a physical basis for the frictional melting of some rock forming minerals abstract under conditions of coseismic slip 0 1 2 0 m s 1 within the upper crust differences in the shear yield strengths fracture toughnesses melting points and to a lesser extent thermal conductivities of rock forming minerals result in a hierarchy of friction melting susceptibilities in many so called pseudotachylytes it is observed that phyllosilicates and amphiboles are preferentially consumed to form the melt phase whilst quartz and feldspar tend to survive as clasts this can be explained in terms of the disequilibrium flash melting of those mineral phases possessing the lower shear yield strengths fracture toughnesses and thermal conductivities rather than by eutectic melting that would otherwise generate equilibrium minimum melts indicative of igneous processes at high strain rates typical of coseismic faulting the mechanical behaviour of minerals is primarily controlled by their fracture toughnesses with progressive comminution during slip leading to fusion the friction melting dependence on the mechanical properties of minerals indicates that there may be an important relationship between wall rock lithology and earthquake magnitude if frictional melting results in fault lubrication

strength and anisotropy of foliated rocks with varied mica contents abstract we have shortened 15 schists and gneisses with varying compositions 15 75 mica by volume at angles of 45 and 90 to foliation s to investigate the influence of micas on the strength and anisotropy of foliated rocks at the conditions tested t 25 c p c 200 mpa and e 10 5 s 1 compressive strengths vary by a factor 4 in the 45 and 90 orientations and individual rock types exhibit directional responses ranging from isotropic to strongly anisotropic trends of decreasing strength decreasing anisotropy and increasing ductility are observed with increasing mica content strains in all samples were localized within inclined shear zones accommodated by dislocation slip and grain scale microkinking in micas and microcracking in all other silicates stress strain response and grain scale deformation microstructures both indicate that mechanical behavior is strongly influenced by the concentration and spatial arrangement of micas shear zone formation is associated either with discrete brittle fracture transitional strain softening or steady strength ductile shear in both brittle and transitional samples extensive microcracking of strong quartzo feldspathic bridges occurs and stress drop magnitudes decrease with decreasing spacing and increasing overlap of adjacent critically oriented micas steady strength ductile shear zones develop in samples containing domains of interconnected micas and isotropic mechanical response is observed in those in which micas are contiguous in nearly all directions

ultracataclasite structure and friction processes of the punchbowl fault san andreas system california abstract the punchbowl fault is an exhumed 40 km displacement fault of the san andreas system in the devil s punchbowl the fault contains a continuous ultracataclasite layer along which the punchbowl formation sandstone and an igneous and metamorphic basement complex are juxtaposed the fabric of the ultracataclasite layer and surrounding rock indicate that nearly all of the fault displacement occurred in the layer by analogy with nearby active faults we assume that the punchbowl fault was seismogenic and that the ultracataclasite structure records the passage of numerous earthquake ruptures we have mapped the ultracataclasite layer at 1 1 and 1 10 to determine the mode of failure and to constrain the processes of seismic slip on the basis of color cohesion fracture and vein fabric and porphyroclast lithology two main types of ultracataclasite are distinguished in the layer an olive black ultracataclasite in contact with the basement and a dark yellowish brown ultracataclasite in contact with the sandstone the two are juxtaposed along a continuous contact that is often coincident with a single continuous nearly planar prominent fracture surface pfs that extends the length of the ultracataclasite layer in all exposures no significant mixing of the brown and black ultracataclasites occurred by offset on anastomosing shear surfaces that cut the contact or by mobilization and injection of one ultracataclasite into the other the ultracataclasites are cohesive throughout except for thin accumulations of less cohesive reworked ultracataclasite along the pfs structural relations suggest that 1 the black and brown ultracataclasite were derived from the basement and sandstone respectively 2 the black and brown ultracataclasites were juxtaposed along the pfs 3 the subsequent final several kilometers of slip on the punchbowl fault occurred along the pfs and 4 earthquake ruptures followed the pfs without significant branching or jumping to other locations in the ultracataclasite by comparison with rock friction experiments the slip localization along the pfs in the ultracataclasite implies rate weakening behavior with a critical slip distance similar to laboratory values and thus relatively small nucleation and breakdown dimensions for earthquake ruptures of the various mechanisms proposed to explain the low strength of the san andreas and to produce dynamic weakening of faults those that require or assume extreme localization of slip are most compatible with our observations

