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

pseudotachylite generated as a result of a drilling burn in abstract material similar to naturally occurring pseudotachylite was generated in proterozoic lava as a result of frictional overheating of a diamond drill dh 5000 impregnated crown at a depth of 2 4 km below surface bit rotation equation to slip rates of about 1 m s is thought to have caused severe overheating to last for 3 10 s estimates show that melting occurred in the temperature range 800 1100 c under wet conditions and at comparatively low effective normal and shear stresses of about 1 9 107 and 9 7 106 pa respectively associated breccia formed as a result of thermal decrepitation rather than dynamic comminution and as a corollary it is suggested that some cataclasites associated with pseudotachylite bearing faults may also form by the mechanism this implies that the presence of both cataclasite and pseudotachylite on a fault plane does not necessarily indicate reactivation

an evaluation of criteria to deduce the sense of movement in sheared rocks some of the most useful criteria for the deduction of the sense of shear are summarized for use in areas where unequivocal field evidence is lacking apparently conflicting evidence from rotated pressure shadow regions around porphyroclasts and porphyroblasts is clarified the use of quartz crystallographic fabric asymmetry to deduce the shear sense in the bulk rock should be treated with caution and used only together with detailed microstructural observations

viscosity determinations of some frictionally generated silicate melts implications for fault zone rheology at high strain rates analytical scanning electron microscopy has been used to determine the major element compositions of some natural and artificial silicate glasses and their microcrystalline equivalents derived by the frictional melting of intermediate to acid protoliths the data show that the matrices of the friction melts which cool to form pseudotachylytes are relatively basic and hydrous even when their protoliths are intermediate to acid this is because frictional fusion involves the selective comminution and nonequilibrium melting of minerals based on their individual mechanical properties and melting points not the formation of minimum melts through equilibrium mineral interaction this means that hydrous ferromagnesian minerals e g micas and amphiboles melt preferentially to form the liquid matrix while feldspars and especially quartz more readily survive as clasts pseudotachylytes generated by frictional melting are therefore not bulk melts and as clast melt suspensions they cannot be considered as simple newtonian fluids the calculated viscosities of the friction melts are low for example at 1200 c most friction melts possess zero shear suspension viscosities of 102 104 dpa s 1 dpa s 1 p this is equivalent to the viscosities of tholeiitic and alkaline basaltic magmas at the same temperature these viscosities are maximum determinations because as clast melt suspensions friction melts may undergo shear thinning and exhibit pseudoplasticity at high shear rates i e during slip on a fault surface contrary to earlier suggestions where the bulk melting of intermediate to acid protoliths was believed to result in the generation of viscous friction melts that could act to inhibit continued sliding this work shows that most pseudotachylytes are partial melts possessing low viscosities the formation of highly fluid suspensions during slip may have profound effects on the dissipation of stored strain energy in the rocks surrounding a fault interface lubrication could facilitate an increase in the slip rate and the rate of energy dissipation this would be manifest as an increase in high frequency seismic wave radiation and vibrational

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

the use of a high resolution 3d chirp sub bottom profiler for the reconstruction of the shallow water archaeological site of the grace dieu 1439 river hamble uk the remains of henry v s flagship the grace dieu currently lie buried within the inter tidal sediments of the river hamble s england previous archaeological investigations have been hindered by difficult excavation conditions resulting in a poor understanding of the dimensions shape and degradation state of the hull s deeper structure this study therefore aimed to image characterize and reconstruct the buried remains of this vessel using a high resolution 3d acoustic sub bottom chirp system with rtk gps positioning capability the accurate navigation and high resolution data that were acquired enabled the construction of a full 3d image of the site that not only identified the remains of the wooden hull but also features buried within it in addition the degradation state of these buried wooden remains were investigated by calculating reflection coefficients while a hypothetical larger reconstruction of the grace dieu s hull was achieved through the use of the shipshape ship design software package the results of this project demonstrate that i acoustic data can be used to successfully image buried wooden shipwrecks ii artefacts are buried within the hull of the grace dieu iii there is variation in the degradation state of the buried timbers as calculated from the acoustic data with the shell of the vessel being moderately well preserved and iv the grace dieu was a very large ship for its time possibly over 60 m long and 16 m wide the outcomes of this research not only have considerable implications for the management and monitoring of submerged and buried archaeological sites but also for planning intrusive surveys should they be required

frictional melting of gabbro under extreme experimental conditions of normal stress acceleration and sliding velocity 1 with the advent of high velocity shear apparatus several experimental studies have been performed in recent years improving our understanding of the evolution of fault strength during seismic slip however these experiments were conducted under relatively low normal stress 20 mpa and using small cylindrical samples where a large gradient in slip velocity exists across the sliding surface given the above limitations the extrapolation of these experimental results to natural conditions is not trivial here we present results from an experimental study on gabbroic rocks using a newly developed rotary shear apparatus capable of reaching higher normal stress up to 50 mpa on ring shaped samples 30 50 mm internal external diameter and allowing precise control of the imposed slip velocity function the results confirm that steady state shear stress during the melt lubricated phase of the experiment depends on normal stress in the form of a power law equation as predicted by theoretical models however the exponent appears closer to 0 5 contrary to the theoretical prediction of 0 25 we observe no systematic dependence of shear stress on acceleration but increasing deceleration drastically decreases the recovery of friction during final slip we find that the slip weakening distance decreases inversely with increasing normal stress in agreement with theoretical considerations and decreases with increasing slip rate extrapolation of the slip weakening distance to natural conditions predicts a slip velocity for ancient seismic events of 0 3 1 m s when compared with field estimates these values compare well with seismological estimates

decimeter resolution 3d seismic volume in shallow water a case study in small object detection the 3d chirp subbottom profiler provides high resolution imaging of coastal and inshore seabed and subseabed structure by combining the known highly repeatable source waveformofchirpprofilerswiththecoherentprocessingand interpretation afforded by true 3d seismic volumes comprising 60 hydrophone groups arranged around a maltese cross of four chirp transducers 3d chirp permits acquisition of a true 3d volume with a horizontal resolution of 12 5 cm providing an excellent base for shallow water engineering archaeological military and geologic studies here we presentresultsfromsurveyinganatidalbasinonthesouthern coastofenglandtomapbedrockprotrusionsandthesizeand distributionofburiedobjects thestudyareaof150250 m provided a series of unique challenges including a large number of discrete objects ranging from tens of centimeters toseveralmetersinsize buriedinathinveneer0 5to1 5 m of unconsolidated silt overlaying a flat bedrock surface that showed high acoustic contrast and short wavelength roughness by comparing comprehensive postsurvey dredging of the entire site with a prestack time migrated 3d volume it is possibletoconfirma100 detectionrateforalldiscreteburied objects larger than 0 300 30 m in an illuminated area although one acoustic anomaly could not be accounted for in thedredgingresults

