JOURNAL OF GEOPHYSICAL
RESEARCH, VOL. 100, NO. B8, PAGES 15,101-15,116, AUGUST 10, 1995
A seismicreflection profile acrossthe Cascadia subductionzone
offshore central Oregon: New constraints on methane
distribution
and crustal
structure
AnneM. Tr6hu,GuibiaoLin, EdwardMaxwell, andChrisGoldfinger
Collegeof OceanicandAtmospheric
Sciences,
OregonStateUniversity,Corvallis
Abstract. In 1989, we conductedan onshore/offshore
seismicexperimentto imagethe crustal
structureof the Cascadiaforearc.In thispaper,we discussthe processing
andinterpretation
of a
multichannelseismicreflectionprofile acrossthe continentalmarginthatwascollectedaspart of
thiseffort.This profilerevealsseveralfeaturesof the forearcthatwerenot apparentin an earlier,
coincidentreflectionprofile.Oneof themostimportantof thesefeaturesis a very strongbottom
simulatingreflection(BSR) beneaththemidsloperegionthatis nearlycontinuous
from water
depthsof about1500m to 600 m, whereit appearsto cropout on the seafloor.The pressureand
temperature
conditionsat the BSR derivedfrom ourobservations
areremarkablyconsistent
with
the experimentallydeterminedphasediagramfor a methanehydrate/seawater
systemover a
broadrangeof temperatures
andpressures,
assuminghydrostaticpressureandthe temperature
gradiantmeasurednearthe baseof the continentalslopeduringOceanDrilling Program(ODP)
leg 146.Intervalvelocitiesandreflectioncoefficients
derivedfromthedataindicatethatthe
BSR represents
a contrastbetweensedimentwith a smallamountof hydrateoverlyingsediment
containingfree gas,consistent
with resultsobtainedduringleg 146.Althoughtheregional
distributionof the anomalouslystrongBSR beneaththe midslopeis poorlyknown,we speculate
thatit may be relatedto apparentslopeinstability.The dataalsoprovideconstraints
on the
thicknessandgeometryof the Siletzterrane,whichis the basementbeneaththe shelfandactsas
the subduction
zonebackstop.A deepreflection,whichmightmistakenlybe interpretedto be
Moho if coincidentlarge-aperturedatawere not available,is interpretedto be the baseof the
Siletzterrane.A "recently"activestrike-slip(?) fault zonethatoverliesthe seawardedgeof the
Siletzterranesuggests
thatthe Siletzterranecontrolsthe locationof decouplingof the subduction complexfrom the restof the forearc.
Introduction
ties and intrusions,reflecting episodicconvergenceand forearc
volcanism. Seaward of the Siletz terrane is a well-developed
The Cascadia subduction zone has received considerable atten-
tion recently, both because of a large uncertainty about the
seismichazardit posesto the inhabitantsof the PacificNorthwest
region[e.g., Heaton and Hartzell, 1987;Atwater, 1987; Grant et
al., 1989; Shedlockand Weaver, 1991] and as a laboratoryto
studythe effectsof fluid flow in accretionaryenvironments[e.g.,
Kulrn et al., 1986; Moore et al., 1990, 1991; Davis et al., 1990].
The data presentedhere provideconstraintson the deepcrustal
accretionary
prismthathasformedsinceconvergence
wasinitiatedseawardof the Siletzterraneduringthe late Eocene.
Recent offshore bathymetric, side scan sonar and seismic
reflection data provide a clear image of the subductionzone
deformationfront. Thosedata indicatesignificantalong-strike
variationin structuralstyle as the dominantmodeof deformation
changesfrom seawardvergingthrustfaultingsouthof 44ø52'Nto
landwardvergingthrustfaultingnorthof 44ø52'N[MacKayet al.,
structure, which are needed to evaluate the seismic hazard, and
1992]. The transition occurs where the deformation front is cut
on the distributionof methaneand thermalgradientswithin the
by a northwesttrendingleft-lateral strike-slipfault that extends
suductioncomplex.
from the abyssalplain seawardof the deformationfront to the
Snavely[1987] and Snavelyet al. [1980] comprehensively continentalshelf(fault B of Goldfingeret al., [ 1992],now known
summarizethe availableinformationaboutthe geologichistory as the Daisy Bank fault). This fault represents
oneof a seriesof
and crustal structureof the continentalmargins of Oregon and
suchfaultsthat cut acrossthe deformationfront andaccretionary
Washingtonprior to recentacquisitionof new data.Stratigraphic prism[Applegateet al., 1992; Goldfingeret al., 1992] andmay
relationshipsdemonstratethat the early Eocene-agedoceanic representconduitsfor fluid flow within the prism [ Tobin et al.,
crustal rocks of the Siletz terrane, which forms the basement of
1993].
the Oregoncontinentalshelf as well as of the Oregonand southern Washington Coast Ranges,were suturedto North America
approximately50 m.y. ago. Overlying this terraneare sedimentary basinsup to 7 km thick thatcontainmany folds,unconformi-
In 1989, we conductedan onshore/offshorelarge-aperture
crustalimagingexperimentacrossthe Cascadiasubductionzone
offshorecentral Oregon to obtain constraintson the velocity
structureof the forearc(Figure 1). The westernendof the profile
is within a dense grid of high-resolution seismic reflection
profiles[MacKay et al., 1992] that was shotas a site surveyfor
OceanDrilling Program(ODP) leg 146 (Figure 1). Drilling was
undertakenin September1992, and was focusedon examining
fluid expulsionwithin the deformationfront. On the continental
Copyright1995 by the AmericanGeophysical
Union.
Papernumber95JB00240.
0148-0227/95/95JB-00240505.00
15,101
15,102
TRl•HU ET AL.' SEISMICREFLECTIONPROFILEACROSSCASCADIA
,/?
•
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TP•HU ET AL.' SEISMIC REFLECTION PROFILE ACROSS CASCADIA
shelf, the age and compositionof the sedimentsand intrusive
rocks overlying the Siletz basementare constrainedby a deep
industrytesthole locatedalongthe profile [Snavely,1987].
For the profile discussedin this paper, a tuned airgun array
15,103
OFFSET (km)
3.8
.3
.3
3.8
.3
3.8
6
witha totalvolumeof 128L (7800in3) wasdeployed
fromthe
R/V Geotide,operatedby DIGICON, andwasfired at 30-s intervals. These shots were recorded by six ocean bottom and 10
onshore seismometersas well as by the 3833-m-long, 144channelhydrophonecable towed by the R/V Geotide. In this
paper, we present the multichannel seismic (MCS) reflection
profile acquiredby the R/V Geotide. The large-aperturedata
collectedby the oceanbottom and onshoreseismometershave
beenpresentedelsewhere[Trdhuand Nakarnura, 1993; Brocher
et al., 1993]. The velocity modelderivedfrom the large-aperture
data [Trdhuet al., 1994; alsoA.M. Tr6hu, manuscriptin preparation, 1995] is used to help constraininterpretationof the reflection profile.
The new MCS profile presentedhere was shotcoincidentwith
an existingMCS profile acquiredby the U.S. GeologicalSurvey
(USGS) in 1974 (profile OR-5 [Snavely, 1987]). Many features
of the new profile are similar to featuresobservedon the USGS
profile, and we refer the readerto Snavely [1987] for a detailed
discussionof that profile. In this paper,we concentrateon several
features of the new data that are not apparent in the USGS
profile. The mostimportantof thesefeaturesare an anomalously
strongbottom simulatingreflection (BSR) associatedwith the
presenceof gashydratebeneaththe continentalslope,a shallow
fault overlyingthe seawardedgeof the Siletz terrane,anda deep
reflectionunderlyingthe Siletz terrane.Thesefeaturesare probably observablein the new data but not in the older USGS data
becauseof the greater bandwidth an.dstrengthof the seismic
sourcein our experiment.
