143

Brown, E.H., Housen, B.A., and Schermer, E.R., 2007, Tectonic evolution of the San Juan Islands thrust system, Washington, in Stelling, P., and Tucker, D.S., eds., Floods, Faults, and Fire: Geological Field Trips in Washington State and Southwest British Columbia: Geological Society of America Field Guide 9, p. 143–177, doi: 10.1130/2007.fl d009(08). For permission to copy, contact editing@geosociety.org. ©2007 The Geological Society of America. All rights reserved.

INTRODUCTION

Rocks and structures of the San Juan Islands of northwest Washington record a long and complex history related to Cor-dilleran 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; Bran-don et al., 1988). The nappes have an oceanic history, indicat-ing accretion to the edge of the North American continent, but they also bear clear evidence of interaction with the continen-tal 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 fi nal 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 Cordil-lera; other aspects may be unique to the San Juan Islands. The east-west transect across the San Juan Islands during this fi eld 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 ear-lier work that identifi ed 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

The Geological Society of AmericaField Guide 9

2007

Tectonic evolution of the San Juan Islands thrust system, Washington

E.H. BrownB.A. Housen

E.R. SchermerDepartment of Geology, Western Washington University, Bellingham, Washington 98225, USA

ABSTRACT

The mid-Cretaceous San Juan Islands–northwest Cascades thrust system is made up of six or more nappes that are a few kilometers or less thick, up to one hundred kilometers in breadth, and that were derived from previously accreted Paleozoic and Mesozoic terranes. This fi eld trip addresses many questions regard-ing the tectonic evolution of this structural complex, including the homeland of the terranes and the process of post-accretionary dispersal that brought them together, how thrusting in the San Juan Islands might have been related to coeval orogenic activity in the neighboring Coast Plutonic Complex, and the origin of blueschist metamorphism in the thrust system relative to subduction and nappe emplacement. The geology of this trip has many counterparts in other outboard regions of the Cordillera, but some aspects of the tectonic processes, as we understand them to date, seem to be unique.

Keywords: San Juan Islands, thrust faults, terranes, blueschist metamorphism, kine-matic analysis, paleomagnetism.

144 Brown et al.

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 paleo geog-raphy of the terranes, and the evolution of deformation before, during, and after emplacement in their current location.

TECTONIC SETTING

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, cur-rent indicators and stratigraphy in the Methow sequence indicate absence of an outboard sediment source until ca. 110 Ma (Ten-nyson 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)

B.C.Wash.

GV

OR

ID

NVCA

AXCH

AX

cc

mc

cc

cc

mc

cc

mc

mc

CC QScc

EK

FR

SC

-FR

faul

t

200 kilometers

WA

Cz

Cz

46

Blue Mtns.

Columbia

Embayment

GVS

B.C.

AK

ST

56135

56

128

50116

13050

42116

42125

CO

AS

T P

LU

TO

NIC

CO

M

PLEX

WR

WR

MT

IZ

BA

H

WJ

WTr

Pz

KlamathMountains

No.Sierra

QS

CPC

Fig. 2

ST

MT

QS

Yalacom fault

YT

SF

Figure 1. Regional setting of the San Juan Islands—northwest Cas-cades 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: Burchfi el et al. (1992a); Gehrels and Kapp (1998); Wheeler and McFeely (1991). B.C.—British Columbia; CA—Cali fornia; cc—Cache Creek belt; Cz—Cenozoic rocks and surfi cial deposits; ID—Idaho; mc—McCloud belt of Miller (1987); OR—Oregon; NV—Nevada; Wash.—Washington.

LS

CH

T

EA

CZNK

CH

EA

BP

HZ

Ross Lake fault

CORE

Str

aigh

t Ck.

- F

rase

r R

. fau

lt

INGT

Me

lan

ge

Be

lts

CPC

VC

EA

HS

LM

FC

LS

COOC

GA

NA

ESTB

WR

B.C.WA

T

N

30 kilometers

123

12149

48

Windy PassThrust

PUGET

SOUND

TS

YA

YAA'

A

88-96 Maplutons

EA

SAN JUAN ISLANDS

NORTHWEST

CRYSTALLINE

EM

EM

HH

HH

WM

WM

DDMF

MS

CN

CN

T

Q

Q

CASCADES

TG

CW

Mt Baker

NK

CH

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.

