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

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

the evolution of forearc structures along an oblique convergent margin central aleutian arc multichannel seismic reflection data were used to determine the evolutionary history of the forearc region of the central aleutian ridge since at least late miocene time this sector of the ridge has been obliquely underthrust 30 west of orthogonal convergence by the northwestward converging pacific plate at a rate of 80 90 km m y our data indicate that prior to late eocene time the forearc region was composed of rocks of the arc massif thinly mantled by slope deposits the forearc region probably lacked both major depositional basins and a tectonically attached accretionary prism of offscraped oceanic deposits beginning in latest miocene or earliest pliocene time a zone of outer arc structural highs and a forearc basin began to form formation of these companion intraarc structures may be linked to the late neogene growth of an accretionary wedge that formed as the result of the deposition of a thick turbidite wedge in the aleutian trench initial structures of the zone of outer arc highs formed as the thickening wedge underran compressively deformed and uplifted the seaward edge of the arc massif above a landward dipping backstop thrust forearc basin strata ponded arcward of the elevating zone of outer arc highs however most younger structures of the zone of outer arc highs cannot be ascribed simply to the orthogonal effects of an underrunning wedge oblique convergence created a major right lateral shear zone the hawley ridge shear zone that longitudinally disrupted the zone of outer arc highs truncating the seaward flank of the forearc basin and shearing the southern limb of hawley ridge an exceptionally large antiformal outer arc high structure slivers of forearc basement rocks and overlying strata have been transported along the shear zone that is flanked by differentially elevated structures attributed to localized transpressive and transtensional processes uplift of hawley ridge may be related to the thickening of the arc massif by westward directed basement duplexes in addition the forearc is disrupted by structures transverse to the margin that occur where unusually high stress accumulations have resulted in the rupture of repeated great earthquakes it is likely that many ancient active margins evolved in tectonic and depositional settings similar to those of the central aleutian ridge great structural complexity including the close juxtaposition of coeval structures recording compression extension differential vertical movements and strike slip displacement should be expected even within areas of generally kindred tectonostratigraphic terranes

a slice and view fib sem study of clay gouge from the safod creeping section of the san andreas fault at 2 7 km depth abstract the san andreas fault is one of the most studied earthquake generating structures on earth but the reason that some sections are anomalously weak and creep without apparent seismicity remains poorly understood here we present results from nanoscale fib sem 3d microstructural observations of weak friction coefficient of 0 095 safod clay fault gouge containing serpentinite clasts recovered from the active central deforming zone at 2 7 km vertical depth our nanoscale observations confirm that frictional slip and extreme weakness occur via deformation of smectite clay that forms a shear fabric within the fault zone we infer that creep initiates by fracture controlled substrate growth of oriented mg smectite on r p and y shears followed by clay smearing and ductile flow of an evolving and expanding clay matrix at the crystal scale pervasive sliding occurs along hydrated smectite interlayers and surfaces occupied by exchangeable mg and ca ions with slip typically spaced at 3 5 lattice layers apart we conclude that the strength and seismic behaviour of major tectonic faults at shallow crustal levels evolves as clay fabric develops with accumulated fault slip

production of nanoparticles during experimental deformation of smectite and implications for seismic slip abstract nanoparticles and amorphous materials are common constituents of the shallow sections of active faults understanding the conditions at which nanoparticles are produced and their effects on friction can further improve our understanding of fault mechanics and earthquake energy budgets here we present the results of 59 rotary shear experiments conducted at room humidity conditions on gouge consisting of mixtures of smectite ca montmorillonite and quartz experiments with 60 50 25 0 wt ca montmorillonite were performed to investigate the influence of variable clay content on nanoparticle production and their influence on frictional processes all experiments were performed at a normal stress of 5 mpa slip rate of 0 0003 v 1 5 ms 1 and at a displacement of 3 m to monitor the development of fabric and the mineralogical changes during the experiments we investigated the deformed gouges using scanning and transmission electron microscopy combined with x ray powder diffraction quantitative phase analysis this integrated analytical approach reveals that at all slip rates and compositions the nanoparticles grain size of 10 50 nm are partly amorphous and result from cataclasis wear and mechanical solid state amorphization of smectite the maximum production of amorphous nanoparticle occurs in the intermediate slip rate range 0 0003 v 0 1 ms 1 at the highest frictional work and is associated to diffuse deformation and slip strengthening behavior instead the lowest production of amorphous nanoparticles occurs at co seismic slip rates v 1 3 ms 1 at the highest frictional power and is associated with strain and heat localization and slip weakening behavior our findings suggest that independently of the amount of smectite nanoparticles they produce fault weakening only when typical co seismic slip rates 0 1 ms 1 are achieved this implies that estimates of the fracture surface energy dissipated during earthquakes in natural faults might be extremely difficult to constrain

focused ion beam fib combined with sem and tem advanced analytical tools for studies of chemical composition microstructure and crystal structure in geomaterials on a nanometre scale abstract in order to make meaningful interpretations of trace element and isotopic information from geomaterials prior characterization of the material of interest with respect to structural and chemical homogeneity radiation damage and microstructure is often required transmission electron microscopy tem is a powerful analytical tool that may be used to fully characterise a wide range of synthetic and natural materials the focused ion beam technique fib is an ideal tool for tem sample preparation that allows for the fabrication of electron transparent foils with typical dimensions of 15 10 0 150 m from any region of interest i e site specific and in virtually any material fib sample preparation consumes only a small volume of material approximately 2300 m 3 for a standard foil with the dimensions 15 10 0 150 m leaving most of the material essentially unaffected by the sampling process modern tem facilitates the measurement of chemical composition with unsurpassed spatial resolution e g nanometer scale high resolution imaging and electron diffraction provide important complimentary structural information it is the combination of crystal structure information and chemical composition that allows for the complete characterization of the volume of interest high resolution elemental mapping or line scans may be used to acquire complete energy dispersive x ray analysis edx spectra of a material pixel by pixel subsequent evaluation of the data allows for the correlation of the edx spectrum with the respective elemental map or line scan with an image thus illustrating spatially resolved chemical composition variations at the 0 5 1 0 wt level frequently minerals enclose sub micrometer or even nanometre sized inclusions of minerals fluids or silicate melt which are of great interest to geoscientists nanoinclusions e g in diamond or olivine are of particular interest because they may have preserved their original crystal structure thus providing a geobarometer recording the pressure conditions at the time of formation of the inclusion fib tem are appropriate techniques to sample and subsequently define the chemical composition and the structural state of mineral inclusion on the nanometre scale the combination of fib with an sem allows for 3d information to be obtained from samples including 3d imaging e g phase distribution and volume of the individual phases in symplectites 3d distribution of elements in a specific volume 3d elemental maps and three dimensional texture analysis using electron backscatter diffraction ebsd

vegetation change in equatorial west africa time slices for the last 150 ka abstract drawing on marine and terrestrial palynological data vegetation maps of equatorial west africa are presented for eleven time slices over the last 150 000 years long marine records are used as a basis for the regional picture and provide a chronology for the last glacial cycle much shorter terrestrial records help fill in the picture for the most recent periods and facilitate interpretation of regional patterns for the longer timescale temporal and spatial variation is revealed in relation to global patterns of climate change rain forest was widespread during oxygen isotope stages 1 and 5 but strongly reduced during stages 3 and 4 and especially during stages 2 and 6 when open grass rich vegetation prevailed glacial rain forest refuges are found in the southwest of the guinean mountains and along the eastern coast of the gulf of guinea miombo woodland of the zambezian vegetation zone expanded during stage 5 especially during the first half podocarpus forest had its largest expansion during substages 5d 115 to 105 ka and 5b 95 to 85 ka the last occurrence of podocarpus in the guinean mountains is during stage 5a the distribution of mangrove swamps was extremely reduced during glacial times during stage 6 savanna and open dry forest covered large areas along the northern coast of the gulf of guinea the southern saharan desert reached far to the south and the namib desert far to the north the area of rain forest was restricted mangrove swamps were strongly reduced and the area of podocarpus forest fluctuated during stage 5 large changes in the area of afromontane forest rain forest dry open forest and savanna occurred rain forest was widespread and mangroves were extensive along the coast during the last interglacial substage 5e podocarpus forest area strongly extended during substages 5d and 5b in substages 5c and 5a rain forest reclaimed areas it had lost in the previous substages 5d and 5b respectively mangrove swamps were less widespread in the later substages of stage 5 than during substage 5e during stage 4 the rain forest area was again strongly reduced and recovered only slightly during the following stage 3 also the mangrove swamp area was reduced except along the ivory coast and along the northwestern coast of the gulf of guinea podocarpus forest only occurred in angola and may be in congo during stage 4 again forest was much reduced during stage 2 and open vegetation types covered large parts of equatorial west africa mangrove swamps must have been rare at the beginning of the holocene mangrove swamps had recovered and reached their largest extension also the rain forests area increased in the early holocene and guinean and congolian rain forests were probably not separated by a savanna corridor now existing in togo and benin

environmental change and human occupation of southern ethiopia and northern kenya during the last 20 000 years abstract our understanding of the impact of climate driven environmental change on prehistoric human populations is hampered by the scarcity of continuous paleoenvironmental records in the vicinity of archaeological sites here we compare a continuous paleoclimatic record of the last 20 ka before present from the chew bahir basin southwest ethiopia with the available archaeological record of human presence in the region the correlation of this record with orbitally driven insolation variations suggests a complex nonlinear response of the environment to climate forcing reflected in several long term and short term transitions between wet and dry conditions resulting in abrupt changes between favorable and unfavorable living conditions for humans correlating the archaeological record in the surrounding region of the chew bahir basin presumably including montane and lake marginal refugia for human populations with our climate record suggests a complex interplay between humans and their environment during the last 20 ka the result may contribute to our understanding of how a dynamic environment may have impacted the adaptation and dispersal of early humans in eastern africa

