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

quaternary landscape evolution in the san jacinto fault zone peninsular ranges of southern california transient response to strike slip fault initiation well constrained case studies of transient landscape response to external forcing are needed to improve our understanding of erosion processes in tectonically active mountain belts the peninsular ranges portion of the san jacinto fault zone sjfz is an excellent location for such a study because it displays pronounced geomorphic disequilibrium resulting from initiation of a major strike slip fault in the past 1 0 to 2 5 million years we recognize two geomorphic domains in this region 1 a relict low relief upland domain consisting of broad flat valleys and low gradient streams and 2 very steep rough topography with deeply incised canyons and retreating erosional knickpoints pleistocene sediments exposed along and near the sjfz include fluvial conglomerate sandstone and mudstone with weak paleosols and west to nw directed paleocurrents these sediments accumulated in a lowgradient stream system represented by domain 1 during an early phase of slip in the sjfz prior to the modern phase of erosion and degradation domain 2 late pliocene or early pleistocene initiation of the sjfz triggered a wave of headward erosion and stream capture that is still migrating nw along the fault zone using the total distance that capture points have migrated along the fault zone and a range of possible ages for fault initiation the rate of knickpoint retreat is estimated at 12 to 44 km my to explore the signal of transient geomorphic response to fault initiation we analyzed 23 tributaries along an 20 km portion of the main fault valley within domain 2 the analysis reveals three zones with distinctive morphologies 1 strongly convex longitudinal profiles in the nw 2 a large ca 5 6 km 2 landslide in the central zone and 3 concave tributaries in the se with profile complexity decreasing and catchment area increasing from nw to se the distribution of these zones suggests close spatial and temporal association of active fault slip bedrock incision deep seated landslides and erosional modification the fundamental driving force behind these processes is profound geomorphic disequilibrium resulting from initiation of the sjfz we suggest that landslides may have played a significant role in shaping the morphology of this fault zone and that the influence of landslides may be underestimated in areas where characteristic landforms and deposits are obscured by later erosion and faulting d 2005 elsevier b v all rights reserved

seismic images of continental lithosphere seismic reflection profiles in usa and west germany over thirty years ago showed strong reflections from the lower crust but technical advances through the 1970s were needed for the method to image the continental lithosphere the british institutions reflection profiling syndicate birps found advantage in working offshore and has obtained over 12 000 km of profiles around britain during the past decade giving the densest regional coverage anywhere reflections are observed from strata in sedimentary basins from low angle faults that can be followed from surface to mid crust from a laterally variable reflective zone in the lower crust from the moho and from fault like features in the upper mantle there is evidence around britain that faults bounding sedimentary basins are older structures which have been reactivated it is argued that lower crustal reflectivity around britain is mainly due to anastomosing ductile shears which accommodate extension in the lower crust commensurate with extension in the upper crust by faulting and block rotation there is also evidence of reflections preserved from older orogenic features so that reflections interpreted as extensional ductile shears in the lower crust may also derive from older features that have been reactivated most reflections in the lower crust arise from rocks that have been highly strained evidence of lateral variability in the reflective character of the moho suggests that the moho has been actively involved in the structural evolution of the lithosphere upper mantle reflections are interpreted as shear zones which transfer crustal deformation through the lithosphere

evolution and thickness of the lithosphere beneath coastal california many of the tectonic features of the california coast ranges are directly related to the northward migration of the mendocino triple junction along the western edge of north america during the past 20 to 30 m y at the triple junction the western edge of the north american plate slides off the relative northward moving and subducting gorda plate leaving the thin western edge in direct contact with asthenosphere upwelling to fill the space vacated by the underlying gorda plate two dimensional time dependent thermal modeling of this process constrained by teleseismic delay studies is used to construct a map of lithospheric thickness of coastal california among the implications of this map are that 1 the high heat flow in the coast ranges can be almost entirely accounted for by the asthenospheric upwelling associated with the migrating mendocino triple junction 2 the general elevation and the late cenozoic volcanism in the california coast ranges are responses to a zone of unusually thin 20 to 45 km lithosphere that extends southward behind the mendocino triple junction and 3 the course of some segments of the san andreas fault in both central and southern california appear to deviate from a deeper more fundamental transform boundary separating the pacific and north american plates

