no flat wadati benioff zone in the central and southern central andes abstract the wadati benioff zone wbz is an approximate plane defined by earthquakes hypocentres observed in convergent plate boundaries and that usually dips at angles greater than 30 in some areas of the andes where there are gaps in volcanic activity and where heat flow is abnormally low this plane in most studies has nearly horizontal dip at a depth of about 75 100 km and it has been associated to flat subduction of the oceanic lithosphere this situation has been taken as the present day analogue of the laramide orogeny of western north america for which a flat slab episode has been proposed in the past years in this work the observed low heat flow in areas of the andes is assumed to be due to low radiogenic heat generation in geologically old and allochthonous terranes constituting large regions of western south america on the basis of geotherms obtained for areas of ecuador peru chile and argentina and of rheological results describing the partition between brittle and ductile regimes the seismic activity observed both in the lower crust and at depths of about 75 100 km is thoroughly explained at these depths earthquakes occur within the subcontinental upper mantle and then there is no flat wbz associated to subduction of the oceanic lithosphere there is evidence from recent seismological observations that the real wbz lies not horizontally and deeper in the tectonosphere

geotraverses across the swiss alps the alpine chain marks the collision zone between the european and african lithospheric plates the earth s crust below the alps has been thickened by tectonic activity to form a root of crustal material which extends to a depth of approximately 60 km here we present the initial results of three seismic reflection traverses across the alps which show large scale crustal indentations resulting from collision between continental plates as well as the nappeforming style of deformation produced by compression of attenuated ocean crust the data suggest that the gross structural deformation of the collision zone results from large scale crustal indentations the descending european crust is delaminated and deformed by wedges of the african crust protruding northwards below the alps the nappe forming deformation seems to be restricted to the upper 10 20 km of the penninic domain in the core of the alps

on the thermal structure of the southern tibetan crust summary we present a quantitative analysis of the heat flow measurements recently made in southern tibet by francheteau et al the data document a sharp heat flow anomaly of 54 12 mw m 2 the sediment ages now available and reported here show that thermal effects due to sedimentation are negligible three physical effects must be taken into account in the interpretation heat refraction across the complex structures of the tibetan crust erosion in transient thermal conditions and unsteady lateral heat transfer we show that the refraction of heat may be responsible for up to 30 per cent of heat flow variations we derive a new formula for the heat flow erosion correction and show that the correction which is usually applied can be a gross overestimate in thermally perturbed regions accounting for the possible magnitude of heat refraction across the geological features of southern tibet the amplitude of the anomaly could be as large as 70 mw m 2 the anomaly can only be due to a shallow and recent heat source we show that in such a case erosion effects are small less than 10 per cent whatever the erosion rate is and therefore do not affect the above values significantly using both the amplitude and the spatial characteristics of the anomaly we show that the source must be younger than 1 myr and shallower than 10 km this is in good agreement with the results of magnetotelluric soundings carried out in the same area the source could well be as young as 50 000 yr which would make it a likely origin for the widespread geothermal activity observed further north on the plateau these results are evidence that melting processes are active today in the tibetan crust near the yalu zangbo suture zone more than 45 myr after the india asia collision and more than 200 km away from the site of underthrusting of the indian lithosphere

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

mapping the lithosphere asthenosphere boundary through changes in surface wave anisotropy abstract the alignment of olivine crystals is considered as the dominant source of seismic anisotropy in the subcrustal lithosphere and asthenosphere different components of large scale anisotropy can be traced in depth distributions of the radial and azimuthal anisotropy of surface waves we propose a global model of the lithosphere asthenosphere boundary lab as a transition between a frozen in anisotropy in the lithosphere to anisotropy in the sublithospheric mantle related to the present day flow due to different orientations of velocity maxima in the anisotropic subcrustal lithosphere and the asthenosphere the velocity contrast related to the lab can increase in particular directions because of their long wavelengths and horizontal propagation surface waves suffer from poor lateral resolution however surface waves with various wavelengths allow us to map gross features of the lab with a good vertical resolution we estimate depths to the lab to be between 200 and 250 km for the precambrian shields and platforms around 100 km for the phanerozoic continental regions and 40 70 km beneath oceans from the world wide depth distribution of the radial and azimuthal anisotropy of surface waves

oriented grain growth and modification of frozen anisotropy in the lithospheric mantle abstract seismic anisotropy throughout the oceanic lithosphere is often assumed to be generated by fossilized texture formed during deformation at asthenospheric temperatures close to the ridge here we investigate the effect of high temperature and high pressure static annealing on the texture of previously deformed olivine aggregates to simulate residence of deformed peridotite in the lithosphere our experiments indicate that the orientation and magnitude of crystallographic preferred orientation cpo will evolve due to the preferential growth of grains with low dislocation densities these observations suggest that texture and stored elastic strain energy promote a style of grain growth that modifies the cpo of a deformed aggregate we demonstrate that these microstructural changes alter the orientation distributions and magnitudes of seismic wave velocities and anisotropy therefore static annealing may complicate the inference of past deformation kinematics from seismic anisotropy in the lithosphere

global anisotropic phase velocity maps for higher mode love and rayleigh waves summary it is well established that the earth s uppermost mantle is anisotropic but there are no clear observations of anisotropy in the deeper parts of the mantle surface waves are well suited to observe anisotropy since they carry information about both radial and azimuthal anisotropy fundamental mode surface waves for commonly used periods up to 200 s are sensitive to structure in the first few hundred kilometres and therefore do not provide information on anisotropy below higher mode surface waves have sensitivities that extend to and beyond the transition zone and should thus give insight about azimuthal anisotropy at greater depths we have measured higher mode love and rayleigh phase velocities using a model space search approach which provides us with consistent relative uncertainties from measurement to measurement and from mode to mode from these phase velocity measurements we constructed global anisotropic phase velocity maps prior to inversion we determine the optimum relative weighting for anisotropy we present global azimuthal phase velocity maps for higher mode rayleigh waves up to the sixth higher mode and love waves up to the fifth higher mode with corresponding average model uncertainties the anisotropy we derive is robust within the uncertainties for all modes given the ray theoretical sensitivity kernels of rayleigh and love wave modes the source of anisotropy is complex but mainly located in the asthenosphere and deeper our models show a good correspondence with other studies for the fundamental mode but we have been able to achieve higher resolution

imaging pacific lithosphere seismic discontinuities insights from ss precursor modeling oceanic lithosphere provides an ideal location to decipher the nature of the lithosphere asthenosphere system which is vital to our understanding of plate tectonics it is well established that oceanic lithosphere cools thickens and subsides as it ages according to the conductive cooling models yet this simple realization fails to explain various observations for example old oceanic lithosphere does not subside as predicted further precise imaging of the lower boundary of the oceanic lithosphere has proven challenging here we use ss precursors to image the discontinuity structure across the pacific ocean using 24 years of teleseismic data we image a sharp pervasive velocity discontinuity 3 15 drop over 36 my there is no age depth dependence and we image the discontinuity at an average depth of 60 1 5 km the amplitude and sharpness of the boundary suggests a compositional variation and or layered carbonatitic melt may be required to explain our observations rather than temperature alone the strength and pervasiveness of the boundary suggests that it is likely related to the lithosphere asthenosphere boundary an additional deeper discontinuity at 80 120 km depth is imaged intermittently that in most cases likely represents a continuing negative velocity gradient in depth

a bayesian approach to assess the importance of crustal corrections in global anisotropic surface wave tomography several recent studies have demonstrated the importance of crustal corrections when inverting surface wave data to model lateral variations in mantle radial anisotropy it has also been shown that the choice of the prior crustal model to correct the data can strongly influence the anisotropy model and potentially lead to different geodynamic interpretations in comparing tomographic models of radial anisotropy obtained from different crustal corrections these studies did not however determine quantitative model uncertainties nevertheless mantle models resulting from different prior crustal corrections are statistically different only if the posterior model errors stemming from the non uniqueness of the inverse problem are smaller than the effect of the crustal correction itself here we applied a model space search approach to global fundamental and higher mode rayleigh and love wave phase velocity maps to determine reliable quantitative model uncertainties on seismic velocities and radial anisotropy the technique employed enabled us to describe the model space with a posterior probability density function and therefore to test whether models obtained from different crustal corrections are statistically different we thus assessed the significance of the choice of the crustal model by comparing the posterior model errors to the differences in mantle structure resulting from different crustal corrections we tested prior crustal models crust2 0 crust1 0 and 3smac our study shows that the use of prior crustal corrections from different crustal models yields significant discrepancies in mantle velocities around 50 km depth and in radial anisotropy down to 100 km the impact of the crustal correction on radial anisotropy can extend down to 250 km in some locations we found that choosing 3smac instead of the other crustal models has a stronger influence on the mantle model but that crust1 0 and crust2 0 yield statistically identical anisotropy models at all depths except at a few grid cells importantly the effect of the crustal model is most significant in continental regions and not so much beneath oceans which has important consequences for determining the depth of continental roots our results therefore suggest that improving constraints on crustal structure in continents is essential for our understanding of continent formation our work also demonstrates that the prior crustal model does not significantly affect radial anisotropy and velocities at depths greater than 100 km this implies that if geodynamic interpretations of radial anisotropy below 100 km depth were to account for tomographic model uncertainties they would not depend on the choice of the prior crustal model it is therefore important for geodynamicists and seismologists to work in concert and to put effort into determining quantitative tomographic model uncertainties before interpreting the results our results also caution against the use of 3smac to correct surface wave data for studies of the continental lithosphere and suggest that the solid earth community would benefit from putting some efforts toward building a revised 3smac the discrepancies between mantle models built based on 3smac crustal corrections and those based on crust1 0 or crust2 0 should also help shed light on the validity of the geodynamical assumptions made in the construction of models like 3smac

