anisotropy in the indian ocean upper mantle from rayleigh and love waveform inversion a tomographic method is applied to love and rayleigh wave seismograms in order to address the problem of upper mantle elastic anisotropy in the indian ocean the first step in our approach is a waveform inversion for each path of the study a 1 d depth dependent model compatible with the waveforms of the fundamental mode and several higher modes of love and rayleigh waves is obtained then the models related to the different paths are inverted in order to retrieve 3 d velocity heterogeneities and anisotropy in this paper both the radial anisotropy and the azimuthal anisotropy of s waves are investigated in detail we find a significant radial anisotropy in the uppermost 300 km of the mantle with an overall amplitude smaller than what is found in global studies azimuthal s wave anisotropy is also present the directions of fast s velocities show rather simple patterns from 100 to 300 km depth and strong correlations with the direction of absolute plate motion apm are found in the uppermost 100 km the pattern of fast velocity directions is complex and does not correlate simply with plate motions from 100 to 200 km depth a correlation with apm is found in most oceanic regions the main lack of correlation is located in the vicinity of la reunion and mauritius islands in this region the anisotropy exhibits a complex pattern between 100 and 150 km possibly due to a disturbing effect of the hotspot at 200 km depth and deeper azimuthal anisotropy vanishes progressively beneath most of the oceanic regions no significant azimuthal anisotropy is found beyond 300 km depth the depth extent of both azimuthal and radial anisotropy the amplitude of radial anisotropy and the pattern of azimuthal anisotropy support the idea of a preferential orientation of olivine crystals in a low viscosity zone beneath the lithosphere this region would be located between 100 and 200 km in oceanic areas and possibly deeper between 200 and 300 km in continental areas not well resolved in this study

abrupt change in mantle fabric across northern apennines detected using seismic anisotropy 1 we estimated sks splitting parameters for 18 earthquakes observed over 2 years on a dense linear seismometer array crossing the apennines orogen northern italy an abrupt within 30 km change in fast polarization occurs near the crest of the apennines southwest of the crest fast polarization parallels the apennines strike wnw ese rotating to ew orientation near the tyrrhenian sea northeast of the crest toward the po plain fast polarization is nne ssw roughly normal to the orogen s strike delay times 0 6 2 9s do not show a remarkable pattern splitting parameters beneath the apennines argue against anisotropy restricted to the crust and suggest that the mantle region deformed by apennines slab rollback is spatially limited we hypothesize that the rollback process has been uneven evolving to a contortion of the northern edge of the apennines slab and perhaps incipient detachment lithospheric anisotropy beneath the po plain may also be significant

seismic evidence for sharp lithosphere asthenosphere boundaries of oceanic plates the mobility of the lithosphere over a weaker asthenosphere constitutes the essential element of plate tectonics and thus the understanding of the processes at the lithosphere asthenosphere boundary lab is fundamental to understand how our planet works it is especially so for oceanic plates because their relatively simple creation and evolution should enable easy elucidation of the lab data from borehole broadband ocean bottom seismometers show that the lab beneath the pacific and philippine sea plates is sharp and age dependent the observed large shear wave velocity reduction at the lab requires a partially molten asthenosphere consisting of horizontal melt rich layers embedded in meltless mantle which accounts for the large viscosity contrast at the lab that facilitates horizontal plate motions

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

bundled turbidite deposition in the central pandora trough gulf of papua since last glacial maximum linking sediment nature and accumulation to sea level fluctuations at millennial timescale since last glacial maximum 23 19 ka earth climate warming and deglaciation occurred in two major steps bolling allerod and preboreal interrupted by a short cooling interval referred to as the younger dryas 12 5 11 5 ka b p in this study three cores mv 33 mv 66 and md 40 collected in the central part of pandora trough gulf of papua have been analyzed and they reveal a detailed sedimentary pattern at millennial timescale siliciclastic turbidites disappeared during the bolling allerod and preboreal intervals to systematically reoccur during the younger dryas interval subsequent to the final disappearance of the siliciclastic turbidites a calciturbidite occurred during meltwater pulse 1b the holocene interval was characterized by a lack of siliciclastic turbidites relatively high carbonate content and fine bank derived aragonitic sediment the observed millennial timescale sedimentary variability can be explained by sea level fluctuations during the last glacial maximum siliciclastic turbidites were numerous when the lowstand coastal system was located along the modern shelf edge although they did not occur during the intervals of maximum flooding of the shelf during meltwater pulses 1a and 1b siliciclastic turbidites reappear briefly during the younger dryas an interval when sea level rise slowed stopped or perhaps even fell the timing of the calciturbidite coincides with the first reflooding of eastern fields reef an atoll that remained exposed for most of the glacial stages

global anisotropy in the upper mantle inferred from the regionalization of phase velocities a data set of 2600 paths for rayleigh waves and 2170 paths for love waves was selected in order to resolve azimuthal anisotropy of surface waves in the period range 70 250 s the epicenters were chosen so that the paths provide uniform spatial and azimuthal coverage of earth a synthetic seismogram is computed for each path and compared to the observed seismogram in order to infer the phase velocity along each path to process such a large data set a new tomographic technique is designed a good correlation is found with surface tectonics in the whole period range for love waves and at shorter periods for rayleigh waves the directions of maximum velocities are also well correlated with plate tectonics motions for the 2 azimuthal term of rayleigh waves when the power spectrum of heterogeneities is computed it is found that for love waves the even orders are usually larger than the odd orders for rayleigh waves the degrees 2 and 6 dominate only at long periods suggesting a deep origin for these components

changes in seismic anisotropy shed light on the nature of the gutenberg discontinuity the boundary between the lithosphere and asthenosphere is associated with a platewide high seismic velocity lid overlying lowered velocities consistent with thermal models seismic body waves also intermittently detect a sharp velocity reduction at similar depths the gutenberg g discontinuity which cannot be explained by temperature alone we compared an anisotropic tomography model with detections of the g to evaluate their context and relation to the lithosphere asthenosphere boundary lab we find that the g is primarily associated with vertical changes in azimuthal anisotropy and lies above a thermally controlled lab implying that the two are not equivalent interfaces the origin of the g is a result of frozen in lithospheric structures regional compositional variations of the mantle or dynamically perturbed lab

