using drill cutting separates to estimate the strength of narrow shear zones at safod 1 a technique is presented for estimating frictional strength of narrow shear zones based on hand selection of drillhole cuttings separates tests were conducted on cuttings from the safod scientific drillhole near parkfield california since cuttings are mixed with adjacent material as they travel up the drillhole these fault derived separates give a better representation of the frictional properties of narrow features than measurements from the bulk material alone cuttings from two shear zones one an active trace of the san andreas fault contain a significant weight percent of clay rich grains that exhibit deformation induced slickensides in addition cuttings from the active saf trace contain around 1 serpentine coefficients of friction for clay rich and serpentine grains were 0 3 0 5 and 0 4 0 45 respectively these values are around 0 12 lower than the friction coefficient of the corresponding bulk cuttings providing an improved estimate of the frictional strength of the san andreas fault

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

origin and significance of clay coated fractures in mudrock fragments of the safod borehole parkfield california 1 the clay mineralogy and texture of rock fragments from the safod borehole at 3067 m and 3436 m measured depth md was investigated by electron microscopy sem tem and x ray diffraction xrd the washed and ultrasonically cleaned samples show slickenfiber striations and thin films of ca k bearing smectite that are formed on polished fault surfaces along freshly opened fractures and within adjacent mineralized veins the cation composition and hydration behavior of these films differ from the na montmorillonite of the fresh bentonite drilling mud although there is more similarity with circulated mud recovered from 3479 m md we propose that these thin film smectite precipitates formed by natural nucleation and crystal growth during fault creep probably associated with the shallow circulation of low temperature aqueous fluids along this shallow portion of the san andreas fault

synoptic imaging of nearshore bathymetric patterns 1 a new technique for near real time optical imaging of surf zone morphology synoptic imaging implemented at scripps beach la jolla california permits simultaneous measurements of the time varying position of individual linear and lunate megaripples fields of bed forms rocks cobble patches beach steps and sandbars and troughs a varying threshold filter for brightness is applied to video frames sampled at 1 hz to remove pixels corresponding to surface foam and suspended bubbles or sediment remaining fragments of the video frames are averaged over time intervals generally ranging from 5 to 10 min to produce images of the surf zone sand bed to facilitate accurate measurement of the position of sand bed features these images are enhanced to increase feature visibility transformed to plan view and corrected for refraction using surveyed bathymetry and tide level contrast in the images results from differences in water depth shading variations from tilted bed surfaces and changes in bed material the surveyed positions of megaripples and sandbars correspond to the positions of persistent features in the images a ray path model quantifying sand bed contrast as a function of water depth angle of line of sight and light attenuation coefficient in water predicts that for the scripps beach field site bed forms with height as small as 0 03 0 2 m can be detected in water depths up to 2 5 m in accord with observations and that visibility can be improved by changing the line of sight toward overhead viewing bed visibility is reduced by persistent surface foam and water turbidity generated by waves with period 8 s and height exceeding 1 5 m example images showing transitions between linear and lunate megaripples location of megaripple crestlines and the interaction between sandbars bed form fields and cobble patches illustrate the potential of synoptic imaging

mapping stress and structurally controlled crustal shear velocity anisotropy in california we present shear velocity anisotropy data from crustal earthquakes in california and demonstrate that it is often possible to discriminate structural anisotropy polarization of the shear waves along the fabric of major active faults from stress induced anisotropy polarization parallel to the maximum horizontal compressive stress stress directions from seismic stations located near but not on the san andreas fault indicate that the maximum horizontal compressive stress is at a high angle to the strike of the fault in contrast seismic stations located directly on one of the major faults indicate that shear deformation has significantly altered the elastic properties of the crust inducing shear wave polarizations parallel to the fault plane