cenozoic tectonism in the central basin and range magnitude rate and distribution of upper crustal strain a wealth of pre cenozoic geologic markers across the central basin and range province defined mainly by thrust faults developed within a miogeoclinal wedge makes it one of the best areas on earth to reconstruct large scale continental extension we use this 100 000 km 2 region bounded by the southern sierra nevada on the west and the colorado plateau on the east to illustrate the concepts of correlation and misalignment for regional strain markers and of strain compatibility and kinematic viability in palinspastic reconstructions the results indicate 250 to 300 km of west northwest motion of the sierra away from the plateau accommodated by both crustal thinning and north south shortening tertiary intermontane basin deposits and mineral cooling ages of deeply exhumed rocks constrain the overall kinematics of motion most of the westward motion occurred between 16 and 5 ma at rates near 2 cm yr slowing to 1 to 1 5 cm yr in the last 5 ma we have quantified the partitioning of strain between vertical crustal thinning via normal faults and map view plane strain via conjugate strike slip faults by placing a grid of 10 x 10 km elements on a retrodeformed map of the region and measuring the increase in area of grid elements between the undeformed and present day grids this analysis yields a maximum finite elongation of the basin and range at 36 n to 37 n of 3 4 oriented n73 w map view area balance shows that 20 percent of this elongation is compensated by north south shortening and 80 percent by crustal thinning this yields an average thinning factor for the upper crust of 2 7 between the sierra and plateau inconsistent with the hypothesis that neogene deformation in the central basin and range is predominantly dextral shear plane strain these results in concert with the observation that the southern sierra nevada has similar crustal thickness to the central basin and range support the hypothesis of large scale eastward flow of sierran deep crust during extension

transtensional model for the sierra nevada frontal fault system eastern california active strike slip and normal faults along the eastern margin of the sierra nevada primarily accommodate northwestward translation of the sierra nevada central valley i e sierran microplate with respect to stable north america strike slip faults bordering the eastern sierran microplate are subparallel to small circles about the sierra nevada north american euler pole normal faults of the sierra nevada frontal fault system strike 45 clockwise of the small circle trajectories and exhibit well defined left stepping en echelon patterns consistent with formation in dextral transcurrent regime major graben bordering the northeastern sierran microplate are located in regions where the locus of range front deformation steps abruptly eastward in a releasing geometry relative to sierra nevada north american motion crustal shortening occurs at the northern end of the sierran microplate where a component of northwest dextral shear steps westward in a left restraining geometry across the sierran crest to the sacramento valley kinematic inversions of earthquake focal mechanisms from the walker lane belt bordering the eastern sierra nevada indicate that seismogenic deformation primarily is characterized by horizontal shearing and oblique crustal thinning directions of macroscopic dextral shear inferred from the inversions are subparallel to the trajectories of small circles about the sierra nevada north american euler pole normal faulting along most of the eastern sierran range front thus appears to primarily accommodate microplate translation rather than sierran uplift or regional basin and range extension

3d structural model of the polish basin abstract a 3d structural modelling of the permian mesozoic polish basin was performed in order to understand its structural and sedimentary evolution which led to basin maturation permian cretaceous and its tectonic inversion late cretaceous paleogene the model is built on the present day structure of the basin and comprises 13 horizons within the permian to quaternary rocks the analysis is based on 3d depth views and thickness maps the results image the basin scale symmetry the perennial localization of the nw se oriented basin axis the salt movements due to tectonics and or burial and the transverse segmentation of the polish basin from these observations we deduce that salt structures are correlated to the main faults and tectonic events from the model analysis we interpret the stress conditions the timing and the geometry of the tectonic inversion of the polish basin into a nw se oriented central horst mid polish swell bordered by two lateral troughs emphasis is placed on the zechstein salt considering its movements during the mesozoic sedimentation and its decoupling effect during the tectonic inversion moreover we point to the structural control of the paleozoic basement and the crustal architecture teisseyre tornquist zone on the geometry of the polish basin and the mid polish swell

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

moho orientation beneath central california from regional earthquake travel times table al is available with entire article on microfiche order from american geophysical union 2000 florida avenue n w washington dc 20009 document b84 011 2 50 payment must accompany order analysis of relative pn travel times recorded by 238 stations of the u s geological survey central california seismic network calnet from 77 regional earthquakes and explosions shows that arrivals are progressively delayed to the northeast in both the coast ranges and sierra nevada foothills by dividing the calnet into 10 subarrays and assuming laterally uniform velocities for a crust over an upper mantle half space the orientation of a planar moho beneath each subarray is calculated by inversion of relative pn arrival times the results show that the moho dips to the northeast between 3 and 6 beneath the coast ranges and 8 17 beneath the great valley and is horizontal beneath the foothills pn residuals west of the san andreas fault are relatively early by 0 35 s but are interpreted as resulting from a lateral velocity contrast across the fault rather than variations in crustal thickness unexplained pn travel time variance improves by approximately 60 after correction for the travel time due to dip and mean pn residual at each station the pn data corrected for the travel time due to a dipping moho indicate an isotropic upper mantle velocity of 7 97 0 05 km s beneath the coast ranges great valley and foothills evidence of upper mantle velocity anisotropy in the azimuthal variation in pn apparent velocity corrected for dip is only marginally significant at the 5 level however large azimuthal variations in corrected apparent velocity do exist and may be attributed to velocity heterogeneities in the crust unmodeled higher order mantle structure and gradients in upper mantle velocity arrival times of cape mendocino earthquakes recorded in the coast ranges south of san francisco bay indicate lateral variations in upper mantle velocity the 7 4 km s apparent velocity estimate from travel times of the oroville earthquake recorded in the coast ranges out to distances greater than 400 km is anomalous in that pn is normally observed at distances greater than 200 km these arrivals may originate from updip propagation along a 6 9 km s layer in the lower crust of the coast ranges subparallel to the moho

