co2 depleted fluids from mid ocean ridge flank hydrothermal springs hydrothermal spring fluids were collected from baby bare a basement outcrop on the sediment covered eastern flank of the juan de fuca ridge overlying 3 5 ma old crust these waters are venting at 25 c but have cooled from 64 c within the upper 200 300 m of the upflow zone during their ascent unlike ridge axis hydrothermal vent fluids previously sampled the total co2 content of the endmember spring fluids 0 85 mol kg 1 is depleted with respect to bottom seawater the very low alkalinities 0 43 meq l 1 and high ca2 concentrations 55 mmol kg 1 of endmember spring fluids suggest that the removal of c in the spring fluids is associated with carbonate precipitation in the igneous basement assuming that 8 20 of the total ridge flank heat loss rate of 50 1018 cal yr 1 6 6 tw is removed by porewater advecting from the sediment at 25 c an upper limit dictated by the global input rate of mg 2 to the oceans mottl and wheat 1994 a maximum global carbon sink of 1 0 2 6 1011 mol yr 1 1 2 3 1 mton c yr 1 and a maximum global alkalinity sink of 140 340 109 eq yr 1 are calculated for warm 25 c ridge flank hydrothermal circulation this carbon flux is only 2 5 of current estimates of subaerial and submarine volcanic co2 emissions 50 1011 mol yr 1 indicating that hydrothermal alteration of oceanic crust on young mid ocean ridge flanks at temperatures of a few tens of degrees celsius is only a relatively minor sink for carbon on a global basis it is still possible however that ridge flank alteration at lower temperatures 25 c is an important component of the global carbon budget

late cenozoic structure and tectonics of the northern mojave desert in the fort irwin region of the northern mojave desert late cenozoic east striking sinistral faults predominate over northwest striking dextral faults of the same age kinematic indicators and offset marker units indicate dominantly sinistral strike slip on the east striking portions of the faults and sinistral thrust slip on northwest striking moderately dipping segments at the east ends of the blocks crustal blocks 7 10 km wide by 50 km long are bounded by complex fault zones up to 2 km wide at the edges and ends of each block faulting initiated after 11 ma and quaternary deposits are faulted and folded we document a minimum of 13 km cumulative sinistral offset in a north south transect from south of the bicycle lake fault to north of the drinkwater lake fault paleomagnetic results from 50 sites reveal two direction groups in early and middle miocene rocks the north to northwest declinations of the first group are close to the middle miocene reference pole however rock magnetic studies suggest that both primary and remagnetized directions are present in this group the northeast declinations of the second group are interpreted as primary and 63 5 7 6 clockwise from the reference pole and suggest net post middle miocene clockwise rotation of several of the east trending blocks in the northeast mojave domain the jurassic independence dike swarm in fort irwin may be rotated 25 80 clockwise relative to the swarm north of the garlock fault thus supporting the inference of clockwise rotation using a simple shear model that combines sinistral slip and clockwise rotation of elongate crustal blocks we predict 23 clockwise rotation using the observed fault slip or one third that inferred from the paleomagnetic results the discrepancy between slip and rotation may reflect clockwise bending at the ends of fault blocks where most of our paleomagnetic sites are located however at least 25 40 of clockwise tectonic rotation is consistent with the observed slip on faults within the domain plus possible rigid body rotation of the region evidenced by clockwise bending of northwest striking domain bounding faults our estimates of sinistral shear and clockwise rotation suggest that approximately half of the 65 km of dextral shear in the eastern california shear zone over the last 10 m y occurred within the northeast mojave domain the remainder must be accommodated in adjacent structural domains e g east of the avawatz mountains and west of the goldstone lake fault supporting appendices 1 and 2 are available on diskette or via anonymous ftp from kosmos agu org directory apend username anonymous password guest diskette may be ordered from american geophysical union 2000 florida avenue n w washington dc 20009 or by phone at 800 966 2481 15 00 payment must accompany order

block models of crustal motion in southern california constrained by gps measurements 1 we estimate slip rates on major active faults in southern california using a block model constrained by global positioning system measurements of interseismic deformation the block model includes the effects of block rotation and elastic strain accumulation consistent with a simple model of the earthquake cycle our estimates of the right lateral strike slip rate on the san andreas fault vary by at least a factor of 5 from a high of 35 9 0 5 mm yr in the carrizo plain to a low of 5 1 1 5 mm yr through the san bernadino segment shortening across the puente hills thrust and left lateral slip on the raymond hill fault are consistent with both thickening and escape tectonics in the los angeles basin discrepancies between geodetic and geologic slip rate estimates along the san andreas and san jacinto faults as well as in the eastern california shear zone may be explained by a temporal change in fault system behavior we find no substantial evidence for long term postseismic relaxation and infer that the viscosity of the lower crust upper mantle may be relatively high 1019 pa s

dislocation models of interseismic deformation in the western united states 1 the gps derived crustal velocity field of the western united states is used to construct dislocation models in a viscoelastic medium of interseismic crustal deformation the interseismic velocity field is constrained by 1052 gps velocity vectors spanning the 2500 km long plate boundary zone adjacent to the san andreas fault and cascadia subduction zone and extending 1000 km into the plate interior the gps data set is compiled from u s geological survey campaign data plate boundary observatory data and the western u s cordillera velocity field of bennett et al 1999 in the context of viscoelastic cycle models of postearthquake deformation the interseismic velocity field is modeled with a combination of earthquake sources on 100 known faults plus broadly distributed sources models that best explain the observed interseismic velocity field include the contributions of viscoelastic relaxation from faulting near the major plate margins viscoelastic relaxation from distributed faulting in the plate interior as well as lateral variations in depth averaged rigidity in the elastic lithosphere resulting rigidity variations are consistent with reduced effective elastic plate thickness in a zone a few tens of kilometers wide surrounding the san andreas fault saf system primary deformation characteristics are captured along the entire saf system eastern california shear zone walker lane the mendocino triple junction the cascadia margin and the plate interior up to 1000 km from the major plate boundaries

mantle flow under the western united states from shear wave splitting we show that sks splitting in the westernmost united states polarization of the fastest shear waves and splitting times including their back azimuthal dependence can be explained by a geodynamic model that includes a continuum mechanics description of plate motions and underlying asthenospheric circulation models that include a counterflow at depths of 300 km are preferred which may indicate a far field effect of the farallon slab anomaly sinking underneath the central continental united states this finding is broadly consistent with earlier suggestions and we demonstrate that a mechanically coupled system though with a strong viscosity contrast with depth is consistent with the data we explore the depth dependence of predicted anisotropy by means of computing seismogram synthetics and comparing synthetic splits with observations some patterns in the data including null observations are matched well linked models of geodynamic flow and mineral alignment in the mantle provide a means to test the relationship between strain and the saturation of texturing lower fabric saturation strains than for global models are preferred by the data which may reflect the relatively active tectonic setting and thin asthenosphere of the study region in general our results show that seismic anisotropy when interpreted jointly with mineral physics theories may be used to quantitatively constrain the spatial character of flow and the degree of force coupling at depth

the mechanics of continental transforms an alternative approach with applications to the san andreas system and the tectonics of california article i nfo displacement on intracontinental transforms is commonly distributed over a zone several hundred km wide and may incorporate large regions of transtensional and transpressional strain but no consensus exists on what controls the distribution and style of this deformation we model the transform boundary as a weak shear zone of finite length that exerts shear stress on the deformable continental lithosphere on either side strain rate decreases away from the shear zone on a scale related to its length force balance in this system requires lateral gradients in shear strain rate to be balanced by longitudinal gradients in stretching rates which create zones of lithospheric thickening and thinning distributed anti symmetrically about the shear zone simple analytical estimates two dimensional 2d spectral models and 2d 3d numerical models are employed to study the spatial scales and magnitudes of the zones of uplift and subsidence using reasonable parameter values the models yield geologically relevant rates strain rate components inferred from the gps determined 2 d velocity field and analysis of seismicity using kostrov s method taken together with the geological data on the distribution of active faults uplift and subsidence suggest that the distribution and rates of active deformation in california are consistent with our predictions this validates the assumptions of the continuum approach and provides a tool for predicting and explaining the tectonics of california and of other intracontinental transform systems

interseismic strain accumulation and the earthquake potential on the southern san andreas fault system the southern section of the san andreas fault is the only part of the fault that hasn t ruptured in historic times for this reason it is believed to pose the largest seismic risk in california although whether tectonic stress is actually accumulating there is a topic of heated debate using data from radar interferometry satellites yuri fialko has produced a detailed map of surface deformation in the area that shows that elastic stress is accumulating at a fairly high rate during the past 300 years a displacement of 6 to 9 metres has built up this suggests that the southern san andreas fault is nearing the end of the interseismic phase of the earthquake cycle and that increased seismic activity of some kind can be expected the san andreas fault in california is a mature continental transform fault that accommodates a significant fraction of motion between the north american and pacific plates the two most recent great earthquakes on this fault ruptured its northern and central sections in 1906 and 1857 respectively the southern section of the fault however has not produced a great earthquake in historic times for at least 250 years assuming the average slip rate of a few centimetres per year typical of the rest of the san andreas fault the minimum amount of slip deficit accrued on the southern section is of the order of 7 10 metres comparable to the maximum co seismic offset ever documented on the fault1 2 here i present high resolution measurements of interseismic deformation across the southern san andreas fault system using a well populated catalogue of space borne synthetic aperture radar data the data reveal a nearly equal partitioning of deformation between the southern san andreas and san jacinto faults with a pronounced asymmetry in strain accumulation with respect to the geologically mapped fault traces the observed strain rates confirm that the southern section of the san andreas fault may be approaching the end of the interseismic phase of the earthquake cycle

reconciliation of san andreas slip discrepancy by a combination of interior basin and range extension and transrotation near the coast measurements of net translation along the neogene san andreas transform boundary between pacific and north american plates as indicated by the global correlation of sea floor magnetic anomalies and by total neogene displacements along strands of the onshore san andreas fault system differ by hundreds of kilometres the apparent motion discrepancy is reconciled for post mid miocene time if both transrotational deformation along the coastal belt of california and basin range extension east of the sierra nevada are taken into account

