Continental-crust structures on the continental margin of western North America

Lecture Notes in Earth Sciences Editors: S. Bhattacharj1, Brooklyn G. M. Friedman, Brooklyn and Troy H. J. Neugebaner, Bonn A. Sellacher, Tuebingen and Yale

62

Henry V. Lyatsky

Continental-Crust Structures on the Continental Margin of Western North America

Springer

Au~or Dr. Henry V Lyatsky Lyatsky Geoscience Research & Consulting Ltd. 4827 Nipawin CR. NW Calgary, Alberta, Canada T2K 2H8

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Die Deutsche Bibliothek - CIP-Etnheitsaufnahme Lyatsky, Henry V.: C o n t i n e n t a l c r u s t s t r u c t u r e s o f tile c o n t i n e n t a l m a r g i n o f w e s t e r n N o r t h A m e r i c a / H e n r y V. L y a t s k y - B e r l i n ; Heidelberg ; New York ; Barcelona ; Budapest ; Hong Kong ; London ; Milan ; Paris ; Santa Clara ; Singapur ; Tokyo : S p r i n g e r , 1996 (Lecture notes ill earth sciences ; 62) ISBN 3-540-60842-7 NE: GT

"For all Lecture Notes m Earth Sciences published till now please see final pages of the book" ISBN 3-540-60842-7 Springer-Verlag Berlin Heidelberg New York This work is subject to copyright. All rights are reserved, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, re-use of illustrations, recitation, broadcasting, reproduction on microfilms or in any other way, and storage in data banks. Duplication of this publication or parts thereof is permatted only under the provisions of the German Copyright Law of September 9, 1965, in its current version, and permission for use must always be obtained from Sprlnger-Verlag. Violations are hable for prosecution under the German Copyright Law. © Sprmger-Verlag Berlin Heidelberg 1996 Printed m Germany Typesetting" Camera ready by author SPIN: 10528995 32/3142-543210 - Printed on acid-free paper

PREFACE

The

a i m of this volume is two-fold.

At the more pragmatic level,

it is to help answer the many questions about the structure of the Pacific

continental

over the years geophysical

as

margin a

surveys.

of

result

North America, which have arisen

of

continuing

field

mapping

and

The second objective is methodological - to

illustrate the irreplaceable role of geological information

among

the various data sets used in earth-science studies.

The

need

to

address

these issues became apparent to the author

during the several years he spent taking part geophysical

studies

results

geologic

of

predictions

in

on the west coast of Canada. field

mapping

disagreed

geological

and

All too often, with

tectonic

from t o o - s t r a i g h t f o r w a r d local applications of global

plate reconstructions, which due to their g e n e r a l i t y do not always take a full account of specific character of particular regions.

To

be

sure,

the

global

approach

has during the last q~/arter-

century greatly expanded the vision of restricted to continental regions.

geoscientists,

However, a negative by-product

of this expansion has been a decline of attention information,

as

tectonic

previously

studies

have

paid

increasingly

to

local

relied on

simply fitting the development of a particular region into this or that prefabricated tectonic template.

Direct

geological observations have limitations of their own. The

observer in most cases deals with products of geologic rather than with the processes themselves.

processes,

Field mapping provides

VJ

local information,

and many years of effort are

regional

overview

restricted

to the ground surface,

cannot

sample

factual

becomes

more

than

of

geologic

determination

of

rock

Conclusions

incorporation

and even the deepest

mapping

quickly

usually

are still mostly

into geological

assisted by e v e r - m o r e - s o p h i s t i c a t e d

methods

is

drillholes

afford regional

to

in areas of

structure

of

a

inferential.

modern computers,

data,

provides

in other ways.

coverage

The

limited

studies of geophysical

unobtainable

a is

shell of the Earth.

about the three-dimensional

huge volume of information

before mapping

types and their relationships

region and its evolution

Broad

Geologic

the outermost

side

exposure.

possible.

needed

a

Geophysical

or images of the

Earth's

deep interior.

Geophysical sciences

methods

have

of methodologies

mathematics quantitative parameters

and

borrowed

physics.

modeling, of

prompted the application

a

The

of

system

this

to

predict others.

or

characteristics

of a geologic

requires

never

or

perfect.

physical

phenomenon, imitation

one to rely on simplifying

better than the assumptions

of

has the

as been

known

pitfall.

a

natural

To

phenomenon.

is relative, incorporate

in a parametrized is

assumptions,

at its base.

to use

such

But in taking this

that representation

is

numerical

a dangerous

representation

representation

into a

important

which allows a scientist

is a simplified

quality

sciences,

Particularly

approach too far, one encounters

A model

from exact

in geological

impossible. and a model

and a all form, This is no

VII

Unrealistic

assumptions

lead

disagreement

arises

such as t h o s e

from g e o l o g i c

tempted

to

between

downplay

observations.

role

an

of

realism

means

methodological American

predictions mapping

It b e c o m e s

-

When

a

and o b s e r v a t i o n s a

modeler

may

or the s i g n i f i c a n c e

tempting

geologist

data

of

arbiter

be

of the

to u n d e r e s t i m a t e

as a p r i n c i p a l

Geological

the

of the

The

the

From western

this North

as follows:

available

models

attention

into

of

the

from

field

mapping

and

and s u m m a r i z e d .

available

synthesized

that p r o v i d e

models.

study

is o r g a n i z e d

information,

geophysical

with particular

control

abstract

the p r e s e n t

margin

is g a t h e r e d

Current

and g e o l o g i c a l

testing

position,

continental

drilling,

3.

models.

of a model.

ultimate

2.

model

differences

experienced

But it is g e o l o g i c a l

i.

unrealistic

field

the

offending

to

for

to t h e i r

data,

an

this

region

underlying

geological

internally

are

considered,

assumptions.

and

consistent

geophysical,

are

geologic-evolution

concept. 4.

This

concept

is t e s t e d

observations

from

Because

current

most

Washington

that

help

American decades. problems,

field m a p p i n g

data

and w e s t e r n

paid to t h e s e

areas.

continental

sets

British

the

margin,

author

does

but he does b e l i e v e

with direct

geological

and drilling.

and

models

Columbia,

Fortunately,

understand

The

by c o m p a r i s o n

these

structure which

particular areas

baffled to have

he has m a d e

northwestern attention

contain

of the e n t i r e

has

not c l a i m

cover

many

keys

western

North

scientists

resolved

a useful

was

for

all t h e s e

contribution

to

VIII

understanding continental

continental-oceanic

of

current

models

lithospheric

ridges.

with two plates

centers, the

mantle

boundaries

If

both

the

sliding creation

lithosphere

as

it

is

regimes

in the

from

occur

of

other.

a

evolution late

at some plate boundaries

Barr and Chase, the

principles

along

a

single

of the boundary,

thereafter

(Atwater,

1974; R i d d i h o u g h of

plate Unless it

rigid-plate

and Hyndman, tectonics,

reconstructions

rigid-plate between

boundary,

must

be

associated

plates.

and tectonic

margin

and

in

et al.,

1976).

To

the 1972;

satisfy

both regimes have to

is

a

plate

the areas of proven ongoing

(in Oregon and southern Washington)

the

it can be tied to

1970; McManus

Also needed

somewhere

into

At such plate

continental

margin.

junction

descends

the structure

exist along this continental

in

to

zones.

of the western North American and

type,

at spreading

by other plates.

template was used to interpret

1960s

of

interact

However,

lithosphere

between them must be abrupt.

in orientation

can

are of strike-slip

each

new

system

the place of its birth

with which it

past

in

of a plate is

global

with a junction of not two, but three different

Such

at this

assumption

The lithophere

away

overriden

lie subduction

transition a change

moves

critical

Some interactions

simply

for

older

a

with other plates,

a variety of ways.

compensate

is

centers m a n i f e s t e d It

towards boundaries in

plates

of plate evolution.

at spreading

mid-ocean

interrelations

margin.

Rigidity

created

plate

transform

triple

subduction

plate

motion

IX

(along

the

southeastern Alaska margin; Atwater,

al., 1972).

Such a triple junction

has

been

1970; McManus et

placed

off

Queen

Charlotte Sound offshore British Columbia (Keen and Hyndman, Riddihough et al., postulated

1983),

between

where

the

a

Pacific

(Hyndman et al. 1979; Riddihough,

spreading and

center

Explorer

1984).

Off

1979;

has

oceanic

northern

been plates

Vancouver

Island, a transform boundary between the Explorer and Juan de Fuca oceanic plates has been postulated,

but

both

these

plates

are

assumed to be subducting beneath Vancouver Island (Hyndman et al., 1979; Riddihough and Hyndman,

1989)o

With the assumed universality of similarity"

has

been

the

suggested

rigid-plate

between

model,

"broad

the geology of western

Oregon and that of western British Columbia, and the Cascadia zone of

active

of

Queen

subduction has been extended as far north as the mouth Charlotte

accretionary

Sound

sedimentary

(Riddihough,

prism

(Yorath,

1979, 1980)

accretionary complex containing several exotic and Hyndman,

Geological

-

1984).

An

or

an

even

"terranes"

(Davis

1989) - has been postulated off Vancouver Island.

observations

onshore

and

offshore (Shouldice,

1971;

Tiffin et al., 1972) have come to be considered too "surficial" to be

of major consequence for large-scale tectonic m o d e l i n g (Yorath

et

al.,

1985a,b;

geophysical

Yorath,

1987).

Variants

of

the

principal

model for this a r e a during the last decade (Clowes et

al., 1987; Hyndman et alo, 1990; Spence et al. 1991; Yuan et 1992;

Dehler

and

Clowes,

1992) have become increasingly distant

from geological observations. were

checked

for

internal

neighboring local models tectonic picture.

al.,

and

As new model variants emerged, they consistency, fidelity

to

compatibility the

overall

with

assumed

However,

detailed

geological

work

continued,

and

many of its

results proved incompatible with the conventional wisdom (Gehrels, 1990;

Babcock

1993a).

et

al.,

1992, 1994; Allan et al., 1993; Lyatsky,

Importantly, questions arose about the

applicability

in

this region of the conventional, simple rigid-plate assumption, as it was shown to be unable to account for all geophysical

peculiarities

in

the

geological

and

some areas (Carbotte et al., 1989;

Allan et al., 1993; Davis and Currie,

1993).

New

solutions

were

made necessary by new findings and by rediscovery of forgotten old data (see Lyatsky et al., 1991; Lyatsky,

Without aiming to resolve all the implications

outstanding

debates,

integrated

with

geochemical

chapters. and

These

are

In

has

structures

observations

other

geologically plausible extrapolations from these observations.

author

geological

by

by

for

by

and

data.

and

searching

verified

results

geophysical

Interpretations of these data, made by this author workers,

tectonic

of the geologic mapping and drilling results in this

region are considered in the following are

1993b).

solutions consistent with all the information, the

restricted along

himself

to

analyzing

this continental margin.

that future models for the offshore regions

continental-crust

He believes, however, of

the

Pacific should consider the results obtained herein.

northeastern

Acknowledgments

Through

the

support

of

the

Universities of British Columbia author

has

been

able

Geological and

Survey

Victoria,

and

of

Canada,

NSERC,

the

to spend a total of several months in the

field, examining first-hand the geology of the Canadian

and

~U.S.

Cordillera and continental-margin areas.

The

author

is grateful to his colleagues whose encouragement and

insightful questions helped him understand the geologic of

the

margin.

Jim

Monger,

Bob Thompson and Glenn Woodsworth

introduced him to the regional geology of the western and

Jim

Haggart,

Cathie

structure

Hickson

Cordillera,

and Peter Mustard (all at the

Geological Survey of Canada) acquainted him with specific problems in

individual

areas.

Jim

Murray (University of Alberta),

Dick

Chase (University of British Columbia), Dave Brew (U.S. Geological Survey)

and Bob Crosson

discussions. Art

Haynes

volume.

offered useful

Technical help was provided, at different times, (Geological

(University of Calgary). this

(University of Washington)

The

Survey

of

Gerry Friedman

Canada)

and

Brian

by Fong

(Brooklyn College) edited

responsibility for the scientific conclusions

presented here, however, rests with the author alone.

CHAPTER

1

-

OUTSTANDING

ISSUES

IN

STUDIES

OF

CONTINENTAL

MARGINS

Basic terminology r e l a t e d t o m a r g i n s of c o n t i n e n t s ......... Definition of c r u s t a l t y p e a t c o n t i n e n t a l margins .......... Historical outline of perspectives on Cordilleran geology .. Shortcomings o f c u r r e n t m o d e l s of C o r d i l l e r a n evolution .... S t r u c t u r e of w e s t e r n N o r t h A m e r i c a p l a t e b o u n d a r y in current models .............................................

CHAPTER

2

-

EVALUATION

OF

THE

DATA

3 -

PRE-CENOZOIC

GEOLOGIC

FRAMEWORK

OF

WESTERN

4

- TERTIARY PROVINCES

STRATIGRAPHIC IN WASHINGTON

FRAMEWORK OF AND BRITISH

21 22 22 24 25 25 26 26 28 30 31 31 32 35

CORDILLERA

Pre-Tertiary stratigraphic record .......................... Paleozoic ............................................... Mesozoic ................................................ T e c t o n i c s t a g e s of p r e - T e r t i a r y geologic evolution ......... Paleozoic interval ...................................... Late Triassic to Early Jurassic interval ................ Mid-Jurassic episode of tectonism ....................... Late Jurassic to Late Cretaceous interval ............... Latest Cretaceous(?) to earliest Tertiary tectonism ..... Timing of terrane accretion in t h e w e s t e r n C o r d i l l e r a ...... P l a c e o f t h e C o a s t B e l t o r o g e n in t h e t e c t o n i c e v o l u t i o n of western Cordillera ...................................... Local uncommon rock complexes on the western and southern p e r i p h e r y of V a n c o u v e r Island .............................. Pacific Rim m~lange complex (including Pandora Peak unit) ................................................... Leech River metamorphic complex .........................

CHAPTER

14

BASE

Direct geological observations - t h e m a i n s o u r c e of information ................................................ Physical parameters of r o c k s - c o n s t r a i n t s on interpretation of p o t e n t i a l - f i e l d data ..................... Rock magnetization ................................ Rock density ............................................ Processing of p o t e n t i a l - f i e l d data ......................... Fundamental notions ..................................... Magnetic and gravity coverage ........................... Reductions of g r a v i t y d a t a .............................. Horizontal-gradient maps ................................ Upward continuation of p o t e n t i a l - f i e l d data ............. Assessment of s e i s m i c d a t a ................................. O v e r v i e w of t h e d a t a .................................... Ambiguities in s e i s m i c i n t e r p r e t a t i o n ................... Methodological principles of this study ....................

CHAPTER

1 3 4 7

37 37 37 42 42 42 43 45 46 47 49 51 51 53

COASTAL COLUMBIA

Early Tertiary paleoenvironments ........................... Early Paleogene ......................................... Early Tertiary basaltic magmatism ....................... Relationship of C r e s c e n t F o r m a t i o n m a s s i f s w i t h e a r l y Tertiary sedimentary sequences ..........................

54 54 55 58

XIV

S t r a t i g r a p h i c r e c o r d of m i d - E o c e n e t o M i o c e n e s e d i m e n t a r y basins ..................................................... S t r a t i g r a p h i c r e c o r d of l a t e T e r t i a r y s e d i m e n t a r y b a s i n s ... O v e r v i e w of T e r t i a r y g e o l o g i c e v o l u t i o n of c o a s t a l provinces .................................................. Two main geologic provinces along the continental margin from Oregon to southeastern Alaska ...................... V a r i a t i o n s in t e c t o n o - m a g m a t i c style along the continental margin .................................................. D i s t r i b u t i o n of T e r t i a r y s e d i m e n t a r y b a s i n s a l o n g t h e m a r g i n in t i m e a n d s p a c e ................................

