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1、JOURNALOFGEOPHYSICALRESEARCH,VOL.103,NO.B10,PAGES23,813-23,825,OCTOBER10,1998EvolutionoftheGordaEscarpment,SanAndreasfaultandMendocinotriplejunctionfrommultichannelseismicdatacollectedacrossthenorthernVizcainoblock,offshorenorthernCaliforniaNicolaJ.Godfrey1,2,A
2、nneS.Meltzer3,SimonL.Klemperer1,AnneM.Tr•hu4,BeateLeitner4,5,SamuelH.Clarke,Jr6andAmyOndrus3Abstract.TheGordaEscarpmentisanorthfacingscarpimmediatelysouthoftheMendocinotransformfault(theGorda/JuandeFuca-Pacificplateboundary)between126øWandtheMendocinotriplejunc
3、tion.ItelevatestheseaflooratthenorthernedgeoftheVizcainoblock,partofthePacificplate,-1.5kmabovetheseaflooroftheGorda/JuandeFucaplatetothenorth.StratigraphyinterpretedfrommultichannelseismicdataacrossandclosetotheGordaEscarpmentsuggeststhattheescarpmentisarelati
4、velyrecentpop-upfeaturecausedbynorth-southcompressionacrosstheplateboundary.Closeto126øW,theVizcainoblockacousticbasementshallowsandisoverlainbysedimentsthatthinnorthtowardtheGordaEscarpment.Thesesedimentsaretiltedsouthandtruncatedattheseafloor.Bycontrast,inalo
5、calizedregionattheeasternendoftheGordaEscarpment,closetotheMendocinotriplejunction,thetopofacousticbasementdipsnorthandisoverlainbya2-km-thickwedgeofpre-11Masedimentaryrocksthatthickensnorth,towardtheGordaEscarpment.Thiswedgeofsedimentsisrestrictedtothenortheas
6、tcorneroftheVizcainoblock.UnlessthewedgeofsedimentswasapreexistingfeatureontheVizcainoblockbeforeitwastransferredfromtheNorthAmericantothePacificplate,thestrongspatialcorrelationbetweenthesedimentarywedgeandthetriplejunctionsuggeststheentireVizcainoblock,withth
7、eSanAndreasatitseasternboundary,hasbeenpartofthePacificplatesincesignificantlybefore11Ma.1.Introductionplatetothesouthfrombathymetricallyelevated(young)Gordaplatetothenorth.Between126øWandtheMendocinoThreeplateboundariesintersectattheMendocinotripletriplejuncti
8、on,inadditiontothetopographichighofthejunction(Figurela,inset).TheGorda/JuandeFucaplateisMendocinoRidge,thereistheGordaEscarpment,anorthsubductingbeneaththeNorthAmericanplat