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

full waveform tomography for radially anisotropic structure new insights into present and past states of the australasian upper mantle article i nfo we combine spectral element simulations and adjoint techniques in a non linear full seismic waveform tomography for the radially anisotropic structure of the australasian upper mantle our method allows us to explain 30 s waveforms in detail and it yields tomographic images with locally 2 lateral resolution in the course of 19 conjugate gradient iterations the total number of exploitable waveforms increased from 2200 to nearly 3000 the final model amsan 19 thus explains data that were not initially included in the inversion this is strong evidence for the effectiveness of the inversion scheme and the physical consistency of the tomographic model amsan 19 confirms long wavelength heterogeneities found in previous studies and it allows us to draw the following inferences concerning the past and present states of the australian upper mantle and the formation of seismic anisotropy 1 small scale neutral to low velocity patches beneath central australia are likely to be related to localised paleozoic intraplate deformation 2 increasing seismic velocities between the moho and 150 km depth are found beneath parts of proterozoic australia suggesting thermochemical variations related to the formation and fragmentation of a centralian superbasin 3 radial anisotropy above 150 km depth reveals a clear ocean continent dichotomy we find strong vshnvsv beneath the coral and tasman seas the anisotropy is strongest at the top of the inferred asthenospheric flow channel where strain is expected to be largest radial anisotropy beneath the continent is weaker but more variable localised patches with vshbvsv appear in accord with small scale intraplate deformation 4 the ocean continent dichotomy disappears gradually between 150 and 250 km depth where the continental lithospheric mantle and the oceanic asthenosphere pass into the underlying convecting mantle 5 significant anisotropy exists below 250 km depth its character can be explained by sublithospheric small scale convection and a change in the dominant glide system of olivine

on the robustness of global radially anisotropic surface wave tomography a number of recent global tomographic studies have modeled three dimensional variations in the parameters of radial anisotropy as yet there is limited agreement among such studies suggesting significant uncertainties in the models which could lead to divergent geodynamical interpretations in this study we assess the robustness of lateral variations in radial anisotropy globally in the upper mantle and in the transition zone to determine the extent to which anisotropic parameters are constrained by a data set of over 10 000 000 fundamental and higher mode surface wave dispersion measurements we carry out inversions for isotropic and radially anisotropic shear wave velocity systematically changing regularization and using three different crustal models to remove the effects of the crust on the data using crustal corrections from different crustal models has an impact on the data fit comparable or larger than that obtained by including lateral variations of radial anisotropy in the modeling moreover the use of crustal corrections from different a priori crustal models may lead to different images of radial anisotropy suggesting divergent geodynamical interpretations this work suggests that the three dimensional determination of global radial anisotropy in the earth s mantle using surface wave dispersion data is still an ongoing experiment

petrological constraints on seismic anisotropy the purpose of this paper is to investigate the correlations between anisotropic parameters for different orientations and mineralogies for realistic mineralogical and petrological models of the upper mantle such correlations make it possible to reduce the number of independent parameters required in seismic modelling of the upper mantle the variation with depth and with degree of random orientation is also investigated it is shown that the anisotropic combinations involved in an equivalent transversely isotropic medium azumithally averaged velocities with a vertical symmetry axis are strongly correlated but the combinations involved in azimuthal anisotropy are more weakly correlated comparison with available seismic data does not yet allow discrimination based on anisotropy between different petrological models it is shown that even if a proportion of random orientation is introduced it should be possible to achieve petrological discrimination from seismic data for some orientation configurations if the inverse problem is well posed i e takes account of the correlations between anisotropic parameters these correlations are incorporated in an inversion process to show that it is possible to explain the dataset used to derive the prem by alternative anisotropic models for the uppermost 220 km

origin of azimuthal seismic anisotropy in oceanic plates and mantle abstract seismic anisotropy is ubiquitous in the earth s mantle but strongest in its thermo mechanical boundary layers azimuthal anisotropy in the oceanic lithosphere and asthenosphere can be imaged by surface waves and should be particularly straightforward to relate to well understood plate kinematics and large scale mantle flow however previous studies have come to mixed conclusions as to the depth extent of the applicability of paleo spreading and mantle flow models of anisotropy and no simple globally valid relationships exist here we show that lattice preferred orientation lpo inferred from mantle flow computations produces a plausible global background model for asthenospheric anisotropy underneath oceanic lithosphere the same is not true for absolute plate motion apm models a 200 km thick layer where the flow model lpo matches observations from tomography lies just below the 1200 c isotherm of a half space cooling model indicating strong temperature dependence of the processes that control the development of azimuthal anisotropy we infer that the depth extent of shear and hence the thickness of a relatively strong oceanic lithosphere can be mapped this way these findings for the background model and ocean basin specific deviations from the half space cooling pattern are found in all of the three recent and independent tomographic models considered further exploration of deviations from the background model may be useful for general studies of oceanic plate formation and dynamics as well as regional scale tectonic analyses

high resolution global phase velocity distributions we present global phase velocity distributions for fundamental mode love and rayleigh waves in the period range 40 to 150 seconds the models expressed in terms of spherical harmonic expansions up to degree and order 40 have beed derived from 28 479 love wave and 33 662 rayleigh wave measurements the measurements were made using an automatic procedure based on non linear waveform inversion we show that the results are characterized by lateral resolving radii between 500 and 850 km a significant reduction from previous work the improved global resolution is largely due to the inclusion of many more major arc measurements the power spectra of the models are much whiter than has previously been found the results show great similarity to the phase velocity distributions derived from the recent a priori model 3 smac in which the primary relevant features are the crustal thickness and crustal velocity distributions the cooling oceanic lithosphere and the thick 300km lithosphere of the continental cratons the results thus confirm these features differences for example in the strengths of the crust ridge and craton signatures should lead to refinement in the thermal and constitutive parameters upon which the a priori model depends

monte carlo analysis of seismic reflections from moho and the w reflector near normal incidence reflections have been used to image the moho and the w reflector structure in the lithosphere offshore northern scotland to determine the impedance variations at these reflectors we use a monte carlo technique which allows incorporation of geologically realistic a priori information as well as an extensive exploration of the model space after testing it on a synthetic data set the method is based on bayesian inversion theory the modeled moho consists of a series of layers with a total thickness of 2 4 0 3 km with an overall positive impedance contrast inversion of the w reflector results in a model of five seven layers with a total thickness of about 3 7 0 6 km and mostly nonpositive impedance contrasts the implied fine scale impedance structure of the moho is consistent with the broader velocity structure determined from previous wide angle reflection refraction profiles however the overall nonpositive impedance contrast at the w reflector requires a structure which is overlain or underlain by a broad increase in velocity in order to match amplitudes of reflected phases observed at large offsets interpreted previously to originate at a similar depth

radial anisotropy in the crust of se tibet and sw china from ambient noise interferometry 1 we use rayleigh and love wave green s functions estimated from ambient seismic noise to study crustal structure and radial anisotropy in the tectonically complex and seismically active region west of the sichuan basin and around the eastern himalaya syntaxis in agreement with previous studies low velocity zones are ubiquitous in the mid lower crust with substantial variations both laterally and vertically discrepancies between 3 ds hear velocity from either rayleigh vsv o r love vsh waves are examined both in view of non uniqueness of tomographic solutions and radial anisotropy low shear wave speed and radial anisotropy with vsh vsv are most prominent in mid lower crust in area northwest to the lijiang muli fault and around the red river and xiaojiang faults this anisotropy could be caused by sub horizontal mica fabric and its association with low velocity zones suggests mica alignment due to flow in deep crustal zones of relatively low mechanical strength citation huang h h yao andr d vanderhilst 2010 radialanisotropyinthecrust of se tibet and sw china from ambient noise interferometry geophys res lett 37 l21310 doi 10 1029 2010gl044981