CHAPTER

5-

64 64 65 69

SIGNIFICANCE OF THE TRANS-CORDILLERAN OLYMPICWALLOWA ZONE IN GEOLOGIC EVOLUTION OF THE WASHINGTON AND BRITISH COLUMBIA COASTAL REGIONS

R e c o g n i t i o n of t h e O l y m p i c - W a l l o w a Z o n e of c r u s t a l weakness ................................................... T h e O W S Z in e a s t e r n O r e g o n a n d W a s h i n g t o n .................. T h e O W S Z in c e n t r a l W a s h i n g t o n ............................. T h e O W S Z as a b o u n d a r y b e t w e e n N o r t h a n d S o u t h W a s h i n g t o n Cascades ................................................... T h e O W S Z w e s t of t h e W a s h i n g t o n C a s c a d e s ................... B o u n d a r y f a u l t s y s t e m s of w e s t e r n O W S Z ..................... South Vancouver Island fault system ..................... North Olympic fault system .............................. Central Olympic Basin ...................................... Hoh Basin .................................................. D e e p s t r u c t u r e of t h e O l y m p i c P e n i n s u l a a r e a f r o m g r a v i t y data ....................................................... O n t h e n a t u r e of c r y s t a l l i n e b a s e m e n t of t h e O l y m p i c Peninsula .................................................. D e e p s t r u c t u r e of s o u t h e r n V a n c o u v e r I s l a n d f r o m s e i s m i c data ....................................................... T i m i n g of i n v e r s i o n of t h e C e n t r a l O l y m p i c B a s i n a n d u p l i f t of t h e O l y m p i c M o u n t a i n s ................................... P o s s i b l e c a u s e s of O l y m p i c M o u n t a i n s u p l i f t ................

CHAPTER

59 62

73 76 78 81 85 90 90 98 102 106 108 116 117 124 126

6 - CONTINENTAL MARGIN OFF SOUTHEASTERN ALASKA, THE QUEEN CHARLOTTE ISLANDS, AND NORTHERN VANCOUVER ISLAND

S c o p e of i d e a s r e g a r d i n g t e c t o n i c n a t u r e of t h e N o r t h America-Pacific plate boundary ............................. General structural characteristics of t h e p l a t e b o u n d a r y along the southeastern Alaska margin ....... . . . . . . . . . . . . . . . . C o n c e r n s a b o u t f i d e l i t y of g e o p h y s i c a l m o d e l s a l o n g t h e western Canada continental margin .......................... Models of western Canada continental margin based on gravity data ............................................... B a t h y m e t r y of t h e B r i t i s h C o l u m b i a c o n t i n e n t a l m a r g i n ...... D e e p s t r u c t u r e of t h e c o n t i n e n t - o c e a n plate boundary off Queen Charlotte Islands .................................... S o u t h w a r d e x t e n s i o n of p l a t e b o u n d a r y o f f Q u e e n C h a r l o t t e Sound ...................................................... C o n c e p t of p l a t e r i g i d i t y as a p p l i e d t o n o r t h e r n J u a n d e Fuca oceanic plate off western Canada ...................... Plate-boundary zone off northern Vancouver Island and the Winona Basin ...............................................

133 136 138 141 143 147 156 162 166

XV

I n t e r l o c k i n g of c o n t i n e n t a l a n d o c e a n i c c r u s t a l b l o c k s in the Brooks-Estevan Embayment ...............................

179

CHAPTER 7 - CRUSTAL BLOCKS U N D E R V A N C O U V E R ISLAND A N D THE EXTERIOR SHELF V a r i a t i o n s in c r u s t a l t h i c k n e s s a l o n g t h e I n s u l a r B e l t ..... G e o l o g i c a l s h o r t c o m i n g s of e x i s t i n g s e i s m i c m o d e l s of Vancouver Island crust ..................................... A n a l y s i s of g r a v i t y a n o m a l i e s o n V a n c o u v e r I s l a n d .......... Seismic refraction constraints on deep crustal structure ... I n i t i a l i n t e r p r e t a t i o n s of V a n c o u v e r I s l a n d s t r u c t u r e from seismic reflection data ............................... G e o l o g y - b a s e d i n t e r p r e t a t i o n of V a n c o u v e r I s l a n d s e i s m i c data ....................................................... I n c o n s i s t e n c i e s in c u r r e n t t e c t o n i c m o d e l s of e v o l u t i o n of Vancouver Island and adjacent submerged margin ............. G e o l o g i c s k e t c h of t h e V a n c o u v e r I s l a n d e x t e r i o r s h e l f ..... T e c t o n i c i n f o r m a t i o n f r o m d e e p d r i l l i n g in t h e T o f i n o Basin ...................................................... T r a n s v e r s e f a u l t s a n d c r u s t a l s t r u c t u r e of t h e e x t e r i o r shelf ...................................................... I d e n t i f i c a t i o n of b l o c k s a n d b o u n d i n g f a u l t s ............ Cove block .............................................. Vargas block . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ..... Ucluth block ............................................ Bamfield block .......................................... Clo-oose block .......................................... Flattery block .......................................... C r u c i a l r o l e of g e o l o g i c a l i n f o r m a t i o n in v a l i d a t i o n of geophysical models .........................................

186 188 191 195 198 202 205 210 217 222 222 226 228 229 232 236 237 238

CHAPTER 8 - STRUCTURE OF CONTINENTAL SLOPE OFF V A N C O U V E R ISLAND R e g i o n a l o v e r v i e w of V a n c o u v e r I s l a n d c o n t i n e n t a l s l o p e .... Gravity and magnetic anomalies along the continental slope off n o r t h e r n W a s h i n g t o n a n d s o u t h e r n B r i t i s h C o l u m b i a ...... Deep structure of the southern Vancouver Island continental slope ...................... . ................... Sediment-filled Juno depression on lower continental slope ...................................................... L i m i t e d n o r t h w a r d e x t e n t of s u b d u c t i o n - r e l a t e d t h r u s t faults and m~lange along the continental margin ............ Zoning in the distribution of continental and oceanic crust on Vancouver Island continental slope ......................

244 245 249 256 263 271

C H A P T E R 9 - INTERLOCKING O F C O N T I N E N T A L AND O C E A N I C C R U S T A L BLOCKS ALONG T H E C O N T I N E N T A L M~%RGIM AND N O N - R I G I D B E H A V I O R OF N O R T H E R N J U A N DE F U C A P L A T E P l a c e o f b l o c k i n t e r l o c k i n g in t h e p l a t e - b o u n d a r y zone ..... G e o m o r p h o l o g i c a l e x p r e s s i o n of b l o c k i n t e r l o c k i n g .......... M a g n e t i c a n d g r a v i t y e x p r e s s i o n of b l o c k i n t e r l o c k i n g ...... E a r t h q u a k e s e i s m i c i t y in t h e z o n e of b l o c k i n t e r l o c k i n g .... E v i d e n c e for l a c k of r i g i d i t y of n o r t h e r n J u a n d e F u c a plate ......................................................

275 276 277 279 284

XVl

C o n s t r a i n t s on t h e t i m i n g of i n t r a p l a t e d e f o r m a t i o n in t h e J u a n de F u c a p l a t e ......................................... G e n e t i c a s p e c t s of t h e g e o l o g y of w e s t e r n N o r t h A m e r i c a continental margin ......................................... C o n t i n u i t y of c o n t i n e n t a l - c r u s t s t r u c t u r e s on t h e submerged continental margin ............................ Regional seismicity and questions about the subduction megathrust .............................................. A b s e n c e of m e g a t h r u s t e a r t h q u a k e s ....................... S e g m e n t a t i o n of v o l c a n i c c h a i n s in w e s t e r n Cordillera .............................................. D r i l l i n g t e s t s of g e o p h y s i c a l m o d e l s of c o n t i n e n t a l - m a r g i n structure ............................................... T h e p r o s a n d c o n s of d y i n g s u b d u c t i o n ...................... S l o w c h a n g e s in t e c t o n i c r e g i m e at t h e c o n t i n e n t a l m a r g i n .. I m p o r t a n c e of g e o l o g i c a l p a r a d i g m as a g u i d e f o r geophysical interpretation ................................. O t h e r e v i d e n c e f o r n o n - r i g i d b e h a v i o r of t h e J u a n d e F u c a p l a t e a n d g r a d u a l c h a n g e of t e c t o n i c r e g i m e a l o n g t h e continental margin ......................................... A b s e n c e of b a t h y m e t r i c t r e n c h ........................... Isometric geoid anomaly off the western North America continental margin ...................................... D e f o r m a t i o n of t h e b a s a l t i c b a s e m e n t in t h e a b y s s a l Cascadia Basin .......................................... S h e a r i n g of t h e J u a n de F u c a p l a t e in t h e t h i r d dimension ............................................... G r a d u a l c h a n g e in t e c t o n i c r e g i m e a l o n g t h e c o n t i n e n t a l m a r g i n a n d d e e p s t r u c t u r e of t h e m a r g i n r e g i o n f r o m teleseismic data ........................................

CHAPTER

i0 - C O N C L U D I N G

REFERENCES

REMARKS

............................

.................................................

288 290 290 293 295 299 301 304 305 309

317 317 317 318 319

321

322

328

LIST

OF

FIGURES

AND

TABLES

F i g u r e i. G e n e r a l z o n i n g of the C a n a d i a n C o r d i l l e r a ....... F i g u r e 2a° G e o g r a p h i c a l i n d e x m a p of s o u t h e a s t e r n Alaska and western British Columbia ..................... F i g u r e 2b. G e o g r a p h i c a l i n d e x m a p of w e s t e r n W a s h i n g t o n and Oregon .............................................. F i g u r e 2c. P r i n c i p a l g e o l o g i c f e a t u r e s in t h e w e s t e r n C o r d i l l e r a f r o m t h e K l a m a t h M o u n t a i n s to V a n c o u v e r Island .................................................. F i g u r e 3a. Conventionally assumed plate boundaries off western North America and major late Cenozoic v o l c a n i c b e l t s in t h e w e s t e r n C o r d i l l e r a ................ F i g u r e 3b. M a g n e t i c s t r i p e s in o c e a n i c r e g i o n s off western North America ................................... F i g u r e 4. conventionally assumed plate boundaries and m a j o r s e a - f l o o r f e a t u r e s off w e s t e r n C a n a d a ............. T a b l e I. G e n e r a l i z e d s t r a t i g r a p h i c c o l u m n s for t h e Q u e e n Charlotte Islands and northern Vancouver Island ......... F i g u r e 5. G e o l o g i c m a p of t h e E o c e n e M e t c h o s i n i g n e o u s m a s s i f o n t h e s o u t h e r n t i p of V a n c o u v e r I s l a n d .......... F i g u r e 6. L o c a t i o n of p r i n c i p a l L a t e C r e t a c e o u s a n d T e r t i a r y s e d i m e n t a r y b a s i n s a l o n g the w e s t e r n Canada continental margin ............................... F i g u r e 7. R e g i o n a l s k e t c h of t h e O l y m p i c - W a l l o w a s t r u c t u r a l zone (OWSZ) l o c a t i o n a n d e x t e n t .............. F i g u r e 8. S t r u c t u r a l m a p of t h e a r e a n e a r t h e s o u t h e a s t e r n e n d of t h e O W S Z ............................ F i g u r e 9. D i s t r i b u t i o n a n d o r i e n t a t i o n of f a u l t s a n d f o l d s in t h e Y a k i m a B e l t ................................ F i g u r e i0. D i s t r i b u t i o n of Q u a t e r n a r y v o l c a n o e s a l o n g the western North America continental margin ............ F i g u r e ii. G e o l o g i c p r o v i n c e s of W a s h i n g t o n a n d adjacent regions ........................................ F i g u r e 12. M a g n e t i c a n o m a l y m a p of t h e S t r a i t of J u a n d e F u c a and v i c i n i t y .................................... F i g u r e 13. B o u g u e r g r a v i t y a n o m a l y m a p of t h e S t r a i t of J u a n de F u c a a n d v i c i n i t y ............................ F i g u r e 14. L o c a t i o n of L I T H O P R O B E s e i s m i c r e f l e c t i o n profiles 84-01 to 84-04 ................................. F i g u r e 15. F a u l t m a p of n o r t h w e s t e r n O l y m p i c P e n i n s u l a and southwestern Vancouver Island ....................... F i g u r e 16. G e o l o g i c m a p of V a n c o u v e r I s l a n d a n d t h e Gulf Islands ............................................ F i g u r e 17a. D i s t r i b u t i o n of f a u l t s and C r e s c e n t F o r m a t i o n b a s a l t i c m a s s i f s in t h e s o u t h e r n s t r a n d of t h e O W S Z and elsewhere on the Olympic Peninsula .................. F i g u r e 17b. D i s t r i b u t i o n a n d a g e of C r e s c e n t F o r m a t i o n m a s s i f s in w e s t e r n W a s h i n g t o n a n d B r i t i s h C o l u m b i a ...... F i g u r e 18. G e o l o g i c m a p of t h e O l y m p i c M o u n t a i n s area, s h o w i n g f a u l t s a n d t e c t o n i c s l i c e s in t h e C e n t r a l Olympic Basin ........................................... F i g u r e 19. B o u g u e r g r a v i t y a n o m a l y m a p of n o r t h w e s t e r n Washington and adjacent regions ......................... F i g u r e 20. B o u g u e r g r a v i t y a n o m a l y m a p of n o r t h w e s t e r n Washington and adjacent regions, u p w a r d c o n t i n u e d to i00 k m .............................. F i g u r e 21. B o u g u e r g r a v i t y a n o m a l y m a p of n o r t h w e s t e r n Washington and adjacent regions, u p w a r d c o n t i n u e d t o 20 k m ...............................

I0 ii 12

13

15 16 17 39 57

60 74 77 79 84 86 88 89 91 95 96

99 i00

104 ii0

113

114

XVIII

F i g u r e 22. S t r u c t u r e of the n o r t h e r n s t r a n d of the O W S Z i m a g e d in t h e d e e p s e i s m i c r e f l e c t i o n line 8 4 - 0 2 across southern Vancouver Island ........................ 119 F i g u r e 23. S t r u c t u r e of t h e n o r t h e r n s t r a n d of t h e O W S Z i m a g e d in the d e e p s e i s m i c r e f l e c t i o n line 8 4 - 0 4 across southern Vancouver Island ........................ 120 F i g u r e 24. M e t a m o r p h i c a u r e o l e in t h e O l y m p i c M o u n t a i n s ... 125 F i g u r e 25. P o s i t i o n of b i g g e s t f a u l t s in s o u t h e r n a n d southeastern Alaska onshore and offshore ................ 134 F i g u r e 26. S t r a n d s of t h e p l a t e - b o u n d a r y f a u l t s y s t e m off s o u t h e a s t e r n A l a s k a ................................. 135 F i g u r e 27. Magnetic anomalies offshore British Columbia ... 140 F i g u r e 28a. B a t h y m e t r y of t h e w e s t e r n C a n a d a s u b m e r g e d continental margin ...................................... 144 F i g u r e 28b. B a t h y m e t r y of t h e V a n c o u v e r I s l a n d a n d Washington submerged continental margin ................. 145 F i g u r e 29. S t r u c t u r e of t h e Q u e e n C h a r l o t t e T e r r a c e a n d T r o u g h i m a g e d in an o l d s e i s m i c r e f l e c t i o n p r o f i l e ...... 149 Figure 30. Gravity anomaly map of the Queen Charlotte Islands continental margin and vicinity: B o u g u e r o n land, f r e e - a i r o f f s h o r e ...................... 150 F i g u r e 31. Crustal seismic refraction model across the Queen Charlotte Terrace ............................. 151 F i g u r e 32. G r a v i t y a n o m a l y m a p of t h e Q u e e n C h a r l o t t e I s l a n d s c o n t i n e n t a l m a r g i n and v i c i n i t y , Bouguer onshore and offshore ............................ 152 F i g u r e 33. G r a v i t y a n o m a l y m a p of t h e Q u e e n C h a r l o t t e Islands continental margin and vicinity, enhanced isostatic onshore and offshore ................. 153 F i g u r e 34. S t r u c t u r e of t h e s u b m e r g e d c o n t i n e n t a l m a r g i n o f f n o r t h e r n Q u e e n C h a r l o t t e S o u n d i m a g e d in seismic reflection line 88-03 ........................... 161 F i g u r e 35. S t r u c t u r e of t h e n o r t h e r n , c e n t r a l a n d southern Winona Basin modeled from gravity data ......... 169 F i g u r e 36. L a r g e n o r m a l o f f s e t s on the steep, w e s t - d i p p i n g S c o t t I s l a n d s f r a c t u r e zone i m a g e d in s e i s m i c r e f l e c t i o n line 88-02: (a) d a t a ................................................ 171 (b) i n t e r p r e t a t i o n ...................................... 172 F i g u r e 37. Faults and folds induced by sediment slumping a n d f l o w a g e in s o u t h e r n W i n o n a Basin, i m a g e d in s e i s m i c r e f l e c t i o n line 8 5 - 0 4 ........................... 174 F i g u r e 38. M a j o r b a t h y m e t r i c f e a t u r e s in t h e W i n o n a Basin ................................................... 176 F i g u r e 39. Seismic reflection profiles across the Winona Basin ............................................ 178 F i g u r e 40. F r e e - a i r g r a v i t y a n o m a l y m a p of t h e Brooks-Estevan embayment ................................ 181 F i g u r e 41. M a g n e t i c a n o m a l y m a p of t h e B r o o k s - E s t e v a n embayment ............................................... 182 F i g u r e 42. S t r u c t u r e of t h e s u b m e r g e d c o n t i n e n t a l m a r g i n o n t h e s o u t h e a s t e r n e n d of t h e B r o o k s - E s t e v a n e m b a y m e n t i m a g e d in s e i s m i c r e f l e c t i o n l i n e 8 9 - 0 9 ....... 184 F i g u r e 43. S e i s m i c r e f r a c t i o n m o d e l of D r e w a n d C l o w e s (1990) f o r t h e L I T H O P R O B E p r o f i l e a c r o s s V a n c o u v e r Island and the adjacent submerged continental margin .... 190 F i g u r e 44. Gravity anomaly map and models of l i t h o s p h e r i c s t r u c t u r e of V a n c o u v e r I s l a n d a n d western Washington and Oregon: (a) g r a v i t y m a p ......................................... 193

