Tectonic evolution of the San Juan Islands thrust system, Washington

Transcription

The Geological Society of America
Field Guide 9
2007
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 regarding 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, kinematic analysis, paleomagnetism.
INTRODUCTION
Rocks and structures of the San Juan Islands of northwest
Washington record a long and complex history related to Cordilleran convergent margin tectonism. The area is underlain by
the San Juan Islands–northwest Cascades thrust system, made
up of nappes a few kilometers or less thick and up to 100 km in
breadth (Figs. 1, 2), thrust onto the continental margin during
mid-Cretaceous time (e.g., Misch, 1966; Brown, 1987; Brandon et al., 1988). The nappes have an oceanic history, indicating accretion to the edge of the North American continent, but
they also bear clear evidence of interaction with the continental margin long preceding their emplacement in Washington.
Their mid-Cretaceous arrival in Washington as thrust sheets
was likely the consequence of some type of post-accretionary
fragmentation and dispersal. The timing and mechanisms of the
accretion, dispersal and final emplacement of terranes of the
San Juan Islands–northwest Cascades thrust system are poorly
known and have been the focus of our recent work.
Many aspects of the lithology, structure, and metamorphism
are similar to the Mesozoic evolution of other parts of the Cordillera; other aspects may be unique to the San Juan Islands. The eastwest transect across the San Juan Islands during this field trip will
highlight the different terranes juxtaposed by the thrust system,
and structures formed before, during and after high-pressure–
low-temperature (HP-LT) metamorphism. The trip builds on earlier work that identified the main terranes and structures in the
San Juan thrust system (e.g., McClellan, 1927; Danner, 1966;
Vance, 1975; Whetten et al., 1978; Brandon et al., 1988). Our
recent results on structure, metamorphism, geochronology, and
paleomagnetism will provide a forum for discussions that bear
on the tectonic history and correlation with other Cordilleran
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 editing@geosociety.org. ©2007 The Geological Society of America. All rights reserved.
143
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Brown et al.
Figure 1. Regional setting of the San Juan Islands—northwest Cascades area in the northwest Cordillera. AX—Alexandria; BA—Baker
terrane; CC—Cache Creek terrane of Miller (1987); CH—Chugach
terrane; EK—Eastern Klamath terrane; FR—Franciscan complex;
GV—Gravina belt; GVS—Great Valley sequence; H—Huntington
terrane; IZ—Izee terrane; MT—Methow basin; QS—Quesnellia;
SC–FR—Straight Creek–Fraser River fault; SF—Shoo Fly complex;
ST—Stikinia; WA—Wallowa terrane; WJ—Western Jurassic belt;
WR—Wrangellia; WTrPz—Western Triassic and Paleozoic belt;
YT—Yukon-Tanana terrane. Sources: Burchfiel et al. (1992a); Gehrels
and Kapp (1998); Wheeler and McFeely (1991). B.C.—British
Columbia; CA—California; cc—Cache Creek belt; Cz—Cenozoic
rocks and surficial deposits; ID—Idaho; mc—McCloud belt of Miller
(1987); OR—Oregon; NV—Nevada; Wash.—Washington.
CH
AX
56
13
5
ST
GV
AX
YT
128
56
AK
B.C
.
WR
CC
QS
cc
COA
ST
units that experienced subduction and HP-LT metamorphism. In
particular, we would like to consider how the geology of the area
relates to various hypotheses regarding the origin and paleogeography of the terranes, and the evolution of deformation before,
during, and after emplacement in their current location.
ST
mc
SC-FR fault
t
IC
ul
fa
MT
m
co
TON
la
Ya
PLU
130
50
C
O
M
WR
P
LE
X
QS
mc
MT
Fig. 2
TECTONIC SETTING
116
B.C.
Q S Wash.
CP
C
ID
Cz
Columbia
Embayment
WA
Cz
BA
OR
Blue Mtns.
WJ
42
125
cc
H
IZ
cc
W
Tr
P
z
200 kilometers
cc
EK
mc
FR
Klamath
Mountains
CA
NV
No.
Sierra
cc SF
GVS
mc
42
116
46
50
The San Juan Islands–northwest Cascades thrust system lies
at the south end of the 1500 km long Coast Plutonic Complex,
a belt of continental arc plutons and metamorphic country rock
that formed from Late Jurassic to Early Cenozoic (Figs. 1, 2).
Outboard of the Coast Plutonic Complex and intruded by it is
the Insular superterrane composed of the co-joined Wrangellia
and Alexander terranes. Inboard of the Coast Plutonic Complex
are rocks of the Early Cretaceous continental margin, including
the Methow stratigraphic sequence in Washington. Detritus, current indicators and stratigraphy in the Methow sequence indicate
absence of an outboard sediment source until ca. 110 Ma (Tennyson and Cole, 1978; Haugerud et al., 2002), thus we view the
locale of the Washington Cascades and San Juan Islands as an
ocean basin until that time. Major orogenic activity characterizes
the region from ca. 110–80 Ma, during which nappes of the San
Juan Islands–northwest Cascades thrust system were emplaced,
the Coast Plutonic Complex was intruded by voluminous arc
plutons, and country rock of the complex was locally buried to
depths of up to 35 km (in the “Cascade crystalline core”; Figs. 2
and 3) and was deformed by orogen-normal and orogen-parallel
displacements. Overlapping the waning stages of this orogenic
pulse was development of the Nanaimo stratigraphic sequence,
bearing detritus from the San Juan Islands–northwest Cascades
thrust system as well as from the Coast Plutonic Complex, in
an elongate basin extending north from the San Juan Islands. In
Eocene time the orogen was cut obliquely and displaced ~170 km
(estimates range from 90 to 190 km; e.g., Vance, 1985; Misch,
1977) by the N-S dextral, strike-slip Straight Creek–Fraser River
fault system. Restoration of the fault shows the San Juan Islands–
northwest Cascades thrust system to have lain along the southern
margin of Wrangellia and the Coast Plutonic Complex (Fig. 3)
123
CPC
Q
VC
B.C.
WA
YA
BP
ES
A
T
121
49
TB
CZ
LM
CO
NK
Mt Baker
EA
FC
EA
NK
e fault
EA
LS
A'
TS YA
s Lak
CH
GA
Ros
OC
HS
HZ
NORTHWEST
NA
WR
CH
CH
SAN JUAN
ISLANDS
CASCADES
LS HH
HH
D
lan
WM
N
elts
ge B
Q
F
DM
Me
EM
EM
CRYSTALLINE
T
PUGET
SOUND
88-96 Ma
plutons
CORE
WM
T
Straight Ck. - Fraser R
. fault
48
30 kilometers
CW
TG
T
CN
Windy
Pass
Thrust
CN
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'
NA
Orcas Island
GA
OC
HS
TB
CO
ES
FC
TB
FC
LM
ES
WRANGELLIA
B
Mt Baker
window
Twin Sisters
Range
VC
YA
Lummi
Island
EA
CN
OC
TS
BP
CH
CH
EA
BP
CH
YA
SL
EA
CH
NK
Chilliwack
batholith
Shuksan
thrust
BP
CC
NK
depositional or intrusive contact
fault contact
Straight
Creek fault
10 km
no vertical exaggeration
Figure 2 (continued). (B) Cross section.
in Late Cretaceous time. South of this orogenic complex is the
Columbia Embayment, an area covered primarily by Cenozoic
volcanic rocks, thought to be underlain by primitive crust, and
considered in some models to be a possible homeland for the
thrust system nappes (e.g., Davis et al., 1978; Vance et al., 1980).
East and south of the Columbia Embayment are accreted terranes
of the Blue Mountains, and Klamath Mountains respectively
(Fig. 1), the latter especially bearing similarities to units of the
San Juan Islands–northwest Cascades thrust system.
Q
Mid-Late
Cretaceous
Plutons
MT
ELLI
A
B.C.
WA
Ro
Jur. -
s
SE
SE
E. Cret.
s
HZ Lake
Fa
ult
N
W
G
RA
NG
eo
N
EL
LI
STRUCTURAL STRATIGRAPHY
A
Plutons
rg
ia
NA
PRC
N
CW
S
tr
ai
t
B.C.
WA
NK EA
NWCS
NAPPES
PACIFIC
OCEAN
N
CZ
Zo
ne
Cascade
Crystalline
Core
HL
E
ANGBELTS
MEL
The nappe pile of the San Juan Islands–northwest Cascades
thrust system (Fig. 2) is characterized by mid to late Paleozoic
terranes overlain by Mesozoic terranes. The structurally lowest
component of the nappe complex is the East Sound Group in the
San Juan Islands and correlative Chilliwack Group in the Cascades. These are island arc derived sedimentary and volcanic rocks
of Devonian–Permian age (Danner, 1966; Vance, 1975; Misch,
1966; Tabor et al., 2003). Calc-alkaline Devonian plutonic rocks
presumed to be related to this arc are the Turtleback and Yellow
Aster Complexes of the San Juan Islands and Cascades, respectively (Mattinson, 1972; Whetten, et al., 1978; Brandon et al.,
1988; Tabor et al., 2003). This assemblage is likely related to arc
rocks that extend from California to northern British Columbia
and mark mid-late Paleozoic convergence along the continental
margin (McCloud belt of Miller, 1987).
Higher in the nappe pile, in both the San Juan Islands and
Cascades, is a disrupted section including Permian to Jurassic ribbon chert, Permian HP-LT schist, ocean island basalt,
Permian limestone bearing Tethyan fusulinids (exotic to North
America), and other materials (Fig. 2, Table 1). In the San Juan
Islands, units are Orcas Chert, Deadman Bay Volcanics, and Garrison Schist (Brandon et al., 1988), observed on this field trip. In
the Cascades, this zone is referred to as the Bell Pass Mélange
and in addition to the above mentioned rock types includes the
10 × 4 km Twin Sisters dunite slab (Tabor et al., 2003). Rocks
and structures of this zone are similar to the “Cache Creek belt”
of Miller (1987) that extends sporadically from northern British
Columbia to California and apparently represents accretionary
mélange of mainly oceanic rocks.
