Coeval high-pressure metamorphism, thrusting, strike-slip, and extensional shearing in the Tauern Window, Eastern Alps

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1 TECTONICS, VOL. 27,, doi: /2007tc002193, 2008 Coeval high-pressure metamorphism, thrusting, strike-slip, and extensional shearing in the Tauern Window, Eastern Alps Johannes Glodny, 1 Uwe Ring, 2 and Alexander Kühn 3,4 Received 2 August 2007; revised 13 December 2007; accepted 15 February 2008; published 23 July [1] Recent findings for a young (31.5 ± 0.7 Ma) age of high-pressure metamorphism at 90 km depths in the Eclogite Zone of the Tauern Window, Eastern Alps, prompt the question about the timing of the structural development of the Tauern Window and its relation to high-pressure metamorphism. We show that all major structures in the Tauern Window, resulting from strong N-S lithospheric shortening and simultaneous minor E-W extension, began developing coevally with high-pressure metamorphism in the Eclogite Zone. Large-scale strike-slip shear zones started to form at Ma and facilitated the spatial accommodation of simultaneous shortening and extension. At least some of the strike-slip and extensional shear zones operated into the Middle Miocene, either continuously or intermittently, with pronounced activity at Ma. The considerable exhumation of the Eclogite Zone from 90 km depths into the middle crust, and the tectonic development of its framework occurred within only 1 2 Ma after eclogitization. This is evidenced by almost identical ages for eclogite facies metamorphism and for the development of the major structures that bound the Eclogite Zone under blueschist- and greenschist facies metamorphic conditions. We discuss a tectonic model in which considerable transpressional shortening and thickening took place in the present central-southern part of the Tauern Window. We propose that the Tauern Window nucleated here and that most of the regional deformation at Ma is today found at the periphery of the window and in the adjacent Austroalpine units. Afterward, transpression continued, the window grew to the E, W, and N, and deformation progressed to those parts of the window. Ductile deformation in the present-day surface level ceased at 15 Ma. Citation: Glodny, J., U. Ring, and A. Kühn (2008), Coeval high-pressure metamorphism, thrusting, strike-slip, and extensional shearing in the Tauern Window, Eastern Alps, Tectonics, 27,, doi: /2007tc GeoForschungsZentrum Potsdam, Potsdam, Germany. 2 Department of Geological Sciences, University of Canterbury, Christchurch, New Zealand. 3 Institut für Geowissenschaften, Johannes Gutenberg-Universität, Mainz, Germany. 4 Now at Gexco AS, Mo i Rana, Norway. Copyright 2008 by the American Geophysical Union /08/2007TC Introduction [2] The interplay between high-pressure deformation related to lithospheric convergence and the development of large-scale thrusts, strike-slip faults and extensional shear zones is of considerable tectonic interest. An intriguing aspect is as to whether high-pressure metamorphism can be coeval with simultaneous motion on all three kinds of large-scale structures and how those structures evolve subsequent to high-pressure metamorphism. A better understanding of the timing of high-grade metamorphism and the structural development can solve this issue, which is important for our understanding of how diffuse continental plate-boundary zones work. [3] The Tauern Window of the Eastern Alps (Figures 1 and 2) offers a superb example to study the above mentioned processes in detail. Recently, we showed that the peak of high-pressure metamorphism in the Eclogite Zone in the south-central Tauern Window occurred at 31.5 ± 0.7 Ma [Glodny et al., 2005], making the Eclogite Zone the youngest high-pressure unit in the Alps. The development of extensional structures related to the formation of the Tauern 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]). The Eclogite Zone itself experienced its high-pressure metamorphism at about 90 km depth (25 kbar, 630 C [Holland, 1979; Hoschek, 2007]). High-pressure metamorphism in the adjacent units is of distinctly lower grade. The directly overlying Glockner nappe was subjected to 7.5 ± 1 kbar, 525 ± 25 C; [Dachs, 1990]; P max here was <12 kbar [Gleissner et al., 2007]. The underlying Venediger nappe experienced P max of kbar at 550 C [Franz et al., 1991; Kurz et al., 1998a, and references therein]. The question is when all three units became juxtaposed and how this relates in time to the formation of the internal architecture of the Tauern Window by large-scale thrusting. The correlation in time of internal thrusting to the development of the fault system bounding the Tauern Window is also unclear. This fault system consists of two normal faults at the western and eastern margin, a prominent sinistral strike-slip fault system in the western half of the window and along its northern flank, and a dextral strike-slip fault system in the eastern half of the window [Genser and Neubauer, 1989; Ratschbacher et al., 1991; Kurz and Neubauer, 1996; Rosenberg et al., 2004]. Displacements along these fault systems have been responsible for the tectonic component of exhumation of the Tauern Window [Neubauer et al., 1999; Frisch et al., 2000], which is 1of27

2 Figure 1. Simplified geologic-tectonic overview of the Central and Eastern Alps and their frame. thought to dominate overall exhumation [cf. Kuhlemann et al., 2001]. [4] Previous geochronologic studies in the Tauern Window have focused on certain areas, and often tried to decipher the thermal instead of the tectonic history. Timing of activity of major shear zones has mainly been inferred from indirect evidence. Tectonochronologic data constraining ages of fabric formation and of deformation along discrete shear zones are very rare. We try to fill this gap by direct isotopic dating of deformation in several key fault zones across the entire Tauern Window. We aim to discuss the timing of tectonometamorphic processes affecting the Tauern Window and its structural framework using published age data, and present 21 new Rb-Sr mineral isochron ages on fabric-forming structures. Integration of the isotopic ages with structural data indicates that nucleation of the Tauern 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, largescale thrusting, extensional shearing and a coordinated system of sinistral and dextral strike-slip shear zones operated simultaneously with each other during overall transpression. Transpressive movement persisted at least until about 15 Ma. 2. Geologic Setting [5] The Alpine orogen (Figure 1) has a protracted tectonic history that involved the closure of three oceanic basins from the Cretaceous through the Cenozoic [Kozur, 1991; Platt et al., 1989; Ring et al., 1989]. In the Eastern Alps, the imprints of two distinct orogenies can be recognized. These are the Eo-Alpine (Cretaceous) event related to the closure of the Meliata ocean, and the Eocene to Oligocene Meso - and Neo-Alpine orogeny as a consequence of collision between the Adriatic and European plates following the closure of the Penninic oceans. The Cretaceous event is mainly recorded in the Austroalpine nappes, which make up the upper (Adriatic) plate during collision. The imprint of the Cenozoic collision processes is ubiquitous in the underlying Penninic and Helvetic nappes, but also evident in the Austroalpine units by local deformation, magmatism, and, regionally, by thermal overprints [cf. Schmid et al., 2004, and references therein]. [6] The Austroalpine units were thrust during the collision process toward the NNE over Penninic and Helvetic units, the latter of which derived from the European, lower plate [e.g., Kurz et al., 1998a; Schmid et al., 2004]. The Tauern Window is the largest of several Penninic/Helvetic windows in the Eastern Alps. The Penninic/Helvetic rocks exposed in the Tauern Window form a series of lithologically distinct nappes. The tectonostratigraphically lowermost major Helvetic unit is the Venediger nappe. It comprises a pre-variscan, European continental basement, intruded by Variscan granitoids, and covered by parautochthonous metasedimentary series [e.g., Frisch, 1980]. This unit is tectonically overlain by basement slices intercalated with mainly Mesozoic passive continental margin successions, now forming the Rote Wand-Modereck and Storz nappes [Kurz et al., 1998a, 2001], also known as Lower Schieferhülle. Oligocene to Miocene Alpine metamorphism of the Venediger/Rote Wand-Modereck/ Storz nappe stack is of blueschist to greenschist grade, regionally reaching amphibolite facies conditions. In the 2of27

