seismic anisotropy tracing plate dynamics in the mantle elastic anisotropy is present where the speed of a seismic wave depends on its direction in earth9s mantle elastic anisotropy is induced by minerals that are preferentially oriented in a directional flow or deformation earthquakes generate two seismic wave types compressional p and shear s waves whose coupling in anisotropic rocks leads to scattering birefringence and waves with hybrid polarizations this varied behavior is helping geophysicists explore rock textures within earth9s mantle and crust map present day upper mantle convection and study the formation of lithospheric plates and the accretion of continents in earth history

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

relative strength of mafic and felsic rocks during amphibolite facies metamorphism and deformation field observations of mafic dykes intruded into felsic rocks from the lewisian complex nw scotland suggest that the mafic rocks are weaker than the felsic ones contrary to experimental results in order to resolve this conflict samples were studied to constrain the deformation mechanisms active under amphibolite facies conditions crystallographic preferred orientation cpo in plagioclase is used to infer deformation by dislocation creep on the 112 11 0 11 2 110 001 11 0 and 001 11 0 slip systems with increasing strain this cpo became weaker due to grain boundary sliding that accompanied diffusion creep in rocks where grain size reduction of plagioclase occurred by chemically dominated recrystallisation there is no cpo suggesting deformation was wholly accommodated by diffusion creep and grain boundary sliding in metamorphosed dykes plagioclase grains have orientations that are not consistent with dislocation creep deformation amphibole has a cpo consistent with either dislocation creep on 100 or deformation by diffusion creep with anisotropic dissolution and precipitation rates it is inferred that metamorphism of the dykes lead to the production of fine grained amphibole and plagioclase both of which deform by diffusion creep but that anisotropic dissolution and precipitation in the amphibole produced a cpo the mafic dykes are weaker than the felsic gneisses because grain size reduction is more extreme in the dykes even though both may be deforming primarily by grain size sensitive mechanisms this work highlights the variation in processes active in polyphase rocks and how these can lead to variations in cpo of the same mineral among different rock types

radial seismic anisotropy as a constraint for upper mantle rheology abstract seismic shear waves that are polarized horizontally sh generally travel faster in the upper mantle than those that are polarized vertically sv and deformation of rocks under dislocation creep has been invoked to explain such radial anisotropy convective flow of the upper mantle may thus be constrained by modeling the textures that progressively form by lattice preferred orientation lpo of intrinsically anisotropic grains while azimuthal anisotropy has been studied in detail the radial kind has previously only been considered in semi quantitative models here we show that radial anisotropy averages as well as radial and azimuthal anomaly patterns can be explained to a large extent by mantle flow if lateral viscosity variations are taken into account we construct a geodynamic reference model which includes lpo formation based on mineral physics and flow computed using laboratory derived olivine rheology previously identified anomalous v sv regions beneath the east pacific rise and relatively fast v sh regions within the pacific basin at 150 km depth can be linked to mantle upwellings and shearing in the asthenosphere respectively continental anisotropy at shallow 50 km depth is under predicted and these deviations are in quantitative agreement with the expected signature of frozen in stochastically oriented anisotropy from past tectonic episodes we also consider two end member models of lpo formation for wet and dry conditions for the asthenosphere 150 km allowing for lateral variations in volatile content the residual signal can be much reduced and the inferred volatile patterns underneath the pacific appear related to plume activity in deeper layers 250 km anisotropy indicates that small scale convection disrupts plate scale shear underneath old oceanic lithosphere we suggest that studying deviations from comprehensive geodynamic reference models or residual anisotropy can provide new insights into the nature and dynamics of the asthenosphere