ultracataclasis sintering and frictional melting in pseudotachylytes from east greenland abstract large volumes of pseudotachylyte an intrusive fault related rock interpreted to form by a combination of cataclasis and melting occur in tertiary normal faults and accommodation zones along 400 km of the east greenland volcanic rifted margin analysis of representative pseudotachylyte samples reveals a wide range of mesoscopic and microscopic textures mineralogies and chemistries in the aphanitic pseudotachylyte matrix three distinct types of pseudotachylyte referred to as angular rounded and glassy are identified based on these characteristics angular pseudotachylyte found primarily in dike like reservoir zones is characterized by angular grains visible on all scales with micron scale fragments of mica and amphibole its matrix is enriched in fe2o3 mgo and tio2 relative to the host rock with minor increases in cao k2o and small decreases in na2o rounded pseudotachylyte is found in reservoir zones injection veins pseudotachylyte filled extension fractures and fault veins small faults with pseudotachylyte along their surfaces it is characterized by smooth surfaced compacted grains on microscopic scales and encloses rounded interpenetrative lithic clasts on outcrop scale its matrix is enriched in fe2o3 mgo tio2 and al2o3 relative to the host rock with minor depletion in na2o and k2o glassy pseudotachylyte is found primarily along fault surfaces its matrix is characterized by isotropic conchoidally fractured material containing microscopic strain free amphibole phenocrysts and is enriched in tio2 al2o3 k2o fe2o3 mgo cao and na2o relative to the host rock these observations suggest that angular pseudotachylyte was produced by cataclasis with enrichment in metallic oxides resulting from preferential crushing of mechanically weak amphibole and mica minerals found in the gneissic host rock cataclasis and concomitant frictional heating resulted in the textural and chemical modification of angular pseudotachylyte by sintering or melting producing rounded and glassy pseudotachylyte respectively compositional and textural observations constrain the temperatures reached during frictional heating 700 900 c which in turn delimit the amount of frictional heat imparted to the pseudotachylytes during slip our results suggest that the east greenland pseudotachylytes formed during small seismic events along faults at shallow crustal levels consistent relative ages and widespread occurrence of pseudotachylyte bearing faults in east greenland suggest that widespread microseismicity accompanied the early development of this volcanic rifted margin

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

earthquake faulting as a structural process abstract structural geology is concerned with the history of movement in the earth s crust and the processes by which displacements occur in the upper one third to one half of deforming continental crust displacement is accommodated largely by seismic slip increments on existing faults it follows that earthquakes and related processes are an integral part of structural geology traditionally structural geologists have been preoccupied with the complexity of the finite deformation within fault zones and with the stress states prevailing at the initiation of faults in intact crust future structural work should be directed more towards understanding the dynamic character of fault reactivation during incremental slip and related effects questions of interest include rheological and geometrical controls on the initiation perturbation and termination of ruptures directivity effects associated with rupture propagation the recognition of structures resulting from repeated stress cycling within seismogenic crust and identification of structural features diagnostic of shear stress levels during faulting structures arising from the inter relationships between slip episodes and induced fluid flow are of special importance because these dynamic fault processes appear influential in the development of much fault hosted mineralization mesothermal gold quartz lodes hosted in high angle reverse shear zones of mixed brittle ductile character form illustrative examples of structures that arguably can only be interpreted by seismo structural analysis embodying the concepts listed above

microstructural and chemical changes associated with cataclasis and frictional melting at shallow crustal levels the cataclasite pseudotachylyte connection in pelitic schists of the nason terrane of washington state pseudotachylyte can be demonstrated to have developed subsequent to the formation of foliation parallel cataclasite zones numerous microstructural lines of evidence point to a genetic link between pseudotachylyte and cataclasite and chemical evidence indicates that cataclasis played an important role in controlling pseudotachylyte chemistry besides optical and textural differences between the cataclasite and pseudotachylyte which may occur together there is unambiguous evidence of a melt origin for the pseudotachylyte while such evidence within the cataclasite is absent tem evidence indicates that glass is present in at least some of the veins as demonstrated by a lack of electron diffraction microlites present in the glass lack dislocations high temperature microlites of a mineralogy and composition not present in the host schist occur in various habits which depend on the distance from the vein margin also present are assimilation textures apparently quenched sulfide droplets and vesicles and amygdules in a pseudotachylyte vein studied in detail five microstructural domains occur these include vein margin cataclasite a transitional dark pseudotachylyte vein interior glass opaque and sulfide rich blebs and translucent haloes surrounding some of the blebs the cryptocrystalline cataclasite matrix grades into the dark pseudotachylyte which optically grades into the glass after adjustment for silica the host schist and cataclasite matrix are chemically very similar the dark pseudotachylyte and glass with the exception of na2o and cao are chemically quite similar to the cataclasite matrix the main difference may be the result of the segregation of the opaque plagioclase blebs thus the pseudotachylyte chemistry is strongly dependent upon the chemistry of the material comprising the cataclasite matrix which in turn is dependent on mineral susceptibility to incorporation into the cataclasite matrix fluids infiltrated along the cataclasite zones prior to pseudotachylyte generation as recorded by hydration reactions sulfide veining na2o loss and fluid inclusions this demonstrates that pseudotachylytes can form under fluid rich conditions as opposed to the commonly assumed dry conditions

