mechanical interactions between rigid particles in a deforming ductile matrix analogue experiments in simple shear flow abstract the mechanical interaction between two or more particles is investigated in newtonian simple shear flow the experimental models allow observation of both the particle rotation and the deformation pattern around the particles the finite and instantaneous strain patterns around rigid particles are strongly heterogeneous and asymmetric with high finite strain zones aligned to the direction of maximum finite stretch these correspond to local flow with low vorticity number heterogeneous strain patterns around rigid particles spread over a distance of 1 2 times the particle length and this distance increases with increasing bulk strain where rigid particles are more concentrated these patterns coalesce and the overall pattern is then strongly controlled by the particles and cannot be simply related to the external boundary conditions in rocks the characteristic asymmetric strain pattern around rigid isolated particles is a reliable shear criterion which becomes unreliable with high concentrations of rigid particles particle rotation is significantly disturbed when the distance between adjacent particles of equal size is shorter than their length that is only in very concentrated suspensions of rigid particles in a composite shape fabric the development of the sub fabric corresponding to the smaller minerals will be more disturbed

the motion of rigid ellipsoidal particles in slow flows abstract preferred orientations of fabric elements in tectonites are usually interpreted with reference to two continuum models namely those of march 1932 and jeffrey 1922 it is argued that the first of these is usually inappropriate if the fabric elements are rigid inhomogeneities embedded in a deformable matrix jeffrey s model is more applicable but until recently its use has been limited to problems involving axisymmetric particles and simple flow geometries in this paper jeffrey s equations have been solved numerically in order to investigate the behaviour of more general systems the motions of two particle shapes one a prolate spheroid the other a triaxial ellipsoid are considered in parallel for four specified flow geometries which involve pure or simple shear and combinations of both any departures from particle axisymmetry or from simple flow geometries have a significant effect on particle behaviour compared with that predicted by spheroid simple flow solutions

flow laws for rocks containing two non linear viscous phases a phenomenological approach abstract mylonitic rocks and rock analogue materials reveal two basic types of structure 1 a load bearing framework lbf of strong phase contains isolated pockets of weak phase 2 interconnected layers of weak phase iwl separate boudins and clasts of strong phase aggregates with the lbf microstructure are characterized by nearly uniform strain rate stress is concentrated in the load bearing framework in aggregates with an iwl microstructure strain rate and sometimes also stress are higher in the interconnected weak phase than in the boudins and clasts of strong phase the degree of stress and strain partitioning depends strongly on the viscous strength contrast and on the relative amounts of the constituent mineral phases based on these observations the rheology of two phase rock is modelled with separate functions for lbf and iwl microstructures a new flow law is derived for rock with iwl structure in which two phases undergo dislocation creep the flow law expresses composite creep strength in terms of temperature bulk strain rate and the volume proportions and creep parameters of the minerals in the rock strain rate and stress are averaged in the constituent phases and slip along phase boundaries maintains strain compatibility within the aggregate composite strengths predicted with the iwl flow law fall well within the uniform stress and uniform strain rate bounds and are generally consistent with the viscous strengths of experimentally deformed bimineralic aggregates a hypothesis of viscous strain energy minimization is used to determine the relative stability of the lbf and iwl microstructures during steady state creep the iwl microstructure is predicted to be stable over a broad range of two phase compositions and mineral strength contrasts whereas the lbf microstructure is stable only in rocks with low volume proportions of weak phase and low to moderate mineral strength contrasts the iwl flow law indicates that rheological stratification in the lithosphere depends strongly on rock composition especially in rocks with low volume proportions of a weak phase and high mineral strength contrasts

torsion experiments on coarse grained dunite implications for microstructural evolution when diffusion creep is suppressed large strain deformation experiments in torsion were conducted on a coarse grained natural dunite with a pre existing lattice preferred orientation lpo experiments were conducted at conditions where deformation by diffusion creep is initially negligible microstructural evolution was studied as a function of strain we observe that the pre existing lpo persists to a shear strain of at least 0 5 at larger strains this lpo is transformed relict deformed grains exhibit lpo with 100 crystallographic axes at high angles to the shear plane unlike previous experimental studies these axes do not readily rotate into the shear plane with increasing strain partial dynamic recrystallization occurs in samples deformed to moderate strains g 0 5 recrystallized material forms bands that mostly transect grain interiors the negligible rate of diffusion creep along relict grain boundaries as well as the limited nature of dynamic recrystallization may account for the relatively large strains required to observe evolution of microstructures our data support hypotheses based on natural samples that microstructures may preserve evidence of complex deformation histories relationships between lpo seismic anisotropy and deformation kinematics may not always be straightforward as the most abundant mineral in the upper mantle olivine plays a key role its rheological behaviour karato wu 1993 hirth kohlstedt 2003 moreover olivine is essential to our interpretation of mantle kinematics because of its strong influence on seismic anisotropy nicolas christensen 1987 karato et al 2008 the generation and evolution of olivine lattice preferred orientation lpo has been well documented in laboratory studies carter ave lallemant 1970 nicolas et al 1973 zhang karato 1995 bystricky et al 2000 zhang et al 2000 jung karato 2001 naturally deformed peridotites nicolas et al 1971 mercier 1985 ben ismail mainprice 1998 warren et al 2008 skemer et al 2010 and numerical simulations wenk et al 1991 wenk tome 1999 tommasi et al 2000 kaminski ribe 2001 blackman kendall 2002 castelnau et al 2009 however there are still substantial differences between the microstructures generated or observed in these studies notably studies of peridotite mylonites suggest that somewhat larger strains may be required to rotate lpo into concordance with deformation kinematics in comparison to experimental studies and numerical simulations that have an initially random texture warren et al 2008 this difference in the influence of strain on lpo may be a consequence of factors such as the initial microstructure and the conditions of deformation furthermore different numerical models show distinct patterns and rates of lpo development which may differ from experimental and geological observations castelnau et al 2009 variations between models arise from differences in numerical techniques but also the inclusion or exclusion of specific dynamic recrystallization mechanisms wenk et al 1991 wenk tome 1999 to expand on existing experimental studies and provide further bases for numerical investigations we conducted a series of deformation experiments on coarse grained natural dunite the objective of these experiments is to improve our understanding of microstructural processes in realistic mantle materials and to help bridge the gap between the laboratory and nature for technical reasons many of the experimental studies of olivine deformation have been conducted on relatively fine grained synthetic materials e g zhang karato 1995 bystricky et al 2000 as a consequence deformation in these experiments typically includes contributions from several parallel mechanisms including dislocation creep diffusion creep and grain boundary sliding fig 1 in contrast many geologically observable mantle rocks are interpreted to have deformed at conditions where dislocation creep is strongly dominant it is therefore important to understand from an from prior d j rutter e h tatham d j eds deformation mechanisms rheology and tectonics microstructures mechanics and anisotropy geological society london special publications 360 211 223 doi 10 1144 sp360 12 the geological society of london 2011 publishing disclaimer www geolsoc org uk pub ethics experimental perspective how mantle materials deform when grain size sensitive creep is more completely suppressed to this end we conducted laboratory deformation experiments on samples cored from a block of balsam gap dunite which was chosen for two reasons first it has a moderately large grain size which allows us to suppress activity of the diffusion creep mechanism secondly it has a moderately strong pre existing lpo this allows us to consider how lpo might evolve in a situation where kinematics are changing for example in a corner flow regime such as a mantle wedge or mid ocean ridge

