Quantitative structural analysis using remote sensing data - Kurdistan, northeast Iraq - 图文

Figure3.GeologicmapsignificantlymodifiedfromBudayandJassim(1984).Thenumbersindicatethelocationofstructuralfieldmeasurementsofforelimbsandbacklimbsofanticlines.Thedataarepresentedinstereographicprojections(equalarea,lowerhemisphere).Bluedotsindicatemeasurementsfromforelimbs;reddotsaremeasurementsfrombacklimbs.Greentrianglesrepresentthecalculatedppolecorrespondingtothebestfitcylindricalfoldaxis.Greatcirclesindicatetheorientationofthecalculatedmeanpcircle.Inthelegend,theincompetentformationsaremarkedwith(IC).ThecrosssectionCC′,supportedbylimitedfielddataandconstructedusingthesoftwaretoolPlaneTrace,isshowninFigure7.

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Figure4.(A)Lithologicboundary(blackdashedline)betweenPilaSpilimestone(shadedred)andunderlyingGercusredsandstone,siltstone,claystone(shadedorange).

Becauseofasignificantdifferentresistancetoerosion,themorecompetentlimestoneformssharpridgeswithsteeplyslopingsidesalongtheerodedflanksoftheanticlines.Theinsetshowsthesatelliteimageofthesamelocationdrapedonthedigitalelevationmodel(DEM).Theredandorangeplanesshowthesedimentarybeddingorientation(blackvalues)ofthePilaSpilimestone(030/60)resp.Gercusredsandstone,siltstone,andclaystone(034/49).BlackdashedlinesvisualizetheintersectionoftheprojectedplaneswiththeDEM.(B)Triangularshapedhogback,consistingofPilaSpiFormation,whichperfectlyout-linesthedipofthestrata.Thebluedashedlineshighlighttheoutcroppingsedimentary

beddings,andtheblueinclinedplaneindicatestheirorientation.

whichactedasductiledetachmentsduringdefor-mation(BahroudiandKoyi,2003;Sherkatietal.,2005;Mouthereauetal.,2007).

Thefoldtrainsintheinvestigatedareacom-priseCretaceoustoCenozoicsediments(Figure2)consistingmainlyoflimestones,dolomites,sand-stones,siltstones,claystones,andconglomerates.Sepehretal.(2006)notedthatintheIranianpartoftheZagrosfoldandthrustbelt,themechanicalanisotropyoftheformationsconsistingofasucces-sionofrelativelycompetent(massivedolomiteandlimestone)andincompetent(claystone,siltstone,andshale)sedimentsessentiallycontrolsthestyleofthefolding(Figure3).Inaddition,thefoldingisinfluencedbyafewlow–shearstrengthlayersthatmayactaslocaldetachmenthorizons:TheUpperTertiaryFatha(i.e.,lowerFars)Formationcontains

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thinlayersofclaystone,gypsum,andanhydrite.OtherlocaldetachmentsarefoundintheLowerJurassicsediments,particularlyintheSehkaniyanandSarkiformations(DeVeraetal.,2009),con-sistingmainlyoflimestoneanddolomite,butalsocontainingthin-beddedshales(Figure2).

STRUCTURALFIELDMEASUREMENTSMorethan500beddingorientationswererecordedintheZagroshighfoldedzoneduringfieldstudies(Figure3),mostlyfromroadcuts.Althoughsecu-rityinthispartoftheZagrosismuchbetterthanelsewhereinIraq,off-roadaccesswasseverelyre-strictedbecauseofseriouscontaminationofthe

areawithlandminesandunexplodedordnance,aproblemdatingbacktotheendofWorldWarII.Thus,onlytwoshortcrosssections,alongmainroadsorientedperpendiculartothetrendofthefoldaxes,couldbemeasured,asaresultofwhichlargepartsofthefoldtrainsremainedunexamined.Thelengthofthesecrosssectionslimitedtheiruseforanyconclusions,andthus,theyarenotpub-lishedinthiswork.

Fieldworkfocusedondetailedstructuralmea-surementsoflithologicboundariesbetweenfor-mationsthathavestrongcompetencecontrasts(Figure4A).Becauseoffastererosionofsofterrocks,themorecompetentlithologiesformsharpridges,withsteeplyslopingsidesalongtheerodedflanksoftheanticlines.Thesehogbacks,asmuchasseveralhundredmetershigh,formtriangular-shapedflanksthatperfectlyoutlinethedipofthestrataandcanbeeasilyidentifiedintheDEM.Hogbackswere,therefore,themajortargetforstructuralfieldmeasurements(Figure4B).

Dipdirectionsandanglesofthelimbsofsixanticlinesweremeasuredindetail(Figure3).Ex-ceptforthesymmetricanduprightPermamandBanaBawianticlines,themeasuredfoldsareclearlyasymmetric,withsteepsouthwest-dippingfore-limbsandshallowernortheast-dippingbacklimbs(Safeen,Hareer,Peris,andMirawaanticlines).Althoughthegeologicmap(BudayandJassim,1984)indicatesathrustcuttingthecoreoftheSafeenanticline,thiscouldnotbeidentifiedinthefieldandisprobablyofminorimportance.AnotherpossiblethrustfaultwasmappedbetweentheShakRookandtheHareeranticline,buttheoffsetisunknownbecausethescarpiscoveredbyQua-ternaryslopedeposits.

