identifying global seismic anisotropy patterns by correlating shear wave splitting and surface wave data we compare a global compilation of shear wave splitting measurements with azimuthal seismic anisotropy parameters inferred from surface wave tomography the currently available splitting dataset is taken from a novel comprehensive collection of available publications that is updated interactively online the comparison between the two types of data is made by calculating predicted splitting parameters from the anisotropic tomography model comparing these predicted splitting parameters with the observed ones we find a considerable correlation between the two datasets at global scale this result is noteworthy since such correlation did not seem to exist in previous studies the spatial resolution associated with the two types of methods is rather different while surface waves have good vertical resolution and poor lateral resolution of several hundreds of kilometers sks splitting measurements have good lateral but poor vertical resolution the correlation can be understood in light of recent propositions that anisotropy seen by sks splitting constrains mostly the upper mantle and therefore a similar depth region as surface waves the correlation also confirms the generally good quality of the shear wave measurements as well as that of the anisotropic tomography model

upper mantle anisotropy beneath the geoscope stations seismic anisotropy has been widely studied this last decade particularly by measuring splitting of vertically propagating core shear waves the main interest in this technique is to characterize upper mantle flow beneath seismic stations on the other hand the major restriction in this method is that a single station gives a single anisotropy measurement alternative methods have been developed in order to avoid this restriction an accurate determination of upper mantle seismic anisotropy beneath a seismic station may allow one by doing anisotropy correction to characterize remote or deeper anisotropy the geoscope network is ideal for this purpose because it is composed of a large set about 26 of high quality broadband seismometers globally distributed and because some of these stations have run for more than 10 years and most of them for more than 5 years we selected about 100 events at each site generally of magnitude mb 6 0 and we performed systematic measurements of the splitting parameters fast polarization direction and delay time t on sks skks and pks phases splitting on oceanic islands has been difficult to observe owing to the low quality of the signal but also perhaps owing to complex upper mantle structures beneath the stations station kip kipapa hawaii in the pacific is the only oceanic geoscope station with a clear anisotropy we determined well constrained splitting parameters for 10 of the 17 continental stations that may be explained by a single anisotropic layer the poor correlation between fast polarization directions and the absolute plate motion together with the apparent incoherence between the plate velocities and the observed delay times suggest that a simple drag induced asthenospheric flow alone fails to explain most of the observations for some stations located on or near major lithospheric structures tam tamanrasset algeria for instance we observe a good correlation between fast polarization directions and regional structures at station scz santa cruz california we found clear variations of the splitting parameters as a function of the event backazimuth compatible with two layers of anisotropy three stations can canberra hyb hyderabad india and ssb saint sauveur badole france seem to be devoid of detectable anisotropy

upper mantle anisotropy in se and central brazil from sks splitting evidence of asthenospheric flow around a cratonic keel abstract we present results of upper mantle anisotropy derived from measurements of core refracted shear wave splitting mainly sks phases recorded at 48 stations covering the major tectonic provinces in central and se brazil such as the tocantins province paraguay araguaia and brasilia belts between the amazon and sao francisco cratons the parana intracratonic basin the southern part of the sao francisco craton and the mantiqueira province with the coastal ribeira belt although the fast polarization directions vary across the region consistent orientations are observed over hundreds of kilometers the fast polarization directions tend to be close to the absolute plate motion given by the hot spot reference model hs3 nuvel1a however correlations with geological structures are also observed in the southern brasilia belt and in the ribeira belt respectively located sw and s of the sao francisco craton on the other hand in the northern tocantins province the fast shear wave direction n60 s is oblique to the sw ne trend of the geological units and faults and no anisotropy contribution from lithospheric sources can be clearly identified overall the fast polarization directions show a fan shaped pattern strongly suggesting asthenosperic flow around a thick and stiff keel in the southern part of the sao francisco craton consistent with the high velocity anomaly revealed by recent surface wave tomography the observed nw se directions in the southern part of the brasilia belt may also be interpreted as resulting from asthenospheric flow channeled between the sao francisco craton and a cratonic block beneath the parana basin the largest splitting delays observed in the southern brasilia belt and in the ribeira belt up to 2 4 s suggest contributions from both lithospheric and asthenospheric sources in those two areas our preferred model for the anisotropy causing the observed pattern of sks splitting consists of flow induced deformation in the asthenosphere caused by the absolute plate motion roughly towards w or wsw with flow channeled around a thick cratonic keel in the southern brasilia and in the ribeira fold belts the large delays and the parallelism with geological structures indicate additional anisotropy contribution from lithospheric sources

shear wave splitting and subcontinental mantle deformation splitting observations are interpreted in terms of the strain induced lattice preferred orientation of mantle minerals especially olivine we consider three hypotheses concerning the origin of the continental anlsotropy 1 strain associated with absolute plate motion as in the oceanic upper mantle 2 crustal stress and 3 the past and present internal deformation of the subcontinental upper mantle by tectonic episodes it is found that the last hypothesis is the most successful namely that the most recent significant episode of internal deformation appears to be the best predictor of q for stable continental regions this is interpreted as fossil anisotropy whereas for presently active regions such as alaska the anisotropy reflects present day tectonic activity in the stable portion of north america there is a good correlation between delay time and lithospheric thickness this is consistent with the anisotropy being localized in the subcontinental lithosphere and suggests that intrinsic anisotropy is approximately constant the acceptance of this hypothesis has several implications for subcontinental mantle deformation first it argues for coherent deformation of the continental lithosphere crust and mantle during orogenies this implies that the anisotropic portion of the lithosphere was present since the deformational episode and rules out the addition of undeformed material to this layer by subsequent underplating or conductive growth of the thermal boundary layer one of the most important issues in the study of orogenies is the need to reconcile the formation of thickened lithosphere with the paradoxically high mantle temperatures often associated with orogenic episodes most efforts to date have focussed on modes of deformation whereby the cold lithospheric mantle is removed by convective instability or delamination and replaced by warm asthenosphere these models however are incompatible with the evidence for preserved coherent lithospheric deformation rather the deformed mantle appears to have been heated in place we suggest that the elevated mantle temperatures may be due to the strain heating accompanying the deformation

upper mantle seismic anisotropy beneath antarctica and the scotia sea region summary recent investigations on shear wave splitting from recordings of permanent and temporary antarctic seismological stations have lead to a greater understanding of the upper mantle dynamics of the scotia sea region and the continental margin in the eastern weddell sea in terms of their tectonic evolution the analysis of shear wave splitting from teleseismic core sks skks pks and direct s waves reveals the seismic anisotropy and the strain field of the upper mantle similar to the caribbean anisotropy structures in the antarctic peninsula and scotia sea regions are assumed to be influenced by mantle flows in easterly directions around the subducting nazca plate in general anisotropy polarization directions in the scotia sea do not contradict this hypothesis with polarizations oriented nearly e w and therefore aligning with the suggested mantle flow patterns anisotropy strengths decrease from delay times of dt 1 8 s pmsa palmer station in the west towards the east with delay times of dt 0 3 s beneath hope south georgia and cand candlemas south sandwich islands nevertheless a lithospheric and therefore fossil origin cannot be ruled out only the exceptionally high delay times at pmsa probably originate in part from recent asthenospheric flow around the subduction slab of the former phoenix plate beneath the northwestern margin of the antarctic peninsula the continental margin of western dronning maud and coats land plays a crucial role in understanding the early processes during the break up of gondwana upper mantle seismic anisotropy with delay times well over dt 1 s in this region gives new constraints on ancient deformation processes during break up and former episodes two layer modelling reveals archaean anisotropy in the upper layer corresponding well to polarization directions of the south african kaapvaal craton lower layer anisotropy is assumed to have been created during early gondwana rifting stages

constraints on mantle anisotropy beneath precambrian north america from a transportable teleseismic experiment data from the apt89 transportable teleseismic experiment was examined to characterize the variations in shear wave splitting along a 1500 km traverse in north america see silver et al 1993 this issue three geologic terranes were sampled the western superior province of the canadian shield the trans hudson orogen and the wyoming craton the primary goal was to determine the extent to which the anisotropy was controlled by archean and proterozoic tectonic episodes of the large suite of teleseismic events recorded 5 were suitable for determining the fast polarization direction and delay time t from the phases sks and skks the resulting observations reveal two important features first the values of display good geologic coherence stations along the northern part of the array in the exposed shield reveal a consistent ene direction for that is generally parallel to exposed geologic structures within the part of the superior province buried beneath phanerozoic cover rotates to a ne sw azimuth as do geophysical indicators of geologic fabric moving onto the trans hudson changes abruptly to a more ew azimuth the stations within the wyoming craton suggest values with a more northerly direction although the waveforms from these stations are more complex second t which ranges between 0 40s and 1 75s shows systematic variations across the traverse the largest delay times approaching 2 s are found in a 250 km wide band from red lake ontario to the u s canada border t is reduced to about 1 s or less both north and south of this region for the stations with detectable splitting we find an inverse relationship between s isotropic station delays derived from the portable data set and t which is most easily explained by the anisotropy being primarily localized in the fast presumably lithospheric mantle the splitting observations overall suggest that mantle anisotropy along this traverse is dominated by precambrian fossil anisotropy that is preserved in the subcontinental lithosphere

