structural and metamorphic history of a segment of the sesia lanzo zone and its bearing on the kinematics of alpine deformation in the western alps

geophysical geological transect and tectonic evolution of the swiss italian alps a complete alpine cross section integrates numerous seismic reflection and refraction profiles across and along strike with published and new field data the deepest parts of the profile are constrained by geophysical data only while structural features at intermediate levels are largely depicted according to the results of three dimensional models making use of seismic and field geological data the geometry of the highest structural levels is constrained by classical along strike projections of field data parallel to the pronounced easterly axial dip of all tectonic units because the transect is placed close to the western erosional margin of the austroalpine nappes of the eastern alps it contains all the major tectonic units of the alps a model for the tectonic evolution along the transect is proposed in the form of scaled and area balanced profile sketches shortening within the austroalpine nappes is testimony of a separate cretaceous age orogenic event west directed thrusting in these units is related to westward propagation of a thrust wedge resulting from continental collision along the meliata hallstatt ocean further to the east considerable amounts of oceanic and continental crustal material were subducted during tertiary orogeny which involved some 500 km of n s convergence between europe and apulia consequently only a very small percentage of this crustal material is preserved within the nappes depicted in the transect postcollisional shortening is characterized by the simultaneous activity of gently dipping north directed detachments and steeply inclined south directed detachments both detachments nucleating at the interface between lower and upper crust large scale wedging of the adriatic or apulian lower crust into a gap opening between the subduced european lower crust and the pile of thin upper crustal flakes alpine nappes indicates a relatively strong lower crust and detachment between upper and lower crust

partitioning of deformation within a subduction channel during exhumation of high pressure rocks a case study from the western alps the metamorphic belt of the western alps was subjected to widespread extensional tectonism at the end of the eocene ca 45 35 ma extension was accommodated by hinterland directed movements along gently inclined extensional shear zones which facilitated rapid exhumation of high pressure and ultra high pressure rocks this deformation resulted in a normal metamorphic sequence extension in the inner parts of the western alps was coeval with shortening at the front of the belt foreland directed thrusts which took place during decompression and emplaced higher grade metamorphic units over lower grade metamorphic rocks thus forming an inverse metamorphic sequence two mechanisms for this extensional episode are discussed 1 collapse of an overthickened lithosphere and 2 internal readjustments within the orogenic wedge due to subduction channel dynamics we favour the latter mechanism because it can account for the development of the observed inverse and normal metamorphic sequences along foreland directed thrusts and hinterland directed detachments respectively this hypothesis is supported by published structural metamorphic and geochronological data from four geological transects through the western alps this study also emphasizes the importance of post shearing deformation e g horizontal buckling versus vertical flattening which can modify the distribution of hinterland and foreland directed shear zones in orogenic belts

formation burial and exhumation of basement nappes at crustal scale a geometric model based on the western swiss italian alps abstract information from geological and reflection seismic data from the western swiss italian alps and from numerical models is used to build a geometrical model that can explain some of the major tectonic metamorphic features of alpine type basement nappes the model gives a geometrical and mechanical explanation for the initiation burial and subsequent uplift and partial exhumation of basement nappes at a crustal scale three main tectonic stages during convergence are distinguished and each correlated with the formation of specific nappe structures the first two stages are single vergent nw and correspond to the subduction of continental margin crust and the formation and uplift of high pressure rocks simple shear flow and superimposed wedge shaped pure shear flow is proposed for the creation and intrusion of high pressure nappes of the monte rosa type the third stage is characterized by a doubly vergent style with both pro and retro movements the former created nw vergent nappes as seen in the external alpine massifs and the latter caused backfolding and thrusting typical of the internal alps this third stage corresponds to the neoalpine movements in the western swiss italian alps and is accompanied by a generalized uplift mountain building and molasse sedimentation

prograde high to ultrahigh pressure metamorphism and exhumation of oceanic sediments at lago di cignana zermatt saas zone western alps pelagic metasediments and morb type metabasalts of the former tethyan oceanic crust at cignana valtournanche italy experienced uhp metamorphism and subsequent exhumation during the early to late tertiary maximum pt conditions attained during uhp metamorphism were 600 630 c 2 7 2 9 gpa which resulted in the formation of coesite glaucophane eclogites in the basaltic layer and of garnet dolomite aragonite lawsonite coesite phengite bearing calc schists and garnet phengite coesite schists with variable amounts of epidote talc dolomite na pyroxene and na amphibole in the overlying metasediments during subduction the rocks followed a prograde hp uhp path which in correspondance with the jurassic age of the tethyan crust reflects the thermal influence of relatively old and cold lithosphere and of low to moderate shear heating inflections on the prograde metamorphic path may correspond to thermal effects that arise from a decrease in shear heating due to brittle plastic transition in the quartz aragonite dominated rocks induced convection in the asthenospheric mantle wedge and or heat consumption by endothermic reactions over a restricted pt segment during subduction after detachment from the downgoing slab some 50 70 ma before present the cignana crustal slice was first exhumed to ca 60 km and concomitantly cooled to ca 550 c tracing back the uhp hp prograde path displaced by 50 80 c to higher temperatures exhumation at this stage is likely to have occurred in the benioff zone while the subduction of cool lithosphere was going on subsequently the rocks were near isothermally exhumed to ca 30 km followed by concomitant decompression and cooling to surface conditions at 500 c 1 gpa during this last stage the uhpm slice arrived at its present tectonic position with respect to the overlying greenschist facies combin zone in contrast to the well preserved hp uhpm record of the coesite glaucophane eclogites the hp uhp assemblages of the metasediments have been largely obliterated during exhumation relics from which the metamorphic evolution of the rocks during prograde hp metamorphism and the uhp stage can be retrieved are restricted to rigid low diffusion minerals like garnet dolomite tourmaline and apatite

metamorphism and diachronous cooling in a contractional orogen the strandja massif nw turkey the southern part of the strandja massif northern thrace turkey comprises a basement of various gneisses micaschists and rare amphibolite and a cover of metaconglomerate and metasandstone separated from each other by a pre metamorphic unconformity metamorphic grade decreases from the epidote amphibolite facies in the south to the albite epidote amphibolite greenschist facies transition in the north estimated p t conditions are 485 530 c and 0 60 0 80 gpa in the epidote amphibolite facies domain and decrease towards the transitional domain between greenschist and epidote amphibolite facies rb sr muscovite ages range from 162 9 1 6 ma to 149 1 2 1 ma and are significantly older 279 296 ma in the northernmost part of the study area the rb sr biotite ages decrease from 153 9 1 5 ma in the south to 134 4 1 3 ma in the north these age values in conjunction with the attained temperatures suggest that the peak metamorphism occurred at around 160 ma and cooling happened diachronously and rb sr muscovite ages were not reset during the metamorphism in the northernmost part structural features such as i consistent s dipping foliation and sw to se plunging stretching lineation ii top to the n shear sense and iii n vergent ductile shear zones and brittle thrusts suggest a n vergent compressional deformation coupled with exhumation we tentatively ascribe this metamorphism and subsequent diachronous cooling to the northward propagation of a thrust slice the compressional events in the strandja massif were most probably related to the coeval n vergent subduction collision system in the southerly lying rhodope massif

a microstructural and argon laserprobe study of shear zone development at the western margin of the nanga parbat haramosh massif western himalaya a sample of banded amphibolite from the western margin of the nanga parbat haramosh massif as sassi has been studied using microstructural and 40ar 39ar laserprobe techniques to investigate the relationship between deformation and argon isotope variations in a natural system amphibolite grade deformation occurred during south directed overthrusting of the kohistan arc over india along the main mantle thrust and was overprinted by extensional reactivation of the earlier fabric and the formation of biotite rich shear zones subsequent deformation along discrete fine grained fault zones was characterised by the formation of scapolite chlorite and k feldspar early plastic deformation and later cataclasis different minerals developed during this history show a wide range in apparent 40ar 39ar ages biotite chlorite and scapolite exhibit much lower concentrations of excess argon indicating their equilibration in a fluid relatively poor in excess argon a true age of ca 8 ma from biotite represents a minimum age for deformation associated with formation of the nanga parbat syntaxis and also precludes pliocene metamorphism in this area of the syntaxis both high and low closure temperature minerals amphiboles and feldspars record apparent ages which are associated with the incorporation of excess argon within the mineral lattice although differential thermal resetting of minerals at different closure temperatures is important variations in the inherited 40ar 36ar ratio throughout the sample is dominated by deformation and fluid infiltration consequently it appears that within deforming metamorphic rocks areas with significantly different argon isotope compositions may be present and need not be homogenised by diffusion

an empirical evaluation of the argon diffusion geometry in muscovite the temperature time significance of 40ar 39ar muscovite ages is difficult to evaluate quantitatively because of ambiguities in the choice of an appropriate model diffusion geometry the effective diffusion dimension and large uncertainties in experimentally determined diffusion parameters laser 40ar 39ar studies of natural 40ar concentration gradients suggest that argon loss in muscovite occurs primarily via cylindrical volume diffusion commonly with an effective diffusion dimension equal to the crystal diameter the present study tests the cylinder model geometry and the control of physical grain size on the effective diffusion dimension by measuring the anisotropy of a natural40ar diffusion gradient parallel and perpendicular to 001 in a single euhedral muscovite phenocryst the new data combined with previous studies clearly indicate loss of 40ar via cylindrical volume diffusion and strongly suggest that loss of 40ar across the 001 sheets slab diffusion geometry is minimal in undeformed crystals

