Coeval high-pressure metamorphism, thrusting, strike-slip, and extensional shearing in the Tauern...

Coeval high-pressure metamorphism, thrusting, strike-slip, and

extensional shearing in the Tauern Window, Eastern Alps

Johannes Glodny,1 Uwe Ring,2 and Alexander Kuhn3,4

Received 2 August 2007; revised 13 December 2007; accepted 15 February 2008; published 23 July 2008.

[1] Recent findings for a young (31.5 ± 0.7 Ma) ageof high-pressure metamorphism at �90 km depths inthe Eclogite Zone of the TauernWindow, Eastern Alps,prompt the question about the timing of the structuraldevelopment of the Tauern Window and its relation tohigh-pressure metamorphism. We show that all majorstructures in the Tauern Window, resulting from strongN-S lithospheric shortening and simultaneous minorE-W extension, began developing coevally withhigh-pressure metamorphism in the Eclogite Zone.Large-scale strike-slip shear zones started to form at�32–30 Ma and facilitated the spatial accommodationof simultaneous shortening and extension. At leastsome of the strike-slip and extensional shear zonesoperated into the Middle Miocene, either continuouslyor intermittently, with pronounced activity at �21–15 Ma. The considerable exhumation of the EclogiteZone from�90 km depths into the middle crust, and thetectonic development of its framework occurred withinonly 1–2 Ma after eclogitization. This is evidenced byalmost identical ages for eclogite facies metamorphismand for the development of the major structuresthat bound the Eclogite Zone under blueschist- andgreenschist facies metamorphic conditions. We discussa tectonic model in which considerable transpressionalshortening and thickening took place in the presentcentral-southern part of the Tauern Window. Wepropose that the Tauern Window nucleated here andthat most of the regional deformation at �32–30 Ma istoday found at the periphery of the window and in theadjacent Austroalpine units. Afterward, transpressioncontinued, the window grew to the E, W, and N, anddeformation progressed to those parts of the window.Ductile deformation in the present-day surface levelceased at�15Ma. Citation: Glodny, J., U. Ring, and A. Kuhn

(2008), Coeval high-pressure metamorphism, thrusting, strike-slip,

and extensional shearing in the Tauern Window, Eastern Alps,

Tectonics, 27, TC4004, doi:10.1029/2007TC002193.

1. Introduction

[2] The interplay between high-pressure deformationrelated to lithospheric convergence and the developmentof large-scale thrusts, strike-slip faults and extensional shearzones is of considerable tectonic interest. An intriguingaspect is as to whether high-pressure metamorphism canbe coeval with simultaneous motion on all three kinds oflarge-scale structures and how those structures evolvesubsequent to high-pressure metamorphism. A better under-standing of the timing of high-grade metamorphism and thestructural development can solve this issue, which is im-portant for our understanding of how diffuse continentalplate-boundary zones work.[3] The Tauern Window of the Eastern Alps (Figures 1

and 2) offers a superb example to study the above men-tioned processes in detail. Recently, we showed that thepeak of high-pressure metamorphism in the Eclogite Zonein the south-central Tauern Window occurred at 31.5 ±0.7 Ma [Glodny et al., 2005], making the Eclogite Zone theyoungest high-pressure unit in the Alps. The developmentof extensional structures related to the formation of theTauern Window is commonly believed to be Miocene in age(�22–12 Ma [Ratschbacher et al., 1991; Frisch et al.,2000; Kuhlemann et al., 2001, and references therein]). TheEclogite Zone itself experienced its high-pressure metamor-phism at about 90 km depth (�25 kbar, 630�C [Holland,1979; Hoschek, 2007]). High-pressure metamorphism in theadjacent units is of distinctly lower grade. The directlyoverlying Glockner nappe was subjected to 7.5 ± 1 kbar,525 ± 25�C; [Dachs, 1990]; Pmax here was <12 kbar[Gleissner et al., 2007]. The underlying Venediger nappeexperienced Pmax of 10–11 kbar at �550�C [Franz et al.,1991; Kurz et al., 1998a, and references therein]. Thequestion is when all three units became juxtaposed andhow this relates in time to the formation of the internalarchitecture of the Tauern Window by large-scale thrusting.The correlation in time of internal thrusting to the develop-ment of the fault system bounding the Tauern Window isalso unclear. This fault system consists of two normal faultsat the western and eastern margin, a prominent sinistralstrike-slip fault system in the western half of the windowand along its northern flank, and a dextral strike-slip faultsystem in the eastern half of the window [Genser andNeubauer, 1989; Ratschbacher et al., 1991; Kurz andNeubauer, 1996; Rosenberg et al., 2004]. Displacementsalong these fault systems have been responsible for thetectonic component of exhumation of the Tauern Window[Neubauer et al., 1999; Frisch et al., 2000], which is

TECTONICS, VOL. 27, TC4004, doi:10.1029/2007TC002193, 2008

1GeoForschungsZentrum Potsdam, Potsdam, Germany.2Department of Geological Sciences, University of Canterbury,

Christchurch, New Zealand.3Institut fur Geowissenschaften, Johannes Gutenberg-Universitat,

Mainz, Germany.4Now at Gexco AS, Mo i Rana, Norway.

Copyright 2008 by the American Geophysical Union.0278-7407/08/2007TC002193

TC4004 1 of 27

thought to dominate overall exhumation [cf. Kuhlemann etal., 2001].[4] Previous geochronologic studies in the Tauern Win-

dow have focused on certain areas, and often tried todecipher the thermal instead of the tectonic history. Timingof activity of major shear zones has mainly been inferredfrom indirect evidence. Tectonochronologic data constrain-ing ages of fabric formation and of deformation alongdiscrete shear zones are very rare. We try to fill this gapby direct isotopic dating of deformation in several key faultzones across the entire Tauern Window. We aim to discussthe timing of tectonometamorphic processes affecting theTauern Window and its structural framework using pub-lished age data, and present 21 new Rb-Sr mineral isochronages on fabric-forming structures. Integration of the isotopicages with structural data indicates that nucleation of theTauern Window, of large parts of its internal structural grain,and of its confining fault systems occurred at �32 to 30 Ma.Our data show that high-pressure metamorphism, large-scale thrusting, extensional shearing and a coordinatedsystem of sinistral and dextral strike-slip shear zonesoperated simultaneously with each other during overalltranspression. Transpressive movement persisted at leastuntil about 15 Ma.

2. Geologic Setting

[5] The Alpine orogen (Figure 1) has a protracted tec-tonic history that involved the closure of three oceanicbasins from the Cretaceous through the Cenozoic [Kozur,1991; Platt et al., 1989; Ring et al., 1989]. In the EasternAlps, the imprints of two distinct orogenies can be recog-

nized. These are the ‘‘Eo-Alpine’’ (Cretaceous) event relatedto the closure of the Meliata ocean, and the Eocene toOligocene ‘‘Meso’’- and ‘‘Neo-Alpine’’ orogeny as a conse-quence of collision between the Adriatic and Europeanplates following the closure of the Penninic oceans. TheCretaceous event is mainly recorded in the Austroalpinenappes, which make up the upper (Adriatic) plate duringcollision. The imprint of the Cenozoic collision processes isubiquitous in the underlying Penninic and Helvetic nappes,but also evident in the Austroalpine units by local defor-mation, magmatism, and, regionally, by thermal overprints[cf. Schmid et al., 2004, and references therein].[6] The Austroalpine units were thrust during the colli-

sion process toward the NNE over Penninic and Helveticunits, the latter of which derived from the European, lowerplate [e.g., Kurz et al., 1998a; Schmid et al., 2004]. TheTauern Window is the largest of several Penninic/Helveticwindows in the Eastern Alps. The Penninic/Helvetic rocksexposed in the Tauern Window form a series of lithologi-cally distinct nappes. The tectonostratigraphically lower-most major Helvetic unit is the Venediger nappe. Itcomprises a pre-Variscan, European continental basement,intruded by Variscan granitoids, and covered by parau-tochthonous metasedimentary series [e.g., Frisch, 1980].This unit is tectonically overlain by basement slices inter-calated with mainly Mesozoic passive continental marginsuccessions, now forming the Rote Wand-Modereck andStorz nappes [Kurz et al., 1998a, 2001], also known as‘‘Lower Schieferhulle’’. Oligocene to Miocene Alpinemetamorphism of the Venediger/Rote Wand-Modereck/Storz nappe stack is of blueschist to greenschist grade,regionally reaching amphibolite facies conditions. In the

Figure 1. Simplified geologic-tectonic overview of the Central and Eastern Alps and their frame.

TC4004 GLODNY ET AL.: TAUERN WINDOW EVOLUTION

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south-central Tauern Window, the Eclogite Zone occursbetween the Venediger and the Rote Wand-Moderecknappe, forming a separate unit [cf. Kurz et al., 1998b] thatexperienced near-UHP metamorphism (25 kbar, 630�C)[Hoschek, 2007] in the Oligocene, at 31.5 ± 0.7 Ma [Glodnyet al., 2005]. The structurally uppermost unit in the TauernWindow is the Glockner nappe (‘‘Upper Schieferhulle’’), aseries of Mesozoic sedimentary and mafic metaigneousrocks with oceanic Penninic affinity.[7] With Oligocene continent collision, the overall kine-

matic field in the Eastern Alps shifted from WNW- toNNE-directed shortening [e.g., Ratschbacher, 1986; Ringet al., 1988; Peresson and Decker, 1997]. In this context,the Eastern Alps experienced continuous transpressionbetween the obliquely converging European and Adriaticplates. Transpression is often partitioned into orogen-normal shortening, strike-slip movements and orogen-parallel extension, as exemplified in the Alps and in otherorogens [Sanderson and Marchini, 1984; Ratschbacher,1986; Tikoff and Teyssier, 1994; Neubauer et al., 1999]. Inthe Tauern window area, transpression resulted in the devel-opment of a complex system of kinematically linked faults(Figure 1), facilitating tectonic exhumation and finallyexposure of the rocks of the Tauern Window [Genser andNeubauer, 1989; Ratschbacher et al., 1991; Neubauer et al.,1999; Frisch et al., 2000].[8] Transpressive N-S shortening and E-W extension was

associated with a sinistral strike-slip fault system north (andwest) and a dextral system south (and east) of the TauernWindow (Figure 2) [Ratschbacher et al., 1991]. The strike-slip fault systems formed both within the Tauern Windowand in the overlying Austroalpine nappes, and are, ingeneral, oriented orogen-parallel (E-W). Kinematic linkagebetween E-W extension and strike-slip faults indicates thatthe latter were formed simultaneously with the extensionaldisplacements [Ratschbacher et al., 1991]. We describe themain features of the faults, especially those in the Austro-alpine, here and follow with a more detailed section on thefaults and shear zones in the Tauern Window that includesour own observations.[9] Themost prominent of the sinistral faults are the Salzach-

Ennstal-Mariazell-Puchberg fault (SEMP) [Ratschbacher et al.,1991], and the Inntal fault [Fugenschuh et al., 1997;Ortner etal., 2006] (Figures 1 and 2). The near-vertical SEMP strikes

along more than 300 km from the Vienna Basin to thenorthern margin of the Tauern Window, accommodating asinistral displacement of 60 km during Cenozoic time [Linzeret al., 2002]. The SEMP partly forms the northeastern edge ofthe Tauern Window. In the western Tauern Window it nolonger exists as a discrete fault but is most likely splayed intoand replaced by a system of near-vertical, ductile shear zones[Behrmann and Frisch, 1990; Wang and Neubauer, 1998;Frisch et al., 2000; Linzer et al., 2002; Rosenberg andSchneider, 2008]. Among these dominantly sinistral, ENE-striking splay faults are, from N to S, the Ahorn, Olperer,Greiner, and Ahrntal shear zones (Figure 2). The Inntal fault(Figures 1 and 2) records brittle subhorizontal sinistral shearin a transpressive regime at the present-day exposure level.Sinistral transpression here brought the southern block(including the Tauern Window) up, relative to the northernblock [Fugenschuh et al., 1997;Ortner et al., 2006]. Seismicdata were recently interpreted to indicate that the Inntalfault changes at depth into a transcrustal ramp, dipping30�S, with the western Tauern Window forming a hanging-wall anticline [Luschen et al., 2004].[10] The dominantly dextral fault system south of the

Tauern Window strikes E-W to NW-SE and consists of thePeriadriatic line and several smaller splay faults. ENE-striking conjugate faults show sinistral offset (Figure 2).The Periadriatic line forms a system of thrusts SW of theTauern Window (Giudicarie, Jaufen, and Passeier faults,Figure 2), while south of the Tauern Window its Pustertaland Gailtal segments are steeply dipping strike-slip faults[Polinski and Eisbacher, 1992]. A precollision, pre-Oligo-cene history can be inferred for parts of the fault systemfrom geochronologic data [Muller et al., 2001; Mancktelowet al., 2001]. Syn- to postcollisional reactivation is charac-terized by syntectonic magmatic activity and an abruptchange from sinistral to dextral motion at �30 Ma in thearea south of the Tauern Window [Muller et al., 2001;Mancktelow et al., 2001]. Among the dextral, NW-strikingsplay faults of the Periadriatic line, the Molltal fault(Figure 2) is the only major one, which continues into thePenninic/Helvetic units of the Tauern Window [cf. Linzer etal., 2002]. The Molltal fault is a subvertical lineament witha dextral offset of up to 30 km [Kurz and Neubauer, 1996].In its northwestern part, it splays into several steeplydipping ductile shear zones within Penninic units of the

