the p t d development at the basement cover boundary in the north eastern tauern window eastern alps alpine continental collision at the basement cover boundary of the north eastern tauern window eastern alps the following alpine p t d development has been reconstructed on the basis of macro and micro structures as well as preferred crystallographic orientations mineral parageneses and compositions during increasing p t conditions in the greenschist facies a first period of deformation produced imbrication of the basement gneisses and cover sediments and then monoclinal folds up to the kilometre scale tectonic transport was continuously top to the ene a second period of deformation began at about peak p t conditions of 9 kbar and c 540 560 c in the south and about 7 9 kbar and 490 500 c in the north this continued locally to lower temperature during the second period transport was continuously top to the se crystallographic orientations of white mica and plagioclase give particularly useful information on the kinematic framework in addition data on the ductile behaviour of dolomite and plagioclase can be inferred at c 7 9 kbar dolomite recrystallization starts at 450 480 c and the beginning of plagioclase recrystallization coincides with the oligoclase boundary in general the alpine geodynamic history of the basement cover boundary may be related to continental collision processes between a northerly plate european or brianconnais and a southerly adriatic one the first deformation period possibly reflects subduction of the gneiss sediment boundary toward the wsw to a depth of 31 32 km the second period may be a result of obduction toward the nw followed by late stage uplift most of the basement domes of the eastern tauern window appear as a result of the final stage of the first deformation formed prior to the peak of metamorphism possibly partly influenced by the final collision between the northern and the southern continents

metamorphic interpretation of high pressure temperature metapelites with preserved growth zoning in garnet eastern grenville province canadian shield high pressure temperature metapelites that occur in close proximity to eclogitized mafic rocks in the southern part of the gagnon terrane parautochthonous belt eastern grenville province were investigated in order to constrain depths of burial and p t paths mineral assemblages and partial melting relationships in these metapelites are consistent with peak temperatures in the range between 700 and 800 c however growth zoning is apparently well preserved in garnets and only narrow rims width 100 500 m are obviously affected by diffusional retrograde resetting despite uncertainties regarding mineral assemblages and compositions of matrix minerals at early stages of garnet growth it can be shown that the observed growth zoning profiles of garnets imply increase of both pressure and temperature up to a common maximum at pressures between 1300 and 1600 mpa and that thermal relaxation did not occur during the initial stages of unloading on the other hand calculated retrograde p t conditions are consistent with steep decompression paths the inferred hair pin shaped p t path is consistent with independent evidence of rapid tectonically driven exhumation resulting in the preservation of growth zoning in garnets from such a high temperature regime

contrasting zoning profiles in high grade garnets evidence for the allochthonous nature of a grenville province terrane in the central part of the grenville province proterozoic metasediments of the reservoir cabonga terrane arc underlain by gently dipping archean migmatites a major mylonite zone located within the migmatites is the base of a large thrust sheet made up of the reservoir cabonga terrane and a migmatitic sole garnets from meta aluminous assemblages in proterozoic metasediments mylonites and migmatites display contrasting types of ca zoning patterns garnets with ca rich cores are found along the western edge of the reservoir cabonga terrane whereas garnets with flat ca profiles are found in the remaining proterozoic metapelites in the mylonites and in the archean migmatites considering samples in which an infinite reservoir of ca ions is warranted by abundant plagioclase in the matrix pressures well in excess of 1200 mpa at temperatures between 700 and 800 c are indicated for a metamorphic event recorded in the cores of garnets from the western edge of the reservoir cabonga terrane the inner rim of garnet records pressures of around 1000 mpa for the same temperature range in homogeneous garnets a single p t set around 800 mpa and 750 c is recorded in the migmatites some of the mylonites initiate their recrystallization under relatively high pressures which is continued during decompression and record the lowest p t values these metamorphic signatures suggest that the proterozoic metapelites were already metamorphosed under relatively high pressure when emplaced on to archean migmatites and that the entire area was subsequently subjected to metamorphism at a lower pressure

alpine metamorphism of calcareous metasediments in the western hohe tauern tyrol mineral equilibria in cohs fluids during tertiary regional metamorphism in the western hohe tauern reaching maximum p t conditions around 6 kb 550 c in calcareous metasediments reaction of pyrite to pyrrhotite is suggested by regional distribution and textural relations in rocks without graphite pyrite is common at all metamorphic grades in graphite bearing rocks however the dominant fe sulfide is pyrite at lower grade and pyrrhotite at higher grade furthermore in graphite bearing high grade rocks pyrite is restricted to assemblages with mg rich silicates several factors control pyrite pyrrhotite relations increase of temperature is most effective by increase of pyrrhotite vs pyrite stability field shift of silicate sulfide reactions toward the stability field of pyrrhotite creation of sulfur free fluids from devolatilization reactions and increase in the proportions of sulfur bearing fluid species presence of graphite also favours progress of pyrite to pyrrhotite reaction as shown by different f s 2 stabilities and changes in the amount of minerals and fluid during metamorphic heating of graphite bearing and graphite free assemblages an opposite effect is shown by assemblages with low fe contents in fe mg silicates due to the enlarged stability field of such minerals with increasing mg and f content another inhibition of pyrite to pyrrhotite reaction is suggested to be due to relatively high sulfur contents of h2o rich infiltrating fluids

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

empirical garnet biotite plagioclase quartz gbpq geobarometry in medium to high grade metapelites on the basis of the net transfer reactions among garnet biotite plagioclase and quartz for both mg and fe end member models the garnet biotite plagioclase quartz gbpq geobarometer was empirically calibrated under physical conditions of p 1 0 11 4 kbar and t 515 878 c based on the input garnet biotite temperatures and garnet aluminosilicate plagioclase quartz gasp pressures of 224 natural aluminosilicate bearing metapelitic samples collated from the literature the calibrations are internally consistent with the asymmetric quaternary solid solution model of garnet the symmetric quaternary solid solution model of biotite and the al avoidance ternary solid solution model of plagioclase in calibrating the garnet biotite geothermometer and the gasp geobarometer the resulting two gbpq barometer formulae reproduce the input gasp pressures well within 1 0 kbar mostly within 0 5 kbar for both aluminosilicate bearing and aluminosilicate absent metapelites the two gbpq barometry formulae yielded identical pressures whether the sample was included or not included in calibrating the gbpq barometry the random error of the gbpq barometry may be expected as 1 2 kbar the dp dt slopes of these two gbpq formulae are close to that of the gasp barometer in p t space applications of the gbpq barometry of aluminosilicate absent metapelites to the rocks within a thermal contact aureole or rocks within a limited geographical area without post metamorphic structural discontinuity show no obvious pressure change it may be concluded that the two gbpq barometry formulae derived in this study may be used as practical tools for metamorphic pelites under the conditions of 515 878 c and 1 0 11 4 kbar in the composition range of xgros 3 in garnet xan 17 in plagioclase and x bio 3 in biotite application of the gbpq barometer beyond the calibration range should be undertaken with caution