experimental studies of shear deformation of mantle materials towards structural geology of the mantle a brief outline is given on experimental studies carried out in the minnesota mineral and rock physics laboratory of microstructural evolution and rheology of mantle mineral aggregates or their analogues using a simple shear deformation geometry a simple shear deformation geometry allows us to unambiguously identify controlling factors of microstructural evolution and to obtain large strains at high pressures and temperatures and thus provides a unique opportunity to investigate the structural geology of the mantle we have developed a simple shear deformation technique for use at high pressures and temperatures pressure up to 16 gpa and temperature up to 2000 k in both gas medium and solid medium apparati this technique has been applied to the following mineral systems i olivine aggregates ii olivine basaltic melt iii catio3 perovskite aggregates the results have provided important data with which to understand the dynamics of the earth s mantle including the geometry of mantle convection mechanisms of melt distribution and migration beneath mid ocean ridges and the mechanisms of shear localization limitations of laboratory studies and future directions are also discussed

orthopyroxene oikocrysts in the mg1 chromitite layer of the bushveld complex implications for cumulate formation and recrystallisation two typical mineral textures of the mg 1 chromitite of the bushveld complex south africa were observed one characterised by abundant orthopyroxene oikocrysts and the other by coarse grained granular chromitite with only minor amounts of interstitial material oikocrysts form elongate clusters of several crystals aligned parallel to the layering and typically have subhedral almost chromite free core zones containing remnants of olivine the core zones are surrounded by poikilitic aureoles overgrowing euhedral to subhedral chromite chadacrysts chromite grains show no preferred crystal orientation whereas orthopyroxene grains forming clusters commonly share the same crystallographic orientation oikocryst core zones have lower mg and higher concentrations of incompatible trace elements compared to their poikilitic aureoles core zones are relatively enriched in ree compared to a postulated parental magma b1 and did not crystallise in equilibrium with the surrounding minerals whereas the composition of the poikilitic orthopyroxene is consistent with growth from the b1 magma these observations cannot be explained by the classic cumulus and post cumulus models of oikocryst formation instead we suggest that the oikocryst core zones in the mg1 chromitite layer formed by peritectic replacement of olivine primocrysts by reaction with an upwards percolating melt enriched in incompatible trace elements poikilitic overgrowth on oikocryst core zones occurred in equilibrium with a basaltic melt of b1 composition near the magma crystal mush interface finally adcumulus crystallisation followed by grain growth resulted in the surrounding granular chromitite

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

magmatic fabric acquisition mechanisms in a syenite results of a combined anisotropy of magnetic susceptibility and image analysis study the structural significance of the anisotropy of magnetic susceptibility ams and acquisition mechanisms of silicate and ferrimagnetic mineral fabrics are investigated in the archean lebel syenite intrusion ontario canada bulk mineral subfabrics are determined by intercept counting of mineral phases in classified digital images of thin sections cut parallel to the principal planes of the magnetic fabric ellipsoids of the samples intensities shapes and principal direction orientations of the resulting intercept ellipsoids are generally similar to the corresponding ams ellipsoids differences are due mainly to bimodal distribution of mineral fabrics observed on foliation planes of some samples showing pre full crystallization textures and structural overprinting in samples deformed after crystallization subfabrics of clinopyroxene and magnetite in samples showing pre full crystallization microstructure are investigated by measuring inertia tensors of individual grain shapes the resulting inertia tensor ellipsoids indicate that magnetic fabrics in these rocks are strongly correlated to the shape preferred orientation of magnetite which is similar to the overall petrofabric of the samples crystal size distributions of clinopyroxene and magnetite indicate that they formed during in situ crystallization and that magnetite probably appeared late in the crystallization and flow history of the magma analysis of preferred orientations of these grains suggests that the petrofabrics of the samples were acquired during flow with strongly interacting grains analogous to a logjam in a river the final characteristics of the ams and silicate petrofabric are the end points of a continuous crystallization and flow history in the magma rather than transient late features

correlating mechanical data with microstructural observations in deformation experiments on synthetic two phase aggregates abstract a simple method of analysing the results of deformation experiments on synthetic two phase aggregates is presented and applied to some tests on calcite halite aggregates the approach involves replacing the observed mechanical behaviour of real two phase materials of coarse microstructure with a phase volume fraction weighted mechanically equivalent representation in which each phase is assigned its own single valued stress and its own single valued strain such a representation can be solved for the stresses and strains in the two phases and the results compared with the stresses and strains as determined from the deformation microstructures in principle it is then possible to constrain which of the features of the microstructure are mechanically significant the stresses supported by the synthetic calcite halite aggregates suggest a strong partitioning of the deformation into the halite at all volume fractions of calcite and yet analysis of the halite strains from the deformation microstructures indicates that there is little or no strain partitioning between the phases however these conflicting results may be reconciled by using the calcite contiguous volume rather than the actual calcite volume fraction to characterize the strength of the aggregates with respect to those of their component phases

