Peak‐temperature patterns of polyphase metamorphism resulting from accretion,subduction and collision (eastern Tauern Window,European Alps) – a study with Raman microspectroscopy on carbonaceous material (RSCM)

Raman microspectroscopy on carbonaceous material (RSCM) from the eastern Tauern Window indicates contrasting peak‐temperature patterns in three different fabric domains, each of which underwent a poly‐metamorphic orogenic evolution: Domain 1 in the northeastern Tauern Window preserves oceanic units (Glockner Nappe System, Matrei Zone) that attained peak temperatures (T_p) of 350–480 °C following Late Cretaceous to Palaeogene nappe stacking in an accretionary wedge. Domain 2 in the central Tauern Window experienced T_p of 500–535 °C that was attained either within an exhumed Palaeogene subduction channel or during Oligocene Barrovian‐type thermal overprinting within the Alpine collisional orogen. Domain 3 in the Eastern Tauern Subdome has a peak‐temperature pattern that resulted from Eo‐Oligocene nappe stacking of continental units derived from the distal European margin. This pattern acquired its presently concentric pattern in Miocene time due to post‐nappe doming and extensional shearing along the Katschberg Shear Zone System (KSZS). T_p values in the largest (Hochalm) dome range from 612 °C in its core to 440 °C at its rim. The maximum peak‐temperature gradient (≤70 °C km^?1) occurs along the eastern margin of this dome where mylonitic shearing of the Katschberg Normal Fault (KNF) significantly thinned the Subpenninic‐ and Penninic nappe pile, including the pre‐existing peak‐temperature gradient.     

Evolution of Alpine eclogites in the Eastern Alps: Implications for Alpine geodynamics

In the Eastern Alps Alpine eclogites are generally associated with rocks of continental lithosphere, while eclogites that are associated with oceanic assemblages are restricted to minor exposures. Such eclogites are exposed both in the Penninic unit of the Tauern Window and in the Austroalpine nappe complex. (1) In the central southern part of the Tauern Window (Eclogite Zone) eclogites and associated high pressure metasediments of a distal continental margin are intercalated between Penninic basement units. A mylonitic eclogitic foliation and stretching lineation are contemporaneous to the high pressure metamorphism and are related to the subduction of distal Penninic continental margin sequences. Continuous subduction of cool lithosphere resulted in blueschist facies overprint of the whole Penninic nappe pile. (2) Within the Middle-AustroAlpine Koralm/Saualm region most eclogites are eclogitic mylonites documenting plastic deformation of omphacite and garnet. The meso- and macroscale structures indicate an overall extensional regime possibly related to a large-scale SE-directed ductile low-angle normal shear zone. The eclogites are associated with migmatite-like structures and are intruded by pegmatites. This indicates decreasing pressure, but isothermal or even increasing temperature conditions during exhumation.These relationships argue for the subduction of Penninic continental lithosphere in the foot-wall of the Austroalpine unit at the time of exhumation of the Koralm/Saualm eclogites. Formation of the Austroalpine eclogites is explained by subduction of continental lithosphere, and subsequent, rapid exhumation in an upper plate tectonic position within an extensional regime.     

Eoalpine tectonics of the Eastern Alps: implications from the evolution of monometamorphic Austroalpine units (Schneeberg and Radenthein Complex)

