seismic structure of the crust and uppermost mantle of north america and adjacent oceanic basins a synthesis we present a new set of contour maps of the seismic structure of north america and the surrounding ocean basins these maps include the crustal thickness whole crustal average p wave and s wave velocity and seismic velocity of the up permost mantle that is pn and sn we found the following 1 the average thickness of the crust under north america is 36 7 km standard deviation s d 8 4 km which is 2 5 km thinner than the world average of 39 2 km s d 8 5 for continental crust 2 histograms of whole crustal p and s wave velocities for the north amer ican crust are bimodal with the lower peak occurring for crust without a high velocity 6 9 7 3 km sec lower crustal layer 3 regions with anomalously high average crustal p wave velocities correlate with precambrian and paleozoic orogens low average crustal velocities are correlated with modern extensional regimes 4 the average pn velocity beneath north america is 8 03 km sec s d 0 19 km sec 5 the well known thin crust beneath the western united states extends into north west canada 6 the average p wave velocity of layer 3 of oceanic crust is 6 61 km sec s d 0 47 km sec however the average crustal p wave velocity under the eastern pacific seafloor is higher than the western atlantic seafloor due to the thicker sediment layer on the older atlantic seafloor

seismic anisotropy tracing plate dynamics in the mantle elastic anisotropy is present where the speed of a seismic wave depends on its direction in earth9s mantle elastic anisotropy is induced by minerals that are preferentially oriented in a directional flow or deformation earthquakes generate two seismic wave types compressional p and shear s waves whose coupling in anisotropic rocks leads to scattering birefringence and waves with hybrid polarizations this varied behavior is helping geophysicists explore rock textures within earth9s mantle and crust map present day upper mantle convection and study the formation of lithospheric plates and the accretion of continents in earth history

localized imaging of the uppermost mantle with usarray pn data 1 usarray has now provided several years of high quality seismic data and improved ray coverage for much of the western united states this allows increased resolution for regional studies of the lithosphere and deeper structure of the north american continent in this study we use pn phases in the usarray data set to solve for velocity structure in the uppermost mantle in the western united states this article focuses on localized imaging techniques that complement traditional pn tomography analysis we apply waveform cross correlation to obtain inter station travel times between the closely and uniformly spaced usarray stations this allows us to use traces without phase picks and reduces errors associated with the picking we obtain differential times that can directly be used to fit locally for slowness and depending on the approach for the direction and curvature of the incoming wavefront the various measurements of incoming wavefronts at different sub arrays provide constraints on azimuthal variations in velocity the traditional tomography approach and the local fitting method reveal similar large scale features no regularization is applied with the local method and the resulting velocity maps reveal smaller scale structures than the tomographic images

mantle deformation and tectonics constraints from seismic anisotropy in the western united states abstract we have examined shear wave splitting in teleseismic shear waves sks skks s from 15 stations in the western united states based on analysis of 123 records from 67 events the varied past and present tectonic styles in this region subduction transform faulting extension and stable domains are expected to cause a wide variety of anisotropic behavior and therefore make it an excellent natural laboratory fast polarization azimuths vary from e w to nearly n s and time delays t range from being barely detectable less than 0 6 s to 1 6 s most stations yielded consistent measurements independent of station event geometry the exceptions were stations situated very close to the san andreas fault which yielded well constrained but inconsistent splitting parameters these have been successfully modeled by two anisotropic layers with different horizontal symmetry axes the upper layer has a fast direction parallel to the fault and t of about 1 s the lower layer with t from 0 6 to 0 9 s is oriented e w near the san francisco bay area and ne sw in the mojave desert other measurements of e w fast are observed as far east as western nevada with large delay times of 1 3 1 5 s but disappear to the north stations in the northwestern basin and range have values of oriented at about 70 with delay times ranging from 0 7 to 1 2 s varies in other regions of the basin and range from 70 in eastern nevada to 20 in the transition zone between the colorado plateau and the basin and range with t of 1 0 s measurements are geographically coherent and in many cases changes correspond to geologic or tectonic boundaries the e w fast feature extends across several tectonic provinces but disappears near the southern boundary of the gorda plate and may therefore be related to the dynamics of plate interactions regional variations in the fast directions of anisotropy within the basin and range suggest that the present extension has not been strong enough to align the fast directions over a great depth extent fast polarization directions within the transition zone between the basin and range and colorado plateau are nearly parallel to the geologic and geophysical boundaries between the two regions and to other geophysical boundaries the size of the delay times suggests that in many of the areas studied either the mantle is almost twice as anisotropic as we estimate or the anisotropy and therefore mantle deformation extends into the sublithospheric mantle we used the two layer method to examine the effect of a postulated lower layer with fast direction parallel to the absolute motion of the north american plate and found the best estimate of contribution from a uniform layer is 0 1 s of t although a contribution of up to 0 7 s cannot be ruled out a value as low as 0 1 s suggests either that olivine is not strongly aligned in a horizontal direction in this layer or that the layer thickness is of the order of 10 km