A

146 Brown et al.

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.

STRUCTURAL STRATIGRAPHY

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 Cas-cades. 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, respec-tively (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 Juras-sic 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 Gar-rison Schist (Brandon et al., 1988), observed on this fi eld 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.

The highest nappes in the San Juan Islands–northwest Cas-cades thrust system are Late Jurassic rocks that include ophio-litic 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 Meta-morphic 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

A

Orcas IslandTwin Sisters Range

StraightCreek fault

CHEA

TS

BPCH

NK

BP

YA

CNOC

COFC

ES OC

FC

Lummi Island

Shuksan thrust

Chilliwack batholith

ES BPCH

CHNK

YA

TBLM

NA

TB

A'

10 km

VCGA

CC

depositional or intrusive contactfault contact

HS

WRANGELLIA

EAEA

no vertical exaggeration

Mt Baker window

SL

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

B

SE

CWHL

SC-FR fault restored

PACIFICOCEAN

N

N

100 KM

N

WRANGELLIA

MT

N

B.C.WA

NWCSNAPPES

QUESNELLIA

NA

HZ

HB

EA

CZ CZ

PRC

Georgia Strai t

NK

B.C.WA

Jur. -

E. Cret.

Plutons

SE

Mid-Late CretaceousPlutons

ING

EA

MN

ME

LAN

GEBE

LTS

Ross Lake Fault Zone

CascadeCrystallineCore

WPT

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—Pacifi c Rim Complex, SE—Settler Schist. See Table 1 for other unit abbreviations.

TA

BLE

1. K

EY

TO

UN

ITS

Thr

ust s

yste

m u

nits

EA

ST

ON

ME

TA

MO

RP

HIC

SU

ITE

(E

A)—

Late

Jur

assi

c oc

ean

floor

and

tren

ch d

epos

its, w

ell-r

ecry

stal

lized

Ear

ly C

reta

ceou

s bl

uesc

hist

. F

IDA

LGO

CO

MP

LEX

(F

C)—

Late

Jur

assi

c ar

c-re

late

d op

hiol

ite, m

inim

al fa

bric

, inc

ipie

nt p

rehn

ite-p

umpe

llyite

met

amor

phis

m.

CO

NS

TIT

UT

ION

FO

RM

AT

ION

(C

O)—

Late

Jur

assi

c tr

ench

dep

osits

, min

imal

fabr

ic, i

ncip

ient

blu

esch

ist m

etam

orph

ism

. LU

MM

I FO

RM

AT

ION

(L

M)—

Late

Jur

assi

c oc

ean

floor

and

tren

ch d

epos

its, p

enet

rativ

e fa

bric

, inc

ipie

nt b

lues

chis

t met

amor

phis

m.

LOP

EZ

ST

RU

CT

UR

AL

CO

MP

LEX

(L

S)—

Jura

ssic

to E

arly

Cre

tace

ous

ocea

n flo

or a

nd tr

ench

dep

osits

, inc

ipie

nt b

lues

chis

t met

amor

phis

m.

TW

IN S

IST

ER

S D

UN

ITE

(T

S)—

Man

tle-d

eriv

ed u

ltram

afic

tect

onite

. T

UR

TLE

BA

CK

CO

MP

LEX

(T

B)

and

corr

elat

ive

YE

LLO

W A

ST

ER

CO

MP

LEX

(Y

A)—

early

to m

iddl

e P

aleo

zoic

gab

bro/

tona

lite,

and

par

agne

iss

in Y

A, m

inim

al

fabr

ic, a

mph

ibol

ite, g

reen

schi

st a

nd p

rehn

ite-p

umpe

llyite

faci

es m

etam

orph

ism

. G

AR

RIS

ON

SC

HIS

T (

GA

) an

d co

rrel

ativ

e V

ED

DE

R C

OM

PLE

X (

VC

)—oc

ean

floor

dep

osits

, Per

mia

n ep

idot

e-am

phib

olite

and

blu

esch

ist m

etam

orph

ism

. O

RC

AS

CH

ER

T in

clud

ing

DE

AD

MA

N B

AY

VO

LCA

NIC

S (

OC

) an

d co

rrel

ativ

e B

ELL

PA

SS

ME

LAN

GE

(B

P)—

Tria

ssic

-Jur

assi

c ch

ert,

less

er o

cean

ic-is

land

ba

salt

in O

C a

nd B

P; e

xotic

blo

cks

of E

arly

Cre

tace

ous

sand

ston

e-ar

gilli

te, T

win

Sis

ters

Dun

ite, Y

ello

w A

ster

Com

plex

, and

Ved

d er

Com

plex

in B

P; G

arris

on

Sch

ist a

nd li

mes

tone

with

Per

mia

n T

ethy

an fu

sulin

ids

in O

C.