joint inversion of long period magnetotelluric data and surface wave dispersion curves for anisotropic structure application to data from central germany geophysical datasets sensitive to different physical parameters can be used to improve resolution of earth s internal structure herein we jointly invert long period magnetotelluric mt data and surface wave dispersion curves our approach is based on a joint inversion using a genetic algorithm for a one dimensional 1 d isotropic structure which we extend to 1 d anisotropic media we apply our new anisotropic joint inversion to datasets from central germany demonstrating the capacity of our joint inversion algorithm to establish a 1 d anisotropic model that fits mt and seismic datasets simultaneously and providing new information regarding the deep structure in central germany the lithosphere asthenosphere boundary is found at approx 84 km depth and two main anisotropic layers with coincident most conductive seismic fast axis direction are resolved at lower crustal and asthenospheric depths we also quantify the amount of seismic and electrical anisotropy in the asthenosphere showing an emerging agreement between the two anisotropic coefficients

a three dimensional radially anisotropic model of shear velocity in the whole mantle summary we present a 3 d radially anisotropic s velocity model of the whole mantle saw642an obtained using a large three component surface and body waveform data set and an iterative inversion for structure and source parameters based on non linear asymptotic coupling theory nact the model is parametrized in level 4 spherical splines which have a spacing of 8 the model shows a link between mantle flow and anisotropy in a variety of depth ranges in the uppermost mantle we confirm observations of regions with vsh vsv starting at 80 km under oceanic regions and 200 km under stable continental lithosphere suggesting horizontal flow beneath the lithosphere we also observe a vsv vsh signature at 150 300 km depth beneath major ridge systems with amplitude correlated with spreading rate for fast spreading segments in the transition zone 400 700 km depth regions of subducted slab material are associated with vsv vsh while the ridge signal decreases whilethemid mantlehasloweramplitudeanisotropy vsv in the lowermost 300 km which appears to be a robust conclusion despite an error in our previous paper which has been corrected here the 3 d deviations from this signature are associated with the large scale low velocity superplumes underthecentralpacificandafrica suggestingthatvsh vsv isgeneratedinthepredominant horizontal flow of a mechanical boundary layer with a change in signature related to transition to upwelling at the superplumes

waveform tomography reveals channeled flow at the base of the oceanic asthenosphere understanding the relationship between different scales of convection that drive plate motions and hotspot volcanism still eludes geophysicists using full waveform seismic tomography we imaged a pattern of horizontally elongated bands of low shear velocity most prominent between 200 and 350 kilometers depth which extends below the well developed low velocity zone these quasi periodic fingerlike structures of wavelength 2000 kilometers align parallel to the direction of absolute plate motion for thousands of kilometers below 400 kilometers depth velocity structure is organized into fewer undulating but vertically coherent low velocity plumelike features which appear rooted in the lower mantle this suggests the presence of a dynamic interplay between plate driven flow in the low velocity zone and active influx of low rigidity material from deep mantle sources deflected horizontally beneath the moving top boundary layer

depth variable crustal anisotropy patterns of crustal weakness and destructive earthquakes in canterbury new zealand abstract low strain rate areas of the earth are often host to long recurrence but damaging earthquake cycles in many cases these events occur on reactivated and previously unrecognized faults noise based imaging of seismic anisotropy is capable of revealing the seismic fabric of inherited structures as well as the preferred orientation of pervasive cracking in the upper crust both features with a potential relationship to failure on faults causing catastrophic earthquakes by understanding the orientation of seismic weaknesses seismic hazard in areas of low strain rate can be better understood the geometric relation of crustal anisotropy and the 3 d crustal stress tensor has the potential to qualitatively inform us of the location and or orientation of large crustal earthquakes in regions with little previously recorded seismicity but known tectonic loading directions in this study noise cross correlation techniques were used to measure surface wave dispersion these measurements were inverted to solve for azimuthal anisotropy of fundamental mode rayleigh waves in the canterbury region of the south island of new zealand the results of passive imaging show a distinct difference in magnitude and azimuth of surface wave anisotropy at different depths within the crust across the region we suggest that the approximately east west fast axis orientation at upper crustal depths reflects the cretaceous faulting of the impacting chatham rise and the approximately northeast southwest fast axis orientation at lower crustal depths reflects the present plate boundary strain direction the upper crust fast axis parallels the surface rupturing greendale fault which gave rise to the ongoing destructive canterbury earthquake sequence we suggest that the upper crust azimuthal anisotropy measured using ambient noise is capable of revealing dominant patterns of crustal weaknesses in regions like canterbury which are prone to low recurrence but highly damaging earthquakes

seismic evidence for stratification in composition and anisotropic fabric within the thick lithosphere of kalahari craton 1 based on joint consideration of s receiver functions and surface wave anisotropy we present evidence for the existence of a thick and layered lithosphere beneath the kalahari craton our results show that frozen in anisotropy and compositional changes can generate sharp mid lithospheric discontinuities mld at depths of 85 and 150 200 km respectively we found that a 50 km thick anisotropic layer containing 3 s wave anisotropy and with a fast velocity axis different from that in the layer beneath can account for the first mld at about 85 km depth significant correlation between the depths of an apparent boundary separating the depleted and metasomatised lithosphere as inferred from chemical tomography and those of our second mld led us to characterize it as a compositional boundary most likely due to the modification of the cratonic mantle lithosphere by magma infiltration the deepening of this boundary from 150 to 200 km is spatially correlated with the surficial expression of the thabazimbi murchison lineament tml implying that the tml isolates the lithosphere of the limpopo terrane from that of the ancient kaapvaal terrane the largest velocity contrast 3 6 4 7 is observed at a boundary located at depths of 260 280 km beneath the archean domains and the older proterozoic belt this boundary most likely represents the lithosphere asthenosphere boundary which shallows to about 200 km beneath the younger proterozoic belt thus the kalahari lithosphere may have survived multiple episodes of intense magmatism and collisional rifting during the billions of years of its history which left their imprint in its internal layering

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

seismic velocities and anisotropy of the lower continental crust a review seismic anisotropy is often neglected in seismic studies of the earth s crust since anisotropy is a common property of many typically deep crustal rocks its potential contribution to solving questions of the deep crust is evaluated the anisotropic seismic velocities obtained from laboratory measurements can be verified by computations based on the elastic constants and on numerical data pertaining to the texture of rock forming minerals for typical lower crustal rocks the influence of layering is significantly less important than the influence of rock texture surprisingly most natural lower crustal rocks show a hexagonal type of anisotropy maximum anisotropy is observed for rocks with a high content of aligned mica it seems possible to distinguish between layered intrusives and metasediments on the basis of in situ measurements of anisotropy which can thus be used to validate different scenarios of crustal evolution