Figure 2. Simplified tectonic sketch map of the Tauern Window area, showing the syn- and postcollisional fault system(modified after Ratschbacher et al. [1991],Kurz et al. [2001], Linzer et al. [2002]), and selected age data [in Ma]. AF, Ahrntalfault; AFZ, Ahorn fault; ATD, Ahorn-Tux dome; BF, Brenner fault; DAV, Defereggen-Antholz-Vals fault; GF, Greiner fault;HF, Hochstuhl fault; HD, Hochalm dome; IF, Inntal fault; ISF, Isel fault; JPF, Jaufen-Passeier fault; KF, Katschberg fault;MM, Mallnitzer Mulde; MMF, Mur-Murztal fault; MoF: Molltal fault; OF, Olperer fault; PL, Periadriatic line; RE, Rensenpluton; RF, Rieserferner pluton; SD, Sonnblick Dome; SEMP, Salzach-Ennstal-Mariazell-Puchberg fault system; TF, Telfsfault. ZVD, Zillertal-Venediger dome. References: (1) this work, (2) Cliff and Meffan-Main [2003], (3) Cliff et al. [1985],(4)Deckert [1999], (5)Christensen et al. [1994], (6) Reddy et al. [1993], (7) Liu et al. [2001], (8)Muller et al. [2001], (9)Cliffet al. [1998], (10) Inger and Cliff [1994], (11) Gleissner et al. [2007], (12) Barth et al. [1989], (13) Romer and Siegesmund[2003], (14) Blanckenburg et al. [1989], (15) Satir and Morteani [1982], (16) Barnes et al. [2004], (17) Satir [1975], (18)Schneider et al. [2007], (19) Raith et al. [1978], (20) Lambert [1970], (21) Eichhorn et al. [1995], (22)Urbanek et al. [2002],(23) Laufer et al. [1997], (24) Muller et al. [2000], (25) Mancktelow et al. [2001], and (26) Thoni [1980].


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southeastern Tauern Window [Kurz and Neubauer, 1996],pretty much in a similar way as the SEMP fault in the thewestern Tauern Window.[11] The syn- to postcollisional tectonic evolution of the

Eastern Alps involved a component of Oligocene to Mio-cene E-W extension, which aided the final exhumation ofthe Tauern Window [Behrmann, 1988; Selverstone, 1988;Genser and Neubauer, 1989; Ratschbacher et al., 1991;Neubauer et al., 1999]. E-W extension resulted in theformation of two conjugate normal fault systems at thewestern and eastern edges of the Tauern Window, respec-tively (Figures 1 and 2). The western margin is formed bythe W-dipping Brenner shear zone and the eastern edge ofthe Tauern Window is defined by the E-dipping Katschbergzone [Genser and Neubauer, 1989; Becker, 1993].[12] Assuming plane-strain constant-volume deformation

and an initial crustal thickness of 35–40 km, the barometricestimates of �25 kbar in the Eclogite Zone suggest thatOligocene NNE-directed shortening in the Tauern Windowregion exceeded 60%. The amount of Oligocene/MioceneE-W extension in the Eastern Alps is a controversialissue. Some workers inferred 40 to 50% of extension[Ratschbacher et al., 1991; Frisch et al., 1998]. However,these studies neglect that even modest erosion of high-amplitude antiforms can cause significant exhumation [cf.Schmid et al., 2004], which makes especially the extensioncalculation from the palinspastic restoration of Frisch et al.[1998] questionable. In contrast, results of numerical mod-eling [Robl and Stuwe, 2005] point to only a minor role ofextensional faulting in the post-Oligocene deformationhistory of the Eastern Alps. Huismans et al. [2001] esti-mated 0 to 16% of extension by subtracting the inferredamount of Miocene extension in the Pannonian Basin fromthe restored amount of Miocene convergence in the Carpa-thians. Rosenberg et al. [2007] recently reviewed theamount of E-W extension in the Eastern Alps and convinc-ingly showed that it did not exceed 20%. If so, N-Sshortening exceeds E-W extension by a factor of �3.[13] Sedimentologic data constraining the unroofing of

the Eastern Alps are scarce. Kuhlemann et al. [2001]presented a denudation budget for the Eastern Alps, inparticular for the Tauern Window, based on the sedimentaryrecord from circum-Alpine sediment traps. A patternemerges in which a first significant increase in sediment

discharge from the Eastern Alps orogen is seen at �32–30 Ma. Subsequently, sediment discharge slightly increaseduntil �21 Ma. At that time, a significant drop in sedimen-tation rates occurred [Kuhlemann et al., 2001], followed byan increase to a distinct, major pulse at �17 Ma. Later on,the Alpine orogen continued to discharge eroded material,with another maximum in the Pleistocene.

3. Structural Data

[14] The overall structure of the Tauern Window is that ofa huge elongated antiform that is bounded on its westernand eastern side by oppositely dipping normal shear zones(Figure 2). The antiform is made up by a northwardimbricated stack of Penninic/Helvetic nappes that is foldedinto a number of upright to NNE-vergent, but also SSW-vergent large-scale folds with E-trending axes. In the limbsof those folds a number of strike-slip shear zones formed. Inthe western part of the Tauern Window these shear zoneshave dominantly sinistral kinematics. In the eastern part ofthe window the dextral Molltal and Mur-Murz faults occur(Figures 2 and 3).[15] Our structural data from the Eclogite Zone show

that the contact of the latter with the underlying Venedigernappe is marked by mylonite with top-NNE shear bands(Figure 3a) [see also Behrmann and Ratschbacher, 1989].In the Dorfertal, mylonite with S-plunging downdip stretch-ing lineations is only a few meters away from mylonite withgently E-plunging stretching lineations that are associatedwith sinistral S-side-down kinematic indicators. Occasion-ally, gently W-plunging lineations associated with dextralS-side-down kinematics have been reported by Hawkins etal. [2007]. No consistent overprinting relationships betweendowndip and the lateral kinematic indicators occur and ourdating (see below) indeed shows that both sets of kinematicindicators formed essentially at the same time. In the upperVenediger nappe the same alternating pattern of S-plungingdowndip and gently plunging stretching lineations wasmapped (Figure 3).[16] The upper contact of the Eclogite Zone with the Rote

Wand Modereck and Glockner nappes is marked by asteeply S-dipping mylonite belt. The mylonite foliationcontains a gently (�20�) E-plunging stretching lineationassociated with sinistral S-side-down kinematic indicators

Figure 3. Simplified structural map of the Tauern Window showing major shear zones and faults. The arrows indicate thekinematics of each individual fault segment. The stereographic projections show great circles of the mylonitic foliation, thestretching lineation on this foliation is shown by a dot and the arrow through this dot gives the sense of relative movement.The data show top-WSW extension across the Brenner fault system and top-ESE extension across the Katschbergextensional fault system. The western tip of the Olperer shear zone shows sinistral strike-slip faulting and top-WSWextension, whereas the Ahrntal fault shows sinistral strike-slip faulting and NNE-SSW shortening. The Greiner shear zoneshows sinistral faulting, a minor set of dextral faults and top-NNE reverse faulting. The top of the Eclogite Zone showssinistral strike-slip faulting associated with a component of S-side-down motion, whereas the bottom of the Eclogite Zoneshows top-NNE thrusting combined with localized sinistral shearing. The Molltal fault shows dextral strike-slip faultingassociated with a component of top-NNE reverse faulting. The Katschberg extensional fault system shows top-ESEextension. The Eclogite Zone and the basal thrust of the Glockner nappe (dashed line with barbs on Glockner nappe) areshown for reference.


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(Figure 3b). This sinistral S-side-down shear zone can befollowed further to the SW where it eventually forms theboundary of the Tauern Window (Figure 3).[17] Along the NE edge of the Tauern Window, the

SEMP forms a system of dominantly sinistral, ductile tobrittle deformation structures [Ratschbacher et al., 1991;Neubauer et al., 1999]. Rosenberg and Schneider [2008]showed that the sinistral, brittle SEMP grades westward intothe ductile Ahorn shear zone. The mylonitic foliation in theAhorn shear zone contains a gently W-plunging stretchinglineation associated with sinistral, S-side-up shear senseindicators. The Ahorn shear zone separates an area withAlpine amphibolite-facies metamorphism in the south froman area in the lowest greenschist facies (just above thebrittle-ductile transition in quartz) in the north due to avertical offset of �7 km [Rosenberg and Schneider, 2008].This offset could result from sinistral shearing parallel to thewest-dipping stretching lineations; however, given the trans-pressive character of the shear zone, the transport directionmay have been steeper than the stretching lineations[cf. Robin and Cruden, 1994]. As a consequence, the lateraloffset of 60 km, which affected the SEMP east of the TauernWindow [Linzer et al., 2002], is partly transferred into avertical one in the Ahorn shear zone. Further west, about15 km east of the Brenner fault, lateral shearing is taken upby upright folding due to N-S shortening [Rosenberg andSchneider, 2008].[18] The Olperer shear zone formed in the northern limb

of the Tux orthogneiss anticline. We found that myloniticdeformation in the subvertical shear zone was pervasive inthe metasediments and much more localized in the orthog-neiss. The shear-zone-related foliation in the metasedimentsis associated with a strong, gently WSW-plunging stretch-ing lineation (Figure 3c). Kinematic indicators indicatesinistral shear. The foliation and stretching lineation arefolded about lineation-parallel tight to isoclinal folds. Thesefolds are locally associated with a strong crenulation cleav-age in hinge zones of the folds. Lineation-parallel foldingcaused strongly prolate strain geometries, especially in themetasediments. The lineation-parallel folding did not invertthe sinistral kinematic indicators in overturned limbs sug-gesting that folding and stretching were coeval. The foldingalso caused the foliation to attain a local WSW-dippingattitude. In these segments of the shear zone the kinematicindicators yielded a top-WSW shear sense (Figure 3c). If thesinistral shear sense was folded after shearing then the shearsense in the hinge zones of those folds should be top-ENE.The fact that the shear sense is top-WSW strongly suggeststo us that sinistral and top-WSW shearing, and also thelineation-parallel folding, occurred simultaneously. Allmentioned structures are then folded about large-scale opento tight folds with WSW-plunging axes. The axial planes ofthese late folds are moderately to steeply dipping to the S.[19] The Greiner shear zone is a steeply S-dipping, ENE-

striking structure. A mylonitic, S-dipping foliation is asso-ciated with a pervasive gently WSW-plunging stretchinglineation (Figure 3d). The Greiner shear zone caused perva-sive deformation in the sedimentary cover rocks, whereas aseries of up to 30 m wide anastomosing splays occur in the

Zentralgneis, indicating lithology-controlled heterogeneousdeformation [cf. Steffen and Selverstone, 2006]. Kinematicindicators supply a dominantly sinistral, S-side-up shearsense [De Vecchi and Baggio, 1982; Behrmann and Frisch,1990]. Nonetheless, the Greiner shear zone is very hetero-geneous and in the Zentralgneiss there are two sets of ductileshear zones with ENE-striking zones showing sinistraldisplacement and SE-striking shear zones recording dextralmovement (Figure 3d). Yet other sections of the shear zonerecord flattening strains and volume loss [Selverstone et al.,1991; Selverstone and Hyatt, 2003], and even dextral over-printing of originally sinistral fabrics has been described[Barnes et al., 2004]. Ductilely deformed feldspar and PTestimates show that shearing commenced under amphibolitefacies conditions [Selverstone et al., 1984, 1991]; however,there are localized shear zones characterized by stronggreenschist facies retrogression.[20] The Ahrntal shear zone is a NE-striking, steeply

dipping shear zone [Reicherter et al., 1993]. The myloniticfoliation contains a penetrative ENE-plunging stretchinglineation (Figure 3e). Ductilely deformed asymmetric tailsaround feldspar indicate that sinistral shear commenced atleast during uppermost greenschist facies metamorphism.The ductile feldspar fabrics are locally overprinted by strongretrogressive greenschist to subgreenschist facies sinistralshearing. In its southwestern part, the Ahrntal shear zoneseparates an area with Oligocene/Miocene uppermostgreenschist to amphibolite facies metamorphism in the northfrom an area in the lowest greenschist facies in the south.This pattern is a mirror image to that at the Ahorn shearzone, creating a pop-up structure bounded by the Ahornshear zone in the north and the Ahrntal shear zone in thesouth.[21] The Brenner shear zone below the brittle Brenner

fault is marked by an at least �400 m thick mylonite zone inits central part [Behrmann, 1988]. The Brenner fault systemhas a two-stage deformation history. Selverstone [1988]showed that normal shearing was syn-peak-metamorphicat deep crustal levels in the bottom parts of the shear zone.Toward the brittle Brenner fault top-WSW normal shearing(Figures 3f and 3g) took place during progressively lower,generally greenschist facies conditions. A discrete brittlefault is superimposed on the normal shear zone andaccounts for a vertical movement of <5 km since �13 Ma,as deduced from fission track data [Fugenschuh et al.,1997]. Seismological data imply that E-W normal faultingeven continues today, associated with NNE shortening[Reiter et al., 2005].[22] Dextral shear at the Molltal fault commenced during

amphibolite facies conditions. The subvertical shear zonecontains a steeply dipping mylonitic foliation which yieldsdextral shear sense indicators (Figures 3h and 3i). Thekinematic indicators in orthogneiss include strain shadowsaround potassium feldspar with recrystallised feldspar inthe asymmetric wings. Minor subvertical dextral shearzones formed in the steep limbs of open to tight, uprightto slightly N-vergent large-scale folds with ESE-trendingaxes. The folds are associated with zones of top-NNE shear(Figures 3h and 3i). The kinematic indicators have not been


