pre alpine history of the pennine zone in the tauern window austria u pb and rb sr geochronology zircon ages from major lithologies of the zentralgneis suggest that much of the variscan magmatism in the tauern window is older than previously suggested in the southeast tauern window a tonalite has been dated at 314 7 m y and a granodioritic biotite augen gneiss at 313 10 m y two granodiorites from the granatspitzkern yielded zircon data consistent with a similar age these zircon data require re interpretation of some previously published rb sr whole rock ages and raise the possibility that alpine metamorphism caused more widespread disturbance of rb sr whole rocks than commonly supposed

formation subduction and exhumation of penninic oceanic crust in the eastern alps time constraints from 40ar 39ar geochronology abstract 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

carbo permian pteridophyll leaf fragments from an amphibolite facies basement tauern window austria graphitic schists from the pennine basement tauern window austria contain fragments of pteridophyll leaves they are preserved despite the overall strong deformation and metamorphism at amphibolite facies conditions 525 c 750 100 mpa due to a particular strain path with volume loss by pressure solution they were determined as cf callipteridium sp indicating a permo carboniferous age this depositional age is similar to radiometric age data obtained for the intrusion of nearby metagranitoids indicating either tectonic contacts between the metagranitoids and the graphite schists or a very active and rapidly changing tectonic regime during the hercynian event

a new detrital mica 40ar 39ar dating approach for provenance and exhumation of the eastern alps detrital 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

eastern alpine tectono metamorphic evolution constraints from two dimensional p t t modeling we use two dimensional 2 d p t t modeling to constrain the thermal and rheological aspects of different scenarios for the late mesozoic and cenozoic tectonic evolution of the eastern alps inferred from excellent data sets from the tauern window tw models invoking subduction of the south penninic sp oceanic lithosphere during thrusting and subsequent erosion of the austro alpine aa upper plate nappe stack are inconsistent with the observed thermal evolution within the aa and penninic units in these models predictions for the aa peak thermal conditions are lower than observed after exhumation and cooling to midcrustal levels and subduction of the continental middle penninic mp block the aa undergoes a phase of renewed heating to almost the previous peak temperatures simultaneously the penninic units experience a phase of heating upon subduction followed by cooling after onset of subduction of the north penninic np basin the model predictions are inconsistent with the observed nearly isothermal uplift path of the sp after subduction and cannot explain observed inverted metamorphic peak conditions in the deeper aa amphibolite facies down to the higher penninic unit greenschist facies a model with the beginning of subduction of the sp occurring after crustal thickening of the aa and subsequent return to normal crustal thicknesses is compatible with the p t t data in this model peak temperature conditions are higher in the aa followed by a phase of strong cooling in the aa upper plate with the onset of underthrusting this model also explains successfully nearly isothermal exhumation of the mp and inverted metamorphic peak conditions in the deeper aa material accreted to the hanging wall from the oceanic crust sp experiences a phase of cooling during ongoing subduction of oceanic lithosphere and begins to heat up to its thermal climax after the subduction of trailing continental lithosphere the subsequent pt path of the penninic units strongly depends on the timing and rates of underthrusting by the foreland observed pt paths in the mp within the tw require continuous subduction of the np and the trailing european foreland under the exhuming mp block documented rapid cooling in the final uplift phase of the penninic units in the tw requires exhumation rates up to approximately 4 mm yr predictions of slightly elevated present day geothermal gradients in the tw area are consistent with available heat flow data as a result of mesozoic rifting followed by late mesozoic crustal thickening of the aa paleorheological reconstructions are characterized by a contrast between relatively strong oceanic lithosphere and adjacent weak continental lithosphere predicted decoupling of weak continental and oceanic lithosphere during subsequent subduction of the penninic units can explain observed late cretaceous crustal extension in the aa units in terms of gravitational spreading ongoing subduction leads to an overall strength increase due to underthrusting of cool oceanic lithosphere whereas subduction of continental lithosphere causes a strength decrease in the upper levels of the lithosphere continuous crustal thickening and relaxation of the depressed isotherms reduce the strength of the lower lithospheric mantle beneath the central orogen further enhanced by rapid late stage uplift predictions for the present day rheological structure of the eastern alps support the existence of a strong upper crustal layer two wedge shaped strong upper mantle layers to the north and the south of the orogen and a weak upper mantle underlying the central orogen

lu hf dating petrography and tectonic implications of the youngest alpine eclogites tauern window austria abstract isotopic dating of metamorphic minerals places fundamental constraints on the rates and mechanisms of burial and exhumation in collisional orogens the eclogite zone in the tauern window has been the focus of many studies on subduction related high pressure metamorphism however the age and duration of the high pressure stage remains the subject of ongoing debate 32 ma sr rb ages interpreted to date eclogite facies metamorphism glodny et al 2005 appear too young in traditional tectonic reconstructions of plate collision in the alps these ages have either been interpreted to indicate extremely rapid exhumation from more than 60 kilometre depth to mid crustal levels within 1 ma years or to date retrogression subsequent to high pressure metamorphism we present element distribution maps and lutetium hafnium lu hf garnet ages of three samples from the eclogite zone all samples display almost unaltered eclogite facies assemblages and garnets preserve growth zoning lu hf ages are thus considered as formation ages recording metamorphism towards peak pressure conditions in the sample with the smallest grain size garnet shows regular bell shaped element distributions with respect to manganese and the iron magnesium ratio a six point isochron of this sample yields 32 8 0 5 ma mswd 1 06 interpreted as the age of alpine eclogite facies metamorphism in one of the other two coarser grained samples garnet chemistry is identical the third sample however shows complex zoning in large garnet crystals cores with a very low iron magnesium ratio are surrounded by a second garnet generation which is very similar to the alpine generation in the other two samples the two coarser grained samples yield scattered ages between 26 9 9 8 ma and 62 7 1 8 ma for individual garnet whole rock pairs as the analysed garnet fractions display very different 176hf 177hf vs 176lu 177hf ratios this scatter reflects varying degrees of mixing between alpine and pre alpine garnet fractions as represented by the cores of the third sample the results confirm the rb sr whole rock ages of glodny et al 2005 despite the problems this result causes for conventional tectonic reconstructions the eclogites from the eclogite zone in the tauern window have to be considered as lower oligocene in age and are thus the youngest eclogites of the alps identified so far