3d shear velocity structure beneath the gulf of california from rayleigh wave dispersion active extension in the gulf of california is characterized by the transition from continental rifting to seafloor spreading puzzling variations in the patterns of both tectonics and magmatism are observed along the length of the gulf and are likely to be related to mantle heterogeneity regional scale mantle structure however has been difficult to constrain due to the lack of broadband seismic stations in the region in this study we utilized new data from the deployment of the nars baja array and other networks and computed a three dimensional shear speed model of the upper mantle beneath the region applying a combination of cross correlation analysis and multimode waveform inversion we measured interstation rayleigh wave dispersion for 450 pairs of stations in a broad period range of 9 250 s we computed phase velocity maps and then inverted the phase velocity data for shear speed structure our results suggest that the location of the transition from seafloor spreading south to continental rifting north in the gulf of california as well as differences in volcanism across baja california and the gulf can be explained by heterogeneity in the upper mantle in particular by the presence of a slab remnant beneath the south central part and an absence of such a slab remnant beneath the northern part of the gulf

seismic anisotropy beneath the northern apennines italy mantle flow or lithosphere fabric shear wave splitting estimates from recordings of 10 portable seismographic stations during the first year of the retreat seismic deployment in combination with broadband data from the italian national seismic network are associated with seismic anisotropy within the upper mantle beneath the northern apennines anisotropic parameters derived from both shear wave splitting and p travel time residuals vary geographically and depend on event back azimuth reflecting complexity in the underlying mantle strain field variations of the splitting time delays and fast polarization seem to exclude a 2 d sublithosphere corner flow associated with the apennines subduction as the main source of the inferred anisotropy the anisotropic signal may be generated by a frozen in fabric of the adriatic and tyrrhenian lithosphere domains or by flow variations induced by episodic and fragmentary slab rollback

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

inferring upper mantle structure by full waveform tomography with the spectral element method summary mapping the elastic and anelastic structure of the earth s mantle is crucial for understanding the temperature composition and dynamics of our planet in the past quarter century global tomography based on ray theory and first order perturbation methods has imaged long wavelength elastic velocity heterogeneities of the earth s mantle however the approximate techniques upon which global tomographers have traditionally relied become inadequate when dealing with crustal structure as well as short wavelength or large amplitude mantle heterogeneity the spectral element method on the other hand permits accurate calculation of wave propagation through highly heterogeneous structures and is computationally economical when coupled with a normal mode solution and applied to a restricted region of the earth such as the upper mantle sem importantly sem allows a dramatic improvement in accounting for the effects of crustal structure here we develop and apply a new hybrid method of tomography which allows us to leverage the accuracy of sem to model fundamental and higher mode long period 60 s waveforms we then present the first global model of upper mantle velocity and radial anisotropy developed using sem our model semum confirms that the long wavelength mantle structure imaged using approximate semi analytic techniques is robust and representative of the earth s true structure furthermore it reveals structures in the upper mantle that were not clearly seen in previous global tomographic models we show that semum favourably compares to and rivals the resolving power of continental scale studies this new hybrid approach to tomography can be applied to a larger and higher frequency data set in order to gain new insights into the structure of the lower mantle and more robustly map seismic structure at the regional and smaller scales

structure of the european upper mantle revealed by adjoint tomography the structure of the european crust and upper mantle is precisely known only in limited regions a new tomographic model for the entire european upper mantle identifies northeastward subduction of the adria plate beneath the dinarides mountains volcanism related to the upwelling eifel hotspot and mantle delamination beneath scandinavia

three dimensional variations in love and rayleigh wave azimuthal anisotropy for the upper 800 km of the mantle we present a new mantle model yb14shani of azimuthal anisotropy for horizontally polarized shear waves sh in parallel with our previously published vertically polarized shear wave sv anisotropy model yb13svani yb14shani was obtained from higher mode love wave phase velocity maps with sensitivity to anisotropy down to 1200 km depth sh anisotropy is present down to the mantle transition zone mtz with an average amplitude of 2 in the upper 250 km and 1 in the mtz consistent with yb13svani changes in sv and sh anisotropy were found at the top of the mtz where olivine transforms into wadsleyite which might indicate that mtz anisotropy is due to the lattice preferred orientation of anisotropic material beneath oceanic plates sv fast axes become subparallel to the absolute plate motion apm at a depth that marks the location of a thermally controlled lithosphere asthenosphere boundary lab in contrast sh anisotropy does not systematically depend on ocean age moreover while upper mantle sv anisotropy is anomalously high in the middle of the pacific as seen in radial anisotropy models sh anisotropy amplitude remains close to the average for other oceans based on the depth at which sv fast axes and the apm direction begin to align we also found that the average thickness of cratonic roots is 250 km consistent with yuan and romanowicz 2010 for north america here we add new constraints on the nature of the cratonic lab and show that it is characterized by changes in both sv and sh anisotropy 2014 american geophysical union all rights reserved

mapping the upper mantle three dimensional modeling of earth structure by inversion of seismic waveforms a method is presented for the inversion of waveform data for the three dimensional distribution of seismic wave velocities the method is applied to data from the global digital networks international deployment of accelerometers global digital seismograph network the selected data set consists of some 2000 seismograms corresponding to 53 events and 870 paths the moment tensors of the earthquakes are determined through an iterative procedure which minimizes the corrupting influence of lateral heterogeneity a global model is constructed for shear wave velocity expanded up to degree and order 8 in spherical harmonics and described by a cubic polynomial in depth for the upper 670 km of the earth s mantle although no a priori information is incorporated the model predictions reproduce much of what is known about the dispersion of mantle waves for example high phase velocities for shields low velocities at ridges and a strong degree 2 pattern for rayleigh waves since the method makes use of complete waveforms overtone data are also included it is shown that the model is reproducible in that substantially the same model can be constructed from each half of the total data set considered independently the model shows that shields and ridges are major features in the depth interval 25 250 km the ridges of the southern pacific and the larger shields persist to 350 km but the southeast indian rise is underlain by a high velocity anomaly at this depth as is much of the mid atlantic ridge at 450 650 km the major features are a broad region of high velocities incorporating south america much of the south atlantic and parts of west africa a broad region of low velocities in the central and eastern pacific high velocities in the western pacific and a low velocity anomaly beneath the red sea and the gulf of aden in the absence of a crustal correction degrees 2 and 3 show a high positive correlation with the geoid paradoxically this is largely destroyed when the distribution in crustal thickness is taken into account spherical harmonic degrees 4 7 show a significant negative correlation

