role of subsurface loads and regional compensation in the isostatic balance of the transverse ranges california evidence for intracontinental subduction bouguer gravity anomalies in the transverse ranges of southern california show a pronounced asymmetry in north south profiles across the mountains and a characteristic shape which is largely insensitive to variations in the topographic load along strike while individual gravity topography profile pairs could be consistent with a number of isostatic compensation mechanisms the unusual relationship between gravity and topography in the san bernardino mountains severely limits the class of acceptable models if the bouguer gravity field is caused by flexure of the crust mantle boundary the simplest solution consistent with the data in the western transverse ranges the san gabriel mountains and the san bernardino mountains models the great valley and mojave provinces to the north as a 50 km thick elastic plate supporting the mountains which are underthrust by a much weaker plate to the south the best fit to the gravity data in the san bernardino mountains requires a terminal bending moment on the southern plate beneath the mountains the magnitude of the terminal bending moment decreases to the west in the san gabriel and western transverse ranges we equate the load responsible for this terminal moment with the high velocity slablike feature mapped in the upper mantle beneath the transverse ranges using seismic tomography the coupling between the subsurface load and the overlying lithosphere suggests that the slab represents lithospheric subduction with an extremely small density contrast rather than a small scale convective feature

chronology and rates of faulting of ventura river terraces california new evidence concerning the chronology of four late pleistocene terraces of the ventura river near oak view california that are vertically offset and tilted by reverse and reverse flexural slip faults provides a means of estimating rates of fault movement and downcutting by the ventura river radiocarbon ages of charcoal contained within terrace deposits date two of the terraces correlation of soils that have developed on terrace deposits and extrapolation of the rate of vertical displacement of the arroyo parida santa ana fault not a flexural slip fault date the others resulting age estimates for the four main late pleistocene terraces are qt5b 30 000 qt6a 38 000 qt6b 54 000 and qt6c 92 000 yr b p vertical slip rates on flexural slip faults range from 0 3 to 1 1 mm yr and apparently are related to the rate form and mechanics of folding of the north limb of the ayers creek canada larga syncline average rates of downcutting of the ventura river for several intervals during the late pleistocene estimated from the chronology and relative elevation of river terraces north of the arroyo panda santa ana fault upstream from the zone of faulting vary from 0 5 to 1 3 mm yr the range in rates probably reflects variations in local uplift as well as adjustments to changing eustatic sea level climatic conditions and or regional deformation of the western transverse ranges the average rate of downcutting of the ventura river north of the zone of flexural slip faulting is 0 8 mm yr compared with 1 2 to 2 2 mm yr in the deformation zone immediately to the south this apparently indicates that during tectonic deformation there is an approximate balance between the rate of uplift due to faulting and folding and the rate of downcutting by the fluvial system

mineralogy and composition of the upper mantle seismic velocities are calculated for two petrological models of the upper mantle an olivine rich assemblage pyrolite and a garnet clinopyroxene rich olivine bearing assemblage piclogite these are compared with recent seismic profiles for various tectonic provinces the shield data is most consistent with a cold olivine and orthopyroxene rich lid the seismic lithosphere extending to 150 km followed by a high temperature gradient and or a change in mineralogy that serves to decrease the velocity from 200 to 400 km the velocities follow a 1400 c adiabat the rise tectonic mantle is much slower presumably hotter and is likely to be above the solidus to depths of at least 300 km the high v p v s ratio of the lower oceanic lithosphere in the western pacific is most consistent with eclogite

in situ measured current structures of the eddy field in the mozambique channel abstract circulation and the related biological production have been studied during five cruises conducted in the mozambique channel mzc between 2005 and 2010 the circulation in the mzc is known to be highly turbulent favouring enhanced primary production as a result of mesoscale eddy dynamics and connectivity throughout the channel due to the variable currents associated with migrating eddies this paper presents the results of in situ measurements that characterize the horizontal and vertical currents in the surface and subsurface layers 0 500 m the in situ data were analysed together with the geostrophic eddy field observed from satellite altimeter measurements different circulation regimes were investigated including the classical anticyclonic eddy generated at the channel narrows 16 s the enhancement of southward migrating eddies by merging with structures both cyclonic and anticyclonic formed in the east of the channel and the presence of a fully developed cyclonic eddy at the channel narrows comparison between in situ measurements s adcp and velocities derived from surface drifters and the geostrophic current derived from sea surface height measurements indicated that the latter can provide a reliable quantitative description of eddy driven circulation in the mzc with the exception that these currents are weaker by as much 30 it is also suggested from in situ observation drifters that the departure from geostrophy of the surface circulation might be linked to strong wind conditions finally our observations highlight that a geostrophic currents need to be considered in future research to facilitate a more comprehensive description of the circulation in this area

