making diagenesis obey thermodynamics and kinetics the case of quartz cementation in sandstones from offshore mid norway calculation of the quantity and distribution of quartz cement as a function of time and temperature depth in quartzose sandstones is performed using a coupled dissolution diffusional transport precipitation model this model is based on the assumptions that the source of the silica cement is quartz surfaces adjoining mica and or clay grains at stylolite interfaces within the sandstones and the quantity of silica transport into and out of the sandstone by advecting fluids is negligible integration of the coupled mass transfer transport equations over geologically relevant time frames is performed using the quasi stationary state approximation results of calculations performed using quartz dissolution rate constants and aqueous diffusion coefficients generated from laboratory data are in close agreement with both the overall porosity and the distribution of quartz cement in the middle jurassic garn formation only after optimizing the product of the effective surface area and quartz precipitation rate constants with the field data when quartz precipitation rate constants are fixed to equal corresponding dissolution rate constants the effective surface area required to match field data depends on the choice of laboratory generated quartz rate constant algorithm and ranges from 0 008 cm 1 to 0 34 cm 1 in either case these reactive surface areas are 2 to 4 orders of magnitude lower than that computed using geometric models

experimental pressure solution in halite the effect of grain interphase boundary structure to investigate the mechanisms and kinetics of pressure solution we measured deformation at contacts between polished convex lenses of halite and polished flat lenses of either halite or fused silica in saturated brine at 50 2 0 2 c and fluid pressures of 0 1 mpa loads applied using small springs ranged from 0 1 4 2 n mean effective normal stresses within the contact zone ranged from 1 14 mpa the geometry and diametric growth rate of the contact spot neck growth and the rate at which the lenses approached one another convergence were monitored during deformation using transmitted and reflected light photomicrography time dependent convergence did not occur at measurable rates when two halite lenses were pressed together in saturated brine but did occur when halite and fused silica lenses were pressed together in brine convergence rates in the halite silica experiments were 0 01 0 05 m day although the initial deformation during loading involved elastic and plastic processes control experiments without brine showed no time dependent convergence indicating that dislocation creep did not contribute to the observed rates no undercutting or cataclasis was observed in any experiments residual fluid inclusions were formed along grain boundaries between two halite lenses loaded in brine indicating non zero wetting angles

quartz microstructures developed during non steady state plastic flow at rapidly decaying stress and strain rate abstract synseismic loading to very high stresses 0 5 gpa and subsequent creep during stress relaxation in the uppermost plastosphere at temperatures of ca 300 350 c near the lower tip of an inferred once seismically active crustal scale fault was proposed based on peculiar microstructures identified in rocks exposed over 100 km 2 in the sesia zone european western alps here we discuss the conspicuous and highly heterogeneous microstructural record of quartz in disseminated small scale shear zones sub basal deformation lamellae and arrays of elongate subgrains on the tem scale indicate an early stage of glide controlled deformation at high stresses distributed brittle failure is indicated by healed microcracks very fine grained recrystallised aggregates with a pronounced crystallographic preferred orientation reflect intense plastic flow by dislocation creep locally a fine grained foam microstructure indicates a final stage of static grain growth at low differential stress for the previously inferred peak stresses of about 0 5 gpa and given temperatures initial strain rates on the order of 10 10 s 1 are predicted by available flow laws for dislocation creep of quartz we emphasise the importance of short term non steady state deformation in the uppermost plastosphere underlying seismically active upper crust the related heterogeneous record of quartz is governed by the local stress history at constant temperature

exhumation of eclogites insights from depth time path analysis abstract literature data on geochronology and pressure temperature estimates have been used to assess the depth time paths for high pressure units in the alps and the variscan belt these depth time paths consistently show that exhumation rates decrease through time the first stages of the exhumation are rapid 1 mm yr 1 and last less than a few million to a few tens of million years in contrast the final 10 20 km ascent to the surface is at least one order of magnitude slower for the deepest and most important part of the exhumation trajectory the chronology of the retrogressive metamorphism excludes gravitational collapse as the major driving force material balance and structural arguments also exclude isostatically assisted erosion processes prism dynamics and tectonically assisted erosion are the most likely mechanisms accounting for the kinetics of exhumation isostasy assisted erosion and shallow level extension are nevertheless required to eventually bring the high pressure rocks to exposure

the pyrope coesite rocks and their country rocks at parigi dora maira massif western alps detailed petrography mineral chemistry and pt path both the coarse and fine grained varieties of the partly coesite bearing pyrope quartzites their interlayered jadeite kyanite rocks and the biotite phengite gneiss country rock common to all of them were subjected to detailed petrographic and textural studies in order to determine the sequence of crystallisation of their mineral constituents which were also studied analytically by microprobe prior to pyrope and coesite growth the mg rich metapelites were talc kyanite chlorite rutile ellenbergerite schists which upon continued prograde metamorphism developed first pyrope megacrysts in silica deficient local environments at the expense of chlorite talc kyanite and subsequently the smaller pyrope crystals with coesite inclusions from reacting talc kyanite based on geobarometrically useful mineral inclusions as well as on experimentally determined phase relations a prograde pt path simplified for water activity 1 is constructed which passes through the approximate pt conditions 16 kbar and 560 c 29 kbar and 720 c and finally up to 37 kbar at about 800 c where the mg rich metapelite was a pyrope coesite rock with phengite kyanite and talc still present during the retrograde path pyrope was altered metasomatically either into phlogopite kyanite quartz or at a later stage to chlorite muscovite quartz both assemblages yield pt constraints the latter about 7 9 kbar 500 600 c the country rock gneisses have also endured high pressures of at least 15 kbar but they provide mostly constraints on the lowest portion of the uplift conditions within the greenschist facies about 5 kbar 450 c microprobe data are presented for the following minerals pyrope ellenbergerite dumortierite unusually mgti rich jadeite vermiculite formed after na phlogopite paragonite and for several generations of phengite chlorite talc phlogopite dravite and glaucophane in the high pressure rocks as well as for biotite chlorite phengites epidote garnet albite and k feldspar in the country rock gneisses an outstanding open problem identified in this study is the preservation of minerals as inclusions within kyanite and pyrope beyond their pt stability limits

chemical and physical responses to deformation in micaceous quartzites from the tauern window eastern alps micaceous quartzites from a subvertical shear zone in the tauern window contain abundant quartz clasts derived from dismembered quartz tourmaline veins bulk plane strain deformation affected these rocks at amphibolite facies conditions shape changes suggest net shortening of the clasts by 11 64 with a mean value of 35 quartz within the clasts accommodated this strain largely via dislocation creep processes on the high stress flanks of the clasts however quartz was removed via solution mass transfer pressure solution processes the resulting change in bulk composition allowed growth of porphyro blastic staurolite chlorite kyanite on the clast flanks matrix sio2 contents decrease from c 83 wt away from the clasts to 49 58 in the selvages on the clast flanks the chemical changes are consistent with c 70 volume loss in the high stress zones calculated shortening values within the clast flanks are similar to the volume loss estimates and are greatly in excess of the shortening values calculated from the clasts themselves flow laws for dislocation creep versus pressure solution imply large strain rate gradients and or differential stress gradients between the matrix and the clast selvages in a rock containing a large proportion of semirigid clasts weakening within the clast flanks could dominate rock rheology in our samples however weakening within the selvages was self limiting 1 growth of strong staurolite porphyroblasts in the selvages protected remaining quartz from dissolution and 2 overall flattening of the quartz clasts probably decreased the resolved shear stress on the flanks to values near those of the matrix which would have reduced the driving force for solution transfer creep extreme chemical changes nonetheless occurred over short distances the necessity of maintaining strain compatibility may lead to significant localized dissolution in rocks containing rheologic heterogeneities and overall weakening of the rocks may result solution transfer creep may be a major process whereby weakening and strain localization occur during deep crustal metamorphism of polymineralic rocks

on the structure of grain boundaries in tectonites abstract grain boundaries in several tectonites have been examined using high voltage and scanning transmission electron microscopy on a gross scale grain boundaries are observed to differ from the host crystal in exhibiting faster rates of preferential etching during ion beam thinning and faster rates of ionization damage during exposure to the electron beam grain boundary features can be grouped according to decreasing size as follows grain junction voids that can form linked networks of tubules arrays of isolated voids and inclusions along the grain boundary surface and a narrow grain boundary film of distorted crystal structure the presence of inclusions in voids and the propensity for electron damage along the boundary suggests that the former were initially fluid filled and that the latter may be hydrated initial microchemical analyses using stem indicate that no compositional difference exists between the grain boundaries and host crystals the overall complex grain boundary structure in tectonites suggests that the effective grain boundary width for intercrystalline diffusion is much greater than that proposed based on metals and that diffusion path lengths along grain boundaries are much less than the theoretical maximum the net effect is to greatly enhance deformation mechanisms dependent on intercrystalline mass transport

open fissure mineralization at 2600 m depth in long valley exploratory well california insight into the history of the hydrothermal system abstract long valley exploratory well drilled into the resurgent dome at long valley caldera california to explore the potential of geothermal power in an active magmatic system achieved temperatures of only ca 100 c at 2500 3000 m depth well below the range expected atop an active magma chamber open fissures encountered at 2600 m depth are coated by mm sized idiomorphic quartz crystals with first and second order growth discontinuities specific growth defects indicating rapid crystallization reflect sudden changes in sio 2 supersaturation fluid inclusions contain low salinity 0 5 wt nacl and low co 2

local origin of high pressure vein material in eclogite facies rocks of the zermatt saas zone switzerland prograde dehydration reactions within metabasalts at the blueschist eclogite facies transition produce kyanite veins usually with quartz sometimes with omphacite or chloritoid of up to several meters length and 20 cm width bulk rock analyses show clear depletion haloes for al2o3 and sio2 near the veins mass balance calculations support the suggestion that the vein material must have been derived from the immediate surroundings these veins are interpreted as local segregation phenom ena and not as fluid dehydration channels of the eo alpine subduction zone calcula tions have been performed for a stagnant vein formation model that considers the rate of the dehydration reactions k responsible for providing the h2o for al and si transport and the rate of diffusion d of these from wall rock to growing vein the model is satisfied for quite a wide range of values for k d and upper limits on the duration of diffusional vein formation of about 1 ma are indicated