a 3 d model of the crust and uppermost mantle beneath the central and western us by joint inversion of receiver functions and surface wave dispersion 1 rayleigh wave phase velocity maps from ambient noise and earthquake data are inverted jointly with receiver functions observed at 828 stations from the usarray transportable array west of 100 w longitude for data recorded in the years 2005 through 2010 to produce a 3 d model of shear wave speeds beneath the central and western us to a depth of 150 km eikonal tomography is applied to ambient noise data to produce about 300 000 rayleigh wave phase speed curves and helmholtz tomography is applied to data following 1550 ms 5 0 earthquakes so that rayleigh wave dispersion maps are constructed from 8 to 80 s period with associated uncertainties across the region harmonic stripping generates back azimuth independent receiver functions with uncertainty estimates for each of the stations a nonlinear bayesian monte carlo method is used to estimate a distribution of shear wave speed vs models beneath each station by jointly interpreting surface wave dispersion and receiver functions and their uncertainties the assimilation of receiver functions improves the vertical resolution of the model by reducing the range of estimated moho depths improving the determination of the shear velocity jump across moho and improving the resolution of the depth of anomalies in the uppermost mantle a great variety of geological and tectonic features are revealed in the 3 d model that form the basis for future detailed local to regional scale analysis and interpretation

crustal shear wave velocity structure of the western united states inferred from ambient seismic noise and earthquake data 1 surface wave dispersion measurements from ambient seismic noise and array based measurements from teleseismic earthquakes observed with the earthscope usarray transportable array ta are inverted using a monte carlo method for a 3 d vs model of the crust and uppermost mantle beneath the western united states the combination of data from these methods produces exceptionally broadband dispersion information from 6 to 100 s period which constrains shear wave velocity structures in the crust and uppermost mantle to a depth of more than 100 km the high lateral resolution produced by the ta network and the broadbandedness of the dispersion information motivate the question of the appropriate parameterization for a 3 d model particularly for the crustal part of the model we show that a relatively simple model in which vs increases monotonically with depth in the crust can fit the data well across more than 90 of the study region except in eight discrete areas where greater crustal complexity apparently exists the regions of greater crustal complexity are the olympic peninsula the mendocinotriple junction the yakima fold belt the southern cascadia back arc the great central valley of california the salton trough the snake river plain and the wasatch mountains we also show that a strong rayleigh love discrepancy exists across much of the western united states which can be resolved by introducing radial anisotropy in both the mantle and notably the crust we focus our analysis on demonstrating the existence of crustal radial anisotropy and primarily discuss the crustal part of the isotropic model that results from the radially anisotropic model by voigt averaging model uncertainties from the monte carlo inversion are used to identify robust isotropic features in the model the uppermost mantle beneath the western united states is principally composed of four large scale shear wave velocity features but lower crustal velocity structure exhibits far greater heterogeneity we argue that these lower crustal structures are predominantly caused by interactions with the uppermost mantle including the intrusion and underplating of mafic mantle materials and the thermal depression of wave speeds caused by conductive heating from the mantle upper and middle crustal wave speeds are generally correlated and notable anomalies are inferred to result from terrane accretion at the continental margin and volcanic intrusions

vertical mantle flow associated with a lithospheric drip beneath the great basin rapid surface uplift or subsidence and voluminous magmatic activity have often been ascribed to regional scale downwelling of lithospheric mantle however because lithospheric drips sinking plumes of cold and dense lithosphere are relatively small and transient features direct evidence of their existence has been difficult to obtain moreover the significant vertical mantle flow that should be associated with such structures has not been detected here we integrate seismic anisotropy data with tomographic images to identify and describe a lithospheric drip beneath the great basin region of the western united states the feature is characterized by a localized cylindrical core of cooler material with fast seismic velocities and mantle flow that rapidly shifts from horizontal to vertical our numerical experiments suggest that the drip can be generated by gravitational instability resulting from a density anomaly of as little as 1 and a localized temperature increase of 10 the drip tilts to the northeast opposite to the motion of the north american plate in the hotspot reference frame and thereby indicates northeast directed regional mantle flow seismic anisotropy data for the great basin region of the western united states coupled with tomographic images help delineate a northeast dipping lithospheric drip numerical experiments suggest that the drip could have formed owing to gravitational instability triggered by a density increase of about 1 and a temperature increase of about 10

effect of pore geometry on vp vs from equilibrium geometry to crack seismic wave velocities of melt or aqueous fluid containing systems are studied over a wide range of pore shapes including oblate spheroids tubes cracks and an equilibrium geometry controlled by a dihedral angle the relative role of liquid compressibility and pore geometry on the v p v s velocity ratio is clarified the result clearly indicates that p and s velocity structures determined by seismic tomography can be used to verify whether interfacial energy controlled melt or fluid geometry equilibrium geometry is achieved relationships between the diverse models are clearly established by relating each model to the oblate spheroid model in terms of the equivalent aspect ratio as a function of the aspect ratio a significant effect of pore geometry on d in v s d in v p the ratio of the fractional changes in v s and v p is shown equilibrium geometry of the partially molten rocks characterized by a dihedral angle of 20 40 corresponds to an aspect ratio of 0 1 0 15 the value of d in v s d in v p expected for the texturally equilibrated partially molten rocks is shown to be 1 1 5 which is much smaller than that expected for cracks and dikes with an aspect ratio of 10 2 10 3 in the upper mantle low velocity regions the seismologically obtained value of d in v s d in v p is within this range beneath the bolivian andes 1 1 1 4 but is as high as 2 beneath iceland 1 7 2 3 and beneath northeastern japan 2 0 the former region can be regarded as a region where equilibrium geometry is achieved and the latter regions can be regarded as regions where dikes and veins typical of a system far from the textural equilibrium dominate

