deformation fabrics and strain localization mechanisms in graphitic carbon bearing rocks from the ailaoshan red river strike slip fault zone abstract graphitic carbon bearing rocks generally occur in low to high grade metamorphic units in many brittle faults graphitic carbon is often associated with gouge or low grade metamorphic rocks whereas in ductile faults graphitic carbon commonly occurs in marble schist or gneiss carbonaceous material gradually transforms from an amorphous into an ordered crystalline structure by increasing thermal metamorphism the degree of graphitization is believed to be a reliable indicator of peak temperature conditions in the metamorphic rock in this contribution based on detailed field observations the variably deformed and metamorphosed graphitic gneisses to phyllites located within the footwall and hangingwall unit of the cenozoic ailaoshan red river strike slip shear zone are studied according to lithological features and temperatures determined by raman spectra of carbonaceous material these graphitic rocks and deformation fabrics are divided into three types type i is represented by medium grade metamorphism and strongly deformed rocks with an average temperature of 509 c and a maximum temperature of 604 c type ii is affected by low grade metamorphism and deformed rocks with an average temperature of 420 c type iii is affected by lower grade metamorphism and occurs in weakly deformed undeformed rocks with an average temperature of 350 c slip localized micro shear zones and laterally continuous or discontinuous slip planes constituted by graphitic carbon aggregates are developed in types i and ii the electron back scattered diffraction ebsd lattice preferred orientation lpo patterns of graphitic carbon grains were firstly observed in comparison with lpo patterns of quartz and switch from basal rhomb to prism slip systems which indicate increasing deformation temperatures comparison of quartz and graphite lpos indicates that graphite lpos show higher temperature conditions according to the graphitic slip planes micro shear zones with grain size reduction and mylonitic foliation constituted by graphitic carbon minerals we also propose that the development of fine grained amorphous carbon plays an important role in rheological weakening of the whole rock during progressive ductile shearing

frictional sliding of dolomite a variation in constitutive behavior fault slip occurs via earthquakes or stable creep depending on the manner in which frictional resistance varies with sliding velocity and displacement if the form of this dependence is a function of rock type observed spatial variations in slip behavior along faults could be explained by lithologic variation we have used a rotary shear apparatus to investigate the frictional response of dolomite marble to changes in sliding velocity we have modeled the results using a state variable constitutive law for rock friction and a numerical method to solve for the interaction between the friction law and the compliance of our machine dolomite marble displays steady state velocity strengthening as contrasted with granite and quartzite which show velocity weakening at the same conditions to model the dolomite results it is necessary to use two state variables and surprisingly the parameters specifying the decay magnitudes associated with these state variables differ in sign from each other the magnitude of the parameter associated with the longer decay is negative leading to a downward decay of frictional strength associated with a decrease in velocity if the shorter decay is of sufficient magnitude as we observe at low velocity it is possible for the machine rock system to exhibit instability and oscillatory behavior even though the steady state response is velocity strengthening another way in which dolomite contrasts with granite and quartzite is that the friction parameters vary strongly with velocity a transition from unstable to stable sliding with increasing velocity has been observed previously and our observed variation of the constitutive parameters with velocity can explain this behavior there is some indication that the evolution of the state variables for dolomite occurs as a function of time rather than displacement in contrast with granite and quartzite the frictional behavior of dolomite shows that the constitutive laws that have been developed based on results from granite and quartzite are inadequate to describe all rock types and that more general laws are required

a new mechanism of slip weakening and strength recovery of friction associated with the mechanical consolidation of gouge i present results of a series of slide hold slide tests on granite surfaces separated by a layer of simulated granite gouge the shear stress during the hold period hold was controlled and used as a major control parameter of the experiments in addition to the time of hold time dependent fault healing which has been observed in many previous similar experiments was observed only at very high hold above 90 of the sliding friction a time independent strengthening termed consolidation strengthening occurred at all hold with a magnitude proportional to the reduction of hold the magnitude of this strengthening was as large as 25 of the residual friction the consolidation strengthening was found to be erased by subsequent slip in quite a different manner than is time dependent healing the friction decreased linearly with slip displacement at a rate independent of the magnitude of the strengthening and hence larger slip displacement is required to erase larger consolidation strengthening on the basis of these results together with earlier authors results of the measurements of porosity change under similar conditions i conclude that the time independent strengthening is due to the occurrence of gouge particle rearrangement to tighter packing enabled by decreased shear stress under a constant normal stress and that the subsequent slip weakening is due to unconsolidation of gouge enabled by particle rearrangement dilatancy during shearing the rate and state dependent friction rsf law was extended to incorporate the present findings

on the role of phyllosilicates on fault lubrication insight from micro and nanostructural investigations on talc friction experiments 1 exposures of mature faults at the earth s surface show that in the shallow crust some of co seismic slip occurs within a narrow phyllosilicate rich gouge material mechanical behavior of phyllosilicates sheared at seismic slip velocities remains a complex issue as suggested by the experimental and theoretical studies reported so far talc represents a simple case for common phyllosilicates in natural faults and therefore we chose talc as an analogue to better understand the mechanical behavior of clay rich mature crustal faults we present a series of high velocity friction experiments conducted on talc at 1 31 m s 1 and normal stresses of 0 3 1 8 mpa for wet and dry conditions at 1 mpa normal stress both wet and dry experiments show a slip weakening behavior however with a higher decreasing rate in wet conditions dynamic shear stress evolves from a peak value of 0 24 0 52 mpa down to a residual state value of 0 08 0 18 mpa in wet conditions and from a peak value of 0 46 1 17 mpa down to a residual state value of 0 12 0 62 mpa in dry conditions based on a detailed microstructural analysis down to the nanoscale we propose thermal pressurization as a possible slip weakening mechanism for wet conditions on the contrary in dry conditions the long lasting weakening is interpreted to be due to a combination of i progressive disappearance of geometrical incompatibilities ii solid lubrication of talc lamellae and iii powder lubrication by nanometric aggregates we conclude that initial gouge humid conditions and inherited fabric from past sliding may have large influence on the slip weakening for subsequent slip