Monometamorphic metasediments of Paleozoic or Mesozoic age constituting Schneeberg and Radenthein Complex experienced coherent deformation and metamorphism during Late Cretaceous times. Both complexes are part of the Eoalpine high-pressure wedge that formed an intracontinental suture and occur between the polymetamorphosed Ötztal–Bundschuh nappe system on top and the Texel–Millstatt Complex below. During Eoalpine orogeny Schneeberg and Radenthein Complexes were south-dipping and they experienced a common tectonometamorphic history from ca. 115 Ma onwards until unroofing of the Tauern Window in Miocene times. This evolution is subdivided into four distinct tectonometamorphic phases. Deformation stage D1 is characterized by WNW-directed shearing at high temperature conditions (550–600°C) and related to the initial exhumation of the high-pressure wedge. D2 and D3 are largely coaxial and evolved during high- to medium-temperature conditions (ca. 450 to ≥550°C). These stages are related to advanced exhumation and associated with large-scale folding of the high-pressure wedge including the Ötztal-Bundschuh nappe system above and the Texel–Millstatt Complex below. For the area west of the Tauern Window, F2/F3 fold interference results in the formation of large-scale sheath-folds in the frontal part of the nappe stack (formerly called “Schlingentektonik” by previous authors). Earlier thrusts were reactivated during Late Cretaceous normal faulting at the base of the Ötztal–Bundschuh nappe system and its cover. Deformation stage D4 is of Oligo-Miocene age and accounted for tilting of individual basement blocks along large-scale strike-slip shear zones. This tilting phase resulted from indentation of the Southern Alps accompanied by the formation of the Tauern Window.     

Reconciling late faulting over the whole Alpine belt: from structural analysis to geochronological constrains

The significance of late-stage fracturing in the European Alps in a large geodynamic context is reappraised by studying brittle deformations over the entire belt. In the internal Western Alps, paleostress datasets display a major occurrence of orogen-parallel extension resulting in normal faulting and associated strike-slip mode. There the direction of subhorizontal extension rotates with the bending of the Alpine belt. In the Central Alps, paleostress tensors also indicate orogen-parallel extensional regimes, both in the Bergell area and the Lepontine Dome, where the brittle structures are associated with ductile structures related to the formation of large-scale upright folds that accommodate most of the collisional shortening due to the north-directed component of the movement of the South-Alpine indenter. This brittle deformation phase is of Miocene age and is coeval with the propagation of the Alpine front toward the external Alpine domains. In the Eastern Alps, brittle deformation of the Tauern Window displays an overwhelming occurrence of orogen-parallel normal faulting and associated strike-slip regimes again, which is inferred to be driven by lateral extrusion of the orogenic wedge toward the Pannonian basin, partly due to indentation on the Dolomites indenter. The major orogen-parallel extensional signal of the brittle Cenozoic deformations appears remarkably stable all over the internal Alps. Extensional brittle structures are part of a late phase of collisional deformation, during which the propagation of the Alpine front of the Western Alps and the northward movement of the Southern Alpine and the Dolomites indenters in the Central and Eastern Alps were accommodated by orogen-parallel extension in the inner zones, at the scale of the entire chain.     

Formation, subduction, and exhumation of Penninic oceanic crust in the Eastern Alps: time constraints from Ar/Ar geochronology

New ⁴⁰Ar/³⁹Ar geochronology places time constraints on several stages of the evolution of the Penninic realm in the Eastern Alps. A 186±2 Ma age for seafloor hydrothermal metamorphic biotite from the Reckner Ophiolite Complex of the Pennine–Austroalpine transition suggests that Penninic ocean spreading occurred in the Eastern Alps as early as the Toarcian (late Early Jurassic). A 57±3 Ma amphibole from the Penninic subduction–accretion Rechnitz Complex dates high-pressure metamorphism and records a snapshot in the evolution of the Penninic accretionary wedge. High-pressure amphibole, phengite, and phengite+paragonite mixtures from the Penninic Eclogite Zone of the Tauern Window document exhumation through ≤15 kbar and >500 °C at 42 Ma to 10 kbar and 400 °C at 39 Ma. The Tauern Eclogite Zone pressure–temperature path shows isothermal decompression at mantle depths and rapid cooling in the crust, suggesting rapid exhumation. Assuming exhumation rates slower or equal to high-pressure–ultrahigh-pressure terrains in the Western Alps, Tauern Eclogite Zone peak pressures were reached not long before our high-pressure amphibole age, probably at ≤45 Ma, in accordance with dates from the Western Alps. A late-stage thermal overprint, common to the entire Penninic thrust system, occurred within the Tauern Eclogite Zone rocks at 35 Ma. The high-pressure peak and switch from burial to exhumation of the Tauern Eclogite Zone is likely to date slab breakoff in the Alpine orogen. This is in contrast to the long-lasting and foreland-propagating Franciscan-style subduction–accretion processes that are recorded in the Rechnitz Complex.     

