Structure and Deformational Character of Strik

PAGEOPH,Vol.124,No.1/2,(1986)0033-4553/86/020203-2151.50+0.20/091986BirkhiiuserVerlag,BaselStructureandDeformationalCharacterofStrike-SlipFaultZonesQIDONGDENG,1DANINGWu,1PEIZHENZHANG2andSHEFACHEN3Abstract--Strike-slipfaultzonesobservedeitherinthefieldorinmodelexperimentsgenerallyconsistofseveralsubparallelfaultswhichmakethesezonescomplicatedingeometryandkinematics.Thegeo-metryofastrike-slipfaultorshearzoneisdependentonarrangement(pinnateorenechelon),onstep(leftsteporrightstep),andontherank(smallerfaultswithinlargerfaults)ofthesubparallelfault.Therelationsandinteractionsofthesethreefactorscreateavarietyofdynamiccircumstancesandtectonicsettingswithinthestrike-slipfaultzones.Theseincludepull-apartsinthereleaseareabetweensubparallelfaults,push-upsinthejogswherethesubparallelfaultsoverlap,andpivotalmovements,orrotation,ofsinglefaultsalongthewholefaultzone.Eachkindoftectonicsettingisinitselfcharacteristic,eachsettingconsistsofmanysubtypes,whicharecontrolledchieflybythegeometricparametersofthesubparallelfaults.Oneofthemostimportantphenomenarevealedinthefieldworkistwodifferentkindsofevolutionofstrike-slipfaultzones:oneistheevolutionofazonewithatensilecomponent,whichisrelatedtothegrowthofrockbridges,andtheother,ofonewithacompressionalcomponent,whichdevelopsbythedestructionofrockbridges.Inthispaperwediscuss,onthebasisofrecentresearchonfourstrike-slipfaultzonesinChina,theessentialcharacteristicso(strike-slipfaultsandthepossiblecausesoftheobservedstructuralphenomena.Attentionisfocussedonthedeformation,development,anddistributionofhori-zontaldisplacementswithinstrike-slipfaultzones.Keywords:Pinnate,en-echelon,pull-apart,push-up,pivotalmovement.LIntroductionStrike-slipfaultsoftenplayanimportantroleintheintraplatetectonicsofcon-tinents.Therearemanystrike-slipfaultsinChina.SomeofthemactedasthekeystructuresduringCenozoicdeformationinCentralAsia(MOLNARandTAPI'ONNIER,1975),andmanyofthemstillshowintenseactivity.Earthquakesofmagnitudegreaterthan7areoftenrelatedtothem,directlyorindirectly.Recentlywestudiedseveralmajorstrike-slipfaultzonesinChina,suchastheFuyunfaultinXinjiang,theHaiyuanfaultinNinxia,theXianshuihefaultinSichuan,andapull-apartareawithinalargestrike-slipfaultzoneinnorthwesternYunnan;seeFigure1.Onthe'InstituteofGeology,StateSeismologicalBureau,Beijing,China.2DepartmentofEarth,Atmospheric,andPlanetarySciences,MassachusettsInstituteofTechnology,Cambridge,Massachusetts02139.3WuhanGeologicalCollege,Wuhan,China.204Q.Dengetal.PAGEOPH,.,0~..\o~-~~-~l',,~2-~iivoEOOONe~oO[-.-,Vol.124,1986StructureandDeformationofStrike-SlipFaultZones205basisofthegeologywithinandaroundthesefaultzones,weshalldescribesomeofthecharacteristicsthatareofimportanceinunderstandingthedeformationmech-anismofthistypeoffault.II.Geometryofstrike-slipfaultzonesAsrevealedbothbyQuaternaryactivefaultsandbysurfacerupturezonesassociatedwithorrelatedtoearthquakeevents,strike-slipfaultsoftenconsistofaseriesofsubparallelshearfaults.DENGetal.(1966)andDENGandZHANG(1984)showedthatthearrangementofthesubparallelfaultsisoftwotypes,pinnateanden-echelon.Inthepinnatearrangementtheanglebetweentheorientationofthewholefaultzoneandthatoftheindividualsubparallelfault,whichiscalledthepinnateangle,islessthan25~(Fig.2a),andthesubparallelfaultsareshearorshear-plus-tensilefractures.Intheen-echelonarrangementtheso-calleden-echelonangle,whichalsoistheanglebetweentheorientationofthewholezoneandthesinglesubparallelfault,isusuallygreaterthan25~(Fig.2b),andthesubparallelfaultsaretensionalfractures.Figure3showspartofthesurfacerupturezonegeneratedbythe1981DaofuearthquakealongtheXianshuihefault;segmentsA,B,andCarecomposedofen-echelonextensionalfractures,whilesegmentDiscomposedofpinnateones.Withinlargestrike-slipfaultzones,subparallelfaultsareoftenarrangedinapinnatepattern,theen-echelonpatternbeingrare.Forexample,Figure4showseightsubparallelshearfaults1to4kmwideintheHaiyuanfaultzone,andthepinnateangleisonly10~to15~Thelengthofthesubparallelfaultsvariesfrom70toonly10km(DENGandZHANG,1984).AnanalogousexampleappearsintheXianshuihefaultzone.Whenthestrike-slipfaultpossessesarelativelylargeexten-sionalcomponent,thepinnateanglewillbegreater,asitinnorthwestYunnan.Wewilldiscussthislateron.Overlappingbetweentwosubparallelfaultsisacommonfeatureinboththepinnateanden-echelonarrangements.Thepatternofoverlappingconsistsofrightstepsandleftsteps(SEGALLandPOLLARD,1980).Theoreticalanalysisandmodelexperimentswouldshowthatrightstepsoccurinleft-lateralfaultzonesandleftstepsinright-lateralfaultzones(Fig.2),butinthefieldanomaloussituationsarecommon,leftstepsappearinginleft-lateralfaultzonesandrightinright.Wecallthisanomalouspatternreverseoverlapping(DENGetal.,1966,DENGandZHANG,1984).Withintheleft-lateralHaiyuanfaultzone,forexample,aresevensubparallelshearfaults(SFztoSFs)reverse-overlappingeachother,andonlySF1andSF2overlapinthenormalmanner(Fig.4).BOHAa(1982)alsonoticedthisphenomenon.Hesuggestedtwomechanismsthatcanleadtotheformationofreverseoverlapping:theinteractingofen-echelonornoncoplanarfracturesundershearandexte