diffuse interseismic deformation across the pacific north america plate boundary crustal movements and deformation within the diffuse pacifi c north america pa na plate boundary are dominated by the right lateral motion between the two plates by using the pa na pole of rotation por spherical coordinate system we decompose observed crustal movements into parallel and normal components to the pa na plate motion we transformed the 840 velocity vectors of the southern california earthquake center scec 3 0 velocity fi eld into the pa na por system in order to characterize the interseismic velocity across the plate boundary our results show that despite the very different deformation styles occurring across the san andreas fault the fault trace follows the half plate motion contour deviation occurs in the southern section where the half motion contour correlates with the san jacinto and imperial fault segments our analysis yields interesting asymmetric patterns in both parallel and normal components the parallel component shows asymmetrical velocity gradients across the san andreas fault and the normal component indicates compression southwest of the big bend but not northeastward the observations are compared with viscoelastic modeling results which show a similar velocity fi eld the main disagreements between the observations and the model are in a narrow band along the san andreas fault and in the mojave block suggesting that crustal heterogeneities and additional unmodeled fault segments should be considered in future models

progressive localization of deformation during exhumation of a major strike slip shear zone norumbega fault zone south central maine usa detailed structural studies along with 40ar 39ar thermochronology provide important constraints on the temporal spatial and structural evolution of a major orogen parallel strike slip shear zone the norumbega fault zone in maine detailed structural analysis in south central maine reveals two very different styles of dextral noncoaxial deformation a wide zone 25 km of heterogeneously distributed ductile shear structures and a relatively narrow zone 1 km of lower greenschist facies high strain mylonitization 40ar 39ar thermochronology indicates that these two zones of deformation developed during a prolonged period of regional exhumation following middle devonian amphibolite facies metamorphism the wide zone of dextral shear is interpreted to reflect a major episode of moderate temperature post amphibolite facies metamorphism but pre regional cooling below 320 c late devonian to early carboniferous transcurrent tectonism the narrow zone of dextral shear represents both a younger latest carboniferous and significantly cooler 320 c deformational event occurring at much higher structural levels a general model of increasingly narrow but more highly focused noncoaxial deformation during progressive regional exhumation is supported by the observations

crustal rheology and faulting at strike slip plate boundaries 1 an analytic model this paper investigates how rheologic stratification within the crust affects the formation and long term evolution of fault systems at a strike slip plate boundary we present an analytic model of deformation at a strike slip plate margin within a two layer viscoelastic crust with fixed shear modulus but varying viscosity in each layer faulting is represented by static elastic dislocations of fixed shallow depth extent imposed at a high critical stress threshold for fracture of a new fault and a lower one for sliding on a preexisting fault to drive crustal deformation we impose basal velocity boundary conditions at the moho representing a narrow zone of high shear in the mantle in this study we restrict attention to deformation at the surface where simple analytic solutions exist for velocities and stresses our results suggest that when a primarily elastic brittle upper crust is underlain by a low viscosity lower crust the deformation zone in the upper crust widens in time at steady state the surface width of the deformation zone may be significantly greater than the prescribed narrow mantle shear zone the long term width of the deformation zone increases with the viscosity contrast within the crust and with the thickness of the low viscosity lower crust widening of the deformation zone is accompanied by the fracture of new faults in the upper crust leading to the formation of a system of parallel strike slip faults surrounding the plate boundary for fixed plate velocity and fracture criteria we find that the width of the brittle fault network within the total distributed deformation zone is primarily governed by strength properties of the upper crust and by the viscosity contrast within the crust

block kinematics of the pacific north america plate boundary in the southwestern united states from inversion of gps seismological and geologic data 1 the active deformation of the southwestern united states 30 41 n is represented by a finite number of rotating elastic plastic spherical caps gps derived horizontal velocities geologic fault slip rates transform fault azimuths and earthquake derived fault slip vector azimuths are inverted for block angular velocities creep on block bounding faults permanent strain rates within the blocks and the rotations of 11 published gps velocity fields into to a common north american reference frame gps velocities are considered to be a combination of rigid block rotations recoverable elastic strain rates resulting from friction on block bounding faults and nonrecoverable strain rates resulting from slip on faults within the blocks the resulting pacific north america angular velocity is similar to some published estimates and satisfies transform azimuths and one spreading rate in the gulf of california earthquake slip vectors in the gulf of california and alaska and gps velocities along coastal california and within the pacific basin published fault slip rates are satisfied except in the southern mojave desert where the motion of the mohave block relative to north america is faster than can be explained by mapped faults the largest blocks the sierra nevada great valley and the eastern basin and range show permanent strain rates after removing elastic strain of only a few nanostrain per year demonstrating approximately rigid behavior observed horizontal strain rates correlate strongly with predicted strain rates from known faults suggesting that the short term strains evident in gps velocities are largely elastic in only about 20 of the region is distributed deformation needed to match the data indicating that a plate tectonic style description of the deformation of the western united states is plausible most blocks rotate about vertical axes at approximately the same rate as the pacific relative to north america suggesting that locally spin rates are communicated from block to block arguing against both floating block and ball bearing mechanisms of block rotation the similarities of the blocks spin rates to that of the pacific suggests that the pacific strongly influences their motions through edge tractions however it is shown that the blocks cannot rotate about the pacific north america pole without spinning counter to the sense of pacific north america shear unlike some other broad plate boundaries in the western united states vertical axis rotations take up very little of the slip rate budget across the region

distribution of aseismic slip rate on the hayward fault inferred from seismic and geodetic data 1 we solve for the slip rate distribution on the hayward fault by performing a least squares inversion of geodetic and seismic data sets our analysis focuses on the northern 60 km of the fault interferometric synthetic aperture radar insar data from 13 independent ers interferograms are stacked to obtain range change rates from 1992 to 2000 horizontal surface displacement rates at 141 bench marks are measured using gps from 1994 to 2003 surface creep observations and estimates of deep slip rates determined from characteristic repeating earthquake sequences are also incorporated in the inversion the fault is discretized into 283 triangular dislocation elements that approximate the nonplanar attributes of the fault surface south of the city of hayward a steeply east dipping fault geometry accommodates the divergence of the surface trace and the microseismicity at depth the inferred slip rate distribution is consistent with a fault that creeps aseismically at a rate of 5 mm yr to a depth of 4 6 km the interferometric synthetic aperture radar insar data require an aseismic slip rate that approaches the geologic slip rate on the northernmost fault segment beneath point pinole although the insar data might be complicated by a small dip slip component at this location a low slip rate patch of 1 mm yr is inferred beneath san leandro consistent with the source location of the 1868 earthquake we calculate that the entire fault is accumulating a slip rate deficit equivalent to a mw 6 77 0 05 per century however this estimate of potential coseismic moment represents an upper bound because we do not know how much of the accumulated strain will be released through aseismic processes such as afterslip

width of mantle deformation across a continental transform evidence from upper mantle pn seismic anisotropy measurements seismic anisotropy v p of at least 6 5 2 5 in the upper mantle pn phase is reported 200 km east of the alpine fault in south island new zealand when these new data are combined with a previous pn study and published sks splitting results two distinct domains of mantle deformation are inferred within south island in southern south island the deforming zone in the mantle lithosphere is as wide as 335 km whereas in mid south island the deforming zone narrows to 200 km both regions display wider zones of seismic anisotropy than found at other continental transforms of the world the width and orientation of mantle anisotropy in southern south island are consistent with a model of simple shear in a zone that has undergone 800 200 km of right lateral shear displacement a value close to that predicted by geological reconstructions hence displacement can be accommodated by distributed shear without faulting being required in the mantle

slip rate of the calico fault implications for geologic versus geodetic rate discrepancy in the eastern california shear zone 1 long term 10 5 years fault slip rates test the scale of discrepancy between infrequent paleoseismicity and relatively rapid geodetic rates of dextral shear in the eastern california shear zone ecsz the calico fault is one of a family of dextral faults that traverse the mojave desert portion of the ecsz its slip rate is determined from matching and dating incised pleistocene alluvial fan deposits and surfaces displaced by fault slip a high resolution topographic base acquired via airborne laser swath mapping aids in identification and mapping of deformed geomorphic features the oldest geomorphically preserved alluvial fan unit b is displaced 900 200 m from its source at sheep springs wash in the northern rodman mountains this fan deposit contains the first preserved occurrence of basalt clasts derived from the pipkin lava field and overlies quaternary conglomerate deposits lacking these clasts the 40 ar 39 ar dating of two flows from this field yields consistent ages of 770 40 ka and 735 9 ka an age of 650 100 ka is assigned to this fan deposit based on these ages and on the oldest cosmogenic 3 he exposure date of 653 20 ka on a basalt boulder from the surface of unit b this assigned age and offset together yield a mid pleistocene to present average slip rate of 1 4 0 4 mm yr ayounger fan surface unit k records 100 10 m of dextral displacement and preserves original depositional morphology of its surface granitic boulders and pavement samples from this surface yield an average age of 56 4 7 7 ka after taking into account minimal cosmogenic inheritance of granitic clasts the displaced and dated k fans yield a slip rate of 1 8 0 3 mm yr distributed deformation of the region surrounding the fault trace if active could increase the overall displacement rate to 2 1 0 5 mm yr acceleration of slip rate from an average of 1 4 mm yr prior to 50 ka to 1 8 mm yr since 50 ka is possible though a single time averaged slip rate of 1 6 0 2 mm yr satisfies the data these rates are faster than any other paleoseismic or long term slip rate yet determined for other dextral faults in the mojave desert and imply that fault slip rates and earthquake productivity are heterogeneous across this portion of the ecsz total displacement across the calico fault diminishes northward as shear is distributed into folding and sinistral faults in the calico mountains this pattern is consistent with an approximately threefold drop in geologic slip rate as the calico fault steps over onto the blackwater fault and demonstrates the significance of fault interaction for understanding the pattern of present day strain accumulation in the ecsz