albite crystallographic preferred orientation and grain misorientation distribution in a low grade mylonite implications for granular flow abstract crystallographic orientation data from pure albite domains in a low grade sheared metagabbro from the combin zone of the western italian alps were measured by electron backscatter diffraction crystallographic preferred orientations cpos in four high strain domains are non random and have a triclinic symmetry the clusters of 100 010 and 001 show an angular relationship that corresponds to that of the albite crystal lattice however the orientations of axis clusters to the kinematic axes vary from domain to domain cpos from a low strain domain also show clustered axes with triclinic symmetry but with more intense clustering than those in the high strain domains grain misorientation distributions are presented both for the low strain domain and one of the high strain domains in the high strain domain the distribution of misorientation angles between neighbouring grains displays a peak at about 70 the equivalent distribution in the low strain domain has a peak at 30 for both domains the misorientation axis distributions between neighbouring and non neighbouring grains are random except for some of axes with 160 180 misorientation that exhibit a slight concentration around 010 the diversity of cpos among the domains suggests that these cpos could not be produced by dislocation creep they are likely to have been inherited from plagioclase parents as a result of host control on the nucleation of the new albite grains these cpos do not contain any direct information about the deformation kinematics we interpret that deformation of these domains occurred by granular flow crystallographic axis dispersion due to grain boundary sliding gbs caused weakening of cpos modification of misorientation angle distributions and randomisation of misorientation axis distributions the fact that a cpo can survive gbs even after a high strain indicates that cpo is not always a sensitive indicator of deformation mechanisms misorientation distribution may provide a complementary and possibly a more sensitive indicator of deformation mechanisms

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

constraints on plate tectonics initiation from scaling laws for single cell convection abstract the earth is the only planet known to have plate tectonics while other planets are covered with a stagnant lid on the earth the initiation of subduction which is thought to be the fundamental process for plate tectonics initiation is caused not only by the negative buoyancy of the lithosphere but also by the forces from plate motions however for planets which do not have plate tectonics the very first episode of lithospheric failure has to be caused by forces other than plate motions sublithospheric convection has been proposed as a possible mechanism that provides lithospheric instability through inducing stresses in the lithosphere and lithospheric failure can occur when the yield stress is below a critical value we test the applicability of scaling laws for the critical yield stress obtained in single cell convection simulations to strongly time dependent multi cell systems we show that with an appropriate choice of characteristic aspect ratio for the convective system the scaling laws from single cell simulations can be used to evaluate the conditions on the terrestrial planets in the inner solar system for plate tectonics to exist in agreement with previous studies the estimated values for critical yield stress and coefficient of friction are much lower than the expected values for the earth s lithosphere

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

the preservation potential of microstructures during static grain growth grain growth simulations using the microstructure simulation system elle have been performed in materials with a pre existing grain shape foliation as might be expected the foliation is destroyed by the end of the experiment and grain areas have increased by a factor of seven the area of material swept by the migrating grain boundaries was monitored and it was found that at every stage virtually all of the grains which survived the grain growth process contain one and only one core of unswept material remarkably these remnant unswept cores preserve a useable record of the initial grain size and the orientation of the grain shape foliation this work suggests that even for samples where no equivalent protolith can be found it may be possible to see past a grain growth episode to estimate the original grain shape and grain size of the rock and perhaps even reconstruct the grain boundary kinematics in addition the identification of unswept cores has the potential to help unravel the evolution of grain boundary chemistry in rocks during metamorphism as an example of a natural system showing these microstructures we describe a peridotite from almklovdalen norway this peridotite was infiltrated by aqueous fluids at several stages during late caledonian exhumation and retrogressive metamorphism grain boundary migration associated with the last of these infiltration events swept off abundant intragranular fluid inclusions in the original chlorite peridotite at the grain scale microstructural mapping of the fluid inclusion rich areas shows that as with the numerical simulations many of the grains retain exactly one core of unswept material examples of other natural systems discussed include dislocation density distributions and trace element zoning

rheology of the upper mantle a synthesis rheological properties of the upper mantle of the earth play an important role in the dynamics of the lithosphere and asthenosphere however such fundamental issues as the dominant mechanisms of flow have not been well resolved a synthesis of laboratory studies and geophysical and geological observations shows that transitions between diffusion and dislocation creep likely occur in the earth s upper mantle the hot and shallow upper mantle flows by dislocation creep whereas cold and shallow or deep upper mantle may flow by diffusion creep when the stress increases grain size is reduced and the upper mantle near the transition between these two regimes is weakened consequently deformation is localized and the upper mantle is decoupled mechanically near these depths