linking itcz migrations to the amoc and north atlantic pacific sst decadal variability abstractwe examine the link between migrations in the intertropical convergence zone itcz and changes in the atlantic meridional overturning circulation amoc atlantic multidecadal variability

pressure temperature and time indicators in mafic schist their application to reconstructing the polymetamorphic history of vermont mineral chemistry and overgrowth relationships in mafic schist from vermont are shown to be sensitive indicators of pressure temperature and relative time and to be good chronicles of the paleozoic history of this polymetamorphic terrane within the common assemblage amphibole chlorite epidote plagioclase quartz ti phase carbonate k mica fe 3 oxide electron microprobe analyses show that increasing metamorphic grade as defined by intercalated pelitic schist is recorded by an increase in the edenite glaucophane and tschermakite contents of amphibole in the anorthite content of plagioclase and in the substitution of al vi f 2 ti al iv for fe 2 mg mn si in biotite chlorite and muscovite with increasing pressure the glaucophane component of amphibole increases for medium pressure metamorphism the albite oligoclase gap is in the garnet zone where amphibole has between 1 2 and 1 8 formula proportion al iv and al vi fe 3 ti cr this gap is below the garnet isograd in low pressure mafic schist where al iv and al vi fe 3 ti cr in the amphibole are both less than 0 6 mineral growth periods observed are characterized by metamorphic grade and facies series and are assigned to two ordovician taconic and two devonian acadian events silurian devonian mafic schist in northeastern vermont records two periods of low pressure devonian metamorphism these events are expressed by medium pressure and low pressure metamorphism in the cambrian and ordovician rocks to the west and south in the pre silurian rocks high pressure and medium pressure metamorphism is assigned to the ordovician the observed high pressure metamorphism is confined to a 110 by 40 km area along the green mountain anticlinorium axis in north central vermont

metamorphic p t paths from pelitic schists and greenstones from the south west tauern window eastern alps petrological data from intercalated pelitic schists and greenstones are used to construct a pressure temperature path followed by the upper schieferhulle ush series during progressive metamorphism and uplift in the south west tauern window italy pseudomorphs of al epidote fe epidote albite oligoclase chlorite after lawsonite and data on amphibole crystal chemistry indicate early metamorphism in the lawsonite albite chlorite subfacies of the blueschist facies at p 7 8 kbar geothermometry and geobarometry yield conditions of final equilibration of the matrix assemblage of 475 25 c 5 6 kbar calculations with plagioclase and phengite inclusions in garnet indicate early garnet growth at pressures of 7 5 kbar garnet zoning patterns are complex and reversals in zoning can be correlated between samples thermodynamic modelling of these zoning profiles implies garnet growth in response to four distinct phases of tectonic activity fluid inclusion data from coexisting immiscible h2o co2 nacl fluids constrain the uplift path to have passed through temperatures of 380 30 c at 1 3 0 2 kbar there is no evidence for metamorphism of ush at pressures greater than 7 5 kbar in this area of the tauern window this is in contrast to pressures of 10 kbar recorded in the lower schieferhulle only 2 3 km across strike a history of differential uplift and thinning of the intervening section during metamorphism is necessary to reconcile the p t data obtained from these adjacent tectonic units

effects of diffusional modification of garnet growth zoning on p t path calculations the effect of intragranular diffusion on chemical zoning in garnet and on p t paths calculated from that zoning was evaluated using a numerical model of multicomponent diffusion in combination with simulations of garnet growth syn and post growth diffusion of mg fe mn ca species in garnet was calculated for a model pelitic assemblage over a range of temperatures from 485 to 635 c compositions from zoned garnet as modified by diffusion hypothetical inclusions of plagioclase within garnet and matrix phases were used to reconstruct pressure temperature p t paths from isobaric and polybaric model histories p t path calculations based on numerical simulations conducted over an input isobaric heating path that reached peak temperatures between 585 and 635 c show that relaxation of garnet compositional gradients by diffusion can induce modest to appreciable curvature in the inferred paths retrieved paths also indicated somewhat smaller overall temperature changes relative to the actual temperature difference of the input path the magnitude of these distortions is shown to depend upon the heating and cooling rate and garnet crystal size as well as the actual peak temperature condition the effect of diffusion on path trajectories in simulations with thermal histories that also included cooling were comparable to heating only models that reached peak temperatures approximately 15 30 c higher compositions of garnets with radii less than 1 mm that reached actual peak temperatures of 605 c along temperature time histories characteristic of regional metamorphism experienced sufficient diffusional relaxation to introduce errors of hundreds of bars to in excess of one kilobar in path trajectories path distortions were significant at heating cooling rates less than 10 c ma but rapidly diminished for rates faster than this in polybaric simulations diffusion effects were least noticeable when the actual pressure temperature conditions changed in a clockwise sense i e convex to higher p and higher t but apprecciable modification was seen in path models that underwent counterclockwise changes in p and t reequilibration of garnet rim compositions occurred during cooling on all paths and temperature maxima obtained from garnet biotite geothermometry underestimated actual peak conditions by 40 to 70 c calculations suggest that p t path trajectories calculated from garnets of at least 1 mm size and that experienced actual thermal maxima below 585 c are not likely to be distorted by diffusional effects during regional metamorphism however p t path reconstructions based on garnet zonation with smaller grains or higher temperatures may lead to misinterpretation of crystallization history the partitioning record of peak metamorphic temperatures may be destroyed by diffusional reequilibration of garnet rim compositions during cooling seriously complicating the task of quantitatively estimating diffusion effects on path calculations

uncertainties in differential thermodynamic gibbs method p t paths a combined analytical and numerical evaluation of the uncertainties in p t paths is made for three assemblages that propagates errors in the parameters initial pressure initial temperature initial composition change of composition of monitor parameters endmember entropy and endmember volume propagated errors along an isobaric heating path t 77 c for assemblage 1 grt bt pl qtz ms chl h2o using as monitor parameters the mole fractions of almandine spessartine grossular and anorthite and ignoring uncertainties in thermodynamic properties are approximately 320 bars 1 and 8 3 c 1 if rim compositional uncertainties of 5 in major cations are assumed or 2 5 c 1 and 50 bars 1 if electron microprobe analytical uncertainties are assumed for compositions the largest source of uncertainty is from the errors in the monitor parameters and p t path uncertainties can depend critically on which monitor parameters are used if the mole fraction of annite is used as a monitor parameter in place of the anorthite content of plagioclase then propagated uncertainties are worse than 29 c 1 and 5800 bars 1 p t path uncertainties also depend on assemblage 1 precisions in assemblages 2 grt bt pl qtz ms sil h2o and 3 grt bt qtz kfs sil h2o using as monitor parameters the mole fractions of almandine spessartine grossular and anorthite are calculated to be 267 bars and 43 4 c and 372 bars and 8 2 c respectively estimates of the accuracies in p t paths that include potential errors in endmember entropy of 1 j mol k and in endmember volume of 1 cm3 mol are 324 bars and 8 5 c assemblage 1 341 bars and 48 6 c assemblage 2 and 388 bars and 9 7 c assemblage 3 use of different garnet plagioclase and muscovite activity models can change the length of a p t path by as much as 15 but does not typically change directions in p t space significantly models that incorporate changes of fluid composition shorten p t paths in assemblages 1 and 3 but do not change trajectories significantly assemblage 2 is virtually unaffected by fluid phase models for the mineral assemblages considered here and using appropriate monitor parameters propagated errors are small compared to the total path length suggesting that the differential thermodynamic approach is a precise and accurate method for determining amounts of heating or thickening during metamorphism and hence for interpreting orogenic processes

