kinematic evolution of the junction of the san andreas garlock and big pine faults california if the san andreas fault with about 300 km of right slip the carlock fault with about 60 km of left slip and the big pine fault with about 15 km of left slip are considered to have been contemporaneously active a space problem at their high angle junctions becomes apparent large crustal masses converge in the area of the junctions as a result of the simultaneous large displacements on the faults we present here a model in which an early straight north northwest trending san andreas deforms to its present bent configuration in response to a westward displacement of crust north of the garlock fault during this deformation the crust north of the garlock in the vicinity of the junction undergoes north south shortening while the fault junction migrates along the trace of the san andreas fault to the southeast relative to its original position as a result of this migration the mojave area is displaced to the east relative to the original junction position we suggest a similar history in mirror image for the big pine fault and the areas of crust adjacent to it

palinspastic reconstruction of southeastern california and southwestern arizona for the middle miocene a paleogeographic reconstruction of southeastern california and southwestern arizona at 10 ma has been made based on available geologic and geophysical data clockwise rotation of 39 has been reconstructed in the eastern transverse ranges consistent with paleomagnetic data from late miocene volcanic rocks and with slip estimates for left lateral faults within the eastern transverse ranges and nw trending right lateral faults in the mojave desert this domain of rotated rocks is bounded by the pinto mountain fault on the north the model requires that the western part of the late miocene pinto mountain fault was a thrust fault which gained displacement to the west because of the absence of evidence for rotation of the san bernardino mountains or for significant right slip faults within the san bernardino mountains the squaw peak thrust system of meisling and weldon 1989 may be a western continuation of this fault system the sheep hole fault bounds the rotating domain on the east east of this fault an array of nw trending right slip faults and south trending extensional transfer zones has produced a basin and range physiography while accumulating up to 16 km of right slip this maximum is significantly less than the 37 5 km of right slip required in this region by a recent reconstruction of the central mojave desert dokka and travis 1990a geologic relations along the southern boundary of the rotating domain are poorly known but this boundary is interpreted to involve a series of curved strike slip faults and linking normal faults that accommodated noncoaxial extension quaternary movement on the pinto mountain and nearby faults is unrelated to the rotation of the eastern transverse ranges and a hiatus during part of pliocene time followed the deformation which produced the rotation the reconstructed clemens well fault in the orocopia mountains proposed as a major early miocene strand of the san andreas fault projects eastward towards arizona where early miocene rocks and structures are continuous across its trace making large displacements on this structure unlikely the model predicts a 14 clockwise rotation and 32 extension during late miocene and early pliocene time along a nw trending line parallel to the present trace of the san andreas fault palinspastic reconstructions of the san andreas system based on this proposed reconstruction may be significantly modified from current models

paleomagnetism and neogene clockwise rotation of the northern cady mountains mojave desert of southern california the early to middle miocene hector formation of the northern cady mountains consists of about 400 m of syntectonic alluvial fan and volcaniclastic sediments deposited in the central mojave desert paleomagnetic samples were collected from 60 sites throughout this unit samples from each site were treated using both alternating field and thermal demagnetization these studies and isothermal remanent magnetization experiments indicate that the natural remanent magnetization nrm is complex and that characteristic directions can be carried in either high coercivity or continuous unblocking temperature spectra depending upon the sample a positive reversals test and concordant directions of the alternating field af and low and high temperature components suggest a stable magnetization although a unique magnetostratigraphic correlation to the time scale is difficult the section studied may represent chrons c6b to c5c after screening of individual components and calculation of site means using fisher statistics 48 of the original 60 sites were used to calculate an overall formation mean direction i e declination 18 6 inclination 45 4 95 6 0 using a 20 m y paleomagnetic pole for north america irving and irving 1982 the hector formation has undergone 20 6 7 6 r r of clockwise rotation around a vertical axis this amount of rotation is considerably less than reported for some early miocene rocks elsewhere in the mojave block least squares regression of the 48 site mean declinations versus stratigraphic position indicates no trend throughout the section suggesting that there was no syndepositional rotation of this unit the late cenozoic structural history of the mojave block is considerably more complex than was originally believed and currently available paleomagnetic data indicate that many blocks within this region have experienced different degrees of rotation there were at least two phases of tectonic rotation in this region 1 early miocene about 25 ma probably related to the initiation of extension within the mojave rift and 2 post middle miocene pliocene between about 16 ma to the present possibly related to movement along principal structural features such as the san andreas fault zone the exact timing of the younger phase of tectonic activity cannot be better constrained until paleomagnetic data become available for late cenozoic rocks within the mojave block

present day stress orientations adjacent to active strike slip faults california and sumatra present day stress directions interpreted from well bore breakouts adjacent to two crastal scale active strike slip faults the san andreas fault in california and the great sumatran fault in sumatra indicate that the maximum horizontal stress direction sh is oriented at a high angle 70 90 to both faults the regionally defined stress fields spanning these faults show that adjacent young or actively growing folds have formed orthogonal to sh and are therefore in the thrust fault orientation these observations indicate a decoupling of the strike slip and compressional components of the deformation within these broadly transpressive zones borehole breakouts in 118 wells in western california indicate a regionally consistent stress pattern with sh generally oriented ne sw nearly perpendicular 80 90 to the strike of the san andreas fault the orientation of sh nearly perpendicular to the san andreas fault implies low shear stress on the fault and is consistent with geological interpretations of the coast and transverse ranges indicating active compressional deformation fault plane solutions for recent dip slip style earthquakes principal stress directions determined from inversion of earthquake focal mechanisms and induced hydraulic fracture orientations a stress trajectory map for western california is computed using an iterative statistical algorithm in which observed directional data such as breakout directions are used to obtain a model regional stress field analysis of well bore breakouts in 25 wells within the central and southern oil districts of sumatra indicates that the regionally defined sh adjacent to the active great sumatran strike slip fault is oriented at a high angle 70 80 to the fault this orientation of sh is consistent regionally with geologic stress indicators and focal mechanisms of dip slip earthquakes preliminary analysis of the stress field in the vicinity of the philippine and alpine faults suggests sh is also oriented at a high angle to these active strike slip faults similarly the minimum horizontal stress sh is oriented at a high angle to the the kane and dead sea transforms the observation of sh and sh in the vicinity of active strike slip faults being oriented nearly perpendicular and parallel to the faults suggests that large crustal scale strike slip faults may in general be inherently weak surfaces

lateral velocity variations in southern california i results for the upper crust from pg waves the plate boundary and major crustal blocks in southern california are imaged by a tomographic backprojection of the pg first arrivals recorded by the southern california array the method formulated specifically for local earthquake arrival times is a fast iterative alternative to direct least squares techniques with it we solve the combined problem of determining refractor velocity perturbations and source and station delays resolution and variance are found empirically by using synthetic examples a map showing lateral velocity variations at a depth of approximately 10 km is presented the results show a strong correlation with surface tectonic features clear velocity contrasts exist across the san andreas the san jacinto and the garlock faults the mojave region has the slowest velocities while the peninsula ranges have the highest the san jacinto block has velocities intermediate between mojave and peninsula range velocities and also has early station delays this may indicate that the san jacinto block has overridden mojave material on a shallow detachment surface no velocity variations are found associated with the transverse ranges which we interpret to mean that the surface batholithic rocks in this area do not extend to pg depths

weakening of rock salt by water during long term creep the rheological properties of rock salt are of fundamental importance in predicting the long term evolution of salt based radioactive waste repositories and strategic storage caverns and in modelling the formation of salt diapirs and associated oil traps1 2 the short term high stress rheology of rock salt is well known from laboratory experiments however extrapolation to appropriately low stresses fails to predict the rapid flow seen in certain natural structures furthermore experiments have failed to reproduce the recrystallized microstructure of naturally deformed salt here we report experiments indicating that the above discrepancies can be explained by taking into account the influence of trace amounts of brine trace brine is always present in natural salt but sometimes escapes during experiments our tests on dry dilated salt show more or less conventional dislocation creep behaviour but brine bearing samples show marked weakening at low strain rates this is associated with dynamic recrystallization and a change of deformation mechanism to solution transfer creep because natural rock salt always contains some brine these results cast substantial doubt on the validity of presently accepted dislocation creep laws for predicting the long term rheological behaviour of salt in nature

