Can belemnite distribution reveal pressure-solutio

Canbelemnitedistributionrevealpressure-solutionprocessesalongfaults?AcasestudyinthechalkoftheMonsBasin,BelgiumJ.Angeliera,*,S.Vandyckeb,F.Bergeratc,P.Gavigliod,C.Schroedere,M.CoulonfaObservatoireOce´anologiquedeVillefranche,UFR939Universite´Pierre-et-MarieCurie&UMR6256Ge´osciencesAzur,BP48,06235Villefranche-sur-Mercedex,andInstitutUniversitairedeFrance,FrancebFNRS,Faculte´PolytechniquedeMons,Ge´ologiefondamentaleetapplique´e,9ruedeHoudain,7000Mons,BelgiumcLaboratoiredeTectonique,Universite´PierreetMarieCurie,FR32CNRS-UPMC,CEPAGE,case117,4placeJussieu,75252Paris,FrancedEA2642Ge´osciences,Universite´deFranche-Comte´,16RoutedeGray,25030Besanc¸oncedex,FranceeGEOMAC,Universite´deLie`ge,ChemindesChevreuils,1,B52/3,4000Lie`ge;ULB,Lab.JacquesVerdeyen,av.Roosevelt,1050Brussels;UCL,DeptGe´nieCivil&Environnement,1459Louvain-La-Neuve,BelgiumfLaboratoiredesSciencesdelaTerre,CRA,2,esplanadeRolandGarros,51100Reims,FranceReceived8November2004;receivedinrevisedform4August2005;accepted8August2005Availableonline10October2005AbstractAdetailedstudyofnormalfaultsinthecalcarenitesandchalksoftheMonsBasinrevealsthatpressure-solutionprocessesplayasignificantroleinfaultzones,whichaffectsthedeterminationoftheamountsofextension.Dragfoldingassociatedwithnormalsimpleshearandrockdissolutionconcurredtoproducelayerflexuringinthehangingwallofthenormalfaults,whereasthefootwallremainedalmostintact.Analysingthedistributionoffossilbelemniterostrainandawayfromfaultzonesenabledustoevaluatetheseeffects.Thesenseandamountofbelemnitetiltconstraintheminimumthicknessofdissolvedhangingwallfringe,revealingsignificantpressure-solutioninadditiontodragfolding.Wedeveloped2-Dand3-Danalyticalmodelsofalong-faultbelemniterotationsinducedbydragfoldinganddissolution,andweappliedthesemodelstoourdatafromtheMonsBasin.Theresultsshowthatthepressure-solutioneffectissignificant,implyingasmallerratiobetweenhorizontalandverticalcomponentsofmotionthanexpected.Itfollowsthattheacross-faultamountofextensionissmallerthanthatpredictedfromthesinglefaultgeometry,suggestingthatalthoughtheexistenceofCampanian–Maastrichtian–DanianextensionintheMonsBasinareaisbeyonddoubt,itsamplitudeshouldnotbeoverestimated.q2005ElsevierLtd.Allrightsreserved.Keywords:Normalfaults;Dragfolding;Simpleshear;Pressure-solution;Dissolution;Belemnite;Chalk;Calcarenite;Cretaceous;MonsBasin1.IntroductionNumerousstudieshavehighlightedcloseinteractionbetweenfaultingandpressure-solutionprocesses(Ramsay,1977;Hancock,1985;Gratier,1987;Carrio-SchaffhauserandGaviglio,1990;DunneandHancock,1994;Peacocketal.,1998).Inchalk,thedissolutionandmigrationofcalciumcarbonateanditsrelationtostresswasdiscussedbyMimran(1977),basedonexamplesfromtheIsleofWightandDorset.However,relativelylittleisknownconcerningthequantificationofdissolvedmaterialalongfaultsandfractures.Whetherornotpressure-solutionphenomenaarecloselyrelatedto,andquantitativelyinfluence,thedevelopmentoftectonicbasinsisgenerallypoorlydocumented.InwesternBelgium,theMonsBasinareaexperiencedsimultaneouschalksedimentdepositionandnormalfaultingduringthelatestCretaceous(lateCampanian–lowerMaastrichtian).Notonlydidtectonicanalysesinthechalklayersrevealwidespreadnormalfaulting(althoughmostoffsetsremainedsmall),buttheyalsoprovidedevidenceofsyndepositionaltectonism(Vandyckeetal.,1991).Typicalpressure-solutionfeatures,suchasstyloliticjoints,areuncommoninthechalkoftheMonsBasin.Fewstyloliticpeakscouldbefoundbutwereverysmall.Thispoordevelopmentofstylolitesisaconsequenceofspecificchalkproperties,especiallyhighporosity.JournalofStructuralGeology28(2006)64–82:10.1016/j.jsg.2005.08.007*Correspondingauthor.Tel.:C33493763754;fax:C33493763766.E-mailaddress:[e-mail](J.Angelier).Pressure-solutionremainsdiffuseinchalk,incontrastwithcompactlimestoneswherewatercirculationcommonlyoccursalongjoints.Qualitativedata,however,revealedsyntectonicpressure-solutionphenomenainthewhitechalkformation,Campa-nianinage.Somepartlydissolvedbelemniteswerefoundinfaults(Marlie`re,1970).Later,petrophysicalanalysesrevealedthatsolutionphenomenaaffectnarrowzones,afewcentimetreswide,alongfaults.Itcouldthusbesuggestedthat‘theextensionaldeformationoftheMonsBasinmighthavebeenpartiallyhinderedbychemicallyinducedmatrixstrains’(Gaviglioetal.,1999).Theseanalysesfurtherprovidedsomeevaluationoftheminimumpressure-solutionshorteningbasedonthereductionofporosityobservedinafault-perpendicularthicknessfrom0.09to0.18m(seeTable3inGaviglioetal.,1999).Dissolutionprobablyoccurredacrosssomesetsofjointsbutpressure-solutionseamsdonotshowstructuresthatwouldallowquantificationofdissolutionprocesses.Inthispaperweaimtobettercharacterisethebalancebetweenthepressure-solutionphenomenaandtheslip-relateddeformationalongfaults,basedonconsiderationofspecificbenchmarks:thebelemnitefossils.Ithasbeenobservedthatclosetofaultsbelemnitesoftenshowsteeptiltanglesthatareuncommonatlargerdistancesfromfaults(Gaviglioetal.,1999).Weconsequentlycarriedoutsystem