Age constraints on the evolution of the Austroalpine basement to the south of the Tauern Window

The Austroalpine basement to the south of the Tauern Window once was part of the northern margin of Gondwana. It includes the “Altkristallin” and the phyllitic Thurntaler Complex. In the Altkristallin (AMU, MPU), suites of arc-related metamafic sequences occur together with calc-alkaline metagranite. SHRIMP U–Pb dating of zircon from calc-alkaline metagranite associated with an eclogitic amphibolite give an age of 470 ± 3 Ma interpreted as the age of protolith emplacement. In the Thurntaler Complex, metaporphyroids occur together with tholeiitic as well as alkaline within-plate basalt-type metabasite. The metaryholites of this association give a crystallization SHRIMP age of 477 ± 4 Ma, which suggests contemporaneity of arc-related and extensional settings in the Austroalpine basement units. The age data demonstrate widespread magmatic activity associated with the Early-Ordovician amalgamation at the end of the 550–470 Ma subduction–accretion–collision cycle. The Pb–Pb and U–Pb systematics of step-wise leached staurolite and kyanite from the peak-metamorphic assemblage of the Altkristallin indicate that (1) step-wise leaching of staurolite and kyanite yields the age of inclusions rather than the host; (2) zircon inclusions in staurolite suggest an Ordovician or older age for the precursor of the staurolite-schists; (3) the weighted average of the ²⁰⁶Pb/²³⁸U data of the various leaching steps yields a Variscan age for the inclusions (ilmenite, biotite, and andesine). Since these inclusions are part of the metamorphic mineral assemblage, this age provides a minimum estimate for staurolite growth, i.e., metamorphism. Thus, the Pb–Pb and U–Pb systematics of staurolite provide evidence for a Variscan metamorphism of the Austroalpine basement, e.g., MPU, AMU and Thurntaler Complex, to the south of the Tauern Window.     

Thrust-regime and transpression-regime tectonics in the Tauern Window (Eastern Alps)

Below the Penninic nappes of the Tauern-Window, the deepest level of the Alpine thrust system is exposed in the parautochthonous sedimentary sequence of Helvetic facies.During and after nappe emplacement and imbrication (D1), large scale tight to isoclinal recumbent folds formed (D2). They were accompanied by an axial surface schistosity and a stretching lineation in the fold limbs at a high angle to its axes. Refolding by slightly inclined large scale folds (D3) included all the exposed crystalline basement of the »Zentralgneis«. Stretching lineation was now east-west directed and parallel to the D3-fold axes, due to a transpressional regime, which acted during Neogene time. By superposition with D2, oblate or prolate finite strain ellipsoids formed in dependence of local heterogeneities. While the Insubric Lineament system is the brittle expression of the westward movement of the Italian Peninsula relative to the rest of Europe north of it, the Tauern-Window seems to have acted as ductile coupling between the two plates.

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Zusammenfassung Unter den penninischen Decken des Tauernfensters sind als tiefste Einheiten des Alpinen Deckenstapels parautochthone Sedimente in helvetischer Fazies exponiert. Während und nach der Deckenüberschiebung und einer damit in Zusammenhang stehenden lokalen Verschuppung (D1) bildeten sich enge bis isoklinale liegende Falten mit großer Aplitude (D2). Eine kräftige Achsenflächenschieferung und eine Streckungslineation etwa senkrecht zur Faltenachse entwickelten sich. (D3)-Wiederfaltung durch leicht südvergente Falten großer Amplitude erfaßte auch die tiefen Bereiche der Zentralgneise. Sie ist als Folge einer transpressiven Deformation zu sehen, da die begleitende Streckungslineation parallel zur Faltenachse ist. Oblate bzw. prolate finite Strainellipsoide bildeten sich durch Überlagerung und in Abhängigkeit von lokalen Heterogenitäten. Während am Insubrischen Lineamentsystem die Westbewegung der Italienischen Halbinsel relativ zu Europa nördlich davon in spröder Deformation erfolgte, scheinen die Gesteine des Tauernfenster-Inhaltes als duktile »Kupplung« zwischen den beiden Kontinentalplatten reagiert zu haben.

