directed deep-subduction model

directed deep-subduction model directed deep-subduction model GEOSCIENCEFRONTIERS2《l1(2011)l—l5 availableatwww.sciencedirect.tom ChinaUniversityofGeosciences(Beijing) GEoSCIENCEFRoNT皿RS journalhomepage:www.elsevier.confflocate/gsf ?ORIGINALARTICLE SeismotectonicsinthePamir:AnobliquetranspreSSi0nal shearandsouth—directeddeep-subductionmodel JiashengZhang水,XinjianShan,XiongnanHuang InstituteofGeology,ChinaEarthquakeAdministration,Beijing100029,China Received20May2010;accepted15September2010 Availableonline28December2010 KEYWORDS Earthquake; Seismotectonics; Deep-subductionof continentalcrust; Transformfault; Rampthrusting; CentralAsia AbstractThe3一 DgeometryoftheseismicityinHinduKush-Pamir--westemChinaregionhasbeen definedbyseismicrecordsfor1975-1999fromtheNationalEarthquakeInformationCenter.t heU.S. GeologicalSurvey,andover16,(M】 0relocatedearthquakessince1975recordedbvtheXinjiangseismic networkofChina.Theresultsshowthatmost>5.0hypocentersintheareaareconfinedtoamaior intracontinentalseismicshearzone(MSSZ).TheMSSZ.whichdipssouthwardsinPamirhasanorth— dippingcounterpartjntheHinduKushtothewest;thetwotectonicrealmsareseparatedbythesinistra1 ChamantransformfaultoftheIndia-Asiacollisionalzone.WedemonstratethattheMSSZconstitutes theupperboundaryofasouth— dipping,activelysubductingPamircontinentalplate.Threeseismic concentrationsarerecognized{ustabovethePamirMSSZatdepthsbetween45-65km.95-120km. and180-220km,suggestingdifferentstructura1relationshipswhereeachoccurs.Resultsfromfocal mechanismsolutionsinallthreeseismologicalconcentrationsshoworientationsoftheprincipal maximumstresstobenearlyhorizontalinanNNW—SSEdirection.Thesouth— dippingPamirsubduction slabiswedge— shapedwithawideuppertopandanarrowdeeperbottom;theslabhasagentleangleofdip intheupperpartandsteeperdipsinthelowerpartbelowanelbowdepthofca.80-120km.Mostofthe deformationrelatedtotheearthquakesoccurswithinthehangingwal1ofthesubductingPamirslab. PublishedgeologicdataandrepeatedGPSmeasurementsinthePamirdocumentabroadsupra—subduc. tion,uppercrustalzoneofevolvingantitheticri.e.noah,dipping)back— thruststhatcontributetonorth— SOUthcrustalshorteningandareresponsibleforexhumationofsomeultrahigh— pressurerocksformed duringearlierTethyanplateconvergence.AnalternatingoccurrenceinactivityofPamirandC haman Correspondingauthor.Te1.:+861062009207. E?mailaddress:jszhang@public3.bta.net.cn(J.Zhang). 1674—9871@2011.ChinaUniversityofGeosciences(Beijing)andPeking University.ProductionandhostingbyElsevierB.VAllfightsreserved. Peer-reviewunderresponsibilityofChinaUniversityofGeosciences (Bering). doi:10.1O16/j.gsf.2010.11.002 J.ZhangetaL/GeoscienceFrontiers2(1)(2011)1--15 Figure1Flowchartshowingseismicdatasourcesandprocessing 匡雪Majorandsecondaryfault囫Intermediate?depthseismiczone图 Meso-Cenozoicbasin目section Figure2Sketchmapshowingtectonicsinthestudyareaandlocationsofprofilelines 4J.Zhangeta1./GeoscienceFrontiers2(1){2011)l—l5 Figure3Geographic—depthprojections(a,b,C,d)ofearthquakesinthestudyarea.