distribution of saline permafrost in the northwest territories canada saline permafrost has a detrimental impact on the performance of foundations increased pore water salinity decreases the strength and increases the deformation susceptibility of permafrost and foundations installed in permafrost a data base of the distribution of saline permafrost in the northwest territories shows high pore water salinities beneath coastal communities and at locations along the upper mackenzie valley and within the mackenzie delta salinity of inland sites is usually low high salinities are usually associated with ice poor materials and are linked with previous marine submergence associated with glacial events key words permafrost saline distribution measurements quaternary geology

imaging permafrost velocity structure using high resolution 3d seismic tomography a 3d seismic survey mallik 3d covering 126 km2 in the mackenzie delta area of canada s north was conducted by industry in 2002 numerous lakes and marine inundation create a complex near surface structure in the permafrost terrain much of the near subsurface remains frozen but significant melt zones exist particularly from perennially unfrozen water bodies this results in an irregular distribution of permafrost ice creating a complex pattern of low and high frequency near surface velocity variations which induce significant traveltime distortions in surface seismic data a high resolution 3d traveltime tomography study was employed to map the permafrost velocity structure utilizing first arrival traveltimes picked from 3d seismic shot records approximately 900 000 traveltime picks from 3167 shots were used in the inversion tomographic inversion of the first arrival traveltimes resulted in a smooth velocity model for the upper 200 m of the subsurface ray coverage in the model is excellent down to

extensional reactivation of an abandoned thrust a bound on shallowing in the brittle regime abstract shallow dip angles 45 suggested by field observations of continental extensional faults are not predicted by classical isotropic mohr coulomb anderson theory earthquake data indicate that normal faults exist in the upper crust with dip angles commonly as shallow as 30 some structural evidence suggests brittle normal faulting with dip angles as shallow as 10 one explanation of the apparent conflict between theory and structural seismic observations is that intrinsically weak shallow dipping pre existing faults are preferentially reactivated any reduction in frictional strength of the pre existing structure below that of surrounding rock increases the likelihood that such structures will be first breaking when extension is initiated enhanced fluid pore pressure on the pre existing fault reduces the effective strength and can further enhance shallow fault reactivation an analytical treatment clarifies the roles of geometry intact pre existing fault strengths and fluid pore pressures frictional strength ratios of 3 or greater could account for extremely shallow normal faults dips 10 20 without consideration of pore pressures in excess of the least principal stress or of principal stress systems rotated away from the gravity vector moderate reduction in friction 3 4 with respect to wall rock can reduce the dip to 30 and can account for shallow normal slip earthquakes

seismic images of the deep structure of the san andreas fault system central coast ranges california three dimensional inversion of teleseimic p wave travel time residuals recorded at the u s geological survey central california array has resolved small scale approx tens of kilometers crustal and upper mantle heterogeneity down to depths of 90 km beneath the california coast ranges upper crustal lateral velocity variations of 8 correlate closely with surface geology lower than average velocities are associated with thick tertiary sedimentary fill and higher than average velocities with basement exposures lower crustal velocity heterogeneity of 4 appear to reflect crustal thickness variations a thinner crust is indicated southwest of the san andreas fault and northwest of san pablo bay a linear zone of low velocities 0 to 40 subparallel to the san andreas fault was resolved in the upper mantle the preferred interpretation is that the low velocities indicate a narrow upwarp of asthenosphere to unusually shallow depths approx 45 km beneath the coast ranges such an unusual upper mantle structure may have been produced by the northwestward migration along the california coast of a transiently unstable mendocino triple junction the inversion results also indicate the possibility of partial decoupling of the crust from the upper mantle

strain on the san andreas fault near palmdale california rapid aseismic change frequently repeated strain measurements near palmdale california during the period from 1971 through 1980 indicate that in addition to a uniform accumulation of right lateral shear strain engineering shear 0 35 microradian per year across the san andreas fault a 1 microstrain contraction perpendicular to the fault that accumulated gradually during the interval 1974 through 1978 was aseismically released between february and november 1979 subsequently november 1979 to march 1980 about half of the contraction was recovered this sequence of strain changes can be explained in terms of south southwestward migration of a slip event consisting of the south southwestward movement of the upper crust on a horizontal detachment surface at a depth of 10 to 30 kilometers the large strain change in 1979 corresponds to the passage of the slip event beneath the san andreas fault