structural transect of the western transverse ranges california implications for lithospheric kinematics and seismic risk evaluation a retrodeformable cross section that integrates surface and subsurface data across the western transverse ranges california illustrates an actively developing fold and thrust belt that began forming at 2 3 ma high level thrusts are interpreted to root in a mid crustal detachment at 12 15 km depth which coincides with the maximum depth of earthquakes the cross section documents 53 km of convergence above the mid crustal detachment dividing this by the time since onset of deformation yields convergence rates of 17 6 26 5 mm yr the high density lithospheric anomaly beneath the transverse ranges is related to subduction of lower crust and lithosphere below the mid crustal detachment to balance the shallow crustal shortening thrust ramps coincide with zones of high seismicity in the transverse ranges this suggests that ramp regions have the highest potential for compressive earthquake events e g the recent whittier narrows earthquake of october 1 1987 m sub l 5 9 occurred along the eastern part of a ramp zone

geodetic measurement of deformation in the central mojave desert california data from triangulation and trilateration surveys made during 1934 1982 are used to calculate shear strain rates in the central mojave desert of california for the region between the helendale and camp rock faults the shear strain rate was determined to be 0 16 0 03 strain yr with maximum right lateral shear strain occurring on a plane oriented n41 w 5 if we assume that this deformation is due to right lateral motion across the northwest trending local faults the average shear straining corresponds to a relative displacement of 6 7 1 3 mm yr across this portion of the network accounting for 12 of the predicted 56 mm yr of relative motion between the north american and pacific plates from the camp rock fault eastward across the network there is a transition from significant to very low strain rates examination of nine focal mechanisms and their relation to the local geology and the strain data suggests that most of the long term displacement occurs on the major northwest trending faults oriented nearly along the direction of relative motion between the north american and pacific plates secondary faulting controlled by a coulomb anderson failure mechanism or by slip on preexisting faults can account for the occurrence of earthquakes on faults of other orientations

active faulting near the cajon pass well southern california implications for the stress orientation near the san andreas fault the dosecc cajon pass well is located 4 km ne of the san andreas fault in a region that includes a diversity of secondary structures active faults close to the well are subparallel to the right lateral san andreas fault but exhibit left lateral and normal slip the stress orientation measured in the well is not consistent with right lateral slip on the san andreas fault or the stresses inferred from focal mechanisms of earthquakes within 10 km of the fault but may drive the e striking slightly normal left lateral cleghorn fault the closest active fault to the well if the in situ stress is everywhere consistent with the highly variable styles of active secondary faulting like the cleghorn the orientation of stresses along the san andreas fault may be quite variable

forecast model for moderate earthquakes near parkfield california earthquake instability models have possible application to earthquake forecasting because the models simulate both preseismic and coseismic changes of fault slip and ground deformation in the forecast procedure proposed here repeated measurements of preseismic fault slip and ground deformation constrain the values of model parameters the early part of the model simulation corresponds to the available field data and the subsequent part constitutes an estimate of future faulting and ground deformation in particular the time location and size of unstable faulting are estimates of the pending earthquake parameters the forecast accuracy depends on the model realism and parameter resolution the forecast procedure is applied to fault creep and trilateration data measured near parkfield california where at least five magnitude 5 5 to 6 earthquakes have occurred regularly since 1881 the last in 1966 the quasi static model consists of a flat vertical plane embedded in an elastic half space spacially variable fault slip of strike slip sense is driven by an increasing regional shear stress but is impeded by a relatively strong patch of brittle strain softening fault the field data are consistent with these approximate values of patch parameters radius of 3 km patch center 5 km deep and 8 km southeast of the 1966 epicenter and maximum brittle strength of 26 bars fluctuations in the available field data prevent estimating the earthquake time with any more precision than use of the 21 8 year recurrence interval however the model may later give a more precise estimate of the earthquake time if the fault slip rate near the inferred patch increases before the earthquake as predicted by the model