slab breakoff a model for syncollisional magmatism and tectonics in the alps slab breakoff is the buoyancy driven detachment of subducted oceanic lithosphere from the light continental lithosphere that follows it during continental collision in a recent paper davies and von blanckenburg 1994 have assessed the physical conditions leading to breakoff by quantitative thermomechanical modeling and have predicted various consequences in the evolution of mountain belts breakoff will lead to heating of the overriding lithospheric mantle by upwelling asthenosphere melting of its enriched layers and thus to bimodal magmatism breakoff will also lead to thermal weakening of the subducted crustal lithosphere thereby allowing buoyant rise of released crustal slices from mantle depths in this paper we present a test of this model in the tertiary evolution of the european alps in the alps both basaltic and granitoid magmatism occur between 42 and 25 ma following the closure of oceanic basins by subduction and continental collision the granitoids are now well established to result from mixing of basalt with assimilated continental crust to identify the tectonically crucial origin of the partial mantle melts we have compiled all published geochemical and isotopic data of numerous mafic dykes occurring throughout the whole alpine arc their trace element and isotopic composition suggests that they have been formed by low degree melting of the mechanically stable lithospheric mantle we see no evidence for melting of asthenospheric mantle it was thus not decompressed to depths shallower than 50 km once initiated rapid lateral migration of slab breakoff will result in a linear trace of magmatism in locally thermal weakened crust this explains why all alpine magmatic rocks intruded almost synchronously along a strike slip fault the periadriatic lineament a compilation of ages from penninic high pressure rocks subducted to depths of up to 100 km shows that subduction took place at circa 55 40 ma followed by uplift at 40 35 ma from the short time interval between their uplift and the onset of magmatism we infer that both processes have been induced by the breakoff the slab breakoff model fulfills its predictions in the case of the alps and therefore supports the assumptions made in the theoretical model on a geological basis we believe that the characteristic association of magmatic activity with the return of high pressure rocks to the surface allows the identification of this process in the earth s mountain belts

deformation inside a paleosubduction channel insights from microstructures and crystallographic preferred orientations of eclogites and metasediments from the tauern window austria the eclogite zone of the tauern window is an exhumed subduction channel comprising eclogites with different grades of retrogression in a matrix of high pressure metasediments the rocks were exposed to 600 c and 20 25 kbars and then retrogressed during their exhumation first under blueschist facies and later under amphibolite facies metamorphism to gain insights into the deformation within the subduction channel during subduction and exhumation both fresh and retrogressed eclogites as well as the surrounding metasediments were investigated with respect to their deformation microstructures and crystallographic preferred orientations cpos pristine and retrogressed eclogites show grain boundary migration and subgrain rotation recrystallization microstructures in omphacite a misorientation axes analysis reveals the activity of complementary deformation mechanisms including grain boundary sliding and dislocation creep the omphacite cpos of the eclogites correspond to dominant sl fabrics characteristic of plane strain deformation though there are local variations towards flattening or constriction within the paleosubduction channel the glaucophane cpos in retrogressed eclogites match those of omphacite suggesting that a constant strain geometry persisted during exhumation at blueschist facies conditions plastic deformation of the host high pressure metasediments outlasted that of the eclogites as indicated by white mica fabrics and quartz cpo the latter is consistently asymmetric pointing to the operation of non coaxial deformation the microstructures and cpo data indicate a continuous plastic deformation cycle with eclogite and blueschist facies metamorphism related to subduction and exhumation of the different rock units

cooling and exhumation of the rieserferner pluton eastern alps italy austria the defereggen antholz vals dav line marks the southern border of alpine greenschist facies metamorphism within the austroalpine basement to the south of the western tauern window during the oligocene sinistral strike slip coeval with vertical displacement was accompanied by the syntectonic emplacement of the rieserferner pluton along the north side of the fault to the north of the dav line rb sr as well as k ar biotite cooling ages continuously increase from 16 ma at the rensen pluton in the west to 30 ma at the eastern end of the rieserferner pluton similar trends are apparent from zircon and apatite fission track data which increase eastward from 19 to 29 ma and 13 to 16 ma respectively low cooling rates allow a direct transformation into exhumation rates assuming an average geothermal gradient of 28 c km 1 the resulting exhumation rates of less than 1 mm year 1 do not affect the depth of isotherms in a geologically significant manner hence variable cooling rates within the basement north of the dav line are a result of differential exhumation of individual blocks high exhumation rates bracketed between the intrusion age of the rieserferner pluton at 31 3 ma and the apatite partial annealing zone paz range between 0 4 and 0 6 mm year 1 whereas final exposure takes place at reduced exhumation rates of 0 2 mm year 1 east side down tilting of the block north of the dav line as indicated by variations in exhumation rates strongly affects the western part of the area studied between the rensen and the central rieserferner pluton whereas further east block rotation becomes less important a total difference in exhumation of about 2 2 km corresponding to 5 tilting between the western and eastern rieserferner as indicated by k ar biotite and zircon fission track data occurred before going through the paz for apatite fission tracks furthermore the significance of the cooling ages especially for biotite is tested by calculating a finite element approximation different numerical models indicate fast cooling of the intrusive melts prior to approaching the closure temperature of the k ar system in biotite

pressure temperature time evolution of high pressure rocks of the acatlan complex southern mexico implications for the evolution of the iapetus and rheic oceans comment vega granillo et al 2007 1 are to be congratulated for publishing high quality pressure temperature time p t t data on the high pressure hp rocks within the acatlan complex fig 1 2 these data expand on previously published p t data of meza figueroa et al 2005 3 the

and mg fe si al2 compositional control on argon behaviour in high pressure white micas a continuous laser probe study from the dora maira nappe of the internal western alps italy abstract high pressure white micas from a restricted area in the dora maira nappe of the western alps italy were analyzed by the conventional 40 ar 39 ar bulk sample step heating method and by the step heating and the spot fusion 40 ar 39 ar continuous laser probe techniques on single grains high si ferich phengites from fine grained gneisses display homogeneous 35 40 ma plateau and spot fusion ages consistent with the upper eocene 40 ar 39 ar phengite ages documented over the whole internal western alps in contrast phengite separates and single grains of high si and mg rich to intermediate composition selected from metapelitic and mg rich mica schist samples yield older discordant apparent ages even when sampled in the same outcrop as the fine grained gneisses these local age variations are interpreted to reflect primarily an increasing ar retentivity with increasing mg content in phengites in metapelite derived white micas the isotopic discordance is marked by 1 1 a characteristic bulk sample upward convex spectrum shape 2 2 single grain disturbed age spectra 3 3 a systematic radially outward decrease in spot ages across individual mica cleavage planes and 4 4 strong grain to grain spot age variations in rock sections 1 1 in mg rich mica schists the isotopic discordance is observed as a reproducible upward convex bulk sample age spectrum with high 140 ma intermediate apparent ages 2 2 upward convex single grain age spectra and 3 3 irregular intra and intergrain age gradients on geochemical grounds it is argued that the isotopic discordance occurring in the metapelites is mainly controlled by the compositional changes introduced by the mg fe si al2 celadonitic re equilibration of the high pressure phengites during greenschist overprinting in mg rich phengites this chemical re equilibration may induce ar loss below their presumed higher blocking temperature a model of ar behaviour in high pressure white micas is proposed which explains the age variations recorded throughout the different lithologies in terms of a simple interaction between the mg fe compositional control on ar retentivity and the mg fe si al2 modifications occurring in phengites during progressive overprinting