strain accumulation near the epicenters of the 1978 bishop and 1980 mammoth lakes california earthquakes abstract the bishop california earthquake m l 5 8 of 4 october 1978 occurred virtually beneath one of the stations in a trilateration network used by the u s geological survey to monitor strain accumulation in northern owens valley subsequently late may 1980 three earthquakes m l 6 0 6 3 and 6 0 the mammoth lakes sequence occurred on the edge of the trilateration network 10 to 20 km from the bishop earthquake epicenter the deformation of the network observed in the epoch 1976 to mid 1979 was not significantly different from that observed in the epoch 1973 to 1976 thus the bishop earthquake itself did not produce a measurable deformation nor was anomalous deformation detected prior to the mammoth lakes earthquakes the deformation of the trilateration network between july 1979 and september 1980 indicates significant extension 50 mm in 25 km in the direction n65 e essentially perpendicular to the alignment of the epicenters of the mammoth lakes earthquakes this deformation suggests normal faulting whereas the preliminary nodal plane solutions seem to require left lateral slip secular strain accumulation was detected in northern owens valley latitude 37 15 to 37 45 in the 1972 to 1979 interval the principal strain rates with standard deviations were 0 08 0 02 strain a n83 e 15 and 0 02 0 02 strain a n07 w 15 extension reckoned positive the observed strain accumulation is consistent with normal faulting along the sierra escarpment in owens valley north of bishop this accumulation appears to contrast with southern owens valley where strain accumulation is primarily right lateral shear acorss the axis of the valley

tectonic motions in california inferred from very long baseline interferometry observations 1980 1984 very long baseline interferometry vlbi has been used to measure three baselines of 150 300 km length within california and three baselines of 1500 km length between california and texas these measurements which span nearly 4 years beginning in 1980 and have typical individual uncertainties of approximately 2 cm have been fitted to a steady plane strain rotation model after solving for internal network strains and velocities with respect to the north american plate interior the jet propulsion laboratory site 35 km sw of the san andreas fault trace is found to move toward the northwest with respect to points 100 km ne of the fault at an average rate of 25 4 mm yr 1 along an azimuth of n40 w 7 no reliably determined motion is registered between the other sites all of which are 100 km from the inferred pacific north american plate boundary the observed rate of displacement across the san andreas fault agrees with independent determinations of contemporary slip rates but appears to conflict with the rigid global plate tectonic rate differences between this rate determination and those from satellite laser ranging observations provide potentially important constraints on the distribution of tectonic strain in the region these results are taken as evidence for a regional distribution of elastic strain that is offset in approximately the same sense and amount as the big bend of the southern san andreas fault finite element simulations support the conjecture that tractions on the base of the crust that accompany mantle downwelling beneath the transverse ranges could contribute to these strain inhomogeneities

gas hydrate growth and dissociation in narrow pore networks capillary inhibition and hysteresis phenomena marine sediments hosting gas hydrates are commonly fine grained silts muds clays with very narrow mean pore diameters 0 1 mm this has led to speculation that capillary phenomena could play an important role in controlling hydrate distribution in the seafloor and may be in part responsible for discrepancies between observed and predicted from bulk phase equilibria hydrate stability zone hsz thicknesses numerous recent laboratory studies have confirmed a close relationship between hydrate inhibition and pore size stability being reduced in narrow pores however to date the focus has been hydrate dissociation conditions in porous media with capillary controls on the equally important process of hydrate growth being largely neglected here we present experimental methane hydrate growth and dissociation conditions for synthetic mesoporous silicas over a range of pressure temperature pt conditions 273 293 k to 20 mpa and pore size distributions results demonstrate that hydrate formation and decomposition in narrow pore networks is characterized by a distinct hysteresis solid growth occurs at significantly lower temperatures or higher pressures than dissociation hysteresis takes the form of repeatable irreversible closed primary growth and dissociation pt loops within which various characteristic secondary scanning curve pathways may be followed similar behaviour has recently been observed for ice water systems in porous media and is characteristic of liquid vapour transitions in mesoporous materials the causes of such hysteresis are still not fully understood our results suggest pore blocking during hydrate growth as a primary cause