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

convective thermal instabilities in the wake of the migrating mendocino triple junction california whether the high seismic velocity anomalies in the upper mantle under california represent cold downwelling mantle flow or fragments of the subducted farallon plate has important implications for mantle dynamics related to the tectonic history of western north america we investigate the former possibility by simulating the time dependent thermal evolution in the asthenosphere beneath california in the wake of the migrating mendocino triple junction mtj using a two dimensional model that represents a transect parallel to the direction of the mtj migration for viscosity within the typical range for the asthenosphere beneath active tectonic regions 1 1018 2 1019 pa s our model predicts vigorous small scale convection characterized by periodic formation of cold downwelling flows near the southern edge of the migrating gorda plate within the relevant time frame the predicted convective thermal structures are comparable to the major mantle features shown by seismic imaging

pn travel times in southern california the pn arrivals recorded from the southern california array have been used to demonstrate regional variations in crustal thickness and pn velocity and to investigate the presence of anisotropy in the mantle over 2800 travel times were used in the analysis the classical time term method was extended to allow the moho velocity to vary in a block type structure the statistical f test shows these effects to be significant crustal delays determined depend on both crustal velocities and moho depth the ventura and los angeles basins have sediment delays which indicate sediment thicknesses near 10 kin the moho depth itself varies by 10 km over southern california with a mean of about 29 kin slightly thicker crust is prevalent in the northwestern corner of the array near the southern coast ranges five kilometers of rapid thinning between the coast and the channel islands is shown by a sharp gradient in the delay times normal crustal thicknesses occur over the rest of the array except for the area east of the salton trough where an anomalously thin crust of 22 km exists this area extends from near the plate boundary into arizona while the peninsular ranges and the san bernadino mountains have small roots the san gabriel mountains have none pn velocities range from 7 6 to 8 2 km s within the array high velocities are found in the mojave and offshore regions low velocities are found in the peninsular ranges and the transverse ranges the analysis supports a mean anisotropy of 0 15 km s with the fast direction at n 75 o w this value represents an average of a parameter that varies considerably in the region

non transparent uppermost mantle in the island arc region of japan abstract in japan the crust and uppermost mantle seismic character is yet unimaged although many refraction surveys have been recorded the longest seismic profiles are analyzed a remarkable feature a long duration coda wave after the pmp wave reflected wave at the moho boundary is observed on the record sections several possible models are considered to explain the long duration coda wave the model with many scatterers located in the uppermost mantle explains the observed data well while the undulating moho and continuous layering models do not account for some aspects of the observed data the scatterer distributed uppermost mantle is not consistent with that of continental region which is often characterized as transparent we estimate the scattering coefficient of the uppermost mantle and crust using simulations the scattering coefficients obtained for upper crust lower crust and uppermost mantle are 0 01 0 02 and 0 025 respectively the scattering coefficient of the uppermost mantle is slightly larger than that of lower crust which is characterized as being reflective the many scatterers in the uppermost mantle might be related to magmatism in japan this will be one of the important observations for understanding formation processes of the moho boundary and uppermost mantle in the island arc environment

cocorp and the continental crust this is a speculative paper on the continental crust a major frontier of modern earth science seismic reflection profiling data from the cocorp project coupled with other kinds of geophysical and geological observations suggest some fresh perspectives on the crust for example there is evidence for great differences within the basement at a single site and from one cocorp site to another a simple layered model is surely not an adequate representation of the crust in most areas there is however the puzzling and rather common occurrence of limited zones of parallel though not always flat layered features at various depths in certain parts of the crust are the corresponding rocks metasedimentary or igneous or both there are transparent zones from which little or no coherent reflected seismic energy is detected are the corresponding rocks intrusions or highly deformed metasediments can degree of deformation suggest whether rocks were deformed in the hanging wall or footwall of a thrust there is evidence for large scale thrusting and related features that emphasizes the great importance of the wilson cycle to continental tectonics and that raises the possibility that water and rocks of sedimentary origin are trapped deep in the crust by the thrusting process and remain there for long periods with a variety of possible rheological and petrological consequences in some places the modern moho and adjacent rocks may correspond to parts of former oceanic or continental margin sections that have been emplaced at depth as a result of thrusting the widely held concept of the moho as a consistent boundary that is everywhere similar and that always separates the same kinds of rocks may be a major barrier to further progress in understanding the continental crust

