seismic and aseismic slip indicators in serpentinite gouge structures observed within experimentally deformed serpentinite gouges provide information that may be used to identify the seismic behavior of natural fault zones in laboratory friction experiments serpentinite exhibits two modes of behavior one can only result in stable fault creep and the other may result in stable slip but has the potential for earthquakes the microstructures that form during these experiments reflect the deformation style of the serpentinite distributed deformation results from aseismic fault creep and localized deformation results from conditions favorable for seismic slip distributed deformation produces a crystallographic preferred orientation of the serpentine grains s foliation localized deformation forms riedel shears similar structures occur within a natural serpentinite shear zone from monterey county california and suggest a history of stable fault creep with intermittent seismic events

lithology and internal structure of the san andreas fault at depth based on characterization of phase 3 whole rock core in the san andreas fault observatory at depth safod borehole abstract we characterize the lithology and structure of the spot core obtained in 2007 during phase 3 drilling of the san andreas fault observatory at depth safod in order to determine the composition structure and deformation processes of the fault zone at 3 km depth where creep and microseismicity occur a total of approximately 41 m of spot core was taken from three separate sections of the borehole the core samples consist of fractured arkosic sandstones and shale west of the saf zone pacific plate and sheared fine grained sedimentary rocks ultrafine black fault related rocks and phyllosilicate rich fault gouge within the fault zone north american plate the fault zone at safod consists of a broad zone of variably damaged rock containing localized zones of highly concentrated shear that often juxtapose distinct protoliths two zones of serpentinite bearing clay gouge each meters thick occur at the two locations of aseismic creep identified in the borehole on the basis of casing deformation the gouge primarily is comprised of mg rich clays serpentinite lizardite chrysotile with notable increases in magnetite and ni cr oxides hydroxides relative to the surrounding host rock the rocks surrounding the two creeping gouge zones display a range of deformation including fractured protolith block in matrix and foliated cataclasite structure the blocks and clasts predominately consist of sandstone and siltstone embedded in a clay rich matrix that displays a penetrative scaly fabric mineral alteration veins and fracture surface coatings are present throughout the core and reflect a long history of syn deformation fluid rock reaction that contributes to the low strength and creep in the meters thick gouge zones

porous fluid flow enables oceanic subduction initiation on earth 1 although most of the presently active intra oceanic subduction zones are relatively young and initiated during the cenozoic subduction initiation process remains poorly understood previous models of subduction initiation assumed excessive weakening of tectonic plate boundaries that does not reconcile with laboratory rock strength measurements the weakening was assumed to be caused by fluids present along tectonic fractures however no self consistent solid fluid model of subduction initiation has been developed so far here we present new numerical hydro thermo mechanical model of spontaneous intra oceanic subduction initiation where solid rock deformation and fluid percolation are fully coupled based on 2 d numerical experiments we demonstrate that although subduction fails to initiate under fluid absent conditions it can naturally start when porous fluid is present inside oceanic crust and along the plate boundaries fluid percolation is localized along spontaneously forming faults where high fluid pressure compensates lithostatic pressure thus dramatically decreasing friction along the incipient subduction zone through the parametric study we conclude that the most important parameter for subduction initiation is the solid matrix permeability paradoxical at first lowering the permeability indeed favors subduction initiation by maintaining high fluid pressure and thus decreasing friction along active faults

importance of pressure solution and coble creep in the deformation of polymineralic rocks the theoretical framework for modeling pressure solution in polyphase rocks is presented it is often assumed that pressure solution is a deformation mechanism of secondary importance deep in the lithosphere it is shown here that the rheology of rock during pressure solution may be drastically reduced in a bimineralic and polymineralic rock relative to its monomineralic counterparts and therefore the importance of this deformation mechanism importance is perhaps underestimated this behavior contrasts with that of a polymineralic rock undergoing dislocation creep where existing models show that the rheology will be some average of the rheologies of individual phases for instance the rheology of a dunite in pressure solution is governed by the diffusion of both mgo and sio2 whichever is the slowest will control the viscosity of the rock conversely in a harzburgite with a specific texture flow will be governed by the fastest diffusing species thus its strength may be as low as that of a quartz rock of comparable grain size given the comparative strength of many phases undergoing dislocation creep relative to quartz this prediction implies a larger range of conditions under which pressure solution forms an important part of bulk rock deformation than inferred previously predictions on textural evolution of polymineralic aggregates are also made and it is shown that some minerals grains may become elongate parallel to the maximum principal stress during deformation even though the rock as a a whole shortens in this direction

quantitative determination of chrysotile in massive serpentinites using dta implications for asbestos determinations there is increasing concern about the health hazard of asbestos from natural geologic deposits such as greenstones natural occurring asbestos noa therefore quantitative determination of the chrysotile asbestos content within massive serpentinites is a recurrent requirement of recent asbestos inherent law regulations due to the possible health hazard associated with the release of chrysotile fibers unfortunately the obtainment of accurate and precise quantitative figures of the actual chrysotile content is strongly complicated by typical serpentinite textures consisting of fine to ultrafine intergrowths of fibrous and non fibrous serpentine minerals often difficult to identify by conventional methods such as x ray diffraction or microanalytical approaches in this paper we propose a reliable and straightforward method for the quantitative determination of chrysotile asbestos within bulk massive serpentinites based on thermal analysis data and specifically on the distinctive thermal behavior of chrysotile lizardite and antigorite during dehydroxylation at 500 800 c deconvolution processing of dta endothermic signals in the dehydroxylation temperature range revealed good linear correlation between peak area ratios and chrysotile content for both lizardite chrysotile and antigorite chrysotile samples the dta correlation curves have been used to determine the chrysotile content in two test serpentinites revealing surprisingly high chrysotile content this novel method is of vast importance as it represents one of the most promising tools for chrysotile quantitative determinations in massive serpentinites providing unbiased and accurate responses to recent asbestos related law requirements

the frictional behavior of lizardite and antigorite serpentinites experiments constitutive models and implications for natural faults laboratory studies of the frictional behavior of rocks can provide important information about the strength and sliding stability of natural faults we have conducted friction experiments on antigorite and lizardite serpentinites rocks common to both continental and oceanic crustal faults we conducted both velocity step tests and timed hold tests on bare surfaces and gouge layers of serpentinite at room temperature we find that the coefficient of friction of lizardite serpentinite is quite low 0 15 0 35 and could explain the apparent low stresses observed on crustal transform faults while that of antigorite serpentinite is comparable to other crustal rocks 0 50 0 85 the frictional behavior of both types of serpentinite is well described by a two mechanism model combining state variable dominated behavior at high slip velocities and flow dominated behavior at low velocities the two mechanism model is supported by data from velocity step tests and timed hold tests the low velocity behavior of serpentinite is strongly rate strengthening and should result in stable fault creep on natural faults containing either antigorite or lizardite serpentinite

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 stress state constraints on the saf

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

imaging capability of cross hole seismic reflection surveys1 in seismic migration it is important to sample a range of dips around the local structural dip at each image point meaningful images are obtained only where this condition holds for cross hole seismic reflection surveys the distribution of dips sampled at each image point is controlled principally by the survey geometry including source and receiver array lengths and their element spacings using a real data set as an example the authors show how survey geometry can limit imaging capability close to the boreholes and even in the middle of the section between the boreholes at the processing stage effective removal of direct waves and accurate estimation of the velocity field are essential for optimizing image quality for migration they propose a generalized berryhill gb scheme which is based on the kirchhoff integral and takes into account both the near field and far field terms this should improve the ability to image close to source and receiver arrays provided that the element spacing in the nearby array is small enough

extreme dynamic weakening of faults during dehydration by coseismic shear heating the dynamic strength of seismogenic faults has a critical effect on earthquake slip instability and seismic energy release high velocity friction experiments on simulated faults in serpentinite at earthquake slip conditions show a decrease in friction coefficient from 0 6 to 0 15 as the slip velocity reaches 1 1 m s at normal stresses up to 24 5 mpa the extraordinary reduction in fault strength is attributed to flash heating at asperity contacts of gouge particles formed during sliding the rapid heating at asperities causes serpentine dehydration in impermeable fault zones in nature serpentine dehydration and subsequent fluid pressurization due to coseismic frictional heating may promote further weakening this dynamic fault weakening mechanism may explain the lack of pronounced heat flow in major crustal faults such as the san andreas

elastic reverse time migration elastic prestack reverse time finite difference migration of two component seismic surface data requires data extrapolation and application of an imaging condition data extrapolation involves synchronous driving of the vertical component and horizontal component finite difference meshes with the time reverse of the recorded vertical and horizontal traces respectively extrapolation uses the coupled elastic wave equation for variable velocity solved with a second order explicit finite difference scheme the imaging condition at any point in the grid is the one way traveltime from the source to that point elastic migrations of both synthetic test data and real two component common source gathers produce simpler images than acoustic migrations because of the coalescing of double reflections compressional waves and shear waves into single loci