I0
Figure 2. Examplesof resorted,unstackedCMP gathersprior to
applicationof automaticediting basedon a specifiedthreshold
for the rms amplitudeof the signalin the last 6 s of the recorded
data. A seriesof very faint, but distinct,reflections(DCR) at 88.5 s twtt is indicatedby arrows.These eventsare interpretedto
be primary eventsfrom the lower crustbecauseof the very small
moveout, in contrast to the event at 6-s twtt, which may be a
multiple of the basementreflection. CMPs 1075-1079, 11001104, and 1125-1129 are shown.
plottedwith only an approximatesphericaldivergencecorrection
applied to the amplitudes and no normal moveout correction.
Reflections at 6- to 7-s two-way travel time (twtt) require a
moveout similar to that of the basement arrival at 3-s twtt,
suggestingthat they may be multiplesof the basementreflection.
The arrival
time of these reflections
is also consistent with this
interpretation.Faint reflectionsat 8.5- to 9-s twtt, on the other
hand, show no measurable move out, implying high seismic
velocitiesand suggesting
that they are primaryeventsfrom the
Data Processing
Sixteensecondsof datawere collectedat a samplingrate of 4
ms. The data were then sorted and binned into common
DCR
mid-
points (CMPs) spaced 12.5 m apart. Becausethe shot interval
was 30 s (resultingin a shotspacingof 66_+8m), the fold of the
data varies between 27 and 28, and trace spacingwithin CMPs
averages66 m. Shootingon time ratherthandistancewasnecessary in order to maximize the amount of data that could be
recordedby oceanbottomseismometers
(OBSs) with a recording
capacityof 5 MBytes, and representsa compromisebetweenthe
MCS andlarge-apertureobjectivesof the experiment.Fortunately
suchcompromisesshouldnot be necessaryin the future, as the
storagecapacityof OBSshasincreasedby ordersof magnitudein
the pastfew years.
Velocity analysis was conducted on every 50th CMP by
manuallypickingthe peakson plotsof semblance(an estimateof
signalcoherencebetweentraces[Sheriffand Geldart, 1982]) asa
functionof velocity and time. For data seawardof the midslope
region(CMP6500), velocitiesobtainedby Cochranet al. [1994]
were used.Not surprisingly,velocitiesare well definedin regions
with numerousstrongreflections;in regionswith few reflections,
velocitiesare poorly defined, and stackingvelocitieswere estimated based on the OBS data. These velocities were then used to
producea preliminarystackedseismicsection.
To constrainthe nature of faint deep reflectionsobservedin
the preliminarystackedsection,the datawere resortedinto 50-m
bins,yielding 108-fold data with an averagetracespacingof 33
m within each CMP gather. Severalexamplesfrom the region
from which deepreflectionsare observedare shownin Figure2,
lower crust or Moho.
Figure 2 also shows that the amplitude of the background
noise varies considerablyfrom trace to trace and that the noise
level exceedsthe amplitudeof the reflectionsat 8.5- to 9-s twtt
on approximatelyonethird of the traces.The noisychannelsvary
from shotto shot,complicatingthe editingprocess.We therefore
designedan algorithmto automaticallyzero all tracesthat have
an averageamplitudein the lower 6 s of data falling above an
empiricallydeterminedthreshold.The amplitudesof the remaining traceswere then scaledto compensate
for the reducedfold.
This processingtechniquewas quitesuccessful
in improvingthe
signal-to-noise
ratio of datafrom the lower crust(Figure3).
The data (sorted into 12.5-m bins of 27- to 28-fold data) were
thenstackedand migratedusinga 45øfinite differencealgorithm
and a velocity model derivedfrom the stackingvelocities.The
datawereband-passfilteredwith a passbandof 5 to 25 Hz above
3-s twtt and a passbandof 5 to 20 Hz below 3.5-s twtt, and four
adjacenttraceswere stackedtogetherfor thisdisplay.Amplitudes
werescaledby a factorof (traveltime)2'5to approximately
correctfor sphericaldivergence.The migrateddatafor the entire
profile are shownin Figure4a. Amplitudesin the easternhalf of
the profile (lower panel) have been increasedby a factor of 2
comparedto amplitudesin the westernhalf (upperpanel). The
annotated profile is shown in Figure 4b. Contours from the
velocity model obtainedfrom the large-aperturedata [ Trdhuet
al., 1994] are shownin Figure4c. The samevelocitymodeland
coincidentpotential field anomaliesare shown as a function of
depthin Figure 4d.
TR]•HUET AL.: SEISMICREFLECTIONPROFILEACROSSCASCADIA
15,104
CMP
A 6
8
12
Figure3. Unmigrated,
stacked
seismic
reflection
section
showing
thedeepreflections
(DCR)beneath
thecontinentalshelf(a)beforeapplication
of automatic
traceeditingand(b)afterapplication
of automatic
traceediting.
WEST
CMP 10000
0
CMP
0
i
5000
I
9000
8000
7000
6000
5000
i
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4000
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EAST
I
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i
i
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i
70
80
90
1O0
110
120
i 12
130
Distance (km)
Figure4a. Finitedifference
migrated
stacked
seismic
section.
A band-pass
filterwitha passband
of 5-25Hz
above
3-stwttand5-20Hz below3.5-stwttwasapplied,
andfouradjacent
traces
werestacked
together
forthis
display.
Amplitudes
arescaled
byafactor
of(travel
time)
zj . Amplitudes
intheeastern
halfoftheprofile
(lower
panel)havebeenincreased
bya factor
of 2 compared
toamplitudes
inthewestern
half(upper
panel).
Seetextfor
furtherdescription
of processing
procedure.
TP•HU ET AL.' SEISMICREFLECTIONPROFILEACROSSCASCADIA
15,105
WEST
CMP 10000
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8000
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6000
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--8
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t 12
70
80
90
1 O0
110
120
130
Distance (km)
Figure 4b. Annotatedseismicsection.BSR, bottom simulatingreflections;BT, basementtopography;CF,
crustalfault; D, diffractionfrom topographyor shallowstructure;DCR, deepcrustalreflection;FF, Fulmarfault;
FT, frontal thrust;LSB, lower slopebasin;OC, top of oceaniccrust;OR, outer ridge; SB, Siletz basement;u,
unconformity.
Discussion
The Deformation
of the tectoniccomplexitywithin the youngaccretionary
prism
Front
Proceedingfrom the abyssalplain to the continentalshelf,we
now discussthe major featuresof this profile. The sedimentsof
the abyssalplain are fiat lying, with a slightthickeningobserved
as the deformationfront is approached.
Althoughfew faultsare
observedin the abyssalplain sedimentson this profile, manyof
the seismicprofiles in the high-resolutionODP site-surveydata
showsmallthrustfaultsin the abyssalplain sediments,
representing a protodeformation
from [MacKay et al., 1992].
The top of oceaniccrustis well imagedbeneaththe abyssal
plain.Note the presenceof significanttopographyon the oceanic
crust near km 2-9 and km 12-18 (BT on Figure 4b). That the
sedimentslap ontothe sidesof thesebasementhills suggests
that
the topography represents constructional volcanic features
formedat or nearthe spreadingcenter.Using a sedimentvelocity
of 3 km/s derivedfrom the OBS data, we calculatea height of
about 450 m for these hills. There is also tentative evidence for
faultingwithin the oceaniccrust(labeledCF). Additionaldataare
neededto determinethe three-dimensionalconfigurationof the
basementtopographyand intracrustaldeformation,but the presenceof thesefeaturesis mentionedbecauseit suggests
that some
may be due to topographyon the subductedoceaniccrust.
