seismic and aseismic slip indicators in serpentinite gouge structures observed within experimentally deformed serpentinite gouges provide information that may be used to identify the seismic behavior of natural fault zones in laboratory friction experiments serpentinite exhibits two modes of behavior one can only result in stable fault creep and the other may result in stable slip but has the potential for earthquakes the microstructures that form during these experiments reflect the deformation style of the serpentinite distributed deformation results from aseismic fault creep and localized deformation results from conditions favorable for seismic slip distributed deformation produces a crystallographic preferred orientation of the serpentine grains s foliation localized deformation forms riedel shears similar structures occur within a natural serpentinite shear zone from monterey county california and suggest a history of stable fault creep with intermittent seismic events

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

origin of siderite concretions in microenvironments of methanogenesis developed in a sulfate reduction zone an exception or a rule abstract the mississippian shales from wardie scotland were deposited in a large normally brackish water basin they host siderite concretions with 13c values ranging from 3 5 to 12 1 which show that sulfate reduction and methanogenesis contributed to the production of bicarbonate fossils are found within each of the concretions suggesting that the decay of these organic remains provided a catalyst for concretionary growth the pore water was freshened which is indicated by the very negative 18o values from 12 8 to 4 7 many features indicate that the concretions are very early diagenetic precompactional precipitates the concretions are mostly homogeneous only a few exhibit complex concentric zoning the microstructure of the homogeneous concretions indicates a pervasive growth however their isotopic composition reveals a clear trend towards lighter 13c and 18o values from the center to the edge it is proposed that the concretions grew due to methanogenesis in microenvironments around large organic remains which acted as nuclei we suggest that this process may be one of the reasons for the very common occurrence of cements as mineral concretions not as disseminated precipitates along beds these microenvironments developed entirely within the sulfate reduction zone which resulted in the establishment of isotopic gradients in bicarbonate dissolved in the pore water across these microenvironments these gradients were caused by fast consumption of sulfate by the intensive oxidation of organic remains this resulted in the isotopic trends preserved in the homogeneous concretions the formation of the zoned concretions proceeded generally concentrically but detailed observations suggest that pervasive growth also contributed to their formation afterwards all the concretions were deeply buried probably within the oil window where late diagenetic phases crystallized mainly kaolinite quartz and sulfides

friction of mineralogically controlled serpentinites and implications for fault weakness serpentines are common minerals in several major tectonic faults in a variety of geodynamic settings and have variable frictional strength and complex deformation processes here we present friction experiments carried out on a suite of serpentine samples that include veins of antigorite lizardite and fibrous serpentine chrysotile and polygonal serpentine together with massive samples of retrograde lizardite and chrysotile rich and prograde antigorite rich serpentinites these samples were characterized from the hand specimen down to the nanoscale to precisely constrain their mineralogical composition and are interpreted to represent typical fault rocks and host rocks in serpentine bearing shear zones respectively experiments were performed at effective normal stress from 5 to 120 mpa at temperatures of 25 c and 170 c and water saturated i e under the faulting conditions of the brittle upper lithosphere friction of antigorite samples either massive or vein is relatively high 0 53 retrograde massive serpentinites constituted primarily of lizardite and fibrous serpentines are frictionally weak 0 30 end members lizardite and fibrous serpentines are even weaker 0 15 0 19 and this weakness is unchanged at high temperature we document deformation of lizardite and fibrous serpentines occurring predominantly via mode ii cracking crystal fiber folding and frictional sliding which account for the observed mechanical weakness when combined with frictional reactivation analysis our data provide mechanical evidence for fault weakness inferred from earthquake dip distributions at oceanic outer rises and low angle normal faults beneath rifted continental margins and at slow ultraslow spreading mid ocean ridges

antigorite crystallographic preferred orientations in serpentinites from japan abstract foliated antigorite serpentinites are contributing to seismic anisotropy in subduction zones however uncertainty remains regarding the development of antigorite crystal preferred orientations cpo this study analyzes the cpos of antigorite and olivine in variably serpentinized samples of antigorite serpentinite from three localities in southwest japan using u stage ebsd and synchrotron x rays in all samples 010 poles show a maximum parallel to the lineation and 001 poles concentrate normal to the foliation with a partial girdle about the lineation 100 poles are less distinct and scatter widely the three measurement methods give similar results though orientation distributions from x ray analysis which averages over larger sample volumes are more regular and weaker than u stage and ebsd which focuses on local well crystallized grains in olivine bearing serpentinite the olivine cpo does not appear to control the antigorite cpo though there are some topotactic relationships the antigorite cpo formed contemporaneously with serpentinization and shear deformation based on our analyses in a subduction zone mantle wedge the 010 pole of antigorite is parallel to the direction of the subduction and 001 lattice planes are parallel to the interface of the subducting oceanic plate by averaging single crystal elastic properties over orientation distributions wave velocities have been calculated fast p velocities are parallel to the subduction direction and slow velocities are perpendicular to the subducting plate with an anisotropy of 10 15

