JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 102,NO. B4, PAGES 8217-8243,APRIL 10, 1997
Oblique strike-slip faulting of the central Cascadia
submarine
forearc
Chris Goldfinger and LaVeme D. Kulm
Collegeof OceanicandAtmospheric
Sciences,
OregonStateUniversity,Corvallis
Robert S. Yeats, Lisa McNeill, and Cheryl Hummon
Departmentof Geosciences,
OregonStateUniversity,Corvallis
Abstract. At leastnineWNW trendingleft-lateralstrike-slipfaultshavebeenmappedon the
Oregon-Washington
continental
marginusingsidescan
sonar,seismicreflection,andbathymetric
data,augmentedby submersible
observations.The faultsrangein lengthfrom 33 to 115 km and
crossmuchof the continental
slope.Five faultsoffsetboththeJuande FucaplateandNorth
Americanplatesand crossthe plateboundarywith little or no offsetby the frontal thrust. Leftlateralseparation
of channels,
folds,andHolocenesediments
indicateactiveslipduringthe
HoloceneandlatePleistocene.Offsetof surficialfeaturesrangesfrom 120to 900 m, anddisplaced
subsurface
piercingpointsat the seawardendsof the faultsindicatea minimumof 2.2 to 5.5 km
of total slip. Near their westerntips,fault agesrangefrom 300 ka to 650 ka, yieldinglate
Pleistocene-Holocene
slip ratesof 5.5 + 2 to 8.5 + 2 mrn/yr. The geometryand slip directionof
thesefaultsimpliesclockwiserotationof fault-boundedblocksaboutverticalaxeswithin the
Cascadiaforearc. Structuralrelationships
indicatethatsomeof thefaultsprobablyoriginatein the
Juande Fucaplateandpropagate
intotheoverlyingforearc.The basement-involved
faultsmay
originateas shearsantitheticto a dextralshearcouplewithin the slab,asplate-couplingforcesare
probablyinsufficientto rupturethe oceaniclithosphere.The setof sinistralfaultsis consistent
with a modelof regionaldeformation
of the submarine
forearc(definedto includethedeforming
slab)by right simplesheardrivenby obliquesubduction
of theJuande Fucaplate.
junctions and includes the smaller Explorer plate to the north,
Introduction
whichmaynst be presently
subducting
[Rohrand Furlong,
The anelasticresponse
of forearcsto obliquesubduction
can
be highly variable. The mostcommonlyrecognizedform is
arc-parallel strike-slip faulting. This type of strain
partitioning has been recognizedin Sumatra [Fitch, 1972;
Jarrard, 1986;McCaffrey, 1991], the Kurils [Kimura, 1986],
1995]. JDF-NOAM convergence is estimated as 40 mm/yr,
directed 062 ø at 45øN along the deformation front (rotation
poles of DeMets et al. 1990]). No active arc-parallel faults
equivalent to the MTL or Great Sumatran fault have been
identified onshorein Cascadia. Snavely [1987] inferred that
the Philippines[Karig et al., 1986],SouthAmerica[Dewey the Fulmar fault, a north striking dextral strike-slip fault,
and Lamb, 1992], and other forearcs[Beck, 1983]. The offsets the continental slope and outer shelf in Oregon by
Sumatranforearc translatesalong the arc-parallel Great about 200 km and attributed an abrupt truncation of the
Sumatran
fault, with the slip rate controlledby convergencebasaltic Siletzia terrane to this fault (Figure 2). The Fulmar
obliquity[Fitch, 1972;Jarrard, 1986;McCaffrey, 1991]. In fault exhibits small offsets of Quaternary strata in southern
Japan,the Median TectonicLine (MTL) servesa similar role Oregon but was mainly active in the Eocene [Snavely, 1987].
in the subductionof the PhilippineSea plate [Fitch, 1972; Some discontinuousarc-parallel faults have been identified in
Sangawa,1986]. The Aleutianarc exhibitsbotharc-parallel the Cascadia forearc, both onshore and offshore which may
translationand rotation of obliquely orientedblocks about accommodate some northward translation of the Cascadia
vertical axes [Geist et al., 1988; Ryan and Scholl, 1993]. forearc (Figure 2) [Weaver and Smith, 1983; Blake et al.,
Both of thesemechanisms
are favoredin obliquesubduction 1985; Niem et al., 1992; Kelsey and Bockheim, 1994;
because the high-angle faults concentratehorizontal shear Goldfinger, 1994; Goldfinger et al., 1992a].
more effectivelythan the dippingsubductioninterface[Fitch,
Paleomagnetically determined clockwise rotations of
1972].
The Cascadiasubduction
zoneconsists
of two smallplates,
theGordaandJuande Fuca(JDF),subducting
to thenortheast
beneaththe North Americanplate(NOAM) (Figure1). The
subduction
systemis bounded
to thenorthandsouthby triple
Copyright1997 by the AmericanGeophysical
Union.
Paper number 96JB02655.
0148-0227/9 7/96JB-02655509.00
coastal basalts in Oregon and Washington suggest that a
processof dextral shearof the forearchas operatedthroughout
the Tertiary. Miocene (12-15 Ma) Columbia River Basaltsin
western Oregon are rotated 10-30ø clockwise, and Eocene
Siletz River Volcanics are rotated up to 90ø clockwise [Wells
and Heller, 1988; England and Wells, 1991]. Mechanisms
proposed to explain these rotations include microplate
rotation during terrane accretion,basin and range extension,
distributed small block rotation, or a combination of these
(see Wells and Heller [1988] for summary).
8217
8218
GOLDFINGER ET AL.: CASCADIA STRIKE-SLIP FAULTING
-130 ø
-125 o
-120 o
deep-tow sidescan processing system. Processing included
I
I
towfish positioning, geometric 'and speed corrections,
correctionfor towfish attitude, georeferencingof image pixels
to a latitude-longitudegrid, histogramequalization,and image
enhancement. The final imagery was then integrated with
other data layers in a raster/vector Geographical Information
System(GIS) for analysis[Goldfinger and McNeill, 1996].
BC
50 ø
Regional.sidescandata [EEZ-SCAN 84 ScientificStaff,
1986] using the (GLORIA) shallow-towed system were also
used in interpretation of margin structure. The GLORIA
imagery was georeferenced to Hydrosweep bathymetry
collected by us in Washington (1993), or existing National
Oceanic and Atmospheric Administration (NOAA) SeaBeam
bathymetryin Oregon in order to producea consistentspatial
Seattle
WA
Juan
de Fuca
Plate
45 ø
--
...L
'.!
-----'
data set.
Portland
D
In our structural interpretationswe have used about 30,000
km of seismic reflection profiles collected by academic
institutions,the U.S. GeologicalSurvey,NOAA, and the
petroleumindustry[Goldfingeret al., 1992a]. The seismic
AMS-150
:.
Pacific
•
Plate •
o
-,•[i
....
"•,
.--SeaMARCIA
channel
digitalprofiles
navigated
withGPS. An important
Gorda •'
Plate \
400 I
reflection profiles vary widely in quality, depth of
penetration, and navigation accuracy and range from singlechannel sparker records navigated with LORAN A to 144-
datasetis a multichannel
seismic
surveyconducted
off central
Oregonin 1989[MacKayet al., 1992]. Approximately
2000
CA
Me,
i
I
km of NAVSTAR-navigated
144-channel
reflection
profiles
were collected and processed through time migration at the
University of Hawaii. Three of the Oregon strike-slip faults
Figure 1. Tectonicmap of the Cascadiasubductionzonefront withinthis survey(Figure3),
Juande Fuca plate system. ApproximateSeaMARC 1A intersectthe deformation
allowing
detailed
structural
analysis,
as well asdetermination
sidescan
coverage
from ThomasThompson
cruiseTT 020
shownshaded,boxesindicateapproximateAlpha Marine
Systems(AMS) 150-kHz surveyareas.
of fault slip rates.
Fault
Retrodeformation
The abundance
of seismicandsidescan
dataallowedus to
determinethe slip rates on five transversestrike-slip faults on
Wehave
identified
nineWNWtrending
transverse
structures
theOregon
andWfishington
margin.Themethod
forsliprate
deformingthe Cascadiaaccretionary
prismOff Oregonand
Washington. Eight 0f thesestructuresare left-lateral strike-
determinationis describedhere, as it is similar for all five
faults. We utilized the simple geometry of trenchward
slip faults based on measuredoffsetsof surfaceand subsurface
thickening
abyssal
plainsediment
wedges,
asdetermined
by
features.
Five of the transverse faults can be traced across the
at leastonetrench-parallel
andtwo trench-normal
reflection
plateboundaryinto the subducting
Juande Fucaplate,up to 21
profiles adjacent to each strike-slip fault (Figure 4). If the
km seaward of the deformation front. Documentation
of these
geometry of the pre-faulting trenchwardthickening wedges is
faults comesfrom sidescansonardata, swath bathymetry, simple: three profiles can define the wedge geometry
seismic reflection
data, and field observations from
sufficiently to use them as three-dimensional(3-D) piercing
submersibles.
In thispaperwe summarize
detailedsurveysof
points,the offsetsof which representsthe net slip of the
these faults on the central Cascadia margin of Oregon and
Washington and discusstheir origin, structural significance,
and implications for deformationof the submarineforearc.
fault. The three Oregon faults used many more profiles than
did the Washingtonfaults, which had the minimum numberof
profiles needed. Reflection data indicate that the geometry of
the wedges is simple; and the layers that bound them are
approximatelyplanar over the distancesinvolved. Goldfinger
Methods
et al. [1996a] also calculatedthe net slip of one fault (the
Data Acquisition and Processing
High-resolution sidescansonar data were collected with a
deep-towed
SeaMARC1A 30-kHzsystem
capable
of imaginga
2-kmor a 5-km swathwidthwithspatialresolutions
of 1 and
2.5 hi, respectively.An AlphaMarineSystems
(AMS) 150kHz system was i•sed to collect sidescan data on the
continentalshelvesof Oregonand Washington,with a 1-km
Wecoma fault) using isopach plots of two subsurfaceunits.
