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

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

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

high pressure metamorphism of the lanzo peridotite and its oceanic cover and some consequences for the sesia lanzo zone northwestern italian alps abstract this paper presents new data on the alpine high pressure evolution of the lanzo peridotite massif and its metasedimentary cover western alps field relations in the northwestern lanzo massif show that the serpentinized part is covered by a metasedimentary sequence including local occurrences of metaophicarbonate metabasalt calcschist metaquartzite and gneiss remnants of granulite facies mafic gneiss associated with metaophicarbonate are found within the serpentinite major element garnet zonation profiles in metaquartzite and mafic gneiss were used to explore the contrasting origin of metasediments bell shaped mn zoning in garnet from metaquartzite indicates a one stage alpine metamorphic evolution while inverse zoning in garnet of the mafic gneiss shows that the protolith equilibrated under high grade metamorphic conditions in pre alpine times similar to kinzigites within the neighbouring sesia zone phase equilibria in metaquartzite and mafic gneiss point to 500 50 c at pressures of 0 9 to 1 3 gpa for the alpine metamorphic equilibration that contrasts with the 550 620 c and pressures in excess of 2 0 gpa obtained for kyanite talc chloritoid and talc chloritoid garnet domains in a metagabbro dike from the internal part of the lanzo massif these different p t trajectories can be explained by a much stronger retrograde overprint at the basement cover contact than in the internal part of the lanzo massif

late cretaceous exhumation of the oman blueschists and eclogites a two stage extensional mechanism the continental material of the saih hatat window has been affected by a late cretaceous obduction related hp lt metamorphism below the oman ophiolite a high rate exhumation process is witnessed by the maastrichtian palaeocene onlap onto the blueschist facies rocks drastic metamorphic omissions are documented between the lowest eclogitic units and the overlying blueschist facies ones widespread late metamorphic shear structures point to a top to the nne detachment in opposition to the sense of the late cretaceous obduction the inversion of the shearing sense occurred under similar low temperature conditions in both the blueschist and eclogite facies units admitting that these hp lt metamorphic rocks formed progressively at various depth in the subducting arabian margin a two stage extensional mechanism of exhumation is suggested i early uplift of the eclogitic rocks up to the blueschists depth by ductile thinning or squeezing of a blind extensional allochthon ii exhumation of the whole hp lt metamorphic core complex by inversion of the obduction sole thrust and isostatic rebound of the lower plate

extension in compressional orogenic belts the eastern alps extension in the eastern alps encompasses the following 1 extension accompanying simple shear crustal stacking subhorizontal stretching occurs along flat lying foliations and regionally consistent stretching lineations with associated high strains on a local scale stretching results from extrusion of ductile sediments squeezed between rigid basement blocks basement blocks during thrust sheet imbrication the overall geometric effect is crustal thickening 2 classical extensional tectonics extension first occurs as a consequence of accretion and underthrusting of continental and oceanic material and represents gravitational adjustment of an unstable orogenic wedge extension then occurs as a consequence of terminal continental collision and is accommodated by lateral extrusion of crustal of crustal blocks the overall geometric effect is crustal thinning extension is generally oriented subparallel to the strike of the orogen

east west extension in the nw indian himalaya explaining the presence of normal faults in overall compressive settings is a challenging problem in understanding the tectonics of active mountain belts the himalayan tibetan orogenic system is an excellent setting to approach this problem because it preserves one of the most dramatic records of long term contemporaneous shortening and extension over the past decades several studies have described extensional features not only in the tibetan plateau but also in the himalaya for a long time the favored model explained the function of the southern tibetan detachment system a major fault zone in the himalaya as a decoupling horizon between the regime of crustal shortening forming the himalayan wedge to the south and the extensional regime of the tibetan plateau to the north however in recent years increasing evidence has shown that n s trending normal faults in the central himalaya crosscut not only the southern tibetan detachment system but also the main central thrust here we present new structural data and geologic evidence collected within the nw indian himalaya and combine them with previously published seismicity data sets in order to document pervasive e w extension accommodated along n s trending faults extending as far south as the footwall of the main central thrust we conducted a kinematic analysis of fault striations on brittle faults documented and mapped fault scarps in quaternary sedimentary deposits using satellite imagery and made field observations in the greater sutlej region spiti lahul kinnaur and the garhwal himalaya studies of extensional features within the regionally nw se trending nw indian himalaya provide the advantage that arc parallel and e w extension can be separated in contrast to the central himalaya therefore our observations of e w extension in the indian nw himalaya are well suited to test the applicability of current tectonic models for the whole himalaya we favor the interpretation of e w extension in the nw indian himalaya as a propagation of extension driven by collapse of the tibetan plateau