3 Figure 2 3of27

4 south-central Tauern Window, the Eclogite Zone occurs between the Venediger and the Rote Wand-Modereck nappe, forming a separate unit [cf. Kurz et al., 1998b] that experienced near-uhp metamorphism (25 kbar, 630 C) [Hoschek, 2007] in the Oligocene, at 31.5 ± 0.7 Ma [Glodny et al., 2005]. The structurally uppermost unit in the Tauern Window is the Glockner nappe ( Upper Schieferhülle ), a series of Mesozoic sedimentary and mafic metaigneous rocks with oceanic Penninic affinity. [7] With Oligocene continent collision, the overall kinematic field in the Eastern Alps shifted from WNW- to NNE-directed shortening [e.g., Ratschbacher, 1986; Ring et al., 1988; Peresson and Decker, 1997]. In this context, the Eastern Alps experienced continuous transpression between the obliquely converging European and Adriatic plates. Transpression is often partitioned into orogennormal shortening, strike-slip movements and orogenparallel extension, as exemplified in the Alps and in other orogens [Sanderson and Marchini, 1984; Ratschbacher, 1986; Tikoff and Teyssier, 1994; Neubauer et al., 1999]. In the Tauern window area, transpression resulted in the development of a complex system of kinematically linked faults (Figure 1), facilitating tectonic exhumation and finally exposure of the rocks of the Tauern Window [Genser and Neubauer, 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 (and west) and a dextral system south (and east) of the Tauern Window (Figure 2) [Ratschbacher et al., 1991]. The strikeslip fault systems formed both within the Tauern Window and in the overlying Austroalpine nappes, and are, in general, oriented orogen-parallel (E-W). Kinematic linkage between E-W extension and strike-slip faults indicates that the latter were formed simultaneously with the extensional displacements [Ratschbacher et al., 1991]. We describe the main features of the faults, especially those in the Austroalpine, here and follow with a more detailed section on the faults and shear zones in the Tauern Window that includes our own observations. [9] The most prominent of the sinistral faults are the Salzach- Ennstal-Mariazell-Puchberg fault (SEMP) [Ratschbacher et al., 1991], and the Inntal fault [Fügenschuh et al., 1997; Ortner et al., 2006] (Figures 1 and 2). The near-vertical SEMP strikes along more than 300 km from the Vienna Basin to the northern margin of the Tauern Window, accommodating a sinistral displacement of 60 km during Cenozoic time [Linzer et al., 2002]. The SEMP partly forms the northeastern edge of the Tauern Window. In the western Tauern Window it no longer exists as a discrete fault but is most likely splayed into and 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 and Schneider, 2008]. Among these dominantly sinistral, ENEstriking 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 shear in a transpressive regime at the present-day exposure level. Sinistral transpression here brought the southern block (including the Tauern Window) up, relative to the northern block [Fügenschuh et al., 1997; Ortner et al., 2006]. Seismic data were recently interpreted to indicate that the Inntal fault changes at depth into a transcrustal ramp, dipping 30 S, with the western Tauern Window forming a hangingwall anticline [Lüschen et al., 2004]. [10] The dominantly dextral fault system south of the Tauern Window strikes E-W to NW-SE and consists of the Periadriatic line and several smaller splay faults. ENEstriking conjugate faults show sinistral offset (Figure 2). The Periadriatic line forms a system of thrusts SW of the Tauern Window (Giudicarie, Jaufen, and Passeier faults, Figure 2), while south of the Tauern Window its Pustertal and Gailtal segments are steeply dipping strike-slip faults [Polinski and Eisbacher, 1992]. A precollision, pre-oligocene history can be inferred for parts of the fault system from geochronologic data [Müller et al., 2001; Mancktelow et al., 2001]. Syn- to postcollisional reactivation is characterized by syntectonic magmatic activity and an abrupt change from sinistral to dextral motion at 30 Ma in the area south of the Tauern Window [Müller et al., 2001; Mancktelow et al., 2001]. Among the dextral, NW-striking splay faults of the Periadriatic line, the Mölltal fault (Figure 2) is the only major one, which continues into the Penninic/Helvetic units of the Tauern Window [cf. Linzer et al., 2002]. The Mölltal fault is a subvertical lineament with a dextral offset of up to 30 km [Kurz and Neubauer, 1996]. In its northwestern part, it splays into several steeply dipping 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, Ahrntal fault; 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-Mürztal fault; MöF: Mölltal fault; OF, Olperer fault; PL, Periadriatic line; RE, Rensen pluton; RF, Rieserferner pluton; SD, Sonnblick Dome; SEMP, Salzach-Ennstal-Mariazell-Puchberg fault system; TF, Telfs fault. 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) Müller et al. [2001], (9) Cliff et 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) Läufer et al. [1997], (24) Müller et al. [2000], (25) Mancktelow et al. [2001], and (26) Thöni [1980]. 4of27

5 Figure 3 5of27

6 southeastern Tauern Window [Kurz and Neubauer, 1996], pretty much in a similar way as the SEMP fault in the the western Tauern Window. [11] The syn- to postcollisional tectonic evolution of the Eastern Alps involved a component of Oligocene to Miocene E-W extension, which aided the final exhumation of the Tauern Window [Behrmann, 1988; Selverstone, 1988; Genser and Neubauer, 1989; Ratschbacher et al., 1991; Neubauer et al., 1999]. E-W extension resulted in the formation of two conjugate normal fault systems at the western and eastern edges of the Tauern Window, respectively (Figures 1 and 2). The western margin is formed by the W-dipping Brenner shear zone and the eastern edge of the Tauern Window is defined by the E-dipping Katschberg zone [Genser and Neubauer, 1989; Becker, 1993]. [12] Assuming plane-strain constant-volume deformation and an initial crustal thickness of km, the barometric estimates of 25 kbar in the Eclogite Zone suggest that Oligocene NNE-directed shortening in the Tauern Window region exceeded 60%. The amount of Oligocene/Miocene E-W extension in the Eastern Alps is a controversial issue. 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 highamplitude antiforms can cause significant exhumation [cf. Schmid et al., 2004], which makes especially the extension calculation from the palinspastic restoration of Frisch et al. [1998] questionable. In contrast, results of numerical modeling [Robl and Stüwe, 2005] point to only a minor role of extensional faulting in the post-oligocene deformation history of the Eastern Alps. Huismans et al. [2001] estimated 0 to 16% of extension by subtracting the inferred amount of Miocene extension in the Pannonian Basin from the restored amount of Miocene convergence in the Carpathians. Rosenberg et al. [2007] recently reviewed the amount of E-W extension in the Eastern Alps and convincingly showed that it did not exceed 20%. If so, N-S shortening 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, in particular for the Tauern Window, based on the sedimentary record from circum-alpine sediment traps. A pattern emerges in which a first significant increase in sediment discharge from the Eastern Alps orogen is seen at Ma. Subsequently, sediment discharge slightly increased until 21 Ma. At that time, a significant drop in sedimentation rates occurred [Kuhlemann et al., 2001], followed by an 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 of a huge elongated antiform that is bounded on its western and eastern side by oppositely dipping normal shear zones (Figure 2). The antiform is made up by a northward imbricated stack of Penninic/Helvetic nappes that is folded into a number of upright to NNE-vergent, but also SSWvergent large-scale folds with E-trending axes. In the limbs of those folds a number of strike-slip shear zones formed. In the western part of the Tauern Window these shear zones have dominantly sinistral kinematics. In the eastern part of the window the dextral Mölltal and Mur-Mürz faults occur (Figures 2 and 3). [15] Our structural data from the Eclogite Zone show that the contact of the latter with the underlying Venediger nappe 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 stretching lineations is only a few meters away from mylonite with gently E-plunging stretching lineations that are associated with sinistral S-side-down kinematic indicators. Occasionally, gently W-plunging lineations associated with dextral S-side-down kinematics have been reported by Hawkins et al. [2007]. No consistent overprinting relationships between downdip and the lateral kinematic indicators occur and our dating (see below) indeed shows that both sets of kinematic indicators formed essentially at the same time. In the upper Venediger nappe the same alternating pattern of S-plunging downdip and gently plunging stretching lineations was mapped (Figure 3). [16] The upper contact of the Eclogite Zone with the Rote Wand Modereck and Glockner nappes is marked by a steeply S-dipping mylonite belt. The mylonite foliation contains a gently (20 ) E-plunging stretching lineation associated 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 the kinematics of each individual fault segment. The stereographic projections show great circles of the mylonitic foliation, the stretching 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 Katschberg extensional fault system. The western tip of the Olperer shear zone shows sinistral strike-slip faulting and top-wsw extension, whereas the Ahrntal fault shows sinistral strike-slip faulting and NNE-SSW shortening. The Greiner shear zone shows sinistral faulting, a minor set of dextral faults and top-nne reverse faulting. The top of the Eclogite Zone shows sinistral strike-slip faulting associated with a component of S-side-down motion, whereas the bottom of the Eclogite Zone shows top-nne thrusting combined with localized sinistral shearing. The Mölltal fault shows dextral strike-slip faulting associated with a component of top-nne reverse faulting. The Katschberg extensional fault system shows top-ese extension. The Eclogite Zone and the basal thrust of the Glockner nappe (dashed line with barbs on Glockner nappe) are shown for reference. 6of27