contemporaneous plutonism and strike slip faulting a case study from the tonale fault zone north of the adamello pluton italian alps 1 a field microstructural and geochronological study of contemporaneous plutonism and strike slip faulting along the eastern tonale fault zone provides new insights into the interrelationships between magmatic emplacement contact metamorphism and shearing and it places new time constraints for the periadriatic fault system although pluton emplacement and shearing were not caused by each other they mutually interacted during contact metamorphism the character of the composite tonale fault zone varies considerably with its proximity to the northern margin of the adamello pluton from a paired greenschist facies mylonite belt northern mylonite zone and cataclastic fault zone in the west to a fault zone consisting of northern mylonite zone cataclastic fault zone and a second mylonite belt southern mylonite zone formed by contemporaneous shearing and contact metamorphism in the east ongoing strike slip motion led to telescoping and advective cooling of the deforming contact aureole new u pb zircon ages for the northernmost adamello intrusions the avio at 34 6 1 0 ma and the presanella at 32 0 2 3 ma date the onset of contact metamorphism rb sr and k ar cooling ages and new zircon fission track ages provide evidence that postintrusive cooling to below 300 c was achieved rapidly i e approximately 30 myr ago being consistent with a shallow crustal emplacement at ambient temperatures of approximately 250 c cataclastic dextral strike slip faulting continued until about 20 ma when the tonale fault zone became offset by a number of minor sinistral shears and by the giudicarie fault hence dextral strike slip motion along the periadriatic fault system east of the bergell pluton lasted from approximately 35 20 myr ago

insights on the thermal evolution of the ligurian apennines italy through fission track analysis apatite fission track analysis has been applied on 24 samples from the geometrically upper units internal and external ligurids of the northern apennines nappe pile they yield ages between c 20 ma and c 6 ma two samples were also dated with fission tracks in zircon ages of c 155 ma and c 199 ma together the data reveal that during post late cretaceous compression the internal ligurids reached temperatures to completely anneal fission tracks in apatite t c 120 c but not in zircon t 300 c after eoccne continental collision these rocks were cooled below c 120 c and started again to retain fission tracks in apatite since the oligocene a further burial under episutural sediments ensued that caused a partial to total apatite fission track annealing during a final phase of cxhumation the sedimentary cover was removed this exhumation can be linked to late miocenc extensional tectonics that started at c 8 9 ma in the central and southern portions of the study area

cataclastic rocks of the san gabriel fault an expression of deformation at deeper crustal levels in the san andreas fault zone the san gabriel fault a deeply eroded late oligocene to middle pliocene precursor to the san andreas was chosen for petrologic study to provide information regarding intrafault material representative of deeper crustal levels cataclastic rocks exposed along the present trace of the san andreas in this area are exclusively a variety of fault gouge that is essentially a rock flour with a quartz feldspar biotite chlorite amphibole epidote and fe ti oxide mineralogy representing the milled down equivalent of the original rock anderson and osborne 1979 anderson et al 1980 likewise fault gouge and associated breccia are common along the san gabriel fault but only where the zone of cataclasis is several tens of meters wide at several localities the zone is extremely narrow several centimeters and the cataclastic rock type is cataclasite a dark aphanitic and highly comminuted and indurated rock the cataclastic rocks along the san gabriel fault exhibit more comminution than that observed for gouge along the san andreas the average grain diameter for the san andreas gouge ranges from 0 01 to 0 06 mm for the san gabriel cataclastic rocks it ranges from 0 0001 to 0 007 mm whereas the san andreas gouge remains particulate to the smallest grain size the ultra fine grain matrix of the san gabriel cataclasite is composed of a mosaic of equidimensional interlocking grains the cataclastic rocks along the san gabriel fault also show more mineralogiec changes compared to gouge from the san andreas fault at the expense of biotite amphibole and feldspar there is some growth of new albite chlorite sericite laumontite analcime mordenite and calcite the highest grade of metamorphism is laumontite chlorite zone zeolite facies mineral assemblages and constrained uplift rates allow temperature and depth estimates of 200 30 c and 2 5 km thus suggesting an approximate geothermal gradient of 50 c km such elevated temperatures imply a moderate to high stress regime for the san andreas which is consistent with experimental rock failure studies moreover these results suggest that the previously observed lack of heat flow coaxial with the fault zone may be the result of dissipation rather than low stress much of the mineralogy of the cataclastic rocks is still relict from the earlier igneous or metamorphic history of the protolith porphyroclasts even in the most deformed rocks consist of relict plagioclase oligoclase to andesine alkali feldspar quartz biotite amphibole epidote allanite and fe ti oxides ilmenite and magnetite we have found no significant development of any clay minerals illite kaolinite or montmorillonite for many sites the compositions of these minerals directly correspond to the mineral compositions in rock types on one or both sides of the fault whole rock major and trace element chemistry coupled with mineral compositions show that mixing within the zone of cataclasis is not uniform and that originally micaceous foliated or physically more heterogeneous rock units may contribute a disproportionally large amount to the resultant intrafault material as previously found for the gouge along the san andreas chemical mobility is not a major factor in the formation of cataclastic rocks of the san gabriel fault we see only minor changes for si and alkalies however there is a marked mobility of li which is a probable result of the alteration and formation of new mica minerals the gouge of the san andreas and san gabriel faults probably formed by cataclastic flow there is some indication presently not well constrained that the fine grained matrix of the cataclasite of from the san gabriel fault formed in response to superplastic flow