postseismic and interseismic displacements near a strike slip fault a two dimensional theory for general linear viscoelastic rheologies 1 we present an analytic solution for the deformation near an infinite strike slip fault in an elastic layer overlying a linear viscoelastic half space the theory is valid for any linear viscoelastic rheology and any earthquake sequence this is a generalization of the work of j c savage and colleagues which only holds for models with a uniform shear modulus maxwell viscoelastic lower half space and periodic rupture recurrence we demonstrate the theory for models of an elastic layer over a maxwell standard linear solid burgers and triviscous half space for each of these models we calculate example postseismic displacements interseismic displacements in a periodic earthquake sequence and interseismic displacements for a nonperiodic earthquake sequence our solution is for a simple geometry however the model presents an elegant tool to explore the evolution of displacements for relatively complex rheologies and rupture recurrence histories

creep and quakes on the northern transition zone of the san andreas fault from gps and insar data 1 the san juan bautista sjb segment forms the northern transition zone of the creeping section of the san andreas fault it is an area of moderate seismicity the largest instrumentally recorded earthquakes have been m5 5 however historic records suggest six m 6 earthquakes occurred near the sjb segment between 1840 and 1899 it is also an area that has experienced several slow earthquakes we perform a joint inversion of gps and insar data to determine the current rate and distribution of interseismic creep we find two low slip locked segments at mid seismogenic depths that may represent source regions for the 19th century earthquakes we find that the sjb segment is currently accumulating strain energy at the rate of one m6 3 6 7 earthquake per century at this rate the 1840 1899 sequence released 135 510 years of accumulated moment deficit

source parameters for the 1952 kern county earthquake california a joint inversion of leveling and triangulation observations coseismic leveling and triangulation observations are used to determine the faulting geometry and slip distribution of the july 21 1952 mw 7 3 kern county earthquake on the white wolf fault a singular value decomposition inversion is used to assess the ability of the geodetic network to resolve slip along a multisegment fault and shows that the network is sufficient to resolve slip along the surface rupture to a depth of 10 km below 10 km the network can only resolve dip slip near the fault ends the preferred source model is a two segment right stepping fault with a strike of 51 and a dip of 75 sw the epicentral patch has deep 6 27 km left lateral oblique slip while the northeastern patch has shallow 1 12 5 km reverse slip there is nearly uniform reverse slip epicentral 1 6 m northeast 1 9 m with 3 6 m of left lateral strike slip limited to the epicentral patch the seismic moment is m0 9 2 0 5 1019 n m mw 7 2 the signal to noise ratio of the leveling and triangulation data is reduced by 96 and 49 respectively the slip distribution from the preferred model matches regional geomorphic features and may provide a driving mechanism for regional shortening across the comanche thrust and structural continuity with the scodie seismic lineament to the northeast

crustal rheology and faulting at strike slip plate boundaries 2 effects of lower crustal flow we present a numerical model of deformation at a strike slip plate boundary within a linear viscoelastic crust which is driven by far field plate motions and basal mantle velocities the crust is assumed to have uniform elastic properties but continuously varying viscosity as a function of depth brittle faulting is represented by static elastic dislocations that are imposed when stresses exceed a critical threshold for fracture or frictional sliding the locations and depth extents of faults in this model are not prespecified but instead are governed by stress evolution within the crust we find that when a primarily elastic upper crust is underlain by a low viscosity lower crustal layer the deformation zone broadens in time to encompass many parallel strike slip faults in an interacting network in contrast when the entire crust behaves elastically the deformation zone remains narrow and focused on a single plate bounding fault reflecting imposed mantle motions surface strain rate patterns within the interacting fault network are complex and reflect significant faulting related strain rate perturbations that decay over timescales of postseismic relaxation in the lower crust 10 100 years the fault network has a characteristic spacing with complex fault interactions and with the depth extents of faults increasing with time to a maximum depth governed by crustal rheology the maximum depth of faults is limited by stress relaxation and large scale viscous flow in the lower crust which confines brittle failure to shallow and midcrustal levels

exhumation of the inyo mountains california implications for the timing of extension along the western boundary of the basin and range province and distribution of dextral fault slip rates across the eastern california shear zone 1 new geologic mapping tectonic geomorphologic 10be terrestrial cosmogenic nuclide and u th he zircon and apatite thermochronometric data provide the first numerical constraints on late cretaceous to late quaternary exhumation of the inyo mountains and vertical slip and horizontal extension rates across the eastern inyo fault zone california the east dipping eastern inyo fault zone bounds the eastern flank of the inyo mountains a prominent geomorphic feature within the western basin and range province and eastern california shear zone u th he zircon and apatite thermochronometry yield age patterns across the range that are interpreted as indicating 1 two episodes of moderate to rapid exhumation associated with laramide deformation during the late cretaceous early tertiary 2 development of a slowly eroding surface during a prolonged period from early eocene to middle miocene 3 rapid cooling exhumation and initiation of normal slip along the eastern inyo fault zone accommodated by westward tilting of the inyo mountains block at 15 6 ma and 4 rapid cooling exhumation and renewed normal slip along the eastern inyo fault zone at 2 8 ma fault slip continues today as indicated by fault scarps that cut late pleistocene alluvial fan surfaces the second episode of normal slip at 2 8 ma also signals onset of dextral slip along the hunter mountain fault yielding a pliocene dextral slip rate of 3 3 1 0 mm a where a is years summing this dextral slip rate with estimated dextral slip rates along the owens valley death valley and stateline faults yields a net geologic dextral slip rate across the eastern california shear zone of 9 3 2 2 1 4 to 9 8 1 4 1 0 mm a

uncertainties in slip rate estimates for the mission creek strand of the southern san andreas fault at biskra palms oasis southern california this study focuses on uncertainties in estimates of the geologic slip rate along the mission creek strand of the southern san andreas fault where it offsets an alluvial fan t2 at biskra palms oasis in southern california we provide new estimates of the amount of fault offset of the t2 fan based on trench excavations and new cosmogenic 10be age determinations from the tops of 12 boulders on the fan surface we present three alternative fan offset models a minimum a maximum and a preferred offset of 660 m 980 m and 770 m respectively we assign an age of between 45 and 54 ka to the t2 fan from the 10be data which is significantly older than previously reported but is consistent with both the degree of soil development associated with this surface and with ages from u series geochronology on pedogenic carbonate from t2 described in a companion paper by fletcher et al this volume these new constraints suggest a range of slip rates between 12 and 22 mm yr with a preferred estimate of 14 17 mm yr for the mission creek strand of the southern san andreas fault previous studies suggested that the geologic and geodetic slip rate estimates at biskra palms differed we find however that considerable uncertainty affects both the geologic and geodetic slip rate estimates such that if a real discrepancy between these rates exists for the southern san andreas fault at biskra palms it cannot be demonstrated with available data

fault slip rates and earthquake histories for active faults in southern california abstract within the context of the historical record of seismicity we review the geological data bearing on the quaternary slip rates and paleoearthquake histories of active faults in southern california

lithospheric shear zones and mantle crust connections abstract a crustal scale ductile shear zone network in the precambrian granulite facies crust of madagascar is examined to determine the nature of the connections between the mantle and lower crust based on three independent data sets field and satellite mapping c and o isotope geochemistry and gravimetry this crust is divided into three zones 1 outside of shear zones 2 minor shear zones that are 350 km long up to 1000 km and 20 35 km wide the mantle is uplifted by about 10 km beneath the major shear zones the major shear zones are rooted in and are inferred to be controlled by the mantle they directly tapped mantle derived co 2 the small scale minor shear zones were controlled by crustal processes and focused crustally derived h 2 o rich co 2 fluids the regular distribution of the shear zones on a crustal scale is in agreement with models of buckling of the continental lithosphere in a compressional context the propagation of these mechanical instabilities promoted and channelled fluid flow these major pan african shear zones thinned the crust and were reactivated during the subsequent drifting of madagascar and opening of the indian ocean during jurassic to cretaceous times they also controlled many of the brittle fault zones in the overlying sedimentary basins mantle rooted large scale shear zones are inferred to be a general feature of cratonic areas reactivated by shear zone systems

the motion of crustal blocks driven by flow of the lower lithosphere and implications for slip rates of continental strike slip faults geodetic measurements in actively deforming areas of the continents reveal the pattern of deformation in the lithosphere if the dominant forces acting on crustal blocks are tractions at their bases then the long term motion of each block will be given by the average velocity of the underlying lithosphere slip rates between blocks estimated in this way from recent geodetic measurements across fault zones in the south island of new zealand and southern california are in good agreement with slip rates estimated geologically

the in situ ph of hydrothermal fluids at mid ocean ridges abstract here we report the first in situ ph of hydrothermal vent fluids at mid ocean ridges measurements were made during dives with dsv alvin to the main endeavour field juan de fuca ridge and the east pacific rise at 21 n and 9 10 n using solid state electrochemical sensors vent fluid temperature and pressure ranged from 180 to 384 c and 220 to 250 bar respectively ph in situ of the highest temperature vent fluid is only slightly acidic 5 1 5 4 although sharply lower values occur at the seawater vent fluid interface knowledge of ph in situ is essential for unravelling the complex geochemical and biogeochemical processes controlling the evolution of seafloor hydrothermal systems