deformation mechanisms and phase distribution in mafic high temperature mylonites from the jotun nappe southern norway abstract shear zones and mylonites in anorthositic to gabbroic rocks from the jotun nappe in southern norway have been studied as an example of high temperature deformation in polyphase rocks the deformation has taken place at approximately 700 c and maximum confining pressures of 900 mpa the constituent phases amphibole pyroxene and plagioclase have recrystallized synkinematically reducing the grain size of the rock to about 20 to 70 m two different microstructures develop depending on the recrystallization mechanism dynamic recrystallization of plagioclase hornblende and pyroxenes produces monophase layers which extend from porphyroclasts heterogeneous nucleation of phases due to chemical disequilibrium produces phase mixtures of hornblende and plagioclase or clinopyroxene and plagioclase these phase mixtures extend from hornblende or clinopyroxene porphyroclasts the product of the deformation and simultaneous recrystallization is a well laminated microstructure of some monophase hornblende pyroxene and plagioclase layers which alternate with abundant layers of phase mixtures different deformation mechanisms operate in the different types of microstructures dislocation substructures dynamic recrystallization microstructures and a crystallographic preferred orientation measured by ji and mainprice 1988 on samples of the same shear zone indicate crystal plastic deformation in monophase plagioclase layers dynamic recrystallization in monophase hornblende layers indicates also crystal plastic deformation the phase distribution of plagioclase and hornblende grain shapes and lower dislocation densities in the phase mixtures suggest that the deformation mechanism is not crystal plasticity but rather a granular flow grain boundary sliding probably accommodated by diffusional mass transfer in the plagioclase hornblende mixtures the phases show an anticlustered non random distribution grain boundary sliding may contribute to phase mixing but cannot be the main reason for the anticlustering of phases and the complete phase mixing directly at the boundaries of the porphyroclasts the reason for the anticlustered distribution is not clear but could be explained by preferential heterogeneous nucleation of hornblende at plagioclase grain boundaries

implications of the mechanical and frictional behavior of serpentinite to seismogenic faulting the widespread occurrence of serpentinite along major strike slip seismogenic faults warrants systematic investigation to determine how its frictional characteristics may affect slip along the fault different locations along the motagua fault zone in guatemala were sampled to investigate the sliding mode as a function of composition and texture confining pressure and displacement rate air dried right circular cylinders 7 1 cm in length and 3 3 cm in diameter with a precut 35 to the cylinder and load axes were deformed at confining pressures up to 200 mpa room temperature and displacement rates of 10 4 and 10 7 cm s compositional analyses of specimens cored from five blocks of serpentinite that were tested show that the serpentinite can be divided into two groups one is a mesh textured serpentinite containing up to 70 serpentine mostly lizardite 11 enstatite phenocrysts 19 oxides and minor amounts of olivine and carbonate undifferentiated the other is a flare textured serpentinite containing up to 84 serpentinte mostly antigorite 10 oxides 6 magnesite and dolomite and almost no enstatite or olivine the flare serpentinite undergoes the transition from stable sliding to stick slip sliding at confining pressures as low as 10 mpa the mesh serpentinite results only in stable sliding up to confining pressures of 200 mpa and a displacement rate of 10 4 cm s at a displacement rate of 10 7 cm s only one of the three mesh blocks exhibits stick slip the shear stress required to initiate sliding for the flare textured serpentinites is given by 0 77 n for the mesh textured serpentinites at a displacement rate of 10 4 cm s it is given by 0 56 n and at a displacement rate of 10 7 cm s by 0 50 n differences in fracture strength between the five specimen blocks correlate with differences in frictional strength differences in the sliding behavior and in the deformation observed along the sliding surfaces can be related to the mineralogical components of each serpentinite type extensive deformation occurs in the mesh serpentinites as a result of hard oxide and enstatite indenters ploughing through a soft lizardite substrate lizardite lizardite contacts prevail and these deform by plastic flow the stick slip behavior in the flare textured serpentinites is explained by brittle failure of antigorite antigorite contacts the smaller grain size of the flare serpentinites relative to the mesh serpentinites may also contribute to the higher frictional strength of the flare serpentinites we discuss the implications of the antigorite rich flare textured serpentinite to seismogenic faulting in particular we consider the case of antigorite forming by shear heating induced progressive metamorphism of lizardite chrysotile serpentinites if serpentine dehydration temperatures are approached talc may form having a lower coefficient of friction than serpentine talc may act as a lubricant along the fault plane

frictional properties of saponite rich gouge from a serpentinite bearing fault zone along the gokasho arashima tectonic line central japan abstract we studied a serpentinite bearing fault zone in gokasho arashima tectonic line mie prefecture central japan characterizing its internal structures mineral assemblage permeability and frictional properties the fault core situated between the serpentinite breccia and the adjacent sedimentary rocks is characterized by a zone locally altered to saponite the clayey gouge layer separates fault rocks of serpentinite origin containing talc and tremolite from fault rocks of sedimentary origin containing chlorite but no quartz the minerals that formed within the fault are the products of metasomatic reaction between the serpentinite and the siliceous rocks permeability measurements show that serpentinite breccia and fault gouge have permeability of 10 14 10 17 m 2 and 10 15 10 18 m 2 respectively at 5 120 mpa confining pressure frictional coefficient of the saponite rich clayey fault gouge ranged between 0 20 and 0 35 under room dry condition but was reduced to 0 06 0 12 when saturated with water the velocity dependence of friction was strongly positive mostly ranging between 0 005 and 0 006 in terms of a b values the governing friction law is not constrained yet but we find that the saponite rich gouge possesses an evolutional behavior in the opposite direction to that suggested by the rate and state friction law in addition to its direct velocity dependence

evidence for deformation induced k feldspar replacement by myrmekite several examples of deformation induced myrmekite have been found in two amphibolite facies mylonites derived from granitic protoliths namely a muscovite poor s c mylonite and a single foliation muscovite poor mylonitic gneiss back scattered sem and conventional optical microscopy show that in both rock types syntectonic myrmekitic intergrowths of oligoclase and quartz formed on the two sides of k feldspar grains that faced the local inferred incremental shortening direction for the mylonite myrmekite does not occur on the two ends of the grain that faced the incremental stretching direction the replacement of k feldspar by plagioclase and quartz results in a volume decrease and is favoured on high normal stress sites around the grains we suggest that the ambient temperature pressure and chemical activities were such that the replacement reaction was favoured but the addition of extra strain energy along the high pressure sides of the grains localized the reaction at these sites this energy could arise from elastic strain or strain associated with tangled dislocations or twin boundaries the relative roles of stress and strain energy concentrations in driving the replacement reaction are not known but both were probably important

an earthquake instability model based on faults containing high fluid pressure compartments it has been proposed that large strike slip faults such as the san andreas contain water in seal bounded compartments arguments based on heat flow and stress orientation suggest that in most of the compartments the water pressure is so high that the average shear strength of the fault is less than 20 mpa we propose a variation of this basic model in which most of the shear stress on the fault is supported by a small number of compartments where the pore pressure is relatively low as a result the fault gouge in these compartments is compacted and lithified and has a high undisturbed strength when one of these locked regions fails the system made up of the neighboring high and low pressure compartments can become unstable material in the high fluid pressure compartments is initially underconsolidated since the low effective confining pressure has retarded compaction as these compartments are deformed fluid pressure remains nearly unchanged so that they offer little resistance to shear the low pore pressure compartments however are overconsolidated and dilate as they are sheared decompression of the pore fluid in these compartments lowers fluid pressure increasing effective normal stress and shear strength while this effect tends to stabilize the fault it can be shown that this dilatancy hardening can be more than offset by displacement weakening of the fault i e the drop from peak to residual strength if the surrounding rock mass is sufficiently compliant to produce an instability slip will propagate along the fault until the shear fracture runs into a low stress region frictional heating and the accompanying increase in fluid pressure that are suggested to occur during shearing of the fault zone will act as additional destabilizers however significant heating occurs only after a finite amount of slip and therefore is more likely to contribute to the energetics of rupture propagation than to the initiation of the instability

how phyllosilicate mineral structure affects fault strength in mg rich fault systems the clay mineralogy of fault gouges has important implications for the frictional properties of faults often identified as a major factor contributing to profound fault weakness this work compares the frictional strength of a group of mg rich minerals common in the mg al si o compositional space talc saponite sepiolite and palygorskite by conducting triaxial frictional tests with water or argon as pore fluid the studied minerals are chemically similar but differ in their crystallographic structure results show that fibrous mg rich phyllosilicates are stronger than their planar equivalents frictional strength in this group of minerals is highly influenced by strength of the atomic bonds continuity of water layers within the crystals and interactions of mineral surfaces with water molecules all of which are dictated by crystal structure the formation and stability of the minerals studied are mainly controlled by small changes in pore fluid chemistry which can lead to significant differences in fault strength