A clearseawardvergingfault (FT on Figure4b) is observedat
the deformationfront. Althoughthis profile was locatedvery
closeto the transitionfrom a narrowseawardvergingdeformation front to a broad landward verging deformation front
[MacKay et al., 1992], it shows structurescharacteristic of a
seawardvergingdeformationfront,with the frontalthrustclearly
visible.Projectingthe frontalthrustontothe velocitymodelindicatesthat it dips approximately30øto the eastand that it comes
within 1-2 km of the top of the subductingoceaniccrust.The
data quality on this profile is not adequateto determinewhether
all sedimenton the subductingplateis beingaccretedor whether
the primary subductiondecollementis located within the sedimentarycolumn. Based on the high-resolutionsite-surveydata,
however,MacKay et al. [1992] have estimatedthat the decollement is approximately1.4 km abovethe oceaniccrustin this part
of the subduction zone.
Two outer ridges (OR1, OR2) and a relatively large lower
slopebasin(labeledLSB) are observedimmediatelylandwardof
the deformationfront (Figure4b). The lower slopebasinrecords
an episode of relatively rapid uplift, which is recorded as an
unconformity(u on Figure 4b), as well as gradualcontinuous
TRl•HU ET AL.' SEISMICREFLECTIONPROFILEACROSSCASCADIA
15,106
WEST
CMP
0
10000
9000
8000
7000
6000
5000
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0
2 i
•2
4--
4
6--
6
8
CMP
0
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I
10
20
30
40
50
60
70
5000
4000
I.
3000
!
2000
1000
!
EAST
0
2 i
•2
4 i
•4
6--
6
8--
8
10 --
--10
12
•
70
80
"i
•
•
•
90
100
110
120
• 12
130
Distance (km)
Figure4c. Velocitycontours
obtained
frominversion
of traveltimesof thelargeaperture
dataoverlain
onthe
MCS profile.
0
25
50
75
100
125
300
150
0
-150
Q 30
0
,
,
,
,
,
25
50
75
100
125
DISTANCE (km)
Figure4d. Velocitymodelshownasa function
of depthwithnoverticalexaggeration.
Magnetic
(solidline)and
free-airgravity(dashedline) anomalies
arealsoshown.The seaward
edgeof theSiletzterrane(SES)is evidentin
the magneticandgravityanomalies.
TRI•HU ET AL.' SEISMIC REFLECTION PROFILE ACROSS CASCADIA
15,107
observablein this region are diffractionsfrom topographic
featuresoutof theplaneof theprofile(labeledD in Figure4b).
Wide-angle
reflections
fromthebaseof thecrustobserved
in the
uplift[Snavely,1987]. The timingof the uplift eventis not
constrained
by thesedata, but probablyreflectsa periodof
sedimentary
underplating
beneaththebasin.
largeaperturedata,however,confirmthatoceaniccrustof the
Thelarge-aperture
dataindicatehighervelocities
beneath
the
outerridges(Figure4c) thanare foundin the abyssalplain
subducted
Juande Fucaplateunderliesthisregion.
sediments
[Trdhuet al., 1994],suggesting
sediment
compaction
anddewatering
duringaccretion
to theupperplate.At firstsight,
thisresultappears
to disagree
withtherelativevelocitydecrease
The BSR
in thisregionreportedby Lewis[1992]andCochrane
et al.
[1994]andinterpreted
to indicateanincrease
in porositydueto
the seafloor near CMP 5900 and shallows gradually until it
deformation of the sedimentslandward of the deformation front.
Plate l a). This reflectionis similarto reflectionsobservedon
A strongreflectionis observedapproximately
0.3 s beneath
appears
to outcropat theseafloornearCMP 5100(Figure4 and
Theseresults,however,canbe reconciledby considering
thatthe
measurements
of Lewis [1992] and Cochraneet al. [1994] were
manycontinental
marginsandknownfor sometimeto berelated
thepresence
of methane
hydrate
in thesediments
[e.g.,Shipley
et
primarily
sensitive
to theupper2 kmof theaccretionary
prism. al., 1979].Theseseismiceventsaregenerallyreferredto asBSRs
Theapparent
discrepancy
between
theirmodelandoursmaybe (bottomsimulating
reflections)
because
theytendto beparallelto
dueto poorresolution
of thevelocitygradient
in theuppermost the seafloor.A strongBSR is widespread
on the lowerslopeof
sedimentsin our model and limited offsets recordedby Lewis
theOregoncontinental
marginandwasdrilledapproximately
17
[1992]andCochrane
et al. [1994].Figure5 shows
twovelocity km southof CMP 2500 duringODP leg 146 [ODP Leg 146
modelsthat fit the travel time data for OBSs 4 and 5 (Figure 1)
ScientificParty, 1993].It wasfoundto resultfromthepresence
equallywell,withintheresolution
of thedata,andindicates
that of free methanegas below a layer with a small amountof
if thevelocitywithintheridges
justlandward
of thedeformation methanehydrate,whichproduced
a velocitydecrease
withdepth
frontdecreases,
thenthe velocitydeeperwithinthe accretionary from 1.75krn/sjust abovetheBSR to 1.25krn/sbelowit. On the
prismmustincrease
abruptly
tomatch
thearrivaltimesobserved profilediscussed
here,BSRis a misnomer,
asthiseventgradually
in our data.
shallows landward from about 270 m below the seafloor at a
waterdepthof 1512m untilit appears
to outcropontheseafloor
The Subducted Oceanic Crust
at a waterdepthof about600 m.
Becauseof the wide rangeof water depthsover which the
BSR is observed(Plate lb), these data provide an excellent
The topof oceaniccrustcanbe tracedfor approximately
30
km east of the deformationfront. Becauseof the complexityof
to testhowcloselytheobservations
followtheexpertheoverlyingstructure,
theimageof thissurface
is discontinuous opportunity
and the detailed structure of the subducted oceanic crust cannot
imentalphasediagramfor thetransition
frommethane
hydrateto
freegas.A numberof parameters,
includingseafloor
temperature,
be resolved.This reflection was useful, however, for helping to
temperature
gradient,
andwaterandsediment
velocity
constrain
theposition
of thetopof theoceanic
crustin a modelof subsurface
and
density
structure
are
needed
to
derive
an
apparent
phase
the velocitystructure
of the marginobtainedfrom traveltime
boundaryfrom the BSR observations.In the absenseof
inversion
of large-aperture
data[Trdhuet al., 1994].
subsurfacetemperaturemeasurements,
severalworkershave
No coherent reflections are observed from the subducted
oceaniccrustof the Juande Fucaplatebeneaththe middleand
assumedthat the BSR observations
follow the phasediagramand
uppercontinentalslope,in spiteof our carefulattemptsto
increase
thesignal-to-noise
ratiothrough
theprocedure
described
have used these observations to derive estimates of heat flow.
above.In fact,verylittle seismicenergyis returnedfromgreater
than 1 s after the seafloorreflection,andthe only coherentevents
A
Seafloor temperatureas a function of water depth was
obtainedfrom monthly averagesof physicaloceanographic
measurements
in an 0.5ø squarecenteredon 45øN,125.5' W.
B
i
i
i
i
i
i
i
i
2
L!.l
25
50
DISTANCE(km)
25
50
DISTANCE(km)
Figure5. (a)Detailofthevelocity
model
ofFigure
4cintheregion
ofthedeformation
front.(b)Analternative
velocity
model
thatalsofitscoincident
ocean
bottom
seismometer
dataandisconstrained
tobeconsistent
with
theresults
of Lewis[1992]in theupperkilometer
beneath
theseafloor.