microstructures and rheology of hydrous synthetic magmatic suspensions deformed in torsion at high pressure the relationship between magma rheology and characteristic magmatic microstructures was investigated by performing high temperature high pressure deformation experiments on hydrous synthetic magmatic suspensions in the range of 0 to 76 solid fraction alumina grains torsion experiments were conducted at 300 mpa confining pressure temperatures ranging from 475 c to 1000 c and shear strain rates ranging from 2 0 10 5 to 2 1 10 3 s 1 up to total strains of 21 3 flow is newtonian for a solid fraction of s 0 0 0 16 with a log dynamic viscosity 10 3 pa s t 500 c a deviation from newtonian behavior is observed for s 0 16 with an increase in apparent viscosity of about 1 order of magnitude between s 0 16 and 0 54 the shape fabric of the solid phase is characterized by a unimodal orientation that is almost stable and nearly parallel to the shear direction both shape fabric and deviation from newtonian behavior originate from the increase in the number of particle clusters in the suspension the apparent viscosity increases by 1 5 orders of magnitude between s 0 54 and 0 65 and extrapolation of the data suggests a very sharp increase in apparent viscosity for s 0 65 at t 550 c and s 0 54 the solid phase forms an almost entirely connected network composed of two alternating orientation domains at t 550 c and s 0 65 intragranular fracturing and tensile fractures result from high local stresses at contact points between neighboring particles the resulting bulk extensional fabric is almost parallel to the shortening direction

strain and fabric analyses based on porphyroclast interaction abstract two dimensional computer models of the interaction of elliptical clasts in a variety of kinematic conditions are used to study how clast imbrication and shape fabric development relate to strain and strain history the models provide templates for a known clast density and clast ellipticity that correlate clast imbrication angle of collision and clast shape fabric to strain and kinematics of deformed rocks modeling also shows that the mode of clast interaction plays a significant role in the accumulation of imbricated clasts and the development of shape fabric upon collision between clasts the trains formed can remain fixed or rotate if they rotate imbricated clasts can become independent when they are no longer forced together by the flow based upon the mode of clast and train rotation using the march rotation of passive lines and the jeffery rotation of rigid particles as end members three models are defined in an attempt to simulate the conditions of clast interaction under varying conditions the march fixed train march rotating train and jeffery rotating train models simulate solid state intermediate and magmatic conditions respectively modeling shows that the two march models produce similar results but the jeffery model of train rotation always produces a lower proportion of imbricated clasts and a much weaker shape fabric application of the modeling to the study of a syntectonic granite in the sierra nevada suggests that deformation was dominated by simple shear some k feldspar imbrication and fabric development took place initially in the magmatic stage and culminated in the solid state to produce a large proportion of imbricated k feldspar porphyroclasts which define a strong shape fabric this type of analysis provides a rationale to determine syn vs post emplacement deformation in the granite

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

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

some mineral physics constraints on the rheology and geothermal structure of earth s lower mantle we explore the implications of recent mineral physics measurements of diffusion coefficients and melting temperatures of lower mantle materials on the rheological and geothermal structure of earth s lower mantle we show that mgsio3 perovskite is significantly stronger than mgo periclase and therefore the rheology of the lower mantle depends strongly on the geometry of a weaker phase periclase we calculate viscosities of the lower mantle for two cases 1 where periclase occurs as isolated grains and 2 where periclase occurs as continuous films using mineral physics data and models of two phase rheology we find that the effective viscosity for the former is about 10 1000 times higher than the latter we therefore suggest that the rheology of the lower mantle is structure and hence strain dependent leading to weakening at large strains due to the formation of continuous films of periclase overall depth variation of viscosity depends not only on the pressure dependence of creep but also on the geothermal gradient both mgsio3 perovskite and periclase have relatively small activation energies e g r t m with g 10 14 where r is the gas constant and t m is melting temperature and therefore the depth variation of viscosity is rather small even for a nearly adiabatic temperature gradient however the geothermal gradients consistent with the geodynamical inference of nearly depth independent viscosity are sensitive to the pressure dependence of viscosity which is only poorly understood a superadiabatic gradient of up to 0 6 k km is also consistent with mineral physics and geodynamical observations

fabric development in mg fe o during large strain shear deformation implications for seismic anisotropy in earth s lower mantle abstract large strain shear deformation experiments were performed on mg 1 x fe x o x 0 25 1 0 one of the important minerals in earth s lower mantle deformation experiments were made on coarse grained 15 20 m grain size hot pressed aggregates at conditions of t t m 0 46 0 65 t temperature t m melting temperature and 0 4 10 3 to 0 9 10 3 differential stress shear modulus up to the shear strain of 7 8 under these conditions deformation occurs by dislocation creep the microstructural development in mg fe o is found to be sluggish and the complete dynamic recrystallization and nearly steady state fabric lattice preferred orientation are achieved only after shear strains of 4 at nearly steady state mg fe o shows strong fabrics characterized by the 1 1 0 axes being parallel to the shear direction and the poles of the 1 0 0 planes and to a lesser extent the poles of the 1 1 1 planes normal to the shear plane the seismic anisotropy corresponding to the deformation fabrics in mg fe o was calculated the nature of anisotropy corresponding to a given flow geometry changes significantly with strain as a result of fabric evolution anisotropy changes also with depth pressure due to the large variation of elastic anisotropy of mg fe o with depth seismic anisotropy caused by the deformation fabric of mg fe o is very weak in the shallow lower mantle v sh v sv anisotropy assuming that mg fe o occupies 20 volume fraction of the lower mantle and little shear wave splitting of vertically travelling waves a result that is consistent with the seismological observations in the d layer of the circum pacific regions thus the deformation fabric of mg fe o is a vital candidate of the cause of seismic anisotropy in these regions anisotropy caused by the lattice preferred orientation of mg fe o has a distinct azimuthal anisotropy with a strong 4 term azimuth the direction of propagation of the fastest slowest sh or p wave is parallel perpendicular to the flow direction and the slowest fastest sh or p wave is at 45 from these two directions

grain size stabilisation by dispersed graphite in a high grade quartz mylonite an example from naxos greece high grade quartz mylonites from naxos greece consist of alternating thin layers of pure quartz and quartz layers with 0 3 3 vol finely dispersed graphite particles graphite free layers are coarse grained 100 300 mm show undulose extinction subgrains lobate grain boundaries and have a strongly developed crystallographic preferred orientation in these layers dislocation flow is interpreted to be the dominant deformation mechanism in contrast graphite rich layers are fine grained 30 70 mm have equant quartz grain shapes and have a crystallographic preferred orientation that becomes progressively weaker with decreasing quartz grain size given the high temperature of deformation and the need for a c axis fabric destroying mechanism grain boundary sliding is interpreted to be important in these layers analysis shows an inverse relationship between quartz grain size and the graphite dispersion suggesting stabilization of quartz grain size by graphite particles graphite particles larger than 5 mm are concentrated along quartz boundaries suggesting that stabilisation only operates above a certain critical graphite particle size this study shows that a dispersed second phase such as graphite in a naturally deforming rock can inhibit grain boundary migration stabilise the grain size and enhance grain boundary sliding at the expense of dislocation flow q 2002 elsevier science ltd all rights reserved