processes controlling the shrinkage of porphyroclasts in gabbroic shear zones abstract this paper presents microstructural and geochemical data on amphibole and clinopyroxene porphyroclasts collected in amphibolite facies ultramylonites of the ivrea verbano zone italy the well rounded porphyroclasts show a concentration of defects including 100 twins stacking faults free dislocations subgrains and microcracks within their rims that indicate stress concentrations localised deformation within the margins of porphyroclasts increases reaction kinetics and supports amphibole clinopyroxene replacement reactions and heterogeneous plagioclase nucleation in addition to microcracking and or subgrain rotation these metamorphic reactions lead to a marginal grain size reduction and the shrinkage of porphyroclasts in order to correlate the defect concentration at the rim of porphyroclasts with stresses a finite element inclusion matrix model is used it depends on the degree of bonding between inclusion and matrix whether strong stress concentrations within the rim of the inclusion occur for well coupled two phase systems shear stresses within the rim of the inclusion can reach a magnitude many times the magnitude of the matrix stresses marginal stress concentrations do not evolve if inclusion and matrix are decoupled this may indicate that mechanically decoupled porphyroclasts can survive to very large strains

the frictional behavior of serpentinite implications for aseismic creep on shallow crustal faults serpentine is common in many active faults and may be responsible for aseismic creep along segments of these faults to test this we have conducted a series of velocity stepping experiments to determine the frictional velocity dependence of serpentinite we slid initially bare rough surfaces of antigorite serpentinite at room temperature with velocities from 0 0032 to 10 0 m s 1 0 102 to 3 2 105 mm yr and normal stresses from 25 to 125 mpa we find that the velocity dependence of serpentinite undergoes a transition from velocity weakening at fast loading velocities to velocity strengthening at slow velocities and that this change is accompanied by other changes in the constitutive behavior these results suggest that serpentinite should not be the site of instability initiation during sliding at plate velocities but may permit propagation of unstable slip initiated elsewhere

flow stresses from microstructures in mylonitic quartzites of the moine thrust zone assynt area scotland recrystallized grain sizes subgrain sizes and dislocation densities of quartz grains in quartzose mylonitic rocks have been examined using optical and transmission electron microscopy tem the samples come from the moine thrust zone in the assynt district scotland they had been studied previously and described in detail with respect to their structural position in relation to the various thrusts in the region and to their preferred orientation stresses were derived from these samples using empirical and theoretical equations relating flow stress to the scale of the microstructures the stresses determined 43 244 mpa from recrystallized grain size 9 13 mpa from etched subgrain size 50 95 mpa from tem scale subgrain size and 75 147 mpa from dislocation density are not constant in individual samples simultaneous formation of the quartz fabrics and of the dynamically recrystallized grains during the period of mylonitization may have occurred under flow stresses ranging from 43 to 244 mpa a later dynamic recovery event reset subgrain sizes and dislocation densities to a constant value for each microstructure throughout the area there is insufficient empirical information available on the flow stress subgrain size relationship and on the effects of annealing recovery and annealing recrystallization to allow for a more detailed interpretation recrystallized grain size is still the most easily measured microstructural feature and the relationship of recrystallized grain size to flow stress has a sounder experimental basis than subgrain size or dislocation density the effect of chemical environment on the behaviour of all the microstructures is still unknown

frictional resistance of a fault zone with strong rotors as a possible mechanism to explain the lack of a heat flow anomaly along the creeping section of the san andreas fault we have determined the effect of placing hard rotors along a surface between two deformable media in this case styrofoam balls between two blocks of foam rubber the probable presence of franciscan rocks at depth along the creeping section of the san andreas fault suggested this mechanism since the franciscan is characterized not only by basic serpentinous rocks which are weak but also by embedded more or less equidimensional blocks of very strong rocks knockers e g high grade blueschists and eclogites which might act as rotors the results suggest that knocker rotation may be a viable mechanism for reduction of friction on the creeping section of the san andreas fault and thus be at least a partial explanation of the lack of any observed frictional heat flow anomaly there

multigrid migration reducing the migration aperture but not the migrated dips given a frequency f cdp interval x and upper surface velocity v 0 the spatial nyquist criterion defines a range of unaliased emergence angles for seismic reflection events for migration events outside that range should be discarded beforehand by dip filtering or the migration operator should be trained to ignore them in this paper we express the spatial nyquist criterion as a limitation on the cdp interval rather than on emergence angle which allows x to be adjusted as a function of emergence angle increasing with decreasing emergence angle we exploit this fact by creating two separate migration grids a fine grid with trace spacing x for migrating events with large emergence angles and a coarse grid with trace spacing 2 x for migrating events with small emergence angles combining the two migrations is straightforward the coarse grid migration involving one half the number of input traces and one half the number of output traces represents a considerable time savings over the smal

crustal strain dependent serpentinisation in the porcupine basin offshore ireland abstract mantle hydration serpentinisation at magma poor rifted margins is thought to play a key role in controlling the kinematics of low angle faults and thus hyperextension and crustal breakup however because geophysical data principally provide observations of the final structure of a margin little is known about the evolution of serpentinisation and how this governs tectonics during hyperextension here we present new observational evidence on how crustal strain dependent serpentinisation influences hyperextension from rifting to possible crustal breakup along the axis of the porcupine basin offshore ireland we present three new p wave seismic velocity models that show the seismic structure of the uppermost lithosphere and the geometry of the moho across and along the basin axis we use neighbouring seismic reflection lines to our tomographic models to estimate crustal stretching c of 2 5 in the north at 52 5 n and 10 in the south at 51 7 n these values suggest that no crustal embrittlement occurred in the northernmost region and that rifting may have progressed to crustal breakup in the southern part of the study area we observed a decrease in mantle velocities across the basin axis from east to west these variations occur in a region where c is within the range at which crustal embrittlement and serpentinisation are possible c 3 4 across the basin axis the lowest seismic velocity in the mantle spatially coincides with the maximum amount of crustal faulting indicating fault controlled mantle hydration mantle velocities also suggest that the degree of serpentinisation together with the amount of crustal faulting increases southwards along the basin axis seismic reflection lines show a major detachment fault surface that grows southwards along the basin axis and is only visible where the inferred degree of serpentinisation is 15 this observation is consistent with laboratory measurements that show that at this degree of serpentinisation mantle rocks are sufficiently weak to allow low angle normal faulting based on these results we propose two alternative formation models for the porcupine basin the first involves a northward propagation of the hyperextension processes while the second model suggests higher extension rates in the centre of the basin than in the north both scenarios postulate that the amount of crustal strain determines the extent and degree of serpentinisation which eventually controls the development of detachments faults with advanced stretching

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

slip localization on the southern alpine fault new zealand 1 results of a detailed field study of the southern onshore portion of new zealand s alpine fault reveal that for 75 km along strike dextral normal slip on this long lived structure is highly localized in phyllosilicate rich fault core gouges and along their contact with more competent rocks at three localities martyr river mckenzie creek and hokuri creek we document complete cross sections through the fault new 40ar 39ar dates on mylonites combined with microstructural and mechanical data on phyllosilicate rich fault core gouges show that modern slip is localized onto a single steeply dipping 1 to 12 m thick fault core composed of impermeable k 10 20 to 10 22 m2 frictionally weak s 0 12 0 37 velocity strengthening illite chlorite and saponite chlorite lizardite fault gouges fault core materials are 1 comparable to those of other major weak cored faults e g san andreas fault and 2 most compatible with fault creep despite paleoseismic evidence of quasiperiodic large magnitude earthquakes mw 7 on this portion of the alpine fault we conclude that frictional properties of gouges at the surface do not characterize the overall seismogenic behavior of the southern alpine fault

