Tectonic evolution of the San Juan Islands thrust system ...

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 field 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
Washington record a long and complex history related to Cor- Many aspects of the lithology, structure, and metamorphism
dilleran convergent margin tectonism. The area is underlain by are similar to the Mesozoic evolution of other parts of the Cordil-
the San Juan Islands–northwest Cascades thrust system, made lera; other aspects may be unique to the San Juan Islands. The east-
up of nappes a few kilometers or less thick and up to 100 km in west transect across the San Juan Islands during this field trip will
breadth (Figs. 1, 2), thrust onto the continental margin during highlight the different terranes juxtaposed by the thrust system,
mid-Cretaceous time (e.g., Misch, 1966; Brown, 1987; Bran- and structures formed before, during and after high-pressure–
don et al., 1988). The nappes have an oceanic history, indicat- low-temperature (HP-LT) metamorphism. The trip builds on ear-
ing accretion to the edge of the North American continent, but lier work that identified the main terranes and structures in the
they also bear clear evidence of interaction with the continen- San Juan thrust system (e.g., McClellan, 1927; Danner, 1966;
tal margin long preceding their emplacement in Washington. Vance, 1975; Whetten et al., 1978; Brandon et al., 1988). Our
Their mid-Cretaceous arrival in Washington as thrust sheets recent results on structure, metamorphism, geochronology, and
was likely the consequence of some type of post-accretionary paleomagnetism will provide a forum for discussions that bear
fragmentation and dispersal. The timing and mechanisms of the on the tectonic history and correlation with other Cordilleran
accretion, dispersal and final emplacement of terranes of the terranes. We will compare and contrast units from the external,
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.fld009(08). For permission to copy, contact [email protected]. ©2007 The Geological Society of America. All rights reserved.

143

144 Brown et al.

CH ST Figure 1. Regional setting of the San Juan Islands—northwest Cas-
AX cades area in the northwest Cordillera. AX—Alexandria; BA—Baker
128 terrane; CC—Cache Creek terrane of Miller (1987); CH—Chugach
56 56 terrane; EK—Eastern Klamath terrane; FR—Franciscan complex;
135 GV—Gravina belt; GVS—Great Valley sequence; H—Huntington
terrane; IZ—Izee terrane; MT—Methow basin; QS—Quesnellia;
GV SC–FR—Straight Creek–Fraser River fault; SF—Shoo Fly complex;
ST—Stikinia; WA—Wallowa terrane; WJ—Western Jurassic belt;
AX YT WR—Wrangellia; WTrPz—Western Triassic and Paleozoic belt;
YT—Yukon-Tanana terrane. Sources: Burchfiel et al. (1992a); Gehrels
AK and Kapp (1998); Wheeler and McFeely (1991). B.C.—British
B.C. Columbia; CA—California; cc—Cache Creek belt; Cz—Cenozoic
rocks and surficial deposits; ID—Idaho; mc—McCloud belt of Miller
WR (1987); OR—Oregon; NV—Nevada; Wash.—Washington.

CC QS

cc

COAST units that experienced subduction and HP-LT metamorphism. In
ST particular, we would like to consider how the geology of the area
PLUTONIC fault relates to various hypotheses regarding the origin and paleogeog-
mcYalacom raphy of the terranes, and the evolution of deformation before,
130 during, and after emplacement in their current location.

50 M T TECTONIC SETTING

WR COM PLEX SC-FR fault The San Juan Islands–northwest Cascades thrust system lies
at the south end of the 1500 km long Coast Plutonic Complex,
QS a belt of continental arc plutons and metamorphic country rock
that formed from Late Jurassic to Early Cenozoic (Figs. 1, 2).
mc Outboard of the Coast Plutonic Complex and intruded by it is
the Insular superterrane composed of the co-joined Wrangellia
MT Q S WBa.Csh.. 50 and Alexander terranes. Inboard of the Coast Plutonic Complex
CPC 116 are rocks of the Early Cretaceous continental margin, including
the Methow stratigraphic sequence in Washington. Detritus, cur-
Fig. 2 rent indicators and stratigraphy in the Methow sequence indicate
absence of an outboard sediment source until ca. 110 Ma (Ten-
ID nyson and Cole, 1978; Haugerud et al., 2002), thus we view the
Cz locale of the Washington Cascades and San Juan Islands as an
ocean basin until that time. Major orogenic activity characterizes
Columbia the region from ca. 110–80 Ma, during which nappes of the San
Juan Islands–northwest Cascades thrust system were emplaced,
Embayment WA 46 the Coast Plutonic Complex was intruded by voluminous arc
plutons, and country rock of the complex was locally buried to
Cz BA depths of up to 35 km (in the “Cascade crystalline core”; Figs. 2
and 3) and was deformed by orogen-normal and orogen-parallel
OR IZ cc displacements. Overlapping the waning stages of this orogenic
H pulse was development of the Nanaimo stratigraphic sequence,
bearing detritus from the San Juan Islands–northwest Cascades
42 WJ Blue Mtns. thrust system as well as from the Coast Plutonic Complex, in
125 an elongate basin extending north from the San Juan Islands. In
cc 200 kilometers Eocene time the orogen was cut obliquely and displaced ~170 km
(estimates range from 90 to 190 km; e.g., Vance, 1985; Misch,
WTrPz 1977) by the N-S dextral, strike-slip Straight Creek–Fraser River
cc fault system. Restoration of the fault shows the San Juan Islands–
Klamath 11642 northwest Cascades thrust system to have lain along the southern
EK Mountains margin of Wrangellia and the Coast Plutonic Complex (Fig. 3)
mc NV
CA
FR

No.
Sierra

cc SF

GVS

mc

123 Q CPC HZ
V C CH
B.C. 121
WA NORTHWEST 49

NA BP YA T
WR CZ
ES TB A'
HS

A

OC CO LM NK
GA EA Mt Baker

LS FC Ross Lake faultCH TS YAEA

SAN JUAN LS HH DMF NK
ISLANDS
Melange BeltsEA

CH

CASCADES

EM HH

D

N Q WM EM CRYSTALLINE

PUGET WM T
SOUND T
CORE 88-96 Ma
30 kilometers
CW plutons
48

TGStraight Ck. - Fraser R. fault
CN

Windy
CN Pass

Thrust

T ING MS
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'

Orcas Island Lummi Twin Sisters Mt Baker Shuksan Chilliwack
GA Island Range window thrust batholith

NA OC FC FC YA
CO LM
YA VC
HS TB ES TB ES TS
EA CN EA NK CH EA SL
WRANGELLIA OC CH
BP BP CC
B CH BP
CH

NK

depositional or intrusive contact 10 km Straight
fault contact Creek fault
no vertical exaggeration

Figure 2 (continued). (B) Cross section.

in Late Cretaceous time. South of this orogenic complex is the Mid-Late MQTUESNELLBI.AC.
Columbia Embayment, an area covered primarily by Cenozoic Cretaceous WA
volcanic rocks, thought to be underlain by primitive crust, and Plutons
considered in some models to be a possible homeland for the Jur. - Ross Lake
thrust system nappes (e.g., Davis et al., 1978; Vance et al., 1980). N E. Cret. HZ
East and south of the Columbia Embayment are accreted terranes
of the Blue Mountains, and Klamath Mountains respectively SSEE Fault
(Fig. 1), the latter especially bearing similarities to units of the
San Juan Islands–northwest Cascades thrust system. Zone

