coesite and pure pyrope in high grade blueschists of the western alps a first record and some consequences a pyrope quartzite originally described by vialon 1966 from the dora maira massif was resampled and reinvestigated garnet up to 25 cm in size phengite kyanite talc and rutile are in textural equilibrium in an undeformed matrix of polygonal quartz the garnet is a pyrope almandine solid solution with 90 to 98 mol mg end member it contains inclusions of coesite which has partially inverted to quartz resulting in a typical radial cracking of the host garnet around the inclusions several lines of evidence show that coesite crystallised under nearly static pressure conditions and that the whole matrix has once been coesite

the continental margin of the mesozoic tethys in the western alps abstract most of the structural units of the western alps were derived from the european continental margin of the ligurian ocean a segment of the mesozoic tethys ocean their mesozoic palaeotectonic and stratigraphic evolution bears witness of the following main stages 1 deposition of the triassic platform carbonates essentially prerift but nevertheless bearing the imprint of some extensional movements 2 the liassic middle jurassic rifting corresponds to the creation of a horst and graben system with especially tilted blocks and hence a shoal and basin palaeogeography but without a preliminary doming event 3 the late jurassic early cretaceous opening and spreading of the ligurian ocean began with a general collapse of the continental margin thermal subsidence latest middle jurassic and earliest late jurassic in this paper emphasis is given to the refting derived structures more especially to the major tilted crustal blocks a few tens of kilometres wide which can be either reconstructed or directly observed the rifting stage lasted roughly 40 m y with the alternation of extensional tectonic phases and of relatively quiet periods during the extensional phases movements along fault planes and related episodes of tilting were followed by sudden and rapid episodes of subsidence the latter may be interpreted as resulting from the isostatic and thermal readjustment that follows a phase of stretching of the crust and of the lithosphere the tectonic evolution of the margin continued but decreased during the late jurassic early cretaceous oceanspreading stage tectonic activity resumed in the late cretaceous this probably resulting from the beginning of contraction of both the ocean and the continental margin leading progressively to the continental collision in the tertiary

a quantification of the glacial imprint on relief development in the french western alps abstract the morphology of the western alps has been strongly influenced by quaternary glaciations on the basis of observations of glacial morphology in the belledonne grandes rousses taillefer and pelvoux ecrins massifs south eastern france we reconstitute the glacial trimline and equilibrium line altitude ela during the most extensive glaciation meg our best estimate of the meg ela is 1800 100m using digital elevation models we compare our glacial reconstruction with the relief structure of nine major catchments draining the massifs modal elevations of the largest catchments occur at 2000 2500m and coincide with minima in plots of mean slope angles as a function of elevation modal elevations and slope minima occur between the modern and meg elas confirming a strong glacial imprint on relief in order to quantify glacial valley carving in the massifs we isolated high elevation low relief surfaces that form rock shoulders adjacent to the glacial valleys from a digital elevation model and constructed an interpolated surface passing through these subtracting the present day topography from this surface allows us to quantify the maximum glacial valley depths maximum valley depths determined in this manner are typically 1000m with spatial maxima occurring around the location of the meg ela in most valleys these numbers do not take into account glacial valley widening and local glacial overdeepenings the also neglect however potential pre glacial fluvial valley incision which could account for 20 50 of the measured valley depths in spite of these problems inferred valley depths are reasonably well correlated with the mean reconstructed ice thickness and constitute about half of the sub ridgeline relief of the studied catchments these results lead us to propose a significant quaternary increase in the relief of the french western alps controlled by climate and associated with the initiation of alpine glaciations for reasonable values of the effective elastic thickness of the lithosphere the isostatic response to glacial valley carving reaches values of 300m across the massifs this number is insufficient to substantially offset topographic lowering due to regional denudation and we conclude that the isostatic response to glacial valley carving has not increased peak elevations significantly

pbsl dating of garnet and staurolite constraints on the paleoproterozoic crustal evolution of the eastern block north china craton abstract it is well accepted that the north china craton formed by assembly of the eastern and western blocks along the trans north china orogen with cratonization in the paleoproterozoic 1 85 ma however there is controversy about the formation of the eastern and western blocks themselves especially the timing of cratonization of the eastern block the jiao liao ji orogenic belt is the most important paleoproterozoic orogenic belt in the eastern block and consists mainly of the north and south liaohe groups and the liaoji granitoids the timing of metamorphism of the liaohe group is the key to understanding the evolution of the eastern block of the north china craton because the isotope systematics of metamorphic minerals can be used to date parts of the prograde thermal histories of metamorphic belts dating of minerals such as garnet and staurolite has the distinct advantage that time information can be related directly to the pressure temperature deformation history recorded by the same mineral in this paper we present results of precise 207pb 206pb stepwise leaching pbsl dating of garnet and staurolite from the metasedimentary rocks of the south liaohe group the primitive pb and radiogenic pb are selectively extracted from garnet and straurolite by leach steps and determined by multiple collector mc icp ms the garnet and staurolite record two groups of 207pb 206pb ages at 1 93 1 91 ga and 1 86 ga respectively these constrain the time of peak metamorphism of the liaohe group during collision between the longgang block in the north and the liaonan nangrim block in the south and a subsequent post collisional event respectively our data for the south liaohe group and previously published data for the liaoji granite combined with key lithological structural and metamorphic information indicate that the cratonization of the eastern block of the north china craton occurred at 1 93 1 91 ga