clay fabrics in safod core samples with optical microscopy scanning electron microscopy sem transmission electron microscopy tem and synchrotron x ray diffraction measurements we analyzed clay microfabrics in ultracataclastic gouge and cataclastic core samples obtained from the main bore hole of the san andreas fault observatory at depth safod the analysis reveals a significant contrast between weak clay fabrics observed in the core samples with synchrotron x ray fabric measurements and strong degree of preferred alignment for clay particles documented with the optical microscope tem and sem observations also show distinct zones of locally aligned and randomly oriented clay minerals the lack of a strong fabric may be attributed to randomly oriented matrix sheet silicates dominating the fault rocks the presence of weak fabrics in intensely strained ultracataclasites fault gouges is attributed to 1 newly formed clay minerals that grew in many orientations 2 folded and kinked clay minerals and 3 clay particles that are wrapped around grains in addition the locally aligned clay particles may act as barriers to fluid flow which in turn decrease porosity expel intergranular pore fluids and consequently may increase fluid pressure

evidence of thermal pressurization in high velocity friction experiments on smectite rich gouges terra nova 22 347 353 2010 abstract thermal pressurization of pore fluid is one of the possible mechanisms responsible for dynamic weakening in landslides and earthquakes but to date has not been reproduced in the laboratory here we report high velocity experiments performed in a rotary shear friction apparatus on smectite rich gouges from the 1963 vaiont landslide italy the gouges were slid under 1 mpa normal stress for displacements up to 30 m and a slip rate of 1 31 m s 1 under room humidity and water saturated conditions sample dilatancy was observed in room humidity runs after 3 4 m of slip concomitant with an increase in normal stress and a decrease in shear stress mineralogical and microstructural investigations suggest that dilatancy resulted from expansion of the h2o released by the collapse of the smectite structure due to frictional heating of the slipping zone at t 200 c we conclude that sample dilatancy is due to thermal pressurization of the clay rich gouge

nondestructive continuous physical property measurements of core samples recovered from hole b taiwan chelungpu fault drilling project 1 the taiwan chelungpu fault drilling project was undertaken in 2002 to investigate the faulting mechanism of the 1999 mw 7 6 taiwan chi chi earthquake hole b penetrated the chelungpu fault and core samples were recovered from between 948 42 and 1352 60 m depth three major zones designated fzb1136 fault zone at 1136 m depth in hole b fzb1194 and fzb1243 were recognized in the core samples as active fault zones within the chelungpu fault nondestructive continuous physical property measurements conducted on all core samples revealed that the three major fault zones were characterized by low gamma ray attenuation gra densities and high magnetic susceptibilities extensive fracturing and cracks within the fault zones and or loss of atoms with high atomic number but not a measurement artifact might have caused the low gra densities whereas the high magnetic susceptibility values might have resulted from the formation of magnetic minerals from paramagnetic minerals by frictional heating minor fault zones were characterized by low gra densities and no change in magnetic susceptibility and the latter may indicate that these minor zones experienced relatively low frictional heating magnetic susceptibility in a fault zone may be key to the determination that frictional heating occurred during an earthquake on the fault

thirty five year creep rates for the creeping segment of the san andreas fault and the effects of the 2004 parkfield earthquake constraints from alignment arrays continuous global positioning system and creepmeters we present results from differential global positioning system gps surveys of seven alignment arrays and four continuous gps sites along the creeping segment of the san andreas fault surveys of four alignment arrays from the central creeping segment yield 33 to 36 year average minimum slip rates of 21 26 mm yr these rates are consistent with previous alignment array surveys spanning a 10 year period and with rates determined by creepmeters indicating approximate steady state creep along the central creeping segment for at least 35 years motion between continuous gps sites that span the central creeping segment is 28 2 0 5 mm yr for two sites that are 1 km apart and 33 6 1 mm yr for two sites that are 70 km apart slip rates therefore increase with distance from the creeping segment of the san andreas fault all rates reported here are significantly slower than the 39 2 mm yr rate predicted for motion between the sierra nevada great valley block and the pacific plate repeat surveys of three alignment arrays following the 2004 parkfield earthquake demonstrate that its coseismic and short term postseismic offsets decrease rapidly with distance from the epicenter from 150 mm to 15 mm to 5 mm at respective distances of 9 36 and 54 km to the northwest continuous gps data confirm that little coseismic and postseismic slip occurred along the central creeping segment geodetic and geologic slip rates are compared and different models for the accommodation of transcurrent deformation across the creeping segment are discussed

acid sulfate alteration of fluorapatite basaltic glass and olivine by hydrothermal vapors and fluids implications for fumarolic activity and secondary phosphate phases in sulfate rich paso robles soil at gusev crater mars 1 phosphate rich rocks and a nearby phosphate rich soil paso robles were analyzed in gusev crater mars by the mars exploration rover spirit and interpreted to be highly altered possibly by hydrothermal or fumarolic alteration of primary phosphate rich material to test mineral phases resulting from such alteration we performed hydrothermal acid vapor and acid fluid experiments on olivine ol fluorapatite ap and basaltic glass gl as single phases and a mixture of phases minerals formed include ca al fe and mg sulfates with different hydration states anhydrite bassanite gypsum alunogen hexahydrite and pentahydrite phosphate bearing minerals formed included monocalcium phosphate monohydrate mcp acid vapor and acid fluid alteration of fluorapatite only and ferrian giniite acid fluid alteration of the ol gl ap mixture mcp is likely present in paso robles if primary ca phosphate minerals reacted with sulfuric acid with little transport of phosphate under fluid rock ratios allowing transport of phosphate a ferric phosphate phase such as ferrian giniite might form instead mossbauer measurements of ferrian giniite bearing alteration products and synthetic ferrian giniite are consistent with spirit s mossbauer measurements of the ferric bearing phase in paso robes soil but are also consistent with ferric sulfate phases in the low p soil arad samra therefore mossbauer data alone do not constrain the fluid rock ratio however the excess iron hematite in paso robles soil which implies aqueous transport combined with our laboratory experiments suggest acid sulfate alteration in a hydrothermal fumarolic environment at fluid rock ratios sufficient to allow dissolution transport and precipitation of secondary chemical components including a ferric phosphate such as ferrian giniite

mass removal and clay mineral dehydration rehydration in carbonate rich surface exposures of the 2008 wenchuan earthquake fault geochemical evidence and implications for fault zone evolution and coseismic slip 1 we report variations in the mineralogical geochemical and isotopic 13c 18o composition of fault rocks sampled in transects across the zhaojiagou and pingxi exposures of the wenchuan earthquake or longmenshan fault zone where the gouge rich fault core and principal slip surface cuts through carbonate rich strata pervasive fluid infiltration was found to modify the mineralogical and geochemical architecture of the fault zones studied enrichment depletion patterns element partitioning and a very large implied volume loss are quite different from those characterizing faults in granites and clastic sedimentary rocks and can be explained by a mass removal model involving dissolution and advective transport enhanced by pressure solution an increasing enrichment in smectite observed toward the principal slip surface a high abundance of elements such as ba mg and f the deposition of minerals such as barite and fluorapatite as well as the distinct depletion in 13c in vein material consistently suggest reactions involving a hydrothermal fluid originating at depth illitization of black gouges caused by coseismic frictional heating was found to be widespread we propose that coseismic frictional heating along with the action of postseismic hydrothermal fluids controlled the transformation and distribution of smectite and illite within the fault core of the longmenshan fault zone the coseismic dewatering reactions are expected to have been more extensive at depth possibly helping generate excess pore pressure assisting dynamic slip weakening during the wenchuan earthquake

hydraulic properties of a low permeable rupture zone on the yingxiu beichuan fault activated during the wenchuan earthquake china implications for fluid conduction fault sealing and dynamic weakening mechanisms abstract fluid transport properties of fault rocks are crucial parameters that affect earthquake nucleation and rupture propagation in this study we examined the internal structure mineral composition and fluid transport properties of fault rocks collected from two shallow boreholes penetrating a granitic rupture zone on the yingxiu beichuan fault ybf that was activated during the 2008 wenchuan earthquake fluid transport properties were measured using water as pore fluid at effective pressures p e ranging from 10 mpa to 165 mpa permeabilities of fault rocks exhibit a wide variation from 2 1 10 22 m 2 to 4 6 10 17 m 2 strongly depending on rock types and overburden pressure specifically at p e of 165 mpa the damage zone samples have permeabilities from 5 0 10 21 m 2 to 1 2 10 17 m 2 and the fault gouges are between 2 1 10 22 m 2 and 3 1 10 19 m 2 thus the ybf consists of a low permeability fault core acting as fluid barrier and surrounding high permeability damage zones acting as fluid conduits combining the structural and compositional results and transport data together we propose that the interplay between cataclasis and fluid rock interactions controls the hydraulic properties and their response to the fault zone evolution it is noteworthy that we measured extremely low permeabilities but high porosities and high specific storages for the gouges the cemented cataclasites which are inferred to be equivalent to the rocks in which the wenchuan earthquake nucleated also have low permeabilities suggesting the fault zone is a potential area for fluid storage and capable of generating high pore pressure at depths according to our laboratory data we found fluid pressurization could occur at depths below 2 7 km we suggest thermal pressurization has played an important role in causing the dynamic weakening of the wenchuan earthquake