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inverted by folding indicating that folding (Figure 3h)(dated at �28 Ma [Inger and Cliff, 1994]) occurred eitherprior to or during dextral shearing.[23] The Katschberg shear zone at the eastern end of the

Tauern Window is a low-angle ductile to brittle top-ESEnormal shear zone (Figure 3j) [Genser and Neubauer, 1989;Becker, 1993]. The shear zone is up to 200 m thick andgrades upwards into a zone of intense brittle normal fault-ing. Normal shearing in the lower parts of the shear zoneprogressed during greenschist-facies metamorphism as in-dicated by strongly recrystallised quartz ribbons in top-ESEshear bands and asymmetric quartz-c-axis fabrics indicatingtop-ESE shear [Becker, 1993].[24] In summary, the data show that strong NNE-directed

shortening occurred coevally with E-W normal and sinistral(in the western Tauern Window and at its northern margin)and dextral (in the eastern Tauern Window) strike-slipshearing (Figure 3). The strike-slip shear zones are trans-pressive, except the segment above the Eclogite Zone,which is transtensional. The strike-slip shear zones prefer-entially formed in the limbs of large antiforms and basicallyaccommodated ‘‘excess’’ NNE shortening and transferred itsideway where the two normal shear zones formed. Thefootwalls of the latter expose amphibolite-facies rocks in thewest and greenschist-facies rocks in the east, consistent witha generally asymmetric exhumation pattern in the TauernWindow.

4. Previous Dating of Alpine Events in the

Tauern Window Area

[25] Collision of the Adriatic and European plates causeddeep underthrusting of the leading edge of the Europeancontinent and occurred at or slightly before 31.5 ± 0.7 Ma,which is the age of the eclogites in the Eclogite Zone[Glodny et al., 2005]. In this paper, we focus on geochro-nologic data for the syn-to postcollisional history, as thishistory involves formation of the Tauern Window and itsstructural framework. From a large body of literature (sum-marized in Table 1) it is evident that most existing age datafor Alpine metamorphic, magmatic and ductile deformationprocesses fall in the age range between 32 and �13 Ma(Table 1). Apparent ages older than �32 Ma mostly derivefrom K-Ar-based isotopic data for high-pressure metamor-phic rocks. These ‘‘old’’ ages are likely to be biased bythe documented ubiquity of excess Ar in these rocks[Blanckenburg and Villa, 1988; Zimmermann et al., 1994;Ratschbacher et al., 2004].[26] Unfortunately, for a number of published age data it

remains unclear what the exact significance of the data is.This is in part a result of lack of structural and microtexturalinformation, and also an outcome of variable assumptions on‘‘closure temperatures.’’ Given the recent findings that purediffusional isotope redistribution particularly between whitemica and its surroundings is extremely sluggish even atamphibolite facies temperatures (see below), many white-mica-based dates previously interpreted as cooling ages mayin fact represent fabric formation, dynamic recrystallizationand, consequently, deformation ages. In addition, the focus in

many previous geochronologic studies has been on thethermal instead of the tectonochronologic record. This pre-sumably is the reason that strongly deformed rocks fromdiscrete shear zones and faults have rarely been investigated.We evaluate the existing geochronologic database as to itsrelevance for the structural development of the TauernWindow in the context of our new deformation age databelow.

5. Methodology: Rb-Sr Dating of Deformation

[27] For isotopic dating of deformation, we employed theRb-Sr internal mineral isochron approach, with bulk mineralseparation from small samples [Freeman et al., 1997; Hetzeland Glodny, 2002; Reddy et al., 2003; Glodny et al., 2002,2005]. Samples (<100 g) were chosen for which the assemb-lages and textures can be tied to certain tectonic or metamor-phic events. Use of small samples for mineral separationminimizes the risk of bias by possible intra-sample isotopicgradients. A main advantage of the Rb-Sr internal mineralisochron approach is that isotopic equilibrium-disequilibriumrelationships between the different assemblage-formingminerals are revealed. These relationships potentially allowone to constrain the reaction history of a rock, to identifyisotopic relics, and to distinguish between diffusion- andrecrystallization-induced isotopic resetting [Glodny et al.,2008].[28] Some key requirements have to be met for isotopic

dating of deformation. First, intermineral isotopic equilibra-tion during the deformation event is mandatory. Carefulstudy of the correlation between microtextures and isotopicsignatures, both by conventional mineral separation techni-ques [Muller et al., 1999] and by Rb-Sr microsampling[e.g., Muller et al., 2000; Cliff and Meffan-Main, 2003] hasshown that full synkinematic recrystallization in mylonitesis usually accompanied by Sr-isotopic reequilibration.Isotopic reequilibration between mica and coexistingphases during mylonitization is viable at temperatures aslow as 300�C [Muller et al., 1999; Reddy et al., 2003 andreferences therein]. The close link between textural andisotopic equilibration implies that in samples free ofobvious predeformative textural relics (like feldspar augenor mica fish), most probably an assemblage in isotopicequilibrium has been frozen in at the end of deformation.Rb-Sr isotopic mineral data from such rocks should thusrecord the waning stages of deformation-induced recrys-tallization [Freeman et al., 1998]. We therefore sampledintensely and pervasively deformed mylonitic rocks andschists.[29] Second, thermally driven diffusion should not alter

the Sr-isotopic mineral signatures once adjusted duringdeformation. We preferred white-mica-bearing assemblagesfor geochronology, because the Rb-Sr system of white micais thermally stable to amphibolite facies temperatures(>550–600�C [Inger and Cliff, 1994; Freeman et al.,1997; Villa, 1998; Glodny et al., 2005]). Provided thatmodally controlled closed system behavior applies [cf.Glodny et al., 2003], the Rb-Sr system of white mica maypersist through even higher temperatures. The high temper-


8 of 27

TC4004

Table 1. Published Isotopic Age Data on Alpine Processes, Tauern Window

Authors Sampling Area Method Age, Age Range, Ma Original Interpretation

Western Tauern WindowSatir [1975] Western TW Rb-Sr, phengite

Rb-Sr, K-Ar, biotite�30�15

crystallization agecooling

Satir [1975] SW margin of TW K-Ar, phengite 31 ± 2 peak metamorphism in calcschistSatir and Morteani [1982] Greiner SZ and

migmatites, W’TWRb-Sr phengiteRb-Sr biotite

21 ± 214 ± 1

coolingcooling

Blanckenburg et al. [1989] Pfitscher Joch area,western Tauern window

K-Ar, Ar-Ar, hblRb-Sr, white micaRb-Sr, K-Ar, biotite

1820–15�13

coolingdeformationcooling

Zimmermann et al. [1994] south-central Tauernwindow (Eclogite Zone)

Ar-Ar phengiteAr-Ar phengite

32–3632–36

cooling of EZmetam. crystallization in Venediger& Glockner nappes

Christensen et al. [1994] Pfitscher Joch area, western TW Rb-Sr, garnet zonation �30 ± 1 Alpine thermal maximum in schistsBarnes et al. [2004] Greiner SZ, Stillup

valley, western TWU-Th-Pb monazite �26 switch from sinistral to dextral shear,

at amphibolite faciesSchneider et al. [2007] Stillup valley,

W’ Tauern windowRb-Sr microsampling,mylonite

�19–17 Ma sinistral shear in W’ continuation ofSEMP; form. of uprightantiforms of W’ TW

Central Tauern WindowLambert [1970] southern margin of TW,

near DollachK-Ar, white mica 27.6–34.6a greenschist facies mica growth

cf. Cliff et al. [1985]Raith et al. [1978] west-central Tauern window K-Ar, mica 30–20 cooling; older ages toward southern

window marginInger and Cliff [1994] EZ + Glockner nappe,

south-central TWRb-Sr,white mica-based

(26) 28–31 end of greenschist facies deformation

Eichhorn et al. [1995] Felbertal, central Tauern window U-Pb zircon 31 ± 4 Alpine metamorphismGlodny et al. [2005] Eclogite zone, central Tauern W. Rb-Sr multimineral 31.5 ± 0.7

31.5 ± 0.5eclogite facies metamorphismgs facies retrogression

Ratschbacher et al. [2004] Eclogite zone, central Tauern W. Ar-Ar, white mica 42–35 coolingGleissner et al. [2007] Glockner nappe S of

Eclogite ZoneRb-Sr multimineral 29.8 ± 0.5 deformation and near-peak

metamorphism

Eastern Tauern WindowCliff et al. [1985] Ankogel-Hochalm area

SE TWK-Ar white micaRb-Sr biotite

28–1716.5

coolingcooling

Reddy et al. [1993] Sonnblick dome,SE Tauern window

Rb-Sr, mica- fspRb-Sr, bt-based

30–2623.5–19

synkinematic crystallizationcooling

Inger and Cliff [1994] SE Tauern windowMallnitzer Mulde/Molltal line

U-Pb, aln, titaniteRb-Sr mineral dataRb-Sr minerals

�2828–2932–22

peak metamorphismformation of Sonnblick domecontinuous deformation

Cliff et al. [1998] Mallnitzer Mulde/Molltalline, SE TW

Th-Pb allanite 27.7 ± 0.3 fabric formation, peak metamorphism

Liu et al. [2001] uppermost Penninic, NE TW Ar-Ar white micaAr-Ar white mica

�30�22

coolingactivity of Katschberg fault

Deckert [1999] footwall of KatschbergFZ, NW TW

K-Ar, white mica �25 cooling after extensional deformation

Cliff andMeffan-Main [2003]

Sonnblick gneiss,SE Tauern window

Rb-Srmicrosampling

27.3 ± 0.825.5 ± 0.3

late gneissic fabric formationSonnblick dome formation

MagmatismDeutsch [1984] Alkalibasaltic dykes S of TW K-Ar, Rb-Sr �30 intrusion of alkalibasaltic dykesBarth et al. [1989] Rensen pluton U-Pb, zircon, allanite 31.7–31.1 magmatic crystallizationMuller et al. [2001] Periadriatic Fault system SW of TW U-Pb zircon 33–30 magmatismRomer and

Siegesmund [2003]Rieserferner pluton U-Pb allanite 32.4 ± 0.4

31.8 ± 0.4magmatic crystallization

Fault Systems Related to Tauern Window FormationLaufer et al. [1997] Periadriatic Fault, SE of TW

(Eder unit, Carnic Alps)K-Ar, Ar-Ar 32–28

18–13dextral transpression along PLexhumation, shear along PL

Muller et al. [2000] DAV fault zone, S of SWTauern W.

Rb-Srmicrosampling

33–30<30

sinistral strike slip deform.dextral transpressive deform.

Muller et al. [2001] Periadriatic Fault system,SW of TW

Rb-Sr, Ar-ArAr-Ar

32–2921–17

ductile deformationbrittle movements, pseudotachylites

Mancktelow et al. [2001] Periadriatic Fault systemS of Tauern W.

Rb-Sr microsampling +microtectonics

�30 Ma change from sinistral-transtensive todextral-transpressive deformation

Urbanek et al. [2002] SEMP, NE Tauern window Ar-Ar, white mica 28–35 sinistral transpression,associated with TW exhum.

aRecalculated using Steiger and Jager [1977] decay constants. TW, Tauern Window; DAV, Defereggen-Antholz-Vals; EZ, Eclogite Zone; PL,Periadriatic line; SZ, shear zone.


9 of 27

TC4004

Table

2.CharacterizationofDated

Sam

plesa

Sam

ple

Coordinates

Rock;Assem

blage

Metam

.Grade

Rb-Srage,

Ma

Tectonic

Position

Tectonochronologic

Significance

Brenner

Line

BRE2

47�00.6980N,11�30.3990E

400m

NW

ofBrennerpass

carbonatemylonitecal,wm,

qtz,py,

hem

atite

gs

21±2

Brenner

faultzone

top-W

extensional

shear

TF04-30a

46�58.5020N,11�28.4660E

1km

SW

TermediBrennero

carbonateschistcal,wm,qtz,ab,

chl,graphite

gs

18.3

±2.6

Brenner

faultzone

top-W

extension

PON1

46�57.8160N,11�27.8220E

2km

SW

TermediBrennero

graphiteschistab,qtz,wm,graphite

gs

17.8

±1.8

Brenner

faultzone

top-W

extensional

shear

TF04-32

46�58.7080N,11�28.2510E

1km

WNW

TermediBrennero

quartzitic

myloniteqtz,wm,

ap,fsp,tur(?)

gs(?)

