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 defor- mationfront. 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 boldarrows 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, de- watering, 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