p wave tomography of the western united states insight into the yellowstone hotspot and the juan de fuca slab abstract we used 190 947 high quality p wave arrival times from 8421 local earthquakes and 1 098 022 precise travel time residuals from 6470 teleseismic events recorded by the earthscope usarray transportable array to determine a detailed three dimensional p wave velocity model of the crust and mantle down to 1000 km depth under the western united states us our tomography revealed strong heterogeneities in the crust and upper mantle under the western us prominent high velocity anomalies are imaged beneath idaho batholith central colorado plateau cascadian subduction zone stable north american craton transverse ranges and southern sierra nevada prominent low velocity anomalies are imaged at depths of 0 200 km beneath snake river plain which may represent a small scale convection beneath the western us the low velocity structure deviates variably from a narrow vertical plume conduit extending down to 1000 km depth suggesting that the yellowstone hotspot may have a lower mantle origin the juan de fuca slab is imaged as a dipping high velocity anomaly under the western us the slab geometry and its subducted depth vary in the north south direction in the southern parts the slab may have subducted down to 600 km depth a slab hole is revealed beneath oregon which shows up as a low velocity anomaly at depths of 100 to 300 km the formation of the slab hole may be related to the newberry magmatism the removal of flat subducted farallon slab may have triggered the vigorous magmatism in the basin and range and southern part of rocky mountains and also resulted in the uplift of the colorado plateau and rocky mountains

lithospheric structure of tibet and western north america mechanisms of uplift and a comparative study we seek to determine the mantle lid thickness of the lithosphere and to constrain the mechanisms that are responsible for the cenozoic uplift of the tibetan plateau located behind the himalaya collision zone and the western united states located behind the san andreas transform convergent plate boundary selected first p arrivals of international seismological centre isc data at distances less than 22 are analyzed by the two station method to constrain the upper mantle structure of the basin and range br colorado plateau cp great plains gp and tibetan plateau tp the data are presented as plots of apparent velocity versus distance to the second station we show that these observations of apparent velocities versus distance as well as those that concern the attenuation of sn provide estimates of the mantle lid thicknesses we obtain the following mantle lid thicknesses br 20 40 km cp 35 50 km gp 150 195 km and tp 135 180 km sn is not observed for paths beneath the br and cp while sn efficiently propagates beneath the gp sn is also observed beneath most of tibet except in the north central part of the tp a possible interpretation of our observations is that indian continental lithosphere underthrusts only about the southern 2 3 of tibet

upper mantle velocity temperature conversion and composition determined from seismic refraction and heat flow 1 we compile upper mantle pn velocities from seismic refraction wide angle reflection surveys in the southern superior province of the canadian shield and compare them with temperatures at the moho deduced from heat flow data calculated moho temperatures and pn velocities correlate well showing that in this area pn depends primarily on temperature the obtained values of v pn t depend weakly on the assumed value of moho heat flow and are on the order of 6 0 10 4 10 km s 1 k 1 within the range of temperature derivatives obtained in laboratory studies of ultramafic rocks comparison between observed pn velocities and predicted values for several mineralogical models at moho temperatures allows constraints on both the moho heat flow and the shallow mantle composition for all moho heat flows undepleted clinopyroxene rich mantle compositions do not allow a good fit to the data for depleted mantle compositions temperatures consistent with the observed pn velocities correspond to values of moho heat flow larger than 12 mw m 2 for our preferred moho heat flow of 15 mw m 2 the best fit mantle composition is slightly less depleted than models for average archean subcontinental lithospheric mantle this may be due to rejuvenation by melt related metasomatism during the keweenawan rifting event the similarity in pn t conversion factors estimated from this empirical large scale geophysical study and those from laboratory data provides confidence in the absolute temperature values deduced from heat flow measurements and seismic studies

three dimensional seismic velocity structure of the northwestern united states 1 we present a high resolution three dimensional p wave tomography model for the northwestern united states major features of the model include clear well resolved images of the juan de fuca gorda slab to depths of at least 500 km reduced velocities beneath newberry volcano and north central oregon increased velocities beneath the idaho batholith and a deep slab like eastward dipping feature of increased velocities beneath nevada we find no evidence for a mantle plume in this region and demonstrate that suggested disruption of the juan de fuca slab beneath oregon in the vicinity of newberry volcano is an inversion artifact we interpret the primary features of this new model within the framework of post laramide tectonomagmatism and the continuing tectonic evolution of the cascadia subduction zone a model explaining the regional tectonomagmatic history consistent with the imaged seismic velocity structure is one driven by mantle flow responding to the evolving subduction system