vertical tectonics at a continental crust oceanic plateau plate boundary zone fission track thermochronology of the sierra nevada de santa marta colombia 1 the topographically prominent sierra nevada de santa marta forms part of a faulted block of continental crust located along the northern boundary of the south american plate hosts the highest elevation in the world 5 75 km whose local base is at sea level and juxtaposes oceanic plateau rocks of the caribbean plate quantification of the amount and timing of exhumation constrains interpretations of the history of the plate boundary and the driving forces of rock uplift along the active margin the sierra nevada province of the southernmost sierra nevada de santa marta exhumed at elevated rates 0 2 km my during 65 58 ma in response to the collision of the caribbean plateau with northwestern south america a second pulse of exhumation 0 32 km my during 50 40 ma was driven by underthrusting of the caribbean plate beneath northern south america subsequent exhumation at 40 25 ma 0 15 km my is recorded proximal to the santa marta bucaramanga fault more northerly regions of the sierra nevada province exhumed rapidly during 26 29 ma 0 7 km my further northward the santa marta province exhumed at elevated rates during 30 25 ma and 25 16 ma the highest exhumation rates within the sierra nevada de santa marta progressed toward the northwest via the propagation of nw verging thrusts exhumation is not recorded after 16 ma which is unexpected given the high elevation and high erosive power of the climate implying that rock and surface uplift that gave rise to the current topography was very recent i e 1 ma and there has been insufficient time to expose the fossil apatite partial annealing zone

subduction of the caribbean plate and basement uplifts in the overriding south american plate the new tectonic interpretations presented in this paper are based on geologic field mapping and gravity data supplemented by well logs seismic profiles and radiometric and earthquake data the present caribbean south american plate boundary is the south caribbean marginal fault where subduction is indicated by folding and thrusting in the deformed belt and a seismic zone that dips 30 to the southeast and terminates 200 km below the maracaibo basin the caribbean south american convergence rate is estimated as 1 9 0 3 cm yr on the basis of the 390 km length of the seismic zone and a thermal equilibration time of 10 m y the caribbean south american convergence has produced a northwest southeast maximum principal stress direction 1 in the overriding south american plate the mean 1 direction for the maracaibo santa marta block is 310 10 based on earthquake focal mechanism determinations and structural and gravity data on the overriding south american plate basement blocks have been uplifted 7 12 km in the last 10 m y to form the venezuelan andes sierra de perija and the colombian santa marta massif crystalline basement of the venezuelan andes has been thrust to the northwest over tertiary sediments on a fault dipping about 25 and extending to the mantle in the sierra de perija mesozoic sediments have been thrust 16 26 km to the northwest over tertiary sandstones along the cerrejon fault a thrust fault dipping 15 10 to the southeast is consistent with field mapping and gravity and density data the santa marta massif has been uplifted 12 km in the last 10 m y by northwest thrusting over sediments the basement block overthrusts of the perijas venezuelan andes and the santa marta massif are pliocene pleistocene analogs for laramide orogenic structures in the middle and southern rocky mountains of the united states the nonmagmatic basement block uplifts along low angle thrust faults reveal horizontal compression in the overriding plate over 500 km from the convergent margin present day east northeast west southwest 080 compression is indicated by earthquake focal mechanisms and strike slip motion on the bocono fault these earthquakes are intraplate deformation associated with east west 080 nazca south american convergence

topographic gravity modeling for global bouguer maps to degree 2160 validation of spectral and spatial domain forward modeling techniques at the 10 microgal level over the past years spectral techniques have become a standard to model earth s global gravity field to 10 km scales with the egm2008 geopotential model being a prominent example for some geophysical applications of egm2008 particularly bouguer gravity computation with spectral techniques a topographic potential model of adequate resolution is required however current topographic potential models have not yet been successfully validated to degree 2160 and notable discrepancies between spectral modelling und newtonian numerical integration well beyond the 10 mgal level have been reported here we rigorously compute and validate gravity implied by a degree 2160 model of earth s topographic masses our experiments are based on two key strategies both of which require advanced computational resources first we construct a spectrally complete model of the gravity field which is generated by the degree 2160 earth topography model this involves expansion of the topographic potential to the 15th integer power of the topography and modelling of short scale gravity signals to ultra high degree of 21 600 translating into unprecedented fine scales of 1 km second we apply newtonian integration in the space domain with high spatial resolution to reduce discretisation errors our numerical study demonstrates excellent agreement 8 microgal rms between gravity from both forward modelling techniques and provides insight into the convergence process associated with spectral modelling of gravity signals at very short scales few km as key conclusion our work successfully validates the spectral domain forward modelling technique for degree 2160 topography and increases the confidence in new high resolution global bouguer gravity maps

