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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
Cataloging-m-Publication data applied for
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, <}. "!
t't
OLYMPIC M OUNTAI N S
".\..-...
.....
,,
WILLAPA ~f
:'. "m !
-8 )>
ft 'd WASHINGTON
"I]
g
I ,O1 ~' ASTOR,,, / ~'
~ m~- P <
I~
t ".,r: ....
I Z
.
0 U
........, + ..... i
0 C) rn
I (D
>
(
z iI
/
r"
i $
'---b,
~g;. 4<..I+.>... 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. ¢, <
,
I~
,,^,,c.e~
Z ~
....
e e = .
FM.
Volcanic and Sedimentary Strata
/ SKONUN % ~/
NORTHERN VANCOUVER ISLAND
FM. ++++++
~ 9=_.+_e_+_+_~ " _eO Unnamed Volcanic/ ~- ~ Sedimentary Strata ~I/I/7/7II////7I/////////f//////W//I///I~
++++++ ++++++
o_
io_
CO
O
NANAIMO GROUP
L
~,oc
O_
o~
©
LU O -< I-ILl
QUEEN CHARLOTTE GROUP
~:
LONGARM FORMATION
Unnamed Upper $ Jurassic-L0wer o_ Cretaceous Strata __-"~/~//'~E~//~/727/~/Z"~ / / ~ 2 ~ + + + + + +
O ,!U
03 ~
QUEEN CHARLOTTE GROUP equivalents LONGARM FORMATION equivalents
and
O J
i
"." c0
KYUQUOT GROUP
MORESBY GP,
P++++++ P++++++
~++++++
~///////////////////~//////////////sll//~ ~-~ $
MAUDE GROUP
_~o ~ ~
KUNGA GROUP
r n 09 ~3-
O. t%
D
KARMUTSEN FM.
= ~--
t HARBLEDOWN FM.
o
PARSON BAY FM. QLAT$1NO FM. KARMUTSEN FM.
=ii <
I--
d
:~='~"• U n n a m e d
=~ ~_ o. < o LUZ~
Carbonate-
Chert Unit; Volcanics
Carbonates and Volcanics
Table 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 I n s u l a r Belt: the Queen Charlotte Islands and northern Vancouver Island (based o n T h o m p s o n et al., 1991; L e w i s et al., 1991a; L y a t s k y , 1993a).
40
fundamental changes in lithofacies, on V a n c o u v e r Island. Lower
Jurassic
sedimentary.
rocks
on
Abundance
is
largely
increases
volcanic
1976; Andrew and Godwin,
marine
basin
are
upsection,
1989a; Desrochers,
quiescent
This region
differences
which
Island (Jeletzky, attest
to
broad
the
and
the
Bonanza
sedimentation
took
place
in
a
covered not only much of the Insular the
northwestern
and shallow basin lay in a tectonically
(Tipper
between
partly
1989).
Belt but also extended far into the interior of Cordillera.
only
(Muller et al., 1974, 1981; Jeletzky,
Late Triassic to Early Jurassic single
Island
of volcanic flows, tuffs and breccias of
intermediate composition Group
Vancouver
However,
and
Richards,
Queen
Charlotte
1976).
Stratigraphic
Islands and Vancouver
1976; Muller, 1977a; Cameron and
Tipper,
1985)
migration of Early and Middle Jurassic tectonism along
the continental margin.
The Middle Jurassic Bajocian Yakoun Group on the Islands
is
made
up
of
volcanic
Queen
Charlotte
and associated clastic rocks.
Unconformities separate it from from underlying older units. unconformably
overlying
Middle Jurassic volcanic-derived clastic
rocks of the Moresby Group and Upper strata meters
are
poorly preserved.
Jurassic
siliceous
clastic
Their thickness is several hundred
(Cameron and Tipper, 1985; Thompson et al., 1991).
These rocks do not extend on Muller,
The
1977a),
where
Vancouver
tectonic
Island
movements
(Jeletzky,
1976;
and the accompanying
erosion were apparently stronger.
Cretaceous rocks, in contrast, are widespread across
the
Insular
41
Belt.
Their
is about Albian
cumulative
3000 m. to
The
Lower
Maastrichtian
fine clastic rocks, subarkoses
and
(Yagishita,
rocks
on
Haggart,
1981; Jeletzky,
The area of sedimentation the
Early
Jurassic.
from the present-day Island
(Lyatsky
when
the
Honna
Longarm
1991).
1976; Nixon et al.,
Queen Charlotte Haggart,
conglomerate
laid
Belt apparently continental paleogeographic tectonic
the
area
Formation
Cretaceous fragments
orogenic
belt
was
1985).
1993).
of
the
Mesozoic
Continuity
of this basin suggest
that
a
and similar
regime existed all along this belt.
Only in Late Cretaceous (regarded
thereafter
and volcanic
development
(Haggart,
uniformity
but
basin evolved along much of the Insular
in response to the margin
in
Coniacian,
resumed.
Andean-type
(Yagishita,
than
generally quiescent
down,
rocks contain abundant granitic
long-lived Cretaceous
was narrower
The
sediments
an
1995).
in the Turonian and
was
(Muller
Islands to northern V a n c o u v e r
sedimentary
The
finer-grained
at that time
sedimentary
covering the Insular Belt
1993).
were disturbed
and
including
Island are less well studied
of
developing
locally
Equivalent
in the Cretaceous
from the east, where
Formation
are minor and occur only in
deposition
derived
Islands
Group comprise coarse to
wackes,
It was elongated,
and
tectonic conditions
lithic Volcanics
northern Vancouver
1974,
Cretaceous
primarily
1985;
on the Queen Charlotte
Queen Charlotte
arenites.
places
et al.,
thickness
sometimes from
time did separate
as sub-basins
eastern
of the Georgia
Vancouver
of these and other
Nanaimo
small
Island
to
and Comox basins Basin)
develop
in
the Gulf Islands.
intracontinental
depressions
42
was followed by local marine and submarine 1989)
fans,
suggests
but presence
that non-marine
to 4000 m of conglomerate, with
coal,
Hiscott,
1984). only
Rocks correlative in local grabens
Charlotte
of local
sourcing
on the mainland Abrupt
Paleozoic Several
in
on northern Vancouver
horsts
and
1991; from
England
and
surrounding
depressions are
Island,
(Pacht,
represented and the Queen
grabens
is
suggested
by
the
Island to the west.
variations,
lithofacies
reflectance
changes,
are
geologic
and variations
consistent
in actively subsiding
stages of p r e - T e r t i a r y
with
an
grabens.
evolution
interval of t e c t o n i s m occurred before the
of Upper Triassic Karmutsen Alaska
are
considerably 1980a;
1989; Gehrels,
and
on
more
complex
Woodsworth
than
pronounced
basalts.
Vancouver
metamorphosed.
In many localities
Island,
Structures structures
and Orchard,
Paleozoic
in in
1985; Massey
them
younger and
and are
rocks Friday,
1990).
Late Triassic to Early Jurassic The
intercalated
to the east and from V a n c o u v e r
of these basins
rocks
(Muller,
Up
from the Coast Belt
southeastern
older
derived
in fault-bounded
poorly resolved episodes
extrusion
mudstone,
(Mustard,
Bickford,
also existed.
with the Nanaimo Group
in coal rank and vitrinite
Tectonic
and
of Nanaimo Group sediments
thickness
evolution
Group
of deltas
Islands.
Development proximal
and
sediments
areas were deposited
(Kenyon
paleoenvironments
sandstone
Clastic
and construction
of coals
make up the Nanaimo
1992).
uplifted
incursions
interval
angular u n c o n f o r m i t y
at the base of the Karmutsen
43
Formation suggests that an episode of before
the
eruption
of
Upper
strong
Triassic
tectonism
basalts.
occurred
However, the
Karmutsen and especially the conformably overlying Upper to
Lower
Jurassic
formations
relative tectonic quiescence. time
in
much
Richards,
of
the
accumulated
under
Triassic
conditions of
A quiescent regime existed at
western
Canadian
1976; Cameron and Tipper,
Cordillera
that
(Tipper and
1985; Miller et al., 1992).
Mid-Jurassic episode of t e c t o n i s m Regional magmatism, metamorphism and deformation occurred Jurassic.
However,
its
region.
Vancouver Island
(Muller
et
al.,
onset was
1974,
was
the
not simultaneous across the
affected
1981;
in
in
Jeletzky,
the
Early
1976),
but
Jurassic the Queen
Charlotte Islands were affected only later, in the Middle Jurassic (Cameron and Tipper,
Voluminous Belt.
On
1985; Thompson et al., 1991).
magmatism Vancouver
granodioritic
occurred Island,
Island
at it
that is
Intrusions
time across the Insular
represented and
comagmatic
extrusive rocks of the Bonanza Group (Armstrong, Godwin, Island
1989a;
Andrew
Intrusions
(Archibald
and
are
Nixon,
et al., 1991). scattered 1995).
(Jeletzky, 1976) and magnetic data 1988),
many
granitoid
plutons
widespread intermediate
1988; Andrew
and
Ar/Ar cooling ages of the
widely From
by
around
170-175
geological
(Arkani-Hamed
Ma
field evidence and
Strangway,
probably merge at shallow depths
into broad batholiths.
Similar Jurassic magmatism
Charlotte
represented by the Bathonian-Oxfordian San
Islands
is
Christoval and Burnaby composition,
and
by
Island
plutonic
comagmatic
(Anderson and Reichenbach,
suites
on
of
the
Queen
intermediate
Bajocian Yakoun Group volcanics
1991; Woodsworth et al., 1991).
44
No strong regional episode.
In
metamorphism
most
accompanied
the Jurassic
parts of the Insular Belt,
Mesozoic
rocks are m e t a m o r p h o s e d
usually
no higher than prehnite-pumpellyite.
is a belt of high-grade western
Vancouver
15-30 km across, the
In
Triassic-Early
Jurassic
grade
around
boundary.
been recognized
A notable
the
Along the western Island,
uplift
near
equivalents
1987). surface
The
which
Islands,
southern
the
form a fault-bounded
though geophysical
In this belt,
a series
from mid-crustal
of
al.,
folds and thrust
1991a).
south.
The
diminishes
were
folding
compression
faults
Island,
correspondingly:
of
the
a
Vancouver
though prominent
of
Isachsen, to
the
thrusting was much Islands,
to 50% shortening
et al.,
declines
Middle
exposed
series
brought
accommodated
(Thompson
The amount of shortening significance
of
1977a;
Charlotte led
just
deep faults.
and
On northern Queen
Bajocian
in
(Muller,
blocks
Graham Island and northern Moresby trending
complex
depths along steep,
mid-Jurassic
greater to the north. Jurassic
of
complex
chapters).
periphery
belt
to
Early-Middle
in the Early Tertiary
rocks of the Westcoast
intensity
Middle
partly
just
of this complex have
(see following
and rapid unroofing
the metamorphic blocks
and
belt,
of
and older rocks native to
i.e.
on the Queen Charlotte
islands offshore
exception
of 15-25 km and m e t a m o r p h o s e d
Ma,
No high-grade
grades
length
narrow Westcoast
Jurassic
180
entire
data suggest they may lie on trend with the Westcoast west of these
and older
not at all or to very low
island were buried to depths
amphibolite
Jurassic
rocks along almost the
Island.
tectonic
by
the on
NE-SW-
1991; Lewis et
rapidly
to
the
Jurassic u n c o n f o r m i t y on Graham Island
and
45
northern Island and
Moresby
(Lewis,
exact
al.,
1991).
age
Cretaceous
Island,
it is less apparent
On northern Vancouver
of Jurassic
tectonic compression
Vertical
block
movements
large enough to bring
protoliths
complex to mid-crustal
levels.
Late Jurassic
to Late Cretaceous
block
Insular Belt Islands,
the
1991;
block
since that time
faults
grain which has
The Coast
sediments
(¥agishita,
1985;
strandlines
of
the
Queen
throughout
for
offsets
basin
1991,
Charlotte
influenced Thompson
et
on a number of
are more than spread
i000
from
m.
local
into the Cretaceous.
NNW trend.
since
that
The
new
of a regional time.
The
in the Insular Belt follows the
rising
1991).
parallel
to
it,
provided
Tracing
the
mainland
Cretaceous
shows that they m i g r a t e d very slowly
1993). Islands
the Cretaceous.
of the
Charlotte
this basin from the present-day
Haggart,
this
(Haggart,
Belt,
Queen
the establishment
predominated basin
most
1991a,b;
faults have a predominant
sedimentary
Island were
movements
sedimentation
structural
eastward
the
block
in the Late Jurassic
of this trend reflected
large Cretaceous
et
metamorphic
across
On
The reported
prominence
same trend.
dominant
into broader areas and continued
Major Late Jurassic
(Nixon
but
interval
and
ceased,
amount
are unclear,
Westcoast
(Lewis et alo,
1991).
movements
depocenters
abundant
the
on Vancouver
the
Jurassic.
faulting
Haggart,
of
became
Late
high-angle
large dip-slip As
movements
in
sedimentation al.,
Island,
rocks overlie the older units unconformably
1995).
Vertical
on southern Moresby
However, and
they mostly remained
northern
Vancouver
on
Island
46
A markedly
different
Vancouver
Island
tectonic
and
the neighboring
i00 and 84 Ma, tectonic orogens
on
the
compression,
affected
Northwest
Cascade
(Brandon et al.,
thrust
1988).
a series of grabens
system
southernmost
San Juan Islands.
Juan
was
In contrast,
created
Between of
where
Turonian
(Pacht,
to the
during that time
and eastern Vancouver
Group
continental-crustal
Islands,
Late Cretaceous
filled with thick
of the Nanaimo
evolution
on
caused by the development
the area of the San
t e c t o n i s m on the Gulf Islands
Thus,
existed
site of the Coast and North Cascade mountains
the east,
sediments
regime
to
extensional
Island created Maastrichtian
1984).
tectonics
dominated
the
geologic
of the region during most of the Mesozoic.
Latest Cretaceous(?)
to earliest Tertiary
tectonism
The next significant
episode of tectonism
in the Insular Belt took
place
in the latest Cretaceous(?)
older high-angle
faults were rejuvenated,
graben movements. by the regional (Woodsworth
That these
association
et al.,
of Cretaceous
on
Queen
shortening Similar
in
Vancouver
Island
In contrast,
the
of Tertiary
igneous rocks with
as well
Late
indicate
et al., as
on
the
1991;
local
Cretaceous
(Nixon et al.,
compression
rocks on reactivated
Islands
(Thompson
compression,
occurred
horst-and-
faults are mostly steep is confirmed
sedimentary
Charlotte
of SI0%
controlling
Many
faults
1991).
Offsets the
and/or earliest Tertiary.
earliest
Lewis et al.,
right-lateral
old faults Tertiary 1991a,b). movements,
to early Tertiary on northern
1995).
Gulf
Islands
and
southeastern
47
Vancouver (England
Island and
sedimentary
created the west-vergent Cowichan thrust system
Calon,
1991)
whose
east-dipping
rocks of the Nanaimo Group.
system does not extend
on
Vancouver
faults
cut
This west-vergent thrust
Island
past
the
Alberni-
Cowichan Lake system of high-angle faults, which divides Vancouver Island into the eastern and western blocks.
Timing of terrane accretion in the western Cordillera The Cordilleran orogenic system evolved under a internal
and
external
factors,
the
interactions at the continental margin.
twin
latter
control
including
The relationship
of
plate between
the internal and external factors, however, remains unclear (Cowan and Bruhn, 1992).
Convergence of oceanic plates with North America the
Mesozoic
during
role
of
"exotic"
terranes
thought
to
Less
clear
et al., 1982; Saleeby and Busby-Spera,
1992).
Wrangellia.
1977;
Most of the
Insular Belt in these models is occupied by the Wrangell or
is
be accreted to the
western edge of the North American continent (Jones et al., Monger
of
might have produced subduction-related magmatic-arc
formations on the site of the western Cordillera. the
much
terrane,
Its current definition is stratigraphic, based on
the presence of Karmutsen basalts and
overlying
Upper
Triassic-
Lower Jurassic limestone and clastic rocks, underlain by Paleozoic units (Monger, 1991). fragments
have
In
this
definition,
Wrangellia
or
its
been identified in southeastern Alaska, the Queen
Charlotte Islands and Vancouver
Island.
Presence
of
Karmutsen
basalts on a small island near the mainland shore of Hecate Strait (Woodsworth,
1988) indicates that
Wrangellia
probably
beneath most of the British Columbia interior shelf.
continues
48
Most
terranes
regarded as travel
recognized
"suspect",
paths
remain
because unclear
particularly true of the Alexander
-
Wrangellia Alexander
presumed was
terrane,
The
rock
evolution
stitches
origins
and Bruhn, 1992).
largest
terranes
-
fundamentally
This is
Wrangell
exist in the Insular Belt. as
and
and
Initially,
distinct
from
the
(Gehrels and Saleeby,
1987b; Brew
et
al.,
assemblage typical of the Alexander terrane is variable, is
Gehrels and Saleeby, Alaska
boundaries,
which is recognized as dominant in many parts
complex and laterally tectonic
to
their (Cowan
two
regarded
of southeastern Alaska 1991).
in the western Cordillera are commonly
sketchy
1987a).
the
and
the
understanding
(Woodsworth
and
Orchard,
A Pennsylvanian granitic
formed
a
its 1985;
pluton
A l e x a n d e r and Wrangell terranes,
that at least since that time they have
of
in
indicating
single
entity
(Gardner et al., 1988).
The docking of Wrangellia to the North American continent was once thought to have taken place
as
recently
as
in
the
Cretaceous
(Monger et al., 1982), but this contention has been questioned all along (Brew and Ford, 1983; van der Heyden, boundary uncertain.
of
the
1992).
The
eastern
composite A l e x a n d e r - W r a n g e l l i a terrane is still
Its suture with
crustal
blocks
in
the
Cordilleran
interior was previously thought to lie in the Coast Belt, where it is masked by vast plutons studies,
however,
a
(e.g., Crawford et al., 1987). single
From new
Alexander-Wrangellia-Stikinia
"megaterrane" docked with North America in
the
Middle
Jurassic,
causing regional compression and m a g m a t i s m (van der Heyden,
For
the
late
Mesozoic
and
Cenozoic,
at
any
1992).
rate, magmatic,
49
evidence
metamorphic and structural accretion
of
offers
no
indication
that
blocks of exotic origin took place in
lithospheric
this region.
Place of the Coast
Belt
orogen
in
the
tectonic
evolution
of
western Cordillera A
major
formative
Cordillera tectonotype
was
the
in
event Late
in
the
history of the North American
Jurassic
Nevadan
orogeny.
the Sierra Nevada in California,
To
its
it has a complex
tectonic expression including regional contractional faulting.
In
folding
and
the west, at the continental margin, subduction of
oceanic lithosphere at that time and later resulted in emplacement of ophiolitic and magmatic-arc sequences.
In particular, the huge
ophiolitic Franciscan complex, made up of mostly Cretaceous
rocks
with oceanic-crustal affinities, was emplaced at California's late Mesozoic continental margin
(Saleeby and Busby-Spera,
1992;
Cowan
and Bruhn, 1992).
North
of
the
Klamath
Mountains
block on the California-Oregon
state boundary, the youngest Mesozoic Middle
to
Late
Jurassic.
Cascade fold-and-thrust mafic
and
belt
ophiolitic-type
are
The Fidalgo complex in the Northwest contains
an
assemblage
including
ultramafic rocks but also quartz diorite and tonalite,
whose presence complicates the interpretation of this an
rocks
ophiolite°
complex
as
These rocks lie in a thrust slice on the San Juan
Islands (Whetten et al., 1980; Brandon et al., 1988).
Early
Jurassic
regionally.
It
magmatism,
in
contrast,
manifested
itself
affected large parts of both western and central
Canadian Cordillera, and intermediate-composition plutons of
that
50
age
are
found
from
the
Intermontane
to
the
Insular
Belt
Late Early to early Middle Jurassic tectonism in the Insular
Belt
(Woodsworth et al., 1991).
in
British
Columbia,
according
to Thompson et al.
into a coherent eastward-younging trend.
From
Late
(1991), fits Jurassic
to
early Tertiary time, emplacement, cooling and uplift of plutons on the western mainland also shifted gradually 1982; van der Heyden,
In
the
eastward
(Hutchison,
1992).
Cretaceous, the narrow Coast Belt orogen was superimposed
on the previous grain of the northwestern Cordillera.
This
event
marked the final partitioning of the once-single geologic province into
three
contrasting
Intermontane,
The
major
tectonic
belts
observed
Coast and Insular.
orogeny
which
created
the
Coast Belt as a distinct
tectonic entity took place in the mid-Cretaceous. crustal
today:
contraction
verging thrust belts
developed,
A new
zone
of
flanked on both sides by outward-
(Rusmore and Woodsworth,
1991,
1994).
The
Coast Belt orogen stretches from the North Cascades in Washington, through the British Columbia mainland, to
Alaska
(Brew
et
al.,
1991; Gabrielse et al., 1991; Brown et al., 1994).
Large
mid-Cretaceous to early Tertiary granodioritic plutons make
up 80% of the exposed Coast Belt rock 1987;
Monger,
1991).
volume
(Crawford
1993).
to
the
point
al.,
Vigorous m a g m a t i s m took place mostly after
the mid-Cretaceous compressional orogenesis, which crust
et
of
thickened
the
melting at its lower levels (Hollister,
Buoyant uplift and unroofing of deep crustal
horizons
in
51
the
Coast
(Parrish,
Belt 1983;
occurred during the Late Cretaceous van der
depressions
developed
which
were
they
sedimentary
Heyden,
formed
front of the mountainous basins;
Monger,
1991;
Local
uncommon
unusual
Mesozoic
orogen
complex of
It lies in a narrow - a few of
(Muller,
1977a).
different
(the Georgia
Barkley
from
detritus.
from Large
and Tertiary time in and
Queen
Charlotte
1993a).
on
the
(including
western
and
southern
Pandora Peak unit~
volcanic
and
has been d i s t i n g u i s h e d
northwest
compensatory
Island
assemblage
age
abundant
in Late Cretaceous
complexes
periphery of V a n c o u v e r
An
with
Lyatsky,
rock
Pacific Rim m~lange
Elongated
along the flanks of the rising orogen,
supplied
basins
1992).
and Tertiary
These rocks,
rocks
on western Vancouver
kilometers
Sound,
sedimentary
across
-
of
Island.
coastal
strip
near the towns of Ucluet and Tofino denoted Pacific
Rim
complex,
are
rocks on the rest of the island and are separated
from them by a large steep fault.
From recent detailed geologic mapping, the
Pacific
comprises and
Rim
unstratified
chert.
Fossils
Carnian-early Jurassic.
complex
Norian
Formation,
andesitic
composition.
Next
upsection
in
but u n m e t a m o r p h o s e d
in
limestone
the and this
those unit
in are
A
divided
lower
unit
limestone
point to a Late Triassic
the Pacific Rim complex rocks.
The
subordinate
the
chert
are
Early
partly coeval with the
but unlike the Karmutsen
clastic
(1989a,b)
units.
rocks with
of
Karmutsen
three
volcanic
age,
Volcanics
into
Brandon
basalts,
they
have
is a unit of deformed
mudstone-sandstone
matrix
52
hosts discrete blocks of p i l l o w rarely,
of ultramafic
Radiolaria
basalt
material.
-
from the chert give Late Jurassic
mudstone,
2000 m thick.
sandstone,
Blocks derived
ribbon
Some blocks
The top unit is Lower Cretaceous rocks
with
in age.
and,
are as big as 300 m. ages.
It contains
conglomerate
from the
chert
sedimentary
and chert - more than
lower
units
also
occur.
This unit is contorted but still partly coherent.
Rocks
similar
in isolated where
they
1985).
to those
localities
of
Upper
were m e t a m o r p h o s e d in
near the southern
have been named Pandora
It comprises
greenstone
in the Pacific Rim complex
black mudstone,
to lawsonite
the Pandora
tip of Vancouver
age.
tuff
and
These rocks
99 and 83
Ma,
i.e.
Like the Pacific Rim slices;
small
fault system on the southern
Island.
Close lithological Pandora
time.
chert,
Peak unit lies in fault-bounded
thrust splays also occur in a complex
have
Cretaceous
grade between
Island,
(Rusmore and Cowan,
greywacke,
late Albian to S a n t o n i a n - C a m p a n i a n
complex,
tip of Vancouver
Peak unit
Jurassic-Lower
are also found
similarities
of the Pacific Rim complex
and the
Peak unit with the Fidalgo complex on the San Juan Islands
been
1989a,b).
noted The
previously
(Brandon
Pacific Rim complex
to be pieces of the Northwest
displaced
from their original
(Brandon,
1989a).
al.,
1988;
Brandon,
and the Pandora Peak unit are
considered
strike-slip movements
et
Cascade
thrust
sheets,
position on the San Juan Islands by
in Late Cretaceous
or
early
Tertiary
time
53
Leech River metamorphic complex In fault contact with the Pandora Peak unit on the southern tip of Vancouver Island lies another unusual local rock assemblage Leech
River
complex.
mapped by Muller Cowan
(1982)
Once
known as Leech River Schist, it was
(1977a) and later re-examined
and
Rusmore
age.
by
and Cowan (1985).
mostly sandstone and basalt, suspected to in
the
Fairchild
and
Its protoliths are
be
Jurassic-Cretaceous
From available descriptions, they seem to be similar to
rocks widespread elsewhere on Vancouver Island.
These rocks were buried to mid-crustal depths K/Ar
dates,
by
41-39
to
At the same time, they
were
intruded
by
Metamorphic foliation in the Leech River complex is
parallel to the Mountain
according
Ma reached the greenschist to amphibolite
grade of metamorphism. felsic sills.
and,
faults
closely (see
spaced
bounding
San
Juan
also Mayrand et al., 1987).
and
Survey
This suggests
that metamorphism was probably synkinematic.
This metamorphic complex is sandwiched between the steep San Juan, Survey
Mountain
and
Leech
River
faults.
The first two faults
separate it from Wrangellian rocks, and locally from Peak
the
Pandora
unit, to north; and the Leech River fault juxtaposes it with
the Tertiary Metchosin igneous complex to probable
origin
the
south.
The
most
of the Leech River complex is by m e t a m o r p h i s m of
protoliths of Wrangellian affinity, displaced and uplifted the South Vancouver Island fault system.
within
CHAPTER4
- TERTIARY STRATIGRAPHIC FRAMEWORK
OF COASTAL PROVINCES ~
WASHIq~GTONAND BRITISH COLUMBIA
Early Tertiary paleonvironments Early Paleogene By the end of the Mesozoic, continental crust was underlying of
the
continental
marginal
region
from
Oregon
to
most
Alaska.
Prominent landmasses existed on the sites of the Klamath Mountains in northern California and southern Oregon and of Vancouver Island and perhaps the Queen Charlotte Islands in British Columbia. position Klamath
of
the
early
Mountains
sedimentological
and
Tertiary
submarine
Vancouver
evidence
Island
indicates
margin
is
that
less
during
Tertiary, on the site of the Olympic Peninsula lay a
The
between the clear, most
but
of
large
the deep-
marine embayment (Heller et al., 1992; Niem et al., 1992a-c).
With
the
possible exception of the Olympic Peninsula (Babcock et
al., 1992, 1994), Paleocene sedimentary rocks are virtually absent between
western
Oregon
and
western
British
Columbia.
suggests that broad regional uplift occurred at that time westernmost Cordillera
This in
the
(Miller et al., 1992).
Partly because much of the older Pacific oceanic crust bearing the magnetic-stripe record has been subducted, are
unclear
interactions
about
the
(Riddihough,
Paleogeographic
details 1982a;
reconstructions
of
local
Stock based
current
and on
plate
history
of
Molnar,
geologic
plate 1988).
mapping are
hampered by the scarcity of Paleocene sedimentary rocks. to
models
The
key
interpretation therefore lies in studies of the volcanic rocks
of early Tertiary age, which are widespread across the region.
55
Early Tertiary basaltic m a g m a t i s m Manifestations and
of Eocene mafic m a g m a t i s m
Washington,
distinguished
where
(Duncan
large
Coast Range igneous province
and
Kulm,
1989).
is still controversial.
oceanic
crust
(MacLeod
them
North America. et
al.
Oregon
1987;
Best-studied uplifted
Babcock
is
crustal
represented southern
lenses
diachronous
these
Basalts of the
1992,
Vancouver
western
Island,
(Muller,
and
ages of eruption between
(Babcock et al.,
as
of
as Eocene (1982)
and
Babcock
products
of
Formation
in
are generally
volcanism
which is exposed
Washington.
where
1977a-c).
radiometric
these
(cp.
1994).
Formation,
in
(1992)
River
style
is
accreted to western
in Washington
and
of
Duncan
basalts
Siletz
Formation
composition
blocks
and
of seamounts
viewed
the Early and Middle Eocene. found
1977),
Brandon and Vance
the Crescent
Formation
carbonate
series
et al.,
nature
on the rim of the Olympic Mountains,
tip of
Metchosin
in
The
They were once treated
al.,
of the Crescent
coeval but differ Snavely,
a
1994)
rifting.
and
et
In contrast,
(1992,
continental
as
Oregon
a
basalts
interpreted
abound in western
dates
it
is
well
as well as on the is
known
Sparse
as
fossils
the from
basalts
suggest
about 57 and 45 Ma,
spanning
No regional
from
It
in many
age progression
has been
1994).
Crescent
basalts
are
not
geochemically
subaerial
flows have been mapped,
facies
substitutions.
These
centers
(Babcock et al.,
1994).
Submarine
and
with many lateral variations
and
basalts
uniform.
erupted from many discrete
58
Separate
basalt bodies are exposed
Willapa
Hills
Dosewallips Puget
south of the Olympic
massif,
Sound.
Fuca.
Several
From
includes
the
topographic
Peninsula.
highs
narrow basaltic
the
Port
Peninsula,
northwest Ludlow,
bodies
parallel
shore
Marmot
the
is the
Mountains
and
form a W N W - t r e n d i n g
to the Strait of Juan
of Puget Sound,
Pass,
in
Much bigger
which lies between the Olympic
belt on northern Olympic de
in
Hurricane
this belt Ridge
and
Crescent Lake massifs.
The
Metchosin
massif
(Muller,
parallel
to the Hurricane
on
north
the
exposed
metamorphosed,
known
as
is
mostly
grade. Sooke
upsection,
Ridge and Crescent
to
m.
Metchosin
the
of
this
and
abundance
in a feeder system.
1986)
(Fig.
5).
massif
by
small
are
plutons
of dikes decreases
The plutons
are mainly
10% to 30% quartz
(Muller,
Hurricane
1977c).
Ridge and
Crescent
Lake
south side of the Strait of Juan de Fuca and the on
the
north
similarities.
side
Continuity
are
indicated
by
anomalies
and by gravity modeling
(MacLeod
et al.,
1977)
their
of basalt under the Strait
from
examination
of
magnetic (Dehler
1992).
In the scope of the complex
Its
and in places to
of Juan de Fuca has been suggested
and Clowes,
is
but some of these stocks contain quartz diorite
massif
compositional
Rocks
The
Links between the Marmot Pass, on
Massey,
Lake massifs but lies
prehnite-pumpellyite
Intrusions.
as expected
and tonalite with
7000
They are cut by dikes
made up of gabbro,
massifs
1980b;
shore of the Strait of Juan de Fuca
thickness
greenschist
1977a-c,
prevailing
has been interpreted
tectonic
models,
the
Metchosin
as a piece of obducted oceanic crust
57
A
~
A~
B
B'
C
~'
METCHOSIN VOLCANICS: amygdaloldai lavas
SOOKE BAY FORMATION conglomerote, sondstone SOOKE INTRUSIONS:metagobbro, quortz diorite, aplite
METCHOSIN VOLCANICS2 pillow lava, breccia, tuff
SOOKE INTRUSIONS:gabbro SOOKE INTRUSIONS:basalticdykes
C
[
~
METCHOSINVOLCANICS: amphibolite, chlorite schist
00~ ""
,~. ~ m
•
T.~,
-'4.'~,..
1
" " "-. • •
'..
0 I ~
Albe~,
15 km 'l
~
,~ EEMOYK PASSAGE
o
Figure 5. Geologic map of the Eocene Metchosin igneous massif on the southern tip of Vancouver Island (modified from Muller, 1977c). The massif is broken into blocks bounded by steep faults (broken wavy lines). The Leech River fault consists of two straight, steep segments meeting at an angle of about 25 ° . Such a structural configuration is inconsistent with emplacement of the Metchosin massif along a Leech River "thrust", as suggested in some tectonic models. The Metchosin massif is one of many massifs in the Eocene Crescent Formation, whose basalts erupted from many discrete centers in a rift setting (Babcock et al., 1992, 1994). The Metchosin massif lies within the Olympic-Wallowa structural zone (OWSZ).
58
(Massey, 1986). No
This contradicts several lines of field evidence.
ophiolite
characteristics
are
observed
in
complex, and the interpretation of feeder roots as is
tenuous.
Subaerial
Metchosin massif crust.
volcanics
(Muller, 1977c),
3000 are
m
not
the
Metchosin
sheeted
dikes
thick, mapped in the expected
in
oceanic
Presence of felsic rocks is incompatible with an oceanic-
crust
interpretation.
oriented
folds
and
The steep
fault-block
mosaic
with
variously
faults mapped in the Metchosin massif
(Fig. 5) is not expected in a unit accreted compressionally.
The reported radiometric ages of Metchosin rocks vary depending on the
method:
57.8±0.8
Ma
Babcock et al., 1994). scattered
around
by Ar/Ar,
52 Ma by U/Pb (Duncan, 1982;
K/Ar dates from the Sooke
45 Ma (Muller, 1977c).
Intrusions
The Hurricane Ridge and
Crescent Lake massifs can be correlated with the Metchosin temporally
and
geochemically,
though
are
the
Metchosin
massif
massif is
eroded to deeper levels due to greater uplift.
Relationship of Crescent Formation massifs with early Tertiary sedimentary sequences In the Deer Park area on northeastern Olympic of
the
Crescent
Peninsula,
basalts
Formation are underlain, with a hot contact, by
sedimentary rocks of the Blue Mountain unit (Tabor and Cady, 1978; Babcock
et
al., 1994).
This sedimentary unit consists of thinly
bedded mudstone and massive sandstone sourced from proximal such
as
areas
the Coast Mountains and the San Juan Islands (Babcock et
al., 1994).
The Blue Mountain unit is unfossiliferous. its
upper
part
is
Syn-volcanic
age
of
suggested by interfingering of Blue Mountain
59
sedimentary rocks with Crescent basalts through the entire section of
the
Hurricane Ridge and Dosewallips massifs, up to the Middle
Eocene.
The Middle Eocene Adwell Formation overlies Blue Mountain
and
Crescent
contact.
rocks
conformably,
Sedimentation
began
in
in
places
the
with a gradational
Paleocene
and
continued
simultaneously with Crescent volcanism (Babcock et al., 1994).
The
fact
that
the Crescent Formation has stratigraphic contacts
with older, coeval and younger sedimentary sequences idea
that
these
basalts
as
pieces
of
negates
the
oceanic crust. Layered
material revealed by seismic data beneath the Siletz River basalts in Oregon may also be sedimentary (Keach et al., 1989).
Structural
position
of
Crescent
basalts likewise suggests that
eruptions occurred in a continental setting. faults
in
western
Washington
is
The
anomalies
seems
to
of
steep
well expressed in gravity and
magnetic maps (Finn, 1990), as a rectangular fractures
grid
pattern
of
crustal
control the distribution of potential- field
(see also McCrumb et al., 1989a,b).
Stratigraphic record of mid-Eocene to Miocene sedimentary basins The most complete Tertiary stratigraphic section in the region
is
found on the south shore of the Strait of Juan de Fuca (see Fig. 6 for basin locations). the
Fuca
Basin,
There, a conformable sedimentary package of
about
8
km
thick,
mid-Eocene to the mid-Miocene
(Niem and
al.,
begins
1992a).
Formation of siltstone
The Middle
with
section Eocene
age.
Snavely,
with It
interbedded sandstone,
mud flows on a continental slope.
spans the period from the 1991;
Niem
et
the 900-m-thick Adwell
consists
of
thin-bedded
interpreted as products of
60
CAN40A'
E
0 L
MILES
120
(~
KILOMETERS
193
=EN C H A R L O T T E BASIN
QUEEN CHARLOTTE IS[ANOS
BRITISH COLUMBIA
W I N O N A BASJ
P~ c~ f ~ c
TOF
Figure 6. Location of principal Late Cretaceous and Tertiary sedimentary basins along the western Canada continental margin. The Tofino and Fuca basins are regarded together in much of the current literature, but the Winona Basin is distinct. The Nanaimo and Comox basins both lie in the Georgia depression.
61
The overlying thick, and
is
Lyre
made up of sandstone
breccia.
continental
The
early Late Eocene
Late
It
slope,
Eocene
is
Hoko
sandstone
to
have
and conglomerate
m thick.
with abundant
to thin-bedded
rich in phyllite
occasionally
The
pebble
The sandstone
Late Oligocene
conglomerate
a shelf and/or upper slope. to
up
of
shallow-water subsidence
The
sandy siltstone
sandstone marine
roughly coeval
is mostly
than
interbedded
turbiditic
and
Muller,
with conglomeratic Formation,
subordinate
on
comprises
m
deposits. thick,
is
abundant
indicate that
basin
rocks of the Carmanah Group lie on
1977a-c;
of Vancouver
Muller et al.,
Island
1981).
coast
where
reduced
to
Paleogene
in channels
channel 800
with
by sedimentation.
sedimentary
is
begins
conglomerate;
fauna and wood fragments
thickness
Pysht Formation It
on the south
erosion.
more
siltstone
just a few partial exposures their
is
probably deposited
Clallam
and
was compensated
1972;
with lesser
and basalt fragments.
environments.
the southern and western periphery al.,
a
- 1600 to
The rest of this formation
The overlying Lower Miocene made
on
slumping.
siltstone
to Early Miocene
in shallower-water
and boulder
mudstone
m
olistostromal.
1400-m-thick,
was deposited
600
deposited
Formation
It is made up of thin-bedded sandstone.
to
is much thicker
The Late Eocene to Late Oligocene Makah 2800
been
fans or by sediment
River Formation
2300 m - and contains massive
300
(lithic arenite and lithic wacke)
thought
in submarine
Formation,
sedimentary
due
(Tiffin
et
They occur in
of
several
the
island,
episodes
rocks are absent in this area,
of and
62
the
Miocene
to
shell-bearing
Pliocene
sandstone
Sooke
coast of Vancouver
Peninsula.
(Shouldice,
rocks
have
1971).
Oligocene contains
These
at
its
Hesquiat
are
base.
sandstone
in very shallow water
Sediments
drilled
on
Island,
entire continental
of
km
the
along
the
on Hesquiat of
similar
exterior
the Carmanah
shelf
Group
sandstone
is
with
into the Late Eocene to
which is some 1200 m thick and
also
and conglomerate.
overlain by the Sooke Formation. formations,
are
throught
it was laid down
1981).
Group were derived
and throughout
which
environment,
(Muller et al.,
margin
4
calcareous
grades
in a bathyal
of the Carmanah
of Vancouver
almost
It
and Hesquiat
have been deposited
outcrops
in it is the Late Eocene Escalante
unconformably
Unlike the Escalante to
and
of 150 m
Formation,
siltstone,
rocks
been
The oldest
which consists
conglomerate
west,
On Hesquiat Peninsula,
about 1400 m thick. Formation
in discontinuous
Island and is most complete
It dips to the
sedimentary
is made up of coarse,
and conglomerate.
The Carmanah Group is exposed western
Formation
from
the Cenozoic
in this area must
have
elevated
parts
the shelf and the been
close
to
their present position.
Stratigraphic Subsidence resumed Juan
record of late Tertiary
between
Vancouver
in the Pliocene,
Island
and
the Olympic Peninsula
when sedimentation
began in the Strait of
de Fuca and a new W N W - t r e n d i n g
Fuca Basin. narrower.
The new depocenter Changes
sedimentary basins
graben developed over the old
is parallel
in depocenter
to the old one,
but is
shape and location occurred
also
63
on the sites of other Olympic basins
Peninsula
old
Tertiary
(Snavely,
developed
in
southeastern Alaska
the
1987;
Neogene
sedimentary Niem on
(Bruns and Carlson,
on
the
et al., 1992a,c).
New
the
basins
exterior
shelf
off
1987).
On the interior shelf between the mainland and the Queen Charlotte Islands, the Queen Charlotte Basin is up to 6 km thick (Shouldice, 1971;
Rohr
and Dietrich,
1992; Lyatsky,
1993a).
On the islands,
Tertiary volcanic rocks as old as Paleocene crop out sporadically, and
Early
Eocene
to
Early
Oligocene
black
shale,
sandstone, conglomerate and coal are found in a few drillholes
(White,
1990).
They
outcrops
of
the
Tertiary normal
Queen
faulting
Charlotte also
and
seem to have been deposited in
fault-bounded structural depressions whose appearance beginning
mudstone,
marked
the
Basin (Lewis et al., 1991a).
occurred
on
northern
Vancouver
Island (Nixon et al., 1995).
Much
more
widespread
on the Queen Charlotte Islands is the Late
Oligocene to Early Pliocene Masset Formation, mafic
and
1991).
felsic
These
lava
volcanics,
which
consists
of
flows and pyroclastic deposits (Hickson, which
erupted
from
several
distinct
centers, are in places up to 3000 m thick.
The
Queen Charlotte Basin lies mostly beneath the interior shelf.
It consists principally of Neogene sedimentary rocks of the Skonun Formation, eastern
which and
is
up
northern
sedimentological paleoenvironments
to
6
km thick.
Graham
characteristics
Island,
In a few exposures on these
suggesting
(Sutherland Brown, 1968; Higgs,
rocks
have
marginal-marine 1991).
64
On the shelf beneath Hecate Strait and Queen Charlotte Sound, Skonun
Formation
in
eight deep wells is non-marine to marginal-
marine in the north and marine Higgs,
1991).
mudstone
the
in
the
south
(Shouldice,
1971;
The Skonun Formation contains marine to continental
and
consolidated.
lithofeldspathic Coal
seams,
sandstone,
generally
semi-
volcaniclastic beds and basalt flows
are found in it at different stratigraphic levels.
Lithofeldspathic sediments derived
of
the
Queen
Alaska. a
Basin
were
from neighboring continental areas - the Coast Mountains,
the Queen Charlotte Islands, and uplifted
to
Charlotte
Numerous
by
unstable
differential
Tertiary basement
of
southeastern
local facies changes and unconformities attest
tectonically
controlled
parts
(Lyatsky,
setting,
as
subsidence
of
sedimentation blocks
in
was
the pre-
1993a).
Overview of Tertiary geologic evolution of coastal provinces Two main geologic provinces along the continental margin from Oregon to southeastern Alaska Two main geologic
provinces
are
usually
distinguished
Pacific continental margin of northwestern North America. western Oregon and Washington,
is the Coast Range
on
the
One, in
province.
The
other, in western British Columbia and southeastern Alaska, is the Insular Belt.
These two provinces (Gabrielse
differ
the
many
aspects
of
their
geology
and Yorath, eds., 1991; Burchfiel et al., eds., 1992).
The Insular Belt is made rocks,
in
Coast
up
mostly
of
Paleozoic
Range province of Cenozoic rocks.
Belt experienced large-amplitude
local
block
and
Mesozoic
The Insular
movements
in
the
65
Tertiary,
whereas
evolution of the Coast Range province was more
uniform: general subsidence during much of the Tertiary,
followed
by invertsion.
During the Cenozoic, the Coast Range province has continually been affected by subduction along the 1989),
and
a
megathrust
continental plate and the Kulm,
1989;
McCrumb
has
Cascadia existed
undergoing
(Atwater,
between
oceanic
et al., 1989b)o
the
slab
1970,
overriding (Duncan
and
In contrast, the nature of
plate interactions
off
(Cousens
1984, 1985; Allan et al., 1993; vs. Riddihough
et
and Hyndman, Oregon
al.,
British
zone
Columbia
1989; Hyndman et al.,
continental
margin,
no
is
1990).
still
In
ODP
dispute
contrast
manifestations
related thrusting have been found by
in
to
the
of a subduction-
drilling
off
southern
Vancouver Island (Carson et al., 1993; MacKay et al., 1994).
Variations in tectono-magmatic style along the continental margin Pre-Tertiary
rocks
of
the Insular Belt have been proposed to be
exotic, accreted to the North fashion
after
traveling
American
long
continent
distances
in
a
chaotic
from remote regions of
South America or southeast Asia (e.g., Jones et al., 1977)o recent
investigators
western Cordilleran considerable
generally suspect
displacements
favor
terranes of
some
a
More
native origin for most
(this
does
crustal
not
blocks
preclude along
the
margin, but in a regular fashion, as can be inferred from geologic mapping).
Paleozoic and Mesozoic rocks, variously metamorphosed,
form the basement for Tertiary formations in the Insular Belt.
Scarcity of pre-Tertiary basement exposures province
leaves
room
for
a
variety
in
the
Coast
Range
of speculations about the
66
nature of the crystalline basement
in
western
gravity models Oregon
Oregon and Washington
(Couch and Riddihough,
as stratified
Crescent basalts continental
ages correspond
in the Cordillera arc,
Crescent
origin
(Cheney,
and Challis m a g m a t i s m
132),
but at any rate,
have
begun
in
western
Patterns irregular
of
volcanism
after
chain consists, northern
36
west-central
along
variously,
California
origin
In current models, is
linked
to
the
(Green,
regional
1994). Challis
(1978)
occurred
connection
between
(op. cit.,
is
now
p. 131,
thought
and Oregon only at 36 Ma, (Brandon and Vance,
continental
of a cluster
of
(Read,
i.e.
margin
remained
to Recent volcanic centers
in
in western Oregon to 1990;
1990)
and Mesozoic
to
1992).
eruption
and southwestern
zone in the Paleozoic
an extensional
The
volcanism
the
cones
1992,
suite is now thought to have an
Even the Pleistocene
Washington
of Eocene rifting
to Armstrong
(Sherrod and Smith,
belt of isolated volcanic fracture
more
and a belt of volcanoes
Washington
northwestern
the
is still unclear
boundary
Ma.
1989).
Once treated as a subduction-
Washigton
near the Eocene-Oligocene
from
a wide zone of reflections
(Babcock et al.,
1994).
arc-related
pre-Tertiary
Deep seismic data in
as products
which according
from 55 to 36 Ma.
a
has been inferred
1989).
to
the Challis volcanic
intracontinental
of
(Keach et al.,
crust or lithosphere
episode of volcanism,
related
rocks
are now interpreted
Their radiometric
In
Existence
show under Siletz River basalts
interpreted
of
basement.
Scott,
1990).
British Columbia, follows
basement
a
a
linear
and may have
1990).
the vigorous Crescent m a g m a t i s m appearance
under
the
in
marginal
the
Eocene
part of the
67
continent
of
Farallon
oceanic
mantle
a
caused
rifting
and
slab
window
plates.
extension basaltic
around 60 Ma,
between
Creation in
the
the
subducting
of such a w i n d o w continental
magmatism.
allowing
the
volcanism
57 and 50 Ma (Babcock et al.,
Coast
volcanism
voluminous 1987)
in western
and
in
geochemical areas
the
Oregon area
differences
in
blocks
centers. in
1985),
but such
(Babcock et al.,
Eocene
mafic
exterior Crescent
of occurring
t e c t o n i s m caused
rotations
(Babcock et al.,
Vancouver
magmatism producing
on
also
distinguish
the
basaltic
1987).
may continue to very deep crustal The near-surface
bodies have an average thickness et al.,
1989),
1977c;
Babcock
both
rotation
of
many
(Wells and
Peninsula
were
the southern part of the
igneous massif
massif Thus,
1994).
on
southern
on the Vancouver presence
Island
or absence
by itself,
of
allow to
from the Insular Belt.
levels,
Crescent
in Washington
to depths of up to 30 km and Siletz River volcanic
of just 5 km (Duncan,
though some massifs et al.,
in
some
from many distinct
Washington
bodies does not,
the Coast Range province
1990).
Snavely,
Despite
Gravity data suggest that some of the igneous bodies
(Finn,
1994).
especially
eruptions
Olympic
affected
the Metchosin
(see Snavely,
Formation
was
to
1994).
Island and the Prometheus shelf
1992),
Oregon and southwestern
minimal
Insular Belt,
Eocene
of the Olympic Peninsula.
Subsequent
western
the
1992,
(the Siletz River Formation;
had a common characteristic
volcanic
Coe,
mafic
producing
rifting began
reach its peak between
Range
and
in the upper
crust,
In these models,
in the Late Paleocene,
Kula
exceed
7
or
8
1982; Keach km
(Muller,
Unexposed deep feeder systems
and
68
frozen magma chambers may be present beneath some volcanic
bodies,
Importantly, restricted have
to mafic eruptions, found.
Metchosin
complex,
tonalite
stocks
them
(Muller,
Sooke
with
and
western
Godwin,
intrudes around
suite,
et al., such
to but
and
felsic
rocks
31 Ma in age,
in the
quartz
diorite
tonalite
and granite
and among
of
the
in the
(Muller et al., 1991).
On
1981; Andrew and
the
west
stock,
(Brandon,
southern
dated
1989a).
coast
of
at 52 Ma,
Felsic
sills
40 Ma in age have been found in the Leech River Complex on
Peninsula
Island
(Snavely,
have been mapped volcanic
(Fairchild
also
1987).
been
and Cowan,
found
reported
province,
and
Olympic
sedimentary
rocks
of
They
older Tertiary
cut
units.
dated at 44 Ma, have
from Striped Peak on the northern Olympic coast and Olympic
Peninsula.
predominantly felsic
the Insular Belt are
northern
in the core of the Olympic Mountains.
from northeastern
Unlike the
on
1982).
Quartz diorite dikes dated at 41 Ma
Dikes and small plugs of dacite and andesite,
age
was not
have been reported
granitoid
the Pacific Rim complex
Felsic Eocene rocks have
been
provinces
They lie along large faults
one
southern Vancouver
both
larger
dated at 52 to 35 Ma, are w i d e s p r e a d
parts of the island
Island,
47
gabbro,
Granodiorite,
1989c; Woodsworth
Vancouver
plutons,
mafic
10% to 30% quartz
1977c).