microstructural observations on natural syntectonic fibrous veins implications for the growth process abstract syntectonic antitaxial and ataxial fibrous veins were investigated using sem microprobe cathodoluminescence cl and optical microscopy in antitaxial calcite veins fibres and surrounding selvage grew simultaneously with similar growth rates of crystallographically differently oriented grains new material precipitated at the vein margin in antitaxial and bi mineralic ataxial microstructures bridges of country rock material formed during vein growth in an initial en echelon vein system in our antitaxial and bi mineralic ataxial samples the spacing of solid inclusions does not reflect individual crack seal openings

weakening of rock salt by water during long term creep the rheological properties of rock salt are of fundamental importance in predicting the long term evolution of salt based radioactive waste repositories and strategic storage caverns and in modelling the formation of salt diapirs and associated oil traps1 2 the short term high stress rheology of rock salt is well known from laboratory experiments however extrapolation to appropriately low stresses fails to predict the rapid flow seen in certain natural structures furthermore experiments have failed to reproduce the recrystallized microstructure of naturally deformed salt here we report experiments indicating that the above discrepancies can be explained by taking into account the influence of trace amounts of brine trace brine is always present in natural salt but sometimes escapes during experiments our tests on dry dilated salt show more or less conventional dislocation creep behaviour but brine bearing samples show marked weakening at low strain rates this is associated with dynamic recrystallization and a change of deformation mechanism to solution transfer creep because natural rock salt always contains some brine these results cast substantial doubt on the validity of presently accepted dislocation creep laws for predicting the long term rheological behaviour of salt in nature

salt rich aqueous fluids formed during eclogitization of metabasites in the alpine continental crust austroalpine mt emilius unit italian western alps abstract the metabasite bodies of the mt emilius continental unit western italian alps underwent a stage of alpine eclogite facies metamorphism 1 1 1 3 gpa and 450 550 c accompanied by polyphase deformation and recrystallization the metabasites consist of two main rock types 1 eclogites omphacite garnet glaucophane epidote phengite paragonite preserving no relics of their precursors 2 eclogitized granulites i e rocks whose incomplete eclogitic recrystallization clinopyroxene ii garnet ii epidote amphibole chlorite allowed survival of textural and mineralogical relics of pre alpine granulitic assemblages clinopyroxene i garnet i plagioclase amphibole in this latter case the pre alpine granulites were converted to eclogites as the result of infiltration of aqueous fluids during eclogitization in both eclogites and eclogitized granulites hydrated high pressure foliations are cut by eclogitic metamorphic veins the bulk rock chemistry of the metabasites influenced the compositions of both the vein and rock forming clinopyroxenes and the compositional correlation between the vein and rock forming clinopyroxenes indicates that the syn eclogitic fluids have re equilibrated with the metabasite hosts the predominant vein minerals omphacite epidote and garnet contain primary high salinity fluid inclusions the fluids consist of two phase liquid vapour and of multiphase liquid vapour salt additional quartz salty aqueous inclusions containing nacl cacl 2 and mgcl 2 as the main chloride species the vein inclusions show a salinity range from 17 to 45 wt salts in eclogites and from 20 to 50 wt salts in eclogitized granulites in contrast fluid inclusions in matrix minerals of the eclogitized granulites contain primary two phase inclusions displaying a salinity range between 10 and 25 wt salts the marked difference in fluid salinities recorded by the inclusions in the eclogitic veins and those in the partially re equilibrated eclogitized granulites is interpreted in terms of progressive hydration during eclogitization of granulite facies rocks which caused an increase in the salt content of the residual inclusion fluids

influence of pore fluid salinity on pressure solution creep in gypsum abstract the uniaxial compaction creep behaviour of wet granular gypsum is investigated under chemically closed drained conditions known to favour pressure solution the experiments were performed using applied stresses of 1 to 4 mpa and grain sizes in the range 50 to 280 m at room temperature the tests were carried out using saline pore fluids with a nacl concentration of 0 01 to 6 m saturated with respect to the unstressed gypsum starting material since gypsum is almost always associated with other evaporite minerals such as halite pore fluid compositions in gypsum will generally be highly saline data on the effect of pore fluid salinity on pressure solution in gypsum are thus directly relevant to the behaviour of gypsum in geological situations the results obtained in terms of mechanical behaviour and microstructures developed are in good agreement with the results obtained in previous experiments performed using nacl free pore fluids however the absolute creep rates obtained here are up to 50 times faster than in the nacl free tests comparison with theoretical models for pressure solution creep and crystal growth data reveals that diffusion is unlikely to be the rate controlling mechanism of pressure solution in the present experiments since the growth rate of gypsum increases with nutrient salinity the observed effect of nacl on pressure solution is consistent with the possibility of rate control by the precipitation reaction however the absolute creep rates observed are one to two orders of magnitude slower than creep rates predicted by a precipitation rate limited model so that the detailed mechanism remains unclear

experimental deformation of synthetic wet jadeite aggregates 1 predictions on interplate coupling and shear heating in the deeper levels of subduction zones require an understanding of the rheology of eclogite the strength of eclogite is probably limited by the flow strength of its major constituent omphacite representing a solid solution between the clinopyroxene end members diopside and jadeite here we report the results of deformation experiments on the end member jadeite naalsi2o6 carried out on fine grained synthetic aggregates crystallized in the stability field of jadeite from a synthetic glass precursor a standard procedure was established to yield samples with a uniform microstructure and an average grain size of approximately 10 m the presence of micropores and the detection of small amounts of freezable water by fourier transform infrared spectroscopy ftir in both undeformed and deformed samples indicates the presence of a free aqueous fluid hence wet conditions deformation experiments were carried out in a modified griggs type apparatus at a confining pressure of 2 5 gpa temperatures between 800 c and 1100 c and at strain rates of 4 10 4 s 1 to 2 10 6 s 1 a molten eutectic cscl nacl mixture was used as pressure medium the microfabrics of the deformed samples and the mechanical data indicate deformation in the dislocation creep regime the mechanical data are fit by a power law using a global inversion method yielding an activation energy of q 326 27 kj mol 1 a stress exponent n 3 7 0 4 and a preexponential factor of ln a 3 3 2 0 extrapolation of this flow law for synthetic jadeitite to low geological strain rates and comparison with available flow laws for diopside indicates jadeite to be significantly weaker than diopside in the dislocation creep regime

morphology of intergranular pores and wetting angles in pelitic schists studied by transmission electron microscopy in pelitic schists composed mainly of quartz and albite grains the morphology of intergranular pores which were filled with water was studied by transmission electron microscopy tem although some pores are defined by crystallographic planes f face most of their form has an ideal shape determined by interface tensions between grains and fluid high resolution tem observations demonstrate that pore free regions at grain boundaries are tight even at the nanometer scale showing that the wetting angle is larger than 0 in this rock the pore distribution in two grain junctions can be compared to a necklace microstructure developed by instability of a fluid film along the boundary induced by microcracking wetting angles for pores located at grain edges of quartz and albite decrease in the order albite albite quartz quartz and quartz albite the quartz quartz wetting angle in a calcite free sample is smaller than that in a calcite containing sample this angle also changes due to grain misorientation our results confirm that solid solid and solid fluid interfacial energies control the geometry of intergranular fluid in natural rocks

apparent activation energy and rate limiting process estimation from natural shale deformed by pressure solution in shallow subduction zone pressure solution is one of the main deformation mechanisms for lithify and stress release in the rocks from shallow subduction zones we observed temperature dependent pressure solution development in naturally deformed shale in shimanto accretionary complex in southwest japan the pressure solution develops with shear dominated or co axial dominated deformation we evaluated apparent activation energy by applying the constitutive equation of pressure solution creep to the temperature dependent relations the activation energy of each deformation type was estimated as 18 kj mol 1 for shear dominated and 45 kj mol 1 for co axial dominated shale the energies enable us to speculate rate limiting processes of pressure solution i e dissolution diffusion and precipitation by comparing the energies obtained in this study with energies had been measured by experiments the lower activation energy estimated here was similar with that of diffusion the similarity indicates that possible rate limiting process of shale deformation in shallow subduction zone would be diffusion the difference of energy between deformation types can be explained by distinctive grain boundary structure

dissolution and replacement creep a significant deformation mechanism in mid crustal rocks abstract zoning patterns and zoning truncations in metamorphic minerals in a granodioritic orthogneiss indicate that strain and s c fabrics in these rocks were produced by dissolution precipitation and replacement processes even at epidote amphibolite facies metamorphic conditions the metamorphic fabric is defined by alternating layers and folia dominated by quartz feldspars and biotite epidote zoning patterns in most metamorphic plagioclase orthoclase epidote and sphene are truncated at boundaries normal to the shortening direction suggesting dissolution interfaces of relict igneous orthoclase phenocrysts that face the shortening direction are embayed and replaced by biotite epidote and myrmekitic intergrowths of plagioclase and quartz metamorphic plagioclase grains are also replaced by epidote we interpret these microstructures to reflect strain enhanced dissolution the cores of many grains show asymmetric overgrowths with at least two generations of beards all oriented on the ends of grains that face the extension direction we interpret these textures to reflect precipitation of components dissolved by deformation enhanced dissolution while biotite and quartz probably deformed by dislocation creep the overall deformation was accommodated by dissolution perpendicular to the shortening direction and precipitation parallel to it these chemical processes must have been activated at lower stresses than the dislocation creep predicted from extrapolations of data from experiments in dry rocks thus wet crust is likely to be weaker than calculated from these experimental studies