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

toward the reconciliation of seismological and petrological perspectives on oceanic lithosphere heterogeneity the character of the high frequency seismic phases po and so observed after propagation for long distances in the oceanic lithosphere requires the presence of scattering from complex structure in 3 d current models use stochastic representations of seismic structure in the oceanic lithosphere the observations are compatible with quasi laminate features with horizontal correlation length around 10 km and vertical correlation length 0 5 km with a uniform level of about 2 variation through the full thickness of the lithosphere such structures are difficult to explain with petrological models which would favor stronger heterogeneity at the base of the lithosphere associated with underplating from frozen melts petrological evidence mostly points to smaller scale features than suggested by seismology the models from the different fields have been derived independently with various levels of simplification fortunately it is possible to gently modify the seismological model toward stronger basal heterogeneity but there remains a need for some quasi laminate structure throughout the mantle component of the oceanic lithosphere the new models help to bridge the gulf between the different viewpoints but ambiguities remain

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

subduction zones an introduction to comparative subductology abstract the subduction model for trench arc back arc systems was deduced more or less as a logical consequence of sea floor spreading many features in these systems such as thrust type inter plate earthquakes and deep earthquakes along wadati benioff zones are readily explained by the subduction model but other features such as extensional spreading and high heat flow in back arc regions and arc volcanism are difficult to explain an approach to solve these problems may be to recognize the existence of the two basically different modes of subduction one chilean type causing a compressional stress regime in the arc and back arc regions the other mariana type causing a tensional stress regime implications of the two types of the mode of subduction for some tectonic problems such as vertical movement and sediment accretion at trenches are discussed with special reference to the results of recent dsdp active margin drilling in discussing the possible causes of the two modes it is suggested that the large scale thermal and stress regimes in the back arc areas may not be the direct products of simple subduction but of some additional factors involving much greater energy such as mantle flow associated with the motion of major plates finally the possible great importance of various types of collision accretion and erosion at subduction zones is emphasized

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

global crustal thickness from neural network inversion of surface wave data summary we present a neural network approach to invert surface wave data for a global model of crustal thickness with corresponding uncertainties we model the a posteriori probability distribution of moho depth as a mixture of gaussians and let the various parameters of the mixture model be given by the outputs of a conventional neural network we show how such a network can be trained on a set of random samples to give a continuous approximation to the inverse relation in a compact and computationally efficient form the trained networks are applied to real data consisting of fundamental mode love and rayleigh phase and group velocity maps for each inversion performed on a 2 2 grid globally we obtain the a posteriori probability distribution of moho depth from this distribution any desired statistic such as mean and variance can be computed the obtained results are compared with current knowledge of crustal structure generally our results are in good agreement with other crustal models however in certain regions such as central africa and the backarc of the rocky mountains we observe a thinner crust than the other models propose we also see evidence for thickening of oceanic crust with increasing age in applications characterized by repeated inversion of similar data the neural network approach proves to be very efficient in particular the speed of the individual inversions and the possibility of modelling the whole a posteriori probability distribution of the model parameters make neural networks a promising tool in seismic tomography

deformation mechanisms and flow regimes in limestones from the helvetic zone of the swiss alps abstract this paper is based on a combined field transmission electron tem and transmission optical tom microscope study of limestones from the helvetic zone swiss alps and discusses the deformation mechanisms and flow regimes that governed the deformation of these rocks during pre metamorphic regional ductile deformations the limestones deformed by power law dislocation creep with differential stresses probably not exceeding 1 kbar dynamic recrystallization with grain boundary sliding and grain boundary migration allowed the grains to be less elliptical than the strain ellipse a characteristic of the structure is the existence of dislocation free subgrains in the footwall of and approaching the lochseiten calc mylonite along the glarus overthrust grain boundary sliding becomes more important shift to diffusional creep or superplastic flow during a syn and post metamorphic deformation dynamic recovery seems to have become less competitive no dislocation free sub grains and along thrust faults twinning indicates a shift to higher differential stresses at the close of the deformation it was not possible to separate these deformation phases on the basis of the dislocation debris sub grain sizes as observed in tem and tom were identical in limestones that underwent cataclastic deformation the rocks seem to have started breaking up along the grain boundaries the new grain fragments are very small 0 1 0 3 m and are heavily twinned in tem the old large grains show very long straight glide dislocations cleavage and when shattered ring patterns in diffraction

the kefalonia transform fault a step fault in the making abstract vertical edges along subducted slabs have been recognized in the majority of subduction zones surprisingly slab edges evolved into subduction transform edge propagator step faults in only a few regions the conditions under which steps form are special it is relevant to constrain the conditions that facilitate step fault initiation because they leave a clear geological footprint in the overriding plate whereas vertical tears generally do not we therefore study a candidate region for step fault initiation in the western hellenic subduction zone we investigate the structure and seismicity of the shallow western hellenic subduction zone using a recent full waveform inversion model which both captures details of crustal and upper mantle structure yielding constraints in the depth interval from 10 to 200 km where lithosphere mantle interactions have tectonic expressions the western end of the hellenic subduction zone is fragmented near the kefalonia transform fault we identify a separate epirus lithospheric fragment that is roughly vertical below the southern albanides we also identify a new and major contrast within the lithospheric mantle of the ionian ocean basin which aligns with a gradient in free air anomalies in the overriding plate the kefalonia transform fault zone accommodates right lateral strike slip deformation we interpret this fault zone as a proto step fault that formed simultaneously with pliocene fragmentation of the epirus fragment comparing the recent evolution of the nw hellenic slab edge with currently active steps indicates that along trench variations in convergence velocity are a prerequisite for step fault initiation such velocity variations may result from subduction of continental crust along part of the trench resistance to sea ward tear propagation by a mechanically strong subducting plate may prevent variations in convergence velocity to occur and thus step fault initiation the amount of time over which a velocity contrast persists will also be relevant for step fault initiation mechanical coupling between upper and lower plate and the deformability of the upper plate appear to also play a role in the initiation of the step fault once a slab is fragmented

digital isochrons of the world s ocean floor we have created a digital age grid of the ocean floor with a grid node interval of 6 arc min using a self consistent set of global isochrons and associated plate reconstruction poles the age at each grid node was determined by linear interpolation between adjacent isochrons in the direction of spreading ages for ocean floor between the oldest identified magnetic anomalies and continental crust were interpolated by estimating the ages of passive continental margin segments from geological data and published plate models we have constructed an age grid with error estimates for each grid cell as a function of 1 the error of ocean floor ages identified from magnetic anomalies along ship tracks and the age of the corresponding grid cells in our age grid 2 the distance of a given grid cell to the nearest magnetic anomaly identification and 3 the gradient of the age grid i e larger errors are associated with high age gradients at fracture zones or other age discontinuities future applications of this digital grid include studies of the thermal and elastic structure of the lithosphere the heat loss of the earth ridge push forces through time asymmetry of spreading and providing constraints for seismic tomography and mantle convection models

time dependent crustal temperature modeling central alps abstract the conventional modeling of crustal temperatures is based on the steady state approach which is reliable in tectonically stable regions only the present work focuses on the assessment of the influence of the transient effects on the thermal field at regions of ongoing continental collision in the first part of this study the thermal effects of most evident processes operating in collision zones thickening and flaking of the crust frictional heating on the surfaces of flakes bulk replacement of mantle lithosphere by hotter rocks and also uplift and erosion are estimated on the basis of 1 d time dependent synthetic models the results show that surface heat flow caused by the most of investigated transient processes may differ from the steady state one by quantities of the order of magnitude larger than the uncertainties of the heat flow measurements these differences may exist over tens of millions of years in the second part three 2 d time dependent temperature sections gradually including the above assessed effects have been calculated for the central alps the results show that time dependent models give better agreement of the calculated and measured heat flow patterns than previously developed for the same region steady state model cermak et al 1990 best agreement is achieved when only the transient effects of thickening and flaking and increased heat inflow from the mantle are taken into account a poorer coherence of the other versions with the measured heat flow is probably attributable to uncertain knowledge and consequently great simplification of spacing and time dependence of erosion and or frictional heating

