exhumation of the dora maira ultrahigh pressure unit by buoyant uprise within a low viscosity mantle oblique slip shear zone the brossasco isasca subunit biu of the dora maira massif is currently the only known continental crustal ultrahighpressure uhp unit in the western alps the peak pressure temperature conditions are 3 5 4 5 gpa 730 c exhumation from 3 5 gpa to 1 gpa occurred within 2 2 1 8 ma but the exhumation mechanism is incompletely understood we present a conceptual model for the buoyancy driven exhumation of the biu inside a low viscosity dense mantle shear zone weakened by increased strain rates due to simultaneous strike slip and subduction oblique slip of the european plate two dimensional thermo mechanical models simulate such a buoyant uprise of an ellipse inside an inclined layer simulations i show the feasibility of the conceptual model ii fit the pressure temperature time record and iii constrain effective viscosities the model is compatible with the i small volume of continental crustal uhp rock in the western alps ii minor erosion during exhumation and iii strike slip deformation during the exhumation period

alpine and pacific styles of phanerozoic mountain building subduction zone petrogenesis of continental crust a broad continuum exists between two distinct end member types of mountain building alpine type orogenic belts develop during subduction of an ocean basin between two continental blocks resulting in collision they are characterized by an imbricate sequence of oceanward verging nappes some alpine belts exhibit superimposed late stage backthrusting sediments are chiefly platform carbonates and siliciclastics in some cases associated with minor amounts of bimodal volcanics pre existing granitic gneisses and related continental rocks constitute an autochthonous parautochthonous basement metamorphism of deeply subducted portions of the orogen ranges from relatively high pressure hp to ultrahigh pressure uhp calcalkaline volcanic plutonic rocks are rare and have peraluminous s type bulk compositions in contrast pacific type orogens develop within and landward from long sustained oceanic subduction zones they consist of an outboard oceanic trench accretionary prism and an inboard continental margin island arc the oceanic assemblage consists of first cycle in part melanged volcaniclastics and minor but widespread cherts deep water carbonates intimately mixed with disaggregated ophiolites the section recrystallized under hp conditions recumbent fold vergence is oceanward a massive slightly older to coeval calcalkaline arc is sited landward from the trench complex on the stable non subducted plate it consists of abundant dominantly intermediate metaluminous i type volcanics resting on old crust both assemblages are thrown into open folds intruded by comagmatic i type granitoids and metamorphosed locally to regionally under high t low p conditions in the subduction channel of collisional and outboard circumpacific terranes combined extension above and subduction below allows buoyancy driven ascent of ductile thin aspect ratio slices of hp uhp complexes to midcrustal levels where most closely approached neutral buoyancy exposure of rising sheets caused by erosion and gravitational collapse results in moderate amounts of sedimentary debris because exhumed sialic slivers are of modest volume at massive sialic buildups associated with convergent plate cusps syntaxes tectonic aneurysms may help transport hp uhp complexes from mid to upper crustal levels the closure of relatively small ocean basins that typify many intracratonic suture zones provides only limited production of intermediate and silicic melts so volcanic plutonic belts are poorly developed in alpine orogens compared with circumpacific convergent plate junctions generation of a calcalkaline arc mainly depends on volatile evolution at the depth of magma generation phase equilibrium studies show that under typical subduction zone p t trajectories clinoamphibole ca al hydrous silicates constitute the major hydroxyl bearing phases in deep seated metamorphic rocks of morb composition other hydrous minerals are of minor abundance ca and na clinoamphiboles dehydrate at pressures of above approximately 2 gpa but low temperature devolatilization may be delayed by pressure overstepping thus metabasaltic blueschists and amphibolites expel h2o at melt generation depths and commonly achieve stable eclogitic assemblages partly serpentinized mantle beneath the oceanic crust dehydrates at roughly comparable conditions for reasonable subduction zone geothermal gradients however white micas biotites remain stable to pressures 3 gpa accordingly attending descent to depths of 100 km mica rich quartzofeldspathic lithologies that constitute much of the continental crust fail to evolve substantial amounts of h2o and transform incompletely to stable eclogite facies assemblages underflow of amphibolitized oceanic lithosphere thus generates most of the deep seated volatile flux and the consequent partial melting to produce the calcalkaline suite along and above a subduction zone where large volumes of micaceous intermediate and felsic crustal materials are carried down to great depths volatile flux severely diminishes thus continental collision in general does not produce a volcanic plutonic arc whereas in contrast the long continued contemporaneous underflow of oceanic lithosphere does

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

deformation history of a subducted continental crust gran paradiso western alps continuing crustal shortening during exhumation eclogite facies continental basement in the western alps outcrops as tectonic windows below the oceanic units in the gran paradiso massif eclogite facies assemblages in mafic rocks display a n s stretching lineation da1 the main phase foliation formed under epidote amphibolite facies conditions da2 and defines a regional dome structure for the whole gran paradiso massif structural data including new detailed mapping microstructural analyses and metamorphic studies in the northern part of the gran paradiso cogne valley reveal the occurrence of major thrusts in this continental basement associated with the da2 deformation e w trending stretching lineations and fold axes are associated with this second and main deformation nappe stacking of the different units was achieved by top to the west shearing during exhumation of the eclogite facies basement the antiformal doming of the gran paradiso basement is thus produced by the nappe stacking during exhumation under epidote amphibolite facies

evidence for protracted prograde metamorphism followed by rapid exhumation of the zermatt saas fee ophiolite major and trace element zoning in garnet in combination with rb sr sm nd and lu hf geochronology provide evidence for a protracted garnet growth history for the zermatt saas fee zsf ophiolite western alps four new lu hf ages from pfulwe c 52 46 ma and one from chamois c 52 ma are very similar to a previously published lu hf age from lago di cignana overall the similarity of geochronological and garnet zoning patterns suggests that these three localities had a similar prograde tectonic history commensurate with their similar structural position near the top of the zsf samples from the lower part of the zsf at saas fee and st jacques however produced much younger lu hf ages c 41 38 ma neither differences in whole rock geochemistry which might produce distinct garnet growth histories nor rare earth element zoning in garnet can account for the age differences in the two suites this suggests a much later prograde history for the lower part of the zsf supporting the idea that it was subducted diachronously such a model is consistent with changes in subduction vectors based on plate tectonic reconstructions where early oblique subduction which produced long prograde garnet growth changed to more orthogonal subduction which corresponds to shorter prograde garnet growth six new rb sr phengite ages range from c 42 to 39 ma and in combination with previously published rb sr ages constrain the timing of the transition from eclogite to upper greenschist facies p t conditions the proximity of the zsf in the saas fee region to the underlying continental monte rosa unit and the similarity of peak metamorphic ages suggest these two units were linked for part of their tectonic history this in turn indicates that the monte rosa may have been partly responsible for rapid exhumation of the zsf unit

geological evolution of the late mesozoic continental margin of northwestern anatolia abstract the armutlu peninsula is a composite tectonic entity made up of sections of the sakarya continent the rhodope pontide fragment and an ophiolite these were assembled following a continental collision between gondwanaland and laurasia during the late cretaceous the northern margin of the sakarya continent underwent progressively increasing deformation prior to and during the advancing collision due to continued convergence between the two continents initially the leading edge of the continent subsided under the load of an approaching ophiolitic slab following this a north directed thrusting and folding occurred during the turonian progressive elimination and eventual closure of the ocean preceded the thrusting of northerly situated collisiondashinduced nappe packages over the leading edge of the sakarya continent the nappe laden edge of the continental margin then collapsed and steadily subsided under the heavy load of the ophiolitic slab and the northern continental fragment consequently the nappe packages and the ophiolite were collectively metamorphosed during the coniaciandashsantonian interval during the subsidence the main body of the sakarya continent partially detached from its collapsed edge along a fault zone and thus suffered an independent but less severe deformation which lasted until the uplift of the collapsed edge in the campanian from the late campanian onward throughout later orogenic stages the metamorphic and nondashmetamorphic units amalgamated into a single tectonic entity forming a basement for younger cover rocks

