the Lopez Structural Complex thought to have occurred (Brandon et al., 1988) (Fig. 15), one of the major fault zones in the San Juan thrust Recent structural analysis of the Lopez Structural Complex system (Brandon et al., 1988). The Lopez Structural Complex (Gillaspy, 2004; Schermer et al., 2007), reveals a sequence of is an ~2.5 km wide imbricate zone composed of northwest- events that provide insight into accretionary wedge mechanics elongated, relatively coherent lenses separated by sheared and regional tectonics. After formation of regional ductile fl at- mudstone-rich fault zones. The large lenses are predominantly tening and shear-related fabrics (the thrusts and strike slip faults ocean fl oor clastic and volcanic rocks and Constitution terrane illustrated in Fig. 5), the area was crosscut by brittle structures sandstone; smaller lenses include Turtleback terrane and exotic including: (1) southwest-vergent thrusts, (2) extension veins and material not found elsewhere in the region. The magnitude of normal faults related to northwest-southeast extension, and (3) offset along the Lopez Structural Complex is unknown, but conjugate strike-slip structures recording northwest-southeast the inclusion of exotic material such as the Early Cretaceous extension and northeast-southwest shortening. Aragonite-bearing Richardson complex (stop 2-3) suggests tens of kilometers of veins are associated with thrust and normal faults, but only rarely movement similar to other terrane bounding faults in the San with strike-slip faults (Fig. 16). High-pressure low-temperature Juan Islands (Brandon et al., 1988; Cowan and Brandon, 1994). (HP-LT) minerals constrain brittle deformation to have occurred Foliation and fault contacts in the Lopez Structural Complex at ≥20 km and ~200–300 °C. The presence of similar structures dip moderately to steeply northeast (Maekawa and Brown, elsewhere indicates the brittle structural sequence is typical of the

lt u a F N y a B ?

k c u B t 43 LO 39 P Const. EZ 40 0 12 km Terrane FA 50 U L 55 Shark 28 T 47 Reef 40 Fidalgo Complex Cattle Pt. Stop 2-2 51 55 57 Richardson Ocean Floor Stop 2-3 70 45 Complex (undivided) John's Pt. 69 68 74 55 ? ? Watmough 49 75 64 Head Ocean Floor Complex 60 45 ? Stop 2-4 62 Clastic sequences 70 47 55 56 Point Iceberg Pt. ? Volcanic rocks and 60 t Colville associated sedimentary rocks "Exotic" and Other slices Approximate Constitution Terrane Richardson Basalt Complex fault contact Sandstone with Major terrane t Turtleback Complex chert and volcanic rocks boundary Mudstone-rich assemblages Imbricate Zones 55 Strike and dip of foliation Figure 15. Generalized geology and terrane map of the Lopez Structural Complex. Open circles show locations of fi eld trip stops 2-2 (Cattle Point), 2-3 (Richardson) and 2-4 (Iceberg Point). Modifi ed from Brandon et al. (1988), Burmester et al. (2000), and M.C. Blake (2000, written commun.). Eastern extension of Lopez thrust (from Brandon et al. 1988) may not coincide with a terrane boundary (from Schermer et al., 2007). Tectonic evolution of the San Juan Islands thrust system, Washington 165

? Aragonite found N A * in vein sample

LO P Lopez Island Const. EZ

Terrane * FA 0 12 km U San Juan Island LT ** Fidalgo Complex * * Ocean Floor * * Complex * * ? 100 * * ? B 6 * * 8 * * 80 8 * * ? ** 13 6 * 60 % 40

20 1

0 Deformed Early Thrusts Extension Normal Late Veins Shear Veins Faults Strike-slip Veins Faults n = 12 n = 10n = 7 n = 22 n = 10 n = 8

Figure 16. (A) Map of the Lopez Structural Complex illustrating locations of samples containing aragonite. See Fig. 15 for rock types. (B) Bar chart showing percent occurrence of aragonite in vein carbonate samples classifi ed by structure type. Deformed veins are veins shortened perpendicular to the solution mass transfer cleavage; early shear veins are reactivated cleavage planes (D2); other structures cut the cleavage, generally at high angles. Number below each bar is total number of samples; number above each bar is number of samples with aragonite. From Schermer et al. (2007).

