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

can we probe the conductivity of the lithosphere and upper mantle using satellite tidal magnetic signals thesis s m massachusetts institute of technology department of earth atmospheric and planetary sciences 2015

use of extension testing to investigate the influence of finite strain on the theological behaviour of marble most mechanical testing of rocks at high temperatures is carried out under axisymmetric triaxial shortening to small 20 strains however the most significant geological deformations involve much higher strains especially in plastic shear zones where there has been strain localization highly strained rocks commonly are microstructurally modified by dynamic recrystallization so that the sheared rock is microstructurally quite different from its protolith high strain deformation in experiments can be induced by direct shear or through torsion or by taking advantage of the tendency for extensional deformations to result in necking it is important to be able to separate out the geometrical factors favouring localization from those arising from strain dependent microstructural changes here we present the results of high strain 1000 axisymmetric extension tests on carrara marble at 700 c and 800 c in the form of strain time plots for the evolution of material sections through the sample at different distances from the neck at 800 c it is clear that total dynamic recrystallization by grain boundary migration is accompanied by substantial mechanical weakening but sufficient strain could not be applied to reach the steady state condition for the recrystallized material the recrystallization is interpreted to produce weakening because the migration of grain boundaries sweeps grains clear of accumulating dislocation density faster than recovery by dislocation climb in nature such mechanical weakening would be expected to favour the stable development of a localized zone of deformation

texture development of calcite by deformation and dynamic recrystallization at 1000 k during torsion experiments of marble to large strains abstract torsion deformation experiments were performed on solid cylinders of carrara marble at high temperature 1000 k and constant twist rate about 4 10 4 rad s 1 to large twist angles between 80 and 840 these conditions correspond to simple shear deformation at constant shear strain rate 3 10 4 s 1 and variable amounts of shear strain of 1 2 5 and 11 at the outer mantle of the sample cylinders lattice preferred orientations lpo were measured on polished thin sections using automated electron backscatter diffraction ebsd to analyze the microtextures in orientation imaging micrographs oim shear deformation produced first a microstructure of elongated grains with shape and orientation close to the finite strain ellipse of the imposed shear strain 1 and 2 a deformation texture developed with monoclinic sample symmetry and an oblique c axis distribution relative to the shear plane with increasing shear the marble recrystallized by subgrain rotation and nucleation of small grains starting along grain boundaries which ultimately replaced the whole deformation microstructure a steady state recrystallization microstructure evolved with nearly equant grains of about 10 m in size 5 and 11 the lpo changed completely from the monoclinic deformation texture into a distribution with a sharp single orientation component with r 10 1 4 parallel to the shear plane and a 1 2 1 0 parallel to the shear direction this recrystallization texture has orthorhombic symmetry with respect to shear plane and shear direction during the transition the applied flow stress decreased simple shear deformation of calcite was modeled with the self consistent polycrystal plasticity theory it included a model for dynamic recrystallization based on a balance between growth and nucleation model results agree with experimental data if slip on r 10 1 4 a 1 2 1 0 is introduced as a potential slip system for calcite in that case the steady state recrystallization texture has an easy slip preferred orientation the hypothesis remains speculative until this slip system is actually confirmed by direct observation

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

deformation of two phase systems with contrasting rheologies abstract rheological model experiments are used as analogues for foliation development in quartz mica rocks in a regime of pure shear in situ observations involving ice containing varying proportions and distributions of naphthalene and camphor illustrated two contrasting types of behaviour a matrix controlled versus a particle controlled deformation the matrix controlled deformation accommodates shortening predominantly by intracrystalline slip in the ice accompanied by some recrystallization while the more rigid naphthalene plates undergo rotation and mechanical alignment where shortening is normal to an initial platelet preferred orientation the foliation is enhanced and the matrix material aggregates between the rigid plates whereas a conjugate foliation is produced during shortening parallel to an initial platelet preferred orientation during particle controlled deformation strain is localized in particle concentrates the matrix areas and particles isolated in the matrix show little deformation at shortenings 10 dissolution of selected particles is achieved through an artificial fluid transport mechanism to create voids further shortening does not create closure of the fluid filled holes where shape is supported by the rigid matrix