Island.
in the coastal
and both
are largely
early Tertiary Catface on Vancouver
the
such as Metchosin.
Paleogene m a g m a t i s m
been
of
present
mafic
magmatism (Woodsworth in
many
magmatism
in
the
Coast
Range
was the norm during the Paleogene et al.,
1991).
Felsic rocks of
areas in western British Columbia
in
that and
69
southeastern in
detail
Alaska
on the Queen Charlotte
suite contains diorite
(Brew et al.,
and
granite.
U/pb
and
ages
in three pulses:
Thus,
the
mafic
approximately
1991).
Pacific
margin
two
36 to 32 Ma,
and
Intrusive bodies of
and dikes,
some
of
which
igneous domains correspond
geologic
provinces
of
the
A broad boundary between these
zones lies in the area of southern Vancouver Olympic
some
volcanism.
principal
of North America.
with
rocks suggest that
46 to 39 Ma,
and felsic Paleogene
to the
studied
where the Kano plutonic
these
the Kano suite are mostly small plutons for comagmatic
been
mondodiorite,
of
(Anderson and Reichenbach,
may have been feeders
They have
Islands,
quartz monzodiorite
magmatism occurred 28 to 27 Ma
1991).
Island
and
northern
Peninsula.
Distribution
of Tertiary
sedimentary
basins along the margin in
time and space On
the
basalts
northeastern
Olympic
of the Crescent Formation
a hot contact,
older Tertiary
Throughout
the entire Crescent
sediments
are intercalated
context,
indications
basalts
in
al.,
Peninsula,
from
in the Deer Park area,
overlap stratigraphically,
sediments section
of the Blue Mountain in that area,
(Babcock et al., seismic
Oregon are underlain
data
1992,
that
by stratified
1989) may suggest presence of sedimentary
basalts.
Seismic reflections
complex on southern V a n c o u v e r circumstantial began to develop the Paleocene.
evidence
with unit.
basalts
1994).
In this
equivalent material
and
Eocene
(Keach et
units beneath these
are also present under the M e t c h o s i n Island
suggests
(Clowes et al.,
that Tertiary
in the continental-margin
1987).
Thus,
sedimentary
basins
region as early
as
in
70
After
the
early
Middle
Eocene,
two
subparallel
sedimentary basins formed along the margin (Fig. 6). belt,
well
(the Tyee Formation;
as in several basins in Washington
Since the Oligocene, and
of
In the inner
abundant arkosic sediments were deposited in western Oregon
north of the Klamath Mountains as
belts
southern
Puget
along the Cascade basins
(see also Dickinson, 1976). Georgia
and
Queen
1993a).
(Niem et al., 1992a).
magmatic
arc,
Willamette
have developed in a fore-arc setting To the
Charlotte
north,
basins
Neogene
the
northern
evolved
depressions in front of the mountains of (Lyatsky,
Snavely, 1987)
the
subsidence
in
compensatory
Coast
along
Puget,
Belt
parts
orogen
of
these
m o u n t a i n ranges created the western Canada interior shelf.
Outboard of these basins lies a series of islands stretching northern
Olympic
island chain Vancouver hereinafter
Peninsula
along
and
the
Queen
as
to
coast
Charlotte
Western
Canada
Southeastern of
British
islands,
from
Alaska.
A single
Columbia,
including
is jointly referred to
Archipelago.
This
archipelago
separates the interior and exterior continental shelves.
Eocene
to
mid-Miocene
marine
clastic
deposits
western Olympic Peninsula (Central Olympic and laid
down
in
a
basins)
were
continental-slope and trench setting (Tabor and
Cady, 1978; Niem et al., 1992b). entire
Hob
on central and
Such conditions existed
in
the
Tertiary Olympic embayment, which was at least i00 km wide
and lay between the Dosewallips igneous massif and the western end of
the Hoh Basin on the present-day shelf.
fairly uniform slope or trench
conditions
Such broad areas with of
sedimentation
are
uncommon at continental margins. Tectonic evolution of the Central
71
Olympic and Hoh basins is also unusual: these basins are metamorphosed, chapter).
compressionally
All
other
slightly
shortened, and inverted (see next
Tertiary
basins
in
the
region
are
unmetamoprphosed and considerably less deformed.
In the Tertiary depressions which developed along the rising Coast Belt,
subsidence
nonetheless
was
considerable,
but
predominated throughout the Tertiary.
depression between the Coast Mountains succession
non-marine
of
and
conditions
In the Georgia
Vancouver
Island,
a
conglomerate, sandstone, mudstone and coal is 6 km
thick.
Marine
derived
from the surrounding mountains accumulated in large delta
complexes
incursions
were
minor,
and
fluvial
sediments
(Mustard, 1991).
To the south, clastic sediments of the Middle to Late Eocene Puget Group, some 3500 m thick, were also laid down in a deltaic setting by
streams
interior.
draining These
granite-rich
deposits
were
areas then
in
the
covered
Cordilleran
by submarine fan
complexes containing Oligocene volcanic-rich sediments subduction-related
arcs
in
the
Miocene sediments are non-marine,
South
Cascades.
containing
shed
from
The overlying
detritus
from
the
Olympic Mountains to the west (Niem et al., 1992a,c).
On the exterior shelf, a series of Tertiary sedimentary basins has been identified from gravity
and
seismic
data,
and
in
places
confirmed by drilling, along the submerged Pacific margin of North America from Oregon to southeastern Washington
and
Oregon,
Basin
Off
the
coast
of
such basins are 3-4 km deep but some are
6-8 km deep (Snavely, 1987; Tofino
Alaska.
Couch
and
Riddihough,
1989).
The
on the exterior shelf off Vancouver Island has been
72
drilled to almost fault-bounded southeastern which
Alaska
m of stratified
Most
(Bruns
and
sediments,
crustal
several
faulting
basins
the
1987).
causes
Cady,
are
varied
Oregon
sediments
1972;
deep
-
basins
deformation
contractional
(as
Niem in
1987)
basins:
the
Olympic et al., Tofino
1990;
Most
the
basins are not metamorphosed, only slightly.
is penetratively Brandon
Anomalous
and Vance,
discordant
margin and the geologic been
satisfactorily
the trans-Cordilleran
the
1992a-c), and
Rohr
Hoh (as and
and the Hoh
Central
sheared and m e t a m o r p h o s e d
Olympic
(Tabor and Cady,
1992).
in the structural
orientation,
Only
has
Basin;
or fault movements Finn,
of
folding and
1993a).
Basin is m e t a m o r p h o s e d
on
between
subsidence
1992; Lyatsky,
Tertiary
wide and
lie
Dietrich,
1978;
off
basins,
Farther outboard,
Where
1987;
Snavely,
in the Puget and Queen Charlotte
Basin
shelf
These
and in the Central
flowage of overpressured Tiffin et al.,
margin.
faults.
Snavely,
of
undrilled
fault system.
1978;
basins;
on
were formed by differential
(as in western
and
Several
tens of kilometers
other
blocks bounded by steep its
found
Carlson,
to the continental
slope,
these
occurred,
also
are several
of the plate-boundary
of
Tabor
are
1971).
to seismic and gravity data contain at least 3000
parallel
the continental strands
(Shouldice,
depressions
according
elongated
4 km depth
sense is
the
Fuca
Basin.
Its
WNW
with the general trend of the continental grain of coastal
explained.
provinces,
has
not
yet
The Fuca Basin lies on trend with
Olympic-Wallowa
zone of crustal
weakness.
CHAPTER
5
SIGNIFICANCE
-
ZONE IN GEOLOGIC
OF THE T R A N S - C O R D I L L E R A N
OLYMPIC-WALLOWA
EVOLUTION OF THE W A S H I N G T O N AMD BRITISH COLUMBIA COASTAL REGIONS
Recognition Observed
of the O l y m p i c - W a l l o w a
in
regional
Zone of crustal weakness
topographic
maps is a long lineament
the Cordillera.
It runs from the Pacific
and northwestern
Oregon
(Fig.
7).
was traced from northern Olympic Plateau, line.
into
Raisz
Lineament
the
(OWL)
Peninsula,
speculated
Washington
across
(1945),
the
it
Columbia
near the Oregon-Idaho
named this topographic
and
into
First noted by Raisz
Wallowa Mountains
(1945)
coast
across
state
feature Olympic-Wallowa
that it may be coincident with a
large fault.
Geological, the
OWL
geophysical
is
not
and geographical
unique,
surveys have
N-S grain of the
King,
1994).
name
Reidel
just a few,
northern
Idaho,
et al.,
Other WNW-ESE
zone in central
in southern Oregon and northern Nevada. curved
Snake
orientation, Meyer,
The
Cordillera
River
plain
in
southern
as do many valleys
and
Oregon,
(e.g.,
lineaments,
are Lewis and Clark zone in western Brothers
that
but rather one of many large lineaments
that trend across the general 1969;
shown
to
Montana
and
Eugene-Denio
zone
The western branch of the Idaho also has a WNW-ESE
mountain
ranges
(Mann
and
1993).
scale of the structural
apparent
zone along the topographic
in the 1970s and 1980s,
and temporal
continuity
and Hooper,
eds.,
folds
parallel
run
1989). to
as mapping revealed
and internal A the
structural
OWL became
its
complexity
spatial (Reidel
series
of
WNW-trending
faults
and
OWL,
in
places coinciding with it
74
49"
47"
45"
43"
125"
121"
117"
Figure 7. Regional sketch of the O l y m p i c - W a l l o w a structural zone (OWSZ) approximate location and extent (Reidel et al., 1994). The Olympic-Wallowa Lineament is the topographic expression of the OWSZ. The OWSZ is in fact tens of kilometers wide, much broader than sketched by these workers.
75
exactly.
The OWL was proposed
Olympic-Wichita
Lineament
to be
presumably
Oklahoma to the Pacific margin
More evidently, (OWSZ),
batholith, Cascade changes
herein,
distinct
structural
system of closely and includes,
with
zone.
spaced
besides
areas,
the
OWSZ
Vancouver province
Island,
and
in central
western
part
the
evidence
upper
tectonic
the
its
the OWSZ
elements
are
form
a continuous,
is
just
a
15-km-wide
it widens to 50-70 km folds,
raised blocks
and depressions Fuca
(Pine Valley
Basin).
eruptions,
Miocene
the
to
Elsewhere,
In
some
such as those of
Peninsula
and southern
Columbia River basaltic
of
to be one of the major shear
the North American (Saltus,
(Catchings
that different
mobility
continent.
1993),
and Mooney,
throughout
is that this zone of
transects
Cordillera
zones
1988).
in
Geophysical
possibly
parts of the OWSZ were
remains unnoted the
it
volcanic
and
Washington.
mantle
suggests
but
on northern Olympic of
province
Along its trend,
another
uplifts)
data show it to be deep-seated
high
coast.
as well as the
The OWSZ is now recognized
into
volcanic
faults and anastomosing
Eocene Crescent basalts
the
Idaho
faults.
controlled
from
runs from the Late Cretaceous
In places,
and Grande Ronde grabens,
longer
zone
one
(Wallowa and Cuddy Mountains
much
structural
to the Pacific
connected
a
1978).
its width and surface expression,
everywhere
of
Olympic-Wallowa
across the Columbia River
Ranges,
part
crossing the Cordillera
(Baars,
the interregional
as designated
a
reaching Structural zones
of
at least the Tertiary.
What
crustal
only
weakness
not
but also controls the position of some
parts of the Pacific continental
margin.
76
The OWSZ in eastern Oregon and Washington West of the huge, the
OWSZ
closely of
is
N-S-trending
Late
Cretaceous
batholith,
expressed m a i n l y as a series of horsts,
spaced faults whose orientation
the
Idaho
faults
bound coherent
crustal
grabens
is p e r s i s t e n t l y blocks,
WNW.
variously
and Some
uplifted
and downdropped.
One
uplifted
expressed rocks.
block
in
as the Wallowa Mountains,
It was raised
flows
northeastern
of
the
in the Neogene
mapped
Mountains
Wallowa
horst:
the
on
topographically
is made up largely of Mesozoic and so was bypassed
Columbia River province
Large faults have been
Oregon,
(Hooper and Conrey,
both
flanks
of
the
fault on the northeast,
fault on the southwest
(Fig.
Grande
and Baker V a l l e y to the south,
Ronde
graben
with the OWSZ. the
1993),
Long Valley fault system
right-lateral
Hooper
and Conrey
transverse
the
west,
disrupted, without 1989,
lateral
systems
the Eagle
are on trend
at least as far
some
(1993).
Idaho
as
(Mann
Miocene
- cross the OWSZ
pluton
that
intrudes
rocks lie on both
(Frizzell
et
1989).
local in scope and limited
at
al.,
by
This idea is
earthquakes.
- Long Valley in western
boundary
offset
of
Price and Watkinson,
were
Wallowa
as well as the
in western
and Mann and Meyer
and other Miocene
1994;
movements
a
1989).
on the OWSZ have been proposed
mechanisms
the O r e g o n - W a s h i n g t o n To
movements
(1989)
focal fault
lava
and on the WNW it runs into Washington.
Neogene
supported by
These structures,
On the ESE, the OWSZ continues
N-S-oriented
and Meyer,
8).
by
However,
Idaho,
Hite on
high
angles.
the OWSZ is not
side
of
1984; Reidel
This suggests in magnitude.
the
OWZ
et al.,
strike-slip
7Z
Figure 8. Structural map of the area near the southeastern end of the OWSZ, showing the Wallowa Mountains horst and b o u n d i n g faults, as well as the Long Valley and Hite fault zones which transect the OWSZ (modified from Mann and Meyer, 1993).
78
The OWSZ truncates the Blue Mountain block of Permian and Triassic rocks and Oregon
the
NE-trending
(Hooper
and
Conrey,
apparently active in abundant
Klamath-Blue
the
1989).
Miocene,
Mountain
Lineament
in
In Washington, the OWSZ was providing
conduits
for
the
lavas of the Columbia River flood basalt province; these
basalts are estimated to be up to 4500 m thick (Reidel and Hooper, eds., 1989; Saltus, 1993).
The OWSZ in central Washington North
of
the O r e g o n - W a s h i n g t o n boundary, the OWSZ is a series of
closely spaced, WNW-trending fault zones and long anticlines.
The
Wallupa Gap fault zone, Umtanum and Manashtash ridges, Rattlesnake Mountain anticline, White River-Naches River fault zone, all a
WNW
orientation (Reidel and Hooper, eds., 1989; Reidel et al.,
1994).
The Pasco Basin in the northwestern part of
River
province
owes
its structure to the OWSZ.
belt contains long, fault-controlled anticlinal stable
blocks,
and
Belt,
faults
and
the
Columbia
The Yakima fold folds
separating
its structural grain changes across the OWSZ
(Reidel and Campbell,
1989).
folds
In the central part
are
oriented WNW, along the OWSZ
spaced
of
particularly
the
reverse
faults.
Rocks
(Price
splays and
displacements
of
between
some
Watkinson, of
small
them, though tilted, are mostly 1994).
Shallow
near-
of the reverse faults dip at low angles 1989).
Yet,
evidence
of
strike-slip
a few kilometers is found in several localities
(Reidel et al., 1994). of
and
(Fig. 9).
undeformed (Watters, 1989; Reidel et al., surface
Yakima
closely
Anticlines in the OWSZ are asymmetric and commonly cored by
many
have
Analysis of
gravity
data
suggests
that
the faults reside only in the basalt succession (Saltus,
79
Nil IIII
\%.1
I
% \ "o~
¢o,~ cow.L~x
f",,
/
~
.,.
/
\ f~
J ""
% "+ " k V
/
L
,,,oo.,
.,o,.
\ ~
k
.S
w ~
pASS
/
,
~'
KILOMETER5 0
SO
0
30 MIL~S
/
/
//
I~F)
Figure 9. Distribution and orientation of faults and folds in the Yakima Belt in southern Washington (modified from Reidel et al., 1994). OWL - Olympic-Wallowa Lineament (topographic m a n i f e s t a t i o n of the OWSZ). Structures along the OWSZ trend WNW.
80
1993).
Seismic reflection
et al.,
1994; Lutter et al.,
However,
bigger
Magnetic
anomaly
OWSZ.
crossing
Seismic refraction
(Saltus,
1993)
the OWSZ lower
drillhole
and
maps
(Johnson
et
the
Columbia
River province
even
of
the
uppper
Paleogene
within the OWSZ.
of
sedimentary
1990)
1988)
of
show
both
along the
Pasco
across
upper
and
Geophysical
Miocene
basalts
increases
a
and gravity
the
mantle.
rocks
The post-Miocene
the pre-
large perturbations
and density structure
data show that thickness
underlying
al.,
and Mooney,
have revealed
in the velocity
crust
in scale and penetrate
(Catchings
surveys
(Jarchow
1994).
faults are crustal
basalt basement. linear
data show them to dip 15°-45 °
and and
dramatically
sedimentary
basin
also
trends WNW and lies in the OWSZ tract.
Seismicity
on
the
have NW-SE dextral and
Reidel,
belt
shows
throughout
OWSZ occurs
focal mechanisms
1989). that the
However,
N-S
al.,
evidence
stresses
Microseismicity
N-S compression
and some earthquakes
(Ludwin et
structural
compressive
Neogene.
still experiencing
in clusters,
were
oriented
Straight
kilometers Tertiary motion Fraser dextral
it
Creek and
(Reidel et al.,
Fraser
across W a s h i n g t o n accommodated
(Monger,
1991).
of
continuation
fault system is found south motions
seem
to
the
have
of been
the
Straight
A system of N-S-
extends
kilometers
this area is
1994).
into British Columbia.
tens No
faults
predominant
suggests
fault system merges with the OWSZ°
Tolan
from the Yakima
At the western edge of the Columbia River province, Creek-Fraser
1989;
of
hundreds
of
In the early right-lateral
of the Straight CreekOWSZ,
dissipated
and
Paleogene
at least partly
81
through side
At
a set of l a r g e N N W - o r i e n t e d
(Evans,
its
a
w i t h t h e OWSZ,
Creek-Fraser
distance
Whereas
of
south
of
Ranges
and beyond.
the
junction,
T h e OWSZ
is c l e a r l y
in
N-S-trending
White River-Naches distance
of
Campbell,
1989),
and
domains
River
North
contrasting throughout movements Mesozoic
east
as s t r u c t u r a l ranges
of t h e C a s c a d e s . faulting,
types
in W a s h i n g t o n . straight
1990).
Cascades and
are
is e x p o s e d
Cenozoic
The
stratigraphic
for
et al.,
a
1984;
between
Cedar
the
and
To t h e south, are
more
than
also by their
South
Cascades
by broad crustal
blocks
low amplitude.
o n l y in a f e w p l a c e s ,
to t h e s u r f a c e
magmatic
The narrow
the
distinguished
slow and had a relatively basement
breaks
1989).
structures.
rocks were brought
unconformity-bounded
of
but they
et al.,
Cascades
mapped
r u n a l o n g t h e OWSZ trend.
(Galster
1994).
These two topographic
segments
2000 m high,
over
extend
and topographic
boundary
which
crystalline
of t h e
WNW across the Cascade
physiographic
by
not
North and South Washington
the T e r t i a r y w e r e c h a r a c t e r i z e d
no d e e p c r u s t a l (Evarts,
continues
does
the
Cascades
which were
trends
et al.,
(Frizzell
South
rock
system
zone of s e v e r e
are less t h a n
and
fault
Reidel
of k i l o m e t e r s
2500 m h i g h in t h e n o r t h
The
1989;
tens
marks
rivers,
the m o u n t a i n s
between
mountain
are s e p a r a t e d
Snoqualmie
t h e OWSZ
expressed
several
South
the N - S s t r u c t u r a l
(Campbell,
Creek-Fraser
T h e O W S Z as a b o u n d a r y
North
system's
f a u l t s y s t e m s w i t c h t h e i r t r e n d to ESE
o n l y i0 k m
the Straight
the
on t h i s
1994).
junction
Straight
faults
suites
sequences
in t h e
(Armstrong, (Cheney,
The and
Tertiary 1978)
1994)
and have
82
been
correlated
hundreds
south of the OWSZ across much of Washington,
over
of kilometers.
The North Cascades, southeastern (Haugerud, orogen,
1989;
Brown
North
and plutonic
et
al.,
Cascades
rocks exhumed
and
remobilized
are a direct
Alaska and British Columbia,
the
Igneous
in contrast,
at pressures
Cretaceous.
1994).
As
>9 kbar
of
(depth
plutons,
in
~
Mesozoic 30
km)
depths.
ages
were
the
Late
in
and early Tertiary
whose buoyancy
this
metamorphic
lower-crustal
mostly
In the latest Cretaceous
intruded by large granitic
elsewhere
are cored by polygenetic
rocks
from
of the Coast Belt orogen
from middle- to
sedimentary
continuation,
they were
caused them
to
rise back to the surface.
The
style,
sharply
rate and volume of Late Cenozoic m a g m a t i s m
across the OWSZ.
several
times
Estimated
1990).
in
southwestern
volcanoes
British
produced
lavas
material
from
the
of
in
Columbia
Juan
de
continental
composition,
crust
arc
plate
is
Washington
isolated
(Taylor,
Washington cones
are
Typical
Fuca
northwestern
are
intermediate
and
that
have
contaminated
with
(Sherrod and Smith,
1990; Read,
1990).
At 36 Ma, arc m a g m a t i s m began continental
of
western Oregon and southwestern
In contrast,
1990; Green,
eruption rates
lower north of Mt. Rainier than south.
m a g m a t i s m related to subduction occurring
Quaternary
also differ
margin Vance,
from
along
the
california
(Brandon
and
1992).
strongly
segmented
developed
south and north of the
(Guffanti
North
America
to southern British Columbia
Later, and
western
however,
Weaver,
OWSZ
(cp.
the
1988), Sherrod
arc
became
and contrasts and
Smith,
83
1990).
This
segmentation
of
the arc probably reflects ongoing
fragmentation of the Juan de Fuca oceanic inferred
from
teleseismic
data
slab,
(Michaelson
VanDecar et al., 1990; Dueker and Humphreys,
From field mapping, geochemical potential-field
data,
analyses
several
Sherrod
and
igneous
are
rocks
characterized
with
different
southern ends of the northern
Cascade
by
Weaver,
1986;
domains
been
(Guffanti and Weaver,
1988;
1990; Scott, 1990).
diachronous
in
eruptions
The
northern
and
northern and
Washington
and
California, are characterized by intermediate to silicic
which
contrast,
erupted
the
from
middle
part
distinct of
composite
the
arc
volcanoes.
(basaltic) lava.
North of the OWSZ, the volume of Pleistocene-Quaternary decreases
substantially
(Sherrod and Smith, 1990).
to silicic lavas are contaminated with material (Green,
1990).
Unlike
in
In
in Oregon and southern
Washington contains overlapping fields of mafic
crust
of
have
compositions. arc,
been
interpretation
and Smith, 1990; Blakely and Jachens,
These domains
lavas
and
has
1994a,b).
igneous
distinguished along the South Cascades
which
from
volcanism
Intermediate continental
areas to the south, volcanoes
north of the OWSZ are considered dormant.
Several separate Quaternary Cascades
volcanoes
runs
NNW
probably indicates Garibaldi basalt,
the
Washington
North
(Glacier Peak, Mt. Baker) and the Garibaldi volcanic belt
in southwestern British Columbia (Fig. which
in
belt,
(Read,
1990;
control which
by
contains
Green, a
long
i0)
form
linear
zone
1990; Smith, 1990).
This
crustal
andesite,
a
fracture.
The
dacite, rhyolite and
is made up of stratovolcanoes, volcanic domes and isolated
84
1
British Columbia ~yJey ,
~
EXPLORER~>< PLATE r~
•
:0 o
.~J Mou~ C~rfb~dl.. Garibaldi Lake
\
\ \
Washington
\
:;
.
_ •
uaunt ~*ar~et
JUAN DE
I
FUCA
I Wlour~
I
PLATE
I
|
Mou.t M a z a . ~
•~
GORDA "~ Mendoeln-g~
~k,.Moum Shi~tl
La-,, P,,*
' --140°
I
California PACIFIC PLATE
I 130 °
San A n d r e a ' s ~ i " fault "'" ,. ? I
[
120 °
Figure lO. Distribution of Quaternary volcanoes along the western North America continental margin (mofified from Scott, 1990; cp. Fig. 3a). Segmentation of the High Cascade arc and the Garibaldi belt (segments 1 to 5) according to Guffanti and Weaver (1988). Recent evidence (Green, 1990) suggests the Garibaldi belt (segment I) formed in an extensional tectonic regime: if so, it is not part of the current subduction-related arc. The c o n v e n t i o n a l l y assumed plate boundaries are shown offshore.
85
lava
flows.
Continuing
tectono-magmatic
activity
at
depth is
suggested by the presence of hot springs.
Geochemical and petrological evidence suggests the Garibaldi was
created
in
belt
an intracontinental extensional tectonic regime.
Even the basaltic
rocks
in
this
belt
"most
closely
resemble
magmatic associations considered to characterize regions of recent uplift, extensional tectonism, and high heat flow" p.
173).
These
(Green,
features are similar to those in other Cenozoic
volcanic belts across the Cordillera in British Columbia Yukon,
which
1990,
were
and
the
related to deep fracturing of the crust in an
extensional tectonic regime (Souther, 1990).
The OWSZ west of the Washington Cascades The Puget and Georgia continuation
sedimentary
basins
cover
eastern
near Everett (Adair et al., 1989).
shore
Puget and Georgia basins
al.,
(Figs. 6, ii).
accommodating Late Cretaceous movements have
Faults
of
Puget
depocenters
on trend with the OWSZ near Seattle (Lees and Crosson, The San Juan Rise (Galster et
the
From geophysical data,
a buried basement horst between sediment-filled
1990).
up
of the OWSZ west of the Washington Cascades.
trending WNW have been described on the Sound,
partly
1989)
lies
1990; Finn,
separates
the
Faults trending WNW, some (e.g., the Lopez
thrust),
been described along its southern flank (see Whetten et al.,
1980; Brandon et al., 1988).
Running SE from the city of Victoria, Juan
de
Fuca,
is
a
series of strong,
magnetic anomalies up to +800 nT in band
of
anomalies
across
forms
a
Strait
of
linear, short-wavelength
amplitude
magnetic
eastern
(Fig.
12).
This
domain boundary: negative
88
IO
60 S¢41e
100 MIhn
Figure Ii. Geologic provinces of Washington and adjacent regions (modified from Galster et al., 1989). The OWSZ (whose approximate position is marked by the two parallel, solid lines) separates the North and South Cascades; on trend with it lies the Juan de Fuca Trough. The southern boundary of the San Juan Rise lies on trend with the OWSZ. Detailed mapping (Muller et al., 1977a; Monger, 1991) shows the northern boundary of the San Juan Rise is a NEtrending system of faults beginning on southern vancouver Island, so unlike in this map, the rise is actually a trianglar block.
87
values as low as -300 nT are observed to the north, south
the
values
Peninsula,
are strongly positive.
short-wavelength
volcanics,
which
Dungeness
Spit well.
Fuca
(Fig.
decline
have
13)
from
anomalies
been
also
Vancouver
drilled
oriented
Island
On northeastern
in the Strait of WNW,
towards
the
off
Whidbey
Island in eastern
revealed
several
subrarallel
disturbance
in sediments
Juan
Olympic
Marine
have
Peninsula
of
structural
(Atwater,
The long Strait of Juan de Fuca and the Fuca sedimentary
filled
with
Gravity
anomaly values
southern coast,
Vancouver
nT
anomalies
(Fig.
12).
Island
to
Strait
Clowes, Port
1992).
Angeles
wavelength Juan
de
similarity volcanic
Juan
on
the
over the basin from >+60
1991).
mGal
WNW
(Fig.
of these positive magnetic continuity
Fuca
without
of Eocene
(MacLeod et al.,
as between the opposite
on
<-20 mGal on the northern Olympic
cities
shores
are similar to those
though
suggests
rocks.
de
Locally,
anomalies Fuca,
graben
13).
in the basin trend WNW and are generally Presence
of
basin lie
(Niem and Snavely,
trend overwhelmingly
far as Cape Flattery suggests the
sediments
decrease
and anomaly contours
Magnetic
1994).
This basin is a long W N W - t r e n d i n g
8 km of Tertiary
de
Strait of Juan de
zones
as young as Quaternary
on trend with the OWSZ.
by
as anomaly magnitides
1977).
Fuca
Olympic
at a depth of 1660 m in the
(MacLeod et al.,
surveys
to the
of this band are sourced
Gravity anomalies
are
whereas
the
of in
strong
anomalies
basalts
under
Victoria Strait,
eastern
Strait
contain
and
short-
linear fabric.
that the Fuca Basin may also
as
1977; Dehler and of
the
>+400
of This
shallow
I
124000 '
Figure 12. Magnetic anomaly map of the Strait of Juan de Fuca and ;icinity, superimposed on a map of faults of the northern and ~outhern strands of the w e s t e r n OWSZ (modified from MacLeod et ~I., 1977). Anomalies are in nT. The predominant WNW orientation Df anomalies reflects the orientation of the OWSZ. Letters A to L cepresent anomalies discussed in detail by MacLeod et al. (1977).
125000 '
I
123000~
~801s'
4803o •
CO O0
Figure 13. Bouguer gravity anomaly map of the Strait of Juan de Fuca and vicinity, superimposed on a map of faults of the northern and southern strands of the western OWSZ (modified from MacLeod et al., 1977). Anomalies are in mGal. The main WNW orientation of anomalies reflects the orientation of the OWSZ. Letters A to L represent anomalies discussed in detail by MacLeod et al. (1977).
90
The
Fuca graben
Olympic
is bounded by long,
Peninsula,
the boundary
trending volcanic massifs belts of sedimentary trending
faults
the
been
fault,
exterior continental
shelf
massif
from
(see
also
These
fault system
(Fig.
South
jointly,
One
of
faults
Leech
separate
River
formations Leech
River,
South
the
metamorphic
to
throughout
as
1987).
the
north
San Juan and the
Tertiary
relationship,
Vancouver
Island
14).
Island fault system
Vancouver
Island fault system represents Because,
OWSZ
Peninsula,
on the Olympic
tectonic history
At the surface, (Fairchild
unlike the southern it is exposed
is still understood
and Cowan,
1982).
(MacLeod
et al.,
strand
of
in a small area,
is
It consists
a
few
the its
kilometers
of straight
with dip angles ranging 1977)
the northern
incompletely.
the Leech River fault zone
dipping steeply to the north, northward
Ridge and
Snavely,
To stress their genetic
strand of the OWSZ.
wide
WNW-
fault systems of western OWSZ
South Vancouver The
-
complexly
1991).
they are herein designated
Boundary
the
faults
- interacted
Monger,
big
1978).
1977;
steep
and the latter from Wrangellian
Survey Mountain
WNW-
and by outcrop
Several
Cady,
(MacLeod et al.,
Metchosin
1977a-c).
by
runs WNW through Cape Flattery onto the
large,
(Muller,
marked
Formation
(Tabor and
Island,
basaltic
On northern
mapped along the Hurricane
On southern Vancouver
complex,
is
rocks of the Fuca Basin.
have
Calawah
faults.
fault system
of the Crescent
Crescent Lake basaltic massifs them,
straight
to subvertical
faults
from 36°-70 °
(Muller,
1977c).
91
BOWENE '.~,_ Porksvilte
20Krn
o t
,,
1
I
o,<,
SALTSPRING
Port
SonJuon
/
%\
AMERICA
~
.PLATE
|
............... |
/~
~ ( -5o , Explorer R=dge / ~ I ~, VANCOUVI.' q _ k ~ 2'/ P~PLORER ~ v ....~,,,,"a.,-~,',,~ ~ / I ~PLATE .':';'%'~k"o'~
"~"
°°=%',~IlooA. OE,OCA\
L "--'~%,I
Figure 14. Location of LITHOPROBE 84-01 to 84-04 (lines 1 to 4) and Vancouver Island, superimposed on a that area (modified from Green, geologic map of the entire Vancouver 16. Inset shows the conventionally rates of motion.
~TERT
'
~:.-::..l sad. liiii~i:'.-'il Metosed.
~
vo,o.,,o,.~.O.Me,o
seismic reflection profiles magnetotelluric (Mr) sites on simplified geologic map of ed., 1990). A more detailed Island is presented in Fig. assumed plate boundaries and
92
Movements
on
Tertiary,
these
if
faults
protoliths
might of
were related to Wrangellian rocks
foundered
amphibolite
to
have
even
before
the Leech River metamorphic units
on
mid-crustal
the M e t c h o s i n basaltic massif,
Vancouver
complex
Island.
These
They were
juxtaposed
these
two
rock
to
against
whose grade of m e t a m o r p h i s m
and together
the
depth and were m e t a m o r p h o s e d
grade around 41-38 Ma.
exceed greenschist,
begun
did not
complexes
were
brought to the surface.
The
exact age of m e t a m o r p h i s m
Olympic
Peninsula,
metamorphic as clasts also
and 42
to
probably
occurred 1982),
40
of the Fuca
On northern
Basin
Because
Leech
41-38 Ma
(Babcock et
contains
River
displacement
al.,
as well
1994;
see
the age of the Lyre complex
metamorphism
(the age proposed by Fairchild
and the uplift must have been
north-side-up
in metamorphic
1994).
Ma,
before
is unclear.
from the Leech River complex,
Peak unit
Brandon,
is
Large
derived
from the Pandora
Formation
and Cowan,
the Lyre Formation
clasts
Garver
and uplift
extremely
rapid.
is indicated by the sharp break
grade across the Leech River fault.
The Leech River fault zone strikes WNW on its east end and E-W the
west.
(Fig. faults
5).
These
two
straight
fault
segments meet at an angle of 25 °
In the eastern part of the massif,
are tilted i0 ° to 30 ° NE towards
River fault and are subparallel the
structures
blocks between
local
the m a s s i f - b o u n d i n g
Leech
to it, but just
i0
tilts are in the opposite direction.
broad antiformal in
fold. the
several transverse,
The
Leech
Metchosin
in
River
massif,
mostly N N E - o r i e n t e d
fault
km
from
the
This indicates truncates
a
these
but is slightly offset by faults
(Muller,
1977c).
93
The Leech River metamorphic sheared.
Foliation
steep and parallel Survey
Mountain
in
to
complex
it,
the
is
being
narrowest
Survey Mountain complex widens
The
amplitude
grade, the
bounding
faults.
part occurs
faults approach
of
(Monger,
Johnson, data, these
fault-controlled
another
1991)
in
downdip continuations al.,
these differences,
itself.
or
This complex
strike-slip
and Cowan,
seismic of the
1985a;
the
Leech
River
width
should therefore
of be
zone on southern Vancouver in this zone is so
various
ideas invoke dip-
(Fairchild and Cowan,
1985) movements.
From
Fairchild
Leech
and
thrust-like
faults.
Detailed
River
et al.,
no comprehensive
Locally,
faults postulated
(Clowes et al.,
sections
Green
Steep dips of these faults 1977a;
across.
1982;
seismic
idea holds that in the Late Eocene or Oligocene(?),
events
et
km
is similar to or greater than the
faults also acted as thrusts
(Yorath
15
of the
uplift of these amphibolite-
Based on field mapping,
1984; Rusmore
low-angle
but
exact history of fault movements
far unresolved. slip
River,
where the Leech River and
each other,
as part of the deep structural The
Leech
is
to the west.
Leech River complex
Island.
The
just 2 and
in the east,
and
with uplift,
and San Juan faults restrict the outcrops
foliated rocks
regarded
disrupted
synkinematic
Leech River complex to an area between The
strongly
have been and 1985,
San
Juan
1987).
interpretation
splays
1982; are
reinterpretation
Rusmore
associated
as
faults
Because
of
is available.
and with
(Muller,
Cowan,
1985).
steep
master
of seismic data has shown that
to be thrusts are in fact steep 1987).
Several
interpreted
are indicated by field mapping
Cowan,
1987; vs. Clowes et al.,
1987).
(Mayrand et al.,
94
Magnetic
and gravity anomalies
Leech River fault continues (MacLeod
et
boundary
al.,
1977)
suggest the eastern
across the
(Brandon
Washington
mainland,
were active
in the Late Oligocene
in the Neogene: massif
down
magnetic
whereas
truncated
off
et
offshore
extension
sediments
(Muller,
the
commonly assumed to swing to Island coast line
The W N W - o r i e n t e d
1989).
Last
overlap the Leech
1977a).
(Figs.
South
12, 15), the
north-side-
with the Leech River fault
Vancouver
similar
the
high-grade
1977a;
metamorphosed
Isachsen,
Middle Eocene. movement,
large
1987)
strike-slip
1987).
The
western
Vancouver
NW
and
1977;
follow
Snavely,
the 1987).
fault is a link between the Leech
Wark
it may be the oldest
It truncates complex.
in
the
Early
sometime
episodes
of
displacements
in
from the south
These
rocks of the W e s t c o a s t
and uplifted
Between these
anomaly coincident
the
Though short,
as
It
but it is
Island system.
known locally
16), which were
(Snavely,
1977).
San Juan fault is unknown,
Survey Mountain
the gneisses to
fault
(MacLeod et al.,
River and San Juan faults. the
the
overlap the Metchosin
sediments
zone associated
Calawah of
On
faults
al.,
Cape Flattery by a magnetic
with the W N W - o r i e n t e d
Fuca
parallel
turns slightly to the SW (anomaly C of MacLeod et al., is
de
1988).
of Everett,
Oligocene
Island offshore
gradient
Juan
segment of the Leech River fault occurred
River complex to the north
West of V a n c o u v e r
al.,
(Adair
flat-lying Miocene
to the south,
et
in the vicinity
on the western
of
and meets faults that form the southern
of the San Juan Rise
movements
Strait
segment of the
rocks
complex
Jurassic
are (Fig.
(Muller,
before or during the downward
and
upward
might have occurred
on
95
~-
,/,% " ~t<,~<~<,b.w - ~,,~;';,;'~,;'~,:.,'7~, ~,~Juan;$?£<>2 .......;.,'2;.42;.,%-,'
"-1
\\_
,.
"~
J
jy~
~
~ .......... ~
~
,
-,~-~,Rlver'~
4e~
-----'--"" "':::i!!iii!!!i{!!ii?ii{!{{!{{i{!{ii ..... . . . . . . . . . . . . . . . . ~ s%~ ~ ! ! i i i i "iiiiiii!!iii ",,,
~,
"~:::~::::::::~:
TC
F i g u r e 15o Fault map of northwestern Olympic Peninsula and southwestern Vancouver Island, according to Snavely (1987). Offshore, the c o n t i n u a t i o n of the C a l a w a h fault had been inferred previously, from gravity and magnetic data, by M a c L e o d et al. (1977). D e t a i l s of fault c o n f i g u r a t i o n in the P o i n t of the A r c h e s area are p r o b a b l y u n j u s t i f i e d , as this i n t e r p r e t a t i o n a s s u m e s the M e s o z o i c g r a n i t i c rocks in that area are part of an accreted terrane rather than basement (see text). O c e a n w a r d d i p of the t h r u s t f a u l t a l o n g the w e s t e r n O l y m p i c Peninsula coast suggests that o b d u c t i o n , as well as s u b d u c t i o n , has occurred. Tc - h i g h l y d e f o r m e d T e r t i a r y s e d i m e n t a r y rocks in the O l y m p i c M o u n t a i n s core; Tm - t h r u s t - f a u l t e d p r e - T e r t i a r y to O l i g o c e n e b r o k e n f o r m a t i o n ; To Upper Oligocene sandstone and conglomerate; Ts - Tertiary s e d i m e n t a r y rocks ( u n d i f f e r e n t i a t e d ) ; T v - T e r t i a r y v o l c a n i c s . -
96
LEGEND
CARMANAH
~..~,
GROUP
MIDOLE" TERTIARY EARLY TO M}DDL~
CATFACE INTRUSIONS
TERTIARY
METCHOS~N VOLCAN~CS
EARLY TERTIARY
NANA~MO GROUP
LATE CRETACEOUS
QUEEN CHARLOTTE GROUP
KYUQUOT
GROUP
J
LEECH RIVER F O R M A T I O N PACIFIC R~M COMPLEX
ISLAND ~NTRUStOhtS
EARLY ANO (?} M~DD£E JURASSIC
BONANZA
~,ARLY .~URASSIC
GROUP
VANCOUVR R GROUP i
LATE JURASSIC TO EARLY C R E T A C E O U S
t
PARSON RAY FORMATION QUATS~NO FORMATION
LATE AND (?] MIDDLE TRIASSIC
KARMUTSEN EORMAT)ON
~',
S~CKER GROUP
PALEOZOIC
METAMORPHIC COMPLEXES
JURASSIC AND OLDER
MILES
20 ,
;o
Figure 16. Geologic map of Vancouver Island and the Gulf Islands (after Muller et al., 1981). Location of LITHOPROBE seismic lines in relation to the Vancouver Island geology is shown in Fig. 14.
97
the Survey Mountain in
the
fault,
Late Cretaceous
movements
apparently
tectonic
slices
as a part of a regional
and early Tertiary
resulted
in the
from the Northwest
(Johnson,
dispersal
1985).
western
zone of the Lyre Formation
(Babcock
Presence
et al.,
1994)
1984).
Such
Pandora
Peak
along the San
(Rusmore and Cowan,
Peninsula
of
system,
Cascade thrust belt on the San
Juan Islands to their current positions
lithofacies
fault
of Pandora
suggests
Juan
fault
Peak clasts
in the
on northern
Olympic
these offsets occurred
prior to 40-42 Ma.
Steep foliation to
the
Survey
indicates
in the Leech River metamorphic Mountain,
complex
is parallel
San Juan and Leech River faults.
the three faults acted in tandem during
the
This
uplift
of
the Leech River complex.
The eastern part of the San Juan fault, the Survey Mountain the
Orcas
fault,
runs NE.
Columbia mainland,
(Monger,
of the San Juan Rise.
to have accommodated
linked it with
of laterally displaced
this fault is steep. of this fault as a (Clowes
On the
and
British
normal movements
in the
1991).
the San Juan fault zone
complex
(1977a)
the Vedder fault on trend with this NE-oriented
fault belt is thought
Presence
Muller
fault belt which runs across the Strait of Georgia
forms the northern boundary
Neogene
from its merger point with
slices of the Pandora
(Rusmore and Cowan,
This evidence 10w-angle
et al.,
that this fault remains
contradicts
thrust
1987).
1985)
in
a
steep in the subsurface,
resolved with seismic data.
suggests
that
the interpretation
Tertiary
Mayrand et al.
Peak unit in
subduction
(1987) have shown as far as
it
is
98
From
the
description
OWSZ extends Strait
of
in this section,
from the interior Juan
de Fuca,
the exterior continental
North Olympic The southern Olympic
Calawah
strand of
faults
basaltic
is
on
fault in
the
OWSZ
consists
well
of
interrelated
12, 13, 15,
and
the
area
are
(Tabor and Cady,
The Hurricane
subparallel
Ridge
Crescent and
Crescent
fault,
of
thrusting
stratigraphic
basalts with sediments of
the
Fuca
to the south. a
local
continuity
et al.,
and
no
distinct
1994).
about N70°W.
It
contains
dipping about 60 ° to the north. interior
Hurricane
Ridge fault.
is
(see
Reidel
insignificant.
relationships
to the north
that allows
gap
across this part of the
et
As al.,
fault,
The
recently
of the Crescent
Formation
of the Olympic Mountains
Basin
Lake
on both sides of the
similarly,
zone strikes
in the Cordilleran
extent
established
reveal
and
successions
observed
Babcock
strands
evolution.
small thrust splays are also found along the master
but the
and
is
their Ridge,
Ridge
Sediments
deformed
1978;
Ridge fault
along the OWSZ 1994),
during
sedimentary
boundary
northern
17).
the basaltic buttress. this
several
exposed
several major W N W - o r i e n t e d
lie between the Hurricane
structural-domain fault
onto
shelf.
It
(Figs.
massifs
in
eastern
Island,
Near the eastern end of the Hurricane occurs
across
southern Vancouver
prominent of them are the Hurricane
These faults
strand of the
fault system
Peninsula.
most
of the Cordillera,
through
faults which were complexly The
the northern
core to the
(see Babcock et al.,
only small displacements
across
south 1994) the
99
0/,,
I
48° t <.
"/
Mt ~" Olympus
%
?
0
10
0
10
20 MILES 20 KILOMETERS
EXPLANATION
Basaltic rocks of the Crescent Formation
Folded thrust fault Queried where uncertain. Sa wteeth on upper plate
High-angle fault Dashed where inferred
I,
~5"20
Syncline
Anticline
Overturned anticline
Large*scale drag fold
St, owing direction o f plunge
Show~,~g direction o/plunge
Showing direction o f dip o/limbs
Showing plunge o f axi~
Figure 17a° Distribution of faults and Crescent Formation basaltic massifs in the southern strand of the OWSZ and elsewhere on the Olympic Peninsula (modified from Tabor and Cady, 1978).
100
g
~'~ "-< ~ - ~
~A,=46.7to s2.3
Figure 17b. Distribution and age of Crescent Formation massifs in western Washington and British Columbia (modified from Babcock et al., 1994). The northward swing of the Leech River fault along Puget Sound is not supported by the data presented in this volume: rather, the Leech River fault appears to be part of the northern strand of the OWSZ.
101
The
Crescent
Hurricane angle
some
The Crescent
a
more
which
northwesterly
25 ° (Tabor and Cady,
which
it
and Calawah
1978;
at
than the
124°W at a an
Babcock et al.,
to the closely
merges
faults
trend
it meets near longitude
fault runs parallel
with
Crescent
has
Ridge fault,
of
fault,
fault
adjacent
some localities.
1994). Calawah
Between the
lies a narrow strip of Eocene
clastic
rocks with minor basalt.
Like
in the Hurricane
Ridge fault
plane dips 50°-60 ° to the north, described Snavely,
along 1987).
Calawah
trend
Cady,
fault is straight
Motion
1978)
but thrust sheets have also
been
the
on
in the late Middle fault
1977;
and
by
Snavely,
once
interpreted thought
1987).
North
relationship,
details
to
of
Eocene
along its entire Flattery
as reverse have
and
(Tabor and
been
sinistral
movements
of young deposits
shelf bathymetry
have
1987).
to Cape
Post-Pleistocene
1977;
on this
onshore
and
(MacLeod et al.,
1987).
The Hurricane Ridge, herein
(Snavely,
and near-vertical
fault are indicated by deformation offshore,
(MacLeod et al.
Peninsula
was
(Snavely,
west
Olympic
but is now usually
strike-slip
fault
in
on the Crescent
length from north-central beyond.
the master Crescent
Thrust movements
been interpreted
The
its
zone,
Crescent
Olympic
and Calawah
fault
have existed
to
served as magma conduits.
Pass,
Hurricane
Crescent
together
emphasize
since at least the Early
they
Ridge,
system
faults,
named
their genetic Eocene,
Several basaltic massifs Lake - lie along these
when
- Marmot
faults.
102
A long history of the southern strand Olympic
Peninsula
across
this
controlled
is
fault the
apparent
system.
position
of
the
OWSZ
on
also from the geologic contrasts
The
North
Olympic
fault
of
site
of
the
Their beginnings may lie in the Paleocene
(the Blue Mountain unit), record
system
of different sedimentary basins, one on
the site of the Olympic Mountains, the other on the Strait of Juan de Fuca.
northern
but
the
well-preserved
stratigraphic
the Fuca Basin spans the Middle Eocene to the Miocene.
In the late Tertiary, the North Olympic fault system served as important
structural
boundary.
To
deformed
(Central
Olympic
and
Hoh
slightly
(Central
Olympic
Basin).
an
the south, rocks are highly basins)
and
To
north, in the Fuca
the
metamorphosed
Basin, they are deformed only mildly and unmetamorphosed.
Central Olympic Basin A large
sedimentary
Basin,
existed
on
basin,
designated
herein
Central
Olympic
the site of the present-day Olympic Mountains
from Paleocene to Miocene.
Fairly stable paleogeological settings
resulted
of
in
accumulation
thick,
subordinate sandstone and conglomerate, with
basalt
(Tabor
and
Cady,
1978).
monotonous
occasionally
environments,
early
large
existed
in
a
with
interbedded
Sedimentation took place
dominantly in continental-slope Tertiary
mudstone
area
which
during
the
from the site of the
Dosewallips basaltic massif outboard.
Despite the control, into four material
generally
monotonous
lithology
and
scarce
fossil
sediments of the Central Olympic Basin have been grouped assemblages
based
on
relative
abundance
and fossil ages (Tabor and Cady, 1978).
of
coarse
They were later
103
interpreted
in
principally
the
mostly 32 Ma
two
main
successions,
ages
(Brandon and Vance,
(Middle Eocene to Early Oligocene), (Late Oligocene
of the Central the interior
1992).
Rocks of
slightly
reaching
the
were
older
Olympic
sedimentary
rocks
at 30-29 Ma by
Mountains.
Even
more
Basin
is
well
though
now
(Tabor,
rocks
are
rise as high as 2427 m at Mt. Olympus.
are characterized relative
faults divide sedimentary
by different
proportion
these rocks reacted stresses
tectonic
of
sand
slices
its
in
1992).
exhumed
(Fig.
soft
and
and easily eroded,
rocks
rock assemblages.
in
18).
the
Olympic
These slices
Distinguished
and shale and abundance
slightly differently
Later,
metamorphism,
rather
which makes them fissile
several
1972).
in the core of the Olympic
the mountains
into
metamorphosed
and
sheared,
Mountains
(Heller et
(Brandon and Vance,
penetratively
Large NW-trending
source
This event occurred
inverted,
exposed
these
were
intense
grade.
around 14-13 Ma
are
to
and the younger one from
succession
affected
the late Early Miocene,
48
Basin have been traced to
prehnite-pumpellyite
The Central
has ages from
of southern Canadian Cordillera
in the mid-Oligocene,
both successions
which occur
to Early Miocene).
Olympic
the
1992).
lie chiefly
and blocks of basalt,
The older succession
in
distinguished
between these two successions
of stringers
in older rocks.
Sediments
al.,
differences
abundance
27 to 19 Ma
areas
of
by their fission-track
Lithological in
terms
by
of basalt,
to compressional
tectonic
in late Tertiary time.