The Tauern Window (Eastern Alps, Austria): a new tectonic map, with cross-sections and a tectonometamorphic synthesis

We present a tectonic map of the Tauern Window and surrounding units (Eastern Alps, Austria), combined with a series of crustal-scale cross-sections parallel and perpendicular to the Alpine orogen. This compilation, largely based on literature data and completed by own investigations, reveals that the present-day structure of the Tauern Window is primarily characterized by a crustal-scale duplex, the Venediger Duplex (Venediger Nappe system), formed during the Oligocene, and overprinted by doming and lateral extrusion during the Miocene. This severe Miocene overprint was most probably triggered by the indentation of the Southalpine Units east of the Giudicarie Belt, initiating at 23–21 Ma and linked to a lithosphere-scale reorganization of the geometry of mantle slabs. A kinematic reconstruction shows that accretion of European lithosphere and oceanic domains to the Adriatic (Austroalpine) upper plate, accompanied by high-pressure overprint of some of the units of the Tauern Window, has a long history, starting in Turonian time (around 90 Ma) and culminating in Lutetian to Bartonian time (45–37 Ma).     

P-T data of Ky-St-Grt gneisses hosting HP amphibolites and eclogites from the Austroalpine Polinik complex, Kreuzeck Mountains, Eastern Alps, Austria

This study focuses on metapelites of the Polinik complex in the Kreuzeck Mts. southeast of the Tauern Window, Eastern Alps, where kyanite — staurolite — garnet gneisses host eclogites and high pressure (HP) amphibolites of the Austroalpine basement. The stable mineral assemblage is garnet — staurolite — biotite — kyanite — quartz. Estimated metamorphic conditions from conventional geothermobarometry are 654±30 °C and 0.9±0.08 GPa, and Average P-T values calculated by THERMOCALC, are 665±15 °C at 0.77±0.09 GPa. Formation of the present mineral association in gneisses is related to the exhumation (D2) stage of hosted eclogites/HP amphibolites within a lateral strike-slip zone.     

Late stage differential exhumation of crustal blocks in the central Eastern Alps: evidence from fission track and (U–Th)/He thermochronology

Thermal history modelling based on zircon‐ and apatite fission track and apatite (U–Th)/He data constrain and refine the near‐surface exhumation of the south‐eastern Tauern Window (Penninic units) and neighbouring Austroalpine basement units in the Eastern Alps. Fast exhumation on both sides of the Penninic/Austroalpine boundary coincides with a period of lateral extrusion and tectonic denudation of the Penninic units in Miocene time (22–12 Ma). The jump to older ages occurs within the Austroalpine unit along the Polinik fault, which therefore defines the boundary between the tectonically denuded units and the hangingwall at that time. According to the different (U–Th)/He ages between the Penninic Hochalm‐ and Sonnblick Domes we demonstrate a differential cooling history of these two domes in the latest Miocene and early Pliocene.     

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 Ötztal 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.     

Rb/Sr geochronology in the Eastern Alps

New Rb/Sr data on mineral and whole rock samples from in and around the south-east corner of the Tauern Window are presented. Pennine orthogeneisses yield an Rb/Sr whole rock age of 279±9 m.y., while orthogneiss samples from the Altkristallin Sheet near Innerkrems, Carinthia, yield an age of 381±30 m.y. by the same technique. The apparent mineral age break across the margins of the Tauern Window is investigated in an area of good structural and petrofabric control. The post-Palaeozoic history of the Eastern Alps is then discussed in the context of the available Rb/Sr data. It is argued that the bulk of the Katschberg Phyllites are of pre-Mesozoic age; that the major overthrusting movements of the Austroalpine Units were completed by 60–65 m.y.; and that the Peri-Adriatic intrusives can be little older than middle Tertiary.     