STRUCTURAL STRATIGRAPHY WRANGELLIA N Georgia Plutons CW Cascade
PRC HL Crystalline
The nappe pile of the San Juan Islands–northwest Cascades NA B.C. Core
thrust system (Fig. 2) is characterized by mid to late Paleozoic Strait WA ING
terranes overlain by Mesozoic terranes. The structurally lowest EA WPT
component of the nappe complex is the East Sound Group in the N NK EA
San Juan Islands and correlative Chilliwack Group in the Cas-
cades. These are island arc derived sedimentary and volcanic rocks PACIFIC NWCS HB
of Devonian–Permian age (Danner, 1966; Vance, 1975; Misch, OCEAN NAPPES MN
1966; Tabor et al., 2003). Calc-alkaline Devonian plutonic rocks
presumed to be related to this arc are the Turtleback and Yellow N MELANGBEELTS CZ
Aster Complexes of the San Juan Islands and Cascades, respec- CZ SC-FR fault
tively (Mattinson, 1972; Whetten, et al., 1978; Brandon et al., restored
1988; Tabor et al., 2003). This assemblage is likely related to arc 100 KM
rocks that extend from California to northern British Columbia
and mark mid-late Paleozoic convergence along the continental Figure 3. Regional geology shown with hypothetical restoration of the
margin (McCloud belt of Miller, 1987). Straight Creek–Fraser River fault. (SC-FR fault) based on ~170 km of
displacement (e.g., Umhoefer and Schiarizza, 1996). Abbreviations:
Higher in the nappe pile, in both the San Juan Islands and HB—Hicks Butte inlier, HL—Harrison Lake stratigraphic sequence,
Cascades, is a disrupted section including Permian to Juras- MN—Manastash Ridge inlier, PRC—Pacific Rim Complex, SE—
sic ribbon chert, Permian HP-LT schist, ocean island basalt, Settler Schist. See Table 1 for other unit abbreviations.
Permian limestone bearing Tethyan fusulinids (exotic to North
America), and other materials (Fig. 2, Table 1). In the San Juan The highest nappes in the San Juan Islands–northwest Cas-
Islands, units are Orcas Chert, Deadman Bay Volcanics, and Gar- cades thrust system are Late Jurassic rocks that include ophio-
rison Schist (Brandon et al., 1988), observed on this field trip. In litic plutonic rocks, mid-oceanic-ridge basalt, ribbon chert, and
the Cascades, this zone is referred to as the Bell Pass Mélange arc-derived mudstone-sandstone. Units of these upper nappes
and in addition to the above mentioned rock types includes the that we will examine include rocks in the Lopez fault zone, the
10 × 4 km Twin Sisters dunite slab (Tabor et al., 2003). Rocks Constitution Formation, Fidalgo Ophiolite and Easton Meta-
and structures of this zone are similar to the “Cache Creek belt” morphic Suite. These units are closely similar to terranes in the
of Miller (1987) that extends sporadically from northern British western Jurassic belt, Franciscan Complex and Coast Range
Columbia to California and apparently represents accretionary Ophiolite of the Klamath Mountains and California Coast
mélange of mainly oceanic rocks. Range (e.g., Brown and Blake, 1987; Garver, 1988; Blake and
Engebretson, 1994; J.S. Miller et al., 2003).

The nappe geometry portrayed in Figure 2B and described
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
boundaries are inherited from events pre-dating assembly of
Peripheral to the San Juan Islands nappe pile along its nappes in their present setting (Brown et al., 2005). However,
northwest flank are the arc-derived Late Triassic Haro Forma- there is good evidence for major mid-Cretaceous assembly.
tion, the Late Jurassic–Early Cretaceous Spieden Group, and the This deformation is referred here to as the thrusting event.
Late Cretaceous Nanaimo Group bearing detritus from the thrust Nappes of the thrust system were emplaced and unroofed in
system. These units lack evidence of HP-LT metamorphism and the San Juan Islands vicinity by the time of deposition of the
penetrative tectonite fabric, and thus are considered to be “exter- Nanaimo Group (Vance, 1975); the oldest part of the Nanaimo
nal” to the thrust system (Brandon et al., 1988). Owing to the dif- known to bear detritus from the thrust system is ca. 85 Ma
ferent tectonic and metamorphic histories, a fault is assumed to (latest Campanian-earliest Santonian; Brandon et al., 1988). A
separate the nappe pile from the external units. This fault, named maximum age for thrusting in the San Juan Islands is given by
the Haro fault, cannot be directly observed, but is inferred to dip fault juxtaposition of Late Aptian (112–115 Ma) fossiliferous
under the nappe pile based on regional dips and a gravity survey rock with 124 Ma HP-LT metamorphic rock on Lopez Island
(Johnson et al., 1986; Palumbo and Brandon, 1990). The Haro (one of our field trip stops). In the Cascades, a population of
fault may have been reactivated during south-vergent thrusting in detrital zircons in the Nooksack Formation (footwall to the
the Cowichan fold and thrust belt (England and Calon, 1991). nappes) gives a maximum depositional age of 114 Ma, and a
large sandstone raft in the Bell Pass Mélange bears 119 Ma
The ultimate footwall to nappes of the San Juan Islands– detrital zircons (Brown and Gehrels, 2007).
northwest Cascades thrust system is problematic in the San Juan
Islands, but clearer in the Cascades. Based on arguments given More precise ages of thrusting are known for two localities:
above that external units underlie the San Juan Island nappes and K-Ar whole rock ages of 87 and 93 ± 3 Ma were obtained for two
observation that Wrangellia underlies Nanaimo Group units on mylonite samples from the west flank of the Twin Sisters Dunite
Vancouver Island, one could infer that Wrangellia is basement (Armstrong in Brown, 1987). Movement on the Windy Pass
to the San Juan Island nappes (e.g., Cowan and Bruhn, 1992). thrust is dated at ca. 94 Ma by relationships with U-Pb zircon-
In the Cascades, evidence indicates that nappes are thrust over dated plutons that predate, postdate and are involved in thrusting
the southern end of the Coast Plutonic Complex. In the central (R.B. Miller et al., 2003). Thus, major displacement is broadly
Cascades, the Ingalls Complex, a component of the San Juan bracketed between ca. 115 and 85 Ma based on youngest terranes
Islands–northwest Cascades thrust system, is thrust over Chi- involved and the age of rocks bearing detritus of the nappes, and
waukum Schist and Mount Stuart batholith along the Windy a more limited time frame is suggested to be ca. 90–95 Ma from
Pass Thrust (Figs. 2, 3; Miller, 1985). In the northwest Cascades, dated rocks in two fault zones.
the relatively undeformed Jurassic-Cretaceous Nooksack Group
which underlies the nappe pile (e.g., Misch, 1966) appears to be a METAMORPHISM
southern extension of the Harrison Lake stratigraphic sequence in
the southern British Columbia Coast Plutonic Complex (Fig. 3; Most units in the San Juan Islands–northwest Cascades
Monger and Journeay, 1994). Along its western flank, the Coast nappe pile show effects of Cretaceous HP-LT metamorphism.
Plutonic Complex is intrusive into Wrangellia. Thus, one inter- The degree of recrystallization and metamorphic fabric devel-
pretation for the regional structure is that Wrangellia and the opment varies greatly, even within the same units. In the Cas-
Coast Plutonic Complex constituted a structural block in mid- cades, evidence of HP-LT metamorphism is found virtually
Cretaceous time that served as footwall to the San Juan Islands– in all thrust system units of Jurassic or older age. The blue-
northwest Cascades thrust system in both the San Juan Islands schist facies Easton Metamorphic Suite in the Cascades bears
and Cascades (e.g., Brown, 1987; McGroder, 1991; Monger and synkinematic metamorphic minerals dated at 120–130 Ma by
Brown, 2008). Other interpretations place nappes of the San Juan K-Ar and Rb-Sr (Brown et al., 1982; Armstrong and Misch,
Islands–northwest Cascades thrust system within, and as part of, 1987). Rock units younger than 120 Ma (Nooksack Forma-
the country rock of the Coast Plutonic Complex (Monger and tion and sandstone in the Bell Pass Mélange) lack definitive
Journeay, 1994; Cowan and Brandon, 1994).