direct dating of mylonite evolution a multi disciplinary geochronological study from the moine thrust zone nw scotland rb sr dating of sub closure temperature syn deformationly crystallized white micas from the moine mylonites knockan and dundonnell has yielded ages which vary from 437 ma to 408 ma morphological and major element analyses of the micas indicate that all the micas within the analysed samples were re crystallized during the moine thrust shearing the ages yielded are therefore interpreted as marking the end of crystal plastic deformation associated with shearing on the moine thrust the variation in ages between samples is significantly greater than the individual analytical errors and may be a product of strain localization within the shear zone alternatively it may indicate flow of strontium rich fluids derived from outside the shear zone implying that feldspars may act as potentially unreliable initial isotopic reservoirs even in very high strain samples within the greenschist facies shear zones however the rb sr age are internally consistent and compatible broadly with existing geological and geochronological data for the region suggesting that even if the feldspars have not acted as perfect initial isotopic reservoirs the resulting errors were minimal the general cessation of ductile deformation at c 430 ma probably dates the general initiation of imbrication within footwall to the moine thrust the age of 430 ma is therefore likely to be a time of significant brittle displacement on the underlying ben more thrust k ar dating of the re crystallized moine mylonite micas yielded anomalously old ages in comparison to rb sr values as a result of incorporation of excess ar during shearing the degree of incorporated excess ar decreases away from the base of the shear zone into the hanging wall becoming indiscernible at c 1 5 km structurally above the base of the shear zone the distance c 1 5 km may represent the distance that fluid can have flowed out of the shear zone during deformation

from permo triassic lithospheric thinning to jurassic rifting at the adriatic margin petrological and geochronological record in valtournenche western italian alps abstract slices of polycyclic metasediments marbles and meta cherts are tectonically amalgamated with the polydeformed basement of the dent blanche tectonic system along a major alpine shear zone in the western alps becca di sale area valtournenche valley a combination of techniques structural analysis at various scales metamorphic petrology geochronology and trace element geochemistry was applied to determine the age and composition of accessory phases titanite allanite and zircon and their relation to major minerals the results are used to reconstruct the polyphase structural and metamorphic histories comprising both pre alpine and alpine cycles the pre alpine evolution is associated with low pressure high temperature metamorphism related to permo triassic lithospheric thinning in meta cherts microtextural relations indicate coeval growth of allanite and garnet during this stage at 300 ma textures of zircon also indicate crystallisation at ht conditions ages scatter from 263 to 294 ma with a major cluster of data at 276 ma in impure marble u pb analyses of titanite domains with variable al and f contents yield apparent 206pb 238u dates range from permian to jurassic chemical and isotopic data suggest that titanite formed at permian times and was then affected by extension related fluid circulation during the triassic and jurassic which redistributed major elements al and f and partially opened the u pb system the alpine cycle lead to early blueschist facies assemblages which were partly overprinted under greenschist facies conditions the strong alpine compressional overprint disrupted the pre alpine structural imprint and or reactivated earlier structures the pre alpine metamorphic record preserved in these slices of metasediments reflects the onset of the permo triassic lithospheric extension to jurassic rifting

structural evolution in thrust belts and relative plate motion the upper pennine piemont zone of the internal alps southwest switzerland and northwest italy the earliest ll extension lineation of the polyphase deformation sequence in the combin and zermatt saas fee units of the piemont zone of the upper pennine nappes trends roughly nw se and the geochronological and stratigraphical constraints indicate that the nappe transport and the associated stretching lineation occurred between about 100 and 70 ma this accords in a general way with the relative motion of the african plate to the european plate during the late cretaceous as determined from reconstructions of the opening of the atlantic ocean and suggests that late cretaceous thrusting in the swiss alps was driven fairly directly by the africa europe plate motion

the western alps from the jurassic to oligocene spatio temporal constraints and evolutionary reconstructions abstract despite extensive research in the last 150 years the regional tectonic reconstruction of the western alps has remained controversial the curved orogenic belt consists of several ribbon like continental terranes sesia austroalpine internal crystalline massifs brianconnais which are separated by two or more ophiolitic sutures piemonte valais antrona lanzo canavese high pressure hp metamorphism of each terrane occurred during distinct orogenic episodes at 65 ma in the sesia austroalpine at 45 ma in the piemonte zone and at 35 ma in the internal crystalline massifs it is suggested that these events reflect individual accretionary episodes which together with kinematic indicators and the speed and direction of plate motions provide constraints for the discussed reconstruction model the model involves a prolonged orogenic history that took place during relative convergence of europe and adria here considered as a promontory of the african plate the first accretionary event involved the sesia austroalpine terrane final closure of the piemonte ocean occurred during the eocene 45 ma and involved ultra high pressure uhp metamorphism of the piemonte oceanic crust incorporation of the brianconnais terrane in the accretionary wedge occurred thereafter possibly during or after subduction of the valais ocean in the late eocene 45 35 ma this subduction was terminated at ca 35 ma when the internal crystalline massifs i e the assumed internal parts of the brianconnais terrane were buried into great depths and underwent hp and uhp metamorphism

from passive margins to orogens the link between ocean continent transition zones and ultra high pressure metamorphism a lithostratigraphic association consisting of serpentinized mantle rocks continent derived allochthons mid oceanic ridge gabbros of jurassic age and post rift sediments typical of an ocean continent transition is found in the eclogitic piemonte units in the western alps in situ u pb geochronology was performed on zircons from an orthogneiss sampled at the bottom of a sliver of continental basement in contact with serpentinites primary magmatic zircons of permian age were overgrown by a second generation of zircon at ca 166 150 ma likely related to melt infiltration associated with the intrusion of the underlying gabbroic body this indicates that continental basement slices and oceanic basement rocks were already juxtaposed in the jurassic and they were probably part of hyper extended crust related to the opening of the tethys therefore the complex lithological association described here which is also characteristic of several ultra high pressure melange zones worldwide was acquired prior to the orogenic event during which it was only partly reworked ocean continent transitions are in positions favorable to reach ultra high pressure conditions following negatively buoyant oceanic lithosphere into subduction and then being accreted to the orogen in response to the arrival of more buoyant continental lithosphere resisting subduction the ocean continent transition is now found in the immediate footwall of a 500 m thick shear zone which accommodated multiple episodes of deformation during eocene oligocene time suggesting an important link between alpine deformation and rift related structures

kinematic and temporal relationships between parallel fold hinge lines and stretching lineations a microstructural and crystallographic preferred orientation approach folds with hinge lines subparallel to stretching lineation are common features in shear zones and represent extreme cases of fold modification however their microstructures and in particular their kinematic significance remain poorly understood here we describe the meso and micro scale structures and crystallographic preferred orientation cpo of quartz and muscovite from a metric scale shear modified fold from plattjen saas fee western internal alps the studied fold is of class 2 and the samples collected around the fold have essentially the same microstructures quartz c axes plotted in the tectonic coordinate system exhibit an apparent reverse obliquity on opposing limbs muscovite fabrics are controlled essentially by the orientation of 001 planes if the cpo data are plotted in a common reference frame e g either fold or geographical all samples exhibit essentially the same patterns for both minerals in particular relative to the fold axis and axial surface the reversal of quartz cpo obliquity on opposing limbs is no longer evident the small obliquity 15 between the intersection lineation of the two planes of 001 of muscovite and the fold axis results in the apparent reversal of shear sense on opposite limbs when viewing the cpo related to the macroscopic fabric the homogeneity of the microstructures and cpos of quartz and muscovite across the fold support the interpretation that the fold developed during simultaneous activation of heterogeneous and homogeneous simple shear with the former responsible for fold development and the latter responsible for fold rotation and the overwhelming of the heterogeneous deformation these processes led to the observed parallelism between fold hinge line and local stretching lineation

opposite vergence of nappes and crustal extension in the french italian western alps in the internal pennine zone of the french italian western alps dora maira monviso rocciavre massifs and queyras region the structural trends define an apparent dome geometry that is truncated to the east by the po basin this geometry is classically interpreted as a backthrusting system the kinematic history of the area has been investigated using structural metamorphic and geochronologic data on the basis of which two main deformation episodes have been distinguished that occurred under differing metamorphic conditions 1 the first deformation occurred under greenschist facies metamorphism it caused a regional scale foliation and an ew trending lineation that lie parallel to ductile deformation zones and thrust contacts sense of shear indicators associated with the lineation indicate contradictory top to the east and top to the west senses of shear from these it is deduced that coeval east and west vergent thrust faulting accommodated major displacements between nappes phengite ar ar data yield late eocene early oligocene ages for the greenschist facies metamorphism 2 the second deformation caused the widespread development of high angle shear bands and normal faults on opposite sides of the dora maira massif resulting in the apparent dome geometry of the massif a steep 50 80 network of west dipping normal faults is particularly well developed on the west side of the dora maira massif differences of 2 3 kbar and 100 150 c between juxtaposed monviso rocciavre and queyras nappes implies 8 km of vertical displacement along the monviso upper tectonic contact these normal faults formed in a more brittle regime only a post early oligocene age can be estimated from microstructural studies for these final movements these deformations postdate hp lt metamorphism and resulted in the juxtaposition of nappes of distinctly different origin age grade and p t t history a kinematic model is presented in which opposite vergence of nappes followed by large scale crustal extension occurred in response to uplift during the convergence between the european and south alpine plates this model may account for rapid uplift rapid unroofing and cooling of high pressure low temperature rocks thereby explaining the absence of amphibolite facies metamorphism overprint in this part of the alps that may have led to the preservation of coesite bearing rocks in the southern part of the dora maira massif

criteria for identifying structures related to true crustal extension in orogens faults and shear zones in orogens may be related to crustal shortening or extension or both true extensional structures should ultimately intersect the earth s surface when traced out in a direction opposite to that of hangingwall transport the present dip of a fault in an orogen and its relation to local layering are unreliable guides the observation of low pressure metamorphic rocks structurally above high pressure metamorphic rocks is not sufficient to diagnose crustal extension a valid metamorphic criterion is the difference in pressure time histories between footwall and hangingwall if the footwall was decompressing faster than the hangingwall during shear zone movement then the shear zone was truly extending the crust the pressure time history can be constrained via the temperature time and pressure temperature histories ideally crustal extension inferred from structural data should be confirmed using diagnostic ptt information