quantifying the partition between seismic and aseismic deformation along creeping and locked sections of the north anatolian fault turkey shallow aseismic creep is a key deformation component along plate boundaries that contributes to the energy budget during the seismic cycle several major active continental faults show spatial alternation of creeping and locked sections the present study focuses on the evaluation of the aseismic part of the total displacement along the north anatolian fault in turkey detailed microstructural analyses of finite strain were performed using various methods based on change of length or angle on six representative samples collected over 32 outcrops along locked and creeping sections of the fault chemical analyses were used to map mineral composition of fault rocks and to calculate relative volume changes associated with creep the relationship between finite strain and volume change allowed quantifying the evolution of the penetrative pressure solution cleavage mechanism of creep in volcanic and analogous creeping rocks finite strain measurements revealed two spatial scales of strain that correspond to the alternation of two types of shear zones with cleavages either oblique or sub parallel to the fault displacement using geodetic and geologic data cumulative aseismic displacement was calculated in the range 9 49 of the total 80 km displacement in the creping sections and was negligible in locked sections the large uncertainty in the kilometer width creeping sections was related to the difficulty of quantifying the high strain values associated with high shear displacement and for which measurement uncertainties are large a promising way to improve such quantification would be to develop reliable statistical analysis of cleavage orientation in the field

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

serpentine in active subduction zones abstract serpentinization is a key phenomenon for understanding the geodynamics of subduction zones in the 10 200 km depth range serpentines are a major water carrier and their rheological properties have a strong influence on deformation partitioning and seismicity at depths i review experimental investigations that have been conducted on serpentines with emphasis on the large body of data acquired over the past decade determinations of physical properties at the pressure and temperature conditions of subductions allow interpreting geophysical data in active subduction in terms of mineralogy and petrology and to link the presence of serpentinites with deformation and fluid circulation the fluid budget can be partially constrained from geophysical data elasticity data provide a quantitative basis for mapping serpentinization in the mantle wedge and slab from seismic tomography anisotropy suggests the existence of thin serpentinite channels above the plate interface that account for mechanical decoupling inferred from down dip limit of the seismogenic zone and heat flow strain rate dependent rheology of antigorite serpentine is consistent with stable deformation of this thin layer or channel over timescales ranging from those of the seismic cycle to those of thermal equilibration and exhumation of high pressure rocks and with the geological record of subduction related deformation circulation of serpentinizing fluids depends on the permeability structure and is imaged by electrical conductivity tomography it could be controlled by fracturing in the undeformed cold nose of the mantle wedge and by plastic deformation along the plate interface fluid migration mechanisms are similar to those inferred from petrological and geochemical data on exhumed serpentinites estimation of the fluid budget associated with serpentine formation will rely on numerical simulations for which coupling of kinetics of hydration and dehydration at scales ranging from grain size up to faulting pattern needs to be established especially for water cycling to the transition zone in the core of the slab

discrimination between pressure and fluid saturation changes from time lapse seismic data explicit expressions for computing saturation and pressure related changes from time lapse seismic data have been derived and tested on a real time lapse seismic data set necessary input is near and far offset stacks for the baseline seismic survey and the repeat survey the method has been tested successfully in a segment where pressure measurements in two wells verify a pore pressure increase of 5 to 6 mpa between the baseline survey and the monitor survey estimated pressure changes using the proposed relationships fit very well with observations between the baseline and monitor seismic surveys 27 of the estimated recoverable hydrocarbon reserves were produced from this segment the estimated saturation changes also agree well with observed changes apart from some areas in the water zone that are mapped as being exposed to saturation changes which is unlikely saturation changes in other segments close to the original oil water contact and the top reservoir interface are also estimated and conf

development of crystal morphology during unitaxial growth in a progressively widening vein ii numerical simulations of the evolution of antitaxial fibrous veins the development of fibrous morphology and capability of fibres for tracking the opening trajectory were investigated using numerical simulations of a natural antitaxial fibrous vein starting from a non unique best case variation of fracture opening velocity grain size wall roughness growth anisotropy and crystal growth velocity shows that these parameters differ in importance for crystal morphology and tracking capability fibrous veins can be simulated using crack seal opening of the fracture grain boundaries track the opening trajectory if the wall roughness is high opening increments are small and crystals touch the wall before the next crack increment starts

fluid overpressure from chemical reactions in serpentinite within the source region of deep episodic tremor slow fault slip includes a range of transient phenomena that occur over timescales longer than those of standard earthquakes slow slip events are often closely associated with swarms of tectonic tremor deep episodic tremor and slip close to the slab mantle interface in subduction zones has been linked to high fluid pressures produced by dehydration of the subducting slab at greater depths the slab mantle interface is a fundamental chemical boundary where mantle rocks are sheared and mixed with oceanic slab lithologies in a highly reactive environment to form serpentinite here we present field and microstructural observations from the plate boundary scale crustal livingstone fault in new zealand that suggest chemical reactions involving serpentinite can promote rock hardening and generate in situ fluid overpressures we infer that these processes collectively can result in hydrofracturing and a transition from distributed creep to localized brittle failure and faulting serpentinite related reactions occur over a wide range of pressure and temperature conditions that overlap with those in many forearc mantle wedges we conclude that the release of fluids derived from such reactions may be an additional and widespread mechanism to generate high fluid pressure patches and brittle failure in the source region of deep tremor along the slab mantle interface chemical reactions between slab and mantle rocks may lead to brittle failure where deep episodic tremor occurs in subduction zones according to field and microstructural observations of a shear zone in new zealand