deformation mechanisms in second phase affected microstructures and their energy balance based on the relationship zener parameter z second phase size second phase volume fraction vs calcite grain size dg second phase controlled aggregates and microstructures that are weakly affected by second phases are discriminated the latter are characterized by large but constant grain sizes high calcite grain boundary fractions and crystallographic preferred orientations cpo while calcite grain size and calcite grain boundary fraction decrease continuously and cpo weakens with decreasing z in second phase controlled microstructures these observations suggest that second phase controlled microstructures predominantly deform via granular flow because pinning of calcite grain boundaries reduces the efficiency of dynamic recrystallization favoring mass transfer processes and grain boundary sliding in contrast the balance of grain size reduction and growth by dynamic recrystallization maintains a steady state grain size in microstructures that are only weakly affected by second phases promoting a predominance of dislocation creep with increasing temperature the relationship between z and dg persists but the calcite grain size increases continuously based on microstructures the energy of each modifying process is calculated and its relative contribution is compared with energies of the competing processes surface energy dragging energy dynamic recrystallization energy the steady state microstructures result from a temperature dependent energy minimization procedure of the system

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

an olivine fabric database an overview of upper mantle fabrics and seismic anisotropy we present a unique database of 110 olivine petrofabrics and their calculated seismic properties the samples come from a variety of the upper mantle geodynamic environments ophiolites subduction zones and kimberlites with a wide range of micro structures a phenomenological relationship is established between p and s wave seismic anisotropy and the degree of crystal alignment fabric strength seismic anisotropy increases rapidly at low fabric strength before reaching a near saturation level of 15 to 20 for p waves and 10 to 15 for s waves despite a large variation in the symmetry of fabric patterns the average seismic properties of the different fabric micro structural and geodynamic settings have similar anisotropies in both magnitude and symmetry hence it would seem possible to determine some measure of fabric strength from seismic anisotropy if the dimensions of the anisotropic region are known but not geodynamic environment or details of the petrofabric pattern a simple pattern of seismic anisotropy characterises the average sample of the database which has the following features the polarisation plane of the fastest s wave is parallel or sub parallel to the foliation plane the maximum shear wave splitting is parallel to the y structural direction in foliation plane and normal to the lineation the maximum of the p wave velocity is parallel to the high concentration of 100 axes which is sub parallel to the lineation the 100 orientation distribution has the greatest influence on the p wave seismic anisotropy the 100 and 001 orientation distributions have the greatest influence on the symmetry of s wave anisotropy although the magnitude of anisotropy is influenced by the distribution of all three principal axes although the database only contains olivine petrofabrics this statistical study clearly shows that seismic anisotropy can be used to deduce the orientation of the structural frame in the upper mantle 1998 elsevier science b v all rights reserved

shape fabrics of particles in low concentration suspensions 2d analogue experiments and application to tiling in magma abstract jeffery s equations ascribe a theoretical cyclic nature to the shape fabric of non interacting rigid particles immersed in a viscous fluid undergoing simple shear flow this theoretical behaviour is confirmed at low shear strains 5 first experiment 134 identical particles or 1 second experiment 178 particles with two size classes then become stable at 17 first experiment and at more than 50 second experiment of the population of particles due to the contribution of the tiled particles the shape fabric becomes asymmetrical in its orientation distribution with a maximum lying above the shear plane the latter result provides a new shear sense indicator in addition to the statistical determination of the tiled features the study also suggests that crystalline fabrics in magmas could be acquired at high melt fractions i e early in the crystallization history of the magma

anisotropy of the reflected radiation field over melting glacier ice abstract in this article we analyze the anisotropy of the reflected radiation field over melting glacier ice using 118 ground based radiance measurements in landsat tm bands 2 and 4 the measurements were carried out on the tongue of the morteratschgletscher switzerland on the basis of directional measurements several bidirectional reflectance distribution functions brdfs were derived for dirty and clean glacier ice and for solar zenith angles between 46 and 60 the anisotropic reflectance factor ranges between 0 70 and 1 60 for tm 2 and between 0 56 and 1 72 for tm 4 the brdfs exhibit nadir darkening and maximum brightening near the forward limb dirty and wet ice reflect more anisotropically than clean ice we give an example to illustrate the effect of anisotropic reflectance on the tm derived albedo along a cross section of the glacier for solar zenith angles around 48 and in the nadir viewing direction we found differences of 0 05 in tm 2 and 0 08 in tm 4 between the albedo and bidirectional reflectance r the differences prove to be more or less uniform for both clean ice and dirty ice it is therefore suggested that one can convert r to without having to classify the glacier surface

an electron microscope study of the stability field and degree of nonstoichiometry in olivine the stability of san carlos olivine fo 90 5 relative to changes in oxygen fugacity at 1300 sup 0 c has been determined by studying the microstructural changes in single crystals using transmission and analytical electron microscopy the first detectable metallic phase heterogeneously nucleated along dislocations within 100 and 001 subboundaries had a composition ni sub 65 fe sub 35 equilibrated with a co co sub 2 gas mixture having an oxygen fugacity f sub 02 of 10 sup 5 6 pa although the 100 subboundaries contained a high density of dislocations with burgers vector 100 the nucleation sites were restricted to 010 and 001 dislocations a result consistent with the theory of heterogeneous nucleation along dislocations under the reducing conditions of an f2 of 10 sup 12 pa a forsteritic olivine is produced at the gas mineral interface with a high density of metallic precipitates fe sub 96 ni sub 4 homogeneously nucleated in the olivine nonstoichiometry of the olivine was inferred from the absence of any additional silica rich phases the nonstoichiometry was analyzed in terms of possible crystalline defect structures especially vacancies assuming no evaporative losses from the surface except an oxygen exchange reaction on the basis of the model proposed the highly reduced olivine would contain an excess of sio sub 2 x2more 0 355 compared with a stoichiometric value of 0 333 from the surface to a depth of several micrometers less

