shear velocity structure radial anisotropy and dynamics of the tibetan crust geophysical and geological data suggest that tibetan middle crust is a partially molten mechanically weak layer but it is debated whether this low viscosity layer is present beneath the entire plateau what its properties are how it deforms and what role it has played in the plateau s evolution broad band seismic surface waves yield resolution in the entire depth range of the tibetan crust and can be used to constrain its shear wave velocity structure indicative of crustal composition temperature and partial melting and radial anisotropy indicative of the patterns of deformation we measured love and rayleigh wave phase velocity curves in broad period ranges up to 7 200 s for a few tens of pairs and groups of stations across tibet combining in each case hundreds of interstation measurements made with cross correlation and waveform inversion methods shear velocity profiles were then determined by extensive series of non linear inversions of the data designed to constrain the depth dependent ranges of isotropic average shear speeds and radial anisotropy shear wave speeds within the tibetan middle crust are anomalously low and also show strong lateral variations across the plateau the lowest mid crustal shear speeds are found in the north and west of the plateau 3 1 3 2 km s 1 within a pronounced low velocity zone in southeastern tibet crustal shear wave speeds increase gradually towards southeast whereas in the north the change across the kunlun fault is relatively sharp the lateral variations of shear speeds within the crust are indicative of those in temperature a mid crustal temperature of 800 c reported previously can account for the low shear velocities across lhasa in the north the temperature is higher and exceeds the solidus resulting in partial melting that we estimate at 3 6 per cent strong radial anisotropy is required by the data in western central tibet 5 per cent but not in northeastern tibet the amplitude of radial anisotropy in the crust does not correlate with isotropic average shear speed and by inference with crustal rock viscosity or with surface elevation instead radial anisotropy is related to the deformation pattern and is the strongest in regions experiencing extension crustal flattening as noted previously the growth of tibet by the addition of indian crustal rocks into its crust from the south is reflected in the higher crustal seismic velocities and thus lower temperatures in the southern compared to northern parts of the plateau more recently added rocks having had less time to undergo radioactive heating within the thickened tibetan crust gravity driven flattening the basic cause of extension and normal faulting in the southern western and central tibet is evidenced by pervasive radial anisotropy in the middle crust beneath the regions undergoing extension the overall eastward flow of the crust is directed by the boundaries and motions of the lithospheric blocks that surround tibet

continental vs oceanic lithosphere beneath the eastern mediterranean sea implications from rayleigh wave dispersion measurements abstract tectonics of the eastern mediterranean are strongly influenced by subduction of oceanic lithosphere with slab pull as a prominent driving force the current distribution of remnants of oceanic lithosphere and the properties of african mantle lithosphere beneath the eastern mediterranean basin have been a matter of debate previous studies variously found continental as well as oceanic lithosphere in this region a set of 1 d s wave velocity models is determined in order to characterize the mantle lithosphere in the area between crete and the middle east for seven paths starting from cyprus and running to crete to anatolia as well as to the middle east broad band average phase velocity curves of the fundamental rayleigh mode are obtained in the frequency range of about 3 mhz to 90 mhz by a two station method in addition an east west oriented path crossing southern anatolia and a path running parallel to the dead sea fault zone are considered phase velocity curves are inverted for path average 1 d s wave velocity models resolution is investigated with the neighbourhood algorithm comparing the resulting 1 d models cold oceanic mantle lithosphere showing s wave velocities above 4 6 km s 1 can be distinguished from mantle lithosphere with low s wave velocities oceanic lithosphere is found west of cyprus beneath the herodotus basin as indicated by benioff zones it is subducting towards northwest in the southeastern aegean and towards northeast beneath southwestern anatolia to the east of cyprus the mantle lithosphere beneath the easternmost mediterranean sea shows low s wave velocities no indications for cold oceanic mantle lithosphere are found beneath the levant basin in the region of the dead sea fault zone and the levant basin the lithosphere asthenosphere boundary is located at shallow depths of about 60 km to 80 km the dsfz is situated in a region of thinned lithosphere underlain by an asthenosphere with anomalous low s wave velocities low s wave velocities beneath southern anatolia extend down to at least 150 km depth as expected for a back arc setting inferred moho depths vary for the different paths between 28 km and 41 km