frictional properties of natural gouges from longmenshan fault zone ruptured during the wenchuan mw7 9 earthquake abstract we investigated the properties of frictional sliding of natural gouges collected from longmenshan fault zone which ruptured during the wenchuan mw7 9 earthquake based on xrd analysis five gouge samples of different mineralogical assemblies and mineral contents are chosen for frictional sliding experiments with initial confining pressure of 50 mpa and at different temperatures from 25 c to 150 c to obtain the rate dependence of friction shearing rates are stepped up and down in the range of 0 0488 to 6 1 m s while the friction coefficient of most gouge samples decreases with the clay content following a general trend consistent with previous studies the organic matter bearing sample px 3 shows a much lower value than the prediction by the general trend indicating further strength reduction due to the 9 organic matter included we also found a significant effect of temperature on the sliding behavior of organic matter bearing and carbonate rich gouges when temperature increases to 150 c the px 3 sample with organic matter exhibits unstable oscillation that indicates velocity weakening frictional sliding behavior in contrast to the velocity strengthening behavior at lower temperatures the velocity dependence of carbonate rich samples px 1 px 2 with more than 50 wt carbonates is also sensitive to temperature and shearing rates and both samples have smaller velocity dependence of friction a b at higher temperatures the a b value of the px 2 sample with 80 carbonates even decreases to the velocity neutral point at 150 c and slow shearing rates based on the rate and state friction framework fast slip nucleation is possible for faults associated with the organic matter bearing gouge at 150 c whereas triggered seismic slips are possible in faults that include the carbonate rich samples which have a small a b value at 150 c the temperature dependent sliding behavior of carbonate dominant and organic matter bearing gouges probably provides a new clue to the problem of transition from shallow aseismic zone to seismic zone in subduction interfaces of the oceanic plate

a viscoelastic shear zone model of compressional and extensional plate boundaries a model is proposed describing the mechanical evolution of a shear zone along compressional and extensional plate boundaries subject to constant strain rate the shear zones are assumed as viscoelastic with maxwell rheology and with elastic and rheological parameters depending on temperature and petrology stress and strain are computed as functions of time and depth for both kinds of boundaries the model reproduces the existence of a shallow seismogenic zone characterized by a stress concentration the thickness of the seismogenic layer is evaluated considering the variations of shear stress and frictional strength on faults embedded in the shear zone assuming that a fault dislocation takes place the brittle ductile transition is assumed to occur at the depth at which the time derivative of total shear stress changes from positive to negative values the effects of different strain rates and geothermal gradients on the depth of the brittle ductile transition are studied the model predictions are consistent with values inferred from seismicity data of different boundary zones

crystal growth in cataclasites diagnostic microstructures and implications abstract microstructures in cataclasites from low and high angle fault zones in granitic basement rocks of the scandinavian caledonides indicate the operation of several processes intensive formation of microcracks filled with fine grained mineral fragments indicates brittle deformation the formation of quartz crystals with euhedral terminations suggests crystal growth occurrence of undulose extinction deformation lamellae and grain boundary recrystallization in other quartz grains illustrates plastic deformation crystal growth causes micro cracks to heal and thereby changes the behaviour of rocks undergoing brittle deformation the crystallization products vary in size from c 10 m to c 5 mm narrow banding parallel to rhomb and prism planes is indicative of crystal growth the growth bands show slightly different crystallographic misorientation in some cases the boundaries of these bands could be identified as dislocation arrays decorated by bubbles the occurrence of fibrous chalcedony aggregates and transitions from extremely fine grained quartz to large grains with euhedral terminations suggests the presence of colloidal silica bearing fluids during growth the crystallization process leads to the formation of vein like structures after crystallization plastic deformation may take place followed by brittle deformation and new crystal growth it is concluded that during the formation of the cataclasites a cyclic operation of brittle failure crystal growth followed by plastic deformation took place and this cycle was repeated several times

cataclasites along the saltville thrust u s a and their implications for thrust sheet emplacement abstract cataclasis and frictional wear are the primary bulk deformation mechanisms along steeply dipping portions of the saltville thrust in the southern appalachian foreland zone u s a fault character ranges from a single discrete sliding surface with negligible gouge to a zone of several discrete sliding surfaces or a zone up to 0 3 m thick of pervasive cataclasite marked fracturing occurs up to 20 m above the fault whereas minimal deformation is found in the footwall rocks hanging wall dolomites range from crush breccias less than 5 matrix to ultracataclasites with 90 matrix although cataclasites 50 70 matrix are predominant foliated cataclasites occur where dolomite is thrust over shale progressive development of cataclastic fabrics is due to comminution by fracturing and grinding along intersecting fractures continued frictional grinding results in complete disruption of the original fabric to produce cataclasite and minor ultracataclasite grain alignment occurs by rigid body rotation with subsequent local enhancement by pressure solution microstructural relations of the fault gouge suggest periodic fluctuations in fluid pressure where v ratio of fluid to overburden pressure probably ranged between 0 45 and 1 the saltville thrust sheet emplacement must have occurred in a caterpillar like fashion involving aseismic and seismic shear shear stresses accompanying fault motion as determined from dolomite twin lamellae are in the order of 65 mpa