EA

ST

SO

UN

D G

RO

UP

(E

S)

and

corr

elat

ive

CH

ILLI

WA

CK

GR

OU

P in

clud

ing

Cul

tus

For

mat

ion

(CH

)—S

iluria

n to

Jur

assi

c is

land

arc

, McC

loud

faun

a, m

inim

al

fabr

ic, i

ncip

ient

blu

esch

ist m

etam

orph

ism

. N

OO

KS

AC

K F

OR

MA

TIO

N (

NK

)—Ju

rass

ic to

Ear

ly C

reta

ceou

s is

land

arc

pos

sibl

y fo

rmed

con

tiguo

us w

ith W

rang

ellia

. Sla

ty fa

bric

, inc

ipie

nt p

rehn

ite-

pum

pelly

ite m

etam

orph

ism

. IN

GA

LLS

TE

CT

ON

IC C

OM

PLE

X (

ING

)—E

arly

to L

ate

Jura

ssic

oce

an fl

oor

and

fore

arc

or b

acka

rc–r

elat

ed o

phio

lite,

pre

hnite

-pum

pelly

ite m

etam

orph

ism

and

th

erm

al a

ureo

le. O

ccur

s ea

st o

f the

Str

aigh

t Cre

ek–F

rase

r R

iver

faul

t, bu

t is

corr

elat

ive

with

the

high

er n

appe

s in

the

thru

st s

yste

m.

Mél

ange

bel

tsH

ELE

NA

-HA

YS

TA

CK

ME

LAN

GE

(H

H)—

serp

entin

ite m

atrix

, blo

cks

of g

rayw

acke

, mud

ston

e, c

hert

, bas

alt-

rhyo

lite

and

150–

170

Ma

gabb

ro-t

onal

ite.

WE

ST

ER

N M

ELA

NG

E B

ELT

(W

M)—

scal

y ar

gilli

te m

atrix

, blo

cks

are

mos

tly L

ate

Jura

ssic

–ear

liest

Cre

tace

ous

lithi

c sa

ndst

one/

silts

tone

, som

e 15

0–16

0 M

a ga

bbro

-ton

alite

blo

cks.

E

AS

TE

RN

ME

LAN

GE

BE

LT (

EM

)—m

ostly

met

a-ch

ert a

nd g

reen

ston

e, D

evon

ian-

Jura

ssic

foss

ils, 1

65–1

90 M

a to

nalit

e-ga

bbro

, Per

mia

n T

ethy

an fu

sulin

ids.

Foo

twal

l uni

ts to

the

San

Jua

n Is

land

thru

st s

yste

mH

AR

O F

OR

MA

TIO

N a

nd S

PIE

DE

N G

RO

UP

(H

S)—

Tria

ssic

to E

arly

Cre

tace

ous

arc-

deriv

ed s

edim

enta

ry r

ocks

, zeo

lite

faci

es m

etam

orph

ism

. W

RA

NG

ELL

IA (

WR

)—P

aleo

zoic

arc

and

Tria

ssic

oce

an p

late

au c

ompl

ex, m

icro

cont

inen

t, ze

olite

faci

es m

etam

orph

ism

.

Cas

cade

cry

stal

line

core

,par

t of t

he C

oast

Plu

toni

c C

ompl

exT

ON

GA

FO

RM

AT

ION

(T

G)—

Ear

ly C

reta

ceou

s tr

ench

dep

osits

and

arc

vol

cani

clas

tic r

ock,

gre

ensc

hist

and

am

phib

olite

faci

es.

CH

IWA

UK

UM

SC

HIS

T (

CW

)—E

arly

Cre

tace

ous

accr

etio

nary

com

plex

, Bar

rovi

an a

mph

ibol

ite fa

cies

met

amor

phis

m.