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Résumé Dans la fenêtre des Hohe Tauern, sous les nappes penniques, affleurent des sédiments parautochtones de facies helvétique qui constituent l'unité la plus inférieure de l'édifice tectonique alpin.Pendant et après la mise en place des nappes et leur écaillage (D1), des plis couchés serrés à isoclinaux, de grande dimension, ont été engendrés (D2). A ces plis sont associées une schistosité de plan axial et une linéation d'étirement subperpendiculaire aux axes. Des plis D3 de grande amplitude, à faible vergence sud, ont ensuite repris les structures D2; ils affectent également les domaines plus profonds, constitués par le «Zentralgneiss«. Ces plis sont accompagnés d'une linéation d'étirement E-W, parallèle à leurs axes, et liée à un régime de décrochement d'âge néogène. Les ellipsoïdes des déformations finies présentent des formes en galette ou en cigare, résultant de la superposition des structures D2 et D3, jointe à des hétérogénéités locales.La fenêtre des Tauern semble s'être comportée comme une jonction ductile entre la péninsule italienne et l'Europe septentrionale, tandis que le linéament insubrien est l'expression cassante d'un mouvement vers l'Ouest de la péninsule italienne.

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Concepts of the evolution of the Austroalpine basement complex (Eastern Alps) during the Caledonian-Variscan cycle

The Austroalpine basement complex has a complicated pre-Alpidic history which begins with the Caledonian era. In the late Precambrian (?) and early Paleozoic a magmatic-sedimentary rock sequence is formed presumably in an island-arc or active continental margin environment. Subduction with eclogite formation is followed by collision, high-grade metamorphism and anatexis in the Ordovician. This Caledonian basement is preserved in parts of the Austroalpine crystalline mass. The post-Caledonian deposits are mainly shelf type sediments with intercalated volcanics, although there is evidence for an oceanic basin to the south. The Variscan facies zones are arranged in SW-NE direction, oblique to the Alpidic trend. In a first stage of Variscan orogeny in the Carboniferous, south(east)-vergent decollement nappes, syntectonic flysch deposits, and granitoids are formed along with regional metamorphism. This is followed by a second stage in the Permian with north(west)-vergent thrusting, renewed granite formation, and metamorphism. The Variscan nappe pile is today exposed in a deeper level in the west or northwest than in the east or southeast.     

Crystallization and very rapid exhumation of the youngest Alpine eclogites (Tauern Window,Eastern Alps) from Rb/Sr mineral assemblage analysis

Multimineral Rb/Sr internal isochrons from eclogite facies rocks of the Eclogite Zone (Tauern Window, Eastern Alps) consistently yield an Early Oligocene age of 31.5±0.7 Ma. This age has been obtained both for late-prograde, dehydration-related eclogitic veins, and for rocks variably deformed and recrystallized under eclogite facies conditions (2.0–2.5 GPa, 600°C). Initial Sr-isotopic equilibria among all phases indicate absence of significant post-eclogitic isotope redistribution processes, therefore the ages date eclogite facies assemblage crystallization. Equilibria also prove that no prolonged pre-eclogite facies history is recorded in the rocks. Instead, subduction, prograde mineral reactions, and eclogitization proceeded rapidly. Fast exhumation immediately after eclogitization, with minimum rates >36 mm/a is inferred from a 31.5±0.5 Ma internal mineral isochron age of a post-eclogitic greenschist facies vein assemblage. Such rates equal typical subduction rates. Late Eocene to Early Oligocene subduction of the European continental margin, with subsequent rapid exhumation of high-pressure nappe complexes has previously been recognized only in the Western Alps. The new data signify synchronous continental collision all along the Alpine belt. Our results demonstrate the unique potential of Rb/Sr assemblage system analysis for precise dating of both eclogite facies and post-eclogitic events, thus for precisely constraining exhumation rates of deep-seated rocks, and for straightforward linkage of petrologic evidence with isotopic ages.     

Characterization of polymetamorphism in the Austroalpine basement east of the Tauern Window using garnet isopleth thermobarometry

Garnet in metapelites from the Wölz and Rappold Complexes of the Austroalpine basement east of the Tauern Window typically shows two distinct growth zones. A first garnet generation usually forms the cores of garnet porphyroblasts and is separated by a prominent microstructural and chemical discontinuity from a second garnet generation, which forms rims of variable width. Whereas the rims were formed during the Eo-Alpine metamorphic overprint, the garnet cores represent remnants of at least two pre-Eo-Alpine metamorphic events. The pressure and temperature estimates obtained from garnet isopleth thermobarometry applied to the first growth increments of the pre-Eo-Alpine garnet cores from the Wölz and Rappold Complexes cluster into two distinct domains: (i) in the Wölz Complex, incipient growth of the first-generation garnet occurred at 4 ± 0.5 kbar and 535 ± 20 °C, (ii) in the Rappold Complex, incipient growth of the oldest garnet cores took place at 5.3 ± 0.3 kbar and 525 ± 15 °C. The Eo-Alpine garnet generation started to grow at 6.5 ± 0.5 kbar and 540 ± 10 °C. According to radiometric dating, the low-pressure garnet from the Wölz complex was formed during a Permian metamorphic event. The first-generation garnet of the Rappold Complex is probably of Variscan age.     