UE
SN
ING
EA
WPT
HB
MN
CZ
SC-FR fault
restored
100 KM
Figure 3. Regional geology shown with hypothetical restoration of the
Straight Creek–Fraser River fault. (SC-FR fault) based on ~170 km of
displacement (e.g., Umhoefer and Schiarizza, 1996). Abbreviations:
HB—Hicks Butte inlier, HL—Harrison Lake stratigraphic sequence,
MN—Manastash Ridge inlier, PRC—Pacific Rim Complex, SE—
Settler Schist. See Table 1 for other unit abbreviations.
The highest nappes in the San Juan Islands–northwest Cascades thrust system are Late Jurassic rocks that include ophiolitic plutonic rocks, mid-oceanic-ridge basalt, ribbon chert, and
arc-derived mudstone-sandstone. Units of these upper nappes
that we will examine include rocks in the Lopez fault zone, the
Constitution Formation, Fidalgo Ophiolite and Easton Metamorphic Suite. These units are closely similar to terranes in the
western Jurassic belt, Franciscan Complex and Coast Range
Ophiolite of the Klamath Mountains and California Coast
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
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.
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.
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.
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.
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 prehnitepumpellyite 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.
TABLE 1. KEY TO UNITS
148
Brown et al.
to be approximately at the same level and laterally contiguous.
This is based in part on correlations of terranes between the two
regions (as shown in Fig. 2 and Table 1). The structural model
also assumes a simple in-sequence assembly of the nappe pile.
Because the stratigraphy is not exposed under the broad nappe of
Easton Suite between the Cascades and San Juan Islands, out-ofsequence thrust models relating nappes in these two areas could be
viable. Cowan and Bruhn (1992) proposed that Cascades nappes
lie at a higher structural level than those in the San Juan Islands.
McGroder (1991) favors a break in continuity of nappes in the
hidden zone between the Cascades and San Juan Islands caused
by out-of-sequence thrusting and folding of the nappe pile.
Peripheral to the San Juan Islands nappe pile along its
northwest flank are the arc-derived Late Triassic Haro Formation, the Late Jurassic–Early Cretaceous Spieden Group, and the
Late Cretaceous Nanaimo Group bearing detritus from the thrust
system. These units lack evidence of HP-LT metamorphism and
penetrative tectonite fabric, and thus are considered to be “external” to the thrust system (Brandon et al., 1988). Owing to the different tectonic and metamorphic histories, a fault is assumed to
separate the nappe pile from the external units. This fault, named
the Haro fault, cannot be directly observed, but is inferred to dip
under the nappe pile based on regional dips and a gravity survey
(Johnson et al., 1986; Palumbo and Brandon, 1990). The Haro
fault may have been reactivated during south-vergent thrusting in
the Cowichan fold and thrust belt (England and Calon, 1991).
The ultimate footwall to nappes of the San Juan Islands–
northwest Cascades thrust system is problematic in the San Juan
Islands, but clearer in the Cascades. Based on arguments given
above that external units underlie the San Juan Island nappes and
observation that Wrangellia underlies Nanaimo Group units on
Vancouver Island, one could infer that Wrangellia is basement
to the San Juan Island nappes (e.g., Cowan and Bruhn, 1992).
In the Cascades, evidence indicates that nappes are thrust over
the southern end of the Coast Plutonic Complex. In the central
Cascades, the Ingalls Complex, a component of the San Juan
Islands–northwest Cascades thrust system, is thrust over Chiwaukum Schist and Mount Stuart batholith along the Windy
Pass Thrust (Figs. 2, 3; Miller, 1985). In the northwest Cascades,
the relatively undeformed Jurassic-Cretaceous Nooksack Group
which underlies the nappe pile (e.g., Misch, 1966) appears to be a
southern extension of the Harrison Lake stratigraphic sequence in
the southern British Columbia Coast Plutonic Complex (Fig. 3;
Monger and Journeay, 1994). Along its western flank, the Coast
Plutonic Complex is intrusive into Wrangellia. Thus, one interpretation for the regional structure is that Wrangellia and the
Coast Plutonic Complex constituted a structural block in midCretaceous time that served as footwall to the San Juan Islands–
northwest Cascades thrust system in both the San Juan Islands
and Cascades (e.g., Brown, 1987; McGroder, 1991; Monger and
Brown, 2008). Other interpretations place nappes of the San Juan
Islands–northwest Cascades thrust system within, and as part of,
the country rock of the Coast Plutonic Complex (Monger and
Journeay, 1994; Cowan and Brandon, 1994).
AGE OF THRUSTING
The age of assembly of the nappes is uncertain because
observed structures could potentially have formed during one
of many tectonic events, including initial accretion going back
to the Paleozoic for the older terranes, post-accretionary terrane translation of at least hundreds of kilometers, emplacement of nappes into the regional geologic setting of northwest Washington, and deformation related to the Eocene and
younger fold and thrust belt affecting the Nanaimo Group
and Chuckanut Formation (England and Calon, 1991). Certainly some metamorphic fabric and possibly some fault
boundaries are inherited from events pre-dating assembly of
nappes in their present setting (Brown et al., 2005). However,
there is good evidence for major mid-Cretaceous assembly.
This deformation is referred here to as the thrusting event.
Nappes of the thrust system were emplaced and unroofed in
the San Juan Islands vicinity by the time of deposition of the
Nanaimo Group (Vance, 1975); the oldest part of the Nanaimo
known to bear detritus from the thrust system is ca. 85 Ma
(latest Campanian-earliest Santonian; Brandon et al., 1988). A
maximum age for thrusting in the San Juan Islands is given by
fault juxtaposition of Late Aptian (112–115 Ma) fossiliferous
rock with 124 Ma HP-LT metamorphic rock on Lopez Island
(one of our field trip stops). In the Cascades, a population of
detrital zircons in the Nooksack Formation (footwall to the
nappes) gives a maximum depositional age of 114 Ma, and a
large sandstone raft in the Bell Pass Mélange bears 119 Ma
detrital zircons (Brown and Gehrels, 2007).
More precise ages of thrusting are known for two localities:
K-Ar whole rock ages of 87 and 93 ± 3 Ma were obtained for two
mylonite samples from the west flank of the Twin Sisters Dunite
(Armstrong in Brown, 1987). Movement on the Windy Pass
thrust is dated at ca. 94 Ma by relationships with U-Pb zircondated plutons that predate, postdate and are involved in thrusting
(R.B. Miller et al., 2003). Thus, major displacement is broadly
bracketed between ca. 115 and 85 Ma based on youngest terranes
involved and the age of rocks bearing detritus of the nappes, and
a more limited time frame is suggested to be ca. 90–95 Ma from
dated rocks in two fault zones.
METAMORPHISM
Most units in the San Juan Islands–northwest Cascades
nappe pile show effects of Cretaceous HP-LT metamorphism.
The degree of recrystallization and metamorphic fabric development varies greatly, even within the same units. In the Cascades, evidence of HP-LT metamorphism is found virtually
in all thrust system units of Jurassic or older age. The blueschist facies Easton Metamorphic Suite in the Cascades bears
synkinematic metamorphic minerals dated at 120–130 Ma by
K-Ar and Rb-Sr (Brown et al., 1982; Armstrong and Misch,
1987). Rock units younger than 120 Ma (Nooksack Formation and sandstone in the Bell Pass Mélange) lack definitive
evidence of high-pressure metamorphism. In the San Juan
Islands, aragonite (Fig. 4) and lawsonite are widely developed in Jurassic and older rocks that are otherwise relatively
unaltered (Vance, 1968; Glassley et al., 1976). This incipient
HP-LT metamorphism has been considered to be related to
mid-Cretaceous thrusting (Brandon et al., 1988; Maekawa
and Brown, 1991) but so far the only isotopic ages available,
Ar-Ar muscovite, indicate metamorphism at 124 Ma (Brown
et al., 2005) and ca. 137–154 Ma (Lamb, 2000), older than the
emplacement phase of thrusting.
The age of blueschist metamorphism relative to thrusting
is critical to understanding the tectonics of the thrust system.
If aragonite was formed during thrusting, burial on the order
of 20 km is required at the ~200 °C temperature estimated for
metamorphism (Brandon et al., 1988), indicating a great thickness of overlying nappes. An alternative concept that blueschist
metamorphism in the thrust system is inherited from an event
predating nappe emplacement may be possible for the older
terranes. However, Schermer et al. (2007) showed that HP-LT
metamorphism lasted during several phases of brittle deformation that followed juxtaposition of the internal San Juan Island
nappes, including the late Aptian Richardson rocks. If all of
the HP-LT metamorphism in the San Juan Islands is related to
the same subduction zone, the time span of deformation and
metamorphism in that subduction zone could be several tens
of millions of years (at least from 124 Ma to some time after
112 Ma, but likely beginning earlier). The subduction zone
model requires emplacement in the San Juan Islands vicinity
after HP-LT conditions ended, and on structures that are not
exposed in the internal nappe pile (Schermer et al., 2007). Figure 5 summarizes various interpretations of the age of metamorphism relative to deformation.
149
TECTONIC EVOLUTION
A number of features and arguments point to primary accretion and residence of terranes of the San Juan Islands–northwest
Cascades thrust system along the continental margin prior to
mid-Cretaceous assembly in the present nappe pile. As Brandon
et al. (1988) note, the presence of detritus in sandstones from
diverse sources, including metamorphic rock, chert, and silicic
arc volcanic rock (e.g., Constitution Formation) suggests proximity to a “continent-like” landmass. They also note that elsewhere in the Cordillera correlatives of Paleozoic terranes of the
San Juan Islands–northwest Cascades thrust system (e.g., East
Sound Group) accreted long before the mid-Cretaceous. Additional arguments and evidence are provided by the: (1) the Yellow
Aster Complex (Figs. 2 and 6; Table 1), a pre-Devonian terrane
with links to the continent indicated by beds of quartz arenite
and a suite of detrital zircons that match those of the miogeocline (Brown and Gehrels, 2007); and (2) Permian blueschist
metamorphism in some units (Garrison Schist, Vedder Complex;
Armstrong et al., 1983), indicating that these rocks were involved
in convergent margin tectonics long before thrusting in the San
Juan Islands–northwest Cascades system.
