on the rheology of partially molten synthetic granite abstract in an attempt to obviate some of the difficulties inherent in using natural rocks to study experimentally the rheology of partially molten granitic rocks we report some preliminary experiments using hot pressed synthetic aggregates with quartz as the solid phase and a quartz albite glass to form the melt phase undrained constant displacement rate and stress relaxation experiments were performed at 1100 c and 300 mpa total confining pressure using nominally 16 and 28 wt anhydrous melt and at 980 c with 12 wt of water saturated melt strain rate varies non linearly with stress corresponding to a stress exponent of 3 6 for the anhydrous melt tests and 1 9 with the wet melt strain rate is enhanced about 160 fold through doubling the melt fraction from 16 wt intracrystalline plasticity appears to contribute insignificantly to the observed deformation and the preservation of relatively equant grains implies a granular flow process microcracking occurs during isostatic hot pressing and also contributes to subsequent non isostatic deformation at high stresses and strain rates models of flow of partially molten rock that assume diffusive transfer accommodation of granular flow predict linear viscosity but only at lower strain rates than accessed experimentally it is suggested that the non linear granular flow observed in these experiments involved the episodic unsticking of sintered contacts between grains by a rate dependent cracking and or dislocation accommodation process

a pressure solution creep law for quartz from indentation experiments 1 indenter experiments have been performed on quartz crystals in order to establish a pressure solution creep law relevant at upper to middle crustal conditions this deformation mechanism contributes to earth s crust geodynamics controlling processes as different as fault permeability strength and stress evolution during interseismic periods or mechanochemical differentiation during diagenesis and metamorphism indenter experiments have been performed at 350 c and 20 120 mpa during months with differential stress varying from 25 to 350 mpa several experimental parameters were varied nature of quartz synthetic or natural nature of fluid manner in which the solid solution solid interface was filled and orientation of the indented surfaces versus quartz crystallographic c axis significant strain rates could only be obtained when using high solubility solutions naoh 1 mol l 1 displacement rates of the indenter were found activated by differential stress with exponential dependence as theoretically predicted the mean thickness of the trapped fluid phase below the indenter was estimated in the range 2 10 nm moreover the development of this trapped fluid phase was relatively fast and allowed fluid penetration into previously dry contact regions by marginal dissolution the indenter displacement rate was driven by differential stress and its kinetics was controlled by diffusion along the trapped fluid and the development of a morphological roughness along the interface conversely marginal strain energy driven dissolution was observed around the indenter and its kinetics was controlled by free surface reaction these experimental results are applied to model the interactions between pressure solution and brittle processes in fault zones providing characteristic time scales for postseismic transitory creep and sealing processes in quartz rich rocks

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

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

deformation mechanisms in a high temperature quartz feldspar mylonite evidence for superplastic flow in the lower continental crust abstract microstructures and crystallographic preferred orientations in a fine grained banded quartz feldspar mylonite were studied by optical microscopy sem and tem mylonite formation occurred in retrograde amphibolite facies metamorphism interpretation of the microstructures in terms of deformation mechanisms provides evidence for millimetre scale partitioning of crystal plasticity and superplasticity strain incompatibilities during grain sliding in the superplastic quartz feldspar bands are mainly accommodated by boundary diffusion of potassic feldspar the rate of which probably controls the rate of superplastic deformation there is evidence for equal flow stress levels in the superplastic and crystal plastic domains in this case mechanism partitioning results in strain rate partitioning fast deformation in the superplastic bands therefore dominates flow and deformation is probably best modelled by a superplastic law if this deformational behaviour is typical shearing in mylonite zones of the lower continental crust may proceed at exceptionally high rates for a given differential stress or at low differential stresses in case of fixed strain rates

frictional viscous flow of phyllosilicate bearing fault rock microphysical model and implications for crustal strength profiles it is widely believed that around the brittle ductile transition crustal faults can be significantly weaker than predicted by conventional two mechanism brittle ductile strength envelopes factors contributing to this weakness include the polyphase nature of natural rocks foliation development and the action of fluid assisted processes such as pressure solution recently ring shear experiments using halite kaolinite mixtures as an analogue for phyllosilicaterich rocks for the first time showed frictional viscous behavior i e both normal stress and strain rate sensitive behavior involving the combined effects of pressure solution and phyllosilicates this behavior was accompanied by the development of a mylonitic microstructure a quantitative assessment of the implications of this for the strength of natural faults has hitherto been hampered by the absence of a microphysical model in this paper a microphysical model for shear deformation of foliated phyllosilicate bearing fault rock by pressure solution accommodated sliding along phyllosilicate foliae is developed the model predicts purely frictional behavior at low and high shear strain rates and frictional viscous behavior at intermediate shear strain rates the mechanical data on wet halite kaolinite gouge compare favorably with the model when applied to crustal materials the model predicts major weakening with respect to conventional brittle ductile strength envelopes in particular around the brittle ductile transition the predicted strength profiles suggest that in numerical models of crustal deformation the strength of high strain regions could be approximated by an apparent friction coefficient of 0 25 0 35 down to depths of 15 20 km

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

compaction creep of simulated anhydrite fault gouge by pressure solution theory v experiments and implications for fault sealing the sealing and healing behaviour of faults filled with anhydrite gouge by processes such as pressure solution is of interest in relation both to the integrity of faults cutting geological storage systems sealed by anhydrite caprocks and to seismic events that may nucleate in anhydrite bearing sequences such as those present in the seismogenic zone beneath the apennines we have developed a detailed series of kinetic models for pressure solution in anhydrite fault gouge allowing for dissolution diffusion and precipitation control to estimate the time scale on which such sealing and healing effects occur we compare the models obtained with previously reported experimental data on compaction creep rates in simulated anhydrite fault gouge tested under wet upper crustal conditions the results confirm earlier indications that compaction under these conditions likely occurs by diffusion controlled pressure solution applying our most rigorous model for diffusion controlled pressure solution constrained by the fit to the experimental data we infer that anhydrite fault sealing will occur in a few decades at most which is rapid compared with both co2 storage time scales and with the recurrence interval for seismicity in the apennines

experimental deformation of partially molten westerly granite under fluid absent conditions with implications for the extraction of granitic magmas the mechanical behavior of partially molten westerly granite was investigated in the temperature range 800 1100 c 250 mpa confining pressure by means of constant strain rate creep and stress relaxation tests the only water in the samples came from the breakdown of hydrous phases biotite minor chlorite and muscovite and alteration products of feldspars thus the amount of melt was controlled by the test temperature and ranged from 3 at 800 c to 50 at 1100 c over that temperature range strength decreased from 500 mpa to less than 1 mpa and a preliminary constitutive flow law for the partially molten rock was obtained to allow extrapolation to low strain rates the comparative viscosity of the melt alone was estimated at 950 and 1000 c from the distance it could be made to penetrate into a porous sand under a known pressure gradient under all conditions deformation of the matrix of solid grains was by brittle fracture only samples containing up to 10 vol melt failed with the formation of a shear fault zone at higher melt fractions melt filled pores collapsed by shear enhanced compaction squeezing the melt into axial cracks above 40 vol melt unfractured solid grains were carried about passively in the flowing liquid there was no evidence of a rheologically critical melt percentage in this system by analogy with the uniaxial compaction of water saturated soils a simple model is erected to describe a two stage process for the extraction of granitic melts from their protoliths with the aid of nonhydrostatic stress shear enhanced compaction is inferred to drive melt into a network of melt filled veins whereupon porous flow through the high permeability vein network allows rapid drainage of melt to higher crustal levels

evolution of mechanical state in carrara marble during deformation at 400 to 700 c the evolution of the mechanical state of carrara marble during deformation up to strains of 0 2 at 400 to 700 c and strain rates of 10 4 to 10 6 s 1 has been investigated by applying hart s 1976 state variable description of inelastic deformation to the results of isothermal constant displacement rate experiments performed at these conditions at t 600 c the magnitude of the mechanical state increases over the full range of strains investigated even when the deformation occurs at almost constant stress and strain rate although the rate of increase at given strain decreases with increasing temperature and decreasing strain rate the deformation microstructures suggest that under these conditions mechanical twinning controls the deformation behavior with twin boundary migration becoming significant at t 450 c as a dynamic recovery process twin boundary migration is as efficient as strain induced grain boundary migration at t 600 c the magnitude of the mechanical states attained is difficult to evaluate reliably using the method employed but the continuing evolution of the deformation microstructures suggests that a steady state is not attained by strains of 0 2 at these conditions mechanical twinning is replaced by other intracrystalline slip processes and by grain boundary bulge nucleated dynamic recrystallization hart s description of inelastic deformation as previously evaluated for carrara marble at t 400 c may be extrapolated to 600 c provided the mechanical state evolution equation is modified to accommodate twin boundary migration and the influence of annealing type recovery processes extrapolation to higher temperatures may require modifications to the equation of state in that description although at the conditions investigated steady state appears to be a poor approximation steady state flow laws of exponential and power law type do provide a good empirical description of the data however the observation that steady state is not attained indicates that any microphysical interpretation of the magnitude of the material parameters in such steady state flow law fits should be treated with caution