Ove

rlap

units

NA

NA

IMO

GP

. (N

A)—

Late

Cre

tace

ous

epic

ontin

enta

l mar

ine

sedi

men

tary

roc

k, z

eolit

e fa

cies

. C

HU

CK

AN

UT

FO

RM

AT

ION

and

rel

ated

uni

ts (

CN

)—E

ocen

e flu

viat

ile s

edim

enta

ry r

ock,

virt

ually

unm

etam

orph

osed

.

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-of-sequence 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 fl ank are the arc-derived Late Triassic Haro Forma-tion, 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 “exter-nal” to the thrust system (Brandon et al., 1988). Owing to the dif-ferent 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 Chi-waukum 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 fl ank, the Coast Plutonic Complex is intrusive into Wrangellia. Thus, one inter-pretation for the regional structure is that Wrangellia and the Coast Plutonic Complex constituted a structural block in mid-Cretaceous 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 ter-rane translation of at least hundreds of kilometers, emplace-ment of nappes into the regional geologic setting of north-west Washington, and deformation related to the Eocene and younger fold and thrust belt affecting the Nanaimo Group and Chuckanut Formation (England and Calon, 1991). Cer-tainly some metamorphic fabric and possibly some fault bound aries 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 fi eld 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 fl ank 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 zircon-dated 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 devel-opment varies greatly, even within the same units. In the Cas-cades, evidence of HP-LT metamorphism is found virtually in all thrust system units of Jurassic or older age. The blue-schist 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 Forma-tion and sandstone in the Bell Pass Mélange) lack defi nitive

Tectonic evolution of the San Juan Islands thrust system, Washington 149

evidence of high-pressure metamorphism. In the San Juan Islands, aragonite (Fig. 4) and lawsonite are widely devel-oped 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 thick-ness 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 deforma-tion 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). Fig-ure 5 summarizes various interpretations of the age of meta-morphism relative to deformation.

TECTONIC EVOLUTION

A number of features and arguments point to primary accre-tion 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 prox-imity to a “continent-like” landmass. They also note that else-where 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. Addi-tional 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 miogeo-cline (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 assem-bly of the terranes is a few tens of millions of years at most,

A B

1.0 mm

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

150 Brown et al.

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 Cre-taceous blueschist metamorphism. Thus, the building process of the San Juan Island nappe pile is different than that under-stood for other parts of the Cordilleran margin.

Maekawa &Brown, 1991

Schermeret al, 2007

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Brown et al.,2005

penetrative cleavage

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rotation of SJI rocks after remagnetization

Ma

remagnetization

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.

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SPIEDEN GROUPSentinal Island Fm.

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Figure 6. Detrital zircon age distributions in terranes of the San Juan Islands–northwest Cascades thrust system (Spieden Group from Housen and Fanning, unpublished; other units from Brown and Gehrels, 2007).

152 Brown et al.

Notwithstanding the important contributions of many previ-ous 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 micro-continent and North America (Brandon and Cowan, 1985; Brandon et al., 1988; Rubin et al., 1990; McGroder, 1991; Burchfi el et al., 1992b; Cowan and Brandon, 1994; Monger and Journeay, 1994).

(2) A transcurrent-transpressional model in which the nappe ter-ranes originally accreted or were deposited south (or north?) along the margin from their present location and then moved coastwise, fi nally stacking up in a reentrant of the continen-tal 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 fi rst 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 Pacifi c North-west, including: the cause of crustal thickening and Barrovian metamorphism in the crystalline core, the origin of the Nanaimo basin, and the confi guration 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 sys-tem 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 kine-matic 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 linea-tions in the Easton Suite coeval with 120–130 Ma blueschist min-erals (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 northwest-directed thrusting. Brandon et al. (1993) disputed this conclu-sion 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 use-ful 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 pene-trative fabric formation. The different interpretations are summa-rized 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 emplace-ment 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 Pacifi c 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 fl ank 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 inter-pretation that the Nanaimo Group was deposited in a foreland

EA

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

154 Brown et al.

basin caused by emplacement of San Juan Islands–northwest 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 experi-mentally to invert quickly to calcite outside its stability fi eld at elevated temperature except under conditions of abnor-mally 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 fl anked 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 Cas-cades 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; Burch-fi el 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 sup-port of the model are: (1) thrust emplacement of the Ingalls ophio-lite 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 seis-mic 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 fi eld running along the southern margin of the embayment (Riddi-hough 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 remagne-tized directions from the San Juan Islands are scattered, how-ever, indicating that a signifi cant 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 signifi cant 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 Cre-taceous 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 Forma-tion passes the inclination-only paleomagnetic fold test, with the best-clustered inclinations occurring at 100% untilting. The mean inclination of 64°, α