problems in the extrapolation of laboratory rheological data abstract the many types of variables and deformation regimes that need to be taken into account in extrapolating rheological behaviour from the laboratory to the earth are reviewed the problems of extrapolation are then illustrated with two particular cases in the case of divine rich rocks recent experimental work indicates that within present uncertainties of extrapolation the flow in the upper mantle could be either grain size dependent and near newtonian or grain size independent and distinctly non newtonian both types of behaviour would be influenced by the present of trace amounts of water in the case of quartz rich rocks the uncertainties are even greater and it is still premature to attempt any extrapolation to geological conditions except as an upper bound the fugacity and the scale of dispersion of the water are probably two important variables but the quantitative laws governing their influence are not yet clear

deep subduction and rapid exhumation role of crustal strength and strain weakening in continental subduction and ultrahigh pressure rock exhumation 1 the exhumation of crustal ultra high pressure uhp material depends on temporal and spatial variations in its detachment within the subduction channel this dependence is investigated using numerical models with variable initial crustal strengths representing a range of initial crustal compositions and parameterized strain weakening representing a range of processes that reduce effective crustal viscosity during deformation competition between down channel shear traction favoring subduction and up channel buoyancy favoring exhumation is expressed as the exhumation number e which can vary with time and position along the channel exhumed lower strength crust which resists subduction owing to weak down channel traction records peak conditions 38 kbar given sufficient strain weakening exhumation proceeds at 60 km ma 1 indicating that buoyancy e 1 drives exhumation in these models in all models exhuming uhp material forms a deforming ductile plume with a range of possible structural relationships predicted between exhumed uhp and hp materials

matrix deformation mechanisms in hp lt tectonic melanges microstructural record of jadeite blueschist from the franciscan complex california abstract exhumed high pressure low temperature metamorphic melanges of tectonic origin are believed to reflect high strain accumulated in large scale interplate shear zones during subduction rigid blocks of widely varying size are embedded in a weak matrix which takes up the deformation and controls the rheology of the composite the microfabrics of a highly deformed jadeite blueschist from the franciscan complex california are investigated to help understand deformation mechanisms at depth the specimen shows a transposed foliation with dismembered fold hinges and boudinage structures several generations of open fractures have been sealed to become veins at high pressure metamorphic conditions the shape of these veins frequently restricted to specific layers indicates distributed host rock deformation during and after sealing small cracks in jadeite and lawsonite are healed with tiny quartz inclusions aligned along the former fracture surface large jadeite porphyroblasts show strain caps and strain shadows open fractures are sealed by quartz and new jadeite epitactically grown on the broken host monophase glaucophane aggregates consist of undeformed needles with a diameter between 0 1 and 2 m grown after formation of isoclinal folds only quartz microfabrics indicate some stage of crystal plastic deformation followed by annealing and grain growth aragonite in the latest vein generation shows retrogression to calcite along its rims the entire deformation happened under hp lt metamorphic conditions in the stability field of jadeite and quartz at temperatures between 300 and 450 c and pressures exceeding 1 1 4 gpa the microfabrics indicate that dissolution precipitation creep was the predominant deformation mechanism accompanied by brittle failure and vein formation at quasi lithostatic pore fluid pressure this indicates low flow strength and combined with high strain rates expected for localized deformation between the plates a very low viscosity of material in the interplate shear zone at a depth 30 45 km

structural assemblage of high pressure mantle and crustal rocks in a subduction channel cabo ortegal nw spain 1 high pressure metamorphic fold nappes and ductile thrusts occur in the cabo ortegal upper allochthons they involve arc root lithospheric mantle peridotites high pressure volcanic arc granulites normal mid oceanic ridge basalt eclogites and shallow crustal volcanosedimentary high pressure gneiss units confluence of units from such disparate tectonic settings occurred at an eo hercynian convergent plate margin their internal deformation juxtaposition and high pressure synmetamorphic reworking resulted from kinematically simple but intense and polyphase ductile deformations fold nappes exhibit a remarkable sheath like form and contain mineral and stretching lineations defined by high pressure mineral assemblages subparallel to fold axes we argue that these structures outline the internal organization of high pressure sheets at deep tectonic realms of an oblique subduction collision orogenic channel

decrepitated uhp fluid inclusions about diverse phase assemblages and extreme decompression rates erzgebirge germany minute polyphase inclusions in garnet of quartzofeldspathic rocks saidenbachite from the saxonian erzgebirge germany contain microdiamond or graphite phlogopite quartz paragonite phengite and other minerals in minor amounts these inclusions are interpreted to represent an original dense coh silicate fluid trapped in crystallizing garnet at depths of 150 km inspection of the inclusion population in a single garnet by sem reveals two characteristic features i the shape of most inclusions indicates healed radial cracks in the host garnet and thus the buildup of a significant differential pressure p i e a contrast in pressure between the inclusion pi and the host mineral pe the mineral assemblages sealing the cracks and showing an equilibrated microstructure indicate temperatures of 750 50 c and pressures below 2 5 gpa ii the diverse types of inclusions appear to be randomly distributed in the garnet host thus the variable phase assemblages do not reflect a compositional evolution of the fluid trapped in the garnet combining observations i and ii we propose that the diversity of the phase assemblage in the inclusions is the result of decrepitation at different times and thus of distinct histories of pi as p at decrepitation is primarily controlled by inclusion size and shape applying a flow law for dislocation creep of garnet a low strength of garnet at 750 50 c for low geological strain rates is predicted thus differential pressure should have been kept low i e pi pe by continuous stretching of the inclusion for typical exhumation rates of metamorphic rocks to attain the differential pressure pi pe required for catastrophic brittle failure of the garnet host the decompression rate must have been extremely high as a robust lower bound a minimum exhumation rate on the order of 100 m year 1 is suggested which corresponds to ascent rates of magma

melange rheology and seismic style shear displacements in crustal fault zones are accommodated by a range of seismic styles including standard earthquakes non volcanic tremor and continuous and transitory aseismic slip subduction channel shear zones containing highly sheared fluid saturated trench fill sediments intermingled with fragments of oceanic crust are commonly inferred to occur along active subduction megathrusts if this interpretation is correct these plate boundary faults are not discrete planes but may resemble the melange shear zones commonly found in exhumed subduction related rock assemblages melange deformation appears to depend critically on the ratio of competent to incompetent material with shear surfaces localized along lithological contacts or within competent domains while matrix flow accommodates shearing by distributed strain if the style of strain and or displacement accommodation in a melange reflects the partitioning between aseismic and seismic slip the proportion of competent material seems likely to be a significant factor affecting seismic style within subduction channel shear zones and along comparable mixed lithology fault zones

coesite in clinopyroxene in the caledonides and its implications for geodynamics coesite is reported for the first time from the caledonide orogen it occurs as inclusions in clinopyroxene in the dolomite eclogite at grytting norway and provides valuable new evidence to support the hypothesis of extremely high pressure 30 kbar in certain norwegian eclogites this finding has implications for the geodynamics and kinetics of the concepts of microscale intracrystalline tectonic overpressure and of macroscale local or regional tectonic overpressure and for the hypotheses of subduction and obduction involving a more than 100km vertical component of transport

on volume change and mass transport during the development of crenulation cleavage abstract a comparison of the bulk chemistry between isolated zones of crenulation cleavage and the surrounding relatively homogeneous slates from three localities boscastle sw england moselle valley germany lac de roselend french alps demonstrates that marked bulk volume change is not a prerequisite for the development of crenulation cleavage x ray powder diffraction results also demonstrate that there is no significant mineralogical change between crenulated and uncrenulated zones for the samples studied mineralogical differentiation and mass transfer takes place on a local scale between the limb and hinge domains of individual crenulation folds and of larger scale folds in layering but can occur without significant overall volume change on the scale of crenulated zones containing many microlithon and cleavage domains relative volume changes between crenulated and uncrenulated regions do not exceed ca 5 at boscastle and the majority of samples from the moselle valley at one locality in the moselle valley however a volume loss of around 35 40 occurred from the crenulated zone produced almost entirely by loss of sio2 as quartz from the system isolated parallel sided bands of crenulation cleavage such as the examples from boscastle and the moselle valley are geometrically constrained to represent some combination of volume change and heterogeneous simple shear superimposed upon a background homogeneous strain in examples with minimal volume changes the heterogeneous strain component must be one of simple shear

construction of crossed girdles by superposing four subfabrics each with a single maximum commonly basal glide is the predominant deformation mechanism of quartz in tectonites therefore local deformation is probably mostly progressive simple shear rotating the sheared domains as well as deforming them

very low temperature record of the subduction process a review of worldwide lawsonite eclogites abstract lawsonite eclogites preserve a record of very low temperature conditions in subduction zones all occur at active margin settings typically characterized by accretionary complexes lithologies and as tectonic blocks within serpentinite matrix melange peak lawsonite eclogite facies mineral assemblages garnet omphacite lawsonite rutile typically occur in prograde zoned garnet porphyroblasts their matrix is commonly overprinted by higher temperature epidote bearing assemblages greenschist or amphibolite facies conditions erase former lawsonite eclogite relics various pseudomorphs after lawsonite occur particularly in some blueschist eclogite transitional facies rocks coesite bearing lawsonite eclogite xenoliths in kimberlitic pipes and lawsonite pseudomorphs in some relatively low temperature ultrahigh pressure eclogites are known using inclusion assemblages in garnet lawsonite eclogites can be classified into two types l type such as those from guatemala and british columbia contain garnet porphyroblasts that grew only within the lawsonite stability field and e type such as from the dominican republic record maximum temperature in the epidote stability field formation and preservation of lawsonite eclogites requires cold subduction to mantle depths and rapid exhumation the earliest occurrences of lawsonite eclogite facies mineral assemblages are early paleozoic in spitsbergen and the new england fold belt of australia this suggests that since the phanerozoic secular cooling of earth and subduction zone thermal structures evolved the necessary high pressure temperature conditions buoyancy of serpentinite and oblique convergence with a major strike slip component may facilitate the exhumation of lawsonite eclogites from mantle depths

