two phase orogenic convergence in the external and internal sw alps the nw se trending sector of the sw alps includes the dora maira massif where tertiary eclogites record ultrahigh pressures and rapid exhumation along a ne sw crustal cross section italy france compiled pressure temperature time data in internal zones are correlated with tertiary stratigraphy in external zones to reconstruct orogen evolution revealing a coherent two phase convergence history during the first subduction accretion phase eocene 55 34 ma rapid north south plate convergence caused the subduction and exhumation of high pressure and ultrahigh pressure uhp rocks in a steady state subduction channel this coincided with the north to nnw migration of an underfilled flexural basin across the european foreland nappe stacking within the subduction channel did not create significant relief implying that primarily subduction forces generated this flexural basin from 34 ma onward the second collisional phase was characterized by slower nw se plate convergence the internal units of the sw alps underwent considerable anticlockwise rotation as they became involved in a nw se oriented sinistral transpression zone between the european and adriatic plates to the north of the orogen the north alpine foreland basin became overfilled as a result of high sediment supply from increasing orogen relief in contrast in se france active flexure of the european plate appears to have ceased and sedimentation became limited to small thrust sheet top basins created by continuing gentle ne sw shortening internal units were exhumed slowly from depths of c 20 km principally by erosion in the sw alps the transition between these two phases was marked by the rapid subduction and exhumation of the dora maira uhp unit assuming lithostatic pressure this unit would have been exhumed from 100 km depth requiring a rate that exceeds that generated by plate convergence therefore either exhumation was accelerated by additional stresses locally generated by transpression slab breakoff or high density contrasts or more controversially the ultrahigh pressure occurred at a considerably shallower depth as a result of local overpressure

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

exhumation of the schistes lustres complex in situ laser probe 40ar 39ar constraints and implications for the western alps the schistes lustres sl suture zone occupies a key position in the alpine chain between the high pressure hp brianconnais domain and the ultrahigh pressure uhp dora maira massif and reached subduction depths ranging from c 40 65 km cottian alps in order to constrain the timing of hp metamorphism and subsequent exhumation several phengite generations were differentiated on the basis of habit texture paragenesis and chemistry as belonging to the first or second exhumation episode respectively d2 or d3 or to earlier stages of the tectono metamorphic evolution ten carefully selected samples showing d2 d3 d2 d3 or earlier mostly peak temperature phengite population s were subjected to laser probe 40ar 39ar analysis the data support the results of the petrostructural study with two distinct age groups crystallization ages for d2 and d3 phengite at 51 45 and 38 35 ma respectively the data also reveal a coherent age cluster at 62 55 ma for peak temperature phengite associated with chloritoid which were preserved in low strain domains the age of the d3 event in the sl complex appears very similar to ages recently obtained for greenschist facies deformation on the border of most internal crystalline massifs exhumation rates of the order of 1 2 mm yr 1 are obtained for the sl complex which are compatible with velocities documented for accretionary wedge settings similarly cooling velocities are only moderate c 5 c myr 1 which is at variance with recent estimates in the nearby uhp massifs

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

cenozoic k rich adakitic volcanic rocks in the hohxil area northern tibet lower crustal melting in an intracontinental setting it is generally accepted that the cenozoic potassic volcanic rocks of northern tibet were derived from a lithospheric mantle source here we report new chronological geochemical and isotopic data for the miocene ca 18 15 ma k rich adakitic volcanic rocks from the hohxil area of the songpan ganzi block in northern tibet we contend that these rocks were generated by partial melting of the mafic lower crust in an intracontinental setting unrelated to subduction of oceanic crust the hohxil rocks exhibit high sr y and la yb ratios high sr and la contents but low yb and y concentrations similar to adakites formed by slab melting associated with subduction however their relatively low e n d values 2 09 to 4 58 high 8 7 sr 8 6 sr 0 7072 0 7075 th u th ba and rb ba ratios and distinctive potassium enrichments k 2 o na 2 o are very different from the composition of typical adakites in addition those k rich adakitic rocks with the highest sio 2 contents 61 wt exhibit the lowest 8 7 sr 8 6 sr ratios and highest e n d values and are the oldest cenozoic volcanic rocks exposed in the songpan ganzi block suggesting that they were derived neither directly from a mantle source nor by differentiation of a shoshonitic magma taking into account the composition of lower crustal mafic xenoliths in tibet as well as the tectonic and geophysical evidence we conclude that the hohxil adakitic magmas were produced by partial melting of amphibole bearing eclogites with a k rich mafic bulk composition in the lower part 55 km of the thickened northern tibetan crust partial melting of the lower crust may have been triggered by dehydration release of fluids from sedimentary materials in the southward subducted continental lithosphere