seismic structure beneath the tonga arc and lau back arc basin determined from joint vp vp vs tomography the tonga arc and associated lau basin exhibit many geologically important processes that link subduction and mantle flow with plate separation and crustal production we create seismic tomograms of the tonga lau region by jointly inverting for vp and vp vs structure using data from the labatts ocean bottom seismograph experiment and several island deployments to better constrain dynamic processes in the mantle wedge jointly using p and s data can help distinguish between the various mechanisms responsible for seismic velocity anomalies such as temperature and the presence of melt and or volatiles because high attenuation in the wedge limits the s wave data set we focus on 2 d inversions beneath the linear obs array where resolution is best and also parameterize the solution in terms of the vp vs ratio as expected the subducting slab has fast vp and vs and a low vp vs ratio consistent with the cold downgoing plate the central lau spreading center clsc exhibits stronger anomalies in vp vs than in vp with the anomalies larger than would be predicted purely by temperature variations the clsc anomaly extends 100 km to the west of the axis suggesting a broad region of melt production driven by passive upwelling from plate separation rather than active upwelling mechanisms the anomaly is asymmetric about the axis suggesting that slab induced corner flow possibly influences mantle dynamics several hundred kilometers away from the arc there is a strong anomaly beneath the volcanic arc that gradually deepens as it trends toward the back arc likely outlining a hydrated region of melt production that feeds the volcanic front hydration possibly continues throughout the wedge to at least 400 km depth the lau ridge exhibits a thicker lithosphere relative to the rest of the basin while the fiji platform likely has a thinner lithosphere than the lau ridge from more recent extension there is also a reasonable likelihood of a small degree of partial melt in the uppermost mantle beneath the platform

anisotropy and vp vs in the uppermost mantle beneath the western united states from joint analysis of pn and sn phases pn and sn phases are valuable for resolving velocity structure in the mantle lid as they propagate horizontally right below the moho relatively few sn tomography attempts have been made compared to pn because sn is often highly attenuated or buried in p wave coda usarray has greatly increased data coverage for regional phases and both pn and sn are routinely picked by network analysts here we jointly invert pn and sn arrival time residuals with a modified time term analysis and a regularized tomography method and present new maps of crustal thickness uppermost mantle p velocity perturbations vp vs ratios and azimuthal anisotropy strength and orientation beneath the western united states the results indicate partially molten mantle below the snake river plain and the colorado plateau the seismic structure of the top 40 km of the mantle below the colorado plateau differs from that seen at greater depths in other studies such as surface wave or teleseismic body wave tomography whereas the snake river plain anomaly just below the moho is comparable to structures seen at about 200 km depth pn fast axes provide complementary information to sks shear wave splitting observations and our analysis indicates that in several regions in the western united states the orientation of azimuthal anisotropy changes with depth in the upper mantle however we have so far been unable to resolve shear wave splitting directly in sn waveforms which seem to be dominated by sn sv energy

an olivine fabric database an overview of upper mantle fabrics and seismic anisotropy we present a unique database of 110 olivine petrofabrics and their calculated seismic properties the samples come from a variety of the upper mantle geodynamic environments ophiolites subduction zones and kimberlites with a wide range of micro structures a phenomenological relationship is established between p and s wave seismic anisotropy and the degree of crystal alignment fabric strength seismic anisotropy increases rapidly at low fabric strength before reaching a near saturation level of 15 to 20 for p waves and 10 to 15 for s waves despite a large variation in the symmetry of fabric patterns the average seismic properties of the different fabric micro structural and geodynamic settings have similar anisotropies in both magnitude and symmetry hence it would seem possible to determine some measure of fabric strength from seismic anisotropy if the dimensions of the anisotropic region are known but not geodynamic environment or details of the petrofabric pattern a simple pattern of seismic anisotropy characterises the average sample of the database which has the following features the polarisation plane of the fastest s wave is parallel or sub parallel to the foliation plane the maximum shear wave splitting is parallel to the y structural direction in foliation plane and normal to the lineation the maximum of the p wave velocity is parallel to the high concentration of 100 axes which is sub parallel to the lineation the 100 orientation distribution has the greatest influence on the p wave seismic anisotropy the 100 and 001 orientation distributions have the greatest influence on the symmetry of s wave anisotropy although the magnitude of anisotropy is influenced by the distribution of all three principal axes although the database only contains olivine petrofabrics this statistical study clearly shows that seismic anisotropy can be used to deduce the orientation of the structural frame in the upper mantle 1998 elsevier science b v all rights reserved

tomography of crustal p and s travel times across the western united states 1 data from the usarray combined with local regional and national networks provide an unprecedented opportunity for imaging the crust of the western united states we employ a simple tomography scheme to invert pg and sg travel times for velocity structure the approach is analogous to pn tomography we assume a great circle arc between source and receiver and solve for station and event terms along with 2 d velocity structure pg velocities are high in the snake river plain western washington and the columbia plateau and in central california the colorado plateau falls midrange while the basin and range is slow as are the geysers region north of the san francisco bay and the salton trough for sg the snake river plain is fast as is the entire cordillera from the southern sierra nevada up through the cascades both the colorado plateau and most of the rocky mountains are also fast at the highest point of the rocky mountains the aspen anomaly is a low velocity region the basin and range is a composite of slower to the west abutting the sierra nevada and faster in eastern nevada coastal california is slow with the exception of the san francisco bay region and far northern california the continental crust of the north american craton appears slightly fast for both pg and sg our observations and their trends compare well with middle to upper crustal velocity estimates from previous studies they also compare reasonably well with average crustal velocities from the same studies