Regions
withvelocitylessthan2.5krn/s
are shaded.
15,108
TRl•HU ET AL.' SEISMIC REFLECTION PROFILE ACROSS CASCADIA
Water temperaturesdo not show significant annual variation
below 100 m (Figure 6a) and can be preciselyestimatedby a
simplefunction[seafloortemperature(øC)= 2.6 + A + B, where
A = (1510 - waterdepth)0.002;B = 0 for waterdepthgreaterthan
800 m; and B = (800- water depth)0.0013for water depthless
than 800 m].
Travel times to the seafloor and to the BSR were digitized
depth.Velocitiesfor the waterand sedimentwerebasedon interval velocities obtainedfrom detailed semblanceanalysesof the
MCS data (Figure 7), which indicatewater velocitiesof 1.4851.5 km/s and velocities for the sediment lying between the
seafloorand the BSR of 1.52-1.67 krn/s,with velocityestimates
increasingas the water and sedimentthicknessesincrease.While
the interval velocity estimatesmay be biasedby seafloorand
from the stackedand migrated record sectionand convertedto
BSR dips of up to 8ø (3ø in the regionof anomalouslylarge
amplitude BSR), these results suggestlow interval velocities
abovethe BSR, indicatingthat relativelylittle hydrateis present.
Figure 6b showsthe water depthfor assumedwater velocitiesof
1.485 and 1.5 krn/s (indistinguishableat the scaleof the figure)
monthly
averages
of
and sediment
thicknesses
for velocities
of 1.6 and 1.8 km/s.
Assuming that the temperaturegradient of 0.051 øK/m (as
measuredduringleg 146 andreportedby ODP Leg 146 Scientific
Party[1993])anda density
of 1.0g/cm3 forwater,wederivethe
temperatureandhydrostaticpressureat the BSR shownin Figure
400
800
1200
6c.
1600
Additional considerationsin deriving the pressureand temperatureat the BSR from the seismicdata concernwhetherthe
thermal gradientis constantand whetherthe pressureis hydro-
DEPTH (m)
400
H20
_
800
,'•.• 300 m
.... '"
200 •
1200
100 •
1600
5200
5600
6000
v
CMP
static or lithostatic.
Several studies have used BSR observations
combined with experimental observationsof the hydrate/gas
phase boundary to derive estimatesof thermal gradient and
consequently
heatflow [e.g., Shipleyet al., 1979; Yamanoet al.,
1982; Davis et al., 1990; Hyndmanand Davis, 1992; Hyndmanet
al., 1993; Zwart and Moore, 1993]. Although most of those
studieshave assumedthat pressureat the BSR is lithostatic,
Hyndrnanet al. [ 1992] have arguedthat the hydrostaticpressure
should be used for these calculations.
To illustrate the effect of
variationsin the thermalgradient,we showpressureandtemperature conditionsat the BSR calculatedfor a rangeof temperature
gradientsassuminglithostaticpressure(Figure 6d, assuminga
'" •1.8
I
crY20
I
I
5200
sediment
density
of 2.0 g/m3) andhydrostatic
pressure
(Figure
I
5600
6000
CMP
tx•
6e). Water and sedimentvelocitiesof 1.5 and 1.8 krn/s,respectively, were used in these calculations.These curves are comparedto experimentallydeterminedphasediagramsfor methane
P fithostatic
HYDRATE
S•W_
Figure 6. (Opposite)(a) Monthlyaverages
of temperature
within
the water column obtainedfrom physicaloceanographic
mea-
I--
•
•
.051
surements(D. Pillsbury, personal communication,1994). (b)
Water depth and thicknessof sedimentsoverlying the BSR.
Curvesfor waterdepthcalculatedassumingacousticvelocitiesin
water of 1.490 and 1.50 krn/s are identical at this scale. Further
22
-
'
6
10
20
PRESSURE(Mpa)
-.
P hydrostatic
_x•..,
HYDRATE
calculationswere done assuminga water velocity of 1.5 krn/s.
Sedimentthicknesses
calculatedassumingsedimentvelocitiesof
1.6 and 1.8 krn/sare shown.(c) Pressure
andtemperature
conditionsat the BSR calculatedassuminga thermalgradientof 0.051
øK/m,hydrostatic
pressure,andsediment
velocities
of 1.6and1.8
krn/s.(d) Derivedpressureandtemperature
conditionsat theBSR
comparedto experimentallydeterminedpressure
andtemperature
conditionsfor the methanegas/hydratephaseboundaryin pure
water and in a solution of water with 3.5% NaC1. Curves are
shownassuminglithostaticpressure,sedimentvelocityandden•
sityof 1.8krn/sand2.0g/cm
3, respectively,
andseveral
tempera-
14
,,,16
•
18
_
-
• 20
'J '• •'•sw
.061 FW•
22
6
10
20
PRESSURE(Mpa)
turegradients.(e) Derived pressureandtemperatureconditionsat
the BSR comparedto experimentallydeterminedpressureand
temperature
conditionsfor the methanegas/hydrate
phaseboundary in pure water and in a solutionof water with 3.5% NaC1.
Curves are shown assuminghydrostaticpressure,sediment
velocity
anddensity
of 1.8km/sand2.0g/cm3,respectively,
and
severaltemperaturegradients.
A
•
•
!
....
I
....
I
....
I
....
I
....
I
....
I
....
I
....
i
I
I
[ ,i
I
I
i
•
•
[
i
.
20
15
10
-5
-10
-15
-20
I•,: -- "i½•"•" .- .,.t-...",'e½. ':'.:,.:':--' ', . '. ' '
tee ,•',,
-
6100
' ....
.,•z,,:•.,,.,
6000
,.,,',,
-'"
,'.,,
5900
"'
•,-,,,
5800
' -;'"½ ,., ,.,
5700
-
, ' • ....
5600
• ...."
5500
• ....
5400
,
• ....
5300
• ....
5200
t
5100
CMP
Plate ]a. Detail of the seismicsectionacrossthe midslopere•ion where a stroh8BSR is observed.Amplitudes
havebeenscal• b• a factorof (time)•'5, andalltraces
amno•aliz• tothesamescale.
Thecolor-coded
amplitudescaleindicatesrelativeamplitudes.•e BSR and seafloormuldple(M) am labeled.•uestion marksindicate
pa•Jculafi• bdsht reflections/diffractions
from beneaththe BSR. One hundredCMPs am equalto approximatel•
1250 m. Ve•ica] exa•eration is approxJmate]•3, assu•n• a velocJt•of ]500 •s.
B
-125 ø 10'
-125 ø 05'
-125 ø 00'
-124 ø 55'
44
ø55'•
AR?
DEPTH (m)
AR
200
400
44ø50'
600
,
800
mud
V•AR
?
volcano?
1000
1200
1400
1600
0
44 ø 45'
5
kilometers
1800
2000
Plate lb. Topographyof the continentalslopedisplayedas a color image with illuminationfrom the west.
Bathymetricdata were collectedand processed
by NOAA staff at PacificMarine EnvironmentalLaboratory
(PMEL), Newport,Oregon,and at Rockville,Maryland.Bold line showsthe positionof the seismicprofile
shownin Plate l a. The arcuateridgesdiscussed
in the text are indicatedby "AR." A possiblemud volcanonear
wherethe BSR outcropson the seaflooris alsoindicated.Faint north-south
trending"ridges"suchas thosenear
44ø48', -125 ø08' probablyrepresent
errorsin thedata.
15,110
TR.•HU ET AL.: SEISMIC REFLECTION PROFILE ACROSS CASCADIA
A
0.5
CMP
B
CMP
5354
5344
5349
1.0
5513
5508
5503
2.0
1.5
D
c
0.5
1.0
1.5
CMP 5503
CMP 5344
0.5 03
1.0
,•
• •
CMP 5354
.-
•,
CMP5513
OFFSET (m)
. •
,- ••.
