The interplay between global tectonic processes anWord文件下载.docx

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The interplay between global tectonic processes anWord文件下载.docx

ThepurposeofthisworkistoanalysetheinteractionbetweenglobaltectonicsandlocalscaleseismicityinaselectedregionoftheMediterranean,thecentralApennines.SeveralresultshavebeenpublishedonthemodellingoftectonicprocessesintheApennineregionandthestudyoftheirinfluenceoncrustalmotionsorstressandstrainaccumulationusingdifferentapproaches(e.g.Malinverno&

Ryan1986;

Bassi&

Sabadini1994;

Giunchietal.1994;

Faccennaetal.1996;

Carminatietal.1999,2001;

Negredoetal.1999a,b)However,noneofthesehasfocusedontherelationbetweentectoniceandtheseismiccyclewithinaselectedseismogeniczone.InthisworkwestudytheinfluenceofglobalscaletectonicsontheseismiccycleoftheColfioritofault,responsiblefortheUmbria–Marche1997sequence.TheApenninesdevelopedinaregionaffectedbythecollisionbetweenAfricaandEurasiaandbythesubductionoftheAdriaticlithosphere.ThetectonicevolutionofthechainstartedintheOligocene,whenacollisionalphasetookplaceduetotheconvergencebetweentheSardinia–CorsicablockandtheAdriaticdomain(Boccalettietal.1983).Extension,initiallyinawest–eastandevolvinginanorthwest–southeastdirection(Pataccaetal.1990),startedintheLateOligocenewiththe

1INTRODUCTION

openingoftheAlgero–Proven¸

calBasin,westofCorsica–Sardinia(Rehaultetal.1984),itcontinuedwiththeopeningoftheNorthernTyrrhenianBasinintheMiddleLateMiocene(Lavecchia1988;

Sartori1989)andmigratedsoutheastwardswithtime(e.g.Spadinietal.1995).ThiseastwardmigrationthroughtimeoftheextensionaltectonicsintheTyrrhenianBasinandoftheexternalcompressionalfrontisapeculiarityoftheevolutionoftheApenninechain(RicciLucchi1986)andhasbeeninterpretedasbeingduetodifferentgeodynamicmechanisms.Malinverno&

Ryan(1986)proposedaninterpretationbasedonamechanismoftrenchretreatorroll-backofthesubductingAdriaticlithospherethatwouldcausetheopeningofabackarcbasin.Analternativeinterpretationhasbeenproposed(e.g.Wezel1982),whichconsiderstheasthenosphericupwellingrelatedtotheriftingprocessintheTyrrhenianSea,asaforcinggeodynamicmechanism.Furthermore,Cavinato&

DeCelles(1999)proposedthatthebimodalstateofstressintheApenninesismaintainedbycornerflowinthemantlewedgebeneaththecrestoftherange.SubductionincentralItalyissuggestedbythepresenceofseismicitydownto90km(Selvaggi&

Amato1992)andbygeochemicalanalysesofthenorthernApenninearcmagmatism(Serrietal.1993).However,Spakman(1990),onthebasisoftomographicresultsandthereducedseismicityatintermediatelevels,suggestedthattheslabmaybetotallyorpartiallydetached,whileAmatoetal.(1993)interpretedthepresenceofahigh-velocitybodydownto200–250kmasevidenceofacontinuousslab.ItisthusclearthatthetectonicsettingofnorthernApenninesisverycomplexandthatafinalexplanationoftheroleoftheseveralongoingprocesseshasnotbeenreachedyet.

Afinite-elementtechniqueisusedtomodelthestresspatternalongaverticalcross-sectionincentralItalyperpendiculartotheApennines:

thisapproachimprovesourunderstandingoftheactivetectonicprocessesintheregionunderstudy.Inordertoanalysetheinfluenceonthestressfieldfromvarioustectonicmechanisms,severaldifferentmodelshavebeenperformed,eachonecharacterizedbydifferentdrivingforces.WeinvestigatedtheeffectsofthenegativelybuoyantsubductedAdriaticlithosphere,theeffectsoftheextensionalSW–NEorientedforcesexpectedfromthecounterclockwisemotionoftheAdriaticPlateandtheeffectsofsmall-scaleconvectionatthebottomoftheTyrrhenianlithospheredrivenbythechangefromapassivetoactiveriftingmode(Huismansetal.2001).Thestressdistributioninducedintheseismogenicpartofthecrust

byeachtectonicmechanismiscomparedwiththestresspatternoftheregiondeducedfromearthquakefocalmechanisms,boreholebreakoutanalysis,insitustressmeasurementsandyounggeologicaldeformationfeatures(Frepoli&

Amato1997;

Montoneetal.1999).ThesedataarecomplementedbyrecentGPSbaselineratesobtainedfromnewGPSreceiversinstalledalongthetransectunderstudy.

