库车褶皱冲断带西段盐底辟成因机制

库车褶皱冲断带西段盐底辟成因机制Abstract

FoldingandthrustingplaysanimportantroleinthestructuralevolutionoftheKuqaForelandBasininNorthwestChina.TheKuqaForelandBasinislocatedintheeasternendoftheTianshanMountainsandiscomposedofaseriesofCenozoicsedimentaryrocks.Inthisstudy,wefocusonthewesternsegmentoftheKuqaFold-and-ThrustBelt,whichischaracterizedbythepresenceoftheKucheFold-and-ThrustBelt,saltstructuresanddetachmentfaults.Inparticular,weinvestigatetheformationmechanismofthesaltstructuresinthewesternsegmentandproposeamodelthatcombinestheinfluenceofthebasementstructureandthesedimentaryevolutiontoexplaintheobservedpatterns.

Introduction

TheKuqaForelandBasinisamajorpetroleumprovinceinChina,withprovenoilandgasreservesofabout10billionbarrelsofoilandabout30trillioncubicfeetofnaturalgas.ThebasinislocatedintheeasternendoftheTianshanMountains,whichareacomplexsystemoffoldsandthrustsformedduringtheIndia-Asiacollision.TheKuqaBasiniscomposedofaseriesofCenozoicsedimentaryrocksandisgenerallydividedintofourtectonicunits:theNorthTarim,SouthTarim,KuqaDepressionandPiedmontFoldBelt.ThePiedmontFoldBeltincludestheKuqaFold-and-ThrustBelt,whichisthefocusofthisstudy.

TheKuqaFold-and-ThrustBeltisacomplextectoniczonethatcomprisesaseriesofthrustfaults,folds,andsaltstructures.Overtheyears,manystudieshavefocusedonthegeometryandkinematicsofthethrustfaultsandfoldsinthiszone.However,theoriginandevolutionofthesaltstructuresarepoorlyunderstood.Therefore,weconductedadetailedstudyonthesaltstructuresinthewesternsegmentoftheKuqaFold-and-ThrustBelt,aimingtoclarifytheirformationmechanism.

Geologicalsetting

ThewesternsegmentoftheKuqaFold-and-ThrustBeltislocatedinthesouthwesternpartoftheTarimBasin,coveringanareaofapproximately31,000squarekilometers.ThesegmentisboundedtothenorthbytheNorthTarimBasin,tothesouthbytheSouthTarimBasin,totheeastbytheKuqaDepression,andtothewestbytheNorthPamir.ThewesternsegmentischaracterizedbythepresenceoftheKucheFold-and-ThrustBelt,whichisamajorpartoftheKuqaFold-and-ThrustBelt,andisfurtherdividedintothreestructuralzones:thenorthern,central,andsouthernzones.

TheKucheFold-and-ThrustBeltisatypicalthin-skinnedfold-and-thrustbelt.ThedeformationmainlyoccurredduringthePaleogene,withthrustfaultsandfoldsformingduringtheOligocene-Miocene.Thesedimentaryrocksinthewesternsegmentcanbedividedintotwomajorunits:theKuqaFormationandtheTertiary-Paleogene.TheKuqaFormationiscomposedofmudstone,sandstone,andconglomerate,whiletheTertiary-Paleogeneiscomposedofathicksequenceofsaltdeposits,withthicknessesrangingfrom800metersto1,000meters.

Methodology

Weusedacombinationofgeologicmapping,fieldobservation,andlaboratoryanalysistostudythesaltstructuresinthewesternsegment.Inthefield,wemeasuredtheorientationsandgeometriesofthesaltstructuresandthesurroundingsedimentaryrocks.Wealsocollectedsamplesofthesedimentaryrocksandsaltdepositsforlaboratoryanalysis,includingpetrographicanalysis,X-raydiffractionanalysis,andgeochemicalanalysis.Wealsoconductedseismicreflectionsurveysandgravitysurveystodelineatethestructuralfeaturesofthesubsurfacestructures.

ResultsandDiscussion

ThesaltstructuresinthewesternsegmentoftheKuqaFold-and-ThrustBeltcanbedividedintotwotypes:saltanticlinesandsaltdomes.Saltanticlinesarecharacterizedbygentletomoderatedips,whilesaltdomesarecharacterizedbysteepdips.ThesaltdepositsmainlyoccurintheTertiary-Paleogene,withthicknessesrangingfrom800metersto1,000meters.Thesaltlayersarecomposedofhalite,anhydrite,andgypsum.

Basedonourobservations,weproposeamodelthatcombinestheinfluenceofthebasementstructureandthesedimentaryevolutiontoexplaintheformationmechanismofthesaltstructuresinthewesternsegment.Themodelsuggeststhatthesaltstructuresinthewesternsegmentformedthroughthreemainstages:(1)thedepositionofthesedimentaryrocks;(2)thedeformationofthebasementandtheoverlyingsedimentaryrocks;and(3)thedissolutionandmovementofthesaltdeposits.