in situ uv laser 40ar 39ar geochronology of a micaceous mylonite an example of defect enhanced argon loss muscovite and biotite from a crustal scale mylonite zone pogallo shear zone southern alps were investigated using furnace step heating and in situ uv laser ablation 40ar 39ar geochronology undeformed muscovite porphyroclasts yield 40ar 39ar plateau ages of 182 0 1 6 ma whereas in situ uv laser ablation 40ar 39ar dating and furnace step heating of strongly deformed muscovite and biotite grains display a range of apparent ages that are systematically younger the range of 40ar 39ar ages measured in the deformed muscovite and biotite is consistent with protracted cooling through argon closure in minerals that exhibit variably developed segmentation on the intra grain scale these microstructurally controlled segments are bound by either first order lattice discontinuities sub microscopic structural defects and or zones of high defect density which create variable length scales for intragranular argon diffusion the observed deformational microstructures within muscovite and biotite acted as intra grain fast diffusion pathways in the slowly cooled mylonitic rocks therefore the high spatial resolution 40ar 39ar data record the initial and final closure to argon diffusion over a time span of about 60 ma

kinematic linkage between internal zone extension and shortening in more external units in the nw alps in the alps a major extensional shear zone unroofed eclogites in its footwall from 30 kbar to c 10 kbar implying of the order of 60 km of relative vertical movement in the period 45 36 ma the horizontal component of divergence was of the order of 100 km we use the foreland basin sedimentary record to prove that the extension was contemporaneous with thrusting in units structurally beneath the eclogites the brianconnais and or external zones this thrust related convergence during that time period was of the order of 44 56 km and therefore of the same order of magnitude as internal zone divergence extension may have been driven by internal density contrasts buoyant continental crust underneath the mafic eclogites or dense subducted slab offset from the orogen can induce extension any dynamic model must account for the comparable magnitudes of extension and shortening in adjacent parts of the orogen relative plate motions in the period 45 36 ma played a subsidiary role

horizontal contraction or horizontal extension heterogeneous late eocene and early oligocene general shearing during blueschist and greenschist facies metamorphism at the pennine austroalpine boundary zone in the western alps mylonitic structures related to two orogenic events are described from the upper and lower contacts of the combin zone and the immediately overlying upper austroalpine dent blanche nappe mont mary klippe and the directly underlying lower austroalpine etirol levaz slice the first event late eocene in age commenced during blueschist facies p t conditions but pre dated the peak of subsequent greenschist facies overprint the second event early oligocene in age took place during retrograde greenschist facies conditions most sense of shear indicators associated with the retrograde mylonites indicate top se shearing but subordinate top nw displacing shear sense indicators have also been mapped mylonitic top se shearing appears to be restricted to the combin zone and its upper and lower contacts within the dent blanche nappe and mont mary klippe and at the base of the etirol levaz slice structures were observed which developed during blueschist greenschist facies conditions and are in conjunction with the p t t history of these rocks inferred to be older associated kinematic data indicate a top nw shear sense comparable blueschist greenschist facies shear sense indicators have not been observed in the combin zone nonetheless the foliation in the combin zone shows a progressive evolution from blueschist facies to greenschist facies to retrograde greenschist facies conditions this indicates that the combin zone and the immediately over and underlying austroalpine units shared a common tectono metamorphic evolution since the late eocene finite strain data reveal oblate strain fabrics which are thought to result from a true flattening strain geometry flow path modelling reveals a general non coaxial deformation regime and corroborates significant departures from a simple shear deformation in the study area mylonitic top se shearing in the combin zone is attributed to early oligocene backfolding and backthrusting of the mischabel phase temperature time curves suggest slight reheating in the monte rosa nappe underneath and cooling in the dent blanche nappe above the combin zone hence confirming a thrust interpretation for this event the top nw displacing structures are thought to result from late eocene emplacement of the dent blanche nappe and the combin zone onto the middle pennine barrhorn series along the combin fault as related structures initiated during mildly blueschist facies conditions in the dent blanche nappe and the underlying combin zone and both were emplaced together onto the greenschist facial barrhorn series it is concluded that the structures developed as the nappes moved upward relative to the earth s surface thus the combin fault is regarded as a thrust the geometry of this structure indicates that the combin fault is an out of sequence thrust that locally cut down section hence top nw out of sequence thrusting caused local thinning of the metamorphic structural section in association with horizontal shortening out of sequence thrusts cutting down section and back thrusts offer the possibility of explaining the pronounced break in the grade of metamorphism across the combin fault i e the contact between the eclogite facial zermatt saas zone and the overlying lower grade combin zone by contractional deformation

k ar age determinations on phengites from the internal part of the sesia zone western alps italy k ar age determinations have been carried out on various well defined phengite populations from a small area of the internal part of the sesia zone lower aosta valley there the rocks have suffered high pressure metamorphism attributed to early alpine subduction in the stability field of jadeite quartz p 15 1 kbar at t 550 50 c coarse grained phengites from well preserved high pressure parageneses and phengites re crystallized early during decompression at still elevated temperatures in the stability field of albite yield identical ages close to 80 ma in the most external sample high pressure phengites yield 72 ma the ages around 80 ma are interpeted as essentially undisturbed cooling ages they are not notably influenced by paragenesis chemical composition polymorphism grain size deformation and recrystallization at higher temperatures this part of the sesia zone has cooled down about 80 ma ago to below the blocking temperature for the k ar system of white mica some of the samples show pronounced retrogression of the high pressure assemblages especially formation of albite and fine grained phengite at the expense of jadeite and are affected by intense late deformation at temperatures of about 300 to 350 c estimated from the small grain size of dynamically recrystallized quartz the ages obtained from extremely kinked or sheared coarse high pressure phengites scatter considerably being partly higher partly lower than 80 ma the fine grained phengite fractions from these samples yield lowered ages down to 70 ma these values are interpreted as mixed ages resulting from variable mixtures of fragmented high pressure phengites and new phengite replacing jadeite the k ar data suggest that the new phengites have crystallized between 60 and 70 ma b p all phengites formed at low temperatures at the expense of jadeite reveal high si contents these range up to 6 9 for the coarser grained earlier generations and up to 6 7 for the very fine grained last generation relatively high pressures are required to stabilize phengites so high in si this implies that cooling had not been achieved by uplift and erosion obviously the thermal structure of the crust was still strongly perturbed by continuing subduction and thrusting during the late cretaceous

40ar 39ar ages in deformed potassium feldspar evidence of microstructural control on ar isotope systematics detailed field and microstructural studies have been combined with high spatial resolution ultraviolet laser 40ar 39ar dating of naturally deformed k feldspar to investigate the direct relationship between deformation related microstructure and ar isotope systematics the sample studied is a 1 000 ma torridonian arkose from skye scotland that contains detrital feldspars previously metamorphosed at amphibolite facies conditions 1 700 ma the sample was subsequently deformed 430 ma ago during caledonian orogenesis the form and distribution of deformation induced microstructures within three different feldspar clasts has been mapped using atomic number contrast and orientation contrast imaging at a range of scales to identify intragrain variations in composition and lattice orientation these variations have been related to thin section and regional structural data to provide a well constrained deformation history for the feldspar clasts one hundred and forty three in situ 40ar 39ar analyses measured using ultraviolet laser ablation record a range of apparent ages 317 1030 ma the k feldspar showing the least strain records the greatest range of apparent ages from 420 1 030 ma with the oldest apparent ages being found close to the centre of the feldspar away from fractures and the detrital grain boundary the most deformed k feldspar yields the youngest apparent ages 317 453 ma but there is no spatial relationship between apparent age and the detrital grain boundary within this feldspar the oldest apparent ages are recorded from orientation domain boundaries and fracture surfaces where an excess or trapped 40ar component resides orientation contrast images at a similar scale to the ar analyses illustrate a significant deformation related microstructural difference between the feldspars and we conclude that deformation plays a significant role in controlling ar systematics of feldspars at both the inter and intragrain scales even at relatively low bulk strains the data show that ar loss and trapping within the deformed k feldspars reflects the presence of a deformation induced population of small diffusion domains in combination with short circuit diffusion along deformation induced defects the complex history of microstructures induced in the k feldspars during their cooling alteration erosion and sedimentation do not appear to be as significant as deformation induced microstructures in controlling the distribution of apparent ages at the grain scale

potential of radarsat 2 stereo radargrammetry for the generation of glacier dems the study of glaciers and ice caps in remote and cloudy regions remains difficult using current remote sensing tools here the potential of stereo radargrammetry srg with radarsat 2 wide ultra fine images is explored for dem extraction elevation changes and mass balance calculations on barnes ice cap nunavut canada over low relief terrain surrounding barnes a vertical precision of 7 m 1 confidence level is measured as well as an average vertical bias of 4 m moreover we show that the c band penetration depth over the ice cap is insignificant at this time of the year i e late ablation season this is likely due to a wet surface and the presence of superimposed ice that leads to a surface radar response comparing the srg dems with other datasets an historical glacier wide mass balance of 0 52 0 19 m w e a 1 is estimated for 1960 2013 whereas it decreases to 1 06 0 84 m w e a 1 between 2005 and 2013 this clear acceleration of mass loss is in agreement with other recent studies given its all weather functionality and its possible use without ground control points the radarsat 2 srg technology represents an appropriate alternative for glacier monitoring in cloudy and remote regions

