Western Tauern Window Research Articles

Constraining the slip history of the Katschberg normal fault (Eastern Tauern Window) by thermo-kinematic modeling: Implications for the tectonic evolution of the Eastern European Alps in the late Cenozoic

The Katschberg normal fault borders the Tauern Window to the east and played a crucial role during Miocene lateral tectonic extrusion in the Eastern European Alps. In this study, we present new cooling ages from low-temperature thermochronology as well as thermo-kinematic models, which constrain the exhumation history of the Penninic units in the footwall of the Katschberg normal fault and its slip history. Zircon and apatite fission track and apatite (U–Th)/He ages from footwall units range from 16.0 ± 1.9 Ma to 12.8 ± 1.4 Ma, 10.4 ± 1.8 Ma to 7.9 ± 1.3 Ma and 8.2 ± 0.8 Ma to 3.9 ± 0.4 Ma, respectively. Thermo-kinematic modeling indicates that the Katschberg normal fault was active between 21.1 ± 1.8 Ma and 12.2 ± 1.3 Ma and accommodated 27 ± 6 km of crustal extension at a total rate of 3.5 ± 0.3 km/Myr. After the end of normal faulting, exhumation continued with a rate of 0.21 ± 0.06 km/Myr until 2.0 ± 0.5 Ma and with a rate of 0.84 ± 0.08 km/Myr until present. A comparison with another Miocene low-angle normal fault in the Eastern Alps – the Brenner fault – reveals that the amount of extension accommodated by these faults decreases from west to east, which is consistent with an eastward decrease in N-S shortening. Therefore, Miocene deformation is greatest in the western Tauern Window near the Brenner normal fault where shortening in front of the Adriatic Indenter is at its maximum.

The detrital zircon record of Variscan to post-Variscan tectonosedimentary and magmatic processes in the Tauern Window (Eastern Alps)

Coupled U–Pb and Lu–Hf LA-ICP-MS detrital and igneous zircon data were obtained from metasedimentary sequences (Kaserer Formation, Schmirntal Quartzite, Seidlwinkel Formation, Bündnerschiefer Basin, Riffler Basin) of the western Tauern Window (Eastern Alps). Results show maximum deposition ages between the Late Permian and the Triassic, indicating protracted sedimentation and magmatism between the Late Paleozoic and the Mesozoic. The Lu–Hf fingerprint shows a change from subchondritic to variable subchondritic to suprachondritic compositions at ca. 290 Ma, possibly documenting the transition from Late Paleozoic Variscan post-collisional processes to intracontinental extension. Lithospheric thinning and magmatic underplating may explain the observed Hf isotopic evolution as the result of mixing of crustal and mantellic sources. From a paleogeographical perspective, results confirm that the Tauern Window was situated between Alpine basement units (South Alpine, Austroalpine and External Massifs) and the Bohemian Massif during the Permian–Triassic.

New constraints on the exhumation history of the western Tauern Window (European Alps) from thermochronology, thermokinematic modeling, and topographic analysis

The Brenner normal fault bounds the Tauern Window to the west and accommodated a significant portion of the orogen-parallel extension in the Eastern Alps. Here, we use zircon (U–Th)/He, apatite fission track, and apatite (U–Th)/He dating, thermokinematic modeling, and a topographic analysis to constrain the exhumation history of the western Tauern Window in the footwall of the Brenner fault. ZHe ages from an E–W profile (parallel to the slip direction of the fault) decrease westwards from ~ 11 to ~ 8 Ma and suggest a fault-slip rate of 3.9 ± 0.9 km/Myr, whereas AFT and AHe ages show no spatial trends. ZHe and AFT ages from an elevation profile indicate apparent exhumation rates of 1.1 ± 0.7 and 1.0 ± 1.3 km/Myr, respectively, whereas the AHe ages are again spatially invariant. Most of the thermochronological ages are well predicted by a thermokinematic model with a normal fault that slips at a rate of 4.2 km/Myr between ~ 19 and ~ 9 Ma and produces 35 ± 10 km of extension. The modeling reveals that the spatially invariant AHe ages are caused by heat advection due to faulting and posttectonic thermal relaxation. The enigmatic increase of K–Ar phengite and biotite ages towards the Brenner fault is caused by heat conduction from the hot footwall to the cooler hanging wall. Topographic profiles across an N–S valley in the fault footwall indicate 1000 ± 300 m of erosion after faulting ceased, which agrees with the results of our thermokinematic model. Valley incision explains why the Brenner fault is located on the western valley shoulder and not at the valley bottom. We conclude that the ability of thermokinematic models to quantify heat transfer by rock advection and conduction is crucial for interpreting cooling ages from extensional fault systems.