95 = 7.8°, suggests an Early Cretaceous paleo-

latitude 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 paleo-magnetic 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 defi ned magnetizations (~60% “failure rate” reported for most sample collections). However, a signifi cant number of samples in all of these studies (a few 100 out of ~1000 samples collected) have well-defi ned magnetizations that pass a reversals or fold test. Studies of inclination error, notably Kim and Kodama (2004), suggest that inclination error in these sedi-ments is moderate (8–10°), and that when corrected for the paleo-magnetic 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 subtropi-cal reef forming bivalves, and are common in a number of Late Cretaceous marginal basin rocks from Baja California to Central California. Using estimated locations of rudist-bearing basins, and the locations of anoxic black shales (Marca Shale) that mark

Tectonic evolution of the San Juan Islands thrust system, Washington 155

the presence of a cold-water upwelling zone along the ancient 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 con-straint 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 Forma-tion from central California (Nicholls and Meckert, 2002).

Another set of data, detrital zircon age distributions, has also been used to test paleogeographic constraints on the location of the Nanaimo Group rocks. Mahoney et al. (1999) used the pres-ence of several Archean-aged zircons to indicate that the Nanaimo Group rocks had been located no more than 500 km south of its 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 posi-tion within the Nanaimo Group. They argued that variations in Protero zoic-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 deposi-tion 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 “moder-ate” 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 strati-fi ed rocks of Wrangellia/Insular affi nity (Wynne et al., 1995, Enkin et al., 2003), or barometrically corrected plutonic rocks (Housen et al., 2003) both indicate more southerly paleolatitudes (36 N, and 3000 ± 700 km of translation) for these units during mid-Cretaceous time (93–88 Ma).

Modern Analogues?

Modern tectonic regimes along the western North American margin (Fig. 8) that serve as possible analogues for emplacement of the San Juan Islands–northwest Cascades thrust system via coastwise movement are collision zones formed by northward displacement of: (1) Siletzia against the south end of Wrangellia (e.g., Wells et al., 1998), and (2) the Yakutat terrane against the southeast corner of Alaska in the Saint Elias orogen (Plafker et al., 1994). Siletzia lies in the Cascade forearc, driven by a combina-tion of oblique plate convergence and Basin and Range extension (Wells et al., 1998). Seismic refl ection allows identifi cation 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–

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Figure 8. Modern-day analogues of orogen-parallel thrusting in the Pacifi c 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 signifi cant 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 fi ts the San Juan Islands–northwest Cascades thrust system, is not a tectonic anomaly.

FIELD TRIP GUIDE

The fi eld 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 fi eld boots.

Please do not use rock hammers or collect specimens any-where on this trip unless specifi cally 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 penetra-tive tectonite fabric. These units are important to understanding the younger portion of the tectonic history of the San Juan Islands. The fi eld 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 pub-lic property, and that the uplands are privately owned. Access to Spieden Island in particular is restricted by its owner.

Directions and Other InstructionsBefore 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 fi eld 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 fi nished the Stuart Island stop, participants will re-embark 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 mag-netostratigraphy (Enkin et al., 2001). These rocks were deposited in a large marginal basin, extending ~175 km from its southern-most 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 Forma tion that are found in conglomerates of the Extension Forma-tion 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) pri-vately owned island in the San Juan archipelago. It has a color-ful 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 ani-mals (mostly Asian and African deer, goat, sheep, and antelope

FridayHarbor

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Arg

yle

Ave

Pic

kett'

s Ln

.

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

158 Brown et al.

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 forma-tions, 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 fi ner-grained turbidite deposits characterizing the Spieden Bluff Formation, and volcaniclastic-rich sandstone, mudstone, and con-glomerates 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 fi eld 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 dis-play 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 por-tion 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, exam-ining 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 1-3From 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.

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Figure 10 (on this and following page). Geology of Stuart Island, from Mercier (1977). (A) Geologic map.

A

Tectonic evolution of the San Juan Islands thrust system, Washington 159

The Haro Formation has only experienced zeolite facies meta-morphism, 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).