Although terranes of the thrust system are similar to
other outboard units of the Cordillera, especially those in the
Klamath Mountains with which they have been correlated
(see below), some aspects of the thrust system are unique.
The stacking sequence of the San Juan Islands–northwest
Cascades thrust system is older on the bottom, younger on
top, approximately reversed from that generally understood
for primary accretion, as in the Klamath Mountains where the
oldest rocks are on top (Irwin, 1981). The duration of assembly of the terranes is a few tens of millions of years at most,
1.0 mm
A
B
Figure 4. Aragonite in the San Juan Islands. (A) Coarse aragonite from marble in the Orcas Chert unit, McGraw-Kittinger quarry, Orcas Island
(Vance, 1977, p. 194). The sample shown is a single crystal exhibiting twin lamellae on a cleavage surface that extends across the entire specimen. (B) Aragonite veins crossing foliation in the Constitution Formation, South Beach, San Juan Island.
150
Brown et al.
SW directed
thrusting
Brown, 1987
NW Cascades
HP-LT
NW directed
thrusting
87-90 Ma
120-130 Ma
San Juan Is.
NW directed
thrusting
Maekawa &
Brown, 1991
HP-LT
penetrative cleavage
SW directed
thrusting
HP-LT
Cowan &
Brandon, 1994
penetrative
cleavage
local
cataclasis
D1
D2
SW-NE
contraction
NW
thrusting
NW-SE
strike-slip
Bergh 2002
penetrative cleavage
HP-LT
penetrative
cleavage
Brown et al.,
2005
thrusting
HP-LT 124 Ma
SW-NE contraction
NW-SE extension
Schermer
et al, 2007
penetrative
cleavage
emplacement
in SJI
veins & brittle
faulting
HP-LT
remagnetization
Burmester
et al. 2000
remagnetization in the eastern SJI sometime during K normal chron
rotation of SJI rocks after remagnetization
140
130
120
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
the Klamath complex (Irwin, 1981). Cretaceous blueschist
metamorphism in the San Juan Islands–northwest Cascades
thrust system affects not only Jurassic-Cretaceous Franciscan
type rocks as in the Klamath Mountains, but also apparently
all the Paleozoic rocks. We are not aware of anywhere else
along the Cordillera that Paleozoic rocks are affected by Cretaceous blueschist metamorphism. Thus, the building process
of the San Juan Island nappe pile is different than that understood for other parts of the Cordilleran margin.
Cretaceous Rocks
Jurassic Rocks
Number
70
Number
fossil age
25
155 Ma
60
177 Ma
30
EASTON SUITE
50
20
SPIEDEN GROUP
Sentinal Island Fm.
15
10
40
224 Ma
30
5
20
0
80 100 120 140 160 180 200 220 240 260 280 300
238 Ma
10
119
0
80
120
60
160
200
148 Ma
240
280
25
165
Number
Number
20
50
FIDALGO COMPLEX
40
10
20
5
237Ma
80
120
70
160
240
200
143
233
0
80 100 120 140 160 180 200 220 240 260 280 300
0
280
ss in BPM
3
1
148 Ma
600
200
60
LUMMI FORMATION
50
40
1000 1400
20
2600
TONGA
FORMATION
30
10
1800 2200
152 Ma
125
30
Number
Number
15
30
10
sandstone in
BELL PASS
MELANGE
20
0
80
120
160
200
149 Ma
240
10
280
0
80 100 120 140 160 180 200 220 240 260 280 300
Number
40
CONSTITUTION
FORMATION
30
153 Ma
80
70
20
NOOKSACK
GROUP
Number
60
10
0
80
120
160
240
200
50
40
30
280
Ma
20
114 Ma
10
0
80 100 120 140 160 180 200 220 240 260 280 300
Early Paleozoic Rock
25
1825
YELLOW ASTER
COMPLEX
20
number
Ma
15
2069 2321
10
2528
1404
5
3316
960
0
800
1200
1600
2000
Ma
2400
2800
3200
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 previous studies of the San Juan Islands–northwest Cascades thrust
system, the homeland of the nappes and the tectonic process of
their transport and emplacement remain unresolved issues. Three
published interpretations (Fig. 7) are:
(1) An orogen-normal contractional model in which the nappes
formed as continental borderland terranes that were caught
in a collision zone between the offshore Wrangellian microcontinent and North America (Brandon and Cowan, 1985;
Brandon et al., 1988; Rubin et al., 1990; McGroder, 1991;
Burchfiel et al., 1992b; Cowan and Brandon, 1994; Monger
and Journeay, 1994).
(2) A transcurrent-transpressional model in which the nappe terranes originally accreted or were deposited south (or north?)
along the margin from their present location and then moved
coastwise, finally stacking up in a reentrant of the continental margin formed by the south end of Wrangellia (Brown,
1987; Maekawa and Brown, 1991; Brown and Dragovich,
2003; Monger and Brown, 2008).
(3) A two-phase model in which terranes were first juxtaposed
by orogen-normal thrusting along the continental margin
south of Wrangellia, and then underwent orogen-parallel
thrusting and strike-slip faulting (Bergh, 2002).
Resolution of the emplacement history of the San Juan
Islands–northwest Cascades thrust system is central to our
understanding of mid-Cretaceous orogeny in the Pacific Northwest, including: the cause of crustal thickening and Barrovian
metamorphism in the crystalline core, the origin of the Nanaimo
basin, and the configuration of terranes along the North American
margin in the Early Cretaceous. On a broader regional scale, the
San Juan Islands–northwest Cascades thrust system is relevant
to understanding evolution of the 1500-km-long Coast Plutonic
Complex which extends from northwest Washington to Alaska.
Based on their interpretation as orogen-normal contractional
features, thrusts of the San Juan Islands and northwest Cascades
have been correlated with thrusts in northern British Columbia
and Alaska and cited as evidence for a west-vergent thrust system that extends virtually the entire length of the Coast Plutonic
Complex and has accommodated many hundreds of kilometers
of mid-Cretaceous shortening between the Insular superterrane
and North America (Rubin et al., 1990).
Kinematics of Outcrop Scale Structures
One approach to understanding displacement of nappes in
the San Juan Islands–northwest Cascades thrust system is kinematic analysis of outcrop scale structures. Such studies to date
yield somewhat disparate results (Fig. 5). Brown (1987), working
in the Cascades, reported a set of orogen-normal stretching lineations in the Easton Suite coeval with 120–130 Ma blueschist minerals (see above). Younger orogen-parallel lineations were found
in mylonite zones separating Cascades nappes (ca. 90 Ma, see
above). Smith (1988), and Maekawa and Brown (1991) mapped
orogen-parallel stretching lineations in the Cascades and San Juan
Islands, respectively, that they interpreted to indicate northwestdirected thrusting. Brandon et al. (1993) disputed this conclusion for the San Juan Islands, suggesting that lineations mapped
by Maekawa and Brown (1991) are the product of differential
solution-mass-transfer, not thrusting. Cowan and Brandon (1994)
described folds and Riedel shears in the Lopez and Rosario fault
zones that they interpret to indicate southwest transport of the
nappes (orogen-normal). In the eastern San Juan Islands, Lamb
(2000) reported northeast vergent (orogen-normal) isoclinal folds
dated by synkinematic mica at ca. 137–154 Ma (see above) in
rocks inferred to be related to the Easton Suite. Bergh (2002)
observed folds, stretching lineations, and shear zones in the
Lopez and Rosario fault zones supporting both orogen-normal
and orogen-parallel displacement and conceived the two-stage
model described above and shown in Figures 5 and 7. Burmester
et al. (2000) found that many of the rocks in question have been
reoriented after acquiring their magnetization, which developed
during or after the fabric was formed; therefore they suggested
that the orientation of the fabrics cannot be used to determine
direction of transport in the present frame of reference. Brown
et al. (2005) determined that fabric in blueschist tectonite of the
Lopez fault zone predates thrusting and they suggested that much
of the kinematic analysis in the San Juan Islands has been carried
out on similar pre-thrust fabric and therefore may not be useful in understanding emplacement of the nappes. Gillaspy (2004)
and Schermer et al. (2007) found that faults and extension veins
indicate a protracted period of orogen-normal shortening coupled
with orogen-parallel extension during aragonite metamorphism
that postdates thrusting, juxtaposition of the terranes, and penetrative fabric formation. The different interpretations are summarized in Figure 5. To more effectively make use of these structural
observations, the challenge for future workers is to understand
the age of outcrop-scale structures relative to the age of emplacement of the nappes.
Regional Considerations
Another strategy for establishing nappe displacements is
consideration of regional geology. Because units of the San
Juan Islands–northwest Cascades thrust system bear evidence
of residence along the continental margin prior to emplacement
in the present day setting, direct accretion of these rocks from
the west, the Pacific basin, seems improbable. Derivation of
the nappes from the northeast is envisaged in the contractional
model of Brandon and Cowan (1985) and McGroder (1991)
which invokes a root zone for the nappes along the northeastern
edge of the Cascade core in the approximate area of the Ross
Lake fault zone (Figs. 2 and 3). In this view, during Wrangellia
collision the nappes were driven to the southwest, riding over
the Cascade core and the northeastern flank of Wrangellia.
Regional geologic features cited as supportive of this model are:
coeval crustal thickening in the Cascade core suggesting thrust
loading, contractional structures in the Cascade core, and interpretation that the Nanaimo Group was deposited in a foreland
McGroder, 1991
Late Jurassic
ST
98 Ma
WR
CPC
CC
WR
MT
SK
NWCS
QS
QS
CORE
terranes
A
94 Ma
CH
NK
CPC
NWCS
MT
90-95 Ma
MT
Early Cretaceous
WR
CPC QS
NK
Plutons
CORE
B
NWCS
F
QS
NWCS
terranes
Brown, 1987
WR
MT
SK
?
NWCS
F
100 km
Late Cretaceous
E. to mid-Cretaceous
Bergh, 2002
CPC
WR
CPC
WR
F
EA
area of
San Juan
Islands
C
EA
F
100 km
Rosario
fault
zone
Lopez
fault
zone
Lopez
Rosario fault
zone
fault
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 continental 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. Postaccretionary 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
Cascades nappes. However, several aspects of regional geology
pose problems for this interpretation.