scales of equilibrium and disequilibrium during cleavage formation in chlorite and biotite grade phyllites se vermont detailed electron microprobe analyses of phyllosilicates in crenulated phyllites from south eastern vermont show that grain scale zoning is common and sympathetic zoning in adjacent minerals is nearly universal we interpret this to reflect a pressure solution mechanism for cleavage development where precipitation from a very small fluid reservoir fractionated that fluid multiple analyses along single muscovite biotite and chlorite grains 30 200 m in length show zoning patterns indicating tschermakitic substitutions in muscovite and both tschermakitic and di trioctahedral substitutions in biotite and chlorite using cross cutting relationships and mineral chemistry it is shown that these patterns persist in cleavages produced at metamorphic conditions of chlorite grade chlorite grade overprinted by biotite grade and biotite grade zoning patterns are comparable in all three settings requiring a similar cleavage forming mechanism independent of metamorphic grade moreover the use of 40ar 39ar geochronology demonstrates this is true regardless of age furthermore samples with chlorite grade cleavages overprinted by biotite porphyroblasts suggest the closure temperatures for the diffusion of al si mg and fe ions are greater than the temperature of the biotite isograd 400 c parallel and smoothly fanning tie lines produced by coexisting muscovite chlorite and muscovite biotite pairs on compositional diagrams demonstrate effectively instantaneous chemical equilibrium and probably indicate simultaneous crystallization these results do not support theories suggesting cleavages form in fluid dominated systems if crenulation cleavages formed in systems in which the chemical potentials of all major components are fixed by an external reservoir then the compositions of individual grains defining these cleavages would be uniform on the contrary the fine scale chemical zoning observed probably reflects a grain scale process consistent with a pressure solution mechanism in which the aqueous activities of major components are defined by local dissolution and precipitation thus the role of fluids was probably limited to one of catalysing pressure solution and fluids apparently did not drive cleavage development

tectonic significance of low temperature blueschist blocks in the franciscan melange at san simeon california abstract the blueschist bearing franciscan melange exposed near san simeon is very similar to the melange terrane forming the central belt north of san francisco epidote bearing low t blueschist blocks at san simeon n 3 are the same age as high t garnet bearing blocks in the northern and diablo range melange terranes 155 150 ma this age approximates the time of initiation of franciscan subduction during the initial stages of franciscan subduction part of the top volcanic sequence from the subducting oceanic plate and small volumes of overlying sediments were imbricated at the bottom of the overriding plate and metamorphosed under high p conditions this underplated material was variably retrograded as subduction refrigerated the forearc blocks actinolitic rinds formed at the contacts between blueschists and intercalated mantle under relatively static conditions as the base of the overriding plate became serpentinized a model is proposed in which exhumation was facilitated by gravity driven seaward oriented extensional thinning of the forearc prism caused by the decrease in subducting plate dip starting around 80 ma that caused the laramide orogeny as this occurred blueschist and graphite schist blocks were plucked from the bottom of the hanging wall incorporated into the shale and water rich shear zone at the plate interface subduction channel shear zone and exhumed during the upward flow of melange driven by the movement of the downgoing plate while subduction was still active

fluid inclusions in coesite bearing eclogites and jadeite quartzite at shuanghe dabie shan china fluid inclusions in coesite bearing eclogites and jadeite quartzite at shuanghe in dabie shan east central china have preserved remnants of early prograde and or peak metamorphic fluids reset during post uhp ultrahigh pressure metamorphic uplift inclusions occur in several minerals e g omphacite epidote notably as isolated primary inclusions in quartz included in various host minerals two major fluid types have been identified non polar fluid species n2 or co2 and aqueous the latter is by far the most predominant aqueous fluids cover a wide range of salinity from halite bearing brines to low salinity fluids for non polar fluids few n2 inclusions occur in undeformed eclogite whereas a number of co2 rich inclusions have been found in microshear zones of eclogite or jadeite quartzite in close proximity to marble occurrences the primary character of n2 and high salinity aqueous inclusions indicates that they are remnants from uhp metamorphic fluids and for some there is the distinct possibility that they are ultimately derived from pre metamorphic fluids this conclusion is supported by the preservation in some samples of microdomains containing synchronous inclusions of variable salinities which tend to relate to the chemical composition of the host crystal carbonic fluids may be derived from neighbouring rocks notably marble and carbonate bearing metasediments during post metamorphic uplift during post uhp exhumation a limited decrease of the fluid density has occurred with formation of new sets of fluid inclusions fluid movements however remained exceedingly limited at the scale of the enclosing crystal

deformation induced garnet zoning compositional zoning patterns in garnet porphyroblasts from kyanite bearing samples of the devonian littleton formation north central massachusetts reveal complex patterns of growth that are related to multiple deformation and metamorphic events garnet porphyroblasts exhibit asymmetrical and irregular zoning patterns in xmn xca and fe fe mg zoning reversals in mn and fe fe mg and patch distribution in ca appear to occur around the boundaries of the textural zones also the compositions of the garnet at the textural boundaries are variable for all traverses these observations suggest that the garnet zoning was not only modified from diffusion processes but was also influenced by pre existing microfabrics through the effects of preferential dissolution and resorption in partial disequilibrium relationships between chemical and textural truncations indicate that the zoning patterns of garnet were strongly modified from preferential dissolution and precipitation during the development of successive foliations that occurred in zones of high strain stress cleavage seams and zones of low strain stress respectively

thermobarometric data from a fossil zircon partial annealing zone in high pressure low temperature rocks of eastern and central crete greece abstract a fossil partial annealing zone of fission tracks in zircon is described from high pressure low temperature hp lt rocks of the phyllite quartzite unit pq on the island of crete greece correlation of regional trends in fission track age populations with independent thermobarometric and microstructural data and with new experimental annealing results allows a calibration of this low temperature thermochronological method to a degree hitherto not available from other field examples the zircon fission track ft ages of samples from the pq across crete range from original detrital signature through reduced to completely reset the annealing is the result of a single heating period related to the hp lt metamorphism with near peak temperatures lasting for only a few million years some time between 24 1 and 20 1 ma in eastern crete where rocks have experienced temperatures of 300 50 c and pressures of 0 8 0 3 gpa zircon ft ages range from 414 24 to 145 10 ma ages above 300 ma occur mostly near the east coast of the island in rocks which have not been heated to above ca 280 c and probably represent a pre variscan source track lengths are already indicative of a substantial annealing at this temperature most of the zircon ft ages from eastern crete scatter within error around the stratigraphic age samples with apparent zircon ft ages significantly younger than the depositional age are only observed in areas where temperatures exceeded ca 320 c towards the west a sudden decrease to very young ages ranging from 17 2 to 18 1 ma reflects a complete resetting at ca 350 c short tracks however are still observed throughout the central and western part of the island ages are consistently below 22 ma thermobarometric data for this area indicate maximum temperatures of 400 50 c and pressures of 1 0 3 gpa only samples from western crete which have been exposed to 400 50 c show exclusively long tracks consequently the high temperature limit of the zircon partial annealing zone zpaz appears to be between 350 and 400 c a significant influence of elevated confining pressure on the stability of fission tracks in zircon is ruled out by the results of annealing experiments at 0 5 gpa and at different temperatures which fit the curves previously obtained by other authors at ambient pressure

low stress deformation of garnet by incongruent dissolution precipitation creep abstract microstructures indicating incongruent dissolution precipitation creep of garnet in eclogite facies graphitic micaschist tauern window eastern alps are investigated garnet dissolution is observed where garnet poikiloblasts grown at eclogite facies metamorphism approached each other as a consequence of progressive deformation during exhumation with estimated p t conditions between 570 c 1 7 gpa and 470 c 0 9 gpa the poikiloblasts are separated by a dissolution seam and flanked by strain shadows filled with quartz white mica and chlorite there is no evidence for crystal plastic deformation of garnet two cases are investigated a stylolitic contact zone b smooth contact zone in both cases internal fabrics of the poikiloblasts and concentric chemical zoning are truncated material previously forming inclusions in the garnet poikiloblasts is now passively enriched in a dissolution seam the original microstructure of fine grained mica graphite aggregates remaining preserved though microstructures suggest that garnet dissolution was driven by local stress concentration the level of differential stress remained too low for plastic deformation of the fine grained white mica graphite aggregates set free from the stress supporting garnet incongruent dissolution precipitation creep appears to be a particularly effective deformation mechanism at low stress in a subduction channel

omphacite fabric variation in the cabo ortegal eclogite nw spain relationships with strain symmetry during high pressure deformation petrostructural study of cabo ortegal eclogite tectonites provides an insight into the relationships between omphacite lattice preferred orientation and the fundamental deformation parameters in particular strain symmetry microstructural evidence is consistent with a tectonic origin of eclogite fabrics by coeval pyroxene plastic deformation and recrystallization under eclogite facies metamorphic conditions omphacite shape fabrics and garnet spatial distributions are anisotropic and representative of the fields of either apparent flattening or constriction omphacite lattice preferred orientations are dominated by 010 axis maxima normal to the foliation s type or flattening fabrics by 001 axis maxima parallel to the lineation l type or constriction or correspond to other fabric patterns described in the literature omphacite crystallographic fabrics are consistently asymmetric with respect to the structural framework defined by foliation and lineation this suggests that non coaxial deformation components either simple or general shear accompanied eclogite fabric development in addition to any flattening or constriction strain paths and permit shear sense determination these petrostructural features were acquired in the course of eo hercynian ductile deformation probably during deep subduction under 1 5 1 7 gpa confining pressure temperatures above 600 700 c low differential stress 15 20 mpa and slow strain rate 10 12 10 15s 1

on the rheologically critical melt fraction abstract with increasing melt fraction the strength of partially molten granite decreases from a value characteristic of solid competent rock to a value nearly equal to that of the melt previously published mechanical and microstructural data indicate that deformation in partially molten rocks often involves brittle processes thus the pressure in the melt is expected to be important in determining strength when the volume changes during deformation strength and fluid flow will be coupled by such parameters as permeability k storage capacity per unit volume or storativity s melt compressibility f grain size d fluid viscosity and strain rate experiments on brittle rock at low temperatures show that the strain rate at which the internal fluid pressure can be maintained constant is approximately proportional to k s an evaluation of published experimental data suggests that this relation also holds for partially molten granites indicating that the strength of these rocks depends on their transport properties since the permeability is related to below a critical melt fraction rcmf the melt pressure will change if deformation is not iso volumetric by assuming a power law relationship between k and we estimate that rcmf f d m 1 n 1 where m and n relate permeability to grain size and respectively