the relationship between slickenside surfaces in fine grained quartz and the seismic cycle abstract well polished or reflective fault surfaces with or without linear striae indicating the sliding direction are commonly referred to as slickensides this study examines the development of slickensides which are now exposed along the surface trace of a large seismically active normal fault zone in dixie valley nevada u s a geologic and mineralogic constraints indicate the slickenside surfaces formed at depths of less than 2 km and temperatures less than 270 c the slickenside surface material is composed of greater than 98 quartz with less than 2 kaolinite and iron oxide transmission electron microscope tem observations reveal that the slickenside surface material has an extremely fine yet variable grain size 0 01 1 m and an unusual nonequilibrium texture characterized by irregular grain boundaries and low dislocation density angular fragments in the cataclasite to either side of the slickensides provide clear evidence of cataclasis crystallographic preferred orientation in the slickenside surface material indicates that non brittle continuous deformation occurred within 0 1 10 mm of the fault surfaces in many areas non brittle continuous deformation must have alternated with cataclasis because some fragments in the cataclasite have strong preferred orientations we suggest that in the dixie valley slickensides continuous deformation and the development of crystallographic preferred orientations occurred at relatively low strain rates during the interseismic period while cataclasis occurred at higher strain rates associated with seismic events the wide range of strain rates occurring within a fault zone during the earthquake cycle may be an essential element in the formation of fine grained glassy slickensides both at dixie valley nevada and many other areas

fluidization and melting of fault gouge during seismic slip identification in the nojima fault zone and implications for focal earthquake mechanisms 1 the record of physical processes that occur during seismic slip events is well preserved in fault rocks from the active nojima fault in japan the fault rocks formed at about 3 km depth and comprise thin alternating layers of very fine gouge and pseudotachylyte derived from granite each layer is thinner than a few millimeters and corresponds to one seismic slip event the very thin slip zone width suggests that some mechanisms of slip weakening operated and our studies of the fault rocks suggest that fluidization and melting of gouge were particularly important fluidized and nonfluidized gouges were distinguished using the detection probability of fragmented counterparts method it is known from granular material science that the phase transition from a grain friction regime to fluidization of granular materials can occur only by a very small decrease in volume fraction of solid grains once fluidization occurs the frictional resistance decreases abruptly to nearly zero even before thermal pressurization reaches its extreme state when fault gouge is melted frictional resistance is governed by the viscosity of melt which depends mainly on temperature chemical composition h2o concentration and the volume fraction of unmelted grains for each pseudotachylyte layer we estimated the temperature of melt using various temperature indices and measured volume fraction of unmelted grains we synthesized these data to reconstruct the change in viscosity during seismic slip events the melt viscosity is very high 107 9 pas during initial melting due to the combined effect of low temperature 750 800 c and large volume fraction of solid grains thus seismic slip is inevitably restrained instantaneously once this mechanical barrier is overcome the viscosity reduces continuously and dramatically as slip increases at 1000 c the viscosity reduces to 104pas and eventually to 10pas at 1280 c thus stress drops almost completely and rupturing tends to run away

the nature and significance of pseudotachylite from the nason terrane north cascade mountains washington abstract pseudotachylite has been recently discovered at numerous localities in the nason terrane of the north cascade mountains washington the pseudotachylite is widespread not associated with any single major fault or shear zone and tends to occur in narrow foliation parallel zones single pseudotachylite veins range in thickness from less than 0 1 to 10 cm veins and networks of veins occur in six distinct types and these form the basis of a suggested classification scheme multiple generations of pseudotachylite are present in some samples host lithologies include quartz diorite semipelitic schist tonalite amphibolite and especially graphitic pelitic schist glassy veins are rare most pseudotachylite is apparently holocrystalline spherulites vesicles and dendritic microphenocrysts are present and indicate a frictional melt origin the ubiquitous occurrence of cataclasite xenoliths within the pseudotachylite along with other textural observations indicates that in the nason terrane the development of cataclasite zones was the precursor to pseudotachylite generation it is inferred that cataclastic deformation of the schist provided the strain softening which resulted in faulting and pseudotachylite generation the water rich and ultra fine grained matrix of the cataclasite was an ideal starting material for pseudotachylite generation based upon bulk chemistry of host lithologies and microprobe analyses of the pseudotachylite it appears that the pseudotachylite formed as a result of a virtually complete melting of the pelitic i e non quartz vein component of the schist where pseudotachylite occurs within pelitic schist alone the composition of the pseudotachylite is very similar to the pelitic component of the schist while the composition of veins cutting tonalite gneiss in addition to pelitic schist appears to define a mixing line between these two lithologies this suggests either participation of the tonalite during initial melting or subsequent assimilation of gneiss into the melt