hydrocarbons in the sediments of port valdez alaska consequences of five years permitted discharge abstract hydrocarbons have been investigated in the sediments of port valdez alaska after three to five years of oil terminal operation with a routine daily discharge of 170 kg of petroleum residue in an otherwise undeveloped area surficial benthic sediments 0 5 cm and core segments down to 30 cm have been analyzed information about total hydrocarbons unresolved complex mixture normal alkanes pristane phytane hopanes and polycyclic aromatic hydrocarbons indicates that petroleum biogenic hydrocarbons and combustion derived aromatic hydrocarbons are present in sediments around the terminal at concentrations up to 218 g g 1 dry sediment the vertical horizontal and temporal distributions of anthropogenic hydrocarbons indicate that most of the sedimentary hydrocarbon accumulation has occurred within 1 km of the terminal a simple calculation suggests that less than 3 of the total oil discharged during routine operations of the terminal has entered the sediments of port valdez for comparison sediment hydrocarbon accumulation associated with a nearby small boat harbor was also examined

complex continental growth along the proto pacific margin of east gondwana we use ambient noise recordings from the largest transportable seismic array in the southern hemisphere to image azimuthal variations in rayleigh wave phase anisotropy in the crust beneath southeast australia this region incorporates a transition from the precambrian shield region of australia in the west to younger phanerozoic terranes in the east which are thought to have been formed by subduction accretion processes our results which span the shallow to lower crust show a strong and consistent pattern of anisotropy that is oriented north south approximately parallel to the former margin of east gondwana however significant deviations from this trend persist through the period range 2 5 s to textgreater10 s one of the most notable deviations occurs along the edge of cratonic australia where the curnamona province forms a salient into the younger accretionary terrane here the fast axis of anisotropy follows the boundary almost exactly and is virtually coincident with magnetic lineations extracted from aeromagnetic data to the east of this boundary beneath the lachlan orogen a region masked by the cenozoic murray basin the fast axis of anisotropy becomes strongly curved and traces out a semicircular pattern with a radius of 200 250 km farther east the fast axis of anisotropy returns to a dominantly north south orientation these new findings provide strong observational support to recent geodynamic modeling results that demonstrate how large scale oroclinal structures can become embedded in accretionary mountain belts 2014 geological society of america

cambrian ordovician orogenesis in himalayan equatorial gondwana an early paleozoic tectonic event the kurgiakh orogeny has long been known from the western tethyan himalaya and it is conspicuously recorded by an angular unconformity between cambrian marine shelf deposits and coarse ordovician conglomerate as well as widespread granitic plutons although the lateral extent of this event is poorly known two regions in the central and eastern himalaya the kumaon of india and klippen of bhutan contain conglomerate units that may be correlative with this event in the kumaon the ralam formation which contains a basal polymictic conglomerate unit with detrital zircon grains as young as ca 512 ma is overlain by sandstone containing the arthropod walking trace diplichnites gouldi an ichnospecies known from ordovician and younger strata the late early cambrian trilobite redlichia sp recovered from the underlying martoli group indicates that the conglomerate is post early cambrian and based on stratigraphic analysis also likely records the kurgiakh orogeny in central bhutan the lower paleozoic succession contains conglomerate units with monomictic quartz sandstone clasts and less mature sandstone matrix these units have also been assigned to a variety of ages but rocks that apparently underlie these strata contain the furongian late cambrian zonal trilobite kaolishania granulosa here we show that detrital zircon ages of a conglomerate clast no grains younger than ca 781 ma are distinctly older than those of the matrix which contains a large peak at 490 500 ma thus these conglomerate units are also potential lateral correlatives of the ordovician conglomerate deposits of the western himalaya our paleontological and geochronologic data suggest that the kumaon and bhutan conglomerate units and possible equivalents in tibet correlate with those of the western himalaya this increases their known along strike extent by over 1000 km bringing the total extent to 1200 km thus the kurgiakh orogeny spanned much of the himalayan orogen along the northern gondwanan margin and reflected a widespread tectonic reorganization of the peri gondwanan realm to andean type margins recent work indicates that it also likely extended into the lhasa block and both the north and south china blocks this event was associated with widespread ca 500 ma arc related plutonism recorded in the himalaya and with abundant detrital zircon grains of that age in younger himalaya strata based on current fossil data the duration of the hiatus associated with the cambrian ordovician boundary which separates pre and postdeformational strata ranges from 22 to 36 m y available geochronological data do not provide greater constraint if the event was close in age to the depositional age of the ordovician molasse then the main phase of tectonic uplift erosion and deposition may have been much later than the presumed cambrian ordovician boundary interval age of ca 485 ma