40ar 39ar dating of white micas from an alpine high pressure metamorphic belt on naxos greece the resetting of the argon isotopic system overprinting of white micas from high pressure low to medium temperature m1 metamorphic assemblages in pelitic schists on naxos during subsequent thermal dome m2 metamorphism ranges from minor in the southeast of the island to complete recrystallization in the amphibolite facies rocks near the migmatites in the centre of the dome the original m1 minerals are phengites si4 6 7 7 0 and the overprinting minerals are muscovites si4 6 0 6 45 40ar 39ar step heating analyses of white mica separates from rocks in the area where phengite and muscovite occur together yield complex age spectra characterized by low apparent ages in the first and the last stages of gas release and high apparent ages in between these upward convex age spectra are shown to be caused by mixing of two generations of micas each of which has a different age spectrum and argon release pattern seemingly good plateaus in some age spectra from white micas of the area must be interpreted as providing meaningless intermediate ages further the upward convex age spectra have been used to trace the isotopic signature of phengites toward increasing m2 metamorphic grade and suggest that as long as phengites can be observed in the rocks upward convex age spectra occur on naxos crystallization of muscovite at the expense of phengite appears to be the main mechanism of resetting argon isotopic ages in white micas however there is also good evidence for argon loss by volume diffusion from phengites simple diffusion calculations suggest that the m2 metamorphism was caused by a shortlived heat source

post collisional orogen parallel large scale extension in the eastern alps abstract large scale extension affected the eastern alps during post collisional lateral tectonic extrusion in early and middle miocene times the tauern window was mainly tectonically exhumed by the 160 km pullapart of the rigidly behaving austroalpine basement blocks forming the tectonic lid of the penninic contents of the window an evaluation of the syn extrusion fault pattern reveals displacements of several tens of kilometers along the low angle extensional shear zones at the western and eastern margins of the window and along the important strike slip fault zones north and south of the window large scale shear is reflected by discontinuities in mineral cooling patterns across important shear zones the cooling histories of the tectonic blocks show that the window boundaries during exhumation coincide only partly with the classical boundaries of the window defined by thrust units reconstruction of the exhumation history shows an asymmetric evolution of the tauern window we argue that the eastern and western low angle extensional shear zones formed in sequence which is reflected by different evolutions of the fragmentation and uplift history of the austraolpine tectonic blocks to the east and west of the window in the central and eastern alps three structural domes the lepontin dome in the central alps and the tauern and rechnitz domes in the eastern alps were formed by large scale extension in miocene times total e w stretch amounts to more than 300 km and results in 70 extension we propose that the central and eastern alps are part of an extensional province that also includes the pannonian basin and has a counterpart separated by the moesian indenter in the rhodope mountains the aegean sea and the menderes massif in asia minor in these regions a number of mainly tectonically exhumed domes or deep sedimentary basins formed by miocene large scale extension

single grain argon laser probe dating of phengites from the blueschist to greenschist transition on sifnos cyclades greece measurements of 40ar 39ar age spectra on single white mica crystals of tertiary age is a new means of obtaining detailed geochronological information of alpine blueschists internal consistency of the data set as well as excellent agreement with previous conventional k ar results demonstrates that this new technique can be used with confidence to obtain information on the thermal evolution of young metamorphic belts

auriferous arsenopyrite pyrite and stibnite mineralization from the siflitz guginock area austria indications for hydrothermal activity during tertiary oblique terrane accretion in the eastern alps

constraints on the duration of high pressure metamorphism in the tauern window from diffusion modelling of discontinuous growth zones in eclogite garnet an eclogite sample from the grossgockner region of the hohe tauern austria contains garnet with a pronounced compositional discontinuity between a mn rich core and an fe rich rim this jump in composition was caused by a garnet consuming reaction followed by growth of the garnet rim omphacite and marks the prograde transition from epidote amphibolite to eclogite facies metamorphism garnet growth ended at peak metamorphic conditions of 570 c 17 kbar but intracrystalline diffusion continued until about 450 c 4 kbar on the retrograde path this garnet overgrowth texture represents a natural diffusion couple and a time span of 1 myr was calculated from the diffusion profile developing out of the original sharp compositional step for typical crustal densities this time corresponds to a minimum average velocity in the range 4 6 7 4 cm yr 1 for vertical movement which is one of the fastest exhumation rates reported the diffusion of all divalent cations of four profiles was modelled both analytically and numerically both approaches gave comparable results but the times computed for each element were always discrepant up to a factor of 2 variations of diffusion coefficients within 2 in analytical calculations remedied this and gave consistent upper time limits numerical modelling does not require the simplifications introduced in the analytical approach on the other hand error propagation was computationally unfeasible with this method

thermal effects of exhumation of a metamorphic core complex on hanging wall syn rift sediments an example from the rechnitz window eastern alps the removal of overburden by tectonic processes displaces the rock masses and their temperature with respect to the surface consequently high cooling rates and increased heat flow have been reported from rapidly exhuming terrains previous studies were mainly focused on the greenschist and amphibolite facies tectono thermal evolution of the footwall a two dimensional numerical thermal model presented in this study concentrates on the thermal history of the hanging wall and the footwall blocks and evaluates the variation of the near surface heat flow above an evolving metamorphic core complex based on structural thermobarometric and geochronological constraints from the rechnitz core complex of the eastern alps the modelling suggests that rapid movement along the extensional fault took place between 22 and 17 ma during this period the heat flow reached values of 140 150 mw m 2 in the hanging wall in a 10 15 km wide zone parallel to the fault trace the increased heat flow caused the high coal rank in the miocene syn rift sediments deposited on the hanging wall apatite fission track geochronology of the thermally overprinted sedimentary deposits gives the time at which the bottom of the syn rift sediment pile passed through 110oc 13 6 0 9 ma the vitrinite reflectance data rr 0 67 1 04 suggest significant burial of the margin of the hanging wall at the time of core complex formation considering the coal rank data and the thermal conductivity of the sedimentary deposits by modelling of the organic maturation we calculate syn rift early middle miocene burial of the hanging wall margin of the rechnitz core complex to be 1100 1600 m this value shows the importance of the post middle miocene erosion of the region which was related to the inversion of the pannonian basin 1998 elsevier science b v all rights reserved

coeval high pressure metamorphism thrusting strike slip and extensional shearing in the tauern window eastern alps 1 recent findings for a young 31 5 0 7 ma age of high pressure metamorphism at 90 km depths in the eclogite zone of the tauern window eastern alps prompt the question about the timing of the structural development of the tauern window and its relation to high pressure metamorphism we show that all major structures in the tauern window resulting from strong n s lithospheric shortening and simultaneous minor e w extension began developing coevally with high pressure metamorphism in the eclogite zone large scale strike slip shear zones started to form at 32 30 ma and facilitated the spatial accommodation of simultaneous shortening and extension at least some of the strike slip and extensional shear zones operated into the middle miocene either continuously or intermittently with pronounced activity at 21 15 ma the considerable exhumation of the eclogite zone from 90 km depths into the middle crust and the tectonic development of its framework occurred within only 1 2 ma after eclogitization this is evidenced by almost identical ages for eclogite facies metamorphism and for the development of the major structures that bound the eclogite zone under blueschist and greenschist facies metamorphic conditions we discuss a tectonic model in which considerable transpressional shortening and thickening took place in the present central southern part of the tauern window we propose that the tauern window nucleated here and that most of the regional deformation at 32 30 ma is today found at the periphery of the window and in the adjacent austroalpine units afterward transpression continued the window grew to the e w and n and deformation progressed to those parts of the window ductile deformation in the present day surface level ceased at 15 ma