structurally controlled growth of fibrous amphibole in tectonized metagabbro constraints on asbestos concentrations in non serpentinized rocks the nucleation and concentration of asbestos in rocks are mostly associated with mechanisms of fibre formation combined with the water dependent mineralogical alteration produced during the serpentinization of ultramafic masses very little is known about the structural settings and tectonic histories that influence and control the occurrence of asbestos in non serpentinized rocks diffusely embedded within tectonized ophiolitic suites focusing on a case history provided by a tectonized metagabbro from the ligurian alps northern italy a multiscale structural petrographic approach is used to investigate the relationships between rock fabric and fibrous amphibole growth within the metagabbro meso to microstructural observations are used to document the role of structurally controlled fluid rock interactions in localizing the growth of fibrous amphibole during ductile to brittle shearing mylonitic foliation to shear veins a qualitative structural scenario is provided to illustrate the growth of asbestos amphiboles in shear veins during the progression of shear deformation towards semi brittle rheological conditions the mechanisms of the structurally controlled growth of fibrous amphibole in non serpentinized rocks imply an examination of the tectonic boundary conditions that are at the origin of the concentration of asbestos in ophiolitic rocks involved in orogenic belt construction

asbestos fibre identification vs evaluation of asbestos hazard in ophiolitic rock melanges a case study from the ligurian alps italy in recent years the high incidence of harmful health effects through inhalation of airborne asbestos from amphibole bearing rock melanges has been thoroughly documented here we present a field based multi scale geological approach aimed at illustrating the occurrence of amphibole fibrous mineralisation in an ophiolitic suite from the ligurian alps italy and discussing the implication on in situ determination of the asbestos hazard the rock melange is composed of plurimetre sized blocks of different lithotypes metagabbro serpentinite chloritoschist juxtaposed by the meaning of tectonic structures the geological structural survey revealed that the fibrous mineralisation is localised in specific structural sites of the rock volume including veins and schistosity both micro chemical and crystal structure analyses on selected fibrous samples revealed that actinolite fibres grow in veins within the metagabbro and in chloritoschists while fibrous tremolite occurs in serpentinite schistosity the morphological features of these amphibole fibres have been analysed in tem images and used for classifying them as asbestiform or non asbestiform the results show that the asbestos hazard determination is not unequivocally identified when different procedures for asbestos fibre identification and classification are applied this may have impact on normatives and regulations in defining environmental hazards due to asbestos occurrence

time lapse monitoring of fluid induced geophysical property variations within an unstable earthwork using p wave refraction abstracta significant portion of the uk s transportation system relies on a network of geotechnical earthworks cuttings and embankments that were constructed more than 100 years ago whose stability is affected by the change in precipitation patterns experienced over the past few decades the vulnerability of these structures requires a reliable cost and time effective monitoring of their geomechanical condition we have assessed the potential application of p wave refraction for tracking the seasonal variations of seismic properties within an aged clay filled railway embankment located in southwest england seismic data were acquired repeatedly along the crest of the earthwork at regular time intervals for a total period of 16 months p wave first break times were picked from all available recorded traces to obtain a set of hodocrones referenced to the same spatial locations for various dates along the surveyed period of time traveltimes extracted from each acquisition were then compared to trac

earthquake potential along the northern hayward fault california the hayward fault slips in large earthquakes and by aseismic creep observed along its surface trace dislocation models of the surface deformation adjacent to the hayward fault measured with the global positioning system and interferometric synthetic aperture radar favor creep at 7 millimeters per year to the bottom of the seismogenic zone along a 20 kilometer long northern fault segment microearthquakes with the same waveform repeatedly occur at 4 to 10 kilometer depths and indicate deep creep at 5 to 7 millimeters per year the difference between current creep rates and the long term slip rate of 10 millimeters per year can be reconciled in a mechanical model of a freely slipping northern hayward fault adjacent to the locked 1868 earthquake rupture which broke the southern 40 to 50 kilometers of the fault the potential for a major independent earthquake of the northern hayward fault might be less than previously thought

structures in natural serpentinite gouges abstract serpentinite gouges from cataclastic fault zones in the voltri massif nw italy are dominated by few discrete boundary parallel y shears and oblique synthetic riedel r 1 shears separating domains of massive gouge unlike natural clay rich fault gouges the cohesive microbreccia domains only rarely show foliations these structures dominated by discrete shears indicate a significant degree of localization of the deformation at the scale of the gouge the dominant gouge forming mechanisms were fracturing of olivine and pyroxene grains along serpentinized grain boundaries and initiation of transgranular fractures in pyroxene grains in domains of lattice bending and grain indentation syn tectonic serpentinization added to the weakening of fragments textural similarities with experimentally formed gouges suggest that the voltri fault zones were dominated by stick slip sliding hence that they may represent fossil seismogenic faults a notable aspect of the gouge texture in the voltri fault zones is a sigmoidal shape of the riedel shears which bend into parallelism with the y shears it is argued that this geometry was largely controlled by local perturbations of the stress field near heterogeneities on y or p shears the average spacing between the r 1 shears increases linearly with 0 47 times the distance between bounding y shears this proportionality may reflect a balance between the cohesive strength of the gouge material and the shear stresses acting on the bounding slip planes

