an assessment of dynamically recrystallized grainsize as a palaeopiezometer in quartz bearing mylonite zones abstract recrystallized grainsize has been measured in quartz and in some cases feldspar in about 200 samples from 22 mylonite zones from 3 discrete tectonic settings 1 1 supracrustal low temperature shallow dipping thrusts 2 2 deeper level major 100 km thrusts in high grade metamorphic rocks and 3 3 steeply dipping fault zones in granite within a low grade palaeozoic fold belt the concept of a steady state dynamically recrystallized grainsize largely unaffected by post creep processes e g static grain growth receives strong support especially from the microstructures and the independence of finite strain and grainsize in most mylonite zones however the usefulness of this grainsize for predicting stress magnitudes is limited by the apparently independent influence of metamorphic environment on grainsize the available theoretical and empirical stress grainsize relationships do not account for these effects the concept of a steady state grainsize and the ease of accurate measurement offer a potentially powerful method for predicting stress intensity during deformation predicted deviatoric stresses have been computed using twiss theoretical model for comparison with other crustal stress estimates and most samples fall in the range from 20 to 150 mpa in a few cases where discrete high strain mylonite zones occur within contemporaneously regionally deformed rocks the recrystallized grainsize indicates that stresses were 1 5 to 2 times higher within the zones there is a clear need for improved theoretical and or empirical stress grainsize relationships for quartz and other minerals before extensive further application of this technique

a thermodynamically self consistent damage equation for grain size evolution during dynamic recrystallization summary we employ basic non equilibrium thermodynamics to propose a general equation for the mean grain size evolution in a deforming medium under the assumption that the whole grain size distribution remains self similar we show that the grain size reduction is controlled by the rate of mechanical dissipation in agreement with recent findings our formalism is self consistent with mass and energy conservation laws and allows a mixed rheology as an example we consider the case where the grain size distribution is lognormal as is often experimentally observed this distribution can be used to compute both the kinetics of diffusion between grains and of dynamic recrystallization the experimentally deduced kinetics of grain size coarsening indicates that large grains grow faster than what is assumed in classical normal grain growth theory we discuss the implications of this model for a mineral that can be deformed under both dislocation creep and grain size sensitive diffusion creep using experimental data of olivine our predictions of the piezometric equilibrium in the dislocation creep regime are in very good agreement with the observations for this major mantle forming mineral we show that grain size reduction occurs even when the average grain size is in diffusion creep because the largest grains of the grain size distribution can still undergo recrystallization the resulting rheology that we predict for olivine is time dependent and more non linear than in dislocation creep as the deformation rate remains an increasing function of the deviatoric stress this rheology is not localizing

experimental grain size sensitive flow of hot pressed brazilian quartz aggregates abstract grain size sensitive flow may be of particular importance in the flow of rocks in natural high strain zones hot pressed aggregates of synthetic ultrafine grained brazilian quartz of nominal grain sizes 0 4 1 3 and 4 5 m were deformed at 300 mpa confining pressure and temperatures of 1273 1473 k fully ductile flow at differential stresses below 200 mpa followed 0 4 exp 220000 r t d 2 where is strain rate s 1 is stress mpa d is grain size microns and r is the gas constant grains remained equant and mostly dislocation free without significant crystallographic fabric water pressure 25 50 mpa was estimated from water adsorbed onto grain boundaries during specimen fabrication flow was inferred to be controlled by silicon volume diffusion and to be sensitive to water fugacity at differential stresses 200 mpa non linear flow involved dislocation motion extrapolation to natural strain rates and temperatures that predict grain size sensitive flow in quartz should only be important at 800 k and very small grain sizes 1 m flow law parameters may change over the extrapolation interval e g through a transition to control by silicon grain boundary diffusion but geological evidence suggests that flow of even very fine grained quartzites commonly occurs by recrystallization accommodated dislocation creep rather than by diffusional accommodation

influence of deformation conditions on the development of heterogeneous recrystallization microstructures in experimentally deformed carrara marble recrystallized grains are potentially useful as indicators of palaeostress in naturally deformed rocks providing that well calibrated relationships palaeopiezometers exist between the recrystallized grain size and stress rocks can exhibit microstructures that are heterogeneous that is containing recrystallized as well as deformed grains and showing subgrains within grains that differ in size and character from the grain core to the mantle previous studies on palaeopiezometers only rarely took into account such heterogeneous microstructure we used electron backscattered diffraction ebsd to accurately quantify the heterogeneous microstructures in experimentally deformed carrara marble flow stress 15 85 mpa temperature 700 990 8c and natural strain 0 15 0 90 the sizes of bulges recrystallized grains and deformed grains have been measured we found that the overall character of the microstructures varies as a function of deformation conditions in heterogeneous samples showing core mantle microstructures the sizes of the bulges and recrystallized grains are independent of strain and show an inverse dependency on stress the recrystallized grains have been found to nucleate at grain boundary bulges our study illustrates that very different microstructures may develop in relation to the complexity of the recrystallization mechanisms we therefore suggest that piezometers should be calibrated and applied for a single type of overall microstructure

tectonic plate generation and two phase damage void growth versus grain size reduction 1 the generation of plate tectonics from mantle convection requires shear localization in order to yield narrow weak plate boundaries separating broad strong plates a plausible shear localizing mechanism involves damage e g distributed microcracking one theoretical model of which involves two phase mechanics two phase damage theory employs a nonequilibrium relation between interfacial surface energy pressure and viscous deformation thereby providing a description of void generation and microcracking and hence weakening failure and shear localization here we examine the application of this theory to the problem of generating plate like behavior from convective type divergent convergent poloidal motion through a source sink formulation we extend the previous damage theory to consider two possible damage effects 1 growth and nucleation of voids associated with dilation of the host matrix and 2 increasing fineness i e reducing coarseness of the mixture by for example grain size reduction void generating damage is found to be poor at plate generation because of the predominance of dilational motion that is adverse to the development of plate like flow fineness generating damage is found to be very efficient at generating plate like behavior if we assume that the matrix viscosity is a simple function of grain void size as is typical for diffusion creep the implied grain size reduction mechanism is different from that of dynamic recrystallization and appears to be more capable of generating the requisite shear localization for forming tectonic plates from mantle flow

grain growth kinetics in olivine aggregates abstract grain growth kinetics were studied in hot pressed fine grained olivine aggregates experimental conditions include pressures of 0 1 mpa 300 mpa and 1 gpa and temperatures of 1473 1573 and 1673 k with or without the presence of water at the initial stage of grain growth growth of larger grains occurs rapidly consuming smaller grains which results in significant pore entrapment almost homogeneous grain size distribution is established in this process after this stage grain size distribution remains almost homogeneous in most cases it was found that the presence of water enhances the grain growth kinetics when the amount of water is limited however when the amount of water is large the water containing pores significantly inhibit the grain boundary migration therefore the enhancement of grain growth due to the presence of water is due to the role of dissolved water in olivine rather than the mass transport through water abnormal grain growth was found in some samples in these cases normal grain growth was inhibited by the presence of a significant amount of water filled pores or grains with significantly larger size than the rest occurred in the starting materials in 0 1 mpa runs significant porosity developed and the grain growth rates were significantly lower than in those at high confining pressures these results show that both water and pores have important effects on grain growth kinetics in olivine when the effects of pores or secondary phases in inhibiting grain growth are unimportant the grain growth rate in olivine is very fast growth to 100 m size will occur in 102 to 103 hours at 1500 k thus fine grain size in olivine will be maintained only for a short geological time unless the grain boundaries are effectively pinned by secondary particles

cooling induced strain localization in carbonate mylonites within a large scale shear zone glarus thrust switzerland abstract the glarus thrust switzerland offers a great field example of strain localization under retrograde conditions along the thrust mylonitic microfabrics are characterized by a temperature stress controlled balance of grain size reducing mechanisms and grain growth consequently mean grain sizes decrease along the thrust with decreasing metamorphic conditions and towards the thrust contact in an opposite manner calcite twin densities increase towards the contact cpos are strongest between 0 5 and 15 m away from the thrust but become generally weaker in the last centimeters where also cataclasites occur the cpo weakening and grain size reduction towards the shear zone point to a change from predominant dislocation creep to granular flow finally ending in cataclastic deformation the microfabric changes correlate with a decrease in 13c and 18o towards the thrust contact indicating the presence of fluids during cycles of brittle and plastic deformation based on the microfabric changes variations in stable isotopes and cross cut relationships a subdivision into a low and high temperature shear zone can be made the microfabric modifications resulted from changes in deformation conditions due to ongoing thrusting and exhumation induced cooling promoting further strain localization of an existing high strain shear zone