mapping the moho with seismic surface waves a review resolution analysis and recommended inversion strategies abstract the strong sensitivity of seismic surface waves to the moho is evident from a mere visual inspection of their dispersion curves or waveforms rayleigh and love waves have been used to study the earth s crust since the early days of modern seismology yet strong trade offs between the moho depth and crustal and mantle structure in surface wave inversions prompted doubts regarding their capacity to resolve the moho here we review surface wave studies of the moho with a focus on early work and then use model space mapping to establish the waves sensitivity to the moho depth and the resolution of their inversion for it if seismic wavespeeds within the crust and upper mantle are known then moho depth variations of a few kilometres produce large 1 perturbations in phase velocities however in inversions of surface wave data with no a priori information wavespeeds not known strong moho depth shear speed trade offs will mask 90 of the moho depth signal with remaining phase velocity perturbations 0 1 only in order to resolve the moho with surface waves alone errors in the data must thus be small up to 0 2 for resolving continental moho with larger errors moho depth resolution is not warranted and depends on error distribution with period an effective strategy for the inversion of surface wave data alone for the moho depth is to first constrain the crustal and upper mantle structure by inversion in a broad period range and then determine the moho depth in inversion in a narrow period range most sensitive to it with the first step results used as reference prior information on crustal and mantle structure reduces the trade offs and thus enables resolving the moho depth with noisier data such information should be used whenever available joint analysis or inversion of surface wave and other data receiver functions topography gravity can reduce uncertainties further and facilitate moho mapping

upper mantle seismic wave attenuation effects of realistic partial melt distribution frequency dependence of seismic velocity and attenuation resulting from viscoelastic relaxation of partially molten mantle is estimated we consider the contribution of the melt squirt mechanism through which pressure differences between disk shaped inclusions are equalized by melt passing through connecting tubes the pressure differences arise as a result of shear strain compressing disk shaped pores differently on the basis of disk orientation with respect to the applied shear the frequencies over which the transition from the unrelaxed to the relaxed states occurs are determined by representing the melt as a network of tubes connecting oblate ellipsoidal pores the pressure equalization process is modeled by a system of first order linear differential equations whose eigenvalues are the characteristic frequencies for melt squirt relaxation it is shown that in this framework the set of frequencies is invariant to the absolute scale of the system but is sensitive to melt bulk modulus and viscosity as well as distribution of melt inside pores and conduits use of realistic solid and melt physical properties and pore and conduit geometries demonstrates that it is the relaxed modulus that is most likely excited in the seismic band and that melt mobility has little effect on seismic attenuation some conceivable melt distributions however would result in detectable attenuation in the seismic band in all cases investigated attenuation increases with frequency indicating that melt squirt is not responsible for global upper mantle q observations