metamorphic evolution of the northwestern ogcheon metamorphic belt south korea the ogcheon metamorphic belt omb comprises late proterozoic to paleozoic metasedimentary and metavolcanic sequences which are intruded by mesozoic granitoid plutons to delineate the metamorphic evolution of the omb we have investigated mineral parageneses metamorphic reactions and p t conditions of pelitic and mafic schists in the chungju area northwestern part of the omb the regional metamorphic grade increases north westward from biotite to garnet zones although low pressure assemblages have developed in contact aureoles around jurassic granites garnet crystals show chemical zoning typical for prograde metamorphism with decreasing mn and increasing fe and mg from core to rim compositions of amphiboles constituting the common assemblage in the mafic schist are consistent with those of biotite and garnet zones documented in the medium pressure metabasites p t conditions of the garnet zone estimated from garnet biotite garnet chlorite and amphibole plagioclase geothermometers together with garnet plagioclase biotite muscoviterquartz geobarometers are in the range of 5 8 kbar and 520 5908c retrograde p t path based on fluid inclusion studies suggests that the exhumation of the omb has passed through the p trange of 1 3 kbar and 350 5008c following the isochore curves of the co inclusions in conjunction with structural and geochronologic data we conclude 2 that the omb has experienced a polycyclic p t evolution characterized by 1 crustal thickening during the middle paleozoic time and 2 regional retrograde metamorphism in the triassic our result further suggests that the triassic collision belt in east central china does not pass through the omb q 1998 elsevier science b v

petrologic constraints on imbrication metamorphism and uplift in the sw tauern window eastern alps the pressure temperature time deformation pttd history of two tectonic units in the southwest tauern window austria italy is evaluated from p t paths of metamorphism and uplift geochronologic data and observations on fabric development petrologic data indicate burial of the lower schieferhulle lsh to depths of 35 km and the adjacent upper schieferhulle ush to only 24 28 km the units are now separated by only 2 km of section metamorphism was postkinematic in the lsh and synkinematic in the ush correlation of p t path reversals calculated from zoned garnets in the two units however indicates that juxtaposition of the lsh and ush occurred prior to metamorphism temperature time and depth time diagrams for both units show increasing cooling rates and decreasing uplift rates as the rocks approached the surface both the petrologic data and the depth time profiles for the lsh and ush point to a history of differential uplift in the early stages of decompression textural features indicate that the lsh behaved as an essentially rigid body during uplift whereas the ush experienced extensive ductile shearing the pttd data for the lsh and ush are incorporated into a model for continental collision in the eastern alps initial tectonic uplift of the lsh was accompanied by ductile thinning of the ush and overlying austroalpine units final uplift and erosion affected all of the units en masse

fluid inclusion constraints on the kinematics of footwall uplift beneath the brenner line normal fault eastern alps dynamic models of isostatic footwall uplift in response to normal faulting can be divided into those in which uplift is accomplished by flexural failure and those in which uplift occurs via subvertical simple shear each class of model predicts a different incremental strain history that should be recorded in the footwall in the tauern window eastern alps postmylonitic structures in the footwall of the brenner line normal shear zone predominantly consist of closely spaced steep west down and east down microfaults formation of the west down faults before and at greater depths than the east down faults would be consistent with unroofing via subvertical simple shear in contrast formation of the two fault types as a conjugate set would be more indicative of unroofing via elastic processes the field data alone do not provide a sufficient test of the two hypotheses because crosscutting relations are only rarely observed and there is no control on the depth at which the structures formed however both depth and timing constraints on the formation of the late structures can be obtained by correlating the orientations of fluid inclusion lined microfaults with the macroscopic west down and east down faults obtaining density data for the inclusions and correlating these data with previously obtained geochronologic data the results indicate that the west down structures formed at depths of 10 20 km and temperatures 450 c in the mid to late oligocene and that the east down structures formed at 2 to 10 km depth and temperatures of 300 50 c in the mid miocene these data support the hypothesis that a rolling hinge was present in the footwall of the brenner line and that isostatically driven footwall deformation was accomplished predominantly by subvertical simple shear the depths at which west down and east down faulting occurred coupled with the angle of dip of the brenner line yield a minimum lateral displacement on the fault of 15 26 km approximately coeval ductile shearing and brittle faulting at depths of 15 20 km and temperatures in excess of 400 c may reflect local variations in strain rate as the footwall rocks entered the zone of rolling hinge deformation

metamorphic petrology of a high pressure low temperature subduction complex in west central baja california mexico the western baja terrane wbt of west central baja california is an uplifted subduction complex that is divided into smaller subterranes on the basis of bounding faults and petrological differences each subterrane contains coherent early jurassic to early cretaceous sedimentary and mafic volcanic rocks not melange that have been metamorphosed under blueschist facies conditions key phases in metabasites and metaturbidites include jadeitic to acmitic clinopyroxene sodic amphibole lawsonite aragonite chlorite titanite and white mica pressure indicators include the jadeite content of clinopyroxene and the presence of aragonite temperature indicators include the presence of lawsonite the absence of greenschist facies minerals and results from vitrinite reflectance studies conditions at the peak of metamorphism were 8 kbar 225 325 c for subterrane 1 7 8 kbar 170 220 c for subterrane 2 and 5 6 kbar 175 200 c for subterrane 3 these correspond to cold geothermal gradients 6 9 km vein assemblages that include aegerine jadeite and aegerine albite aragonite lawsonite and sodic amphibole indicate uplift during continued cold conditions probably during steady state subduction

the p t evolution of the middle koli nappe complex scandinavian caledonides 68 n and its tectonic implications near 68 n the scandinavian caledonides are composed of 3 tectonic domains each of which has a different tectonostratigraphy the lower 2 domains can be related stratigraphically to scandinavia prior to caledonian deformation whereas the highest domain the middle koli nappe complex mknc represents a fore arc accretionary complex that was accreted to scandinavia during caledonian deformation subsequent to accretion the flyschoid sediments that dominate the mknc were metamorphosed to the amphibolite facies in the area covered by this study the mknc is composed of two nappes a lower langvatn nappe and an upper marko nappe each of which has a unique early metamorphic history pelitic mineral assemblages in the marko nappe constrain the peak p t to be 625 7 0 kbars whereas ultramafic mineral assemblages in the lower langvatn nappe constrain its peak temperature to be 580 c p t estimates from garnet biotite and garnet plagioclase geothermobarometry for both nappes overlap ranging from 528 c and 6 6 kbars to 620 c and 8 8 kbars with an average of 567 32 c and 8 0 0 9 kbar

