strain heating as a mechanism for partial melting and ultrahigh temperature metamorphism in convergent orogens implications of temperature dependent thermal diffusivity and rheology 1 we explore the importance of strain heating in ductile shear zones for production of leucogranites and high temperature metamorphism in collisional orogens with temperature dependent thermal diffusivity d and rock rheology new measurements on metamorphic rocks from the bohemian massif show that d is 1 3 mm2 s at 25 c and decreases exponentially to as low as 0 4 mm2 s at 600 c this temperature dependence causes model lithospheric geotherms to be straighter compared to geotherms calculated with a constant d using power law parameters for rheology we show that deformation of quartzite at strain rate of 3 10 13 s 1 produces 100 w m3 at 550 c and 7 w m3 at 800 c deformation of stronger clinopyroxenite at this strain rate produces 8 w m3 even at 1050 c when strain heating is introduced by a 3 km thick ductile shear zone with quartz rheology at the depth of 35 km in a lithosphere that has 70 km thick crust and 60 km thick lithospheric mantle the schist solidus is reached by 8 myr in the vicinity of the shear zone deforming at the strain rate of 3 10 13 s 1 for clinopyroxenite rheology ultrahigh temperature uht metamorphic conditions are reached in 40 myr after initiation of strain heating two dimensional models that replicate the exhumation of the greater himalaya crystalline rocks above the main central thrust produce extensive partial melting and an inverted metamorphic field gradient occurrences of leucogranites within crustal shear zone systems may be evidence of the coupling between deformation and heat production in the crust

fault and fold growth of the amenthes uplift implications for late noachian crustal rheology and heat flow on mars abstract determining the rheologic conditions that control growth of compressive structures on mars is an elusive problem one limited by the lack of seismologic and other data commonly available for comparable active uplifts on earth in some instances however the geometry of faults on mars that offset impact craters can be deduced from surface topography alone with this aim construction of a balanced and restorable structural cross section across the late noachian amenthes uplift or fault related fold suggest it forms above a deeply penetrating blind thrust with a gently curved or listric geometry that flattens downward preferred structural solutions suggest the thrust dips between 41 5 and 56 1 at the surface and flattens into a horizontal decollement at depths of 33 48 km the range of values for depth to detachment are greater than previous estimates for the amenthes thrust based on elastic modeling for a range of planar thrust geometries using the inference that the decollement corresponds to the onset of plasticity in the crust the depth to detachment is consistent with surface heat flow of 24 33 mw m 2 based on average values for heat production and the temperature threshold for the transition from brittle faulting to ductile shear on mars this suggests the crust at amenthes during the late noachian may be slightly cooler than previously thought similar to recent estimates derived from studies of lithospheric strength

importance of pressure solution and coble creep in the deformation of polymineralic rocks the theoretical framework for modeling pressure solution in polyphase rocks is presented it is often assumed that pressure solution is a deformation mechanism of secondary importance deep in the lithosphere it is shown here that the rheology of rock during pressure solution may be drastically reduced in a bimineralic and polymineralic rock relative to its monomineralic counterparts and therefore the importance of this deformation mechanism importance is perhaps underestimated this behavior contrasts with that of a polymineralic rock undergoing dislocation creep where existing models show that the rheology will be some average of the rheologies of individual phases for instance the rheology of a dunite in pressure solution is governed by the diffusion of both mgo and sio2 whichever is the slowest will control the viscosity of the rock conversely in a harzburgite with a specific texture flow will be governed by the fastest diffusing species thus its strength may be as low as that of a quartz rock of comparable grain size given the comparative strength of many phases undergoing dislocation creep relative to quartz this prediction implies a larger range of conditions under which pressure solution forms an important part of bulk rock deformation than inferred previously predictions on textural evolution of polymineralic aggregates are also made and it is shown that some minerals grains may become elongate parallel to the maximum principal stress during deformation even though the rock as a a whole shortens in this direction

effect of quartz inclusions on plastic flow in marble to investigate the effect of variations in volume fraction of a second phase on the strength and microstructure of rocks we produced synthetic marbles with a grainsize of 5 25 m containing quartz of similar size with volume fractions of 0 5 1 and 20 4 vol and between 3 to 9 porosity these rocks were deformed in triaxial compression at a strain rate of 3 x 10 5 s 1 confining pressure of 200 mpa and 600 c marbles containing 20 4 vol quartz were almost 5 times stronger than those containing no quartz the work hardening rate at given strain increased with quartz content marbles containing 20 4 vol quartz work hardened up to 10 strain whereas in single phase samples work hardening was negligible beyond 3 strain the dislocation density in the calcite increased with increasing quartz content the data are consistent with models for the strength of ceramic matrix composites or with the hypothesis that strength is increased by decreasing porosity

deformation mechanisms in a high temperature quartz feldspar mylonite evidence for superplastic flow in the lower continental crust abstract microstructures and crystallographic preferred orientations in a fine grained banded quartz feldspar mylonite were studied by optical microscopy sem and tem mylonite formation occurred in retrograde amphibolite facies metamorphism interpretation of the microstructures in terms of deformation mechanisms provides evidence for millimetre scale partitioning of crystal plasticity and superplasticity strain incompatibilities during grain sliding in the superplastic quartz feldspar bands are mainly accommodated by boundary diffusion of potassic feldspar the rate of which probably controls the rate of superplastic deformation there is evidence for equal flow stress levels in the superplastic and crystal plastic domains in this case mechanism partitioning results in strain rate partitioning fast deformation in the superplastic bands therefore dominates flow and deformation is probably best modelled by a superplastic law if this deformational behaviour is typical shearing in mylonite zones of the lower continental crust may proceed at exceptionally high rates for a given differential stress or at low differential stresses in case of fixed strain rates

surface deformation and lower crustal flow in eastern tibet field observations and satellite geodesy indicate that little crustal shortening has occurred along the central to southern margin of the eastern tibetan plateau since about 4 million years ago instead central eastern tibet has been nearly stationary relative to southeastern china southeastern tibet has rotated clockwise without major crustal shortening and the crust along portions of the eastern plateau margin has been extended modeling suggests that these phenomena are the result of continental convergence where the lower crust is so weak that upper crustal deformation is decoupled from the motion of the underlying mantle this model also predicts east west extension on the high plateau without convective removal of tibetan lithosphere and without eastward movement of the crust east of the plateau