azimuthal anisotropy beneath southern africa from very broad band surface wave dispersion measurements summary seismic anisotropy within the lithosphere of cratons preserves an important record of their ancient assembly in southern africa anisotropy across the archean kaapvaal craton and limpopo belt has been detected previously by observations of sks wave splitting because sks splitting measurements lack vertical resolution however the depth distribution of anisotropy has remained uncertain end member interpretations invoked the dominance of either anisotropy in the lithosphere due to the fabric formed by deformation in archean or palaeoproterozoic orogenies or that in the asthenosphere due to the fabric formed by the recent plate motion each with significant geodynamic implications to determine the distribution of anisotropy with depth we measured phase velocities of seismic surface waves between stations of the southern african seismic experiment we applied two complementary measurement approaches very broad band cross correlation and multimode waveform inversion robust rayleigh and love wave dispersion curves were derived for four different subregions of the archean southern africa in a period range from 5 s to 250 400 s rayleigh and 5 s to 100 250 s love depending on the region rayleigh wave anisotropy was determined in each region at periods from 5 s to 150 200 s sampling from the upper crust down to the asthenosphere the jackknife method was used to estimate uncertainties and the f test to verify the statistical significance of anisotropy we detected strong anisotropy with a n s fast propagation azimuth in the upper crust of the limpopo belt we attribute it to aligned cracks formed by the regional e w extensional stress associated with the southward propagation of the east african rift our results show that it is possible to estimate regional stress from short period surface wave anisotropy measured in this study using broad band array recordings of teleseismic surface waves rayleigh wave anisotropy at 70 120 s periods shows that the fabric within the deep mantle lithosphere of the limpopo belt and northern kaapvaal craton is aligned parallel to the archean palaeoproterozoic sutures at block boundaries this confirms that the fabric within the lithosphere created by pervasive ancient deformation is preserved to this day suture parallel fabric is absent however in the deep lithosphere of the western kaapvaal craton suggesting that it was not reworked in the collision with the craton s core either due to its mechanical strength or because the deformation mechanism was different from those that operated in the north anisotropy at periods greater than 120 130 s shows fast directions parallel to the plate motion and indicates shear wave anisotropy in the asthenosphere the depth distribution of anisotropy revealed by surface wave measurements comprises elements of both end member models proposed previously anisotropy in the asthenosphere shows fast propagation directions parallel to the plate motion anisotropy in the limpopo and northern kaapvaal lithosphere shows fast directions parallel to the archean palaeoproterozoic sutures the distribution of sks splitting orientations across southern africa reflects anisotropic fabric both within the lithosphere dominating the splitting beneath the limpopo belt and northern kaapvaal craton and within the asthenosphere dominating beneath the western kaapvaal craton

a global shear velocity model of the upper mantle from fundamental and higher rayleigh mode measurements we present dr2012 a global sv wave tomographic model of the upper mantle we use an extension of the automated waveform inversion approach of debayle 1999 which improves our mapping of the transition zone with extraction of fundamental and higher mode information the new approach is fully automated and has been successfully used to match approximately 375 000 rayleigh waveforms for each seismogram we obtain a path average shear velocity and quality factor model and a set of fundamental and higher mode dispersion and attenuation curves we incorporate the resulting set of path average shear velocity models into a tomographic inversion in the uppermost 200 km of the mantle sv wave heterogeneities correlate with surface tectonics the high velocity signature of cratons is slightly shallower approximate to 200 km than in other seismic models thicker continental roots are not required by our data but can be produced by imposing a priori a smoother model in the vertical direction regions deeper than 200 km show no velocity contrasts larger than 1 at large scale except for high velocity slabs within the transition zone comparisons with other seismic models show that current surface wave datasets allow to build consistent models up to degrees 40 in the upper 200 km of the mantle the agreement is poorer in the transition zone and confined to low harmonic degrees 10

large scale waveform inversions of surface waves for lateral heterogeneity 2 application to surface waves in europe and the mediterranean linear surface wave scattering theory is used to reconstruct the lateral heterogeneity under europe and the mediterranean using surface wave data recorded with the network of autonomously recording seismographs nars the waveform inversion of the phase and the amplitude of the direct surface wave leads to a variance reduction of approximately 40 and results in phase velocity maps in the period ranges 30 40 s 40 60 s and 60 100 s a resolution analysis is performed in order to establish the lateral resolution of these inversions using the phase velocity perturbations of the three period bands a two layer model for the s velocity under europe and the mediterranean is constructed the s velocity perturbations in the deepest layer 100 200 km are much more pronounced than in the top layer 0 100 km which confirms that the low velocity zone exhibits pronounced lateral variations in both layers the s velocity is low under the western mediterranean while the s velocity is high under the scandinavian shield in the deepest layer a high s velocity region extends from greece under the adriatic to northern italy several interesting smaller features such as the massif central are reconstructed one of the spectacular features of the reconstructed models is a sharp transition in the layer between 100 and 200 km near the tornquist tesseyre zone this would indicate that there is a sharp transition at depth between central europe and the east european platform the waveform inversion of the surface wave coda leads to good waveform fits but the reconstructed models are chaotic this is due both to a lack of sufficient data for a good imaging of the surface wave energy on the heterogeneities and to an appreciable noise component in the surface wave coda

joint inversion for global isotropic and radially anisotropic mantle structure including crustal thickness perturbations we present a new global whole mantle model of isotropic and radially anisotropic s velocity structure sglobe rani based on 43 000 000 surface wave and 420 000 body wave travel time measurements which is expanded in spherical harmonic basis functions up to degree 35 we incorporate crustal thickness perturbations as model parameters in the inversions to properly consider crustal effects and suppress the leakage of crustal structure into mantle structure this is possible since we utilize short period group velocity data with a period range down to 16 s which are strongly sensitive to the crust the isotropic s velocity model shares common features with previous global s velocity models and shows excellent consistency with several high resolution upper mantle models our anisotropic model also agrees well with previous regional studies anomalous features in our anisotropic model are faster sv velocity anomalies along subduction zones at transition zone depths and faster sh velocity beneath slabs in the lower mantle the derived crustal thickness perturbations also bring potentially important information about the crustal thickness beneath oceanic crusts which has been difficult to constrain due to poor access compared with continental crusts

seismic anisotropy changes across upper mantle phase transitions the mantle transition zone is believed to play an important role in the thermochemical evolution of our planet and in its deep water cycle constraining mantle flow at these depths can help elucidate its nature and better understand mantle dynamics and the history of plate tectonics seismic anisotropy i e the directional dependence of seismic wave velocity provides us with the most direct constraints on mantle deformation its detection below 250 km depth is challenging and it is often assumed that the deep upper mantle is seismically isotropic due to a change in mantle deformation mechanism here we present a global model of azimuthal anisotropy for the top 1000 km of the mantle we used a dataset composed of fundamental and higher mode rayleigh wave phase velocity maps which provides resolution of azimuthal anisotropy to much greater depths than in previous studies our model unravels the presence of significant anisotropy in the transition zone challenging common views of mantle deformation mechanisms and reveals a striking correlation between changes in anisotropy amplitudes and in the fast direction of wave propagation where upper mantle phase transitions occur the newly found relation between anisotropy changes and phase transformations gives new insight on the nature of the mtz and suggests that the anisotropy originates from the lattice preferred orientation of anisotropic material while the interpretation of our results in terms of mantle deformation is not straightforward due to many possible scenarios possible explanations for our findings include recrystallization during phase transformations or changes in the slip system across the mtz boundaries which in turn could be explained by changes in the water content of mantle material consistent with the idea that the transition zone acts as a water filter 2013