complex flow in lowest crustal anastomosing mylonites strain gradients in a kohistan gabbro northern pakistan 1 strain localization in near paleo moho metagabbros of the kohistan arc northern pakistan produced anastomosing shear zones the two dimensional 2 d analysis of strain gradients in planes parallel to the general flow direction reveals simple shear strain 7 combined with about 50 surface loss the pargasite garnet assemblage of the mylonite has a density 2 8 higher than the protolith which is insufficient to account for the measured surface loss furthermore major and trace element compositions of the gabbro and the mylonite indicate isochemical deformation average vp and acoustic impedance measured at room temperature and up to 300 mpa increase from gabbro through the gradient zone to mylonite they are consistent with density measurements the three dimensional analysis of shape preferred orientation and lattice preferred orientation of sheared and synkinematic minerals indicates that the 2 d surface loss reflects sideways and lengthwise material transfer sideways and lengthwise material transfers take place in widening and lengthening mylonites respectively the anastomosing shear zone pattern impels this complexity of the regional flow we conclude that shear zones with a thinning component are likely representative of deep crustal flow in other tectonic environments

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

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

post deformational annealing of calcite rocks abstract the evolution of microstructure and crystallographic preferred orientation cpo during post deformational annealing was studied on three calcite rock types differing in purity and grain size carrara marble 98 calcite mean grain size of 115 m solnhofen limestone 96 5 m and synthetic calcite aggregates 99 7 m samples were first deformed in torsion at 727 c at a shear strain rate of 3 10 4 s 1 to a shear strain of 5 and subsequently heat treated at 727 c for various durations between 0 and 24 h microstructures and cpos were analysed by optical microscopy image analysis and electron backscatter diffraction ebsd all rock types deformed in the dislocation creep field at the same applied conditions but their microstructures and cpos after deformation and after annealing differed depending on starting grain size and material composition in carrara marble and in the synthetic calcite aggregate a strong cpo developed during deformation accompanied by dynamic recrystallisation with significant changes in grain size during annealing widespread grain growth and subtle changes of cpo occurred and equilibrated foam microstructures were approached after long annealing times the cpo is the only feature in annealed samples indicating an earlier deformation phase although it is not always identical to the cpo formed during deformation in the more impure solnhofen limestone secondary phases on grain boundaries suppressed grain boundary mobility and prevented both the formation of a recrystallisation cpo during deformation and grain size modification during deformation and annealing

experimental investigation into the microstructural and mechanical evolution of phyllosilicate bearing fault rock under conditions favouring pressure solution abstract mature crustal fault zones are known to be zones of persistent weakness this weakness is believed to result from microstructural modifications during deformation such as grain size reduction and foliation development around the brittle ductile transition phyllosilicates are expected to have a significant effect on fault strength in particular under conditions favouring pressure solution to study such effects we performed rotary shear experiments on brine saturated halite kaolinite mixtures aimed at investigating the relation between microstructural and mechanical evolution in a system where pressure solution and cataclasis dominate the results show significant strain weakening and a transition with progressive strain towards more rate sensitive and less normal stress sensitive behaviour this was accompanied by a microstructural evolution from a purely cataclastic microstructure to a mylonitic microstructure consisting of elongate asymmetric clasts in a fine grained foliated matrix the results demonstrate that strain weakening and the development of a typical mylonitic microstructure can occur as a consequence of grain size reduction by cataclasis and a transition to pressure solution accommodated deformation even in the absence of dislocation creep the data raise questions regarding the reliability of microstructures as rheology indicators as well as on the use of low strain monomineralic flow laws for modelling crustal dynamics

interlayer material transport during layer normal shortening part i the model to analyse mass transfer during deformation the case is considered of a multilayer experiencing a layer normal shortening that is volume constant on the scale of many layers strain rate is homogeneously distributed on the layer scale if diffusion is absent when transport of matter between the layers occurs strain rate and dilatation rate are found to vary with distance from the contacts between layers an instantaneous model for differentiation is developed on the assumption that the sole driving force for diffusion is the difference in the hydrostatic component of stress caused by flow of theologically contrasting layers physical properties incorporated in the model are viscosity mobility of the migrating component m and a factor for an isotropy in removal or addition of material each is found to influence flow behaviour and diffusion to a degree depending on scale the magnitude of the characteristic diffusion length b 2 0 m where 0 is the specific volume of the diffusing component is essential in this respect mass removal and mass addition contribute significantly to deformation of relatively thin layers in thick layers beyond about six times the characteristic diffusion length away from a contact deformation proceeds as in the absence of diffusion the theory further predicts a dynamically maintained stable layer thickness to which thicker and thinner layers converge during prolonged deformation the question how periodically layered gneiss may develop by deformation of directionless igneous rock is discussed in the light of this conclusion