footwall uplift in an orogenic wedge the tauern window in the eastern alps of europe abstract in the tauern window tw the deepest structural units of the eastern alps are exposed in a 30 160 km antiform european basement and cover occur under nappes of oceanic penninic units and continental origin austroalpine nappes the tertiary structural history of the tw is related to a rapid uplift of 20 30 km since oligocene times structural mapping revealed 1 detachment and folding of the cover units and stacking of orthogneiss sheets 2 constrictional folding of the entire duplex with amplitudes and half wavelengths of several kilometres 3 backthrusting and backfolding along the northern margin of the tw and 4 displacement along steep dip divergent strike slip faults the strain history is complex and related to all these deformation phases the most uniform structural element is a stretching parallel to the long axis of the tw and its major fold axes strain did not contribute to tw uplift because ductile n s shortening was compensated by e w extension restoration of the tw shows that the exposed structures are not sufficient to explain the tw uplift by analogy with the engadine window and the western alps where seismic data are available a deep seated duplex is suspected the tw uplift is best explained in a dextral transpressive regime

constriction during exhumation evidence from eclogite microstructures the eclogite zone tauern window eastern alps austria represents one of only a few examples of high pressure units providing both the prograde and the retrograde metamorphic evolution the eclogites are associated with rocks originating from continental and transitional crust that also have been affected by high pressure metamorphism eclogites that formed along the prograde path indicate metamorphic conditions of 17 20 kbar 1700 2000 mpa at 550 580 c the peak assemblages formed at 21 25 kbar 2100 2500 mpa at 600 620 c omphacite microstructures in particular shape fabrics and crystallographic preferred orientations indicate that the final phases of the prograde evolution were characterized by flattening strain the evolution at the pressure peak and along the exhumation path shows a constrictional strain geometry this change is interpreted to have been controlled by the force balance between slab pull related to an oceanic slab and the buoyancy of subducted adjacent continental crustal material including the eclogites at a certain lithospheric level where the subducted continental rocks were entirely surrounded by high density lower crustal and upper mantle material the buoyancy forces exceeded the slab pull forces this force change resulted in the buoyancy driven extrusion of continental material between two lithospheric plates the part of the subducted slab that remained in buoyant equilibrium was therefore affected by constriction moreover this process may have resulted in breakoff of the subducted oceanic slab if so the cessation of slab pull resulted in accelerated buoyancy driven extrusion of the continental material extrusion of these high pressure sheets would have been accompanied by constriction subparallel to the subduction channel as well

low stress deformation of garnet by incongruent dissolution precipitation creep abstract microstructures indicating incongruent dissolution precipitation creep of garnet in eclogite facies graphitic micaschist tauern window eastern alps are investigated garnet dissolution is observed where garnet poikiloblasts grown at eclogite facies metamorphism approached each other as a consequence of progressive deformation during exhumation with estimated p t conditions between 570 c 1 7 gpa and 470 c 0 9 gpa the poikiloblasts are separated by a dissolution seam and flanked by strain shadows filled with quartz white mica and chlorite there is no evidence for crystal plastic deformation of garnet two cases are investigated a stylolitic contact zone b smooth contact zone in both cases internal fabrics of the poikiloblasts and concentric chemical zoning are truncated material previously forming inclusions in the garnet poikiloblasts is now passively enriched in a dissolution seam the original microstructure of fine grained mica graphite aggregates remaining preserved though microstructures suggest that garnet dissolution was driven by local stress concentration the level of differential stress remained too low for plastic deformation of the fine grained white mica graphite aggregates set free from the stress supporting garnet incongruent dissolution precipitation creep appears to be a particularly effective deformation mechanism at low stress in a subduction channel

40ar 39ar muscovite ages from the penninic austroalpine plate boundary eastern alps the radstadt mountains eastern alps expose the tectonic boundary between the base of the austroalpine continental plate hanging wall and the penninic oceanic units foot wall parts of the austroalpine basement were penetratively deformed because of ongoing rifting of the continental crust during the permian 290 250 ma austroalpine permo mesozoic cover rocks were deformed during the cretaceous 80 ma and the paleogene 55 50 ma these ages are interpreted as representing two distinct events of nappe stacking the younger ages represent the collision of austroalpine and penninic tectonic units the penninic nappe complex displays a successive decrease of ages from 37 to 25 ma from high to deep tectonic levels a second age group of 22 ma was found both in low temperature release steps and as plateau ages close to the penninic austroalpine boundary it is attributed to a thermal overprint due to ductile extension of the overthickened orogenic wedge

time calibration of a pt path from the western tauern window eastern alps the problem of closure temperatures new hornblende k ar and 39ar 40ar and mica rb sr and k ar ages are used to place specific timemarks on a well constrained pressure temperature path for the late alpine metamorphism in the western tauern window after identification of excess 40ar the closure behavior of ar in hornblende is compared with that of sr and ar in phengite and biotite samples were collected in three locations whose maximum temperatures were 570 c zemmgrund 550 c pfitscher joch and 500 540 c landshuter hutte

on uplift and exhumation during convergence a simple kinematic model is used to illustrate the vertical motions of rocks during convergent deformation and erosion at the surface it is shown that rocks may move upward or downward in the crust depending on the relative rates of erosion and thickening and depending on their initial depth in the crust exhumation during thickening can only occur if rapid denudation accompanies the thickening process during homogeneous thickening with erosion that is elevation dependent the initial depth from which rocks can be exhumed is only determined by the density distribution in the column and is independent of erosion or thickening rates the maximum initial depth from which rocks can be exhumed is of the order of 30 km or less there is therefore no conflict in the observation of synchronous exhumation and convergent deformation if the peak pressure of rocks is equivalent to no more than 30 km of burial it is also shown that uplift defined as vertical motion of the surface with respect to a reference level for example the geoid and exhumation defined as vertical motion of rocks with respect to the surface may follow different patterns in time and that the difference between the two evolutions may be a useful indicator of the exhumation process the model serves to emphasize the important differences between uplift and exhumation which are often not distinguished in the literature

p t evolution of metasediments from the eclogite zone south central tauern window austria abstract petrologic data on the paragenesis of i kyanite zoisite marbles and ii garnet chloritoid quartz mica schists are presented with the goal of providing constraints on the pressure temperature evolution of the eclogite zone tauern window austria the peak metamorphic assemblages in the two rock types are i kyanite zoisite dolomite quartz zoisite muscovite dolomite calcite quartz and ii garnet chloritoid kyanite muscovite quartz epidote dolomite zn staurolite the estimated peak metamorphic conditions are 19 2 kbar 590 20 c secondary alteration of the kyanite zoisite marbles was accomplished in two stages the early stage resulted in the production of margarite paragonite secondary muscovite and chlorite and the later stage resulted in the formation of sudoite a di trioctahedral mg al layer silicate and kaolinite the early alteration is bracketed at conditions between 3 and 10 kbar 450 550 c and the later alteration between 200 and 350 c p 3 kbar the p t path is characterized by maximum burial to approximately 19 kbar 60 70 km at 590 c followed by nearly isothermal decompression to approximately 10 kbar 30 km and then more gradual decompression with cooling to approximately 3 kbar 10 km alteration was apparently accomplished by the influx of h 2 o rich fluids with the composition of the fluid locally buffered by the mineral assemblage