complex and variable crustal and uppermost mantle seismic anisotropy in the western united states deformation in the crust and mantle measured using seismic anisotropy is poorly constrained in the western united states because of inconsistencies in the existing data a three dimensional model that reconciles these discrepancies reveals that seismic anisotropy in the crust correlates with different geological provinces but is unrelated to anisotropy in the underlying mantle

complex subduction and small scale convection revealed by body wave tomography of the western united states upper mantle abstract new high resolution p and s wave tomography of the united states upper mantle from the pacific coast to the great plains reveals strong multi scale heterogeneity closely correlated with tectonic and magmatic activity we invert teleseismic travel time residuals from the earthscope transportable array and more than 1700 additional temporary and permanent stations for 3 d velocity perturbations to a depth of 1000 km the inversion uses recent advances in western u s crust models to better isolate the mantle component of travel time residuals and frequency dependent 3 d sensitivity kernels to map travel time residuals measured in multiple frequency bands into velocity structure in addition to separate v p and v s models we jointly invert the two datasets for v p v s perturbations by imposing a smoothness constraint on the ln v s ln v p field the joint inversion helps us identify regions where partial melt is probable the amplitude of v p v s and v p v s variations is greatest in the upper 200 km of the mantle and the form of velocity anomalies suggests a provincially heterogeneous lithosphere and the occurrence of widespread small scale convection partially molten mantle is inferred beneath yellowstone and the eastern snake river plain srp the salton trough and the clear lake volcanic field the inferred depth extent of partial melt is consistent with a generally hydrated upper mantle and elevated temperatures beneath the eastern srp and yellowstone despite continuous subduction since the cretaceous the distribution of sub lithospheric high velocity anomalies is dissected similar to other recent studies based on our new tomography models western u s geologic history and plate tectonic reconstructions we infer patchy and incomplete removal of the flat subducting laramide slab and slab tearing associated with eocene accretion in the northwestern u s

shear and compressional velocity models of the mantle from cluster analysis of long period waveforms summary we present a new technique for the efficient measurement of the traveltimes of long period body wave phases the technique is based on the fact that all arrivals of a particular seismic phase are remarkably similar in shape for a single event this allows the application of cross correlation techniques that are usually used in a regional context to measure precise global differential times the analysis is enhanced by the inclusion of a clustering algorithm that automatically clusters waveforms by their degree of similarity this allows the algorithm to discriminate against unusual or distorted waveforms and makes for an extremely efficient measurement technique this technique can be applied to any seismic phase that is observed over a reasonably large distance range here we present the results of applying the algorithm to the long period channels of all data archived at the iris dmc from 1976 to 2005 for the seismic phases s and p from 23 to 100 and ss and pp from 50 to 170 the resulting large data sets are inverted along with existing surface wave and updated differential traveltime measurements for new mantle models of s and p velocity the resolution of the new model is enhanced particularly in the mid mantle where ss and pp turn we find that slow anomalies in the central pacific and africa extend from the core mantle boundary to the upper mantle but their direct connection to surface hotspots is beyond our resolution furthermore we find that fast anomalies that are likely associated with subducting slabs disappear between 1700 and 2500 km and thus are not continuous features from the upper to lower mantle despite our extensive coverage and high resolution of the mid mantle

uppermost mantle seismic velocity and anisotropy in the euro mediterranean region from pn and sn tomography summary in the last 10 15years the number of high quality seismic stations monitoring the euromediterranean region has increased significantly allowing a corresponding improvement in structural constraints we present here new images of the seismic velocity and anisotropy variations in the uppermost mantle beneath this complex area compiled from inversion of pn and sn phases sampling the whole region the method of hearn has been applied to the traveltime arrivals of the international seismological center catalogue for the time period 1990 2010 a total of 579 753 pn arrivals coming from 12 377 events recorded at 1 408 stations with epicentral distances between 220km and 1400km have been retained after applyingstandardqualitycriteria maximumdepth minimumnumberofrecordings maximum residual values our results show significant features well correlated with surface geology and evidence the heterogeneous character of the euro mediterranean lithosphere the station terms reflect the existence of marked variations in crustal thickness consistent with available moho depths inferred from active seismic experiments the highestpnvelocities are observed along a continuous band from the po basin to the northern ionian sea other high velocity zones include the ligurian basin the valencia trough the southern alboran sea and central part of the algerian margin most significant low velocity values are associated to orogenic belts betics pyrenees alps apennines and calabrian arc dinarides hellenides and lowvelocityzonesarealsoidentifiedbeneathsardiniaandthebalearicislands theintroductionof an anisotropic term enhances significantly the lateral continuity of the anomalies in particular in the most active tectonic areas pn anisotropy shows consistent orientations subparallel to major orogenic structures such as betics apennines calabrian arc and alps the sn tomographic image has lower resolution but confirms independently most of the features evidenced in the pn tomography