((a):P— Pamir;TN—NorthTianshanMountains TS—SouthTianshanMountains;K—Karakorum;Xl,X2一 thesurfacetraceoftheChamanfaultzone). E . 一 , n n) 凸 ? . — C O- 0 o N 圈M<5.0囤M?5.0目MSSZ日LSSZ Figure4SectionalprojectionsofearthquakesinthePamirregion. 日Reversedthrust —E兰u1&?D j.Zhangeta1./GeoscienceFrontiers2(1)《2011)1-15 E ,一 t- ? ? 0 ? ,', , ? o E 邑 t- 苞 m 0 E 兰 , 0 逸堡三 一 , , /. ,I, . 一Section9\{n. 团M<5.0 圈Ms?50 日MSSZ 目LSSZ 日ReVersedthrust Figure5SectionalprojectionsofearthquakesintheChamanandHindu--Kushregions magnitudesduring2000.1-2003.7(Fig.11.BecausetheXJSN consistsof40regionalseismicstationsand1ltelemetry stations,theprecisionoflocationhasgraduallyimprovedsince the1970s.SomeoftheearthquakerecordsintheXJSNmaior seismiccataloguewithunclearphasesandinitialmotiondirec— tionshavebeenrelocatedusingtheprogramBLOC96(Zhao eta1..1993).Thiswasdonebycheckingtheirphases,arrival timesandamplitudesonoriginalseismicrecordsfromtheXJSN andChinaseismicnetworks(csN,includingdataobtained fromexchangesbetweenChinaandthethreeaacentmid—Asian countries(Kazakhstan,KyrgyzstanandTajikistan).Besides,the micro—earthquakesfromtheXJSNcatalogueareselectedto havehorizontalandverticalerrorslessthan5and10km, respectively.Forthisstudy,3364relocatedmajorea~hquakes (1975--1999),6402micro—earthquakesand7258earthquakes of1stand2ndclassrecords(2000.1-2003.71fromtheXJSN 6j.Zhangeta1./GeoscienceFrontiers2(1){2011)i一15 a E C t- 苞 0 o Figure6(a)Two—dimensional,and(b)three— dimensionaltopographicdistributionofthemainseismicshearzone(MSSZ),and(c)the geometryofthedeep—subductionslab. catalogueswereconsideredacceptable.Aftercheckingover— lappingrecordsbetweentheNEICandtheXJSNcatalogues. 22,200earthquakerecordswereused.Inaddition,thisseismic analysisused46ea~hquakefoCHSmechanismsolutionscalcu— latedbymeasuringinitialmotiondirectionsofP—wavesacquired fromtheXJSNandCSN.243Centroid—Moment一IIensor(CMT, datawerecollectedfromtheHarvardCMTPmiect.and17 earthquakefclcalmechanismsfromtheUnitedStatesGeologica1 SurveyfUSGS). 3.Analysisof3-Dgeometryofseismicity Theearthquakeswereplottedaccuratelyonplanesatdifierent depthsandprojectedinto42vertica1profileswithdifferent orientations(Fig.21.Oursubsequentanalysisoftheseplots showednotonlythe3.Dgeometryoftheseismicity,butalsothe specificseismotectonicsdefinedbyseismicshearzonesand seismicconcentrations.Ithasbeen,therefore,possibletodefine subductingslabs.andtodiscusstheseis. theshapeofthedeep— motectonicsanddynamicsofdeep—subductioninthisregionin combinationwithfocalmechanismdata,surfacegeologyand seismicvelocitystructureinthestudyarea. Geographic—depthproJectionsoftheearthquakesinthe Pamir—HinduKush--westernChinaregionshowthatshallow earthquakesabove70kmdepthoccurdiffusely(Fig.3.0—70km) andcanberoughlygroupedintoouterandinnerseismiczones. Theearthquakesintheouterzoneareprimarilydistributedalong theSW-trendingTianshanMountainsrangewithintensity