from faulted blocks to megamullions and megaboudins tethyan heritage in the structure of the western alps along the grenoble pelvoux viso transect in the western alps we schematize the geometrical relationships between 1 ductile marly calcareous sedimentary covers and 2 their more rigid substratum this substratum may be the pre alpine granitic gneissic basement the triassic dolomites or even the jurassic ophiolites basement mullions a few kilometer size are characteristic of the external part of the western alps arc dauphine zone they strongly contrast with the dolomitic or ophiolitic boudins also a few kilometer size in the inner part of the arc piedmont zone the dauphine megamullions formed and then became accentuated during the oligocene and then the neogene tectonic phases which correspond to the last major alpine contractions at relatively shallow structural levels in the sedimentary cover upright or inclined folds were associated with slaty or strain slip cleavage development and low grade metamorphism the corresponding stretching directions locally subvertical trend normal to the regional fold and mega mullions axes and to the mountain belt itself the piedmont megaboudinage took place during the earlier major eocene alpine phase which but for some exeptions did not affect the dauphine zone in the very ductile schistes lustres that surround the dolomitic and ophiolitic megaboudins generalized intrafolial recumbent folding was associated with blueschist metamorphism the deformation characteristic of a deep structural level was a strong flattening within a subhorizontal plane and a preferential elongation parallel to the fold axes owing to regional horizontal festooning those axes trend normal oblique or even parallel to the piedmont zone i e to the incipient mountain belt restored initial trends coeval megaboudinage was probably omnidirectional chocolate tablet pattern the eocene piedmont structure has not been really obliterated during the later alpine deformations leading to megamullions development in the dauphine zone we summarize the very numerous stratigraphical sedimentological and tectonic field observations which lead us to regard each dauphine megamullion as directly inherited from a previous pre alpine faulted block each piedmont megaboudin has a similar origin the corresponding paleofaults are in fact normal and or strike slip faults of truly tethyan age their synsedimentary offset occurred either during the liassic middle jurassic passive margin rifting evolution of the southern edge of the european continent or during the late jurassic opening up of the ligurian oceanic tethys we analyze how those tethyan faults partly induced the style of the alpine collisional structures according to the regional characters of the successive synmetamorphic deformations i e to their localization in the western alps mountain belt

late quaternary hydrological changes in the gulf of carpentaria the gulf of carpentaria is a shallow epicontinental sea between australia and papua new guinea fig 1 it has a maximum depth of 69 m and is connected to the arafura sea across a 53 m sill1 in the east the connection to the coral sea through torres straits is 12 m ref 1 we propose here that low sea levels during the pleistocene2 3 fig 2 resulted in subaerial exposure of the gulf of carpentaria thereby reducing the regional input of moisture and latent heat to the atmosphere and tropical cyclogenesis as a result the climate of the carpentaria basin may have been significantly drier than at present4 5 even so previous workers6 7 have reported the presence of lacustrine sediments in the gulf that could have been deposited when the sea level dropped below the 53 m contour1 during 1982 the extent of lake carpentaria was defined by high resolution seismic profiles and cores the size and development of lake carpentaria and its dependence on the hydrological balance are discussed below

deformation and motion in the western alpine arc we attempt to reconstruct the kinematics deformation and motion of the western alpine arc between the mercantour and aar massifs first we compile a map showing principal extension x direction within three major structural units in the external zone x directions are broadly radial to the alpine arc in the internal zone they are uniform or smoothly varying in some domains whereas in the intermediate zone they often show much scatter next we use geochronological data and p t conditions estimated from metamorphic mineral reactions to time the deformation on each of a series of interval maps we show x directions for strain accumulated during a chosen time interval we argue that these incremental x directions are approximately equivalent to segments of particle paths showing displacements relative to stable europe for 120 100 ma the displacement direction was about n150 and coeval with eclogite facies metamorphism it is interpreted as a direction of overthrusting by the african plate around 40 ma the displacement in the south part of the arc became westerly finally between 25 and 15 ma displacement directions were generally radial to the arc

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

downstream changes in sand composition roundness and gravel size in a short headed high gradient stream northwestern italy abstract we assessed changes in grain composition and roundness of the 1 2 and 2 3 size fractions of sand along a 16 km length of the graveglia river liguria italy to its mouth at the tyrrhenian sea and then for 1 km along the beach rocks exposed in the drainage area include ophiolites ribbon chert limestone shale sandstone and quaternary sands the erosion of shale and limestone bedrock yields large amounts of sand but sand grains of these rock types decrease rapidly in abundance downstream chiefly through dilution rather than abrasion carbonate rock fragments that reach the beach survive wave abrasion but most shale grains do not predictably monocrystalline and polycry talline quartz and chert are the most durable grain types but serpentine basalt and unidentified rock fragments also show significant resistance to both stream and wave abrasion polycrystalline quartz feldspar basalt serpentine sandstone and shale grains are more abundant in the coarser sand size fraction monocrystalline quartz carbonate rock fragments and unidentified rock fragments are more abundant in the finer sand size fraction and chert and plutonic rock fragments have no size preference a comparison of the outcrop areas of the different rock types with the corresponding percentages of different grain types present in each of three drainage areas indicates that shale which breaks down readily to silt and clay size particles and chert which does not readily break down into sand size fragments are underrepresented in the sands limestone and serpentine however are overrepresented in comparison to outcrop area in certain drainage areas the overrepresentation of limestone reflects in part a sampling bias sand was sampled adjacent to limestone outcrops but the overrepresentation of serpentine reflects its resistance to weathering compared to other aluminosilicates and its durability shale limestone and serpentine increase in roundness downstream and along the beach to a maximum limiting value of 5 rho units medium grained shale clasts attain high rounding in only a few kilometers of transport downstream decrease in gravel size follows an exponential formula that can be expressed in linear form using phi size size 0 1167 distance km 10 29