two dimensional geoelectrical structure of the goynuk geothermal area northwest anatolia turkey abstract a detailed survey with magnetotelluric measurements covering the period range 0 04 500 s was carried out to define the geoelectrical structure of goynuk geothermal area northwest anatolia the study area is not far from the well known geothermal field around northwest anatolia that has several potentially valuable minerals and hot springs the resulting two dimensional 2 d geoelectric structures with true resistivity values obtained from transverse electric te transverse magnetic tm and joint te tm data inversion were interpreted by considering geological cross sections gravity anomaly and direct current dc schlumberger vertical electrical sounding ves data the 2 d models of geoelectric structures obtained from these inversions clearly display the existence of three electrically conductive 4 6 m zones z1 z2 and sz the origin of remarkably good conductivity within the two large zones z1 and z2 located from the depth of about 5 km to depths of more than 15 km and the existence of a smaller zone sz at between 4 7 km are explained by the circulation of hydrothermal fluids with low resistivity values 0 28 0 52 m and by the effects of a strong hydrothermal alteration in the rocks a low magnetic anomaly with low intensity of about 50 under the middle of the goynuk area is ascribed to the hydrothermal demagnetisation of the rocks due to geothermal activity the sharp decrease in resistivity to very low values probably suggests that both zones z1 and z2 are highly permeable porosity about 30 and therefore saturated with underground geothermal fluids the upper zone sz may act as a corridor for hydrothermal circulation between both of the zones z1 and z2 and it is a prime target for further geothermal exploration

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

subduction and exhumation of continental crust insights from laboratory models summary when slivers of continental crust and sediment overlying oceanic lithosphere enter a subduction zone they may be scraped off at shallow levels subducted to depths of up to 100 200 km and then exhumed as high pressure hp and ultra high pressure uhp rocks or subducted and recycled in the mantle to investigate the factors that influence the behaviour of subducting slivers of continental material we use 3 d dynamically consistent laboratory models a laboratory analogue of a slab upper mantle system is set up with two linearly viscous layers of silicone putty and glucose syrup in a tank a sliver of continental material also composed of silicone putty overlies the subducting lithosphere separated by a syrup detachment the density of the sliver viscosity of the detachment geometry of the subducting system attached plate versus free ridge and dimensions of the sliver are varied in 34 experiments by varying the density of the sliver and viscosity of the detachment we can reproduce a range of sliver behaviour including subduction subduction and exhumation from various depths and offscraping sliver subduction and exhumation requires sufficient sliver buoyancy and a detachment that is strong enough to hold the sliver during initial subduction but weak enough to allow adequate sliver displacement or detachment for exhumation changes to the system geometry alter the slab dip subduction velocity pattern of mantle flow and amount of rollback shallower slab dips with more trench rollback produce a mantle flow pattern that aids exhumation steeper slab dips allow more buoyancy force to be directed in the up dip direction of the plane of the plate and aide exhumation of subducted slivers slower subduction can also aide exhumation but if slab dip is too steep or subduction too slow the sliver will subduct to only shallow levels and not exhume smaller slivers are most easily subducted and exhumed and influenced by the mantle flow

slab breakoff a model for syncollisional magmatism and tectonics in the alps slab breakoff is the buoyancy driven detachment of subducted oceanic lithosphere from the light continental lithosphere that follows it during continental collision in a recent paper davies and von blanckenburg 1994 have assessed the physical conditions leading to breakoff by quantitative thermomechanical modeling and have predicted various consequences in the evolution of mountain belts breakoff will lead to heating of the overriding lithospheric mantle by upwelling asthenosphere melting of its enriched layers and thus to bimodal magmatism breakoff will also lead to thermal weakening of the subducted crustal lithosphere thereby allowing buoyant rise of released crustal slices from mantle depths in this paper we present a test of this model in the tertiary evolution of the european alps in the alps both basaltic and granitoid magmatism occur between 42 and 25 ma following the closure of oceanic basins by subduction and continental collision the granitoids are now well established to result from mixing of basalt with assimilated continental crust to identify the tectonically crucial origin of the partial mantle melts we have compiled all published geochemical and isotopic data of numerous mafic dykes occurring throughout the whole alpine arc their trace element and isotopic composition suggests that they have been formed by low degree melting of the mechanically stable lithospheric mantle we see no evidence for melting of asthenospheric mantle it was thus not decompressed to depths shallower than 50 km once initiated rapid lateral migration of slab breakoff will result in a linear trace of magmatism in locally thermal weakened crust this explains why all alpine magmatic rocks intruded almost synchronously along a strike slip fault the periadriatic lineament a compilation of ages from penninic high pressure rocks subducted to depths of up to 100 km shows that subduction took place at circa 55 40 ma followed by uplift at 40 35 ma from the short time interval between their uplift and the onset of magmatism we infer that both processes have been induced by the breakoff the slab breakoff model fulfills its predictions in the case of the alps and therefore supports the assumptions made in the theoretical model on a geological basis we believe that the characteristic association of magmatic activity with the return of high pressure rocks to the surface allows the identification of this process in the earth s mountain belts

late stage deformation in a collisional orogen western alps nappe refolding back thrusting or normal faulting nappe refolding back thrusting and normal faulting frequently cause severe late stage overprinting of the architecture of an orogen a combined investigation of nappe stack polarity kinematics of shearing and metamorphic gradients in the western alps develops criteria for distinguishing between these three modes of late stage deformation this distinction is a prerequisite for any retro deformation necessary for understanding the main tectonic and metamorphic evolution of collisional orogens in the case of the western alps overprint was by mega scale nappe refolding in the oligocene this implies exhumation of the hp rocks prior to postnappe folding i e during nappe stacking and by foreland directed ascent within a subduction channel

electrical conductivity structure of the purcell anticlinorium in southeast british columbia and northwest montana magnetotelluric data from almost 200 sites were acquired by a commercial contractor over the precambrian purcell anticlinorium west of the rocky mountains of canada and the united states fifteen east west profiles cross the anticlinorium between latitudes of 48 and 49 5 n and provide a grid suitable for a regional three dimensional study of the electrical structure of predominantly the upper crust the data show essentially a resistive uppermost crust varying from 2 to 6 km in thickness over a strongly conductive widespread electrical basement the general electrical strike of this conductive basement is found to be n30 w which concurs with the surface geological trend of the region the data from all profiles were inverted one dimensionally and from two of the profiles two dimensionally using different algorithms to test the accuracy of the one dimensional images the main features found are i the sediments in the upper and middle belt purcell strata are in general more conducting than thos

40ar 39ar laser probe dating by step heating and spot fusion of phengites from the dora maira nappe of the western alps italy 40ar 39ar laser probe dating of phengitic micas has been carried out by step heating and spot fusion procedures these micas represent successive deformation stages in the structural evolution of the internal dora maira nappe western alps single phengites from a gneiss affected by a single ductile strain under retrogressive conditions sample 99 1 display complete isotopic resetting with nearly homogeneous intracrystalline ar distribution and yield plateau ages of about 40 ma small clusters of phengites from an earlier foliation were selected from a polydeformed mica schist sample ptx3 they show a partial isotopic resetting in response to overprinting during the retrogressive deformation stage with a concentric age zoning from 68 ma on the rim to 87 ma in the core on one cleavage plane this zonation is fully consistent with the laser derived discordant age spectrum which ranges from 40 to 90 ma from low to high temperatures according to the deformation history of both samples these preliminary data suggest a deformation control on ar migration during recrystallization processes and they are consistent with the timing of the collisional evolution previously reported for southern dora maira units this study shows that the 40ar 39 ar continuous laser probe dating technique produces data accurate enough to discriminate several tectonometamorphic episodes recorded in single hand samples

