Seismic Structure of the Southern Cascadia Subduction Zone and Accretionaryprism North of the Mendocino Triple Junction

JOURNALOF GEOPHYSICALRESEARCH, VOL. 1031 NO. Bll, PAGES27,207-27,222, NOVEMBER 10, 1998

Seismic structure of the southern Cascadia subduction zone and accretionaryprism north of the Mendocino triple junction

Sean P.S. Gulick and Anne M. Meltzer Departmentof Earthand Environmental Sciences, Lehigh University, Bethlehem, Pennsylvania

Samuel H. Clarke, Jr. Coastaland Marine Geology,United StatesGeological Survey, Menlo Park, California

Abstract. Fourmultichannel-seismic reflection profiles, collected as part of theMendocino triple junctionseismic experiment, image the toe of the southernCascadia accretionary prism. Today, 250-600m of sedimentis subductingwith the Gordaplate, and 1500-3200m is accretingto the northernCalifornia margin. Faultsimaged west and east of the deformationfront show mixed structuralvergence. A north-southtrending, 20 km longportion of the centralmargin is landward vergentfor theouter 6-8 km of thetoe of theprism. Thisregion of landwardvergence exhibits no frontalthrust, is unusuallysteep and narrow, and is likely causedby a seaward-dippingbackstop closeto thedeformation front. The lackof margin-widepreferred seaward vergence and wedge- taperanalysis suggests the prism has low basalshear stress. The three southern lines image wedge-shapedfragments of oceaniccrust 1.1-7.3 km in widthand 250-700 m thicknear the deformationfront. Thesewedges suggest shortening and thickening of the upperoceanic crust. Dis- continuitiesin theseafloor west of theprism provide evidence for masswasting in the formof slumpblocks and debris fans. The southernmostprofile extends 75 km westof theprism imag- ingnumerous faults that offset both the Gorda basin oceanic crust and overlying sediments. Thesehigh-angle faults, bounding basement highs, are interpreted as strike-slip faults reactivating structuresoriginally formed at thespreading ridge. Northeast or northwesttrending strike-slip faultswithin the basin are consistent with publishedfocal mechanism solutions and are likely causedby north-southGorda-Pacific plate convergence.

1. Introduction deformationfront, and thrust motion within the accretionary complexand North Americanplate [McPherson,1992; Smith Offshore northern California lies a remnant of the Farallon et al., 1993]. Our study examinesthe accretionaryprism and platecalled the Gordaplate. The Gordaplate, a southernex- Gorda crust for deformation or other features related to this ac- tension of the Juande Fuca-Explorermicroplate system, is tive seismicity. sometimesreferred to as the southernJuan de Fuca plate or the The Cascadiasubduction zone and accretionary prism lie Gordadeformation zone [Wilson, 1989]. Prior to 30 Ma the between Vancouver, Canada and northern California. Seismic- Farallonplate formed a continuoussubduction zone alongthe reflectionmethods have been usedto studythe Cascadiaprism westernmargin of North America.Today, the Gordaplate is off Vancouver [Davis et al., 1990; Hyndman et al., 1993a; subductingslowly beneath North Americaalong the southern Yuanet al., 1994], Oregon[Tobin et al., 1993; MacKay et al., Cascadiasubduction zone (CSZ)as well as experiencing a 1994; Cochrane et al., 1994a, b, 1996; Moore et al., 1995; componentof north-southcompression across the Mendocino Trehu et al., 1995b], and Washington [Shayely, 1987; Lewis, transformfault (MTF)(Figure 1) [Wilson, 1989]. The south- 1991; Flueh et al., 1996; Fisher et al., 1996]. On the basis of easterncorner of this systemis the Mendocinotriple junction thesestudies the Cascadiaaccretionary prism has been classi- (MTJ)where the Gorda, Pacific, and North American plates in- fiedin thoselocales in termsof its structuralvergence. The tersect in a fault-fault-trenchtriple junction (Figure 1) term "vergence"was used by Seely [1977] to describethe [Atwater,1970; McKenzie and Morgan, 1969]. dominant direction of movement of the fold and thrust sheets Theregion near the Mendocinotriple junction is seismi- within a prism. Seawardvergence, the more commoncase, oc- cally active[Oppenheimer et al., 1993; Smith et al., 1993]. curswhere the prismis a hangingwall being thrust over in- Seismicityis concentratedwithin the Gordaplate, along the comingsediments above a d•collement. A seaward-vergent Mendocino transformfault, and in the triple junction region accretionaryprism typically comprises landward-dipping (Figure 2a). Focal-mechanismsolutions show primarily (seaward-vergent)faults within the prismand a frontalthrust strike-slipmotion within the Gordaplate, west and east of the at the baseof slopeof the prismand commonlyhas a seriesof blind thrusts, known as a protothrust zone, seawardof the baseof slope [MacKayet al., 1992]. In the caseof landward Copyright1998 by the AmericanGeophysical Union vergencethe incomingsediments above the d•collementare scrapedoffthe incomingplate and thrust landwardonto the Papernumber 98JB02526. prism[MacKay et al., 1992]. A landward-vergentprism con- 0148-0227/98/98JB-02526509.00 sists of primarily seaward-dipping(landward-vergent) thrust

27,207 27,208 GULICK ET AL.: SOUTHERNCASCADIA SUBDUCTION ZONE

128øW 126øW 124øW 122øW 120øW 48øN 48øN -i-

... / • / -I- -I- -I- -I- -I- -I- / 47øN 47øN x - + + Washington " x \ \ JUAN DE • ..... + + + + + + FUCA PLATE 46øN 46øN

45øN 45øN + + + + +

+ +

+ + + 44øN 44øN + + Oregon

+ + + +

43øN 43ON - PACFIC NORTH + AMERICAN PLATE•7 PLATE 42ON _ 42øN + + + + + + +

41ON - /] GORDA + + + 41øN _• PLATE+ + California + + + + Mendocino FZ 40øN 40øN •, + + + + + PACFIC PLATE + + + + + +

39 ø N 39 ø N 128øW 126øW 124øW 122øW 120øW

Figure 1. Platetectonic setting of the studyregion. The westernpatterned area is the Cascadiaaccretionary prism;the bold arrows show the primarystructural vergence direction of the prismand previous lack of data on the vergenceof the southernprism [MacKay et al., 1992]. The dashedline betweenthe Gordaridge and the accretionaryprism is the locationof the kink in Gordaplate magneticanomalies [Atwater and Severinghaus, 1988]. The Mendocinotriple junction (MTJ) is the broadzone (dashed-in)of intersectionbetween the Gorda, North American,and Pacific plates.

faults, has no frontal thrustbeneath the base of slope, and has plate and suggestedthat the southernpart of the plate rotated no protothrustzone [MacKay et al., 1992]. clockwise between 2.5 and 1.5 Ma to produce the curved The Cascadiaprism changesvergence in several locations anomalies. Wilson[1989] suggestedthat the Gorda plate has along its strike. From Vancouverto northernOregon (latitude beendeforming internally for at leastthe past 5 Myr. Accord- 45ø14'N)the prismis landwardvergent, and from 45ø14'N to ing to Wilson'smodel, between 3.5 and 5 Ma, a zone of right southof the Blancofracture zone the prism is seawardvergent lateralshearing formed nearly parallel to the Mendocinofrac- (Figure 1) [MacKay et al., 1992]. At the 45ø14'N changein ture zone. This shear zone was later translated and rotated to a vergencea local regionof mixed vergenceis present [MacKay northwest-southeast orientation close to the center of the et al., 1992]. Biddle and Seely [1983] interpretedan industry Gordaridge and boundedto the eastby northeast-southwest line off northernCalifornia as being seawardvergent and this trendingstrike-slip faulting [Wilson, 1989]. The northeast- interpretationhas commonly[e.g., Clarke, 1992] been extrapo- southwest oriented strike-slip faults are consistent with lated as the principle vergence direction throughout the north-south compressionacross the Mendocino transform northern California prism. This study examinesvergence di- fault becauseof the unstablegeometry of the Mendocino triple rectionfor the northern California prism and discussesimpli- junction [Wilson, 1989] and are supportedby strike-slip focal cations for the shear stress condition of the southern Cascadia mechanismsolutions found for the Gorda plate [McPherson, subduction zone. 1992; Smith et al., 1993]. Riddihough's [1984] rigid plate Complexitiesin the Gorda plate magneticanomalies, in- requiresnorthward motion relative to North America at 42-46 cludinga southwardcurve or kink in anomalies2A, 3, and 3A +7mmyr4, while Wilson's[1993] best estimate is northeast- [Silver, 1971; Atwater and Severinghaus, 1988] have re- wardmotion of <30mmyr -• relativeto North America. sulted in two differing plate tectonic reconstructionsand High-quality multichannel seismic-reflectionprofiles col- plate motion estimates [Riddihough, 1984; Wilson, 1989, lected during the Mendocino triple junction seismic experi- 1993] (Figure 1). Riddihough [1984] assumeda rigid Gorda ment(MTJSE)image the complexitiesof the deformationfront GULICK ET AL.' SOUTHERN CASCADIA SUBDUCTION ZONE 27,209

regionoffnorthern California (Figure 2b). Thesedata image faulting of the crustand sediments,suggesting that the Gorda seafloordiscontinuities are caused by debrisfans and slump plate is internally deforming. Differencesin the structure of blocksthat may resultfrom seismicshaking. General trends of the subducting crust along strike of the margin suggest a velocity changewithin the sedimentsshow that the seaward change in the stressregime between northernmostCalifornia limit of the protothrustzone is markedby an increasein veloc- and the more southernportions of the Gorda plate (Figure 1). ity likely due to a combinationof increasedcompaction, dewatering, and cementation. The MTJSE transects show the Cascadia accretionarymargin between 40.4ø and 41.6øN 2. Data and Processing switches vergence,at the deformationfront, from seaward to The MTJSE [Trehu et al., 1995a] included the collection of landwardand backto seaward(Figures 2 and 3). This obser- over 1500 km of marinemultichannel seismic (MCS) data (A. vation along with supportingevidence from bathymetric and S. Meltzer et al., manuscriptin preparation, 1998). The MCS basaltaper anglessuggests that southof 41.6øN, the southern componentwas collected aboard the R/V Maurice Ewing in Cascadiaaccretionary prism is underlainby a weak d6colle- 1994 using the Ewing's 8385 cu. in. tuned airgun array, fired ment.The landward-vergentregion of the prismis apparently every 50 m, as the source. The shotswere received by the Ew- causedby a local seaward-dippingbackstop (Figure 3). A ing's 4.2 km, 160 channelstreamer, with hydrophones located singlecrossing of the Gordaplate shows extensive strike-slip every 25 m,a near offsetof 235 m, and a far offsetof 4210 m. Data were recordedfor 14 s at a sampling interval of 2 ms. This geometryresults in 40-fold data with commonmidpoints 1260 1250 1240 (CMP) locatedevery 12.5 m. Our study used four MCS lines, a 420 o MTJ-6, MTJ-8, MTJ-12, and MTJ-14, crossing the transition øc•P o from the Gordabasin to the Cascadiaaccretionary prism (Fig- o o ø • o ure 2b). o o o o oOo o Processingof the MCS data presentedhere was completed oo ? øgoo oø o at Lehigh University using the Omega© seismic processing o system. Prestack processingincluded sorting to CMP gath- 0 _ øoo00• 0 oO%o o o • 0 0•0 o õocp ers, frequency-wavenumbermultiple suppression,geometric spreading correction, predictive aleconvolution, normal moveout correction (NMO), outside mute to remove NMO stretching and refractions,and an inside mute for additional 41o multiple suppression. The stacked data were time-migrated o using a two-step migration consisting of an extended Stolt o 0%0 migration,using minimum velocities, and a finite differencere- sidual migration, using smoothedstacking velocities [Bea- sley and Klotz, 1991]. Depth sections were created by con- vertingthe time-migratedsections into depth using smoothed stacking velocities converted to interval velocities [Dix, 1955]. oo000%oo o o Detailed velocity analyses were completedusing Omega's 0o interactivevelocity processing(IVP) package. The computed 1250 1240 velocitieswere used both to stack and migrate the data and to 420