dolomite microstructures between 390 and 700 c indications for deformation mechanisms and grain size evolution abstract dolomitic marble on the island of naxos was deformed at variable temperatures ranging from 390 c to 700 c microstructural investigations indicate two end member of deformation mechanisms 1 diffusion creep processes associated with small grain sizes and weak or no cpo c rystallographic p referred o rientation whereas 2 dislocation creep processes are related with larger grain sizes and strong cpo the change between these mechanisms depends on grain size and temperature therefore sample with dislocation and diffusion creep microstructures and cpo occur at intermediate temperatures in relative pure dolomite samples the measured dolomite grain size ranges from 3 to 940 m grain sizes at tmax 450 c show an arrhenius type evolution reflecting the stabilized grain size in deformed and relative pure dolomite the stabilized grain size is five times smaller than that of calcite at the same temperature and shows the same arrhenius type evolution in addition the effect of second phase particle influences the grain size evolution comparable with calcite calcite dolomite mixtures are also characterized by the same difference in grain size but recrystallization mechanism including chemical recrystallization induced by deformation may contribute to apparent non temperature equilibrated mg content in calcite

combining natural microstructures with composite flow laws an improved approach for the extrapolation of lab data to nature so far rheological predictions that are based on experimentally derived flow laws appear inconsistent with microstructural observations in natural calcite mylonites because grain size sensitive gss diffusion creep is predicted while microstructural criteria suggest grain size insensitive gsi dislocation creep as the dominant deformation mechanism a way out of this discrepancy between experiment and nature can be found by combining full grain size distributions with composite gsscgsi flow laws we applied this approach to natural carbonate mylonites from the helvetic alps switzerland for steady state microstructures our calculations indicate an increasing gss component with increasing temperature at geologically constrained strain rates of 10 k10 10 k11 s k1 the modeling results are consistent with microstructural observations of the natural mylonites in these rocks dynamically steady state microfabrics show a systematic change in grain size grain size distribution and grain aspect ratio with temperature while crystallographic preferred orientations remain of similar strength when single value mean grain sizes are used rather than grain size distributions rheological modeling fails to give reasonable results further paleostresses estimated from conventional recrystallized grain size piezometers were found to be unrealistically high but were reduced to 80 100 mpa when results of the composite modeling were used hence combining composite flow laws with microstructural data from natural mylonites forms a promising approach for a better extrapolation of experimental data to nature and therefore provides new insight into changes in rheology during large scale geodynamic processes q 2005 elsevier ltd all rights reserved

the role of recrystallisation on the deformation behaviour of calcite rocks large strain torsion experiments on carrara marble abstract the rheological microstructural and textural evolution of carrara marble was studied during experimental deformation in torsion up to very large shear strains 0 4 50 experiments were performed at constant angular displacement rates corresponding to shear strain rates of 6 10 5 3 10 3 s 1 a confining pressure of 300 mpa and temperatures of 500 600 and 727 c microstructures and crystallographic preferred orientations cpo were analysed using light microscopy and electron backscatter diffraction ebsd at all conditions deformation occurred dominantly by dislocation creep even up to very large shear strains after peak stresses 2 recrystallisation mostly by subgrain rotation caused weakening 5 20 grain size reduction and a change in the cpo a monoclinic cpo that formed at small strains at all temperatures evolved into two different large strain cpos at different temperatures at large strains even after complete recrystallisation the cpo continued to strengthen and a secondary foliation was formed at a steep angle to the shear zone boundary this secondary foliation was continuously reset by both subgrain rotation and grain boundary migration recrystallisation and eventually overprinted the primary foliation misidentifying the primary and secondary foliations could result in misinterpretation of the applied strain field more generally information about the amount of strain is lacking for microstructures and cpos of highly deformed ultramylonites

exhumation of high p marbles of the samana terrane northern hispaniola insights from paleostress and microstructural imprints abstract paleostress variations and microstructural imprints of a subducted carbonate slab record changes in mechanical strength during its exhumation the slab studied here forms part of the high p samana terrane located on the north eastern margin of the hispaniola island cold cathodoluminescence images reveal relict cataclastic fabrics within the highest pressure marbles of the punta balandra and santa barbara schists structural units formed in the early stages of exhumation at p t conditions ca 2 0 gpa 500 c cataclastic flow was triggered after a moderate increase of water content 1 2

relationships between dynamically recrystallized grain size and deformation conditions in experimentally deformed olivine rocks new microstructural data on experimentally deformed wet and dry natural olivine rocks anita bay and aheim dunite together with the other reliable experimental data indicate that the experimental stress recrystallized grain size relationship in olivine rocks is largely independent of water content and temperature and is only slightly dependent on the flow properties of the material the experimental data cover a stress range of 30 300 mpa water contents from 30 ppm to 300 ppm and temperatures in the range 1100 1650 c local melt contents of up to 10 volume cannot be demonstrated to have a significant effect on the stress grain size relationship

plate generation and two phase damage theory in a model of mantle convection summary the formation of narrow rapidly deforming plate boundaries separating strong plate interiors are integral components of the generation of plate tectonics from mantle convection the development of narrow plate boundaries requires the interaction of a non linear rheology and convection one such non linear rheology is two phase damage theory which employs a nonequilibrium relation between interfacial surface energy pressure and viscous deformation thereby forming a theoretical model for void generation two phase damage theory was recently extended to allow for deformational work to increase the fineness reduce the grain size of the matrix phase we present results testing two phase damage theory in a 2 d convectively driven system where we allow for 1 pure void generating damage 2 pure fineness generating damage and 3 combined void and fineness generating damage pure void generating damage is found to be unsuccessful at producing plate like features finenessgenerating damage is successful at inducing plate like behaviour in certain circumstances including increasing viscosity sensitivity to fineness and certain regimes of damage input and healing rate cases with combined void and fineness generating damage produce significantly more localization than the end members due to the apparent increase of deformational work input into fineness generation the interaction of microcracks and grain size reduction in two phase damage theory suggests a rheological model for shear localization necessary for the formation of plate tectonic boundaries

the normal grain growth behaviour of nominally pure calcitic aggregates the normal grain growth behaviour of four different but all nominally pure calcite powders 99 analytic grade calcite 99 7 chalk 99 97 crushed iceland spar 99 95 chelometric grade calcite has been investigated as a function of temperature 550 600 650 700 c and confining pressure 100 190 mpa under both dry and hydrostatic pfluid ptotal conditions the initial particle size of both the analytic grade and chelometric grade calcite was about 5 m and that of the chalk was about 3 m while the experiments on the iceland spar were conducted on powders of three different initial particle sizes 3 4 7 5 38 5 m on each material at each pressure temperature condition 6 to 15 experiments equally spaced in log time from 15 minutes to 50 days were conducted under dry conditions all four materials recrystallized to aggregates which contained less than 2 porosity and which had a grain size of between 4 and 20 m depending on the initial particle size subsequently the aggregates coarsened by normal grain growth with the kinetics of the growth process being controlled by the rate at which the grain boundaries could drag the residual pores with them as they migrated under nominally identical conditions both the mechanism and rates of pore drag differed greatly for the different materials implying that this process is highly sensitive to trace solute impurity concentrations this sensitivity renders the task of providing a systematic account of dry calcite grain growth kinetics highly problematic under hydrostatic conditions all the powders followed the same normal grain growth kinetics in which the growth process was rate controlled by diffusion through the pore fluid on the grain boundaries an activation enthalpy of 162 6 kj mol 1 and an activation volume of 34 35 cm3 mol 1 was obtained for this process

faults shear zones in the earth s mantle abstract geodetic data support a short term continental deformation localized in faults bounding lithospheric blocks whether major faults observed at the surface affect the lithospheric mantle and if so how strain is distributed are major issues for understanding the mechanical behavior of lithospheric plates a variety of evidence from direct observations of deformed peridotites in orogenic massifs ophiolites and mantle xenoliths to seismic reflectors and seismic anisotropy beneath major fault zones consistently supports prolongation of major faults into the lithospheric mantle this review highlights that many aspects of the lithospheric mantle deformation remain however poorly understood coupling between deformation in frictional faults in the uppermost crust and localized shearing in the ductile crust and mantle is required to explain the post seismic deformation but mantle viscosities deduced from geodetic data and extrapolated from laboratory experiments are only reconciled if temperatures in the shallow lithospheric mantle are high 800 c at the moho seismic anisotropy especially shear wave splitting provides strong evidence for coherent deformation over domains several tens of km wide in the lithospheric mantle beneath major transcurrent faults yet it cannot detect narrow strain localization zones or shallowly dipping faults seismic profiling images shallow dipping seismic reflectors in the lithospheric mantle interpreted as the continuation of normal or less frequently inverse faults in the mantle however the nature of these reflectors is poorly understood there is a large discrepancy between the scale at which deformation processes are studied a few kilometers at most due to the limited size of continuous mantle exposures at the earth s surface and the scale inferred for strain localization in the mantle from geophysical observations tens of kilometers combining data on deformation microstructures and crystal preferred orientations in naturally deformed peridotites and seismologic observations allows nevertheless discussing strain localization processes that may play a role on the development of mantle shear zones