(1) The contractional model invokes transit of nappes of the
San Juan Islands–northwest Cascades thrust system over
the Cascade crystalline core (Fig. 3) at precisely the time
of great magmatic arc activity in that region. No rocks
related to this arc activity are found in the San Juan Islands
or Cascades, except where nappes lap onto the southern
edge of the Cascade core in the vicinity of the Windy Pass
thrust (Figs. 2 and 3).
(2) Nappes of the San Juan Islands–northwest Cascades thrust
system carry metamorphic aragonite acquired prior to (as
well as after) thrusting. Aragonite has been shown experimentally to invert quickly to calcite outside its stability field
at elevated temperature except under conditions of abnormally low T/P, less than 10 °C/km (Carlson and Rosenfeld,
1981). Transit of the thrust system nappes over the active
arc would place them in a region of abnormally high T/P,
precluding preservation of aragonite.
(3) The elongate, orogen-parallel Nanaimo basin is flanked not
by terranes of the San Juan Islands–northwest Cascades
thrust system, but by plutonic rocks of the Coast Mountains.
Thrust system terranes occur south along strike from the
Nanaimo (Figs. 2 and 3), and thus the basin is not likely a
consequence of nappe loading.
Many workers have envisaged a southerly origin of some
or all of the terranes of the San Juan Islands–northwest Cascades thrust system, in the Columbia embayment, Klamath
Mountains, or California Coast Range (e.g., Davis et al., 1978;
Vance et al., 1980; Brown and Blake, 1987; Garver, 1988; Burchfiel et al., 1992b). Davis et al. (1978) and Vance et al. (1980)
proposed that the Mesozoic ophiolitic terranes of the San Juan
Islands–northwest Cascades system formed in a “pull-apart gap”
in southeastern Oregon and subsequently moved northward and
were obducted onto the continent. Geologic features cited in support of the model are: (1) thrust emplacement of the Ingalls ophiolite over the south edge of the Cascade core, (2) absence from
eastern Oregon and western Idaho of some continental margin
terranes that are part of the Mesozoic assemblage to the north
and south along the Cordillera, and (3) Sr isotope ratios and seismic velocities indicating primitive crust underlying the Columbia
embayment. More recent geophysical evidence for a deep crustal
rift in the Columbia embayment is a linear break in the gravity
field running along the southern margin of the embayment (Riddihough et al., 1986).
Paleomagnetic and Other Constraints of Paleogeography
Paleomagnetic studies of the rocks in the San Juan Islands
have had mixed success in constraining their tectonic history,
with the main complication being an extensive remagnetization
that has affected all of the “internal” units that have experienced
high P-T metamorphism. Burmester et al. (2000) found that these
rocks had all been remagnetized during or after folding, and that
the predominantly normal polarity of the remagnetized directions
indicated to them that this remagnetization occurred during the
Cretaceous Long-Normal Chron (116–83.5 Ma). The remagnetized directions from the San Juan Islands are scattered, however, indicating that a significant amount of rotation and/or tilt
occurred after this remagnetization event.
Paleomagnetic studies of the unmetamorphosed “external”
units of the San Juan Islands have more promising results. The
exception is the Haro Formation; Hults and Housen (2000) have
found that these rocks were also remagnetized prior to folding,
despite their lack of any significant metamorphism.
The rocks of the Spieden Group have complex magnetizations,
with the majority of these clastic rocks having poorly resolved
magnetizations. Dean (2002) found three magnetic components in
most of the Late Jurassic Spieden Bluff Formation samples, which
yielded an inconclusive paleomagnetic fold test. The Early Cretaceous Sentinel Island Formation has a simpler, two-component
magnetization in some of the rocks. Dean (2002) found that the
second-removed component from the Sentinel Island Formation passes the inclination-only paleomagnetic fold test, with the
best-clustered inclinations occurring at 100% untilting. The mean
inclination of 64°, α95 = 7.8°, suggests an Early Cretaceous paleolatitude of 46° N. Comparing this direction with that expected for
the present-day location of Spieden Island calculated from a stable
North America reference pole (Housen et al., 2003), a latitudinal
translation of 1500 ± 1000 km is estimated for these rocks.
The Nanaimo Group has been the subject of extensive paleomagnetic study, primarily from outcrops in the Canadian Gulf
Islands (Ward et al., 1997; Enkin et al., 2001; Kim and Kodama,
2004), with limited work from Orcas Island (Housen et al.,
1998). All of these studies have found that most Nanaimo Group
rocks have poorly defined magnetizations (~60% “failure rate”
reported for most sample collections). However, a significant
number of samples in all of these studies (a few 100 out of ~1000
samples collected) have well-defined magnetizations that pass a
reversals or fold test. Studies of inclination error, notably Kim
and Kodama (2004), suggest that inclination error in these sediments is moderate (8–10°), and that when corrected for the paleomagnetic inclinations in these rocks place the Nanaimo Basin at
a paleolatitude of 41° N during Campanian-Maastrichtian time.
Using a Late Cretaceous North American reference pole for
comparison, a translation of 1600 ± 900 km is indicated for these
rocks since ca. 75 Ma.
Related constraints on the Late Cretaceous paleogeography
of the San Juan Islands also come from paleofaunal data from
the Nanaimo Group rocks. Kodama and Ward (2001) argued
that the lack of rudistid bivalves in the otherwise well-preserved
paleofauna of the Nanaimo Group can be used to constrain the
paleolatitude of these rocks. Rudistids are tropical to subtropical 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
14
KA
AS
AL
F
ell
P-MF
50 mm/yr
Yakutat
terrane,
transform
displacement
F-QCF
North
America
plate
60
12
0
Pacific
plate
Casc
300 KM
LRF
Juan
de Fuca
plate
ade arc
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 combination 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–
W
ra
ng
Modern Analogues?
Ale utian -
D
the presence of a cold-water upwelling zone along the ancient
California margin, Kodama and Ward (2001) suggested that the
Nanaimo Group rocks were located at or north of the location
of the Moreno Basin (central California, 42° N reconstructed
paleolatitude) at 75 Ma. Some additional support for this constraint comes from the recognition of a marine reptile fauna from
Nanaimo Group rocks on Vancouver Island, which share some
provinciality with the marine reptile fauna of the Moreno Formation from central California (Nicholls and Meckert, 2002).
Another set of data, detrital zircon age distributions, has also
been used to test paleogeographic constraints on the location of
the Nanaimo Group rocks. Mahoney et al. (1999) used the presence of several Archean-aged zircons to indicate that the Nanaimo
Group rocks had been located no more than 500 km south of its
present-day location, during Late Cretaceous time. Using the same
set of data, Housen and Beck (1999) compared variations in the
detrital zircon age distributions as a function of stratigraphic position within the Nanaimo Group. They argued that variations in
Proterozoic-aged zircons support a source of detritial zircons from
the Mazatzal and Yavapai orogens in southwest North America, and
that northward migration of the Nanaimo Basin during its deposition was consistent with other paleomagnetic evidence, and plate
motion estimates. The analyses of Kodama and Ward (2001), and
Kim and Kodama (2004) also supported the conclusion of Housen
and Beck (1999), that the Nanaimo Group reached the “moderate” paleolatitude of ~43° N at 75 Ma, consistent with the so-called
“Baja-BC” (Baja–British Columbia) hypothesis.
Taken together, these paleogeographic data would be most
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 stratified 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).
155
0
CAN 50
U.S .
.
8 mm/yr
Siletzia,
fore-arc displacement
Figure 8. Modern-day analogues of orogen-parallel thrusting in the
Pacific Northwest. F-QCF—Fairweather-Queen Charlotte fault,
DF—Denali fault, LRF—Leech River fault. References: Plafker et al.
(1994), Wells et al. (1998); Bruhn et al. (2004).
156
Brown et al.
400 km. The current rate of arc-parallel transport is 6–8 mm/yr at
the northern end of the terrane (Wells and Simpson, 2001).
The Yakutat terrane is moving north along the Fairweather–
Queen Charlotte transform fault at 45–50 mm/yr relative to
North America (Plafker et al., 1994; Bruhn et al., 2004). At the
corner area in southern Alaska where plate interaction changes
from transform to convergent, the Yakutat terrane is colliding
with the continent (Fig. 8). A north-dipping Benioff zone and the
Wrangell magmatic arc in this region both testify to significant
subduction of the Yakutat terrane (and probably other materials).
The convergent zone is marked by a thin-skinned accretionary
complex of Cretaceous and younger rocks displaced northward
on gently to moderately dipping thrust faults (Bruhn et al., 2004).
Displacements are strongly partitioned between strike-slip faults
and thrusts. Both analogues are characterized by low-dip thrusts
accommodating margin-parallel displacement indicating that
such structure, as possibly fits the San Juan Islands–northwest
Cascades thrust system, is not a tectonic anomaly.
FIELD TRIP GUIDE
The field trip guide begins at Friday Harbor, San Juan Island
(Fig. 9). Before departing, be certain that you have brought along
warm clothes, raingear, and good field boots.
Please do not use rock hammers or collect specimens anywhere on this trip unless specifically advised.
DAY 1
Day 1 is spent primarily on the terranes “external” to the San
Juan Islands thrust system. These units are the Haro Formation,
Spieden Group, and Nanaimo Group. They broadly overlap in age
with rocks in the nappe pile but are distinguished by their absence
of, or very low-grade (zeolite facies), metamorphism, and, in the
case of the Spieden and Nanaimo Groups, an absence of penetrative tectonite fabric. These units are important to understanding the
younger portion of the tectonic history of the San Juan Islands.
The field trip will begin with a drive from Friday Harbor across
San Juan Island to picturesque Roche Harbor, on the northern end
of San Juan Island. We will depart from the boat ramp at Roche
Harbor, taking a chartered craft to Stuart and Spieden Islands.
We will be landing on public access beach areas, but please note
that only the intertidal zone in these areas is considered to be public property, and that the uplands are privately owned. Access to
Spieden Island in particular is restricted by its owner.