Geochemistry and paleogeographic implications of Permo-Triassic metasedimentary cover from the Tauern Window (Eastern Alps)

Abstract. The chemical composition of metasediments is a valuable source of paleogeographic information about the protolith's sedimentary environment. Here, we compile major- and trace-element whole-rock data, including B contents, and 10/11B-isotope ratios from the Permo-Triassic metasedimentary cover of the Pfitsch–Mörchner basin, overlying the Variscan basement in the western Tauern Window, Eastern Alps (Austria and Italy). The basement consists of orthogneiss (“Zentralgneis”, metamorphosed Variscan granitoids with intrusion ages between 305 and 280 Ma), and the roof pendant consists of granites (amphibolites, paragneiss, and minor serpentinites). The Zentralgneis is partly hydrothermally altered into pyrite quartzite with high Al–S contents, low Na–Sr–Ca–Mg contents, and very strong depletion of the light rare earth elements. Comparison with published detailed mapping of this and other time-equivalent basins in the western Tauern Window, with radiometric age data in the literature, and with unmetamorphosed basins in the South Alpine realm yields a late Permian to Early Triassic age of sedimentation. Although during Alpine metamorphism all rocks were strongly deformed, the whole-rock chemical compositions of the metasediments were not pervasively changed during deformation. We show that the sediments were deposited in a small, probably lacustrine–fluviatile, intramontane basin, under arid to semi-arid climatic conditions. The sequence starts with metaconglomerates, which can be interpreted as a mixture of the different basement rocks, based on a combination of major-element ratios Na2O / (Na2O + K2O) and MgO / (MgO + Fe2O3) with concentrations of trace elements Cr, V, and Ni. The sequence is overlain by a fining-upwards sequence of clastic sediments, in which the behavior of K, Rb, and Sr allows the reconstruction of intense diagenetic K–B metasomatism, which raised the K2O contents up to ∼ 10 wt %. The average B content of 218 µg g−1 is well above the B content of common sediments, and the B-isotope composition reaches extremely low values of down to −33 ‰ δ11B. The top of the sequence is a lazulite quartzite, interpreted as a former conglomeratic phosphatic sandstone, which marks the transition from a closed Permian basin to an open Triassic basin. Within the clastic sequence, the presence of hydrothermal tourmalinite veins documents a hydrothermal event after deposition but before the onset of Alpine metamorphism. A metamorphosed mafic dike swarm in the orthogneiss indicates a post-Variscan event of basaltic magmatism, and this event is tentatively correlated with increased heat flow in the Triassic basin and hydrothermal activity. A consistent conceptual model of this basin and its diagenetic modifications, based on a combination of geochemical data with petrographical and field information, provides the geodynamic context of the European margin at the onset of the Alpine orogeny.

Evaluation and application of the quartz-inclusions-in-epidote mineral barometer

AbstractWe have examined the suitability of a quartz-inclusions-in-epidote (qtz-in-ep) mineral barometer to better constrain P-T histories of epidote-bearing lithologies. Theoretical calculations applying an isotropic elastic model suggest that the qtz-in-ep barometer exhibits minimal temperature dependence, and thus, offers the potential to constrain growth conditions of epidote in various geologic environments, including skarn deposits, epidote-bearing granitoids, and metamorphic rocks.To test if the applied equations of state and isotropic elastic model reasonably simulate the elastic evolution of two anisotropic minerals, we measured Raman shifts of the 464 cm–1 band of quartz inclusions relative to that of an unencapsulated quartz standard. We calculated a quartz inclusion pressure (Pincl464) at various temperatures and compared these values with temperature-dependent Pincl predicted by elastic modeling (Pinclmod) at elevated temperatures. Three epidote-bearing samples with reasonably well-constrained P-T histories were also examined: (1) sample HF14C from the Upper Schieferhuelle in the Western Tauern Window, Italy (Pincl464=0.01 GPa); (2) sample LdC-31C from Lago di Cignana, Italy (Pincl464≈0.16 GPa); and (3) sample FT1E from the Frosnitz Tal in the Western Tauern region, Austria (Pincl464=0.57 GPa).Entrapment pressures (Pent464) calculated from Pincl464 determined at various temperatures show nominal differences from Pent calculated from Pinclmod, suggesting that for qtz-in-ep pairs, the calculated Pent does not significantly vary with the temperature of measurement. Furthermore, our calculated Pent464 for a sample from the Upper Schieferhuelle is in agreement with petrographic context and previously established P conditions, and the Pent464 determined for the Frosnitz Tal sample closely approximate previously reported pressures. The Lago di Cignana sample is derived from an epidote vein that is encased in a high-P foliation, and the calculated Pent464 is consistent with early, low-P epidote vein formation that pre-dates high-P metamorphism, or alternatively, late vein formation during exhumation, and confirms that the epidote did not form at or near peak conditions (~2.0 GPa). The results of this study indicate that the qtz-in-ep barometer potentially provides another tool that geoscientists can employ to better constrain P-T conditions in some epidote-bearing environments, where conventional thermobarometric techniques cannot be applied.