fracture geometry of pseudotachylyte generation zones a study of shear fractures formed during seismic events abstract the orientation and movement sense of shear fractures in pseudotachylyte generation zones developed in the ikertoq shear belt west greenland are described and the fracturing sequence is inferred the generation zones occur in strongly foliated gneisses and consist of concordant usually paired principal displacement shear fractures paired shears on which most melt was generated and a system of minor shear fractures minor shear fracturing was most intense between principal displacement shear fractures and most melt intruded from these paired shears inwards with respect to the zone intrusion of injection veins of pseudotachylyte off the principal displacement shear fractures is believed to be contemporaneous with minor shear fracturing in the generation zones generation zones affect intact rocks and individual principal displacement shear fractures have a maximum displacement of 4 6 m paired principal displacement shear fractures are up to 3 m apart and have a maximum observed length of 1 km in all the zones described the fracturing appears to be the result of a single episode of slip both dextral and sinistral generation zones occur within the study area and have different geometries in each case the displacement is strike slip

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

sidewall ripouts in strike slip faults abstract distinctive splay fault configurations in wallrocks associated with pseudotachylyte bearing strike slip fault structures at three different localities are recognized as sidewall ripouts sidewall ripout structures occur as asymmetric doubly tapered fault bounded lenses or slabs adjacent to a dominant planar fault surface scallop shaped lens ripouts develop in isotropic or weakly anisotropic host rocks and elongate slab ripouts develop in strongly anisotropic host rocks the tapered ends of these structures consist of leading contractional p type ramp structures at a low angle to the dominant planar fault surface and trailing extensional r or x type ramp structures at higher angles to the dominant fault surface the formation of ripout structures is attributed to transient variations in the coefficient of friction that cause adhesion between fault surfaces leading to rupture of the sidewalls ripouts are interpreted as large outcrop scale examples of adhesive wear during rupture of the sidewalls developed along planar fault surfaces during slip the asymmetry of the ripout structures in these exposures allows their use as a kinematic indicator where the narrow tapered leading end points in the direction of movement of the ripout lens or slab during adhesion to the opposing fault block wall

fault plane processes and mesoscopic structure of a strong type seismogenic fault in tonalites adamello batholith southern alps abstract the gole larghe fault is an exhumed paleoseismic fault crosscutting the adamello tonalites italian southern alps ambient conditions of faulting were 9 11 km in depth and 250 300 c in the study area the fault accommodates 1100 m of dextral strike slip over a fault thickness of 550 m displacement is partitioned into three hierarchically different sets of discrete subparallel cataclastic horizons faults1 2 3 fault displacement is in the range of few centimeters faults3 to a maximum of a few tens of meters in major faults1 faults1 2 nucleated on pre existing joints whereas faults3 are newly generated fractures produced during slip along faults1 2 each fault within the gole larghe fault records the same evolution with development of indurated cataclasites precursory to pseudotachylyte production pseudotachylytes are usually generated at the host rock cataclasite boundary and within cataclasites the mean clast size decreases getting closer to pseudotachylyte fault veins pseudotachylytes and cataclasites have a similar chemical composition which is enriched in loss on ignition k rb ba u and fe3 compared to host rock we envision two models for the evolution of the gole larghe fault in both models synkinematic fluid rock interaction along a fault causes fault hardening by precipitation of abundant k feldspar epidote and minor chlorite in the cataclasite matrix conducive to final production of pseudotachylyte in the first model induration occurs progressively by differential precipitation related to fabric evolution in cataclasites in the second model induration occurs abruptly dependent on the development of full connectivity within the fault network and to fluid reservoir whatever the model the gole larghe fault represents a strong fault where hardening processes resulted in a low displacement fault thickness ratio and contrast with many mature weak faults where localized repeated seismic slip along the same weak horizons yields high displacement fault thickness ratios

effects of lithology cataclasis and melting on the composition of fault generated pseudotachylytes in lewisian gneiss scotland abstract petrological studies of fault generated pseudotachylytes formed during coseismic slip need to consider cataclastic processes as well as rapid closed system melting and freezing in order to account for the observed similarities and the variations between the chemical compositions of pseudotachylytes and their host rocks whole rock analyses of pseudotachylytes and their gneissic host rocks from the lewisian complex of northwest scotland indicate that the composition of the pseudotachylyte is sensitive not only to the bulk host rock composition but also to its mineralogy and fabric in anisotropic e g gneissic rocks selective cataclastic and melting effects may both contribute to determining the whole rock composition of pseudotachylytes electron microprobe analyses of pseudotachylyte matrices indicate more rapid decomposition and fusion of hydrous ferromagnesian phases such as biotite and amphibole and to a lesser extent feldspar relative to quartz the pseudotachylytes of this study appear to have formed neither by ultracataclasis alone nor by classical equilibrium partial melting or total fusion but through the interaction of selective cataclastic and melting processes controlled by the lithological layering and mineralogical heterogeneity in their host rocks this interpretation may be widely applicable to pseudotachylytes generated by faulting in foliated host rocks