upper mantle seismic wave velocity effects of realistic partial melt geometries we investigate seismic wave velocity reduction resulting from the presence of partial melt in the upper mantle the amount of shear and bulk modulus reduction produced by the presence of a connected network of realistically shaped and naturally organized melt inclusions is found using finite element calculations the geometries of the inclusions are taken directly from laboratory experiments of mantle melting with finite element meshes constructed to conform to these shapes the shear and bulk moduli of the composite material are found for both the unrelaxed isolated inclusions and relaxed pressure equalized inclusions cases by assigning appropriate material properties to the fluid modulus reduction from deformation simulations of a solid containing realistically shaped and ellipse shaped melt inclusions quantify the effect of melt pocket cuspateness and melt pocket organization on seismic velocity reduction the three dimensional response is estimated from two dimensional distributions of the melt phase by determining the mode ii and mode iii components of elastic modulus reduction separately and summing their effects in general cuspate and naturally organized melt inclusions cause greater velocity reduction it is shown that v and vs reduction per percent partial melt are at least 3 6 and 7 9 respectively even higher values for velocity reduction are possible above 1 melt fraction if melt exists only in tubules below 1 melt fraction the lower more conservative values of velocity reduction are 070 greater for vp and 84 greater for vs than the analytically determined values for ellipsoidal inclusions somewhat greater effects are possible if nonrandom organization of melt occurs on scales greater than our model

adiabatic temperature profile in the mantle abstract the temperature at the 410 km discontinuity is re evaluated by comparing the depth of the discontinuity with the olivine wadsleyite transition pressure obtained using in situ x ray diffraction experiments by katsura et al 2004a and equation of state eos of mgo by tange et al 2009 tange scale and matsui et al 2000 the newly estimated temperature is 1830 48 k 70 k higher than that by our previous estimation the eoses of the major mantle minerals olivine wadsleyite ringwoodite and perovskite are also recalculated using the tange scale the adiabatic temperature gradient is calculated using the thermal expansion coefficient obtained from these eoses the adiabatic temperature gradient gradually decreases with increasing depth without a phase transition and abruptly increases in association with phase transitions the adiabatic temperature gradients are found to be 04 0 5 and 0 3 k km in the upper and lower parts of the mantle respectively the temperatures at a depth of 200 km the bottom of the mantle transition zone the top of the lower mantle and a depth of 2700 km are found to be 1720 40 2010 40 1980 40 and 2730 50 k the mantle potential temperature is found to be 1610 35 k

topography of the 410 km discontinuity from pp and ss precursors a number of previous studies have confirmed the global existence of a transition zone discontinuity at 410 km depth by aligning large numbers of long period seismograms on a surface reflection phase before stacking discussion of ss and pp precursors from the 410 km discontinuity termed p410p and s410s has largely focused on the long wavelength topography of this discontinuity recent models suggest that the precursor derived topography of the 410 km discontinuity is dominated by low degree spherical harmonics however there remain some discrepancies as to the details of the topography and its relation to mantle processes here we present a new model for 410 topography which is constrained using both p410p and s410s measurements and examine the relationship between our observations of 410 topography and seismic velocity anomalies in the transition zone we find a moderate negative correlation between discontinuity topography and transition zone seismic velocity anomalies at very low spherical harmonic degrees this is consistent with the expected relationships between the position of the olivine phase boundary seismic velocity anomalies and mantle temperatures however when shorter wavelengths are included the relationship becomes more complex in smaller regions beneath northeast asia and the western pacific we observe positive correlations between seismic velocities and discontinuity topography which we interpret as evidence for the effects of chemical heterogeneities on seismic velocities and the 410 km discontinuity our results suggest there is a long wavelength temperature pattern in the mantle however at shorter wavelengths the influence of chemical heterogeneities becomes visible in seismic velocity anomalies and mantle discontinuity topography this separation of the spectra for thermal and chemical heterogeneities suggests that short wavelength chemical heterogeneities can survive in convecting mantle