Three generations of monazite in Austroalpine basement rocks to the south of the Tauern Window: evidence for Variscan, Permian and Eo-Alpine metamorphic events

Three monazite generations were observed in garnet-bearing micaschists from the Schobergruppe in the basement to the south of the Tauern Window, Eastern Alps. Low-Y monazite of Variscan age (321?±?14?Ma) and high-Y monazite of Permian age (261?±?18?Ma) are abundant in the mica-rich rock matrix and in the outer domains of large garnet crystals. Pre-Alpine monazite commonly occurs as polyphase grains with low-Y Variscan cores and high-Y Permian rims. Monazite of Eo-Alpine age (112?±?22?Ma) is rarer and was observed as small, partly Y-enriched grains (3?wt. %?Y₂O₃) in the rock matrix and within garnet. Based on monazite-xenotime thermometry, Y?+?HREE values in monazite indicate minimum crystallization conditions of 500?°C during the Variscan and 650?°C for the Permian and Alpine events, respectively. Garnet zoning and thermobarometric calculations with THERMOCALC 3.21 record an amphibolite facies, high-pressure stage of ~600?°C/13?C16?kbar, followed by a thermal maximum at 650?C700?°C and 6?C9?kbar. The Eo-Alpine age for these two events is supported by inclusions of Cretaceous monazite in the garnet domains used for thermobarometric constraints and through the high growth temperatures of Eo-Alpine monazite, which is consistent with that of the thermal maximum (~700?°C). The age and growth conditions of a few Mn-rich garnet cores, sporadically present within Eo-Alpine garnet, are unclear because inclusions of monazite, plagioclase and biotite necessary for thermobarometric- and age constraints are absent. However, based on monazite thermometry, Permian and Variscan metamorphic conditions were high enough for the growth of pre-Alpine garnet. The formation of Variscan garnet and its later resorption, plus Y-release, would also explain the high Y in Permian monazite, which cannot originate from preexisting Variscan monazite only. Monazite of Variscan, Permian and/or Eo-Alpine ages were also observed in other garnet-bearing micaschists from the Schobergruppe. This suggests that the basement of the Schobergruppe was overprinted by three discrete metamorphic events at conditions of at least lower amphibolite facies. While the Variscan event affected all parts of this basement, the younger events are more pronounced in its structurally lower units.     

Geochemistry and metamorphic evolution of the Pohorje Mountain eclogites from the easternmost Austroalpine basement of the Eastern Alps (Northern Slovenia)

Kyanite-rich and quartz-rich eclogites occur as lenses within amphibolite-facies quartzo-feldspathic gneisses in the Pohorje Mountains, Northern Slovenia, that form the easternmost Austroalpine basement. Major and trace elements indicate that the kyanite-rich eclogites were derived from plagioclase-rich gabbroic cumulates, whereas the quartz-rich eclogites represent more fractionated basaltic compositions. Both varieties are characterized by a LREE-depleted N-MORB type REE signature. Geothermobarometry and P-T pseudosections indicate that eclogites equilibrated at 1.8-2.5 GPa and 630-700 °C, consistently with the lack of coesite and with equilibration conditions of the chemically similar eclogites from the adjacent basement units at Koralpe and Saualpe type localities. Decompression reaction textures include (i) clinopyroxene-plagioclase intergrowths after omphacite, (ii) replacement of kyanite by corundum-plagioclase-spinel±sapphirine symplectites, (iii) breakdown of phengite to biotite-plagioclase sapphirine symplectites. The results of this study indicate that Koralpe, Saualpe and Pohorje high-pressure rocks represent former MORB-type oceanic crust that was subducted in the course of the late Cretaceous (approximately 100 Ma ago) collision between the European and the Apulian plates.     