extracting coherent coda arrivals from cross correlations of long period seismic waves during the mount st helens 2004 eruption 1 we computed cross correlations of seismic waves generated by long period lp events in the frequency band 0 8 2 2 hz the data were recorded in the vicinity of mount st helens msh during an eruptive period from september to december 2004 the time symmetric coherent coda arrivals of the time derivative of the cross correlation function dcf resulting from scattering and topographic effects in msh area correspond to arrivals of the green s function gf between the two stations the dcf waveforms computed on an hourly basis show a remarkable consistency over time periods of several hours but show temporal variations on longer time scales we investigated the emergence rate of the symmetric arrivals of the dcf and discussed their potential applications for volcano monitoring

mineralogical characterization of protolith and fault rocks from the safod main hole 1 washed cuttings provide a continuous record of the rocks encountered during drilling of the main hole of the san andreas fault observatory at depth safod both protolith and fault rocks exhibit a wide variety of mineral assemblages that reflect variations in some combination of lithology p t conditions deformation mechanisms and fluid composition and abundance regions of distinct neomineralization bounded by faults may record alteration associated with fluid reservoirs confined by faults in addition both smectites occurring as mixed layer phases and serpentine minerals are found in association with active strands of the san andreas fault that were intersected during drilling although their rheological influence is not yet fully known faults containing these mineralogical phases are prime candidates for continuous coring during phase 3 of safod drilling in the summer of 2007

water rock interactions in fault gouge measurements were made of the amounts of d 18o and h2o in fault gouge collected over a depth of 400 m in the san andreas fault of california the amounts and isotopic compositions of the pore fluids also analyzed suggest that formation waters from adjacent franciscan rocks have migrated into the gouge and mixed with local meteoric water thus the gouge is an open system permeable to fluid flow this permeability has important implications concerning heat flow along the fault zone

constraining the exhumation and burial history of the safod pilot hole with apatite fission track and u th he thermochronometry 1 the san andreas fault observatory at depth safod pilot hole traverses the upper 2 km of a site 1 8 km west of the san andreas fault saf near parkfield california in order to evaluate the burial and exhumation history of the site and its relationship to the kinematics and mechanics of the saf we use 15 apatite fission track ft and 5 u th he analyses from pilot hole samples to document their thermal history sample ages decrease with depth ft and u th he ages range from 60 and 31 ma respectively in the upper 800 m of the hole to 3 and 1 ma at the base of the hole 2 2 km depth 93 c thermal modeling of the distribution of ft lengths indicates three events in the last 80 ma 1 cooling and exhumation of 60 c that culminated at 30 ma 2 reheating of 50 c from 30 to 8 4 ma probably as the result of basin subsidence and burial by 1 1 5 km of sediments and 3 cooling of 30 c and estimated coast range exhumation of 1 km since 8 4 ma

high resolution x ray texture goniometry high resolution x ray texture goniometry hrxtg significantly improves peak background ratios and allows for the determination of crystallographic preferred fabrics of small regions 1 mm2 in heterogeneously deformed rocks the basic instrument is a commercially available automated single crystal x ray diffractometer equipped with a mo source a stage for transmission mode was constructed that is driven by the original motors so the unit is fully computer controlled absorption corrections are calculated which removes the need for generally difficult to obtain empirical corrections this combined with equipment optimization enables us to use small irradiated volumes of rocks with variable compositions the advantage of hrxtg in heterogeneously deformed rocks is demonstrated in a cleaved mudstone analysis of 1 mm2 areas in a 2 cm bed showing a gradation from micaceous sandstone to slate reveals that fabric intensity of both mica and chlorite increases significantly dm from 0 64 to 1 03 and from 0 45 to 0 78 respectively without appreciable change in the oblate shape km of the fabric thus hrxtg can record a strain state at the mm scale of heterogeneously deformed rocks