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

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

strain insensitive preferred orientation of porphyroclasts in mont mary mylonites abstract the shape preferred orientation spo of porphyroclasts was determined in high temperature mylonites the porphyroclasts approach rhomboidal sillimanite or elliptical garnet plagioclase sillimanite shapes and exhibit aspect ratios r as high as 11 particles with r 3 are dominantly rhomboidal the long axis of the best fit ellipse defines a very strong spo inclined at 5 10 to the mylonitic foliation in an antithetic sense with respect to the shear direction this angle is independent of r the inclination of the long sides of rhomboidal sillimanite increases from 10 to 20 with decreasing r in contrast the short sides have a constant orientation of 15 to 17 irrespective of r and are parallel to extensional crenulation cleavage low aspect ratio mainly elliptical objects show low intensity spo close to the shear plane the two spos appear strain insensitive in the case of r r 3 a stable position is acquired this is not explained by any of the current theoretical and experimental models of spo

the effect of pre tectonic reaction and annealing extent on behaviour during subsequent deformation insights from paired shear zones in the lower crust of fiordland new zealand granulite facies pargasite orthogneiss is partially to completely reacted to garnet granulite either side of narrow 20 mm felsic dykes in fiordland new zealand forming 10 80 mm wide garnet reaction zones the metamorphic reaction changed the abundance of minerals and their shape and grain size distribution the extent of reaction and annealing temperature related coarsening and nucleation is greatest close to the dykes whereas further away the reaction is incomplete as a consequence grain size and the abundance of garnet decreases away from the felsic dykes over a few centimetres the aspect ratios of clusters of s1 pyroxene and pargasite in the orthogneiss which are variably reacted to post s1 garnet decrease from high in the host to near equidimensional close to the dyke post reaction deformation localized in the fine grained partially reacted areas this produced a pattern of paired shear zones located at the outer parts of the garnet reaction zone our study shows that grain size sensitive deformation occurs where the grain size is sufficiently reduced by metamorphic reaction the weakening of the rock due to the change in grain size distribution outweighs the addition of nominally stronger garnet to the assemblage

the origin of shape preferred orientations in mylonite inferences from in situ experiments on polycrystalline norcamphor abstract polycrystalline norcamphor c 7 h 10 o undergoing plane strain simple shear in a see through deformation rig develops intergranular microshears whose activity and orientation is closely related to shape preferred orientation spo of dynamically recrystallized grains and grain aggregates intergranular microshears nucleate at 80 90 to the sample s shear zone boundary szb and rotate synthetically toward this boundary they accommodate increasing amounts of incremental shear strain at angles ranging from 40 to 60 s b orientation with respect to the szb with progressive simple shear both the rotation rate and the amount of strain accommodated by these microshears decrease but strain accommodation increases as the microshears attain a lower angle 10 30 s a orientation to the szb the microshears gradually deactivate as they acquire inclination angles to the szb of less than 10 the deactivation of such microshears is accompanied by the nucleation of fresh high angle microshears this heterogeneous deformation is associated with two shape preferred orientations in the norcamphor mylonite a steep oblique grain spo comprising the long axes of dynamically recrystallized grains 40 60 subparallel to the s b oriented microshears and gently inclined domainal spo 20 subparallel to the s a oriented microshears the domain spo is defined by the length axes of grain aggregates with a uniform crystallographic preferred orientation the angle between domainal and grain spos is a potential measure of the bulk vorticity during mylonitization

numerical study of creep in two phase aggregates with a large rheology contrast implications for the lower mantle it is generally accepted that the earth s lower mantle is dominated by two minerals magnesiowustite mg fe o mw and mg fe sio3 perovskite mg pv which are thought to exhibit very different rheological properties in order to assess the respective role of those phases in the mechanical properties of the assemblage we have carried out 3d finite element modelling of a model two phase aggregate an isotropic random polycrystal has been built from a voronoi mosaic then each grain has been attributed a hard or weak behavior in such a way that the hard phase represents a volume fraction of 70 the creep law introduced for both phases is a simple power law creep without hardening a contrast of 35 is chosen between the strain rates of both phases under a shear stress of 10 mpa a representative volume element of 470 grains has been shown to provide a satisfactory description of the mechanical response of the aggregate with a relative precision equal to 3 numerical creep experiments conducted under a constant macroscopic shear stress of 10 mpa suggest that the creep rate of lower mantle assemblages would be dominated by the mechanical behavior of the harder phase mg pv the finite element model allows one to extract the individual behavior of the two phases within the aggregate it is shown that during creep the harder mg pv phase carries most of the stress whereas the softer mw phase is responsible for most of the accumulated strain this result should have implications for the development of shape and lattice preferred orientations of mw within the aggregate thus for seismic anisotropy of the lower mantle

large strain deformation and strain partitioning in polyphase rocks dislocation creep of olivine magnesiowustite aggregates aggregates composed of olivine and magnesiowustite have been deformed to large strains at high pressure and temperature to investigate stress and strain partitioning phase segregation and possible localization of deformation in a polyphase material samples with 20 vol of natural olivine and 80 vol of mg0 7fe0 3 o were synthesized and deformed in a gas medium torsion apparatus at temperatures of 1127 c and 1250 c a confining pressure of 300 mpa and constant angular displacement rates equivalent to constant shear strain rates of 1 3 3 10 4 s 1 the samples deformed homogeneously to total shear strains of up to 15 during constant strain rate measurements the flow stress remained approximately stable at 1250 c while it progressively decreased after the initial yield stress at the lower temperature mechanical data microstructures and textures indicate that both phases were deforming in the dislocation creep regime the weaker component magnesiowustite controlled the rheological behavior of the bulk material and accommodated most of the strain deformation and dynamic recrystallization lead to grain refinement and to textures that were not previously observed in pure magnesiowustite and may have developed due to the presence of the second phase at 1127 c olivine grains behaved as semi rigid inclusions rotating in a viscous matrix at 1250 c some olivine grains remained largely undeformed while deformation and recrystallization of other grains oriented for a slip on 010 resulted in a weak foliation and a texture typical for pure dry olivine aggregates both a slip and c slip on 010 were activated in olivine even though the nominal stresses were up to 2 orders of magnitude lower than those needed to activate these slip systems in pure olivine at the same conditions