The oldest,
Lower-Middle
Eocene rocks make up the Elwha assemblage
30'
¢7°
~8 °
A
10 KILOMETERS
...."C-
10 MILES
Western Olympic litlfic assemblage
Peg,
Pet,
Tgss,
phyllitestoneand fotiated thick-bedded sand-
Grand Valley lithi¢ assemblage sandstone and foliated saodstone with dO 70 percent siltstone, slate, and Tgs,
greenstone and greenschlat
glomerate,schistSandst°ne*thick'beddedandCOn'f°liatedsemi- sandstone
['ess~ foliated semischtst with 10 - 5 0 percent slate and phyllite
Elwha lithic assemblage Fes, s l a t e a n d p h y l l i t e with less than 20 percent/oliated sandstone and semischist
basalt, minor diabase, and gabbro thin.bedded Mare a~ld siltstone with less than 30 percent sandstone
Tnb, TnL
Trims, thick.bedded micaceous sandstone
Tnm, micaceous sandstone with less than 60 percent slate
Needles - Gray Wolf lit hie assemblage
Twos, slate and phyllite with less than 30 percent sandstone arid semi~chtst phacoids T ~ o t , thlck.bedded sandstone with less than 20 percent slate
Tx~o, sandstone, foliated sandstone, and semiscbist with less than 40 per. cent siltstone and slate
"cb,
"s,
Cre*eent Formation; basalt, diabase, a n d gabbro .j
shale, sandstone, and conglomerate
Sedimentary and igneous rocks
Peripheral r o c k s
F~ocks of the Southern fault zone 7late. phyllite, foliated sandstone, semischist, basalL and diabase
I
_]
EXPLANATION Core rocks
Figure ~i~ 18. Geologic map of the Olympic Mountains area, showing ~aults and tectonic slices in the Central Olympic Basin (modified ~rom Tabor and eddy, 1978).
O
~.~ ...__
123~30 '
~o
,x
105
in the fault-bounded slice
next
slice in the center of the mountains°
to it on the east,
In
a
Middle and Upper Eocene as well as
Lower Miocene rocks are contained
in the Grand Valley
In
Upper Eocene and Lower Oligocene
a
slice
still further east,
are found in the N e e d l e s - G r a y older
core
Oligocene Middle
lies
Wolf assemblage.
the Western Olympic
to Lower Miocene
sediments
Eocene to Lower Oligocene
regular
E-W age progression
northern Olympic
Peninsula,
usually
with
and Vance, Valley and
slices a
The most
Wolf
These
more-deformed
partly
of
tectonic
rock
thrust-sheet found
of tectonic
stresses
intensity
successions
over
one
(Tabor and Cady, are
slices rocks
assemblages
depocenters.
deformation
However,
Thus,
no
rocks exists across the
thrusted
deformed
less-deformed
several
compression
older,
may
slices.
the
lie
another,
1978;
Elwha
Brandon
and
the Western
are
Grand Olympic
relatively
less
on both sides of the
having
and degrees of deformation
compressional
farther west.
and rocks of different
slices suggests that the original contained
and
rocks in the middle.
The coincidence lithologies
which contains Upper
whereas to the west and east,
Needles-Gray
deformed.
sediments
westerly vergency
1992).
slices,
are
To the west of the
near Mt. Olympus
of sedimentary
be found in the four tectonic
These tectonic
slice,
assemblage.
with identifiable
induced
1987), by
the lack of a systematic also suggests
tectonic
They were later telescoped Evidence
in the Olympic Mountains
probably
variable
Central Olympic Basin might have
complex.
(Snavely,
slightly
of
into a
polyphase
indicates variability
though the strongest was E-W
Juan de Fuca plate subduction. E-W progression
other influences.
in
deformation
106
Hob Basin A
different
stratigraphy
characterizes
exposed west of the Olympic Mountains Basin.
Common
of the Olympic are
rocks in outcrops Peninsula
turbiditic
(Snavely, been
deposited
et al.,
1992b).
Snavely
(1987)
units
tentatively by
and Late Oligocene
(1993)
were
later
unable
coastal
lowlands
shelf
offshore
siltstone
and mudstone have
or trench environments
(Niem
Middle
thought
in the Hoh succession hiatus:
Miocene.
two
Middle to Late Orange
to confirm the existence
et
al.
of these two
but the age span of the succession was not challenged.
Basin
was largely coeval with the Central Olympic
paleoenvironments massif
basins,
were strikingly
similar
from
to at least the edge of the present-day
be possible
if a large embayment
forming
Niem et al.,
Miocene,
Basin,
where sedimentation
This would
margin
two (cp.
ended in the
Hob
Basin continued to develop much later
Its
structural
evolution
In the late Middle Miocene,
by
faulting,
continental
and
Dosewallips
shelf.
in the continental
different. thrust
Basin,
1992a-c).
the
1992c).
the
The
existed on the site of these
a deep re-entrant
Unlike the Central Olympic
al.,
Olympic
to
distinguished
to
assemblages
Central
on the
They are
a large stratigraphic
Eocene,
Hoh
1993).
in continental-slope
separated
units,
on the western
and organic-rich
1987; Orange et al.,
rock
and of the
and in drillholes
sandstone
the
induced
and oceanic plates.
partial underthrusting
perhaps
was
also
(Niem et
considerably
the Hoh Basin was affected by
interactions
This tectonic
of pre-Late Miocene
episode
rocks
of
the
resulted
(Niem
et
in
al.,
107
1992b),
but
in
many
thrusted
over other rocks
1993).
Both
faults
both the continent thrust
The
is d i p p i n g
places
(Snavely,
and
mapping
and
a n d the ocean. oceanward
in
structural M~langes
several
are
of
s t e e p faults.
strata
areas
(e.g., has
revealed
zones
a N-S-oriented
1987).
large
is u n e v e n
N o t all the d e f o r m a t i o n
areas
had tectonic
sediments
are
origins.
diapirism
were
in r o c k d e f o r m a t i o n .
early
Miocene
by tectonic
was
uplift
interrupted
and erosion.
resulted
in a l a r g e s t r a t i g r a p h i c
gap spanning
between
about
15
and
7
When
Quinault
Formation
was
laid
Pliocene,
The
at
over a pronounced
Quinault
deposits,
Formation
as c o n d i t i o n s
present.
P-0141
Ma.
angular
contains
in
latest
years
resumed,
the
Miocene
and
u p to 2000 m of s h a l l o w m a r i n e were comparable
25 k m f r o m t h e coast,
a similar
the
This event
eight million
sedimentation the
in
mud
unconformity.
of s e d i m e n t a t i o n
O n t h e shelf,
well encountered
down
Sediment
and p o s t - d e p o s i t i o n a l
of t h e u p p e r H o h a s s e m b l a g e
Middle
with and
setting
Accumulation
1990;
uniform,
in a c o n t i n e n t a l - s l o p e factors
and the
are c o r r e l a t e d
where
in
Peninsula.
(Orange,
mobility
important
Detailed
diapiric,
in the H o h B a s i n w a s n o t broad
mostly
variations
Olympic
z o n e s of s h e a r i n g
lie
al.,
towards
as c o n s i s t i n g
including
and sheared rocks
Deformation
et
are d i p p i n g
Snavely,
origins
Many
Orange
as
15).
has b e e n d e s c r i b e d
rocks
1993).
sheared
coherent.
320;
A l o n g t h e coast,
(Fig.
different
of b r o k e n
et al.,
between
p.
s t y l e s a l o n g t h e w e s t c o a s t of t h e
distribution Orange
broken
1987,
sedimentary
entire Hob assemblage
of m ~ l a n g e
the s a m e r o c k s h a v e b e e n m a p p e d
thickness
to
those
the Pan-American
of P l i o c e n e
and
Upper
108
Miocene
clastic
Oligocene(?)
rocks,
Therefore,
the
the
early
Hoh
Basin
has evolved
on
the
site
Peninsula.
By Late Miocene
shallowed,
and shelf environments
of western Olympic
The boundary a
basement
understood,
the
with
data
a
of
the
Hoh
and
Central
this
area
is
in
shelf.
known
of
Mesozoic
(MacLeod et al.,
New gravity data for western Washington
to date and examine
in
line
large-scale
with
recent
crustal
interpretations
reflection
profiles
and
is from
poorly
in opposite of
orogens.
of two distinct plain
are separated by
area,
which
crystalline
contains
rocks on the 1992a).
area from gravity data and
southwestern
these
elevations
were constrained facts.
British
interpretations
geological
structure,
and geological
basins
under the coastal
to bring geophysical
continued to a range of nominal Final
belts
1977; Niem et al.,
of the Olympic Peninsula
it possible
basin
Peninsula
complex
These two depressions
the
make
Olympic
the
Olympic
fold-and-thrust
high in the Point of the Arches exposure
in
in large parts
variety of thrust faults dipping
a basement
Columbia
however,
were established
which lie in large depressions
Deep structure
well
existed
present-day
suggest that the Hoh Basin consists
Olympic Peninsula
the
and adjacent submerged margin.
in
common
and the continental
only
slope
high inferred on the Olympic
Structure
as is
depocenters
to Upper
since Middle Eocene time.
to Pliocene time,
Peninsula
between
gravity data.
Gravity
Miocene
on a very wide continental
Tertiary
directions
Middle
1987).
environments
probably
by
sediments which continued to the b o t t o m of
at 3160 m (Snavely,
Bathyal
underlain
information.
up To
data were upward
from
by seismic
5
to
i00
refraction
km. and
109
In the Bouguer anomaly map to
large
basaltic
massifs.
anomaly of >+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 <-80 mGal in the free-air reduction (Finn Dehler
lows,
Clowes,
1992),
Between
these
lies two
just
west
negative
et
of
al.,
Cape
gravity
1991;
Flattery
anomalies,
a
positive anomaly, with values higher than -50 mGal, lies northern
corresponds
to
west the
coast
Point
Mesozoic
crystalline
analoqy,
the
gravity
of
of
rocks
Olympic
Peninsula.
It
the Arches structural high, where
are
high
the
exposed
between
at
the
surface.
By
the Central Olympic and Hoh
basins may represent a buried basement arch.
Straight gravity gradient zones separating the Central
Olympic lows trend NW and NE.
these basins
have
boundary
the
of
the
same
Central
trends
Olympic
southern
Hoh
and
In outcrop, faults between (Fig.
gravity
17). low
The
southern
and the eastern
boundary of the southern Hoh low are in part oriented NW
and
lie
on trend with the Nisqually lineament at the southern end of Puget Sound (McCrumb et al., 1989b). zone
is
part
This NW-trending gravity
of a larger set of anomalies suggested by regional
isostatic maps to begin in northern Oregon 1990).
gradient
(Blakely
and
Jachens,
The Nisqually fault zone thus extends from the Washington
Cascades into the olympic Peninsula and even beyond,
towards
the
submerged Pacific margin.
Many
straight
gradient zones with various orientations, bounding
112
polygonal
gravity
Many
those
of
and the g r a d i e n t gravity
data
persistent system
upward
only
eastward
the
margin
anomaly oceanward
the m a p
lies over the g r a n i t e - r i c h ,
discernible
Regional gravity the
from g r a v i t y
structure data
North
gradient
Cascades
is
The
as it is in the causative Metchosin
Metchosin
high-density
magma
chamber
massif,
whose mapped
highs
in
southern
Coast
Range
basalt
to 20 km
in Fig.
Finn,
100-km
map
are revealed. across
the
of the
thick
crust
slab
of the
is
not
1990).
expressed 21).
in the m a p of The
low
Creek-Fraser
and c e n t r a l l y
19.
This
directly lateral
over
is only Finn
7 km. (1990)
to be up to 30 km thick.
beneath
levels
fault
symmetrical,
indicates
offset
lie at m i d - c r u s t a l
Washington,
the g r a v i t y
the
Fuca
(Fig.
is s t r o n g
thickness
and
in the n o r t h e a s t e r n
Straight
significant
may
Calawah
-
on the w e s t by an e a s t - s i d e - d o w n
body extends
without
frozen
map
de
is well
large
high
low-level
massif,
(see also
flanked
to the
anomaly
Juan
the c r u s t
continued
zone r e l a t e d
system.
data
of
upward
negative
subducted
fault
attenuation
corner
The
and
values
The large
Cascades.
Prominent
of the OWSZ
features
crust.
North
the
area,
In
continental of
when
the
of this
regional
the
strands
i00 km.
Bouguer
reflects
shape,
Peninsula.
to 20 and
of
polygonal
Nisqually
Island,
structure
largest
decline
the
and s o u t h e r n
Olympic
faults.
less distinct,
mark
Vancouver
large-scale continued
sharply
with
to just 5 km.
however,
on n o r t h e r n
upward
continental
lose t h e i r
continued
on
associated
considerably
zones,
fault
faults
20),
probably
as the n o r t h e r n
River
data w e r e
Gradual
highs
are
investigate
(Fig.
gravity
gradient
the L e e c h
To
are
zones b e c o m e
as well
Crescent
highs,
that the
the e x p o s e d
at depth. beneath
From similar has m o d e l e d
A this
gravity
bodies
of
¢¢)
-t
115
Of particular -55
mGal
over
rectangular Olympic
interest the
Olympic
Peninsula
N-S-trending
(Fig.
17).
corresponding This
zones
Fault Zone,
Peninsula
crust.
as a boundary negative
of
the
in
hypocenters
northeastern between
though
large,
zone
(Finn,
the
suggests 1987).
corner
on the scale
though
Deflection
at
depth,
of seismicity
been considered
puzzling
to
which
are
resist
distribution
with
the
of
slightly
change
in
fault system:
the
strike than the Crescent
trend of the OWSZ. block
of
of the OWSZ around this
parts of the North Olympic
of the OWSZ and the North Olympic anomalies
the
consistent
faults and the general
in
thought to be rooted
in may be arching upward
is
block
is not apparent
structures
Ridge fault has a more westerly
concentration
on
anomaly.
This block is therefore
(cp. Crosson and Owens,
field
lies
crustal
OWSZ,
reworking
and Calawah
massif.
Puget Sound
tectonic
Hurricane
the
gravity gradient
It has been rigid enough
orientation
the
is
which
above the base of the crust.
block's
a
of this block
it from the Dosewallips
the 100-km gravity map, which reflects
earthquake
on
boundary
fault that crosses
by the rectangular
entire
fault
north-side-down
This North Olympic crustal block,
the
of
fault is shown by the 20-km gravity map to continue
across the Olympic delineated
major
is the Southern
to the Seattle
low
This anomaly outlines
Its eastern
fault which separates
with an E-W-oriented,
elongated
The northern boundary
fault system.
Its southern boundary
1990).
Peninsula.
feature bounded by known
is the North Olympic
trend
in the 20-km map is the
may
be
Interaction
causing
reflected
in
stressa
strong
in Puget Sound whose origins have long
(e.g., McCrumb et al.,
1989b).
116
On the nature of crystalline basement of the Olympic Peninsula In
the
1970s,
Crescent Formation basalts, presumed to be Eocene
oceanic crust, were regarded as the crystalline basement for thick Tertiary 1977a). Clowes
sedimentary
successions
in
this region (e.g., Muller,
Though it remained popular in the et
al.,
1980s
(Duncan,
1982;
1987), this interpretation has been disproved by
recent findings.
The Crescent Formation does not have an oceanic-crustal origin and cannot
be
regarded
as
the basement.
Many discontinuous Eocene
basaltic massifs were produced by eruption from different volcanic centers.
These
basalts overlie and interfinger with sedimentary
rocks of the Blue Mountain unit, overlain
conformably
and
this
package
is
in
turn
by sediments of the Adwell Formation of the
Fuca Basin (Babcock et el., 1992, 1994).
Mesozoic crystalline rocks are exposed on the northern west of
the
Olympic
Peninsula,
at Point of the Arches.
coast
Though these
old granitoids are only known from a small area several kilometers across, their presence requires an explanation.
An
outcrop
of
crystalline rocks at Point of the Arches contains
gneissic diorite and gabbro of Late Jurassic K/Ar
radiometric
1977). and
age,
chert
reported
dates on the diorite of 144 Ma (MacLeod et al.,
Also exposed in that locality are greywacke,
minor
with
believed to be Lower Cretaceous.
pillow basalt This Mesozoic
assemblage is covered, apparently with a depositional contact, Tertiary
sediments
and basalts
1991; Niem et al., 1991b).
by
(Snavely, 1987; Niem and Snavely,
117
In the context of the oceanic-crust model, were
these
Mesozoic
rocks
presumed to be an olistostromal block derived from Vancouver
Island and incorporated into the sedimentary matrix by unspecified processes
in
interpretation
the
Middle
Miocene
(Niem et al., 1992b).
Another
(Snavely, 1987) regarded Point of the Arches
as
a
separate terrane juxtaposed against other small terranes in a very complex manner: numerous, variously oriented sheets separated b y m u t u a l l y
3,
vergent
thrust
truncating local faults were proposed
in this small area (Fig. 15). Figs.
and
Brandon
and
Vance
(1992,
their
15) linked all these "terranes" on northwestern Olympic
Peninsula,
including the one containing the Mesozoic
rocks,
with
the rock assemblage in the core of the Olympic Mountains.
A
layer
of
crystalline
crust
under
the Olympic Peninsula was
included in several gravity models (R,W. Couch in MacLeod
et al., 1977; Riddihough,
1979).
this crystalline crust was 2,900-2,920 boundary
1977a;
The density assigned to
kg/m3.
The
crust-mantle
was shown to lie variously at 20 or 27 km.
The totality
of such characteristics is best explained if area is continental
(see chapter I).
the
crust
in
this
Though no seismic reflection
data are available in the rugged Olympic profiles
Muller,
Mountains,
modern
deep
on southern V a n c o u v e r Island help elucidate the stucture
of the continental crust.
Deep structure of southern vancouver Island from seismic data Two reflection profiles Island
(Fig. 14).
cross
the
OWSZ
on
southern
vancouver
Though short and lacking a tie, they provide a
partial insight into the crustal structure
in
this
area.
Line
118
84-02
is
basalts, Leech
24.4
km
crosses
River
begins
Mountain
and S u r v e y
fault,
Mountain
et
al.
and
postulated the
Line
84-04
and ends
Metchosin
massif
seismic
at long t r a v e l t i m e s
equivalent beneath
This
to
those
Vancouver
model
subduction
southern Farther
This
Such
fault
south,
the
with
in t e r m s
in t h e s e
exotic
were
interpreted
Olympic
Peninsula,
base
of
profiles complex
"terranes"
Broad bands as
of
sediments
underthrusted
undulating, Island
at
and
is
of the OWSZ -
that
a
depth km
under
and M i d d l e
as
the
speculative,
information
deep-rooted,
fossil
massif
Island
steep,
and
to
be
Fuca Ridge
Basin. fault.
insufficiently
discussed
conduits
Eocene.
a
of 6 to 13 km u n d e r
w i t h the H u r r i c a n e
rather
acted
requires
hypothesized
- the S o u t h V a n c o u v e r are
in the E a r l y
was
8-10
it was c o r r e l a t e d
1992)
of the M e t c h o s i n
megathrust
by the g e o l o g i c a l
systems
junction
The L e e c h R i v e r as
23).
to the S u r v e y
lines
in the Eocene.
and Clowes,
at
a
interpretation
discontinuities magmas
and
Vancouver
constrained strands
Dehler
megathrust
subhorizontal
the
faults
(Fig.
basalts.
seismic
All
the
Island.
(see also
Wrangellia.
Wrangellia
on
its
in K a r m u t s e n
treated
beneath
between
in W a r k g n e i s s e s
runs p a r a l l e l
thrusts.
underthrusted events
a n d ends
these
were
complex
fault n e a r
structure.
to be l o w - a n g l e
in the M e t c h o s i n
is 20.8 km long
complex,
interpreted
thrust
It b e g i n s
faults,
the San J u a n
fault,
(1987)
subduction-related were
Mountain
complex.
crosses
22).
River metamorphic
in the L e e c h R i v e r
the S u r v e y
Clowes
(Fig.
the L e e c h
of the W e s t c o a s t It
10ng
above.
and N o r t h
Both Olympic
long-lived
crustal
for C r e s c e n t
Formation
From
the
Middle
Eocene
119
LINE 2
S
N
I-
Figure 22. Deep structure of the northern strand of the OWSZ on southern Vancouver Island, imaged in the seismic reflection line 84-02 (modified from Clowes et al., 1987). Note the arcuate events around 4 s and the basal north-dipping events below 8 s. Profile location is given in Fig. 14; profile length is 24.4 km.
120
I l l q l ~ 4,
SE
Figure 23. Deep structure of the northern strand of the OWSZ on southern Vancouve Island, imaged in the seismic reflection line 84-04 (modified from Clowes et al., 1987). The events labeled "B" and "F" are discussed separately in Chapter 9. Profile location is given in Fig. 14; profile length is 20.8 km.
121
until
the
Early
Miocene,
the Fuca graben
fault systems,
creating the 8-km-thick
northern
and
southern
Vancouver
Island fault system,
uplift
from
complex,
Protoliths in
the
Eocene,
complexes
juxtaposed.
Eocene Lyre Formation
Cowan,
idea
(Muller,
these
However,
sedimentary variously
successions metamorphosed
stratigraphic plutons,
these
movements.
diffractions
Steep
buried,
metamorphosed
the Leech River
and
sediments
Metchosin
for the late Fuca
Fairchild 1987).
1994).
1982; Rusmore
is generally
flatten
of low-angle
and Cowan,
For detecting
profiling
faults
out events
high-angle
a poor tool.
at depth relies in lines
84-02
events may have other origins. on southern V a n c o u v e r and deformed. contacts,
faults
and sideswipe.
Many
and
Island are
Reflections metamorphic
structural
Mesozoic
rocks
and
and
Volcanodiverse,
fronts,
tops of
flanks of plutons may cause
events
stratigraphic
on a
may arise from
are
observed
84-04, whose SE part runs along the steep Survey Mountain
Great
Basin
the San Juan and Leech River faults
et al.,
and structural
sills.
The South
accommodated
and younger units of the
1977c;
interpretation
has
the
the Leech River metamorphic
They provided
seismic reflection that
structural 84-04.
of
1994; Garver and Brandon,
1985; Mayrand
structures,
Faults on
along faults of the South Vancouver
to the thrust model,
are subvertical
The
particular,
also strike-slip
During the uplift,
(Babcock et al.,
and
depths
and uplifted
were
Contrary
in
of the Leech River complex were
Island system.
Middle
Fuca Basin.
strands of the OWSZ are steep.
mid-crustal
and perhaps
subsided between these
complexity
is revealed by geologic mapping
in
fault.
in Paleozoic
(see
Line
Chapter
and 3).
122
The Paleozoic Sicker Group, made up of many units with contrasting lithologies limestone)
(basalt,
sandstone,
siltstone,
up
to
Triassic Karmutsen basalts
6 km thick, and together these two successions form a
broad anticlinorium (Muller,
in
1977a,b,
the
1980a;
eastern
half
of
a
synclinorium
Vancouver
Massey and Friday, 1989).
flank of this positive structure, north of lies
where
the
Sicker
the
rocks.
Upper
Triassic
about
1
with
km, are exposed in places.
Juan
fault,
Group and the Karmutsen
limestone
Quatsino and Parson Bay formations,
Island
On the west
San
Formation are covered by up to 3.5 km of younger volcanic
chert,
is heavily faulted, folded and in places metamorphosed.
These rocks are at least 5.5 km thick. are
argillite,
a
sedimentary
and total
shale
and
of
the
thickness
of
More widespread is the Lower
Jurassic Bonanza Group, which contains up to 2.5 km of
volcanics,
conglomerate, sandstone and shale.
Because
deformation
the Bonanza Group is rocks,
and
in
many
on Vancouver Island occurred in many pulses, deformed areas
less its
than
bedding
the
underlying
older
remains subhorizontal.
Whereas the Bonanza Group is unmetamorphosed, metamorphic grade in Karmutsen
basalts is usually no higher than prehnite-pumpellyite,
and in the Sicker Group as high as greenschist.
Further complexity is provided by abundant plutons
(Muller, 1977a-
c; Woodsworth et al., 1991) and sills (Fairchild and Cowan, 1982). Jurassic Island Intrusions are widespread across Vancouver Island. From
geologic
field
mapping and analysis of magnetic anomalies,
many of the exposed pluton are thought to be batholiths 1988).
at
depth (Jeletzky,
Another pulse
of
apophyses
of
large
1976; Arkani-Hamed and Strangway,
intrusive
magmatism
occurred
in
the
123
Tertiary, sills
producing
in t h e L e e c h
bodies
and
Catface River
sills
may
and Sooke plutons
complex. serve
Tops
as l o w - a n g l e
different
levels
about
s, w h i c h m a y b e c o r r e l a t e d
4
in t h e crust.
feature
complexes,
km.
Subhorizontal,
according
to
Clowes
s
(28
et al.,
to
10.7
north-dipping profile.
events
continental which
Vancouver
Island
On Vancouver been
diorite dikes Catface
of
nature
on
mapped
in
suite
Vancouver
in
the
about
conversions
northward,
lie
this band with
south
end
below about
of
that
i0 s in b o t h
Such a Moho depth
is a l s o s u g g e s t e d
Tertiary
Olympic
on
Peninsula
country
intrude the
intrude pre-Tertiary These plutons
n o s i g n of c o m p r e s s i o n a l
dipping
Moho.
rocks:
is
magmatic
4).
episode
gabbro and quartz massif,
granitoid
intrude deformed themselves
southern
(see C h a p t e r
felsic
plutons
rocks throughout
deformation
by felsic
both
Metchosin
in t h e L e e c h R i v e r c o m p l e x ,
Island.
depth
and sills from that
various
of t h e S o o k e s u i t e
lies a t
is c o n t i n e n t a l .
and northern stocks
and Wark
lie o n b o t h e n d s of
km;
the
reflections
of the c r u s t
occurred
Island,
occur
pluton-like
fault.
s e e m to c o n n e c t
at 7.5 t o i0 s
t h a t the c r u s t
magmatism
have
events,
Weak events
at
O n t h e n o r t h e n d of L i n e 84-
probably marks the reflection
an i n d i c a t i o n
The
s.
Disappearance
profiles
a
Island
events 35
1987).
02, a d e e p b a n d of h i g h - a m p l i t u d e to
events
at
t h e n o r t h e n d of L i n e
indicate
t i p of V a n c o u v e r
discontinuous
at 8.5 to 10.5
9.5
might
reflectors
arcuate
on b o t h s i d e s of t h e S u r v e y M o u n t a i n
Line 84-04
at
seismic
igneous
at a d e p t h of 1 0 - 1 5 k m in t h e L e e c h R i v e r
The Moho under the southern 35
between
as f e l s i c
intrusive
Concave-upward,
8 2 - 0 2 a n d t h e SE e n d of L i n e 84-04, domal
of
as w e l l
the
of t h e
rest
of
rocks but bear
(cp.
Massey
and
124
Friday,
1989).
Rather, they are cut by high-angle normal faults
(Muller et al., 1981).
Timing of inversion of the Central Olympic Basin and uplift of the Olympic Mountains Though
soft, sheared clastic rocks make up the Olympic Mountains,
the topographic relief is high and rugged, rising at to as much as 2427 m.
Mt.
Olympus
To sustain such a high relief despite rapid
erosion, the uplift must be rapid.
The work of Tabor Calderwood
(1990),
centered on Mt. terrain
and
Cady
showed
Olympus.
(1978), the
supported
Olympic
This
rising
by
Mountains dome
and
Brandon
to be a dome the
mountain
it creates are fairly isometric in map view (Finn et al.,
1991).
In contrast, metamorphic isograds outline a smaller
30
long
km
with
and
pumpellyite
contours.
grade
are
found
Rocks in
metamorphosed this
oval
to
on
the
structural
prehnite-
(Brandon and Vance,
This metamorphic pattern in the Central Olympic
superimposed
oval,
15 km wide, elongated E-W (Fig. 24), discordant
topographic
1992).
and
Basin
is
pattern of NW-trending tectonic
slices (Fig. 17).
The tectono-metamorphic and tectono-magmatic history of this basin can
be
restored
from
the
available data with a fair degree of
confidence.
An early episode of widespread rock recrystallization
took
at
place
30-29
Ma (Tabor, 1972; Brandon and Vance,
This rock-alteration event seems to have accompanied deformation
the
pulse
of
which affected the entire northern Olympic Peninsula.
General restructuring led to the creation of distinct where
a
1992).
younger
Basin was laid down.
depocenters
sedimentary succession of the Central Olympic
125
i ......
.Es~
sAMPLEs
i
•
o.
i
.,,. L \(~'z(%-~zJW~l-__ ~,~. i 50km I
I
\ ~ ~ m ~ ~ '
""--'----~"
Figure 24. Metamorphic aureole in the Olympic Mountains, based on fission-track data from detrital zircons (modified from Brandon and Vance, 1992). The numbers indicate fission-track ages of zircons. Rocks within the small oval aureole are m e t a m o r p h o s e d to p r e h n i t e - p u m p e l l y i t e grade, and zircon ages within it are reset due to m e t a m o r p h i s m at 13-14 Ma.
126
Sedimentation about
19
ended with a new pulse of heating
Ma,
when isoclinal
which affected tectonic
all the rocks,
slices
(Fig.
rocks of all ages, 1978).
determined
Inversion
Uplift
in
core
metamorphism
(Fig.
of the basin
by
the
the
of detrital
present-day
12 Ma fission-track 1990;
and
Cady,
24)
14-13 zircons;
(the
Late
Miocene
1990;
Babcock
(Niem et al.,
Mt. Olympus,
was
Ma
(as
Brandon
Montesano et al.,
1992c).
is continuing
of
tectonic
Olympic
buried episode.
Mountains
is
cooling age of these rocks 1992).
Formation;
1994)
Erosion
Brandon
and
and in the Puget Basin to
This rapid,
domal uplift,
to the present day.
centered
Its deep roots
zone of earthquake
some 40 km under northern Olympic
Crosson and Owens,
of
for the Grays Harbor basin to the
are suggested by the c o n c a v e - u p w a r d depth
exhumation
Brandon and Vance,
area provided detritus
Calderwood,
a
at
after the mid-Miocene
of
(Brandon and Calderwood,
at
of
age 17 Ma, penetrated (Tabor
analysis
thrusting, pattern
zones
Olympic Basin and
immediately
the
indicated
the east
veins,
shear
on the new structure
of the Central
began
that
present-day
at
1992).
rocks
on
Quartz
along
by fission-track
and Vance,
south
formed the
Prehnite-pumpellyite
superimposed
in
folding and w e s t - v e r g e n t
17).
mostly
and deformation
hypocenters
Peninsula
(cp.
1987).
Possible causes of Olympic Mountains uplift Buoyant rise of extremely as
a
Cady,
possible 1978).
cause
thick young sediments
of the Olympic Mountains
Another hypothesis
ascribed
the
has been uplift uplift
proposed (Tabor and to
upward
127
arching
of
the
subducting
Juan
de
Fuca
plate.
An eastward-
plunging arch in the slab under the peninsula is supposedly caused by
curvature
Owens, margin
1987).
of the continental margin in this area
(Crosson and
Brandon and Calderwood (1990) speculated
curvature
and
the
arch
in
the
that
the
slab had resulted from
outward push of the western U.S. continental margin due
to
Basin
and Range extension in the Cordilleran interior in the Miocene.
Field mapping in the western Cordillera, however, does not support such a
model.
bending
have
No
structures
yet
accommodating
Neogene
oroclinal
1991).
The domal
been identified (cp. Monger,
uplift of the Olympic Mountains is hard to explain by the rise an
elongated
arch underneath.
uplift may be the rise of a Olympic Mountains.
of
An alternative cause of the domal
buoyant
granitic
massif
under
the
Such an explanation is consistent with all the
available geological and geophysical data.
A broad, negative magnetic anomaly with amplitudes of -300 to -600 nT lies over the Olympic Mountains
(Finn, 1990).
Because Tertiary
sediments are not magnetic, this anomaly is probably caused deeper
reversely
magnetized
provinces of western
U.S.
occasionally
associated
which cooled
in
a
source.
and with
Canada,
negative
a
in the coastal anomalies
are
reversely magnetized igneous rocks
reverse-polarity
(Arkani-Hamed and Strangway,
Elsewhere
by
ambient
geomagnetic
field
1988; Finn, 1990).
Stark, negative gravity anomalies also mark the Olympic Mountains. Appearance of these anomalies in both Bouguer and (Blakely
and Jachens,
isostatic
maps
1990; Finn et al., 1991) suggests that 10w-
density material is abundant in this area and/or
that
the
North
128
Olympic
crustal
latter case,
block is below its isostatic
a negative
imbalance
gives
equilibrium.
this
block
In the
a
natural
tendency to rise.
An isostatic
imbalance might have been created by emplacement
large amount of continental
light
material,
reducing
crust above the level of isostatic
mechanisms
may be envisaged
formation
of
for
such
a subduction-related
The other is emplacement
first m e c h a n i s m
the
Juan
de
buttress
is appealing
Fuca
plate
(e.g.,
of felsic igneous rocks of various that
area.
the 30-km thickness
oceanic
by this scenario
(Brandon and
other subduction
zones.
estimates
were made
in gravity models.
km-thick
sediment
2,900 crustal kg/m3.
kg/m3.
layer
crystalline
Calderwood,
of
Cloos,
the prism
1993).
by
Tertiary crust
subduction 1989).
and
the
ages preclude
underlies
sediments
1990)
area
is
this
required
unusual
at
in this area
(1977) modeled
a
15-
with an average density of 2,700 kg/m3,
an
rocks with
average
section under the Olympic Mountains adjacent
is known to be much thinner
Peninsula
sediment thickness
MacLeod et al.
implies
By comparison,
(e.g.,
of accreted
layer of crystalline
This
accretionary
origin of Crescent basalts
the possibility
over a 12-km-thick
Two
is
Couch and Riddihough,
presence
conservative
One
setting characterized
the continental-rift
More
the
compensation.
since the Olympic
However,
Besides,
of
of a large body of granitoids.
lies in a continental-margin of
density
emplacement.
sedimentary
piled up against a rigid continental
The
the
of a
density
a
density
of
for the whole
to be only about
2,790
areas where the sedimentary
cover
and lying over continental
crust,
have
129
in
that model an average density of about 2,850 kg/m3.
model of Riddihough of
2,600 kg/m3,
a density oceanic
(1979)
showed a sediment
up to i0 km thick,
of 2,920 kg/m3,
at least
A gravity
layer with a
density
over crystalline
material with
i0
A
km
thick.
subducting
slab was modeled below the base of the continental
and a wedge of continental oceanic
slab
and
the
upper mantle was
continental
cautioned that a subducting
modeled
Moho.
Finn
between
(1990),
slab cannot be resolved
crust, the
however,
from
gravity
data in western Washington.
Largely
following
Brandon
and Calderwood
model
for the Olympic Mountains
Fig.
14)
showed
oceanic crust.
A Crescent
(Dehler and
rocks
"terrane"
comparison,
Vancouver
their
on top of a slab of
assumed
in that model
are
was assumed to be a panel about
measured
surface densities
in Washington
Island
(Currie
(Finn,
discontinuous
basaltic
Measured densities
massifs
of the Central
are 2,400 to 2,600 kg/m3
of low-density
material
1990)
1976).
et
al.,
are
kg/m3
Rather being a to consist 1992,
Olympic and Hoh basin
of
1994).
sediments
2).
is a simpler way to put a large volume
into the continental
region offers many examples and Coast mountains,
and 2,950±60
is recognized
(Babcock
(see chapter
Intrusion of granitic magma
of Crescent basalts
and Muller,
the Crescent Formation
Cascade
1992,
were assigned a high density of
uniform panel,
this
Clowes,
km of sediments
rock parameters
a recent gravity
with an extremely high density of 3,200 kg/m3.
2,200 to 2,950 kg/m3 on
20
Sedimentary
2,700 kg/m3.
By
to
However,
unrealistic.
5 km thick,
up
(1990),
crust.
of such intrusion.
Mesozoic
Geology
of
In the North
and Cenozoic granitoid
rocks
130
occur
in
measured
Felsic
abundance densities
intrusive
Vancouver Puget
elongated
the
19.7 and 17 Ma
Reidel
et
of
the
Charlotte
1994).
(Lyatsky,
1991a)
sedimentary e.g.,
in
veins
magmatism
may
the Cordillera
17 Ma.
center
aureole
connection.
If the oval m e t a m o r p h i c
also
of
and refraction
data,
1984; from flank
and Queen
in
the
Olympic
been
Mt.
the overall
episode related responsible
for
many
mountainous
1983; Monger,
1991).
a topographic
In
dome.
this dome with the center of the Olympus) aureole
the a p o p h y s i s closest to the surface,
crust
is
ages are
et al.,
1990)
A thermal
(cp. Parrish,
metamorphic
continental
quartz
is known from
of
The
It
on the west
(Finn,
such rise has produced
(at
near
at 13-14 Ma.
Coincidence
domain may represent
basins
of
have
of basin sediments
the
Frizzell
on
over the apex of another such pluton.
is around
the Olympic Mountains,
batholith.
Its radiometric
the Puget
Buyoant rise of granitoid massifs regions
widespread
basins.
may have developed
metamorphism
are
Snoqualmie
the OWSZ.
hydrothermal
intrusive
age
similar plutons have been inferred
Coast Belt orogen,
The age of these veins to
Their
2,700 kg/m3.
(late Early Miocene;
data beneath
The conspicuous Mountains
felsic
intrudes
between
potential-field
1994).
On the west flank of the North Cascades,
and
al.,
Brown et al.,
rocks of early Tertiary
lies
N-S
1982;
are mostly around
Island.
Sound,
(Hutchison,
suggests
a
reflects
the more
genetic
the shape of
isometric
mountain
shape of the pluton.
in this area,
is attenuated
as suggested
by teleseismic
somewhat but still about
30
km
131
thick
(Taber
1990).
and
Lewis,
1986; Owens et al., 1988; Lapp et al.,
This provides a sufficient crustal
granitic
magmas.
thickness
to
produce
Such magmas, commonly formed by partial melting
at depths of 20-30 km, may rise
to
solidifying
(Hollister,
Petford
al., 1994).
Because felsic rocks are light, their b u o y a n c y causes
uplift
involving
1993;
both
the
as
little
as
7
km
before
et al°, 1993; Grocott et
plutons and the country rocks of the
upper crust.
Hot granitic magmas cause metamorphism of country rocks, including roof
rocks over large intrusive bodies.
The prehnite-pumpellyite
grade of metamorphism is usually reached at 2.5
kbar
(Digel
and
Ghent,
240°-245 ° (Brandon and Vance, geothermal
gradient
of
pressures
the
exposed
rocks
1992).
for the Olympic Peninsula,
might
have
been
metamorphism
km.
However,
sufficiently
high
have existed at shallower depth if a magmatic
massif supplied local heat. might
of
the core of the Olympic Mountains was
reached at a depth of about 12 temperatures
to
Assuming a very low Tertiary
19.4±1.7°/km
in
up
1994) and temperatures no less than
Brandon and Vance (1992) concluded that the peak of
of
Besides, pressure-depth relationships
complicated
if
the
adjacent
subduction zone
provided additional compressive stress.
In these circumstances, rocks of the have
Central
Olympic
Basin
may
reached the prehnite-pumpellyite metamorphic conditions at a
depth of 7-10 km.
Such thickness is
not
unreasonable
for
this
basin in Middle Miocene time, when both the Middle Eocene to Early Oligocene and Late Oligocene already
been
deposited.
to After
Early the
Miocene
successions
emplacement
of
had
a buoyant
granitic pluton and prehnite-pumpellyite metamorphism in the
late
132
Early
and
Middle Miocene,
inversion
and uplift of the Olympic Mountains surrounding has
been
basins eroded
metamorphosed
in
provided
the Late Miocene.
from
rocks,
of the Central
the the
Olympic
sediments
Basin
for
the
If 7-10 km of material
Mountains
crystalline
Olympic
to
basement
unroof
containing
the the
felsic pluton must now be close to the surface.
Such a scenario of
explains
the Olympic Mountains.
isostatic gravity
gravity low
peninsula
over
remanent
may
the
also
metamorphic
This
as
Olympic
it
be
would
also
a
terrain.
pronounced
an unrealistic
anomaly over the
for by a pluton with a negative thin
cover
of
non-magnetic
explain the lack of correlation
isograds with the fault-bounded
mountain
the
Peninsula without
accouted under
peculiarities
with Bouguer and
explains
The strong negative magnetic
magnetization,
sediments.
It is also consistent
anomaly data,
sediment thickness.
the
all the diverse geological
No arbitrary
of
slices or the shape of
external
tectonic
needed to drive the uplift of the Olympic Mountains.
forces are
CHAPTER 6 - CONTINENTAL MARGIN OFF SOUTHEASTERN ALASKA, THE QUEEN CHARLOTTE ISLANDS, A~{D NORTHER~ VANCOUVER ISLAND
Scope of ideas ~egarding tectonic nature
of
the
North
America-
Pacific plate boundary since
the
inception
between the Pacific
of the plate tectonics theory, the boundary
continental
plate
has
North
been
placed
American continental margin. predominantly
America
transform,
along
the
and
the
oceanic
northwestern
North
This boundary has been classified as
dominated
Pacific plate past the western edge 1970, 1989).
plate
by strike-slip motion of the of
the
continent
(Atwater,
By definition, such a margin must be associated with
a system of faults of lithospheric scale, across which the are
juxtaposed.
plates
Such faults have been detected by marine seismic
surveys along the western North America margin all in southeastern Alaska
and
farther SSE (Figs. 25, 26).
These faults control the
geologic structure of the continental margin (yon Huene, 1989).
Off southeastern Alaska, the plate-boundary fault zone, von
Huene
et al. (1979) Chichagof-Baranof,
the lower continental British 1987). the
Columbia
slope
border
from
in
Cross
Dixon
Sound
to
the
Alaska-
Entrance (Bruns and Carlson,
This fault system has been noted to continue northward, as
Fairweather
fault,
into
Chichagof-Baranof-Fairweather
continental-crust
regions
of
Dixon
Entrance,
(Sutherland
Brown,
A
inside single
fault system can thus be inferred.
off the Queen Charlotte Islands, this
plate-boundary fault system has been named Queen zone
by
has been traced along
Alaska, where it separates large continental terranes.
South
named
1968;
Chase
and
Charlotte
Tiffin, 1972).
fault With
134
!
200 kJ
0
i
t
,
62
l
\ \
\
\
NORTH
\
,\
AMERICAN
\
t',,
, "¢,~\
\
PLATE
\\ Juneau .
P A CIFIC
............ 140 °
\\
I
58 °
~,
\ ~
s4° 130 °
Figure 25. Position of biggest faults in southern and southeastern Alaska onshore and offshore (modified from Bruns and Carlson, 1987). QCF - Q u e e n C h a r l o t t e fault; F W F - F a i r w e a t h e r f a u l t ; T F - T r a n s i t i o n fault.
135
\, $ W/SW
~ '
to KILOMETERS E/NE
J
LINE 961
4.0
fl" "
~]'~ ?
LINE 959
/
LINE 9R7
LiNE 953
LINE g51
ZONE (OSZ}
Figure 26. Strands of the plate-boundary fault system off southeastern Alaska. Structure of this system is illustrated in the line drawings of marine seismic reflection profiles identified in the map (modified from Bruns and Carlson, 1987).
136
modern data,
a single
(Figs.
26) is known to control the position
25,
continental Brew et al.,
Fairweather-Queen
margin of northwestern 1991).
Charlotte
North America
Still unresolved
Vancouver
1979; Riddihough alternative
ideas
structural
southeastern The
margin
involve
which
Yakutat terrane
to the west
From the Alaskan
interior,
Pacific.
which
later
(Plafker,
al.,
evolution
1969),
and
1993).
Many
of the Canadian
interpretations.
by
this
(Plafker,
along the
1987;
1987;
its
from
Brew et al.,
it
Tertiary 1991).
dives
shelf and slope.
location
beneath There it
fault
off Cross Sound,
system,
shift of the Yakutat Islands
off southern Alaska
1987).
major (Fig.
be
fault runs SSE into the
This composite
present
Alaska
the
lay off the Queen Charlotte
show several
slope off southeastern
to
The mostly Mesozoic
the northward
Bruns and Carlson,
Seismic profiles
are recognized
fault.
accommodated
to
considered
fault
the Fairweather
in the Miocene
moved
now
Alaska.
waters onto the continental
in current models,
and
et
In the Cross Sound area,
meets the Chichagof-Baranof
terrane,
Sound
of the plate boundary
compose
is separated
ocean
Charlotte
conflicting
(King,
Chugach terrane
the
continuation
has long been known as a boundary between
domains
terranes,
northeastern
tectonic
characteristics
fault
geologic
accreted
the
1989;
1975; Keen and Hyndman,
1989; Allan
about
(von Huene,
Alaska m a r g i n
Fairweather
distinct
(Chase et al.,
and Hyndman,
Pacific continental
General
Island
system
of the submerged
is the southern
of this fault system off the mouth of Queen northern
fault
faults 26).
on
the
continental
Two main parallel
three south of
Sitka
faults
Sound.
In
137
many
places
of seismic floor.
they are covered by thick sediments,
sections
Though
seismic
from limited signal reaches
these
several
faults have broken through to the images of the stratified
penetration,
kilometers.
of
the
Huene et al.,
underlying
1979;
where the Though
sediments
The intensity
massive
shelf and slope,
outer
older
fault
scarps
far
shaped
and ridges
as Sitka Sound.
of coherent crustal
this broad structural
deep
and
the
and
is obscure
(von
1989).
are
largely
increases to the south, also
the
becomes
wider.
southeastern
Alaska
south of Sitka Sound.
Sound,
it becomes
up
plate motion
America (e.g.,
and remain
fault close
to
40
km
wide.
slivers bounded by fault strands of
in
sedimentary
basin off Dixon Entrance may be many kilometers 1979;
Bruns and Carlson,
from NNW-SSE
margin changes
to NW-SE.
plates DeMets
has et al.,
slightly
south
between the Pacific
been suggested 1990),
1987).
Off the Queen Charlotte
a small amount of oblique convergence
North
in a
There this fault system widens to
of the continental
of Dixon Entrance, Islands,
The
margin
zone caused the development
(von Huene et al.,
The orientation
but
Miocene.
many of the faults are expressed
20-30 km, and off Dixon Entrance
basins,
thickness
and inner fault strands of the Chichagof-Baranof
south
Subsidence
the
system
system are only about 8 km apart at Cross as
then
1987; yon Huene,
along
suffer
are undrilled,
basement
of deformation
sedimentation
series of sea-floor
no
crystalline
Chichagof-Baranof
continental
The
be
ocean
packages
sediment
sediments
Bruns and Carlson,
North of Sitka Sound, undisturbed.
the resolved These
their age is often assumed to nature
but in a number
but
from estimates
seismic
of
refraction
138
surveys
have
et al.,
1989;
concerns
failed to identify Spence and Asudeh,
geophysical
scale plate-tectonic is
the Queen Charlotte shelf
off
However, transition Canadian
models
along
the
models relying primarily
reconstructions
western
the continental
Islands
the
Island
local
between
as well
on large-
in the northeastern
presumed to underlie
Vancouver
from
(Mackie
margin
In the conventional
crust
oceanic slab
1993).
about fidelity of geophysical
Canada continental
oceanic
a subducted
as
slope
(Riddihough
geological
and
continental
and
and
slope off
and
exterior
Hyndman,
geophysical oceanic
segment of the western North America
Pacific,
1989).
data,
crust
the
along
continental
the
margin
may be much more complex.
The
striped pattern
of linear magnetic
oceanic crust worldwide, continental oceanic
margins,
magnetic
margins
stripes
Off southeastern of
the
a
associated
with
of this type of crust.
At
prime
is commonly
Alaska,
At
interpreted
magnetic
continent
fault the
system
Washington
gradually,
1991).
be two-fold:
Reasons subduction
to the anomaly source, rocks at d e p t h destroys
Deep
of how far the structure
based on the extent of
1990).
and
do
margin,
not they
reach
slope
and
dissipation
shelf
the source altogether.
have
(Finn,
of stripes may
of the oceanic plate increases alteration
the
dissipate
and their faded extensions
for such gradual
and thermal
of
stripes end at the outer strands
been noted even on the upper continental 1990,
indicator
the continent.
(e.g., Johnson et al.,
slope.
the
is
towards
plate-boundary
continental towards
it
crust extends
continental
is diagnostic
anomalies,
the
depth
of m a g n e t i t e - b e a r i n g
139
At the British Columbia continental margin, stripes are terminated very abruptly west of the continental slope off northern Vancouver Island (Fig. 27). km
off
the
To the north, they disappear gradually some
Queen
Charlotte
Islands,
without
40
reaching
the
continental slope (Currie et al., 1983a,b).
The absence of oceanic-crustal magnetic signatures magnetic
zone
along
the
western
disparate
explanations.
led
An
early
explanation
reworking of the oceanic crust 1972;
Srivastava,
by
1973).
invoked faulting
Another
hydrothermal
alteration
ocean floor (Levi and Riddihough,
These explanations are not Vancouver
Island,
where
borne
under
by
intense
held
destroyed
out
by
secondary structural
and
basalts
that
original
by
unusually
facts.
Off
northern
oceanic-crust
revealed by
by
seismic
ocean
data.
water
hydrothermal fluids is a common phenomenon worldwide, but where
Tiffin,
the blank magnetic zone is particularly
Alteration
extent
complex
1986).
is
the
in
a sediment blanket on the
well defined, no severe faulting of
to
(Chase
idea
magnetization of oceanic-crust rocks was strong
blank
Usually it is assumed that magnetic
stripes existed there originally but were destroyed processes.
the
Canada continental margin,
areas supposedly underlain by oceanic crust, has and
in
and
not
to
magnetic stripes are completely destroyed.
On
the ocean floor off western North America, high heat flow has been recorded in many places, usually where warm fluids are vented near faults (Davis and Riddihough, from
these
local
thermal
1982; Moran and Lister, 1987).
Away
anomalies, however, the measured heat
140
%
/A
Figure 27. Magnetic anomalies offshore British Columbia (after Currie et al., 1983a,b). Magnetic stripes are absent in a zone some 60 km wide along the submerged margin. Farther west, the stripes are clearly expressed, but broken due to internal deformation of the northern Juan de Fuca plate (compare with the regional magnetic anomaly map in Fig. 3b). The stripes stop abruptly at the Revere-Dellwood fault.
