comparison of smectite and illite rich gouge frictional properties application to the updip limit of the seismogenic zone along subduction megathrusts along plate boundary subduction thrusts the transformation of smectite to illite within fault gouge at temperatures of 150 c is one of the key mineralogical changes thought to control the updip limit of seismicity if correct this hypothesis requires illite rich gouges to exhibit frictionally unstable velocity weakening behavior here we report on laboratory experiments designed to investigate the frictional behavior of natural and synthetic clay rich gouges we sheared 5 mm thick layers of commercially obtained pure ca smectite a suite of smectite quartz mixtures and natural illite shale grain size ranging from 2 to 500 m in the double direct shear geometry to shear strains of 7 30 at room humidity and temperature xrd analyses show that the illite shale contains dominantly clay minerals and quartz within the clay sized fraction 40 mpa our data specifically the velocity strengthening behavior of illite shale under a wide range of conditions do not support the hypothesis that the smectite illite transition is responsible for the seismic aseismic transition in subduction zones we suggest that other depth and temperature dependent processes such as cementation consolidation and slip localization with increased shearing may play an important role in changing the frictional properties of subduction zone faults and that these processes in addition to clay mineralogy should be the focus of future investigation

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

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

the evolution of illite to muscovite an example of the behaviour of isotopes in low grade metamorphic terrains abstract a review of the possibilities and limitations of the use of radiogenic isotopes in low grade terrains i e in the temperature range below the blocking temperature of potassic white mica is discussed using a background of a data base of over 500 isotope analyses on more than 250 illitic muscovitic samples published in various papers special stress is laid on the main problems 1 distinction between detrital and neoformed minerals 2 the slow evolution of minerals without pronounced breaks and 3 retention problems of radiogenic daughter products a brief description of low grade metamorphism is given and criteria enabling a subdivision into low to very low grade metamorphism are discussed comprising illite crystallinity mineralogy fluid inclusions and coal rank measurements the useful type of lithologies for this kind of work ranges from shales slates through volcaniclastic tuffs with high primary glass contents and limestone dolomites marbles to mylonites it can be concluded that carefully selected illitic muscovites yield geologically meaningful ages provided that more than one isotopic method is applied

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

frictional properties of megathrust fault gouges at low sliding velocities new data on effects of normal stress and temperature abstract friction data used in modelling studies of subduction zone seismogenesis are often poorly representative of in situ conditions we investigated the influence of in situ effective stresses and temperatures on the frictional properties of simulated fault gouges prepared either from nankai odp material or illite shale at sliding velocities approaching those relevant for earthquake nucleation and sses biaxial double direct shear experiments were performed at room temperature normal stresses of 5 30 mpa and sliding velocities of 0 16 18 m s all materials exhibited velocity strengthening under these conditions along with an increase in the friction coefficient and slip hardening rate with increasing normal stress illite gouge showed increased velocity strengthening towards higher normal stresses the effect of temperature was investigated by means of ring shear experiments on illite gouge at 200 300 c an effective normal stress of 170 mpa a pore fluid pressure of 100 mpa and sliding velocities of 1 100 m s these experiments showed a transition from velocity strengthening to velocity weakening at 250 c our results provide a possible explanation for the updip seismogenic limit within subduction zone megathrusts and imply an enhanced tendency for earthquake nucleation and sses at low effective normal stresses

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

the volcanoseismic swarms of 1981 1983 in the tahiti mehetia area french polynesia during the years 1981 1983 three intense seismic swarms took place in the tahiti mehetia area of french polynesia at the presumed location of the society island hot spot the 1981 swarm featured 4000 earthquakes with a maximum magnitude ml 4 3 in the immediate vicinity of the island of mehetia the 1982 swarm occurred along the flank of the major teahitia seamount and involved more than 9000 events a second swarm occurred at teahitia in 1983 and involved 3000 events although no precise constraint can be placed on the depth of individual events from their travel times to polynesian stations features in the evolution of the mehetia swarm are generally consistent with the probable ascent of a magma body toward the surface in the case of teahitia the recording of abundant tremors of both high and low frequency particularly intense during the 1983 swarm is directly similar to cases of documented volcanic eruptions the swarms are interpreted as episodes of active volcanism part of the process of building the next island in the chain appendix tables are available with the entire article on micro fiche order from american geophysical union 2000 florida avenue n w washington d c 20009 document b84 005 2 50 payment must accompany order