XlX

(b) g r a v i t y m o d e l s a c r o s s the c o n t i n e n t a l m a r g i n (by R i d d i h o u g h , 1979) ............................... F i g u r e 45. L o c a t i o n of L I T H O P R O B E and U.S. G e o l o g i c a l S u r v e y s e i s m i c r e f l e c t i o n and r e f r a c t i o n p r o f i l e s on and off s o u t h e r n V a n c o u v e r Island .................... F i g u r e 46. Deep s t r u c t u r e of V a n c o u v e r I s l a n d i m a g e d in the s e i s m i c r e f l e c t i o n p r o f i l e 84-01 ................. F i g u r e 47. The o r i g i n a l s e i s m i c r e f r a c t i o n m o d e l of S p e n c e et al. (1985) for the L I T H O P R O B E p r o f i l e a c r o s s V a n c o u v e r Island and the a d j a c e n t s u b m e r g e d continental margin ...................................... F i g u r e 48. An a l t e r n a t i v e s e i s m i c r e f r a c t i o n m o d e l of M e r e u (1990) for the L I T H O P R O B E p r o f i l e across V a n c o u v e r I s l a n d and the a d j a c e n t s u b m e r g e d continental margin ...................................... F i g u r e 49. F r e e - a i r g r a v i t y a n o m a l y m a p of the s u b m e r g e d c o n t i n e n t a l m a r g i n off V a n c o u v e r I s l a n d and Q u e e n C h a r l o t t e S o u n d ........................ Figure 50. M a i n b l o c k s and t h e i r b o u n d i n g faults on the V a n c o u v e r I s l a n d c o n t i n e n t a l shelf .................. F i g u r e 51. A b u r i e d i g n e o u s b o d y in the T o f i n o B a s i n off c e n t r a l V a n c o u v e r Island, imaged in s e i s m i c r e f l e c t i o n line 89-06 ................................... F i g u r e 52. S u b m e r g e d c o n t i n e n t a l m a r g i n off s o u t h e r n V a n c o u v e r Island, i m a g e d in s e i s m i c r e f l e c t i o n p r o f i l e 85-01 ........................................... Figure 53. G r a v i t y a n o m a l y m a p of s o u t h e r n and c e n t r a l V a n c o u v e r I s l a n d and a d j a c e n t s u b m e r g e d c o n t i n e n t a l margin; B o u g u e r on land, f r e e - a i r o f f s h o r e .............. F i g u r e 54. C r u s t a l s t r u c t u r e of the c e n t r a l W a s h i n g t o n c o n t i n e n t a l margin, in an E-W g r a v i t y a n d m a g n e t i c model of Finn (1990) .................................... Figure 55. V e l o c i t y s t r u c t u r e of the s o u t h e r n V a n c o u v e r I s l a n d c o n t i n e n t a l s l o p e and a d j a c e n t areas, as m o d e l e d f r o m the L I T H O P R O B E r e f r a c t i o n d a t a by W a l d r o n et al. (1990) ................................... Figure 56. C o n t i n e n t a l slope off s o u t h e r n V a n c o u v e r I s l a n d and n o r t h e r n O l y m p i c P e n i n s u l a , i m a g e d in the U.S. G e o l o g i c a l S u r v e y s e i s m i c r e f l e c t i o n line 76-19 .............................................. F i g u r e 57. S t r u c t u r e of the s u b m e r g e d c o n t i n e n t a l m a r g i n off V a n c o u v e r Island, from the shelf to the abyssal plain, i m a g e d in s e i s m i c r e f l e c t i o n line 85-02 .......... F i g u r e 58. S t r u c t u r e of the n o r t h e r n p a r t of the J u n o d e p r e s s i o n on the lower c o n t i n e n t a l s l o p e off s o u t h e r n V a n c o u v e r Island, imaged in s e i s m i c r e f l e c t i o n line 89-07 ................................... F i g u r e 59. Local and r e g i o n a l d i s t r i b u t i o n of e a r t h q u a k e e p i c e n t e r s a l o n g the B r i t i s h C o l u m b i a and s o u t h e a s t e r n Alaska continental margin ............................... F i g u r e 60. P r e s e n t - d a y rates of c o a s t a l s u b s i d e n c e and u p l i f t in w e s t e r n W a s h i n g t o n and O r e g o n .................

194

197 199

208

209

214 215 220

221

231 247

251

255

259

260

283 298

CHAPTER

i - OUTSTANDING

Basic terminology

margin

geoscience

literature.

is one of

a simple way,

of

submarine

of

(1952,

definition

(1919),

processes,

activity

landmasses

(continents)

in

and deep-ocean

terrace

to an ocean.

Sedimentologically,

and Sanders, extensions

1978).

of various margin

the

them

zones

continental

This limitation

margins

viewed

transition

term.

whether

to be products as

results

between

a continental

belongs

m a r g i n or

to a continent as

masses or parts of ocean basins

of continental

of

continental

they may be regarded

In tectonic terms,

But

pelagic plains.

(shelf plus slope)

continental

rim

rises.

continental

Dietz

a continental

of

modern

this term to a set

who had originated

It is only a matter of perspective

margins

in

of continental

that

slopes and deep-water

tectonic

terms

restricted

features

Johnson had considered

accumulative

common

despite the existence

1964)

geomorphological

had begun with Johnson

MARGINS

and this term is used loosely.

Dietz

shelves,

OF CONTINENTAL

of continents

most

no comprehensive

In

whereas

the

However,

has yet been formalized,

landmasses:

IN S T U D I E S

related to margins

Continental

classifications,

ISSUES

they

commonly

lithosphere from interiors

or

either

(Friedman represent

of continents

to their periphery.

In the conventional inboard,

upper boundary

lower boundary or,

where

sometimes

nomenclature,

of the continental

of the terrace

present,

the shoreline

is regarded

terrace.

The outboard,

is the foot of the continental

the trench.

also used to describe

Confusingly,

as the

slope

the term terrace

any step-like bathymetric

is

feature.

Continental

shelves

continuity

with

less than 200 m slopes,

are

coastal water

which

shallow

lie

lowlands

depth

outboard,

steep

marginal

movements position

"passive"

inclined

is

and

used

though,

eustatic

to

over long periods

inflexibly.

Atlantic margin.

continental back-arc

plate.

domains

comprehensive back-arc

of

at mid-

submerged

two

changes for

make

by

demarcation

are applied

Where a magmatic

(1973),

an

in front of a continental

lithospheric

yet exists

margin of

as opposed to the

Dickinson

an

Where a magmatic

the

of geologic time.

these features

oceanic

and emergent

classifications

As defined

with

also regarded together.

as "active",

can be d i s t i n g u i s h e d

domains.

separated

synonymous

arc onshore;

demarcation

3°-10 ° but in

The Pacific continental

is usually c l a s s i f i e d

subduction

Continental

of

and thus unreliable

may arise if generalized

and a magmatic

lie at

composed

as

sea-level

active margin normally has a deep trench slope,

are

lower part,

are sometimes

ephemeral,

to local situations North America

are usually

Confusingly,

of features that evolved

Complications

deeper.

margin

parts of continents

Tectonic shoreline

terrace.

direct

They usually

be

Most slopes

in

break.

Often the term continental continental

may

upper part and gentle

slope by a morphological

plateaus

onshore.

but

some areas much more steeply. parts:

submarine

are created plate

arc is present,

due

beneath

fore-arc

a and

in relation to it, though no for the inboard arc is absent,

boundary

of

these domains

lose their distinction.

In the absence of

terminological

consensus,

the

geologic

term

"continental margin" is used herein to include,

in addition to the

submerged areas outboard, onshore areas at least as far as the end of

coastal

plains.

The

uplands

that

begin there are usually

created by cratonic or orogenic processes unrelated to present-day interactions

of

continental

and

oceanic

plates.

However,

active subduction settings where magmatic arcs have been zones

of

continent-ocean

transition

Where practical,

erected,

may be extended inland far

beyond coastal plains, to include the arcs and even regions.

in

the

back-arc

in this volume specific qualifiers are

added for clarity, such as "submerged continental margin".

Definition of crustal type at continental margins Continents are characterized by a specific type of lithosphere and crust,

distinguished

composition,

layering,

geophysical

and

differences

between

composition

from

oceanic lithosphere and crust by rock

structure

and

thickness.

Geological,

geochemical surveys the world over have revealed continental

and

oceanic

crust

in

rock

(sialic vs. simatic), age (Early Archean to Recent vs.

middle Mesozoic to Recent),

and styles of

structural

deformation

and reworking°

Variable types of magmatism occur in regions of continental crust. Felsic magmatism, which is completely absent in oceanic crust, diagnostically

continental.

A broad range of metamorphic grades,

from as low as zeolite to as high as granulite, continental altered.

and

more

oceanic

is found

crust, by contrast,

only

in

is only slightly

A wide diversity in styles of deformation is typical for

continental regimes.

crust;

is

regions, varying widely between cratonic and orogenic

Oceanic crust, by all structural parameters, uniform.

It

is

typically

characterized

is by

simpler linear

magnetic

anomalies,

Continental

crust

crystalline velocity

of

and

generic descriptions

common

in

rock

permit

unequivocal

margins,

continental

the

ocean

Rosendahl

at

(Couch

geophysical and

crust,

transitions properties

typically

oceanic

crust.

North American

Historical

more

kg/m3

and

are

velocity

structure

1989).

and thin,

blocks

averages

of

sometimes

Bathymetric

by itself may not

crustal

depths

Elsewhere,

type.

At

continues

(Grant,

some

towards

1980,

1987;

of oceanic-type

crust

slope and even shelf

(Finn,

1990).

occupy broad zones where crust has between typically

The conventional

between

Blocks

1989;

which

margins

of

water

modified

term

continental

"transtional

continental

and

crust"

modified

of both types make up large parts of the

Pacific margin.

outline of perspectives

Pacific coastal

uncertain

attenuated

may

these

and Riddihough,

intermediate

oceanic.

fails to distinguish

or

determination

may lie under the continental

Crustal-type

of

from a multitude

zones,

parts of continental

1992).

of 2,900-3,000

from

transition

considerable

et al.,

eds.,

crust is generally

averages

deviations

continent-ocean

zoning of submarine

density

and seismic P-wave

and Mooney,

Oceanic

are global

local

properties

Pakiser

regions.

6.5 km/s.

contain crust with intermediate and

with

kg/m3

with densities

exceeding

Large

2,900

1995).

5 to i0 km thick,

observations.

to

(e.g.,

Mooney,

in continental-crust

20 to 45 km thick,

2,700

of 6 to 7.5 km/s

P-wave velocities

Such

is usually

rocks

Christiansen uniform,

which are absent

on Cordilleran

areas of the North American

geology

continent

are

a

part

of

the

1991;

Cordilleran

Burchfiel

orogenic

et al.,

been

zones

described

(Fig.

I).

with

series Belt,

of

(2) Omineca Belt,

Belt,

once regarded

uplands

the

five physiographic they are:

a fold-and-thrust

links to the Archean

Intermontane

and

most

rugged

of

metamorphic

rocks;

the

Insular

islands

of southeastern

orogenic (e.g.,

system

was

Douglas,

demarcated

Vancouver

1970).

clear

field

in

began with

the northeastern

linear and stripe-like,

remain parallel

within

abruptly

across

mainland

Islands,

and

Cordilleran synopses

boundary

was

part of the Insular Belt

discovery

that

the

magnetic

Pacific Ocean has a regular character, different

anomalies

broad

and reverse

from

thousands

domains,

well-defined

These magnetic

records of normal

a

distinctly

Linear magnetic

1961).

outboard

80%

1969).

in perception

Mason,

the

to

as well as

the

Queen Charlotte

at that time for the offshore

Change

change

off

a

(4) Coast

up

diorites)

Belt

(3)

containing

in the early geological No

from

craton;

with

Thus subdivided,

described

ed.~

(see also King,

continent.

Island,

Alaska.

extending

exhumed Precambrian

all,

leucocratic

including

Mountains

subdued topography;

and

coast,

belt)

as a median massif,

(granodiorites (5)

and geologic

to Early Proterozoic

granitoids

and

eds.,

Cordillera

(i) Rocky

containing

with relatively

tallest

and Yorath,

much of the Canadian

as comprising

literature,

Mexico to Alaska; rocks

surveys,

From east to west,

(in the current

(Gabrielse

1992).

Since the first regional has

system

whereas

that

on

the

of kilometers

long

stripe

patterns

domain boundaries

(Raff and

lineations

were

soon

explained

as

polarity of the changing geomagnetic

field at the time of cooling of ocean-floor

basalts

erupted at and

moving

away

from

spreading

centers

(Vine and Matthews,

Considered to be symmetrical relative to these

magnetic

lineations

came

to

the

spreading

1963).

centers,

be interpreted as isochrons

which permit restoration of the history of sea-floor spreading and plate

motions

over

Vine and Wilson,

hundreds of millions of years (Wilson, 1965;

1965).

Early reconstructions Pacific

Ocean

of

plate

movements

(Atwater,

1970)

were

in

northeastern

pivotal to the revision of

Cordilleran geology (e.g., Price and Douglas, idea

the

eds.,

1972).

The

that offshore plate interactions influenced the continental-

margin geology has been accepted broadly and applied productively. At

the

extreme,

however,

plate

movements

have sometimes been

considered the main factor in the genesis of continental orogens.

This ocean-based, Poseidonian perspective on came

to

dominance

continental

geology

during the 1980s (see Burchfiel et al., eds.,

1992).

Now the development of the Cordillera is sometimes treated

simply

as

a

passive

(Monger, 1993). Cordilleran

result

of

plate

motions

This approach has the advantage

geology

in the Pacific of

putting

the

into a global plate-tectonic context, but it

risks ignoring self-development of continental lithosphere.

Off Vancouver Island, all rocks on the shelf and slope included

by

some

workers

into

a

complex

of

sedimentary

Dehler

and

and

volcanic

Clowes,

(Duncan,

1982).