kinematic inversion of physically plausible earthquake source models obtained from dynamic rupture simulations one approach to investigate earthquake source processes is to produce kinematic source models from inversion of seismic records and geodetic data the setup of the inversion requires a variety of assumptions and constraints to restrict the range of possible models here we evaluate to what extent physically plausible earth quake scenarios are reliably restituted in spite of these restrictions we study which characteristics of ruptures such as rupture velocity slip distribution stress drop rise time and slip function can be reliably determined from the inversion of near field seismic and geodetic data using spontaneous dynamic rupture simulations we gen erate five earthquake scenarios each of which has different characteristics of the source process then we conduct a blind test by modeling the synthetic near source data using a standard inversion scheme that optimizes the fit to the observations while searching for solutions with minimum roughness the inversion procedure assumes a rupture front propagating away from the hypocenter with variable rupture velocity and a simple cosine slip time function our results show that overall slip distribution and stress drop are reasonably well determined even for input models with relatively complex histories such as a subshear rupture transitioning to supershear speeds depth averaged rupture velocities are also reasonably well resolved although their estimate progressively deteriorates away from the hypocenter the local rise time and slip function are not well resolved but there is some sensitivity to the rupture pulse width which can be used to differentiate between pulse like and crack like ruptures our test for understanding the inaccuracies in green s functions shows that random 3d perturbations of 5 standard deviation do not lead to significant degradation of the estimation of earthquake source parameters as remedies to the current limitations we propose smoothing slip function parameters and using more complicated inversion schemes only if data necessitates them online material figures showing snapshots of forward and inverse modeling of rupture l curves slip models and waveform fits

the brittle plastic transition and the depth of seismic faulting a simple rheological model of shearing of the lithosphere that has gained wide acceptance is a two layer model with an upper brittle zone in which deformation takes place by frictional sliding on discrete fault surfaces and a lower plastic zone in which deformation takes place by bulk plastic flow the two are separated by an abrupt brittle plastic transition which is assumed to be indicated by the lower limit of seismicity experimental studies however as well as the deformation structures of mylonites indicate that a broad transitional field of semi brittle behavior lies between these extremes this is a field of mixed mode deformation with a strength that can be expected to be considerably higher than that predicted from the extrapolation of high temperature flow laws for quartzofeldspathic rocks the semi brittle field lies between t1 the onset of quartz plasticity at about 300 c and t2 feldspar plasticity at about 450 c a model is presented in which the transition t1 does not correspond to a transition to bulk flow but to a change from unstable velocity weakening friction to stable velocity strengthening friction t1 thus marks the depth limit of earthquake nucleation but large earthquakes can propagate to a greater depth t3 t3 t2 which corresponds to the lower limit of dynamic frictional behavior in the semi brittle field and approximately to the peak in strength the zone between t1 and t3 is one of alternating behavior with flow occurring m the interseismic period and with co seismic dynamic slip occurring during large earthquakes this zone is characterized by mylonites interlaced by pseudotachylytes and other signs of dynamic faulting the transition t1 is also marked by a change in the generation mechanism of fault rocks from abrasive wear above which produces cataclastites to adhesive wear below which is proposed as an important generation mechanism of mylonites in the upper part of the semi brittle field

kinetics of pressure solution at halite silica interfaces and intergranular clay films pressure solution is widely regarded as a potentially important deformation mechanism along crustal faults and during aliagenesis yet the mechanisms and kinetics of this process remain highly controversial to better understand the fundamental factors controwing the rates of pressure solution at the grain to grain scale we conducted experiments in which convex halite lenses were pressed against fiats of fused silica in brine fluid pressures were maintained at 0 1 mpa temperatures and mean contact normal stresses ranged from 8 3 o to 90 2oc and 0 5 to 13 5 mpa respectively the geometry and growth rate of the contact spot between the two lenses and the rate at which the lenses approached one another convergence were monitored using reflected light interferometry and transmitted light photomicrography convergence occurred when halite and silica lenses were pressed together in brine halite silica experiments no undercutting was observed and dry control experiments indicated negligible dislocation creep convergence rates in experiments at 50 2oc ranged from 0 01 to 0 05 m d depending on mean normal stress and contact spot radius the data are consistent with intergranular pressure solution ips rate limited by diffusion through an intergranular film with a very high diffusion coefficient 10 5 10 7 cm2 s the data further suggest that the diffusion coefficient and or thickness of this film increases with decreasing normal stresses at least for normal stresses less than about 4 mpa as no island channel boundary structures were observed we propose that this film consists of a continuous layer of strongly adsorbed i e structured water that is maintained between the halite and silica lenses during deformation convergence rates in similar experiments conducted at a constant load of 0 11 n but at 8 3 o 50 2 o and 90 2oc were approximately constant at a given normal stress and contact spot size the cause of this temperature insensitivity is unknown but might result from changes in interphase boundary structure or thickness with increasing temperature that are sufficient to offset the expected thermal activation an experiment was also conducted in which a halite lens was pressed against a fused silica fiat coated with an 0 8 m thick film of na montmorillonite in brine this clay film produced an approximately fivefold increase in convergence rates over those observed in a halite silica experiment conducted without clay at the same load and temperature the strong sensitivity of ips rates both to contact spot radius and to the presence of second phases along grain boundaries suggests that fine grained clay rich fault gouges and multiphase granular aggregates should be particularly susceptible to pressure solution creep in the middle to upper crust

dehydration induced damage and deformation in gypsum and implications for subduction zone processes 1 experimental heating tests were performed on volterra gypsum to study the micromechanical consequences of the dehydration reaction the experimental conditions were drained at 5 mpa fluid pressure and confining pressures ranging from 15 to 55 mpa one test was performed with a constantly applied differential stress of 30 mpa the reaction is marked by 1 a porosity increase and homogeneous compaction 2 a swarm of acoustic emissions 3 a large decrease in p and s wave velocities and 4 a decrease in vp vs ratio wave velocity data are interpreted in terms of crack density and pore aspect ratio which modeling pores as spheroids is estimated at around 0 05 crack like spheroid complementary tests performed in an environmental scanning electron microscope indicate that cracks first form inside the gypsum grains and are oriented preferentially along the crystal structure of gypsum most of the visible porosity appears at later stages when grains shrink and grain boundaries open extrapolation of our data to serpentinites in subduction zones suggest that the signature of dehydrating rocks in seismic tomography could be a low apparent poisson s ratio although this interpretation may be masked by anisotropy development due to preexisting crystal preferred orientation and or deformation induced cracking the large compaction and the absence of strain localization in the deformation test suggests that dehydrating rocks maybe seen as soft inclusions and could thus induce ruptures in the surrounding nonreacting rocks

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

the role of transiently fine grained reaction products in syntectonic metamorphism natural and experimental examples it is proposed that one of the most important mechanisms of interrelationship between deformation and metamorphism is the facilitation of one of several grain size sensitive deformation mechanisms resulting from the formation of fine grained products of a metamorphic reaction during prograde metamorphism such effects are likely to be transient because grain coarsening and textural equilibration are likely in response to rising temperature conditions thus deformation mechanisms are often difficult to infer from such naturally deformed rocks in localized shear zones exhibiting retrogressive metamorphism evidence of enhanced deformability by such mechanisms is most likely to be preserved because cooling conditions inhibit grain growth and both deformed and undeformed rocks are likely to be preserved an experimental study has been made of the effects of deformation on serpentinite under conditions of progressive dehydration but with controlled pore pressure a marked weakening near linear viscous rheol

brittle ductile deformation in the glarus thrust lochseiten lk calc mylonite the glarus thrust accommodated at least 30 km of northward displacement strongly localized within a 1 m layer of lochseiten lk calc mylonite this layer displays veins in various states of plastic deformation and a wildly refolded foliated gouge texture lattice and shape preferred orientations are observed within the fine grained recrystallized matrix these features indicate the alternate activity of brittle and ductile deformation mechanisms in contrast to the classical view that grain boundary sliding superplasticity is the dominant deformation mechanism it is advocated that fluids derived from the footwall and expelled along the thrust are responsible for hydrofracturing and cataclastic deformation in periods between fracture events deformation was ductile in this new interpretation a substantial amount of the total thrust displacement was accommodated by numerous short lived and strongly localized fracture events at the base of the verrucano thrust sheet rather than a permanently weak decollement lithology

quantifying creep behaviour of clay bearing rocks below the critical stress state for rapid failure mam tor landslide derbyshire england abstract the mam tor landslide derbyshire england slips downhill at up to 0 5 m a 1 and offers an excellent opportunity to study time dependent creep in mudstones under in situ conditions annual surveys show that creep rates increase following heavy rainfall but to establish detailed correlation between creep and pore water pressure required spatially and temporally higher resolution monitoring we acquired 3 years of such data at 3 h intervals from wire creep meters borehole piezometers and rainfall monitoring showing that creep is strongly focused into the winter months and correlates well with pore water pressure summer grass and fern growth strongly influence rainfall infiltration recycling much of the rainfall back to the atmosphere explaining the seasonal variability of creep the dependence of creep rate on pore pressure can be recast as a rheological model relating shear strain rate to shear stress using the friction angle for the creeping mudstone to link effective normal stress to shear stress the creep is very non linear following a power law with a stress exponent of 48 this flow law may apply also to shear failure in poorly consolidated mudrocks under high pore pressure conditions for example within some landslides subglacial sediments tectonic fault zones tectonic melanges and accretionary complexes

large scale folding in the upper part of the ivrea verbano zone nw italy new geological mapping has led to a new interpretation of the large scale superimposed folding in the upper part of the ivrea verbano zone italian alps the region is widely held to represent an upended section through lower continental crust of northern italy the dominant fold structure extending some 40 km along strike is the massone tight to isoclinal antiform with a hinge line strongly curved through 115 this folds pre existing large scale folds that formed during regional migmatization probably during the hercynian orogeny to form a type 2 interference geometry the region then suffered post orogenic mafic magmatic underplating and other magmatism accompanied by crustal stretching with contact metamorphism and migmatization causing the imposition of the final pattern of metamorphic isograds the ivrea verbano zone was brought into contact with the overlying metamorphic rocks of the serie dei laghi on a major shear zone sub solidus stretching continued though displacements on low angle high temperature shear zones most of the ivrea verbano zone was finally tilted to the vertical and emplaced into its present position after the mesozoic era and probably during alpine orogenesis forming the vertical limb of a crustal scale double kink