Evolution of quartz microstructures and textures during polyphase deformation within the Tauern Window (Eastern Alps)

The interior of the Tauern Window exposes underplated Penninic continental lithosphere and the overlying obducted Penninic oceanic crust within a large antiformal dome in the internal zone of the Eastern Alps. These units have been affected by a polyphase deformation history. Generally, three deformation events are distinguished. D₁ is related to underplating of, and top-to-the-N nappe stacking within, the Penninic continental units of the Tauern Window. Deformation stage D₂ is interpreted to reflect the subsequent continent collision between the Penninic continental units and the European foreland, D₃ is related to the formation of the dome structure within the Tauern Window. During thickening of continental lithosphere and nappe stacking (D₁), and subsequent intracontinental shortening (D₂), these tectonic units have been ductilely deformed close to a plane strain geometry. Conditions for the plastic deformation of the main rock-forming mineral phases (quartz, feldspar, dolomite, calcite) have prevailed during all three phases of crustal deformation. Generally, two types of quartz microstructures that are related to D₁ are distinguished within the Tauern Window: (a) Equilibrated and annealed fabrics without crystallographic preferred orientations (CPO) have only been observed in the central part of the southeastern Tauern Window, corresponding with amphibolite-grade metamorphic conditions. (b) In the northeastern and central part of the Tauern Window microstructures are characterized by quartz grains that show equilibrated shape fabrics, but well preserved CPO with type-I cross girdle distributions, indicating a deformation geometry close to plane strain. During D₂, two types of quartz microstructures are distinguished, too: (a) Quartz grains that show equilibrated shape fabrics, but well-preserved CPO. The c-axes distributions generally are characterized by type-I cross girdles, locally by type-II cross girdles, and in places, oblique single girdle distributions. (b) A second type of quartz microstructure is characterized by highly elongated grains and fabrics typical for dislocation creep and grain-boundary migration, and strong CPO. This type is restricted to the southern sections of the western and eastern Tauern Window. The c-axis distributions show type-I cross girdles in the western part of the Tauern Window and single girdles in the southeastern part. In the western part of the Tauern Window, a continuous transition from type (b) microstructures in the south to type (a) microstructures in the north is documented. The microstructural evolution also documents that the dome formation in the southeastern and western Tauern Window has already started during D₂ and has continued subsequent to the equilibration during amphibolite to greenschist facies metamorphism. D₃ is restricted to distinct zones of localized deformation. D₃-related quartz fabrics are characterized by the formation of ribbon grains; the c-axes show small-circle distributions around the Z-axis of the finite-strain ellipsoid. During exhumation and doming (D₃), deformation occurred under continuously decreasing temperatures.     

Palaeontological and facial features of the Upper Jurassic Hochstegen Marble (Tauern Window, Eastern Alps)

Micropalaeontological, microscopic and mineralogical investigations of the ductily deformed and greenschist-facies metamorphic Hochstegen Marble in the Tauern Window shed new light on its stratigraphy and fades.
New radiolarian and sponge spicule discoveries have been made in cherty limestone marbles. They confirm previous age assignments and permit for the first time a more exact micropalaeontological age determination of early Tithonian for the upper parts of the marble. Forty morphotypes of radiolarians could be distinguished; in one sample a Fisher diversity index of 6 is reached indicating deeper marine conditions. The spicule fauna is also diverse and shows affinity to the S-German Malm. In respect to all the data it can be presumed that carbonate sedimentation of the Hochstegen Marble took place in a deeper marine environment at the southern margin of the European continent (Helvetic realm) during the whole Late Jurassic.     

Understanding the pre-Variscan and Variscan basement components of the central Tauern Window, Eastern Alps (Austria): constraints from single zircon U-Pb geochronology

