TectonometamorphicevolutionoftheKoralmComplex(EasternAlps): constraintsfrommicrostructuresandtexturesofthe`Plattengneis' shearzone WalterKurz * ,HaraldFritz,VeronikaTenczer,WolfgangUnzog Institut fu Èr Geologie und Pala Èontologie, Karl-Franzens-Universita Èt Graz, Heinrichstrab e 26, 8010 Graz, Austria Received13June2001;receivedinrevisedform19December2001;accepted10January2002 Abstract Latticepreferredorientations(LPO)ofquartzhavebeeninvestigatedalongasouth±northorientedsectionacrossthePlattengneisofthe KoralmComplex(EasternAlps).ThePlattengneisformsanimportantshearzonewithintheAustroalpinenappecomplexoftheEastern Alps,whichhasdevelopedduringtheCretaceousevolutionoftheAlpineorogen.Thequartz c-axesformsmallcircledistributionsinthe southernmostpartsoftheKoralmComplex,whichrepresenttheuppermoststructurallevelofthePlattengneis.FurthertotheNorthtwo maximabetweenthe Y and Z directionsofthe®nitestraincanbeinterpretedintermsofpreferredslipontherhombplanes.Thesefabrics continuouslygradeinto(typeIandtypeIIcrossed)girdledistributionsinanorthwarddirection.Astrongmaximumnearthe Y-axiswiththe tendency to be distributed along a single girdle, with three corresponding maxima of a-axesnearthemarginofthepole®gure,canbe observedinthecentralandnorthernparts.SuchLPOarecharacteristicforbothhighgrademetamorphicconditionsandhigh®nitestrain.The microstructures display that the deformation within the Plattengneis shear zone was synmetamorphic. A continuous increase of peak temperatures(andpressure)fromapproximately550 8Ctoapproximately750 8CfromtheSouthtothecentralpartscanbeinferredfrom geothermometric calculations. The temperatures then decrease to approximately 650 8CfromthecentralpartstotheNorth.Therelated pressuresincreasefrom8to16kbar,andthendecreaseto10kbar.TheLPOchangesthathavebeenobservedinthestudyareaarebest interpretedintermsoftemperaturedependenceoftheactivationofglidesystemswithinquartzaggregates.Thetemperatureandpressure evolutionmayindicatethatthecentralpartsoftheKoralmComplexhavebeenexhumedbylargeramountsthanthenorthernandsouthern parts.ThisisalsodocumentedbytheLPOevolution.Therefore,weassumethatthePlattengneisshearzoneformedduringtheexhumationof theKoralmComplex,andisrelatedtotheexhumationofhigh-pressureunitsinthefootwallofthisshearzone.Accordingly,thekinematics ofthePlattengneisshearzoneisratherextensionalthanthrust-related.TheimplicationsforthestructuralevolutionoftheEasternAlpsare shortlydiscussed. q 2002ElsevierScienceLtd.Allrightsreserved. Keywords:Plattengneisshearzone;Latticepreferredorientations(LPO);Microstructure;Exhumation;KoralmComplex;EasternAlps 1. Introduction The Plattengneis (or so-called `Stainzer Plattengneis') forms an important shear zone within the Austroalpine nappe complex of the Eastern Alps (Fig. 1). This major shear zone has been interpreted to have developed during the Early Cretaceous collisional event within the Austro- alpineunit(Frank,1987;Krohe,1987;Stu ÈweandPowell, 1995).Thus,thePlattengneiswasthesubjectofmanystruc- tural and microstructural investigations for several years (e.g. Frank et al., 1983; Flo Èttmann et al., 1986; Krohe, 1987). In particular the Plattengneis has been investigated inordertoobtaininformationabouttheevolutionofmicro- fabricsandthedevelopmentoflatticepreferredorientations (LPO; textures) in high-grade, amphibolite facies crustal shear zones (De Roo, 1983; Frank et al., 1983; Simpson and Schmid, 1983; Flo Èttmann et al., 1986; Schmid and Casey,1986;Krohe,1987;KurzandUnzog,1999;Brosch et al., 2000). However, these investigations were often restrictedbothtosingleoutcropsandfromamethodological pointofview.Therefore,wewilldiscusstheevolutionof thePlattengneisshearzonealongaS±Nsectionacrossthe KoralmComplexinordertogetinformationonthehomo- geneityofdeformationandonthestraingeometry,basedon therelationshipsbeweenthemicrostructures,LPO,andthe metamorphic ®eld gradient (Tenczer and Stu Èwe, 2001, JournalofStructuralGeology24(2002)1957±1970 0191-8141/02/$-seefrontmatter q 2002ElsevierScienceLtd.Allrightsreserved. PII:S0191-8141(02)00008-1 www.elsevier.com/locate/jstrugeo * Corresponding author. Present adress: Geologisches Institut, Rhei- nische Friedrich-Wilhelms-Universita Èt Bonn, Nussallee 8, 53115 Bonn, Germany.Tel.: 149-228-73-4812;fax: 149-228-73-9037. E-mail addresses: walter.kurz@uni-bonn.de(W.Kurz), walter.kurz@uni-graz.at(W.Kurz).