joint hypocenter velocity inversion for the eastern tennessee seismic zone a joint hypocenter velocity inversion for the eastern tennessee seismic zone etsz has resolved velocity features in basement rock below detached appalachian thrust sheets p and s wave arrival times from 492 earthquakes have been inverted for one 1 d and three dimensional 3 d velocity models to midcrustal depths the 3 d p and s wave velocity solutions are computed independly and are very similar in relation to the 1 d model velocity anomalies range from 8 to 16 in the first layer upper 5 km and between 7 in deeper layers prominent velocity anomalies parallel the seismic zone and are consistent from layer to layer the most persistent anomaly is a low velocity region that borders the seismic zone to the northwest and is flanked on either side by regions of anomalously high velocity the new york alabama ny al magnetic lineament coincides with or lies close to the southeast boundary of the prominent velocity low in both the p and s wave velocity images the spatial coincidence between velocity gravity and magnetic gradients suggests that major discontinuities are present in the basement relocation in the 3 d velocity model reduced the number of very deep earthquakes below 20 km and further accentuated differences in seismogenic properties on either side of the ny al lineament after relocation most earthquakes occur in a vertically bounded region roughly 30 km wide extending from 4 to 22 km in depth most earthquakes occur in regions characterized by either average velocity or small velocity anomalies

frictional strengths of talc serpentine and talc quartz mixtures 1 talc is a constituent of faults in a variety of settings and it may be an effective weakening agent depending on its abundance and distribution within a fault we conducted frictional strength experiments under hydrothermal conditions to determine the effect of talc on the strengths of synthetic gouges of lizardite and antigorite serpentinites and of quartz small amounts of talc weaken serpentinite gouges substantially more than predicted by simple weight averaging in comparison mixtures of quartz and talc show a linear trend of strength reduction at talc concentrations 15 wt and enhanced weakening at higher concentrations all of the strength data are fit by a modified version of the reuss mixing law that allows for the dominance of one mineral over the other the difference in the behavior of serpentinite talc and quartz talc mixtures at low talc concentrations is a reflection of their different textures lizardite antigorite and talc all have platy habits and displacement within gouges composed of these minerals is localized to narrow shears along which the platy grains have rotated into alignment with the shear surfaces the shears in the mixed phyllosilicate gouges maximize the proportion of the weaker mineral within them when mixed with a strong rounded mineral such as quartz some minimum concentration of talc is needed to form connected pathways that enhance strength reductions the typical development of talc by the reaction of si rich fluids with serpentinite or dolomite would tend to localize its occurrence in a natural fault and result in enhanced weakening

crustal structure of the appalachian highlands in tennessee abstract crustal structure of the southern appalachians and adjacent interior low plateaus in tennessee is derived from seismic refraction measurements observed by the u s geological survey in 1965 along reversed lines normal nw se and parallel ne sw to the structure of the appalachian highlands major geologic divisions its easternmost part is located approximately 80 km southwest of the westernmost part of the cocorp seismic reflection traverse within the blue ridge province the velocity depth models derived for both observational directions consist of three crustal layers with surprisingly high velocities being about 6 1 6 2 km s in the upper crust down to 7 10 km depth 6 7 6 8 km s for the middle crust between about 17 and 34 km and varying from 7 1 to 7 4 km s for the lower crust at about 40 47 km depth the boundaries between the three crustal layers as well as the crust mantle boundary are transition zones of up to 11 km thickness similar to old orogens in other parts of the earth the main result is a thick crust at places in excess of 50 km with high average velocity and a broad crust mantle transition zone

rock and mineral transformations in a fault zone leading to permanent creep interactions between brittle and viscous mechanisms in the san andreas fault creep processes may relax part of the tectonic stresses in active faults either by continuous or episodic processes the aim of this study is to obtain a better understanding of these creep mechanisms and the manner in which they change in time and space results are presented from microstructural studies of natural samples collected from san andreas fault observatory at depth borehole drilled through the san andreas fault which reveal the chronology of the deformation within three domain types i a relatively undeformed zone of the host rock reflects the first step of the deformation process with fracturing and grain indentations showing the coupling between fracturing and pressure solution ii shear deformation development that associates fracturing and solution cleavage processes leads to profound changes in rock composition and behavior with two types of development depending on the ratio between the amount of dissolution and deposition abundant mineral precipitation strengthens some zones while pervasive dissolution weakens some others iii zones with mainly dissolution trended toward the present day creeping zones thanks to both the passive concentration of phyllosilicates and their metamorphic transformation into soft minerals such as saponite this study shows how interactions between brittle and viscous mechanisms lead to widespread transformation of the rocks and how a shear zone may evolve from a zone prone to earthquakes and postseismic creep to a zone of steady state creep in parallel the authors discuss how the creeping mechanism mainly controlled by the very low friction of the saponite in the first 3 4 km depth may evolve with depth

three dimensional p and s wave velocity structure of redoubt volcano alaska the three dimensional p and s wave structure of redoubt volcano alaska and the underlying crust to depths of 7 8 km is determined from 6219 p wave and 4008 s wave first arrival times recorded by a 30 station seismograph network deployed on and around the volcano first arrival times are calculated using a finite difference technique which allows for flexible parameterization of the slowness model and easy inclusion of topography and source receiver geometry the three dimensional p wave velocity structure and hypocenters are determined simultaneously while the three dimensional s wave velocity model is determined using the relocated seismicity and an initial s wave velocity model derived from the p wave velocity model assuming an average vp vs ratio of 1 78 convergence is steady with approximately 73 and 52 reduction in p and s wave arrival time rms respectively after 10 iterations the most prominent feature observed in the three dimensional velocity models derived for both p and s waves is a relative low velocity near vertical pipelike structure approximately 1 km in diameter that extends from 1 to 6 km beneath sea level this feature aligns axially with the bulk of seismicity and is interpreted as a highly fractured and altered zone encompassing a magma conduit the velocity structure beneath the north flank of the volcano between depths of 1 and 6 km is characterized by large lateral velocity variations high velocities within this region are interpreted as remnant dikes and sills and low velocities as regions along which magma migrates no large low velocity body suggestive of a magma chamber is resolved in the upper 7 8 km of the crust

new views on the structure and rheology of the lithosphere over the last 10 years a series of developments have led to a new understanding of what controls the variations in lithosphere strength structure and evolution that produce dramatic contrasts between the geological histories of oceans ancient shields and young orogenic belts those developments involve a wide range of observations from a great diversity of geological geophysical and geochemical disciplines that none the less provide a mutually consistent and coherent overall picture this paper summarizes in one place the essential stages in the evolution of the relevant ideas and observations that have led to this situation

preferred crystallographic orientation development during the plastic and superplastic flow of calcite rocks abstract grain size sensitive flow of fine grained rocks such as are often found in mylonitic shear zones is commonly inferred to be accomplished by grain boundary sliding so that no preferred crystallographic orientation is expected to develop pre existing fabrics are supposed to weaken and grain shapes to remain equant however many fine grained mylonites commonly exhibit marked preferred crystallographic orientation whilst displaying a stable but weak grain shape fabric we have examined preferred crystallographic orientation and shape fabric development during the flow of synthetic ultrafine grained 700 c the strong crystal plastic fabric develops more slowly but is not eliminated through the broad transition into true superplastic flow this is interpreted to reflect an important role of intracrystalline plasticity in the accommodation of grain boundary sliding from experiments designed to investigate the extent of survival or modification of an initial twinning fabric through recrystallization and subsequent hot working it was shown that although the microstructure could be totally annealed the early formed fabric survived even subsequent high temperature crystal plastic or superplastic flow this may explain the frequent occurrence of strong low temperature fabrics in calcite ultramylonites

dramatic effects of stress on metamorphic reactions thermodynamic calculations predict mineralogy from temperature and pressure and vice versa such calculations assume that stress is isotropic despite the fact that differential stresses prevail in earth resulting from large scale tectonics and or differences between fluid and rock pressures in porous rocks new calculations show that differential stress can have significant effects on thresholds for metamorphic reactions depending on the grain scale reaction pathways a differential stress may depending on the reaction pathway have an effect equivalent to a pressure difference on the order of assemblage volume reaction volume change differential stress the multiplying factor is typically 10 or more for example the onset of a garnet clinopyroxene breakdown reaction may be offset up or down by the equivalent of 500 mpa in pressure for a 50 mpa differential stress the effect is equivalent to a temperature difference on the order of assemblage volume reaction entropy change differential stress for example the onset of muscovite quartz breakdown may be offset up or down by the equivalent of 130 c for a 50 mpa differential stress much of earth is under differential stress so the new calculations invite a reappraisal of metamorphic mineralogy and microstructure indicating that new insights into stresses and fluid pressures on earth can be gained

origin of reflections from the brevard fault zone the brevard fault zone is a belt of mylonitic metamorphic rocks that extends northeast over 700 km from alabama into virginia seismic reflection studies show that the fault zone produces excellent reflections which dip gently to the southeast and sole at depth the origin of these reflections has been investigated by measuring compressional wave velocities to 800 mpa confining pressure in 127 samples obtained from drill core which continuously sampled the fault zone to a depth of over 300 m densities are reported as well as chemical analyses at selected depths anisotropy at elevated pressures ranges within the fault zone from less than 5 to over 26 similarly the fault zone rocks show significant chemical variability with sio2 ranging from 58 0 to 70 4 by weight synthetic seismograms generated from the velocity and density measurements show that the reflections originate within the brevard fault zone from the complex interaction of compositional variations and seismic anisotropy resulting from ductile strain essential to this study has been the continuous coring of a major section of the brevard fault zone which has provided the necessary stratigraphic control as well as unweathered samples for laboratory studies