the effect of the extent of freezing on seismic velocities in unconsolidated permafrost we develop a model to relate velocities of seismic waves in unconsolidated permafrost is idealized as an assemblage of spherical quartz grains imbedded in a matrix composed of spherical water inclusions in ice the theory of kuster and toksoz based on wave scattering considerations is used to determine the effective elastic moduli and hence the wave speeds the hasin shtrikman theoretical bounds on the elastic moduli of heterogeneous materials and considerations establish the plausibility of the model the model predicts vp and vs to be decreasing functions of both the porosity and the water to ice ratio and the ratio vp vs to be an increasing function of these two parameters the theory is then applied to laboratory measurements of shear and compressional wave velocities in 23 permafrost samples from different sites in the beaufort sea mackenzie river valley and canadian arctic islands although no direct measurements were made of the extents of freezing in these samples the data are consistent w

an effective medium model for p wave velocities of saturated unconsolidated saline permafrost abstractto better understand the relationship between p wave velocities and ice content in saturated unconsolidated saline permafrost we constructed an effective medium model based upon ultrasonic p wave data that were obtained from earlier laboratory studies the model uses a two end member mixing approach in which an ice filled fully frozen end member and a water filled fully unfrozen end member are mixed together to form the effective medium of partially frozen sediments this mixing approach has two key advantages 1 it does not require parameter tuning of the mixing ratios and 2 it inherently assumes mixed pore scale distributions of ice that consist of frame strengthening i e cementing and or load bearing ice and pore filling ice the model predicted p wave velocities agree well with our laboratory data demonstrating the effectiveness of the model for quantitatively inferring ice content from p wave velocities the modeling workflow is simple and is largely free of calibration paramete

seismic wave attenuation and dispersion resulting from wave induced flow in porous rocks a review one major cause of elastic wave attenuation in heterogeneous porous media is wave induced flow of the pore fluid between heterogeneities of various scales it is believed that for frequencies below 1 khz the most important cause is the wave induced flow between mesoscopic inhomogeneities which are large compared with the typical individual pore size but small compared to the wavelength various laboratory experiments in some natural porous materials provide evidence for the presence of centimeter scale mesoscopic heterogeneities laboratory and field measurements of seismic attenuation in fluid saturated rocks provide indications of the role of the wave induced flow signatures of wave induced flow include the frequency and saturation dependence of p wave attenuation and its associated velocity dispersion frequency dependent shear wave splitting and attenuation anisotropy during the last four decades numerous models for attenuation and velocity dispersion from wave induced flow have been developed with varying degrees of rigor and complexity these models can be categorized roughly into three groups according to their underlying theoretical framework the first group of models is based on biot s theory of poroelasticity the second group is based on elastodynamic theory where local fluid flow is incorporated through an additional hydrodynamic equation another group of models is derived using the theory of viscoelasticity though all models predict attenuation and velocity dispersion typical for a relaxation process there exist differences that can be related to the type of disorder periodic random space dimension and to the way the local flow is incorporated the differences manifest themselves in different asymptotic scaling laws for attenuation and in different expressions for characteristic frequencies in recent years some theoretical models of wave induced fluid flow have been validated numerically using finite difference finite element and reflectivity algorithms applied to biot s equations of poroelasticity application of theoretical models to real seismic data requires further studies using broadband laboratory and field measurements of attenuation and dispersion for different rocks as well as development of more robust methods for estimating dissipation attributes from field data

seismicity focal mechanisms and tectonics in the last four years there has been a tremendous increase in the detail of tectonic processes reported in the typical seismotectonic study new data from many local and regional networks of seismographs established during the previous four year period plus the now routine application of improved hypocentral location procedures long period body wave modelling and moment tensor inversion using digitally recorded data have increased resolution of both the hypocentral location and the nature of the faulting process consequently research emphasis has been on local tectonics or those aspects of regional and global tectonics requiring high resolution i briefly describe below some of the highlights of these studies