crystallographic preferred orientations of plagioclase via grain boundary sliding in a lower crustal anorthositic ultramylonite to clarify the rheological properties and deformational mechanisms that operate within lower crustal shear zones we analyzed deformation microstructures and fabrics in plagioclase grains from high strain regions of intrafolial drag folds of an anorthositic ultramylonite from langoya vesteralen northern norway these grains have developed weak crystallographic preferred orientations cpos other than deformation twinning they do not show any indications of intracrystalline plasticity or dynamic recrystallization in some domains of the drag fold plagioclase cpos are more distinct but the majority of inferred slip systems are not consistent with the main slip systems in plagioclase these suggest that neither dislocation creep nor dislocation accommodated grain boundary sliding gbs produced the plagioclase cpos but rather grain rotation via gbs along specific grain boundaries parallel or subparallel to crystallographic planes of 010 100 or 110 which led to an alignment of the easy slip grain boundaries in the flow direction that produced the observed cpo patterns the anisotropy of dissolution and growth rates may have been responsible for the development of the specific grain boundaries

microstructural evolution during strain localization in dolomite aggregates abstract dolomite aggregates deformed by dislocation creep over a wide range of conditions t 700 1000 c effective pressure of 900 mpa strain rates of 10 7 10 4 s strain weaken by up to 75 of the peak differential stress microstructural study of samples shortened to different finite strains beyond the peak differential stress shows that strain becomes highly localized within shear zones by high temperature creep processes with no contribution of brittle cracking at low strains 8 dolomite deforms homogeneously by recrystallization accommodated dislocation creep at progressively higher sample strains deformation is localized into narrow shear zones made up of very fine 3 m recrystallized grains and relict porphyroclasts 20 100 m finely recrystallized dolomite grains in the shear zones are largely dislocation free and localized shear is facilitated by diffusion creep in contrast original dolomite grains and porphyroclasts in shear zones have high dislocation densities and do not deform after shear zone formation calculated strain rates in the shear zones are two to three orders of magnitude faster than the imposed bulk strain rate of the samples and these strain rates are consistent with predictions of the diffusion creep flow law for fine grained dolomite

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

generation of plate tectonics with two phase grain damage and pinning source sink model and toroidal flow abstract the grain damage and pinning mechanism of bercovici and ricard 2012 for lithospheric shear localization is employed in two dimensional flow calculations to test its ability to generate toroidal strike slip motion and influence plate evolution this mechanism posits that damage to the interface between phases in a polycrystalline material like peridotite composed primarily of olivine and pyroxene increases the number of small zener pinning surfaces which then constrain mineral grains to ever smaller sizes regardless of creep mechanism this effect allows a self softening feedback in which damage and grain reduction can co exist with a grain size dependent diffusion creep rheology moreover grain growth and weak zone healing are greatly impeded by zener pinning thereby leading to long lived relic weak zones the fluid dynamical calculations employ source sink driven flow as a proxy for convective poloidal flow upwelling downwelling and divergent convergent motion and the coupling of this flow with non linear rheological mechanisms excites toroidal or strike slip motion the numerical experiments show that pure dislocation creep rheology and grain damage without zener pinning as occurs in a single phase assemblages permit only weak localization and toroidal flow however the full grain damage with pinning readily allows focussed localization and intense plate like toroidal motion and strike slip deformation rapid plate motion changes are also tested with abrupt rotations of the source sink field after a plate like configuration is developed the post rotation flow and material property fields retain memory of the original configuration for extensive periods leading to suboptimally aligned plate boundaries e g strike slip margins non parallel to plate motion oblique subduction and highly localized weak and long lived acute plate boundary junctions such as at what is observed at the aleutian kurile intersection the grain damage and pinning theory therefore readily satisfies key plate tectonic metrics of localized toroidal motion and plate boundary inheritance and thus provides a predictive theory for the generation of plate tectonics on earth and other planets

on topotaxy and compaction during antigorite and chlorite dehydration an experimental and natural study dehydration reactions result in minerals replacement and a transient fluid filled porosity these reactions involve interface coupled dissolution precipitation and might therefore lead to fixed crystallographic orientation relations between reactant protolith and product phases i e topotaxy we investigate these two phenomena in the dehydration of a foliated antigorite atg serpentinite by comparing the crystallographic preferred orientation cpo developed by olivine ol orthopyroxene opx and chlorite chl during high pressure antigorite and chlorite dehydration in piston cylinder experiments and in natural samples recording the dehydration of antigorite cerro del almirez betic cordillera spain experiments were performed under undrained conditions resulting in fluid filled porosity and in strong cpo of the prograde minerals controlled by the pre existing antigorite cpo in the reactant foliated serpentinite the orientation of aol opx and c text chl is parallel to c text atg from the protolith the cerro del almirez samples show similar locally well developed topotactic relations between orthopyroxene chlorite and antigorite but the product cpos are weaker and more complex at the thin section scale in contrast to the experiments olivine from natural samples shows a weak correlation between bol and the former c text atg we relate the strengthening of local topotactic relations and the weakening of the inherited cpo at a larger scale in natural samples to compaction and associated fluid migration microstructural features that might be related to compaction in the natural samples include 1 smooth bending of the former foliation 2 gradual crystallographic misorientation up to 16 of prismatic orthopyroxene due to buckling by dislocation creep 3 inversion of enstatite to low clinoenstatite p21 c along lamellae and 4 brittle fracturing of prismatic orthopyroxene enclosed by plastically deformed chlorite the coexistence of orthopyroxene buckling and clinoenstatite lamellae enables estimating the local strain rates and shear stresses generated during compaction an lower bound for the strain rates in the order of 10 12 to 10 13 s 1 and shear stresses of 60 70 mpa are estimated based on creep data lower shear stresses 20 40 mpa are retrieved using a theoretical approach these data point to slow compaction and fluid extraction in nature if the system is not perturbed by external forces with rates only marginally higher than the viscoplastic deformation of the solid matrix

the recrystallized grain size piezometer for quartz an ebsd based calibration we have re analyzed samples previously used for a quartz recrystallized grain size paleopiezometer using electron backscatter diffraction ebsd recrystallized and relict grains are separated using their grain orientation spread gos which acts as a measure of intragranular lattice distortion and a proxy for dislocation density for ebsd maps made with a 1 m step size the piezometer relationship is d 103 91 0 41 1 41 0 21 for rms mean diameter values we also present a sliding resolution piezometer relationship d 104 22 0 51 1 59 0 26 that combines 1 m step size data at coarser grain sizes with 200 nm step size data at finer grain sizes the sliding resolution piezometer more accurately estimates stress in fine grained 10 m samples the two calibrations give results within 10 of each other for recrystallized grain sizes between 10 m and 100 m both piezometers match the original light optical microscopy quartz piezometer within error

the preservation of seismic anisotropy in the earth s mantle during diffusion creep summary seismic anisotropy in the earth particularly in the mantle is commonly interpreted as the result of solid state deformation by dislocation creep that induces a lattice preferred orientation lpo diffusion creep operates where stress levels are lower and or grain sizes smaller it is often assumed that diffusion creep induces grain rotations that eventually destroy any existing lpo a new numerical test of this assumption shows that it is not necessarily the case diffusion creep will create some relative grain rotations but rotation rates decrease through time hence when microstructural change due to diffusion creep dominates that due to grain growth defined here as type p behaviour the converse being type o behaviour the model indicates that lpo will be weakened but preserved for a variety of strain paths including both pure and simple shear one measure of anisotropy is the proportional difference in shear wave velocities for different polarization vectors avs a model olivine microstructure with equant grains and initial maximum avs of 10 0 percent has this value reduced to 6 7 per cent when rotational steady state is attained other models with different initial maximum avs values exhibit final maximum avs values more than half the initial values if the grains are initially elongate by a factor of 2 maximum avs is reduced just slightly to 8 5 per cent thus when grain growth plays a subordinate role to the deformation diffusion creep weakens seismic anisotropy by a factor of less than 2 using maximum avs as a measure and olivine as an example consequently the link between seismic anisotropy and deformation mechanism in the mantle requires reappraisal regions with lpo may comprise material which once deformed by dislocation creep but is now deforming by diffusion creep in a rotational steady state

influence of grain size evolution on convective instability 1 grain size one of the most important microstructural properties of materials evolves during creep deformation to minimize the free energy of polycrystalline aggregates we apply a model of grain size evolution to the study of convective instability of cooling boundary layers the grain size evolution model is coupled to a composite rheology where the deformation rate is the sum of that due to dislocation creep and grain size sensitive diffusion creep the onset of convection is sensitive to grain growth rates and the initial grain size the formation of convective instabilities is enhanced by stresses induced by plate motions therefore small scale convection is more likely to occur beneath fast moving plates in finite amplitude convection grain size evolution leads to high viscosity in regions where convective stresses are low and can induce viscosity contrasts exceeding one order of magnitude such viscosity contrasts are sufficient to influence the dynamics of convection often leading to domains which remain isolated from the well mixed convecting fluid a composite viscosity including diffusion creep which has a lower activation energy than dislocation creep reduces the effective temperature dependence of viscosity