crustal earthquake instability in relation to the depth variation of frictional slip properties recent stability studies using constitutive relations of the type found by dieterich ruina and others to describe frictional slip of rocks in the laboratory have provided a new explanation of the depth cutoff of shallow crustal earthquakes the class of friction laws discussed has the property that the sliding stress depends on normal stress temperature slip rate and slip history for sliding at a fixed slip rate v and fixed environment e g normal stress temperature etc the shear strength evolves toward a steady state value ss v stability analysis show that for d ss v dv 0 velocity strengthening implies stable steady state sliding at least for a broad class of constitutive laws more fully described in the paper experiments by dieterich and by tullis and weeks on westerly granite with mature sliding surfaces indicate that d ss v dv is negative at room temperature whereas higher temperature experiments by stesky show that d ss v dv becomes positive above approximately 300 c therefore in the case of the earth where temperature increases with depth the above observations seem to suggest that the depth cutoff of crustal earthquake activity can be understood in terms of the variation of the frictional response with depth from a regime with d ss v dv 0 this is not inconsistent with but rather refines the suggestion by sibson and others that the depth cutoff is due to a transition from brittle friction to ductile flow further our results show definitively that the shallow depth confinement of seismicity is compatible with a model in which deformation is localized to a fault zone extending well below the seismogenic depth the depth confinement does not require a model showing transition to a zone of broadly distributed creep flow beneath the seismogenic zone following mavko a two dimensional quasi static strike slip fault model is analyzed but using different numerical procedures and a depth variation of frictional properties based on the laboratory data mentioned above and the lachenbruch sass depth variation of temperature for the san andreas fault the resulting predictions of such features as the confinement of crustal earthquakes to shallow depths the development of locked patches the recurrence time for the seismic cycle the seismic stress drop and displacement etc are generally in agreement with the observed characteristics of large scale strike slip earthquakes along the san andreas fault calculations based on the laboratory data predict for example that stick slip nucleates around 5 7 km that large seismic motion occurs around the nucleation depth and above but diminishes gradually to zero at 13 15 km depths that rapid postseismic creep occurs over another 3 4 km depth and that at greater depths steady slip consistent with the average plate velocity is only modestly perturbed by the earthquakes occurring above the most uncertain parameter is the slip weakening distance l for evolution of fault surface state we illustrate how features of the predicted earthquake cycles vary with l over the range for which calculations are feasible

electron backscatter diffraction ebsd analysis of bassanite transformation textures and crystal structure produced from experimentally deformed and dehydrated gypsum dehydration reactions have important effects on fluid flow porepressure and brittle failure in the earth the rates and mechanismsof reaction are relevant to understanding those effects the micro structures ofa set ofsamples containing bassanite and gypsum fromdehydration experiments on volterra gypsum have been analysed byconventional transmitted light microscopy and electron backscatterdiffraction ebsd these techniques have been used to character ize the varying crystal morphology and to measure the crystallo graphic preferred orientation cpo of the samples in somesamples bassanite grains show two distinct morphologies large 0 5 2 mm euhedral crystals and small 550mm acicular crys tals these represent the two different bassanite crystal habits bassanite was indexed using ebsd for thefirsttime and problemsrelated tosignificant mis indexing resulting from the pseudotrigonalsymmetry of bassanite are resolved the original starting materialhas a strong local and measurable cpo a strong cpo inpost experiment bassanite is found to mimic the original gypsumc axis orientation bassanite grain size is related to the degree ofreaction overstep spatial heterogeneity in bassanite distributionmay indicate the development of fluid pathways within the samples

deformation mechanism maps for feldspar rocks abstract deformation mechanism maps for feldspar rocks were constructed based on recently published constitutive laws for dislocation and grain boundary diffusion creep of wet and dry plagioclase aggregates the maps display constant temperature contours in stress grain size space for strain rates ranging from 10 16 to 10 12 s 1 two fields of dominance of grain boundary diffusion controlled creep and dislocation creep are separated by a strongly grain size sensitive transition zone for wet rocks diffusion controlled creep dominates below a grain size of about 0 1 1 mm depending on temperature stress strain rate and feldspar composition plagioclase aggregates containing up to 0 3 wt water as often found in natural feldspars are more than 2 orders of magnitude weaker than dry rocks the strength of water bearing feldspar rocks is moderately dependent on composition and water fugacity for a grain size range of about 10 50 m commonly observed in natural ultramylonites the deformation maps predict that diffusion controlled creep is dominant at greenschist to granulite facies conditions low viscosity estimates of 10 18 10 19 pa s from modeling postseismic stress relaxation and channel flow of the continental lower crust can only be reconciled with laboratory experiments assuming dislocation creep at high temperatures 900 c or at lower temperatures diffusion creep of fine grained rocks possibly localized in abundant high strain shear zones for similar thermodynamic conditions and grain size lower crustal rocks are predicted to be less than order of magnitude weaker than upper mantle rocks

the nature and importance of phyllonite development in crustal scale fault cores an example from the median tectonic line japan abstract like many large crustal scale faults the median tectonic line mtl in sw japan has a long history of movement having been active predominantly as a strike slip fault since the mid cretaceous fault rock exposures in the core of the mtl preserve a history of deformation at a range of mid to shallow crustal depths ryoke mylonites 1 4 km north of the main contact record deeper level cretaceous top to the south sinistral movements the remainder of the fault zone core is surprisingly narrow exhibiting a wide variety of fault rocks that illustrate both the interaction and effects of syn tectonic fluid influx over a range of deformation conditions exposures within 50 m of the central slip zone display a progressive sequence in fault rock evolution from ultramylonite cataclasite foliated cataclasite phyllonite breccia gouge this sequence occurs because cataclasis in the vicinity of the fault core creates permeable pathways for the ingress of chemically active fluids into the fault zone this leads to the replacement of load bearing phases such as feldspar by fine grained foliated aggregates of intrinsically weaker phyllosilicates such as white mica and chlorite the grain size reduction associated with both cataclasis and mineral alteration creates conditions ideal for the operation of fluid assisted stress induced diffusive mass transfer mechanisms comparison with the findings of recent experimental studies suggest that the fault zone processes observed in the core of the mtl will lead to long term weakening provided the network of phyllosilicate rich fault rocks are able to form an interconnected thin layer of weak material on kilometre to tens of kilometre length scales

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

palaeostress estimation using calcite twinning experimental calibration and application to nature abstract by means of high temperature deformation experiments on calcite rocks of different grain sizes we have established the relationships between differential stress and twinning incidence twin density and volume fraction and the way that these relationships depend on grain size both twinning incidence and volume fraction increase with grain size at a constant level of differential stress log twin density is proportional to stress but is independent of grain size at a given grain size volume fraction of twinning increases non linearly with differential stress twinning activity measured by twinning incidence increases linearly with stress the behaviour of each of these parameters is independent of temperature strain and strain rate these relationships can be used as a basis for the estimation of palaeostress values in naturally deformed calcite rocks this is illustrated by a study of twinning activity in limestones close to thrust faults in the cantabrian zone of the variscan orogenic belt in northwestern spain high peak differential stresses were measured ranging from 150 to 250 mpa using the turner dynamic analysis technique however it is clear that shear stresses resolved parallel to the fault planes were very low the attainment of high stress differences when the resistance to faulting was low is attributed either to bending forces induced during the motion of the non planar surfaces relative to each other or to the stress peak being attained prior to the localization of deformation onto the fault surfaces

structure porosity and stress regime of the upper oceanic crust sonic and ultrasonic logging of dsdp hole 504b abstract the layered structure of the oceanic crust is characterized by changes in geophysical gradients rather than by abrupt layer boundaries correlation of geophysical logs and cores recovered from dsdp hole 504b provides some insight into the physical properties which control these gradient changes borehole televiewer logging in hole 504b provides a continuous image of wellbore reflectivity into the oceanic crust revealing detailed structures not apparent otherwise due to the low percentage of core recovery physical characteristics of the crustal layers 2a 2b and 2c such as the detailed sonic velocity and lithostratigraphic structure are obtained through analysis of the sonic borehole televiewer and electrical resistivity logs a prediction of bulk hydrated mineral content consistent with comparison to the recovered material suggests a change in the nature of the alteration with depth data from the sonic borehole televiewer electrical resistivity and other porosity sensitive logs are used to calculate the variation of porosity in the crustal layers 2a 2b and 2c several of the well logs which are sensitive to the presence of fractures and open porosity in the formation indicate many zones of intense fracturing interpretation of these observations suggests that there may be a fundamental pattern of cooling induced structure in the oceanic crust

a new fully three dimensional numerical model for ice dynamics the problem of describing ice dynamics has been faced by many researchers in this paper a fully three dimensional model for ice dynamics is presented and tested using an approach followed by other researchers ice is considered a non linear incompressible viscous fluid so that a fluid dynamic approach can be used the model is based on the full three dimensional stokes equations for the description of pressure and velocity fields on the saint venant equation for the description of the free surface time evolution and on a constitutive law derived from glen s law for the description of ice viscosity the model computes the complete pressure field by considering both the hydrostatic and hydrodynamic pressure components it is time evolutive and uses high order numerical approximation for equations and boundary conditions moreover it can deal with both constant and variable viscosity three theoretical tests and two applications to priestley glacier antarctica are presented in order to evaluate the performance of the model and to investigate important phenomena of ice dynamics such as the influence of viscosity on pressure and velocity fields basal sliding and flow over perturbed bedrocks all these applications demonstrate the importance of treating the complete pressure and stress fields