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

global azimuthal seismic anisotropy and the unique plate motion deformation of australia differences in the thickness of the high velocity lid underlying continents as imaged by seismic tomography have fuelled a long debate on the origin of the roots of continents1 2 3 4 5 some of these differences may be reconciled by observations of radial anisotropy between 250 and 300 km depth with horizontally polarized shear waves travelling faster than vertically polarized ones2 this azimuthally averaged anisotropy could arise from present day deformation at the base of the plate as has been found for shallower depths beneath ocean basins6 such deformation would also produce significant azimuthal variation owing to the preferred alignment of highly anisotropic minerals7 here we report global observations of surface wave azimuthal anisotropy which indicate that only the continental portion of the australian plate displays significant azimuthal anisotropy and strong correlation with present day plate motion in the depth range 175 300 km beneath other continents azimuthal anisotropy is only weakly correlated with plate motion and its depth location is similar to that found beneath oceans we infer that the fast moving australian plate contains the only continental region with a sufficiently large deformation at its base to be transformed into azimuthal anisotropy simple shear leading to anisotropy with a plunging axis of symmetry may explain the smaller azimuthal anisotropy beneath other continents

s velocity structure and radial anisotropy in the aegean region from surface wave dispersion summary the lithospheric structure of the aegean region is investigated by analysis of rayleigh wave fundamental mode dispersion measurements isotropic 1 d models for almost 100 two station ray paths across the region display distinct variations in the moho depth and crustal s wave velocities the descending slab of the subducting african plate can be resolved down to 120 km depth beneath the volcanic arc three different regions are distinguished in terms of moho depth 1 the forearc with large crustal thicknesses between 38 and 48 km and an average of 43 km 2 the northern aegean with an average moho depth of 28 km and 3 the southern aegean central volcanic arc i e cyclades and sea of crete with an even thinner crust of around 25 km lateral variations in structure between 25 and 55 km depth indicate a marked difference between the western and eastern forearc collocated with pronounced changes in trench and slab geometry as well as published deformation rates s velocities between 25 and 55 km depth are low everywhere beneath the forearc but increase from the peleponnesus to crete an abrupt change occurs between western and central crete in terms of the visibility of the aegean moho and the seismic structure of the lithospheric mantle wedge an aegean mantle wedge with s velocities above 4 4 km s 1 is only observed to the east of central crete whereas to the west velocities of less than 4 0 km s 1 occur down to the plate contact these low velocities above the slab may indicate the presence of a melange of metamorphic rocks at the depths a low velocity asthenospheric layer is observed beneath the sea of crete and the cyclades below 40 km depth between the thinned lithosphere above and the slab below the observed radial anisotropy in the northern part of the aegean is likely to be due to preferred orientation of anisotropic minerals within the lower crust possibly caused by lateral ductile flow associated with recent lithospheric extension

heterogeneity and anisotropy of the lithosphere of se tibet from surface wave array tomography 1 understanding the geotectonic evolution of the southeastern tibetan plateau requires knowledge about the structure of the lithosphere using data from 77 broadband stations in sw china we invert rayleigh wave phase velocity dispersion curves from ambient noise interferometry t 1 0 40 s and teleseismic surface waves t 2 0 150 s for 3 d heterogeneity and azimuthal anisotropy in the lithosphere to 150 km depth our surface wave array tomography reveals 1 deep crustal zones of anomalously low shear wave speed and 2 substantial variations with depth of the pattern of azimuthal anisotropy upper crustal azimuthal anisotropy reveals a curvilinear pattern around the eastern himalayan syntaxis with fast directions generally parallel to the main strike slip faults the mantle pattern of azimuthal anisotropy is different from that in the crust and varies from north to south the tomographically inferred 3 d variation in azimuthal anisotropy helps constrain the source region of shear wave splitting south of 26 n off the high plateau most of the observed splitting can be accounted for by upper mantle anisotropy but for stations on the plateau proper with thick crust crustal anisotropy cannot be ignored on long wavelengths the pattern of azimuthal anisotropy in the crust differs from that in the mantle this is easiest explained if deformation varies with depth the deep crustal zones of low shear wave speed and presumably mechanical strength may represent loci of ductile deformation but their lateral variation suggests that in se tibet localized crustal channel flow and motion along the major strike slip faults are both important

savani a variable resolution whole mantle model of anisotropic shear velocity variations based on multiple data sets we present a tomographic model of radially anisotropic shear velocity variations in the earth s mantle based on a new compilation of previously published data sets and a variable block parameterization adapted to local raypath density we employ ray theoretical sensitivity functions to relate surface wave and body wave data with radially anisotropic velocity perturbations our database includes surface wave phase delays from fundamental modes up to the sixth overtone measured at periods between 25 and 350 s as well as cross correlation traveltimes of major body wave phases before inversion we apply crustal corrections using the crustal model crust2 0 and we account for azimuthal anisotropy in the upper mantle using ray theoretical corrections based on a global model of azimuthal anisotropy while being well correlated with earlier models at long spatial wavelength our preferred solution savani additionally delineates a number of previously unidentified structures due to its improved resolution in areas of dense coverage this is because the density of the inverse grid ranges between 1 25 in well sampled and 5 in poorly sampled regions allowing us to resolve regional structure better than it is typically the case in global s wave tomography our model highlights i a distinct ocean continent anisotropic signature in the uppermost mantle ii an oceanic peak in above average 1 which is shallower than in previous models and thus in better agreement with estimates of lithosphere thickness and iii a long wavelength pattern of 1 associated with the large low shear velocity provinces in the lowermost mantle

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

one dimensional models of shear wave velocity for the eastern mediterranean obtained from the inversion of rayleigh wave phase velocities and tectonic implications summary on a sw ne profile from the libyan coast towards central turkey phase velocity curves of the fundamental rayleigh mode were measured using a two station method the inversion of phase velocity curves yields 1 d models of shear wave velocity down to approximately 200 km depths that may be interpreted as estimates of average models between neighbouring stations on the profile strong lateral variations in the shear wave velocity structure are imaged along the profile the subducted oceanic african mantle lithosphere is indicated in 1 d models for the region around crete by significantly enlarged shear wave velocities it is also imaged by an average model of the structure between stations on crete and santorini on a path crossing the libyan sea south of crete the resulting model is slower than a model expected for 110 myr old oceanic lithosphere the passive african margin is thus assumed to extend northwards beneath the libyan sea anomalous low shear wave velocities are found for the uppermost mantle beneath central turkey down to a depth of approximately 130 km using two stations on crete the average depth of the oceanic moho within the subducting slab is estimated to be at approximately 50 km beneath crete for this arc parallel path an enlarged standard deviation of the measured phase velocities of approximately 0 2 km s 1 between 10 and 30 mhz is observed that is probably caused by strong lateral heterogeneity related to the subducting slab in addition in this frequency range an anomalous propagation of the fundamental rayleigh mode is detected that is indicated by measured phase velocities that are approximately one standard deviation faster than phase velocities expected from a great circle approximation an average shear wave velocity of approximately 3 5 km s 1 is observed above the oceanic moho in order to explain the recent lithospheric structure of the hellenic subduction zone a tectonic model is assumed for the ne sw striking profile considered it is based on the calculated 1 d models tectonic reconstructions and on a model derived from the metamorphic history of rocks exposed on crete the suggested model summarizes the tectonic development at a lithospheric scale starting in the late cretaceous accretion of crustal material of two microcontinents to eurasia is assumed while continuous subduction of the oceanic lithosphere of different ocean basins and possibly of the mantle lithosphere of the microcontinents resulted in a single slab the length of the oceanic lithosphere that was subducted south of crete is estimated to be not greater than approximately 550 km