the frictional behavior of serpentinite implications for aseismic creep on shallow crustal faults serpentine is common in many active faults and may be responsible for aseismic creep along segments of these faults to test this we have conducted a series of velocity stepping experiments to determine the frictional velocity dependence of serpentinite we slid initially bare rough surfaces of antigorite serpentinite at room temperature with velocities from 0 0032 to 10 0 m s 1 0 102 to 3 2 105 mm yr and normal stresses from 25 to 125 mpa we find that the velocity dependence of serpentinite undergoes a transition from velocity weakening at fast loading velocities to velocity strengthening at slow velocities and that this change is accompanied by other changes in the constitutive behavior these results suggest that serpentinite should not be the site of instability initiation during sliding at plate velocities but may permit propagation of unstable slip initiated elsewhere

rock and mineral transformations in a fault zone leading to permanent creep interactions between brittle and viscous mechanisms in the san andreas fault creep processes may relax part of the tectonic stresses in active faults either by continuous or episodic processes the aim of this study is to obtain a better understanding of these creep mechanisms and the manner in which they change in time and space results are presented from microstructural studies of natural samples collected from san andreas fault observatory at depth borehole drilled through the san andreas fault which reveal the chronology of the deformation within three domain types i a relatively undeformed zone of the host rock reflects the first step of the deformation process with fracturing and grain indentations showing the coupling between fracturing and pressure solution ii shear deformation development that associates fracturing and solution cleavage processes leads to profound changes in rock composition and behavior with two types of development depending on the ratio between the amount of dissolution and deposition abundant mineral precipitation strengthens some zones while pervasive dissolution weakens some others iii zones with mainly dissolution trended toward the present day creeping zones thanks to both the passive concentration of phyllosilicates and their metamorphic transformation into soft minerals such as saponite this study shows how interactions between brittle and viscous mechanisms lead to widespread transformation of the rocks and how a shear zone may evolve from a zone prone to earthquakes and postseismic creep to a zone of steady state creep in parallel the authors discuss how the creeping mechanism mainly controlled by the very low friction of the saponite in the first 3 4 km depth may evolve with depth

applications of algebraic reconstruction techniques to crosshole seismic data tomographic imaging techniques were applied to two crosshole data sets to determine the velocity structures and the reliability and resolution of the algorithms on real data the experiments were carried out at the retsoff salt mine in new york and at the underground radioactive waste study site at the stripa mine facility in sweden the traveltimes at restoff were high quality and were obtained over raypaths of up to 500 m in length the structure was quite complicated with velocity contrasts up to 50 percent the stripa site was in granitic rock with velocity contrasts of only a few percent the dimensions of the experiment were small with maximum ray lengths of just over 10 m the data at this site were collected with very high accuracy source and receiver locations were measured to better than 1 0 mm and traveltimes were read to 0 001 ms a number of algorithms similar to the algebraic reconstruction techniques art used in medical imaging have been applied to the data some modifications of the algorithms such as the application of weighting schemes damping parameters and curved raypaths were performed the resulting velocity fields were compared to the known fields and with each other to determine an optimal method the algorithms were found to be a rapid reliable means of reconstructing the slowness field of real data low velocity zones were recovered with accuracy in location and value it was also found that great care was necessary in application of the techniques to ensure that proper damping parameters are used and the proper number of iterations taken otherwise poor reconstructions will result

sequential deformation from serpentinite mylonite to metasomatic rocks along the sashu fault sw japan the deformation of serpentinites along the sashu fault saganoseki peninsula sw japan involved both mylonitization and brecciation the brecciation was accompanied by metasomatism and the metasomatized rocks record additional ductile deformation serpentinite mylonite is composed mainly of antigorite with minor magnetite and cr spinel foliation and lineation in this rock type are defined by the shape preferred orientation of antigorite and the alignment of fine grained magnetite and carbonate minerals the antigorite also shows a lattice preferred orientation lpo as measured with a universal stage with c axes oriented near perpendicular to the foliation and b axes oriented parallel to the lineation brecciation is concentrated in the reaction zone along the boundary between serpentinites and surrounding country rocks talc and chlorite schists developed in this zone as a result of associated metasomatism and deformation consequently they contain composite planar fabrics and drag folds the breccias themselves are composed of serpentinite fragments cut by veins of calcite and talc analyses of fluid inclusions in calcite from the breccia indicate that brecciation occurred at 200 300 c at depths of 3 8 11 8 km metasomatic reactions particularly the formation of talc contributed to weakening of this serpentinite bearing fault zone

long term fault creep observations in central california the u s geological survey usgs has been monitoring aseismic fault slip in central california for more than 10 years as part of an earthquake prediction experiment since 1968 the usgs creep network has grown from one creep meter at the cienega winery south of hollister to a 44 station network that stretches from hayward east of san francisco bay to palmdale in southern california in general the long term slip pattern is most variable on sections of the faults where several magnitude 4 and larger earthquakes occurred during the recording period e g calaveras fault near hollister and san andreas fault between san juan bautista and bear valley these fault sections are the most difficult to characterize with a single long term slip rate in contrast sections of the faults that are seismically relatively quiet e g san andreas fault between bear valley and parkfield produce the steadiest creep records and are easiest to fit with a single long term slip rate appendix is available with entire article on microfiche order from the american geophysical union 2000 florida avenue n w washington d c 20009 document j82 004 1 00 payment must accompany order