San Juan Island nappes, at least for the Constitution and struc- Figure 7, would be unlikely to have associated HP-LT metamor- turally higher terranes. Sustained HP-LT conditions are possible phism or along-strike (NW-SE) extension. It is possible that the only if structures formed in an accretionary prism during active unexposed Haro fault (stop 1-3) is one of the main emplacement subduction, suggesting brittle structures record internal wedge related structures, but the timing and kinematics of this fault are deformation at depth and early during uplift of the San Juan poorly understood. Island nappes. The structures are consistent with orogen-normal shortening and vertical thickening followed by vertical thinning Stop 2-2. Cattle Point Park, San Juan Island (Figs. 9, 15) and along-strike extension. The change in vein mineralogy indi- cates exhumation occurred prior to the strike-slip event. The P-T At Cattle Point, highly sheared mudstones with disrupted conditions, and spatial and temporal extent of small faults associ- and elongated sandstone beds and clasts form a NW-striking, ated with fl uid fl ow suggests a link between these structures and steeply dipping shear zone adjacent to less-deformed coarse the silent earthquake process. grained sandstones and chert-pebble conglomerates. We will Given that these latest identifi ed structures likely formed in examine early ductile and late brittle deformation. In the sheared an accretionary wedge setting, we are faced with the dilemma mudstone, which is interpreted by Bergh (2002) to contain a of not having found the Late Cretaceous structures related to composite S1-S2 fabric, there is evidence of NW-SE shearing, emplacement in northwest Washington. These emplacement interpreted as top to the NW thrusting by Maekawa and Brown structures, if they formed by any of the models illustrated in (1991) and sinistral reactivation of SW-vergent thrusts by Bergh 166 Brown et al.

