late tectonic and metamorphic evolution of the piedmont accretionary wedge queyras schistes lustres western alps evidences for tilting during alpine collision this paper addresses the contribution of extension in the upper crust during syncollisional exhumation of subducted material we present structural data based on the analysis of ductile brittle ductile and brittle deformation features in metapelitic schists and meta basalts from the schistes lustres nappe stack in the piedmont zone of the western alps to link the structures with synkinematic pressure and temperature estimates geothermobarometry was performed using fluid inclusion investigation in quartz together with temperature estimations based on metamorphic chlorite composition samples used for geothermobarometry were collected taking into account the structural framework within veins associated with extensional structures observed at the regional scale the early generation of veins associated with boudinage occurred at 415 345 c and 6 2 5 kbar and subsequent incipient normal faulting occurred at temperatures of 315 235 c and pressures 1 7 kbar the transition from ductile to brittle extension occurred between ca 27 ma and ca 10 ma these results indicate a continuous extension in the upper crust during exhumation and continuous deformation associated with regional scale tilting of the exhumed schistes lustres tectonic pile the extensional tectonic regime in greenschistfacies conditions remains active today in most of the internal alpine zones as the alpine collision continues

crustal strain in the southern alps france 1948 1998 active tectonics in the western alps is revealed by a moderate level of seismic activity and geological evidences for quaternary deformation we present new geodetic determinations of the current strain rates in the southern part of the western alps that complement existing results in provence the jura and the subalpine and belledone massifs around grenoble we combined first and second order triangulation data collected in 1948 with global positioning system gps data collected in 1998 we estimate shear strain rates of 0 1 0 2 mrad yr 1 over distances on the order of 30 km significantly diverent from zero in most of the network we obtain nw se to n s compressive shear strain directions over most of the area in agreement with the seismological and geological data these geodetic estimates correspond to long term overall n s to nw se shortening of 2 4 mm yr 1 over the study area on the same order as the far field shortening measured by continuous gps on the grasse torino baseline 2 0 0 5 mm yr 1 that incorporates our geodetic network these results provide new constraints on interseismic strain in the western alps 2000 elsevier science b v all rights reserved

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 crustal scale cross section of the south western alps combining geophysical and geological imagery a geotransect across the south western alps from the pelvoux massif external french alps to the dora maira massif internal italian alps through the monviso ophiolitic complex was investigated in the framework of the geo france 3d alpes programme a new interpretative crustal scale section across the south western alps is proposed combining geological and geophysical 2d 3d data the apulian mantle i e the ivrea body might be divided into two rigid pieces separated by the downward prolongation of the penninic frontal thrust these mantle indenters drive the decoupling of the european crust beneath the high to ultra high pressure metamorphic nappes the deep structure results from the stacking of crustal slices extracted from the european lithosphere the proposed struc tural model provides a basis for discussing the evidence of the crustal scale partitioning of the current strain pattern as well as the location of the seismicity

origin of exotic clasts in the central southern apennines clues to the cenozoic fold and thrust collisional belt in the central mediterranean area

compression versus extension in the exhumation of the dora maira coesite bearing unit western alps italy abstract several recent papers suggest that uplift and exhumation of high pressure low temperature hp lt metamorphic rocks are due to crustal extension coeval with and or post dating the contraction responsible for the metamorphism a detailed survey of the coesite bearing unit of the southern dora maira massif offers an opportunity to discuss the relative importance of extension and compression in the exhumation tectonics of ultra high pressure uhp rocks this uhp unit consists of pre alpine basement rocks which equilibrated at a depth of 100 km probably 120 100 ma ago it is overlain by two units with a similar lithology and eoalpine metamorphic age and high p t gradients which equilibrated at 60 and 50 km respectively this eoalpine basement complex is intercalated between blueschist facies units the whole pile was affected by a mesoalpine greenschist facies mylonitic deformation during which the early contacts were reactivated as low angle shallow dipping shear zones extension is documented only during the mesoalpine collisional period and is referred to the gravity collapse of a thickened orogenic wedge with an exhumation rate of about 1 mm yr the most important part of the exhumation of the uhp rocks occurred earlier at a higher rate close to 4 mm yr a forced return flow or extrusion tectonics of imbricate slices in an on going subduction setting seems appropriate to account for this early stage the fact that the omission of intermediate pressure rocks between juxtaposed units is an unreliable criterion for demonstrating extension is discussed