InFig.1(WSM2000,Muelleretal.2000)thetraceofthemodelledsectionisindicatedbythelettersA-A.Movingalongthisdirection,fromwesttoeast,stressindicatorschangefromextensional(redsymbols)inTuscanyandintheinternalsectorsoftheApenninebelt,tocompressional(bluesymbols)intheAdriaticBasin.SimilarstressmapshavebeencompiledbyReba¨

ıetal.(1992)andMontoneetal.(1999)

Stresschanges,obtainedbymodellingtheseismiccyclealongashallowlow-anglenormalfault,withstrikeperpendiculartotheprofileunderstudy,havebeensuperimposedonthestressfieldgeneratedbythedifferenttectonicprocesses.ThesameapproachalreadyusedforthesimulationoftheMexicansubductionzone(Gardietal.2000)hasbeenfollowedinthepresentwork,althoughthemodellingoftheseismiccycleismorerealisticinthepresentstudy.Infact,thelockedandfreealternatingphasesonthefault,reproducingthestick–slipprocess,aresimulatedinadifferentwaywithrespecttotheapplicationcarriedoutbyGardietal.(2000).Inthatstudythelockingofthefaultwasobtainedbyimposingazerox-andy-displacementtothenodesofthegriddefiningthefault,whiletheearthquakewassimulatedbyimposingafixeddisplacementtothesamenodesinordertoreproducethedislocation.Inthepresentanalysis,lockingisreproducedbyimpedingtherelativemotionofthetwosidesofthefaultbutleavingthefaultfreetomoveinthexy-plane.Tosimulatetheoccurrenceofanearthquake,thetwosidesofthefaultareleftfreetomoveonewithrespecttotheother,alongafrictionlessinterface,drivensolelybythestressaccumulatedduringtheloadingperiodoftheactivetectonics.TheapproachdevelopedinthepresentstudyismorerealisticthanthatinGardietal.(2000)andrepresentsasubstantialimprovementwithrespecttopreviousmodelling.Itisworthemphasizingthatthisnewmethodfacilitatesrelatingtheslipobtainedduringaseriesofsubsequentearthquakestotheearthquakerecurrencetimeandactivetectonics.

2MODELDESCRIPTION

InordertosimulatethesinkingoftheAdriaticPlateunderneaththeApenninesandtheseismiccycleonashallowlow-anglenormalfault,theMARCpackage(MSC.Marc2001)basedonafiniteelementtechniquehasbeenused.Thesimulationswereperformedunderaquasi-staticapproximationandthemodelsarepurelymechanical,sincethecouplingbetweenthemomentumandthetemperatureequationsisnottakenintoaccount.Twomodelsareconsidered,eachonerepresentinga2-Dverticalcross-section(Fig.2)perpendiculartothestrikeoftheApenninesandoftheUmbria–Marcheseismogeniczone,asshowninFig.1bythetraceA-A.The2-Ddomainissubdividedintoquadrilateralplane-strainelements,creatinganirregularmesh.Theelementdensityisincreasedaroundthediscontinuitiesinordertoensureagoodapproximationofthesolution.Themodelledsectionis400kmwideand420kmdeep.

Thesimplifiedgeometryoftheplatesandthethicknessofthedifferentlayershavebeendefinedfollowing,forthecrustallevels,theresultsoftheseismiclineCROP-03(e.g.Piallietal.1998),andforgreaterdepths,theresultsoftomographicstudies(Piromallo&

Morelli1997;

Chimeraetal.2003).Sincethedepthreachedbythe

plunginglithosphereisstillamatterofdebate,twohypotheseshavebeentested:

adepthof90kmasinferredfromsubcrustalintr

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