strong velocity weakening and powder lubrication of simulated carbonate faults at seismic slip rates 1 high velocity friction tests were conducted on solid and hollow cylinders of carrara calcite marble dolomite marble silicate bearing calcite marble and calcite gouge to investigate the strength of carbonate faults during seismic slip the experiments performed at normal stresses of 0 6 14 7 mpa slip rates of 0 03 1 60 m s and room temperature in a rotary shear friction testing machine yielded an extraordinarily low steady state friction coefficient 0 1 at slip rates of 1 1 1 2 m s the slip weakening distance of 4 28 m became shorter at higher normal stress or frictional work rate strong velocity weakening was observed not only in steady state but also in nonsteady state friction while the slip rate was changing thus slip deceleration was accompanied by fault strength recovery large rapid temperature rises in narrow shear localization zones less than a few micrometers induced carbonate decomposition such as the breakdown of calcite into aggregates of cao nanograins and co2 in carrara marble scanning electron microscope observation revealed that the shear localization zone in the highly porous decomposition product was a layer of scattered small grains mostly 1 m in diameter these microstructures and the measured high permeability 10 14 m2 of the decomposed marble indicate that the dominant weakening mechanism in our experiments was possibly powder lubrication powder rheology at high slip rates is not yet well understood but the frictional behavior of nanograins appears to be strongly velocity dependent if decarbonation occurs during seismic slip in natural carbonate faults powder lubrication may make the faults slippery even under fluid drained conditions

electron microscopy study of hyalomylonites evidence for frictional melting in landslides abstract hyalomylonites from langtang nepal and kofels otz valley austria were investigated by transmission electron microscopy tem the matrix is glassy at both localities rarely it is crystallized and devitrified to dendrites and skeletal microlites of plagioclase alkali feldspar and biotite the matrix is chemically heterogeneous with schlieren and formed by partial to almost complete melting of host rocks of granitic to granodioritic composition locally glasses with pure quartz plagioclase and alkalifeldspar composition were found from sio 2 glass minimum temperatures of 1520 c are estimated at both hyalomylonite localities melting appears to be the result of frictional heating on the gliding plane of a large landslide the host rocks were deformed in a brittle mode microstructures typical of plastic deformation at high temperatures are displayed by inclusions and were probably inherited from previous tectonic events

experimental investigation of flash weakening in limestone abstract flash heating and weakening could operate during earthquake nucleation and propagation we performed 27 friction experiments in a compression torsion apparatus on ring shaped limestone samples at sub seismic to seismic slip rates 340 mm s centimetric displacements and normal stresses of 8 mpa friction decreases dramatically at slip rates of 50 150 mm s flash weakening was contemporaneous with a peripheral temperature rise of 90 c measured with an infrared camera the peripheral temperature yields a lower limit to the slipping zone temperature the decrease in friction may result from weakening of the asperity contacts due to decarbonation of calcite induced by 1 flash heating or 2 mechanically activated reactions however x ray powder diffraction and raman spectroscopy analyses do not reveal the presence of decarbonation products in the slipping zone instead white light interferometry and field emission scanning electron microscope observations reveal the presence of a smooth sliding surface made of nanometric particles the mechanical data can be fit by the rate and state dependent friction model or by a quadratic model the latter proposed for powder lubrication we conclude that flash heating and weakening and powder lubrication may operate together to decrease dynamic friction in limestone in experiments and for the conditions investigated here in nature

constitutive behavior and stability of frictional sliding of granite an understanding of the frictional sliding on faults that can lead to earthquakes requires a knowledge of both constitutive behavior of the sliding surfaces and its mechanical interaction with the loading system we have determined the constitutive parameters for frictional sliding of initially bare surfaces of westerly granite using a recently developed high pressure rotary shear apparatus that allows long distances of sliding and therefore a greater assurance of attaining steady state behavior from experiments conducted at room temperature and normal stresses of 27 84 mpa several important results have been found 1 a gouge layer 100 to 200 m thick was developed from the initially bare rock surfaces after 18 to 70 mm of sliding 2 the steady state frictional resistance attained after about 10 mm of sliding is proportional to the negative of the logarithm of the sliding velocity 3 abrup changes in the velocity of sliding result in initial changes in the frictional resistance which have the same sign as the velocity change and are followed by a gradual decay to a new steady state value over a characteristic distance of sliding this velocity weakening behavior is essentially identical with that found by several previous workers on the same material at lower normal stress 4 our results are well described by a two state variable constitutive law the values of the constitutive parameters are quite similar to those found previously at low normal stress but the characteristic distance is about an order of magnitude smaller than that found at 10 mpa normal stress with thicker layers of coarser gouge 5 we have approximated our results with a one state variable constitutive law and compared the results with the predictions of existing nonlinear stability analysis in addition we have extended the stability analysis to systems possessing two state variables with such formulations good agreement is found between the experimentally observed and theoretically predicted transitions between stable and unstable sliding these results allow a better understanding of the instabilities that lead to earthquakes

neogene to quaternary broken foreland formation and sedimentation dynamics in the andes of nw argentina 25 s 1 the northwest argentine andes constitute a premier natural laboratory to assess the complex interactions between isolated uplifts orographic precipitation gradients and related erosion and sedimentation patterns here we present new stratigraphic observations and age information from intermontane basin sediments to elucidate the neogene to quaternary shortening history and associated sediment dynamics of the broken salta foreland this part of the andean orogen which comprises an array of basement cored range uplifts is located at 25 s and lies to the east of the arid intraorogenic altiplano puna plateau in the salta foreland spatially and temporally disparate range uplift along steeply dipping inherited faults has resulted in foreland compartmentalization with steep basin to basin precipitation gradients sediment architecture and facies associations record a three phase 10 5 and 2 ma east directed yet unsystematic evolution of shortening foreland fragmentation and ensuing changes in precipitation and sediment transport the provenance signatures of these deposits reflect the trapping of sediments in the intermontane basins of the andean hinterland as well as the evolution of a severed fluvial network present day moisture supply to the hinterland is determined by range relief and basin elevation the conspiring effects of range uplift and low rainfall help the entrapment and long term storage of sediments ultimately raising basin elevation in the hinterland which may amplify aridification in the orogen interior