DESCRIPTIONANDUSEOFTHEPLANETRACETOOL

PlaneTrace,anadd-ontoolfortheremotesensingsoftwarepackageWinGeol,wasdevelopedfortheanalysisandvisualizationofintersectionsofplanarstructuresofanyspatialorientationwithirregularandruggedsurfaces(e.g.,thetopography).Suchintersectionscanbeusedtocalculatethespatial

orientationofthefeature,constrainedbythedipdirectionanddipangle,ifthetopographyisrug-gedandthefeaturecutsacrossaridgeoravalleyformingatriangularorV-shapedintersection(theV-ruleintextbooks)orifitcropsoutasaresultofselectiveerosionoflessresistantsurroundingli-thologies.PlaneTraceisdesignedtodisplaytheintersectionsbetweenaruggedtopographicsurface(e.g.,theDEM)andavirtualtransparentplaneintwo-dimensional([2-D]crosssectionview)or3-D(blockdiagramview).Thevirtualplaneshavethefollowingparameters:geographiccoordinatesandelevationofthecenter,thedimensionsoftherec-tangledefiningtheplane,thedipangleanddirec-tion,andaBooleanvariablerepresentingthepo-larityoftheplane(uprightoroverturned).Theplanescanbetranslatedandrotatedinanydirec-tionuntilitfitsthespatialorientationofthegeo-logicplanarfeature(i.e.,fault,sedimentarybed-ding),recordingthegeographicposition,thedipdirection,andthedipangle.

ToincreasethegeologicinformationoftheDEM,anybitmaplikereferencedgeologicmaporrectifiedaerialorsatelliteimagecanbedrapedoverthetopography.ThegeographiclocationandtheanglesspecifyingthespatialorientationofthesurfacescanbeexportedintomostcommondatabaseformatsorintoanASCIIfile,andtheintersectionlinesofthesurfaceswiththeDEMcanbeexportedintoAutoCADdxffiles,ArcGisshapefilesorintoanASCIIfile.

AnimportantfeatureofPlaneTraceisthatanychangeinthepositionandorientationofthemea-suredplaneinstantlyupdatestheshapeandposi-tionofthepolylinerepresentingtheintersectionofthemeasuredsurfacewiththeDEM.Asaresult,thelateralcontinuationoftheplanarfeatureorthedeviationofthisfeaturefromanidealplanegeo-metrycanbedirectlycomparedwithindependentdatasuchasageologicmaporfieldphotographs.Ifthefeatureisnottrulyplanar(e.g.,thelimbsofanoncylindricalfold),thewholestructurecanbesubdividedintoseveralsmalleressentiallyplanarelements,approximatingthe3-Dsurface.Thus,thesizeofindividualplanescanbevariedde-pendingonthesizeofthestructuralmodel.

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Figure5.Flowchartandscreen-shotsoftheindividualworkingstepsinPlaneTracedemonstratedwithdatafromtheinvestigatedareanortheastofErbil:(A)as-semblageofremotesensingdataandavailablemaps;(B)mappingandcalculationofthedipdirec-tions(q)anddipangles(d)ofgeologicplanarstructures(givenexampleisfromthenortheastlimboftheSafeenanticline);(C)furtherprocessingofmea-surementswithWinGeol(e.g.,interpolationofsubsurfacestruc-turesinathree-dimensionalmodelbetweentheSafeenandPerisanticlinesbasedonsurfacedipmeasurements);(D)balancedsouthwest-northeastcrosssec-tionoverthePermamandSafeenanticlinesbasedonprojectionofthecalculateddipvaluesintoatwo-dimensionalsection.DEM=digitalelevationmodel.

Figure5showstheextendedworkflowofthequantitativespatialanalysisofgeologicplanarstruc-tures(inthiscase,sedimentarybedding)depictedfromDEMdata.Thefirststepcomprisesgeoref-erencing,rectifying,merging,andbandstackingofimagerythatwillbedrapedontheDEM(Figure5A).AlthoughthenecessaryfunctionsareavailableinWinGeol,theseprocedurescanbealsoaccom-plishedbyothersoftwarepackagesandimported.Thesecondstepincludesresamplingofthereso-948

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lutionofotherbitmapdata(e.g.,thegeologicmap,satelliteimage)tomatchtheresolutionoftheDEM.Thisstepshouldalsobeusedtotestthequalityandthecontrastofthegeologicplanarfeatures.Itisalsousefultocroptheimagestothedimensionsofthestudyareatoreduceprocessingtime.Inthethirdstep,theprepareddataareloadedintotheWinGeolpackage.WinGeolworksasasimplegeographicin-formationsystemapplicationthatallowsthedrap-ingofadditionalbitmapdataontotheDEM.Using