elevated pore pressure and anomalously low stress in regions of low frequency earthquakes along the nankai trough subduction megathrust 1 recent seismic reflection and ocean bottom seismometer obs studies reveal broad regions of low seismic velocity along the nankai subduction plate boundary megathrust offshore sw japan these low velocity zones lvz s extend 55 km landward from the trench corresponding to depths of 10 km below sea floor here we estimate the in situ pore pressure and stress state within these lvz s by combining p wave velocities obtained from the geophysical surveys with new well constrained empirical relations between p wave velocity porosity and effective mean stress defined by laboratory deformation tests on drill core samples of the incoming oceanic sediment we document excess pore pressures of 17 87 mpa that extend 55 km into the subduction zone indicating that trapped pore fluids support 45 91 of the overburden stress along the base of the upper plate and surrounding major fault zones the resulting effective stresses in the lvz are limited to 1 3 of the values expected for non overpressured conditions these low effective stresses should lead to a mechanically weak and predominantly aseismic plate boundary the region of lowest effective stress coincides with precisely located very low frequency earthquakes providing the first quantitative evidence linking these anomalous slip events to low stress and high pore pressure

experimental compaction of quartz sand at low effective stress and temperature conditions experiments have been carried out on dry and fluid saturated quartz sands to investigate the behaviour during compaction under diagenetic conditions and to evaluate the importance of stress induced solution transfer 9pressure solution9 as a compaction mechanism the experiments were performed at temperatures t in the range 150 350 c applied effective stresses e up to 20 7 mpa and pore fluid pressures p 1 of 12 5 and 15 5 mpa using material with a grain size d in the range 20 100 m and an initial porosity of 45 52 dry quartz sands underwent significant compac tion during the loading stage but showed very little compaction creep once full load was achieved i e essentially time independent compaction in contrast fluid saturated material at constant applied effective stress showed substantial time dependent compaction i e creep with increasing temperature there is a decreasing number of grains in the wet compacted sand which show intragranular cracks and an increasing number of grains which show dissolution features at contacts with adjacent grains in addition there is a decreasing dependence of the volumetric strain rate on e with both increasing volumetric strain e v and increasing e and a decreasing dependence of on e v with increasing temperature these observations suggest that for wet quartz sand a gradual change might occur from compaction creep controlled by time dependent microcracking at t 250 300 c to compaction creep controlled by stress induced intergranular solution transfer at t 300 350 c

crystallographic controls on the frictional behavior of dry and water saturated sheet structure minerals 1 we compare the frictional strengths of 17 sheet structure mineral powders measured under dry and water saturated conditions to identify the factors that cause many of them to be relatively weak the dry coefficient of friction ranges upward from 0 2 for graphite leveling off at 0 8 for margarite clintonite gibbsite kaolinite and lizardite the values of dry correlate directly with calculated 001 interlayer bond strengths of the minerals this correlation occurs because shear becomes localized along boundary and riedel shears and the platy minerals in them rotate into alignment with the shear planes for those gouges with dry 0 8 shear occurs by breaking the interlayer bonds to form new cleavage surfaces where dry 0 8 consistent with byerlee s law the interlayer bonds are sufficiently strong that other frictional processes dominate the transition in dry friction mechanisms corresponds to calculated surface energies of 2 3 j m2 adding water causes to decrease for every mineral tested except graphite if the minerals are separated into groups with similar crystal structures wet increases with increasing interlayer bond strength within each group this relationship also holds for the swelling clay montmorillonite whose water saturated strength is consistent with the strengths of nonswelling clays of similar crystal structure water in the saturated gouges forms thin structured films between the plate surfaces the polar water molecules are bonded to the plate surfaces in proportion to the mineral s surface energy and wet reflects the stresses required to shear through the water films