Crustal-scale extension in a convergent orogen : the Sterzing-Steinach mylomte zone in the Eastern Alps

Abstract

The western margin of the Tauera Window (Eastern Alps) is defined by a low angle westward dipping fault zone of potently We disp lacement. Ductile deformation of the fault rocks results in a carpet of mylonites up to 400 metres thick. Evidence from shear criteria and the excision of part of the Cretaceous-Tertiary metamorphic edifice both indicate normal displacements, and relative movement of Austroalpine nappe complex towards the west. The Sterzing-Steinach mylonite zone overprints the Alpine nappe edifice. Movements occurred on the cooling path of the Tauern metamorphism, and may be as recent as Middle Miocene.

The Kinematics and geometry of the mylonite zone constrain two likely t ectonic explanations that are both compatible with secondary thining of a thick orogenic wedge. (1) Ute the Austroalpine nappe pile due to tectonic unroofing of the Tauern window. (2) Continental escape by east-west stretching of the Alpine orogenic wedge in response to continental collision.     

Mapping of the post-collisional cooling history of the Eastern Alps

We present a database of geochronological data documenting the post-collisional cooling history of the Eastern Alps. This data is presented as (a) georeferenced isochrone maps based on Rb/Sr, K/Ar (biotite) and fission track (apatite, zircon) dating portraying cooling from upper greenschist/amphibolite facies metamorphism (500–600 °C) to 110 °C, and (b) as temperature maps documenting key times (25, 20, 15, 10 Ma) in the cooling history of the Eastern Alps. These cooling maps facilitate detecting of cooling patterns and cooling rates which give insight into the underlying processes governing rock exhumation and cooling on a regional scale.The compilation of available cooling-age data shows that the bulk of the Austroalpine units already cooled below 230 °C before the Paleocene. The onset of cooling of the Tauern Window (TW) was in the Oligocene-Early Miocene and was confined to the Penninic units, while in the Middle- to Late Miocene the surrounding Austroalpine units cooled together with the TW towards near surface conditions.High cooling rates (50 °C/Ma) within the TW are recorded for the temperature interval of 375–230 °C and occurred from Early Miocene in the east to Middle Miocene in the west. Fast cooling post-dates rapid, isothermal exhumation of the TW but was coeval with the climax of lateral extrusion tectonics. The cooling maps also portray the diachronous character of cooling within the TW (earlier in the east by ca. 5 Ma), which is recognized within all isotope systems considered in this study.Cooling in the western TW was controlled by activity along the Brenner normal fault as shown by gradually decreasing ages towards the Brenner Line. Cooling ages also decrease towards the E–W striking structural axis of the TW, indicating a thermal dome geometry. Both cooling trends and the timing of the highest cooling rates reveal a strong interplay between E–W extension and N–S orientated shortening during exhumation of the TW.     

Fault slip analysis in the Koralm Massif (Eastern Alps) and consequences for the final uplift of “cold spots” in Miocene times

The Paleogene and Neogene evolution of Austroalpine basement units east of the Tauern Window is characterised by the formation of two major sets of faults: (1) ESE–WNW- to E–W-trending faults, associated with ENE- and NNW-trending conjugate structures and (2) N–S to NNE-SSW striking structures, mainly acting as high-angle normal faults, often associated with E-dipping low-angle normal faults along the western margin of the Styrian Basin.Together with the stratigraphic evolution of the Styrian and Lavanttal Basins and the related subsidence histories a tectonic evolution may be reconstructed for this part of the Eastern Alps. In the southern part of the Koralm Massif, WNW-trending fractures were activated as dextral strike-slip faults, associated with the evolution of WNW-trending troughs filled up with coarse block debris. W- to WNW-trending fractures were reactivated as normal faults, indicating N–S extension. It is assumed that these phases resulted in subsidence and block debris sedimentation in Karpatian and Badenian times (ca. 17–13 Ma).In the Western Styrian Basin no Sarmatian (13–11.5 Ma) sediments are observed; Pannonian (11.5 to 7.1 Ma) sediments are restricted to the Eastern Styrian Basin. This indicates, that the Koralm basement and the Western Styrian Basin were affected by post-Sarmatian uplift, coinciding with a re-activation of N-trending normal faults along the eastern margin of the Koralm Massif. Therefore, we suggest that the final uplift of the Koralm Complex, partly together with the Western Styrian Basin, occurred during the early Pannonian (at approximately 10 Ma). The elevation of clastic deposits indicates that the Koralm Complex was elevated by approximately 800 m during this phase, associated with an additional phase of E–W-directed extension accommodated by N–S striking normal faults.     