decreasingtowardtheNNW(Fig.3,O一7Okm,A卜3).Meanwhile, theconvexnorthwardsinnerzoneiscomposedofthreeparts. including:anarc—shapedseismiczone(asthecentralpart)along thenorthernrimofthePamirplateau(Fig.3,0—70km,P):the SE—strikingseismiczone(astheSEsegment)oftheKarakorum (Fig.3,0—70km,K):andtheNE—strikingseismiczone(asthe SWsegment)neartheHinduKush(Fig.3,0--70km,XI一2). Accordingt0surfaceinvestigationsofthebroaderdeformation zoneinthisregioncausedbycontinental-continentalcollision followingtheclosureofTethys(Windley,1988;Faneta1.,l994; Searle,1996).theshallow—foCHSearthquakesoccurringinthe outerzonearethoughttoberelatedtoaseriesofactivesouth— dippingthrustfaultsintheSWTianshanMountainsorogenicbelt, whichdisappearatdepthsbelow70km(Fig.3).wesuggestthat theshallow—focusearthquakesintheSWandNEsegmentsofthe innerzonecorrespond.respectively,toleft—latera1strike—slip displacementalongtheChamantransformfaultinthewest,and 一,兰Sclm0 j.ZhangetaL/GeoscienceFrontiers2(1)(2011)1--157 80~142.80~/40.80~38.80~/36.8o~E/34~N I n——I400.nce(mj600J800一plJ,200 一…啾一 I—T^,:,D,,:1Section80\1Qn口 j'."...".一I… 79~/42.79~/40.79938.79936.79~E/34~N ….. I.一一l400,,l600I800l9( , 一, —一Jd..9,180. {一一TarimBasin一 N 一 I一.J,,,,殳l— l一600fI800{9 囱嚣I毒——,"一埔蕊?一p,'一 一 .,U0. 一 s 小… Figure7SectionalprojectionsofearthquakesintheTarimBasin right—lateralstrike.snipdisplacementalongtheKarakol-tlmfault intheeast:earthquakesinthemid—frontarcarerelatedtosouth— ward—dippingthrustingbeneaththePamir.Inmapview,the intermediate—todeep—foCUScarthquakesbelow70kmdepth aredistributedinanS—shapezone(Fig.3and71—130and 13l一200km).consistingofacentralstraightNE—trendingseismic zonewithadjoiningEW-trendingseismiczonesatitsnortheastern andSOUthwesternends.ThemiddlesegmentoftheSshaped seismiczonecorrespondstoacontinuationdownwardsofthe shallowleveloftheSWsegmentoftheinnerzone(Fig.3a,X1一,). Itis,therefore,probablyrelatedtotheChamanstrike—slipfault. Thes~ongerE--W-trendingintermediate..tOdeep..foCUSseismic 8J — .Zh — angeta1./GeoseienceFrontiers2(1)(2011)l一15 ? . ,一 , ? ? 0 Amountofearthquakes Figure8Distributionof>5.0earthquakesatdepth zoneatthenortheasternendofthemid—segmentextendsdown— wardstotheshallow—foCUSPamirseismiczone,anddisappears below200kmdepth.WhereasthesmallerHinduKushseismic zoneattheSWendofthemid—segmentextendsdownwards.both thePamirandHinduKushseismiczonesshrinktowardstomiddle NE—trendingseismiczone.TheS-shapedintermediate—todeeD. 1ocusseismiczone,therefore,mayindicatethattheChaman transformfaultmotivatesE-W-trendingthrustingoneitherside ofthezone. Forprofileplotting(Figs.4,5and71,earthquakesoccurring wlthinverticalcorridorsabout80kmwide, andextending0.5.to eithersideoftheselectedsectionlines, wereselectedfromthe seismicdatabase.Errorscausedbytheearth'scurvaturealongthe directionoftheprofilewerecorrectedusingtheplottingprogram, andIon.gitudeandlatitudecoordinatesoftheearthquakeSOHI'Ces weretransformedintodistances(km)fromthestartingpointofthe profilewhenplottedontothecentraIprofile.withdepthsinkilo meters.Theplottedprofiles(Figs.4,5and7)areinkm—scale.and aremarkedbyreferencepointsoflongitude/latitude. 