on two dimensional interpretation of magnetotelluric soundings summary the paper presents the views of the 2 d interpretation of magnetotelluric mt data that are characteristic of the russian magnetotelluric school discussing the strategy of 2 d interpretation of mt data we have to answer three questions in decreasing order of importance these questions are as follows 1 which field mode is more sensitive to the near surface and deep structures that are the targets of mt surveys 2 which field mode is more robust to the 3 d evects caused by real geological bodies 3 which field mode is more susceptible to the static shift induced by nearsurface inhomogeneities we examine the transverse magnetic tm and transverse electric te modes of the 2 d magnetotelluric field and show that they satisfy the principle of information complementarity 1 while the tm mode is more sensitive to the near surface structures the te mode may be more sensitive to the deep structures 2 while the tm mode is more robust to the 3 d evects caused by conductive structures the te mode may be more robust to the 3 d evects caused by resistive structures and 3 while the tm mode is more susceptible to the static shift the te mode may be almost undistorted thus the gaps left by one mode can be filled by another mode if so the most comprehensive and reliable information on the conductivity of the earth s interior can be obtained using both modes i e the transverse and longitudinal mt curves the general scheme of this bimodal mt inversion is rather simple the transverse curves provide details of near surface structures e g in the sediments and allow one to evaluate the lithosphere resistance and outline the deep conductive faults while the longitudinal curves help one to detect the conductive zones in deep layers of the lithosphere and in the asthenosphere an eycient two level algorithm for the bimodal mt inversion realizing this scheme is suggested as an illustration the paper presents the geoelectrical model of the kirghiz tien shan constructed by means of the bimodal mt inversion

exhumation of ultrahigh pressure metamorphic oceanic crust from lago di cignana piemontese zone western alps the structural record in metabasites the metamorphic and deformational history of coesite bearing ultrahigh pressure metamorphic oceanic crust from lago di cignana valtoumanche western alps as recorded along the decompression and cooling path has been resolved to derive constraints on the physical state of the crust during exhumation the following history is proposed eclogites derived from pillow basalts were deformed by dislocation creep of pyroxene at a depth of more than 80 km in the stability field of coesite the deformation was progressively localised into shear bands over the first 40 km of subsequent exhumation the rocks remained essentially unmodified any deformation must have been localised beyond the limits of the present outcrop retrogression first in a closed system started without concomitant deformation at a depth of about 40 km and temperatures of 550 to 500 c subsequently the system size increased fluid infiltration focussed along tensile fractures is documented by the formation of veins vein shapes and orientations indicate high pore fluid pressures and low differential stress after cooling to about 350 450 c at depths of 12 to 18 km localised ductile deformation transformed the retrogressed eclogites into greenschists for that stage the microstructure of the quartz veins indicates deformation by dislocation creep at falling temperatures and increasing differential stress final cooling to below ca 300 c took place at depths of 7 to 12 km as indicated by the density of fluid inclusions later deformation in the brittle field cannot be correlated with the p t path

methods used in the u s s r to reduce near surface inhomogeneity effects on deep magnetotelluric sounding abstract the effects of near surface inhomogeneities on deep magnetotelluric sounding are considered methods are suggested allowing for and ruling out the near surface effects examples for practical application of these methods are given

distortion of magnetotelluric sounding curves by three dimensional structures distortions of magnetotelluric fields caused by three dimensional 3 d structures can be severe and are not predictable using one dimensional or two dimensional modeling i used a 3 d modeling algorithm based upon an extension of a generalized thin sheet analysis due to ranganayaki and madden 1980 to examine field distortions in crustal environments three major physical mechanisms cause these distortions these mechanisms are resistive coupling between the electrical mantle and upper crust resistive coupling between conductive features within the upper crust and local induction of current loops within good conductors each mechanism produces different spatial and frequency effects upon the background field so identification of the dominant mechanism can be used as an interpretational aid i finally use this analysis to identify distortion mechanisms seen in magnetotelluric data from beowawe nevada to aid in an interpretation of that area

geochemistry and 40ar 39ar geochronology of pseudotachylyte associated with uhp whiteschists from the dora maira massif italy abstract in situ uv laser ablation 40 ar 39 ar geochronological and geochemical data together with rock and mineral compositional data have been determined from pseudotachylyte and surrounding mylonitic gneiss associated with the uhp whiteschists of the dora maira massif italy several generations of fresh pseudotachylyte occur as irregular veins up to a few cm thick both parallel and at high angles to the foliation whole rock xrf data collected from representative lithologies of mylonitic gneiss are uniformly consistent with a mildly alkalic granitic protolith minimal compositional variation is observed between the pseudotachylyte and its surrounding mylonitic gneiss the pseudotachylyte contains newly crystallized grains of biotite and k feldspar in a matrix of glass with partially fused grains of quartz zircon apatite and titanite electron microprobe analyses of the glass show significant compositional variation that is probably strongly influenced by micrometer scale changes in mineralogy uv laser ablation icp ms traverses across the mylonitic gneiss pseudotachylyte contact are consistent with cataclastic communition of ree carriers such as epidote monazite allanite zircon and apatite before melting as an efficient mechanism of ree homogenization in the pseudotachylyte the 40 ar 39 ar data from one band of pseudotachylyte indicate formation at 20 1 0 5 ma when the mylonitic gneisses were already in a near surface position the variable effects of top to the west shear deformation within outcrops of the coesite bearing unit are reflected in localized zones of protomylonite cataclasite ultracataclasite and pseudotachylyte preservation of several generations of pseudotachylyte suggests that seismic events may have played a significant role in triggering late unroofing of the uhp rocks it is speculated that deeper crustal seismic events potentially played a role in the unroofing of the uhp rocks at earlier stages in their exhumation history

u pb zircon rb sr and sm nd geochronology of high to very high pressure meta acidic rocks from the western alps three meta acidic rocks from the western italian alps a magnesiochloritoid bearing metapelite from the monte rosa massif a coesite pyrope quartzite from the dora maira massif and the monte mucrone granite in the sesia zone have been studied by u pb zircon rb sr on whole rock apatite and phengite and sm nd wholerock methods the mineral parageneses of the investigated rocks indicate high to very high pressure and medium to high temperature metamorphism this combined isotopic study has enabled us to constrain the ages of magmatic and metamorphic events and also to compare the behaviour of u pb zircon systems in three intensely metamorphosed areas of the pennine domain the u pb zircon data have yielded a magmatic age for the monte mucrone granite at 286 2 ma this result confirms the occurence of late hercynian magmatism in the sesia zone as in other austro alpine units and in other areas of the european crystalline basement in the monte rosa massif a geologically meaningless lower intercept age of 192 2 ma has been interpreted as an artefact due to a complex evolution of the u pb zircon system the magmatic shape of the zircons implies a magmatic or volcano sedimentary protolith for this rock originally considered as a metasediment the very high pressure metamorphism in the dora maira quartzite has produced an opening of the u pb zircon system at 121 12 29 ma the rb sr data support the occurence of high grade metamorphism during cretaceous times phengites model ages are slightly younger than the u pb zircon lower intercept ages due to cooling phenomena or possible response of the phengites to a later deformation the nd model ages from the whole rock samples as well as the u pb upper intercept ages from zircons of all three investigated rocks indicate the presence of proterozoic crustal components inherited from the precursors of these meta acidic rocks the studied zircon populations and their u pb systems apparently showed open system behaviour only when affected by extreme metamorphic conditions 700 750 c 28 kbar whereas eclogite facies conditions of 500 550 c and 14 16 kbar were not enough to disturb significantly the u pb zircon evolution it is also probable that the sedimentary or magmatic origin of the protoliths of these meta acidic rocks which involved different characteristics such as grain size and fluid phase concentration and composition could be another important factor controlling the u pb zircon system behaviour during metamorphic events