artificial generation of pseudotachylyte using friction welding apparatus simulation of melting on a fault plane a 150 m thick fused layer of rock has been produced by rotating two metadolerite core faces against each other at 3000 r p m under an axial load of 330 kg for 11 s using friction welding apparatus scanning electron microscopy and electron microprobe analysis reveal that the melt layer comprises sub angular to rounded porphyroclasts of clinopyroxene feldspar and ilmenite 20 m diameter derived from the host metadolerite set within a silicate glass matrix thermal calculations confirm that melting occurred at the rock interface and that mean surface temperatures in excess of 1400 c were attained the fused layer shows many textural similarities with pseudotachylyte described from fault zones morphologically the fused layer consists of a series of stacks of porphyroclasts welded together by melt to form build ups oriented at right angles to the friction surface there is also evidence of gouging ploughing and plucking as well as transfer and adhesion of material having occurred between the rock faces the mean surface velocity attained by the metadolerite 0 24 m s 1 and duration of the experiment are comparable with velocities and rise times of typical single jerk earthquakes occurring during stick slip seismic faulting within brittle crust i e slip rates of 0 1 0 5 m s 1 for say 1 10s in these respects the experiment successfully simulated frictional fusion on a fault plane in the absence of an intergranular fluid power dissipation during the experiment was about mw m 2 comparable only to very low values for earthquakes e g 1 100 mw m 2 for displacement rates of 0 1 0 5 m s 1 at shear stresses of 100 1000 bars this indicates that melting on fault planes during earthquakes should be commonplace field evidence however does not support this contention either pseudotachylyte is not being recognized in exhumed ancient seismic fault zones or melting only occurs under very special circumstances

dilatancy induced fluid pressure drop during dynamic rupture direct experimental evidence and consequences for earthquake dynamics abstract fluid pressure and flow in the crust are key parameters controlling earthquake physics since earthquake slip is linked to spatio temporal localisation of deformation it is expected that the local fluid pressure around the fault plane could potentially impact the dynamic strength of the slipping fault zone coseismic fluid pressure drops have been inferred from field studies notably in gold deposits which are thought to be formed by this process but reliable quantitative predictions are still lacking here experimental results are presented where local on and off fault fluid pressure variations were measured in situ during dynamic rock fracture and frictional slip under upper crustal stress conditions during the main rupture the on fault fluid pressure dropped rapidly to zero indicating partial vaporisation and or degassing further deformation produced stick slip events systematically associated with near instantaneous drops in fluid pressure providing direct experimental support of the concept of seismic suction pump in situ fluid volume and wave speed measurements together with microstructural investigations show that dilatancy is the process driving fluid pressure drops during rupture and slip extrapolation of the laboratory results indicate that dilatancy induced fluid pressure drops might be a widespread phenomenon in the crust counteracting thermal pressurisation as a weakening mechanisms in freshly fractured rock

semi brittle flow of granitoid fault rocks in experiments field studies and seismic data show that semi brittle flow of fault rocks probably is the dominant deformation mechanism at the base of the seismogenic zone at the so called frictional viscous transition to understand the physical and chemical processes accommodating semi brittle flow we have performed an experimental study on synthetic granitoid fault rocks exploring a broad parameter space temperature t 300 400 500 and 600 c confining pressure pc 300 500 1000 and 1500 mpa shear strain rate 10 3 10 4 10 5 and 10 6 s 1 to finite shear strains 0 5 the experiments have been carried out using a granular material with grain size smaller than 200 m with a little h2o added 0 2wt only two experiments performed at the fastest strain rates and lowest temperatures have failed abruptly right after reaching peak strength 1400 mpa all other samples reach high shear stresses 570 1600 mpa then weaken slightly by 10 190 mpa and continue to deform at a more or less steady state stress level clear temperature dependence and a weak strain rate dependence of the peak as well as steady state stress levels is observed in order to express this relationship the strain rate stress sensitivity has been fit with a stress exponent assuming n and yields high stress exponents n 10 140 which decrease with increasing temperature the microstructures show widespread comminution strain partitioning and localization into slip zones containing at first nanocrystalline and partly amorphous material later during continued deformation fully amorphous material in slip zones despite the mechanical steady state conditions the fabrics in the slip zones and outside continue to evolve and do not reach a steady state microstructure below 5 within the slip zones the fault rock material progressively transforms from a crystalline solid to an amorphous material we present and interpret the experimental results both in terms of sliding friction and viscous flow and we discuss the possible effect that the formation of nanocrystalline and amorphous layers may have on earthquake nucleation