4?Three_dimensionalgeometryofthesubducting Pamircontinentalplate Earthquakesplottedonthecorridorprofilesathighanglestothe Famirseismiczonelieprimarilywithintwoseismicshearzones andInvertedtriangularareasabovethezones(Fig.4).Onall profilesofFig.4perpendiculartothePamirearthquakezone. adiscontinuouslydevelopedmajorseismicshearzone(MSSZ) separatesanupperdomainthatischaracterizedbymajorquakes (?5.0)fromalowerdomainwithafewsmallearthquakes.It. theMSSZ,isinterpretedasdefiningthe,0p0ffbesubducting continentalPamirplate.The1owerseismicshearzonefLSSZ)lies withintheplateandisroughlyparalleltotheMSSZ;bothshear zonesareinclinedtothesouthintheNEsegment(Pamir1ofthe — shapedintermediatedepthearthquakezone(Fig. 4).Inthe middlesegmentoftheS-shapedintermediate—depthearthqua】(e zone,theearthquakeprojectionpresentsanearlyverticalstrong— earthquakezonewithadepthof300km(Fig.5,Section5),which isgraduallysubstitutedbyasouth—dippingearthquakezone (Fig.5,Sections6—9).IntheSWsegment(HinduKush1oftheS— sbapedintermediate—depthearthquakezone,earthquakesdecrease andthedoubleseismicshearzoneshowsanNWinclination (Fig.5,Section3and4). Thesouth—dippingsubductionslabexhibitsadownwardna卜 rowinggeometryaswellasapronouncedsteepeningbelow adepthof80--120km(Figs.6andl1).Thisslabextends continuouslytotheeastfromthePamir,butitssubductiondepth decreasesrapidlyanditissplitintotwoindependentsegments slipfault.One aRerltcrossestheKarakorumright—lateralstrike— belt1slocatedonthenorthsideofthesouthwestTianshan MountainsandtheotheronthenorthsideoftheWestKunlun MountainsinChina.Thedepthsofbothsubductionsegmentsare lessthan120km(Fig.7).ThePamirdeepsubductingslabstoDs westwardattheNE_strikingChamantransformfault.Thereis asmallNW—dippingseismiczoneintheHinduKush,westofthe Chamanfault,whichprobablyrepresentsatectonicresponseto theleft—lateralmovementofthatfaultasdiscussedinmoredetail elsewhere. Besidesthedoubleearthquakeshearzonesmentionedabove , threeearthquakeconcentrations(upper,middleandlower)callbe distinguishedfromthesectionalprojectionsofmostoftheM>5 earthquakes(Figs.4,5,7and8).Theuppermostearthquake concentration(UEC)occursatadepthofca.50km(45-65kin). ItextendscontinuouslytotheeastalongthestrikeofthePamir subductionslab,butabruptlyandmarkedlyveerstothesouthin theHinduKushaftercrossingtheChamanstrike—slipfaults.The middleearthquakeconcentration(MEC)liesroughlyatadepthof 100km(95-120kin),whereasthelowerearthquakeclustering occursnearerto200kmdepth(180-220hn).Thelower concentration(LEC)becomesshalloweralongstrikebefore merginglntothemiddleearthquakeclusterandfinallydis— appearmgaftercrossingtheChamanfault. A儿threeoftheearthquakeconcentrationsoccuralongthe MSSZandinrocksofitshangingwallneartheboundary.Their distributionisrelatedtothenatureofcrustalstructuresatthese J.ZhangetaL/GeoscienceFrontiers2{i)(2o/i)1--159 aUpperseismicconcentration(N=75) Figure9Focalmechanismsofearthquakesin(a)theupperand(b)thelowerseismicconcentrat