laboratory constraints on the rheology of the upper mantle the dynamics of convection and the mechanical behavior of the lithosphere are controlled by the rheology of upper mantle rocks for this reason experimental and theoretical studies on the rheology of olivine aggregates have been fields of active research for at least the last 35 years in this short course paper i briefly review some experimental and theoretical constraints on the rheology of upper mantle rocks and minerals and then discuss the application of these data for understanding the rheology of the upper mantle in different tectonic environments there is an expansive literature on the subject of mantle rheology extensive reviews and background can be found in several recent articles and books poirier 1985 ranalli 1995 karato and wu 1993 kohlstedt et al 1995 drury and fitzgerald 1998 hirth and kohlstedt 2003 as well as numerous references made throughout this chapter i focus the discussion of this chapter on the application of experimental data for constraining the rheology of the oceanic lithosphere and mantle constraints on mantle rheology based on extrapolation of laboratory experiments to deformation conditions in the oceanic lithosphere and asthenosphere are illuminating for several reasons first the composition of the mantle is relatively well constrained by analyses of peridotites from ophiolites and mid ocean ridges as well as chemical analyses of basalts second the oceanic lithosphere is comprised of rock that cooled from high temperature conditions at high pressure and is therefore not previously fractured in this way the lithosphere is similar to the ideal rocks we use in our experiments third the temperature of the lithosphere is constrained by a number of geophysical observations fourth microstructural observations on naturally deformed mantle rocks justify applying experimental flow laws at geologic conditions finally several independent geophysical observations such as constraints on viscosity based on analysis

role of fluids in faulting inferred from stress field signatures the stress orientation signature of weak faults containing high pressure fluids has been observed for segments of the san andreas fault system in southern california the inferred lithostatic fluid pressures extend into the surrounding relatively intact rock in a zone scaling with the width of the interseismic strain accumulation repeated strain related fracturing and crack sealing may have created low permeability barriers that seal fluids into the network of currently active fractures

development of schistosity by dissolution crystallization in a californian serpentinite gouge to understand the behaviour and deformation mechanisms of serpentinites in the seismogenic zone we study the deformation macro and microstructures of serpentinites along the santa ynez fault in the san andreas system at the outcrop scale deformation is localized in a gouge zone that shows three different structures 1 micrometric undeformed fragments clasts of the previously serpentinized peridotite 2 localized shear planes y and r and 3 a penetrative schistosity s observations under sem and tem reveal that the schistosity corresponds to serpentine fibres parallel to each other and whose orientation varies as they wrap around clasts tem micro textures indicate that these long fibres result from continuous syntectonic growth rather than from reorientation of pre existing fibres implying a slow transfer process that occurs at short distances we propose a dissolution diffusion crystallization process for the formation of the schistosity that corresponds to a low strain rate creeping process of deformation that can be effective in aseismic fault segments

chemical controls on fault behavior weakening of serpentinite sheared against quartz bearing rocks and its significance for fault creep in the san andreas system 1 the serpentinized ultramafic rocks found in many plate tectonic settings commonly are juxtaposed against crustal rocks along faults and the chemical contrast between the rock types potentially could influence the mechanical behavior of such faults to investigate this possibility we conducted triaxial experiments under hydrothermal conditions 200 350 c shearing serpentinite gouge between forcing blocks of granite or quartzite in an ultramafic chemical environment the coefficient of friction of lizardite and antigorite serpentinite is 0 5 0 6 and increases with increasing temperature over the tested range however when either lizardite or antigorite serpentinite is sheared against granite or quartzite strength is reduced to 0 3 with the greatest strength reductions at the highest temperatures temperature weakening and slowest shearing rates velocity strengthening the weakening is attributed to a solution transfer process that is promoted by the enhanced solubility of serpentine in pore fluids whose chemistry has been modified by interaction with the quartzose wall rocks the operation of this process will promote aseismic slip creep along serpentinite bearing crustal faults at otherwise seismogenic depths during short term experiments serpentine minerals reprecipitate in low stress areas whereas in longer experiments new mg rich phyllosilicates crystallize in response to metasomatic exchanges across the serpentinite crustal rock contact long term shear of serpentinite against crustal rocks will cause the metasomatic mineral assemblages which may include extremely weak minerals such as saponite or talc to play an increasingly important role in the mechanical behavior of the fault our results may explain the distribution of creep on faults in the san andreas system

magmatic underplating extension and crustal reequilibration insights from a cross section through the ivrea zone and strona ceneri zone northern italy the thermal impact of magmatic underplating at various crustal levels is studied along a traverse through the ivrea verbano zone and strona ceneri zone in northern italy geochronological and petrologic data are compared to a two dimensional thermal kinematic model field data and numerical simulation show the strong disturbance of the temperature field in the lower and intermediate crust in relation to magmatic underplating leading to granulite to amphibolite facies metamorphism as well as reequilibration of mineral chemical and isotopic systems magmatic underplating leaves a crust with an apparently heterogeneous tectonometamorphic evolution as information on the earlier history is preserved only at upper crustal levels

wavefront sets analysis of limited aperture migration sections although geophysical literature is quite rich in papers on migration there are only a few papers that treat lam limited aperture migration deterministic and stochastic criteria for the reconstruction of reflection interfaces in prestack migration were derived and demonstrated on synthetic and real data examples a real data set was acquired offshore the brazilian coast courtesy of petrobras the reconstruction of reflection interfaces in lam depends on the relative position of normal bundles as defined here and gradients to the reflection interfaces the derived stochastic criterion relates the variance of normal bundles to the variance of slowness in the medium these criteria will help the geophysicist to identify directly distorted regions caused by lam and those regions that are correctly migrated

deformation around rigid particles the influence of slip at the particle matrix interface abstract theoretical and previous experimental studies of rigid particle structures have only considered the behaviour of rigid particles for which the interface between particle and matrix is coherent in natural rocks the boundary behaviour may cover the whole range from free slip to complete coherence examples where slip may be promoted include crack seal deformation during the development of pressure shadows core mantle structures where the mantle is composed of much weaker fine recrystallized material and crystallizing magmas or partially molten rocks where grain boundaries are lubricated by the melt phase analogue scale model experiments have been performed under pure shear and simple shear conditions to compare the particle rotation and the flow in the matrix for both slipping and non slipping particle matrix interfaces when slip does occur particles rotate faster in pure shear and slower in simple shear than for non slip leading to the more rapid development of a shape preferred orientation the preferred orientation tends to be bimodal in simple shear flow the flow and finite strain patterns in the matrix are also strongly influenced by slip at the interface and porphyroclast systems can develop geometries with ambiguous or contradictory shear senses dependent on the original particle orientation in multi particle experiments slip on the interfaces enhances strong strain localization in non linear viscous material to produce patterns very similar to natural s c fabrics

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

the concept of steady state foliation abstract a steady state foliation is thought of as a foliation with constant orientation and intensity in a rock undergoing steady state flow the problem discussed is whether such straininsensitive foliations exist in rocks and how they might be distinguished from the more familiar strain sensitive foliations steady state foliations would be useful indicators of stress or of strain rate yet misleading if used as indicators of finite strain one requirement for maintenance of steady state foliations is operation of processes that locally weaken grain shape preferred orientation or reduce grain aspect ratios or destroy lithological layering during flow these are the inverses of the processes normally discussed in connection with foliation development many such processes are known though the details of how and when they operate efficiently enough to stabilize a foliation are unknown there is however a tendency for some of them to operate more efficiently at high strains high temperatures and in flow regimes with high vorticity natural and experimental high temperature shear zones are accordingly one environment in which steadystate foliations may be encountered an experiment is described in which a thin specimen of para dichlorobenzene was given a large shearing deformation at 90 of its absolute melting point a steady state foliation was not produced but the technique is a promising one for further microstructural studies where in situ observation of material undergoing large deformations is desirable

mineral precipitation as a mechanism of fault core growth abstract faults vary in structural style from simple planes to complex systems composed of fault cores and damage zones increased fault complexity results from the interaction of mechanical and chemical processes including fracture growth shear and linkage and mineral dissolution and precipitation although water rock interaction is traditionally associated with fault rock weakening and shear localization we investigate processes of fault core widening by water rock interactions that resulted in quartz precipitation we combine field and petrographic observations with prior mechanical characterization to assess the impact of alteration and cementation on fault architecture at the dixie comstock epithermal gold deposit nevada usa mineralized portions of the fault contain strong thick silicified fault cores and wide weak damage zones with evidence for widening of the core through entrainment of damage zone material and repeated cycles of embrittlement dilation and cementation we present a model of fault zone evolution in which the hydrothermal regimes favoring either alteration weakening or precipitation strengthening result in distinct fault zone architecture and mechanical and flow properties of fault systems alteration weakening favors localization of the fault into thinner clay rich low permeability fault cores precipitation strengthening promotes thick strong and low permeability fault cores with mineralization embrittlement enhancing transient permeability following coseismic failure

exploring fault rocks at the nanoscale abstract this paper aims to demonstrate the potential role of micro nanoscale investigations in the study of fault rocks given that a multiscale and multidisciplinary approach linking structural geology petrology and mineralogy from the outcrop to the unit cell scale is essential to the comprehensive characterization of geological materials to explore fault rocks at the sub micrometer scale a fundamental contribution is provided by the transmission electron microscope tem which offers a real time combination of high resolution images diffraction patterns and chemical data for extremely small volumes providing accurate mineralogical and nanostructural determinations this paper focuses on a few selected examples that are particularly appropriate for tem investigation in particular 1 grain size determinations on ultrafine grained fault rocks 2 detection and characterization of poorly crystalline minerals e g clays and amorphous phases within fault rocks 3 identification of deformation induced mineral reactions occurring within fault zones e g dolomite decarbonation and serpentine dehydration and 4 observations of recurrent deformation nanotextures in phyllosilicates tem investigation can provide an unexpected amount of data much of which cannot be obtained by conventional techniques and it can therefore significantly contribute to an understanding of the physico chemical conditions for faulting and fault mechanics