changes in vp vs with depth implications for appropriate velocity models improved earthquake locations and material properties of the upper crust properties associated with the arrival times of shallow microearthquakes are determined for three tectonically different sites soviet central asia the central united states and southern california simple graphical presentations of the earthquake data including wadati and riznichenko diagrams are used to resolve the trade off in origin time and focal depth estimates of the average half space velocities for p and s waves and of the travel time ratio ts tp are also obtained that are relatively free of any model constraints used to initially locate the earthquake in all cases p and s arrivals exhibit a systematic decrease in ts tp with depth in the upper few kilometers this decrease is significant and matches similar variations observed in situ for the velocity ratio vp vs based on laboratory and theoretical studies such observations are consistent with the closing of saturated microcracks with increasing confining pressure independent determinations of velocity structure and rock properties from both refraction studies and borehole experiments show analogous results by specifying separate velocity models for both p and s waves that reflect this variation rather than assuming a constant value of vp vs we find improved stability in hypocenter determination and increased resolution of shallow seismicity patterns

the deep crustal structure of the mojave desert california from cocorp seismic reflection data cocorp seismic reflection profiling in the western and northern mojave desert of southern california has revealed the presence of numerous major low angle reflecting horizons within the crust these complex though laterally continuous horizons are interpreted to represent major southwesterly dipping crustal fault zones and as such they place important constraints on the tectonic evolution of the region the upper most horizon is interpreted to be the rand thrust which where exposed places precambrian and late cretaceous crystalline rocks over possibly younger pelona rand orocopia schist this reflecting horizon extends for 25 km southwest of the rand mountains where it appears to be truncated at a depth of about 7 4 km by another horizon which may be a later low angle normal fault the other reflecting horizons are not traceable to the surface and so greater ambiguity remains in their interpretation the most prominent of these horizons occurs at midcrustal depths 15 6 km exhibits a ramp and flat geometry and extends over the northern area of the mojave survey into the basin and range province a lower horizon at depths of 20 30 km in the northern part of the survey area is antiformal and appears to terminate above a flat and relatively continuous moho depth horizon the crustmantle transition appears to be represented by a continuous series of reflections which occur at about 10 s 33 km in the north of the survey and at about 8 9 s 26 29 km in the south these reflections are offset in the vicinity of the town of mojave the deep intracrustal fault zones inferred from the cocorp survey may represent 1 the deep crustal continuation of the system of mesozoic thrusts which crops out in southern nevada and southeastern california 2 late cretaceous to early cenozoic northeast vergent thrusts related to the uplift of the pelona orocopia rand schist or 3 low angle normal faults related to early miocene northeast southwest directed crustal extension the cocorp survey also traversed the major strike slip faults that bound the mojave block the san andreas fault zone appears to truncate reflectors at depths of 6 8 and 20 km within the mojave basement suggesting that it is a major vertical feature which extends to at least 20 km depth conversely the garlock fault does not offset an underlying reflecting horizon which occurs at 9 km depth and therefore appears to be a relatively shallow crustal feature

fracture strength of tourmaline and epidote by three point bending test application to microboudin method for estimating absolute magnitude of palaeodifferential stress abstract three point bending tests are conducted on millimetre scale tourmaline and epidote samples at room temperature and pressure to determine fracture strengths suitable for use with the microboudin method of palaeostress analysis the fracture strength of the samples is obtained from the weibull scale and shape parameters using a size effect model three different size effect models are evaluated and it is found that the effective length model is the most appropriate for describing the fracture strength of tourmaline and epidote based on geological constraints the palaeodifferential stresses determined by this method for samples from wadi tayin sultanate of oman greenbushes australia and four high pressure metamorphic belts in japan are in the range of 10 300 mpa

computer simulations of california tectonics confirm very low strength of major faults recent improvements in the technique of modeling fault networks with thin plate finite elements yield full convergence of the solutions without any compromise in the representation of the frictional rheology of the upper crust or the transition to dislocation creep in the lower crust we apply these techniques to model the california region incorporating all faults with estimated slip rate over 1 mm yr as well as variations in elevation heat flow and crustal thickness velocity boundary conditions on the model sides are based on the nuvel 1 plate model and an approximation of deformation in the great basin the frictional and dislocation creep rheologic constants of the crust are calibrated to reproduce the observed variations in the maximum depth of seismicity which occurs at model temperatures of 350 410 c this leaves two free parameters the time averaged coefficient of friction on faults and the apparent activation energy for creep in the lower crust these parameters are systematically varied in three sets of 10 81 and 64 experiments respectively the predictions of each model are tested against three published data sets a set of 79 geologic limits on average slip rate on faults a set of 221 principal stress directions and a set of 841 secular rates of geodetic baselines both trilateration and vlbi very long baseline interferometry the patterns of model scores indicate that the time averaged friction coefficient of major faults is only 0 17 0 25 or only 20 30 of the value 0 85 that is assumed for the friction in the intervening blocks the laser trilateration data taken alone would indicate a preference for a high stress model but this model is one with very little fault slip and seismicity which can be rejected on other grounds in the final three parameter set of 64 models we test whether the anomalous weakness of faults is uniform or proportional to net slip with slip dependent weakening there is a general reduction in prediction errors which supports the hypothesis the geologic data and geodetic data agree in implying that fault weakness that is friction of 0 17 is general with an additional 30 slip dependent weakening of the san andreas to friction of 0 12 or 14 of normal the stress data however are fit best if all weakness is slip dependent effectively only the san andreas is weak thus the hypothesis that all weakening is slip dependent should not be rejected the best models predict all available data with rms root mean square mismatch of as little as 3 mm yr 6 of the relative plate velocity so their predictions may be useful for seismic hazard estimation at least along faults where no data are available this study extends the finding of very low friction from subduction zones where it was previously documented to a dominantly strike slip system in which underthrusting of wet sediment is not widespread return of previously subducted water from the mantle and its chemical or physical binding in fine grained gouge are possible explanations for the very low friction

lithosphere strength inferred from fracture strength of rocks at high confining pressures and temperatures abstract a model is proposed for the strength of the crust or lithosphere in which the peak strength is in the brittle regime but not on the brittle ductile transition the model is based on experimental evidence which suggests that the fracture mechanism changes from ordinary brittle fracture to a high pressure type when the compressive strength equals the frictional strength the model assumes that a high pressure type of fracture is dominant in the earth s lower crust except where very high pore pressures exist it takes account of the scale effect on the strength of rocks the strength of the high pressure regime is estimated as a function of depth or pressure and temperature using experimental results derived at high confining pressures and temperatures in the upper part of the brittle zone where the frictional strength is lower than the compressive strength byerlee s friction law is used in the ductile regime the steady state creep law is used two cases of lithospheric strength dry and wet hydrostatic water pressure respectively are evaluated for thrust strike slip and normal faulting mechanisms with the exception of a wet lithosphere and a normal faulting mechanism the strength between the frictional and creep regimes is approximately constant with depth although the peak strength is in this part

cenozoic crustal rotations in the mojave desert from paleomagnetic studies around barstow california paleomagnetic data from the mojave desert around barstow california suggest 55 to 75 of clockwise rotation of this area in earliest miocene time followed by 23 of counterclockwise rotation in the early miocene and no rotation after 18 ma earliest miocene clockwise rotation of the barstow area is suggested by data from nine sites in the oligocene miocene lane mountain quartz latite and jackhammer formation and may be related to oroclinal bending of the southern sierra nevada batholith data from 13 sites in the early miocene pickhandle formation indicate counterclockwise rotation which coincides with early miocene e ne w sw extension and detachment faulting in the mojave desert other parts of the mojave desert experienced variable senses and amounts of rotation at this time which may be related to drag along transfer zones within the upper plate while the lower plate was not rotated these results suggest that miocene extension in the mojave desert is related to similarly oriented extension in the colorado river extensional corridor early miocene to pliocene rocks of the andesite and dacite of the calico mountains barstow formation andesite of murphy s well and black mountain basalt show no evidence for significant rotation of the barstow area after 18 ma most other paleomagnetic data from elsewhere in the mojave are consistent with a lack of significant post 18 ma rotations suggesting that post 10 ma right lateral faulting there has produced relatively little crustal rotation the exception to this generalization is the northeast corner of the mojave desert which underwent substantial post 18 ma clockwise rotation paleomagnetic flattening data also indicate that post oligocene n s translation of crustal blocks in the mojave desert has been insignificant