tomographic image of the southern california mantle p wave teleseismic travel time delays recorded by the southern california array are inverted by the method of backprojection tomography to obtain images of variations in the p wave velocity structure to a depth of 750 km two major upper mantle features arc resolved one beneath the transverse ranges region and another beneath the salton trough region the transverse ranges feature appears as a curtainlike east trending high velocity anomaly this feature is 60 km thick extends most deeply on its eastern end to 250 km and attains a maximum velocity 3 greater than average southern california mantle of the same depth the salton trough feature which is not as well resolved as the transverse ranges feature is composed of low velocities in the upper 70 100 km these p wave velocities arc depressed 3 4 compared to average southern california mantle tests of the inversion indicate the major aspects of the imaged structure are authentic

paleomagnetism and neogene tectonics of the northern channel islands california geologic observations and previous paleomagnetic studies have suggested that the western transverse ranges arrived at their anomalous east west orientation by clockwise tectonic rotation a paleomagnetic study of the northern channel islands was undertaken in order to test the extent of rotated areas and to develop constraints on tectonic models concerning the formation of the southern california borderland the islands of anacapa santa cruz santa rosa and san miguel form an east west trending chain and are considered part of the transverse range physiographic province oligocene through miocene volcanic and intrusive rocks occur on these islands and were sampled for this paleomagnetic study eocene sandstones were sampled on san miguel island and eocene sandstones and miocene siltstones were sampled on santa cruz island paleomagnetic results from igneous units are characterized by declinations deflected clockwise by 69 to 81 from expected directions and inclinations which are too shallow by 10 to 25 normal and reversely magnetized units yield antipodal mean directions the mean result is concordant to the directions obtained from individual units which passed fold and baked contact tests these results suggest that the northern channel islands have been tectonically rotated into place since early miocene time as the outer borderland area translated northwestward within a large shear zone between the pacific and north american plates the data from the northern channel islands when averaged with the data from the santa monica mountains yield a result of i 36 1 5 1 d 72 6 6 3 for the southern part of the western transverse ranges 52 units 405 samples the shallow paleomagnetic inclinations suggest a northward latitude translation of 14 0 3 7 or 3 9 for the western transverse ranges however most plate tectonic reconstructions suggest only 4 3 of northward translation the discrepancy may be due to initial dips of the volcanic flows which would cause an artificial shallowing of inclination when the structural dip of the units was corrected to an assumed original horizontal other causes for the shallow inclinations could be the effects of an offset dipole field or nondipole field components or motions of small tectonic plates between the north american and pacific plates as the farallan plate broke up and was subducted

kinematics of present crust and mantle flow in southern california in the southern california region published geologic studies of prehistoric fault offsets yield at least 81 rates of slip covering various periods of time chiefly since the opening of the gulf of california these rates together with the known geometries of active faults are used to constrain a hand fit block model for present horizontal velocity of the crust the assumptions employed are that these rates apply to the present that the blocks are internally rigid down to the greatest depth of earthquakes and that the over all pacific north american plate motion is correctly given by global models the resulting model fits 67 data within their uncertainties 10 more with a small discrepancy 1 mm yr and fails to accommodate only 4 data 2 of which are inconsistent with other data nominal precision is good 3 mm yr but accuracy and uniqueness are both suspect because many block boundaries are questionable despite its many local defects this model suggests four reliable regional conclusions first the rate of shortening on the south side of the transverse ranges varies from 6 to 14 mm yr increasing eastward which implies an unrecognized seismic hazard unless the motion is taken up by folding or fault creep second the pattern of block rotation rates is quite different from the pattern of rotations shown by tertiary paleomagnetic declinations which implies some major pliocene reorganization of blocks third the great bend left step of the plate boundary fault system in the transverse ranges appears to be straightening gradually by a 3 m y clockwise rotation relative to both plates finally if it is assumed that plate shortening has been occurring at this bend since the early pliocene and that crust and upper mantle velocities are similar then this model predicts mantle lithosphere downwelling to at least 250 km depth beneath the transverse ranges this prediction is consistent with upper mantle seismic velocity structure