implications of grain size evolution on the seismic structure of the oceanic upper mantle abstract we construct a 1 d steady state channel flow model for grain size evolution in the oceanic upper mantle using a composite diffusion dislocation creep rheology grain size evolution is calculated assuming that grain size is controlled by a competition between dynamic recrystallization and grain growth applying this grain size evolution model to the oceanic upper mantle we calculate grain size as a function of depth seafloor age and mantle water content the resulting grain size structure is used to predict shear wave velocity v s and seismic quality factor q for a plate age of 60 myr and an olivine water content of 1000 h 10 6 si we find that grain size reaches a minimum of 15 mm at 150 km depth and then increases to 20 30 mm at a depth of 400 km this grain size structure produces a good fit to the low seismic shear wave velocity zone lvz in oceanic upper mantle observed by surface wave studies assuming that the influence of hydrogen on anelastic behavior is similar to that observed for steady state creep further it predicts a viscosity of 10 19 pa s at 150 km depth and dislocation creep to be the dominant deformation mechanism throughout the oceanic upper mantle consistent with geophysical observations we predict larger grain sizes than proposed in recent studies in which the lvz was explained by a dry mantle and a minimum grain size of 1 mm however we show that for a 1 mm grain size diffusion creep is the dominant deformation mechanism above 100 200 km depth inconsistent with abundant observations of seismic anisotropy from surface wave studies we therefore conclude that a combination of grain size evolution and a hydrated upper mantle is the most likely explanation for both the isotropic and anisotropic seismic structure of the oceanic upper mantle our results also suggest that melt extraction from the mantle will be significantly more efficient than predicted in previous modeling studies that assumed grain sizes of 1 mm

hot upper mantle beneath the tristan da cunha hotspot from probabilistic rayleigh wave inversion and petrological modeling understanding the enigmatic intraplate volcanism in the tristan da cunha region requires knowledge of the temperature of the lithosphere and asthenosphere beneath it we measured phasevelocity curves of rayleigh waves using cross correlation of teleseismic seismograms from an array of ocean bottom seismometers around tristan constrained a region average shear velocity structure and inferred the temperature of the lithosphere and asthenosphere beneath the hotspot the ocean bottom data set presented some challenges which required data processing and measurement approaches different from those tuned for land based arrays of stations having derived a robust phase velocity curve for the tristan area we inverted it for a shear wave velocity profile using a probabilistic markov chain monte carlo approach the model shows a pronounced low velocity anomaly from 70 to at least 120 km depth vs in the low velocity zone is 4 1 4 2 km s not as low as reported for hawaii 4 0 km s which probably indicates a less pronounced thermal anomaly and possibly less partial melting petrological modeling shows that the seismic and bathymetry data are consistent with a moderately hot mantle mantle potential temperature of 1 410 1 4308c an excess of about 50 1208c compared to the global average and a melt fraction smaller than 1 both purely seismic inversions and petrological modeling indicate a lithospheric thickness of 65 70 km consistent with recent estimates from receiver functions the presence of warmer than average asthenosphere beneath tristan is consistent with a hot upwelling plume from the deep mantle however the excess temperature we determine is smaller than that reported for some other major hotspots in particular hawaii

anisotropic rayleigh wave tomography of ireland s crust implications for crustal accretion and evolution within the caledonian orogen 1 the irish landmass now at the western extremity of the eurasian plate was formed in the caledonian orogeny during the palaeozoic assembly of pangea the associated closure of the iapetus ocean is recorded in the ne sw structural trends that dominate the tectonic set up of ireland today the deep crustal dynamics of the orogeny and the effect on the crust of the subsequent extension and magmatism in the north atlantic are debated fabrics within deep crustal rocks preserve a record of deformation during and after the continental collisions here we measured rayleigh wave phase velocities using seismograms recorded by permanent and temporary intermediate band stations in ireland and inverted the data for phase velocity maps including azimuthal anisotropy the observed isotropic phase velocity heterogeneity reflects moderate crustal thickness and seismic velocity variations across ireland anisotropy of rayleigh waves at 10 20 s periods shows a ne sw fast propagation direction and is largest up to 2 at a 15 s period at which rayleigh waves sample primarily the middle and lower crust the ne sw trend of the deep crustal anisotropic fabric is parallel to tectonic trends in particular the iapetus suture zone which indicates that suture parallel flow in the middle and lower crust accommodated the continental collision the apparent preservation of the caledonian age fabric also shows that the deep crust of the eurasian margin in ireland was neither stretched by the nw se extension associated with the opening of the north atlantic nor modified significantly by the cenozoic magmatism in the region

