mechanisms of shear localization in the continental lithosphere inference from the deformation microstructures of peridotites from the ivrea zone northwestern italy the ultramafic massif of balmuccia northwestern italy shows a variety of deformation fabrics including some localized shear zones that resulted in nearly complete melting pseudotachylyte a series of peridotite specimens were collected near one of the pseudotachylyte fault veins to investigate the mechanisms of shear localization the microstructural analyses show at least three deformation stages the first stage i is nearly homogeneous deformation at low stress 3 mpa and high temperature 1300 1500 k followed by localized deformation stage ii at a higher stress 60 mpa and moderate temperature 1000 1150 k and finally semi brittle deformation stage iii at a very high stress 400 mpa that resulted in the formation of pseudotachylyte the stage ii deformation resulted in relatively small strains in most areas but significant localized deformation leading to dynamic recrystallization occurred the degree of which increases systematically toward the fault zone based on this observation together with the observation that the brittle deformation post dated dynamic recrystallization we conclude that the shear localization in this locality occurred as a result of ductile deformation rather than brittle deformation various mechanisms of shear localization in the ductile regime are examined and we conclude that the grain size reduction due to dynamic recrystallization at relatively high stresses and moderate temperatures is a probable mechanism of shear localization the intermediate regime between dislocation and diffusion creep where rheology is grain size sensitive yet continuous recrystallization occurs due to dislocation creep is suggested to play an important role in shear localization in the upper mantle the conditions of shear localization due to this mechanism are examined based on the laboratory data on creep and dynamic recrystallization it is shown that shear localization occurs at relatively low temperatures and or high stresses localized deformation in the continental lithosphere will occur well below the brittle ductile transition 10 km depth down to 40 80 km depth depending upon the geothermal gradient and water fugacity

grain growth in partially molten olivine aggregates grain growth rates in partially molten olivine aggregates were determined experimentally at a pressure of 1 gpa the aim of the experiments with fine grained starting material solution gelation derived fo90 olivine with a grain size of 1 m with 2 and 4 wt added basaltic glass was to determine growth rates during steady state grain growth a series of experiments was conducted at a fixed temperature of 1 350 c for durations from 2 to 700 h and a second series at fixed duration 168 h at temperatures from 1 250 to 1 450 c the resulting activation energy e 390 kj mol and growth exponent n 4 3 are comparable to melt free results nichols and mackwell 1991 the observed grain size distributions are broader than those predicted for normal grain growth or ostwald ripening and most similar to the log normal distribution the observed melt distribution is influenced by the continuous rearrangement of neighboring grains during growth the relatively large growth exponent observed in this study and by nichols and mackwell 1991 where secondary phases are present suggests that grain growth in the upper mantle is slower than predicted by karato 1989

dislocation interactions during low temperature plasticity of olivine and their impact on the evolution of lithospheric strength abstract the strength of the lithosphere is typically modelled based on constitutive equations for steady state flow however strain hardening may cause significant evolution of strength in the colder load bearing portion of the lithosphere recent rheological data from low temperature deformation experiments on olivine suggest that strain hardening occurs due to the presence of temperature independent back stresses generated by long range elastic interactions among dislocations these interpretations provided the basis for a flow law that incorporates hardening by the development of back stress here we test this dislocation interaction hypothesis by examining the microstructures of olivine samples deformed plastically at room temperature either in a deformation dia apparatus at differential stresses of 4 3 gpa or in a nanoindenter at applied contact stresses of 10 2 gpa high angular resolution electron backscatter diffraction maps reveal the presence of geometrically necessary dislocations with densities commonly above 10 14 m 2 and intragranular heterogeneities in residual stress on the order of 1 gpa in both sets of samples scanning transmission electron micrographs reveal straight dislocations aligned in slip bands and interacting with dislocations of other types that act as obstacles the resulting accumulations of dislocations in their slip planes and associated stress heterogeneities are consistent with strain hardening resulting from long range back stresses acting among dislocations and thereby support the form of the flow law for low temperature plasticity based on these observations we predict that back stresses among dislocations will impart significant mechanical anisotropy to deformed lithosphere by enhancing or reducing the effective stress therefore strain history with associated microstructural and micromechanical evolution is an important consideration for models of lithospheric strength the microstructural observations also provide new criteria for identifying the operation of back stress induced strain hardening in natural samples and therefore provide a means to test the applicability of the flow law for low temperature plasticity

