seismic detection of folded subducted lithosphere at the core mantle boundary seismic tomography of the d region beneath the caribbean reveals a 100 km vertical step in an otherwise flat d shear velocity discontinuity and localized low velocities at the edge of the slab material which may involve upwellings caused by the slab laterally displacing a thin hot thermal boundary layer seismic tomography has been used to infer that some descending slabs of oceanic lithosphere plunge deep into the earth s lower mantle1 2 the fate of these slabs has remained unresolved but it has been postulated that their ultimate destination is the lowermost few hundred kilometres of the mantle known as the d region relatively cold slab material may account for high seismic velocities imaged in d beneath areas of long lived plate subduction and for reflections from a seismic velocity discontinuity just above the anomalously high wave speed regions3 4 the d discontinuity itself is probably the result of a phase change in relatively low temperature magnesium silicate perovskite5 6 here we present images of the d region beneath the cocos plate using kirchhoff migration of horizontally polarized shear waves and find a 100 km vertical step occurring over less than 100 km laterally in an otherwise flat d shear velocity discontinuity folding and piling of a cold slab that has reached the core mantle boundary as observed in numerical and experimental models can account for the step by a 100 km elevation of the post perovskite phase boundary due to a 700 c lateral temperature reduction in the folded slab we detect localized low velocities at the edge of the slab material which may result from upwellings caused by the slab laterally displacing a thin hot thermal boundary layer

complex anisotropy in d beneath the eastern pacific from sks skks splitting discrepancies article i nfo although observations of seismic anisotropy in the lowermost mantle are abundant the mechanism which generates anisotropy in the d layer is not well understood most observational constraints on d anisotropy come from the splitting of shear waves that propagate nearly horizontally through the lowermost mantle however anisotropy in the d layer can also give rise to discrepancies in shear wave splitting for sks and skks phases for the same event station pair as these phases sample different regions of the lowermost mantle here i report observations of strongly discrepant sks skks splitting observed at broadband stations in western mexico and california in particular strong skks splitting with fast polarization directions near 60 and delay times up to 3 s is observed for a group of raypaths that sample a region of the d layer beneath the eastern pacific ocean a comparison of skks splitting with sks splitting observed from the same events as well as with sks splitting observed at a variety of backazimuths at each station suggests that the anomalous anisotropic structure is in the lowermost mantle my preferred model for the unusual anisotropic geometry in this region is large shear deformation in d at the edge of a region where slab material impinges upon the core mantle boundary resulting in lattice preferred orientation of lower mantle minerals

high velocity anomaly adjacent to the western edge of the pacific low velocity province koki idehara 1 satoru tanaka2 and nozomu takeuchi3 1department of earth and planetary science university of tokyo 7 3 1 hongo bunkyo ku tokyo 113 0033 japan e mail idehara eps s u tokyo ac jp 2institute for research on earth evolution japan agency for marine earth science and technology 2 15 natsushima cho yokosuka 237 0061 japan 3earthquake research institute university of tokyo 1 1 1 yayoi bunkyo ku tokyo 113 0032 japan

lowermost mantle anisotropy and deformation along the boundary of the african llsvp shear wave splitting of sk k s phases is often used to examine upper mantle anisotropy in specific cases however splitting of these phases may reflect anisotropy in the lowermost mantle here we present sks and skks splitting measurements for 233 event station pairs at 34 seismic stations that sample d beneath africa of these 36 pairs show significantly different splitting between the two phases which likely reflects a contribution from lowermost mantle anisotropy the vast majority of discrepant pairs sample the boundary of the african large low shear velocity province llsvp which dominates the lower mantle structure beneath this region in general we observe little or no splitting of phases that have passed through the llsvp itself and significant splitting for phases that have sampled the boundary of the llsvp we infer that the d region just outside the llsvp boundary is strongly deformed while its interior remains undeformed or weakly deformed