New single-grain and within-grain U-Pb zircon ages from the central Tauern Window help sorting out the time dimension among the various Variscan and pre-Variscan basement components that were strongly overprinted by Alpine orogeny. Single-grain isotope dilution (ID-TIMS) U-Pb zircon geochronology of three Basisamphibolit samples yield protolith formation ages of 351±2, 349±1 and 343±1 Ma. Laser ablation ICP-MS and ID-TIMS U-Pb detrital zircon dating of the Biotitporphyroblastenschiefer constrained the maximum time of sedimentation to between 362±6 Ma and 368±17 Ma. Paragneisses from the Zwölferzug yield maximum sedimentation ages from 345±5 Ma (ion microprobe data) to 358±10 Ma. Zircons from gabbroic clasts and detrital zircons from a meta-agglomerate from the Habach Phyllite give an upper intercept age of 536±8 Ma and a near-concordant age of 506±9 Ma, respectively. Hence, apart from the Habach Phyllite, the maximum sedimentation ages of the metasediments investigated range from Upper Devonian to Lower Carboniferous. Consequently, the Basisamphibolit, the Biotitporphyroblastenschiefer, and the paragneisses of the Zwölferzug form parts of the Variscan basement series. The Basisamphibolit (351-343 Ma) is distinct both in space and time of formation from the Zwölferzug garnet amphibolite (c. 486 Ma), which forms part of the pre-Variscan basement.     

P -T-t history of the Lower Austroalpine Nappe Complex in the “Tarntaler Berge” NW of the Tauern Window: implications for the geotectonic evolution of the central Eastern Alps

New petrologic and ⁴⁰Ar/³⁹Ar geochronologic data constrain conditions of Alpine metamorphism along the northwestern border of the Tauern Window. The P-T estimations based on phengite barometry were determined for samples from units of the Lower Austroalpine nappe complex exposed above the Southpenninic interior of the Tauern Window, and from upper parts of the Southpenninic “Bündner Schiefer” sequence. Results suggest that both Mesozoic metasedimentary nappe units (Reckner and Hippold Nappes) and an ophiolitic nappe (Reckner Complex) of the Lower Austroalpine nappe complex have been metamorphosed at pressures between 8 and 10.5 kbar and temperatures around 350 °C. The structurally highest Lower Austroalpine unit (Quartzphyllite Nappe) was not affected by high-pressure metamorphism and records maximum P-T conditions of approximately 4 kbar and 400 °C. Highest parts of the structurally underlying Southpenninic Bündner Schiefer sequence were metamorphosed at intermediate pressures (6–7 kbar). Temperatures increased in all structural units during decompression. Whole-rock ⁴⁰Ar/³⁹Ar plateau ages of silicic phyllites and cherts with abundant high-Si phengites record ages around 50 Ma in the Reckner Nappe, and 44–37 Ma in the Hippold Nappe and Southpenninic Bündner Schiefer sequence. These ages are interpreted to date closely the high-pressure metamorphism. The Lower Austroalpine-Southpenninic border area in the NW Tauern Window appears to have evolved along an indented, fragmented active continental margin where the Reckner Complex represents one of the oldest sections of the Southpenninic (Piemontais) Oceanic tract that was originally situated close to, or even within, the Lower Austroalpine continent. During closure of the Piemontais Ocean, the resultant subduction zone did not entrain components of the Reckner Complex or its cover sequences (Reckner and Hippold Nappes): therefore “Eoalpine” high-pressure metamorphism did not occur. Sequences exposed within the study area were subducted to relatively shallow depths during the last stage of consumption of oceanic crust and immediately prior to final continental collision. Received: 30 July 1996 / Accepted: 7 April 1997     

Chemical and physical responses to deformation in micaceous quartzites from the Tauern Window, Eastern Alps

Micaceous quartzites from a subvertical shear zone in the Tauern Window contain abundant quartz clasts derived from dismembered quartz‐tourmaline veins. Bulk plane strain deformation affected these rocks at amphibolite facies conditions. Shape changes suggest net shortening of the clasts by 11–64%, with a mean value of 35%. Quartz within the clasts accommodated this strain largely via dislocation creep processes. On the high‐stress flanks of the clasts, however, quartz was removed via solution mass transfer (pressure solution) processes; the resulting change in bulk composition allowed growth of porphyroblastic staurolite + chlorite ± kyanite on the clast flanks. Matrix SiO₂ contents decrease from c. 83 wt% away from the clasts to 49–58% in the selvages on the clast flanks. The chemical changes are consistent with c. 70% volume loss in the high‐stress zones. Calculated shortening values within the clast flanks are similar to the volume‐loss estimates, and are greatly in excess of the shortening values calculated from the clasts themselves. Flow laws for dislocation creep versus pressure solution imply large strain‐rate gradients and/or differential stress gradients between the matrix and the clast selvages. In a rock containing a large proportion of semirigid clasts, weakening within the clast flanks could dominate rock rheology. In our samples, however, weakening within the selvages was self limiting: (1) growth of strong staurolite porphyroblasts in the selvages protected remaining quartz from dissolution; and (2) overall flattening of the quartz clasts probably decreased the resolved shear stress on the flanks to values near those of the matrix, which would have reduced the driving force for solution‐transfer creep. Extreme chemical changes nonetheless occurred over short distances. The necessity of maintaining strain compatibility may lead to significant localized dissolution in rocks containing rheologic heterogeneities, and overall weakening of the rocks may result. Solution‐transfer creep may be a major process whereby weakening and strain localization occur during deep‐crustal metamorphism of polymineralic rocks.     