development of schistosity by dissolution crystallization in a californian serpentinite gouge to understand the behaviour and deformation mechanisms of serpentinites in the seismogenic zone we study the deformation macro and microstructures of serpentinites along the santa ynez fault in the san andreas system at the outcrop scale deformation is localized in a gouge zone that shows three different structures 1 micrometric undeformed fragments clasts of the previously serpentinized peridotite 2 localized shear planes y and r and 3 a penetrative schistosity s observations under sem and tem reveal that the schistosity corresponds to serpentine fibres parallel to each other and whose orientation varies as they wrap around clasts tem micro textures indicate that these long fibres result from continuous syntectonic growth rather than from reorientation of pre existing fibres implying a slow transfer process that occurs at short distances we propose a dissolution diffusion crystallization process for the formation of the schistosity that corresponds to a low strain rate creeping process of deformation that can be effective in aseismic fault segments

plastic deformation and development of antigorite crystal preferred orientation in high pressure serpentinites we have inferred the deformation mechanisms of antigorite by high resolution ebsd mapping of samples from cerro del almirez ultramafic massif betic cordillera se spain textural relations and phase diagram calculations constrain the foliation development conditions to the subduction prograde path at up to 600 630 c and 1 6 1 9 gpa deformation was followed by static annealing of antigorite at ca 680 c the crystal preferred orientation cpo of antigorite is characterised by a strong alignment of 001 poles normal to the foliation plane and a weaker but clear parallelism between 100 axes and the macroscopic lineation defined by the elongation of magnetite aggregates analysis of misorientations across subgrains shows predominance of 010 rotation axis consistent with activation of the 100 001 slip system however tilt subgrain boundaries subparallel to 100 probably formed by edge dislocations of this system are subsidiary most subgrain boundaries are subparallel to 001 planes they are interpreted as 001 twins wherein continuing viscoplastic deformation resulted in a slight increase of the misorientation between the twins modelling of the evolution of the antigorite cpo using a lower bound approach and considering different deformation regimes and sets of basal and non basal slip systems has shown that intensities of 100 and 010 maxima reflect the relative strength of the antigorite 100 001 and 010 001 systems activation of other basal or non basal slip systems does not change significantly the cpo patterns but results in less concentrated cpo 3d transpression models better reproduce the cpo of natural antigorite serpentinites we propose that the widespread occurrence of strong cpo in high pressure antigorite serpentinite is consistent with deformation by dislocation creep with dominant glide on hk0 001 together with the activation of twinning implying that a power law rheology would better account for the mechanical behaviour of antigorite serpentinite deep in the subduction channel and mantle wedge

chemical controls on fault behavior weakening of serpentinite sheared against quartz bearing rocks and its significance for fault creep in the san andreas system 1 the serpentinized ultramafic rocks found in many plate tectonic settings commonly are juxtaposed against crustal rocks along faults and the chemical contrast between the rock types potentially could influence the mechanical behavior of such faults to investigate this possibility we conducted triaxial experiments under hydrothermal conditions 200 350 c shearing serpentinite gouge between forcing blocks of granite or quartzite in an ultramafic chemical environment the coefficient of friction of lizardite and antigorite serpentinite is 0 5 0 6 and increases with increasing temperature over the tested range however when either lizardite or antigorite serpentinite is sheared against granite or quartzite strength is reduced to 0 3 with the greatest strength reductions at the highest temperatures temperature weakening and slowest shearing rates velocity strengthening the weakening is attributed to a solution transfer process that is promoted by the enhanced solubility of serpentine in pore fluids whose chemistry has been modified by interaction with the quartzose wall rocks the operation of this process will promote aseismic slip creep along serpentinite bearing crustal faults at otherwise seismogenic depths during short term experiments serpentine minerals reprecipitate in low stress areas whereas in longer experiments new mg rich phyllosilicates crystallize in response to metasomatic exchanges across the serpentinite crustal rock contact long term shear of serpentinite against crustal rocks will cause the metasomatic mineral assemblages which may include extremely weak minerals such as saponite or talc to play an increasingly important role in the mechanical behavior of the fault our results may explain the distribution of creep on faults in the san andreas system

exploring fault rocks at the nanoscale abstract this paper aims to demonstrate the potential role of micro nanoscale investigations in the study of fault rocks given that a multiscale and multidisciplinary approach linking structural geology petrology and mineralogy from the outcrop to the unit cell scale is essential to the comprehensive characterization of geological materials to explore fault rocks at the sub micrometer scale a fundamental contribution is provided by the transmission electron microscope tem which offers a real time combination of high resolution images diffraction patterns and chemical data for extremely small volumes providing accurate mineralogical and nanostructural determinations this paper focuses on a few selected examples that are particularly appropriate for tem investigation in particular 1 grain size determinations on ultrafine grained fault rocks 2 detection and characterization of poorly crystalline minerals e g clays and amorphous phases within fault rocks 3 identification of deformation induced mineral reactions occurring within fault zones e g dolomite decarbonation and serpentine dehydration and 4 observations of recurrent deformation nanotextures in phyllosilicates tem investigation can provide an unexpected amount of data much of which cannot be obtained by conventional techniques and it can therefore significantly contribute to an understanding of the physico chemical conditions for faulting and fault mechanics