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

the influence of strain localisation on the rotation behaviour of rigid objects in experimental shear zones abstract mica fish and tourmaline fish from natural mylonites were analysed in thin section to determine their orientation distribution they are oriented with their long axes tilted with respect to the mylonitic foliation and fish with a small aspect ratio exhibit a slightly larger angle than fish with a large aspect ratio this orientation seems to be a stable orientation for the mica and tourmaline fish analogue experiments with two rheologically different matrix materials were performed to explain the data one material was pdms a linear viscous polymer the other was tapioca pearls a granular material with low cohesion and mohr coulomb type behaviour in contrast to a fairly homogeneous strain distribution in pdms distinct small scale shear bands developed in tapioca pearls during deformation experiments modelled different vorticity numbers and parallelogram shaped rigid objects with different aspect ratios were used rotation rates of objects in a viscous matrix are very similar to analytical solutions for ellipses in viscous flow but stable orientations differ from data of natural examples in all experiments with a mohr coulomb matrix elongated objects had a stable orientation due to small scale strain localisation we therefore suggest that small scale strain localisation mm that might be hidden by ongoing deformation and recrystallisation processes is an important characteristic of the rheology of mylonites

rheology of the lithosphere selected topics we review recent results concerning the rheology of the lithosphere with special attention to the following topics 1 the flexure of the oceanic lithosphere 2 deformation of the continental lithosphere resulting from vertical surface loads and forces applied at plate margins 3 the rheological stratification of the continents 4 strain localization and shear zone development and 5 strain induced crystallographic preferred orientations and anisotropies in body wave velocities we conclude with a section citing the 1983 1986 rock mechanics literature by category

the role of pre existing mechanical anisotropy on shear zone development within oceanic mantle lithosphere an example from the oman ophiolite structural and fabric analysis of the well exposed hilti mantle section oman ophiolite suggests that shear zone development which may have resulted from oceanic plate fragmentation was influenced by pre existing mantle fabric present at the paleoridge detailed structural mapping in the mantle section revealed a gently undulating structure with an east west flow direction a nw se strike slip shear zone cuts across this horizontal structure the crystal preferred orientation cpo of olivine within the foliation is dominated by 010 axial patterns rather than more commonly observed 010 100 patterns suggesting that the horizontal flow close to the moho involved non coaxial flow olivine cpo within the shear zone formed at low temperature is characterized by 001 100 patterns and a sinistral sense of shear the olivine cpo becomes weaker with progressive mylonitization and accompanying grain size reduction and ultimately develops into an ultra mylonite with a random cpo pattern the olivine 010 axis is consistently sub vertical even where the horizontal foliation has been rotated to a sub vertical orientation within the shear zone these observations suggest that the primary mechanical anisotropy mantle fabric has been readily transformed into a secondary structure shear zone with minimum modification this occurred as a result of a change of the olivine slip systems during oceanic detachment and related tectonics during cooling we propose that primary olivine cpo fabrics may play a significant role in the subsequent structural development of the mantle thus the structural behavior of oceanic mantle lithosphere during subduction and obduction may be strongly influenced by initial mechanical anisotropy developed at an oceanic spreading center

strain localisation in bimineralic rocks experimental deformation of synthetic calcite anhydrite aggregates abstract deformation of synthetic calcite anhydrite aggregates to large shear strains up to 12 4 at 600 c 300 mpa confining pressure and a constant angular displacement rate corresponding to a shear strain rate of 10 3 s 1 resulted in the first experimental observation of strain localisation from initially homogeneous rocks in contrast to experiments on pure calcite and anhydrite which deformed homogeneously to large strains 5 all experiments on calcite anhydrite mixtures resulted in heterogeneous deformation at 1 and the formation of narrow localised bands in the microstructures at 4 in these bands the amount of strain is at least twice as large as in the rest of the sample and individual grains of the same phase cluster and align thereby forming microstructural layering similar to planar fabrics in natural mylonites a switch in deformation mechanism in anhydrite from dislocation creep to diffusion creep and or grain boundary sliding occurs simultaneously with strain localisation it is concluded that deformation induced heterogeneous phase distributions cause local strength differences initiating strain localisation in the calcite anhydrite mixtures the study suggests that the presence of two phases in combination with a change in deformation mechanism may be responsible for strain localisation in natural poly mineralic mylonites

localization in rate dependent shearing deformation with application to torsion testing abstract the localization of plastic deformation in rock is of particular interest in geology in connection with the formation of ductile shear zones it is commonly conjectured that strain softening as evidenced by a falling stress strain curve at constant strain rate is likely to lead to strain localization yet observations in torsion tests at constant twist rate fail to show such an effect however a more sophisticated analysis using the theory of fressengeas and molinari j mech phys solids 1987 35 185 211 for material showing strain rate dependence of the flow stress does show that when the boundary conditions are specified in terms of displacements no localization is predicted in case of strain softening in contrast if the boundary conditions are set in terms of forces localization can be expected for a strain softening material this prediction needs experimental testing

viscoplastic self consistent and equilibrium based modeling of olivine lattice preferred orientations implications for the upper mantle seismic anisotropy anisotropy of upper mantle physical properties results from lattice preferred orientation lpo of upper mantle minerals in particular olivine we use an anisotropic viscoplastic self consistent vpsc and an equilibrium based model to simulate the development of olivine lpo and hence of seismic anisotropy during deformation comparison of model predictions with olivine lpo of naturally and experimentally deformed peridotites shows that the best fit is obtained for vpsc models with relaxed strain compatibility slight differences between modeled and measured lpo may be ascribed to activation of dynamic recrystallization during experimental and natural deformation in simple shear for instance experimental results suggest that dynamic re crystallization results in further reorientation of the lpo leading to parallelism between the main 010 100 slip system and the macroscopic shear thus modeled simple shear lpos are slightly misoriented relative to lpos measured in natural and experimentally sheared peridotites this misorientation is higher for equilibrium based models yet seismic properties calculated using lpo simulated using either anisotropic vpsc or equilibrium based models are similar to those of naturally deformed peridotites errors in the prediction of the polarization direction of the fast s wave and of the fast propagation direction for p waves are usually 15 moreover overestimation of lpo intensities in equilibrium based and vpsc simulations at high strains does not affect seismic anisotropy estimates because these latter are weakly dependent on the lpo intensity once a distinct lpo pattern has been developed thus both methods yield good predictions of development of upper mantle seismic anisotropy in response to plastic flow two notes of caution have nevertheless to be observed in using these results 1 the dilution effect of other upper mantle mineral phases in particular enstatite has to be taken into account in quantitative predictions of upper mantle seismic anisotropy and 2 lpo patterns from a few naturally deformed peridotites cannot be reproduced in simulations these abnormal lpos represent a small percent of the measured natural lpos but the present sampling may not be representative of their abundance in the earth s upper mantle