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

inhibition of subduction thrust earthquakes by low temperature plastic flow in serpentine abstract serpentine is considered a candidate hydrous mineral in explaining aseismic zones along faults however recent frictional experiments have shown a marked increase in its shear strength with increasing depth implying its deformation via unstable sliding at a plate interface in subduction zones here we report the results of a series of simple shear experiments designed to determine the dominant deformation style of low temperature serpentine species under p t conditions that correspond to the mantle wedge corner in cool subduction zones ca p 1 gpa and t 200 c we found that with increasing shear strain the dominant flow mechanism of the serpentinite evolves from 1 inhomogeneous semi brittle flow by strain localization into sample scale shear bands to 2 homogeneous ductile flow by intracrystalline deformation within individual serpentine grains with the development of a planar shape fabric and a strong crystallographic preferred orientation tem observations revealed that grain size reduction in lizardite crystals occurs due to 001 interlayer glide represented by the occurrence of tiny sheet like sectors oriented parallel to 001 liz lattice fringes which evolved from large elongated sectors with long axes oriented normal to 001 liz given that a low viscosity serpentinized layer upon a subducting plate interface produces strain localization within the layer and subsequent large bulk shear strains the above results indicate that the presence of low temperature serpentine species at the plate interface in cool subduction zones inhibits the initiation of subduction thrust earthquakes as stress is preferentially accommodated by plastic flow this hypothesis explains the occurrence of an anomalous non seismic region devoid of large thrust earthquakes in locally hydrated forearc mantle within northeast japan

strength of slightly serpentinized peridotites implications for the tectonics of oceanic lithosphere we deformed cores of peridotite with 10 15 lizardite and chrysotile serpentine to determine the influence of serpentine content on the strength and the style of deformation the strength the pressure dependence of strength and the nominally nondilatant mode of brittle deformation of slightly serpentinized peridotites are comparable to those of pure serpentinites these results indicate that deformation is accommodated primarily by serpentine while olivine despite being the more abundant component remains nominally undeformed on the basis of these data and previous work we determine that the transition from a strong dilatant dunite rheology to a weak nondilatant serpentinite rheology is not a linear function of the degree of serpentinization instead an abrupt transition in strength is observed at low degrees of serpentinization the pressure of the transition from localized to distributed deformation also decreases abruptly from 1000 mpa to 150 350 mpa the change in rheological behavior occurs at a serpentine content of 10 15 or less which corresponds to published compressional seismic velocity of 7 8 7 5 km s at a pressure of 200 mpa the seismic velocity of the oceanic lithosphere particularly of that formed at slow spreading ridges can thus provide constraints on its mechanical properties at depth because slightly serpentinized peridotites have a rheology similar to that of pure serpentinite significant lithospheric weakening may occur after the onset of alteration near or at the ridge axis

strengths of serpentinite gouges at elevated temperatures serpentinite has been proposed as a cause of both low strength and aseismic creep of fault zones to test these hypotheses we have measured the strength of chrysotile lizardite and antigorite rich serpentinite gouges under hydrothermal conditions with emphasis on chrysotile which has thus far received little attention at 25 c the coefficient of friction of chrysotile gouge is roughly 0 2 whereas the lizardite and antigorite rich gouges are at least twice as strong the very low room temperature strength of chrysotile is a consequence of its unusually high adsorbed water content when the adsorbed water is removed chrysotile is as strong as pure antigorite gouge at room temperature heating to 200 c causes the frictional strengths of all three gouges to increase limited data suggest that different polytypes of a given serpentine mineral have similar strengths thus deformation induced changes in polytype should not affect fault strength at 25 c the chrysotile gouge has a transition from velocity strengthening at low velocities to velocity weakening at high velocities consistent with previous studies at temperatures up to 200 c however chrysotile strength is essentially independent of velocity at low velocities overall chrysotile has a restricted range of velocity strengthening behavior that migrates to higher velocities with increasing temperature less information on velocity dependence is available for the lizardite and antigorite gouges but their behavior is consistent with that outlined for chrysotile the marked changes in velocity dependence and strength of chrysotile with heating underscore the hazards of using room temperature data to predict fault behavior at depth the velocity behavior at elevated temperatures does not rule out serpentinite as a cause of aseismic slip but in the presence of a hydrostatic fluid pressure gradient all varieties of serpentine are too strong to explain the apparent weakness of faults such as the san andreas

on the transient response of serpentine antigorite gouge to stepwise changes in slip velocity under high temperature conditions 1 shear sliding tests were conducted on serpentine antigorite gouge to understand the rheology of serpentine bearing faults the experiments were carried out using a constant confining pressure 100 mpa a constant pore water pressure 30 mpa and a range of temperatures from room temperature to 600 c the transient response in frictional behavior following stepwise changes in the slip velocity were documented at each temperature slip rates varied between 0 0115 and 11 5 m s both the general level of frictional strength and the transient responses changed drastically at around 450 c as the temperature increased from 400 c to 450 c the strength of antigorite rose sharply the transient response also indicated a change in the mode of deformation from flow type behavior at temperatures below 400 c to frictional behavior stick slip at temperatures above 450 c 500 c although only a limited volume of serpentine was involved in the dehydration reaction x ray diffraction analyses and scanning electron microscopy observations showed that forsterite had nucleated in the experimental products at the higher temperatures that were associated with frictional behavior submicron sized streaky forsterite masses in shear localized zones may be evidence of shear induced dehydration that caused strengthening and embrittlement of the gouge although antigorite rheology is complicated the subsequent change in friction coefficient per order of magnitude change in sliding velocity increased with both increasing temperature and decreasing velocity implying that a possible flow mechanism of intragranular deformation became activated

deformation of single crystal clinopyroxenes 1 mechanical twinning in diopside and hedenbergite laboratory deformation experiments were carried out on two single crystal clinopyroxenes chrome diopside and hedenbergite tests were made in a griggs solid medium triaxial hot creep tester a confining pressure of 1000 mpa was applied in all experiments crystals were deformed at strain rates from 10 4 to 10 8 s 1 and at temperatures from 400 c to 1200 c two orientations of the crystals with respect to the maximum principal compressive stress were tested the first orientation subjects the 100 001 and 001 100 mechanical twinning systems in clinopyroxene to a high resolved shear stress in the sense appropriate for mechanical twinning to occur the second orientation subjects these systems to an equal resolved shear stress but in the opposite sense so that mechanical twinning is not possible effects of temperature and strain rate on the flow stress are observed for the two clinopyroxene compositions mechanical twinning on the system 100 001 is observed to be the primary deformation mechanism in crystals oriented favorably for twinning the resolved shear stress required for mechanical twinning on this system is 140 mpa for hedenbergite and 100 mpa for chrome diopside the smaller twinning stresses for diopside are considered to be due to abundant inclusions in the crystals the mechanical twinning stress is nearly independent of temperature from 400 c to 850 c and strain rate from 10 4 to 10 8 s 1 above 850 1000 c for diopside and 1000 c for hedenbergite deformation occurs by dislocation glide and is strongly temperature and strain rate dependent crystals tested in the orientation for which mechanical twinning is not possible deform by two different dislocation glide mechanisms depending on temperature details of these experiments are reported in part 2 results for mechanical twinning predict that this is not an important deformation mechanism in the earth unless resolved shear stress on the twinning system exceeds 140 mpa lack of dependence of twinning stress on strain rate implies that large rapid strains will occur in this mineral if the twinning stress is reached independence of twinning stress in clinopyroxenes from temperature strain rate or composition allows use of this mineral as a geopiezometer at plate margins where stresses are high presence of mechanically twinned clinopyroxene grains allows determination of the orientation and magnitude of stresses responsible for the deformation

strength of chrysotile serpentinite gouge under hydrothermal conditions can it explain a weak san andreas fault chrysotile bearing serpentinite is a constituent of the san andreas fault zone in central and northern california at room temperature chrysotile gouge has a very low coefficient of friction 0 2 raising the possibility that under hydrothermal conditions might be reduced sufficiently to 0 1 to explain the apparent weakness of the fault to test this hypothesis we measured the frictional strength of a pure chrysotile gouge at temperatures to 290 c and axial shortening velocities as low as 0 001 m s as temperature increases to 100 c the strength of the chrysotile gouge decreases slightly at low velocities but at temperatures 200 c it is substantially stronger and essentially independent of velocity at the lowest velocities tested we estimate that pure chrysotile gouge at hydrostatic fluid pressure and appropriate temperatures would have shear strength averaged over a depth of 14 km of 50 mpa thus on the sole basis of its strength chrysotile cannot be the cause of a weak san andreas fault however chrysotile may also contribute to low fault strength by forming mineral seals that promote the development of high fluid pressures