cooling history of the dauphinoise zone western alps france deduced from the thermopaleomagnetic record geodynamic implications a post tectonic thermoremanent magnetization carried by pyrrhotite has been previously demonstrated in the dauphinoise zone of the french western alps the paleomagnetic signal is interpreted to record successive partial thermoremanent magnetizations acquired in normal and reverse geomagnetic fields during the slow cooling of the epimetamorphic marly limestones a comparison between the derived magnetic polarity sequence versus temperature thermopaleomagnetic record and the magnetostratigraphic time scale taking into account radiochronological constraints allows us to derive 21 age temperature couples between 24 1 ma 302 c and 20 2 ma 182 c for the bourg d oisans area combining these thermopaleomagnetic data with other p t t and structural data a geodynamical model is proposed for the tectono metamorphic evolution of the dauphinoise zone the thickening contemporaneous with the main syncleavage folding occurs around 26 5 ma depending on the hypothesis considered to explain the excess of heat the bourg d oisans area was buried to 8 6 km fluids hypothesis or 12 5 km tectonic hypothesis in both cases these depths imply the existence of overlying units which might come from the internal zones actually cropping out eastward of the penninic front a tectonic denudation is proposed just before 24 1 ma in order to explain our observations after this denudation a faster cooling occurs it is probably related to the dissipation of the thermal anomaly induced by the tectonic denudation and with the increase of the erosion due to the underthrust of a more external crustal slice this interpretation is supported by the similar fluctuations of cooling rate derived here and subsidence of the molasse basin afterwards the cooling slowed down during this time the studied area seems to be passively moved westward in the hanging wall of the external crystalline massifs thrust without significant deformations

glacial effects limiting mountain height the height of mountain ranges reflects the balance between tectonic rock uplift crustal strength and surface denudation tectonic deformation and surface denudation are interdependent however and feedback mechanisms in particular the potential link to climate are subjects of intense debate spatial variations in fluvial denudation rate caused by precipitation gradients are known to provide first order controls on mountain range width crustal deformation rates and rock uplift moreover limits to crustal strength are thought to constrain the maximum elevation of large continental plateaus such as those in tibet and the central andes there are indications that the general height of mountain ranges is also directly influenced by the extent of glaciation through an efficient denudation mechanism known as the glacial buzzsaw here we use a global analysis of topography and show that variations in maximum mountain height correlate closely with climate controlled gradients in snowline altitude for many high mountain ranges across orogenic ages and tectonic styles with the aid of a numerical model we further demonstrate how a combination of erosional destruction of topography above the snowline by glacier sliding and commensurate isostatic landscape uplift caused by erosional unloading can explain observations of maximum mountain height by driving elevations towards an altitude window just below the snowline the model thereby self consistently produces the hypsometric signature of the glacial buzzsaw and suggests that differences in the height of mountain ranges mainly reflect variations in local climate rather than tectonic forces

fluvial incision into bedrock insights from morphometric analysis and numerical modeling of gorges incising glacial hanging valleys western alps france bedrock gorges incising glacial hanging valleys potentially allow measurements of fluvial bedrock incision in mountainous relief using digital elevation models topographic maps and field reconnaissance we identified and characterized 30 tributary hanging valleys incised by gorges near their confluence with trunk streams in the romanche watershed french western alps longitudinal profiles of these tributaries are all convex and have abrupt knickpoints at the upper limit of oversteepened gorge reaches we reconstructed initial glacial profiles from glacially polished bedrock knobs surrounding the gorges in order to quantify the amount of fluvial incision and knickpoint retreat from morphometric analyses we find that mean channel gradients and widths as well as knickpoint retreat rates display a drainage area dependence modulated by bedrock lithology however there appears to be no relation between horizontal retreat and vertical downwearing of knickpoints assuming a postglacial origin of these gorges our results imply high postglacial fluvial incision 0 5 15 mm yr 1 and knickpoint retreat 1 200 mm yr 1 rates that are however consistent with previous estimates numerical modeling was used to test the capacity of different fluvial incision models to predict the inferred evolution of the gorges results from simple end member models suggest transport limited behavior of the bedrock gorges a more sophisticated model including dynamic width adjustment and sediment dependent incision rates predicts present day channel geometry only if a significant supply of sediment from the gorge sidewalls 10 mm yr 1 is triggered by gorge deepening combined with pronounced inhibition of bedrock incision by sediment transport and deposition

significant increase in relief of the european alps during mid pleistocene glaciations the effect of expanded glaciation on the relief of mountain ranges is debated chronometric and model data from the european alps suggest that relief in the rhone valley was enhanced as a result of the increased incision of the valley following the mid pleistocene climate transition

diachronous exhumation of hp lt metamorphic rocks from south western alps evidence from fission track analysis new fission track ages on zircon and apatite zft and aft from the southwestern alpine paleo accretionary wedge document a contrasting cooling history from east to west in the eclogitic monviso ophiolites the zft ages are 19 6 0 8 ma and the aft ages are 8 6 1 7 ma in the ht blueschist eastern queyras zft ages range from 27 0 1 5 ma to 21 7 1 6 ma and aft ages from 14 2 2 0 to 9 4 1 1 ma in the lt blueschist western queyras zft ages are between 94 7 3 1 ma and 63 1 2 9 ma and aft ages are between 22 2 1 6 and 22 6 1 5 ma the chenaillet ophiolite yields ages of 118 1 3 7 ma on zft and of 67 9 8 5 ma on aft these new ft data combined with petrological and geochronological constraints record a diachronous exhumation in the paleo accretionary wedge during subduction and collision