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-
where in the Cordillera correlatives of Paleozoic terranes of the
The age of blueschist metamorphism relative to thrusting San Juan Islands–northwest Cascades thrust system (e.g., East
is critical to understanding the tectonics of the thrust system. Sound Group) accreted long before the mid-Cretaceous. Addi-
If aragonite was formed during thrusting, burial on the order tional arguments and evidence are provided by the: (1) theYellow
of 20 km is required at the ~200 °C temperature estimated for Aster Complex (Figs. 2 and 6; Table 1), a pre-Devonian terrane
metamorphism (Brandon et al., 1988), indicating a great thick- with links to the continent indicated by beds of quartz arenite
ness of overlying nappes. An alternative concept that blueschist and a suite of detrital zircons that match those of the miogeo-
metamorphism in the thrust system is inherited from an event cline (Brown and Gehrels, 2007); and (2) Permian blueschist
predating nappe emplacement may be possible for the older metamorphism in some units (Garrison Schist, Vedder Complex;
terranes. However, Schermer et al. (2007) showed that HP-LT Armstrong et al., 1983), indicating that these rocks were involved
metamorphism lasted during several phases of brittle deforma- in convergent margin tectonics long before thrusting in the San
tion that followed juxtaposition of the internal San Juan Island Juan Islands–northwest Cascades system.
nappes, including the late Aptian Richardson rocks. If all of
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

AB

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.

Brown, 1987 SW directed NW directed
thrusting thrusting

NW Cascades HP-LT 120-130 Ma 87-90 Ma

San Juan Is. NW directed
thrusting
Maekawa &
Brown, 1991 penetrative cleavage HP-LT

Cowan & SW directed
Brandon, 1994 thrusting HP-LT

Bergh 2002 local penetrative
cataclasis cleavage
Brown et al.,
2005 D1 D2

Schermer SW-NE NW
et al, 2007 contraction thrusting
NW-SE
strike-slip

penetrative cleavage

HP-LT

penetrative thrusting
cleavage

HP-LT 124 Ma

penetrative SW-NE contraction
cleavage NW-SE extension emplacement

in SJI

veins & brittle
faulting
HP-LT

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

140 130 120 Ma 110 100 90 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

Number Jurassic Rocks Number 30 177 Ma
fossil age SPIEDEN GROUP
155 Ma
25 Sentinal Island Fm.
70
20
60 EASTON SUITE
15
50
40 10
30
20 5 224 Ma

10 238 Ma 0
80 100 120 140 160 180 200 220 240 260 280 300
0
119
80 120 160 200 240 280 25 sandstone in
148 Ma
165 BELL PASS
60 20 MELANGE

Number 50 FIDALGO COMPLEX Number 15

40 143 233

30 10

20 5

10 237Ma 0
80 100 120 140 160 180 200 220 240 260 280 300
0
3 ss in BPM
80 120 160 200 240 280
70 148 Ma 1
200 600 1000 1400 1800 2200 2600
60
Number 152 Ma
50 LUMMI FORMATION
30 125
40
Number TONGA
30 FORMATION

20 20
10
0 10

80 120 160 200 240 280
149 Ma

Number 40 0
80 100 120 140 160 180 200 220 240 260 280 300
CONSTITUTION Number
30 FORMATION 80 153 Ma

20 70

10 60 NOOKSACK
GROUP
0
50
80 120 160 200 240 280
Ma 40

Early Paleozoic Rock 30

20 114 Ma

10
0
80 100 120 140 160 180 200 220 240 260 280 300

Ma

25 1825
YELLOW ASTER
COMPLEX

20

number 15

10 2069 2321
2528
5
1404
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
zones that they interpret to indicate southwest transport of the
formed as continental borderland terranes that were caught nappes (orogen-normal). In the eastern San Juan Islands, Lamb
in a collision zone between the offshore Wrangellian micro- (2000) reported northeast vergent (orogen-normal) isoclinal folds
continent and North America (Brandon and Cowan, 1985; dated by synkinematic mica at ca. 137–154 Ma (see above) in
Brandon et al., 1988; Rubin et al., 1990; McGroder, 1991; rocks inferred to be related to the Easton Suite. Bergh (2002)
Burchfiel et al., 1992b; Cowan and Brandon, 1994; Monger observed folds, stretching lineations, and shear zones in the
and Journeay, 1994). Lopez and Rosario fault zones supporting both orogen-normal
(2) A transcurrent-transpressional model in which the nappe ter- and orogen-parallel displacement and conceived the two-stage
ranes originally accreted or were deposited south (or north?) model described above and shown in Figures 5 and 7. Burmester
along the margin from their present location and then moved et al. (2000) found that many of the rocks in question have been
coastwise, finally stacking up in a reentrant of the continen- reoriented after acquiring their magnetization, which developed
tal margin formed by the south end of Wrangellia (Brown, during or after the fabric was formed; therefore they suggested
1987; Maekawa and Brown, 1991; Brown and Dragovich, that the orientation of the fabrics cannot be used to determine
2003; Monger and Brown, 2008). direction of transport in the present frame of reference. Brown
(3) A two-phase model in which terranes were first juxtaposed et al. (2005) determined that fabric in blueschist tectonite of the
by orogen-normal thrusting along the continental margin Lopez fault zone predates thrusting and they suggested that much
south of Wrangellia, and then underwent orogen-parallel of the kinematic analysis in the San Juan Islands has been carried
thrusting and strike-slip faulting (Bergh, 2002). out on similar pre-thrust fabric and therefore may not be use-
Resolution of the emplacement history of the San Juan ful in understanding emplacement of the nappes. Gillaspy (2004)
Islands–northwest Cascades thrust system is central to our and Schermer et al. (2007) found that faults and extension veins
understanding of mid-Cretaceous orogeny in the Pacific North- indicate a protracted period of orogen-normal shortening coupled
west, including: the cause of crustal thickening and Barrovian with orogen-parallel extension during aragonite metamorphism
metamorphism in the crystalline core, the origin of the Nanaimo that postdates thrusting, juxtaposition of the terranes, and pene-
basin, and the configuration of terranes along the North American trative fabric formation. The different interpretations are summa-
margin in the Early Cretaceous. On a broader regional scale, the rized in Figure 5. To more effectively make use of these structural
San Juan Islands–northwest Cascades thrust system is relevant observations, the challenge for future workers is to understand
to understanding evolution of the 1500-km-long Coast Plutonic the age of outcrop-scale structures relative to the age of emplace-
Complex which extends from northwest Washington to Alaska. ment of the nappes.
Based on their interpretation as orogen-normal contractional
features, thrusts of the San Juan Islands and northwest Cascades Regional Considerations
have been correlated with thrusts in northern British Columbia
and Alaska and cited as evidence for a west-vergent thrust sys- Another strategy for establishing nappe displacements is
tem that extends virtually the entire length of the Coast Plutonic consideration of regional geology. Because units of the San
Complex and has accommodated many hundreds of kilometers Juan Islands–northwest Cascades thrust system bear evidence
of mid-Cretaceous shortening between the Insular superterrane of residence along the continental margin prior to emplacement
and North America (Rubin et al., 1990). in the present day setting, direct accretion of these rocks from
the west, the Pacific basin, seems improbable. Derivation of
Kinematics of Outcrop Scale Structures the nappes from the northeast is envisaged in the contractional
model of Brandon and Cowan (1985) and McGroder (1991)
One approach to understanding displacement of nappes in which invokes a root zone for the nappes along the northeastern
the San Juan Islands–northwest Cascades thrust system is kine- edge of the Cascade core in the approximate area of the Ross
matic analysis of outcrop scale structures. Such studies to date Lake fault zone (Figs. 2 and 3). In this view, during Wrangellia
yield somewhat disparate results (Fig. 5). Brown (1987), working collision the nappes were driven to the southwest, riding over
in the Cascades, reported a set of orogen-normal stretching linea- the Cascade core and the northeastern flank of Wrangellia.
tions in the Easton Suite coeval with 120–130 Ma blueschist min- Regional geologic features cited as supportive of this model are:
erals (see above). Younger orogen-parallel lineations were found coeval crustal thickening in the Cascade core suggesting thrust
in mylonite zones separating Cascades nappes (ca. 90 Ma, see loading, contractional structures in the Cascade core, and inter-
above). Smith (1988), and Maekawa and Brown (1991) mapped pretation that the Nanaimo Group was deposited in a foreland
orogen-parallel stretching lineations in the Cascades and San Juan

Late Jurassic McGroder, 1991

ST 98 Ma QS
WR CC
WR CPC
QS MT

SK NWCS CORE A

terranes

CH MT 94 Ma
NK
NWCS

CPC SK MT QS
NWCS
terranes

Brown, 1987 90-95 Ma

Early Cretaceous MT

WR CPC QS WR Plutons CORE
NK

NWCS B

F?