deformation mechanisms and reaction of hornblende examples from the bergell tonalite central alps abstract hornblendes in the bergell tonalite begin to crystallize relatively early during the crystallization history of the pluton and become resorbed during the late stages of crystallization during the crystallization of the magma deformation commences by magmatic flow in this stage the hornblendes behave as rigid particles in a viscous matrix melt the rotation and alignment of hornblende as elongate rigid particles have produced a strong preferred orientation of both particle long axes and crystallographic directions with progressive solidification of the melt there is a gradual transition from magmatic flow to solid state deformation in the bergell tonalite the crystallographic and shape preferred orientation that have developed during solid state deformation are identical to those of magmatic flow this identical fabric development during solid state and magmatic flow deformation can be explained by the same deformation mechanism that has prevailed in hornblends during both deformation periods i e hornblendes have always acted as rigid particles in a viscous matrix tem observations show no evidence for intracrystalline plasticity in hornblende grains the modal abundance of hornblende decreases progressively with increasing solid state deformation to yield more biotite epidote and quartz small hornblende matrix grains form by cataclastic processes the observed changes in grain size shape and abundance of hornblende occur mainly by fracturing dissolution and reaction so that the solid state deformation of the tonalite is a complex process to which fracturing dissolution of hornblende and metamorphic reactions all contribute the relationship between deformation and reaction represents an example of incongruous pressure solution or diffusive mass transfer involving reaction it is inferred that generally hornblende does not appear to deform significantly by intracrystalline plasticity at temperatures below 650 750 c in the presence of an aqueous fluid crystal plasticity could become dominant at higher temperatures and or lower aqueous fluid activities

a few remarks on the kinetics of static grain growth in rocks static grain growth is a relatively simple transformation in which grain size increases under driving forces caused by grain and interphase boundary curvature given the relative simplicity of the protocol for grain growth experiments measurements of grain boundary mobility show surprising variations boundary mobilities during grain growth are affected by solute and impurity chemistry chemical fugacity of trace and major elements pore size and number pore fluid chemistry the presence of melts and the presence of solid second phases as well as temperature and pressure all of these factors may exert influence on grain growth of rocks in natural situations and many are also present during the laboratory experiments provided that the necessary kinetics parameters are known bounds may be placed on the interface mobility when pores partial melts or solutes are present to predict the rate of grain growth in natural situations will require improved laboratory data and careful consideration of the thermodynamic conditions likely to be encountered in nature

rheology of the plate interface dissolution precipitation creep in high pressure metamorphic rocks abstract subduction zone models invoke deformation to be concentrated along the plate interface in a region of particularly low temperature geophysical observations do not provide constraints on temperature stress and deformation patterns with desired resolution in contrast the record of high pressure metamorphic rocks exhumed from subduction zones provides details on p t history deformation mechanisms and stress state albeit not readily correlated with the former dynamic situation on larger scale here we review available information on dissolution precipitation creep dpc in high pressure metamorphic rocks which if representative for subduction zones in general can pose constraints on conditions rheology and flow patterns along the plate interface the key observations and conclusions are that 1 deformation is typically highly inhomogeneous and localized into shear zones 2 stresses are generally too low to drive crystal plastic deformation 3 microfabrics suggest dissolution precipitation creep to be the predominant deformation mechanism 4 an aqueous fluid at quasi lithostatic pressure is available throughout allowing for tensile fracturing and crack healing or sealing 5 low stress combined with high strain rates required for localized deformation at typical subduction rates implies low viscosity and 6 contribution of shear heating to the thermal budget of subduction zones should be moderate the dominant deformation mechanism dpc is reviewed in some detail including experimental and theoretical approaches various examples of dpc in high pressure metamorphic rocks are illustrated emphasizing the role of interphase boundaries as sites of dissolution rheology governed by dpc is proposed to control interplate coupling and development of a subduction channel with return flow being a likely candidate for rapid exhumation of high pressure metamorphic rocks

crystallographic preferred orientation development by dissolution precipitation creep abstract crystallographic preferred orientations cpos in deformed rocks are commonly interpreted as resulting from crystal plastic deformation mechanisms where deformation is achieved by the movement of dislocations in this paper we investigate the possibility of cpo development by dissolution precipitation creep or pressure solution a numerical model is presented which simulates the development of a grain aggregate that deforms by reaction controlled dissolution precipitation creep grains are simulated as rectangular boxes that change their shape by growth or dissolution of their surfaces depending on the normal stresses acting on the individual surfaces grains can also rotate due to an applied vorticity for non coaxial deformation and if they have a non equidimensional shape for each strain increment stress that is applied to the grains is the same for all grains while individual grains deform and rotate by different amounts a variety of cpos develop at moderate strains depending on the reaction rates of the different crystal surfaces and type of deformation uni axial shortening plane strain pure shear and simple shear the modelling results confirm that dissolution precipitation creep may play a role in cpo development in rocks

the role of pressure solution seam and joint assemblages in the formation of strike slip and thrust faults in a compressive tectonic setting the variscan of south western ireland abstract the ross sandstone in county clare ireland was deformed by an approximately north south compression during the end carboniferous variscan orogeny the initial assemblage consists of mutually abutting orthogonal arrays of 170 oriented set 1 joints veins jvs and approximately 75 oriented set 1 pressure solution seams psss formed under the same stress conditions orientations of splay jvs and psss set 2 suggest a clockwise remote stress rotation of about 35 responsible for the contemporaneous shearing of the set 1 arrays among these nearly orthogonal strike slip faults the prominent set is sub parallel to set 1 jvs these faults are formed by the linkage of en echelon segments with broad damage zones responsible for right lateral offsets of hundreds of meters thrust faults with up to 30 m of offset initiate within shale horizons and follow either the psss in the sandstones or high angle shales within tilted sequences within the large thrust fault zones compartmentalised blocks of rocks are bounded by thrust faults segments with various dip angles strike slip and thrust faults are contemporaneous and owe their existence to initial weaknesses in the form of jvs and psss rather than by switching relative stress magnitudes and orientations associated with andersonian models of faults and related stress orientations

double seismic zone and dehydration embrittlement of the subducting slab 1 dehydration embrittlement of metamorphosed oceanic crust and mantle in the subducting slab may be responsible for the occurrence of intermediate depth earthquakes we explore the possibility that this hypothesis can explain the morphology of the double seismic zones observed in northeast japan southwest japan northeast taiwan northern chile cape mendocino and eastern aleutians we calculate transient temperature structures of slabs based on geologically estimated subduction histories of these regions we then determine dehydration loci of metamorphosed oceanic crust and serpentinized mantle using experimentally derived phase diagrams the depth range of the dehydration loci of metamorphosed oceanic crust and serpentine is dependent on slab age the dehydration loci of serpentine produce a double layered structure because the upper dehydration loci of serpentine are mostly located in the wedge mantle above the slab we regard the upper plane seismicity representing dehydration embrittlement in the oceanic crust and we fix the slab geometry so that the upper plane seismicity is just below the upper surface of the slab we find that the lower plane seismicity is located at the lower dehydration loci of serpentine which indicates that the morphology of the double seismic zones is consistent with the dehydration embrittlement

kinetic effects of microscale plasticity at grain boundaries during pressure solution 1 it is generally assumed in kinetic models for pressure solution in materials such as quartz that the effective dissolution rate coefficient in grain boundaries is equal to the conventional geochemical dissolution rate coefficient on a free surface however predictions based on this assumption usually overestimate both natural and experimental pressure solution rates even when evidence for dissolution rate control is strong a possible explanation for this discrepancy is that grain boundary structure and dissipative processes such as microscale plasticity in the grain boundary can decrease this effective rate coefficient on the basis of a simple grain boundary model assuming an island channel structure we have derived a preliminary model for the effect that dissipation by plastic deformation work hardening flow and creep of grain boundary islands has on dissolution controlled pressure solution rates comparing the predictions of this model with the experimental data on quartz pressure solution rates we see that microscale plasticity at grain boundary islands does slow down pressure solution and can help explain the discrepancies between observed and theoretical pressure solution rates when applied to pressure solution creep of sandstones or fault rocks in nature our model predicts that grain boundary plastic deformation in quartz might have a significant effect at depths beyond 9 10 km

double seismic zone of the nazca plate in northern chile high resolution velocity structure petrological implications and thermomechanical modeling this paper presents an interdisciplinary study of the northern chile double seismic zone first a high resolution velocity structure of the subducting nazca plate has been obtained by the tomodd double difference tomography method the double seismic zone dsz is observed between 80 and 140 km depth and the two seismic planes is 20 km apart then the chemical and petrologic characteristics of the oceanic lithosphere associated with this dsz are deduced by using current thermal petrological seismological models and are compared to pressure temperature conditions provided by a numerical thermomechanical model our results agree with the common hypothesis that seismicity in both upper and lower planes is related to fluid releases associated with metamorphic dehydration reactions in the seismic upper plane located within the upper crust these reactions would affect material of basaltic morb composition and document different metamorphic reactions occurring within high p 2 4 gpa and low t 130 km lawsonite amphibole eclogite conditions the lower plane lying in the oceanic mantle can be associated with serpentinite dehydration reactions the vp and vs characteristics of the region in between both planes are consistent with a partially 25 30 vol antigorite 0 10 vol brucite and 4 10 vol chlorite hydrated harzburgitic material discrepancies persist that we attribute to complexities inherent to heterogeneous structural compositions while various geophysical indicators evidence particularly cold conditions in both the descending nazca plate and the continental fore arc thermomechanical models indicate that both seismic planes delimit the inner slab compressional zone around the 400 c 50 c isotherm lower plane earthquakes are predicted to occur in the slab s flexural neutral plane where fluids released from surrounding metamorphic reactions could accumulate and trigger seismicity fluids migrating upward from the tensile zone below could be blocked in their ascension by the compressive zone above this plane thus producing a sheeted layer of free fluids or a serpentinized layer therefore earthquakes may present either downdip compression and downdip tensile characteristics numerical tests indicate that the slab s thermal structure is not the only factor that controls the occurrence of inner slab compression 1 a weak ductile subduction channel and 2 a cold mantle fore arc both favor inner slab compression by facilitating transmission of compressional stresses from the continental lithosphere into the slab 3 decreasing the radius of curvature of the slab broadens the depth of inner slab compression whereas 4 decreasing upper plate convergence diminishes its intensity all these factors indicate that if dszs indeed contour inner slab compression they cannot be linked only to slab unbending but also to the transmission of high compressional stresses from the upper plate into the slab