multimode rayleigh wave inversion for heterogeneity and azimuthal anisotropy of the australian upper mantle summary we present an azimuthally anisotropic 3 d shear wave speed model of the australian upper mantle obtained from the dispersion of fundamental and higher modes of rayleigh waves we compare two tomographic techniques to map path average earth models into a 3 d model for heterogeneity and azimuthal anisotropy method i uses a rectangular surface cell parametrization and depth basis functions that represent independently constrained estimates of radial earth structure it performs an iterative inversion with norm damping and gradient regularization method ii uses a direct inversion of individual depth layers constrained by bayesian assumptions about the model covariance we recall that bayesian inversions and discrete regularization approaches are theoretically equivalent and with a synthetic example we show that they can give similar results the model we present here uses the discrete regularized inversion of independent path constraints of method i on an equal area grid with the exception of westernmost australia we can retrieve structure on length scales of about 250 km laterally and 50 km in the radial direction to within 0 8 per cent for the velocity 20 per cent for the anisotropic magnitude and 20 for its direction on length scales of 1000 km and longer down to about 200 km there is a good correlation between velocity heterogeneity and geologic age at shorter length scales and at depths below 200 km however this relationship breaks down the observed magnitude and direction of maximum anisotropy do not in general appear to be correlated to surface geology the pattern of anisotropy appears to be rather complex in the upper 150 km whereas a smoother pattern of fast axes is obtained at larger depth if some of the deeper directions of anisotropy are aligned with the approximately n s direction of absolute plate motion this correspondence is not everywhere obvious despite the fast 7 cm yr 1 northward motion of the australian plate more research is needed to interpret our observations in terms of continental deformation predictions of sks splitting times and directions an integrated measure of anisotropy are poorly matched by observations of shear wave birefringence

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

intrinsic versus extrinsic seismic anisotropy the radial anisotropy in reference earth models 1 seismic anisotropy usually arises from different mechanisms which include lattice or crystallographic preferred orientation lpo cpo alignment of cracks with or without fluid inclusions fine layering or partial melting this makes the interpretation of anisotropy in terms of intrinsic produced by lpo cpo versus extrinsic produced by other mechanisms properties difficult and nonunique the radial anisotropy in the one dimensional global spherically symmetric reference earth is usually claimed to be intrinsic here we explore whether the radial anisotropy in one dimensional reference earth models including preliminary reference earth model prem and the constrained reference earth model acy400 contains extrinsic anisotropy especially in relation to fine layering we conclude that as well as intrinsic anisotropy extrinsic anisotropy introduced by finely layered models can be considered to explain the lithospheric anisotropy in prem but cannot explain alone its asthenospheric anisotropy we also find that radial anisotropy in model acy400 is mainly intrinsic due to its petrological constraints

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

on the relationship between azimuthal anisotropy from shear wave splitting and surface wave tomography 1 seismic anisotropy provides essential constraints on mantle dynamics and continental evolution one particular question concerns the depth distribution and coherence of azimuthal anisotropy which is key for understanding force transmission between the lithosphere and asthenosphere here we reevaluate the degree of coherence between the predicted shear wave splitting derived from tomographic models of azimuthal anisotropy and that from actual observations of splitting significant differences between the two types of models have been reported and such discrepancies may be due to differences in averaging properties or due to approximations used in previous comparisons we find that elaborate full waveform methods to estimate splitting from tomography yield generally similar results to the more common simplified approaches this validates previous comparisons and structural inversions however full waveform methods may be required for regional studies and they allow exploiting the back azimuthal variations in splitting that are expected for depth variable anisotropy applying our analysis to a global set ofskssplitting measurements and two recent surface wave models of upper mantle azimuthal anisotropy we show that the measures of anisotropy inferred from the two types of data are in substantial agreement provided that the splitting data is spatially averaged so as to bring it to the scale of long wavelength tomographic models and reduce spatial aliasing observed and tomography predicted delay times are significantly correlated and global angular misfits between predicted and actual splits are relatively low regional anisotropy complexity notwithstanding our findings imply that splitting and tomography yield a consistent signal that can be used for geodynamic interpretation

continuous record of microparticle concentration and size distribution in the central greenland ngrip ice core during the last glacial period 1 a novel laser microparticle detector used in conjunction with continuous sample melting has provided a more than 1500 m long record of particle concentration and size distribution of the ngrip ice core covering continuously the period approximately from 9 5 100 kyr before present measurements were at 1 65 m depth resolution corresponding to approximately 35 200 yr particle concentration increased by a factor of 100 in the last glacial maximum lgm compared to the preboreal and sharp variations of concentration occurred synchronously with rapid changes in the 18o temperature proxy the lognormal mode of the volume distribution shows clear systematic variations with smaller modes during warmer climates and coarser modes during colder periods we find 1 7 m diameter during lgm and 1 3 m during the preboreal on timescales below several 100 years and the particle concentration exhibit a certain degree of independence present especially during warm periods when generally is more variable using highly simplifying considerations for atmospheric transport and deposition of particles we infer that 1 the observed changes of in the ice largely reflect changes in the size of airborne particles above the ice sheet and 2 changes of are indicative of changes in long range atmospheric transport time from the observed size changes we estimate shorter transit times by roughly 25 during lgm compared to the preboreal the associated particle concentration increase from more efficient long range transport is estimated to less than one order of magnitude

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

from mantle to crust stretching the mediterranean abstract the origin of forces driving the deformation of the continental crust near subduction zones and especially in backarc regions is debated this work is based on a compilation of sks fast splitting directions that give an image of flow lines in the mantle around the mediterranean subduction zones and a comparison with stretching and shear directions in metamorphic core complexes that show the pattern of deformation at the scale of the middle and lower crusts we find that 1 the two sets of directions are parallel in the three main backarc regions namely the alboran sea the tyrrhenian sea and the aegean sea showing that the lithosphere deforms with the same direction of stretching in the crust and the mantle suggesting that 2 crustal deformation is mainly driven from below by slab retreat and 3 the lithospheric fabric is reset within a few millions of years in backarc environments