NWCS

F
100 km

E. to mid-Cretaceous Late Cretaceous

CPC Bergh, 2002

WR CPC
WR

FC

area of

EA San Juan EA
Islands

F

Lopez Lopez
fault fault
zone Rosario zone
Rosario fault
100 km fault zone
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-
tized directions from the San Juan Islands are scattered, how-
San Juan Islands–northwest Cascades thrust system over ever, indicating that a significant amount of rotation and/or tilt
the Cascade crystalline core (Fig. 3) at precisely the time occurred after this remagnetization event.
of great magmatic arc activity in that region. No rocks
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 significant metamorphism.
system carry metamorphic aragonite acquired prior to (as
well as after) thrusting. Aragonite has been shown experi- The rocks of the Spieden Group have complex magnetizations,
mentally to invert quickly to calcite outside its stability field with the majority of these clastic rocks having poorly resolved
at elevated temperature except under conditions of abnor- magnetizations. Dean (2002) found three magnetic components in
mally low T/P, less than 10 °C/km (Carlson and Rosenfeld, most of the Late Jurassic Spieden Bluff Formation samples, which
1981). Transit of the thrust system nappes over the active yielded an inconclusive paleomagnetic fold test. The Early Cre-
arc would place them in a region of abnormally high T/P, taceous Sentinel Island Formation has a simpler, two-component
precluding preservation of aragonite. magnetization in some of the rocks. Dean (2002) found that the
(3) The elongate, orogen-parallel Nanaimo basin is flanked not second-removed component from the Sentinel Island Forma-
by terranes of the San Juan Islands–northwest Cascades tion passes the inclination-only paleomagnetic fold test, with the
thrust system, but by plutonic rocks of the Coast Mountains. best-clustered inclinations occurring at 100% untilting. The mean
Thrust system terranes occur south along strike from the inclination of 64°, α95 = 7.8°, suggests an Early Cretaceous paleo-
Nanaimo (Figs. 2 and 3), and thus the basin is not likely a latitude of 46° N. Comparing this direction with that expected for
consequence of nappe loading. the present-day location of Spieden Island calculated from a stable
Many workers have envisaged a southerly origin of some North America reference pole (Housen et al., 2003), a latitudinal
or all of the terranes of the San Juan Islands–northwest Cas- translation of 1500 ± 1000 km is estimated for these rocks.
cades thrust system, in the Columbia embayment, Klamath
Mountains, or California Coast Range (e.g., Davis et al., 1978; The Nanaimo Group has been the subject of extensive paleo-
Vance et al., 1980; Brown and Blake, 1987; Garver, 1988; Burch- magnetic study, primarily from outcrops in the Canadian Gulf
fiel et al., 1992b). Davis et al. (1978) and Vance et al. (1980) Islands (Ward et al., 1997; Enkin et al., 2001; Kim and Kodama,
proposed that the Mesozoic ophiolitic terranes of the San Juan 2004), with limited work from Orcas Island (Housen et al.,
Islands–northwest Cascades system formed in a “pull-apart gap” 1998). All of these studies have found that most Nanaimo Group
in southeastern Oregon and subsequently moved northward and rocks have poorly defined magnetizations (~60% “failure rate”
were obducted onto the continent. Geologic features cited in sup- reported for most sample collections). However, a significant
port of the model are: (1) thrust emplacement of the Ingalls ophio- number of samples in all of these studies (a few 100 out of ~1000
lite over the south edge of the Cascade core, (2) absence from samples collected) have well-defined magnetizations that pass a
eastern Oregon and western Idaho of some continental margin reversals or fold test. Studies of inclination error, notably Kim
terranes that are part of the Mesozoic assemblage to the north and Kodama (2004), suggest that inclination error in these sedi-
and south along the Cordillera, and (3) Sr isotope ratios and seis- ments is moderate (8–10°), and that when corrected for the paleo-
mic velocities indicating primitive crust underlying the Columbia magnetic inclinations in these rocks place the Nanaimo Basin at
embayment. More recent geophysical evidence for a deep crustal a paleolatitude of 41° N during Campanian-Maastrichtian time.
rift in the Columbia embayment is a linear break in the gravity Using a Late Cretaceous North American reference pole for
field running along the southern margin of the embayment (Riddi- comparison, a translation of 1600 ± 900 km is indicated for these
hough et al., 1986). rocks since ca. 75 Ma.

Paleomagnetic and Other Constraints of Paleogeography Related constraints on the Late Cretaceous paleogeography
of the San Juan Islands also come from paleofaunal data from
Paleomagnetic studies of the rocks in the San Juan Islands the Nanaimo Group rocks. Kodama and Ward (2001) argued
have had mixed success in constraining their tectonic history, that the lack of rudistid bivalves in the otherwise well-preserved
with the main complication being an extensive remagnetization paleofauna of the Nanaimo Group can be used to constrain the
that has affected all of the “internal” units that have experienced paleolatitude of these rocks. Rudistids are tropical to subtropi-
high P-T metamorphism. Burmester et al. (2000) found that these cal reef forming bivalves, and are common in a number of Late
Cretaceous marginal basin rocks from Baja California to Central
California. Using estimated locations of rudist-bearing basins,
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 Aleutian - Wra D F ngell ALASKA
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- P-MF North 60
straint comes from the recognition of a marine reptile fauna from 50 mm/yr America
Nanaimo Group rocks on Vancouver Island, which share some plate
provinciality with the marine reptile fauna of the Moreno Forma- Yakutat F-QCF
tion from central California (Nicholls and Meckert, 2002). terrane,
transform 120
Another set of data, detrital zircon age distributions, has also displacement
been used to test paleogeographic constraints on the location of
the Nanaimo Group rocks. Mahoney et al. (1999) used the pres- Pacific
ence of several Archean-aged zircons to indicate that the Nanaimo plate
Group rocks had been located no more than 500 km south of its
present-day location, during Late Cretaceous time. Using the same 300 KM Cascade arc
set of data, Housen and Beck (1999) compared variations in the
detrital zircon age distributions as a function of stratigraphic posi- LRF
tion within the Nanaimo Group. They argued that variations in
Proterozoic-aged zircons support a source of detritial zircons from Juan UC.ASN. . 50
the Mazatzal andYavapai orogens in southwest NorthAmerica, and de Fuca
that northward migration of the Nanaimo Basin during its deposi- plate 8 mm/yr
tion was consistent with other paleomagnetic evidence, and plate
motion estimates. The analyses of Kodama and Ward (2001), and Siletzia,
Kim and Kodama (2004) also supported the conclusion of Housen fore-arc displacement
and Beck (1999), that the Nanaimo Group reached the “moder-
ate” paleolatitude of ~43° N at 75 Ma, consistent with the so-called Figure 8. Modern-day analogues of orogen-parallel thrusting in the
“Baja-BC” (Baja–British Columbia) hypothesis. Pacific Northwest. F-QCF—Fairweather-Queen Charlotte fault,
DF—Denali fault, LRF—Leech River fault. References: Plafker et al.
Taken together, these paleogeographic data would be most (1994), Wells et al. (1998); Bruhn et al. (2004).
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-
fied rocks of Wrangellia/Insular affinity (Wynne et al., 1995,
Enkin et al., 2003), or barometrically corrected plutonic rocks
(Housen et al., 2003) both indicate more southerly paleolatitudes
(36 N, and 3000 ± 700 km of translation) for these units during
mid-Cretaceous time (93–88 Ma).

Modern Analogues?