7 (Figure 3b). This sinistral S-side-down shear zone can be followed further to the SW where it eventually forms the boundary of the Tauern Window (Figure 3). [17] Along the NE edge of the Tauern Window, the SEMP forms a system of dominantly sinistral, ductile to brittle deformation structures [Ratschbacher et al., 1991; Neubauer et al., 1999]. Rosenberg and Schneider [2008] showed that the sinistral, brittle SEMP grades westward into the ductile Ahorn shear zone. The mylonitic foliation in the Ahorn shear zone contains a gently W-plunging stretching lineation associated with sinistral, S-side-up shear sense indicators. The Ahorn shear zone separates an area with Alpine amphibolite-facies metamorphism in the south from an area in the lowest greenschist facies (just above the brittle-ductile transition in quartz) in the north due to a vertical offset of 7 km[rosenberg and Schneider, 2008]. This offset could result from sinistral shearing parallel to the west-dipping stretching lineations; however, given the transpressive character of the shear zone, the transport direction may have been steeper than the stretching lineations [cf. Robin and Cruden, 1994]. As a consequence, the lateral offset of 60 km, which affected the SEMP east of the Tauern Window [Linzer et al., 2002], is partly transferred into a vertical one in the Ahorn shear zone. Further west, about 15 km east of the Brenner fault, lateral shearing is taken up by upright folding due to N-S shortening [Rosenberg and Schneider, 2008]. [18] The Olperer shear zone formed in the northern limb of the Tux orthogneiss anticline. We found that mylonitic deformation in the subvertical shear zone was pervasive in the metasediments and much more localized in the orthogneiss. The shear-zone-related foliation in the metasediments is associated with a strong, gently WSW-plunging stretching lineation (Figure 3c). Kinematic indicators indicate sinistral shear. The foliation and stretching lineation are folded about lineation-parallel tight to isoclinal folds. These folds are locally associated with a strong crenulation cleavage in hinge zones of the folds. Lineation-parallel folding caused strongly prolate strain geometries, especially in the metasediments. The lineation-parallel folding did not invert the sinistral kinematic indicators in overturned limbs suggesting that folding and stretching were coeval. The folding also caused the foliation to attain a local WSW-dipping attitude. In these segments of the shear zone the kinematic indicators yielded a top-wsw shear sense (Figure 3c). If the sinistral shear sense was folded after shearing then the shear sense in the hinge zones of those folds should be top-ene. The fact that the shear sense is top-wsw strongly suggests to us that sinistral and top-wsw shearing, and also the lineation-parallel folding, occurred simultaneously. All mentioned structures are then folded about large-scale open to tight folds with WSW-plunging axes. The axial planes of these late folds are moderately to steeply dipping to the S. [19] The Greiner shear zone is a steeply S-dipping, ENEstriking structure. A mylonitic, S-dipping foliation is associated with a pervasive gently WSW-plunging stretching lineation (Figure 3d). The Greiner shear zone caused pervasive deformation in the sedimentary cover rocks, whereas a series of up to 30 m wide anastomosing splays occur in the Zentralgneis, indicating lithology-controlled heterogeneous deformation [cf. Steffen and Selverstone, 2006]. Kinematic indicators supply a dominantly sinistral, S-side-up shear sense [De Vecchi and Baggio, 1982; Behrmann and Frisch, 1990]. Nonetheless, the Greiner shear zone is very heterogeneous and in the Zentralgneiss there are two sets of ductile shear zones with ENE-striking zones showing sinistral displacement and SE-striking shear zones recording dextral movement (Figure 3d). Yet other sections of the shear zone record flattening strains and volume loss [Selverstone et al., 1991; Selverstone and Hyatt, 2003], and even dextral overprinting of originally sinistral fabrics has been described [Barnes et al., 2004]. Ductilely deformed feldspar and PT estimates show that shearing commenced under amphibolite facies conditions [Selverstone et al., 1984, 1991]; however, there are localized shear zones characterized by strong greenschist facies retrogression. [20] The Ahrntal shear zone is a NE-striking, steeply dipping shear zone [Reicherter et al., 1993]. The mylonitic foliation contains a penetrative ENE-plunging stretching lineation (Figure 3e). Ductilely deformed asymmetric tails around feldspar indicate that sinistral shear commenced at least during uppermost greenschist facies metamorphism. The ductile feldspar fabrics are locally overprinted by strong retrogressive greenschist to subgreenschist facies sinistral shearing. In its southwestern part, the Ahrntal shear zone separates an area with Oligocene/Miocene uppermost greenschist to amphibolite facies metamorphism in the north from an area in the lowest greenschist facies in the south. This pattern is a mirror image to that at the Ahorn shear zone, creating a pop-up structure bounded by the Ahorn shear zone in the north and the Ahrntal shear zone in the south. [21] The Brenner shear zone below the brittle Brenner fault is marked by an at least 400 m thick mylonite zone in its central part [Behrmann, 1988]. The Brenner fault system has a two-stage deformation history. Selverstone [1988] showed that normal shearing was syn-peak-metamorphic at 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 brittle fault is superimposed on the normal shear zone and accounts for a vertical movement of <5 km since 13 Ma, as deduced from fission track data [Fügenschuh et al., 1997]. Seismological data imply that E-W normal faulting even continues today, associated with NNE shortening [Reiter et al., 2005]. [22] Dextral shear at the Mölltal fault commenced during amphibolite facies conditions. The subvertical shear zone contains a steeply dipping mylonitic foliation which yields dextral shear sense indicators (Figures 3h and 3i). The kinematic indicators in orthogneiss include strain shadows around potassium feldspar with recrystallised feldspar in the asymmetric wings. Minor subvertical dextral shear zones formed in the steep limbs of open to tight, upright to slightly N-vergent large-scale folds with ESE-trending axes. The folds are associated with zones of top-nne shear (Figures 3h and 3i). The kinematic indicators have not been 7of27

8 inverted by folding indicating that folding (Figure 3h) (dated at 28 Ma [Inger and Cliff, 1994]) occurred either prior 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-ese normal shear zone (Figure 3j) [Genser and Neubauer, 1989; Becker, 1993]. The shear zone is up to 200 m thick and grades upwards into a zone of intense brittle normal faulting. Normal shearing in the lower parts of the shear zone progressed during greenschist-facies metamorphism as indicated by strongly recrystallised quartz ribbons in top-ese shear bands and asymmetric quartz-c-axis fabrics indicating top-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-slip shearing (Figure 3). The strike-slip shear zones are transpressive, except the segment above the Eclogite Zone, which is transtensional. The strike-slip shear zones preferentially formed in the limbs of large antiforms and basically accommodated excess NNE shortening and transferred it sideway where the two normal shear zones formed. The footwalls of the latter expose amphibolite-facies rocks in the west and greenschist-facies rocks in the east, consistent with a generally asymmetric exhumation pattern in the Tauern Window. 4. Previous Dating of Alpine Events in the Tauern Window Area [25] Collision of the Adriatic and European plates caused deep underthrusting of the leading edge of the European continent 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 geochronologic data for the syn-to postcollisional history, as this history involves formation of the Tauern Window and its structural framework. From a large body of literature (summarized in Table 1) it is evident that most existing age data for Alpine metamorphic, magmatic and ductile deformation processes fall in the age range between 32 and 13 Ma (Table 1). Apparent ages older than 32 Ma mostly derive from K-Ar-based isotopic data for high-pressure metamorphic rocks. These old ages are likely to be biased by the 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 microtextural information, and also an outcome of variable assumptions on closure temperatures. Given the recent findings that pure diffusional isotope redistribution particularly between white mica and its surroundings is extremely sluggish even at amphibolite facies temperatures (see below), many whitemica-based dates previously interpreted as cooling ages may in fact represent fabric formation, dynamic recrystallization and, consequently, deformation ages. In addition, the focus in many previous geochronologic studies has been on the thermal instead of the tectonochronologic record. This presumably is the reason that strongly deformed rocks from discrete shear zones and faults have rarely been investigated. We evaluate the existing geochronologic database as to its relevance for the structural development of the Tauern Window in the context of our new deformation age data below. 5. Methodology: Rb-Sr Dating of Deformation [27] For isotopic dating of deformation, we employed the Rb-Sr internal mineral isochron approach, with bulk mineral separation from small samples [Freeman et al., 1997; Hetzel and Glodny, 2002; Reddy et al., 2003; Glodny et al., 2002, 2005]. Samples (<100 g) were chosen for which the assemblages and textures can be tied to certain tectonic or metamorphic events. Use of small samples for mineral separation minimizes the risk of bias by possible intra-sample isotopic gradients. A main advantage of the Rb-Sr internal mineral isochron approach is that isotopic equilibrium-disequilibrium relationships between the different assemblage-forming minerals are revealed. These relationships potentially allow one to constrain the reaction history of a rock, to identify isotopic relics, and to distinguish between diffusion- and recrystallization-induced isotopic resetting [Glodny et al., 2008]. [28] Some key requirements have to be met for isotopic dating of deformation. First, intermineral isotopic equilibration during the deformation event is mandatory. Careful study of the correlation between microtextures and isotopic signatures, both by conventional mineral separation techniques [Müller et al., 1999] and by Rb-Sr microsampling [e.g., Müller et al., 2000; Cliff and Meffan-Main, 2003] has shown that full synkinematic recrystallization in mylonites is usually accompanied by Sr-isotopic reequilibration. Isotopic reequilibration between mica and coexisting phases during mylonitization is viable at temperatures as low as 300 C [Müller et al., 1999; Reddy et al., 2003 and references therein]. The close link between textural and isotopic equilibration implies that in samples free of obvious predeformative textural relics (like feldspar augen or mica fish), most probably an assemblage in isotopic equilibrium has been frozen in at the end of deformation. Rb-Sr isotopic mineral data from such rocks should thus record the waning stages of deformation-induced recrystallization [Freeman et al., 1998]. We therefore sampled intensely and pervasively deformed mylonitic rocks and schists. [29] Second, thermally driven diffusion should not alter the Sr-isotopic mineral signatures once adjusted during deformation. We preferred white-mica-bearing assemblages for geochronology, because the Rb-Sr system of white mica is thermally stable to amphibolite facies temperatures (> C [Inger and Cliff, 1994; Freeman et al., 1997; Villa, 1998; Glodny et al., 2005]). Provided that modally controlled closed system behavior applies [cf. Glodny et al., 2003], the Rb-Sr system of white mica may persist through even higher temperatures. The high temper- 8of27