constraints on the late thermotectonic evolution of the western alps evidence for episodic rapid uplift forty two new apatite and zircon fission track ages are presented for samples from the western alps in southern switzerland northern italy and southeastern france measured ages plotted against assumed closure temperatures yield cooling patterns for the final cooling uplift and exhumation of the western alps similar fission track zircon ages in the penninic gran paradiso massif dent blanche nappe sesia lanzo zone and ivrea zone indicate cooling of all four units to 225 c by 33 ma differences in apatite ages reveal differential cooling of the four blocks between 33 ma and the present in the sesia lanzo zone similarity of apatite ages regardless of elevation together with near volcanic confined fission track length patterns suggest rapid cooling and uplift at 25 ma compared with slow cooling of other western alps units around 12 ma uplift is thus not continuous but episodic often over a short time interval beyond the resolution of other methods such episodes of uplift as revealed here in the sesia lanzo zone may be the rule rather than the exception

subduction and eduction of continental crust major mechanisms during continent continent collision and orogenic extensional collapse a model based on the south norwegian caledonides during continental collision in the middle silurian the thickness of the lithosphere under the caledonides of s norway was doubled by subduction of the western margin of baltica including the western gneiss region under laurentia crustal rocks of the baltic plate reached sub moho depths of near 100 km or more as inferred from the presence of coesite in eclogites isostatic calculations indicate an average elevation of the mountain chain of about 3 km at this stage the subducted lithosphere experienced vertical constrictional strains as a result of slab pull by its heavy and cold root eduction of the deeply buried crustal material was initiated by decoupling of the thermal boundary layer in the subducted lithosphere isostatic rebound resulted in very rapid uplift 1 2 mm yr 1 and the deep crust was exhumed mainly by tectonic extensional stripping over a period of 30 40 myr the eduction was probably related to a rolling hinge footwall uplift mechanism and the early high pressure coaxial fabrics were overprinted by extensional simple shear as the deep crust reached middle and upper crustal levels the model explains the present day normal crustal thickness under the exhumed deep rocks without necessarily invoking large scale lateral flow of material in the lower crust or igneous underplating

albite crystallographic preferred orientation and grain misorientation distribution in a low grade mylonite implications for granular flow abstract crystallographic orientation data from pure albite domains in a low grade sheared metagabbro from the combin zone of the western italian alps were measured by electron backscatter diffraction crystallographic preferred orientations cpos in four high strain domains are non random and have a triclinic symmetry the clusters of 100 010 and 001 show an angular relationship that corresponds to that of the albite crystal lattice however the orientations of axis clusters to the kinematic axes vary from domain to domain cpos from a low strain domain also show clustered axes with triclinic symmetry but with more intense clustering than those in the high strain domains grain misorientation distributions are presented both for the low strain domain and one of the high strain domains in the high strain domain the distribution of misorientation angles between neighbouring grains displays a peak at about 70 the equivalent distribution in the low strain domain has a peak at 30 for both domains the misorientation axis distributions between neighbouring and non neighbouring grains are random except for some of axes with 160 180 misorientation that exhibit a slight concentration around 010 the diversity of cpos among the domains suggests that these cpos could not be produced by dislocation creep they are likely to have been inherited from plagioclase parents as a result of host control on the nucleation of the new albite grains these cpos do not contain any direct information about the deformation kinematics we interpret that deformation of these domains occurred by granular flow crystallographic axis dispersion due to grain boundary sliding gbs caused weakening of cpos modification of misorientation angle distributions and randomisation of misorientation axis distributions the fact that a cpo can survive gbs even after a high strain indicates that cpo is not always a sensitive indicator of deformation mechanisms misorientation distribution may provide a complementary and possibly a more sensitive indicator of deformation mechanisms