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

frictional strength of cuttings and core from safod drillhole phases 1 and 2 1 we investigated the frictional properties of drill cuttings and core obtained from 1 85 3 1 km true vertical depth in the safod scientific borehole in central california triaxial frictional sliding experiments were conducted on samples from primary lithologic units and significant shear zones including the inferred active trace of the san andreas fault the samples were deformed at room temperature under constant effective normal stresses of 10 40 and 80 mpa with axial shortening rates of 0 01 1 0 m s 1 the weakest samples were from shale claystone and siltstone units with friction coefficient 0 4 0 55 stronger samples were from quartzo feldspathic rocks with 0 6 materials tested from two shear zones at 2560 and 3067 m measured depth had 0 4 0 55 and velocity strengthening behavior consistent with fault creep at depths 4 km the coefficient of friction for bulk samples from the inferred trace of the san andreas fault was 0 6

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

control on seafloor spreading geometries by stress and strain induced lithospheric weakening abstract seafloor spreading typically occurs along ridge segments oriented at right angles to plate motion and offset by orthogonal transform faults few regions exhibit different patterns such as the east pacific rise epr which additionally displays overlapping spreading centres oscs and microplates we introduce a dynamical model using two independent scalar types of damage in an elastic plate that generates most observed spreading geometries as natural failure modes suggesting that the dynamics of the underlying mantle have only a minor influence on accretionary plate margins the elastic modulus that is affected by the damage determines the type of localized deformation damage reducing the bulk modulus tends to result in tensile fractures while a reduction in shear modulus leads to shear fractures the damage source determines the fracture orientation material weakening in tension results in fractures perpendicular to the most tensile principal stress while shear weakening results in two conjugate fractures at 45 relative to the applied stress strain or energy dependent damage results in propagating localized fractures stress dependent damage tends to evolve into diffuse regions that may eventually focus into narrow zones starting from small perturbations with reduced elastic moduli as nucleation points all ridge geometries start with ridge propagation caused by tensile energy reducing both elastic moduli by a model of damage caused by tensile energy reducing both moduli orthogonal transform faults develop in regions between offset segments if there is an additional reduction in shear modulus due to shear stress the orthogonality of the transform faults does not derive from the local stress orientation but from the dynamics of damage focusing which causes the fault to converge towards an optimal geometry that concentrates nearly all deformation into damaged zones oscs form when the shear damage leading to transform faults is suppressed while microplate formation requires additional reduction of the shear modulus by tensile energy oblique spreading at 45 recently discovered along ultraslow spreading ridges occurs when both moduli are weakened by shear energy a parameter regime exists in which ridge transform patterns develop at low applied tension while microplates form at higher stresses these results indicate that at least three different micromechanical processes operate with different evolution rates oscs and oblique spreading require different material properties

ductile creep and compaction a mechanism for transiently increasing fluid pressure in mostly sealed fault zones a simple cyclic process is proposed to explain why major strike slip fault zones including the san andreas are weak field and laboratory studies suggest that the fluid within fault zones is often mostly sealed from that in the surrounding country rock ductile creep driven by the difference between fluid pressure and lithostatic pressure within a fault zone leads to compaction that increases fluid pressure the increased fluid pressure allows frictional failure in earthquakes at shear tractions far below those required when fluid pressure is hydrostatic the frictional slip associated with earthquakes creates porosity in the fault zone the cycle adjusts so that no net porosity is created if the fault zone remains constant width the fluid pressure within the fault zone reaches long term dynamic equilibrium with the hydrostatic pressure in the country rock one dimensional models of this process lead to repeatable and predictable earthquake cycles however even modest complexity such as two parallel fault splays with different pressure histories will lead to complicated earthquake cycles two dimensional calculations allowed computation of stress and fluid pressure as a function of depth but had complicated behavior with the unacceptable feature that numerical nodes failed one at a time rather than in large earthquakes a possible way to remove this unphysical feature from the models would be to include a failure law in which the coefficient of friction increases at first with frictional slip stabilizing the fault and then decreases with further slip destabilizing it

nearly frictionless faulting by unclamping in long term interaction models in defiance of direct rock friction observations some transform faults appear to slide with little resistance in this paper finite element models are used to show how strain energy is minimized by interacting faults that can cause long term reduction in fault normal stresses unclamping a model fault contained within a sheared elastic medium concentrates stress at its end points with increasing slip if accommodating structures free up the ends then the fault responds by rotating lengthening and unclamping this concept is illustrated by a comparison between simple strike slip faulting and a mid ocean ridge model with the same total transform length calculations show that the more complex system unclamps the transforms and operates at lower energy in another example the overlapping san andreas fault system in the san francisco bay region is modeled this system is complicated by junctions and stepovers a finite element model indicates that the normal stress along parts of the faults could be reduced to hydrostatic levels after 60 100 k y of system wide slip if this process occurs in the earth then parts of major transform fault zones could appear nearly frictionless

dehydration reactions and micro nanostructures in experimentally deformed serpentinites high t torsion experiments on lizardite chrysotile serpentinites produced mineralogical and micro nanostructural changes with important implications in rheological properties high resolution tem showed that specimens underwent ductile by microkinking and 001 interlayer glide and brittle deformation by microfracturing together with dehydration and break down reactions lizardite is affected by polytypic disorder and microkinking kink axial planes at high angle with respect to 001 planes that were not present in the initial ordered 1t lizardite chrysotile fibres are deformed resulting in elliptical cross sections with strong loss of interlayer cohesion both lizardite and chrysotile break down to a fine intergrowth of olivine up to 200 nm talc up to 30 nm and poorly crystalline material lizardite out reaction preferentially occurs at kink axial planes representing sites of preferential strain and enhanced reactivity conversely chrysotile break down is a bulk process resulting in large healed olivine aggregates up to micrometric in size overall observations suggest that dehydration and break down reactions are more advanced in chrysotile than in lizardite

stabilizing effect of high pore fluid pressure on slip behaviors of gouge bearing faults this work is supported by the department of energy office of science the office of basic energy sciences chemical sciences geosciences biosciences division under award no de fg 0207er15916 and by the national science foundation under grant no ear 1761912

crustal stress field in southern california and its implications for fault mechanics we present a new high spatial resolution image of stress orientation in southern california based on the inversion of earthquake focal mechanisms we use this image to study the mechanics of faulting in the plate boundary region the stress field contains significant spatial heterogeneity which in some cases appears to be a result of the complexity of faulting and in other cases appears to be a cause temporal changes in the stress field are also observed primarily related to major earthquakes the observed 15 10 rotation of the stress axes due to the 1992 m7 3 landers mainshock implies that the deviatoric stress magnitude in the crust is low of the order of 10 mpa this suggests that active faults in southern california are weak the maximum principal stress axis near the san andreas fault is often at 50 to the fault strike indicating that the shear stress on the fault is comparable to the deviatoric stress the san andreas in southern california may therefore be a weak fault in a low strength crust

3d development of detachment faulting during continental breakup abstract the developing asymmetry of rifting and continental breakup to form rifted margins has been much debated as has the formation mechanics and role of extensional detachments bespoke 3d seismic reflection data across the galicia margin west of spain image in unprecedented detail an asymmetric detachment the s reflector mapping s in 3d reveals its surface is corrugated proving that the overlying crustal blocks slipped on s surface during the rifting crucially the 3d data show that the corrugations on s perfectly match the corrugations observed on the present day block bounding faults demonstrating that s is a composite surface comprising the juxtaposed rotated roots of block bounding faults as in a rolling hinge system with each new fault propagation moving rifting oceanward changes in the orientation of the corrugations record the same oceanward migration however in contrast to previous rolling hinge models the slip of the crustal blocks on s occurred at angles as low as 20 requiring that s was unusually weak consistent with the hydration of the underlying mantle by seawater ingress following the embrittlement of the entire crust as the crust only becomes entirely brittle once thinned to 10 km the asymmetric s detachment and the hyper extension of the continental crust only developed late in the rifting process which is consistent with the observed development of asymmetry between conjugate magma poor margin pairs the 3d volume allows analysis of the heaves and along strike architecture of the normal faults whose planes laterally die or spatially link together implying overlaps in faults activity during hyper extension our results thus reveal for the first time the 3d mechanics and timing of detachment faulting growth the relationship between the detachment and the network of block bounding faults above it and the key processes controlling the asymmetrical development of conjugate rifted margins