spatial variability in microscopic deformation and composition of the punchbowl fault southern california implications for mechanisms fluid rock interaction and fault morphology abstract we examine the distribution and nature of microstructures geochemistry and mineralogy along two traverses across the punchbowl fault southern california to determine the morphology and deformation mechanisms of the fault zone in schistose rocks the punchbowl fault is an exhumed fault that has two main strands of slip localization and has a total of 44 km of right slip protolith in the study area consists of the pelona schist which is primarily a quartz albite muscovite actinolite schist with thin to medium banding and rare metabasalts the traverses are 1 3 km apart and were conducted at a site with a single fault strand and a site where both principal strands of the fault are exposed the fault zone thickness is a function of the type of measurement that is used to define it for the single strand site analysis of the distribution of microfractures shows that the fault zone consists of a roughly 40 m thick damaged zone adjacent to the fault core the damaged zone is marked by an increase in veins thin cataclasite bands inter and intragranular fracturing and alteration relative to the country rock brittle grain size reduction occurs in a zone 10 m thick as measured from the fault core which consists of a continuous 10 cm thick very fine grained cataclasite that experienced repeated alteration vein injection and grain size reduction whole rock geochemical analyses of the fault related rocks suggest that the geochemically defined fault zone is less than 10 m thick volume loss at the site with one fault strand appears to have been small the dominant alteration reactions associated with the fault core are the hydration of hornblende and actinolite accompanied by the alteration of muscovite to produce a quartz chlorite albite epidote ultracataclasite the composition of the fault core is variable and locally influenced by one of the adjacent protoliths the examination of the two fault strand sites shows that two damaged zone fault core structures are present the region between the two strands experienced a greater degree of deformation than the protolith but the total deformation is much less than immediately adjacent to the fault cores the total thickness of the damaged zone around the two strands is less than 200 m the fault core enveloped by a damaged zone morphology as well as the textures of the fault core rocks are similar to rocks associated with the north branch san gabriel fault which formed in crystalline rocks with a total displacement of 22 km fault thickness is less in the pelona schist than in the crystalline rocks perhaps owing to more efficient strain localization in the schists thus the faults of the san andreas system may be thinner in regions where schists or franciscan rocks are the protolith but the main fault core may be a constant feature of the fault zone transformation induced weakening is less important in this part of the punchbowl fault since the protolith has a large amount of mica the structure of the fault zone with two principal slip surfaces is marked by both chemical changes and microstructures and indicates that some parts of the san andreas fault system may consist of multiple slip surfaces each with a damaged zone that together may create a fault zone 100 m thick and in which slip is localized to zones meters to decimeters thick

coulomb constitutive laws for friction contrasts in frictional behavior for distributed and localized shear we describe slip rate dependent friction laws based on the coulomb failure criteria frictional rate dependence is attributed to a rate dependence of cohesionc and friction angle we show that differences in the stress states developed during sliding result in different coulomb friction laws for distributed shear within a thick gouge layer versus localized shear within a narrow shear band or between bare rock surfaces for shear within gouge shear strength is given by c cos n sin whereas for shear between bare rock surfaces the shear strength is c cos n tan where and n are shear and normal stress respectively in the context of rate dependent coulomb friction laws these differences mean that for a given material and rate dependence of the coulomb parameters pervasive shear may exhibit velocity strengthening frictional behavior while localized shear exhibits velocity weakening behavior we derive from experimental data the slip rate dependence and evolution ofc and for distributed and localized shear the data show a positive rate dependence for distributed shear and a negative rate dependence for localized shear indicating that the rate dependence ofc and are not the same for distributed and localized shear even after accounting for differences in stress state our analysis is consistent with the well known association of instability with shear localization in simulated fault gouge and the observation that bare rock surfaces exhibit predominantly velocity weakening frictional behavior whereas simulated fault gouge exhibits velocity strengthening followed by a transition to velocity weakening with increasing displacement natural faults also exhibit displacement dependent frictional behavior and thus the results may prove useful in understanding the seismic evolution of faulting