texture of solnhofen limestone deformed to high strains in torsion solnhofen limestone was deformed in torsion to shear strains ranging from 1 to 12 at a temperature of 750 c 300 mpa confining pressure and a maximum strain rate of 10 3 s 1 these deformation conditions correspond to the intracrystalline power law dislocation creep field close to the boundary to the grain size sensitive superplastic creep field the grain shape microstructure was observed using orientation contrast by backscattered electrons in the sem the grains remain sub equant with an average grain size of around 4 m even to the highest strains lattice preferred orientation was determined using both x ray texture goniometry and automated electron back scatter diffraction the c axis preferred orientation develops from two main maxima with a weak sub maximum through two maxima to a single maximum perpendicular to the shear plane the rate of increase of the intensity of the single maximum with increasing strain diminishes and it appears that there is a tendency to a steady state texture the final single c axis maximum is displaced slightly towards the shortening direction of the applied simple shear the a axes tend to a girdle perpendicular to the c axis maximum it is proposed that the partitioning of deformation between intra and inter crystalline mechanisms results in a pulsating strain state in the grains contributing to the maintenance of sub equant grains it is argued that the lattices of constituent grains rotate continuously with no stable end orientation and that this can lead to a steady state texture the experimental preferred orientation compares well with that of natural calcite mylonites in the position of the c axis maximum and the a axis girdle

obliteration of olivine crystallographic preferred orientation patterns in subduction related antigorite bearing mantle peridotite an example from the higashi akaishi body sw japan

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

microstructural evolution and rheology of quartz in a mid crustal shear zone abstract we present microstructural and crystallographic preferred orientation cpo data on quartz deformed in the middle crust to explore the interaction and feedback between dynamic recrystallization deformation processes and cpo evolution the sample investigated here is a moderately deformed quartz rich mylonite from the blue ridge in virginia we have created high resolution crystallographic orientation maps using electron backscatter diffraction ebsd of 51 isolated quartz porphyroclasts with recrystallized grain fractions ranging from 10 to 100 recrystallized grains are internally undeformed and display crystallographic orientations dispersed around the orientation of the associated parent porphyroclast we document a systematic decrease in fabric intensity with recrystallization suggesting that progressive deformation of the recrystallized domains involves processes that can weaken a pre existing cpo relationships between recrystallization fraction and shear strain suggest that complete microstructural re equilibration requires strains in excess of 5 variation in the degree of recrystallization implies that strain was accumulated heterogeneously and that a steady state microstructure and rheology were not achieved

d rex a program for calculation of seismic anisotropy due to crystal lattice preferred orientation in the convective upper mantle summary models of development of lattice preferred orientation lpo of crystals aggregates in convec tive flow are necessary to interpret the anisotropic seismic signature of the earth s upper mantle for that purpose we previously developed a model of lpo evolution in olivine aggregates by plastic deformation and dynamic recrystallization by subgrain rotation and grain boundary migration this paper presents a refined version of that model called d rex for dynamic recrystallization induced lpo a public version of which is made available on our web site the code displays two new features 1 enstatite is incorporated in the aggregates and 2 grain boundary sliding gbs of small grains is taken into account enstatite is incorporated on the assumption of no direct interaction with olivine the fast a axis of enstatite grains tend to be parallel to the slow c axis of olivine which dilutes the total anisotropy grain boundary sliding is included using a threshold dimensionless volume fraction defined as the ratio of the initial size of the grains over the size for which gbs is the dominant mechanism of deformation grains with a dimensionless volume smaller than do not rotate by plastic deformation and their strain energy is set to zero comparison with torsion experiments at very large strain constrains the threshold dimensionless volume to 0 3 0 1 the incorporation of grain boundary sliding prevents the lpo from becoming singular at large strains and yields more realistic predictions our kinematic formalism and the model s semi analytical character insures that it is fast robust and stable it can be applied efficiently to arbitrary 3 d convective flows