thickening of young pacific lithosphere from high resolution rayleigh wave tomography a test of the conductive cooling model direct seismic measurements of the thickening of oceanic lithosphere away from the spreading axis are rare due to the remoteness of mid ocean ridges on the east pacific rise at 17 s there were two long term broadband ocean bottom seismometer deployments the melt and glimpse experiments these arrays spanned young pacific plate seafloor ranging in age from 0 to 8 ma combining these two data sets we describe the increase in rayleigh wave phase velocities for 16 100 s period as function of distance from the ridge with two parameterizations an arbitrary function of seafloor age and a simple polynomial function of age on the pacific and nazca plates although resolution analysis shows that 10 to 15 independent pieces of information about the age variation of phase velocity on the pacific plate can be resolved at periods of 50 s the three parameter polynomial model fits the data almost as well indicating a simple pattern of the evolution of structure with age to compare our observations to the predictions for the conductive cooling of the lithosphere we convert a thermal half space cooling model to shear velocity using anharmonic and anelastic contributions the patterns in the velocities are not consistent with simple conductive cooling the conductive cooling model under predicts the changes in phase velocity with age observed in the 20 to 78 s period range we observe significant changes in shear velocity in the asthenosphere at depths greater than conductive cooling should extend the asthenospheric low velocity zone is asymmetric dipping to the west with the lowest velocities beneath the pacific plate west of the spreading center the conductive cooling model also over predicts the shear velocities in the lithosphere by 0 2 km s these anomalies indicate that more complicated mantle flow and significant mantle heterogeneities such as melt are required

rayleigh wave phase velocities in the atlantic upper mantle phase velocity in the period range 30 130 s is measured for approximately 10 000 fundamental mode rayleigh waves traversing the atlantic basin in order to isolate the signal of the oceanic upper mantle paths with 30 of their length through continental upper mantle are excluded the lateral distribution of rayleigh wave phase velocity in the atlantic upper mantle is explored with two approaches one phase velocity is allowed to vary only as a function of seafloor age two a general two dimensional parameterization is utilized in order to capture perturbations to age dependent structure in both scenarios phase velocity shows a strong dependence on seafloor age at all periods with higher velocity associated with older seafloor removing age dependent velocity from the 2 d phase velocity maps highlights areas of anomalously low velocity almost all of which are proximal to locations of hotspot volcanism the age dependent phase velocities for the atlantic are not consistent with a half space cooling model and are best explained by a plate cooling model with thickness of 75 km and mantle temperature of 1400 c in contrast age dependent phase velocities for the pacific basin determined by nishimura and forsyth 1989 can be fit reasonably well by a half space cooling model with mantle temperature approximately 50 c warmer than the atlantic comparison of rayleigh wave phase velocity and fractionation corrected na concentrations in mid ocean ridge basalts erupted at 87 axial ridge segments reveals a positive correlation coefficient that increases with period as expected if along ridge variations in mantle potential temperature are controlling both quantities

tibetan and indian lithospheres in the upper mantle beneath tibet evidence from broadband surface wave dispersion broadband seismic experiments over the last two decades have produced dense data coverage across tibet yet the mechanism of the india asia lithospheric convergence beneath it remains a puzzle with even its basic features debated and with very different end member models advocated today we measured highly accurate rayleigh and love wave phase velocity curves in broad period ranges up to 5 200 s for a few tens of pairs and groups of stations across tibet combining in each case hundreds to thousands of interstation measurements made with cross correlation and waveform inversion methods robust shear velocity profiles were then determined by extensive series of nonlinear inversions of the data designed to constrain the depth dependent ranges of isotropic average shear speeds and radial anisotropy temperature anomalies in the upper mantle were estimated from shear velocities using accurate petrophysical relationships our results reveal strong heterogeneity in the upper mantle beneath tibet very large high velocity anomalies in the upper mantle are consistent with the presence of underthrust beneath southwestern tibet and subducted beneath central and eastern tibet indian lithosphere the corresponding thermal anomalies match those estimated for subducted indian lithosphere in contrast to the indian lithosphere tibetan lithosphere and asthenosphere display low to normal shear speeds tibetan lithosphere is thus warm and thin radial anisotropy in the upper mantle is weak in central and strong in northeastern tibet possibly reflecting asthenospheric flow above the subducting indian lithospheric slab