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

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

detailed kinematics structure and recurrence of micro seismicity in the safod target region 1 large numbers of small earthquakes recorded over 2 decades and analyzed with advanced techniques are used to characterize the detailed kinematics structure and recurrence interval scaling properties of micro seismicity in a 4 4 km lateral and 6 km deep crustal volume encompassing the region of the safod deep drilling experiment the characterization reveals that the seismically active san andreas fault in the vicinity of safod s repeating magnitude 2 target earthquakes is composed of two sub parallel fault strands that are creeping at comparable rates and that one of the strands lies between the safod drilling platform and safod s target events in the region 55 of the earthquakes are members of 52 characteristically repeating earthquake sequences the recurrence intervals of the repeating target events are consistent with the interval scaling of the other sequences however this scaling is contrary to that expected from standard constant stress drop theory

continental transforms a view from the alpine fault abstract continental transform faults are dominantly highly localized strike slip shear zones hundreds of kilometers long that accumulate tens to hundreds of kilometers of displacement from work on the alpine fault we pose the questions what is the deep structure of a continental transform and how does the displacement become localized we review research on the alpine fault and propose a model in which the fault partitions at depth into a steep zone extending into the mantle with largely fault parallel motion and a flat ductile decollement in the lower crust the fault localizes around two thirds of the plate motion within a 100 km wide zone of distributed deformation a review of other active continental fault systems suggests that variation between them may reflect their tectonic origins the nature of the crust in which they develop the presence of a significant oblique component of motion and the displacement rate all however have evidence for the development of a single principal fault zone that carries 50 of the total displacement and extends as a localized zone of shear into the upper mantle we review mechanisms of strain weakening and suggest that localization of a principal fault may be initiated in the seismogenic crust and through a series of positive feedbacks eventually extend through the lower crust into the upper mantle

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

geophysical properties within the san andreas fault zone at the san andreas fault observatory at depth and their relationships to rock properties and fault zone structure 1 we examine the relationships between borehole geophysical data and physical properties of fault related rocks within the san andreas fault as determined from data from the san andreas fault observatory at depth borehole geophysical logs cuttings data and drilling data from the region 3 to 4 km measured depth of the borehole encompass the active part of the san andreas fault the fault zone lies in a sequence of deformed sandstones siltstone shale serpentinite bearing block in matrix rocks and sheared phyllitic siltstone the borehole geophysical logs reveal the presence of a low velocity zone from 3190 to 3410 m measured depth with vp and vs values 10 30 lower than the surrounding rocks and a 1 2 m thick zone of active shearing at 3301 3303 m measured depth seven low velocity excursions with increased porosity decreased density and mud gas kick signatures are present in the fault zone geologic data on grain scale deformation and alteration are compared to borehole data and reveal weak correlations and inverse relationships to the geophysical data in places vp and vs increase with grain scale deformation and alteration and decrease with porosity in the fault zone the low velocity zone is associated with a significant lithologic and structural transition to low velocity rocks dominated by phyllosilicates and penetratively foliated sheared rocks the zone of active shearing and the regions of low sonic velocity appear to be associated with clay rich rocks that exhibit fine scale foliation and higher porosities that may be a consequence of the fault related shearing of foliated and fine grained sedimentary rocks

constraints on mineralization fluid rock interaction and mass transfer during faulting at 2 3 km depth from the safod drill hole 1 mineralogical and geochemical changes in mudrock cuttings from two segments of the san andreas fault observatory at depth safod drill hole 3066 3169 and 3292 3368 m measured depth are analyzed in this study bulk rock samples and hand picked fault related grains characterized by polished surfaces and slickensides were investigated by x ray diffraction electron microscopy and geochemical analysis the elemental changes in fault related grains along the sampled san andreas fault are attributed to dissolution of detrital grains particularly feldspar and quartz and local precipitation of illite smectite and or chlorite smectite mixed layers in fractures and veins assuming zro2 and tio2 to be immobile elements systematic differences in element concentrations show that most of the elements are depleted in the fault related grains compared to the wall rock lithology calculated mass loss between the bulk rock and picked fault rock ranges from 17 to 58 with a greater mass transport in the shallow trace of the sampled fault that marks the upper limit the fault core the relatively large amount of element transport at temperatures of 110 114 c recorded throughout the core requires extensive fluid circulation during faulting whereas dissolution precipitation may be partly induced by the disequilibrium between fluids and rocks during diagenetic processes stress induced dissolution at grain contacts is proposed as the main mechanism for extensive mineral transformation in the fault rocks and localization of neomineralization along grain interface slip surfaces