OFFSET
(m)
F
E
• •48o
•
.-.
1460
,.
1480
1500
•1500
o•1520
ß
1520,
> 1540
0.0
0.5
1540
1.0
• .•
2.0
0.0
3.0
0.5
I
,
1.0
2.5
5.0
2.0
2.5
3.0
.
1500
••5oo •
O•1520
o
'"
> 1540 CMP5354 •
i
0.0
2.0
•(s)
Tw'n' (s)
• 1480
,
1.5
0.5
8.
i
i
1.0
1.5
Tw'r'r (s)
/),
2.0
'
CMP 5513
-1520
> 1540
'"
I'!
2.5
,
3.0
0.0
0.5
,
1.0
1.5
Tw'r'r (s)
Figure 7. Examplesof theBSR wavelet.Datahavenotbeenprocessed
exceptfor anti-aliasfilteringdoneprior
to recording 4 ms andresamplingto 8 ms andcorrectionfor normalmoveoutassuminga velocityof 1490 krn/s.
(a andb) Near traceof adjacentCMPs for two portionsof theprofile.The questionmarkon Figure7b indicatesa
strongreflection/diffraction
from a brightspotbeneaththeBSR. (c andd) CMP gatherscorresponding
to four of
the tracesshownin Figures7a and 7b. Amplitudeshavebeenapproximatelycorrectedfor sphericaldivergence
bya factorof time]. All traces
areplotted
atthesamescale.
At largeoffsets,
astheseafloor
andBSRreflections
come together,the waveformsare distortedbecauseof the normal moveout.Velocity analysesof the data in
Figures7c and7d, displayedascontoursof semblanceasa functionof velocityandtime, are shownin Figures7e
and 7f, respectively.
TRI•HU ET AL.: SEISMIC REFLECTIONPROFILEACROSSCASCADIA
in pure water and in a solution of water with 3.5% NaC1
[Hyndrnanet al., 1992;HyndrnanandDavis, 1992].
Figure 6e shows that the observedshallowing of the BSR
follows the expected phase relationship for methane hydrate
stabilityremarkablywell comparedto observationsin many other
regions[e.g., Shipleyet aL, 1979] for a constantthermalgradient
of 0.051 øK/m,assuminghydrostaticpressure.Oscillationsin the
predictedstabilitycurve are associatedwith small-scaleseafloor
topographythat may resultfrom landsliding,as discussed
below.
Most of theseoscillationsdo not appearto correspondto faults
that reachthe seafloorand may reflect smoothingof the subsurface pressureand temperature fields due to the shearstrengthof
the sedimentand lateralheatconduction,respectively,ratherthan
to upwelling of warm fluids along faults, as has been inferred
nearthedeformationfront [Zwart and Moore, 1993].The largest
amplitudeoscillations,between CMPs 5417-5529 and upslope
from CMP 5155, however,do appearto correspondto a fault and
to an apparentseaflooroutcropof the BSR at 5110. Strongbut
discontinuous
reflectionsimmediatelybeneaththe BSR between
CMP 5417-5528 and CMP 5240-5390 and deeper within the
sectionmay alsobe relatedto fluid migration.
Prior to consideringthe effect of hydrostaticratherthan lithostaticpressureon the derived phasediagram,we had tentatively
interpretedthe observationsof Figure 6d to indicatea landward
increasein heat flow of similar magnitudeto that derived from
BSR observations
offshore
Vancouver
Island
and modeled
flow.
_
-ø12
•4--
ß
_
gs
no gs
o 0.8
cr
0.4I
m 0.0
I
I
I
5200
information
on the nature of the BSR is contained
in the wavelet shape(Figure 7) and in the large variationsin
amplitude of the BSR along the profile (Plate la). Whether
amplitudevariationsin BSRs are due primarily to variationsin
the amountof gashydratewithin the sedimentabovethe BSR or
to variationsin the amountof underlyingfree gas has been the
subjectof severalrecentpapers[e.g., Hyndrnanand Spence,
1992; Bangset al., 1993; Katzmanet al., 1994; Woodet al.,
1994]. Along our profile, the waveletshapeassociated
with the
BSR and recorded at near-vertical offset is consistentlyvery
simple and closely resemblesa mirror image of the seafloor
reflection,indicatingthatthe impedanceconstrast
generatingthis
reflectionis quitesharprelativeto thewavelengthof the seismic
dataand that if discretelayersof free gasor hydrateare present,
theymusteitherbe verythinor havegradational
upper(hydrate)
andlower(gas)boundaries.
Figure7 alsoillustrates
variabilityin
I
5600
CMP
B
sfrc =
-0.4
0.25,
ø'2øf
-
0.15,t'.
",1,^,
'./,'?""1
©
, ',
O0
ß
r•
5200
5600
CMP
C
A
--
sfrc
= 0.20
•• I•'
-
-
_
'5
o
•
>
v• =
!1•
•
I t, l'•'•
•,,: ,, .'/•'•,,
;Itf,',¾,,f,
,t; ', w,,'•,
-
1.8"
'•'
I
I1•Ix•
I1• I.,
4•', .'A
i'-•Y•/'.-,•.",•;'.
,, •
7.'7,
2 2,'1; '•"A[,,'RJ ','•
2:o5'
1.2 08ß
!
i\1•
,•,',•'
'•L-'
•
•!
"
'
'
I
I
I
5200
In a study of pressureand temperatureconditionsat a BSR
that was drilled in the Nankai trough,Hyndrnanet aL [1992]
concludedthat the in situ hydrate/freegas phaseboundarywas
bestdecribedby the experimentalphaseboundaryfor methanein
freshwater.Althoughour resultssuggestthat the phaseboundary
for a salinesolutionsimilar to seawateris more appropriate,the
data in Figure 6e could also be interpretedto indicatea slight
landward increasein thermal gradient (and consequentlyheat
flow) from about0.055 øK/mto about0.065 øK/m,assumingthe
freshwaterstabilitycurve.
Additional
A
as
being the result of sediment thickening and fluid expulsion
[Hyndmanet aL, 1993].However,basedon observations
of nearhydrostaticpressures
in the drill holesduringleg 146 [ ODP Leg
146 Scientific Party, 1993], we concludethat the hydrostatic
pressureis the appropriatepressureto usefor thiscalculationand
thatthe datado not indicateany overalllandwardincreasein heat
15,111
5600
CMP
•20
'
-
-
sfrc 0.15
o.w
-sfrc 0.15.•1
0.27
1.2 _sfrc/to0.20
e
\",/•'/,,,•
-
V/,.v.,•
08.
5200
5600
CMP
Figure 8. (a) Ratio of the peak-to-peakamplitudeof the BSR to
the amplitude of the seafloorreflection measuredfrom raw data
traces(curve labeled"no gs"). The effect of correctingfor geo-
metricalspreading(curvedlabeled"gs")is alsoshown,assuming
velocitiesof 1.5 and 1.8 km/s for calculatingpath lengthin the
water and sediments,respectively.(b) The reflection coefficient
at the BSR derivedfrom the amplituderatioscorrectedfor geometrical spreading assuming several different values for the
seafloorreflection coefficient (sfrc). The effect of attenuationhas
not been includedbut may be quite large, especiallyfor CMPs
greaterthan5400. (c) Velocity beneaththe BSR derivedfrom the
reflection coefficients,assuminga seafloorreflectioncoefficient
of 0.2 and several different velocities for the sediments above the
theamplitude-versus-offset
behaviorof theBSRreflection.