mechanism of deformation in a dry porous basalt at high pressures abstract the deformation behavior of a dry fine grained porous basalt with 7 porosity has been studied up to a confining pressure of 2 5 gpa at room temperature microscopic observations for triaxially compressed specimens have been compared with the results on the measurements of strain and activity of acoustic emission ae for the same rock initial pores of 100 200 m size begin to collapse and the compaction starts at a mean pressure of about 400 mpa as confining pressure and differential stress increase compaction is followed by crystals colliding with each other undergoing deformation with microcracks crushing and finally granulation during compaction the rock is compressed beyond the hydrostat and the ae activity is relatively high as confining pressure and differential stress increase to much higher levels e g above 1 5 gpa confining pressure or above 1 5 gpa differential stress at 1 gpa confining pressure most pores collapse areas where the crystals are deformed with numerous microcracks crushed and granulated begin to pervade the specimen further deformation is due to the pervasion of such areas and to the closure of pores or voids a few m in size among granulated grains in this stage of deformation the rock is compressed in the same way as in hydrostatic compression in spite of the uniaxial compression and the ae activity is very low and almost constant the deformation at lower confining pressures is also discussed for a brittle fractured specimen and a bulged specimen with luders bands in the ductile regime cataclastic flow is predominant during the compaction whereas intragranular flow and grain sliding and or a combination of both with cataclastic flow seem to govern the behavior in the post compaction state at higher stress levels the results have interesting implications on studies of the structures of aseismic zones and fault zones in the crust and the upper mantle although further work is necessary to examine whether the governing processes producing ductile flow correspond between the present experimental conditions and those in the earth s interior

paleomagnetic evidence for neogene clockwise tectonic rotations in the central mojave desert california paleomagnetic directions from early miocene volcanic flows indicate that a large part of the central mojave desert has been tectonically rotated stratigraphic relations and isotopic ages suggest that two separate clockwise rotations have occurred an early miocene rotation of 50 15 6 is coincident in time and space with opening of the daggett extensional terrane a post early miocene rotation of 30 to 50 may have affected the alvord mountains and the immediate vicinity

displacements on late cenozoic strike slip faults of the central mojave desert california field studies demonstrate that displacements on northwest striking wrench faults of the central mojave desert are too small to support hypotheses suggesting large interior translations and associated rotation of the province during late cenozoic time the margin of an early miocene structural belt provides the marker with which to establish lateral displacement on individual faults displacement values for faults are as follows lenwood fault 1 5 3 km camp rock fault 1 6 4 0 km calico fault 8 2 km and rodman pisgah faults 6 4 14 4 km cumulative displacement on all the major northwest striking faults of the mojave desert is about 26 7 38 4 km most if not all regionally distributed right shear presumably related to pacific north american plate interaction developed in the central mojave desert after 20 m y b p right shear was preceded by significant amounts of kinematically unrelated northeast southwest crustal extension

two types of brittle fracture of silicate rocks under confining pressure and their implications in the earth s crust abstract four silicate rocks granite gabbro dunite and eclogite were triaxially compressed up to a confining pressure of 3 gpa at room temperature and were found to exhibit brittle fracture behaviour from the measurements of the compressive and frictional strengths and the activity of acoustic emission ae and the observations of the microstructure of fractured specimens the pattern of fracture was found to change when the compressive strength became equal to the frictional strength two types of fracture mechanism low pressure and high pressure types were inferred the low pressure type fracture exhibits features of brittle fracture equivalent to those recorded by previous workers the features of high pressure type fracture are similar to those observed between brittle fracture and ductile creep in high temperature triaxial experiments microcracks are not concentrated close to the main fault and the main fault is sharp and oriented at approximately 45 to the stress direction this suggests that the high pressure type fracture might correspond to the transitional fracture type between the brittle and ductile regimes at room temperature for granite and gabbro samples of a few centimeters size high pressure type fracturing begins at 0 8 gpa confining pressure while for dunite and eclogite it begins at 1 0 and 2 0 gpa respectively the size effect on compressive strength under confining pressure was estimated based on the suggestion that the uniaxial strength decreases with increasing specimen size and ceases to decrease for specimens 1 m this estimation suggests that the compressive strength for a granite specimen a few meters in size would become equal to the frictional strength at confining pressures as low as 60 mpa the significance of this is that it may be necessary to model earthquakes using the high pressure type fracturing because in the absence of pore pressure 60 mpa confining pressure corresponds to a depth of 3 km faulting processes in the crust including both the initial faulting and the movement of existing faults were inferred using the experimental results and the estimated size effect on strength

late cenozoic tectonic evolution of southwestern california we synthesize available geologic and geophysical data from southern california and adjoining areas in order to develop a reconstruction of the tectonic evolution of the southern coast ranges western transverse ranges and borderland regions of california since 30 ma premises include specified relative plate motions for times prior to 10 5 ma and after 3 ma a mid tertiary bight in the continental margin of southern california midcrustal detachment faults under much of california and adjoining regions rotation of the western transverse ranges about an eastern pivot and specified fault displacement histories prior to 18 ma about 90 of the tangential component of pacific north america relative motion was accommodated on an offshore dextral fault system near the toe of the continental slope from 18 ma to 5 5 ma dextral slip was accommodated predominantly on the offshore system but also on a second inboard system that included the san andreas fault fault bounded crustal blocks between the offshore and inboard fault systems underwent translation rotation and extension from 18 ma to 11 5 ma but not from 11 5 ma to 5 5 ma since 5 5 ma dextral slip has been accommodated predominantly on the inboard fault system dextral slip ceased on the offshore fault system north of the transverse ranges but continues on faults west of or within baja california the san gregorio san simeon hosgri fault system has accommodated less than 5 km of the late cenozoic dextral slip the distribution of dextral displacement in the plate boundary zone has been controlled by abrupt jumps of the position of a southern triple junction and to a lesser degree changes in magnitude of pacific north america relative motion based on our reconstuction the tangential component of pacific north america relative motion was 90mm yr for the period 10 5 ma to 5 5 ma and 60mm yr for the period 5 5 ma to 3 ma

ductile behavior of a fine grained porous basalt at room temperature and pressures to 3 gpa abstract a dry fine grained porous rock yakuno basalt with grain size of 0 05 0 4 mm and 7 porosity was triaxially compressed up to 3 gpa confining pressure at room temperature the axial differential stress was increased stepwise at fixed intervals at each step in the ductile regime of the rock above 300 mpa confining pressure the observed strain exhibited time dependent creep behavior the linear time dependent strain was well represented by a power law constitutive equation the stress exponent had a value of 3 at 1 5 gpa and lower confining pressures and decreased as confining pressure increased above 1 5 gpa asymptotically approaching 1 this suggests there is a pressure induced transition of the cataclastic flow mechanism even at room temperature microstructures showed highly granulated grains in former pore spaces in the host areas as well as in the granulated areas extensive microcrack formation was observed at lower confining pressures but there was less microcrack formation at higher confining pressures this contrast probably corresponds to the change in the flow mechanism the equivalent viscosity was estimated as a function of differential stress and strain rate

interaction of folding and thrusting during fold and thrust belt evolution insights from numerical simulations and application to the swiss jura and the canadian foothills abstract we use two dimensional numerical simulations to investigate the interaction of folding buckling and thrusting localized shearing during the structural evolution of fold and thrust belts ftb we model layer parallel shortening of mechanically strong layers resting above a weaker layer referred to as detachment we consider a viscoplastic rheology with linear viscosity and frictional plastic drucker prager yield criterion the detachment thickness is larger in one model half and only this geometrical thickness perturbation triggers folding and thrusting in the simulations we conduct a systematic analysis to quantify the impact of model parameters such as magnitude of the thickness perturbation or detachment rheology frictional or viscous on the ftb evolution the magnitude of the perturbation determines whether or not folding is favoured over thrusting and hence controls the progressive interaction of folding and thrusting during ftb evolution the asymmetry of modelled anticlines is proportional to the relative height of the initial perturbation we also adapt the model configuration to specific mechanical stratigraphies of two regions in natural ftb namely the haute chaine of the jura mountains switzerland and the foothills of alberta canada the modelled fold and thrust geometries agree with different types of observed anticlinal structures such as symmetric detachments folds and asymmetric thrust folds our results suggest that an initial stage of low amplitude buckling likely occurred prior to the main folding and thrusting phase we argue that folding and thrusting are equally important and interacting processes during the structural evolution of many natural ftb