paleoclimate of the past 4000 years at lake turkana kenya based on the isotopic composition of authigenic calcite abstract authigenic calcite was analyzed for 13 c and 18 o in a 12 m long core from lake turkana that spans the last 4200 yr the 18 o record is interpreted to reflect lake level history and indicates high stands prior to 4000 yr b p from 1800 to 2000 yr b p and within the last century this coincides better with the history of lake level change reported for other lakes in africa than did the previous lake level curve for lake turkana based on 14 c dates of exposed shoreline deposits the 13 c curve roughly parallels the 18 o curve and may reflect the influence of the omo river plume both of the isotope records are strongly cyclic with periodicites common to both at 80 120 150 and 225 yr other somewhat longer periodicites are nearly equal in the two records and there is close agreement with previously published cycles based on carbonate and lamination thickness analyses in the same turkana core

low frictional strength of quartz rocks at subseismic slip rates 1 laboratory experiments on rocks at low sliding velocity typically yield values of the friction coefficient of 0 6 0 85 here we demonstrate that an extraordinary decrease in friction coefficient accompanies sliding of room dry quartz rocks at rates faster than in most laboratory experiments but slower than seismic slip rates in some cases the friction coefficient decreases from low speed values by more than a factor of 3 this extraordinary weakening likely results from the formation of finely comminuted amorphous wet material on the sliding surface

clay clast aggregates in fault gouge an unequivocal indicator of seismic faulting at shallow depths abstract a common problem encountered in studies of gouge bearing natural faults is the difficulty of ascertaining whether the observed gouge was sheared seismically or aseismically this problem arises because of the scarcity of indicators of fault slip rates for gouge recently clay clast aggregates ccas a cca comprises a clastic core mantled by a rim of ultrafine particles were proposed as a possible indicator of seismic slip in gouge on the basis of shear experiments on gouge at seismic slip rates to examine the processes and conditions of cca formation we conducted rotary shear experiments on quartz and quartz bentonite gouges under normal stresses 0 3 3 0 mpa and slip rates 0 0005 1 3 m s 1 and in both room humidity room dry and water saturated wet conditions we found that ccas could be produced in room dry gouges even at the lowest slip rates which are considerably slower than actual seismic slip rates this finding demonstrates that thermal pressurization and fluidization at elevated temperature during seismic slip are not necessarily needed for the formation of ccas contrary to previous views given the occurrence of ccas over a wide range of slip rates we suggest that the presence of ccas is not an unequivocal indicator of fault slip at seismic slip rates

microstructures and their implications for faulting processes insights from dglab core samples from the gulf of corinth abstract we have examined microstructures mineralogical composition geochemical alteration and texture of four selected fault rock samples from the deep geodynamical laboratory dglab gulf of corinth project using optical microscopy cathodoluminescence microscopy cl scanning electron microscopy sem transmission electron microscopy tem and synchrotron x ray diffraction measurements the fault core is composed of red and gray clayey gouge material and surrounded by a damage zone of brecciated limestones pressure solution features calcite veins and calcite clasts in the breccia and gouge material attest the presence of paleo fluids and fluid driven mass transfer during deformation differences in cl colors between the matrix and calcite vein cement and inside the vein cement suggest repeated infiltration of fluids with different composition from various sources formation water and meteoric water twin lamellae densities estimated in calcite veins are used as paleo piezometer the deduced differential stress is 140 70 mpa for the older vein generation and appears to be higher than stress for the youngest veins 45 23 mpa in spite of the relatively small clay content in both samples newly formed clay minerals have been observed in gray as well as red clayey gouge material differences between gray and red clay gouge material are found in fault rock composition porosity and clay fabric the proportion of chlorite in the red gouge is significantly less than that in the gray gouge whereas the initial porosity is significantly higher than in the gray gouge material the detection of a well oriented clay fabric in red clay gouge samples is unique in comparison to other major fault zones

frictional melting of clayey gouge during seismic fault slip experimental observation and implications clayey gouges are common in fault slip zones at shallow depths thus the fault zone processes and frictional behaviors of the gouges are critical to understanding seismic slip at these depths we conducted rotary shear tests on clayey gouge 41 wt clay minerals at a seismic slip rate of 1 3 m s here we report that the gouge was melted at 5 mpa of normal stress and room humidity conditions the initial local melting was followed by melt layer formation clay minerals e g smectite and illite and plagioclase were melted and quenched to glass with numerous vesicles both flash heating and bulk temperature increases appear to be responsible for the melting this observation of clayey gouge melting is comparable to that of natural faults e g chelungpu fault taiwan due to heterogeneous fault zone properties e g permeability frictional melting may be one of the important processes in clayey slip zones at shallow depths

coseismic fluid rock interactions at high temperatures in the chelungpu fault chemical and isotopic variations in crushed material along the chelungpu fault in taiwan are suggestive of interactions at high temperatures with coseismically generated aqueous fluids high fluid pressures along the fault zone during the magnitude 7 6 chi chi earthquake in 1999 could have reduced friction and fault strength aqueous fluids are thought to have an essential role in faulting and the dynamic propagation of earthquake rupture fluid overpressure can affect earthquake nucleation1 2 and in a process termed thermal pressurization pore fluid pressure produced by frictional heating can reduce the effective normal stress acting on the fault surface3 4 5 this may lead to a marked reduction in fault strength during slip however the coseismic presence of fluids within slip zones and the role of fluids in dynamic fault weakening is still a matter of debate here we present compositions of major and trace elements as well as isotope ratios of core samples representing relatively undamaged as well as very fine grained deformed material from three active zones of the chelungpu fault taiwan depth profiles across the most intensely sheared bands that range in thickness from 2 15 cm exhibit sharp compositional peaks of fluid mobile elements and of strontium isotopes we suggest that high temperature fluids 350 c derived from heating of sediment pore fluids during the earthquake interacted with material within the fault zone and mobilized the elements the coseismic presence of high temperature fluids under conditions of low hydraulic diffusivity6 within the fault zone is favourable for thermal pressurization this effect may have caused a dynamic decrease of friction along the chelungpu fault during the 1999 magnitude 7 6 chi chi earthquake

correlation of clayey gouge in a surface exposure of serpentinite in the san andreas fault with gouge from the san andreas fault observatory at depth safod abstract magnesium rich clayey gouge similar to that comprising the two actively creeping strands of the san andreas fault in drill core from the san andreas fault observatory at depth safod has been identified in a nearby outcrop of serpentinite within the fault zone at nelson creek each occurrence of the gouge consists of porphyroclasts of serpentinite and sedimentary rocks dispersed in a fine grained foliated matrix of mg rich smectitic clays the clay minerals in all three gouges are interpreted to be the product of fluid assisted shear enhanced reactions between quartzofeldspathic wall rocks and serpentinite that was tectonically entrained in the fault from a source in the coast range ophiolite we infer that the gouge at nelson creek connects to one or both of the gouge zones in the safod core and that similar gouge may occur at depths in between the special significance of the outcrop is that it preserves the early stages of mineral reactions that are greatly advanced at depth and it confirms the involvement of serpentinite and the mg rich phyllosilicate minerals that replace it in promoting creep along the central san andreas fault

strong coherence between solar variability and the monsoon in oman between 9 and 6 kyr ago variations in the amount of solar radiation reaching the earth are thought to influence climate but the extent of this influence on timescales of millennia to decades is unclear a number of climate records show correlations between solar cycles and climate1 but the absolute changes in solar intensity over the range of decades to millennia are small2 and the influence of solar flux on climate is not well established the formation of stalagmites in northern oman has recorded past northward shifts of the intertropical convergence zone3 whose northward migration stops near the southern shoreline of arabia in the present climate4 here we present a high resolution record of oxygen isotope variations for the period from 9 6 to 6 1 kyr before present in a th u dated stalagmite from oman the 18o record from the stalagmite which serves as a proxy for variations in the tropical circulation and monsoon rainfall allows us to make a direct comparison of the 18o record with the 14c record from tree rings5 which largely reflects changes in solar activity6 7 the excellent correlation between the two records suggests that one of the primary controls on centennial to decadal scale changes in tropical rainfall and monsoon intensity during this time are variations in solar radiation

nanocoatings of clay and creep of the san andreas fault at parkfield california mudrock samples were investigated from two fault zones at 3066 m and 3296 m measured depth md located outside and within the main damage zone of the san andreas fault observatory at depth safod drillhole at parkfield california all studied fault rocks show features typical of those reported across creep zones with variably spaced and interconnected networks of polished displacement surfaces coated by abundant polished films and occasional striations electron microscopy and x ray diffraction study of the surfaces reveal the occurrence of neocrystallized thin film clay coatings containing illite smectite i s and chlorite smectite c s minerals 40 ar 39 ar dating of the illitic mix layered coatings demonstrated miocene to pliocene crystallization and revealed an older fault strand 8 1 3 ma at 3066 m md and a probably younger fault strand 4 4 9 ma at 3296 m md today the younger strand is the site of active creep behavior reflecting a possible re activation of these clay weakened zones we propose that the majority of slow fault creep is controlled by the high density of thin

temperature estimates of coseismic heating in clay rich fault gouges the chelungpu fault zones taiwan abstract to investigate the coseismic frictional temperature in seismogenic fault zones we examine the characteristics of clays in the chelungpu fault zones with isothermal heating experiments scanning electron microscope coupled to an energy dispersive spectrometer sem edx and thermogravimetry analysis tga in the tcdp case taiwan chelungpu fault drilling project three fault zones of the chelungpu fault system were identified at the depth of 1111 m 1153 m and 1222 m described as fz1111 fz1153 and fz1222 hereafter respectively the clay mineral assemblages of fz1111 show evidence of melting and the temperature in a 2 cm band within the black gouge zone is estimated to be from 900 c to 1100 c by comparing the sem images of in situ natural samples with those of heated materials and the finding of no recrystallization of kaolinite amorphous aluminosilicates spinel in the fault samples the clay mineral assemblages of fz1153 suggested that kaolinite has been broken down by the thermal decomposition dehydroxylation but chlorite has not the clay characteristics and results of sem edx and tga constrain the faulting temperature from 500 c to 900 c with a spatial distribution up to 1 3 m the clay characteristics of fz1222 indicated that clays were changed by experiencing high temperature acid fluids instead of thermal decomposition dehydroxylation processes and that the temperature is localized in 2 cm and ranges from 350 c to 500 c the lowest temperature among three fault zones the estimates of temperature ranges and thermal anomaly intervals among three fault zones provide important information and constraints on the physical and chemical processes coseismic dynamic weakening mechanism and earthquake energy budget in the future