Cenozoic deformation in the Tauern Window (Eastern Alps) constrained by in situ Th-Pb dating of fissure monazite

Abstract. Thorium–lead (Th-Pb) crystallization ages of hydrothermal monazites from the western, central and eastern Tauern Window provide new insights into Cenozoic tectonic evolution of the Tauern metamorphic dome. Growth domain crystallization ages range from 21.7 ± 0.4 to 10.0 ± 0.2 Ma. Three major periods of monazite growth are recorded between ∼ 22–20 (peak at 21 Ma), 19–15 (major peak at 17 Ma) and 14–10 Ma (major peak around 12 Ma), respectively, interpreted to be related to prevailing N–S shortening, in association with E–W extension, beginning strike-slip movements and reactivation of strike-slip faulting. Fissure monazite ages largely overlap with zircon and apatite fission track data. Besides tracking the thermal evolution of the Tauern dome, monazite dates reflect episodic tectonic movement along major shear zones that took place during the formation of the dome. Geochronological and structural data from the Pfitschtal area in the western Tauern Window show the existence of two cleft generations separated in time by 4 Ma and related to strike-slip to oblique-slip faulting. Moreover, these two phases overprint earlier phases of fissure formation. Highlights. In situ dating of hydrothermal monazite-(Ce). New constraints on the exhumation of the Tauern metamorphic dome. Distinct tectonic pulses recorded from east to west.

On the petrology of brittle precursors of shear zones – An expression of concomitant brittle deformation and fluid–rock interactions in the ‘ductile’ continental crust?

AbstractThe inherited localization model for shear zone development suggests that ductile deformation in the middle and lower continental crust is localized on mechanical anisotropies, like fractures, referred to as shear zone brittle precursors. In the Neves area (Western Tauern Window, Eastern Alps), although the structural control of these brittle precursors on ductile strain localization is well established, the relative timing of the brittle deformation and associated localized fluid flow with respect to ductile deformation remains in most cases a matter of debate. The present petrological study, carried out on a brittle precursor of a shear zone affecting the Neves metagranodiorite, aims to determine whether brittle and ductile deformations are concomitant and therefore relate to the same tectonic event. The brittle precursor consists of a 100–500 µm wide recrystallized zone with a host mineral‐controlled stable mineral assemblage composed of plagioclase–garnet–quartz–biotite–zoisite±white mica±pyrite. Plagioclase and garnet preserve an internal compositional zoning interpreted as the fingerprint of Alpine metamorphism and fluid–rock interactions concomitant with the brittle deformation. Phase equilibrium modelling of this garnet‐bearing brittle precursor shows that metamorphic garnet and plagioclase both nucleated at 0.6 ± 0.05 GPa, 500 ± 20°C and then grew along a prograde path to 0.75 ± 0.05 GPa, 530 ± 20°C. These amphibolite facies conditions are similar to those inferred from ductile shear zones from the same area, suggesting that both brittle and ductile deformation were active in the ductile realm above 500°C for a depth range between 17 and 21 km. We speculate that the Neves area fulfils most of the required conditions to have hosted slow earthquakes during Alpine continental collision, that is, coupled frictional and viscous deformation under high‐fluid pressure conditions ~450°C. Further investigation of this potential geological record is required to demonstrate that slow earthquakes may not be restricted to subduction zones but are also very likely to occur in modern continental collision settings.

Deformation of feldspar at greenschist facies conditions – the record of mylonitic pegmatites from the Pfunderer Mountains, Eastern Alps