Directions and Other Instructions
Before departing on the Humpback Hauling vessel, be certain
that you have brought warm clothes and your lunch. Even if the
weather appears to be sunny, raingear is recommended. A lifejacket
(provided on the vessel) is required at all times, and please do not
forget yours on the beach. If you are prone to seasickness, please
take appropriate precautions. The vessel has a landing-craft type
ramp, so we will be able to disembark on relatively dry land. How-
ever, caution must be exercised to avoid a nasty fall on the slick
seaweed-covered rocks that may be present. Please pay attention to
the field trip guides as the departure time draws near, to ensure you
are on the vessel, and the trip can run in a safe and timely fashion.
After we have finished the Stuart Island stop, participants will reembark for a ~45 min trip to Spieden Island.
Stop 1-1. Fossil Cove, Stuart Island, Nanaimo Group
(Fig. 10)
The Nanaimo Group comprises a set of 11 formations, ranging
from Turonian to Maastrichtian in age, composed of clastic marine
and deltaic sedimentary deposits (Fig. 10). The ages of these rocks
are constrained by biostratigraphy (e.g., Haggart, 1994), and magnetostratigraphy (Enkin et al., 2001). These rocks were deposited
in a large marginal basin, extending ~175 km from its southernmost extent in the San Juan Islands to its northernmost extent on
Vancouver Island. The Nanaimo Group contains several elements
that are of tectonic interest. Structurally, the Nanaimo Group
rocks (along with the Paleocene-Eocene Chuckanut Formation)
are folded as part of the Cowichan fold and thrust belt (England
and Calon, 1991; see also Mustoe et al., this volume, and Blake
and Engebretson, this volume). One of the primary constraints on
the age of uplift and thrusting of the metamorphosed “interior”
domain of the San Juan Islands is the presence of metamorphosed
sandstone clasts interpreted as being derived from the Constitution
Formation that are found in conglomerates of the Extension Formation of the Nanaimo Group on Orcas and Stuart Islands (Brandon
et al., 1988). On a larger scale, age distributions of detrital zircons
(Housen and Beck, 1999; Mahoney et al., 1999), paleomagnetism
(Ward et al., 1997; Housen et al., 1998; Enkin et al., 2001; Kim and
Kodama, 2004), and fossil assemblages (Kodama and Ward, 2001)
have been used to evaluate possible large-scale displacements of
the Nanaimo Group rocks.
On Stuart Island, the turbidites and sandstones of the Haslam
Formation, the conglomerates of the Extension Formation, and
the sandstones and siltstones of the Pender Formation can be
found (Fig. 10). A stop at Fossil Cove, on the NW end of Stuart
Island (a boat trip of ~45 minutes), allows for examination of the
bedding and sedimentary structures in these rocks, as well as the
many fossils (primarily Inoceramus). Time permitting, we may
stop at a beach where the Extension Formation crops out, in order
to examine the conglomerate clasts of this interesting unit.
Stop 1-2. North Shore Spieden Island, Spieden Group
(Fig. 11)
Spieden Island is one of the largest (perhaps the largest) privately owned island in the San Juan archipelago. It has a colorful history, most notably as “Safari Island,” when in the 1970s
a group of investors purchased the island with the bright idea
of transforming it into a private exotic game hunting reserve.
The island was stocked with many species of exotic game animals (mostly Asian and African deer, goat, sheep, and antelope
70
1-1
Kn
Kn
65
70
Kn
STUART
ISLAND
Kn
ORCAS
ISLAND
75
Kn
55 84
Kn
30
Pe
Pt
1-2
JKs
Ha
ro fault
SPIEDEN
ISLAND
Trh
Or
1-3
Pt
ca
TrJo
Pt
s t
hrus
t
TrJo
TrJo
Ro
JKc
TrJo
sa
Jc
SAN JUAN
ISLAND
rio
R
oc
th
ru
st
Jc
h
e
H arbor
y rd
rd.
averton Val
le
Be
SHAW
ISLAND
.
PTrd
Jc
Pg
Bailer Hill rd.
TrJo
Buc
Kn = Nanaimo Group
JKs = Spieden Group
JKl = Lopez Structural Complex
Jc = Constitution Formation
TrJo = Orcas Chert
Trh = Haro Formation
PTrd = Deadman Bay Volcanics
Pg = Garrison Schist
Pe = East Sound Group
Pt = Turtleback Complex
rd.
American
Camp
N
2-1
2.0 km
Pickett's Ln.
es
tS
i de
kB
ay
fau
lt
W
1-4
San Juan Valley rd.
Cattle Point rd.
Lime Kiln Point
Argyle Ave
Friday
Harbor
JKl
Cattle
Point
2-2
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.
74
Turn
Point
50
70
65
Kne
78
48
50
34
65
Kne
Knh
64
Prevost 28
Harbor
62
76
Fossil
Cove
80
83
Kne
78
Stop 1-1
72 72
50
63
Knp
Knh
76
83
55
Satellite
Island
48
81
58
71
83
65
64
62
71
56
Knp
A
Knh
70
65
53
Nanaimo Group: 56
Knp: Pender Fm 75
Kne: Extension Fm
Knh: Haslam Fm
85
54
70
75
55
Reid
50
61
Knp
Knp
Kne
45
Stu
or
Kne
50
art
61
Harb
63
32
Isla
nd
14
N
22
55
26
scale 1 km
Figure 10 (on this and following page). Geology of Stuart Island, from
Mercier (1977). (A) Geologic map.
24
22
species). Needless to say, the concept of hunting exotic game in
the midst of an ecological paradise did not work out; the island
reverted to Spieden Island, and the descendants of the surviving
creatures can be seen cavorting around the island today.
Geologically, Spieden Island, and nearby Sentinel Island, are
the only known occurrences of the late Jurassic–early Cretaceous
Spieden Group. The Spieden Group is composed of two formations, the Oxfordian-Kimmeridgian Spieden Bluff Formation, and
the uppermost Valanginian Sentinal Island Formation (Fig. 11).
The ages of these units are constrained by biostratigraphy
(McClellan, 1927, Haggart, 2000), primarily via fossils of
Buchia. The rocks of both formations are clastic sediments, with
finer-grained turbidite deposits characterizing the Spieden Bluff
Formation, and volcaniclastic-rich sandstone, mudstone, and conglomerates characterizing the Sentinel Island Formation. The rocks
also display some soft-sediment deformation features; some have a
very weak anastomosing scaly cleavage, and have been folded.
Our field trip stop will be located on a wave-cut bench,
exposed at low tide, on the north shore of Spieden Island. Here
we will see outcrops of both formations, and localities that display the locally abundant macrofossils. We will have ~30 min
at this location; please follow the instructions of the trip leaders
closely. After we re-embark, the vessel will take us on a ~40 min
trip back to the Roche Harbor boat ramp, where the seaborne portion of this trip will end.
After leaving the Roche Harbor boat ramp, we will drive to
Davidson Head, parking on the shoulder of the road at the “neck”
of the head. We will then walk northwest along the beach, examining the exposures of the Haro Formation in the intertidal zone.
Fans of fresh oysters will be certain to notice the abundant (likely
seeded) oysters present on the Haro Formation outcrops.
Directions to Stop 1-3
From Roche Harbor waterfront, drive southwest on Reuben
Memorial Drive.
0.2 mi
Go left on Roche Harbor Road.
0.9
Go left (NW) on Afterglow Drive.
1.8
Neck of Davidson Head; park on gravel shoulder on
right side of road.
Stop 1-3. Davidson Head, San Juan Island, Haro Formation
The north shore of San Juan Island is home to one of the
most geographically restricted units in the San Juan Islands—the
Late Triassic (Norian) Haro Formation. This unit crops out on
Davidson Head, and is a 700-m-thick mixed volcaniclastic unit.
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 metamorphism, and thus the contact between the Haro Formation and
the high-P, low-T metamorphic rocks immediately to the south
of Davidson Head represents a fundamental structural boundary
in the San Juan Islands. This contact is nowhere exposed, but is
inferred to be a thrust fault (the Haro Thrust), based primarily
on the large-scale structural architecture of the San Juan–north
Cascades nappes (e.g., Brandon et al., 1988).
0.0
Return on Afterglow drive to Roche Harbor Road;
reset odometer and go left (east-southeast).
1.3 mi
Go right (south) on West Valley Road.
2.8
Go right (west) on Mitchell Bay Road.
5.6
Go left (south) on Westside Road.
9.7
Turn in at entrance to Lime Kiln Point Park and follow
trail to coast.
A
Sp
ied
en
Isla
nd
65 80 63 42
U
D
Sentinel
Island
10
U
D
5
45
15
33
22
24
Lower Cretaceous Sentinel Island Formation
upper member
lower member
N
Upper Jurassic Spieden Bluff Formation
upper member
lower member
faults
anticline
strike/dip
contour interval = 10 meters
B
AGE
GROUP
U
D
FORMATION
200
0
200
400
meters
MEMBER THICKNESS
LITHOLOGY
DESCRIPTION
sandstone
conglomerate and minor
Crudely stratified volcanic
600 m +
Upper member
Sentinel Island Formation
Spieden Group
Haurterivian
Early Cretaceous
not exposed
Lower
Lowermember
member
Lower Upper
member member
Spieden Bluff
Formation
Valanginian
Oxfordian or
Kimmeridgian
Late
Jurassic
Unconformity
Massive
andbedded
thinly
Massive
and thinly
fossiliferous
sandstone
and siltstone
bedded
fossiliferous
sandstone and siltstone
140 mm
140
Unconformity
Massive and thinly
bedded fossiliferous
sandstone and siltstone
20 m
80 m
not exposed
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.
Stop 1-4. Lime Kiln Point State Park, San Juan Island
(Fig. 12)
Lime Kiln Point is a famous venue for orca whale spotting.
For geologists, the locality is important for its exposures of limestone that bears Permian Tethyan Fusulinids, known to have
grown at a tropical latitude and suggesting large displacements
of the terrane (Danner, 1966, 1976; Monger and Ross, 1971).
The limestone occurs as layers and irregular masses within a
sequence of ocean island pillow basalt flows and breccias, named
the Deadman Bay Volcanics (Brandon et al., 1988). The age of the
unit as a whole ranges from Early Permian to Late Triassic based
161
on fusulinids, conodonts and radiolaria. The Tethyan fusulinids
link this unit to the “Cache Creek belt” of mélanged oceanic rock
extending along the Cordilleran margin from California to northern British Columbia (Miller, 1987). The limestone is largely
recrystallized to aragonite marble (Vance, 1968).