Using the wedge geometry capturestotal offset by the fault,
which
often
is underestimated
in
measurements
of
offset
isopachsdue to drag folding and local velocity effectsnear the
fault. The five faultsfor whichWeusedthistechnique
havea
verticalcomponent
of offset and showpronounced
growth
swathwidth and0.5-m resolution.Approximate
coverageof
strata on the downthrowh block; thus we were able to easily
distinguishsynfaultingand prefaultingunits (Figure 3). We
identified the point in the section at which interval thickness
thesesurveysis shownin Figure 1. All sidescandata were
located using Global Positioning System (GPS), then
processedusing Oregon State University's (OSU's) Sonar
block to thinner on the downthrown block. Synfaulting
sedimentary units could not be used for this determination
across the fault
reversed
from
thicker
on the downthrown
-127 ø
-126 ø
-125 ø
-124 ø
Vancouver
:123 ø
-122 ø
8219
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t
Ql.tcnpic
Peninsula
48 ø
shelf edge
5.5 _+
/i
6.7+_3
Gra• Harbor
47 ø
;)
juande '
z
Fuca
Willapa
Bay
plate
46 ø
-'-"::::•"-<::•:::
:8.5._+2.
.....
.......
::•;!:':5,7
+_
NorthAmerican
plate
S?':w':•:."'i
;"' 6.6 :.ñ........
45 ø
,Yaquina Bay
Alsea Bay
"::::::::::::::::::::::::::
. ":.:•: 0
:
'.::<.
.i'. :';'i"2':":::.-:.-:-.:ii
.-- .. •:i?•:;'-..
.. :;:!:.i•11:•:
' ";::.i;:L."..
'•i western
edge
of
Siletzia terrane
44 ø
' ":'.-:.:'-,w
::::"":.::F.;:•:-:;;11:'-:::'.-".9<:-'...
z
..........
;i:::•'•
.•"•.• ;S::•;:;:•/•::
-'•:.';::i
=======================':.'::!':.5'
:Z;:..:-':"-:i--"::**
Coos
Bay
43 ø
....
........
Cape Blanoo
0 - M>3,5
= M<3,5
shelf edge
":'.....
..:.:
.
....
42 ø
Figure 2. Active tectonicmap of the Oregon-Washington
continentalmargin Faultsand anticlines
shown;synclinesdeletedfor clarity. JDF-NOAM vectorof DeMetset al. [1990] is shown. Siletzia
terraneboundarybasedon magneticsandreflection-refraction
data. Strike-slipfaultsshownwith slip
rates.inmrn/yr. NNF, North Nitinat fault; SNF, SouthNitinat fault; WCF, Willapa Canyonfault; WF,
Wecomafault; DBF, Daisy Bank fault; ACF, Alvin Canyonfault; HSF, HecetaSouthfault; CBF, Coos
Basinfault;TRF, Thompson
Ridgefault.NF,NitinatFan;AF,Astoria
Fan.Majordepocenters
are
shownby stipple:WB, Willapa Basin;AB, AstoriaBasin;NB, NewportBasin;CBB, CoosBay Basin.
Submarine Banks are NB, Nehalem Bank; HB, Heceta Bank; CB, Coquille Bank. All offshore
seismicityis shown,with BlancoFractureZoneandGordaplateeventsremoved.Focalmechanisms
1,
2, and 3, are M 3.3, M 7.1, andM 5.8 slabearthquakes,
respectively,andare discussed
in the text.
8220
GOLDFINGER ET AL.: CASCADIA STRIKE-SLIP FAULTING
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GOLDFINGER ET AL.: CASCADIA STRIKE-SLIP FAULTING
a. Restored to pre-faulting geometry (600 ka)
...
.•.......
•' -.-•:.•::•....•:•-•);•:•::;:..
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8221
determinedthe fault offset required to producethe observed
thicknesschange for two pre-faulting units (units 2 and 3 in
Figure 3). If Z 1 and Z 2 are the thicknessesof the offset unit
acrossthe fault, at is the angle betweenthe bottom and top
contactsof the unit (defining the eastwardthickeningwedge),
and 13is the plan view angle betweenthe seismicprofile and
the fault, net fault slip (S) is then given by S = [Z1-Z2/tan at]
cos 13.For the three Oregon faults, we also used a trial and
error restorationusing the reflection profiles directly. To find
the best fit offset required to restore the abyssal plain
geometry, we iteratively tested the offset to find the best
match
of between
10 and 18 individual
reflectors
within
the
prefaulting section using the margin-parallel and marginnormal reflection profiles and correcting for the difference
betweenfault strike and profile strike (as with 13above). The
geometryused in both methodsis illustratedschematicallyin
b. Unrestored,post-faultinggeometry(present-day)
Figure 4. The error estimate includes the minimum and
seismic
profile
[5
Wecoma
fault
maximum fault separationthat could be accommodatedwithin
the interpretationof the seismicdata.
We estimatedthe age of faulting by convertingthe two-way
travel time to the base of the growth stratato depthin meters.
This conversion used an average velocity of 1680 m/s,
calculatedfor the upper 400 m of the Nitinat fan from Ocean
Drilling Program(ODP) drilling and reflectiondata [Hyndman
and Davis, 1992; Shipboard Scientific Party, 1994]. We
derived fault age by using a net sedimentation rate of 100
crn/1000 years for the Nitinat fan calculated from the 1993
ODP drill sites [Shipboard Scientific Party, 1994] and 110
cm/1000 years for the Astoria fan [Goldfinger, 1994;
Figure 4. Cartoon diagram illustrating strike-slip fault Goldfinger et al., 1996a] calculatedusing the thicknessand
restoration. (a) Abyssal plain section restored to age of the fan. To establish the age of the Astoria fan, we
prefaultingconfiguration(600 ka). (b) Unrestoredsection correlated a prominent seismic reflector at the base of the fan
as imaged on MCS line 37 (present-day). The three- from the Wecoma fault to Deep Sea Drilling Project (DSDP)
dimensional (3-D) wedgeboundingunits 2 and 3, as well Site 174A (70 km southeast)usinga seismicreflectionprofile
as individual reflector-bound intervals within the units,
connecting the drill site to the fault [Kulm et al., 1973a, b;
form the 3-D piercing points which were matchedacross Goldfinger et al., 1996a]. A profound lithologic change
the fault to determinefault slip. Unit 2 resolvedthe fault
offset somewhatbetter becauseit thickenedmore rapidly
to the eastthanunit 3. Relationusedto calculatenet slip is
given in the text. The variablesusedare shownhere:c• is
the angle betweenthe boundingreflectorsof the principal
seismicunits and B is the angle betweenthe strike of the
seismicprofile used in restoration,and the strike of the
fault. Z1 and Z2 are thicknesses
of the restoredunit in the
upthrownand downthrownblocks,respectively.Example
between
is for the Wecoma
section is subperpendicular to the sediment transport
direction.
Thus it is unlikely that significant time
transgressionhas occurredbetween Site 174A and the Wecoma
fault. For the other two Oregon faults (Daisy Bank and Alvin
Canyonfaults) we also usethe baseof the fan to estimatefault
age, although it is likely to be slightly older due to the more
southerly (distal) position of these faults.
Although
additional uncertainties exist in sedimentation rates, age of
fault.
because the presence of growth strata invalidates the
assumptionthat measuredgeometric changeson the profiles
result from horizontal fault offset only. In practice,we used
two methods to determine fault slip. For all five faults, we
determined the wedge geometry from the seismic grid, then
sand turbidites
of the fan and silt turbidites
of the
abyssal plain sequence was observed at the depth of this
reflector in the drill cores. Biostratigraphic analysis of the
coresfrom Site 174 yields an age of 760 + 50 ka (J. C. Ingle,
Stanford University, written communication, 1995). We
infer that the age of the base of the fan is approximatelythe
same at the Wecoma fault becausewe observeno significant
onlap or offlap relationshipswithin several hundred vertical
meters
of the base of the fan and the trend
of the seismic
the fan, and seismic velocities, we use the same estimate of
Figure 3. The (a) Wecoma,(b) DaisyBank,and(c) Alvin Canyonfaultsimagedon a N-S 144-channel
migrated reflectionprofile (MCS line 37) approximately3 km seawardof the deformationfront.
Seismicunitbounding
reflectors
usedfor faultrestoration
areshownin white. Baseof growthstratais
shownwith whitearrows.Growthstratathickenon thedownthrown
block;prefaultingunits2 and3
thin on the downthrownblock as a resultof strike-slipmotion. (d) Regionalseismicreflectionline
showingthe basementpopupstructures
(P) thatsimilarlyoffsetoceaniccrustat the threefaults. Fourth
faultunderthekilometer
scaleisa tearfaultin thesedimentary
section
only.
8222
GOLDFINGERET AL.:CASCADIASTRIKE-SLIP
FAULTING
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GOLDFINGER ET AL.: CASCADIA STRIKE-SLIP FAULTING
error (+ 50 kyr) here becausewe are unable to quantify these
errors independently.
The age of the fault is then simply the depth in meters of
the oldest growth strata, divided by the sedimentationrate in
meters per 1000 years. The age of first vertical motion for
each fault is calculatedin this way, and the net slip can then be
divided by the age to obtainthe slip rate of the fault. Any pure
strike-slipmotion (no vertical component)that occurredprior
to vertical displacementwould result in an underestimateof
the age of the fault, and thusour derivedslip rate valueswill be
too high.
In two cases(Wecoma and North Nitinat faults) we were able
to calculate slip rates independentlyusing measuredoffsetsof
late Pleistocenechannels(120 m and 150 m, respectively)and
estimated channel ages. Seismic reflection records showing
the truncation of deep-sea channel walls and numerous
sediment hiatuses in cores from the axial part of these
channelsdocument the erosive characterof the coarse-grained
late Pleistoceneturbidity currents in this region [Griggs and
Kulm, 1973]. We estimatethe age of the offset channelwalls
to be 12-24 ka, consistent with incision during the last
episode of high turbidity-current activity during the
Wisconsin low stand [Goldfinger et al., 1992a]. We assume
fault motion was constant during the Holocene and late
Pleistocene. Using this age range, we calculated a latest
Pleistocene-Holocene
slip rate of 8.3 + 4 and 8.5 + 4 mm/yr
for the North Nitinat and Wecoma faults, respectively. These
rates were similar to, but somewhathigher than, the estimates
from retrodeforming the fault. The higher late Pleistocene
slip rate is expected, since the point at which the growth
stratawere measured,originally locatedat the fault tip, is now
20 km landwardof the tip. Since fault slip ratesdecreasefrom
the centerof a fault toward its tip [Bilham and Bodin, 1992;
$cholz et al., 1993], the retrodeformation method using the
entire movement history includes low slip rates from the
fault's early history and shouldyield a lower averagerate. The
slip rate may also have varied in time over the life of the fault.