frictional melting processes and products in geological materials introduction and discussion this special issue is the outcome of a theme session convened by the above authors entitled frictional melting processes and products in geologic materials held at the 1990 geological society of america annual meeting in dallas texas u s a the goals of the session were threefold to bring together researchers working on various aspects of frictional melting in rock to provide a forum for the exchange of new ideas and to encourage the production of a thematic set of papers dedicated to the topic of the eight papers presented here the first two spray swanson are primarily processand mechanism oriented the next magloughlin maddock o hara deal primarily with chemical aspects of host rock pseudotachylyte relations the last three mainly provide descriptions of naturally occurring koch and masch techmer et al and artificially produced kennedy and spray friction melts natural pseudotachylytes are described from strike slip zones magloughlin swanson thrusts and their listric counterparts koch and masch techmer et al and connecting lateral ramps o hara thus it appears that there exists an association between pseudotachylyte generation and relatively long lived largedisplacement faulting and shearing the prime objective of the meeting was to promote the integration of different approaches

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

the experimental deformation of dunite deformation experiments have been carried out on two dunites anita bay of 100 m grain size and aheim of 900 m grain size at strain rates from 10 3 to 10 6 s 1 and temperatures from 1000 c to 1300 c in a gas medium deformation apparatus at 300 mpa confining pressure most of the tests were under wet conditions defined by the presence of small amounts of water from hydrous minerals initially present constant strain rate and relaxation experiments covering ranges of flow stress down to about 70 mpa and 7 mpa respectively show that there is a change in flow law in going below about 100 mpa differential stress and that the coarser grained rock is stronger than the finer grained one power law parameters above the transition are n 4 48 0 31 and q 498 38 kj mol 1 for aheim dunite and n 3 35 0 17 and q 444 24 kj mol 1 for anita bay dunite while below the transition relaxation tests on anita bay dunite give n 2 44 0 18 and q 386 27 kj mol 1 it is concluded that there is a weakening effect of water that this effect is mainly in the grain boundaries and that grain boundary sliding is probably a significant deformation mechanism at the lower stresses under wet conditions

low temperature plasticity of olivine during high stress deformation of peridotite at lithospheric conditions an experimental study abstract deformation experiments on natural peridotite from the almklovdalen complex norway were carried out in a griggs type apparatus at temperatures of 20 300 and 600 c confining pressures of 1 0 to 2 5 gpa and strain rates of 3 10 6 to 8 10 5 s 1 the experiments yield maximum differential stresses in the range of 1 0 to 2 9 gpa falling mostly between byerlee s law and goetze s criterion and thus indicating semi brittle behaviour whereas strength of samples deformed at 20 c increases significantly with increasing confining pressure a systematic pressure dependence of strength is not obvious at 300 and 600 c the intracrystalline deformation features of the main constituent olivine were analysed by light and electron microscopic techniques sem ebsd tem deformation microstructures systematically vary with temperature but are insensitive to confining pressure samples deformed at 20 c reflect predominantly brittle failure by intragranular microcracks and shear zones microstructures from samples deformed at higher temperatures show evidence of low temperature plasticity of olivine in the form of pronounced undulatory extinction associated with high dislocation densities pile up of dislocations leads to the formation of either fracture arrays at 300 c or deformation lamellae parallel 100 and cellular structures at 600 c indicating intragranular work hardening a gradual increase in glide controlled crystal plastic deformation of olivine at increasing temperature is interpreted to be responsible for the variation in mechanical behaviour and microstructural characteristics the mechanical data and microstructural observations consistently suggest a temperature for the transition from the strength controlling dominance of brittle to crystal plastic deformation mechanisms close to 600 c the tested peridotite samples show a lower strength than quartzite samples at comparable experimental conditions possibly related to crystallographic differences of olivine and quartz the agreement between microfabrics of experimentally and naturally deformed peridotites demonstrates the importance of low temperature plasticity of olivine during high stress deformation at lithospheric conditions related in particular to seismic activity in the mantle