source tracing of detrital serpentinite in the oligocene molasse deposits from the western alps barreme basin implications for relief formation in the internal zone we present the first contribution of tracing the source area of ophiolitic detritus in the alpine molasse deposits by raman spectroscopy the lower oligocene molasse deposits preserved in the barreme basin in the sw foreland of the western alpine arc are known for the sudden arrival of the first exotic detritus coming from the internal alpine zones among them the pebbles of serpentinized peridotites have so far not been studied we show that they only consist of antigorite serpentinite implying that they originate from erosion of high temperature blueschists in contrast the upper oligocene lower miocene molasse shows mixed clasts of serpentine including antigorite and lizardite without any evidence of chrysotile this suggests that they were derived from a less metamorphosed unit such as the low temperature blueschist unit taking into account the sediment transport direction in the basin and the varied metamorphic characteristics of the other ocean derived detritus we constrain the lithologic nature of the source zones and the location of the relief zones identified as the internal alps se of the pelvoux external crystalline massif available structural data and in situ thermochronological data allow the reconstruction of the oligocene to early miocene collisional geometry of the palaeogene subduction wedge this phase corresponds to two major phases of uplift evolving from a single relief zone located above the ivrea body during early oligocene times and persisting up to early miocene times then during late oligocene early miocene times a second relief zone developed above the brianconnais zone at that time the internal western alps acquired its double vergency

deep water sedimentation in the alpine basin of se france new perspectives on the gres d annot and related systems the gres d annot formation of se france constitutes a classic and outstanding example of a sand rich turbidite system controlled by synsedimentary tectonics and has often been used as an outcrop analogue for deep water hydrocarbon reservoirs over the last 10 years research efforts by different academic and industrial teams have led to important reappraisals of prevailing ideas on the gres d annot particularly on the nature and controlling factors of the depositional processes the small scale architecture and sequence organization of the deposits the tectonic regime of the basin and its impact on palaeogeography and the interaction between sediment gravity flows and basin floor topography this volume offers a multidisciplinary overview and draws up the state of the art of scientific knowledge on this influential turbidite system complementary aspects are covered from structural geology sedimentology and sequence stratigraphy to modelling of sedimentary processes and architectures geochemistry reservoir characterization seismic modelling and uses as analogues for deep marine hydrocarbon fields this book will be of use to both academic researchers geologists and geophysicists and industry professionals dealing with the characterization and modelling of deep water sediments

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

from oceanic closure to continental collision a synthesis of the schistes lustres metamorphic complex of the western alps recent lithostratigraphic and structural studies made in the western alps along the valais savoy and queyras schistes lustres transects indicate that there are strong similarities in the nature of the sedimentary series and in the general structural edifice between these areas the main new conclusion is that a significant part of the schistes lustres complex is made up of syn orogenic upper cretaceous deposits a first type present in all three transects consists of lower upper cretaceous metasedimentary rocks containing oceanic and continental detrital materials which were deposited on the western edge of the northwest tethyan paleomargin and in the oceanic realm undergoing tectonization a second type is made up of upper cretaceous calcareous gritty metaflysch known only in the savoy transect which is interpreted as a sedimentary consequence of the complete or partial closure of the tethyan ocean

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

exhumation of alpine high pressure rocks insights from petrology of eclogite clasts in the tertiary piedmontese basin ligurian alps italy the study of late and postorogenic sedimentary basins is a powerful tool to understand uplift exhumation and erosion of an orogen in the ligurian alps high pressure ophiolitic rocks are directly overlain by the sediments of the tertiary piedmontese basin tpb the conglomerates at the bottom of the tpb succession contain clasts of metaophiolites and metasediments which display deformation structures acquired at peak metamorphic conditions ranging from eclogite to blueschist facies the deformation events prevalently recorded by the high pressure rocks of the ligurian alps are either related to subduction and peak metamorphism or to the late stage greenschist collision the main structures which accomplish the early exhumation of such high pressure terrains are only locally recorded and poorly explored further information on these early tectonic events can be gathered through the study of the clasts in the conglomerates in this paper we present a textural and petrologic outline and some thermobarometric estimates of the main types of high pressure clasts sampled and their comparison with the high pressure equivalents presently exposed in the ligurian alps in particular some clasts displaying a na amphibole white mica epidote sphene blueschist foliation superimposed to an eclogitic garnet na clinopyroxene rutile tectonitic assemblage have been studied in detail this kind of rock records a pressure temperature p t path implying cooling during exhumation whereas the high pressure bedrocks are characterized by either isothermal decompression or initial heating and subsequent cooling we conclude that the blueschist foliation of these clasts most likely developed along shear zones and or contacts among different slices formed during tectonic coupling of warm uprising eclogite units with cooler blueschist slices during the early stages of exhumation