Abstract. Deformation microstructures of albitic plagioclase and K-feldspar were investigated in mylonitic pegmatites from the Austroalpine basement south of the western Tauern Window by polarized light microscopy, electron microscopy and electron backscatter diffraction to evaluate feldspar deformation mechanisms at greenschist facies conditions. The main mylonitic characteristics are alternating almost monophase quartz and albite layers, surrounding porphyroclasts of deformed feldspar and tourmaline. The dominant deformation microstructures of K-feldspar porphyroclasts are intragranular fractures at a high angle to the stretching lineation. The fractures are healed or sealed by polyphase aggregates of albite, K-feldspar, quartz and mica, which also occur along intragranular fractures of tourmaline and strain shadows around other porphyroclasts. These polyphase aggregates indicate dissolution–precipitation creep. K-feldspar porphyroclasts are partly replaced by albite characterized by a cuspate interface. This replacement is interpreted to take place by interface-coupled dissolution–precipitation driven by a solubility difference between K-feldspar and albite. Albite porphyroclasts are replaced at boundaries parallel to the foliation by fine-grained monophase albite aggregates of small strain-free new grains mixed with deformed fragments. Dislocation glide is indicated by bent and twinned albite porphyroclasts with internal misorientation. An indication of effective dislocation climb with dynamic recovery, for example, by the presence of subgrains, is systematically missing. We interpret the grain size reduction of albite to be the result of coupled dislocation glide and fracturing (low-temperature plasticity). Subsequent growth is by a combination of strain-induced grain boundary migration and formation of growth rims, resulting in an aspect ratio of albite with the long axis within the foliation. This strain-induced replacement by nucleation (associated dislocation glide and microfracturing) and subsequent growth is suggested to result in the observed monophase albite layers, probably together with granular flow. The associated quartz layers show characteristics of dislocation creep by the presence of subgrains, undulatory extinction and sutured grain boundaries. We identified two endmember matrix microstructures: (i) alternating layers of a few hundred micrometres' width, with isometric, fine-grained feldspar (on average 15 µm in diameter) and coarse-grained quartz (a few hundred micrometres in diameter), representing lower strain compared to (ii) alternating thin layers of some tens of micrometres' width composed of fine-grained quartz (<20 µm in diameter) and coarse elongated albite grains (long axis of a few tens of micrometres) defining the foliation, respectively. Our observations indicate that grain size reduction by strain-induced replacement of albite (associated dislocation glide and microfracturing) followed by growth and granular flow simultaneous with dislocation creep of quartz are playing the dominating role in formation of the mylonitic microstructure.

A new detrital mica 40Ar/39Ar dating approach for provenance and exhumation of the Eastern Alps

AbstractDetrital thermochronology can be used as a tool to quantitatively constrain exhumation rates and its spatial variability from active mountain belts. Commonly used methods for this purpose assume a steady state relationship between tectonic uplift and erosion. However, this assumption does not account for the transitory response of a dynamic orogenic system to changes in the boundary conditions. We propose a different approach that uses the observed detrital age distributions as “markers” of the past exhumation and of the present‐day erosion and mixing occurring in a river system. In this paper, we present new 40Ar/39Ar biotite and white mica age distributions for 19 modern river sands from the Eastern Alps north of the Periadriatic line. The results present three main clusters of ages at ~0.5–50, ~60–120, and ~250–350 Ma that record the main orogenic phases in this sector of the Alps. We have applied two numerical methods to the cooling ages to (a) linearly compute the spatial variability of the relative present‐day erosion of a set of four detrital mineral samples from drainage basins along the Inn River and (b) quantify the rates of the cooling and erosion in the Tauern Window during Paleocene‐Miocene time of the Alpine orogeny. Our results suggest a 0.34–0.84 mm/yr range of exhumation rates for the Tauern Window since the Miocene. Our estimates of exhumation rates of the western Tauern Window are higher than those for the eastern Tauern Window, which is consistent with the previous studies.

Complicated secondary textures in zircon record evolution of the host granitic rocks: Studies from Western Tauern Window and Ötztal-Stubai Crystalline Complex (Eastern Alps, Western Austria)

Samples of metamorphosed and deformed granitic rocks were collected from two Alpine complexes with well-constrained metamorphic history: Western Tauern Window and Ötztal-Stubai Crystalline Complex. Zircon grains from these samples were investigated in situ by a combination of scanning electron microscope techniques, cathodoluminescence (CL) imaging and Raman spectroscopy. The aims were: to describe and interpret complicated secondary textures and microstructures in zircon; based on cross-cutting relationships between secondary microstructures, reconstruct the sequence of processes, affecting zircon crystals; link the evolution of zircon with the history of the host rocks.The results indicate that zircon in the sampled granitic rocks forms growth twins and multi-grain aggregates, which are unusual for this mineral. Moreover, various secondary textures have been found in the sampled zircon, often cross-cutting each other in a single crystal. These include: distorted oscillatory CL zoning with inner zones forming inward-penetrating, CL-bright embayments, which are the evidence of dry recrystallization via annealing/lattice recovery; CL mosaicism with no preservation of growth zoning, but abundant nano- and micro-scale pores and mineral inclusions, which are the evidence of recrystallization by coupled dissolution–reprecipitation and/or leaching; embayed zircon boundaries filled with apatite, monazite, epidote and mylonitic matrix, indicating mineral-fluid reactions resulting in zircon dissolution and fragmentation; overgrowth CL-dark rims, which contain nano-pores and point to transport and precipitation of dissolved zircon matter. We conclude that zircon in our meta-granites is sensitive to metamorphism/deformation events, and was reactive with metamorphic fluids. Additionally, we have found evidence of crystal-plastic deformation in the form of low angle boundaries and bent grain tips, which is a result of shearing and ductile deformation of the host rock. We suggest that the observed complicated secondary textures in zircon can be linked to the evolutionary stages of the host rocks such as magmatic crystallization, prograde metamorphism, peak of amphibolite-facies metamorphism, post-peak cooling and exhumation, formation of ductile shear zones and final cooling to ~250°C.