fission track geochronologic evidence for late cretaceous mylonitization and early paleocene uplift of the northeastern peninsular ranges california fission track dating of minerals from the santa rosa mylonite belt and adjoining rocks of the northeastern peninsular ranges batholith indicates that the area was affected by a brief but intense cooling event approximately 61 m y ago concordant ages from different mineral species suggest a minimum temperature drop of 120 c during this event this rapid cooling event probably reflects major uplift and unroofing of the batholith at the beginning of the paleocene these new data coupled with other published isotopic data firmly establishes that the santa rosa mylonite belt formed during the late cretaceous

temperature fields generated by the elastodynamic propagation of shear cracks in the earth w 1 the temperature maximum shifts toward the crack tip for faults having slip zone thickness of the order of centimeters or less the onset of thermally induced phenomena e g frictional melting thermal pressurization etc may occur at any point along the rupture depending on the degree of slip localization and rupture duration in the absence of significant increases in the pore fluid pressure localized fault slip may raise temperature by several hundred degrees sufficient to cause melting the onset of frictional melting may give rise to substantial increases in the effective fault strength due to an increase in the effective fault contact area and high viscosity of silicate melts near solidus the inferred transient increases in the dynamic friction viscous braking are consistent with results of high speed rock sliding experiments and might explain field observations of the fault wall rip out structures associated with pseudotachylites possible effects of viscous braking on the earthquake rupture dynamics include 1 delocalization of slip and increases in the effective fracture energy 2 transition from a crack like to a pulse like rupture propagation or 3 ultimate rupture arrest assuming that the pulse like ruptures heal by incipient fusion the seismologic observations can be used to place a lower bound on the dynamic fault friction this bound is found to be of the order of several megapascals essentially independent of the earthquake size further experimental and theoretical studies of melt rheology at high strain rates are needed to quantify the effects of melting on the dynamic fault strength index terms 3210 mathematical geophysics modeling 5104 physical properties of rocks fracture and flow 7209 seismology earthquake dynamics and mechanics 7260 seismology theory and modeling 8010 structural geology fractures and faults

pseudotachylyte controversy fact or friction high speed slip experiments performed on westerly granite using friction welding apparatus reveal that comminution is an essential precursor to melting by friction observations of slip surfaces via analytical scanning electron microscopy sem document the following sequence of events occurring in 2 s with increasing velocity up to 2 m s fracture progressive comminution surface melting of mineral fragments fragment to fragment adhesion and finally production of a fragment laden melt supported suspension explosive dehydration and melting of the epidote group mineral allanite indicates that temperatures of at least 1000 c were realized at the interface this is corroborated by calculation of the temperature rise for the known operating conditions contrary to earlier proposals these results show that comminution and frictional melting are complementary and not mutually exclusive processes depending on the velocity shear stress displacement relations prevailing during frictional slip rocks produced in seismogenic zones can be predominantly comminuted wall rock or fragment melt mixes pseudotachylytes

a late triassic age for the rochechouart impact structure france argon 40 argon 39 laser spot fusion dating of pseudotachylyte from the 25 km diameter roche chouart impact structure of western central france yields a matrix age of 214 c 8 ma 20 field evidence indicates that the pseudotachylyte was generated during the modification stage of the impact process prob ably during transient cavity collapse this new age is considerably older than the previously accepted age of 186 8 ma for this structure which was obtained from hydrothermally altered melt sheet samples the new age is in accordance with earlier paleomagnetic and fission track data which indicated that rochechouart was formed during the late triassic moreover the new determination is in agreement with the regional geolog ical setting and field relations of the structure the new age of 214 2 8 ma falls within the norian stage of the triassic system

high localization of primary slip zones in large earthquakes from paleoseismic trenches observations and implications for earthquake physics 1 paleoseismic exposures excavated across relatively straight sections of major faults in southern california display a high degree of localization at depths of only a few meters below the surface in some cases the width of the slip zone in events with multimeter displacement is on the order of 1 2 mm to a cm which is the resolution of the observations repetitive slip events in the same zone increase the observed width of the faulting as expressed at trench depths but the superposed slip again tends to be highly localized these observations are probably representative of 80 of the length of the faults studied based on these results combined with the expected tendency for narrower slip zone with depth observations from exhumed faults and high localization of seismicity along large faults we hypothesize that the majority of slip carried by large faults occurs in very narrow zones if correct the emerging integrated view of high slip localization during earthquake ruptures places important constraints on many issues of earthquake dynamics

the geochemistry of pseudotachylyte and its host rocks from the west rand goldfield witwatersrand basin south africa implications for pseudotachylyte genesis abstract the chemical composition of the pseudotachylyte in the west rand goldfield of the witwatersrand basin south africa is closely related to the composition of the host rocks and this is reflected in the colour of the pseudotachylyte grey pseudotachylyte is generally hosted by and similar in composition to quartzites of the witatersrand supergroup whereas maroon pseudotachylyte has a similar relationship to the mafic lava of the ventersdorp supergroup in some instances the composition of the pseudotachylyte is intermediate between these two host rock types and a mixing process is proposed a study of the ferrous to ferric iron ratio provides limited evidence that pseudotachylyte is slightly more reduced than the rocks from which they have been derived the only elements that are consistently enriched in the pseudotachylyte irrespective of host rock composition are s pb and au it is speculated that this indicates the existence of a sulphide bearing fluid phase along the fault zone either prior to or during pseudotachylyte formation geochemical and petrographic evidence favour an origin by frictional fusion rather than ultracomminution for the pseudotachylyte from the west rand goldfield