coupling and decoupling of crust and mantle in convergent orogens implications for strain partitioning in the crust topographic relief strain partitioning and crust mantle coupling within convergent orogens may be systematically related to the distribution of strength as a function of depth within continental crust this result is obtained by solving analytically for basally driven three dimensional flow within an idealized crust with a laterally invariant depth dependent newtonian viscosity two end member flow states are shown to occur in the first crustal flow is directly coupled to the underlying mantle and is controlled by stresses transmitted vertically through the crust in the second upper crustal to midcrustal flow is decoupled from the underlying mantle and is controlled by stresses transmitted horizontally through the crust this model is applied to deformation of continental crust above plate like mantle that undergoes oblique convergence across a narrow suture if no significant low viscosity zone develops within the crust crustal deformation is coupled to motion of the underlying mantle and a triangular mountain range develops surface shortening occurs at the foot of the mountains normal to the trend of the range while strike slip and extension are partitioned along the axis of the orogen if a low viscosity zone is initially absent but develops during crustal thickening a steep sided flat topped plateau forms crustal deformation is coupled to mantle motions on the plateau flanks but may be strongly decoupled beneath the plateau surface shortening normal to the plateau occurs at the foot of the plateau while surface extension normal to the plateau occurs at the plateau edge if a low viscosity zone is present in the lower crust prior to convergence a wide orogen with low topographic relief develops crustal deformation is decoupled from the mantle except at the edges of the deforming region and surface shortening is partitioned uniformly across the belt this study shows that horizontal and vertical partitioning of strain occur even in the absence of lateral strength variations and that shortening and extension may exist simultaneously in different parts of an orogen similar morphology and strain partitioning can be observed in many real orogenic belts including the eastern alps the tibetan plateau and the lachlan tasman fold belt

the europium anomalies in banded iron formations and the thermal history of the oceanic crust abstract the ree rare earth element patterns in banded iron formations bif s from various sites in north america two sites in finland and three drillcores of the hamersley basin in western australia have been investigated using icp ms inductively coupled plasma mass spectrometry and inaa instrumental neutron activation analysis the results of this study as well as existing data show trends in the eu anomalies bif s older than 2 5 ga exhibit positive eu anomalies while younger ones exhibit none in chemical sediments between 2 5 and 2 3 ga both trends can be found because of the worldwide distribution of bif s this feature can be considered as a global change in the conditions controlling ree mobilization consideration of the redox conditions of bif formation in comparison with the calculated and measured eu redox equilibria suggests that a eu anomaly points to a dominantly hydrothermal source of ree s with solution temperatures above 175 c we suggest that the disappearance of the eu anomaly towards the proterozoic marks a change from mainly high temperature hydrothermal to dominantly low temperature alteration

modelling the bulk viscosity of two phase mixtures in terms of clast shape abstract the rheological behaviour of many rock types will depend on how mixtures of different minerals and rock clasts behave together this paper develops two dimensional modelling of the bulk viscosity of two phase viscous mixtures with different shape fabrics in pure shearing based on self consistent mechanics and behaviour of inclusion matrix systems an equation is presented for the bulk viscosity in terms of the phase viscosity contrast phase fractions and clast shape results are given for idealised two dimensional mixtures of circles squares or ellipses with a variety of phase viscosity contrasts in pure shearing parallel perpendicular to the shape fabric the normalised bulk viscosities of all these mixtures fall between the theoretical upper and lower viscous bounds given by homogeneous strain rate and homogeneous stress respectively but nearer to the upper bound with increasing shape fabric mixtures with a particular geometry are shown to have a constant relative horizontal distance between these two bounds as proposed by p d bons using graphs for the normalised bulk viscosities of mixtures with variably elliptical aligned shape fabrics the results for fabrics orthogonal to pure shearing can be extrapolated to a diagonal fabrics in pure shearing and b orthogonal or diagonal fabrics in simple shearing these reveal the bulk viscous anisotropy of mixtures with shape fabric in two dimensions it is speculated that during a progressive finite deformation two phase mixtures might undergo changes in bulk viscosity over time with steady stiffening during pure shear and perhaps cyclic stiffening and softening during simple shear the two dimensional models provide an approximation to the three dimensional properties of mixtures with shape fabrics with potential applications to rocks including conglomerates

strain localization along the atlantis bank oceanic detachment fault system southwest indian ridge microstructural observations and mineral thermometry from in situ samples collected from the atlantis bank oceanic core complex sw indian ridge indicate that detachment faulting was initiated under hypersolidus conditions in the ductile regime and continued through subgreenschist temperatures through the ductile semibrittle and brittle regimes as strain localized along the exposed now subhorizontal fault surface ductile semibrittle and brittle fabrics are developed within dominantly gabbroic rocks footwall rocks exhibit crystal plastic fabrics distributed over a structural thickness up to 400 m below the denuded fault surface exposed at the seafloor whereas semibrittle and brittle fabrics are concentrated in the 80 and 30 m immediately below the principal slip surface of the detachment fault respectively sample fabrics suggest that strain localization was achieved by dynamic recrystallization of plagioclase at temperatures between 910 c and 650 c by amphibole accommodated dissolution precipitation creep at temperatures 750 c 450 c by chlorite accommodated reaction softening at temperatures 450 c 300 c and by brittle fracturing and cataclasis at temperatures 300 c

deformation mechanisms and phase distribution in mafic high temperature mylonites from the jotun nappe southern norway abstract shear zones and mylonites in anorthositic to gabbroic rocks from the jotun nappe in southern norway have been studied as an example of high temperature deformation in polyphase rocks the deformation has taken place at approximately 700 c and maximum confining pressures of 900 mpa the constituent phases amphibole pyroxene and plagioclase have recrystallized synkinematically reducing the grain size of the rock to about 20 to 70 m two different microstructures develop depending on the recrystallization mechanism dynamic recrystallization of plagioclase hornblende and pyroxenes produces monophase layers which extend from porphyroclasts heterogeneous nucleation of phases due to chemical disequilibrium produces phase mixtures of hornblende and plagioclase or clinopyroxene and plagioclase these phase mixtures extend from hornblende or clinopyroxene porphyroclasts the product of the deformation and simultaneous recrystallization is a well laminated microstructure of some monophase hornblende pyroxene and plagioclase layers which alternate with abundant layers of phase mixtures different deformation mechanisms operate in the different types of microstructures dislocation substructures dynamic recrystallization microstructures and a crystallographic preferred orientation measured by ji and mainprice 1988 on samples of the same shear zone indicate crystal plastic deformation in monophase plagioclase layers dynamic recrystallization in monophase hornblende layers indicates also crystal plastic deformation the phase distribution of plagioclase and hornblende grain shapes and lower dislocation densities in the phase mixtures suggest that the deformation mechanism is not crystal plasticity but rather a granular flow grain boundary sliding probably accommodated by diffusional mass transfer in the plagioclase hornblende mixtures the phases show an anticlustered non random distribution grain boundary sliding may contribute to phase mixing but cannot be the main reason for the anticlustering of phases and the complete phase mixing directly at the boundaries of the porphyroclasts the reason for the anticlustered distribution is not clear but could be explained by preferential heterogeneous nucleation of hornblende at plagioclase grain boundaries