velocity dependence of strength and healing behaviour in simulated phyllosilicate bearing fault gouge abstract despite the fact that phyllosilicates are widespread in fault zones little is known about the strength of phyllosilicate bearing fault rocks under brittle ductile transitional conditions in this study we explored the steady state strength and healing behaviour of a simulated phyllosilicate bearing fault rock i e muscovite plus halite and brine at room temperature normal stresses of 1 9 mpa atmospheric fluid pressure and sliding velocities of 0 001 13 m s using a rotary shear apparatus while 100 halite and 100 muscovite samples exhibit rate independent frictional brittle behaviour the strength of mixtures containing 10 50 muscovite is both normal stress and sliding velocity dependent at low velocities 1 m s velocity weakening frictional behaviour occurs along with the development of a structureless intermixed cataclastic microstructure the steady state porosity of samples deformed in this regime increases with increasing sliding velocity we propose that this behaviour involves competition between dilatation due to granular flow and compaction due to pressure solution towards higher sliding velocities dilatation increasingly dominates over pressure solution compaction so that porosity increases and frictional strength decreases during periods of zero slip pressure solution compaction occurs causing a significant strength increase on reshearing our results imply that cataclastic overprinting of mylonitic rocks in natural fault zones does not require any changes in temperature or effective pressure conditions but can simply result from oscillating fault motion rates our healing data suggest that foliated aseismically creeping fault segments will remain weak and aseismic whereas segments that have slipped seismically will rapidly re strengthen and remain in the unstable velocity weakening regime

friction on subduction megathrust faults beyond the illite muscovite transition abstract previous experimental studies addressing subduction megathrust friction have focused on shallow fault gouges dominated by smectite illite and quartz here we aim to determine the effect of the transition to muscovite rich fault rock at depths beyond the illite dominated region of the seismogenic zone to achieve this rotary friction experiments were performed on simulated muscovite quartz 65 35 gouge at 100 600 c an effective normal stress of 170 mpa pore pressures of 100 and 200 mpa and sliding velocities of 1 100 m s we isolated the effects of muscovite by shearing pure muscovite gouge at 200 600 c under otherwise identical conditions at low shear strains 1 muscovite quartz gouge showed a friction coefficient of 0 4 0 5 while pure muscovite gouge exhibited 0 3 both gouges showed an increase in with and temperature the muscovite quartz gouge was characterised by velocity strengthening or neutral behaviour at 100 350 c velocity weakening at 350 500 c and velocity strengthening at 500 600 c in contrast pure muscovite gouge showed predominantly velocity strengthening or neutral behaviour demonstrating a key role of quartz in producing velocity weakening the velocity weakening regime in the mixed gouge extends that of 250 400 c recently reported for illite quartz gouge up to 500 c taking into account the observed effect of sliding velocity on a b the data for illite quartz and muscovite quartz together help explain the depth extent of seismogenesis on subduction megathrusts interestingly comparison with previous microphysical models suggests that the mechanism causing velocity weakening in phyllosilicate quartz gouges is one of granular flow involving competition between shear induced dilation and compaction through thermally activated quartz deformation

on the mechanical role of smectite in subduction zones smectites a group of hydrated phyllosilicate minerals are often a main component of sediments entering a subduction zone they are common in hemipelagic sediments and consist of an authigenetic component that arises from the alteration of volcanic ash intercalated in the sediment and a detrital component smectite rich strata are structurally weak retain water and are highly porous a review of sediment lithology in subduction zones around the world suggests that decollement zones may preferentially form in smectite rich horizons one consequence of this observation is that the assignment of high frictional resistance to materials in a decollement may be unwarranted at deeper structural levels smectite is transformed to illite a mechanically stronger mineral estimates of the depth of this transition coincide with the depth at which earthquakes are first detected along plate interfaces i speculate that the smectite illite transition in subduction zones may be an important component of sediment consolidation processes that trigger interplate seismicity