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

an earthquake model with interacting asperities summary a model is presented that treats an earthquake as the failure of asperities in a manner consistent with modern concepts of sliding friction the mathematical description of the model includes results for elliptical and circular asperities oblique tectonic slip static and dynamic solutions for slip on the fault stress intensity factors strain energy and second order moment tensor the equations that control interaction of asperities are derived and solved both in a quasi static tectonic mode when none of the asperities are in the process of failing and a dynamic failure mode when asperities are failing and sending out slip pulses that can trigger failure of additional asperities the model produces moment rate functions for each asperity failure so that given an appropriate green function the radiation of elastic waves is a straightforward calculation the model explains an observed scaling relationship between repeat time and seismic moment for repeating seismic events and is consistent with the properties of pseudo tachylites treated as fossil asperities properties of the model are explored with simulations of seismic activity that results when a section of the fault containing a spatial distribution of asperities is subjected to tectonic slip the simulations show that the failure of a group of strongly interacting asperities satisfies the same scaling relationship as the failure of individual asperities and that realistic distributions of asperities on a fault plane lead to seismic activity consistent with probability estimates for the interaction of asperities and predicted values of the gutenberg richter a and b values general features of the model are the exterior crack solution as a theoretical foundation a heterogeneous state of stress and strength on the fault dynamic effects controlled by propagating slip pulses and radiated elastic waves with a broad frequency band

pseudotachylite from the main zone of the hidaka metamorphic belt hokkaido northern japan pseudotachylite veins have been found in the mylonite zone of the hidaka metamorphic belt hokkaido northern japan they are associated with faults with wnw ese to ene wsw or ne sw trends which make a conjugate set cutting foliations of the host mylonitic rocks with high obliquity the mylonitic rocks comprise greenschist facies to prehnite pumpellyite facies mineral assemblages the mode of occurrence of the pseudotachylite veins indicates that they were generated on surfaces of the faults and were intruded as injection veins along microfractures in the host rocks during brittle deformation in near surface environments an analysis of the deformational and metamorphic history of the hidaka main zone suggests that the ambient rock temperature was 200 300 c immediately before the formation of the hidaka pseudotachylite three textural types of veins are distinguished cryptocrystalline microcrystalline and glassy the cryptocrystalline or glassy type often occupies the marginal zones of the microcrystalline type veins the microcrystalline type is largely made up of quench microlites of orthopyroxene clinopyroxene biotite plagioclase and opaque minerals with small amounts of amphibole microlites the interstices of these microlites are occupied by glassy and or cryptocrystalline materials the presence of microlites and glasses in the pseudotachylite veins suggests that the pseudotachylites are the products of rapid cooling of silicate melts at depths of less than 5 km the bulk chemical composition of the pseudotachylite veins is characterized by low sio2 and a high water content and is very close to that of the host mylonitic rocks this indicates that the pseudotachylite was formed by virtual total melting of the host rocks with sufficient hydrous mineral phases local chemical variation in the glassy parts of the pseudotachylite veins may be due to either crystallization of quench microlites or the disequilibrium nature of melting of mineral fragments and incomplete mixing of the melts pyroxene microlites show a crystallization trend from hypersthene through pigeonite to subcalcic augite with unusually high al contents the presence of pigeonite and high al pyroxene microlites of hornblende and biotite microlites and rare plagioclase microlites may indicate the high temperature and high water content of the melt which formed the pseudotachylite veins the melt temperatures were estimated to be up to 1100 c using a two pyroxene geothermometer using published data relating water solubilities in high temperature andesitic magmas to pressure a depth estimate of about 4 km is inferred for the hidaka pseudotachylites evidence derived from pseudotachylites in the hidaka metamorphic belt supports the conclusion that pseudotachylite is formed by frictional melting along fault surfaces at shallow depths from rocks containing hydrous minerals

pseudotachylites in the eastern peninsular ranges of california abstract a continuous zone containing pseudotachylites in the eastern peninsular ranges of california extends over more than 15 km from deep canyon in the north to at least toro canyon in the south pseudotachylites are found in rocks of tonalitic to dioritic composition while the overall compositions of host rock and pseudotachylite veins are similar veins are characteristically enriched in fe and ti and depleted in si many veins are cataclastic and all contain fragments but a large number have a groundmass with skeletal and spherulitic microlites of calcic plagioclase an 40 50 biotite and ilmenite indicative of partial melting the pseudotachylites formed mainly during a late brittle event 56 62 ma and postdate the high strain ductile mylonitic deformation in the santa rosa mylonite zone 65 87 ma as ascertained by 40ar 39ar ages the size of veins ranges from less than 1 mm to more than 10 cm in thickness and aspect ratios display a broad log normal distribution three quarters of the veins are more or less parallel to the foliation the orientation of the veins is consistent with a top to the west displacement similar to that for the ductile mylonites from the sizes energies required for melting are estimated to range between 105 j for smaller veins and 1010 j for larger veins interestingly the energy distribution and geometry of these pseudotachylites correspond closely with energy distributions of current microseismic events along the san andreas fault at parkfield suggesting that basic mechanisms may be similar and due to intrinsic mechanical properties of rocks in particular these indicate that local stress concentrations rather than average crustal stresses are relevant for earthquakes

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

laser probe 40ar 39ar investigation of a pseudotachylyte and its host rock from the outer isles thrust scotland the formation of a friction melt or pseudotachylyte in the footwall of the outer isles thrust scotland during thrust movement in the early paleozoic caledonian orogeny caused argon loss from biotites in the host gneiss mean 40 ar 39 ar ages for biotite grains decrease from 1450 to 923 ma in a zone 730 c in the host gneiss these high temperatures may have caused catastrophic argon loss rather than loss by volume diffusion an 40 ar 39 ar age traverse across the pseudotachylyte vein revealed old ages adjacent to the margin reflecting the incorporation of partially outgassed host rock clasts apparent ages for the pseudotachylyte decrease unevenly toward the center of the vein a weighted mean age of 430 6 ma 2 obtained in the center corresponds closely to movement ages derived for the associated moine thrust zone

evidence for regional dauphine twinning in quartz from the santa rosa mylonite zone in southern california a neutron diffraction study preferred orientation in granitic mylonites from the santa rosa mylonite zone in southern california is investigated with time of flight neutron diffraction quartz and biotite display strong preferred orientation whereas feldspar alignment is weak for quartz a c axis maximum in the intermediate fabric direction is consistent with dynamic recrystallization pole figures for positive and negative rhombs such as rz 10 11 and zz 01 11 are distinct over a wide region suggesting that the pattern reflects tectonic deformation and was not

evidence for late cretaceous crustal thinning in the santa rosa mylonite zone southern california the santa rosa mylonite zone is part of a 100 km long belt of deformation along the eastern margin of the northern peninsular ranges batholith southern california in the northern santa rosa mountains complexly deformed mylonitic plutonic and metasedimentary tectonites are cut by a stacked sequence of five low angle faults evidence from field relationships and the analysis of minor structures microstructures and textures preferred orientation suggest that the coaxial component of deformation pure shear dominated over a noncoaxial simple shear component it is proposed that mylonitic deformation and low angle faulting which appear to be kinematically associated are the result of late cretaceous extension and thinning of the continental margin