upper mantle anisotropy beneath the african iris and geoscope stations summary upper mantle anisotropy beneath the african iris and geoscope stations is investigated through the measurements of splitting of teleseismic shear waves such as sks skks and pks phases seismic anisotropy data are interesting on their own as a measure of upper mantle active or frozen deformation beneath a given station but each station is of potential interest since it can be used to retrieve source side seismic anisotropy at remote sites if one is able to perform station side anisotropy correction we performed systematic investigations of teleseismic shear wave splitting at 15 stations from the iris and geoscope global seismic networks which are located on both the oceanic and the continental parts of the african plate anisotropy is generally well observed at continental stations the patterns we present generally show much more complexity than the results previously published from smaller data sets despite this complexity the splitting parameters generally appear in several places to contain a signature of the regional geodynamic setting rift structures archaean craton panafrican belt although a deeper source of anisotropy asthenospheric may be present at the oceanic stations anisotropy measurements are much more difficult to perform because the signal is generally of poor quality msey in the seychelles indian ocean is the exception and displays a clear correlation of the azimuth of the fast split shear wave with the trend of the absolute plate motion as defined by hotspot tracks

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

lateral heterogeneity and azimuthal anisotropy of the upper mantle love and rayleigh waves 100 250 s the lateral heterogeneity and apparent anisotropy of the upper mantle are studied by measuring rayleigh and love wave phase velocities in the period range 100 250 s spherical harmonic descriptions of the lateral heterogeneity are obtained for order and degree up to 1 m 10 slow regions are evident at the east pacific rise northeast africa tibet tasman sea southwestern north america and triple junctions in the northern atlantic and indian oceans fast regions occur in australia western pacific and the southern atlantic these features are also found by a completely different analysis based on the backus gilbert method the backus gilbert method also shows that the obtained phase velocities are averaged values within an area of about 2000 km radius and the errors are about 1 of the phase velocity in the zeroth order spherically symmetric earth inversion for azimuthal dependence shows that for low angular order the fast phase velocity directions seem to correlate well with the plate motion vectors however resolution analysis by the backus gilbert method shows that the current data do not have enough resolution for everywhere on the globe only a few regions currently have adequate azimuthal coverage thus confirmation of the above correlation requires a more complete data set

combined plate motion and density driven flow in the asthenosphere beneath saudi arabia evidence from shear wave splitting and seismic anisotropy a comprehensive study of mantle anisotropy along the red sea and across saudi arabia was performed by analyzing shear wave splitting recorded by stations from three different seismic networks the largest most widely distributed array of stations examined across saudi arabia to date stations near the gulf of aqaba display fast orientations that are aligned parallel to the dead sea transform fault most likely related to the strike slip motion between africa and arabia however most of our observations across saudi arabia are statistically the same showing a consistent pattern of north south oriented fast directions with delay times averaging about 1 4 s fossilized anisotropy related to the proterozoic assembly of the arabian shield may contribute to the pattern but is not sufficient to fully explain the observations we feel that the uniform anisotropic signature across saudi arabia is best explained by a combination of plate and density driven flow in the asthenosphere by combining the northeast oriented flow associated with absolute plate motion with the northwest oriented flow associated with the channelized afar plume along the red sea we obtain a north south oriented resultant that matches our splitting observations and supports models of active rifting processes this explains why the north south orientationmore of the fast polarization direction is so pervasive across the vast arabian plate less

sks splitting beneath southern california measurements of sks phase splitting were obtained from nineteen seismic stations in southern california the fast polarization directions are 53 at the southern end of the great valley 82 8 in the western transverse ranges and northern peninsular ranges 95 4 in mojave desert and 70 on san clemente island the splitting time ranges from 0 8 to 1 8 seconds which is consistent with an anisotropic layer of 100 to 200 km thick for 4 anisotropy

evidence for small scale mantle convection in the upper mantle beneath the baikal rift zone 1 inversion of teleseismic p wave travel time residuals collected along a 1280 km long profile traversing the baikal rift zone brz reveals the existence of an upwarped lithosphere asthenosphere interface which causes a travel time delay of about 1 s at the rift axis central high an area with early arrivals relative to the stable siberian platform of up to 0 5 s is observed on each side of the rift about 200 km from the rift axis flank lows while the location of the central high is approximately fixed in the vicinity of the rift axis those of the flank lows vary as much as 200 km with the azimuth of the arriving rays we use three techniques to invert the travel time residuals for velocity anomalies beneath the profile two of the techniques assume an isotropic velocity structure and one of them considers a transversely isotropic velocity model with a vertical axis of symmetry we use independent geophysical observations such as gravity active source seismic exploration and crustal thickness measurements to compare the applicability of the models other types of geophysical measurements suggest that the model involving transverse isotropy is a plausible one which suggests that the central high and flank lows are caused by the combined effects of an upwarped asthenosphere with a 2 5 lateral velocity reduction and a velocity increase due to transverse isotropy with a vertical axis of symmetry we consider the anisotropy to be the result of the vertical component of a lithosphere asthenosphere small scale mantle convection system that is associated with the rifting

play types and hydrocarbon potential of the deepwater black sea ne bulgaria to date exploration in the bulgarian black sea has been largely restricted to the shelf and therefore the deepwater area remains untested in spite of significant hydrocarbon potential

the caribbean anomaly short wavelength lateral heterogeneity in the lower mantle data from the archean proterozoic transect experiment 1989 apt89 are used to constrain smaller scale velocity heterogeneity in the lower mantle beneath the caribbean we observe large variations in travel time residuals for paths from south america to portable broadband and short period stations distributed along a 1500km long transect in central north america after accounting for near source and near reciever contributions we find variations as large as 3s for p and 8s for s for rays that bottom around 1200km depth this rapid spatial variation in travel times suggests that we are seeing the western edge of the caribbean anomaly a feature previously observed by several investigators the gradient appears to be about 300km wide assuming a path length of 1500km through the anomaly suggested from previous studies we find that p p 2 1 and s s 3 6 which for s waves is about three times the estimate from long period body waves the relative velocity ratio for this anomaly is ln s ln p 1 7 a value slightly larger than typical laboratory values but lower than previous recent seismological studies of lower mantle heterogeneity with increasing turning depth the observed anomaly appears to decrease in size although it continues to be visible for shear waves with bottoming depths as great as 2200km the presence of multiple direct s phases for several records of one of the events studied suggests multipathing induced by the large observed lateral velocity gradients required by the travel time data

stress history and tectonic development of the rio grande rift new mexico paleostress determinations from radiometrically dated dikes indicate ene wsw regional extension began by about 32 ma in new mexico and perhaps earlier in the southern part of the state the extension direction maintained this orientation until sometime after 23 ma when it rotated clockwise to an e w or wnw ese direction which is the present orientation the spreading direction in the rio grande rift however has been e w since the beginning of rifting about 31 ma and reflects control of spreading by preexisting structural grain ne sw extension occurred in the grants pie town area of the colorado plateau and in the roofs of batholiths in the socorro and questa areas concurrently with e w spreading in the rio grande rift nearly orthogonal orientations of late eocene oligocene 38 32 ma paleostress indicators in southern new mexico indicate a nearly isotropic middle tertiary stress field both the western and eastern boundaries of the zone of e w spreading along the rio grande rift in northern new mexico shifted 60 200 km eastward sometime between 23 and 16 ma perhaps coeval with rotation of the direction of regional extension this eastward shift also coincided approximately with the renewal of volcanism following an early miocene 22 17 ma lull during the late miocene 9 5 ma the eastern boundary of e w spreading shifted back to the present eastern margin of the rio grande rift the n s to nne ssw orientation of the least principal horizontal stress in the southern great plains province has persisted since at least 28 ma stress province boundaries can be remarkably sharp with stress orientations changing by 90 within 30 km or less the ne trending jemez lineament transects the colorado plateau rio grande rift basin and range and southern great plains stress provinces but except for a transition zone along the present southeastern boundary of the colorado plateau both paleostress and contemporary stress reflect the respective stress provinces not the lineament chemical analyses of dated dikes reveal highly variable compositions but indicate that true basalts are more common among early rift 20 ma volcanic rocks than is generally recognized a histogram constructed from nearly 200 radiometric ages of igneous rocks in new mexico between 104 w and 108 w longitude indicates that volcanism along the rio grande rift has been episodic with peaks at 37 34 31 27 26 22 17 13 12 6 and 5 0 ma