Duringthefirststage,thesedimentaryrocksweredepositedinasubsidingbasin,whichwasmainlycontrolledbythebasementstructure.Thesedimentaryrocksweremainlycomposedofmudstone,sandstone,andconglomerate,withathicknessofabout8,000meters.Duringthesecondstage,thebasementstructurewassubjectedtotectonicdeformation,leadingtotheformationofthrustfaultsandfolds.Thedeformationofthebasementandtheoverlyingsedimentaryrockscreatedthenecessaryconditionsforsaltmovementanddeposition.Duringthethirdstage,thesaltdepositsdissolvedandmovedupwardalongthrustfaultsanddetachmentfaults,formingsaltstructuressuchassaltanticlinesandsaltdomes.

Inconclusion,ourstudyprovidesnewinsightsintotheformationmechanismofthesaltstructuresinthewesternsegmentoftheKuqaFold-and-ThrustBelt.Theproposedmodelcombinestheinfluenceofthebasementstructureandthesedimentaryevolutiontoexplaintheobservedpatterns.OurresultshaveimportantimplicationsforunderstandingthestructuralevolutionandhydrocarbonaccumulationintheKuqaForelandBasin.ThestudyoftheKuqaFold-and-ThrustBeltisimportantforbothgeologyandindustry.ThisregionisnotonlyamajorpetroleumprovinceinChina,butalsoarepresentativenaturallaboratoryforstudyingthetectonicevolutionoftheTianshanMountainsandthenorthernedgeoftheTibetanPlateau.

Theformationofsaltstructuresplaysanimportantroleinthetectonicevolutionofthisregion.TheproposedmodelfortheformationmechanismofsaltstructuresinthewesternsegmentoftheKuqaFold-and-ThrustBelthighlightstheinteractionbetweenthebasementstructureandthesedimentaryevolution.Itprovidesacomprehensiveexplanationforthespatialdistributionandgeometryofsaltstructures,andcontributestoourunderstandingofthecomplexdeformationandhydrocarbonaccumulationintheregion.

Moreover,thestudyofsaltstructureshaspracticalimplicationsforpetroleumexplorationandexploitation.Saltstructurescanactasbothtrapsandbarriersforhydrocarbons,andareassociatedwithreservoirqualityandfracturedevelopment.UnderstandingtheformationanddistributionofsaltstructuresisthereforeessentialforpredictingandevaluatingthepotentialofhydrocarbonreservoirsintheKuqaForelandBasin.

Insummary,thisstudyprovidesanewperspectiveontheformationmechanismofsaltstructuresinthewesternsegmentoftheKuqaFold-and-ThrustBelt,highlightingtheimportanceoftheinteractionbetweenthebasementstructureandthesedimentaryevolution.Itshedslightonthetectonicevolutionandhydrocarbonaccumulationinthisregion,andhaspracticalimplicationsforpetroleumexplorationandexploitation.Inaddition,theKuqaFold-and-ThrustBeltalsohassignificantimplicationsforunderstandingthebroadertectonicevolutionoftheTianShanMountainsandtheCentralAsianOrogenicBelt(CAOB).TheTianShanMountainsandtheCAOBareimportantgeologicalfeaturesthathaveplayedacriticalroleinshapingthetopographyandgeologyofmuchofAsia.

Fordecades,geologistshavedebatedthetectonicevolutionoftheTianShanMountainsandtheCAOB,withsomescholarssuggestingthattheprocesswasdrivenbysubductionandaccretionofoceanicplates,andotherssuggestingthatitwasdrivenbycollisionofcontinentalfragments.ThestudyoftheKuqaFold-and-ThrustBeltprovidesnewinsightintothislong-standingdebate.

TheformationofsaltstructuresintheKuqaFold-and-ThrustBeltisconsistentwithamodelofcompressionaltectonicsdrivenbythecollisionofcontinentalfragments.Thisprovidessupportforthecollisionmodelandsuggeststhatthetectonicevolutionoftheregionwasacomplexinterplayofcollisionalandcompressionalforces.

Overall,thestudyoftheKuqaFold-and-ThrustBeltisimportantforbothscientificandpracticalreasons.ItshedslightonthetectonicevolutionoftheTianShanMountainsandtheCAOB,andprovidesessentialinsightsintotheformationanddistributionofhydrocarbonreservesintheregion.Thesefindingshaveimportantimplicationsforthepetroleumindustryandcanhelpinformfutureexplorationanddevelopmentactivitiesinthearea.ThestudyoftheKuqaFold-and-ThrustBeltalsohasbroaderimplicationsforunderstandingthemechanismsofmountainbuildingandplatetectonics.ByinvestigatingtheprocessesthatledtotheformationoftheKuqaFold-and-ThrustBelt,geologistscanbetterunderstandhowmountainrangesworldwidehavebeenformedandcontinuetoevolve.

Inaddition,theKuqaFold-and-ThrustBeltprovidesauniqueopportunityforscientiststostudytheeffectsofsalttectonicsonthedeformationofsedimentaryrocks.SalttectonicsisageologicalphenomenonwheresaltdepositsdeepbeneaththeEarth'ssurfacearesqueezedupwardsduetotheextremepressureandheatoftectonicactivity.Thiscanhavesignificantimpactsonthesurroundingsediment