Figure 2. (a) Distributionof more than 6000 seismicevents of magnitude2.5 or greater occurring from January 1, 1975, to May 29, 1997, within the MTJ region, as recorded by the Northern California Seismic Network. Seismicity for the Gorda plate increasestoward the boundary with the Pacific plate (the Mendocino fracturezone) and is most concentrated offshoreCape Mendocino. (b) The southernCascadia subduction zone study area. Thin black solid lines are faults [Clarke, 410 1992], thin black dashed lines are folds [Clarke, 1992], thick black lines are multichannel seismic profiles used in this study, and thick black circles are ocean bottom seismometers coincidentwith the prof'fies. The square outline shows the part of the four transectsdiscussed in this paper. Abbrevia- tions are CSZ, Cascadia subduction zone; MFZ, Mendocino fracturezone; and MTJ-#, refersto the MCS lines used in this study. Onshoregeology includes Cenozoic sediments(Cs), CoastalBelt Franciscanterranes (Tkfcc and Tfcy), Central Belt terrane(KJfc), EasternBelt terranes(KJfey and KJfep),and the variousmetamorphic and igneousrocks of the Klamath Moun- 400 tains [Clarke, 1992]. Sedimentarybasins in this region are the Gorda basin west of the subduction zone and the forearc 50 0 50 100 basin, the Eel River basin, to the east. 27,210 GULICK ET AL.' SOUTHERN CASCADIA SUBDUCTION ZONE

1.o • MTJ-14 SeawardVergent Dual-vergence 1.0 NORTH 2.0 0 3 Ion Frontal Thrust Deformation Proto-thrusts •3.0 3.0 • "a.. • • 4.0• _•5.0 OceanicCrust 5.0• •6.0 7.0 D6collement 7.0 8.0 8.0

1.0 • MTJ-8 Landward SeawardVergent Strata 1.0 2.0 0 Zoneof 3Extension Ion No Proto-thrusts VergentDeformation ß ,, • • • ,• • , 2.0 - - -• = = = • ...... •?•••q•e zo.en 2 .. -• - 4.0 •a.o • • • 5.0 7.0 * * * 7.o s.o D•collement s.o

1.o • MTJ-12 VergentLandward SeawardVergent Strata-- 1.o 2.0 0 3Ion Zone of Extension No Proto-thrusts Deformatio_.n.•-- -- ,w-- ,•, • • • • 2.0 t, 3.0 • • • "• •' • • • 3.0t, _•5.0 • • --'...... • • -- --•=•OpaqueZone • •6.0•4.0 (LocalBackstop?) • • • 5.oj6.0• 7.0 '- wrusta•weages•--"-- ? ? __ • 7.0 8.o D•collement OceanicCrust 8.0

1.0 • MTJ-6 SeawardVergent Dual-vergence 1.0 2.0 0 3Ion Proto-thrustsFrontal Thrust 2.0 • 3.0 3.0 • 4.0 ..... •. •.. •_. •. '• ...... 5o0 • s.o_• • 6.0 7.0 CrustalWedges OceanicCrust 7.0 D6collement 8.0 8.0 WEST EAST SOUTH

Figure 3. A line drawingin depth(vertical exaggeration is 0.75X) summarizingthe principle interpretations for the deformationfront of the northernCalifornia prism. Observethe similarityin prismdeformation between MTJ-14 andMTJ-6 andthe similarityof deformationbetween MTJ-8 andMTJ-12. The smoothoceanic crust on MTJ-14 is shownto contrastwith the rough/faultedcrust of the southernthree transects.

3 mi. limit (CA waters) •0 km 60•m 5(•!on 4(•km 3(•km 2•km l(•km 0 km LINE 14 Forearc 3.0 High •

LINE 8 Forearc 3.0 High• • km • • • ForearcBasin

_ LINE 12 ), Forearc VEEAST = 4X I km3.0 Hlg••,-- ForeaSe_ Basi•n ----• •'

LINE 6 2.9 ForearcHigh • • km DisruptedForearc Basin

Figure 4. Bathymetricprofiles off northernCalifornia. The horizontaldistance is shown relative to the 3 mile buffer(position of Canadianversus U.S. waters). The horizontalline aboveeach profile is sealevel, and the valueslisted down the left sideare waterdepths. The maximumslope for transectslisted in the text was calculatedfrom the steepestportions of the outer slopes. The transectsare •90, 90, 60, and 125 km long, respectively, andall four transectsimage the Gordabasin, the Cascadiaaccretionary prism, the Eel River basin,and the continentalshelf to within 3 miles of California,with the exceptionof MT!-12 which stops short of the continentalshelf. This studyfocuses on the portionof lhesetransects west of the forearchigh. GULICK ET AL.' SOUTHERN CASCADIA SUBDUCTION ZONE 27,211

CMP 7200 7000 6800 6600 6400 6200 6000 5800 5600 5400 5200

VE=2.75X ---2 IMTJ-141 e 3 km

Baseof Slope

Frontal Thrust

D6collement Oceanic Crust i I , I , I , I , I • I , I • I , I , I , , .4.

"'•i•' •'"'"'•'-' '" '•,-'•' ' G'-•-•"'

-5 •,:,•;•', - " •',• ...... ,,, •...... '""?:;:,...',•,,...•:,..•,,.•.•,,,•.,...... :;...... ,•!•-•. •,•,,•:i,.,•.-.,.•"',"',:,•:;,t'../:.-,..,'-'%...•.,,.•...-_,.,,•.•,•,.:,,,.,' • ' ' '.a._"W•.:.•,',,•' •,, .' •._•.,.,,.,•

' ' "1" ' - I '. - ' I '" '1 ' I ' ! ' ' I ' I ' I ' i' ' ' 7200 7000 6800 6600 6400 6200 6000 5800 5600 5400 5200

Figure 5. Interpretedand uninterpretedtime-migrated sections of MTJ-14. The heavy solid lines are thrust faults,the heavylines of shortdashes are normalfaults, the light solid lines are oceaniccrustal reflectors, the light lines of shortdashes are sedimentaryreflectors, and the light line of long dashesis a bottom-simulating reflector. Note the prominentprotothrust zone, seaward-vergentfrontal thrust, and series of thrust faults within the prism. Verticalexaggeration for this andall seismicfigures in this paperwere calculatedat the seafloor using water velocity.

computeinterval velocities to examinechanges in physical reflector at the farthest offsets. Normal moveout at the far offset propertieswithin the sedimentsof the Gorda basin and southof the deepestreflector is still an averageof 360 ms, allowing ern Cascadia accretionaryprism. To ensure geologically us to discriminatein picking stackingvelocities for the entire meaningfulinterval velocities, we first identified a series of sedimentarysection. Given the lack of reflectionsfrom within continuous horizons on each seismic line. These horizons the small amountof subducting sediments,interval velocities were then overlaid on semblanceplots constructedfrom seven of the sedimentarypackage below the d6collementare deter- adjacentsummed CMP gatherscentered at 20 CMP intervals minedby velocity picks fromthe d6collementand picks from (the sumof 75 m of amplitudesevery 250 m), and the highest the top of the oceaniccrust. Semblancecontours from oceanic semblancevalue for each horizon was picked. This method crustal reflections are more smearedbecause of the longer pe- providesmore meaningful velocities, for conversionto inter- riod of these reflections, causing the calculated interval ve- val velocitiesthan simply picking the highest semblanceval- locities for the subducting sedimentsto be highly variable. ues without regardto location within the section. A primary We calculatederror in our stacking to interval velocity con- sourceof error in converting stacking velocities to interval versionby examiningthe effect on our final interval velocities velocitiesis that semblanceplots are createdby summingthe of slightly underestimatingand overestimatingthe stacking energyat eachtime for a range of velocities and then contour- velocitiesfor the horizonsbounding a given layer. These cal- ing the amplitudeson a time-velocityplot. Each semblance culations show the maximum error in the final interval veloci- contouror peak has built into it a range of velocities which ties is _+100ms '• for the shallowest sedimentsand _+250ms -• producea high degreeof coherency.In our analysiswe fine- for the deepestsediments. These errors are the maximumwe tunedpicks of maximumsemblancevalues using a movedout couldobtain if we bias our picking to maximizeand minimize compositegather display to test for completeflattening oft he our stackingvelocities. Our averageerror is less. 27,212 GULICK ET AL.' SOUTHERNCASCADIA SUBDUCTIONZONE

CMP 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 , I • I i I • I • I • I , BSR VE=2.75X .!!•...::- ,....,,;:.c..•.•.•.•, ,... .•'"•...... •..•,• Baseof_Slope SeafloorSteps i

•5

['6 ...... • •....,.-•.-•.• •-•...... ß;•..,- ß'.•. ....ß •...• :;-•• •:.,,:• t•..., "•n• ..,:;•.•.,•,•- . .. • .... • ,.•a• . ..,.•. ' I ' I • I ' I ' I ' I Crustal Wedge OceanicCrust Opaque Zorn '(•ocalBackstop ?)

ß.-...:...• ;.•',5.•.•' '•,,•,.'•.-_•.. •.;•,•.,.•..

.•-•...... :.-•:•-, •;f'•i•E.•'•-•'.5 .....'-• ...... ,'_.•'•.'•."• ....

• ß.'*•-• .;;.•...... • -.•-•-.'•_..;•.•,:•:-?•,.-..•,,•.•..•...... -- ...... '.• ..... i*• :.._•.._•,.•.•-. •,:C'•,•...• '.-• ,.:•_.• 4

...... ':• ...... --•-•..-...... • ...... -•.."•.--'•- - '•.• ,•-•.•7.,•• . :•c • ...... -.,. ,,. •:.•..;•.-.-. %,='• .-. '•...- ,.. - .. ; ,,. ,.--.•- -• ,•..

...... :•-..•. :-_...•• '-.-=;'•-"w..7:' _...... •-.....•----='-•'"•• ...... ::•U••'-'.'E••?-:".';L•..,•'•J•:./.?.'.•,;•:'•'f•. -•._--'•.•...•.•...... '• ...... •-.;.E'L-•:_'-•.•:.•.•i , •'-. '?"'•.";•'•.•.!•'.•.•L•'.•,';E:.....,. '-...... ' .'.-•:•"•..,.•.'• ....•..• I ...... '•••'- --•_'-•.-;.• .....•:...... •'•E:•...... '•:.,,.. ":.•,;.•_•,,.-.-.•,.,,:., :::-.•..,,•--•':.•,..-•.• ,-.-:,•.•. ,,.i-;:•::•-.•.,,,..•-•-•;.•;.•. • .,'"• '•..,-'"',-•e•..:• - '•:''•• •- ...... -•.'"•-.1.• •-- •.-.:---,.,-:,-•...... •':,"...... ' ...... •,:•.•..' ....•,.• ..•.•_,• ,-'-.•-•<.'.-...... _•.•...-•.• •.•.• , "'•',,•.•.•;.-• ...,'"•:'?"•...... '.,.,-.c' .•---•' .•,-•;:,,;_,,•:...--'•'L•''-".-•'•..•;-' i'•.•,•.,,'.':,',";•'•"•""•?. •••?•_e.•.•...•.•L.'-•7•'...;.,',, ß ? ."-,av•,;•.•.