rheology of the plate interface dissolution precipitation creep in high pressure metamorphic rocks abstract subduction zone models invoke deformation to be concentrated along the plate interface in a region of particularly low temperature geophysical observations do not provide constraints on temperature stress and deformation patterns with desired resolution in contrast the record of high pressure metamorphic rocks exhumed from subduction zones provides details on p t history deformation mechanisms and stress state albeit not readily correlated with the former dynamic situation on larger scale here we review available information on dissolution precipitation creep dpc in high pressure metamorphic rocks which if representative for subduction zones in general can pose constraints on conditions rheology and flow patterns along the plate interface the key observations and conclusions are that 1 deformation is typically highly inhomogeneous and localized into shear zones 2 stresses are generally too low to drive crystal plastic deformation 3 microfabrics suggest dissolution precipitation creep to be the predominant deformation mechanism 4 an aqueous fluid at quasi lithostatic pressure is available throughout allowing for tensile fracturing and crack healing or sealing 5 low stress combined with high strain rates required for localized deformation at typical subduction rates implies low viscosity and 6 contribution of shear heating to the thermal budget of subduction zones should be moderate the dominant deformation mechanism dpc is reviewed in some detail including experimental and theoretical approaches various examples of dpc in high pressure metamorphic rocks are illustrated emphasizing the role of interphase boundaries as sites of dissolution rheology governed by dpc is proposed to control interplate coupling and development of a subduction channel with return flow being a likely candidate for rapid exhumation of high pressure metamorphic rocks

eclogite meso and microfabrics implications for the burial and exhumation history of eclogites in the tauern window eastern alps from p t d paths abstract the eclogite zone of the central southern tauern window comprises eclogites and associated high pressure metasediments that are intercalated between penninic basement units in the foot wall and an imbricate stack in the hanging wall this nappe stack consists of continental basement units cover sequences of a distal continental slope and the main part of the ophiolitic glockner nappe textures and microfabrics of eclogites from the central southern tauern window allow the establishment of the eclogitic and post eclogitic deformation histories from burial by subduction to subsequent exhumation the metamorphic evolution of the eclogites is documented by 1 initial greenschist to blueschist facies 2 eclogite facies at ca 550 630 c and 20 kbar 3 a second blueschist facies overprint ca 450 c 10 15 kbar and 4 exhumation to pressures of ca 6 7 kbar at 500 550 c upper greenschist to lower amphibolite facies an eclogitic penetrative foliation s 1 and s dipping stretching lineation l 1 formed during the final stages of subduction of the eclogite bearing unit the p t evolution of the eclogites during d 1 documents the final increment of the prograde p t path later blueschist facies metamorphism is contemporaneous to top to the n emplacement of the eclogite bearing unit onto continental basement units within a subduction zone we discuss two models of eclogite emplacement most probably emplacement is achieved by a thrust along the base of the eclogite bearing unit alternatively the p t path of the eclogite facies rocks suggests an emplacement model similar to corner flow subsequent to eclogite and nappe emplacement the nappe stack is entirely affected by a penetrative deformation d 2 that resulted in the development of a mylonitic foliation s 2 and an e w oriented stretching lineation l 2 within lower amphibolite to upper greenschist facies this deformation is related to a crustal scale detachment zone that formed during the exhumation of the penninic units

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 tp of 350 480 c following late cretaceous to palaeogene nappe stacking in an accretionary wedge domain 2 in the central tauern window experienced tp 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 peaktemperature 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 tp 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

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

quantification of carbon dioxide sourced by mineral reactions in ultradeep sedimentary basins abstract the accumulation of co 2 in some natural gas fields represents a risk for exploration and calls for a way of integrating its occurrence in basin modelling among several possible sources we investigate mineral reactions able to take place within bottom sediments of particularly deep and hot sedimentary basins i e at conditions of low grade metamorphism up to 500 c and 2500 bar these reactions involve lithologies where carbonate and alumino silicate minerals are combined to form a mixed composition for appraising decarbonation quantitatively we propose a method founded on thermodynamic calculations that follows a modelling approach commonly used in metamorphic petrology at given bulk composition and temperature pressure t p conditions the composition and proportions of solid and fluid phases that coexist at equilibrium are computed using a gibbs energy minimization gem algorithm on the basis of r g berman s database we incorporate thermodynamic parameters for a variety of solid solutions including clay minerals and for a ch 4 co 2 h 2 o fluid mixture represented by the cpa electrolyte equation of state a large range of decarbonation reactions is explored through a systematics involving 85 model compositions and a unique t p path typical of the targeted basins we confirm that pure carbonate lithologies do not behave as co 2 source in the conditions explored in contrast we demonstrate that mixed compositions are likely to yield co 2 from successive mineral reactions in which clay minerals play a prominent role we also show that the temperature of incipient decarbonation staggered over 325 425 c depends on the ca mg fe composition ratio of the bulk sediment co 2 prone reactions occurring from a selection of natural sediments are then presented the co 2 prolific character of particular facies associated to the rift phase of a basin evolution is illustrated notably in the case of lacustrine oil shales finally we indicate a protocol that integrates the decarbonation scheme derived from thermodynamics into a co 2 generation formalism compatible with the usual formalism of a basin model

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 bundner 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 bundner 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 bundner schiefer sequence these ages are interpreted to date closely the high pressure metamorphism

denudation rates thermal evolution and preservation of blueschist terrains the erosion of low temperature high pressure blueschist facies metamorphic rocks presents a number of problems if their rate of erosion is not sufficiently rapid then the rise in temperature caused by thermal relaxation and radioactive self heating is great enough to obliterate the original low temperature mineral assemblages the problem has been investigated using a one dimensional two layer model to mimic an accretionary prism overlying oceanic lithosphere the results suggest that if no erosion takes place the maximum time that blueschist conditions may exist is about 26 m y if levels of radioactive self heating are high and approximately 100 m y if levels are very low if erosion takes place the maximum temperature reached by an element of rock is at a higher level in the earth s crust than its initial depth of burial rocks may re equilibrate at this temperature or be overprinted by incomplete reactions depending on the temperature rise and rocks involved this could result in the geotherm

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

u pb zircon and sm nd geochronology of mafic and ultramafic rocks from the central part of the tauern window eastern alps geochronological and geochemical analyses were carried out in order to identify the pre variscan basement of the tauern window eastern alps maficultramafic rocks from the central part of the tauern window have been studied by ree analysis and u pb and sm nd isotopic analyses on whole rock zircons garnets and sphene u pb and sm nd zircon dating define both magmatic pan african and cambro ordovician events from 650 ma to 486 ma within the alpine fold belt this indicates a time span of 150 ma for magmatic activities in the tauern window of the eastern alps the ages of 657 ma u pb zircon and 644 sm nd zireon obtained from an amphibolite are the oldest dates of the eastern alps they may be related to the pan african orogeny and imply an early cycle of magmatic intrusion before major activity started at around 500 ma sm nd whole rock analyses of the precambrian rocks do not define an isochron reflecting heterogenities within the mantle source the initial end values 1 2 to 4 7 are very low implying an enrichment of the magma source the second main phase of magmatic activity 539 486 ma is characterized by the emplacement of mafic ultramafic rock sequences as no ophiolitic relies are observed in these domains the early paleozoic magmatism was likely associated with extensional tectonics obtained ages of 301 3 and 314 4 3 ma point to a variscan metamorphism the first combined u pb zircon sm nd zircon data for an amphibolite from the basal amphibolite formation baf favoured the sm nd zircon isochron age as a magmatic age whereas the low initial end value point to an enriched magma source as well as to heterogenities within the magma source the obtained ages suggest that parts of the pre variscan basement within the alpine fold belt were formed during the pan africa cycle the detection of pre variscan ages within the alpine basement must reffect a complex history involving significant pre variscan activity