late neogene motion of the pacific plate oceanic crust between 38 s and 65 s along the pacific antarctic ridge records a complete history of neogene pacific antarctic motion we have modeled magnetic stages corresponding to portions of isochrons 1 2 2a 3 3a 4 and 5 from 17 high quality marine magnetic traverses of the ridge to determine the neogene finite rotation poles for this plate pair we chose to model velocities over time intervals of about 0 5 m y because this greatly increased the number of estimates of pacific antarctic plate velocity our analysis shows that a change in pacific antarctic relative motion occurred between chron 2a 3 40 ma and the beginning of chron 3 time 3 86 ma we used 203 estimates of pacific antarctic spreading velocity to calculate a 0 3 4 ma and 3 86 10 3 ma euler pole combined with tracks of seamounts that record the passage of the pacific plate over mantle plumes our data show that this change in relative motion corresponds to a change in the absolute motion of the pacific plate with respect to the hotspot frame of reference using these euler poles to calculate pacific antarctic africa north america circuit euler poles show that this recent change in pacific plate absolute motion corresponds to a change from strike slip to transpressive motion along the coastal california pacific north america plate boundary interestingly this age of increased tectonism agrees in time with deformation in new zealand and northern japan

san luis obispo transform fault and middle miocene rotation of the western transverse ranges california a fault disrupted linear belt of lower middle miocene pyroclastic and volcanic rocks was erupted 15 17 m y b p along a trend 140 to 200 km long in the southern coast ranges and along the northern flank of the western transverse ranges of california offset segments of the belt are coincident with the oceanic west huasna santa maria river little pine and lompoc solvang fault zones the volcanic rocks thin eastward and also vary in thickness along strike the latter reflecting the undulation of porpoising highs and lows of basins to account for the linear distribution of over 3000 km2 of bimodal volcanic rock types it is proposed that they were intruded extruded and ejected into a continental submarine margin environment along a feature here named the san luis obispo transform slot the fault was near the mid miocene continental margin and evidently joined the early mid miocene mendocino triple junction with the crest of the east pacific rise subduction of the farallon plate had stopped along the adjacent continental margin before miocene magmatism in this region the inferred position of the east pacific rise crest was too far to the south and the inferred position of the mendocino triple junction was too far to the north between 15 and 20 m y b p to have been directly related to the miocene volcanic event near 35 n latitude at that time a part of the western transverse ranges was transported northwestward immediately following miocene volcanism earlier movement is possible but evidence is inconclusive microplate transport was concurrent with the development of pull apart structures and the 12 to 16 m y old period of volcanism along the southern margin of the microplate within the los angeles basin variable paleomagnetic directions that display clockwise deflections in the oligocene e g morro rock islay hill complex and miocene igneous rocks e g western transverse ranges in the vicinity of slot can be accounted for by extension and rotational tectonics during pull apart development rotation of blocks between faults during the northwestward transport of the western transverse ranges along slot and by rotation and possible northward translation of a part of the western transverse ranges

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

seismic anisotropy and mantle flow seismic anisotropy in the mantle is primarily due to deformation induced lattice preferred orientation lpo of olivine crystals the aim of this paper is to better understand how such lpo is produced by flow and to learn whether observations of seismic anisotropy can be used to infer the local direction of flow within the mantle the basis of the work is a two dimensional theory which describes how lpo evolves during an arbitrary time dependent deformation ribe 1989 i investigate the evolution of lpo beneath ocean ridges above subducted slabs and in thermal convective flows the ocean ridge model predicts that the seismic anisotropy produced at a fast spreading ridge should be greater in magnitude than that produced at a slow spreading ridge and that the fast axis should be more nearly horizontal the subduction zone model predicts that strong seismic anisotropy should be produced above subducted slabs with the fast axis aligned with the slab regardless of the subduction angle the evolution of lpo in typical convective flows is extremely complex and depends strongly on such factors as the mode of heating and the surface boundary conditions in general there is no simple relation between the lpo and the local flow direction in any of the models considered the attempt to use seismic anisotropy to map flow patterns on a global scale is therefore probably unwarranted however the lpo does become aligned with the flow direction when the flow is a progressive simple shear such a flow occurs in limited portions of the mantle beneath the lithosphere at a fast spreading ridge above subducted slabs and in boundary layers beneath rigid surfaces such as continents observations of seismic anisotropy may therefore indicate the sense of shear at shallow depths in the mantle