deformation and structural evolution of mantle peridotites during exhumation on transform faults a forced transition from ductile to brittle regime abstract the exhumation of peridotite rocks in oceanic transform zones passes by the rheological transition between the ductile and brittle deformation until the complete emplacement in the oceanic lithosphere the sao pedro and sao paulo archipelago spspa in the equatorial atlantic records the deformational products of ductile brittle and the rocks fluid interaction generating specific structures in each domain the deformational stages are related to the transpressional and transtensional geodynamics of sao paulo transform fault firstly during transpression exhumation occurs associated with the ductile domain causing intense mylonitization in temperatures between 700 and 800 c defined by olivine and orthopyroxene recrystallization the interaction with fluids initially originated from the mantle generates amphibole and oxide rich layers marking the passage to a semi brittle deformation the continuation of peridotite exhumation associated with an nw se shortening and transpressional led to a higher availability of hydrothermal fluids as a consequence four serpentinization episodes are recorded which are associated with semi brittle to brittle transition under temperatures between 300 and 400 c finally the complete exhumation and establishment of brittle mechanisms led to carbonatation phase near the surface with temperatures ranging from 300 to 150 c the active nw se tectonic stress generated e w strike slip faults that were filled by carbonates recording the final exhumation stage

practical techniques for 3d resistivity surveys and data inversion1 techniques to reduce the time needed to carry out 3d resistivity surveys with a moderate number 25 to 100 of electrodes and the computing time required to interpret the data have been developed the electrodes in a 3d survey are normally arranged in a square grid and the pole pole array is used to make the potential measurements the number of measurements required can be reduced to about one third of the maximum possible number without seriously degrading the resolution of the resulting inversion model by making measurements along the horizontal vertical and 45 diagonal rows of electrodes passing through the current electrode the smoothness constrained least squares inversion method is used for the data interpretation the computing time required by this technique can be greatly reduced by using a homogeneous half space as the starting model so that the jacobian matrix of partial derivatives can be calculated analytically a quasi newton updating method is then used to estimate the partial derivatives for subsequent iterations this inversion technique has been tested on synthetic and field data where a satisfactory model is obtained using a modest amount of computer time on an 80486dx2 66 microcomputer it takes about 20 minutes to invert the data from a 7 by 7 electrode survey grid using the techniques described below 3d resistivity surveys and data inversion can be carried out using commercially available field equipment and an inexpensive microcomputer

strengths of serpentinite gouges at elevated temperatures serpentinite has been proposed as a cause of both low strength and aseismic creep of fault zones to test these hypotheses we have measured the strength of chrysotile lizardite and antigorite rich serpentinite gouges under hydrothermal conditions with emphasis on chrysotile which has thus far received little attention at 25 c the coefficient of friction of chrysotile gouge is roughly 0 2 whereas the lizardite and antigorite rich gouges are at least twice as strong the very low room temperature strength of chrysotile is a consequence of its unusually high adsorbed water content when the adsorbed water is removed chrysotile is as strong as pure antigorite gouge at room temperature heating to 200 c causes the frictional strengths of all three gouges to increase limited data suggest that different polytypes of a given serpentine mineral have similar strengths thus deformation induced changes in polytype should not affect fault strength at 25 c the chrysotile gouge has a transition from velocity strengthening at low velocities to velocity weakening at high velocities consistent with previous studies at temperatures up to 200 c however chrysotile strength is essentially independent of velocity at low velocities overall chrysotile has a restricted range of velocity strengthening behavior that migrates to higher velocities with increasing temperature less information on velocity dependence is available for the lizardite and antigorite gouges but their behavior is consistent with that outlined for chrysotile the marked changes in velocity dependence and strength of chrysotile with heating underscore the hazards of using room temperature data to predict fault behavior at depth the velocity behavior at elevated temperatures does not rule out serpentinite as a cause of aseismic slip but in the presence of a hydrostatic fluid pressure gradient all varieties of serpentine are too strong to explain the apparent weakness of faults such as the san andreas

strength of chrysotile serpentinite gouge under hydrothermal conditions can it explain a weak san andreas fault chrysotile bearing serpentinite is a constituent of the san andreas fault zone in central and northern california at room temperature chrysotile gouge has a very low coefficient of friction 0 2 raising the possibility that under hydrothermal conditions might be reduced sufficiently to 0 1 to explain the apparent weakness of the fault to test this hypothesis we measured the frictional strength of a pure chrysotile gouge at temperatures to 290 c and axial shortening velocities as low as 0 001 m s as temperature increases to 100 c the strength of the chrysotile gouge decreases slightly at low velocities but at temperatures 200 c it is substantially stronger and essentially independent of velocity at the lowest velocities tested we estimate that pure chrysotile gouge at hydrostatic fluid pressure and appropriate temperatures would have shear strength averaged over a depth of 14 km of 50 mpa thus on the sole basis of its strength chrysotile cannot be the cause of a weak san andreas fault however chrysotile may also contribute to low fault strength by forming mineral seals that promote the development of high fluid pressures

detecting clandestine tunnels using near surface seismic techniques abstractgeophysical detection of clandestine tunnels is a complex problem that has met with limited success multiple methods have been applied spanning several decades but a reliable solution has yet to be found we evaluated shallow seismic data collected at a tunnel test site representative of geologic settings found along the southwestern u s border our results demonstrated the capability of using p wave diffraction and surface wave backscatter techniques to detect a purpose built subterranean tunnel near surface seismic data were also collected at multiple sites in afghanistan to detect and locate subsurface anomalies including data collected over the escape tunnel discovered in 2011 at the sarposa prison in kandahar afghanistan which allowed more than 480 prisoners to escape and data from another shallow tunnel recently discovered at an undisclosed location the final example from afghanistan was the first time surface based geophysical methods have detected a tunnel whose presence and loca