experimental investigations of blueschist greenschist transition equilibria pressure dependence ofal2o3 contents in sodic amphiboles a new geobarometer

geochemical and stable isotope resetting in shear zones from taschalp constraints on fluid flow during exhumation in the western alps fluid flow at greenschist facies conditions during exhumation of the western alps occurred in several penecontemporaneous systems including shear zones at lithological contacts deformed contacts between serpentinite bodies and metabasalts albite veins within metabasalts and calcite quartz veins within calcareous schists fluid flow in shear zones that juxtapose metasediments and ophiolitic rocks within the piemonte unit reset o and h isotope ratios 18o values are buffered by the wall rocks however calculated fluid 2h values are similar within all the shear zones suggesting that they formed an interconnected network the similarity of 2h values of the sheared rocks and those of unsheared calcareous schists suggests that the fluids were derived from or had equilibrated with the schists that envelop the ophiolite rocks time integrated fluid fluxes at the sheared contacts estimated from changes in si in metabasalts were up to 105 m3 m 2 with the fluid flowing up temperature driven either by topography or seismic pumping individual shear zones were active for c 2 3 myr implying average fluid fluxes of up to 10 9 m3 m 2 s 1 rocks in shear zones within the ophiolite away from contacts with the metasediments show much less marked isotopic and geochemical changes implying that fluid volumes decreased into the ophiolite unit consistent with the source of fluids being the metasediments fluids were generated by dehydration reactions that were intersected during exhumation and while many rocks show the affects of fluid rock interaction large scale fluid flow between major units was not common

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

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

stable isotope geochemistry of alpine ophiolites a window to ocean floor hydrothermal alteration and constraints on fluid rock interaction during high pressure metamorphism the subduction of hydrated oceanic lithosphere potentially transports large volumes of water into the upper mantle however despite its potential importance fluid rock interaction during high pressure metamorphism is relatively poorly understood the stable isotope and major element geochemistry of pennine ophiolite rocks from italy and switzerland that were metamorphosed at high pressures are similar to that of unmetamorphosed ophiolites suggesting that they interacted with little pervasive fluid during high pressure metamorphism cover sediments also have oxygen isotope ratios within the expected range of their protoliths in the rocks that escaped late greenschist facies retrogression different styles of sub ocean floor alteration may be identified using oxygen isotopes petrology and major or trace element geochemistry within the basalts zones that have undergone high and low temperature sub ocean floor alteration as well as relatively unaltered rocks can be distinguished serpentinites have 18o and 2h values that suggest that they were formed by hydration on or below the ocean floor the development of high pressure metamorphic mineralogies in metagabbros occurred preferentially in zones that underwent sub ocean floor alteration and which contained hydrated fine grained reactive assemblages given that the transformation of blueschist facies metabasic rocks to eclogite facies assemblages involves the breakdown of hydrous minerals e g lawsonite zoisite and glaucophane and will thus liberate considerable volumes of fluids metamorphic fluid flow must have been strongly channelled high pressure quartz calcite omphacite glaucophane titanoclinohumite veins that cut the ophiolite rocks represent one possible channel however stable isotope and major element data suggest that they were not formed from large volumes of exotic fluids fluids were more likely channelled along faults and shear zones that were active during high pressure metamorphism such strong fluid channelling may cause fluids to migrate toward the accretionary wedge especially along the slab mantle interface which is probably a major shear zone this may preclude all but a small fraction of the fluids entering the mantle wedge to flux melting additionally because fluids probably interact with relatively small volumes of rock in the channels they cannot scavenge elements from the subducting slab efficiently