deformation of granitoids at low metamorphic grade i reactions and grain size reduction abstract deformation and chemical reaction in granitoids at greenschist facies conditions were investigated in samples from three shear zones two from the alps corvatsch granodiorite and aiguilles rouges granite switzerland and one from eastern australia wyangala granite when examined by light and electron microscopy all show similar features of deformation that progressed from coarse parent rock through to fine grained mylonite quartz deformed plastically while both k feldspar and plagioclase fractured and recrystallized in conjunction with chemical change which had as its end point an assemblage albite quartz white mica caal silicates k feldspar and calcic plagioclase depending on fluid chemistry from the parent rock were unstable at low metamorphic grades in the presence of aqueous fluid since ca bearing plagioclase was not stable in these environments myrmekite did not replace k feldspar in any of the rocks examined recrystallization of feldspar which should perhaps be termed neocrystallization was initiated predominantly at clast margins or along microfractures that are marked by fluid inclusions and twin offsets in old feldspar grains particularly in plagioclases dislocations exist in wall like structures that are commonly associated with voids suggesting origins in microfractures these dislocations had severely limited mobility and subgrain rotation does not appear to have contributed to recrystallization we conclude that recrystallization mainly occurred via a classical nucleation mechanism with minor contributions from twin boundary migration we argue that the 450 550 c lower limit for recrystallization of feldspar referenced throughout the literature is not applicable to the rock systems under investigation and should be discarded as a universal limit microfracturing microboudinage and reaction are seen as the prerequisites for mylonite formation from coarse grained granitic rocks at low metamorphic grades see part ii of this paper neocrystallization fluid access and nucleation appear to have accompanied or immediately followed the first deformation increments and were primarily responsible for continued grain size reduction transgranular fracture was comparatively important only at the onset of deformation since the formation of shear zones in granitoids commonly takes place under conditions of low metamorphic grade in the presence of aqueous fluids in the earth s crust breakdown reactions and nucleation recrystallization in feldspars may be important influences upon localization of deformation in granitoids

amazonian aged fluvial valley systems in a climatic microenvironment on mars melting of ice deposits on the interior of lyot crater valley networks regional drainage patterns suggesting liquid water stability at the surface are confined to early in the history of mars the noachian hesperian boundary and before prior to a major climate transition to the hyperarid cold conditions of the amazonian several later fluvial valley systems have been documented in specific hesperian and early amazonian environments and are thought to have formed due to local conditions here we describe fluvial valley systems within lyot crater that have the youngest well constrained age reported to date middle or late amazonian for systems of this size tens of km these valleys are linked to melting of near surface ice rich units extend up to 50 km in length follow topographic gradients and deposit fans the interior of lyot crater is an optimal micro environment since its low elevation leads to high surface pressure and temperature conditions at its location in the northern mid latitudes are sufficient for melting during periods of high obliquity this micro environment in lyot apparently allowed melting of surface ice and the formation of the youngest fluvial valley systems of this scale yet observed on mars

shear zone and nappe formation by thermal softening related stress and temperature evolution and application to the alps we present the results of two dimensional thermo mechanical numerical simulations of the formation of kilometre scale shear zones and tectonic nappes which are generated by thermal softening during horizontal shortening this model is thermo mechanically coupled and conserves energy by converting mechanical work into heat the results show that in homogeneous material the temperature increase in shear zones caused by viscous heating can be 100 c the characteristic width of the thermally perturbed part of the deforming zone is three times larger than the width of the corresponding shear zone because of diffusive heating of the wall rock therefore temperature variations initiated by shear heating may be difficult to identify based on petrological inferences of field data during lithospheric shortening with a bulk rate of 2 5 10 15 s 1 a nappe forms in the crust at a depth of 45 km peak temperatures in the nappe reach 600 c 100 c increase due to shear heating significant shear heating occurs only locally and is not active continuously in time along the entire crustal shear zone peak values of the maximal principal stress locally reach 2 5 gpa in the nappe peak values of temperature and maximal principal stress increase locally to 770 c 300 c increase due to shear heating and 3 8 gpa respectively if the applied bulk shortening rate is 5 10 15 s 1 and if higher effective viscosities for the crust are applied in the model lithosphere high differential stresses 500 mpa and significant stress deviations from the lithostatic pressure 50 occur only locally and are transient the highest average effective viscosity of the model lithosphere is 2 1022 pa s which is in agreement with the estimates for the continental lithosphere a characteristic feature of the presented dynamic nappe model is a significant stress decrease and cooling within a hundred thousand to 2 million years these rates of decompression and cooling are discussed in the context of microstructural and metamorphic observations the results also show that differential stresses in mature shear zones can be several hundred mpa smaller than at the onset of shear zone formation which suggests that estimates based on microstructural analysis of rocks in ductile shear zones provide minimum values the presented model could explain the formation of coherent alpine nappes with inferred high pressure 1 5 2 5 gpa and ultrahigh pressure 3 gpa conditions in 40 60 km depth at the base of an orogenic wedge