paleomagnetism geochronology and possible tectonic rotation of the middle miocene barstow formation mojave desert southern california the barstow formation of the central mojave desert in the mud hills consists of 965 m of epiclastic and volcanic sediments deposited in an extensional rift this widespread unit is of importance because its mammalian faunas characterize the barstovian land mammal age previous isotopic ages on interbedded tuffs have been used to calibrate middle miocene time and it was deposited in a tectonically active region volcanic ash samples were collected from the barstow formation for isotopic dating using conventional 40 k 40 ar and 40 ar 39 ar single crystal laser fusion methods multiple analyses on minerals from 5 superposed tuffs yielded the following mean 40 k 40 ar ages rak tuff at 16 3 0 3 m y oreodont tuff at 15 9 0 2 m y dated tuff at 14 8 0 2 m y hemicyon tuff at 14 0 0 1 m y and lapilli tuff at 13 4 0 2 m y similar mean ages were also obtained on the following tuffs using single crystal 40 ar 39 ar laser fusion methods oreodont tuff at 15 9 0 06 m y dated tuff at 14 8 0 06 m y and hemicyon tuff at 14 0 0 09 m y in order to develop a local magnetostratigraphy 272 paleomagnetic sites were collected from 7 important sections within the mud hills saturation isothermal remanence experiments suggested the presence of either hematite or magnetite as carriers of remanence after either stepwise alternating field or thermal demagnetization characteristic directions were usually isolated between 30 and 50 mt or 300 and 500 c positive reversal and fold tests indicate a stable remanence acquired prior to folding there are 21 magnetic polarity zones interpreted from the continuous part of the barstow formation in the study area by using the 5 isotopic ages determined from interbedded tuffs the local composite polarity pattern is correlated to the top of chron c5d through the bottom of c5ab on the magnetic polarity time scale or an age range from 17 6 to 13 4 m y the boundary between hemingfordian and barstovian land mammal ages herein occurs 30 m below the oreodont tuff 15 8 0 2 m y within c5b and is taken at about 16 0 m y ago the barstow formational mean declination is 347 1 inc 47 5 95 6 51 d 7 97 suggesting negligible or possibly slight counterclockwise rotation r 4 1 r 10 5 for this part of the mojave block during the past 18 m y a 40 ar 39 ar mean age of 19 3 0 02 m y for the red tuff which is near the local base of the barstow formation and unconformably underlying our composite section constrains a minimum age for the beginning of local basin filling with principally epiclastic sediments

seismic evidence for conjugate slip and block rotation within the san andreas fault system southern california the pattern of seismicity in southern california indicates that much of the activity is presently occurring on secondary structures several of which are oriented nearly orthogonal to the strikes of the major through going faults slip along these secondary transverse features is predominantly left lateral and is consistent with the reactivation of conjugate faults by the current regional stress field near the intersection of the san jacinto and san andreas faults however these active left lateral faults appear to define a set of small crustal blocks which in conjunction with both normal and reverse faulting earthquakes suggests contemporary clockwise rotation as a result of regional right lateral shear other left lateral faults representing additional rotating block systems are identified in adjacent areas from geologic and seismologic data many of these structures predate the modern san andreas system and may control the pattern of strain accumulation in southern california geodetic and paleomagnetic evidence confirm that block rotation by strike slip faulting is nearly ubiquitous particularly in areas where shear is distributed and that it accommodates both short term elastic and long term nonelastic strain a rotating block model accounts for a number of structural styles characteristic of strike slip deformation in california including variable slip rates and alternating transtensional and transpressional features observed along strike of major wrench faults domains of evenly spaced antithetic faults that terminate against major fault boundaries continued development of bends in faults with large lateral displacements anomalous focal mechanisms and differential uplift in areas otherwise expected to experience extension and subsidence since block rotation requires a detachment surface at depth to permit rotational movement low angle structures like detachments of either local or regional extent may be involved in the contemporary strike slip deformation of southern california a block nature of the crust also implies that not only will strains be inhomogeneous and likely concentrated along edge bounding faults but that local stress orientations will largely be responding to local kinematic constraints of block rotation and fault interaction this behavior coupled with the presence of possible regional detachments accounts for some of the precursory changes observed at considerable distances prior to large earthquakes and the triggering of seismicity or slip on nearby faults or around adjacent block edges although fault displacements along secondary structures associated with block rotations remain small they may still influence the nucleation and the characteristic rupture length of large earthquakes a more complete description of what these structures are and how they interact may prove critical to any fundamental understanding of the earthquake process and any realistic assessment of the regional seismic hazard

state of stress from focal mechanisms before and after the 1992 landers earthquake sequence the state of stress in the eastern california shear zone ecsz changed significantly because of the occurrence of the 1992 m w 6 1 joshua tree and the m w 7 3 landers earthquakes to quantify this change focal mechanisms from the 1975 galway lake sequence the 1979 homestead valley sequence background seismicity from 1981 to 1991 and the 1992 landers sequence are inverted for the state of stress in all cases the intermediate principal stress axis s2 remained vertical and changes in the state of stress consisted of variations in the trend of maximum and minimum principal stress axes s 1 and s 3 and small variations in the value of the relative stress magnitudes in general the stress state in the ecsz has s 1 trending east of north and 0 43 to 0 65 suggesting that the ecsz is a moderate stress refractor and the style of faulting is transtensional south of the pinto mountain fault in the region of the 1992 joshua tree earthquake the stress state determined from the 1981 to 1991 background seismicity changed on 23 april and again on 28 june 1992 in the central zone s 1 rotated from n14 5 e to n28 5 e on 23 april and back again to n16 5 e on 28 june thus the landers mainshock in effect recharged some of the shear stress in the region of the m w 6 1 joshua tree earthquake comparison of the state of stress before and after 28 june 1992 along the landers mainshock rupture zone showed that the mainshock changed the stress orientation the s1 trend rotated 7 to 20 clockwise and became progressively more fault normal from south to north along the emerson camp rock faults the variation was so prominent that the focal mechanisms of aftershocks could not be fit by a single deviatoric stress tensor the complex distribution of p and t axes suggests that most of the uniform component of the applied shear stress along the northern part of the rupture zone was released in the mainshock the san bernardino mountains region of the m w 6 2 big bear earthquake has a distinctively different state of stress as compared to the landers region with s 1 trending n3 5 w this region did not show any significant change in the state of stress following the 1992 m w 6 2 big bear sequence

how strong is an asperity a recent study of repeating earthquakes on the san andreas fault in central california by nadeau and johnson 1998 found that the smallest events occurred on patches having a linear dimension of the order of 0 5 m displacements of about 2 cm and stress drops of the order of 2000 mpa roughly 10 times larger than rock strengths measured in the laboratory the stress drop for larger events was observed to decrease as a power law of the seismic moment reaching the commonly observed value of 10 mpa at about magnitude 6 these large strengths are shown here to be consistent with laboratory data if the preexisting microcracks are all healed a hierarchical fractal asperity model is presented which is based on recent laboratory observations of contact distributions in sliding friction experiments this cantor dust model is shown to be consistent with the observed power law decrease in stress drop and increase in displacement with increasing event size the spatial distribution of hypocenters in the parkfield area is shown to be consistent with this simple fractal model and with a hierarchical clustering of asperities having a fractal dimension of d 1 and discrete rescaling factor of about 20

reply to what do the cajon pass stress measurements say about stress on the san andreas fault comment on in situ stress measurements to 3 5 km depth in the cajon pass scientific research borehole implications for the mechanics of crustal faulting by mark d zoback and john h healy

water enhanced dynamic recrystallization and solution transfer in experimentally deformed carnallite cylindrical samples of polycrystalline carnallite kmgcl3 6h2o were deformed in a triaxial apparatus at 60 c at confining pressures between 0 1 and 31 mpa and at strain rates between 10 4 and 10 8 s 1 in a number of cases small amounts of saturated carnallite brine were added samples without added brine deform by intracrystalline slip mechanical twinning cracking and by frictional sliding on crack surfaces stress strain curves of these samples are strongly dependent on confining pressure addition of brine has a dramatic effect on both microstructural development and mechanical properties grain boundary migration is strongly enhanced at lower strain rates additional intracrystalline effects start to appear together with the onset of solution transfer rapid compaction in samples deformed with added brine causes high fluid pressures to develop at higher strain rates addition of brine results in a decrease of the flow stress by a factor of two this weakening will increase even further at strain rates below about 10 9 s 1 when solution transfer becomes rate controlling it is argued that deformation of carnallite in nature is adequately described by the flow law found for samples deformed with added brine

a model for neogene crustal rotations transtension and transpression in southern california two episodes of crustal rotation have occurred in the regions of southern california and the mojave desert since the beginning of miocene time an early miocene episode of clockwise crustal rotation is associated with widespread north south to north northeast south southwest crustal extension throughout southeast california and western arizona this episode is apparently related to changing subduction parameters of the farallon plate a middle miocene and later episode of clockwise rotation in the transverse ranges and bordering areas is related to shear between the pacific and north american plates in both episodes clockwise crustal rotations first occur in a deforming zone which has a component of extension across its trend and shear along its trend the effects of extension are most pronounced in the earlier episode but clear in the later one also the pacific north american deforming zone began contracting in width during pliocene time but clockwise rotations continued and are probably in progress today