9 Table 1. Published Isotopic Age Data on Alpine Processes, Tauern Window Authors Sampling Area Method Age, Age Range, Ma Original Interpretation Western Tauern Window Satir [1975] Western TW Rb-Sr, phengite Rb-Sr, K-Ar, biotite crystallization age cooling Satir [1975] SW margin of TW K-Ar, phengite 31 ± 2 peak metamorphism in calcschist Satir and Morteani [1982] Greiner SZ and migmatites, W TW Rb-Sr phengite Rb-Sr biotite 21 ± 2 14 ± 1 cooling cooling Blanckenburg et al. [1989] Pfitscher Joch area, western Tauern window K-Ar, Ar-Ar, hbl Rb-Sr, white mica Rb-Sr, K-Ar, biotite cooling deformation cooling Zimmermann et al. [1994] south-central Tauern window (Eclogite Zone) Ar-Ar phengite Ar-Ar phengite cooling of EZ metam. crystallization in Venediger & Glockner nappes Christensen et al. [1994] Pfitscher Joch area, western TW Rb-Sr, garnet zonation 30 ± 1 Alpine thermal maximum in schists Barnes et al. [2004] Greiner SZ, Stillup U-Th-Pb monazite 26 switch from sinistral to dextral shear, Schneider et al. [2007] valley, western TW Stillup valley, W Tauern window Rb-Sr microsampling, mylonite at amphibolite facies Ma sinistral shear in W continuation of SEMP; form. of upright antiforms of W TW Central Tauern Window Lambert [1970] southern margin of TW, near Döllach K-Ar, white mica a greenschist facies mica growth cf. Cliff et al. [1985] Raith et al. [1978] west-central Tauern window K-Ar, mica cooling; older ages toward southern window margin Inger and Cliff [1994] EZ + Glockner nappe, Rb-Sr, (26) end of greenschist facies deformation south-central TW white mica-based Eichhorn et al. [1995] Felbertal, central Tauern window U-Pb zircon 31 ± 4 Alpine metamorphism Glodny et al. [2005] Eclogite zone, central Tauern W. Rb-Sr multimineral 31.5 ± ± 0.5 eclogite facies metamorphism gs facies retrogression Ratschbacher et al. [2004] Eclogite zone, central Tauern W. Ar-Ar, white mica cooling Gleissner et al. [2007] Glockner nappe S of Eclogite Zone Rb-Sr multimineral 29.8 ± 0.5 deformation and near-peak metamorphism Eastern Tauern Window Cliff et al. [1985] Ankogel-Hochalm area SE TW K-Ar white mica Rb-Sr biotite cooling cooling Reddy et al. [1993] Sonnblick dome, SE Tauern window Rb-Sr, mica- fsp Rb-Sr, bt-based synkinematic crystallization cooling Inger and Cliff [1994] SE Tauern window Mallnitzer Mulde/ Mölltal line U-Pb, aln, titanite Rb-Sr mineral data Rb-Sr minerals peak metamorphism formation of Sonnblick dome continuous deformation Cliff et al. [1998] Mallnitzer Mulde/Mölltal Th-Pb allanite 27.7 ± 0.3 fabric formation, peak metamorphism line, SE TW Liu et al. [2001] uppermost Penninic, NE TW Ar-Ar white mica Ar-Ar white mica cooling activity of Katschberg fault Deckert [1999] footwall of Katschberg K-Ar, white mica 25 cooling after extensional deformation FZ, NW TW Cliff and Meffan-Main [2003] Sonnblick gneiss, SE Tauern window Rb-Sr microsampling 27.3 ± ± 0.3 late gneissic fabric formation Sonnblick dome formation Magmatism Deutsch [1984] Alkalibasaltic dykes S of TW K-Ar, Rb-Sr 30 intrusion of alkalibasaltic dykes Barth et al. [1989] Rensen pluton U-Pb, zircon, allanite magmatic crystallization Müller et al. [2001] Periadriatic Fault system SW of TW U-Pb zircon magmatism Romer and Siegesmund [2003] Rieserferner pluton U-Pb allanite 32.4 ± ± 0.4 magmatic crystallization Fault Systems Related to Tauern Window Formation Läufer et al. [1997] Periadriatic Fault, SE of TW (Eder unit, Carnic Alps) K-Ar, Ar-Ar dextral transpression along PL exhumation, shear along PL Müller et al. [2000] DAV fault zone, S of SW Tauern W. Rb-Sr microsampling <30 sinistral strike slip deform. dextral transpressive deform. Müller et al. [2001] Periadriatic Fault system, SW of TW Rb-Sr, Ar-Ar Ar-Ar ductile deformation brittle movements, pseudotachylites Mancktelow et al. [2001] Periadriatic Fault system Rb-Sr microsampling + 30 Ma change from sinistral-transtensive to S of Tauern W. microtectonics dextral-transpressive deformation Urbanek et al. [2002] SEMP, NE Tauern window Ar-Ar, white mica sinistral transpression, associated with TW exhum. a Recalculated using Steiger and Jäger [1977] decay constants. TW, Tauern Window; DAV, Defereggen-Antholz-Vals; EZ, Eclogite Zone; PL, Periadriatic line; SZ, shear zone. 9of27

10 Table 2. Characterization of Dated Samples a Sample Coordinates Rock; Assemblage Metam. Grade Rb-Sr age, Ma Tectonic Position Tectonochronologic Significance BRE N, E 400 m NW of Brennerpass TF04-30a N, E 1 km SW Terme di Brennero PON N, E 2 km SW Terme di Brennero TF N, E 1 km WNW Terme di Brennero carbonate mylonite cal, wm, qtz, py, hematite carbonate schist cal, wm, qtz, ab, chl, graphite Brenner Line gs 21 ± 2 Brenner fault zone top-w extensional shear gs 18.3 ± 2.6 Brenner fault zone top-w extension graphite schist ab, qtz, wm, graphite gs 17.8 ± 1.8 Brenner fault zone top-w extensional shear quartzitic mylonite qtz, wm, ap, fsp, tur (?) gs (?) 39.3 ± 3.6 immediate hangingwall of Brenner line ductile deformation (?) TF N, E 1500 m NNW of Geraer Hütte TF N, E 1 km NE Geraer Hütte SEG N, E 3.5 km SW of Ginzling PFI1 1.3 km NNW of Pfitscher Joch, Zamser Bach at 2500 m altitude HAU3 700 m ENE of Rotbachlspitze, 2.5 km E of Pfitscher Joch mylonitized augengneiss wm, qtz, fsp, pg, ep, zrn mylonitic paragneiss qtz, bt, wm, ep, zrn granite, almost undeformed pl (clear), qtz, bt, ap, zrn, grt mylonitic gneiss fsp, qtz, wm, ap, zrn, bt/chl quartz-ky-wm-mobilisate qtz, ky, wm, staurolite, garnet PFI2 300 m W of Rifugio Passo di Vizze phosphate metaquartzite qtz, lazulite, ky, wm PFI3 400 m WSW of Rifugio Passo di Vizze fine-grained schist wm, qtz, fsp, bt/chl, ap, zrn TF N, E 3 km SSE of Lake Zillergründl TF N, E E 0 shore of Lake Zillergründl HWZ N, E 4 km N of Pfunders mylonitic gneiss fsp, qtz, grt, bt, ep, wm, ap, cal muscovite schist qtz, wm, bt/chl, fsp, ap, zrn Western Tauern Window - SEMP Splays af-gs (?) 15.7 ± 1.3 SZ zone along NW margin of Tux gneiss (Ahorn SZ) af 31.2 ± 0.4 Olperer shear zone [cf. Lammerer and Weger [1998]] af gs (?) 17.0 ± 5.3 almost undeformed Variscan granitoid (Tuxer Kern) upper gs (?) 17.6 ± 1.7 small fault zone within granite, parallel to Greiner shear zone sinistral shearing prolate strain, sinistral shearing decompression related mineral reactions ductile shear - main foliation of granitoid af 15.0 ± 0.4 within Greiner shear zone metamorphic mobilisate crystallization, decompression af 19 ± 4 Greiner shear zone ductile shear gs (?) 16.9 ± 0.6 Greiner shear zone ductile shear af 26.7 ± (bt) Eastern continuation of Greiner SZ upper gs (?) 21.5 ± 0.8 Eastern continuation of Greiner SZ/Habachtal Mulde carbonate mylonite cal, qtz, wm, bt gs 19.8 ± 0.4 Ahrntal fault zone - immediate footwall of Glockner nappe sinistral shearing decompression (biotite age ) sinistral shearing sinistral (?) ductile deformation EIS14a N, E 700 m S of Johannishütte EIS14b N, E 700 m S of Johannishütte EIS N, E 500 m NNE of Eisseehütte EIS N, E Seekopfscharte (3059 m) EIS N, E Mittlerer Seekopf (3221 m) calcschist cal, wm, pg, grt, tur, graphite calcschist cal, wm, pg, grt, ap, tur, graphite carbonate mylonite cal, amp, ep, wm, qtz mafic schist amp, ep, cal, ap, ttn, bt/chl mafic schist ep, amp, grt, qtz, wm, ap, bt Central Tauern Window-Eclogite Zone and Environs af-gs 30.4 ± 0.4 contact EZ-Venediger nappe 30 m N of sample EIS14b gs (?) 30.6 ± 0.9 contact EZ-Venediger nappe 30 m S of sample EIS14a gs (?) 31.2 ± 0.6 contact Eclogite Zone - Rote Wand-Modereck nappe bs-gs 31.4 ± 0.4 contact Glockner nappe - Rote Wand-Modereck nappe top-n shear sinistral top-w shearing sinistral shearing (top-w?) sinistral top-w shearing bs-gs 31.1 ± 0.4 within Eclogite Zone retrograde overprint within EZ, associated with ductile shear TF04-1b N, E 2 km NNW of Obervellach TF N, E Oberkolbnitz, Mölltal quartz mylonite fsp, qtz, wm, zrn, tur, amp(?) carbonate mylonite cal, ankerite, qtz, wm, py Eastern Tauern Window (?) 25.3 ± 2.9 Mölltal line ductile shear gs (?) 20.7 ± 2.3 Mölltal line dextral shearing a Wm, muscovitic to phengitic white mica; gs, greenschist; af, amphibolite facies; bs, blueschist facies; EZ, Eclogite Zone. 10 of 27

11 Table 3. Rb/Sr Analytical Data a Sample No. Analysis No. Material Rb, ppm Sr, ppm 87 Rb/ 86 Sr 87 Sr/ 86 Sr 87 Sr/ 86 Sr 2s m,% Brenner Line BRE2 (21 ± 2 Ma, MSWD = 1.1, Sr i = ± ) PS1566 calcite PS1565 wm mm PS1560 wm mm PS1561 wm mm TF04-30A (18.3 ± 2.6 Ma, MSWD < 1, Sri = ± ) PS1299 calcite PS1322 wm nm 1.3A PS1324 wm mm PS1329 wm mm PON1 (17.8 ± 1.8 Ma, MSWD = 1.6, Sr i = ± ) PS1558 quartz-feldspar PS1562 wm mm PS1571 wm <125 mm PS1572 wm mm TF04-32 (39.3 ± 3.6 Ma, MSWD = 4.9, Sr i = ± ) PS1262 wm mm PS1263 wm mm PS1264 wm mm PS1265 apatite PS1441 feldspar Western Tauern Window SEMP Splays TF04-26 (15.7 ± 1.3 Ma, MSWD = 24, Sr i = ± ) PS1289 paragonite PS1292 wm mm PS1296 wm mm PS1297 epidote-conc TF04-25 (31.2 ± 0.4 Ma, MSWD <1, Sr i = ± ) PS1267 wm mm PS1268 wm mm PS1270 epidote SEG2 (17.0 ± 5.3 Ma, MSWD = 447, Sr i = ± 0.016) PS1559 apatite PS1564 biotite mm PS1573 biotite >3 mm PS1567 feldspar PS1563 biotite mm PFI1 (17.6 ± 1.7 Ma, MSWD = 37, Sr i = ± ) PS1431 feldspar PS1433 wm mm PS1434 wm mm m = 0.75A PS1435 apatite PS1436 wm mm m = 0.5A PS1440 wm mm HAU3 (15.0 ± 0.4 Ma, Sr i = ± ) PS1576 wm > 3 mm PS1575 garnet PFI2 (19 ± 4 Ma, MSWD = 1.6, Sr i = ± ) PS1438 lazulite PS1439 wm >250 mm m = 0.5A PS1447 wm nm 1.2A PFI3 (16.9 ± 0.6 Ma, MSWD = 122, Sr i = ± ) PS1402 wm mm m = 0.8A PS1400 apatite PS1399 wm mm m = 0.8A PS1393 K-feldspar PS1392 wm mm nm = 0.8A TF04-15 (excl. bi: 26.7 ± 1.2 Ma, MSWD = 2.9, Sr i = ± ) (bt + ep: 16.4 ± 0.2 Ma) PS1285 apatite PS1287 epidote PS1294 wm mm PS1395 wm mm PS1291 wm mm PS1298 biotite of 27