shear localisation in upper mantle peridotites upper mantle peridotite bodies at the earth s surface contain relict structures and microstructures which provide direct information on the role and the mechanisms of shear localisation in the upper mantle deformation which occurred at high temperatures t 950 50 c is relatively homogeneous within domains ranging in scale from a few kilometres to a few tens of kilometres below 950 50 c strain is localised into centimetre to several hundred metre wide shear zones which commonly contain hydrated mylonitic peridotites the microstructures developed in the peridotites suggest there is a correlation between the occurrence of shear localisation and the occurrence of strain softening and brittle deformation processes the most important strain softening processes are inferred to be structural and reaction induced softening structural softening processes include dynamic recrystallisation and strain induced transitions from dislocation creep to some form of grain size sensitive gss creep reaction induced softening is related to the formation of fine grained polyphase reaction products which deform by gss creep and the formation of weak sheet silicates such as phlogopite chlorite talc and antigorite from experimental studies these softening processes and brittle deformation processes are inferred to occur mainly at temperatures less than about 910 160 c this temperature range is inferred to be a significant rheological transition in the upper mantle below 910 160 c deformation during orogenesis may be accommodated by an anastomosing network of hydrated mylonitic shear zones with a distinct perhaps weak rheology at higher temperatures strain is accommodated in much wider deformation zones

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

grain coarsening maps a new tool to predict microfabric evolution of polymineralic rocks polymineralic rocks undergo grain coarsening with increasing temperature in both static and deformational environments as long as no mineral reactions occur the grain coarsening in such rocks is complex because the different phases influence each other and it is this interaction that controls the rate of grain coarsening of the entire aggregate we present a mathematical approach to investigate coupled grain coarsening using a set of microstructural parameters including grain size and volume fraction of both second phases and matrix mineral in combination with temperature information based on samples from polymineralic carbonate mylonites that were deformed at different temperatures we demonstrate how the mathematical relation can be calibrated for this natural system using such data sets for other lithologies grain coarsening maps can be generated which allow the prediction of microstructural evolution in polymineralic rocks such predictions are crucial for all subdisciplines in the earth sciences that require fundamental knowledge about microstructural changes and rheology of an orogen at different depths such as structural geology geophysics geodynamics and metamorphic petrology

frictional viscous flow in mylonite with varied bimineralic composition and its effect on lithospheric strength abstract a theoretical composite flow law is presented for mylonite containing interconnected layers of a weak mineral undergoing power law creep and porphyroclasts of a stronger mineral undergoing fracture and frictional sliding such mylonite termed clastomylonite is said to undergo frictional viscous fv mylonitic flow its bulk strength is expressed as a function of bimineralic composition temperature effective pressure and shear strain rate as well as the material parameters for the constituent minerals the fv flow law predicts that the rheology of clastomylonite is predominantly non linear viscous and only slightly pressure sensitive for most bimineralic compositions fv mylonitic flow is shown to occupy a depth interval between cataclastic flow involving adhesive wear in the upper crust and fully viscous mylonitic flow at greater depths the transition from adhesive wear to fv mylonitic flow is related to the onset of dislocation creep glide plus climb in the weakest phase and is inferred to coincide with a crustal strength maximum a peak strength of about 80 mpa is calculated with a combination of byerlee s constants for frictional sliding of granite and the fv flow law for granitic clastomylonite 30 vol quartz at elevated fluid pressures this value falls within the range of flow stresses independently obtained from quartz palaeopiezometry in many greenschist facies granitic mylonite zones

experimental observations on the effect of interface slip on rotation and stabilisation of rigid particles in simple shear and a comparison with natural mylonites for axial ratios r not vert similar3 porphyroclasts from three mylonites all show a very strong spo with the long axis of the best fit ellipse at an antithetic angle of 5 10 to the shear direction this is more consistent with a stable end orientation than with the transient fabrics predicted by theory for elliptical rigid particles in simple shear the cause of this divergence is investigated in a series of high simple shear strain 15 analogue experiments performed in a ring shear machine couette flow using a linear viscous matrix pdms the rotational behaviour of elongate r not vert similar5 rigid particles with elliptical and rhomboidal shapes comparable with the natural examples is modelled for both coherent and slipping particle matrix interfaces interface slip causes a dramatic reduction in the rotation rate of the elliptical particle compared with theory when the long axis is close to the shear direction but not stabilisation interface slip does result in stabilisation of the rhomboidal particle with the long diagonal oriented at a small antithetic angle to the shear direction for monoclinic particles mirror image shapes referred to here as types 1 and 2 show different rotational behaviour for the type 1 particle with a shape comparable to porphyroclast systems and mica fish the long side rotates asymptotically into parallelism with the shear direction natural examples of type 1 particles such as hornblende and olivine porphyroclasts measured from the finero mylonites southern alps show a very strong preferred orientation for r not vert similar3 with the long side parallel or at a small 5 antithetic angle to the mylonitic foliation for type 2 particles the short side stabilises close to the shear direction or at a small synthetic angle as also observed for sillimanite porphyroclasts from the mont mary mylonites western alps in this natural case stabilisation of the short sides is against an extensional crenulation cleavage rather than the mylonitic foliation the analogue experiments establish that interface slip is one mechanism for stabilisation of elongate rhomboidal particles in natural examples decoupling from the matrix may be affected by extensional crenulation cleavage or c planes in s c fabrics

the deformational behaviour of bimineralic limestone halite aggregates abstract a compositional foliation banding develops within synthetic limestone halite aggregates experimentally deformed in axially symmetric compression and in simple shear the strength of the bulk aggregate decreases as the foliation becomes stronger foliation weakening in calcite rich specimens the development of foliation starts with the formation of cataclastic shear systems in calcite domains between isolated halite domains with increasing deformation halite is sheared into these systems the halite domains coalesce into halite rich shear systems and accommodate an increasing amount of the bulk deformation the bulk strength of bimineralic aggregates at a given amount of strain increases with decreasing halite content the influence of halite on the bulk strength is much greater than would be expected from its volume fraction and becomes dominant with increasing deformation due to the development of the foliation it is postulated that such processes also occur during natural deformation of polyphase rocks competent clasts within a weaker ductile matrix showing no recrystallization differ in shape and rotational behaviour from recrystallizing clasts such as feldspar augen these competent clasts denoted as type clasts may cause problems in deducing the sense and direction of shear when formed as almost exact mirror images of type recrystallizing clasts