metal transports and enrichments in iron depositions hosted in basaltic rocks ii metal rich fluids and fe origin abstract this study focuses on revealing the mechanism of metal transport enrichment and fe origin of iron deposition during water basalt interactions occurred in basaltic rocks observations of the iron deposits anhydrite magnetite pyroxene type deposits hosted in k rich basaltic rocks in the mesozoic volcanic area of the middle lower yangtze river valley china indicate that the mechanism of metal transport and enrichment for those deposits are significant objective to scientists and the fe origin problem is not well resolved here the metal transport enrichment and iron origin have been investigated in high temperature experiments of water basaltic interactions these deposits were accompanying a wide zone with metal alteration the effects of hydrothermal alteration on major rock forming element concentrations in basaltic rock were investigated by systematically comparing the chemical compositions of altered rocks with those of fresh rocks in the deposits these metals are distributed throughout altered rocks that exhibit vertical zoning from the deeper to the shallow then combined with the investigations of the metal alterations we performed kinetic experiments of water basaltic rock interactions using flow through reactors in open systems at temperatures from 20 c to 550 c 23 34 mpa release rates for the rock forming elements from the rocks have been measured experiments provide the release rates for various elements at a large temperature range and indicate that the dissolution rates release rates for various elements vary with temperature si al and k have high release rates at temperatures from 300 c to 500 c the maximum release rates rmx for si are reached at temperatures from 300 c to 400 c the rmxs for ca mg and fe are at low temperatures from 20 c to 300 c results demonstrate that fe is not released from 400 c to 550 c and indicate that when deep circling fluids passed through basaltic rocks fe was not mobile and fixed in the rocks at temperatures from 400 c to 550 c significance of the results is to provide evidence that the fe of ores originated from basalt and fe oxides precipitated across the critical state of water simultaneously ca mg and fe are fixed in the deeper altered rocks mafic minerals but fe was dissolved at relatively low temperatures 100 300 c si al and k were easily mobile from basalt by upward flowing fluids from 300 c to 400 c and transported to the upper part silicified and argillized rock

strain modelling seismic anisotropy and coupling at strike slip boundaries applications in new zealand and the san andreas fault despite similar surface transform faulting behaviour observed shear wave splitting patterns in the california and new zealand plate boundary regions are markedly different to better understand the origin of the anisotropy in these regions we model mantle flow and strain for a variety of strike slip plate boundary scenarios simple relations between the flow or strain and elastic anisotropy are assumed to determine the integrated splitting in shear particle motion along teleseismic paths straincontrolled models fit the observations in new zealand and california better than simplified flow controlled models fast shear polarizations are progressively rotated toward the shear plane over time and even a constant viscosity model provides a good fit to the fast directions in new zealand and southern california the constant viscosity implies strong coupling between the surface and the deeper mantle to fit the lack of decrease in delay times with distance from the fault the relationship between delay time and strain must saturate at small strains if this is the case then southern new zealand and southern california both fit models in which strain is smaller than in nearby regions equivalent to that achieved along an infinite fault by about 3 10 my of their present motion stratified viscosity allows more rapid rotation of fast directions toward fault parallel than occurs in isoviscous models and can explain the nearly fault parallel fast directions in the central south island different aspects of the northern california results are fit with different models but a rapid change in viscosity with depth is needed to produce the full effects of the behaviour previously modelled as two layers of anisotropy noted in the area suggesting vertical decoupling

the internal structure of an active sea floor massive sulphide deposit the hydrothermal circulation of sea water through permeable ocean crust results in rock water interactions that lead to the formation of massive sulphide deposits these are the modern analogues of many ancient ophiolite hosted deposits1 4 such as those exposed in cyprus here we report results obtained from drilling a series of holes into an actively forming sulphide deposit on the mid atlantic ridge a complex assemblage of sulphide anhydrite silica breccias provides striking evidence that such hydrothermal mounds do not grow simply by the accumulation of sulphides on the sea floor indeed the deposit grows largely as an in situ breccia pile as successive episodes of hydrothermal activity each form new hydrothermal precipitates and cement earlier deposits during inactive periods the collapse of sulphide chimneys dissolution of anhydrite and disruption by faulting cause brecciation of the deposit the abundance of anhydrite beneath the present region of focused hydrothermal venting reflects the high temperatures 150 c currently maintained within the mound and implies substantial entrainment of cold sea water into the interior of the deposit these observations demonstrate the important role of anhydrite in the growth of massive sulphide deposits despite its absence in those preserved on land

geodetic constraints on areal changes in the pacific north america plate boundary zone what controls basin and range extension using 1500 geodetic velocities we model the present day spatial patterns of areal changes inside the pacific north america plate boundary zone from this model we show that between the central gulf of california and the queen charlotte islands there is no significant net change in surface area this zero net areal change result allows us to relate regions of areal growth to areas of equivalent contraction elsewhere within the plate boundary zone we find that areal growth of the basin and range province brp and its eastern margin 5 2 0 1 10 3 m 2 yr is balanced by areal reduction near northwestern california between 38 n and 42 n the san andreas fault system south of 38 n and the plate boundary zone north of 42 n including the juan de fuca and gorda ridge systems each have no significant net areal change our results suggest a kinematic relationship between extension in the brp and contraction near the northern california coast ranges and klamath mountains from these observations we propose that although brp extension may be caused by internal forces the southernmost cascadia subduction zone provides a window of escape that acts as a stress guide to brp extension as well as northwestward sierra nevada motion such a dynamic model is consistent with independent findings that 1 the least principal horizontal stress orientations in the brp are toward northern california 2 extension directions in the brp have changed orientation to track the northward migration of the mendocino triple junction and 3 the southernmost cascadia subduction zone is a relatively weak plate boundary

large magnitude transient strain accumulation on the blackwater fault eastern california shear zone we investigate the quaternary slip rate for the blackwater fault eastern california shear zone through mapping and geochronology of offset volcanic rocks basalt flows of the black mountains support the presence of faulting at 3 77 0 11 ma 1 8 0 1 km of subsequent slip and a well constrained long term slip rate of 0 49 0 04 mm yr total slip diminishes northward evidenced by a 0 3 1 8 km offset of a 7 23 1 07 ma dacite flow in the black hills and fault termination in the lava mountains 5 km short of the garlock fault slow long term slip rate together with sparse evidence for holocene rupture contradict predictions of rapid slip rate from tectonic geodesy these results support the conclusion that as much as 95 of geodetic strain accumulation across the blackwater fault and thus from 1 to 6 mm yr of geodetic strain measured across the eastern california shear zone is a transitory phenomenon discrepant geologic and geodetic results may indicate an increased near term seismic hazard but merit caution for interpretation of fault slip rates from geodesy alone

slip rate of the western garlock fault at clark wash near lone tree canyon mojave desert california the precise tectonic role of the left lateral garlock fault in southern california has been controversial three proposed tectonic models yield significantly different predictions for the slip rate history orientation and total bedrock offset as a function of distance along strike in an effort to test these models we present the first slip rate estimate for the western garlock fault that is constrained by radiocarbon dating a channel referred to here as clark wash incised into a latest pleistocene alluvial fan has been left laterally offset at least 66 6 m and no more than 100 m across the western garlock fault indicating a left lateral slip rate of 7 6 mm yr 95 confidence interval of 5 3 10 7 mm yr using dendrochronologically calibrated radiocarbon dates the timing of aggradational events on the clark wash fan corresponds closely to what has been documented elsewhere in the mojave desert suggesting that much of this activity has been climatically controlled the range front fault located a few hundred meters northwest of the gar lock fault has probably acted primarily as a normal fault with a holocene rate of dip slip of 0 4 0 7 mm yr the record of prehistoric earthquakes on the garlock fault at this site though quite possibly incomplete suggests a longer interseismic interval 1200 2700 yr for the western garlock fault than for the central garlock fault the relatively high slip rate determined here indicates that the western and central segments of the garlock fault show similar rates of movement that are somewhat faster than rates inferred from geodetic data the high rate of motion on the western garlock fault is most consistent with a model in which the western garlock fault acts as a conjugate shear to the san andreas fault other mechanisms involving extension north of the garlock fault and block rotation at the eastern end of the fault may be relevant to the central and eastern sections of the fault but they cannot explain a high rate of slip on the western garlock fault

present day kinematics of the eastern california shear zone from a geodetically constrained block model we use global positioning system gps data from 1993 2000 to determine horizontal velocities of 65 stations in eastern california and western nevada between 35 and 37 n we relate the geodetic velocities to fault slip rates using a block model that enforces path integral constraints over geologic and geodetic time scales and that includes the effects of elastic strain accumulation on faults locked to a depth of 15 km the velocity of the sierra nevada block with respect to nevada is 11 1 0 3 mm yr with slip partitioned across the death valley 2 8 0 5 mm yr panamint valley 2 5 0 8 mm yr and airport lake owens valley 5 3 0 7 4 6 0 5 mm yr faults the western mojave block rotates at 2 1 0 8 my clockwise with 3 7 0 7 mm yr of left lateral motion across the western garlock fault we infer 11 2 mm yr of right lateral motion across the mojave region of the eastern california shear zone

coulomb stress accumulation along the san andreas fault system 1 stress accumulation rates along the primary segments of the san andreas fault system are computed using a three dimensional 3 d elastic half space model with realistic fault geometry the model is developed in the fourier domain by solving for the response of an elastic half space due to a point vector body force and analytically integrating the force from a locking depth to infinite depth this approach is then applied to the san andreas fault system using published slip rates along 18 major fault strands of the fault zone gps derived horizontal velocity measurements spanning the entire 1700 200 km region are then used to solve for apparent locking depth along each primary fault segment this simple model fits remarkably well 2 43 mm yr rms misfit although some discrepancies occur in the eastern california shear zone the model also predicts vertical uplift and subsidence rates that are in agreement with independent geologic and geodetic estimates in addition shear and normal stresses along the major fault strands are used to compute coulomb stress accumulation rate as a result we find earthquake recurrence intervals along the san andreas fault system to be inversely proportional to coulomb stress accumulation rate in agreement with typical coseismic stress drops of 1 10 mpa this 3 d deformation model can ultimately be extended to include both time dependent forcing and viscoelastic response

mesozoic metallogeny in east china and corresponding geodynamic settings an introduction to the special issue abstract the giant east china mesozoic metallogenic province hosts some of the world s largest resources of tungsten tin molybdenum antimony and bismuth ores of gold silver mercury lead zinc copper uranium and iron are also of major importance the province and its constituent metallogenic belts or regions south china middle lower yangtze river valley east qinling dabie interior of north china craton yan liao and north east china are the products of several pulses of igneous activity and mineralisation between 240 and 80 ma each successive stage has produced a distinctive suite of deposits that can be readily related to the geodynamic evolution of the region during the mesozoic this geodynamic evolution is linked to a complex series of tectonic events involving far field subduction plate collisions crustal thickening post collision collapse and rifting