complex layered deformation within the aegean crust and mantle revealed by seismic anisotropy continental lithosphere can undergo pervasive internal deformation often distributed over broad zones near plate boundaries however because of the paucity of observational constraints on three dimensional movement at depth patterns of flow within the lithosphere remain uncertain endmember models for lithospheric flow invoke deformation localized on faults or deep shear zones or alternatively diffuse viscous fluid like flow here we determine seismic rayleigh wave anisotropy in the crust and mantle of the aegean region an archetypal example of continental deformation our data reveal a complex depth dependent flow pattern within the extending lithosphere beneath the northern aegean sea fast shear wave propagation is in a north south direction within the mantle lithosphere parallel to the extensional component of the current strain rate field in the south central aegean where deformation is weak at present anisotropic fabric in the lower crust runs parallel to the direction of palaeo extension in the miocene the close match of orientations of regional scale anisotropic fabric and the directions of extension during the last significant episodes of deformation implies that at least a large part of the extension in the aegean has been taken up by distributed viscous flow in the lower crust and lithospheric mantle

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

global upper mantle tomography with the automated multimode inversion of surface and s wave forms summary we apply the automated multimode inversion of surface and s wave forms to a large global data set verify the accuracy of the method and assumptions behind it and compute an s vvelocity model of the upper mantle crust 660 km the model is constrained with 51 000 seismograms recorded at 368 permanent and temporary broadband seismic stations structure of the mantle and crust is constrained by waveform information both from the fundamentalmode rayleigh waves periods from 20 to 400 s and from s and multiple s waves higher modes in order to enhance the validity of the path average approximation we implement the automated inversion of surface and s wave forms with a three dimensional 3 d reference model linear equations obtained from the processing of all the seismograms of the data set are inverted for seismic velocity variations also relative to a 3 d reference in this study composed of a 3 d model of the crust and a one dimensional 1 d global average depth profile in the mantle below waveform information is related to shear and compressional velocity structure within approximate waveform sensitivity areas we use two global triangular grids of knots with approximately equal interknot spacing within each a finely spaced grid for integration over sensitivity areas and a rougher spaced one for the model parametrization for the tomographic inversion we use lsqr with horizontal and vertical smoothing and norm damping we invert for isotropic variations in s and p wave velocities but also allow for s wave azimuthal anisotropy in order to minimize errors due to possible mapping of anisotropy into isotropic heterogeneity the lateral resolution of the resulting isotropic upper mantle images is a few hundred kilometres varying with data sampling we validate the imaging technique with a spectral element resolution test inverting a published global synthetic data set computed with the spectral element method using a laterally heterogeneous mantle model we are able to reconstruct the synthetic model accurately this test confirms both the accuracy of the implementation of the method and the validity of the jwkb and path average approximations as applied in it reviewing the tomographic model we observe that low s v velocity anomalies beneath mid ocean ridges and backarc basins extend down to 100 km depth only shallower than according to some previous tomographic models this presents a close match to published estimates of primary melt production depth ranges there in the seismic lithosphere beneath cratons unambiguous high velocity anomalies extend to 200 km pronounced low velocity zones beneath cratonic lithosphere are rare where present south america tanzania they are neighboured by volcanic areas near cratonic boundaries the images of these low velocity zones may indicate hot material possibly of mantle plume origin trapped or spreading beneath the thick cratonic lithosphere