(2002). Strain in the early thrusting event(s) is strongly parti- 2.1 mi Turn left (east) on Fisherman Bay Road. tioned into the mudstone-rich units, as seen here and throughout 2.3 Go right (south) on Center Road. the Lopez structural complex. Foliation in the sandstone unit is 7.7 Turn right (west) on Lopez Sound Road. dominated by pressure solution and volume loss (Feehan and 7.9 Turn left (south) on Richardson Road continue south Brandon 1999). The later brittle structures studied by Gillaspy to coast. (2004) and Schermer et al. (2007) are present in both sandstone 9.6 End of road at fuel terminal; park here. and mudstone units, but best observed in the sandstone, where several generations of faults and extension veins cross cut the Stop 2-3. Richardson, Lopez Island dominant foliation. These structures include rare SW-vergent thrusts subparallel to foliation, followed by extension veins and Geologic relations at Richardson on Lopez Island (Figs. 18 normal faults, then conjugate strike slip faults. Analysis of these and 19) have played an important role in understanding San Juan structures in outcrops throughout the Lopez structural complex Island evolution since the discovery there of Cretaceous micro- and eastern San Juan Islands indicates a prolonged episode of fossils by Ted Danner of the University of British Columbia brittle deformation at the base of the accretionary wedge that (Danner, 1966), establishing a maximum age limit on thrusting. resulted in N-S to NW-SE extension (Figs. 5 and 17). Until recently the accepted age for these rocks was late Albian (ca. 100 Ma), determined by Bill Sliter of the U.S. Geologi- Directions to Stop 2-3 cal Survey (in Brandon et al., 1988) based on microfossils in a Return to Friday Harbor and take the ferry to Lopez Island. mudstone collected by John Whetten, University of Washing- 0.0 Ferry terminal on Lopez Island, drive south on Ferry ton, in 1977. Map relations displayed at this site show a layered Road. sequence of pillow basalts, pillow breccias, tuff and mudstone (Fig. 19). All these rocks were considered to represent a coherent mid-Cretaceous stratigraphic assemblage (Brandon et al., 1988). 122 W However, recent Ar-Ar analysis of blueschist facies phengitic mica in the pillow breccias (Fig. 20) yielded an age of 124.43 N 73 53 48 ± 0.72 Ma (Brown et al., 2005). Revisitation of the fossil ages 1O km in the Whetten sample indicates a late Aptian age (112–115 Ma) (Fig. 21). Remapping the structural features demonstrates that the K-is K-bj fossiliferous mudstones (Fig. 21) are faulted into the sequence. These fi ndings broaden the age brackets for thrusting, and sug- G gested to Brown et al. (2005) that San Juan Islands blueschist metamorphism is older than thrusting. But, a recent fi nding of F aragonite veins in the late Aptian mudstones by Schermer et al. (2007) indicates that the blueschist metamorphism continued to at least that time and was apparently coeval with and outlasted E thrusting, as interpreted by earlier workers (Brandon et al., 1988; Maekawa and Brown, 1991). B H D Figure 17. Generalized map of the eastern San Juan Islands with paleo- A magnetic results of Burmester et al (2000) and kinematics of late brittle 48 deformation from Schermer et al. (2007), Gillaspy (2004), and Lamb (2000). Small grey arrows indicate direction of magnetic vector from Burmester et al (2000); inclination values omitted; small arrowheads indicate upward inclination. K-is and K-bj show expected directions C for in situ and Baja-BC terrane models of the Cretaceous location of San Juan terranes. Other arrows indicate kinematic directions of brittle structures as defi ned in key. If no arrow is shown for brittle structures at a site, data are too few to conclude kinematic signifi cance. Circles indicate subhorizontal extension in several directions during normal faulting or extension veining. Foliation symbols show average thrust vergence T axis from normal folia tion direction and are located at reconnaissance study sites: at all faults, extension veins sites, the same sequence of faulting is observed, but not all sites have T axis from enough data to conclude kinematic signifi cance of all phases of fault- subhorizontal ing. A—San Juan Island; B—N. Lopez Island; C—Watmough Head; strike slip faults extension D—Guemes Island; E—Jack Island; F—Lummi Island; G—Eliza Island; H—Samish Island. Tectonic evolution of the San Juan Islands thrust system, Washington 167

BLAKELY TrJo Jc ISLAND Jf Jf ORCAS

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A N Jl = Lummi Formation JKl Jf = Fidalgo Complex 2-4 Jc = Constitution Fm. JKl = Lopez Structural Complex TrJo = Orcas Chert 2.0 km

Figure 18. Map of Lopez Island. Based on Brandon et al. (1988) and Burmester et al. (2000) and M.C. Blake Jr. (2000, written commun.).

The fault that juxtaposes the mudstone and volcanic rocks areas at a county park picnic site with parking for 2 or 3 cars each, bears slickenlines trending N30° W and plunging 20°, seen below available on the left (south) side of the road ~50 m before the the road at this locality. This lineation is part of the data set used by end. After parking, walk to the end of the public road, go straight Maekawa and Brown (1991) as a basis for their inference of domi- through the open wooden gate onto the private road, take the nantly orogen-parallel transport of the San Juan Island nappes. right-hand fork through private land, pass through a metal gate and follow the path ~15 min to Bureau of Land Management land Directions to Stop 2-4 at Iceberg Point. Please respect private property. 9.6 mi Drive north from end of Richardson Road. 9.9. Vista Road. Turn right (east). Stop 2-4. Iceberg Point, Lopez Island 11.4 Mud Bay Road. Go right (south). 12.5 MacKaye Harbor Road. Turn right (west). At Iceberg point, we will examine interbedded sandstones 14.6 End of road. and mudstones with several generations of brittle structures. If There is no parking at the end of the road; note a sign indi- time and tide permits, we will also examine a shear zone between cating “private road” at the end of the public road. There are two these rocks (of ocean-fl oor affi nity) and a fault slice of Consti- 168 Brown et al.