geothermometry geobarometry and the al2sio5 triple point at mt moosilauke new hampshire the distribution of aluminum silicate minerals in pelitic schists constrains pressure and temperature conditions during the acadian orogeny to have been approximately those of the al 2 sio 5 invariant point thus providing an excellent opportunity to evaluate the internal consistency of a variety of published geothermometers and geobarometers with the al 2 sio 5 invariant point of holdaway 1971 four fluid independent equilibria yield p t estimates consistent with holdaway 1971 two fluid dependent equilibria yield internally consistent but unrealistically high temperatures our data are consistent with the conclusions that 1 only pyrope grossular mixing in the quaternary garnet system is significantly non ideal and 2 the activity coefficient for the anorthite component in plagioclase is approximately 2 at triple point p t conditions modified journal abstract

controls on p t t deformation path from amphibole zonation during progressive metamorphism of basic rocks estuary of the river vilaine south brittany france edenite tremolite and edenite magnesio hornblende in equilibrium with plagioclase chlorite epidote quartz and vapour involve several types of reactions for which kd can be related to t and p thermodynamic calculation of these equilibria leads to isopleth systems given knowledge of the progressive changes of end member activities in zoned ca mg amphiboles based on microprobe analyses it is possible to construct precise pressure temperature time paths p t t paths which have been followed by metabasites during polyphase metamorphism when applied to basic rocks from the river vilaine area this method allows us to construct a p t t path that can be compared directly to the p t t path constructed from interbedded acid rocks aluminous micaschists in the same structural unit through time both basic and acid rocks underwent the same complex deformation history that can be described conveniently in the l s fabric system of flinn this allows us to construct a p t t deformation path for this structural unit these paths are interpreted in terms of an under overthrusting continental collision belt the hercynian belt and represent an illustration of the time delay caused by stacking of more than two crustal units

extension and inversion structures in the tethys atlantic linkage zone algarve basin portugal the algarve basin is a meso cenozoic sedimentary basin overlying carboniferous basement located in the southwestern margin of the iberian peninsula its structure reveals a protracted tectonic history comprising various pulses of mesozoic extension followed by cenozoic compression this work deals with the structure along the northern margin where the mesozoic extensional structures and cenozoic inversion structures crop out the strike of the extensional structures ranges from e w to n s as controlled by a shift from tethyan dominated extension in the east to atlantic dominated extension in the west contractional structures are inverted extensional structures following their same trends it is argued that the thickness of the hettangian evaporite layer exerts a strong control on the structural style throughout the basin during the extensional and inversion episodes the basin is affected by thick skinned deformation along the northern margin where salt is thin or absent basement involved fault systems and short cut structures basinward as the hettangian salt thickens the margin is affected by thin skinned deformation with listric and down to the basin growth faults diapirism and salt cored detachment folds the aim was to discuss the key tectonic features the relevance of salt and understand the nature timing and significance of all these structures in the regional tectonic evolution

fluid inclusion constraints on the uplift history of the metamorphosed massive sulphide deposits at ducktown tennessee standard petrographic microthermometric and raman spectroscopic analyses of fluid inclusions from the metamorphosed massive sulphide deposits at ducktown tennessee indicate that fluids with a wide range of compositions in the c o h n s salt system were involved in the syn to post metamorphic history of these deposits primary fluid inclusions from peak metamorphic clinopyroxene contain low salinity h2o ch4 fluids and calcite quartz and pyrrhotite daughter crystals many of these inclusions exhibit morphologies resembling those produced in laboratory experiments in which confining pressures significantly exceed the internal pressures of the inclusions secondary inclusions in metamorphic quartz from veins pods and host matrix record a complex uplift history involving a variety of fluids in the c o h n salt system early fluids were generated by local devolatilization reactions while later fluids were derived externally isochores calculated for secondary inclusions in addition to the chronology of trapping and morphological features of primary and secondary fluid inclusions suggest an uplift path which was concave toward the temperature axis over the p t range 6 3 kbar and 550 225 c immiscible h2o ch4 n2 nacl fluids were trapped under lithostatic to hydrostatic pressure conditions at 3 0 5 kbar and 215 20 c entrapment occurred during alleghanian thrusting and the fluids may have been derived by tectonically driven expulsion of pore fluids and thermal maturation of organic material in lower plate sedimentary rocks which are thought to underlie the deposits episodic fracturing and concomitant pressure decreases in upper plate rocks which host the ore bodies would have allowed these fluids to move upward and become immiscible post alleghanian uplift appears to have been temperature convex uplift rates of 0 10 0 05 mm year 1 from middle ordovician to middle silurian late devonian and 0 07 0 12 mm year 1 from middle silurian late devonian to late permian are suggested by our uplift path and available geochronological data

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

p t paths from garnet zoning a new technique for deciphering tectonic processes in crystalline terranes a procedure to calculate quantitative rock pressure temperature p t paths based on chemical zoning profiles in garnet combined with mineral chemistry from the other minerals in the assemblage has been applied to samples from diverse tectonic settings with the result that the calculated p t path is shown to be a sensitive monitor of tectonic processes terranes where metamorphic recrystallization paths are controlled by uplift and erosion show heating during decompression in the prograde p t path and cooling during decompression in the retrograde path in nappe terranes where hot rocks are emplaced over cool rocks the upper plate shows cooling during decompression whereas the lower plate shows heating during compression in nappe terranes where cool rocks are emplaced over hot the lower plate shows cooling during compression calculation of p t paths from mineral zoning should provide a powerful new tool for deciphering tectonic processes in crystalline terranes