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

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

microstructure and mechanical properties of halite coarse muscovite synthetic aggregates deformed in torsion abstract we investigated the behavior of a synthetic two phase aggregate composed of 80 halite 20 coarse muscovite under torsion deformation at 100 200 and 300 c a confining pressure of 250 mpa and a constant strain rate of 3e 4 s 1 at all temperatures the two phase aggregate deformed homogeneously at the sample scale the strength of the aggregate and muscovite deformation depended on temperature strain rate and initial orientation of muscovite greatest dimension relative to the shear plane with muscovite initially parallel to the shear plane halite flowed plastically and long muscovite grains were not strong enough to behave as a rigid inclusion rotating in a ductile matrix instead high aspect ratio grains deformed by folding which was visibly accomplished by slip along mica 001 cleavage and by great flattening of halite in the core of the folds when initially normal to the shear plane high aspect ratio muscovite grains passively rotated towards the shear plane in both cases small and low aspect ratio muscovite grains behaved mostly as mica fish with mica grains tilted opposite to shear sense similarly to natural mylonites porphyroclast systems and rolling structures were also common in the microstructure a strong foliation made of halite ribbons and aligned muscovite flakes rapidly developed but did not make the composite aggregate weaker when compared with single phase halite comparison with synthetic aggregates of single phase halite shows that the two phase aggregate was much stronger than single phase halite in all cases comparison with a synthetic aggregate with calcite porphyroclasts shows that the strength of the aggregate with muscovite at 100 c was lower below a strain of ca 2 6 and higher beyond this value and that the aggregate with calcite was stronger at 200 c strain rate stepping tests 1e 5 s 1 2e 3 s 1 indicate that the two phase aggregate behaved as power law viscous with stress exponents of ca 12 and 10 at 100 and 200 c respectively the mechanical data obtained in this study represent the actual rheological behavior of the aggregates because we used soft polymer jackets

texture development in experimentally deformed two phase aggregates of calcite and anhydrite abstract fully quantitative textures have been measured from experimentally deformed two phase aggregates of calcite and anhydrite two sets of deformation experiments were performed firstly to investigate the development of microstructure and texture with composition samples with varying volume proportions of calcite and anhydrite were deformed to 20 strain at identical conditions secondly to study the texture development of the two materials with strain and explore the possible use of texture index as a strain gauge in polyphase materials pure calcite and pure anhydrite were deformed at the same conditions as the two phase rocks to strains of 6 12 and 20 the textures in both calcite and anhydrite were found to be similar but not identical to those reported in previous studies for pure samples our experimental investigations and measured textures show that the textures both of the pure materials and of the two phase rocks depend not only on strain but also on stress and grain size through their influence on the deformation mechanisms the dependence of texture index with change in deformation mechanism is particularly pronounced in the anhydrite component where the texture index decreases with grain size which decreases with anhydrite content the texture in the pure calcite and in the calcite component of the two phase rocks was influenced by a preexisting fabric the intensity of the texture in the deformed samples seems to correlate with the stress in the calcite rather than with strain

texture development of polycrystalline anhydrite experimentally deformed in torsion a polycrystalline aggregate of anhydrite was deformed in torsion to a maximum shear strain of 8 1 at 700 c and a maximum shear strain rate of 5 10 3 s 1 the crystallographic preferred orientation cpo or texture was investigated as a function of shear strain shear strain rate in a radial profile from the centre to the edge of the sample a deformation texture developed at shear strains of 1 5 2 corresponding to shear strain rates of 1 to 1 3 10 3 s 1 and reached a stable position relative to the kinematic frame at a shear strain of 3 7 2 3 10 3 s 1 further shear strain only led to a small increase in texture strength but no change in the orientation relative to the kinematic frame the cpo is very similar to naturally observed textures and can be explained by the activity of the 001 and 012 slip systems although independent mechanical data indicate that a change of mechanism from dislocation to diffusion controlled creep occurred at a shear strain of approximately 1 5 the texture does not weaken but rather increases in strength with higher shear strains