the relationship of strain and preferred orientation of phyllosilicate grains in rocks a review abstract the calculation of strain from the preferred orientation of platy phyllosilicate grains taking strain to be homogeneous is surprisingly often successful from a pragmatic point of view although numerous theoretical objections to this procedure can be and have been raised for the calculation to succeed the original distribution of mica or clay mineral grains must be known or assumed the usual assumption that the starting distribution was random and therefore nearly uniform implies that the strain history of pelitic sediments is taken to start when the watery sediment was last stirred by waves or bioturbation or if it was laid down in still anoxic waters from the moment of first deposition it therefore includes compaction as part of the strain history which is usually neglected by structural geologists originally random distribution of clay flake orientations has been observed in recently deposited sediments in many instances and has been theoretically explained in various ways exceptions probably exist but their frequency has not been estimated biologically disturbed sediments may be brought nearer to a state of random orientation of clay flakes than they were at deposition with the exception of whole beds or concretions in which pores are early filled with cement mudrocks are almost always compacted by the weight of later deposits the strain so induced is axially symmetric about the vertical and thus about the pole of the bedding planes compaction acts on soft unconsolidated rocks but may continue with increasing overburden on rocks strengthened by cementation and by increasing interlocking of grains tectonic deformation may start early before the rock is fully compacted or it may begin only much later and the mechanisms of deformation increase in complexity with the increasing temperature and hydrostatic stress of deeper burial up to a certain level of mild metamorphism however phyllosilicate grains seem to be rather permanent if not as mineral species then at least by preservation of their constituent sheets of silica tetrahedra given this permanence the mechanisms of rock deformation whether intergranular slip cataclasis or dissolution and reprecipitation and various intragranular processes in the non phyllosilicate matrix hardly matter as far as the recording of strain is concerned even a certain amount of stress solution at especially heavily loaded points of the phyllosilicate grains themselves does not destroy their value as strain markers as long as the dissolved material is disposed of mainly by crystallographically continuous overgrowth on existing grains the same holds for local dissolution of phyllosilicates in portions of old grains that have become thermodynamically unstable by bending widespread irregular or random nucleation and growth of newly nucleated phyllosilicate grains at intermediate or high grades of metamorphism can reduce the reliability of the strain record and so can domainal structure of the deformed rock if both the volume fraction of phyllosilicate grains and the intensity of total strain differ sharply from one domain to another if distinct domains are not equant and have themselves a strong preferred orientation of shape nevertheless rocks containing an anastomosing network of zones rich in phyllosilicate grains as slates and other deformed mudrocks commonly do have in many cases been found to have average preferred orientations of mica or chlorite which if used to estimate the strain give results similar to those obtained from independent strain markers in the same rocks thus the most significant verification of the usefulness of strain estimates by measurement of preferred orientation is the mutual confirmation with independently determined strain the limit to such verification is the rarity of strain markers so that the coexistence of two independent ones is exceptionally rare for this reason however it is important to make full use of the orientation of phyllosilicate grains where they are available the alternative to have no strain measurements at all causes structural geologists to miss potential information

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

texture development in naturally compacted and experimentally deformed silty clay sediments from the nankai trench and forearc japan the petrophysical properties of fine grained marine sediments to a large extent depend on the microstructure and crystallographic preferred orientations cpos in this contribution we show that rietveld based synchrotron texture analysis is a new and valuable tool to quantify textures of water saturated fine grained phyllosilicate rich sediments and assess the effects of compaction and tectonic deformation we studied the cpo of compositionally almost homogeneous silty clay drillcore samples from the nankai accretionary prism slope and the incoming philippine sea plate offshore sw japan basal planes of phyllosilicates show bedding parallel alignment increasing with drillhole depth thus reflecting progressive burial and compaction in some samples calcite and albite display a cpo due to crystallographically controlled non isometric grain shapes or nannofossil tests consolidated undrained experimental deformation of a suite of thirteen samples from the prism slope shows that the cpos of phyllosilicate and calcite basal planes develop normal to the experimental shortening axis there is at least a qualitative relation between cpo intensity and strain magnitude scanning electron micrographs show concurrent evolution of preferred orientations of micropores and detrital illite flakes normal to axial shortening this indicates that the microfabrics are sensitive strain gauges and contribute to anisotropic physical properties along with the cpo