12 Table 3. (continued) Sample No. Analysis No. Material Rb, ppm Sr, ppm 87 Rb/ 86 Sr 87 Sr/ 86 Sr 87 Sr/ 86 Sr 2s m,% TF04-18 (21.5 ± 0.8 Ma, MSWD = 10, Sr i = ± ) PS1281 apatite PS1282 wm mm PS1290 wm mm PS1327 feldspar + qtz HWZ3 (19.8 ± 0.4 Ma, Sr i = ± ) PS1568 white mica PS1569 calcite Central Tauern Window Eclogite Zone and Environs EIS14a (30.7 ± 0.7 Ma, MSWD = 3.3, Sr i = ± ) PS1015 carbonate PS1016 wm > 500 mm PS1117 wm mm PS1116 wm mm PS1135 wm m = 0.9A, mm PS1137 wm m = 0.9A, mm PS1136 wm m = 0.9A, mm PS1138 paragonite mm PS1017 paragonite < 500 mm EIS14b (30.6 ± 0.9 Ma, MSWD = 7.4, Sr i = ± ) PS1131 wm > 500 mm PS1133 wm m = 1.2 A, mm PS1128 wm m = 0.9 A, mm PS1129 wm m = 0.9 A, mm PS1132 wm m = 0.9 A, mm PS1127 wm m = 0.9 A, mm PS1130 paragonite > 355 mm PS1134 apatite PS1125 carbonate EIS6 (31.2 ± 0.6 Ma, MSWD = 9.8, Sr i = ± ) PS1091 wm mm PS1092 wm mm PS1093 wm mm PS1094 wm > 500 mm PS1090 carbonate PS1088 epidote PS1089 amphibole PS1172 wm mm EIS11 (31.4 ± 0.4 Ma, MSWD = 1.6, Sr i = ± ) PS1095 carbonate PS1096 amphibole PS1097 epidote PS1099 wm <200 mm PS1113 wm bulk EIS13 (31.1 ± 0.4 Ma, MSWD = 32, Sr i = ± ) PS1076 wm mm PS1082 wm mm PS1083 wm mm PS1084 wm mm PS1087 epidote PS1085 apatite PS1068 amphibole Eastern Tauern Window TF04-1b (25.3 ± 2.9 Ma, MSWD = 25, Sr i = ± ) PS1449 wm nm = 1.8A PS1451 wm mm m = 1.15A PS1455 feldspar PS1529 whole rock TF04-5 (20.7 ± 2.3 Ma, MSWD = 9.8, Sr i = ± ) PS1283 wm mm PS1284 ankerite PS1323 wm mm PS1391 wm mm PS1286 calcite a An uncertainty of ±1.5 % is assigned to Rb/Sr ratios. Wm, white mica; bt, biotite; ep, epidote; m/nm, magnetic/nonmagnetic on Frantz magnetic separator, 13 tilt, at electric current as indicated. 12 of 27

13 atures required for potential thermally induced resetting of the Rb-Sr system in phengite ensures that phengite ages from greenschist to amphibolite facies mylonites can, as a rule, be regarded as deformation ages. A criterion for the activity of diffusive partial Sr-isotope reequilibration is a positive correlation between grain size and apparent age for micas, combined with excess 87 Sr in high-diffusivity phases like apatite or calcite [Glodny et al., 2008]. We therefore analyzed different white mica grain size fractions, and apatite or calcite whenever separable, to check for potential partial thermal resetting. Analysis of white mica in different grain size fractions also serves to detect possible mixed white mica populations or Sr-isotopic inhomogeneities within mica grains. Such inhomogeneities may originate from long-term, incomplete dynamic recrystallization, or from possible presence of unequilibrated, predeformational white mica relics, and would lead to mixed, geologically meaningless ages if overlooked [cf. Freeman et al., 1998; Müller et al., 1999]. [30] Last not least, isotopic signatures may only record deformation if minerals escaped from postkinematic recrystallization processes, and from deuteric alteration by weathering or low-temperature fluids. Feldspar and biotite are potential products of decompression-related metamorphic mineral reactions (see below; Cesare et al. [2001]). These phases are also particularly susceptible to low-temperature alteration of isotopic systems [Parsons et al., 1999; Jeong et al., 2006]. Alteration products generally start to form along fluid pathways, i.e., along grain boundaries and cracks. For isotopic analysis we therefore did not utilize altered whole rocks but only used well-defined and clean mineral separates. Whenever viable we avoided analysis of potentially altered material, like biotite-chlorite intergrowths or turbid feldspar. 6. Analytical Procedures [31] Rb/Sr analyses were performed, as a rule, on all separable and unaltered Sr-bearing phases of small, lithologically homogeneous rock samples. For white mica, we separated subpopulations with distinct magnetic properties and grain sizes, to discriminate between phengite and paragonite, and to detect possible Sr-isotopic heterogeneity. Traces of secondary (Fe, Mn) hydroxides on some amphibole, epidote and garnet separates were removed with a 5% aqueous solution of oxalic acid. White mica fractions were ground in ethanol in an agate mortar and then sieved in ethanol to obtain pure, inclusion-free separates. All mineral concentrates were checked and finally purified by handpicking under a binocular microscope. [32] Rb and Sr concentrations were determined by isotope dilution using mixed 87 Rb- 84 Sr spikes. Samples were weighed into Savillex screw-top containers. After addition of a suitable spike, they were dissolved in a mixture of HF + HNO 3, and then converted into chlorides. Solutions were processed by standard, HCl-based cationexchange techniques. Determinations of Rb and Sr isotope ratios were carried out on a VG Sector 54 multicollector TIMS instrument (GeoForschungsZentrum Potsdam). Sr was analyzed in dynamic mode. The value obtained for 87 Sr/ 86 Sr of NBS standard SRM 987 was ± (n = 19). The observed ratios of Rb analyses were corrected for 0.25% per a.m.u. mass fractionation. Total procedural blanks were consistently below 0.15 ng for both Rb and Sr. Because of generally low and highly variable blank values, no blank correction was applied. Isochron parameters were calculated using the Isoplot/Ex program of Ludwig [1999]. Standard errors, as derived from replicate analyses of spiked white mica samples, of ± 0.005% for 87 Sr- 86 Sr ratios and of ±1.5% for Rb-Sr ratios were applied in isochron age calculations. Individual analytical uncertainties were in most cases smaller than these values. If otherwise, individual errors have been used for age calculation. 7. Characterization of Dated Samples [33] For isotopic dating of deformation events we selected samples that are penetratively deformed, and that show the general microstructural and petrological characteristics of the respective shear zones. In this context, we occasionally had to compromise between suitability for isotopic dating at one side, and clearness of microstructural and metamorphic signatures at the other side. In absence of clear indicators of metamorphic grade and shear sense of deformation we inferred these parameters from rocks in the immediate outcrop context. Characteristics of the investigated samples are presented in Table Results and Significance of Isotopic Data [34] Rb-Sr data for minerals from 21 samples of mostly highly strained/mylonitic rocks from the Tauern Window area are presented in Table 3. By abundance, the age data broadly define two groups, the first at 32 to 30 Ma, and a second one at Ma, with a few results in between the two clusters. The significance of individual data sets is discussed below Brenner Fault Mylonites [35] We analyzed three mylonite samples from the Brenner shear zone. For the two carbonate mylonites (samples BRE2, TF04-30a) and one quartz-albite dominated graphite schist (PON1) we obtained valid isochron ages (MSWD < 2.5 [cf. Wendt and Carl, 1991]). Ages are between 21 ± 2 Ma and 17.8 ± 1.8 Ma, all of which are identical within limits of error (Table 3 and Figure 4). The greenschist facies deformation fabrics consistently indicate top- WSW shear. We therefore interpret the white-mica-based ages as dating the end of top-wsw ductile deformation in the investigated samples. [36] A quartzitic mylonite from the Austroalpine unit of the direct hangingwall of the Brenner line yielded an age value of 39.3 ± 3.6 Ma (sample TF04-32, Table 3). While the mylonitic texture suggests that in general deformation has set this age value, the MSWD of 4.9 may indicate some disturbance or incomplete resetting. The latter appears more likely given the Cretaceous metamorphism and the dominantly Cretaceous to Paleocene zircon fission 13 of 27

