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

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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