Modern tectonic regimes along the western North American
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
(e.g., Wells et al., 1998), and (2) the Yakutat terrane against the
southeast corner of Alaska in the Saint Elias orogen (Plafker et al.,
1994). Siletzia lies in the Cascade forearc, driven by a combina-
tion of oblique plate convergence and Basin and Range extension
(Wells et al., 1998). Seismic reflection allows identification of
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 field trip guides as the departure time draws near, to ensure you
The Yakutat terrane is moving north along the Fairweather– are on the vessel, and the trip can run in a safe and timely fashion.
Queen Charlotte transform fault at 45–50 mm/yr relative to After we have finished the Stuart Island stop, participants will re-
North America (Plafker et al., 1994; Bruhn et al., 2004). At the embark for a ~45 min trip to Spieden Island.
corner area in southern Alaska where plate interaction changes
from transform to convergent, the Yakutat terrane is colliding Stop 1-1. Fossil Cove, Stuart Island, Nanaimo Group
with the continent (Fig. 8). A north-dipping Benioff zone and the (Fig. 10)
Wrangell magmatic arc in this region both testify to significant
subduction of the Yakutat terrane (and probably other materials). The Nanaimo Group comprises a set of 11 formations, ranging
The convergent zone is marked by a thin-skinned accretionary from Turonian to Maastrichtian in age, composed of clastic marine
complex of Cretaceous and younger rocks displaced northward and deltaic sedimentary deposits (Fig. 10). The ages of these rocks
on gently to moderately dipping thrust faults (Bruhn et al., 2004). are constrained by biostratigraphy (e.g., Haggart, 1994), and mag-
Displacements are strongly partitioned between strike-slip faults netostratigraphy (Enkin et al., 2001). These rocks were deposited
and thrusts. Both analogues are characterized by low-dip thrusts in a large marginal basin, extending ~175 km from its southern-
accommodating margin-parallel displacement indicating that most extent in the San Juan Islands to its northernmost extent on
such structure, as possibly fits the San Juan Islands–northwest Vancouver Island. The Nanaimo Group contains several elements
Cascades thrust system, is not a tectonic anomaly. that are of tectonic interest. Structurally, the Nanaimo Group
rocks (along with the Paleocene-Eocene Chuckanut Formation)
FIELD TRIP GUIDE are folded as part of the Cowichan fold and thrust belt (England
and Calon, 1991; see also Mustoe et al., this volume, and Blake
The field trip guide begins at Friday Harbor, San Juan Island and Engebretson, this volume). One of the primary constraints on
(Fig. 9). Before departing, be certain that you have brought along the age of uplift and thrusting of the metamorphosed “interior”
warm clothes, raingear, and good field boots. domain of the San Juan Islands is the presence of metamorphosed
sandstone clasts interpreted as being derived from the Constitution
Please do not use rock hammers or collect specimens any- Formation that are found in conglomerates of the Extension Forma-
where on this trip unless specifically advised. 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 (Housen and Beck, 1999; Mahoney et al., 1999), paleomagnetism
(Ward et al., 1997; Housen et al., 1998; Enkin et al., 2001; Kim and
Day 1 is spent primarily on the terranes “external” to the San Kodama, 2004), and fossil assemblages (Kodama and Ward, 2001)
Juan Islands thrust system. These units are the Haro Formation, have been used to evaluate possible large-scale displacements of
Spieden Group, and Nanaimo Group. They broadly overlap in age the Nanaimo Group rocks.
with rocks in the nappe pile but are distinguished by their absence
of, or very low-grade (zeolite facies), metamorphism, and, in the On Stuart Island, the turbidites and sandstones of the Haslam
case of the Spieden and Nanaimo Groups, an absence of penetra- Formation, the conglomerates of the Extension Formation, and
tive tectonite fabric. These units are important to understanding the the sandstones and siltstones of the Pender Formation can be
younger portion of the tectonic history of the San Juan Islands. found (Fig. 10). A stop at Fossil Cove, on the NW end of Stuart
The field trip will begin with a drive from Friday Harbor across Island (a boat trip of ~45 minutes), allows for examination of the
San Juan Island to picturesque Roche Harbor, on the northern end bedding and sedimentary structures in these rocks, as well as the
of San Juan Island. We will depart from the boat ramp at Roche many fossils (primarily Inoceramus). Time permitting, we may
Harbor, taking a chartered craft to Stuart and Spieden Islands. stop at a beach where the Extension Formation crops out, in order
We will be landing on public access beach areas, but please note to examine the conglomerate clasts of this interesting unit.
that only the intertidal zone in these areas is considered to be pub-
lic property, and that the uplands are privately owned. Access to Stop 1-2. North Shore Spieden Island, Spieden Group
Spieden Island in particular is restricted by its owner. (Fig. 11)

Directions and Other Instructions Spieden Island is one of the largest (perhaps the largest) pri-
Before departing on the Humpback Hauling vessel, be certain vately owned island in the San Juan archipelago. It has a color-
ful history, most notably as “Safari Island,” when in the 1970s
that you have brought warm clothes and your lunch. Even if the a group of investors purchased the island with the bright idea
weather appears to be sunny, raingear is recommended. A lifejacket of transforming it into a private exotic game hunting reserve.
(provided on the vessel) is required at all times, and please do not The island was stocked with many species of exotic game ani-
forget yours on the beach. If you are prone to seasickness, please mals (mostly Asian and African deer, goat, sheep, and antelope
take appropriate precautions. The vessel has a landing-craft type
ramp, so we will be able to disembark on relatively dry land. How-

ORCAS Kn
ISLAND
75

Kn Kn

70 65 Kn
Kn 55 84
1-1 70 Kn
30
STUART
ISLAND

Pe Pt

1-2

Haro fault JKs SPIEDEN
ISLAND
TrJo
Trh Orcas thrust TrJo

1-3 TrJo

Pt Pt

JKc TrJo Jc Rosario thrust Jc

SAN JUAN Roch
ISLAND Harbor

rd.Beaverton Valley rd. SHAW
e ISLAND
Friday
W Harbor

PTrd San Juan Valley rd. Argyle Ave

Lime Kiln Point Pg Jc

1-4 est Side rd. Bailer Hill rd.

Kn = Nanaimo Group TrJo Cattle Point rd.

JKs = Spieden Group Buck Bay fault

JKl = Lopez Structural Complex

Jc = Constitution Formation

TrJo = Orcas Chert

Trh = Haro Formation N American JKl
Camp
PTrd = Deadman Bay Volcanics Pickett's Ln. Cattle
2-1 Point
Pg = Garrison Schist
2-2
Pe = East Sound Group 2.0 km
Pt = Turtleback Complex

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

158 Brown et al.

Turn Point 74
50
70

Kne 65

48 65 78 34 50
83
Knh Kne 85

64 76 80 62 Prevost 28
83 Harbor
Kne Knh Satellite
Fossil 50 76 55 Island 48
Cove Knp 65 81
78 72 72 83 71 Knh
63 62
58

Stop 1-1 64

A Knp 71
56
53 65
70

Nanaimo Group: 56 55 75 70 54
Knp: Pender Fm 75 61
Kne: Extension Fm 61 Kne 50 Reid Harbor
Knh: Haslam Fm Kne 63
Knp Knp
14 N
45 scale 1 km

Stuart Island 50
32
22

55

26

Figure 10 (on this and following page). Geology of Stuart Island, from 24
Mercier (1977). (A) Geologic map. 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
creatures can be seen cavorting around the island today. After leaving the Roche Harbor boat ramp, we will drive to
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
finer-grained turbidite deposits characterizing the Spieden Bluff Memorial Drive.
Formation, and volcaniclastic-rich sandstone, mudstone, and con- 0.2 mi Go left on Roche Harbor Road.
glomerates characterizing the Sentinel Island Formation. The rocks 0.9 Go left (NW) on Afterglow Drive.
also display some soft-sediment deformation features; some have a 1.8 Neck of Davidson Head; park on gravel shoulder on
very weak anastomosing scaly cleavage, and have been folded.
right side of road.
Our field trip stop will be located on a wave-cut bench,
exposed at low tide, on the north shore of Spieden Island. Here Stop 1-3. Davidson Head, San Juan Island, Haro Formation
we will see outcrops of both formations, and localities that dis-
play the locally abundant macrofossils. We will have ~30 min The north shore of San Juan Island is home to one of the
at this location; please follow the instructions of the trip leaders most geographically restricted units in the San Juan Islands—the
closely. After we re-embark, the vessel will take us on a ~40 min Late Triassic (Norian) Haro Formation. This unit crops out on
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 figures, 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
of Davidson Head represents a fundamental structural boundary reset odometer and go left (east-southeast).
in the San Juan Islands. This contact is nowhere exposed, but is 1.3 mi Go right (south) on West Valley Road.
inferred to be a thrust fault (the Haro Thrust), based primarily 2.8 Go right (west) on Mitchell Bay Road.
on the large-scale structural architecture of the San Juan–north 5.6 Go left (south) on Westside Road.
Cascades nappes (e.g., Brandon et al., 1988). 9.7 Turn in at entrance to Lime Kiln Point Park and follow

trail to coast.