141
flow is regionally
lower
than
predicted
by
theory
for
young
oceanic plates such as Juan de Fuca (Riddihough et al., 1983). sediments are indeed acting as a partial effect
is
insufficient
thermal
blanket,
If
their
to destroy magnetic stripes elsewhere in
the Juan de Fuca plate.
Off western blanketing
Canada, did
other
evidence
also
shows
that
not create the blank magnetic zone.
boundary of this
zone
off
northern
Vancouver
sediment
The outboard
Island
is
very
abrupt, whereas sediments thicken towards the continent gradually. The abrupt termination of magnetic stripes in this area
coincides
with a major NW-trending regional fault (Revere-Dellwood) which is well expressed in bathymetry and in seismic profiles.
The Revere-Dellwood fault separates blocks of crust with different composition
and of different nature.
British Columbia is interpreted continental
margin
of
to
The blank magnetic zone off reflect
foundered
blocks
presence of
along
continental crust,
juxtaposed by faults against the oceanic crust of the Pacific Juan
de
Fuca plates.
the
and
The transition from continental to oceanic
crust therefore lies at the outer
strands
of
the
broad
plate-
boundary fault system.
Models of western Canada continental margin based on gravity data An
early
interpretation
(Stacey and Stephens, prominent
anomalies
of
1969) with
gravity involved known
analysing the overall Bouguer gravity Belt
in
data
in
the Insular Belt
correlating geologic signature
some
features. of
the
of
the
Later, Insular
comparison with that of other parts of southern Canadian
Cordillera,
Stacey
(1973)
concluded
that
Vancouver
Island
is
142
characterized Riddihough
by abnormally
(1979)
distribution: presence
considered
abnormally
of a subducted
The first explanation seismic
refraction
km/s,
et al.,
mid-1980s
and
conventional
mafic
oceanic
crust
on
light upper mantle. for such
a
Vancouver
density
Island
or
slab underneath.
is appealing
due to subsequent
that
from
in the Insular Belt
layers of P-wave velocity
up to and exceeding
is up to i0 km (Spence et al.,
Because these their
the
findings
crust
whose thickness 1992)o
two explanations
surveys
contains mid-crustal 7
dense crust and/or
layers were not
nature
interpretation
remains
1985;
Yuan
till
the
inferred
unclear
to
this day,
has come to hold that a subducted
slab
of oceanic
lithosphere
Riddihough
(1979) used this idea as a basis for his gravity models
of deep
structure
regions.
He
is present beneath Vancouver
the
of
the
postulated
western British Columbia,
Washington
m a r g i n as far north as Vancouver zone was later extended
and these models
seismic
prifiles
1985a,b;
Spence et al.,
recently
the
1985;
plate
oceanic
similarities
between
assumed
is occurring Island.
that
along the
The Cascadia
to the mouth of Queen Charlotte subsequent
continental Clowes
et al.,
interpretations
margin
of
(Yorath et al.,
1987; Hyndman
et al.,
1992).
which
(Pakiser and Mooney, new
influenced
across
Yuan et al.,
Non-uniqueness,
and
Fuca
adjacent
and Oregon and
continental
1990;
de
and
geological
of
Sound,
Juan
broad
Belt
subduction
subduction
the
Insular
Island.
geological
is
eds., data
normal 1989), cast
in geophysical makes
these
interpretations
models
ambiguous,
doubt on their validity.
It was
shown by m o d e l i n g that gravity data by themselves
cannot
143
resolve
a subducted
and c o n s t r a i n t s Geological
oceanic
from other
control models,
information
is d i s c u s s e d
The Queen
and
Charlotte
Charlotte their
western
faults
of the Q u e e n
Cenozoic
Hickson,
1991).
form
a
huge
ocean
floor d e e p e n s
At the b o t t o m step-like wide,
water
the
depth
the
Columbia
continental
on
fault
the almost
abruptly
lies the
of the
A smaller
increases
abruptly
1982;
is b o u n d e d
scarp
across
Farther
west,
in the p e l a g i c
Kodiak-Bowie
which
which
m
basement
chain
islands scarp
Currie
area,
m
these
faults
where
Terrace, Just
on
Charlotte
et al.,
1983b).
20-30
floor rises
the
This
about
The b r o a d
a km
where
terrace
Trough
(Chase trough, still
200 m.
gentle,
Its s l i g h t l y
by T e r t i a r y
the
slightly
side by another,
lies a broad,
1977).
1968;
it on the west,
2000
Rise.
Mesozoic
Brown,
inclined
Queen
as O s h a w a
by s t e e p
28).
slope.
and is
to
sea level.
all
Charlotte
flanks
from some
is c r o w n e d
(Chase,
cut
(Fig.
Queen
the o c e a n
known
Entrance
abruptly
(Sutherland
on its o u t b o a r d
smaller
crustal
i000 m above
to 1 0 0 0 - 2 0 0 0
Seemann,
oceanic
than
on the lower c o n t i n e n t a l
feature
geophysical
next to the shoreline,
et al.,
ocean-floor
testing
margin
no r o o m for a shelf,
scarp
of the s c a r p
wide,
and
from D i x o n
islands
2800 m in the a d j a c e n t
km
1990).
in
extend
zone,
edge to a b o u t
20-30
(Finn,
importance
geological
which
ocean.
1975;
required
t h e y are t r u n c a t e d
Charlotte
length
N o r t h America,
together.
bathymetric
feature
are
rise no h i g h e r
Leaving
it runs the
towards
below
shore,
rocks
northwestern
particular
Islands,
Sound,
Along
and
of
of the B r i t i s h
Queen
sources
is
geophysical
Bathymetry
slab under
positive elevated
seamounts ocean-floor
of
the
uplift
144
145
Figure 28b. Bathymetry (in meters) of the Vancouver Island and Washington submerged continental margin and vicinity (from J. Mammerickx and I.L. Taylor, 1971, Scripps Institute of Oceanography, Geological Data Center, Special Chart #i).
146
and the localized activity et al.,
seamounts
in the oceanic crust, 1985).
The basement
across the Gulf of Alaska,
No
on it are products
extension
unrelated
obliquely
where the continental
blanketing
thick
1979).
because
the
subsiding
for detritus
continental
slope
which probably
interior
On
the
sedimentary
here
abyssal and Tuzo
cover
is
elevations
locally exceeding
south,
(the Queen Charlotte
but because in
only
to
(Yon Huene et
Charlotte
shelf provided
Islands, an inboard
terrace
ends.
by many turbidite faults
small,
that
a
The
channels
transect
isolated mounds,
Seamounts,
protrude
the
known as
above
the
1989).
1500 m. narrow
Island becomes
and the interior
coastline
fan
is higher than the Queen Charlotte
Vancouver
The exterior
auxiliary
(Carbotte et al.,
where
NW
margin.
is very gentle due
the
dissected
Wilson
Island
mainland.
Sound,
plain,
Vancouver
middle,
run
shed from the Coast Mountains.
follow local,
Dellwood Knolls
the
slope
(Cousens
is observed north of
of the Barancf
Off the mouth of Queen Charlotte
slope.
Terrace
No such fan exists off the Queen
there
catchment
sediments
chain
to the continental
Dixon Entrance,
al.,
to the continent
rise and the seamount
of the Queen Charlotte
by
of tectono-magmatic
The island
Islands,
its
is the widest
in
seaway separates narrower
in
the
it from the north
and
shelf between the island and the m a i n l a n d
Strait and the Strait of Georgia)
shelf off V a n c o u v e r
widens.
Island widens to the south.
curves eastward towards the southern the shelf edge retains
the south to as much as 80 km.
its SE trend,
The
tip of the island, the shelf
widens
Off northern Vancouver
Island,
147
however, the
the exterior
Queen
steep,
Charlotte
apparently
1972).
shelf is only about 20 km Islands,
controlled by a
On the other hand,
Width
of
the
Brooks-Estevan
the
major
embayment
slope (new
varies
name)
irregularly.
Several
contains
large
many
off
km
wide
in the south is gentler
small
underwater
35-40
it
central
but it is 60-70 km wide to the north and 50-60 km
and
al.,
The large
embayment,
north
and
is broad and gentle.
disrupts
only
off
et
Island.
The upper slope
is
(Tiffin
Vancouver
to the south.
slope
Like
slope is straight
fault
the lower slope
continental
The
upper
wide.
terraces
canyons dissect
and
in the wide
than in the
sediment
it and continue
ponds. to the
base of the lower slope.
The Juan de Fuca canyon, Juan de Fuca, margins. and
the V a n c o u v e r
Both are broad,
slope
shelf
separates
originating
in
them
at the mouth of the Strait of Island and Olympic
up to 120-140 km, but the width of shelf
varies.
Off southern Vancouver
is 80 km wide and the slope about 60 km
Olympic
Peninsula
Peninsula
margin
has a 90-km-wide
wide,
Island, whereas
the the
slope and a 30-40-km-
wide shelf.
Deep structure Charlotte The
1989),
plate
boundary
off
Queen
Islands
Chichagof-Baranof
continues, margin
of the c o n t i n e n t - o c e a n
off
fault
as the Queen Charlotte British Columbia
where deep structure
system
off
fault zone,
southeastern
into the continental
(Bruns and Carlson,
of the plate-boundary
revealed by modern seismic surveys.
Alaska
1987; yon Huene, fault system
is
148
The
deep
levels
of the Queen Charlotte
from seismic refraction
Trough have been modeled
and gravity data as containing
6-km-thick
crystalline
oceanic crust tilted towards the continent
al.,
1989).
Seismic reflection
Fig.
29)
reveal
sedimentary
above
the
profiles
landward dips increase with depth et al.,
1980;
Gradual
eastward dissipation
trough
(Fig.
Davis and Seemann,
27)
outboard
the
fault
(1984)
side, crust
the
deepening
structure layer,
has
Snavely
the
(Fig.
29).
the terrace
west
survey,
data
show
decrease
been
with velocities
the
modeled
Terrace the
are
ponds
between
and grabens.
at 13
km
and at 20 km on the east
(Fig.
31).
The
5.3 km/s at the top, of
to be
and Horn et al.
the Moho has been modeled
depth
notably
sediments
(19~3)
as a set of horsts
i0
km.
in the s e d i m e n t a r y
of 2-3 km/s,
gradually
areas to near -50
sediment
Srivastava
side of the terrace
below
crust.
30).
towards the continent
km/s
oceanic
disturbs the magnetic
on the Queen Charlotte
has variable velocities:
more than 7.3
trough
with local u n d i s t u r b e d
interpreted
on
(Fig.
reflection
From a deep refraction depth
the
from near zero in outboard
Seismic
blocks
1972;
whose
stripes over
the
Free-air gravity values
signatures
strongly deformed, uplifted
reflections
1981).
bounding
trough,
different.
a 1.5-km-thick
(Chase and Tiffin,
wedge blanketing
mGal at the foot of the slope
Geophysical
pattern of
(e.g.,
may be a result of increasing depth to anomaly
stripes only slightly. across
basement
of oceanic magnetic
source below the sedimentary The
across the trough
crystalline
wedge with a fan-shaped
(Mackie et
crystalline increasing to A
cover:
is some 2 km thick;
bipartite the upper the
lower
149
GSC
0
SO !
I
D~STANCEFROM COAST (kin} 40 I
!
2O !
|
Queen Charlotte Tertece
//
~.~/
/ / I ! /I
~ p
,I J
g" i! 2'
t
.r[
': :Y.!, ;
B 5
Figure 29. S t r u c t u r e of the Q u e e n C h a r l o t t e Terrace and Trough imaged in an old seismic reflection p r o f i l e (after Davis and Seemann, 1981; R i d d i h o u g h and Hyndman, 1989): (a) g e o g r a p h i c a l index map w i t h profile location; (b) s e i s m i c data. Two s t e e p scarps b o u n d the Queen C h a r l o t t e T e r r a c e on the east and west.
o~
151
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5 " 6 ::::;J~::]
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======================= I I I I I I TERRACE
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.....................
[...
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u.J
0
VELOCITY
(km/s)
Figure 31. Crustal seismic refraction model across the Queen Charlotte Terrace (modified from Dehler and Clowes, 1988). The terrace is a d i s t i n c t , f a u l t - b o u n d e d c r u s t a l block. Bold numbers - v e l o c i t i e s in km/s; s m a l l n u m b e r s - d o w n w a r d v e l o c i t y gradients in k m / s / k m .
152
153
154
layer,
with velocities
(Dehler
and Clowes,
terrace
most
continental
This
Queen
are
Ocean.
+15 mGal, seamounts
Islands
outboard
Rise
highs
chain.
in H e c a t e
The c o n t i n e n t a l (Fig.
31).
related
crust
to a b r u p t
(Stacey,
thickens
changes
Also
faults 32,
33).
free-
outboard,
by a b r o a d
Inboard,
are well
gentle
the
about
values
free-
northeastern high
+40
of a b o u t
mGal
over
gravity
anomalies
Queen
Charlotte
block.
52°N,
by a local
In the B o u g u e r
over
over the u p l i f t e d
at l a t u t u d e
Strait
27).
anomaly
30,
Further
of
of the
as a d o w n d r o p p e d
gravity
(Figs.
zero
is m a r k e d
local
are p o s i t i v e
low is d i s r u p t e d
map
magnetic
parts
(Fig.
and b o u n d i n g
Trough.
near
the K o d i a k - B o w i e
Island,
isostatic
lies on t r e n d w i t h a f a u l t - b o u n d e d Ridge,
the
foundered
the
the e a s t e r n
of the t e r r a c e
Charlotte
crustal
of
lies the c o m p a r a t i v e l y
generally
continental
Moresby
gravity
3 km
suggests
of a t t e n u a t e d ,
terrace
in the B o u g u e r
with superimposed of
includes
-80 mGal,
The O s h a w a
in all r e d u c t i o n s
Off
than
as g r a d i e n t s
air a n o m a l i e s
zone
and the entire
low over the Q u e e n
Pacific
of a b o u t
structure
w i t h the c h a r a c t e r
and e n h a n c e d
lower
N e x t to the t e r r a c e air
on a b l o c k
interpretation
free-air
are
expressed
Trough
w i t h the
both it
lies
is c o n s i s t e n t
Charlotte
over
Such a velocity
the b r o a d b l a n k
consistent
has a t h i c k n e s s
crust.
as
block,
1988).
probably
conclusion
field,
of 4-5 km/s,
map
the Q u e e n
relative basement
Charlotte
Terrace
high oriented high,
known
NE°
It
as M o r e s b y
1975).
towards (Fig.
in c r u s t a l
32),
the steep
properties
continent gradient across
stepwise zones
deep
are
faults
155
bounding
the Queen Charlotte
Terrace.
reported at the outer bounding flow
on
the
terrace
Charlotte
fault of the terrace:
(Hyndman
Islands,
et
Hecate
al.,
slightly
but
remain
et
Crustal
al.,
km
stepwise
on
the
eastward the
Islands
increase
margin
Brew et al.,
1991)
and
gravity
et
to 16 km on the
of southeastern
Alaska
and southern Vancouver
1987). the
offsets
However,
outer
This
increasing
features
oceanic crust are juxtaposed
The Queen Charlotte
the
noted
though it is now
but
1972;
1990).
off
is well al.,
(Bruns and Carlson,
that historically
south,
been
B~rub4 et
properties
is deformed variably,
from
Terrace,
Islands
along the southeastern
to
confirm
fault system
1986;
in crustal
suggests
zone
Islands margins
deformation
(Rogers,
bathymetric
von
A similar
(Mereu,
Charlotte
most of the displacement,
plate-boundary
Charlotte
of
the main changes
strand.
accommodated The
Queen
has
Island
Alaska
and
to
1993).
(Johnson et al.,
southeastern
in patterns of seismicity
1981).
thickness
inner strand of the p l a t e - bo u n d a r y
1989)
the
1972;
across big faults:
The landward,
expressed
al.,
(Hyndman and Ellis,
the
the
data
of seismicity data
Trough,
in crustal
and
Under
Spence and Asudeh,
of the Moho occurs
13 km in eastern Queen Charlotte
across
1993;
models based on interpretation
27
1982).
(Johnson
1979; Hole et al.,
that eastward deepening
to
heat
in the depth range of 25 to 32 km,
much deeper than under the terrace Huene
measured
Strait and Dixon Entrance,
Moho has been shown from seismic refraction undulate
break has been
has been found to be closer to that on the
islands than in the trough Queen
A geothermal
occur
at
this strand inactive.
Alaska and Queen
with the amount of the continental
and
across the outer strand.
Terrace must be
cored
by
crystalline
crust
156
composed
of
rigid rocks, which enabled it to maintain structural
integrity in such a complex zone. are
High-grade
metamorphic
rocks
exposed in the Westcoast belt on Vancouver Island, which runs
along the western periphery of the island and beneath its northern exterior
shelf.
This belt and the terrace have a similar width,
about 20-30 km, and they lie on suggests
their
continuity.
trend In
with
this
one
case,
another.
the
This
terrace has a
Mesozoic metamorphic basement, covered by compacted late
Mesozoic
or early Tertiary sedimentary rocks and younger sediments.
Southward extension of plate b o u n d a r y off Queen Charlotte Sound similarity
of
Mesozoic
formations
on
the
Vancouver islands suggests old geologic links Canada
Archipelago.
Queen Charlotte and along
the
Western
According to plate reconstructions,
since 42
Ma dextral strike-slip movements have been occurring off the Queen Charlotte Islands, and the Cascadia subduction zone has existed to the south. zone
is
Though in practice the exact extent of this subduction unclear
(Lister,
models its northern (Riddihough,
This
area
end
is
1991; Allan et al., 1993), placed
1984; Riddihough and Hyndman,
is
modeled
as
Queen
Charlotte
1989).
the
north,
subduction
postulated Hyndman, Fuca
in
1989).
this
area (Keen and Hyndman,
the
has
been
1979; Riddihough and
The Explorer plate, detached from the larger Juan
plate around 3.5 Ma, is supposedly converging with North
America at a rate and angle distinct plate
in
A plate triple junction between North America, Pacific and
Juan de Fuca (or its northern segment, Explorer) plates
de
Sound
a point which separates two different
tectonic regimes: strike-slip in south.
off
in current
farther
from
those
of
its
south (Riddihough et al., 1980; Riddihough,
parent 1984).
157
A left-lateral been
transform boundary
proposed
spreading
between
Studies
of
validity
of
and Tuzo Wilson
Sound their
Explorer
and
Seamounts
(Figs.
geochemistry
led
to
in this area
et
1993).
(Chase,
1984; Michael
Dellwood
Knolls
features they
et al.,
and
sediments
kg/m3,
differences
hundreds the
foot
in
basalts
(Cousens
are
et
not prominent
Bathymetrically, in a broad volcanic
Charlotte
Sound
on a subdued background,
(Fig.
28).
correspond
on the ocean floor
(Fig.
to
27),
occur in Dellwood lava
to the knolls has been inferred
from
(Davis,
1982).
A
low
bulk
density,
with low magnetization,
(Riddihough of
meters
of
the have
to
of D e l l w o o d - s o u r c e d
coupled
Knolls have different mantle
seem
Interbedding
next
profiles
for the knolls
from
Queen
sediments
with
with
sea-floor
are draped by sediments.
flows
Knolls
solutions
spreading.
highs scattered
in equal proportion.
filled
et al.,
Seamounts
sea-floor
Knolls
suggested
adequate
Wilson
anomalies,
basalts
2,500±100
of
the
1989).
Tuzc
and some volcanic mounds
just
products
about
simple
and
reflection
basalts off
Even for the Explorer Ridge to the west,
the many bathymetric
Intercalated
at
3, 4).
doubts
just a few of the many small mounds
magnetic
seismic
as
and
Allan
field on the ocean floor off Local
has
1985;
related to active
are
-
plates
of submarine
1977; Cousens
spreading models do not provide al.,
zone
Pacific
long-standing
interpretation
spreading al.,
the
volcanic-rock
Queen Charlotte
fault
the Explorer and Juan de Fuca plates,
centers between
Dellwood Knolls
- the Nootka
et al.,
1980).
of sediments
continental led
sources
A
has been
depression
separates
Dellwood
slope.
Geochemical
to a suggestion
that Dellwood
(Cousens et al.,
1984).
158
Even smaller are Tuzo Wilson Seamounts small
volcanic
mounds
Their
basalts,
like
geochemical
almost
isometric
mounds
bathymetric (Carbotte
Absence Sound
margin Winona
mGal,
The
the
structural
Queen
characterizes
isostatic remain
around
isostatic
-40 mGal
anomalies
(Srivastava,
and enhanced suggests
lie
1973,
isostatic regional
if sea-floor
along
the
anomalies
with
are observed
locally drop to
(Figs.
30, 32,
in -60
33).
on the Queen Charlotte
been cited as evidence compensated
large negative
to the north and the
anomaly values
be
probably be disrupted
Whereas
only local
isostatic c o m p e n s a t i o n
Sound
from the
which run NW, NE, N-S
Terrace
mouth of the sound,
Charlotte
features.
increases
from -40 to +i0 mGal,
that area is isostatically
large free-air
They are
linear
gravity anomalies
Charlotte
usually
regional
good"
origin
contradicting
pattern
data.
basin to the south,
Enhanced
but generally
"fairly
exhibit
breaks off northern Queen Charlotte
isostatic
Sound shelf has previously in
Knolls,
Also
lineaments,
by geophysical
amplitudes,
of
of only 12 km3.
of these mounds.
irregular
very
1989).
sedimentary
lack
1993).
thickness
An
These
with spreading-ridge
show that sediment
indicated
this area.
not
and p o t e n t i a l - f i e l d
over
Dellwood
coherent
and enhanced
moderate
do
volume
form
of dramatic
free-air
the
inconsistent
directions.
et al.,
is
in
is the morphology
and
surveys
in all
edifice
1985; Allan et al.,
spreading
Detailed
an
those
properties
(Cousens et al., sea-floor
have
to the north.
that
(Stacey,
the
1975).
crust At the
has also been found
p. 1669).
anomalies
The absence
in and
equilibrium,
to of
off
Queen
which
would
spreading w e r e occurring.
159
A
prominent
continental (Fig.
east-side-down slope
off
32) may reflect
continent.
On
gradient
Queen
interior
gravity and seismic refraction depths
of
23
to
27
similar to that at abrupt
continentward
along
the
lower
Sound in the Bouguer map
in Moho depth
towards
shelf under Queen Charlotte models
show
km (YUan et al.,
the
the Bouguer gradient north
Charlotte
a sharp increase
the
zone
Queen
the
1992).
Charlotte
deepening
to
Sound, lie
at
This situation
Islands
margin,
of the Moho also occurs.
zone off Queen Charlotte
with the similar gradient
Moho
the
is
where Indeed,
Sound merges
in
the
zone along the outer flank of the
Queen Charlotte Terrace.
Off Queen Charlotte towards
the
Sound,
continental
from it in the south. seems
narrower
Vancouver
this slope
gradient
in the north,
The blank magnetic
(Fig.
These
magnetic
anomalies
reentrant
in the c o n t i n e n t a l - o c e a n i c
examples the
may
be
are
27).
occupied
of interlocking
plate
interpreted
boundary
by
turns
slightly
and then turns away
zone off the sound
than off the Queen Charlotte
Island
reentrant
zone
also
Islands and northern
patterns
of
gravity
and
herein to be due to a small plate-boundary
zone.
This
a block of oceanic
crust.
Other
blocks
along
of oceanic and continental
off central Vancouver
Island are discussed
in
the next chapter.
Despite the local c o m p l e x i t y crustal blocks, continental
related to interlocking
the p l a t e - b o u n d a r y
margin off southeastern
Islands continues
of dissimilar
fault zone recognized Alaska
past Queen Charlotte
along the
and the Queen Charlotte
Sound.
The inner strand of
160
this
fault
zone h a s b e e n
reflection
data
identified
(Line
88-03;
detailed
studies
far
Dellwood Knolls.
as
Wilson
Seamounts,
Dellwood
In
of t h e o c e a n
fault
Line
where
(Fig.
Rohr
and
e t al.,
34),
the pelagic
foot of t h e slope.
probably
structural
zone.
lower slope observed
form
the
Between
is s m o o t h ,
bathymetric
break
these
outer
to weakly
branch
of
at SP 1 2 0 0 - 1 3 0 0 .
the
Charlotte
plate-boundary Soundr
o u t e r strand.
northern
indicated 4,
is
similar
up
with oceanic
to
lies a and
faults,
plate-boundary f l o o r on the
reflections
are
with the sharp Charlotte
f a u l t strand. from
that
If
downdropped
sliver between to
The
strands
under
Queen
crust occurs
at the
of the p l a t e - b o u n d a r y
zone in
this
15 km.
Vancouver
Island,
by the steepness
28) s i m i l a r
by this
the crustal
and the contact
The total width
a r e a is o n l y a b o u t
Off
zone
steep
S t r a t a of the Q u e e n
lower slope are
of
the
and dipping
the short reflections
Basin strata,
gently
reflections
the ocean
abruptly
Charlotte
Tuzo
Revere-
offsets
suggest
B a s i n to the e a s t are t r u n c a t e d
Queen
as
stratified.
inner strand coincides
under the
towards
SP 900 a n d 1050,
ridges
SP 1050 a n d 1200,
The
at least
floor shallows
Discontinuous
but many short
underneath.
and
in the t o p 300 to 700 ms
between
ridges.
below
1992),
subparallel
with many abrupt
Inboard,
s e r i e s of s m a l l b a t h y m e t r i c
which
Sediments
is i r r e g u l a r ,
3 0 0 - 5 0 0 m s in a m p l i t u d e .
the
ocean
t h e o c e a n f l o o r are t r a n s p a r e n t
diffractions
Dietrich,
seismic
1989).
beneath
numerous
from
The outer strand continues
the
basement
area
f l o o r s h o w it c o n t i n u e s
towards
underlying
this
it n e a r l y m e e t s
(Carbotte
88-03
in
continuity
of t h e
of the u p p e r c o n t i n e n t a l
t o t h a t off t h e Q u e e n C h a r l o t t e
inner strand slope
Islands
is
(Figs.
( T i f f i n et
Figure 34. Structure of the submerged continental margin (from the abyssal plain to the outer shelf) off northern Queen Charlotte Sound, imaged in seismic reflection line 88-03 (the box gives the location; the data after Rohr and Dietrich, 1990). The interpretation, including two strands of the plate-boundary fault system, is discussed in text. The inner strand coincides with the abrupt steepening of the upper continental slope.
162
al.,
1972; Chase et al.,
side of
of
the Queen Charlotte
Dellwood
(Carbotte
The
1975).
Knolls,
et al.,
Terrace,
where
the
strand,
fault
is
floor ridge up to 500 m high. oceanward-side-down,
1982;
represented
separates
far
years,
as
Tuzo
et
and
the
Sound
(e.g.,
Cousens
al.,
1991;
Lyatsky,
Sound.
shown to continue
features
1989).
are known
Recent
NW
at
where only a small gap
expression
of the outer boundary
Terrace.
geological
scenarios
continental
1993b; idea
and
sea-floor
fault continues
Wilson Seamounts,
et al.,
support the original Charlotte
by a sea-
tectonic models have been proposed
involving complex plates
begins
it by up to 6-8 km (Davis
al.,
it from the bathymetric
alternative
oceanic
fault
prominently
bathymetric
across
Carbotte
fault the Queen Charlotte
Various
west
Seismic data show this fault has an
mapping has shown that the R e v e r e - D e l l w o o d as
the
ends only about 50 km north
Revere-Dellwood
and sea-floor
to be offset r i g h t - l a t e r a l l y
least
on
1989).
Revere-Dellwood
Riddihough,
The outer
1984,
Instead,
1985;
Lister,
1989;
1993).
sea-floor
the
between
the
northern
et
New data do not off
Queen
fault system is
through this area towards Vancouver
to
Lyatsky
spreading
plate-boundary
Concept of plate rididity as applied
for
margin off Queen Charlotte
Allan et al.,
about
in recent
Island.
Juan
de
Fuca
oceanic plate off western Canada Two basic assumptions is that magnetic movements
underlie
anomaly
patterns
of slabs of oceanic
in which anomaly sources
the theory of plate tectonics. in
oceanic
lithosphere
reside,
regions
One
reflect
that include the crust,
as well as the lithospheric
upper
163
mantle.
At what crustal
is still unclear, blocking
anomalies
are sourced
but the sources must lie between the
remanence-
isotherm
levels the magnetic
somewhere
near the crust-mantle
the shallow sedimentary
cover.
The oceanic
crystalline
crust has a normal
thick,
on
thin. are
so
Still,
lithospheric
a
records
ridges,
single
reconstructing
are
magnetic
on
stiff.
as rigid slabs,
the
into
subduction
zones,
diffuse 1990).
rigid
lithosphere plate
are a powerful
moved
tool
for
plate Evidence
plates to
or
bend
be
strong
boundaries
To
explain
paid to deformation (Stock and Molnar,
for such phenomena
of
become
downward
at
enough
to
may be misleading. a
which
these
more
omit inin
plate
discrepancies,
within plates and to 1988;
DeMets
is now acknowledged
generalized
of analysis,
may
generally
they contain gaps and overlaps
application
data and methods
lithospheric
They
reconstructions
unacceptable.
local circumstances
that
faulting or warping.
plate
is increasingly
Uncritical
lithospheric
deformation.
but they are assumed
available
are
attention
of the oceanic
assumption:
internal
smaller
plate deformation, tectonics
layer is
sheeted dikes and basalts
anomalies
second
resist in-plate delamination,
the
of about 7 km
They are assumed to interact with one another
without
fragmented
Because
and
plate motions.
This tool relies plates
of production
and that the entire
entity,
thickness
scale the anomaly-causing
to the extent that crustal
undisturbed
at spreading as
a
interface,
assumptions
to
et al.,
worldwide.
specific
With the benefit of modern sophisticated
approach
is
164
called
for.
The
assumption
that
plates are rigid needs to be
checked in each case.
Unusual characteristics of the Juan de Fuca plate have been
noted
for
1983;
decades
Acharya,
(e.g.,
1992).
McManus,
1971;
Riddihough
Particularly puzzling has
et
been
al.,
the
absence
of
seismicity at the Juan de Fuca spreading ridge, or of Benioff-zone thrust seismicity along the Cascadia subduction that
zone.
Accepting
this subduction zone is either aseismic or completely locked
does not eliminate its specificity.
Stacey (1973) considered the western Canada continental margin be
different
respects:
lack
magmatic
arc
from of
typical a
active
deep-sea
activity
margins
bathymetric
slabs.
He
three important
trench,
absence
of
in southwestern British Columbia, and the
lack of earthquakes on a megathrust between oceanic
in
to
speculated
the
continental
and
that subduction might have taken
place earlier in the Tertiary, but has now stopped.
In
the
excitement
acceptance
of
plate
undue conservatism. western
Canada
postulate
of
novelty
that
tectonics,
surrounded
the
general
these local caveats seemed like
R i d d i h o u g h and Hyndman (1976) argued that the
margin
subduction,
possesses and
this
all the required attribUtes to idea
has
guided
subsequent
geophysical interpretations.
New
data,
however,
Cascadia subduction character
of
confirm zone
onshore
are
that the Juan de Fuca plate and the "unusual"
magmatism,
(Acharya,
particularly
British Columbia and northwestern Washington,
in
1992).
The
southwestern
is atypical
(Green,
165
1990;
Sherrod
and Smith, 1990).
The Cascadia subduction zone is
remarkably quiet seismically (McCrumb et al., 1989a,b). seismicity
occurs
in
clusters
(Crosson and Owens, 1987). Sovanco,
Explorer
and
Largely
even
1993).
According
"considerable
as
the one in Puget Sound
aseismic
Juan
generally regarded as active between oceanic plates
such
de
Fuca
spreading
Onshore,
or
offshore ridges,
transform
are
the
which
are
boundaries
(Riddihough et al., 1983; Davis and Currie,
to
WahlstrSm
seismicity
and
occurs
Rogers
inside
(1992,
the
p.
953),
Explorer
plate,
indicating internal deformation".
Allan et al.
(1993) have
transferred
off Queen Charlotte Sound from the inner fault of the
Queen Charlotte Terrace sea-floor spreading.
proposed
to
the
that
tectonic
Revere-Dellwood
movements
fault,
and
diffuse
boundary
without
R.P. Riddihough (in: Allan et al., 1993) has
acknowledged that a transfer zone may connect the Queen Terrace
are
Charlotte
northern Vancouver Island continental margins, and a has
been
proposed
between
the
Pacific
and
northern Juan de Fuca, or Explorer, plates (Furlong et al., 1994). More importantly, rigidity
is
it
is
now
inappropriate
accepted
that
the
assumption
of
for the oceanic lithosphere off Queen
Charlotte Sound (Carbotte et al., 1989; Davis and Currie,
A broad zone of deformation extends from the northern end
1993).
of
the
Juan de Fuca Ridge to southern Moresby Island (Milne et al., 1978; Wahlstr~m and Rogers,
1992),
requiring
alternative
models
for
interaction of the Pacific and Juan de Fuca plates with each other (Furlong et al., 1994) and with North 1991;
Lyatsky,
Charlotte
Sound
1993). is
America
(Lyatsky
et
al.,
Evidence for sea-floor spreading off Queen lacking,
and
the
same
continental-margin
structures continue all along the Western Canada Archipelago.
166
Plate-boundary
zone
off northern V a n c o u v e r
Island and the Winona
Basin The structural American Island. been
zone demarcating
continental
plate
there
(Dehlinger et al.,
1970).
continental
which
continues
slope,
as
faults,
sedimentary
basin
of
Scott
is straight
(Srivastava
and
et al.,
the
North American
east
and
side.
whereas
shale
ridge crest, al.,
The fault
In
thickness
is 520 m.
grained,
consolidated,
layers of sand,
has
fracture
zone
the
upper
The outer strand, fault.
zone,
Between
lies the Winona
fault
1975).
mark
the
crust inboard with oceanic
(or Explorer)
plate outboard.
its west side is steeper than
lies along the steep west side of the sediments.
part of the ridge,
Lower Pliocene
sediments
1973; Kulm et al.,
The lower 460 m contain massive
Sandstone
have been dredged
sandstone,
1973).
from the
(Chase
drilling
et
at the
underlain
by
Total sediment
a basal unit of fineoverlain by rhythmic
silt and clay likely of turbiditic
succession
Vancouver
1971; Chase et al.,
continental
the northern
(Couch and Chase,
Pliocene
and steep.
North
system
and basalt and gabbro from the west flank
1975).
the
the position of
plant fragments
DSDP site 177 encountered basalt
Islands
the east side is covered by
containing
fault
Revere-Dellwood
The Paul Revere Ridge is asymmetric:
of
northern
is the R e v e r e - D e l l w o o d
crust of northern Juan de Fuca
ridge,
edge
off
and within the plate-boundary
The Paul Revere Ridge contact
the
It controls
lying some 60 km outboard,
its
western
The inner strand of the plate-boundary recognized
these
the
origin.
is covered by 60 m of hemipelagic
This
silty clay
167
of Pleistocene
age, which
contains
water fauna found in the Pliocene displaced.
Unlike typical
deep-water
turbidite
oceanic crust,
Paul Revere Ridge are not magnetic
Conglomerate
with
Kwakiutl Ridge to
the
slope
quartzite
Vancouver
Island,
and
have
(Tiffin et al.,
succession
basalts
Shallowis probably
that
core
(Davis and Riddihough,
and
Hyndman,
conglomerate, been dredged
1972).
A
from the
part of the Winona Basin
(Davis
close
1989).
similar
to
the
1982).
of soft mud has been dredged
in the southeastern
continental
Cretaceous
east
pebbles
fauna.
Lower
those
on
from the upper slope to the
bathymetric
break
at
mid-slope
(1600-1800 m water depth)
marks the eastern boundary of the basin.
An
showed the Winona Basin as a deep graben
early
gravity
model
(Srivastava,
1973).
be
(Tiffin
complex
Riddihough,
Seismic data show its internal et
al.,
for
Chase et al.,
speculation.
of the crystalline
Plate
plate
was
initially considered
(Davis and Riddihough,
status
of a separate
Models
of
required
plate the
age
lithospheric
of subsidence
by
Davis and
Recently plate
(Davis
has
developed
oceanic
a broken-off
1982).
restructuring
basement
reconstructions
1970s suggested this basin is underlain block
1975;
to
1982).
Lack of direct observations room
1972;
structure
crust.
in the This
part of the Explorer it was raised
(Davis and Currie, and
left
Riddihough,
to
a
1993). 1982)
of the Winona block to be latest
Pliocene to Pleistocene.
This
interpretation
difficulties.
Seismic
encounters refraction
geological and gravity
and
geophysical
surveys
show that
168
the crust thins oceanward under the continental (Fig.
35).
gradually
To the east, Moho depth is between about 40 km under
Vancouver Island (McMechan and 1990)
margin
Spence,
1983;
Drew
and
Clowes,
and as little as 23 km under Queen Charlotte Sound (Yuan et
al., 1992). eastern
The crystalline crust is 10-15
part
of
the
km
thick
and
Riddihough,
Such a gradual attenuation of crustal thickness indicates
that the Winona Basin is probably underlain by continental This
the
Winona Basin and only 7 km thick under the
basin's western part (Clowes et al., 1981; Davis 1982).
under
conclusion
is
in
agreement
with
crust.
the absence of magnetic
stripes over the basin.
The Moho is mostly flat west of the Revere-Dellwood separates
attenuated
outboard. which
continental
This fault abruptly
are
present
over
crust
truncates the
from
the
oceanic
crust
is
unusually
large,
crust
strong
negative
crust
stripes,
throughout
the
Thickness of
the
1982).
The Winona Basin is generally thought to contain A
oceanic
varying between 7 and ii km
(Malecek and Clowes, 1978; AU and Clowes,
sediments.
which
magnetic
northeastern Pacific region outboard (Fig. 27). oceanic
fault,
gravity
about
8
33).
This
minimum
is offset slightly to the NE.
the
of
the
basin.
south,
and
The Bouguer map (Fig.
32) shows a major east-side-down gradient zone along flank
(Figs.
anomaly delineates the shape of the Winona Basin
and reveals its asymmetry: the anomaly widens to its
of
anomaly, as low as -130
mGal, lies over it in free-air and enhanced isostatic maps 30,
km
the
western
Seismicity is concentrated in the northern
part of the R e v e r e - D e l l w o o d fault (WahlstrSm
and
Rogers,
indicating ongoing activity in the plate-boundary zone.
1992),
169
SW ok~
,
0
/~ /5o\
,
_~.7~dV, 1.110
•
;/
1oo
~ I '~1.03' . . , ~ . 4 5 ' - - . - ' ~ 2 . ~ / l
20
3,291
"
\X 1~'.
;/
' < - ' ~ ,~ 07' "~ " ~ - i ~
~ "'\[ 4
~
0
i -3 3O
3O
:P%. •
+
0 I "%
,:.-
t
~
..~,~
-
G3
z
,
.............
/
,
I t'l
,
......... I
/i
o
",' ?'--., \j/I i1.03
-
l
-
-
~+
...........
G2
0
-,
...................... :. . . . . . . .
"l
,.,,,
3~
......., observed'i
j"\
- , ,~'
,o.
I0-
NE -~ +
~,
_
/ ~
i ~ t,
'-.-.__
0
2.87
!
.
30
Figure 35. Structure of the northern, central and southern Winona Basin modeled from gravity data (modified from Davis and Riddihough, 1982). Profile locations are given in Fig. 38. Oceanward dip of the basement on the basin's east side is confirmed by seismic refraction data (dotted lines; Clowes et al., 1981). The cup shape of the Winona Basin is especially pronounced in the south.
170
In
seismic
reflection
stratified profile
sediments
the
Fig.
Island Winona
producing
increases
listric
Basin.
The
Gretener,
dips
seismic
Fig.
A westward
Clowes with
of
symmetric
reflection
37) reveals
and deepens
1150,
apparent
new
Island
rocks dip towards
downsection
seismic
the
in extensional
of
gradually,
reflections.
result from syndepositional
i000)
Such
movements
regimes
et al., the idea
underlying
profile
stepwise
on
(e.g.,
1981;
The
(steps at SP
south.
acoustic
550,
(Line
basement (at SP 450
650,
800;
and
(around SP 900).
on the Winona Basin's east flank is
refraction
and gravity data
Davis and Riddihough,
(Davis and Riddihough,
this basin has
the
off Brooks Peninsula
to about 6 s in the graben axis
from seismic
to
on the flanks of the basin
dip of the basement also
and bowl-shaped
some new details.
lies at about 4 s traveltime
1200,
A
of
1986).
modern
and 1250)
increase
are expected
The Winona Basin becomes
85-04;
eastward.
margin north of Vancouver
pattern
which typically faults,
thickness
zone, which forms the inboard boundary
fan-shaped
normal
the basin,
dramatically
36) shows that stratified
fracture
a
patterns,
A
across
crossing the continental
(Line 88-02; Scott
profiles
been
1982)
1982).
This
(Fig.
contrasts
that the crustal
u n d e r t h ru s t i n g
35;
Vancouver
block Island
during the last 1-2 Ma.
Sedimentary
rocks of Pleistocene
upper part of this basin, are
untested
and Pliocene
but the age and nature of
deeper
and from some early plate reconstructions
proposed to be no older than Late Pliocene 1982).
age indeed form the
Extremely
(Davis and
high rates of sedimentation
rocks
were also
Riddihough,
w o u l d be required to
Figure 36a. Large normal offsets on the steep, west-dipping Scott Islands fracture zone, imaged in seismic reflection line 88-02 (after Rohr and Dietrich, 1990), with location given in box. The fan-shaped pattern of reflections in the Winona Basin, with dips towards the normal fault increasing with depth, indicates syndepositional normal faulting. Such normal faults and fan-shaped reflection patterns in the hanging walls are typically associated with extensional tectonic regimes.
' ~.o~,...
' i
%'
0
~igure 36b.
~%A/
Structural
interpretation of Line 88-02.
j~
173
fill an 8-km-deep basin in so short Basin,
of similar depth,
sedimentation
a
time:
to
fill
the
took about 30 million years.
rates at the base of the continental
Fuca
The modern
slope
in
this
region are up to 1 m per thousand years,
or 1 km per million years
(Barnard,
1979),
1978;
see also Hyndman et al.,
to fill the Winona Basin in just 1-2 Ma. of alteration them
the
seismic
extremely
here,
(1986)
turbidites high
processes
velocities
from reflection
Davis and Clowes young
have
degree.
is
al.,
Davis and Clowes,
was
by
foundering
and from north to south.
(Fig.
lithification
rapidly
The
and
reached
invokes
an
uncommon
of
>5
km/s
(Clowes et
correlated
rocks on Vancouver
Terrace,
of
scenario proposed
velocities
continental
Basin,
with
Island. which
crust similar to that
eliminates
the need for
in the Winona Basin increases downsection Broad
folds
Most faults are fairly steep,
directions.
SP 600)
and ramps
faults have been inactive recently Synclines
are
apparent
and reverse
Some breach the sea floor,
37, Line 85-04,
sediments.
that
1986) may easily be
of
km/s -
data.
speculation
sedimentary
5
give
assumptions.
Intensity of deformation
various
of
such rocks into deep parts of the Winona
geological
profiles.
excess
In an alternative
which created the Queen Charlotte abnormal
an unusual kind
seismic data for deep parts of the basin
and early Tertiary
formed
a
older.
by
Projecting
unusually
Such
suggested
Mesozoic
in
speculated
rocks.
the Winona Basin
-
and refraction
proceeded
in unsampled
1981;
Besides,
of buried young sediments would be required to
high
determined
still much too low
producing
(SP 850).
(SP 1050-1150)
seismic
faults dip in mounds
Many folds and
and are covered by
and anticlines
in
undisturbed
affect all but
174 S'W 200
0,~
SHOT POINTS 400 I
LJ-INE_8.5~O~
600 I
800 t
NE 1000 I
0
1200 I
10
2
i Figure 37. Faults and folds induced by sediment slumping and flowage in southern Winona Basin, imaged in seismic reflection line 85-04 (data after Yorath et al., 1987; Davis and Hyndman, 1989). Line location is given in Fig. 45.
175
the shallowest 1050-1150).
rocks. They
were
related to downslope underlying stepwise
Some folds are detached
to the west,
Sediment disturbances Davis
and
sliding
basement
of
a
sediment
in the
Winona
(1982)
Basin
were
Later
hold that convergence
however,
with North America has virtually
caused by
stopped
Presence of a deep graben beneath
younger
a
in two stages.
sedimentary
loading-related
basin axis, Line 85-04
pressure
where sediments (Fig.
in the location of sedimentary Kwakiutl
Other ridges graben.
lie over
deformation
causes and was partially
Dellwood
probably
anticlinal
the
suggests
deformation
overpressured
sediments.
highest along the
much of the
flowage
and fault movements is reflected
ridges on the sea
flank of the basin western
in
Some of it could
The same phenomenon
flank
floor
(Fig.
38).
the
deep
of
in the Winona Basin had a variety of
controlled
Oblique to the predominant
the
which
the graben might be a
irregularities
- near the eastern
(SP 600)
Thus,
is
1993).
succession
37) occurred near basin flanks.
on the sides of the deep graben.
Haida,
of the Winona block
Much of
are thickest,
have been triggered by basement
-
convergence.
(Davis and Currie,
unit overlying
by
(1989) to be
plate
stratified
result of flowage of semi-consolidated, Though
The
interpreted
the folding and faulting mostly remain coherent,
that the basin developed the
stresses
mass.
and Davis and Hyndman
shortening
in
(SP
in this area near the slope deepens
results only of tectonic
despite
s
towards the axis of the graben.
Riddihough
models,
4.5-5.0
probably created by compressive
gravity
acoustic
at
by deep,
steep faults.
NW trend of the b a s i n - b o u n d i n g
fault and some of the sea-floor
ridges,
Revere-
the Winona Ridge
176
Figure 38. Major bathymetric features in the Winona Basin (in meters; modified from Davis and Riddihough, 1982), as well as the DSDP site 177 and locations of old seismic (Fig. 39) and gravity (Fig. 35) profiles.
177
trends NNW. boundary
Its trend is in
fault
the south.
line
with
system to the north.
The
axis
of
tilting
strands
of
the
plate-
The Winona Basin deepens to
of
the
continentward-dipping
basement block is therefore not parallel to the Paul Revere Ridge, but rather trends NHW. of
this
block's
Tilting in the Pliocene caused the
western
flank,
upturn
creating the Paul Revere ridge
which was then onlapped from the east by Pleistocene sediments.
An interplay of NNW and NW structural trends
is
typical
of
the
continental margin all along Vancouver Island.
Sediment
deformation
in
the
Winona
Basin might have also been
related to compressional shortening in amount
the
past.
However,
the
of shortening was not large, and strata generally remained
continuous along
Line
85-04.
No
low-angle
thrust
faults
or
pervasive oceanward-vergent asymmetric structures are found in old seismic sections through this basin
(Davis
and
Seemann,
1981).
The reverse fault at SP 800 (Line 85-04) is steep and has and dips oceaward. between
In wide areas throughout the basin, deformed
zones
sedimentary
are undisturbed altogether.
Revere Ridge and along the upper
continental
slope,
rocks
At the Paul the
Winona
Basin is bounded by two steep, west-dipping normal faults.
Localized
magnetic
anomalies
as
high as +500 nT on the basin's
eastern flank are parallel to the continental slope (Fig. 27) are
probably
controlled disturbance
caused
igneous of
by
igneous
activity
buried
might
rocks be
along the
a fault. reason
for
and
Faultthe
sediments on the NE end of seismic llne 7
(Fig. 39) in the area of these anomalies.
178
Figure 39. Seismic reflection profiles across the Winona Basin (data after Davis and Seemann, 1981; Davis and Riddihough, 1982). Line locations are given in Fig. 38. The DSDP site 177 is marked on line 3.
179
The southern part of the Winona Paul
Basin
Revere Ridge loses bathymetric
the R e v e r e - D e l l w o o d sediments. side-down magnetic
stripes
Interlocking
Bathymetric
continental
maps
(Fig.
continental
is the Brooks
the other,
faults
beginning
fault
zone
in
an
Nootka
In
Brooks-Estevan
and
oceanic
systems
Island
from Vancouver
(Tiffin et al.,
area
between
embayment,
several
notice
of the Vancouver
fracture
(Muller et al., 1975).
tectonic
to
in the modern
important observations
The northern boundary
al.,
in the
strands
Island.
narrows
into
the
One of them et
al.,
Peninsula
30-35
km,
its existence this
and
that
help
the from
is hard
embayment
literature.
However,
understand
has it the
margin.
embayment
northern
is the broad
Vancouver
(Tiffin et al.,
of this fracture
floor
named herein,
just
of the Brooks-Estevan
and the shelf
continue
is
Island continental
zone that runs across 1974)
ocean
1972; Muller
it
models,
Brooks
4.5 s), and
1981).
as
slope narrows d r a m a t i c a l l y
escaped
the
Hesquiat
to reconcile with
structure
of
is the system of NE-trending
(Muller et al.,
simple
of
that
Because
offers
cover
crustal blocks
images
about 60 km to the north and south.
generally
Still,
in that area.
named here Estevan,
southern
continental
a
slope south of Brooks Peninsula
margin
fracture
1974);
the
there beneath
SP 420, traveltime
28) and sonar
two N E - t r e n d i n g
submerged
to the SE.
the
Embayment
that the continental
between
and
seismic data show a buried west-
(Line 85-04,
are terminated
of
Broo k s - E s t e v a n
show
fault
deeper,
definition
fault may continue
Off Brooks Peninsula, normal
subsided
Island
1972; Chase et
zone bound Brooks Peninsula.
180
The
oceanward
projection
of the Brooks
south end of the principal basin seems to dissipate bounded depressions. <-i00 mGal,
The
southern
to the southeast
(Fig.
part
embayment
form
40).
(Fig.
the
41).
Magnetic Bathymetric
southern
beneath the shelf,
for detailed
at
end
the
(Fig.
by Tertiary
sedimentary
the
is
shelf
trends
the submerged
The Brooks-Estevan
basement
that continue
from oceanic
has
Brooks
fracture
Muller et al.
mapped
basaltic Outboard,
(1974)
flows
and
been
1972;
zone and
NE in
to
the
a
shown
Island,
Basin.
the
Armstrong
rocks
are covered
The
crust
continue
on into
Island.
variety
trends
with
see subsequent
and sedimentary
into it both continental The
127°W
N-S trends are observed
margin from Vancouver
regions outboard.