the geochemical signature caused by earthquake propagation in carbonate hosted faults friction laboratory experiments have been performed at sub seismic 0 01 m s to seismic slip rates 1 m s on dolomite gouges of the triassic evaporites which hosted the five mainshocks 5 mw 6 of the 1997 colfiorito earthquakes in the northern apennines italy experimental faults are lubricated as marked falls of the steady state sliding friction coefficients ss 0 2 are observed at seismic slip rates as opposed to values of ss 0 6 attained for sub seismic slip rates at seismic slip rates decarbonation reactions triggered by frictional heating in the experimental slip zone produced 1 new fluid co2 and mineral phases e g mg calcite periclase brucite lime portlandite 2 isotopic fractionation between the reaction products and the reactant mineral phases the variations of total dissolved inorganic carbon tidc in concentration tdic and isotopic composition 13ctidc in a carbonate aquifer with geochemical parameters similar to those of an aquifer located in the seismic belt of the northern apennines have been modelled after an input of earthquake produced co2 modelling results show that variation in 13ctidc can be detected in volumes of groundwater which are about three times larger than those calculated for the variations in tdic for amounts of co2 produced by coseismic decarbonation of 5 wt of the slip zone gouge modelling results show that a detectable geochemical anomaly is obtained if the produced co2 is dissolved into volumes of water comparable to those of the shallower aquifers feeding the springs in the 1997 colfiorito earthquakes area we conclude that the integration of results from laboratory experiments performed at seismic condition and geochemical analyses can potentially aid in the calibration of monitoring strategies of geochemical properties of water in seismically active areas and provide insights into seismic fault zone processes e g constraints on the temperature rise during earthquake propagation

deformation mechanisms and rheology why marble is weaker than quartzite when deformed together in nature calcite rocks invariably appear weaker and more ductile than quartz rocks this can be reconciled with experimental flow data only by taking into account grain size sensitive gss flow in calcite rocks which is predicted to dominate even at grain sizes on the order of 1mm at middle metamorphic grades using new experimental data that demonstrate the transition between intracrystalline plastic and gss flow of quartz rocks we predict that unnaturally small grain sizes at temperatures of 700 c or higher are required for gss flow of quartz in nature thus natural flow of quartzite is expected to occur by intracrystalline plastic processes even after recrystallization to a fine grain size

rock mechanics observations pertinent to the rheology of the continental lithosphere and the localization of strain along shear zones emphasized in this paper are the deformation processes and rheologies of rocks at high temperatures and high effective pressures conditions that are presumably appropriate to the lower crust and upper mantle in continental collision zones much recent progress has been made in understanding the flexure of the oceanic lithosphere using rock mechanics based yield criteria for the inelastic deformations at the top and base at mid plate depths stresses are likely to be supported elastically because bending strains and elastic stresses are low the collisional tectonic regime however is far more complex because very large permanent strains are sustained at mid plate depths and this requires us to include the broad transition between brittle and ductile flow moreover important changes in the ductile flow mechanisms occur at the intermediate temperatures found at mid plate depths two specific contributions of laboratory rock rheology research are considered in this paper first the high temperature steady state flow mechanisms and rheology of mafic and ultramafic rocks are reviewed with special emphasis on olivine and crystalline rocks rock strength decreases very markedly with increases in temperature and it is the onset of flow by high temperature ductile mechanisms that defines the base of the lithosphere the thickness of the continental lithosphere can therefore be defined by the depth to a particular isotherm tc above which at geologic strain rates the high temperature ductile strength falls below some arbitrary strength isobar e g 100 mpa for olivine tc is about 700 800 c but for other crustal silicates tc may be as low as 400 600 c suggesting that substantial decoupling may take place within thick continental crust and that strength may increase with depth at the moho as suggested by a number of workers on independent grounds put another way the moho is a rheological discontinuity a second class of laboratory observations pertains to the general phenomenon of ductile faulting in which ductile strains are localized into shear zones ductile faults have been produced in experiments of five different rock types and is generally expressed as strain softening in constant strain rate tests or as an accelerating creep rate stage at constant differential stress a number of physical mechanisms have been identified that may be responsible for ductile faulting including the onset of dynamic recrystallization phase changes hydrothermal alteration and hydrolytic weakening microscopic evidence for these processes as well as larger scale geological and geophysical observations suggest that ductile faulting in the middle to lower crust and upper mantle may greatly influence the distribution and magnitudes of differential stresses and the style of deformation in the overlying upper continental lithosphere

reduction of friction on geological faults by weak phase smearing most common crustal rock types display friction coefficients of 0 6 or higher but some faults must be frictionally weak as they slip when the stress state is unfavourably oriented i e the resolved shear stress is low for a given normal stress across the fault surface a role for low friction minerals and high pore fluid pressures either separately or in combination is frequently invoked to explain such slip but volume fractions of dispersed weak phases often seem to be present in fault gouge in amounts too small to produce significant mechanical weakening by means of mechanical tests on synthetic fault gouge and microstructural study of run products we show that the effective area of an embedded weak phase graphite on a slip plane can be substantially increased by mechanical smearing and that the enlarged area of the weak phase on the slip plane follows a linear mixing law this allows a relatively small volume fraction of the initially dispersed weak phase to have a disproportionately large effect provided the smearing is concentrated into a narrow planar slip zone or into an interconnected network of them

reaction induced weakening of plagioclase olivine composites abstract the localisation of strain into natural ductile shear zones is often associated with the occurrence of metamorphic reactions in order to study the effects of solid solid mineral reactions on plastic deformation of rocks we have investigated the shear deformation of plagioclase olivine composites during the reaction plagioclase olivine orthopyroxene clinopyroxene spinel garnet microstructures of plagioclase olivine composites were studied after shear deformation experiments in a griggs apparatus experiments were performed on anorthite forsterite an fo and labradorite forsterite lab fo composites at 900 c confining pressures between 1000 1600 mpa and with constant shear strain rates of 5 10 5 s 1 in absence of reaction lab fo composites are stronger than pure olivine and labradorite end members that deform with a high temperature plasticity mechanism lab fo composites strain harden due to the inhibition of extensive recrystallisation by interphase boundaries in an fo composites the reaction induces strain weakening by a switch from dislocation creep to grain size sensitive deformation mechanisms through the development of fine grained size

three dimensional p and s wave velocity structures beneath taiwan deep structure beneath an arc continent collision the island of taiwan is a byproduct of one of the few active collisions between a continent represented by the eurasian continental shelf and an island arc represented by the northern extension of the luzon arc on the philippine sea plate to understand better the evolution and current tectonics of this collision we selected 1260 well recorded earthquakes from an initial set of 50 000 located by the taiwan telemetered seismograph network for the determination of one and three dimensional p and s wave velocity structures beneath the island the results for both structures and earthquake relocations reveal that the island is divided tectonically into three distinct zones in the east the philippine oceanic plate is underthrusting the eurasian plate east of a north south boundary that is well defined by both seismic activity and a region of high velocity in the south the eurasian continental plate is underthrusting the philippine sea plate south of an east west boundary at about 23 n that is sharply defined by both subcrustal seismicity and a zone of relatively low velocities the dip of the subducted continent is shallow until it reaches the luzon island arc 50 km east of the main island structure under the main part of the island north of 230n reveals a shallow dipping zone of low velocities in the west above 25 km depth that narrows and steepens below that depth to at least 50 km beneath the central range the dip of this low velocity region is outlined by a narrow zone of seismicity that extends to depths of 100 km this seismic zone lies in a velocity saddle and marks an apparent offset in the central low velocity region at 24 n evidence for the subduction of the eurasian continent beneath taiwan therefore exists everywhere beneath the island and the low velocity regions in the mantle support but do not require the subduction of some 6 16 km of lower continental crust to depths of at least 50 km the distinct change in both the velocity structure and the seismic activity at 23 n and the offset of the low velocity regions at 24 n argue for abrupt changes in the nature of subduction across these latitudes these changes may be caused by an interaction of the luzon island arc with the subducted continental shelf that mimics a similar interaction evident at the surface finally the velocity structure of the subducting philippine sea plate suggests that the earthquakes beneath 70 km depth are not occurring within the highest velocity regions of the plate but are probably located near the upper edge of the plate there is also evidence from earthquake locations and velocity structure that suggests that the subducting plate is segmented and that subduction is presently occurring as far south as 22 n

large scale crustal deformation of the tibetan plateau the topography velocity and strain fields calculated from a three dimensional newtonian viscous model for large scale crustal deformation are generally in good agreement with results from geological geodetic and earthquake studies in and around the tibetan plateau provided that the model rheology incorporates a weak zone within the deep crust beneath the plateau equivalent to a viscosity of 1012 pa s within a 250 m thick channel or 1018 pa s within a 15 km thick channel model studies and observations show a plateau at 5 km elevation with steep topographic gradients across the southern and northern plateau margins and more gentle gradients across the southeastern and northeastern margins rapid shortening strain is concentrated along the lower portions of the northern and southern plateau margins at rates 20 mm yr model results show north south shortening 10 mm yr in reasonable agreement with gps data 5 8 mm yr of north south shortening across the northern two thirds of the plateau and east west stretching 10 15 mm yr across the eastern half of the high plateau in reasonable agreement with seismic geologic and gps data upper crustal material moves eastward from the plateau proper into a lobe of elevated topography that extends to the south and east clockwise rotation of material around the east himalayan syntaxis at rates up to 10 mm yr occurs partly as a result of dextral shear between indian and asian mantle at depth and partly as a result of gravitational spreading from the high plateau to the south and east there is little difference in model surface deformation for assumptions of moderately weak or extremely weak lower crust except in southern and northern tibet where margin perpendicular extension occurs only for the case of an extremely weak lower crust our results suggest that the tibetan plateau is likely to have gone through a two stage development the first stage produced a long narrow high orogen whose height may have been comparable to the modern plateau the second stage produced a plateau that grew progressively to the north and east east west stretching eastward plateau growth and dextral rotation around the east himalayan syntaxis probably did not begin until well into the second stage of plateau growth perhaps becoming significant after 20 m y of convergence