strain localization in olivine aggregates at high temperature a laboratory comparison of constant strain rate and constant stress boundary conditions we performed high strain torsion experiments on aggregates of fo50 olivine to test the influence of imposed boundary conditions on localizing deformation we deformed both solid and thin walled cylinders of fo50 either at constant strain rate or at constant stress samples deformed in constant strain rate experiments reached a peak stress followed by weakening at a continually decreasing weakening rate in contrast samples deformed in constant stress experiments weakened at an accelerating weakening rate localization is manifested in samples deformed at constant stress as irregularities along strain markers s c foliations and torsional buckling of thin walled cylinders in contrast samples deformed at constant strain rate deformed homogeneously grain boundary maps created with electron backscatter diffraction data indicate that high strain regions in constant stress samples correlate with finer grain sizes and stronger crystallographic fabrics since the dominant deformation mechanism is grain size sensitive heterogeneous recrystallization leads to strain localization in finer grained regions however variations in strength are not large enough to initiate localization in constant strain rate experiments the magnitude of grain size heterogeneity remains relatively constant with increasing strain implying that shear zones are maintained throughout the experiments even as non localizing regions recrystallize based on our results we propose that deformation driven at constant stress in earth s lithosphere will easily localize even if structural heterogeneities are not initially present

computational study of deformation mechanisms and grain size evolution in granulites implications for the rheology of the lower crust abstract most of granulite terrains worldwide are characterized by large mean grain sizes of 1 mm or more an important exception are the high pressure felsic granulites in the bohemian massif the european variscan belt there recrystallization of original coarse grained ternary feldspar led to formation of a fine grained 100 m mixed matrix dominated by plagioclase and k feldspar this change occurred at temperatures of 850 c and was probably caused by chemically induced decomposition related to slight cooling and enhanced by deformation during continental collision the resulting microstructure shows indications of diffusion creep assisted by melt enhanced grain boundary sliding further on minor coarsening occurred associated with deformation by dislocation creep and aggregation of mineral phases using a thermodynamics based model of grain size evolution we show that stability of the fine grained microstructure crucially depends on zener pinning in the two phase mineral matrix pinning efficiently hinders grain growth and the small grain size that resulted from the ternary feldspar decomposition can be stable even at high temperatures the late switch from the grain size sensitive creep to dislocation creep is rather difficult to explain by temperature and strain rate or stress changes only however a simple incorporation of melt solidification can successfully simulate this behavior alternatively the switch and the associated grain size growth can be related to mineral phase aggregation at lower pressure temperature conditions resulting into a decrease of pinning efficiency this study suggests that the fine grain size of the bohemian granulites in contrast to the common coarse grained type stems from abrupt recrystallization during the high pressure high temperature conditions and pinning in the fine grained matrix such a process may in some cases significantly and suddenly reduce the strength of the lower continental crust and allow for its efficient redistribution

intermediate depth earthquake generation and shear zone formation caused by grain size reduction and shear heating the underlying physics of intermediate depth earthquakes have been an enigmatic topic several studies support either thermal runaway or dehydration reactions as viable mechanisms for their generation here we present fully coupled thermomechanical models that investigate the impact of grain size evolution and energy feedbacks on shear zone and pseudotachylite formation our results indicate that grain size reduction weakens the rock prior to thermal runaway and significantly decreases the critical stress needed for thermal runaway making it more likely to result in intermediate depth earthquakes at shallower depths furthermore grain size is reduced in and around the shear zone which agrees with field and laboratory observations where pseudotachylites are embedded in a simultaneously formed mylonite matrix the decrease in critical stress to initialize localization has important implications for large scale geodynamics as this mechanism might induce lithosphere scale shear zones and subduction initiation we suggest that the combination of grain size reduction and shear heating explains both the occurrence of intermediate depth earthquakes and the formation of large scale shear zones

fluid migration in the mantle wedge influence of mineral grain size and mantle compaction mineral grain size in the mantle affects fluid migration by controlling mantle permeability the smaller the grain size the less permeable the mantle is mantle shear viscosity also affects fluid migration by controlling compaction pressure high mantle shear viscosity can act as a barrier to fluid flow here we investigate for the first time their combined effects on fluid migration in the mantle wedge of subduction zones over ranges of subduction parameters and patterns of fluid influx using a 2 d numerical fluid migration model our results show that fluids introduced into the mantle wedge beneath the forearc are first dragged down dip by the mantle flow due to small grain size 1 mm and high mantle shear viscosity that develop along the base of the mantle wedge increasing grain size with depth allows upward fluid migration out of the high shear viscosity layer at sub arc depths fluid introduced into the mantle wedge at post arc depths migrate upwards due to relatively large grain size in the deep mantle wedge forming secondary fluid pathways behind the arc fluids that reach the shallow part of the mantle wedge spread trench wards due to the combined effect of high mantle shear viscosity and advection by the inflowing mantle and eventually pond at 55 65 km depths these results show that grain size and mantle shear viscosity together play an important role in focusing fluids beneath the arc

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

a phenomenological numerical approach for investigating grain size evolution in ductiley deforming rocks abstract the sizes of recrystallised grains in exhumed ductile shear zones are often used to infer conditions of deformation i e stress strain rate and temperature here we present a simple numerical method of calculating the dynamic evolution of grain size during ductile deformation our phenomenological method is based on the fact that the dynamic competition between grain growth and recrystallisation will drive grains towards a steady state size at each time increment grain growth and reduction contributions are calculated with magnitudes which depend on the difference between the current grain size and a desired steady state grain size in our models we use a recrystallised grain size piezometer to calculate the steady state grain size for a given stress our numerical routine is incorporated into the sulec finite element package allowing us to explore spatial and temporal changes in grain size as a test we compare model results to measured grain sizes in quartz layers thinned and recrystallised around rigid garnet porphyroclasts under simple shear dominated deformation in the alpine fault zone of new zealand numerical models are able to replicate observed grain size variations with boundary conditions consistent with those constrained for the central alpine fault zone

focusing of upward fluid migration beneath volcanic arcs effect of mineral grain size variation in the mantle wedge we use numerical models to investigate the effects of mineral grain size variation on fluid migration in the mantle wedge at subduction zones and on the location of the volcanic arc previous coupled thermal grain size evolution t gse models predict small grain size 1 mm in the corner flow of the mantle wedge a down dip grain size increase by two orders of magnitude along the base of the mantle wedge and finer grain size in the mantle wedge for colder slab subduction zones we integrate these t gse modeling results with a fluid migration model in which permeability depends on grain size and fluid flow through a moving mantle matrix is driven by fluid buoyancy and dynamic pressure gradients induced by mantle flow our modeling results indicate that fluids introduced along the base of the mantle wedge beneath the forearc are initially dragged down dip by corner flow due to the small grain size and low permeability immediately above the slab as grain size increases with depth permeability increases resulting in upward fluid migration fluids released beneath the arc and the backarc are also initially dragged down dip but typically are not transported as far laterally before they begin to travel upward as the fluids rise through the backarc mantle wedge they become deflected towards the trench due to the effect of mantle inflow the combination of down dip migration in the forearc and trench ward migration in the backarc results in pathways that focus fluids to the arc this article is protected by copyright all rights reserved

paleowattmeters a scaling relation for dynamically recrystallized grain size during dislocation creep mineral grains often evolve to a stable size dictated by the deformation conditions we suggest that grain size evolution during deformation is determined by the rate of mechanical work provided that other elements of microstructure have achieved steady state and that the dissipation rate is roughly constant then changes in internal energy will be proportional to changes in grain boundary area if normal grain growth and dynamic grain size reduction occur simultaneously then the steady state grain size is determined by the balance of those rates a scaling model using these assumptions and published grain growth and mechanical relations matches stress grain size relations for quartz and olivine rocks with no fitting for marbles the model also explains scatter not rationalized by assuming that recrystallized grain size is a function of stress alone when extrapolated to conditions typical for natural mylonites the model is consistent with field constraints on stresses and strain rates

using short term postseismic displacements to infer the ambient deformation conditions of the upper mantle 1 to interpret short term postseismic surface displacements in the context of key ambient conditions e g temperature pressure background strain rate water content creep mechanism we combined steady state and transient flow into a single constitutive relation that can explain the response of a viscoelastic material to a change in stress the flow law is then used to investigate mantle deformation beneath the eastern california shear zone following the 1999 m7 1 hector mine earthquake the flow law parameters are determined using finite element models of relaxation processes constrained by surface displacement time series recorded by 55 continuous gps stations for 7 years following the earthquake results suggest that postseismic flow following the hector mine earthquake occurs below a depth of 50 km and is controlled by dislocation creep of wet olivine diffusion creep models can also explain the data but require a grain size 3 5 mm that is smaller than the inferred grain size 10 20 mm based on the mantle conditions at these depths in addition laboratory flow laws predict dislocation creep would dominate at the stress grain size conditions that provide the best fit to diffusion creep models model results suggest a transient creep phase that lasts 1 year and has a viscosity 10 times lower than subsequent steady state flow in general agreement with laboratory observations the postseismic response is best explained as occurring within a relatively hot upper mantle e g 1200 1300 c at 50 km depth with a long term background mantle strain rate of 0 1 0 2 mstrain yr consistent with the observed surface strain rate long term background shear stresses at the top of the mantle are 4 mpa then decrease with depth to a minimum of 0 1 0 2 mpa at 70 km depth before increasing slowly with depth due to the pressure dependence of viscosity these conditions correspond to a background viscosity of 10 21 pa s within a thin mantle lid that decreases to 5 10 19 pa s within the underlying asthenosphere this study shows the utility of using short term postseismic observations to infer long term mantle conditions that are not readily observable by other means