wet phases in the sahara sahel region and human migration patterns in north africa the carbon isotopic composition of individual plant leaf waxes a proxy for c3 vs c4 vegetation in a marine sediment core collected from beneath the plume of sahara derived dust in northwest africa reveals three periods during the past 192 000 years when the central sahara sahel contained c3 plants likely trees indicating substantially wetter conditions than at present our data suggest that variability in the strength of atlantic meridional overturning circulation amoc is a main control on vegetation distribution in central north africa and we note expansions of c3 vegetation during the african humid period early holocene and within marine isotope stage mis 3 50 45 ka and mis 5 120 110 ka the wet periods within mis 3 and 5 coincide with major human migration events out of sub saharan africa our results thus suggest that changes in amoc influenced north african climate and at times contributed to amenable conditions in the central sahara sahel allowing humans to cross this otherwise inhospitable region

mechanical twinning in quartz shock experiments impact pseudotachylites and fault breccias abstract increasing use of diffraction methods to study preferred orientation of minerals has established that quartz in deformed rocks not only displays characteristic c axis orientation patterns but that there is also generally a distinct difference in the orientation of positive and negative rhombs in the trigonal quartz crystal structure positive and negative rhombs are structurally different and particularly negative rhombs e g 01 1 1 are much stiffer than positive rhombs e g 10 1 1 here we focus on the role of mechanical dauphine twinning under stress as a cause of this difference and illustrate with ebsd measurements ubiquitous twinning in quartz bearing rocks subjected to high stresses characteristic twinning is observed in experimentally shocked sandstones and stishovite bearing quartzites from the vredefort meteorite impact site in south africa similar twinning is documented for quartz associated with pseudotachylites from the santa rosa mylonite zone in southern california whereas quartz in underlying ductile mylonites are more or less twin free it suggests that twinning was produced by local seismic stresses that caused fracture and frictional melting on fault surfaces quartz bearing breccias from the safod san andreas fault observatory at depth drilling project also show evidence of twinning and suggest high seismic stresses in the currently creeping segment of the san andreas fault at parkfield from these observations it appears that dauphine twin microstructures can be diagnostic of high local and transient stresses

evidence of frictional melting from disk shaped black material discovered within the taiwan chelungpu fault system 1 the taiwan chelungpu fault drilling project penetrated three fault zones as the chelungpu fault system which slipped during the 1999 chi chi earthquake discovering disk shaped black material bm disk within the middle and lower fault zones in hole b the microscopic features of the bm disks indicated that they were pseudotachylytes and they showed high magnetic susceptibility possibly the result of intense shearing or high temperature conditions inorganic carbon content of the bm disks was low possibly because of thermal decomposition of carbonate minerals the high temperatures might be related to frictional heating during the earthquake implying that the bm disks were produced under intense shearing with frictional heating that reached melting temperature because the disks which provide the only evidence of melting pre date the 1999 earthquake we concluded that frictional melting did not occur during the earthquake

ultrafine spherical quartz formation during seismic fault slip natural and experimental evidence and its implications abstract in recent works on the determination of pseudotachylyte within the principal slip zone psz of the chelungpu fault taiwan we demonstrated that frictional melting occurred at shallow depths during the 1999 mw 7 6 chi chi earthquake thus the characteristics of melts are of paramount importance to investigate processes controlling dynamic fault mechanics during seismic slips we conducted rock friction experiments on siltstone recovered from the taiwan chelungpu fault drilling project tcdp at a slip rate of 1 3 m s and a normal stress of 1 mpa here we not only target to characterize experimental pseudotachylyte and evaluate the associated frictional behavior but also compare it with natural frictional melts of tcdp our results show that 1 initial shear stress drop was related to the generation of low viscosity melt patches 2 the evolution of shear stress in the postmelting regime was congruent with frictional melt rheology and 3 the slip strengthening presumably resulted from the extremely low content of water of the frictional melt in particular the state of art of in situ synchrotron analyses x ray diffraction and transmission x ray microscope determine the presence of ultrafine spherical quartz usq grains 10 nm to 50 nm in the glassy matrices presumably produced at high temperature our observations confirm that the usq grains formed in rock friction experiments do occur in natural faults we surmise the usq is the result of frictional melting on siltstone and represents the latest slip zones of the chelungpu fault and further infer that the viscous melts may terminate seismic slips at shallow crustal conditions

microstructures in landslides in northwest china implications for creeping displacements abstract microstructures mineralogical composition and texture of selected landslide samples from three landslides in the southern part of the gansu province china were examined with optical microscopy transmission electron microscopy tem x ray diffraction xrd and synchrotron x ray diffraction measurements common sheet silicates are chlorite illite muscovite kaolinite pyrophyllite and dickite other minerals are quartz calcite dolomite and albite in one sample graphite and amorphous carbon were detected by tem edx analyses and tem high angle annular dark field images the occurrence of graphite and pyrophyllite with very low friction coefficients in the gouge material of the suoertou and xieliupo landslides is particularly significant for reducing the frictional strength of the landslides it is proposed that the landslides underwent comparable deformation processes as fault zones the low friction coefficients provide strong evidence that slow moving landsliding is controlled by the presence of weak minerals in addition tem observations document that grain size reduction in clayey slip zone material was produced mainly by mechanical abrasion for calcite and quartz grain size reduction was attributed to both pressure solution and cataclasis therefore besides landslide composition the occurrence of ultrafine grained slip zone material may also contribute to weakening processes of landslides tem images of slip zone samples show both locally aligned clay particles as well as kinked and folded sheet silicates which are widely disseminated in the whole matrix small newly formed clay particles have random orientations based on synchrotron x ray diffraction measurements the degree of preferred orientation of constituent sheet silicates in local shear zones of the suoertou and duang he ba landslide is strong this work is the first reported observation of well oriented clay fabrics in landslides

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

evidence of transient increases of fluid pressure in safod phase iii cores 1 the san andreas fault observatory at depth safod in parkfield central california has been drilled through a fault segment that is actively deforming through creep and microearthquakes creeping is accommodated in two fault strands the southwest and central deforming zones embedded within a damaged zone of deformed shale and siltstone during drilling no pressurized fluids have been encountered even though the fault zone acts as a permeability barrier to fluid circulation between the north american and pacific plates microstructural analysis of sheared shales associated with calcite and anhydrite bearing veins found in safod cores collected at 1 5m from the southwest deforming zone suggests that transient increases of pore fluid pressure have occurred during the fault activity causing mode i fracturing of the rocks such build ups in fluid pressure may be related to permeability reduction during fault creep and pressure solution processes resulting in localized failure of small fault zone patches and providing a potential mechanism for the initiation of some of the microearthquakes registered in the safod site

frictional strengths of talc serpentine and talc quartz mixtures 1 talc is a constituent of faults in a variety of settings and it may be an effective weakening agent depending on its abundance and distribution within a fault we conducted frictional strength experiments under hydrothermal conditions to determine the effect of talc on the strengths of synthetic gouges of lizardite and antigorite serpentinites and of quartz small amounts of talc weaken serpentinite gouges substantially more than predicted by simple weight averaging in comparison mixtures of quartz and talc show a linear trend of strength reduction at talc concentrations 15 wt and enhanced weakening at higher concentrations all of the strength data are fit by a modified version of the reuss mixing law that allows for the dominance of one mineral over the other the difference in the behavior of serpentinite talc and quartz talc mixtures at low talc concentrations is a reflection of their different textures lizardite antigorite and talc all have platy habits and displacement within gouges composed of these minerals is localized to narrow shears along which the platy grains have rotated into alignment with the shear surfaces the shears in the mixed phyllosilicate gouges maximize the proportion of the weaker mineral within them when mixed with a strong rounded mineral such as quartz some minimum concentration of talc is needed to form connected pathways that enhance strength reductions the typical development of talc by the reaction of si rich fluids with serpentinite or dolomite would tend to localize its occurrence in a natural fault and result in enhanced weakening

nano crystal formation of tio2 polymorphs brookite and anatase due to organic inorganic rock fluid interactions the occurrence of the titania polymorphs brookite and anatase as nano crystals in organic matter rich sediments of differing age and thermal maturity has been investigated by means of a multidisciplinary analytical approach fib tem organic geochemistry and petrography it was the aim of the study to analyze the formation mechanisms fate and behavior of the titania nano crystals as a result of organic inorganic rock fluid interactions brookite nano crystals have been detected in immature mediterranean sapropels of quaternary age but anatase also occurs in deeper and older black shales furongian alum shale sweden devonian to carboniferous bakken shale williston basin usa whereas anatase prevails as single crystals brookite nano crystals often are agglomerated single brookite nano crystals from posidonia shale lower jurassic northern germany have increasing crystal diameters with increasing thermal maturity exclusively anatase nano crystals both as single crystals or as agglomerates have been detected at oil water contacts in oilfields and along fractures with fluid flow enriched in dissolved organic carbon titania nano crystal precipitation growth and agglomeration takes place in the pore water of micro environments at low to high temperatures and where low ph is coupled to the occurrence of dissolved organic components low sedimentation rates preserving a critical geochemical environment or higher temperatures seem major controls for the precipitation of anatase and its tendency not to agglomerate