multiple seismic reflectors in earth s lowermost mantle the modern view of earth s lowermost mantle considers a d region of enhanced seismologically inferred heterogeneity bounded by the core mantle boundary and an interface some 150 300 km above it with the latter often attributed to the postperovskite phase transition in mgsio3 seismic exploration of earth s deep interior suggests however that this view needs modification so called scs and skks waves which probe the lowermost mantle from above and below respectively reveal multiple reflectors beneath central america and east asia two areas known for subduction of oceanic plates deep into earth s mantle this observation is inconsistent with expectations from a thermal response of a single isochemical postperovskite transition but some of the newly observed structures can be explained with postperovskite transitions in differentiated slab materials our results imply that the lowermost mantle is more complex than hitherto thought and that interfaces and compositional heterogeneity occur beyond the d region sensu stricto

a doubling of the post perovskite phase boundary and structure of the earth s lowermost mantle a new model of the deep earth allows geophysicists to measure variations in earth s temperature near the boundary between the rocky mantle and core using a recently discovered phase change in the lowest part of the earth s mantle where atoms rearrange themselves in crystals under extreme pressure and temperature the model also explains odd features of this part of the mantle such as patchy regions that reflect seismic waves generated by large earthquakes and inferences of dense partial melting combined with observations the results imply very large variations in temperature providing further evidence for circulation of material throughout the earth s mantle the thermal structure of the earth s lowermost mantle the d layer spanning depths of 2 600 2 900 kilometres1 is key to understanding the dynamical state and history of our planet earth s temperature profile the geotherm is mostly constrained by phase transitions such as freezing at the inner core boundary or changes in crystal structure within the solid mantle that are detected as discontinuities in seismic wave speed and for which the pressure and temperature conditions can be constrained by experiment and theory a recently discovered phase transition at pressures of the d layer2 3 4 is ideally situated to reveal the thermal structure of the lowermost mantle where no phase transitions were previously known to exist here we show that a pair of seismic discontinuities observed in some regions of d can be explained by the same phase transition as the result of a double crossing of the phase boundary by the geotherm at two different depths this simple model can also explain why a seismic discontinuity is not observed in some other regions and provides new constraints for the magnitude of temperature variations within d

the origin of d reflections a systematic study of seismic array data sets regional seismic studies of the earth s deep mantle indicate the presence of p and s wave reflectors at the top of the d layer such reflectors appear to be laterally intermittent occur at a range of depths and produce waveforms whose amplitude and polarity differ at different locations a number of possible explanations for d reflections have been proposed including a phase change from perovskite to post perovskite thermal or thermochemical discontinuities generated by subducted slabs alignment of anisotropic minerals and global or small scale chemical heterogeneity arising perhaps from core mantle interactions or primitive material however some of the observed heterogeneity in d structure may be related to lack of consistency between the style quality and number of seismic observations in different regions as well as complexity in wave propagation due to 3 d structure in this study we present a compilation of observations of d reflections for regions in which 1 a reflector is seen in both p and s wave arrivals and 2 consistent array seismology techniques have been applied to infer the properties of the reflectors using a recent monte carlo thermodynamic modelling method to convert changes in chemistry and mineralogy into changes in seismic velocity we compare the fit of different isotropic thermochemical structures to the observed seismic properties of d reflections namely the polarity and amplitude of the waveforms our results indicate that it may be possible to discriminate between different possible origins for d reflections on the basis of polarity observations alone amplitude measurements may provide further constraints but must be treated with caution due to a large number of factors which can influence waveform amplitudes in regions such as the caribbean which are characterized by high amplitude negative polarity p waveforms and positive polarity s waveforms a phase change from perovskite to post perovskite provides the most likely explanation for the discontinuity however in other regions notably siberia and the central pacific p and s waveforms both display a positive polarity and multiple explanations for the discontinuity are possible including subduction related chemical heterogeneity and anisotropy