ß I ' I ' I ' I ' I ' I ' I ' I ' I ' I ' 200 400 600 800 1000 1200 1400 1600 1800 2000 2200 2400 Figure6. Interpretedand uninterpreted time-migrated sections of MTJ-8. Thekey is the sameas Figure5 with the additionof nearvertical solid lines that are suspectedstrike-slip faults and a heavy line of long dashes that is the unidentifiedboundary discussed in the text. Note the zoneof extensionwith the Gordabasin, the lackof a protothrustzone, the lackof a seawardvergent frontal thrust, and the largelandward-vergent faults. Betweenthe most landwardthrust fault and the startof landward-dippingreflectors lies the opaquetriangular zonediscussed in the text. A set of overlappingcrustal reflectors define a wedgeof crustalmaterial at the top of the oceaniccrust seaward of the prism.

3. MCS Data Observations 3.1. Bathymetry, Sediments,and Seafloor Discontinuities The bathymetryof the four MCS transectsvaries fromnorth The four MCS lines presentedhere image portions of the to southalong the strikeof the subductionzone (Figure4). In Gorda plate and the transition from the Gorda basin to the the north,MTJ-14 showsthe outer sloperising from the Gorda Cascadiaaccretionary prism. From northto souththe lines are basin floor to the northernEel River basin at an averageangle MTJ-14, MTJ-8, MTJ-12, and MTJ-6, which at the base of slope of 5.5ø andlocally up to 8-12ø (Figures4-5). Furthereast, the are 40, 18, and 34 km apart, respectively(Figure 2). The three seafloorrises gradually (<1ø) acrossthe forearcbasin with an northern transectsextend •15 km west of the base of slope, increasein slope (20-3ø ) at the transition to the continental whereasMTJ-6 extends75 km into the Gorda basin (Figure 2). shelf(Figure 4). In contrast,the centraltransects, MTJ-8 and Overall, bathymetryvaries along the margin as shown in Fig- MTJ-12, show a steeperouter slope (locally up to-•15ø-20ø) ure 4. Figure3 summarizesand Figures5-8 showin detail the risingfrom the Gordabasin to the Eel River basin (Figures4, changesin deformationalstyle betweenthe four crossingsof 6, and7). Eastof theforearc high on profilesMTJ-8 and MTJ- the southernCascadia margin. Interestingfeatures imaged by 12 the outerEel River basinconsists of a 30 km wide flat pla- theseprofiles include seafloor discontinuities (Figures 6-8), teau, namedthe Klamathplateau by Silver [1971], beneath 1 opaquezones (Figures 6 and 7), and wedgesof oceaniccrust km of water(Figure 4). The seafloorthen rises sharply across a (Figures6-8). Figure9 showsthe singlecrossing of the distal 9ø slope,known as the plateauslope, to the continentalshelf Gorda basinthat we use to examinethe southern Gokda plate on its easternedge. On the southernprofile, MTJ-6, the outer for evidenceof strike-slip deformationcaused by north-south slope, averages-•5ø, similar to MTJ-14 (Figures 4 and 8). Pacific-Gordaconvergence and to examinethe structureof the However,this profile shows a narrower continentalmargin, oceaniccrust prior to deformationassociated with subduction with the coastlinelying 30 km fromthe base of slope, versus processes. 75 km on the three northerntransects (Figure 4). GULICK ET AL.' SOUTHERN CASCADIA SUBDUCTION ZONE 27,213

CMP 4800 4600 4400 4200 4000 3800 3600 3400 3200 3000 2800 2600 2400 • UE=2.75X BS •2 IMTJ-Zl 8 3 km

Truncated

ic Crust Crus ges OpaqueZone (Local Backstop?)

•2 o , I , I , I , I , I , I , I , I , I..... , Iß , I•[ß •3 ß

•4 ...... - • ...... •.•,. . _ • ...... •,•...... •:•:•::•,....• •_ •.•...• ,•.• . ?-•-• ...:...... :: ...:::. •:.:_...... ,..... - ...... ::...... =...... ?•- ...... •sae•, •5

•...... ,•... • ,.•,'•,, •. ... .•._ ..• , . ß' ..... :..,?;,•,,,,,, ...... •.-• ' ...... , •- .- • •--•-,,.•. ?•c•. •...• ..•:..:. -..... '-•-•,.. . • ...... -';:•.•?• ...... '•••••••'•••/

4800 4600 4400 4200 4000 3800 3600 3400 3200 3000 2800 2600 2400

Figure7. Interpretedand uninterpretedtime-migrated sections of MTJ-12. The key is the sameas Figures5 and6. Note,as in Figure6, that thereis no protothrustzone and no seaward-vergentfrontal thrust; instead, thereare two largelandward-vergent thrusts followed by an opaquezone similar to the one imagedon MTJ-8. Thereare three wedge-shaped crustal fragments like the oneobserved on MTJ-8 (Figure6).

No distinct bathymetrictrench exists along the southern 8), has two 20-30 m high steps similar to the seafloor steps Cascadiasubduction zone. Regionswest of the base of slope seen on MTJ-8 and MTJ-12. are entirely filled with sedimentsthat vary in thickness from All four MTJSE transectsshow a strong negative-polarity 1.5 km on MTJ-14 to 3.2 km on MTJ-12 and MTJ-8 (Figures 3 bottom-simulatingreflector (BSR) paralleling the prism slope. and 5-8). Sedimentthickness in the Gorda basin increases BSRs indicatethe presenceof methanegas hydratesand/or free dramaticallyupon approachto the prism,from only 300 m on gas within the sediments[e.g., Shipleyet al., 1979]. the westernedge of MTJ-6 to 3 km at the baseof slope(Figures 8 and 9). All four transectsshow a strong bottom-simulating 3.2. Structure and Deformation reflector (BSR) beneath the outer slope of the accretionary Deformation of the sediments and crust seaward of the base prism (Figures5-8). of slopeand within the southernCascadia prism varies along The Gorda basin seafloor near the accretionaryprism, on the northern California margin(Figure 3). There exists a pro- MTJ-8, has three discontinuities west of the base of slope tothrustzone seawardof the baseof slopeon the northern and (Figure6 and 10). Two of these are steps in the seafloor,-25 southern MTJSE transects,MTJ-14 and MTJ-6 (Figures 3,5, and 60 m in height, that are underlain by unbroken sedimen- and 8). Sedimentsat the baseof the continental slope on MTJ- tary reflectors(Figure 10). The third seafloordiscontinuity is 14 and MTJ-6 are offset by seaward-vergentfrontal thrusts a nearly vertical 150 m high scarp. Beneaththis 150 m scarp (Figure3, 5, and 8). East of the frontal thrusts, on these two and for 1.25 km to the west of it the sediments between 4.2 and lines,are a seriesof additional seawardand landward vergent 4.4 s appearto be disrupted(Figure 10). MTJ-12 shows a se- thrustfaults (Figures 3, 5, and 8). In contrast,the centertran- ries of three seafloordiscontinuities west of the base of slope, sects,MTJ-8 and MTJ-12, show normal faulting of the sedi- similarto the two steps in the seaflooron MTJ-8, that are ap- mentsseaward of the baseof slope(Figures 3, 6, and 7). At the proximately10, 40, and 20 m high from westto east (Figures 7 baseof the slope on MTJ-8 and MTJ-12 the sedimentsare not and 10). In addition,-700 m of the upper portion or'the sedi- offsetby a frontal thrust but instead are obducted onto the mentarysection within the Gorda basin is truncated against prism along large landward-vergent thrusts. Triangular- the slope of the accretionaryprism on MTJ-12 (Figures 3, 7, shaped opaque zones lie east of these landward vergent and 10). The Gordabasin seafloor,imaged on MTJ-6 (Figure thrusts, and to the east of the opaque zones lie a landward- 27,214 GULICK ET AL.: SOUTHERN CASCADIA SUBDUCTION ZONE

4800 5000 5200 5400 5600 5800 6000 6200 6400 6600 6800 7000 CMP ubeMTJ- (S6 3kmVE=2.75X Base of Slope 3 Frontal Thrust Proto-thrustsSeaflo•r ••.

Crustal Wedges Oceanic Crust I • I • , I • I •

' I 4800 5000 5200 5400 5600 5800 6000 6200 6400 6600 6800 7000

Figure 8. Interpretedand uninterpretedtime-migrated sections of the 31 km portion of MTJ-6 crossingthe baseof slope. The key is the sameas Figures5-7. Similarto MTJ-14 (Figure5), thereis a protothrustzone and seaward-vergentfrontal thrust but no opaquezone as imagedon MTJ-8 and MTJ-12 (Figures6 and 7). There are overlappingcrustal reflections defining wedge-shapedsections of oceaniccrust similar to those on MTJ-8 and MTJ-12 (Figures6 and 7). dippingreflector that may be either structuralor stratigraphic large faults (locatedat CMPs 1000 and 2850) with the down- in nature. The landward-dipping reflectorsform a westward thrown side to the east(Figure 9). Betweenthese larger faults, boundaryto a thick sequenceof landward-dipping sediments faults with smaller offsets cut both sediments and basement within the prism(Figures 6 and 7). There is no evidenceof an into horst and graben type structures. In general, the smaller acoustically opaque zone within the accretionaryprism on faultsare high-angle,show no preferreddip direction or sense MTJ-6 or MTJ-14 (Figures5 and 8). of offset,and vary in amountof basementinvolvement (Figure All four MTJSE transectsimage a d6collement 100-400 ms 9). (250-600 m) abovethe oceaniccrustal reflection. Thrust faults 3.3. Structure of the SubductingCrust west and east of the base of slope sole into this d&ollement (Figures 3 and 5-8). The d6collementreflection beneath the At the toe of the accretionaryprism, MTJ-14 images4-5 Myr northern California prism is not distinct in contrast to old oceaniccrust of the Gorda plate [Atwater and Severing- collementreflections in many prisms [e.g., Bangs and West- haus, 1988]. Oceaniccrust is relatively smoothand unde- brook, 1991]. The d6collement on each transect is a reflector formedfor the entire length of this profile. The oceaniccrust that separatessediments that bend upward into the prism east imagedon MTJ-8, MTJ-12, and MTJ-6 (Figures 6-8) is much of the deformationfront from thosethat continueto parallel the rougherthan that imagedon MTJ-14. MTJ-8 imagesa promi- oceaniccrust beneath the prism (Figure 11). nent crustal reflection located 10.6 km west of the base of The westemportion of MTJ-6 (Figure 9) imagesGorda ba- slope(Figure 6). This reflectionoverlaps the crustalreflection sin sedimentsand oceaniccrust that have not yet been affected fartherto the west,defining a wedge-shapedpackage of crustal by accretionary processesalong the continental margin. material 2.5 km wide and 300-700 m thick. The oceanic crust Thesesediments and oceaniccrust are cut by faults that pass beneathMTJ-12 has shallow,landward-dipping reflectors that throughthe entire section. The Gorda crust imagedby MTJ-6 define three moreunusual crustal wedges (Figure 7). The is separatedinto three regionsfrom west to east by two large, western two wedges are each 1.1 km wide and 250-350 m high-angle faults that cut the overlying sedimentsup to the thick and are centered 12.5 and 11.25 km west of the base of seafloor(Figure 9). The oceaniccrust is offsetacross the two slope,respectively. The third wedge, locatedbeneath the base GULICK ET AL.: SOUTHERN CASCADIA SUBDUCTION ZONE 27,215