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

mantle flow beneath a continental strike slip fault postseismic deformation after the 1999 hector mine earthquake two recent large earthquakes in the mojave desert california the magnitude 7 3 1992 landers and magnitude 7 1 1999 hector mine earthquakes have each been followed by elevated crustal strain rates over periods of months and years geodetic data collected after the hector mine earthquake exhibit a temporally decaying horizontal velocity field and a quadrant uplift pattern opposite to that expected for localized shear beneath the earthquake rupture we interpret the origin of this accelerated crustal deformation to be vigorous flow in the upper mantle in response to the stress changes generated by the earthquake our results suggest that transient flow in the upper mantle is a fundamental component of the earthquake cycle and that the lower crust is a coherent stress guide coupling the upper crust with the upper mantle

the effect of dissolved magnesium on diffusion creep in calcite we experimentally tested a series of synthetic calcite marbles with varying amounts of dissolved magnesium in a standard triaxial deformation machine at 300 mpa confining pressure temperatures between 700 and 850 c stresses between 2 and 100 mpa and strain rates between 10 7 and 10 3 s 1 the samples were fabricated by hot isostatic pressing of a mixture of calcite and dolomite at 850 c and 300 mpa the fabrication protocol resulted in a homogeneous fine grained high magnesian calcite aggregate with minimal porosity and with magnesium contents between 0 07 and 0 17 mol mgco3 at stresses below 40 mpa the samples deformed with linear viscosity that depended inversely on grain size to the 3 26 0 51 power suggesting that the mechanisms of deformation were some combination of grain boundary diffusion and grain boundary sliding because small grain sizes tended to occur in the high magnesium calcite the strength also appeared to vary inversely with magnesium content however the strength at constant grain size does not depend on the amount of dissolved magnesium and thus the impurity effect seems to be indirect at stresses higher than 40 mpa the aggregates become non linearly viscous a regime we interpret to be dislocation creep the transition between the two regimes depends on grain size as expected the activation energy for diffusion creep is 200 30 kj mol and is quite similar to previous measurements in natural and synthetic marbles deformed at similar conditions with no added magnesium

plagioclase microfabrics in a ductile shear zone from the jotun nappe norway abstract gabbroic and anorthositic rocks from the jotun nappe are transected by small ductile shear zones in which a high grade paragenesis and a new foliation are formed most plagioclase grains show evidence of plastic deformation and have recrystallized by subgrain rotation and bulge processes to fine grained mylonite during these processes a plagioclase grain shape fabric was destroyed and with increasing strain a pre existing plagioclase crystal fabric was successively modified enhanced and finally obliterated this could be evidence of superplastic flow inverse pole figure analysis of the typical plagioclase crystal fabric in the shear zone indicates that 021 a could be an operative slip system and planes close to 132 could be possible slip planes

monitoring stress changes in a concrete bridge with coda wave interferometry coda wave interferometry is a recent analysis method now widely used in seismology it uses the increased sensitivity of multiply scattered elastic waves with long travel times for monitoring weak changes in a medium while its application for structural monitoring has been shown to work under laboratory conditions the usability on a real structure with known material changes had yet to be proven this article presents experiments on a concrete bridge during construction the results show that small velocity perturbations induced by a changing stress state in the structure can be determined even under adverse conditions theoretical estimations based on the stress calculations by the structural engineers are in good agreement with the measured velocity variations

the rheology of polymineralic rocks an approach deduced from empirical and theoretical work a model is proposed for the rheology of bi and polyphase rocks based on the rheology of the components their volume fractions and their geometrical distribution the model is formulated for the common case where some minerals deform by crystall plasticity at steady state and others stronger ones deform in the vincinity of the brittle ductile transition

high temperature plastic deformation of quartz plagioclase multilayers by layer normal compression it is a common practice in rheological modeling to take the extrapolated compressive flow strengths of quartzite and plagioclase rock as the bulk flow strengths of the upper and lower continental crusts respectively such a practice implies that the bulk flow strength of polyphase rocks is identical to that of the pure weak phase to test this assumption we performed deformation experiments on synthetic layered and particulate quartz anorthite 50 50 composites and aggregates of the end members in a paterson gas medium apparatus at a confining pressure of 300 mpa a constant strain rate of 10 5 s 1 and temperatures from 1273 to 1473 k under these conditions anorthite deforms by recrystallization accommodated dislocation creep while quartz is semibrittle electron backscatter diffraction measurements show a strong lattice preferred orientation lpo of anorthite developed in deformed specimens and the lpo pattern is interpreted as a result of dislocation slip on the 010 100 system our experimental results show that the bulk flow strength of layered composites increases with decreasing thickness of the layers thinlayered composites are significantly stronger than pure anorthite aggregates and a homogeneous quartz anorthite mixture i e particulate composites with the same modal composition but weaker than pure quartz aggregates our experimental results together with the theoretical overview presented previously in the materials science literature demonstrate that 1 compressive flow strength of a polyphase rock cannot be represented by that of the weak phase and 2 thin layered rocks compressed normal to the layering are rheologically stronger than homogeneous isotropic mixtures under the same deformation conditions thus weak phase based rheology will lead to an underestimation of the bulk flow strength of the continental crust in which polyphase rocks dominate

investigation of the strength contrast at the moho a case study from the oman ophiolite we investigate the viscosity contrast between the crust and mantle by analysis of deformed gabbronorite dikes in the mantle section of the oman ophiolite our study provides a rare opportunity to directly constrain the rheological contrast between a plagioclase rich lithology typical of the lower crust and an olivine dominated harzburgite we constrain deformation conditions using field relationships geothermometry grain size piezometry and electron backscatter diffraction the viscosity of gabbronorite was at least two orders of magnitude lower than the viscosity of the harzburgite consistent with the rheological contrast between plagioclase and olivine calculated via experimental flow laws these flow laws predict that a significant viscosity contrast exists at the crust mantle boundary where the crustal lithology is dominated by plagioclase or where deformation localizes within plagioclase rich layers

the effect of melt distribution on the rheology of plagioclase rocks abstract synthetic plagioclase aggregates were fabricated by hot isostatic pressing glass powder with an60ab40 composition at a confining pressure of 300 mpa and a temperature of 1223 k the residual glass content 15 vol was reduced to 1 3 vol by annealing the specimens at temperatures between 1423 and 1523 k at 0 1 mpa high resolution electron microscopy showed that the residual glass was confined to grain triple junctions when these samples were annealed at 0 1 mpa and above the solidus 1620 k they also contained silica rich amorphous phases at grain boundaries other samples were hot pressed at 1423 k they often have grain boundaries containing thin amorphous films 5 50 nm wide the chemical composition of the films differs from pure labradorite with up to 80 wt sio2 the volume fraction of si rich glass increases up to 2 vol for samples that contain 1 wt water uniaxial creep experiments were performed at temperatures of 1373 1638 k and stresses between 2 and 50 mpa the plagioclase aggregates deformed by grain size sensitive creep with a stress exponent n 1 the activation energy for grain boundary diffusion controlled creep of the partially molten aggregates was 400 20 kj mol 1 it remained unchanged with varying glass content and glass topology the strength of samples containing glass along grain boundaries is 4 10 times less than without amorphous grain boundary films the transition between diffusion and dislocation creep is shifted towards higher stresses for samples that contain glass films at grain boundaries