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

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

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

normal faulting at convergent plate boundaries mylonitic extensional fabrics in the franciscan subduction complex in del puerto canyon california revisited 1 using a strain and rotation analysis we tested the hypotheses that top east mylonitic extensional structures in the uppermost franciscan subduction complex in del puerto canyon california accomplished exhumation of the franciscan blueschists we found no evidence of strongly noncoaxial deformation instead our data indicate overall coaxial deformation in the proposed zone of mylonitic extensional deformation there are no extensional strains moderate vertical shortening occurred without horizontal extension and was compensated by modest deformation related volume loss there is also no strain gradient toward and within the proposed shear zone therefore the results of our work indicate that mylonitic extensional structures in del puerto canyon do not exist overall we conclude that there is no currently identifiable structure adjacent to or within the franciscan in california that could have accommodated large scale horizontal extension exhumation of the franciscan blueschists was mainly achieved by erosion of an emergent forearc high our strain results and published data are also inconsistent with the development of a supercritically tapered franciscan wedge that ultimately should have triggered normal faulting strain magnitudes are low despite the relatively long residence time of the rocks in the accretionary wedge from this we infer that strain rates were low and that the wedge material flowed with a relatively high viscosity therefore ductile flow probably was not fast enough to form a supercritically tapered franciscan wedge

can crustal extension be distinguished from thrusting in the internal parts of mountain belts a case history of the entrelor shear zone western alps abstract criteria for the distinguishing between structures that accommodate crustal scale extension or crustal scale shortening may be applied to late orogenic shear zones developed in the internal parts of mountain belts this is done for a large shear zone that has been mapped in the nw italian alps around the internal basement massif of gran paradiso the structure involved is termed the entrelor shear zone it carries greenschist facies european continental rocks the brianconnais grand st bernard unit onto an old subduction complex of oceanic piemonte and continental gran paradiso material that preserve eclogitic assemblages evidence for the map scale continuity of the entrelor shear zone is presented together with kinematic data to show how folds stretching lineations and shear criteria relate to a regional episode of ese directed shearing however folds initiated systematically oblique to the shear direction on a ne sw axis with continued shearing the fold population remains skewed e w of the population of stretching lineations the shear zone may be traced for 70 km around the dome of the gran paradiso massif and displays a minimum displacement of 20 km this entrelor structure shows variable offset of metamorphic grade however for much of its length greenschist facies rocks are carried in its hanging wall onto eclogites in the footwall tracing these units regionally shows that this apparent extensional offset of metamorphic facies is the result of restacking of an originally inverted metamorphic sequence i e hp piemonte on lp brianconnais this contractional behavior is consistent with the profile of the shear zone relative to the modern surface the entrelor shear climbs up from generally buried in the west to generally eroded in the east the conclusion that the entrelor shear zone is probably a thrust rather than an extensional structure relies on linking outcrop data into a semi regional context false interpretations may result from examining only small parts of the structure however although these map and structural criteria are consistent with contractional kinematics associated with crustal shortening confirmation is required through linked petrological and isotopic studies the entrelor shear zone is thus a good illustration of the need for caution in inferring the contractional or extensional nature of structures developed late in the history of the internal parts of mountain belts

thrust tectonics deep structure and crustal subduction in the alps and himalayas the structural restoration of collision orogenic belts onto crustal templates provides important insights into the tectonic evolution deep structure and amounts of plate convergence after the initial contact between two continental masses balanced cross sections have been constructed parallel to the local displacement directions across the western alps and western himalayas and demonstrate very large amounts of crustal shortening above intra crustal detachments to achieve a balance substantial volumes of lower crust must have been subducted beneath the two tectonic hinterlands a model of eclogite metamorphism is invoked to facilitate this subduction and to explain the varying isostatic responses of the alpine and himalayan hinterlands patterns of eclogite metamorphism are controlled by the geometry of thrust profiles on a lithospheric scale the development of crust mantle detachments being of crucial importance such a profile is proposed for the himalayas suggesting that relatively small volumes of the footwall crust succumbed to eclogite metamorphism in the alps however a steeper thrust profile apparently developed emplacing mantle onto crust causing wholescale eclogite metamorphism in the underlying franco swiss crust the resultant density increase would provide a mechanism of isostatic collapse and flexural subsidence in the po plain region