velocity and resistivity changes during freeze thaw cycles in berea sandstone we report on simultaneous measurements of resistivity compressional vp and shear vs velocities made during the temperature cycling of berea sandstone while at elevated pressure temperatures were cycled through the freezing point of the saturating brine the responses of resistivity and velocities are in phase but are asymmetrical as the brine freezes and thaws limited freezing causes no discernible damage to the core free water is present below the nominal freezing point of the brine vp vs and resistivity change by 16 24 and 500 respectively over the temperature range of 4 c to 6 c at 6 9 mpa confining pressure

slip along the san andreas fault associated with the great 1857 earthquake historical records indicate that several meters of lateral slip along the san andreas fault accompanied the great 1857 earthquake in central and southern california these records together with dendrochronological evidence suggest that the rupture occurred along 360 to 400 km of the fault including several tens of kilometers of the currently creeping reach in central california geomorphic expressions of late holocene right lateral offsets are abundant along the 1857 rupture along 300 kilometers of the 1857 rupture between cholame and wrightwood the youngest discernible offset ranges from 3 to 9 1 2 meters dormancy of the fault since 1857 almost certainly indicates that this latest offset was created in 1857 fault slip apparently associated with the 1857 earthquake varies in a broadly systematic way along the trace of the fault it is relatively uniform along each of several long segments but changes rather abruptly in value between these segments this nonuniform displacement pattern may imply that some segments of the fault rupture more frequently or experience a slower long term slip rate than others the 1857 offsets indicate a seismic moment m o between 5 3 and 8 7 10 27 dyne cm assuming a 10 to 15 km depth of rupture and relatively uniform slip as a function of depth a comparison with the rupture length average slip value and tectonic setting of the california earthquake of 1906 m s 8 1 4 indicates a value of m 8 1 4 for the 1857 event

full wavefield inversion of surface waves for mapping embedded low velocity zones in permafrost abstractsurface waves are advantageous for mapping seismic structures of permafrost in which irregular velocity gradients are common and thus the effectiveness of refraction methods are limited nevertheless the complex velocity structures that are common in permafrost environments often yield unusual dispersion spectra in which higher order and leaky modes are dominant such unusual dispersion spectra were prevalent in the multichannel surface wave data acquired from our permafrost study site at barrow alaska owing to the difficulties in picking and identifying dispersion curves from these dispersion spectra conventional surface wave inversion methods become problematic to apply to overcome these difficulties we adopted a full wavefield method to invert for velocity models that can best fit the dispersion spectra instead of the dispersion curves the inferred velocity models were consistent with collocated electric resistivity results and with subsequent confirmation cores which indicated the re

a rock physics investigation of unconsolidated saline permafrost p wave properties from laboratory ultrasonic measurements abstractsaline permafrost is sensitive to thermal disturbances and is prone to subsidence which renders it a major source of geohazard in arctic coastal environments seismic methods could be used to map and monitor saline permafrost at scales of geotechnical interests because of the ice content dependencies of seismic properties we have developed a comprehensive study of the ultrasonic p wave properties i e velocity and attenuation of synthetic saline permafrost samples for a range of salinities and temperatures and measurements conducted on a fine grained permafrost core obtained from barrow alaska the resulting data consist of p wave properties presented as functions of temperature and salinity notable observations include the following p wave velocities showed marked reductions in the presence of dissolved salts and complex variations resulting from the water to ice phase transitions strong p wave attenuation was present in the temperature intervals in which the samples were partially froz

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

timing and tectonic setting of tertiary low angle normal faulting and associated magmatism in the southwestern united states major episodes of tertiary low angle normal faulting in the southwestern united states south of latitude 37 n moved northward with time occurring about 25 million years ago in southern arizona 20 million years ago in the mojave desert region and 10 to 15 million years ago in the las vegas area in most areas faulting and associated volcanism occurred when the mendocino triple junction lay to the west this correspondence probably resulted from the unstable configuration of the triple junction east west to northeast southwest horizontal stretching as plates pulled apart at the triple junction prevented development of a hole in the lithosphere at the triple junction the regional strain history predicted by this model is consistent with observed tertiary low angle faults strike slip faults and folds

