a pressure solution creep law for quartz from indentation experiments 1 indenter experiments have been performed on quartz crystals in order to establish a pressure solution creep law relevant at upper to middle crustal conditions this deformation mechanism contributes to earth s crust geodynamics controlling processes as different as fault permeability strength and stress evolution during interseismic periods or mechanochemical differentiation during diagenesis and metamorphism indenter experiments have been performed at 350 c and 20 120 mpa during months with differential stress varying from 25 to 350 mpa several experimental parameters were varied nature of quartz synthetic or natural nature of fluid manner in which the solid solution solid interface was filled and orientation of the indented surfaces versus quartz crystallographic c axis significant strain rates could only be obtained when using high solubility solutions naoh 1 mol l 1 displacement rates of the indenter were found activated by differential stress with exponential dependence as theoretically predicted the mean thickness of the trapped fluid phase below the indenter was estimated in the range 2 10 nm moreover the development of this trapped fluid phase was relatively fast and allowed fluid penetration into previously dry contact regions by marginal dissolution the indenter displacement rate was driven by differential stress and its kinetics was controlled by diffusion along the trapped fluid and the development of a morphological roughness along the interface conversely marginal strain energy driven dissolution was observed around the indenter and its kinetics was controlled by free surface reaction these experimental results are applied to model the interactions between pressure solution and brittle processes in fault zones providing characteristic time scales for postseismic transitory creep and sealing processes in quartz rich rocks

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

comparison of brittle and viscous creep in quartzites implications for semi brittle flow of rocks abstract the co existence and interaction between brittle and viscous deformation processes contributes to the integrated strength of the crust and results in a wide range of energy release mechanisms ranging from earthquakes to creep here we compare flow laws derived for quartz rich rocks deforming by brittle creep wet dislocation power law creep and dissolution precipitation creep we investigate theoretically the conditions when both brittle and viscous processes contribute significantly to deformation provided that all processes act independently and in parallel utilizing a comprehensive data set for deformation experiments in quartz rich rocks we find that the transition between deformation mechanisms is strongly dependent on input variables such as initial flaw size and grain size the transition can occur abruptly or over hundreds of mpa in differential stress and hundreds of degrees kelvin at a constant strain rate the transition is strongly dependent on grain size and confining pressure limitations to this work are first that all three flow laws are poorly constrained by experimental data for conditions relevant for the comparison secondly a need exists for new experiments to infill the knowledge gaps between high temperature and low temperature deformation experiments and deriving quantitative flow laws for low temperature plasticity and high temperature brittle creep

experimental deformation of a glassy basalt abstract deformation experiments were conducted with the intention of identifying how the mechanical behavior and deformation microstructures of a basalt are influenced by the presence of glass that can crystallize to metamorphic minerals or flow a glassy plagioclase rich basalt was tested for periods of up to 649 hours at temperatures of 675 875 c a confining pressure of 1 0 gpa and strain rates of 10 4 10 7 s 1 over the whole range of experiments the basalt deformed ductilely in mechanical and textural steady state at low temperatures 675 775 c plagioclase crystals deformed by microcracking and twinning and the glass did not flow into intracrystalline cracks at higher temperatures 825 875 c the samples were notably weaker plagioclase crystals deformed by twinning and dislocation glide and the glass flowed into cracks in all samples that were tested for more than 30 hours neoblasts of dark mica and quartz crystallized at the expense of k rich glass enstatite h2o this neomineralization did not affect the strength of the basalt presumably because it affected only 50 vol of each sample changes in plagioclase deformation mechanism from twinning and microcracking at fast strain rates to dislocation glide at slow strain rates may however reflect an increase in effective pressure caused by crystallization of the glass

roughness at the base of the seismogenic zone contributing factors the cutout depth of microseismic activity in continental fault zones appears to correspond to the onset of greenschist metamorphic conditions at about 300 c it can generally be modeled as the transition from frictional to quasi plastic behavior in quartzofeldspathic crust shear resistance increases with depth through the frictional regime to peak at the transition beneath which it falls off exponentially with increasing temperature larger earthquake ruptures ml 5 5 nucleate around this transition depth where the highest concentrations of strain energy may accumulate varying depth and amplitude of the peak shear resistance along strike induce fluctuations in strain energy concentration at the base of the seismogenic zone factors affecting the depth of the transition include crustal composition geometry and mode of faulting fluid pressure levels in the frictional regime and water content in the quasi plastic regime quasi plastic strain rate and geothermal gradient evaluation of their relative importance is complicated because several are interdependent however compositional change may cause abrupt irregularities in seismogenic depth and peak shear resistance while regional variations in heat flow look to be particularly effective in creating long wavelength heterogeneities in strain energy concentration affecting faulting style

influence of microscale weak zones on bulk strength shear zones have different rheological properties than the surrounding rocks indicating that the bulk strength of regions containing shear zone networks cannot be determined by considering the the host rock rheology alone we demonstrate the value of this concept at the microscale we first consider the phase arrangements in naturally deformed rocks and document that weak phases exhibit little interconnection within a microstructure rather three dimensional weak zones analogous to viscous shear zones can interconnect or bridge weak phases these zones typically form at high stress sites comprise multiple minerals and deform by mechanisms independent of those in the surrounding minerals the presence of weak zones strongly affects the bulk strength of the rock disproportionate to the mode of the weak zones for example the development of 1 mode of a weak zone at a high stress site can reduce the bulk strength of the rock nearly an order of magnitude calculation of the bulk strength of the rock by some averaging algorithm of the deformation mechanisms operating outside the weak zones will overestimate strength instead accurate calculations and predictions of bulk strength require accounting for the presence and geometry of weak zones for this reason we advocate use of the scale independent conceptual rheological model of interconnected weak zones or layers rather than that of interconnected weak phases more generally the way forward in improving quantification of the mechanical properties of the lithosphere requires recognizing and explicitly accounting for the spatial and temporal distribution of deformation mechanisms operating throughout a rock this article is protected by copyright all rights reserved

gravity anomalies and the structure of western tibet and the southern tarim basin gravity anomalies across the western part of the tarim basin and the kunlun mountain belt show that this area is not in local isostatic equilibrium these data can be explained if a strong plate underlying the tarim basin extends southwestward beneath the belt at least 80 km and supports part of the topography of northwest tibet this corroborates norin s inference that late tertiary crustal shortening has occurred in this area by southward underthrusting of the tarim basin beneath the kunlun this study places a lower bound on the amount of underthrusting

low frictional strength of quartz rocks at subseismic slip rates 1 laboratory experiments on rocks at low sliding velocity typically yield values of the friction coefficient of 0 6 0 85 here we demonstrate that an extraordinary decrease in friction coefficient accompanies sliding of room dry quartz rocks at rates faster than in most laboratory experiments but slower than seismic slip rates in some cases the friction coefficient decreases from low speed values by more than a factor of 3 this extraordinary weakening likely results from the formation of finely comminuted amorphous wet material on the sliding surface