Directions to Stop 1-40.0 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.

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

B

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and conglomerate,minor sandstone,siltstone and tuff

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AGE GROUP FORMATION MEMBER THICKNESS LITHOLOGY DESCRIPTION

Figure 11. Geology of Spieden and Sentinel Islands, after Johnson (1981) and Dean (2002). (A) Geologic map. (B) Schematic section of the Spieden Group.

A

B

Tectonic evolution of the San Juan Islands thrust system, Washington 161

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 lime-stone 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 fl ows 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

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 north-ern British Columbia (Miller, 1987). The limestone is largely recrystallized to aragonite marble (Vance, 1968).

The Deadman Bay Volcanics are separated from the over-lying 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.

Directions to Stop 2-10.0 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 fi nally 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 sug-gested dominantly northwest thrusting (Fig. 14A). Cowan and

Brandon (1994) applied a “symmetry based statistical analy-sis” 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 volcanic-rich 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 signifi cantly HP-LT greenschist-amphibolite of the Permian Garrison Schist unit.

Amount and timing of displacement on the Rosario thrust are diffi cult questions. Vance (1975) noted that the overlying Consti-tution Formation bears detritus that appears to be derived from the underlying Orcas Chert, Garrison Schist, and Deadman Bay Vol canics 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).

Directions to Stop 2-28.7 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 orien-tations statistically indicate southwest-vergent thrusting (i.e., toward the viewer with respect to Fig. 14B).

164 Brown et al.

Lopez Structural Complex

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 northwest-elongated, relatively coherent lenses separated by sheared mudstone-rich fault zones. The large lenses are predominantly ocean fl oor 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 fl at-tening 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

Ocean Floor Complex

Clastic sequences

Volcanic rocks andassociated sedimentary rocks

Constitution TerraneSandstone with chert and volcanic rocks

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"Exotic" and Other slices

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ault

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Figure 15. Generalized geology and terrane map of the Lopez Structural Complex. Open circles show locations of fi eld trip stops 2-2 (Cattle Point), 2-3 (Richardson) and 2-4 (Iceberg Point). Modifi ed from Brandon et al. (1988), Burmester et al. (2000), and M.C. Blake (2000, written commun.). Eastern extension of Lopez thrust (from Brandon et al. 1988) may not coincide with a terrane boundary (from Schermer et al., 2007).

Tectonic evolution of the San Juan Islands thrust system, Washington 165

San Juan Island nappes, at least for the Constitution and struc-turally 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 indi-cates exhumation occurred prior to the strike-slip event. The P-T conditions, and spatial and temporal extent of small faults associ-ated with fl uid fl ow suggests a link between these structures and the silent earthquake process.

Given that these latest identifi ed 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 metamor-phism 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

0 1 2 km

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

166 Brown et al.

(2002). Strain in the early thrusting event(s) is strongly parti-tioned 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).

Directions to Stop 2-3Return to Friday Harbor and take the ferry to Lopez Island.

0.0 Ferry terminal on Lopez Island, drive south on Ferry Road.

2.1 mi Turn left (east) on Fisherman Bay Road.2.3 Go right (south) on Center Road.7.7 Turn right (west) on Lopez Sound Road.7.9 Turn left (south) on Richardson Road continue south

to coast.9.6 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 micro-fossils 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. Geologi-cal Survey (in Brandon et al., 1988) based on microfossils in a mudstone collected by John Whetten, University of Washing-ton, 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 fi ndings broaden the age brackets for thrusting, and sug-gested to Brown et al. (2005) that San Juan Islands blueschist metamorphism is older than thrusting. But, a recent fi nding 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).

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thrust vergence T axis from normalfaults, extension veins

T axis fromstrike slip faults

subhorizontal extension

Figure 17. Generalized map of the eastern San Juan Islands with paleo-magnetic 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 defi ned in key. If no arrow is shown for brittle structures at a site, data are too few to conclude kinematic signifi cance. Circles indicate subhorizontal extension in several directions during normal faulting or extension veining. Foliation symbols show average folia tion 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 signifi cance of all phases of fault-ing. A—San Juan Island; B—N. Lopez Island; C—Watmough Head; D—Guemes Island; E—Jack Island; F—Lummi Island; G—Eliza Island; H—Samish Island.