earthquake focal mechanisms in the eastern transverse ranges and san emigdio mountains southern california and evidence for a regional decollement earthquake focal mechanisms obtained from p wave first motions are presented for the eastern transverse ranges and the san emigdio mountains in southern california the former region shows a predominance of strike slip faulting whereas quaternary faults in the region show thrust motion we suggest that the observed strike slip mode of deformation cannot continue indefinitely without the occurrence of more thrust faulting fault deformation in the san emigdio mountains inferred from focal mechanisms is in accord with displacements across quaternary faults in the area this study and a search of the literature has yielded 19 mechanisms with shallow dipping nodal planes previous workers have interpreted such mechanisms as evidence for a regional decollement if such a regional decollement exists these data give some indication of its regional extent slip directions inferred from the focal mechanisms with shallow dipping nodal planes show some regional consistency but this pattern cannot be entirely explained with current tectonic models a comparison of the stress drop of an event having a shallow dipping nodal plane with an event with steeper planes gave inconclusive results

rheology of the plate interface dissolution precipitation creep in high pressure metamorphic rocks abstract subduction zone models invoke deformation to be concentrated along the plate interface in a region of particularly low temperature geophysical observations do not provide constraints on temperature stress and deformation patterns with desired resolution in contrast the record of high pressure metamorphic rocks exhumed from subduction zones provides details on p t history deformation mechanisms and stress state albeit not readily correlated with the former dynamic situation on larger scale here we review available information on dissolution precipitation creep dpc in high pressure metamorphic rocks which if representative for subduction zones in general can pose constraints on conditions rheology and flow patterns along the plate interface the key observations and conclusions are that 1 deformation is typically highly inhomogeneous and localized into shear zones 2 stresses are generally too low to drive crystal plastic deformation 3 microfabrics suggest dissolution precipitation creep to be the predominant deformation mechanism 4 an aqueous fluid at quasi lithostatic pressure is available throughout allowing for tensile fracturing and crack healing or sealing 5 low stress combined with high strain rates required for localized deformation at typical subduction rates implies low viscosity and 6 contribution of shear heating to the thermal budget of subduction zones should be moderate the dominant deformation mechanism dpc is reviewed in some detail including experimental and theoretical approaches various examples of dpc in high pressure metamorphic rocks are illustrated emphasizing the role of interphase boundaries as sites of dissolution rheology governed by dpc is proposed to control interplate coupling and development of a subduction channel with return flow being a likely candidate for rapid exhumation of high pressure metamorphic rocks

the stress heat flow paradox and thermal results from cajon pass conventional friction models predict a substantial thermal anomaly associated with active traces of strike slip faults but no such anomaly is observed from over 100 heat flow determinations along 1 000 km of the san andreas fault the cajon pass well is being drilled to bring deep heat flow and stress data to bear on this paradox preliminary heat flows to 1 7 km in the neighboring arkoma well indicated a positive anomaly of about 30 as would be expected if frictional resistance of the fault were 50 mpa or so but the pliocene and pleistocene history of the transverse ranges particularly high rates of uplift and erosion could also explain the anomaly preliminary stress results from cajon pass and a new interpretation of regional data by mark d zoback and colleagues suggests that the maximum compressive stress near the fault is almost normal to the trace and hence the resolved shear stress is low and the fault weak although this is consistent with long standing evidence from low heat flow vexing questions remain regarding the physics of strike slip faulting the heat flow data show large variability with depth probably from three dimensional structure and an overall decrease from over 90 mw m in the upper kilometer to less than 80 mw m in the lower 300 m with no evidence of advective heat transfer whether this decreasing trend is significant and whether it continues downward to the background value 70 mw m less the radiogenic heat from overlying rocks as expected for a weak fault must be verified with data from greater depth

the mojave extensional belt of southern california it is proposed that the regional pattern of early miocene low and high angle normal faulting and dike swarm emplacement in the western and central mojave desert is the result of regional extension developed within a roughly east trending zone named here the mojave extensional belt this zone of extension although now disrupted by late cenozoic right slip faulting can be traced from the intersection of the san andreas and garlock faults to the eastern mojave desert near bristol lake and the granite mountains fault restoration of post oligocene movement along the san andreas fault system places the now buried western part of the mojave extensional belt opposite a similar age extended area of west central california surface mapping and limited subsurface data suggest that the dominant tectonic process responsible for development of the mojave extensional belt was low angle normal sense simple shear detachment faulting strain within the mojave extensional belt is partitioned between four domains edwards waterman daggett and bullion terranes that each consist of one or more half grabens each half graben is composed of tilted normal fault bounded blocks that lie above a rooted low angle brittle ductile normal sense shear zone tilting and extension of the upper plate resulted from the superposition of several generations of originally steeply dipping 80 planar to slightly curviplanar normal faults at least one generation of high angle faults postdates the formation of the detachments differential extension between the domains was accommodated by strike slip faults transfer zones similarly the lack of early miocene extensional elements north of the western segment of the garlock fault suggests that the garlock may have been active in the early miocene and served as the northern transfer boundary of the mojave extensional belt the mojave extensional belt initially began to open about 22 m y ago with the major phase of extension occurring between 22 and 20 ma local high angle normal faults cut older detachments and were active between 20 and 17 ma kinematic indicators suggest that the western part of the mojave extensional belt opened in a ne sw direction whereas the eastern portion extended in an ene wsw direction present day reference frame rotation of the region about vertical axes as revealed by paleomagnetic studies suggests that the direction of extension within the mojave extensional belt was originally oriented n s and was rotated as much as 50 clockwise between 20 and 18 ma early miocene

neogene clockwise tectonic rotation of the eastern transverse ranges california suggested by paleomagnetic vectors results from a paleomagnetic study of neogene volcanic rocks situated within and around the pinto basin region of the eastern transverse ranges suggest that the ranges have undergone a clockwise tectonic rotation of 41 4 7 7 since late miocene time both normal and reversed polarities are observed and inclination data indicate no significant flattening the rotation was accompanied by progressive sinistral slip on the several east west trending faults which bound and transect the eastern transverse ranges a computation using the observed amount of cumulative left slip on the east west trending faults of the eastern transverse ranges 50 km and the spacing between the pinto mountain and salton creek faults which bound the region 70 km indicates that 35 of clockwise rotation is required which roughly agrees with the paleomagnetic data removal of the eastern transverse ranges rotation in a reconstruction shows that the distributed dextral shear across the mojave desert region to the north must have been on the order of 100 km since late miocene time from the results of this study we infer that crustal rotation of the eastern transverse ranges began later and is of less magnitude than that of the western transverse ranges

the method of compression test under high pressures in a cubic press and the strength of granite abstract a cubic press was used for the triaxial test on rocks under very high confining pressures up to 4 gpa for a specimen of 16 mm length and 8 mm diameter in a silver chloride sleeve with a pressure medium of pyrophyllite the axial and the confining pressures were calibrated by means of the phase transitions in bismuth thallium and barium the compressive strength of granite was determined up to 3 gpa of confining pressure in the cubic press the results were compared with those from the ordinary triaxial testing machine with a pressure medium of oil up to 450 mpa for the whole pressure range the compressive strength was expressed by c c0 1 4 33 p c0 0 47 where c is the differential compressive strength p is the confining pressure and 0 is the uniaxial compressive strength which was 180 mpa for the man nari granite used in this work

fracture strength of dry silicate rocks at high confining pressures and activity of acoustic emission abstract three dry silicate rocks gabbro dunite and eclogite were triaxially compressed up to a confining pressure of 3 gpa at room temperature these rocks exhibited brittle fracture behavior up to the highest confining pressure the change of the mechanism of fracture in the brittle region is suggested from the measurement of the compressive fracture strength and the activity of acoustic emission the existence of the high pressure brittle fracture phase is proposed the fracture strength increased with increase of confining pressure the increasing rate of strength was lowered at a value of confining pressure at about 0 8 gpa on gabbro at about 1 0 gpa on dunite and at about 1 5 gpa on eclogite at lower confining pressures than the above value the acoustic emission rate began to increase at the onset of dilatancy and increased rapidly followed by fracture as the axial stress was increased at the higher confining pressures however the acoustic emission rate did not increase rapidly before final fracture and stayed constant to the fracture the similar behavior was shown on the granite studied previously it is interesting that the frictional strength forms the boundary between low and high pressure brittle fracture phases