simple shear experiments on calcite rocks rheology and microfabric ideal simple shearing of carrara marble and solnhofen limestone was achieved by inserting calcite rock slices into sandstone cylinders precut at 35 the experiments covered a wide range in temperatures 25 900 c at confining pressures of 200 and 250 mpa and at strain rates between 10 3 and 10 5 s 1 shear strains of up 2 7 were achieved the stress strain data for simple shear favourably compare with predictions on the basis of existing flow laws established in coaxial testing this comparison demonstrates that the simple shear experiments covered most of the previously established flow regimes for calcite rocks at the highest temperatures carrara marble exhibited substantial work softening after high shear strains a complete texture analysis was carried out with the u stage and a texture goniometer the following four distinct microfabric regimes were found 1 twinning regime an oblique c axis maximum deflected from the pole to the flattening plane towards 1 and against the imposed sense of shear results directly from e twinning extensive twinning some grains are completely twinned rapidly rotates the c axes into near parallelism with 1 this regime was observed for carrara marble deformed at temperatures below 700 c and for solnhofen limestone at room temperature 2 intracrystalline slip regime three c axis maxima were observed in both rock types at more elevated temperatures and in the absence of twinning these c axis orientations result from an alignment of slip planes and slip directions into parallelism with the shear zone boundary and the shear direction respectively this finding supports the idea of an orientation for easy slip resulting from simple shearing in addition to r and f glide basal glide is inferred to have been operative 3 grain boundary sliding regime a great circle distribution of c axes approximately perpendicular to the direction of finite shortening is observed this texture is weaker compared to the other regimes it is interpreted to result from minor contributions of f slip towards total strain this regime is restricted to fine grained solnhofen limestone deformed at the highest temperatures 4 grain boundary migration regime only in carrara marble deformed at the highest temperatures is there substantial grain boundary migration recrystallization the texture consists of a dominant c axis maximum oriented perpendicular to the shear zone boundary this single maximum corresponds to an orientation for easy slip for basal slip grain boundary migration assists rapid lattice reorientation and leads to geometrical work softening quantitative analysis of grain boundary surface orientations gave the following results within the twinning and the intracrystalline slip regimes the characteristic grain shape is elliptical for both rock types the strain ellipses derived from this analysis closely correspond to the known finite strain at the lowest temperatures at intermediate temperatures and especially in carrara marble there is a consistent trend for the aspect ratios to indicate too little flattening and for the angles between the long axis of the strain ellipse and the shear zone boundary to be too small in the absence of grain boundary migration recrystallization this finding indicates minor translations of grains with respect to one another grain boundary sliding or translations accommodated in the mantle regions of grains at the highest temperatures where extensive grain boundary migration occurs in carrara marble grain boundary migration regime two preferred orientations of grain boundaries are observed 1 parallel to the shear zone boundary and 2 parallel to an oblique direction deflected 20 away from the normal to the shear zone boundary in the sense of the imposed shear the same pattern of grain boundary alignment is observed in solnhofen limestone deformed in the grain boundary sliding regime the geological applications of this experimental work are discussed in terms of calcite tectonites in particular and sheared rocks in general

experimental syntectonic hydration of basalt abstract this paper outlines in general terms the role of metamorphic transformations during rock deformation long duration triaxial stress relaxation tests at 600 c 160 mpa confining pressure on wet crushed basalt sheared in sawcuts made in intact basalt cylinders are reported mechanical data are interpreted in terms of a material which shows a small strain rate sensitivity to large variations in applied stress but which work hardens dramatically as a result of marked microstructural changes which occur during straining permanent distortions of c 20 were produced in the samples tests at 100 mpa pore water pressure resulted in the formation of oriented overgrowths of amphibole preferentially developed on less stressed interfaces between original grains and in pore spaces substantial redistribution of feldspar also occurred at low pore pressures no amphibole formed and only feldspar was recrystallized much of the original porosity of the rock was eliminated in all cases the mechanical and microstructural data are interpreted in terms of flow by some combination of grain sliding and diffusional accommodation with the reprecipitated diffused material being the product of a chemical reaction involving all original solid phases plus water from a consideration of the effects of the reaction on the chemical potential gradients driving diffusion it is concluded that although the syntectonic hydration exerts a profound effect on the microstructural and mineralogical development of the rock during deformation there is no substantial enhancement of the rate of strain similar microstructural developments also occur in natural rocks deformed during retrogressive metamorphic events and examples are described for comparison

constraints on the structure of the himalaya from an analysis of gravity anomalies and a flexural model of the lithosphere the intracontinental subduction of india beneath the himalaya presents several similarities to that occurring at island arcs we study one of those similarities by analyzing gravity anomalies across the himalaya assuming that the topography is supported by the indian elastic plate flexed under the weight of both the overthrust mountains and the sediments in the ganga basin we first examine in detail the effects of each of the following parameters on the configuration of the elastic plate and on the gravity anomalies the flexural rigidity the position of the northern end of the elastic plate the amount of underthrusting of such a plate beneath the range and the density contrasts between the crust and mantle and between the sediments and the crust a plate with a constant flexural rigidity of about 0 7 1025 n m between 0 2 and 2 0 1025 n m allows a good fit to the data from the lesser himalaya and the ganga basin such a plate however cannot underthrust the entire himalaya instead the gravity anomalies show that the moho steepens from only about 3 beneath the lesser himalaya to about 15 beneath the greater himalaya this implies a smaller flexural rigidity beneath the greater himalaya 0 1 to 1 0 1023 n m than beneath the ganga basin and the lesser himalaya even with a thin weak plate beneath the greater himalaya the weight of the mountains depresses the plate too much unless an additional force or moment is applied to the plate the application of a bending moment unit length to the end of the plate of about 0 6 1018 n m is adequate to elevate the indian plate and to bring the calculated gravity anomalies in agreement with those observed both the smaller flexural rigidity and the bending moment can be understood if we assume that part or all of the indian crust has been detached from the lower lithosphere that underthrusts the greater himalaya we study the tectonic implications of these results by means of a series of idealized balanced cross sections from the collision to the present that reproduce several important features of the geology of the himalaya and predict an amount of eroded material comparable to that in the ganga basin and the bay of bengal these cross sections include high grade metamorphic rocks near the main central thrust and a steeper dip of it there than in the lesser himalaya they predict rapid uplift only in the greater himalaya and at the foot of the lesser himalaya

the role of fluid pressure on frictional behavior at the base of the seismogenic zone to characterize stress and deformation style at the base of the seismogenic zone we investigate how the mechanical properties of fluid rock systems respond to variations in temperature and strain rate the role of fluids on the processes responsible for the brittle ductile transition in quartz rich rocks has not been explored at experimental conditions where the kinetic competition between microcracking and viscous flow is similar to that expected in the earth our initial analysis of this competition suggests that the effective stress law for sliding friction should not work as efficiently near the brittle ductile transition as it does at shallow conditions

stresses and pressures at the quartz to coesite phase transformation in shear deformation experiments coesite was found in quartz aggregates experimentally deformed at confining pressures of 1 0 1 5 gpa and temperatures between 600 c and 900 c the confining pressure pc and in most cases the mean stress m of the experiments were below those of the quartz to coesite phase transformation yet coesite formed when the maximum principal stress 1 was within the p t range of the coesite stability field in one sample the euhedral coesite grains were corroded indicating that coesite started to transform back to quartz it is inferred that this sample started to deform with 1 above the quartz to coesite phase transformation and with on going deformation 1 decreased to values in the quartz stability field due to strain weakening in all cases 1 triggered the quartz to coesite reaction as well as the reverse reaction suggesting that 1 is the critical parameter for the quartz to coesite transformation not pc or m with progressive deformation the coesite laths rotated towards the shear plane as more rigid particles with the sense of shear in case of back reaction new quartz grains exhibit no systematic crystallographic relationship with respect to old coesite the experiments cover different degrees of pressure overstepping different temperatures and different experimental durations at p and t and deformation always enhances the reaction kinetics the observation that 1 is critical for a pressure dependent phase transformation also for reversals poses questions for the thermodynamic treatment of such phase transformations

creep and strain dependent microstructures of synthetic anorthite diopside aggregates abstract we investigated plastic deformation of fine grained synthetic diopside and two phase anorthite diopside aggregates in triaxial compression and torsion up to 5 temperature confining pressure and stress ranged between 950 1180 c 200 400 mpa and 5 500 mpa respectively water content of samples ranged between 0 005 0 002 and 0 075 0 025 wt h 2 o all samples deformed in linear viscous creep with a stress exponent of 1 0 0 2 the activation energy ranged between 571 53 and 290 28 kj mol depending on mineralogy and water content sample strength depended on water fugacity with an exponent of 1 55 0 25 samples deformed in torsion and coaxial compression gave similar flow laws in spite of significant differences in the corresponding microstructures scanning and transmission electron microscopy of two phase samples deformed in torsion showed phase mixing cavitation and dislocation processes we suggest that linear viscous creep of fine grained two phase aggregates involved grain boundary sliding accommodated by grain boundary diffusion and significant dislocation accommodation at high strains we also observed that cavity coalescence and microcracking led to sample failure hence dynamic instabilities may exist in high strain shear zones accommodating viscous deformation in the lower continental crust