neogene to ongoing normal faulting in the inner western alps a major evolution of the late alpine tectonics 1 widespread brittle extension occurs in the internal zones of the southwestern alps forming a dense fault network which overprints the compressional structures the extension has been followed by transcurrent motions in terms of paleostress this extension is radial to the belt close to the crustal penninic front and multidirectional in the eastern part of the internal zones the paleostress field of the strike slip phase is coherent with the extensional one and compatible with dextral shear along longitudinal faults globally we propose that the internal zones of the southwestern alps underwent a single neogene transtensive tectonic regime to strengthen our structural results the current tectonics has been analyzed using seismotectonic tools this approach allowed the recognition of several active faults moreover the ongoing tectonics is coherent with the observed brittle deformation the fault network controls the present day seismicity the combined seismotectonic and structural approaches give coherency and continuity from the neogene to ongoing extensional tectonics in the southwestern alps in the framework of the whole western alps a synthesis of our results with neotectonic related data allowed us to provide an accurate map of the neogene to present kinematics of the belt extension which appears as a major feature of the internal zones has been coeval with the propagation of thrusts in the outer zones during neogene times the geodynamic processes which rule this major tectonic evolution of the belt remain a matter of debate

active deformation in the inner western alps inferred from comparison between 1972 classical and 1996 gps geodetic surveys abstract eighteen geodetic points surveyed in 1972 by the french national geographic institute ign were remeasured by gps in 1996 in the brianconnais and piemont zones east of the pelvoux massif french western alps a displacement vector set was determined for the two surveys common points calculations of the strain rate tensors associated with 15 triangular cells of the network have been performed only four of them show a strain rate significant at a 95 level of confidence these data suggest an e w extension of about 2 4 mm yr between the western and eastern part of the network pelvoux external crystalline massif and queyras blueschists respectively associated with n s shortening this active deformation agrees with neotectonic and seismotectonic data the measured tectonic motion seems to be distributed throughout the central part of the brianconnais zone where the seismic activity is concentrated the local seismicity has been precisely surveyed since 1989 it is moderate ml

the southern ocean surface between marine isotope stages 6 and 5d shape and timing of climate changes summer sea surface temperature ssst and sea ice extent obtained at millennial to centennial resolution are presented for seven cores recovered in the atlantic and western indian antarctic zone from termination ii and the last interglacial s s i e marine isotope substage mis 5e the diatom based ssst estimates show that during the late mis 6 intensified export of cold waters from the weddell sea resulted in sssts close to 0 c in the central and eastern atlantic sector due to northward expansion of the winter sea ice edge by 3 5 latitude the sea ice seasonality was reduced such conditions have great potential for efficient year round reduction in air sea gas exchange and glacial co2 draw down the warming at termination ii started at 132 131 ka marked by rapid retreat of the sea ice edge the ssst increased within a few thousands of years to reach maximum interglacial summer temperatures ranging between 2 5 and 3 5 c in the earliest mis 5e 128 125 ka besides a distinct cold reversal around 129 5 ka the warming was punctuated by short lived temperature rebounds with amplitudes of 1 1 5 c occurring at 200 300 yr frequency thus in the range of the periodicity of solar input increased chaetoceros spp abundance suggests meltwater events as the primary cause of the coolings the ssst optimum characterised by strongly enhanced biogenic sedimentation rates ended at ca 125 ka with the full re establishment of the global conveyor belt and beginning of the northern hemisphere climatic optimum at this time sssts decreased stepwise and stabilised on values slightly warmer than today showing in the following period of mis 5e only low amplitude oscillations during mis 5d a northward expansion of the sea ice field took place but sssts only slightly colder than modern ones and a weakened meridional temperature gradient indicate that the southern ocean did not fall in a true glacial mode the tight correlation between southern ocean ssst and winter sea ice records and vostok atmospheric temperature and co2 records support the idea that the antarctic winter sea ice and southern ocean temperature exert a primary control on the atmospheric ocean co2 exchanges and hence on global climate reduction of the antarctic sea ice field and increase of ssst at the onset of termination ii could have been triggered by precessional changes influencing southern high latitude summer insolation positive feedback mechanisms reduction of surface albedo and release of co2 into the atmosphere and changes in global ocean circulation affected by a collapse or strong reduction in north atlantic deep water nadw production could have induced further reinforcement of the southern ocean warming