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

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

factors controlling the evolution of the perdido fold belt northwestern gulf of mexico determined from numerical models 1 the perdido fold belt pfb is a prominent saltcored deep water structure in the northwestern gulf of mexico it is characterized by symmetric kink banded folds of a 4 5 km thick prekinematic layer and its vicinity to the extensive sigsbee salt canopy we use 2 d finite element numerical models to study the evolution of the pfb as a gravity driven fold belt both in a local context and in the context of the larger scale passive margin influenced by adjacent allochthonous salt structures we show that parameters such as overburden strength salt geometry or salt viscosity determine timing extent and location of the modeled fold belt simplified models of the gulf of mexico show that toe of slope folding is a viable mechanism to develop diapirs in the deep salt basin and to delay folding of the distal overburden in this scenario the pfb likely represents the terminal folding of a much larger diachronously formed fold belt system citation gradmann s c beaumont and m albertz 2009 factors controlling the evolution of the perdido fold belt northwestern gulf of mexico determined from numerical models tectonics 28 tc2002 doi 10 1029 2008tc002326

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

tectonostratigraphy of the west alpine foreland correlation of tertiary sedimentary sequences changes in eustatic sea level and stress regimes abstract analysis of the tertiary depositional history of the west alpine foreland shows that its different basins were governed by essentially synchronous tectonostratigraphic scenarios the tectono sedimentary records of the west alpine foredeep and molasse basin and the rhone and massif central grabens demonstrate successions of tectonic deformation events and so called cenozoic rift and foredeep crf sequences which correlate to both changes in eustatic sea level and coeval modification in foreland stress regime since correlative successions developed in the north alpine foreland it seems likely that the western and northern alpine foreland stress fields were repeatedly modified at the same time compressional stresses associated with the intermittent tectonic and sedimentary changes in the foreland originated firstly from the alpine and pyrenean orogens in conjunction with the episodic history of the foreland stress regime the syn orogenic accumulation of widespread tectono sedimentary sequences reflects sequential kinematic events in the adjacent mountain chains the apparent synchroneity of the tectonostratigraphic events in the principal basins of the west alpine foreland testifies for a common geodynamic cause for the occurrence of correlative series of widespread tectonic phases and crf subdivisions

subduction polarity reversal at the junction between the western alps and the northern apennines italy abstract the controversial relationship between the orogenic segments of the western alps and the northern apennines is here explored integrating recently published 3d tomographic models of subduction with new and re interpreted geological observations from the eclogitic domain of the voltri massif ligurian alps italy where the two belts joint each other the voltri massif is here described as an extensional domain accommodating the opposing outward migration of the alpine and apennine thrust fronts since about 30 35 ma using tomographic images of the upper mantle and paleotectonic reconstructions we propose that this extensional setting represents the surface manifestation of an along strike change in polarity of the subducted oceanic slab whose polarity changed laterally in space and in time our tectonic model suggests that the westward shift of the alpine thrust front from the oligocene onward was the consequence of the toroidal asthenospheric flow induced by the retreat of the apenninic slab

strain partitioning in the axial nw alps since the oligocene 1 this work analyzes the postmetamorphic evolution of the northwestern alps thanks to a multiscale field geology study integrated by fission track analysis it illustrates how the fission track method constraining throws along major faults and age of mesoscale deformation provides interpretation keys to detangle complex structural data sets results are discussed within the framework of the alps apennines knot and give new insights on strain partitioning within transpressional orogenic belts the axial nw alps acted since the late oligocene as an orogen scale shear zone accommodating in the aosta valley dextral strike slip coeval with forward propagation of external thrusts within this transcurrent belt right lateral deformation was partitioned between the fast exhuming western block and the already unroofed eastern block these blocks show contrasting structural characters the former is cut by first order longitudinal structures bounding major tectonostratigraphic units the latter is fragmented by complex patterns of lower order faults interpreted as subsidiary shears the onset of mesoscale extension widespread in the eastern block is diachronous across the belt and migrated through time toward the european foreland transition from syncollisional shortening to orogen scale transcurrence observed in the nw alps corresponds to a regional strain field variation documented in the mid oligocene all around the alps apennines knot which is possibly related to a change of the adria trajectory relative to europe since the late oligocene deformation associated with oblique plate convergence was partitioned between strike slip in the axial belt and thrusting in external areas most of the shortening was accommodated in the helvetic chains 102 km deformation was limited elsewhere to an order of 101 km