electric lithosphere of the slave craton the archean slave craton in northwestern canada is an ideal natural laboratory for investigating lithosphere formation and evolution and has become an international focus of broad geoscientific investigation following the discovery of economic diamondiferous kimberlite pipes three deep probing magnetotelluric surveys have recently been carried out on the craton using novel acquisition procedures the magnetotelluric responses reveal an unexpected and remarkable anomaly in electrical conductivity collocated with the kimberlite field that is modeled as a spatially confined upper mantle region of low resistivity 30 m at depths of 80 100 km and is interpreted to be due to dissolved hydrogen or carbon in graphite form this geophysically anomalous upper mantle region is also spatially coincident with a geochemically defined ultradepleted harzburgitic layer the tectonic processes that emplaced this structure are possibly related to the lithospheric subduction and trapping of overlying oceanic mantle at 2630 2620 ma

thermal effects of thrust faulting abstract calculations based on simple models of overthrust sheets in crystalline basement rocks show that significant thermal effects may result from their movements if rates are sufficiently high e g plate tectonic rates the thrust sheets sufficiently thick 5 10 and 15 km are modelled here the distances moved sufficiently large and for reasonable values of the coefficient of friction along the thrust plane overthrusting can cause metamorphic mineral zonations and heat flow anomalies observable in the field regions where large scale overthrusting has occurred should be characterized by a decrease with depth of grade of metamorphic mineral assemblages and anomalously low heat flow the theoretical effects are presented as a series of maximum temperature vs depth and heat flow vs time plots

mechanisms of crustal deformation thirty sixthwilliam smith lecture the energy source for crustal deformation is isotopic heating and secular cooling of the mantle in a true solid heat would be lost to the surface by conduction however solid state creep processes allow the earth s solid mantle to exhibit a fluid behaviour thus thermal convection can convert heating into directed motion variations in temperature lead to variations in densitythrough thermal expansion and contraction although the resultant body forces are vertical horizontal variations in temperature lead to horizontal body forces at an ocean ridge the variations in the thickness of the lithosphere lead to the elevation of the ridge the result is a substantial horizontal ridge push force this force along with the trench pull force on the descending lithosphere is the primary force driving plate tectonics lithospheric thinning must occur beneath continental rifts and plateau uplifts i suggest that lithospheric thinning is caused by the diapiric penetration of hot asthenospheric rock to the crust mantle boundary this penetration may also cause lithospheric stoping and plateau uplifts lithospheric thinning leads to large tensional stresses above the zone of thinning these forces are generally much larger than the compressional forces generated by crustal thinning the forces generated by lithospheric thinning are quite large and may be responsible for the propagation of continental rifts in island arcs and andean type orogenic belts both lithospheric thinning and crustal thickening result in large tensional forces probably the most complex zone of crustal deformation is the continental collision variations in types of continental collisions are discussed

atmospheric boundary waves excited by the tsunami generation related to the 2011 great tohoku oki earthquake 1 atmospheric pressure changes caused by the 2011 off the pacific coast of tohoku japan earthquake mw 9 0 are investigated sensitive microbarographs in and around japan recorded unequivocal signals associated with the tsunami we identify them as atmospheric boundary waves excited by the uplift and subsidence of the ocean surface on the basis of the waveform characteristics as well as similarity with the data from ocean bottom pressure gauges potential usefulness of an observation network of atmospheric pressure is discussed regarding the improvement of the tsunami warning system

anisotropic tomography around la reunion island from rayleigh waves in the western indian ocean the reunion hot spot is one of the most active volcanoes on earth temporal interactions between ridges and plumes have shaped the structure of the zone this study investigates the mantle structure using data from the reunion hotspot and upper mantle reunions unterer mantel rhum rum project which significantly increased the seismic coverage of the western part of the indian ocean for more than 1 year 57 ocean bottom seismometer stations and 23 temporary land stations were deployed over this area for each earthquake station path the rayleigh wave fundamental mode phase velocities were measured for the periods from 30 s to 300 s and group velocities for the period from 16 s to 250 s a three dimensional model of the shear velocity in the upper mantle was built in two steps first the path mean phase and group velocities were inverted to obtain regionalized velocity maps for each separate period then all of the phase and group velocity maps were combined and inverted at each grid point to obtain the local s wave velocity as a function of depth using a transdimensional inversion scheme the three dimensional anisotropic s wave velocity model has resolution down to 300 km in depth the tomographic model and surface tectonics are correlated down to 100 km in depth starting at 50 km in depth a slow velocity anomaly beneath rodrigues ridge and the east west orientation of the azimuthal anisotropy show a connection between reunion upwelling and the central indian ridge the slow velocity signature beneath la reunion is connected at greater depths 150 300 km with a large slow velocity zone beneath the entire mascarene basin this develops along a northeast direction following the general motion direction of the african plate these observations indicate nonisotropic spreading of hot plume material and dominant horizontal flow in the upper mantle beneath this area

seismic azimuthal anisotropy in the oceanic lithosphere and asthenosphere from broadband surface wave analysis of obs array records at 60 ma seafloor we analyzed seismic ambient noise and teleseismic waveforms of nine broadband ocean bottom seismometers deployed at a 60 ma seafloor in the southeastward of tahiti island the south pacific by the tomographic investigation by seafloor array experiment for the society hotspot project we first obtained one dimensional shear wave velocity model beneath the array from average phase velocities of rayleigh waves at a broadband period range of 5 200 s the obtained model shows a large velocity reduction at depths between 40 and 80 km where the lithosphere asthenosphere boundary might exist we then estimated shear wave azimuthal anisotropy at depths of 20 100 km by measuring azimuthal dependence of phase velocities of rayleigh waves the obtained model shows peak to peak intensity of the azimuthal anisotropy of 2 4 with the fastest azimuth of nw se direction both in the lithosphere and asthenosphere this result suggests that the ancient flow frozen in the lithosphere is not perpendicular to the strike of the ancient mid ocean ridge but is roughly parallel to the ancient plate motion at depths of 20 60 km the fastest azimuths in the current asthenosphere are subparallel to current plate motion at depths of 60 100 km additional shear wave splitting analysis revealed possible perturbations of flow in the mantle by the hot spot activities and implied the presence of azimuthal anisotropy in the asthenosphere down to a depth of 190 210 km

oceanic lithosphere asthenosphere boundary from surface wave dispersion data abstract according to different types of observations the nature of lithosphere asthenosphere boundary lab is controversial using a massive data set of surface wave dispersions in a broad period range 15 300 s we have developed a three dimensional upper mantle tomographic model first order perturbation theory at the global scale this is used to derive maps of the lab from the resolved elastic parameters the key effects of shallow layers and anisotropy are taken into account in the inversion process we investigate lab distribution primarily below the oceans according to different kinds of proxies that correspond to the base of the lithosphere from the shear velocity variation at depth the amplitude radial anisotropy and the changes in azimuthal anisotropy g orientation the estimations of the lab depth based on the shear velocity increase from a thin lithosphere 20 km in the ridges to a thick old ocean lithosphere 120 130 km the radial anisotropy proxy shows a very fast increase in the lab depth from the ridges from 50 km to the older ocean where it reaches a remarkable monotonic subhorizontal profile 70 80 km the lab depths inferred from the azimuthal anisotropy proxy show deeper values for the increasing oceanic lithosphere 130 135 km the difference between the evolution of the lab depth with the age of the oceanic lithosphere computed from the shear velocity and azimuthal anisotropy proxies and from the radial anisotropy proxy raises questions about the nature of the lab in the oceanic regions and of the formation of the oceanic plates

evolution of the lithosphere in the indian ocean from combined earthquake and ambient noise tomography rayleigh wave dispersion extracted from ambient seismic noise has been widely used to image crustal and uppermost mantle structure applications of this approach in continental settings are abundant but there have been relatively few studies within ocean basins by cross correlating ambient noise recorded at broadband seismic stations around the indian ocean we demonstrate the feasibility of extracting high quality long period 10 30 mhz rayleigh waves that traverse the entire ocean basin high quality rayleigh wave cross correlation functions can be obtained from stacking waveforms over less than two years at land stations and less than four years at island stations we show that adding the dispersion information extracted from ambient noise to a global earthquake data set can improve the resolution of phase velocity maps by about 20 in the northern indian ocean where the station distribution is the best we find that a plate cooling model with a potential temperature of 1450 c and plate thickness of 125 km can fit both the seismic observations and seafloor topography the seychelles mascarene plateau is characterized by anomalously slow velocity at 30 mhz the inclusion of ambient noise data in the tomographic inversion shifts the slow velocity anomaly into better agreement with the topographic relief allowing us to estimate its crustal thickness and confirm that the plateau s elevation is supported by thick crust the 10 and 20 mhz phase velocity maps show a strong asymmetry across the central indian ridge that is best explained by eastward asthenospheric flow emanating from nearby hotspots