textural evolution of synthetic anhydrite halite mylonites the rheology and textural evolution of mylonites for both crustal and mantle rocks are generally poorly understood stress and strain partitioning between shear zones and wall rock foliation development flow mechanisms and constitutive relations are little known to address these problems we have begun an experimental study in which anhydrite and halite aggregates are deformed in simple shear homogeneous mixes at intervals of 10 vol of anhydrite and halite powders of 90 to 180 m grain size comprise the 1 mm thick zone between the 35 precut surfaces in 2 by 4 cm right circular cylinders of anhydrite all specimens are confined under 200 mpa pc and tested at 300 c t producing an aggregate of near zero porosity all specimens are shortened at an axial displacement rate of 5 l0 6 cm s to shear strains of about 2 0 most stress strain curves have a similar form they are characterized by an initial linear response that is followed by a work hardening region that leads to a plateau of constant stress whilst strain continues to accumulate this apparent steady state behavior is followed by a drop in stress to a second apparent steady state stress the strain at this stress drop and the magnitude of the drop increase with increasing anhydrite content the second lower steady state stress for all mixes approaches that of sheared pure halite optical examination of the fabrics shows that straining under the initial apparent steady state is accommodated by both mineral phases a foliation evolves as defined by mineral segregation and grain elongation of halite at 15 to 30 to the shear zone walls anhydrite grains rotate until their 001 010 slip system parallels the foliation the anhydrite also exhibits twin lamellae kinks and microfractures following the stress drop rheologic behavior is dominated by dynamic recovery in halite the recrystallized grain size increasing consistent with the lower stress level

the autocorrelation function an image processing tool for fabric analysis abstract the use of the autocorrelation function acf is proposed as a method for fabric and or strain analysis its chief purpose is to extend fabric analysis to fine grained materials such as clays micrites etc or to materials where the determination of individual grain shapes is tedious or impossible the main advantage of the method is that no segmentation of the image prior to analysis is necessary in other words any bias associated with identifying grain boundaries and the like is avoided in as much as the recognition of grains or grain boundaries is a matter of scale the acf can be used without loss of generality whenever the grain size is small in comparison with the size of the analysed area the method is computer based it requires a scanner for the digitization of the images and sufficient storage space and computing power for digital image processing the method uses half tone images as input these can be obtained from thin sections acetate peels polished surfaces photographic negatives or positives etc the acf is a two dimensional function that is related to the average shape that occurs in the fabric it is scaled with respect to the original image and allows simultaneous analysis of small and large features the acf enables a a quantitative description of the fabric in general terms b an efficient grain size determination and c if strain is a valid interpretation of the fabric the derivation of the finite strains in the plane of observation in the first part of the paper the acf method is introduced by way of a few numerically generated fabrics and compared to other methods of fabric and strain analysis in the second part an application to three samples of experimentally deformed solnhofen limestone is demonstrated

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

modelling of porphyroclasts in simple shear and the role of stress variations at grain boundaries abstract grain scale numerical experiments involving simple shear of a two phase non linear viscous material are described and compared with mineral fish or lozenge shaped porphyroclasts such as muscovite two types of 2d models are considered either a single elongate grain or two parallel elongate grains in both cases supported by a weaker polygonal grain matrix the relative viscosities of the contrasting grain structures were systematically varied allowing us to observe the effects of non linear viscous rheology on the resulting microstructure and flow patterns the results show that the finite rotation of the hard elongate grain was similar within any one experiment but was largely influenced by viscosity contrast the geometry of the model and the imposed shear strain models involving single elongate hard grains show increasing instability at their ends and less strain compatibility with the deforming matrix grains as the viscosity contrast is increased in the paired grain models the greatest variation in the matrix grain microstructure is seen in the region where the two hard grains are oriented at a high angle to the direction of shear finally we consider the changes in intragranular stress by comparing microstructural observations using different viscosities with the distribution of stress in space and during progressive shear in the plane approximately parallel to the maximum principal stress direction 1 a localised change of stress occurs across and along the interface between the hard and soft grains variations in the mean stress at these boundaries are directly attributable to changes in the minimum principal stress we propose that with shear strains greater than 2 it is the minimum principal stress that can control diffusion processes at the grain boundary rather than mean stress in conclusion we suggest that our models have the potential for providing useful insights into why metamorphic reactions can occur at the interface between a porphyroclast and matrix at high shear strains and how stress distribution can control the initiations of such reactions