strength evolution and the development of crystallographic preferred orientation during deformation of two phase marbles abstract we conducted conventional triaxial compression and confined torsion tests on calcite rocks at 1023 k and equivalent strain rates between 10 6 and 10 3 s 1 to investigate the influence of variations in volume fraction and geometry of a second phase on the evolution of strength and cpo nominally dense two phase marbles were made by hot isostatic pressing of mixtures of calcite and amorphous carbon the carbon grains were either spheres or splinters with known size distributions were chemically inert and are nearly rigid adding 1 vol spheres caused only small changes in strength or cpo strengths were consistent with the activation of slip on f 10 1 1 the hardest system in calcite adding 10 vol splinters increased the strength at low strains and strain rates but had smaller effects at high strains or rates the cpo of rocks containing splinters were similar to those in pure samples but with reduced intensity when 10 vol spheres were added the aggregate strength under confined torsion was less than that of the pure material distinct and intense cpos developed the reduced stresses were consistent with those necessary for slip on systems weaker than f viscoplastic self consistent calculations used to model the texture evolution required slip on f 10 1 1 and c 1 2 1 0 to match the cpo of pure samples results were consistent with the inclusion of slip on r 1 2 1 0 in samples containing 10 vol spheres deformed in torsion slip activity on the harder systems was decreased the exact cause of the strength reduction in these experiments is not understood but it is possible that adding smooth spheres enhanced grain boundary sliding thereby decreasing activity on f 10 1 1 thus shape and volume fraction of second phases may be important in determining cpo and strength and it is important to consider the loading geometry

analogies between the high temperature deformation of polyphase rocks and the mechanical behavior of porous powder metal abstract in some rock types one or more mechanically similar minerals may constitute a loadcarrying framework while other subordinate phases carry little load if incompetent phases are more than one or two orders of magnitude weaker than the competent phases the load carrying framework behaves much like a porous solid the deformational properties of porous powder metal provide analogies for the deformation of such porous like rock types 1 the yield strength of porous powder metal is analogous to the power law flow stress for rock deformed at high temperatures 2 the correspondence principle relating elastic and viscoelastic deformation may be used to relate the young s modulus of porous powder metals to the flow stress for nabarro herring creep in rock where the incompetent fraction is 15 of the rock there should be a 50 reduction in power law flow stress for an incompetent fraction of 40 the power law stress is reduced by an order of magnitude and the nabarro herring creep stress is reduced by 80

strain localization during high temperature creep of marble the effect of inclusions abstract the deformation of rocks in the earth s middle and lower crust is often localized in ductile shear zones to better understand the initiation and propagation of high temperature shear zones induced by the presence of structural and material heterogeneities we performed deformation experiments in the dislocation creep regime on carrara marble samples containing weak limestone or strong novaculite second phase inclusions the samples were mostly deformed in torsion at a bulk shear strain rate of 1 9 10 4 s 1 to bulk shear strains between 0 02 and 2 9 using a paterson type gas deformation apparatus at 900 c temperature and 400 mpa confining pressure at low strain twisted specimens with weak inclusions show minor strain hardening that is replaced by strain weakening at 0 1 0 2 peak shear stress at the imposed conditions is about 20 mpa which is 8 lower than the strength of intact samples strain progressively localized within the matrix with increasing bulk strain but decayed rapidly with increasing distance from the inclusion tip microstructural analysis shows twinning and recrystallization within this process zone with a strong crystallographic preferred orientation dominated by r and c slip in recrystallization induced weakening starts at local shear strain of about 1 in the process zone corresponding to a bulk shear strain of about 0 1 in contrast torsion of a sample containing strong inclusions deformed at similar stress as inclusion free samples but do not show localization the experiments demonstrate that the presence of weak heterogeneities initiates localized creep at local stress concentrations around the inclusion tips recrystallization induced grain size reduction may only locally promote grain boundary diffusion creep accordingly the bulk strength of the twisted aggregate is close to or slightly below the lower isostress strength bound determined from the flow strength and volume fraction of matrix and inclusions

quantitative analysis of trace oh in garnet and pyroxenes to calibrate infrared ir spectroscopy for quantitative analysis of trace structural oh in specific minerals we have determined concentrations of h in pure separates of mantle derived pyrope garnet 56 6 ppm h 2o by weight augite 268 8 ppm h 2o and enstatite 217 11 ppm h 2o by manometry after heating the samples and extracting h 2 gas under vacuum ir spectroscopy confirmed the presence of intrinsic oh in these samples prior to extraction and indicated between 86 and 100 removal of h during the extraction procedure the integral specific absorption coefficients of 1 39 0 14 l 7 09 0 32 and 15 6 0 94 ppm h 2o cm 2 for pyrope augite and enstatite respectively allow precise spectroscopic determination of the oh content of upper mantle garnets and pyroxenes to concentration levels of a few parts per million uncertainties in accuracy depend on mineral composition and characteristics of the oh absorption spectrum and are estimated to range between 10 and 50

flow laws of multiphase rocks calculated from experimental data on the constituent phases abstract an attempt has been made to develop expressions for the flow laws of polyphase rocks in terms of the volume fractions and flow laws of their constituent phases the flow strengths predicted by the proposed model for granite diorite diabase and lherzolite are in good agreement with previously determined experimental values the study allows one to calculate to the first approximation the flow laws of a large number of rocks at geologic conditions based on the experimentally determined flow laws of a relatively small number of monomineralic aggregates

bulk flow strength of forsterite enstatite composites as a function of forsterite content creep experiments have been conducted to investigate the effect of varying forsterite content vfo on the bulk flow strength of dry forsterite enstatite fo en aggregates in order to evaluate the applicability of existing theoretical models to two phase rocks as well as to understand the rheology of polyphase systems in general the experiments were performed at temperatures of 1423 1593 k stresses of 18 100 mpa oxygen fugacities of 10 14 10 2 5 mpa and 0 1 mpa total pressure the fine grained fo 10 17 mm en 14 31 mm composites of various fo volume fractions vfo 0 0 2 0 4 0 5 0 6 0 8 and 1 were synthesized by isostatically hot pressing in a gas medium apparatus at 1523 and 350 mpa our experiments show that flow strength contrasts between fo and en are in the range of 3 8 at the given experimental conditions with fo as the stronger phase the measured stress exponent n and activation energy q values of the fo en composites fall between those of the endmembers the n values show a nearly linear increase from 1 3 to 2 0 while the q values display a non linear increase from 472 to 584 kj mol with en volume fraction from 0 to 1 0 there is no clear dependence of creep rates on oxygen fugacity for the fo en composites the mechanical data and tem microstructural observations suggest no change in deformation mechanism of each phase when in the composites compared to when in a single phase aggregate the en deformed mainly by dislocation creep while the fo deformed by dislocation accommodated diffusion creep for our grain sizes and experimental conditions comparisons between the measured composite strengths and various theoretical models indicate that none of the existing theoretical models can give a precise predication over the entire vfo range from 0 to 1 however the theoretical models based on weak phase supported structures wps yield a good prediction for the flow strengths of the composites with vfo 0 6 no model gives a good prediction for the bulk strength of two phase composites in the transitional regime vfo 0 4 0 6 applications of the wps and sps based models in the transitional regime result in under and over estimations for the composite flow strength respectively thus the effect of rock microstructure should be taken into consideration in modeling the bulk flow strengths of the crust and upper mantle using laboratory determined flow laws of single phase aggregates d 2001 elsevier science b v all rights reserved