crystal growth in cataclasites diagnostic microstructures and implications abstract microstructures in cataclasites from low and high angle fault zones in granitic basement rocks of the scandinavian caledonides indicate the operation of several processes intensive formation of microcracks filled with fine grained mineral fragments indicates brittle deformation the formation of quartz crystals with euhedral terminations suggests crystal growth occurrence of undulose extinction deformation lamellae and grain boundary recrystallization in other quartz grains illustrates plastic deformation crystal growth causes micro cracks to heal and thereby changes the behaviour of rocks undergoing brittle deformation the crystallization products vary in size from c 10 m to c 5 mm narrow banding parallel to rhomb and prism planes is indicative of crystal growth the growth bands show slightly different crystallographic misorientation in some cases the boundaries of these bands could be identified as dislocation arrays decorated by bubbles the occurrence of fibrous chalcedony aggregates and transitions from extremely fine grained quartz to large grains with euhedral terminations suggests the presence of colloidal silica bearing fluids during growth the crystallization process leads to the formation of vein like structures after crystallization plastic deformation may take place followed by brittle deformation and new crystal growth it is concluded that during the formation of the cataclasites a cyclic operation of brittle failure crystal growth followed by plastic deformation took place and this cycle was repeated several times

quartz grain size reduction in a granitoid rock and the transition from dislocation to diffusion creep abstract in the gran paradiso metagranodiorite western alps small scale lower amphibolite facies shear zones record the transition from a mylonite composed of polycrystalline mineral aggregates to a homogeneous ultramylonite with a grain scale phase mixture polycrystalline quartz aggregates in the mylonite deform by dislocation creep developing a crystallographic preferred orientation cpo and a monoclinic surface orientation distribution function odf the polymineralic matrix of the mylonite and the ultramylonite deform by diffusion creep in the ultramylonite the quartz cpo is randomized and the surface odf becomes orthorhombic the transition from mylonite to ultramylonite is accompanied by a grain size decrease and a disintegration of quartz aggregates concomitant with the precipitation of k feldspar biotite between quartz grains in quartz reduction from the dynamically recrystallized grain size in the aggregates 110 m to the size of the dispersed grains in the ultramylonite 25 m occurs through the following processes k feldspar precipitates at opening sites along grain boundaries strain incompatibility pinning the grain size in quartz aggregates coalescence of k feldspar leads to enhanced grain boundary sliding and disintegration of the quartz aggregates solution precipitation reduces the size of the dispersed grains to less than subgrain size 45 m

gravity anomalies and flexure of the lithosphere at mountain ranges the bouguer gravity anomaly over the himalayan alpine and appalachian mountains is characterized by a generally asymmetric gravity low which spans the mountains and associated foreland basins the minimum of the gravity low is generally systematically displaced from the region of greatest topographic relief and shows no obvious relationship to surface geology in addition the alps and appalachians are associated with a generally symmetric gravity high that is unrelated to the topographic relief together the gravity low and high form a characteristic positive negative anomaly couple the steep gravity gradient between the positive and negative couple correlates with the insubric line and its equivalents in the alps and the brevard zone in the southern appalachians this gravity anomaly couple is interpreted as evidence for flexure of the continental lithosphere by subsurface buried and surface topographic loading the magnitude of the subsurface load is estimated from the integrated excess mass represented by the positive anomaly and the magnitude of the surface load from the topography by combining these loads the flexure of the lithosphere and the associated bouguer gravity anomaly are calculated for different assumed values of the elastic thickness te of continental lithosphere the best fitting value of te for the himalayas is 80 100 km for the alps 25 50 km and for the appalachians 80 130 km the model calculations suggest that the main contribution to the gravity couple in the alps and appalachians arises not from surface loads such as thrust sheets and nappes but from subsurface loads however in the case of the himalayas the topography appears to be a sufficient load to explain the observed asymmetric gravity low when subsurface loads do exist therefore we would not expect a close correlation between mountain topographic relief and depth to the m discontinuity as required by classical models of isostasy rather the m discontinuity is expected to relate to the center of mass of all the loads acting on the lithosphere the surface and subsurface loads inferred from the gravity anomaly play a major role in the development of mountains and foreland basins the emplacement of large subsurface loads represents the primary event in the alps and appalachians while in the himalayas the emplacement of the surface load is the primary event the emplacement of the subsurface load probably marks the initiation of foreland basin development the regional characteristics of the basin are controlled by the primary load while local variations are controlled by the secondary thrust sheet loads subsequent movement of either load will produce migration of the basin depocenter the origin of the subsurface load is not clear but its association with the insubric line in the alps and the brevard zone in the appalachians suggests that it may be related to the obduction of crustal blocks flakes onto the underlying plate during continental collision or possibly to the preloaded crustal structure of the underlying plate

in situ and step heating 40ar 39ar dating of white mica in low temperature shear zones tenda massif alpine corsica france in order to clarify the link between 40ar 39ar record in white mica and deformation we performed in situ and bulkwise 40ar 39ar dating over the east tenda shear zone tenda massif alpine corsica white micas from 11 samples were selected and extensively analyzed using in situ techniques across nested scales of strain intensity gradients developed at the expense of a late variscan protolith 40ar 39ar systematics are unaffected by inherited ar and directly linked to deformation with little or no ar lattice volume diffusion extensive sampling allows constraining the end of deformation related to burial and exhumation respectively at 34 and 22 ma bracketing the duration of regional extensional shear to 12 myr results also highlight a regional strain localization toward the upper contact of the unit with smaller scale localization in specific lithologies notably meta aplites second order complications exist such as local ill defined correlations between ages and finite strain microstructures thus the use of a strain gradient as a proxy for strain localization in time is regionally valid but sometimes locally too complex to track or resolve strain partitioning localization trends at the meter outcrop scale and below age mixing and incomplete isotopic homogenization by dissolution precipitation are identified as the main causes of local discrepancies that complicate the link between age and microstructure and the derivation of strain localization rates tracking temporal trends in shear distribution across regional scale deformation gradients in such settings is possible but requires a multi scale approach as implemented here to reveal younging patterns associated to strain localization

deformation of granitoids at low metamorphic grade i reactions and grain size reduction abstract deformation and chemical reaction in granitoids at greenschist facies conditions were investigated in samples from three shear zones two from the alps corvatsch granodiorite and aiguilles rouges granite switzerland and one from eastern australia wyangala granite when examined by light and electron microscopy all show similar features of deformation that progressed from coarse parent rock through to fine grained mylonite quartz deformed plastically while both k feldspar and plagioclase fractured and recrystallized in conjunction with chemical change which had as its end point an assemblage albite quartz white mica caal silicates k feldspar and calcic plagioclase depending on fluid chemistry from the parent rock were unstable at low metamorphic grades in the presence of aqueous fluid since ca bearing plagioclase was not stable in these environments myrmekite did not replace k feldspar in any of the rocks examined recrystallization of feldspar which should perhaps be termed neocrystallization was initiated predominantly at clast margins or along microfractures that are marked by fluid inclusions and twin offsets in old feldspar grains particularly in plagioclases dislocations exist in wall like structures that are commonly associated with voids suggesting origins in microfractures these dislocations had severely limited mobility and subgrain rotation does not appear to have contributed to recrystallization we conclude that recrystallization mainly occurred via a classical nucleation mechanism with minor contributions from twin boundary migration we argue that the 450 550 c lower limit for recrystallization of feldspar referenced throughout the literature is not applicable to the rock systems under investigation and should be discarded as a universal limit microfracturing microboudinage and reaction are seen as the prerequisites for mylonite formation from coarse grained granitic rocks at low metamorphic grades see part ii of this paper neocrystallization fluid access and nucleation appear to have accompanied or immediately followed the first deformation increments and were primarily responsible for continued grain size reduction transgranular fracture was comparatively important only at the onset of deformation since the formation of shear zones in granitoids commonly takes place under conditions of low metamorphic grade in the presence of aqueous fluids in the earth s crust breakdown reactions and nucleation recrystallization in feldspars may be important influences upon localization of deformation in granitoids