Cascadia Transverse Strike-Slip Faults
North
Nitinat
Fault
Two prominent Washington margin faults, the North
8223
fan (Figure 2). We surveyedthe NNF in 1993 usingSeaMARC
1A
with
both
2-km
and
5-km
swaths
and
coincident
Hydrosweepswathbathymetry. The 5-km swathsurveyshows
that the fault
intersects the deformation front at 47ø24'N,
strikes 282 ø, and extends 20 km seaward of the deformation
front into the Juan de Fuca plate. The NNF intersectsand
crossesthe deformationfront at a large slumpon the landward
vergent frontal-thrust anticline. The NNF cuts and offsets
slump debris within the arcuateslump, as well as the slump
scarpand the crestof the marginal ridge, 150 m left laterally
(Figure 5). At the deformationfront the fault offsetslandward
vergent protothrustsby 300-400 m. Figure 5 shows that
these structures and the frontal thrust anticline
bend 10-15 ø to
the left where they intersectthe NNF. A mud volcanostraddles
the fault near a right (restraining) bend 9 km seawardof the
deformation front (Figure 6). The 2 km swath revealed a
submarine channel parallel to the base of the continental
slope on the abyssalplain. This channel is cut and offset 150
m left-laterally by the NNF (Figures 5 and 6). The offset
channel wall has subsequentlyslumped (Figure 6), but the left
separationis still apparentusing the wider swath in Figure 5
to map the channel wall. We estimatethe age of the offset
channel wall to be 16-20 ka, spanningthe last Pleistocene
lowstand at 18 ka, when turbidite activity and channel
downcutting were at a maximum [Nelson, 1968; Griggs and
Kulm, 1973]. This timing is consistentwith activity in other
submarine channels on the Cascadia margin, although minor
channel cutting continuedinto the Holocene [Nelson, 1968].
The Holocene-latePleistoceneslip rate of this fault, basedon
the observed offset and inferred channel age, is 8.3 + 1.0
mm/yr, the error range reflecting uncertainty in the timing of
the channel cutting episode. Further evidence of the leftlateral motion on this fault is the presenceof a mud volcanoat
a restraining bend in the fault (Figure 6). We infer that the
mud volcano results from elevated pore fluid pressuresdue to
the transpressional
fault geometry,resultingin the extrusion
of deeply buried abyssalplain sediments.
Restoration of fault motion on the abyssal plain using
three seismicprofiles (University of Washington(UW) cruise
TT-063, 1971) yielded a net left-lateral slip of 2.2 + 0.3 km
for the NNF. The inferred age of the oldest growth strata
adjacentto the fault is 400 + 50 ka, which we take as the time
of initial fault motion on the abyssal plain near the
front.
We assume that horizontal
and vertical
Nitinat fault (NNF) and South Nitinat fault (SNF), were deformation
initially recognizedby one of us (L.D. Kulm) in a 1971 air gun motion began concurrentlyand were continuousover the life
seismicsurveyon the southernflank of the Nitinat submarine of the fault. From the calculatednet slip and age for the fault
Figure 5. (top) SeaMARC 1A sidescansonarimageof the intersectionof the North Nitinat fault and
the plateboundaryoff centralWashington.Light areasare highbackscatter.Fault offsetsthe landwardvergentthrustridge, slumpdebris,slumpheadwallscarps,andan abyssalplain channel. The fault is not
offsetlaterallyat the plateboundarythrust.Swathwidth is 5 km. (bottom)Interpretationof the sidescan
image. A local 20ø bend in the strike of accretionarywedge anticlinesoccursacrossthe fault, which
widensand branchesinto multiple tracesat the deformationfront. Offset of abyssalplain channelis
shownin detail in Figure 6. Extensiveslumpingof the backlimbof the landwardvergentfrontal
anticlineof the accretionarywedgemay be triggeredby strike-slipfaulting. Two episodesof slumping
are indicatedby the glide pathsof the youngerslump blockswhich have overriddenearlier slump
debris. Someinterpretationdetailsarebasedon 3-D stereovisualizationof the imageryandcoregistered
swathbathymetry.
8224
GOLDFINGER
ET AL.: CASCADIA
STRIKE-SLIP FAULTING
GOLDFINGER El' AL.: CASCADIA STRIKE-SLIP FAULTING
-125.98
-125.96
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:-:-5
•-....:.
•:...:::::::::::::::::::::::::::
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-125.94
8225
-125.92
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Kilometers
1
0
SLUMP
SCAR
47.22
47.2O
-125.28
-125.26
-125.24
-125.22
47.O8
47.O6
b
Figure 7. Two SeaMARC 1A 5-km swathsalong the North Nitinat fault showing the variable
deformation styles along this structure. (a) The third and fourth thrust ridges landward of the
deformationfront. The westernridge bendssharplyleft and is offset by individual E-W left-lateral
faults. The easternridge is also offset along a larger scarpsubparallelto the main trend of the NNF,
shownby white dashes. (b) The easternfault tip, showingen echelonfracturesat lower left, and a
northerntracethatwe interpretascontrollinga headwarderodingchannel.Overalltrendof fault zoneis
shownby white dashes.
we calculate
a sliprateof 5.5 + 2 mrn/yrfor theNorthNitinat continental slope was variable. We observed accretionary
fault. Air gun records from the University of Washington
surveydid not penetrateto oceanicbasement;thus we do not
know if the NNF
or SNF offset the basaltic oceanic crust.
We traced the NNF
southeastward
across the continental
slopewith sidescan
sonarusinga 5-kin swath. Evidenceof
faultingwas observedover a total lengthof 115 km from the
westerntip of the NNF to the upper continentalslope. The
expression of the NNF in the fold-thrust belt of the
wedge folds acrossthe lower and middle continentalslope
that were offset left-laterally, as well as sharplybent, and in
several casesoffset left-laterally by possibleR shears(black
arrows in Figure 7a). Subduedlinear traceswere observedin
the intervening synclinal basins. We also observedregions
of multiple parallel scarps,en echelon fault strands(Figure
7b), and sigmoidal bends in accretionary wedge thrust
anticlines. Determinationof net slip on the continentalslope
8226
GOLDFINGER
ET AL.: CASCADIA
STRIKE-SLIP
FAULTING
portion of the fault is problematic,as we know of no datable
piercing points. Based on the observation that net slip
We have mappedtwo major active reversefaults on the SW
Washington shelf that appear to control the location of the
reaches a maximum
Willapa Basin depocenter(Figure 2) [Cranswickand Piper,
1992] and Willapa Bay onshore (L. McNeill et al., Listric
normal faulting of the northern Oregon and Washington
continental margin, Cascadia subductionzone, submittedto
Journalof GeophysicalResearch,1996). Older structures
bend
sharplyseawardjust westof the mappednorthernfault (Figure
2). We investigated the northern of these E-W to WNW
trending structureswith the AMS 150 sidescansystem in
1994, suspectingthat this structuremight be related to the
SNF. We observedlinear WNW trendingbackscatter
patterns
in the sedimentbut found no surface faulting in the thick
at a fault's center, we infer that the
maximum horizontal slip is larger than the 2.2-km slip
observednear the seawardfault tip. The NNF is bestexpressed
in the accretionary wedge thrust ridges and is subdued or
absent in the intervening basins. This morphology is also
characteristicof the South Nitinat, Willapa Canyon, Wecoma,
and Alvin Canyon faults and, to a lesser extent, the Daisy
Bank fault. However, the strike-slipfaults in southernOregon
are well imaged even in the synclinal basins. We infer that
the principal reason for this difference in surficial expression
is probably related to a different origin for the southern
Oregon faults, discussedin a subsequentsection. Second,the
low vertical separation across the faults results in little
expression of faulting in the broad flat-bottomed basins in
northern Oregon and Washington as comparedwith the steep
continental slope of southernOregon. We speculatethat the
latitudinal difference in surfaceexpressionmay also be due to
the relatively high sedimentation rates in northern Oregon
and Washington as compared with southern Oregon [e.g.,
Sternberg, 1986; Barnard, 1978], which would tend to bury or
subdue fault traces in the synclinal basins. The greater
sediment supply is suggestedby the greater thickness of
abyssalplain sediment cover due to the Astoria, Nitinat and
Willapa submarine fans: 2.5-2.8 s two-way travel time off
northern Oregon and central Washington, 2.0 s off southern
Oregon.
South
Nitinat
Fault
The South Nitinat fault (SNF, Figure 2) intersects the
deformation front at 47ø05.3'N,
19 km south of the North
Nitinat fault. The SNF strikes 283ø, intersectingthe base of
slope at a 7.7-km left step in the deformation front, and
extends19 km seawardof the deformationfront on the abyssal
plain. The base of slope channel offset by the North Nitinat
fault also crosses the trace of the SNF adjacent to the
accretionarywedge, where the seawardflank of the first thrust
ridge forms its easternbank. The channel bends sharply left
as it crossesthe fault, suggestiveof left-lateral offset, but the
channel bathymetry is so subdued that offsetting
relationships could not be clearly distinguished in the
sidescandata. In a N-S seismicprofile (UW TT-063, line 32)
the SNF appearsvirtually identical to the NNF, with a downto-the-south vertical separationand thickened growth strata
on the downthrown block. Eastward thickening prefaulting
abyssal plain units thin abruptly from north to south across
the fault, indicating left-lateral slip on the SNF. By restoring
fault motion, we obtain a net slip for the SNF of 2.0 _+0.8 km.
The restoration reflects greater uncertainty than the NNF
restorationdue to the somewhatpoorer seismic record for the
SNF. The depth in the Nitinat fan section at which growth
strata appear on the downthrownside of the fault is 302 m;
thus, using the Nitinat Fan sedimentationrate of 100 cm/1000
years, the age of the fault is - 300 _+40 ka. From the age and
net left-lateral separationwe calculate a slip rate of 6.7 _+3
mm/yr for the South Nitinat fault. We obtainedsidescanand
Hydrosweep bathymetric data for the seaward40 km of the
fault and attempted, unsuccessfully,to survey the landward
part of the fault based on evidencefrom GLORIA regional
sidescandata. GLORIA-sidescanimagery suggeststhat this
fault may extend70 km fartherlandward.
Holocene
sediments.
Willapa
Canyon
Fault
The Willapa Canyonfault (WCF) intersectsthe deformation
front at 46ø18'N, 8 km south of the outlet of Willapa
submarine canyon onto the abyssal plain (Figure 2). This
fault cuts the accretionarywedge but doesnot extendinto the
abyssalplain as a detectable surface rupture. The seaward
projection of the WCF is crossed 4.7 km west of the
deformation front by University of Washington reflection
line TT 79-1, which showsno disruptionof the abyssalplain
section,confirming the lack of lower plate involvementwest
of the plate boundary. On the accretionarywedge,the Willapa
Canyonfault is poorly expressedover much of its lengthbut
is well expressednear its center,on the mid continentalslope,
where it offsetsleft-laterally a northwesttrendingchannelby
approximately900 m. The overall strike of the fault is 280ø.