the thermodynamics of deformed metamorphic rocks a review abstract the deformation of rocks is a disequilibrium and strongly non linear phenomenon with a number of interacting chemical thermal and microstructural processes operating simultaneously we review progress in this area over the past 30 years deforming chemically reacting systems are dissipative systems and hence are characterised by highly ordered structures that develop through cooperative processes once parameters such as critical strains strain rates fluid infiltration rates damage densities or temperatures are attained such criticality is the hallmark of deformed rocks at all length scales and is the basis for a diverse range of structures such as foliations and lineations produced by metamorphic differentiation rotation recrystallisation folding boudinage and micro to regional scale fracture systems criticality is identified with classical criticality and not self organised criticality the first and second laws of thermodynamics are used to show that such structural diversity arises from reaction diffusion deformation equations criticality of the system is associated with the stored energy becoming non convex and structures arise in order to minimise this non convex energy these structures are scale invariant and hence are characterised by fractal and minimal surface geometries thermodynamics is a powerful discipline to integrate seemingly unrelated processes in structural geology and produce an integrated approach to the subject that crosses all length scales

reconciling mantle attenuation temperature relationships from seismology petrology and laboratory measurements seismic attenuation measurements provide a powerful tool for sampling mantle properties laboratory experiments provide calibrations at seismic frequencies and mantle temperatures for dry melt free rocks but require 102 103 extrapolations in grain size to mantle conditions also the effects of water and melt are not well understood at the same time body wave attenuation measured from dense broadband arrays provides reliable estimates of shear wave attenuation qs 1 affording an opportunity for calibration we reanalyze seismic data sets that sample arc and back arc mantle in central america the marianas and the lau basin confirming very high attenuation qs 25 80 at 1 hz and depths of 50 100 km at each of these sites independent petrological studies constrain the temperature and water content where basaltic magmas last equilibrated with the mantle 1300 1450 c the qs measurements correlate inversely with the petrologically inferred temperatures as expected however dry attenuation models predict qs too high by a factor of 1 5 5 modifying models to include effects of h2o and rheology dependent grain size shows that the effects of water enhanced dissipation and water enhanced grain growth nearly cancel so h2o effects are modest therefore high h2o in the arc source region cannot explain the low qs nor in the back arc where lavas show modest water content most likely the high attenuation reflects the presence of melt and some models of melt effects come close to reproducing observations overall body wave qs can be reconciled with petrologic and laboratory inferences of mantle conditions if melt has a strong influence beneath arcs and back arcs

fabric development as the key for forming ductile shear zones and enabling plate tectonics abstract lithospheric deformation on earth is localized under both brittle and ductile deformation conditions as high temperature ductile rheologies are fundamentally strain rate hardening the formation of localized ductile shear zones must involve a structural or rheological change or a change in deformation conditions such as an increase in temperature in this contribution i develop a localization potential that quantifies the weakening associated with these changes the localization potential corresponds to the increase in strain rate resulting from that change under constant stress conditions i provide analytical expressions for the localization potential associated with a temperature increase grain size reduction an increase in water fugacity melt content or the abundance of a weak mineral phase i show that these processes cannot localize deformation from a mantle convection scale 103 km to a ductile shear zone scale 1 km to achieve this is it necessary to invoke a structural transition whereby the weak phase in a rock forms interconnected layers this process is efficient only if one phase is much weaker than the others or if the weakest phase has a highly non linear rheology micas melt and fine grained aggregates unless dry rheologies are used have the necessary characteristics as none of these phases is expected to be present in the dry lithosphere of venus this concept can explain why venus unlike the earth does not display a global network of plate boundaries the diffuse plate boundary in the central indian ocean may be as yet non localized because serpentinization has not reached the ductile levels of the lithosphere

the lateral boundary of a metamorphic core complex the moutsounas shear zone on naxos cyclades greece we describe the structure microstructures texture and paleopiezometry of quartz rich phyllites and marbles along n trending moutsounas shear zone at the eastern margin of the naxos metamorphic core complex mcc fabrics consistently indicate a top to the nne non coaxial shear and formed during the main stage of updoming and exhumation between ca 14 and 11 ma of the naxos mcc the main stage of exhumation postdates the deposition of overlying miocene sedimentary successions and predates the overlying upper miocene pliocene conglomerates detailed microstructural and textural analysis reveals that the movement along the moutsounas shear zone is associated with a retrograde greenschist to subgreenschist facies overprint of the early higher temperature rocks paleopiezometry on recrystallized quartz and calcite yields differential stresses of 20 77 mpa and a strain rate of 10 15 10 13 s 1 at 350 c for quartz and ca 300 c for calcite chlorite geothermometry of the shear zone yields two temperature regimes 300 360 c and 200 250 c the lower temperature group is interpreted to result from late stage hydrothermal overprint

graphite and quartz petrofabrics examples from the ediacaran black quartzites of the ossa morena zone sw iberia abstract we study with the electron back scattered diffraction ebsd technique the fabric of metamorphosed and ductilely deformed phtanites or graphitic cherts a common lithotype in ediacaran supracrustals of the west european cadomian orogen currently they are black quartzites with a planolinear tectonite structure microscopically they present a sub mm scale alternation of coarse and fine grained dynamically recrystallized quartz bands we attribute intracrystalline plasticity partitioning to variations in graphite inclusion concentration constraining quartz grain boundary mobility during dynamic recrystallization under non coaxial strain regimes and moderate to elevated temperatures 400 650 c lattice preferred orientations of quartz c and axes are geometrically related to the external reference frame provided by foliations and lineations we also identify the involvement of 0001 r m and m c quartz intracrystalline slip systems deformation modes operated simultaneously and under identical temperatures in interleaved parallel domains mm to cm thick in adjacent coarse and fine grained bands medium to high t plasticity is congruent with the syntectonic temperature gradients associated with the amphibolite facies metamorphism of the country rocks we also present the first specific study published so far on natural deformation graphite lattice preferred orientation graphite inclusions as well as those of mica exhibit mineral shape fabrics that suggest operation of 0001 slip however ebsd measurements also record fabrics suggestive of m slip in spite of a rather small volumetric proportion graphite spatial organization at increased shear strains facilitated ductile deformation if a graphite network is established in the rock it can potentially increase rock electrical conductivity thus accounting for mid and lower crust anomalous electric conductivity

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

experimental study of the influence of stress temperature and strain on the dynamic recrystallization of carrara marble carrara marble has been deformed experimentally at temperatures ranging between 500 and 1000 c at confining pressures of 200 and 300 mpa and up to very large strains in extension and compression in order to study the microstructural and mechanical property changes associated with dynamic recrystallization microstructural studies were made by optical microscopy on ultrathin sections and automatic grain size measurement techniques were used when the temperature is sufficiently high 600 c and the stresses are high enough for deformation twinning to occur twin boundary migration is a powerful recrystallization mechanism that does not modify the grain size at stress levels too low to activate twinning recrystallization occurs in two stages the formation of nuclei by grain boundary bulging and subgrain rotation recrystallization in the grain boundary regions followed by a second stage of grain boundary migration recrystallization to a larger grain size that eventually overprints the entire rock volume the recrystallization process requires larger prestrains at the lower temperatures the size to which migration recrystallized grains grow seems to be limited by the stress and grain size dependent twinning field boundary or by the boundary between the dislocation creep and grain size sensitive flow mechanisms within the range of experimental observations dynamically recrystallized grain size does not depend on strain rate or temperature separate empirical stress versus grain size relations are presented for rotation and migration recrystallization that may be used for palaeopiezometry no unequivocal experimental evidence of weakening resulting from recrystallization to finer grain sizes at laboratory strain rates was found however extrapolation of the experimental data to low natural strain rates suggests that weakening following recrystallization may occur in nature

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

grain size measurements of natural gas hydrates abstract methane hydrates are present in marine seep systems and occur within the gas hydrate stability zone very little is known about their crystallite sizes and size distributions because they are notoriously difficult to measure crystal size distributions are usually considered as one of the key petrophysical parameters because they influence mechanical properties and possible compositional changes which may occur with changing environmental conditions variations in grain size are relevant for gas substitution in natural hydrates by replacing ch 4 with co 2 for the purpose of carbon dioxide sequestration here we show that crystallite sizes of gas hydrates from some locations in the indian ocean gulf of mexico and black sea are in the range of 200 400 m larger values were obtained for deeper buried samples from odp leg 204 the crystallite sizes show generally a log normal distribution and appear to vary sometimes rapidly with location