observations of changing anisotropy across the southern margin of the african llsvp we present evidence for the presence of complex anisotropy in the lowermost mantle from 3 d waveform modelling of observed core diffracted shear waves that sample the southern edge of the african large low shear velocity province llsvp the anomalously strong amplitude of the sv component for the shear core diffracted phase at large distances indicates the presence of anisotropy we measure shear wave splitting parameters to determine which part of the elastic tensor is constrained by this particular data set the modelling is performed using the spectral element method the anisotropy is strong outside the llsvp weakens or rotates close to its boundary and appears to be absent inside the llsvp the presence of the llsvp margin may cause flow in the mantle to change direction the occurrence of strong anisotropy in the region of fast seismic velocities is compatible with lattice preferred orientation in post perovskite due to accommodation of flow through dislocation creep

closure of the mongol okhotsk ocean insights from seismic tomography and numerical modelling abstract the existence of the palaeozoic and mesozoic mongol okhotsk ocean is evident from the mongol okhotsk suture which stretches from central mongolia to the sea of okhotsk a lack of sufficient palaeomagnetic data and an otherwise diffuse suture with an abrupt termination to the west has led to difficulties in reconstructing the history geometry and closure of this ocean both the timing and style of the ocean s closure are unclear and have led to several alternative reconstructions closure timing ranges between the late jurassic 155 ma and beginning of the early cretaceous 120 ma and the proposed kinematics include contemporaneous subduction along two opposite margins subduction along only one margin or with a component of left lateral shear in the present study numerical models of mantle convection are coupled with global plate reconstructions to investigate ambiguities regarding the closure of the mongol okhotsk ocean in order to decipher the tectonic history of this enigmatic region two end member scenarios of subduction location either along the present day northern or the southern margins of the mongol okhotsk ocean are imposed as kinematic surface boundary conditions for the past 230 myrs through a comparison to seismic tomography the results indicate a preferred subduction history along the siberian margin relative northern margin of the mongol okhotsk ocean at present day the slab remnant is predicted to be located farther west than previously proposed furthermore we find that the subducting slabs in this region generate a hot dense pile at the same location and with a similar shape as the perm anomaly

shear velocity variation within the d region beneath the central pacific 1 small scale variability of shear velocity vs structure in the d region beneath the central pacific is imaged using 442 broadband tangential component s waveforms recorded in western north america for 37 intermediate and deep focus tonga fiji earthquakes double array stacking of spatially binned subsets of data reveals lateral variations in the relative timing and amplitude of deep mantle discontinuity reflections on scale lengths of about 130 km across the 6 8 region of d sampled waveform modeling using localized one dimensional structures indicates variations of the vs increase at the d discontinuity ranging from 0 5 to 2 3 and discontinuity depths ranging from 2490 to 2735 km deepening and weakening from southwest to northeast across the study area two abrupt vs reductions are also detected within the d layer we thus present a laterally variable three layer model of the d region beneath the central pacific the complex structure may be associated with lateral thermal and chemical gradients that produce a lens of postperovskite material above an ultralow velocity zone on the margin of a large low shear velocity province

towards absolute plate motions constrained by lower mantle slab remnants the movement of tectonic plates before the cretaceous period is poorly understood a global mantle tomography model suggests that the longitude of oceanic subduction zones up to 300 million years ago was offset by up to 18 compared with tectonic reconstructions for the same period