middle stone age msa site distributions in eastern africa and their relationship to quaternary environmental change refugia and the evolution of homo sapiens this paper considers the evolution of homo sapiens in eastern africa in relation to refugia and bottlenecks around 200 ka bp at a macro scale middle stone age msa lithics site distributions and locations are analysed in relation to palaeovegetation maps of the last glacial interglacial cycle which are used as a proxy for earlier climate cycles a push and pull model is then postulated for the spread of homo sapiens out of refugia in eastern africa involving both volcanism push and habitat availability pull a date within ois 5 is suggested for this expansion to other parts of the continent and potentially further afield contrary to a frequently proposed expansion within ois 3

nanoscale porosity in safod core samples san andreas fault article i nfo with transmission electron microscopy tem we observed nanometer sized pores in four ultracataclastic and fractured core samples recovered from different depths of the main bore hole of the san andreas fault observatory at depth safod cutting of foils with a focused ion beam technique fib allowed identifying porosity down to the nm scale between 40 and 50 of all pores could be identified as in situ pores without any damage related to sample preparation the total porosity estimated from tem micrographs 1 5 is comparable to the connected fault rock porosity 2 8 6 7 estimated by pressure induced injection of mercury permeability estimates for cataclastic fault rocks are 10 21 10 19 m 2 and 10 17 m 2 for the fractured fault rock porosity and permeability are independent of sample depth tem images reveal that the porosity is intimately linked to fault rock composition and associated with deformation the tem estimated porosity of the samples increases with increasing clay content the highest porosity was estimated in the vicinity of an active fault trace the largest pores with an equivalent radiusn 200 nm occur around large quartz and feldspar grains or grain fragments while the smallest pores equivalent radiusb 50 nm are typically observed in the extremely fine grained matrix grain sizeb 1 m based on pore morphology we distinguish different pore types varying with fault rock fabric and alteration the pores were probably filled with formation water and or hydrothermal fluids at elevated pore fluid pressure preventing pore collapse the pore geometry derived from tem observations and bet brunauer emmett and teller gas adsorption desorption hysteresis curves indicates pore blocking effects in the fine grained matrix observations of isolated pores in tem micrographs and high pore body to pore throat ratios inferred from mercury injection suggest elevated pore fluid pressure in the low permeability cataclasites reducing shear strength of the fault 2010 elsevier b v all rights reserved

analyses of pseudotachylyte from hole b of taiwan chelungpu fault drilling project tcdp their implications for seismic slip behaviors during the 1999 chi chi earthquake abstract the seismic slip behavior during the 1999 chi chi taiwan earthquake m w 7 6 was contrastive between the northern and southern segments of the activated chelungpu fault large fast and smooth slips with large stress drop in the north while smaller slower and irregular slips with smaller stress drop in the south we analyzed the pseudotachylyte samples recovered from 1194 m 1243 m and 1314 m depths of hole b of taiwan chelungpu fault drilling project tcdp to reveal the spatial difference in friction mechanism all pseudotachylyte layers are thin 0 7 2 8 cm the volume fraction of protoliths is very large more than 63 and the estimated temperature distribution is very heterogeneous from ca 750 1750 c these observations suggest that these pseudotachylyte melts were in the partial melting regime of montgomery montgomery r s 1976 friction and wear at high sliding speeds wear 36 275 298 where friction coefficient is abnormally large similar pseudotachylyte was found already in the core sample from 175 m depth of the nanto borehole penetrating the southern fault since both pseudotachylyte samples from the two boreholes are older than the 1999 chi chi event and have been uplifted from depths farther down dip of their current locations it is likely that recent seismic ruptures also would have encountered these mechanical barriers of viscous melt patches at deeper parts in the north than in the south elastohydrodynamic lubrication of clayey gouge worked effectively at the shallower parts of the northern segment however there is no evidence that it played an important role in the south these differences are the plausible causes of the contrastive local slip behaviors during the chi chi earthquake

comparative mineral chemistry and textures of safod fault gouge and damage zone rocks abstract creep in the san andreas fault observatory at depth safod drillhole is localized to two foliated gouges the central deforming zone cdz and southwest deforming zone sdz the gouges consist of porphyroclasts of serpentinite and sedimentary rock dispersed in a foliated matrix of mg smectite clays that formed as a result of shearing enhanced reactions between the serpentinite and quartzofeldspathic rocks the cdz takes up most of the creep and exhibits differences in mineralogy and texture from the sdz that are attributable to its higher shearing rate in addition a 0 2 m wide sector of the cdz at its northeastern margin ne cdz is identical to the sdz and may represent a gradient in creep rate across the cdz the sdz and ne cdz have lower clay contents and larger porphyroclasts than most of the cdz and they contain veinlets and strain fringes of calcite in the gouge matrix not seen elsewhere in the cdz matrix clays in the sdz and ne cdz are saponite and corrensite whereas the rest of the cdz lacks corrensite saponite is younger than corrensite reflecting clay crystallization under declining temperatures and clays in the more actively deforming portions of the cdz have better equilibrated to the lower temperature conditions

fluid rock interactions in seismic faults implications from the structures and mineralogical and geochemical compositions of drilling cores from the rupture of the 2008 wenchuan earthquake china we describe the structural features and mineralogical and geochemical compositions of the fault rocks recovered from boreholes at the golden river site on the yingxiu beichuan fault which activated and slipped along a 240 km long main surface rupture zone during the 2008 wenchuan earthquake the fault which accommodated co seismic slip cuts granitic rocks from the pengguan complex in which this earthquake most likely nucleated fault rocks including cohesive cataclasite unconsolidated breccia and three fault gouges with distinct colors were identified from the drilling cores on going uplift and erosion in the area means that the fault rocks formed at different depth were exhumed to the shallow surface during the uplift history of the longmenshan fault zone a clear change from fracturing and comminution in the cataclasites and breccia to more pervasive shear formation of fine grained materials in the gouges has been observed the gouges are distinct and have accommodated significant displacement in multiple increments of shear furthermore fault rocks recovered from the boreholes display numerous features indicative of fluid infiltration and fluid rock interaction toward the fault core clay minerals have replaced feldspars the element enrichment depletion patterns of the fault rocks show general fluid infiltration trends such as 1 mobile elements are generally depleted in the fault rocks 2 the microstructural mineralogical and geochemical results of the fault rocks consistently indicate that pervasive fluid infiltration and fluid rock interactions altered feldspars and mafic minerals to clay minerals the fluid was mg2 and fe2 rich facilitating formation of chlorite isocon analyses further reveal that a large rock volume has been lost which is attributed to the removal of mobile elements associated with fluid infiltration and perhaps enhanced by pressure solution these results reflect the accumulated effects of cataclasis and fluid infiltration fluid rock interaction during the long evolution history of the longmenshan fault zone fluid rock interactions in seismic faults implications from the structures and mineralogical and geochemical compositions of drilling cores from the rupture of the 2008 wenchuan earthquake china

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

clay clast aggregates in gouges new textural evidence for seismic faulting 1 spherical aggregates named clay clast aggregates ccas have been reported from recent investigations on retrieved clay bearing fault gouges from shallow depth seismogenic faults and rotary shear experiments conducted on clay bearing gouge at seismic slip rates the formation of ccas appears to be related to the shearing of a smectite rich granular material that expands and becomes fluidized we have conducted additional high velocity rotary shear experiments and low velocity double shear experiments we demonstrate that a critical temperature depending on dynamic pressure temperature conditions is needed for the formation of ccas this temperature corresponds to the phase transition of pore water from liquid to vapor or to critical which induced gouge pore fluid expansion and therefore a thermal pressurization of the fault a detailed examination by energy dispersive x ray spectrometry edx sem element mapping sem and transmission electron microscopy tem shows strong similar characteristics of experimental and natural ccas with a concentric well organized fabric of the cortex and reveals that their development may result from the combination of electrostatic and capillary forces in a critical reactive medium during the dynamic slip weakening accordingly the occurrence of ccas in natural clay rich fault gouges constitutes new unequivocal textural evidence for shallow depth thermal pressurization and consequently for past seismic faulting

talc bearing serpentinite and the creeping section of the san andreas fault the section of the san andreas fault located between cholame valley and san juan bautista in central california creeps at a rate as high as 28 mm yr 1 ref 1 and it is also the segment that yields the best evidence for being a weak fault embedded in a strong crust serpentinized ultramafic rocks have been associated with creeping faults in central and northern california and serpentinite is commonly invoked as the cause of the creep and the low strength of this section of the san andreas fault however the frictional strengths of serpentine minerals are too high to satisfy the limitations on fault strength and these minerals also have the potential for unstable slip under some conditions here we report the discovery of talc in cuttings of serpentinite collected from the probable active trace of the san andreas fault that was intersected during drilling of the san andreas fault observatory at depth safod main hole in 2005 we infer that the talc is forming as a result of the reaction of serpentine minerals with silica saturated hydrothermal fluids that migrate up the fault zone and the talc commonly occurs in sheared serpentinite this discovery is significant as the frictional strength of talc at elevated temperatures is sufficiently low to meet the constraints on the shear strength of the fault and its inherently stable sliding behaviour is consistent with fault creep talc may therefore provide the connection between serpentinite and creep in the san andreas fault if shear at depth can become localized along a talc rich principal slip surface within serpentinite entrained in the fault zone