No age information is available for the channel; thus we are
unableto determinethe age or slip rate for the WCF.
Wecoma
Fault
We surveyedthe entire length of the Wecoma fault using
SeaMARC 1A with a 5-km swathwidth. We investigatedthe
continental slope and outer shelf sectionsof the fault, as well
as the abyssal plain portion from a different illumination
angle than the 1989 survey. The new data supportearlier
interpretationsof the continuity of the Wecoma fault as an
active structureacrossthe slope [Goldfinger et al., 1992b;
1996a], and we were able to trace this fault to its apparent
eastern tip over a total length of 95 km (Figure 2). The
Wecoma, Daisy Bank, and Alvin Canyon faults each offset the
abyssalplain sedimentarysectionand the underlyingbasaltic
oceanic crust (Figure 3) [Goldfinger et al., 1992b, 1996a;
Appelgateet al., 1992; MacKay, 1995]. Near the deformation
front, all three fault zones have two main strandsdipping
steeplytowardeach otherthat definepopupstructures
(Figure
3d). The Wecomafault stepsto the right on the abyssalplain,
forming a restraining step. A doubly plunging anticline
between the two fault strands appearsto be causedby this
restrainingstep(left end of Figure 8). This upwarpis coredby
oceaniccrust, basedon the reflectiondata and modelingof a
magneticprofile over the upwarp [Appelgateet al., 1992;
Goldfingeret al., 1992b, 1996a].
Goldfingeret al. [1992b, 1996a] estimatethe ageof the
Wecomafaultto be 650 + 50 ka andthenetleft-lateralslipto
be 5.5 + 0.8 km. Fromthe ageandnetleft-lateralseparation,
they calculatea slip rate of 8.5 + 2 mrn/yrfor the Wecoma
fault. Offset of a dated channel 5 km west of the deformation
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8228
GOLDFINGER ET AL.: CASCADIA STRIKE-SLIP FAULTING
front allowedindependentcalculationof a slip rate of 8.5 _+4
mm/yr for the Wecoma fault [Appelgate et al., 1992;
Goldfingeret al., 1992b, 1996a]. Figure8 showsa SeaMARC
1A 5-km sidescanswath over the Wecoma fault on the abyssal
plain and lower continental slope. The surficial fault trace
splitsinto two faults defining a popupstructurejust seawardof
the fault and associated positive flower structure, which
abruptly truncate an adjacent syncline (Figure 10). OSU
sparker line SP-54 shows a similar truncationby the highangle fault, and a possiblesecondvertical strand2.2 km to the
north (Figure 11; 8 km easton the E-W profile).
the deformation
records terminated at 44ø54.8'N, 124ø35.93'W.
front.
Both strands extend across the frontal
two thrust anticlines, beyond which the northern strand may
die out. Between the two strands,the frontal thrust vergence
reverses to seaward from the regional landward vergence
direction. Multichannel reflection data do not clearly image
thesefaults in the complexaccretionarywedge. Nevertheless,
the sonarimagery showsshearzonesexposedat the crestsof
thesetwo ridges,as well as bendsand offsetsof the anticlinal
axes(Figure8). The secondthrustridgeis offsetby a seriesof
NE-SW trendingleft-lateralen echelonfaultsbetweenthe two
strandsof the Wecoma fault, and the secondanticlinal ridge
bendssharplyto the left as it crossesthe fault (Figure 8). The
third ridge terminates at the fault. Extensive methane-rich
fluid venting was observedalong the southernstrandon the
frontal thrust anticline [Tobin et al., 1993]. Bedding attitudes
taken from the submersibleAlvin show that dips are rotated
into parallelismwith the Wecoma fault within this samearea,
which is on the seaward limb of the seawardvergent frontal
thrust(Figure 8) [Tobinet al., 1993].
The Wecoma fault, like the Daisy Bank, Alvin Canyon, and
the three Washingtonfaults, is subduedin surfaceexpression
in the lower slope basins but prominent on the anticlinal
ridges and on the middle and upper continental slope.
SeaMARC 1A imagery showsthat the Wecoma fault consists
of multiple surficial fault tracesin a zone of deformationthat
increasesin width landward. Young folds on the lower slope
are commonly deflected or offset to the left at the fault and
show marked increased elevation near the fault.
On the middle
to upper slope, several prominent folds bend sharply to an
orientation parallel to the Wecoma fault from the regional
NNW fold trend. These and several other folds parallel the
Wecoma fault for a distanceof 15 km (Figure 9). The area
shown in Figure 9 appears to be either a large right
(compressional)step or complex deformation between two
strandsof the Wecoma fault. The right side of Figure 9 is near
the eastern tip of the Wecoma fault, and the sonar image
shows the fault bending toward the northeastin a probable
horse-tail splay. Fold axes in the southeastand northwest
parts of Figure 9 are offset left-laterally. Additionally, leftsteppingen echelon folds along the fault indicate the overall
shear senseof the Wecoma fault is sinistral [e.g., Wilcox et
al., 1973; Harding and Lowell, 1979; Sylvester, 1988], in
agreementwith the observedseparations.The entire Wecoma
fault zone lies in a broad parallel swale visible in SeaBeam
bathymetry. A proprietary seismic reflection profile shows
The
surface
trace of the Wecoma
fault
on the sidescan
This area of
the outermostshelf was covered with ripples and sandwaves
in unconsolidatedsand, confirmed by two Delta submersible
dives
in
1992
and
1993.
The
unconsolidated
and mobile
sedimentmade locatingthe easternfault tip problematic. The
Wecoma fault appearsto terminate near the inferred western
limit of the Siletzia terrane shown in Figure 2. The terrane
boundary is inferred on the basis of the strong magnetic
signature of the underlying Siletz block and by velocity
analysisof a wide-angle reflection profile just to the southof
the Wecoma fault [Trdhu et al., 1995]. SeaBeambathymetry
data and numerousreflection profiles show this boundaryto
mark a profoundchangein structuralstyle acrossthe forearc.
The rapidly deforming fold-thrust belt of the active
accretionarywedgegivesway to gentleopenfolds landwardof
the Siletzia boundary. SeaBeamdata reveal a seriesof short
NW trending en echelon folds just seawardof the boundary,
suggestiveof right-lateral shear. Snavely [1987] infers that
the dextral
Fulmar
fault controls
the location
of the seaward
edgeof the Siletziablock, and Trdhuet al. [1995] suggestthat
a fault observedoverlying the terrane boundary may be the
active Fulmar fault.
Our data also show a fault near the western
edge of Siletzia (eastern fault in Figure 11), although the
SeaMARC 1A datashowthis fault to be NW trending.Thuswe
are uncertain
whether
this structure
is related
to the $iletzia
boundaryor is the easterntip of the Wecoma fault.
Daisy Bank Fault
We surveyedthe entire lengthof the Daisy Bank fault (DBF;
fault B of Goldfingeret al. [1992b] using the SeaMARC 1A
system on the abyssal plain and slope and the AMS 150
system on the upper slope and shelf. Figure 3 shows the
Daisy Bank fault immediately west of the deformationfront.
The DBF extends 21 km seaward of the deformation
front onto
the abyssalplain where surfaceand subsurfaceexpressiondie
out. The main fault trace intersectsa 150-m-highridge along
the boundarybetweenthe landwardvergentthrustrampandthe
fault. MCS lines 37 (Figure 3b) and 19 (not shown)show
this ridge to be a southwestvergentthrust ridge boundedby
the DBF
on its southern flank.
The main strand of the DBF
stepsto the right at the westernend of this anticlinalridgeand
continuesto the northwest.We interpretthe fold as a pressure
ridge developed between the two overlapping fault strands
similar to the fold alongthe Wecomafault [Goldfingeret al.,
1992a; 1996b].
Basement reflectors show two offsets, one
Figure 8. (top) SeaMARC 1A sidescanimage of the Wecoma fault zone on the abyssalplain and
crossingthe frontal and secondaccretionarywedge anticlines. (bottom)Interpretation. The singlestrandedfault bifurcates,defininga popupstructurejust seawardof the frontalthrust. Both fault strands
crossboth ridgesand the interveningbasin,althoughthey are faint in the basin. The secondridge is
offsetby a seriesof possiblesyntheticleft-lateralfaults. The secondridgebendssharplyat the southern
fault strand,forming a steep,highly reflectivescarpat right. The third ridge (not shown)terminatesat
the southernfault strand. Strike and dip symbolsin and north of western shear zone from Alvin
observations [Tobin et al., 1993].
GOLDFINGER ET AL.: CASCADIA STRIKE-SLIPFAULTING
8229
8230
GOLDFINGER ET AL.: CASCADIA STRIKE-SLIP FAULTING
Figure 10. Line drawingof Shell Oil Companysparker
line 7380 on the middle continental slope off central
Oregon. Locationis shownon Figure 9. The Wecoma
fault truncatesa NNW trendingsyncline.We interpretthis
structure as a transpressionaloblique-slippingfault
segment
thatis accommodating
a regionalrightstepof the
Using SeaMARC 1A sidescanimagery,we tracedthe Daisy
Bank fault zone acrossthe lower continentalslope (Figure
12). As with the other northern Oregon and Washington
faults, the fault morphologyis subduedon the lower slope
relative to the upper slope or abyssal plain. The DBF is
characterizedby discontinuous
fault tracesthat disruptthrust
anticlinesand, to a lesserdegree,the interveningbasins. One
3 to 4-km long strandterminatesat the foot of a thrustridge,
producing gullies and a prominent slump scarp. Farther
seaward,severalsplaysof the DBF truncatethe frontal thrust
anticline of the accretionary wedge. Reversals of vertical
separationoccur along this ridge, with several tens of meters
of relief evident along the main splay. The DBF crossesthe
boundarybetweenthe Juande Fuca and North Americanplates
in a 1 km-wide fault zone that appears to have localized
slumpingof the seawardlimb of the frontal thrust.
Daisy Bank, on the upper continental slope, is one of
several uplifted Neogene structuralhighs off Oregon [Kulm
Wecoma
and Fowler, 1974]. The DBF bounds the southernflank of
Vertical
exaggeration
= 3.3:1
WECOMAFAULT
fault zone.
Daisy Bank; a second less prominent strand of the fault
boundsthe northernflank. SeaMARC 1A sidescanimagery
up-to-the-north
andoneup-to-the-south,
thatdefinea "popup" and multichannel (MCS) and single channel (SCS) seismic
reflection data show that the Daisy Bank fault is a wide
of thebasement
acrosstheDBF on MCS line 37 (Figure3b).