39.3

±3.6

immediate

hangingwall

ofBrenner

line

ductiledeform

ation(?)

Western

TauernWindow-SEMPSplays

TF04-26

47�03.415N,11�37.4080E

1500m

NNW

ofGeraerHutte

mylonitized

augengneiss

wm,qtz,

fsp,pg,ep,zrn

af-gs(?)

15.7

±1.3

SZzonealongNW

margin

ofTuxgneiss

(Ahorn

SZ)

sinistral

shearing

TF04-25

47�02.89700 N

,11�38.2370

E�1km

NEGeraerHutte

mylonitic

paragneiss

qtz,bt,wm,ep,zrn

af31.2

±0.4

Olperer

shearzone[cf.Lammerer

andWeger

[1998]]

prolate

strain,sinistral

shearing

SEG2

47�05.2130N,11�46.7380E

3.5

km

SW

ofGinzling

granite,

almostundeform

edpl(clear),qtz,bt,ap,zrn,grt

af–gs(?)

17.0

±5.3

almostundeform

edVariscan

granitoid

(Tuxer

Kern)

decompressionrelated

mineral

reactions

PFI1

1.3

km

NNW

ofPfitscher

Joch,

Zam

serBachat

2500m

altitude

mylonitic

gneiss

fsp,qtz,

wm,ap,zrn,bt/chl

upper

gs(?)

17.6

±1.7

smallfaultzonewithin

granite,

parallelto

Greiner

shearzone

ductileshear-mainfoliation

ofgranitoid

HAU3

700m

ENEofRotbachlspitze,

2.5

km

EofPfitscher

Joch

quartz-ky-w

m-m

obilisateqtz,ky,

wm,

staurolite,garnet

af15.0

±0.4

within

Greiner

shearzone

metam

orphic

mobilisate

crystallization,decompression

PFI2

300m

WofRifugio

Passo

diVizze

phosphatemetaquartziteqtz,

lazulite,ky,

wm

af19±4

Greiner

shearzone

ductileshear

PFI3

400m

WSW

ofRifugio

Passo

diVizze

fine-grained

schistwm,qtz,fsp,

bt/chl,ap,zrn

gs(?)

16.9

±0.6

Greiner

shearzone

ductileshear

TF04-15

47�05.0620N,12�07.020E

3km

SSEofLakeZillergrundl

mylonitic

gneiss

fsp,qtz,grt,bt,

ep,wm,ap,cal

af26.7

±1.2

16.4

(bt)

Eastern

continuationof

Greiner

SZ

sinistral

shearingdecompression

(biotite

‘‘age’’)

TF04-18

47�07.116N,12�05.0420E

E0shore

ofLakeZillergrundl

muscoviteschistqtz,wm,

bt/chl,fsp,ap,zrn

upper

gs(?)

21.5

±0.8

Eastern

continuationofGreiner

SZ/HabachtalMulde

sinistral

shearing

HWZ3

46�55.3280N,11�42.5250E

4km

NofPfunders

carbonatemylonitecal,qtz,wm,bt

gs

19.8

±0.4

Ahrntalfaultzone-im

mediate

footwallofGlockner

nappe

sinistral

(?)ductiledeform

ation

CentralTauernWindow-EclogiteZoneandEnvirons

EIS14a

47�03.2240N,12�19.9360E

�700m

SofJohannishutte

calcschistcal,wm,pg,grt,

tur,graphite

af-gs

30.4

±0.4

contact

EZ-Venediger

nappe

30m

Nofsample

EIS14b

top-N

shear

EIS14b

47�03.2240N,12�19.9360E

�700m

SofJohannishutte

calcschistcal,wm,pg,grt,

ap,tur,graphite

gs(?)

30.6

±0.9

contact

EZ-Venediger

nappe

30m

Sofsample

EIS14a

sinistral

top-W

shearing

EIS6

47�03.3650N,12�22.3150E

500m

NNEofEisseehutte

carbonatemylonitecal,am

p,

ep,wm,qtz

gs(?)

31.2

±0.6

contact

EclogiteZone-Rote

Wand-M

oderecknappe

sinistral

shearing(top-W

?)

EIS11

47�03.768N,12�24.4580E

Seekopfscharte

(3059m)

mafic

schistam

p,ep,cal,

ap,ttn,bt/chl

bs-gs

31.4

±0.4

contact

Glockner

nappe–-Rote

Wand-M

oderecknappe

sinistral

top-W

shearing

EIS13

47�04.2480N,12�24.4510E

Mittlerer

Seekopf(3221m)

mafic

schistep,am

p,

grt,qtz,wm,ap,bt

bs-gs

31.1

±0.4

within

EclogiteZone

retrogradeoverprintwithin

EZ,

associated

withductileshear

EasternTauernWindow

TF04-1b

46�56.6060N,13�12.4280 E

�2km

NNW

ofObervellach

quartz

mylonitefsp,qtz,wm,

zrn,tur,am

p(?)

(?)

25.3

±2.9

Molltalline

ductileshear

TF04-5

46�53.1560N,13�18.5260E

Oberkolbnitz,

Molltal

carbonatemylonitecal,ankerite,

qtz,wm,py

gs(?)

20.7

±2.3

Molltalline

dextral

shearing

aWm,muscovitic

tophengitic

whitemica;

gs,greenschist;af,am

phibolite

facies;bs,blueschistfacies;EZ,EclogiteZone.


10 of 27

TC4004

Table 3. Rb/Sr Analytical Dataa

Sample No.Analysis No. Material Rb, ppm Sr, ppm 87Rb/86Sr 87Sr/86Sr 87Sr/86Sr 2sm, %

Brenner LineBRE2 (21 ± 2 Ma, MSWD = 1.1, Sri = 0.708296 ± 0.000034)

PS1566 calcite 0.07 1428 0.00014 0.708300 0.0014PS1565 wm 500–355 mm 394 783 1.46 0.708728 0.0018PS1560 wm 355–250 mm 398 770 1.50 0.708752 0.0020PS1561 wm 250–125 mm 355 859 1.20 0.708627 0.0014

TF04-30A (18.3 ± 2.6 Ma, MSWD < 1, Sri = 0.709023 ± 0.000034)PS1299 calcite 0.11 596 0.0005 0.709023 0.0012PS1322 wm nm 1.3A 344 777 1.28 0.709356 0.0016PS1324 wm 355-250 mm 261 846 0.892 0.709253 0.0014PS1329 wm 250–160 mm 300 865 1.00 0.709285 0.0016

PON1 (17.8 ± 1.8 Ma, MSWD = 1.6, Sri = 0.708111 ± 0.000034)PS1558 quartz-feldspar 18.7 322 0.168 0.708158 0.0018PS1562 wm 355–250 mm 411 640 1.86 0.708603 0.0027PS1571 wm <125 mm 230 740 0.901 0.708336 0.0013PS1572 wm 250–125 mm 377 654 1.67 0.708509 0.0014

TF04-32 (39.3 ± 3.6 Ma, MSWD = 4.9, Sri = 0.71449 ± 0.00014)PS1262 wm 250–160 mm 170 156 3.23 0.716261 0.0014PS1263 wm 160–125 mm 186 157 3.43 0.716432 0.0014PS1264 wm 355–250 mm 249 223 3.22 0.716248 0.0014PS1265 apatite 4.51 1495 0.00874 0.714479 0.0014PS1441 feldspar 8.36 10.2 2.38 0.715875 0.0016

Western Tauern Window – SEMP SplaysTF04-26 (15.7 ± 1.3 Ma, MSWD = 24, Sri = 0.74148 ± 0.000084)

PS1289 paragonite 316 52.3 17.6 0.745585 0.0014PS1292 wm 355–315 mm 475 19.9 69.6 0.757174 0.0024PS1296 wm 180–125 mm 472 20.7 66.3 0.756051 0.0014PS1297 epidote-conc. 62.7 307 0.592 0.741492 0.0014

TF04-25 (31.2 ± 0.4 Ma, MSWD <1, Sri = 0.711094 ± 0.000035)PS1267 wm 355–250 mm 261 72.1 10.5 0.715743 0.0034PS1268 wm 160–125 mm 254 68.1 10.8 0.715879 0.0014PS1270 epidote 18.7 1248 0.0433 0.711113 0.0014

SEG2 (17.0 ± 5.3 Ma, MSWD = 447, Sri = 0.704 ± 0.016)PS1559 apatite 1.05 382 0.0079 0.709781 0.0022PS1564 biotite 350–250 mm 692 11.9 169 0.741391 0.0013PS1573 biotite >3 mm 679 5.12 387 0.800660 0.0019PS1567 feldspar 94.5 785 0.348 0.708870 0.0012PS1563 biotite 160–90 mm 744 20.3 106 0.727270 0.0016

PFI1 (17.6 ± 1.7 Ma, MSWD = 37, Sri = 0.7184 ± 0.0012)PS1431 feldspar 33.8 76.4 1.28 0.718623 0.0014PS1433 wm 500–355 mm 443 23.2 56.3 0.733276 0.0018PS1434 wm 250–180 mm m = 0.75A 443 21.5 59.8 0.733346 0.0018PS1435 apatite 3.35 455 0.0214 0.718425 0.0014PS1436 wm 250–180 mm m = 0.5A 443 20.7 62.0 0.733776 0.0016PS1440 wm 100–125 mm 443 18.9 68.1 0.734718 0.0012

HAU3 (15.0 ± 0.4 Ma, Sri = 0.710822 ± 0.000052)PS1576 wm > 3 mm 186 25.4 21.2 0.715336 0.0028PS1575 garnet 12.6 8.00 4.56 0.711795 0.0012

PFI2 (19 ± 4 Ma, MSWD = 1.6, Sri = 0.712936 ± 0.000035)PS1438 lazulite 5.54 2148 0.0075 0.712940 0.0014PS1439 wm >250 mm m = 0.5A 232 822 0.818 0.713166 0.0020PS1447 wm nm 1.2A 172 703 0.710 0.713106 0.0018

PFI3 (16.9 ± 0.6 Ma, MSWD = 122, Sri = 0.7470 ± 0.0011)PS1402 wm 160–90 mm m = 0.8A 674 8.60 229 0.802191 0.0018PS1400 apatite 12.7 859 0.0430 0.746740 0.0014PS1399 wm 90–63 mm m = 0.8A 666 10.6 184 0.790285 0.0028PS1393 K-feldspar 354 324 3.17 0.748006 0.0024PS1392 wm 160–90 mm nm = 0.8A 623 15.4 118 0.775710 0.0016

TF04-15 (excl. bi: 26.7 ± 1.2 Ma, MSWD = 2.9, Sri = 0.709931 ± 0.000085) (bt + ep: 16.4 ± 0.2 Ma)PS1285 apatite 0.16 624 0.00074 0.709948 0.0010PS1287 epidote 10.1 4053 0.0072 0.709913 0.0014PS1294 wm 180–125 mm 289 123 6.82 0.712515 0.0016PS1395 wm 125–80 mm 283 115 7.13 0.712594 0.0014PS1291 wm 355–250 mm 312 138 6.52 0.712458 0.0016PS1298 biotite 375 10.7 101 0.733468 0.0040


11 of 27

TC4004

Table 3. (continued)

Sample No.Analysis No. Material Rb, ppm Sr, ppm 87Rb/86Sr 87Sr/86Sr 87Sr/86Sr 2sm, %

TF04-18 (21.5 ± 0.8 Ma, MSWD = 10, Sri = 0.71939 ± 0.00020)PS1281 apatite 1.09 475 0.0066 0.719339 0.0016PS1282 wm 180–125 mm 291 25.6 33.0 0.729397 0.0014PS1290 wm 315–200 mm 296 26.8 32.0 0.729191 0.0036PS1327 feldspar + qtz 125 186 1.95 0.720046 0.0018

HWZ3 (19.8 ± 0.4 Ma, Sri = 0.707747 ± 0.000035)PS1568 white mica 342 79.3 12.5 0.711244 0.0016PS1569 calcite 5.47 208 0.0514 0.707761 0.0012

Central Tauern Window – Eclogite Zone and EnvironsEIS14a (30.7 ± 0.7 Ma, MSWD = 3.3, Sri = 0.709079 ± 0.000046)

PS1015 carbonate 1.76 1248 0.0041 0.709111 0.0014PS1016 wm > 500 mm 418 125 9.71 0.713286 0.0018PS1117 wm 500–355 mm 427 117 10.6 0.713686 0.0084PS1116 wm 355–250 mm 396 190 6.08 0.711715 0.0012PS1135 wm m = 0.9A, 180–125 mm 396 274 4.17 0.710925 0.0014PS1137 wm m = 0.9A, 125–80 mm 284 359 2.29 0.710107 0.0016PS1136 wm m = 0.9A, 250–180 mm 392 211 5.38 0.711481 0.0018PS1138 paragonite 250–180 mm 90.3 279 0.938 0.709431 0.0014PS1017 paragonite < 500 mm 60.0 503 0.315 0.709194 0.0016