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

implications of grain size evolution on the seismic structure of the oceanic upper mantle abstract we construct a 1 d steady state channel flow model for grain size evolution in the oceanic upper mantle using a composite diffusion dislocation creep rheology grain size evolution is calculated assuming that grain size is controlled by a competition between dynamic recrystallization and grain growth applying this grain size evolution model to the oceanic upper mantle we calculate grain size as a function of depth seafloor age and mantle water content the resulting grain size structure is used to predict shear wave velocity v s and seismic quality factor q for a plate age of 60 myr and an olivine water content of 1000 h 10 6 si we find that grain size reaches a minimum of 15 mm at 150 km depth and then increases to 20 30 mm at a depth of 400 km this grain size structure produces a good fit to the low seismic shear wave velocity zone lvz in oceanic upper mantle observed by surface wave studies assuming that the influence of hydrogen on anelastic behavior is similar to that observed for steady state creep further it predicts a viscosity of 10 19 pa s at 150 km depth and dislocation creep to be the dominant deformation mechanism throughout the oceanic upper mantle consistent with geophysical observations we predict larger grain sizes than proposed in recent studies in which the lvz was explained by a dry mantle and a minimum grain size of 1 mm however we show that for a 1 mm grain size diffusion creep is the dominant deformation mechanism above 100 200 km depth inconsistent with abundant observations of seismic anisotropy from surface wave studies we therefore conclude that a combination of grain size evolution and a hydrated upper mantle is the most likely explanation for both the isotropic and anisotropic seismic structure of the oceanic upper mantle our results also suggest that melt extraction from the mantle will be significantly more efficient than predicted in previous modeling studies that assumed grain sizes of 1 mm

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

quartz veins deformed by diffusion creep accommodated grain boundary sliding during a transient high strain rate event in the southern alps new zealand abstract the crystallographic preferred orientations cpos and microstructures of deformed quartz veins were measured for four samples in the hanging wall of the alpine fault in the southern alps new zealand their deformation and exhumation has occurred since 4 ma the quartz veins have been ductilely sheared to finite shear strains of 5 15 in late cenozoic shear zones at 450 50 c 310 90 mpa and strain rates between 2 10 11 and 2 10 9 s 1 the sheared veins have a polygonal microstructure with few subgrains and an average grain size of 100 m the cpo of the veins is random to very weak within the shear zones we suggest that dislocation creep accommodated initial shear deformation at high stresses and strain rates the deformation must have created a strong cpo and concomitant dynamic recrystallization reduced the grain size significantly dissipation of stresses during initial deformation lead to a stress and strain rate drop required for a switch to diffusion creep accommodated grain boundary sliding gbs continued shearing accommodated by gbs destroyed the cpo post deformational grain growth gave rise to a final polygonal microstructure with a similar grain size in veins and in the wall rocks analysis of existing experimental data suggest that this sequence of events is possible in the time available rates of all processes may have been enhanced by the presence of a water rich fluid within the shear zones these observations of naturally deformed rocks provide a model for the processes that may occur during short lived deformation at transiently high stresses at mid crustal depths or deeper

impact of grain size evolution on necking in calcite layers deforming by combined diffusion and dislocation creep abstract natural pinch and swell structure in centimetre thick calcite layers shows a reduction of grain size from swell towards pinch however the impact of grain size evolution on necking and pinch and swell formation is incompletely understood we perform zero dimensional 0d and 2d thermo mechanical numerical simulations of calcite layer extension to quantify the impact of grain size evolution on necking for bulk extension rates between 10 12 s 1 and 10 14 s 1 and temperatures around 350 c for a combination of diffusion and dislocation creep we calculate grain size evolution according to the paleowattmeter grain size is proportional to mechanical work rate or the paleopiezometer grain size is proportional to stress numerical results fit three observations i significant thickness variations along the layer after extension ii grain size reduction from swells towards pinches and ii dislocation creep dominated deformation in swells and significant contribution of diffusion creep in pinches modelled grain size in pinches 10 60 m and swells 70 800 m is close to observed grain size in pinches 21 6 m and in swells 250 1500 m in the models grain size evolution has a minor impact on necking and viscous shear heating and grain size evolution have a negligible thermal impact

grain size sensitive rheology of orthopyroxene the grain size sensitive rheology of orthopyroxene is investigated using data from rheological and microstructural studies a deformation mechanism map is constructed assuming that orthopyroxene deforms by two independent mechanisms dislocation creep and diffusion creep the field boundary between these mechanisms is defined using two approaches first experimental data from lawlis 1998 which show a deviation from non linear power law behavior at low stresses are used to prescribe the location of the field boundary second a new orthopyroxene grain size piezometer is used as a microstructural constraint to the field boundary at constant temperature both approaches yield sub parallel field boundaries separated in grain size by a factor of only 2 5 extrapolating to lithospheric conditions the deformation mechanism transition occurs at a grain size of 150 500 m consistent with observations from nature as the transition from dislocation to diffusion creep may promote shear localization grain size reduction of orthopyroxene may play a prominent role in plate boundary deformation

the role of second phases for controlling microstructural evolution in polymineralic rocks a review abstract we present a state of the art review of the microstructural evolution in rocks under static and deformational conditions first the general concepts and processes are introduced using monomineralic aggregates then they are expanded into the more complex context of polymineralic rocks with a dominant matrix phase the first part of this contribution delivers information on sample strategies to quantify polymineralic microfabrics based on comparisons between microfabrics of monomineralic and polymineralic rocks we use the common knowledge collected over the past decades for monomineralic systems and discuss the differences to polymineralic ones in terms of microstructures modal compositions spatial distribution of phases crystallographic preferred orientations and associated processes the article puts particular emphasis on the effect of coupled grain growth mass transfer processes and deformation mechanisms we speculate on the effect of mineral reactions during the evolution of microstructures and rheology in polymineralic aggregates at different metamorphic conditions at the end of the article we demonstrate the great potential of grain size evolution maps as microstructural tool to unravel the geological history of polymineralic rocks that evolved under a variety of geodynamic situations

evidence of phase nucleation during olivine diffusion creep a new perspective for mantle strain localisation for the past decades grain size reduction leading to diffusion creep in olivine is believed to be a very important process for strain localisation in the lithospheric mantle however the mechanisms of grain size reduction in this regime are still poorly understood e g platt 2015 here we show new experimental results that document grain size reduction and material weakening during wet olivine diffusion creep while occurring for both mono phase and two phase aggregates grain size reduction is coeval with strain localisation and local phase mixing in olivine pyroxene aggregates based on evidence of fluid inclusions and cracks filled with a fine grained phase mixture we conclude that grain size reduces as a result of fluid assisted nucleation that takes place in the presence of an aqueous fluid during diffusion creep cavitation induced by grain boundary sliding creep cavitation can be inferred and may play a critical role for olivine grain size reduction amongst their implications for rock rheology in general our findings highlight a key process for strain localisation in the ductile uppermost mantle

stress dependence of microstructures in experimentally deformed calcite abstract optical measurements of microstructural features in experimentally deformed carrara marble help define their dependence on stress these features include dynamically recrystallized grain size dr subgrain size sg minimum bulge size l and the maximum scale length for surface energy driven grain boundary migration l taken together with previously published data dr defines a paleopiezometer over the range 15 291 mpa and temperature over the range 500 1000 c with a stress exponent of 1 09 ci 1 27 to 0 95 showing no detectable dependence on temperature sg and dr measured in the same samples are closely similar in size suggesting that the new grains did not grow significantly after nucleation l and l measured on each sample define a relationship to stress with an exponent of approximately 1 6 which helps define the boundary between a region of dominant strain energy driven grain boundary migration at high stress from a region of dominant surface energy driven grain boundary migration at low stress

grain growth kinetics of quartz and calcite aggregates grain growth rates for novaculite flint and jasper have been determined between 1000 and 800 c and 15 to 2 kb water pressure a pre treatment established an equilibrium microstructure and a nearly uniform mean grain diameter bar d 0 at 1000 c and 15 kb the grain growth data are linear on a log bar d bar d 0 vs log time plot and there is no significant difference between the three samples although the jasper contained 5 10 vol iron oxide at lower pressures and temperatures the growth curves are approximately parallel to the 1000 c and 15 kb data but growth is slower and that for jasper is slower than for the other two the growth rate is strongly dependent on the solubility of quartz in the intergranular fluid and the activation energy is about 11 kcal mole the data are used to estimate the growth rate of quartz at metamorphic conditions and to help evaluate the conditions where growth of dynamically recrystallized grains may be significant the growth rate of calcite grains s

a new perspective on paleopiezometry dynamically recrystallized grain size distributions indicate mechanism changes the dynamically recrystallized grain size is a material parameter associated with dislocation creep of crystalline solids that is especially important as a flow stress indicator via piezometer calibrations grain sizes have been measured in many studies of deformed rocks as well as metals and ceramics but global analyses of the frequency distribution of dynamically recrystallized grain sizes are lacking here we present the first systematic investigation of the recrystallized grain size distribution for quartz the grain diameters compiled from 555 samples of 31 studies of quartz mylonites deformed over a wide range of conditions extend from 3 m to 3 mm with distinct maxima at 10 20 m and 70 80 m and minima at 35 40 m and 120 m the frequency maxima correlate with distinct microstructures and the minima with the transitions between these microstructures which we interpret to result from the dominance of the recrystallization mechanisms of bulging subgrain rotation and grain boundary migration recrystallization these results demonstrate the necessity of distinct piezometer calibrations for different recrystallization mechanisms and highlight the importance of the recrystallized grain size for theoretical models of dynamic recrystallization