the oligocene alps domal unroofing and drainage development during early orogenic growth abstract the source of huge volumes of quartzofeldspathic detritus feeding oligo miocene foredeep turbidites of the apennines has long remained an unsolved geological enigma in this article we reinterpret the available sandstone petrography fission track stratigraphic and geomorphological evidence to demonstrate that the adriatic foredeep was dominantly sourced by a himalayan type tectonic aneurysm growing in the central alps since the late oligocene the lepontine dome the lepontine dome is the envelope of distinct subdomes that formed stepwise from east to west during northwestward indentation of adria beneath the axial alps focused erosion of domal culminations cut by antecedent drainage started suddenly in the late oligocene with unroofing of recently emplaced plutons and young amphibolite facies gneisses exhumation was tectonically driven and possibly accelerated by positive feedback between erosion and uplift of hot weak rocks from depth the huge volumes of resulting detritus gonfolite macigno clastic wedge were transported southward for hundreds of kilometers along the rapidly subsiding foredeep formed since middle eocene times in front of the alpine retrobelt and nascent apennine forebelt such massive sediment influx following 15 2 myr of starved foredeep sedimentation testifies to rapid formation of mountain relief in the central alps synchronous to dextral activity along the insubric fault the lepontine dome remains today as the only deeply dissected region in the alps where medium grade metamorphic rocks of the axial belt are extensively exposed the relationships between geological structures and drainage patterns still mirror those formed originally at oligocene times river headwaters have not been significantly displaced since then and their thalwegs constrain finite lateral displacement relative to the southalpine retrowedge to a maximum of 10 20 km our reconstruction has wide implications not only for a better understanding of the tertiary evolution of the alps apennines orogenic couple but also for general relationships among tectonic activity relief formation drainage development erosion distribution and long distance sediment transfer during continental collision and orogenic growth

40ar 39ar dating of penninic front tectonic displacement w alps during the lower oligocene 31 34 ma direct absolute dating of the penninic frontal thrust tectonic motion is achieved using the 40 ar 39 ar technique in the pelvoux crystalline massif western alps the dated phengites were formed syn kinematically in shear zones they underline the brittle ductile stretching lineation pressure shadow fibres and slickensides consistent with underthrusting of the european continental slab below the propagating penninic thrust chlorite phengite thermobarometry yields 10 15 km and t 280 c while 40 ar 39 ar phengite ages mainly range between 34 and 30 ma with one younger age at 27 ma this early oligocene age range matches a major tectonic rearrangement of the alpine chain preservation of prograde 40 ar 39 ar ages is ascribed to passive exhumation of the pelvoux shear zone network sandwiched between more external thrusts and the penninic front reactivated as an e dipping detachment fault partial resetting in the low temperature part of argon spectra below 24 ma is ascribed to brittle deformation and alteration of phengites

multidisciplinary tephrochronological correlation of marker events in the eastern tyrrhenian sea between 48 and 105 ka abstract the basal portion 2 92 5 08 m from core top of the cet1 core located in a bathyal area of the eastern tyrrhenian sea is the subject of a multidisciplinary investigation encompassing tephrostratigraphy quantitative analyses of planktonic foraminifera and calcareous nannofossil assemblages and 18o measurements supported by a 40ar 39ar age determination calcareous nannofossil assemblages allowed the attribution of the analyzed sediments to the biozone mmn21a and the succession spans from more than 48 to ca 105 ka although there are at least two stratigraphic gaps and minor resedimentation episodes a paleoclimatic reconstruction obtained via 18o and planktonic foraminifera data identifies several of the major climatic events that occurred in the investigated time span a total of 13 visible tephra layers and cryptotephras are recognized and correlated with their volcanic sources campanian volcanic zone and pantelleria volcano and when possible with well known and dated events or with widespread marker tephras using a combination of biostratigraphic oxygen isotope and tephrostratigraphic correlations we develop a chronostratigraphy for the main climatic events and tephra layers in the core and pinpoint tephra markers for the climatic events comprised between gs 22 and gi 24 in the eastern tyrrhenian sea

widespread extension in the core of the western alps revealed by earthquake analysis the western alps are an active collision belt whose current stress field is inhomogeneous muller et al 1992 we report new seismological data which significantly improve our knowledge of this stress field about 1600 earthquakes which occurred in the western alps during the last 10 years were precisely located and 79 new focal solutions were computed the analysis of this database shows that widespread extension affects all the internal zones of the belt to better constrain the associated stress regime six stress tensors were computed using the gephart and forsyth 1984 method they show that the current tectonics of the western alps are contrasted with close variation in the stress regime transpression to the front of the belt contrasting with extension in the core of the belt the extensional direction is radial to the arcuate geometry of the belt and bounded outboard by the former thrust of the internal zones onto the external zone suggesting extensional reactivation of this inherited crustal discontinuity such widespread extension within the inner part of an actually ongoing collision belt cannot be explained by simple collision related tectonics we propose that intrabelt buoyancy forces such as those produced by a slab retreat or slab break off interfere with the boundary forces driven by the ongoing europe africa convergence