pseudotachylyte bearing strike slip duplex structures in the fort foster brittle zone s maine abstract detailed 1 60 scale mapping of the fort foster brittle zone in the mylonitic rye formation of southernmost maine has revealed the intricate internal duplex structure of a system of probable paleozoic age dextral strike slip faults that have produced abundant pseudotachylyte and minor breccia the internal configuration of this brittle zone consists of a mosaic of individual pseudotachylyte generation zones as slab duplex structures individual duplex zones are up to 100 m in length and 1 m or less in width and are defined by pairs of layer parallel slip surfaces along which frictional melts were produced these slab duplex structures are interpreted as zones of displacement transfer between long overlapping layer parallel en echelon strike slip fault surfaces contractional duplexes develop layer parallel compressional structures that tend to shorten and thicken the fault bounded slabs by the formation of lateral ramps and conjugate faults kinks and asymmetric folds extensional duplexes develop layer parallel stretching and thinning by the formation of oblique dextral shears high angle conjugate pairs and localized fault breccias the production of pseudotachylyte by friction melting along layer parallel fault surfaces in these exposures is attributed to rapid slip during paleoseismic events the rupture structures developed during these events may be characteristic of fault structure and mechanics at near focal depths in a strike slip seismogenic zone

an argument against the crush origin of pseudotachylytes based on the analysis of clast size distribution abstract the origin of pseudotachylytes has been controversial since wenk cast doubt on the melt origin of the matrix of pseudotachylytes in 1978 the matrix of this rock is so fine grained that the crush origin of pseudotachylytes revived by wenk cannot easily be denied this paper presents a new line of argument based on the size analysis of clasts contained in pseudotachylytes in felsic granulite from the musgrave range central australia these clasts are definitely crush products produced during the pseudotachylyte generation their sizes as measured on photomicrographs of thin sections obey the size distribution n n r d where n is the cumulative number of clasts with sizes greater than r d is the fractal dimension and n is a constant that depends on the number of measurements d was found to be 1 5 0 05 for the size ranges of 10 2000 m if the matrix of the pseudotachylytes consists mostly of ultrafine crush products they must have formed simultaneously with those coarse crush products the proportion of fine products relative to the coarse clasts can be estimated assuming that a similar size distribution also holds for the fine products the estimated area occupied by fine products in a thin section is of the order of only several percent whereas the measured area of the matix is about 60 thus the major part of the matrix of the pseudotachylytes cannot be regarded as crush products it is also shown that the number of clasts smaller than about 5 m becomes very small perhaps as a result of nearly complete dissolution of fine clasts in a melt however if the ultrafine grained matrix of the pseudotachylytes had formed by crushing during seismogenic fault motion the grain size refinement during the crushing should have occurred jumping the size range of at least 1 5 m this is quite unreasonable and disproves the crush origin for the matrix of pseudotachylytes

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

coeval formation of cataclasite and pseudotachylyte in a miocene forearc granodiorite southern kyushu japan abstract cataclastic rocks and pseudotachylytes are exposed along the uchinoura shear zone a normal fault zone cutting the middle miocene 14 ma osumi granodiorite in southern kyushu japan cataclastic rocks include non foliated clast supported to matrix supported cataclasite and foliated clast supported cataclastic granodiorite in these rocks fracturing and comminution played a major role but dissolution and recrystallization of quartz and plastic deformation of quartz and biotite were also active processes especially in foliated granodiorite two types of pseudotachylyte are distinguished a foliated type characterized by a planar arrangement of clasts and microlites and a spherulitic type characterized by clasts surrounded by microlite overgrowths both types are of melt origin as attested by the presence of microlites and rounded or embayed clasts and by the scarcity of biotite clasts unlike spherulitic type pseudotachylyte which solidified without being deformed the foliated type pseudotachylyte underwent flow before complete solidification this deformation is thought to reflect post seismic strain accommodation immediately following the main slip episode kinematic indicators which consist of riedel type secondary fractures branching on primary fractures shear bands offsetting the foliation of foliated granodiorite or asymmetrical porphyroclast systems within pseudotachylyte veins show that all fault rocks were generated during n s to nw se directed extensional deformation pseudotachylyte is closely associated both in time and space with cataclastic rocks thus indicating that the behaviour of the uchinoura fault zone alternated between comminution and frictional melting given the slow strain rates which characterize dissolution and recrystallization processes detected in cataclasites the juxtaposition of pseudotachylytes and foliated cataclasites provides an example of aseismic and seismic displacements within the same shear zone

fault structure wear mechanisms and rupture processes in pseudotachylyte generation abstract fault generated pseudotachylyte is found within both cataclastic and mylonitic host rocks suggesting that rapid catastrophic displacements have occurred at a variety of depths within paleoseismogenic zones pseudotachylyte bearing fault zones represent a composite of structural features associated with the process of earthquake rupture propagation and coseismic slip the development of multiple pseudotachylyte veins in fault linkages duplexes sidewall ripouts en echelon arrays and brittle zones suggests repeated rupturing during a series of characteristic earthquakes each earthquake as a coseismic slip event can be subdivided into initial rupture acceleration stable sliding and final deceleration stages these evolve through distinctive sequences of wear and deformation mechanisms that vary with sliding velocity duration of slip total displacement and the hydrodynamics of the developing fault zone slip is thought to proceed toward surface refinement and possible frictional melting following the propagation of leading shear fracture process zones the passage of the initial process zone of oblique fracturing would be followed by linkage to a throughgoing structure with asperity reduction through brecciation comminution and refined cataclastic flow for frictional melting in an abrasive wear dominant model at greater depths in the presence of mylonitic anisotropy slip would proceed through initial layer parallel surfaces and duplex linkages with rapid surface refinement through plastic smearing and laminar flow for frictional melting in an adhesive wear dominant model