co seismic geomorphic and geologic fold growth associated with the 1978 tabas e golshan earthquake fault in eastern iran we describe the seismicity and late cenozoic deformation associated with a blind thrust fault at tabas e golshan hereafter referred to as tabas eastern iran which generated a devastating mw 7 3 earthquake on the 16th september 1978 measurements from a structural transect through the sardar anticline segment indicate fault propagation folding above a gently 20 eastward dipping blind thrust fault the thrust flattens into a horizontal detachment at a depth of only 2 km tightening of the fold forelimb is accommodated by flexural slip along numerous bedding planes with many of the slip surfaces showing fresh striations with a large component of right lateral strike slip a steeply dipping fault zone showing almost pure strike slip is also developed within the forelimb of the fold our field observations are consistent with the source parameters of the 1978 tabas earthquake and additional events in 1979 and 1980 which all involved slip on a shallowly dipping thrust with a significant component of right lateral slip the surface of an alluvial fan which is likely to have been abandoned at 8 10 ka has been folded as it crosses the sardar anticline the age constraints combined with topographic profiles along the deformed fan surface and constraints on the dip of the fault at depth provide an approximate rate of horizontal shortening of 1 5 mm yr shortening at tabas appears to result from transpressional bending at the north end of the nayband strike slip fault a northward continuation of the nayband fault which may be slipping at rates of 2 mm yr is identified along the base of the shotori mountains 10 20 km east of the tabas thrust the range front fault did not move in 1978 and constitutes an additional threat to local populations

effects of simulated clay gouges on the sliding behavior of tennessee sandston abstract the effects of simulated fault gouge on the sliding behavior of tennessee sandstone are studied experimentally with special reference to the stabilizing effect of clay minerals mixed into the gouge about 30 specimens with gouge composed of pure clays of homogeneously mixed clay and anhydrite or of layered clay and anhydrite along a 35 precut are deformed dry in a triaxial apparatus at a confining pressure of 100 mpa with a shortening rate of about 5 10 4 sec and at room temperature pure clay gouges exhibit only stable sliding and the ultimate frictional strength is very low for bentonite mont morillonite intermediate for chlorite and illite and considerably higher for kaolinite anhydrite gouge shows violent stick slip at 100 mpa confining pressure when this mineral is mixed homogeneously with clays the frictional coefficient of the mixed gouge determined at its ultimate frictional strength decreases monotonically with an increase in the clay content the sliding mode changes from stick slip to stable sliding when the frictional coefficient of the mixed clay anhydrite gouge is lowered down below 90 95 of the coefficient of anhydrite gouge the stabilizing effect of clay in mixed gouge is closely related to the ultimate frictional strength of pure clays that is the effect is conspicuous only for a mineral with low frictional strength only 15 20 of bentonite suppresses the violent stick slip of anhydrite gouge in contrast violent stick slip occurs even if the gouge contains as much as 75 of kaolinite the behavior of illite and chlorite is intermediate between that of kaolinite and bentonite bentonite anhydrite two layer gouge exhibits stable sliding even when the bentonite content is only 5 thus the presence of a thin clay rich layer in a fault zone stabilizes the behavior much more effectively than do the clay minerals mixed homogeneously with the gouge this result brings out the mechanical significance of internal structures of a fault zone in understanding the effects of intrafault materials on the fault motion based on the present experimental results incorporated with some other experimental data it is argued that although the stabilizing effect of montmorillonite and vermiculite is indeed remarkable at room temperature the effect should be much less pronounced at elevated temperatures due perhaps to the dewatering of the clays in most geological environments where shallow earthquakes occur the stabilizing effect of clays is probably not so conspicuous as to completely suppress the unstable motion of a fault

transition from dislocation creep to melt enhanced diffusion creep in fine grained granitic aggregates abstract sintered aggregates composed of quartz albite and microcline of two different grain sizes 2 10 and 10 50 m each with two different melt contents

amorphous material in experimentally deformed mafic rock and its temperature dependence implications for fault rheology during aseismic creep and seismic rupture abstract amorphous materials are frequently observed in natural and experimentally produced fault rocks their common occurrence suggests that amorphous materials are of importance to fault zone dynamics however little is known about the physico chemical impact of amorphous materials on fault rheology here we present deformation experiments on mafic fault rock where amorphous material forms due to intense mechanical wear during the experiments the experiments are run at temperatures from 300 to 600 c confining pressures of 0 5 or 1 0 gpa and at constant displacement rates of d ax 2 10 7 2 10 8 or 2 10 9 ms 1 resulting in bulk strain rates of 3 10 4 3 10 5 and 3 10 6 s 1 at these conditions the mafic rock material undergoes intense brittle deformation and cataclastic flow but sample strength significantly decreases with increasing temperatures a feature commonly attributed to viscous deformation processes microstructural analyses show that after an initial stage of homogeneous cataclastic flow strain localizes into narrow 2 10 m wide ultra cataclastic bands that evolve into amorphous shear bands with the data presented in this research paper we argue that the temperature sensitivity recorded in the mechanical data is caused by viscous deformation of the amorphous material we suggest that with the formation of amorphous materials during brittle deformation fault rheology becomes significantly temperature sensitive this has important implications for our understanding of fault strength and weakening due to the presence of amorphous materials in addition weak material along faults will lead to stress concentrations that may trigger seismic rupture

amorphous material formed by the mechanochemical effect in natural pseudotachylyte of crushing origin a case study of the iida matsukawa fault nagano prefecture central japan abstract pseudotachylytes from the iida matsukawa fault central japan iida pseudotachylytes are described down to the nanometer scale using high resolution electron microscopy scanning electron microscopy analyses of chemical compositions and mercury intrusion porosimetry the pseudotachylyte matrix chiefly consists of nanoscale particles and elongated submicron fragments with biotite composition amorphous material of several tens of nanometers in size is also observed despite the absence of melt textures lattice fringe images reveal that the amorphous phase coexists with lattice distortion in deformed biotite fragments that are several hundreds of nanometers in size the presence of these submicrostructures indicates that the amorphous material formed by mechanical stress during the comminution process that is via the mechanochemical effect we conclude that amorphous material does not always provide evidence of the rapid cooling of melt in pseudotachylyte the amorphous materials present in the iida pseudotachylyte resulted from the mechanochemical effect due to both shear stress and normal stress during the comminution process that accompanied fault motion