a bayesian approach to calibrating apatite fission track annealing models for laboratory and geological timescales abstract we present a new approach for modelling annealing of fission tracks in apatite aiming to address various problems with existing models we cast the model in a fully bayesian context which allows us explicitly to deal with data and parameter uncertainties and correlations and also to deal with the predictive uncertainties we focus on a well known annealing algorithm laslett g m green p f duddy i r gleadow a j w 1987 thermal annealing of fission tracks in apatite 2 a quantitative analysis chem geol 65 1 1 13 and build a hierachical bayesian model to incorporate both laboratory and geological timescale data as direct constraints relative to the original model calibration we find a better in terms of likelihood model conditioned just on the reported laboratory data we then include the uncertainty on the temperatures recorded during the laboratory annealing experiments we again find a better model but the predictive uncertainty when extrapolated to geological timescales is increased due to the uncertainty on the laboratory temperatures finally we explictly include a data set vrolijk p donelick r a quenq j cloos m 1992 testing models of fission track annealing in apatite in a simple thermal setting site 800 leg 129 in larson r lancelet y eds proceedings of the ocean drilling program scientific results vol 129 pp 169 176 which provides low temperature geological timescale constraints for the model calibration when combined with the laboratory data we find a model which satisfies both the low temperature and high temperature geological timescale benchmarks although the fit to the original laboratory data is degraded however when extrapolated to geological timescales this combined model significantly reduces the well known rapid recent cooling artifact found in many published thermal models for geological samples

diachronous late stage exhumation across the western alpine arc constraints from apatite fission track thermochronology between the pelvoux and dora maira massifs we present new apatite fission track aft data from the central western alps that confirm the synchronicity and high cooling rates during latest miocene pliocene final exhumation of the external crystalline massifs but also provide evidence for diachronous neogene evolution along and across the internal arc to the se of the pelvoux massif across the front of the internal arc penninic frontal thrust the jump in aft ages c 22 ma and in final cooling rates is significantly larger than further north this difference results from reversal of movement along a major oligocene thrust in its hanging wall the western brianconnais zone provides a mean aft age of c 27 ma which is older than further north early cooling in the southern brianconnais zone would result from rapid erosion of the compressional fan structure built during the oligocene across the entire brianconnais and piemont nappe stack with the exception of the dora maira massif aft ages young eastward and span the entire miocene a period during which this structure underwent extension further north a reverse gradient with ages younging northwestward has been described prompting the question of the asymmetry of the internal western alpine arc during its late stage tectonic and morphological evolution

exhumation and relief development in the pelvoux and dora maira massifs western alps assessed by spectral analysis and inversion of thermochronological age transects 1 we have used dedicated sampling and analysis of apatite fission track aft and apatite u th he ahe thermochronological data sets in an attempt to quantify relief evolution and exhumation rates in the pelvoux and dora maira massifs western european alps a dual approach comparing spectral analysis and thermal kinematic model inversion was applied we sampled age elevation relationships at a range of topographic wavelengths along two north south transects crossing these massifs for the 40 km long pelvoux transect we report 35 new aft ages that range between 3 0 0 4 ma and 12 6 1 0 ma and 8 new ahe ages between 3 5 1 5 and 4 7 0 7 ma the dora maira transect spans a distance of 60 km and includes 28 23 new and 5 previously published significantly older aft ages which vary between 13 1 0 2 and 27 3 0 3 ma inferred exhumation rates of 0 7 km m y 1 since 8 ma in the pelvoux and only 0 1 km m y 1since 20 ma in dora maira are consistent between methods as well as with previous estimates although they are associated with large uncertainties due to imperfect sampling and analytical errors neither massif displays clear evidence for relief change during the intervals constrained by the data data from the dora maria massif suggest moderate recent relief increase whereas the results from the pelvoux are inconclusive but could imply relief decrease our study highlights the difficulties of applying thermochronology techniques to constrain relief changes we show that an unreasonable number of samples is needed to perform reliable spectral analysis of age elevation transects and discuss the overall limitations of both techniques to address relief evolution we suggest that a more efficient approach would be to apply thermal kinematic model inversion to relatively small study areas using spatially distributed sampling and multiple thermochronometer analyses per sample

extensional neotectonics around the bend of the western central alps an overview the western alps active tectonics is characterized by ongoing widespread extension in the highest parts of the belt and transpressive compressive tectonics along its borders we examine these contrasting tectonic regimes using a multidisciplinary approach including seismotectonics numerical modeling gps morphotectonics fieldwork and brittle deformation analysis extension appears to be the dominant process in the present day tectonic activity in the western alps affecting its internal areas all along the arc shortening in contrast is limited to small areas located along at the outer borders of the chain strike slip is observed throughout the alpine realm and in the foreland the stress orientation pattern is radial for 3 in the inner extensional zones and for 1 in the outer transcurrent tranpressional ones extensional areas can be correlated with the parts of the belt with the thickest crust quantification of seismic strain in tectonically homogeneous areas shows that only 10 20 of the geodesy documented deformation can be explained by the alpine seismicity we propose that alpine active tectonics are ruled by isostasy buoyancy forces rather than the ongoing shortening along the alpine europe adria collision zone this interpretation is corroborated by numerical modeling the neogene extensional structures in the alps formed under increasingly brittle conditions a synthesis of paleostress tensors for the internal parts of the west alpine arc documents major orogen parallel extension with a continuous change in 3 directions from ene wsw in the simplon area to n s in the vanoise area and to nnw sse in the briancon area minor orogen perpendicular extension increases from n to s this second signal correlates with the present day geodynamics as revealed by focal plane mechanisms analysis the orogen parallel extension could be related to the opening of the ligurian sea during the early middle miocene and to compression rotation of the adriatic indenter inducing lateral extrusion