moho beneath the north sea compared on normal incidence and wide angle seismic records the mohorovicic discontinuity the moho is properly defined as the step in the velocity of horizontally travelling p waves from about 7 km s 1 to about 8 1 km s 1 its detailed structure varies from place to place but it is universally interpreted as the transition from crust to mantle normal incidence reflections originating at about moho depth have been reported sporadi cally during the past 20 yr during the past 10 yr continuous seismic reflection profiles have been obtained to this depth both on the continents and in the deep sea and at the same time striking improvements have been made in the methods available for interpretation of refraction experiments here we report a comparison between the moho determined by a long range refraction experiment across the north sea and the results of a short normal incidence reflection profile shot along the same line fig 1 we believe this to be the first time that such a direct comparison has been made

lithospheric instability beneath the transverse ranges of california recent high resolution seismic experiments reveal that the crust beneath the san gabriel mountains portion of the transverse ranges thickens by 10 15 km contrary to earlier studies associated with the transverse ranges there is an anomalous ridge of seismically fast upper mantle material extending at least 200 km into the mantle this high velocity anomaly has previously been interpreted as a lithospheric downwelling both lithospheric downwelling and crustal thickening are associated with the oblique convergence of pacific and north america plates across the san andreas fault though it seems likely that the lithospheric downwelling is driven at least partly by gravitational instability of the cold lithospheric mantle we show by means of numerical experiment that the balance between buoyancy forces that drive deformation and viscous stresses that resist deformation determines the geometry of crustal thickening and mantle downwelling we use a simple two layered lithospheric model in which dense lithospheric mantle overlies relatively inviscid and less dense asthenosphere and is overlain by buoyant crust external plate motion drives convergence which is constrained by boundary conditions to occur within a central convergent zone of specified width a fundamental transition in the geometry of downwelling is revealed by our experiments for slow convergence or low crustal viscosity downwelling occurs as multiple sheets on the margins of the convergent zone for fast convergence or crust that is stronger than mantle lithosphere a single downwelling occurs beneath the center of the convergent zone this complexity in the evolution of the system is attributed to the interaction of crustal buoyancy with the evolving gravitational instability in order for a narrow downwelling slab to have formed beneath the transverse ranges within the last 5 myr the effective lithospheric viscosity of the convergent region is at most about 10 20 pa s

lithospheric evolution of the pacific north american plate boundary considered in three dimensions abstract tomographic images developed using the combined seismic networks of california and nevada provide a three dimensional view into neogene pacific north american plate boundary evolution images reveal structures similar in size and spatial distribution to the large scale structures observed at the surface a prominent linear anomaly in the mantle is imaged beneath the western foothills to a depth of 70 90 km called the sierran foothill anomaly sfa this 1 2 high velocity structure extends the length of the physiographic sierra nevada mountains much of the velocity increase is thermal in origin with a first episode of cooling during the same late cretaceous to paleogene low angle subduction that affected most of western north america post laramide resumption of subduction continued chilling the western sfa small thrust earthquakes at the contact between the juan de fuca slab and the sfa indicate that the contact is still relatively cold to depths approaching 90 km we propose that this chilled lithospheric section as responsible for the block like integrity of the core of the sierra nevada the peninsular ranges in southern california have also maintained a high degree of integrity at least until recent times because they shared a similar batholithic generation and laramide chilling as the sierra nevada the tomographic image includes other large scale features west of the sfa is a low velocity region interpreted as upwelling mantle filling the window behind the nw translating juan de fuca slab this low velocity feature appears to be offset by the san andreas fault which would indicate transform relations extend into the mantle compression and shortening between the peninsular ranges and the sierra nevada blocks began when the transform boundary shifted east of the peninsular ranges and baja california around 5 5 ma high velocity structures in the upper mantle developed beneath the transverse ranges and the southern great valley sgv to accommodate crustal shortening alternative explanations for the sgv anomaly are explored by comparing the volume of proximal sources to the volume of the sinking sgv anomaly no local source such as convective overturning of sierran block eclogitic roots seems adequate to provide the volume needed to explain the anomaly as a purely local feature surface and subsurface views of the plate margin are complimentary but consistent and each contributes to resolving problems not accessible to the other

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

the seismic reflection character of the continental mohorovicic discontinuity the continental mohorovicic discontinuity is most often interpreted as a step function velocity boundary however on deep crustal seismic reflection profiles reflections at depths where refraction data places the moho have laminated character and are laterally discontinuous these observations on continental reflection data point to a model of the moho that has thin layered rather than block structure previous workers who have synthetically generated seismic responses from various crust mantle boundary models have shown that thin layered models of alternating high and low velocity generate responses that best emulate observed moho arrivals geologic interpretations of the thin layering include relatively undeformed metasediments cumulate layering tectonic banding and lenses of partial melt to obtain more direct geologic evidence a synthetic seismogram is generated from geologic cross sections of the ivrea verbano zone an exposed section through the lower crust and upper mantle the synthetic shows laminations similar to those observed on reflection data without more direct evidence no single geologic interpretation of the moho is reasonable indeed the moho may be a laterally variable boundary its composition and structure dependent upon the geologic history of the overlying crust