sulfur isotopes and pyrite anhydrite equilibria in a volcanic basin hydrothermal system of the middle to lower yangtze river valley anhydrite pyrite mineral pairs from a paleohydrothermal system of the volcanic area of the middle to lower yangtze river valley have been quantitatively interpreted within the constraints of sulfur isotope and thermochemical data on relevant mineral assemblages the delta 34 s values of pyrite in this deposit range from 11 to 9 per mil and those of anhydrite from 14 to 20 per mil the fractionation between coexisting pyrite and anhydrite delta 34 s anhydrite pyrite ranges from 9 to 25 per mil if sulfur isotopes equilibrated between pyrite and anhydrite then these fractionations imply mineralization temperatures of 550 degrees to 310 degrees c these temperatures are roughly 100 degrees c higher than fluid inclusion homogenization temperatures for the same anhydrites suggesting probable sulfur isotope disequilibrium between coexisting pyrite and anhydrite the chemistry of the ore forming fluid is constrained by thermodynamic data and observed mineralization for example early stage ph ranged from 6 4 to 5 0 sigma m s was roughly 0 02 and m sub ca super 2 0 17 whereas late stage values of the same variables were 6 0 to 3 0 0 02 to 0 01 and 0 17 to 0 1 respectively the sulfur isotope composition of the ore forming fluid remained relatively constant with delta 34 s 15 5 to 16 0 per mil from early to late stage deposition the heavy sulfur isotope signature of the ore fluid was presumably acquired by interaction of isotopically lighter more magmatic fluids with basal sediments including anhydrite the delta 34 s values of pyrite decrease from roughly 10 per mil in the early ore stages to roughly 10 per mil in the late stages this is attributed primarily to increasing oxygen fugacity and decreasing temperature throughout ore deposition at the same time the delta 34 s values of coexisting anhydrites increase from roughly 15 to 20 per mil apparently in response to continuing fractionation of heavier sulfur into later stage sulfates which would also decrease the delta 34 s value of coprecipitated sulfides

boundary layer mantle flow under the dead sea transform fault inferred from seismic anisotropy lithospheric scale transform faults play an important role in the dynamics of global plate motion near surface deformation fields for such faults are relatively well documented by satellite geodesy strain measurements and earthquake source studies and deeper crustal structure has been imaged by seismic profiling relatively little is known however about deformation taking place in the subcrustal lithosphere that is the width and depth of the region associated with the deformation the transition between deformed and undeformed lithosphere and the interaction between lithospheric and asthenospheric mantle flow at the plate boundary here we present evidence for a narrow approximately 20 km wide subcrustal anisotropic zone of fault parallel mineral alignment beneath the dead sea transform obtained from an inversion of shear wave splitting observations along a dense receiver profile the geometry of this zone and the contrast between distinct anisotropic domains suggest subhorizontal mantle flow within a vertical boundary layer that extends through the entire lithosphere and accommodates the transform motion between the african and arabian plates within this relatively narrow zone

southern san andreas san jacinto fault system slip rates estimated from earthquake cycle models constrained by gps and interferometric synthetic aperture radar observations we use ground geodetic and interferometric synthetic aperture radar satellite observations across the southern san andreas saf san jacinto sjf fault systems to constrain their slip rates and the viscosity structure of the lower crust and upper mantle on the basis of periodic earthquake cycle maxwell viscoelastic finite element models key questions for this system are the saf and sjf slip rates the slip partitioning between the two main branches of the sjf and the dip of the saf the best fitting models generally have a high viscosity lower crust 10 21 pa s overlying a lower viscosity upper mantle 10 19 pa s we find considerable trade offs between the relative time into the current earthquake cycle of the san jacinto fault and the upper mantle viscosity with reasonable assumptions for the relative time in the earthquake cycle the partition of slip is fairly robust at around 24 26 mm a for the san jacinto fault system and 16 18 mm a for the san andreas fault models for two subprofiles across the saf sjf systems suggest that slip may transfer from the western coyote creek branch to the eastern clark superstition hills branch of the sjf from nw to se across the entire system our best fitting model gives slip rates of 2 3 12 9 12 9 and 17 3 mm a for the elsinore coyote creek clark and san andreas faults respectively where the large uncertainties in the slip rates for the sjf branches reflect the large uncertainty in the slip rate partitioning within the sjf system

modeling of periodic great earthquakes on the san andreas fault effects of nonlinear crustal rheology we analyze the cycle of great earthquakes along the san andreas fault with a finite element numerical model of deformation in a crust with a nonlinear viscoelastic rheology the viscous component of deformation has an effective viscosity that depends exponentially on the inverse absolute temperature and nonlinearly on the shear stress the elastic deformation is linear crustal thickness and temperature are constrained by seismic and heat flow data for california the models are for anti plane strain in a 25 km thick crustal layer having a very long vertical strike slip fault the crustal block extends 250 km to either side of the fault during the earthquake cycle that lasts 160 years a constant plate velocity p 2 17 5 mm yr 1 is applied to the base of the crust and to the vertical end of the crustal block 250 km away from the fault the upper half of the fault is locked during the interseismic period while its lower half slips at the constant plate velocity the locked part of the fault is moved abruptly 2 8 m every 160 years to simulate great earthquakes the results are sensitive to crustal rheology models with quartzite like rheology display profound transient stages in the velocity displacement and stress fields the predicted transient zone extends about 3 4 times the crustal thickness on each side of the fault significantly wider than the zone of deformation in elastic models models with diabase like rheology behave similarly to elastic models and exhibit no transient stages the model predictions are compared with geodetic observations of fault parallel velocities in northern and central california and local rates of shear strain along the san andreas fault the observations are best fit by models which are 10 100 times less viscous than a quartzite like rheology since the lower crust in california is composed of intermediate to mafic rocks the present result suggests that the in situ viscosity of the crustal rock is orders of magnitude less the rock viscosity determined in the laboratory

plagioclase and epidote buffering of cation ratios in mid ocean ridge hydrothermal fluids experimental results in and near the supercritical region abstract experiments have been performed with na ca k cl fluids of seawater chlorinity and diabase basalt and plagioclase bearing mineral mixtures at 350 425 c and 250 400 bars to help constrain hydrothermal alteration processes at mid ocean ridges dissolved ca na and ph for all experiments responded systematically to differences in dissolved sio2 concentrations and the compositions of plagioclase reactants diabase alteration at low fluid rock mass ratios 0 5 to 1 produces fluids undersaturated with respect to quartz during hydration of primary olivine and orthopyroxene whereas basalt alteration under similar conditions yields fluids slightly supersaturated with respect to quartz during breakdown of glass to smectite and amphibole fluid chemistry in all experiments appears to approach a partial equilibrium state with the albite and anorthite components in plagioclase and approaches a ph consistent with plagioclase alteration to epidote trace element data from vent fluids specifically b and sr together with major element chemistry ca na sio2 ph provides evidence that the reaction zone for black smoker fluids at mid ocean ridges is composed of only slightly altered diabase and is characterized by small amounts of epidote nearly fresh plagioclase and clinopyroxene and partially to completely hydrated olivine and orthopyroxene fluids reacting with this rock may be undersaturated with respect to quartz so pressure estimates based on the quartz geobarometer should be regarded as minimums using equilibrium between plagioclase the dominant reactant and epidote the dominant reaction product in experiments we estimate that temperatures in reaction zones are in excess of 375 c for most vent systems these temperatures are higher than measured vent temperatures suggesting that hotspring fluids commonly loose heat during ascent to the seafloor

interseismic strain accumulation spin up cycle invariance and irregular rupture sequences using models of infinite length strike slip faults in an elastic layer above linear viscoelastic regions we investigate interseismic deformation in the models we investigate interseismic strain accumulation on mature faults is the result of the cumulative effects of all previous ruptures and is independent of the fault loading conditions the time for a fault to spin up to a mature state depends on the rheologies and the fault loading conditions after the model has spun up the temporal variation of shear stresses is determined by the fault slip rate and model rheologies the change in stress during spin up depends on the slip rate rheologies and fault loading conditions but is independent of the magnitude of the initial stress over enough cycles such that the cumulative deformation is block like the average mature interseismic velocities are equal to the interseismic velocities of an elastic model with the same geometry and distribution of shear moduli in a model that has spun up with the fault rupturing periodically the cumulative deformation is block like at the end of each seismic cycle and the interseismic deformation is cycle invariant i e the same in all cycles when the fault ruptures nonperiodically the fault spins up to a mature state that is the same as if the fault had ruptured periodically with the mean slip rate when the fault slip rate within each cycle varies the interseismic deformation evolves toward the cycle invariant deformation determined by the most recent fault slip rate around a fault whose slip rate has been faster slower than average interseismic velocities are larger smaller than the cycle invariant velocities and increase decrease from cycle to cycle

continental fault structure and the shallow earthquake source plate boundaries in continental crust are generally less sharply defined than in the oceans with seismicity spread over broad areas interplate displacements appear to be largely accommodated by networks of major fault zones a simple 2 level model for these important structures accounts for the depth distribution of most continental earthquakes and for the observed range of faulting styles and associated rock deformation textures the model consists of a seismogenic frictional slip regime overlying quasi plastic mylonite belts wherein shearing is largely accommodated aseismically due mainly to the changing response of quartz to deformation with increasing temperature shear resistance increases with depth to a peak value in the vicinity of the frictiona1 quasi plastic transition and then decreases rapidly the depth to which microseismic activity extends appears inversely related to regional heat flow and can be satisfactorily modelled as the frictional quasi plastic transition for different geotherms using laboratory determined flow laws for quartz bearing rocks larger earthquake ruptures m 5 5 tend to nucleate near the base of the seismogenic regime in the region inferred to have the highest shear resistance and concentration of distortional strain energy consideration is also given to the depression of isotherms and seismic activity in regions of thrusting and to the question of the downward continuation of major fault zones through the lithosphere decoupling of the upper crust on flat lying shear zones may accompany higher level dip slip and perhaps in some circumstances strike slip faulting being favoured by above average continental heat flow and a high quartz content in the middle or deep crust the average level of deviatoric stress within the seismogenic regime remains an outstanding problem

where is the real transform boundary in california 1 the zone of highest geodetically defined strain rate in california does not everywhere coincide with the surface trace of the san andreas fault saf to determine whether this reflects the pattern of long term permanent deformation we analyzed the velocity field on swaths across the transform located so as to avoid intersections among the major fault strands slip rates and flexural parameters for each fault were determined by finding the best fit to the velocity profile using a simple arctan model representing the interseismic strain accumulation our slip rates compare well with current geologic estimates which suggests that the present day velocity field is representative of long term motions we find that the transform is a zone of high strain rate up to 80 km wide that is straighter than the saf and has an overall trend closer to the relative plate motion vector than the saf most sections of the saf take up less than half of the total slip rate and slip is transferred from one part of the system to another in a way that suggests that the saf should not be considered as a unique locator of the plate boundary up to half of the total displacement takes place on faults outside the high strain rate zone distributed over several hundred kilometers on either side our findings substantiate previous suggestions that the transform has the characteristics of a macroscopic ductile shear zone cutting the continental lithosphere around which stress and strain rate decrease on a length scale controlled by the length of the transform