frost cracking control on catchment denudation rates insights from in situ produced 10be concentrations in stream sediments ecrins pelvoux massif french western alps the potential tectonic and climatic controls on erosion rates in the european alps and other mountain belts remain strongly debated we have quantified denudation rates at catchment scales using in situ produced cosmogenic nuclides 10be in stream sediments sampled at the outlets of twelve variously sized 27 1072 km2 catchments of the ecrins pelvoux massif french western alps with average elevations ranging from 1700 to 2800 m spatially averaged denudation rates corrected for potential shielding by little ice age glaciers vary from 0 27 0 05 to 1 07 0 20 mm yr on millennial timescales our results exhibit a correlation 2 0 56 between denudation rate and mean catchment elevation in the absence of significant correlation with any other morphometric parameters relief slope catchment size hypsometry etc although such variations in erosion rates have been previously linked to variations in tectonic uplift rate the relatively small size and tectonic homogeneity of our study area exclude a strongly variable tectonic control we interpret the increase in erosion rate with elevation as the effect of frost controlled processes which are strongly temperature dependent we use a one dimensional heat flow model driven by high resolution instrumental temperature records from the study area to correlate the variability in denudation rates with the integral of the absolute temperature gradient within the frost cracking window 3 to 8 c a proxy of the frost cracking intensity for each catchment the results imply that the efficiency of frost cracking constitutes a major control on catchment wide denudation rates in the study area explaining more than half the measured variability in these rates our study shows that present day denudation of the ecrins pelvoux massif is controlled by a climatically driven factor and suggests that frost cracking processes impose an important control on the post glacial topographic evolution of mid latitude mountain belts

inversion of thermochronological age elevation profiles to extract independent estimates of denudation and relief history ii application to the french western alps abstract thermochronologic data collected along age elevation profiles are commonly interpreted as recording temporally variant but spatially constant exhumation rates however thermochronologic age elevation relationships are known to be perturbed by topographic effects and potential changes in relief which are neglected in the inherently 1 d interpretation commonly applied such data thus potentially record both the denudation and relief history of the sampled region but extracting this information is challenging in a companion paper we develop a methodology for rigorously interpreting thermochronologic age elevation profiles in terms of exhumation rates and relief development through time and to independently quantify the resolution of these constraints here we test this approach using a thermochronological dataset consisting of apatite and zircon fission track and u th he data collected at la meije peak in the pelvoux ecrins massif french western alps our data and models suggest a three phase exhumation history in the pelvoux ecrins massif including a pulse of rapid exhumation at 6 5 5 ma preceded and followed by more moderate rates of denudation in the order of 0 3 0 4 km myr 1 this rapid exhumation event appears to occur coevally in other external crystalline massifs in the alps but is not detected by qualitative inspection of the age elevation relationships both our synthetic results and inversion of the la meije data strongly suggest that apatite fission track age elevation relationships alone cannot resolve both denudation and relief histories independently and that multiple thermochronometers are required combining apatite fission track and u th he ages and particularly including fission track length data greatly improves the resolution of the inferred exhumation histories although denudation rates through time and the timing of rate changes are generally well resolved our data have insufficient resolution to satisfactorily constrain the relief history synthetic results reported in the companion paper suggest that the reason for this limitation is that relief increase through valley carving has been insufficient with respect to the regional denudation rates to be unambiguously extracted from the data

thermal structure of a fossil subduction wedge in the western alps new peak metamorphic temperatures are obtained by raman spectroscopy of carbonaceous material to document the thermal structure of the central western alps with high sampling resolution we show that peak metamorphic t gradually increases eastward from 330 to 350 c ultra dauphinois to subbrianconnais units 350 to more than 400 c brianconnais domain including the zone houillere where metamorphic index minerals are rare and from 350 to more than 500 c liguro piemontese domain combined with other constraints on the metamorphic evolution this dataset reveals a good preservation of the overall thermal structure of the fossil subduction wedge with no particular thermal overprint during collision however local confrontation with p t estimates and radiometric ages reveals more subtle variations within tectonic units and across the main contacts that are linked to the past activity of the major thrusts and extensional shear zones during subduction and exhumation

relation between rock uplift and denudation from cosmogenic nuclides in river sediment in the central alps of switzerland 1 a north south traverse through the swiss central alps reveals that denudation rates correlate with recent rock uplift rates in both magnitude and spatial distribution this result emerges from a study of in situ produced cosmogenic 10be in riverborne quartz in central alpine catchments as a prerequisite we took care to investigate the potential influence of shielding from cosmic rays due to snow glaciers and topographic obstructions to calculate a possible memory from last glacial maximum lgm glaciation and to identify a watershed size that is appropriate for systematic sampling mean denudation rates are 0 27 0 14 mm a for the alpine foreland and 0 9 0 3 mm a for the crystalline central alps the measured cosmogenic nuclide derived denudation rates are in good agreement with post lgm lake infill rates and are about twice as high as denudation rates from apatite fission track ages that record denudation from 9 to 5 ma in general denudation rates are high in areas of high topography and high crustal thickness the similarity in the spatial distribution and magnitude of denudation rates and those of rock uplift rates can be interpreted in several ways 1 postglacial rebound or climate change has introduced a transient change in which both uplift and denudation follow each other with a short lag time 2 the amplitude of glacial to interglacial changes in both is small and is contained in the scatter of the data 3 both are driven by ongoing convergence where their similarity might hint at some form of long term quasi steady state or 4 enhanced continuous quaternary erosion and isostatic compensation of the mass removed accounts for the distribution of present day rock uplift