14 Figure 4. Rb-Sr isochron plots for selected samples from ductile parts of the Brenner line fault system, Western Tauern Window. track ages in the Austroalpine nappes west of the Brenner fault [Fügenschuh et al., 1997]. In any case, this sample does not contribute to evaluation of the postcollisional deformation history and its correct interpretation requires further research Western Tauern Window [37] From the western Tauern Window, we selected 2 samples of undeformed rocks and 8 samples of mylonites and schists for isotopic analysis. Mylonites and schists come from different, WSW-ENE striking, mostly sinistral shear zones in the area, interpreted to represent splays of the sinistral SEMP. We therefore discuss the results together here, starting in the NW 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 and Schneider, 2008] along the northwestern margin of the Tux gneiss (see Figure 8 for location of Tux gneiss). Two phengite-muscovite sieve fractions together with paragonite and epidote data yield an age of 15.7 ± 1.3 Ma (Table 3), which we interpret as dating deformation at amphibolite- to greenschist facies conditions. [39] A mylonitic paragneiss (TF04-25) from the amphibolite facies Olperer shear zone yields a well-defined threepoint mineral isochron age of 31.2 ± 0.4 Ma (Table 3 and Figure 5). Consistency between the isotopic data for the two different white mica sieve fractions, together with the rocks texture, indicates absence of predeformational relics, and absence of postrecrystallizational isotopic resetting. Amphibolite facies deformation here, in the investigated domain of a paragneiss lamella inside the Tux orthogneiss, was thus terminated already in Oligocene times, significantly earlier than in the adjacent Ahorn shear zone which is only 1 km away. It needs to be stressed that coexistence of this Oligocene white-mica-based age in close proximity with Miocene ages (also found in the adjacent Greiner shear zone, see below) precludes interpretation of the data as regional cooling ages. [40] In the Pfitscher Joch segment of the Greiner shear zone, deformation largely occurred under amphibolite facies conditions. For the here studied phosphate metaquartzite (sample PFI2), deformation conditions of 550 C, 7 kbar were inferred, in line with a number of other P,T estimates in the area [Morteani and Ackermand, 1996; Selverstone and Hyatt, 2003]. Unfortunately, the extremely Sr-rich composition of sample PFI2 has led to low Rb/Sr ratios in all minerals, which inhibits high-precision Rb-Sr dating. We interpret the Rb-Sr mineral isochron age of 19 ± 4 Ma from this sample (Figure 5) as dating deformation at amphibolite facies conditions. A few hundred meters away from sample site PFI2, sample PFI1 was collected at a location within the Tux orthogneiss. Shearing of PFI1 occurred at least at upper greenschist facies conditions, as indicated by ductile behavior of feldspar. Some feldspar augen imply that deformation was not entirely penetrative. This is reflected in the isotopic data, which show a clear correlation between white mica grain sizes and apparent ages (see Figure 5; fsp + wm mm yields 19.1 ± 0.3 Ma; fsp + wm mm: 17.0 ± 0.3 Ma). We interpret the isotopic data from this sample as reflecting amphibolite to upper greenschist facies deformation at 17.6 ± 1.7 Ma ± 0.3 Ma is a maximum age for the end of deformation in this sample. [41] Detailed thermobarometric work on deformation fabrics has shown that the rocks from the western Greiner shear zone document a P-T path roughly from 10 kbar, C to 4 kbar, 500 C. The actual P-T conditions recorded in a specific lithology or domain largely depend upon the mineral reaction history and the shear strength of 14 of 27

15 Figure 5. Rb-Sr isochron plots for selected samples from splays of the sinistral SEMP fault system, Western Tauern Window. 15 of 27

16 the rocks [cf. Steffen and Selverstone, 2006]. The finegrained mica rich schist we analyzed (sample PFI3), unsuitable for precise thermobarometry, is by its composition inherently weaker than the adjacent more coarse-grained, porphyroblast-rich or feldspar-rich rocks. It would thus act to localize strain during decreasing PT conditions [cf. Janecke and Evans, 1988]. This schist therefore most likely records and dates the latest increments of ductile deformation in the western Greiner shear zone, probably at greenschist facies conditions. The deformation age for this sample 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 undeformed amphibolite facies mobilisate, collected 2.5 km east of Pfitscher Joch (HAU3). This quartz-kyanite-phengite mobilisate formed at amphibolite facies conditions, as indicated by the presence of kyanite+staurolite+garnet in its wallrock and the absence of any alteration reactions between mobilisate and wallrock. We estimate the age of the mobilisate by combining Rb-Sr data for garnet of the sidewall of the mobilisate, with phengite data from the interiors of the mobilisate, which results in an apparent age of 15.0 ± 0.4 Ma (Table 3). As we have no control on initial isotopic equilibrium between the garnet and phengite, this age value only provides a vague hint on the age of mobilisate formation. We speculate that the mobilisate was formed by fluids released by decompression-related dehydration reactions in the rocks nearby [cf. Cesare et al., 2001], at about 15 Ma. [43] The eastern segment of the Greiner shear zone, 30 km ENE of the Pfitscher Joch area, is represented in our sample set by a mylonitic gneiss from the northern limbs of the Zillertal gneiss (sample TF04-15) and by a muscovite schist from a schist belt on the eastern side of the Zillergründl reservoir (sample TF04-18). Sinistral ductile shear in the muscovite schist came to an end at 21.5 ± 0.8 Ma (Table 3). The high MSWD of 10.1 for this sample is related to slight disequilibrium between feldspar and apatite, probably caused by weathering. It does not affect the 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 size fraction data (Table 3 and Figure 5). Given the dominantly amphibolite facies assemblage of this sample, we interpret the age as dating a waning stage of sinistral shear at amphibolite facies conditions. The Rb-Sr signature of biotite plots off the above isochron line. Its significance is discussed below. [44] In the area north of Pfunders (Figure 2), a branch of the broad, complex Ahrntal fault zone separates the Glockner nappe from Rote Wand-Modereck and Venediger nappe lithologies. The Ahrntal fault zone, so far undated, is the southernmost SEMP-correlated fault zone of the western Tauern Window (Figure 2). In a reconnaisance analysis we studied a strongly foliated carbonate mylonite (HWZ3) in the immediate footwall of the Glockner nappe. The twopoint calcite-white mica age of 19.8 ± 0.4 Ma (Table 3) is, on textural grounds, interpreted as approximating the age of late increments of sinistral shear Biotite Data [45] Rb-Sr isotopic data for biotite in the Western Tauern Window have been obtained for samples SEG2 (Tux orthogneiss, Pfitscher Joch area) and TF04-15 (from the eastern part of the Greiner shear zone). Biotite grain size fractions from granitoid sample SEG2 show a systematic decrease of Rb-Sr ratios and apparent feldspar-biotite ages with grain size (Figure 5). The apparent fsp-bi age for bi >3 mm is 16.7 ± 0.3 Ma. The corresponding apparent age for biotite mm is 12.3 ± 0.2 Ma. For the mylonitic gneiss sample TF04-15, the apparent epidote-biotite age calculates as 16.4 ± 0.2 Ma, despite the much higher whitemica-based deformation age of 26.7 ± 1.2 Ma (Table 3). The here obtained biotite age estimates are consistent with Rb-Sr and K-Ar literature data of Ma for biotite from the area [Satir, 1975; Satir and Morteani, 1982; Blanckenburg et al., 1989]. While the existing biotite data were interpreted as cooling ages, we suggest an alternative hypothesis. It has recently been shown that volume diffusion under dry, static conditions does not effectively reset Rb-Sr signatures of biotite in high-grade metamorphic rocks, even during several Myr of T >600 C [Glodny et al., 2008]. The marked contrast between white mica-based and biotite-based ages is inconsistent with an interpretation of both as deformation ages. Instead, it has been recognized that granitoids from the Tauern Window experienced posttectonic partial recrystallization, even those which preserved igneous textures (as our sample SEG2) [Morteani, 1974; Morteani and Raase, 1974]. Cesare et al. [2001] deduced that this posttectonic recrystallization was related to decompression, which triggered dehydration reactions in the metamorphosed granitoids and schists of the type Ms þ Grt þ Ep þ PlðÞþQtz I ðþcalþ¼plðiiþþbt þ H 2 OðþCO 2 Þ [46] Such decompression reactions generate biotite and feldspar as reaction products, and intergranular fluid that may enable Sr-isotopic exchange between preexisting biotite and its matrix. We hypothesize that the biotite age data, if unaffected by deuteric alteration, may in fact nearly date substantial decompression and the associated mineral reactions at Ma in the Tauern Window Eclogite Zone and Environs [47] To constrain the age of exhumation-related greenschist facies shear within the EZ (Figure 6), we analyzed a sample of greenschist facies mafic schist, which we interpret as pervasively retrogressed former eclogite (sample EIS13). The isotopic data (Table 3) show slight initial isotopic disequilibria between epidote, apatite and amphibole, the origin of which remains unclear. Nevertheless, the deformation age is well-defined at 31.1 ± 0.4 Ma. This age value is identical within limits of error to the age of greenschist facies 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 mylonite zones with different senses of shear (see above). For a 16 of 27