48O 27.0' 1818 6 1010 5' N 35 Pl

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15fenceAttitude of meta- 1212 FUEL morphic foliation TANKS Mudstone

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Figure 19. Map of Richardson locality, stop 2-3. From Brown et al. (2005). tution terrane to the north. Late brittle structures include SW- DAY 3 vergent thrusts subparallel to foliation, abundant extension veins and normal faults showing predominantly NW-SE extension, and Day 3 is mostly devoted to the Fidalgo Complex on Fidalgo conjugate strike slip faults. Because the late brittle structures are Island. On the last stop of the day we observe outcrops of the broadly distributed across the Lopez Structural Complex, we may Easton Metamorphic Suite exposed at the south end of Chucka- not be able to see all generations of structures and cross-cutting nut Mountain. relations between them. Fidalgo Complex Directions to Overnight Lodging Return to ferry landing and take the ferry to Anacortes. Drive The Fidalgo Complex (Fig. 22) consists of a stratigraphic on Washington State Highway 20 spur to the intersection with the sequence distinctive of ophiolite. From the base upward in the main route of highway 20, at Sharps Corner. section are: ultramafi c tectonite, cumulate gabbro, a sheeted Tectonic evolution of the San Juan Islands thrust system, Washington 169

lated (Garver, 1988; Blake and Engebretson, 1994). This unit also bears some affi nity to the Ingalls Complex in the central Cascades (described by Miller, 1985, and Metzger et al., 2002).

Directions to Stop 3-1 Drive into Anacortes on Washington 20 Spur and follow signs toward the ferry terminal. Continue past the turnoff to the ferries on Sunset Ave. to Washington Park. 0.0 Entrance to Washington Park 0.2 mi Begin one-way loop drive. 0.7 Park on left, cement stairs to beach on right.