The Deadman Bay Volcanics are separated from the overlying Orcas Chert unit by an east-dipping thrust fault; however
these two units are regarded by Brandon et al. (1988) as parts of
a single terrane based on their mutual similarity of age, lithology,
and chemical signature of ocean island basalts.
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
On day 2, we will examine the Rosario and Lopez fault
zones on San Juan and Lopez Islands, two of the major structures
in the San Juan Islands thrust system.
0.0
Intersection of Argyle Ave and Spring Street in Friday
Harbor; head south on Argyle Ave.
1.0 mi
Beginning of Cattle Point Road.
7.1
Turn right (south) on Pickett’s Lane in American
Camp Park.
7.6
Go right (west) on Salmon Banks Road (dirt road).
7.9
End of road; park.
Stop 2-1. South Beach, American Camp National Park,
San Juan Island (Fig. 13)
Americans and British disputed the boundary between their
respective territories in the early 1800s and set up military camps
on San Juan Island, which both sides claimed. War nearly broke
out in 1859 when an American settler shot a pig belonging to
the British Camp. The international boundary dispute was finally
resolved by arbitration in 1872, in favor of the Americans.
This part of the Rosario thrust, well exposed at the water’s
edge, has been a key locality for interpretations of San Juan
Islands structural evolution (summarized in Figs. 5 and 7).
Maekawa and Brown (1991) observed shear zones with fault
drag and northwest trending lineations at this locality and suggested dominantly northwest thrusting (Fig. 14A). Cowan and
Brandon (1994) applied a “symmetry based statistical analysis” of asymmetric folds and Riedel shears, concluding that the
structures formed by southwest thrusting. Bergh (2002) divided
structures into an early set of folds, foliation and lineations
related to southwest contraction (D1), and a later set of lineations
and shears (D2) formed by northwest displacement as exhibited
at this locality (Fig. 14B).
The Rosario thrust at this locality dips northeast. Footwall to
the thrust is the Triassic-Jurassic Orcas Chert which is dominantly
composed of ribbon chert with lesser pillow basalt, mudstone,
and limestone (Vance, 1975). In the hanging wall is the Late
Jurassic Constitution Formation, mostly composed of volcanicrich graywacke sandstone. The fault at this locality (mapped in
detail by Brandon et al., 1988) is marked by an imbricate zone
~100 m wide bearing lenses and rafts of ribbon chert, sandstone,
mudstone, greenstone, and most significantly HP-LT greenschistamphibolite of the Permian Garrison Schist unit.
Amount and timing of displacement on the Rosario thrust are
difficult questions. Vance (1975) noted that the overlying Constitution Formation bears detritus that appears to be derived from
the underlying Orcas Chert, Garrison Schist, and Deadman Bay
Volcanics and proposed that the contact is an unconformity. The
imbricate structure and inclusion of the Garrison Schist in the
deformation zone, however, suggested a fault of large displacement
to Maekawa and Brown (1993) and Cowan and Brandon (1994).
8.7 mi
Retrace route to Cattle Point Road. Turn right (east)
and drive to Cattle Point.
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 orientations statistically indicate southwest-vergent thrusting (i.e., toward the viewer with respect to Fig. 14B).
164
Brown et al.
Lopez Structural Complex
aul
t
At Cattle Point and subsequent stops on Lopez Island, we
will see rocks and structures of the Lopez Structural Complex
(Fig. 15), one of the major fault zones in the San Juan thrust
system (Brandon et al., 1988). The Lopez Structural Complex
is an ~2.5 km wide imbricate zone composed of northwestelongated, relatively coherent lenses separated by sheared
mudstone-rich fault zones. The large lenses are predominantly
ocean floor clastic and volcanic rocks and Constitution terrane
sandstone; smaller lenses include Turtleback terrane and exotic
material not found elsewhere in the region. The magnitude of
offset along the Lopez Structural Complex is unknown, but
the inclusion of exotic material such as the Early Cretaceous
Richardson complex (stop 2-3) suggests tens of kilometers of
movement similar to other terrane bounding faults in the San
Juan Islands (Brandon et al., 1988; Cowan and Brandon, 1994).
Foliation and fault contacts in the Lopez Structural Complex
dip moderately to steeply northeast (Maekawa and Brown,
1991; Cowan and Brandon 1994; Bergh, 2002) (Fig. 15). These
structures are subparallel to the northern boundary of the Lopez
Structural Complex, the Lopez fault, where most of the offset is
thought to have occurred (Brandon et al., 1988)
Recent structural analysis of the Lopez Structural Complex
(Gillaspy, 2004; Schermer et al., 2007), reveals a sequence of
events that provide insight into accretionary wedge mechanics
and regional tectonics. After formation of regional ductile flattening and shear-related fabrics (the thrusts and strike slip faults
illustrated in Fig. 5), the area was crosscut by brittle structures
including: (1) southwest-vergent thrusts, (2) extension veins and
normal faults related to northwest-southeast extension, and (3)
conjugate strike-slip structures recording northwest-southeast
extension and northeast-southwest shortening. Aragonite-bearing
veins are associated with thrust and normal faults, but only rarely
with strike-slip faults (Fig. 16). High-pressure low-temperature
(HP-LT) minerals constrain brittle deformation to have occurred
at ≥20 km and ~200–300 °C. The presence of similar structures
elsewhere indicates the brittle structural sequence is typical of the
ay F
N
Buc
kB
?
t
Const.
Terrane
43
LO
PE
Z
39
40
55
47
Cattle Pt.
Shark
Reef
28
0
FA
UL
T
50
40
51
55
Richardson
Ocean Floor
Complex
(undivided)
45
Stop 2-3 70
John's Pt.
49
Ocean Floor Complex
Volcanic rocks and
associated sedimentary rocks
Iceberg Pt.
Mudstone-rich assemblages
?
68 74
75 64
60
45
70
60
62
56
?
t
? Watmough
Head
?
47
Point
Colville
"Exotic" and Other slices
Constitution Terrane
Sandstone with
chert and volcanic rocks
69
55
Stop 2-4
55
Clastic sequences
2 km
Fidalgo Complex
Stop 2-2
57
1
t
Richardson Basalt Complex
Approximate
fault contact
Turtleback Complex
Major terrane
boundary
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).
*
?
A
LO
*
Const.
Terrane
San Juan Island
B 100
N
Aragonite found
in vein sample
Lopez Island
PE
Z
0
FA
UL
T
**
* *
*
6
80
8
13
2 km
*
Ocean Floor
Complex
* **
**
?
?
?
%
6
1
Fidalgo Complex
*
** **
* * *
8
60
165
40
20
1
0
Deformed
Veins
Early
Shear
Veins
n = 12
n = 10
Thrusts Extension
Veins
n=7
n = 22
Normal
Late
Faults Strike-slip
Faults
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 structurally higher terranes. Sustained HP-LT conditions are possible
only if structures formed in an accretionary prism during active
subduction, suggesting brittle structures record internal wedge
deformation at depth and early during uplift of the San Juan
Island nappes. The structures are consistent with orogen-normal
shortening and vertical thickening followed by vertical thinning
and along-strike extension. The change in vein mineralogy indicates exhumation occurred prior to the strike-slip event. The P-T
conditions, and spatial and temporal extent of small faults associated with fluid flow suggests a link between these structures and
the silent earthquake process.
Given that these latest identified structures likely formed in
an accretionary wedge setting, we are faced with the dilemma
of not having found the Late Cretaceous structures related to
emplacement in northwest Washington. These emplacement
structures, if they formed by any of the models illustrated in
Figure 7, would be unlikely to have associated HP-LT metamorphism or along-strike (NW-SE) extension. It is possible that the
unexposed Haro fault (stop 1-3) is one of the main emplacement
related structures, but the timing and kinematics of this fault are
poorly understood.
Stop 2-2. Cattle Point Park, San Juan Island (Figs. 9, 15)
At Cattle Point, highly sheared mudstones with disrupted
and elongated sandstone beds and clasts form a NW-striking,
steeply dipping shear zone adjacent to less-deformed coarse
grained sandstones and chert-pebble conglomerates. We will
examine early ductile and late brittle deformation. In the sheared
mudstone, which is interpreted by Bergh (2002) to contain a
composite S1-S2 fabric, there is evidence of NW-SE shearing,
interpreted as top to the NW thrusting by Maekawa and Brown
(1991) and sinistral reactivation of SW-vergent thrusts by Bergh
166
Brown et al.
(2002). Strain in the early thrusting event(s) is strongly partitioned into the mudstone-rich units, as seen here and throughout
the Lopez structural complex. Foliation in the sandstone unit is
dominated by pressure solution and volume loss (Feehan and
Brandon 1999). The later brittle structures studied by Gillaspy
(2004) and Schermer et al. (2007) are present in both sandstone
and mudstone units, but best observed in the sandstone, where
several generations of faults and extension veins cross cut the
dominant foliation. These structures include rare SW-vergent
thrusts subparallel to foliation, followed by extension veins and
normal faults, then conjugate strike slip faults. Analysis of these
structures in outcrops throughout the Lopez structural complex
and eastern San Juan Islands indicates a prolonged episode of
brittle deformation at the base of the accretionary wedge that
resulted in N-S to NW-SE extension (Figs. 5 and 17).
Return to Friday Harbor and take the ferry to Lopez Island.
0.0
Ferry terminal on Lopez Island, drive south on Ferry
Road.
122
W
N
73 53
1O km
48
K-is K-bj
G
F
E
2.1 mi
2.3
7.7
7.9
9.6
Turn left (east) on Fisherman Bay Road.
Go right (south) on Center Road.
Turn right (west) on Lopez Sound Road.
Turn left (south) on Richardson Road continue south
to coast.
End of road at fuel terminal; park here.
Stop 2-3. Richardson, Lopez Island
Geologic relations at Richardson on Lopez Island (Figs. 18
and 19) have played an important role in understanding San Juan
Island evolution since the discovery there of Cretaceous microfossils by Ted Danner of the University of British Columbia
(Danner, 1966), establishing a maximum age limit on thrusting.