the determination of earthquake hazard parameters deduced from bayesian approach for different seismic source regions of western anatolia the bayesian method is used to evaluate earthquake hazard parameters of maximum regional magnitude mmax value and seismic activity rate or intensity and their uncertainties for the 15 different source regions in western anatolia a compiled earthquake catalog that is homogenous for ms 4 was completed during the period from 1900 to 2013 the computed mmax values are between 6 00 and 8 06 low values are found in the northern part of western anatolia whereas high values are observed in the southern part of western anatolia related to the aegean subduction zone the largest value is computed in region 10 comprising the aegean islands the quantiles of functions of distributions of true and apparent magnitude on a given time interval 0 t are evaluated the quantiles of functions of distributions of apparent and true magnitudes for future time intervals of 5 10 20 50 and 100 years are calculated in all seismogenic source regions for confidence limits of probability levels of 50 70 and 90 according to the computed earthquake hazard parameters the requirement leads to the earthquake estimation of the parameters referred to as the most seismically active regions of western anatolia the aegean islands which have the highest earthquake magnitude 7 65 in the next 100 years with a 90 probability level is the most dangerous region compared to other regions the results found in this study can be used in probabilistic seismic hazard studies of western anatolia

diffusion creep modelling of fibrous pressure shadows abstract grain scale deformation of a bimineralic composite by normal stress driven grain boundary diffusive mass transfer has been modelled using diffusion creep software that for the first time allows different diffusion coefficients for interphase boundaries and grain boundaries as well as for different components by selecting suitable coefficients we have obtained a fully coupled mechanical and mass transfer numerical solution for the formation of fibrous pressure shadows or strain fringes at the interface between a rigid object and a deforming rock matrix of different compositions the model is two dimensional and run in pure shear velocity mode to a stretch ratio of 1 4 the first results are reported which show dissolution and face controlled antitaxial growth of a soluble polycrystalline calcite matrix at the interface with a circular and relatively insoluble pyrite object the ratio of caco3 diffusion along calcite pyrite versus calcite calcite boundaries strongly influences the extent of pressure shadow growth and the pattern of strains in the matrix a low ratio produces little growth significant perturbation and deflection of total strain axes around the pyrite an intermediate ratio produces appreciable growth little perturbation and near parallel total strain axes a high ratio produces larger pressure shadows and total strain axes that bend in towards the pyrite as if it were a weak object sliding and rotation of matrix grains are also affected these results differ fundamentally from purely physical models of ductile flow around a rigid object and demonstrate the influence of interface diffusional properties on the resulting behaviour in particular assumptions such as perturbation of the flow by the object deflection of external foliation around pre deformational porphyroblasts and relationships of pressure shadow strain to bulk strain may need to be re examined in the light of this finding

interaction of chemical and physical processes during deformation at fluid present conditions a case study from an anorthosite leucogabbro deformed at amphibolite facies conditions we present microstructural and chemical analyses of chemically zoned and recrystallized plagioclase grains in variably strained samples of a naturally deformed anorthosite leucogabbro southern west greenland the recorded microstructures formed in the presence of fluids at mid crustal conditions 620 640 c 7 4 8 6 kbar recrystallized plagioclase grains average grain size 342 m with a random crystallographic orientation are volumetrically dominant in high strain areas they are characterized by asymmetric chemical zoning an80 cores and an64 rims that are directly associated with areas exhibiting high amphibole content and phase mixing analyses of zoning indicate anisotropic behaviour of bytownite plagioclase with a preferred replacement in the left langle 0 10 right rangle direction and along the 001 plane in areas of high finite strain recrystallization of plagioclase dominantly occurred by bulging recrystallization and is intimately linked to the chemical zoning the lack of cpo as well as the developed asymmetric zoning can be explained by the activity of grain boundary sliding accommodated by dissolution and precipitation creep dpc in low strain domains grain size is on average larger and the rim distribution is not related to the inferred stress axes indicating chemically induced grain replacement instead of stress related dpc we suggest that during deformation in high strain areas pre existing phase mixture and stress induced dpc caused grain rotations that allowed a deformation enhanced heterogeneous fluid influx this resulted in local plagioclase replacement through interface coupled dissolution and precipitation and chemically induced grain boundary migration accompanied by bulging recrystallization along with neocrystallization of other phases this study illustrates a strong interaction and feedback between physical and chemical processes where the amount of stress and fluids dictates the dominant active process the interaction is a cause of deformation and external fluid infiltration with a result of strain localization and chemical re equilibration at amphibolite facies conditions