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

metamorphic p t paths from calcic pelitic schists from the strafford dome vermont usa metamorphism of the gile mountain formation and waits river formation in the strafford dome and townshend brownington syncline in east central vermont records two nappe style events d1 and d2 followed by doming d1 formed a muscovite biotite ilmenite schistosity subparallel to compositional layering so and was followed by heating to garnet grade the temperature and pressure at the end of d1 are estimated to be c 450d c and 6 8 kbar d2 variably crenulated and folded s1 during a nearly isothermal pressure increase of 1 2 kbar calculated from compositions of garnet which have inclusions trails with progressive crenulation and rotation of the s1 fabric similar p t paths are computed for most of the area suggesting that the later schistosity developed during emplacement of a regional nappe 3 6 km thick there is a general lack of d3 dome stage microstructures near the strafford willoughby arch staurolite and kyanite overgrew s2 in pelites and plagioclase with increasing xan overgrew s2 in calcic pelites reflecting post d2 heating to a maximum of 550 600d c metamorphic pressures at the end of d2 are fairly constant on the west side of the dome indicating minor dome stage uplift in contrast pressures at the thermal peak of metamorphism decrease by more than 4 kbar east of the dome the observed pattern of isotherms and isobars is mainly the result of post metamorphic differential uplift and unroofing finally a minor retrograde metamorphism produced the assemblage albite epidote k feldspar muscovite chlorite with grade increasing east toward the connecticut river

heat sources of cretaceous metamorphism in the eastern alps a discussion abstract the alpine orogenic cycle is characterised by a series of early metamorphic events that occurred in the cretaceous the eoalpine events a collection of data on peak metamorphic conditions pt paths and on relationships between structure and metamorphism for these events in the eastern alps shows that 1 eoalpine metamorphism occurred largely simultaneously with pervasive deformation 2 the eoalpine thermal peak was reached near the pressure peak 3 lower grade structurally higher rocks reached their metamorphic peak earlier than higher grade structurally lower rocks and there is a wide spread of metamorphic ages 4 low grade metamorphic conditions are probably more perturbed than high grade conditions and 5 there is a conspicuous regional distribution of metamorphic grade with amphibolite and eclogite facies metamorphism occurring predominantly in two regions located symmetrically west and east of the tauern window possible heating mechanisms are discussed that can satisfy these observations and a review of previous thermal models is provided it is shown that models which rely on conductive heat transfer cannot account for all of these observations in order to assess other heat sources the effects of shear heating are evaluated here with simple model calculations and a straightforward comparison of length and time scales of dissipative heat production the results suggest that shear heat episodically released during the stacking of the austroalpine nappe pile may be a significant contributor to the observed thermal perturbation

structural expression of a rolling hinge in the footwall of the brenner line normal fault eastern alps the kinematic and temporal sequence of structures observed to overprint mylonites along the brenner line low angle normal fault may record passage of the footwall through two rolling hinges at the top and bottom of a ramp in the shear zone the structures comprise west down brittle and brittle ductile structures and east down brittle structures pt conditions of formation 250 to 400 c and 2 23 km depth obtained from analysis of oriented fluid inclusion planes indicate that west down structures were formed at greater depths and temperatures and therefore earlier than the east down structures these data suggest that the brittle structures formed under conditions that permit crystal plastic deformation at long term geologic strain rates and therefore probably reflect transient rapid strain rates and or high fluid pressure structures inferred to have formed at a lower hinge are consistent with viscous flow models of rolling hinge deformation and support the concept of a crustal asthenosphere such high temperatures at shallow crustal depth also suggest significant upward advection of heat by extensional unroofing of warm rocks which may have reduced the flexural rigidity of the footwall and thus affected mechanical behavior at the upper rolling hinge exposed mylonitic foliation within a few hundred meters of the brenner line and on top of the east west trending anticlines in the footwall dips 15 west our data favor a ramp dip of 25 but permit a dip as great as 45 fluid inclusion data suggest that structures related to the hinge at the base of the ramp formed at depths of 12 25 km if the average dip of the brenner shear zone to those depths was 20 intermediate between the favored ramp dip and the dip of exposed foliation then the horizontal component of slip could be as high as 33 63 km the two discrete sets of structures with opposite shear senses formed in the temporal sequence indicated by pt data are consistent with subvertical simple shear models of rolling hinge strain this kinematic pattern is not predicted by the flexural failure model for rolling hinges however the predominance of normal slip at the upper hinge which extends rather than shortens the mylonitic foliation fails to match the subvertical simple shear model which predicts shortening of the foliation there one possible solution is that superposition of regional extension upon hinge related stresses modified the rolling hinge kinematics such a modified subvertical shear model can account for the observed small foliation parallel extensional strains if the foliation was bent 5 10 passing through the upper hinge if more bending than that occurred the data suggest rolling hinge kinematics in which deformation is achieved by uniform sense simple shear across the shear zone as in the subvertical simple shear model but in which material lines parallel to the shear zone foliation and the detachment fault undergo very small length changes presumably indicating that footwall rocks retained significant resistance to shear and underwent minimal permanent strain the mechanics that would generate such a rolling hinge are uncertain but may incorporate aspects of both subvertical simple shear and flexural failure an important kinematic consequence of such a rolling hinge is that all of the net slip across a normal fault not only its horizontal component is converted into horizontal extension this implies a significantly larger magnitude of crustal extension across dipping normal faults whose footwalls passed through a rolling hinge than for those that did not develop along with a hinge

empirical garnet muscovite plagioclase quartz geobarometry in medium to high grade metapelites abstract based on the net transfer reactions among garnet muscovite plagioclase and quartz for both mg and fe end member models the garnet muscovite plagioclase quartz gmpq geobarometry was empirically calibrated under the physical conditions of p 1 0 11 4 kbar and t 505 745 c for 128 natural metapelitic rock samples collected from the literature the input temperatures and pressures were simultaneously determined by the garnet biotite thermometer and the garnet aluminosilicate plagioclase quartz gasp barometer the gmpq calibrations adopted the same asymmetric quaternary solid solution model of garnet and the same al avoidance asymmetric ternary model of plagioclase as the calibrations of the garnet biotite geothermometer and the gasp geobarometer a symmetric fe mg al vi ternary solid solution model of muscovite was adopted and the margules parameters of muscovite were obtained through regression the mg and fe model reactions along with the assumption of whether the ferric iron content in muscovite is 0 or 50 resulted in four gmpq barometry formulae the gmpq barometry formulae reproduce the input gasp pressures well within 1 0 kbar mostly within 0 5 kbar for both aluminosilicate bearing and aluminosilicate absent samples the gmpq barometry formulae yield identical pressures for every sample whether the sample was included or not in calibrating the barometers for each of the mg or fe model reaction the formulae gave identical pressures within 40 bars the random error of the gmpq barometry may be expected as 1 4 kbar the d p d t slopes of these gmpq formulae are close to that of the gasp barometer in the p t space applications of the gmpq barometry to aluminosilicate absent metapelites within a limited geographic area without postmetamorphic structural discontinuity generally show no pressure difference it may be concluded that the gmpq barometry formulae derived in this work may be used as practical tools for metamorphic pelites under the conditions of 505 745 c and 1 11 4 kbar in the composition range of x gros 3 in garnet and x an 17 in plagioclase

retrograde evolution of eclogite facies rocks from ne oman evidence from fluid inclusions and petrological data eclogite facies rocks from eastern saih hatat oman metamorphosed under peak conditions of p 12 kbar and t of 550 580 c have several types of fluid inclusions in quartz clinopyroxene and garnet most of these inclusions are two phase at room t and contain an aqueous fluid belonging to the system nacl mgcl2 h2o final melting temperatures for most inclusions range from 4 to 0 c but values as low as 13 c have been recorded indicating salinities of 490 c inclusions with three phases at room t also contain a predominantly low salinity aqueous fluid with xco2 12 kbar to 4 kbar or less at t 500 550 c this conclusion is supported by the scatter in homogenization temperatures for these inclusions and their large variety of shapes and sizes which range from irregular with hair like fractures surrounded by satellite inclusions to negative crystals the replacement of ferroan dolomite ankerite and quartz by calcite talc and of rutile calcite by titanite or rutile glaucophane by ilmenite albite in these eclogites are also consistent with a stage of isothermal decompression following near isothermal decompression to depths of 10 15 km brittle deformation prevailed leading to the formation of layer parallel and crosscutting fractures that were filled by quartz calcite this stage was followed by near isobaric cooling recorded by the isochores for fluid inclusions in vein quartz and by the occurrence of dendritic networks of decrepitated and imploded fluid inclusions in these veins