The intersection
of the DBF and the accretionary
wedge structuralzone, within which Daisy Bank is upliftedas a horst
marks an abrupt transition in structural domains between between two strands of the main fault. The main fault zone is
seawardvergentthrusts(seawarddirectedthrusting)southward 5-6 km wide northwestof Daisy Bank, wideningaroundthe
to 42ø10'N and landwardvergentthrusts(landwarddirected oblong bank, then narrowing to a single strand to the
thrusting)
northward
to BarkleyCanyonat 48ø12'N[MacKay southeast. The traces of the fault strands are straight,
et al., 1992; MacKay, 1995; Goldfinger et al., 1996b]. implying a near verticalfault. Probabledrag foldsof exposed
Goldfinger et al. [1996b] interpret a progressivevergence strata, with a left-lateral sense of motion, are visible in
reversalfrom southto north along a 15-km distancealong the sidescanimagerysoutheast
of the bank (Figure 12). Mapping
margin south of the DBF. The 1989 MCS data suggest from seismicreflectionprofilesindicatesleft-lateraloffsetsof
progressiveundercuttingof an originally landwardvergent NNW trending accretionarywedge fold axes at Daisy Bank
ridge by a younger seawardvergent thrust. The vergence (Figure 12). Scarp heights measured from the Delta
reversal that appears to be localized by the DBF has submersiblerange from tens of centimetersto 47 m. The net
apparently progressedfrom south to north, suggesting uplift of the southernflank of Daisy Bank by both foldingand
faulting is about 130 m. From Delta, we traced one of the
passageof the DBF beneaththe margin.
We determined the geometry of pre-faulting eastward Daisy Bank scarps into an area of low relief and mud
thickeningabyssalplain sedimentwedgesusing MCS lines deposition. We observeda fresh scarpstriking290ø across
05-08 for the Daisy Bank fault. Retrodeformationof the the unconsolidated Holocene mud, which is visible in the
abyssalplain sectionyielded a net left-lateral separationof AMS 150-kHz sidescanimages(Figure 12). Stratigraphic
2.2 +_0.5 km. The depthto the oldestgrowthstratais 360 m, relationships
indicate
post12-kamotionon thissegment
of
determined from MCS line 37. From the sedimentation rate of
94 cm/1000 years,we obtainan age of 380 + 50 ka for the
DBF. The age andnet slip valuesyield a slip rateof 5.7 + 2
mm/yr for the Daisy Bank fault.
WEST
0.4-
A_
wecøma
fa
theDaisyBankfault[Goldfinger
et al., 1996b].
The mapped
eastern
tip of theDaisyBankfaultoverlies
the
Siletziaterraneboundary
as doestheWecomafault,thuswe
inferthattheDaisyBankfaultprobably
terminates
against
EAST
ult
- 0.4
0.5-
0.6-
0.6 _•
0.7-
0.7•,
0.8-
0.8
Figure 11. OSU single-channel
sparkerprofileSP-54. A prominent
strandof the Wecomafault
juxtaposes
ananticline
andsyncline
at left. Thesecond
subvertical
faultat rightis probably
alsoa
strike-slipstructure
andmayeitherbe thedyingnorthernstrandof theWecomafaultor theFulmarfault.
SeeFigure 16 for locationandtext for discussion.
GOLDFINGER
ETAL.:CASCADIA
STRIKE-SLIP
FAULTING
o
o
8231
8232
GOLDFINGERET AL.: CASCADIA STRIKE-SLIPFAULTING
the Siletziaterrane,givinga mappedlengthof 94 km. Further
detailsof the DaisyBankfault are givenby Goldfingeret al.
[1996b].
Alvin
Canyon
Fault
The Alvin Canyonfault (ACF), namedfor its proximityto a
series of Alvin dive sites, is similar in structuralstyle to the
DaisyBankfault. Its structural
detailsarenot aswell known,
since it has not been surveyedwith sidescansonaron the
continental slope. The fault was mapped from 1989
SeaMARC 1A sidescandata on the abyssalplain and from
seismicreflectionprofiles and SeaBeamswathbathymetric
dataon the continentalslope.The Alvin Canyonfault extends
approximately
7 km seawardof the deformation
front,based
on sidescandata and two reflectionprofile crossingson the
abyssal plain. The intersectionof the fault with the
deformationfront is markedby a 3.7-km left stepin the basal
thrust and initial ridge of the accretionarywedge. Linear
patternsof authigeniccarbonates
mappedon the lowerslope
extend landward from the intersection of the ACF and the base
of slope,parallelto the fault trace [Carsonet al., 1994].
Carson et al. [1994] used reprocessedGLORIA sidescan
imagerywith the topographic
effectsof reflectivitysubtracted
from the image to identify zonesof carbonateprecipitation.
The highly reflectivecarbonate-bearing
sedimentsare the
productof fluid ventingof methane-rich
porefluidsthathave
been sampleddirectlyby submersible
in the immediatearea
[Kulrn and Suess,1990] and are commonlyassociated
with
fault zoneselsewhereon the slope [Kulrn and Suess,1990;
Tobin et al., 1993; Goldfingeret al., 1996b].
We retrodeformedprefaultingstratalwedgesusingMCS
lines 01-05 to define the geometryof prefaultingunits. We
calculatea net 2.2 + 0.5 km of left-lateral separationfor the
Alvin Canyonfault. We estimatea sedimentation
rate of 79
cm/1000yearsfor the Astoriafan sectionat the Alvin Canyon
fault. A depth of 310 m for the oldestgrowth stratawas
determinedfrom MCS line 37 usingthe depthin two-waytime
and the sedimentation
rate. Using the sedimentation
rate and
depth values, we obtain an age of 380 + 50 ka for the
initiation of motion on the fault. These values yield a slip
rate of 6.2 + 2 mm/yr for the Alvin Canyonfault.
Like the Daisy Bank fault, the Alvin Canyonfault widens
from a singlestrandon the abyssalplain to a 6-km-widefault
zoneon the upperslope. The Alvin Canyonfault hasstrong
morphological
similarities
to thebettersurveyed
Wecomaand
DaisyBankfaults. An unnamed
submarine
bankis upliftedas
a horst between two strands of the fault, similar to the
structuralsettingof Daisy Bank. Like Daisy Bank, this horst
hasbeentruncatedby erosionduringPleistocene
low sealevel
stands.On the abyssalplain, Holoceneseafloorsediments
are
offsetby the Alvin Canyonfault, suggesting
that it is likely
to be an active structureon the slope, but this cannot be
confirmed without further investigation.
Heceta
Bank
Structure
and Heceta
South
Fault
The Heceta Bank structure is centered at approximately
uplifted bank. Swathbathymetricdata from a NOAA shallow
water system (BSSS) revealed that Heceta Bank has at least
one submergedPleistocene lowstand shoreline flinging its
western and northwesternflanks (C. Goldfinger, unpublished
data, 1993). A smooth wave-cut platform seaward of the
former shorelineis clearly visible for a length of 45 km in the
bathymetric data, as are former beach berms and probable
subaerialdrainagefeatures. The submergedshorelineis tilted
to the south, deepeningfrom 115 m at the northernend of
HecetaBank to 225 m adjacentto the HecetaBank structure.A
possiblecontinuationof the submergedshorelineand wavecut platform southof the fault suggests> 200 m of vertical
separationacrossthis structure. However, a Klein 50-kHz
sonar survey in 1992 failed to identify any offset surficial
featuresfrom which to characterizethe horizontal separation
(if any) acrossthis structure. Sidescandata from this survey
and subsequentdives with the submersibleDelta identified
areas of tabular carbonatedepositionindicative of methanebearingfluid venting at the top of the scarp,but this evidence
was subduedin comparisonto other active faults. The scarp
itself had a relatively low slope, and no evidence of surface
rupturewas found in severalobliquetraverseswith Delta, nor
on the sidescan imagery. An unmigrated multichannel
reflection profile (USGS line WO 77-05) shows only weak
evidence for faulting and suggestsa principally monoclinal
structure. The present surface morphology may in part be
erosional accentuation of the monocline by wave action
during Pleistocenelow stands. We concludethat either the
Heceta Bank structureis a monocline(presumablyoverlyinga
more deeply buried fault) or a strike-slipfault in which lateral
motion is not resolved on the reflection profile.
The Heceta South fault lies 15 km west of the Heceta Bank
fault and was originally mappedas being the same structure
[Goldfinger et al., 1992a]. Subsequentinterpretationof
SeaMARC 1A sidescanand swathbathymetrydata showsthat
they are two separatestructures.The Heceta Southfault is 35
km in length and is composedof multiple segmentsstriking
2930-325ø (Figure 13). The scarpof the seawardfault segment
forms the northern rim of a slump scar on the lower
continentalslope (Figure 13). This slump,approximately7.5
km in diameter,
involvedapproximately
14 km3 of accreted
material from the accretionmywedge. Of three known slumps
of this magnitude on the Oregon and Washington margins,
two occur at the intersectionof WNW trending strike-slip
faults with the deformation front (North Nitinat fault, Figure
5; Heceta South fault, Figure 13). The Heceta South fault
appearsto extend several kilometers into the abyssal plain,
suggestedby the linear truncationof debris from the slump
along the projection of the fault, although no seismic
reflection data crossingthe possibleabyssalplain extension
of the fault are available to confirm this. Alternatively, the
linear trendsin the slump debrismay have originatedalong
the fault prior to slumpingand were then translatedonto the
plain duringthe slumpevent. Sidescanandbathymetricdata
revealed no piercing points from which to determine
horizontalseparationfor this fault.
43ø56'Non the continentalslope(Figure2), strikes308ø, and Coos Basin Fault
hasa minimumlengthof 16-20 km (Figure13). This structure
The Coos Basin fault intersectsthe base of slopeat a 1.2bounds the southeasternflank of Heceta Bank, a complex
frontat 44ø04'N(Figure2).
anticlinorium that has undergoneup to 1000 m of post- km left stepin the deformation
Mioceneuplift [Kulrnand Fowler, 1974]. The HecetaBank We observedno evidencefor abyssalplainruptureseawardof
structureis a strikinglinearfeaturethat abruptlytruncates
the the intersection of the fault and the deformation front in
GOLDFINGER
ET AL.:CASCADIASTRIKE-SLIP
FAULTING
o
8233
8234
GOLDFINGER
ET AL.: CASCADIA STRIKE-SLIP FAULTING
sidescanimages. Near-surfacefaulting approximatelyalong
the seaward
extension
of both
the Coos
Basin
fault
and
Thompson Ridge faults is visible on two-channel seismic
reflection records on the abyssal plain [EEZ-SCAN 84
Scientific Staff, 1986]. These faults could not be tied to the
observedupperplate faults,nor are their strikesknown from
the single seismic profile. The surficial expressionof the
Coos Basin fault is an approximately 5-km-wide zone of
deformation, the major elements of which are visible in
GLORIA sidescanimagery. Two main scarpsoffsetanticlinal
ridgeson the lower slopeleft-laterally,and the broadzone is
expressedbathymetricallyas linear trendsstriking2880-293ø.