deep intraplate seismicity in the western sierra nevada central california abstract a microearthquake monitoring program was conducted from 1976 to 1979 in the eastern san joaquin valley and sierran foothills of central california this program revealed one of the deepest occurrences of intraplate earthquakes observed in the conterminous united states earthquakes in the m l range of approximately 0 to 3 2 were confined to the lower crust of the sierran block at depths from 12 to 38 km the distribution of epicenters was diffuse and widespread with some clustering in space and time two particulary active areas that were consistent with the historical seismicity record were recognized an area 15 km south of the town of madera and an area 15 km south of mariposa no association of the seismicity with faults or other geologic structures expressed at the surface was apparent based on regional seismic monitoring by the u s geological survey and the detailed monitoring performed in this study this occurrence of deep seismicity does not appear to extend farther north along the foothills than jamestown although two deep events have been observed near oroville the southern limit is not as well defined due to the lack of seismographic coverage focal mechanisms of these deep crustal earthquakes exhibited 1 strike slip faulting combined with some reverse and normal components and 2 a regional tectonic stress field characterized by a maximum compressive stress generally trending north south and a minimum compressive stress trending east west these observations support the view that 1 the sierra nevada represents a transition between a region of primarily shear deformation along the san andreas fault zone and extensional tectonics of the basin and range province and 2 the orientation of the tectonic stress field is generally uniform throughout the crust of the sierran block

a deep seismic reflection profile across the northern north sea the viking graben is a major mesozoic rift basin in the northern north sea1 seismic refraction data2 gravity surveys3 and theoretical modelling4 have led to the general acceptance of an extensional tectonic origin for the viking graben involving an early mesozoic phase of extension and fault controlled subsidence followed by a more regional thermal contraction subsidence the early success of the birps deep seismic reflection profiling5 and the results from cocorp6 led to the planning and acquisition during 1983 of a deep seismic line recorded to 15 s twt two way time across the north viking graben the aims of the present study were to provide constraints on the geometry of the viking graben and its extensional faults to provide data on the geometry of crustal thinning beneath the graben and to provide an insight into the relation between the shallow extensional features of the sedimentary basin and the deep crustal thinning here we present a specific interpretation of this seismic profile further details will be given elsewhere7

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

lithospheric structure of northern california from teleseismic images of the upper mantle teleseismic p wave travel time residuals from 120 earthquakes recorded across the u s geological survey california seismic network were used to determine the lithosphere p wave velocity structure beneath northern california a region characterized by complex interactions between the pacific north american and gorda plates lateral p wave velocity variations beneath the array were determined by inversion of 9383 travel time residuals inversion results for the crust show strong correlations to volcanic features the active volcanic fields shasta medicine lake lassen and clear lake are characterized by crustal low velocity anomalies that average approximately 6 possibly identifying partially molten magma bodies cooled solidified magma bodies beneath the extinct volcanic fields sonoma southern clear lake and sutler buttes are denoted by relative velocity highs averaging 3 the largest upper mantle velocity variations occur in the depth range 30 110 km where velocities vary from 5 5 to 9 5 these velocity variations reflect changes in the thickness and geometry of the pacific north american and gorda plates where they interact at the mendocino triple junction north of the mendocino triple junction the steep 70 east dipping portion of the gorda plate is imaged as a 5 velocity high to depths near 270 km a presumed segment of the gorda plate observed beneath the northern great valley and south of the inferred edge of the plate is characterized by a 9 velocity high in the depth range 30 70 km beneath the northern coast ranges shallow asthenosphere is imaged in the depth range 30 100 km as a pronounced southward tapering 4 low velocity zone which we interpret as the slab window results from this study provide improved constraints on gorda plate subduction evolution of the san andreas fault system and development of the lithosphere beneath western north america

evidence for a 180 km long subducted slab beneath northern california we present new seismic data for subduction of a 180 km long segment of the gorda plate beneath cape mendocino in northern california the inclined seismic zone dips 10 to 11 in the direction s68 e for a length of 120 km to depths of 30 to 35 km then steepens in dip to 25 and plunges to depths of at least 60 km p wave focal mechanisms of two earthquakes at depths of 30 and 58 km are consistent with downdip tension and imply that these events resulted from stresses within the slab and not from gorda north american plate interaction