Island,
age.
Crystalline
contains
but in
in that area also
and the main structures
embayment
between
28).
rocks of the Tofino
continental
vary
in the area of
similar to those on Vancouver
continental,
by several
longitude
anomalies
NW/NE pair;
review).
age,
the
fault-
are mostly NW-SE,
embayment,
an orthogonal
of Mesozoic
Along
the
low, with amplitudes
seismic data to dip to the SW (Tiffin et al.,
trends
elongated,
amplitudes
Their trends
run N-S.
near the embayment's
chapters
where
is characterized
and lows whose
of
anomalies
have N-S trends
Inboard,
into a series of small,
relative highs
127°40"W,
depocenter,
The main Winona gravity
domain
-70 and -20 mGal the
Basin
zone lies at the
also ends there.
gravity
localized
Winona
fracture
of
structural
regions
inboard and
are
continental.
interior of Vancouver et
al.
(1985)
have
dikes of latest Miocene and Pliocene
faults of this fracture
zone
juxtapose
Mesozoic
181
Figure 40. Free-air gravity anomaly map of the Brooks-Estevan embayment (in mGal; modified after Hyndman et al., 1979). Heavy lines indicate anomaly axes. Dashed lines indicate the location of the presumed "Nootka fault zone".
182
Figure 41. Magnetic anomaly map of the Brooks-Estevan embayment (in nT; modified from Hyndman et al., 1979; compare with regional magnetic anomaly maps in Figs. 3b, 27). Solid lines indicate some of the breaks in the anomaly pattern. Dashed lines indicate the presumed location of the broad "Nootka fault zone". In the southern part of the map area, note the partial coincidence of N-S-oriented magnetic anomalies with gravity anomalies in Fig. 40.
183
rocks
exposed
on Brooks
Basin
on the s u b m e r g e d
On the south, projection western
breaks
of
et
shelf
the
al.,
et al.,
embayment
NE-trending cuts both
1981).
Several
lie on t r e n d w i t h N E - o r i e n t e d
NW-SE
the B r o o k s - E s t e v a n
embayment
are
the
and
on
of
However,
the N-S
from o c e a n i c
regions.
seismic
profile
across
on
t r e n d w i t h the E s t e v a n
shows
folds
700)
with
km. 37),
in s t r a t i f i e d an a m p l i t u d e
T h e y are s i m i l a r which
Several (Fig.
are
reverse 42).
traveltime
Further
at
SP
to the
folds to
cut the
outboard
crystalline
crust,
slope.
on the
89-09;
lower
of them,
400
(based
traveltime which
in s o u t h e r n downslope
Fig.
slope
Winona
slumping
at the
thrust
on
of
offset
margin 42).
It
(SP 500
to
of about
Basin
4
(Fig.
of sediments.
foot of the
a low-angle
the
slope
w i t h up to 1
reflections
at
the ocean.
lie u n d i s t u r b e d ,
to
(Line
sediments
of 4 s) dips t o w a r d s
corresponds
lower
sediments
attributed faults
system
the
of up to 0.5 s and a w a v e l e n g t h
The b i g g e s t
km s h o r t e n i n g
fault
runs
in
strands
into the e m b a y m e n t
reflection
Island.
trends
trends
marine
to the n o r t h w e s t .
rocks
again.
and p o t e n t i a l - f i e l d with
on
bathymetric
Vancouver
slope w i d e n s
aligned
which
Cenozoic
fault
A modern
offshore
system,
plate-boundary continue
system
fault
faults
structural
at
and l o w e r - s l o p e
the c o n t i n e n t a l
bathymetric,
of the T o f i n o
1972).
Mesozoic mid-
rocks
ends
Estevan
Island
South of the e m b a y m e n t ,
The
and C e n o z o i c
(Tiffin
the B r o o k s - E s t e v a n
Vancouver
(Muller
Peninsula
of
in this
bedded 1.5
s.
seismic
sediments They
whose
thickness
overlie
oceanic
line c o n t i n u e s
under
the
184
UNE 8 9 - 0 9
600
7~
2 T E (s)
B 2
8
Figure 42. Structure of the submerged continental margin on the southeastern end of the Brooks-Estevan embayment imaged in seismic reflection line 89-09 (after Spence et al., 1991). The box gives line location; shaded areas represent magnetic highs; labeled dots represent drillholes.
185
Magnetic
anomalies
Estevan
embayment
(Fig.
41).
elongated
trending N-S from
Near
the
free-air
N-S
(Fig.
40).
at
longitude
the
28).
embayment
gravity
anomalies,
127°-127°40"W,
Thus,
the
oceanic-crust
embayment,
oceanic
however,
basement
crust. the
In
crust
and continental
crust are complexly
convergence
crustal
the
such processes
chapters Vancouver drillhole
will
are occurring
examine
information
ridges
slope (Fig.
of
This
the
42), this
oceanic-crust
northern
areas
embayment
part
of
is the
with N W - t r e n d i n g
indicates
juxtaposed
that
the
the
as
evolution
and interlocked.
embayment.
of
scale,
block
submerged margin, as geophysical
of this region.
and
North
of the oceanic plate
on a large
character
well
Fuca
for the invasion of
Brooks-Estevan
Island and adajacent
model of geologic
from
of the Juan de
might account
whether
and
structures.
leading to local compression
into
such as
is an area where small blocks of oceanic
against the continent, blocks
ramps
is continental,
embayment
plates,
also coincide,
of southern B r o o k s - E s t e v a n
the
anomalies.
with
is also
features
in Line 89-09
anomalies
Brooks-Estevan
America
anomalies
part of the Brooks-Estevan
faults and p o t e n t i a l - f i e l d
Some sort of previous
coincide
under the lower continental
crystalline
Brooks-
whose orientation
slope and mid-slope
of p o t e n t i a l - f i e l d
by
they
with N-S bathymetric
configuration
that the southern
underlain
southern
of the Juan de Fuca plate
slope,
is influenced by N - S - o r i e n t e d
continuity
into
These N-S p o t e n t i a l - f i e l d
coupled with continuity
suggests
interior
continental
the foot of the continental (Fig.
continue
oceanicTo assess the
movements
next on
using outcrop and data to develop a
CHAPTER
7 - CRUSTAL AND
Variations
is
seismic that
km thick
THE
profile
1983;
along
Vancouver
by continental
Island
crust
Drew and Clowes,
36-42
1990).
It
i0 to 15 km thicker than the crust under the Olympic Mountains
to the south and the Queen Charlotte
Refraction the
Islands to the north.
and gravity data show that under Queen Charlotte
Moho may be as shallow as 23 km
north,
it deepens
Islands
and
again,
27-29
1993).
Under
southeastern
Thus,
along
the
Strait
(Mackie et al.,
and Asudeh,
Queen
Farther
1993;
Charlotte
Hole
1989;
et
thickness
and the Moho becomes
1990; Brew et al.,
and
from
1991).
is attenuated
shelf
al.,
Moho lies at 25-30 km depth
and similar values have been reported
The crust
under the continental
1992).
Sound
under
the
(Brew et al.,
ocean stepwise,
Mereu,
Spence
the Insular Belt,
varies greatly.
km
Entrance,
1972),
Alaska
29
(Yuan et al.,
under Hecate
1991;
Dixon
(Johnson et al.,
to
km
Sweeney and Seemann,
the
ISLAND
SHELF
is underlain
(McMechan and Spence,
VANCOUVER
along the Insular Belt
refraction
the island
UNDER
EXTERIOR
in crustal thickness
A regional confirms
BLOCKS
slope
of the continental
crust
from the mainland towards shallower, (Johnson
step by step,
et
al.,
1972;
1991).
The dramatic Moho relief along the Insular Belt in Canada has been interpreted Neogene.
as a recent phenomenon, Large
extension
a product of tectonism
(factor 1.5 to 3.3) was proposed
the cause of the Moho rise under Queen Charlotte Hyndman,
1983; Hyndman
in
and Hamilton,
1993).
Sound
the to be
(Yorath and
This idea arose from
187
applying to the overlying Queen Charlotte Basin in this theoretical
basin-formation
model
of
McKenzie
area
(1978),
the which
suffers from unrealistic assumptions about properties and dynamics of
the
lithosphere leading to exaggerated estimates of extension
(Lyatsky and Haggart,
1993; Lyatsky, 1994).
No tectonic sutures (Yorath and (Yorath
and
Hyndman,
Chase,
1983),
1981)
predicted
or
regional
for the Queen Charlotte
Islands in models of Mesozoic terrane accretion and late extension 1991;
and
subduction,
Thompson
subsidence
et
al.,
are
expressed
1991;
Haggart,
the
1991).
Differential
of crustal blocks bounded by pre-existing faults, with
Queen
Charlotte Basin (Lyatsky, 1993a).
been shown on other continental margins that the passive
Cenozoic
in outcrops (Hickson,
extension no greater than 10%, was responsible for of
tilt
marker
that
simply
rises
in
the
formation
Besides, it has Moho
proportion
is to
not
a
crustal
extension (Rosendahl et al°, 1992).
Large Tertiary extension and associated strike-slip faulting proposed
for
were
the Queen Charlotte Basin (Yorath and Chase, 1981).
However, subsequent detailed mapping has revealed no major strikeslip
faults
or faults accommodating large extension (Thompson et
al., 1991; Lewis et al., 1991b; Tribe, 1993). is
indicated
by
modern
seismic
basin on the interior shelf. faults
and
dike
swarms
Only ~i0% extension
reflection profiles across the
Cross-cutting relationships of major
on
the Queen Charlotte Islands and the
interior shelf also preclude large strike-slip movements (Lyatsky, 1993a).
In
a
modified model, Rohr and Dietrich (1992) proposed
that right-lateral movements were distributed into the basin the Queen Charlotte fault.
from
However, strands of the plate-boundary
188
fault system lie west of distinct Lyatsky,
Unlike
from
the
the
Queen
Charlotte
fault network on the interior
gradually
in
western
contours
run parallel
(Mooney
and Weaver,
Crosson and Owens,
the continental
shelf
Washington to
the
1989).
and
margin
crust thins
Oregon, without
In W a s h i n g t o n
are
(see also
1987; Owens et al.,
1988;
the crust is about 40 km thick
about
30 km thick under the Olympic Mountains,
km of sedimentary
or
the
km,
slab over the
and
in the North Cascades,
undergoing
and Oregon was smoothed
The lack of smoothness Columbia
1990;
and
about
20
km
under about 2-3
the leading edge of the
plate is thus fairly blunt.
British
al.,
7
continental
subduction.
et
1986;
crust
to
The continental-crust
even
disruptions
crystalline
gradually,
Washington
major
On the shelf and upper slope,
cover,
i0
where Moho-depth
Lapp
1990),
thick at the coast.
oceanward
(Taber and Lewis,
Finn,
western
and
1993a,b).
in British Columbia,
western
Islands
suggests
thins
oceanward
oceanic
slab
in
from below by Cenozoic
along the base of the crust in that
in
that
area,
such a
process did not operate.
Geological
shortcomings
of existing seismic
models
of
Vancouver
Island crust The crust-mantle
interface
as a flat-lying,
5-km-thick
(McMechan km thick, 1985;
and
Spence,
with velocities
Island was
zone with velocities
1983).
with velocities
Drew and Clowes,
under Vancouver
1990).
around
It was later remodeled
from 7.4 to 7.9
km/s
once
7.5
km/s
as a zone 2
(Spence
This zone separates
modeled
et
al.,
the lower crust
6.4 to 6.95 km/s from the continental
upper mantle
189
whose
velocities
are ~7.9 km/s.
modeled by McMechan from 17 to 22 km,
In
and Spence
5.3
Belt on the mainland, Tofino
Basin
applied
to
lies
to 6.4 km/s
on the
the
variable
in
Muller, basalts
probably
(1990),
Vancouver
It extends
Thus,
layer
with
from the Coast
Island,
across the
statics corrections
unfortunately
the modeled velocity
undermined
overlain
and
on
Vancouver
succession
Island,
which
are
2-4; Fig.
Island
is
up
is more than
to
around
16).
6
km
Geophysically rocks of
thick.
2,950 kg/m3
sedimentary 5.5
km
by Upper Triassic Strong
Their
(Currie and
inversion
Mesozoic
and volcanic
(Massey
and
rocks
Friday,
in
acoustic
rocks
impedance
Formation,
and at
is expected.
succession
is
thus contains many acoustic-impedance and clastic units,
These
to Lower Cretaceous
contrasts
the
at the top of the crust
mark the top and base of the Karmutsen
The post-Karmutsen
the
structure with the
so their seismic velocity must also be high.
its base a velocity
carbonate
surface
are Upper Triassic mafic volcanic
Formation,
more than 4 km thick.
and
data
are underlain by Paleozoic
and
and Drew and Clowes
43).
(see Chapters
whose total thickness 1989),
island
shelf.
general
distinctive
1976),
the
3-km-thick
(Fig.
volcano-sedimentary
is about 15 km thick
on
along
surface geology.
In the Insular Belt
density
a
refraction
observable,
the Karmutsen
(1985)
exterior
of correlating
most
vary
over the entire Vancouver
possibility
Phanerozoic
of the upper crust was
from 5.5 to 6.75 km/s.
the models of Spence et al.
velocities
the
(1983) to
its velocity
at the top of the crust
Thickness
as well
as
lithologically contrasts. volcanic
diverse
It includes
rocks
of
the
190
West
DISTANCE (KM)
Vancouver Island N - S Lln,
0
40
80
120
East
Mainland
t60
320
360
0-
20i
30-
1.1 tm
40-
50~0
~\ V,£. 2.5:1
\
\
~11~
Figure 43. Seismic refraction model of Drew and Clowes (1990) for the LITHOPROBE profile across Vancouver Island and the adjacent submerged continental margin. Profile location is given in Fig. 45; numbers give P-wave velocities in km/s. The steep crustalscale boundary in the middle of Vancouver Island coincides with the Alberni-Cowichan Lake structural zone (see geologic maps in Figs. 14, 16). To illustrate the non-uniqueness of this model, some of the existing alternative refraction models along this profile are presented in Figs. 47, 48.
191
Early Jurassic Bonanza Group. widespread granitoids mapping
(Jeletzky,
of
Comagmatic with these volcanics are
the
1976)
(Arkani-Hamed and Strangway,
Island
and
Intrusions
analysis
1988)
of
suggest
suite.
Field
aeromagnetic
these
data
plutons
might
merge in the subsurface into bigger bodies.
The
Georgia
Basin,
which
lies between Vancouver Island and the
mainland and on parts of eastern Vancouver Island, velocity
contains
low-
clastic sedimentary rocks intercalated with coals.
This
succession,
of
Late
Cretaceous
and
Tertiary
kilometers thick (Pacht, 1984; Mustard, 1992).
In
the
consolidated
Tofino
Basin
on
usually
Clowes,
is
many
1991; England and Hiscott,
the
exterior
shelf,
semi-
Tertiary mudstone and sandstone have been drilled to
a depth of almost 4 km (Shouldice, are
age,
1971); their seismic velocities
less than 2.5 km/s (Yorath et al., 1987; Calvert and
1991).
To represent such diverse rock types as a
uniform
crustal layer covering the entire region is misleading.
A
diverse
volcano-sedimentary package 15 km thick may cause many
seismic reflections. shown
that
a
Drilling in the Sevier Desert
stratigraphic
contact
reflections previously interpreted (Anders
and
stratigraphic
Christie-Blick, contacts
in
in
as
a
1994).
seismic
in
Utah
has
that area accounts for large On
sections
low-angle Vancouver may
fault Island,
similarly
be
confused with low-angle faults.
Analysis
of g r a v i t y
Comparison
of
anomalies
gravity
on V a n c o u v e r
Island
signatures in the Insular Belt with those
elsewhere in the Canadian Cordillera suggested
to
Stacey
(1973)
that, overall, the Vancouver Island crust may be abnormally dense.
192
Indeed, seismic refraction and parts
of
the
Insular
Belt
gravity
models
mid-crustal
layers
may
many
Yuan
et
al.,
1992).
be oceanic-lithosphere material underplated by
tectonic processes related to subduction. shape
in
layers of high seismic
velocity and density (Spence et al., 1985; These
suggest
On the other hand,
the
of many of these lenses suggests that magmas from the upper
mantle might have produced sill-like bodies in the crUst.
Consistent with variations in the geologic structure of the
upper
crust, three principal domains are seen in Bouguer gravity maps of Vancouver Island (Fig. 44).
From north to south, they are:
(I) positive (0 to +30 mGal), roughly north of latitude 50°N; (2) negative (0 to -30 mGal), roughly between 50 °
and
49~N;
and
(3) positive (0 to +30 mGal), south of 49°N.
In
the
northern
domain,
strong (>+60 mGal) gravity Sound
(stacey
basement
and
block,
underlies
positive anomalies are extensions of a high
Stephens,
probably
over 1969;
southern Lyatsky,
containing
thick
Queen
Charlotte
1991a).
A raised
Karmutsen
basalts,
this part of the interior shelf, and Tertiary strata in
marine seismic profiles are thin (Rohr and northern
Vancouver
Island,
the
Dietrich,
gravity
in
The central domain, where Bouguer anomaly values are negative,
is
dominated
by
low-density rocks.
w i d e s p r e a d in outcrop in its
splits
On
up
accordance with the Karmutsen outcrop pattern
high
1992).
(Fig. 16).
Granitoid Island Intrusions are
western
part.
Incongruously,
the
dense Karmutsen basalts are common at the surface on the east side of this
domain,
Occurrence
in
but that
their
thickness
is
reduced
by
erosion.
area of outcrops of Paleozoic rocks prompted
193
130o
Figure 44a. Gravity anomaly map and models of lithospheric structure of Vancouver Island and western Washington and Oregon (in mGal; Bouguer on land, free-air offshore; after Riddihough, 1979). Dark triangles represent Quaternary volcanoes. Dark lines mark locations of the 2-D gravity models in Fig. 44b.
194
t,
+
+
:*
SW
N£
W
--
..
o.-T.
....
°
E
Figure 44b. Gravity models of lithospheric structure of Vancouver Island and western Washington and Oregon continental margin by Riddihough (1979). These models were seminal for postulation of ongoing subduction at the western North America continental margin, but subsequent modeling tests reported by Finn (1990) have shown that presence of a subducted slab in this area cannot be established from gravity data.
195
Stacey
and S t e p h e n s
Karmutsen
their
is
thinned
Formation
anticlinorium.
Existence
field m a p p i n g
The
(1969,
(Massey
boundary
anomalies
Island
two
of
observed anomalies
areas w h e r e
On the east, are
coast,
are also present;
all rocks
of the N a n a i m o
The g r a v i t y
are c o v e r e d
to the M e t c h o s i n
Seismic
refraction
At lower-crustal
p.
Positive
over
the
Jurassic
of
if t h e i r
eroded
an volume
further.
by light U p p e r
block to
or lie c l o s e
Bonanza and
Group.
Paleozoic
increases
Along
are
correspond
anticlinorium.
Island
at
depth,
the i s l a n d ' s
Cretaceous
igneous
constraints
levels,
geophysical
data
at the s o u t h e r n
east
sediments
where
41) n o t e d
that
constraints are m o r e
from
data
conjunction
with other
information, the
and
Warner
alone do not r e s o l v e
and
and
geology
The q u a l i t y
and M o r g a n
if i n t e r p r e t e d
Island
surface
speculative.
w i t h depth,
"the r e f r a c t i o n
Vancouver
is
structure
But still,
southern
island
massif.
on deep crustal
models
also d e c r e a s e s
tip of the
structure".
across
Eastern
in o u t c r o p s
the deep in
anomalies
are e x p o s e d
are l a r g e l y
Vancouver
Group.
related
(1990,
by
runs WNW,
on s o u t h e r n
blocks.
whereas
core
h i g h of >+60 mGal
are lacking,
zone of an
domain
anomalies
is r e d u c e d
the
is c o n f i r m e d
Bouguer
of o n l y L o w e r
the
that
axial
The p o s i t i v e
basalts
in
of the c r u s t
of s e i s m i c
block,
basalts
Karmutsen
positive
crustal
a cover
exposed
Intrusions density
under
propose
1989).
the e x i s t e n c e
negative.
Karmutsen
to the s u r f a c e
rocks
reflect
distinct
remain
to
in an u p l i f t e d
and Friday,
over the W e s t e r n
5)
of such an a n t i c l i n o r i u m
w i t h the s o u t h e r n
and g r a v i t y
Fig.
with
due
caution
the s e i s m i c
transect
adjacent
submerged
196
continental
margin
the s t r u c t u r e
(Figs.
of this
The M o h o u n d e r
km.
Iwasaki
crystalline velocities with
were
(1990),
the
crust
along
crustal
modeled
areas
mainland
at d e p t h s
the
velocity
Similar
is p l a c e d
layered,
between
(1990).
blocks
on s o u t h e r n
The s t e e p
of the
Drew and C l o w e s
as d i f f e r e n t
east,
the m o d e l e d
albeit
to
km/s,
island (1990
lower
crust
between
structural (Figs.
modeled
to the east
Island,
in the r e f r a c t i o n
boundary Lake
Vancouver
Mereu and the
model
them
is
zone w h i c h
14,
43).
Eastern
a wedge-shaped
four thin p r i s m s
with a combined
thickness
of up to 20 km.
with
7.1-7.2
with velocities
km/s.
interpretation,
which
high
of 7.7 km/s
velocity
of the c r u s t
The c o m p o s i t e
(Spence
This m o d e l
assumed
et al.,
lower-crustal
exists
is d i f f e r e n t
in this
set
of of
Two of
as low as 6.35 km/s,
that a single
the
with velocities
it c o n t a i n s
were modeled
lower
To
To the west,
prisms
other
Island
and w e s t of this boundary. block
located
unlike
the V a n c o u v e r
is a s i n g l e
of D r e w
runs N W - S E
6.4 to 6.95 km/s°
others
with
horizontal
According
of 6.5-6°6
40
continental
parameters,
inversions
and P a n d i t
Alberni-Cowichan
crust
these
into
of a b o u t
entire
to be h o r i z o n t a l l y
found an e x p r e s s i o n
the m i d d l e
workers,
1990)
c r u s t has v e l o c i t i e s
(1990).
the
through
insights
of 6.8 km/s.
Western,
and Clowes
(1990)
u s e d by F o w l e r
The two m a i n and
ed.,
6.4 and 6.85 km/s.
upper
m a n y useful
and the w e s t e r n
(Green,
for t h e s e
intracrustal
layers,
lower
models
between
some
Island
and S h i m a m u r a crust
45) o f f e r s
region.
Vancouver
in m o s t r e f r a c t i o n
14,
two
from an e a r l i e r
b o d y w i t h an e x t r e m e l y area
in the m i d d l e
and
Clowes
part
1985).
wedge
of D r e w
(1990)
is
197
Figure 45. Location of LITHOPROBE and U.S. Geological Survey seismic reflection and refraction profiles on and off southern Vancouver Island (modified from Green, ed., 1990). Heavy lines indicate reflection profiles or their segments reproduced in other figures. Some of the alternative models for the refraction profile I are shown in Figs. 43, 47, 48, 55. Refraction profile IV along Vancouver Island was used by McMechan and Spence (1983) and Drew and Clowes (1990) to model the main characteristics of the Vancouver Island crust.
198
about
90
km
wide
and
along-island
refraction
just
thick,
2
km
distance
averages
eds.,
1989).
a dynamic
To e x p e c t
near the refraction
profile
f r o m c o a s t to coast, 84-03
line.
Line
is s h o w n
u p p e r crust,
crust
are
underplated
and
of a w e d g e
with
Mooney,
produced
to c o n t i n u e
by
a l o n g the
is u n r e a l i s t i c .
Island structure
available
(Figs.
roughly
separated
(e.g.,
in Fig.
two broad
Clowes
The mid-crustal
14,
parallel
Juan
de
Fuca
upper
reflection
Wrangellia,
by another et al.,
basalts
plate
on
45).
from seismic
Vancouver
band was which
runs NE-SW
to the refraction
transect.
interpreted
keeping
the
bands
the
intersect
mostly
of s e i s m i c
transparent
zone
the
long
transparent
events have been in
the
middle
1987).
zone w a s
interpreted
and sediments et al.,
interpreted
was
Island
Line 84-01
Under
1984,
(Yorath
lower band was with
46.
low-angle
transparent
thrust-imbricated
of
some
Island for a
as
(Pakiser
is s h o r t a n d r u n s N-S b u t d o e s n o t
reflection
detected,
wedge,
data profiles
84-01
crust
of k i l o m e t e r s
of V a n c o u v e r
In t h e i r
k m / s are c o m p a t i b l e
like u n d e r p l a t i n g
Two deep reflection
Line
of 7 . 1 - 7 . 2
for h u n d r e d s
this
interpreted
the fine s t r u c t u r e
process
margin
of
all a l o n g V a n c o u v e r
layers were
velocities
interpretations
reflection
four layers
for l o w e r c o n t i n e n t a l
tectonic
continental
Initial
These
However,
normal
model,
were extended
of 350 km.
material.
t h i n s to zero u n d e r t h e shelf.
assumed
s c r a p e d off the u n d e r g o i n g 1985b;
Yorath,
as a t h r u s t to be
1987).
model,
The
zone at t h e b a s e
just 15 k m thick.
as the s u b d u c t i o n
subduction-complex
as " u n d e r p l a t e d " ,
megathrust
zone.
east-dipping
The In
seismic
i0 s e c
Figure 46. Deep structure of Vancouver Island imaged in the seismic reflection profile 84-01 across the island (after Green, ed., 1990). Profile location in relation to geology is given in Fig. 14. Note the two broad bands of seismic events in the middle and lower crust. The deep event "F" had been interpreted variously as the top (Hyndman et al., 1990) or base (Clowes et al., 1987) of the subducting Juan de Fuca slab, but processing tests showed it to probably be off-line noise (Hawthorne, 1990; Levato et al., 1990). Its correlation with the event labeled "F" in the unconnected Line 84-04 on southern Vancouver Island (Fig. 23), suggested by Clowes et al. (1987), is highly questionable.
o &J
m
>
5 sec
200
events were emphasized
and
dipping events received
less attention.
In
a
later
interpreted
interpretation
subducting
slab
was
reflection
band.
down,
to
subducted
in
the
were presumed oceanic
transparent
blueschist-grade Island,
sediments
Line
the
lower
(event
"F").
and
ultramafic
rocks
are
(Hawthorne,
exposed
1990;
profile
grade.
bands
rocks
and
However, on
no
Vancouver
processing
tests to
Levato et al.,
1990).
soon changed again.
All
were assumed to be scraped off the incoming Juan de Fuca
idea
of underplating
two main reflection representing containing
water
prism on the continental
was rejected bands
metamorphic (Hyndman,
the subducting
were
interpreted
fronts 1988;
slope,
(Davis and Hyndman,
and
as
porous
Hyndman
and the
1989).
The
non-structural,
intracrustal
et al.,
1990).
The
zones top
slab was moved still further down and placed at
"F", which is probably noise.
Structure
of the upper crust was interpreted
data
geological
in
wedge of Metchosin of
Line
km
wide
terms by Yorath
from
the
(1987, his Fig.
84-01
(where complex,
these
rocks
1974,
A thick
on the west
are not exposed).
mapped in outcrop as
(Muller et al.,
reflection
13).
basalts was shown in the subsurface
Westcoast metamorphic 15-30
84-01
to blueschist
of this reflection
into an accretionary
end
of
oceanic Moho was placed at the
mafic
or ophiolitic
plate
event
west-
the top of the
base
and event "F" was shown by subsequent
Interpretation
of
of
metamorphosed
probably be off-line noise
but
zone between two main reflection
to be slices
sediments,
1987),
the
short event at i0 s on the west end of Rocks
thrusts,
(Clowes et al.,
moved
The
as
1981),
a
belt
was interpreted
The just as a
201
broad,
east-dipping
Island.
In the shallow
upper reflection were
shown
Jurassic and
zone
slicing
beneath
(<4 s) transparent
many low-angle, through
Island Intrusions.
Mesozoic
basalts,
band,
continuing
volcanic
an
seismic
thrust
faults
crust made up chiefly of
The entire
succession
and sedimentary
Vancouver
zone above the
west-vergent
upper
was shown as small pendants,
central
rocks,
of
Paleozoic
including Karmutsen
only about
2
km
thick,
on
top of these granitoids.
In
the
refraction
continental even
in
models,
thicknesses
the
lower
the Vancouver
(about 40 km) and velocities
crust;
Green,
ed.,
suggest that the original
continental
place,
full thickness
and a more-or-less
under Vancouver
are
reflection
profiles.
NW-trending
fault systems
faults of
in reflection
levels.
system,
is located
these
are
pattern
dips towards
is
still
of Wrangellian
imaged
with
at various
depths.
belt,
where
crust
46).
in lies
the
seismic
high-angle,
diffractions
Alberni-Cowichan
Lake
(Figs.
between
fault
16, 45).
at about Events
blocks
from mid-
the two main reflection
The upper band,
and
One set of such
its strands bound
is almost horizontal.
The lower band,
the continent.
in
rocks brought to the surface
steep fault systems, (Fig.
well
Island two major,
in the interior of the island
(4-5 s traveltime),
discontinuous.
Such parameters
crust
associated
The other set,
in Line 84-01 diverge depth
lower
always
On Vancouver
metamorphic
crustal
Between
not
lies in the Westcoast
high-grade
1990).
(about 7 km/s
Island.
Steep discontinuities
changes
Island crust has normal
bands 15
km
in it are
22 and >30 km (7 to i0 s),
202
Amphibolite-
and
granulite-grade
middle and lower crust, metamorphic events
fronts.
with local presence
Geology-based The
crustal-scale
lesser
and separates
the A l b e r n i - C o w i c h a n
may
1991;
disrupt
Vancouver
England,
(Fig.
uplift,
Triassic
1977a,b). Cretaceous
This time
system,
saw the creation
Clowes, fault
a
steep
1984).
system
On
is
of
Island blocks.
exposed 1980;
on
the
Branden
of the cowichan
and eastern Vancouver
1991).
San
et al.,
Juan 1988).
thrust system, Island
(England
Thrusts related to these systems
reflection
patterns
in the crust of eastern
46).
East of the A l b e r n i - C o w i c h a n
Upper
Lake
revealed
In the mid- to Late Cretaceous,
(Whetten et al.,
seismic
Island
geanticlinal
and
lies under the
that might have affected the Eastern block
on the Gulf Islands
and Calon,
consistent
as it lies entirely within the Insular Belt
Cascade thrust
The early Tertiary exposed
have
the Eastern and Western Vancouver
was created
of
Island seismic data
data
(White
are revealed by geologic mapping.
Islands,
represent
be
the Coast and Insular belts
Two phases of thrusting
the Northwest
might
1990) may
of V a n c o u v e r
discontinuity
significance,
bands
in the
water.
where refraction
Island,
is common
spots and discontinuity
et al.,
of metamorphic
between
Strait of Georgia,
of bright
(Milkereit
interpretation
boundary
Vancouver
and these seismic
Presence
in these bands
metamorphism
Lake fault system,
lie exposures
basalts
of
the
anticlinorium
of Paleozoic
Karmutsen
developed
(Massey and Friday,
in the core
1989).
of
a
rocks flanked by
Formation
(Muller,
in Late Jurassic Its eastern
to mid-
limb
was
203
affected
by block movements
in the Late Cretaceous
where thick Nanaimo Group sediments deposited
Blocks
by
Lake
a narrow raised
geologic maps
(Fig.
Basin
were
Line 84-01
by a central
(Fig.
events
fairly coherent. synclinorium by thrusts.
structures.
zone,
NE-oriented
events on the
Continentward
Bonanza Group
basalts.
from
the southern
east
fronts
above 4 s
both
sides.
flank
of
represented
the
by
the
ignore these shallower
line 84-03,
the synclinorium
Nitinat
from
perhaps due to disruption
at 4-5 s (about 15 km)
similar events
because
is apparent
the west end of the profile are weak but
In the short seismic of
synclinorium
is indicated
center
brighter but shorter,
band
flank
structural
on
its
The inferred m e t a m o r p h i c
subhorizontal
a
belt of Karmutsen
46), this synclinorium
West-dipping
are
system,
zone in the middle
by dips of seismic events towards East-dipping
fault
are also dowdropped.
16): two belts of Upper Jurassic
rocks are separated
unclear
Georgia
on the west side of the anticlinorium
disrupted
eastern
the
1984),
in a series of grabens.
West of the A l b e r n i - C o w i c h a n
In
of
(Pacht,
which
crosses
the
and the A l b e r n i - C o w i c h a n
Lake
lie at 5-6 s, but their nature
is
half of this profile runs along the
fault system.
deepening of the lower reflection
band in
Line
84-
01, from 7 s (22 km) on its SW end to i0 s (>30 km) on the NE, may suggest regional of
the
tilting of Vancouver
continent.
laterally variable along the profile. Eastern block,
This
band,
amplitudes
Island towards the
about
1
s
(Milkereit
et al.,
Tilting of Vancouver
Island,
wide,
may be related to t e c t o n o - m a g m a t i c
interior
despite the
1990),
persists
especially loading
of the of
the
204
crust
in
the
1987;
Rusmore
thrusting
on
Coast Belt during the Cretaceous and the
early Tertiary
Woodsworth, island's
1991),
(Crawford et al.,
and/or
to
west-vergent
east side in the Late Cretaceous
(Brandon et al.,
1988;
England and Calon,
reflection
meta m o r p h i c
that it ignores this tilt is not surprising.
The deep seismic events, fronts,
shear
lower crust,
zones
are tilted with Vancouver
models
structural
(Fig.
43; Drew and Clowes,
zone continues,
46), many short events,
In the
dipping
events at 2-3 s dip m o s t l y towards
the
north
synclinorium not
of
Line
to a traveltime
intersect,
presence
end
and
seismic
of 5 s.
events
in the
Lake
synclinorium
Westcoast
the lower crust.
Alberni-Cowichan
reflection
between
Line
84-01
directions,
2 and i0 s.
lie
Shallow
short events dip towards the However,
faults
these
(Figs.
Westcoast
structural
and the a n t i c l i n o r i u m
in the
to the base of the crust
are o b s e r v e d between traveltimes
and
variations
in opposite
M o s t l y they dip towards the synclinorium. Cowichan
metamorphic
profiles
between them are c o m p l i c a t e d
of both NW- and N - S - o r i e n t e d
On the SW end of Line 84-01,
recent
the synclinorium.
84-03,
correlations
a
Island.
steeply,
1990).
has
older
suggest that the
on the east flank of the s y n c l i n o r i u m
On
s
primary compositional
Lake
(Fig.
4-5
which might represent
or
At least some refraction
at
1991).
If the subhorizontal origin,
band
and
Thus,
belt
do by
14, 16).
area,
short
of 3.5 and 7.5 s~ both the Alberni-
zones,
w h i c h they bound,
as well as the are rooted in
205
This
conclusion is consistent with the suggestion from refraction
models that the continental crust of Wrangellia thick.
It
is
is
about
40
km
diffucult to reconcile with those interpretations
which assume a thin wrangellia - modeled to be just 12 km thick on the west side of the island and 18 km on the east side (Dehler and Clowes,
1992).
Presence on Vancouver Island of Phanerozoic rocks of various rules
out large thinning of the crust by massive surface erosion.
Though the island was uplifted during most of rate
ages
of uplift in most areas was slow.
was anomalously thin to begin with abraded
from
below
by
of
Tertiary,
the
The ideas that Wrangellia
(Drew
and
Clowes,
1990)
or
subducting oceanic slabs (Yorath et al.,
1985b; Clowes et al., 1987) underplating
the
later
to
be
thickened
again
by
oceanic-derived material - are inconsistent with
refraction models showing the Vancouver island lower crust to have continental-type properties.
These
and
other
inconsistencies of Vancouver Island geophysical
models with each other and with observable cardinal
rule
of
interpretation:
geology
illustrate
a
geophysical solutions are not
unique, and they need to be tested by
joining
various
geophysical data with direct geological constraints
types
of
(e.g., Pakiser
and Mooney, eds., 1989).
Inconsistencies
in
current
tectonic
models
of
evolution
of
Vancouver Island and adjacent submerged margin Models
of the structure of the upper crust can be tested directly
by geological observations at the surface and deeper
crust
and
mantle,
by
drilling.
For
verification of geophysical models is
206
more difficult. velocity,
proved
high-density
depths under margin.
This
western
to
body
be
the
sometimes
Vancouver
case
with
modeled
Island
and
the
at
high-
mid-crustal
adjacent
submerged
Existence of a body was suspected originally from gravity
data (Stacey, 1973), but to this day its
presence
has
not
been
confirmed unequivocally.
Early gravity models of Riddihough lithosphere underthrusted continental
lithosphere
Washington.
This
subsequent
beneath of
general
(1979) showed a slab of oceanic the
largely
Vancouver
idea
became
undifferentiated
Island the
and
basis
western
for
many
interpretations of regional geophysical data (Keen and
Hyndman, 1979; Clowes et al., 1984, 1987; Yorath et al., 1985a,b). Though
it
has
been
demonstrated in western Washington that the
gravity data by themselves cannot resolve a subducted slab 1990),
in
British
Columbia
its
presence
is
(Finn,
still considered
evident (Hyndman et al., 1990; Dehler and Clowes, 1992).
Plate reconstructions indeed require convergence of Fuca though
and
North
America
"unusual"
developed
in
many
plates. respects
Juan
de
The Cascadia subduction zone, (Acharya,
1992),
is
well
in western Oregon and most of western Washington, where
internal disruptions of the Juan de contrast,
the
on
Fuca
plate
are
small.
By
both the southern and northern ends of this oceanic
plate, magnetic stripes are broken or curved and earthquakes occur in the plate interior (Wilson, 1986; Stoddard, and Severinghaus,
1989; WahlStr~m and Rogers,
1987, 1991; Atwater 1990, 1992).
parts of the Juan de Fuca plate may no longer be subducting and Riddihough,
1989; Davis and Currie, 1993).
These (Couch
207
In a seismic Spence
et
refraction model al.
(1985)
across the Vancouver
included,
of material
This body, and
reflection sliver
bands
in Line 84-01,
1984,
1988),
1987).
later
Mere u
a
(1990) modeled
(see papers
single
Vancouver
as far as the mid-shelf,
and upper slope
high-velocity in
many
(Fig.
48).
other models,
Pandit,
1990;
Iwasaki
body was reinterpreted
a
of
four
7.1-7.2 km/s
(Fig.
Island
layers 43).
the
with
crust
of intrusive
note,
yet been reliably
ed.,
Island
premature
1990).
crustal
outer
no distinct
Island crust to
no higher than 6.95
and Shimamura,
1990).
by Drew and Clowes
seismic
velocities
(1990)
km/s This as
from 6.35 to
under central
Lake fault system,
igneous
block
crust beneath the
Vancouver
but to the west
Such a wedge might be explained
to, say, the Karmutsen
On a cautionary
(see also
No such body was included
It is terminated
by the A l b e r n i - C o w i c h a n
it thins gradually. in
judged
which showed the Vancouver
high-velocity wedge
slab
This model contains
body in the middle crust.
and
was
underplated
where a steep fault separates
have a layered structure with velocities (Fowler
47).
Wrangellia
as an
in Green,
it from an outboard block of transitional shelf
of
from a subducted idea
(Fig.
a
zone between the two
was interpreted
This
shelf,
and from the same seismic data many alternative
models were developed
continuing
crust
roughly to the transparent
oceanic crust detached
et al.,
(Sobczak,
of 7.7 km/s
placed beneath a thin continental
of
Clowes
Island and the exterior
with a very high velocity
corresponding
margin,
at a depth of 17 to 29 km within
the crust under western V a n c o u v e r body
Island
by
presence
rocks with parameters
similar
Formation.
even the postulated
imaged with refraction
subducted
slab
seismic data.
has
not
Though the
208
DISTANCE
West 40
o 0
80
(KM)
120
~ ~ / / / / / / ~ 8 . 8 5 - e . 6 ~ I I
-1i-o. i,i c~
3o-
.
--"~-~
40,
I I 118.0-8.~1
L I I'T-I--F[ "<.[ t t I I
=---= I/./I
5o-
.
.
.
.
o.,,- . . . .
. . . I I'Y4r/J2,,t i I l
360
=
. . . . . .I . .11,-L-~ ... i ~J' ¢i , ' K , ' . ' ~ 6
'
20-
Mainland 320
160
4 , 0 - 6 . 8 5 ~
IO
East
Vancouver Island n - s LIn,
"9
s ~
I.
"-_, , ~'-J I I ~
s.95-s.zk,W'kW"~
-. J ".
.~
I
"d "
•
I I I I I I '~ i ~ I i t
"ti l l u ,,.I
.16.6-8.o, I[-.
~-
I
I
IlII I
,7,.9.5"7t2 Iiil
V.E. 2.5:1 60
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 c o n t i n e n t a l m a r g i n (profile l o c a t i o n is given in Fig. 45). N u m b e r s g i v e P - w a v e v e l o c i t i e s in km/s. N o t e the m i d - c r u s t a l b o d y w i t h v e l o c i t y of 7.7 km/s, and the s t e e p c r u s t a l scale boundary in the m i d d l e of V a n c o u v e r I s l a n d c o i n c i d i n g w i t h the Alberni-Cowichan Lake structural zone (geologic maps of V a n c o u v e r I s l a n d are p r e s e n t e d in Figs. 14, 16).
209
E
W P[9
OBSI PI3
P8
oBs3
0BS5 iS0
200
3OO
250
3501
6.5
G 6.O
£
v
DISTANCE (KM) I
Jl
2O
6.8
- -
K
-'ft.-C.L
4O P 8.3
A. - - Pacific O c e a n
C. - - Intermediate continental crust E. - - Y o u n g low velocity sediments G. - - Basement rocks J. - - Intermediate discontinuity M. - - Moho V. - - V a n c o u v e r Island.
8. -- O c e a n i c s e d i m e n t s and crust -- D e f o r m a t i o n front - - Folded/faulted sediments - - U p p e r continental crust - - L o w e r crust with zero gradient - - J u a n d e F u c a plate - - Tofino fault
D. F. H. K. P. 1".
Figure 48. An alternative seismic refraction 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 a c r o s s V a n c o u v e r I s l a n d and the adjacent submerged c o n t i n e n t a l m a r g i n (profile l o c a t i o n is g i v e n in Fig. 45). N u m b e r s give P - w a v e v e l o c i t i e s in km/s. The outer c o n t i n e n t a l shelf and u p p e r slope in this m o d e l are u n d e r l a i n by a b l o c k of t r a n s i t i o n a l crust. The crust thickens continentward stepwise, across s t e e p faults.
210
reason for this may be poor data Warner,
1990;
at
depth
and Shimamura,
1990),
noted that "the velocity
structure
this
cannot
from
be
constrained slab
is
Iwasaki
quality
determined
existing
mainly by the assumption
continuous
interesting, subducting
of
in
this
slab
regard,
was
modeled
towards the continent
stepwise
that by
Thybo part
(1990, of
refraction
[sic]
between different
(Morgan
that
the
disagreements
seismic
reflection
beneath
Vancouver
1985b;
Clowes
the
top
of
(1990)
and that a
fronts
constraints
on
Geologic
Calvert
the
of
structure
the
Stephens,
transitional (Fig.
48).
reflection
origin
bands
(Yorath et al.,
1990)
Vancouver
of
or represent 1990).
Island
Thus,
and
the
Island exterior shelf
crust under the shelf was initially considered
also
in
the
1969; Riddihough,
continental
presumed
from seismic data are only loose.
of the Vancouver
suggested
broad
and Clowes,
of
It is
in the interpretation
1988; Hyndman et al.,
sketch of the V a n c o u v e r
an extension was
1987;
margin available
Geologically,
the
is
to be deepening
block
have a tectonic
(Hyndman,
the
also
whether
Island
et al.,
meta m o r p h i c
continental
data,
and
parts of the model".
Mereu
remain
model
subducting
crust was modeled under the outer shelf and upper slope
Fundamental
p. 69)
the
data
and
(Figs.
43,
Island crust early
1979).
47)
or
(Shouldice,
gravity
1971),
models
(Stacey and
In current refraction transitional
(Fig.
as
models,
48) crust is
shown under the shelf and even the upper slope.
In other interpretations, ending
at
the W e s t c o a s t
Westcoast metamorphic
belt
however, fault,
continental
crust
is
shown
i.e. at the outer b o u n d a r y of the
(Davis and Hyndman,
1989;
Hyndman
et
211
al.,
1990).
parallel
To the west,
to the p r e s e n t - d a y
been postulated: Crescent wedge
and
Hyndman,
Many N W - o r i e n t e d
(Fig.
sedimentary
overlying the
presumably
Snavely,
accreted
(e.g.,
a local branch of the Westcoast
magnetic
Rim
rocks,
Peninsula
Brandon,
fault.
1989a)
A larger fault
offshore,
anomaly and farther WNW
rocks
narrow tectonic
slice
are
exposed
(Brandon,
unit on southern Vancouver the San Juan fault zone
towards
(MacLeod et al.,
the 1977;
rocks probably
along
the Westcoast
1989a,b),
Island
(Rusmore
of exotic terranes.
and
is confined and Cowan,
the
1985).
Emplacement
faulting
or early Tertiary.
Basaltic massifs
Formation
are distinct bodies
produced by eruptions
1992,
1994).
(Brandon and
Seismic
refraction
of
Vance,
1992;
models
the
from separate Babcock
and geophysical
this crust
evidence
is broken
et
show the Vancouver
Various
Island,
of
Crescent
by uniform crust of continental
like on Vancouver
in
in the Late
Island shelf to be underlain lines of geological
Peak
largely to slices
occurred by strike-slip
in a rift setting
fault in a
Pandora
Cretaceous
al.,
The
1987).
Pacific
centers
1992).
faults are indeed present on the coast and on the
But presence of faults does not imply presence
these
the
basin.
15) runs from the Olympic
Prometheus
have
accretionary
Dehler and Clowes,
The Tofino fault on the inner shelf
is probably
zones
the Pacific Rim "terrane",
1989;
as a fore-arc
Island roughly
three tectonic
and the still-growing
Tertiary Tofino Basin, was interpreted
shoreline,
from east to west,
"terrane",
(Davis
shelf.
running along Vancouver
type.
suggest that
into blocks.
That a large raised block lies in southern Queen
Charlotte
Sound
212
is
suggested
by
exposures
of
Jurassic
Bonanza Group rocks on the small Scott 1974),
by
free-air
Islands
Intrusions and
(Muller
et
1991a), and
sediments in marine seismic lines.
by
thinness
the
edge
of
the
shelf.
of
This block, named by
Rohr and Dietrich (1992) Cape Scott High, extends westward as as
al.,
gravity anomaly values in excess of +60 mGal
(Stacey and Stephens, 1969; Lyatsky, Tertiary
Island
far
There it is truncated by the Scott
Islands fracture zone (Dehlinger et al., 1970), which the
seismic
line in Fig. 36 shows to be a west-dipping normal fault.
West of northern Vancouver Island, on the exterior shelf north and south of Brooks Peninsula, (Tiffin
et
al., 1972).
lie two blocks tilted towards the ocean The Northern block
by I00 to 400 m of Neogene
sedimentary
(new name) is covered
rocks.
The
rectangular
Broooks Peninsula, bounded by strands of the Brooks fracture zone, is composed of Jurassic metamorphic rocks of the Westcoast complex (Muller,
1977a,b;
Nixon
et
al.,
1995).
The submerged Kyuquot
block to the south, extending as far as Nootka Island,
is
covered
by folded Paleogene rocks of unknown thickness and more than 600 m of west-dipping Neogene strata Yorath,
1987).
Magnetic
(see
also
Chase
et
al.,
1975;
maps and geologic correlations suggest
the basement in both these shelf
blocks
contains
rocks
of
the
Westcoast complex.
The outer boundary of these blocks is a NW-trending fault parallel to the Scott Islands fracture zone, which also controls of
the
shelf.
the
edge
All along the shelf edge, seismic data show that
sediments have slumped down (Tiffin et al., 1972).
the
steep
upper
continental
slope
213
In
the
Northern block,
sections, 400
m
on
the outer shelf.
the
slightly
In seismic
similar to Brooks Peninsula;
low-density
cover
irregular basement
the slope are similar (Tiffin
to Pliocene
to
et al.,
at
High free-air gravity anomaly values
is
thin.
Fig.
Quartzite Lower
1972;
sedimentary-rock
49)
confirm
In seismic sections,
and the overlying
dip towards the shelf edge.
Island
is very shallow.
it lies at i00 m below seabed on the inner shelf and
(+15 to +30 mGal, that
the basement
sedimentary
and conglomerate
Cretaceous
rocks
Chase et al.,
1975).
the
strata
dredged on
on
Vancouver
Lower Miocene
ages have been confirmed by
dredging
on the upper slope.
Free-air
gravity
to -20 mGal
anomalies
(Fig.
49).
over the Kyuquot block are lower,
The sedimentary
lines is thicker
and consists
Oligocene
(Tiffin et al.,
strata
primarily
1981).
rocks are overlain, and Pliocene
Near
in
marine
seismic
of folded Upper Eocene
1972).
25 ° to the SW are exposed onshore (Muller et al.,
cover
Oligocene
rocks
shelf
edge,
with an angular unconformity,
strata more than 600 m thick.
The
and
dipping
locally north of Esperanza
the
-i0
Inlet
Eocene-Oligocene by Upper Miocene unconformity
and
the Neogene beds dip >5 ° towards the shelf edge.
The
southern
end
of the Kyuquot block,
which runs onto the shelf reflected
from
in p o t e n t i a l - f i e l d
Esperanza
maps.
well
penetrated
3095
failed to reach the basement
Jurassic metamorphic
rocks,
m
of
(Fig.
(Fig.
on
1971;
Brooks
Yorath,
fault
50),
is
free-air
49), and the
Neogene sedimentary
(Shouldice,
exposed
Inlet
South of this fault,
gravity anomaly values drop to -30 mGal J-14
at the NE-trending
Apollo
rocks but
1980).
Peninsula,
cause
214
. . . . .