kinematic analysis of serpentinite structures and the manifestation of transpression in southwestern puerto rico abstract faults and shear zones recorded in the monte del estado and rio guanajibo serpentinite masses in southwestern puerto rico show previously unrecognized southwestward tectonic transport the orientations of planar and linear structures and the sense of slip along faults and shear zones determined by offset rock layers drag folds in foliations and steps in slickensided surfaces and or s c fabrics from 1846 shear planes studied at more than 300 stations reveal two predominant groups of faults 1 northwesterly striking thrust faults and easterly striking left lateral faults and 2 northwesterly striking right lateral faults and easterly striking thrust faults shortening and extension p and t axes calculated for geographic domains within the serpentinite reveal early north trending shortening followed by southwestward directed movement during which older structures were re activated the sw directed shortening is attributed to transpression that accompanied late eocene left lateral shearing of the serpentinite a third younger group comprising fewer faults consists of northwesterly striking left lateral faults and north directed thrusts that also may be related to the latest transpressional deformation within puerto rico deformational events in puerto rico correlate to tectonic events along the caribbean north american plate boundary

deformation mechanisms and rheology of serpentines in experiments and in nature microstructures in serpentine samples recovered from deformation experiments performed at high pressure 1 8 gpa high temperature 150 500 degrees c and laboratory strain rates 4 10 4 10 6 s 1 were studied using transmission electron microscopy on thin sections prepared by focused ion beam lizardite crystals deform by easy glide along the basal planes associated with microkink this mechanism is associated with a plastic strength of similar to 100mpa that defines the upper bound of lizardite strength in natural conditions antigorite crystals deform essentially by conjugated slip along 101 and 10 1 over bar planes observed in sections close to 010 this conjugate system results in an apparent global slip system akin to 100 001 in all samples delamination and comminution produce fine grained interconnected regions at grain boundaries because intracrystalline deformation mechanisms are insufficient to satisfy the von mises criterion the deformation laws of lizardite plastic flow and antigorite strain rate dependent differ because of their differing intracrystalline deformation mechanisms subgrain boundary geometry in natural samples and 100 001 crystal preferred orientations indicate the activation of slip systems similar to those observed in experimentally deformed samples suggesting that the strain rate dependent rheology of antigorite derived from experiments applies to subduction zone conditions delamination of antigorite crystals of a few tens to hundreds of nanometers is not observed in natural samples from subduction zones suggesting plastic deformation of serpentinites at natural low strain rates and stresses is likely accompanied by recrystallization through dissolution precipitation processes that act as a low temperature equivalent of dynamic recrystallization through diffusion at high temperature

dehydration reactions and micro nanostructures in experimentally deformed serpentinites high t torsion experiments on lizardite chrysotile serpentinites produced mineralogical and micro nanostructural changes with important implications in rheological properties high resolution tem showed that specimens underwent ductile by microkinking and 001 interlayer glide and brittle deformation by microfracturing together with dehydration and break down reactions lizardite is affected by polytypic disorder and microkinking kink axial planes at high angle with respect to 001 planes that were not present in the initial ordered 1t lizardite chrysotile fibres are deformed resulting in elliptical cross sections with strong loss of interlayer cohesion both lizardite and chrysotile break down to a fine intergrowth of olivine up to 200 nm talc up to 30 nm and poorly crystalline material lizardite out reaction preferentially occurs at kink axial planes representing sites of preferential strain and enhanced reactivity conversely chrysotile break down is a bulk process resulting in large healed olivine aggregates up to micrometric in size overall observations suggest that dehydration and break down reactions are more advanced in chrysotile than in lizardite

seismic modeling to monitor co2 geological storage the atzbach schwanenstadt gas field 1 we develop a petro elastical numerical methodology to compute realistic synthetic seismograms and analyze the sensitivity of the seismic response when injecting carbon dioxide co2 in a depleted gas reservoir the petro elastical model describes the seismic properties of the reservoir rock saturated with co2 methane and brine and allows us to estimate the distribution and saturation of co2 during the injection process the gas properties as a function of the in situ pressure and temperature conditions are computed with the peng robinson equation of state taking into account the absorption of gas by brine wave attenuation and velocity dispersion are based on the mesoscopic loss mechanism which is simulated by an upscaling procedure to obtain an equivalent viscoelastic medium corresponding to partial saturation at the mesoscopic scale having the equivalent complex and frequency dependent bulk dilatational modulus we include shear attenuation and perform numerical simulations of wave propagation at the macroscale by solving the viscoelastic differential equations using the memory variable approach the pseudo spectral modeling method allows general material variability and provides a complete and accurate characterization of the reservoir the methodology is used to assess the sensitivity of the seismic method for monitoring the co2 geological storage at the atzbach schwanestadt depleted gas field in austria the objective of monitoring is the detection of the co2 plume in the reservoir and possible leakages of co2 the leakages are located at different depths where the co2 is present as gaseous liquid and supercritical phases even though the differences can be very subtle this work shows that seismic monitoring of co2 from the surface is possible while the identification of shallow leakages is feasible the detection of the plume and deep leakages located in the caprock just above the injection formation is more difficult but possible by using repeatability metrics such as the normalized rms nrms images considering real data conditions affected by random noise a reference detection threshold for deep leakages and the co2 plume in the reservoir corresponds to a signal to noise ratio of about 10 db