the provenance of sediments in the barreme thrust top basin haute provence france abstract this paper records the results of an integrated analysis of heavy minerals sedimentary facies palaeocurrents and structural data the objective of the study was to evaluate the provenance of the sediments of the barreme thrust top basin this basin contains a late eocene to late oligocene succession of shallow marine alluvial and lacustrine sediments late eocene marine transgression resulted in starvation of clastic supply to the basin hence the calcaires nummulitiques received only a limited influx of heavy minerals derived from local sedimentary sources augmented by the first appearance of assemblages with andesitic affinities at c 42 41 ma clastic members associated with the overlying marnes bleues record a significant diversification of provenance and sediment transport paths clastic detritus supplied to the shallow marine gres de ville were derived from permo triassic sediments to the south conglomeratic submarine channels of the la poste member mark the introduction of alpine ophiolite blueschist derived minerals at c 37 35 ma eroded from the embrun ubaya thrust complex to the ne the deltaic st lions member is dominated by volcaniclastic assemblages completing the marine fill the shoreface gres de senez received detritus from the southerly maures esterel crystalline massif the succeeding oligocene continental sediments record an increasing supply of minerals from alpine lithologies the alluvial molasse rouge contains reworked sedimentary detritus including material cannibalized from the underlying marine tertiary and first cycle alpine minerals the succeeding lacustrine serie saumon serie grise show the increasing diversity of alpine suites the youngest sediments the fluvial gres verts signal a major influx of alpine derived assemblages related to reactivation of the embrun ubaye thrusts between c 28 27 and 25 24 ma

antigorite high pressure stability in the system mgosio2h2o msh abstract the breakdown reactions of antigorite 1 forming talc forsterite water at low pressures and 2 forming forsterite clinoenstatite water at high pressures were determined in reversed equilibrium experiments results on reaction 1 were found to be in good agreement with former experimental determinations by both johannes johannes w 1975 zur synthese und thermischen stabilitat von antigorit fortschr mineral beih 53 36 and evans et al evans b w johannes w oterdoom h trommsdorff v 1976 stability of crysotile and antigorite in the serpentinite multisystem schweiz mineral petrogr mitt 56 79 93 from our experiments the invariant point i1 interconnecting the two reactions can be located at about 15 kbar 650 c this is consistent with the thermodynamic calculations using the dataset of berman berman r g 1988 internally consistent thermodynamic data for minerals in the system j petrol 29 445 522 however it is in contrast to recent experimental studies of ulmer and trommsdorff ulmer p trommsdorff v 1995a serpentine stability to mantle depths and subduction related magmatism science 268 858 861 who determined i1 at 21 kbar 730 c our pt conditions for i1 could be confirmed by equilibrium experiments on reaction 10 talc forsterite clinoenstatite water which is generated at i1 as well up to about 25 kbar the breakdown reaction 2 is nearly pressure independent towards still higher pressures the dp dt slope of reaction 2 bends and becomes negative schreinemakers analysis as well as thermodynamic calculations of the upper pressure stability of antigorite show that the possible antigorite breakdown reaction 3 antigorite clinoenstatite brucite water and reaction 4 brucite clinoenstatite forsterite water could originate at a new invariant point i2 provided that the reactions 2 and 11 brucite antigorite forsterite water intersect bracketing equilibrium 4 and combining these results with those on reaction 2 i2 was located at only about 51 kbar 490 c compared to 77 kbar 680 c according to berman s data however when taking into account the dense hydrous magnesium silicate dhms phase a mg7si2o8 oh 6 the phase relations of antigorite are changed resulting i in the metastability of i2 and reaction 3 and ii in a new invariant point i7 at about 44 kbar and 580 c generating the new antigorite breakdown reaction 16 antigorite phase a clinoenstatite water on the basis of these new data on the stability of antigorite earlier conclusions about dehydration depths in subducted serpentine bearing oceanic lithosphere have to be reconsidered the maximum pressure stability of antigorite according to reaction 16 extends between 44 and 55 kbar that is between about 130 and 160 km depths as opposed to about 75 kbar 220 km following ulmer and trommsdorff see above because many different thermal regimes are possible in subduction zones no specific dehydration depth can be expected but rather more continuous dehydration fronts in space and time

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

dynamic control on serpentine crystallization in veins constraints on hydration processes in oceanic peridotites deformation and hydration processes are intimately linked in the oceanic lithosphere but the feedbacks between them are still poorly understood especially in ultramafic rocks where serpentinization results in a decrease of rock density that implies a volume increase and or mass transfer serpentinization is accompanied by abundant veining marked by different generations of vein filling serpentines with a high variety of morphologies and textures that correspond to different mechanisms and conditions of formation we use these veins to constrain the role of deformation and mass transfer processes during hydration of oceanic peridotites at slow spreading ridges we have selected a representative set of veins from ocean floor serpentinites of the mid atlantic ridge near kane transform fault 23 n and characterized these in detail for their microstructures and chemistry by coupling optical and electron microscopy sem tem with electron microprobe analyses four main veining episodes v1 to v4 accompany the serpentinization the first episode identified as vein generation v1 is interpreted as the tectonically controlled penetration of early seawater dominated fluid within peridotites enhancing thermal cracking and mesh texture initiation at 3 4 km up to 8 km depth and at t 300 350 c the two following vein stages v2 and v3 formed in a closed diffusive system and accommodate volume expansion required to reach almost 50 serpentinization of the protolith the cracks exploited by these veins were caused by the progressive unroofing at depths of 4 to 2 km along a detachment fault degree and rate of serpentinization seem to be controlled by the capacity of the system to create space and to drive the mass transfer needed for ongoing serpentinization and this capacity is in turn linked to the exhumation rate and local tectonics during this period water consumed by hydration may prevent the establishment of convective hydrothermal cells the onset of an open hydrothermal system in the shallow lithosphere 2 km where brittle fracturing and advective transfer dominate and enable the completion of serpentinization is marked by the last vein generation v4 these results show a complete history of alteration with the crystallization of different types of serpentine recording different tectonic events chemical conditions and modes of hydrothermal alteration of the lithosphere