quantitative measure of the variation in fault rheology due to fluid rock interactions we analyze friction data from two published suites of laboratory tests on granite in order to explore and quantify the effects of temperature t and pore water pressure pp on the sliding behavior of faults rate stepping sliding tests were performed on laboratory faults in granite containing gouge granite powder both dry at 23 to 845 c lockner et al 1986 and wet pp 100 mpa at 23 to 600 c blanpied et al 1991 1995 imposed slip velocities v ranged from 0 01 to 5 5 m s and effective normal stresses were near 400 mpa for dried granite at all temperatures and wet granite below 300 c the coefficient of friction shows low sensitivity to v t and pp for wet granite above 350 drops rapidly with increasing t and shows a strong positive rate dependence and protracted strength transients following steps in v presumably reflecting the activity of a water aided deformation process by inverting strength data from velocity stepping tests we determined values for parameters in three formulations of a rate and state dependent constitutive law one or two state variables were used to represent slip history effects each velocity step yielded an independent set of values for the nominal friction level five constitutive parameters transient parameters a b1 and b2 and characteristic displacements dc1 and dc2 and the velocity dependence of steady state friction ss ln v a b1 b2 below 250 data from dry and most wet tests are adequately modeled by using the slip law ruina 1983 and one state variable a 0 003 to 0 018 b 0 001 to 0 018 dc 1 to 20 m dried tests above 250 can also be fitted with one state variable in contrast wet tests above 350 require higher direct rate dependence a 0 03 to 0 12 plus a second state variable with large negative amplitude b2 0 03 to 0 14 and large characteristic displacement dc2 300 to 4000 m thus the parameters a b1 and b2 for wet granite show a pronounced change in their temperature dependence in the range 270 to 350 c which may reflect a change in underlying deformation mechanism we quantify the trends in parameter values from 25 to 600 c by piecewise linear regressions which provide a straightforward means to incorporate the full constitutive response of granite into numerical models of fault slip the modeling results suggest that the succeptibility for unstable stick slip sliding is maximized between 90 and 360 c in agreement with laboratory observations and consistent with the depth range of earthquakes on mature faults in the continental crust

effects of smectite to illite transformation on the frictional strength and sliding stability of intact marine mudstones 1 at subduction zones earthquake nucleation and coseismic slip occur only within a limited depth range known as the seismogenic zone one leading hypothesis for the upper aseismic seismic transition is that transformation of smectite to illite at 100 150 c triggers a change from rate strengthening frictional behavior that allows only stable sliding to rate weakening behavior considered a prerequisite for unstable slip previous studies on powdered gouges have shown that changes in clay mineralogy alone are unlikely to control this transition but associated fabric and cementation developed during diagenesis remain possible candidates we conducted shearing experiments designed specifically to evaluate this hypothesis by using intact wafers of mudstone from ocean drilling program site 1174 offshore sw japan which have undergone progressive smectite transformation in situ we sheared specimens along a sawcut in a triaxial configuration oriented parallel to bedding at normal stresses of 20 150 mpa and a pore pressure of 1 mpa during shearing we conducted velocity stepping tests to measure the friction rate parameter a b friction coefficient ranges from 0 28 0 40 and values of a b are uniformly positive both are independent of clay transformation progress our work represents the most direct and comprehensive test of the clay transformation hypothesis to date and suggests that neither illitization nor accompanying fabric development and cementation trigger a transition to unstable frictional behavior we suggest that strain localization in combination with precipitation of calcite and quartz is a viable alternative that is consistent with both field observations and recent conceptual models of a heterogeneous seismogenic zone