effect of hydration state on the frictional properties of montmorillonite based fault gouge 1 we report on laboratory experiments examining the effect of hydration state on the frictional properties of simulated clay and quartz fault gouge we tested four mixtures of ca montmorillonite and quartz 100 70 50 and 30 montmorillonite at four hydration states dry 16 0 wt water we controlled the hydration state using either oven drying for 13 wt h2o under conditions of controlled relative humidity for each clay quartz mixture and hydration state we measured frictional properties over a range of normal stresses 5 100 mpa and sliding velocities 1 300 m s we observe a systematic decrease in the coefficient of friction with increasing water content normal stress and clay content values of for 50 50 mixtures range from 0 57 to 0 64 dry and decrease to 0 21 0 55 for the most hydrated cases wet for layers of 100 montmorillonite ranges from 0 41 0 62 dry to 0 03 0 29 wet as water content is increased from 0 to 20 0 wt the friction rate parameter a b becomes increasingly positive variation in a b values decreases dramatically as normal stress increases if our experimental results can be applied to natural fault gouge the combination of stress state hydration state and quartz content that facilitates unstable fault behavior implies that the onset of shallow seismicity in subduction zones is more complicated than a simple transition from smectite to illite

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

upper crustal structure of the western colombian basin caribbean sea multichannel seismic reflection profiles show that most of the western colombian basin is underlain by a large oceanic plateau the crust of this plateau has a smooth upper surface an oceanic velocity structure and a thickness at least twice that of typical oceanic crust locally well defined reflections within the plateau crust are related to layered basalt flows and interbedded sediments a mounded reflection pattern within acoustic basement at the crest of the plateau mono rise may represent volcanic knolls smooth surfaced basement of the western colombian basin is probably equivalent to the upper cretaceous basement complex below seismic reflection horizon b in the central venezuelan basin and resembles the basements of some cretaceous oceanic plateaus and aseismic rises in the western pacific an area of apparently typical oceanic crust lies adjacent to the colombian basin plateau and may represent relatively unmodified oceanic crust upon which the plateau was constructed reflections within the flank of the colombian basin plateau appear to overlap this crust indicating that the plateau is younger than the oceanic crust oceanic plateau crust extends to panama and costa rica without apparent structural break and may be continuous with the basement complex of southern central america

carbon chain length distribution in n alkyl lipids a process for evaluating source inputs to lake qinghai abstract lake sediments generally contain a mixture of terrestrial and aquatic source inputs and determining the major inputs is important for understanding geological records in paleoenvironment and paleoclimate research in this study we describe the distribution of n alkanes and n fatty acids fas in representative modern plants from around lake qinghai we found a significant difference in the average length of n fa carbon chains acl fa 16 32 in terrestrial 23 3 and aquatic plants 18 6 the results reveal that acl fa 16 32 may essentially serve as a proxy for evaluating the major source inputs to lake sediments assessment of surface sediments from the lake showed that the fas originated from a mixture of inputs with the aquatic source input predominant at most sites additionally the d values of sediment mid chain n acids c 22 showed a relationship with the acl fa 16 32 proxy an increased fa acl corresponded to more negative hydrogen isotope ratio values we suggest that different sources should be considered and acl fa 16 32 could be a potential calibration proxy before using d values to extract reliable isotopic information from lake water more attention should be paid to source inputs and their relationship to other geochemical proxies in future studies of lake sediments