Until recently the accepted age for these rocks was late Albian
(ca. 100 Ma), determined by Bill Sliter of the U.S. Geological Survey (in Brandon et al., 1988) based on microfossils in a
mudstone collected by John Whetten, University of Washington, in 1977. Map relations displayed at this site show a layered
sequence of pillow basalts, pillow breccias, tuff and mudstone
(Fig. 19). All these rocks were considered to represent a coherent
mid-Cretaceous stratigraphic assemblage (Brandon et al., 1988).
However, recent Ar-Ar analysis of blueschist facies phengitic
mica in the pillow breccias (Fig. 20) yielded an age of 124.43
± 0.72 Ma (Brown et al., 2005). Revisitation of the fossil ages
in the Whetten sample indicates a late Aptian age (112–115 Ma)
(Fig. 21). Remapping the structural features demonstrates that the
fossiliferous mudstones (Fig. 21) are faulted into the sequence.
These findings broaden the age brackets for thrusting, and suggested to Brown et al. (2005) that San Juan Islands blueschist
metamorphism is older than thrusting. But, a recent finding of
aragonite veins in the late Aptian mudstones by Schermer et al.
(2007) indicates that the blueschist metamorphism continued to
at least that time and was apparently coeval with and outlasted
thrusting, as interpreted by earlier workers (Brandon et al., 1988;
Maekawa and Brown, 1991).
B
D
H
A
48
C
thrust vergence
T axis from
strike slip faults
T axis from normal
faults, extension veins
subhorizontal
extension
Figure 17. Generalized map of the eastern San Juan Islands with paleomagnetic results of Burmester et al (2000) and kinematics of late brittle
deformation from Schermer et al. (2007), Gillaspy (2004), and Lamb
(2000). Small grey arrows indicate direction of magnetic vector from
Burmester et al (2000); inclination values omitted; small arrowheads
indicate upward inclination. K-is and K-bj show expected directions
for in situ and Baja-BC terrane models of the Cretaceous location
of San Juan terranes. Other arrows indicate kinematic directions of
brittle structures as defined in key. If no arrow is shown for brittle
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 faulting. 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.
BLAKELY
ISLAND
Jc
Ferry Road
TrJ o
167
ORCAS
ISLAND
Jf
Jf
y
SAN JUAN
ISLAND
Jf
Road
Road
Jc
Jf
LOPEZ ISLAND
DECATUR
ISLAND
Jl
Center
an
Fisherm
Ba
ud
M
JKl
Ba
y
2-2
2-3
Jl = Lummi Formation
Jf = Fidalgo Complex
Jc = Constitution Fm.
JKl = Lopez Structural Complex
TrJo = Orcas Chert
Road
JKl
Aleck Bay Rd
JKl
Richardson
Jf
Jl
N
2-4
2.0 km
Figure 18. Map of Lopez Island. Based on Brandon et al. (1988) and Burmester et al. (2000) and
M.C. Blake Jr. (2000, written commun.).
The fault that juxtaposes the mudstone and volcanic rocks
bears slickenlines trending N30° W and plunging 20°, seen below
the road at this locality. This lineation is part of the data set used by
Maekawa and Brown (1991) as a basis for their inference of dominantly orogen-parallel transport of the San Juan Island nappes.
9.6 mi
Drive north from end of Richardson Road.
9.9.
Vista Road. Turn right (east).
11.4
Mud Bay Road. Go right (south).
12.5
MacKaye Harbor Road. Turn right (west).
14.6
End of road.
There is no parking at the end of the road; note a sign indicating “private road” at the end of the public road. There are two
areas at a county park picnic site with parking for 2 or 3 cars each,
available on the left (south) side of the road ~50 m before the
end. After parking, walk to the end of the public road, go straight
through the open wooden gate onto the private road, take the
right-hand fork through private land, pass through a metal gate
and follow the path ~15 min to Bureau of Land Management land
at Iceberg Point. Please respect private property.
Stop 2-4. Iceberg Point, Lopez Island
At Iceberg point, we will examine interbedded sandstones
and mudstones with several generations of brittle structures. If
time and tide permits, we will also examine a shear zone between
these rocks (of ocean-floor affinity) and a fault slice of Consti-
168
Brown et al.
O
48 27.0'
122 53.5'
18
10
6
Pb
O
N
35 Pl
26
RIC
HARD
SON
35
20
26
Pl
0
Pb
5
10
meters
12 = elevation in feet
above mean low tide
ROAD
35
30
Bay
Pb
34
55
Fau
25
Pb
mailbox
lt z
one
22
Pb
2
Pb
15
Ar samples
Fossil
sites
35 Attitude of fault
55
surface
6
20
Pl
12
FUEL
TANKS
Attitude of slickenlines in fault zone
15fenceAttitude of metamorphic foliation
Mudstone
Tuff
Pl
18
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 SWvergent thrusts subparallel to foliation, abundant extension veins
and normal faults showing predominantly NW-SE extension, and
conjugate strike slip faults. Because the late brittle structures are
broadly distributed across the Lopez Structural Complex, we may
not be able to see all generations of structures and cross-cutting
relations between them.
DAY 3
Day 3 is mostly devoted to the Fidalgo Complex on Fidalgo
Island. On the last stop of the day we observe outcrops of the
Easton Metamorphic Suite exposed at the south end of Chuckanut Mountain.
Fidalgo Complex
Directions to Overnight Lodging
Return to ferry landing and take the ferry to Anacortes. Drive
on Washington State Highway 20 spur to the intersection with the
main route of highway 20, at Sharps Corner.
The Fidalgo Complex (Fig. 22) consists of a stratigraphic
sequence distinctive of ophiolite. From the base upward in the
section are: ultramafic tectonite, cumulate gabbro, a sheeted
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).
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 Richardson, sample R3, from which phengite gives an Ar-Ar age of 124.75
± 0.87 Ma (Brown et al., 2005). Phengite and chlorite crystallized
from volcanic glass, plagioclase is altered to fine-grained aggregate
of Ca-Al silicates. Aragonite and pumpellyite (not visible), as well as
chlorite and phengite shown here, are synkinematic minerals.
intrusive complex of mostly plagiogranite (diorite, tonalite,
trondjemite, albite granite) and hypabyssal equivalents, a volcanic sequence of mainly dacitic to andesitic breccias and interlayered tuffaceous argillite, coarse sedimentary breccia that bears
clasts of all the underlying units, pelagic argillite, and volcanicrich graywacke at the top of the section. U-Pb zircon ages of the
plagiogranites on Fidalgo Island are 167 ± 5 Ma, and elsewhere
are 160 ± 3 Ma on Lummi Island and 170 ± 3 on Blakely Island
(Whetten et al., 1978, 1980). Radiolaria in the pelagic argillite
are late Kimmeridgian–early Tithonian ca. 150 Ma (Gusey, 1978;
Brandon et al., 1988). The U-Pb age pattern of detrital zircons
from a sample of the graywacke unit bears a single prominent
peak at 148 Ma, considered to represent a nearby volcanic provenance (Brown and Gehrels, 2007). All these rocks are affected
by prehnite-pumpellyite metamorphism.
The plutonic part of the Fidalgo Complex is interpreted to
be a remnant arc. An arc origin of the ophiolite is indicated by
the abundance of intermediate to felsic igneous rocks (Brown,
1977; Gusey and Brown, 1987; Burmester et al., 2000). The
coarse breccia and overlying radiolarian argillite stratigraphically
above the igneous rocks indicate that the 160–170 Ma arc was
rifted and terminated as a volcanic center prior to deposition of
the sedimentary part of the section. Thus the arc was faulted and
shifted off its magmatic axis before attaining much crustal thickness or subaerial exposure. The old eroded arc was then buried
at ca. 148 Ma by younger clastic arc detritus from an adjacent
volcanic axis. This evolution is similar to that of modern remnant
arcs (e.g., Karig, 1972).
The Fidalgo complex is similar in age and lithology to the
California Coast Range ophiolite with which it has been corre-
Ultramafic rock here is interpreted to be basement to the
Fidalgo ophiolite (Fig. 22) based on its position structurally
beneath the other parts of the ophiolite; however, the contact is
covered by Quaternary materials. Minerals are serpentinite (after
olivine), relict pyroxene, and chromite. Protolith rock ranges from
dunite to peridotite. Rock that was originally peridotite is marked
by significant amounts of relict pyroxene, together with serpentine, whereas the original dunite is virtually free of pyroxene.
The meta-peridotite and meta-dunite are thus distinguishable and
can be seen as irregular layers through this exposure. Pyroxenite
veins exhibiting comb structure cross the other lithologies. We
will speculate about the origin of these layers and veins and what
information they might provide about mantle deformation and
basalt genesis.
Continue around the “loop road”; exit Park back to
Sunset Ave.
3.1 mi
Turn right on Anaco Beach Road and continue on to
merge with Marine Drive.
Stop 3-2. Private Property along Marine Drive,
Fidalgo Island (Fig. 22)
(Access is not guaranteed as of this writing.)
Cumulate gabbro displays layering formed by differential
settling of pyroxene and plagioclase crystals in the melt (Fig. 23).
Dikes of plagiogranite and keratophyre occur locally in the gabbro, and exclusively as a sheeted complex higher in the section.
The orientation of bedding in the gabbro and direction of grading are consistent with its mapped structural position low in the
ophiolite stratigraphy, but above the ultramafic rock.
Continue south on Marine Drive
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
A
B
foliated rind
S2
bedding
foliation
S1
S 2 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
9.3
Turn right (south) on Erie Mountain Drive.
Park in pull-out on right; cross the highway to see
outcrops.
Stop 3-3. Roadcut along Erie Mountain Drive,
Anacortes City Park, Fidalgo Island (Fig. 22)
Observe green volcanic breccia. The lithology here is keratophyre (= meta-dacite). The volcanic section of the ophiolite
ranges from 48 to 74 wt% SiO2 (Brown et al., 1979). K2O is typi-
cally <1.0% through the suite, anomalously low for calc-alkaline
rocks. Primary textures are well preserved, as in this exposure,
and do not support a hypothesis of postmagmatic chemical alteration. Igneous minerals observable in thin-section are plagioclase,
clinopyroxene, quartz, and opaques. Metamorphic minerals, in
veins and incipiently developed in the igneous matrix, are chlorite, epidote, pumpellyite, prehnite, albite, and quartz. Identification of aragonite at one locality (Gusey, 1978) has not been confirmed by X-ray analysis of many other carbonate samples from
the Fidalgo ophiolite (M.C. Blake, 2006, personal commun.).