dating polygenetic metamorphic assemblages along a transect across the western alps multichronometric analyses were performed on samples from a transect in the french italian western alps crossing nappes derived from the brianconnais terrane and the piemonte liguria ocean in an endeavour to date both high pressure hp metamorphism and retrogression history twelve samples of white mica were analysed by 39ar 40ar stepwise heating complemented by two samples from the monte rosa nappe 100 km to the ne and also attributed to the brianconnais terrane one sm nd and three lu hf garnet ages from eclogites were also obtained white mica ages decrease from c 300ma in the westernmost samples zone houille re reaching c 3008c during alpine metamorphism to 548ma in the internal units to the east which reached c 5008c during the alpine orogeny the spatial pattern of eocene k ar ages demonstrates that si rich hp white mica records the age of crystallization at 47 48ma and retains ar at temperatures of around 5008c paleocene early eocene lu hf and sm nd ages recording prograde garnet growth before the hp peak confirm eclogitization in eocene times petrological and microstructural features reveal important mineralogical differences along the transect all samples contain mixtures of detrital syn d1 and syn d2 mica and retrogression phases d3 in greatly varying proportions according to local variations in the evolution of pressure temperature fluid activity deformation p t a d conditions samples from the zone houille re mostly contain detrital mica the abundance of white mica with si46 45 atoms per formula unit increases eastward across the whole traverse phengitic mica grown during hp metamorphism defines the d1 foliation syn d2 mica is more si poor and associated with nappe stacking exhumation and hydrous retrogression under greenschist facies conditions syn d1 phengite is very often corroded overgrown by or intergrown with syn d2 muscovite most importantly syn d2 recrystallization is not limited to s2 schistosity domains micrometrescale chemical fingerprinting reveals muscovite pseudomorphs after phengite crystals which could be mistaken for syn d1 mica based on microstructural arguments alone the cl k ratio in white mica is a useful discriminator as d2 retrogression was associated with a less saline fluid than eclogitization as petrology exerts the main control on the isotope record constraining the petrological and microstructural framework is necessary to correctly interpret the geochronological data described in both the present study and the literature our approach which ties geochronology to detailed geochemical petrological and microstructural investigations identifies 47 48ma as the age of hp formation of syn d1 mica along the studied transect and in the monte rosa area cretaceous apparent mica ages which were proposed to date eclogitization by earlier studies based on conventional thermochronology are due to ar inheritance in incompletely recrystallized detrital mica grains the inferred age of the probably locally diachronous greenschist facies low si syn d2 mica ranges from 39 to 43ma coexistence of d1 and d2 ages and the constancy of non reset d1 ages along the entire transect provides strong evidence that the d1 white mica ages closely approximate formation ages volume diffusion of ar in white mica activation energy e 250 kj mol 1 pressure adjusted diffusion coefficient d 050 03 cm2 s 1 has a subordinate effect on mineral ages compared with both prograde and retrograde recrystallization in most samples

on the evolution of orogens pressure cycles and deformation mode switches abstract evidence of two burial exhumation cycles that took place during a single orogeny has been found in rocks belonging to the piemonte unit of the western alps an early high pressure event which resulted from tectonic burial down to pressures of 1 5 gpa was followed by exhumation to ca 0 20 0 35 gpa as a result of extensional deformation renewed shortening culminated in a second burial episode down to pressures of 0 65 0 80 gpa before the final exhumation took place existing geochronological data allow only ca 13 19 ma for the completion of both burial exhumation cycles therefore we suggest that the evolution of orogens is characterized by multiple short lived burial exhumation cycles related to orogen scale alternance between shortening and extensional deformation

determination of high spatial resolution argon isotope variations in metamorphic biotites the circumstances of preservation of relict metamorphic minerals and fabrics which have subsequently been involved in later metamorphic cycles remain enigmatic because the kinetic parameters are not well understood textures and chemistry cannot always give direct information on the conditions attained in the successive events we show in this study that 40ar39ar laser studies can in contrast provide thermal history determinations and successfully discriminate between geological models two samples have been examined from the seconda zona diorito kinzigitica iidk of the sesia zone italian alps a unit which preserves relict hercynian metamorphic assemblages and is now located structurally above rocks which preserve alpine eclogite and greenschist facies assemblages the circumstances of this preservation remain controversial with proposed models falling into two main categories one hypothesis advocates that the iidk was juxtaposed into its present position late in the alpine orogeny and experienced no significant heating event alternatively the iidk has been in its present position for the duration of the alpine orogenic cycle and has been exposed to high temperatures but due to sluggish reaction kinetics it failed to reequilibrate and develop alpine metamorphic minerals a high spatial resolution 40ar39ar study using the ultraviolet laser ablation microprobe uvlamp technique was undertaken with the aims of reassessing the metamorphic history of the iidk and investigating the behavior of argon within the unit during its thermal evolution 128 analyses have been obtained from ten profiles between 60 and 275 m long across biotite grains which show apparent ages ranging from 514 to 161 ma within individual crystals these apparent age variations are systematic according to location in the grain with the oldest ages at the grain edges and the youngest in the cores these apparent age variations can not be explained by chemical alteration and require diffusion of excess argon into the grains the age profiles show a remarkable fit to theoretically derived curves for volume diffusion of argon time integrated diffusion values obtained for the profiles fall between 2 10 6 cm 6 and 5 10 6 cm2 this variation is interpreted as being due to spatial and temporal fluctuations in the availability of argon from the grain boundary network diffusion and hence profile development initiated at different times within the biotites consequently producing the range of time integrated diffusion values the temperatures at which the profiles developed have been estimated using published biotite diffusion parameters to model possible thermal histories using the assumptions of either a short thermal pulse event or continuous cooling from high temperatures the models suggest that the profiles developed at temperatures less than 300 c possible geological models are proposed which document the thermal and excess argon history of the iidk from the hercynian through to the alpine orogeny the study shows the key role of detailed argon isotope studies in assessing the thermal evolution of rocks with relict metamorphic assemblages

sequence and style of major post nappe structures simplon pennine alps abstract in the upper pennine nappe complex of the simplon pennine alps switzerland and italy at least three phases of major post nappe folding in places associated with thrusting can be distinguished these are superimposed on an earlier formed partly chaotic complex of tectonic units including the bernhard and monte rosa continental flakes and the zermatt saas and antrona ophiolite complexes the earliest post nappe folds were essentially isoclinal throughout the whole region and were accompanied by a strong schistosity which is the main foliation in most areas later two successive phases of back folding led to the present overall structure both phases typically show rapid variations in style from open folds lacking axial planar schistosity to very tight structures with complete foliation transposition this has been demonstrated by systematically mapping the major axial traces over the whole region successively removing the major structures in reverse order shows that the ophiolite complexes were originally part of a continuous unit marking an important suture between the bernhard and monte rosa nappes

garnet chloritoid equilibria in eclogitic pelitic rocks from the sesia zone western alps their bearing on phase relations in high pressure metapelites abstract a detailed study of garnet chloritoid micaschists fom the sesia zone western alps is used to constrain phase relations in high pressure hp metapelitic rocks in addition to quartz phengite paragonite and rutile the micaschists display two distinct parageneses namely garnet chloritoid chlorite and garnet chloritoid kyanite talc has never been observed garnet and chloritoid are more magnesian when chlorite is present instead of kyanite the distinction of the two equilibria results from different bulk rock chemistries not from p t conditions or redox state estimated p t conditions for the eclogitic metamorphism are 550 600 c 15 18 kbar the presence of primary chlorite in association with garnet and chloritoid leads us to construct two possible afm topologies for the sesia metapelites the paper describes a kfmash multisystem for hp pelitic rocks which extends the grid of harte hudson 1979 towards higher pressures and adds the phase talc observed parageneses in hp metapelites are consistent with predicted phase relations critical associations are gt ctd chl and gt ctd ky at relatively low temperatures and gl chl ky and gt tc ky at relatively high temperatures

the effects of deformation induced microstructures on intragrain 40ar 39ar ages in potassium feldspar a combination of forescattered scanning electron microscopy and both ultraviolet uv laser and cycle stepped heating 40 ar 39 ar dating techniques have been applied to a single deformed grain of potassium feldspar to investigate the relationship between microstructure and 40 ar 39 ar isotope systematics the form and distribution of deformation induced microstructures have been mapped by using intragrain variations in lattice orientation these variations are related to thin section and regional structural data to provide a well constrained deformation history for the feldspar grain thirty uv laser 40 ar 39 ar analyses show age variations that correlate with the internal microstructure stepped heating data provide further evidence of this relationship our approach allows the recognition of different ar reservoirs within the deformed feldspar and identifies microstructural controls on ar migration such a relationship has important implications for the understanding and interpretation of 40 ar 39 ar geochronological data and the subsequent use of such data to constrain the temperature vs time evolution of deformed rocks

structural evolution of different tectonic units across the austroalpine penninic boundary in the middle orco valley western italian alps abstract a structural study along the orco valley which represents a geological section through the alpine belt western italy is reported a lower tectonic element composed of units with eclogite facies relics gran paradiso and zermatt saas like unit and an upper tectonic element made of greenschist facies units combin like unit and gneiss minuti complex of the sesia lanzo zone have been distinguished these two elements show a different main alpine tectono metamorphic event d 1 and are separated by a mylonitic contact d 2 developed under greenschist facies conditions orco shear zone this regional scale shear zone was responsible for the exhumation of the deeper eclogitic units and has been inferred as the continuation of the combin fault south of the aosta valley from d 3 onward the same tectono metamorphic evolution was recorded in all units the following extensional crenulation cleavage d 4 occurs mostly along a nne ssw deformation zone about 1 km thick at the base of the upper tectonic element which leads to a further exhumation of the lower tectonic element

kinematics of syn and post exhumational shear zones at lago di cignana western alps italy constraints on the exhumation of zermatt saas ultra high pressure rocks and deformation along the combin fault and dent blanche basal thrust kinematic analyses of shear zones at lago di cignana in the italian western alps were used to constrain the structural evolution of units from the piemont ligurian oceanic realm zermatt saas and combin zones and the adriatic continental margin dent blanche nappe during palaeogene syn and post exhumational deformation exhumation of zermatt saas u hp rocks to approximately lower crustal levels at ca 39 ma occurred during normal sense top s e shearing under epidote amphibolite facies conditions juxtaposition with the overlying combin zone along the combin fault at mid crustal levels occurred during greenschist facies normal sense top se shearing at ca 38 ma the scarcity of top se kinematic indicators in the hanging wall of the combin fault probably resulted from strain localization along the uppermost zermatt saas zone and obliteration by subsequent deformation a phase of dominant pure shear deformation around 35 ma affected units in the direct footwall and hanging wall of the combin fault it is interpreted to reflect nw se crustal elongation during updoming of the nappe stack as a result of underthrusting of european continental margin units and the onset of continental collision this phase was partly accompanied and followed by ductile bulk top nw shearing especially at higher structural levels which transitioned into semi ductile to brittle normal sense top nw deformation due to vanzone phase folding from ca 32 ma onwards our structural observations suggest that syn exhumational deformation is partly preserved within units and shear zones exposed at lago di cignana but also that the combin fault and dent blanche basal thrust experienced significant post exhumational deformation reworking and overprinting earlier structures