microearthquake evidence for extension across the kane transform fault we report the results of a microearthquake experiment conducted in 1987 in the active transform portion of the kane fracture zone the kane is a slow slipping 25 mm yr large offset 150 km transform delineated by a pronounced transform valley the experiment site lies in the eastern half of the transform 40 km west of the eastern ridge transform intersection in a region of marked transform parallel topography the network consisted of 12 ocean bottom hydrophones and seismometers and extended for 25 km along the transform and 10 km in width spanning the transform valley and a narrow median ridge to the north hypocentral parameters were determined for 86 earthquakes epicenters within the network define a narrow transform parallel trend along the steep southern wall of the transform valley maximum focal depths increase from 6 to 9 km from east to west across the network significant activity was also detected outside the transform tectonized zone to the southeast this activity may be associated with the inside corner of the ridge transform intersection while to the north about 20 events occurred in 7 m y old crust in a region of ridge parallel bathymetry six focal mechanisms were obtained from p wave first motions for events within the transform valley the best constrained solutions for four earthquakes within the network show normal faulting on fault planes subparallel to the trend of the transform all the mechanisms indicate a tension axis perpendicular to the trend of the transform these results together with a significant historical record of large earthquakes near the experiment site lead us to conclude that the principal transform displacement zone was inactive during our experiment and that the activity we recorded is the result of extension in the adjacent lithosphere the observed focal mechanisms and the inference that the axis of least compressive stress is approximately perpendicular to the transform provide direct evidence that the transform fault is mechanically weak relative to the surrounding lithosphere potential sources of extension across the transform include thermal stress in the young oceanic lithosphere topographic loading a small component of plate divergence normal to the transform and northward motion of the asthenosphere relative to the surface plate

vertical variations of mercury in pennsylvanian coal beds from indiana abstract twenty three in situ coal sections were sampled in indiana mines to investigate mercury hg concentrations with a special reference to in seam vertical variations in hg distribution in addition to raw coal corresponding float fractions were also analyzed to evaluate the effectiveness of hg reduction during conventional pre combustion washing hg content in indiana coals in the locations studied averages 0 11 mg kg which after recalculation into equal energy units corresponds to 9 2 lb hg 1012 btu 39 5 kg hg 1010 mj hg content is reduced after washing to 0 07 mg kg on average corresponding to an equal energy hg loading of 5 2 lb hg 1012 btu 22 3 kg hg 1010 mj a large portion of hg appears to be associated with pyrite as in the springfield coal member of the petersburg formation pennsylvanian in the samples from the danville coal member of the dugger formation pennsylvanian hg is associated to a larger extent with organic matter large differences in hg content may exist between different portions of a coal bed in a single location indicating that very careful complete sampling is necessary to get representative hg concentration data

semi brittle behavior of wet olivine aggregates the role of aqueous fluid in faulting at upper mantle pressures the role of aqueous fluid in fracturing in subducting slabs was investigated through a series of deformation experiments on dunite that was undersaturated i e fluid free or saturated with water i e aqueous fluid bearing at pressures of 1 0 1 8 gpa and temperatures of 670 1250 k corresponding to the conditions of the shallower regions of the double seismic zone in slabs in situ x ray diffraction radiography and acoustic emissions aes monitoring demonstrated that semi brittle flow associated with aes was dominant and the creep failure strength of dunite was insensitive to the dissolved water content in olivine in contrast aqueous fluid drastically decreased the creep failure strength of dunite up to 1 gpa of weakening over a wide range of temperatures in the semi brittle regime weakening of the dunite by the aqueous fluid resulted in the reduction of the number of ae events i e suppression of microcracking and shortening of time to failure the ae hypocenters were located at the margin of the deforming sample while the interior of the faulted sample was aseismic i e aseismic semi brittle flow under water saturated conditions a faulting slip rate of 10 3 to 10 4 s 1 associated with a large drop of stress 0 5 to 1 gpa and or pressure p 0 5 gpa was dominant in fluid free dunite while a slow faulting slip rate 8 10 5 s 1 without any stress pressure drop was common in water saturated dunite aseismic semi brittle flow may mimic silent ductile flow under water saturated conditions in subducting slabs