present day geodynamics in the bend of the western and central alps as constrained by earthquake analysis summary the contrasted tectonics of the western central alps is examined using a synthesis of 389 reliable focal mechanisms the present day strain regime is mapped and interpolated for the entire alpine belt based on a newly developed method of regionalization the most striking feature is a continuous area of extension which closely follows the large scale topographic crest line of the alpine arc thrusting is observed locally limited to areas near the border of the alpine chain a majority of earthquakes within the alps and its forelands are in strike slip mode stress inversion methods have been applied to homogenous subsets of focal plane mechanisms in order to map regional variations in stress orientation the stress state is confirmed to be orogen perpendicular both for 3 in the inner extensional zones and 1 in the outer transcurrent transpressional zones extensional areas are well correlated with the part of the belt which presents the thickest crust as shown by the comparison with the bouguer anomaly and the average topography of the belt in the northwestern swiss alps extension is also correlated with currently uplifting zones these observations and our strain stress analyses support a geodynamic model for the western alps in which the current activity is mostly a result of gravitational body forces earthquakes do not provide any direct evidence for ongoing convergence in the alpine system but a relationship with ongoing activity of complex block rotations of the apulian microplate cannot be ruled out

from passive margin to continental collision a tectonic scenario for the western alps general transverse section just south of the pelvoux massif european margin of tethys an ancient atlantic type margin it was subject to extension from triassic to the beginning of late cretaceous times and to collision thereafter as the result of convergence of the eurasian and african continents paleogeographic evolution is outlined a new alpine tectogenetic reconstruction is proposed an attempt is made to establish how the previous extensional structures influenced the later tectonic evolution modified journal abstract

metamorphic alteration mineral paragenesis and geochemical re equilibration of early archean quartz amphibole pyroxene gneiss from akilia southwest greenland quartz amphibole pyroxene gneiss from the island of akilia southwest greenland has been claimed to contain the earliest traces of life on earth in the form of biogenic carbonaceous matter encapsulated as inclusions in apatite crystals various lines of evidence including petrography geochronology field relations and geochemistry have however been presented that challenge this interpretation textural relationships and geochemical signatures in this controversial gneiss presented here manifest a complex spatially variable metamorphic history that includes granulite and amphibolite facies overprints and metasomatism a peak metamorphic granulite facies quartz orthopyroxene clinopyroxene amphibole magnetite assemblage is preserved in only a few centimeter scale layers within the 5 m thick quartz amphibole pyroxene gneiss unit calcite veinlets that appear to postdate the peak metamorphism occur in pyroxene the quartz amphibole pyroxene gneiss unit has subsequently experienced isochemical except hydration amphibolite facies alteration during which pyroxenes were retrogressed to amphiboles and magnetite and calcite was consumed parts of the quartz amphibole pyroxene gneiss that contain texturally late hornblende have experienced metasomatic alteration by al carrying fluids these fluids controlled the alteration of pyroxenes and amphiboles to hornblende and modified the trace element composition by remobilizing lree and eu apatite has variable ree composition and 87sr 86sr in the quartz amphibole pyroxene gneiss but on the local scale cm is in equilibrium with co existing silicates effective recrystallization of apatite crystals as well as co existing silicates during several stages of the metamorphic history makes the intact preservation of diagenetic apatite with encapsulated primary carbonaceous matter implausible hence it is highly unlikely that akilia apatite could serve as repository of the earliest traces of life on earth

litho structural control morphotectonics and deep seated gravitational deformations in the evolution of alpine relief a case study in the lower susa valley italian western alps abstract an integrated study of quaternary geology and geomorphology structural geology and morphotectonic lineament analysis has been performed for the lower susa valley in order to evaluate the relationships between neotectonics of the central internal side of the italian western alps and the morphodynamics of the mountain relief a regional nne ssw fault system the colle delle finestre fault zone cffz separates two sectors the middle and the lower susa valleys characterized by different fault patterns ene wsw regional faults represent the main brittle structures in the middle susa valley while e w faults prevail in the lower susa valley geomorphic anomalies plio quaternary deformed deposits and distribution of glacial deposits suggest differential uplift of fault bounded sectors distribution of deep seated gravitational slope deformations and landslides depends on lithological and structural controls suggesting a close relationship between post glacial morphodynamic evolution and quaternary fault activity this activity is also indicated by instrumental data and historical earthquakes ms ranging from 4 to 5 that struck the lower susa valley

quaternary erosion induced isostatic rebound in the western alps the high elevation and deep incision of the alps have traditionally been used as an argument for recent tectonic activity that has elevated the belt and increased erosion rates normal faulting and horizontal extension however dominate current tectonic activity and isostatic compensation of thinning crust should lead not to increased but to decreased mean elevations here we test the idea that enhanced quaternary erosion of the alps and isostatic compensation of the mass removed can account for the distribution of present day geodetically measured rates of vertical movement in the western alps using geophysical relief and kuhlemann s estimated average erosion rate for the alps we quantify the spatial distribution of erosion and the volume of eroded rock respectively from these we obtain a map of rock eroded within a given time span the calculated isostatic response of the alpine lithosphere to erosional unloading for a variety of values of the flexural rigidity of the alpine lithosphere reaches a maximum of 500 m since 1 ma in the inner swiss alps and vertical movement extends across the entire belt including peri alpine basins assuming a steady erosion rate since 1 ma this rebound accounts for half of the measured vertical motion of 1 1 mm yr in the southern valais thus a significant fraction 50 of the present day vertical movement results from the isostatic response to enhanced erosion during plio quaternary time