large scale quaternary detachments in ventura basin southern california the ventura basin is an east north east trending trough in the california transverse ranges which records major quaternary detachment faulting at three levels the earliest thrusting occurred along weak siltstone interbeds in a sequence dominated by competent basin plain turbidite sandstone because sedimentation continued during thrusting the age rate and direction of thrusting can be worked out faulting began 1 3 m y ago and ceased 0 65 m y ago with a maximum slip rate of 2 8 mm yr to the southeast the fault set moved up a 45 ramp and ended as a blind thrust the ramp had topographic expression on the seafloor diverting turbidites around the ramp and preserving ash beds along with other hemipelagic sediments on its crest following the end of deposition 0 2 m y ago the competent basin plain turbidites underwent flexural slip folding over an incompetent miocene sequence dominated by shale underlying competent paleogene strata were not folded the south flank of the ventura avenue anticline tilted at 3 4 rad yr the anticlinal crest rose at a rate of 15 16 mm yr decelerating to 4 3 5 2 mm yr and the anticline and an adjacent syncline shortened at a rate of 20 mm yr the high rate of folding in the ventura avenue oil field resulted in overpressured sandstone reservoirs and oil water interfaces which have not had time to reach gravity equilibrium the red mountain san cayetano and santa susana faults mark the surface expression of a seismically active midcrustal detachment which produced convergence across the ventura basin at rates as high as 23 mm yr total convergence across the eastern san cayetano fault near fillmore is 11 600 2000 m in the last million years

late cenozoic sedimentation in california continental borderland basins as revealed by seismic facies analysis the california continental borderland began to form during late miocene time only the latest miocene and younger sediments are ponded in the contemporary basins upper miocene strata are characterized by widespread diatomaceous deposits that conformably drape all of the underlying topographic irregularities the pliocene quaternary sediments occur as flat lying ponded clastic sequences that fill the contemporary basins a seismic facies analysis has defined facies patterns in the pliocene quaternary basin fill sequences of each basin in the offshore basins patton tanner santa cruz and san nicolas fine grained mass flow and hemipelagic sediments derived from the local insular sources dominate the basin fill sequences santa catalina basin is a transitional basin filled with fine grained turbidites and hemipelagic sediments derived from both continental and local insular sources the nearshore basins santa barbara santa monica and san pedro basins and san diego trough are filled by thick sequences of continental sediments that form prominent deep sea fans the borderland was dominated by biogenic pelagic sedimentation in the late miocene and by detrital sediment gravity flow deposition in the pliocene and quaternary accumulation rates estimated from pliocene quaternary sediment thickness for each basin show that in the offshore basins as well as in san diego trough rates have remained fairly constant since the pliocene in contrast the nearshore basins exhibit an increasing rate of accumulation versus time that is related to the progradation of continent derived sediments outer and intermediate basin accumulation data indicate decreasing contributions with time from offshore highs because of erosion and regional subsidence intermediate basins show an increase in mass wasting deposits slope facies with time which is the result of increasing fine suspension slope deposition and eventual failure

a revised estimate of pacific north america motion and implications for western north america plate boundary zone tectonics for the past 20 years much effort has been directed to determining the present day relative motion of the pacific and north american plates using two independent approaches one uses geologic observations and geodetic measurements along the san andreas fault and other faults in the plate boundary zone the other is based on plate motion models that incorporate spreading rates from marine magnetic anomalies transform azimuths and earthquake slip vectors geologic and geodetic studies find two principal elements of deformation slip along the san andreas of 34 mm yr directed n41 w and extension across the basin and range province of about 10 mm yr directed n56 w in contrast plate motion studies find 56 60 mm yr directed n35 w the discrepancy between these estimates a vector of about 15 mm yr oriented nearly due north is often attributed to a combination of slip along faults parallel to the san andreas and shortening normal to it here we revise the estimate of pacific north america motion by analyzing marine magnetic profiles from the gulf of california since 3 ma spreading has averaged 48 mm yr 10 mm yr slower than estimated before and consistent with the 49 mm yr spreading predicted by a new global plate motion model derived without any data along the pacific north america boundary the discrepancy with geodetic and geologic estimates is thus reduced to only 5 mm yr parallel to the san andreas 60 less than estimated before and 7 mm yr of shortening across the san andreas similar to prior estimates these results suggest that strike slip motion on faults west of the san andreas is less than thought before a conclusion consistent with geodetic seismological and other geologic observations