faulting processes in active faults evidences from tcdp and safod drill core samples abstract the microstructures mineralogy and chemistry of representative samples collected from the cores of the san andreas fault drill hole safod and the taiwan chelungpu fault drilling project tcdp have been studied using optical microscopy tem sem xrd and xrf analyses safod samples provide a transect across undeformed host rock the fault damage zone and currently active deforming zones of the san andreas fault tcdp samples are retrieved from the principal slip zone psz and from the surrounding damage zone of the chelungpu fault substantial differences exist in the clay mineralogy of safod and tcdp fault gouge samples amorphous material has been observed in safod as well as tcdp samples in line with previous publications we propose that melt observed in tcdp black gouge samples was produced by seismic slip melt origin whereas amorphous material in safod samples was formed by comminution of grains crush origin rather than by melting dauphine twins in quartz grains of safod and tcdp samples may indicate high seismic stress the differences in the crystallographic preferred orientation of calcite between safod and tcdp samples are significant microstructures resulting from dissolution precipitation processes were observed in both faults but are more frequently found in safod samples than in tcdp fault rocks as already described for many other fault zones clay gouge fabrics are quite weak in safod and tcdp samples clay clast aggregates ccas proposed to indicate frictional heating and thermal pressurization occur in material taken from the psz of the chelungpu fault as well as within and outside of the safod deforming zones indicating that these microstructures were formed over a wide range of slip rates

a vertical exposure of the 1999 surface rupture of the chelungpu fault at wufeng western taiwan structural and paleoseismic implications for an active thrust fault we mapped and analyzed two vertical exposures exposed on the walls of a 3 to 5 m deep 70 m long excavation and a smaller 3 m deep 10 m long excavation across the 1999 rupture of the chelungpu fault the primary exposure revealed a broad anticlinal fold with a 2 5 m high west facing principal thrust scarp contained in fluvial cobbly gravel beds and overlying fine grained overbank deposits sequential restoration of the principal rupture requires initial failure on the basal east dipping thrust plane followed by wedge thrusting and pop up of an overlying symmetrical anticline between two opposing secondary thrust faults net vertical offset is about 2 2 m across the principal fault zone from line length changes we estimate about 3 3 m of horizontal shortening normal to fault strike the ratio of these values yields a total slip of 4 0 m and an estimate of about 34 for the dip of the fault plane below the excavation this value is nearly the same as the 35 average dip of the fault plane from the surface to the hypocenter restoration of the exposed gravelly strata and adjacent overbank sediments deposited prior to the 1999 event around the principal rupture suggests the possible existence of a prior event a buried 30 m wide anticlinal warp beneath the uplifted crest of the 1999 event is associated with three buried reverse faults that we interpret as evidence for an earlier episode of folding and faulting in the site the prior event is also recorded in the smaller excavation which is located 40 m south and is oriented parallel to the larger excavation radiocarbon dating of samples within the exposed section did not place tight constraints on the date of the previous event available data are interpreted as indicating that the previous event occurred before the deposition of the less than 200 14 c yr b p overbank sands and after the deposition of the much older fluvial gravels we interpret the previous event as the penultimate event relative to the 1999 chi chi earthquake we estimated the long term slip rate of the chelungpu fault to be 10 15 mm yr during the last 1 ma based on previously published retrodeformable cross sections this rate is however significantly higher than geodetic rates of shortening across the chelungpu thrust where two pairs of permanent global positioning system stations suggest 7 10 mm yr of shortening across the fault given the 4 m of average slip the long term slip rate yields an interseismic interval of between 267 and 400 yr for the chelungpu fault

clay mineral anomalies in the fault zone of the chelungpu fault taiwan and their implications 1 the taiwan chelungpu fault drilling project tcdp hole a recovered continuous core samples across the rupture zone of the 1999 chi chi earthquake mw7 6 studying in situ chemical properties sequentially from fresh fault zone materials of the chelungpu fault provides insight into possible faulting mechanisms distinct anomalies of mineral assemblages at the 1111 m fault zone of tcdp hole a are found to be 1 a decrease in clay content within the primary slip zone psz and 2 a significant decline of illite disappearance of chlorite and kaolinite and spike in smectite within the psz meanwhile features relating to melting or amorphous material in the psz have been observed by sem and tem the results suggest that the psz might have experienced generation of glassy materials such as pseudotachylyte by the expense of clay minerals due to strong shear heating then prompt alteration of pseudotachylyte into smectite characteristics of clay minerals and images obtained from electronic microscopes in the psz thus imply that pseudotachylyte possibly developed during the 1999 chi chi earthquake but quickly altered into smectite this particular phenomenon may explain why pseudotachylyte is rarely found in exhumed hydrated fault zones

fault zone geology lessons from drilling through the nojima and chelungpu faults several drilling projects have been conducted through active faults with the aim of learning about the geology of the fault zones and tentatively correlating the structure and mineralogy of the fault zones with their seismological behaviour during recent earthquakes here we present the major results obtained from structural and mineralogical studies of core samples retrieved from the dextral reverse strike slip nojima fault japan within granitic rocks following the kobe earthquake 1995 and from the chelungpu thrust fault taiwan within alternating silts and shales following the chi chi earthquake 1999 we show how these projects despite not fullfilling all their objectives have still contributed to a better geological knowledge of the fault zones to a better characterization of the slip zones related to the recent earthquakes particularly of their thickness microstructures and deformation mechanisms and to a better understanding of the nature and role of fluids within the fault zone they have also led to new questions and to new approaches for studying fault rock samples for all of these reasons they have stimulated international scientific research into fault zone geology

coseismic thickness of principal slip zone from the taiwan chelungpu fault drilling project a tcdp a and correlated fracture energy abstract direct observations of the physical structures of the seismogenic zones of active faults are rare due to the difficulty in reaching the fault zone at depth current geological evidences mostly from the surface suggest that principal slip zone psz accommodated most shear displacement and was the place where physico chemical processes occurred during an individual coseismic event and the thickness of psz is a few millimeter to tens of centimeter wide however the actual thickness of psz of a large earthquake a key parameter of seismology in understanding energy dissipation and rupture processes remains largely unknown the chelungpu fault that ruptured during the 1999 mw 7 6 chi chi earthquake taiwan was drilled to a depth of 2003 m providing a unique opportunity to sample an active fault that slipped in a recent large earthquake the psz corresponding to the 1999 chi chi earthquake was well characterized within cores at a borehole depth of 1111 m from the taiwan chelungpu fault drilling project a tcdp a here we determine the interval of clay anomaly that resulted from frictional melting thermal decomposition process by state of art in situ synchrotron xrd analysis providing very high spatial resolution for mineralogy combined with the interval of the presence of vesicles from microstructural observation the thickness of chi chi psz is estimated to be 1 mm thus the correlated contribution of surface fracture energy to earthquake breakdown work at least in this locality is quantified to be 1 9 the huge remaining part of the breakdown work seems to be turned into heat associated with fault dynamic processes during the 1999 chi chi earthquake

on the origin of mixed layered clay minerals from the san andreas fault at 2 5 3 km vertical depth safod drillhole at parkfield california a detailed mineralogical study is presented of the matrix of mudrocks sampled from spot coring at three key locations along the san andreas fault observatory at depth safod drill hole the characteristics of authigenic illite smectite i s and chlorite smectite c s mixed layer mineral clays indicate a deep diagenetic origin a randomly ordered i s mineral with ca 20 25 smectite layers is one of the dominant authigenic clay species across the san andreas fault zone sampled at 3 066 and 3 436 m measured depths md whereas an authigenic illite with ca 2 5 smectite layers is the dominant phase beneath the fault sampled at 3 992 m md the most smectite rich mixed layered assemblage with the highest water content occurs in the actively deforming creep zone at ca 3 300 3 353 m true vertical depth of ca 2 7 km with i s 70 30 and c s 50 50 the matrix of all mudrock sam ples show extensive quartz and feldspar both plagioclase and k feldspar dissolution associated with the crystalli zation of pore filling clay minerals however the effect of rock deformation in the matrix appears only minor with weak flattening fabrics defined largely by kinked and fractured mica grains adopting available kinetic models for the crystallization of i s in burial sedimentary envi ronments and the current borehole depths and thermal structure the conditions and timing of i s growth can be evaluated assuming a typical k concentration of 100 200 ppm for sedimentary brines a present day geothermal gradient of 35c km and a borehole temperature of ca 112c for the sampled depths most of the i s minerals can be predicted to have formed over the last 4 11 ma and are probably still in equilibrium with circulating fluids the exception to this simple burial pattern is the occurrence of the mixed layered phases with higher smectite content than predicted by the burial model these minerals which characterize the actively creeping section of the fault and local thin film clay coating on polished brittle slip surfaces can be explained by the influence of either cooler fluids circulating along this segment of the fault or the flow of k depleted brines

elastic anisotropy modeling of kimmeridge shale 1 anisotropy of elastic properties in clay rich sedimentary rocks has been of long standing interest these rocks are cap rocks of oil and gas reservoirs as well as seals for carbon sequestration elasticity of shales has been approached by direct velocity measurements and by models based on microstructures here we are revisiting the classical kimmeridge shale studied by hornby 1998 by first quantifying microstructural features such as phase volume fractions grain shapes and grain orientations and pore distributions with advanced analytical methods and then using this information in different models to explain bulk elastic properties it is shown that by application of a self consistent algorithm based on eshelby s 1957 model of inclusions in a homogeneous medium it is possible to explain most experimental elastic constants though some discrepancies remain which may be due to the interpretation of experimental data using a differential effective medium approach an almost perfect agreement with experimental stiffness coefficients can be obtained though the physical basis of this method may be questionable the influence of single crystal elastic properties grain shapes preferred orientation and volume and shapes of pores on elastic properties of shale is explored