coeval plutonism and metamorphism in a latest oligocene metamorphic core complex in northwest turkey a metamorphic core complex of latest oligocene age crops out in the kazdag mountain range in northwest turkey the footwall of the core complex consists of gneiss amphibolite and marble metamorphosed at 5 1 kbar and 640 50 c the average muscovite and biotite rb sr ages from the gneisses are 19 ma and 22 ma respectively and imply high temperature metamorphism during latest oligocene times the hangingwall is made up of an unmetamorphosed lower tertiary oceanic accretionary melange with upper cretaceous eclogite lenses the hangingwall and footwall are separated by an extensional ductile shear zone two kilometres thick mylonites and underlying high grade metamorphic rocks show a n trending mineral lineation with the structural fabrics indicat ing down dip top to the north shear sense the shear zone the accretionary melange and the high grade metamorphic rocks are cut by an undeformed granitoid with a 21 ma rb sr biotite age analytically indistinguishable from the rb sr biotite ages in the surrounding footwall gneisses the estimated pressure of the metamorphism and that of the granitoid emplacement indicate that the high grade metamorphic rocks were rapidly exhumed at 24 ma from a depth of 14 km to 7 km by activity along the shear zone the subsequent exhumation of the metamorphic rocks to the surface occurred during pliocene quaternary times in a transpressive ridge between two overstepping fault segments of the north anatolian fault zone the high grade metamorphic rocks of the kazdag range are surrounded by voluminous calc alkaline volcanic and plutonic rocks of late oligocene early miocene age which formed above the northward dipping hellenic subduction zone the magmatic arc setting of the core complex and stratigraphic evidence for subdued topography in northwest turkey prior to the onset of extension suggest that the latest oligocene regional extension was primarily related to the roll back of the subduction zone rather than to the gravitational collapse

ultra high pressure uhp terrains lessons from thermal modeling the thermal history of subducted exhumed crustal fragments is explored to depths of 125 km using a forward modeling approach the finite element code uses an adaptive gridding technique that allows deformation and differential movement of subgrids to simulate tectonic mass flow the geometry and velocity of plate convergence and the large scale strain partitioning for example subduction channel mobile tectonic fragments are described explicitly as functions of time the evolution of metamorphic conditions during descent and subsequent exhumation of a terrane within the subduction channel is computed for different kinematic scenarios these include a range of velocities relative to a stable continental block for the subducting slab the exhuming uhp fragment and a subduction channel of given width though essentially generic in emphasis our models are designed to help understand real uhp occurrences most especially their exhumation history boundary conditions and the geometric set up were chosen according to three case studies of known uhp terranes eastern china western norway western alps simulations indicate that the temperature reached at a given depth during the subduction stage is crucially dependent on the convergence rate width of the subduction channel location of the fragment within the subduction channel and on rates of shear heating and convection of the subcontinental mantle the p t t path experienced by a tectonic fragment during its exhumation from pmax is determined chiefly by its size thickness and exhumation velocity the rate of ongoing subduction is less relevant given the rapid exhumation required to preserve uhp assemblages the depth at which tmax is reached by an exhuming uhp fragment and the dp dt slope of its exhumation path may critically affect the extent of irreversible phase transitions during decompression comparing data for the three uhp case studies with corresponding simulation results indicates general agreement of the p t paths predicted t t evolution paths are also compatible with well constrained mineral chronometry from the study areas this agreement between field data and simulation results leads to a number of observables that may be useful in relating uhp occurrences to their evolution for example no metamorphic field gradients are expected for fragments of less than 5 km thickness and the spread in mineral ages reflecting the high pressure stage is predicted to be less than 1 ma therefore observed variations in metamorphic conditions for the stage near tmax and p tmax may reflect real differences of evolution in very large fragments only and the same is valid for a substantial spread in metamorphic age data pertaining to stages near pmax or tmax tectonic extrusion of fragments along a subduction channel is not the only mechanism proposed to explain uhp occurrences however the broad agreement between several test cases and the present simulation results lend support to the viability of the kinematic scenario adopted where field evidence indicates a similar emplacement mechanism and allows the initial size of the uhp fragment to be inferred the melonpit model provides a powerful integrative tool to produce a quantitative picture of the dynamics of uhp terranes

ductile extrusion of the higher himalayan crystalline in bhutan evidence from quartz microfabrics quartz textures measured from deformed quartz tectonites within the lesser himalaya and higher himalaya crystalline of bhutan show similar patterns orientation and distribution of the quartz crystallographic axes were used to confirm the regional shear sense the asymmetry of c axis and a axis patterns consistently indicates top to the south shearing the obliquity of the texture and the inferred finite strain plane strain to moderately constrictional suggest the strain regime had a combination of rotational and irrotational strain path in most of the samples from the bhutan himalaya the inferred deformation mechanisms suggest moderate to high temperature conditions of deformation that produced the observed crystallographic preferred orientation much higher temperature of deformation is indicated in the quartz veins from a leucogranite the observed ductile deformation is pervasively developed in the rocks throughout the investigated area the intensity of deformation increases only slightly in the vicinity of the main central thrust simultaneous southward shearing within a large part of the higher himalaya crystalline near and above the main central thrust and normal faulting across the south tibetan detachment is explained by the tectonically induced extrusion of a ductily deforming wedge the process of extrusive flow suggested here can be approximated quantitatively by channel flow models that have been used to describe subduction zone processes channel flow accounts for some observed phenomena in the himalayan orogen such as inverted metamorphic sequences near the main central thrust not related to an inversion of isotherms and the syntectonic emplacement of leucogranites into the extruding wedge locally leading to an inversion of isotherms due to heat advection

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

photometry and polarimetry of jupiter at large phase angles ii polarimetry of the south tropical zone south equatorial belt and the polar regions from the pioneer 10 and 11 missions abstract the imaging photopolarimeter ipp experiment on the pioneer 10 and 11 missions to jupiter measured the intensity and linear polarization of red and blue sunlight reflected from the planet over a range of phase angles inaccessible from the earth we give an overview of the polarization data obtained in the two jupiter encounters at phase angles from 43 to 117 and briefly describe the photometry data from the pioneer 11 encounter at phase angles between 34 and 80 which partially fill a gap in the phase coverage from pioneer 10 m g tomasko r a west and n d castillo 1978 icarus 33 558 592 the polarimetry and photometry of the south tropical zone strz the north component of the south equatorial belt sebn and a north south cut extending to the south pole are given in detailed tables comparison of the data to multiple scattering models yields several details of the distribution and single scattering properties of the clouds and aerosols on jupiter the observed polarization in blue light at latitudes less than about 40 shows only small variations between belts and zones simple models indicate that the tops of the belt and zone clouds are reached at nearly the same pressure level of about 320 mb and that the polarization differences are a result of the lower cloud albedo in the belt the optical thickness of the belt as well as the zone clouds at this level must be at least 1 5 to prevent the polarization produced by underlying gas from being seen in the data the polarization rises abruptly toward the limb and terminator in red light indicating a haze of positively polarizing particles with an optical thickness of a few tenths at a pressure level of about 120 mb the polarization in both colors increases abruptly from latitudes north of 40 n and south of 48 s to values as high as 60 at high latitudes this effect is not due to a longer slant path but must be due to a large increase in the optical thickness of the polarizing haze at high latitudes there is some indication that the size of the haze aerosols grows with increasing latitude as well the photometry data indicate little change in the brightness of planetary features in the year between the two pioneer encounters photometric models that fit the pioneer 10 data fit the pioneer 11 data remarkably well with essentially the same phase functions using a two cloud model we find that our models best fit the limb darkening at 12 phase when the belt absorbers are evenly distributed in both the clouds there is no evidence for rainbow like bumps on the single scattering phase functions in the range of scattering angles from 120 to 140 as might result from scattering by spherical particles