a reassessment of the rupture characteristics of oceanic transform earthquakes 1 we investigate the long period source spectra of oceanic transform earthquakes and find that previously proposed slow rupture components can be explained as artifacts generated by the modeling procedure we use low frequency 20 mhz rayleigh and love waves to calculate the amplitude spectra of five earthquakes on the romanche and chain transform faults in the equatorial mid atlantic ocean we find that errors and approximations in the centroid depth focal mechanism and earth structure at the source have significant effects on the shape of the source spectra if global catalog values and an average crustal model are assumed the spectra exhibit apparent anomalous energy at long periods which has previously been interpreted as a result of slow rupture we recalculate the source spectra using precise independently determined depths and moment tensors and a more realistic oceanic crustal structure in the source region the resulting source spectra are flat at long periods with no indication of anomalous long period energy our results imply that oceanic transform earthquakes do not commonly have detectable slow rupture components

pseudotachylytes in the balmuccia peridotite ivrea zone as markers of the exhumation of the southern alpine continental crust terra nova 22 70 77 2010 abstract two types of pseudotachylytes are observed in the balmuccia peridotite of the ivrea zone southern alps italy a type pseudotachylytes correspond to previously studied occurrences and were formed under temperatures comprising between 550 and 900 c and pressures comprising between 0 6 and 1 2 gpa these conditions were met in the ivrea crust between 350 and 270 ma suggesting that a type pseudotachylytes were formed during variscan tectonics or permian transtensional tectonics b type pseudotachylytes post date a type pseudotachylytes textural characteristics of b type veins suggest a formation in the upper continental crust at depths of about 5 10 km or less petrological constraints indicate that the exhumation of the ivrea crust at such shallow depths was achieved later than c 70 ma thus providing a maximum age of 70 ma for b type veins pseudotachylytes appear as markers of the poly orogenic evolution of the alpine belt

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

dislocation recovery in olivine under deep upper mantle conditions implications for creep and diffusion the kinetics of dislocation recovery in olivine have been studied up to 10 gpa and 1500 c using a multianvil apparatus dislocations in the 010 100 slip system were generated by high temperature deformation at room pressure and then the specimens were statically annealed at high pressures and temperatures the rate of dislocation recovery was measured as a function of pressure at controlled oxygen fugacity and oxide activity under anhydrous conditions microstructural observations indicate that dislocation recovery occurs through the climb of edge dislocations and hence is controlled by diffusion a small activation volume v 6 1 cm3 mol was obtained which is interpreted to be due to an interstitial mechanism of diffusion for the relevant transport limiting species oxygen or silicon combined with the previous observation of a small activation volume for mg fe diffusion we conclude that ionic diffusion in the olivine lattice in general has small activation volumes and therefore will not vary significantly with depth in the deep portions of the upper mantle we have estimated the relative importance of dislocation and diffusion creep in the upper mantle using the present result and the published results on creep parameters such as stress exponents grain size exponents and activation volume for dislocation creep the results suggest that the dominant creep mechanism may change from dislocation creep in the shallow upper mantle to diffusion creep in the deep upper mantle

a servohydraulically controlled deformation apparatus for rock deformation under conditions of ultra high pressure metamorphism a deformation apparatus has been developed to study the mechanical behaviour of high pressure and ultra high pressure metamorphic rocks it is based on the conventional griggs design and the molten salt cell concept introduced by h e green ii both the axial loading and the confining pressure are servohydraulically controlled alternatively a self made multilayer pressure vessel or a commercial stripwound construction are used the pressure cell is improved with respect to systems described previously by the use of different salt mixtures with low eutectic temperatures by a mechanically stable arrangement of the thermocouples and by an optimization of the frictional characteristics of the axial loading system the apparatus has been successfully used in deformation experiments on cylindrical aragonite and coesite samples 3 to 4 mm in diameter and 6 to 10 mm in length at confining pressures up to 3 7 gpa and temperatures up to 1170 c