effect of clay content and mineralogy on frictional sliding behavior of simulated gouges binary and ternary mixtures of quartz illite and montmorillonite 1 we investigated the frictional sliding behavior of simulated quartz clay gouges under stress conditions relevant to seismogenic depths conventional triaxial compression tests were conducted at 40 mpa effective normal stress on saturated saw cut samples containing binary and ternary mixtures of quartz montmorillonite and illite in all cases frictional strengths of mixtures fall between the end members of pure quartz strongest and clay weakest the overall trend was a decrease in strength with increasing clay content in the illite quartz mixture the trend was nearly linear while in the montmorillonite mixtures a sigmoidal trend with three strength regimes was noted microstructural observations were performed on the deformed samples to characterize the geometric attributes of shear localization within the gouge layers two micromechanical models were used to analyze the critical clay fractions for the two regime transitions on the basis of clay porosity and packing of the quartz grains the transition from regime 1 high strength to 2 intermediate strength is associated with the shift from a stress supporting framework of quartz grains to a clay matrix embedded with disperse quartz grains manifested by the development of p foliation and reduction in riedel shear angle the transition from regime 2 intermediate strength to 3 low strength is attributed to the development of shear localization in the clay matrix occurring only when the neighboring layers of quartz grains are separated by a critical clay thickness our mixture data relating strength degradation to clay content agree well with strengths of natural shear zone materials obtained from scientific deep drilling projects

talc friction in the temperature range 25 400 c relevance for fault zone weakening abstract talc is one of the weakest minerals that is associated with fault zones triaxial friction experiments conducted on water saturated talc gouge at room temperature yield values of the coefficient of friction shear stress effective normal stress n in the range 0 16 0 23 and increases with increasing n talc gouge heated to temperatures of 100 400 c is consistently weaker than at room temperature and talc has a temperature pressure range of stability that extends from surficial to eclogite facies conditions making it of potential significance in a variety of faulting environments talc has been identified in exhumed subduction zone thrusts in fault gouge collected from oceanic transform and detachment faults associated with rift systems and recently in serpentinite from the central creeping section of the san andreas fault typically talc crystallized in the active fault zones as a result of the reaction of ultramafic rocks with silica saturated hydrothermal fluids this mode of formation of talc is a prime example of a fault zone weakening process because of its velocity strengthening behavior talc may play a role in stabilizing slip at depth in subduction zones and in the creeping faults of central and northern california that are associated with ophiolitic rocks

influence of subduction zone conditions and gouge composition on frictional slip stability of megathrust faults to understand the temperature depth distribution of destructive earthquakes in subduction megathrusts and the mechanisms of nucleation of these events data on the frictional behaviour of phyllosilicate quartz rich megathrust fault gouges under in situ conditions are needed we performed rotary shear friction experiments at effective normal stresses of 25 200 mpa pore fluid pressures of 50 200 mpa at 140 600 c and sliding velocities of 1 100 m s using gouge mixtures with an illite quartz ratio between 65 35 and zero experiments on 65 35 mixtures deformed at an effective normal stress neff of 170 mpa a pore fluid pressure pf of 100 mpa and 150 500 c provided a reference dataset this showed three temperature dependent slip stability regimes with potentially unstable velocity weakening behaviour at 250 400 c and velocity strengthening at lower and higher temperatures the velocity weakening regime was found to shift towards higher temperatures with decreasing neff being located at 350 600 c at 50 mpa increasing quartz content and decreasing sliding velocity also displaced the velocity weakening regime towards lower temperatures increasing pf increased a b at all temperatures narrowing the temperature extent of the velocity weakening regime we explain our results qualitatively in terms of a microphysical model in which changes in friction coefficient and a b with velocity and temperature are brought about by changes in the relative importance of deformation of the clast phase by thermally activated stress corrosion cracking and pressure solution versus athermal granular flow of the mixture accompanied by dilatation our results imply that the depth extent of the seismogenic zone on subduction megathrusts depends not only on temperature and that to predict its location it is essential to have well constrained depth profiles for pore fluid pressure and effective normal stress