plastic deformation and development of clinopyroxene lattice preferred orientations in eclogites abstract we use an anisotropic viscoplastic self consistent vpsc model to simulate the development of omphacite lattice preferred orientations lpos in response to deformation by dislocation glide in these simulations we consider slip systems identified either in naturally deformed omphacite or in experimentally deformed diopside simulated lpos reproduce very well the characteristic omphacite lpo pattern in naturally deformed eclogites a strong concentration of 001 axes sub parallel to the lineation and of 010 poles sub perpendicular to the foliation these models reconcile the interpretation of omphacite lpos in eclogites and tem observations of naturally deformed omphacite since they show that omphacite lpos in naturally deformed eclogites may develop by dislocation glide on 1 2 110 110 001 110 and 001 100 we also investigate the effect of the strain regime on the omphacite lpo development the simulations show that changes in deformation regime lead to second order variations in omphacite lpo patterns similar to those observed in eclogites such as an asymmetry of the lpo relative to the structural frame a stronger concentration of the 001 axes relative to the 010 pole concentration or a dispersion of 001 axes in the foliation plane this suggests that omphacite lpo patterns may carry information on the deformation regime simple shear transtension active during the high pressure events

arc generated blocks with crustal sections in the north atlantic craton of west greenland crustal growth in the archean with modern analogues abstract the ca 700 km long archean craton of west greenland consists of six meso neoarchean ca 3000 2720 ma shear zone bounded crustal blocks that display similar cross sections from south to north ivittuut kvanefjord bjornesund sermilik fiskefjord maniitsoq each block has a southerly upper and a northerly lower zone thus each faces upwards to the south upper zones have prograde amphibolite facies mineralogy and have never been in the granulite facies whereas lower zones reached granulite facies and were partly retrogressed to amphibolite facies upper and lower zones consist predominantly of tonalite trondhjemite granodiorite ttg orthogneisses geochemistry suggests generation by slab melting in subduction settings of island arcs and active continental margins the gneisses contain km thick metavolcanic amphibolite layers typically bordered by km thick layers containing anorthosite and leucogabbro most upper zones contain upper greenschist to amphibolite facies metavolcanic belts including volcaniclastic andesitic rocks the two most prominent metavolcanic belts in the fiskefjord block at qussuk andesitic volcaniclastic rocks garde a a 2007 a mid archaean island arc complex in the eastern akia terrane godthabsfjord southern west greenland journal of the geological society london 164 565 579 and ivisaartoq mafic ultramafic rocks and anorthosite leucogabbro from upper and lower parts of a supra subduction zone system polat a frei r appel p w u dilek y fryer b ordonez calderon j c yang z 2008 the origin and compositions of mesoarchean oceanic crust evidence from the 3075 ma ivisaartoq greenstone belt sw greenland lithos 100 293 321 have island arc geochemical signatures the 2 km thick fiskenaesset complex bjornesund block comprises chromite layered anorthosites leucogabbros and gabbros and local pillow bearing roof pendants from overlying metavolcanic amphibolite the style of deformation changes downwards within crustal blocks upper zones are characterised by linear metavolcanic belts deformed by mostly one major phase of isoclinal folding and lower zones by kilometre scale double triple fold interference patterns everywhere ttg protoliths have intruded anorthositic and volcanic rocks typically along ductile shear zones often so extensively that only anorthositic or amphibolitic lenses are preserved the meso neoarchean crust was thickened by a combination of thrusting isoclinal folding and continued ttg injection dissimilarities in the proportions of anorthositic and metavolcanic rocks in the six blocks suggest that they evolved in several different microcontinents but by similar processes these crustal blocks provide an exceptional example of how continents evolved in the meso neoarchean comparable archean examples in kapuskasing and pikwitonei canada and modern analogues in fiordland new zealand kohistan himalayas southern california batholith peruvian andes and hidaka japan demonstrate that processes of continental growth from island arc to continental arc magmatism and by implication to continental collision were broadly similar throughout most of earth history

vein quartz microfabrics indicating progressive evolution of fractures into cavities during postseismic creep in the middle crust discordant quartz veins in metamorphic rocks formed at temperatures similar to 300 c in an extensional tectonic regime the vein shape is characterized by an aperture to length ratio exceeding the maximum value feasible for purely elastic deformation by orders of magnitude non elastic ductile deformation of the host rock must have controlled widening of the arrested cracks fracture parallel shortening is low due to the low accumulated strain the microstructural record of this stage of deformation cannot be isolated in the polyphase host rock information is exclusively obtained from the microfabrics of the vein sealing quartz there the density of geometrically necessary dislocations derived from orientation maps created by electron backscatter diffractometry ebsd decreases markedly from the vein wall to the centre the gradient is attributed to progressive cavity formation and buckling of the vein walls during sealing early grown crystals at the vein margins record a prolonged deformation history compared to lately grown grains in the centre of the vein the gradient and the lack of significant crystal plastic deformation of the crystals in the vein centre show that deformation of the completed vein is insignificant compared to deformation during progressive opening and sealing in view of the precursory brittle deformation and the intense crystal plastic deformation at the vein margins the monogenetic veins are interpreted to record a single mid crustal stress transient related to the earthquake cycle fracturing is proposed to be a consequence of coseismic loading while progressive opening and sealing record the deformation during postseismic stress relaxation by creep

primary fluids entrapped at blueschist to eclogite transition evidence from the tianshan meta subduction complex in northwestern china the tianshan is the first locality worldwide where primary fluids at blueschist to eclogite transition have been documented veins containing high pressure minerals in massive host eclogites transitional to blueschists or eclogite boudins surrounded by blueschists indicate that a free fluid phase was present at the time of eclogitization the high pressure veins are predominantly composed of omphacite fibers with minor quartz or calcite the transition from blueschist to eclogite facies parageneses occurs as dehydration halos around these veins clinozoisite paragonite glaucophane and omphacite inclusions preserved in garnet porphyroblasts in wall eclogites document the transformation of blueschist to eclogite c axis parallel non planar populations of fluid inclusions were trapped during the growth of omphacite in high pressure veins and dehydrated wall rocks low salinity h2o nacl solid bearing inclusions are preserved in omphacite fibers in veins and matrix omphacite of wall rocks none of the isochores of these low salinity aqueous fluid inclusions intersect peak eclogite facies metamorphic conditions suggesting that although the textural evidence constrains the entrapment of fluid inclusions to peak metamorphic conditions their densities must have been modified during exhumation the fluids are interpreted to have been derived from the host blueschist as a result of dehydration reactions such as 13 gln 5 czo 9 prp 26 jd 12 di 19 qtz 15 h2o and gln pg prp 3 jd 2 otz 2 h2o the similarity of vein and wall rock mineral compositions fluid inclusion characteristics and o isotope data also favor an internal source for the fluids the major element composition of veins indicate that si na and ca rich aqueous fluids were released during dehydration at a depth of 50 10 km within a paleozoic subduction zone

experimentally based water budgets for dehydrating slabs and consequences for arc magma generation phase diagrams of hydrous mid ocean ridge mor basalts to 330 km depth and of hydrous peridotites to 250 km depth are compiled for conditions characteristic for subduction zones a synthesis of our experimentally determined phase relations of chlorite lawsonite epidote zoisite amphibole paragonite chloritoid talc and phengite in basalts and of phase relations from the literature of serpentine talc chlorite amphibole and phase a in ultramafics permits calculation of h2o contents in hydrous phase assemblages that occur in natural compositions this yields the information necessary to calculate water budgets for descending slabs starting from low grade blueschist conditions 10 20 km depth with h2o contents between 5 and 6 wt for hydrated oceanic crust complete dehydration is achieved between 70 and 300 km depth as a function of individual slab geotherms hydrous phases which decompose at depth below volcanic arcs are lawsonite zoisite chloritoid and talc phengite in mafic compositions and chlorite and serpentine in peridotite approximately 15 35 of the initially subducted h2o are released below volcanic arcs the contribution of amphibole dehydration to the water budget is small 5 20 and occurs at relatively shallow depth 65 90 km in any predicted thermal structure dehydration is a combination of a stepwise and a continuous process through many different reactions which occur simultaneously in the different portions of the descending slab such a dehydration characteristic is incompatible with single phase dehydration models which focus fluid flow through a unique major dehydration event in order to explain volcanic fronts as a consequence of continuously progressing dehydration water ascending from the slab will be generally available to depth of ca 150 200 km the fluid rising from the subducting lithosphere will cause partial melting in the hot portion of the mantle wedge we propose that the volcanic front simply forms above the mantle wedge isotherm where the extent of melting is sufficient to allow for the mechanical extraction of parental arc magmas thermal models show that such an isotherm ca 1300oc locates below volcanic fronts slab surface depths below such an isotherm are compatible with the observed depths of the slab surface below volcanic fronts 1998 elsevier science b v all rights reserved