1992).

rocks presumably

evolved as a result of accretion caused by subduction plates

been

a Tertiary accretionary complex

(Yorath, 1980; Hyndman et al., 1990; Such

have

of

oceanic

On the Washington and Oregon continental

margin, this complex was presumed to be

very

wide

and

even

to

extend

into

coastal

thought to underlie

areas onshore.

the Olympic

Crust of oceanic origin was

Peninsula

Fig.

2;

Figs.

17 and 18 and in the corresponding continental

50; the reader

diagrams

before

chapter;

margin off Vancouver

is encouraged

reading

the

following

Recent geologic

summaries

point to continental

western

North

American

coastal

1992).

New

geologic

lithosphere

origin

field

of

regions

evidence

and Oregon

(Babcock

et al.,

considered

the Insular Belt and,

et al.,

Shortcomings Disputes

presumably,

the

a

This model-based reconstructions, investigations in

remote

observations basis

of

most

et al.,

eds.,

oceanic-

areas in western Other workers

(von

Huene,

crust 1989;

side

restricted approach at

the

effect

of

geophysical

relies

mostly

expense

in areas of interest oceanic

the

on

obtained by m a p p i n g

on land,

global-scale

more-local

onshore. rather

tectonic

models

data sets far offshore.

of

regions,

regions.

evolution

nature of the crust in western North America

for reconstructions

continental

acquaint

the continental

slope

other

1991).

an undesirable

from

to

an

1994).

of current models of Cordilleran

about

illustrate

(Burchfiel

1992,

and

affinities

precludes

to extend to the foot of the continental Gabrielse

the fault map of

these

the crust in basalt-rich

Washington

are shown in

chapters,

with the region).

1977;

Island is presented

to examine

himself/herself

deduced

al.,

details of geology of the Olympic Peninsula

the submerged in Fig.

(MacLeod et

than

Magnetic local

plate

geological anomalies geological

all too often serve as a

of the evolutionary

history

of marginal

A casualty has been studies,

which

outcrops

and

cornerstone

the

relied

old

method

principally

drillholes. of

of

on factual

This

geoscience

into

only a few parameters,

integrated

shortcomings. everywhere be

model-based First,

conclusive.

substantial

Stock and Molnar, America,

oceanic

and deformed plate.

Magnetic

reconstructions, crust

in

the

Errors

1989)

stripes,

A

critical

assumption

in

Carbotte

et al.,

these

and

Off

as a

are not

are

known

et al.,

western

is apparently

1985; North

too fragmented

rigid,

coherent

for plate-motion

in deformed oceanic

parts of the Juan de Fuca plate

Couch and Riddihough,

reconstructing

1989; Allan et al.,

crust

is the

1989;

Davis

is that

cannot

lithosphere

be

(e.g.,

1993).

that

of the North America

been

motion

tectonics

oceanic

supposition

interactions have

plate

of rigid-plate

regions of deformed

plate

studies

in

Principles

The second shortcoming

tectonic

important

motions

which serve as a basis

1989;

for

1993).

applied

passively to

1990)o

to be regarded

Gorda and Explorer

plates are rigid.

lithosphere

two

(Engebretson

are strongly curved or broken

(Atwater and Severinghaus, and Currie,

in reconstructed

DeMets et al.,

take

observations.

the existing plate reconstructions

crust in many places

(Atwater,

can

it is no substitute

approach has at least

even for big plates

1988;

by the much

because modeling

studies based above all on factual

The conventional

to

However,

from

methodological

has partly been displaced

approach.

geological

observations

traditional

simpler model-based consideration

continental

through

the

continental

plate only responded

time.

reduced to accounting

As

a

result,

of presumably

arbitrary events: terranes docking, rock

deformation

induced

by

stresses transmitted from far away.

Continental crust is rich in radioactive elements and thus has its own sources

for

self-development.

manifestations,

Where

continental tectonism,

continental regions, cannot plate motions.

always

studied

in

be

correlated

with

modeled

Rapid vertical movements that occurred in the Late

Washington

North

Coast Mountains to

movements

including

Cascade Mountains and the British Columbia

(Figs. i, 2; also Fig. II) are not simply of

the

Juan

related

de Fuca plate (Muller et al., 1992):

geobarometry studies show that rocks of surface origin were buried

rapidly

to

depths

as

exhumed (Brown et al., 1994). Olympic

Peninsula

much

Peninsula

first

as 30 km, then uplifted and

Tertiary felsic

magmatism

on

the

is also puzzling if the crust in that area has

an oceanic origin (Snavely, 1987). Olympic

its

including that in marginal

Cretaceous and early Tertiary in the western Cordillera, the

all

crust

This puzzle is resolved if the

has continental affinities,

as does the

crust farther north, where felsic magmatism was widespread.

Horst-and-graben depressions

in

tectonics

and

development

the Late Cretaceous

was

not

sections

for

geologic

correlative with the plate convergence usually

modeled for that time (Pacht, 1984). was

fault-bounded

(e.g., the Nanaimo Basin; see

the upcoming Fig. 6 and the corresponding details)

of

The

Queen

Charlotte

Basin

presumed in Poseidonian models to have been stretched greatly

in the Tertiary (Yorath and Hyndman, 1993),

but

large

inconsistent (Thompson

et

with

extension

in

geological

al., 1991; Lyatsky,

1983; Hyndman

and

Hamilton,

that area was later shown to be and

geophysical

1993a).

observations

The unusual pattern of

10

(a)

(b) /

•

Granitic rock

~----~ Greenschist facies Figure i. General morphogeologic belts; J.W.H° Monger, 1992).

•

Amphibolite facies

[-~

81ueschist facies

zoning of the Canadian Cordillera: (a) (b) simplified metamorphic map (courtesy

|, / ,

.

rm- "

C,

......

Figure 2a. Geographical index map western British Columbia (from W.H, / Geological Survey of Canada Map 1701A).

I

~f s o u t h e a s t e r n Alaska and ~athews, compiler, 1986,

%

12

\\

IOFINO

~'~..,

BASIN ~

"-,.., \

"~L,'ANO

~C" K~'""..,/" .....

,,

...........

VANCOUVER

"'..!........ :.

\ 48"

\'~

OLYMPIC """ k, ..........'}A BASINC_,~'~ , .,......~ .: ~ I,

(

z iI

/

r"

i $

'---b,

~g;. 4... coos BAY i

OREGON

/~YIU ,

L

i

i

i lo~)KILOMETERS (/

KLAMATH

MOUNTAINS

i

Figure 2b. Geographical index map of western Washington and Oregon, with locations of offshore wells and old seismic reflection p r o f i l e s s o m e of w h i c h a r e d i s c u s s e d in t e x t ( m o d i f i e d f r o m S n a v e l y , 1987). The small islands between the southern Vancouver Island and the mainland are called the Gulf Islands on t h e C a n a d i a n s i d e of t h e b o r d e r , a n d t h e S a n J u a n I s l a n d s on the U.S. side.

13

~

\k ..\.

+

O

\ O f

O O ,I

r l"-t

i

D ...........

Z

\ o

i;o

t o o KM

8¢eh~

i

~2t*

Figure 2c. Principal geologic features in the western Cordillera from the Klamath Mountains to Vancouver Island (modified from Snavely, 1987). Details of the geology of specific areas are presented in the subsequent chapters and figures.

14

seismicity

onshore

continental

margin

explanation

Structure

and offshore still

awaits

(e.g., Acharya,

of w e s t e r n

along the western a

North

compelling,

American

comprehensive

1992).

North

America

plate

boundary

in

current

models At

present,

the

North

American

continent

western margin with two main oceanic the

much

smaller

Juan

de Fuca plate

plate is the largest in the world, most of the Pacific Ocean. past the North American their shared boundary: Fairweather-Queen In

the

1989;

north,

von Huene,

continent

subducting

is

a

Farallon

during the Tertiary, They

were

floors

segments

fault in California

beneath A l a s k a

between

remnant

together with the Kula plate The

crust

of

and the Alaska.

(Atwater,

1970,

1989).

Columbia,

region.

oceanic

The Pacific

fault system off southeastern

The Juan de Fuca oceanic plate, British

3, 4).

at two NNW-trending

the San Andreas

is

the Pacific plate and

(Figs.

and its

along its

The Pacific plate is sliding d e x t r a l l y

Charlotte it

plates:

interacts

once

of

the

dominated

by

plate

the Pacific

California

Farallon plate,

plate was fragmented

and the Kula

replaced

northern

the

Pacific

Ocean

completely.

Fragmentation

plate,

which began at around

50

present

(also

Stock and Lee,

The number of small oceanic

blocks or microplates

From detailed the

Juan

de

continues

studies, Fuca

(Johnson and Holmes,

Pacific

1989).

continuing

at

to grow.

sea-floor

and

is

of the

Farallon

1994).

Ma,

which

and mostly subducted

disappeared

plate.

and

spreading plates

Subduction

is

occurring

between

at the Juan de Fuca Ridge of

the

Juan

de

Fuca

15

/~/%

//

I KODIAK

o"

;~0~ V'~,:

/

/ INLET

/

(

/

f//

)

\

WILLIAM " ~ SOUND

/

~¢30

! /

~;$"• YAKUTAT BA

GULF

CROSS SOUN~

-I_FAULT

OF ALASKA

"-..,

STIKINE r

8 ~ ~f~~

, • VOLCAN|C

mou,Een J. TUZO WILSON KNOLLS DELLWOOD KNOLLS ,~t.~ EXPLORER Pt EXPLORER R I D G E .

/ //

AMERICA PLATE

"

~_

" .] ~ , ~ '

BELT ALEXANDER /)~ARCHIPELAGO, I'\ •

/

/ /

CHARLOTTE rlSLANDS

/

]

•VOLCANIC BELT t~ ?•

PACIFIC PLATE

50 ~

CO|,UMBIA \ PLATEAU ' I

~J

I

/ I

/ I

o

# /

t 7GSC

Figure 3a. Conventionally assumed plate boundaries off western North America and major late Cenozoic volcanic belts in the western Cordillera (after Riddihough and Hyndman, 1989, 1991). Distribution of Quaternary volcanoes in coastal areas is shown in more detail in Fig. i0.

";6

~0 °

45 °

135 °

130 °

1250

40 °

Figure 3b. Magnetic stripes in oceanic regions off western North America (after Raff and Mason, 1961). Shading marks positive anomalies. Chaotic anomalies mark the northern (Explorer; Fig. 3a) and southern (Gorda) ends of the oceanic Juan de Fuca plate, reflecting intraplate deformation in these areas.

17

Figure 4. Conventionally assumed plate boundaries and major seafloor features off western Canada (bathymetry in meters; modified from Riddihough and Hyndman, 1989).

18

oceanic

plate

under the North American

at the Cascadia

Complications it

is

subduction

southern

Columbia

(Gorda)

zone.

internal

- off northern California

in the

north

continent

(Riddihough,

deformation

is apparently

is the geodynamics

the

plate

proposed plate

de

Fuca

previously

exists

in

off Vancouver

area,

Pacific plate was p o s t u l a t e d (Hyndman et al.,

The

middle

without

of

the

Plate-tectonic

declined

Juan

de

beneath the continent,

thrust seismicity

1992).

lie

has increased

(or

its

Explorer

northern V a n c o u v e r models,

(Fig.

small

Explorer

off

to

direction,

be

moving

part

4).

It was oceanic

Charlotte

but

plate in

is

an

Sound

still being

unusual

trench

manner,

(e.g., Acharya,

with

1992,

the

America

perhaps with a very small component

has

and the obliquity 1994).

of northern Juan

The Pacific plate North

of

1979).

(Babcock et al.,

past

in

1989).

suggest the rate of convergence

fragment)

Island.

entirely

boundary with the

Queen

Fuca

The least resolved are the interactions plate

northern

during the last several million years,

of convergence

with

Island

or a bathymetric

models

convergence

and subduction

northern

1979; Keen and Hyndman,

part

underthrusted

whose

to

the

accommodated

of the

that an independent, that

In

(Couch and Riddihough,

Less well understood Juan

where

1984).

of the oceanic plate,

that area is thought to have stopped

plate,

in the south and

part of the Juan de Fuca plate,

the North American by

is taking place

occur at the ends of the Juan de Fuca

most fragmented

off British

continent

de

Fuca

Pacific plate off

is believed, dextrally

in

all

in a NNW

of convergence

off

19

the

Queen

Charlotte

DeMets et al., 1990). simpler:

no

Islands

(e.g.,

Minster

The situation off

convergence

has

and Jordan,

southeastern

1978;

Alaska

is

been inferred there, and the North

America and Pacific plates are assumed to be separated by a rightlateral transform boundary.

In reality, a complex fault system in

a broad structural zone has been found along the plate boundary in that area (von Huene, 1989).

The

logic

of rigid-plate tectonics requires a ridge-trench-fault

triple junction between the three plates, which has off

Queen

Charlotte

Sound.

Sea-floor

modeled

spreading

between

the

Explorer

fragment)

Pacific and Juan de Fuca plates

(or

supposedly

two parallel ridges oriented at a

taking

place

from

the

been

right angle to the continental margin off

Queen

Charlotte

is

Sound

(Riddihough et al., 1980).

During the Cenozoic, periods of transtension were proposed to have caused rifting and large stretching of the

continental

crust

in

the Insular Belt, resulting in the creation of the Queen Charlotte Basin.

By contrast, uplift of the

Queen

Charlotte

Islands

ascribed to late Cenozoic transpression (Yorath and Hyndman, Hyndman and Hamilton, Vancouver

Island

1993).

margin

Deep have

seismic been

profiles

interpreted

was 1983;

across in

terms

the of

continentward-dipping thrust slices that presumably developed as a result

of

Cenozoic

subduction

(Yorath,

1980;

Yorath

et al.,

1985a,b; Clowes et al., 1987; Hyndman et al., 1990).

Major pitfalls occur models,

which

are

in

uncritical

application

of

theoretical

based on generalized assumptions, to specific

local geologic situations.

Large uncertainties still bedevil

the

20

available and

plate

Molnar,

1988;

interactions

Geologic

mapping

thrust belts

mainland

(Brandon

island itself faults

DeMets

et

(Engebretson al.,

shows

that

et

al.,

1988; by

1991;

1976; Muller,

correlated Queen

Lewis

This suggests of tectonism entire

et

between the

Charlotte

Late

and

al.,

1991a,b).

Vancouver

reactivation

data

to

bounding

crustal

be blocks

(Brew et al.,

Subsequent

chapters will show that two

structural

zones meet off V a n c o u v e r

margin off southeastern

Islands.

The O l y m p i c - W a l l o w a

interior

in eastern Washington

de Fuca.

This structural

continental

crust,

with the adjacent

of

steep

1981),

can

mainland,

network

1991;

be the

main

mode of the

magnetic

and

of steep faults

Lyatsky,

prominent,

Island.

and

shelf.

Structure

gravity,

1993a).

inter-regional

The Fairweather-Queen interior

along

the

Alaska and the Queen Charlotte

zone continues

from the

Cordilleran

and Oregon into the Strait of Juan

configuration

the existing tectonic models. in the Cordilleran

The

and the interior

fault system runs from the Alaskan

continental

related

a

1991).

networks

Columbia

from by

the

(Thompson et

of old steep faults was the

controlled

and

pattern

is similar

islands,

plate

and early

Island

in this region during the Tertiary.

seismic

in

Cretaceous

Fault

British

Insular Belt was interpreted

Charlotte

inferred

Muller et al.,

Islands

Stock

means.

regular

1977a-c;

western

1985;

England and Calon, a

the geology of the Queen Charlotte al.,

so

lie only between Vancouver

is characterized

(Jeletzky,

et al.,

1990),

need to be tested by independent

field

Tertiary

reconstructions

is not taken into

Still,

these two fault systems,

interior to zones of

also control

oceanic plates.

account

weakness

in

the

large parts of the plate boundary

CHAPTER

2 - EVALUATION

OF

THE

DATA

BASE

Direct geological observations - the main source of information Only

geological

rocks, their yields

observation can provide direct information about

properties

the

most

and

field

inferences

geophysical

Observation

reliable controls on any models, qualitative or

numerical, used to predict unknown Because

relationships.

data

parameters

from

known

ones.

about structure and composition of rocks from are

non-unique,

geological

observations

are

irreplaceable as a controlling tool.