extension versus shortening models for hinterland directed motions in the southern quebec appalachians abstract in the taconian internal zone of southern quebec structures related to hinterland directed motion are found on both limbs of the sutton notre dame mountains anticlinorium d3west and d3east structures and along the st joseph fault these ductile to brittle ductile structures were formed during upper to mid greenschist grade metamorphic events thus suggesting that the area occupied a mid crustal position during hinterland motion layer extensional fabrics along the eastern limb of the sutton notre dame mountains anticlinorium can be interpreted either as structures contemporaneous to backthrusting deformation rooted on the western limb of the anticlinorium or as structures formed during normal faulting along the st joseph fault the study of layer extensional structures found in the taconian internal zone of the southern quebec appalachians shows that it is not obvious to infer wether normal sense fabrics that occur in orogenic hinterlands are related to regional crustal extension or to local syn collisional extensional faulting

incision and morphologic evolution of an alluvial channel recovering from hydraulic mining sediment hydraulic gold mining in the sierra nevada california produced such a large volume of sediment from 1853 to 1884 that channels could not carry a substantial proportion of the delivered sediment the lower bear river aggraded as much as 5 m and underwent a major avulsion during the 1870s channel morphology continues to respond to these changes more than 100 years after the cessation of most hydraulic mining textures of mine tailings and older alluvium are contrasted and channel incision and channel shape changes from 1930 to 1985 are documented unlike the yuba and sacramento rivers the lower bear river had not returned to pre mining base levels by 1950 as sediment loads decreased nor were incision rates or morphological change governed by reduced sediment loads below dams built in 1928 and in the mid 1960s channel stability was due to resistance of a cohesive stratum on the channel bed an episode of channel down cutting was instigated by a moderately large flood in december 1955 that pierced the cohesive layer and was sustained through the 1970s by moderate magnitude floods the 1955 flood did not directly erode large volumes of sediment but it destabilized the channel allowing smaller floods to erode the channel at an accelerated rate episodic channel bed incision is contrasted to progressive morphologic changes cross section shape narrowed and deepened steadily from 1930 through the mid 1970s as the channel was superimposed onto cohesive older alluvium progressive deepening set up a positive feedback that ensured the ultimate penetration of the paved bed channel morphologic readjustment from the avulsion thus was both progressive and episodic

extensional faulting in an intraoceanic subduction complex working hypothesis for the palaeogene of the alps apennine system abstract within the ophiolite complexes exposed in the ligurian alps and apennines nw italy several extensional detachment faults are identified which overprint late cretaceous high pressure metamorphic fabrics developed in an intraoceanic subduction system one of these detachment faults is the sestri voltaggio line which marks an 8 kbar pressure gap between blueschist facies ophiolites and calcareous sediments of the sestri voltaggio zone and underlying eclogite fades ophiolites and metasediments of the voltri massif radiometrie and stratigraphie constraints suggest that extension has occurred during the late paleocene to possibly early eocene from the middle eocene onward compressional tectonics prevailed again resulting in westerly directed imbrications in the ligurian alps and easterly directed folding and thrusting in the ligurian apennines a strong correlation is identified between the africa europe convergence rate and the stability of the accretionary wedge i e the paleocene extension coincides with a period of strongly reduced convergence velocities whilst the middle eocene transition to compressional tectonics concurs with an acceleration of the convergence rate this inferred relation between convergence rate and style of internal deformation matches very closely the results from mechanical models addressing the stability of orogenic wedges with a strain rate dependent rheology and provides the basis for a working hypothesis on the palaeogene evolution of the alps apennine orogenic system