pattern of lobate scarps on mercury s surface reproduced by a model of mantle convection numerical simulations suggest that lobate scarps on the surface of mercury could have resulted from compressive stresses induced by convection within the planet s mantle in addition to those generated by global contraction mercury is the smallest and least tectonically active of the terrestrial planets1 2 although mercury s ancient cratered surface resembles the moon it has the largest ratio of metallic core to silicate mantle among the terrestrial planets3 as well as an internal magnetic field4 images from the mariner 10 spacecraft reveal lobate scarps so called because of their curved or scalloped edges which have been interpreted to be high angle thrust faults5 6 7 8 resulting from a period of global contraction a range of mechanisms has been invoked to explain the stresses leading to global contraction including cooling and core formation5 9 tidal effects due to gravitational interactions with the sun10 mantle convection11 and the impact that formed the caloris basin12 here i present numerical simulations of the three dimensional nature of convection within mercury s silicate mantle the model yields a regularly spaced pattern of convection in which upwelling regions of the mantle assume linear sheet like shapes at low latitudes and a nearly hexagonal pattern near the poles the distribution of resultant surface stresses is consistent with the observed pattern of lobate scarps suggesting that the compressive features record an ancient pattern of mantle convection11 in addition to global contraction the gravity and topographic data returned from the messenger 11 mission13 will help test this hypothesis

pre pliocene extension around the gulf of california and the transfer of baja california to the pacific plate late miocene 12 5 ma extension around the edges of the gulf of california has been alternatively attributed to basin and range extension back arc extension or development of the pacific north america plate boundary this extension was ene directed and similar in structural style to extension in the basin and range province timing constraints permit nearly synchronous onset of this deformation in a belt extending sse from northernmost baja california to the mouth of the gulf where this extensional faulting continued through pliocene time to the present synchronous with motion on the modern transform plate boundary in the gulf of california no change in direction of extension can be resolved revised constraints on pacific north america plate motion support the development of this late miocene extension as a component of pacific north america displacement that could not be accommodated by strike slip displacement along the existing plate boundary west of the baja california peninsula this scenario implies that transfer of baja california from the north america plate to the pacific plate was a gradual process beginning about 12 10 ma when motion of the pacific plate relative to north america was partitioned into separate regimes of strike slip and dip slip displacement on opposite sides of baja california

a kinematic model for the late cenozoic development of southern california crust and upper mantle a model is presented for the young and ongoing kinematic deformation of southern california crust and upper mantle that accounts for both the known late cenozoic geologic record and upper mantle structure in this model the high velocity upper mantle transverse ranges anomaly is created through the convergence and sinking of the entire thickness of subcrustal lithosphere using typical scaling relations we estimate this anomaly to be 500 c colder and 1 more dense than average southern california mantle of the same depth we infer that subcrustal lithosphere from both sides of the convergence zone participates nearly equally in the construction of the anomaly the low velocity upper mantle anomaly beneath the salton trough region is attributed to high temperatures and 1 4 partial melt related to adiabatic decompression during mantle upwelling our model has crustal motions in the central transverse ranges region differing from mantle motions while the subcrustal lithosphere south of the transverse ranges moves toward and sinks beneath the transverse ranges the corresponding crust largely avoids convergence by adopting a more westerly direction of motion the differing velocity fields necessitates of a decoupling zone immediately south of the transverse ranges near the moho we interpret the upper mantle anomalies as results of small scale convection beneath southern california to which we attribute much of the recent tectonic activity in the region

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

geodetic measurement of tectonic deformation in the santa maria fold and thrust belt california we have analyzed geodetic observations to resolve tectonic deformation across the santa maria fold and thrust belt northwest of santa barbara california the geodetic network forms a braced quadrilateral with 40 km sides whose southwest corner is the vandenberg very long baseline interferometry station the data include 1 historical triangulation from the 1880s 1920s and 1950s 2 electronic distance measurement trilateration from 1971 and 1985 and 3 global positioning system observations from 1986 and 1987 we combine the three different types of data to estimate two dimensional station positions and strain rate parameters simultaneously using a model which constrains the relative velocity field to be linear in space and constant in time we find significant strain rates the maximum compressive strain is oriented n17 e 5 e and the compressive strain rate in this direction is 0 13 0 03 strain yr under the assumption that the unresolved rotational component of the velocity field is zero we estimate that the integrated rate of deformation across the basin is 7 1 mm yr oriented at n03 e 13 this vector can be decomposed into 6 2 mm yr of crustal shortening on the general structural trend of n30 e and 3 1 mm yr of right lateral shear across this axis on the basis of these values and earthquake focal mechanisms in the area we infer that the deformation occurs on northwest trending folds and thrusts within the belt these results are consistent with the rate and direction of deformation across the central california coast ranges inferred by balancing pacific north america plate motion against san andreas slip and basin and range extension they imply that the santa maria fold and thrust belt is the primary active element in transforming motion from the coast ranges to the western transverse ranges and the santa barbara channel