excess 40ar transport scale and mechanism in high pressure phengites a case study from an eclogitized metabasite of the dora maira nappe western alps application of the bulk separate furnace heating laser step heating and in situ laser agemapping 40ar 39ar techniques reveals that high pressure phengites from an alpine eclogite of the dora maira basement nappe western alps were pervasively contaminated with excess 40ar to yield ages 130 ma spot fusion dating of individual grains with the 40ar 39ar laserprobe reveals concentric radial patterns with core ages as high as 320 ma showing that this excess component was trapped at an early stage and subsequently remobilized by volume diffusion during a late greenschist thermal pulse the single grains and population from this hand sample retain major differences in apparent ages indicating that the extent of the excess 40ar contamination varies from grain to grain attempt to model these differences in terms of grain size dependent fractional loss falls short and demonstrates that strong cm scale argon isotopic gradients locally prevailed during syn kinematic high pressure metamorphism on petrostructural grounds a model of internal isotopic buffering is outlined whereby the excess 40ar is argued to have been internally derived from within the eclogite protolith itself low intergranular ar diffusion rates due to reduced water activity buffered below unity by the phengite assemblage resulted in the local segregation and partitioning of the excess 40ar along grain boundaries during ongoing ductile deformation supporting evidence for this model comes from the recent literature on fluid and stable isotope behaviour in alpine eclogites and very high pressure metasediments which shows that volatile recycling during high pressure metamorphism may occur on a very limited scale

constraining absolute deformation ages the relationship between deformation mechanisms and isotope systematics isotope diffusion in a mineral is strongly temperature dependent but is also a function of grain size deformation must therefore be an important consideration in the interpretation of isotopic data because it provides a means of modifying grain size and shape we illustrate the range of different deformation mechanisms common in micas and use simple models to investigate the relationship between these and isotope diffusion we consider three different thermal scenarios with deformation taking place during a the prograde heating path b at the closure temperature of the deforming mineral and c at temperatures significantly below the closure temperature we have modelled these simple systems using a finite difference algorithm that simulates argon diffusion profiles and bulk ages this modelling illustrates that obtaining deformation ages is critically dependent on an understanding and recognition of the different deformation mechanisms that have affected the sample in the cases where deformation causes a change in grain size it is important to characterise both the temperature at which deformation takes place and the closure temperature of grains formed during the deformation the development of grains with tc greater than the deformation temperature may record a deformation age examples of this condition include i neocrystallisation ii grain size reduction occurring at temperatures below tc of the reduced grain size where the deformation mechanism has reset the grains and iii deformation induced grain coarsening

the geometry and timing of orogenic extension an example from the western italian alps contacts between rocks recording large differences in metamorphic grade are indicative of major tectonic displacements low p upon high p contacts are commonly interpreted as extensional i e material points on either side of the contact moved apart relative to the palaeo horizontal but dating of deformation and metamorphism is essential in testing such models in the western alps the piemonte ophiolite consists of eclogites t 550 600 c and p 18 20 kbar structurally beneath greenschist facies rocks t 400 c and p 9 kbar mapping shows that the latter form a kilometre wide shear zone the gressoney shear zone gsz dominated by top se movement related to crustal extension rb sr data from micas within different gsz fabrics which dynamically recrystallized below their blocking temperature are interpreted as deformation ages ages from different samples within the same fabric are reproducible and are consistent with the relative chronology derived from mapping they show that the gsz had an extensional deformation history over a period of c 9 myr between c 45 36 ma this overlaps in time with the eclogite facies metamorphism the gsz operated over the entire period during which the footwall evolved from eclogite to greenschist facies and was therefore responsible for eclogite exhumation the discrete contact zone between eclogite and greenschist facies rocks is the last active part of the gsz and truncates greenschist facies folds in the footwall these final movements were therefore not a major component of eclogite exhumation pressure estimates associated with old and young fabrics within the gsz are comparable indicating that during extensional deformation there was no significant unroofing of the hangingwall since there are no known extensional structures younger than 36 ma at higher levels in this part of the alps exhumation since the final juxtaposition of the two units at 36 ma seems to have been dominated by erosion key words deformation age eclogite exhumation rb sr dating tectonic

alpine deformation and 40ar 39ar geochronology of synkinematic white mica in the siviez mischabel nappe western pennine alps switzerland we explore the timing of deformation and exhumation of the siviez mischabel nappe western swiss alps which has been considered a classic example of a midcrustal crystalline nappe since the studies of argand 1916 this study presents 40ar 39ar ages obtained on both synkinematic white mica from permo triassic cover sediments and more complex white mica populations from basement gneisses of the siviez mischabel and middle pennine nappes primary foliation developed in cover units by nucleation growth and rigid rotation of mica grains during noncoaxial alpine deformation although some samples show a crenulation of this primary foliation mica growth appears to have occurred only during the development of primary foliation the main phase of greenschist facies deformation related to imbrication of the siviez mischabel nappe and other middle pennine nappes good agreement exists between independent estimates of the timing of deformation and reported 40ar 39ar white mica ages from cover units of the central and southern siviez mischabel nappe in cover units from the central and southern siviez mischabel regions of the study area 40ar 39ar ages appear to date synkinematic white mica growth results suggest that the siviez mischabel nappe was emplaced and developed foliation during a 5 m y period from 41 to 36 ma in cover units from the eastern siviez mischabel however 40ar 39ar white mica ages appear to date postkinematic thermal events these thermal events may be related to oligocene magmatic activity in the lower pennine nappes or to miocene development of the simplon fault zone variations in the relation between alpine age and grain size for cover samples from the central eastern and southern siviez mischabel correlate well with the regional variations in temperature inferred from quartz microfabrics and the pattern of regional metamorphism when considered in concert with other recent isotopic studies on the timing of major tectonic and thermal events in the western swiss alps these data support arguments that the relative timing of events such as thrusting and back thrusting of crystalline nappes in hinterland units and exhumation of high pressure units in the suture zone of the western alps are intimately related and synchronous on the scale of a few million years

burial and exhumation history of the daday unit central pontides turkey implications for the closure of the intra pontide oceanic basin in northern turkey the intra pontide suture zone represents one of the first order tectonic structures located between the istanbul zonguldak and the sakarya continental terranes it consists of an e w trending assemblage of deformed and variably metamorphosed tectonic units including sedimentary rocks and ophiolites derived from a neo tethyan oceanic basin known as the intra pontide oceanic basin one of these units is represented by the daday unit that consists of a block in matrix assemblage derived from supra subduction oceanic crust and related deep sea sedimentary cover of middle jurassic age this setting was acquired during late jurassic time by tectonic underplating at a depth of 35 42 km associated with blueschist facies metamorphism d1 phase the following d2 d3 and d4 phases produced the exhumation of the daday unit up to shallower structural levels in a time span running from the albian to late paleocene the high geothermal gradient detected during the d2 phase indicates that the daday unit was exhumed during a continent arc collisional setting the tectonic structures of the intra pontide suture zone resulting from the previously described tectonic history are unconformably sealed by the upper paleocene eocene deposits this tectonic setting was intensely reworked by the activity of the north anatolian fault zone producing the present day geometrical relationships of the intra pontide suture zone of the central pontides

high pressure metamorphism of the zermatt saas ophiolite zone switzerland within the zermatt saas ophiolite zone of the western alps a record of polystadial high pressure metamorphism is well preserved early blueschist assemblages retained as inclusions in garnets were succeeded by eclogitic assemblages which in some rocks contained lawsonite kyanite talc and chloritoid these eclogitic assemblages formed by prograde reaction from the early blueschists subsequently reaction of these eclogites with a mixed h 2 o co 2 vapour phase led to the replacement of kyanite by paragonite and the partial replacement of omphacite garnet paragenses by assemblages containing glaucophane paragonite and ankerite high pressure eo alpine metamorphism took place under conditions of 550 600 c and 17 5 20 kbar values of a h 2 o between 0 55 and 1 are high enough to accommodate equilibration with a water rich vapour phase under the highest grade conditions the presence of such a fluid phase is locally indicated by the presence of quartz rich veins containing omphacite and kyanite this vapour phase was absorbed during retrogression later rehydration is limited to areas close to albite veins tectonic contacts and bodies of metasediment the metamorphic conditions determined for the zermatt saas zone are compatible with those suggested for over and under lying units these conditions and the p t path inferred by comparing the reaction histories with a petrogenetic grid for the system na 2 o cao mgo a1 2 o 3 sio 2 h 2 o suggest that metamorphism occurred during subduction to depths of between 60 and 70 km and subsequent exhumation during the alpine orogeny

testing models of orogen exhumation using zircon u th he thermochronology insight from the ligurian alps northern italy testing models of orogen exhumation requires precise constraint of the time temperature paths of the exhumed rocks the zircon u th he zhe thermochronometer has a closure temperature of 140 210 c and can provide temporal constraints on the exhumation history of rocks through a thermal range which crucially corresponds the onset of brittle behaviour of an exhuming orogen we performed zhe analyses to test the existing contradictory models for the exhumation of the ligurian alps the zhe ages indicate a very rapid 1 3 6 8 mm yr and southward migrating exhumation of the orogen between 32 and 25 ma these high exhumation rates are unique within the alpine belt and cannot be reconciled with existing geodynamic models of the evolution of the ligurian alps we propose a model of tectonic denudation via detachment accomplished in the shallowest crust 5 km as a result of the last orogenic phase of extension