Tourmaline as a petrogenetic indicator in the Pfitsch Formation, Western Tauern Window, Eastern Alps

Tourmaline is a common accessory mineral in the metasedimentary Pfitsch Formation located in the Pfitscher Joch (Passo de Vizze) area in the Tauern Window of the Eastern Alps. These post-Variscan metasedimentary units experienced peak metamorphic conditions of ~550°C, 1.0GPa during the Alpine orogeny. Tourmaline is most abundant in a ~25m thick unit of feldspathic gneiss (~20–200μg/g B), where it occurs as idiomorphic crystals typically 10mm in length. The abundance and size of the tourmaline crystals increase near coarse-grained quartzofeldspathic segregations (~1200μg/g B), reflecting the mobilization and concentration of B by metamorphic fluids. Near segregations, individual tourmaline crystals have up to three growth zones, recording pro- (~350–500°C, 0.7–1.0GPa) and retrograde (~400°C, 0.2GPa) growth as determined by combining textural information and sector-zoning thermometry. Retrograde tourmaline occurs as individual crystals as well as overgrowths on prograde tourmaline crystals, especially on the surface of extensional fractures formed by E–W extension during regional decompression.Tourmaline in the Pfitsch Formation is dravitic with a variable Fe content that correlates with the Fe content of its respective host unit. Charge balance calculations suggest that a significant proportion of Fe in tourmaline is ferric, supporting the interpretation of a subaerial continental sedimentary protolith. Tourmaline near segregations has the highest inferred ferric iron content, which decreases across growth zones, potentially reflecting a reduction of the fluid during metamorphism. The Mg/(Mg+Fe) ratio increases with prograde tourmaline growth and decreases in retrograde overgrowths. In contrast, the Ca/(Ca+Na) ratio increases gradually from 0.05 to 0.20 with prograde growth and continues to increase up to 0.25 in the retrograde overgrowths, recording the maximum Ca/(Ca+Na) ratio of the fluid during Alpine metamorphism.The metasediments of the Pfitsch Formation have very low and variable whole-rock δ11B values (−14.1 to −33.6‰), with the highest values (−17.7 to −14.1‰) found in B-rich samples (165–1200μg/g B) containing abundant tourmaline, and the lowest values (−24.2 to −33.6‰) in B-depleted samples (21–40μg/g), which lack tourmaline. This observation supports preferential loss of 11B from the rocks during prograde metamorphism. Zoned tourmaline crystals in the Pfitsch formation show successively decreasing B isotope ratios from −7.8 to −11.2‰ in their cores and −17.3 to −20.3‰ in their rims. As supported by a Rayleigh fractionation model, the B-isotope values of the host rocks and the tourmaline crystals are most easily explained by the internal redistribution of B from a B-rich precursor mineral (e.g. mica) to the tourmaline during Alpine metamorphism.

U–Pb ages of apatite in the western Tauern Window (Eastern Alps): Tracing the onset of collision-related exhumation in the European plate

We performed single spot U–Pb analyses of zircons and apatites from eighteen samples distributed along three sections crossing the western Tauern Window (Eastern Alps). Zircon analyses yield concordant ages of ∼295 Ma dating the Variscan emplacement of the Zentralgneiss batholith. Common Pb-corrected apatite U–Pb ages show two age groups; the first is characterized by apparent 206Pb/238U ages scattering between ∼300 and 40 Ma and a second having a relatively uniform cluster of age values around 30 Ma. We applied an age extractor algorithm (TuffZirc ISOPLOT) to all apatite analyses of each sample to calculate median ages from the younger age cluster and asymmetric 2σ errors as their uncertainties, which are mostly in the range of ∼10%. These median ages are interpreted to date cooling below ∼450–500 °C during Alpine orogeny after the thermal climax of Barrovian metamorphism in the western Tauern Window. Cooling ages give a consistent pattern over the study area of ∼31–29 Ma. Along each section younger cooling ages were obtained in the center compared to its margins. The eastern section shows older cooling ages of ∼36–34 Ma whereas the western section younger cooling ages of ∼26–25 Ma. The reliability of both age trends, (1) younging towards the centers of the sections and (2) younging towards the west across all three sections were verified by six independent two-sample Student's t-tests. Spatially the cooling age pattern resembles those observed from geochronometers having lower closure temperatures (∼375–110 °C), that are largely related to doming of the Tauern Window. Cooling rates of five samples, where additional fission track ages are available, yielded uniform results and were averaged to 15.3±1.9 K/Myr (2σ). Linear extrapolation using the specific cooling rates and the estimated maximum temperature (Tmax) indicates a time range of ∼37–34 Ma, interpreted to date the thermal climax of Barrovian metamorphism in the western Tauern Window.