EIS14b (30.6 ± 0.9 Ma, MSWD = 7.4, Sri = 0.708898 ± 0.000078)PS1131 wm > 500 mm 352 121 8.43 0.712569 0.0024PS1133 wm m = 1.2 A, 500–250 mm 411 119 9.96 0.713256 0.0016PS1128 wm m = 0.9 A, 500–250 mm 398 126 9.19 0.712975 0.0012PS1129 wm m = 0.9 A, 250–180 mm 368 164 6.47 0.711649 0.0014PS1132 wm m = 0.9 A, 180–125 mm 261 156 4.85 0.710954 0.0014PS1127 wm m = 0.9 A, 125–80 mm 329 186 5.13 0.711066 0.0012PS1130 paragonite > 355 mm 23.2 504 0.134 0.708908 0.0014PS1134 apatite 1.40 592 0.007 0.708981 0.0016PS1125 carbonate 0.51 402 0.0036 0.708933 0.0014

EIS6 (31.2 ± 0.6 Ma, MSWD = 9.8, Sri = 0.708113 ± 0.000082)PS1091 wm 125–160 mm 358 131 7.89 0.711532 0.0014PS1092 wm 250–355 mm 362 71.3 14.7 0.714601 0.0012PS1093 wm 355–500 mm 367 55.6 19.1 0.716581 0.0014PS1094 wm > 500 mm 361 48.0 21.8 0.717832 0.0012PS1090 carbonate 1.36 3323 0.0012 0.708135 0.0016PS1088 epidote 2.65 3536 0.0022 0.708218 0.0012PS1089 amphibole 10.6 25.4 1.21 0.708567 0.0018PS1172 wm 125–80 mm 294 210 4.03 0.709892 0.0014

EIS11 (31.4 ± 0.4 Ma, MSWD = 1.6, Sri = 0.703991 ± 0.000019)PS1095 carbonate 0.03 613 0.00013 0.703980 0.0014PS1096 amphibole 5.52 78.2 0.204 0.704113 0.0018PS1097 epidote 0.06 689 0.00027 0.703971 0.0014PS1099 wm <200 mm 233 39.3 17.1 0.711620 0.0028PS1113 wm bulk 177 107 4.77 0.706120 0.0018

EIS13 (31.1 ± 0.4 Ma, MSWD = 32, Sri = 0.70440 ± 0.00015)PS1076 wm 180–250 mm 205 10.0 59.3 0.730615 0.0020PS1082 wm 500–355 mm 189 14.7 37.3 0.720755 0.0054PS1083 wm 355–250 mm 206 10.8 55.2 0.728778 0.0016PS1084 wm 125–180 mm 207 10.8 55.7 0.729017 0.0064PS1087 epidote 0.46 456 0.0029 0.704290 0.0014PS1085 apatite 0.03 183 0.0005 0.704338 0.0018PS1068 amphibole 1.13 19.6 0.167 0.704657 0.0016

Eastern Tauern WindowTF04-1b (25.3 ± 2.9 Ma, MSWD = 25, Sri = 0.7170 ± 0.0011)

PS1449 wm nm = 1.8A 486 34.2 41.2 0.732141 0.0014PS1451 wm 125–160 mm m = 1.15A 486 38.7 36.4 0.729674 0.0012PS1455 feldspar 55.5 176 0.913 0.717251 0.0012PS1529 whole rock 51.1 156 0.949 0.717393 0.0016

TF04-5 (20.7 ± 2.3 Ma, MSWD = 9.8, Sri = 0.70801 ± 0.00011)PS1283 wm 355–250 mm 443 188 6.80 0.710020 0.0014PS1284 ankerite 0.50 1047 0.0014 0.707949 0.0014PS1323 wm 180–80 mm 380 405 2.71 0.708797 0.0016PS1391 wm 250–180 mm 405 467 2.51 0.708754 0.0020PS1286 calcite 0.65 259 0.0073 0.708085 0.0012

aAn uncertainty of ±1.5 % is assigned to Rb/Sr ratios. Wm, white mica; bt, biotite; ep, epidote; m/nm, magnetic/nonmagnetic on Frantz magneticseparator, 13� tilt, at electric current as indicated.


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atures required for potential thermally induced resetting ofthe Rb-Sr system in phengite ensures that phengite agesfrom greenschist to amphibolite facies mylonites can, as arule, be regarded as deformation ages. A criterion for theactivity of diffusive partial Sr-isotope reequilibration is apositive correlation between grain size and apparent agefor micas, combined with ‘‘excess’’ 87Sr in high-diffusivityphases like apatite or calcite [Glodny et al., 2008]. Wetherefore analyzed different white mica grain size frac-tions, and apatite or calcite whenever separable, to checkfor potential partial thermal resetting. Analysis of whitemica in different grain size fractions also serves to detectpossible mixed white mica populations or Sr-isotopicinhomogeneities within mica grains. Such inhomogeneitiesmay originate from long-term, incomplete dynamic recrys-tallization, or from possible presence of unequilibrated,predeformational white mica relics, and would lead tomixed, geologically meaningless ages if overlooked [cf.Freeman et al., 1998; Muller et al., 1999].[30] Last not least, isotopic signatures may only record

deformation if minerals escaped from postkinematic recrys-tallization processes, and from deuteric alteration by weath-ering or low-temperature fluids. Feldspar and biotite arepotential products of decompression-related metamorphicmineral reactions (see below; Cesare et al. [2001]). Thesephases are also particularly susceptible to low-temperaturealteration of isotopic systems [Parsons et al., 1999; Jeonget al., 2006]. Alteration products generally start to formalong fluid pathways, i.e., along grain boundaries andcracks. For isotopic analysis we therefore did not utilizealtered whole rocks but only used well-defined and cleanmineral separates. Whenever viable we avoided analysis ofpotentially altered material, like biotite-chlorite inter-growths or turbid feldspar.

6. Analytical Procedures

[31] Rb/Sr analyses were performed, as a rule, on allseparable and unaltered Sr-bearing phases of small, litho-logically homogeneous rock samples. For white mica, weseparated subpopulations with distinct magnetic propertiesand grain sizes, to discriminate between phengite and para-gonite, and to detect possible Sr-isotopic heterogeneity.Traces of secondary (Fe, Mn) hydroxides on some amphi-bole, epidote and garnet separates were removed with a 5%aqueous solution of oxalic acid. White mica fractions wereground in ethanol in an agate mortar and then sieved inethanol to obtain pure, inclusion-free separates. All mineralconcentrates were checked and finally purified by hand-picking under a binocular microscope.[32] Rb and Sr concentrations were determined by

isotope dilution using mixed 87Rb-84Sr spikes. Sampleswere weighed into Savillex screw-top containers. Afteraddition of a suitable spike, they were dissolved in amixture of HF + HNO3, and then converted into chlorides.Solutions were processed by standard, HCl-based cation-exchange techniques. Determinations of Rb and Sr isotoperatios were carried out on a VG Sector 54 multicollectorTIMS instrument (GeoForschungsZentrum Potsdam). Sr

was analyzed in dynamic mode. The value obtained for87Sr/86Sr of NBS standard SRM 987 was 0.710268 ±0.000015 (n = 19). The observed ratios of Rb analyseswere corrected for 0.25% per a.m.u. mass fractionation.Total procedural blanks were consistently below 0.15 ngfor both Rb and Sr. Because of generally low and highlyvariable blank values, no blank correction was applied.Isochron parameters were calculated using the Isoplot/Exprogram of Ludwig [1999]. Standard errors, as derivedfrom replicate analyses of spiked white mica samples, of± 0.005% for 87Sr-86Sr ratios and of ±1.5% for Rb-Srratios were applied in isochron age calculations. Individualanalytical uncertainties were in most cases smaller thanthese values. If otherwise, individual errors have been usedfor age calculation.

7. Characterization of Dated Samples

[33] For isotopic dating of deformation events we selectedsamples that are penetratively deformed, and that show thegeneral microstructural and petrological characteristics ofthe respective shear zones. In this context, we occasionallyhad to compromise between suitability for isotopic dating atone side, and clearness of microstructural and metamorphicsignatures at the other side. In absence of clear indicators ofmetamorphic grade and shear sense of deformation weinferred these parameters from rocks in the immediateoutcrop context. Characteristics of the investigated samplesare presented in Table 2.

8. Results and Significance of Isotopic Data

[34] Rb-Sr data for minerals from 21 samples of mostlyhighly strained/mylonitic rocks from the Tauern Windowarea are presented in Table 3. By abundance, the age databroadly define two groups, the first at 32 to 30 Ma, and asecond one at 21–15 Ma, with a few results in between thetwo clusters. The significance of individual data sets isdiscussed below.

8.1. Brenner Fault Mylonites

[35] We analyzed threemylonite samples from the Brennershear zone. For the two carbonate mylonites (samplesBRE2, TF04-30a) and one quartz-albite dominated graphiteschist (PON1) we obtained valid isochron ages (MSWD< �2.5 [cf. Wendt and Carl, 1991]). Ages are between21 ± 2 Ma and 17.8 ± 1.8 Ma, all of which are identicalwithin limits of error (Table 3 and Figure 4). The greens-chist facies deformation fabrics consistently indicate top-WSW shear. We therefore interpret the white-mica-basedages as dating the end of top-WSW ductile deformation inthe investigated samples.[36] A quartzitic mylonite from the Austroalpine unit of

the direct hangingwall of the Brenner line yielded an agevalue of 39.3 ± 3.6 Ma (sample TF04-32, Table 3). Whilethe mylonitic texture suggests that in general deformationhas set this age value, the MSWD of 4.9 may indicatesome disturbance or incomplete resetting. The latterappears more likely given the Cretaceous metamorphismand the dominantly Cretaceous to Paleocene zircon fission


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track ages in the Austroalpine nappes west of the Brennerfault [Fugenschuh et al., 1997]. In any case, this sampledoes not contribute to evaluation of the postcollisionaldeformation history and its correct interpretation requiresfurther research.

8.2. Western Tauern Window

[37] From the western Tauern Window, we selected 2samples of undeformed rocks and 8 samples of mylonitesand schists for isotopic analysis. Mylonites and schistscome from different, WSW-ENE striking, mostly sinistralshear zones in the area, interpreted to represent splays of thesinistral SEMP. We therefore discuss the results togetherhere, starting in the NW.8.2.1. White-Mica-Based Ages: Mylonites and Schists[38] The mylonitic augengneiss sample TF04-26 is from

the continuation of the Ahorn shear zone [cf. Rosenberg andSchneider, 2008] along the northwestern margin of the Tuxgneiss (see Figure 8 for location of Tux gneiss). Twophengite-muscovite sieve fractions together with paragoniteand epidote data yield an age of 15.7 ± 1.3 Ma (Table 3),which we interpret as dating deformation at amphibolite- togreenschist facies conditions.[39] A mylonitic paragneiss (TF04-25) from the amphib-

olite facies Olperer shear zone yields a well-defined three-point mineral isochron age of 31.2 ± 0.4 Ma (Table 3 andFigure 5). Consistency between the isotopic data for the twodifferent white mica sieve fractions, together with the rocks’texture, indicates absence of predeformational relics, andabsence of postrecrystallizational isotopic resetting. Amphib-olite facies deformation here, in the investigated domain of aparagneiss lamella inside the Tux orthogneiss, was thusterminated already in Oligocene times, significantly earlierthan in the adjacent Ahorn shear zone which is only �1 kmaway. It needs to be stressed that coexistence of this

Oligocene white-mica-based age in close proximity withMiocene ages (also found in the adjacent Greiner shear zone,see below) precludes interpretation of the data as regionalcooling ages.[40] In the Pfitscher Joch segment of the Greiner shear

zone, deformation largely occurred under amphibolite faciesconditions. For the here studied phosphate metaquartzite(sample PFI2), deformation conditions of �550�C, �7 kbarwere inferred, in line with a number of other P,T estimates inthe area [Morteani and Ackermand, 1996; Selverstone andHyatt, 2003]. Unfortunately, the extremely Sr-rich compo-sition of sample PFI2 has led to low Rb/Sr ratios in allminerals, which inhibits high-precision Rb-Sr dating. Weinterpret the Rb-Sr mineral isochron age of 19 ± 4 Ma fromthis sample (Figure 5) as dating deformation at amphibolitefacies conditions. A few hundred meters away fromsample site PFI2, sample PFI1 was collected at a locationwithin the Tux orthogneiss. Shearing of PFI1 occurred atleast at upper greenschist facies conditions, as indicated byductile behavior of feldspar. Some feldspar augen implythat deformation was not entirely penetrative. This isreflected in the isotopic data, which show a clear correla-tion between white mica grain sizes and apparent ages (seeFigure 5; fsp + wm 500–355 mm yields 19.1 ± 0.3 Ma;fsp + wm 100–125 mm: 17.0 ± 0.3 Ma). We interpret theisotopic data from this sample as reflecting amphibolite toupper greenschist facies deformation at 17.6 ± 1.7 Ma.17.0 ± 0.3 Ma is a maximum age for the end ofdeformation in this sample.[41] Detailed thermobarometric work on deformation

fabrics has shown that the rocks from the western Greinershear zone document a P-T path roughly from 10 kbar,550–600�C to 4 kbar, 500�C. The actual P-T conditionsrecorded in a specific lithology or domain largely dependupon the mineral reaction history and the shear strength of

Figure 4. Rb-Sr isochron plots for selected samples from ductile parts of the Brenner line fault system,Western Tauern Window.