Tectonic evolution of the San Juan Islands thrust system, Washington 167

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 domi-nantly orogen-parallel transport of the San Juan Island nappes.

Directions to Stop 2-49.6 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 indi-cating “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-fl oor affi nity) and a fault slice of Consti-

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LOPEZ ISLAND

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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.).

168 Brown et al.

tution terrane to the north. Late brittle structures include SW-vergent 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.

Directions to Overnight LodgingReturn 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.

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 Chucka-nut Mountain.

Fidalgo Complex

The Fidalgo Complex (Fig. 22) consists of a stratigraphic sequence distinctive of ophiolite. From the base upward in the section are: ultramafi c tectonite, cumulate gabbro, a sheeted

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Pl

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Pl

Pb

Pb

Fault zone

20

20Attitude of slicken-lines in fault zone

35

Pillow lavaPl

Pillow breccia

Tuff

Mudstone

Pb

35 Attitude of faultsurface

12 = elevation in feet above mean low tide

Figure 19. Map of Richardson locality, stop 2-3. From Brown et al. (2005).

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

intrusive complex of mostly plagiogranite (diorite, tonalite, trondjemite, albite granite) and hypabyssal equivalents, a vol-canic sequence of mainly dacitic to andesitic breccias and inter-layered tuffaceous argillite, coarse sedimentary breccia that bears clasts of all the underlying units, pelagic argillite, and volcanic-rich 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 prov-enance (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 thick-ness 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-

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

Directions to Stop 3-1Drive 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 Park0.2 mi Begin one-way loop drive.0.7 Park on left, cement stairs to beach on right.

Stop 3-1. Washington Park, Fidalgo Island

Ultramafi c rock here is interpreted to be basement to the Fidalgo ophiolite (Fig. 22) based on its position structurally beneath the other parts of the ophiolite; however, the contact is 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 signifi cant amounts of relict pyroxene, together with serpen-tine, whereas the original dunite is virtually free of pyroxene. The meta-peridotite and meta-dunite are thus distinguishable and can be seen as irregular layers through this exposure. Pyroxenite 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.

Directions to Stop 3-2Continue 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 gab-bro, and exclusively as a sheeted complex higher in the section. The orientation of bedding in the gabbro and direction of grad-ing are consistent with its mapped structural position low in the ophiolite stratigraphy, but above the ultramafi c rock.

Directions to Stop 3-3Continue 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.

Figure 20. Photomicrograph of metamorphosed pillow breccia at Rich-ardson, 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 fi ne-grained aggregate of Ca-Al silicates. Aragonite and pumpellyite (not visible), as well as chlorite and phengite shown here, are synkinematic minerals.

170 Brown et al.

9.1 Turn right (south) on Erie Mountain Drive.9.3 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 kera-tophyre (= meta-dacite). The volcanic section of the ophiolite ranges from 48 to 74 wt% SiO

2 (Brown et al., 1979). K

2O 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 altera-tion. Igneous minerals observable in thin-section are plagioclase, clinopyroxene, quartz, and opaques. Metamorphic minerals, in veins and incipiently developed in the igneous matrix, are chlo-rite, epidote, pumpellyite, prehnite, albite, and quartz. Identifi ca-tion of aragonite at one locality (Gusey, 1978) has not been con-fi rmed by X-ray analysis of many other carbonate samples from the Fidalgo ophiolite (M.C. Blake, 2006, personal commun.).

C

A B

bedding

foliation

1.0 cm

foliated rind S2

S1

S2 shear fabric in tuff

0.20 mm

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

Hav

ekos

t R

oad

41st St.

20 Spur

Com

mer

cial

Ave

.

12th St.

Oaks Ave

WA

20

SharpsCorner

7070

45

1 km

Anacortes

SERPENTINITE

CUMULATE GABBRO

PLAGIOGRANITEdikes

KERATOPHYRE AND SPILITE flows

SEDIMENTARY BRECCIA

PELAGIC ARGILLITE

SILTSTONE AND GRAYWACKE

unexposed

Detrital zircon samplepeak age 148 Ma

Zircon age167 5 Ma

Radiolarian agesl. Kim.- e. Tith. ~ 150 Ma

SCHEMATIC SECTION OF FIDALGO OPHIOLITE

1000m

Detrital zirconsample

Q

Q

Q

85

75

65 60

45

75 60

3065

40

25

WashingtonPark

Mt Erie

Allan Is.