CMP 500 1000 1500 2000 2500 3000 3500 4000 45OO 3

;,?•,.:,'i:.•,...,,,,,,.?.,,,,.:4;?.,•""",1¾,*• ....!,'•:'• 1•'•,•,'•.;•".".,,'. , ,..,,,•.,.,r•. :. • ,,",',',?,";,.•% ...... ;•;½. ;.•. ".•'-;.,:•;4,.',.;.% .... ,.•....,'','-,. ,, •,: •; ' •..,,._..• • %•,'.."• . , ß.• ,. ,•. ,.fag.',•,.=.,,?•,l,..'k,;, ,•,,.•) .:,,.•, ... i ,,,•;i' ,%'r2,,:"It,' -,',•' •';:, ');4L.;2r'•,;-T.-.',,,. "-,",,t, k•.; i, ,'':.'• g,,,2E•g•'• •,;d;',•.•;•).,., •,•:•. ;,?'..',•; .... 5.,,•:•,.:,•.;':•2• • ,•,•"2.'"'""•'• ,',;,:;,•,',•.,'"".•"'•'.• 4Y&'•:&h'•".:'%•- , ',•': ,,g,.L?- ...'.4'•.;;3•.m.,,;:.'•r,,•'!,' s ' •,",•,".h,

:•r.::•,,..½",•"•, ;• :,:•,.,,, C,7•,• "•.' •:•.., ,,• • :.•'" ,• .....,,• ,..•• .,..;• • ,':l,,.,,..•':',.'• ,,• •,•.•,m•,•, ',;,.,,,•.,••' , ;, ,,,•,•

,',.:,,2,,"",:•..:.-,,•,,:•!!,-,•,;•.,., ,• :.:.",:T,,T. ß,, '.".,:: :,•.."';,,':;•;,.:,'.., .,:,•,•...,,,,',;,,•;,.'•; .,.•:::,,,.•,,.,• ,'.2,,•L.',,.,:,.. , ,;.,•.,•. , -..,•:,,, •, ,,.,.•;...... b,:,,a,.•,. •.* ,,.••, •,,i,,;:,;:;•,,,•;;.,•..•%.,•, .•,., ..• .•,:, • ,,, •e•,,,:,::•,,i[,,:,;:•,• , ,,.,,,:r..•..l,• ,.•;..•.,,,.,l'"'•l,..r•e•,,•;•,,,,L..:.,,:F•i., .•Z;,•'";• ?...,:,,•}3•"• ,.ll.•.,,.,,,.L?,:.,.::,,?,..,. ,•.,, '""'•: •i";..... 'f'h •11lift" Y'"". •""" ' '"' i ß Crustal Region Cr•l Region ,, I , I I I , I : I , I : I

, t43

1 •?,•:,,,,?.•.".•!•.:•:,,',""....•,';::•..•. ':...'•..,"i!:•,;,'?..-":•'7•"r:T-;*•,s? '•.a-,,-.;-'.•.•,-,'"-•,;,.7/o.•,,r,,;,, 7,'s"•'"4•:',•,•:• ß'•:• •.•,?-. •"'-...... •::•.-..•":,..."•..,4•.=.;,.•:'•:;3.-::•_-,,•,...,,:,'a,•.-'.,j/:y.?. ':.•:t,:t.•':'s:•Z.•:•:,.. ;%•'..'•..'.L3;:',•::,',.•,•.:,•:•.,:•,•;;;.

.....;:Y.•'J.•e ,•"•'!':'•.?;• I,,:::;I',%•;'•,½ ,•:"•'.i; :'""','?:,i:,:•:;•:?•.".' %':. "•'•'½•'"•.• ' '.:." "•'• ,.',,; ,,•"½,,;•:'","'.r,",,;•;, i' .•'i' ,' ",.h';':,•..,..•,. i:.;::::'•'•};•::.",g, ' ,'5,',.":'.;;?,,,;'•.':','.:"!'"• ,".,;•;i•:',•.•i,',"?•!•,".'"."'"t:.".,;!,-:,,'.l::---,r,";;i;'•,::•'?..'.:;•,-.-•2. ,"'•.'.•.:.•.•'..•,m•'•'-'• 5 •5 -t.,•,•:.'r.• Z'.'••,?.':.:•:::,;, •.•,•,,i, ::'..•',,:'•,:.:',%.•;:.:,•,,'; 1.:;,•:.•,•;,,,,.;•,• .,.•:".:'.•'?'m;'4,,.•,.-.:.•:'.?....•.:4: ..""•;•:'•"' "'""•.:' :''•,,:"'"•'•i.•"1.•;;•,,. •.,• ,;., ;hv,.:fg• ,•l ,, ,,,•,•:; i.•,',.;l::•;,,•,.,. :,;', .•.,,:•:.',:.;Z;.ii, .,-,',, ::':,,.'•.:,;•."•! >":.' •;:;,;.::: •,..'.•:,;,:'.•.,,..•.•.l, :: ;::• ,: :::".!..-,,!.',::•.',:•:•:,: .-., •;•'4•: '•':::::?...•:-..:•"l:l ::'"":l .•,,'::';•',•'..'•,,,'• ,,., ;?4:: ".:,":•.::,,.;,i•:•; ?'*':•'' ;.•:,';•ff•!ii,.•: ':,,''.. ''•':',"'," ."':;;;..•:•::•,• •"","',',,,,•. ,•:•:•;•:'•.•,•':•,::.,-.•;:•:r..:•':,;...... ,..,•,•, ...... ,, ,"•,:',•'.•.', • :,',••;.%: •.,• :""½',.2, ;•,½ I,. ',. %. ,.,,•,1,•..•., • ,•,,•.: ,,.,,4.,, .,.•,•..,, •,r• ,',•..;, •,',', ..... •. •,•:,:r:;..,,1.,•b.•,•,,;.'.: r..:-',,•.. "'•..:.;4i;•'. ,."' .?'"-:".,:.• ' ß,"•...•. ",;:',,'7,.i, .•'.:'•.' ... "•..... • ,, .' :' ,•?.. ,,i, .4•*•" . ,•";,. '"'4,: "•,'F'.,,•,•,l ':"'•:. .•':'e;.'•,;,.'• • ' •' •'l,,. "•': ,.,,... i, •,.;• i•,,,.• • i•',,,.. • ,,.,'u,, . i•.:,. ,,,•-ß ,,'.• ' •i,' ,,1 •""'.'--•,•','"',,' ...,...,• .,.•., '•. '",,,•'...... ,,•.:•l,• r,".•.-,,, ,,h;,,,•,•,•",•' '.... ,,.•;,,...;%•1 •. ' '" ß.'• ,• •,•.•:.'. -'•-•-,.,•",'t,•.., ,. ,,.,.•,,•. ;i, '•.. •,'•,,.,,,,•,-,.' '.' '-'.,'",.•,.'- 4. • ,•.•,. .*•.•.2.:•,•'* ß, ,,•... ,•..,,,., ,, ':•.,•w;•r.•,'-,•l ' ...,,, '... , • .,,,,' ,,,;•,':,,:,'!•..'•.,•' •--,', '•'x:.•;•,:•... ;:r,•.,,, '"•.'. .... ;:':': "' ß',4;. '".".•':'•"•"','i,'..•, ,• '•'"'.,'.•% ',",.',,:'• ,'..... ,-'•.:.•":'• ;:,;,,.,'•: ,r,,• .,: ','%•,,;,!', ',,.,,•'t, ,,.. ,' :',"•"•"'':"'".""'• ,'•",'"'""'.:•-':.' "-i,',,.;'•:'"'•;""•'•',Y-'!,'""•;". ,',•,,'•:,•...4•,• 6 '•.,. •,.-.•". ,,' ß;,h,,..,',%,.,•l .... ,..•,•.,•.;..•,.. •,,• ..... ,,.,,,...•: ... ,:,:.'_,•.•,.,.. ,,v I.,•.' •;,.,..,..•,,',,•,.'•,• I,•.."•"v','%' ;,.3. •.6",•.'",,,'•.."..)..'.. '•,,.',,,.:) ..,.'"•', ,.. ,,' ' ,,. ,,•,'.' .>..,'••J ',• "', ',,','•,",.• ,.%.,•,..,.•.,•,,..',.¾•,'1,.'"',,%.,' ;r-'½,';•,•,•' •:,,½•-¾ "•,.•,.";.....,',;•;: ½..•,.: •."-i•. ' ., ß ' •.'• '•-'• ' "•.•, ;.•.' ,?,,•,,•"•'....,•. ";,•.'•¾", ,.'":'",.',-"".½•"'",.- •'•;,,.,",)' '• ~',•'.,,;'"','•.,•"'.;•;,."•"•" ,", ,'•,,;;N•'.,••'. '"'H•: •;• ß' ..'-,'--'.,,...... ?j •.•,•,-,,-'•, ,-'," ' .,••h,.<'. ;• ,,'•' .'-;,.,•,',. ,'•'•," ß 0'•.,.,•. ,"•'-•- - .... ,•J,'•'.•' •.,•.,' ,,.,, •' ..,..;.,,•./,•.,• ,'•,• .' , ,"L•,.. ,,u,"',: •,.... -•,;"•,•t• .-:.;•, • • , ,.** ,,,, ...... ,',•.,..,•-,•,-, ;1• • f'...•,.,;,• •;½,..-.;' ,;•.... . •,,,...•,. ,i.).,•...q, •,,,,•.,..•-;,•4,•,1:....", .,•,•:.:,.:,,. % ;'•'"'"' 1..... •-',,;,,'• ...... r" '•,"•,,••'•'•mcr"' ...... "..'"' •.1',,:•'•'"'"'"•," •;.. ".•"• ,,im• •.lq.•R,,•. , ,.',';.•:,•.•,"• •'.' ½,4;-'"'"',•N:-".•.,•;'":'.•.;':•.'½.', ",•',"'. •;., '-•:•., ,'.•,, .•' • .,"•" '•,'""•?•"" ""•"•.'"-,;",,' ...... :',', ,, ß :"',•'.:;,'"•w' "r'l;•'..•,;'"',•':; .,",•:h"';;• "a•'% ": '.:'•;, .;'"""'"'"•,,½, "'r,• b"'" ;•;•! "'•" ,..;:•-" ,•*;:"', •.•":'• • ",:',;; •:,• ;"' •' .' '"•r.,,,•r•. :,:';•.... • .;:,',';-'-',;•:''"!'.'.• ' , •;'2;;;•;•."•;'• ',,H...... r,;,.•.,,,:,,,.',,;,.,•, •,..r ,,.: 'h;•';•,.".: '½%f•!.', ,•.';'"" ':;• ..•.•: •'" "',,• '"'.'.• 4;I ;,, .,,;...... ,2,•,.,', ;;,,. ,',,i"?;';::: .... ';•: •':•,;' '",•" -,;',•" ,•",.: ...... 'V • ,5.:,.,;'•. ;i, • ,•,;:,::,.,•.,•,•,:,:, .""'•,,, •., ,,, ':';.r•::•'•..,..,:2....:•,ß ,, r,, .,;.,,,.., ,•...,,1•-•.:•.... ' .•:;•,%";'•,,'[.:.,,.".•';,:.•, ,• .... .,,,,,.•,,,,,•,,,•"'•,,• ,,:'?: •.... ,i!,*.;,"..•, r•..;,•,,,•,,. ,.,.'• .•,'2,;;•.;;.',_,.;• .. •,,,,;....,.i.,,•...f4.,.,,. ,;,,,,.' ,,: .,,,•,..,..:;"' ,, .. ,:r,:•, •:, •.q',..•;;.•, ..,., ,,u•,,• •..,.,'•.,,•.1.., ,,;:.... •,.r•;:;:,:,•:,'. , ,,.,,,,.,•, ..,,;,•.•...,•,,•',., ' ...... '.',; .,,,,•,•' •,•,,,1•=,.•.,•:#,",:-:;;.;,,; :' •,,;'•.,•.•.1o,.,.• ',,'.,..',.•,;;,,,,,..i.,¾1• ,,;;,.,,., "r:•'".:,j;,,•L-•"ß ß ', •'., ':',,,.•,.",,,•.,.•',"1,•,• ,! "•',,. 7 ' •'.:•,'.,,"*'": ' ß .,•';•,•. '•"•,,'-, •;,:"• •, ,,. •'" ' •.-'"•%•" ,:' ,•. •".1',•. •' ,:,'"' .,,"'.u• .,•,..... ,, .,m•.,'e '..•l.:i•,J,. ,.,,"•.•.•..',..!,, ' ' .'• ,'•h,,.',r';- •. •,,,,'".,",v ,-....' . , •.•.,. '•.• •,,,",, i-i7 ...."%-,•i,%:'•..,4.,,;..-5...•.,-,.•,:•."",• '*•,;"•' ,"•,;•,.. ".b•,'",.,.-,','," ;,:•.''"'•";:,."",.....;,:'mr.."."" •;•...',,.•;;,': "" '.'"•:','"k'•'"•,•,:- ,,,,,,....""=,'-.'•.'.I:'"'• .... r,'"*"'•',f'',.,,•,'-" .•,;",:.* ..•.•.';.. •'..:'•,,.'•,,,i "•.',;.,*•,.•,•'"',•.'•,.•4 •.,,;'•-,7,;•i;•': ,:;;•;',•,,.....,:,,%.•'• ,'t.•'.".-:,, ,• ',:','...',%.,;•,,,,.",•,,:.".*•,".,-•.,: ',, ,. z' • •'""•,.;.;;:;t:•r,':'•,""'.'. .,,,'"':•,-.'..'i,:.•'.,,.-.•,;;'."•,:".'4•,:,•-? .....•;.•.,•,,, '., '_"',::.,,,,'•'.•,..,•,:•,"'.,,.?.','*":,.,,'" :.,,.,••.,',,•!'-•.:'"..• •;,.•. •,•,4, ',*,fit"•'-,"E',"?•.,,,½",4,•!;;, ..,;.•,.. ,.,-.r;, ,::.,•,, ...... •,. ' ,½.•,,:,?,;.,,•.":,•'.•, ,',,,, •',..,, •,,•...•,.• ,,:,!"'<.;'•'•.;.,"'i,•..•'•';-,•,,..,;c'..',I...... ;,:,.••,•,,%•:;. ,,,,• ;,,. '::,•.',,• ...... ,.,•..;'•,.,.,"-,,.:•,..,.4,..-,::;,',,,,•;,•,,•: •I.,..,,.•. ,,,:., '•,,q'.,,, ...... ;[!!•'*1'2',,•.•,...,,•'. .•' ""•,•;",'-";-. •",•-..;.•,•.,'":• "..;.,•.:,.:;,..,, '-.""•-",.: "•"-'"'[a";':,•,.,?,';::•,,,,,,;;•;'.,•;,,;-,,,.•.fi•..•.•,:.-;, .... '• •".•" ' , :..",v,'-"i,,,• ,. :,.,-r,.?,.,,.... ,.,,• ..•,"....: •: ,, '4.,..,;",...,,,.,•:•::,- ..:..r'.,....,,.•?.., •, .,•,....l•,,.,••,,•..,.,,:,;;,..,;•,,"'•t•,.,%':! .,,..:,',:,.,,•;,;;., •.... •..... ;:.½,;.,.,•./.,,.-.,,''•'•,,...,.•,.,",",,'•,.,.;,,,;;, "".•...... •,•"!:'":'"•;'•;..".;;4,".,,,•..,,.•,,,,•.•,•,,,•r ,..,:.'":';.;•:,,".,:,:,.'...,:•,•.;,..,,,._-?,,•.. ;,•:"'•..,,,•""• , ,.r,;',.;2,,':,",,•:•""',T,,•-'•'...".;;;;"•,L.... •,•,,,,. .,,'""',:•,', •,....•,.;;.. .,:.•.•:','.,,.;•,'".?-:',,2;;.-'";,,"'r,,•, ,,:....•.!,.,,., '• "..--'""'-,,.,•r,'•,,-'", ,•..,...,..•,.•....r ,,:;•,-."'5,...•.,•,r•4,.,,';• .... .;.'",,•,,,,-:•,..... - ...... •,•"!,,•,•'•;',',,•,,"""•.,,•'•,''*,,;.,, .,...•,,,..-,.•,,,,,e ,'..,i;,:;r•,,:;•,.,,,,.;,?,-, '.•',.,•:,.,,, ..... '..i'"",."'•,,,,,.-..•,•;;•.•..• "•"""'"'•.,.-; ',".•,•,• !'"•""',;•.½: ..;.r..,'•, :I'7.",,,,•,.•;-?:• '!,•.... .4;== ...... ;,i".:,""•'•,;,.,•'• ,"-":'.•,"•-"""•"""•-,: :"'•.'::,,., ...... ,: .,,,f•, '",,•,.•.'•=:- ' '.•,,;;.',,-:-,,.. ",[;•ß...... ,,.•.,..,,.• •;',:!.,•.'•,,,,•:., :',;:;'•r.r,.,.•'•"""•,, ,, •;;>'•,,,,,,; .?.,,,,,"":•.i:;;,..... •, -'-,,.-,.-.,',' '•,,-,".::';': '•','•,,'," .... -",:;,',,,.-"•""• '""'.'.?"":::'•,.---•.. :,,',,',:". ,. •;,',: ..... t::'::,.,,,,'" ',,•.,",.":?,,," , •:;, ':--,:,,,, •,,:I ,•;•.... •.,,.,,.' :..... i, '•"'hl', ;...... ' '"'.,,, ','I ihi" ;l'•111h;,llllh'*" ' •lm''""•"""•',,,I ß.,'.,.•w.• ' ,. ,l•l.I ,,,.,.. I ' ,,•!11•i::l•,. ,' ,,.,, ...... ,,,t....I •lr,•- ,.,..t4• ' ..,,,..,,.-,.::-.- ...... I - .,..' ...... ,...... I ...... ' ..,. I...... , ...... I ., ,,...... ,..... 8 500 1000 1500 2000 2500 3000 3500 4000 45( •0