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Figure 5. Rb-Sr isochron plots for selected samples from splays of the sinistral SEMP fault system,Western Tauern Window.


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the rocks [cf. Steffen and Selverstone, 2006]. The fine-grained mica rich schist we analyzed (sample PFI3), un-suitable for precise thermobarometry, is by its compositioninherently weaker than the adjacent more coarse-grained,porphyroblast-rich or feldspar-rich rocks. It would thusact to localize strain during decreasing PT conditions[cf. Janecke and Evans, 1988]. This schist therefore mostlikely records and dates the latest increments of ductiledeformation in the western Greiner shear zone, probably atgreenschist facies conditions. The deformation age for thissample is 16.9 ± 0.6 Ma (Table 3 and Figure 5).[42] The above deformation ages from the Pfitscher

Joch area are complemented by data from an undeformedamphibolite facies mobilisate, collected 2.5 km east ofPfitscher Joch (HAU3). This quartz-kyanite-phengitemobilisate formed at amphibolite facies conditions, asindicated by the presence of kyanite+staurolite+garnet inits wallrock and the absence of any alteration reactionsbetween mobilisate and wallrock. We estimate the age ofthe mobilisate by combining Rb-Sr data for garnet of thesidewall of the mobilisate, with phengite data from theinteriors of the mobilisate, which results in an apparent ageof 15.0 ± 0.4 Ma (Table 3). As we have no control oninitial isotopic equilibrium between the garnet and phen-gite, this age value only provides a vague hint on the ageof mobilisate formation. We speculate that the mobilisatewas formed by fluids released by decompression-relateddehydration reactions in the rocks nearby [cf. Cesare etal., 2001], at about 15 Ma.[43] The eastern segment of the Greiner shear zone,

�30 km ENE of the Pfitscher Joch area, is represented inour sample set by a mylonitic gneiss from the northernlimbs of the Zillertal gneiss (sample TF04-15) and by amuscovite schist from a schist belt on the eastern side of theZillergrundl reservoir (sample TF04-18). Sinistral ductileshear in the muscovite schist came to an end at 21.5 ±0.8 Ma (Table 3). The high MSWD of 10.1 for this sampleis related to slight disequilibrium between feldspar andapatite, probably caused by weathering. It does not affectthe validity of the age constraint. From gneiss sample TF04-15 we obtained a well defined isochron age of 26.7 ±1.2 Ma, based on epidote, apatite, and white mica grain sizefraction data (Table 3 and Figure 5). Given the dominantlyamphibolite facies assemblage of this sample, we interpretthe age as dating a waning stage of sinistral shear atamphibolite facies conditions. The Rb-Sr signature ofbiotite plots off the above isochron line. Its significance isdiscussed below.[44] In the area north of Pfunders (Figure 2), a branch of

the broad, complex Ahrntal fault zone separates the Glock-ner nappe from Rote Wand-Modereck and Venediger nappelithologies. The Ahrntal fault zone, so far undated, is thesouthernmost SEMP-correlated fault zone of the westernTauern Window (Figure 2). In a reconnaisance analysis westudied a strongly foliated carbonate mylonite (HWZ3) inthe immediate footwall of the Glockner nappe. The two-point calcite-white mica age of 19.8 ± 0.4 Ma (Table 3) is,on textural grounds, interpreted as approximating the age oflate increments of sinistral shear.

8.2.2. Biotite Data[45] Rb-Sr isotopic data for biotite in the Western Tauern

Window have been obtained for samples SEG2 (Tuxorthogneiss, Pfitscher Joch area) and TF04-15 (from theeastern part of the Greiner shear zone). Biotite grain sizefractions from granitoid sample SEG2 show a systematicdecrease of Rb-Sr ratios and apparent feldspar-biotite ageswith grain size (Figure 5). The apparent fsp-bi age for bi>3 mm is 16.7 ± 0.3 Ma. The corresponding apparent agefor biotite 160–90 mm is 12.3 ± 0.2 Ma. For the myloniticgneiss sample TF04-15, the apparent epidote-biotite agecalculates as 16.4 ± 0.2 Ma, despite the much higher white-mica-based deformation age of 26.7 ± 1.2 Ma (Table 3).The here obtained biotite age estimates are consistent withRb-Sr and K-Ar literature data of �15–13 Ma for biotitefrom the area [Satir, 1975; Satir and Morteani, 1982;Blanckenburg et al., 1989]. While the existing biotite datawere interpreted as cooling ages, we suggest an alternativehypothesis. It has recently been shown that volume diffu-sion under dry, static conditions does not effectively resetRb-Sr signatures of biotite in high-grade metamorphicrocks, even during several Myr of T >600�C [Glodny etal., 2008]. The marked contrast between white mica-basedand biotite-based ages is inconsistent with an interpretationof both as deformation ages. Instead, it has been recognizedthat granitoids from the Tauern Window experienced post-tectonic partial recrystallization, even those which preservedigneous textures (as our sample SEG2) [Morteani, 1974;Morteani and Raase, 1974]. Cesare et al. [2001] deducedthat this posttectonic recrystallization was related to decom-pression, which triggered dehydration reactions in themetamorphosed granitoids and schists of the type

MsþGrtþ Epþ Pl Ið Þ þQtz þCalð Þ ¼ Pl IIð Þ þBtþH2O þCO2ð Þ

[46] Such decompression reactions generate biotite andfeldspar as reaction products, and intergranular fluid thatmay enable Sr-isotopic exchange between preexisting bio-tite and its matrix. We hypothesize that the biotite age data,if unaffected by deuteric alteration, may in fact nearly datesubstantial decompression and the associated mineral reac-tions at �16–13 Ma in the Tauern Window.

8.3. Eclogite Zone and Environs

[47] To constrain the age of exhumation-related greens-chist facies shear within the EZ (Figure 6), we analyzed asample of greenschist facies mafic schist, which we interpretas pervasively retrogressed former eclogite (sample EIS13).The isotopic data (Table 3) show slight initial isotopicdisequilibria between epidote, apatite and amphibole, theorigin of which remains unclear. Nevertheless, the defor-mation age is well-defined at 31.1 ± 0.4 Ma. This age valueis identical within limits of error to the age of greenschistfacies mobilisate formation in the EZ (31.5 ± 0.5 Ma[Glodny et al., 2005]).[48] The contact between the Eclogite Zone and the

Venediger nappe is characterized in the field by mylonitezones with different senses of shear (see above). For a


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calcschist with top-N shear sense indicators (sampleEIS14a) we obtained a well-defined age of 30.7 ± 0.7 Ma(Table 3 and Figures 6 and 7). Grain size and apparent agefor 6 white mica grain size fractions are not correlated,which signifies dynamic recrystallization in a short-termevent with no later resetting. A remarkable observation isthe systematic variation of mica Rb-Sr ratios with grain size:Large grains show high Rb/Sr ratios, while smaller grainshave lower Rb/Sr ratios due to their considerably higher Srconcentration (Table 3 and Figure 7). Since all analyzedwhite mica fractions were pure separates, this correlationmust reflect variations in mica crystal chemistry imposed bydeformation-related processes. Assuming that grain-sizesensitive diffusion creep [e.g., Herwegh et al., 2005] wasan important deformation mechanism, the interiors oflarge mica grains would preserve a record of earlier stagesof deformation compared to the rims of smaller grains.We therefore postulate that the above effect relates to amarked change in Sr equilibrium partitioning between micaand calcite in these rocks during progressive deformation.Increasing availability of Sr for white mica with progressivedeformation is possibly linked to passage of the calcschiststhrough conditions of the aragonite-calcite phase transition.This would imply deformation conditions at 30.7 ± 0.7 Maof <9 kbar at <600�C [cf. Salje and Viswanathan, 1976],consistent with the above independent constraint onexhumation to greenschist facies from the interiors of theEclogite Zone.

[49] A similar calcschist (EIS14b) at the Eclogite Zone-Venediger nappe contact, collected a few meters away fromsample EIS14a, records sinistral S-side-down shear. Theisotopic data similarly show a correlation between micagrain size and Rb-Sr ratios (Figure 7 and Table 3). The agevalue deduced from that sample is 30.6 ± 0.9 Ma,numerically indistinguishable from the EIS14a deformationage. However, isotopic disequilibria exist (MSWD ofregression is 7.4). Detailed inspection reveals that in thissample there is also a covariation of mica apparent ageswith grain size. Combining isotopic data for white mica>250 mm with apatite and calcite data leads to a welldefined apparent age of 30.5 ± 0.8 Ma, while an equallywell defined younger age of 29.1 ± 0.5 Ma is obtained forthe mica fractions <250 mm (Figure 7). Absence of thesame effect in the nearby sample EIS14a implies that thisfeature is not temperature but deformation induced. Weinterpret the pattern to reflect protracted deformation inthis sample, lasting out to <29.1 ± 0.5 Ma. In other words,top-N and sinistral shear at the lower contact of theEclogite Zone commenced simultaneously at about31 Ma, but fabric development in the top-N mylonitewas completed earlier than in the sinistral mylonite.[50] To define the age of fabric formation along the steeply

dipping tectonic contacts of the Rote-Wand-Moderecknappe (Figure 6), we sampled a carbonate mylonite fromits contact with the Eclogite Zone (sample EIS6) and ablueschist- to greenschist facies mafic schist from itscontact with the Glockner nappe (EIS11). Both samples

Figure 6. Map of the Eclogite Zone and adjacent units, with sample locations and age data. See Figure 2for regional context. Map base: Geologische Karte der Republik Osterreich, 1:50000, Blatt 152 Matrei,Geologische Bundesanstalt [1987]. Inset profile after Raith et al. [1980]. G2005: Glodny et al. [2005],(11) Gleissner et al. [2007], and (10) Inger and Cliff [1994].


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record sinistral S-side-down shear. The obtained multi-mineral isochron ages are identical within limits of error,with 31.2 ± 0.6 Ma (EIS6) and 31.4 ± 0.4 Ma (EIS11),respectively (Figure 7 and Table 3). Sample EIS6 againreveals a clear correlation between mica grain size and Srconcentration.

8.4. Eastern Tauern Window

[51] The age of dextral shear in mylonites from thedextral Molltal fault in the southeastern Tauern Window isconstrained by two samples. Sample TF04-1b is a quartz-feldspar mylonite, characterized by internal isotopic dis-equilibria. Using data for whole rock, feldspar, and twowhite mica fractions we obtained an age value of 25.3 ±2.9 Ma (Table 3), which is imprecise but, due to high Rb/Srratios in the mica, considered reliable within limits of error.The other sample is a carbonate mylonite (TF04-5). Whilethree different white mica grain size fractions together with

ankeritic carbonate define a valid isochron with an agevalue of 21.5 ± 0.6 Ma, calcite does not fall on thatregression line, possibly due to weathering effects. Collec-tively, the mineral data result in an age of 20.7 ± 2.3 Ma(Table 3), again considered reliable within limits of error.

9. Discussion

9.1. High-Pressure Metamorphism and Thrusting

[52] The earliest stage of formation of the Alpine struc-tural architecture within and around the Tauern Windowwas early collisional deep underthrusting of, and incipienttop-N nappe stacking within the Penninic units. Its timing isbest constrained by the age of 31.5 ± 0.7 Ma for eclogitefacies metamorphism in the Eclogite Zone [Glodny et al.,2005]. Emplacement of the Eclogite Zone on top of theVenediger nappe and its iuxtaposition with the Rote Wand-Modereck nappe must have been associated with exhuma-

Figure 7. Rb-Sr isochron plots for selected mylonite samples from the Eclogite Zone and environs,central Tauern Window.


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tion of the Eclogite Zone, and occurred in the course of top-N nappe stacking and sinistral S-side-down shearing(Figure 10a). Within the Eclogite Zone, the age of exhu-mation from 90 km depths to the middle crust is constrainedby greenschist facies mobilisate formation at 31.5 ± 0.5 Ma[Glodny et al., 2005]. Our new data for exhumation-relatedgreenschist facies deformation within the Eclogite Zone of31.1 ± 0.4 Ma (sample EIS13) is fully consistent with thepublished results. This strongly corroborates the previousassertion of transiently very rapid exhumation of the Eclo-gite Zone at 32–30 Ma at minimum rates of 36 mm/a[Glodny et al., 2005]. An additional constraint on the age oftop-N thrusting comes from calcschist sample EIS14a. Itswell-defined deformation age of 30.7 ± 0.7 Ma directlydates thrusting of the EZ onto the Venediger nappe.Although PT conditions of shearing cannot be defined withprecision for that sample, the observed inverse correlationbetween Sr content and grain size of white mica (Figure 7and Table 3) probably indicates that latest deformationincrements occurred under amphibolite- to greenschist fa-cies conditions (see above). Thus top-N thrusting, eclogiti-zation, internal deformation and rapid exhumation of the EZall occurred in a narrow time frame at 32–30 Ma.[53] Notably, this time frame is identical to the timing of

granitoid and alkalibasaltic magmatism south of the TauernWindow (32–30 Ma; see Table 1 for references), which hasbeen explained by slab breakoff [Blanckenburg and Davies,1995]. This chronologic coincidence possibly points toimportant syncollisional rebound of the subducted Europeancontinental crust as a consequence of slab breakoff, con-tributing to Penninic top-N nappe thrusting. The sedimen-tologic data of Kuhlemann et al. [2001] indicate thatconsiderable relief existed in the Tauern Window area at�30 Ma.