the evolution of illite to muscovite mineralogical and isotopic data from the glarus alps switzerland thirty five illite and muscovite concentrates were extracted from triassic and permian claystones shales slates and phyllites along a cross section from the diagenetic alpine foreland tabular jura and borehole samples beneath the molasse basin to the anchi and epimetamorphic helvetic zone of the central alps concentrates and thin sections were investigated by microscopic x ray infrared mossbauer thermal dta and tg wet chemical electron microprobe k ar rb sr 40ar 39ar and stable isotope methods with increasing metamorphic grade based on illite crystallinity data xrd and ir the following continuous changes are observed i the 1md 2m1 polymorph transformation is completed in the higher grade anchizone ii k2o increases from 6 8 wt diagenetic zone to 8 5 10 anchizone to 10 11 5 epizone reflecting an increase in the total negative layer charge from 1 2 to 2 0 iii a decrease of the chemical variation of the mica population with detrital muscovite surviving up to the anchizone epizone boundary iv a shift of an endothermic peak in differential thermal curves from 500 to 750 c v k ar and rb sr apparent ages of the fraction 2 m decrease from the diagenetic zone to the epizone k ar ages being generally lower than rb sr ages the critical temperature for total ar resetting is estimated to be 260 30 c k ar and rb sr ages become concordant when the anchizone epizone boundary is approached the stable isotope data on the other hand show no change with metamorphic grade but are dependent on stratigraphic age these results suggest that the prograde evolution from 1 md illite to 2m1 muscovite involves a continuous lattice restructuration without rupture of the tetrahedral and octahedral bonds and change of the hydroxyl radicals however this is not a recrystallization process this restructuration is completed approximately at the anchizone epizone boundary the isotopic data indicate significant diffusive loss of 40ar and 87sr prior to any observable lattice reorganization the restructuration progressively introduces a consistent repartition of ar and k in the mineral lattices and is outlined by the 40ar 39ar age spectra concordant k ar and rb sr ages of around 35 30 ma with concomitant concordant 40ar 39ar release spectra are representative for the main phase of alpine metamorphism calanda phase in the glarus alps a second age group between 25 and 20 ma can probably be attributed to movements along the glarus thrust ruchi phase while values down to 9 ma in regions with higher metamorphic conditions suggest thermal conditions persisting at least until the middle tortonian

phase relations among smectite r1 illite smectite and illite a variety of smectitic and illitic clays were studied by tem and aem following ex pansion by l r white resin to define phase relations for clay minerals undergoing di genesis and low grade metamorphism samples included a prograde shale sequence from the gulf coast a hydrothermal bentonite from zempleni hungary and shales from the nankai trough japan michigan and the welsh sedimentary basin all samples were dominated by various proportions of only three kinds of clay minerals smectite having no 10 alayers r1 illite smectite i s 50 illite and illite with only small proportions of smectite like interlayers mixed layer phases with intermediate ratios of i s were observed only as minor components of other layer sequences lattice fringe images show that the common layer spacing of r1 i s is 21 aand that it has 0 7 0 8 k pfu its properties are not an average of those of smectite and illite which is consistent with the uniqueness of the r1 i s structure a prograde sequence of clay mineral transitions in the studied samples can be charac terized by five stages with different combinations of the three major phases i e smectite r1 i s and illite corresponding to different grades the sequence from low to high grade is 1 pure smectite 2 smectite with small proportions of discrete r1 i s and illite 3 r1 i s with small proportions of smectite and illite 4 illite with some r1 i s and smectite and 5 illite common exceptions to this scheme are inferred to be caused by the inherent metastability of all phases in occurrences that are rate or path dependent