stress orientation profile to 3 5 km depth near the san andreas fault at cajon pass california a detailed profile of the orientation of the apparent maximum horizontal compressive stress was constructed over the depth interval 1 75 3 46 km in the cajon pass drill hole located 4 2 km from the san andreas fault in southern california the profile is based on the analysis of stress induced well bore breakouts and consists of 32 616 orientation determinations at a basic sampling interval of 4 cm the azimuth of the apparent maximum horizontal compressive stress around the borehole is 057 s d 19 depth dependent variations in breakout azimuths from a few degrees to as much as 100 occur over depth intervals of several centimeters to hundreds of meters in the borehole with wavelengths showing a self similar distribution over a range of depth intervals the variations in breakout orientation at depth seem to reflect stress fluctuations associated with active faults penetrated by the drill hole assuming linear elastic behavior perturbations in stress magnitude and orientation were computed for predrilling slip on these faults fault stress drops limited to the ambient shear stress i e up to total stress drop along with occasional local extreme stress drops and geometrical complications can explain the variations in breakout orientations in the drill hole the penetrative stress inhomogeneity may suggest that the average orientation of borehole breakouts in cajon pass which indicates left lateral shear on planes parallel to the san andreas fault may not be representative of the state of stress near the fault at depth in this region rather the stress orientation profile indicates the superposition of numerous local stress perturbations on an average stress field which is characterized by an absence of appreciable right lateral shear on planes parallel to the san andreas combining these observations with the highly variable stress state in shallow boreholes in the western mojave desert it is suggested that the heterogeneous stress field in this region representing a spectrum of seismic events may extend over substantial distances across and along the san andreas fault zone if the average measured breakout orientation in cajon pass is representative of the state of stress near the san andreas in this region then substantial left lateral shear stress is resolved on planes parallel to the fault in contradiction to the sense of its long term motion we use an elastic dislocation model to compute the net stress change near the cajon pass drill site since just prior to the large 1812 earthquake taking into account coseismic slip in major earthquakes and aseismic accumulation of slip on the san andreas and san jacinto faults the model suggests that the net change in fault parallel shear stress during the current earthquake cycle in the cajon pass area is nearly negligible the difference between the measured and computed results requires that additional left lateral shear stress be superimposed on a generally fault normal maximum horizontal compressive stress the results suggest that during the great fort tejon earthquake of 1857 left lateral shear stress parallel to the san andreas fault in the region of cajon pass may have played a role in terminating the southeastward extent of the rupture propagation

modeling of neogene crustal block rotation case study of southeastern california numerical modeling using a two dimensional distinct element method dem was performed to give an additional understanding of the mechanism of crustal block rotation in a highly active portion of the pacific north america transform plate boundary paleomagnetic and structural investigations have shown that large 30 50 horizontal rotations of crustal blocks occurred in the eastern transverse zone and the northeast mojave block southeastern california since the end of early miocene the main aspect of the modeling was to investigate whether an anisotropic horizontal stress state can be responsible for the rotation of crustal blocks with a length of 100 km or not the simulations were performed using different stress boundary conditions and different parameters of the fault strength applying an assumed ratio of 2 for the horizontal stress components as boundary condition block rotations in the order of 40 only occur if 1 the major horizontal stress sh is oriented n20 e with a maximal deviation of 15 to the east or west and 2 the fault strength described by the friction coefficient is smaller than 0 12 or 14 only of the value 0 85 for intact rocks byerlee s law for a weakening model of the fault strength an initial friction coefficient of 0 85 allows also rotations of about 40 if the friction coefficient decreases rapidly due to an increasing shear displacement to a basic friction coefficient of 0 12 or smaller for the same amount of rotation 40 a decreasing stress ratio requires a lowering of the friction coefficient used whereas an increase of the ratio allows a higher friction coefficient the simulation also show that the modeling with elastic blocks can adequately describe the observed main features of the crustal deformation especially the cumulative slip values discrepancies in the observations and modeling results for the western parts of the mojave desert are the result of a neglect of the three dimensional deformation in our two dimensional modeling

clockwise rotation of the western mojave desert paleomagnetic results from miocene volcanic rocks in the western mojave desert suggest about 25 of clockwise rotation for this region twelve sites 110 oriented cores were collected from the 20 ma saddleback basalt and seven sites 56 oriented cores from the red buttes quartz basalt around boron california structural corrections were made from the attitude of the basalt or the tropico group sedimentary rocks in which the sampled basalts are interbedded paleomagnetic directions are coincident for the two basalts and show both normal and reversed polarity after demagnetization to 40 or 60 mt application of structural corrections and inversion of reversed sites the data yield an average direction of i 51 6 and d 15 6 k 19 95 8 2 comparison with the expected direction for miocene rocks for stable north america shows a clockwise rotation of 23 8 11 3 and a flattening of 2 1 7 4 clockwise rotation of the western mojave agrees in direction with oroclinal bending of the southern sierra nevada due to right lateral shear along the western margin of north america most of this rotation is constrained by other paleomagnetic and structural information to have occurred soon after the sampled basalts were deposited about 20 ma and before about 16 ma these clockwise declination anomalies also indicate that any subsequent counterclockwise rotation of the mojave is small and or compensated by previous clockwise rotation

the 1992 landers earthquake sequence seismological observations the m w 6 1 7 3 6 2 1992 landers earthquakes began on april 23 with the m w6 1 1992 joshua tree preshock and form the most substantial earthquake sequence to occur in california in the last 40 years this sequence ruptured almost 100 km of both surficial and concealed faults and caused aftershocks over an area 100 km wide by 180 km long the faulting was predominantly strike slip and three main events in the sequence had unilateral rupture to the north away from the san andreas fault the m w6 1 joshua tree preshock at 33 n58 and 116 w19 on 0451 ut april 23 was preceded by a tightly clustered foreshock sequence m 4 6 beginning 2 hours before the mainshock and followed by a large aftershock sequence with more than 6000 aftershocks the aftershocks extended along a northerly trend from about 10 km north of the san andreas fault northwest of indio to the east striking pinto mountain fault the m w7 3 landers mainshock occurred at 34 n13 and 116 w26 at 1158 ut june 28 1992 and was preceded for 12 hours by 25 small m 3 earthquakes at the mainshock epicenter the distribution of more than 20 000 aftershocks analyzed in this study and short period focal mechanisms illuminate a complex sequence of faulting the aftershocks extend 60 km to the north of the mainshock epicenter along a system of at least five different surficial faults and 40 km to the south crossing the pinto mountain fault through the joshua tree aftershock zone towards the san andreas fault near indio the rupture initiated in the depth range of 3 6 km similar to previous m 5 earthquakes in the region although the maximum depth of aftershocks is about 15 km the mainshock focal mechanism showed right lateral strike slip faulting with a strike of n10 w on an almost vertical fault the rupture formed an arclike zone well defined by both surficial faulting and aftershocks with more westerly faulting to the north this change in strike is accomplished by jumping across dilational jogs connecting surficial faults with strikes rotated progressively to the west a 20 km long linear cluster of aftershocks occurred 10 20 km north of barstow or 30 40 km north of the end of the mainshock rupture the most prominent off fault aftershock cluster occurred 30 km to the west of the landers mainshock the largest aftershock was within this cluster the m w6 2 big bear aftershock occurring at 34 n10 and 116 w49 at 1505 ut june 28 it exhibited left lateral strike slip faulting on a northeast striking and steeply dipping plane the big bear aftershocks form a linear trend extending 20 km to the northeast with a scattered distribution to the north the landers mainshock occurred near the southernmost extent of the eastern california shear zone an 80 km wide more than 400 km long zone of deformation this zone extends into the death valley region and accommodates about 10 to 20 of the plate motion between the pacific and north american plates the joshua tree preshock its aftershocks and landers aftershocks form a previously missing link that connects the eastern california shear zone to the southern san andreas fault

creep of rocksalt abstract a review is presented of the fundamental flow properties and processes in experimentally deformed natural and synthetic halite single crystals and polycrystalline aggregates included in the summary are discussions of a microstructures induced during steady state creep b creep rupture of rocksalt c experiments associated with project salt vault and more recent field studies and d brine migration a representative steady state flow law determined for natural aggregates and maximum natural deviatoric stresses deduced from subgrain sizes are applied briefly to considerations of creep in waste repositories and of salt dome dynamics while the mechanical behavior of rocksalt is probably better understood than for all other rock types further investigations especially on load path stress history and creep rupture are clearly mandated furthermore additional investigations of brine migration and of bench and field scale deformations are needed the latter incorporating realistic rocksalt flow properties into numerical simulations of natural rock mass response