high temperature rheology of enstatite implications for creep in the mantle high temperature deformation experiments have been performed on oriented single crystals of enstatite at 1 atm under controlled conditions of oxygen fugacity and silica activity the results indicate that creep on the weakest high temperature slip system in enstatite within the protoenstatite stability field is independent of oxygen fugacity and obeys a power law relation with a stress exponent of n 3 8 0 5 and activation energy of q 820 80 kj mol a decrease in creep rate with increasing aluminum and iron content suggests that aluminum or iron ions play an active role in maintaining charge neutrality within the crystals the single crystal strengths are greater than those reported for polycrystalline enstatite deforming predominantly on the same slip system suggesting that the polycrystalline samples may have been water weakened a comparison of the single crystal results with those for olivine deformed at 1 atm indicates that enstatite is probably stronger than olivine under upper mantle conditions unless the presence of water results in a greater weakening of enstatite than olivine

semi brittle flow of granitoid fault rocks in experiments field studies and seismic data show that semi brittle flow of fault rocks probably is the dominant deformation mechanism at the base of the seismogenic zone at the so called frictional viscous transition to understand the physical and chemical processes accommodating semi brittle flow we have performed an experimental study on synthetic granitoid fault rocks exploring a broad parameter space temperature t 300 400 500 and 600 c confining pressure pc 300 500 1000 and 1500 mpa shear strain rate 10 3 10 4 10 5 and 10 6 s 1 to finite shear strains 0 5 the experiments have been carried out using a granular material with grain size smaller than 200 m with a little h2o added 0 2wt only two experiments performed at the fastest strain rates and lowest temperatures have failed abruptly right after reaching peak strength 1400 mpa all other samples reach high shear stresses 570 1600 mpa then weaken slightly by 10 190 mpa and continue to deform at a more or less steady state stress level clear temperature dependence and a weak strain rate dependence of the peak as well as steady state stress levels is observed in order to express this relationship the strain rate stress sensitivity has been fit with a stress exponent assuming n and yields high stress exponents n 10 140 which decrease with increasing temperature the microstructures show widespread comminution strain partitioning and localization into slip zones containing at first nanocrystalline and partly amorphous material later during continued deformation fully amorphous material in slip zones despite the mechanical steady state conditions the fabrics in the slip zones and outside continue to evolve and do not reach a steady state microstructure below 5 within the slip zones the fault rock material progressively transforms from a crystalline solid to an amorphous material we present and interpret the experimental results both in terms of sliding friction and viscous flow and we discuss the possible effect that the formation of nanocrystalline and amorphous layers may have on earthquake nucleation

amorphous material in experimentally deformed mafic rock and its temperature dependence implications for fault rheology during aseismic creep and seismic rupture abstract amorphous materials are frequently observed in natural and experimentally produced fault rocks their common occurrence suggests that amorphous materials are of importance to fault zone dynamics however little is known about the physico chemical impact of amorphous materials on fault rheology here we present deformation experiments on mafic fault rock where amorphous material forms due to intense mechanical wear during the experiments the experiments are run at temperatures from 300 to 600 c confining pressures of 0 5 or 1 0 gpa and at constant displacement rates of d ax 2 10 7 2 10 8 or 2 10 9 ms 1 resulting in bulk strain rates of 3 10 4 3 10 5 and 3 10 6 s 1 at these conditions the mafic rock material undergoes intense brittle deformation and cataclastic flow but sample strength significantly decreases with increasing temperatures a feature commonly attributed to viscous deformation processes microstructural analyses show that after an initial stage of homogeneous cataclastic flow strain localizes into narrow 2 10 m wide ultra cataclastic bands that evolve into amorphous shear bands with the data presented in this research paper we argue that the temperature sensitivity recorded in the mechanical data is caused by viscous deformation of the amorphous material we suggest that with the formation of amorphous materials during brittle deformation fault rheology becomes significantly temperature sensitive this has important implications for our understanding of fault strength and weakening due to the presence of amorphous materials in addition weak material along faults will lead to stress concentrations that may trigger seismic rupture

deformation of mafic schists from subducted oceanic crust at high pressure and temperature conditions abstract we conducted triaxial deformation experiments using a griggs type solid pressure medium apparatus on two mafic schists that experienced different peak metamorphic conditions from the sambagawa metamorphic belt a greenschist and an epidote amphibole schist at a confining pressure of 1 gpa 400 c and strain rate of 10 5 1 s differential stresses for all mafic schists were higher than 1 gpa the greenschist samples were weaker than epidote amphibolite samples under all experimental conditions microstructures of recovered samples of greenschist indicate that deformation of mafic schist is accommodated by semi brittle deformation samples of the epidote amphibole schist deformed at 500 c show shear localization along a single fault oriented parallel to the foliation while mechanical data shows no rapid stress drop on the surface of the fault we observe the formation of slickensides with little gouge all sample types exhibit a high stress exponent n 15 and strain rate strengthening i e positive a b values frictional behavior that inhibits earthquake nucleation differential stress increased with increasing confining pressure while the friction coefficient decreased with increasing confining pressure and temperature greenschist and epidote amphibolite facies metamorphism typically occurs at hot subduction zones such as cascadia and southwest japan the lack of seismicity within the subducting oceanic crust at hot subduction zones could be explained by semi brittle deformation or frictional sliding with a positive a b during deformation of these types of metamorphic rocks weak and aseismic fault zones in subducting slabs might promote slab mantle decoupling at the corner of the mantle wedge and detachment and underplating of oceanic crust from the subducting slab to forearc crust

water weakening of clinopyroxene in the dislocation creep regime we performed a series of triaxial compressive creep experiments at two different water fugacities to investigate the effect of water on the creep strength of a natural clinopyroxenite samples were deformed under water saturated conditions at temperatures between 1373 and 1473 k confining pressures of 150 and 300 mpa and differential stresses from 34 to 261 mpa strain rates were in the range 10 7 to 10 5 s 1 water fugacity was controlled at either 140 or 280 mpa the creep results yield a stress exponent of 2 7 0 3 and an activation energy of 670 40 kj mol compared to dry clinopyroxene wet samples creep over 100 times faster at a given temperature confining pressure water fugacity and differential stress the creep rate of clinopyroxene is proportional to the water fugacity to the 3 0 0 6 power with an activation volume of 0 m 3 mol one possible water weakening mechanism is an enhancement of the rate of dislocation climb associated with increases in the concentration of jogs and the diffusivity of silicon ions compared to other major minerals in earth s lower crust specifically olivine and plagioclase the water weakening effect is most significant for clinopyroxene under hydrous conditions the strengths of clinopyroxene and anorthite are comparable over the investigated stress range and both phases are weaker than olivine since the mineral assemblages in earth s lower continental crust are often dominated by plagioclase and pyroxene in places where a wet flow law applies the mechanical behavior of clinopyroxene will have a substantial effect on creep strength