fluid regime in faulting deformation of the waratah fault zone australia as inferred from major and minor element analyses and stable isotopic signatures abstract geochemical studies have been conducted on the waratah fault zone a steep ne sw trending brittle fault zone in the southeastern lachlan fold belt australia strike slip movement along the fault juxtaposed an early devonian turbidite sequence liptrap formation against heavily veined early devonian limestones the fault zone consists of the faulted liptrap formation and faulted limestones and a fault core composed of a fault melange zone and strongly brecciated limestones the fault zone records extensive mineralogical and stable isotope alteration within the deformed turbidites of the liptrap formation and within the limestone sequence for the liptrap formation a volume loss of 28 was calculated for the transition from unaltered sediments to the fault melange zone the volume loss in the limestone gouge is much higher 60 the calculated fluid rock ratios in the limestone sequence are at least an order of magnitude lower than in the fault melange zone stable isotopic and rare earth elemental data document the infiltration of near surface meteoric water during brittle deformation and alteration of the fault zone the interaction of fluids with contrasting lithologies led to different alteration processes affecting fault weakening and or strengthening the cyclic change between large scale fluid infiltration due to brittle failure open system and fault rock cementation closed system may represent a combined conduit barrier system in the limestone sequence within the liptrap formation no cementation took place as the conditions for quartz precipitation were unsuitable hence the fault system remains open in respect to fluids conduit system only additionally the permanent fluid infiltration dissolved silica and altered detrital feldspar resulting in subsequent porosity increase within the fault melange zone our results are consistent with investigations in modern accretionary complexes where major decollements and subsidiary faults have been shown to be important fluid conduits

phyllosilicate mineral assemblages of the safod pilot hole and comparison with an exhumed segment of the san andreas fault system 1 this paper establishes a reference phyllosilicate data set from the safod pilot hole for future safod drilling and presents an application of these data for studies of exhumed fault segments the chlorite assemblages in cuttings from two intervals of the safod pilot hole are separated into two populations based on x ray diffraction characteristics the first population is found in granite in both the deeper and shallower interval whereas the second population occurs only in clastic sedimentary rocks in the shallower interval the characteristics of the first population match those for protolith and cataclasite of the exhumed punchbowl fault whereas samples from intensely deformed ultracataclasite are most similar to the second this supports previous findings that the mineral assemblages in the ultracataclasite formed after the cessation of motion along the fault above a depth of 2 km and that mineral assemblages in these exhumed fault rocks have been overprinted by post faulting alteration

experimental constraints on the relationship between clay abundance clay fabric and frictional behavior for the central deforming zone of the san andreas fault the presence of smectite saponite in fault gouge from the central deforming zone of the san andreas fault at parkfield ca has been linked to low mechanical strength and aseismic slip however the precise relationship between clay mineral structure fabric development fault strength and the stability of frictional sliding is not well understood we address these questions through the integration of laboratory friction tests and fib sem analysis of fault rock recovered from the san andreas fault observatory at depth safod borehole intact fault rock was compared with experimentally sheared fault gouge and different proportions of either quartz clasts or safod clasts extracted from the sample nano textural measurements show the development of localized clay particle alignment along shear folia developed within synthetic gouges such slip planes have multiples of random distribution mrd values of 3 0 4 9 the mrd values measured are higher than previous estimates mrd 1 5 that show lower degrees of shear localization and clay alignment averaged over larger volumes the intact fault rock exhibits less well developed nano clay fabrics than the experimentally sheared materials and mrd values decrease with smectite content we show that the abundance strength and shape of clasts all influence fabric evolution via strain localization quartz clasts yield more strongly developed clay fabrics than serpentine dominated safod clasts our results suggest that 1 both clay abundance and the development of nano scale fabrics play a role in fault zone weakening and 2 aseismic creep is promoted by slip along clay shears with 20 wt smectite content and mrd values 2 7

detailed kinematics structure and recurrence of micro seismicity in the safod target region preparing for the san andreas fault observatory at depth earthquakes and crustal structure large numbers of small earthquakes recorded over 2 decades and analyzed with advanced techniques are used to characterize the detailed kinematics structure and recurrence interval scaling properties of micro seismicity in a 4 x 4 km lateral and 6 km deep crustal volume encompassing the region of the safod deep drilling experiment the characterization reveals that the seismically active san andreas fault in the vicinity of safod s repeating magnitude 2 target earthquakes is composed of two sub parallel fault strands that are creeping at comparable rates and that one of the strands lies between the safod drilling platform and safod s target events in the region 55 of the earthquakes are members of 52 characteristically repeating earthquake sequences the recurrence intervals of the repeating target events are consistent with the interval scaling of the other sequences however this scaling is contrary to that expected from standard constant stress drop theory