Burrows Is.

N

35

Sunset Ave.

20 Spur

Ferr

ies

Anaco Road

Marine D

rive

Rosario Road

Hea

rt L

ake

Roa

d

3-2

3-5

3-4

3-3

3-1

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

172 Brown et al.

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 3–4Continue 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 fi ne-grained 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.

Directions to Stop 3-5Retrace 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 offi ce. 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 brec-cia and overlying pelagic argillite. The coarse breccia consists of clasts of all lithologies of the underlying plutonic section includ-ing ultramafi c 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 ultramafi c detritus (Gusey, 1978). Volcanic-rich graywacke overlies these sedimentary rocks and bears detrital zircons with a 148 Ma age peak, younger than the breccia detritus which is derived from the 160–170 Ma under-lying arc (Fig. 22). The Fidalgo ophiolite is interpreted to be a remnant arc, and the overlying graywacke to have been deposited in either a fore-arc or backarc basin.

Figure 23. Cumulate bedding in layered gabbro, Fidalgo Island.

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

Tectonic evolution of the San Juan Islands thrust system, Washington 173

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

shellfi sh 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 fl ats 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 well-recrystallized 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 car-bonaceous phyllite, derived from mudstone, named the Dar-rington 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 defi nes 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.

Cz

CZ

FC

meta-tonalite

155 Ma peak

130 5 Mablueschist

144-160 Maamphibolite

CZ

TS

NK

NK

CH

BP

BP

CH

HH

HH

CH

BP

BP

CH

WMEM

CC

CZ

LM

HH

10 kmN

Detritalzircon

blueschist

MountBakerBellingham

meta-gabbro163 2 Ma

128 4 Ma

Edison

164 2 Mameta-gabbro

Bay163 2 Ma

Far

m to

Mar

ket r

d.

WA 20

I - 5

Anacortes

Chuckanut Dr.

I - 5

meta-peliteDarington Phyllite

meta-oceanic basaltShuksan Greenschist

amphibolite andrare eclogite

Easton Metamorphic Suite

meta-pelite and-graywacke

3-6

237

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

174 Brown et al.

An ocean fl oor stratigraphy is evident where the Shuksan Greenschist is stratigraphically overlain by a thin zone of metal-liferous quartzose rock which is in turn overlain by Darrington Phyllite (Haugerud et al. 1981). The metagraywacke unit is inter-layered with Darrington Phyllite in the western part of the Shuk-san Nappe and represents volcanic arc and fl ysch 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 gray-wacke. These relations and the older age of the gabbro-tonalite bodies imply that they were faulted or slid into the graywacke depositional basin.

Metamorphic ages known from Rb-Sr and K-Ar ages of musco vite and amphibole (Armstrong in Brown et. al. 1982; Armstrong and Misch, 1987) date regional blueschist meta-morphism at 120–130 Ma and the higher grade localized amphibolite-eclogite metamorphism at 144–160 Ma.

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

This outcrop is near the western margin of exposure of the Easton Suite, which comprises the “Shuksan Nappe.” Meta-morphic mineralogy and structure point to continuation of the Shuksan Nappe somewhat beyond this point into small islands of the eastern San Juan archipelago (Lamb, 2000). Some workers have considered that the Shuksan Nappe possibly extended as 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 semi-schist 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.

The fi eld 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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META -GABBRO~ 163 Ma

70

45

5040

5

010

locallatedeformation

SEMISCHIST<155 Ma

t i d e f l a t s

colluvium 30 m

N

foliation

stretching lineation

Oys

ter C

k

Figure 26. Map of the outcrop area of stop 3-6, near the mouth of Oyster Creek.

Figure 27. Photomicrograph of semischist showing stretched chert clasts at stop 3-6.

Tectonic evolution of the San Juan Islands thrust system, Washington 175

Armstrong, R.L., Harakal, J.E., Brown, E.H., Bernardi, M.L., and Rady, P.M., 1983, Late Paleozoic high-pressure metamorphic rocks in northwest-ern Washington and southwestern British Columbia: The Vedder Com-plex: Geological Society of America Bulletin, v. 94, p. 451–458, doi: 10.1130/0016-7606(1983)94<451:LPHMRI>2.0.CO;2.

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