thrusting and extension in the southern dora maira ultra high pressure massif western alps view from below the coesite bearing unit the dora maira ultra high pressure uhp unit exposed in the internal western alps is a 1 km thick continental slice sandwiched between lower pressure rock units the contact at the base of the uhp unit is a major contractional shear zone that thrust the uhp unit and the overlying continental and oceanic units onto the sanfront pinerolo unit the latter metamorphosed under blueschist greenschist conditions is the lowest structural unit exposed in the western alps our analysis indicates that all rock units of the dora maira massif were sheared and juxtaposed together within a top to the sw large scale medium temperature low pressure ductile shear zone in the middle crust thrust faulting was accompanied and followed by vertical thinning that progressively reduced the thickness of the orogenic wedge and brought the ultra high pressure uhp unit into the brittle crust contractional tectonic contacts in the southern dora maira were concealed by late extensional structures the majority of the extensional structures postdated the emplacement of the uhp unit over the sanfront pinerolo unit implying that extension lagged behind much of the exhumation of the uhp unit extensional shear zones in the dora maira and elsewhere in the western alps are parallel to the strike of the orogen their overall effect was to widen the orogenic belt in the north east south west direction the sense of extensional movement was toward the frontal part of the orogen so that rather than defining tectonic wedges formed during extrusion the extensional structures represent lateral spreading of the mid to upper crust of the internal western alps toward the foreland rapid compression of the dora maira was coeval with the transition from marine flysch deposits to molasse sedimentation in the foreland of the western alps the switchover from burial to exhumation and the lateral spreading of the upper crust in the internal western alps were coupled by readjustment of the entire orogenic wedge

two dimensional interpretation of three dimensional magnetotelluric data an example of limitations and resolution summary interpretation of magnetotelluric mt data for three dimensional 3 d regional conductivity structures remains uncommon and two dimensional 2 d models are often considered an adequate approach in this paper we examine 2 d interpretation of 3 d data by considering the synthetic responses of a 3 d structure chosen specifically to highlight the advantages and limitations of 2 d interpretation 2 d models were obtained from inversion of the synthetic 3 d data set with different conditions noise and distortion applied to the data we demonstrate the importance of understanding galvanic distortion of the data and show how 2 d inversion is improved when the regional data are corrected prior to modelling when the 3 d conductive structure is located below the profile the models obtained suggest that the effects of finite strike are not significant if the structure has a strike extent greater than about one half of a skin depth in this case the use of only tm mode data determined better the horizontal extent of the 3 d anomaly when the profiles are located away from the 3 d conductive structure the use of only tm mode data can imagine phantom conductive structures below the profile in this case the use of both polarizations produced a better determination of the subsurface structures it is important to note that the main structures are identified in all the cases considered here although in some cases the large data misfit would cause scepticism about features of the models

slab breakoff a model of lithosphere detachment and its test in the magmatism and deformation of collisional orogens we present a model proposing that oceanic lithosphere detaches from continental lithosphere during continental collision slab breakoff allowing an explanation of syn to post collisional magmatism and metamorphism continental collisions are preceded by subduction of dense oceanic lithosphere and followed by attempted subduction of buoyant continental lithosphere this situation of opposing buoyancy forces leads to extensional deformation in the subducted slab a narrow rifting mode of deformation will result if there is strain localization slab breakoff results we have assessed the plausiblity of this process by quantitatively evaluating an upper bound for the strength of the lithosphere and have compared it with the change in buoyancy force during continental sbduction whether breakoff will occur and the depth at which it will occur is a strong function of temperature and hence the subduction velocity for a subduction velocity of 1 cm yr breakoff could occur at depths of between 50 and 120 km while at higher velocities it is still likely to occur but at deeper depths as a result of the rifting during breakoff the asthenosphere upwells into the narrow rift and following breakoff it impinges on the mechanical lithosphere of the overriding plate the resulting conducted thermal perturbation leads to melting of the metasomatised overriding mantle lithosphere producing basaltic magmatism that leads to granitic magmatism in the crust dry asthenospheric mantle will melt only if breakoff occurs at a depth shallower than ca 50 km breakoff removes the force at the downdip side of the continental crust while the enhanced heating leads to a reduction of the strength of the underlying crust both effects facilitate the freeing of buoyant crustal sheets which can then rise towards the surface leading to the rapid exhumation of eclogite facies continental crust the cessation of subduction and replacement of the cold oceanic lithosphere by asthenosphere leads to rapid uplift of the orogen we have tested the variety of predicted expressions of slab breakoff in the geological record of the alps the aegean islands and the dabie shan a comparison of the various belts highlights 1 that the magmatism and metamorphism are found near the suture and the centre of the orogen demonstrating the general steepening of suture during collision 2 that a vague inverse correlation exists between maximum depth of metamorphism and volume of syn orogenic magmatism and 3 that the melts can be emplaced in both compressive and extensional environments we suggest that slab breakoff is an important step in the evolution of many orogenic belts and it allows an explanation for the combined presence in collisional orogens of magmatism with a mantle parentage and ultra high pressure metamorphics

three dimensional elastic wave velocity structure of the western and central tien shan arrival times of compressional p and shear s waves generated by earthquakes at local and teleseismic distances and recorded by seismographs located in the western and central tien shan are used to determine one and three dimensional elastic wave velocity structures of the crust and upper mantle beneath the mountain belt the best fit one dimensional structures suggest that the average depth of the mohorovicic discontinuity in this area is 50 km the three dimensional structure of the upper crust reveals thick sediments within each of the major depressions in the region a 7 km thick wedge of sediment beneath the chu depression is outlined at depth by a south dipping plane of seismic activity suggesting the presence of an active decollemont these low velocities extend continuously to the southeast toward issyk kul suggesting a structural relationship between the two however rather than being consumed it appears that issyk kul is overthrusting the surrounding ranges the low velocity sediments in the fergana basin reach depths of 10 km and are bounded on three sides by amorphous bands of seismicity velocities at midcrustal depths generally are lower beneath the central tien shan than beneath the western tien shan this pattern becomes more evident in the uppermost mantle with p velocity contrasts of as much as 10 across a boundary that corresponds roughly to the geographical position of the talasso fergana fault the low velocities beneath the central tien shan exceed 150 km depth but do not appear to be deeper than 300 km depth there is no evidence for a lithospheric root beneath this part of the range rather the low velocities imply the presence of a positive buoyancy force uplifting the mountains evidence that this low velocity region existed before the collision suggests that the tien shan may not owe its rejuvenation simply to its location at the northern edge of a strong tarim basin but rather to an anomalous upper mantle that was easier to deform than the surrounding lithosphere

coronitic reactions and ductile shear zones in eclogitised ophiolite metagabbro western alps north italy abstract the objective of this work is to study the most significant static and syn tectonic transformations which accompanied the eclogite metamorphism 450 50 c 12 15 kbar of an ophiolite metagabbro from the western alps in partly undeformed volumes abundant igneous mineral relics occur and the eclogite metamorphism produced coronitic or incomplete pseudomorphic reactions the disequilibrium or local equilibrium conditions implied by these features suggest that slow diffusion and interface kinetics were the rate limiting factors which prevented a large scale eclogite equilibration the undeformed volumes are bounded by syn eclogite ductile shear zones where the magmatic minerals and textures are obliterated and a new fabric foliation develops the enlargement of equilibrium domains observed in the shear zones indicates that a close approach to eclogite equilibration of these metagabbros was not attained without the catalytic effect of pervasive deformation the bulk rock chemistry also played an important role in determining the kind of eclogite assemblage in particular only sheared gabbros rich in fe can produce bimineralic eclogites