Figure 9. Interpretedand uninterpretedtime-migrated sections of the outer 58 km of MTJ-6. The key is the sameas Figures5-8. Note the two largefaults separatingthe three regions of horsts and grabenscut by a series of nearvertical faults. The thickness of sedimentincreases stepwise toward the east and the subduction zone acrossthese larger faults. The scale and vertical exaggerationare greaterthan that of the Figures 5-8. Note the contrastin deformationstyle betweenthe eastern(Figure 8) and western(this figure) portion of MTJ- 6.

of slope,is 2.5 km wide and450-550 m thick (Figure7). MTJ- Velocitiesof the upper accretingsediments on all four tran- 6 also imagesoverlapping crustal reflectorsthat define three sectsare <2500 ms '• andare commonly near water velocity wedgesof crustalmaterial-•400-500 m thick and 3-4 km wide &1500ms4), while sediment velocities in the lowerpackage each(Figure 8). A blowup of the westerntwo wedges on of accretingmaterial range from 2000 to 5000ms 4 (Figure13). MTJ-6 is shownin Figure 12. Laterally, the averagevelocity of the upper sedimentseither remains constant or shows a very small increase upon ap- 3.4. MCS Velocity Observations proach to the prism. The averagevelocity of the lower sedi- In orderto examinethe changesin physical propertiesof mentson all transectsincreases toward the prism (Figure 13). the sedimentsas they are transported into the subduction On MTJ-14 andMTJ-6 the lower sedimentvelocity appearsto zone we examinedthe interval velocities computedby our in- increaseat a greaterrate within the protothrust zone (Figure teractivevelocity analysis. For the purpose of interpreting 13). Though the velocity data frombeneath the d6collement the interval velocity of the sedimentsthe section was verti- (subductingsediments) vary widely, there is a generaltrend of cally dividedinto three packages:upper accretingsediments, increasingvelocity toward the prism on MTJ-6, MTJ-12, and MTJ-8, and decreasingvelocity towards the prism on MTJ-14 lower accretingsediments, and subductingsediments. The accretingsediments were separatedalong a continuous,high- (Figure 13). No evidenceis found for low-velocity zones in amplitudereflector traceable on all four transectsthat coin- the subductingsediments on the southernthree lines, though cided with a distinct increasein velocity. Velocities within someevidence pointed to a <2 kmwide, 500 ms '• low-velocity eachsediment package at each CMP location were averaged zone existingin the subductingsediments on MTJ-14. vertically and then smoothedlaterally over three velocity analyses(60 CMPs) (Figure 13). The amountof smoothing 4. Discussion was intentionally larger than the maximumwidth of the Fresnelzone of the data (400-600 m or 32-48 CMPs) in order Observationsfrom the seismic-reflectiondata provide in- to account for the horizontal resolution of the MCS data. formationon mass-wasting,sedimentary, structural, and tec- 27,216 GULICK ET AL.: SOUTHERN CASCADIA SUBDUCTION ZONE

WEST Seafloor MTJ-8MIGRATED Seafloor EAST Step8 8 8 Scarp

3.2 I I I I I I I I I I I I I I I•kl I I I I i I I I I I I I I I I I I I I I I I I I I1•!'1 I I I I I I I 3 Y

3. 1 111111111111Illl 11 ]•11111111111Illl I1 II111 lt111l•Ill I lilttill/ 3 8 III II ItIIII IIIII IIIII IIIIIIIIII IIII IIIII1•1111111IIIIIIII Ill 111 III I IIIIIlYlIIIIIIIIIIIIIIIII•I IIIIIIII II IIIII/ illtl• 3 8

•.0 • 0 •••••••••I••r••II•I•$•I•• 1

MTJ-12 MIGRATED Seafloor

__.3. Y

__3. $

__3. 8

/ Truncated Reflections __3. 6 •3. 2 __3. 8 3.8 .0 .1 .2

__•. 5

2 1.25 km 0 km 1.25 km ] VE- 3.7X

Figure10. Blowupof theseafloor steps and truncated sediments on MTJ-12and the seafloorsteps and scarp on MTJ-8. Note the continuousnature of the reflectorsbeneath the seafloorsteps and the chaoticnature of the reflectorsbeneath the seafloorscarp. Approximately 580 rn of sedimentis truncatedagainst the slopeon MTJ- 12. tonic processes.These processes occur within the sediments 4.1. Mass-Wasting Processes as they are transportedfrom the Garda basin into or beneath the Cascadiaaccretianary prism and within the oceaniccrust The three southernlines imagetwo types of seafloordis- in both the Garda deformation zone and the Cascadia subduc- continuities. Thesediscontinuities do not appearto be fault tian zone. related,as the reflectorsbeneath them appearto be continuous GULICK ET AL.' SOUTHERN CASCADIA SUBDUCTION ZONE 27,217

WEST MTJ-14 MIGRATED EAST

Faults

____q. 5

__q. 6

__5. 2

•. S

•q. 8

5 .0

5.1______5. 1

___5. P

5.3__ 3

5.4__ •5. •

5.5__ __5. 5

5. .6

5. P 5.2

5. __5. $

5.8___

1.25 km 0 km 1.25 km ] D6co!!ement VE- 2.8X

Figure 11. Blowup ofthe region beneaththe baseof slope on MTJ-14 showingthe split betweenthe sedi- mentsbeing incorporatedinto the prismand the sedi•nentssubducting beneath it. The low-amplituded6- collementon MTJ-14 lies around5.4 s two-way travel time.