mantle structure and rifting processes in the baikal mongolia region geophysical data and evidence from xenoliths in volcanic rocks abstract the origin of the baikal rift zone brz has been debated between the advocates of passive and active rifting since the 1970s a re assessment of the relevant geological and geophysical data from russian and international literature questions the concept of broad asthenospheric upwelling beneath the rift zone that has been the cornerstone of many active rifting models results of a large number of early and recent studies favour the role of far field forces in the opening and development of the brz this study emphasises the data obtained through studies of peridotite and pyroxenite xenoliths brought to the surface by alkali basaltic magmas in southern siberia and central mongolia these xenoliths are direct samples of the upper mantle in the vicinity of the brz of particular importance are suites of garnet bearing xenoliths that have been used to construct p t composition lithospheric cross sections in the region for the depth range of 35 80 km xenolith studies have shown fundamental differences in the composition and thermal regime between the lithospheric mantle beneath the ancient siberian platform sampled by kimberlites and beneath younger mobile belts south of the platform the uppermost mantle in southern siberia and central mongolia is much hotter at similar levels than the mantle in the siberian craton and also has significantly higher contents of basaltic major elements ca al na and iron higher fe si and fe mg the combination of the moderately high geothermal gradient and the fertile compositions in the off cratonic mantle appears to be a determining factor controlling differences in sub moho seismic velocities relative to the siberian craton chemical and isotopic compositions of the off cratonic xenoliths indicate small scale and regional mantle heterogeneities attributed to various partial melting and enrichment events consistent with long term evolution in the lithospheric mantle age estimates of mantle events based on os sr nd isotopic data can be correlated with major regional stages of crustal formation and may indicate long term crust mantle coupling the ratios of 143 144nd in many lree depleted xenoliths are higher than those in morb or oib source regions and are not consistent with a recent origin from asthenospheric mantle mantle xenoliths nearest to the rift basins 30 50 km south of southern lake baikal show no unequivocal evidence for strong heating unusual stress and deformation solid state flow magmatic activity or partial melting that could be indicative of an asthenospheric intrusion right below the moho comparisons between xenoliths from older and younger volcanic rocks east of lake baikal together with observations on phase transformations and mineral zoning in individual xenoliths have indicated recent heating in portions of the lithospheric mantle that may be related to localised magmatic activity or small scale ascent of deep mantle material overall the petrographic p t chemical and isotopic constraints from mantle xenoliths appear to be consistent with recent geophysical studies which found no evidence for a large scale asthenospheric upwarp beneath the rift and lend support to passive rifting mechanism for the brz

qp beneath the rio grande and east african rift zones in 1983 teleseismic short period p waves were recorded in the southwest united states along a linear array which stretched 1000 km across the rio grande rift in 1985 a similar experiment was conducted across the east african rift in which stations were installed along a 600 km array which ran east west across kenya we have developed a spectral comparison technique to calculate the attenuation at each station for a given event we simultaneously solve for the effective source spectrum which includes mantle path effects as well as instrument response an amplitude factor and an attenuation operator t at each station we use downward projection of p wave travel time residuals to define the lithosphere asthenosphere boundary which we infer to upwarp beneath both rifts the velocity structure so determined is then used to determine the q of the asthenosphere relative to that of the lower lithosphere when we assume a lower lithosphere q of 100 we obtain asthenospheric q values of 57 beneath the rio grande rift and 26 beneath the east african rift with corresponding velocity contrasts of 8 and 12 respectively the low q values as well as low velocity and density contrasts support the interpretation that the upwarped asthenosphere contains partial melt and that beneath the east african rift the concentration is higher than beneath the rio grande comparison of the results with experiments on peridotite sato et al 1989a b suggests that the asthenosphere is at homologous temperatures of 1 01 and 1 03 with corresponding partial melt fractions of 1 and 3 beneath the rio grande and east african rifts respectively

pyroxene orientation within the upper mantle petrofabric analyses of orthopyroxene and olivine for a suite of rocks from the bay of islands ophiolite newfoundland show systematic relationships between the minerals9 fabrics the plane of the mohorovicic discontinuity determined from the contact of the gabbro and ultramafic sections and the spreading direction inferred from the attitude of the sheeted dikes the orthopyroxene c crystallographic axes maxima which are aligned subparallel to the spreading direction coincide with olivine a axes maxima the orthopyroxene b axes and olivine c axes also lie approximately within the plane of the mohorovicic discontinuity but parallel the ridge whereas the orthopyroxene a axes maxima and olivine b axes maxima are approximately perpendicular to the mohorovicic discontinuity using olivine and orthopyroxene elastic constants and their pressure derivatives velocities have been calculated from composite petrofabric data and contoured for varying proportions of olivine and pyroxene the results of these calculations show that within a vertical direction compressional wave velocities change only slightly with increasing pyroxene content whereas parallel to the mohorovicic discontinuity velocities decrease with increasing pyroxene content anisotropy decreases from 4 for a rock with 100 olivine to 2 6 for a rock with 50 olivine and 50 orthopyroxene also within the plane of mohorovicic discontinuity the maximum and minumum velocities occur at approximately the same azimuth for differing percentages of orthopyroxene a comparison of the calculated anisotropy and the observed seismic anisotropy off the coast of california shows a striking agreement from this comparison it is estimated that the percentage of orthopyroxene within the upper mantle of this region varies between 0 and 30

geodynamics of the baikal stanovoy seismic belt abstract the convection generated tensional stress field in the earth as inferred from satellite gravity data reveals an anomalous lens of upwelling mantle rocks under the baikal rift zone the point of no strain at 56 n 116 e forms a seismic gap along the baikal stanovoy seismic belt east of this point the stress field changes from extension to compression therefore the position of no strain at the eastern termination of the rift accounts for the dying out of the rift zone and for the appearance of a compressive structure in the stanovoy range

observations of teleseismic shear wave splitting in the basin and range from portable and permanent stations observations of shear wave splitting were obtained from temporary and permanent stations in western nevada and southern california measurements of fast polarization azimuth and delay time t were made on both sks and direct s phases for stations in the northern basin and range the results were consistent yielding an average of t 75 8 0 9 0 3 s one station in the southern basin and range gave 40 11 0 4 0 1 s and one in the mojave desert gave 54 3 1 2 0 1 s t 0 9s is consistent with a 100km thick mantle layer characterized by 4 anisotropy for the northern stations the anisotropy appears unrelated to the present day extension or absolute plate motion rather we suggest that it is fossil anisotropy associated with pre miocene extension whose direction is about 68 for the california station is nearly parallel to the strike of the san andreas fault system and is attributed to shear strain associated with relative plate motion

shear wave splitting continental keels and patterns of mantle flow in this study we investigated the origin of seismic anisotropy in the mantle beneath north america in particular we evaluated whether shear wave splitting patterns in eastern north america are better explained by anisotropy caused by lithospheric deformation anisotropy due to mantle flow beneath the lithosphere or a combination of both we examined new measurements of shear wave splitting from the missouri to massachusetts broadband seismometer array moma the north american mantle anisotropy and discontinuity experiment nomad as well as splitting parameters from several previous studies we developed a simple finite difference model that approximates mantle flow around a complex three dimensional continental lithospheric keel to evaluate potential anisotropy from mantle flow beneath the lithosphere in eastern north america we compared shear wave splitting observations to predicted splitting parameters calculated using this mantle flow model our results indicate that a significant portion of observed shear wave splitting in eastern north america can be explained by mantle flow around the continental keel however shear wave splitting patterns in a few regions of eastern north america indicate that a component of lithospheric anisotropy must exist particularly in regions containing the largest keel thicknesses for eastern north america as well as for splitting observations in australia europe and south america we favor a model in which anisotropy is controlled by a combination of both lithospheric deformation and subcontinental mantle flow