A 65 80 63 42 Spieden Island

U 10 U
D 5 D

Sentinel 200 15

Island 22
24
45
33

Lower Cretaceous Sentinel Island Formation U
upper member D

lower member N

Upper Jurassic Spieden Bluff Formation
upper member

lower member 0 200 400
meters
faults

anticline

strike/dip
contour interval = 10 meters

B AGE GROUP FORMATION MEMBER THICKNESS LITHOLOGY DESCRIPTION

not exposed

Early Cretaceous Crudely stratified volcanic
Haurterivian conglomerate and minor
Spieden Group
sandstone
Sentinel Island Formation
Upper member
600 m +

Valanginian Lower member member Lowermembmeermber 114400mm Unconformity
MaMssiavessanivd ethiannlydbetdhdiend ly
Late Spieden Bluff Lower Upper 20 m fossilifebreoudsdsaenddsftoonsesainldifseilrtsotounse
Jurassic Formation sandstone and siltstone
Oxfordian or 80 m
Kimmeridgian Unconformity
not exposed Massive and thinly
bedded fossiliferous
sandstone and siltstone

Massive and crudely
stratified volcanic breccia

and conglomerate,
minor sandstone,
siltstone and tuff

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
grown at a tropical latitude and suggesting large displacements The Deadman Bay Volcanics are separated from the over-
of the terrane (Danner, 1966, 1976; Monger and Ross, 1971). lying Orcas Chert unit by an east-dipping thrust fault; however
The limestone occurs as layers and irregular masses within a these two units are regarded by Brandon et al. (1988) as parts of
sequence of ocean island pillow basalt flows and breccias, named a single terrane based on their mutual similarity of age, lithology,
the Deadman Bay Volcanics (Brandon et al., 1988). The age of the and chemical signature of ocean island basalts.
unit as a whole ranges from Early Permian to Late Triassic based
Return to Friday Harbor via West Side Road and Bailer Hill
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
Jurassic Constitution Formation, mostly composed of volcanic-
Camp Park. rich graywacke sandstone. The fault at this locality (mapped in
7.6 Go right (west) on Salmon Banks Road (dirt road). detail by Brandon et al., 1988) is marked by an imbricate zone
7.9 End of road; park. ~100 m wide bearing lenses and rafts of ribbon chert, sandstone,
mudstone, greenstone, and most significantly HP-LT greenschist-
Stop 2-1. South Beach, American Camp National Park, amphibolite of the Permian Garrison Schist unit.
San Juan Island (Fig. 13)
Amount and timing of displacement on the Rosario thrust are
Americans and British disputed the boundary between their difficult 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 finally 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
to Maekawa and Brown (1993) and Cowan and Brandon (1994).
This part of the Rosario thrust, well exposed at the water’s
edge, has been a key locality for interpretations of San Juan Directions to Stop 2-2
Islands structural evolution (summarized in Figs. 5 and 7). 8.7 mi Retrace route to Cattle Point Road. Turn right (east)
Maekawa and Brown (1991) observed shear zones with fault
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
system (Brandon et al., 1988). The Lopez Structural Complex Recent structural analysis of the Lopez Structural Complex
is an ~2.5 km wide imbricate zone composed of northwest- (Gillaspy, 2004; Schermer et al., 2007), reveals a sequence of
elongated, relatively coherent lenses separated by sheared events that provide insight into accretionary wedge mechanics
mudstone-rich fault zones. The large lenses are predominantly and regional tectonics. After formation of regional ductile flat-
ocean floor clastic and volcanic rocks and Constitution terrane tening and shear-related fabrics (the thrusts and strike slip faults
sandstone; smaller lenses include Turtleback terrane and exotic illustrated in Fig. 5), the area was crosscut by brittle structures
material not found elsewhere in the region. The magnitude of including: (1) southwest-vergent thrusts, (2) extension veins and
offset along the Lopez Structural Complex is unknown, but normal faults related to northwest-southeast extension, and (3)
the inclusion of exotic material such as the Early Cretaceous conjugate strike-slip structures recording northwest-southeast
Richardson complex (stop 2-3) suggests tens of kilometers of extension and northeast-southwest shortening. Aragonite-bearing
movement similar to other terrane bounding faults in the San veins are associated with thrust and normal faults, but only rarely
Juan Islands (Brandon et al., 1988; Cowan and Brandon, 1994). with strike-slip faults (Fig. 16). High-pressure low-temperature
Foliation and fault contacts in the Lopez Structural Complex (HP-LT) minerals constrain brittle deformation to have occurred
dip moderately to steeply northeast (Maekawa and Brown, at ≥20 km and ~200–300 °C. The presence of similar structures
elsewhere indicates the brittle structural sequence is typical of the

Buck Bay Fault N

?

Const. t 40 LOPEZ FAULT 0 1 2 km
Terrane 43
Shark 50 40
39 Reef 28

55 Fidalgo Complex
47

Cattle Pt. Stop 2-2 51 55

Ocean Floor Complex 57 Richardson Ocean Floor
Complex
Clastic sequences Stop 2-3 70 45 (undivided)
Volcanic rocks and
associated sedimentary rocks John's Pt. 55 69 ? 68 74 ? Watmough

Constitution Terrane 49 60 75 64 Head
45
Stop 2-4 70 62 ?
55 56
47

Iceberg Pt. 60 t ? Point
Colville

"Exotic" and Other slices

Richardson Basalt Complex Approximate
fault contact

Sandstone with t Turtleback Complex Major terrane
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 field trip stops 2-2 (Cattle Point),
2-3 (Richardson) and 2-4 (Iceberg Point). Modified 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

A *? Aragonite found N

in vein sample

Const. * LOPEZ Lopez Island
Terrane
FAULT 0 1 2 km
San Juan Island **
Ocean Floor
* * Fidalgo Complex Complex
**

B 100 8 6 * *** ** * ** ?
8 * * ?
80 ? **
13 6
60

%

40

20 1

0 Early Thrusts Extension Normal Late
Deformed Shear Veins Faults Strike-slip
Veins Veins
n=7 n = 22 n = 10 Faults
n = 12 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 classified 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 fluid flow 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
examine early ductile and late brittle deformation. In the sheared
Given that these latest identified structures likely formed in mudstone, which is interpreted by Bergh (2002) to contain a
an accretionary wedge setting, we are faced with the dilemma composite S1-S2 fabric, there is evidence of NW-SE shearing,
of not having found the Late Cretaceous structures related to interpreted as top to the NW thrusting by Maekawa and Brown
emplacement in northwest Washington. These emplacement (1991) and sinistral reactivation of SW-vergent thrusts by Bergh
structures, if they formed by any of the models illustrated in

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-
ton, in 1977. Map relations displayed at this site show a layered
0.0 Ferry terminal on Lopez Island, drive south on Ferry sequence of pillow basalts, pillow breccias, tuff and mudstone
Road. (Fig. 19). All these rocks were considered to represent a coherent
mid-Cretaceous stratigraphic assemblage (Brandon et al., 1988).
1O km 122 W N 48 However, recent Ar-Ar analysis of blueschist facies phengitic
G mica in the pillow breccias (Fig. 20) yielded an age of 124.43
73 53 ± 0.72 Ma (Brown et al., 2005). Revisitation of the fossil ages
K-is K-bj in the Whetten sample indicates a late Aptian age (112–115 Ma)
(Fig. 21). Remapping the structural features demonstrates that the
F fossiliferous mudstones (Fig. 21) are faulted into the sequence.
These findings broaden the age brackets for thrusting, and sug-
E gested to Brown et al. (2005) that San Juan Islands blueschist
H metamorphism is older than thrusting. But, a recent finding of
aragonite veins in the late Aptian mudstones by Schermer et al.
D (2007) indicates that the blueschist metamorphism continued to
at least that time and was apparently coeval with and outlasted
48 thrusting, as interpreted by earlier workers (Brandon et al., 1988;
Maekawa and Brown, 1991).