the thermal response of a metamorphic belt to extension constraints from laser ar data on metamorphic micas abstract metamorphic rocks may be exhumed to the surface either by isostatic rebound uplift and erosion or tectonically via thrusting or extension the latter mechanism is particularly important in the himalayas where a large extensional detachment system appears to be responsible for very fast unroofing at 20 ma here we present the results of a laser argon study of micas and quartz aimed at investigating the thermal response of an orogenic belt to such extension the data when combined with existing constraints on the prograde evolution lead to a uniquely complete thermal history for a metamorphic belt laser argon data are reported for quartz and micas from semi pelitic metamorphic rocks from the zanskar himalayas nw india the samples derive from three traverses extending from low grade rocks near the detachment to high grade rocks in the core of the belt most samples demonstrably contain no excess component and the ages of muscovite and biotite show a correlation with distance from the extensional detachment the ages themselves as well as the difference between the muscovite and biotite ages both increase as the detachment is approached muscovite and biotite ages from within the shear zone date deformation associated with extension at 20 25 ma the remaining data are most readily interpreted in terms of early slow cooling of the lower grade rocks near the detachment in response to erosional denudation before the detachment became active rocks in the core of the orogen were still heating at this time later fast cooling of the high grade rocks in the core of the belt ensued when the extensional regime became active simple heat transfer calculations based on the distance of the high grade rocks from the fault and the argon ages of their micas provide further constraints on the timing of extension and suggest that the fault first moved 23 25 ma ago and that it abruptly terminated metamorphism in the core of the metamorphic belt our analysis of the thermal history suggests that cooling and exhumation rates determined from metamorphic belts beneath extensional detachments will often be minima

the ophiolite bearing schistes lustres nappe in alpine corsica a model for the emplacement of ophiolites that have suffered hp lt metamorphism

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

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

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

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

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

structural evolution of a subducted continental sliver the northern dora maira massif italian alps continental crust of the dora maira massif was subducted in part to pressures of 30 kbar during early stages of the alpine orogeny a new investigation in a transect across the north of the massif has shown that much of the dominant foliation in the upper dora maira pellice unit was formed at high pressures of the order of 11 kbar but at lower pressure than the locally preserved relicts of eclogite facies metamorphism this dominant deformation phase produced large recumbent isoclinal folds with intensely flattened limbs in the pellice unit basement and the overlying permian mesozoic cover now white augen gneisses and calcschists the rocks were thus part of the upper crust prior to their subduction unless an unrealistically large thickness of mesozoic rocks now removed is hypothesized the pellice unit was later emplaced over the chisone unit in greenschist facies conditions all shear sense indicators are top to west implying a simple but prolonged history in which high pressure units were emplaced westwards over lower pressure ones whilst being exhumed the intense horizontal stretching recorded in the basement gneisses may have helped bring them closer to the surface by thinning indirect arguments suggest that such tectonic thinning is not on its own sufficient to explain the present setting of the eclogite units one or more sheets of eclogite material may have been extruded from the eoalpine subduction zone in addition such slabs would be bounded below by west directed and above by east directed shears as the dora maira massif forms the structurally lowest eoalpine eclogite unit in the alps the observed history is not incompatible with this model east or southeast directed structures in the alps are commonly identified as backthrusts but an origin for some as the extensional shears predicted by this model should not be excluded

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

probing the transition between seismically coupled and decoupled segments along an ancient subduction interface the transition zone at the downdip end of seismic coupling along subduction interfaces is often the site of megathrust earthquake nucleation and concentrated postseismic afterslip as well as the focus site of episodic tremor and slip features exhumed remnants of the former alpine subduction zone found in the swiss alps allow analyzing fluid and deformation processes near the transition zone region 30 40 km paleodepth the dent blanche thrust dbt is a lower blueschist facies shear zone interpreted as a fossilized subduction interface where granitic mylonites overlie a metamorphosed accretionary wedge we report field observations from the dbt region where multiple several tens of meters thick foliated cataclastic networks are interlayered within the basal dbt mylonites petrological results and microstructural observations indicate that the various cataclasis events took place at near peak metamorphic conditions 400 500 c 1 1 1 3 gpa during subduction of the tethyan seafloor in eocene times 42 48 ma some of these networks exhibit mutual crosscutting relationships between mylonites foliated cataclasites and vein systems indicating mutual overprinting between brittle deformation and ductile creep whole rock chemical compositions in situ 40ar 39ar age data of recrystallized phengite and sr isotopic signatures reveal that dbt rocks also underwent multiple hydrofracturing and metasomatic events via the infiltration of fluids mainly derived from the oceanic metasediments underneath the dbt from the rock fabrics we infer strain rate fluctuations of several orders of magnitude beyond subduction strain rates 10 12 s 1 accompanied by fluctuation of supralithostatic and quasi lithostatic fluid pressures 1 0 95 dbt brittle plastic deformation switches highlight the diversity of deformation processes and fluid rock interactions in the transition zone region of the subduction interface