heat focussing granite genesis and inverted metamorphic gradients in continental collision zones abstract the thermal conductivity of sedimentary rocks is usually much lower than that of crystalline rocks gneisses and granites continental collision zones are characterized by large scale thrusts which superpose crystalline basement and sedimentary layers this leads to heat refraction effects of significant magnitude sedimentary layers act as a barrier to the transport of heat we show that the peculiar geometry of a gently sloping sedimentary layer thrust into more conductive rocks is very efficient in diverting the vertical flux of heat temperatures can vary by more than 50 along the horizontal and reach a marked maximum at the top of crystalline layers these results are in general agreement with the main features of orogenic belts as exemplified by the himalayas they explain why the young leucogranites of the main central thrust mct are confined to a narrow belt at the top of the basement slab they also account for the inverted metamorphic gradient observed there in a crustal accretionary wedge folding and thrusting modify the structure of the upper crust by thickening the sedimentary cover and by introducing sedimentary rocks at depth this leads to high temperatures at shallow depths in specific places les zones de collision continentale se caracterisent par de grands chevauchementsafaible pendage qui empilent les unes sur les autres des couches de socle cristallin et des couches sedimentaires cette structure particuliere engendre d importants effets de refraction thermique en effet leur conductibilitethermiqueetant normalement bien plus faible que celle des roches cristallines les formations sedimentaires font office de barrieres pour le transfert thermique le flux de chaleur se trouve fortement devie dans un plan horizontal la temperature peut varier de plus de 50 les maximaetant situesal interface superieur des dalles cristallines en ces endroits precis les isothermes profonds remontent tres significativement ces resultats rendent bien compte de plusieurs faits essentiels observes dans l himalaya zoneographie inverse du metamorphisme localisation des leucogranites le long d une bandeetroite situee au sommet de la dalle du tibet

seafloor age dependence of rayleigh wave phase velocities in the indian ocean variations in the phase velocity of fundamental mode rayleigh waves across the indian ocean are determined using two inversion approaches first variations in phase velocity as a function of seafloor age are estimated using a pure path age dependent inversion method second a two dimensional parameterization is used to solve for phase velocity within 1 25 1 25 grid cells rayleigh wave travel time delays have been measured between periods of 38 and 200 s the number of measurements in the study area ranges between 4139 paths at a period of 200 s and 22 272 paths at a period of 40 s at periods 100 s the phase velocity variations are strongly controlled by seafloor age and shown to be consistent with temperature variations predicted by the half space cooling model for a mantle potential temperature of 1400 c the inferred thermal structure beneath the indian ocean is most similar to the structure of the pacific upper mantle where phase velocities can also be explained by a half space cooling model the thermal structure is not consistent with that of the atlantic upper mantle which is best fit by a plate cooling model and requires a thin plate removing age dependent phase velocity from the 2 d maps of the indian ocean highlights anomalously high velocities at the rodriguez triple junction and the australian antarctic discordance and anomalously low velocities immediately to the west of the central indian ridge

computation of large anisotropic seismic heterogeneities clash summary a general tomographic technique is designed in order i to operate in anisotropic media ii to account for the uneven seismic sampling and iii to handle massive data sets in a reasonable computing time one modus operandi to compute a 3 d body wave velocity model relies on surface wave phase velocity measurements an intermediate step shared by other approaches consists in translating for each period of a given mode branch the phase velocities integrated along ray paths into local velocity perturbations to this end we develop a method which accounts for the azimuthal anisotropy in its comprehensive form the weakly non linear forward problem allows to use a conjugate gradient optimization the earth s surface is regularly discretized and the partial derivatives are assigned to the individual grid points possible lack of lateral resolution due to the inescapable uneven ray path coverage is taken into account through the a priori covariances on parameters with laterally variable correlation lengths this method allows to efficiently separate the 2 and the 4 anisotropic effects from the isotropic perturbations fundamental mode and overtone phase velocity maps derived with real rayleigh wave data sets are presented and compared with previous maps the isotropic models concur well with the results of trampert woodhouse large 4 heterogeneities are located in the tectonically active regions and over the continental lithospheres such as north america antarctica or australia at various periods a significant 4 signature is correlated with the hawaii hotspot track finally concurring with the conclusions of trampert woodhouse our phase velocity maps show that rayleigh wave data sets do need both 2 and 4 anisotropic terms

the lithosphere asthenosphere boundary and cratonic lithospheric layering beneath australia from sp wave imaging sp and ps scattered wave receiver functions were calculated for nineteen stations across australia and the island of tasmania in order to image the lithosphere asthenosphere boundary and layering within the lithosphere within phanerozoic eastern australia and the eastern margin of the south australia craton prominent sp phases from a negative velocity contrast were found at depths of 61 11 km to 131 9 km consistent with the lithosphere asthenosphere boundary depth range from surface wave tomography these phases imply significant velocity drops over depth ranges of 30 40 km or less and thus cannot be explained by a lithosphere asthenosphere boundary that is controlled by temperature alone rather they imply that the asthenosphere is hydrated with respect to a drier depleted lithosphere or contains a small amount of partial melt the shallowest sp phases have the largest amplitudes and occur in regions with the most recent voluminous volcanism strengthening the link to partial melt at the base of the lithosphere in contrast no significant negative sp phases were found at the base of the thick cratonic lithosphere at the stations in central and western australia implying that the cratonic lithosphere asthenosphere velocity gradient is distributed over more than 50 70 km in depth this gradient may be purely thermal in origin although gradational changes in composition or melt content cannot be ruled out a negative sp phase was observed at depths of 69 8 km to 85 14 km at stations in central and western australia indicating the presence of a drop in velocity internal to the lithosphere this interface within the lithosphere may be a relic of cratonic mantle formation or the result of alteration by melt and metasomatism

earth science the slippery base of a tectonic plate high resolution imaging of the base of the pacific plate as it descends beneath new zealand discloses a 10 kilometre thick channel that decouples the plate from underlying upper mantle see letter p 85

quantifying the relationship between short wavelength dynamic topography and thermomechanical structure of the upper mantle using calibrated parameterization of anelasticity oceanic residual depth varies on 5000 km wavelengths with amplitudes of 1 km a component of this short wavelength signal is dynamic topography caused by convective flow in the upper 300 km of the mantle it exerts a significant influence on landscape evolution and sea level change but its contribution is often excluded in geodynamic models of whole mantle flow using seismic tomography to resolve buoyancy anomalies in the oceanic upper mantle is complicated by the dominant influence of lithospheric cooling on velocity structure here we remove this cooling signal from global surface wave tomographic models revealing a correlation between positive residual depth and slow residual velocity anomalies at depths 300 km to investigate whether these anomalies are of sufficient amplitude to account for short wavelength residual depth variations we calibrate an experimentally derived parameterization of anelastic deformation at seismic frequencies to convert shear wave velocity into temperature density and diffusion creep viscosity asthenospheric temperature anomalies reach 150 c in the vicinity of major magmatic hotspots and correlate with geochemical and geophysical proxies for potential temperature along mid ocean ridges locally we find evidence for a 150 km thick low viscosity asthenospheric channel incorporating our revised density structure into models of whole mantle flow yields reasonable agreement with residual depth observations and suggests that 30 km deviations in local lithospheric thickness account for a quarter of total amplitudes these predictions remain compatible with geoid constraints and substantially improve the fit between power spectra of observed and predicted dynamic topography this improvement should enable more accurate reconstruction of the spatio temporal evolution of cenozoic dynamic topography

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

imaging the colliding indian and asian lithospheric plates beneath tibet 1 a high resolution image of the base of the lithosphere from s to p converted seismic waves revealed the collision architecture of the indian and asian continental plates beneath the tibetan plateau the base of the indian lithosphere dips northward from a depth of 160 km beneath the himalayas to a depth of 220 km just south of the bangong suture the base of the asian lithosphere is nearly horizontal at the depth of 160 180 km from central to northern tibet there is a vertical gap of about 50 km between indian and asian lithospheres our observation of a well defined thick lithosphere throughout the entire plateau is not consistent with models of wholesale convective instability of a thickened mantle lithosphere which would predict a very thin asian lithosphere the hypothesized sequential southward subduction of tibetan and asian continental lithospheres leading to the growth of the tibetan plateau if correct cannot be occurring below 180 km depth our results along with available geological and geophysical data strongly support that the plateau is predominantly formed by a relatively coherent north dipping subducted indian continental lithosphere in the south which presently can be traced to the middle of the plateau and a south dipping subducted asian lithosphere in the north at a shallower depth

anisotropic models of the upper mantle long period rayleigh wave and love wave dispersion data particularly for oceanic areas have not been simultaneously satisfied by an isotropic structure in this paper available phase and group velocity data are inverted by a procedure which includes the effects of transverse anisotropy anelastic dispersion sphericity and gravity we assume that the surface wave data represents an azimuthal average of actual velocities thus we can treat the mantle as transversely isotropic the resulting models for average earth average ocean and oceanic regions divided according to the age of the ocean floor are quite different from previous results which ignore the above effects the models show a low velocity zone with age dependent anisotropy and velocities higher than derived in previous surface wave studies the correspondence between the anisotropy variation with age and a physical model based on flow aligned olivine is suggestive for most of the earth sh sv in the vicinity of the low velocity zone neat the east pacific rise however sv sh at depth consistent with ascending flow anisotropy is as important as temperature in causing radial and lateral variations in velocity the models have a high velocity nearly isotropic layer at the top of the mantle that thickens with age this layer defines the lid or seismic lithosphere in the pacific the lid thickens with age to a maximum thickness of 50 km this thickness is comparable to the thickness of the elastic lithosphere the lid thickness is thinner than derived using isotropic or pseudo isotropic procedures a new model for average earth is obtained which includes a thin lid this model extends the fit of a prem type model to shorter period surface waves