Fluid heterogeneities and hornblende stability in interlayered graphitic and nongraphitic schists (Tauern Window,Eastern Alps)

The assemblage hornblende+white mica occurs in graphite-free schists at two localities in the southwest corner of the Tauern Window, Eastern Alps. In interbedded graphitic layers (1 mm to 1 m thick), however, hornblende is typically replaced by pseudomorphs of biotite+plagioclase +epidote±chlorite+staurolite in the presence of white mica. Garnets adjacent to these pseudomorphs have pronounced growth discontinuities near their rims, in contrast to the continuously zoned garnets in nongraphitic layers. These observations imply that reactions of the type hbl+white micagar+bio+plag+epid±chl±staur +H₂O occurred in the graphitic samples, but that hbl+white mica remained stable in graphite-free layers.Calculation of the equilibrium constants for solid phases in five dehydration equilibria at locality 1 indicates thata(H₂O) in the nongraphitic layers was 6 to 11 times greater thana(H₂O) in the graphitic layers. Similar calculations involving six dehydration equilibria at locality 2 show no difference ina(H₂O) between layers at the conditions of final equilibration. Initial differences in fluid composition maintained between the graphitic and nongraphitic layers caused the hbl+white mica reaction to occur at differentP-T conditions in different horizons of the schists.These data indicate that systematic differences in fluid composition were generated during metamorphism of the interlayered graphitic and non-graphitic schists but were subsequently homogenized at locality 2. The heterogeneities could initially have been produced while the rocks were in theP-T field of CO₂-H₂O immiscibility. Development of a penetrative, layer-parallel shear foliation at this time would have prevented subsequent mixing of the fluids across layers after temperatures exceeded the consolute temperature in the CO₂-H₂O system. Late-stage homogenization of fluids at locality 2 is thought to reflect loss of the buffer capacity of the mineral assemblage in response to total consumption of hornblende.     

The P-T-d development at the basement-cover boundary in the north-eastern Tauern Window (Eastern Alps): Alpine continental collision

Abstract At the basement-cover boundary of the north-eastern Tauern Window (Eastern Alps), the following Alpine P-T-d development has been reconstructed on the basis of macro- and micro-structures as well as preferred crystallographic orientations, mineral parageneses and compositions.
During increasing P-T conditions in the greenschist facies a first period of deformation produced imbrication of the basement gneisses and cover sediments, and then monoclinal folds up to the kilometre scale. Tectonic transport was continuously top-to-the-ENE. A second period of deformation began at about peak P-T conditions of 9 kbar and c. 540–560°C in the south, and about 7–9 kbar and 490–500° C in the north; this continued locally to lower temperature. During the second period, transport was continuously top-to-the-SE. Crystallographic orientations of white mica and plagioclase give particularly useful information on the kinematic framework. In addition, data on the ductile behaviour of dolomite and plagioclase can be inferred. At c. 7–9 kbar, dolomite recrystallization starts at 450–480° C, and the beginning of plagioclase recrystallization coincides with the oligoclase boundary.
In general, the Alpine geodynamic history of the basement-cover boundary may be related to continental collision processes between a northerly plate (European or Briançonnais) and a southerly (Adriatic) one. The first deformation period possibly reflects subduction of the gneiss-sediment boundary toward the WSW, to a depth of 31–32 km. The second period may be a result of obduction toward the NW, followed by late-stage uplift. Most of the basement domes of the eastern Tauern Window appear as a result of the final stage of the first deformation, formed prior to the peak of metamorphism, possibly partly influenced by the final collision between the northern and the southern continents.     