This

study

of

the

western

North

American

continental margin

benefited from combining geological observations with data

onshore

and

offshore.

Constrained

by

geophysical

geophysical data,

geologic relationships observed on land by mapping were into

submerged

parts

of

the

continental

geological information, obtained by areas

and

by

offshore

America.

margin.

A wealth of

mapping

in

coastal

well drilling and sea-floor dredging,

available along the Pacific North

outcrop

projected

continental

margin

of

is

northwestern

Onshore geology provided the primary constraints

on geophysical interpretations and plate-tectonic models.

Geologic mapping has been carried out in many parts of Oregon Washington,

and

of the region. reports help

ongoing programs offer a new look on the geology Though results are not yet summarized

the

regional

and

local

geologic

geological information is provided b y w e l l s

hydrocarbon exploration previous

everywhere,

of previous and recent surveys and of industrial drilling

elucidate

Offshore,

and

Deep

Sea

and

research,

including

structure. drilled for

those

of

the

Drilling Program (DSDP) and the ongoing ocean

Drilling Program (ODP).

22

Reconnaissance Charlotte

mapping was carried

islands

out

on

Vancouver

in the 1960s and 1970s.

Samples

and

Queen

of sedimentary

and igneous rocks on the ocean floor were obtained by dredging the

shelf,

slope

and

wells were drilled

abyssal

plain.

Fourteen deep exploration

in the 1960s on the interior

of western British Columbia. by the DSDP and the ODP.

In bathyal

Important

on

areas,

new

and exterior

shelf

wells were drilled

drilling

results

were

provided by ODP Leg 146.

Detailed Island. Charlotte resulted published

In

geologic

reports

are

available

A program of detailed mapping Islands, in many

in

conjunction

reports

by

the

for parts of Vancouver

of large parts of the Queen

with geophysical Geological

surveys,

Survey

of

has

Canada

in the late 1980s and early 1990s.

southeastern

Alaska,

has led to revision

of

evolution

of that area.

scarce.

Still,

comprehensive

ongoing detailed

the

a

number

However, new

of the entire continental

Physical parameters potential-field

earlier

offshore

data

understanding

of

have

geologic mapping onshore ideas

geologic

already

about

the

information

produced

of geology of southeastern

a

is

more

Alaska and

margin.

of rocks ~ constraints

on

interpretation

of

data

Rock magnetization Magnetization

is the rock property

to its geologic magnetic mineral

source

(Reynolds

that relates

et

in the study region

al.,

a magnetic

1990).

The

anomaly

principal

is evidently magnetite

(Coles

23

and Currie, 1977; Arkani-Hamed and Strangway,

1988).

It is mostly

associated with igneous rocks, whose distribution largely controls the magnetic anomaly pattern.

Magnetic susceptibilities of volcanic rocks been reported by Currie and Muller Clowes

(1992).

the

region

rarely

more

have

(1976), Finn (1990), Dehler and

Paleozoic volcanics in the Insular Belt

susceptibility, units).

in

than

100xl0-6

emu

have

low

(i,250xi0-6 SI

Triassic and Jurassic volcanics, with values between only

40xi0-6 and 2,000xi0-6 emu (500xi0-6 to 25,000xi0-6 SI units), are variously magnetic. (>12,500xi0-6

SI

Usually units),

highly

magnetic,

>l,000x10-6

emu

are Eocene basalts on Vancouver Island

and in Washington, which are commonly marked by strong anomalies.

Most of the exposed granitoid plutons in the region are marked prominent positive and negative magnetic anomalies.

by

This makes it

possible to locate, by analogy with such anomalies, plutons hidden under

roof

rocks or sea water (Arkani-Hamed and Strangway,

Finn, 1990; Lyatsky,

1991a).

such

the

as

those

Island, are others,

such

of

associated as

the

Many high-grade

Jurassic with Leech

metamorphic

1988; rocks,

Westcoast complex on Vancouver

positive

magnetic

anomalies,

but

River complex on southern Vancouver

Island, are consistently associated with negative anomalies.

Interpretation is complicated because magnetization of

rocks

may

be induced or remanent, normal or reverse, and sometimes different magnetization vectors from the same source body interfere. result,

As

a

causative bodies produce a variety of anomaly forms which

may be difficult to interpret in detail.

Alignment

of

magnetic

anomalies and presence of steep linear gradient zones may indicate

24

faults.

In oceanic

regions,

magnetic-anomaly

presence

of blocks of oceanic crust.

lineations

indicate

Rock density Information

about

publications

(Stacey,

1975;

al.,

Anderson

and

1977;

Seemann,

1991;

information Olympic

rock

Dehler,

is well

Peninsula

densities

was

Currie and Muller, Greig,

1991).

logs.

obtained

1989;

Another

and southeastern

1976;

Finn,

northern

Alaska are produced by

density

i.

densities

(e.g.,

2.

Tertiary

Volcanic Charlotte Jurassic

Jurassic

are denser

clastic

have

a

rocks of Tertiary Islands

have

volcanics

on

a

Vancouver

about

higher

2,700

density

of

and Middle Jurassic

age

density

Vancouver

characterize

Island.

of

2,650

Island

are

Upper Triassic

light,

~2,640

kg/m3.

Upper

~2,760

kg/m3.

on

the

kg/m3,

Queen

but Lower

usually

heavier

between

2,200 and 2,950 kg/m3, Island.

and grade of metamorphism,

2,730 to 2,900 kg/m3o

Karmutsen basalts:

Islands and around 2,950

Eocene basalts

kg/m3 on southern Vancouver lithology

kg/m3).

kg/m3).

High densities

densities

shelf

In the core of the Olympic

are relatively

are

2,880 kg/m3 on the Queen Charlotte on

continental

(2,400-2,600

rocks

clastics

limestones

(2,700-2,800 3.

sediments

but Cretaceous

Triassic

1993a).

increasing with depth)

on the interior Basin).

(Lyatsky,

onshore and in the Hoh and Tofino Basins on the exterior

Lower

kg/m3,

sediments

in the Queen Charlotte

Mountains shelf,

(1,800 to 2,500 kg/m3,

Neogene

Sweeney and

between

between rocks of three main categories

characterize

et

source of density

anomalies

contrasts

Low

numerous

MacLeod

1990;

important

Most gravity

from

in western Washington and a

Paleozoic

density rocks,

of

kg/m3 have 2,950

depending

have variable densities

on

from

25

Plutonic

massifs

in

the

Insular and Coast belts generally have

densities between 2,600 kg/m3 for diorite.

Depending

granite

interpereted

2,820

from

magnetic

anomalies

for

maps.

are

more

Confusingly,

some

plutons cause negative gravity anomalies

similar

Tertiary

Prominent

sediment-filled

depressions.

along the entire length of the western North margin

kg/m3

on their country rocks, many plutons in this

region are not marked by strong gravity readily

and

and

to

those

over

gravity

lows

America

continental

are associated with Tertiary sedimentary basins (couch and

Riddihough,

1989).

Yet,

plutonic

and

metamorphic

rocks

of

continental-crust crystalline basement may also contribute to some of those pronouced anomalies.

Processing of potential-field data Fundamental notions Magnetic and gravity data may reveal composition given

different

and structure of the region.

locality

magnetic-field

is

the

difference

aspects

of

rock

Magnetic anomaly at any between

the

recorded

intensity and the theoretical one predicted by the

International Geomagnetic Reference Field.

Magnetic maps

reflect

rock properties no deeper than the Curie isotherm, whereas gravity maps represent density contrasts at both shallow and

deep

levels

in the lithosphere.

Gravity

anomaly

is

the

field and a field computed

difference between the measured gravity for

a

given

location

from

theory,

assuming an idealized rotating, spheroidal Earth (Goodacre et al., 1987a).

Density

contrasts

which

cause

gravity

anomalies

are

26

located crust;

at various

(2) in the crystalline

sedimentary affected

supracrustal

also

elevation

by

and

Magnetic

Geomagnetic (i.e.,

as

Desirable

well

1988)

1977;

are

Currie

gravity

as

by

volcano-

values

are

recording-site

for geological

Reference

et

now available

and Washington

magnetic

station

km on average.

data

al.,

interptetation at

1983).

crustal

acquired by High-quality

in British Columbia

(Finn,

1990).

were corrected

Field and r e s a m p l e d

areas

spacing varies

(Currie

For the purpose of

for the International

at

a

812.8-m

interval

are

covered

Finn et

1991).

data

were

British Columbia

is available

unevenly

1983;

These

across the region and is about i0

The best coverage

(Currie et al.,

al.,

and 1991;

Sweeney

gridded at an optimal

and a 5-km interval

Dependence

of gravity values on the distance

reduction (Goodacre

hence

on elevation,

(Goodacre et al.,

of the rock mass,

et

1987c)

al., onshore

whereas

in less detail and

Seemann,

2-km interval

in

in Washington.

of gravity data

Earth,

offshore,

generally

Reductions

the

the

two samples per mile).

Gravity

land

Measured

in

in the region were based on profiles

data

study,

(3)

coverage

(MacLeod et al.f

this

and

levels.

Old magnetic maps

and Teskey,

(i) below the base of the

which reflect the Earth structure

and aravity

aeromagnetic

cover.

latitude.

and supracrustal

crust;

topography,

are those anomalies

ship

levels in the Earth:

from

is accounted

1987b). takes

The

the

center

of

for by the free-air Bouguer

reduction

into account the attraction

assumed to be a horizontal

slab,

density

2,670

27

kg/m3, between the recording station and the sea level. the Bouguer reduction 1,030

kg/m3)

kg/m3).

involves

"replacing"

sea

Offshore,

water

(density

with an equivalent thickness of rock (density 2,670

Thus, anomalies in a Bouguer map mainly

reflect

crustal

structure and variations in Moho depth.

Gravitational

effects

of

variations in crustal thickness may be

partly attenuated by the isostatic reduction. isostasy,

assumed

The Airy

model

of

typically for the Earth's crust, requires that

areas of positive topography,

if in equilibrium, be

underlain

by

crust of increased thickness; areas of negative topography must be underlain by

abnormally

thin

crust.

The

isostatic

reduction

accounts for the gravitational attraction of these assumed crustal roots and antiroots, and an isostatic map represents crust-sourced anomalies

better

than

does a Bouguer map (Simpson et al., 1986;

Goodacre et al., 1987d; Simpson and Jachens,

However, those

isostatic maps may still

sourced

contain

1989).

anomalies

other

than

by intracrustal or supracrustal density contrasts.

These components of the gravity field may be

related

to

crustal

flexure, local variations in mantle density and heat balance, etc. Their wavelengths usually exceed those interest.

Many

such

This

algorithm

geologic

features

of

anomalies are correlative with topography,

and they can be attenuated by (Sobczak and Halpenny,

of

the

enhanced

isostatic

reduction

1990).

employs

a

least-squares

procedure

to linearly

correlate conventional isostatic gravity values in a map area with topography offshore.

onshore

and

imaginary

rock-equivalent

topography

The latter is computed by "replacing" the mass

of

sea

28

water

(density

1,030

kg/m3)

with

an

equivalent

(density 2,670 kg/m3) and adding the thickness rock

layer

to

the

existing bathymetry.

relationship is used produce map

an

are,

to

enhanced in

correct

the

of

caused

the

isostatic

largely

simulated

The topography-anomaly

isostatic anomaly map.

theory,

mass of rock

data

and

thus

Anomalies in such a

by

intracrustal

and

supracrustal sources, with other influences minimized.

Interpretation caution.

of maps resulting from gravity reductions requires

These

procedures

rely

on

specific

assumptions,

for

example, that: the geoid is represented by the reference ellipsoid in the map area; isostatic compensation is one-dimensional, Airy;

mantle

density

is

constant

beneath

the

map

sensu

area;

a

horizontal slab of uniform density 2,670 kg/m3 represents the rock mass

between

the sea level and the ground surface; lower-crustal

roots and antiroots are plane masses at a depth of 30 km; flexure

produces

isostatic

a

anomaly

linear values.

crustal

relationship

between topography and

Fortunately,

errors

variations on these assumptions are usually small

arising

from

(Simpson et al.,

1986; Goodacre et al., 1987a-d).

Along the continental margin, any

interpretation

errors

due

to

edge effects are minimized by calibrating the interpretations with seismic refraction and gravity models. is

large,

only

coarse

crustal

Where

structure

bathymetric

relief

is interpreted. Such

interpretations are robust enough, and constrained well enough, to be relatively insensitive to edge effects.

Horizontal-gradient maps Horizontal-gradient

maps

enhance

short-wavelength

features

in

29

gravity and magnetic data. magnitude

They reflect lateral variations in the

of a potential field; abrupt variations are emphasized.

Horizontal-gradient maps help interpret shallow crustal structure.

Different methods of

generating

different

The

workers.

such

maps

finite-difference

have

been

used

by

method estimates the

horizontal gradient at a grid node from differences

with

anomaly

values at neighboring grid nodes (Cordell and Grauch, 1985).

Another

common technique (Sharpton et al., 1987; Goodacre et al.,

1987e) relies on fitting of a planar surface to a potential-field

values.

The

slope

of

this

window

5x5

plane is a scalar

quantity considered to represent the magnitude of gradient

of

the

horizontal

of the potential field in the center of the window.

Yet

another method (Lyatsky et al., 1992a,b) involves fitting a thirdorder

surface

to

a

window

of

5x5

potential-field values.

A

higher-order surface offers a more realistic representation of the field within the window, while the third order is still low enough for the best-fit spurious

data

surface points.

not The

to

be

greatly

affected

by

any

horizontal gradient computed at the

center of the window is treated as a vector and displayed on a map as an arrow whose azimuth represents the direction of the gradient and whose length is proportional to the gradient's magnitude.

Contouring or color coding scalar gradient values can be produce

maps,

as

was

1992).

to

done with gravity data for Washington and

southwestern British Columbia Clowes,

used

(Finn

et

al.,

1991;

Dehler

and

An aeromagnetic horizontal-gradient vector map has

been produced for western British Columbia from northern Vancouver Island to Dixon Entrance (Lyatsky et al., 1992a,b).

30

Upward continuation To

investigate

magnetic from

large geologic

5 to I00 km.

ground

or

sea

Washington

two

a

were

maps

selected

meaning

level.

state

presented

requires

This procedure

data recorded

more

is more

Blakely

by

and

onshore

the

al.

(1989),

potential

maps

(1991),

Upward

no

rocks

are

who

continental-margin

high

Coast

sea

to

are

are

high

only

interest,

Regardless,

the tallest mountain

Upward continuation wavelength: If

(Lyatsky,

1991a),

and

elevations (>2400

m)

attained on a in

the

North

the most geologically

gravity maps in the region were found to be to 20 km

No

exist

level,

region topographic

elevations

Mountains.

(1989a).

assumed

above

The Olympic Mountains

Such

West

At sea, this assumption

protrude

and

preferentially.

of

continuation

Teskey et al.

field

Cascade

upward continued

the

was discussed by Grant and

scale inland from the areas of

anomaly

above

gravity

et

cut-off.

regional

informative

elevation

of the

filter but produces maps whose physical

Connard

i000 m.

localized.

ranging

intuitive.

because

rarely exceed

elevations

and

longer and shorter than

the real and nominal map levels.

in

gravity

the appearance

Finn

anomalies

wavelength

complex

is justified

but

simulates

produced

sources or sinks of the between

to nominal

Wavelength-filtered

The theory of upward continuation (1965),

in the region,

at a specified

containing

100-km

data

features

data were upward continued

potential-field

the

of p o t e n t i a l - f i e l d

the

ones

many times higher than

peaks.

involves

filtering the data on

short-wavelength the

potential

anomalies field

is

the are

basis

of

attenuated

measured

on

a

31

horizontal

plane and desired on a higher horizontal plane, upward

continuation is given by (Blakely and Connard,

1989):

6z>O,

F[hU(x,y)] = F[h(x,y)]exp(-k6z)

where k is the

anomaly

inverse of wavelength),

wavenumber

(the

quantity

F[hU(x,y)]

is

is

the

6z is the distance of upward continuation,

F[h(x,y)] is the recorded potential field in the and

k/2~

the

upward-continued

Fourier

domain,

potential field in the

Fourier domain.