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

pressure temperature time paths the past decade has been a time of unprecedented improvement in our ability to reconstruct the thermal evolution of orogenic belts like many instances of rapid advancement in science this achievement can be traced to the convergence of disparate research directions the new field of thermochronology the study of the time temperature evolution of rocks derived from the realization by geochronologists that many mineral iso topic systems could be used to determine the time of cooling of a terrain through a series of predictable temperatures e g armstrong 1966 wagner et ai 1977 harrison mcdougali 1980 berger york 1981 petrologists have made great progress in their ability to discern the pressure tem perature history of metamorphic rocks through the improvement of con ventional thermobarometric techniques see bohlen lindsley 1987 and essene 1989 for reviews and the development of important new approaches to relating changes in mineral chemistry to changes in environ mental parameters spear selvcrstonc 1983 st onge 1987 from a theoretical standpoint several geophysical papers in the midto late 1970s and early 1980s explored heat transfer processes in orogenic belts and the role of these in the development and preservation of metamorphic mineral assemblages oxburgh england 1974 bickle et a1 1975 bird et a1 1975 england richardson 1977 toksoz bird 1977 england thompson 1984 among others the integration of geochronologic and petrologic techniques aliows us

deformation partitioning during updoming of the sonnblick area in the tauern window eastern alps austria the sonnblick dome forms a ne vergent dome structure cored by basement gneisses within the southeastern tauern window of the eastern alps austria a succession of ductile and brittle deformation stages documents doming and exhumation subsequent to the thermal peak of metamorphism contrasting deformation geometries within internal parts and along the margins of the dome are explained in terms of deformation partitioning subhorizontal shortening is documented by subvertical en echelon extensional gashes within central parts of the dome subhorizontal as well as subvertical flattening is also documented by fold structures during dextral transpression strike slip is accommodated along the ne margin sonnblick lamella moll valley fault and the southern margin moser fault of the sonnblick dome while vertical thickening occurred within the interior of the dome crustal thickening triggered unroofing and extension parallel to the dome axis which is documented by ductile low angle normal faults in the uppermost structural sections of the dome this normal fault system contributed to footwall uplift and exhumation of the dome structure the sonnblick lamella associated with the dextral moll valley fault forms a stretching fault where deformation is concentrated along a potential zone of weakness this fault is interpreted to represent the transition from vertical thickening within the dome to vertical thinning at the dome margins during upbending the dome structure passed the isotropic stress surface that is characterized by equality of 1 and 2 this is documented in a perturbation of the orientations of principal stress axes 1 and 2 while 3 remains constant in orientation transpression contributed substantially to updoming and to rapid nearly isothermal exhumation subsequent to the thermal peak of metamorphism

pre variscan back arc and island arc magmatism in the tauern window eastern alps abstract pre variscan sequences of the central and eastern part of the tauern window consist of amphibolites plagioclase gneisses and minor intercalations of metapelites and metacherts they were analyzed geochemically and in addition morphological studies of their zircon crystals were carried out the metabasaltic rocks in the eastern tauern window storz group geochemically represent a transitional island arc back arc geotectonic setting they are intercalated with acid plagioclase gneisses in some places which as is implied from the characteristics of their zircon crystal faces are most probably mantle derived metarhyolites biotite plagioclase gneisses with detrital zircon populations predominate over the amphibolites and are interpreted as metagreywackes derived from the nearby volcanic arc a dismembered ophiolite complex stubach group in the central tauern window shows chemical characteristics of a back arc oceanic crust it is overlain mainly by metabasalts metaandesites and metarhyolites habach group with low k to calc alkaline geochemistry representing a primitive island arc in conclusion the pre variscan sequences of the tauern window were formed in an oceanic setting and represent supra subduction oceanic crust and a primitive island arc cratonization of the area did not occur until the intrusion of large masses of s and i type granitoids during the variscan orogeny

the control of precursor brittle fracture and fluid rock interaction on the development of single and paired ductile shear zones abstract ductile shear zones can occur as relatively isolated single structures as arrays or as characteristic paired zones in continuous glaciated exposures of metagranodiorites from the tauern window eastern alps the control of initial dilatant brittle fracture and associated fluid rock interaction on the geometry of subsequent ductile shear zones can be unequivocally established shearing occurred under amphibolite facies conditions fractures in weakly deformed metagranodiorites are often less than 1 mm thick but extend for tens of metres many are healed joints without shear offset others show minor mm cm discrete dextral offset such brittle faults commonly display a low angle en echelon arrangement with displacement transferred between discrete fracture segments by ductile compressive bridges the geometry of more strongly reactivated zones depends on the degree and heterogeneity of fluid rock interaction which is related to fluid infiltration and veining along the primary fractures with little fluid rock interaction reactivation produces single heterogeneous ductile shear zones centred on and immediately flanking the pre existing fracture with increased fluid rock interaction a bleached halo is developed symmetrically to either side of a central epidote quartz garnet calcite vein ductile shear zones commonly flank this bleached zone to develop a characteristic paired pattern strain is partitioned localizing in the central fracture vein and the flanking shear zones paired zones may anastomose in accordance with changes in the width of the central bleached zone but are always symmetrically spaced with regard to the central fracture vein with increasing deformation the ductile shear zones broaden into the adjacent metagranodiorite but not into the bleached zone which remains preserved as a low strain region paired shear zones can also develop to either side of aplite dykes examples of characteristic paired shear zones usually with a clear central vein are found in many areas ranging from greenschist to eclogite facies suggesting that the mechanism of their formation is quite general