17 Figure 6. Map of the Eclogite Zone and adjacent units, with sample locations and age data. See Figure 2 for regional context. Map base: Geologische Karte der Republik Österreich, 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]. calcschist with top-n shear sense indicators (sample EIS14a) we obtained a well-defined age of 30.7 ± 0.7 Ma (Table 3 and Figures 6 and 7). Grain size and apparent age for 6 white mica grain size fractions are not correlated, which signifies dynamic recrystallization in a short-term event with no later resetting. A remarkable observation is the systematic variation of mica Rb-Sr ratios with grain size: Large grains show high Rb/Sr ratios, while smaller grains have lower Rb/Sr ratios due to their considerably higher Sr concentration (Table 3 and Figure 7). Since all analyzed white mica fractions were pure separates, this correlation must reflect variations in mica crystal chemistry imposed by deformation-related processes. Assuming that grain-size sensitive diffusion creep [e.g., Herwegh et al., 2005] was an important deformation mechanism, the interiors of large mica grains would preserve a record of earlier stages of deformation compared to the rims of smaller grains. We therefore postulate that the above effect relates to a marked change in Sr equilibrium partitioning between mica and calcite in these rocks during progressive deformation. Increasing availability of Sr for white mica with progressive deformation is possibly linked to passage of the calcschists through conditions of the aragonite-calcite phase transition. This would imply deformation conditions at 30.7 ± 0.7 Ma of <9 kbar at <600 C [cf. Salje and Viswanathan, 1976], consistent with the above independent constraint on exhumation to greenschist facies from the interiors of the Eclogite Zone. [49] A similar calcschist (EIS14b) at the Eclogite Zone- Venediger nappe contact, collected a few meters away from sample EIS14a, records sinistral S-side-down shear. The isotopic data similarly show a correlation between mica grain size and Rb-Sr ratios (Figure 7 and Table 3). The age value deduced from that sample is 30.6 ± 0.9 Ma, numerically indistinguishable from the EIS14a deformation age. However, isotopic disequilibria exist (MSWD of regression is 7.4). Detailed inspection reveals that in this sample there is also a covariation of mica apparent ages with grain size. Combining isotopic data for white mica >250 mm with apatite and calcite data leads to a well defined apparent age of 30.5 ± 0.8 Ma, while an equally well defined younger age of 29.1 ± 0.5 Ma is obtained for the mica fractions <250 mm (Figure 7). Absence of the same effect in the nearby sample EIS14a implies that this feature is not temperature but deformation induced. We interpret the pattern to reflect protracted deformation in this sample, lasting out to <29.1 ± 0.5 Ma. In other words, top-n and sinistral shear at the lower contact of the Eclogite Zone commenced simultaneously at about 31 Ma, but fabric development in the top-n mylonite was 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-Modereck nappe (Figure 6), we sampled a carbonate mylonite from its contact with the Eclogite Zone (sample EIS6) and a blueschist- to greenschist facies mafic schist from its contact with the Glockner nappe (EIS11). Both samples 17 of 27

18 Figure 7. Rb-Sr isochron plots for selected mylonite samples from the Eclogite Zone and environs, central Tauern Window. record sinistral S-side-down shear. The obtained multimineral 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 again reveals a clear correlation between mica grain size and Sr concentration Eastern Tauern Window [51] The age of dextral shear in mylonites from the dextral Mölltal fault in the southeastern Tauern Window is constrained by two samples. Sample TF04-1b is a quartzfeldspar mylonite, characterized by internal isotopic disequilibria. Using data for whole rock, feldspar, and two white mica fractions we obtained an age value of 25.3 ± 2.9 Ma (Table 3), which is imprecise but, due to high Rb/Sr ratios in the mica, considered reliable within limits of error. The other sample is a carbonate mylonite (TF04-5). While three different white mica grain size fractions together with ankeritic carbonate define a valid isochron with an age value of 21.5 ± 0.6 Ma, calcite does not fall on that regression line, possibly due to weathering effects. Collectively, 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 structural architecture within and around the Tauern Window was early collisional deep underthrusting of, and incipient top-n nappe stacking within the Penninic units. Its timing is best constrained by the age of 31.5 ± 0.7 Ma for eclogite facies metamorphism in the Eclogite Zone [Glodny et al., 2005]. Emplacement of the Eclogite Zone on top of the Venediger nappe and its iuxtaposition with the Rote Wand- Modereck nappe must have been associated with exhuma- 18 of 27

19 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 exhumation from 90 km depths to the middle crust is constrained by greenschist facies mobilisate formation at 31.5 ± 0.5 Ma [Glodny et al., 2005]. Our new data for exhumation-related greenschist facies deformation within the Eclogite Zone of 31.1 ± 0.4 Ma (sample EIS13) is fully consistent with the published results. This strongly corroborates the previous assertion of transiently very rapid exhumation of the Eclogite Zone at Ma at minimum rates of 36 mm/a [Glodny et al., 2005]. An additional constraint on the age of top-n thrusting comes from calcschist sample EIS14a. Its well-defined deformation age of 30.7 ± 0.7 Ma directly dates thrusting of the EZ onto the Venediger nappe. Although PT conditions of shearing cannot be defined with precision for that sample, the observed inverse correlation between Sr content and grain size of white mica (Figure 7 and Table 3) probably indicates that latest deformation increments occurred under amphibolite- to greenschist facies conditions (see above). Thus top-n thrusting, eclogitization, internal deformation and rapid exhumation of the EZ all occurred in a narrow time frame at Ma. [53] Notably, this time frame is identical to the timing of granitoid and alkalibasaltic magmatism south of the Tauern Window (32 30 Ma; see Table 1 for references), which has been explained by slab breakoff [Blanckenburg and Davies, 1995]. This chronologic coincidence possibly points to important syncollisional rebound of the subducted European continental crust as a consequence of slab breakoff, contributing to Penninic top-n nappe thrusting. The sedimentologic data of Kuhlemann et al. [2001] indicate that considerable relief existed in the Tauern Window area at 30 Ma Incipient Strike-Slip and Extensional Shear [54] The best constraints on the age of incipient strike slip and extensional shear come from the framework of the Eclogite Zone (Figure 6). Three mylonite samples from the contact between the Glockner nappe and the Rote-Wand- Modereck nappe (EIS11, 31.4 ± 0.4 Ma), from the contact between the Rote-Wand-Modereck nappe and the Eclogite Zone (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 distribution among different white mica grain size fractions probably indicates deformation of these mylonites at decreasing pressures and temperatures (see above), ceasing at greenschist facies conditions. Isotopic disequilibria in sample EIS14b suggest that sinistral S-side-down shear lasted until 29.1 ± 0.5 Ma and outlasted top-n thrusting in this locality. In this context it is important to note that the deformation 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 published 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 and neighboring units sinistral S-side-down deformation started at Ma, coeval with high-pressure metamorphism and rapid exhumation of the Eclogite Zone. In other words, high-pressure metamorphism in the Eclogite Zone, the immediately following thrusting of the latter onto the Venediger nappe and blueschist- to greenschist facies sinistral S-side-down shearing within parts of the Eclogite Zone, in the Rote-Wand Modereck and lowermost Glockner nappes occurred, within error, at the same time. Strikeslip S-side-down ductile deformation was completed at 29 Ma. This also indicates that the formation of the nappe pile in the Tauern Window was completed in this narrow time span between Ma (Figure 10a). [55] In the following we suggest that essential parts of the normal and strike-slip fault system in the entire Tauern Window region started to form at Ma, synchronous with or immediately subsequent to Penninic/Helvetic nappe stacking. A review of published and new isotopic age data for deformation shows that Oligocene ages of Ma have been obtained from a number of the shear zones considered responsible for exhumation of the Tauern Window (Figure 2). In addition, a number of age data close to 30 Ma exist for Penninic and Helvetic units, the exact tectonic significance of which remains unclear but which nevertheless indicate that fabric formation in large parts of the Tauern Window was already completed at that time (see Figure 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 Window. A number of white-mica-based mylonitization ages have been published for various segments of the Periadriatic line, generally documenting dextral shear around 30 Ma (Figure 2) [Läufer et al., 1997; Müller et al., 2000, 2001; Mancktelow et al., 2001]. An important observation is the change from sinistral strike slip to dextral transpressive motion for the Defereggen-Antholz-Vals fault system at 30 Ma [Müller et al., 2000; Mancktelow et al., 2001]. This age directly reflects a major tectonic reorganisation and the establishment of the Tauern Window-formation/ exhumation regime. The 32.4 ± 0.4 Ma Rieserferner pluton [Romer and Siegesmund, 2003] was syntectonic with mylonitic deformation along the Defereggen-Antholz-Vals fault system and with E-W directed horizontal extension in its Austroalpine country rocks [Wagner et al., 2006]. Similar results were obtained for deformation patterns in and around the Rensen pluton, SW of the Tauern Window [Barth et al., 1989; Müller et al., 2000; Krenn et al., 2003]. [57] Published Rb-Sr data on schists from the wider Mölltal 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 clustering around Ma. In addition, Cliff et al. [1998] documented deformation at 27.7 ± 0.3 Ma, based on Th-Pb data for allanite crystals, which grew during the formation of a crenulation cleavage resulting from large-scale folding. Although protracted ductile deformation along the Mölltal fault 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, 19 of 27