Anacortes
Fer
ries
12th St.
Commercial Ave.
ve
ks A
Oa
ur
20
Sp
Sunset Ave.
d
oa
aco R
An
Washington
Park
3-1
Q
Q
20
u
Sp
41st St.
r
Hav
eko
st
70
M
rive
Allan Is.
85
D
ine
ar
3-2
Ro
ad
45
70
3-5
60
40
35
Detrital zircon
45 sample
60
75
75
3-3
Ro
s
Q
R
ake oad
Heart L
N
1 km
65
ari
oR
oa
d
3-4
65
Sharps
Corner
30
WA 20
Burrows Is.
25
Mt Erie
SCHEMATIC SECTION OF FIDALGO OPHIOLITE
Detrital zircon sample
peak age 148 Ma
SILTSTONE AND GRAYWACKE
PELAGIC ARGILLITE
Radiolarian ages
l. Kim.- e. Tith. ~ 150 Ma
SEDIMENTARY BRECCIA
KERATOPHYRE
AND SPILITE flows
Zircon age
167 5 Ma
PLAGIOGRANITE
dikes
CUMULATE
GABBRO
unexposed
1000m
SERPENTINITE
Figure 22. Map and schematic stratigraphic section of northern Fidalgo Island. Rocks of Fidalgo Island are interpreted to represent an ophiolite
sequence based on the stratigraphy shown here. The abundance of felsic igneous rock and absence of mid-oceanic-ridge basalts precludes origin
of the ophiolite as sea 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.
Fifty meters down the road, and structurally below the
volcanic rock, is dark brown, manganese-rich, radiolarian
argillite. This rock unit, termed “pelagic argillite,” is as much
as 500 m thick and forms the second sedimentary layer up in
the ophiolite section (Figs. 22 and 24). An unexposed thrust
fault separates these rocks. This structure as well as other
shear zones in the Fidalgo ophiolite have not been analyzed
but have potential for addressing the kinematics of the San
Juan Islands thrust system.
Directions to Stop 3–4
Continue up Erie Mountain Drive.
10.7 mi Summit of Mount Erie.
Stop 3-4. Mount Erie Summit, Anacortes City Park,
Fidalgo Island (Fig. 22)
Massive diorite of the sheeted zone is intruded by finegrained green dike rock (keratophyre and spilite). See views of
the Olympic Mountains Tertiary subduction complex, Admiralty
Inlet to Puget Sound, glacial drift from the Puget lobe, Eastern
and Western Mélange belts in the Cascade foothills.
Retrace route down the Erie Mountain Drive.
12.3
Go south on Heart Lake Road.
13.5
Turn right (west) on Rosario Road.
14.4
Turn right (north) on Havekost Road, past intersection
with Marine Drive.
16.2
Entranceway to the Lakeside Industries quarry is on
the right.
Obtain permission at the office. Hard hats and vests are
required. Avoid quarry slopes, which are unstable and dangerous.
Stop 3-5. Lakeside Industries Quarry, Fidalgo Island
(Fig. 22)
Here we observe non-faulted stratigraphic contacts between
the plagiogranite and sedimentary breccia, and between the breccia and overlying pelagic argillite. The coarse breccia consists of
clasts of all lithologies of the underlying plutonic section including ultramafic rock, and therefore indicates uplift and exposure
of the deeper levels of the section, presumably by faulting. The
breccia represents slide and/or talus deposition. Presence of
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 underlying 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.
Directions to Stop 3-6 (See Also Fig. 25)
Return to highway 20 spur by the following route:
• Turn right out of the Lakeside Industries driveway to go
north on Havekost Road.
• 41st Street: Go right (east) on 41st St.
• O avenue: Jog north one block then east one block.
• Commercial Avenue: Go north.
• Highway 20 spur: Drive east on highway 20
0.0
Sharps Corner, main highway 20, reset odometer;
continue east.
6.8 mi
Highway 237 (Farm to Market Road); go north to the
village of Edison.
14.6
Bow Hill Road; go right (east).
15.6
Chuckanut Drive; go left (north).
19.6
Cross Oyster Creek (at hairpin turn).
19.7
On the left is Oyster Creek Inn and the road to Taylor
shellfish farm (sign). Head down this one-lane road,
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.
Easton Metamorphic Suite
The Easton Metamorphic Suite (formerly known as the
Shuksan Metamorphic Suite; Misch, 1966) is a mostly wellrecrystallized blueschist terrane with close similarities to the
Pickett Peak terrane of the Franciscan Complex (Brown and
Blake, 1987). A variety of lithologic components are found
in this unit (Fig. 25): (1) blueschist and greenschist derived
from mid-oceanic-ridge basalt (Dungan et al., 1983) known
as the Shuksan Greenschist (Misch, 1966); (2) quartzose carbonaceous phyllite, derived from mudstone, named the Darrington Phyllite; (3) metagraywacke semischist derived from
sandstone with abundant chert and dacitic-andesitic clasts;
(4) local pods of metamorphosed plutonic rock of tonalitic to
gabbroic composition; and (5) a local zone of high-pressure
amphibolite and eclogite. The suite as a whole defines the
“Shuksan Nappe” of Tabor et al. (2003), a sheet some 100 km
in length and breadth exposed across much of the northwest
Cascades and breached in an anticlinal structure known as the
“Mt Baker window” (Misch, 1966) where underlying nappes
can be observed.
N
10 km
173
NK
CZ
CZ
Bellingham
Bay
metagabbro
163 2 Ma
LM
BP
CH
TS
CH
.
Dr
BP
130 5 Ma
blueschist
HH
BP
CH
237
Farm to Market rd.
NK
Detrital
zircon
155 Ma peak
I-5
ut
an
Chuck
Edison
FC
BP
meta-gabbro
164 2 Ma
3-6
Anacortes
meta-tonalite
163 2 Ma
Mount
Baker
128 4 Ma
blueschist
I-5
WA 20
CZ
HH
HH
144-160 Ma
amphibolite
Easton Metamorphic Suite
meta-pelite
Darington Phyllite
meta-oceanic basalt
Shuksan Greenschist
meta-pelite and
-graywacke
amphibolite and
rare eclogite
CC
Cz
WM
CH
EM
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.
Brown et al.
META -GABBRO
~ 163 Ma
N
30 m
colluvium
SEMISCHIST
<155 Ma
70
45
5
tide flats
0
10
50
foliation
local
late
deformation
Ck
40
stretching
lineation
te
r
An ocean floor stratigraphy is evident where the Shuksan
Greenschist is stratigraphically overlain by a thin zone of metalliferous quartzose rock which is in turn overlain by Darrington
Phyllite (Haugerud et al. 1981). The metagraywacke unit is interlayered with Darrington Phyllite in the western part of the Shuksan Nappe and represents volcanic arc and flysch detritus. Based
on the above relations, Gallagher et al. (1988) proposed a back
arc setting for the Easton Metamorphic Suite.
Protolith ages are indicated by U-Pb zircon ages of the
tonalite and gabbro bodies at 163–164 Ma (Fig. 25; Walker in
Gallagher et al. 1988, Dragovich et al. 1998, 1999) and detrital
zircons in a sample of the graywacke yielding a prominent age
peak at 155 Ma (Brown and Gehrels, 2007). The gabbro-tonalite
bodies occur within the graywacke stratigraphy and bear the
same metamorphic mineralogy and tectonite fabric as the graywacke. These relations and the older age of the gabbro-tonalite
bodies imply that they were faulted or slid into the graywacke
depositional basin.
Metamorphic ages known from Rb-Sr and K-Ar ages of
muscovite and amphibole (Armstrong in Brown et. al. 1982;
Armstrong and Misch, 1987) date regional blueschist metamorphism at 120–130 Ma and the higher grade localized
amphibolite-eclogite metamorphism at 144–160 Ma.
Oy
s
174
Figure 26. Map of the outcrop area of stop 3-6, near the mouth of
Oyster Creek.
Stop 3-6. Semischist and Gabbro of the Easton Suite
at the Mouth of Oyster Creek, Private Land
(Fig. 26)
This outcrop is near the western margin of exposure of the
Easton Suite, which comprises the “Shuksan Nappe.” Metamorphic mineralogy and structure point to continuation of the
Shuksan Nappe somewhat beyond this point into small islands
of the eastern San Juan archipelago (Lamb, 2000). Some workers
have considered that the Shuksan Nappe possibly extended as
a structurally high unit across the San Juan Islands contributing
to the 20-km-thick burial required for aragonite metamorphism
(Brandon et al., 1988).
The semischist exposed here is chert rich (Fig. 27) and is
interbedded with carbonaceous phyllite. Stretched chert clasts
mark a northeast trending shallow lineation of similar orientation
to that found regionally in the Easton Suite and interpreted to
represent orogen-normal displacement during Early Cretaceous
subduction zone metamorphism (Brown, 1987).
A short distance along the tidelands to the north is a body of
metagabbro (Fig. 26) similar to others in the Easton Suite dated
to be 163–164 Ma (Fig. 25). The contact of the gabbro and semischist along the beach is covered by colluvium, but in roadcuts
along the highway above, serpentine is seen to intervene between
the units. The origin of the gabbro bodies in the Easton is an
interesting problem. They have apparently either slid or been
faulted into the graywacke section (see above). The gabbro ages
are similar to plutonic rocks in the Fidalgo Complex and the
Ingalls Complex (Fig. 2A), which therefore could conceivably
have been a source for these materials.
Figure 27. Photomicrograph of semischist showing stretched chert
clasts at stop 3-6.
The field trip ends here. Return to Chuckanut Drive and go
north to Bellingham or south to I-5.
ACKNOWLEDGMENTS
Clark Blake and Eric Force, both retired from the U.S.
Geological Survey, reviewed and considerably improved the
manuscript.
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Tectonic evolution of the San Juan Islands thrust system, Washington

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