kinematics and dynamics of the southeastern margin of the tibetan plateau summary on the southeastern margin of the tibetan plateau lies a large region which seismicity and gps data show to be actively deforming this paper describes the active faulting in the region and how it relates to the velocity field observed with gps in places the velocity field is accommodated by rotations about vertical axes and most or all of the strain at the surface in the region appears to be released seismically gps velocities are then compared to velocities calculated using a model for deformation driven by gravitational driving forces using rheologies estimated from experimentally derived mineral flow laws the model provides velocities that are in good agreement with observed gps velocities it is not possible to uniquely determine the rheology or flow velocity at depth and there are two forms of model solution which match the observed horizontal surface velocities in one of these vertical planes deform by pure shear and in the other vertical gradients of horizontal velocity are present within the crust two distinct regions of normal faulting earthquakes are present in the region and have mechanisms which are most easily explained by gravity driven deformation

the interplay and effects of deformation and crystallized melt on the rheology of the lower continental crust fiordland new zealand abstract microstructural electron backscatter diffraction ebsd and misorientation analyses of a migmatitic granulite facies orthogneiss from the exhumed lower crust of a cretaceous continental arc in fiordland new zealand show how deformation was accommodated during and after episodes of melt infiltration and high grade metamorphism microstructures in garnet omphacite plagioclase and k feldspar suggest that an early stage of deformation was achieved by dislocation creep of omphacite and plagioclase with subsequent deformation becoming partitioned into plagioclase continued deformation after melt infiltration resulted in strain localization in the leucosome of the migmatite where a change of plagioclase deformation mechanism promoted the onset of grain boundary sliding most likely accommodated by diffusion creep in fine recrystallized plagioclase grains our results suggest three distinctive transitions in the rheology of the lower crust of this continental arc where initial weakening was primarily achieved by deformation of both omphacite and plagioclase subsequent strain localization in plagioclase of the leucosome indicates that the zones of former melt are weaker than the restite and that changes in deformation mechanisms within plagioclase and an evolution of its strength primarily control the rheology of the lower crust during and after episodes of melting and magma addition

transition from dislocation creep to melt enhanced diffusion creep in fine grained granitic aggregates abstract sintered aggregates composed of quartz albite and microcline of two different grain sizes 2 10 and 10 50 m each with two different melt contents

anisotropic horizontal thermal contraction of young oceanic lithosphere inferred from stress release due to oceanic intraplate earthquakes how freely the oceanic lithosphere contracts horizontally due to thermal contraction is important information because it reflects the boundary condition of the oceanic lithosphere which includes information regarding the magnitude of driving resisting forces of plate tectonics we investigated the horizontal thermal contraction of young oceanic lithosphere using an analysis of the intraplate stress release due to oceanic intraplate earthquakes oceqs and numerical simulations the stress release due to oceqs in young oceanic lithosphere 5 15 ma shows significant differences between the spreading directional component and the ridge parallel component the extensional stress release of the ridge parallel component is six times as large as that of the spreading directional component while the compressional stress release of the ridge parallel component is one seventh that of the spreading directional component we conducted a numerical simulation of the thermal stress evolution of the oceanic lithosphere to investigate how the difference in the horizontal contraction rates between the spreading direction and the ridge parallel direction can explain the observed anisotropic stress release the result indicates that young oceanic lithosphere 5 15 ma barely contracts in the ridge parallel direction only 0 30 of the spreading directional contraction rate while it contracts freely in the spreading direction due to the weakness of the oceanic ridge strength and the low viscosity asthenosphere from the results we constrained the magnitude of the basal traction working on the bottom of the oceanic lithosphere to be smaller than 0 44 mpa

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

influence of shear heating on microstructurally defined plate boundary shear zones abstract in plate boundary scale ductile shear zones defined by microstructural weakening shear heating may lead to a temperature increase over 5 m y of up to 80 c just below the brittle ductile transition up to 120 c just below the moho and to thermal boundary zones tens of km wide on either side of the shear zone where rock strength is highest shear zones are narrow 1 km and thermal gradients within the shear zone itself are low so there is no tendency for increased localization heating results in thermal weakening but this is partly offset by grain growth related to the decrease in stress in shear zones of the order of 10 km width shear stress strain rate and hence heat generation are lower and thermal gradients are insufficient to cause additional strain localization temperature increases in the mid crust are of the order of 10 c insufficient to cause partial melting or an increase in metamorphic grade in the upper mantle shear zones may be 50 km or more wide and the temperature increase is less than 20 c in 5 m y but temperature differences between center and margin may enhance the strain rate at the center by up to 18

grain growth in synthetic marbles with added mica and water evolution of grain size in synthetic marbles was traced from compaction of unconsolidated powder through primary recrystallization and normal grain growth to a size stabilized by second phases to form the marbles reagent grade caco3 was mixed with 0 1 and 5 volume mica and heat treated under pressure with added water densification with negligible recrystallization occurred within one hour at 500 c and 500 mpa confining pressure primary recrystallization occurred at 500 550 c causing increases of grain size of factors of 2 5 resulting samples had uniform grain size gently curved grain boundaries and near equilibrium triple junctions they were used subsequently for normal grain growth studies normal grain growth occurred above 550 c at 800 c grain size d increased from 7 m d0 to 65 m in 24 hours growth rates fit the equation d n d 0 n kt where k is a constant and n 2 6 minor amounts of pores or mica particles inhibit normal grain growth and lead to a stabilized grain size dmax which depends on the size of the second phases and the inverse of their volume fraction raised to a power between 0 3 and 1 once dmax is reached normal growth continues only if second phases are mobile or coarsen or if new driving forces are introduced that cause unpinning of boundaries normal grain growth in solnhofen limestone was significantly slower than in pure synthetic marble suggesting that migration is also inhibited by second phases in the limestone