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 similar to 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 pb 206 u 238 ages scattering between similar to 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 2 sigma errors as their uncertainties which are mostly in the range of similar to 10 these median ages are interpreted to date cooling below similar to 450 500 degrees 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 similar to 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 similar to 36 34 ma whereas the western section younger cooling ages of similar to 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 similar to 375 110 degrees 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 2 sigma linear extrapolation using the specific cooling rates and the estimated maximum temperature t max indicates a time range of similar to 37 34 ma interpreted to date the thermal climax of barrovian metamorphism in the western tauern window

grain size dependence of garnet composition revealed by provenance signatures of modern stream sediments from the western hohe tauern austria abstract heavy minerals are valuable indicators about the geological framework in the source area the heavy mineral garnet is one of the most widespread heavy minerals in orogenic sediments and its geochemistry provides important information about metamorphic conditions the application of heavy minerals and garnet geochemistry for sedimentary provenance analysis is tested for modern stream sediments collected along three rivers draining the eclogite zone and adjacent geological source units of the western hohe tauern area in the central eastern european alps for comparison with the stream sediments rock outcrops exposed in this area were also sampled the chosen area is very well investigated and provides an excellent place to constrain the relations between source rocks and sediment in first order drainages the influence of grain size is studied in detail by considering grain size fractions ranging from coarse silt to coarse sand 32 to 1000 m in all grain size fractions the heavy mineral assemblages are characterised to a variable extent by epidote zoisite garnet and green calcic amphibole in the smaller grain size fraction apatite is more frequent whereas in the coarser grain size fractions an increase of green calcic amphibole and garnet can be observed electron microprobe analysis of detrital garnet shows the dominance of almandine rich garnet stream sediments within and downstream of the eclogite zone show an increase of pyrope rich garnets interestingly in all samples grossular rich garnets are more frequent in the smaller grain sizes and pyrope rich garnets are more frequent in the coarser grain sizes this is controlled by the original finer size distribution of grossular in the source rocks rather than being a hydraulic effect the heavy mineral assemblages and garnet geochemical data reflect the geological setting of the study area hence confirming the general strength of these methods in sedimentary provenance analysis however the data underline strong grain size control on sediment composition including single grain compositional variations

dissolution kinetics of basaltic glasses control by solution chemistry and protective effect of the alteration film basaltic glasses are considered as natural analogs for industrial nuclear aluminoborosilicate glasses alteration experiments were conducted in closed and open systems at 90 c with a synthetic basalt glass doped with 1 lithium dissolution tracer the evolution of the alteration kinetics over time was assessed by comparison of reaction progress at different degrees in closed system experiments the maximum dissolution rate initial rate r0 was comparable to the value observed for an son68 type nuclear glass the basaltic glass alteration rate subsequently dropped by four orders of magnitude the kinetic models currently proposed in the literature to account for the alteration kinetics of basaltic glasses nuclear glasses or aluminosilicate minerals are based on the concept of chemical affinity the chemical affinity alone is assumed to control the dissolution kinetics when applied to the experimental data for the closed system tests with basaltic glass these models failed to account for the low measured rates an inhibiting effect of dissolved silica was then investigated through open system basaltic glass alteration experiments with silicon enriched solutions the basaltic glass dissolution rate dropped by a factor not exceeding 200 or about two orders of magnitude compared with r0 at the high imposed silicon concentrations 130 ppm a protective effect of the alteration film was advanced to account for the four orders of magnitude rate drop observed in closed system experiments based notably on an examination of natural basaltic glasses ranging in age from several thousand to a few million years the mean alteration rates can be estimated from the measured palagonite thicknesses taking into account the age of the natural glasses the rates were comparable to those measured in the laboratory for high reaction progress

metamorphism of metasediments at the scale of an orogen a key to the tertiary geodynamic evolution of the alps major discoveries in metamorphic petrology as well as other geological disciplines have been made in the alps the regional distribution of late cretaceous tertiary metamorphic conditions documented in post hercynian metasediments across the entire alpine belt from corsica tuscany in the west to vienna in the east is presented in this paper in view of the uneven distribution of information we concentrate on type and grade of metamorphism and we elected to distinguish between metamorphic paths where either pressure and temperature peaked simultaneously or where the maximum temperature was reached at lower pressures after a significant temperature increase on the decompression path the results show which types of process caused the main metamorphic imprint a subduction process in the western alps a collision process in the central alps and complex metamorphic structures in the eastern alps owing to a complex geodynamic and metamorphic history involving the succession of the two types of process the western alps clearly show a relatively simple picture with an internal high pressure dominated part thrust over an external greenschist to low grade domain although both metamorphic domains are structurally very complex such a metamorphic pattern is generally produced by subduction followed by exhumation along a cool decompression path in contrast the central alps document conditions typical of subduction and partial accretion followed by an intensely evolved collision process often resulting in a heating event during the decompression path of the early subducted units subduction related relics and collisional decompressional heating phenomena in different tectonic edifices charac terize the tertiary evolution of the eastern alps the tuscan and corsica terrains show two differ ent kinds of evolution with corsica resembling the western alps whereas the metamorphic history in the tuscan domain is complex owing to the late evolution of the apennines this study confirms that careful analysis of the metamorphic evolution of metasediments at the scale of an entire orogen may change the geodynamic interpretation of mountain belts

evolution of an eclogitized continental fragment in the eastern alps sieggraben austria abstract the sequence of tectonometamorphic events that developed in continental basement rocks during the alpine orogeny was studied in the eclogitic sieggraben structural complex ssc in the austrian eastern alps the eclogitization of a pre alpine basement fragment occurred during early cretaceous when this was collisionally buried at the depth of ca 50 55 km with an estimated mean temperature of 720 c at about 15 kbar d1 thermal relaxation under increasing temperatures to ca 770 c and slightly decreasing pressures resulted in rapid exhumation recorded by symplectitic intergrowth of cpx2jd3 22 and pl2 in eclogites and hp amphibolites the exhumation was enhanced by the dynamic strain softening of prograde metamorphic omjd38 and zo1 into an aggregate of minor cpx2jd18 31 and zo2 dynamic recrystallization of hbl1 feldspars and qtz ribbons at high to medium temperatures was followed by medium to low temperature qtz recrystallization and cal twinning the textures indicate dislocation creep as the principal micromechanism of ductile deformation of cpx am pl and qtz or dominated mechanical twinning in cal textural patterns are mainly related to high medium temperature mesoscopic fabrics d2 compressional exhumation along a detachment fault initiated top to sse extensional unroofing of the eclogitized ssc now overlying the low grade mp hp lower austro alpine structural complexes

archimedes revisited a structural test of eclogite emplacement models in the austrian alps high pressure rocks of the eclogite zone tauern window eastern alps formed at 20 kb 600 c uplift was fast without post burial heating extensive near field and far field deformation of the eclogite zone and its surroundings provide evidence for a tectonic uplift mechanism published emplacement models buoyant rise underplating and extension wrench faulting create distinct patterns of crustal deformation and are therefore testable by structural analysis we show that emplacement related deformation and its kinematics are consistent with underplating and extension extension is two phased the first phase may be driven by changes in rate or direction of plate convergence the second phase is due to large scale underplating of continental crust