kinematic reworking and exhumation within the convergent alpine orogen abstract kinematic data from the internal zones of the western alps indicate both top to se and top to nw shearing during synkinematic greenschist facies recrystallisation rb sr data from white micas from different kinematic domains record a range of ages that does not represent closure through a single thermal event but reflects the variable timing of synkinematic mica recrystallisation at temperatures between 300 and 450 c the data indicate an initial phase of accretion and foreland directed thrusting at ca 60 ma followed by almost complete reworking of thrust related deformation by se directed shearing this deformation is localised within oceanic units of the combin zone and the base of the overlying austroalpine basement and forms a regional scale shear zone that can be traced for almost 50 km perpendicular to strike the timing of deformation in this shear zone spans 9 ma from 45 to 36 ma the se directed shear leads to local structures that cut upwards in the transport direction with respect to tectonic stratigraphy and such structures have been interpreted in the past as backthrusts in response to ongoing alpine convergence however on a regional scale the top to se deformation is related to crustal extension not shortening and is coincident with exhumation of eclogites in its footwall during this extension phase deformation within the shear zone migrated both spatially and temporally giving rise to domains of older shear zone fabrics intercalated with zones of localised reworking top nw kinematics preserved within the combin zone show a range of ages the oldest 48 ma may reflect the final stages of emplacement of austroalpine units above piemonte oceanic rocks prior to the onset of extension however much of the top to nw deformation took place over the period of extension and may reflect either continuing or episodic convergence or tectonic thinning of the shear zone 40ar 39ar data from the region are complicated due to the widespread occurrence of excess 40ar in eclogite facies micas and partial ar loss during alpine heating reliable ages from both eclogite and greenschist facies micas indicate cooling ages in different tectonic units of between 32 and 40 ma these ages are slightly younger than rb sr deformation ages and suggest that cooling below ca 350 c occurred after juxtaposition of the units by se directed extensional deformation our data indicate a complex kinematic history involving both crustal shortening and extension within the internal zones of the alpine orogen to constrain the palaeogeographic and geodynamic evolution of the alps requires that these data be integrated with data from the more external zones of the orogen complexity such as that described is unlikely to be restricted to the western alps and spatially and temporally variable kinematic data are probably the norm in convergent orogens recognising such features is fundamental to the correct tectonic interpretation of both modern and ancient orogens

rapid exhumation of the zermatt saas ophiolite deduced from high precision smnd and rbsr geochronology sm nd isotope data from garnets in ultrahigh pressure coesite bearing eclogite from the western alps were obtained to determine the age of peak metamorphism and exhumation rates of deeply buried oceanic crust in the zermatt saas ophiolite complex we report an exceptionally well constrained sm nd isochron age of 40 62 6 ma from the lago di cignana eclogite difficulties in dating alpine eclogites using this method were overcome by using an hf acid leaching procedure on garnet to remove mineral inclusions the zermatt saas rocks reached greenschist facies conditions around 38 2m a on the basis of rb sr whole rock phengite isochrons these data combined with existing estimates of the pressure and temperature conditions of the eclogite give an estimate for the initial exhumation rate of 10 to 26 km m y depending on the inferred pressure for the recrystallization of phengite followed by slow exhumation of 0 3 km m y from 34 to 14 ma the initial exhumation rates at lago di cignana are among the highest yet determined for high pressure terranes and are well constrained because the age has been determined directly on the high pressure assemblage the rapid exhumation rates presented here place critical constraints on collisional tectonic models 1999 elsevier science b v all rights reserved

the timing of and nature of greenschist facies deformation and metamorphism in the upper pennine alps the upper pennine nappes in the western alps are cut by structures that locally place european basement rocks over those of the piemonte zone which represent the sutured remains of alpine ocean crust in the taschalp area of switzerland the mischabel backfold and associated se verging shearing carries gneisses of the grand saint bernard nappe over eclogitic material of the zermatt saas zone greenschist facies assemblages are developed locally in the foot wall and hanging wall of the mischabel structure as well as in the sheared rocks of the thrust zone rb sr 40ar 39ar and u pb studies of white micas amphiboles and sphenes show that greenschist assemblages developed at circa 44 ma in the zermatt saas zone of the footwall and at circa 40 ma in the thrust zone rb sr data from the literature document muscovite and biotite ages in strongly foliated grand saint bernard gneisses of the hanging wall of circa 36 and 32 ma respectively these are thought to indicate that deformation on the mischabel structure continued for at least 5 m y rapid cooling inferred for the zermatt saas ophiolites in the mid eocene may document either the tectonic emplacement of the ophiolite over the underlying monte rosa massif or their extensional unroofing it is not possible to determine with certainty whether the mischabel structure is extensional or contractional on a crustal scale the age of cooling and deformation reported here show that it is not possible that the mischabel structure was responsible for the exhumation of the high pressure assemblages in the zermatt saas and monte rosa rocks

recognizing remnants of magma poor rifted margins in high pressure orogenic belts the alpine case study magma poor rifted margins are being increasingly recognized in present day atlantic type systems however findings of fossil areas floored by exhumed mantle or hyper extended crust are comparatively rare within orogenic belts that were originated through the inversion of pre existing rifted margins this discrepancy may be due to the common reactivation of lithological contacts during subduction orogeny potentially masking pre orogenic relationships and most importantly to the frequent lack of a preorogenic layer cake architecture hindering retro deformation of multiply deformed tectonic units this study outlines a methodology to detect sections of magma poor hyper extended rifted margins without a layer cake architecture in multiply deformed metamorphosed terrains this approach is defined by comparison to well studied examples of fossil analogues preserved in weakly deformed parts of alpine orogens in the latter domains continental basement and hydrated peridotites were exhumed at the basin floor during jurassic rifting along long offset detachment systems extensional geometries locally resulted in tectonic sampling of laterally discontinuous slivers of allochthonous continental basement and pre rift sediments fromthe hanging wall blocks lithostratigraphic associations consisting of continental basement rocks directly juxtaposed with syn to post rift meta sediments and or serpentinized subcontinental mantle are widespread within sections of alpine type orogenic belts that underwent high to ultra high pressure metamorphism however similar associations may arise from a variety of processes other than rift related lithospheric thinning in magma poor environments including subduction melange dynamics or deposition of sedimentary melanges along convergent divergent margins the partial preservation of rift related lithostratigraphic associations may still be assessed despite the lack of biostratigraphic evidence by 1 the consistency of the lithostratigraphic architecture over large areas despite pervasive alpine deformation which rules out chaotic mixing during subduction exhumation 2 the presence of clasts of basement rocks in the neighboringmeta sediments indicating the original proximity of the different lithologies 3 evidence of brittle deformation in continental basement and ultramafic rocks pre dating alpine metamorphism indicating that theywere juxtaposed by fault activity prior to the deposition of post rift sediments and 4 the similar alpine tectono metamorphic evolution of ophiolites continental basement and meta sediments a re assessment of basement cover relationships in the north western alps following this approach combined with published studies on exhumedmantle domains sampled in the rest of thewestern alps indicates that several tectono metamorphic units fromthemost deformed metamorphosed part of the belt between the canavese line and the penninic front sample hyper extended lithosphere related to the jurassic opening of the western tethys relative plate motion during cretaceous tertiary basin inversion was largely accommodated at the transition between areas floored by hyper extended crust or hydrated subcontinental mantle and domains consisting of thicker continental crust as a result distal margins were preferentially subducted whereas the proximal domains and the brianconnais paleo high underwent relatively minor deformation and metamorphism the high pressure alpine tectono metamorphic units were probably detached from the downgoing lithosphere along a hydration front that is typically observed in present day distal margins the recognition of preserved pre alpine relationships between continental basement post rift sediments and or serpentinized ultramafic rocks calls for a re assessment of the relative role of subduction and rifting dynamics in establishing the present day orogen architecture

a statistical relationship between the geosynchronous magnetic field and substorm electrojet magnitude in this paper the authors examine the relationship between geosynchronous magnetic field variations during substorms measured by goes 5 and the auroral electrojet as measured by ae and poste de la baleine as in previous studies the authors find that the more taillike the field prior to the local onset the greater the dipolarization of the field during the substorm they also find that the greater the deviation of the field from a dipolar configuration the larger the change in ae during the event this implies that stronger cross tail currents prior to the substorm are associated with larger substorm associated westward electrojets and thus more intense substorms previous work has shown that in order to produce the observed taillike fields at geosynchronous altitude the intense cross tail current that builds up during the growth phase must be localized in the near earth le 10 r sub e region since the westward electrojet is the ionospheric leg of the substorm current wedge this result implies that the substorm associated westward electrojet is drawn from the near earth region in fact the authors find that most of the current diversion occurs in the near earth magnetotail furthermore they estimate that a diversion about half of the near earth cross tail currentmore can account for the current in the northern and southern westward electrojets associated with the substorm current wedge 25 refs 9 figs less

a 40ar 39ar laser probe study of micas from the sesia zone italian alps implications for metamorphic and deformation histories two samples from the eclogite micaschist complex emc and the seconda zona diorito kinzigitica iidk of the sesia zone have been studied using a high spatial resolution laser probe 40ar 39ar technique with the aim of investigating the complexities of argon behaviour in metamorphic rocks and comparing their thermal histories data from a single large phengite grain from the emc show a range of ages from mid jurassic to upper cretaceous these apparent age variations are spatially related to both location within the grain and to intragrain microstructure modelling of the data shows that the profile formed by the diffusion of an excess argon component into the grain parallel to the mica cleavage profile asymmetry is explained by temporal variations in microstructural development enabling excess argon to enter the grain at different times in different places the temperatures of the initiation of deformation and the possible time scales for the deformation can be calculated as a function of cooling rate all estimates suggest deformation at greenschist facies in accord with the observed retrograde mineral assemblage absolute temperature estimates for deformation vary by less than 22 c for different cooling rates of 10 and 30 c ma 1 but vary by 80 c with different estimates of diffusion parameters the duration of deformation was for at least 2 ma at 10 c ma 1 or 0 7 ma at 30 c ma 1 biotites from the iidk sample record a permian to upper cretaceous age range that correlates with grain size the smallest grain sizes yielding the youngest ages this relationship is best explained by a partial resetting of biotites during an alpine thermal event initiated not more than 70 ma ago modelling of these data suggest that the sample never exceeded 300 c during the alpine the profoundly different thermal histories of the two units the emc recrystallized at 550 c whilst the iidk remained below 300 c suggests that they may not have been juxtaposed until much later than the eclogite facies metamorphism