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

dating offset fans along the mojave section of the san andreas fault using cosmogenic 26al and 10be analysis of cosmogenic 1 0 be and 2 6 al in samples collected from exposed boulders n 20 and from buried sediment n 3 from offset fans along the san andreas fault near little rock california yielded ages ranging from 16 to 413 ka which increase with dis lance from their source at the mouth of little rock creek in order to determine the age of the relatively younger fans the erosion rate of the boulders and the cosmogenic nuclide inheritance from exposure prior to deposition in the fan were established cosmogenic nuclide inheritance values that range between 8 5 x 10 3 and 196 10 3 atoms 1 0 be g 1 quartz were determined by measuring the concentrations and ratios of 1 0 be and 2 6 al in boulders n 10 and fine sediment n 7 at the outlet of the present active stream boulder erosion rate ranging between 17 and 160 mm k y 1 was estimated by measuring 1 0 be and 2 6 al concentrations in nearby bedrock outcrops n 8 since the boulders on the fans represent the most resistant rocks in this environment we used the lowest rate for the age calculations monte carlo simulations were used to determine ages of 16 5 and 29 7 ka for the two younger fan surfaces older fans older than 100 ka were dated by analyzing 1 0 be and 2 6 al concentrations in buried sand samples the ages of the three oldest fans range between 227 242 and 413 185 ka although fan age determinations are accompanied by large uncertainties the results of this study show a clear trend of increasing fan ages with increasing distance from the source near little rock creek and provide a long term slip rate along this section of the san andreas fault slip rate along the mojave section of the san andreas fault for the past 413 k y can be determined in several ways the average slip rate calculated from the individual fan ages is 4 2 0 9 cm yr 1 a linear regression through the data points implies a slip rate of 3 7 1 0 cm yr 1 a most probable slip rate of 3 0 1 0 cm yr 1 is determined by using a 2 test these rates suggest that the average slip along the mojave section of the san andreas fault has been relatively constant over this time period the slip rate along the mojave section of the san andreas fault determined in this study agrees well with previous slip rate calculations for the quaternary

strain transfer at continental scale from a transcurrent shear zone to a transpressional fold belt the patos serido system northeastern brazil during the brasiliano pan african orogeny a complex continental scale pattern of east west transcurrent shear zones and northeast trending fold belts formed in the northern and central borborema province of northeastern brazil the east west shear zones have been usually regarded as slightly younger features but the study of the most spectacular case of intersection between these two structures the patos shear zone and the serido transpressional belt leads to a different tectonic model satellite imagery and structural petro logical and geophysical data support the interpretation that these structures 1 are in structural continuity and 2 formed simultaneously under amphibolite facies metamorphic conditions that led to partial melting this suggests a model of strain transfer at the scale of the orogen at the eastern end of the patos east west dextral shear zone the strain that accommodated the relative motion of the northern block was transferred to the northeast trending serido belt where it resulted in folding strike slip faulting and stretching parallel to the strike of the belt

chemical controls on the composition of vent fluids at 13 11 n and 21 n east pacific rise six vent fields sampled at 13 11 n east pacific rise epr in may 1984 exhibit large interfield variations and a much wider range of chemical compositions than previously observed at 21 n measured ph at 25 c are acidic ranging from 3 1 to 3 7 sodium and chloride vary from 40 lower to 30 higher than seawater iron concentrations range from 2 to 10 mmol kg compared with 0 7 2 5 mmol kg at 21 n other sulfide forming metals cu zn cd and pb are generally lower at 11 13 n than at 21 n reliable temperature measurements were obtained at only two of the six vents and both were 350 5 c the vent fields at 21 n epr were resampled in august 1985 thus extending to almost 6 years the time period over which they have been monitored previous expeditions were made in november 1979 and november 1981 campbell et al this issue have shown that the chemistry of the hydrothermal fluids from these fields has been very stable over the period of repeated observation equilibrium calculations for the fluids from the fields at 13 n and 21 n using a greatly improved thermodynamic data base are described in this report they indicate that the chemistry is rock buffered and that the stability of these systems over time is a result of equilibrium control with respect to a greenschist type mineral assemblage at depth calculated high temperature ph of the fluids range from 4 1 to 4 7 with those from 13 11 n at the more acidic end of the range calculated affinities show that the fluids are close to or at saturation with respect to quartz albite muscovite smectite chlorite epidote and pyrrhotite the computations imply that lower temperature vents such as national geographic smoker ngs 273 c may have their silica concentrations controlled by equilibrium with respect to a phase other than quartz a comparison of fluid chemistry between ngs and vent 5 at 11 n suggests that the latter vent may also have a temperature 300 c while the fluid chemistry of a given field is compositionally stable the compositions themselves are unique for a particular field and cover a wide range between fields the processes controlling this variability probably include the depth of reaction i e pressure which is difficult to treat thermodynamically at present in addition increasing depth in the crust may be accompanied by decreasing permeability and hence a probable decrease in the water rock ratio it may be difficult to separate these two effects if the age differences between the fields at present unknown are of the order of decades then their temporal histories may have an important influence on their particular chemistries

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

opening mode fracture in siliceous mudstone at high homologous temperature effect of surface forces in analogy to high temperature sintering of ceramics and metal powder compacts the formation of opening mode fractures in siliceous mudstone during natural in situ combustion of hydrocarbons is attributed to contractile surface forces between mineral grains and an interstitial melt phase a comparison between bulk density increase during sintering and created fracture space indicates that fracturing resulted from contraction of the rock matrix due to porosity reduction grain scale mass transfer and high temperature mineral formation it is suggested that contractile surface forces between mineral grains and between mineral grains and pore fluid contribute to subcritical fracture formation under a wide range of subsurface conditions

stress evolution of the san andreas fault system recurrence interval versus locking depth 1 major ruptures along the san andreas fault system safs are driven by stress that has accumulated in the upper locked portion of the crust the present day stress accumulation rate on any given fault segment is fairly well resolved by current geodetic measurements model stress accumulation rates vary between 0 5 and 7 mpa per century and are inversely proportional to earthquake recurrence intervals in contrast the total accumulated stress on a given fault segment is poorly resolved since it depends on the uncertain rupture history of each fault over the past few thousand years we simulate accumulated stress at crustal depths for both past and present day conditions by assuming complete release of accumulated slip deficit during major ruptures these speculative results indicate that the southern san andreas which has not ruptured in a major earthquake in over 300 years is currently approaching a threshold stress level

deformation kinematics in the western united states determined from quaternary fault slip rates and recent geodetic data we estimate the horizontal velocity gradient tensor field from quaternary fault slip rates and recent global positioning system gps and very long baseline interferometry vlbi velocity solutions in the western united states transform plate boundary zone the total velocity obtained from the quaternary fault slip rate data across the entire plate boundary is within 1 mm yr of the nuvel 1a predicted pacific pa north american na plate motion velocity but directions are 5 6 anticlockwise of directions given by nuvel 1a the total velocity obtained from inversion of recent geodetic data is 2 3 anticlockwise from the nuvel 1a na pa velocity but the difference between the two is not significant at the 95 confidence level the discrepancy between the total pa na motion obtained from the geological data and nuvel 1a indicates that a marginally significant amount of ne sw shortening possibly as much as 5 mm yr is missing overall in the geologic data shortening may occur in the long term in the offshore and coastal areas of california where such shortening is required in the shorter term geodetic solution the seismic moment released in the last 148 years is 59 of the total moment release rate expected from long term strain rate field including both seismic and aseismic deformation derived from the inversion of geological data with nuvel 1a far field pa na motion constraints the accumulated strain in the areas containing the southern san andreas fault san jacinto fault the san francisco bay area and the area containing the ventura basin in the western transverse ranges in the last 148 years is the equivalent to that which could be released by an mw 7 0 earthquake in each 50 100 15 km3 crustal volume if strain is to be released seismically in these areas