detrital high pressure low temperature minerals in a late turonian flysch sequence of the eastern alps western austria implications for early alpine tectonics along the northern margin of the eastern alps structurally intercalated between the main austroalpine thrust and the pennine rhenodanubic flysch nappe a thin tectonically complex zone occurs that comprises a series of tectonic melanges and imbricates of oceanic south pennine and distal continental margin austroalpine origin this zone is the lateral continuation of the ophiolite bearing arosa zone of the swiss alps we refer to it as the walsertal zone in a broken flysch formation of the walsertal zone turbiditic sandstones yield detrital grains of the high p low t metamorphic minerals lawsonite and glaucophane crossite ferroglaucophane the sandstones are dated as late turonian to earliest coniacian about 87 to 89 ma this age falls into the time span of radiometric ages of subduction related high p low t metamorphism along the south pennine austroalpine convergent margin and of coeval cooling ages of medium p medium t metamorphism in the middle austroalpine nappes if the high p low t minerals are related to mid cretaceous high p low t metamorphism as we think the high p metamorphic rocks must have been brought up from about 20 km depth within about 10 m y or even less as isostatic uplift alone would have been too slow alternative tectonic mechanisms such as corner flow must be responsible for rapid uplift and erosion in a fore arc environment

alpine metamorphism in the south east tauern window austria 1 p t variations in space and time the pennine rocks exposed in the south east tauern window austria contain mineral assemblages which crystallized in the mid tertiary late alpine regional metamorphism the pressure and temperature conditions at the thermal peak of this event have been estimated for rocks at four different structural levels using a variety of published and thermochemically derived geobarometers and geothermometers the results are a in the garnet chlorite zone 2 5 km structurally above the staurolite biotite isograd t 490 50 c p 7 1 kbar b within 0 5 km of the staurolite biotite isograd t 560 300c p 7 1 kbar c in the staurolite biotite zone c 2 5 km structurally below the staurolite biotite isograd t 610 30 c p 7 6 1 2 kbar d in the staurolite biotite zone 3 4 km structurally below the staurolite biotite isograd t 630 40 c p 6 6 1 2 kbar the pressure estimates imply that the total thickness of overburden above the basement cover interface in the mid tertiary was c 26 4 km this overburden can only be accounted for by the austro alpine units currently exposed in the vicinity of the tauern window if the altkristallin the middle austro alpine nappe was itself buried beneath an upper austro alpine nappe or nappe pile which was 7 4 km thick at that time the occurrence of epidote margarite quartz pseudomorphs after lawsonite in garnet indicates that part of the mesozoic pennine cover sequence in the south east tauern experienced blueschist facies conditions t 450 c p 12 kbar in early alpine times evidence from the central tauern is used to argue that the blueschist facies imprint post dated the main phase of tectonic thickening d1a and was thus a direct consequence of continental collision combined oxygen isotope and fluid inclusion studies on late stage veins thought to have been at lithostatic pressure and in thermal equilibrium with their host rocks during formation suggest that they crystallized from aqueous fluids at 1 1 0 4 kbar and 420 20 c early alpine late alpine and vein formation p t constraints have been used to construct a p t path for the base of the mesozoic cover sequence in the south east tauern window the prograde part of the p t path between early and late alpine metamorphic imprints is unlikely to have been a smooth curve and may well have had a low dp dt overall the decompression presumably due to erosion which occurred immediately before the thermal peak and possibly also earlier in the tertiary was probably partly or completely cancelled by the effects of early to mid tertiary d2a tectonic thickening the thermal peak of metamorphism was followed by a phase of almost isothermal decompression which implies a period of rapid uplift in the middle tertiary the peak metamorphic p t estimates are compared with the solutions of england s 1978 one dimensional conductive thermal model of the eastern alps and are shown to be consistent with the idea that the late alpine metamorphism was caused by tectonic burial of the pennine zone beneath the austro alpine nappes in the absence of extraneous heat sources such as large intrusions at depth

geothermobarometry in metasediments of the southern grossvenediger area tauern window austria metasediments in the southern grossvenediger area tauern window austria were studied along a cross section through rocks of increasing metamorphic grade from the margin of the tauern window in the south to the base of the upper schieferhulle including the eclogite zone in the north in the southern part of the cross section there is no evidence for a pre late alpine metamorphic history in the form of high pressure relics or pseudomorphs mineral assemblages are characterized by the stability of tremolite calcite biotite calcite and biotite chlorite calcite in the northern part a more complete alpine metamorphic evolution is preserved primary high pressure assemblages are dolomite quartz tremolite zoisite zoisite dolomite quartz phengite i and probably tremolite dolomite phengite i secondary post kinematic assemblages tremolite calcite talc calcite phengite ii chlorite calcite quartz biotite chlorite calcite biotite zoisite calcite formed as a result of the dominant late alpine metamorphic overprint the occurrence of biotite zoisite calcite is confined to the northernmost area and defines a biotite zoisite calcite isograd p t estimates based on standard thermobarometric techniques and on stability relationships of tremolite calcite dolomite quartz and zoisite give consistent results p t conditions of the main tertiary metamorphic overprint were 525 c p 7 5 1 kbar in the northern part of the cross section the southern part was metamorphosed at lower temperatures of 430 470 c the si content of phengites from this area is almost as high as that of phengites from the eclogite zone simax 3 4 pfu pressures 10 kbar at 420 c are suggested by phengite barometry according to massone schreyer 1987 in the absence of high pressure relics or pseudomorphs these phengites which lack late alpine re equilibration are the only record that rocks of the southern part probably also experienced an early non eclogitic high pressure metamorphism

balancing lateral orogenic float of the eastern alps abstract oligocene to miocene post collisional shortening between the adriatic and european plates was compensated by frontal thrusting onto the molasse foreland basin and by contemporaneous lateral wedging of the austroalpine upper plate balancing of the upper plate shortening by horizontal retrodeformation of lateral escaping and extruding wedges of the austroalpine lid enables an evaluation of the total post collisional deformation of the hangingwall plate quantification of the north south shortening and east west extension of the upper plate is derived from displacement data of major faults that dissect the austroalpine wedges indentation of the south alpine unit corresponds to 64 km north south shortening and a minimum of 120 km of east west extension lateral wedging affected the eastern alps east of the giudicarie fault west of the giudicarie fault north south shortening was compensated by 50 to 80 km of backthrusting in the lombardian thrust system of the southern alps the main structures that bound the escaping wedges to the north are the inntal fault system ca 50 km sinistral offset the konigsee lammertal traunsee klt fault 10 km and the salzach ennstal mariazell puchberg semp fault system 60 km these faults as well as a number of minor faults with displacements less than 10 km root in the basal detachment of the alps the thin skinned nature of lateral escape related structures north of the semp line is documented by industry reflection seismic lines crossing the northern calcareous alps nca and the frontal thrust of the eastern alps complex triangle zones with passive roof backthrusts of middle miocene molasse sediments formed in front of the laterally escaping wedges of the northern eastern alps the aim of this paper is a semiquantitative reconstruction of the upper plate of the eastern alps most of the data is published elsewhere