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

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

relating experimental and geological rheology this paper is concerned with the question of how to relate laboratory measurements of the rheology of rocks to the rheological assumptions that need to be made in geodynamic modelling first there is a brief resume of the principal types of rheological behaviour that have been studied in the laboratory both pressure dependent strain rate independent and pressure independent strain rate dependent then the generalization of the results from the relatively simple stress states of the experiments to general stress states is discussed followed by consideration of the extrapolation of the experimental results to geological strains and strain rates finally the problems associated with spatial scale are considered leading to the question of how to model the rheological behaviour of large scale rock masses using the rheological measurements of laboratory specimens and taking into account the heterogeneity of geological scale rock masses

weakening and strain localization produced by syn deformational reaction of plagioclase there are many observations in naturally deformed rocks on the effects of mineral reactions on deformation but few experimental data in order to study the effects of chemical disequilibrium on deformation we have investigated the hydration reaction plagioclase h2o more albitic plagioclase zoisite kyanite quartz we utilized fine grained 2 6 m plagioclase aggregates of two compositions an54 and an60 both dried and with 0 1 0 4 wt h2o present in shear deformation experiments at two sets of conditions 900 c 1 0 gpa in the plagioclase stability field and 750 c 1 5 gpa in the zoisite stability field dry samples and those deformed in the plagioclase stability field underwent homogeneous shearing by dislocation creep but samples with 0 1 to 0 4 wt water deformed in the zoisite stability field showed extreme strain localization into very narrow 1 3 m shear bands after low shear strain in these samples the microstructures of reaction products in the matrix differ from those in the shear bands in the matrix large up to 400 m zoisite crystals grew in the direction of finite extension and relict plagioclase grains are surrounded by rims of recrystallized grains that are more albitic in the shear bands the reaction products albitic plagioclase zoisite white mica and traces of kyanite form polyphase aggregates of very fine grained 0 1 m dislocation free grains most of the sample strain after 2 has occurred within the shear bands within which the dominant deformation mechanism is inferred to be diffusion accommodated grain boundary sliding dagbs the switch from dislocation creep in dry samples deformed without reaction to dagbs in reacted samples is associated with a decrease in flow stress from 800 to 200 mpa these experiments demonstrate that heterogeneous nucleation driven in part by chemical disequilibrium can produce an extremely fine grained polyphase assemblage leading to a switch in deformation mechanism and significant weakening thus localization of deformation in polyphase rocks may occur on any pressure p temperature t path where the equilibrium composition of the constituent minerals changes

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

numerical study on stress heterogeneity development in two phase rocks with large rheological contrast and implications in the lower crust development of stress heterogeneity in two phase rocks was investigated via a finite element method at 1000 1200 k and 100 mpa two groups of rock models were considered anorthite diopside and anorthite clinopyroxene with a phase volume ratio of 1 1 in each group and different dislocation creep rates between phases 4 8 orders of magnitude our numerical results indicate that the stress inside the model can be several times higher than the differential stress applied to the model and stress will tend to concentrate in hard phase especially near the sharp boundaries with soft phase moreover large stress gradient in hard phase and nearly homogeneous stress in soft phase will lead to the initialization of localized dynamic recrystallization or fracture these numerical observations suggest that the rheological contrast between two phases plays a crucial role in stress heterogeneity rather than other factors such as grain size the boundary conditions or mesh density which may eventually accelerate development of stress heterogeneity in the lower crust our study provides new insights into the dynamic processes of grain size reduction in the lower crust which may cause the transformation from dislocation creep to diffusion creep and enable the weakened shear zones

rock strength and texture evolution during deformation in the earth s ductile lithosphere a two phase thermodynamics model building upon the two phase and grain damage theory we propose a new formulation allowing to track the evolution of phase mixing segregation during ductile deformation of a two phased aggregate our model is based on a set of variables characterizing a rock texture the mean grain sizes and the phase proportion during ductile deformation activation of different micro mechanical processes impacts the aggregate texture dislocation and diffusion creep are the two main deformation processes considered we only account for the effect of zener pinning in slowing down grain growth and allow for active grain size reduction mechanisms in the diffusion creep domain for this purpose an equation is proposed to track the phase mixing evolution during ductile deformation numerical application using anorthite rheology shows that any grain reduction mechanisms that could be active in the diffusion creep regime requires a very high partition fraction in order to reach the grain size size predicted by the feldspar piezometer application of this model to gabbroic composition relevant for the ductile crust demonstrates that the strong coupling between phases grain sizes and interface evolution results in steady state grain sizes far below the field boundary this effect is coeval with an important increase of mixing between the two phases in addition accounting for the phase mixing results in a drop of the global aggregate stress during deformation this model allows for further comparison of mylonitized textures evolution with natural shear zones at the local and regional scales

microstructures and lattice preferred orientations in experimentally deformed clinopyroxene aggregates microstructures and lattice preferred orientations lpo were analysed on experimentally deformed natural and hot pressed clinopyroxene aggregates in order to understand the relationship between deformation processes and evolving microstructures the lpo was measured using electron backscatter diffraction techniques in the scanning electron microscope sem microstructures were observed by polarized light microscopy and by orientation contrast in the sem natural samples sleaford bay pyroxenite were deformed in axial compression stepping tests up to 16 shortening these samples deformed mainly by twinning and dislocation glide with very little recrystallization no clear lpo evolution apart from the initial lpo could be attributed to deformation synthetic clinopyroxenite samples were hot pressed from powders of the same material with three different grain size ranges and deformed in compression stepping experiments up to 28 shortening in the samples with coarse 30 m and intermediate 20 m grain sizes deformation was dominated by dislocation creep accommodated by subgrain rotation recrystallization the recrystallized grains have sizes up to 8 m and represent 15 25 of the sampled area the sample with the finest initial grain size 5 m deformed dominantly by diffusion creep accompanied by grain boundary migration all of the hot pressed samples deformed in compression have a similar texture consisting of a girdle of c 001 axes normal to compression and a point maximum of b 010 axes and a 100 poles parallel to the compression direction the lpo of recrystallized grains was separated on spatially resolved orientation maps and shows an s type fabric with b 010 parallel to the compression axis and a random a 100 distribution the bulk fabric was largely present in the hot pressed starting material and is interpreted as the result of compaction during cold pressing at moderate strains in axial compression the initial compaction fabric has been weakly overprinted by the recrystallization fabric one hot pressed sample coarse grain size was deformed in torsion to a shear strain of 0 1 the microstructure indicates dominant dislocation creep and contains about 20 recrystallized grains the texture evolved from the rotationally symmetric compaction lpo to an oblique fabric

compositional dependence of lower crustal viscosity we calculate the viscosity structure of the lower continental crust as a function of its bulk composition using multiphase mixing theory we use the gibbs free energy minimization routine perple x to calculate mineral assemblages for different crustal compositions under pressure and temperature conditions appropriate for the lower continental crust the effective aggregate viscosities are then calculated using a rheologic mixing model and flow laws for the major crust forming minerals we investigate the viscosity of two lower crustal compositions i basaltic 53 wt sio2 and ii andesitic 64 wt sio2 the andesitic model predicts aggregate viscosities similar to feldspar and approximately 1 order of magnitude greater than that of wet quartz the viscosity range calculated for the andesitic crustal composition particularly when hydrous phases are stable is most similar to independent estimates of lower crust viscosity in actively deforming regions based on postglacial isostatic rebound postseismic relaxation and paleolake shoreline deflection