As a first attemptto quantifytheseobservations,
we measured BSR (V]). (d) Effect on predictedsub-BSRvelocitiesof varying
the amplituderatiobetweentheBSR andthe seafloorreflection the seafloorreflection coefficientand includinga densityinver(Figure8a) in orderto derivelimitson the reflectioncoefficient sion(Ap) beneaththe BSR.
15,112
TRI•HU ET AL.: SEISMIC REFLECTION PROFILE ACROSSCASCADIA
at the BSR. Again, this estimatedependson manyparameters
in
additionto observedamplitudes,includingthe seafloorreflection
coefficient,geometricalspreadingof the wavefront,sourceand
receiver array directivity, intrinsic attenuationwithin the sediments, attenuation due to scattering at a rough surface, and
focussingand defocussingof energy due to lateral velocity
heterogeneity.
We attemptedto derivean estimateof the seafloor
reflection coefficient from measurementsof the amplituderatio
betweenthe seafloorreflectionand the first seafloormultiple as
recordedat twice the offsetof the directarrival.Theseamplitudes
were corrected assuming a spherical divergence correction
proportional
to traveltime. Becausetheamplitudeof theseafloor
multiplewasdifficult to pick on individualtracesbecauseof its
low amplitudeandinterferencewith otherarrivals,our measurements show considerablescatter,but range between 0.16 and
0.22. A seafloor reflection coefficient of 0.2 was similarly
derivedby Hyndmanand Spence[ 1992]in theirstudyof theBSR
offshore Vancouver.
Peak-to-peakamplitudesof the seafloorreflection and and
BSR were pickedfrom unprocessed
near-tracedatafilteredfor
anti-aliasingbeforerecordingandresamplingat 8 ms. The measured amplitudes were corrected for geometrical spreading
proportionalto path lengthassumingvelocitiesof 1.5 and 1.8
km/s in the water and sediment.Observedamplituderatios are
shownin Figure8a, with andwithoutthe geometrical
spreading
correction.
Assumingverticalincidenceandfollowing Claerbout
[1976],thereflection
coefficient
attheBSRisA(1-c2),whereA
is thepressure
amplituderatiobetweentheBSR andthe seafloor
reflection and c is the seafloor reflection coefficient. Reflection
coefficients for the BSR calculated for assumed seafloor reflec-
tion coefficientsof 0.15, 0.20, and 0.25 are shownin Figure 8b.
Becausethe water depthsin this studyand the source-receiver
offset for the near trace of 258 m imply incidentanglesof less
than 15øon the BSR, assumingverticalincidenceandneglecting
sourceandreceiverarraydirectivityaremuchsmallersourcesof
uncertaintythanuncertaintyin the seafloorreflectioncoefficient.
The maximum reflection coefficient is very dependenton the
assumedseafloorreflection coefficientand rangesfrom 0.25 to
0.43.
Attenuation and geometricalfocussingare poorly known
processes
thatmayalsohavea largeeffecton apparent
reflection
coefficients. That intrinsic attenuation in the sediments, which
dependson waveletfrequencyand pathlength,is importantis
indicatedin Figures7a and7b, wherethe frequencyof theBSR
wavelet decreasesas BSR depth increases.Quantifying the
effectsof attenuationon measurements
of peak-to-peakamplitudeandseparating
thiseffectfrominterference
effectsdueto the
characteristics
of reflectinginterfacesis difficult. For this study,
we neglected attenuation and note that this implies that our
reflection coefficientsmay be considerablyunderestimatedand
that the amountby which the reflectioncoefficientsare underestimatedincreasesas the BSR deepens.
In orderto placeconstraints
on velocitiesaboveandbelowthe
BSR that are compatiblewith the derivedreflectioncoefficients,
we calculated the sub-BSR velocity implied by a range of
assumedvalues for velocity and density above the BSR. For
these estimates, we used assumedvelocities above the BSR that
are higherthan thoseindicatedby the intervalvelocitiesbecause
it is likely that the velocity within the sedimentsabovethe BSR
increaseswith depth.Becauseof the absenceof additionalcoherent reflections within these sedimentsthat would permit more
detailedintervalvelocityestimates,however, we do not haveany
independentconfirmationof the velocity above the BSR. The
resultsof thesecalculationsare summarizedin Figure 8c, which
showsthe effectof differentvelocitiesabovetheBSR assuminga
constantseafloor reflection coefficient (0.2) and density (2.0
g/m3),andFigure8d,whichillustrates
theadditional
effectof
varyingthe seafloorreflectioncoefficientand includinga density
inversion
of 0.2 g/m3 beneath
theBSR.In Figure8c,velocities
lower
than
1.2 km/s
are obtained
beneath
the BSR
between
CMPs 5250 and 5400 for assumed velocities above the BSR that
rangefrom 1.8 to 2.2 km/s. Suchvelocitiesimply the presenceof
a few percent,by volume, of free gas in the sedimentsbeneath
the BSR [Hyndman and Spence, 1992; Bangs et al., 1993;
MacKayet al., 1994]. Even if oneconsiders
the extremecaseof a
seafloorreflectioncoefficientof 0.15, a densitydecreasebeneath
theBSRfrom2.0to 1.8g/m3, anda velocity
of 2.0km/sabove
the BSR, a sub-BSRvelocity lower thanthat of seawateris indicated.Althoughthe apparentsub-BSRvelocitieswest of CMP
5400 are considerablyhigher and do not requirethe presenceof
free gas, these estimatesmay be considerablyunderestimated
becausewe have neglectedthe effect of attenuation,which may
decreasethe apparentreflection coefficientby as muchas 40%.
We conclude that the large amplitude of the BSR observed
beneaththe mid-sloperegionalongthisprofile suggests
the presence of methane,both in the form of free gas and gas hydrate,
within the older parts of the accretionarycomplex offshore
Oregon.
In an attempt to understandthe origin of the methane,we
lookeddeeperin the seismicsection.Few coherentreflectionsare
observedeither aboveor just below the BSR. However,a locally
strongbut discontinuous
reflectivezoneis observed0.5 to 1.0 s
beneaththe BSR (Figure4 andPlate la). The oceanbottomseismometer data indicate that the average velocity between this
reflective"surface"and the seaflooris lessthan 2.0 km/s (Figure
4c), suggestingseveralexplanationsfor this zone.One explanation is that it representsthe base of a midslopebasin formed
eitherin situ or as a lower slopebasin(similarto the onelabeled
LSB on Figure 4b) and subsequentlytilted and uplifted. The
absenceof any coherentlayeredstructurewithinthisbasinandits
discontinuous
nature,however,argueagainstthis interpretation.
We believe that this reflective zone may representthe baseof a
large debrisflow. High-resolutiontopographyfrom the region
showsseveralarcuateridgesthat are concavelandward(AR on
Plate lb) andmay representthe downslopesignatureof sediment
instability over a large (approximately15x20 km) region. The
seismicreflectiondataalsosuggestthat midslopesedimentsmay
overly the flat lying sedimentsof the lower slopebasinbetween
CMP 5910 and 6010, althoughthis might also be an artifact of
incompletemigration.
Exceptionallybright spotswithin this region (questionmarks
on Plate 1a andFigure7b), whichshowbothnormalandreversed
polarity, may be reflectionsand diffractionsfrom the top and/or
bottomof diffusepocketsof free gasor overpressured
fluids.We
speculatethat thesefluidsand/orgasmigrateupwardthroughthe
sedimentcolumnand are trappedbeneaththe BSR, wherethey
migratelaterally until they are ventedat the surfacewhere the
BSR outcropson the seafloorand along small faults. A small
circularhill observedabout0.5 km southof CMP 5350 (Plate lb)
may be a mud volcano,providingfurtherevidenceof activefluid
movement(and densityinversions)within the older part of the
accretionaryprism.