viscoelastic shear zone model of a strike slip earthquake cycle i examine the behavior of a two dimensional 2 d strike slip fault system embedded in a 1 d elastic layer schizosphere overlying a uniform viscoelastic half space plastosphere and within the boundaries of a finite width shear zone the viscoelastic coupling model of savage and prescott 1978 considers the viscoelastic response of this system in the absence of the shear zone boundaries to an earthquake occurring within the upper elastic layer steady slip beneath a prescribed depth and the superposition of the responses of multiple earthquakes with characteristic slip occurring at regular intervals so formulated the viscoelastic coupling model predicts that sufficiently long after initiation of the system 1 average fault parallel velocity at any point is the average slip rate of that side of the fault and 2 far field velocities equal the same constant rate because of the sensitivity to the mechanical properties of the schizosphere plastosphere system i e elastic layer thickness plastosphere viscosity this model has been used to infer such properties from measurements of interseismic velocity such inferences exploit the predicted behavior at a known time within the earthquake cycle by modifying the viscoelastic coupling model to satisfy the additional constraint that the absolute velocity at prescribed shear zone boundaries is constant i find that even though the time averaged behavior remains the same the spatiotemporal pattern of surface deformation particularly its temporal variation within an earthquake cycle is markedly different from that predicted by the conventional viscoelastic coupling model these differences are magnified as plastosphere viscosity is reduced or as the recurrence interval of periodic earthquakes is lengthened application to the interseismic velocity field along the mojave section of the san andreas fault suggests that the region behaves mechanically like a 600 km wide shear zone accommodating 50 mrn yr fault parallel motion distributed between the san andreas fault system and eastern california shear zone in the viscoelastic coupling model savage and prescott 1978 time dependent deformation around a strike slip fault zone is realized by loading subsequent rupture and relaxation of a fault present in a schizosphere elastic upper crust overlying a ductile plastosphere assuming a two dimensional 2 d geometry periodic rupture of the fault and identical slip interseismic deformation is obtained as the superposition of viscoelastic relaxation of the plastosphere summed over the infinity of past periodic earthquakes this model yields far field interseismic motions equal to 1 2 of the average slip rate v0 imposed on the fault thus even when no external mechanism is invoked to load the fault i e when the locked portion of the fault penetrates the entire schizosphere the model satisfies the expectation of rigid plate like behavior in the far field in addition when periodic coseismic offsets are factored in the model predicts that the time averaged fault parallel velocity of each point is vol2 depending on location

nonlinear strain buildup and the earthquake cycle on the san andreas fault two contrasting models of the earthquake deformation cycle on strike slip faults predict significant temporal declines in shear strain rate near the fault accompanied by a progressive broadening of the zone of deformation adjacent to it in the thin lithosphere model transient deformation results from flow in the asthenosphere due to stress relaxation following faulting through most or all of the lithosphere for an earth model with a thick elastic lithosphere plate thickness depth of seismic slip transient motions are due to postearthquake aseisrnic slip below the coseismic fault plane data from the san andreas fault indicate a long term temporal decrease in strain rate that persists for at least 30 years and may extend through the entire earthquake cycle observations support a cycle long rate decrease and a temporal spreading of the deformation profile only if movement cycles on the northern and southern locked sections of the fault are basically similar if so the usually lower strain rates and broader deformation zone currently observed on the southern san andreas represent a later evolutionary stage of the northern locked section where a great earthquake is a more recent occurrence although the data allow some extreme models to be discarded no sufficiently strong constraints exist to decide between the thin and thick lithosphere models regardless of the appropriate model the geodetic observations themselves indicate that strain buildup is sufficiently nonlinear to cause significant departures from recurrence estimates based on linear strain accumulation and the time predictable model

constraints on fault slip rates of the southern california plate boundary from gps velocity and stress inversions summary we use global positioning system gps velocities and stress orientations inferred from seismicity to invert for the distribution of slip on faults in the southern california plate boundary region of particular interest is how long term slip rates are partitioned between the indio segment of the san andreas fault saf the san jacinto fault sjf and the san bernardino segment of the saf we use two new sets of constraints to address this problem the first is geodetic velocities from the southern california earthquake center s scec crustal motion map version 3 by shen et al which includes significantly more data than previous models the second is a regional model of stress field orientations at seismogenic depths as determined from earthquake focal mechanisms while gps data have been used in similar studies before this is the first application of stress field observations to this problem we construct a simplified model of the southern california fault system and estimate the interseismic surface velocities using a backslip approach with purely elastic strain accumulation following meade et al in addition we model the stress orientations at seismogenic depths assuming that crustal stress results from the loading of active faults the geodetically derived stressing rates are found to be aligned with the stress orientations from seismicity we therefore proceed to invert simultaneously gps and stress observations for slip rates of the faults in our network we find that the regional patterns of crustal deformation as imaged by both data sets can be explained by our model and that joint inversions lead to better constrained slip rates in our preferred model the sjf accommodates 15 mm yr 1 and the indio segment of the saf 23 mm yr 1 of right lateral motion accompanied by a low slip rate on the san bernardino segment of the saf anomalous fault segments such as around the 1992 m w 7 3 landers surface rupture can be detected there observed stresses deviate strongly from the long term loading as predicted by our simple model evaluation of model misfits together with information from palaeoseismology may provide further insights into the time dependence of strain accumulation along the san andreas system

evidence of power law flow in the mojave desert mantle studies of the earth s response to large earthquakes can be viewed as large rock deformation experiments in which sudden stress changes induce viscous flow in the lower crust and upper mantle that lead to observable postseismic surface deformation laboratory experiments suggest that viscous flow of deforming hot lithospheric rocks is characterized by a power law in which strain rate is proportional to stress raised to a power n refs 2 3 most geodynamic models of flow in the lower crust and upper mantle however resort to newtonian linear stress strain rate relations here we show that a power law model of viscous flow in the mantle with n 3 5 successfully explains the spatial and temporal evolution of transient surface deformation following the 1992 landers and 1999 hector mine earthquakes in southern california a power law rheology implies that viscosity varies spatially with stress causing localization of strain and varies temporally as stress evolves rendering newtonian models untenable our findings are consistent with laboratory derived flow law parameters for hot and wet olivine the most abundant mineral in the upper mantle and support the contention that at least beneath the mojave desert the upper mantle is weaker than the lower crust

pyrite anhydrite magnetite pyroxene type deposits and coexisting hydrothermal fluids in mesozoic volcanic basins yangtze river valley china abstract anhydrite pyrite magnetite pyroxene type deposits occur in the mesozoic volcanic areas of the middle lower yangtze valley in china these deposits are hosted in alkaline basaltic rocks and are generally accompanied by melanocratic and leucocratic alteration zones both of which are characterized by a distinct vertical zonation pattern investigation of these zones indicates that the chemical compositions of solid solutions and polymorphs of various minerals vary spatially in the alteration profile upwards from the lowest level and outwards from the center here we report a case study on the luohe deposit in the melanocratic alteration zone the composition of magnetite including trace elements ti v mg mn pyroxene mg fe 2 fe 3 al 2 o 3 plagioclase an x ab 1 x pyrite co ni and apatite f ceo 2 y 2 o 3 la 2 o 3 changes with depth the isotherms of hydrothermal fluids determined from fluid inclusion data including homogenization temperature and salinity also vary with depth activity diagrams were constructed from mineral and isotherm analysis to estimate the chemical constraints on the alteration mineral assemblages and the coexisting hydrothermal solutions for the na 2 o k 2 o cao mgo feo fe 2 o 3 a1 2 o 3 sio 2 h 2 so 4 h 2 s hci h 2 o system at 350 to 600 c and 500 bars 50 mpa assuming that the major alteration mineral assemblages along the profile reflect the nature of the coexisting hydrothermal solutions the activity diagrams adopted the major minerals as buffers to fix the activities of the aqueous species in the system simulating the physicochemical conditions of the magnetite anhydrite pyroxene equilibrium and of solid solutions of diopside hedenbergite grossular andradite and anorthite albite found in the profile this study provides an approach to modeling the chemical constraints of coexisting fluids in ore alteration zones based on field observations

elevated shear zone loading rate during an earthquake cluster in eastern california we compare geodetic velocity to geologic fault slip rates to show that tectonic loading was doubled across the eastern california shear zone ecsz during a cluster of major earthquake activity new slip rates are presented for six dextral faults that compose the ecsz in the central mojave desert these rates were determined from displaced alluvial fans dated with cosmogenic 10 be and from a displaced lava flow dated with 40 ar 39 ar we find that the sum geologic mojave ecsz slip rate 6 2 1 9 mm yr is only half the present day geodetically measured velocity of 12 2 mm yr these rates account for cumulative fault slip and geodetic observations that span the 60 km wide shear zone therefore this difference cannot be attributed to postseismic relaxation redistribution of tectonic loading over the earthquake cycle at a regional scale suggests that earthquake clustering may be enhanced via feedback with weakening of ductile shear zones

strain accumulation across the carrizo segment of the san andreas fault california impact of laterally varying crustal properties 1 major strike slip faults juxtapose geologically dissimilar terrain which may vary in mechanical properties leading to an asymmetric pattern of strain accumulation we present new gps data on the carrizo segment of the san andreas fault separating the salinian block southwest of the fault from franciscan terrane northeast of the fault to better quantify asymmetric strain accumulation we also present a series of finite element models to investigate the possible role of variable elastic layer thickness and material properties of the upper crust the geodetic data are well fit with a simple model comprising a weak upper crustal zone 10 25 km wide northeast of the fault this model is also consistent with geologic data on the distribution of major rock types and corresponding laboratory data on their material properties as well as paleoseismic seismic and magnetotelluric data using this model we estimate a long term average over several seismic cycles slip rate for the san andreas fault of 36 1 5 2 mm yr in agreement with the known holocene rate within uncertainties and a viscosity for the combined lower crust upper mantle of 2 5 1019 pa s

timing and extent of neogene tectonic rotation in the western transverse ranges california paleomagnetic data collected from lower miocene rocks throughout the western transverse ranges province have consistently clockwise deflected magnetic declinations which suggests that the entire province has experienced clockwise tectonic rotation during neogene time paleomagnetic declinations in lower miocene rocks increase westward from 33 5 c 11 in the san gabriel block to 91 7 7 in the western santa ynez range suggesting a westward increase in the net amount of clockwise tectonic rotation paleomagnetic data that constrain the tuning of rotation suggest that the entire western transverse ranges rotated uniformly 50 60 clockwise during the middle miocene this implies a minimum of 200 km of distributed dextral shear along the california coast and offshore during middle miocene time little tectonic rotation appears to have occurred in the western transverse ranges during late miocene time since miocene time continued clockwise rotation in the western santa ynez range 31 3 combined with counterclockwise rotation in the san gabriel block 16 has oroclinally bent the transverse ranges to the west of the san andreas fault the pliocene pleistocene clockwise rotation implies an additional 30 60 km of distributed dextral shear across the southern california coast ranges during post miocene time