Finite lattice distortion patterns in plastically deformed zircon grains

Abstract. This study examines finite deformation patterns of zircon grains from high-temperature natural shear zones. Various zircon-bearing rocks were collected in the Western Tauern Window, eastern Alps, where they were deformed under amphibolite facies conditions, and in the Ivrea–Verbano Zone (IVZ), southern Alps, where deformation is related with granulite-facies metamorphism. Among the sampled rocks are granitic orthogneisses, metalamprophyres and paragneisses, all of which are strongly deformed. The investigated zircon grains ranging from 10 to 50 μm were studied in situ using a combination of scanning electron microscope (SEM) techniques, backscattered electron (BSE) imaging, forward-scattered electron (FSE) imaging, cathodoluminescence (CL) imaging, and crystallographic orientation mapping by electron backscatter diffraction (EBSD), as well as micro-Raman spectroscopy. Energy-dispersive X-ray spectrometry (EDS) was applied to host phases. Microstructural analysis of crystal-plastically deformed zircon grains was based on high-resolution EBSD maps. Three general types of finite lattice distortion patterns were detected: type (I) is defined by gradual bending of the zircon lattice with orientation changes of about 0.6–1.8° per micrometer without subgrain boundary formation. Cumulative grain-internal orientation variations range from 7 to 25° within single grains. Type (II) represents local gradual bending of the crystal lattice accompanied by the formation of subgrain boundaries that have concentric semicircular shapes in 2-D sections. Cumulative grain-internal orientation variations range from 15 to 40° within single grains. Type (III) is characterized by formation of subgrains separated by a well-defined subgrain boundary network, where subgrain boundaries show a characteristic angular closed contour. The cumulative orientation variation within a single grain ranges from 3 to 10°. Types (I) and (II) predominate in granulite facies rocks, whereas type (III) is restricted to the amphibolite facies rocks. The difference in distortion patterns is controlled by strain rate and by ratio between dislocation formation and dislocation motion rates, conditioned by the amount of differential stress. Investigated microstructures demonstrate that misorientation axes are usually parallel to the < 001 > and < 100 > crystallographic directions; dominant slips are < 010 > {001}, < 010 > {100} and < 001 > {010}, whereas in some grains cross-slip takes place. This study demonstrates that activation of energetically preferable slip systems is facilitated if zircon grain is decoupled from the host matrix and/or hosted by a soft phase.

Geology of the Brenner Pass-Fortezza transect, Italian Eastern Alps

We present a 1:30,000 geological map resulting from detailed geostructural surveys carried out along the Italian segment of the design corridor for the Brenner Pass railway base tunnel (BBT), extending from Fortezza (Italy) to Innsbruck (Austria). The map covers the southern part of the Austroalpine-Penninic collisional wedge, the Periadriatic Fault System, associated Oligocene igneous bodies (Periadriatic magmatism) and part of the Southalpine basement. The Penninic Zone in the western Tauern Window is represented by the double domal structure of the Europe-derived Tux and Venediger-Zillertal basement and cover nappe system, capped by the ophiolitic Glockner nappe. The overlying Austroalpine nappe system is here represented by the polymetamorphic Merano-Mules basement and minor cover sheets. The Southern Alps domain includes the Bressanone Granite and pre-granite quartz-phyllites. Four Alpine ductile deformation phases have been recognized, followed by ductile-brittle shear zones, and finally brittle deformations along faults with extensional and strike-slip kinematics. The Quaternary is characterized by glacial deposits, large gravitational mass movements and landslides.