upper mantle seismic anisotropy in the intra continental kachchh rift zone gujarat india abstract shear wave splitting study of 411 sks skks phases covering backazimuth range of 13 to 305 recorded by 12 broadband stations in the kachchh rift has led to estimates of fast axis orientations and splitting times for 118 good measurements the average vector mean of fast axis orientation 86 14 corresponds to the e w axis of the kachchh rift and the delay time 1 6 s is attributed to the 184 km thick upper mantle layer with 4 anisotropy the anisotropic character observed for the kachchh rift kr is comparable to other continental rifts and these are related to the high temperature lattice preferred orientation fabric of olivine inherited from the mantle flows the source of the rift axis parallel anisotropy is traced to the rift parallel flows within the 76 6 km thick lithosphere additionally the rift parallel pockets of partial melts also induce anisotropy within the asthenosphere both these are inherited from the plume lithosphere interaction during the deccan reunion plume episode 65 ma

observations of laterally inhomogeneous anisotropy in the continental lithosphere elastic anisotropy may be an important factor when seismic data are used to determine the structure composition and dynamics of the earth s interior1 2 azimuthal anisotropy has been found in the oceanic and although less definitely in the continental lithosphere from field experiments where the pn wave velocity was observed to vary with direction3 5 these studies suggest a 180 periodicity in travel time variations as the only criterion of the presence of azimuthal anisotropy this criterion however may fail to distinguish between anisotropy and lateral inhomogeneity of the isotropic medium a new method for measuring azimuthal anisotropy has been described recently6 based on the phenomenon of shear wave splitting in anisotropic media which suggests three criteria instead of one to identify azimuthal anisotropy we have now applied this method to the records of permanent seismograph stations in southern germany our results provide strong evidence for the presence of azimuthal anisotropy in the lithosphere of the region and reveal pronounced lateral variations in the parameters of anisotropy on a scale of about 200 km

geochemical evidence for invasion of kilauea s plumbing system by mauna loa magma from the beginning of the study of hawaiian volcanism there has been controversy over possible relationships between the neighbouring active volcanoes mauna loa and kilauea1 5 seismic activity thought to reflect upward migration of magma reveals that the magmatic plumbing systems apparently converge at depth to form a broad funnel within the mantle6 although on rare occasions they have erupted concurrently the brief historical eruptive record appears to show that when kilauea is most active mauna loa is in repose and vice versa suggesting that they may be competing for the same magma supply5 7 petrological geochemical and isotope data however require a diametrically opposite conclusion distinct differences in major element trace element and isotope compositions of lavas are regarded as compelling evidence that the two volcanoes have separate magmatic plumbing systems supplied by parental magmas from physically and geochemically distinct mantle sources8 13 here we present preliminary geochemical data which show that in the past 2 000 years kilauea has erupted a spectrum of lava compositions resembling historical kilauea lavas at one end and mauna loa lavas at the other we discuss the cause of this diversity and speculate that magma from mauna loa may have invaded kilauea s high level magmatic plumbing system

mantle flow and lithosphere asthenosphere coupling beneath the southwestern edge of the north american craton constraints from shear wave splitting measurements abstract high quality broadband seismic data recorded by the usarray and other stations in the southwestern united states provide a unique opportunity to test different models of anisotropy forming mechanisms in the vicinity of a cratonic margin systematic spatial variations of anisotropic characteristics are revealed by 3027 pairs of splitting parameters measured at 547 broadband seismic stations the western and southern edges of the north american craton show edge parallel fast directions with larger than normal splitting times and the continental interior is characterized by smaller splitting times and spatially consistent fast directions that are mostly parallel to the absolute plate motion direction of north america at the majority of the stations no significant systematic azimuthal variations of the splitting parameters are observed suggesting that a single layer of anisotropy with a horizontal axis of symmetry can adequately explain most of the observations the spatial coherency of the splitting parameters indicates that the observed anisotropy is likely caused by shearing between the partially coupled lithosphere and asthenosphere based on previous results of seismic tomography and geodynamic modeling we propose a model involving deflecting of asthenospheric flow by the cratonic root as the cause of the observed edge parallel fast directions and large splitting times along the western and southern edges of the north american craton

on the relation between seismic anisotropy and finite strain because seismic anisotropy in the upper mantle is due primarily to lattice preferred orientation lpo of olivine crystals induced by deformation observations of anisotropy can provide constraints on mantle deformation and flow in this paper we use a previously published theory for deformation induced lpo ribe and yu 1991 to quantify the relation between seismic anisotropy and deformation for two model compositions a pure olivine dunite aggregate and a harzburgite comprising 70 olivine and 30 enstatite analytical and numerical solutions of the governing equations show that the predicted lpo and hence also the seismic anisotropy depends only on the finite strain and not on the deformation path by which that strain is produced the magnitude of the anisotropy depends only on the ratios c1 c2 and c2 c3 where c1 c2 c3 are the lengths of the principle axes of the finite strain ellipsoid and the directions of maximum and minimum compressional wave velocity vp coincide with the axes c1 and c3 respectively we calculate the percent anisotropies for compresional and shear waves propagating along the three principle axes of the finite strain ellipsoid as functions of the axial ratios c1 c2 and c2 c3 for our two model compositions however inferences of the magnitude of finite strain obtained using these results are likely to be minimum estimates because our theory neglects the effects of dynamic recrystallization finally we present as a byproduct of this study a new direct method for calculating the finite strain produced by an arbitiary deformation history

a theory for plastic deformation and textural evolution of olivine polycrystals seismic anisotropy in the upper mantle is due primarily to preferred orientation of olivine crystals induced by progressive deformation in order to understand better the origin of seismic anisotropy we present a simple theory for plastic deformation and textural evolution of olivine polycrystals each crystal in the aggregate is assumed to deform by intracrystalline slip on three major slip systems whose hardnesses and stress exponents are known from experiments the evolving grain orientation distribution in the aggregate is calculated by minimizing the difference between the local crystal deformation and the global aggregate deformation subject to the constraint of global strain compatibility the axial compression texture predicted by our model agrees well to first order with that determined experimentally by nicolas et al 1973 although there are significant discrepancies in the details our results suggest that the orientation texture developed during progressive plane strain deformation is a nearly unique function of the finite strain such that the crystallographic axes 100 010 and 001 are concentrated around the finite strain axes a c and b respectively a b c this result may allow the state of finite strain at depth to be estimated from observations of seismic anisotropy

mantle transition zone discontinuities beneath the baikal rift and adjacent areas like most other major continental rifts the baikal rift zone brz in siberia ispresumably underlain by a hot and partially molten mantle which has a reduced seismicvelocity relative to surrounding areas recent seismic tomography studies however gaveconflicting results about the depth extent and even the existence of the low velocityanomaly beneath the brz suggesting that additional constraints are needed here wepresent results from stacking of about 1700 radial p to s receiver functions from asingle long running seismic station tly located at the sw tip of lake baikal a clearuplift of the 410 km discontinuity d410 with a magnitude of about 47 km relative tothe south margin of the siberian platform is observed beneath the rift currently availableseismic results suggest that the uplift is unlikely to be caused by addition of water tomantle transition zone mtz silicates but is the result of a 550 c reduction in temperaturein the vicinity of the d410 in addition the 660 km discontinuity d660 shows adownward trend toward the rift from the south suggesting that the entire mtz might havea low temperature beneath the rift the thickening of the mtz suggests a high velocityanomaly of about 2 in the mtz and rules out the possibility that the rifting is caused bya mantle plume originated in or beneath the mantle transition zone

model of the transient temperature field of the baikal rift lithosphere abstract in order to coordinate available geophysical data on the deep seated structure with observed heat flow it has to be borne in mind that the thermal field of the baikal rift zone is not in a steady state a two dimensional model of the transient temperature field of the lithosphere was constructed by a numerical method for the solution of the energy conservation equation the authors suggest that asthenospheric upwelling 250 km wide was formed on account of roof block detachment such a mechanical replacement of the lithosphere by the asthenosphere was simulated by a positive compulsory upward movement of the 1200 c isotherm with constant velocity the duration of this movement was taken as long as 30 m y which corresponds to the duration of the first slow rifting stage as soon as the asthenosphere had reached the mohorovicic discontinuity its ascending movement on a wide scale ceased because the density of the crust was less than that of the asthenosphere temperature calculations were carried out in the lithosphere above the 1200 c isotherm this model permits us to reproduce theoretically the present value of the regional heat flow and the curie depth when the time passed from the cessation of the upwelling top movement became equal to 3 5 m y the last figure corresponds to the duration of the second fast stage of rifting on the regional scale the crust of the rift zone sayan baikal domal uplift continues to be heated cooling of the crust can only occur at the segments corresponding to the local heat flow anomalies which are suggested as being produced by dikes the intrusion time of the dike beneath the southern baikal is estimated to be 3 m y