lithosphere asthenosphere interaction beneath the western united states from the joint inversion of body wave traveltimes and surface wave phase velocities summary the relation between the complex geological history of the western margin of the north american plate and the processes in the mantle is still not fully documented and understood several pre usarray local seismic studies showed how the characteristics of key geological features such as the colorado plateau and the yellowstone snake river plains are linked to their deep mantle structure recent body wave models based on the deployment of the high density large aperture usarray have provided far more details on the mantle structure while surface wave tomography ballistic waves and noise correlations informs us on the shallow structure here we combine constraints from these two data sets to image and study the link between the geology of the western united states the shallow structure of the earth and the convective processes in mantle our multiphase dna10 s model provides new constraints on the extent of the archean lithosphere imaged as a large deeply rooted fast body that encompasses the stable great plains and a large portion of the northern and central rocky mountains widespread slow anomalies are found in the lower crust and upper mantle suggesting that low density rocks isostatically sustain part of the high topography of the western united states the yellowstone anomaly is imaged as a large slow body rising from the lower mantle intruding the overlying lithosphere and controlling locally the seismicity and the topography the large e w extent of the usarray used in this study allows imaging the slab graveyard a sequence of farallon fragments aligned with the currently subducting juan de fuca slab north of the mendocino triple junction the lithospheric root of the colorado plateau has apparently been weakened and partly removed through dripping the distribution of the slower regions around the colorado plateau and other rigid blocks follows closely the trend of cenozoic volcanic fields and ancient lithospheric sutures suggesting that the later exert a control on the locus of magmato tectonic activity today the dna velocity models are available for download and slicing at http dna berkeley edu

a 3 d shear velocity model of the crust and uppermost mantle beneath the united states from ambient seismic noise summary in an earlier study bensen et al measured surface wave dispersion curves from ambient noise using 203 stations across north america which resulted in rayleigh and love wave dispersion maps from 8 70 s period and 8 20 s period respectively we invert these maps in a two step procedure to determine a 3 d shear wave velocity model v s of the crust and uppermost mantle beneath much of the contiguous us the two steps are a linearized inversion for a best fitting model beneath each grid node followed by a monte carlo inversion to estimate model uncertainties in general a simple model parametrization is sufficient to achieve acceptable data fit but a rayleigh love discrepancy at periods from 10 to 20 s is observed in which simple isotropic models systematically misfit rayleigh and love waves in some regions crustal features observed in the model include sedimentary basins such as the anadarko green river williston basins as well as california s great valley and the mississippi embayment the east west velocity dichotomy between the stable eastern us and the tectonically deformed western us is shown to be abrupt in the crust and uppermost mantle but is not coincident in these regions crustal high velocity material tends to lap over the high velocities of the uppermost mantle the rayleigh love discrepancy between 10 and 20 s period is crustal in origin and is observed in a number of regions particularly in extensional provinces such as the basin and range it can be resolved by introducing radial anisotropy in the lower or middle crust with v sh v sh by about 1 per cent

a numerical investigation of a convectively generated inertially stable extratropical warm core mesovortex over land part i structure and evolution abstract a 36 h nested grid numerical simulation of the life cycle of a convectively generated inertially stable warm core mesovortex is presented the vortex evolved from a mesoscale convective complex that developed from a squall line over oklahoma during 7 8 july 1982 a modified version of the pennsylvania state university national center for atmospheric research mesoscale hydrostatic model with a fine mesh grid resolution of 25 km is utilized for this study the model simultaneously incorporates parameterized convection and a grid resolved convective scheme containing the effects of hydrostatic water loading condensation evaporation freezing melting and sublimation genesis intensification and maintenance of a low to midtropospheric closed meso scale cyclone as well as the associated surface pressure perturbations the evolution of moist convection and the distribution and magnitude of total rainfall are simulated by the model similarly the observed amplification of a 700 mb meso sc

seismic properties of the supra subduction mantle constraints from peridotite xenoliths from the avacha volcano southern kamchatka 1 this work presents seismic properties derived from a detailed microstructural study of supra subduction mantle xenoliths extracted by the active avacha volcano in the southern kamchatka arc these peridotites underwent low stress deformation and syn to late kinematic reactive percolation of hydrous melts or fluids leading to local orthopyroxene enrichment and dispersion of the olivine cpo in the shallow mantle wedge all samples show an axial 100 olivine cpo implying fast s wave polarization parallel to the flow direction in the mantle orthopyroxene enrichment by reactive fluids melts percolation lowers the vp vs ratio but cannot explain the vp vs ratios 1 7 mapped in the japan and andes forearc mantle these low vp vs ratios may however be explained by considering the intrinsic anisotropy of the peridotites