high velocity frictional properties of gabbro high velocity friction experiments have been performed on a pair of hollow cylindrical specimens of gabbro initially at room temperature at slip rates from 7 5 mm s to 1 8 m s with total circumferential displacements of 125 to 174 m and at normal stresses to 5 mpa using a rotary shear high speed friction testing machine steady state friction increases slightly with increasing slip rate at slip rates to about 100 mm s velocity strengthening and it decreases markedly with increasing slip rate at higher velocities velocity weakening steady state friction in the velocity weakening regime is lower for the non melting case than the frictional melting case due perhaps to severe thermal fracturing a very large peak friction is always recognized upon the initiation of visible frictional melting presumably owing to the welding of fault surfaces upon the solidification of melt patches frictional properties thus change dramatically with increasing displacement at high velocities and such a non linear effect must be incorporated into the analysis of earthquake initiation processes

deep crustal extensional faulting in the ivrea zone of northern italy abstract the ivrea zone in northern italy consists of amphibolite and granulite facies metasedimentary metabasic and ultrabasic rocks and is thought to represent a section through the deep continental crust detailed mapping of the high grade part of this zone has revealed a series of high temperature shear zones which after the effects of later low temperature faulting and folding have been removed appears to accommodate extension of the sequence the shear zones formed at temperatures similar to those at which thermal equilibration occurred in the host rocks and locally show evidence for development under prograde conditions these observations fit well with thermal effects calculated from a simple heat flow model for extensional faulting in this model the effects of shear heating and displacement of cool hanging wall rocks against hotter footwall rocks are calculated these observations indicate that faulting as a mode of failure may be important during extension of the lower continental crust especially when basic rocks are the dominant components of the rock pile a restored section through the extended lower crust as observed in the ivrea zone is presented and the extent to which the features observed are able to explain seismic reflectivity of the lower crust is discussed

direct calculation of rupture depth for an exhumed paleoseismogenic fault from mylonitic pseudotachylyte multiple generations of pseudotachylyte within granulite facies orthogneiss northern norway caledonides are cut by ultramylonitic shear bands that contain the mineral assemblage garnet clinopyroxene plagioclase quartz mineralogic thermobarometry yields pressure p 950 150 mpa at temperature t 625 675 c for equilibration conditions of the ultramylonite assemblage requiring minimum depths of 35 5 km for coseismic slip that generated pseudotachylyte these depths are well below the base of the standard seismogenic zone 15 20 km ambient temperatures are 200 300 c higher than those commonly cited for the onset of ductile creep in quartzofeldspathic rocks but they are similar to those in modern intraplate settings of higher than average heat flow that have experienced deep crustal earthquakes the occurrence of overprinting mylonites and pseudotachylytes requires tens of kilometers of dynamic downward rupture propagation from the brittle frictional regime into the ductile regime plastic instabilities do not explain the relationship between ultramylonites and mylonitized pseudotachylytes in this setting

experimental sytectonic dehydration of serpentinite under conditions of controlled pore water pressure we have studied the mechanical behavior of serpentinite under conditions favouring dehydration to olivine talc water at temperatures ranging between 300 and 600 and at total pressures from 100 to 300 mpa constant stress stress relaxation and constant displacement rate testing techniques permitted a wide range of deformation rates and paths to be accessed effective confining pressure was held constant during the dehydration process by means of a controlled pore water pressure system stress supported by the samples was almost independent of deformation rate at temperatures below those required for the onset of the breakdown reaction at 500 and 600 c however the strength was linearly dependent on the inverse of the deformation rate and independent of effective confining pressure over the range investigated microstructural studies revealed that the weakening was associated with sliding in narrow shear zones containing ultrafine grained 0 1 m olivine produced in the breakdown reaction the enhanced deformability is interpreted in terms of flow of the olivine in these shear zones by diffusion accommodated grain boundary sliding these experimental results suggest that metamorphic transformations affect the deformability of rocks and demonstrate enhancement of deformability through the transient existence of fine grained reaction products the results may also be applicable to the mechanics of deformation processes in oceanic transform faults and give some indication of the degree of weakening that might be expected from grain size reduction by dynamic recrystallization in plastically deforming rocks

a thermal tectonic model for high pressure metamorphism and deformation in the sesia zone western alps the sesia zone western alps is a slice of austroalpine continental crust that was once metamorphosed at t 500 560 c and p 14 16 kbar at 130 100 ma b p pervasive blueschist eclogite facies assemblages record this event and a later greenschist facies overprint is only well developed in the nw part of the zone on the basis of thermal calculations it is difficult to explain the preservation of high pressure assemblages in a simple model of underthrusting and subsequent uplift erosion an alternative model is proposed in which the sesia zone was emplaced by underthrusting of the austroalpine continental margin prior to subduction of oceanic lithosphere of the piemonte basin subduction of relatively cool oceanic lithosphere below the underthrust sesia zone then continued for a period of time e g 20 ma thus suppressing thermal recovery of this latter unit exhumation of sesia zone rocks began during this subduction episode by uplift on the insubric line erosion and by underplating of oceanic

structure and geological history of the carboneras fault zone se spain part of a stretching transform fault system abstract the carboneras fault zone cfz is a major ne sw trending tectonic lineament in se spain active from upper miocene to recent times it separates the volcanic cabo de gata terrain to the se accumulated over 18 6 ma bp from the tract of uplifted alpine metamorphic basement blocks and post orogenic basins that comprise the betic cordilleras lying to the nw new geological mapping and age determinations have been used to constrain the geometry and geological history of the fault zone the cfz consists of left lateral strike slip faults bearing fault gouge formed in the uppermost 5 km of the crust the faults cut metamorphic basement and folded post orogenic sediments and volcanic rocks and acted as a conduit for calc alkaline volcanic rocks rising to the surface nw of the cfz a series of unconformities and deformation episodes affect successive sedimentary formations of upper miocene age the cfz is interpreted as part of a transform fault system separating ne sw stretched and nw se shortened crust deformed above a south westward retreating subducted slab from a less deformed terrain lying to the south east total offset on the cfz may be up to 40 km but is at least 15 km

towards an improved understanding of the mechanical properties and rheology of the lithosphere an introductory article to rock deformation from field experiments and theory a volume in honour of ernie rutter ernie rutter s influential contribution to our understanding of rock deformation spans the whole continental crust from deformation just under the grass e g rutter green 2011 to melt migration in the lower crust e g rutter neumann 1995 ernie has also worked on many aspects of the deformation of oceanic crust rutter brodie 1988 however here we reflect on the wide range of deformation conditions ernie has studied by presenting key aspects of the strength and rheology of the continental lithosphere as we understand it today with the aim of providing the context for the contributions included within this volume the continental crust forms just over a third of the earth s surface and it is different from the crust of any other planet in our solar system it is rich in those elements that partition in a silicate melt and its formation and evolution determined the composition of the mantle and that of the atmosphere hawkesworth 2006 thanks to its physical and chemical fingerprint the earth s crust has sustained life uninterrupted for 3 8 ga and it is the source of major natural resources such as hydrocarbons and mineral deposits and others such as geothermal energy the continental crust is less dense than its oceanic counterpart and is therefore more buoyant it forms a substantial part of the cold thermal layer of the earth and its rheology from greek r1v rhe flow and logia logia study the study of the flow of matter is a key factor controlling plate tectonic processes at the earth s surface and in its interior davies 2011 beneath the continental crust lies the lithospheric mantle and together they form the continental lithosphere considerable variations in structure thickness and composition of the continental lithosphere mean that despite its importance we currently have limited understanding of the rheology of this complex system thatcher pollitz 2008 significant advances in our knowledge of continental rheology have been possible through

dynamic weakening of serpentinite gouges and bare surfaces at seismic slip rates to investigate differences in the frictional behavior between initially bare rock surfaces of serpentinite and powdered serpentinite gouge at subseismic to seismic slip rates we conducted single velocity step and multiple velocity step friction experiments on an antigorite rich and lizardite rich serpentinite at slip rates v from 0 003 m s to 6 5 m s sliding displacements up to 1 6 m and normal stresses n up to 22 mpa for gouge and 97 mpa for bare surfaces nominal steady state friction values nss in gouge at v 1 m s are larger than in bare surfaces for all n tested and demonstrate a strong n dependence nss decreased from 0 51 at 4 0 mpa to 0 39 at 22 4 mpa conversely nss values for bare surfaces remained 0 1 with increasing n and v additionally the velocity at the onset of frictional weakening and the amount of slip prior to weakening were orders of magnitude larger in gouge than in bare surfaces extrapolation of the normal stress dependence for nss suggests that the behavior of antigorite gouge approaches that of bare surfaces at n 60 mpa x ray diffraction revealed dehydration reaction products in samples that frictionally weakened microstructural analysis revealed highly localized slip zones with melt like textures in some cases gouge experiments and in all bare surfaces experiments for v 1 m s one dimensional thermal modeling indicates that flash heating causes frictional weakening in both bare surfaces and gouge friction values for gouge decrease at higher velocities and after longer displacements than bare surfaces because strain is more distributed