holocene activity of the rose canyon fault zone in san diego california the rose canyon fault zone in san diego california has many well expressed geomorphic characteristics of an active strike slip fault including scarps offset and deflected drainages and channel walls pressure ridges a closed depression and vegetation lineaments geomorphic expression of the fault zone from mount soledad south to mission bay indicates that the mount soledad strand is the most active a network of trenches excavated across the mount soledad strand in rose creek demonstrate a minimum of 8 7 m of dextral slip in a distinctive early to middle holocene gravel filled channel that crosses the fault zone the gravel filled channel was preserved within and east of the fault but was removed west of the fault zone by erosion or possibly grading during development consequently the actual displacement of the channel could be greater than 8 7 m radiocarbon dates on detrital charcoal recovered from the sediments beneath the channel yield a maximum calibrated age of about 8 1 0 2 kyr the minimum amount of slip along with the maximum age yield a minimum slip rate of 1 07 0 03 mm yr on this strand of the rose canyon fault zone for much of holocene time other strands of the rose canyon fault zone which are east and west of our site may also have holocene activity based on an analysis of the geomorphology of fault traces within the rose canyon fault zone along with the results of our trenching study we estimate the maximum likely slip rate at about 2 mm yr and a best estimate of about 1 5 mm yr stratigraphie evidence of at least three events is present during the past 8 1 kyr the most recent surface rupture displaces the modern a horizon topsoil suggesting that this event probably occurred within the past 500 years stratigraphie and structural relationships also indicate the occurrence of a scarp forming event at about 8 1 kyr prior to deposition of the gravel filled channel that was used as a piercing line a third event is indicated by the presence of several fault strands that displace the channel but did not move during the most recent event other events may also have occurred but these data suggest that the return time for surface rupturing earthquakes is no more than about 4 kyr

kinematics of the pacific north america plate boundary zone northern california we measured motions of 54 sites in an east west transect across northern california at 38 40 north by global positioning system gps observations over a 4 year we estimate the total slip rate on the san andreas fault system to be 39 6 0 6 1 5 mm yr 68 6 upper and lower confidence intervals from a nonlinear inversion are indicated by superscripts and subscripts slip rates on the individual faults are determined less precisely due to the high correlations between the estimated parameters our best fitting model fits the fault parallel velocities with a mean square error of 1 04 and the following estimated fault slip rates all in mm yr san andreas 17 4 3 1 2 5 ma acama 13 9 2 8 4 1 and bartlett springs 8 2 1 9 2 1 the data are fit best by models in which the san andreas fault is locked to 14 9 7 1 12 5 km the ma acama fault locked to 13 4 4 8 7 4 km except for shallow creep in the upper 5 km and the bartlett springs fault creeping at all depths the ma acama fault most likely poses a significant seismic hazard as it has a high slip rate and has accumulated a slip deficit large enough to generate a magnitude 7 earthquake we find little evidence for contraction across coast ranges except at western edge of great valley where 1 3 mm yr of shortening is permitted by the data no strain is observed within the great valley or sierra nevada except that associated with right lateral strike slip on the san andreas fault system this is consistent with models of the pacific north america plate boundary zone in which the relative plate motion is partitioned into two domains one strike slip and one dominantly extensional separated by the elastically deforming sierra nevada great valley block

teleseismic shear wave splitting and lithospheric anisotropy beneath and across the altyn tagh fault sixteen 3 component 5 sec stations along a profile crossing the altyn tagh fault atf at 90 e were used to measure shear wave splitting beneath tibet s northern edge splitting delay times t of 0 3 to 1 3 sec show a marked increase in a narrow zone centered on the fault implying anisotropy in the lithospheric mantle fast polarization directions fpd swing counterclockwise from a mean azimuth of 110 e in the qaidam basin south of the fault to a mean azimuth of 60 e in the altyn mountains this swing occurs in less than 40 km both the t increase and fpd swing are consistent with ductile sinistral shear along the 40 km wide low p wave velocity anomaly 6 8 imaged to a depth of 140 km beneath the fault by teleseismic tomography this confirms that the atf guides the northeastwards extrusion of the tibet qaidam s lithosphere relative to the tarim s

length scales for continental deformation in convergent divergent and strike slip environments analytical and approximate solutions for a thin viscous sheet model the deformation of a thin viscous layer that has a moving boundary is investigated for comparison with zones of deformation in the continental lithosphere exact analytical solutions for the case of a newtonian fluid and approximate solutions for the case of fluid with power law rheology show that when the imposed velocity vector is normal to the boundary congressional or extensional regime the deformation field decays away from the boundary with a characteristic length scale 1 3 to 1 10 the wavelength of the imposed boundary velocity distribution for n between 1 and 10 where n is the stress strain exponent in the rheology in contrast when the imposed velocity vector is parallel to the boundary transcurrent regime the length scale of the deformation field is approximately 4 times smaller in each case these length scales decrease approximately as n the difference in length scales arises even in the absence of any buoyancy forces acting on thickened or thinned crust such forces would modify the ratio of length scales but not sufficiently to affect this result

distributed deformation in the lower crust and upper mantle beneath a continental strike slip fault zone marlborough fault system south island new zealand converted phases from teleseisms recorded by a seismic array spanning the northern half of the marlborough fault system south island new zealand show a continuous unbroken moho underlying a seismically anisotropic lower crust beneath the two north ernmost faults of the fault system these observations suggest that distributed deforma tion not slip on a narrow vertical fault accommodates displacement in the lower crust below the 120 480 km of right lateral slip across the wairau fault one splay of the marl borough fault system and the northward continuation of the alpine fault beneath the wairau fault the moho dips 258 308 southeast from a depth of 26 km northwest of the fault to a depth of 34 km southeast of the fault farther to the southeast ps conversions from the moho continue under the awatere fault 34 6 10 km of slip with a constant amplitude and depth of 34 km across the two faults converted energy from 16 20 km depth varies with back azimuth in a manner suggesting the presence of anisotropy in the lower crust these observations imply that one of the tenets of plate tectonics that faults defining plate boundaries pass through both crust and upper mantle does not apply to new zealand or to continents in general

global positioning system constraints on fault slip rates in southern california and northern baja mexico we use global positioning system gps estimates of horizontal site velocity to constrain slip rates on faults comprising the pacific north america plate boundary in southern california and northern mexico we enlist a simple elastic block model to parameterize the distribution and sum of deformation within and across the plate boundary we estimate a pacific north america relative plate motion rate of 49 3 mm yr one standard deviation consistent with nuvel 1a estimates we are able to resolve robust slip rate estimates for the southernmost san andreas san jacinto and elsinore faults 26 2 9 2 and 6 2 mm yr respectively and for the imperial and cerro prieto faults 35 2 and 42 1 mm yr respectively accounting for about 86 of the total plate motion the remaining 14 appears to be accommodated to the west of these fault systems probably via slip along the san clemente fault and or the san miguel vallecitos rose canyon and newport inglewood fault systems these results are highly consistent with paleoseismic estimates for slip rates implying that off fault strain accumulation within the deforming zone of the plate boundary is largely elastic we estimate that the seismically quiescent southernmost san andreas fault has incurred about 8 2 m of slip deficit over the last few hundred years presumably to be recovered during a future large earthquake

relation between surface velocity field and shear wave splitting in the south island of new zealand 1 the relative motion between the pacific and australian plates across the central south island of new zealand as measured by repeated gps measurements is distributed over a region more than 80 km wide the surface distribution of velocity is fit almost equally well by a model in which deformation of the lower lithosphere is concentrated into very narrow slip zones below one or two major faults and by a model in which the lower lithosphere deforms in a broad shear zone observations of the splitting of sks waves permit distinction between the two models on the basis of the finite strain they predict deformation in the deep lithosphere that was limited to narrow shear zones would not produce measurable splitting hence any splitting observed should not correlate with surface tectonics however the distribution of fast propagation directions revealed by observations of shear wave splitting do reflect surface tectonics in particular the orientations of fast propagation directions are consistent with the finite strain calculated by assuming that the distribution of velocity at depth within the lithosphere resembles the distribution of velocity measured at the surface by gps the observed distribution of splitting can be reproduced with an rms misfit of 15 provided that the velocity field is integrated over at least the time interval since significant convergence across the southern alps began at about 6 5 ma

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

slicing up the san francisco bay area block kinematics and fault slip rates from gps derived surface velocities received 26 october 2004 revised 11 february 2005 accepted 10 march 2005 published 16 june 2005 i observations of surface deformation allow us to determine the kinematics of faults in the san francisco bay area we present the bay area velocity unification bavu bay view a compilation of over 200 horizontal surface velocities computed from campaign style and continuous global positioning system gps observations from 1993 to 2003 we interpret this interseismic velocity field using a three dimensional block model to determine the relative contributions of block motion elastic strain accumulation and shallow aseismic creep the total relative motion between the pacific plate and the rigid sierra nevada great valley sngv microplate is 37 9 0 6 mm yr 1 directed toward n30 4 w 0 8 at san francisco 2 fault slip rates from our preferred model are typically within the error bounds of geologic estimates but provide a better fit to geodetic data notable right lateral slip rates in mm yr 1 san gregorio fault 2 4 1 0 west napa fault 4 0 3 0 zone of faulting along the eastern margin of the coast range 5 4 1 0 and mount diablo thrust 3 9 1 0 of reverse slip and 4 0 0 2 of right lateral strike slip slip on the northern calaveras is partitioned between both the west napa and concord green valley fault systems the total convergence across the bay area is negligible poles of rotation for bay area blocks progress systematically from the north america pacific to north america sngv poles the resulting present day relative motion cannot explain the strike of most bay area faults but fault strike does loosely correlate with inferred plate motions at the time each fault initiated