B 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
deformation from Schermer et al. (2007), Gillaspy (2004), and Lamb
C (2000). Small grey arrows indicate direction of magnetic vector from
Burmester et al (2000); inclination values omitted; small arrowheads
thrust vergence T axis from normal indicate upward inclination. K-is and K-bj show expected directions
faults, extension veins for in situ and Baja-BC terrane models of the Cretaceous location
T axis from of San Juan terranes. Other arrows indicate kinematic directions of
strike slip faults subhorizontal brittle structures as defined in key. If no arrow is shown for brittle
extension structures at a site, data are too few to conclude kinematic significance.
Circles indicate subhorizontal extension in several directions during
normal faulting or extension veining. Foliation symbols show average
foliation direction and are located at reconnaissance study sites: at all
sites, the same sequence of faulting is observed, but not all sites have
enough data to conclude kinematic significance of all phases of fault-
ing. A—San Juan Island; B—N. Lopez Island; C—Watmough Head;
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

TrJo Jc BLAKELY
ISLAND
Ferry Road
Jf
Jf

ORCAS
ISLAND

Jc Road
Road
SAN JUAN Jf Jf
ISLAND
Ba

LOPEZ ISLAND
Fisherman Jl DECATUR
y ISLAND

Center

Jf

JKl RichardsonMud

2-2 JKl Bay

2-3 Aleck Bay Rd Jl

Road

Jl = Lummi Formation JKl N
Jf = Fidalgo Complex
Jc = Constitution Fm. 2-4 2.0 km

JKl = Lopez Structural Complex
TrJo = Orcas Chert

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
time and tide permits, we will also examine a shear zone between
There is no parking at the end of the road; note a sign indi- these rocks (of ocean-floor affinity) and a fault slice of Consti-
cating “private road” at the end of the public road. There are two

168 Brown et al.

122O53.5'48O27.0' 10 18 N
6

Pb 35 Pl 26

35 RICHARDSON 26

20 Pb ROAD Pl
35 0 5 10
meters
30 12 = elevation in feet
Bay above mean low tide

25 Pb 34

55

Faultmazilobonxe

Pb 22 Pb

2 Fossil
Pb sites
Ar samples
15
55
6 35 Attitude of fault

Pl surface
12
20 Attitude of slicken-
Pl
18 lines in fault zone

FUEL 15fenceAttitude of meta-
TANKS morphic foliation
Mudstone

Tuff
Pb Pillow breccia

Pl Pillow lava

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
sequence distinctive of ophiolite. From the base upward in the
on Washington State Highway 20 spur to the intersection with the section are: ultramafic tectonite, cumulate gabbro, a sheeted
main route of highway 20, at Sharps Corner.

Tectonic evolution of the San Juan Islands thrust system, Washington 169

lated (Garver, 1988; Blake and Engebretson, 1994). This unit
also bears some affinity 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

Figure 20. Photomicrograph of metamorphosed pillow breccia at Rich- Ultramafic rock here is interpreted to be basement to the
ardson, sample R3, from which phengite gives an Ar-Ar age of 124.75 Fidalgo ophiolite (Fig. 22) based on its position structurally
± 0.87 Ma (Brown et al., 2005). Phengite and chlorite crystallized beneath the other parts of the ophiolite; however, the contact is
from volcanic glass, plagioclase is altered to fine-grained aggregate covered by Quaternary materials. Minerals are serpentinite (after
of Ca-Al silicates. Aragonite and pumpellyite (not visible), as well as olivine), relict pyroxene, and chromite. Protolith rock ranges from
chlorite and phengite shown here, are synkinematic minerals. dunite to peridotite. Rock that was originally peridotite is marked
by significant amounts of relict pyroxene, together with serpen-
intrusive complex of mostly plagiogranite (diorite, tonalite, tine, whereas the original dunite is virtually free of pyroxene.
trondjemite, albite granite) and hypabyssal equivalents, a vol- The meta-peridotite and meta-dunite are thus distinguishable and
canic sequence of mainly dacitic to andesitic breccias and inter- can be seen as irregular layers through this exposure. Pyroxenite
layered tuffaceous argillite, coarse sedimentary breccia that bears veins exhibiting comb structure cross the other lithologies. We
clasts of all the underlying units, pelagic argillite, and volcanic- will speculate about the origin of these layers and veins and what
rich graywacke at the top of the section. U-Pb zircon ages of the information they might provide about mantle deformation and
plagiogranites on Fidalgo Island are 167 ± 5 Ma, and elsewhere basalt genesis.
are 160 ± 3 Ma on Lummi Island and 170 ± 3 on Blakely Island
(Whetten et al., 1978, 1980). Radiolaria in the pelagic argillite Directions to Stop 3-2
are late Kimmeridgian–early Tithonian ca. 150 Ma (Gusey, 1978; Continue around the “loop road”; exit Park back to
Brandon et al., 1988). The U-Pb age pattern of detrital zircons
from a sample of the graywacke unit bears a single prominent Sunset Ave.
peak at 148 Ma, considered to represent a nearby volcanic prov- 3.1 mi Turn right on Anaco Beach Road and continue on to
enance (Brown and Gehrels, 2007). All these rocks are affected
by prehnite-pumpellyite metamorphism. merge with Marine Drive.

The plutonic part of the Fidalgo Complex is interpreted to Stop 3-2. Private Property along Marine Drive,
be a remnant arc. An arc origin of the ophiolite is indicated by Fidalgo Island (Fig. 22)
the abundance of intermediate to felsic igneous rocks (Brown,
1977; Gusey and Brown, 1987; Burmester et al., 2000). The (Access is not guaranteed as of this writing.)
coarse breccia and overlying radiolarian argillite stratigraphically Cumulate gabbro displays layering formed by differential
above the igneous rocks indicate that the 160–170 Ma arc was settling of pyroxene and plagioclase crystals in the melt (Fig. 23).
rifted and terminated as a volcanic center prior to deposition of Dikes of plagiogranite and keratophyre occur locally in the gab-
the sedimentary part of the section. Thus the arc was faulted and bro, and exclusively as a sheeted complex higher in the section.
shifted off its magmatic axis before attaining much crustal thick- The orientation of bedding in the gabbro and direction of grad-
ness or subaerial exposure. The old eroded arc was then buried ing are consistent with its mapped structural position low in the
at ca. 148 Ma by younger clastic arc detritus from an adjacent ophiolite stratigraphy, but above the ultramafic rock.
volcanic axis. This evolution is similar to that of modern remnant
arcs (e.g., Karig, 1972). Directions to Stop 3-3
Continue south on Marine Drive
The Fidalgo complex is similar in age and lithology to the
California Coast Range ophiolite with which it has been corre- 5.8 mi Turn right (south) on Havekost to the intersection with
Rosario Road.

6.8 Rosario Road: turn right (east).
7.8 Heart Lake Road: turn left (north).
9.0 Go right (east) on Ray Auld Drive.

170 Brown et al.

0.20 mm B

A foliated rind
S2
bedding
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) Unidentified
foram in mudstone. Age definitive microfossils were not found in this specimen; however a sample from an unspecified 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 defines 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 defined by flattened 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,
and do not support a hypothesis of postmagmatic chemical altera-
outcrops. tion. Igneous minerals observable in thin-section are plagioclase,
clinopyroxene, quartz, and opaques. Metamorphic minerals, in
Stop 3-3. Roadcut along Erie Mountain Drive, veins and incipiently developed in the igneous matrix, are chlo-
Anacortes City Park, Fidalgo Island (Fig. 22) rite, epidote, pumpellyite, prehnite, albite, and quartz. Identifica-
tion of aragonite at one locality (Gusey, 1978) has not been con-
Observe green volcanic breccia. The lithology here is kera- firmed by X-ray analysis of many other carbonate samples from
tophyre (= meta-dacite). The volcanic section of the ophiolite the Fidalgo ophiolite (M.C. Blake, 2006, personal commun.).
ranges from 48 to 74 wt% SiO2 (Brown et al., 1979). K2O is typi-

Ferries Oaks Ave Anacortes

12th St.