low differential stress during high pressure metamorphism the microstructural record of a metapelite from the eclogite zone tauern window eastern alps constraints on the state of stress at deep levels of convergent plate boundaries can be obtained from the microstructural record of high pressure metamorphic rocks provided that this record has remained unchanged during exhumation the inclusion patterns in poikiloblastic garnet of a metapelite from the eclogite zone of the tauern window eastern alps reveal a foam microstructure of quartz controlled by grain boundary free energy the garnet blasts show concentric chemical zonation thermobarometry based on correlation between compositional domains in garnet chloritoid and phengite coexisting with kyanite rutile and quartz shows that garnet growth commenced before peak p t conditions close to 25 kbar and 600 c had been reached and continued during the early stage of decompression this implies that the microstructure of quartz controlled by grain boundary free energy persisted continuously during the latest stage of burial to ca 80 km depth and the early stage of exhumation involving detachment from the downgoing slab and release into the subduction channel the foam structure of the overgrown quartz indicates that the differential stress was too low to drive dislocation creep during that period when the rock crossed the interface between the plates with a high differential stress resulting in tectonic overpressure or significant shear heating being ruled out outside the protective garnet the interfacial free energy controlled quartz microstructure has been destroyed completely due to dynamic recrystallization during deformation by dislocation creep this happened while the crustal slice was exhumed to a depth of about 45 km the preservation of this synkinematic quartz microstructure indicates a tectonic scenario where the final cooling was rapid and no marked stress peak was associated with the brittle plastic transition

alpine metamorphism in the south east tauern window austria 2 rates of heating cooling and uplift existing geochronological data are reviewed and new rb sr k ar and 39ar 40ar ages are presented including a suite of 33 mica ages from a 20 km north south tunnel section these data are discussed in relation to the thermal history from the overthrusting of the autroalpine nappes c 65 myr ago to the present the earliest phase of metamorphism involving lawsonite crystallization is associated with emplacement of these nappes subsequently temperatures in the rocks beneath rose at a mean rate of 3 6 c myr until the climax of metamorphism at high structural levels published data indicate an age 35 myr for the metamorphic climax in contrast a new 39ar 40ar step heating age of 23 8 0 8 myr on amphibole from near the base of peripheral schieferhulle closely approximates the age of metamorphism and provides the first clear indication that the climax of metamorphism occurred later at deeper structure levels following the climax near isothermal uplift and erosion reduced pressure to c 1 kbar before white mica closure at 19 myr this implies uplift at 3 mm yr along the tunnel section white mica k ar ages vary systematically from 24 myr to 16 5 myr with position relative to a late 4 km amplitude dome whereas biotite rb sr ages are uniform at 16 5 myr across the whole profile doming is thus dated at 16 5 myr with transient uplift rates 5 mm yr at other times uplift rates were 1 mm yr

eocene high pressure metamorphism in the penninic units of the tauern window eastern alps evidence from 40 ar 39 ar dating and petrological investigations the40ar 39ar degassing spectra of white micas and amphiboles from three tectonic units of the central tauern window pennine basement and cover in the eastern alps have been measured white micas are classified as 1 pre alpine low si relic micas with an age value of 292 ma variously disturbed by the alpine metamorphism 2 alpine phengitic micas of variable composition with an age between 32 and 36 ma 3 alpine low si micas with a maximum age of 27 ma we attribute the higher alpine ages to a blueschist facies event whereas the lower age reflects the late cooling of the nappe pile blueschist facies phengites from the basement lower schieferhulle and the tectonic cover upper schieferhulle crystallized at a temperature below the closure temperature tc for argon diffusion in white mica and record ages of 32 to 36 ma at the same time a thin eclogite facies unit eclogite zone was thrust between the lower and the upper schieferhulle and cooled from eclogite facies conditions at about 600 c at 20 kbar to blueschist facies conditions at 450 c or even 300 c at 10 kbar eclogite facies phengites closed for argon diffusion and record cooling ages coinciding with the crystallization ages in the hanging and the footwall unit amphibole age spectra actinolite glaucophane barroisite are not interpretable in terms of geologically meaningful ages because of excess argon

the tectono metamorphic history of the valaisan domain from the western to the central alps new constraints on the evolution of the alps petrographic and structural studies on metasedimentary rocks of the valaisan domain from the engadine window switzerland to the petit st bernard france show that this entire domain was subjected to high pressure low temperature conditions during the alpine orogeny the pressure and temperature conditions increased from east to west the metasedimentary rocks of the engadine window and the grisons area are characterized by blueschist facies conditions 12 13 kbar 350 400 c defined by mg carpholite and chloritoid occurrences toward the west in the petit st bernard and versoyen units the metamorphic conditions reached the eclogite facies 15 16 kbar 500 c the shortening direction associated with the high pressure metamorphic event was oriented northwest southeast all over the valaisan domain this direction is compatible with the direction of convergence between the european and apulian plates in early tertiary time 65 35 ma in the eastern and central alps blueschist facies rocks occur below the austroalpine nappes over an area of 300 20 km 2 from the tauern window to the grisons area and have a thickness around 10 km this volume of blueschist facies rocks is in contrast with eclogite facies rocks of western alps that form only a small slice with a thickness of 2 to 5 km we interpret the difference in volume and metamorphic conditions from east to west as being due to a change of the subduction style in the east we assume the formation of a large wedge with a thickness of 40 50 km such that rocks underwent blueschist metamorphism and were exhumed before collision between the european and apulian plates in the west the petit st bernard and the versoyen units were metamorphosed and extruded in a subduction channel above the subducting slab the data further allow us to conclude that the valaisan domain was an ocean that was independent of the piemontese ocean and that had an important place in the youngest phase of the evolution of the alps before the collision

alpine geodynamic evolution of passive and active continental margin sequences in the tauern window eastern alps austria italy a review the penninic oceanic sequence of the glockner nappe and the foot wall penninic continental margin sequences exposed within the tauern window eastern alps have been investigated in detail field data as well as structural and petrological data have been combined with data from the literature in order to constrain the geodynamic evolution of these units volcanic and sedimentary sequences document the evolution from a stable continent that was formed subsequent to the variscan orogeny to its disintegration associated with subsidence and rifting in the triassic and jurassic the formation of the glockner oceanic basin and its consumption during the upper cretaceous and the paleogene these units are incorporated into a nappe stack that was formed during the collision between a penninic zentralgneis block in the north and a southern austroalpine block the venediger nappe and the storz nappe are characterized by metamorphic jurassic shelf deposits hochstegen group and cretaceous flysch sediments kaserer and murtorl groups the eclogite zone and the rote wand modereck nappe comprise permian to triassic clastic sequences wustkogel quartzite and remnants of platform carbonates seidlwinkl group as well as jurassic volcanoclastic material and rift sediments brennkogel facies covered by cretaceous flyschoid sequences nappe stacking was contemporaneous to and postdated subduction related high pressure eclogite and blueschist facies metamorphism emplacement of the eclogite bearing units of the eclogite zone and the glockner nappe onto penninic continental units zentralgneis block occurred subsequent to eclogite facies metamorphism the eclogite zone a former extended continental margin was subsequently overridden by a pile of basement cover nappes rote wand modereck nappe along a ductile out of sequence thrust low angle normal faults that have developed during the jurassic extensional phase might have been inverted during nappe emplacement