channel flow and the himalayan tibetan orogen a critical review the movement of a low viscosity crustal layer in response to topographic loading provides a potential mechanism for 1 eastward flow of the asian lower crust causing the peripheral growth of the tibetan plateau and 2 southward flow of the indian middle crust to be extruded along the himalayan topographic front thermomechanical models for channel flow link such extrusion to focused orographic precipitation at the surface isotopic constraints on the timing of fault movement anatexis and thermobarometric evolution of the exhumed garnet to sillimanite grade metasedimentary rocks support mid crustal channel flow during the early to mid miocene exhumed metamorphic assemblages suggest that the dominant mechanism of the viscosity reduction that is a requirement for channel flow was melt weakening along the upper surface defined by the south tibetan detachment system and strain softening along the base bounded by the main central thrust neotectonic extrusion bounded by brittle quaternary faults south of the main central thrust is positively correlated with the spatial distribution of precipitation across a north south transect suggesting climate tectonic linkage over a million year time scale a proposed orogen wide eastward increase in extrusion rate over 20 ma reflects current precipitation patterns but climate tectonic linkage over this time scale remains equivocal

lower crustal relaxation beneath the tibetan plateau and qaidam basin following the 2001 kokoxili earthquake summary in 2001 november a magnitude 7 8 earthquake ruptured a 400 km long portion of the kunlun fault northeastern tibet in this study we analyse over five years of post seismic geodetic data and interpret the observed surface deformation in terms of stress relaxation in the thick tibetan lower crust we model gps time series first year and insar line of sight measurements years two to five and infer that the most likely mechanism of post seismic stress relaxation is time dependent distributed creep of viscoelastic material in the lower crust since a single relaxation time is not sufficient to model the observed deformation viscous flow is modelled by a lower crustal burgers rheology which has two material relaxation times the optimum model has a transient viscosity 9 1017 pa s steady state viscosity 1 1019 pa s and a ratio of long term to maxwell shear modulus of 2 3 this model gives a good fit to gps stations south of the kunlun fault while displacements at stations north of the fault are over predicted we attribute this asymmetry in the gps residual to lateral heterogeneity in rheological structure across the southern margin of the qaidam basin with thinner crust higher viscosities beneath the basin than beneath the tibetan plateau deep afterslip localized in a shear zone beneath the fault rupture gives a reasonable match to the observed insar data but the slip model does not fit the earlier gps data well we conclude that while some localized afterslip likely occurred during the early post seismic phase the bulk of the observed deformation signal is due to viscous flow in the lower crust to investigate regional variability in rheological structure we also analyse post seismic displacements following the 1997 manyi earthquake that occurred 250 km west of the kokoxili rupture we find that viscoelastic properties are the same as for the kokoxili area except for the transient viscosity which is 5 1017 pa s the viscosities estimated for the manyi and kokoxili areas are consistent with constraints obtained from other earthquakes in the northwest and south central parts of the tibetan plateau

high pressure high temperature seismic velocity structure of the midcrustal and lower crustal rocks of the ivrea verbano zone and serie dei laghi nw italy the ivrea verbano zone and the adjacent serie dei laghi in the inner arc of the western alps nw italy display an upended cross section through much of the thickness of the continental crust as it existed in that region in triassic jurassic time we collected a suite of oriented rock samples that represent most of the volume of the rocks of the region p and s wave velocity measurements were made in three orthogonal directions related to the mesoscopic fabric of the rocks at room temperature and up to 550 mpa confining pressure combined high temperature and high pressure measurements were made up to 700 c on a subset of the samples the lithologic units were divided into 23 different principal rock types and we present velocity data averaged in terms of these groups vertical and horizontal velocity and anisotropy sections are computed from the measured velocity data based on a restored geologic section these show that vp and vs both increase and anisotropy decreases systematically with depth of burial reflecting variations in rock type and metamorphic grade with depth vp and vs anisotropy arises mainly because of crystallographic preferred orientation or mineralogical banding with the slowest direction tending to be normal to the foliation or banding the vp velocity sections were used to compute synthetic seismic reflection profiles for the region when it lay at the bottom of the continental crust these correspond well with contemporary deep reflection profiles for regions of comparable tectonic evolution

himalayan tectonics explained by extrusion of a low viscosity crustal channel coupled to focused surface denudation recent interpretations of himalayan tibetan tectonics have proposed that channel flow in the middle to lower crust can explain outward growth of the tibetan plateau1 2 3 and that ductile extrusion of high grade metamorphic rocks between coeval normal and thrust sense shear zones can explain exhumation of the greater himalayan sequence4 5 6 7 here we use coupled thermal mechanical numerical models to show that these two processes channel flow and ductile extrusion may be dynamically linked through the effects of surface denudation focused at the edge of a plateau that is underlain by low viscosity material our models provide an internally self consistent explanation for many observed features of the himalayan tibetan system8 9 10

seismic evidence for an ancient rift beneath the cumberland plateau tennessee a detailed analysis of broadband teleseismic p waveforms broadband receiver functions developed from teleseismic p waveforms recorded on the midperiod passband of regional seismic test network station rscp are inverted for vertical velocity structure beneath the cumberland plateau tennessee the detailed broadband receiver functions are obtained by stacking source equalizd horizontal components of teleseismic p waveforms the resulting receiver functions are most sensitive to the shear velocity structure near the station a time domain inversion routine utilizes the radial receiver function to determine this structure assuming a crustal model parameterized by many thin flat lying homogeneous layers lateral changes in structure are identified by examining azimuthal variations in the vertical structure the results reveal significant rapid lateral changes in the midcrustal structure beneath the station that are interpreted in relation to the origin of the east continent gravity high located northeast of rscp the results from events arriving from the northeast show a high velocity midcrustal layer not present in results from the southeast azimuth this velocity structure can be shown to support the idea that this feature is part of a keweenawan rift system another interesting feature of the derived velocity models is the indication that the crust mantle boundary beneath the cumberland plateau is a thick probably laminated transition zonemore between the depths of 40 and 55 km a result consistent with interpretations of early refraction work in the area less

direct observation of fault zone structure at the brittle ductile transition along the salzach ennstal mariazell puchberg fault system austrian alps 1 structural analysis of two key exposures reveals the architecture of the brittle ductile transition bdt of the subvertical strike slip salzachtal fault at lichtensteinklamm the fault zone is dominantly brittle with a 70 m wide high strain fault core highlighted by a 50 m thick highly foliated gouge zone in contrast at kitzlochklamm deformation is dominantly ductile albeit with relatively low strain indicated by weak lattice preferred orientations lpos the marked contrast in structural style indicates that these sites span the bdt the close proximity of the outcrops coupled with raman spectroscopy indicating similar maximum temperatures of 400 c suggests that the difference in exhumation depth is small with a commensurately small difference in total downdip width of the bdt the small strains indicated by weak lpos at kitzlochklamm coupled with evidence for brittle slip at the main fault contact and along the sides of a 5 m wide fault bounded sliver of klammkalk exposed 30 m into the grauwacken zone rocks suggest the possibility that this exposure may record hybrid behavior at different times during the earthquake cycle with ductile deformation occurring during slow interseismic slip and brittle deformation occurring during earthquakes as dynamic coseismic stresses induced a strain rate dependent shift to brittle fault behavior within the nominally ductile regime in the lower part of the bdt a key aspect of both outcrops is evidence of a high degree of strain localization through the bdt with high strain fault cores no wider than a few tens of meters

implications for mechanical properties of brittle faults from observations of the punchbowl fault zone california field observations of the punchbowl fault zone an inactive trace of the san andreas are integrated with results from experimental deformation of naturally deformed punchbowl fault rocks for a qualitative description of the mechanical properties of the fault and additional information for conceptual models of crustal faulting the punchbowl fault zone consists of a single continuous gouge layer bounded by zones of extensively damaged host rock fault displacements were not only localized to the gouge layer but also to discrete shear surfaces within the gouge deformation in the exposure studied probably occurred at depths of 2 to 4 km and was dominated by cataclastic mechanisms textural data also suggest that significant amounts of pore fluids were present during faulting and that fluid assisted mechanisms such as dissolution diffusion and precipitation were operative the experimental data on specimens collected from the fault zone suggest that there is a gradual decrease in strength and elastic modulus and an increase in relative ductility and permeability toward the main gouge zone the gouge layer has fairly uniform mechanical properites and it has significantly lower strength elastic modulus and permeability than both the damaged and the undeformed host rock for the punchbowl fault and possibly other brittle faults the variations in loading of the gouge zone with time are primarily governed by the morphology of the fault and the mechanical properties of the damaged host rock in addition the damaged zone acts as the permeable unit of the fault zone and surrounding rock it appears that the gouge primarily governs whether displacements are localized and it therefore may have a significant influence on the mode of slip

cl scapolite cl amphibole and plagioclase equilibria in ductile shear zones at nusfjord lofoten norway implications for fluid compositional evolution during fluid mineral interaction in the deep crust abstract cl rich scapolite and amphibole formed during ductile shear deformation associated with the infiltration of an externally derived cl bearing fluid in a gabbroanorthosite of the flakstadoy basic complex lofoten norway amphibole and scapolite formed along the contacts between incompletely altered igneous mafic minerals orthopyroxene clinopyroxene biotite and ilmenite and plagioclase and internally in the grains of primary igneous plagioclase the secondary minerals show large compositional variations on thin section scale the cl content of scapolite varies between 0 3 and 0 95 apfu atoms per formula unit whereas amphibole shows cl concentrations from 0 to 1 5 apfu the primary igneous plagioclase an50 an60 underwent extensive recrystallization and alteration during the fluid rock interactions secondary plagioclase shows compositions in the range from an20 to an55 in general plagioclase did not equilibrate with the fluid phase during alteration due to the sluggishness of the cation exchange reactions between plagioclase and fluid occasionally however equilibrium among plagioclase scapolite and the fluid phase was attained the compositional variations of amphibole scapolite and plagioclase that equilibrated with the fluid are principally related to variations in the fluid activity ratios acl a co3 2 and acl aoh the large compositional variations of the minerals on thin section scale thus indicate steep gradients in the fluid activity ratios acl a co3 2 and acl aoh the activity gradients of the fluid phase developed as a result of the preferential extraction of water from the grain boundary fluid during the formation of hydrous silicates scapolite and the most cl enriched amphibole formed in equilibrium with an evolved fluid phase enriched in cl and co2