9.2. Incipient Strike-Slip and Extensional Shear

[54] The best constraints on the age of incipient strikeslip and extensional shear come from the framework of theEclogite Zone (Figure 6). Three mylonite samples from thecontact between the Glockner nappe and the Rote-Wand-Modereck nappe (EIS11, 31.4 ± 0.4 Ma), from the contactbetween the Rote-Wand-Modereck nappe and the EclogiteZone (EIS6, 31.2 ± 0.6 Ma) and from the Eclogite Zone-Venediger nappe contact (EIS 14b, 30.6 ± 0.9 Ma)constrain activity of sinistral S-side-down shear. Sr distri-bution among different white mica grain size fractionsprobably indicates deformation of these mylonites atdecreasing pressures and temperatures (see above), ceasingat greenschist facies conditions. Isotopic disequilibria insample EIS14b suggest that sinistral S-side-down shearlasted until 29.1 ± 0.5 Ma and outlasted top-N thrusting inthis locality. In this context it is important to note that thedeformation ages date the last pervasive recrystallization,i.e., the end of ductile deformation [cf. Freeman et al.,1997]. The above results are fully consistent with pub-lished greenschist facies deformation ages from the area(31–28 Ma [Inger and Cliff, 1994; Gleissner et al.,2007]). We conclude that in the Eclogite Zone andneighboring units sinistral S-side-down deformation started

at 32–31 Ma, coeval with high-pressure metamorphismand rapid exhumation of the Eclogite Zone. In otherwords, high-pressure metamorphism in the Eclogite Zone,the immediately following thrusting of the latter onto theVenediger nappe and blueschist- to greenschist faciessinistral S-side-down shearing within parts of the EclogiteZone, in the Rote-Wand Modereck and lowermost Glock-ner nappes occurred, within error, at the same time. Strike-slip S-side-down ductile deformation was completed at�29 Ma. This also indicates that the formation of thenappe pile in the Tauern Window was completed in thisnarrow time span between 32–29 Ma (Figure 10a).[55] In the following we suggest that essential parts of the

normal and strike-slip fault system in the entire TauernWindow region started to form at 32–30 Ma, synchronouswith or immediately subsequent to Penninic/Helvetic nappestacking. A review of published and new isotopic age datafor deformation shows that Oligocene ages of 32–30 Mahave been obtained from a number of the shear zonesconsidered responsible for exhumation of the TauernWindow (Figure 2). In addition, a number of age data closeto 30 Ma exist for Penninic and Helvetic units, the exacttectonic significance of which remains unclear but whichnevertheless indicate that fabric formation in large parts ofthe Tauern Window was already completed at that time (seeFigure 2) [Satir, 1975; Lambert, 1970; Raith et al., 1978;Cliff et al., 1985; Inger and Cliff, 1994; Reddy et al., 1993;Liu et al., 2001].[56] We start our discussion with the fault system in the

overlying Austroalpine nappes south of the Tauern Win-dow. A number of white-mica-based mylonitization ageshave been published for various segments of the Periadri-atic line, generally documenting dextral shear around 30 Ma(Figure 2) [Laufer et al., 1997; Muller et al., 2000, 2001;Mancktelow et al., 2001]. An important observation is thechange from sinistral strike slip to dextral transpressivemotion for the Defereggen-Antholz-Vals fault system at�30 Ma [Muller et al., 2000; Mancktelow et al., 2001].This age directly reflects a major tectonic reorganisationand the establishment of the Tauern Window-formation/exhumation regime. The 32.4 ± 0.4 Ma Rieserferner pluton[Romer and Siegesmund, 2003] was syntectonic withmylonitic deformation along the Defereggen-Antholz-Valsfault system and with E-W directed horizontal extension inits Austroalpine country rocks [Wagner et al., 2006].Similar results were obtained for deformation patterns inand around the Rensen pluton, SW of the Tauern Window[Barth et al., 1989; Muller et al., 2000; Krenn et al., 2003].[57] Published Rb-Sr data on schists from the wider

Molltal fault area, namely from the Mallnitzer Mulde(Figure 2), are between 32 and 20 Ma. Inger and Cliff[1994] reported a number of deformation ages clusteringaround 30–28 Ma. In addition, Cliff et al. [1998] docu-mented deformation at 27.7 ± 0.3 Ma, based on Th-Pb datafor allanite crystals, which grew during the formation of acrenulation cleavage resulting from large-scale folding.Although protracted ductile deformation along the Molltalfault until �20 Ma is indicated by our new data (25.3 ±2.9 Ma, sample TF04-1b; 20.7 ± 2.3 Ma, sample TF04-5,


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Table 3) and by literature data (see Figure 2) [Inger andCliff, 1994], there is clear evidence that dextral tectonicmovements related to the Molltal fault system started duringearly exhumation [cf. Kurz and Neubauer, 1996] in theOligocene. The above range of deformation ages suggeststhat deformation along the Molltal fault was continuously orat least intermittently active in the entire period between�30 and �20 Ma.[58] For the Katschberg normal shear zone isotopic age

data are scarce. Normal shearing in late Oligocene times isindicated by K-Ar data for fine-grained white mica fromthe NE edge of the Tauern Window (Figure 2). The age of�25 Ma was interpreted as cooling postdating normalshearing [Deckert, 1999]. From an area nearby, Liu et al.[2001] reported a number of Ar-Ar white mica plateau agesclose to 30 Ma for Penninic rocks in the immediate footwallof the Katschberg normal shear zone. However, it remainsuncertain whether these ages reflect early nappe stacking,cooling, or normal, Katschberg-fault-related deformation.Some Ar-Ar dates at �22 Ma were interpreted as directlydating Katschberg normal fault activity [Liu et al., 2001].The Katschberg fault connects at its southern terminationwith the dextral Molltal fault [Kurz and Neubauer, 1996;Frisch et al., 2000]. Thus kinematic and chronologiclinkage between the two faults can be inferred, withinitiation of normal faulting on the Katschberg fault in theOligocene and lasting into the Early Miocene.[59] The SEMP shows strong evidence for nucleation

contemporaneous with Oligocene thrusting and crustalthickening. For the SEMP along the northeastern TauernWindow, sinistral transpression has been directly dated(Ar-Ar, white mica) to between 35 and 28 Ma [Urbaneket al., 2002]. Given that the interpretation of the WSW-ENE-striking ductile shear zones in the western TauernWindow as deep-crustal equivalents of the SEMP fault[Behrmann and Frisch, 1990; Linzer et al., 2002] iscorrect, our new deformation age for the Olperer shear

zone (sample TF04-25, 31.2 ± 0.4 Ma; Table 3 andFigures 2, 5, and 8) confirms activation of sinistral shearat depths of �35 km (as inferred from the P-T data ofSelverstone et al. [1984]) also within the western TauernWindow in the 32–30 Ma time frame. This deformationobviously continued in the Late Oligocene to Early Miocene,as indicated by our data for the age of sinistral shear in theeastern part of the Greiner shear zone (26.7 ± 1.2 Ma, sampleTF04-15; 21.5 ± 0.8 Ma, sample TF04-18; Table 3 andFigures 2, 5, and 8). Late Oligocene to Early Miocenedeformation in the Greiner shear zone is also recorded bythe monazite U-Th-Pb data of Barnes et al. [2004].[60] For the Brenner normal shear zone we obtained

three independent and concordant deformation ages for theend of ductile top-W shearing in the investigated samples(18.3 ± 2.6 Ma, 21 ± 2 Ma, and 17.8 ± 1.8 Ma, Table 3and Figures 2 and 4). These ages record late-stage,greenschist facies deformation. The kinematic relationshipof the Brenner fault with the sinistral strike-slip shearzones and faults is complicated and controversially dis-cussed. The following five arguments suggest an Oligo-cene initiation of top-W ductile shear at the Brenner shearzone. (1) Thoni [1980] reported four K-Ar white micaages, between 32.2 ± 1.8 and 20.3 ± 1.4 Ma, forpervasively deformed quartzites of Austroalpine origin.These quartzites form tectonic lenses within the Brennerfault zone north of Brennerpass (Figure 2). In line with theoriginal interpretation of Thoni [1980], we regard theabove age data as early, previously largely unnoted agesfor Brenner fault tectonic movements. (2) At its northerntermination the Brenner fault is transformed into thesinistral Inntal fault [Fugenschuh et al., 1997; Frisch etal., 2000; Ortner et al., 2006]. For the Inntal fault, pre-Late Oligocene movements have been reported [Ortnerand Stingl, 2001; Ortner et al., 2006]. In addition, theInntal fault belongs to the same sinistral fault system asthe �30 Ma SEMP fault. The proposed kinematic linkage

Figure 8. NNW-SSE profile across the Western Tauern Window, crossing the Pfitscher Joch area(modified after Lammerer and Weger [1998], GBA 2006), with projected age data (in Ma) on ductiledeformation processes. See Figure 2 for location of the profile. (1) This study, (14) Blanckenburg et al.[1989], (15) Satir and Morteani [1982], (16) Barnes et al. [2004], (17) Satir [1975], (18) Schneider et al.[2007], and (24) Muller et al. [2000].


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between the Inntal and Brenner faults thus suggestscontemporaneous initiation. (3) In the south, the motionof the Brenner fault is transformed into dextral displace-ment along the Periadriatic line [Frisch et al., 2000], theestablishment of which is constrained to �30 Ma. (4) Inthe westernmost Tauern Window amphibolite facies top-WSW normal shear is documented, which started atconditions close, maybe even prior to the thermal peakand promoted initial decompression [Selverstone, 1988,1993]. Given that the thermal peak in the western TauernWindow occurred at 30 ± 1 Ma [Christensen et al., 1994],top-WSW normal shear contemporaneous with N-S short-ening thus commenced at around 30 Ma [Selverstone,1993; Selverstone et al., 1995]. Protracted exhumation-related cooling probably controlled the development ofearly, diffuse top-WSW shear into the later, discrete,greenschist facies Brenner fault zone [Selverstone et al.,1995; Axen et al., 1995]. (5) Our structural and geochro-nologic work in the Olperer shear zone suggests thatamphibolite facies sinistral and top-WSW normal shearingoccurred simultaneously at 31.2 ± 0.4 Ma. We suggest thatour age data from the Brenner fault reflect the end of aprotracted history of ductile normal shear at �21–18 Ma.[61] Nonetheless, Rosenberg and Schneider [2008] argued

that if the Greiner shear zone continued westward until theBrenner fault without a marked change in strike, it wouldreach the Brenner Fault at its southern end (Figure 2). In thisarea the kinematics of the west-dipping Brenner extensionalfault would predict a dextral shear zone, associated withexhumation of the footwall of the Brenner Fault [Fugenschuhet al., 1997] and not a sinistral shear zone. Therefore a directkinematic link between the Greiner shear zone and theBrenner fault seems unlikely [Rosenberg and Schneider,2008]. We believe that further detailed field work is neededto clarify the kinematic relationship between the Greinershear zone and the Brenner fault.[62] The above discussion shows that a large number of

individual postcollisional faults of the Tauern Window areaformed at different lithospheric levels (from the base of thecrust or even the upper lithospheric mantle to the semi-brittle regime) in the Oligocene, at �32–30 Ma. Thisholds both for the transpressional strike-slip systems northand south of the Tauern Window, and for the normal shearzones west and east of the Window. Considering the entirefault pattern as a kinematically interconnected systemresponsible for the tectonic exhumation of the TauernWindow [Ratschbacher et al., 1991; Neubauer et al.,1999; Frisch et al., 2000] implies that the formation ofthe Tauern Window was coeval with rapid exhumation ofthe deep-seated Eclogite Zone at 32–30 Ma. In otherwords, we propose that the structures responsible for theformation of the Tauern Window started to develop about10 Ma earlier than hitherto thought.[63] No ages <29 Ma occur in the Eclogite Zone and in

the units directly adjacent to it. This indicates that this mostdeeply exhumed segment of the Tauern Window underwentvery rapid tectonism in only �3 Ma in the Oligocene. Thelack of ages <29 Ma in and around the Eclogite Zone couldmean that the transpressive regime nucleated in the area of

the present Eclogite Zone. Subsequently the orogenicwedge grew in N-S and got slightly extended in E-W andin the course of this growing wedge deformation is succes-sively transferred to the N, W and E. If this speculativeinterpretation is correct, it would imply that the �30 Mastructures in the Katschberg and Olperer shear zones aretransported structures. Interestingly, the �30 Ma deforma-tion ages tend to occur at the periphery of the window andin the adjacent Austroalpine nappes.[64] It is also conceivable that the �30 Ma deformation

occurred in the fault systems in the surroundings of theTauern Window since the window just started to form bythis time. Much of the strong N-S shortening at �30 Maseems to have been accommodated in the Penninic units ofthe developing Tauern Window into vertical extension, nothorizontal E-W extension. Vertical extension appears to berelated to the development of the antiforms and strongthinning of the limbs of the folds. It is in these limbs wherethe strike-slip shear zones preferentially nucleated andwhere the deeply exhumed units, especially the EclogiteZone, occur. Note that both options are not mutuallyexclusive and might have operated in concert with eachother.