constraints on the stress state of the san andreas fault with analysis based on core and cuttings from san andreas fault observatory at depth safod drilling phases 1 and 2 1 analysis of field data has led different investigators to conclude that the san andreas fault saf has either anomalously low frictional sliding strength 0 6 arguments for the apparent weakness of the saf generally hinge on conceptual models involving intrinsically weak gouge or elevated pore pressure within the fault zone some models assert that weak gouge and or high pore pressure exist under static conditions while others consider strength loss or fluid pressure increase due to rapid coseismic fault slip the present paper is composed of three parts first we develop generalized equations based on and consistent with the rice 1992 fault zone model to relate stress orientation and magnitude to depth dependent coefficient of friction and pore pressure second we present temperature and pressure dependent friction measurements from wet illite rich fault gouge extracted from san andreas fault observatory at depth safod phase 1 core samples and from weak minerals associated with the san andreas fault third we reevaluate the state of stress on the san andreas fault in light of new constraints imposed by safod borehole data pure talc 0 1 had the lowest strength considered and was sufficiently weak to satisfy weak fault heat flow and stress orientation constraints with hydrostatic pore pressure other fault gouges showed a systematic increase in strength with increasing temperature and pressure in this case heat flow and stress orientation constraints would require elevated pore pressure and in some cases fault zone pore pressure in excess of vertical stress

simulating retrogressive slope failure using two different smoothed particle finite element methods a comparative study abstract various smoothed particle finite element methods spfems have been developed to simulate large deformation problems but their efficiency and accuracy in simulating progressive landslides in sensitive clays have remained unclear in this study a series of numerical analyses are carried out to investigate the development of retrogressive landslides by two spfems an edge based strain smoothing pfem espfem and a node based strain smoothing pfem nspfem in view of their outstanding performance in large deformation analysis a strain softening mohr coulomb mc model is adopted to simulate the behaviour of sensitive clays during landslide assuming a poisson s ratio of 0 49 to ensure undrained conditions the influence of mesh density on the development of retrogressive failure is evaluated for two spfems numerical analyses of three mesh sizes 0 2 m 0 15 m and 0 12 m are carried out sequentially with all results demonstrating that 1 the spread retrogressive landslides with horsts and grabens can be achieved by both spfems with the adopted soil model 2 run out and retrogression distances decrease as mesh density increases for both methods 3 retrogressive collapse occurs earlier for espfem but is delayed for nspfem with increased mesh density 4 nspfem allows faster calculations and reduces mesh dependency problems when compared with espfem and 5 the increase of shape factor can accelerate retrogressive evolution of landslides finally a real landslide in sensitive clay at sainte monique quebec is simulated to demonstrate espfem s computational efficiency and accuracy

temperature and isotopic composition of seawater in the epicontinental sea central paratethys during the middle miocene climate transition based on mg ca δ 18 o and δ 13 c from foraminiferal tests abstract details regarding seawater temperature and isotopic composition of seawater in the central paratethys during the miocene epoch are not well known here we present an insight based on results from a combination of mg ca 18o and 13c on foraminifera from the shallow core lom 1 representing the time interval of 14 4 to 14 36 ma globigerinoides trilobus globigerina bulloides cibicidoides spp and melonis pompilioides were chosen for analysis due to their variable ecological preferences within the water column and on the sea floor we assessed problems concerning diagenetic alteration of foraminiferal calcite by using a single test approach when analysing 18o and 13c paleotemperature ranges turned out to be comparable with modern subtropical regions with no cooling trend as was expected at this period following the middle miocene climatic optimum however we find an interesting offset between the benthic species temperature response that is likely related to the different life strategies of those species based on a combination of mentioned proxies it was possible to partly reconstruct 18oseawater for the studied locality and compare it with mg ca derived temperature to obtain a possible correlation between temperature and salinity we also investigated differences in surface and bottom water hydrography in comparison with the mediterranean and distinguish their different hydrographies in addition it was possible to identify dissimilar origins of surface and bottom water masses based on comparison of their isotopic composition