tertiary faulting and pseudotachylytes in the east greenland volcanic rifted margin seismogenic faulting during magmatic construction volcanic rifted margins have voluminous magmatic constructions at continent ocean transitions including thick sequences of flood basalts dike swarms and mafic plutons developed in crustal scale flexures in the tertiary east greenland volcanic rifted margin upper crustal dilation and flexure were accommodated by extensive faulting locally pervasive cataclastic deformation and pseudotachylyte generation the widespread distribution and large volumes of pseudotachylyte suggest that regional seismogenic faulting was vigorous during rapid crustal subsidence dike intrusion and the start of sea floor spreading

seismically induced serpentine dehydration as a possible mechanism of water release in subduction zones insights from the alpine corsica pseudotachylyte bearing monte maggiore ophiolitic unit abstract the monte maggiore ophiolitic unit of alpine corsica consists of an intact to variably hydrated spinel lherzolite intruded by jurassic gabbro dykes a widespread serpentinization marked by low pressure serpentine polymorphs results from sea water interaction with mantle rocks during jurassic sea floor spreading of the piemonte liguria ocean a second serpentinization event marked by the presence of antigorite is related to the cretaceous to paleogene subduction of the piemonte liguria oceanic lithosphere tectonic pseudotachylyte veins locally crosscutting the unit display several original characteristics 1 they are hosted by serpentinite showing that seismic ruptures can propagate through hydrated mantle rocks 2 the host serpentinite in contact with pseudotachylyte veins shows a thin 500 m or less rim of secondary olivine newly crystallized at the expense of serpentine the heat necessary for serpentine dehydration is likely provided by the frictional melt antigorite bearing clasts themselves partly dehydrated along their boundaries reworked in pseudotachylyte veins on one hand and antigorite veins crossing pseudotachylyte veins on the other hand show that frictional melting took place at pressure and temperature conditions compatible with antigorite stability this indicates that frictional meting occurred at pressures between 0 7 and 0 85 gpa that is at 20 to 30 km depths in the subducting piemonte liguria oceanic slab since the dehydration of serpentine into olivine is only observed at the host rock selvages of pseudotachylyte veins it cannot be related to the crossing by the subducting slab of the regional 610 100 c isotherm of the subduction zone temperature at which antigorite starts to dehydrate into olivine an estimate of the water released by the dehydration reaction suggests that for a magnitude 6 earthquake the average amount is about 1 36 l m2 or 1 36 105 m3 for the entire rupture surface assumed to be 100 km2 if the released water is not incorporated in the nearby frictional melt it can flow away from the slip zone and contribute to the various fluid rock interactions active in subduction zones alternatively if the water cannot escape from the slip zone and if the pore pressure is locally high enough it could trigger aftershocks following the initial seismic rupture

chemical heterogeneity in an enderbite hosted pseudotachylite eastern india evidence for syn deformation multi reaction melting in pseudotachylite the origin of chemical and mineralogical heterogeneity in tens of microns wide layers and domains in enderbite hosted couple of centimeters wide pseudotachylite vein is examined based on the results of bse and x ray element imaging and electron probe microanalyses of major elements of host rock minerals clasts microphenocrysts and pseudotachylite matrix the pseudotachylite layers and domains containing variable proportions of orthopyroxene and magnetite microphenocrysts continue as mantles around quartz k feldspar plagioclase and garnet clasts the clasts are chemically modified along margins and intra clast pseudotachylite injections the chemical modifications are extensive in smaller clasts 5 m diameter at least three chemically distinct layers and domains in the pseudotachylite and their fine grained matrices plot in sharply defined well segregated and non overlapping fields in feo mgo al2o3 sio2 feo cao mgo and cao na2o k2o and feo vs feo feo mgo diagrams the compositions of the layers and domains smeared between a feldspar quartz component and a ferromagnesian component of garnet fe ti oxides orthopyroxene possibly correspond to fractionated quenched melts or admixtures of microphenocrysts that cannot be resolved by the microbeam techniques employed the compositional variations are incompatible with deformation driven crystal fractionation in melt instead the layers and domains possibly are crystal melt mushes produced by syn deformation ultra high temperature 1 250 1 375 c melting reactions involving variable proportions of host rock minerals determined by time transient local phase aggregates experiencing strain the similar element variation trends in pseudotachylite examined here and those reported from anorthosite metapelite and charnockite elsewhere suggests local phase aggregate controlled multi reaction melting is a phenomenon commoner than hitherto realized in pseudotachylites

glassy pseudotachylyte veins from the fuyun fault zone northwest china glassy pseudotachylytes occur in granitic mylonites from the fuyun fault zone northwest china powder x ray diffraction data indicate that the fine grained matrices of the fuyun pseudotachylytes vary from almost crystalline to complete glass a few to 90 wt glass powder x ray diffraction patterns also indicate that clasts in the glass type pseudotachylyte veins mostly consist of quartz grains microlites displaying quenched or rapid cooling textures vesicles and amygdules flow structures and rounded fragments are also common in the fuyun pseudotachylytes the average bulk chemical compositions of these pseudotachylytes are similar to those of the host rocks but the glass matrix has a low sio2 component which is 5 15 wt lower than that of the host rocks the pseudotachylytes contain 2 3 wt water h2o using experimental data which relate the solubility of water to pressure approximately 400 bars lithostatic pressure corresponding to approximately 1 5 km depth of pseudotachylyte formation is estimated a minimum temperature estimate of 1450 c can be estimated due to the presence of the pure sio2 glass the x ray diffraction and chemical data indicate that the fuyun pseudotachylytes formed during seismic faulting at shallow depths by selective melting under water saturated conditions rather than by total melting or by crushing of the host rocks along fault surfaces

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

development of phyllonite from granodiorite mechanisms of grain size reduction in the santa rosa mylonite zone california abstract field and laboratory investigations indicate that phyllonite within the santa rosa mylonite zone developed from granodiorite without hydrous alteration of the original mineral assemblage initial grain size reduction within zones of phyllonite was accomplished through whole rock cataclasis subsequent deformation occurred through ductile flow the original cataclastic failure is recorded by sharp boundaries with the surrounding rock which are retained through the ductile overprint biotite in phyllonite has a mean grain size of 3 m and exhibits straight boundaries and a strong crystallographic preferred orientation trace analyses of hvem images indicate that the largest biotite grains are most strongly oriented and the grains are elongate parallel to lineation tem observations indicate that typical grains have irregularly spaced stacking faults and twins evidence for intracrystalline folding and cataclasis present in protomylonite and mylonite is absent bending or kinking of grains is rare these observations suggest that biotite in phyllonite accommodated deformation through dynamic recrystallization in concert with intracrystalline slip and mechanical rotation grain size reduction to form phyllonite is accompanied by macroscopically and microscopically visible changes in the character of foliations in the rock we suggest that these changes are linked to the deformation mechanisms operating within the fine grained phyllonite