rheological weakening of olivine orthopyroxene aggregates due to phase mixing 1 mechanical behavior to understand the processes involved in rheological weakening due to phase mixing in olivine orthopyroxene aggregates we conducted high strain torsion experiments on two phase samples at a temperature of 1200 c and a confining pressure of 300 mpa samples composed of iron rich olivine plus 26 orthopyroxene were deformed to outer radius shear strains of up to 26 values for the stress exponent n and grain size exponent p were determined based on least squares fits of the strain rate stress and grain size data to a power law creep equation both at smaller strains 3 and at larger strains 24 microstructural observations demonstrate that with increasing shear strain grain size decreased and mixtures of small equant grains of olivine and pyroxene developed the values of n and p combined with associated changes in microstructure demonstrated that our samples deformed i by dislocation accommodated grain boundary sliding with subgrains present at lower strains and ii by dislocation accommodated grain boundary sliding with subgrains absent at higher strains the evolution of both the mechanical and the microstructural properties observed in this study provide insights into the dynamic processes associated with rheological weakening and strain localization in the plastically deforming portion of the lithosphere

rheological and microstructural evolution of carrara marble with high shear strain results from high temperature torsion experiments abstract this study investigated the rheological and microstructural evolution of carrara marble deformed to large shear strain to understand how dynamic recrystallization and lattice preferred orientation lpo are related to strain softening processes solid cylinders of carrara marble were deformed in torsion up to a shear strain of 11 at constant twist rates which correspond to a shear strain rate of 3 10 4 s 1 at the outer surface and at temperatures of 1000 and 1200 k 727 and 927 c respectively for the initial grain size of 150 m these conditions are within the dislocation creep regime substantial changes in both rheology and microstructure were observed as the marble deformed to high shear strain at 1000 k 1200 k a peak stress was reached at about 1 0 5 followed by moderate strain weakening an apparent steady state flow stress was obtained at high shear strain of 5 2 the stress exponent n decreased slowly with strain from 10 1 to 6 9 at 1000 k but it remained approximately constant at 1200 k n around 10 at the maximum reached shear strain of 11 8 5 the marble had almost completely recrystallized to a fine grain size of about 10 m 20 m a secondary foliation developed in the recrystallized matrix which is at a large oblique angle to the shear zone boundary szb lpo was measured by electron backscatter diffraction ebsd for both temperatures the lpo evolved from an oblique deformation texture to a very sharp and symmetric single orientation component with r 1014 parallel to the shear plane and a 1210 parallel to the shear direction it is concluded that strain weakening was associated with the development of a strong lpo during dynamic recrystallization to a finer grain size mechanical and microstructural steady state is only reached at large shear strain the steady state lattice and grain shape fabrics can hardly be used as shear sense indicators in such recrystallized calcite mylonites

microstructural tectonometamorphic processes and the development of gneissic layering a mechanism for metamorphic segregation the mary granite in the east athabasca mylonite triangle northern saskatchewan provides an example and a model for the development of non migmatitic gneissic texture gneissic compositional layering developed through the simultaneous evolution of three microdomains corresponding to original plagioclase orthopyroxene and matrix in the igneous rocks plagioclase phenocrysts were progressively deformed and recrystallized first into core and mantle structures and ultimately into plagioclase rich layers or ribbons garnet preferentially developed in the outer portions of recrystallized mantles and with further deformation produced garnet rich sub layers within the plagioclase rich gneissic domains orthopyroxene was replaced by clinopyroxene and garnet and hornblende if sufficient water was present which were in turn drawn into layers with new garnet growth along the boundaries the igneous matrix evolved through a number of transient fabric stages involving s c fabrics s c c fabrics and ultramylonitic domains in addition quartz veins were emplaced and subsequently deformed into quartz rich gneissic layers moderate to highly strained samples display extreme mineralogical compositional segregation yet most domains can be directly related to the original igneous precursors the mary granite was emplaced at approximately 900 c and 1 0 gpa and was metamorphosed at approximately 750 c and 1 0 gpa the igneous rocks crystallized in the medium pressure granulite field opx pl but were metamorphosed on cooling into the high pressure grt cpx pl granulite field the compositional segregation resulted from a dynamic mutually reinforcing interaction between deformation metamorphic and igneous processes in the deep crust the production of gneissic texture by processes such as these may be the inevitable result of isobaric cooling of igneous rocks within a tectonically active deep crust