9.3. Significance of Miocene Deformation Ages

[65] For the period between �29 and 15 Ma, the databaseof ductile deformation ages (Figure 2) shows scattered databetween 29 and �21 Ma, and a large number of age valuesbetween 21 and 15 Ma. Although this "statistics’ is far frombeing representative, we hypothesize that it reflects pro-nounced reactivation of preexisting structures at �21–15 Ma. Cliff et al. [1985] also argued that their geochrono-logic data reflect rapid exhumation of the Tauern Windowbetween �20–15 Ma. Kuhlemann et al. [2006] interpreteddetrital apatite fission track data from Alpine Molasse basinsto be related to an increased rate of tectonic denudation of theTauern Window at the same time.[66] The sinistral shear zones in the western Tauern

Window show a striking pattern of amphibolite/greenschistfacies and brittle reactivation, or continued activity, between21–15 Ma (Figures 2, 8, and 10b). Our interpretation of thebiotite ages indicates considerable decompression, thusexhumation, at �15 Ma. Our age data and their relationto the metamorphic conditions during shearing indicate thatshearing under amphibolite- and greenschist facies condi-tions occurred at about the same time in the currentlyexposed parts of the shear zones indicating that those shearzone segments were subsequently juxtaposed. In large partsof the Greiner shear zone no evidence for annealing of thedated ductile fabrics occurs. The same holds true for theAhorn shear zone [Rosenberg and Schneider, 2008]. There-fore a lot of the ductile deformation in the western TauernWindow is Miocene in age, which is in contrast to previousassertions for a prepeak metamorphism (i.e., pre-30 Ma) ageof deformation [cf. Behrmann and Frisch, 1990]. In somesegments of the heterogeneous shear zones older ages of30–26 Ma are preserved.[67] The Ahorn shear zone accommodated �7 km of

S-side-up vertical displacement. At the western termination


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Figure

9.

NNW-SSE

profile

across

theWestern

Tauern

Window,crossing

thePfitscher

Joch

area

(modified

after

Lammerer

andWeger

[1998],GBA

2006),withpublished

zirconandapatitefissiontrackagedata.

See

Figure

2for

locationoftheprofile.(1)Most

[2003],(2)Stockhertet

al.[1999],(3)Fugenschuhet

al.[1997],(4)Grundmannand

Morteani[1985],and(5)Steenkenet

al.[2002].


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of this fault, part of the sinistral lateral displacement alongthe SEMP probably is transferred into vertical displacement,a tectonic setting linked with formation of the large-scaleENE-WSW striking upright folds of the western TauernWindow (Figures 8 and 9) [Rosenberg and Schneider,2008]. This indicates that the Oligocene transpressiveregime persisted into the Miocene (Figure 10b). As aconsequence, the �30–26 Ma ages from the Olperer andGreiner shear zones could reflect ,frozen-in’ ages fromsegments of the shear zone that were active in the Oligoceneand ceased activity then, whereas in other segments ofthe same shear zone pronounced ductile shearing in theMiocene until 17–15 Ma is recorded. Whether or not

shearing in the heterogeneously deforming shear zoneswas continuous or intermittent in the period from �30–15 Ma cannot be decided from our data set.[68] The Ahrntal shear zone shows a component of

N-side-up motion of probably similar magnitude as theAhorn shear zone (Figures 8 and 9) in the Miocene. Thetwo shear zones created a pop-up wedge in the westernTauern Window. This wedge controlled part of the exhu-mation of the central parts of the western Tauern Window.The overall antiformal structure requires that the deepestexhumation of �25 km occurs along the central axis of thewestern Tauern Window [Fugenschuh et al., 1997], between21 and 15 Ma. As a consequence, the central parts were still

Figure 10. Illustration of envisaged tectonic development of the Tauern Window region. (a) Nucleationof the Tauern Window coeval with deep underthrusting and high-pressure metamorphism in the EclogiteZone and initial normal shearing at Katschberg and Brenner normal faults as well as shearing on majorlateral shear zones. Map on the left shows Tauern Window dashed implying that Penninic/Helvetic rocksare not yet at the surface, as indicated on schematic N-S cross section on the right. Intrusion of plutonsand also alkalibasaltic volcanism along major strike-slip faults are thought to be due to slab breakoff,implying that strike-slip faulting played a major role during deep continental underthrusting. (b) Miocenesituation. Tauern Window grew to the N, W and E and large-scale folding occurred along with the growthof the lateral faults in and around the Tauern Window. Note that relative scales for the maps and crosssections are distinctly different. Abbreviations: DAV, Defereggen-Antholz-Vals fault; EZ, Eclogite Zone;GN, Glockner nappe; RW, Rote Wand-Modereck nappe; SEMP, Salzach-Ennstal-Mariazell-Puchbergfault; VN, Venediger nappe.


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at amphibolite facies conditions while toward the N and Slower temperatures and higher shear strengths can beexpected [Rosenberg and Schneider, 2008]. Zircon andapatite fission track ages show that the brittle part of thepop-up wedge formed after �15 Ma. The rather flat zirconand apatite fission track age profiles between the Ahorn andAhrntal faults (Figure 9) corroborate that no major differ-ential vertical movements occurred in the Venediger nappeafter �15 Ma. The contrasting apatite fission track agesacross the two faults indicate that brittle movements (S-sideup at the Ahorn fault and N-side up at the Ahrntal fault)continued until at least the Late Miocene (�8 Ma)[cf. Most, 2003].[69] Similar Miocene ages of 21–18 Ma date the end of

greenschist facies mylonitic deformation in the Brennershear zone. About 20 km to the E of the Brenner shearzone, rocks in the Greiner shear zone were still at >500�C at17 Ma, indicating considerable exhumation at the brittleBrenner fault after cessation of ductile shearing at 18 Ma.The fission track data of Fugenschuh et al. [1997] show that<5 km of exhumation since �13 Ma was accommodated bybrittle motion on the Brenner fault which is continuing tothe present-day [Reiter et al., 2005]. The problem is thatthese data do not fully account for the exhumation of thefootwall of the Brenner fault system because the >500�Crocks demand an overburden of nearly �20 km that musthave been removed after 17 Ma. Two options might beconsidered: (1) There is another so far undetected top-Wnormal shear zone below the Brenner fault system, an ideafirst considered by Axen et al. [1995]. In fact, geomorphicevidence for the existence of such a structure has recentlybeen described [Pazzaglia et al., 2007]. (2) Penetrative top-W normal shear in the Brenner fault system took placeunder lowermost greenschist facies conditions just abovethe closure temperature of fission tracks in zircon of�280�C. The problem is that the parageneses of thesamples we dated at 17 Ma do not supply precise PTconditions and leave the option open that pronouncedexhumation occurred above �280�C. In the intervalbetween 17 Ma and 13 Ma �15 km or more of exhumationmust have occurred at rates of �4 km/Ma.[70] The 21–15 Ma ages mainly occur in the western

Tauern Window but are also present in the eastern part ofthe window [Cliff et al., 1985; this work]. Although thewestern Tauern Window is studied in much more detail, weconclude from the isotopic data that the entire TauernWindow experienced a major episode of ductile shear andexhumation in the Miocene.

10. Major Open Questions

[71] We argue for a general transpressive deformationregime in the Oligocene and Miocene in the Tauern Win-dow [cf. Genser et al., 1996; Kurz and Neubauer, 1996] inwhich N-S shortening exceeded E-W extension by a factorof �3 [cf. Huismans et al., 2001; Rosenberg et al., 2007]. Inthis regard the argument of Schmid et al. [2004] that erosionof the high-amplitude antiforms is sufficient to exhume theTauern Window almost to its presently exposed structural

level is important because it shows that no large-scaleextension is needed for exhuming the core of the TauernWindow. The 70 km of exhumation of the Eclogite Zone inthe Oligocene remains a problem. If this exhumation wasresolved by S-side-down sinistral shear in the mylonite zoneabove the Eclogite Zone then a displacement of >150 kmwould be needed. This displacement would in turn result ina time-averaged slip rate of >70 mm/a in this mylonite zone.There is no evidence that would support such great figures.Slip rates of the order of 70 mm/a exceed the greatestreported rates of 15–>20 mm/a [Sorel, 2000; Ring andReischmann, 2002; Little et al., 2007] we are aware of by afactor of at least 3.[72] Nonetheless, given the local transtensive character of

the S-side-down shear zone at the top of the Eclogite Zone,the transport direction may have been steeper than thestretching lineation [e.g., Robin and Cruden, 1994], whichwould bring the displacement and slip rate estimates of>150 km and >70 mm/a somewhat down. The minimumexhumation rate of 36 mm/a reported by Glodny et al.[2005] would demand a slip rate >36 mm/a if exhumationwas accomplished by a single, nonvertical down-dip shearzone.[74] We regard the option that the S-side-down shear

zone at the top of the Eclogite Zone had more or less down-dip displacement more likely. If so, the Eclogite Zone wouldrepresent a lateral extrusion wedge as proposed by Kurz andFroitzheim [2002]. The S-side-down normal displacementwould not be due to N-S extension but would be ageometric effect, i.e., normal displacement would accom-modate movement at the top of the upward extrudingwedge. The importance of exhumation of high-pressurenappes soon after their metamorphic climax in extrusionwedges has recently been stressed in the Hellenic subduc-tion zone in the Aegean, where coeval motion on the basalthrust and the normal fault at the top of two extrusionwedges has been demonstrated [Ring et al., 2007a, 2007b].For the Eclogite Zone extrusion wedge the exhumation rateof >36 mm/a demands great slip rates exceeding 25 mm/aon the faults bounding the wedge. We envisage that fastnormal slip associated with the development of a large-amplitude antiform and concomitant erosion may haveaccomplished the extremely fast exhumation of the EclogiteZone at 32–30 Ma.

11. Conclusions

[75] From our data discussed above we draw the follow-ing conclusions.[76] (1) High-pressure metamorphism, top-N thrusting

and sinistral, S-side-down shearing in the Eclogite Zoneand the directly neighboring units, sinistral shearing at theSEMP, top-E normal shearing at the Katschberg shear zonein the eastern Tauern Window, sinistral shearing in theOlperer shear zone and associated normal shearing at theBrenner shear zone, and also lateral shearing in the Austro-alpine nappes all initiated at 32–30 Ma.[77] (2) This has one major tectonic implication: The

pattern of deep burial and high-pressure metamorphism, as


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well as simultaneous top-N thrusting, sinistral shearing andlateral extension was a phenomenon that affected the entireTauern Window from the very start of deformation at 32–30 Ma onwards. In other words, high-pressure metamor-phism in the Eclogite Zone and initial normal shearingcommenced largely coevally in the Early Oligocene and thestrike-slip shear zone apparently transferred part of theshortening deformation in the central part of the developingTauern Window into E-W normal shearing at its eastern andwestern periphery.[78] (3) The transpressive deformation regime persisted,

either continuously or intermittently, at least into the MiddleMiocene (�15 Ma) as indicated by the formation of a pop-up structure in the central western Tauern Window andbrittle extension across the Brenner fault. After the Middle

Miocene, only brittle deformation is recorded in the TauernWindow area. Seismological data suggest that transpressivedeformation persists until today.[79] (4) Deformation ages from shear zones bounding the

Eclogite Zone imply that emplacement of the Eclogite Zonein its present-day structural position was accomplishedalready at�29 Ma, i.e., only about 3 Ma after eclogitization.[80] (5) The very fast exhumation of the Eclogite Zone

demands high strain rates in the shear zones that bound theEclogite zone.

[81] Acknowledgments. Support of this study by the Deutsche For-schungsgemeinschaft (DFG grant RI 538/25-1 to U.R. and J.G.) isgratefully acknowledged. We thank W. Kurz and J. Selverstone for theircareful and constructive reviews.

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��J. Glodny, GeoForschungsZentrum Potsdam, Tele-

grafenberg C2, 14473 Potsdam, Germany. ([email protected])

A. Kuhn, Gexco AS, Postbox 500, N-8601 Mo iRana, Norway. ([email protected])

U. Ring, Department of Geological Sciences,University of Canterbury, Private bag 4800, Christch-urch 8140, New Zealand. ([email protected])


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Coeval high-pressure metamorphism, thrusting, strike-slip, and extensional shearing in the Tauern...

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