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

introduction to special section preparing for the san andreas fault observatory at depth 7209 seismology earthquake dynamics andmechanics 7230seismology seismicityandseismotectonics 8010structural geology fractures and faults citation hickman s m zoback and w ellsworth 2004 introduction to specialsection preparing for the san andreas fault observatory atdepth geophys res lett 31 l12s01 doi 10 1029 2004gl020688

low temperature deformation in calcite veins of safod core samples san andreas fault microstructural analysis and implications for fault rheology abstract the microstructures of four core samples from the san andreas fault observatory at depth safod were investigated with optical and transmission electron microscopy these samples consisting of sandstone siltstone and fault gouge from phase iii of the drilling campaign 3141 3307 m md show a complex composition of quartz feldspar clays and amorphous material microstructures indicate intense shearing and dissolution precipitation as main deformation processes the samples also contain abundant veins filled with calcite within the inspected veins the calcite grains exhibit different degrees of deformation with evidence for twinning and crystal plasticity dislocation densities ranging from 3 1012 m 2 to 3 1013 m 2 and twin line densities 22 mm 1 165 mm 1 are used as paleo piezometers the corresponding estimates of differential stresses vary between 33 and 132 mpa deduced from dislocation density and 92 251 mpa obtained from twin density possibly reflecting chronologically different maximum stress states and or grain scale stress perturbations mean values of stress estimates are 68 46 mpa and 168 60 mpa respectively where estimates from dislocation density may represent a lower bound and those from twin density an upper bound the stress estimates are also compatible with residual lattice strains determined with microfocus laue diffraction yielding equivalent stresses of 50 300 mpa in twinned calcite the lower stress bound agrees with stress estimates from borehole breakout measurements performed in the pilot hole from these data and assuming hydrostatic pore pressure and a low intermediate principal stress close to the overburden stress frictional sliding of the san andreas fault at the safod site is constrained to friction coefficients between 0 24 and 0 31 these low friction values may be related to the presence of clays talc and amorphous phases found in the fault cores and support the hypothesis of a weak san andreas fault

pseudotachylytes in an ancient accretionary complex and implications for melt lubrication during subduction zone earthquakes abstract pseudotachylyte bearing fault zones found in the shimanto accretionary complex southwest japan developed during subduction or underplating at seismogenic depths the pseudotachylytes occur in narrow dark veins less than a few millimeters thick that are sharply bounded by foliated cataclasites that originated from a melange the microstructures of pseudotachylytes are represented by a fragment laden glass supported texture resulting from the rapid cooling of the frictional melt transmission electron microscopy reveals the presence of glass in which euhedral microcrystals of mullite are locally developed the compositions of the pseudotachylyte matrix and the characteristics of the unmelted grains and microlites in the matrix suggest that frictional melting occurred in an illite rich slip zone with a minimum melting temperature of 1100 c the viscosities of the frictional melt were calculated from the pseudotachylyte matrix composition as well as from the volume fraction and aspect ratio of the unmelted grains the viscosities at 1100 c range from 85 to 290 pa s and the corresponding shear resistance along a 1 mm thick melt layer at a slip rate of 1 m s was 0 1 0 3 mpa the formation of a melt layer in an illite rich slip zone can possibly induce large stress drops increase the slip rate and enhance rupture propagation which together could affect the earthquake magnitude in a subduction zone

shear zones in clay rich fault gouge a laboratory study of fabric development and evolution abstract clay rich fault rocks have long been recognized to host distinctive fabric elements and fault rock fabric is increasingly thought to play a fundamental role in fault mechanical behaviour in the brittle regime although the geometries of fabric elements in fault gouges have been described for almost a century the genesis and evolution of these elements during shear and their links to bulk mechanical properties remain poorly understood we characterize the development and evolution of fabric elements with increasing shear in a variety of clay rich experimental gouges over shear strains of 20 and at normal stresses of 2 150 mpa in the double direct shear configuration in addition to sem observations of experiment products at a variety of shear strains we quantified clay fabric intensity and the degree of grain size reduction using x ray texture goniometry xtg and particle size distribution psd measurements we also measured p and s wave velocities during shear to further probe the evolution of shear fabric and gouge properties we find that clay fabric elements develop in a systematic manner regardless of the gouge material riedel shears in the r1 orientation and boundary parallel shears are the dominant fabric elements riedel shears nucleate at layer margins and propagate into the layer shortly after reaching yield stress clay particles rotate into the p orientation shortly after riedels propagate through the layer the riedel shears are through going but are 10 thinner than similar zones observed in coarser granular materials our results suggest that the weakness of clay rich fault gouge may be less a function of anisotropic crystal structure as has been suggested previously and more a consequence of very thin shear surfaces permitting deformation in clay rich materials with minimal dilation or cataclasis the very thin shear surfaces are a function of the fine grain size of the materials and possibly polymodal psd s

micromechanisms of creep in clay rich gouge from the central deforming zone of the san andreas fault we report the strength and constitutive behavior of gouge sampled from the central deforming zone cdz of the san andreas fault layers of flaked cdz gouge were sheared in the triaxial saw cut configuration using the stress relaxation technique to measure the gouge strength over 4 orders of magnitude in shear strain rate and at rates as low as 5 10 10s 1 and within an order of magnitude of in situ rates deformation conditions correspond to the in situ effective normal stress 100 mpa and temperature 65 to 120 c at the sampling depth of 2 7 km gouge was sheared dry and with brine pore fluid at 25 mpa pore pressure dry gouge is stronger and more rate strengthening than brine saturated gouge brine saturated cdz gouge strengthens with increasing strain rate and decreasing temperature and the dependencies of strength on strain rate and temperature increase at rates below 5 10 9s 1 at strain rates greater than 5 10 9s 1 the rate dependence is consistent with previous studies on the cdz gouge conducted at even higher rates the increase in rate dependence below 5 10 9s 1 indicates a change in the rate controlling deformation mechanism the magnitude of the friction rate dependence parameter a and the temperature sensitivity of a are consistent with crystal plasticity of the phyllosilicates we hypothesize a micromechanical model for the cdz gouge whereby a transition from fracture and delamination accommodated frictional flow to crystal plasticity accommodated frictional flow occurs with decreasing strain rate

the 2003 2004 seismic swarm in the western corinth rift evidence for a multiscale pore pressure diffusion process along a permeable fault system microseismic multiplets occurring in the western corinth rift greece during a large swarm are analyzed to retrieve their spatiotemporal characteristics these multiplets activated small subfaults at depth 7 km up to 1 km long at the root of two parallel active normal faults the swarm migrates westward nearly horizontally over 10 km at an average velocity of 50 m d with a diffusivity of 0 5 m2 s 1 it successively activates the aigion fault a relay zone in its hanging wall and the fassouleika fault within each multiplet hypocenters also migrate with diffusivities ranging from 0 001 to 0 4 m2 s 1 the largest internal diffusivities appear at the core of the layer defined by the clusters these results are interpreted as a hydroshear process caused by pore pressure migration within permeable corridors resulting from the intersection of the major faults with a brittle geological layer inherited from the hellenic nappe stack

structural geology natural fault lubricants motion along faults can occur in sudden earthquakes or through steady aseismic creep rock samples retrieved by drilling deep into a creeping section of the san andreas fault show that clay minerals in fault rock promote creep behaviour

clay clast aggregates a new textural evidence for seismic fault sliding to determine the processes responsible for slip weakening in clayey gouge zones rotary shear experiments were conducted at seismic slip rates equivalent to 0 9 and 1 3 m s at 0 6 mpa normal stress on a natural clayey gouge for saturated and non saturated initial conditions the mechanical behavior of the simulated faults shows a reproducible slip weakening behavior whatever initial moisture conditions examination of gouge obtained at the residual friction stage in saturated and non saturated initial conditions allows the definition of two types of microstructures a foliated type reflecting strain localization and a non foliated type composed of spherical aggregates friction experiments demonstrate that liquid vapor transition of water within gouge due to frictional heating has a high capacity to explain the formation of spherical aggregates in the first meters of displacement this result suggests that the occurrence of spherical aggregates in natural clayey fault gouges can constitute a new textural evidence for shallow depth pore water phase transition at seismic slip velocity and consequently for past seismic fault sliding

quantification of fabrics in clay gouge from the carboneras fault spain and implications for fault behavior abstract clays in fault rocks have the potential to control fault behavior the formation of frictionally weak clays in fault rocks can lead to fault zone weakening and the development of fabrics through authigenesis and mechanical rotation in clay rich fault rocks can influence fault zone permeability structures the left lateral carboneras fault of southwestern spain is a ca 1 km wide fault zone with anastomosing layers of clay rich gouge up to 50 m thick the gouge derived largely from mica schist is composed of neoformed illite and chlorite anisotropy of magnetic susceptibility 201 measurements and x ray pole figures 144 measurements show comparable oblate clay fabrics with the axis of maximum anisotropy sub parallel to strike this dimensional fabric contrasts with previously published maximum permeability results that are sub vertical this observation coupled with the generally poor clay fabric measured in the samples indicates that the orientation of clays is not likely to be the primary control of the permeability anisotropy of clay rich fault gouge becoming important only when gouge zones are 10s of meters thick which is very unusual even in a fault zone like that of the carboneras fault with thick very clay rich gouge zones with a visible foliations and slip surfaces the degree of clay preferred orientation is low above the millimeter scale showing that any strong clay fabric associated with the foliation are extremely localized phenomena this suggests that processes operating below the millimeter scale may significantly influence fault behavior a result compatible with earlier characterizations of faults in a variety of geologic settings highlighting the importance of understanding how the formation and fabric of fault related authigenic clays influence fault behavior