3d seismic imaging of buried younger dryas mass movement flows lake windermere uk abstract windermere is a glacially overdeepened lake located in the southeastern lake district uk using the three dimensional 3d chirp subbottom profiler we image mass movement deposits related to the younger dryas yd within a decimetre resolution 3d seismic volume documenting their internal structure and interaction with preexisting deposits in unprecedented detail three distinct flow events are identified and mapped throughout the 3d survey area package structures and seismic attributes classify them as a small total volume of c 1500 m 3 debris flow containing deformed translated blocks a large inferred total volume of c 500 000 m 3 homogeneous fine grained mass flow deposit and a debris flow inferred total volume of c 60 000 m 3 containing small c 8 0 2 0 m deformed translated blocks geomorphological mapping of their distribution and interaction with preexisting sediments permit the reconstruction of a depositional history for the stratigraphic units identified in the seismic volume

al rich calcic pyroxene in pseudotachylite an indicator of high pressure and high temperature abstract an aluminous pyroxene with an approximate formula ca 0 60 na 0 05 0 20 mg 0 58 fe 0 11 2 al 0 60 vi si 1 68 a1 0 32 o 6 or jd 0 05 ts 0 32 es 0 20 wo 0 10 en 0 27 fs 0 06 has been found in a pseudotachylite vein from gabbros in the musgrave ranges central australia textural evidence indicates that it formed by breakdown of calcic plagioclase augite and olivine which constitute the isochemical country rock there is a large amount of m 2 vacancy compared to most pyroxenes the vein mineral is most similar in composition to natural pyroxenes in kimberlites and eclogites and especially to synthetic pyroxenes which formed at pressures in excess of 3 gpa 30 kb we suggest that these high pressures were not lithostatic but represent dynamic stresses released during catastrophic brittle rupture either at tips of propagating fractures or spallation of unsupported crackwalls such transient shear stresses may reach the shock range we propose such a fracture mechanism as an alternative to frictional melting for the productoin of pseudotachylite veins

seismological studies at parkfield vi moment release rates and estimates of source parameters for small repeating earthquakes abstract waveform data from a borehole network of broadband seismographic stations have been used to study microearthquakes along the parkfield segment of the san andreas fault saf analysis of almost 10 years of such data demonstrates that much of the seismicity in this region consists of repeating sequences quasiperiodic sequences of earthquakes that are essentially identical in terms of waveform size and location scalar seismic moments have been estimated for 53 of these repeating sequences and combined with equivalent estimates from 8 similar but larger event sequences from the stone canyon section of the fault and the main parkfield sequence these estimates show that seismic moment is being released as a function of time in a very regular manner measurements of the moment release rate combined with an assumed tectonic loading rate lead to estimates of the seismic parameters source area slip and recurrence interval such parameters exhibit a systematic dependence upon source size over a range of 10 10 in seismic moment that can be described by three simple scaling relationships several implications of these scaling relationships are explored including the repeat time of earthquakes average stress drop strength of the fault and heat generated by earthquakes what emerges from this analysis of moment release rates is a quantitative description of an earthquake process that is controlled by small strong asperities that occupy less than 1 of the fault area this means that the fault is highly heterogeneous with respect to stress strength and heat generation such heterogeneity helps to explain many of the apparent contradictions that are encountered in the study of earthquakes such as why faults appear weak why significant heat flow is not observed how significant high frequencies can be generated by large earthquakes and how various geologic features such as pseudotachylytes might form

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

magmatic arc asymmetry and distribution of anomalous plutonic belts in the batholiths of california effects of assimilation crustal thickness and depth of crystallization in order to better understand geologic fac tors controlling pronounced regional variations in whole rock chemistry mineralogy and mineral chemistry in the batholiths of california we calculate the magmatic intensive variables f hf f h 2 o f hf f hcl and f o 2 regional scale west to east increases in f oh in mafic silicates corresponding to the systematic i wc to i mc and i sc progression reflect orders of magnitude increase in f hf f h 2 o attending pluton crystallization low f hf f h 2 o of formation of western i wc types is consistent with their derivation from low fluorine source rocks in subducted oceanic slabs or the upper mantle in contrast higher f hf f h 2 o of crystallization of i mc and i sc types to the east implies the involvement of 1 progressively greater amounts of continental crustal source material such as biotite bearing metamorphic rocks their unweathered sedimentary derivatives retaining f rich mafic minerals or their fusion products and or 2 source materials which become more f rich toward the continental interior the regional distribution of i mc and i sc types suggests that the precambrian craton of western north america or derivative sediments may extend farther north in california and be morphologically more complex than previously thought from seemingly out of place occurrences of i wc plutons on the eastern slopes of the sierra nevada batholith we infer the existence of regions where precambrian basement was thin or absent in mesozoic time which prevented extensive cratonal contamination of subducted slab or upper mantle derived magmas new methods for estimating t f o 2 relations in the magmas demonstrate that i scr granites crystallize at oxygen fugacities as much as five orders of magnitude lower than those of i wc i mc and i sc types under conditions at or below the maximum stability limit of graphite in equilibrium with a c o h s gas phase the local scale formation of i scr granites in plutonic belts within specific wall rock terranes containing highly reducing sediments or metasediments may occur by contamination of i types with graphitic pelite or in some cases by the direct fusion of this reducing pelitic wall rock the spatial distribution of i scr granite provinces therefore is controlled simply by wall rock lithology amphibole geobarometry demonstrates a general west to east decrease in crystallization pressure across the sierra nevada batholith in contrast the peninsular ranges batholith displays a west to east crystallization pressure increase the bulk of the california batholiths crystallized at pressures less than 4 5 kb and depths less than about 15 19 km in the southern sierra nevada batholith the san gabriels and the eastern peninsular ranges however plutons crystallized at pressures exceeding 6 kb at deep crustal levels 23 km reconstruction of the pre erosion top of the batholith shows that in an east west cross section the central sierra nevada batholith was a horizontal tabular body with an aspect ratio of at least five to one