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

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

slow and fast deformation in the dora maira massif italian alps pseudotachylytes and inferences on exhumation history abstract the dora maira massif dm western alps is a type example of an ultra high pressure uhp metamorphic terrain the occurrence of pseudotachylytes within the val gilba area of dm led to speculation regarding the role of seismic slip in the exhumation of the massif in this study new field microstructural and geochemical data on the pseudotachylyte veins and their mylonitic gneiss host are presented the formation of pseudotachylyte veins occurred late in the exhumation history at 20 1 0 5 ma and at shallow depths

effects of three dimensional structure on magnetotelluric sounding curves significant errors may result when applying one dimensional 1 d or two dimensional 2 d interpretation methods to magnetotelluric mt data collected in three dimensional 3 d environments both depths and resistivities can be grossly incorrect if 1 d or 2 d methods are applied in a 3 d setting we present examples of mt sounding curves generated using a 3 d modeling program madden and park 1982 and illustrate some interpretation pitfalls if 3 d effects are not considered the 3 d effects discussed herein are attributed to a surface layer heterogeneity and can be readily identified in mt data from a well designed mt survey the mt survey must include stations chosen to yield information about regional structure alternatively carefull examination of a geologic map will help the intepreter estimate the regional effects present in the data

femg partitioning between coexisting garnet and phengite at high pressure and comments on a garnet phengite geothermometer abstract the fe mg exchange reaction between coexisting garnet and phengite has been studied by reacting a natural phengite mg 67 in the presence of quartz and water at pressures of 20 35 kb and temperatures of 800 1000 c compositions of coexisting garnet and mica indicate a linear relation between both the inkd fe mg garnet fe mg phengite and temperature and inkd and pressure in the above p t range this fe mg exchange reaction between garnet and phengite is shown to be dependent on the ca content of the garnet and on the mg number of the bulk composition these two composition effects have been studied by usin phengitic mica mixes with mg numbers of 23 and 46 and by using a synthetic basaltic composition the overall results allow broad empirical calibration of separate geothermometers for pelitic and basaltic systems respectively however because of non ideality in the exchange reaction this geothermometer should not be used in any practical application outside the composition ranges studied also careful consideration of the presence of fe3 in phengite must be made if the fe3 content of the natural phengite is unknown then the temperatures obtained will be maximum temperatures only

buoyancy driven rapid exhumation of ultrahigh pressure metamorphosed continental crust abstract preservation of ultrahigh pressure uhp minerals formed at depths of 90 125 km require unusual conditions our subduction model involves underflow of a salient 250 150 km wide 90 125 km long of continental crust embedded in cold largely oceanic crust capped lithosphere loss of leading portions of the high density oceanic lithosphere by slab break off as increasing volumes of microcontinental material enter the subduction zone buoyancy driven return toward midcrustal levels of a thin 2 15 km thick low density slice finally uplift backfolding normal faulting and exposure of the uhp terrane sustained over 20 million years rapid 5 mm year exhumation of the thin aspect ratio uhp sialic sheet caught between cooler hanging wall plate and refrigerating downgoing lithosphere allows withdrawal of heat along both its upper and lower surfaces the intracratonal position of most uhp complexes reflects consumption of an intervening ocean basin and introduction of a sialic promontory into the subduction zone uhp metamorphic terranes consist chiefly of transformed yet relatively low density continental crust compared with displaced mantle material otherwise such complexes could not return to shallow depths relatively rare metabasaltic metagabbroic and metacherty lithologies retain traces of phases characteristic of uhp conditions because they are massive virtually impervious to fluids and nearly anhydrous in contrast h2o rich quartzofeldspathic gneissose schistose more permeable metasedimentary and metagranitic units have backreacted thoroughly so coesite and other uhp silicates are exceedingly rare because of the initial presence of biogenic carbon and its especially sluggish transformation rate uhp paragneisses contain the most abundantly preserved crustal diamonds

hydrogeochemical outline of thermal waters and geothermometry applications in anatolia turkey abstract the chemical compositions of a total of 120 thermal water samples from four different tectonically distinct regions central north east and west anatolia of turkey are presented and assessed in terms of geothermal energy potential of each region through the use of chemical geothermometers na ca hco3 type waters are the dominant water types in all the regions except that na cl type waters are typical for the coastal areas of west anatolia and for a few inland areas of west and central anatolia where deep water circulation exists the discharge temperature of the springs ranges up to 100 c and the bottom hole temperatures in drilled wells up to 232 c geothermometry applications yield reservoir temperatures of about 125 c for central anatolia 110 c for north anatolia 136 c for east anatolia and 251 c for west anatolia the latter agreeing with some of the bottom hole temperatures measured in drilled wells the results reveal that the highest geothermal energy potential in turkey is associated with the west anatolian extensional tectonics which provides a regional deep seated heat source and a widespread graben system allowing deep circulation of waters the north anatolian region bounded to the south by the dextral north anatolian fault along which most of the geothermal sites are located has the lowest energy potential probably due to the restriction of the heat source to local magmatic activities confined to pull apart basins the east anatolian region undergoing contemporary compression and the central anatolian region where the compressional regime in the east is converted to the extensional regime in the west have moderate energy potential although the recently active volcanoes suggest the presence at depth of still cooling magma chambers that are potential heat sources the lack of well developed fault systems is probably responsible for the comparatively low energy potential of these regions almost all the thermal waters of turkey are saturated with respect to calcite and hence have a significant calcite scaling potential which is particularly high for west anatolian waters

dating deformation in the gran paradiso massif nw italian alps implications for the exhumation of high pressure rocks in a collisional belt the gran paradiso massif situated in the internal part of the western italian alps records a complex tectono metamorphic history involving high pressure metamorphism and subsequent exhumation dur

a model of palaeozoic obduction and exhumation of high pressure low temperature rocks in the southern urals abstract a new evolutionary geodynamic model of devonian continental subduction and exhumation of high pressure low temperature metamorphic rocks in the southern urals is proposed based on the results of physical modelling geological and geophysical data and on the comparison of this belt with other orogens the model includes the following principal phases 1 closure of the western uralian ocean by eastward intraoceanic subduction associated with the magnitogorsk volcanic arc 2 subduction of the european continental margin causing failure of the overriding plate in the arc area along a fault dipping eastward under the arc 3 subduction into the mantle of the fore arc block together with the underlying continental crust and sedimentary layer the sedimentary cover as well as the crustal and mantle fragments of this block are scraped off and accreted in front of and under the arc the continental crust shielded from the hot mantle by the fore arc block subducts to depths of more than 150 km remaining at a relatively low temperature the crust then fails at depths of several tens of kilometres 4 the subducted crustal slice starts to rapidly rise and intrude the interplate zone scraping and pushing up the sediments and slivers of the oceanic fore arc block previously dragged down to different depths 5 exhumation of the rising high pressure low temperature rocks within the uraltau dome separating the subduction accretionary complex from the magnitogorsk arc the eastern border of the uraltau dome coincides with the main uralian fault this fault represents a complex suture zone which at the stage of deep crustal subduction was associated with a major thrust subduction fault and at the exhumation stage with a major normal fault corresponding to the upper surface of the rising crustal sedimentary body

relation between electrical resistivity and earthquake generation in the crust of west anatolia turkey abstract in this paper we present a relation between the earthquake occurrence and electric resistivity structures in the crust in west anatolia and the thrace region of turkey the relationship between magnetotelluric georesistivity models and crustal earthquakes in west anatolia during a period from 1900 to 2000 is investigated it is found that most of the large crustal earthquakes occurred in and around the areas of the highest electrical resistivity in the upper crust although rare small magnitude earthquakes are observed in some parts of the conductive lower crust in west anatolian extensional terrain the high resistivity zones may represent rocks that are probably mechanically strong enough to permit sufficient stress to accumulate for earthquakes to occur in western anatolia and the thrace region however some recent studies state that the generation of a large earthquake is not only a pure mechanical process but is closely related to fluid existence we also reviewed recent world wide researches including results from the anatolian data for the first time and discussed all general findings in combination our findings show that the boundary between the resistive upper crust and the conductive lower crust correlates well with the cutout depth of the seismicity in west anatolia and thrace this boundary is also attributed to the fluid bearing brittle ductile transition zone in world literature fluid migration from the conductive lower crust to the resistive upper crust may contribute the seismicity in resistive zones alternatively the upper lower crust boundary may act as a stress concentrator and fluids may help to release strain energy in brittle parts of lower crust by small magnitude earthquakes whereas they may help in focusing strain in mechanically strong and electrically resistive zones for large earthquakes to occur