unusual marbles in a non metamorphic succession of the sw alps valdieri italy due to early oligocene hydrothermal flow in the sw alps at the ne margin of the argentera massif the occurrence of isolated bodies of silicate bearing marbles valdieri marble laterally passing to non metamorphic mesozoic limestones of the dauphinois succession appears enigmatic since their origin cannot be related to the regional metamorphism the stratigraphic geometric petrographic and geochemical features of the valdieri marble indicate that it originated from the upflow of hydrothermal fluids which deeply recrystallised the limestone and provoked the abundant neoblastesis of silicate minerals at an estimated crystallisation temperature of ca 350 c this paper provides for the first time an absolute u pb age comprised between 30 and 31 6 ma early oligocene for the valdieri marble which is affected by a syn genetic shear foliation in turn crosscut by the main alpine tectonic foliations the hydrothermal fluids that formed the valdieri marble migrated along low angle shear zones mostly parallel to the sedimentary primary bedding which were downward connected with high angle master faults of the alpine sw transfer a main oligocene early miocene transcurrent shear zone at the southern termination of the western alps the localized heat flow sustaining the hydrothermal system could be related to the early oligocene thermal event well documented by magmatic activity in the western and central alps

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

quantifying sand provenance and erosion marsyandi river nepal himalaya we use petrographic and mineralogical data on modern sediments to investigate erosion patterns in the marsyandi basin of the central himalaya a privileged natural laboratory in which a series of multidisciplinary geomorphological sedimentological geochemical and geochronological studies have been recently carried out to unravel the interrelationships between tectonic climatic and sedimentary processes in high relief orogenic belts although relative erosion patterns are effectively constrained by analyses of replicate samples along six successive tracts of the marsyandi river uncertainties are caused by potential compositional variation between the monsoon and post monsoon season estimates of erosion rates are significantly affected by poor knowledge of total sediment flux through the basin ourresults support focusederosion of the southern tectonically lower part of the greater himalayain the hangingwall of the mct zone where thesummer monsoon reachesitspeak intensity up to 5 m a andsediment yieldsanderosion ratesreach14 100 3400t km 2 and 5 1 1 2 mm a erosion rates sharply decrease southward in low relief lesser himalayan units 1 6 0 6 mm a and also progressivelydecreasenorthwardsinthehigh altitude tectonically upperpartofthegreaterhimalaya whererainfalldecreasesrapidly to b2 m a even areas of extreme topography such as the manaslu granite are characterized by relatively low erosion rates 2 4 0 9 mm a because precipitations become too scarce to feed significant ice flux and glacial activity monsoonal rainfall decreases further tob0 5 m a in the tethys himalayan zone farther north where erosion rates are 1 mm a coupling between erosion and peak monsoonal rainfall along the southern front of the greater himalaya is consistent with both channel flow models of tectonic extrusion

kinematics of the alpine arc and the motion history of adria the alps have been interpreted traditionally in terms of north or northwest directed convergence between africa and europe1 with adria the continental underpinnings of the adriatic sea and northern italy acting as a promontory of the african continent2 this has been supported by recent analyses of africa europe relative plate motions3 5 which indicate dominantly north south directed convergence during the tertiary phase of intracontinental shortening we show here however that kinematic indicators from alpine tectonites are difficult to reconcile with this motion history figs 1 and 2 and are explained best by roughly west northwest directed adria europe motion since about 70 million years ago combined with an arc normal pattern of motion driven by local body forces this implies that adria has moved independently of both africa and europe since at least the late cretaceous