seismic anisotropy in the upper mantle 2 predictions for current plate boundary flow models 1 the anisotropic seismic structure due to flow induced mineral alignment is investigated for a series of models designed to simulate deformation in the upper mantle within a few hundred kilometers of a plate boundary the orientation distributions of olivine enstatite aggregates evolve along streamlines of each flow model based on each grains plastic response to the local stress strain field the effective elastic tensor for these textured aggregates provides predictions of p wave anisotropy and shear wave splitting throughout the model space p and s travel time delay patterns and fast shear wave polarization angles are found to vary significantly with incidence angle for a given model comparison of predicted fast p direction for our method versus a finite strain based estimate shows that agreement is acceptable for much of the model space but notable differences occur in regions up to several tens of kilometers in size two dimensional models of spreading center flow are presented for slow and fast rates and for several cases in which the ridge migrates over the deeper mantle the effect of flow in the third dimension is addressed in a few calculations for one comparison of flow in the mantle wedge at a subduction zone the introduction of trench parallel flow causes significant changes in the predicted patterns of p wave anisotropy magnitude more than orientation and sks splitting

shear waves from isotropically dominated sources comparison of the 2013 rudna poland and 2007 crandall canyon utah mine collapses comparison of observed and synthetic radiation patterns suggests that love waves generated by the 2007 crandall canyon mine collapse m w 4 1 are the result of shear waves generated by a relatively large compensated linear vector dipole component at the source whereas the love waves associated with a 2013 rudna mine collapse m w 4 2 are minimal except where amplified by a deep sedimentary basin we examine the source of the 19 march 2013 rudna mine poland collapse with a full moment tensor inversion method using the complete waveforms of regional distance seismic data the computed source is primarily composed of a dominant and statistically significant implosive component the rudna source is more similar to previous mine collapses than to double couple tectonic earthquakes although source mechanisms are similar there are clear differences in love wave amplitudes online material tables of velocity models and figures of moment tensors and waveform fits

age independent seismic anisotropy under oceanic plates explained by strain history in the asthenosphere the depth of the oceanic lithosphere asthenosphere boundary lab as inferred from shear wave velocities increases with lithospheric age in agreement with models of cooling oceanic lithosphere on the other hand the distribution of radial anisotropy under oceanic plates is almost age independent in particular radial anisotropy shows a maximum positive gradient at a depth of 70 km which if used as a proxy indicates an age independent lab depth these contrasting observations have fueled a controversy on the seismological signature of the lab to better understand the discrepancy between these observations we model the development of lattice preferred orientation lpo in upper mantle crystal aggregates and predict the seismic anisotropy produced by plate driven mid ocean ridge flows the model accounts for the progressive cooling of the lithosphere with age and can incorporate both diffusion and dislocation creep deformation mechanisms we find that an age independent distribution of radial anisotropy is the natural consequence of these simple flows the depth and strength of anisotropy is further controlled by the deformation regime dislocation or diffusion creep experienced by crystals during their ascent towards and subsequent motion away from the ridge axis comparison to surface wave tomography models yield constraints on rheological parameters such as the activation volume although not excluded additional mechanisms proposed to explain some geophysical signatures of the lab such as the presence of partial melt or changes in water content are not required to explain the radial anisotropy proxy our prediction that the age independent radial anisotropy proxy marks the transition to flow induced asthenospheric anisotropy provides a way to reconcile thermal mechanical and seismological views of the lab

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

modeling olivine cpo evolution with complex deformation histories implications for the interpretation of seismic anisotropy in the mantle relating seismic anisotropy to mantle flow requires detailed understanding of the development and evolution of olivine crystallographic preferred orientation cpo recent experimental and field studies have shown that olivine cpo evolution depends strongly on the integrated deformation history which may lead to differences in how the corresponding seismic anisotropy should be interpreted in this study two widely used numerical models for cpo evolution d rex and vpsc are evaluated to further examine the effect of deformation history on olivine texture and seismic anisotropy building on previous experimental work models are initiated with several different cpos to simulate unique deformation histories significantly models initiated with a preexisting cpo evolve differently than the cpos generated without preexisting texture moreover the cpo in each model evolves differently as a function of strain numerical simulations are compared to laboratory experiments by boneh and skemer 2014 in general the d rex and vpsc models are able to reproduce the experimentally observed cpos although the models significantly over estimate the strength of the cpo and in some instances produce different cpo from what is observed experimentally based on comparison with experiments recommended parameters for d rex are m 10 5 and 0 3 and for vpsc 10 100 numerical modeling confirms that cpo evolution in olivine is highly sensitive to the details of the initial cpo even at strains greater than 2 these observations imply that there is a long transient interval of cpo realignment which must be considered carefully in the modeling or interpretation of seismic anisotropy in complex tectonic settings