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

conditions for a crustal block to be sheared off from the subducted continental lithosphere what is an essential factor to cause features associated with collision 1 crustal blocks are detached from the subducting lithosphere at various depths i call such phenomena shearing off and investigate the mechanical conditions for shearing off to occur in the continental lithosphere the buoyancy of the crustal block does not work selectively as a driving force until it has been sheared off and i suppose that shearing off occurs when the shear traction at the thrust whose maximum value is calculated from the strength along the thrust overcomes the strength of the boundary faults of the block and determine the depth of the leading edge of the subducted crustal block when shearing off occurs the temperatures of the slab and at its surface are calculated by the steady state formula of molnar and england 1990 as a function of the surface heat flow dip angle and convergence velocity of the subducted continental plate as the shear stress and temperature at the ductile part of the thrust zone are consistent with the flow law the shear stress at the brittle part is a strong function of the pore fluid pressure ratio i show that shearing off of an upper crustal block is possible only for a small value of such as 0 4 when the surface heat flow value is 70 mw m2 i suggest that the essential factor to cause shallow offscraping of crustal blocks in collision zones is not a buoyancy of the continental crust but a low value of which would result from a little amount of dehydration from the subducted continental plate

coupling of oceanic and continental crust during eocene eclogite facies metamorphism evidence from the monte rosa nappe western alps high precision u pb geochronology of ru tile from quartz carbonate white mica rutile veins that are hosted within eclogite and schist of the monte rosa nappe western alps italy indicate that the monte rosa nappe was at eclogite facies metamorphic conditions at 42 6 0 6 ma the sample area indren glacier furgg zone dal piaz 2001 geology of the monte rosa massif historical review and personal comments smpm consists of eclogite boudins that are exposed inside a south plunging overturned synform within micaceous schist associated with the eclogite and schist are quartz carbonate white mica rutile veins that formed in tension cracks in the eclogite and along the contact between eclogite and surrounding schist intrusion of the veins at about 42 6 ma occurred at eclogite facies metamorphic conditions 480 570 c 1 3 1 4 gpa based on textural relations oxygen isotope thermometry and geothermobarometry the

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

exhumation of uhp lt rocks due to the local reduction of the interplate pressure thermo mechanical physical modelling abstract spatial distribution of uhp lt terrains suggests that their exhumation is essentially a three dimensional process that occurs only locally in specific sites along mountain belts on the other hand the continental subduction resulting in the formation of uhp lt rocks takes place along the whole belt the previously performed by the authors 2 d thermo mechanical laboratory modelling of continental subduction has shown that exhumation of deeply subducted continental crust is possible only when the effective interplate pressure p n is lower than the lithostatic pressure low compression subduction regime at the same time this modelling showed that for the deeply subducted continental crust to be preserved at low temperature at great depth the continental subduction should be accompanied by the subduction of the fore arc block or the arc plate the latter process occurs only when p n is high high compression regime to reconcile both processes we suggest that within the background of a generally high compression regime the interplate pressure can be locally reduced in some specific situations which would then allow the local exhumation of uhp lt material using physical modelling technique we investigate one of such situations that occurs when the frontal part of the overriding plate undergoes subduction induced or not extension parallel to the plate boundary with activation or formation of a strike slip transform fault oblique to the plate boundary to the interplate zone the displacement along this fault results in a local reduction of the interplate pressure at the intersection of the fault with the interplate zone this pressure reduction permits the rise of the deeply subducted low density continental crust and sediments submitted to uhp lt conditions under buoyancy force a 10 km thick slice of crust detaches at 150 km depth and moves up along the interplate zone with a starting rate of ca 3 cm yr the ascent rate reduces when the unit reaches crustal depth the deformation of this unit is small when it first detaches and starts rising but increases when reaching the base of the overriding plate the exhuming material is sheared and stretched in a narrow passage at 70 km depth therefore in real conditions the deformation is mainly recorded when the unit is submitted to hp and not uhp conditions

seasonal variations in the north south asymmetry of polarized light of jupiter abstract over an 18 year period seasonal changes in the north south asymmetry of polarization at high latitudes of jupiter have been revealed by polarimetric observations in blue light the average seasonal difference in the polarization degree between north and south is positive and equal to about 0 5 there is some relationship between seasonal variations in the observed difference and the seasonal north south asymmetry in solar radiation incident on jupiter s atmosphere there are two maxima on the observed seasonal curve falling on the jovian spring and autumn and coinciding correspondingly with positive and negative maxima of the heliographic latitude of jupiter two possible explanations are discussed seasonal changes in insolation and or time dependent magnetospheric influence on the polar events

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

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

electrical resistivity structure of the northwestern anatolia and its tectonic implications for the sakarya and bornova zones abstract the crustal structure of the northwestern anatolia was studied by magnetotelluric measurements with periods up to 500 s at 20 locations along a line line ks crossed istanbul sakarya and bornova zones to investigate deep electrical resistivity patterns that contribute to the understanding of tectonic setting of the region the results obtained from the two dimensional 2d inversion of magnetotelluric data for the modes transverse electric te and transverse magnetic tm show that the tectonic structure up to 5 km depth was comparatively complex and that the structural pattern was generally disturbed by the grabens and neogene basins of low resistivity feature the low resistivity

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

exhumation of early tertiary coesite bearing eclogites from the pakistan himalaya early tertiary coesite bearing eclogites crop out in the internal zones of the indian plate in the kaghan valley of north pakistan field and geochronological data suggest that uhp metamorphism at c 46 ma accompanied subduction of the leading edge of continental india beneath kohistan a new u pb rutile age of 44 1 1 1 ma can be interpreted as either a growth or a cooling age given the uncertainty of the closure temperature of rutile for pb but clearly documents a very short lived metamorphic peak with cooling occurring between 44 and 40 ma eclogitic assemblages are preserved in permo triassic mafic rocks in both cover and basement sequences with the best preserved eclogites hosted in the anhydrous basement the eclogite facies rocks lie on the hanging wall of a major south directed ductile thrust zone which placed basement rocks that contain coesite eclogites southward onto lower pressure cover metasediments shear criteria show that those sequences that contain eclogite facies rocks are flanked above by a zone of pervasive north directed extensional shearing mineral assemblages contained within both the thrust and extensional shear zones are consistent with them both having operated synchronously during the amphibolite to greenschist facies transition amphibole ar ar cooling history data suggest that this was at c 40 42 ma as in the alps this implies exhumation from the metamorphic peak to greenschist facies conditions within a few million years this short time scale has important implications as it is likely that both the thrust and extensional shear zones represent the late stage of a deformation continuum that commenced at uhp and brought the uhp rocks back to mid crustal levels this late eocene extensional shearing and associated exhumation of hp or uhp rocks has not previously been recorded in the pakistan himalaya the thrusts and shears were later folded by major east trending folds themselves cut by miocene aged north vergent extensional faults that currently separate the indian plate from the structurally overlying kohistan island arc