the magnitude symmetry and origin of upper mantle anisotropy based on fabric analyses of ultramafic tectonites summary seismic anisotropy within the upper mantle originates from the preferred orientation of highly anisotropic single crystals the symmetry and magnitude of anisotropy depend upon 1 the volume percentages of the minerals constituting the upper mantle 2 the degree and symmetry of preferred orientation of each mineral and 3 the alignment of the minerals crystallographic axes relative to one another the nature of upper mantle anisotropy can be examined by studying mineral orientations within ultramafic rocks which were once part of the mantle petrofabric data for olivine and pyroxene have been used to obtain velocity anisotropy patterns over large regions of ultramafic rocks from the samail ophiolite oman the troodos ophiolite cyprus the bay of islands ophiolife newfoundland the twin sisters ultramafic washington usa the dun mountain ophiolite new zealand the red hills ophiolite new zealand and the red mountain ophiolite new zealand the compressional wave anisotropy calculated for these massifs ranges from 3 to 8 per cent in excellent agreement with observed seismic anisotropy in the upper continental and oceanic mantle the symmetry varies from orthorhombic to axial with the axial symmetry axis corresponding to the olivine a axes maxima and subparallel to spreading directions in oceanic upper mantle pyroxene a b and c axes maxima generally parallel olivine b c and a axes respectively and anisotropy decreases with increasing pyroxene content shear wave splitting is predicted for all propagation directions within the upper mantle symmetry is also orthorhombic or axial with the minimum difference in velocity between the two shear waves parallel to the maximum compressional wave velocity

pn tomography of the western united states using usarray 1 usarray has now provided several years of high quality seismic data and improved ray coverage for much of the western united states which will enable increased resolution for studies of the lithospheric and deeper structure of the north american continent here we analyze pn arrival times from the transportable stations of usarray to resolve crustal thickness and uppermost mantle structure we use 123 008 pn picks from april 2004 to october 2009 as measured by the array network facility at epicentral distances from 180 to 1450 km these picks are derived from 778 stations at 70 km spacing and 7903 earthquakes and quarry blasts applying the classic time term method we use a regularized least squares inversion to estimate crustal thickness variations and image velocity perturbations in the uppermost mantle just below the moho we also consider upper mantle anisotropy and describe the velocity perturbations with a cos 2 azimuthal variation our crustal thickness map generally agrees with receiver function results from other researchers but differs in some details we obtain an average upper mantle velocity of 7 93 km s with higher velocities beneath eastern washington and northern idaho and lower velocities near the california mexico border the sierra nevada the northern coastal california region and the greater yellowstone area we observe large anisotropic anomalies in southern california as well as in the snake river plain area these results should complement other seismic studies e g body and surface wave tomography and shear wave splitting to provide information about composition temperature and tectonic processes in the western united states

seismic evidence for widespread western us deep crustal deformation caused by extension morgan moschetti and colleagues use ambient noise tomography data from the western united states to show that strong radial seismic anisotropy exists in the deep crust confined mainly to the geological provinces that have undergone significant extension during the cenozoic era since about 65 million years ago the coincidence of crustal radial anisotropy with these extensional provinces supports the hypothesis that the deep crust within these regions has undergone widespread and relatively uniform strain in response to crustal thinning and extension here a method of seismic wave imaging known as ambient noise tomography is used to generate high resolution images of seismic wave speeds in the crust and uppermost mantle the observations reveal strong and uniform anisotropy where waves travel through rock at different speeds depending on direction in the deep crust in areas of the western united states that have undergone significant extension during the past 65 million years laboratory experiments have established that many of the materials comprising the earth are strongly anisotropic in terms of seismic wave speeds1 observations of azimuthal2 3 and radial4 5 anisotropy in the upper mantle are attributed to the lattice preferred orientation of olivine caused by the shear strains associated with deformation and provide some of the most direct evidence for deformation and flow within the earth s interior although observations of crustal radial anisotropy would improve our understanding of crustal deformation and flow patterns resulting from tectonic processes large scale observations have been limited to regions of particularly thick crust6 here we show that observations from ambient noise tomography in the western united states reveal strong deep middle to lower crustal radial anisotropy that is confined mainly to the geological provinces that have undergone significant extension during the cenozoic era since 65 myr ago 7 8 the coincidence of crustal radial anisotropy with the extensional provinces of the western united states suggests that the radial anisotropy results from the lattice preferred orientation of anisotropic crustal minerals caused by extensional deformation these observations also provide support for the hypothesis that the deep crust within these regions has undergone widespread and relatively uniform strain in response to crustal thinning and extension9 10 11

sn velocities in western and eastern north america we have investigated the use of the new delay time data set of engdahl et al 1997 for the construction of tomographic images of sn velocity in north america the delays show a significant difference in average delay between paths located west and east of 105 w this difference is larger when paths longer than 10 are included indicating a positive gradient for the s velocity in eastern north america discarding outliers two separate regions in the western and eastern united states and canada where imaged low velocities were found under the colorado plateau extending westwards towards the gulf of california in the eastern u s the michigan basin stands out with low sn velocity and the grenville province is characterized by a positive anomaly

anisotropic pn tomography in the western united states pn travel times are affected not only by lateral variations in crust and mantle velocity but also by significant amounts of laterally varying anisotropy to investigate uppermost mantle anisotropy a tomography algorithm was reformulated to include lateral variations in both velocity and horizontal anisotropy and it was applied to pn travel time data from the western united states results show that anisotropy is as important in explaining the travel time residuals as are the velocity variations a detailed resolution study examined the trade off between the velocity variations and the anisotropy variations and showed that both could be resolved for regions with good ray path coverage pn anisotropy beneath the western united states has maximum amplitudes of 0 3 km s 7 5 when resolved on a length scales of around 3 the orientations of pn anisotropy often correlate well with those inferred from shear wave splitting results this correlation suggests that orientation of mantle anisotropy does not change significantly with depth for many regions the orientation of the pn anisotropy can be correlated with some of the tectonic processes which occur within the western united states for example the fast direction of pn anisotropy parallels the northeast direction of subduction of the juan de fuca plate beneath the northwest pacific coast suggesting that there the anisotropy is related to subduction driven deformation the fast direction of pn anisotropy also parallels the strike of the san andreas fault system in central california indicating that shear strains along the plate boundary may be responsible for the anisotropy there within the great basin the pn anisotropy varies substantially and both partial melting and small scale convection within the uppermost mantle could be responsible for these anisotropy variations