3 d modelling of alpine mohos in southwestern alps summary we present and discuss a 3 d geometrical model of the moho topography in the southwestern alps to achieve this objective we used the potential of 3 d modelling software i e a 3 d geomodeller to combine gravity seismic and seismological constraints in a same and coherent 3 d space a new regional bouguer gravity anomaly map of the southwestern alps was calculated and filtered to isolate the moho signature in the southwestern alps then two alternative 3 d models were computed with data from the literature based on seismic and seismological constraints the first one represents an european lithospheric mantle decoupled from the european orogenic crust which is back thrusted by the ivrea body whereas the second 3 d model illustrates the subduction of a 20 km thick piece of lower continental crust coupled with the european upper mantle beneath the ivrea body according to the geological knowledge of these units we then assigned a density value to each modelled layer and we computed their gravity effects to compare them with the filtered moho map of the southwestern alps first of all the significant discrepancies of the moho gravity signature generated by these two geometrical models reveal that subduction of continental crust is no more active in the present day configuration of the southwestern alps therefore the first 3 d model was refined by a stochastic 3 d gravity inversion based on this processing our investigations confirm the presence of three superposed mohos in the southwestern alps and underline that

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

geometry and structural evolution of ultra high pressure and high pressure rocks from the dora maira massif western alps italy abstract the crystalline nappes of the dora maira massif western alps essentially made of continental material from the upper crust show petrological relics of an ultra high pressure uhp to high pressure hp cold eclogite eoalpine metamorphism they also display relics of uhp hp structures preserved in boudins and or within large uhp porphyroclasts in a retrograde greenschist facies regional deformation fabric the greenschist facies overprint has the character of a shallow dipping mylonitic foliation s m bearing a penetrative stretching lineation l m which roughly parallels the axes of coeval isoclinal folds shear sense markers indicate a w verging overthrust mechanism the uhp and hp relic structures arc of variable nature the coexistence of equant and inequant either symmetric or asymmetric fabrics indicates that the deformation at uhp hp conditions was strongly heterogeneous and partitioned this is also supported by the local preservation of hercynian magmatic fabrics the uhp and hp deformation involved at least locally rotational components although less intensive than during the latter retrograde stage the regional structural evolution is envisaged as follows i the eoalpine subducted crust was subdivided into lenticular bodies surrounded by uhp hp shear zones the main part of the exhumation processes remains unconstrained due to the sparseness and late rotation of the uhp hp structural relics conflicting models are possible depending on the interpretation of the early sense of movement normal vs reverse along the faults that limit the lens shaped units and ii the late heterogeneous regional greenschist deformation can be attributed to the eocene collapse of the alpine orogenic wedge

a three dimensional crustal velocity model of the southwestern alps from local earthquake tomography a temporary network of 65 short period seismological stations was installed in the southwestern alps during the second half of 1996 it complemented the permanent monitoring networks obtaining an average interstation distance of 10 km travel time data from 446 local earthquakes and 104 quarry blasts were inverted simultaneously for hypocenter parameters and three dimensional velocity structure the p wave velocity model displays strong lateral contrasts both at shallow and deeper levels a low velocity anomaly stands out at shallow depths beneath the digne and castellane nappes in the southwestern part of the investigated area farther east the monviso ophiolitic massif appears to have a much larger extension at depth than previously assumed the largest and strongest anomaly is located under the dora maira massif and the westernmost po plain it correlates with the well known ivrea body which is classically interpreted as a wedge of adriatic upper mantle at the best resolved depths 10 and 15 km it appears as a rather thin 10 to 15 km north south elongated high velocity 7 4 to 7 7 km s 1 anomaly with very sharp edges extending to the south as far as 10 km north of the surface trace of the frontal penninic thrust special care was taken with regard to the quantitative estimation of the resolution for the main anomalies using the inversion of synthetic travel time data