-
/
'I
Figure 49. Free-air gravity anomaly map of the submerged continental margin off Vancouver Island and Queen Charlotte Sound (in mGal; after Currie et al., 1983b).
F i g u r e 50. Main blocks and their bounding faults on the Vancouver Island continental shelf. Bathymetry is given in meters. Dark 9vals indicate elongated diapirs as mapped by T i f f i n e t al. (1972). AD - Apollo diapir; BFZ - Brooks fracture zone; BKS 8arkley Sound; CF Calawah fault; CS - Clayoquot Sound; EsF ~stevan fault; EzF - Esperanza fault; FI Flores Island; HP ~esquiat Peninsula; NFS Nitinat fault system; NI - Nootka Island; MCF - Millar Channel fault; TF Tahsis fault; WF ~estcoast fault; BB Bamfield block; CB - Cove block; CoB ~lo-oose block; FB - Flattery block; KB Kyuquot block; NB ~orthern block; UB - Ucluth block; VB - Vargas Block.
216
magnetic
anomalies
Strangway, magnetic
1988).
NM-9-10-M)
probably
onshore,
indicates
with
Westcoast
Island Intrusions
belt. and
Bonanza
of
when
levels,
this
volcanics faults
Survey of Canada
Map
and
tectonic however, not
the Jurassic known
complex
accounts
to
be
1981).
It
fault took place
in the
was still at deep
but prior to the Jurassic magmatism.
fault in the Tertiary
reappear
displacement
(Muller et al.,
Westcoast
complex.
complex metamorphic
are
on the Esperanza
the
NW-trending and
Inlet
inland,
and
onto the Northern
Westcoast
Farther
of
of the Westcoast
a 15-km left-lateral
offset by NE-trending
Jurassic,
crustal
Peninsula
of
(Arkani-Hamed
continuity
continuity
outcrops
thus seems that movements Early
reason,
27; see also Geological
This suggests
similarly
this
Island
stop abruptly near Esperanza
along
the
on V a n c o u v e r
across Brooks
(Fig.
These anomalies
of
For
anomalies
Kyuquot blocks
rocks.
elsewhere
Reactivation
for dissimilar
subsidence
of shelf blocks to the north and south.
The Westcoast has
as
belt thus consists of two parts.
its outer boundary
the Scott Islands
The northern fracture
is also the inner strand of the plate-boundary the southern part of the Westcoast Westcoast the
fault,
OWSZ.
anomaly, al.,
1977;
which
onto the shelf runs
Snavely,
of the OWSZ.
to it, the Calawah
Thus,
along
Vancouver
where these two very large structural plate-boundary
the
Prometheus
fault system,
Island
meet.
In
is the
strand
fault continues
This fault forms the
the central
system.
northern
along southern Vancouver
1987).
zone, which
the outer boundary
which is connected with the
Parallel
Olympic Peninsula
belt,
fault
part
of
from the magnetic
(MacLeod et
southern
strand
Island shelf is the area
zones,
the
OWSZ
and
the
2t7
Tectonic Six
information
deep
from deep drilling
hydrocarbon-exploration
Shell Canada Ltd. on coastal
outcrops,
the
sedimentary
package
wells
outcrops,
and
consolidated A
major
difference in the wells,
coastal
outcrops.
are
Sandstone
are
correlative
Large stratigraphic
wells
1-65,
though
off
only
lenses,
sections.
to
those
unconformities
in
Pliocene
(1200 m) strata;
the
Well
and contains
section,
two
rocks:
mainly
and conglomerate
In
in
the
wells,
because of poor fossil 50 and
200
m
thick,
the six wells reflect
Sound,
Pluto 1-87,
the
Zeus D-14 and Zeus
have encountered
very
different
The Zeus D-14 well penetrated
about 2200
Eocene
rocks,
sandstone
underlain
Crescent
Miocene
up
(about
bodies occur at
penetrated
five sandstone higher
Three 1800
(1700 m), and Upper Miocene
(in the Lower Miocene) Zeus 1-65
by basalts
Formation.
in
about
intervals: the
the
base
m and of
and at the Miocene-
and about 500 m of Lower Miocene rocks.
conformable of
between
and Upper Miocene
section
sandstone.
and cannot serve as markers.
separate Lower and Middle
Pliocene boundary.
- semi-
in the Tofino Basin.
the
Middle
the sedimentary
broadly similar
location.
to Plio-Pleistocene
depth),
Oligocene
wells
differences
Clayoquot
m of Early Miocene
In the
the shoreline during at
usually between
between
as
1971).
tZhe Neogene
that
well
about the Tertiary
subordinate
approximate
just 10-15 km apart,
stratigraphic
similar
confirms
of block movements
Three
in
lay near its present
control.
complexity
is with
only
as
(Shouldice,
as opposed to sandstone
This
determinations
not
and mudstone,
in the 1960s by
shelf,
information
lithology
occurs
mudstone
age
direct
the
drilled
Island
in the Tofino Basin
siltstone
least the Neogene
wells
Vancouver
provide
in the Tofino Basin
2500
m
This section
of is
one at the base
Oligocene,
one in the
218
u n d i f f e r e n t i a t e d Upper Oligocene-Lower Miocene, Lower
Miocene.
The
Pluto
1-87
Eocene and Oligocene sedimentary
and
one
rocks,
overlain
by
m) rocks.
the Eocene, Oligocene,
a
similar
Unconformities occur
between Eocene and Oligocene (depth 3200 m) and Lower (600
the
well contains almost 2000 m of
thickness of Lower and Middle Miocene rocks.
Miocene
in
and
Middle
Three thin sandstone bodies are found in
and at the Lower-Middle Miocene boundary.
The Prometheus H-68 well has a section similar to that in the Zeus D-14 drillhole, except that the intra-Miocene unconformities merge and the Middle Miocene is absent. Lower
and
The
unconformity
Upper Miocene lies at about 1600 m.
between
The Upper Miocene
section ends at an unconformity at about 1200 m, above which a conformable Plio-Pleistocene section.
sedimentary
encountered: Miocene,
in
rocks.
Four
Upper
Miocene,
beneath
sandstone intervals have been
at the base of the sedimentary section in the
lies
No Paleogene rocks occur,
and at about 1800 m depth crescent volcanics lie directly Neogene
the
in
the
the
Pliocene,
Lower
and in the
u n d i f f e r e n t i a t e d Upper Pliocene-Pleistocene.
On the outer shelf off southern Nootka well
penetrated
Island,
a Pleistocene mud diapir.
the
the
Lower
and
Middle
Miocene
Miocene and Lower
Pliocene
(1500
undifferentlated
Plio-Pleistocene
m),
(about and
(900
J-14
It encountered nearly
3100 m of Early Miocene tQ Pleistocene rocks, with between
Apollo
Lower m).
unconformities 2600 m), Middle Pliocene Two
and
sandstone
intervals lie in the Lower Miocene, one in the Middle Miocene, and one
at
the
Middle
Miocene-Lower Pliocene boundary.
The Cygnet
J-100 well, on the outer shelf off Cape Flattery, penetrated m
of
2460
Upper Miocene to Early Pliocene rocks, with an unconformity
219
between the Upper Miocene bodies
in the Upper Miocene
Such a stratigraphic develop broad and
shallow,
deep.
The
sedimentary
s
whereas
suggest
Fore-arc
basins
1-87
sediment
traveltime
Reflections
well
penetrated
almost
thickness
1990;
of
4
in Line 89-06
under Crescent volcanics
The Tofino Basin is graben-like,
(Fig.
al., 52).
1972),
which
seismic
A similar to 3-3.5 Fig.
(1900 ms at SP 1840)
51).
suggest
rocks.
flanked by raised crustal blocks. a
buried
in Line 85-01
basement
is apparent
Island.
Marking the graben axis,
the gravity
southern
Vancouver
at
gradient
zones are associated
lies
ridge
around SP
Inboard of the basin lies the uplifted
Island
of
Egger and Ansorge,
(SP 1700 and 2050;
Seismic data show under the shelf edge et
narrow km
Several
about 5 km.
these basalts may be underlain by sedimentary
1400
usually
is indicated by presence of seismic reflections
two-way
(Tiffin
are
the Tofino Basin is relatively
(Iwasaki and Shimamura,
a
boundary.
the Tofino Basin did not
rocks and did not reach the basement.
refraction models
thickness
suggests
setting.
Pluto
(1900 m) and two sand
and at the Miocene-Pliocene
variability
in the fore-arc and
1990)
and Lower Pliocene
Vancouver
low off central
mid-shelf.
The
and
bounding
with major faults some of which are
related to the OWSZ.
Well data show 1971), al.,
and
which
(Figs.
postulated
presumably
51,
mud
Basin
these
cut
the
overpressured
Tofino
were
(Tiffin et
model,
east-dipping
Basin
Neither
(Shouldice,
in it trend NNW
subduction-complex
that shallow,
anticlines.
52) indicate
is
anticlines
In defense of the
1987)
creating
Tofino
elongated
1972).
(1980,
the
Yorath
thrust faults
responsible
for
wells nor seismic profiles
a thrust structure
of this basin.
220
LINE 8 9 - 0 6
300 --
SP
,40o
Magnetic Anomaly (nT)
,50o
18~
iz~
18~
19,oo
2opo
2,~
,,
22,oo
2%00
T I M E (s)
Figure 51. A buried igneous body in the Tofino Basin off central Vancouver Island, imaged in marine seismic reflection line 89-06 (modified from Spence et al., 1991). The box gives line location; shading represents magnetic highs; labeled dots represent wells. Reflections at 1900 ms beneath a body of Eocene Crescent Formation basalt (SP 1800-1840) suggest the basalt may be underlain by stratified sedimentary rocks. Similar stratigraphic relationship of Crescent volcanics with sedimentary rocks has been mapped on the Olympic Peninsula onshore (Babcock et al., 1994).
I
o
600
Kllometres
800
~
[
1000
1200
1400
1600
1800
2000
Figure 52. Submerged continental margin off southern Vancouver Island, from the shelf to the abyssal plain, imaged in seismic reflection line 85-01 (data after Yorath et al., 1987; Hyndman et al., 1990). Line location is given in Fig. 45; interpretation is discussed in text. Note the absence of thrust faults in the Tofino Basin, and a steep reverse fault on the outboard side of the Juno depression.
10
8--
bJ =~ 6 m I--
4m
2~
O--
Line200 85-01 400 2200
2400
SHOT POINT 2600
M
2800
222
The coherent NNW orientation of mud anticlines that
diapiric
faults.
movements
were
(Fig. 50) indicates
probably triggered by deep, steep
The Apollo J-14 well, for which detailed
available,
was
is
drilled into such an anticline on the outer shelf
(Shouldice, 1971; Yorath, sedimentary
information
1980).
It penetrated 3095 m of
Neogene
rocks divided by unconformities into four sedimentary
units: Lower Miocene (below 2561 m), Middle Miocene (2561 to
1540
m), Lower Pliocene (1540 to 896 m) and Plio-Pleistocene (above 896 m).
This might indicate a complicated
subsidence
and
uplift.
But
history
of
jerky,
rapid
despite the stratigraphic gaps, no
major breaks are observed in wireline logs even at unconformities. The
main
lithology
grey mudstone locally
and
throughout minor
glauconite.
the section is semi-consolidated
siltstone, This
with
suggests
rapidly, allowing shelfal conditions
abundant
pyrite
that
movements
(probably
below
and
occurred the
storm
wave base) to be re-established quickly.
Tectonic instability is also indicated by the many faults that cut this
3-4-km-wide,
1980).
From
old
Apollo structure is
17-km-long, seismic
data,
controlled
faults (Shouldice, 1971).
NNW-trending
by
anticline
(Yorath,
it has been suggested that the west-dipping,
steep
reverse
On trend with this anticline to the NNW
lies the straight shelf edge of the Brooks-Estevan embayment.
Transverse faults and crustal structure of the exterior shelf Identification of blocks and bounding faults Transverse faults, commonly recognized on Vancouver Island (Muller et
al., 1974, 1981; Jeletzky,
1976; Nixon et al., 1995), continue
223
on t h e shelf, blocks
characterized
uplift.
Large
wells
make
Basin
faults
of
Vancouver
Island,
crustal
blocks
end
continental
slope
of
faults are indicated
long,
narrow fiords
accentuated
transverse outcrop
in
adjacent
of t h e T o f i n o
the
Brooks
and
Peninsula,
zone e x t e n d i n g
Northern
outboard,
NE
this
and
all
Kyuquot
zone c o n t r o l s
embayment,
where
the
narrow.
by bays with straight or Alberni
or C l a y o q u o t
shear
fracture
Brooks-Estevan
the
even
and
models.
the
Farther
sounds.
Quaternary
faults on Vancouver
patterns,
between
features.
separates
- Esperanza
Barkley
of s u b s i d e n c e
Island control
NE-trending
is a b n o r m a l l y
continental-crustal
the development
coastline
the
Other
were
variations
on the shelf.
north
of
histories
to explain
many
across
Lake,
dissimilar
on western Vancouver
a broad,
Nitinat
series
of s i m p l e t w o - d i m e n s i o n a l
lies w i t h i n
the
a
stratigraphic
orientation
which
by
it i m p o s s i b l e
in t e r m s
Transverse N-S
where they bound
coastlines
inlets,
Millar
Topographic
by glaciers.
I s l a n d are m a n i f e s t e d
zones,
or
by
Channel,
lineaments
Geologically, as b r e a k s
or l i n e a r b e l t s of v o l c a n i c
in
rocks
and plutons.
Many
large transverse
margin. Esperanza by
fault, 40
for e x a m p l e ,
This shelf embayment,
(Fig.
28).
embayment
o n the s u b m e r g e d
which
ouboard,
continental
in t h e b a t h y m e t r y .
the shelf deepens
m, as d e p t h c o n t o u r s
Brooks-Estevan Sound
continue
Some of t h e m are e x p r e s s e d
about
inland.
faults
Across
abruptly
the
to t h e SE
f r o m 60 t o 160 m jump 1 0 - 1 5 k m bears
no
continues
relation as
to
the
far as B a r k l e y
224
Two SW-tilted blocks on the shelf off northern Northern
and
(Tiffin
et
Kyuquot, al.,
1972),
separated are
by
cored
Vancouver
the
Brooks
by
high-grade
metamorphic rocks of the Westcoast complex.
fracture zone
However,
Cenozoic history of their subsidence and uplift
is
Island,
Jurassic
a dissimilar reflected
in
their different Tertiary stratigraphy.
Across the Esperanza fault, to the south, the Westcoast complex is offset inland, and a different basement inderlies the central southern
parts
of the shelf.
and
Six continental-crustal blocks are
distinguished there.
These blocks are most apparent on the inner shelf. Vancouver
Island,
of the OWSZ. expressed
It is represented by the WNW-trending Calawah fault,
as
3-5).
a
west-side-down gravity gradient zone and a westand
Clowes,
of
1992,
their
It separates inner-shelf blocks from the deeper part
of the Tofino and northern Hob basins at mid-shelf. some
southern
their outboard boundary is the southern strand
side-up magnetic gradient zone (Dehler Figs.
Off
the
transverse
faults
Nevertheless,
bounding can be traced far into
these basins, across the shelf.
Unlike on the Northern and Kyuquot blocks, magnetic anomalies central
and
southern
commonly less than positive
anomalies
-i00
that
nT.
Island
Only
are
narrow,
mostly variously
Wells
Zeus
D-14
and
have
oriented
Prometheus
they are caused by Eocene volcanic rocks.
the Metchosin massif on southern Vancouver Island, bodies
negative,
up to +150 nT in amplitude lie between Flores
Island and Nitinat Lake. confirmed
Vancouver
off
these
H-68 Unlike
igneous
no strong gravity signature and may lack deep roots.
225
Reflections under the basalt in seismic line
suggest
89-06
they
may be underlain by stratified sedimentary rocks.
The crystalline basement rocks on central and southern shelf cause no magnetic highs. metamorphic
complex
magnetic anomalies with
On Vancouver is
Island,
consistently
only
the
associated
(Arkani-Hamed and Strangway,
Leech with
River
negative
1988).
Material
seismic velocities virtually identical to those in the Leech
River complex was included in some refraction models
(Waldron
et
al., 1990; Drew and Clowes, 1990; cp. Mayrand et al., 1987) across the submerged continental margin off Barkley Sound.
Admittedly, K/Ar dates from the Leech River Complex of only 40
Ma
(Fairchild
and
Cowan,
1982) make it difficult to extend
these rocks onto the shelf, because the unmetamorphosed, Middle
Eocene
volcanics
in
rather
clasts from Formation
than metamorphism:
the whose
Leech age
Early
the Tofino Basin are older.
other hand, these K/Ar dates may reflect the age uplift
about
River
of
to
On the
cooling
and
in the Fuca Basin to the south,
complex
are
found
in
the
is 42-40 Ma (Babcock et al., 1994).
Lyre At any
rate, presence under the Vancouver Island upper slope and shelf of similar
but
older metamorphic rocks would be consistent with all
the geophysical evidence.
The existence of several basement blocks with dissimilar history
of
information. data, Basin.
relative
movements
is
Based on wells, coastal
suggested outcrops,
by and
Tertiary
the drillhole geophysical
six crustal blocks have been newly recognized in the Tofino They are reviewed below, from north to south.
226
Cove block The Cove block is separated trending
Esperanza
characteristics
Still, On
from the
fault,
and
as far SE as Flores
with
the
Estevan
Estevan Cove
the
embayment.
block
Island,
Estevan
fault,
fault
other
it follows Tahsis
Intrusions occurred
and the Westcoast
in the Neogene,
boundary
of Hesquiat
Carmanah Group
In coastal
predominate, Tertiary
are separated
Peninsula,
rocks
settings. overlain small
islands.
the
locally,
unconformably
seaward promontory
Westcoast
and Eocene-Oligocene
This
on
western
complex
granitoids.
older
rocks
or
fault.
formations
sedimentary
(belonging Peninsula
to
the
as well as
"unsorted slump-conglomerate",
deposited
western
by the Neogene named
on it
1981).
rocks of
and Hesquiat
Island
rocks of the
Upper Eocene and Oligocene
and shale was probably Only
basalts,
Oligocene
have been m a p p e d on Hesquiat
on Nootka and Flores sandstone
truncating
from them by the W e s t c o a s t
Group)
On Vancouver
The latest movements
in places overlap the
Up to 1300 m of conformable
Carmanah
runs into the
offsets.
cutting Karmutsen
1974,
metamorphic
rocks of the Escalante
Farther
because this N-S fault forms the
intruded by Jurassic
sedimentary
as its edge
Island.
oriented N-S,
complex.
(Muller et al.,
outcrops,
faults.
from the south the Brooks-
significant
Inlet,
transverse
Nootka
bounds
causing
NE-
geophysical
the shelf widens
Another big fault,
without
the
Island.
steps about i0 km outboard off southern outboard,
by
similar
retains
this block is crosses by several
trend
block
Kyuqout
Bajo
in shelf to upper-slope
Nootka
Island,
Sooke Formation, Point.
These
are
they
found on a well-sorted
227
conglomerate deposited
and
old
seaward
seismic
et al.,
1972,
sedimentary outer
their Fig.
shelf,
well
1971;
well,
unconformities Miocene
and
Pleistocene several
Yorath,
1555
1980).
Pliocene,
and
Pliocene
uplift.
Vancouver
floor.
Island
may
overpressuring availability faults
to
of basement
played
alignment
due
Apollo
to -40 mGal at mid-shelf
Plio-
to
reached
Presence of Middle Plio-
interrupted formed
with
the
1972).
by
in the
great
development
They have been (e.g.,
of the diapirs by
impermeability
shelf are negative,
of
that involved all older strata.
caused
faults
diapir
trigger
mud
Apollo)
off central
basin
of sedimentary
depth,
rocks,
flowage.
by a common NNW elongation
and That and
50).
Free-air gravity anomaly values over this part Island
m
On the
Miocene,
subsidence was
Concentration
(Fig.
Pliocene
rocks were
but some of them
a role is suggested
of the diapirs
of
(Tiffin
and undifferentiated
(Tiffin et al.,
be
m
1540
Middle
is consistent
mapped mostly with seismic data, sea
and
The
creating an anticline
the
Inlet
rocks are missing.
Lower
in this area of mud diapirs
breach
No Paleogene
the
of the Tofino Basin
dips
m of Middle and Lower Miocene rocks
indicates that Neogene
Pleistocene,
Group
older rocks.
penetrated
between
of
200-250
overlying
and Upper Miocene
pulses
Deepening
7) showed about
J-14
were
1981).
the shelf off Esperanza
Apollo and
just 20 m thick,
The entire Carmanah
unconformably
rocks
(Shouldice, this
this area
(Muller et al.,
line across
rocks
Pleistocene
in
in
in very shallow water.
and thickens
An
sandstone,
of
the
Vancouver
-20 to -30 mGal near the coast and -30
(Fig.
49).
This also indicates
that
the
228
Cove
block
is
downdropped
relative to the Kyuquot block, where
anomaly values are only -i0 to -20 mGal.
Magnetic anomalies over the Cove block are mostly negative, to
<-i00 nT (Fig. 27).
from 0
This suggests that rocks causing magnetic
highs, such as Jurassic Island Intrusions or Westcoast complex
on
Vancouver Island, are probably absent beneath the sediments on the shelf.
A small WNW-trending positive anomaly (0 to
Flores
Island
and
nT)
off
is probably caused by igneous rocks similar to the
Eocene basalts that lie succession
+50
are
within
known
the
thick
Tertiary
sedimentary
to cause the Prometheus magnetic high
farther south.
A WNW-trending
linear
Peninsula
runs
and
magnetic
low
begins
of
Hesquiat
past Flores Island to Clayoquot Sound.
narrow low and the high off Flores Island projection
south
of the Tahsis fault.
end
on
the
This
southward
The magnetic low might be caused
by metamorphic basement rocks, which on the flank
of
the
Tofino
graben lie close to the sea bottom.
Vargas block North-south
faults
probably separate the Cove and Vargas blocks.
On both sides of Flores Island lie N-S inlets, and on them
in
the
volcanics
interior
and
Island
trend
with
of Vancouver Island outcrops of Karmutsen Intrusions
trend
N-S
along
longitudes
126°05"W to 126°25'W (Fig. 16; Muller et al., 1981).
The Carmanah
Group crops out on southern Flores Island,
Pacific
complex
is
southeast.
exposed
but
the
Rim
on Vargas Island and Ucluth Peninsula to the
229
The N-S Millar Channel
fault zone
on
the
east
Island even seems to offset the W N W - o r i e n t e d laterally
about 5 km (cp. Brandon,
SE,
Westcoast
the
complexes.
1989a,
fault separates
his Figs.
the Westcoast
The latter was believed by Brandon
Pacific
Rim
strong signature 1988;
Dehler
block,
magnetic
shelf, Well
and
complex,
exposed
in magnetic
and
Clowes,
maps 1992,
become
positive mid-shelf
Miocene
strata
lie at the top of the that
Oligocene:
the
Vargas
anomaly
values
to the north and south
<-30 mGal
3042
has no
Strangway,
Over the Vargas the
m
of
inner
(Fig.
27).
sedimentary
about 500 m of Lower
section block
a
faults.
outcrops, and
be
(Shouldice,
was
1971).
downdropped
in the
and early Neogene but is now uplifted.
Relative uplift of this block
-I0 mGal
and Tofino
12).
only
To the
to
km from the coast
penetrated
are
indicates
mostly
10-15
which
free-air
i, 2).
anomaly values remain around -50 nT on
Zeus 1-65 at
Paleogene
fault left-
(1989a)
coastal
their Fig.
Flores
and Pacific Rim
(Arkani-Hamed
rocks,
This
in
of
Westcoast
tectonic slice 15 km wide between the Westcoast
The
side
are higher than over the adjacent
(Fig.
lies off Clayoquot in amplitude,
Ucluth block,
is consistent with gravity data,
49).
A broad relative
Sound,
blocks
high of
0
and a narrow and segmented
lies at mid-shelf.
gravity values are lower,
To the south,
as
to low,
over the
declining rapidly from -i0
mGal at the coast to -40 and -50 mGal at mid-shelf.
Ucluth block The boundary between the Vargas Large stratigraphic -
Pluto 1-87,
differences
and
blocks
between three closely
Zeus D-14 and Zeus 1-65
that this area was tectonically
Ucluth
active
(Shouldice,
is
unclear.
spaced wells
1971)
-
during the Tertiary.
confirm
230
Zeus
D-14
Neogene and
encountered,
sedimentary
Middle
1-87,
in
Paleogene
contrast,
turn
sedimentary
bottomed
similar
out
rocks
in
of
Neogene
reflections
the
and Upper Miocene
are
absent.
to Lower
rocks.
2 km thick,
Seismic
covered by a
line 89-06 in this
stratified
rocks to
of 3-3.5 s, SP 1700 and 2050,
Such
of
great
thickness
Ucluth block is relatively Eocene basalts basement. and
Magnetic
89-06).
The
51).
that the shallow
are
NNW in the Vargas block,
Presence
of
stratified
is suggested by reflections
rocks
narrow
mostly WNW
beneath
at 1900 ms at SP 1840
The boundary between these magnetic
highs,
and
coincides
the (Line
between
with a fault
in the are of the three wells.
junction
southeast, amplitude, off Barkley grad i e n t
relatively
volcanics
Eocene volcanic bodies they represent,
junction
depth
Fig.
rocks confirms
highs related to these
change their orientation:
volcanics
This
downdropped.
a
at the bottom of the Zeus D-14 well do not form the
in the Ucluth block.
the
sedimentary
rocks,
Paleogene
of about 5 km (traveltimes
a
Pluto
Miocene,
by the Middle Miocene.
suggesting
Lower
Eocene sedimentary
in this well are almost
thickness
area contains
rocks
some 2200 m of
between
from the Oligocene
overlain u n c o n f o r m a b l y
sedimentary
volcanics,
M i d d l e and Upper Miocene,
separated by an unconformity in
Eocene
rocks with u n c o n f o r m it i e s
Miocene,
and Pliocene.
above
is also indicated
the
Tofino
in gravity maps
free-air
low
broadens
from about -30 mGal off Clayoquot Sound.
Its western b o u n d a r y
zone w h i c h continues
(Fig.
To the
and increases
Sound
to
is a gentle,
to the fault
53).
junction.
<-50
in
mGal
NNW-trending Outboard
of
231
50°N
~2e°w
127 °
'~26"
125 ~
124~
123°
Figure 53. Gravity anomaly map of southern and central Vancouver Island and adjacent submerged continental margin; Bouguer on land, free-air offshore (in mGal; simplified from Fig. 3 of Dehler and Clowes, 1992).
232
the
junction
lie
local
gravity
anomalies oriented NE and N-S,
suggesting that faults with these trends may continental
slope
and
outer
shelf.
be
present
on
the
The junction of all these
faults was an area of increased tectonic activity in the Tertiary, perhaps accounting for the large stratigraphic differences between three neigbcring
wells.
In
this
junction
lies
the
boundary
between the Ucluth and Vargas blocks.
Magnetic
anomaly
some
km
12
relatively
values over the Ucluth block drop b e l o w -i00 nT the
from
coast
non-magnetic
rocks
confined to a narrow slice. the
Eocene
volcanic
(Fig. of
27). the
This
Pacific
confirms
Rim complex are
Between the Pacific Rim
bodies
that
complex
and
indicated by magnetic highs, strong
negative anomalies suggest presence of metamorphic rocks.
Bamfield block The uplifted Bamfield block lies between central Barkley Sound and Nitinat
Lake.
Inboard,
Jurassic
metamorphic
rocks
Westcoast complex dominate coastal outcrops, and the complex
is
absent.
of
the
Pacific
Rim
The Bamfield block, likes the blocks to the
SE, lies on the inner shelf between strands of the OWSZ.
A
steep
magnetic
magnetic low
over
gradient this
near
block
the
coastline
shelf.
the
Westcoast
fault
the
from the high over the W e s t c o a s t
complex onshore (Arkani-Hamed and Strangway, follows
separates
under
The Westcoast fault in this
1988).
shallow area
is
This gradient
water on the inner in
line
with
the
northern strand of the OWSZ.
Outboard
of
the
Bamfield
block
lies
the mid-shelf Prometheus
233
magnetic
high of +150 nT.
volcanics al.,
It is caused by a buried body of Eocene
whose northern boundary
1977;
Snavely,
1987).
separates
this
<-i00 nT.
This gradient
the southern
A straight gradient
magnetic
Bamfield block,
is the Calawah
fault
zone trending
high from the inner-shelf zone,
coincides
(MacLeod et
linear low of
marking the outer boundary
with the Calawah
strand of the OWSZ
of
the
fault, which is part of
(see Chapter
The Bamfield block is relatively
WNW
uplifted,
5).
and
free-air
gravity
anomalies
on the inner shelf are as high as 0 mGal.
At mid-shelf,
anomalies
drop to -50 mGal over the Tofino graben.
The
between
the
Bamfield block and the graben
gradient
zone associated with the Calawah
At mid-shelf,
is marked by a gravity
fault.
on the outer flank of the Tofino gravity
the area of the magnetic
high,
the Prometheus
at a depth of only about 1800 m,
them
total
its
depth
encountered
in this well,
directly.
Unconformities
Upper Miocene that
occasional
remained
No Paleogene
strata overlie
(Shouldice,
1971).
in
rocks were
the
basalts
Lower and Upper Miocene,
the Neogene
sedimentary
from Upper Miocene
reached
continuous.
in
and
and
This suggests
and was interrupted
the Cygnet J-100 well penetrated
Plio-Pleistocene
Pleistocene
rocks
m.
in
by
pulses of uplift.
unconformity well
separate
occurred
On the outer shelf, of
2335
and Neogene
and Pliocene
subsidence
of
low and
H-68 well penetrated
Eocene basalts to
boundary
its
total
sequence It
had
sedimentary
by
in
m
by
an
which
the
The conformable
that subsidence preceded
separated rocks
depth of 2460 m.
suggests been
rocks,
about 1900
Plio-
during that time was
alternative
pulses
of
234
subsidence J-100
and uplift in the Miocene.
well
encounter
penetrated
bedded
sedimentary
reflection
margin
profile
offshore continuation
in this area
85-01
(Fig.
Sound to pelagic
the continental
On the
Basin
structures.
slope° to
1500
the
shelf,
Locally, magnetic
top
of
the
at SP 2320 to 2450,
than
s
which
not
in
the
forms
an
Island.
It
areas outboard of
shows
between
high,
strata
of
the
folded over deeper
deep,
probably
resolved
raised Bamfield block,
SP 2100 and 2320,
the
strong
in
SW-dipping
with diffractions
of the
This event
the Prometheus
lies a fault-bounded
associated
stratified
due
H-68 well,
lies at SP 2280.
The gravity m i n i m u m occurs over this the
52),
though
basaltic body:
Inboard,
it,
did
imaged
along tops of igneous horizons.
where volcanics were penetrated,
3
it
to 2200 ms is associated
kind commonly observed marks
Cygnet
The floor of the Tofino Basin is mostly undefined
the area of the Prometheus at
is
oceanic
be fairly continuous,
to poor signal penetration.
event
but
of Line 84-01 across Vancouver
runs NE-SW from Barkley
Tofino
rocks
the
any m~lange.
The submerged continental seismic
Significantly,
package
to about 500 ms
half-graben
with the Calawah half-graben.
fault
zone.
Inboard
from
thins dramatically on
the
NE
more
end
over the of
this
seismic profile.
The
outer
parts of the Tofino Basin,
resolved
stratified
sedimentary
gravity
minimum.
Increase
side of the basin may be (compared
to
that
in
dense Eocene volcanics.
rocks,
containing
up to 2500 ms of
lie o u t b o a r d
of
in gravity anomaly values
caused
by
reduced
the half-graben)
main
on the west
sediment
or presence
the
thickness
in places
of
235
Besides the O W S Z - r e l a t e d
faults
is also bounded by NE-oriented this block, orientation
presence
28).
shelfal
such
depression
The middle
Outboard,
continental
In the
to
of
the
end of
is
suggested
Clowes,
1992,
The southeastern
NE-trending
Barkley
Sound,
Fig.
an elongated <-30 mGal
NE
Inlet ends off Barkley
channels
cut across the
magnetic
complex
and Strangway, This suggests
anomalies
jump about 15 km 1988;
Dehler
and
that the W e s t c o a s t
faults.
boundary of the Bamfield block is the N E - t r e n d i n g which begins margin.
on Vancouver
relative
free-air gravity
trending
Nitinat
Lake,
parallel,
narrow relative
free-air gravity
and the more isometric
slope and outer
it is represented
low, some 20
km
wide
between highs to the north and south
On western Vancouver
projection
Island and crosses
On the continental
south of the Cygnet J-100 well,
in amplitude,
its
5).
by transverse
the entire continental just
NE
On the inner shelf,
positive
metamorphic
(Arkani-Hamed
fault system,
shelf,
the
to less than 80 m, and the
several
their
belt is disrupted
Nitinat
by
as low as 200 m is also oriented
shelf shallows
Westcoast
inland to the north
Tofino
On the northwestern
slope.
middle
related
shelf,
faults
block
embayment which begins at Esperanza
Sound.
53).
faults.
Bamfield
of the Barkley Sound coastlines.
a local bathymetric (Fig.
of
trending WNW, the
Island, this
Bouguer
system is associated
gravity
low.
On
passes between the elongated,
the
NE-
with a middle
NW-trending
low of about -55 mGal off V a n c o u v e r low of <-80 mGal off the Olympic
and (Fig.
in the area of the narrow,
fault
by
Island
Peninsula.
236
This low is associated with the northern Basin
(see
considered
Chapter
Thus,
to be a boundary
On the inner shelf, reflection al.,
5).
the
Nitinat
1987).
the Nitinat
Though
fault
signal
and
sedimentary
rocks
the underlying
the
section deepens
(Yorath et
of
deepening
Consistent
with
along the profile
are here),
from
about
Dip of some of the suggests
may be draped over some step-like
free-air gravity values
seismic
the line is not reproduced
direction
basement.
is
resolution
on the NW to about 1200 ms on the SE. in
Hob
system
to the coast
penetration
(for t h a t reason,
reflections
the
fault system is imaged in
at SP 950-1000 the r e s o l v e d stratified ms
of
between the Tofino and Hob basins.
Line 85-05, which runs parallel
extremely poor
900
depocenter
this
that
structure
in
interpretation,
drop from about -15 mGal
on the NW of the step to -30 mGal on the SE.
Clo-oose block The C l o - o o s e block, Nitinat
fault system.
30 km long and amplitude,
Coastal
downdropped
15
km
outcrops
in
complex
anomalies
associated
along the coast, up
SE
This
low,
area
Island
-30
to
of
gravity
-40
mGal
contain
Jurassic
Intrusions.
outboard of which lies
to -150 nT in amplitude.
low in
rocks of the
Positive
a
WNW-oriented
The Nitinat
block terminates
the
zones trending WNW and NE.
magnetic
with these rocks end at a steep gradient
NW side of the Clo-oose anomaly,
wide.
this
and
lies
It is outlined by a box-shaped
is flanked by steep gradient
Westcoast
low
and rectangular,
the
which is found only to the north.
zone
magnetic
fault system on the Prometheus
magnetic
237
The
outboard boundary
A steep W N W - t r e n d i n g strong
inner-shelf
amplitude Calawah
of the Clo-oose block is the Calawah
magnetic gradient
low from moderate
is only 0 to -50 nT. fault
inner shelf
separates
from
the
zone along it separates
anomalies
at mid-shelf,
A gravity gradient
the rectangular
strong,
fault.
negative
the
whose
zone along
the
free-air
anomaly on the
anomaly
at
mid-shelf,
related to the Hoh Basin.
Flattery block This
uplifted,
rectangular
partly on the exterior Fuca.
Gravity
WNW-trending
zone in the Strait of Juan
de
of the Flattery block,
The southern boundary
of this block
is
the
lies
Juan
over it are as high as -i0 mGal.
forms the northern boundary OWSZ.
35 km long and 15 km wide,
shelf and partly in the Strait of
anomalies
gradient
block,
de
A sharp
Fuca,
which
lies within the gradient
zone
along the Calawah fault.
The
western boundary
by NE-trending gradient Clo-oose zone
gravity and magnetic
zone
separates
the
and Flattery blocks.
marks
shelf,
of this block,
the
near Cape Flattery, gradient
zones.
rectangular
end of the negative magnetic
and anomalies
The
anomalies
The NE-trending
magnetic
over the Strait of Juan de Fuca are
(>+500 nT) due to presence
trending
fault is inferred to run across the continental
An
isolated
Flattery, may
separating
the Flattery
magnetic
high
of
on trend but unconnected
represent
a
small
gravity over
the
gradient
domain on the inner
positive
this area,
is m a r k e d
of Metchosin
strongly
volcanics.
A NE-
shelf
in
and Clo-oose blocks.
>+i00
nT
is found west of Cape
with the Prometheus
body of volcanics
high.
It
in the Hob Basin.
The
238
western boundary on
of this magnetic
trend with the inferred
Flattery block. depocenter
of
fault on the northwestern
the Hoh Basin.
Its basement to
the
block
is probably
and
south.
underlain
outcrops
The
by
a
contain
Jurassic
crust basement
(see Chapter
Crucial
of
role
geophysical the
rocks
intrusive
of
Island
well
and
is
To the south, the
Olympic
rocks of the continental-
information
plates,
surface
in
validation
whose thickness
outcrops
continental
(1980),
(Apollo
related thrusting
diapir,
to the north
5).
lithospheric
old paper by Yorath
sedimentary
body.
high.
exposures
1977c)
on the west coast of
rocks
in
of
in
were interpreted
drillholes
a
few
information
A notable
from
exception
to interpret
the is an
the
in terms of ongoing subduetion
Tofino
this
may reach
In many recent papers on
which attempted
J-14)
the
or
margin,
Tofino Basin wells is rarely mentioned.
one
massif
(Muller,
deep may seem insignificant.
the Vancouver
from
structural
as indicated by
crystalline
geological
of kilometers,
kilometers
margin.
models
scale
hundreds
the large sea-floor
continental
Metchosin
large
at Point of the Arches
Peninsula
flank of the
SW into the northern
a fault-bounded
at least 7 km of volcanic
probably
On
submerged
has a complex composition,
north
contains
It may control
lies
at the mouth of the Strait of Juan de Fuca
and runs SW across the entire
Flattery
is oriented NE and
This fault may thus continue
canyon which originates
The
anomaly
Basin.
well,
as low-angle
which
Minor
distuptions
was drilled
data and in
into a mud
thrusts.
Such shallow thrusts were shown by later surveys
(mostly
seismic;
239
Figs.
51,
52) to be absent.
Perhaps
it was this initial
success that led to disuse of the well offered
tectonic
barely mentioned
models
for
(Hyndman et
Dehler
and
Clowes,
because
it provides
of the subsurface.
In
cases,
some
information Vancouver
even
46).
Still,
drilling
Island,
the
Spence
well control
in
In the
(Kurtz
may provide crust
al.,
has been
1986,
in Line 84-01
1990) (Fig.
hypothesized
that the high reflectivity
and the high conductivity
have a common
cause:
fluids,
Pacific
Ocean,
Fuca slab, fronts
saline
and then expelled
(1988)
reservoirs estimate
estimated
thought
(e.g.,
speculations notoriously graphite
regions Bailey,
about loose.
(Jones
to
1989;
derived
the
zone with the Juan de
zones
along
metamorphic
crust.
porosity
in
these lower-crustal
in the range of 1 to 4 percent.
be
This
but small amounts of m e t a m o r p h i c
present
1993).
the
in
the
But due to lack
structure
of
the
A common cause of electrical et al.,
Meissner
from
lower
crust
of
that have nothing to do with subduction
1!994), and intracrustal
have a variety of geologic Meissner,
the
is probably exaggerated,
intracontinental processes
into porous
continental
to be very high,
water are now
presumably
carried down the subduction
in the overriding
Hyndman
Hyndman
under
(1988)
intracrustal
coincidence,
about the
conductivity et
1991;
is indispensable
lower
deeper band of seismic reflections
To explain this
papers
al.,
regions
a zone of high electrical surveys
et
information
remote
relevant to local models.
revealed by m a g n e t o t e l l u r i c along
1990;
the only direct geologic
structure
Subsequent
this area where the well data were al.,
1992).
data.
lack of
causes
and Wever,
(Smithson 1992).
and
of lower
constraints, crust
are
conductivity
is
reflectivity
may
Johnson,
1989;
240
In
the upper crust, however, geophysical models are in some areas
constrained
by
Fennoscandian
The
drilling. Shield,
Kola
away
far
superdeep
from
any
well
in
the
continental margins,
encountered several zones in
the
continental
crystalline
crust
where fluids migrate freely.
Intervals of crystal dissolution and
"hydraulic disaggregation" were found at depths of 7-10 km. zones
are
assiciated
with
some of which had initially
These
seismic reflections and refractions, been
misinterpreted
as
fundamental
compositional boundaries in the continental crust, such as between the hypothetical "granitic" and "basaltic" layers (Kozlovsky, ed., 1984;
Pavlenkova,
Cordilleran
1989).
metamorphic
on core
the
other
complex
in
hand, drilling into a Arizona
showed
that
reflectivity in that area is caused by thin compositional layering in gneisses (Thompson et al., 1989).
Several continental drilling
projects
showed
that
porosity
in
fracture systems in the upper crust may occasionally be as high as 2-3% at shallow depths, but is usually 1% or less (Kozlovsky, ed., 1984;
Zimmermann
et al., 1992).
Unexpectedly,
it was found that
circulation of fluids, along with variations in thermal properties of rocks, in places causes sharp increases in geothermal gradients which prior geophysical models Huenges,
The
failed
to
predict
and
1993).
Vancouver
Island
continental
margin was modeled to contain
three belts of oceanic-crustal origin outboard Island
(Clauser
"backstop":
the
Pacific
Rim
"terrane", and the m o d e r n sedimentary
of
"terrane", accretionary
the
Vancouver
the prism
Crescent (Davis
241
and Hyndman,
1989; Hyndman et al., 1990; Dehler and Clowes, 1992).
These belts were thought to continue along the island for hundreds of
kilometers.
The Westcoast, Tofino and Hurricane Ridge faults
were postulated continent.
to
The
be
low-angle
small
thrusts
half-graben
associated
fault in the area of the Bamfield block Fig.
52)
was
interpreted
subduction megathrust.
as
a
dipping
towards
the
with the Calawah
(Line 85-01, SP 2320-2450;
"fossil
trench" over a former
The Tofino Basin was regarded as
a
fore-
arc basin lying on top of these deeper structural belts.
In
defense
of
this scenario, several closely spaced gravity and
magnetic models across the southern Vancouver Island presented
by
Dehler and Clowes
margin
were
(1992), but parameters assumed in
these models for the postulated structural belts are questionable. The
Pacific Rim "terrane",
for example, was assigned an extremely
high density of 2,900 kg/m3. outcrop
mapping,
This is incompatible with results of
which show this complex to contain a m~lange of
sedimentary and volcanic rocks cut by ultramafic
rocks
found
(Brandon, 1989a,b). ultramafic
rocks
only
Rim
granitoid
locally
in
are
slice.
Also
displaced
with
fragments that
present in abundance at shallow depth, but
By
subdued
contrast,
magnetic
field
over
the
a presumed sediment-volcanic
m@lange off Washington has been modeled with 2,700-2,750 kg/m3
pluton,
A high density like 2,900 kg/m3 implies
this is inconsistent with the Pacific
a
a
density
of
only
(Finn, 1990).
unrealistic in the models of Dehler and Clowes (1992) is the
presentation of the Crescent "terrane" as basaltic
panel
4
km
thick.
The
a
continuous,
uniform
Crescent Formation along the
margin onshore is known to consist of marly
discontinuous
igneous
242
bodies
with great lateral variations
(Babcock et al., gravity shelf, of
and
1992,
1994),
magnetic maps
isolated magnetic
only localized
anomalies
confirmes
al.,
The
1977),
regional
3),
northwestern
these bodies
highlighted
presence
of several
and NNW,
gravity
Peninsula
Continuity
magnetic
strand
a
highs
volcanic
reflecting
the
were
Prometheus
H-68 well,
10-15
away
penetrated
well
from
running
from
shelf as far as
in Fig.
(Fig.
the
(1992, their
fault on the
shelf
was
15.
27) indicate
trends
interplay
vary
the
between
of faults along which
at
body is local and
a
depth
but at 2200 in the
from the Prometheus
Pluto 1-87 e n c o u n t e r e d Pluto
apparent
Their
(MacLeod
erupted.
Well data show that the Prometheus Volcanics
gravity
strand of the OWSZ.
lineament
shelf
bodies.
a complex
these basalts were probably
km
is
as illustrated
on
Island
presence
fault
along the exterior
of the Calawah
(1987),
in
are local.
gravity map of Dehler and Clowes
also noted by Snavely
WNW
this
the
of correlative
body sits on the Calawah
of
reflected
reflect
which is part of the southern
Distinct
is
and thickness
On the Vancouver
Absence
Olympic
Barkley Sound.
1990).
igneous bodies.
extent
which
(Finn,
probably
horizontal-gradient Fig.
whose distribution
anomalies
The elongated Prometheus et
in composition
of
about 1800 m in the
Zeus
D-14
location,
wells
no igneous rocks at all,
is nearly 4 km deep.
non-uniform.
even
Though basalts
well.
Just
Zeus 1-65 and though
the
in the Prometheus
well are up to 500 m thick,
they have only a local distribution.
In seismic
51),
line 89-06
(Fig.
reflections
under
the
basalts
243
(1900 ms,
SP 1840)
sedimentary confirmed
suggest that volcanic
rocks. that
Field mapping on
sedimentary
(Babcock et al°,
1994).
Existence
on the shelf of
eight
continue
from V a n c o u v e r
this area is probably exposure
Large
relative
differences
contradicting drilled
of
blocks
The
Peninsula
same
whose
that the crust in is
indicated
rocks on Brooks Peninsula,
movements,
recorded
the
Tofino
Basin
in
the subduction-complex
by
and by Island.
in the stratigraphic
are inconsistent a fore-arc
model,
in
bounding
across the margin off southern Vancouver block
has
Crescent volcanics
Island confirms
between the offshore wells,
development
Olympic
crustal
continental.
of Jurassic metamorphic
refraction models
the
rocks underlie
that area
faults
flows may be underlain by
with
the
setting.
Also
the Cygnet J-100
well
on the outer shelf to a depth of 2460 m did not encounter
any m~lange.
Distribution discussed
of
crustal
blocks
in the next chapter.
on
the
continental
slope
is
C H A P T E R 8 - STRUCTURE OF C O N T I N E N T A L
Regional The
overview of V a n c o u v e r
outer
strand
of
structural
zone,
cover
undisturbed
of
embayment. off
the
and
Island continental
the
sediments
fault,
plate-boundary
continues
into northwestern
slope in the
southern Vancouver
is
about
sediment water
60
km
ridges,
depths
embayment
ridges hundreds seabed,
are
Island continental Seismic
and
channels,
slumps,
ponds.
about
sharp
break
locally
by
(Carter,
1974).
The
narrows,
Tertiary
Tofino
Tertiary
strata
slope with only however,
accretionary subducting and Hyndman,
and
1971).
One
continuing
from
minor the
prism Juan
on the
was initially
(Shouldice,
Later,
by
(Fig.
areas
28).
at
of
smooth
The upper slope
disruption
sediments
glacial
scours
to about 80 km.
drilled on the shelf to a upper
continental
slope.
thought to extend to the foot of seismic
profile
showed
late
the outer shelf onto the upper (Snavely
and
continually
Wagner,
1990).
1981).
as a t h r u s t - f a u l t e d scraped
de Fuca plate by the Wrangellian
1989; Hyndman et al.,
A
which is smooth but cut
slope was r e i n t e r p r e t e d of
lie
Many anticlinal
Holocene
succession,
4 km, continues
the slope
NW-SE,
data show numerous
The shelf widens to the south,
Basin
oriented
is steeper than the lower slope.
channels
sedimentary
but
fault.
Broad plateaus
it from the shelf,
Pleistocene
depth of almost The
Island
separates
sonar
surrounded
found on the lower slope
off southern Vancouver
slope,
1400 m and 2000-2200 m.
of meters high,
a
Island the lower slope steps
wide.
of
under
Brooks-Estevan
out and again lies on trend with the R e v e r e - D e l l w o o d
The southern Vancouver
ISLAND
slope
continental-oceanic
Revere-Dellwood
The continental
central
SLOPE OFF V A N C O U V E R
off
backstop
the
(Davis
245
Gravity and magnetic northern Washington Near-zero
anomalies
along
and southern
free-air
gravity
the
continental
anomaly values characterize
slope off northern
Washington
Columbia.
On the middle shelf
lie negative anomalies:
Hoh
Basin.
A
low
Between these lows,
of
to
near-zero
the
Columbia, margin
pelagic northeastern
and thickening over
(Fig.
on
southern
in response
-30 to -55
the
the
upper
Pacific off Washington
northern
slope
and
and British
towards the continental
to downbending
of the overlying
British
53).
free-air anomaly values decrease
gradually,
the upper
-50 mGal marks the lower slope.
anomalies
outer shelf form a relative high
Over
and
less than -80 mGal over
-40
off
British C o l u m b i a
continental
mGal over the Tofino Basin,
slope
sediments.
of the oceanic crust
The sedimentary
layer
the Juan de Fuca plate is 500 m thick on average but as much
as 2500-3000 m thick in the Astoria
and Nitinat
of
1987).
the
slope
(Carlson and Nelson,
is conventionally
A pronounced
denoted Cascadia
Juno structural
under the lower slope.
and magnetic
anomalies;
end
Juno gravity
This sedimentary
layer
Island.
Basin,
depression
faults, this
to the Nitinat
fault system,
and
with
the
sediment-
in the oceanic crust
the Juno depression
expressed as coincident
structural
of the Brooks-Estevan low is terminated
lies along the foot
It is associated with
(new name)
On the north,
the system of N-S-oriented
southern
foot
Basin.
of the slope off southern Vancouver
filled
the
gravity minimum of about -50 mGal
the deepest part of the Cascadia
fans at
zone
embayment.
by a NE-trending
also
ends
gravity
marks
On the south, lineament
which crosses the continental
at
the the
related margin.