rheology of synthetic anorthite diopside aggregates implications for ductile shear zones 1 we investigated the high temperature creep strength of fine grained anorthite diopside rocks at temperatures ranging from 1323 k to 1523 k and at 300 mpa confining pressure in a paterson type gas medium deformation apparatus flow stress varied between 20 and 450 mpa resulting in strain rates between 6 1 10 7 s 1 and 7 5 10 4 s 1 pure diopside and anorthite samples were hot pressed from crushed natural single crystals and glass powders respectively two phase samples were produced by hot isostatic pressing of mechanically mixed powders of anorthite glass with 25 50 and 75 vol diopside particles arithmetic mean grain size of the anorthite matrix is dan 3 5 m three different ranges of diopside particle size were used ddi 25 m 35 m and 45 m water content of as is samples was about 0 05 0 02 wt h2o and predried samples contain about 0 004 0 001 wt h2o at experimental conditions as is samples are assumed to be water saturated water content of predried samples is about 3 times less than that of starting diopside single crystals the specimens contain about 1 vol glass located at fluid inclusions and some multiple grain junctions two grain boundaries examined by high resolution transmission electron microscopy did not show amorphous layers to a resolution of 1 nm at experimental conditions pure diopside aggregates are about 2 3 orders of magnitude stronger than pure anorthite samples for as is and predried specimens respectively in general strength of the two phase aggregates increases with increasing diopside content but remains between isostress and isostrain rate bounds aggregate strengths predicted from continuum mechanics models are in good agreement with the experimental data for dilute diopside particle mixtures and high volume fractions when diopside particles form a load bearing framework at low stresses 100 200 mpa the stress exponent is n 1 suggesting diffusion controlled creep at higher stresses mechanical data and microstructures suggest that samples deformed in the transition region between diffusion controlled creep and dislocation creep for pure anorthite and diopside aggregates deforming in dislocation creep we estimated stress exponents of n 3 and n 5 5 respectively for the two phase aggregates n is between n 3 and n 5 depending on diopside content at low stresses deformation microstructures indicate load transfer from a weak anorthite matrix to stronger diopside particles creep activation energies for pure diopside and anorthite mixtures range from 286 kj mol 1 for wet anorthite deformed at low stresses to 691 kj mol 1 for dry diopside deformed at high stresses activation energies of two phase mixtures are between or close to those of the end members as is samples have significantly lower activation energies than predried samples

accurate differential stress measurement using the molten salt cell and solid salt assemblies in the griggs apparatus with applications to strength piezometers and rheology abstract calibrations have been developed for the molten salt cell and solid salt assemblies used in tri axial griggs type piston cylinder deformation apparatus griggs apparatus that enable accurate differential stress determinations in both assemblies at high pressures and high resolution differential stress measurements in the molten salt cell elastic response of the apparatus was characterized by loading alumina and wc rods and flow strengths were measured for high purity nickel and molybdenum and a ti zr mo alloy in replicate experiments in a griggs apparatus and a heard tri axial gas apparatus gas apparatus at identical pressures temperatures and strain rates the elastic response of the griggs apparatus is nonlinear at low total loads 1 760 mpa on a 5 mm diameter sample but becomes linear at loads greater than 15 kn plastic deformation experiments on water insensitive metals indicate that differential stresses measured in the molten salt cell msc and solid salt ssa assemblies in the griggs apparatus are greater than those measured in the gas apparatus at all conditions but vary systematically and reproducibly a simple linear calibration gas apparatus 0 73 griggsmsc 10 mpa where differential stress 1 3 can be applied to the differential stresses obtained using the msc in the griggs apparatus to obtain sample flow strength a similar linear calibration gas apparatus 0 73 griggsssa 48 mpa 30 mpa can be applied to the differential stresses obtained using the griggs apparatus with a solid salt assembly to obtain accurate sample flow strength these calibrations have been tested by applying them to mechanical data for carbonates deformed in the griggs apparatus and gas apparatus the calibrations are also applied to data from previous studies performed in the griggs apparatus on clinopyroxenites and quartzites and compared to results from similar experiments using the gas apparatus when the experimental starting materials and conditions are the same the recalculated flow strengths measured in the griggs apparatus experiments agree with those measured in the gas apparatus however discrepancies in differential stress are still observed in cases where starting materials differ or water available to silicate samples varies presumably through hydrogen diffusion through sample jackets and hydrogen fugacities of the surrounding confining media

transformation weakening diffusion creep in eclogites as a result of interaction of mineral reactions and deformation abstract the deformation of eclogites and the driving forces for their fabric development are an important topic potentially allowing to determine deformation rates and stresses in subduction zones where the greatest number of large earthquakes occurs here fabric studies of grain size and shape texture and chemical composition from two locations of variscan and alpine eclogites are presented all samples show a well developed crystallographic preferred orientation cpo of omphacite with a strong maximum of 001 in the lineation direction and a weaker maximum of poles to 010 normal to foliation garnet shows no systematic cpo anisotropic chemical zoning developed in omphacite and garnet during growth together with elongated grain shapes and can be related to a prograde in terms of pressure change p t path the individual chemically zoned and elongated grains orientated in the stretching direction are single crystals without major internal misorientations chemical microstructural and cpo data indicate that the deformation microstructure and texture were produced by preferential crystal growth of garnet and omphacite grains in the extension direction dislocation creep can be excluded as a possible fabric formation process by the systematic and oriented chemical zonation of single crystals and absence of dynamic recrystallization microstructures the dominant deformation is inferred to be diffusion creep where dissolution of material took place in reacting mafic phases plagioclase pyroxene and precipitation took place in the form of new eclogite facies minerals omphacite garnet zoisite this type of diffusion creep deformation represents a transformation process involving both deformation and metamorphic reactions it is emphasized that the weakening is directly connected to the transformation and therefore transient the weakening facilitates diffusion creep deformation of otherwise strong minerals pyroxene garnet zoisite at far lower stresses than dislocation creep the results imply low stresses during the deformation of eclogite blocks in subduction zones these results can be applied to other rock types too

experimental investigation of the brittle viscous transition in mafic rocks interplay between fracturing reaction and viscous deformation rock deformation experiments are performed on fault gouge fabricated from maryland diabase rock powder to investigate the transition from dominant brittle to dominant viscous behaviour at the imposed strain rates of view the mathml source 3 10 5 3 10 6 s 1 the transition is observed in the temperature range of 600 c t 800 c at confining pressures of 0 5 gpa pc 1 5 gpa the transition thereby takes place by a switch from brittle fracturing and cataclastic flow to viscous dissolution precipitation creep and grain boundary sliding mineral reactions and resulting grain size refinement by nucleation are observed to be critical processes for the switch to viscous deformation i e grain size sensitive creep in the transitional regime the mechanical response of the sample is a mixed mode between brittle and viscous rheology and microstructures associated with both brittle and viscous deformation are observed as grain size reduction by reaction and nucleation is a time dependent process the brittle viscous transition is not only a function of t but to a large extent also of microstructural evolution

high temperature deformation of diopside single crystal 1 mechanical data laboratory deformation experiments were carried out on diopside single crystals creep tests were made in a dead load apparatus at temperatures t 1020 to 1320 c axial compressive stresses 50 to 170 mpa with strain rates e ranging from 2 10 9 s 1 to 4 10 7 s 1 the specimens were oriented such that mechanical twinning was not possible the experiments were designed to activate 110 1 2 a b slip systems our results demonstrate that they are the major operative systems for t 1000 c and appear to be predominant over the 100 c slip system the mechanical data are fitted to a power law e a n exp e rt an inversion method was used to determine the parameter in a the activation energy e and the stress exponent n below a critical temperature tc 1130 1140 c e is found to be 440 30 kj mol associated with the 110 1 2 a b slip systems activated symmetrically without any contribution from the 100 c system orientation 2 e is found equal to 740 30 kj mol if all three systems have the same resolved shear stresses orientation 1 for t tc values of e drop to 50 15 and 85 30 kj mol for orientations 2 and 1 respectively so that the creep law of diopside becomes nearly temperature independent the law derived from the orientation 2 crystals could be consistent with natural deformations for this orientation and within the range of temperatures covered n is 6 5 0 4 the strong decrease in the activation energy at tc may be ascribed unambiguously to the occurrence of partial melting leading to microdroplets which pin the mobile dislocations ingrin et al this issue the high value found for the stress exponent n in the creep law suggests that clinopyroxene is softer than olivine at high stresses at t 900 c stress 20 mpa and e 10 15 s 1 while olivine is softer at lower stresses this competence inversion is predicted to occur within the ranges of stresses and strain rates expected in the lower crust and upper mantle