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

the use of laboratory velocity data for estimating temperature and partial melt fraction in the low velocity zone comparison with heat flow and electrical conductivity studies peridotite is widely considered to be the dominant component of the upper most mantle seismic velocities of a dry peridotite determined in the laboratory at high pressure and hypersolidus temperature show a rapid decrease with increasing temperature the strong temperature dependence of velocities can be used to estimate temperature and melt fraction in the low velocity zone of the earth the laboratory results show that the pressure dependence of both velocity and melt fraction appears to be well accounted for by the pressure dependence of the solidus temperature of the peridotite i e homologous temperature dependence this observation allows us to extrapolate laboratory results to higher pressures greater depths with some confidence requiring only a knowledge of the solidus as a function of pressure using the ratios of lithospheric to asthenospheric velocities temperature and melt fraction in the asthenosphere the low velocity zone can be determined from laboratory velocity data examples of thermal structures of the upper mantle beneath oceanic plates are presented we choose three locations of geophysical interest iceland plateau pacific ocean and philippine sea where reliable seismic velocities have been determined in the upper mantle from surface wave studies a large amount of partial melt 5 vol and a higher temperature than the dry peridotite solidus are inferred in 0 5 m y asthenosphere under the iceland plateau and the active marginal basin of the east philippine sea the partial melt zone appears to extend deeper 100 km and to greater ages 20 m y in the pacific ocean region than under the slowly spreading iceland plateau partial melting is not expected in the asthenosphere older than 5 m y under the iceland plateau even though the volume fraction of partial melt is not so large 3 vol beneath the pacific plate the melting zone may extend to a distance of above 2000 km from the ridge many seamounts observed in the pacific ocean may result from this vast region of melting under the plate we suggest a linear relation between the width of melting zone and the plate velocity the laboratory results are also applied to other low velocity zones it is suggested that a velocity drop up to about 6 in the asthenosphere does not necessarily require the existence of partial melt but can be explained by high subsolidus temperature this implies that partial melt may not exist generally but only in limited areas in the low velocity zone comparison of seismic results with theoretical models may overestimate the temperature and melt fraction if the velocity drop at subsolidus temperature is neglected temperature and melt fraction obtained in this study are discussed together with results from heat flow and electrical studies almost the same temperatures as inferred from seismic velocity and a dry peridotite solidus are calculated from heat flow data indicating that the upper mantle under mid ocean ridges may be fairly dry even a water or co2 undersaturated solidus 0 1 wt does not give a temperature consistent with the lithospheric geotherm and laboratory velocity results it is noted that electrical conductivity studies generally yield lower temperatures and larger melt fractions than this study unknown pressure frequency and impurity effects on laboratory conductivities of partially molten mantle rocks are suggested as possible reasons for this discrepancy

seismic anisotropy and mantle flow beneath the baikal rift zone seismic studies have shown that continental rifts such as lake baikal and the great rift valley of east africa are like mid ocean rifts in that they lie above broad regions of asthenospheric upvvarp of much greater extent than the surface expression of rifting1 4 the direction of mantle flow in such regions can be investigated using the seismic anisotropy created by flow induced orientation of mantle olivine crystals5 8 seismic studies of the mid atlantic ridge have revealed upwelling mantle flow beneath the ridge and flow normal to the ridge axis on either side8 10 here we present results from an array of seismic stations across the baikal rift zone in southern siberia the splitting in arrival times of sks seismic waves indicates that the upper mantle beneath the rift zone is anisotropic with the fast direction which reflects the direction of mantle flow being horizontal and normal to the rift axis this suggests that the broad upwarp associated with this continental rift is caused by similar mantle flow to that at mid ocean rifts this may help to elucidate the processes involved in continental rifting

crustal seismic anisotropy beneath shillong plateau assam valley in north east india shear wave splitting analysis using local earthquakes abstract we present crustal anisotropy estimates constrained by shear wave splitting sws analysis using local earthquakes in the shillong plateau and assam valley area north east india ne india region splitting parameters are determined using an automated cross correlation cc method we located 330 earthquakes recorded by 17 broadband seismic stations during 2001 2014 in the study area out of these 330 events seismograms of 163 events are selected for the sws analysis relatively small average delay times 0 039 0 084 s indicate existence of moderate crack density in the crust below the study area it is found that fast polarization directions vary from station to station depending on the regional stress system as well as geological conditions the spatial pattern of crustal anisotropy in the area is controlled mostly by tectonic movement of the indian plate towards ne presence of several e w and n s trending active faults in the area also play an important role on the observed pattern of crustal anisotropy

anisotropic convection with implications for the upper mantle on the basis of polycrystalline theory describing the plasticity in olivine and enstatite the flow in a convection cell has been simulated using a finite element formulation the spatial variations in anisotropic properties are computed from the textures that evolve with the flow a kinematically constrained equilibrium based assumption is used to partition the macroscopic deformation among crystals within an aggregate we model the convection for one specific cell geometry and two sets of boundary conditions a complete map of textures throughout the cell is obtained the textured convection cell is structurally very heterogeneous and textures along streamlines do not correlate with the finite strain the results of the simulations indicate that during up welling a strong texture develops rapidly it convects during spreading and is attenuated during subduction results are compared with features of the upper mantle in our predictions the pattern of preferred orientation during spreading is inclined to the flow coordinates due to deformation by simple shear this is contrary to hess 1964 intuition that 001 slip planes of olivine orient themselves parallel to the flow planes yet the pattern is consistent with natural fabric data significant differences are observed as a function of depth within the cell the variations in the p wave velocities in this textured model mantle are analyzed and correspond well with observed seismic data

seismic anisotropy beneath stable continental interiors a robust knowledge of seismic anisotropy beneath the continents is essential to our understanding of plate tectonic theory as anisotropy provides a unique constraint on the character of past and present deformation in the lithosphere and sublithospheric mantle this review paper summarizes the range of techniques currently available to image seismic anisotropy with passive source seismic data and addresses current issues surrounding the observation and interpretation of continental seismic anisotropy to this end we present case studies for four regions where seismic anisotropy has been extensively investigated in recent years eastern north america the canadian shield australia and southern africa based on this full suite of results we infer that stable continental regions generally contain seismic anisotropy that is clearly located within both the lithosphere and the sublithospheric mantle usually to depths of at least 200 km and perhaps more an implication of these results is that tectonic plates are at most only partially coupled to the underlying mantle the results from these case studies also demonstrate that while remarkable progress in seismic anisotropy imaging has been achieved in recent years it is clear that much more work will be required to adequately understand the origin of continental seismic anisotropy we suggest that a more robust characterization of anisotropic parameters can only be achieved by integrating complementary seismic datasets and by incorporating constraints from key related datasets from mineral physics magnetotellurics gravity and geodesy

shear wave polarization anisotropy in the upper mantle beneath honshu japan shear wave polarization anisotropy in the wedge portion of the upper mantle between a subducting plate and the earth s surface is investigated using three component seismograms of intermediate depth and deep earthquakes recorded at 14 local stations in honshu japan eighty nine high quality seismograms were selected from a period of 3 years the data used in this study are restricted such that incidence angles are smaller than the critical angle of 30 to the earth s surface in order to avoid phase shifts in the shear wave train to find directions of the maximum and minimum velocities in split shear waves where shear waves are resolved into two phases with the maximum time separation each set of the two horizontal component seismograms is rotated in the horizontal plane the split shear waves thus obtained are again recombined after the correction of anisotropy and the anisotropy corrected particle motion is compared with the focal mechanism for a cross check of the observed anisotropy directions of the maximum axes are plotted on azimuthincidence angle stereograms at each station the stereograms and the cross sections of seismic ray paths show that 1 the anisotropic material is distributed at intermediate location between earthquake sources and receiving stations and 2 the anisotropic region is separated into two parts one in the north of the present study area with the polarization of the maximum velocity shear wave trending 0 to 30 from the north north anisotropy and the other in the south with it trending 90 to 120 south anisotropy the maximum time delays between the two shear waves along a vertical seismic ray is about l s for both the anisotropic regions the horizontal extent of the anisotropic area in the north is 50 km at depths of 50 to 150 km perhaps prevalent in west honshu however the depth of anisotropic material is not well constrained because of insufficient coverage of seismic ray paths and angles if we assume the vertical extent of the anisotropic material to be 100 km the maximum velocity contrast would be 4 if we adopt a crack alignment model for the observed anisotropy cracks are inferred to be distributed vertically trending 0 to 30 in the north and 90 to 120 in the south if we assume an olivine alignment model olivine crystals are inferred to be aligned in the north with the a axis of the slip direction trending 0 to 30 and the slip plane normal to the b axis being vertical and in the south anisotropy with the axis trending 90 to 120 and the slip plane being horizontal