sequential inversion of local earthquake traveltimes and gravity anomaly the example of the western alps summary we present a joint analysis of gravity anomaly and seismic arrival time data recorded in the western alps seismological data were collected by a network of 126 permanent and temporary stations implemented in 1996 a set of 550 local events has been recorded gravity data result from the addition of two new gravity surveys to an existing data base a published velocity model obtained by local earthquake tomography let was used to construct an initial 3 d gravity model using a linear velocity density relationship birch s law while the synthetic bouguer anomaly field calculated for this model has the same shape and wavelength as the observed anomaly its amplitude is strongly underestimated to derive a crustal velocity density model that accounts for both types of observations we performed a sequential inversion of seismological and gravity data the variance reduction of the arrival time data for the final sequential model was comparable to the variance reduction obtained by simple let moreover the sequential model explained 90 per cent of the observed gravity anomaly the main features of our model compared with the let model are 1 an important broadening of the high velocity anomaly associated with the high velocity high density ivrea body 2 a 10 km thick low velocity zone beneath the nappes of digne and castellane and 3 a high velocity zone at more than 25 km depth under the internal zone of the range

thermo mechanical approach to validation of deep crustal and lithospheric structures inferred from multidisciplinary data application to the western and northern alps current multidisciplinary studies confront the difficult problem of validation and verification of internal compatibility of geophysical and geological data of different nature quality and origin our geodynamic approach exploits the natural interdependence between the data related to different physical properties of geological structures density rheology temperature stress using the latter as input for a thermo mechanical finite element model we can verify whether the inferred properties and geometries i are mechanically stable ii require rheologically consistent stresses strains realistic thermal conditions etc and 3 will evolve in a geologically realistic way in the future this mechanically balanced approach is tested on the alpine orogen ecors and nfp20 profiles in the framework of the geofrance 3d programme the results suggest a number of corrections and adjustments that can be made to existing seismically and gravity predicted geometries of structures such as the ivrea body and those of the depleted subducted crust

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

lithospheric wedging in the western alps inferred from the ecors crop traverse the ecors crop traverse integrates vertical and wide angle seismic reflection gravity and magnetic studies along a 190 km profile across the western alps the combination of geophysical and geologic data constrains the deep structure along the traverse although alternative models are not excluded the model that best fits the data of crustal accretion implies that after the consumption of the piemontese ocean continental collision proceeded by lithospheric wedging of the subducting european continental foreland in the internal zones further shortening may have been achieved by an unknown amount of penetrative deformation and by back thrusting

the cretaceous tertiary deformation of the lhasa block and its implications for crustal thickening in tibet the strong tertiary deformation of the northern margin of the indian plate and of the tsangpo suture zone is not matched by comparable deformation within the lhasa block yet the southern margin of the block was the site of large scale intrusive activity the gangdese or transhimalayan batholith throughout the late cretaceous and was therefore presumably weaker than its surroundings this poses a problem for any approach that interprets the deformation of tibet in terms of diffuse crustal thickening however there is evidence that the lhasa block was in elevation as well as in magma type an andean margin by the end of the cretaceous and we suggest that the buoyancy force associated with the elevated crust was sufficient to inhibit compressional deformation within the southern lhasa block this suggestion is tested quantitatively by treating the continental lithosphere as a thin viscous sheet containing an isostatically compensated elevation contrast when subjected to boundary conditions representing the collision between india and asia the sheet accommodates convergence by diffuse deformation over a region comparable in size with the tibetan plateau in the absence of an elevation contrast this deformation consists of approximately 100 thickening strain over the region a precollision elevation contrast of 3000 m in the lhasa block would have resulted in its experiencing roughly 40 rather than 100 postcollisional thickening smaller elevation contrasts would have resulted in greater thickening and vice versa these calculations have implications for the mode of crustal thickening in northern tibet as the available evidence suggests that the topographic expression of the earlier mesozoic tectonic activity in southern asia was erased before the india asia collision we expect that there has been widespread and relatively homogeneous crustal shortening migrating northward from the lhasa block during the tertiary