Local, short-wavelength anomalies, which would not be observed a

high

recording

level,

are

suppressed.

shallow origin are not excluded, continued

maps

the subsurface. of

the

are

and

most

Broad

at

anomalies of

features

in

upward-

caused by large sources at various depths in

Upward continuation to 20 km gives a good picture

large-scale

structure

of

the upper crust, but is still

detailed enough to permit correlation of anomalies

with

features

in surface geology.

Assessment of seismic data Overview of the data Much

of

have a low

the available reflection and refraction data are old and resolution.

However,

combined

with

the

available

modern seismic profiles in Oregon (Keach et al., 1989), Washington (Taber and Lewis, 1986) and British Columbia Clowes et al., 1987; Rohr and Dietrich,

(Yorath et al., 1987;

1992; and others) and with

the potential-field data, they help interpret the of different parts of the continental margin.

deep

structure

32

Modern

controlled-source seismic profiles are available in places

across the British Columbia continental margin. and

refraction

program.

imaged

seismic

Survey.

best,

Refraction

reflection

data have been acquired largely by the LITHOPROBE

Other

Geological

These

but

As

data

are

expected,

resolution

available

from

the

U.S.

shallow subsurface levels are

decreases

rapidly

with

depth.

data across the margin generally offer reasonably good

constraints for modeling the structure of the upper crust, but the data for the lower crust and upper mantle are of lower quality.

In

the

reflection

surveys, signal penetration is reduced due to

scatter from structural and stratigraphic bodies

with

contrasting lithologies.

contacts

between

rock

Results are poor images of

deep parts of sedimentary basins and uncertain definitions of basement

Coarse

(Bruns and Carlson,

images

of

the

1987; Lyatsky,

lithosphere

1991b).

are provided by inversion of

teleseismic arrivals from distant earthquakes. stations

are

in

operation

in

Numerous recording

western U.S. and Canada, and the

first important summaries of results have already (e.g., Humphreys and Dueker,

the

been

published

1994a,b).

Ambiguities in seismic interpretation Deep seismic data across the Vancouver Island margin are generally of good quality. temptation

But even so, care must be taken

to overinterpret.

to

resist

the

This caveat is important because in

some influential papers these data have been cited as

"the

first

direct evidence for the process of subduction underplating"

(sic!)

beneath the

continental

margin

Island

(Clowes

al.,

p.

et

1987,

Hyndman et al., 1990).

off

southern

Vancouver

31; see also Yorath et al., 1985a,b;

33

Such a view is

overly

optimistic.

During

a

workshop

of

the

International Association of Seismology and Physics of the Earth's Interior in 1987, alternative interpretations and velocity derived

from these data have been presented by investigators from

national and foreign institutions 1990).

This

diversity

of

(see

opinions

restriction on interpretations that plate

models

is

being

papers

Green,

ed.,

arose despite the a priori

"the

Juan

de

Fuca

oceanic

subducted beneath the collage of exotic terranes

that constitute Vancouver Island and the mainland"

in:

western

North

American

(Green, ed., 1990, p. i).

All the same, analysis of the data led different groups of workers to very different conclusions. interpretation

suffered

Many

from

participants

variations

parameters and quality of the data and from deep

velocity

structure.

Most

workers

in poor

observed the

that

recording

constraints

took

care

on

stress the

general problem of non-uniqueness of geophysical interpretations.

Thybo (1990) noted that a exist,

not

subducted

compellingly

slab

resolved.

was

only

assumed

Subjectivity

to

of

seismic

interpretations was also pointed out by Morgan and Warner

(1990),

who cautioned that their own refraction model across the margin is only "one of similarly

a

series

acknowledged

of

solutions"

his

Limited

seismic

coverage

40).

Weber

(1990)

interpretation as tentative because,

"due to the non-uniqueness of explain the observed data"

(p.

modeling,

other

models

may

also

(p. 49).

contributed

to

the

uncertainties in

34

refraction the

models

southern

(Morgan and Warner,

Vancouver

well.

This

leaves

levels,

the position

oceanic

slab.

Still very uncertain density, shelf old

delaminated

Spence et al., (Riddihough, models, Drew

of the Moho,

profile

structure

slice

of

lower-crustal

and the existence

of a subducted

body under Vancouver

shapes.

Clowes

of subducted

oceanic crust

underthrusted

Confirming

(1990)

high-

and

an

1973;

lithosphere

of geophysical

Ansorge

and others has different

be

(Stacey,

oceanic

the non-uniqueness

to

(1990),

velocities

and dip

and

of

the

slab.

(1990)

showed that the high-velocity

not required by the refraction

data at all.

was shown in the model of Iwasaki and Pandit

a

Island and adjoining

These models differ also in the position

subducted

of

This body was once presumed

1985) or newly

of

is constrained

is the nature and even existence

1973).

1979).

the

only the top part

such a slice in the models of Egger

and

Thybo

Island refraction

uncertain

high-velocity

(cp. Stacey,

1990):

(1990)

mid-crustal

No high-velocity

and Shimumara

also noted that such a

sliver

sliver

is

zone

(1990),

and Fowler

under

Vancouver

Island may not exist.

Seismic

images were noted to be especially

shelf and upper slope off Vancouver signal

penetration

to

be

by

multiples.

of the profile refraction

Island.

reduced,

faulting of shallow sediments,

poor on the continental Thybo

probably

ill-constrained.

profile three crustal

(1990)

Mereu zones:

found

due to folding and

and deep reflections

Iwasaki and Shimumara

(1990)

to

be

masked

also found this part

(1990) oceanic,

modeled

in

the

transitional,

and

35

continental. slope,

The transitional

was

shown

zone,

as a coherent

under

crustal

its seismic velocity

increasing downward

properties

for

typical

model deepens

towards the continent

The dependence processing

continental

of seismic

and display

showed that many seismic events

et

deep

al.,

reflection

structures

1990).

Moho

et al.,

1990).

To minimize data,

principles

uncertainties

several

precautions

observed projected

from

surface

geological to

the

in

of

being

data

in

to be

tests on these

the

regarded noise

as

the

(Hawthorne,

subducted

1990;

interpretation

of

Levato

geologic

mapping,

geophysical

Results with

is

The first was

all available

some

floor,

were

offshore, an

information

The then

as far as advantage

onshore

and

inferences.

information

dredging the sea bed and drilling wells. ocean

available.

having

for tectonic

geologic

information,

relationships

thus obtained,

offshore,

the

geophysical

where the most reliable

served as constraints

mostly sonar images of

Vancouver

Stacking and migration

offshore,

direct

on

1990).

diffractions

periphery of the continent

consistent

(Hawthorne,

were taken in this study.

and

marine data allowed.

chosen for data

of this study

to "stand on the continent',, obtained

Such

fact

may be just off-line

Methodological

km/s.

initially considered

are

data showed that the deep event once oceanic

upper

The Moho in Mereu's

is a common complication

Island

(Milkereit

from 6 to 7

images on the parameters

of

and

not smoothly but stepwise.

detailed reprocessing

from

shelf

block about 20 km thick,

crust.

Indeed,

reflections

the

is

available

Geophysical seismic

from

data include profiles

and

36

potential-field results.

(gravity,

magnetic,

electromagnetic)

survey

Non-uniqueness in the interpretation of geophysical data

was reduced in this study by using diverse data types jointly with geological

facts,

interpretation

to

generate

an

(Lyatsky and Lyatsky,

internally

consistent

1990).

In the past, a major hindrance to regional integration of data was jurisdictional border.

and

institutional

Interpretations

barriers

In

this

study,

geology

and

a

more

U.S.-Canada

Washington,

comprehensive

and

vice

of coastal British Columbia is

linked to that of the neighboring regions in obtain

the

in British Columbia often differed from

those in the adjacent parts of Alaska versa.

at

geologic

U.S.,

model

of

in the

order

to

crustal

structure of the western margin of the North American continent.

CHAPTER

3

-

PRE-CEHOZOIC

Pre-Tertiary

GEOLOGIC

stratigraphic

FRAMEWORK

OF

WESTERN

CORDILLERA

record

Paleozoic Early Cambrian plutonic

rocks have been identified

islands of southeastern

Alaska

Islands al.,

suspected

The

in

Gehrels

country

rocks

rocks of various

southeastern

et al.,

abundance

to

Middle(?)

1987; the

Alaska Brew

south,

(Gehrels

et in

Devonian or younger

lithologies

al.,

on the Queen Charlotte

on

western

Vancouver

Island

Islands

(Muller,

and

variously m e t a m o r p h o s e d

plutons,

Island,

Paleozoic

Friday,

1989).

complex

et al.,

by these plutons

are

assemblage

of western W a s h i n g t o n

1988)

are

Saleeby, They

also

1987a,b;

decrease

Columbia. have

(Woodsworth

(Hesthammer

et

been

mapped

and Orchard, al.,

1991)

Massey and Friday,

and deformed. 5.5

km

thick

in Only

They include a variety of stratified

rocks are at least

Paleozoic

and ages

and

rocks

1980a;

1991).

provinces

et

British

Paleozoic

Andrew et al.,

A

(Brandon

1991).

along the mainland coast of Hecate Strait

and

south

and on the San Juan

boundary

intruded

the

to be Precambrian.

Younger Paleozoic

1985),

1990)

near the British C o l u m b i a - W a s h i n g t o n

1988).

found

(Gehrels,

on

1989; units

On V a n c o u v e r (Massey

and

is found locally in the coastal

(on the San Juan Islands;

Brandon

is still poorly studied.

Mesozoic Mesozoic

rocks

stratigraphic

are

hiatus,

more

widespread.

this succession

After

a

begins with massive

regional basalts

38

and

associated

(Table I).

tuffs

These basalts

Alaska

(Jones et al.,

islands

(Sutherland

mainland

The

different volcanic

breccia

on the Queen Charlotte

and Vancouver

and tuff.

pillow

of the Columbia

River province

so

1970s,

the

are

many

crystalline

crust

lavas

and

in

the

is diverse

Cordilleran

(Andrew and Godwin,

of

Formation

(Muller,

1977a),

flows, (Barker basalts

interior 1989b).

(Reidel

the Karmutsen was

the

rocks of

subaerial

are intracontinental

parts

Karmutsen

mafic extrusive

similar to that of Cenozoic

and central W a s h i n g t o n

1994),

near

in places up to 6 km

Their geochemistry

but in general

and

1988).

thick,

It comprises

submarine

the Columbia River basalts

1977a,b)

(Woodsworth,

is extremely

1989)

eastern

southeastern

1968; Muller,

1989d).

Formation

and

Strait

Formation

types:

al.,

also occur in southern

1977),

shore of Hecate

Karmutsen

the Upper Triassic Karmutsen

Brown,

(Andrew and Godwin,

et

of

Since

et

Formation.

considered

to

be

in

al., In the

oceanic

but that idea is inconsistent

with the new evidence.

With a narrow but gradational

contact,

upsection

strata

Jurassic

into

sedimentary

subdivided thickness

On

the

Queen

into two groups, exceeding

Charlotte Kunga and

i000 m.

package

are mainly carbonate,

up

shale

and

(Cameron and Tipper,

These carbonates

sandstone

limestone,

Islands, Maude,

these

with

The lower stratigraphic whereas with

1985; Thompson

and clastics

volcanics

of latest Triassic

age, which consist of shallow marine

sandstone.

of

Karmutsen

a

pass

and Early shale and rocks are cumulative

units

in this

and the upper units are made minor tuff and volcanic

et al.,

flows

1991).

of Late Triassic

age extend,

without

39

QUEEN CHARLOTTE ISLANDS o >- io NI~ ,.,r. ¢,
+60 mGal smaller

gravity

Washington.

(Fig. 19), prominent

over

highs

Northwest of

correspond

The most pronounced is the positive

the

lie

highs

Metchosin

massif.

Similar

but

over basaltic bodies in southwestern

the

Metchosin

massif,

local

gravity

highs north of Barkley Sound are probably related to mafic igneous rocks in the subsurface. values

decrease

In the Strait of Juan de

the

Olympic

Mountains.

OWSZ,

and

become

lies

1991).

along

the

negative

This southward decrease of anomaly

values is consistent with the asymmetry of the Fuca axis

anomaly

southward from the Metchosin high across several

steep gradient zones parallel to the over

Fuca,

northern

Basin,

whose

Olympic coast (Niem and Snavely,

The gravity low over the

Olympic

Mountains

may

reflect

either thick sediments or a low-density crystalline basement.

Despite

a

strong density contrast between the Tertiary sediments

and Crescent basalts (the latter in this area have 2,700

a

density

of

kg/m3, considerably heavier than the surrounding sediments;

Finn, 1990), not all basaltic massifs are associated gravity

anomalies.

Unlike

the

with

strong

Metchosin massif, the Hurricane

Ridge and Crescent Lake bodies are not strongly expressed

in

the

gravity map, probably due to lack of deep roots.

Over

the

Olympic

Peninsula

and the adjacent continental shelf,

three local minima can be distinguished. are

separated

by elongated relative highs.

the Olympic Mountains is about -90 west,

it

Isometric in shape, they

mGal

in

The gravity low over magnitude.

On

the

is flanked by a relative high (-50 to -70 mGal) bounded

by g r a d i e n t zones trending NE and NW.

O

111

The Hoh Basin farther probably

west

corresponding

-75 mGal in amplitude, Olympic

coast.

is

marked

by

two

isometric

to distinct depocenters.

One low, nearly

lies over the central part of

and

offshore. relative on

the

the

western

Another low, also around -70 mGal in the Bouguer

map but 4 s t r a v e l t i m e

is 18

al.

continue

buried

If the d e p r e s s i o n et

and 3900

the p a l e o - s l o p e

inboard

Juno depression,

Sandwiched

represents

down

depression's

3300

however,

zone.

slumped

SP i000,

between

facies,

sediments of

fault-bounded

reflection

it b e g i n s 1700,

but

break

around

lies

at

the Juno

lines

at SP 1870. may

about

85-02

m water

depth.

depression

8 km wide,

about

SP

Fig.

58),

(Fig.

reliably

and

1300

definition

northern

less t h a t h a l f

to

bathymetric

1700

the

imaged

85-02

In the n a r r o w e s t

89-07;

only

inboard

floor between

(also SP 500 to 680 in Line is

In Line

as far as the m i d - s l o p e

The o c e a n

1400

has also b e e n

and 89-07.

It can be t r a c e d

continue

SP 1300.

depression

Juno

its w i d t h

in

the south.

A strong

basement

at

s

5.5

in

bathymetric 500),

this

the C a s c a d i a

The r i d g e

seismic

Line

ridge

event under

89-07.

Despite

at the o u t e r

event Basin

at SP 500

seems

(Line

boundary

similar

outboard.

the d e p r e s s i o n the

disruption

of

the

is

an

inboard

anticlinal

apparent under

depression

to the o c e a n i c - c r u s t

It can be t r a c e d

89-07)

is

basement

the (SP of

to SP 680.

structure

in

I--

ILl

O3

2400

2200

2000

1800

1600

1400

SHOT POINT 1200

1000

300

600

FigUre 57. Structure of the submerged continental margin off Vancouver Island, from the shelf to the abyssal plain, imaged in seismic reflection line 85-02 (data after Yorath et al., 1987; Hyndman et al., 1990). Line location is given in Fig. 45.

2600

Line 8 5 - 0 2 200 101

260

300

SP 200

E

1

I

~

2

-

~

.,

600

1

i. ,

700

I

,

800 ODPsite

I

' i

.

,, "

500

I

illlll I

3 t 4

O

400 ,

"

..

.

.

.

i " J

.

.

'

.

.

_

.

_

.

.

_

.

.

c

k -

g$9/890 '.

tI

-

i

-

,

.

.

.

~

-

~

~ ~ - ~ 7 " ~

5 6 -10

-5

0

5 10 Distance from Toe (kin)

15

20

Figure 58. Structure of the northern part of the Juno depression on the lower continental slope off southern Vancouver Island, imaged in seismic reflection line 89-07 (data after Yuan et al., 1994). This line runs parallel to the reflection line 85-02 (Fig. 57), some 5 km to the south.