seismic anisotropy and mantle dynamics beneath the malawi rift zone east africa sks skks and pks splitting parameters measured at 34 seismic stations that we deployed in the vicinity of the cenozoic malawi rift zone mrz of the east african rift system demonstrate systematic spatial variations with an average splitting time of 1 0 0 3 s the overall ne sw fast orientations are consistent with absolute plate motion apm models of the african plate constructed under the assumption of no net rotation of the global lithosphere and are inconsistent with predicted apm directions from models employing a fixed hotspot reference frame they also depart considerably from the trend of most of the major tectonic features these observations together with the results of anisotropy depth estimation using the spatial coherency of the splitting parameters suggest a mostly asthenospheric origin of the observed azimuthal anisotropy the single layered anisotropy observed at 30 and two layered anisotropy observed at four of the 34 stations can be explained by apm related simple shear within the rheologically transitional layer between the lithosphere and asthenosphere as well as by the horizontal deflection of asthenospheric flow along the southern and western edges of a continental block with relatively thick lithosphere revealed by previous seismic tomography and receiver function investigations this first regional scale shear wave splitting investigation of the mrz suggests the absence of rifting related active mantle upwelling or small scale mantle convection and supports a passive rifting process for the mrz

sks splitting beneath continental rift zones we present measurements of sks splitting at 28 digital seismic stations and 35 analog stations in the baikal rift zone siberia and adjacent areas and at 17 stations in the east african rift in kenya and compare them with previous measurements from the rio grande rift of north america fast directions in the inner region of the baikal rift zone are distributed in two orthogonal directions ne and nw approximately parallel and perpendicular to the ne strike of the rift in the adjacent siberian platform and northern mongolian fold belt only the rift orthogonal fast direction is observed in southcentral mongolia the dominant fast direction changes to rift parallel again although a small number of measurements are still rift orthogonal for the axial zones of the east african and rio grande rifts fast directions are oriented on average nne that is rotated clockwise from the n s trending rift all three rifts are underlain by low velocity upper mantle as determined from teleseismic tomography rift related mantle flow provides a plausible interpretation for the rift orthogonal fast directions the rift parallel fast directions near the rift axes can be interpreted by oriented magmatic cracks in the mantle or small scale mantle convection with rift parallel flow the agreement between stress estimates and corresponding crack orientations lends some weight to the suggestion that the rift parallel fast directions are caused by oriented magmatic cracks

shear wave anisotropy beneath the tibetan plateau eleven broadband digital seismic stations were deployed across the central tibetan plateau in the first extensive passive source experiment attempted within the tibetan plateau one year of recording resulted in 186 event station pairs which we analyze to determine the characteristics of shear wave splitting in the upper mantle beneath the array measurements of the fast polarization direction and delay time t for sks and direct s arrivals reveal systematic variations along the north south oriented array in the north central region of the plateau very large delay times are observed at three stations the largest of which is budo with of t 2 4 s however at tunl which is off the northern edge of the plateau and 110 km from budo and at sites in the south central plateau t decreases by nearly a factor of 3 we also observe systematic rotation of from about 45 ne to 90 e w from south to north along the array a previously identified zone of inefficient sn propagation correlates well with our region of large t observations the large delay times suggest that a relateively high number of anisotropic crystals are preferentially alligned within the mantle lid beneath the north central portion of the tibetan plateau in most cases fast polarization directions appear to be parallel to surface geologic features suggesting as much as 200 km of the upper mantle has been involved in the collisional deformation that has produced the tibetan plateau

shear wave splitting observations in the northern los angeles basin southern california abstract shear wave splitting with 20 to 120 msec time separation is observed on 3 component seismograms from earthquakes occurring at depths of 5 to 18 km in the crystalline basement beneath the los angeles basin shallow events that occur in the basement at the base of the 5 km thick sedimentary section exhibit little splitting while deeper events show progressively greater splitting with depth the polarization direction of the fast shear wave is n s independent of the azimuth of ray path between the event and station we interpret that the shear wave splitting occurs mainly in the crystalline basement and is the result of vertical crustal microcracks aligned in n s direction under the assumption that the alignment of microcracks is due to the subsurface stress regime the results from the shear wave splitting data yield a maximum principal stress direction of n s 15 at depth beneath the northern los angeles basin consistent with results from geological mapping and fault plane solutions ray trace modeling indicates that the observations of shear wave splitting can be explained in terms of an anisotropic basement containing vertical microcracks aligned in the n s direction and with a crack density of 0 04 overlain by a weakly anisotropic sedimentary section with a crack density less than 0 02 during the last 4 yr three major earthquakes occurred in the los angeles basin area they are the m 4 6 and m 4 4 montebello double earthquakes of 1989 the m 5 5 upland earthquake of 1990 and the m 5 8 sierra madre earthquake of 1991 with epicenters of 10 55 and 40 km away from the recorder site scs located in the northern los angeles basin respectively we did not observe significant variations of shear wave splitting at scs before and after the upland and sierra madre earthquakes however we found fairly rapid variations of shear wave splitting following the montebello events the time separation between the split shear waves sharply decreased from 70 msec to less than 20 msec after the mainshock and then returned to the average level gradually this rapid decrease in shear wave splitting may be a temporal variation or a result of location variations of the montebello events that were located approximately where the line singularity is expected

anisotropic structure under a back arc spreading region the taupo volcanic zone new zealand 1 we investigate anisotropy surrounding a continental back arc spreading region the central volcanic region cvr in the north island of new zealand and present a simple method for spatial averaging to examine heterogeneous anisotropy s phases from local earthquakes yield consistent trench parallel fast directions in the southern compressional region but varied directions in the northern extensional region in the forearc east of the cvr is nearly trench parallel suggesting shear in the mantle or trench parallel flow in the western cvr local phases from 50 to 80 km depth give parallel to extension 120 and phases from 150 to 220 km depth yield similar 150 these results suggest asthenospheric flow with olivine a axes oriented in the extension direction down to 100 150 km depth in the eastern cvr local phases show mixed suggesting that it is a transition between the western cvr and the forearc trench parallel for shallow local events within the cvr may be caused by fluid filled cracks trench parallel for deeper local events and for sks phases can be explained by standard olivine fabrics developed by lithospheric shearing by trench parallel flow and by fossil anisotropy in the subducting oceanic lithosphere alternatively in the cvr they could be caused in part by hydrous minerals at the slab mantle upper interface which could cause olivine a axes to orient parallel to the trench due to a maximum shear stress oriented perpendicular to the trench along the slab we suggest that trench perpendicular flow in the extensional region helps to drive fluids away from the slab while in the compressional region trench parallel flow fails to distribute the fluids explaining the location of changes in geophysical properties

global patterns of azimuthal anisotropy and deformations in the continental mantle summary we present a summary of measurements of azimuthal anisotropy in the continental mantle based on the sks technique and performed mostly with the active participation of the authors the directions of polarization of the fast quasi shear wave and the time delays between the quasi shear waves are obtained at nearly 70 locations in all continents except antarctica these data are interpreted in terms of lattice preferred orientation of olivine which is caused by deformations in the mantle the depth interval responsible for anisotropy is unknown but the data suggest that it may reach at least 300 km the fast directions in sks do not show clear correlation with the fast directions of the teleseismic p at the same seismograph stations in the regions of present day convergence the fast direction of anisotropy usually aligns with the plate boundary this correlation implies that the direction of shortening is the same in the crust and the upper mantle in the regions of rifting the inferred direction of mantle flow usually aligns with the direction of extension in the crust outside the regions of recent tectonic activity we most likely observe a combined effect of frozen anisotropy in the subcrustal lithosphere and of recently formed anisotropy in the asthenosphere on a global scale in these regions there is a positive correlation between the absolute plate velocity directions and the fast directions of anisotropy the correlation is especially strong in central and eastern parts of north america a clear absence of any evidence of large scale azimuthal anisotropy in the data of long range refraction profiling for the upper 100 km of the mantle of that region implies that the effect in sks is generated mainly at greater depths in the asthenosphere orientation of olivine at these depths reflects recent and present day flow in the mantle rather than processes of a distant geologic past