simulating coseismic deformation of quartz in the middle crust and fabric evolution during postseismic stress relaxation an experimental study abstract non steady state deformation and annealing experiments on vein quartz are designed to simulate earthquake driven episodic deformation in the middle crust three types of experiments were carried out using a modified griggs type solid medium deformation apparatus all three start with high stress deformation at a temperature of 400 c and a constant strain rate of 10 4 s 1 type a some are followed by annealing in the stability field of quartz for 14 15 h at zero nominal differential stress and temperatures of 800 1000 c type a b or by annealing for 15 h at 900 c and at a residual stress type a c the quartz samples reveal a very high strength 2 gpa at a few percent of permanent strain the microstructures after short term high stress deformation type a record localized brittle and plastic deformation statisc annealing type a b results in recrystallisation restricted to the highly damaged zones the new grains aligned in strings and without crystallographic preferred orientation indicate nucleation and growth annealing at non hydrostatic conditions type a c results in shear zones that also develop from deformation bands or cracks that formed during the preceding high stress deformation in this case however the recrystallised zone is several grain diameters wide the grains are elongate and a marked crystallographic preferred orientation indicates flow by dislocation creep with dynamic recrystallisation quartz microstructures identical to those produced in type a b experiments are observed in cores recovered from long valley exploratory well in the quaternary long valley caldera california with considerable seismic activity the experiments demonstrate the behaviour of quartz at coseismic loading type a and subsequent static annealing type a b or creep at decaying stress type a c in the middle crust the experimentally produced microfabrics allow to identify similar processes and conditions in exhumed rocks

abyssal peridotite mylonites implications for grain size sensitive flow and strain localization in the oceanic lithosphere abstract microstructures preserved in abyssal peridotites dredged from the oceans record several different physical regimes of deformation fabrics associated with deformation processes at slow spreading mid ocean ridges form two major classes of abyssal peridotites based on detailed microstructural observations the most abundant class are medium to coarse grained tectonites with microstructures that reflect deformation processes during mantle upwelling and emplacement to the base of the lithosphere these tectonites give geothermometric temperatures of 755 c or higher interpreted to represent lower temperature limits for diffusive exchange in coarse grained abyssal peridotites during cooling this conclusion is consistent with flow laws for olivine at these temperatures the second class of abyssal peridotites previously largely undescribed for the mid ocean ridge environment include fine grained mylonites associated with faulting and shear zones that develop during extension and cooling of the oceanic lithosphere when the brittle plastic transition extends into mantle rocks these mylonites give temperatures of 600 c which we suggest represent a lower temperature limit for plastic deformation reduced grain size in mylonites allows for diffusive exchange to continue to these low temperatures relict augen in the mylonitic samples preserve equilibration temperatures similar to those exhibited by the coarse grained tectonites based on flow laws for olivine we suggest that deformation in some fine grained mylonites occurred by diffusion creep down to 600 c rheological data for olivine indicate that dislocation creep is not likely to occur at this temperature we conclude that a reduction in grain size by cataclasis or dynamic recrystallization resulted in a transition in deformation mechanisms from dislocation to diffusion creep during uplift and or cooling the observation of these fine grained mylonites indicates that shear zones that extend into the upper mantle will be weaker than expected if deformation was accommodated by brittle processes or dislocation creep weak faults may promote the development of long lived detachments in the upper mantle this inference supports mid ocean ridge tectonic models that suggest that ultramafic rocks exposed at the inside corners of ridge axis discontinuities are exhumed along long lived detachment faults

elevated stresses and creep rates beneath the brittle ductile transition caused by seismic faulting in the upper crust 1 large faults in the brittle upper continental crust are envisaged to terminate near the brittle ductile transition zone at a depth that is a complex function of material properties temperature stress and strain rate here we investigate the effect of coseismic displacement along a fault in the upper crust on the adjacent middle and lower crust using a two dimensional numerical model of faulting within an elastic frictional viscous rheology results demonstrate that a major displacement event can cause a dramatic increase in differential stress and creep strain rates in the crust near and below the lower termination of the fault a transient deflection of the brittle ductile transition zone and a zone of elevated strain rates down to the base of the crust the perturbation decays over timescales consistent with postseismic relaxation the stresses and strain rates predicted in such models are in accord with those inferred from microstructural damage analysis of exhumed metamorphic rocks indicating short term deformation at decaying stress and strain rates

