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

electrical resistivity cross section across northern part of saurashtra region an insight to crystallized magma and fluids abstract saurashtra region forms the western part of the deccan volcanic province dvp in india and is bounded by the cambay rift basin crb in the east narmada rift basin nrb in the south and kachchh rift basin krb in the north this region is uplifted due to different stages of inter continental breakup madagascar india seychelles india different stages of rifting reactivation of fault zones and deccan volcanism the major portion of saurashtra region is covered by deccan basalts underlain by mesozoic sediments detailed magnetotelluric mt and long period magnetotelluric lmt soundings have been carried out in the northern part of saurashtra with an aim to bring out the geoelectrical configuration of different basins jamnagar jasdan and cambay along the profile 2d inversion carried out for different modes transverse electric te transverse magnetic mode tm and te tm independently the inversion of different modes shows distinct electrical signatures due to the 3d nature of the data we have considered the tm mode of interpretation because it is less prone to 3d distortions the final model represents large scale heterogeneities in the crust due to the presence of different resistive and conductive blocks high conductive lithosphere denotes the thermal influx of the mantle material to the base of the crust exchange of heat with the lower crust during basaltic magmatic underplating released carbonate fluids and or dehydration of minerals resulting in high conductivity anomalies a prominent conductive feature on the east side of the profile at upper crustal depths may be due to partial melts associated with mafic ultra mafic intrusions related to the reunion plume activity different resistive blocks in conjunction with seismic tomography studies represent recrystallized older precambrian crust that forms the uplifted blocks of jamnagar jasdan and western part of cambay basins

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

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

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

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

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

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

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

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

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

remotely forced intraseasonal oscillations over the tropical atlantic abstract intraseasonal 20 70 day variability is examined in the atlantic region during northern hemisphere winter using ecmwf analyses and noaa outgoing longwave radiation olr it is found that the dominant 200 mb zonal wind fluctuation over the tropical atlantic a1 is related to global scale circulation anomalies with their origins in the pacific compositing techniques are used to investigate the nature of the pacific atlantic teleconnections and related changes in the tropical olr and moisture convergence the olr anomalies associated with a1 are characterized by eastward propagation over the indian ocean and the western pacific and a standing oscillation over the tropical atlantic the latter extends from northeast brazil to west africa and is the dominant component of the atlantic olr variability on these time scales an analysis of the velocity potential and moisture convergence fields suggests that the fluctuations in convection are coupled between the western pacific and atlantic via large s

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

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

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

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