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

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

reconstructing early himalayan tectonic evolution and paleogeography from tertiary foreland basin sedimentary rocks northern india the latest paleocene middle eocene subathu formation and the oligocene miocene dagshai and kasauli formations of the indian foreland basin record the early evolution of the himalayan fold thrust belt sandstone petrography of the subathu formation shows a predominantly recycled sedimentary source with a distinct ophiolitic and volcanic influence that was drastically reduced by the time of deposition of the dagshai formation sandstones in the dagshai and kasauli formations consist predominantly of metapelitic detritus the metamorphic grade of metapelitic lithic grains increases with time from dominantly very low grade at the base of the dagshai formation to dominantly low grade in the kasauli sandstones mudstone geochemistry documents the presence of a mafic ultramafic source during the time of deposition of the subathu formation that becomes significantly less important by the time of deposition of the dagshai formation compositions of subathu formation detrital spinels show they were either derived from both mid ocean ridge basalt type and arc type ophiolites or from an ophiolite of composite origin and that southerly derivation from the deccan trap continental flood basalts is unlikely kasauli formation garnet compositions suggest derivation from medium grade metamorphic rocks to amphibolite facies subathu formation composition reflects provenance influence from the indus suture zone during latest paleocene middle eocene time indicating initiation of continent continent collision and development of the foreland basin by this time suture zone influence was drastically reduced by the time of deposition of the dagshai formation when the embryonic thrust belt provided a barrier sufficient to partially separate the suture zone from the himalayan foreland basin the first appearance of himalayan metamorphic detritus occurs in the dagshai formation at the close of oligocene time whereas the kasauli formation records erosion to deeper metamorphic levels during earliest miocene time the occurrence of garnet and higher grade metamorphic lithic grains during early neogene time is coincident with the timing of displacement along the main central thrust and south tibetan detachment zone composition of early foreland basin sediments from pakistan balakot murree and kamlial formations to nepal bhainskati and dumri formations and bangladesh kopili barail formations surma group indicates diachronous arrival of ophiolitic to low grade metamorphic detritus derived respectively from the indus suture and early himalayan thrust sheets in the north this is consistent with progressively later closure of neotethys along the suture from latest paleocene time in the west to eocene time or even later in the east

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

evidence of synextension tilting and doming during final exhumation from analysis of multistage faults queyras schistes lustres western alps abstract during the neogene the internal arc of the western alps underwent extension behind its inverted paleogene frontal thrust while shortening affected the external arc in the core of the internal arc doming of eclogite bearing gneissic nappes has formed the dora maira massif the blueschist bearing schistes lustres of queyras which overlie the western flank of the dora maira dome allow observation of how the neogene extension developed in ductile to brittle conditions during the evolution from chocolate tablet boudinage to cross trend normal faulting the extension kept a tendency to be multidirectional the analysis of variably oriented fault families allows considering them as globally associated with the same long lived tectonic regime focusing on faults bearing several generations of slickenlines and using an adapted right dihedra analysis we propose that faulting accompanied the westwards tilting of the eastern queyras structure this tilting is consistent with the doming of the dora maira massif itself associated with tectonic denudation along its western flank therefore two processes operated simultaneously during final exhumation regional thinning through widespread multitrend normal faulting and more localised tectonic denudation and doming both are consistent with a neogene regime dominated by vertical compression the role of an ascending deep indenter below the internal arc remains to be documented

first paleomagnetic data from the sedimentary cover of the french penninic alps evidence for tertiary counterclockwise rotations in the western alps abstract we present a paleomagnetic study performed in the brianconnais sedimentary cover of the western alpine arc france sampling was focused on upper jurassic rocks of the brianconnais cover in the briancon guillestre area more than 100 samples from 11 sites were collected essentially from ammonitico rosso limestones and sedimentary dykes of the upper jurassic thermal and alternating field demagnetizations revealed three components of magnetization carried dominantly by magnetite 1 a low temperature component with present day magnetic field direction of viscous origin 2 a well defined reverse polarity component with a maximum unblocking temperature of 440 c a component and 3 a weak high temperature component b component that experienced a viscous overprint during heating and was difficult to isolate the a component exhibits a negative fold test and shows a better clustering when corrected for tilting associated with late alpine extension this component is interpreted as a secondary remagnetization acquired during the cooling path related to late eocene early oligocene metamorphism the mean a component direction corrected for late alpine extensional tilting is d 142 i 57 k 44 95 8 this direction significantly differs by 47 13 from the expected direction for eurasia we interpret this difference to be related to a post eocene counterclockwise rotation about a vertical axis of the penninic zone relative to stable europe

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

actualistic ophiolite provenance the cyprus case abstract the island of cyprus represents an excellent site to assess quantitatively petrologic clastic response to actively obducting oceanic sources in order to define an actualistic reference for ophiolite provenance in terms of framework composition and heavy mineral suites an improved methodology an extension of the classic ternary qfl logic to include a wider spectrum of key indexes and ratios provides an accurate synthesis of modal data and allows differentiation of three main petrographic provinces and at least seven subprovinces diagnostic signatures of detritus from various levels of an oceanic lithospheric source and criteria for distinguishing provenance from suprasubduction versus mid oceanic ophiolites are also outlined modern sands derived from the troodos ophiolite contain variable proportions of largely pelagic carbonate to chert boninite to basalt diabase to metabasite plagiogranite to gabbroic and cumulate grains supplied from progressively deeper seated levels of the multilay