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

mantle lithospheric deformation and crustal metamorphism with some speculations on the thermal and mechanical significance of the tauern event eastern alps changes in the thickness of the mantle lithosphere beneath convergent orogens through processes such as the pervasive thickening of the lithosphere and the convective thinning of the thermal boundary layer dictate the heat flow through the base of the crust and thus may profoundly influence the thermal evolution of crustal metamorphic terrains calculations based on one dimensional thermal energy balances show that when mantle lithosphere is substantially thickened and remains intact through the orogenic cycle then low temperature facies series including blueschist and eclogite facies may be preserved throughout the crust in contrast crust thickened above attenuated mantle lithosphere will develop much higher temperature facies series potential energy arguments suggest that changes in the thickness in the mantle lithosphere induce changes in the elevation and the potential energy stored within the lithosphere therefore the response to mantle lithospheric deformation observable in crustal metamorphic terrains should not only be recorded in the thermal regime but also in changes in the incremental strain history in as much as it reflects the force balance operating within the orogen for example mantle lithospheric thinning beneath thickened crust may cause an increase in crustal temperatures synchronous with the termination of convergent strain or for large reductions in mantle lithospheric thickness with the onset of extensional deformation such histories are recorded in a number of young metamorphic terrains for example the eastern alps which may provide an important thermal and isostatic record of the mantle lithospheric response to convergent deformation here we show how application of these ideas may lead to new insights into some outstanding problems concerning the thermal and mechanical evolution of the eastern alps during the tertiary tauern metamorphic event

an exotic eclogite blueschist slice in a barrovian style metamorphic terrain alanya nappes southern turkey a 600 m thick more than 40 km long slice of eclogite fades rocks called the sugozu nappe occurs in the alanya area southern turkey sandwiched between two other crystalline nappes which do not show hp lrmetamorphism all three nappes were affected by a later barrovian type metamorphism and penetrative deformation which welded the nappes into a single tectonic unit the sugozu nappe consists predominantly of garnet mica schists with lenses of eclogite and blueschist metabasites the mineral assemblage in the eclogites garnet omphacite glaucophane paragonite quartz phengite rutile is estimated to have formed at 13 5 1 5 kb and 510 25cc the eclogite assemblage is variably overprinted by a later barrovian metamorphism with the development of barroisite chlorite and albite the extent of the barrovian overprint was controlled by the supply of fluid to the rocks the barrovian metamorphism increases in grade downwards in the structural sequence biotite and garnet isograds are mapped in the lowermost mahmutlar nappe made up mostly of metapelites and metapsammites the metapelites in the garnet zone consist of garnet biotite chlorite muscovite albite oligoclase quartz ilmenite metamorphic conditions are estimated as 6 5 10 kb and 469 13 c the hp l7 rocks of the sugozu nappe underwent a cooling of about 100 c during a dry uplift from a depth of about 48 km to 21 km where they were intercalated with the other nappes and affected by a barrovian metamorphism caused by the incoming fluids the case of the alanya nappes illustrates that the barrovian overprint observed in many eclogites and blueschists may not be due to increasing temperature during uplift but simply due to the introduction of a fluid phase during part of the uplift p t path

pressure temperature paths from p t pseudosections and zoned garnets potential limitations and examples from the zanskar himalaya nw india the extraction of p t histories from metamorphic rocks provides a valuable dataset for the elucidation of the tectonic mechanisms for orogeny while continued re equilibration frequently obliterates early information garnet zonation and inclusion assemblages can often surmount this problem the task is more difficult in high variance assemblages or if inclusions are not preserved but one approach is to use pseudosections that are specific to the bulk composition of a given rock in the latter case the compositions and abundances of all the minerals are fixed at a given p t point such that if the effective bulk composition is known the garnet composition alone can be used to reconstruct the history here we explore this approach using examples from the zanskar himalaya nw india pseudosections have been calculated for four pelitic to semipelitic rocks from the zanskar himalaya and have been contoured for garnet composition the calculations adequately model the mineral assemblages in the rocks and predict the presence of chlorite in the early assemblage where chlorite is found as inclusions within garnet moreover the pseudosections successfully model the garnet core compositions with all three independent compositional contours overlapping at a single pressure and temperature this occurs at 550 c and at pressures varying from 3 7 kbar for the four rocks studied we have been less successful however at modelling garnet compositions beyond the cores because fractionation of the effective bulk composition is caused by garnet growth itself however in this case a combination of the pseudosection and conventional thermobarometry using fe ti inclusions and matrix phases allows us to reconstruct the entire p t history the resulting p t paths record burial of 3 5 kbar without significant temperature increase followed by isobaric heating of 50 100 c this evolution is consistent with himalayan collision in the early tertiary but a combination of the p t data presented here and published geochronological data suggests renewed thrusting south of the suture zone in the oligocene in addition the data demonstrate that no extra heat source is required to cause melting of the himalayan crust in the miocene while melting could have occurred both by dehydration during decompression or in the presence of a fluid the lack of garnet resorption does suggest decompression was rapid and followed quickly by cooling this scenario favours melting by decompression

graphite ch4 h2o co2 equilibria at low grade metamorphic conditions equilibrium calculations and published phase equilibria are used to show that at temperatures below 400 c and pressures above 300 bar the fluids coexisting with graphite in the c o h system consist either of co2 h2o mixtures or ch4 h2o mixtures bulk fluid compositions consisting of ch4 h2o will unmix to methane rich fluid and h2o rich liquid at temperatures below about 325 c compositions on the co2 h2o join will unmix only at temperatures below about 275 c oxygen fugacity in the ch4 h2o graphite three phase region is fixed at constant p and t and approximately equal to that of the quartz magnetite fayalite assemblage in the co2 h2o graphite three phase region oxygen fugacity is about three orders of magnitude greater from the end stages of diagenesis to temperatures of 300 c and possibly 400 c in salt rich systems many metamorphic rocks may contain ch4 h2o fluids rather than co2 h2o mixtures thus metamorphic reactions involving carbonate minerals would involve ch4 graphite and h2o rather than co2 the immiscibility between ch4 and h2o could result in the common occurrence of methane natural gas in low grade metamorphic terrains

fluid inclusions as petrogenetic indicators in granulite xenoliths pali aike volcanic field chile granulite xenoliths within alkali olivine basalts of the pali aike volcanic field southern chile contain the mineral assemblage orthopyroxene clinopyroxene plagioclase olivine green spinel these granulites are thought to be accidental inclusions of the lower crust incorporated in the mantle derived basalt during its rise to the surface symplectic intergrowths of pyroxene and spinel developed between olivine and plagioclase imply that the reaction olivine plagioclase al orthopyroxene al clinopyroxene spinel 1 occurred during subsolidus cooling and recrystallization of a gabbroic protolith of the granulites

evidence for east west crustal extension in the eastern alps implications for the unroofing history of the tauern window the contact between the western border of the tauern window and the overlying austroalpine nappes austria and italy is marked by a structural petrologic and geochronologic discontinuity that developed in the early stages of unroofing of the window within the window pressure temperature time paths j selverstone 1985 indicate that significant ductile thinning occurred shortly after the cessation of thrusting paleocene eocene sense of shear indicators rotated porphyroblasts asymmetric augen and pressure shadows s c fabrics associated with a west plunging stretching lineation indicate that this thinning occurred in response to top to the west low angle normal shear during metamorphism the same sense of normal shear is present in the mesozoic members of the austroalpine sequence west of the window in these rocks however west directed shear occurred at low temperatures 300 c after the thermal peak of metamorphism structural data and the regional geometry imply that the western end of the tauern window is a low angle normal fault brenner line that juxtaposed brittlely deformed rocks of the austroalpine nappes against more ductilely deformed rocks from the window biotite k ar and rb sr data suggest a miocene age for final movement on the brenner line these data indicate a prolonged history of east west extension in the eastern alps that affected all crustal levels early ductile thinning of lower crust followed by mid to upper crustal low angle normal faulting a model is presented that relates the young east west extension to displacement transfer from dextral movement on the periadriatic lineament in this model the brenner line and by analogy the simplon line can be thought of as detachment faults that tectonically unroofed the tauern window and lepontine thermal highs respectively