counterclockwise rotation of the western alps since the oligocene new insights from paleomagnetic data 1 a paleomagnetic study of penninic units in the southern part of the alpine arc has been carried out more than 200 samples 23 sites were collected in brianconnais ammonitico rosso limestones of the high ubaye valley and the ligurian alps a characteristic component of magnetization of reverse polarity was isolated on most of the sites this component does not pass the fold test and is interpreted as a tertiary overprint related to alpine metamorphism mean directions in geographic coordinates are d 121 i 52 95 11 and d 72 i 48 95 15 for the ubaye and the liguria localities respectively indicating large counterclockwise rotations about vertical axis of 68 and 117 relative to stable europe these rotations in agreement with a previous study conducted in the briancon area together with other paleomagnetic data from the western alps show that the internal alps suffered a large but nonhomogeneous counterclockwise rotation since the oligocene the rotations are in agreement with the combination of earlier separately proposed processes the rotation of the adriatic plate accounts for about 25 of rotation remaining rotation and southward gradient would be related to left lateral shear accommodating the displacement between adria and europe at the southern border of the western alps furthermore the southward extrusion of the western alps south of the simplon fault zone may account for up to 10 of rotation rotations appear therefore as a major process accommodating deformation in the western alps since the oligocene

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

kinematic thermal and petrological model of the central alps lepontine metamorphism in the upper crust and eclogitisation of the lower crust abstract seismic and seismological studies as well as gravimetric models indicate that a slab of european lithospheric mantle and lower crust is currently underthrust below the apulian crust we assume a simple kinematic model in which the lower and upper subducted european crusts are decoupled along a decollement the lower crust goes into subduction without deformation the upper crust deforms by pure shear with a horizontal compressional axis the total erosional flux is adjusted to balance upper crust input so that the belt keeps the same geometry different distributions of erosion being used the computed temperature field is steady state if the kinematic model applies during a minimum time of 40 myr for a convergence rate of 8 mm yr equilibrium mineral assemblages and densities are determined from the computed p t conditions for a granodioritic chemical composition of the upper crust and an andesitic composition of the lower crust assuming local isostasy the density model fits the average topographic profile across the central alps the p t t paths obtained for the part of the upper crust initially at depths 10 to 16 km are compatible with the medium pressure oligocene metamorphism in the lepontine dome the peak calculated temperature for the deepest non subducted crustal rocks is 600 c for a pressure of 0 8 gpa near the lower limit of high pressure amphibolites we thus propose that the lepontine metamorphism corresponds to the steady state thermal regime however either faster erosion rates in the internal part of the belt or tectonic denudation are required for exhumation of the deeper portion of the belt the computed temperature field implies eclogitisation of the lower crust at a depth of 55 to 60 km we conclude that the moho limiting the deepest part of the root may correspond to the eclogitisation phase change lower crust eclogites have a density comparable to or higher than that of the mantle depending on their chemical composition 3 37 for andesitic eclogites 3 56 for gabbroic eclogites thus andesitic eclogites may stay in gravitational equilibrium in the mantle below the root whereas gabbroic eclogites are gravitationally unstable and should sink

current strain regime in the western alps from continuous global positioning system measurements 1996 2001 four to six years of continuous measurements at 10 permanent global positioning system sites in the western alps show horizontal residual velocities of 2 mm yr with respect to stable europe uncertainties range from 0 3 to 1 4 mm yr these velocities and the associated strain rate field indicate that the central part of the range is currently dominated by east west extension whereas the southern part shows north south to northwest southeast compression the geodetic and seismotectonic data are consistent with a model where strain is essentially controlled by the counterclockwise rotation of the adriatic microplate with respect to eurasia this rotation together with the arcuate shape of the contact between the adriatic microplate and the alps induces dextral shear kinematic boundary conditions across the western alps with an additional divergence component in their central part and in switzerland and a convergence component in their southern part