postseismic pressure solution creep evidence and time dependent change from dynamic indenting experiments active faults in the earth s upper crust can slide either steadily by aseismic creep or abruptly causing earthquakes seismic and aseismic processes are closely related earthquakes are often followed by transient afterslip creep postseismic displacement rates progressively decrease with time over a period of years or decades so seismic fracturing activates the creep rate and various healing processes progressively reduce it this article presents pressure solution indenter experiments on halite calcite and plaster that show how fracturing and comminution processes induced by dynamic stress loading applied by dropping steel balls drastically accelerate the displacement rates accommodated by pressure solution creep by decreasing the dissolution contact area and the diffusive mass transfer distance along this contact however as fractures progressively heal and dissolution contacts flatten these effects disappear and the displacement rates slow down the time dependent change in indenter displacement after dynamic stress loading has been measured and is best fitted by power laws with exponents that change with time from 0 3 to 1 when healing is achieved natural postseismic afterslip displacement time relationships have been analyzed and also show a power law change with a power law exponent in the range of 0 25 0 4 it is proposed that the variation in power law exponent with time is related to the change in morphology of the dissolution contact that is fractured or comminuted during the dynamic event and is then progressively healed and smoothed in natural faults monitoring the power law parameters could give access to the characteristic healing time in the fault

paleomagnetic reorientation of san andreas fault observatory at depth safod core 1 we present a protocol for using paleomagnetic analysis to determine the absolute orientation of core recovered from the safod borehole our approach is based on determining the direction of the primary remanent magnetization of a spot core recovered from the great valley sequence during safod phase 2 and comparing its direction to the expected reference field direction for the late cretaceous in north america both thermal and alternating field demagnetization provide equally resolved magnetization possibly residing in magnetite that allow reorientation because compositionally similar siltstones and fine grained sandstones were encountered in the san andreas fault zone during stage 2 rotary drilling we expect that paleomagnetic reorientation will yield reliable core orientations for continuous core acquired from directly within and adjacent to the san andreas fault during safod phase 3 which will be key to interpretation of spatial properties of these rocks citation pares j m a m schleicher b a van der pluijm and s hickman 2008 paleomagnetic reorientation of san andreas fault observatory at depth safod core geophys res lett 35 l02306 doi 10 1029 2007gl030921 during phase 3 of safod which is being conducted in the summer of 2007 600 m of continuous core will be acquired from directly within and adjacent to the active san andreas fault zone in side tracks drilled off the existing borehole these cores will be extensively tested in the laboratory to determine their mineralogy geochem ical composition deformation mechanisms frictional be havior and physical properties 3 several physical properties measured in the core have a directional nature including bedding faults veins micro fractures seismic velocities and permeability reconstruct ing the in situ orientation of recovered drill core termed here reorientation is hence of great importance when interpreting the structural deformational and physical prop erties of fault and country rocks paleomagnetism can be used to reconstruct the absolute orientation of the core by providing a reference direction relative to geographic coor dinates the paleomagnetic core reorientation method has been successfully used for a number of years e g fuller 1969 kodama 1984 shibuya et al 1991 and offers distinct advantages over other methods measurements are made on samples from the cores following recovery from the borehole and hence do not have any impact on the coring process itself unlike for example scribing techni ques that can lead to core jamming and poor recovery especially in highly fractured rock because cores are oriented one piece at a time paleomagnetic core reorienta tion is generally more reliable than traditional scribed core techniques which require extremely precise correlations between core depths and orientation data acquired with a downhole orientation tool this technique can be applied to recently drilled cores and stored old cores even in the absence of real time core orientation or borehole image log data in this paper we report results of a paleomagnetic and mineralogic sem study aimed at providing a reliable method for safod core reorientation