WEST MTJ-6 MIGRATED EAST

4.5 •4. 5 q. 6__ __4.6

4.7

__.4.8

5. .0

5.1__ •5. 1

5. __5. P

5. .3

5. 5.4

5. __5. 5

5. •5. 6

5. ___5. 2

5. __5. ¾

5.8__ __5.9

6. ___.6. ¸

1.25 km 0 km 1.25 km Wedges VE = 3.7X

Figure 12. Blowupof the westerntwo crustalwedges on MTJ-6. The arrowspoint to the reflectorsat the base of the wedgesthat may be thrustplanes. Observethe shallow landward dip of the overlying and underlying oceanic reflectors. 27,218 GULICK ET AL.' SOUTHERN CASCADIA'SUBDUCTION ZONE

5000 MTJ-6 MTJ-14 4000 .- .-Subducting Sediments 4500 - : U. AccretingSediments _

•'•3500 •o3ooo ,•2500 ...... 1500,.•• •15oo

500 • •' ' B i '_•..• Proto-thrust• •-'-'ri m-• ' -- Prot•-th•st..... 500• ...... ß...... L.. s...... -•GordaBasin• .....Z..o.n.e. ....-•--Accretionary Prism 0 oo•oooo•oooo•oooo•oooo•o

CMP Location (12.5 rn spacing) CMP Location(12.5 m spacing) 5000 MTJ-8 MTJ-12 9000

...... 8000 ...... 7000 6000

2oo 4000 ;>2000 ,000t 2000 I [ 1000• • Gorda Basin • Prism

CMP Location (12.5 rn spacing) CMP Location(12.5 m spacing)

Figure 13. Plotsof averageinterval velocity against common midpoint (CMP) locationfor eachtransect. The thick lines with squares,medium lines with triangles,and thin lineswith diamondsrepresent the averagein- tervalvelocities for the upperpackage of accretingsediments, lower package of accretingsediments, and subductingsediments, respectively. The thin diagonallines show the trendof the subductingsediment velocities for eachtransect. The curveswere generated with a runningaverage of 60 CMPs (threedata points) repre- sentingthe Fresnelzone or limit of horizontalresolution. The graphsare dividedinto Gordabasin sediments, protothrusts,and the prism on MTJ-14and MTJ-6 andinto zones of extension,no deformation,and the prism on MTJ-8 andMTJ-12. Note the lack of changein the uppersediment velocities on all transectsand the increasein lower sedimentvelocities within the protothrustzone on MTJ-14 and 6 but not on MTJ-8 and MTJ- 12. See text for explanationof the subductingsediment velocities.

(Figure 10). The smaller discontinuities, or seafloor steps, tratednear the triple junction (Figure 2a) as the northemmost rangefrom 10 to 60 m in heightand likely representthe edges transectlacks thesemass-wasting features [McPherson, 1992; of turbiditefans or debris aprons that were transporteddown Oppenheimeret al., 1993; Smithet al., 1993]. the slopeof the accretionaryprism. The sedimentin these fans 4.2. Sedimentary Processes or apronsis most likely derived fromfailure or remobilization of sedimentfrom the adjacent outer slope of the accretionary Seismic-velocitydata can be used to infer changesin mate- prism,as evidencedby truncatedreflectors on MTJ-12 (Fig- rial propertieswithin sedimentsas they are transferredfrom ures 7 and 10). The 150 m seafloor scarp on MTJ-8 is inter- the Gorda basin into the accretionaryprism. The interval ve- preted as a slump block that slid en massefrom the nearby locity datatransects from MTJSE that crossthe baseof slopeof slope,on the basis of its larger size, sharpnessof its seaward the accretionaryprism show lateral increasesin the velocity of edge,and chaoticnature of the reflectionsat its base(Figure 6 the lower sedimentarysection toward the prism. This obser- and 10). GLORIA datafor the regionshow a largeslide effect- vation suggeststhat the sedimentsare undergoinga changein ing a 30 km long portionof the baseof slopein the vicinity of materialproperties as they are accreted. MTJ-8, lending support to our interpretation [Hilde et al., Interval velocitiescomputed from stackingvelocities are of- 1984]. The approximatevolume of the slumpblock is 8.4 km3. ten used to infer porosity by assumingthat velocity changes Slumps3 ordersof magnitudegreater in volume have been re- are primarily controlled by porosity [Bangs et al., 1990; ported in the southernOregon portion of the margin [Gold- Bangs and Westbrook, 1991; Cochrane et al., 1996; Hynd- finger et al., 1995]. The slumping(MTJ-8)and the turbidite man et al., 1993a; Hyndman and Wang, 1993; MacKay et al., fans/debrisaprons (seafloor steps) (MTJ-8, MTJ-12, and MTJ- 1995]. These studies use velocity/porosity relationshipsde- 6) may have been triggered by the active seismicityconcen- termined either by laboratory measurementson terrigenous GULICK ET AL.: SOUTHERN CASCADIA SUBDUCTION ZONE 27,219

sediments[Hamilton, 1978] or fromOcean Drilling Program d6collement [Dahlen et al., 1984; Davis et al., 1983; Hubbert (ODP) in situ or laboratorystudies on oceaniccores [Hynd- and Rubey, 1959]. Seely [1977] suggests low basal shear man et al., 1993b; darrard et al., 1995]. Westbrook[1991] stresscan causelandward vergence. However, sandboxmod- andHyndrnan and Wang[1993] furtherinfer the ratio of pore eling and seismicreflection observations suggest that low ba- pressureto lithostaticpressure from the inferredporosities us- sal shearstress does not require landward vergence,it just ing the assumptionthat effectivepressure is a direct function precludespreferred seaward vergence (N. Kukowski, personal of porosity. These studies have shown an increasein basin communication,1997) [Carsonand Bergland,1986; MacKay sedimentvelocities in proximity to, and across the frontal et al., 1992]. The landward-vergentportions of the northern thrustsof, accretionaryprisms. This patternis interpretedas California prism are unusual in that only a single landward- resultingfrom porosity reduction and is used as evidencefor vergent ridge exists and the local slope of the prism is steep sediment dewatering. (up to 15ø-20ø) (Figures 4, 6, and 7). Landward-vergent However, Hamilton [1979] showed in laboratory experi- prisms in Oregon and Washington have several landward- mentsthat the initial stages of cementation,which occur at vergent ridges and are less steep than their seaward-vergent grain contacts,increase velocity by increasingthe rigidity counterparts[Fisher et al., 1996;Flueh et al., 1996; MacKay without significantly changing porosity. Additionally, re- et al., 1992; Seely, 1977]. sultsfrom ODP Leg 146 challengethe assumptionthat poros- One explanationfor the changein vergencewithin the cen- ity variationsare the primary causeof velocity changein MCS tral transectsis a changeto a morerigid material close to the data in prism environments[darrard et al., 1995; MacKay et toe of the prism creating a seaward-dipping backstop and al., 1995]. Most recently,darrard [1997] describesvelocity causing local landward vergence. The seismically opaque variations as resulting fromthe interplay of three factors:(1) zonesjust east of the landward-vergentportion of MTJ-8 and fluid flow-induced cementationwithout porosity loss, (2) MTJ-12 separatethe landward- and seaward-vergentportions compaction-inducedporosity reduction,and (3)compression- of the prism and appearto act as a local backstop(Figures 6 inducedfracturing. Becausevelocity data are unable to differ- and 7). We assumethese opaque backstopsto be genetically entiatebetween these three processes,we interpret the veloc- related,or a singlestructure, given that they exist just east of ity increasetoward the prism as causedby somecombination the landward-vergentfaulting on both centertransects and are of thesethree factorsresulting in dewatering of the sediments. absent on MTJ-14 and MTJ-6. What is significanthere is that an increasein velocity toward The opacityof the zones can either be due to the existence the prism was observedon all four transects,suggesting the of a more homogenousmaterial or to the destruction of seismi- processesstart well seawardof the deformationfront. MTJ-14 cally imageablesedimentary layering by deformation,cementa- and MTJ-6 show an increasein the rate of velocity change tion, and/or metamorphism.The origin of the backstop(s) within the lower accreting sedimentsthat coincides with the could be related to underplating of a morerigid material, en- protothrust zone, suggesting that some or all of the three trenchmentof a subductedseamount, creation of a block of me- processesincrease within the protothrustzone (Figure 13). lange, or fault-channeledfluids causing local metamorphism and cementation. However, there is no evidence in the Decade 4.3. Structural Processes of North American Geology (DNAG) gravity data from the All four transectscollected during the 1994 MTJSE show a Geological Society of America to support a subducted d6collementclose to the oceanic crust, demonstratingthat seamountand no velocity or other data of a fine enough scale most of the incoming sedimentsare accretingto the northern to shedlight on the origin of this feature. California margin. Only 250-600 m of sedimentare subduct- The four MTJSE transectscollectively show that the toe of ing beneaththe North American plate, whereasthe remaining the northern California prism has no preferredstructural ver- 1500-3200 m of sediment is being currently accretedto the gencedirection within the limits of the study area. As shown margin(Figure 3). by sandboxand numericalmodeling, a lack of preferredver- Within the eastern Gorda basin and the Cascadia accretion- gence suggeststhat the prism has low basal shear stress be- ary prismthe four transectsshow two differentstyles of defor- causehigh basal shear stressprisms exhibit preferredseaward mation(Figure 3). The northern and southern transects,MTJ- vergence during periods of accretion [Dahlen et al., 1984; 14 and MTJ-6, have a series of faults that sole into the d6- Davis et al., 1983; Gutscher et al., 1998]. Mohr-Coulomb- collement seaward of the base of slope and seaward-vergent based critical wedge theory [Dahlen et al., 1984; Davis et al., frontal thrusts (Figures 3, 5, and 8). The central transects, 1983; Enlow and Koons, 1998] demonstratesthat the angle of MTJ-8 and MTJ-12, image extensional features 5-15 km sea- the top, ct, and base,[3, of the northernCalifornia prism sug- ward of the base of slope. There is no protothrust zone sea- gest low basal shear stressfor the toe and whole prism. One ward of the base of slope and no frontal thrusts, and all sedi- measureof shear stressis the coefficient of basal friction, gb, ments above the d6collement are thrusting onto the prism given by along landward-vergentthrusts (Figures 3, 6, and 7). Further Net+ (N-1)[•--•, (1) east, MTJ-8 and MTJ-12 show reflectorsdipping landward, suggestinga changein vergencebetween the toe and the more assuminga noncohesive thin planar wedge [Dahlen et al., landwardparts of the prism. Our interpretation is that the 1984; Davis et al., 1983; Enlow and Koons, 1998], where N northern and southern transects, MTJ-14 and MTJ-6, are sea- is the flow value [Terzaghi,1943]. The flow valueN is related ward-vergentand that the central transectsswitch from land- to the internalangle of friction,q•, by ward-vergentat the toe of the prism to seaward-vergent-10 km east of the deformation front. N = (1 + sinqb)/(1- sin (2) Vergencechanges in prismsare relatedto eithera changein the strengthof the material or to a changein the strengthof the Table 1 shows the averagesurface and basal tapers of the 27,220 GULICK ET AL.: SOUTHERN CASCADIA SUBDUCTION ZONE

Table 1. Mohr-Coulomb-Based Shear Stress Calculations for the Northern California Prism

Shear Stress Tests Test 1 Test 2 Test 3 Andersonianatr = 30ø Sand[t = 0.57 Nankaiq• = 24.4ø