tectonics of pliocene removal of lithosphere of the sierra nevada california pliocene ca 3 5 ma removal of dense eclogitic material under the sierra nevada has been proposed from variations in the petrology and geochemistry of neogene volcanic rocks and their entrained xenoliths from the southern sierra the replacement of eclogite by buoyant warm asthenosphere is consistent with present day seismologic and magnetotelluric observations made in the southern sierra a necessary consequence of replacing eclogite with peridotite is that mean surface elevations and gravitational potential energy both increase an increase in potential energy should increase extensional strain rates in the area if these forces are insuffi cient to signifi cantly alter pacifi c north american plate motion then increased extensional strain rates in the vicinity of the sierra must be accompanied by changes in the rate and style of deformation elsewhere changes in deformation in california and westernmost nevada agree well with these predictions existing geologic evidence indicates that a period of rapid uplift along the sierran crest of more than 1 km occurred between 8 and 3 ma most likely as a consequence of removal of lower lithosphere about this same time extensional deformation was initiated within 50 km of the eastern side of the sierra 5 3 ma and regional shortening began to produce the california coast ranges 5 3 ma we suggest that these events were induced by the 1 2 10 12 n m increase of gravitational potential energy generated by the sierran uplift evidence for pliocene uplift adjoining crustal extension and shortening in directly opposing parts of the coast ranges is found along nearly the entire length of the sierra nevada and implies that lithosphere was removed beneath all of the presentday mountain range the uplifted area lies between two large upper mantle high pwave velocity bodies under the south end of the san joaquin valley and the north end of the sacramento valley these high velocity bodies plausibly represent the present position of material removed from the base of the crust lithospheric removal may also be responsible for shifting of the distribution of transform slip from the san andreas fault system to the eastern california shear zone a prediction that awaits better defi ned slip histories on both faults overall the late cenozoic deformational history of the sierra nevada and vicinity illustrates that locally derived forces can infl uence deformation kinematics within plate boundary zones

western u s upper mantle structure regional three dimensional inversions of upper mande p wave velocity structure are created from teleseismic p wave travel time residuals recorded by many of the high density high frequency regional arrays operated within the western united states these inversions are adjusted to a global international seismic centre reference and merged to obtain an image of the upper mantle beneath western united states the p wave velocities in the upper mantle are slow on average and the structure is very heterogeneous where resolution is good coherent upper mantle structures are imaged that extend as deep as 200 km the juan de fuca and gorda slabs which penetrate to greater depths are exceptions and deviate from the average velocity at a given depth by as much as 4 lateral resolution of these structures usually is very good although the magnitude of the actual seismic variations is probably greater than that imaged the long wavelength part of the imaged mantle structure defines coherent elongate features with wavelengths of 200 500 km within 250 km of the pacific coast these structures have a wavelength of 250 km and trend parallel to the surface physiography and young tectonic structures beneath the continental interior where use is made of additional seismic studies to infer average structure in regions of poor teleseismic data coverage structures have a wavelength of 500 km and trend northeasterly this northeast orientation is discordant with young tectonic structures but aligns with young volcanic activity

thermal evolution of the sierra nevada tectonic implications of new heat flow data eight new heat flow measurements in the southern sierra nevada constrain models of thermal evolution and lithospheric structure low reduced heat flows 18 to 21 mw m in the southwest sierra nevada are consistent with previous results from the northwestern and central sierra nevada and extend the known region of linear heat flow heat production correlation an additional 150 km to the south isostatic residual gravity and measured rock densities are also linearly correlated in the central sierra nevada suggesting a general association between the upper crustal distribution of density and heat production the linear residual gravity density relation implies that isostatic residual gravity anomalies have upper crustal sources and therefore is evidence against a flexural model of cenozoic sierra nevada uplift new reduced heat flows measured in the southeast sierra nevada are relatively high 32 to 57 mw m and correlate spatially with a region of high seismicity that includes extensional earthquake swarms this correlation supports the view that basin and range extensional tectonics and associated magmatic processes are encroaching on the eastern sierra nevada in the stable southwest sierra nevada as in the central and northwestern sierra nevada the persistence of low reduced heat flow at the surface is consistent with a thermal origin for the cenozoic uplift of the sierra nevada provided the conductive lithosphere is at least 60 to 90 km thick simple order of magnitude calculations show that thermal uplift models combining mechanisms such as advective warming and thinning of the mantle lithosphere advection of basaltic magma and heat into the crust simple thermal expansion and eclogite to basalt phase conversion can account for the timing and amount of cenozoic sierra nevada uplift thermal models do not need the assumptions of unusual lithospheric strength and crustal buoyancy required by mechanical models for cenozoic uplift driven by a mesozoic crustal root

crustal thinning and subsidence in the north sea we present here the results of a detailed study of subsidence and crystal thinning across the central graben of the north sea basin two independent lines of evidence have been used to test how well the uniform extensional model1 2 can account for the evolution of the north sea basin a long range seismic profile was shot perpendicular to the central graben from edinburgh to southern norway and data were analysed from 11 exploration wells situated close to the seismic line fig 1 the estimates of the increase in surface area due to extension from the crustal structure determined using the seismic data and from the well subsidence patterns are both compatible with a simple uniform extensional mechanism for the post palaeozoic evolution of the north sea