processing of crust in the root of the caledonian continental collision zone the role of eclogitization segments of the root to the caledonian collision zone are exposed along the west coast of norway allowing the study of processes and petrophysical conditions at the deepest crustal levels p t conditions prevailing in these crustal root zones correspond to eclogite facies eclogitization is associated with marked changes in petrophysical properties notably a density 10 15 and vp increase which may give a mantle signature to an eclogitized crust the observed ductility enhancement may be caused by transformation plasticity reduction in grain size formation of new mineral assemblages and presence of fluid in the western gneiss region and the bergen arcs eclogites form along shear zones interpreted as fluid pathways with sharp boundaries to their metastable protolith this makes eclogite shear zones potential deep crustal reflectors dry crust subducted into roots of continental collision zones will undergo eclogitization only if hydrous fluids are available timing of metamorphic reactions does not depend on crossing of equilibrium boundaries in the p t space but rather on the introduction of fluids into the system eclogites are not only passive recorders of p t t paths illustrating very deep subduction of crust 100 km rapid changes in petrophysical properties make fluid induced eclogitization a dynamic process that influences geodynamics stresses set up by the volume changes or by tectonic forces are released by ductile deformation along shear zones and by seismic faulting as evidenced by syn eclogitic pseudotachylytes in the metastable protolith deformation including seismicity enhances fluid infiltration and further eclogitization dense and rheologically weak eclogites may delaminate and sink into the mantle in the absence of fluids crustal doubling may occur without metamorphic re equilibration the evolution of a collision zone depends on the fluid regime which should be considered in geodynamic modelling in addition to p and t

high temperature creep of partially molten plagioclase aggregates we have investigated the high temperature creep of synthetic labradorite an60ab40 between 1323 k and 1523 k at atmospheric pressure and low stresses 2 65 mpa average grain size varies from 12 to 16 m samples contained up to 12 vol melt an60ab40 glass fourier transform infrared measurements indicate that trace amounts of water 0 1 0 15 wt h2o were incorporated during crystallization this water was only partly released at high temperature and atmospheric pressure at low stresses and temperatures deformation of labradorite was controlled by grain boundary diffusion creep yielding a stress exponent of n 1 and an activation energy of 365 25 kj mol regime i with increasing stress and temperature mechanical data and microstructural observations indicate a transition to power law creep with n 3 regime ii this transition occurs in both partially molten and melt free aggregates in regime i the strain rate of samples containing as much as 12 vol melt is between 3 and 5 times higher than in fully crystalline specimens the activation energy is only slightly affected by the melt content between 0 and 10 2 vol transmission electron microscopy shows that the amorphous phase resides in connected triple junctions or in unconnected pockets grain boundaries are not wetted except surrounding large melt pockets the melt distribution of undeformed and deformed samples is similar the strength and activation energy of synthetic labradorite are comparable to those for pure anorthite with trace amounts of water both plagioclase compositions are slightly weaker than synthetic diopside

composite flow laws for crystalline materials with log normally distributed grain size theory and application to olivine abstract conventional steady state flow laws describing combined grain size insensitive dislocation creep and grain size sensitive diffusion creep incorporate grain size as a single mean value however rocks exhibit distributed grain size and variations in the shape of the distribution such as its width will accordingly affect rheological behavior to evaluate this effect we have derived composite dislocation diffusion flow laws describing upper and lower bounds on the rate of deformation of materials showing a grain size distribution of the commonly observed log normal type the upper and lower bound flow laws obtained allow systematic investigation of the influence of the grain size distribution parameters i e the median and standard deviation on composite rheology the results demonstrate major effects of distribution width as well as median value application to polycrystalline olivine deforming by coble creep and power law dislocation creep reveals significant implications for interpreting deformation experiments and modeling flow in the earth s mantle the flow laws also provide an improved basis for modeling the deformation of rocks during geological processes in the crust and mantle

dynamic recrystallization of feldspar a mechanism for ductile shear zone formation feldspar aggregates experimentally deformed in the dislocation creep regime undergo dynamic recrystallization because recovery is difficult due to the limited climb of dislocations recrystallizing aggregates have a lower strength because of the cyclic production of small strain free grains and they develop a strong preferred orientation consistent with that observed in mylonites thus recrystallization accommodated dislocation creep may be responsible for the grain size reduction and strain softening that lead to the formation of many mylonites and ductile shear zones

constraints on the viscosity of the continental crust and mantle from gps measurements and postseismic deformation models in western mongolia 1 we use gps measurements and models of postseismic deformation caused by seven m6 8 to 8 4 earthquakes that occurred in the past 100 years in mongolia to assess the viscosity of the lower crust and upper mantle we find an upper mantle viscosity between 1 1018 and 4 1018 pa s the presence of such a weak mantle is consistent with results from independent seismological and petrological studies that show an abnormally hot upper mantle beneath mongolia the viscosity of the lower crust is less well constrained but a weak lower crust 3 1016 to 2 1017 pa s is preferred by the data using our best fit upper mantle and lower crust viscosities we find that the postseismic effects of viscoelastic relaxation on present day horizontal gps velocities are small 2 mm yr 1 but still persist 100 years after the 1905 m8 4 bolnay earthquake this study shows that the gps velocity field in the baikal mongolia area can be modeled as the sum of 1 a rigid translation and rotation of the whole network 2 a 3 5 mm yr 1 simple shear velocity gradient between the siberian platform to the north and northern china to the south and 3 the contribution of postseismic deformation mostly caused by the 1905 bolnay tsetserleg sequence and by the smaller but more recent 1957 bogd earthquake

stress gradients around porphyroclasts palaeopiezometric estimates and numerical modelling abstract based on strain gradients commonly found around porphyroclasts the mechanical interaction of porphyroclasts and recrystallized mylonite matrix in a retrograde amphibolite facies shear zone of the ivrea verbano zone italy was investigated differential stresses inferred from recrystallized grain size and dislocation density indicate stress concentrations at the porphyroclast matrix interface of about 1 7 2 0 with respect to the average in the matrix the palaeopiezometric stress data are compared to the results of a finite element model composed of two rheologically different phases in this model the local distribution and concentration of stresses around porphyroclasts mostly depend on clast matrix coupling and the effective viscosity contrast of the two phases the bulk strength and work hardening of the composite system increases with interface coupling both the modified stress field and the reduced average grain size around porphyroclasts may result locally in instabilities and promote strain localization and a change in the dominant deformation mechanism