disappearing ink how pseudotachylytes are lost from the rock record abstract melt origin pseudotachylytes are the most widely accepted feature recording earthquake slip in the fault rock record however reports of pseudotachylytes are rare compared to the frequency and distribution of earthquakes in active faults suggesting melting occurs only under exceptional circumstances and therefore that pseudotachylytes are rarely formed in this paper we document the processes whereby pseudotachylytes are overprinted destroyed and otherwise removed from the rock record we present examples of recrystallized altered and cataclastically and crystal plastically deformed pseudotachylytes from a variety of ancient faults based on these observations we identify characteristics of pseudotachylytes that are resistant to change over geologic time and develop criteria to allow recognition of relict pseudotachylytes our results imply that pseudotachylytes are vastly under reported due to their vulnerability to destruction and the resulting difficulty in identification as a consequence the significance of frictional melting is underestimated the criteria we propose to distinguish relict pseudotachylytes can help to reconcile the observed frequency of earthquakes with the difficulty of demonstrating ancient seismic slip in the rock record

identification and tectonic implications of nano particle quartz 50 nm by synchrotron x ray diffraction in the chelungpu fault gouge taiwan abstract to determine the ultrafine nano scale grains in a fault gouge the chelungpu fault gouge was sampled and analyzed it was separated into different particle size ranges and was analyzed by synchrotron x ray diffraction and transmission electron microscopy the minerals of gouge are predominantly composed of quartz plagioclase smectite illite chlorite and kaolinite the mineral association of

cyclic shear zone cataclasis and sintering during lava dome extrusion insights from chaos crags lassen volcanic center usa abstract the ascent and extrusion of crystal rich magma is commonly facilitated by deformation partitioned within annular conduit parallel shear zones the physical properties and textures of the shear zone materials where exposed at surface provide a record of ascent and eruption dynamics we describe the shear zone developed in dome c part of chaos crags in the lassen volcanic center california usa the extruded shear zone comprises volcanic fault gouge and variably densified cataclasites the competent cataclasites evidence deep seated gouge production followed by gouge densification within the conduit on the timescale of lava dome ascent textural geochemical and mineralogical data identify solid state sintering as the densification mechanism at the temperatures and pressures in the volcanic conduit solid state sintering causes rapid porosity and permeability loss within the gouge and concomitant material strengthening longer dwell times i e slower ascent allow for more sintering producing stronger denser and less permeable cataclasites at chaos crags we use the extent of sintering quantified by residual porosity to recover minimum in conduit dwell times necessary to produce the observed cataclasites our analysis of the dome c cataclasites suggests a maximum linear ascent rate of 10 m day and a minimum ascent time of 100 days we evaluate the consequences of shear zone lithification by solid state sintering for the eruption of other crystal rich glass poor magmas chaos crags cataclasites preserve evidence of multiple cycles of fracturing cataclasis and re sintering suggesting a mechanism for transitions between effusive and explosive phases of dome building eruptions

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

experimental investigation of the brittle viscous transition in mafic rocks interplay between fracturing reaction and viscous deformation rock deformation experiments are performed on fault gouge fabricated from maryland diabase rock powder to investigate the transition from dominant brittle to dominant viscous behaviour at the imposed strain rates of view the mathml source 3 10 5 3 10 6 s 1 the transition is observed in the temperature range of 600 c t 800 c at confining pressures of 0 5 gpa pc 1 5 gpa the transition thereby takes place by a switch from brittle fracturing and cataclastic flow to viscous dissolution precipitation creep and grain boundary sliding mineral reactions and resulting grain size refinement by nucleation are observed to be critical processes for the switch to viscous deformation i e grain size sensitive creep in the transitional regime the mechanical response of the sample is a mixed mode between brittle and viscous rheology and microstructures associated with both brittle and viscous deformation are observed as grain size reduction by reaction and nucleation is a time dependent process the brittle viscous transition is not only a function of t but to a large extent also of microstructural evolution

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

major and trace element constraints on the petrogenesis of a fault related pseudotachylyte western blue ridge province north carolina a newly identified 6 cm thick zoned pseudotachylyte from the western blue ridge province of north carolina is interpreted to be the result of episodic melting on a reactivated brittle fault previously active in the ductile regime kinematic indicators in cataclasites associated with the pseudotachylyte indicate top to the northeast shear whereas kinematic indicators in mylonites indicate top to the northwest shear higher concentrations in the pseudotachylyte of al2o3 k2o feo and depletion in sio2 are explained by the total fusion of micas and partial fusion of k feldspar 60 leaving quartz and refractory phases as residual enrichment of the pseudotachylytes in ba2 rb sr2 and eu2 provide additional evidence for the preferential melting of k feldspar the trace element signature of the pseudotachylytes similar ree patterns ba k k rb ba rb zr hf ti cr ratios and entrained lithoclasts identify the source of the melt as cataclastically deformed mylonite of the hangingwall the similarity of the ree patterns in the pseudotachylytes and the source rock is explained by the entrainment and or dissolution of ree bearing phases in the melt in proportion to its volume these data indicate that coseismic flash melting is distinctly different from equilibrium partial melting and that trace elements can be used to identify the source rock and distinguish those phases that melt from those that remain residual