261

semi-consolidated Nearby, to

at SP 650,

0.5

s

disturbance a

but inside

sedimentary

the Juno depression, A narrow

on the i n b o a r d

side of this

seismic

can be t r a c e d

southern

stratified

in amplitude.

subhorizontal

event,

thinly

event

inboard

part of the Juno

lies a s y n c l i n e

anticline

system.

East of it b e g i n s unlike

as far as SP 720. (Fig.

up

at SP 620 is a small

at 4 s which,

depression

rocks.

the b a s e m e n t

Compared

52),

with

the

the n o r t h e r n

part

is m o r e d e f o r m e d .

A drillhole ODP site

near

SP 810 in Line

889/890,

below

is

the

345 m of P l i o c e n e

(Carson

1993;

fractured faults This

be

been

deformation

made.

found

shape

150

(Line data

of

the

is 2.5 to 3 km.

in the Juno shallower line

85-02,

1700

and

depression ocean

floor,

gravity 1300,

of 1 3 0 0 - 1 4 0 0

m.

anomaly

where

Significantly,

Unfortunately,

are

however,

no

images:

no d e e p

maps

in this

(Fig.

effect

about

are about plateau

free-air

area,

can

but some

53).

is

rectangular

It e x t e n d s

Basin,

offset

rises

interpretation

depression

Cascadia

a bathymetric

rocks

of s e i s m i c

in amplitude.

values

in s e d i m e n t s

the q u a l i t y

probably

which

1994).

Island.

The g r a v i t y is

silt,

Vancouver

low over the Juno

deep parts

al.,

Sedimentary

are a v a i l a b l e

and -40 to -50 mGal

et

at

off s o u t h e r n

85-02),

by g r a v i t y

85-02),

to Q u a t e r n a r y

of 104 m, but

m.

degrades

in Line

MacKay

70 ° .

in this well

are p r o v i d e d

the a d j a c e n t thickness

as

below

probably

gravity

a depth

much

No r e f r a c t i o n

The f r e e - a i r

to

as

SP 1700 and 1300

constraints

in

are

pervasively

have

between

et al.,

subhorizontal dips

(SP 1550

penetrated

c l a y and fine sand Bedding

89-07

also

into

where

sediment

of t h i c k e r

sediments

by

the

400 m. -30 mGal

effect

Along

seismic

between

lies at a w a t e r

gravity

profiles

of

SP

depth

inboard

262

from

the

Juno

structure

not

so

as mimic the m o r p h o l o g y

bathymetric boundary

depression

mimic

makes

much

anomalies,

finding

the

are contaminated

w h i c h become

the

of the continental

from gravity data impossible.

gravity signatures

reflect

geologic

slope.

Juno depression's Besides,

This inboard

in this area Juno

by the first appearance

increasingly

prominent

of N-S

towards the Brooks-

Estevan embayment.

Typical

oceanic-crustal

the Juno depression Thickness

magnetic

from

Pacific

of the crystalline

about 7 km (Waldron et al., akin

to

the

Queen

lineations

regions

continue

outboard

1990).

The Juno

Trough

of fault-bounded

into

(Fig.

crust beneath the lower slope

Charlotte

formed by downdropping

anomaly

depression

further north.

27).

is only is

thus

Both were

blocks of oceanic crust of

the Juan de Fuca plate.

Genetic

links

are

Juno depression. seismic

of

the

edge

this

material, fragments

of

section

packages

over transparent),

is about 50% thicker.

in

Upper

(Carson et al.,

1993).

from

Island allowed

all sand

Drilling

levels and

abundant

even

Pleistocene

sedimentation

of the Juno depression,

7

Though dominated

by

coarser

gravel.

Wood

and Holocene

strata

It appears that abundant

and flow over into adjacent

in the Juno

Upper

at

found

though

similar

(ODP site 888) encountered

contains

were

with

west

567 m thick.

coarse

Basin and the

km

sediments

such as fine to

with subsidence

sediment

the depression

to Recent

Vancouver

between the Cascadia

contain

(stratified

each facies

Pleistocene silt,

Both

facies

depression

suggested

sediment

supply

not only to keep pace

but also to

parts of the Cascadia

overshoot

Basin.

it

263

Limited

extent of s u b d u c t i o n - r e l a t e d thrust faults and

northward

m~lange along the continental margin Sedimentary rocks deformed into broken formation or

m~lange

have

previously been described along the entire continental margin from Oregon to southern m~lange

British

Columbia.

Snavely

(1987)

sedimentary rocks from at

beneath the cover of stratified

least five deep wells on the shelf off Washington. and

reported

On the

Oregon

Washington margin, such a style of deformation can readily be

linked with west-vergent thrusting and subduction of the Fuca

plate.

de

Off Oregon, thrusts have been detected by sea-floor

mapping and seismic surveys 1992;

Juan

(Snavely,

1987;

Goldfinger

et

al.

Cochrane et al., 1994), and by recent drilling on the lower

slope during the ODP Leg 146 (Carson et al., 1993).

On the northern Washington broken

formation

previously. Peninsula

may

have

been

development less

doubt

intense

m~lange than

complexity

(Orange, 1990).

Snavely

supposed Olympic

(1987)

noted

in that area and proposed that these rocks

might have been affected by processes not of Recent

and

on the perception of Hob Basin sedimentary

rocks as a simple m~lange

obduction.

of

Results of new field mapping on northwestern cast

structural

margin,

publications

show

subduction,

many

faults

in

but

of

western

Olympic coastal areas are rather steep (Orange et al., 1993).

Many blocks of Eocene basalts in western Oregon Washington rotated

and

southwestern

onshore have been found by paleomagnetic studies to be

clockwise.

These

rotations

have

been

attributed

to

obliquity of plate convergence during the Tertiary (Wells and Coe, 1985). the

The amount of block rotation decreases along the coast

north,

and

to

on the Olympic Peninsula it virtually disappears

(Babcock et al., 1992, 1994).

264

These observations preclude

are circumstantial,

the existence

off

Vancouver

However,

Island

studies of the submerged observations

onshore.

almost

continental

that

No

Juan

lower

On

the

by

geological

shelf,

fractured

stratified

rocks

created

is known on the Vancouver

plate

it.

is "not probable"

greatly

Presence

by

Island

of

continental

outboard

accretionary

prism.

volume

of material

1987;

Davis

limits

Estimates

the

as

the

(Waldron

space

al.,

available

crystalline mid-slope

to

crust off

bathymetric

depression

available

on the

for

an

from various tectonic models of the

that should have been accreted

and Hyndman,

et

scraped off the

space

Juno structural the

suggests

and mafic rocks at

presumably

exceeds

and of the fault-bounded greatly

for accretion

sediments

The volume of material

Island as far

slope,

available

of oceanic

assumed rates

Fuca

accommodate

break,

(except

of material

accretion

p. 112).

Vancouver

Island.

or diapirism)

the volume

continuous

de

and

by detailed

slope.

conventionally 1990,

Island

extent of any such

is severaly constrained margin

not

No m~lange has been found on the shelf or

m~lange

slumping

Estimating

do

rocks as old as Eocene have been drilled to a depth of

4 km.

sediment

the possible

continental

slope off southern Vancouver sedimentary

themselves

of some accreted rocks off Vancouver

and northern Washington. rocks

and by

1989; Waldron

et al.,

(Clowes et 1990)

al.,

differ from

one another,

but all exceed by far any remaining

available

For example,

Davis and Hyndman

that the Cascadia

Basin

sediments,

if

scraped

(1989) off

calculated

space.

the Juan de Fuca plate,

would

265

alone have contributed during

the

l a s t 1.8 Ma,

these sediments enter

the

Waldron off

margin

(1990)

oceanic

prism containing

km

given the shortage be

is

would

a total

supposedly

The estimate

of

a d d 400 km3 p e r 1 k m Nonetheless,

space,

estimate

if c o n s t a n t

this

for

a

in e q u a l p r o p o r t i o n , of 200 km3 p e r

with the current models.

of a v a i l a b l e

of

t h e t o p 2 k m of m a f i c m a t e r i a l

and mafic material

(1990),

length

Thickness

they

km°

exaggerated.

consistent

realistic

where

2.5-3

crust

seems

sediments

supposed by Finn

to

prism,

of m a r g i n

i00 km3 p e r 1 Ma.

that scraping

1 Ma

per 1 Ma appears

high

or about

crystalline

length per

as w a s

for e a c h k i l o m e t e r

at t h e f o o t of t h e slope,

accretionary

e t al.

the

170 km3

number

is

r a t e s of a c c r e t i o n

1

However, much

too

are a s s u m e d

for the C e n o z o i c .

The u p p e r - s l o p e modeled only

body with velocities

by Drew and Clowes

3 km and a maximum

interpreted

by

been

filled

to have

l e n g t h of a b o u t

Waldron

With a cross-section

(1990)

of 4.8 to a b o u t

et

al.

area around

in less t h a n 1 Ma.

this

m~lange,

sedimentary

space

was

prism.

would

have

space between

s l a b in t h i s m o d e l w e r e

a r e a of a b o u t 1200 km2

(Tofino Basin

rocks

would have been filled

in just 6 Ma.

space problems

subtracted)

assumption

of c o n t i n u o u s

e v e n in W a s h i n g t o n ,

t h a t a r e a in

a

penetrations

on

beneath

It

of

the cross-section

W i t h s u c h rates,

m~lange

53).

E v e n if t h e e n t i r e

t h e s e a f l o o r a n d t h e t o p of t h e s u b d u c t e d a

(Fig.

as an a c c r e t i o n a r y

150 km2,

was

an a v e r a g e t h i c k n e s s

50 k m

(1990)

5.5 k m / s

involving

short

pulse

pre-Late

undisturbed

more

shelf

Miocene

younger

but emplacement

seems

the Washington

strata

accretion would

Tertiary

of a m ~ l a n g e

plausible.

indicate

produce

Drillhole

the e x i s t e n c e

r o c k s of v a r i o u s

(Snavely,

1987).

in

of a ages,

Depth extent

266

of this m~lange attenuated

is uncertain,

continental

but

it

crystalline

is

probably

crust.

that created this m~lange was probably

underlain

The accretionary

related to the

by

pulse

mid-Miocene

tectonic episode that also caused sudden contractional

deformation

in the Hob Basin.

No such contractional and

models

of

episode

an accretionary

slope are inconsistent Rim

"terrane"

of

(1992)

encompasses

field

mapping,

its

Hyndman

1989a,b).

many

evidence.

(1990)

1994).

the

origins

and

shelf

Formation

model)

Vance, off

basalts

trending WNW and NNW. the anomalies, correlative small.

sedimentary seismic

From

Rim

complex 1985;

emplaced

by

(another presumed terrane

are

now

centers 1992;

Vancouver

is

recognized in situ,

Babcock Island,

reflected

In the Prometheus

these basalts

basalts

rocks onshore

their

underlain,

(Fig.

al.,

magnetic

H-68 well

(Babcock et al.,

line 89-06 on the shelf

have

probably

et

are about 500 m thick,

are

to

in a 1992,

anomalies

extent.

The

in narrow magnetic highs

strong gravity highs suggests

Crescent

types.

(Rusmore and Cowan,

related to such basaltic bodies have only a limited of

Pacific

and Dehler and Clowes

slices were probably

local volcanic

(Brandon and

distribution

The

not thrusting.

rift setting On

Island,

under the shelf and upper

Peak unit

These

in the a c c r e t i o n a r y - c o m p l e x from

Vancouver

from slices of the Pacific

Pandora

movements,

on

rocks of the Leech River complex are

Eocene basalts of the Crescent

erupted

al.

rocks of different

equivalent

strike-slip

et

metamorphic

Brandon et al.,

complex

with the available

now clearly distinguished and

is recognized

in the area

but absence of

total

volume

with a hot contact, 1994).

of

similarly,

51), b e l o w the volcanics

is by in at

267

1900 ms lie short seismic events (SP 1840) which

may

be

due

to

deep stratified rocks.

By analogy with the interpretation of similar seismic and magnetic anomalies in the Winona Basin (Chapter 5), and consistent with the distribution onshore, are

and

stratigraphic

position

local magnetic anomalies on the

of

Crescent

Vancouver

probably caused by local volcanic bodies.

Island

Instead,

individual

shelf

No single Crescent

"terrane" was accreted to North America in Washington Columbia.

basalts

or

British

igneous massifs of various sizes

were produced in situ, by local eruptions.

Neither does geological evidence confirm the presence of scale

thrust

faults which are the cornerstone of the subduction-

complex models of the Vancouver Island margin. mapped

on

crustal-

the

east

side

Thrusts have

been

of Vancouver Island, and local thrust

splays are found within the OWSZ (see previous chapters).

But

no

thrust-sheet structures are evident on western Vancouver Island or on the submerged margin.

Tectonic

slices

of

the

Pacific

complex were displaced along steep strike-slip faults. wide slice in the Ucluet area on the wedges

west

coast

Brandon,

it

1989a,b).

are

thought

to

merge

the

island

fault zones (Rusmore and Cowan,

right-lateral

1977a,b;

is steep.

Survey

Mountain

1985), which belong to the OWSZ.

movements on the Westcoast fault in the Late

Cretaceous and/or early T e r t i a r y fault

(Muller,

On southern Vancouver Island, small slices of

Pandora Peak rocks are found in the San Juan and

this

The 15-km-

out to the north and south, where the Westcoast and Tofino

faults that bound

Large

of

Rim

(Brandon,

1989a)

confirm

that

The same is indicated by its measured dips

268

(Muller et al., along

1974,

1981),

and its trace

the west coast of V a n c o u v e r

it strikes N50~W, Eocene

and

Oligocene

(east-side-down) exposed,

dips steeply strata

sense.

On

Island.

lateral

east,

fault is unlikely

movements

in

crustal-scale,

of

the Carmanah Group in a normal

to be

a

the Late Cretaceous

Eocene

and

Oligocene

faults

52 Ma,

intrudes the Pacific Rim complex

rules

out the supposed accretion

(1987)

No

big

(Woodsworth

Such Right-

10ng-lived,

of the

Island are aligned

along

1991).

One such stock, (Brandon,

of

apparent

drilled

Eocene

reflection

to

Plio-Pleistocene

Island,

in

the

combined with geophysical the

of fault-bounded

Alon g the egde of the southern

and

Island,

profiles

reflection

are

shelf

from

that the basin evolved by alternating

uplift and subsidence

seismic

et

No thrusts

Differences

faults w h i c h continue on

indicate

rocks

thrusting.

profiles.

between parts of the basin,

for transverse

by Clowes

in the Tofino Basin on the shelf.

and show no signs of significant seismic

This

of the Pacific Rim oceanic-crust

(1990).

are

dated at

1989a).

et al.

differential

not

plutons

and Hyndman

stratigraphy

Vancouver

et al.,

granitoid

42 Ma as was postulated

in

evidence

this

around

thrusts

stratified

thrust.

by thrusting

4 km

is

or early Tertiary were

in the history of

steep

al.

fault

Late

a N50~W strike.

low-angle

suite in many parts of Vancouver

appear

this

offsets

steep fault.

Unmetamorphosed

"terrane"

Peninsula,

the

Island,

and

straight

to

Nootka

only one of the many episodes

Almost

On Hesquiat

but Carmanah Group strata maintain

a straight

Catface

is remarkably

central show

shelf a

blocks.

off buried

Vancouver positive

269

structure over which stratified sedimentary rocks thin zero (Tiffin et al., 1972).

almost

to

Such a basin-bounding structural high

is observed in lines 85-01 (Fig. 52; SP 1300-1400) and 85-02 (Fig. 57;

SP

1050-1150).

Gravity

anomalies over the outer shelf and

upper slope are relatively positive, whereas a strong gravity

low

is found over the Tofino graben at mid-shelf.

Off

Oregon and Washington, similar gravity lows on the shelf have

been

interpreted

Riddihough,

as

sediment-filled

1989).