Dating the longevity of ductile shear zones: Insight from 40Ar/39Ar in situ analyses

We attempt to improve temporal constraints on the longevity and the termination of ductile shear zones by performing texturally-controlled in situ 40Ar/39Ar analyses of pre-kinematic muscovite, biotite and K-feldspars, of syn-kinematic phengite and K-feldspar, and of post-kinematic phengite within the same samples of sinistral shear zones from the western Tauern Window (Eastern Alps). Additionally two samples were dated by the Rb/Sr method (microsampling). Relative sequences of mineral formation based on microstructural, cross-cutting relationships were confirmed by in situ 40Ar/39Ar analyses, showing that syn-kinematic minerals are, in general, younger than pre-kinematic minerals and older or of equal age than the post-kinematic minerals of the same sample.From the rim to the core of the western Tauern Window syn-kinematic phengite and K-feldspar reveal a set of formation ages varying between 33 and 15Ma for the northernmost and peripheral shear zone (Ahorn Shear Zone), between 24 and 12Ma for the intermediate shear zone network (Tuxer Shear Zones), and between 20 and 7Ma for the southernmost and central shear zone (Greiner Shear Zone). The age variation of syn-kinematic phengite and K-feldspar analyses is larger than the analytical error of each age obtained. In addition, isochron calculations of the syn-kinematic minerals reveal atmospheric-like 40Ar/36Ar intercepts. Therefore, the obtained age values of the syn-kinematic minerals are interpreted as formation ages which date increments of a long lasting deformation period. The time range of deformation of each shear zone system is bracketed by the oldest and youngest formation ages of syn-kinematic phengite and K-feldspar.Post-kinematic phengite laths show the youngest formation ages and overlap with the youngest syn-kinematic formation ages. This relationship indicates that post-kinematic growth occurred immediately after syn-kinematic mineral formation at the end of ductile sinistral shear. Hence, the termination of deformation is dated by the ages of these post-kinematic phengite blasts.Pre-kinematic minerals are characterized by break down and exsolution reactions and their age values are heterogeneous and often affected by the presence of extraneous Ar. These age values are usually older than, but sometimes overlapping with, ages of the syn-kinematic minerals.Using the temporal constraints obtained by the ages of pre-, syn-, and post-kinematic minerals, we could assess partly overlapping time intervals of syn-kinematic mineral formation of 19Myr (33–15Ma) in the Ahorn Shear Zone, 13Myr (24–12Ma) in the Tuxer Shear Zones and 14Myr (20–7Ma) in the Greiner Shear Zone. This indicates successive localization and propagation of ductile shear zones in the western Tauern Window from lower metamorphic sites at the rim towards higher metamorphic sites in the center.

Thermochronological record of thrusting and strike-slip faulting along the Giudicarie fault system (Alps, Northern Italy)

The Giudicarie fault system represents the central, approximately NE–SW-trending segment of the Periadriatic fault system. Based on new fission track data the near surface evolution of the units bordering the Giudicarie fault system (GFS) is analysed. New and published zircon fission track (ZFT) data are compiled in an age-contour map of the area, and depict some first order features of cooling and exhumation pattern.Out of the three main Permian plutons located in the footwall of the GFS, the northern (Brixen pluton) and southernmost (Kreuzberg pluton) yielded ZFT ages of about 100Ma, while the intermediate Ifinger pluton cooled through the zircon partial annealing zone only in the Miocene. The Ifinger granodiorite overthrusts the Southalpine basement and the Permian rocks of the Athesian Volcanic District along the NW dipping brittle Naif fault. Across this thrust an important age jump from Miocene to Permian ZFT ages is observed, suggesting that the deformation has jumped towards the southeast, thus deactivating the related segment of the GFS.The most eye-catching feature of the age-contour map is the corridor of young, Miocene ZFT ages from small tonalitic intrusions along the Northern Giudicarie fault. This corridor connects Early Miocene (17–23Ma) ZFT ages of the NE-Adamello with the Middle–Late Miocene (19–9Ma) ZFT ages of the Meran–Mauls basement and the western Tauern window and provides evidence for a polyphase deformation along the GFS. A three-step evolution model is proposed for the GFS: (a) shearing of the northern rim of the Adamello batholith along the dextral strike slip Periadriatic fault system; (b) bending of the central part of the fault to a NE direction together with the adjacent intrusive rocks; (c) Dissection of the bend part (Meran–Mauls fault) by the sinistral transpressive Northern Giudicarie fault and exhumation of small tonalitic bodies along this structure.