The DAV and Periadriatic fault systems in the Eastern Alps south of the Tauern window

Alpine deformation of Austroalpine units south of the Tauern window is dominated by two kinematic regimes. Prior to intrusion of the main Periadriatic plutons at ~30 Ma, the shear sense was sinistral in the current orientation, with a minor north-side-up component. Sinistral shearing locally overprints contact metamorphic porphyroblasts and early Periadriatic dykes. Direct Rb-Sr dating of microsampled synkinematic muscovite gave ages in the range 33-30 Ma, whereas pseudotachylyte locally crosscutting the mylonitic foliation gave an interpreted ⁴⁰Ar-³⁹Ar age of ~46 Ma. The transition from sinistral to dextral (transpressive) kinematics related to the Periadriatic fault occurred rapidly, between solidification of the earlier dykes and of the main plutons. Subsequent brittle-ductile to brittle faults are compatible with N-S to NNW-SSE shortening and orogen-parallel extension. Antithetic Riedel shears are distinguished from the previous sinistral fabric by their fine-grained quartz microstructures, with local pseudotachylyte formation. One such pseudotachylyte from Speikboden gave a ⁴⁰Ar-³⁹Ar age of 20 Ma, consistent with pseudotachylyte ages related to the Periadriatic fault. The magnitude of dextral offset on the Periadriatic fault cannot be directly estimated. However, the jump in zircon and apatite fission-track ages establishes that the relative vertical displacement was ~4-5 km since 24 Ma, and that movement continued until at least 13 Ma.     

Timing of metamorphism in the Tauern Window, Eastern Alps: Rb-Sr ages and fabric formation

The Peripheral Schieferhülle of the Tauern Window of the Eastern Alps represents post-Hercynian Penninic cover sequences and preserves a record of metamorphism in the Alpine orogeny, without the inherited remnants of Hercynian events that are retained in basement rocks. The temperature-time-deformation history of rocks at the lower levels of these cover sequences have been investigated by geochronological and petrographic study of units whose P-T evolution and structural setting are already well understood. The Eclogite Zone of the central Tauern formed from protoliths with Penninic cover affinities, and suffered early Alpine eclogite facies metamorphism before tectonic interposition between basement and cover. It then shared a common metamorphic history with these units, experiencing blueschist facies and subsequent greenschist facies conditions in the Alpine orogeny. The greenschist facies phase, associated with penetrative deformation in the cover and the influx of aqueous fluids, reset Sr isotopes in metasediments throughout the eclogite zone and cover schists, recording deformation and peak metamorphism at 28-30 Ma. The Peripheral Schieferhülle of the south-east Tauern Window yields Rb-Sr white mica ages which can be tied to the structural evolution of the metamorphic pile. Early prograde fabrics pre-date 31 Ma, and were reworked by the formation of the large north-east vergent Sonnblick fold structure at 28 Ma. Peak metamorphism post-dated this deformation, but by contrast to the equivalent levels in the central Tauern, peak metamorphic conditions did not lead to widespread homogenization of the Sr isotopes. Localized deformation continued into the cooling path until at least 23 Ma, partially or wholly resetting Sr white mica ages in some samples. These isotopic ages may be integrated with structural data in regional tectonic models, and may constrain changes in the style of crustal deformation and plate interaction. However, such interpretations must accommodate the demonstrable variation in thermal histories over small distances.     

Modes of orogen-parallel stretching and extensional exhumation in response to microplate indentation and roll-back subduction (Tauern Window, Eastern Alps)