low to high velocity frictional properties of the clay rich gouges from the slipping zone of the 1963 vaiont slide northern italy the final slip of about 450 m at about 30 m s of the 1963 vaiont landslide italy was preceded by 3 year long creeping phase which was localized in centimeter thick clay rich layers 60 70 smectites 20 30 calcite and quartz here we investigate the frictional properties of the clay rich layers under similar deformation conditions as during the landslide 1 5 mpa normal stress 2 10 7 to 1 31 m s slip rate and displacements up to 34 m experiments were performed at room humidity and wet conditions with biaxial torsion and rotary shear apparatus the clay rich gouge was velocity independent to velocity weakening in both room humidity and wet conditions in room humidity experiments the coefficient of friction decreased from 0 47 at v 0 70 m s full lubrication results from the formation of a continuous water film in the gouge the vaiont landslide occurred under wet to saturated conditions the unstable behavior of the landslide is explained by the velocity weakening behavior of the vaiont clay rich gouges the formation of a continuous film of liquid water in the slipping zone reduced the coefficient of friction to almost zero even without invoking the activation of thermal pressurization this explains the extraordinary high velocity achieved by the slide during the final collapse

preseismic deformation and coseismic displacements associated with the 1999 chi chi taiwan earthquake the destructive 1999 chi chi taiwan earthquake m w 7 5 produced an approximately 100 km long surface rupture mostly along the previously recognized north south trending chelungpu fault preseismic deformation in central taiwan is realized from annually repeated global positioning system gps data acquired during the 1992 1999 period the total wnw ese shortening rate in the vicinity of the epicentral region that is from the west coast to the western boundary of the central range is up to 25 mm yr the crustal deformation before the chi chi earthquake was essentially a uniaxial compressional strain of 0 36 strain yr in the direction of 114 the gps measurements taken 0 2 2 7 yr before and within 3 months after the mainshock were utilized to estimate the coseismic displacements horizontal movements of 1 1 9 1 m in the nw nnw directions are observed on the hanging wall eastern side of the fault there is a northward increasing trend in the magnitude of the displacement vectors and a dramatic change in the direction of about 50 toward the east along the fault strike in contrast much smaller se see movements of 0 1 1 5 m are found on the footwall western side of the fault the gps data show 2 4 10 1 m of total horizontal offsets across the chelungpu fault vertical offsets of 1 2 4 4 m with the eastern side up are also observed along the surface rupture the uplift on the hanging wall decreases rapidly toward the east it becomes subsidence at sun moon lake and in the puli wushe area the stations on the footwall show subsidence of 0 02 0 26 m the width of the uplift zone increases from about 10 km in the south to approximately 30 km in the north manuscript received 13 october 2000

a high resolution absolute dated holocene and deglacial asian monsoon record from dongge cave china abstract we present a continuous record of the asian monsoon over the last 16 ka from 18o measurements of stalagmite calcite over 900 oxygen isotopic measurements providing information on shifts in monsoon precipitation are combined with a chronology from 45 precise 230th dates 18o and therefore asian monsoon intensity generally follows changes in insolation although changes in 18o are generally accommodated in abrupt shifts in contrast to smoothly varying insolation indicating that threshold effects may be important 18o decreased dramatically 3 at the start of the holocene 11 5 ka and remained low for 6 ka four positive 18o events centered at 11225 97 yr bp 1 05 10880 117 yr bp 1 15 9165 75 yr bp 1 4 and a double event centered at 8260 64 yr bp 1 1 and 8080 74 yr bp 1 0 punctuated this period of high monsoon intensity all four events correlate within error with climate changes in greenland ice cores thus the relationship between the asian monsoon and the north atlantic observed during the glacial period appears to continue into the early holocene in addition three of the four events correlate within error with outburst events from lake agassiz the decline of monsoon intensity in the mid late holocene is characterized by an abrupt positive shift in 18o which occurs at 3550 59 yr bp 1 1 in 100 yr in addition the holocene is punctuated by numerous centennial and multi decadal scale events amplitudes 0 5 to 1 up to half the amplitude of the glacial interstadial events seen in the last glacial period thus holocene centennial and multi decadal scale monsoon variability is significant although not as large as glacial millennial scale variability the monsoon shows a strong connection with northern south american hydrological changes related by changes in itcz position spectral analysis of the 18o record shows significant peaks at solar periodicities of 208 yr and 86 yr suggesting variation is influenced by solar forcing however there are numerous other significant peaks including peaks at el nino frequencies observed for high resolution portions of the record between 8110 and 8250 yr which suggest that changes in oceanic and atmospheric circulation patterns in addition to those forced by solar changes are important in controlling holocene monsoon climate in addition for this high resolution portion we observe a distinctive biennial oscillation of the asian monsoon which has been associated with the tropospheric biennial oscillation tbo

climatic change recorded in the sediments of the chew bahir basin southern ethiopia during the last 45 000 years abstract east african paleoenvironments are highly variable marked by extreme fluctuations in moisture availability which has far reaching implications for the origin evolution and dispersal of homo sapiens in and beyond the region this paper presents results from a pilot core from the chew bahir basin in southern ethiopia that records the climatic history of the past 45 ka with emphasis on the african humid period ahp 15 5 ka calbp geochemical physical and biological indicators show that chew bahir responded to climatic fluctuations on millennial to centennial timescales and to the precessional cycle since the last glacial maximum potassium content of the sediment appears to be a reliable proxy for aridity showing that chew bahir reacted to the insolation controlled humidity increase of the ahp with a remarkably abrupt onset and a gradual termination framing a sharply defined arid phase 12 8 11 6 ka calbp corresponding to the younger dryas chronozone the chew bahir record correlates well with low and high latitude paleoclimate records demonstrating that the site responded to regional and global climate changes

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

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

co seismic and or a seismic microstructures of jfast 343 core samples from the japan trench abstract the microstructures mineralogy and chemistry of four representative samples collected from cores extracted from the japan trench during integrated ocean drilling project expedition 343 the japan trench fast drilling project jfast have been studied using optical microscopy tem sem xrf xrd and microprobe analyses the samples provide a transect from relatively undeformed marine sediments in the hanging wall to the undeformed footwall material crossing the thrust interface between the pacific and north american plate where the fault slipped during the march 2011 tohoku oki earthquake our preliminary results suggest that the low strength of jfast fault gouge material is caused by the high amount of clay minerals 60 smectite 14 illite clay minerals in the decollement gouge sample are partly replaced by newly formed manganese oxide which precipitated from hydrothermal fluids dauphine twins were found in quartz grains of the decollement sample suggesting local high stress possible during seismic loading other microstructures cannot be assigned unambiguously to co seismic or a seismic faulting processes the observed scaly clay fabric is consistent with observations in many other plate boundary fault zones significant grain size reduction was found in the fault decollement zone sample but a change in lithology of the fault material cannot be ruled out microstructures typical for a seismic deformation like dissolution precipitation features e g dissolved grain boundaries mineral alteration occur in all jfast core samples but more frequently in the decollement sample

origin and behavior of clay minerals in the bogd fault gouge mongolia we analyzed twelve fault gouge samples from the bogd fault in south western mongolia to understand the origin and behavior of clay minerals the investigation relies on x ray powder diffraction xrd scanning electron microscopy sem transmission electron microscopy tem and high energy synchrotron x ray diffraction methods to investigate microstructure and preferred orientation smectite montmorillonite illite smectite mixed layers illite mica and kaolinite are the major clay components in addition to quartz and feldspars which are present in all samples the observations suggest that the protoliths and the fault rocks were highly altered by fluids the fluid rock interactions allow clay minerals to form as well as alter feldspars to precipitate kaolinite and montmorillonite thus newly formed clay minerals are heterogeneously distributed in the fault zone the decrease of montmorillonite component of some of the highly deformed samples also suggests that dehydration processes during deformation were leading to illite precipitation based on synchrotron x ray diffraction data the degree of preferred orientation of constituent clay minerals is weak with maxima for 001 ranging from 1 3 to 2 6 multiples of a random distribution m r d co existing quartz and feldspars have random orientation distributions microstructure and texture observations of the gouges from the foliated microscopic zone alternating with micrometric isotropic clay rich area also indicate that the bogd fault experienced brittle and ductile deformation episodes the clay minerals may contribute to a slip weakening behavior of the fault

preferred orientation of phyllosilicates comparison of fault gouge shale and schist abstract samples of fault gouge from the san andreas fault drill hole safod a shale from the north sea sedimentary basin and schists from metamorphic rocks in the alps have been analyzed with high energy synchrotron x rays to determine preferred orientation of mica and clay minerals the method relies on obtaining 2d diffraction images which are then processed with the crystallographic rietveld method implemented in the software maud allowing for deconvolution of phases and extraction of their orientation distributions it is possible to distinguish between detrital illite muscovite and authigenic illite smectite kaolinite and chlorite and muscovite and biotite with strongly overlapping peaks in the diffraction pattern the results demonstrate that phyllosilicates show large texture variations in various environments where different mechanisms produce the rock microfabrics fault gouge fabrics are quite weak and asymmetric with maxima for 001 in the range of 1 5 2 5 multiples of random distribution m r d this is attributed to heterogeneous deformation with randomization as well as dissolution precipitation reactions shale fabrics have maxima ranging from 3 to 9 m r d and this is due to sedimentation and compaction the strongest fabrics were observed in metamorphic schists 10 14 m r d and developed by deformation as well as recrystallization in a stress field in the analyzed samples fabrics of co existing quartz are weak all phyllosilicate textures can be explained by orientation of 001 platelets with no additional constraints on a axes

natural and experimental evidence of melt lubrication of faults during earthquakes melt produced by friction during earthquakes may act either as a coseismic fault lubricant or as a viscous brake here we estimate the dynamic shear resistance f in the presence of friction induced melts from both exhumed faults and high velocity 1 28 meters per second frictional experiments exhumed faults within granitoids tonalites indicate low f at 10 kilometers in depth friction experiments on tonalite samples show that f depends weakly on normal stress extrapolation of experimental data yields f values consistent with the field estimates and well below the byerlee strength we conclude that friction induced melts can lubricate faults at intermediate crustal depths