hydrological disturbances of the conductive heat flow in stable continental crust abstract geophysical data generally support the presence of free aqueous fluids in the crust to at least the depth of the 700 750 c isotherm where granulite facies conditions are reached which are presumed to have no free water the fluid phase distribution and transport of fluid in rocks strongly depend on the rheological conditions in the lithologically heterogeneous crust changes in rheological regime can produce quite complex sandwich like fluid transport layering including domains of upward migration very impermeable layers or thin reservoirs with nearly horizontal fluid transport in the present study two two dimensional thermal models of the stable continental crust with different spatial configurations of fluid systems have been calculated the results show that such systems redistribute heat in the crust and give rise to zones of concentrated heat energy storage and transfer the pattern of the disturbances to the conductive heat flow depends on the arrangement of the fluid systems which is a function of the crustal structure temperature and regional stress field and shows significant lateral and vertical variations even at very low velocities of the fluid percolation a few millimetres per year the disturbances to the conductive heat flow may reach 30 60

non linear crustal corrections in high resolution regional waveform seismic tomography summary we compare 3 d upper mantle anisotropic structures beneath the north american continent obtained using standard and improved crustal corrections in the framework of non linear asymptotic coupling theory nact applied to long period three component fundamental and higher mode surface waveform data our improved approach to correct for crustal structure in high resolution regional waveform tomographic models goes beyond the linear perturbation approximation and is therefore more accurate in accounting for large variations in moho topography within short distances as observed for instance at ocean continent margins this improved methodology decomposes the shallow layer correction into a linear and non linear part and makes use of 1 d sensitivity kernels defined according to local tectonic structure both for the forward computation and for the computation of sensitivity kernels for inversion the comparison of the 3 d upper mantle anisotropic structures derived using the standard and improved crustal correction approaches shows that the model norm is not strongly affected however significant variations are observed in the retrieved 3 d perturbations the largest differences in the velocity models are present below 250 km depth and not in the uppermost mantle as would be expected we suggest that inaccurate crustal corrections preferentially map into the least constrained part of the model and therefore accurate corrections for shallow layer structure are essential to improve our knowledge of parts of the upper mantle where our data have the smallest sensitivity

thermal structure radial anisotropy and dynamics of oceanic boundary layers defining the oceanic lithosphere as a thermal boundary layer allows to explain to first order age dependent bathymetry and isotropic wave speeds in contrast ss precursors and receiver functions suggest a subhorizontal interface within this layer on top of a radially anisotropic zone comparing a suite of geodynamic scenarios against surface wave dispersion data and seismic discontinuities we find that only weak age dependency of the radially anisotropic zone is compatible with observations we show that this zone is confined from below by a second weaker seismic interface while observed azimuthal anisotropy is consistent with lattice preferred orientation of olivine due to asthenospheric flow underneath the lithosphere radial anisotropy requires additional contributions perhaps from petrological fabrics or melt ponding this implies that seismic reflectors previously associated with the base of the lithosphere are instead associated with preserved structures embedded in it they carry information about plate formation but have little control on plate deformation

3 d shear wave radially and azimuthally anisotropic velocity model of the north american upper mantle summary using a combination of long period seismic waveforms and sks splitting measurements we have developed a 3 d upper mantle model sawum na2 of north america that includes isotropic shear velocity with a lateral resolution of 250 km as well as radial and azimuthal anisotropy with a lateral resolution of 500 km combining these results we infer several key features of lithosphere and asthenosphere structure a rapid change from thin 70 80 km lithosphere in the western united states wus to thick lithosphere 200 km in the central cratonic part of the continent closely follows the rocky mountain front rmf changes with depth of the fast axis direction of azimuthal anisotropy reveal the presence of two layers in the cratonic lithosphere corresponding to the fast to slow discontinuity found in receiver functions below the lithosphere azimuthal anisotropy manifests a maximum stronger in the wus than under the craton and the fast axis of anisotropy aligns with the absolute plate motion as described in the hotspot reference frame hs3 nuvel 1a in the wus this zone is confined between 70 and 150 km decreasing in strengthwithdepthfromthetop fromthermftothesanandreasfaultsystemandthejuande fuca gorda ridges this result suggests that shear associated with lithosphere asthenosphere couplingdominatesmantledeformationdowntothisdepthinthewesternpartofthecontinent the depth extent of the zone of increased azimuthal anisotropy below the cratonic lithosphere is not well resolved in our study although it is peaked around 270 km a robust result radial anisotropy is such that predominantly 1 where vsh vsv 2 under the continent and its borders down to 200 km with stronger in the bordering oceanic regions across the continent and below 200 km alternating zones of weaker and stronger radial anisotropy with predominantly 1 correlate with zones of small lateral changes in the fast axis direction of anisotropy and faster than average vs below the lab suggesting the presence of small scale convection with a wavelength of 2000 km finally in the western united states complex 3 d patterns of isotropic velocity and anisotropy reflect mantle dynamics associated with the rich tectonic history of the region

high resolution seismic constraints on flow dynamics in the oceanic asthenosphere convective flow in the mantle and the motions of tectonic plates produce deformation of earth s interior and the rock fabric produced by this deformation can be discerned using the anisotropy of the seismic wave speed this deformation is commonly inferred close to lithospheric boundaries beneath the ocean in the uppermost mantle including near seafloor spreading centres as new plates are formed via corner flow and within a weak asthenosphere that lubricates large scale plate driven flow and accommodates smaller scale convection seismic models of oceanic upper mantle differ as to the relative importance of these deformation processes seafloor spreading fabric is very strong just beneath the crust mantle boundary the mohorovicic discontinuity or moho at relatively local scales but at the global and ocean basin scales oceanic lithosphere typically appears weakly anisotropic when compared to the asthenosphere here we use rayleigh waves recorded across an ocean bottom seismograph array in the central pacific ocean the nomelt experiment to provide unique localized constraints on seismic anisotropy within the oceanic lithosphere asthenosphere system in the middle of a plate we find that azimuthal anisotropy is strongest within the high seismic velocity lid with the fast direction coincident with seafloor spreading a minimum in the magnitude of azimuthal anisotropy occurs within the middle of the seismic low velocity zone and then increases with depth below the weakest portion of the asthenosphere at no depth does the fast direction correlate with the apparent plate motion our results suggest that the highest strain deformation in the shallow oceanic mantle occurs during corner flow at the ridge axis and via pressure driven or buoyancy driven flow within the asthenosphere shear associated with motion of the plate over the underlying asthenosphere if present is weak compared to these other processes

surface wave tomography for the pacific ocean incorporating seafloor seismic observations and plate thermal evolution abstract using broadband seismic waveforms recorded on the seafloor by more than 200 broadband ocean bottom seismometers as well as those on land we measured the phase speed dispersions of love and rayleigh waves up to the 4th higher mode to determine the three dimensional radially anisotropic shear wave speed structure in the upper mantle beneath the pacific ocean the fastest anomalies at depths shallower than 100 km were located beneath southeast of the shatsky rise and strong radial anisotropy was located in the central pacific at depths of 100 200 km the isotropic shear wave speed structures showed age dependence from the age bin averaged shear wave speed profiles and a half space cooling model we estimated the thermo speed relationship for the pacific plate to construct a reference age dependent shear wave speed model which was further used as an initial model for tomography iteration deviation maps in the pacific ocean from the reference model indicated that large negative residuals which may be due to partial melting anelasticity and or added heat from mantle plumes were located along the ridges and beneath hotspots and that large positive residuals were found beneath the northwestern pacific ocean the use of an age dependent reference model as well as the incorporation of obs data greatly improves the accuracy of local phase speed estimates in tomography as evident from a direct comparison with in situ array measurements

thermal evolution of the oceanic lithosphere an alternative view abstract the most common model used for representing the evolution with age of the oceanic lithosphere is the plate model where the temperature is set at a fixed depth called the base of the plate this base of the plate has no physical meaning but this model provides a mathematical substitute for a system where small scale convection occurs through instabilities growing at the base of the cooling lithosphere and becomes effective only below old ocean another possible view is that convection provides heat at the base of the lithosphere whatever the age of the overlying plate this last process can be modeled by a constant heat flow applied on the bottom lithospheric isotherm chablis model a good fit to the observables bathymetry and geoid as function of age and old age heat flow can be obtained both for plate and chablis models in spite of an experimentally determined thermal expansion coefficient much larger than assumed in previous plate models these models have important consequences for several geodynamic processes the plate at an age of 100 ma is only 80 km thick for both models melting above a hot spot can then occur in the garnet spinel transition field without much plate thinning in the plate model the subsidence is stopped at an age of about 80 ma while according to the chablis model several hundred meters of subsidence are expected after 100 ma thus the two models predict quite a different long term pattern of subsidence in the sedimentary basins finally in the chablis model the global cooling of the mantle coming from cold material eroded by secondary convection at the base of the plates is considerably larger than in plate models it amounts to 40 the remaining 60 being due to the subduction process