growth of contact area between rough surfaces under normal stress the contact area between deforming rough surfaces in marble alabaster and quartz was measured from thin sections of surfaces bonded under load with low viscosity resin epoxy the marble and alabaster samples had contact areas that increased with stress at an accelerating rate this result suggests that the strength of the asperity contacts decreased progressively during the deformation following some form of strain weakening relationship this conclusion is supported by petrographic observation of the thin sections that indicate that much of the deformation was cataclastic with minor twinning of calcite and kinking of gypsum in the case of the quartz the observed contact area was small and increased approximately linearly with normal stress only the irreversible cataclastic deformation was observed however strain induced birefringence and cracking of the epoxy not observed with the other rocks suggests that significant elastic deformation occurred but recovered during unloading

frictional properties of fault zone gouges from the j fast drilling project mw 9 0 2011 tohoku oki earthquake smectite rich fault gouges recovered during integrated ocean drilling program expedition 343 japan trench fast drilling project j fast from the plate boundary slip zone of the 2011 mw 9 0 tohoku oki earthquake were deformed at slip velocities of 10 m s 1 to 3 5 m s 1 and normal stresses up to 12 mpa water dampened gouges 1 are weaker apparent friction coefficient 0 1 m s 1 a significant amount of amorphous material formed in room humidity experiments at low and high slip velocities likely by comminution and disordering of smectite our results indicate that the frictional properties of water dampened gouges could have facilitated propagation of the tohoku oki rupture to the trench and large coseismic slip at shallow depths

phase transformation and nanometric flow cause extreme weakening during fault slip faults weaken during earthquakes laboratory simulations of earthquake rupture show that the nanometric scale fault gouge created during slip is inherently weak and flows by grain boundary sliding providing a mechanism to weaken faults

the effect of normal stress on the real area of contact during frictional sliding in rocks the real area of contact during frictional sliding has been determined as a function of changing normal stress in triaxial experiments through the use of thermodyes utilizing the technique described by teufel and logan in 1978 with saw cut surfaces inclined 35 to the load axis determinations were made for monolithologic sliding of tennessee sandstone and indiana limestone and dilithologic sliding of the same rocks confining pressures to 200 mpa were investigated at a constant shortening rate of 10 2 mm sec and at room temperature direct measurements were made of single asperity areas and the asperity density the product of these measurements gives the percent area of real contact across the sliding surface single asperity area and density are found to remain relatively constant during the displacement single asperity areas are in the ranges of 0 4 to 6 10 2 mm2 for sandstone 0 8 to 2 10 2 mm2 for limestone and 0 2 to 24 10 2 mm2 for sandstone sliding against limestone these values are smaller than the grain size of either rock the values increase with increasing normal stress for both monolithologic and dilithologic sliding in sandstone the asperity density increases from about 0 8 to 2 75 contacts per square millimeter in a logarithmic fashion monolithologic limestone has values of about 0 9 contacts per square millimeter and does not show significant change with increasing normal stress the percent area of real contact increases in all cases with average maximum values of 16 of the apparent area at a normal stress of 374 mpa in sandstone 18 at 25 mpa in limestone and 22 at 123 mpa in the dilithologic specimens the normal stress recalculated for the real area of contact approaches the unconfined compressive strength for sandstone and limestone

stability and localization of rapid shear in fluid saturated fault gouge 1 linearized stability analysis field observations of major earthquake fault zones show that shear deformation is often confined to principal slipping zones that may be of order 1 100 m wide located within a broader gouge layer of order 10 100 mm wide this paper examines the possibility that the extreme strain localization observed may be due to the coupling of shear heating thermal pressurization and diffusion in the absence of a stabilizing mechanism shear deformation in a continuum analysis will collapse to an infinitesimally thin zone two possible stabilizing mechanisms studied in this paper are rate strengthening friction and dilatancy for rate strengthening friction alone a linear stability analysis shows that uniform shear of a gouge layer is unstable for perturbations exceeding a critical wavelength using this critical wavelength we predict a width for the localized zone as a function of the gouge properties taking representative parameters for fault gouge at typical centroidal depths of crustal seismogenic zones we predict localized zones of order 5 40 m wide roughly consistent with field and experimental observations for dilatancy alone linearized strain rate perturbations with a sufficiently large wavelength will undergo transient exponential growth before decaying back to uniform shear the total perturbation strain accumulated during this transient strain rate localization is shown to be largely controlled by a single dimensionless parameter e which is a measure of the dilatancy of the gouge material due to an increase in strain rate

shear zone broadening controlled by thermal pressurization and poroelastic effects during model earthquakes as a result of the comminution that takes place over numerous earthquake cycles mature faults are characterized by thick layers of pulverized gouge with finite porosity that is saturated with water at seismogenic depths the heat generated during earthquakes raises the gouge temperature and thermal expansion of the pore fluid and surrounding solids produce elevated pore pressures that cause fault strength to decrease in the process known as thermal pressurization building upon this framework we describe a model that imposes a plane strain configuration and shows that the stress variations caused by porothermoelasticity promote the mohr coulomb failure of previously undeformed regions except in special cases where the friction is rate strengthening we find that the frictional strength must vary throughout the post failure region which we identify in our model with the shear zone we introduce a strain rate function that describes the overall influence of distributed slip on energy dissipation and fault strength as the shear zone thickness expands using typical fault parameters at 7 km depth the shear zone reaches several millimeters of thickness after 1 s sliding at an overall rate of 1 m s the expansion of the shear zone limits the temperature rise to several hundred degrees celsius and the average fault strength falls to about a tenth of the static frictional strength