abrupt onset of the little ice age triggered by volcanism and sustained by sea ice ocean feedbacks 1 northern hemisphere summer temperatures over the past 8000 years have been paced by the slow decrease in summer insolation resulting from the precession of the equinoxes however the causes of superposed century scale cold summer anomalies of which the little ice age lia is the most extreme remain debated largely because the natural forcings are either weak or in the case of volcanism short lived here we present precisely dated records of ice cap growth from arctic canada and iceland showing that lia summer cold and ice growth began abruptly between 1275 and 1300 ad followed by a substantial intensification 1430 1455 ad intervals of sudden ice growth coincide with two of the most volcanically perturbed half centuries of the past millennium a transient climate model simulation shows that explosive volcanism produces abrupt summer cooling at these times and that cold summers can be maintained by sea ice ocean feedbacks long after volcanic aerosols are removed our results suggest that the onset of the lia can be linked to an unusual 50 year long episode with four large sulfur rich explosive eruptions each with global sulfate loading 60 tg the persistence of cold summers is best explained by consequent sea ice ocean feedbacks during a hemispheric summer insolation minimum large changes in solar irradiance are not required

toward a generalized plate motion reference frame an absolute plate motion apm model is required to address issues such as the thermochemical evolution of earth s mantle all apm models have to rely on indirect inferences including those based on hot spots and seismic anisotropy each with their own set of uncertainties here we explore a seafloor spreading aligned reference frame we show that this reference frame fits azimuthal seismic anisotropy in the uppermost mantle very well the corresponding euler pole is close to those of hot spot reference frames ridge motion minimizing models and geodynamic estimates of net rotation and predicts clear trench motion patterns we conclude that a net rotation pole guided by the spreading aligned model at 64 e 61 s with moderate rotation of 0 2 0 3 myr could indeed represent a standard comprehensive reference frame for present day plate motions with respect to the deep mantle

crustal radial anisotropy across eastern tibet and the western yangtze craton 1 phase velocities across eastern tibet and surrounding regions are mapped using rayleigh 8 65 s and love 8 44 s wave ambient noise tomography based on data from more than 400 program for array seismic studies of the continental lithosphere and chinese earthquake array stations a bayesian monte carlo inversion method is applied to generate 3 d distributions of vsh and vsv in the crust and uppermost mantle from which radial anisotropy and isotropic vs are estimated each distribution is summarized with a mean and standard deviation but is also used to identify highly probable structural attributes which include 1 positive midcrustal radial anisotropy vsh vsv across eastern tibet spatial average 4 8 1 4 that terminates abruptly near the border of the high plateau 2 weaker 1 0 1 4 negative radial anisotropy vsh vsv in the shallow crust mostly in the songpan ganzi terrane 3 negative midcrustal anisotropy 2 8 0 9 in the longmenshan region 4 positive midcrustal radial anisotropy 5 4 1 4 beneath the sichuan basin and 5 low vs in the middle crust 3 427 0 050 km s of eastern tibet midcrustal vs 3 4 km s perhaps consistent with partial melt is highly probable only for three distinct regions the northern songpan ganzi the northern chuandian and part of the qiangtang terranes midcrustal anisotropy provides evidence for sheet silicates micas aligned by deformation with a shallowly dipping foliation plane beneath tibet and the sichuan basin and a steeply dipping or subvertical foliation plane in the longmenshan region near vertical cracks or faults are believed to cause the negative anisotropy in the shallow crust underlying tibet

global radially anisotropic mantle structure from multiple datasets a review current challenges and outlook abstract since the 1960s seismologists have mapped anisotropy in the uppermost mantle the mantle transition zone and the d region when combined with constraints from mineral physics and geodynamics anisotropy provides critical information on the geometry of mantle flow here we review the theory early work recent tomographic models and experimental constraints on radial anisotropy we discuss current challenges in resolving radial anisotropy seismically in particular we show that it is highly beneficial to use multiple datasets in inversions for anisotropy notably short period group velocity data with strong sensitivity to the crust we present a new whole mantle model of radial anisotropy based on surface wave and body wave travel time data along with incorporated moho perturbations our whole mantle model shares common features with previous global models and is consistent with results from several high resolution regional studies

amphibole and lower crustal seismic properties abstract measurements of seismic anisotropy frequently offer insights of the deformation kinematics in situ within the earth in the continental crust seismic anisotropy is commonly ascribed to regionally aligned lattice preferred orientation lpo of mica crystals however determinations of composition suggest that the deep continental crust contains rather little mica in this contribution the role of amphibole a far more plausible constituent especially for deep continental crust of basic composition is assessed a field analogue approach is adopted where the lpo are measured correlated with the strain state of the rocks and used to calculate the seismic properties the seismic properties may be up scaled through the specimen using the modal proportions of the constituent mineral phases to examine the role of composition in controlling the intensity of seismic anisotropy seismic properties are computed using an array of modal compositions so called rock recipes the case history comes from the central block of the lewisian of nw scotland where a regional basic dyke swarm the scourie dykes is locally deformed within amphibolite facies shear zones conditions that are representative of the deep crust shear strain profiles across a 100 m wide shear zone using deflected banding in the host gneisses yield values of 0

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

lithospheric and sublithospheric anisotropy beneath the baltic shield from surface wave array analysis abstract we report measurements of radial and azimuthal anisotropy in the upper mantle beneath southern and central finland which we obtained by array analysis of fundamental mode rayleigh and love waves azimuthally averaged phase velocities were analysed in the period range 15 to 190 s for rayleigh waves and 15 to 100 s for love waves the azimuthal variation of the rayleigh wave phase velocities was obtained in the period range 20 to 100 s the limited depth resolution of fundamental mode surface waves necessitated strong damping constraints in the inversion for anisotropic parameters we investigated the effects of non unicity on the final model by experimenting with varying model geometries the radial anisotropy beneath finland can be explained by a lithosphere at least 200 km thick predominantly 50 by volume composed of olivine crystals having their a axes randomly distributed in the horizontal plane on the contrary the measured lithospheric azimuthal anisotropy is small this can be reconciled with body wave observations made in the area that indicate a complex pattern of rapidly varying anisotropy below 200 250 km depth that is below the petrologic lithosphere as revealed by xenolith analyses conducted in the area the magnitude of the azimuthal anisotropy increases and would be compatible with a mantle containing 15 20 by volume of olivine crystals whose a axes are coherently aligned in the n ne direction the alignment of the a axes is off the direction of present day absolute plate motion in either the no net rotation or hot spot reference frame currently n55 n60 we interpret this mismatch as evidence for a complex convective flow pattern of the mantle beneath the shield which by inference is decoupled from the overlying lithosphere

repeated subaerial exposure of subtidal carbonate platforms triassic northern italy evidence for high frequency sea level oscillations on a 104 year scale triassic carbonate platforms in northern italy ladinian latemar carnian durrenstein norian dolomia principale consist of hundreds of stacked meter scale cycles each made of a subtidal unit with a cap rock that carries vadose cements pisoids and tepee structures these diagenetic caps are interpreted to be due to subaerial exposure of the subtidal sediments the absence of peritidal deposits rules out progradation as the cause of the exposure i e these are not ginsburg autocycles thus the repeated exposure must be the result of relative sea level oscillations which based on the number of cycles and time intervals involved occurred with an average frequency in the 104 year range we postulate a combination of small scale high frequency eustasy and local tectonic movements e g differential subsidence in the norian and infrequent pulses of uplift in the ladinian to explain the cyclicity and its internal variations