Stop 3-1. Washington Park, Fidalgo Island

Ultramafi c rock here is interpreted to be basement to the Fidalgo ophiolite (Fig. 22) based on its position structurally beneath the other parts of the ophiolite; however, the contact is Figure 20. Photomicrograph of metamorphosed pillow breccia at Rich- ardson, sample R3, from which phengite gives an Ar-Ar age of 124.75 covered by Quaternary materials. Minerals are serpentinite (after ± 0.87 Ma (Brown et al., 2005). Phengite and chlorite crystallized olivine), relict pyroxene, and chromite. Protolith rock ranges from from volcanic glass, plagioclase is altered to fi ne-grained aggregate dunite to peridotite. Rock that was originally peridotite is marked of Ca-Al silicates. Aragonite and pumpellyite (not visible), as well as by signifi cant amounts of relict pyroxene, together with serpen- chlorite and phengite shown here, are synkinematic minerals. tine, whereas the original dunite is virtually free of pyroxene. The meta-peridotite and meta-dunite are thus distinguishable and can be seen as irregular layers through this exposure. Pyroxenite intrusive complex of mostly plagiogranite (diorite, tonalite, veins exhibiting comb structure cross the other lithologies. We trondjemite, albite granite) and hypabyssal equivalents, a vol- will speculate about the origin of these layers and veins and what canic sequence of mainly dacitic to andesitic breccias and inter- information they might provide about mantle deformation and layered tuffaceous argillite, coarse sedimentary breccia that bears basalt genesis. clasts of all the underlying units, pelagic argillite, and volcanic- rich graywacke at the top of the section. U-Pb zircon ages of the Directions to Stop 3-2 plagiogranites on Fidalgo Island are 167 ± 5 Ma, and elsewhere Continue around the “loop road”; exit Park back to are 160 ± 3 Ma on Lummi Island and 170 ± 3 on Blakely Island Sunset Ave. (Whetten et al., 1978, 1980). Radiolaria in the pelagic argillite 3.1 mi Turn right on Anaco Beach Road and continue on to are late Kimmeridgian–early Tithonian ca. 150 Ma (Gusey, 1978; merge with Marine Drive. Brandon et al., 1988). The U-Pb age pattern of detrital zircons from a sample of the graywacke unit bears a single prominent Stop 3-2. Private Property along Marine Drive, peak at 148 Ma, considered to represent a nearby volcanic prov- Fidalgo Island (Fig. 22) enance (Brown and Gehrels, 2007). All these rocks are affected by prehnite-pumpellyite metamorphism. (Access is not guaranteed as of this writing.) The plutonic part of the Fidalgo Complex is interpreted to Cumulate gabbro displays layering formed by differential be a remnant arc. An arc origin of the ophiolite is indicated by settling of pyroxene and plagioclase crystals in the melt (Fig. 23). the abundance of intermediate to felsic igneous rocks (Brown, Dikes of plagiogranite and keratophyre occur locally in the gab- 1977; Gusey and Brown, 1987; Burmester et al., 2000). The bro, and exclusively as a sheeted complex higher in the section. coarse breccia and overlying radiolarian argillite stratigraphically The orientation of bedding in the gabbro and direction of grad- above the igneous rocks indicate that the 160–170 Ma arc was ing are consistent with its mapped structural position low in the rifted and terminated as a volcanic center prior to deposition of ophiolite stratigraphy, but above the ultramafi c rock. the sedimentary part of the section. Thus the arc was faulted and shifted off its magmatic axis before attaining much crustal thick- Directions to Stop 3-3 ness or subaerial exposure. The old eroded arc was then buried Continue south on Marine Drive at ca. 148 Ma by younger clastic arc detritus from an adjacent 5.8 mi Turn right (south) on Havekost to the intersection with volcanic axis. This evolution is similar to that of modern remnant Rosario Road. arcs (e.g., Karig, 1972). 6.8 Rosario Road: turn right (east). The Fidalgo complex is similar in age and lithology to the 7.8 Heart Lake Road: turn left (north). California Coast Range ophiolite with which it has been corre- 9.0 Go right (east) on Ray Auld Drive. 170 Brown et al.

0.20 mm A B

foliated rind bedding S2

foliation S1

S2 shear fabric in tuff 1.0 cm C

Figure 21. Illustrations of the single fossiliferous mudstone tectonic fragment collected at Richardson by Brown et al. (2005). (A) Unidentifi ed foram in mudstone. Age defi nitive microfossils were not found in this specimen; however a sample from an unspecifi ed locality in the Richardson vicinity collected by John Whetten and considered by him to be “probably the same bed as in the roadcut” (Whetten et al., 1978) is determined to be late Aptian (112–115 Ma) by Cretaceous foram experts Mark Leckie, University of Massachusetts, and Isabella Premoli-Silva and Davide Verge both of the University of Milan. (B) Photomicrograph of mudstone unit illustrating sedimentary pellet structure. Flattening of pellets in part defi nes the foliation exhibited in the hand specimen. Minerals in this rock are dominantly quartz and chlorite, little or no feldspar or mica. Clay minerals are absent, thus the rock is at least somewhat recrystallized from its protolith. (C) Sketch of hand sample of fossil-bearing mudstone. Bedding is marked by concentrations of pyrite and quartz-hematite laminations. Foliation is defi ned by fl attened pellets, solution mass-transfer residues, shear surfaces and pull-apart structures.

9.1 Turn right (south) on Erie Mountain Drive. cally <1.0% through the suite, anomalously low for calc-alkaline 9.3 Park in pull-out on right; cross the highway to see rocks. Primary textures are well preserved, as in this exposure, outcrops. and do not support a hypothesis of postmagmatic chemical altera- tion. Igneous minerals observable in thin-section are plagioclase, Stop 3-3. Roadcut along Erie Mountain Drive, clinopyroxene, quartz, and opaques. Metamorphic minerals, in Anacortes City Park, Fidalgo Island (Fig. 22) veins and incipiently developed in the igneous matrix, are chlo- rite, epidote, pumpellyite, prehnite, albite, and quartz. Identifi ca- Observe green volcanic breccia. The lithology here is kera- tion of aragonite at one locality (Gusey, 1978) has not been con- tophyre (= meta-dacite). The volcanic section of the ophiolite fi rmed by X-ray analysis of many other carbonate samples from ranges from 48 to 74 wt% SiO2 (Brown et al., 1979). K2O is typi- the Fidalgo ophiolite (M.C. Blake, 2006, personal commun.). Anacortes 12th St.