In an attempt to determinewhether the midslopeBSR we
observeis a widespreadfeatureof the upper slopeor is genetically relatedto the apparentdebrisflow we haveidentifiedin the
bathymetricdata,we scannedthe existingdatabase
in thisregion.
TRI•HU ET AL.: SEISMIC REFLECTIONPROFILEACROSSCASCADIA
Very few seismiclines crossthis part of the slope. A USGS
multichannelseismicreflection profile that crossesthe slope
approximately15 km northof our profile (approximatelyalong
the northernboundaryof Plate lb) doesnot showany signof a
BSR in this region.It is interestingto notethat boththe landslide
andlarge fluid accumulations
in this regionmay be relatedto the
Daisy Bank fault, which passesalong its southwesternedge
(Figure 1). This fault may have contributedto slopeinstability
either throughshakingduringan earthquakeor by servingas a
deepconduitfor fluid transport.We further speculatethat this
processmay contributeto the formationof headlesscanyonson
the continentalslope.
Becausethe BSR appearsto cropouton the seafloorat a depth
whereit canbe relativelyeasilysampledand observed,this site
may be a goodtestsitefor studyingBSR formationandevolution
on activemargins.A recentstudyof an exposedBSR at 540 m
depthin the Gulf of Mexico has documentedthis processon a
passivemargin and shownthat the formation and dissociationof
gas hydrateis a significantfactor in the trappingand releaseof
oil andgas[ MacDonald et al., 1994]. Additionaldata,including
seismicdatato map the regionalextentof the BSR and correlate
it with topographyandsamplingof theheelandtoeof the apparent slump,will alsobe neededto resolvethe multipleuncertainties concerningorigin and amountof methanehydrateand free
gasin the older part of the accretionarycomplexandthe possible
relationshipbetweenmethaneand slopestability.
The Continental Shelf and SubductionZone Backstop
A deep basin that records a complex history of changing
depocenters,
erosion,folding, faulting,and intrusionis foundon
the continentalshelf. Seismicprofiles from this region are well
known as textbookexamplesof unconformities[e.g., Sheriffand
Geldart, 1982]. The basementreflection beneaththis basin (SB
on Figure4b) is shownin the migrateddatato be a block-faulted
surface,probably formed by normal faulting in an extensional
regimesoonafter accretionof the Siletz terraneto North America
[Snavelyet al., 1980]. Althoughsucha structurewas suggested
by the earlierUSGS data,the faultedcharacterof this surfacewas
poorly definedin thoseunmigrateddata.The new data alsomore
clearly define the seawardedge of the Siletz basement(labeled
FF on Figure 4b). We note, however, the presenceof several
diffractionsseawardof this boundary(D on Figure 4b), which
may representout-of-planeeventsresultingfrom possiblealongstrike variations in the westward extent of the Siletz block.
Beneaththe basementreflection,a deepreflectionis observed
at about 8-s twtt. No reflections were observed beneath the Siletz
basement in the earlier data, and the base of the crust was assumed to be at about 16 km beneath the shelf, based on an old
refractionprofile in the onshoreCoastRange[Berget al., 1966].
Evidencethat thesereflections(labeledDCR on Figure 4b) are
primary eventsfrom the lower crustwas discussedabove.In the
absenceof coincidentlarge aperturedata,one might be tempted
to identify theseeventsas the Moho, and the crustalthicknessof
about 16-18 km derivedfrom suchan interpretationwould be
consistentwith the earlier refraction data and with subsequent
estimatesof crustal thicknessderived from gravity modeling
[Couchand Riddihough,1989]. The largeaperturedata[Trdhuet
al., 1994], however,indicatethat the baseof the crustis dipping
eastin this regionand is at a depthof 22 km nearkm 105 and 30
km near the coast(km 135). The twtt to the Moho predictedby
the model derived from the large aperture data is shown on
Figure4c.
15,113
One explanationfor thesedeep reflectionsis that they representreflectionsfrom a velocity inversionunderlyingthe baseof
the Siletz terrane. Alternatively, they may representreflections
from a velocity increaseoccurringat the top of the subducted
oceanic crust (or, more likely, the top of lower oceanic crust,
since the upper oceanic crust seemsto have been significantly
disrupted and incorporatedinto the accretionaryprism); this
explanationrequiresthat a layer of sedimentbe sandwiched
between the base of the Siletz terrane and the subducted oceanic
crust in order to have lower-velocity material overlying the
subductedcrustof the Juande Fucaplate. It is alsopossiblethat
the observedreflectionrepresents
an interference
pattembetween
reflections from the base of the Siletz and the top of oceanic
crust.Becauseof the poor signalto noiseratio of this event,we
did not attemptquantitativemodelingof the waveformandpolarity of this reflection. The uncertaintyaboutwhetherit represents
the baseof the Siletz or the top of the subductedlower oceanic
crust is important for evaluating models of upper plate/lower
plate interaction in this region and will be discussedin detail
elsewhere.
A zone of disrupted reflections immediately overlies the
seawardedge of the Siletz block (labeledFF in Figure 4b). This
regionof the seismicsectionis shownat large scalein Figure 9
and suggestsa two-strandedstrike-slipfault with very little vertical offset. Figure 9 suggeststhat this fault may disrupt the
seafloor, implying recent activity. That this feature of the data
reflects a true disruptionof the strataand is not an artifact of
processingis supportedby a look at the original shot gathers,
which show disruption of refracted arrivals as this region is
crossed.Figure 10 shows three reduced record sectionsof shot
gathers.The first (top, Figure 10) illustratesthe refractedarrival
whenboth the shotand streamerare eastof the disruptedzone,
and the othertwo sectionsshowshotswestof the disruptedzone
when the streamercrossedthe disruptedzone.When sourceand
receiverare both on the sameside of the disruptedzone, the
waveletis impulsiveandsimple.As soonasthe disruptedzoneis
crossed,
thewaveletis complicated
andits amplitudeis small.No
similarzonesof disruptionareobservedfurthereast.
Although it is impossibleto identify the trend of this fault
from a single crossing,and a searchof the available seismic
reflectiondata [ Goldfinger et al., 1992] for a continuationof this
feature was inconclusive,it is temptingto associatethis event
with the Fulmar Fault, identifiedby Shayely[1987] on the basis
of magneticanomaliesandinterpretedto be a strike-slipfault that
resultedin northwardtransportof a portionof the forearcduring
the early mid-Eocene.In Snavely'smodel [Shayely,19871,the
Fulmar Fault is a late-middle to early-late Eocene feature that
resultedin truncationof the seawardedge of the Siletz terrane
andjuxtapositionof early Eocenemelangewith Siletzrocks.Our
data suggestthat northwardtransportof the accretionaryprism
relativeto the restof the forearcmay be continuing.An alternative explanationfor the fault observedin our datais local deformation
associated
with
the anticline
that overlies
the seaward
edgeof the Siletz terrane.
In either case,that the easternboundaryof the Siletz block is
overlainby an activefault supportsthe suggestion
of Trdhuet al.
[ 1994] that this boundaryplaysa major role in localizingalecoupling of the accretionaryprism from the rest of the forearc.
Decoupling between the western and eastern portion of the
Cascadia forearc must occur somewhere beneath the continental
shelf or Oregon Coast Range becausethe offshore left lateral
stike-slip faults of Goldfinger et al. [1992] indicate a stress
regimewherethe major axis of compression
is directedapproxi-
15,114
TRI•HU ET AL.: SEISMIC REFLECTION PROFILE ACROSSCASCADIA
W
CMP
_1.,
2_I00
2_050
2_000
1950
• 250rni
1900
1850
Figure 9. Detail of the seawardedgeof Siletzbasementanddisruptions
in theoverlyingsediments.