N 3 2.93 2.41

N-1 2 1.93 1.41

Averageat (toe) 5.5ø (0.096 radians) 5.5ø (0.096 radians) 5.5ø (0.096 radians)

Average13 (toe) 2ø (0.035 radians) 2ø (0.035 radians) 2ø (0.035 radians)

Averageat (wedge) 1.1ø (0.019 radians) 1.1ø (0.019 radians) 1.1ø (0.019 radians)

Average13 (wedge) 6ø (0.105 radians) 6ø (0.105 radians) 6ø (0.105 radians)

la•(toe) 0.36 0.35 0.28

la• (wedge) 0.27 0.26 0.19

northernCalifornia prism as calculatedfrom the MCS data and zone indeed exhibits low basal shear stress as our data sug- the results of three calculations to solve for the basal coeffi- gest, it is significantbecause a weak d6collementwithin the cient of friction. Each calculationis basedon using a different southemCascadia region may allow for large earthquakesun- assumptionto solvefor the value of N. For these calculations der very low stress. the averagebasal taper, [3,is found by observingthe d6colle- mentparalleling the downgoing Gorda slab nearthe toe and 4.4. Tectonic Processes assumingthis relationshipcontinues beneath the prism(Fig- The seawardportion of the southernmostprofile MTJ-6 im- ures 3-8). The Gorda slab is known to dip 6ø beneaththe ages a seriesof high-angle faults that cut both oceanic crust prism(A.M. Trehu,unpublished data, 1998). The first calcu- and sedimentswithin the southernGorda plate. Thesefaults lation assumesa 30ø angle of fracture,c•A from maximumcom- lie on the edgesof horstsand grabens(Figure 9) and mayhave pressivestress, c•x, that is the theoretical,and empiricallysup- originally formed as ridge-parallel normal faults during ported,angle of fracturefor Earth materials[Anderson, 1951]. spreading[Silver, 1971]. Reactivationof these faults as high- The internal angle of friction is calculatedfrom the angle of angle strike-slip faults is consistent with Wilson's model shearfailure, {x• by [1989; 1993], which calls for internal deformation of the Gorda plate, and with focal mechanismsolutions of Gorda 12cxA= (90- ½)ø (3) plate earthquakes[McPherson, 1992; Smith et al., 1993]. Northeast trending strike-slip faulting within the southern Our second calculation assumesthat the prism material is Gorda plate is likely causedby north-south compression equivalent to sand with a coefficientof internal friction, g, across the Mendocino transform fault. equalto 0.57. The angleof internalfriction is then determined Recentmodeling by Wang and Davis [1997] of the intra- by plate stressregime for the Gorda plate suggeststhat north- tanq• (4) south compressionshould be experiencedby the entire Juan de Fuca-Gordaplate system. However, the northern transect [Twiss and Moores, 1992]. Our third test uses the Ocean MTJ-14 (Figure 5)images smooth and undeformedoceanic Drilling Program(ODP) hole 808 value of 24.4ø for the aver- crust seawardof the deformationfront. A comparisonof this age angle of internal friction found for the Nankai prism, a crustwith the strike-slip faulted crust observedin the Gorda coarseclastic prism similar to Cascadia[Feeset et al., 1993]. basin(Figure 9) and the rough crust imagednear the deforma- The results of all three calculations found values for the coeffi- tion front on the southernthree lines (Figures 6-8) suggests cient of basal friction, gb, of <0.36, suggestingthe northern that triple-junction-relatednorth-south deformationdoes not Californiamargin is under a low basal shearstress condition. propagateto the latitude of MTJ-14. The kink in the Gorda Low basal shearstress for the seismogenicportion of the basin magneticanomalies lies betweenMTJ-14 and the south- northern California subduction zone is also suggested by ern three transects,suggesting some type of boundary may Schwartzand Hubert [1997], who inverted 70 focal mecha- separateMTJ- 14 from the rest of the transects(Figure 1). The nism solutions for the maximumcompressivestress acting on undeformedand contrastingnature of the oceaniccrust on our the southernGorda plate. They found the maximumcompres- northernmostseismic line suggests that it is in a different sive stressfor the Gordaplate to be nearlyparallel to the sub- stressregime than those lines closer to the Mendocino triple duction zone and suggestedthat for the 1992 magnitude7.1 junction. earthqu'aketo have occurredthe d•collementmust be weak. TransectsMTJ-8, MTJ-12, and MTJ-6 image a series of Wang et al. [1995] have concludedthat the entire Cascadia wedge-shapedfeatures bound by high-amplitude reflections subductionzone has low basal shear stress on the basis of, in from apparentoceanic crustal material. Several possibilities part, low heatflow valuessuggesting a low degreeof friction exist for the origin of these features:out-of-plane scattering on the subduction zone. If the southern Cascadia subduction from basementtopography, mid-ocean-ridge-related features of GULICK ET AL.: SOUTHERN CASCADIA SUBDUCTION ZONE 27,221 magmaticor structuralorigin, and/or seaward-vergentthrust a changein stressregime between these profiles. While we slicesof crust. Out-of-plane events sometimesappear in seis- cannot rule out the possibility of contaminationfrom out-of- mic-reflectiondata as dippingevents within the oceaniccrust plane reflections,overlapping crustal reflectionsseaward of [Cohenand Bleistein, 1983; Kent et al., 1997]. Thesespuri- the southern Cascadiaprism appearto image thrust-faulted ous events are causedby energy reflecting from out-of-plane slices of the upper layer of oceanic crust resulting from basementtopography or fault scarpsthat are recordedwith the breakupof the uppermostGorda slab during subductionnear in-planereflected energy. Two-dimensionalmigration of out- the Mendocino triple junction. of-planeevents will often also produce spuriousdouble re- flectionson a reflection section [Cohen and Bleistein, 1983]. Acknowledgments.This work was funded under NSF's continental While we cannotrule out the possibilitythat part of thesefea- dynamicsprogram grants EAR-9219598 and EAR-9526116 to Lehigh University. We would like to thank the captain, crew, and technical tures are, in fact,out-of-plane reflections, an intriguing inter- staffof the R/V Maurice Ewing for their assistancethroughout the data pretation is that they representthrust-faulted slivers of oce- collectionstage. Many ideasexpressed in thismanuscript were the re- anic crust. sultof provocativediscussions with C. Goldfinger,D. Anastasio,and B. A thrust-faultorigin is morelikely than the featuresbeing Carson. We would also like to thank J. Diebold for his aid in the collection of the dataand discussions of the results.This manuscriptbenefited related to ridge processesbecause structures formed at slow fromearly reviews by A. Trehu,B. Beaudoin,D. Anastasio,and B. Car- spreadingridges are commonlyhalf-grabens with ridgeward- sonand final reviewsby A. Levander,N. Bangs,and R. Hyndman. dippingnormal faults of moderateangles [Carbotte and Mac- donald, 1990; Srivastava and Keen, 1995] not ubiquitously landward-dippingshallow features(Figures 6-8). Any ridge- References related structuresshould be observablein the outer portion of MTJ-6, where we observeno landward-dippingwedge-shaped Anderson,E.M., The Dynamicsof Faulting and Dyke Formation with Applicationsto Britain, 206pp.,Oliver and Boyd, White Plains,N.Y., features(Figure 9). The possibility of shorteningwithin the 1951. upper oceaniccrust is supportedby evidencefor a slight Atwater, T., Implicationsof plate tectonicsfor the Cenozoic tectonic thickeningin the crustin a velocity model coincidentwith evolutionof westernNorth America, Geol. Soc. Am. Bull., 81, 3513- MTJ-6 (A.M. Trehu,unpublished data, 1998). Evidencein 3536, 1970. Atwater,T., andJ. Severinghaus,Tectonic Map of the NortheastPacific the form of thickenedsedimentary sections seaward of the base Ocean,Dep. of Geol. Sci., Univ. of Calif.-SantaBarbara, 1988. of slopeon the three southernlines suggeststhat the com- Bangs,N.L.B., andG. K. Westbrook,Seismic modeling of the d6colle- pressivestresses are being accommodated in part by vertical mentzone at the baseof the BarbadosRidge accretionarycomplex, thickeningof the incomingGorda sediments. J. Geophys.Res., 96, 3853-3866, 1991. Bangs,N.L., G. K. Westbrook,J.W. Ladd, and P. Buhl, Seismicveloci- ties from the BarbadosRidge complex:Indicators of high pore fluid pressuresin an accretionarycomplex, J. Geophys.Res., 95, 8767- 5. Conclusions 8782, 1990. Beasley,C., andKlotz, R., Modified residualmigration, 61st Annual In- Evidence exists along the northern California marginfor ternationalMeeting Expanded Abstracts, 91, pp. 1114-1117, Soc. for multiplelower slope mass-wasting events possibly caused by Explor.Geophys., Tulsa, Okla., 1991. the active seismicityof the region. The mass-wastingevents Biddie,K.T., and D.R. Seely, Structureof a subductioncomplex, in areexpressed as both true slumpblocks creating large offsets SeismicExpressions of StructuralStyle, edited by A.W. Bally, pp. in the seafloorand as debris fans that spreadout into the 129-133, Am. Assoc.of Pet. Geol., Tulsa, Okla., 1983. Carbotte,S.M., andK.C. Macdonald,Causes of variationin fault-facing Gordabasin, creating small seafloor offsets at theirtermini. directionon the oceanfloor, Geology,18, 749-752, 1990. Interval velocities converted from stacking velocities on Carson,B., and P. L. Bergland,Sediment deformation and dewatering the northern and southern transects show an increase in the under horizontal compression:experimental results, in Structural velocityof the loweraccreting sediments toward the deforma- Fabric in Deep Sea Drilling Project Coresfrom Forearcs, Geol. Soc. Am. Mere. 166, editedby J.C. Moore, pp. 135-150, 1986. tion front and an increasein the rate of velocity changewithin Clarke, S.H., Jr., Geology of the Eel River Basin and adjacentregion: the protothrustzone. The observationssuggesting the proc- Implicationsfor late Cenozoictectonics of the southerncascadia essesof dewatering,compaction, and cementationstart well subductionzone and Mendocinotriple junction,AAPG Bulletin,76, seawardof the deformationfront and may increasewithin the 199-224, 1992. Cochrane,G.R., M. E. MacKay, G. F. Moore, and J. C. Moore, Consoli- protothrustzone. dationand deformationof sedimentsat the toe of the central Oregon The northernCalifornia accretionary prism south of 41.6øN accretionaryprism from multichannelseismic data, Proc. Ocean exhibitsno preferredstructural vergence direction. These ob- Drill. ProgramInitial Rep., 146part 1,421-426, 1994a. servationand critical-wedgetheory analysis of the topog- Cochrane,G. R., J. C. Moore, M. E. MacKay, and G. F. Moore, Veloc- raphicand basal taper of the prismsuggest that the northern ity and inferred porositymodel of the Oregon accretionaryprism from multichannelseismic data: Implications on sedimentdewatering California margin is underlain by a weak d6collement. and overpressure,J. Geophys.Res., 99, 7033-7043, 1994b. Thoughmixed vergent as a whole,the outermost6.5 kmof the Cochrane,G. R., J. C. Moore, and H. J. Lee, Sedimentpore-fluid over- prismbetween 41.3øN and 41.1 øN arelandward vergent. This pressuringand its affect on deformationat the toe of the Cascadia smalllandward vergent section of the prismwas likely caused accretionaryprism from seismicvelocities, in SubductionTop to Bottom,edited by G. E. Beboutet al., pp. 57-64, 1996. by the presenceof a local seaward-dippingbackstop of un- Cohen,J. K., and N. Bleistein,The influence of out-of-plane surface knownorigin that appearsopaque in the seismicdata. propertieson unmigratedtime sections,Geophysics, 48, 125-132, Numerous basement and sediment faults within the Gorda 1983. Basinsuggest the GordaPlate south of 41.3øN is responding Dahlen, F. A., J. Suppe,and D. Davis, Mechanics of fold-and-thrust to north-southcompression across the Mendocino fracture beltsand accretionarywedges: Cohesive Coulomb theory, J. Geo- phys.Res., 89, 10087-10101,1984. zoneby strike-slipfaulting. Comparisonsin the amountof Davis, D., J. Suppe,and F. A. Dahlen, Mechanics of fold-and-thrust deformationin the oceaniccrust imagedon the northernmost beltsand accretionmywedges, J. Geophys.Res., 88, 1153-1172, line with the crustimaged on the southernthree lines suggest 1983. 27,222 GULICK ET AL.: SOUTHERN C,ASCADIA SUBDUCTION ZONE