budget of crustal shortening and subduction of continental crust in the alps the stratigraphy surface structure and deep structure of the alps permit but do not require that a significant amount of european continental crust was subducted into the mantle beneath adria the fragment of continental lithosphere now comprising much of italy and the adriatic sea floor the present thin crust in much of europe suggests that the crust that was stretched in mesozoic time to form the southern margin of europe was initially thinner than normal the stratigraphic history of sediment deposited on that margin before its collision with adria suggests that the crustal basement on which much of this material was deposited had become particularly thin during the formation of the margin most of the crustal basement underlying the european margin and involved in the collision and the formation of the chain probably was thinner than 20 km and much of it may have been thinner than 10 km profiles of the present deep structure of the alps yield a range of possible values for the the average cross sectional area of crust involved in the collision that permit as much as 280 km of crustal shortening if this crust was as thin as implied by the stratigraphy thus although subduction of hundreds of kilometers of continental crust into the mantle beneath adria could have occurred the budget of crustal material involved in the formation of the alps does not require such crustal subduction

new k ar ages and the geologic evidence against rejuvenated stage volcanism at haleakalā east maui a postshield stage volcano of the hawaiian island chain the postshield and previously inferred rejuvenated stage history of haleakal volcano is reevaluated on the basis of 52 new k ar ages 42 from the postshield kula volcanics and 10 from the overlying h na volcanics postshield extrusion was robust from 0 93 to 0 76 ma a period of low extrusion rate or volcanic quiescence occurred between 0 76 and 0 65 ma well within kula time a chemical change to increasingly alkalic lava occurred at this time as the volcano changed from broadly hawaiitic to basanitic in its eruptive products and robust extrusion resumed a slightly longer period of low extrusion rate or quiescence occurred after ca 0 4 ma but only trifling change in geochemical character is observed geochemically the h na volcanics unit chiefly basanitic overlaps greatly with the upper part of the kula volcanics there is a weak tendency to slightly more alkaline character among the h na volcanics the age of the kula h na boundary is ca 0 15 0 12 ma thus volcanic quiescence of only 0 03 m y separates the two formations much shorter than the previously known limit of 0 25 0 30 m y the brevity of this hiatus coupled with coincident vent loci and broadly similar geochemical characteristics for the h na and the upper part of the kula volcanics indicates that the h na volcanics unit comprises deposits of postshield stage volcanism that has waned substantially since ca 0 4 0 3 ma haleakal has not yet begun a classically defined rejuvenated stage our findings support recent numerical modeling of plume lithosphere interactions that predict that haleakal is near the end of its postshield growth

numerical model of the effect of serpentinites on the exhumation of eclogitic rocks insights from the monviso ophiolitic massif western alps abstract the intimate association of serpentinite melange with eclogites worldwide suggests that serpentinites may have played an important role in the exhumation of eclogites serpentinites may have formed by fluid percolation in the mantle wedge or by underplating of oceanic serpentinites we have tested numerically the role of serpentinites in the exhumation of eclogites using physical parameters obtained in the monviso massif of the western alps our calculation shows that the buoyancy of serpentinized peridotites in dense anhydrous peridotites may contribute to the exhumation of eclogitic blocks small eclogitic blocks 3 may be exhumed in completely hydrated mantle wedge but it is not common considering the absence of totally serpentinized peridotites in mantle wedges the exhumation of large eclogitic blocks 50 km 3 requires a viscosity of the serpentinite wedge to be between 10 20 and 10 21 pa and a density difference between the serpentinite wedge and the surrounding mantle on the order of 100 400 kg m 3 this condition is satisfied by only partially hydrated peridotites containing 10 50 serpentinites such serpentinized peridotites are expected to form at a depth ranging from 20 to 60 km in subduction zones by dehydration of subducting plates our conclusion is supported by the common occurrences of partially hydrated peridotites in close association with high pressure to ultra high pressure metamorphic rocks