weaker than weakest on the strength of shear zones thin laterally continuous ultramylonites within kilometer scale ductile shear zones may control mid lower crustal strength where deformation is localised interconnected phyllosilicate networks are commonly suggested to be the weakest geometry a shear zone can reach yet fine grained polyphase mixtures are commonly found in the cores of high strain zones we study a continental strike slip shear zone which deformed granulite facies quartzofeldspathic migmatitic gneisses at retrograde amphibolite to greenschist facies conditions a brittle feldspar framework and interconnected phyllosilicate networks control the strength of the lower strain protomylonites and mylonites respectively whereas the ultramylonites comprise a fine grained mixture of the host rock minerals the localisation of strain in ultramylonites demonstrates how fine grained polyphase mixtures can be weaker than and supersede interconnected phyllosilicate networks with increasing shear strain this contradicts the common assumption that interconnected layers of phyllosilicates is the weakest state a shear zone can reach

the role of deformation reaction interactions to localize strain in polymineralic rocks insights from experimentally deformed plagioclase pyroxene assemblages abstract in order to study the mutual effect of deformation and mineral reactions we have conducted shear experiments on fine grained plagioclase pyroxene assemblages in a griggs type solid medium deformation apparatus experiments were performed at a constant shear strain rate of 10 5 s 1 a confining pressure of 1 gpa and temperatures of 800 850 and 900 c while the peak stress of plagioclase orthopyroxene assemblages reaches values between those of the end member phases the strength of polymineralic materials strongly decreases after peak stress and reaches flow stresses that stabilize far below those of the weaker phase plagioclase this weakening correlates with the coeval development of high strain shear zones where new phases are preferentially produced including new pyroxene plagioclase and amphibole the reaction products mostly occur as intimately mixed phases within fine grained and interconnected shear bands together with different compositions with respect to the starting material this indicates that deformation significantly enhances the kinetics of mineral reactions which in turn strongly weaken the deforming sample here attributed to a switch to grain size sensitive diffusion creep through phase nucleation and grain size reduction such an interplay between deformation and mineral reactions may have strong implications for the initiation development and durability of shear zones in the lower crust

plagioclase preferred orientation in layered mylonites evaluation of flow laws for the lower crust thesis s m joint program in marine geology and geophysics massachusetts institute of technology dept of earth atmospheric and planetary sciences and the woods hole oceanographic institution 2008

micromechanisms of shear zone propagation at the brittle viscous transition our investigation of progressively strained rock samples from the margins of greenschist facies shear zones utilizes a space for time approach to reveal how the mylonitic overprint of metapsammitic and pelitic host rocks at the cap de creus involved brittle fracturing we present a set of microscale observations indicating that microfractures formed immediately prior to or coevally with a fine grained mylonite microfracturing dominated early stages of strain localization on the scale of the shear zones on the microscale centimeter long fractures facilitated strain softening by allowing enhanced fluid access thereby accelerating the dynamic recrystallization of quartz and a metamorphic reaction of biotite as these two processes produce a polyphase matrix of small dislocation poor grains that eventually form an interconnected rheologically weak phase fractures become inactive this represents a strain dependent brittle viscous transition we outline this transition in a conceptual model for the rheological evolution of mid crustal shear zones

reaction induced weakening of plagioclase olivine composites abstract the localisation of strain into natural ductile shear zones is often associated with the occurrence of metamorphic reactions in order to study the effects of solid solid mineral reactions on plastic deformation of rocks we have investigated the shear deformation of plagioclase olivine composites during the reaction plagioclase olivine orthopyroxene clinopyroxene spinel garnet microstructures of plagioclase olivine composites were studied after shear deformation experiments in a griggs apparatus experiments were performed on anorthite forsterite an fo and labradorite forsterite lab fo composites at 900 c confining pressures between 1000 1600 mpa and with constant shear strain rates of 5 10 5 s 1 in absence of reaction lab fo composites are stronger than pure olivine and labradorite end members that deform with a high temperature plasticity mechanism lab fo composites strain harden due to the inhibition of extensive recrystallisation by interphase boundaries in an fo composites the reaction induces strain weakening by a switch from dislocation creep to grain size sensitive deformation mechanisms through the development of fine grained size

replacement reactions and deformation by dissolution and precipitation processes in amphibolites the deformation of the middle to lower crust in collisional settings occurs via deformation mechanisms that vary with rock composition fluid content pressure and temperature these mechanisms are responsible for the accommodation of large tectonic transport distances during nappe stacking and exhumation here we show that fracturing and fluid flow triggered coupled dissolution precipitation and dissolution precipitation creep processes which were responsible for the formation of a mylonitic microstructure in amphibolites this fabric is developed over a crustal thickness of 500 m in the lower seve nappe scandinavian caledonides amphibolites display a mylonitic foliation that wraps around albite porphyroclasts appearing dark in panchromatic cathodoluminescence the albite porphyroclasts were dissected and fragmented by fractures preferentially developed along the 001 cleavage planes and display lobate edges with embayments and peninsular features two albite oligoclase generations bright in cathodoluminescence resorbed and overgrew the porphyroclasts sealing the fractures electron backscattered diffraction shows that the two albite oligoclase generations grew both pseudomorphically and topotaxially at the expense of the albite porphyroclasts and epitaxially around them these two albite oligoclase generations also grew as neoblasts elongated parallel to the mylonitic foliation the amphibole crystals experienced a similar microstructural evolution as evidenced by corroded ferrohornblende cores surrounded by ferrotschermakite rims that preserve the same crystallographic orientation of the cores misorientation maps highlight how misorientations in amphibole are related to displacement along fractures perpendicular to its c axis no crystal plasticity is observed in either mineral species plagioclase and amphibole display a crystallographic preferred orientation that is the result of topotaxial growth on parental grains and nucleation of new grains with a similar crystallographic orientation amphibole and plagioclase thermobarometry constrains the mylonitic foliation development to the epidote amphibolite facies 600 c 0 75 0 97 gpa our results demonstrate that at middle to lower crustal levels the presence of h2o rich fluid at grain boundaries facilitates replacement reactions by coupled dissolution precipitation and favours deformation by dissolution precipitation creep over dislocation creep in plagioclase and amphibole

the deformation matrix for simultaneous simple shearing pure shearing and volume change and its application to transpression transtension tectonics simultaneous simple shearing and pure shearing with or without additional volume change can be combined into a single upper triangular deformation matrix the off diagonal term is named the effective shear strain and is a simple function of the pure shearing and simple shearing components a three dimensional deformation matrix for the simultaneous combination of coaxial deformation with or without additional volume change and up to three simple shearing systems with mutually orthogonal shear planes is also presented by using this matrix one can easily extract the various properties of incremental as well as finite strain and the progressive as well as finite rotation of passive markers during deformation the case of transpression transtension is revised using the unified deformation matrix the orientation of the major axis of the strain ellipsoid 1 is always horizontal if the deformation is transtensional switches from horizontal to vertical during transpressional wrenching 1 wk 0 81 for constant vorticity deformations and is always vertical for highly transpressional deformations wk 0 81 for transpression material lines initially rotate towards the horizontal shearing direction but generally turn to rotate towards the vertical axis after a certain strain for transtension all material lines rotate towards a direction in the horizontal plane which is oblique to the shearing direction