magnetic properties of fault pseudotachylytes in granites 1 we investigate the petrographic geochemical and magnetic properties of fault pseudotachylytes formed by frictional melting in granitic rocks from southern california the italian alps and kyushyu japan the main magnetic remanence carriers are mixtures of grain sizes of fine grained magnetite these ferrimagnetic grains record a stable multicomponent magnetization that consists of one or more of the following coseismic thermal remanent magnetization coseismic lightning isothermal remanent magnetization and post seismic chemical remanent magnetization fault pseudotachylytes from the three localities display contrasting magnetic properties which suggests that oxygen fugacity and host rock composition ultimately control the magnetic assemblage

borrego springs santa rosa mylonite zone a late cretaceous west directed thrust in southern california the borrego springs shear zone section of the eastern peninsular ranges mylonite zone eprmz contains unequivocal evidence for ductile shearing with an east over west sense of movement field evidence includes the presence of s and c planes and shear bands all lines of microstructural evidence are internally consistent and support the field observations it appears that the data are compatible only with post mid cretaceous west directed thrusting on the eprmz

the geological hubble a reappraisal for shallow water traditional two dimensional 2d seismic acquisition techniques image the subsurface using a grid of orthogonal lines dips are recorded only in the along track direction limiting migration to a 2d along track approximation of the inherently 3d wavefields this produces profiles that are often complicated by out of plane reflections and with resolution for structural interpretation constrained by the line spacing rather than the source wavelength the acquisition of true 3d seismic reflection data in contrast provides dip information for the reflected wavefields in both along and across track directions this allows a full treatment of the 3d wavefields during migration affording accurate 3d structural reconstruction significantly improved resolution theoretically 1 2 source wavelength and increasing signal to noise ratio snr through more effective noise cancellation

formation of alpine mylonites and pseudotachylytes at the base of the silvretta nappe eastern alps optical microscopy and transmission electron microscopy tem have been used to investigate deformation induced microstructures in alpine pseudotachylytes mylonites and ultramylonites from the upper austro alpine silvretta nappe at the southwestern margin of the lower engadine window in the eastern alps of switzerland and austria the earliest alpine stage of deformation d1 produced mylonitic gneisses with dislocation glide being the dominant deformation mechanism in quartz in the second stage d2a mylonites were formed as a result of strain softening in quartz manifest by dynamic recrystallization and a lattice preferred orientation tem observations are used to characterize the initial stages of migration and rotation recrystallization pseudotachylytes were generated during d2b when strain hardening due to dislocation tangling was not outweighed by strain softening several generations of mylonites and pseudotachylytes are observed a model for the propagation of seismic fractures initiated in comparatively rigid amphibolites into an otherwise ductile environment is proposed for the pseudotachylytes as such they are formed as transient discontinuities in the mylonites d1 and d2a b are related to the eo alpine detachment of the silvretta nappe subsequent eo alpine metamorphism in the investigated area reached the stilpnomelane zone and this coincided with the development of ultramylonites from both the mylonites and pseudotachylytes during d3 and peak metamorphism the increased temperature combined with fluid infiltration from the overridden penninic strata favoured grain boundary sliding superplasticity as the dominant deformation mechanism during d3

effects of progressive mylonitization on ar retention in biotites from the santa rosa mylonite zone california and thermochronologic implications the santa rosa mylonite zone developed predominantly from a granodiorite protolith in the eastern margin of the peninsular ranges batholith a wide variation in k ar biotite dates within the zone is shown to reflect the times of cooling through closure temperatures whose variability is chiefly a result of deformation induced reduction in grain size we suggest that such variation generally may be exploited to place constraints on the timing of deformation episodes previous workers have shown that deformation in the santa rosa mylonite zone involved the formation of mylonites and an imbricate series of low angle faults which divide the area into structural units field work petrographic studies and tem analysis of deformation mechanisms in biotite show that the granodiorite mylonite the lowermost structural unit formed below the granodiorite solidus the granodiorite mylonite varies from protomylonite to ultramylonite with regions of high strain distributed heterogeneously within the zone previously reported biotite and hornblende k ar dates from the granodiorite protolith below 82 89 ma and the asbestos mountain granodiorite above 61 68 ma the mylonite zone indicate dramatically dissimilar thermal histories for the lowermost and uppermost structural units other workers fission track dates on sphene zircon and apatite from the granodiorite mylonite and the asbestos mountain granodiorite suggest thermal homogenization and rapid cooling to 100 c by ca 60 ma within and adjacent to the mylonite zone k ar dates on 5 samples of biotite from variably deformed granodiorite range from 62 76 ma dates are not correlated with structural depth but clearly decrease with degree of deformation and concomitant grain size reduction 40ar 39ar incremental heating analyses of biotite from the granodiorite protolith reveals no evidence of excess argon and produces a relatively flat age spectrum 40ar 39ar incremental heating analysis of biotite from the granodiorite mylonite discloses discordance consistent with 39ar recoil loss k analysis of samples allowing k ar dates to be calculated is therefore recommended as an adjunct to 40ar 39ar step heating analysis in rocks that have experienced similar deformation during mylonitization biotite grain size reduction through intracrystalline cataclasis results in estimated grain dimensions as small as 0 05 m locally within porphyroclasts as large as 1 mm because biotite compositions are relatively uniform fe fe mg mn ti alvi 0 47 0 52 and show no systematic variation with grain size compositional dependence of activation energy and diffusivity can be eliminated as sources of variation in ar retention ar closure temperatures calculated with appropriate diffusion parameters for the observed grain sizes are in the range 220 280 c and define a cooling curve consistent with a thermal history intermediate between those of the granodiorite protolith below and the asbestos mountain granodiorite above the mylonite zone changes in the slope of the cooling curve indicate that the main deformation episode initiated at or above ca 330 c 80 ma above the closure temperature for thermally activated diffusion of argon in biotite and continued to a minimum of ca 220 260 c 62 ma

thickness of the seismic slip zone this article reviews geologic and other evidence constraining the thickness of the principal slip zone psz that accommodates the bulk of coseismic shear displacement during an individual rupture event surface deformation from rupturing may occupy swaths tens of meters or more in width but trenches across active faults generally reveal that incremental slip is accommodated by a psz that is tens of centimeters or less in thickness geomorphic evidence coupled with the observations from trenching suggest a psz may stay well localized for distances of several kilometers through many rupture episodes mine exposures and exhumed fault zones demonstrate that pszs separating different lithologies within the fault core although contained within damage zones of variably fractured rock ranging up to hundreds of meters in thickness often comprise just a few centimeters of gouge ultracataclasite that have accommodated large finite displacements 1 km microstructural studies demonstrate incremental slip localized still further down to 1 10 mm as do other fault rock assemblages slickensides and slickenfibers fault veins of pseudotachylyte friction melt intravein septa in hydrothermal fault infills the accumulated evidence indicates that localization of coseismic shearing to less than 10 cm on planar faults is widespread throughout the crustal seismogenic zone with extreme localization to less than 1 cm not uncommon however some distributed coseismic shear may also develop especially at rupture irregularities coseismic reduction of shear resistance from friction melting t 1000 c or from transient thermal pressurization of aqueous fluids t 100 c requires slip during moderate to large earthquakes u 1 m to be restricted to narrow zones respectively a few centimeters or tens of centimeters in thickness given the evidence for slip localization the apparent scarcity of pseudotachylyte suggests either that seismic friction melting is a rare phenomenon or that pseudotachylyte is only rarely preserved in recognizable form within mature hydrated fault zones