246
In
a
gravity
model
across
the central Washington margin, Finn
(1990) included under the shelf and upper slope about 1 to 2 km of sediments
with
a
low
density
of 2,530 kg/m3 (Fig. 54).
These
sediments overlie material which had previously been modeled a
density
of
2,800 kg/m3 (McClain,
1981).
with
Finn considered this
value exaggerated and assigned to this body a density of 2,700 2,750
kg/m3.
slab.
To explain negative gravity anomalies on the Washington and
Oregon
Below
to
this body she placed the undergoing oceanic
shelf, Couch and Riddihough
(1989) had previously proposed
a series of sediment-filled depressions. cause
of
such
Finn (1990) thought
a low off central Washington is the material with
the density of 2,700-2,750 kg/m3, which she
modeled
thickening
the inner shelf.
to
as
much
as
20
km
under
as
interpreted it as a mix of sediments and basalts scraped subducting
the
Juan
de
Fuca
plate.
Confusingly,
a
wedge
off
though,
She the such
densities are also consistent with crystalline continental crust.
In the gravity models of Dehler and Clowes densities
(1992),
a
wedge
with
of 2,410 to 2,610 kg/m3, some I0 km thick, was included
under the outer shelf and slope
off
southern
Vancouver
Island.
Such densities are similar to those measured in the Tofino and Hoh basins, but Dehler and material
Clowes
(1992)
interpreted
as an accretionary sedimentary prism.
this assumed low-density pile, they assigned 2,880
modeled
To compensate for high
density
of
kg/m3 to the underlying body, which they presumed to be the
oceanic Juan de Fuca slab. this
a
this
slab
2,850 kg/m3.
By
comparison,
Finn
(1990)
modeled
under the W a s h i n g t o n margin to have a lower density of
247 1832F ,
i I I I ' ' ' ' [ ' ' ' ' I ' ' I i I ' ' ' i
'
I
' ' ' I-4
,
, , ,
1500"-
~_,-
--
. . . .
,
. . . .
,
. . . .
-5
, ~ , I , , ,
. . . . .
~
-tO0! -140
. ~
~
,
i
I
. ,
i
'
. i
I
. I
i
i
. i
i
. I
,
~
'
. ~
I
~
-
o
Z
. I
i
i
t
i
I
i
i
,
i
1
i
~
i
;
i L3°
kllometer"
V.E.3:l
17 ~
11~I~/ 53
, 28 50~
Figure 54. Crustal structure of central Washington continental margin, in an E-W gravity and magnetic model of Finn (1990). Under the shelf and slope, body 5 (in heavy outline) with density of 2,700-2,750 kg/m3 was interpreted by Finn as a m~lange consisting of sheared sedimentary and volcanic rocks. However, as discussed in text, its modeled parameters are similar to those typical for attenuated continental crust.
248
Previous
chapters
showed that placement
shelf of u n d e r t h r u s t e d geological seismic
oceanic-crustal
observations
refraction
variously
and
continental-crustal
Vancouver
continental Cenozoic
affinity.
Island
margin.
1991a,
Steep faults
in
were inherited A
crust
with
thousands
of crustal
blocks
the inboard parts of the submerged style has
characterized
tectonic
beyond
style
Vancouver
the
long-lived
from at least the Mesozoic
similar
extends
with
blocks
accommodated
uplift and subsidence
and
and
shelf is u n d e r l a i n by
of most of the Insular Belt, where
1993a).
continental
the
Island
is inconsistent with
fault-bounded
Such a tectonic
evolution
fault networks
Rather,
depressed,
of meters of differential on
bodies
from field mapping and drilling,
models.
uplifted
under the Vancouver
(Lyatsky,
indicates
Island
that
into
the
on the lower slope.
Many
submerged margin.
A
different
faults
fault pattern
in that zone are continuations
the p l a t e - b o u n d a r y
Another
from the NNW of
zone continues
Island from the ESE.
of crustal weakness
in Idaho and Oregon,
zone.
The OWSZ,
stretching
an
North
crosses the Washington
Olympic
offshore,
where the Calawah
controls
the
disrupts
Prometheus
Pleistocene
system continues
fault
intracontinental
from the Cordilleran
old faults were incorporated
The
of
onto the sumberged margin
Cascades
along the Strait of Juan de Fuca onto the shelf rejuvenated
strands
fault system.
major structural
off Vancouver zone
is recognized
(Fig.
into the
system
The South
WNW as the W e s t c o a s t
of
this
on the shelf blocks,
rocks,
and even
Vancouver
fault.
New and
crustal
body of Eocene volcanic
deposts.
persists
7).
tract
continues
fault bounds several
and
interior
Island
fault
249
On
the
shelf
eastern
flank of the elongated
lies the W N W - o r i e n t e d
OWSZ-related
Calawah
low lies a gentler, NNW.
gradient
fault.
slightly
Tofino gravity
zone
(Fig.
Eocene
On the western
50).
Positive
basaltic
bodies
narrow
magnetic
The magnetic metamorphic
zone
that
orientations
also trends WNW.
idicates
that
the
west,
lower
continental
longitude
The interplay
branches
suggests southern
Vancouver
most probably
Olympic
oceanic Island,
continental
Peninsula
is
on the submerged
margin
plain to the
low
Their
On the upper slope, of
about
evidence presented
crust underlies
-i00
nT
the at
in this chapter
the lower slope
off
the crust beneath the upper slope
is
continental.
acquired between slope
WNW
of both the plate-boundary
Deep structure of the southern V a n c o u v e r The submerged
trends.
are stripes oriented N-S.
is a faint
Geophysical
that whereas
by
suggesting
of NNW and
slope and on the abyssal
anomalies
anomaly
126°W.
caused
thought to be associated with
range from -300 nT to +400 nT.
only N-S magnetic
anomalies
Island.
the main magnetic
amplitudes
trends
erupted have similar
fault system and the OWSZ have propagated off southern V a n c o u v e r
the
all over the
have both these orientations,
low on the inner shelf, rocks,
with
flank of this gravity
irregular gradient
that faults along which these basalts
On
coinciding
Such an interplay of WNW and NNW trends occurs
shelf
low on the
margin
Vancouver
Island
poor,
with
Signal penetration
on
a notable exception
slope
and
has been imaged in seismic reflection
1976 and 1989.
extremely
off
Island continental
the
profiles
the
upper
of the U.S.
250
Geological
Survey Line 76-19 which is discussed
profiles,
the reflection
Without
good
information a
seismic
(Fig.
profile
45).
contradictory,
However, as
very
these data by different
In the model of Mereu slope
lies
a
slope,
sediments.
and
this
most
the continental
interpretations
velocities similar
and
different models have been produced
from
workers
(Green,
under
of
margin off Barkley are
(1990),
block
is
ed.,
the
covered
this
1990).
outer
shelf
stepwise.
crystalline
sediment
crust velocities
The detailed model of W a l d r o n as
being
is underlain
section
increasing downward 48).
On
the
and the Moho deepens In the model of Weber between
et al°
(Fig.
increase downward by an oceanic
Thybo but
(1990) gave
4°8 with
modeled
a
the underlying
of 6 km/s.
3 to 4 km thick,
dept h from 1.9 to 3.2 km/s velocities
upper
covered by up to 3.5 km of sediments
from <3.3 km/s to 3.9 km/s.
upper-slope
and
by 1-2 km of low-velocity
it are steep,
the continent
is
reliable
profile
The faults b o u n d i n g
km/s
other
of the upper slope comes from
the crust under the upper slope has velocities
5.7
cover
the
to the Moho at about 18 km (Fig.
across them towards (1990),
images,
block of crust with velocities
from 6 to 7 km/s, upper
reflection
across
In
on the upper slope is chaotic.
about the deep structure
refraction
Sound
character
below.
(1990)
showed the sedimentary
with velocities
55).
Below lies
increasing with material
whose
from 4.8 km/s to about 5~5 km/s.
slab,
which deepens
It
from 7 km under the
middle slope to 12 km under the shelf edge.
Such
a
diversity of models
indicates
that the refraction
profile
251
SW
0
20
DISTANCE (km) 40 60
NE
80
I00
O
UO E v
I
O T"
13. W a T"
O ¢W Figure 55. Velocity structure of the southern Vancouver Island continental slope and adjacent areas, as modeled from LITHOPROBE seismic refraction data by Waldron et al. (1990). The first number in each block is the P-wave velocity along the top boundary in km/s, the second is the linear velocity gradient in km/s/km. On the lower slope, the Juno depression between vertical faults is well expressed. Beneath the upper slope, the velocity of 4.8 km/s (gradient 0.i0 km/s/km) is almost identical to that in the Leech River metamorphic complex on southern Vancouver Island. The Juno depression lies between 45 and 63 km.
252
offers no unique solution, and involve
other
a
comprehensive
considerations.
Just
18
km
approach
should
SE of this seismic
profile, on the outer shelf, well Cygnet J-lO0 penetrated of
stratified
(Shouldice,
Late
1971).
Miocene
and
Pliocene
2460
sedimentary
No m~lange or thrust faults,
m
rocks
expected
in
an
accretionary prism, were encountered.
Waldron
et
al.
(1990, p. 112) cautioned that "existing data and
their interpretations are processes
are
4
km
to
C)
interpreted
what
tectonic
Still, these workers
the
undrilled
material
depth, whose velocity they m o d e l e d to be between 4.8
km/s and 5.5 km/s, as an accretionary sediment
clarify
taking, or have taken, place".
(their velocity profile below
inadequate
thickness
of
about
wedge.
i0
km
However,
required
a
by
total
such
an
interpretation (Fig. 55) is inconsistent with other evidence.
By proposing thick sediments off Oregon and Washington, whether in depocenters or in accretionary wedges, Couch and Riddihough (1989) and Finn (1990) sought to explain (commonly
-60
mGal
or
less)
the on
very
the
low
gravity
continental
contrast, on the southern Vancouver Island upper slope
values
shelf. and
In outer
shelf free-air anomaly values are generally near zero; in the area of the refraction profile, they exceed +I0 mGal relative
gravity
high
along
the
outer
shelf
contains several local maxima, of which the refraction
transect
trending band separated
between
is
the
highest~
longitudes
126"W
(Fig.
one
These and
the
The
and upper slope crossed
by
the
maxima form a NW127~W.
They
are
by transverse gradient zones trending NE and N-S, whose
causative faults seem to continue to the mid-shelf and to
53).
long-lived
structural
complexity
indicated
contribute by
large
253
stratigraphic Vargas
differences
and Ucluet blocks
transverse
(see previous
chapter).
faults on the upper slope is
hard
this area is underlain by old continental
On
southern Vancouver
Island,
to those modeled by Waldron have
been
found
rocks.
et al.
1987),
Clowes,
(1990)
1992),
(Fig.
velocities
54).
1986).
Thus,
underneath
parts Island
of
the
(Walcott,
km/s.
Stratified
J-100
well
the
upper
of
(4.6 to
high-grade
see
also
submerged
Dehler
margin
data
margin
near
the
shelf
Miocene
and
show
that (Taber
presence
of
River
complex
-
off
Washington
and
with all the evidence.
edge,
et al.
between
sedimentary
(1990)
shows
just 1.9 and 3.2
rocks
in
the
Cygnet
drilled to a depth of 2460 m,
Plio-Pleistocene.
disconformity
(between the Upper Miocene
no indication
of thrust
conformable
off
sense,
Leech
the model of Waldron
fine-grained
slope
of the Leech River
1967;
suggest that fairly stable marine conditions
The
almost identical
refraction
4 km of sediments with velocities
Late
unless
to the values used by Finn
in the
submerged
is consistent
Above these deep rocks,
the
explain
consists
in the geophysical
rocks - such as those
Vancouver
of old
in the deep parts of this wedge may reach 6 km/s
Lewis,
to
which
Seismic
the
Presence
the Leech River complex
identical
crystalline
up
(1990) under
for the wedge she modeled under the
Washington
and
almost
in
crust.
Surface m e a s u r e d densities
complex are 2,670 to 2,750 kg/m3 and
to
seismic velocities
to characterize
5.5 km/s; Mayrand et al., metamorphic
wells
in the three ne i g h b o r i n g
existed there There
is
during
only
one
and Lower Pliocene),
and
faults or m~lange.
Plio-Pleistocene
succession
in this well
is about
254
1900 m thick. continental
margin
Pleistocene thin
out
Snavely, depth
Seismic
to
zero
only
There,
coincides
over
near
(Fig.
Cygnet
for about
a
for the very youngest deformation,
the
shows that Plio-
(Snavely and Wagner,
1981;
break at about i000 m
late Tertiary
older rocks.
Outboard,
(Quaternary?)
are
thrust
strata are
draped
all sediments
involved
in
water
except
penetrative
and at the foot of the slope even these sediments
cut by a thrust succession
crosses
of c o n t i n e n t w a r d - d i p p i n g
Inboard from this break, in
which
location,
at mid-slope
series
56),
30 km on the upper slope and
a bathymetric
with
structures
west.
the
strata continue
1987).
faults.
line 76-19
fault.
This
from stratified
fault
sediments
separates
the
of the Cascadia
Due to poor signal penetration,
the details
are
lower-slope Basin further
of
deformation
on the Washington
shelf to the
on the lower slope are hard to interpret.
A
m~lange
south. wells
is
present,
The Shell Oil Co. well P-0155, and
seismic
faulted m~lange and
however,
younger
reflection
lies beneath
strata
along
profiles,
with
several
indicates
that a thrust-
1-1.5 km of u n d i s t u r b e d
(Snavely,
1987,
his
Fig.
other
Upper Miocene
9).
Because the
deepest of the wells reached only 4017 m, the depth extent of this m~lange
is unclear.
It was probably thrusted
crystalline
rocks,
timing
contraction
of
The northward there
is
system.
In the
continental
age
is
m~lange
evidence that it continues area
Island,
consistent
with
the
of the Hoh Basin on the Olympic Peninsula.
extent of the W a s h i n g t o n
no
Vancouver
and its Miocene
over any underlying
of
the
presence
refraction under
crust is consistent
is
unclear,
beyond the Nitinat profile
off
but fault
southern
the upper slope of attenuated
with all the geophysical
evidence.
%0
10 20
30
40
DISTANCE (km) 50
60
NE
/ "t
Cygnet Wetl "k~80
\"%'-'7/i'
70
Figure 56. Continental slope off southern Vancouver Island and northern Olympic Peninsula, imaged in the U.S. Geological Survey seismic reflection line 76-19: (a) data; (b) interpretation (both after Snavely and Wagner, 1981). Profile location is given in Figs. 2b, 45. Stratified Plio-Pleistocene sedimentary rocks, penetrated on the outer shelf by the well Cygnet J-100, continue on the upper slope. This well, drilled to a depth of 2460 m, encountered no m~lange.
=
0
r..-
SW o .......
&n
256
Off northern Vancouver
Island,
the narrow shelf
Jurassic
rocks,
and
metamorphic
crustal-scale fault
normal
system.
broader,
fault w h i c h
Off
southern
but the u n d e r l y i n g
foundered
continental
fault separates
underlain
by
its outer edge is defined by a belongs
to
Vancouver
crust
crust
is
is
plate-boundary
Island,
the
continental.
continues
it from a d o w n d r o p p e d
the
shelf
is
Attenuated,
to the mid-slope,
where a
block of oceanic crust under
the Juno depression.
Sediment-filled
Juno d e p r e s s i o n
The
boundary between the continental
structural
is expressed west,
and oceanic
mid-slope
slope,
slope
break.
of
a
structural
sediments
lower continental detailed
To
depression
filled
with
is indicated by seismic profiles
slope south of the B r oo k s - E s t e v a n
refraction
model of W a l d r o n et al.
embayment.
(1990),
this
the lower unit, 1.8 km thick,
from base to top,
with P-wave velocities
with velocities
at
the
uppermost veneer vertical
faults
bottom
(1990)
The
four units are included:
~6.0 km/s;
unit which
to
2.1
two
lower
next
unit,
a
velocities
as
of
and the The
were modeled to lie km
below
units were assigned by Waldron
et al.
to the oceanic c r y s t a l l i n e
depth
has
is only 1.9 km/s°
the Juno depression to
the
km/s at the top;
300 m thick whose velocity
18 k m apart and continue seafloor.
and
bounding
(Fig.
of 6.0 km/s at the base and 4.0 km/s
at the top; the 2.5- to 3-km-thick km/s
In
the Juno
55).
model,
thick
across the
lies between about 45 and 63 km profile distance
2.4
the
lies the Juno structural
depression In
crust
in the oceanic crust.
existence
stratified
the
as the
under the lower continental
depression
The
in the b a t h y m e t r y
on lower continental
much
as
6
crust of the Juan de Fuca plate,
and the two upper units to a b i p a r t i t e
sedimentary
package.
257
Detailed
velocity
analysis
of
modern
seismic
(lines 89-04 and 89-07; Yuan et al., 1994) showed depression
contains
reflection data that
the
a sedimentary package up to 5 km thick, with
velocities increasing from 1600 m/s at the seafloor to 4000
m/s
at
the
base.
more
depth
between
SP
700 and i000.
thick, though the floor of the The
depression
than
In the reflection line 85-01 (Fig. 52)
along the refraction profile, the Juno depression lies at water
Juno
is bowl-shaped.
2000
m
Sediments are about 4 km
sedimentary
package
is
unclear.
Strata dip towards its axis from
the east and west, but continentward dips are predominant.
The western boundary of the Juno depression in this is
a
steep,
east-dipping
seismic
line
fault at SP 700 which cuts the entire
sedimentary section and produces on the ocean floor a ramp 400-500 m
high.
This fault is reverse, but because the sediments thicken
across it to the east, inverted.
Other
it must have intially been normal
reverse
was
faults lie just outboard at SP 600, or
inside the depression at SP 810. the
but
The same faults are observed
nearby, parallel Line 89-04 (see Yuan et al., 1994).
in
A small
west-dipping reverse fault just outboard of the Juno depression in observed in the adjacent Line 89-03 (see Spence et al., 1991).
Shallow deeper
sediments ones
are
in the Juno depression are stratified, whereas semi-transparent
(Fig.
52).
A
similar
distribution of seismic facies is observed in the adjacent eastern parts of the Cascadia Basin, but traveltime through each facies in the Juno depression is about 50% longer.
258
On
the
inboard
side of the
reflections
become
Stratified
seismic
and b e l o w this
this
zone
chaotic
chaotic
probably
dips
seismic
events
but f u r t h e r
boundary km
Waldron
(1990)
suggests,
near
55 km to the north,
seismic
57), SP
wide,
as
bedded
the
seismic SP
900.
both
above
sediments,
slumps.
The
east:
westward
are o b s e r v e d
as far as
is chaotic.
The
85-01 the
is thus
indistinct.
refraction
its i n b o a r d
Juno
boundary
model
of
should
lie
SP ii00.
About in
image
in Line
at
sediment
from
east the s e i s m i c
ms
eastward
between
at >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,
<+150
continuity
of
oceanic
crust.
Off
Finn (1990, 1991) noted that magnetic stripes, though
faded, persist on the continental slope to the shelf, probably due to
underthrusting
America.
of
Magnetic
oceanic
stripes
crust
in
beneath this part of North
the
Juno
depression
are
much
clearer, because the oceanic crystalline crust in this area is not underthrusted but merely covered by Brooks-Estevan
embayment,
sediments.
magnetic
stripes
In
southeastern
also remain strong,
suggesting the sediments are underlain by oceanic crust.
Coincident southeastern
with
the
large
Brooks-Estevan
N-S
magnetic
embayment
is
high-low a
free-air
pair
relative
gravity high of -30 mGal, which lies between relative lows of to
-70
mGal (Fig. 40).
Suggesting
these faults also strongly
influenced the ocean-floor morphology,
in the same area N-S trends
the
origin of such a
lower curious
movements
trending
on
characterize
that
-60
Such large variations in gravity anomaly
values are probably caused by dip-slip offsets on faults N-S.
in
slope
including the toe (Fig. 28).
correlation
between
oceanic
The
magnetic
stripes and large faults is discussed in the following sections.
279
The
northwestern
part
magnetic stripes.
of the Brooks-Estevan embayment lacks N-S
In this area begins the blank
magnetic
domain
which is so well expressed over the Winona Basin.
The negative free-air gravity anomaly over the Winona Basin, which at its minimum drops below -130 mGal, does not continue embayment.
However,
it
Rather,
the
the
does not stop at any strong NE-trending
gradient zone that could be correlated fault.
onto
with
a
large
transverse
Winona low breaks up into several "tongues"
that wedge out to the SE.
Anomaly values decline in the embayment
gradually,
from
mostly below -70 mGal in the northwest to mostly
above
mGal
in
-70
northwestern
part
the of
southeast.
the
These
tongues
Basin
breaks
up
Faults in this area trend NW-SE and are
aligned with the plate-boundary Winona
the
embayment trend mostly NW-SE, in line
with the Winona 10w, suggesting that the Winona into smaller depocenters.
in
structural
zone.
Basin, the crust is probably continental.
of crust underlie the Brooks-Estevan embayment
Like
in
the
Thus, two types
-
continental
in
the northwest, oceanic in the southeast.
Earthquake seismicity in the zone of block interlocking The
Scott
Islands
fracture
boundary fault system, to
be
a
steep,
Brooks Peninsula, this
area
is
the
it meets the
compounded
Brooks-Estevan
which
belongs to the plate-
is shown by marine seismic reflection
west-dipping
Brooks fracture zone.
zone,
by
normal fault (Fig. 36). OWSZ.
Structural
South of
complexity
in
presence of the broad, NE-trending
This zone transects embayment
data
and
the
accounting
margin, for
bounding
numerous NE-
oriented bathymetric features on the continental slope.
280
No direct extensions outboard
have
of this zone
been
detected
closer to shore,
several
zone,
and Estevan
Esperanza
from Vancouver onto
the
magnetic
Island.
abyssal stripes
oceanic crust
plain
Brooks-Estevan this
Vancouver
Island,
epicenters
(e.g., to
oceanward suggested Sound
Barr
separate
protrusion
(Barr,
1974; was
comm.
confirmed to Barr,
Kinematically presumed
more
plausible boundary
or fragments of
magnetic
continue
1974).
North
Fuca
in
of the
other around
Ridge
Initially,
America
it
was
from a triagnular plate,
which
was
I s l a n d and Queen Charlotte
1974).
Existence
such
a
with the usual understanding
of
were
the
separates
of
fault configuration
seismic data
anomalies fragments
- were postulated
to
band of earthquake
(D.L. Tiffin,
later the
ideas Juan
was pers.
that
the
de Fuca and
of the Juan de Fuca plate.
oceanic-lithosphere
plates
they
faulting
de
de Fuca plate
contemporary p. 1197).
studies
Juan
and anyway the supposed
NE-trending
independent Explorer
the
1974,
Pacific plates, regional
Juan
Barr and Chase,
by
fracture
complexity
NE-oriented
Chase,
hard to reconcile
plate kinematics,
but
A plate b o u n d a r y was proposed to
to lie off northern V a n c o u v e r
protrusion
not
of
extensive
northern
and
that
structural
a broad,
the
Brooks
in this area are rigid,
for the
from
along
1982),
1993).
plates
embayment.
area,
-
areas
though strong disruptions
(cp. Davis and Currie,
were proposed
oceanic
and Clowes,
is no evidence
in that area suggests
explanations
believed
Au
deep
faults
directly,
that
the
faults - run onto the shelf and slope
There
assuming
run across
(e.g.,
NE-trending
Previously,
the
into
Following
(Riddihough,
1977),
two
- the Juan de
Fuca
and
to lie, respectively,
south and
281
north of the NE-trending et
al.,
band of earthquakes
1979; Keen and Hyndman,
1979).
variously
Riddihough,
to
to
3.5
Ma
day.
Barr
(1974,
fault from the northern
plate et
running
the
the
Sovanco
fracture
showed
it
as
junction zone,
being
magnetic (1993)
of
towards 15-25
zone
Nootka,
migrated
wide,
Nootka
but
Thus,
who
showed
it
Island.
They
Recently,
Davis
southward
and
confusion
Island.
50-55
km
south
so as to "disappear",
Hyndman,
end
1991,
of
the
p. 445).
of
fracture
the western part of Nootka
surrounds
NE-oriented
the definition
fault
the zone. zone
and a new fault has
Juan
de
Fuca
Ridge
The eastern part of the
Also m i s l e a d i n g
Despite
of a plate boundary
band of earthquakes
off northern
is the early supposition
that no faults related to
island.
and
been
along the coast northward.
the broad,
1979)
(1979),
fault zone in this new model has during the last 5 Ma
migrating
are
having no particular
expression.
beginning
been initiated at the northern
Nootka
1979;
showed the fault boundary between the Juan de Fuca
interpretation,
(Riddihough
al.,
Juan de Fuca Ridge with the
junction of the Juan de Fuca Ridge with the Sovanco
has
been
i0) showed it as a
Hyndman et al.
northern
km
or bathymetric
and Explorer plate fragments
In another
has
1991).
her Fig.
Island. fault
from
of
end of the Juan de Fuca Ridge
named the presumed p l a t e - b o u n d a r y
Currie
Fuca (Hyndman
and Hyndman,
the southern tip of Nootka
gravity,
Hyndman
and even the location of their presumed boundary this
left-lateral to
8
1984; Riddihough
The structure unclear
at
3, 4;
The time of splitting
the Explorer plate from the larger Juan de estimated
(Figs.
several
this
attempts
band
are
along
Vancouver
(Hyndman et al., present
on
the
to constrain the width of this
282
earthquake band (presumably to a single fault broad
(Milne
zone),
it
remains
et al., 1978; WahlstrSm and Rogers, 1992).
Borders
of the Explorer plate are now believed to be "diffuse rather single faults,, (Wahlstr~m and Rogers,
than
1992, p. 960).
Studies of regional seismicity around the Brooks-Estevan embayment (Fig. 59) have led to the now-common conclusion that modern strain in
this
area
is
diffuse.
The band of earthquakes that runs NE
from northern Juan de Fuca Ridge towards Nootka Island and is
about
beyond
40 km wide, much too broad to represent a single fault.
Focal mechanisms throughout this band -
in
the
pelagic
oceanic
areas,
on the submerged margin and in adjacent parts of Vancouver
Island
-
suggest
sinistral
movements
Rogers, 1990). on
dextral
movements
on
NW-SE
faults
and/or
on NE-SW faults (Rogers, 1979; W a h l s t r ~ m and
Faults with both these orientations
are
abundant
western Vancouver Island (Muller et al., 1974, 1981), and they
are also common in the submerged Brooks-Estevan embayment.
Onshore, mapping shows the ages of many such Neogene, of
the
or
even pre-Tertiary.
Insular
reactivated
Belt
were
of
these
faults
outboard, however,
are
to
be
pre-
Faults oriented NE in m a n y parts
inherited
from
Mesozoic
in the Tertiary (Lyatsky, 1991a, 1993a).
on V a n c o u v e r Island and the some
faults
adjacent active
submerged still.
time
and
Earthquakes
margin
indicate
Earthquakes farther
reflect deformation in oceanic lithosphere.
On the shelf, earthquakes seem to be most common near the boundary between
the Northern and Kyuquot blocks, the boundary between the
Kyuquot and Cove blocks, and within the northern part of the block
(Fig.
59).
Cove
The amount of seismicity declines drastically
283
Figure 59. Distribution of earthquake epicenters along the British Columbia and southeastern Alaska continental margin (modified from Riddihough and Hyndman, 1991). The seismicity pattern off Vancouver Island is significantly different from the conventionally assumed plate configuration.
284
off southern Vancouver Island, (McCrumb
and
et al., 1989a,b; Acharya,
to explain in terms of interaction
remains 1992). of
low
off
Washington
Such a pattern is hard
rigid
plates,
but
other
evidence indicates that the behavior of the Juan de Fuca plate off western North America is largely non-rigid.
Evidence for lack of rigidity of n o r t h e r n Juan de Fuca plate From diffuse seismicity and curved and intraplate
deformation
has
broken
previously
magnetic
been
inferred
southern (Gorda) segment of the Juan de Fuca plate Stoddard,
1987,
1991).
(Couch and Riddihough, part
of
the
Juan
stripes, in
(Wilson,
the 1986;
Subduction there appears to have stopped
1989). de
Fuca
Similar phenomena in plate
also
deformation, which has been confirmed by
point
detailed
the to
northern intraplate
local
studies
off the mouth of Queen Charlotte Sound (see Allan et al., 1993).
The
Explorer
and
northern
Juan
de Fuca ridges and the Sovanco
transform between them demarcate the boundary between the northern Juan
de
Fuca
plate
and
the oceanic Pacific plate to the west.
However, the Explorer Ridge and the Sovanco aseismic
and
lacking
et
al., 1989a).
al.,
1975;
Allan
et
Riddihough
al., 1993).
centers
rotated
Davis and Currie,
and
et
al.,
1983;
(Michael
et
al.,
The Sovanco zone is a belt, several
tens of kilometers wide, of fault-bounded variously
remarkably
The Explorer Ridge has some geochemical
characteristics uncommon at spreading 1989;
are
many other indications of ongoing tectonic
activity (e.g., Chase et McCrumb
zone
oceanic-crustal
blocks
tilted (Cowan et al., 1986; Lister,
1991;
1993).
Discordant with the presumed plate boundaries off western
Canada,
285
a
broad
band
of earthquakes
runs NNW from northern Juan de Fuca
Ridge towards the southern tip (Fig.
58;
1992).
also
Milne
et al.,
Focal mechanisms
dextral,
is evidently deformation
has
a physical
reality".
is occurring
direction
perpendicular
velocities
from
dimension
in
the
the
P-wave
oceanic
overlying
the N-S magnetic
lithosphere.
correspondence crust
interlocked
oceanic
velocity Fuca plate
structure (Malecek
1984)
that under
S-wave
seismic
mantle has the fast Ridge.
The
P-
and gravity into
data revealed of and
to the
if the upper mantle
crust,
adding
a
is
third
in northern Juan de Fuca plate
In southeastern
broken
and
of the Juan de Fuca plate.
crust,
rather
Brooks-Estevan
anomalies
blocks,
with blocks of continental
Seismic refraction
intraplate
to the ridge are 8.3 km/s and 4.6
anomalies
of magnetic is
structures
but only 7.5 km/s and 4.5 km/s parallel
the
As
that
and
upper
may reflect the motion of only the oceanic
oceanic
1982,
to the Juan de Fuca
to the fragmentation
the entire
or NW-SE
"lack of correlation
indicates
Such anisotropy m i g h t be explained
decoupled
so,
It
(Au and Clowes,
perpendicular
respectively,
ridge.
p. 960),
1990,
in this area.
also been reported
anisotropy
Islands
embayment.
seismicity with some mapped ocean-bottom
velo c i t y
If
Charlotte
1978; W a h l s t r ~ m and Rogers,
(1992,
the northern Juan de Fuca plate,
km/s,
Queen
similar to those around the B r o o k s - E s t e v a n
of significant
S-wave
the
in this band are NE-SW sinistral
noted by W a h l s t r ~ m and Rogers
It
of
which
than
of
embayment,
suggests
that the
in that area are
crust.
considerable
variations
in
the
the oceanic crust in the northern Juan de Clowes,
1978;
Au
and
Clowes,
1982).
286
Thickness
of the upper lithospheric
layer d i s t i n g u i s h e d
as oceanic
crust was found to vary from 6-7 km to as much as i0-ii km in some places.
The crust was also noted to be thickening
from the Juan de Fuca Ridge. / accommodated in the interpteted sediments
and Clowes
maturing" their
(1982)
to account
layer
varies
3.
"bunching-up"
these
thickness
of the
against
variations
are
layer 3 (beneath the
and sheeted
alluded to unspecified for
away
dikes
and
oceanic
the
reported
large steep faults
Ridge.
Many
processes
velocity
Malecek crust
continental
and clowes to
(Hyndman et al.,
Recent reflection
profiles
of
has repeatedly many
Calvert et al., also
the
Island, 550,
data
of
compressional
buttress.
They had also
Fig.
the
in layer 2 (Rohr et al.,
of
relief
1990; Hasselgren Hasselgren
local east-side-down
700;
reflection Brooks-
1979).
basalts under the Cascadia
meters
of
had
its
been found in seismic reflection
hundreds
(1978)
across northern Juan de Fuca plate also
variations
oceanic-crust
in
in the oceanic crust near the Explorer
Estevan embayment
top
of "crustal
other steep faults were noted in seismic
thickness
layer
variations
profiles between the northern Juan de Fuca Ridge and
reveal
of
from 3.1 to 7.4 km (op. cit).
On the other hand,
ascribed thickening
of
lower-crustal
of layer 1 and the basalts
2), whose thickness
Au
Most
eastward,
56, SP 2000,
deepen towards the continent
Basin sediments have
(Carlson and Nelson,
1987;
1992;
and Clowes,
2400)
The
to
et al.,
offsets
1988).
(Fig.
profiles
Rohr,
1995).
1994;
see
Off Vancouver
52, SP 180,
400,
500,
cause the oceanic basement
to
stepwise.
The Cascadia Basin covers v i r t u a l l y the entire Juan de Fuca plate.
287
From
magnetic
anomalies, the age of the underlying oceanic crust
is estimated at no more than iO Ma (Riddihough, 1984), and the age of
the Cascadia Basin is therefore inferred to be Late Miocene to
Recent (see also Riddihough and Hyndman,
1991).
Sparse data
from
deep-ocean drilling (such as DSDP site 174 off Oregon) and seismic reflection profiles, as summarized by Carlson and
Duncan
and
Kulm
and
Nelson
(1989), show the basin contains two units.
Only Pliocene to Recent sediments have been drilled. unit,
fairly
contains
The
bottom
transparent in seismic sections, consists of thinly
bedded silty hemi-pelagic clay. unit
(1987)
The seismically reflective
upper
mostly thick- to thin-bedded, medium and fine sand
of Upper Pleistocene to Holocene age.
The Cascadia Basin generally thickens eastward, Fuca Ridge towards the continent. 2.5-3 km thick. off
Vancouver
Juan
de
Juan
de
At the foot of the slope, it is
Water depth increases gradually from 2400-2600
m
Island to as much as 3000 off northern California.
Modern channels on the abyssal plain distribute the
from the
Fuca
sediments
plate mostly from north to south.
across
This plate
thus seems to have a slight but persistent southward tilt.
Variations in the thickness of considered
to
extrusive
basalts
in
been
found
magnetic anomalies northern
part
of
Fuca
Ridge
to have a noticeable effect on the character of (Tivey and Johnson,
1993; Tivey, 1994).
In the
the Juan de Fuca plate, magnetic anomalies are
disrupted by many faults (Figs. 28, always
2A,
be the main source of oceanic magnetic stripes, by
several hundred meters in the vicinity of the Juan de have
layer
41),
and
therefore
do
not
provide a reliable framework for plate reconstructions off
western canada
(Davis and Currie,
1993).
288
In the southeastern Brooks-Estevan embayment, magnetic and gravity anomalies
coincide
features
on
gravity
the
with
each
continental
anomalies
over
other slope.
the
polarity.
The
Off
continental
parts of the abyssal plain correspond opposite
and with major bathymetric
two
to
profiles
central
slope and the adjacent magnetic in
anomalies
well
between gravity and magnetic anomalies may reflect
keyboard-style
elongated
expressed.
Such
the
correspondence
of
less
outboard
a
shuffling
is
of
that area are almost
mirror images of one another (Fig. 54), but farther antisymmetry
Washington,
blocks or warping of weak oceanic crust.
If so, the faulting or warping affect the arrangement of stripes.
Plate
reconstructions
may
therefore
magnetic
suffer from the
effects of structural deformation of oceanic crust.
Constraints on the timing of intraplate deformation in the Juan de Fuca plate A
fundamental
assumption
in
plate-motion
reconstructions from
oceanic magnetic lineations and hot-spot tracks is that plates are coherent,
rigid
lithospheric
entities.
interactions at plate boundaries, which
Conventional models of flow
largely
from
reconstructions, usually also assume plates to be rigid.
such
However,
during the Cenozoic the oceanic Farallon plate has been undergoing fragmentation 1994). being
since
about
55
Ma (Atwater,
1989; Stock and Lee,
Its modern remnant, the small Juan de Fuca plate, is still fragmented
into
ever
northern and southern ends.
smaller pieces, particularly on its For a
crumbling
plate,
generalized
models assuming rigid-plate behavior may be inappropriate.
Probably
because the Juan de Fuca plate is young, it is thin (30-
289
35 km lithospheric and
weak.
thickness
at the Washington
Heavy shearing,
vertical
the upper part of this plate's
Parts of the Juan de Fuca support
a
flexural
continental
and horizontal,
are
which
still has
been
gravity data
(Jachens et al.,
the northern
and southern parts of this plate.
distributed
magnetic
phenomena Island. Vancouver
stripes
(Wilson,
The broad plate-boundary
(also Allan et al.,
Juan
de
from
marine
is apparent
Due to
de
intraplate
Fuca
1987,
structural off
in
broadly
plate
1991). off
off
Similar Vancouver
zone continues
to the
southeastern
Alaska
past the mouth of Queen Charlotte
1993).
A diffuse boundary
between
the
Fuca and Pacific plates has been proposed to run through
the "Explorer plate"
(Furlong et al.,
The timing when intraplate fragmentation Cenozoic.
of
the
Magnetic
present-day
deformation
began is hard to
(Duncan and Kulm,
Seismic reflection
pinpoint:
Farallon plate went on through most of the
Juan de Fuca lithosphere
confirms
in
1994).
stripes have been used to estimate the age
the foot of the slope
usually
Stoddard,
Island margin from the margin
Sound
outboard
No such bulge
to
along the
are curved[ and earthquakes
1986;
Islands,
enough
inferred
occur in the northern part of this plate
and the Queen Charlotte
the
1989).
in the southern part of the Juan
northern California
1990)
has affected
coherent
margin off Oregon and southern Washington
deformation,
Finn,
lithosphere.
plate
bulge,
margin;
(Riddihough,
the
1984);
of
as no more than i0 Ma at DSDP drilling
age of the sediments
some 80 km
is at least Pliocene
1989).
profiles
the hundreds
show relief
of meters,
in the basaltic
throughout
the basin
basement, (Carlson
290
and Nelson, Rohr,
1987; Calvert et al., 1990; Hasselgren et
1994).
However,
though
Pliocene
al.,
1992;
sediments are strongly
affected, Pleistocene beds are less disturbed.
This suggests that
some intraplate deformation occurred in the Pliocene in many parts of the Juan de Fuca plate.
At present, strong active
deformation
is taking place in the plate's northern and southern parts.
Genetic
aspects
of
the
geology
of
western
North
America
continental margin Continuity of continental-crust structures on the submerged continental margin North of the Olympic Mountains, the
continental
crust
on
seismic
refraction
the
Insular
Belt,
its
27-29
km
under
1990).
29
km
Hecate
under
the
Strait;
Sweeney
and Seemann,
1992; Spence and Asudeh, varying
along
Queen
25-30
Entrance and southeastern Alaska (Johnson, 1972; 1989;
Father
north
thickness varies considerably:
under Queen Charlotte Sound; Islands;
suggest
Vancouver Island is about 40 km thick
(McMechan and Spence, 1983; Drew and Clowes, in
data
23 km
Charlotte
km under Dixon Mackie
et
al.,
1991; Brew et al., 1991; Yuan et al.,
1993; Hole et
al.,
1993).
Apart
from
the Insular Belt, in many of these areas the crust
thins towards the continental margin stepwise (see Chapters 6, 7).
In
an
E-W
seismic
Washington,
refraction
profile
across
west-central
the crust, with velocities not exceeding 7.1 km/s, is
20 km thick at the coast and more than 30 km thick under the Puget lowlands
(Taber
crust in western Peninsula
is
and
Lewis,
Oregon
still
a
Finn, 1990), but thr Moho
1986).
and
Washington
topic is
continental margin gradually
The composition of the deep south
of
the
Olympic
of discussion (Keach et al., 1989; smooth
and
shallows
(Mooney and Weaver,
towards
1989).
the
291
Still,
thickness
of
the crystalline crust in these areas varies
abruptly, as thickness of the sedimentary cover varies.
Localized
depressions are apparently filled with many kilometers of Tertiary sediments alone. across
and
Several such very deep depressions,
about
200
km
apart,
on
the
100-200
shelf off Oregon and
Washington, might have been created by large-scale shearing the
plate
bounded
boundary (Couch and Riddihough,
Tertiary
depressions,
some
3
km
1989).
km
along
Smaller fault-
deep
and
tens
of
kilometers across, lie on the shelf off southeastern Alaska (Bruns and Carlson, the
1987).
exterior
Such depressions in the continental crust
on
shelf were initiated before the latest Tertiary, as
the many kilometers of sediments in them must have taken
millions
of years to accumulate.
Regional
structural analysis suggests that areas inboard from the
exterior
shelf
propagating 1993a,b). reverse
from
were the
not
significantly
plate-boundary
affected
structural
Modern earthquakes, with right-lateral focal mechanisms,
(Rogers,
1983,
shearing
zone (Lyatsky, and
in
places
along the southeastern Alaska and Queen
Charlotte Islands margin, are concentrated on strand
by
this
1986; B~rub~ et al., 1989).
zone's
inner
In the past,
however, the outer strand was the most active, and no more than small amount of shearing might: have propagated inboard.
a
It is the
outer strand that separates the continental and oceanic crust.
Seismicity is less intense along the margin off southern Vancouver Island
(Fig. 58) as well as off Washington and Oregon (Riddihough
et al., 1983; Taber and Smith,
1985;
Crosson
and
Owens,
1987;
292
McCrumb
et
al.,
Washington interacts
is
1989a,b).
clustered
around
of Puget
the
(see Chapter
Cenozoic,
slip movements. Snoqualmie
whose
strands
of the OWSZ show no evidence
a strike-slip
sense the
by the Calawah on
probably minor indicates,
1994).
fault of Eocene
northwestern
the
southern
massifs
reaches
Olympic
of
Peninsula,
Eocene
subsided
these blocks continue
on
Vancouver
differentially lie transcurrent the
shelf
such as the Nitinat
This N E - o r i e n t e d
in
Crescent
strike-slip
offset
sedimentary is local and
277).
This
strand of the OWSZ.
exterior
faults bound a number of crustal and
Island,
displacing
if real,
1977, p.
the
offset
that this fault was active recently.
Island
northern
large strike-
Vancouver
fault
shelf.
-
Island
Both it and continue
Whereas
fault does not continue directly past Barkley Sound, fault
differently
(Reidel et al.,
of consistently
strand - the Westcoast
Vancouver
OWSZ
rectangular
and even late Pliocene
The Calawah fault is the southern the OWSZ's northern
the
is intruded by
The suspected
(see MacLeod et al.,
nonetheless,
the OWSZ
and
associated
(Babcock et al.,
without
age is 17 to 19.7 Ma
On the Olympic Peninsula
basalts
where
themselves
though apparently
East of Puget Sound,
batholith
in western
5).
1994).
onto
Sound,
parts of the OWSZ have manifested
throughout
rocks
the seismicity
with the eastern side of the crustal-scale,
North Olympic block
Various
Most
(Figs.
the Calawah
the W e s t c o a s t
16, 50).
blocks on the shelf,
from Vancouver
fault zone,
Island,
and
N-S.
to
the
rose
Between They
and some of them,
cut the entire continental
fault zone continues
These
which
since at least the Eocene. faults trending NE
WNW
lower
margin.
continental
293
slope.
There
depression,
it
marks
the
which was formed
block of oceanic crust.
southeastern by
downdropping
clear
Unlike
rectangular the
depression
of
South of this fault zone,
at the foot of the slope off Washington a
boundary
shape
fault-bounded off Washington
rectangular
the gravity
is less regular
(see Finn, Juno
a
of the Juno
]990;
low
and
lacks
Finn et al.,
1991).
depression,
the
along-slope
may simply be an undulation
of the Juan
de Fuca plate.
North of Juno depression, are juxtaposed
blocks
complexly.
oceanic
crust
plat e - b o u n d a r y
Regional
spreading consumed fault
were
ridge
and questions
must
At such
at another plate boundary a
plate.
is best illustrated There,
slabs at megathrusts
which can continue
boundary,
Such a
and subducting
plates,
and Alaska-
occur in W a d a t i - B e n i o f f
to depths of hundreds
Among
subduction
zones
or
thrust
at the Japan-Kurile
focal mechanisms.
earthquakes
great
many strong earthquakes
in many respects
(Acharya,
due to
1992).
the
friction
seismicity
of kilometers.
worldwide,
earthquakes
and
structural
have thrust
deep
a
a
classical
earthquakes
"unusual"
in these parts of the
lithosphere moving away from
develop between the overriding
Aleutian margins.
the
Blocks
about the subduction megathrust
oceanic
dipping under the overriding
of
emplaced
be subducted
into the mantle.
configuration
and off
they are interlocked.
probably
space balance,
must
Sound,
embayment
crust
zone along faults.
seismicity
To maintain
and oceanic
In the Brooks-Estevan
the mouth of Queen Charlotte of
of continental
Many such
Cascadia It
lacks
zones,
zone a
with tlhrust mechanisms.
is
strong At one
294
time, the
this was considered Juan
evidence
against ongoing
de Fuca plate beneath North America,
thought to have ceased
(see McCrumb et al.,
that attitude was rejected,
Keen
and
to
prove
effort
Hyndman,
subduction-zone remains.
generalized
1979).
seismicity of
lithosphere
DeMets et al., rigid the
has
the
typical attributes
incompatible
with
and
the Cascadia
submarine v o l c a n i s m
suggests that
aseismic.
though also largely aseismic, 1989).
is producing
Addition
of
that partly
deformation.
in Pacific p l a t e - t e c t o n i c have been well publicized
1990).
The assumption
reconstructions
continent,
However,
for the
(Stock and Molnar,
1988;
that all oceanic plates
the search for explanation
American
undeniable.
is
behavior,
(Johnson and Holmes,
pushed
North
fits
the Juan de Fuca plate is now accommodated
Large mismatches Cenozoic
at
the large subsequent
zone
offshore
rigid-plate
for Holocene
the Juan de Fuca Ridge,
1970s
(Riddihough,
absence of some essential
evidence
late
Despite
rigid
the
But still,
by intraplate
was
model,
zone is abnormally
to
In the
Cascadia
models
lithosphere
1989b).
the
subduction
oceanic
and subduction was
that
Intraplate
of
but the tectonic models e s t a b l i s h e d
that time assumed the Juan de Fuca plate 1977;
subduction
where
in the direction
internal
deformation
are of is
another part of the answer may be that the
oceanic Juan de Fuca plate is not rigid.
Intraplate d e f o r m a t i o n behavior
of
the
Juan de Fuca plate warping,
occurs elsewhere,
Indian
plate
is deformed
crust-mantle
(e.g.,
and
it
DeMets
et al.,
more penetratively
delamination.
That
complicates
the
by
1990).
the The
faulting, rigid-plate
295
assumption
is
inappropriate
California
is now r e c o g n i z e d
and a similar conclusion mouth al.,
of
Queen
1993).
assumption
The
Sound
character
off
Stoddard, the
northern
1987,
area
(Carbotte et al.,
1991),
off
the
1989; A l l a n et rigid-plate
of the Juan de Fuca plate was acknowledged
plate-tectonic
but was not explained et
were
tectonic models (Riddihough subduction
1986;
area
fails for much of northern Juan de Fuca plate.
(e.g., McManus
over"
(Wilson,
Gorda
has been reached for
Charlotte
since the early
behavior
the
The data presented here suggest that the
atypical
region,
for
al.,
interpretations
not
the
with the data available
1972).
still
of
Later,
at the time
abnormalities
comprehensively
Hyndman,
1989,
1991).
However,
zone "is unique among other subduction
(McCrumb
et
al.,
absence of seismogenic
1989b,
p. 605).
megathrust,
a
of
explained,
for this area relied on the rigid-plate
and
Pacific
plate because
assumption
the Cascadia
zones the
world
It is remarkable
by the
bathymetric
trench,
or
a
coherent magmatic arc.
Absence of megathrust The most
lack
of seismicity
striking
Earthquakes magnitudes Puget
Hypocenters
related to a subduction
abnormality
near
the
are low.
Sound,
earthquakes
earthquakes
of
the
continental
Epicenters
Cascadia margin
al.,
few,
occur in clusters,
and no earthquakes
is the
subduction
are
zone.
and
their
like the one in
are deeper than 80 km.
No thrust
have been r e c o r d e d along the C a s c a d i a subduction of
such
earthquakes
as
do
supposed
east-dipping
1983; Taber and Smith,
megathrust
1985;
zone.
occur lie in the North
America plate and in the inferred descending their
megathrust
slab
boundary
Taber and Lewis,
-
but
not
(Riddihough 1986;
at et
Crosson
296
and
Owens,
1989;
1987;
Rogers et al.,
Normal-fault level
focal mechanisms
striking
crust
orientations Consistent
with
reverse
other
data,
of the Cordillera
ground movements,
Washington
Cenozoic 1989a;
and
Above,
dextral
(e.g., R i d d i h o u g h
the interior recent
and
are typical
1985).
suggest
faults
this
western
Werner et al.,
1991;
et al.,
aseismic
is
prevailing
Reidel
coastal These
in periodic
movements
However,
Cited
option
is
local drowning is tenuous.
Columbia
1990).
breakouts
during
in and
stress
the
late
1987; M c C r u m b et al.,
1994).
conflicting
cataclysms
been
models hold
(see Rogers,
Such phenomena
in
should
support
sea-floor
that
1988).
occasional
the
currently
such as drowned slumps.
triggered by sudden
1987,
1994).
turbidity
and past earthquakes occur in coastal
completely
turbidites,
as
(e.g., Atwater,
be
of
evidence
interpreted
between
of forests,
fault
indirect
swamps,
on the m e g a t h r u s t
a connection
WNW-
faults with various
that the principal
et al.,
translithospheric
have
to
seem artificial.
forests and peat phenomena
NNW-
is aseismic or the stress accumulates
improbable. second
on
(Dragert,
ongoing
active
on
in the
1983; Weaver et al.,
indicates
either
Both these explanations
shallow earthquakes
as well as drillhole
recorded thrust earthquakes,
slowly and is released
seismicity
such as late Cenozoic structures
British
subduction
at the lower
movements
has been N-S compression
an
Couch and Riddihough,
movements
Without
That
1989;
1990).
(Taber and Smith,
continental
in
Mooney and Weaver,
currents
or
on the megathrust,
areas worldwide,
with
297
no
relation
to
subduction
zones.