Two
measured
left-lateral
offsets of surficial
and 158 m are visible in a 10-km-wide
collected
in 1993.
features of 125
SeaMARC
The two well-defined
1A mosaic
strands of the fault
diverge southeastwardinto a broader structuralzone which
loses definition
about
35 km landward
of the base of the
continental slope.
Thompson
Ridge Fault
The Thompson Ridge fault intersects the base of the
continentalslopeat a 6 km left stepin the deformationfront
at latitude 43ø16.5'N (Figure 2), and, like the Coos Basin
fault, does not extend into the abyssalplain as a surficial
featurebasedon SeaMARC 1A imageryandSeaBeamdata. The
ThompsonRidge fault is the best expressedbathymetrically
of the nine Cascadiastrike-slipfaults mappedto date. The
main scarpat the deformationfront along the left step is 650
m in height, north block up. The fault zone is clearly
observedin SeaBeambathymetryas a patternof disruptionof
accretionarywedge thrust ridges,and like many of the other
faults, its component strands diverge slightly to the
southeast.Figure 14 showsa perspectiveshaded-reliefimage
of NOAA SeaBeamswath bathymetrydata in the Thompson
Ridge fault area, gridded to a 100-m point spacing. In map
view, a patternof left stepsand sigmoidalbendingand offset
of crossingfolds indicatesleft-lateral shear. The SeaBeam
bathymetry suggeststhree left stepping anticlinoria (Figure
section and the basement are offset by these structures
[Goldfinger et al., 1992b; 1996a; Appelgate et al., 1992;
MacKay, 1995]. These three faults dip steeply to the
northeast and bound the southern flanks of asymmetrical
basementpop ups (Figure 3d). Magnetic modeling of the
Wecoma
fault zone indicates that the basement is both offset
and significantlyupwarpedbeneatha pressureridge anticline
formed at a right step in the fault trace [Goldfinger et al.,
1992b; 1996a; Appelgate et al., 1992]. The basementpopups
clearly show involvement of the basementin the transverse
deformation of the Cascadia forearc, and indicate that these
three faults are slightly transpressional.The reflection data
for the Washington faults did not reach basement;however,
their striking similarity to the Oregonfaults suggestssimilar
origins and thus probable basementinvolvement.
Although the 1989 reflection data clearly resolvebasement
offseton the threeOregonfaults,we are unableto discriminate
between throughgoingrupture of oceanic crust or a more
superficialdetachmentof upper crustalblocks such as may
have emplaced basaltic blocks in the Franciscansubduction
complex in northern California [e.g., Kirnura et al., 1996].
However,high pore fluid pressureand very low wedgetaperin
northern Oregon and Washington, discussedfurther below,
suggestthat the lower continentalslopeis poorly coupledto
the subductingplate. The basementoffsetsobservedseaward
of the deformation front, therefore, are unlikely to be the
result of detachment
of basaltic
blocks as a direct result of
interactionbetweenthe accretionarywedgeand the slab.
We note a consistentlongitudinal pattern of deformation
along the faults that extend seawardof the base of slope:
strong expressionon the abyssal plain, poor expressionon
the lower slope,and strongexpressionon the upperslopeand
outermost shelf.
For several reasons, we infer that this
pattern is most consistentwith faults that originate in the
slab and propagateup throughthe upper plate. Pore fluid
pressurein accreted sedimentsnear the plate boundary is
known to be high. Negative polarity reflections in
d6collementzones at the toe of the Oregon slope and a
subhorizontalmaximum compressivestresssuggesthighly
14), indicatingthe overallshearsenseof theThompson
Ridge
overpressured
dilatant zones at the plate boundary[Moore et
fault is sinistral. Other related folds parallel the strike of the
al., 1995a]. Fluid pressures in the range of 0.9-0.95
fault zone. Crossing thrust ridges step to the left and are
lithostatic are estimatedfor the Barbadosaccretionarywedge
elevated at the fault zone, a distinctive morphology also
based on recent logging from drilling resultsfrom ODP Leg
observed at the Wecoma and Daisy Bank faults. This
156 [Moore et al., 1995b]. During the 1993 SeaMARC 1A
elevation difference, in the case of the Wecoma fault, is the
survey
we observedmud volcanoeson the lower slope and
result of a compressional
flower structurecomposedof fault
splayswith a small reversecomponentof motion basedon plain off Oregon and Washington, implying lithostatic fluid
pressureswithin the accreted section. The lower slope off
seismic reflection records.
We infer that much of the
bathymetricexpressionof the strike-slipfaults observedon northernOregon and Washingtonhas a very low wedge taper
the slopeis due to the superimposition
of flower structures
on and widely spacedlandwardvergentfolds,furtherindicationof
the coevalor olderthrustridges.Equipmentdamageprevented a very weak d6collementdue to high porefluid pressurein this
the acquisitionof all but a shortsegmentof sidescandataover region [Seely, 1977; Davis et al., 1983; MacKay, 1995]. We
speculate that the longitudinal pattern of morphologic
the ThompsonRidge fault.
expressionalong the transversefaults can be explained by
reduced transmission of lower plate fault slip across the
Discussion
overpressuredand poorly coupled d6collementbeneath the
lower
slope. Rejuvenationof the faults on the upper slope is
Basement
Involvement
consistent with progressive dewatering of the wedge,
Five of the strike-slip faults (North and South Nitinat, resultingin better interplatecouplingas would be expectedfor
Wecoma, Daisy Bank, and Alvin Canyon faults) crossthe the rearwardpart of the wedge. The landwardwidening fault
plateboundary
andareobserved
in bothupperandlowerplates zones(Figure 2) are also consistentwith fault slip transmitted
(Figure2). Reflectiondatafor thethreeOregonfaultssuggest upward through the eastwardthickeningaccretionarywedge.
that seawardof the deformation
front,the entiresedimentary The slip distribution on the Wecoma fault, determined by
GOLDFINGER ET AL.: CASCADIA STRIKE-SLIP FAULTING
8235
':.;!•.?.....,
.. ß
...
ß..•?::•.
....
•.:::...:
::::::::
======================
8236
GOLDFINGER E-'I'AL.: CASCADIA STRIKE-SLIP FAULTING
GOLDFINGER
ET AL.:CASCADIASTRIKE-SLIP
FAULTING
isopachplots of two abyssalplain stratigraphicunits, shows
that slip increaseslandwardalong the fault from its western
tip to the frontal thrust (Figure 15), landward of which
isopachingwas not possible[Goldfinger et al., 1996a]. The
observeddecreasein morphologicalexpressionacrossthe
plate boundary,where slip is still increasingin lower plate
units, is most consistentwith the fault propagatingupward
acrossthe poorly coupled d6collement.Alternatively, older
episodesof slip may be preservedat the landwardends of the
faults but may be overprintedby compressionaldeformation
8237
seaward geometry of the Cascadia subduction zone off
Washington. This geometryis inferred to resultin archingof
the slab beneath Washington, with a more steeply dipping
slab both to the north and to the south [Michaelson and
Weaver, 1986; Crosson and Owens, 1987]. This slab
observedin SeaBeambathymetry,may be sucholder traces.
What mechanisms
mightbe responsible
for transverse
rupture
of the subductingslab? Three modelsthat mightexplainslab
rupture are interplate coupling stresses,membranestrain due
to slab geometry, and slab stressesimposedby slab-mantle
configuration results in strike-parallel membrane strain as
modeled by Creager et al. [1995, Figure 3] for the Bolivian
syntaxis. Subductionof the JDF slab is expectedto result in
similar N-S compression,and this model is supportedby
sparsefocal mechanisms[Spence, 1989]. A strike-slip focal
mechanism indicating right-lateral slip on a NW striking
plane or left slip on a NE striking plane is located 30 km
seawardof the westerntip of the Daisy Bank fault (Figure 2)
[Spence, 1989]. Either nodal plane for this event is
consistentwith N-S compression;however, N-S compression
in the slab is inconsistentwith the nearbyWNW striking leftlateral faults. These faults shouldbe dextralif they are driven
by N-S compression. The evidencefor N-S compressionjust
seaward of the left-lateral faults, though sketchy, suggests
that the principal horizontal stressdirection is significantly
different seawardof the plate boundarythan in the subducting
slab and suggeststo us that membranestraindue to subduction
geometry or other mechanismsmay be operative but that it
must be overprintedby someother mechanismthat drives the
interaction.
sinistral
on the lower slope.
We infer that the significant deformation of the
accretionarywedge is the expressionof faulting originating
in the subductingplate, transmittedto a passivelydeforming
upper plate. The passageof the basementfaults beneaththe
accretionarywedgeshouldleaveolderfault tracesthat may be
progressively overprinted by compressional deformation
(Figure15). The four faultswithoutlowerplateexpression,
as
well
as other
transverse
folds
and linear
features
These models are discussed below.
we have
transverse
faults.
Recently, Scholz and Campos[1995] proposeda dynamic
model of interplate coupling and decoupling at subduction
Origin of the Transverse Faults
zones that incorporates the hydrodynamic resistanceof the
Giventhe evidencefor deformation
of bothplates,in which motion of the slab through the viscous mantle. In their
plate did the deformationoriginate? Wang et al. [1995] model, the mantle is considered stationary in a hot spot
calculate that interplate stresses<10 MPa result from JDF referenceframe. This "sea anchor"force, in a mantle system
platesubduction,
consistent
with low observed
stressdropsin that is fixed to the hot spotframe, shouldhavetrench-parallel
subductionearthquakes,assumingthat stressreleaseis close and trench-normal components in the case of oblique
to complete [e.g., Kanarnori, 1980; Magee and Zoback, subduction[Scholzand Campos,Figure 1] (Figure 15). The
1993]. High heatflow in Cascadiaplacesthe downdipbrittle trench-parallelcomponentof hydrodynamicmantle resistance
ductiletransitionfor crustalrocksin the outer-shelf-upperto subductionis an unbalancedforce that setsup a shearcouple
slope region [Hyndrnan and Wang, 1995]. Between the in the plane of the slab. For Cascadia,the shearcouplewould
poorly coupled accretionarywedge and the shallow brittle be dextral, and antithetic shears should be left-lateral and eastductiletransitionmay be a very narrowzoneof significant west to northwesttrending. We proposethat this shearcouple
interplatefrictional stress(C. Goldfinger,manuscriptin may be responsiblefor the observedtransverseruptureof the
slab we have observed in the central Cascadia forearc. Limited
preparation, 1996). We estimate that this narrow zone of
couplingis unlikely to be responsiblefor ruptureof the slab seismicity is consistent with a model of transverse
oceaniclithosphere,which has a shearstrength,for any shearingin the subductingslab. The largest instrumentally
conditionsof temperature
and porepressurewithin the brittle recordedearthquakein Cascadia(1949, mb= 7.1; 47.13øN,
regime, greater [e.g., Brace and Kohlstedt, 1980] than the 122.95øW [Baker and Langston,1987]) occurredin the Puget
Soundregion in Washington(Figure 2) at a depthof 54 km in
interplatecouplingstresses
inferredby Wanget al. [1995].