orogens and slabs vs their direction of subduction abstract subduction zones appear primarily controlled by the polarity of their direction i e w directed or e to nne directed probably due to the westward drift of the lithosphere relative to the asthenosphere the decollement planes behave differently in the two end members in the w directed subduction zone the decollement of the plate to the east is warped and subducted whereas in the e to nne directed it is ramping upward at the surface there are w directed subduction zones that work also in absence of active convergence like the carpathians or the apennines w directed subduction zones have shorter life 30 40 ma than e or ne directed subduction zones even longer than 100 ma the different decollements in the two end members of subduction should control different ptt paths and therefore generate variable metamorphic assemblages in the associated accretionary wedges and orogens these asymmetries also determine different topographic and structural evolutions that are marked by low topography and a fast eastward migrating structural wave along w directed subduction zones whereas the topography and the structure are rapidly growing upward and expanding laterally along the opposite subduction zones the magmatic pair calc alkaline and alkaline tholeiitic volcanic products of the island arc and the back arc basin characterise the w directed subduction zones magmatic rocks associated with e or ne directed subduction zones have higher abundances of incompatible elements and mainly consist of calc alkaline shoshonitic suites with large volumes of batholithic intrusions and porphyry copper ore deposits the subduction zones surrounding the adriatic plate in the central mediterranean confirm the differences among subduction zones as primarily controlled by the geographic polarity of the main direction of the slab the western margin of the adriatic plate contemporaneously overridden and underthrust europe toward the west to generate respectively the alps and the apennines while the eastern margin subducted under the dinarides hellenides these belts confirm the characters of the end members of subduction zones as a function of their geographic polarity similarly to the pacific subduction zones

rupture process of the 2011 tohoku oki earthquake and absolute elastic strain release 1 on 11 march 2011 the tohoku oki earthquake in eastern japan and the devastating tsunami that followed it caused severe damage and numerous deaths to clarify the rupture process of the earthquake we inverted teleseismic p wave data applying a novel formulation that takes into account the uncertainty of green s function which has been a major error source in waveform inversion the estimated seismic moment is 5 7 1022 nm mw 9 1 associated with a fault rupture 440 km long and 180 km wide along the plate interface the source process is characterized by asymmetric bilateral rupture propagation but we also found continuous slips up dip from the hypocenter which led to a large maximum slip 50 m long slip duration 90 s and a large stress drop 20 mpa the long slip duration large stress drop extensional normal faulting aftershocks in a previously compressional stress regime and low angle normal slips at approximately the depth of the plate interface suggest that the earthquake released roughly all of the accumulated elastic strain on the plate interface owing to exceptional weakening of the fault the stress accumulated on the plate interface was about 20 mpa near the trench and 0 10 mpa in the down dip source region

tsunami source of the 2011 tohoku oki earthquake japan inversion analysis based on dispersive tsunami simulations 1 the large tsunami of the 2011 tohoku oki earthquake was clearly recorded by the ocean bottom pressure and gps wave gauges deployed in and around japan we estimated the initial tsunami water height distribution by inversion analysis of the waveforms based on dispersive tsunami simulations the distribution is characterized by a peak height of 8 m located near the trench and the high water 2m region extending landward with a width of 100 km a series of numerical simulations suggests that a relatively steep peak located near the trench is necessary in order to simultaneously reproduce the dispersive wave at a far field station and the near field waveforms furthermore we estimated the coseismic slip distribution at the plate boundary which indicates that large slip 30 m occurred at a depth of 20 km which corresponds to a large slip deficit area in the interseismic period another slip 25 m occurred at the shallower part 10 km during the rupture

acoustic gravity waves from the source region of the 2011 great tohoku earthquake mw 9 0 1 atmospheric pressure waves were recorded within 5 h after the 2011 great tohoku earthquake mw 9 0 by sensitive microbarographs at four regional stations and eight international monitoring system stations at distances up to 6700 km while its apparent phase velocity between the regional stations is 341 m s the global stations indicate weak dispersive wave trains with low frequencies between 1 6 and 4 8 mhz propagating with an average phase velocity around 364 m s the low frequency waves may be interpreted as acoustic gravity waves excited by upheaval and depression of the sea surface in the source region due to coseismic uplift and subsidence of the sea bottom during this great thrust earthquake assuming the source dimension and the average coseismic vertical displacements of the sea surface with reference to tsunami observations we calculate synthetic waveforms for some of the far field stations by incorporating a standard sound velocity structure in the atmosphere up to an altitude of 220 km the synthetics provide reasonable explanations for the general features of the observed waveforms suggesting possible ranges for the source parameters generating these acoustic gravity waves our analysis suggests that the average initial upheaval of the sea surface in the central zones of the source region may exceed 4 6 m and that the risetime of the coseismic deformation may be in the range between 3 and 4 min in the eastern narrow zone adjacent to the japan trench the deformation has significantly higher initial amplitude and shorter risetime

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

imaging the lithospheric structure beneath the indian continent we present a high resolution 3 d lithospheric model of the indian plate region down to 300 km depth obtained by inverting a new massive database of surface wave observations using classical tomographic methods data are collected from more than 550 seismic broadband stations spanning the indian subcontinent and surrounding regions the rayleigh wave dispersion measurements along 14 000 paths are made in a broad frequency range 16 250 s our regionalized surface wave group and phase dispersion data are inverted at depth in two steps first an isotropic inversion and next an anisotropic inversion of the phase velocity including the sv wave velocity and azimuthal anisotropy based on the perturbation theory we are able to recover most of the known geological structures in the region such as the slow velocities associated with the thick crust in the himalaya and tibetan plateau and the fast velocities associated with the indian precambrian shield our estimates of the depth to the lithosphere asthenosphere boundary lab derived from seismic velocity vsv reductions at depth reveal large variations 120 250 km beneath the different cratonic blocks the lithospheric thickness is 120 km in the eastern dharwar 160 km in the western dharwar 140 200 km in bastar and 160 200 km in the singhbhum craton the thickest 200 250 km cratonic roots are present beneath central india a low velocity layer associated with the midlithospheric discontinuity is present when the root of the lithosphere is deep

geodynamic tomography constraining upper mantle deformation patterns from bayesian inversion of surface waves in the earth s upper mantle seismic anisotropy mainly originates from the crystallographic preferred orientation cpo of olivine due to mantle deformation large scale observation of anisotropy in surface wave tomography models provides unique constraints on present day mantle flow however surface waves are not sensitive to the 21 coefficients of the elastic tensor and therefore the complete anisotropic tensor cannot be resolved independently at every location this large number of parameters may be reduced by imposing spatial smoothness and symmetry constraints to the elastic tensor in this work we propose to regularize the tomographic problem by using constraints from geodynamic modelling to reduce the number of model parameters instead of inverting for seismic velocities we parametrize our inverse problem directly in terms of physical quantities governing mantle flow a temperature field and a temperature dependent viscosity the forward problem consists of three steps 1 calculation of mantle flow induced by thermal anomalies 2 calculation of the induced cpo and elastic properties using a micromechanical model and 3 computation of azimuthally varying surface wave dispersion curves we demonstrate how a fully nonlinear bayesian inversion of surface wave dispersion curves can retrieve the temperature and viscosity fields without having to explicitly parametrize the elastic tensor here we consider simple flow models generated by spherical temperature anomalies the results show that incorporating geodynamic constraints in surface wave inversion help to retrieve patterns of mantle deformation the solution to our inversion problem is an ensemble of models i e thermal structures representing a posterior probability therefore providing uncertainties for each model parameter

measurements of mantle wave velocities and inversion for lateral heterogeneities and anisotropy 3 inversion lateral heterogeneity in the earth s upper mantle is investigated by inverting dispersion curves of long period surface waves 100 330 s models for seven different tectonic regions are derived by inversion of regionalized great circle phase velocity measurements from our previous studies we also obtain a representation of upper mantle heterogeneities with no a priori regionalization from the inversion of the degree 6 spherical harmonic expansion of phase and group velocities the data are from the observation of about 200 paths for love waves and 250 paths for rayleigh waves for both the regionalized and the spherical harmonic inversions corrections are applied to take into account lateral variations in crustal thickness and other shallow parameters these corrections are found to be important especially at low spherical harmonic order the trench region and fast velocities down to 250 km under shields below 200 km under the oceans both s velocity and s anisotropy support a model of small scale convection in which cold blobs detach from the bottom of the lithosphere when its age is large enough the spherical harmonic models clearly demonstrate a posteriori the relation between surface tectonics and s velocity heterogeneities in the first 250 km all shields are fast most ridges are slow below 300 km a belt of fast mantle follows the pacific subduction zones however at greater depths large scale heterogeneities that seem to bear no relationship to surface tectonics are observed the most prominent feature at 450 km is a fast velocity region under the south atlantic ocean smaller scale heterogeneities that are not related to surface tectonics are also mapped at shallower depths an anomalously slow region centered in the south central pacific is possibly linked to intense hot spot activity a very fast region southeast of south america may be related to subduction of old pacific plate between 200 and 400 km a belt of sv sh anisotropy follows part of the ridge and subduction systems indicating vertical mantle flow in these regions the spherical harmonic results open new horizons for the understanding of convection in the mantle perspectives for the improvement of the models presented are discussed