Davis,E.E., R. D. Hyndman,and H. Villinger, Ratesof fluid expulsion prism:Indicators of overpressuring,J. Geophys.Res., 100, 12895- acrossthe northernCascadia accretionary prism: Constraints from 12906, 1995. new heatflow and multichannelseismic reflection data, J. Geophys. Oppenheimer,D., et al., The Cape Mendocino,Califomia, earthquakes Res., 95, 8869-8889, 1990. of April 1992: Subductionat the triple junction,Science, 261, 433- Dix, C.H., Seismicvelocities from surface measurements,Geophysics, 438, 1993. 20, 68-86, 1955. Riddihough,R.P., Recent movements of the Juande Fucaplate system, Enlow, R.L., and P.O. Koons, Critical wedges in three dimensions: J. Geophys.Res., 89, 6980-6994, 1984. Analyticalexpressions from Mohr-Coulombconstrained perturbation Schwartz,S.Y., and A. Hubert, The state of stressnear the Mendocino analysis,J. Geophys.Res., 103, 4897-4914, 1998. triplejunction from inversionof earthquakefocal mechanisms,Geo- Feeser, V., K. Moran, and W. BrOckmann, Stress-regime-controlled phys.Res. Lett., 24, 1263-1266,1997. yield and strengthbehavior of sedimentfrom the frontal part of the See!y,D.R., The significanceof landwardvergence and obliquestruc- Nankai accretionaryprism, in Proc. Ocean Drilling Program Sci. turaltrends on innerslopes, in IslandArcs, Deep Sea Trenches,and Results,131,261-274, 1993. Back-ArcBasins, Maurice Ewing Ser., vol. 1, edited by M. Talwani Fisher, M., E. Flueh, D. Scholl, J. Childs, T. Parsons,D. Klaeschen, U. andW. PitmandIII, pp. 187-198,AGU, Washington,D.C., 1977. ten Brink, and J. Bialas, MCS Data collected aboard the F. S. Sonne Shipley,T.H., M.H. Houston,R.T. Buffier, F.J. Shaub, K.J. McMillen, reveal the detailed structureof the accretionarywedge off Cas- J.W. Ladd, and J.L. Worzel, Seismicevidence for widespreadpossi- cadia,Eos Trans.AGU, 77(46),Fall Meet. Suppl.,F656, 1996. ble gashydrate horizons on continentalslopes and rises,AAPG Bul- Flueh,E., M. Fisher,D. Scholl,T. Parsons,J. Childs,D. Klaeschen,N. letin, 63, 2204-2213, 1979. Kukowski,J. Bialas,N. Vidal, and U. ten Brink, New seismicstudies Silver, E.A., Transitional tectonicsand late Cenozoic structure of the of the Cascadiasubduction zone: Summary of Sonnecruise SO-108, continentalmargin off northemmostCalifomia, Geol. Soc. Am. Bull., Eos Trans.AGU, 77(46),Fall Meet. Suppl.,F655, 1996. 82, 1-22, 1971. Goldfinger,C., L. D. Kulm,and L. C. McNeill, Super-scaleslumping of Smith,S.W., J. S. Knapp, and R. McPherson,Seismicity of the Gorda the southernOregon Cascadia margin: Tsunamis, tectonic erosion, plate and eastwardjump of the Mendocinotriple junction,d. Geo- and extensionof the forearc, Eos Trans. AGU, 77(46), Fall Meet. phys.Res., 98, 8153-8171, 1993. Suppl.,F361, 1995. Snavely,P.D., Jr., Tertiary geologicframework, neotectonics,and pe- Gutscher,M., N. Kukowski,J. Malavieille, and S. Lallemand,Episodic troleumpotential of the Oregon-Washingtoncontinental margin, in imbricatethrusting and underthrusting; Analog experiments and me- Geologyand Resource Potential of theContinental Margin o•f West- chanicalanalysis applied to the Alaskanaccretionary wedge, J. ern North America and Adjacent Ocean Basins: Beaufort Sea to Geophys.Res., 103, 10,161-10,176,1998. Baja, California, editedby D.W. Scholl,A. Grantz,and J. G. Vedder, Hamilton,E.L., Soundvelocity-density relations in sea-floorsediments pp. 305-335, Circum-PacificCouncil for Energy and Mineral Re- androcks, J. Acoust.Soc. of Am., 63, 366-375, 1978. sources,Earth Sci. Ser. v. 6, Houston,Texas, 19'87. Hamilton,E.L., Soundvelocity gradients in marinesediments J. Acoust. Srivastava,S.P., and C. E. Keen, A deep seismic reflection profile Soc.of Am., 65, 909-922, 1979. acrossthe extinctMid-Labrador Sea spreadingcenter, Tectonics, 14, Hilde, T. W. C., W. E. K. Warsi, and R. C. Searle,Gloria long-range, 372-389, 1995. side-scansonograph mosaic, in OceanMargin DrillingProgram Re- Terzaghi, K., TheoreticalSoil Mechanics, 510 pp., John Wiley, New gionalAtlas Series 9, p. 1b, Mar. Sci.Int., WoodsHole, Mass., 1984. York, 1943. Hubbert,M.K., and W. W. Rubey,Role of fluid pressurein the me- Tobin, H.J., J. C. Moore, M. E. MacKay, D. L. Orange,and L. D. Kulm, chanicsof overthrustfaulting, Mechanics of fluid-filledporous solids Fluid flow along a strike-slipfault at the toe of the Oregon accre- and its applicationto overthrustfaulting, Geol. Soc. Am. Bull., 70, tionaryprism: Implicationsfor the geometryof frontal accretion, 115-166, 1959. Geol. Soc. Am. Bull., 105, 569-582, 1993. Hyndman,R.D., andK. Wang,Thermal constraints on the zoneof ma- Trehu, A.M., and the MendocinoWorking Group, Pulling the rug out jor thrustearthquake failure: The Cascadiasubduction zone, J. Geo- from underCalifomia: Seismic images of the Mendocinotriple junc- phys.Res., 98, 2039-2060,1993. tion region,Eos Trans.AGU, 76(38), 369, 380-381, 1995a. Hyndman,R.D., K. Wang,T. Yuan,and G.D. Spence,Tectonic sedi- Trehu,A.M., G. Lin, E. Maxwell, and C. Goldfinger,A seismicreflec- mentthickening, fluid expulsion, and the thermalregime of subduction profile acrossthe Cascadiasubduction zone offshore central tionzone accretionary prisms: The Cascadiamargin offshore Van- Oregon:New constraintson methanedistribution and crustalstruc- couverIsland, J. Geophys.Res., 98, 21,865-21,876,1993a. ture,J. Geophys.Res., 100, 15,101-15,116,1995b. Hyndinan,R.D., G. F. Moore,and K. Moran,Velocity, porosity, and Twiss, R.J., and E. M. Moores, Structural Geology, 532 pp., W. H. pore-fluidloss from the Nankai subduction zone accretionary prism, Freemanand Company,New York, 1992. Prpc.Ocean Drilling Program Sci. Results, 131, 211-220, 1993b. Wang,K., T. Mulder, G. C. Rogers,and R. D. Hyndman,Case for very Jarrard,R.D., Originsof porosityand velocityvariations at Cascadia low couplingstress on the Cascadiasubduction fault, J. Geophys. accretionaryprism, Geophys. Res. Lett., 24, 325-328,1997. Res., 100, 12,907-12,918, 1995. Jarrard,R.D., M. E. MacKay, G. K. Westbrook,and E. J. Screaton, Wang,K., J. He, and E. E. Davis, Transformpush, oblique subduction Log-basedporosity of ODP siteson theCascadia accretionary prism, resistance,and intraplatestress of the Juan de Fuca plate, J. Geo- Proc.Ocean Drilling Program Sci. Results, 146, 313-336, 1995. phys.Res., 102, 661-674, 1997. Kent,G.M., R. S. Detrick,S. A. Swift,J. A. Collins,and I. I. Kim, Evi- Westbrook,G.K., Geophysical evidence for the role of fluidsin accre- dencefrom Hole 504B for the originof dippingevents in oceanic tionary wedge tectonics,Philos. Trans.Royal Soc. London,Ser. A: crustalreflection profiles as out-of-planescattering from basement Math. and Phys.Sci., 335, 227-242, 1991. topography,Geology, 25, 131-134,1997. Wilson, D.S., Deformation of the so-called Gorda Plate, J. Geophys. Lewis,B.T.R., Changes in P and S velocitiescaused by subductionre- Res., 94, 3065-3075, 1989. latedsediment accretion of Washington/Oregon,in Shear Wavesin Wilson, D.S., Confidence intervals for motion and deformation of the Marine Sediments,edited by J. Kloven and R. Stoll,pp. 379-386, Juande Fuca plate,J. Geophys.Res., 98, 16053-16071, 1993. Kluwer Acad., Norwell, Mass., 1991. Yuan, T., G. D. Spence,and R. D. Hyndman,Seismic velocities and in- MacKay,M.E., G. F. Moore,G. R. Cochrane,J. C. Moore,and L. D. ferred porositiesin the accretionarywedge sedimentsat the Cas- Kulm, Landwardvergence and oblique structural trends in the Ore- cadiamargin, J. Geophys.Res., 99, 4413-4427, 1994. gonmargin accretionary prism: Implications and effect on fluid flow, Earth Planet Sci.Lett., 109, 477- 491, 1992. SamuelH. Clarke, Jr., Coastaland Marine Geology,United States MacKay,M.E., G. F. Moore,D. Klaeschen,and R. von Huene,The GeologicalSurvey, 345 MiddlefieldRoad (MS 999), Menlo Park, CA caseagainst porosity change: Seismic velocity decrease at the toe of 94025. (e-mail: sclarke•octopus.wr.usgs.gov) theOregon accretionary prism, Geology, 23, 827-830,1995. SeanP.S. Gulick andAnne S. Meltzer, Departmentof Earthand En- McKenzie,D.P., and W. J. Morgan,Evolution of triplejunctions, Na- vironmentalSciences, Lehigh University, 31 Williams Drive, Bethle- ture, 224, 125-133, 1969. hem,PA 18015. (e-mail:ssg2•lehigh. edu; asm3•lehigh.edu) McPherson,R.C., Seismicityand stressat the southernend of the Cas- cadiasubduction zone, in FieM Trip GuideBook, pp. 19-28, Am. As- soc.of Pet. Geol.-Soc.for Sediment.Geol., Tulsa, Okla., 1992. Moore,J.C., G. F. Moore,G. R. Cochrane,and H. J. Tobin,Negative- (ReceivedJanuary 20, 1998;revised June 12, 1998; polarityseismic reflections along faults of the Oregonaccretionary accepted July 30, 1998.)