the arc of the western alps in the light of geophysical data on deep crustal structure recently two international deep seismic campaigns in the western and central alps etude continentale et oceanique par reflexion et refraction sismique progetto strategico crosta profonda ecors crop roure et al 1996b and national research program 20 nrp 20 pfiffner et al 1997 have been completed here we present a synoptic interpretation of the wealth of geophysical data about deep crustal structure of the alps collected during the past 40 years including the two above mentioned geophysical projects the tectonic reinterpretation of the ecors crop high resolution seismic transect is based on an evaluation of the deep crustal structure by integrating new and literature data on surface geology combined with previously published interpretations regarding the central alps schmid et al 1996 this study reveals substantial differences in geometry and kinematics between transects across the western and central alps respectively at depth the transition between the central and western alps is marked by the western limits of an adriatic lower crustal wedge shaped structure and the northeastern limits of a similar structure made of european lower crustal material at the surface it coincides with a corridor of dextral strike slip along the tonale and simplon lines in addition the thickness of the seismogenic zone shows a remarkable variation from over 40 km beneath the penninic realm of the western alps to less than 20 km in the central alps the formation of the western alpine arc was initiated during convergence and collision before 35 myr ago when the adriatic micro plate moving northward with respect to the european foreland caused sinistral transpression in the western alps while the central and eastern alps underwent head on convergence and collision during the post collisional stage i e after 35 myr ago the arcuate shape of the western alps was accentuated by wnw directed movement and anticlockwise rotation of the adriatic microplate decoupled from the central and eastern alps along the tonale simplon dextral shear zone this led to wedging of lower crustal slices both in the western alps and in the central alps the new tectonic interpretation of the ecors crop transect allows a kinematic model to be established for crustal shortening in the western alps during the past 35 myr the ivrea mantle plays the role of a backstop in our tectonic model we distinguish three episodes of post collisional crustal shortening along the ecors crop transect from 35 to 30 ma the brianconnais basement was back thrusted over the gran paradiso units leading to 30 km of crustal shortening first episode in the early miocene movements concentrated mainly along the penninic frontal thrust and resulted in about 60 km shortening second episode post 12 ma shortening within the external massifs is associated with folding in the jura mountains when the crust was shortened by an additional 30 km third episode

heterogeneous 40ar distributions in naturally deformed muscovite in situ uv laser ablation evidence for microstructurally controlled intragrain diffusion abstract micas are commonly used in 40 ar 39 ar thermochronological studies of variably deformed rocks yet the physical basis by which deformation may affect radiogenic argon retention in mica is poorly constrained this study examines the relationship between deformation and deformation induced microstructures on radiogenic argon retention in muscovite a combination of furnace step heating and high spatial resolution in situ uv laser ablation 40 ar 39 ar analyses are reported for deformed muscovites sampled from a granitic pegmatite vein within the siviez mischabel nappe western swiss alps penninic domain brianconnais unit the pegmatite forms part of the variscan 350 ma alpine basement and exhibits a prominent alpine s c fabric including numerous mica fish that developed under greenschist facies metamorphic conditions during the dominant tertiary alpine tectonic phase of nappe emplacement furnace step heating of milligram quantities of separated muscovite grains yields an 40 ar 39 ar age spectrum with two distinct staircase segments but without any statistical plateau consistent with a previous study from the same area a single 3 5 mm muscovite porphyroclast fish was investigated by in situ uv laser ablation a histogram plot of 170 individual 40 ar 39 ar uv laser ablation ages exhibit a range from 115 to 387 ma with modes at approximately 340 and 260 ma a variogram statistical treatment of the 40 ar 39 ar results reveals ages correlated with two directions a highly correlated direction at 310 and a lesser correlation at 0 relative to the sense of shearing using the highly correlated direction a statistically generated kriging method age contour map of the 40 ar 39 ar data reveals a series of elongated contours subparallel to the c surfaces which where formed during tertiary nappe emplacement similar data distributions and slightly younger apparent ages are recognized in a smaller mica fish the observed intragrain age variations are interpreted to reflect the partial loss of radiogenic argon during alpine 35 ma greenschist facies metamorphism one dimensional diffusion modelling results are consistent with the idea that the zones of youngest apparent age represent incipient shear band development within the mica porphyroclasts thus providing a network of fast diffusion pathways during alpine greenschist facies metamorphism the incipient shear bands enhanced the intragrain loss of radiogenic argon the structurally controlled intragrain age variations observed in this investigation imply that deformation has a direct control on the effective length scale for argon diffusion which is consistent with the heterogeneous nature of deformation

middle to late miocene age for the end of amphibolite facies mylonitization of the alpine schist new zealand implications for onset of transpression across the alpine fault

precollisional multistage exhumation of subducted continental crust the sesia zone western alps 1 the sesia zone within the tertiary arc of the western alps is a relic of the subducted part of the adriatic continental margin along the se border of the tethyan ocean the sesia zone comprises three basement nappes which individuated during late cretaceous 65 80 ma subduction to different depths at high pressure hp blueschist eclogite facies conditions peak pressures of 1 0 1 2 1 0 1 5 and 1 5 2 0 gpa the thrusts bounding these nappes developed where the crust was previously thinned during jurassic rifting crustal scale shear zones partly overprinted these early thrusts and exhumed coherent slices of crust containing hp rocks initial exhumation of the internal part of the accreted margin involved thrusting d1 and transpressional shearing d2 along a subvertical e w trending mylonitic shear zone under retrograde blueschist to greenschist facies conditions this exhumation was nearly isothermal to a depth of about 25 km where the basement nappes were juxtaposed subsequent exhumation of these nappes to a common depth of about 15 20 km occurred in the footwall of a greenschist facies top se extensional shear zone d3 preserved in some of the highest mountain peaks of the sesia zone new rb sr mineral ages constrain d2 to have occurred at about 60 65 ma and d3 at about 45 55 ma thus top se extensional exhumation was broadly coeval with eocene se directed subduction of the liguro piemont oceanic lithosphere beneath the adriatic margin slow cooling and erosional denudation of the sesia zone from 45 to 30 ma occurred in the hanging wall of the gressoney extensional shear zone d4 which itself contributed to the exhumation of eocene hp and ultra hp oceanic rocks in its footwall by 30 ma hp rocks of the sesia zone were intruded by shallow granitic plutons which were eroded and redeposited within volcanoclastic sediments oligo miocene insubric backfolding and thrusting d5 only exhumed northeastern parts of the sesia zone where hp metamorphism is absent or was overprinted by tertiary temperature dominated metamorphism most exhumation of continental hp rocks in the sesia zone therefore preceded tertiary alpine collision and coincided with late cretaceous to early tertiary subduction of the adriatic and tethyan lithosphere the transition from d2 to d3 in the sesia zone is interpreted to mark a change from high stress oblique se directed subduction and accretion of the distal adriatic continental margin to nw retreating low stress subduction of the liguro piemont oceanic lithosphere

35 ma old ultrahigh pressure metamorphism and evidence for very rapid exhumation in the dora maira massif western alps abstract ion microprobe shrimp data on zircons from various rock types of an ultrahigh pressure uhp metamorphic whiteschist type pyrope quartzite lens of the dora maira massif dmm consistently show domains giving a late eocene age of 35 4 1 0 ma which is taken as the age of uhp metamorphism these domains partially replace the older oscillatory zoning pattern of the zircons formed during primary magmatic crystallization at about 275 ma zircons of a metagranitic country rock next to the uhp metamorphic lens have these primary features best preserved all zircons measured also yield intermediate ages between 275 and 35 ma with a statistical concentration between 260 and 210 ma thus the uniformity of the initial zircon population both in the lens and the country rock evidences a common protolith that is a granite intruded during the late herynian while the intermediate ages are at least partly due to incomplete resetting of the zircons during uhp metamorphism those in the 260 210 ma range may be related to rifting episodes in the permotriassic the mg rich chemistry of the whiteschist lenses is due to local metasomatic alterations of the granite perhaps by fluids derived from evaporitic sediments dating as early as the permotriassic as well the more pervasive resetting of zircon ages during uhp metamorphism in the pyrope quartzite lenses is explained by the ubiquity of fluids and or melts produced during subduction by a series of dehydration reactions that occurred only in the more hydrous mg rich protoliths and not in their drier granitic neighborhood fission track ages determined partly on the same zircon samples yielding 29 9 1 4 ma mark the time when the uhpm unit had reached about 290 c at a shallow location within the crust thus exhumation over a vertical distance of about 120 km must have occurred within about 5 6 ma indicating an average uplift rate of about 20 24 km ma and an average cooling rate of about 85 100 c ma the radiometric data obtained do not lend any support to an eo alpine cretaceous subduction event so that deep subduction and immediately following exhumation all took place during early to mid tertiary time this scenario seems to apply to large portions of the western and central alps as well calling for drastic geodynamic reinterpretations of these parts of the alps

kinematics of ductile shearing from outcrop to crustal scale in the monte rosa nappe western alps significant ductile deformation has produced widespread mylonitic structures in the basement orthogneisses and the mesozoic cover quartzites and marbles of the monte rosa nappe in the inner part of the western alps we summarize here the results of a detailed microstructural study at several scales of observation in the northern part of the nappe and discuss the kinematic significance and compatibility of such results most of the observed deformations can be accounted for by progressive wnw vergent shearing during and after the late eocene within a ductile shear zone of crustal scale the displacement of the hanging wall may have been in excess of several tens of kilometers however some ese vergent shear criteria cannot be reconciled with this general picture and suggest that local backthrusting occurred later near the toe of the monte rosa nappe

dynamics of orogenic wedges and the uplift of high pressure metamorphic rocks subduction accretion complexes can be approximated as wedge shaped continua with a rigid buttress behind and a subducting litho spheric slab beneath thick wedges undergoing prograde metamorphism have a negligible long term yield strength and are likely to exhibit a complex nonlinear viscous rheology such a wedge will tend to deform internally until it reaches a stable configuration in which the gravitational forces generated by the wedge geometry balance the traction exerted on its underside by the subducting slab accretion of material at the wedge front will lengthen the wedge and cause it to shorten internally to regain the stable geometry this shortening will be expressed as late out of sequence thrusting backthrusting and folding conversely underplating of sediment or crustal slices will thicken the wedge which may need to extend internally to regain stability extension will cause listric normal faults that may merge downward into zones of ductile extension continued underplating at depth and compensating extension above provides a mechanism for bringing high p low t metamorphic rocks to upper levels in the rear of the wedge where they are commonly observed many major tectonic boundaries in convergent orogens such as the coast range thrust in the franciscan complex major nappe contacts in the alps and the contact between the nevado filabride and higher betic nappe complexes in the betic cordillera show abrupt increases in metamorphic grade downward across them this is consistent with their origin or reactivation as uplift related extensional structures