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

on relating apparent stress to the stress causing earthquake fault slip apparent stress a is defined as where is the average shear stress loading the fault plane to cause slip and is the seismic efficiency defined as ea w where ea is the energy radiated seismically and w is the total energy released by the earthquake the results of a recent study in which apparent stresses of mining induced earthquakes were compared to those measured for laboratory stick slip friction events led to the hypothesis that this hypothesis is tested here against a substantially augmented data set of earthquakes for which can be estimated mostly from in situ stress measurements for comparison with a the expanded data set which includes earthquakes artificially triggered at a depth of 9 km in the german kontinentales tiefbohrprogramm der bundesrepublik deutschland ktb borehole and natural tectonic earthquakes covers a broad range of hypocentral depths rock types pore pressures and tectonic settings nonetheless over 14 orders of magnitude in seismic moment apparent stresses exhibit distinct upper bounds defined by a maximum seismic efficiency of 0 06 consistent with the hypothesis proposed before this behavior of a and can be expressed in terms of two parameters measured for stick slip friction events in the laboratory the ratio of the static to the dynamic coefficient of friction and the fault slip overshoot typical values for these two parameters yield seismic efficiencies of 0 06 in contrast to efficiencies for laboratory events for which is always near 0 06 those for earthquakes tend to be less than this bounding value because ea for earthquakes is usually underestimated due to factors such as band limited recording thus upper bounds on appear to be controlled by just a few fundamental aspects of frictional stick slip behavior that are common to shallow earthquakes everywhere estimates of from measurements of a for suites of earthquakes using are found to be comparable in magnitude to estimates of shear stress on the basis of extrapolating in situ stress data to seismogenic depths

modeling frictional melt injection to constrain coseismic physical conditions abstract pseudotachylyte a fault rock formed through coseismic frictional melting provides an important record of coseismic mechanics in particular injection veins formed at a high angle to the fault surface have been used to estimate rupture directivity velocity pulse length stress drop as well as slip weakening distance and wall rock stiffness these studies have generally treated injection vein formation as a purely elastic process and have assumed that processes of melt generation transport and solidification have little influence on the final vein geometry using a pressurized crack model an analytical approximation of injection vein formation based on dike intrusion we find that the timescales of quenching and flow propagation may be similar for a subset of injection veins compiled from the asbestos mountain fault usa gole larghe fault zone italy and the fort foster brittle zone usa under minimum melt temperature conditions 34 of the veins are found to be flow limited with a final geometry that may reflect cooling of the vein before it reaches an elastic equilibrium with the wall rock formation of these veins is a dynamic process whose behavior is not fully captured by the analytical approach to assess the applicability of simplifying assumptions of the pressurized crack we employ a time dependent finite element model of injection vein formation that couples elastic deformation of the wall rock with the fluid dynamics and heat transfer of the frictional melt this finite element model reveals that two basic assumptions of the pressurized crack model self similar growth and a uniform pressure gradient are false the pressurized crack model thus underestimates flow propagation time by 2 3 orders of magnitude flow limiting may therefore occur under a wider range of conditions than previously thought flow limited veins may be recognizable in the field where veins have tapered profiles or smaller aspect ratios than expected the occurrence and shape of injection veins can be coupled with modeling to provide an independent estimate of minimum melt temperature finally the large aspect ratio observed for all three populations of injection veins may be best explained by a large reduction in stiffness associated with coseismic damage as injection vein growth is likely to far exceed the lifetime of dynamic stresses at any location along a fault

fault rocks within the blueschist metabasalts of the diamante terranova unit southern italy potential fossil record of intermediate depth subduction earthquakess vitale et al fault rocks within blueschist metabasalts

preferred orientation of phyllosilicates in phyllonites and ultramylonites abstract x ray texture goniometry operated in the transmission mode has been applied to fine grained phyllosilicate rich sc mylonites from the santa rosa mylonite zone california the whipple mountains core complex california and the austroalpine nappes of the eastern alps austria basal planes 001 phyllosilicate pole figures depart in two respects from orthorhombic symmetry a the maximum pole density deviates from the pole to the mylonitic foliation and lies along a great circle which is normal to the foliation and contains the mylonitic lineation and b the contours of equal pole density are more widely spaced on one side of the 001 maximum than on the other measured along the mylonitic lineation optical microscopy reveals s and c surfaces and sets of shear bands sb characteristic of non coaxial deformation the pole figures are used for two purposes a the first is strain estimation after the march model by imposing orthorhombic symmetry the resulting march strains are more or less the same in all californian samples this is interpreted as evidence for a steady state foliation b the second is shear sense determination it is demonstrated that consistently asymmetric preferred orientations reflect structures which are typical of non coaxial deformation it is concluded that x ray analysis of phyllosilicate rich mylonites can in addition to providing an estimate of total strain give information about deformation regime and shear sense furthermore the study documents the heterogeneous nature of deformation on macroscopic and microscopic scales and the importance of non coaxial deformation in accommodating large strains

experimentally produced pseudotachylite like veins in gabbro abstract thin veins of mobile microcrystalline particles 50 500 a in size have been produced experimentally during catastrophic failure of gabbroic rocks under rapidly applied stresses in the range 2 0 4 0 gpa optically the veins contain crushed fragments of minerals in an isotropic groundmass and are locally brown in color particularly where they cross pyroxene crystals in appearance they resemble some pseudotachylite veins transmission electron microscopy has been used to characterize the microstructure of the grains and document their crystalline structure