correlation between velocity dependence of friction and strain localization in large displacement experiments on feldspar muscovite and biotite gouge abstract room temperature rotary shear experiments on rock powders of albite feldspar and muscovite and biotite micas show that the degree of localization of shear strain in these materials correlates with the frictional behavior in both feldspar and mica experiments increasing localization correlates with a lower coefficient of friction and a more negative velocity dependence of friction feldspar gouges tend to have localized deformation along one or more boundary parallel shears and a negative velocity dependence more localized deformation correlates with a lower coefficient of friction and a more negative velocity dependence in general micas deform throughout a broader zone than feldspar and have a lower coefficient of friction compared to muscovite biotite is weaker has a negative rather than a positive velocity dependence and shows more localized deformation for materials as diverse as feldspars and micas localization along boundary parallel shears correlates with negative velocity dependence along compliant natural faults a negative velocity dependence would result in a greater tendency for unstable sliding thus localized boundary parallel shears may indicate negative velocity dependence and slip by earthquakes on natural faults

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

frictional slip of granite at hydrothermal conditions sliding on faults in much of the continental crust likely occurs at hydrothermal conditions i e at elevated temperature and elevated pressure of aqueous pore fluids yet there have been few relevant laboratory studies to measure the strength sliding behavior and friction constitutive properties of faults at hydrothermal conditions we slid laboratory granite faults containing a layer of granite powder simulated gouge velocity stepping experiments were performed at temperatures of 23 to 600 c pore fluid pressures ph2o of 0 dry and 100 mpa wet effective normal stress of 400 mpa and sliding velocities v of 0 01 to 1 m s 0 32 to 32 m yr conditions were similar to those in earlier tests on dry granite to 845 c by lockner et al 1986 the mechanical results define two regimes the first regime includes dry granite up to at least 845 and wet granite below 250 c in this regime the coefficient of friction is high 0 7 to 0 8 and depends only modestly on temperature slip rate and ph2o the second regime includes wet granite above 350 c in this regime friction decreases considerably with increasing temperature temperature weakening and with decreasing slip rate velocity strengthening these regimes correspond well to those identified in sliding tests on ultrafine quartz we infer that one or more fluid assisted deformation mechanisms are activated in the second hydrothermal regime and operate concurrently with cataclastic flow slip in the first cool and or dry regime is characterized by pervasive shearing and particle size reduction slip in the second hot and wet regime is localized primarily onto narrow shear bands adjacent to the gouge rock interfaces weakness of these boundary shears may result either from an abundance of phyllosilicates preferentially aligned for easy dislocation glide or from a dependence of strength on gouge particle size major features of the granite data set can be fit reasonably well by a rate and temperature dependent three regime friction constitutive model chester this issue we extrapolate the experimental data and model fit in order to estimate steady state shear strength versus depth along natural slipping faults for sliding rates as low as 31 mm yr we do this for two end member cases in the first case pore pressure is assumed hydrostatic at all depths shallow crustal strength in this case is similar to that calculated in previous work from room temperature friction data while at depths below about 9 13 km depending on slip rate strength becomes less sensitive to depth but sensitive to slip rate in the second case pore pressure is assumed to be near lithostatic at depths below 5 km strength is low at all depths in this case 20 mpa in agreement with observations of weak faults such as the san andreas the predicted depth of transition from velocity weakening to velocity strengthening lies at about 13 km depth for a slip rate of 31 mm yr in rough agreement with the seismic aseismic transition depth observed on mature continental faults these results highlight the importance of fluid assisted deformation processes active in faults at depth and the need for laboratory studies on the roles of additional factors such as fluid chemistry large displacements higher concentrations of phyllosilicates and time dependent fault healing