An old gravity model

1977a) showed the Tofino Basin to (1990)

depressions

interpreted

seismic

be

(Couch

and

(R.W. Couch in: Muller,

8-9

km

thick,

but

Thybo

data to suggest a thickness of 5 km.

In another seismic line (89-06; Fig. 51), the

stratified

package

at SP 1700 and 2050 also continues to 3-3.5 s, or about 5 km.

Poor

seismic-signal

penetration hinders interpretation all along

the submerged margin of western North America 1987; Lyatsky,

1991b).

(Bruns and

For example, the modern seismic line 85-05

resolved only the top 1.5 s (about 2 km) in the Fuca is

as

much as 8 km deep (Niem and Snavely,

Basin, the

basement

diapirs,

folds

degrade

seismic

geophysicists

and

is

usually

faults

images,

that

the

base

Basin

which

In the Tofino

Bedding

planes,

acoustic energy and greatly

leading

to

structure

of

of

1991).

ill-defined.

scatter

interpret (Iwasaki and Shimamura,

Only locally is the

Carlson,

observations

by

many

the shelf is difficult to

1990; Thybo,

penetration

1990).

in

the

Tofino

Basin

associated with clear events reasonably interpretable as volcanicrelated. associated

In Line with

85-01,

SP

2050

characteristic

to

2320

(Fig.

diffractions

due

52), to

it

is

surface

270

roughness

and internal

inhomogeneity

penetrated

by the Prometheus

events do

not

basalts

on

the

into overlying Crescent

mark

Eocene

Formation

it

crystalline

Peninsula

sedimentary is

laterally

Line

51),

suggesting

may also be present beneath

The m u d - d o m i n a t e d overpressured flowage, such

as

Farther

Basin

central

was

at SP 1400-1800

made up of many

found

by

drilling

Overpressuring parts

Island

probably

(Tiffin

caused

and SP 2600-2700

of

by

mud

over

deeper

be

basin, 1972).

flowage (Fig.

(Fig.

structures

57).

(e.g.,

are 52), In Line

SP 1500 to 2000).

A set of small grabens

lies at SP

650-800

half-graben

85-01,

2320-2450,

in

Line

faults w h i c h disturb

interpreted

model,

as a "fossil trench"

and half-grabens

not compressional, are apparent

SP

Line

regimes.

in seismic reflection

85-02.

The

is bounded by steep In

keeping

this structure was previously

(Hyndman et al.,

are normally

tectonic

in

shallow and deep strata.

with the a c c r e t i o n a r y - c o m p l e x

grabens

the

in Line 85-01

are

draped

to

et al.,

places,

normal

rocks

led to sediment

diapir around SP 200 in Line 85-02

85-01,

1994)o

on the shelf were noted in

and a piercement strata

rocks,

(Babcock et al.,

in the deep

Vancouver

anticlines

The

Tofino Basin sedimentary

1971).

which was most common off

and they pass

the volcanics.

(Shouldice,

south,

observed

Tofino

Crescent

with marine sedimentary

below the volcanics

these

gradationally.

discontinuous,

them with a hot contact

(Fig.

indeed

However,

basement.

sequences

Seismic reflections 89-06

were

are diachronous,

It is interbedded

overlies

which

H-68 well at SP 2280.

true

Olympic

volcanic bodies. and

the

of flows,

1990).

However,

associated with extensional, No low-angle

profiles

thrust

on the shelf.

faults

271

Thus,

along the continental

Columbia,

examination

northward

decrease

margin

of

from Oregon to southern British

Tertiary

geology

reveals

in the abundance of phenomena

a

gradual

attributable

to

subduction.

Zoning in the distribution of continental VancouVer Island continental

slope

Structure

Belt

of

the

Insular

systems of crustal detected Georgia

and separates 1984).

the

The

Western

crust

by several One

such

on

fault

system,

lies b e n e a t h the Strait of

and

Coast

Alberni-Cowichan

between the Eastern and

Lake

Vancouver

belts

(White

fault Island

and

system

lies

blocks.

The

separated from the Tofino Basin by the inner strand of

is

the OWSZ - the Westcoast the

scale.

data,

Insular

oceanic

dominated

or even lithospheric

with seismic refraction

Clowes,

latter

is

and

Calawah

fault

fault.

- separates

The outer strand of crustal

the

OWSZ

-

blocks on the inner shelf

from the Tofino graben.

Similarly, The

structural

mid-slope

zoning occurs

bathymetric

break,

upper slope from the relatively Vancouver

island,

also

in the continental between

gentle

lower

marks a structural

the relatively slope

off

boundary.

continental

crust lies inboard,

A different

zoning is found north of the B r o o k s - E s t e v a n

where

the

continental

entire

lower

slope area.

soUthern Attenuated

oceanic crust outboard.

slope

is

underlain

by

embayment, attenuated

crust and the boundary with oceanic crust follows

Revere-Dellwood

fault.

steep

the

272

The

previous

Island some

idea

is underlain

that

the Winona Basin off northern V a n c o u v e r

by a block of

oceanic

8 km during the last 1.5 Ma

shown in Chapter

stated that the basement in line 88-02 subducting believed

(Fig.

oceanic

this basin

36)

Explorer

are entirely absent

sediments

have

unrealistically velocities presence crust,

of deep of

been

high

old

Yuan et al.

Davis

fault

(Fig.

down of

in

in

sedimentary

basin

rocks.

Off southern Vancouver

Island,

by contrast,

boundary slope,

crust

of this depression and

of

the B r o o k s - E s t e v a n

This

embayment

extension

only

27).

For

1.5

observed 8

Ma

requires

High

seismic

consistent

crystalline

the

lower

slope

is

The outer

roughly with the foot of the

fault,

is about 70 km long.

are approximately

if it were projected

In its n o r t h w e s t e r n

under

part,

fault might be one of the

faults near SP 400 in seismic reflection

line 85-04

(Fig.

37).

however,

and

and no clear extension

anomalies

run

N-S,

fault is apparent

in seismic

coincident

line 89-09

an

buried

the southern part of the embayment,

Revere-Dellwood

with

embayment.

of the R e v e r e - D e l l w o o d

magnetic

km of

is probably continental.

both its outer and inner boundaries

in line with the R e v e r e - D e l l w o o d

(1993)

magnetic

the Juno depression.

coincides

the

However, are

are

of

Currie

The underlying

15 km,

oceanic

top

oceanic

sedimentation.

this

p. 1516)

small

just

reaches

by

and

and

whose thickness

underlain

the

by an independent,

laid

strata

represents

in this area

rates

(1992,

under the Winona Basin

has now stopped.

Revere-Dellwood

to

1982) was

plate".

is underlain

stripes

the

(Davis and Riddihough,

"undoubtedly

plate whose subduction

of

subsided

seismic reflection

Winona

west

which

6 to be unrealistic.

crust

In

gravity of the

(Fig.

42).

273

But the plate-boundary kilometers

structural

zone continues

along the continental

for thousands

margin of western North America.

Compared with that distance,

a gap of a few tens of kilometers

minor.

Alignment

the

boundary

faults of the Juno depression

continuity

of

this

the R e v e r e - D e l l w o o d the

Juno

between

system's

depression.

it separates

The inner strand of

straight

shelf

50).

with

branches,

the

overall

which

of

bound

is of most importance,

plate-boundary

Island.

as

disruptions

faults,

in

its

such as Estevan the

widens to the south by about i0 km, on the outer shelf

lies

Apollo anticline 1980).

NNW-oriented oriented

faults

faults

diapirs

the elongated trends NNW.

fault,

fairly

which

Yorath,

Estevan

structural

which remains

Local

to transverse

Just south of the

(Shouldice,

This elongated

the shelf edge to the north,

other

and

The SE extension

of the shelf edge,

be attributed

the NNW-trending 1972;

into two

NNW-trending

the position

can

outer strands.

is

and oceanic crust.

all along Vancouver

position (Fig.

the

fault

is consistent

The inner branch

the continental

zone controls

Revere-Dellwood

fault splits

of

onto

is suggested in the Tofino

which the

may

shelf.

across

1971; Tiffin et al.,

anticline indicate

positive m a g n e t i c

propagation

Presence

by the elongation Basin.

is on trend with

of

and

of

other NNW-

alignment

of

The volcanic body that causes

anomaly off

Clayquot

Sound

Like many other Crescent volcanic bodies,

also

it may have

formed along a fault.

The OWSZ, the

trending WNW,

plate-boundary

Westcoast

also has a regional

fault

system

fault is actually Mesozoic,

off

extent,

Vancouver

but it was

and

it

Island.

reactivated

meets The and

274

incorporated

into

the

OWSZ in the Tertiary.

Vancouver

Island,

as far as the

possibly

beyond

(Muller

also extends Another

et al.,

on the shelf,

WNW-oriented

transverse 1974,

continuing

It continues

Esperanza

1981).

as

far

as

Barkley

and fault

Sound.

the volcanic

body and the magnetic

anomaly

However,

faults are mostly confined to the shelf,

OWSZ-related

only a few WNW bathymetric Brooks-Estevan

The

amount

tectonic

of

Cenozoic

thrusts

Islands

is

1987; von Huene,

zone off Washington

margin

fragmented

found

1989;

on the

slope

in

and the

be

northern

characteristics

No evidence

submerged

Alaska,

Island

and Chapter

6).

(see

Steep,

Bruns

margin

NNW-trending

fault system off southeastern

run into the N - S - t r e n d i n g

the

In contrasts,

continental

Oregon.

Cascadia

Alaska

subduction

and Oregon.

northward decay in the intensity may

along the western

off southeastern

and especially

strands of the p l a t e - b o u n d a r y and British Columbia

increases

and northern Vancouver

off southern Washington

the

Peninsula.

margin from north to south.

are fully developed

The gradual

are found on the

compression

compression

Queen Charlotte and Carlson,

Hesquiat

embayment.

North America continental of

trends

of

fault

The Calawah

fault is the one which controls just south

along

of compression

along

related to non-rigid b e h a v i o r of the heavily Juan

de

of the Cascadia

Fuca plate are discussed

Fuca subduction

plate.

Such

unusual

zone and of the Juan de

in the next chapter.

C H A P T E R 9 - INTERLOCKING

OF C O N T I N E N T A L A N D OCEANIC C R U S T A L BLOCKS

ALONG THE C O N T I N E N T A L M A R G I N A N D N O N - R I G I D BEHAVIOR OF NORTHERN ~VJAN DE FUCA pLATE

Place of block interlocking Vancouver

Island

has

providing

sediments

in the p l a t e - b o u n d a r y

mostly

been

uplifted

zone

in

for the basins on its sides

Cenozoic,

(Muller,

Sea-floor dredging

and analysis

Mesozoic

rocks similar to those on Vancouver

basement

of magnetic

the

anomalies

1977a).

show

that

Island also

underlie Tertiary

strata on the Kyuquot and Northern blocks on the

shelf.

south,

To

the

continental volcanic

crystalline

massifs

(Muller,

1977c;

(Brandon,

of

suggesting

crust, the

Snavely,

1989a).

also

Crescent Formation

1987),

Crustal

Seismic Mereu,

1990)

blocks

were

similar

slope

(Finn,

to those in continental been

containing

sedimentary

Waldron shows crust.

both

et al.,

1990),

1986;

oceanic-crustal

slab

and Riddihough,

1989).

various

Drew and Clowes,

and Washington

with

velocities

crystalline

interpreted and

as

an

but the analysis

this crust forms of the subducting

submerged

and

densities

That material

accretionary rocks

in the

1990;

show under the

crust.

volcanic

that at least some of this material Off Washington,

from

1990) models across the

material

has traditionally

delineated

Island shelf

chapter.

margin off southern British Columbia shelf and upper

including M e t c h o s i n

on the Vancouver

(Taber and Lewis,

and gravity

Eocene

as well as the Pacific Rim complex

in a previous

refraction

of underlying

felsic stocks and dikes cut

and in the Strait of Juan de Fuca lines of evidence

presence

m~lange

(Finn,

previous

1990;

chapters

is probably continental a

slab

on

top

Juan de Fuca plate

Off southern V a n c o u v e r

Island,

the

of

an

(Couch amount

276

of

underthrusting

is

minimal,

d e p r e s s i o n blocks of continental

Off northern V a n c o u v e r

Island,

crust

by

are

separated

Further north, Alaska,

noted

faults

off

in

this

Queen

embayment

off

Revere-Dellwood

continental

Charlotte

and

Vancouver

is disrupted

To the northwest

Island,

the The

latter of

embayment NW

the slope

more clearly expressed.

canyons

comparison,

the slope in the embayment

embayment

inspection, are

in the southeastern in

the Brooks embayment.

better

however,

distinctly

The

defined

many

N-S,

(Fig.

28) is less

alignment

to only 35-40

is more regular

trends

and

embayment.

more isolated.

bathymetric

By

trends

in

the

Most N-S trends occur and

most

The NE trends

zone and the Estevan

WNW

the

appears chaotic.

NE and WNW.

part.

of

Off southern Vancouver

and

part of the embayment,

the northwestern fracture

area is

continental

it is about 60 km wide and gentler than in the

On closer

zone were

Brooks-Estevan

and the slope narrows

and southeast,

are

Local

margins.

The

Transverse

trends

crust.

of block interlocking

regular than to the north and south.

its NW orientation

in

blocks at continental

in the Brooks-Estevan

km.

southeastern

for interlocking

The

slope

and

structural

Island.

expression

continental

(Chapter 5).

oceanic

and

Geomorphological bathymetry

oceanic

fault

Islands

continuous

Sound

proposed here as the tectonotype and oceanic crustal

and

the outer strand of the plate-boundary

otherwise

central

crust are juxtaposed.

blocks of continental

the

in

zone separate

disruptions

and oceanic

off the Queen Charlotte

other

structural

and on the inner side of the Juno

fault,

NE

and

WNW

are aligned with which

bound

the

are local and may be ascribed to the

277

continuity N-S

into this area of the OWSZ.

trends,

Most interesting

which are aligned with magnetic

that c o n t i n u e

into the embayment

are

the

and gravity anomalies

from oceanic regions.

Magnetic and gravity expression of block interlocking The N-S

alignment

of

magnetic

stripes

Pacific Ocean has been recognized Mason

(1961).

abruptly magnetic crust

the

Revere-Dellwood

zone marks foundered

(Fig.

27).

Winona Basin overlies

In contrast, the

Juno

fault, of

inboard

attenuated

of the Brooks-Estevan

stripes persist

of

oceanic

crust

with the Juan de Fuca plate.

is downdropped

but magnetic

from gravity maps:

stripes

because

low-density

sediments,

to -50 mGal

(Fig.

faults

the

of

downdropped.

a

53).

it

areas

is not expressed

Its main elongation

as

structural

is

around

in

show it km

of

clear.

chapter and

NW,

largely

covered

by

low of -40

parallel

zone along which

However,

longitude

Island.

4

end of the Juno depression

fault zone.

clearly:

low

some

it is associated with a free-air

The southeastern Nitinat

by

into

was

Seismic profiles

downdropped

the

crust.

in the previous

is

blank

embayment,

originally

are nevertheless

plate-boundary

the NE-trending

as

which

stop

continental

from outboard

along steep faults and covered

The Juno depression was delineated

shelf.

and

stripes

block on the lower slope off southern Vancouver

continuity

anomalies

northeastern

these

such a block of continental

N-S magnetic

This block is made

sediments,

Island,

blocks

Northwest

the

since the early work of Raff and

Off northern Vancouver

at

over

to

it is

lies

on

its n o r t h w e s t e r n

end

127~W,

many

gravity

-50 mGal run N-S as far as the edge of the

The Juno low ends in that broad zone of N-S anomalies.

278

Magnetic anomalies in the southeastern part of the embayment

also

trend

regions (Fig. 41). anomaly

around

N-S,

continuing from the outboard oceanic

Particularly

127°40'W.

Brooks-Estevan

well

Despite

expressed

is

the

linear

local breaks, it reaches the

latitude of 49~40'N.

This relative high-low pair has amplitudes between >+200

nT.

Continuity

nT

and

of magnetic stripes from oceanic areas is

usually thought to indicate Washington,