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SCHEMATIC SECTION OF FIDALGO OPHIOLITE

Detrital zircon sample SILTSTONE AND GRAYWACKE peak age 148 Ma PELAGIC ARGILLITE Radiolarian ages SEDIMENTARY BRECCIA l. Kim.- e. Tith. ~ 150 Ma KERATOPHYRE AND SPILITE flows

Zircon age PLAGIOGRANITE dikes 167 5 Ma

CUMULATE GABBRO

unexposed 1000m

SERPENTINITE

Figure 22. Map and schematic stratigraphic section of northern Fidalgo Island. Rocks of Fidalgo Island are interpreted to represent an ophiolite sequence based on the stratigraphy shown here. The abundance of felsic igneous rock and absence of mid-oceanic-ridge basalts precludes origin of the ophiolite as sea fl oor crust, and indicates an affi nity with island arc magmatism (from Brown et al. 1979). Ages are from igneous zircons in plagiogranite, radiolaria in pelagic sediment, and detrital zircons in clastic sediments at the top of the section. All are mutually consistent considering their relative position in the stratigraphy, and indicate a Late Jurassic age. References: Whetten et al. (1978); Gusey (1978); Brown and Gehrels (2007). Q—Quaternary deposits. 172 Brown et al.

Figure 24. Photomicrograph of radiolarian argillite in the Fidalgo Complex.

Directions to Stop 3-5 Retrace route down the Erie Mountain Drive. 12.3 Go south on Heart Lake Road. 13.5 Turn right (west) on Rosario Road. 14.4 Turn right (north) on Havekost Road, past intersection with Marine Drive. 16.2 Entranceway to the Lakeside Industries quarry is on Figure 23. Cumulate bedding in layered gabbro, Fidalgo Island. the right. Obtain permission at the offi ce. Hard hats and vests are required. Avoid quarry slopes, which are unstable and dangerous. Fifty meters down the road, and structurally below the volcanic rock, is dark brown, manganese-rich, radiolarian Stop 3-5. Lakeside Industries Quarry, Fidalgo Island argillite. This rock unit, termed “pelagic argillite,” is as much (Fig. 22) as 500 m thick and forms the second sedimentary layer up in the ophiolite section (Figs. 22 and 24). An unexposed thrust Here we observe non-faulted stratigraphic contacts between fault separates these rocks. This structure as well as other the plagiogranite and sedimentary breccia, and between the brec- shear zones in the Fidalgo ophiolite have not been analyzed cia and overlying pelagic argillite. The coarse breccia consists of but have potential for addressing the kinematics of the San clasts of all lithologies of the underlying plutonic section includ- Juan Islands thrust system. ing ultramafi c rock, and therefore indicates uplift and exposure of the deeper levels of the section, presumably by faulting. The Directions to Stop 3–4 breccia represents slide and/or talus deposition. Presence of Continue up Erie Mountain Drive. radiolaria (Fig. 24) and high manganese content of the overlying 10.7 mi Summit of Mount Erie. argillite indicates a marine environment enriched by alteration of volcanic materials and isolated from continent-derived sediment. Stop 3-4. Mount Erie Summit, Anacortes City Park, The argillite is a chloritic mudstone with minor tuffaceous layers Fidalgo Island (Fig. 22) and thin sandstone beds with ultramafi c detritus (Gusey, 1978). Volcanic-rich graywacke overlies these sedimentary rocks and Massive diorite of the sheeted zone is intruded by fi ne- bears detrital zircons with a 148 Ma age peak, younger than the grained green dike rock (keratophyre and spilite). See views of breccia detritus which is derived from the 160–170 Ma under- the Olympic Mountains Tertiary subduction complex, Admiralty lying arc (Fig. 22). The Fidalgo ophiolite is interpreted to be a Inlet to Puget Sound, glacial drift from the Puget lobe, Eastern remnant arc, and the overlying graywacke to have been deposited and Western Mélange belts in the Cascade foothills. in either a fore-arc or backarc basin. Tectonic evolution of the San Juan Islands thrust system, Washington 173