oxygen isotope geochemistry of the amphiboles isotope effects of cation substitutions in minerals abstract the occurrence of coexisting amphiboles in rocks and the likelihood of concurrent isotope closure allows equilibrium oxygen isotope fractionations among the amphiboles to be recovered from natural samples oxygen isotope analyses of mineral separates using laser fluorination show that coexisting amphiboles increasingly partition 18o in the order hornblende gedrite 0 9 previously published data for hornblende actinolite glaucophane and garnet show that act hbl 0 2 gln grt 1 and hbl grt 0 thus glaucophane strongly partitions 18o relative to the calcic amphiboles the fractionation between two amphiboles of arbitrary composition can be predicted from the known fractionations for mica endmembers pyroxene endmembers and exchange components such as caal nasi 1 naal camg 1 camg 1 mgfe 1 femn 1 kna 1 kal si 1 and fe3 al 1 applications of the exchange component method reproduce measured amphibole fractionations to within 0 1 to 0 2 whereas other predictive methods cause misfit for typical metamorphic hornblende of 0 5 at 575 c although the isotope effects of cation exchanges may be small at high t they magnify dramatically for minerals formed in surficial diagenetic and low t metamorphic environments different composition clays are predicted to have equilibrium 18o differences of 2 9 if the isotope fractionation can be determined for one mineral endmember then calibrated exchanges allow accurate prediction of the isotope fractionations for intermediate compositions of most ortho ring chain and sheet silicates

evolution of regional metamorphism in the cape smith thrust belt northern quebec canada interaction of tectonic and thermal processes syn metamorphic re imbrication of the internal part of thrust belts can result in distinct pressure temperature time deformation p t t d pathways for different structural metamorphic domains in the early proterozoic cape smith thrust belt canada an external piggyback sequence thrusting domain is characterized by thermal peak metamorphism occurring after deformation in contrast thermal peak metamorphism in an internal domain occurred during re imbrication by out of sequence thrusting the interactions of tectonic and thermal processes have been studied using three methods i qualitative evaluation of the timing between mineral growth and deformation ii analytical p t paths from growth zoned garnet porphyroblasts and iii numerical modelling of vertical heat conduction derived p t t d pathways suggest that uplift in the external domain resulted in part from erosion and isostatic unloading in contrast paths for the internal domain indicate that the out of sequence portion of the thrust belt may have experienced faster unroofing relative to the external domain this is attributed to thickening by out of sequence thrusting and possibly to extensional faulting at now eroded higher structural levels observations on the timing of metamorphism coupled with numerical modelling suggest that the thermal peak metamorphism documented in the external domain is a consequence of the emplacement of the out of sequence thrusts stack in the internal portion of the thrust belt

inverse ductile thinning via lower crustal flow and fold induced doming in the west carpathian eo alpine collisional wedge 1 continental core complexes are generally interpreted to result from extensional doming due to gravity driven upflow of lower crust in contrast the vepor dome is characterized by the lack of inverted density profile and relatively cold metamorphic field gradient which precludes an activation of rayleigh taylor instability instead the crustal structure of the vepor unit is marked by dense and weak metapelitic lower crust and light and strong granitoid upper crust inherited from variscan nappe stacking it is shown that the cretaceous eo alpine tectonic evolution of the vepor dome is controlled by the dynamics of two neighboring mechanically strong continental blocks i e the overthrusting of the suprastructural gemer unit from the south and the underthrusting of the fatric basement from the north structural metamorphic and geochronological data from the vepor unit imply two main phases of the convergent process 1 lower cretaceous crustal thickening due to overthrusting and internal deformation of the gemer unit together with upper crustal folding in the vepor unit led to the progressive development of the orogenic front parallel pressure gradient the instantaneous response of the lower crustal and low viscosity metapelites led to an along strike lower crustal flow accompanied by prograde barrovian type metamorphism 2 as the south vergent underthrusting of the fatric basement propagated to greater depths during the upper cretaceous the convergent process switched from top driven to bottom driven and the exhumation of the lower crust occurred via polyharmonic folding overall doming of the vepor unit induced upper crustal detachment faulting and eastward unroofing of the dome

relating collisional kinematics to exhumation processes in the eastern alps abstract based on a review of the surface and deep structure of the eastern alps we link the timing and the inferred displacement fields to exhumation of upper and lower crustal units of the orogenic nappe stack during collision the discussion focuses mainly on the tauern window and its country rocks the only area of the eastern alps where the orogenic wedge from its uppermost austroalpine nappes down to its deepest european basement nappes is continuously exposed we summarize and discuss the long standing controversy on the mechanisms of exhumation of this nappe stack on the base of a synthesis of structural and geochronological data and restorations of collisional displacements both in cross sections and map views we conclude that the large amounts of exhumation assessed for the western eastern alps resulted from large amounts of thickening and erosion not observed in the eastern part of the eastern alps extensional faults laterally bounding the area of major thickening and exhumation are inferred to nucleate in order to accommodate displacement around the indenter corner in the west and in order to reduce a large gradient of crustal thickness and surface elevation in the east restorations to the pre indentation stage document an amount of northward increasing orogen parallel extension varying 45 km to 85 km corresponding to 15 of extension that is partly accommodated along normal faults n s shortening between the northern calcareous alps and the dolomites indenter attained 75 km in the west and decreased to 30 km in the east 55 km out of these 75 were accommodated in the area of the tauern window our kinematic model shows that lateral extrusion accommodated along conjugate strike slip faults requires large amounts of north south shortening in the western part of the eastern alps such shortening is consistent with the reconstructed upright folding and erosion of the tauern window thus explaining the largest amount of its exhumation in contrast the eastern termination of the eastern alps represents an area where collisional convergence was barely accommodated by crustal thickening this transition from a highly shortened thickened and exhumed wedge in the west mainly affected by orogen perpendicular displacements to a barely shortened and exhumed wedge in the east mainly characterised by orogen parallel displacements spatially coincides with a change in the deep structure of the european slab indeed the inferred continental european slab imaged in the west disappears into a low velocity anomaly where no slab is detected in the east an inherited step in the geometry in map view of the european passive margin causing its crust to enter the subduction zone earlier than the area east of the tauern window may explain the rapid decrease of shortening of thickening the different syn collisional p t gradients and the disappearance of the continental slab east of the katschberg fault