on the structure of the ivrea verbano zone northern italy and its implications for present day lower continental crust geometry the ivrea verbano zone in northern italy represents a section through the lower continental crust which has been tilted and emplaced into its present position during the alpine orogeny recent and on going structurally oriented geological mapping in this region is providing new information about the geometry of the complex the central part of the zone is dominated by a large basic complex the mafic formation which is intrusive into the surrounding gneisses the foliation within the envelope of gneisses is deflected around the intrusive complex as if by ballooning but in the region south west of monte capio both units are folded together into a tight to isoclinal steeply plunging fold with an amplitude of c 10 km this fold locally inverts the stratigraphy of the layered basic group of the complex and is thought to be the result of gravitational collapse following intrusion and inflation of a large magma body into the lower crust several high temperature shear zones have now been traced within the country rock for distances up to 20 km the geometry of these and their relationship to the basic complex suggests that at least some of the extensional collapse of the mafic body is related to uplift caused by intrusion of this body close parallels can be drawn between the observed structure in the ivrea verbano zone after removing the effects of late low temperature faulting and folding related to emplacement of the rocks into their present position and those inferred from deep seismic reflection profiling in areas of current extension such as parts of the us basin and range province

flow of synthetic wet partially molten granite under undrained conditions an experimental study 1 major problems in experimental studies of the flow of partially molten granitic rocks are 1 maintaining control of the grain size of the matrix of solid grains 2 controlling the melt fraction and 3 controlling the melt viscosity closely linked to water content to overcome these problems we used a synthetic granitoid comprising a solid matrix of quartz grains 50 m grain size mixed with an albite quartz melt prepared from oxides with added water 2 5 wt dissolved in the melt phase viscosity 9 104 pa s experiments were performed undrained with melt fractions of 0 1 0 2 0 25 and 0 3 constant strain rate creep and stress relaxation experiments were carried out in a gas medium apparatus at 1273 and 1173 k mostly at 300 mpa confining pressure up to 15 shortening strain strain rates ranged between 10 4 to less than 10 7 s 1 and fully ductile mechanical behavior was observed melt fraction has a more profound effect on strength than water content temperature or total confining pressure low strain rate 10 6 s 1 data were fitted to an empirical flow law of the form d dt a exp b m exp h rt n with the parameters loga 1 39 m 3 n 1 8 b 192 h 220 kjmol 1 where d dt is strain rate s 1 is flow stress mpa is melt fraction and t is temperature k strength is expected to fall rapidly above about 0 4 melt fraction to levels characteristic of flow of the melt containing suspended crystallites extrapolation to geological strain rates using this flow law shows that in nature migmatites bearing granitic melt will be extremely weak much weaker than silicate rocks deforming by intracrystalline plasticity microstructural study shows grains remain equant at all strains with no discernable formation of a grain flattening fabric nor a large strain contribution from microfracturing thus bulk intracrystalline plasticity is of minor importance and intergranular sliding is implied the nonlinear flow observed is tentatively attributed to a combination of sintering of grain contacts and sliding failure of such contacts with a substantial contribution from diffusive transfer processes

experimental intracrystalline plastic flow in hot pressed synthetic quartzite prepared from brazilian quartz crystals samples of synthetic ultrafine grained quartzite were prepared by hot pressing aggregates of crushed clear brazilian quartz of mean grain size 0 4 m at 300 mpa and 1373 and 1473 k the samples displayed rapid grain growth to ca 12 20 m with 2 porosity at 1473 k facilitated by the 0 6 wt of water adsorbed onto the grain boundaries during sample preparation this water could be driven off by preheating thereby preventing grain growth sufficient water was incorporated during growth into the coarsened samples to render them weak and ductile these were deformed experimentally in the quartz field using an argon gas medium apparatus at 300 mpa confining pressure at temperatures mainly between t 1273 and 1473 k ductile flow was described by the flow law 10 4 9 3 2 9 7 f h 2 o exp 242000 rt with stress a in mpa and grain size d in microns and strain rate in s 1 r is the gas constant and f h 2 o is water fugacity based on observation of grain flattening optical strain features shapes and densities of dislocation arrays and flow law parameters samples deformed dominantly by dislocation creep with some contribution from grain boundary sliding bearing in mind possible changes in the flow law parameters over the extrapolation interval and possible effects of the phase transition extrapolation to natural strain rates and temperatures predicts plastic flow at higher flow stresses at the same water fugacity at greenschist facies temperatures than previously published flow laws

a reappraisal of the sibson scholz fault zone model the nature of the frictional to viscous brittle ductile transition along a long lived crustal scale fault outer hebrides scotland the widely cited sibson scholz conceptual fault zone model suggests that seismically active upper crustal brittle faults pass downward across a predominantly thermally controlled transition at 10 15 km depth into ductile shear zones in which deformation occurs by aseimic viscous creep the crustal scale outer hebrides fault zone ohfz in nw scotland has been described as the type example of such a continental fault zone it cuts precambrian basement gneisses and is deeply exhumed allowing direct study of the deformation products and processes that occur across a wide range of crustal depths a number of fault rock assemblages are recognized to have formed during a long lived displacement history lasting in excess of 1000 myr during caledonian movements that are recognized along much of the 190 km onshore fault trace brittle cataclasite bearing faults in the west of the ohfz are unequivocally overprinted to the east by a younger fabric related to a network of ductile shear zones field observations and regional geochronological data demonstrate that there is no evidence for reheating of the fault zone due to thrust related crustal thickening or shear heating microstructural observations show that the onset of viscous deformation was related to a major influx of hydrous fluids this led to retrogression with the widespread development of new finegrained phyllosilicate bearing fault rocks phyllonites and the onset of fluid assisted grain size sensitive diffusional creep in the most highly deformed and altered parts of the fault zone phyllonitic fault rocks also occur in older more deeply exhumed parts of the fault zone implying that phyllonitization had previously occurred at an earlier stage and that this process is possible over a wide temperature depth range within crustal scale faults our data provide an observational basis for recent theoretical and experimental studies which suggest that crustal scale faults containing interconnected networks of phyllosilicate bearing fault rocks will be characterized by long term relative weakness and shallow 5 km frictional viscous transition zones similar processes acting at depth may provide an explanation for the apparent weakness of presently active structures such as the san andreas fault

fluid assisted healing processes in gouge bearing faults insights from experiments on a rock analogue system on the basis of both experimental and field studies solution transfer processes are expected to be important in contributing to fault strength recovery healing under the hydrothermal conditions prevailing in large parts of the seismogenic zone however most experimental work on healing effects in faults has been done using quartzo feldspathic sliding surfaces or fault gouges under conditions in which solution transfer processes are very slow mechanisms of fluid assisted fault healing are accordingly rather poorly understood we have performed slide hold slide fault healing experiments on simulated faults containing brine saturated granular halite as a fault rock analogue halite was used because solution transfer processes are known to be rapid in this system under easily accessible conditions we studied the effects of hold time duration shear stress during hold pre hold sliding velocity and pore fluid composition on strength evolution the results show rapid fluid assisted compaction and stress relaxation during hold periods and a significant hold time dependent strengthening upon re shear the data reveal that healing resulted from both a packing density increase and a contact area strength increase during hold periods the general type of behaviour observed is similar to that observed in quartzo feldspathic gouges at room temperature although important differences were observed as well notably the time dependence of healing deviates from the log linear trend observed in gouges where solution transfer processes are absent this means that if pressure solution is an important healing mechanism in natural faults applying a log linear trend will underestimate natural fault healing rates

experimental deformation of muscovite shear zones at high temperatures under hydrothermal conditions and the strength of phyllosilicate bearing faults in nature abstract layers of fine grained muscovite were hot pressed then sheared between alumina sliders to shear strains up to 2 at temperatures between 300 and 700 c confining pressures of 206 mpa and various pore water pressures high pore water pressures helped suppress dehydroxylation of the mica and permitted testing at temperatures higher than previously used shear strain rates between 10 3 s 1 and 10 7 s 1 were accessed using constant shear strain rate and stress relaxation testing except for strain rates 5 s 1 at 700 c deformation was strain rate and temperature insensitive but effective normal stress sensitive with a friction coefficient at yield of 0 3 rising with strain to 0 5 steady state sliding was not attained from the mechanical data and microstructural study deformation was inferred to have occurred by a mixture of brittle frictional and crystal plastic processes at 700 c and low strain rates the shear strength falls rapidly with a linear viscous characteristic in a way not previously reported this is tentatively attributed to rate control by viscous glide of basal dislocations extrapolating these results to geological strain rates we expect mica rich fault zones will exhibit frictional behaviour with a low friction coefficient between 0 25 and 0 5 giving way at mid crustal conditions to a rapid strength drop as viscous creep supervenes thus mid to lower crustal mica dominated faults of any orientation and terrains of schistose metapelitic rocks in the cores of orogeneic belts are expected to be very weak supporting shear stresses in the range 1 10 mpa the frictional behaviour of mica rich faults in upper crustal regions is however expected to be too strong to account alone for the proposed weakness of some major fault zones