thermal decomposition of serpentine during coseismic faulting nanostructures and mineral reactions abstract this paper reports a detailed characterization of an antigorite bearing serpentinite deformed at seismic slip rate 1 1 m s in a high velocity friction apparatus micro nanostructural investigation of the slip zone 200 m thick revealed a zonal arrangement with a close juxtaposition of horizons with significantly different strength respectively consisting of amorphous to poorly crystalline phases with bulk anhydrous composition close to starting antigorite and of highly crystalline assemblages of forsterite and disordered enstatite 200 nm in size and in polygonal like nanotextures the slip zone also hosts micro nanometre sized cr magnetite grains aligned at low angle with respect to the slipping surface and inherited from the host serpentinite overall observations suggest that frictional heating at asperities on the slipping surface induced a temperature increase up to 820 1200 c in agreement with flash temperature theory responsible for serpentine complete dehydration and amorphization followed by crystallization of forsterite and enstatite under post deformation static conditions the results of this study may provide important keys for the full comprehension of the mechanical behaviour and of the possible geodynamical role of serpentinite hosted faults through the seismic cycle

nondilatant brittle deformation of serpentinites implications for mohr coulomb theory and the strength of faults we conducted deformation experiments to investigate the strength deformation processes and nature of the brittle ductile transition of lizardite and antigorite serpentinites a transition from localized to distributed deformation occurs as confining pressure increases from 200 to 400 mpa at room temperature deformation in both brittle localized and ductile distributed regimes is accommodated by shear microcracks which form preferentially parallel to the 001 cleavage axial microcracks mode i are infrequently observed volumetric strain measurements demonstrate that brittle deformation is mostly nondilatant consistent with the shear dominated microcracking three observations indicate that deformation in the ductile regime is accommodated by cataclastic flow 1 a lack of evidence for crystal plastic deformation 2 a positive pressure dependence of the maximum differential stress and 3 abundant evidence for brittle microcracking the weakness of serpentinites relative to other brittle rocks is explained by a low fracture strength along the 001 cleavage combined with the low pressure dependence of strength the transition from brittle to ductile deformation occurs at the crossover between the strength of intact serpentinite and the friction law unique to each type of serpentinite rather than the more general byerlee s law if brittle deformation regimes are defined based on the mode of microcracking and on the occurrence of crystal plasticity serpentinites define an end member style of nondilatant brittle deformation this deformation style may result in extremely weak faults in nature and it may also strongly influence the tectonic evolution of the oceanic lithosphere where serpentinite is present

natural deformation related to serpentinisation of an ultramafic inclusion within a continental shear zone the key role of fluids abstract the role of fluids in the deformation of continental serpentinites is investigated from structural microstructural and petrographic analyses applied to a km scale porphyroclast mantled in a viscous matrix of amphibolites the clast is sited within a shear zone of the palaeozoic maures massif france syntectonic fluid rock interactions occurred from km to mm scales at first on the clast borders along the main rheological boundaries then within the clast they are accommodated macroscopically by slickenfibers faults and microscopically by shear microcracks within crack seal veins typifying an intermediate brittle ductile behaviour of serpentinites three main stages of deformation serpentinisation processes occurred in relation with the left lateral movement of the hosted shear zone they developed under metamorphic conditions evolving from amphibolites to green schists facies conditions 400 mpa 550 c to 200 mpa

unique deformation processes involving the recrystallization of chrysotile within serpentinite implications for aseismic slip events within subduction zones serpentinite bodies within the franciscan complex a mesozoic accretionary prism located in california usa display a unique form of deformation that involves the recrystallization of chrysotile and the formation of a block in matrix structure the phacoidal shaped blocks have a preferred orientation and result from the local replacement of serpentine minerals by chrysotile grains that are aligned parallel to ductile shear planes such as s c foliation ultimately some of the rocks evolved into chrysotile schist the relic blocks are also fragmented into multiple parts with the spaces between fragments being infilled by recrystallized chrysotile the low coefficient of friction of chrysotile means that this deformation process acts to suppress the frictional properties of the entire serpentinite body within the forearc mantle this phenomenon can be attributed to the slip style that occurs in aseismic regions of subduction zones in areas shallower than the stability field of antigorite

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

aluminum speeds up the hydrothermal alteration of olivine the reactivity of ultramafic rocks under hydrothermal conditions controls chemical fluxes at the interface between the internal and external reservoirs of silicate planets on earth hydration of ultramafic rocks is ubiquitous and operates from deep subduction zones to shallow lithospheric environments where it considerably affects the physical and chemical properties of rocks and can interact with the biosphere this process also has key emerging societal implications such as the production of hydrogen as a source of carbon free energy to date the chemical model systems used to reproduce olivine hydrothermal alteration lead to the formation of serpentine with sluggish reaction rates here we use in situ diamond anvil cell experiments and show that the presence of aluminum in hydrothermal fluids increases the rate of olivine serpentinization by one to two orders of magnitude aluminum increases the solubility of olivine and enhances the precipitation of aluminous serpentine after two days olivine crystals were fully transformed to aluminous serpentine under conditions typical for natural hydrothermal environments i e 200 and 300 c 200 mpa this result motivates a re evaluation of the natural rates of olivine serpentinization and of olivine hydrolysis in general in a wide range of settings this discovery also opens the potential of the serpentinization reaction for industrial scale production of hydrogen at economically feasible timescales and temperature