The Tauern Window exposes a Paleogene nappe stack consisting of highly metamorphosed oceanic (Alpine Tethys) and continental (distal European margin) thrust sheets. In the eastern part of this window, this nappe stack (Eastern Tauern Subdome, ETD) is bounded by a Neogene system of shear (the Katschberg Shear Zone System, KSZS) that accommodated orogen-parallel stretching, orogen-normal shortening, and exhumation with respect to the structurally overlying Austroalpine units (Adriatic margin). The KSZS comprises a ≤5-km-thick belt of retrograde mylonite, the central segment of which is a southeast-dipping, low-angle extensional shear zone with a brittle overprint (Katschberg Normal Fault, KNF). At the northern and southern ends of this central segment, the KSZS loses its brittle overprint and swings around both corners of the ETD to become subvertical, dextral, and sinistral strike-slip faults. The latter represent stretching faults whose displacements decrease westward to near zero. The kinematic continuity of top-east to top-southeast ductile shearing along the central, low-angle extensional part of the KSZS with strike-slip shearing along its steep ends, combined with maximum tectonic omission of nappes of the ETD in the footwall of the KNF, indicates that north–south shortening, orogen-parallel stretching, and normal faulting were coeval. Stratigraphic and radiometric ages constrain exhumation of the folded nappe complex in the footwall of the KSZS to have begun at 23–21 Ma, leading to rapid cooling between 21 and 16 Ma. This exhumation involved a combination of tectonic unroofing by extensional shearing, upright folding, and erosional denudation. The contribution of tectonic unroofing is greatest along the central segment of the KSZS and decreases westward to the central part of the Tauern Window. The KSZS formed in response to the indentation of wedge-shaped blocks of semi-rigid Austroalpine basement located in front of the South-Alpine indenter that was part of the Adriatic microplate. Northward motion of this indenter along the sinistral Giudicarie Belt offsets the Periadriatic Fault and triggered rapid exhumation of orogenic crust within the entire Tauern Window. Exhumation involved strike-slip and normal faulting that accommodated about 100 km of orogen-parallel extension and was contemporaneous with about 30 km of orogen-perpendicular, north–south shortening of the ETD. Extension of the Pannonian Basin related to roll-back subduction in the Carpathians began at 20 Ma, but did not affect the Eastern Alps before about 17 Ma. The effect of this extension was to reduce the lateral resistance to eastward crustal flow away from the zone of greatest thickening in the Tauern Window area. Therefore, we propose that roll-back subduction temporarily enhanced rather than triggered exhumation and orogen-parallel motion in the Eastern Alps. Lateral extrusion and orogen-parallel extension in the Eastern Alps have continued from 12 to 10 Ma to the present and are driven by northward push of Adria.     

Zircon ages of high-grade gneisses in the Eastern Erzgebirge (Central European Variscides)—constraints on origin of the rocks and Precambrian to Ordovician magmatic events in the Variscan foldbelt

This study is an attempt to unravel the tectono-metamorphic history of high-grade metamorphic rocks in the Eastern Erzgebirge region. Metamorphism has strongly disturbed the primary petrological genetic characteristics of the rocks. We compare geological, geochemical, and petrological data, and zircon populations as well as isotope and geochronological data for the major gneiss units of the Eastern Erzgebirge; (1) coarse- to medium-grained “Inner Grey Gneiss”, (2) fine-grained “Outer Grey Gneiss”, and (3) “Red Gneiss”. The Inner and Outer Grey Gneiss units (MP–MT overprinted) have very similar geochemical and mineralogical compositions, but they contain different zircon populations. The Inner Grey Gneiss is found to be of primary igneous origin as documented by the presence of long-prismatic, oscillatory zoned zircons (540 Ma) and relics of granitic textures. Geochemical and isotope data classify the igneous precursor as a S-type granite. In contrast, Outer Grey Gneiss samples are free of long-prismatic zircons and contain zircons with signs of mechanical rounding through sedimentary transport. Geochemical data indicate greywackes as main previous precursor. The most euhedral zircons are zoned and document Neoproterozoic (ca. 575 Ma) source rocks eroded to form these greywackes. U–Pb-SHRIMP measurements revealed three further ancient sources, which zircons survived in both the Inner and Outer Grey Gneiss: Neoproterozoic (600–700 Ma), Paleoproterozoic (2100–2200 Ma), and Archaean (2700–2800 Ma). These results point to absence of Grenvillian type sources and derivation of the crust from the West African Craton. The granite magma of the Inner Grey Gneiss was probably derived through in situ melting of the Outer Grey Gneiss sedimentary protolith as indicated by geological relationships, similar geochemical composition, similar Nd model ages, and inherited zircon ages. Red Gneiss occurs as separate bodies within fine- and medium-grained grey gneisses of the gneiss–eclogite zone (HP–HT overprinted). In comparison to Grey Gneisses, the Red Gneiss clearly differs in geochemical composition by lower contents of refractory elements. Rocks contain long-prismatic zircons (480–500 Ma) with oscillatory zonation indicating an igneous precursor for Red Gneiss protoliths. Geochemical data display obvious characteristics of S-type granites derived through partial melting from deeper crustal source rocks. The obtained time marks of magmatic activity (ca. 575 Ma, ca. 540 Ma, ca. 500–480 Ma) of the Eastern Erzgebirge are compared with adjacent units of the Saxothuringian zone. In all these units, similar time marks and geochemical pattern of igneous rocks prove a similar tectono-metamorphic evolution during Neoproterozoic–Ordovician time.