fractal dimension of molten surfaces as a possible parameter to infer the slip weakening distance of faults from natural pseudotachylytes high velocity friction experiments on gabbro and monzodiorite using a rotary shear high velocity friction apparatus have revealed that frictional melting and progressive growth of a molten layer along a fault cause slip weakening eventually reaching a nearly steady state the melting surface at the host rock molten layer interface is initially very flat but it becomes more complex and rounded in shape towards the steady state owing to the selective melting of minerals with lower melting points and the gibbs thomson effect this change in the melting surface topography can be quantitatively expressed by the fractal dimension d as determined by the divider method from about 1 0 near the peak friction to around 1 1 near the steady state friction the ultimate fractal dimension at steady state friction tends to decrease with increasing heat production rate presumably due to more rapid and uniform melting a systematic correlation of d with mechanical behavior of the fault during frictional melting may provide a way of estimating slip weakening distance and heat production rate at steady state friction by measuring d for natural pseudotachylytes on slip surfaces with different displacements the weakening distance is of vital significance in relation to fault instability and the heat production rate is related to the fault strength the experimental studies point to ways to estimate these difficult quantities for natural faults

an exceptionally thick middle pleistocene tephra layer from epirus greece abstract a newly recognized 2 m thick trachytic volcanic ash deposit from northwestern greece is dated at 374 000 7000 yr and correlated with the middle pleistocene volcanic activity of central italy the deposit represents ash fallout from one of the largest volcanic eruptions in europe of the past 400 000 yr and should provide an important stratigraphic marker within the poorly dated middle pleistocene deposits of italy and greece

formation of joints in cooling plutons abstract the geometry age and mineralogy of steeply dipping joints in the lake edison granodiorite of california indicate that thermal stresses played a key role in the formation of the joints joint traces curve to approach the pluton boundary at high angles at both large and small scales and they generally terminate at or near the contact with an older pluton radiometric dates and the epidote and chlorite fillings of the joints tie jointing to the initial cooling of the pluton a thermo mechanical stress analysis assuming two dimensional conductive cooling predicts thermal stresses of several tens of mpa the same order of magnitude as plausible fluid pressures regional stresses and lateral normal stresses associated with the overburden the orientation of the joints can be accounted for rather well by the stress field formed by superposing a uniform regional stress field on the predicted thermal stresses the large scale pattern of the early joints in many plutons should be predictable based on a pluton s geometry its age relative to the adjacent rock and knowledge of the regional stress at the time of initial cooling these findings bear on issues pertinent to mining petroleum recovery nuclear waste repository siting and ground water flow

transformation of cataclastically deformed rocks to pseudotachylyte by pervasion of frictional melt inferences from clast size analysis abstract optical contrast between clasts and surrounding dark coloured matrix in thin sections has been used in a computer aided method which has been applied for clast size analysis of pseudotachylyte exposed in a 25 km long brittle deformation zone in the precambrian terrain of western india while the number of clasts n2 equal to or coarser than size u shows a power law distribution n2 u d the area a1 occupied by the clasts of different size fractions shows a log normal distribution the clast size distribution pattern indicates transformation of cataclastically deformed rocks to pseudotachylytes by pervasion of a rock melt generated at the frictional slip planes it is inferred that the melt pervaded the permeable cataclastically damaged zones and so occupied the intergranular space this melt mixed with the comminuted material flowed further as a slurry to form the pseudotachylyte veins the clast laden final product retained the grain size distribution pattern of the parent comminuted material with minor modifications fusion of clast rinds due to heat transfer from the melt slightly modified the initial grain size distribution pattern because of the short duration of the heat transfer process combined with the low conductivity and high specific heat of rocks the temperature at the centre of the coarse grains remained at the initial temperature ti and reduction w of radius r by fusion was a negligible fraction of r as a consequence the population of coarse clasts retained the pre fusion size distribution pattern in contrast w r of the smaller grains was considerable the initial grain size distribution pattern did not survive in the finer fraction of the clasts this difference in thermal response of the smaller versus larger grains to the heat transfer process is revealed in clast size distribution graphs for the pseudotachylytes moreover the observed proportion of matrix can not be explained by cataclasis alone the extent of comminution in the parent material and size of the openings through which the clast laden melt flowed to form veins controlled the clast matrix ratio in the pseudotachylytes