Erratum to: Magmato-sedimentary Carboniferous to Jurassic evolution of the western Tauern window, Eastern Alps (constraints from U-Pb zircon dating and geochemistry)

Erratum to: Magmato-sedimentary Carboniferous to Jurassic evolution of the western Tauern window, Eastern Alps (constraints from U-Pb zircon dating and geochemistry)

Estimating displacement along the Brenner Fault and orogen-parallel extension in the Eastern Alps

The major structure accommodating orogen-parallel extension in the Eastern Alps is inferred to be the Brenner Fault, which forms the western boundary of the Tauern Window. The estimated amount of extension along this fault varies from a minimum of 10–20 km to a maximum of >70 km. All investigations that have attempted to constrain this amount of extension have calculated the fault plane parallel displacement required to restore the difference in structural level between footwall and hanging wall as constrained by geobarometry. However, these calculations neglected the component of exhumation of the footwall resulting from folding and erosion. Therefore, the total amount of extensional displacement was systematically overestimated. In the present study, we project a tectonic marker surface from the footwall and hanging wall of the Brenner Fault onto a N–S-striking cross section. This marker surface, which is the base of the Patscherkofel unit in the footwall and the base of the Otztal basement in the hanging wall, is inferred to have occupied the same structural level in the hanging wall and footwall of the Brenner Fault before its activity. Therefore, the difference in height between the marker projected from the footwall and from the hanging wall is a measure of the vertical offset across the Brenner Fault. This construction shows that the vertical offset of the marker horizon on both sides of the Brenner Fault varies strongly and continuously along strike of the Brenner Fault, attaining a maximum value of 15 km at the hinge of the folded footwall (Tauern Dome). The along-strike change of vertical offset is explained by large-scale upright folding of the footwall that did not affect the hanging wall of the Brenner Fault. Therefore, the difference in vertical offset of 10 km between the area of the Brenner Pass and the area immediately south of Innsbruck corresponds to the shortening (upright folding) component of exhumation of the footwall. The remaining 5 km of vertical offset must be attributed to extensional deformation. The Brenner Fault itself is barely folded, its dip varies between 20 and 70°, and it crosscuts the upright folds of the western Tauern Window. Given the offset of 5 km, the dip of the fault constrains the extensional displacement to be between 2 and 14 km. We conclude that the Tauern Window was exhumed primarily by folding and erosion, not by extensional unroofing.

Magmato-sedimentary Carboniferous to Jurassic evolution of the western Tauern window, Eastern Alps (constraints from U-Pb zircon dating and geochemistry)

New geochronological U-Pb (LA-ICP-MS) zircon data and geochemical analyses from the Variscan orthogneisses and metavolcanic rocks in the western Tauern window are presented and used to reconstruct the pre-Alpine evolution of this area. The late- and post-Variscan stage in the Tauern window was characterised by distinct magmatic pulses accompanied by the formation of volcano-sedimentary basins. The magmatic activity started in the Visean (335.4 ± 1.5 Ma) with the intrusion of a K-rich, durbachitic biotite-granite (protolith of the Ahorn gneiss). Following a period of exhumation and erosion, Westfalian–Stefanian volcanics were deposited (Grierkar meta-rhyodacite: 309.8 ± 1.5 Ma; Venntal meta-rhyolite: 304.0 ± 3.0 Ma). A renewed magmatic pulse occurred in the Early Permian, producing large volumes of tonalites and granodiorites (Tux meta-granodiorite: 292.1 ± 1.9 Ma). The youngest magmatism is characterised by pyroclastic and tuffitic deposits (Pfitsch meta-rhyolite: 280.5 ± 2.6 Ma; Schonach valley meta-andesite: 279.0 ± 4.8 Ma). This volcanism was probably related to crustal extensional faulting within an intra-continental graben and horst setting, asthenospheric upwelling and heat flow increase due to the onset of the Permian rifting. The Permo-Triassic peneplanation and subsidence is documented by shallow marine and evaporitic deposits. Probably in the Middle Jurassic times, the area was flooded and in the Late Jurassic the whole area was covered by limestones, representing post-rift sediments on the southern European continental margin.

The Pfitsch-Mörchner Basin, an example of the post-Variscan sedimentary evolution in the Tauern Window (Eastern Alps)

A review of post-Variscan metasedimentary and metavolcanic successions in the western Tauern Window is presented. U/Pb — datations of zircons in metavolcanic rocks reveal ages between 309 and 280 Ma. Deposition of grey conglomerates and black pelites started before 309 Ma in the northernmost basin of the Tauern, the Riffler-Schonach basin. In the more central Pfitsch-Morchner basin, the onset of conglomerate sedimentation can be dated into the time span between 293 and 280 Ma. The Pfitsch and Windtal Formations are newly defined. The basins were filled with up to 1 km of mainly continental clastics until Early Triassic. Short marine ingressions in Middle-and Late Triassic times flooded only basinal parts of the area where we suppose a more or less continuous sedimentation until the Late Jurassic. Only the Hochstegen Marble documents a nearly complete submergence in the area of the Tauern Window. In spite of the metamorphic overprint, the tentative interpretations of the sedimentary facies give a reasonable picture and allow correlations to nonmetamorphic areas in South Germany or the External Massifs of Eastern Switzerland.