boudinage as a material instability of elasto visco plastic rocks abstract pinch and swell structures are commonly interpreted to evolve out of viscosity contrasts which are induced by geometric interactions and material imperfections from materials science an additional localization phenomenon is well established where localization emerges out of steady state for critical material parameters and or loading rates here we investigate the conditions under which this second type of instabilities prevails and whether geological materials necessarily require a trigger by imperfections in order to generate instability we focus on imperfections in terms of grain size variations embedded in a deformation environment controlled by thermo mechanical feedbacks we introduce a random distribution of grain sizes over two orders of magnitude in a central layer embedded in a matrix with a diffusion creep rheology the rheology of the layer evolves with dislocation and diffusion creep as end member deformation mechanisms applying pure shear extension the 3 layer model is subjected to natural deformation conditions the central layer quickly establishes a viscous steady state as a natural response of the system due to relaxation and energy optimization upon continued loading localization then intriguingly arises out of a homogeneous state we present an analysis which confirms that this type of instability is indeed physically admissible using vibration analysis we verify the robustness of the numerical solution by first identifying the natural mode shapes and frequencies of the simulated structure and material parameters including geometric imperfections in a second step the eigenmodes are perturbed and superposed to the initial conditions we conclude that this pattern of perturbations guides the onset of strain localization boudinage can therefore be seen both as a geometric problem and or a material bifurcation which evolves out of homogeneous state the latter class offers the great possibility of extracting fundamental material parameters out of localized structures directly from field observations

quantitative analysis of crystal grain sizes and their distributions in 2d and 3d abstract we review methods to estimate the average crystal grain size and the crystal grain size distribution in solid rocks average grain sizes often provide the base for stress estimates or rheological calculations requiring the quantification of grain sizes in a rock s microstructure the primary data for grain size data are either 1d i e line intercept methods 2d area analysis or 3d e g computed tomography serial sectioning these data have been used for different data treatments over the years whereas several studies assume a certain probability function e g logarithm square root to calculate statistical parameters as the mean median mode or the skewness of a crystal size distribution the finally calculated average grain sizes have to be compatible between the different grain size estimation approaches in order to be properly applied for example in paleo piezometers or grain size sensitive flow laws such compatibility is tested for different data treatments using one and two dimensional measurements we propose an empirical conversion matrix for different datasets these conversion factors provide the option to make different datasets compatible with each other although the primary calculations were obtained in different ways in order to present an average grain size we propose to use the area weighted and volume weighted mean in the case of unimodal grain size distributions respectively for 2d and 3d measurements the shape of the crystal size distribution is important for studies of nucleation and growth of minerals the shape of the crystal size distribution of garnet populations is compared between different 2d and 3d measurements which are serial sectioning and computed tomography the comparison of different direct measured 3d data stereological data and direct presented 2d data show the problems of the quality of the smallest grain sizes and the overestimation of small grain sizes in stereological tools depending on the type of csd

descending layer variability over arecibo descending layers of ionization over arecibo exhibit very diverse behavior from night to night that does not appear to be strongly correlated to geomagnetic activity solar forcing or average semidiurnal tidal winds on some nights three or more distinct layers are observed to form near 170 km over timescales of approx 2 hours rather than descending smoothly over periods of several hours these layers stall abruptly disappear or even reverse direction in the midst of their descent the time scales for their disappearance are examined and compared to loss rates arising from diffusion and recombination diffusion alone is found to be too slow to account for the observations but recombination is fast enough provided that the convergent wind shear that forms the layer is relatively weak coincident with their disappearance the continuity equation is solved in conjunction with a time sequence of radar profiles to estimate the vertical drift and horizontal neutral wind consistent with the observed behavior the resultant wind field is northward has an average speed of approx 80 m s and varies significantly near the altitude where the layers are observed these inferred winds are consistent with the presence of the observed layers and their magnitudes as obtained from the classical continuity and momentum equations are reasonable for this altitude range

the recrystallized grain size piezometer for quartz d 1 0 3 56 0 27 s 1 26 0 13 with no change in slope at the regime 2 3 transition and no effect of temperature or a b stability field another experimental piezometer relation for regime 1 of hirth and tullis 1992 differs in slope suggesting that different recrystallization mechanisms require different piezometer calibrations index terms 3902 mineral physics creep and deformation 5120 physical properties of rocks plasticity diffusion and creep 8030 structural geology microstructures 8159 tectonophysics rheology crust and lithosphere 8164 tectonophysics stresses crust and lithosphere citation stipp m and j tullis the recrystallized grain size piezometer for quartz geophys res lett 30 21 2088 doi 10 1029 2003gl018444 2003

the effect of dynamic recrystallization on olivine crystal preferred orientations in mantle xenoliths deformed under varied stress conditions abstract spinel peridotite xenoliths from persani mountains southeastern carpathians romania were submitted to deformation by dislocation creep accompanied by dynamic recrystallization under variable stress and temperature conditions predominance of low angle boundaries parallel to 100 with well defined 0vw rotation axes in olivine indicates that subgrain rotation is the main recrystallization mechanism and that dislocation glide occurs mainly in 100 0kl systems analysis of olivine crystal preferred orientations highlights that recrystallization results in dispersion of the orientations of recrystallized grains relatively to the parent grains this dispersion may be quantified by a dimensionless dispersion factor defined as the ratio of the random component of the 100 axis distributions between the recrystallized grains and porphyroclasts this factor is largely independent from the overall fabric strength the number of grains analyzed and the recrystallized grain size the olivine cpo strength and anisotropy at the rock scale quantified by j index are largely controlled by the volume of recrystallized grains which depends on finite strain comparison of olivine cpo in coarse and fine grained porphyroclastic peridotites shows that deviatoric stress and temperature conditions play nevertheless a role on the cpo evolution during recrystallization selective grain growth which is favored at high temperature and low stresses does counteract the dispersion produced by the nucleation processes

using microstructures and titaniq thermobarometry of quartz sheared around garnet porphyroclasts to evaluate microstructural evolution and constrain an alpine fault zone geotherm abstract interpretations of deformation processes within ductile shear zones are often based on the characterisation of microstructures preserved in exhumed rocks however exhumed microstructures provide only a snapshot of the closing stages of deformation and we need ways of understanding how microstructures change through time and at what rate this occurs to address this problem we study optical microstructures and electron backscatter diffraction ebsd data from samples of quartz layers deflected around garnet porphyroclasts which generate local stress and strain rate perturbations during mylonitic deformation in the alpine fault zone of new zealand during shearing around rigid garnet porphyroclasts quartz undergoes grain size reduction in response to locally increased stresses while c axes reveal increasing components of rhomb and prism slip reflecting a local increase in shear strain and strain rate titaniq thermobarometry and quartz microstructures suggest a rather narrow range of recorded quartz deformation temperatures around 450 500 c which we propose reflects the cessation of grain boundary migration driven deformation given that temperatures well above the brittle ductile transition for quartz 350 c are preserved we anticipate that rapid cooling and exhumation must have occurred from the 500 c isotherm ultimately we propose a modified geotherm for the central alpine fault zone hanging wall which raises the 500 c isotherm to 11 km depth near the brittle ductile transition our updated alpine fault zone geotherm implies a hotter and weaker middle to lower crust than previously proposed

microstructures and rheology of a calcite shale thrust fault abstract a thin 2 cm layer of extensively sheared fault rock decorates the 15 km displacement copper creek thrust at an exposure near knoxville tn usa in these ultrafine grained 5 m that exhibit a lattice preferred orientation lpo with pores at twin twin and twin grain boundary intersections and 2 ultrafine grained 0 3 m calcite that exhibits interpenetrating grain boundaries four grain junctions and lacks a lpo coarse calcite layers crosscut ultrafine grained layers indicating intermittent vein formation during shearing calcite in the fault rock layer is derived from vein calcite and grain size reduction of calcite took place by plasticity induced fracture the ultrafine grained calcite deformed primarily by diffusion accommodated grain boundary sliding and formed an interconnected network around shale clasts within the shear zone the interconnected network of ultrafine grained calcite indicates that calcite not shale was the weak phase in this fault zone

stress in recrystallized quartz by electron backscatter diffraction mapping the long term state of stress at middle and lower crustal depths can be estimated through the study of the microstructure of exhumed rocks from active and or ancient shear zones constitutive equations for deformation mechanisms in experimentally deformed rocks relate differential stress to the size of recrystallized grains cross et al 2017 take advantage of electron backscatter diffraction mapping to systematically separate new recrystallized grains from host grains on the basis of the measurable lattice distorsion within the grains they produce the first calibrated piezometer for quartz with this technique reproducing within error a previous calibration based on optical microscopy