determination of slow slip episodes and strain accumulation along the cascadia margin 1 continuous gps stations in the pacific northwest geodetic array network clearly record subduction related strain accumulation and slow slip episodes along the cascadia convergent margin many of the slow slip episodes have been correlated in time and space with seismic evidence for nonvolcanic tremor leading to the previous discovery of episodic tremor and slip ets in this study we use a hyperbolic tangent curve fitting technique for the identification of slow slip times and displacement magnitudes within the gps time series independent of seismic tremor data we then apply this technique to study the patterns of strain accumulation and release associated with ets events and characterize patterns of coupling associated with the locked and transition zones of the plate interface we demonstrate the effectiveness of this automated technique for both identification of slow slip observations and calculation of slow slip displacements recurrence patterns in the distribution of gps observations demonstrate coherence among neighboring stations over time and apparent along strike segmentation of the subduction interface when slow slip events are removed from the time series we can estimate the total site velocities between slow slip events these velocities decay as depth to the subduction interface increases but they diverge from the long term trends expected from the interseismic cycle at 30 60 km above the interface consistent with the location where slow slip displacements occur forward modeling of coupling on the plate interface reveals that in between slow slip events there is a patch of at least 30 coupling from 20 to 35 km depth which is needed to produce the observed back slip displacements intriguingly our best fitting models have a decrease in coupling down to 30 at 20 km depth followed by a peak of greater than 80 coupling at 30 35 km depth suggesting the source zone for ets events acts as a distinct locking zone that releases strain more frequently than the updip seismogenic locked zone although a zone of constant 30 coupling cannot be ruled out with this data set such a scenario indicates that frictional behavior with depth follows a more complex model than a simple temperature controlled transition we propose that coupling initially decreases with depth due to a decrease in strength of the overriding lower crust but then coupling increases again when the subducting plate comes in contact with the stronger overriding mantle

stress gradients around porphyroclasts palaeopiezometric estimates and numerical modelling abstract based on strain gradients commonly found around porphyroclasts the mechanical interaction of porphyroclasts and recrystallized mylonite matrix in a retrograde amphibolite facies shear zone of the ivrea verbano zone italy was investigated differential stresses inferred from recrystallized grain size and dislocation density indicate stress concentrations at the porphyroclast matrix interface of about 1 7 2 0 with respect to the average in the matrix the palaeopiezometric stress data are compared to the results of a finite element model composed of two rheologically different phases in this model the local distribution and concentration of stresses around porphyroclasts mostly depend on clast matrix coupling and the effective viscosity contrast of the two phases the bulk strength and work hardening of the composite system increases with interface coupling both the modified stress field and the reduced average grain size around porphyroclasts may result locally in instabilities and promote strain localization and a change in the dominant deformation mechanism

strain and kinematic analysis in general shear zones we present a unifying theory for the development and distribution of strain markers and kinematic indicators in zones of general shear and thus provide a framework in which data previously considered contradictory may be understood rigid and deformable porphyroclast systems including and complex grains are potential indicators of both strain and flow the shapes and distribution of such porphyroclast systems may be used to distinguish among different tectonic regimes general shear is divided into two fields sub simple shear in which the rotational component of the strain is less than that for simple shear and super simple shear in which the rotational component is greater than for simple shear sub simple shear may involve narrowing or broadening of shear zones super simple shear regimes are possible in local regions such as the vicinities of deformable porphyroclasts but must be enclosed by regimes of sub simple shear the polar mohr constructions for finite deformation and flow are useful to analyze general shear in theory the hyperbolic net is employed for practical plotting of real data and derivation of the kinematic vorticity number wn this number represents the relative contributions of pure and simple shear in steady flow in nature deformation is thought to build up and decay by processes that may invalidate the assumption of constant flow regime we therefore introduce the concept of accelerating deformation and analyze the implications of non steady flow for the shearing histories of deformed objects

potential of phyllosilicate dehydration and dehydroxylation reactions to trigger earthquakes 1 this study experimentally simulated dehydration and dehydroxylation induced weakening of clay rich gouges to investigate the potential of phyllosilicate minerals to generate abnormal pore pressure triggering earthquakes for the experiment the gouge samples were subjected to sliding with heating at 9 6 c min to 500 c under 80 mpa of confining pressure in the undrained condition in which expulsed water was confined in the gouges significant frictional strength reductions were found in the na montmorillonite gouge due to dehydration and in the kaolinite gouge due to dehydroxylation however frictional strength continued to increase in the fully drained condition that allowed released water to move outside the gouge frictional strengths under the two drained conditions were compared during dehydration and dehydroxylation reactions to estimate the pore pressure generated in the gouge zone under the undrained condition the 6 8 wt of dehydroxylated water generated from the kaolinite caused a rapid increase in pore pressure to 68 mpa at 500 c in contrast in the na montmorillonite gouge the pore pressure gradually increased to 72 mpa as the temperature rose to 500 c especially on a coseismic stage these results suggest that dehydration dehydroxylation could advance the frictional sliding acceleration induced by frictional heating even if the fault is in a dry condition

the updip and downdip limits to great subduction earthquakes thermal and structural models of cascadia south alaska sw japan and chile this study examines thermal and structural controls of the updip and downdip rupture limits of great subduction thrust earthquakes data on past great earthquake seismic limits have been compiled for four continental subduction zones cascadia sw japan south alaska and chile these limits have then been compared to the predictions of several models for what constrains great earthquake rupture temperatures on the subduction thrusts have been estimated by finite element numerical models the landward limits of the observed updip aseismic zones correspond to the position where the thrust temperature reaches about 100 c that is depths of about 2 to 10 km for the subduction zones studied this temperature agrees with the dehydration of stable sliding smectite clays to illite chlorite the temperatures in this region are controlled mainly by the thickness of sediment on the incoming crust and by the crustal age and thus heat flow the downdip limits correspond to the depth on the thrust where either 1 the temperature reaches about 350 c which corresponds to thermally activated stable sliding behavior for crustal rocks with a transition to 450 c or 2 about 40 km depth if 350 c is reached at greater depth depths of about 40 km approximately correspond to the intersection of the thrust with the continental forearc moho and this downdip limit may be a consequence of stable sliding serpentinite or talc and other hydrated forearc mantle rocks the primary temperature controls on the downdip region are the age of the subducting oceanic plate and the thrust dip profile secondary control comes from the thickness of incoming sediment the convergence rate and the radioactive heat generation in the overlying forearc the 100 c updip limit occurs near the trench for young subducting plates with a thick sediment section such as cascadia 6 8 ma and up to 80 km landward for older oceanic crust such as south alaska 50 ma the 350 c downdip thermal limit is applicable for young oceanic plates e g cascadia and nankai whereas the forearc mantle limit applies for older plates e g south alaska and chile except near the chile rise for the margins studied that have experienced great earthquakes there is generally good agreement between the postulated thermal and moho limits and the rupture or seismogenic zone as defined by the distribution of aftershocks and by waveform tsunami and dislocation modeling the downdip limit of the interseismic locked zone from dislocation modeling is also in agreement with these limits

the effects of sliding velocity on the frictional and physical properties of heated fault gouge the frictional properties of a crushed granite gouge and of gouges rich in montmorillonite illite and serpentine minerals have been investigated at temperatures as high as 600 c confining pressures as high as 2 5 kbar a pore pressure of 30 bar and sliding velocities of 4 8 and 4 8 10 2 m sec the gouges showed nearly identical strength behaviors at the two sliding velocities all four gouges however showed a greater tendency to stick slip movement and somewhat higher stress drops in the experiments at 4 8 10 2 m sec varying the sliding velocity also had an effect on the mineral assemblages and deformation textures developed in the heated gouges the principal mineralogical difference was that at 400 c and 1 kbar confining pressure a serpentine breakdown reaction occurred in the experiments at 4 8 10 2 m sec but not in those at 4 8 m sec the textures developed in the gouge layers were in part functions of the gouge type and the temperature but changes in the sliding velocity affected among other features the degree of mineral deformation and the orientation of some fractures

experimental modelling of mantled porphyroclasts abstract mantled porphyroclasts are commonly used as sense of shear indicators in mylonites analogue experiments have been carried out to model the development of such objects rigid spheres mantled by a viscous plasticene putty mixture with power law behaviour were suspended in an optically transparent newtonian polymer matrix the samples were deformed between two cylinders one placed within the other and with coinciding axes this induces a non coaxial flow between the cylinders and allows the achievement of high finite strain the viscous mantle around the rigid sphere remained undeformed deformed into an ellipsoid or developed wings the final geometry depends mainly on the induced viscosity contrast between the mantle and the matrix which is a function of the flow induced stress on the surface of the mantle winged mantles only develop if the induced viscosity contrast between the mantle and the matrix polymer is sufficiently small if this critical induced viscosity contrast is exceeded the mantled sphere remains undeformed or obtains a stable ellipsoidal shape even at high finite strain shaped winged mantles only develop in simple shear flow if the mantle around a rigid sphere is thin