evolution in h 2 o contents during deformation of polycrystalline quartz an experimental study shear experiments were performed in a griggs type apparatus at 800 c and 1 5 gpa at a strain rate of 2 1 10 5s 1 using different starting materials i powder grain size 6 10 m of dry brazil quartz with 0 15 wt added h2o ii dry brazil quartz porphyroclasts grain size 100 200 m devoid of fluid inclusions embedded in the same fine grained powder and iii wet porphyroclasts grain size 100 200 m containing initially a high density of m scale fluid inclusions embedded in the same powder after hot pressing samples were deformed to large shear strains 3 to 4 5 in order for the microstructures and h2o distribution to approach some state of equilibrium the h2o content and speciation in quartz were analyzed by fourier transform infra red ftir spectroscopy before and after the experiments mechanical peak strength is generally lower in experiments with 100 hydrated matrix intermediate in experiments incorporating wet porphyroclasts with a proportion of 30 or 70 and highest in those with dry porphyroclasts all experiments with porphyroclasts show pronounced strain weakening and the strengths of most samples converge to similar values at large strain wet porphyroclasts are pervasively recrystallized during deformation while dry porphyroclasts recrystallize only at their rims and remain weakly deformed recrystallization of the initially fluid inclusion rich porphyroclasts results in a decrease in inclusion abundance and total h2o content while h2o content of initially dry clasts increases during deformation h2o contents of all high strain samples converge to similar values for matrix and recrystallized grains in samples with wet porphyroclasts shear bands with high porosity and fluid contents develop and they host the precipitation of euhedral quartz crystals surrounded by a free fluid phase these high porosity sites are sinks for collecting h2o in excess of the storage capacity of the grain boundary network of the recrystallized aggregate the h2o storage capacity of the grain boundary network is determined as a h2o boundary film of 0 7 nm thickness

amphibole fabric formation during diffusion creep and the rheology of shear zones we conducted experiments to examine the effect of amphibole forming hydration reactions on mineral fabric development and the strength of mafic rocks both hydrostatic and general shear deformation experiments were conducted on powdered basalt with added water at lower continental crust conditions 800 c 1 gpa amphibole that formed under hydrostatic conditions exhibits a random lattice preferred orientation lpo in contrast amphibole formed during deformation exhibits both a strong shape preferred orientation spo and lpo with a 001 maximum aligned subparallel to the shear direction plagioclase in both hydrostatic and deformed samples shows a very weak to random lpo at low effective strain rates 10 5 s 1 to 10 6 s 1 the stress exponent is 1 1 5 consistent with deformation accommodated by diffusion creep the correlation of the spo and lpo coupled with the rheological evidence for diffusion creep indicates that the amphibole fabric results from oriented grain growth and rigid grain rotation during deformation the experimentally produced fabrics are strikingly similar to those observed in amphibolite grade natural shear zones supporting interpretations that such rocks deform by diffusion creep in addition the rheology of the fine grained experimental amphibolite is comparable to that predicted using flow laws for wet anorthite thus both our experiments and field analyses indicate that wet plagioclase rheology provides a good constraint on the strength of hydrated lower crust

influence of water fugacity and activation volume on the flow properties of fine grained anorthite aggregates 1 to specify quantitatively the effect of pressure and water weakening on the flow strength of feldspar we performed triaxial creep experiments in a gas deformation apparatus at temperatures of 1000 1150 c confining pressures of 100 450 mpa and axial stresses of 10 400 mpa resulting in strain rates of 6 10 7 to 3 10 3 s 1 dense samples with a grain size of 3 m were prepared by hot isostatic pressing of anorthite glass powder hydrous samples contain about 0 33 0 14 wt h2o and dry specimens 0 0005 0 02 wt h2o the estimated residual glass content of wet samples is 2 vol samples deformed by grain boundary diffusion controlled creep at low stresses and dislocation creep at stresses 150 mpa we estimate an activation volume of v 24 cm3mol 1 for anhydrous samples deforming in diffusion creep for wet samples deformed in hydrous conditions with varying buffers fixing oxygen fugacity the activation volume is about 38 cm3 mol 1 creep rate of hydrous anorthite aggregates depends on water fugacity raised to a power of r 1 0 0 3 suggesting hydrolysis of oxygen bonds considering the effect of activation volume and water fugacity on extrapolation of constitutive laws to conditions prevailing in the continental lower crust viscosities of hydrous feldspar aggregates increase by a factor of 3

evolution of c axis pole figures and grain size during dynamic recrystallization results from experimentally sheared quartzite 1 quartzite samples experimentally sheared at conditions where subgrain rotation and grain boundary migration recrystallization are active show a striking change in the c axis crystallographic preferred orientation cpo with increasing shear strain up to 8 and degree of recrystallization up to 100 we used optical methods to determine the c axis cpo for the bulk samples and for porphyroclasts and recrystallized grains separately and to track the strengths of the cpos in five different orientation domains with increasing the c axis pole figure evolves from a broad peripheral maximum indicative of basal a slip to an inclined single girdle with two maxima indicative of rhomb a slip and finally an elongate single maximum at the girdle center indicative of prism a slip throughout this sequence the fabric skeleton rotates with the sense of shear the bulk preferred c axis orientation results from growth and shrinkage of differently oriented crystallographic domains grain boundary density analysis indicates that the size of prism a recrystallized grains is 1 5 times that of average and 2 times that of rhomb grains indicating that prism a slip is significantly easier at our experimental conditions this inference is supported by autocorrelation function analysis of the various orientation domains and how they evolve with strain similar progressive changes in c axis cpo are observed in quartzites naturally sheared at conditions where grain boundary migration recrystallization operates these results indicate that achievement of microstructural and mechanical steady state requires high strain and complete recrystallization

the control of precursor brittle fracture and fluid rock interaction on the development of single and paired ductile shear zones abstract ductile shear zones can occur as relatively isolated single structures as arrays or as characteristic paired zones in continuous glaciated exposures of metagranodiorites from the tauern window eastern alps the control of initial dilatant brittle fracture and associated fluid rock interaction on the geometry of subsequent ductile shear zones can be unequivocally established shearing occurred under amphibolite facies conditions fractures in weakly deformed metagranodiorites are often less than 1 mm thick but extend for tens of metres many are healed joints without shear offset others show minor mm cm discrete dextral offset such brittle faults commonly display a low angle en echelon arrangement with displacement transferred between discrete fracture segments by ductile compressive bridges the geometry of more strongly reactivated zones depends on the degree and heterogeneity of fluid rock interaction which is related to fluid infiltration and veining along the primary fractures with little fluid rock interaction reactivation produces single heterogeneous ductile shear zones centred on and immediately flanking the pre existing fracture with increased fluid rock interaction a bleached halo is developed symmetrically to either side of a central epidote quartz garnet calcite vein ductile shear zones commonly flank this bleached zone to develop a characteristic paired pattern strain is partitioned localizing in the central fracture vein and the flanking shear zones paired zones may anastomose in accordance with changes in the width of the central bleached zone but are always symmetrically spaced with regard to the central fracture vein with increasing deformation the ductile shear zones broaden into the adjacent metagranodiorite but not into the bleached zone which remains preserved as a low strain region paired shear zones can also develop to either side of aplite dykes examples of characteristic paired shear zones usually with a clear central vein are found in many areas ranging from greenschist to eclogite facies suggesting that the mechanism of their formation is quite general

a possible dependence of tectonic strength on the age of the crust in asia in asia the intensity and style of active tectonics appears to depend on the age of the last orogenic activity the shields have remained essentially undeformed during the india eurasian collision but paleozoic and mesozoic orogenic belts apparently have been reactivated by this collision with a suggestion of greater reactivation and more diffuse deformation of the younger belts if the greater observed heat flow in regions of more recent orogenic activity reflects at least in part a greater heat flow from the mantle beneath the younger belts then the temperatures in the mantle beneath the younger belts should be higher than beneath older belts and shields because of the strong dependence on temperature of the creep strength of minerals particularly of olivine the crust and mantle beneath the hotter younger belts should be much weaker than those beneath older belts this difference in temperature and consequent difference in strength may be the cause of the greater reactivation of younger belts