exposed plate interface in the european alps reveals fabric styles and gradients related to an ancient seismogenic coupling zone 1 we present observations from a continuous exposure of an ancient plate interface in the depth range of its former seismogenic zone in the central alps of europe related to late cretaceous early tertiary subduction and accretion of the south penninic lower plate underneath the adriatic upper plate the material forming the exposed plate interface zone has experienced flow and fracturing over an extended period of time followed by syncollisional exhumation thus reflecting a multistage evolution fabric formation and metamorphism however chiefly record the deformation conditions of the precollisional setting along the plate interface we identify an unstable slip domain from pseudotachylytes occurring in the temperature range between 200 and 300 c this zone coincides with a domain of intense veining in the subduction melange with mineral growth into open cavities indicating fast possibly seismic rupture evidence for transient near lithostatic fluid pressure as well as brittle fractures competing with mylonitic shear zones continues into the region below the occurrence of pseudotachylytes possibly reflecting a zone of conditionally stable slip the zone above the unstable slip area is devoid of veins but displays ample evidence of fluid assisted processes similar to the deeper zone solution precipitation creep and dehydration reactions in the melange matrix hydration and sealing of the base of the upper plate seismic rupture here is possibly expressed by ubiquitous localized deformation zones we hypothesize that trenchward sealing of parts of the plate interface as well as reaction enhanced destruction of upper plate permeability is an important component localizing the unstable slip zone this relation may result from the competition of the pervasive presumably interseismic pressure solution creep destroying permeability and building elevated fluid pressure until the strength threshold is reached with seismic failure

new evidence for polyphase metamorphism of glaucophane schist and eclogite exotic blocks in the franciscan complex california and oregon abstract the early metamorphic history of high grade exotic blocks in the franciscan complex may be more complicated than previously supposed the different assemblages of high grade glaucophane schist eclogite amphibolite and hornblende schist are commonly considered to have formed at the same time from essentially unmetamorphosed oceanic crust however new textural and mineralogical data presented here suggest that high grade glaucophane schist and eclogite have replaced an earlier epidote amphibolite facies assemblage that is identical to the primary assemblages in many of the hornblende rich blocks at least some of the hornblende rich blocks may therefore be well preserved remnants of the earlier metamorphism comparison of the mineral assemblages and element partitioning in the mixed assemblage blocks suggests that the glaucophane schist and eclogite metamorphism took place at slightly lower temperatures but at the same or higher pressures than the earlier hornblende forming stage

exhumation rates of high pressure metamorphic rocks in subduction channels the effect of rheology 1 exhumation of high pressure metamorphic rocks can take place with typical plate velocities of cm year this is consistent with a model of forced flow in a subduction channel the micro structural record of exhumed metamorphic rocks indicates that stresses are generally too low to drive deformation of the bulk material by dislocation creep according to a power law rheology instead deformation appears to be localized in low strength shear zones and is dominated by dissolution precipitation creep or fluid assisted granular flow implying a newtonian rheology 1d modeling shows that the effective rheology of the material in the subduction channel has a significant influence on the rate of exhumation when the subduction flux either equals or exceeds the return flux the maximum exhumation rate for newtonian behavior of the material is at least twice as high 1 3 of the subduction burial rate compared to that for power law creep 1 6 of the subduction burial rate

an integrated model for transitional pressure solution in sandstones abstract a key aspect of the deformation of sedimentary rocks during diagenesis is pressure solution however it is often not clear why some rocks of a given mineralogy depth and pressure exhibit a great deal of pressure solution while very similar rocks under similar conditions do not to address such difficulties which we believe arise from the natural non linear physico chemical processes a quantitative model for pressure solution applied to sandstones has been developed this model subsumes various factors 1 pressure temperature and burial rate 2 grain size shape and packing of the mineral grains 3 the variability of water film thickness and coefficient of diffusion between any two grains in contact as a function of stress across the contact pore fluid pressure salinity ph and temperature in this study we distinguish several types of surface sites over quartz grains in a sandstone with corresponding differences in chemical potential thus allowing the diffusive transfer of matter from one site to another the contact between two grains is divided into two parts the actual true contact between two grains is a site of water film diffusion and the inclusions inside the contact the free face inclusion contacts are sites of free face pressure solution on the pore surface site precipitation occurs it is shown that a kinetics transition from a reaction limited deformation to a diffusion limited deformation occurs in the upper crust a geometric transition occurs too due to the textural evolution of the rock with deformation the main goal of this paper is to show the dynamics of the shifting importance of actual and free face inclusion contacts using a unified model including both features the result is a predictive model for porosity variations with depth due to pressure solution in sandstones

stylolite interfaces and surrounding matrix material nature and role of heterogeneities in roughness and microstructural development rough pressure solution interfaces like stylolites are one of the most evident features of localized slow deformation in rocks of the upper crust there is a general consensus that the development of these rough structures is a result of localized stress enhanced dissolution of material along a fluid filled interface but little is known on the initiation of this roughness the aim of this article is to reveal the role of heterogeneities initially present in the host rock on roughness initiation this should give insights on whether stylolite roughness is generated by a stress induced instability or by the presence of disorder in the material i e quenched noise we use a microstructural approach based on sem ebsd analysis combined with orientation contrast oc image analysis of stylolites in limestones we found that the stylolite roughness is induced by heterogeneities in the host rock clay particles and detrital quartz grains in our case in addition close to mature stylolite interfaces matrix modifications occur which can be attributed to the compaction along the stylolite the grain size decreases by 15 25 and a pre existing shape and lattice preferred orientation spo lpo are significantly modified in the vicinity of the stylolite the results presented here imply that localized pressure solution along stylolites is not necessarily restricted to the actual interface but influences the adjacent matrix the heterogeneity data might serve as a quantitative basis for elaborate numerical models of localized compaction

kinematics of crystal growth in syntectonic fibrous veins abstract detailed observations of a set of fibrous antitaxial calcite veins in a slate reveal that some of the calcite fibres do not connect material markers on both sides of the vein and can therefore not have tracked the full opening trajectory during vein growth this calls for a better understanding of the mechanisms of fibre formation and reliable criteria to test the tracking hypothesis based on surface roughness characteristics of the vein wall we develop a simple model for shape growth of crystals in a crack seal environment which can account for both tracking and non tracking behaviours and propose a set of tracking criteria for antitaxial veins finally we discuss ways by which the model can be tested against natural examples

development of shape and lattice preferred orientations application to the seismic anisotropy of the lower crust abstract we review the physical basis of the development of fabrics in plastic and viscous flow and illustrate the typical fabrics formed by these processes in the main rock forming silicates of the lower crust feldspar quartz pyroxene and amphibole the orientation process in plastic deformation where a single slip is dominant is recalled and the role of the constraint of neighbouring grains is emphasized the fabric development of anisometric crystals in viscous flow is discussed as a function of the main controlling parameters shear strain aspect ratio and interference between crystals the same sense of fabric asymmetry is introduced by plastic and viscous flow between the flow plane and the shape preferred orientation and hence coherent kinematic analysis can be undertaken in both modes of flow in order to assess the role of such fabrics in the seismic laminations of the lower continental crust we have calculated the seismic p wave properties of typical fabrics for hypothetical monomineralic and polymineralic rocks the calculations show that the strongest anisotropies develop in monomineralic rock with values between 5 and 16 compared with 5 and 8 for typical rock compositions the strongest anisotropies for layered monomineralic rocks generated by fabrics is only 6 compared to the 14 suggested by model studies of the observed seismic laminations we suggest that other effects such as compositional layering and or constructive interference of seismic waves are responsible for augmenting the apparent anisotropy

plastic flow of coesite eclogite in a deep continent subduction regime microstructures deformation mechanisms and rheologic implications abstract ultrahigh pressure uhp metamorphic rocks are continental crust that once has been subducted to mantle depth and subsequently exhumed to the surface in collisional orogenic belts despite the long distance transport of the uhp rocks in a deep subduction channel and the fast strain rate required for rapid exhumation little evidence for uhp deformation is found in the exhumed uhp terrains so far identified some uhp rocks have even preserved primary igneous and volcanic structures these observations have led to the postulation that the stress level is low a few mpa in the deep subduction zones deformation microstructures of some eclogite mylonites from a ductile shear complex in the sulu uhp metamorphic belt eastern china provide for the first time convincing evidence that dislocation creep is the predominant deformation mechanism for the major constituent minerals of the uhp rocks in a highly strain localized shear zone at the uhp conditions omphacite phengite and kyanite are the main strain accommodating phases and are dynamically recrystallized through progressive subgrain rotation and grain boundary migration coesite is either fractured with irregular extinction in the moderately deformed eclogite or pseudomorphed by polycrystalline quartz aggregates that have a foam structure garnet does not show optically discernable plastic deformation p t conditions for the deformation are estimated to be p 3 3 gpa and t 624 c using garnet omphacite kyanite phengite coesite quartz thermobarometry suggesting that deeply subducted continental crust remains cold at the upper mantle depths large part of the deeply subducted crust remains relatively undeformed strain is strongly localized into narrow shear zones stress level in the deep subduction channel is extrapolated on the order of tens of mpa much higher than it was previously thought

retrogressive dissolution of garnet effect on garnet biotite geothermometry cation exchange geothermometry is predicated on the basis of chemical equilibration between phases involved in ion exchange reactions if chemical equilibrium is attained between the phases at any temperature and subsequently locked in temperature determinations should be consistent throughout a given sample the attainment of chemical equilibrium is commonly inferred where phases used for thermobarometry are in mutual contact or where sharp straight grain boundaries exist between grains thermometric determinations on garnet biotite pairs in a metapelite from the cheyenne belt wyoming illustrate the problem of using these criteria to infer chemical equilibrium in one example garnet biotite pairs from different parts of a single zoned garnet grain yield apparent temperatures that range over 150 c and are anomalously higher than documented metamorphic conditions almandine garnets from low to intermediate grade pelitic rocks commonly show strong compositional zoning retrograde modification coupled with strain induced dissolution of garnet may result in meaningless temperatures because of nonequilibration of phases used for geothermometry apparent textural equilibrium or mutual contact of grains may not be indicative of chemical equilibrium dissolution of garnet under conditions of biotite stability can produce garnet biotite pairs with sharp straight grain boundaries suggestive of textural equilibrium in the absence of visible truncation of compositional zones compositional maps or microprobe traverses both parallel and normal to schistosity are necessary to evaluate the possibility of dissolution failure to do so may result in the incorrect interpretation of t p determinations