effect of varying enstatite content on the deformation behavior of fine grained synthetic peridotite under wet conditions the effect of varying enstatite content on the deformation behavior of synthetic fine grained 1 to 2 m forsterite enstatite rock with 0 5 wt added water was investigated at temperatures of 900 to 1000 c strain rates between 10 7 and 10 5 s 1 and a confining pressure of 600 mpa the samples exhibited approached steady state flow at stresses 60 mpa the results show that at constant strain rate increasing enstatite content is associated with a sharp decrease in flow strength in the range 0 2 5 vol enstatite with little further change up to 20 vol the observed power law n value of 1 7 and microstructures are similar to those obtained in previous work on material with 2 5 wt enstatite and are consistent with a water enhanced grain boundary sliding gbs dominated deformation mechanism significantly a negative correlation was found between grain size and enstatite content indicating that enstatite content played a role in controlling the grain size of the starting materials moreover a high correlation between measured flow strength and grain size was found consistent with a grain size exponent of 3 in a conventional grain size sensitive flow equation a water enhanced deformation mechanism involving gbs accommodated probably by grain boundary diffusion and or dislocation activity is therefore implied with the effect of enstatite content on flow strength being an indirect physical effect caused by grain size control other effects of second phase content such as weakening caused by interphase boundary diffusion and or migration processes seem to be unimportant under the conditions investigated

the role of tectonic grain size reduction in the rheological stratification of the lithosphere with the aid of published constitutive flow laws for common crustal and upper mantle rock types it is possible to construct curves showing variation of rock strength with depth in the continental lithosphere assuming a particular crustal stratification of rock types and a particular geothermal gradient such exercises lead to the conclusion that marked changes in strength can occur at major petrological interfaces such as the moho discontinuity which are therefore likely to be associated with a tendency for major sub horizontal detachments to form constitutive flow laws used for such purposes are usually assumed to represent steady state flow at constant rock microstructure

recrystallization and fabric development in plagioclase the microstructures and fabrics of naturally deformed intermediate composition plagioclase feldspars from the amphibolite granulitic gneisses from the lower crust have been studied special interest in the effects of dynamic recrystallization and deformation fabrics has been taken the plagioclase feldspars an40 50 have undergone extensive ductile deformation by intracrystalline slip and twinning they show strong crystallographic preferred orientation which was produced by dominant 010 001 slip dynamic recrystallization is indicated by fine grained new grains neoblasts interspersed with deformed relicts porphyroclasts the neoblasts have formed preferentially along grain boundaries kink band boundaries kbbs intracrystalline shear zones and around solid inclusions such as garnet and pyroxene optical microstructural evidence suggests that dynamic recrystallization occurred primarily by grain boundary migration with subsidiary subgrain rotation which appears to have occurred only at low st

fault slip rates for the active external dinarides thrust and fold belt 1 we present estimates of slip rates for active faults in the external dinarides this thrust and fold belt formed in the adria eurasia collision zone by the progressive formation of ne dipping thrusts in the footwalls of older structures we calculated the long term horizontal velocity field slip rates and related uncertainties for active faults using a thin shell finite element method we incorporated active faults with different effective fault frictions rheological properties appropriate geodynamic boundary conditions laterally varying heat flow and topography the results were obtained by comparing the modeled maximum compressive horizontal stress orientations with the world stress map database the calculated horizontal velocities decrease from the southeastern external dinarides to the northwestern parts of the thrust and fold belt this spatial pattern is also evident in the long term slip rates of active faults the highest slip rate was obtained for the montenegro active fault while the lowest rates were obtained for active faults in northwestern slovenia low slip rates influenced by local active diapirism are also characteristic for active faults in the offshore central external dinarides these findings are contradictory to the concept of adria as an internally rigid aseismic lithospheric block because the faults located in its interior release a part of the regional compressive stress we merged the modeling results and available slip rate estimates to obtain a composite solution for slip rates

magnitude of weakening during crustal scale shear zone development we describe and apply a field based approach for calculating the bulk strength of a heterogeneous material to a crustal scale shear zone defining the margin of the ma parry sound domain in the grenville province of southeastern ontario using a numerical method we calculate bulk strength defined as effective viscosity as the ratio between the surface traction needed to deform a square block in simple shear and the velocity gradient across that block we use natural shear zone geometries to define the internal block structure and assign internal relative viscosities based primarily on textural criteria the margin of the parry sound domain developed into the km scale twelve mile bay shear zone accommodating several tens of km of transport while the domain interior remained rigid fracturing and fluid infiltration drove development of an amphibolite facies meter scale shear zone network that evolved into the twelve mile bay structure we analyzed three sites across the w5 km wide strain gradient from near the granulitic domain to the large scale shear zone the rocks at the shear zone margin weakened by approximately 30 those in the core weakened by at least 77 and probably by an order of magnitude these values lie between but differ substantively from the isostress and isostrain rate bounds indicating that a numerical approach such as presented here markedly improves the accuracy of bulk strength calculations

extreme ductility of feldspars from a mylonite parry sound canada abstract single feldspar crystals in mylonites from the grenville structural province of canada have accumulated extreme strains by ductile mechanisms the samples studied are from a deep crustal shear zone and were collected at parry sound ontario the mylonites were derived from granite and leucogabbro and the feldspar crystals originated in late syntectonic pegmatite dykes optical and transmission electron microstructures of microperthitic alkali feldspars show evidence of syntectonic and synchronous dislocation climb leading to recovery si al ordering and na k interdiffusion evidence for the operation of these processes is respectively ubiquitous subgrains development and coarsening of tweed texture and transition of monoclinic to triclinic k feldspar and exsolution domain reorganization these processes occurred concurrently with extreme straining of the host crystals by dislocation and diffusion mechanisms the effects of ductile deformation preserved in these crystals are quite different from those usually observed in feldspars the unusual microstructures are thought to be due to the conditions of high temperature and confining pressure which existed during deformation in concert with a deformation enhancing point defect chemistry possibly associated with incorporation into the crystals of a species of water

high temperature deformation in the neoproterozoic transpressional ribeira belt southeast brazil abstract the neoproterozoic ribeira belt is subdivided in two domains with contrasting tectonic characteristics the northern domain is dominated by shallowly dipping foliations and orogen normal thrust tectonics the southern domain is characterized by a 1000 km long network of anastomosing transcurrent shear zones parallel to the belt this contrast is interpreted as reflecting continent continent convergence that is almost orthogonal to the margins in the northern domain and significantly oblique in the southern domain the central transitional domain of the ribeira belt displays the northern termination of the transcurrent shear zone network the alem paraiba padua shear zone system appss the 250 km long alem paraiba padua system involves granulites facies mylonites deformed through transpression a detailed study of the microstructure and lattice preferred orientation lpo of the rock forming minerals in these granulite mylonites allow a better understanding of deformation mechanisms active at high temperature in the crust plagioclase crystals are plastically deformed they display curved twins and cleavages mechanical twins and evidence of dynamic recrystallization lpo of plagioclase is consistent with activation of the 010 100 and 010 001 slip systems lpo of orthopyroxene and amphibole indicates that these minerals have been deformed through dislocation creep with the activation of the 100 001 slip system quartz in granulite mylonite displays evidence of extensive growth through grain boundary migration the lpo of quartz is therefore the result of a static transformation of an initial syn kinematic lpo and cannot be straightforwardly interpreted in terms of deformation mechanisms active during mylonitization