Their causes may be various:
sliding of rain-drenched sediments, small local storms, are
tsunamis, etc.
generally
Pleistocene
absent
(Acharya, along
wave-cut
1992).
the
coast
of
are
common.
platforms
McCrumb,
1988).
and
coastal
Washington
of
Oregon,
whereas
This
probably
Oregon
margin
(West
On the other hand, the generally irregular
distribution of subsidence belt
sea
Raised Holocene terraces
indicates slow and gradual upwarping of the and
earthquakes,
Oregon,
uplift
zones and
along
the
British
entire
Columbia
is
inconsistent with regional buildup of compressive stress.
Data from leveling surveys and geodetic well
as
tidal
paleoshorelines,
data
and
strain
observations
of
measurements,
raised
or
subsided
are sometimes linked with the presumed kinematics
in the Cascadia subduction zone.
Reliability of these data varies
(Crosson, 1986), and their tectonic interpretations are (e.g.,
Dragert,
1987).
Washington,
western
by
Columbia
(e.g., Riddihough,
irregular vertical ground movements
explicable
British
1982b).
(Fig. 59), not easily
two-dimensional flexure models, have been recorded
along the Oregon, Washington and British Columbia margin (see data
presented
Hyndman, Local
directions. trending
by
Lisowski,
1991; Dragert et blocks
are
al.,
sinking,
the
1985; Dragert, 1987; Riddihough and 1994; rising
Mitchell and
NE but in places also NW.
et
tilting
These blocks are separated by narrow
mostly
and
local block movements and eustatic sea-
level fluctuations remains problematic
Complex,
ambiguous
Discrimination between contributions from
post-gracial isostatic rebound in northwestern
as
al., in
1994).
different
gradient
zones
A broad, E-W-oriented
zone of subsidence near latitude 45~N is closed on the
shelf
but
298
2
1 0
48 o
46 °
44 °
42 °
40 ° 126 °
122 °
Figure 60. Present-day rates of coastal subsidence and uplift in western Washington and Oregon (modified from Mitchell et al., 1994). Subsidence and uplift rates (contoured) are in mm/year. The stippled area represents the conventionally assumed region of elastic-strain accumulation, based on the rigid-plate model. However, the irregular pattern of subsidence and uplift is not consistent with simple 2-D subduction models.
299
open
landward.
In contrast,
a similar subsidence
is closed landward but open to the shelf. bounded
by
subsidence
uplift
near
46~N
These two downwarps and
and uplift are also irregular
Archipelago.
On
NE-oriented
gradient
island
zones
are
at
Vancouver
Island,
zone near 47~N
48~N.
on
crustal
the
Patterns Western
blocks separated by a
being tilted at dissimilar
rates have varied during the last several decades 1985;
Dragert,
Other there
and
discussed
is no extensive
subduction
margin
accumulation
subducted of
in
of
the
Cenozoic v o l c a n i s m subdivided
or
no thrust
stress
In
is
past.
Thus,
occurring,
the
occurred
idea
that
in western Cordillera
(1978)
and Oregon Cordillera
into four episodes,
and Vance,
1992).
Eocene m a g m a t i s m
and Oregon
is now thought to be a result
volcanoes
the
in this entire region will soon
subduction-related
1992,
that
Cascadia
that large earthquakes
v o l c a n i s m before the Oligocene
(also Babcock et al.,
under
the
seismicity
in the western Washington
represent
Cenozoic
Lisowski,
suggests
megathrust
is
beginning no
arc magmatism,
believed to have begun in this region only around
Late
These
is unfounded.
chains
by A r m s t r o n g
However, to
(see
chapters,
Columbia.
recent
earthquake
Sec[mentation of volcanic
thought
slab
British
elastic
lead to a gigantic
at 55 Ma.
previous
is no clear indication
on the megathrust
was
in
zone farther south,
there
rates.
the
1987).
evidence,
continental
of
Canada
zone running through the central part of
present
are
in coastal of
36
Ma
longer which is (Brandon
areas of Washington continental
rifting
1994).
in the western
Cordillera
can be grouped
300
into four main belts. California
to
The Cascade belt runs N-S from northwestern
northwestern
Washington.
Form
there,
the NNW-
trending Garibaldi belt runs along the southern Coast Belt and
loses definition gradually around 50°30'N.
orogen
Inland from Queen
Charlotte Sound, the ENE-oriented Anahim belt crosses much of Canadian
Cordillera.
the
In northwestern British Columbia, near the
border with southeastern Alaska,
lies
the
N-S-oriented
Stikine
volcanic belt.
The
Cascade
Mountains
area contains the Late Pliocene to Recent
(post-2 Ma) volcanic rocks of the High cascade suite, as older,
Oligocene
and Miocene volcanics.
at
1994).
jump
in
the
of volcanic centers and a structural unconformity at the
base of the High Cascade suite. dated
as
Discontinuous character
of volcanism in the late Cenozoic is indicated by a location
well
19
to
The previous
River
sequence,
2 Ma, has a wide regional distribution (Cheney,
In the Cordilleran interior,
Columbia
volcanic
province,
which
it includes
erupted
basalts
of
the
in an intracontinental
setting unrelated to the continental margin (Cheney, 1994;
Reidel
et al., 1994).
Attributes
of a subduction-related magmatic arc are most apparent
in the post-2 Ma High Cascade volcanic suite (e.g., Taylor, 1990), but
in
northwestern Washington and southwestern British Columbia
the mode of volcanism and the composition of lavas change (Sherrod and
Smith,
1990;
Oregon, segmentation teleseismic
Green, of
1990). the
Under
subducted
western Washington and slab
data (Michaelson and Weaver, 1986).
is
inferred
Fault-controlled
segmentation of the High Cascade volcanic chain has been in
part from gravity maps (Blakely and Jachens,
from
1990).
inferred Different
301
segments
of this chain
timing of v o l c a n i s m Weaver,
1988;
built
largely
northern along
i0) are characterized
and composition
Scott,
The High Cascade
(Fig.
further,
High
overlapping,
Quaternary
across the OWSZ
which separates
the High Cascade
North
OWSZ,
parallel the
the
volcanoes
chain
1990; Read,
is
geochemical
1990;
signatures
andesite,
dacite and rhyolite,
extensional
of
material
in the British
Stikine, also
are generally
developed
unrelated
eruptions
inactive
such
away from the continental
the
of
subduction. Mt.
Garibaldi
St.
associations Abundance
in of
presence
Other volcanic as
Anahim
and
margin and are Whereas
Helens
chain
(1990)
chain has
indicates
Cordillera,
in
Green
tectonism.
Columbia
volcanoes
(Souther,
magmatic
crust.
at
along faults
volcanic
continental
recently
1990),
The NNW trend
1990).
besides basalts,
of
decline
They decline
aligned
of the Garibaldi
processes
occurred
Washington,
rates
volcanic chains.
from
far
to
1976,
continental
is
by a series of hot springs
closely resembling
of
belts
and Garibaldi
Souther,
regions
magmas
and
Lavas on its
(Sherrod and Smith,
accentuated
noted that the rock assemblage
in
eruption
to the grain of the Coast Belt orogen.
Garibaldi
(Green,
mafic lava fields.
Cascade chain from south to north.
dramatically,
of
(Guffanti
in Oregon and southern Washington,
end are more felsic.
the
lavas
dissimilar
1990).
arc proper, of
of erupted
by
are
in
large western
currently
1990).
Drilling tests of geophysical
models
of c o n t i n e n t a l - m a r g i n
structure Leg
146
of
the
Ocean
Drilling
Program was planned to test the
302
accretionary-prism Island.
On
models
the
off
Oregon
Oregon
continental
d i p p i n g to the east are a p p a r e n t in (e.g.,
Snavely,
and
southern
slope,
seismic
1987; G o l d f i n g e r et al.,
low-angle
oceanic-crustal
o v e r l y i n g sediments faults
which
basement
are
flatten
cut out
b e t w e e n the d e t a c h m e n t and maintain
subhorizontal
extensive,
1992).
ODP drilling. Island
On
lower
depths of 260, 891.
thrust
above
the
(Cochrane et al.,
1994).
The
this detachment. basement
are
stratification.
No thrust
slope 308,
low-angle
In are
thrust
The sediments
not
affected
contrast, apparent
no
in
and such
seismic
(see C h a p t e r s 6, 8).
faults on the O r e g o n slope was c o n f i r m e d by
(Carson et al.,
the
s
low-angle thrust s t r u c t u r e s
of
profiles
A d e t a c h m e n t is
1.5
distinctive,
into the
lines off V a n c o u v e r Island
Existence
by
thrusts
reflection
c l e a r l y imaged w i t h i n the s e d i m e n t a r y section, basaltic
Vancouver
faults
were
penetrated
1993; M a c K a y et al.,
off Oregon,
off
Vancouver
1994).
fault zones were e n c o u n t e r e d at
375 and 440 m b e l o w seabed
at
the
ODP
site
These faults cut c l a y e y silt and silty clay w i t h s u b o r d i n a t e
sand layers. depth
of
52
At site 892 nearby, m,
followed by shear bands and stratal d i s r u p t i o n s
i n t e r p r e t e d as a n o t h e r interval
with
strongly
fault
as
conduits
for
zone
developed
m~lange) was e n c o u n t e r e d at serve
a fault zone was p e n e t r a t e d at a
106-175
fluids
g r o w i n g a c c r e t i o n a r y wedge,
at
62.5-67
fault-zone m.
expelled
and local
These
m;
yet
fabric faults
another
(including probably
from sediments in the
hydrogeochemical
anomalies
are a s s o c i a t e d w i t h the p e n e t r a t e d fault zones.
No
such p h e n o m e n a were e n c o u n t e r e d off southern V a n c o u v e r Island.
303
Some to
7 k m w e s t o f the f o o t of the slope, a
depth
of
undisturbed. less
than
780,000
sediments,
derived
pieces
in
which
depth
from adjacent
may
or
may
dip
occasionally
slickensided
Significantly,
silt
gravel.
in
not
be
These on t h e section
at a water
ledge,
of
sediments,
in
northern drilled
an Juno
to
a
c l a y e y s i l t a n d fine sand. Between
104
and
B e l o w 127 m, r o c k s a r e
pervasive,
interlocking
and
150 m.
contrast
fault-plane sediments
part
t h e west.
are
below
sharp
slope,
bathymetric
are silt,
to
fractures
Without
to t h e a c c r e t i o n a r y
conduits,
is d i f f u s e
o n the s l o p e o f f
p r i s m off Vancouver
t h e f l o w of f l u i d s e s c a p i n g
( C a r s o n e t al.,
1993;
MacKay
1994).
Deformation
of
sediments
Vancouver
Island
Drilling
results
was
reactivated
on
caused
support the
this area that only sparse,
tectonic
on t h e m i d d l e
no faults have been penetrated
from compacting et al.,
clayey
were deposited
in t h e t o p 104 m.
40°-70 ~
The
Island.
even
sediments,
T h e u p p e r p a r t of the
to Q u a t e r n a r y
is s u b h o r i z o n t a l
fractured.
Oregon,
and
l a n d areas,
T h i s s i t e s i t s on a
which
beds
laminated
to be
of wood.
of 345 m b e l o w s e a b e d ,
m,
Basin sediments
and Holocene
are
sand
currents.
Upper Pliocene
The bedding
a
age,
at s i t e 8 8 9 / 8 9 0 w e r e d r i l l e d
depression.
127
years
w i t h fine to c o a r s e
d e p t h of 1322 m. area
found the Cascadia
floor by turbidity
contains
Wells
m
The bedded Upper Pleistocene
intercalated
ocean
567
at O D P s i t e 888, d r i l l i n g
normal
shortening.
the c o n t i n e n t a l primarily
interpretation
steep reverse
fault,
by
s l o p e off s o u t h e r n
downslope
of s e i s m i c d a t a f r o m
faults,
accommodate
slumping.
no
o n e of w h i c h more
than
is
minor
304
The pros and cons of dying subduction The idea that subduction of the Juan de Fuca plate is stopping an
ongoing
plate
reorganization is voiced at times (Srivastava,
1973; Lister, 1991)o (see
McCrumb
et
However,
al.,
it has
1989b),
as
received the
little
Simply to argue that the subduction of the has
stopped
alternative
yet
tectonics,
would
exists
which
require
the that
general oceanic
assumption
1991).
entire
indeed be difficult. to
attention
rigid-plate
continues to be applied (Riddihough and Hyndman,
plate
Juan
de
Fuca
No comprehensive
principles lithosphere
of
plate
created at
s p r e a d i n g r i d g e s must be disposed of in subduction zones to space
problems.
geological
In the study area, unusual
at
parts
Oregon in
particular,
found
cut
to
1987). is
as
the
the
western
Juan
de
Fuca
is,
Ridge
and
North America margin.
continentward-dipping
thrusts
have
In been
Eocene sedimentary rocks along the coast (Snavely,
Existence of such thrusts on the Oregon continental
confirmed
it
and geophysical observations are consistent with
continuing sea-floor spreading at underthrusting
avoid
In the worldwide practice, these principles have
met considerable success. many
in
by
slope
seismic data (Cochrane et al., 1994) and by ODP
drilling (Carson et al., 1993).
The volcanic arc along the margin
in Oregon and southern Washington is still extant.
it
has
been suggested that subduction may no longer be occurring
in the southern (Gorda) part of the Juan de Fuca plate (Couch Riddihough, internal
and
1989), where plate convergence is probably taken up by
deformation
of
oceanic
lithosphere
(Wilson,
1986;
305
Stoddard,
1987,
1991).
Disrupted
magnetic stripes and diffuse
seismicity are also found in the northern part of the Juan de Fuca plate
off
British
Columbia
Wahlstr~m and Rogers, al.,
1994).
(Atwater
1992; Davis and
Detailed
studies
and
Severinghaus,
Currie,
1993;
1989;
Furlong
et
off the mouth of Queen Charlotte
Sound have found that to apply the standard rigid-plate assumption in that part of the plate is awkward (Carbotte et al., 1989; Allan et al., 1993).
Rather than a comprehensive stoppage of subduction,
it seems
more
realistic that the Cascadia subduction zone is shrinking gradually from the north and south.
The Juan
de
Fuca
plate
increasingly
accommodates convergence by internal deformation.
Slow changes in tectonic regime at the continental margin Early
doubts
whether
the
Juan
de
subduction zone are behaving in a al.,
1972;
Stacey,
1973;
(Riddihough and Hyndman,
"typical"
Srivastava,
1976),
in
presumably
less-than-wholehearted
doubters of
plate
Cascadia
subduction
tectonics zone
Fuca plate and the Cascadia
reflection
seismic,
model (Keen and Hyndman,
(McManus
et
1973) were soon dismissed
part
by
acceptance
(Riddihough
et
being by al.,
ascribed some
of
1983).
to the The
was extended as far as Queen Charlotte
Sound (Hyndman et al., 1979; Riddihough, mostly
manner
1984).
Big
geophysical,
surveys were used to corroborate this
1979; Clowes et al., 1984, 1987;
Hyndman
et al., 1990).
In
the scope of this model, the seismic profiles were interpreted
in terms of subduction-related structures. readily interpreted as thrust faults.
Low-angle events
were
306
However,
these events could also be related to lithologic
in the thick Phanerozoic igneous data
bodies,
showed
structural al.,
metamorphic
that
of
significance
Geologic
the
are
mapping
southern V a n c o u v e r Fairchild
Faults
in
continues
Island margin
in
the
in Alaska.
from Alaska,
crustal
weakness
separates
large Cordilleran
huge structural
Washington
geologic
both ends into the continent,
Vancouver
Island,
zone trends
there.
et
1974,
al., and
1981;
1985).
the
OWSZ,
and steep. The
to
which
Both these
plate-boundary
fault in the Cordilleran
it separates
the
continental
Alaska and British Columbia. which
is
a
long-lived
the principal
Columbia
provinces.
of kilometers
geologic
Cordillera
and
Such a composite,
long and running
on
has no relation to subduction.
the boundary between the North America
Juan de Fuca plates runs southward, found
a
(Milkereit et
al.,
interior.
British
zone, thousands
assigned
Levato
and in
that transects and
of seismic
zone, which continues
are d e e p - s e a te d
it meets the OWSZ,
the
is
and Cowan,
structural
continental
grain of
zone
(Muller et
Farther south,
Island,
1990;
intrusive
that most faults on western
plates off southeastern
Off V a n c o u v e r
initially
fact diffractions
zone begins as the Fairweather
oceanic
of
1982; Rusmore
there from Idaho,
structural interior
shows
succession,
Reprocessing
(Hawthorne,
the p l a t e - b o u n d a r y
zones begin
From
in
etc.
events
Island are steep
and Cowan,
the Vancouver
zone
many
fronts,
1990) or off-line noise
1990).
and
volcano-sedimentary
contacts
and
the
Cascadia
The OWSZ part of the composite
ESE and runs into the continental
interior.
and
subduction structural
307
On the upper continental slope off southern Vancouver the
Olympic
suggests
Peninsula,
the
Vancouver
integrated
presence
of
Island
and
analysis of geophysical data
continental
crust.
Off
northern
Island, attenuated continental crust underlies both the
upper and lower slope (Chapters 5-8).
On the Olympic Peninsula, extensive might
have
Olympic
the
Hoh
Mountains,
Orange, pile,
to
Basin
to
the
obduction
in
rather
developed
shortening.
However,
west, many shear zones are steep and
than
1990; Orange et al., 1993). which
Miocene
compressional structures with a regional
separated by areas of coherent bedding; related
the
In the Central Olympic Basin and
westerly vergency accommodate considerable in
in
been induced by convergence of North America with the
Farallon/Juan de Fuca plate. the
deformation
some of the faults may
subduction
1987;
accretionary
sedimentary
largely in the Late Miocene,
lies farther
outboard, on the submerged margin;
An
(Snavely,
be
in part it overlies
attenuated
continental crust.
On
the lower slope off the Olympic Peninsula, some thrusting went
on till the Quaternary (Line 76-19; Fig. 56). the
m~lange
drilled
on
the
Olympic
However, the age of
Peninsula
shelf
beneath
stratified younger sediments, and the estimates of likely rates of m~lange
growth
(Chapter
8),
suggest
developed in the Late Miocene - around deformation in the Hoh Basin.
that the
most time
of of
the prism uplift
and
Since then, no major compression is
known to have occurred in the Hoh
Basin.
On
the
lower
slope,
c o n t i n e n t w a r d - d i p p i n g thrust faults cut sediments as young as Late Pliocene, but only one, at the foot of the slope, cuts sediments
(Fig. 56; Snavely and Wagner, 1981).
Quaternary
This indicates the
308
intensity of compressional
tectonism
at
the
Olympic
Peninsula
continental margin has probably declined since the Late Miocene.
Many
bathymetric
ridges
on the Washington continental slope are
caused by thrust faults.
A number of
oceanward,
continentward
rather
accretionary indicate
prism
that
than
(Snavely,
conditions
a
are
dipping
expected in a typical
Laboratory
thrusts in
faults
as
1987).
landward-verging
variations in local
these
experiments
may
indeed form due to
thick,
semi-consolidated,
compressed sediment pile (Seely, 1977).
Aside
from
thrusting,
diapirs on the Miocene
to
overpressuring may lead to diapirism,
Washington
Recent
shelf
sedimentary
deform blanket
and
pierce
(Snavely, of
movements
occur
underlying
steep
faults,
Vancouver Island (Shouldice, 1971; Tiffin diapirs
are
et
Upper
1987).
overpressuring
diapirs, probably caused by a combination on
the
and
More and
on the shelf off
al.,
1972).
Such
found worldwide in m u d - d o m i n a t e d basins that have no
relation to subduction.
Also unrelated to subduction are the young have
been
imaged
sediment
slumps
with sonar and seismic techniques on the upper
and lower continental slope off northern
and
southern
Vancouver
Island
(Tiffin
1989).
Buried slumps, like those on the inboard side of the
depression
in
that
et al., 1972; Figs. 5 and 6 of Davis and Hyndman, Juno
seismic line 85-01 (Fig. 52), probably account for
the penetrative yet diffuse deformation of
sediments
encountered
at ODP drilling site 889/890 at mid-slope.
Seismic
profiles
show
that
on the slope off southern Vancouver
309
Island reverse faults are sparse and steep, and no such faults are observed
off
northern
Vancouver Island (see Chapters 6, 8).
accretionary m~lange was penetrated in the Cygnet the
southern
Vancouver
Island
outer
shelf.
above, little room is available for such a Vancouver
Island,
and
none
at
all
J-100
on
As was discussed
m~lange
off
well
No
central
off
southern
and northern
Vancouver Island.
Structural and geophysical evidence from the Olympic Peninsula and the
adjacent
submerged
margin
indicates that manifestations of
subduction in that area have decreased gradually the Late Miocene.
in
time,
since
They also decrease gradually in space, from the
Washington margin northward.
Importance of geological
paradigm
as
a
guide
for
geophysical
interpretation Because
interpretation
of
geophysical
data
yield a unique solution, selection between
generally does not
various
geophysically
permissible options must be guided by geological reasoning.
Based
on the principles of tectonics of rigid plates, a switch of regime along
a
occur
gradually:
between
continental
these
guided
the
continental Hyndman, avoid
a
margin
well-defined
tectonic
domains.
interpretation margin
1989, 1991).
from subduction to transform cannot
of
of
western
triple Such
junction a
required
model has previously
geophysical North
is
America
It is internally logical
data
along
(Riddihough as
it
seeks
the and to
space problems in reconstructing plate motions - as long as
plate rigidity is accepted.
But if the oceanic lithosphere is not
rigid, rethinking of the model is required.
310
Consistent
with
the
gradual
northward loss of stiffness of the
Juan de Fuca plate, manifestations northward.
subduction
(Snavely,
1987;
many
such
thrust
Carlson
detachment
Vancouver Island. cut
by
decrease
continentward-dipping and Nelson,
al., 1992) coalescing into a detachment No
also
Seismic data show the continental slope off Oregon and
southern Washington is cut by faults
of
is
thrust
1987; Goldfinger et
(Cochrane et
al.,
1994).
apparent in seismic profiles off
The idea that the Tofino Basin on the shelf
low-angle thrusts
is
(Yorath, 1980, 1987) is contradicted by
both seismic profiles and well information.
Thrust-sheet structure has been inferred from deep seismic data on Vancouver Island and the adjacent submerged margin (Yorath et al., 1985a,b; Clowes et al., 1987; Hyndman extremely on
the
et
al.,
interpretation
and
upper
difficult.
at imaging low-angle
continental
slope
make
discontinuities
in
the
subsurface,
Geologic mapping
et al., 1974, 1981; Muller, 1977a-c; Fairchild
and
al., 1987) show the major faults on the island to
steep (Muller
Cowan,
and Cowan, 1985) and detailed seismic studies
no
reliable
Besides, whereas seismic data are good
discontinuitues often remain undetected.
well,
However,
poor signal penetration and chaotic character of events middle
Rusmore
1990).
be
1982;
(Mayrand et steep.
As
objective criteria have yet been developed to interpret
deep seismic events as thrusts, rather than as metamorphic
fronts
(Hyndman, 1988), pluton tops, sills, stratigraphic contacts, etc.
All
this
leaves
much
room
for
speculation.
seismic data across the continental margin, since
the
1970s
to
In interpreting
it has been
customary
concentrate on continentward-dipping events
assumed to represent subduction-related thrusts.
311
Recently,
however,
Island previously
some of the deep seismic interpreted
by data-processing 1990).
The
variously al.,
tests to be
event
"F"
as the top
1987)
of
in
the
submerged margin,
workers
and
isolated fault
very
Vancouver
them
("B") was postulated
the
SE
end
the
any case, events
its
at
in the NW
the
The Leech River
accreted
half
model,
to be linked with
(Clowes et al.,
1987).
of
fault,
This
complex massif
oceanic-crustal
adjoins
which
is
the
regarded
Crescent
the
from 2
to
5
the shallowest
of
events
Eocene
from
1977c).
so projecting As well,
In any
the event
in the middle of was
on
But field mapping
the
shown by detailed
(Mayrand et al.,
Metchosin
in some models
"terrane"
short,
the steep San Juan
(e.g., Muller,
A break in it occurs Juan
same
as a reflection
surface trace is conjectural.
San
the
low-angle
and interpreted
analysis of these data to be subvertical
massif.
of
1990).
River fault is steep
"B" is not continuous. profile
ed.,
Line 84-04 does not cross this fault,
to
1990).
have been produced by different
84-04 which crosses
of this profile
Leech
(Clowes et
1990; Levato et al.,
with the thrust-complex
the Leech River "thrust"
al.,
has been shown to
14, 23), events dipping NW are observed
keeping
et
interpreted
or the base
interpretations
data
Island,
line
In
shows
have been shown
46),
oceanic crust,
different
s.
Vancouver
(Milkereit
(Fig.
1990)
on
transect through the island and adjacent
in Green,
reflection
(Figs.
84-01
(Hawthorne,
refraction
(see papers
On southernmost
Line
undergoing
Along the main LITHOPROBE
structures
diffractions
(Hyndman et al.,
probably be off-line noise
reflection
as thrust
events
1987).
igneous
as part of an
(Hyndman et al.,
1990)
312
which
under
panel the
Vancouver
Island
(Dehler and Clowes, 1992).
a 4-km-thick, east-dipping
But field mapping onshore
shows
Crescent Formation comprises many discrete igneous massifs of
various sizes, which erupted setting,
in situ.
from
al.,
1992,
multiple
centers
in
a
rift
Structural evidence from the Olympic Peninsula
argues against their accretion et
forms
1994).
(Brandon and Vance,
1992;
Babcock
Inconsistent with oceanic-crustal origin,
felsic rocks are found in these massifs on the
Olympic
Peninsula
(Snavely, 1987) and southern Vancouver Island (Muller, 1977c).
Crescent
basalts
are
in
stratigraphic contact with sedimentary
rocks of both the Central Olympic and Fuca basins on Peninsula,
and
no
fossil
subduction
megathrust
the
Olympic
is recognized
between them (Babcock et al., 1994; Chapter 5).
Magnetic anomalies Vancouver
Island
over
three
exterior
Eocene
shelf
are
basaltic
erupted.
the
Crescent
bodies
On are
with faults of the OWSZ, along which they seem to have On the shelf,
(Calawah)
on
elongated and narrow.
northern Olympic Peninsula, other elongated associated
bodies
an
extension
of
one
of
these
faults
bounds the Prometheus basaltic body, suggesting in-situ
eruption in that area as well.
Absence of gravity anomalies
over
these bodies confirms that their size is small.
South of the Olympic Peninsula, other Crescent massifs which cause strong gravity anomalies were modeled by Finn (1990) to be 30
km
thick.
up
to
The gravity high over the Metchosin massif is also
strong and centrally symmetrical, and remains so when gravity data are
upward continued to 20 km.
There is no indication of lateral
offset in a thrust slice, but rather a suggestion
that
a
frozen
mafic magma chamber underlies this massif at mid-crustal level.
313
Another
presumed
thrust-bounded
exotic
"terrane"
Rim complex on western and southern Vancouver al.,
1990;
Dehler and Clowes,
1992).
dispersed
into
these
(Rusmore and Cowan,
the
bounding
faults
It is long-lived, history 1974,
1981;
South
of
distinct to
Miocene, of
Brandon,
1989a,b).
During
sense
the
shows
probably
from the San
1989a,b).
Mesozoic
and strike-slip
the Hoh sedimentary
inconformity-bounded Miocene,
and
packages
Pliocene
One
and
of
fault.
Cenozoic
(Muller et al.,
Tertiary,
the accretionary
basin
it
was
in
uplift
blocks,
the
and
and
In the Late when
most
However,
plate
development
of
other
The Tofino Basin was controlled
subsidence
of
fault-bounded
Fuca Basin lies within the OWSZ.
the Puget and Georgia basins
Oligocene
around the time
m~lange was created outboard.
this region.
differential
Upper
Recent ages.
had less direct effect on the
basins
is made up of three
of Eocene,
to
it was deformed and uplifted,
interactions deep
faulting
et
into the OWSZ.
the OWSZ,
Middle
field mapping
a Pacific Rim slice is the W e s t c o a s t
reverse
Brandon,
incorporated
1985;
having acted during its
in a normal,
(Hyndman
slices which were
areas by strike-slip
Juan Islands
Island
However,
it is confined to narrow fault-bounded
is the Pacific
occurred
in conjunction
crustal
Subsidence with
by
of
uplift
of the North Cascade and Coast Mountains.
Fragmentation various Explorer
lines
of
the northern Juan de Fuca plate is indicated by
of
evidence.
The
supposed
boundaries
of
the
fragment with the Pacific plate - the Explorer Ridge and
the Sovanco
fracture
zone -
are
not
active
seismically
(Barr,
314
1974;
Barr and Chase,
along
the
Explorer
unexpected
at
al.,
1993).
and
contains
1991;
Ridge
spreading
many
triple
plates
1994).
rigidly
assumption
off Queen Charlotte
(Carbotte et al.,
North
America
Brooks-Estevan crust
are
magnetic boundary
maps.
From
structural
southern Vancouver
Ongoing
pattern
surveys,
Dragert
Comox
continued and
the
River
The
Fuca
(or
along the
Island,
Juan
or
(Furlong et behaves
1993).
along the
from southeastern
faults
expressed
Brooks-Estevan
where
modern
to
the
western
Alaska,
Island.
in
In the
and oceanic gravity
embayment, submerged
past
and
the platemargin
off
it meets the OWSZ.
Island is inconsistent
ground
movements.
(1987) reported d i s s i m i l a r
Campbell
de
lithosphere
to northern Vancouver
off Vancouver of
and
oceanic
From
River areas,
with the leveling
elevation changes
blocks on the island.
at about 1 mm/year between
1984,
Campbell
and
Sound.
small blocks of continental
rates of tilting of different the
Charlotte
that
zone continues
subduction
observed
Queen
and may be diffuse
margin
Islands,
juxtaposed
Lister,
centers
fault system continues
embayment,
1986;
wide
Sound has been found to be unrealistic
continental
the Queen Charlotte
1989; Allan et
spreading
1989; Allan et al.,
The broad plate-boundary
et al.,
active
Pacific
is still undefined
The
Basalts
characteristics
(Cowan et al.,
off
the
1992).
zone is tens of kilometers
No
exist
between
(Michael
blocks
1993).
and Rogers,
geochemical
centers
rotated
junction
northern boundary Explorer)
have
The Sovanco fracture
Davis and Currie,
plate
al.,
1974; W a h l s t r ~ m
Upwarping
and in
on the island's east side,
1946 and 1977.
Comox area rose at the same rate, just to the north accelerated
to
5
Between
1977
but uplift near mm/year.
The
315
boundary
between
trends NE. suggested
these
To Dragert, regional
al.,
1983;
regime,
Washington subduction,
From detailed patterns deposits,
McCrory
is an
area
processes"
al.,
regime
recent
subsidence
of
and
1987,
the
transition
occurs
off
An end-member
involving
Quaternary
that the northern Washington
between
Canada Archipelago,
subduction
are apparent
thins
towards
blocks
the
"subduction-dominated "margin-dominated
middle
slope
no direct signs
from geological Washington,
margin stepwise.
of attenuated
farther outboard,
off
continental
Island.
Off the Queen Charlotte
the p l a t e - b o u n d a r y
Island is a west-dipping
No compressional
normal
structures
ongoing
or geophysical
the
continental
and oceanic crust runs
southern Vancouver
along the R e v e r e - D e l l w o o d
of
The boundary between
Island and even
fault, Islands,
follows the outer scarp of the Queen Charlotte of
p. 695).
margin
to the south and unspecified
crust
strand
with
of the submerged margin and
uplift
concluded
in western Oregon and
Vancouver
along
(Riddihough
It is "inconsistent
(Dragert,
of the structure
Unlike
the
and Washington
is evident off Oregon and southern Washington.
data.
along
movements
to the north.
Along the Western
juxtaposed
Island,
which has also been inferred
1994).
subduction"
(1994)
tectonic processes"
or
et
block
and southern British Columbia.
studies
of coastal
of modern
British Columbia
change of tectonic
northern
margin
such a pattern
Reidel
simple two-dimensional
Gradual
on central Vancouver
N-S compression,
for the entire western et
two blocks,
off
northern
this boundary
Terrace.
The inner
fault system off northern V a n c o u v e r fault
(Fig.
36).
have yet been found to accommodate
the
316
presumed
very
oblique
Pacific plates modeled uplift
and
convergence between the North America and off
the
continentward
Queen
Charlotte
convergence
Hyndman,
1983;
Riddihough and Hyndman, 1991).
however,
shows
that
began
uplift
1991).
of
the
much earlier.
it was accommodated by many significant
eastward
shelf
in
Hecate
Western
faults
and
Canada
(Thompson
et
Strait
occurred
without
al., 1991; Hickson, are
(Lyatsky,
found
on
1993a).
Vancouver Island was raised throughout the Tertiary. is
and
On the Queen Charlotte Islands,
steep
tilting
(Yorath
Geologic mapping,
entire
Many more faults, normal and reverse,
interior
The
tilt of the islands, supposedly in the
Pliocene, has been cited as evidence of
Archipelago
Islands.
Most Its
the of
uplift
continuing at present (Dragert et al., 1994), but in a complex
fashion suggestive of N-S compression (Dragert, 1987).
Along the plate-boundary fault seismicity
and
system
inner
strand
of
and
Carlson,
linked directly with Terrace.
Alaska,
have
mostly
been
strike-slip.
the fault system is active at present, but
outboard strands are thought to (Bruns
southeastern
offsets of sea-floor bathymetric features suggest
that late Cenozoic displacements The
off
1987; the
von
faults
have
been
active
in
the
past
Huene, 1989).
These faults are
bounding
Queen
the
Charlotte
North of Cross Sound, the Fairweather fault component of
this system runs into the interior of southern Alaska.
Thus, no simple
separation
subduction
strike-slip regimes along the continental margin.
and
division
can
Such a division was made previously on behavior
be
the
made
between
assumption
that
the
the
of oceanic plate in this region is rigid (Riddihough and
Hyndman, 1991).
Instead, the subduction
regime,
well
developed
317
off Oregon and southern Washington, transitional
zone
lies
southern Vancouver
Other
evidence
off
decays northward
northern
Washington
gradually. and
perhaps
Island.
for
non-rigid behavior
of the Juan de Fuca plate
and gradual change of t e c t o n i c regime along the continental Absence
of bathymetric
For decades, Cascadia typical
unusual
the
absence
and
of
at other subduction
(McCrumb et al.,
bathymetric
has remained a mystery.
subduction
zones, But
then,
identified
and
the
mostly
is the presence
beneath
of a fore-trench
thus
Cascadia
are bulge
from gravity data
the
the
Trenches
outboard, bulge.
are
region and zone highly
oceanic
intimately is
only
Cascadia
plates
weakly
Presence
Oregon
entering
associated
(e.g., Jachens
Basin.
off
Such bulges are
rather than from relief of the ocean floor or of the basalts
along
zones in the circum-Pacific
caused by downward bending of
trenches.
trench
1989a).
southern Washington,
typically
a
and absence of a trench makes the Cascadia
All the more strange,
margin
~rench
zone of subduction
elsewhere,
A
with
developed,
et al., top
1989)
of
of a bulge
the
is not
indicated by gravity data farther north.
Such abnormal loss
of
the
rigid behavior
phenomena
are easier to explain by gradual
strength of oceanic
lithosphere
northward
and by general non-
of the Juan de Fuca plate.
Isometric geoid anomaly off the western North America continental margin Active subduction
zones around the perimeter
of the Pacific
Ocean
318
-
Japan,
Kurile,
Aleutian,
geoid lows which run degree-10
residual
km),
(thousands
long
on average;
A
California huge -i0
negative
- are marked by pronounced
subduction-related
trenches.
have been found to be narrow and distinctly
These (200-400
negative
(-20 m
1983).
anomaly
to southeastern
in
etc.
of kilometers)
geoid
diameter m
along
anomalies
Bowin,
different
Chile,
of about
amplitude. anomalies
lies off western North America,
Alaska.
It is nearly
2500 km.
isometric,
It is concentric
Unrelated
to
subduction,
of similar dimensions
from
with a
and only about positive
and
are the norm all across
the Pacific.
Absence low
along the Cascadia
typical
weakness small
of
other
subduction
to
produce
a
trench
of the subducting
lack of earthquakes
Deformation Drillhole
thinly bedded,
or an associated
sandstone
continent-derived, with
an
The
1990; Hasselgren
et al.,
faults.
of the lower,
overlying
is too
low in geoid maps.
in the abyssal
upper
pelagic
average
of meters of relief
sediments
it supports
seismic data show that the Cascadia
hundreds
and
geoid
for
the
on the megathrust.
the top of u n d e r l y i n g
mounds
elongated
plate might also be the reason
However,
al.,
the flexure
of the basaltic basement and
zone of an
zones might be related to the
of the Juan de Fuca plate:
Weakness
and
subduction
(Carlson and Nelson, 1992; Rohr,
Depressions seismically unit
basalts
1994)
Basin
Basin contains
and turbiditic
total thickness
ocean-crust
Cascadia
siltstone
of about 500 m. is rugged, 1987;
caused
with
Calvert et by
buried
in this relief are filled with transparent,
consists
of
sedimentary
turbiditic,
unit.
well-bedded
319
deposits.
Near the continental slope, mostly in the
Astoria
and
Nitinat fans, the Cascadia Basin is up to 2.5-3 km thick.
Along
the lower continental slope off British Columbia, faults of
the plate-boundary blocks
and
structural
formation
accommodated
subsidence
of grabens in both the oceanic
depression) and continental Brooks-Peninsula
zone
(e.g., Winona Basin)
embayment,
fault-related
(e.g., Juno
crust.
In
gravity
outboard
parts
of
the
northern
Juan
reactivation of older faults in the oceanic
area
de Fuca plate.
crust
the
anomalies
coincide with magnetic linear anomalies continuing into this from
of
was
If
involved,
origin of magnetic anomalies in parts of the northern Juan de Fuca plate might be related to local faulting. anomalies
Interpretation of
such
as isochrons related to sea-floor spreading alone would
contaminate local plate reconstructions.
Presence of a broad belt of earthquakes running from northern Juan de
Fuca
Ridge
NNW
(e.g.,
WahlstrSm and Rogers,
considerable in-plate deformation. dike
magmatism,
resulting
1992) suggests
If late-stage faulting
magnetic
caused
anomalies might deceptively
resemble those conventionally related to sea-floor spreading.
Shearing of the Juan de Fuca plate in the third dimension Earthquake data, as well as studies of young geologic modern
ground
motion,
well
break-outs, etc., indicate that the
stress field in Washington varies with depth: N-S the
continental
The
compression
in
crust, vs. extension at deeper levels presumably
associated with the subducted slab. unclear.
structures,
continental
crust
Causes of this in
western
probably thick enough, and rich enough in
contrast
North
sources
of
are
America is radiogenic
320
heat,
to
be
capable of tectono-magmatic and tectono-metamorphic
self-development. fully
explained
1989a). where
At any rate, the N-S compression has never been in
terms of ongoing subduction (McCrumb et al.,
Presence of such stresses in they
manifested
the
themselves
by
Cordilleran creating
interior,
late
Cenzoic
structures in the Yakima belt (Reidel et al., 1994), casts further doubt on a relationship to subduction.
Probably,
these
stresses
are
related to regional adjustment as
plate interactions are changing. of
continental
crust
Jostling of fault-bounded blocks
in a changing stress field is reflected in
the irregular current ground movements.
Reflecting ongoing change in plate recent
compression
interactions,
the
amount
of
at the continental slope and the intensity of
current arc magmatism decrease from south to north.
Most
of
the
change takes place in northern Washington and perhaps southernmost British Columbia. the
Cascadia
The stoppage of subduction
subduction
is
encroaching
on
zone gradually, from the north, and the
plate interactions slowly continue to change.
Jostling of blocks adjusting to a new regime is also the northern Juan de Fuca plate. bands.
variable, and
related
processes, velocity structure of the oceanic crust is
1978;
Au
and
reflection profiles contain crust
some
and its thickness locally seems to reach ii km
Clowes,
in
Earthquakes there occur in broad
Due to structural readjustments and perhaps
deep-seated
apparent
Clowes, dipping
1982,
1984).
events
within
(Maleeek
N e w seismic the
oceanic
and reveal many other variations in crustal structure which
have been ascribed to magmatic underplating
(Hasselgren
et
al.,
321
1992).
Alternatively,
structural
thickening
(the "bunching-up"
of Malecek
which case the dipping reflections intracrustal
structural
of the oceanic crust might be and
Clowes,
may indicate
low-angle
in tectonic
deep S~rU~ture
of the margin region from teleseismic
if coarse,
with
teleseismic
regime along the continental
images of continental-margin data
perpendicular
to
different
(Michaelson
the
dips
Humphreys,
depth
of
1990; VanDecar,
km,
of
and
Overall,
steep, zone
Dueker,
though,
up to 60 ~ just (Rasmussen
1994a,b).
slab in a N-S
in the entire Cascadia
and
A
direction,
tear at
a
(VanDecar et al.,
has
1984).
rate seems to be greater
degradation
from south to the
zone
(Riddihough,
north.
continental-margin
than in the south
in
of the underthrusted
levels to a depth of several hundred different
Tears
1991).
in the south,
structure
1986).
subduction
the last several million years
be proceeding
obtained
Juan de Fuca slab
has also been suggested
decline of the subduction than
data
deteriorating.
slab is remarkably
Humphreys
The rate of subduction in
and Weaver,
through the subducted 150-300
is
margin and
divide the slab into segments with
from the Cascadia
1988;
propagating
Washington
margin
dip of the subducted
a short distance
zones of
structure
show that the subducted
beneath western Oregon and the
in
delamination.
Gradual chanae
Deep,
1978),
(Humphreys
In
the
north,
region,
from
kilometers, and Dueker,
decreased Though the the
north
slab seems to the
entire
the crustal
appears 1994a,b).
to
be
CHAPTER 10 - C O N C L U D I N G R E M A R K S
Continental-crust structures in boundary zones between continental and oceanic plates have received several
decades.
little
attention
in
the
last
Perhaps because the northeastern Pacific is the
cradle of the plate-tectonic theory, a perception has evolved that this
region,
above
all,
should
fit
global models of plate interactions. observations
deviating
perfectly the generalized
As a result, many geological
from the expected "typical" behavior have
been underemphasized.
The temptation has developed even
geologists
field observations to fit their findings to
who
pre-conceived,
But hiding resolves
make
the
simplistic tectonic scenarios.
from the
for
mismatches
between
facts
problem nor makes it go away.
of our knowledge about the nature is
and
models
neither
Room for improvement
endless,
and
no
level
of
understanding we achieve is likely to be final.
The
contacts
between
continental
and oceanic crust off western
North America are of two main types: along steep faults, or thrusts.
In
the
case of thrust emplacement, depending on which
type of crust overrides which, a distinction can be subduction
and
obduction.
made
The former predominates,
lines of evidence indicate that a subduction developed off Oregon and southern Washington. steep faults may also be of two types: fault
along
a
between
and various
megathrust
is
well
Juxtaposition along
single,
long
bounding
may clearly separate oceanic and continental crust, or many
blocks d e t a c h e d or semi-detached from their mother plates interlocked along many local faults.
may
be
323
No
clear
break
is observed
the tectonic~domains predominate. explained plate
The
where lack
juxtaposition of
and thrust
to be "unusual".
high degree of intraplate
Benioff
seismicity
change
along
juxtaposition
the
continental
warm,
and
crust.
and warm,
Acharya
regimes
plate's
Whatever
the
continental
(1992)
for some of these
including
still concluded
Attenuated
shelf and upper
heat,
Fault
Juan
continental The
which
contains
the relationship
found
far offshore.
in
the
A huge
between
those
which
that the Juan Perhaps
it
class.
de Fuca plate,
the role of
of the margin was
crust underlies
capacity
margin and the behavior
networks
continue
slope.
crust,
complicates
continental
the
crust in shaping the structure
considerable.
continental
of
-
Comparing the behavior of this
a member of a special
bahavior
of
weakness
de Fuca plate is unusual even against that background. should be considered
gradual
the interlocking
plate with other oceanic plates worldwide, young
the
the
thin and soft - accounts
continental
are
crust.
Fuca
but largely leaves unexplained
This
remarkable
and
between
of oceanic
for the Juan de
rigid.
the absence of Wadati-
trench,
margin
and u n d e r t h r u s t i n g
that it is young, phenomena
deformation,
break can be
beneath North America
Particularly
or of a bathymetric
The common explanation
are
relationships
a sharp t e c t o n i c - d o m a i n
itself and the mode of its subduction
oceanic
margin between
if the Juan de Fuca plate is not perfectly
are now acknowledged the
along the continental
for
also
parts of the
self-development
of
its own sources of radiogenic
between
the
structure
of
the
of oceanic plates outboard.
continental structural
crust zone,
on
land also
comprising
the
324
OWSZ,
the
Scott Islands
fracture
zone, the Chichagof-Baranof
zone,
fault zone,
the Queen Charlotte and the Fairweather
runs from the interior of the Pacific Northwest submerged margin, Alaska.
This
and
fault
back
into
the
system
has
at
the
plate
controlled
the
Washington,
British Columbia
Continental
position
crust
does
plates,
Cordilleran
The dynamic, though
only
boundary
the
(e.g.,
on the development
off
northern
Forte
understood,
movements
et
al.,
of continental
structure
global
unravel tool,
tectonic
the evolution however,
framework
1993).
crust are
these reconstructions
to
use
it
uncritically,
where sophistication
In the early days of
applying
and finesse
plate
many,
emerged
from
that
plates.
may help
scientific Misuses
the
entire
thinking.
effort:
continental,
lithosphere;
behaves
and
as a stiff,
are
a rather blunt instrument
most
that
of
the
scientific
as to erecting the
Two general
assumptions
oceanic magnetic
stripes
reflect the creation and m o t i o n not just of the oceanic crust of
and
other lines of inquiry,
attention was paid not so much to local details
have
may
are in order.
tectonics,
main pillars of the new global
any
can be misused.
to rely on this one tool whilst neglecting or
Like
of
Potential
from plate reconstructions
of specific regions.
of
of continental
they may have nothing to do with the motion of neighboring
The
in
Alaska.
sketchily
regions
influences
interior
deep thermal
be related to large vertical past
to the
than react passively to motions
nevertheless in
fault,
least in the late Cenozoic
and southeastern
more
adjacent oceanic plates. continental
of
Cordillera,
fault
that
each
plate,
rigid lithospheric
but
oceanic or
entity.
325
In the first approximation, these assumptions have performed well. The
early
large,
plate reconstructions flowing from them seemed, by and
internally
discrepancies
coherent,
and
dealing
with
any
remaining
and improving the resolution of models was expected
to be a matter of reconstructions
mopping
up.
However,
refinements
of
plate
have failed to eliminate the discrepancies.
Ill-
defined plate boundaries and misties in global and regional circuits
have
received
a
plate
lot of attention in the recent years,
because to proper reconstructions they are unacceptable (Stock and Molnar,
1988; DeMets et al., 1990).
Intraplate
deformation
discrepancies: western
is
sometimes
invoked
Basin and Range extension in the
North
American
continent;
or
to
explain
interior
warping
of
of
the
oceanic
lithosphere in the Indian plate.
If a plate
or
magnetic stripes correspondingly
is
delaminated
become less
internally,
reliable
lithospheric plate.
as
indicators
of
expands,
the
motion
of
contracts,
the
entire
Applying erroneous reconstructions might lead
to errors in local tectonic interpretations.
To guard against this danger, specific
regions
through
it pays to test tectonic models geology-based
local
studies.
for Such
studies, of course, are easiest in continental areas onshore.
The
usual
is
the
interpretation
the
advantage
of
detailed
possibility of incorporating
into
local a
investigations
single
results of various geological and geophysical surveys.
NO one method of geoscience inquiry can lead to a unique solution. Plate-motion reconstructions
relying
only
on
oceanic
magnetic
326
stripes
and
hot-spot tracks
reconstruction many
more
are limited.
lines
of
sedimentological, of
In continental
evidence
geochemical,
sedimentary
maturation,
are limited because
basins
may
etc. be
their
magmatism
be
may
deep
to
lithosphere.
Structures
deformation.
The list goes on.
model of a region's
is better constrained evidence.
assumptions provide
The
useful
specific
The natural a
part
of
If rocks are
h i s t o r y offer clues to
exhumation.
the
involving
of thermal
Styles
more
is known,
history
of
their
all such considerations
constraints
the less need there
Global
regions.
A
plate
on models
region
of
of the u n d e r l y i n g
than a model based on just one or two
lines
is to make
reconstructions
of regional
tectonics,
multidisciplinary
can but
analysis
should be treated as a complex
system.
system examined here the
is a small element of the globe -
North American continent.
whole is a much bigger system, The
of rocks.
in rocks reflect
about what is not.
geodynamical
burial histories
and
they should be checked by independent of
For example,
the composition
evolution
contrast,
paleontological,
analysis
burial
related
in
of
available:
studies of their metamorphic
the history of
of
regions,
deduced from patterns
or from fission-track
metamorphosed,
A
are
these methods
of
a
higher
nature - or its parts - is complex,
we can ever know,
interacting
imagine.
One
can
limitless,
in a simplified
The planet Earth as a hierarchical
order.
made up of more bits than
in more ways than we could
possibly
only conceive of something that complex,
that
form.
So we conjure up our inherently
limited models.
They help
us
to
327
understand
the internal
and external
and their evolution.
But abstraction,
also
if
be
Modeling
misleading
it
accurate
reliability does
not
testable
then, begin
indispensable
systems
as it is, can
takes one too far away from reality.
model?
accurate
is its internal the
model its
enough?
is
One
"true".
carefully
a
important than another,
defining
facts.
model's
that
alone
the
system
manifestations. of the study,
simulation.
and all assumptions
to be tested by observable
but
a
1994).
phenomenological
suitable
on
simplistic?
The model must also be
in mind the intended objective
develop
check
consistency,
1994; Lyatsky,
requires
observable
When is a model too
logical
model
(Oreskes et al.,
keeping to
is
prove
Building a examining
of natural
is a means to an end, not an end in itself.
But what is a "realistic" How
structure
and Only
can one
No one step is more and
conclusions
need
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