35 ma old ultrahigh pressure metamorphism and evidence for very rapid exhumation in the dora maira massif western alps abstract ion microprobe shrimp data on zircons from various rock types of an ultrahigh pressure uhp metamorphic whiteschist type pyrope quartzite lens of the dora maira massif dmm consistently show domains giving a late eocene age of 35 4 1 0 ma which is taken as the age of uhp metamorphism these domains partially replace the older oscillatory zoning pattern of the zircons formed during primary magmatic crystallization at about 275 ma zircons of a metagranitic country rock next to the uhp metamorphic lens have these primary features best preserved all zircons measured also yield intermediate ages between 275 and 35 ma with a statistical concentration between 260 and 210 ma thus the uniformity of the initial zircon population both in the lens and the country rock evidences a common protolith that is a granite intruded during the late herynian while the intermediate ages are at least partly due to incomplete resetting of the zircons during uhp metamorphism those in the 260 210 ma range may be related to rifting episodes in the permotriassic the mg rich chemistry of the whiteschist lenses is due to local metasomatic alterations of the granite perhaps by fluids derived from evaporitic sediments dating as early as the permotriassic as well the more pervasive resetting of zircon ages during uhp metamorphism in the pyrope quartzite lenses is explained by the ubiquity of fluids and or melts produced during subduction by a series of dehydration reactions that occurred only in the more hydrous mg rich protoliths and not in their drier granitic neighborhood fission track ages determined partly on the same zircon samples yielding 29 9 1 4 ma mark the time when the uhpm unit had reached about 290 c at a shallow location within the crust thus exhumation over a vertical distance of about 120 km must have occurred within about 5 6 ma indicating an average uplift rate of about 20 24 km ma and an average cooling rate of about 85 100 c ma the radiometric data obtained do not lend any support to an eo alpine cretaceous subduction event so that deep subduction and immediately following exhumation all took place during early to mid tertiary time this scenario seems to apply to large portions of the western and central alps as well calling for drastic geodynamic reinterpretations of these parts of the alps

deformation induced clinopyroxene fabrics from eclogites abstract clinopyroxene omphacite microfabric studies in eclogites are useful in characterizing the deformation that has affected deeper parts of subduction zones we present the microfabrics and tem structures of four samples representative of the various deformation types known in eclogites the 001 axis tends to be parallel to the lineation while the 010 plane is close to the foliation but these general features undergo strong variations in relation to the shape fabric in strongly lineated samples 010 tends to be spread out in a great circle normal to the lineation whereas 001 is dispersed in the foliation in the case of strongly foliated samples the slip systems identified by tem are 001 100 001 110 and 1 2 110 110 operation of dislocation creep is testified by the presence of dislocations and dislocation walls but this process alone cannot explain all the observed features particularly some aspects of the crystallographic preferred orientation hence other deformation mechanisms must be involved these may be dynamic migration recrystallization by grain boundary migration as well as mass transfer processes this study illustrates that not every crystallographic mineral preferred orientation can be explained by dislocation glide and or twinning processes alone in addition it also demonstrates that eclogite microfabrics can be used to unravel deformation parameters in subduction and or collision zones

low differential stress during high pressure metamorphism the microstructural record of a metapelite from the eclogite zone tauern window eastern alps constraints on the state of stress at deep levels of convergent plate boundaries can be obtained from the microstructural record of high pressure metamorphic rocks provided that this record has remained unchanged during exhumation the inclusion patterns in poikiloblastic garnet of a metapelite from the eclogite zone of the tauern window eastern alps reveal a foam microstructure of quartz controlled by grain boundary free energy the garnet blasts show concentric chemical zonation thermobarometry based on correlation between compositional domains in garnet chloritoid and phengite coexisting with kyanite rutile and quartz shows that garnet growth commenced before peak p t conditions close to 25 kbar and 600 c had been reached and continued during the early stage of decompression this implies that the microstructure of quartz controlled by grain boundary free energy persisted continuously during the latest stage of burial to ca 80 km depth and the early stage of exhumation involving detachment from the downgoing slab and release into the subduction channel the foam structure of the overgrown quartz indicates that the differential stress was too low to drive dislocation creep during that period when the rock crossed the interface between the plates with a high differential stress resulting in tectonic overpressure or significant shear heating being ruled out outside the protective garnet the interfacial free energy controlled quartz microstructure has been destroyed completely due to dynamic recrystallization during deformation by dislocation creep this happened while the crustal slice was exhumed to a depth of about 45 km the preservation of this synkinematic quartz microstructure indicates a tectonic scenario where the final cooling was rapid and no marked stress peak was associated with the brittle plastic transition

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

eocene high pressure metamorphism in the penninic units of the tauern window eastern alps evidence from 40 ar 39 ar dating and petrological investigations the40ar 39ar degassing spectra of white micas and amphiboles from three tectonic units of the central tauern window pennine basement and cover in the eastern alps have been measured white micas are classified as 1 pre alpine low si relic micas with an age value of 292 ma variously disturbed by the alpine metamorphism 2 alpine phengitic micas of variable composition with an age between 32 and 36 ma 3 alpine low si micas with a maximum age of 27 ma we attribute the higher alpine ages to a blueschist facies event whereas the lower age reflects the late cooling of the nappe pile blueschist facies phengites from the basement lower schieferhulle and the tectonic cover upper schieferhulle crystallized at a temperature below the closure temperature tc for argon diffusion in white mica and record ages of 32 to 36 ma at the same time a thin eclogite facies unit eclogite zone was thrust between the lower and the upper schieferhulle and cooled from eclogite facies conditions at about 600 c at 20 kbar to blueschist facies conditions at 450 c or even 300 c at 10 kbar eclogite facies phengites closed for argon diffusion and record cooling ages coinciding with the crystallization ages in the hanging and the footwall unit amphibole age spectra actinolite glaucophane barroisite are not interpretable in terms of geologically meaningful ages because of excess argon

crystallographic preferred orientation produced by diffusional creep of harzburgite effects of chemical interactions among phases during plastic flow 1 experimental deformation experiments have been conducted on fine grained two phase aggregates of olivine and orthopyroxene to investigate the role of grain and phase boundary sliding on rheology and fabric development a suite of large strain 1 general shear experiments conducted at t 1200 c and p 1 6 gpa on aggregates ranging from 65 to 0 vol orthopyroxene to characterize the evolution of fabric were complemented by small strain axial compression experiments at t 1200 c and p 0 3 gpa to better constrain the rheology microstructural and rheological data suggest that deformation of these two phase aggregates in the diffusion creep regime occurs via interface reaction controlled diffusion creep that is accompanied by extensive migration of olivine orthopyroxene phase boundaries the resulting rheologies suggest that olivine orthopyroxene composites are weaker than the olivine end member at the conditions tested physically this behavior arises because long range i e grain scale diffusion of si4 is unnecessary in these pseudobinary two phase aggregates we further demonstrate that interface controlled diffusion creep leads to strong crystallographic preferred orientations cpo of the component minerals which develops in the near absence of dislocation activity the cpo formed in these anhydrous low stress experiments has the olivine a axis aligned perpendicular to the flow direction type b fabric argued by some to be the unique result of deformation under conditions of high differential stress and high water fugacity phase boundary dynamics thus are argued as a significant factor in the accumulation of strain in polyphase aggregates

crack seal fibre growth mechanisms and their significance in the development of oriented layer silicate microstructures abstract an analysis of crack seal fibre growth mechanisms has shown two important implications of the crack seal process for the interpretation of microstructures in low grade metamorphic rocks 1 1 the ability of fibre growth to track the incremental separation history of crack walls depends on details of the nucleation and growth mechanisms 2 2 fibrous crystals may develop cristallographic preferred orientations in certain crack seal situations the microstructures of layer silicates and associated phases developed in several examples of syntectonic intragranular microfracture sites and veins indicates that layer silicate 001 and grain shape preferred orientation can develop during crack seal deformation by oriented growth mechanisms during successive crack seal increments preferred orientation may develop in response to an interaction between anisotropic growth kinetics and the displacement history resulting in preferential rejoining by syntaxial overgrowth of pulled apart grains having fast growth directions parallel to the incremental displacement direction across a microcrack preferred orientation may also be developed and enhanced in crack seal growth sites by overgrowth of previously oriented layer silicates in the microcrack walls in this case the crystallographic preferred orientation need not be simply related to the displacement history during crack seal fibre growth since crack seal processes may operate on all scales down to the minimum size of a microfracture in a deforming rock such mechanisms of layer silicate preferred orientation development are expected to be very significant in developing and enhancing foliation during deformation involving microfracture and solution transfer processes

episodic methane concentrations at seep sites on the upper slope opouawe bank southern hikurangi margin new zealand along many active and some passive margins cold seeps are abundant and play an important role in the mechanisms of methane supply from the subsurface into seawater and atmosphere with numerous cold seeps already known the convergent hikurangi margin east of north island new zealand was selected as a target area for further detailed multidisciplinary investigation of cold seeps within the new vents and associated projects methane and temperature sensors mets were deployed at selected seep sites on the opouawe bank off the southeastern tip of north island and near the southern end of the imbricate thrust hikurangi margin together with seismic ocean bottom stations they remained in place for about 48 h while seismic data were collected the seeps were associated with seep related seismic structures methane concentrations were differing by an order of magnitude between neighbouring stations the large differences at sites only 300 m apart demonstrate that the seeps were small scale structures and that plumes of discharged methane were very localised within the bottom water high methane concentrations recorded at active seep sites at anticlinal structures indicate focused fluid flow methane discharge from the seafloor was episodic which may result from enhanced fluid flow facilitated by reduced hydrostatic load at low tides the strong semi diurnal tidal currents also contribute to the fast dilution and mixing of the discharged methane in the seawater despite dispersal by currents fluid flow through fissures fractures and faults close to the mets positions and tidal fluctuations are believed to explain most of the elevated methane concentrations registered by the mets small earthquakes do not appear to be correlated with seawater methane anomalies