thermal decomposition of serpentine during coseismic faulting nanostructures and mineral reactions abstract this paper reports a detailed characterization of an antigorite bearing serpentinite deformed at seismic slip rate 1 1 m s in a high velocity friction apparatus micro nanostructural investigation of the slip zone 200 m thick revealed a zonal arrangement with a close juxtaposition of horizons with significantly different strength respectively consisting of amorphous to poorly crystalline phases with bulk anhydrous composition close to starting antigorite and of highly crystalline assemblages of forsterite and disordered enstatite 200 nm in size and in polygonal like nanotextures the slip zone also hosts micro nanometre sized cr magnetite grains aligned at low angle with respect to the slipping surface and inherited from the host serpentinite overall observations suggest that frictional heating at asperities on the slipping surface induced a temperature increase up to 820 1200 c in agreement with flash temperature theory responsible for serpentine complete dehydration and amorphization followed by crystallization of forsterite and enstatite under post deformation static conditions the results of this study may provide important keys for the full comprehension of the mechanical behaviour and of the possible geodynamical role of serpentinite hosted faults through the seismic cycle

least squares migration of incomplete reflection data a least squares migration algorithm is presented that reduces the migration artifacts i e recording footprint noise arising from incomplete data instead of migrating data with the adjoint of the forward modeling operator the normal equations are inverted by using a preconditioned linear conjugate gradient scheme that employs regularization the modeling operator is constructed from an asymptotic acoustic integral equation and its adjoint is the kirchhoff migration operator we tested the performance of the least squares migration on synthetic and field data in the cases of limited recording aperture coarse sampling and acquisition gaps in the data numerical results show that the least squares migrated sections are typically more focused than are the corresponding kirchhoff migrated sections and their reflectivity frequency distributions are closer to those of the true model frequency distribution regularization helps attenuate migration artifacts and provides a sharper better frequency distribution of estimated reflectivity the least squares migrated sections can be used to predict the missing data traces and interpolate and extrapolate them according to the governing modeling equations several field data examples are presented a ground penetrating radar data example demonstrates the suppression of the recording footprint noise due to a limited aperture a large gap and an undersampled receiver line in addition better fault resolution was achieved after applying least squares migration to a poststack marine data set and a reverse vertical seismic profiling example shows that the recording footprint noise due to a coarse receiver interval can be suppressed by least squares migration

movement along a low angle normal fault the s reflector west of spain 1 the existence of normal faults that moved at low angles less than 20 has long been debated one possible low angle fault is the s detachment at the west galicia spain margin and thought to occur at the top of serpentinized mantle it is unlikely that s was a large submarine slide as it was probably active over several million years without the development of any compressional features such as toe thrusts it appears to have rooted beneath the conjugate flemish cap margin and it is similar to structures elsewhere that also appear to be rooted detachments here we analyze depth images to identify synrift sediment packages above s and use the geometry of these synrift packages to constrain the angle at which s both formed and remained active we find that s must have remained active at angles below 15 too low to be explained simply by the low friction coefficient of partially serpentinized peridotites instead we suggest that transient high fluid pressures must have developed within the serpentinites and propose a model in which anastomosing fault strands are alternately active and sealed enabling moderately high fluid pressures to develop

deformation related recrystallization processes abstract recrystallization is a common microstructural transformation that occurs during deformation metamorphism and diagenesis of rocks studies on minerals and rock analogues have demonstrated that a wide range of recrystallization mechanisms can occur the range of mechanisms is related to the various ways in which two basic processes grain boundary migration and new grain boundary formation combine to transform the microstructure two recent papers drury et al 1985 urai et al 1986 have proposed different schemes for the description of recrystallization mechanisms the purpose of this paper is to provide a unified framework for the description of mechanisms recrystallization mechanisms are divided into three main types rotation mechanisms which principally involve the formation of new grain boundaries migration mechanisms which principally involve grain boundary migration and general mechanisms which involve both basic processes a further distinction is made on the basis of the continuity of the microstructural transformation with respect to time each of the three main types of mechanism can be divided into a number of sub types depending on whether the processes of grain boundary migration new grain boundary formation and new grain formation occur in a discontinuous or continuous manner with respect to time as the terms continuous and discontinuous have been used in the metallurgical literature to signify the spatial continuity of the microstructural transformation the terms discontinuai and continual are used to refer to the temporal continuity of the transformation it is recommended that the following aspects should be specified if possible in a general description of recrystallization mechanisms 1 1 how do the basic processes combine to transform the microstructure 2 2 if new grain development occurs what is the development mechanism and does new grain formation occur in a continual or discontinuai manner 3 3 if grain boundary migration is involved in the transformation what is the migration mechanism i e fast solute escape migration slow solute loaded migration fluid assisted migration etc and is migration a continual or discontinuai process the application of the unified scheme is illustrated by reviewing studies that have provided detailed information on the recrystallization mechanisms involved the complicating effects of solid solution impurities dispersed second phase particles and grain boundary fluid films are also considered and it is demonstrated that variations in content of these types of impurity can significantly effect the types of recrystallization that occur in a given material

the akato tagh bend along the altyn tagh fault northwest tibet 2 active deformation and the importance of transpression and strain hardening within the altyn tagh system we investigated active deformation within the akato tagh restraining double bend to determine the age of the active altyn tagh fault relative to the altyn tagh system and thereby evaluate the extent to which this system evolved by net strain hardening or softening active structures were mapped based on their geomorphology and disruption of quaternary deposits surfaces the style of active faulting is strongly correlated with fault strike 065 070 striking segments show pure left slip whereas faults with more northward or eastward strikes are transtensional or transpressional respectively our mapping further suggests that the 065 to 070 striking western and eastern segments of the akato tagh bend are characterized by pure strike slip motion with partitioned transpression along the 090 striking central segment of the double bend it remains unclear how this active bend perpendicular shortening is absorbed in conjunction with previous work the present study fails to support the commonly held idea that tarim tibet motion is strongly oblique to the altyn tagh system estimates for the age of the akato tagh bend derived in a companion study suggest the bend is only a few million years old the current principal trace is probably similarly young because formation of the bend by recent deformation of an old trace should result in transpression along the western and eastern segments contrary to the pure left slip shown here the altyn tagh system comprises multiple fault strands in a zone 100 km wide across strike because the main trace appears to be much younger than the system in which it is embedded we speculate that this system evolved by the sequential formation and death of short lived fault strands in particular we suggest that geometrically complex strike slip fault systems such as the altyn tagh may form via system strain hardening where this net response reflects a dynamic competition between hardening and softening processes that are active simultaneously within the fault zone hardening mechanisms may include growth of restraining bend topography or material hardening of phyllosilicate rich gouge whereas softening processes might include r p shear linkage or reduction in bend angle by vertical axis rotation our analysis suggests that net strain hardening of a fault system can produce continental deformation that is spatially localized over the 1 5 m y during which an individual strand is active but distributed over the 10 100 m y corresponding to the life span of the whole fault system and the collision zone in which it is contained thus time scale is critically important in determining whether or not continental deformation is spatially distributed or localized

a rheologic model for wet crust applied to strike slip faults the strength and stability of crustal faults are not adequately addressed by the widely used two mechanism rheologic models of the crust based on byerlee s law and power law creep the models neglect the fluid assisted mechanisms of deformation that are important to strength of the crust and do not describe the variation in rate dependence of friction that governs the stability of fault slip several distinct mechanisms of frictional slip are defined on the basis of variations in frictional behavior and microfabric of simulated fault gouge in laboratory experiments state variable constitutive relations are used in a multiple mechanism formulation to describe the rate and temperature dependence of three friction mechanisms in wet quartz gouge at elevated temperatures this multimechanism description of friction is substituted for byerlee s law in the two mechanism models to generate a multimechanism rheologic model for the crust the multimechanism model is used to predict the rate and temperature dependence of crustal strength and the slip rate and depth temperature conditions over which each mechanism dominates application of the model to strike slip faults in the crust illustrates that the thickness of the actively shearing zone within faults is a critical parameter governing fault strength and stability the model predicts that frictional strength at midcrustal depths is significantly reduced relative to byerlee s law only for thick fault zones the reduction in strength is due to the operation of a strain rate sensitive friction mechanism involving combined cataclasis and solution transfer the rheologic model predicts that only very thin faults display the rate dependent characteristics necessary for initiation of seismic slip to any significant depth in the crust