dynamic recrystallization near the brittle plastic transition in naturally and experimentally deformed quartz aggregates the electron backscattering diffraction technique ebsd was used to analyze bulging recrystallization microstructures from naturally and experimentally deformed quartz aggregates both of which are characterized by porphyroclasts with finely serrated grain boundaries and grain boundary bulges set in a matrix of very fine recrystallized grains for the tonale mylonites we investigated a temperature range of 300 380 c 0 25 gpa confining pressure a flow stress range of 0 1 0 2 gpa and a strain rate of 10 13 s 1 were estimated experimental samples of black hills quartzite were analyzed which had been deformed in axial compression at 700 c 1 2 1 5 gpa confining pressure a flow stress of 0 3 0 4 gpa a strain rate of 10 6 s 1 and to 44 to 73 axial shortening using orientation imaging we investigated the dynamic recrystallization microstructures and discuss which processes may contribute to their development our results suggest that several deformation processes are important for the dismantling of the porphyroclasts and the formation of recrystallized grains grain boundary bulges are not only formed by local grain boundary migration but they also display a lattice misorientation indicative of subgrain rotation dynamic recrystallization affects especially the rims of host porphyroclasts with a hard orientation i e with an orientation unsuitable for easy basal slip in addition dauphine twins within porphyroclasts are preferred sites for recrystallization we interpret large misorientation angles in the experimental samples which increase with increasing strain as formed by the activity of fluid assisted grain boundary sliding

kick and cook experiments on peridotite simulating coseismic deformation and post seismic creep terra nova 24 62 69 2012 abstract experiments comprising deformation at 600 c and annealing at 700 1000 c were performed on natural peridotite to examine the microfabric evolution at conditions corresponding to those prevailing just below the seismogenic zone in suboceanic upper mantle the found olivine microstructures indicate that deformation occurs by low temperature plasticity at low and high annealing temperatures zones of high strain are replaced by subgrains and recrystallized grains respectively the microstructures after annealing at 1000 c resemble core and mantle structures from shear zone peridotites often interpreted as imprint of steady state dislocation creep our study shows that such structures can form by a sequence of low temperature plasticity at high stress and subsequent recrystallization at low stress as corresponding to coseismic deformation and post seismic creep in this case palaeopiezometers are not applicable and a local cpo in aggregates of new grains does not necessarily indicate the activated glide system

how vein sealing boosts fracture widening rates the buckling enhanced aperture growth mechanism for syn tectonic veins abstract the paper introduces the mechanism of buckling enhanced aperture growth for syn tectonic veins that formed in simple shear dominated kinematic frameworks in the middle or lower crust apart from the well understood concepts of fracture widening driven by effective tensile stresses buckling enhanced fracture aperture growth relates widening to active outward buckling of more viscous incipient cement layers precipitated as hydrothermal minerals for the pore fluid on the walls of juvenile syn tectonic veins driven by fracture parallel compressive creep strain in the host rocks thus the mechanism proposed here follows similar principles as tectonic folding although important differences exist inspired by the structural record of low aspect ratio veins exposed in hp lt metamorphic rocks cropping out on south evia island greece generic numerical models are calculated to study development of buckling instabilities in such incompletely cemented veins and their impact on aperture growth rates the models indicate 1 that aperture growth rates increase with increasing viscosity contrast between the host rocks and the cement layers 2 an increase in the thickness of the cement layers cause acceleration of aperture growth 3 that support of restraining forces at the vein tips offered by the host rocks against buckling of the cement layers cause fully compressive states of stress ahead the fracture tips and 4 that fracture aperture growth is possible against fully compressive fracture normal stresses the buckling enhanced vein aperture growth mechanism yields important implications for the maintenance and decay of fracture bound permeability and for the mechanical state of the middle and lower crust in seismically active regions

amphibolite facies pseudotachylytes in premosello metagabbro and felsic mylonites ivrea zone italy abstract pseudotachylytes solidified friction induced melts produced during seismic slip that formed under high temperature ambient conditions might retain information on earthquake mechanics beneath the brittle ductile elasto frictional viscous plastic transition in the crust thin sharply bounded pseudotachylytes and ultramylonites occur together inside the granulite facies premosello metagabbro and inside country felsic mylonites of the ivrea zone italian southern alps petrographic and microstructural data indicate that the studied pseudotachylytes and ultramylonites are coeval and developed under amphibolite facies 500 650 c metamorphic conditions the occurrence of foliated pseudotachylytes and of local remnants of cataclastic microstructures within ultramylonites suggests that pseudotachylytes might have represented planar heterogeneities for nucleation of ultramylonites the association of coeval pseudotachylyte and mylonites is interpreted as caused by cyclic stages of coseismic high strain rate transients related to downward propagation of seismic ruptures from the upper brittle crust followed by post seismic stress relaxation by ductile flow

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