seismic images of the continental moho of the indian shield abstract deep seismic reflection profiling has imaged different patterns of the moho across the indian shield with a variety of tectonic environments from archean to recent the character of the moho varies from a discrete strong event the base of strong coherent lower crustal sub horizontal reflections the base of dipping lower crustal reflections into mantle to no clear reflection boundary the seismic reflection data suggest a laminated lower crust in several places and offsets in the moho at others kinematic and dynamic modeling of wide angle reflection data across the mesoproterozoic south delhi fold belt and the central indian tectonic zone suggests a laminated lower crust for these regions in general the lower crust of the indian shield is heterogeneous a clear moho is identified in some of the precambrian orogenic belts and sedimentary basins along with reflective lower crust whereas the cratonic areas exhibit a diffused moho post collisional extensional process such as orogenic collapse delamination magmatic intrusions low viscosity ordering and underplating might have played a role in the generation of lower crustal laminated zone and formation of a younger moho the termination of lower crustal reflectivity at the moho with a transparent upper mantle need not necessarily indicate homogeneous upper mantle

overview tectonic implications of cenozoic volcanism in coastal california cenozoic volcanic rocks of coastal california were erupted west of the magmatic arc trend related to subduction along the continental margin two assemblages representing discrete pulses of mid tertiary oligocene miocene and mid miocene volcanism occupied relatively compact tracts as restored palinspastically prior to disruption and dispersal by tectonic displacements within the neogene san andreas transform system a third assemblage records migratory post mid miocene volcanism at centers located progressively farther north the distribution and petrologic character of all three assemblages reflect slab window magmatism triggered by decompression melting of upwelling mantle paleogeographic reconstructions based on analysis of magnetic anomaly patterns offshore and restoration of on land features prior to san andreas transform slip and associated transrotational deformation indicate the paleotectonic positions of the cenozoic volcanic fields before structural disruption the pulses of mid tertiary and mid miocene volcanism were related to transient episodes of mantle upwelling generated successively by rise trench encounters associated with subduction of the vancouver farallon and monterey arguello plates respectively along the continental margin off southern california migratory post mid miocene volcanism in central california accompanied the incremental expansion of a growing slab window as the mendocino triple junction migrated northward along the continental margin mid miocene columbia river basalt volcanism in the pacific northwest was coeval with the mid miocene pulse of coastal volcanism and may have reflected tectonism induced by the final demise of offshore microplates to allow initial full integration of the san andreas transform as a coherent plate boundary columbia river volcanism may have stemmed from mantle perturbation caused by torsional deformation of the continental block in response to shear imposed by the pacific plate

the seismic reflection moho beneath the united kingdom and adjacent areas abstract onshore and offshore deep seismic reflection data have been integrated to produce two way travel time and depth maps of the seismic reflection moho beneath the uk moho travel times are mostly in the range 9 5 11 5 s showing only weak correlation with specific upper crustal structural features but decreasing generally northwestward towards the atlantic margin moho depths show a much stronger correlation with upper crustal structure than do travel times there is pronounced crustal thinning moho depths 32 km are found generally beneath basement massifs particularly onshore short wavelength topographic features on the moho are quite rare and are generally restricted to ancient zones of continental collision and or subduction some of these ancient collision and or subduction zones are spatially associated with a highly reflective moho this is particularly evident where low angle shear zones in the lower crust or uppermost mantle converge with or intersect the moho it is suggested that development of a moho detachment surface can tectonically smooth the moho and thereby markedly increase its reflectivity a tendency in certain areas for the moho to remain at roughly constant two way travel time though not constant depth beneath both sedimentary basins and intervening basement massifs is interpreted as indicating that structural features on the scale of several tens of kilometres are in regional isostatic equilibrium conversely travel time perturbations beneath smaller features 10 km across indicate that these are at least partially uncompensated the uk crust is therefore weak but not sufficiently weak to attain local airy isostasy basement massifs west of scotland show systematically smaller moho two way travel times than similar structures farther south within the constraints of regional isostasy this can be interpreted in two main ways greater rigidity and strength of lewisian rocks within the west scottish massifs manifest as higher seismic velocity relative to density could be a cause of the observed travel time differences alternatively low density material beneath the crust could buoy up the western scottish massifs if this buoyancy were the consequence of igneous underplating then any underplated material would have to lie beneath the seismic reflection moho