creep and strength testing of frozen saline fine grained soils little creep data is available for frozen fine grained saline soils salinity accelerates the creep rate of a frozen soil under load and reduces its shear strength owing to the higher unfrozen water contents in the pore phase creep data for saline fine grained soils are required by engineers and scientists working in areas of coastal or offshore permafrost the data are also used in projects involving artificial freezing of saline soils this paper describes the results of 34 creep tests and 11 time dependent strength tests carried out on saline soils the tests were carried out for two different research programs consequently two different fine grained soil types were used most of the tests were carried out in a constant stress and constant temperature creep test environment some were completed in a triaxial constant strain rate test format in order to extend the data base to an area of higher strain rates the range of temperatures was 2 3 to 25 c the range of stresses in the creep tests was

the tectonics of precambrian craton mobile belts progressive deformation of polygonal miniplates abstract the archaean proterozoic crust of many precambrian terrains consists of two contrasting tectonic units archaean cratonic blocks made up of granite greenstone terrains and archaean proterozoic mobile zones fold belts and orogens which separate and tend to surround and flow around the cratons the cratons are relatively rigid blocks but have a history of ductile and brittle deformations the surrounding mobile belts are either high strain high grade metamorphic belts or folded basins thus the relatively rigid cratons are surrounded by more ductile zones of mobility it is speculated that the archaean cratons are originally separate although neighbouring ensialic polygonal miniplate blocks of a single continent which have moved relative to one another according to the mantle controls and the prevailing eulerian poles and this mutual jostling has progressively deformed their common boundaries the deformed boundaries are now the sites of the surrounding ductile and higher strain mobile belts which are persistent crustal defects while the cratons represent the more rigid and lower strain cores and relicts which have stabilized after the archaean the mega scale relationships between the cratons and mobile belts e g east africa are compared to the smaller scale micro meso scale porphyroclast matrix structures found in augen gneisses and mylonites these structural relationships are of vastly different magnitudes 108 but as there exists a continuum on all the intermediate scales they may all be related their geometric similarities are interpreted as having a common mechanical rheological origin

contemporary block tectonics california and nevada well determined fault plane solution and the gross pattern of late cenozoic faulting in california and nevada show a systematic relation between the orientation of fault planes and slip directions in general normal faults have northerly strikes reverse faults have easterly strikes and dextral and sinstral strike slip faults have northwesterly and northeasterly strikes respectively kinematically this relation is consistent with the response of clusters of fault bounded crustal blocks to a regional stress field generated by the relative motion between the pacific and north american plates in this stress field the greatest and least principal compressive stresses are restricted to northerly and easterly striking vertical planes respectively clusters of crustal blocks bound by faults having the above attitudes form three basic regimes 1 a spreading regime with a combination of normal and conjugate strike slip faults 2 a converging regime with a combination of reverse and conjugate strike slip faults and 3 a wrench regime with sets of subparallel throughgoing northwest striking dextral faults or northeast striking sinstral faults these three regimes are typified by faulting patterns in the basin and range province spreading the transverse ranges converging and the san andreas garlock fault systems wrench respectively the gross deformation of each regime resulting from relative displacements between individual blocks is characterized by north south shortening and east west extension with the ratio of extensional to shortening strains and the areal dilitation decreasing systematically from spreading to wrench to compressional regimes the wrench regime involves a component of net rotational deformation clockwise for dextral slip and counter clockwise for sinstral slip while deformation of the spreading and converging regimes is irrotational local deviations from regional kinematic directions are concentrated along the boundaries between regimes reflecting the mismatch in gross deformation fields between regimes maximum principal and shear stress magnitudes will increase systematically from spreading to wrench to converging regimes provided that fault slip is controlled by frictional strength byerlee s law along preexisting fractures and that pore pressure in the brittle crust is laterally uniform a minimum strength difference between active block bounding faults and block interiors is 15 30 simple arrangements of such block clusters mimic the gross kinematic pattern of quaternary faulting in california and nevada some implications for contemporary tectonics emphasized by this model involve the westward displacement of the sierra nevada block with respect to the stable interior of the north american plates oblique thrusting of the salinian block over the pacific plate and a progressive increase in the offset of the san andreas fault represented by the big bend through the transverse ranges