living dead slabs in 3 d the dynamics of compositionally stratified slabs entering a slab graveyard above the core mantle boundary abstract segregation of subducted mid ocean ridge basalt morb at the cmb has been identified as a potentially important mechanism in the long term evolution of the mantle and core here three dimensional 3 d and two dimensional 2 d simulations of a compositionally stratified slab reaching the cmb are presented with the goals of characterising the resulting thermo chemical structures for comparison with seismic studies and of quantifying the fraction of morb that is able to segregate and remain near the cmb compositional stratification of the slab results in a strong torque due to the relatively high density of basalt and low density of harzburgite which tends to rotate the slab such that the basalt side faces down slab cmb interaction is characterised by heating up of the slab followed by separation of the basalt and harzburgite layers with harzburgite rising in vigorous plumes plumes form at the edges and sides of slabs at the cmb as well as in their interiors as previously observed for purely thermal slabs with plume heads dominated by depleted harzburgitic material sometimes with small amounts of entrained basalt while plume tails entrain basaltic material segregation of basalt depends strongly on the presence or absence of a preexisting dense layer at the cmb and by dimensionality a preexisting dense layer greatly increases the fraction of basalt that segregates from the slab in 3 d this fraction is in the range 0 5 0 7 with a layer present compared to 0 25 0 45 with no preexisting layer two modes of basalt segregation are observed for slabs that land basalt side up i hot harzburgite extruding from the sides and edges and ii hot harzburgite rich plumes bursting through the basalt layer as previously observed in laboratory experiments whereas for a slab that lands basalt side down iii hot basalt can peel off from its underside displaying fingering instabilities in 3 d furthermore basalt harzburgite segregation is sometimes observed in slab segments that have already been heated at the cmb and risen a few hundred km by a folding mechanism a range of interesting structures are observed in the model cmb region which may be useful in interpreting seismic observations strong lateral gradients in composition and temperature are quite common structures include plumes next to vertical slab segments inverted slab sections perched above the cmb and harzburgite curtains the presence of a stratified layer above the cmb as observed in global simulations in which a layer builds up by basalt segregation strongly suppresses plume formation 2 d simulations give a good first order guide to the dynamics obtained in fully 3 d geometry but inherently 3 d structures are missed plumes versus sheets fingering and there is a quantitative difference in the fraction of basalt segregated

lower mantle seismic discontinuities and the thermal morphology of subducted slabs abstract the nature of the lowermost few 100 km of the mantle holds insights into the scale of convection of the earth s mantle here we investigate the fine structure of this region using seismic migration of shear waves reflecting from velocity discontinuities near the core mantle boundary cmb recorded by a dense network of european seismic stations two main features are found one structure exhibits large scale topography and can be explained by a sharp increase in seismic velocity 206 316 km above the cmb the second structure lies 55 85 km above the cmb and marks a sharp reduction in velocity to explain the origin of these discontinuities we appeal to numerical simulations of three dimensional 3d mantle convection incorporating rigidly moving plates the plates organize the convective flow to allow for the development of large convection cells and cool sheet like downwellings at a convergent plate boundary the results suggest that in places observed seismic complexity in d discontinuity structure may be due to the thermal gradients resulting from the accumulation of cool subducted plate material at the cmb

geographic correlation between hot spots and deep mantle lateral shear wave velocity gradients abstract hot spot volcanism may originate from the deep mantle in regions exhibiting the earth s most pronounced lateral s wave velocity gradients these strong gradient regions display an improved geographic correlation over s wave velocities to surface hot spot locations for the lowest velocities or strongest gradients occupying 10 of the surface area of the core mantle boundary cmb hot spots are nearly twice as likely to overlie the anomalous gradients if plumes arise in an isochemical lower mantle plume initiation should occur in the hottest thus lowest velocity regions or in the regions of strongest temperature gradients however if plume initiation occurs in the lowest velocity regions of the cmb lateral deflection of plumes or plume roots are required the average lateral deflections of hot spot root locations from the vertical of the presumed current hot spot location ranges from 300 to 900 km at the cmb for the 10 30 of the cmb covered by the most anomalous low s wave velocities the deep mantle may however contain strong temperature gradients or be compositionally heterogeneous with plume initiation in regions of strong lateral s wave velocity gradients as well as low s wave velocity regions if mantle plumes arise from strong gradient regions only half of the lateral deflection from plume root to hot spot surface location is required for the 10 30 of the cmb covered by the most anomalous strong lateral s wave velocities we find that strong gradient regions typically surround the large lower velocity regions in the base of the mantle which may indicate a possible chemical in addition to thermal component to these regions