20 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 ductile deformation 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) Müller et al. [2000]. Table 3) and by literature data (see Figure 2) [Inger and Cliff, 1994], there is clear evidence that dextral tectonic movements related to the Mölltal fault system started during early exhumation [cf. Kurz and Neubauer, 1996] in the Oligocene. The above range of deformation ages suggests that deformation along the Mölltal fault was continuously or at 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 is indicated by K-Ar data for fine-grained white mica from the NE edge of the Tauern Window (Figure 2). The age of 25 Ma was interpreted as cooling postdating normal shearing [Deckert, 1999]. From an area nearby, Liu et al. [2001] reported a number of Ar-Ar white mica plateau ages close to 30 Ma for Penninic rocks in the immediate footwall of the Katschberg normal shear zone. However, it remains uncertain whether these ages reflect early nappe stacking, cooling, or normal, Katschberg-fault-related deformation. Some Ar-Ar dates at 22 Ma were interpreted as directly dating Katschberg normal fault activity [Liu et al., 2001]. The Katschberg fault connects at its southern termination with the dextral Mölltal fault [Kurz and Neubauer, 1996; Frisch et al., 2000]. Thus kinematic and chronologic linkage between the two faults can be inferred, with initiation of normal faulting on the Katschberg fault in the Oligocene and lasting into the Early Miocene. [59] The SEMP shows strong evidence for nucleation contemporaneous with Oligocene thrusting and crustal thickening. For the SEMP along the northeastern Tauern Window, sinistral transpression has been directly dated (Ar-Ar, white mica) to between 35 and 28 Ma [Urbanek et al., 2002]. Given that the interpretation of the WSW- ENE-striking ductile shear zones in the western Tauern Window as deep-crustal equivalents of the SEMP fault [Behrmann and Frisch, 1990; Linzer et al., 2002] is correct, our new deformation age for the Olperer shear zone (sample TF04-25, 31.2 ± 0.4 Ma; Table 3 and Figures 2, 5, and 8) confirms activation of sinistral shear at depths of 35 km (as inferred from the P-T data of Selverstone et al. [1984]) also within the western Tauern Window in the Ma time frame. This deformation obviously continued in the Late Oligocene to Early Miocene, as indicated by our data for the age of sinistral shear in the eastern part of the Greiner shear zone (26.7 ± 1.2 Ma, sample TF04-15; 21.5 ± 0.8 Ma, sample TF04-18; Table 3 and Figures 2, 5, and 8). Late Oligocene to Early Miocene deformation in the Greiner shear zone is also recorded by the 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 the end of ductile top-w shearing in the investigated samples (18.3 ± 2.6 Ma, 21 ± 2 Ma, and 17.8 ± 1.8 Ma, Table 3 and Figures 2 and 4). These ages record late-stage, greenschist facies deformation. The kinematic relationship of the Brenner fault with the sinistral strike-slip shear zones and faults is complicated and controversially discussed. The following five arguments suggest an Oligocene initiation of top-w ductile shear at the Brenner shear zone. (1) Thöni [1980] reported four K-Ar white mica ages, between 32.2 ± 1.8 and 20.3 ± 1.4 Ma, for pervasively deformed quartzites of Austroalpine origin. These quartzites form tectonic lenses within the Brenner fault zone north of Brennerpass (Figure 2). In line with the original interpretation of Thöni [1980], we regard the above age data as early, previously largely unnoted ages for Brenner fault tectonic movements. (2) At its northern termination the Brenner fault is transformed into the sinistral Inntal fault [Fügenschuh et al., 1997; Frisch et al., 2000; Ortner et al., 2006]. For the Inntal fault, pre- Late Oligocene movements have been reported [Ortner and Stingl, 2001; Ortner et al., 2006]. In addition, the Inntal fault belongs to the same sinistral fault system as the 30 Ma SEMP fault. The proposed kinematic linkage 20 of 27

21 between the Inntal and Brenner faults thus suggests contemporaneous initiation. (3) In the south, the motion of the Brenner fault is transformed into dextral displacement along the Periadriatic line [Frisch et al., 2000], the establishment of which is constrained to 30 Ma. (4) In the westernmost Tauern Window amphibolite facies top- WSW normal shear is documented, which started at conditions close, maybe even prior to the thermal peak and promoted initial decompression [Selverstone, 1988, 1993]. Given that the thermal peak in the western Tauern Window occurred at 30 ± 1 Ma [Christensen et al., 1994], top-wsw normal shear contemporaneous with N-S shortening thus commenced at around 30 Ma [Selverstone, 1993; Selverstone et al., 1995]. Protracted exhumationrelated cooling probably controlled the development of early, 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 geochronologic work in the Olperer shear zone suggests that amphibolite facies sinistral and top-wsw normal shearing occurred simultaneously at 31.2 ± 0.4 Ma. We suggest that our age data from the Brenner fault reflect the end of a protracted history of ductile normal shear at Ma. [61] Nonetheless, Rosenberg and Schneider [2008] argued that if the Greiner shear zone continued westward until the Brenner fault without a marked change in strike, it would reach the Brenner Fault at its southern end (Figure 2). In this area the kinematics of the west-dipping Brenner extensional fault would predict a dextral shear zone, associated with exhumation of the footwall of the Brenner Fault [Fügenschuh et al., 1997] and not a sinistral shear zone. Therefore a direct kinematic link between the Greiner shear zone and the Brenner fault seems unlikely [Rosenberg and Schneider, 2008]. We believe that further detailed field work is needed to clarify the kinematic relationship between the Greiner shear zone and the Brenner fault. [62] The above discussion shows that a large number of individual postcollisional faults of the Tauern Window area formed at different lithospheric levels (from the base of the crust or even the upper lithospheric mantle to the semibrittle regime) in the Oligocene, at Ma. This holds both for the transpressional strike-slip systems north and south of the Tauern Window, and for the normal shear zones west and east of the Window. Considering the entire fault pattern as a kinematically interconnected system responsible for the tectonic exhumation of the Tauern Window [Ratschbacher et al., 1991; Neubauer et al., 1999; Frisch et al., 2000] implies that the formation of the Tauern Window was coeval with rapid exhumation of the deep-seated Eclogite Zone at Ma. In other words, we propose that the structures responsible for the formation of the Tauern Window started to develop about 10 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 most deeply exhumed segment of the Tauern Window underwent very rapid tectonism in only 3 Ma in the Oligocene. The lack of ages <29 Ma in and around the Eclogite Zone could mean that the transpressive regime nucleated in the area of the present Eclogite Zone. Subsequently the orogenic wedge grew in N-S and got slightly extended in E-W and in the course of this growing wedge deformation is successively transferred to the N, W and E. If this speculative interpretation is correct, it would imply that the 30 Ma structures in the Katschberg and Olperer shear zones are transported structures. Interestingly, the 30 Ma deformation ages tend to occur at the periphery of the window and in the adjacent Austroalpine nappes. [64] It is also conceivable that the 30 Ma deformation occurred in the fault systems in the surroundings of the Tauern Window since the window just started to form by this time. Much of the strong N-S shortening at 30 Ma seems to have been accommodated in the Penninic units of the developing Tauern Window into vertical extension, not horizontal E-W extension. Vertical extension appears to be related to the development of the antiforms and strong thinning of the limbs of the folds. It is in these limbs where the strike-slip shear zones preferentially nucleated and where the deeply exhumed units, especially the Eclogite Zone, occur. Note that both options are not mutually exclusive and might have operated in concert with each other Significance of Miocene Deformation Ages [65] For the period between 29 and 15 Ma, the database of ductile deformation ages (Figure 2) shows scattered data between 29 and 21 Ma, and a large number of age values between 21 and 15 Ma. Although this "statistics is far from being representative, we hypothesize that it reflects pronounced reactivation of preexisting structures at Ma. Cliff et al. [1985] also argued that their geochronologic data reflect rapid exhumation of the Tauern Window between Ma. Kuhlemann et al. [2006] interpreted detrital apatite fission track data from Alpine Molasse basins to be related to an increased rate of tectonic denudation of the Tauern Window at the same time. [66] The sinistral shear zones in the western Tauern Window show a striking pattern of amphibolite/greenschist facies and brittle reactivation, or continued activity, between Ma (Figures 2, 8, and 10b). Our interpretation of the biotite ages indicates considerable decompression, thus exhumation, at 15 Ma. Our age data and their relation to the metamorphic conditions during shearing indicate that shearing under amphibolite- and greenschist facies conditions occurred at about the same time in the currently exposed parts of the shear zones indicating that those shear zone segments were subsequently juxtaposed. In large parts of the Greiner shear zone no evidence for annealing of the dated ductile fabrics occurs. The same holds true for the Ahorn shear zone [Rosenberg and Schneider, 2008]. Therefore a lot of the ductile deformation in the western Tauern Window is Miocene in age, which is in contrast to previous assertions for a prepeak metamorphism (i.e., pre-30 Ma) age of deformation [cf. Behrmann and Frisch, 1990]. In some segments of the heterogeneous shear zones older ages of Ma are preserved. [67] The Ahorn shear zone accommodated 7 km of S-side-up vertical displacement. At the western termination 21 of 27

22 Figure 9. NNW-SSE profile across the Western Tauern Window, crossing the Pfitscher Joch area (modified after Lammerer and Weger [1998], GBA 2006), with published zircon and apatite fission track age data. See Figure 2 for location of the profile. (1) Most [2003], (2) Stöckhert et al. [1999], (3) Fügenschuh et al. [1997], (4) Grundmann and Morteani [1985], and (5) Steenken et al. [2002]. 22 of 27

23 Figure 10. Illustration of envisaged tectonic development of the Tauern Window region. (a) Nucleation of the Tauern Window coeval with deep underthrusting and high-pressure metamorphism in the Eclogite Zone and initial normal shearing at Katschberg and Brenner normal faults as well as shearing on major lateral shear zones. Map on the left shows Tauern Window dashed implying that Penninic/Helvetic rocks are not yet at the surface, as indicated on schematic N-S cross section on the right. Intrusion of plutons and 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) Miocene situation. Tauern Window grew to the N, W and E and large-scale folding occurred along with the growth of the lateral faults in and around the Tauern Window. Note that relative scales for the maps and cross sections are distinctly different. Abbreviations: DAV, Defereggen-Antholz-Vals fault; EZ, Eclogite Zone; GN, Glockner nappe; RW, Rote Wand-Modereck nappe; SEMP, Salzach-Ennstal-Mariazell-Puchberg fault; VN, Venediger nappe. of this fault, part of the sinistral lateral displacement along the SEMP probably is transferred into vertical displacement, a tectonic setting linked with formation of the large-scale ENE-WSW striking upright folds of the western Tauern Window (Figures 8 and 9) [Rosenberg and Schneider, 2008]. This indicates that the Oligocene transpressive regime persisted into the Miocene (Figure 10b). As a consequence, the Ma ages from the Olperer and Greiner shear zones could reflect,frozen-in ages from segments of the shear zone that were active in the Oligocene and ceased activity then, whereas in other segments of the same shear zone pronounced ductile shearing in the Miocene until Ma is recorded. Whether or not shearing in the heterogeneously deforming shear zones was continuous or intermittent in the period from 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 the Ahorn shear zone (Figures 8 and 9) in the Miocene. The two shear zones created a pop-up wedge in the western Tauern Window. This wedge controlled part of the exhumation of the central parts of the western Tauern Window. The overall antiformal structure requires that the deepest exhumation of 25 km occurs along the central axis of the western Tauern Window [Fügenschuh et al., 1997], between 21 and 15 Ma. As a consequence, the central parts were still 23 of 27