Directions to Stop 3-6 (See Also Fig. 25) Easton Metamorphic Suite Return to highway 20 spur by the following route: • Turn right out of the Lakeside Industries driveway to go The Easton Metamorphic Suite (formerly known as the north on Havekost Road. Shuksan Metamorphic Suite; Misch, 1966) is a mostly well- • 41st Street: Go right (east) on 41st St. recrystallized blueschist terrane with close similarities to the • O avenue: Jog north one block then east one block. Pickett Peak terrane of the Franciscan Complex (Brown and • Commercial Avenue: Go north. Blake, 1987). A variety of lithologic components are found • Highway 20 spur: Drive east on highway 20 in this unit (Fig. 25): (1) blueschist and greenschist derived 0.0 Sharps Corner, main highway 20, reset odometer; from mid-oceanic-ridge basalt (Dungan et al., 1983) known continue east. as the Shuksan Greenschist (Misch, 1966); (2) quartzose car- 6.8 mi Highway 237 (Farm to Market Road); go north to the bonaceous phyllite, derived from mudstone, named the Dar- village of Edison. rington Phyllite; (3) metagraywacke semischist derived from 14.6 Bow Hill Road; go right (east). sandstone with abundant chert and dacitic-andesitic clasts; 15.6 Chuckanut Drive; go left (north). (4) local pods of metamorphosed plutonic rock of tonalitic to 19.6 Cross Oyster Creek (at hairpin turn). gabbroic composition; and (5) a local zone of high-pressure 19.7 On the left is Oyster Creek Inn and the road to Taylor amphibolite and eclogite. The suite as a whole defi nes the shellfi sh farm (sign). Head down this one-lane road, “Shuksan Nappe” of Tabor et al. (2003), a sheet some 100 km across Oyster Creek at the bottom of the hill, continue in length and breadth exposed across much of the northwest for ~100 m, and park on the right near the railroad Cascades and breached in an anticlinal structure known as the tracks. Hike across the tracks and north along the tide “Mt Baker window” (Misch, 1966) where underlying nappes fl ats to the mouth of Oyster Creek. can be observed.

N NK 10 km CZ CZ

Mount Bellingham BP Baker Bay meta-tonalite BP meta- 163 2 Ma gabbro CH LM 163 2 Ma TS meta-gabbro CH 164 2 Ma NK BP 3-6 Detrital

zircon BP C

h I - 5 - I 155 Ma peak u Edison c k a n u t D 130 5 Ma r. CH

Anacortes HH blueschist 237 FC Farm to Market rd. Market to Farm 128 4 Ma

WA 20 5 - I blueschist

CZ HH 144-160 Ma HH amphibolite CC

Easton Metamorphic Suite Cz CH meta-pelite meta-oceanic basalt Darington Phyllite Shuksan Greenschist

meta-pelite and amphibolite and EM -graywacke rare eclogite WM

Figure 25. Regional map of the Easton Metamorphic Suite; isotopic ages from Brown et al. (1982), Armstrong and Misch (1987), Gallagher et al. (1988), Dragovich et al. (1988, 1999). Cz—Cenozoic rocks and surfi cial deposits; abbreviations of other units given in Table 1. 174 Brown et al.