strong inheritance of texture between perovskite and post perovskite in the d layer flow in the deep mantle is thought to create textures in the high pressure mineral post perovskite laboratory simulations of the transformation between lower pressure perovskite and post perovskite show that post perovskite can also inherit textures from the perovskite phase and vice versa

fluid present deformation aids chemical modification of chromite insights from chromites from golyamo kamenyane se bulgaria abstract chemical signatures of chromitites are commonly used to track the evolution of the earth s mantle however chemical modification during deformation may have important implications for the interpretation of chromites signatures here we describe the details of how deformation promotes chemical modification in chromite physicochemical characteristics of the chromites were quantified by measuring crystallographic orientation relationships using electron back scattered diffraction ebsd and electron microprobe analysis emp chromites show porphyroclastic textures with coarse grained porphyroclasts ca 0 2 5 mm and fine grained neoblasts 3 and cr and lower mg values than the cores of large grains we interpret f1 and f2 to represent chromite recrystallized by heterogeneous nucleation and subgrain rotation recrystallization respectively crystallographic preferred orientation cpo and misorientation data on the well developed low angle subgrain boundaries in coarse grains and f2 grains indicate that deformation in chromite was accommodated mainly by dislocation creep with the dominant activation of the 111 slip system the retrograde p t exhumation path predicted by thermodynamic and chemical modeling suggests that these fine grained chromites were produced when the initial chromitites reacted with oxidizing fluids during retrograde metamorphism 1 0 gpa and 500 700 c our results show that deformation in the dislocation creep regime in a chemically open system has induced chemical modification and homogenization within chromite aggregates as well as strain localization this close physicochemical link offers new avenues of interpreting the chemical signatures of chromites utilizing their microstructurally controlled variation or lack thereof

using thermo mechanical models of subduction to constrain effective mantle viscosity mantle convection and plate dynamics transfer and deform solid material on scales of hundreds to thousands of km however viscoplastic deformation of rocks arises from motions of defects at sub crystal scale such as vacancies or dislocations in this study results from numerical experiments of dislocation dynamics in olivine for temperatures and stresses relevant for both lithospheric and asthenospheric mantle 800 1700 k and 50 500 mpa gouriet et al 2019 are used to derive three sigmoid parameterizations erf tanh algebraic which express stress evolution as a function of temperature and strain rate the three parameterizations fit well the results of dislocation dynamics models and may be easily incorporated into geodynamical models here they are used in an upper mantle thermo mechanical model of subduction in association with diffusion creep and pseudo brittle flow laws simulations using different dislocation creep parameterizations exhibit distinct dynamics from unrealistically fast sinking slabs in the erf case to very slowly sinking slabs in the algebraic case these differences could not have been predicted a priori from comparison with experimentally determined mechanical data since they principally arise from feedbacks between slab sinking velocity temperature drag and buoyancy which are controlled by the strain rate dependence of the effective asthenosphere viscosity comparison of model predictions to geophysical observations and to upper mantle viscosity inferred from glacial isostatic adjustment shows that the tanh parameterization best fits both crystal scale and earth scale constraints however the parameterization of diffusion creep is also important for subduction bulk dynamics since it sets the viscosity of slowly deforming domains in the convecting mantle within the range of uncertainties of experimental data and most importantly of the actual rheological parameters prevailing in the upper mantle e g grain size chemistry viscosity enabling realistic mantle properties and plate dynamics may be reproduced by several combinations of parameterizations for different deformation mechanisms deriving mantle rheology cannot therefore rely solely on the extrapolation of semi empirical flow laws the present study shows that thermo mechanical models of plate and mantle dynamics can be used to constrain the effective rheology of earth s mantle in the presence of multiple deformation mechanisms