effect of hydration state on the frictional properties of montmorillonite based fault gouge 1 we report on laboratory experiments examining the effect of hydration state on the frictional properties of simulated clay and quartz fault gouge we tested four mixtures of ca montmorillonite and quartz 100 70 50 and 30 montmorillonite at four hydration states dry 16 0 wt water we controlled the hydration state using either oven drying for 13 wt h2o under conditions of controlled relative humidity for each clay quartz mixture and hydration state we measured frictional properties over a range of normal stresses 5 100 mpa and sliding velocities 1 300 m s we observe a systematic decrease in the coefficient of friction with increasing water content normal stress and clay content values of for 50 50 mixtures range from 0 57 to 0 64 dry and decrease to 0 21 0 55 for the most hydrated cases wet for layers of 100 montmorillonite ranges from 0 41 0 62 dry to 0 03 0 29 wet as water content is increased from 0 to 20 0 wt the friction rate parameter a b becomes increasingly positive variation in a b values decreases dramatically as normal stress increases if our experimental results can be applied to natural fault gouge the combination of stress state hydration state and quartz content that facilitates unstable fault behavior implies that the onset of shallow seismicity in subduction zones is more complicated than a simple transition from smectite to illite

fault stability inferred from granite sliding experiments at hydrothermal conditions seismicity on crustal faults is concentrated in the depth interval 1 3 to 12 15km tse and rice 1986 suggested that the lower bound on seismicity is due to a switch with increasing temperature from velocity weakening destabilizing to velocity strengthening stabilizing friction they inferred this transition from friction data for dry granite however pore fluids exist at elevated temperatures throughout the crust and may strongly influence strength and sliding behavior we present new data from sliding experiments on granite at elevated t 23 to 600 plus elevated ph2o 100 mpa our results show velocity strengthening at room temperature but velocity weakening from 100 to 350 c except at 250 from 350 to 600 there are systematic trends from velocity weakening to strong velocity strengthening and from high to low friction neither trend was seen in tests on dry granite the velocity dependence data imply the potential for unstable slip in the interval 100 to 350 using a geotherm to map temperature to depth this interval closely matches the observed earthquake distribution

friction experiments on saturated sediments and their implications for the stress state of the nankai and barbados subduction thrusts the nankai and barbados forearcs have low stress subduction thrusts the sediments entering the subduction zone and namely the material in the decollement zones have been well characterized by numerous deep sea drilling legs and studies of the recovered cores nankai has high heat flow and significant amounts of illite while barbados is a smectite dominated system based on results from ring shear 2 mpa normal stress and direct shear 30 mpa tests on marine sediments and mineral standards this translates into a residual frictional resistance of r 0 25 and r 0 11 in clay horizons respectively such values agree with theoretical estimates from critical wedge theory nankai b 0 16 0 26 and barbados b 0 06 0 09 and fault spacing geometries from seismic profiles nankai b 0 12 0 23 and barbados b 0 11 0 19 maximum pore pressure ratios of 0 85 and 0 73 for nankai and barbados respectively allow us to estimate effective shear stresses as a function of friction coefficient and density of the sediment gouge to reach only 10 mpa or less in the frontal 50 km of the decollement zone respectively our data support the contention that fluid pressure transients and sediment composition contribute equally to the weakness along plate boundary faults down to the seismogenic zone with the first probably dominating the shallow decollement shear velocity stepping tests show that the clay dominated gouges strengthen velocity irrespective of the clay mineralogy and hence suggest that clay transformation does not affect the onset of seismogenesis

the effect of mineralogy and effective normal stress on frictional strength of sheet silicates abstract phyllosilicates are common constituents of upper crustal faults and subduction forearcs we studied the effect of mineralogy and controlled effective normal stress between 5 mpa and 100 mpa on frictional strength although the effect of mineralogy on frictional strength of single phase phyllosilicate gouges has been previously studied the influence of effective normal stress has not we conducted water saturated and vacuum dry frictional tests on single phase phyllosilicate gouges using a triaxial apparatus minerals included talc pyrophyllite kaolinite lizardite illite montmorillonite chlorite muscovite phlogopite and biotite particle size 20 mpa dry friction coefficients are constant and wet friction coefficients show a small increase this is attributed to loss of water and increased contact area the results indicate that frictional strength of clay rich faults increases at depths less than 1 km under hydrostatic pore fluid pressures