identifying deformed pseudotachylyte and its influence on the strength and evolution of a crustal shear zone at the base of the seismogenic zone abstract on a strand of the norumbega fault system a paleozoic subvertical seismogenic fault system in northeastern new england usa we document changes associated with the formation and deformation of pseudotachylyte to form ultramylonite phyllonite layers we consider how those textural and mineralogical changes affected the rheology of the layer and how significant volumes of pseudotachylyte over time may have weakened the shear zone the norumbega fault system is characterized by a number of mylonitic shear zones exhumed from depths of 10 15 km some of which preserve evidence for mutually overprinting pseudotachylyte and mylonite along one of these the sandhill corner shear zone all stages of the pseudotachylyte to ultramylonite phyllonite transformation are preserved from 1 primary pseudotachylyte structures to 2 initial mineral crystallization 3 grain coarsening and reactions and 4 viscous deformation our observations show that ultramylonite layers exhibit identifying features that when present together are distinctive of a pseudotachylyte origin using these features we estimate that 5 50 of the rock volume in the sandhill corner shear zone mean 30 locally 50 is deformed pseudotachylyte suggesting that deformed pseudotachylyte may be more prevalent than previously thought in faults exhumed from the base of the seismogenic zone the sandhill corner shear zone localized along the contact between two rheologically contrasting units mylonite fabric intensity and the occurrence of fresh and deformed pseudotachylyte increase with proximity to the contact and shear zone core indicating that seismic rupture also localized there a decrease in grain size promoting grain size sensitive creep and a progressively interconnected mica network associated with local basal slip within the deformed pseudotachylyte both worked to decrease the strength of those layers the formation of multiple generations of weak deformed pseudotachylyte layers at or near the lithologic contact may have played an important role in the spatiotemporal persistence of the shear zone core

amorphous material in safod core samples san andreas fault evidence for crush origin pseudotachylytes 1 several types of amorphous material in ultracataclastic core samples recovered from 3194 m and 3294 m depth of the main bore hole of the san andreas fault observatory at depth are identified and described with transmission electron microscopy and scanning electron microscopy we observed 1 amorphous material on a slickenside surface 2 glassy bands contained in an ultracataclastic matrix and 3 amorphous rims surrounding quartz or feldspar clasts chemical analyses of the amorphous material reveal that silica content is slightly enriched or similar as in the adjacent matrix we suggest that all amorphous material was formed by comminution of clasts crush origin pseudotachylytes rather than by melting melt origin pseudotachylytes the observed amorphous phases may act as lubricating layers that reduce friction in the san andreas fault

amorphous material in high strain experimental fault gouges the microstructures of gouges produced in room temperature rotary shear sliding experiments were examined by transmission electron microscopy gouges were produced by sliding on ground surfaces of granite quartzite or marble except for one experiment in which a 1 mm thick simulated gouge layer was used water was added to the sliding surfaces of all but one sample crystal plastic processes play no role in the granite and quartzite gouges and a minor role in the marbles all of the gouges consist of mostly submicron crystalline fragments in addition the granite gouges contain 5 60 amorphous material and the quartzite gouge contains 50 amorphous material in the granite samples the composition of the amorphous material commonly lies between k rich and na ca rich feldspars although portions may be silica rich the microstructural relations suggest that the amorphous material forms by comminution of fragments rather than by melting the amount of amorphous material increases and the size of the largest crystalline fragments decreases with an increase in average shear strain although the microstructure is nearly uniform throughout each granite gouge layer these observations suggest that after slip becomes localized on y shear surfaces and or r1 riedel shears the entire gouge layer must continue to undergo deformation it is suggested that cyclic deformation in the gouge must occur to accommodate the passage of geometric irregularities on the active slip surfaces

origin of pseudotachylites in slow creep experiments we conducted a series of experiments on granitoid cataclasites under mid crustal conditions pc 500 mpa t 300 c and slow displacement rates 10 8 ms 1 d 10 7 ms 1 i e within an order of magnitude of plate tectonic velocities the samples reach high peak strength 1250 1375 mpa then weaken slightly by 40 mpa and continue to deform without any abrupt stress drops up to a of 4 microstructural observations show that at peak strength nanocrystalline mean grain size 34 nm smallest crystalline particles 8 nm to partly amorphous material develops weakening occurs once this material is abundant enough to form through going anastomozing slip zones where the strain partitions during higher strain deformation deformation localizes further and 1 to 2 multiply connected layers of fragment loaded amorphous material develop this material shares many similarities with naturally occurring pseudotachylites however was formed at velocities up to 8 orders of magnitude slower than those calculated for earthquakes 1 ms 1 we conclude that the use of pseudotachylites as indicators for paleoseismic events should be reconsidered

possible mechanism of molten sulfur eruption implications from near surface structures around of a crater on a flank of mt shiretokoiozan hokkaido japan abstract shiretokoiozan volcano in northern japan is well known for its eruptions which eject huge amounts of molten sulfur watanabe 1940 reported details of the 1936 eruption but its mechanisms and how and where the huge amount of sulfur is produced and pushed out remain unknown the aim of this study is to elucidate the near surface underground structure of this area and the mechanisms of the molten sulfur eruption we implemented aerial photographic observations geological surveys hot spring analysis self potential survey and dc resistivity surveys at the western flank of mt shiretokoiozan the geology of this area is mostly composed of hydrothermally altered boulders gravels sand and clay some areas of fumaroles are covered by sulfur cement chemical analyses revealed that so42 and cl are rich in hot water which imply an area with upwelling hot water gas below the surface results of dc resistivity surveys conducted at several sites show extremely low resistivity suggesting an aquifer several meters below the surface compiling this evidence we infer a possible mechanism of molten sulfur eruption the sulfur has been produced and stored in an aquifer located at the eastern hill from crater i for several decades by chemical reactions of volcanic gases it gushes out when volcanic activity becomes high

fabric development in brittle to ductile shear zones brittle to ductile shear zones from two separate geological settings are shown to have nucleated on zones of predominantly brittle deformation the shear zones are not simply foliated cataclasites since they contain abundant evidence of dynamic recrystallization of constituent minerals a small quartz diorite lens in the borrego springs shear zone southern california contains centimeter scale cataclasite zones that exhibit a gradual transition into foliated rock alteration of magnesiohornblende to actinolite feldspar to white mica plus quartz and biotite to chlorite produced elongate minerals that define the foliation during the later stages of deformation intracrystalline slip and dynamic recrystallization of quartz and feldspar were important deformation mechanisms

microstructural shear criteria associated with grain boundary sliding during ductile deformation abstract experimental studies on rock analogues are described which establish that certain microstructures diamond grain structures tabular grain structures and asymmetric grain boundary bulges can be used to determine i if deformation was coaxial or non coaxial and ii the sense of shear in zones of non coaxial deformation mechanisms for the development of the structures are described which involve the linked operation of grain boundary sliding and migration

deformation of granitoids at low metamorphic grade ii granular flow in albite rich mylonites abstract the high strain deformation at low metamorphic grades of three investigated granitoids is dominated by the granular flow of albite rich polyphase aggregates these aggregates formed by retrograde breakdown reactions from intermediate plagioclase and k feldspar the predominant deformation mechanism changes in granitoids as the grain size is reduced coarse grained low strain examples are deformed by a combination of intracrystalline plasticity quartz and fracturing feldspar in the mylonites intracrystalline plasticity of quartz plays only a minor role and the dominant deformation mechanism is a non cataclastic granular flow of polyphase aggregates consisting largely of albite and quartz deformation appears to be stable probably because grain growth in albite quartz mixtures is inhibited these fine grained aggregates are mechanically weaker than pure quartz aggregates thus the change in deformation mechanism mainly due to feldspar breakdown reactions appears to be important for the localization of high shear strain deformation at low metamorphic grades in granitoids and other lithologies modally dominated by feldspar the rheological behaviour of albite dominated mineral aggregates may have two consequences for middle to upper crustal deformation of modally feldspar dominated lithologies 1 feldspar in the presence of aqueous fluids can be an important mineral controlling the rheology at low metamorphic grades that is below amphibolite facies p t conditions 2 the occurrence of granular flow of fine grained polyphase aggregates in low grade granitoids is probably common and calls for great care in the modelling of middle to upper crustal rheology based on flow laws for intracrystalline plasticity of single minerals such as quartz