microlite morphology and chemistry in pseudotachylite from the fuyun fault zone china microlites in pseudotachylite veins from the fuyun fault zone northwest china can be divided into four morphological groups simple skeletal dendritic and spherulitic from the margin to the center of microcrystalline and microlitic pseudotachylite veins the size of single crystals or crystal fibers becomes progressively larger from a few m to as much as 15 m the morphologies of microlites vary from simple shapes such as acicular and globular forms to more complex forms such as skeletal dendritic and coarse spherulitic forms in glassy pseudotachylites only simple and fine spherulitic microlites generally smaller than 5 6 m occur these microlites have a varied mineralogy but are mainly made up of plagioclase an20 70 and alkali feldspar including sanidine and anorthoclase as well as clinopyroxene grossular garnet biotite hornblende and some magnetic minerals the biotite and hornblende microlites have a variable ti content generally higher than that of biotite and hornblende frag

melt origin of fault generated pseudotachylytes demonstrated by textures a petrographic and electron microprobe study of fault generated pseudotachylytes from the outer hebrides thrust zone scotland demonstrates that the textures have resulted from the primary crystallization of a clast laden melt rather than the devitrification of a glass or by crushing and cataclasis the observations are consistent with these pseudotachylytes having formed by frictional fusion followed by rapid quenching implying faulting at shallow crustal depths

deformation of mylonites in palm canyon california based on xenolith geometry abstract an analysis of xenoliths in granodiorite and mylonites of the peninsular ranges batholith in southern california documents large deformation with von mises equivalent strains ranging from 1 to 4 from the geometry of xenoliths in mylonites it is concluded that much of the deformation occurred by flattening if deformation had been homogeneous the 800 m thick mylonite sequence would have extended originally over more than 10 km contrary to xenoliths associated with ballooning diapirs these in santa rosa mylonites show much larger strains and are elongated in the lineation direction it is proposed that the overall deformation pattern is consistent with a top to the west shear displacement but most of the strain in the santa rosa mylonites was produced in flattening during emplacement of the partially solidified batholith the large deformation may have produced folds that have coalesced with the foliation to accommodate the strain

amorphous material in safod core samples san andreas fault evidence for crush origin pseudotachylytes 1 several types of amorphous material in ultracataclastic core samples recovered from 3194 m and 3294 m depth of the main bore hole of the san andreas fault observatory at depth are identified and described with transmission electron microscopy and scanning electron microscopy we observed 1 amorphous material on a slickenside surface 2 glassy bands contained in an ultracataclastic matrix and 3 amorphous rims surrounding quartz or feldspar clasts chemical analyses of the amorphous material reveal that silica content is slightly enriched or similar as in the adjacent matrix we suggest that all amorphous material was formed by comminution of clasts crush origin pseudotachylytes rather than by melting melt origin pseudotachylytes the observed amorphous phases may act as lubricating layers that reduce friction in the san andreas fault

rapid emplacement of the oman ophiolite thermal and geochronologic constraints in understanding of ophiolite emplacement requires knowledge of the elapsed time between igneous crystallization and intraoceanic thrusting and the rate and duration of that thrusting hornblende 4oar 39ar ages demonstrate that the igneous oceanic crust in oman crystallized and cooled to 825 k in 1 2 m y hornblende ages from metamorphic rocks and from cross cutting dikes require that the basal metamorphic thrust fault beneath the ophiolite also cooled below 825 k in 1 2 m y motion along the sole thrust accounted for 200 km of displacement at a rate of 100 200 mm yr on the basis of age relationships and thermal considerations we favor a two stage model for the initial stages of samall ophiolite emplacement intraoceanic thrusting over 2 m y old lithosphere at 150 km m y parallel to a spreading ridge for 1 2 m y followed by equally rapid and brief thrusting over cold and old lithosphere preservation of the samail ophiolite is a direct result of its young age and positive buoyancy at the time of ocean closure and we propose that all ophiolites that originated near spreading centers and were emplaced onto continents were young at the time of intraoceanic thrusting

argon retention properties of silicate glasses and implications for 40 ar 39 ar age and noble gas diffusion studies sized aggregates of glasses 47 84 wt sio2 were fused from igneous derived cohesive fault rock and igneous rock and step heated from 400 to 1 200 c to obtain their 39ar diffusion properties average e 33 400 cal mol 1 d o 4 63 10 3 cm2 s 1 at t 1 mm are analyzed glasses with compositions similar to ours may preserve their formation ages

the development of pseudotachylyte in the ivrea verbano zone of the italian alps abstract pseudotachylyte was formed as part of the retrogressive tectonometamorphic evolution of the ivrea verbano zone during its post hercynian uplift from the lower crust during the mesozoic and cenozoic the pseudotachylytes preferentially occur in mafic rocks and in paragneisses and were generated along brittle faults which locally cut older ductile shear zones fracturing and the selective melting of minerals resulted in an aphanitic matrix containing host rock fragments and newly formed minerals e g biotite quartz sericite feldspar friction melt induced hydraulic fracturing of the adjacent rocks facilitated the formation of pseudotachylyte veins the veins occur as networks or as single 0 1 m to several metres thick injections with the geometry of pull apart structures in zones with a high density of contemporaneously formed pseudotachylyte veins the fractured wall rock lost cohesion and became intrusive masses of angular to rounded fragments became mobilized when the macroscopically homogeneous pseudotachylyte matrix exceeded about 30 of the total rock volume geochemical data show that the pseudotachylytes developed from the immediately adjacent host rocks scanning and transmission electron microscopy indicate that the enrichment or depletion of individual elements in the pseudotachylyte relative to the host rock depended on the amount of selectively melted minerals that were incorporated amphiboles plagioclase quartz and or sheet silicates and on the degree of secondary alteration the development of pseudotachylyte took place in the upper crust during retrogressive metamorphism which commenced in the ductile deformation field with the formation of high temperature mylonites followed by the generation of low temperature mylonites and finally by brittle deformation

credible occurrence probabilities for extreme geophysical events earthquakes volcanic eruptions magnetic storms 1 statistical analysis is made of rare extreme geophysical events recorded in historical data counting the number of events k with sizes that exceed chosen thresholds during specific durations of time under transformations that stabilize data and model parameter variances the most likely poisson event occurrence rate k applies for frequentist inference and also for bayesian inference with a jeffreys prior that ensures posterior invariance under changes of variables frequentist confidence intervals and bayesian jeffreys credibility intervals are approximately the same and easy to calculate 1 k z 2 2 k z 2 2 where z is a parameter that specifies the width z 1 z 2 corresponding to 1 68 3 2 95 4 if only a few events have been observed as is usually the case for extreme events then these error bar intervals might be considered to be relatively wide from historical records we estimate most likely long term occurrence rates 10 yr occurrence probabilities and intervals of frequentist confidence and bayesian credibility for large earthquakes explosive volcanic eruptions and magnetic storms