heat flow and geodynamics in the tyrrhenian sea the present heat flow in the southern tyrrhenian sea appears as a transient thermal wave that has migrated eastward in time the higher heat flow in the south eastern side of the basin confirms the suggestion of an eastward migrating rift punctuation of the tyrrhenian backarc extension in lithospheric boudins is accompanied by a concentrated increase in heat flow generated by asthenospheric intrusions and related magmatism progressively moving eastward the migration of the asthenosphere in the same direction could explain these phenomena

age independent seismic anisotropy under oceanic plates explained by strain history in the asthenosphere the depth of the oceanic lithosphere asthenosphere boundary lab as inferred from shear wave velocities increases with lithospheric age in agreement with models of cooling oceanic lithosphere on the other hand the distribution of radial anisotropy under oceanic plates is almost age independent in particular radial anisotropy shows a maximum positive gradient at a depth of 70 km which if used as a proxy indicates an age independent lab depth these contrasting observations have fueled a controversy on the seismological signature of the lab to better understand the discrepancy between these observations we model the development of lattice preferred orientation lpo in upper mantle crystal aggregates and predict the seismic anisotropy produced by plate driven mid ocean ridge flows the model accounts for the progressive cooling of the lithosphere with age and can incorporate both diffusion and dislocation creep deformation mechanisms we find that an age independent distribution of radial anisotropy is the natural consequence of these simple flows the depth and strength of anisotropy is further controlled by the deformation regime dislocation or diffusion creep experienced by crystals during their ascent towards and subsequent motion away from the ridge axis comparison to surface wave tomography models yield constraints on rheological parameters such as the activation volume although not excluded additional mechanisms proposed to explain some geophysical signatures of the lab such as the presence of partial melt or changes in water content are not required to explain the radial anisotropy proxy our prediction that the age independent radial anisotropy proxy marks the transition to flow induced asthenospheric anisotropy provides a way to reconcile thermal mechanical and seismological views of the lab

water rich sublithospheric melt channel in the equatorial atlantic ocean the lithosphere asthenosphere boundary is the most extensive boundary on earth separating the mobile plate above from the convecting mantle below but its nature remains a matter of debate using an ultra deep seismic reflection technique here we show a systematic seismic image of two deep reflectors that we interpret as the upper and lower limits of the lithosphere asthenosphere boundary beneath a 40 70 million year old oceanic lithosphere in the atlantic ocean these two reflections correspond to 1 260 c and 1 355 c isotherms and bound a low velocity channel suggesting that the lithosphere asthenosphere boundary is thermally controlled we observe a clear age dependency of this sublithospheric channel its depth increases with age from 72 km where it is 40 myr old to 88 km where it is 70 myr old whereas its thickness decreases with age from 18 km to 12 km we suggest that partial melting facilitated by water is the main mechanism responsible for the low velocity channel the required water concentration for melting increases with age nevertheless its corresponding total mass remains relatively constant suggesting that most of the volatiles in the oceanic sublithospheric channel originate from a horizontal flux near the ridge axis

seismic and electrical signatures of the lithosphere asthenosphere system of the normal oceanic mantle although plate tectonics started as a theory of the ocean basins nearly 50 years ago the mechanical details of how it works are still poorly known our understanding of these details has been hampered partly by our inability to characterize the physical nature of the lithosphere asthenosphere system las beneath the ocean we review the existing observational constraints on the seismic and electrical properties of the las particularly for normal oceanic regions away from mid oceanic ridges hot spots and subduction zones where plate tectonics is expected to present its simplest form whereas a growing volume of seismic data on land has provided remarkable advances in large scale pictures seafloor observations have been shedding new light on essential details by combing through these observational constraints researchers are unveiling the nature of the enigmatic las future directions for large scale seafloor observations are also discussed

evidence of large scale repeating slip during the 2011 tohoku oki earthquake 1 the repetition of slip during rupture process of earthquake is a debate issue which had never been confirmed clearly in the past big events due to the lack of dense near field observations and limited resolution in time of source model the 2011 m9 0 tohoku oki earthquake generated a wealth seismic records which provided us an unprecedented opportunity to study the rupture evolution of giant earthquake at a high spatio temporal resolution here we use teleseismic local strong motion and near field coseismic geodetic data to investigate the source rupture process of this event based on the parallel inversion technique the results reveal a broad slip zone with remarkable large scale repeating slip during the earthquake the inverted source model shows several time periods of energy release with three main peaks these energy bursts and temporal rupture snapshots suggest repetition of a large scale slip on the biggest asperity this rupture behavior resulted in 50 m slips on the slip zone and prolonged the entire rupture process for a long duration of 160 seconds the proposed source model is in a good agreement with the aftershock distribution and can interpret the characteristics of local strong motions further investigations of repeating slip during this event are crucial which will deeply transform earthquake science from dynamic point of view

the rate of uplift in the alpine mobile belt abstract the maximum rates of uplift in the alpine mobile belt the alps carpathians caucasus pamirs and himalaya are considered geologic and geomorphic methods used to study periods of 20 40 m y and 2 10 m y respectively yield maximum uplift rates on the order of fractions of millimeters per year to 1 5 mm yr with the exception of the nanga parbat massif in the himalaya which is rising as fast as 5 9 mm yr values of the same order have been obtained for recent uplift of the alps and himalayas by means of relevelling in the carpathians caucasus and pamirs the maximum uplift rates obtained by this method are distinctly different being as high as 6 14 mm yr possible causes of such significantly differing values of the rate of recent movements on the one hand and large discrepancies in the uplift rates for the neotectonic and recent time span within the carpathians caucasus and pamirs on the other are discussed it is shown that the maximum rates of the genuine secular tectonic uplift for the mountain systems within the alpine belt hardly exceed 1 2 mm yr and it is only in the himalayas that the rate may be higher

a review of the timing of coalification in the light of coal seam erosion clastic dykes and coal clasts abstract coal seam erosion clastic dykes penetrating coal seams and studies of coal clasts occurring in late paleozoic basins are used to demonstrate that coalification to the rank of subbituminous and bituminous coal occurred rapidly at shallow depth in some parts of the variscan orogen implying a precondition of high paleotemperatures for the coal basins the origin of clastic dykes particularly in the bolsovian westphalian c deposits of the plze and kladno rakovnik continental basins czech republic is shown to be a relevant factor in determining the timing of coalification pesek j 1978 erosion and clastic dikes in coal seams of the central bohemian basins and their significance for determination of plant substance coalification folia musei rerum naturalium bohemiae occidentalis geologica 12 1 34 is of the opinion that the 50 100 cm deep fractures filled with superposed clastics originated in fully ranked high volatile bituminous coal exposed during a hiatus between the early westphalian c early bolsovian and late westphalian d late asturian these dykes have either irregular shape or form conic bodies with sharp boundaries their filling does not show any signs of postsedimentary deformation coal clasts occurring in clastic sediments either close to or far above the roof of coal seams are a few millimetres up to several centimetres in size being subangular or angular however there also exist clasts of which one side is angular to subangular whereas the opposite side is markedly sharp in some basins coal clasts are embedded in units of the same age they are mostly of somewhat lower rank by 0 01 0 03 than the nearest coal seam whereas gayer r a pesek j s korova i valterova p 1996 coal clasts in the upper westphalian sequence of the south wales coal basin implication for the timing of maturation and fracture permeability in gayer r harris j eds coalbed methane and coal geology geological society special publication 109 pp 103 130 presumed in the south wales foreland basin a burial level of about 1 km with an elevated geothermal gradient 60 c km enhanced by hot fluids along thrusts resulting in rapid coalification to the bituminous coal rank no deep burial of organic matter could have occurred in the intramontane basins of the bohemian massif coalification of the late paleozoic seams must have proceeded at a very high rate the only logical reason leading to such rapid coalification of biomass could have been high value of geothermal gradient at least in the basins of the bohemian massif

constraints on the anisotropic contributions to velocity discontinuities at 60 km depth beneath the pacific strong sharp negative seismic discontinuities velocity decreases with depth are observed beneath the pacific seafloor at 60 km depth it has been suggested that these are caused by an increase in radial anisotropy with depth which occurs in global surface wave models here we test this hypothesis in two ways we evaluate whether an increase in surface wave radial anisotropy with depth is robust with synthetic resolution tests we do this by fitting an example surface wave data set near the east pacific rise we also estimate the apparent isotropic seismic velocity discontinuities that could be caused by changes in radial anisotropy in s to p and p to s receiver functions and ss precursors using synthetic seismograms we test one model where radial anisotropy is caused by olivine alignment and one model where it is caused by compositional layering the result of our surface wave inversion suggests strong shallow azimuthal anisotropy beneath 0 10 ma seafloor which would also have a radial anisotropy signature an increase in radial anisotropy with depth at 60 km depth is not well resolved in surface wave models and could be artificially observed shallow isotropy underlain by strong radial anisotropy could explain moderate apparent velocity drops 6 in ss precursor imaging but not receiver functions the effect is diminished if strong anisotropy also exists at 0 60 km depth as suggested by surface waves overall an increase in radial anisotropy with depth may not exist at 60 km beneath the oceans and does not explain the scattered wave observations

