The interplay between global tectonic processes an.docx
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The interplay between global tectonic processes an.docx
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Theinterplaybetweenglobaltectonicprocessesan
TheinterplaybetweenglobaltectonicprocessesandtheseismiccycleintheUmbria–Marcheseismogenicregion
ForthecentralApennines,peninsularItaly,aseriesoftectonicmechanismsarereproducedbymeansoffinite-elementmodels,inordertostudytheeffectsofactivetectonicsontheseismiccycleintheUmbria–Marcheseismogeniczone.Continentalextensionandriftpusheffectsinducedbysmall-scaleconvectionaremodelledwithin2-Dviscoelasticmodelsofthecrust–lithospheresystem,inverticalcross-sectionsperpendiculartothestrikeofthemajortectonicstructuresunderstudy,namelytheApenninesandtheColfioritofaultzone,wherethe1997seismicsequencetookplace.Withtheaimofconstrainingtheactivetectonicmechanismsattheregionalscaleandthebehaviourofthefaultintheseismogeniczoneatthelocalscale,modelledbaselinerateofchangearecomparedwithnewlyacquiredGPSdata,retrievedfromthetwopermanentGPSreceiversofCamerino(CAME)andElba(ELBA),deliberatelyinstalledalongthemodelledtransect.ThesereceiversarelocatedatbothedgesofthecontinentalextensioninthefrontoftheApennines,closetotheAdriaticPlateintheeast,andintherearofthechain,intheTyrrheniandomain.Thedeformationpatterninferredfromseismicityandfromthegeodeticdataisconsistentwithsmall-scaleconvectionintheTyrrheniandomain,whichreproducesextensionintherearoftheApenninesandcompressioninthefrontofthechain.Aconvectivemechanism,associatedwithbackarcopeninganddomingoftheasthenosphere,providesanextensionalrate,alongabaselineconnectingtwositesinthefrontofthechain(Camerino)andinitsrear(Elba),comparabletotheobservedbaselinerateofchange.Theviscosityofthelowercrustplaysafundamentalroleindeterminingthestyleofstressinthecrust–lithospheresystem.OnceconstrainedbymeansoftheextensionalbaselinerateinferredfromGPS,themodelledslipacrosstheColfioritofaultandthemodelledearthquakerecurrencetimeareconsistentwiththe1997normalfaulteventandwithpalaeoseismicity,respectively.
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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