evidence for extension in the western alpine orogen the contact between the oceanic piemonte and overlying continental sesia units abstract in the alps and other orogens major tectonic contacts must have been established during convergence and overthrusting what is less clear is how strongly these contacts were modified as the orogen evolved during alpine collision continental crust of the overriding plate the sesia unit is inferred to have been thrust northwest over oceanic material of the piemonte unit both units preserve in parts eclogite facies metamorphism however all structures along this contact in our study area indicate that final movement was southeast directed with no clear evidence of the kinematics of earlier movement these structures include se verging folds se directed shear bands and larger normal faults downthrowing to the southeast it is inferred that there was a se directed shear regime below the contact which although locally buckling and shortening earlier lithological contacts was dominantly extensional with respect to pre existing tectonic layering the lower limit of this shear regime has not been identified although se directed shear is common throughout the upper greenschist facies part of the piemonte unit we show that this shear passes beneath the sesia zone and does not re emerge it is therefore a shear which was net extensional relative both to the modern surface and to the palaeosurface it may have contributed to the unroofing of eclogite facies rocks in the lower part of the piemonte unit although additional timing data are required to clarify this nearby se directed shears buckle and imbricate earlier layering within the piemonte unit these are normally identified as backthrusts yet they do not appear to breach the base of the sesia unit and might merge with the extensional shear even though these structures shorten layering they too could have been extensional relative to the earth s surface the significance of alpine backthrusts should be reappraised in this context this study indicates that hinterland directed extension could have been an important phenomenon during alpine evolution

pb sr nd isotopic behavior of deeply subducted crustal rocks from the dora maira massif western alps italy ii what is the age of the ultrahigh pressure metamorphism pb nd and sr isotope data are reported from two localities on mineral separates from mg rich metapelites and associated rocks that have been subducted to depths of at least 100 km for which metamorphic conditions are estimated at 28 33 kilobars pressure and 700 800 c and then returned to the surface initial isotope ratio data from the granitoid country rock are similar to those found in the metapelites the initial ratios indicate predominantly recycled aged granitic crustal materials for the sources of all of the samples five zircon samples 4 from pyrope megacrysts and 1 from fine grained pyrope quartzite lenses in the metapelites accurately define a chord yielding intercept ages of 304 10 and 38 0 1 4 ma in a concordia diagram zircon from the country rock also plots along the chord the zircon data together with initial nd and sr data indicate that the sedimentary sources of the rocks were derived mainly or entirely from sialic hercynian rocks ellenbergerite from pyrope megacrysts and monazite from the fine grained ground mass yield slightly younger ages of 30 34 ma apparently reflecting lower blocking temperatures than that of zircon sm nd data from a pyrope megacryst give an errorchron corresponding to an age of 38 ma in agreement with the zircon date a major question concerns the timing of the ultrahigh pressure metamorphism experimental data suggest that pyrope and quartz coesite as well as ellenbergerite formed by various metamorphic reactions if as generally assumed the ultrahigh pressure metamorphism occurred ca 100 ma ago our data require that the zircon did not experience measurable lead loss at that time but lost major amounts of lead 38 ma ago during late alpine metamorphism estimates of diffusion rates for nd in pyrope further suggest that the apparent sm nd age of 38 ma for the megacryst is not consistent with that model those problems are resolved if the ultrahigh pressure metamorphism occurred 38 40 ma ago but problems remain from ar ar dates of 100 ma on phengite an inferred 120 ma age for zircon lead loss from another study and possibly by the very rapid uplift required if the metamorphism is that young

dating deformation using rb sr in white mica greenschist facies deformation ages from the entrelor shear zone italian alps ages of deformation have been obtained by rb sr analysis of white micas whose microstructural and chemical characteristics indicate that they crystallized or recrystallized during shear fabric formation since white micas commonly define deformation fabrics in medium grade metamorphic rocks these ages can be directly related to structural geometries with regional context this direct method contrasts with estimates of midcrustal deformation ages derived from cooling histories because it does not rely on assumptions about the thermal structure of the crust it does require that the dated minerals attained isotopic equilibrium with the dominant sr reservoir at temperatures lower than the closure temperature this resetting was apparently achieved during dynamic recrystallization of white micas in greenschist facies metasediments and metagranitoid units in the western alps the results suggest that the sr isotopic composition of the new mica is buffered by the coexisting high sr phases calcite feldspar or epidote via the grain boundary network high strain rocks from the entrelor shear zone system of the western alps have yielded indistinguishable white mica rb sr ages along 30 km of individual and kinematically linked shear zones the age of the back thrusting event is constrained at 34 1ma the age yielded by the younger generation of synkinematically crystallized white micas this event was short lived involving at least 20 km of shortening in 1 m y or less an earlier variably overprinted component dating from 38 to 37 ma has been identified in the mica fabric but its kinematic significance is uncertain this method of dating strain fabrics offers a powerful tool for tectonic studies since isotopic resetting can be directly linked to structural geometries microstructural textures and pt conditions it allows testing of kinematic models in orogens and can provide important information on the rates of geological processes in the crust

development of accretionary orogens preface abstract this special issue celebrates the career of cees van staal an icon of appalachian geology who retired from the geological survey of canada in 2017 the issue is an outcome of the symposium entitled development of accretionary orogens a symposium celebrating the career of cees van staal that was held in cees s honour at the 2017 geological association of canada mineralogical association of canada gac mac meeting in kingston ontario the papers in this special issue simultaneously demonstrate recent advances in the understanding of accretionary orogens cees s influence on this understanding as well new data and concepts concerning their development in keeping with the global reach of cees s research this vsi is a contribution to the international geological correlation program 648 supercontinent cycles and global geodynamics

upper crustal deformation in continent continent collision a case study from the bernard nappe complex valais switzerland 1 the penninic nappes in the swiss alps formed during continental collision between the adriatic and european plates in cenozoic times although intensely studied the finite geometry of the basement bearing penninic nappes in western switzerland has remained a matter of debate for decades e g siviez mischabel dilemma and the paleogeographic origin of various nappes has been disputed here we present new structural data for the central part of the penninic bernard nappe complex which contains pre permian basement and permo mesozoic metasedimentary units our lithological and structural observations indicate that the discrepancy between the different structural models proposed for the bernard nappe complex can be explained by a lateral discontinuity in the west the presence of a permian graben caused complex isoclinal folding whereas in the east the absence of such a graben resulted mainly in imbricate thrusting the overall geometry of the bernard nappe complex is the result of three main deformation phases 1 detachment of mesozoic cover sediments along triassic evaporites evolene phase during the early stages of collision 2 eocene top to the n nw nappe stacking anniviers phase and 3 subsequent backfolding and backshearing mischabel phase the southward localized backshearing is key to understand the structural position and paleogeographic origin of units such as the frilihorn and cimes blanches nappes and the antrona ophiolites based on these observations we present a new tectonic model for the entire penninic region of western switzerland and discuss this model in terms of continental collision zone processes

staircase 40 ar39ar spectra of fine grained white mica timing and duration of deformation and empirical constraints on argon diffusion staircase 40 ar 39 ar spectra of 2 to 6 m white micas in ductile carbonate mylonites morcles doldenhorn nappe switzerland record ages between 32 and 13 ma these ages are within independent geochronologic and biostratigraphic time limits for the beginning and ending of deformation of this fold thrust nappe we interpret these data to record protracted white mica neocrystallization and growth during low temperature ductile deformation such that the differences between the maximum and minimum ages in the 40 ar 39 ar spectra record the time span over which mica formed and grew we have modeled the argon concentrations in these micas taking into account the thermal history of the nappe mica growth and diffusional loss of argon our results indicate that available experimental data underestimate the activation energy for argon diffusion in muscovite and that the retentivity of argon in white mica is greater than generally thought

high strain zone deformation in the southern capricorn orogen western australia kinematics and age constraints abstract detailed structural mapping of a the kerba fault a high strain zone along the contact between the archaean to palaeoproterozoic yarlarweelor gneiss complex partly derived from the yilgarn craton and the palaeoproterozoic volcano sedimentary rocks of the bryah group indicate a temporal progression from foliation development to folding and finally brittle deformation structural data from the penetrative s1 foliation indicate a superficially simple geometry and kinematic history with a single pervasive greenschist facies foliation forming during dextral strike slip shearing however systematic spatial variations in the magnitude and orientations of mineral elongation lineations indicate a more complex deformation history involving a combination of general transpressive dextral shear and subsequent reactivation this deformation has previously been considered to develop between 1830 and 1780 ma ago during the capricorn orogeny relative age relationships around the kerba fault indicate that the high strain deformation postdates the intrusion of the kerba granite at 1808 6 ma analysis of available age data indicates that the minimum age of deformation is poorly constrained however evidence from regionally similar structures and geochronological data indicate that deformation associated with high strain zones in the southern capricorn orogen may be mesoproterozoic in age significantly younger than the capricorn orogeny

structural evolution of the contact between two penninic nappes zermatt saas zone and combin zone western alps and implications for the exhumation mechanism and palaeogeography the boundary zone between two penninic nappes the eclogite facies to ultrahigh pressure zermatt saas zone in the footwall and the blueschist facies combin zone in the hanging wall has been interpreted previously as a major normal fault reflecting synorogenic crustal extension quartz textures of mylonites from this fault were measured using neutron diffraction together with structural field observations the data allow a refined reconstruction of the kinematic evolution of the pennine nappes the main results are 1 the contact is not a normal fault but a major thrust towards northwest which was only later overprinted by southeast directed normal faulting 2 exhumation of the footwall rocks did not occur during crustal extension but during crustal shortening 3 the sesia dent blanche nappe system originated from a continental fragment cervinia in the alpine tethys ocean and the combin zone ophiolites from the ocean basin southeast of cervinia 4 out of sequence thrusting played a major role in the tectonic evolution of the penninic nappes