Q
Washington Sunset Ave. 20 Spur Havekost Road
Park Commercial Ave.
Anaco Road Q
3-1

41st St. 20 Spur

Burrows Is. 45 65 60
Allan Is. 70
Marine Drive 85 40 35 Detrital zircon
70 75 sample
3-5 45
3-2 3-3 75
60 Sharps
N Heart Lake Road 65 Corner

30
WA 20
25

Q Rosario Road Mt Erie
3-4

1 km

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
167 5 Ma dikes

CUMULATE
GABBRO

1000m unexposed
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 floor crust, and indicates an affinity 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.

Figure 23. Cumulate bedding in layered gabbro, Fidalgo Island. Directions to Stop 3-5
Retrace route down the Erie Mountain Drive.
Fifty meters down the road, and structurally below the
volcanic rock, is dark brown, manganese-rich, radiolarian 12.3 Go south on Heart Lake Road.
argillite. This rock unit, termed “pelagic argillite,” is as much 13.5 Turn right (west) on Rosario Road.
as 500 m thick and forms the second sedimentary layer up in 14.4 Turn right (north) on Havekost Road, past intersection
the ophiolite section (Figs. 22 and 24). An unexposed thrust
fault separates these rocks. This structure as well as other with Marine Drive.
shear zones in the Fidalgo ophiolite have not been analyzed 16.2 Entranceway to the Lakeside Industries quarry is on
but have potential for addressing the kinematics of the San
Juan Islands thrust system. the right.
Obtain permission at the office. Hard hats and vests are
Directions to Stop 3–4 required. Avoid quarry slopes, which are unstable and dangerous.
Continue up Erie Mountain Drive.
Stop 3-5. Lakeside Industries Quarry, Fidalgo Island
10.7 mi Summit of Mount Erie. (Fig. 22)

Stop 3-4. Mount Erie Summit, Anacortes City Park, Here we observe non-faulted stratigraphic contacts between
Fidalgo Island (Fig. 22) the plagiogranite and sedimentary breccia, and between the brec-
cia and overlying pelagic argillite. The coarse breccia consists of
Massive diorite of the sheeted zone is intruded by fine- clasts of all lithologies of the underlying plutonic section includ-
grained green dike rock (keratophyre and spilite). See views of ing ultramafic rock, and therefore indicates uplift and exposure
the Olympic Mountains Tertiary subduction complex, Admiralty of the deeper levels of the section, presumably by faulting. The
Inlet to Puget Sound, glacial drift from the Puget lobe, Eastern breccia represents slide and/or talus deposition. Presence of
and Western Mélange belts in the Cascade foothills. radiolaria (Fig. 24) and high manganese content of the overlying
argillite indicates a marine environment enriched by alteration of
volcanic materials and isolated from continent-derived sediment.
The argillite is a chloritic mudstone with minor tuffaceous layers
and thin sandstone beds with ultramafic detritus (Gusey, 1978).
Volcanic-rich graywacke overlies these sedimentary rocks and
bears detrital zircons with a 148 Ma age peak, younger than the
breccia detritus which is derived from the 160–170 Ma under-
lying arc (Fig. 22). The Fidalgo ophiolite is interpreted to be a
remnant arc, and the overlying graywacke to have been deposited
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
from mid-oceanic-ridge basalt (Dungan et al., 1983) known
0.0 Sharps Corner, main highway 20, reset odometer; as the Shuksan Greenschist (Misch, 1966); (2) quartzose car-
continue east. bonaceous phyllite, derived from mudstone, named the Dar-
rington Phyllite; (3) metagraywacke semischist derived from
6.8 mi Highway 237 (Farm to Market Road); go north to the sandstone with abundant chert and dacitic-andesitic clasts;
village of Edison. (4) local pods of metamorphosed plutonic rock of tonalitic to
gabbroic composition; and (5) a local zone of high-pressure
14.6 Bow Hill Road; go right (east). amphibolite and eclogite. The suite as a whole defines the
15.6 Chuckanut Drive; go left (north). “Shuksan Nappe” of Tabor et al. (2003), a sheet some 100 km
19.6 Cross Oyster Creek (at hairpin turn). in length and breadth exposed across much of the northwest
19.7 On the left is Oyster Creek Inn and the road to Taylor Cascades and breached in an anticlinal structure known as the
“Mt Baker window” (Misch, 1966) where underlying nappes
shellfish farm (sign). Head down this one-lane road, can be observed.
across Oyster Creek at the bottom of the hill, continue
for ~100 m, and park on the right near the railroad
tracks. Hike across the tracks and north along the tide
flats to the mouth of Oyster Creek.

N CZ NK

10 km

CZ

Bay Bellingham BP Mount
meta-tonalite Baker
LM meta-
gabbro 163 2 Ma BP
163 2 Ma CH
TS

3-6 meta-gabbro CH NK BP
164 2 Ma BP
Edison
I-5 Detrital
Chuckanut Dr. zircon
155 Ma peak

Anacortes Farm to Market rd. HH 130 5 Ma CH
237 blueschist

FC I-5 128 4 Ma
blueschist
WA 20 HH 144-160 Ma
amphibolite
CZ Cz

HH WM EM CC

Easton Metamorphic Suite CH

meta-pelite meta-oceanic basalt
Darington Phyllite Shuksan Greenschist

meta-pelite and amphibolite and
-graywacke rare eclogite

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 surficial deposits; abbreviations
of other units given in Table 1.

174 Brown et al.

An ocean floor stratigraphy is evident where the Shuksan META -GABBRO N
Greenschist is stratigraphically overlain by a thin zone of metal- ~ 163 Ma
liferous quartzose rock which is in turn overlain by Darrington 30 m
Phyllite (Haugerud et al. 1981). The metagraywacke unit is inter- colluvium
layered with Darrington Phyllite in the western part of the Shuk-
san Nappe and represents volcanic arc and flysch detritus. Based 70 SEMISCHIST
on the above relations, Gallagher et al. (1988) proposed a back 45 <155 Ma
arc setting for the Easton Metamorphic Suite.
tide flats 5 Oyster Ck
Protolith ages are indicated by U-Pb zircon ages of the
tonalite and gabbro bodies at 163–164 Ma (Fig. 25; Walker in stretching 0 10
Gallagher et al. 1988, Dragovich et al. 1998, 1999) and detrital lineation
zircons in a sample of the graywacke yielding a prominent age foliation 50
peak at 155 Ma (Brown and Gehrels, 2007). The gabbro-tonalite 40
bodies occur within the graywacke stratigraphy and bear the
same metamorphic mineralogy and tectonite fabric as the gray- local
wacke. These relations and the older age of the gabbro-tonalite late
bodies imply that they were faulted or slid into the graywacke deformation
depositional basin.
Figure 26. Map of the outcrop area of stop 3-6, near the mouth of
Metamorphic ages known from Rb-Sr and K-Ar ages of Oyster Creek.
muscovite 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 Figure 27. Photomicrograph of semischist showing stretched chert
Easton Suite, which comprises the “Shuksan Nappe.” Meta- clasts at stop 3-6.
morphic mineralogy and structure point to continuation of the
Shuksan Nappe somewhat beyond this point into small islands The field trip ends here. Return to Chuckanut Drive and go
of the eastern San Juan archipelago (Lamb, 2000). Some workers north to Bellingham or south to I-5.
have considered that the Shuksan Nappe possibly extended as
a structurally high unit across the San Juan Islands contributing ACKNOWLEDGMENTS
to the 20-km-thick burial required for aragonite metamorphism
(Brandon et al., 1988). Clark Blake and Eric Force, both retired from the U.S.
Geological Survey, reviewed and considerably improved the
The semischist exposed here is chert rich (Fig. 27) and is manuscript.
interbedded with carbonaceous phyllite. Stretched chert clasts
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Tectonic evolution of the San Juan Islands thrust system, Washington 175

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