effects of dissolution precipitation processes on the strength and mechanical behavior of quartz gouge at high temperature hydrothermal conditions laboratory experiments on simulated quartz gouges at a temperature of 927 c a confining pressure of 300 mpa and a pore water pressure of 200 mpa demonstrate the interactions between cataclastic and dissolution precipitation processes and their effects on the strength and mechanical behavior significant porosity reduction and microstructures including grain interlocking grain interpenetration and widespread growth of euhedral shaped grains indicate the operation of dissolution precipitation processes under conditions which favor the activity of dissolution precipitation processes i e for small grain size or at slow displacement rates deformation is distributed across the whole gouge layer and strength increases continuously with increasing displacement so that coefficients of friction reach to 0 7 in contrast for specimens with large grain size sheared at fast displacement rates deformation is localized along a gouge forcing block interface after a small amount of displacement and results in slip softening and subsequent quasi stable sliding with coefficients of friction as low as 0 45 analysis of our results indicates that where the kinetics of dissolution precipitation is sufficiently fast to partly accommodate shear strain the mechanical behavior and strength of quartz gouge can be characterized by slip hardening and high strength with velocity strengthening in contrast where cataclastic processes predominantly accommodate shear strain the mechanical behavior and strength can be characterized by slip softening and subsequent localized sliding with low strength and velocity strengthening

frictional behaviour of simulated quartz fault gouges under hydrothermal conditions results from ultra high strain rotary shear experiments abstract numerous experimental studies have been conducted to explore the mechanical behaviour and strength properties of rock materials most of these studies however do not take the effects of fluids into account though it has long been recognized that fluids might play an important role in the deformation of crustal rocks under brittle ductile conditions moreover all these studies are limited in the total shear strain that could be reached in this study we report on ultra high strain experiments performed on simulated quartz fault gouges in a rotary shear apparatus under hydrothermal conditions effective normal stresses of 20 100 mpa a fluid pressure of 200 mpa temperatures of 400 600 c sliding velocities of 0 01 1 m s and total shear strains up to 50 the experiments show strain hardening up to a shear strain of 0 6 1 8 followed by strain weakening of up to 30 towards a steady state value at a strain of 8 12 this strain weakening effect is much higher than previously reported for quartz gouge the steady state shear strength increases with decreasing grain size increasing sliding velocity and decreasing temperature the microstructure of the deformed quartz gouge is characterized by the presence of a through going boundary parallel y shear some samples also show riedel shears oriented oblique to the shear zone boundary deformation in these tests was largely by cataclastic processes with most displacement being accommodated along the boundary parallel y shear causing the marked weakening observed intergranular pressure solution did not accommodate significant shear strain but appears to play a role in smoothing and weakening the localized slip surface and in controlling gouge compaction

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

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

a microphysical model for strong velocity weakening in phyllosilicate bearing fault gouges 1 previous rotary shear experiments performed on a halite muscovite fault gouge analogue system have shown that the presence of phyllosilicates under conditions favoring the operation of cataclasis and pressure solution in the matrix phase can have major effects on the frictional behavior of gouges while 100 halite and 100 muscovite samples exhibit rate independent frictional brittle behavior the strength of mixtures containing 10 30 muscovite is both normal stress and sliding velocity dependent at high sliding velocities 1 m s 1 such mixtures show unusually marked velocity weakening along with the development of a structureless cataclastic microstructure in the present paper a micromechanical model is developed in an attempt to explain this behavior the model assumes a granular flow process involving competition between intergranular dilatation and compaction by pressure solution the predictions of the model agree favorably with the experimental results extension of the model to quartz mica systems implies that the presence of phyllosilicates plus the operation of pressure solution can strongly promote unstable velocity weakening behavior at rapid slip rates on natural faults under midcrustal conditions static stress drop predictions based on the model agree reasonably well with estimates from seismic observations our results may help explain the discrepancy between laboratory derived rate and state friction parameter values obtained for dry low strain and or single phase rock systems and the values for natural fault rocks inferred from seismological data