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

grain boundary sliding in san carlos olivine flow law parameters and crystallographic preferred orientation 1 we performed triaxial compressive creep experiments on aggregates of san carlos olivine to develop a flow law and to examine microstructural development in the dislocation accommodated grain boundary sliding regime gbs each experiment included load and temperature steps to determine both the stress exponent and the activation energy grain boundary maps created with electron backscatter diffraction data were used to quantify grain size distributions for each sample inversion of the resulting data produced the following flow law for gbs gbs 104 8 0 8 2 9 0 3 d0 7 0 1 exp 445 20 kj mol 1 rt with d and gbs in units of mpa m and s 1 respectively although relatively weak crystallographic preferred orientations cpos have 010 maxima parallel to the compression direction along with 100 and 001 girdles perpendicular to the compression direction cpos and subgrain boundary misorientation axes suggest that the 010 100 slip system contributes significantly to deformation we propose that these experimental results are best modeled by a deformation mechanism in which strain is accomplished primarily through grain boundary sliding accommodated by the motion of dislocations extrapolation of our flow laws to mantle conditions suggests that gbs is likely to be the dominant deformation mechanism in both lithospheric shear zones and asthenospheric flow and therefore strong upper mantle seismic anisotropy can not be attributed solely to the dominance of dislocation creep

dislocation creep of magnesiowustite mg0 8fe0 2o abstract magnesiowustite is likely the second most abundant and probably weakest lower mantle phase its deformation properties will therefore influence the rheological and dynamic behaviour of this region in this study magnesiowustite aggregates with a composition of mg0 8fe0 2o have been deformed under dry conditions in axial compression at a confining pressure of 300 mpa and temperatures from 1200 to 1400 k 0 41

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

the role of water in the deformation of dunite specimens of two natural dunites anita bay of 100 m grain size and aheirn of 900 m grain size have been deformed in experiments at temperatures between 1200 and 1400 c and constant strain rates between 10 4 and 10 6 s 1 in a gas medium deformation apparatus at 300 mpa confining pressure prior to these experiments the specimens were given a heat treatment in a controlled oxygen fugacity furnace at 1200 c for 60 hours to dehydrate the trace layer silicate minerals present and drive off adsorbed water the predried specimens are much stronger than specimens tested under wet conditions in an earlier study the predried specimens also do not show the apparent grain size dependence seen under wet conditions infrared absorption measurements on the present specimens and those of the earlier study indicate that for a given rock flow stress can be correlated with the intensity of a residual water related broad absorption the addition of water to a predried specimen results in both a mechanical reweakening and an intensifying of the broadband infrared component the weakening of the dunites under wet conditions is probably strongly influenced by the presence of small amounts of a water related species in the grain boundaries but intragranular weakening may also be contributing it is suggested that the concentration of the water related species in the grain boundaries is governed by its concentration in the small melt fraction present the amounts of water that have been found to be of importance for the strength of these dunites are of the order of 0 01 wt amounts much less than most estimates of the water content of the earth s undepleted upper mantle such water may have an important role in determining upper mantle flow behavior stress strain curves for all experiments are available with entire manuscript on microfiche order from american geophysical union 2000 florida avenue n w washington dc 20009 document b84 008 2 50 payment must accompany order

the effect of second phase particles on stable grain size in regionally metamorphosed polyphase calcite marbles samples of the calcite rich shelburne marble collected at the pfizer quarry in adams massachusetts show an order of magnitude variation in grain size calcite grain size ranges from 94 to 1101 m because these calcite marbles share the same pressure temperature and strain histories some other factor must be responsible for the grain size variation grain size appears to be controlled by the concentration of impurity or second phase particles large calcite grain size occurs where the volume fraction of second phase particles is low and grain size decreases as second phase volume fraction increases the relationship between calcite grain size d second phase grain size d and second phase volume fraction f can be described by the power law d d 1 4 f 0 36 a result that is consistent with models based upon short term hours or days laboratory experiments with metals and ceramics and computer simulations of grain growth grain growth appears to be greatly restricted by as little as a few per cent of second phase particles with a transition from highly restricted to almost unrestricted grain growth occurring at 5 volume of second phase particles these results indicate that second phase particles exercise an important control on grain size and can effectively inhibit grain growth in metamorphic rocks the behaviour of second phases in short term laboratory experiments may closely approximate the behaviour of second phases in grain growth lasting several orders of magnitude longer in the metamorphic environment

flow laws of multiphase materials and rocks from end member flow laws abstract rheological properties of polyphase rocks play an important role in the dynamics of the lithosphere and asthenosphere however flow laws for large portions of the polyphase rocks in the earth s crust and mantle have not been well determined an analysis based on the theory of mixtures has been made to calculate the general flow laws of coarse 5 m nearly equant grained aspect ratio 3 and massive polyphase rocks and materials for which only the flow laws and volume fractions of the constituents are taken into consideration in the modeling of the bulk rheology and effects of microstructure could be ignored the theoretical analysis is based on three assumptions 1 the polyphase composite and the monophase aggregates of its constituents obey the same kind of flow laws linear power or exponential 2 there is no change in the operative deformation mechanism of each phase when it is in the composite as compared to when it is in a monophase aggregate and 3 neither chemical metamorphic reactions take place among the constituent phases nor eutectic melting occurs due to the phase mixing the proposed iterative process allows to predict to the first approximation the flow laws for a large number of polyphase rocks in terms of the experimentally determined flow laws of a relatively small number of monomineralic aggregates applications of this approach to typical polyphase rocks such as granite diorite diabase aplite and websterite as well as to synthetic two phase materials such as forsterite enstatite mixtures and water ice ammonia dehydrate aggregates yield quite accurate approximations to the experimental values