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

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

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

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

compositional and fluid pressure controls on the state of stress on the nankai subduction thrust a weak plate boundary abstract we show that both fault mineralogy and regional excess fluid pressure contribute to low resolved shear stresses on the nankai subduction plate boundary off southwest japan ring and direct shear tests indicate that saturated clay minerals in the fault possess intrinsically low residual friction coefficients r at stress levels between 1 0 and 40 mpa the direct shear r values for purified smectite are 0 14 0 02 for illite 0 25 0 01 and for chlorite 0 26 0 02 for point load velocities of 0 0001 mm s these clay minerals dominate the nankai subduction decollement zone illite plus quartz is mechanically important in the altered incoming muroto section and the predicted decollement r should lie between 0 2 and 0 32 this low residual strength together with elevated fluid pressure limits shear stresses to below 4 mpa within the frontal 50 km of the subduction system consistent with the low wedge taper in this region a higher wedge taper off the ashizuri peninsula indicates basal shear stresses rise slightly along strike towards this region our analysis indicates lower fluid pressures must predominantly be responsible because only small second order along strike variations in r are predicted to occur as a result of variations in smectite and total clay content these variations should be further reduced at depth under the wedge as smectite is diagenetically altered to illite however our data suggest the low r values of the clay rich decollement still limit shear stresses to between 17 and 29 mpa within the frontal 50 km of the wedge consistent with other estimates of plate boundary weakness indeed we propose that it should be expected that subduction plate boundaries like nankai will be weak because of the intrinsic presence of clay rich faults and moderate fluid overpressures our data do not support the hypothesis that the smectite to illite reaction directly controls the onset of seismogenic behavior deep in the nankai system because there is already a mechanical dominance of illite rather than smectite in the shallow decollement zone and we find all the clay phases tend to velocity strengthen however temperature activated clay diagenesis and dehydration may cause secondary changes in the fault properties and state of stress across the up dip limit of the seismogenic zone

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

spacing of consecutive normal faulting in the lithosphere a dynamic model for rift axis jumping tyrrhenian sea we propose that the flexure of a broken elastic plate can determine the spacing of consecutive and localized zones of shearing on a lithospheric scale the model provides a dynamic explanation for rift axis migration and as a consequence for a wide rift mode of extension calculated values for rift axis shifting are consistent with the available observations and kinematic modelling results in the tyrrhenian basin the quantitative relation obtained between lithospheric fault spacing and the effective elastic thickness t of the lithosphere can be used in an inverse modelling approach in which paleo t e e values are calculated for a basin where the diachronous and spaced nature of rifting is known

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

new constraints on megathrust slip stability under subduction zone p t conditions abstract to understand and model subduction zone seismogenesis data are needed on the frictional properties of meta pelitic subduction zone materials under in situ megathrust conditions here we report the results of rotary shear friction experiments on simulated illite quartz fault gouge at an effective normal stress of 170 mpa a pore fluid pressure of 100 mpa at 150 500 c and sliding velocities of 1 100 m s the results show three temperature dependent regimes characterized by velocity strengthening at 150 250 c velocity weakening at 250 400 c and velocity strengthening at 400 500 c the regimes are defined by a decrease in the rate and state friction rsf parameter a b at 150 300 c followed by an increase at 300 500 c these trends correlate with systematic changes in other rsf parameters as well as an increase in friction coefficient above 300 c we explain the effects of increasing temperature in terms of a transition from frictional granular flow 150 250 c through granular flow whereby dilatation is balanced by thermally activated compaction involving stress corrosion cracking scc of quartz clasts 250 400 c to non dilatant slip on the phyllosilicates with accommodation at quartz clasts by scc 400 500 c taking into account the effects of sliding velocity the observed velocity weakening regime broadly explains the extent of the seismogenic zone within subduction megathrusts