Another possible scenariofor intraslab deformation could the upper part of the subductingplate (Figure 2) [Baker and
be the introduction of membrane strain due to the concave Langston, 1987; Ludwin et al., 1991]. The focal mechanism
Figure 15. Blockrotationmodelfor thecentralCascadia
forearc.SeaBeam
bathymetry
shaded
from
the north. The WecomaandDaisyBank faultsare shown,with the DaisyBankfault exposedin
foreground.Well-mapped
faulttracesarein solid;discontinuous
tracesaredashed.The arc-parallel
component
of obliquesubduction
createsa dextralshearcouple,whichis accommodated
by WNW
trendingleft-lateralstrike-slip
faults.We propose
thatshearing
of theslabdueto obliquesubduction
is
responsible
for thefaultsinvolvingoceaniccrust. "Seaanchor"forceandcomponents
shownat lower
right[Scholz
andCampos,
1995].WF, Wecomafault;DBF,DaisyBankfault;FF,Fulmarfault; "pr"
pressureridge;"DB", Daisy Bank. "OT?",possibleold left-lateralfault strand. Arrow headsandtails
showstrike-slip
motion.Whitearrowsat western
endof Wecoma
faultshoweastward
increasing
slip
calculatedfrom isopachoffsets[Goldfingeret al., 1996a]. Dots at easternend of Wecomafault indicate
the locationof the faultsshownon Figure 11. Schematicblockrotationmodelshownat lower left,
showingdeformation
of parallelogram
ABCD in dextralsimpleshear[afterWellsandCoe, 1985].
8238
GOLDFINGER
ET AL.: CASCADIA
A
•k
I
R and
Thrust
Extension
R' Shears
Faults
Fractures
Folds
All
structures
Superimposed
STRIKE-SLIP FAULTING
is strike-slip with a preferredfault plane striking east-west_+
15ø (azimuth 255-285 ø) and has nearly pure left-lateral slip
[Baker and Langston, 1987].
While this strike-slip
mechanismindicatessheafing within the Juan de Fuca plate,
the fault planes of the 1949 event are not parallel to the
convergencedirection, negating a possible tear fault origin
[Weaver and Baker, 1988]. A similar left-lateral strike-slip
solution was determined for a small 1981 earthquake
(magnitude 3.3) in the upper Juan de Fuca plate on the
southernWashingtoncoast (Figure 2) [Taber and Smith,
1985; Weaver and Baker, 1988]. The seismicity plotted on
Figure 2 reveals several other interesting patterns. The
Washingtoncoast strike-slip event lies at the easternend of
an east-westlinear trend of earthquakesthat lie roughlyon the
projectionof the SouthNitinat fault, and are near a major eastwest reversefault mappedon the shelf. Two eventsplot near
the Wecomafault, and severalplot near the landwardend of the
Alvin Canyon and Willapa Canyon faults. The depthsand
locations of all of these events, however, are suspectdue to
poor azimuthal station coverage (R. Ludwin, personal
communication, 1995).
Could the basement involved
C
faults be reactivated structures
in the oceanic basalt? Comparison of the strikes of the
oblique faults to the seafloor magnetic anomaly map of
Wilson [1993] shows that the three Washington faults, with
D
strikes of about 283 ø, could be reactivated small fracture zones
C
A
perpendicularto the magnetic anomalies.The Oregon faults
strike 292 ø to 325ø, deviating from an orthogonal to the
magneticlineationsby 12ø to 45ø and thus are unlikely to be
related to preexisting basement structure. We have also
consideredconjugate shorteningof the forearc of the type
describedby Lewiset al. [1988]; however,extensivemapping
of margin structures[Wagner et al., 1986; Clarke, 1990;
Goldfinger et al., 1992a; C. Goldfinger and L. McNeill,
manuscriptin preparation,1996] has not identified structures
conjugateto the WNW faults. We have mappedtwo majorNE
trendingstructures,one on the slopeoff northernOregonand
one off centralWashington,but both of theseare clearly leftlateral tear faults of the accretionaryfold-thrustbelt.
Kinematic
T1
T1
Model:
Upper
Plate Deformation
Northeasterlydirectedsubductionresultsin a dextral shear
couple in the North American plate and, as suggestedabove,
possiblyin the Juande Fuca plate as well (Figures15 and 16).
Figure 16. Strike-slipmodels,convergence
shownat 062ø
for centralOregonusingpoleof DeMetset al. [1990]. (a)
Simple shearmodel showingR, R', and P shears.(b)
Structuretypesandorientations
expectedin overallright
simpleshear[after Sylvester, 1988]. (c) Block rotation
modelin a dextralshearcouple[afterWellsand Coe,1985]
(d) Map viewinteraction
of basement
strike-slip
faultwith
a growingaccretionary
wedge. Shadingindicateslower
plate; no shadingindicatesupperplate. (top) Time 1,
(bottom)time 2. Remanenttraces,showndashed,
may
remain as active structuresor be overprintedby
compressional
deformation.As the basementfault moves,
theaccretionary
wedgeadvances,
maintaining
theyouthof
the intersection
point.
GOLDFINGER
ET AL.: CASCADIA
The existenceof active sinistralfaults in the upper plate, both
related and unrelated to basementfaulting, suggeststhat the
North American plate may be respondingto both passive
strain transmittedfrom the slab and to dextral shear imparted
STRIKE-SLIP
FAULTING
8239
0
ZZ
ZZZ
by interplate coupling. In the following discussionwe
presenta model for deformationof the upperplate, which is
observable,but a similar model may also be operativein the
subductingslab.
Pure shear deformation of the Cascadia margin is well
expressed
in the fold andthrustbelt of the accretionary
wedge.
For the most part, these structuresare subparallel to the
margin and representthe responseof the upper plate to the
normal component of plate convergence. In model
experiments of simple shear and in earthquake ground
ruptures,five setsof fractureshave been observed:R and P or
synthetic shears, with the same motion sense as the simple
shear couple; R' or antithetic shears, with a motion sense
opposite to the main shear; tension fractures or normal faults
oriented at about 45 ø to the main shear zone; and Y shears or
faults parallel to the shear couple. Strike-slip, reverse, and
normal faults of the orientations shown in Figure 16b are
consistentwith a simple shear model of overall right-oblique
shear [e.g., Wilcox et al., 1973; Sylvester, 1988, and
referencestherein], althoughnone of the experimentalmodels
involved subduction-drivensimple shear. Comparisonof the
right simple shearmodel (Figure 16) with the structuralmap of
the Oregon-Washingtonmargin in Figure 2 suggeststhat the
WNW
left-lateral
faults are consistent with R' shears antithetic
to the right shearcoupledriven by oblique subduction.WNW
to NW trending folds and thrust faults of the middle
continental slope to outer continentalshelf are also expected
in the right simple shearmodel. Y shears,or faults parallel to
the main shear couple, would be difficult to detect within the
similarly oriented structural grain of the accretionarywedge.
We note that the three best mappedfault zonesthat crossthe
deformation front, the North Nitinat, Wecoma, and Daisy
Bank faults, bend 3-6 ø southward on the continental slope
from their abyssal plain strike (Table 1). The south bending
of these two fault zones could be due either to passageof the
basement faults beneath the wedge (Figure 16d) or a
componentof dextral arc-parallelsheardistributedacrossthe
accretionarywedge, similar to that suggestedfor the onshore
forearcby England and Wells (1991).
In the case of the forearc rotation we propose, the shear
couple is set up by oblique insertionof the subductedplate
into the mantle, and thus two margin-parallel faults may not
be required. Linkage betweenthe upperand lower plate faults
for anything but a very short time requires slip along the
inboardedge of the rotatedterranein the upperplate. Trdhu et
al. (1995) suggestthat a small fault observedto overlie the
edge of the Siletzia terranein central Oregon may representan
active dextral fault that decouplesthe active wedge from the
inboard Siletzia terrane. Our data are somewhatsupportiveof
this hypothesis. We observe a vertical, probably strike-slip
fault (easternfault in Figure 11) that also overlies the edge of
the Siletzia terrane (see Figure 2 for location). SeaMARC 1A
data show this fault to be NW trending, and we initially
correlated
-H -H c•. -H -H -H c•. c•.
it as the northern
strand
of the Wecoma
fault.
Alternatively,it could be the samefault shownby Trdhuet al.
[1995], 7 km to the south. SeaBeambathymetryrevealsthat
the edge of the Siletzia terraneis overlainby a north trending
zone of short, doubly plungingen echelonfolds suggestiveof
•g•
•o..•
<<
8240
GOLDFINGER ET AL.: CASCADIA STRIKE-SLIPFAULTING
dextral shear at depth (shown on SeaBeambathymetryin
Figure 15)
A geometricrequirementof a set of parallelstrike-slip
faults with the same motion sense is that the intervening
blocks and the faults themselvesmust rotate (Figure 16) [e.g.,
Freund, 1974]. We infer that the Cascadialeft-slip faults
segmentmuch of the continental slope in Oregon and
Washingtoninto elongateblocks rotatingclockwiseabout
verticalaxes. This styleof deformationhasbeenproposed
for
the Aleutianforearcon a muchlargerscale[Geistet al., 1988;
Ryan and Scholl, 1993] and has been documented
in other
tectonicenvironments[e.g., Cowan et al., 1986;Beck, 1989;
Garfunkel, 1989; Jackson and Molnar, 1990]. Although
Cascadialacks seismicity-definedblocks, paleomagnetically
determined clockwise rotation of basalts in western Oregon
and Washingtonindicatesthat clockwiserotationhasoccurred
throughoutmostof the Tertiary. A smoothcoastwardincrease
in the rotation of the Miocene Gingko and Pomonamembers
of the CRBG
is consistent
with
a model
of distributed
deformation of the forearc and inconsistentwith microplate
docking models [England and Wells, 1991]. The viscous
sheetdeformationmodelof Englandand Wells [ 1991] implies
a northwardtransportof the forearcat 13-15 mm/yr sincethe
Miocene, or about 75% of the presentmargin-parallelplate
convergence. Such northward transport of the forearc is
consistentwith our simple shearmodel.