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

geometry and significance of internal windows and regional isoclinal folds in northeast saih hatat sultanate of oman saih hatat contains tiered sets of low angle structural breaks that reflect a complex stacking of sheets which requires multiple upward movement of units within an unstable orogenic wedge in a convergent setting early recumbent folds within the saih hatat window are subparallel to but cut by a major low angle contractional fault decollement that separates lower plate high pressure rocks some eclogites exposed in two windows hulw and as sifah windows from high pressure upper plate rocks containing carpholite and lawsonite parts of this decollement had been mapped or speculated upon to explain the variation in metamorphic grade across saih hatat upper plate fold nappes are controlled by the mapped decollement surface below the decollement sheath like lower plate closures formed independently in an intense zone of top to the northeast shearing eclogites occur within mafic boudins at the structurally lowest level of the retrograde lower plate shear zone this zone was responsible for the partial exhumation of the lower plate very early in the observed structural history and has a normal movement sense the two lower plate windows are essentially foliation domes reflecting fold interference between regional scale sheath like recumbent isoclines and more open upright northwest north and northeast trending fold sets the early upper plate fold nappes reflect a major top to the north or northeast movement of the rocks away from the oman margin at the same time the ophiolite was being emplaced over the top in the opposite direction top to the southwest or south south directed faulting and shearing is superimposed on the early recumbent folds in the mountain range mapped relationships and structural observations agree with bureau de recherches geologiques et minieres mapping that recognised fold nappes in this part of the oman mountains all of the mapped structures are associated with exhumation of the high pressure rocks making reconstruction of the margin during high pressure metamorphism problematic

voyager 2 imaging eclipse observations of the jovian high altitude haze abstract images of jupiter s northern hemisphere dawn and dusk limbs taken by the voyager 2 wide angle camera were analyzed to explore the nature of the high stratospheric haze layer at most latitudes the scattered intensities at 0 43 and 0 60 m wavlengths are smaller than those for a conservatively scattering molecular atmosphere due to extinction by aerosols in the upper 100 mbar region considerable latitudinal structure is evident with intensities in the latitude range 40 n to 50 n being higher at 0 60 m only than those for a conservatively scattering molecular atmosphere asymmetry in the dawn and dusk limbs is also observed at high latitude these variations are evidence for a dynamically active stratosphere and may be the most visible indicators of processes such as diabatic circulation latitudinally dependent eddy mixing or wave dissipation and breaking two kinds of aerosols models were compared with the observations in the first free parameters of the model were the particle radius constant with altitude the pressure level of the top of the haze the haze optical depth at 0 43 m and the haze scale height although the solutions are multiple the top of the haze must be in the range of a few millibars to a few tenths of a millibar the optical depth above the 100 mbar level can be either small 0 1 or large many times that value at low latitudes but is always large at high latitudes and the most frequent particle radii found were in the range between 0 01 and 0 05 m in the second type of model the haze radius and number density were prescribed by a microphysical model which has particle radius increasing with depth the free parameters were the total column density and the pressure of the haze top sensitivity studies were performed to assess the influence of particle size in the top layers and the refractive index at 0 60 m wavelength again the solutions are not unique but the most economical solution can explain the variations at latitudes less than 50 n by a haze with optical depth above the 100 mbar level near 0 1 at 0 43 m with a minimum particle radius r min of 0 05 m and with variations in haze top pressure from 7 mbar at 5 9 n to 0 5 mbar at 39 7 n producing most of the variation in limb intensity simplistic calculations of particle transport suggest that at low latitudes hydrazine mixed upward from the tens of millibars region where it is produced can account for the inferred particle number densities at high latitudes the same may be true pending the results of a realistic microphysical model

a mechanism for syn collisional rock exhumation and associated normal faulting results from physical modelling abstract properly scaled physical modelling of the subduction of continental lithosphere is performed with a three layer lithospheric model the model includes a strong upper crust a weak ductile lower crust and a strong mantle part the lithosphere is underlain by a low viscosity asthenosphere subduction is produced by a piston push force and the pull force from the mantle lithospheric layer which is slightly denser than the asthenosphere the results of the modelling are tested on the example of the himalayas in the experiments all lithospheric layers subduct into the mantle to a depth corresponding to 200 300 km in nature until the upper crustal layer fails in front of the subduction zone forming a first major thrust fault the mct in our interpretation of the himalayas underthrusting along this fault increases relief which upon reaching a height corresponding to several kilometres in nature is removed mechanically by a blade the blade is moved horizontally back and forth throughout the experiment this erosional unloading mechanism causes the previously subducted segment of the buoyant upper crust to slide upward over the weak lower crust at a rate comparable to the subduction rate the rise of the crustal slice accelerates displacement along the crustal thrust while producing a normal sense motion and exhumation of the material from great depth up to 40 50 km depending on the model parameters along the upper surface of the slice the rapid uplift of the crust then ceases and the orogenesis occurs further due to continuing underthrusting along the first major thrust at a certain stage the crust fails again producing a new major crustal thrust the mbt in the himalayas

analysis of seismicity and micro seismicity associated with the october november 2010 sampeyre swarm southwestern alps abstract the study presents a detailed analysis of the swarm that occurred in the area surrounding sampeyre piedmont italy during the period october november 2010 the crisis caught the attention of seismologists because of its intense seismic productivity that has never been observed in the past during the swarm 550 earthquakes were recorded and located by the regional seismic network of northwestern italy rsni http www distav unige it geofisica in the study area however including the micro seismicity mainly earthquakes of magnitude lower than 0 detected through an sta lta short term average long term average based algorithm the number of events goes up to approximately 3250 among all these events approximately two hundred exceeded a local magnitude m l of 1 0 but only two earthquakes exceeded m l 3 0 reaching up to m l 3 2 following a general description of the swarm evolution the study presents the results of a waveform similarity analysis and double difference relocation aimed at identifying groups of earthquakes belonging to common genetic sources a statistical analysis of the resulting earthquake families has revealed a variation of the scaling exponent b value of the gutenberg and richter size frequency model with depth in particular our results have evidenced a slightly higher b value at shallower depths where a distinct family of earthquakes was identified through seismogram cross correlation this may reflect variability in the distribution of fractures in the rock volume involved in the swarm

thermobaric structure of the franciscan complex in the pacheco pass region diablo range california examination of the thermobaric structure of jadeite glaucophane bearing franciscan rocks in the pacheco pass region of the diablo range show that they are subhorizontal and the structurally intermediate horizon has the highest metamorphic grade mineral parageneses of more than 700 metagraywackes plus 38 metagraywacke samples from up to 1200 vertical feet of drill core were examined modal compositions of about 100 metagraywackes were determined and more than 2600 microprobe analyses were completed to quantify compositions of jadeitic pyroxene from about 80 metagraywackes the jadeite jd zone is defined by the assemblage jadeitic clinopyroxene quartz pm albite lawsonite pm sodic amphibole pm aragonite this zone occurs at an intermediate structural position sandwiched between subhorizontal jadeitic clinopyroxene free albite ab zones transition zones occur both above and below the jd zone the jd zone covers an area 20 km wide and 400 m thick the nearly flat lying boundaries betwee

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

indenter growth in analogue models of alpine type deformation a series of analogue experiments were carried out to simulate continental convergence as seen in a profile through the central alps a rigid indenter representing the adriatic plate was driven laterally into a sand pack representing the brittle upper crust of europe detached and thickening above its subducting ductile lower crust the rigid indenter advanced at the same steady rate in each experiment but the dip of its front face was steepened in 15 increments from 15 to 90 where the rigid indenter face dipped at 45 or less a sand wedge rose and was bound by a series of forekinks that nucleated at the toe of the indenter where the face of the rigid indenter dipped 60 or more the wedge was defined by a single forekink and one or more backkinks that nucleated from a point advancing in front of the indenter toe we interpret these results as indicating that slices of the sand pack and rising wedge are transferred across kink bands to build an effective indenter with a frontal dip closer to that dictated by the changing shear strength of the sand pile which thickens vertically as it shortens laterally one of our models with a rigid indenter dipping 75 simulates most of the major structures shown in recent syntheses of surface geology and deep seismic data in the central alps without the isostatic lithospheric depression this model accounts for the late collisional stage oligocene to present complex strain and metamorphic histories in the core of the orogenic wedge the rapid rise and extrusion of small pips of alpine eclogites and the current passivity of the insubric line it also emphasizes that lateral extension along gently dipping thrusts orogen normal horizontal escape is confined to the extruded portion of the rising wedge