confirmation of two types of fracture in granite deformed at temperatures to 300 c abstract shimada m 1992 confirmation of two types of fracture in granite deformed at temperatures to 300 c in t mikumo k aki m ohnaka l j ruff and p k p spudich editors earthquake source physics and earthquake precursors tectonophysics 211 259 268 dry granite samples were triaxially compressed at confining pressures of up to 2 gpa at temperatures to 300 c the existence of two types of fracture previously observed in room temperature experiments was confirmed the low pressure type occurs when the compressive strength is higher than the frictional strength and the high pressure type occurs when the compressive strength is equal to the frictional strength the rate of increase of strength with confining pressure in the high pressure type regime is lowered as temperature increases as if an ideal coulomb fracture criterion at a fracture angle of 45 might be applicable with increasing temperature near the boundary between the low and high pressure type regimes unusually high values of strength were measured the specimens after such runs gave the suggestion that both low and high pressure types of fracture coexist there are some discrepancies in the values of strength from room temperature and those performed previously which are considered to be due to the experimental variability however the present experimental results to 300 c confirmed that two types of brittle fracture exist and that the fracture type changes when the compressive strength becomes equal to the frictional strength this tends to confirm previous speculations that many crustal fractures are of the high pressure type if so the low pressure type of fracture in which precursory ae acoustic emission activity before final fracturing is observed would be anticipated only at very shallow depth at ordinary pore pressure conditions it would occur at the depth of seismogenic zones only in the case of very high pore pressure

modeling dynamic fracture in granite under in situ conditions at high temperatures and pressures abstract reproducing the in situ mechanical response of granite at a depth of a few kilometer under dynamic loading presents significant challenges we have developed and calibrated a new thermo mechanical model for granite using extensive experimental data from both quasi static and dynamic test data the model includes parameters which can be calibrated for a specific geologic setting using the model developed we have performed a series of 2d and 3d simulations to evaluate effects of in situ stress and temperature on the extent of damage surrounding underground explosions we compare our results with available data for cavity size plastic zone and peak velocity attenuation with range in granite

chasing the garlock a study of tectonic response to vertical axis rotation vertical axis clockwise block rotations in the northeast mojave block are well documented by numerous authors however the effects of these rotations on the crust to the north of the northeast mojave block have remained unexplored in this paper we present a model that results from mapping and geochronology conducted in the north and central owlshead mountains the model suggests that some or all of the transtension and rotation observed in the owlshead mountains results from tectonic response to a combination of clockwise block rotation in the northeast mojave block and basin and range extension the owlshead mountains are effectively an accommodation zone that buffers differential extension between the northeast mojave block and the basin and range in addition our model explores the complex interactions that occur between faults and fault blocks at the junction of the garlock brown mountain and owl lake faults we hypothesize that the bending of the garlock fault by rotation of the northeast mojave block resulted in a misorientation of the garlock that forced the owl lake fault to break in order to accommodate slip on the western garlock fault subsequent sinistral slip on the owl lake fault offset the garlock creating the now possibly inactive mule springs strand of the garlock fault dextral slip on the brown mountain fault then locked the owl lake fault forcing the active leach lake strand of the garlock fault to break

the micromechanics of westerley granite at large compressive loads the micromechanical damage mechanics formulated by ashby and sammis pure appl geophys 133 3 489 521 1990 has been shown to give an adequate description of the triaxial failure surface for a wide variety of rocks at low confining pressure however it does not produce the large negative curvature in the failure surface observed in westerly granite at high confining pressure we show that this discrepancy between theory and data is not caused by the two most basic simplifying assumptions in the damage model 1 that all the initial flaws are the same size or 2 that they all have the same orientation relative to the largest compressive stress we also show that the stress strain curve calculated from the strain energy density significantly underestimates the nonlinear strain near failure in westerly granite both the observed curvature in the failure surface and the nonlinear strain at failure observed in westerly granite can be quantitatively fit using a simple bi mineral model in which the feldspar grains have a lower flow stress than do the quartz grains the conclusion is that nonlinearity in the failure surface and stress strain curves observed in triaxial experiments on westerly granite at low loading rates is probably due to low temperature dislocation flow and not simplifying assumptions in the damage mechanics the important implication is that discrepancies between experiment and theory should decrease with increased loading rates and therefore the micromechanical damage mechanics as formulated can be expected to give an adequate description of high strain rate phenomena like earthquake rupture underground explosions and meteorite impact

compressive failure of rocks by shear faulting a model for the compressive failure of rocks via the process of shear faulting is presented the model addresses the progressive growth of damage that leads to the formation of a critical fault nucleus which grows unstably in its own plane by fracturing the grain boundaries in an increasingly rapid succession the model uses a two parameter weibull type shear strength distribution for defining the nucleation of initial damage followed by the use of stress enhancement factors for addressing the increased probability of failure in the vicinity of already cracked grain boundaries these factors essentially involve surface averaging of enhanced stresses in the neighboring grains with the appropriate strength distribution as the weighting function as the stress is further increased similar correlated fracturing events get preferentially aligned to the crack cluster resulting in en echelon of cracks this crack cluster is modeled as an elliptical inhomogeneity within which the cracks interact and lead to material pulverization the effect of which mechanistically is to lower the shear modulus compared with the uncracked material on the outside the shear stress concentration resulting from this moduli mismatch is calculated and used to compute the stress enhancement factors for defining the nucleation of additional cracking events near the crack cluster eventually the size of the crack cluster becomes sufficiently large that it carries a stress concentration high enough to fracture all grain boundary elements in from of it in an increasingly rapid succession the stress associated with this event is taken as the failure stress since the model allows other cracking events to occur within the material volume in accordance with the assumed strength distribution function formation of other competing but subcritical shear faults naturally occurs besides the faulting stress for a prescribed confinement the model is able to predict the angle of the shear fault fairly well the model is applied to a variety of rock types including granite eclogite gabbro aplite rock salt sandstone dunite limestone and marble and the predicted failure envelopes compare rather well with the failure data available in the literature

late cenozoic tectonics of the northwestern san bernardino mountains southern california the late cenozoic structural and stratigraphic history of the northwestern san bernardino mountains supports two distinct episodes of uplift in late miocene to earliest pliocene and quaternary time that we hypothesize are related to movements on low angle structures beneath the range in this paper we document the nature distribution and timing of late cenozoic deformation and deposition in the northwestern san bernardino mountains and we illustrate the neotectonic evolution of the area in a series of interpretive paleotectonic block diagrams in the first episode of deformation late miocene to earliest pliocene motion on the south southwest directed squaw peak thrust system disrupted drainage in pre existing miocene nonmarine basins and uplifted the western third of the present range to form the ancestral san bernardino mountains crystalline rocks of the san bernardino mountains were thrust southward across the present site of the san andreas fault between 9 5 and 4 1 ma at a time when the san gabriel fault was the active strand of the san andreas transform system we speculate that the liebre mountain crystalline block at the northern margin of the ridge basin may be the missing upper plate of the squaw peak thrust now offset along the san andreas fault the second episode of deformation began with uplift of the northern plateau of the modern san bernardino mountains on north directed range front thrusts in early pleistocene time between 2 0 and 1 5 ma synchronous uplift of the northern plateau recorded in early pleistocene fanglomerates on the northwestern margin of the range is interpreted to be the result of movement of a relatively coherent crustal block northward up a south dipping detachment ramp beneath the central range in middle pleistocene time activity on the northern range front began to wane and the locus of uplift shifted to a narrow zone of arching and northward tilting adjacent to the san andreas fault which subsequently migrated rapidly northwestward along the san andreas fault from the western san bernardino mountains into the northeastern san gabriel mountains we attribute this pattern of deformation to the passage of a bulge or strike slip ramp attached to the southwest side of the san andreas fault at depth

fault mechanics and the kinematics of block rotations in many strike slip tectonic settings large rotations up to 100 of crustal blocks have been inferred from paleomagnetic data these blocks are bounded by sets of parallel faults which accommodate the relative motion between the blocks as regional deformation progresses simple geometrical considerations require that the faults must also rotate in this paper we show that on the basis of mechanical considerations the amount of fault rotation permissible under a stationary stress field is limited to 20 to 45 consequently block rotations that are larger than 40 or 45 require more than one set of accompanying faults to accommodate the block rotation examples of such multiple sets with 40 to 45 between them as predicted by the model were recognized in sistan iran in yerington the lake mead area nevada and in southern california