syn convergent exhumation implied by progressive deformation and metamorphism in the valle dell orco transect nw italian alps structural metamorphic and geochronological data from the valle del9orco are best explained by an orogenic wedge model in which crustal slices from continent and ocean acted as passive blocks at different times in the alpine orogeny deformation generally migrated westwards in this section and metamorphic recrystallization at any given point was largely controlled by the pervasiveness of strain the data suggest a dynamic crustal stacking system lasting from 70 to 35 ma rather than continental collision following long lived b type subduction and offer a relevant and simple model for eclogite exhumation that requires exhumation of ductile shear bounded slices within the orogenic wedge the easternmost internal unit of the sesia lanzo zone the eclogitic micaschist complex was subducted and pervasively recrystallized under eclogite facies conditions during the early stages of convergence in the late cretaceous during the development of the major preserved structures the eastern eclogitic micaschist complex was undergoing general non coaxial shear with an overall top down to the east vorticity suggesting a potential mechanism of exhuming an eclogitic 9pip9 slices of pre alpine basement the vasario sparone subunit were incorporated into the lower western parts of this unit during the latest part of this phase but do not show penetrative effects of the early alpine deformation they were passively folded during the early decompressional part of the p t history prior to the unit being juxtaposed against the underlying gneiss minuti complex under blueschist greenschist facies conditions the gneiss minuti complex itself was then recrystallized at high strain in the greenschist facies in the latest eocene fabrics developed at this stage indicate top down to east extension which may have accommodated the exhumation of eclogitized piemont oceanic material and the european continental material of the gran paradiso massif evidence of northwest directed thrusting within the gran paradiso unit at the same time suggests again that the crustal block was transported on a deeper reverse sense shear zone that presumably links to the more external thrusts now exposed to the west high pressure mineral lineations within the piemont zone cannot be unequivocally assigned to either the burial or exhumation paths but are consistent with the generally east west transport directions observed in the structurally higher units

feldspar fabrics in a greenschist facies albite rich mylonite from electron backscatter diffraction abstract the current database of plagioclase cpos in naturally deformed rocks is small and is not supported by cpos from experimentally deformed rocks or by numerical modelling so that the current understanding of plagioclase cpos is limited electron backscatter diffraction ebsd has the potential of measuring the full crystallographic orientation of small domains of any crystalline material including plagioclase we generate and test a file of the relative electron diffraction intensities for lattice planes in the albite structure this enables rapid and accurate individual orientation measurements of albite using ebsd ebsd is used to collect 360 data points from 7 areas across a 2 cm by 1 mm pure albite band in a greenschist facies albite mylonite from a gabbro protolith the cpos are unlike any published the a b and c axes of albite are not symmetrically distributed with respect to the kinematic axes of deformation and do not possess an internal monoclinic or orthorhombic symmetry there is some microstructural evidence for the operation of dislocation creep although this interpretation conflicts with the generally held view that crystal plastic deformation does not occur in plagioclase below about 500 c an alternative mechanism is one of granular flow accommodated by solution transfer however this mechanism would be expected to give a random cpo for either mechanism we need to explain the triclinic nature of the cpo it is possible that the cpo represents the modification by deformation of a pre existing cpo probably defined by a small number of original plagioclase grains