implications of volcanism in coastal california for the neogene deformation history of western north america 1 the geologic record of coastal california includes evidence of numerous volcanic centers younger than 30 ma that do not appear to have erupted in an arc setting by correlating these volcanic centers with specific slab windows predicted from analysis of magnetic anomalies on the pacific plate we add new constraints to tectonic reconstructions since 30 ma our correlations such as erupting the morro rock islay hill complex south of the pioneer fracture zone and the iversen basalt south of the mendocino fracture zone require larger displacements within western north america than advocated by most previous authors specifically we infer at least 315 km of motion between the sierra nevada and rigid north america at an azimuth of about n60 w and at least 515 km between baja california and rigid north america in a similar direction a consequence of inferring a large displacement of baja california is that the pacific north american plate boundary must have developed most of its current form prior to 10 ma we interpret a slab window developing between cocos and monterey plates after 19 ma that reconstructs under nearly all of the southern california volcanic centers dated at 18 14 ma most of the sedimentary basins associated with volcanic rocks show brief periods of rapid subsidence synchronous with volcanism followed by slow subsidence of variable but often extended duration consistent with rapid extension of cold lithosphere over recently introduced hot asthenosphere

deep structure of the foreland to the caledonian orogen nw scotland results of the birps winch profile the winch marine deep seismic reflection profile crosses the hebridean shelf the proterozoic foreland to the caledonian orogen west of scotland the data quality is very good the upper crust is largely devoid of coherent seismic reflections although this may in part be due to acquisition techniques being inappropriate for this problem in contrast the middle and lower crust 10 25 km depth exhibits good reflections the mid crust contains reflectors which may be relics of early palaeozoic caledonian or earlier grenvillian eastward dipping thrust zones which pass into an acoustically strongly layered lower crust the outer isles thrust is mapped from the surface to the mid crust and tied into its land outcrop on north lewis reactivation of this thrust offshore in a normal sense in mesozoic times caused the formation of the minch and north lewis basins the mono is defined by a strong band of reflections at about 27 km depth which correlates well with results from the hmse explosion seismic survey moho depth is apparently rather uniform in the area of the foreland crossed by winch winch and hmse results together suggest a mean crustal velocity for the hebridean shelf of 6 4 0 1 km s 1 the eastward dipping flannan thrust can be mapped into the upper mantle on three lines from about 15 to 45 km depth well into the upper mantle neither the flannan thrust nor the outer isles thrust appear to pass straight through the reflective lower crust suggesting that the lower crust is a region of high strain the outer hebrides is a positive block probably formed as an isostatic response to mesozoic normal faulting which reactivated the outer isles thrust

a new perspective on the geometry of the san andreas fault in southern california and its relationship to lithospheric structure the widely held perception that the san andreas fault saf is vertical or steeply dipping in most places in southern california may not be correct from studies of potential field data active source imaging and seismicity the dip of the saf is significantly nonvertical in many locations the direction of dip appears to change in a systematic way through the transverse ranges moderately southwest 55 75 in the western bend of the saf in the transverse ranges big bend vertical to steep in the mojave desert and moderately northeast 37 65 in a region extending from san bernardino to the salton sea spanning the eastern bend of the saf in the transverse ranges the shape of the modeled saf is crudely that of a propeller if confirmed by further studies the geometry of the modeled saf would have important implications for tectonics and strong ground motions from saf earthquakes the saf can be traced or projected through the crust to the north side of a well documented high velocity body hvb in the upper mantle beneath the transverse ranges the north side of this hvb may be an extension of the plate boundary into the mantle and the hvb would appear to be part of the pacific plate

mantle instability beneath the sierra nevada mountains in california and death valley extension the southern sierra nevada mountain range rapidly uplifted at 3 5 ma simultaneously with a pulse of basaltic volcanism xenoliths recovered from volcanics indicate that the range lost a dense crustal root after the miocene the vertical motions and removal of the root have been linked to a fast seismic velocity anomaly that extends 200 km into the mantle but is offset to the west of the range with visco elasto plastic thermo mechanical numerical models we have tested the influence of crustal strength on the kinematics of removal and on the amount of associated uplift we find that delamination of the dense root is the most likely mechanism for gravitational instability to occur the model satisfies the plio quaternary vertical motions the shift of the mantle anomaly to the west of the range and intense miocene extension to the east based on those results we propose the existence of a dynamic link between the sierra nevada mantle instability and death valley rifting

a comparison of the moho interpreted from gravity data and from deep seismic reflection data in the northern north sea summary we present the results of gravity modelling of the nsdp 84 deep seismic reflection profiles in the northern north sea the moho depths inferred from forward modelling of marine bouguer gravity anomalies from independent inversion of free air gravity anomalies derived from satellite altimetry and from deep seismic reflection profiling are in good mutual agreement this agreement suggests that the moho discontinuity interpreted from seismic reflection data corresponds to a major jump in density and therefore is likely to represent not only the geophysical but also the petrological boundary between the continental crust and the upper mantle in spite of the significant lateral heterogeneity of the crystalline basement below the northern north sea area as documented by seismic reflection profiling the average density contrast between the basement and the upper mantle is interpreted to show rather little variation this is not true for the deepest part of the viking graben where the nature of the lower crust and upper mantle may have been changed by magmatic processes accompanying the graben formation a relationship between the degree of crustal extension and the volume of magmatic intrusion into the lower crust is tentatively supported by our results