alpine thermal and structural evolution of the highest external crystalline massif the mont blanc 1 the alpine structural evolution of the mont blanc highest point of the alps 4810 m and of the surrounding area has been reexamined the mont blanc and the aiguilles rouges external crystalline massifs are windows of variscan basement within the penninic and helvetic nappes new structural 40ar 39ar and fission track data combined with a compilation of earlier p t estimates and geochronological data give constraints on the amount and timing of the mont blanc and aiguilles rouges massifs exhumation alpine exhumation of the aiguilles rouges was limited to the thickness of the overlying nappes 10 km while rocks now outcropping in the mont blanc have been exhumed from 15 to 20 km depth uplift of the two massifs started 22 myr ago probably above an incipient thrust the alpine sole thrust at 12 ma the ne sw trending mont blanc shear zone mbsz initiated it is a major steep reverse fault with a dextral component whose existence has been overlooked by most authors that brings the mont blanc above the aiguilles rouges total vertical throw on the mbsz is estimated to be between 4 and 8 km fission track data suggest that relative motion between the aiguilles rouges and the mont blanc stopped 4 myr ago since that time uplift of the mont blanc has mostly taken place along the mont blanc back thrust a steep north dipping fault bounding the southern flank of the range the european roof is located where the back thrust intersects the mbsz uplift of the mont blanc and aiguilles rouges occurred toward the end of motion on the helvetic basal decollement hbd at the base of the helvetic nappes but is coeval with the jura thin skinned belt northwestward thrusting and uplift of the external crystalline massifs above the alpine sole thrust deformed the overlying helvetic nappes and formed a backstop inducing the formation of the jura arc in that part of the external alps nw se shortening with minor dextral ne sw motions appears to have been continuous from 22 ma until at least 4 ma but may be still active today a sequential history of the alpine structural evolution of the units now outcropping nw of the pennine thrust is proposed

cosmogenic 10be derived denudation rates of the eastern and southern european alps denudation rates from cosmogenic 10be measured in quartz from recent river sediment have previously been used in the central alps to argue that rock uplift occurs through isostatic response to erosion in the absence of ongoing convergence we present new basin averaged denudation rates from large rivers in the eastern and southern european alps together with a detailed topographic analysis in order to infer the forces driving erosion denudation rates in the eastern and southern alps of 170 1 400 mm ky 1 are within a similar range to those in the central alps for similar lithologies however these denudation rates vary considerably with lithology and their variability generally increases with steeper landscapes where correlations with topographic metrics also become poorer tertiary igneous rocks are associated with steep hillslopes and channels and low denudation rates whereas pre alpine gneisses usually exhibit steep hillslopes and higher denudation rates molasse flysch and schists display lower mean basin slopes and channel gradients and despite their high erodibility low erosion rates exceptionally low denudation rates are also measured in permian rhyolite which has high mean basin slopes we invoke geomorphic inheritance as a major factor controlling erosion such that large erosive glaciers in the late quaternary cold periods were more effective in priming landscapes in the central alps for erosion than in the interior eastern alps however the difference in tectonic evolution of the eastern and central alps potentially adds to differences in their geomorphic response their deep structures differ significantly and unlike the central alps the eastern alps are affected by ongoing tectonic influx due to the slow motion and rotation of adria the result is a complex pattern of high mountain erosion in the eastern alps which has evolved from one confined to the narrow belt of the tauern window in late tertiary time to one affecting the entire underthrust basement orogenic lid and parts of the southern alps today

radiation damage control on apatite u th he dates from the grand canyon region colorado plateau individual detrital apatite grains from the esplanade coconino and moenkopi formations in the grand canyon region of the colorado plateau yield u th he dates from 104 to 5 ma the range of dates within each unit far exceeds analytical uncertainty but correlates with both he concentration he and effective u concentration eu these dates are all signifi cantly younger than the sandstone units indicating partial to complete he loss following deposition recently published laboratory diffusion data suggest that he retentivity in apatite increases with radiation damage forward models predict that the consequences of this effect will be manifested most clearly as a correlation between u th he dates and the he and eu in suites of apatites that 1 are characterized by a large span of eu and 2 had thermal histories in which suffi cient time elapsed for the apatite he diffusion kinetics to diverge prior to reheating and partial resetting apatites in the sedimentary units investigated fi t these cri teria using geologically reasonable deposition burial and unroofi ng histories simulations that include the effect of radiation damage on apatite he retentivity can reproduce the observed distributions of apatite dates and correlations with parent and daughter concentrations these results suggest that a span of u th he dates positively correlated with eu may provide important information regarding a sample s thermal history

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

miocene to present differential exhumation in the western alps insights from fission track thermochronology 1 analysis of cooling age patterns yielded by low temperature thermochronometers provides key information about the role played by tectonic discontinuities during the late stages of exhumation of metamorphic belts in the western alps fission track data published so far are heterogeneously scattered and concentrated in few structural domains preventing analyses at the scale of the whole belt the new apatite fission track data reported in this work obtained with the external detector method as well as the population method in very low u content samples fill this gap they constrain the postmetamorphic evolution of the western alps along two transects from the foreland to the retroforeland unraveling the effective role played by some major faults during the exhumation of the belt at shallow crustal levels a clear regional pattern characterized by decreasing ages moving from the axial sector to the european external sector of the belt and by an along strike gradient with increasing ages from north to south has been unraveled evident breaks in this age pattern have been detected in correspondence of faults that are near parallel to the trend of the belt pointing to the occurrence of active tectonics during and after exhumation the most apparent breaks have been observed in the axial sector of the belt where the postmetamorphic deformation would have been negligible according to classic tectonic models faults located in the axial sector split the belt into two major blocks eastern and western since the miocene the western block experienced higher exhumation rates than the eastern one such differential exhumation was accommodated in the northern portion of the belt thanks to reverse motion along the internal houiller fault which occurred in a convergent transcurrent framework to the south it was accommodated instead by normal reactivation of the brianconnais front and by activity of the longitudinal fault system which occurred in a divergent transcurrent framework the tectonic activity affecting the axial sector of the belt in a context of regional dextral strike slip is coeval with the forward propagation of the external thrusts and of similar magnitude we suggest that the contrasting kinematic regimes i e convergent versus divergent transcurrence observed in the western alps moving along strike were responsible of the increasing exhumation rates toward the north revealed in both blocks by the along strike age gradient the higher exhumation rates recognized northward would be related to an increasing importance of crustal shortening that promoted erosion during the late stages of exhumation of the belt