mantle dynamics and seismic anisotropy abstract observations of seismic anisotropy yield some of the most direct constraints available on both past and present day deformation in the earth s mantle insight into the character of mantle flow can also be gained from the geodynamical modeling of mantle processes on both global and regional scales we highlight recent progress toward understanding mantle flow from both observations and modeling and discuss outstanding problems and avenues for progress particularly in the integration of seismological and geodynamical constraints to understand seismic anisotropy and the deformation that produces it to first order the predictions of upper mantle anisotropy made by global mantle circulation models match seismological observations well beneath the ocean basins but the fit is poorer in regions of greater tectonic complexity such as beneath continental interiors and within subduction systems in many regions of the upper mantle models of anisotropy derived from surface waves are seemingly inconsistent with shear wave splitting observations which suggests that our understanding of complex anisotropic regions remains incomplete observations of anisotropy in the d layer hold promise for improving our understanding of dynamic processes in the deep earth but much progress remains to be made in characterizing anisotropic structure and relating it to the geometry of flow geochemical heterogeneity or phase transitions major outstanding problems related to understanding mantle anisotropy remain particularly regarding the deformation and evolution of continents the nature of the asthenosphere subduction zone geodynamics and the thermo chemical state of the lowermost mantle however we expect that new seismological deployments and closer integration of observations with geodynamical models will yield rapid progress in these areas

evidence for the overflow origin of the grand canyon abstract the colorado river s course through grand canyon and across the topographic high imposed by the kaibab plateau represents one of the most well known transverse drainages only four processes allow a river to flow across a topographic obstruction antecedence superimposition piracy and overflow since powell first described the canyon in 1875 various authors have repeatedly proposed variations of all four mechanisms to explain the formation of the grand canyon and this paper does not review or try to refute this prior scholarship this paper s purpose rests in providing a detailed evaluation of evidence associated with overflow upstream across and downstream of the east kaibab monocline paleodivide upstream of the paleodivide we re interpreted the bidahochi formation the older units of this formation represent a fluctuating system between playa and lacustrine deposition fed by multiple localized tributaries entering a topographically barred basin the younger bidahochi units signify the abrupt arrival of an additional drainage with a distinctive geochemical signature into the basin roughly 6 8 ma supported by three lines of evidence 1 an order of magnitude increase in depositional rates from 3 m ma in the older units to 50 m ma in the younger units that buried the paleo landscape at least locally up to an elevation of at least 2250 m 7380 ft 2 new evidence of an elevated 87sr 86sr ratio in the younger units when compared to the older units and the modern colorado river and 3 fossil evidence of the existence of large freshwater lakes and rivers in the bidahochi basin during deposition of the younger units newly re evaluated evidence at the east kaibab paleodivide focuses on a southern bend of the colorado river s path across the kaibab plateau as well as the confluence of the colorado and the little colorado rivers at the junction of the kaibab plateau and cedar ridge both lines of evidence at the paleodivide focus on the impact of topography on grand canyon incision and both lines of evidence are consistent only with an overflow process evidence downstream of the paleodivide includes the abrupt arrival of river water with an elevated 87sr 86sr ratio followed by colorado river sediment into a series of successively lower downstream basins that integrated via overflow new 10be 9be geochemistry data support the relative timing of deposition of the bidahochi formation followed soon after by deposition of the bouse formation downstream of the grand canyon we conclude that newly presented and re evaluated available evidence supports an overflow process for the colorado river s transverse section across the kaibab plateau ultimately resulting in incision of the grand canyon

na sws 1 1 a uniform database of teleseismic shear wave splitting measurements for north america 1 this version of shear wave splitting sws database db for north america na contains 6224 pairs of splitting parameters fast polarization directions and splitting times measured using all the available data recorded by digital broadband seismic stations over the period of 1980 2007 and archived at the incorporated research institutions for seismology data management center the measurements were produced using a set of robust procedures that involve automated batch processing and manual screening and were ranked quantitatively on the basis of a uniform criterion the result is a homogeneous database of individual rather than station averaged splitting parameters that can be used by a variety of geoscientists for the understanding of the structure and dynamics of the earth s deep interior beneath na this data brief describes the seismic data and techniques used to generate the sws results the spatial and azimuthal coverage and the limitations of the db the db will be periodically updated and enhanced by adding newly recorded data and by measuring splitting parameters using the pks and skks phases in addition to sks the database can be accessed at http www mst edu liukh sws

three dimensional imaging of the p and s wave velocity structure and earthquake locations beneath southwest iceland three dimensional imaging of the p and s wave velocity structure and earthquake locations beneath southwest iceland