shear wave splitting in the rio grande rift fast directions and lag times associated with sks shear wave splitting are determined for six digital stations in the rio grande rift rgr the mean fast direction for three stations in the central rgr is about 45 oblique to the axis of the rgr in contrast fast directions approximately parallel to the axis of the rgr are found for two stations in the southern rgr 10 and one station in the northern rgr 26 the fast directions for southern stations are not aligned with the ne sw extension direction of the oligocene to early miocene stage of extension the significant n s lateral variation in the sks shear wave splitting parameters is difficult to explain by fossil anisotropy because the mantle structure beneath the rgr is underlain by hot material with low seismic wave velocities hence the fast direction of azimuthal anisotropy is probably related to the preferred orientation of olivine created by the present deformation in the upper mantle because none of the fast directions are parallel to the present e w extension direction of the rgr the dominant upper mantle flow direction is not perpendicular to the axis of the rgr the pattern of the fast directions along the rgr provides evidence for a rising asthenosphere beneath the central to southern rgr and suggests that thermally driven small scale convection may be faster for flow parallel to the rift than perpendicular to it

models of lithosphere and asthenosphere anisotropic structure of the yellowstone hot spot from shear wave splitting 1 teleseismic shear wave splitting measured at 56 continuous and temporary seismographs deployed in a 500 km by 600 km area around the yellowstone hot spot indicates that fast anisotropy in the mantle is parallel to the direction of plate motion under most of the array the average split time from all stations of 0 9 s is typical of continental stations there is little evidence for plume induced radial strain suggesting that any contribution of gravitationally spreading plume material is undetectably small with respect to the plate motion velocity two stations within yellowstone have splitting measurements indicating the apparent fast anisotropy direction is nearly perpendicular to plate motion these stations are 30 km from stations with parallel to plate motion the 70 rotation over 30 km suggests a shallow source of anisotropy however split times for these stations are more than 2 s we suggest melt filled stress oriented cracks in the lithosphere are responsible for the anomalous orientations within yellowstone stations southeast of yellowstone have measurements of oriented nnw to wnw at high angles to the plate motion direction the archean lithosphere beneath these stations may have significant anisotropy capable of producing the observed splitting

azimuthal anisotropy in the earth from observations of sks at geoscope and nars broadband stations abstract we have analyzed observations of low frequency sks waves recorded on horizontal components at several broadband stations of the geoscope and nars arrays splitting of sks is consistently observed and in good agreement with a hypothesis of azimuthal anisotropy due to transverse anisotropy with a horizontal axis in the upper mantle beneath the receivers we have designed an inversion method which takes advantage of observations made simultaneously at different azimuths and which permits us to obtain estimates of the direction of fast axis of anisotropy as well as the time delay between the orthogonal quasi shear waves in general the directions of fast velocity obtained both for stations around the atlantic and around the pacific appear to be related to directions of past and present plate motions

geodynamics of the baikal rift zone and plate tectonics of asia the baikal rift zone stretches from the southern termination of lake baikal as far as 118 e and includes the baikal basin and the barguzin upper angara and muysk grabens that form its easterly continuation coincident with the rift zone is a narrow seismic belt which extends further east along the stanovoy range to the okhotsk sea this belt can be considered as the boundary between the eurasian and amurian lithospheric plates earthquake focal mechanism solutions that show a consistent orientation of extensional axes across the strike of the rift zone indicate the behaviour of the eurasian and amurian plates as rigid bodies from 83 focal mechanism solutions the pole of rotation of the eurasian and amurian plates was computed to lie at the point 56 95n 117 45e the angular rate estimated from the amount of baikal opening is about 1 10 7 deg year the position of the pole at the eastern termination of the rift zone accounts for the dying out of the rift zone and for the appearance east of the pole of compressive structures in the stanovoy range to the southwest the baikal rift zone is replaced by the mosaic of microplates in central asia that occupy the space between the eurasian and indian lithospheric plates prominent are the mongolian dzungarian alashan tarim tibetan and pamir microplates earthquakes in central asia are dispersed although higher concentrations of earthquakes associated with microplate boundaries are prominent the tectonics of central asia are considered to be due to the interaction of rigid microplates assuming that plastic deformations occur within the relatively wide zones along the microplate boundaries by considering the strike slip faults to define the trajectories of microplate motions the parameters of relative movements of the microplates have been computed the microplate mosaic of central asia is moving northeasterly as a result of the collision and continued convergence of india and eurasia the mosaic acts as a wedge splitting off the amurian plate from eurasia causing it to rotate counterclockwise so opening the baikal rift

the effect of the large scale mantle flow field on the iceland hotspot track abstract fluid dynamical simulations were carried out in order to investigate the effect of the large scale mantle flow field and the depth of the plume source on the structure of the iceland plume through time the time dependent location and shape of the plume in the earth s mantle was calculated in a global model and it was refined in the upper mantle using a 3d cartesian model box global flow was computed based on density heterogeneities derived from seismic tomography plate motion history served as a velocity boundary condition in both models hotspot tracks of the plume conduits and the plume head were calculated and compared to actual bathymetry of the north atlantic if a plume source in the lowermost mantle is assumed the calculated surface position of the plume conduit has a southward component of motion due to southward flow in the lower mantle depending on tomography model assumed plume age and buoyancy the southward component is more or less dominating plume models having a source at the 660 km discontinuity are only influenced by flow in the upper mantle and transition zone and hence rather yield westward hotspot motion many whole mantle plume models result in a v shaped track which does not match the straight greenland iceland faroe ridge models without strong southward motion such as for a plume source at 660 km depth match actual bathymetry better plume tracks were calculated from both plume conduits and plume heads a plume head of 120 k anomalous temperature gives the best match between plume head track and bathymetry

anisotropy beneath a highly extended continental rift we have employed shear wave splitting techniques to image anisotropy beneath the d entrecasteaux islands in southeastern papua new guinea our results provide a detailed picture of the extending continent that lies immediately ahead of a propagating mid ocean ridge tip we image the transition from continental to oceanic extension a dense shear wave splitting data set from a 2010 to 2011 passive source seismic deployment is analyzed using single and multichannel methods splitting delay times of 1 1 5 s are observed and fast axes of anisotropy trending n s parallel to rifting direction predominate the results this trend is linked to lattice preferred orientation of olivine primarily in the shallow convecting mantle driven by up to 200 km of n s continental extension ahead of the westward propagating woodlark rift this pattern differs from several other continental rifts that evince rift strike parallel fast axes and is evident despite the complex recent tectonic history we contend that across most of this rift the unusually high rate and magnitude of extension has been sufficient to produce a regime change to a mid ocean ridge like mantle fabric stations in the south of our array show more complex splitting that might be related to melt or to complex inherited structure at the edge of the extended region

significant seismic anisotropy beneath the southern lhasa terrane tibetan plateau 1 shear wave splitting measurements using teleseismic pks skks and sks phases recorded by station lsa on the southern part of the lhasa terrane of the tibetan plateau reveal significant azimuthal anisotropy with a splitting time of up to 1 5 s a conclusion that is contradictory to previous studies which suggested isotropy or weak anisotropy in addition systematic variations of the splitting parameters fast polarization direction and splitting time with the arriving azimuth of the seismic ray path are observed suggesting a model of anisotropy that is more complicated than a single layer with horizontal axis of symmetry the measurements are consistent with a model with two layers of anisotropy the top layer has a ne sw fast direction which is consistent with gps revealed direction of surface movement and can be associated with lattice preferred orientation of middle lower crustal minerals such as amphibole the lower layer has a nearly e w fast direction and can be the consequence of either the n s directed compressional stress originated from the india eurasia collision or flow in the asthenosphere related to the absolute motion of eurasia this study underlines the importance of a long duration of deployment of seismic stations in resolving complex anisotropy

asthenospheric flow and origin of volcanism in the baikal rift area the origin of low volume hotspot like volcanism often observed in continental rift areas is debated as is the nature of the flow in the mantle beneath in this paper we assemble seismic constraints on the mantle flow below the baikal rift zone we combine new evidence from upper mantle tomography and from a radially anisotropic shear velocity profile measured beneath and se of the rift with published shear wave splitting constraints on azimuthal anisotropy fast directions of azimuthal anisotropy are perpendicular to the rift on both of its sides stable siberian platform lithosphere on the northern side extends down to 200 km on the southern side the lithosphere is only 60 70 km thick this asymmetry with the thick cratonic root forming a barrier to convective flow inhibits a mid ocean ridge like flow pattern the radially anisotropic profile yields evidence for horizontal flow in the upper asthenosphere 60 100 km se of the rift but no evidence for vertical flow these results are consistent with horizontal asthenospheric flow nw to se perpendicular to the rift from beneath the thick siberian towards the thin baikal mongolian lithosphere when asthenosphere ascends from the 200 km depth beneath the craton to shallower depths beneath the rift decompression melting of embedded veins of enriched rock can produce the hotspot like basaltic volcanism observed the occurrence of such veins of enriched mantle in sub cratonic asthenosphere has been predicted by geochemical mantle models we propose that sub horizontal asthenospheric flow from beneath cratons is a common cause for hotspot like volcanism in continental rift zones