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

exhumation of high pressure rocks a review of concepts and processes the exhumation of high pressure metamorphic rocks requires either the removal of the overburden that caused the high pressures or the transport of the metamorphic rocks through the overburden exhumation cannot be achieved simply by thrusting or strike slip faulting it may be caused by erosion of shortened and thickened crust but this is unlikely to be the only mechanism for exhuming rocks from depths greater than about 20 km one or more of the following additional mechanisms may be involved 1 corner flow of low viscosity material trapped between the upper and lower plates in a subduction zone can cause upward flow of deeply buried rock and may explain some occurrences of high pressure tectonic blocks in melange this process does not however appear to be adequate to explain the exhumation of regional high pressure terrains 2 buoyancy forces acting directly on metamorphic rock bodies may cause them to rise relative to more dense surroundings this is likely to be the most important mechanism of exhumation of crustal rocks subducted into the mantle but cannot explain the emplacement of coherent tracts of high density metamorphic rock into shallow crustal levels some high pressure blocks emplaced at shallow levels in accretionary terrains may have been entrained in diapiric intrusions of low density mud or serpentinite 3 extension driven by the forces associated with contrasts in surface elevation may explain the exhumation and structural setting of many high pressure terrains extension may occur in the upper part of an accretionary wedge thickened by underplating or it may affect the whole lithosphere in a region of intracontinental convergence if surface elevation has been increased by the removal of a lithospheric root in the second case extension may be accompanied by magmatism and an evolution towards higher temperature during decompression of the metamorphic terrain

evolution of a thrust sheet top basin the tertiary barreme basin alpes de haute provence france in the french sector of the tertiary alpine mountain belt of southern europe the early stages of compression established an initial thrust front a long distance into the foreland 30 40 km rearward of the terminal thrust front an extensive region of thrust sheet top basins was established behind this initial thrust front particularly after a regional marine transgression in late eocene time priabonian these basins were initially connected but became increasingly compartmentalized as the thrust belt evolved the basins are now preserved as a series of structurally isolated synclinal remnants in the external zones of the alpine thrust belt one of these remnants known as the barreme basin has a preserved fill that is 900 m thick spans 18 m y and is exposed over 50 km 2 the tertiary basin fill comprises initial nonmarine deposits overlain by a marine succession and subsequent continental strata and includes evidence of submarine gravity slides growth structures and abrupt changes in sediment transport directions and provenance evolution of the basin fill was influenced by both far field and more localized near field deformation events the latter being dominated by the progressive growth of a steep thrust related anticline that defined the eastern margin of the basin this basin fill records several key steps in the evolution of the alpine mountain belt such as the first incursion of clasts derived from the emplacement of major thrust structures in the inner parts of the mountain belt ca 32 30 ma synsedimentary volcanic activity and the growth of an important out of sequence anticline that progressively separated the barreme basin from the gres d9annot basin to the east in addition the basin preserves the progression from an early underfilled flysch basin to a subsequent overfilled molasse basin in early oligocene time rupelian between ca 33 and 30 ma records of these events are not available from the fill of the younger foreland basin preserved beyond the final thrust front and this remnant of the thrust sheet top basin province therefore provides a unique record of the early structural and stratigraphic evolution of the alpine mountain belt

rapid exhumation and mountain building in the western alps petrology and 40ar 39ar geochronology of detritus from tertiary basins of southeastern france 1 tertiary convergence high pressure low temperature hp lt metamorphism and subsequent exhumation in the internal western alps were concurrent with detrital deposition in foreland basins at the external periphery of the orogen in the present work we have probed the geochronology and chemistry of white mica grains and metamorphic pebbles from the foreland basins of se france in order to gain additional perspective on exhumation rates and mountain building processes in the internal domain of the tertiary orogen our data indicate that throughout the tertiary these basins were mainly fed from a relatively low pressure source dominated by si poor micas yielding pre alpine 40ar 39ar ages small amounts of detritus composed of high pressure minerals and pebbles are first detected in early oligocene 30 32 ma strata of the barreme basin the 40ar 39ar geochronology of phengites from garnet blueschist pebbles from these strata yielded 34 3 ma ages indicating rapid ascent and exposure of hp lt rocks in the internal part of the orogen shortly after metamorphism to explain the preponderance of pre alpine micas alongside the rare presence of alpine hp lt detritus we suggest that the general architecture of the western alps whereby inner hp lt metamorphic units are cut off from the se france basins by a low grade lid of brianconnais back thrusts was rapidly built in the early oligocene and did not change significantly since then

subduction of water into the mantle history of an alpine peridotite the erro tobbio peridotite western alps is a slice of subcontinental mantle that underwent pre jurassic passive rifting followed by sea floor hydration and cretaceous alpine subduction analysis of its extension and subduction related structures and metamorphism suggest the dip below the adria plate of both the extensional detachments and the subduction plane high pressure boudinage structures of rigid metarodingite within ductile antigorite serpentinite prove that antigorite survived subduction and remained stable at eclogite facies conditions subduction of serpentinized peridotites from ophiolite slabs is here considered the most effective mechanism of bringing water to great depth within the mantle

episodic exhumation in the western alps oligocene to miocene clastic sediments of the tertiary piedmont basin northwest italy were derived from erosion of western alps source rocks detrital white micas from dif ferent stratigraphic units and from sands of three present day rivers draining the internal western alps have been analyzed by 40 ar 39 ar geochronology our data suggest a wide spread fast cooling and exhumation event prior to ca 38 ma followed by a 30 m y period of slower cooling and exhumation combined with erosion of crustal rocks with uniform 40 ar 39 ar signatures these processes have resulted in a pattern of regularly increasing lag time up section