A block rotation model implies geometricspaceproblems
at the marginsof the rotatedblocks. If the pivotsare fixed to
the North Americanplate, compression
betweenthe blocksis
required. Such compression
is observedalongthe Wecoma,
Daisy Bank, Alvin Canyon, and ThompsonRidge faults off
Oregon but not along the Washington faults. Without
compression
alongthe block edges,the blocksmusttranslate
northwardrelative to the North American plate. Small gaps
and overlapsoccurat the endsof the blocks(Figure 16). On
the abyssalplain, compression
is taken up by anticlinesand
horsetailsplaysnear the westerntips of the Wecoma,Daisy
Bank, and Alvin Canyonfault and by splaysat the North and
South Nitinat
faults.
At the landward
block ends, the
complexity of the accretionarywedge is such that minor
deformation
such as this would be difficult to discern.
small offsets of the frontal thrust and the relatively straight
trends of the fault zones across the slope suggestthat the
accretionarywedge and subductingplate are not converging
along the expectedJDF-NOAM plate vector. The forearcmay
deforming internally and/or translating northward due to
oblique subduction[Pezzopaneand Weldon,1991; Wells and
Weaver, 1992; McCaffrey and Goldfinger, 1995].
Alternatively, subductionhas ceased or dramatically slowed
within the last 0.6 Ma. The latter hypothesisconflicts with
abundantevidence for modem subductionincluding Holocene
shorteningof the accretionarywedge apparentin numerous
seismic reflection records throughoutthe Cascadia margin;
onshoregeodetic data indicating shorteningapproximatelyin
the convergence direction [Savage and Lisowski, 1991;
Dragerr et al., 1994]; and paleoseismologicevidenceof large,
probably subduction-related
earthquakesin the coastalbays of
Oregon, Washington, and California (e.g., Atwater, 1987,
1992; Darienzo and Peterson, 1990; Nelson et al., 1995).
The continuity of the oblique strike-slip faults acrossthe
plate boundaryand the evidencethat subductionhas occurred
through the Holocene to the present suggest to us that the
submarine forearc is translating northward, either as a
microplate or in small block translations and or rotations.
The extensiveinternal deformationof the forearc supportsthe
latter hypothesis. We find supportfor northwardtranslation
in the active structuresof offshore Washington. Structural
trends of late Quaternary folds and thrust faults off western
Washington are nearly north-southon the continentalshelf
(Figure 2) [Wagneret al., 1986; C. Goldfingerand L. McNeill,
manuscript in preparation, 1996], and are parallel to the
margin (~ 020ø) on the continentalslope. Contemporaryfold
and thrust trends in the Juan de Fuca Strait, between
Washingtonand VancouverIsland, strike NW, parallelto the
strait. Active shorteningis apparently occurring acrossthe
strait, in contrast to the east-west shorteningoccurring west
of Washington, suggestingthat the Washington forearc is
colliding with a more rigid Vancouver Island buttress, as
suggestedby Wells and Weaver [1992].
Goldfinger [1994] and McCaffrey and Goldfinger [1995]
calculated
the total rate of forearc
deformation
in the arc-
parallel direction as a result of slip on the nine strike-slip
The fact that five of the nine mappedfaults crossthe plate faults (Table 1). A componentof extensionbetween points
boundarymust be reconciledwith northeasterlysubductionat on any two blocks occursas a result of left-lateral motion on
40 mm/yr. The JDF plate shouldhavetraveled10 to 24 km to the intervening strike-slip fault. Slip on all nine faults
the northeastrelative to the North American plate during the translatesthe northernend of the rotateddomainnorthwardby
0.3 to 0.6 Ma elapsed since the initiation of left-lateral the sum of these arc-parallel components relative to the
faulting (Table 1). Thus we might expect to observe southernend, assumingno shorteningacrossthe blocks. The
horizontaloffsetsof the strike-slipfaults where they crossthe net arc-parallel extensionrate is 10.5 mm/yr from the five
deformation front, but we have observed few such offsets. We
faults with known slip rates. If we assignthe lowest known
suggestthat the interactionbetweenthe obliquefaultsand the slip rate value, 5.5 mm/yr, to the faults with unknownslip
growing accretionarywedge tends to reducethe amountof rates, the extension rate is 17.4 mm/yr, or 87% of the 20
lateraloffsetexpectedat the deformationfront (Figure16d). If
mm/yr tangential componentof convergence[Goldfinger,
the deformation front was fixed in east-westposition during 1994; McCaffrey and Goldfinger, 1995]. If correct, this
the known life span of the faults, the expectednorthward implies that the oblique faults alone can accountfor most of
offset across the plate boundary due to northeasterlyplate the oblique componentof subduction. Forearc deformation
motionwouldbe 4-11 kin. Goldfingeret al. [1996b] estimate rates have recently been linked to subduction earthquake
that the deformationfront advanced14-21 km westwardduring magnitude[McCaffrey, 1993]. Using the observeddifference
thisperiodbasedon the ageof uplift of thrustridges[Kulmet between subductionearthquakeslip vectorsand plate motions
al., 1973a]. The rapid advance of the deformationfront to estimate total rates of forearc deformation, McCaffrey
implies that the point of intersectionbetweenthe strike-slip [ 1993] finds a negativecorrelationbetweenrapidly deforming
faults and the frontal thrustis always very young,reducingthe forearcsand the largestsubductionearthquakes.The physical
expectedoffset of the frontal thrust. Some possibleminor explanationfor this is that earthquakemagnitudeis ultimately
offsets may be interpretedin Figure 5. Nevertheless,the linked to the ability of the forearc to store elastic strain
GOLDFINGER
ET AL.: CASCADIA
STRIKE-SLIP
FAULTING
8241
energy; thus weak forearcs generate smaller earthquakes shorteningand arc-parallelstrike-slipand translation. The
[McCaffrey, 1993]. McCaffrey and Goldfinger [1995] high slip ratesof the strike-slipfaults,coupledwith the lack
calculate that the extensivestrike-slipfaulting in the central of offset of these faults as they cross the plate boundary,
Cascadia forearc may limit subductionearthquakesto about imply that the seawardaccretionarywedgeis not movingat
MW 8.0.
the expectedconvergence
raterelativeto the subducting
plate.
Last, the process of block rotation of the Cascadia
submarineforearc may have occurredfor a longer period of
time than the agesof the datedfaults. We speculatethat new
oblique faults periodically rupture the lower plate, and
sometimesboth plates, then slip for a relatively short period
of time, perhapstaking advantageof basementweaknesses.
Motion on individual faults may ceaseafter a short period of
time, after which the abyssal plain fault traces would be
subducted or accreted, depending on the local vergence
direction and dtcollement position. Accreted upper plate
faults may remain as active deformation zones or may be
altered by mass wasting, deposition,erosion, or subsequent
accretionary wedge faulting. In SeaBeam bathymetry and
GLORIA sidescandata we have observedmany poorly defined
WNW trending lineations that may be the remnants of
previous episodes of strike-slip faulting, one of which is
shown on Figure 15. Alternatively, if the faults are no older
than the datedgrowthstrataon the abyssalplain, the observed
deformationcould be very young, reducingthe effect of JDF
motion on the time history of upper plate deformation.
Conclusions
Using sidescan sonar, seismic reflection profiles, and
swath bathymetric data, we have mapped a set of WNW
trending left-lateral strike-slip faults that deform the Oregon
and Washingtonsubmarineforearc. Evidence for left-lateral
separation includes offset of accretionary wedge folds,
channels,and other surficial features; sigmoidal left bending
of accretionary wedge folds; and offset of abyssal plain
sedimentary units. Five of these faults cross the plate
boundary, extending 5-21 km into the Juan de Fuca plate.
Using offset of subsurface piercing points and offset of
approximately dated submarinechannels, we calculate slip
rates for these five faults of 5.5 to 8.5 mm/yr. Little or no
offset of these faults by the basal thrust of the accretionary
wedge is observed. Holocene offset of submarinechannels
and unconsolidated
sediments
is observed
in sidescan records
and directly by submersible.
The strike-slip faults are most likely driven by dextral
shearingof the subductingslab and propagateupwardthrough
the overlying accretionarywedge. Tangential hydrodynamic
drag causedby obliqueinsertionof the slab into the mantleis
a possibledriving mechanism. Four sinistralfaults observed
in only the upper plate may be reinanenttracesof previous
basement-driven deformation. Alternatively,
a similar,
though unrelated dextral shear couple driven by interplate
couplingmay drive thesefaults and may augmentdeformation
of the upper plate for all the sinistralfaults.
A model of overall right-lateral simple shear of the
submarine
forearc
is consistent
with
the observed
surface
We conclude that the accretionary wedge is rotating and
translatingnorthward,driven by the tangentialcomponentof
Juande Fuca-North American plate convergence.
Acknowledgments. We thank the crews of the research
vesselsThomas Thompson(University of Washington), and
supportvesselsCavalier andJolly Roger; Delta pilots Rich
and Dave Slater,Chris Ijames, and Don Tondrow;membersof
the Scientific Party on cruises from 1992 to 1993 during
whichmos•of thedatawerecollected;
KevinRedman,David
Wilson, Tim McGiness,Wolf Krieger, Chris Center, and Kirk
O'Donnell
from
Williamson
and
Associates
of
Seattle
Washington, our sidescancontractors;Ed Llewellyn for cowriting Sonar, OSU's sidescan-sonar
processingsoftware;and
Chris Fox, and Steve Mutula of NOAA for their assistancewith
the multibeam data. Multibeam bathymetry data were
collectedby NOAA andprocessed
by the NOAA PacificMarine
and EnvironmentalLaboratory,Newport Oregon. Thanks to
Chris Fox and Bob Dziak, also of NOAA Newport, Oregonfor
the T wave earthquakelocationsshownin Figure2. We thank
Eric Geist, Roger Bilham, and Greg Moore for thoroughand
helpful reviews. This researchwas supportedby National
ScienceFoundationgrantsOCE-8812731 and OCE-9216880;
U.S. Geological Survey National Earthquake Hazards
Reduction Program awards 14-08-0001-G1800, 1434-93-G2319, and 1434-93-G-2489; and the NOAA Undersea Research
Programat the West CoastNational UnderseaResearchat the
Universityof Alaska grantsUAF-92-0061 and UAF-93-0035.
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