grain size sensitive seismic wave attenuation in polycrystalline olivine 1 in order to investigate the processes responsible for the attenuation of seismic shear waves in the earth s upper mantle four olivine polycrystals ranging in mean grain size d from 3 to 23 m have been fabricated characterized and mechanically tested in torsion at high temperatures and seismic frequencies both the shear modulus which governs the shear wave speed vs and the dissipation of shear strain energy q 1 have been measured as functions of oscillation period to temperature t and for the first time grain size at sufficiently high t all four specimens display similar absorption band viscoelastic behavior adequately represented for 1000 t 1200 or 1300 c and 1 to 100 s by the expression q 1 a tod 1 exp e rt with a 7 5 102 s m 0 26 and e 424 kj mol 1 this mildly grain size sensitive viscoelastic behavior of melt free polycrystalline olivine is attributed to a combination of elastically and diffusionally accommodated grain boundary sliding the latter becoming progressively more important with increasing t and or to extrapolation to the larger mm cm grain sizes expected in the earth s upper mantle yields levels of dissipation comparable with those observed seismologically implying that the same grain size sensitive processes might be responsible for much of the observed seismic wave attenuation the temperature sensitivity of vs is increased substantially by the viscoelastic relaxation allowing the lateral variability of wave speeds to be associated with relatively small temperature perturbations

low seismic velocities below mid ocean ridges attenuation versus melt retention the first comprehensive seismic experiment sampling subridge mantle revealed a pronounced low velocity zone between 40 and 100 km depth below the east pacific rise epr that has been attributed to substantial retained melt fractions of 0 3 2 such high melt fractions are at odds with low melt productivity and high melt mobility inferred from petrology and geochemistry here we evaluate whether seismic attenuation can reconcile subridge seismic structure with low melt fractions we start from a dynamic spreading model which includes melt generation and migration and is converted into seismic structure accounting for temperature pressure composition phase and melt dependent anharmonicity and temperature pressure frequency and hydration dependent anelasticity our models predict a double low velocity zone a shallow approximately triangular region due to dry melting and a low velocity channel between 60 and 150 km depth dominantly controlled by solid state high temperature seismic attenuation in a damp mantle with only a minor contribution of 0 1 melt we test how tomographic inversion influences the imaging of our modeled shear velocity features the epr experiment revealed a double low velocity zone but most tomographic studies would only resolve the deeper velocity minimum experimentally constrained anelasticity formulations produce vsas low as observed and can explain lateral variations in near ridge asthenospheric vs with 100 k temperature variations and or zero to high water content furthermore such qs formulations also reproduce low asthenospheric vs below older oceans and continents from basic lithospheric cooling models although these structures are compatible with global qs images they are more attenuating than permitted by epr data

crustal anisotropy beneath hudson bay from ambient noise tomography evidence for post orogenic lower crustal flow the crust underlying hudson bay canada records a long and complex tectonic history in this study we investigate this region using tomographic inversion based on continuous ambient noise recordings from 37 broadband seismograph stations that encircle hudson bay the ambient noise data were processed to obtain group velocity dispersion measurements from 10 35 s period which were inverted using an algorithm that incorporates the effects of anisotropy this work is among the first in which ambient noise data have been used to investigate azimuthal anisotropy the inversion method uses smoothing and damping to regularize the solution due to the significantly increased number of model parameters relative to the isotropic case we performed a careful analysis for parameter selection to determine whether leakage occurs between isotropic and anisotropic model parameters we observe a robust pattern of anisotropic fast directions in the mid crust that are consistent with large scale tectonic trends based on magnetic anomaly patterns in particular a distinctive double indentor shape for the superior craton is clearly expressed in both data sets this pattern breaks down deeper in the crust suggesting that some degree of lithospheric decoupling in the lower crust such as channel flow occurred during orogenesis given regional evidence for vertically coherent deformation in the crust and underlying mantle we interpret this pattern in the lower crust as a tectonic overprint that post dates the main phase of trans hudson deformation at most levels in the crust we observe a profound change in direction of anisotropic fast direction across an inferred suture beneath hudson bay