fluids tectonics and crustal deformation abstract in the plate tectonic process lithosphere creation at ocean ridges and its cooling leads to volatile fixation in the oceanic crust the outer 10 km or so of all crust contains abundant water in pores and fractures and variable amounts of volatiles in minerals when surface rocks are buried by tectonic processes fluids must be released and modify the mechanical properties in the subduction process hydrated oceanic crust may be decoupled from the remaining oceanic lithosphere at depth rising aqueous fluids or melts lead to a complex series of mass energy transfer processes which may decouple continental crust near the moho continental crust if subducted may also be decoupled from its lithosphere by degassing fluid release processes which create gas solid mixtures beneath impermeable cover create low strength systems subject to facile deformation hydraulic fracture processes and diapiric phenomena

thermorheological model for the european central alps brittle ductile transition and lithospheric strength a two dimensional thermorheological model of the central alps along a north south transect is presented thermophysical and rheological parameters of the various lithological units are chosen from seismic and gravity information the inferred temperature distribution matches surface heat flow and results in moho temperatures between 500 and 800 c both european and adriatic lithospheres have a jelly sandwich structure with a 15 20 km thick brittle upper crust overlying a ductile lower crust and a mantle lid whose uppermost part is brittle the total strength of the lithosphere is of the order of 0 5 1 0 1013 n m 1 if the upper mantle is dry or slightly less if the upper mantle is wet in both cases the higher values correspond to the adriatic indenter

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

eucrust 07 a new reference model for the european crust 1 we present a new digital model eucrust 07 for the crust of western and central europe and surroundings 35n 71n 25w 35e available results of seismic reflection refraction and receiver functions studies are assembled in an integrated model at a uniform grid 15 0 15 0 the model consists of three layers sediments and two layers of the crystalline crust besides depth to the boundaries we provide average p wave velocities in the upper and lower parts of the crystalline crust the new model demonstrates large differences in the moho depth compared to previous compilations over 10 km in some specific areas e g the baltic shield furthermore the velocity structure of the crust is much more heterogeneous than in previous maps eucrust 07 offers a starting point for numerical modeling of deeper structures by allowing correction for crustal effects beforehand and to resolve trade off with mantle heterogeneities citation tesauro m m k kaban and s a p l cloetingh 2008 eucrust 07 a new reference model for the european crust geophys res lett 35 l05313 doi 10 1029 2007gl032244

the lithosphere asthenosphere transition and radial anisotropy beneath the australian continent a new 3 d s wave speed model for the australian region derived from multimode surface waves allows us to examine the nature of the lithosphere asthenosphere transition lat and its relation to radial anisotropy in eastern phanerozoic australia the estimated depths of the lat tie well with those from receiver functions however in the archean and proterozoic lithosphere in western and central australia the lat derived from the surface wave model is generally much deeper than the discontinuities recognized from receiver functions and shows a smooth transition there is significant radial anisotropy sh sv in the upper lithosphere as well as in the lat and the underlying asthenosphere strong anisotropy in the asthenosphere reflects the effects of present shear flow in the mantle beneath the continent the lateral variation of lithospheric anisotropy correlates well with the suture zones between cratonic blocks representing frozen anisotropy associated with the ancient tectonics of australia

deep structure of the alps what do we really know abstract in the last five decades the deep structure of the alps has been probed by every geophysical method applicable and the resulting amount of data is unmatched for any other orogen in this study an attempt is made to review the data and the proposed structural models with the aim of separating the certain from the probable and from the speculative information this can be achieved by first reviewing the theoretical resolving power and ambiguity of the applied interpretation methods and then analysing the proposed models the methods reviewed are inversion of surface wave data teleseismic and local earthquake seismic tomography near vertical reflection seismology wide angle reflection and refraction seismology and gravity modelling all information about the moho rated as certain is combined to give a moho map of the alpine area the information rated as certain and probable and additional qualitative arguments are used to discuss a crustal model of the western and central alps represented by two cross sections major structural elements in this crustal model are a thick overthrust penninic nappe system wedging at mid to lower crustal levels a discontinuous moho and strong variations along the strike of the orogen whereas the structures of the european upper crust and of the penninic nappe system are well constrained only few and isolated lower crustal structural elements are rated as certain finally the shape of the lower lithosphere in the alps is discussed by review and comparison of the results from surface wave teleseismic travel time residual and tomographic studies qualitative arguments suggest the existence of a lithospheric root or slab beneath the alps probable tomographic information suggests a south vergent european lithospheric slab beneath the southern alps and the po plain despite the considerable number of studies aimed at resolving the deepest part of alpine orogeny the available quantitative information on the sub moho structure cannot be rated as certain

olivine anisotropy suggests gutenberg discontinuity is not the base of the lithosphere tectonic plates are a key feature of earth s structure and their behavior and dynamics are fundamental drivers in a wide range of large scale processes the operation of plate tectonics in general depends intimately on the manner in which lithospheric plates couple to the convecting interior current debate centers on whether the transition from rigid lithosphere to flowing asthenosphere relates to increases in temperature or to changes in composition such as the presence of a small amount of melt or an increase in water content below a specified depth thus the manner in which the rigid lithosphere couples to the flowing asthenosphere is currently unclear here we present results from laboratory based torsion experiments on olivine aggregates with and without melt yielding an improved database describing the crystallographic alignment of olivine grains we combine this database with a flow model for oceanic upper mantle to predict the structure of the seismic anisotropy beneath ocean basins agreement between our model and seismological observations supports the view that the base of the lithosphere is thermally controlled this model additionally supports the idea that discontinuities in velocity and anisotropy often assumed to be the base of the lithosphere are instead intralithospheric features reflecting a compositional boundary established at midocean ridges not a rheological boundary

superweak asthenosphere in light of upper mantle seismic anisotropy earth s upper mantle includes a 200 km thick asthenosphere underneath the plates where viscosity and seismic velocities are reduced compared to the background this zone of weakness matters for plate dynamics and may be required for the generation of plate tectonics itself however recent seismological and electro magnetic studies indicate strong heterogeneity in thinner layers underneath the plates which if related to more extreme global viscosity reductions may require a revision of our understanding of mantle convection here i use dynamically consistent mantle flow modeling and the constraints provided by azimuthal seismic anisotropy as well as plate motions to explore the effect of a range of global and local viscosity reductions the fit between mantle flow model predictions and observations of seismic anisotropy is highly sensitive to radial and lateral viscosity variations i show that moderate sub oceanic viscosity reductions to 0 01 to 0 1 times the upper mantle viscosity are preferred by the fit to anisotropy and global plate motions depending on layer thickness lower viscosities degrade the fit to azimuthal anisotropy localized patches of viscosity reduction or layers of subducted asthenosphere however have only limited additional effects on anisotropy or plate velocities this indicates that it is unlikely that regional observations of sub plate anomalies are both continuous and indicative of dramatic viscosity reduction locally such weak patches may exist and would be detectable by regional anisotropy analysis for example however large scale plate dynamics are most likely governed by broad continent ocean asthenospheric viscosity contrasts rather than a thin possibly high melt fraction layer

sks splitting in the western indian ocean from land and seafloor seismometers plume plate and ridge signatures we present sks splitting measurements in the western indian ocean recorded on 20 land and 57 seafloor seismometers deployed by the rhum rum experiment reunion hotspot and upper mantle reunions unterer mantel we discuss our splitting observations within their geodynamic settings and compare them to sks splitting parameters predicted from an azimuthally anisotropic rayleigh wave tomography model that includes the rhum rum data we find that anisotropic directions poorly correlate with the present day motion of the somali plate which at 2 6 cm yr may be too slow to cause strongly sheared fabric in the asthenosphere fast split directions between la reunion and the central indian ridge cir trend e w and provide strong first seismological evidence for near horizontal flow in the asthenosphere that connects the reunion mantle upwelling with the cir supporting a long standing hypothesis on plume ridge interaction in the vicinity of the reunion hotspot we observe a seismic anisotropy pattern indicative of a parabolic asthenospheric flow controlled by the reunion mantle upwelling and its consecutive asthenospheric spreading we furthermore observe ridge normal along the cir and ridge parallel along the southwest indian ridge swir both mainly attributed to asthenospheric mantle flows in the mozambique channel between east africa and madagascar we attribute e w trending to frozen lithospheric structures recording the paleo orientation of the spreading ridges that enabled madagascar s separation away from africa based on the synopsis of this and previous sks splitting studies at mid ocean ridges we propose that ridge normal may develop at fast and intermediate spreading ridges e g cir and east pacific rise and ridge parallel could be characteristic to slow spreading ridges e g swir mid atlantic ridge and the paleo ridges in the mozambique channel