Miocene emplacement and rapid cooling of the Pohorje pluton at the Alpine-Pannonian-Dinaridic junction,Slovenia

New laser ablation-inductive coupled plasma-mass spectrometry U-Pb analyses on oscillatory-zoned zircon imply Early Miocene crystallization (18.64 ± 0.11 Ma) of the Pohorje pluton at the southeastern margin of the Eastern Alps (northern Slovenia). Inherited zircon cores indicate two crustal sources: a late Variscan magmatic population (~270–290 Ma), and an early Neoproterozoic one (850–900 Ma) with juvenile Hf isotope composition close to that of depleted mantle. Initial εHf of Miocene zircon points to an additional, more juvenile source component of the Miocene magma, which could be either a juvenile Phanerozoic crust or the Miocene mantle. The new U-Pb isotope age of the Pohorje pluton seriously questions its attribution to the Oligocene age ‘Periadriatic’ intrusions. The new data imply a temporal coincidence with 19–15 Ma magmatism in the Pannonian Basin system, more specifically in the Styrian Basin. K-Ar mineral- and whole rock ages from the pluton itself and cogenetic shallow intrusive dacitic rocks (~18–16 Ma), as well as zircon fission track data (17.7–15.6 Ma), gave late Early to early Middle Miocene ages, indicating rapid cooling of the pluton within about 3 Million years. Medium-grade Austroalpine metamorphics north and south of the pluton were reheated and subsequently cooled together. Outcrop- and micro scale structures record deformation of the Pohorje pluton and few related mafic and dacitic dykes under greenschist facies conditions. Part of the solidstate fabrics indicate E–W oriented stretching and vertical thinning, while steeply dipping foliation and NW–SE trending lineation are also present. The E–W oriented lineation is parallel to the direction of subsequent brittle extension, which resulted in normal faulting and tilting of the earlier ductile fabric at around the Early / Middle Miocene boundary; normal faulting was combined with strike-slip faulting. Renewed N–S compression may be related to late Miocene to Quaternary dextral faulting in the area. The documented syn-cooling extensional structures and part of the strike-slip faults can be interpreted as being related to lateral extrusion of the Eastern Alps and/or to back-arc rifting in the Pannonian Basin.     

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     

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-Schönach basin. In the more central Pfitsch-Mörchner 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.     

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.     

Exhumation of the Tauern window,Eastern Alps

The exhumation of metamorphic domes within orogenic belts is exemplified by the Tauern window in the Eastern Alps. There, the exhumation is related to partitioning of final orogenic shortening into deep-seated thrusts, near-surface antiformal bending forming brachyanticlines, and almost orogen-parallel strike-slip faults due to oblique continental plate collision. Crustal thickening by formation of an antiformal stack within upper to middle crustal portions of the lower lithosphere is a prerequisite of late-stage orogenic window formation. Low-angle normal faults at releasing steps of crustal-scale strike-slip faults accomodate tectonic unloading of synchronously thickened crust and extension along strike of the orogen, forming pull-apart metamorphic domes. Initiation of low-angle normal faults is largely controlled by rock rheology, especially at the brittle-ductile transitional level within the lithosphere. Several mechanisms may contribute to uplift and exhumation of previously buried crust within such a setting: (1) Shortening along deep-seated blind thrusts results in the formation of brachyanticlines and bending of metamorphic isograds; (2) oversteps of strike-slip faults within the wrench zone control the final geometry of the window; (3) unloading by tectonic unroofing and erosional denudation; and (4) vertical extrusion of crustal scale wedges. Rapid decompression of previously buried crust results in nearly isothermal exhumation paths, and enhanced fluid circulation along subvertical tensile fractures (hydrothermal ore and silicate veins) that formed due to overall coaxial stretching of lower plate crust.     