plate tectonics damage and inheritance lithospheric damage combined with transient mantle flow and migrating proto subduction is proposed to explain the apparent emergence of plate tectonics three billion years ago modelling confirms that tectonic plate boundaries and fully formed tectonic plates can arise under conditions characteristic of earth but not of venus plate tectonics the division of the earth s surface into rigid plates separated by linear zones of concentrated deformation is unique among known terrestrial bodies as yet there is no consensus view on how the phenomenon arose here david bercovici and yanick ricard outline an explanation that starts with the microscopic properties of deforming minerals when sufficient lithospheric damage accumulates the theory states shear localization occurs and long lived weak zones develop transient mantle flow and migrating proto subduction then lead to the accumulation of plate boundaries and eventually tectonic plates the authors model this process with a grain evolution and damage mechanism coupled to an idealized model of pressure driven lithospheric flow and show that tectonic plate boundaries and fully formed tectonic plates can form under such conditions the initiation of plate tectonics on earth is a critical event in our planet s history the time lag between the first proto subduction about 4 billion years ago and global tectonics approximately 3 billion years ago suggests that plates and plate boundaries became widespread over a period of 1 billion years the reason for this time lag is unknown but fundamental to understanding the origin of plate tectonics here we suggest that when sufficient lithospheric damage which promotes shear localization and long lived weak zones combines with transient mantle flow and migrating proto subduction it leads to the accumulation of weak plate boundaries and eventually to fully formed tectonic plates driven by subduction alone we simulate this process using a grain evolution and damage mechanism with a composite rheology which is compatible with field and laboratory observations of polycrystalline rocks1 2 coupled to an idealized model of pressure driven lithospheric flow in which a low pressure zone is equivalent to the suction of convective downwellings in the simplest case for earth like conditions a few successive rotations of the driving pressure field yield relic damaged weak zones that are inherited by the lithospheric flow to form a nearly perfect plate with passive spreading and strike slip margins that persist and localize further even though flow is driven only by subduction but for hotter surface conditions such as those on venus accumulation and inheritance of damage is negligible hence only subduction zones survive and plate tectonics does not spread which corresponds to observations after plates have developed continued changes in driving forces combined with inherited damage and weak zones promote increased tectonic complexity such as oblique subduction strike slip boundaries that are subparallel to plate motion and spalling of minor plates

grain size distribution in the mantle wedge of subduction zones 1 mineral grain size plays an important role in controlling many processes in the mantle wedge of subduction zones including mantle flow and fluid migration to investigate the grain size distribution in the mantle wedge we coupled a two dimensional 2 d steady state finite element thermal and mantle flow model with a laboratory derived grain size evolution model in our coupled model the mantle wedge has a composite olivine rheology that incorporates grain size dependent diffusion creep and grain size independent dislocation creep our results show that all subduction settings lead to a characteristic grain size distribution in which grain size increases from 10 to 100 m at the most trenchward part of the creeping region to a few centimeters in the subarc mantle despite the large variation in grain size its effect on the mantle rheology and flow is very small as 90 of the deformation in the flowing part of the creeping region is accommodated by grain size independent dislocation creep the predicted grain size distribution leads to a downdip increase in permeability by 5 orders of magnitude this increase is likely to promote greater upward migration of aqueous fluids and melts where the slab reaches 100 km depth compared with shallower depths potentially providing an explanation for the relatively uniform subarc slab depth seismic attenuation derived from the predicted grain size distribution and thermal field is consistent with the observed seismic structure in the mantle wedge at many subduction zones without requiring a significant contribution by the presence of melt

formation of lithospheric shear zones effect of temperature on two phase grain damage abstract shear localization in the lithosphere is a characteristic feature of plate tectonic boundaries and is evident in the presence of small grain mylonites localization and mylonitization in the ductile portion of the lithosphere can arise when its polymineralic material deforms by a grain size sensitive rheology in combination with zener pinning which can impede or possibly even reverse grain growth and thus promotes a self softening feedback mechanism however the efficacy of this mechanism is not ubiquitous and depends on lithospheric conditions such as temperature and stress therefore we explore the conditions under which self weakening takes place and in particular the effect of temperature and deformation state stress or strain rate on these conditions in our model the lithosphere like polymineralic material is deformed in a two dimensional simple shear driven by constant stress or strain rate the mineral grains evolve to a stable size which is obtained when the rate of coarsening by normal grain growth and the rate of grain size reduction by damage are in balance damage involves processes by which some of the deformational energy gets transferred into surface energy this can happen by i dynamic recrystallization grain damage and ii stretching deforming and stirring the material interface interface damage the influence of temperature enters through rheological laws which govern the rate of work and damage grain growth kinetics and the damage partitioning fraction which is the fraction of deformational work that goes into creating new surface energy we demonstrate that a two phase damage model in which the partitioning fraction depends on both temperature and roughness of the interface between the phases can successfully match the field data including the reported correlation of grain size and temperature the increasing dominance of dislocation creep at higher temperatures and a large range of grain sizes observed across the depth of a single shear zone

dislocation creep regimes in quartz aggregates abstract using optical and tem microscopy we have determined that three regimes of dislocation creep occur in experimentally deformed quartz aggregates depending on the relative rates of grain boundary migration dislocation climb and dislocation production within each regime a distinctive microstructure is produced due primarily to the operation of different mechanisms of dynamic recrystallization at lower temperatures and faster strain rates the rate of dislocation production is too great for diffusion controlled dislocation climb to be an effective recovery mechanism in this regime recovery is accommodated by strain induced grain boundary migration recrystallization with an increase in temperature or decrease in strain rate the rate of dislocation climb becomes sufficiently rapid to accommodate recovery in this regime dynamic recrystallization occurs by progressive subgrain rotation with a further increase in temperature or decrease in strain rate dislocation climb remains sufficiently rapid to accommodate recovery however in this regime grain boundary migration is rapid thus recrystallization occurs by both grain boundary migration and progressive subgrain rotation the identification of the three regimes of dislocation creep may have important implications for the determination of flow law parameters and the calibration of recrystallized grain size piezometers in addition the identification of a particular dislocation creep regime could be useful in helping to constrain the conditions at which a given natural deformation has occurred

strain localization in ductile rocks a comparison of natural and simulated pinch and swell structures we study pinch and swell structures in order to uncover the onset of strain localization and the change of deformation mechanisms in layered ductile rocks to this end boudinaged monomineralic veins embedded in an ultramylonitic matrix are analyzed quantitatively the swells are built up by relatively undeformed original calcite grains showing twinning and minor subgrain rotation recrystallization sgr combined with progressive formation of high angle misorientations between grains indicative of sgr severe grain size reduction defines the transition to the pinches accordingly dynamically recrystallized grains have a strong crystallographic preferred orientation cpo toward the necks further grain size reduction increasingly random misorientations nucleation of new grains and a loss of the cpo occur we postulate that this microstructure marks the transition from dislocation to diffusion creep induced by strain localization we confirm that the development of boudins is insensitive to original grain sizes and single crystal orientations in order to test these microstructural interpretations a self consistent numerical grain size evolution is implemented based on thermo mechanical principles end member flow laws and microphysical processes applying constant velocity and isothermal boundary conditions to a 3 layer finite element pure shear box pinch and swell structures emerge out of the homogeneous layer through grain size softening at a critical state viscosity weakening due to elevated strain rates and dissipated heat from grain size reduction promotes strain rate weakening until a critical grain size is reached at this point a switch from dislocation to diffusion creep occurs this state locks in at local steady states and is microstructurally expressed in pinches and swells respectively thus boudinage is identified as an energy attractor identifying the high energy steady state of an extending layered structure we conclude from the similarity between natural observations and numerical results that critical deformation conditions of ductile creep can be derived for the surrounding highly strained host rock matrix

the influence of stress history on the grain size and microstructure of experimentally deformed quartzite deformation of middle crustal shear zones likely varies with time as a result of the stress build up and release associated with earthquakes and post seismic deformation but the processes involved and their microstructural signature in the rock record are poorly understood we conducted a series of experiments on quartzite at 900 c to characterize microstructures associated with changes in stress and strain rate and to investigate the feasibility of carrying out grain size piezometry in natural rocks that experienced analogous changes differential stress referred to simply as stress was varied in two stage experiments by changing strain rate and by stopping the motor and allowing stress to relax the two stage samples preserve a microstructural record that can be interpreted quantitatively in terms of stress history the microstructure associated with a stress increase is a bimodal distribution of recrystallized grain sizes the smaller grains associated with the second deformation stage accurately record the stress of the second stage and the surviving coarse grains remain similar in size to those formed during the earlier stage the transient microstructure associated with stress decrease is a partial foam texture containing a larger concentration of stable 120 triple junctions than occur in samples deformed at a relatively constant strain rate our results indicate that microstructures preserved in rocks that experienced relatively simple two stage deformation histories can be used to quantitatively assess stress histories grain growth rates during deformation are similar to rates observed in previous isostatic growth experiments supporting theoretical approaches to recrystallized grain size such as the wattmeter theory austin and evans 2007 that incorporate static growth rates from an analysis of the experimental data for quartz recrystallized grain size we find 1 recrystallized grain size quickly reaches a value consistent with ambient deformation conditions we argue that this explains a good match between average grain sizes predicted by the wattmeter after complete recrystallization and the recrystallized grain sizes of the experiments 2 the present formulation of the wattmeter overestimates the rates at which porphyroclasts recrystallize by as much as an order of magnitude and 3 owing to problems with extrapolation of grain growth data for quartz the wattmeter is not presently applicable to natural samples deformed at low temperatures we present a simplified flow law for quartz and suggest that the change in slope of the quartz piezometer at high stress regime 1 is related to a switch to a linear viscous rheology