The horizontal blackbar at the top of the figureindicatesthe regioninterpretedto be a recentstrike-slipfault zone,whichis
tentativelyinterpreted
to be a modemmanifestation
of theFulmarFaultof Snavelyet al. [1980;Snavely,1987].
mately east-west, whereas onshore data from the Willamette
Valley indicate north-southdirectedcompression[ Werner et al.,
1991, 1992; Madin et al., 1993]. Other evidenceof the influence
of Siletz
terrane crustal stuctum on forearc deformation
that is
cited by Trdhu et al. [ 1994] includeslarge along-strikevariations
in the width of the postaccretion
subduction
complexandin forearc seismicitythat are correlatedwith along-strikevariationsin
the thicknessof the Siletz terraneand in backstopgeometry.
placement.This fault overlies the seawardedge of the Siletz
terraneand suggeststhat the accretionaryprism seawardof the
Siletz terranemay be at leastpartially decoupledfrom the restof
the forearc. Recent submersible
observation of a carbonate chim-
ney in this fault zone suggeststhat this fault zone may also be
associatedwith fluid flow in the forearc (R. Yeats, personal
communication, 1994).
3. Observationof a very strongBSR beneaththe midslope
region25-35 km landwardof the deformationfront.The inferred
Conclusions
temperatureandpressureat the BSR is consistentwith the experimentally determined phase diagram for a methane/seawater
Several
new constraints
on the crustal structure of the
phasediagramover a wide rangeof temperaturesandpressures
if
Cascadiasubductionzone and on methanewithin the accrationar-y one assumeshydrostaticpressureand the temperaturegradient
prismhave beenobtainedfrom a deepcrustalseismicreflection measuredat the baseof the slopeduringODP leg 146.This result
providesstrongconfirmationof the generallyacceptedhypotheprofile that was collected as part of a comprehensiveonwith the baseof the methanehydrate
shore/offshore
seismicimagingexperimentacrossthe continental sisthat BSRs am associated
margin. These constraintsare summarizedin Figure 11 and stabilityzone. Although usingBSR observationsas a proxy for
heatflow is subjectto severaluncertainties,the datasuggestthat
includethe following.
1. Recognitionof a deep reflectionbeneaththe continental heat flow through the midslope region is relatively uniform,
shelf. That this reflection must come from within the crust rather
exceptfor localizedzonesof higherapparentheatflow relatedto
thanfrom the Moho is requiredby coincidentlargeaperturedata. a small fault and to where the BSR appearsto outcrop on the
The reflectionmay indicateeitherthe baseof the accretedSiletz seafloor.Approximate reflection coefficientsderived from the
terrane,the top of the subducted
loweroceaniccrust,or an inter- data suggestthe presenceof a small amountof methanehydrate
ference pattern between reflectionsfrom both of thesebound- overlying the BSR and of free gas below it, consistentwith
aries.This interpretationsuggests
that the interpretation
of simi- results from ODP leg 146. Multiple uncertaintiesprevent a
lar deep reflections from regions where no coincident large preciseestimateof the relative volumesof gas and hydrate,but
intervalvelocitiesof only 1.5-1.7 for the overlyinglayer indicate
aperturedataam availableis uncertain.
2. Identification of a shallow, recently active fault that that the amountof hydrateis relatively low. It cannotbe deterdisrupts a 0.75-kin-wide region and shows little vertical disminedfrom the limited numberof existingseismicprofilesacross
TRI•HUET AL.: SEISMICREFLECTION
PROFILEACROSSCASCADIA
w
15,115
E
,l•l}ll
lIIII!ll
Ill
!lIIIIll!l
III!ill
II:llllil
Ill
Ill/!Ill
Illill/frill?
500
I000
500
BOO
I000
500
I000
2000
2500
3000
150•
•000
2500
3000
1500
2000
2500
3000
OFFSET
5500
3500
(m)
Figure 10. Recordsectionsof shotgathersasthe shiptraveledfrom eastto westacrossthe disruptedzonetentatively interpretedto be the Fulmar Fault. The horizontalblack bar parallelto the offset axis indicatesthe location
of the fault zone similarlymarkedon Figure 9.
the continentalslope whether the apparentpresenceof free gas
and gashydratein this part of the accretionarycomplexis genetically related to slope instability that has been inferred on the
basis of the new seismic data and high-resolution regional
bathymetry and/or to the Daisy Bank fault, which is a northwest
trendingleft-lateral strike-slipfault that extendsfrom the abyssal
plain to the continentalshelf and crossesthe slopeapproximately
15 km southour seismicprofile.
DaisyBankfault
Fulmar
fault?
ue
to
section)
•
deformation
fro
nt.•.>..._..
•• .... BSR
• ••
-
10-
•_••t
• Sil•etz•
•
fluids
•-•....___ ?
oceanic crust
Coast
-
-20
20-
30
lO •E
no
deep
reflections
observed
• ba•basement
..._.
I
20
I
I
40
I
I
60
I
I
I
80
I
100
I
I
120
Distance along model (km)
Figure 11. Schematicsummaryof constraintson methanedistributionandcrustalstructureon the Oregoncontinentalmarginobtainedfrom the seismicreflectionprofile discussedin this paper.
30
v
ß
Acknowledgments. Many peopleassisted
in the acquisitionandprocessing of thesedata.Vern Kulm, Greg Moore, andCaseyMoore encouraged
this piggybackexperimenton their high-resolutionseismicsurvey,which
was conductedas a site survey for the Ocean Drilling Program.Guy
Cochranespentthe extra day at seaas the scientificobserveron the R/V
Geotideand made surethat data were properlyrecorded.Guy Cochrane
andMary MacKay processed
the seawardportionof the profileaspart of
the ODP sitesurvey.JianingShi andTim Holt assisted
in the early stages
of dataprocessingat OSU. Lili Weldonsuggested
thatthe midslopebasin
might be a landslide,inspiringus to look at the high-resolution
topography for confimation.Discussions
with KristinRohrled to a betterappre-
Lewis, B.T.R., Changesin P and S velocitiescausedby subductionrelated sedimentaccretionof Washington/Oregon,
in Shear Wavesin
Marine Sediments,edited by J. Kloven and R. Stoll, pp. 379-386,
Kluwer Academic,Norwell, Mass., 1992.
ciation of the uncertainties
MacKay, M.E., R.D. Jarrad,G.K. Westbrook,R.D. Hyndman,and the
ShipboardScientificPartyof ODP leg 146, Originof bottomsimulating reflectors:geophysicalevidencefrom the Cascadiaaccretionary
prism,Geology,
22,459-462, 1994.
Madin, I.P., G.R. Priest,M.A. Mabey, S. Malone, T.S. Yedlin, and D.
Meier, March 25, 1993, ScottsMills earthquake--westernOregon's
wake-upcall, Oreg. Geol., 55, 51-57, 1993.
Moore, J.C., D. Orange,andL.D. Kulm, Interrelationship
of fluid venting
andstructuralevolution: Oregonmargin,J. Geophys.Res., 95, 8795-
associated with estimation of reflection coef-
ficients.Nathan Bangsand an anonymousreviewerprovidedinsightful
and constructivereviews of the manuscript.Acquisitionand analysisof
thesedata was supportedby the National ScienceFoundationthrough
grantOCE-8900963.Relatedwork on the coincident
velocitymodelwas
supported
by grantEAR-8905189 andUSGS grant14-08-0001-G2072.
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(ReceivedMay 17, 1994; revisedJanuary20, 1995;
acceptedJanuary20, 1995.)