ions,andastatisticalanalysisofseismotectonic elements. positions.Amongthem,theupperearthquakeconcen~ationmay respondstothebrittle-ductiletransitionandchangingofrock frictionalbehaviorofthefelsiccrust,wherethegenerationof earthquakesobeysthetwo—phasedeformationmechanism(Sibson, 1980,1992;Zhang,1987).Themiddleearthquakeconcentration 1ieswithinadepthrangesimilartothatwherethedipangleofthe Pamirdeep—subducdonslabchangesfromgentletosteep;the lowerearthquakeconcentrationoccursapproximatelyatthefront ofthedeeplysubductedslab.Theoccurrenceoftheearthquakesin themiddleandlowerconcen[ra,ionsmaynotblerelacedsolelY60 themechanicsofdifferentsectionsofthedeep—subductionslab, butmayalsobeaffectedbytheinteractionbetweencold(the J.Zhangeta1./GeoscienceFrontiers2(1)I2011)卜一I5 N W \一一一一 __ ,,,==,:=,:,= :::!:,:;L=:::..ncentra".rc- P3=24n) 0 , , , , , , , 52(710. , l , Figure10Orientationoftheconjugateshearfracturesandstressfieldsderivedfromfocalmec hanismsofearthquakesintheupperandlower seismicconcentrationsandalongtheChamanfault. subductedslab)andhotinsituuppermantlerocks(Vinniketa1., 1977),aswellasbyrheologicalinstabilitiesinducedbythis interaction(Hobbs,1986;Aki,1992;ClarkeandNorman,1993). 5.Relationshipbetweenleft-lateralstrike-slip movementoftheChamanfaultandthePamir earthquakezone SeismicityassociatedwiththeChamantransformfaultisrepre— sentedbytwoseparate.nearlyve~icalseismiczonesatadepthof ca.150km(Fig.5.Section51.Previousstudyoffocalmecha- nismsindicatesthatearthquakesourcesintheChamanfaultzone arepredominantlystrike—slipfaults(PeglerandDas,1998). However,accordingtothepresentlimitedresultsoffoca1mech— anismsolutions.earthquakesourcesin山eupperandlower earthquakeconcentrationsinPamir,nexttotheChamanfault,are predominantlythrusts.Theanalysisoffocalmechanismsforthe upperand1owcrearthquakeconcentrations(Fig.9a.b1indicates thatalmostalloftheprincipalcompressivestressaxesarenearly horizonta1andorientedNNW-SSE.Oneoftheconjugateshear fractures(F,inFig.9a'b),paralleltotheMSSZ,isthoughttohave E t-- ? n) o formedinresponsetomovementinthesouth—dippingsubduction slab.Poleprojectionsshowthatdip—directionsofF2differslightly inbothoftheupperandlowereaghquakeconcentrations,butdip— anglesofF,inlowerseismicconcentrationf155,60areevidently largerthanintheupper(165/33).Theproduceddip—angleshave beenusedtodefineasteeperdownwardsshapeofthesubduction slab(Fig.1O1.Foca1mechanismsfromtheChamantransformfault andthesubductingslabinthePamir,asmentionedabove,show ditterentsynchronouskinematics. Time—spacerelationshipsofearthquakesduring1975—2003. indicatethatM>5ea~hquakeswithafocaldepthover200km arepredominantinthesouthemsegmentoftheChamanearth— quakezone(Figs.3and5).HoweverinadditiontotheChaman seismiczone.intermediate—deepsourceea~hquakeswithdepths 1essthan200kmaremostlyrelatedtothePamirea~hquakezone. Analternatingoccurrenceintimeofthesetwoearthquakeszones indicatesthatthereisinteractionbetweenstrike—slipmovementof theChamantransformfaultsystemandde?