Late Cretaceous, synorogenic, low-angle normal faulting along the Schlinig fault (Switzerland, Italy, Austria) and its significance for the tectonics of the Eastern Alps

The Schlinig fault at the western border of theÖtztal nappe (Eastern Alps), previously interpreted as a west-directed thrust, actually represents a Late Cretaceous, top-SE to -ESE normal fault, as indicated by sense-of-shear criteria found within cataclasites and greenschist-facies mylonites. Normal faulting postdated and offset an earlier, Cretaceous-age, west-directed thrust at the base of theÖtztal nappe. Shape fabric and crystallographic preferred orientation in completely recrystallized quartz layers in a mylonite from the Schlinig fault record a combination of (1) top-east-southeast simple shear during Late Cretaceous normal faulting, and (2) later north-northeast-directed shortening during the Early Tertiary, also recorded by open folds on the outcrop and map scale. Offset of the basal thrust of theÖtztal nappe across the Schlinig fault indicates a normal displacement of 17 km. The fault was initiated with a dip angle of 10° to 15° (low-angle normal fault). Domino-style extension of the competent Late Triassic Hauptdolomit in the footwall was kinematically linked to normal faulting.

The Schlinig fault belongs to a system of east- to southeast-dipping normal faults which accommodated severe stretching of the Alpine orogen during the Late Cretaceous. The slip direction of extensional faults often parallels the direction of earlier thrusting (top-W to top-NW), only the slip sense is reversed and the normal faults are slightly steeper than the thrusts. In the western Austroalpine nappes, extension started at about 80 Ma and was coeval with subduction of Piemont-Ligurian oceanic lithosphere and continental fragments farther west. The extensional episode led to the formation of Austroalpine Gosau basins with fluviatile to deep-marine sediments. West-directed rollback of an east-dipping Piemont-Ligurian subduction zone is proposed to have caused this stretching in the upper plate.     

Using white mica 40Ar/39Ar data as a tracer for fluid flow and permeability under high‐P conditions: Tauern Window,Eastern Alps

New single‐grain‐fusion muscovite and paragonite ⁴⁰Ar/³⁹Ar data from eclogite and blueschist units exposed in the Tauern Window, Eastern Alps yield a range of apparent ages from 90 to 23 Ma. These apparent ages are generally older than expected for ⁴⁰Ar/³⁹Ar cooling ages, given constraints from other geochronological systems such as Rb–Sr and U–Pb. Numerical Ar‐in‐muscovite diffusion models for Tauern Window nappe P–T paths in an open system suggest that ⁴⁰Ar/³⁹Ar ages should lie between 29 and 24 Ma, and that they should constrain cooling and decompression following the post‐high pressure Barrovian overprint. The measured ranges of apparent ⁴⁰Ar/³⁹Ar dates suggest that the assumption of open system behaviour is not valid for this region. The local and/or regional generation of fluid during exhumation promoted pervasive recrystallization of high pressure lithologies throughout the Tauern Window to greenschist and amphibolite facies assemblages. The old apparent ⁴⁰Ar/³⁹Ar white mica dates in all lithologies are therefore interpreted as being due to inefficient removal of grain boundary Ar by the grain boundary fluids during the Barrovian overprint, due to high Ar concentrations or limited connectivity or both. This caused spatially (mm‐scale) and temporally variable fluxes of Ar out of, and probably into, white mica in both metasedimentary and metabasic lithologies.