An ocean fl oor stratigraphy is evident where the Shuksan META -GABBRO N Greenschist is stratigraphically overlain by a thin zone of metal- ~ 163 Ma colluvium 30 m liferous quartzose rock which is in turn overlain by Darrington Phyllite (Haugerud et al. 1981). The metagraywacke unit is inter- layered with Darrington Phyllite in the western part of the Shuk- SEMISCHIST 70 san Nappe and represents volcanic arc and fl ysch detritus. Based <155 Ma on the above relations, Gallagher et al. (1988) proposed a back 45 5 arc setting for the Easton Metamorphic Suite. tide flats 10 Protolith ages are indicated by U-Pb zircon ages of the 0 50 tonalite and gabbro bodies at 163–164 Ma (Fig. 25; Walker in 40 stretching local Gallagher et al. 1988, Dragovich et al. 1998, 1999) and detrital lineation late zircons in a sample of the graywacke yielding a prominent age foliation deformation peak at 155 Ma (Brown and Gehrels, 2007). The gabbro-tonalite Oyster Ck bodies occur within the graywacke stratigraphy and bear the Figure 26. Map of the outcrop area of stop 3-6, near the mouth of same metamorphic mineralogy and tectonite fabric as the gray- Oyster Creek. wacke. These relations and the older age of the gabbro-tonalite bodies imply that they were faulted or slid into the graywacke depositional basin. Metamorphic ages known from Rb-Sr and K-Ar ages of musco vite and amphibole (Armstrong in Brown et. al. 1982; Armstrong and Misch, 1987) date regional blueschist meta- morphism at 120–130 Ma and the higher grade localized amphibolite-eclogite metamorphism at 144–160 Ma.

Stop 3-6. Semischist and Gabbro of the Easton Suite at the Mouth of Oyster Creek, Private Land (Fig. 26)

This outcrop is near the western margin of exposure of the Easton Suite, which comprises the “Shuksan Nappe.” Meta- morphic mineralogy and structure point to continuation of the Shuksan Nappe somewhat beyond this point into small islands of the eastern San Juan archipelago (Lamb, 2000). Some workers have considered that the Shuksan Nappe possibly extended as Figure 27. Photomicrograph of semischist showing stretched chert a structurally high unit across the San Juan Islands contributing clasts at stop 3-6. to the 20-km-thick burial required for aragonite metamorphism (Brandon et al., 1988). The semischist exposed here is chert rich (Fig. 27) and is The fi eld trip ends here. Return to Chuckanut Drive and go interbedded with carbonaceous phyllite. Stretched chert clasts north to Bellingham or south to I-5. mark a northeast trending shallow lineation of similar orientation to that found regionally in the Easton Suite and interpreted to ACKNOWLEDGMENTS represent orogen-normal displacement during Early Cretaceous subduction zone metamorphism (Brown, 1987). Clark Blake and Eric Force, both retired from the U.S. A short distance along the tidelands to the north is a body of Geological Survey, reviewed and considerably improved the metagabbro (Fig. 26) similar to others in the Easton Suite dated manuscript. to be 163–164 Ma (Fig. 25). The contact of the gabbro and semi- schist along the beach is covered by colluvium, but in roadcuts REFERENCES CITED along the highway above, serpentine is seen to intervene between the units. The origin of the gabbro bodies in the Easton is an Armstrong, R.L., and Misch, P., 1987, Rb-Sr and K-Ar dating of mid-Mesozoic interesting problem. They have apparently either slid or been blueschist and Late Paleozoic albite-epidote amphibolite and blueschist faulted into the graywacke section (see above). The gabbro ages metamorphism in the North Cascades, Washington and British Colum- are similar to plutonic rocks in the Fidalgo Complex and the bia, and Sr isotope fi ngerprinting of eugeosynclinal rock assemblages, in Schuster, J.E., ed., Selected papers on the geology of Washington: Ingalls Complex (Fig. 2A), which therefore could conceivably Olympia, Washington Division of Geology and Earth Resources, v. 77, have been a source for these materials. p. 85–105. Tectonic evolution of the San Juan Islands thrust system, Washington 175

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