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

variable late neogene exhumation of the central european alps low temperature thermochronology from the aar massif switzerland and the lepontine dome italy 1 several recent studies proposed an important increase in exhumation rate in the western european alps since circa 5 4 ma in order to assess potential spatial differences in exhumation histories we present new apatite fission track aft and apatite u th he ahe ages from the central aar massif guttannen area switzerland and the western lepontine dome formazza area italy internal u th zoning in apatites explains alpha ejection corrected ahe ages that are older than the corresponding aft ages in this study a qualitative interpretation of aft and ahe age elevation relationships suggests a two phase 9 7 and 5 3 ma exhumation scenario affecting the central alps with a stronger expression of the pliocene signal in the formazza area however a quantitative evaluation of exhumation scenarios using the 3 d heat equation solver pecube highlights the existence of several other likely scenarios casting doubt on the validity of a qualitative interpretation of the age elevation relationships in formazza scenarios suggested by quantitative modeling include continuous denudation at a rate of 750 m ma and a one step exhumation rate change from 300 to 1000 m ma at 5 ma in guttannen they include continuous denudation at a rate of 400 m ma with valley deepening and two periods of higher exhumation rate increasing from 300 to 700 m ma repeatedly at 9 7 and at 5 3 ma contingent upon further flexural isostatic modeling the magnitude of exhumation recorded in the axial region of the alps since circa 5 ma does not appear sufficient to solely explain the denudation recorded in the north alpine foreland basin

messinian climate change and erosional destruction of the central european alps at the end of the miocene the european alps ceased outward expansion and tectonic uplift and exhumation shifted into the orogen interior this shift is consistent with a change from orogenic construction to orogenic destruction reflecting an increase in the ratio of erosional flux to accretionary flux the coincidence of this change with an increase in sediment yield from the alps suggests a climate driven increase in erosional flux the timing of deformation and sediment release from the southern alps indicates that the tectonic change occurred synchronous with the last phase of the messinian salinity crisis we attribute the increase in erosional flux to a climatic shift to wetter conditions throughout europe likely augmented by the base level fall that occurred during the mediterranean dessication this climate change is represented in the stratigraphic record by the lago mare deposits of the mediterranean salinity crisis

apatite u th sm he thermochronology of rapidly cooled samples the effect of he implantation apatite u th sm he ahe thermochronology is increasingly used for reconstructing geodynamic processes of the upper crust and the surface results of ahe thermochronology however are often in conflict with apatite fission track aft thermochronology yielding an inverted age relationship with ahe dates older than aft dates of the same samples this effect is mainly explained by radiation damage of apatite either impeding he diffusion or causing non thermal annealing of fission tracks so far systematic age inversions have only been described for old and slowly cooled terranes whereas for young and rapidly cooled samples too old ahe dates are usually explained by the presence of undetected u and or th rich micro inclusions we report apatite u th sm he results for rapidly cooled volcanogenic samples deposited in a deep ocean environment with a relatively simple post depositional thermal history robust age constraints are provided independently through sample biostratigraphy all studied apatites have low u contents 5 ppm on average while aft dates are largely in agreement with deposition ages most ahe dates are too old for leg 43 where deposition age of sampled sediment is 26 5 29 5 ma alpha corrected average ahe dates are up to 45 ma indicating overestimations of ahe dates up to 50 this is explained by he implantation from surrounding host u th rich sedimentary components and it is shown that ahe dates can be corrected by mechanically abrading the outer part of grains we recommend that particularly for low u th apatites the possibility of he implantation should be carefully checked before considering the degree to which the alpha ejection correction should be applied

are the alps collapsing abstract orogenic collapse is a process that transfers gravitational potential energy from regions of high potential energy to regions of lower potential energy this transfer is classically considered to be accomplished by extension in the orogenic core and by synchronous shortening in foreland regions of the orogen not all extensional features in collisional mountain belts need however reflect orogenic collapse normal faulting thrust faulting and strike slip faulting are all active in different parts of the alps today and reflect complex local responses to ongoing europe adria convergence the western alps is the only area today where extension and shortening radial to orogen trend occur synchronously and where orogenic collapse may be an important process elsewhere in the alps normal faults are oriented at a high angle to orogen trend and were primarily active in oligocene and miocene time most present day activity in the central and eastern alps is on strike slip faults that are accommodati

flexural isostatic response of the alps to increased quaternary erosion recorded by foreland basin remnants se france we test the hypothesis that flexural isostatic compensation of the mass removed by enhanced quaternary erosion is responsible for uplift of the western european alps and their forelands we use two well preserved and well dated 1 8 ma abandonment surfaces of foreland basin remnants in se france the chambaran and valensole plateaux as passive benchmarks for tilting of the foreland estimating their initial slope from morphometric scaling relationships we determine bulk post depositional tilting of 0 5 0 8 for these surfaces the calculated isostatic response of the alpine lithosphere to erosional unloading using the method recently proposed by champagnac et al geology 35 2007 195 198 yields a predicted tilting of 0 3 0 4 in the considered areas explaining approximately half of the determined post depositional tilting such long term deformation being insensitive to cyclic loading unloading because of glaciations we suspect the other half to be related to as yet undetermined long wavelength and long lived tectonic process es