surface albedo observations of hudson bay canada landfast sea ice during the spring melt the shortwave albedo is a major component in determining the surface energy balance and thus the evolution of the spring melt cycle as the melt commences the ice is partitioned into multiple surface types ranging from highly reflective white ice to absorptive blue ice the reflectance from these surfaces shows significant spatial and temporal variability spectral albedo measurements were made at six different sites encompassing these two surface types from 19 march to 3 may 2005 on 1 5 m thick landfast sea ice in southwestern hudson bay canada 588 n furthermore the broadband albedo and the surface energy balance were continuously recorded at a nearby site during the 1 month period rapid changes in the albedo were found to relate to typical subarctic climate conditions i e frequent incursions of southerly air resulting snow and rain events and the generally high maximum solar insolation levels subsequently diurnal variations in snow surface temperature were evident often causing daytime melting and night time refreezing resulting in the formation of ice lenses and superimposed ice after rain events and extensive melting the snowpack was transformed throughout into melt freeze metamorphosed snow and superimposed ice the integrated 350 1050 nm albedo varied between 0 52 and 0 95 at the blue ice sites while it varied between 0 73 and 0 91 at white ice sites variability on the order of 10 in the white ice broadband albedo resulted from the diurnal freeze thaw cycle but also synoptic weather events such as snowfall and rain events could rapidly change the surface conditions

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

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

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

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

retrogressive fabric development during exhumation of the voltri massif ligurian alps italy arguments for an extensional origin and implications for the alps apennines linkage geological mapping coupled with structural investigations carried out in the voltri massif eastern ligurian alps italy provide new data for the interpretation of the tectonic context controlling main fabric development during exhumation of its high pressure core the voltri massif is here interpreted as a c 30 km long eclogite bearing asymmetric dome formed by the progressive verticalisation of the regional second phase mylonitic foliation developed during retrogressive greenschist metamorphic conditions in this light the exhumation history is driven by a ductile to brittle extensional process operating through low angle top to the w multiple detachment systems a late eocene early oligocene age for this extensional episode is proposed on the basis of structural correlations stratigraphic and radiometric constraints in this scenario the voltri massif is interpreted as an extensional domain developed to accommodate the late eocene early oligocene arching of the western alps northern apennines orogenic system

reply to the comment by g capponi et al on subduction polarity reversal at the junction between the western alps and the northern apennines italy by g vignaroli et al tectonophysics 2008 450 34 50

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

retracted structural complexity in the curnamona province south australia polyphase strain partitioning and reactivation in this paper we examine the apparent timing relationships between the structural and metamorphic events of the amphibolite facies sequence in the walter outalpa shear zone wosz area in the southwestern part of the palaeo to mesoproterozoic curnamona province the local structural geometry and style are largely due to heterogeneous non coaxial flow in which foliation development is related to the superposition of two main independent structural events d2 and d3 strain was progressively partitioned into high strain zones in the southern part of the study area in contrast to the low strain areas in the north of the study area d2 deformation developed under amphibolite facies conditions and is partitioned between zones of thrusting and flattening folding related to nappe formation d3 deformation developed under greenschist facies conditions and is partitioned between strike slip faulting and folding d3 high strain zones form discontinuous nw trending anastomosing strands that contain relics of the d2 thrust related foliation these relicts preserve the higher grade mineral assemblages that are partially overprinted by d3 retrograde assemblages strike slip movement along the wosz is therefore interpreted to have re used the pre existing d2 thrust such that d2 strain partitioning influenced renewed d3 strain partitioning the high grade metamorphism and phase of pervasive deformation d2 m2 are ascribed to the d2 mesoproterozoic olarian orogeny that occurred in the curnamona province between ca 1600 and 1590 ma the tectonic context of the d3 m3 shearing event is highly uncertain recent geochronological sm nd isotope ages from garnet from within the wosz shear zone yield ca 520 490 ma delamerian ages dutch r a hand m clark c 2005 cambrian reworking of the southern australian proterozoic curnamona province constraints from regional shear zone systems geol soc london 162 5 763 775 however the d3 m3 shear foliation are truncated by the neoproterozoic unconformity flooring the adelaide rift complex stratigraphy suggesting pre delamerian orogeny activity along the shear zone it is thus likely that the d3 movement along the shear zone records a combination of late olarian orogenic movement with a component of reactivation during the delamerian orogeny 2005 elsevier b v all rights reserved