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

granular nanoparticles lubricate faults during seismic slip nanoparticles are known to form in narrow slip zones in natural and experimental fault zones but their possible role in dynamic weakening of faults during seismic slip remains almost unexplored we conducted friction experiments on periclase mgo nanoparticles 50 nm in average size at rates as high as 1 3 m s 1 a typical speed of seismic slip the nanoparticles were used as the initial gouge to avoid complexities arising from comminution and fluid release the dynamic friction decreased with increasing slip rate and the steady state frictional coefficient reduced to as low as 0 1 at a slip rate of 1 3 m s 1 flash heating is not effective for nanoparticles and we propose that development of slickensides and dominant operation of nanoparticle rolling cause such a weakening nanoparticle lubrication appears to be as effective as melt lubrication and thermal pressurization and the formation of nanoparticles in slip zones may be an important fault lubrication process

amorphous material in high strain experimental fault gouges the microstructures of gouges produced in room temperature rotary shear sliding experiments were examined by transmission electron microscopy gouges were produced by sliding on ground surfaces of granite quartzite or marble except for one experiment in which a 1 mm thick simulated gouge layer was used water was added to the sliding surfaces of all but one sample crystal plastic processes play no role in the granite and quartzite gouges and a minor role in the marbles all of the gouges consist of mostly submicron crystalline fragments in addition the granite gouges contain 5 60 amorphous material and the quartzite gouge contains 50 amorphous material in the granite samples the composition of the amorphous material commonly lies between k rich and na ca rich feldspars although portions may be silica rich the microstructural relations suggest that the amorphous material forms by comminution of fragments rather than by melting the amount of amorphous material increases and the size of the largest crystalline fragments decreases with an increase in average shear strain although the microstructure is nearly uniform throughout each granite gouge layer these observations suggest that after slip becomes localized on y shear surfaces and or r1 riedel shears the entire gouge layer must continue to undergo deformation it is suggested that cyclic deformation in the gouge must occur to accommodate the passage of geometric irregularities on the active slip surfaces

the roles of time and displacement in velocity dependent volumetric strain of fault zones the relationship between measured friction a and volumetric strain during frictional sliding was determined using a rate and state variable dependent friction constitutive equation a common work balance relating friction and volume change and two types of experimental faults initially bare surfaces of westerly granite and rock surfaces separated by a 1 mm layer of 90 m westerly granite gouge the constitutive equation is the sum of a constant term representing the nominal resistance to sliding and two smaller terms a rate dependent term representing the shear viscosity of the fault surface direct effect and a term which represents variations in the area of contact evolution effect the work balance relationship requires that a differs from the frictional resistance that leads to shear heating by the derivative of fault normal displacement with respect shear displacement d n d s an implication of this relationship is that the rate dependence of d n d s contributes to the rate dependence of a experiments show changes in sliding velocity lead to changes in both fault strength and volume analysis of data with the rate and state equations combined with the work balance relationship preclude the conventional interpretation of the direct effect in the rate and state variable constitutive equations consideration of a model bare surface fault consisting of an undeformable indentor sliding on a deformable surface reveals a serious flaw in the work balance relationship if volume change is time dependent for the model at zero slip rate indentation creep under the normal load leads to time dependent strengthening of the fault surface but according to the work balance relationship no work is done because compaction or dilatancy can only be induced by shearing additional tests on initially bare surfaces and gouges show that fault normal strain in experiments is time dependent consistent with the model this time dependent fault normal strain which is not accounted for in the work balance relationship explains the inconsistency between the constitutive equations and the work balance for initially bare surface faults all rate dependence of volume change is due to time dependence similar results are found for gouge we conclude that a reflects the frictional resistance that results in shear heating and no correction needs to be made for the volume changes the result that time dependent volume changes do not contribute to a is a general result and extends beyond these experiments the simple indentor model and particular constitutive equations used to illustrate the principle

experimental evidence for wall rock pulverization during dynamic rupture at ultra high pressure conditions abstract the mechanisms triggering intermediate and deep earthquakes have puzzled geologists for several decades there is still no consensus concerning whether such earthquakes are triggered by brittle or ductile mechanisms we performed a deformation experiment on a synthetic lawsonite bearing blueschist at a confining pressure of 3 gpa and temperatures from 583 to 1 073 k after deformation the recovered sample reveals conjugated shear fractures garnet crystals are dissected and displaced along these narrow faults and reveal micro and nanostructures that resemble natural pulverization structures as well as partial amorphization formation of such structures at low confining pressures is known to require high tensile stresses and strain rates and is explained by the propagation of a dynamic shear rupture the absence of shearing in the pulverized wall rock is taken as evidence that these structures pre date the subsequent heat producing frictional slip in analogy to observations at low pressure we infer that the garnet structures in our experiment result from rapid propagation of a shear fracture even at the high pressure exerted on the sample and thus suggest that brittle deformation is possible at lower crustal to upper mantle depths

silica gel formation during fault slip evidence from the rock record dynamic reduction of fault strength is a key process during earthquake rupture many mechanisms for causing coseismic weakening have been proposed based on theory and laboratory experiments including silica gel lubrication however few have been observed in nature here we report on the fi rst documented occurrence of a natural silica gel coating a fault surface the corona heights fault slickenside in san francisco california is covered by a shiny layer of translucent silica microstructures in this layer show fl ow banding armored clasts and extreme comminution compared to adjacent cataclasites the layer is composed of 100 nm to 1 m grains of quartz hydrous crystalline silica and amorphous silica with 10 100 nm inclusions of fe oxides and ellipsoidal silica colloids kinematic indicators and mixing with adjacent cataclasites suggest the shiny layer was fl uid during fault slip the layer therefore represents a relict silica gel that formed during fault motion and which could have resulted in frictional instability these observations confi rm that the silica gels formed in rock friction experiments do occur in natural faults and therefore that silica gel formation can act as a dynamic weakening mechanism in faults at shallow crustal conditions