quaternary flake tectonics of the california transverse ranges p residual studies suggest that the pacific american plate boundary in the upper mantle lies in the central mojave desert northeast of its crustal position at the san andreas fault focal mechanisms of some relatively deep microearthquakes suggest a flattening with depth of north dipping thrust faults resulting in a mid crustal detachment in which the central transverse ranges move south relative to the mantle this makes sense geologically if the central transverse ranges and western mojave desert are considered as tectonic flakes of brittle crystalline rocks underlain by ductile pelona schist or catalina schist which makes up the zone of decollement such ductile schists would occur today at depths comparable to those at which they were metamorphosed the detachment breaks through to the surface at the point dume malibu coast fault zone and the red mountain san cayetano santa susana sierra madre cucamonga fault zone the latter coinciding with the southern edge of the palmdale bulge the transverse ranges flake now moves west northwest relative to the western mojave flake south southwest relative to the pacific plate and northwest relative to the great valley sierra nevada the mantle boundary may rejoin the san andreas fault north of a southwest trending zone of high seismicity in the tehachapi mountains which may be produced by convergence the age of the present tectonic system may be the same as the age of the break through thrust faults these are probably no older than 2 m y and may be even younger

the role of an intracrustal asthenosphere on the behavior of major strike slip faults measurements of strain accumulation adjacent to the san andreas fault indicate that the zone of strain accumulation extends only a few tens of kilometers away from the fault such a narrow zone of cyclic strain accumulation and release is consistent with the measured coseismic strain and inferred stress drop during the 1906 earthquake the restricted zone of cyclic accumulation and release of elastic energy adjacent to major strike slip faults has been attributed to the damping effect of a viscoelastic asthenosphere however to explain the narrowness of the zone on the san andreas fault it is necessary to conclude that the thickness of the lithosphere is 10 20 km this does not appear to be consistent with the relatively low surface heat flow measurements in this paper an alternative four layer model is proposed to explain the behavior of the san andreas fault an upper elastic plate extends to a depth of about 15 km the depth of the deepest seismicity on and adjacent to the fault beneath this upper elastic plate is a soft intracrustal asthenosphere that exhibits a viscoelastic behavior the viscous behavior of these crustal rocks at relatively low temperatures is attributed to either the presence of quartz which is known to have a very soft rheology or the presence of water that can lead to pressure solution creep or to both effects beneath this soft layer is a second elastic layer made up of the tough lower crustal and upper mantle rocks and beneath this second elastic layer is the asthenosphere there is extensive geological and geophysical evidence for the existence of an intracrustal asthenosphere both in california and elsewhere we show that the damping due to the intracrustal asthenosphere can explain the observed narrow zone of cyclic strain accumulation and release on the san andreas fault

seismicity and fault kinematics through the eastern transverse ranges california block rotation strike slip faulting and low angle thrusts hypocentral locations and first motion data from the southern california seismic network are used to infer the present pattern of deformation along the southern san andreas fault the study area lies between the san jacinto fault and desert hot springs and includes the san bernardino mountains and san gorgonio pass this region has some of the deepest earthquakes observed anywhere along the entire san andreas system exhibits a complex surface geology characterized by both right and left lateral faults has high topographic relief as a result of recent uplift and is a potential site for the nucleation of a great earthquake although this area is unusually seismogenic little activity can be directly associated with major throughgoing faults seismicity is also generally absent in the upper 5 km the predominant style of faulting above 10 12 km is oblique slip with a large reverse component the spatial distribution of relocated hypocenters and first motion data suggests the presence of a system of left slip faults striking northeast this pattern of faulting in conjunction with an unusual set of normal and reverse focal mechanisms is interpreted as the clockwise rotation of a small set of crustal blocks subject to regional right lateral shear at depths greater than about 10 km seismicity defines a wedge shaped volume undergoing pervasive internal deformation on a combination of strike slip and low angle thrust faults velocity structures determined from earthquake arrival times suggest a low velocity zone at about 10 km below the san bernardino mountains but not below the san jacinto mountains this is nearly the same depth as the transition between the two layers of different kinematic behavior seen south of the mill creek mission creek branch of the san andreas fault and the maximum depth of seismicity seen north of that fault the low velocity zone and the transition between block rotations and the deeper deformation may thus correspond to a detachment under much of this region and would imply that the overthrust san bernardino mountains are allochthonous the present pattern of seismic deformation in shocks of small to moderate size may only characterize the interval between large earthquakes and may change systematically as the region prepares to accommodate large right lateral displacements

compressive failure of rocks abstract a model for the compressive failure of rocks via the process of shear faulting is presented the model addresses the progressive growth of damage that leads to the formation of a critical fault nucleus which grows unstably in its own plane by fracturing the grain boundaries in an increasingly rapid succession the model uses a two parameter weibull type shear strength distribution for the defining nucleation of initial damage followed by the use of stress enhancement factors for addressing the increased probability of failure in the vicinity of already cracked grain boundaries as the stress is further increased similar correlated fracturing events gets preferentially aligned to the crack cluster resulting in an echelon of cracks this crack cluster is modeled as an elliptical inhomogeneity with a lower shear modulus compared with the uncracked material on the outside the shear stress concentration resulting from this moduli mismatch was calculated and used to compute the stress enhancement factors for defining the nucleation of additional cracking events near the crack cluster eventually the size of the crack cluster becomes sufficiently large such that it carries a stress concentration high enough to fracture all grain boundary elements in front of it in an increasingly rapid succession the stress associated with this event is taken as the failure stress the model is applied to a variety of rocks including granite eclogite gabbro aplite rocksalt sandstone dunite limestone and marble and the results compare rather well with the failure data from the literature

simple shear of southern california during neogene time suggested by paleomagnetic declinations we have studied the paleomagnetism of various neogene age rocks in southern california as a means of determining the amounts of oligocene and younger tectonic rotation and translation which has occurred in this region our results suggest that fully 25 of this area in particular the transverse ranges has undergone extreme clockwise rotation work in southeastern california implies that 40 of clockwise rotation has occurred here although paleomagnetic declinations adjacent to a major right lateral fault are apparently rotated over 200 the crustal block bounded by the san gabriel and san andreas fault has undergone a net clockwise rotation of 35 although the data here can be interpreted to show that an original early miocene rotation of about 50 was followed by a late miocene or pliocene counterclockwise rotation of 15 paleomagnetic results from the offshore islands suggests that san clemente santa barbara and san nicolas islands have not rotated but santa catalina has undergone about 90 of clockwise rotation all of the northern channel islands including anacapa santa cruz santa rosa and san miguel are implied to have rotated 70 or 80 clockwise this result is also found for the santa monica mountains east of anacapa island in the santa ynez range north of the channel islands paleomagnetic study of the monterey formation also indicates large clockwise rotations of near 90 these data also suggest that the santa maria basin is not rotated and that the north boundary of the rotated region is the santa ynez river fault stratigraphic control on the paleomagnetic data from the monterey formation implies that the rotation began about 16 m y ago and may be continuing today in the western region paleomagnetic inclination data from our study show that the northern channel islands in particular may have translated 15 northward since middle miocene time however equally valid interpretations of these same data are that the low inclinations are due to the combined effects of erroneous structural corrections non dipole magnetic field behavior and right offset on the san andreas fault system palinspastic reconstruction of southern california regions for the early miocene implies that parts of the transverse ranges structures were once aligned with north trending extensional structures in the southwestern united states we propose that pacific american plate interactions both rifted the continental crust to create this pattern and rotated the western most structures within a dextral simple shear zone which had a half width of about 400 km

rotation of the southernmost sierra nevada california structural paleogeographic and magmatic trends of the sierra nevada batholith swing from a characteristic nnw trend in the northern and central parts of the batholith to a nearly e w trend in the tehachapi and san emigdio mountains paleomagnetic results from eight sites in the upper cretaceous 80 86 m y bear valley springs pluton in the tehachapi mountains yield a mean direction d 022 i 56 k 97 95 5 6 this mean direction is significantly different from the expected late cretaceous field direction for this locality and suggests that the pluton has undergone a clockwise rotation of 45 14 in accord with the change in structural trend in that area lower to middle miocene volcanic and sedimentary rocks of the kinnick formation overlie the plutonic rocks and yield a mean direction d 354 i 64 a paleomagnetic pole derived from this mean direction is not significantly different from the expected miocene pole for north america this constrains the time of rotation of the pluton to be between about 80 and 20 m y probably predating establishment of the present san andreas transform in this area we suggest that the observed rotation is part of a large oroclinal bend affecting to different degrees the entire southern sierra nevada the driving mechanism for such a bending may be right lateral shear along a proto san andreas transform for which there is now a significant body of evidence