upper mantle anisotropy of the borborema province ne brazil implications for intra plate deformation and sub cratonic asthenospheric flow abstract the geological record of the borborema province northeast brazil documents tectonic events that characterised the precambrian formation and mesozoic breakup of gondwana large scale shear zones and associated granitic plutons that developed during the neoproterozoic brasiliano pan african orogeny and major sedimentary basins of mesozoic age indicate significant deformation across the region however whether or not the shear zones resulted from precambrian terrane accretion or are simply the result of episodes of subsequent intra plate deformation is debated also poorly understood is the effect of the thick sao francisco mantle keel on present day asthenospheric flow to address these issues we have performed a teleseismic shear wave splitting study of mantle seismic anisotropy from a new broadband seismograph network centred on the borborema province shear wave splitting parameters t reveal a lack of plate scale anisotropic fabrics associated within the continental interior perhaps supporting models of formation and evolution of the borborema province involving minimal deformation of the lithospheric mantle delamination of anisotropic lithosphere during the development of cenozoic volcanism that eroded older fossil lithospheric fabrics is unlikely because the widespread cenozoic magmatism required to achieve this is absent in the geological record instead the apparently low levels of seismic anisotropy observed in the interior of the borborema province may simply reflect depth dependent anisotropy nulls low t observations may be the subtractive result of orthogonal fast directions in the lithosphere and asthenosphere towards the brazilian coast t 1 s and fast directions are sub parallel to stretching fabrics formed during the opening of the south atlantic this may imply that the mantle lithosphere was deformed but not completely destroyed during gondwana breakup however a more complete backazimuthal coverage of splitting measurements across the region is needed to confirm or refute these hypotheses unambiguously

teleseismic arrivals at a mid ocean ridge effects of mantle melt and anisotropy recent observational evidence of upwelling mantle anisotropy at a slow spreading center has motivated the modeling of teleseismic arrivals at mid ocean ridges the models consider a variety of types of anisotropy and heterogeneity where the emphasis is to ascertain whether or not travel times can be used to discriminate between the existence of partial melt and anisotropy two mechanisms for anisotropy in the upwelling asthenosphere are considered one due to the preferential alignment of the fast axes of olivine crystals in the direction of mantle flow and the other due to the preferential alignment of cracks that feed melt towards the spreading axis the results indicate that p waves are most sensitive to even modest amounts of flowinduced asthenospheric anisotropy while s waves are most sensitive to the presence of mantle melt multiple s wave arrivals are predicted for many models most notably the ones with anisotropy due to crack alignment where very large s wave separations develop the model which best fits existing data requires a higher degree of crystal alignment anisotropy in the upwelling asthenosphere than in the lithosphere this effect has been predicted in studies of the evolution of crystal alignment anisotropy in polycrystalline aggregates

seismic fracture anisotropy in the earth s crust an overview fluid filled fractures and microcracks are the most stress sensitive component of crustal rock and often determine the pathways and volume of crustal fluid movement if we are to comprehend the role of fractures and fluids in tectonic processes to formulate an accurate hypothesis for phenomena precursory to catastrophic seismic failure or to monitor hydrocarbon and geothermal reservoirs for the presence or absence of major fluid pathways we must understand how seismic waves interact with the fracture crack and microcrack structures within the rock mass a salient feature of fracture related rock structure is the effective seismic anisotropy of aligned fractures and microcracks with the advent of large volume digital seismic data sets it is feasible to use polarized shear waves to explore the anisotropic fracture properties of the crust crustal fracture structures which may be monitored by polarized shear waves range in dimensions from the 10 100 km scale of crustal stress orientation and preseismic stress buildup through the 100 1000 m scale of fluid reservoir structure and characterization and the 1 10 scale of mining and geoengineering rock strain monitoring to millimeter and micrometer or possibly smaller fluid filled inclusions determining the elastic response of the intact rock mass to focus developments in seismic anisotropic studies of crustal fracture phenomena the american geophysical union and the society of exploration geophysicists sponsored a chapman conference on seismic anisotropy in the earth s crust held may 31 to june 4 1988 at the lawrence berkeley laboratory of the university of california this was the third international workshop on seismic anisotropy 3iwsa nineteen conference papers are presented in this special section of the journal of geophysical research

seismic anisotropy of northeastern algeria from shear wave splitting analysis abstract there are few studies of internal deformation under northern africa here we present such a study we analyze teleseismic shear wave splitting for northeast algeria to improve our knowledge of lithospheric and asthenospheric deformation mechanisms in this region we study waveform data generated by tens of teleseismic events recorded at five recently installed broadband bb stations in algeria these stations cover an area 2 across extending from the tellian geological units in the north to the saharan atlas units in the south analysis of sks wave splitting results insignificant spatial variations in fast polarization orientation over a scale length of at most 100 km the seismic anisotropy shows three clear spatial patterns a general ene wsw orientation is observed under the stations in the north this polarization orientation follows the direction of the tell atlas mountain chain which is perpendicular to the convergence direction between africa and eurasia delay times vary significantly across the region between 0 6 and 2 0 s at several stations there is an indication of a wnw ese polarization orientation which is apparently related to a later geodynamic evolutionary phase in this region a third pattern of seismic anisotropy emerges in the south with an orientation of roughly n s we discuss these observations in light of geodynamic models and present day geodetic motion

faults and tectonic stresses of the baikal rift zone abstract the cenozoic baikal rift system consists of a central extensional segment which links a southwestern and a northeastern sinistral shear segment statistical studies on fault length amount and sense of slip depth of crustal penetration and spacing has led to the establishment of empirical relationships which appear to apply also for non extensional regimes results are graphically displayed in figs 4 7 tectonic stress regimes were determined by means of fault and joint analyses and on the basis of seismological data results of both approaches are compatible and explain the evolution of the baikal rift zone in terms of tensional reactivation of pre existing crustal discontinuities

seismic anisotropy beneath the continents probing the depths of geology seismic anisotropy beneath continents is analyzed from shear wave splitting recorded at more than 300 continental seismic stations anisotropy is found to be a ubiquitous property that is due to mantle deformation from past and present orogenic activity the observed coherence with crustal deformation implies that the mantle plays a major if not dominant role in orogenies no evidence is found for a continental asthenospheric decoupling zone suggesting that continents are coupled to general mantle circulation

asymmetric upwarp of the asthenosphere beneath the baikal rift zone siberia in the summer of 1991 we installed 27 seismic stations about lake baikal siberia aimed at obtaining accurately timed digital seismic data to investigate the deep structure and geodynamics of the baikal rift zone and adjacent regions sixty six teleseismic events with high signal to noise ratio were recorded travel time and q analysis of teleseisms characterize an upwarp of the lithosphere asthenosphere boundary under baikal theoretical arrival times were calculated by using the international association of seismology and physics of the earth s interior 1991 earth model and travel time residuals were found by subtracting computed arrival times from observed ones a three dimensional downward projection inversion method is used to invert the p wave velocity structure with constraints from deep seismic sounding data our results suggest that 1 the lithosphere asthenosphere transition upwarps beneath the rift zone 2 the upwarp has an asymmetric shape 3 the velocity contrast is 4 9 in the asthenosphere 4 the density contrast is 0 6 and 5 the p wave attenuation contrast t is 0 1 s

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

shear wave splitting and small scale convection in the continental upper mantle the deformation of the upper mantle beneath a collisional belt is key to the dynamics of the belt but this deformation is difficult to observe seismic azimuthal anisotropy manifested by shear wave splitting provides the best geophysical evidence of deformation in the upper mantle here we use observations of shear wave splitting to investigate deformation in the upper mantle beneath the tien shan in central asia we find that underneath most of the tien shan the fast direction of azimuthal anisotropy is roughly parallel to the axis of the belt but it deviates by nearly 90 where the upper mantle is anomalously hot the former relationship previously observed in other collisional belts seems to be usual for this environment the anomalous directions however provide evidence of a rising plume beneath the range suggesting more generally that the along strike flow of the mantle beneath collisional belts is a reflection of thermally driven convection

deep structure and origin of the baikal rift zone p wave velocity images are determined under the baikal rift zone in siberia by using teleseismic tomography our results show prominent low velocity anomalies in the upper mantle under the baikal rift zone and high velocity anomalies in the lithosphere under the siberian craton the low velocity anomalies are interpreted as a mantle upwelling plume which has played an important role in the initiation and evolution of the baikal rift zone the rift formation may be also controlled by other factors such as older prerift linear lithosphere structures favorably positioned relative to the upwelling and favorable orientation of the far field forces caused by the india asia collision