effect of fe enrichment on seismic properties of perovskite and post perovskite in the deep lower mantle recent experimental measurements of the equation of state of perovskites and post perovskites in the mg fe sio3 and mg fe al fe al si o 3 systems over a wide range of iron contents are used to constrain the effects of fe and al on density and bulk modulus of these phases at deep mantle pressures the density of fe bearing perovskite follows a linear relationship with fe content at a representative mid mantle depth of 1850 km 80 gpa 80 g cm 3 5 054 1 1 270 3 x fe the bulk modulus of silicate perovskite is not sensitive to fe content and follows the relationship k 80 gpa 546 2 12 25 x fe the velocity heterogeneity parameter partial derivative ln v b partial derivative x fe determined by experimental values for the bulk sound speed is 0 10 1 in agreement with theory and the behaviour of other fe bearing silicates near the core mantle boundary fe rich post perovskite is observed to be more compressible than the mg end member in contrast to theoretical predictions from experimental data the densities of perovskite and post perovskite at 125 gpa 2700 km depth are 125 pv g cm 3 5 426 11 1 38 4 x fe and 125 ppv g cm 3 5 548 1 1 41 3 x fe the density contrast across the post perovskite transition is similar to 2 per cent irrespective of fe content but the contrast in bulk sound speed increases with fe content al rich silicates exhibit no significant differences in density or compressibility relative to al free silicates but may be responsible for seismic heterogeneities due to differences in the depth and width of the post perovskite transition observations of increased densities in large low shear velocity provinces and ultra low velocity zones may be consistent with local iron enrichment from mg 90 to mg 78 88 and mg 50 respectively

temperature profile in the lowermost mantle from seismological and mineral physics joint modeling the internal structure of the core mantle boundary cmb region of the earth plays a crucial role in controlling the dynamics and evolution of our planet we have developed a comprehensive model based on the radial variations of shear velocity in the d layer the base of the lower mantle and the high p t elastic properties of major candidate minerals including the effects of post perovskite phase changes this modeling shows a temperature profile in the lowermost mantle with a cmb temperature of 3 800 200 k which suggests that lateral temperature variations of 200 300 k can explain much of the large velocity heterogeneity observed in d a single crossing phase transition model was found to be more favorable in reproducing the observed seismic wave velocity structure than a double crossing phase transition model

anisotropy in the deep earth abstract seismic anisotropy has been found in many regions of the earth s interior its presence in the earth s crust has been known since the 19th century and is due in part to the alignment of anisotropic crystals in rocks and in part to patterns in the distribution of fractures and pores in the upper mantle seismic anisotropy was discovered 50 years ago and can be attributed for the most part to the alignment of intrinsically anisotropic olivine crystals during large scale deformation associated with convection there is some indication for anisotropy in the transition zone particularly in the vicinity of subducted slabs here we focus on the deep earth the lower mantle and core where anisotropy is not yet mapped in detail nor is there consensus on its origin most of the lower mantle appears largely isotropic except in the last 200 300 km in the d region where evidence for seismic anisotropy has been accumulating since the late 1980s mostly from shear wave splitting measurements recently a picture has been emerging where strong anisotropy is associated with high shear velocities at the edges of the large low shear velocity provinces llsvps in the central pacific and under africa these observations are consistent with being due to the presence of highly anisotropic mgsio 3 post perovskite crystals aligned during the deformation of slabs impinging on the core mantle boundary and upwelling flow within the llsvps we also discuss mineral physics aspects such as ultrahigh pressure deformation experiments first principles calculations to obtain information about elastic properties and derivation of dislocation activity based on bonding characteristics polycrystal plasticity simulations can predict anisotropy but models are still highly idealized and neglect the complex microstructure of polyphase aggregates with strong and weak components a promising direction for future progress in understanding the origin of seismic anisotropy in the deep mantle and its relation to global mantle circulation is to link macroscopic information from seismology and microscopic information mineral physics through geodynamics modeling anisotropy in the inner core was proposed 30 years ago to explain faster p wave propagation along the direction of the earth s axis of rotation as well as anomalous splitting of core sensitive free oscillations there is still uncertainty about the origin of this anisotropy in particular it is difficult to explain its strength based on known elastic properties of iron as it would require almost perfect alignment of iron crystals indeed the strongly anomalous p travel times observed on paths from the south sandwich islands to alaska may or may not be due to inner core anisotropy and will need to be explained before consensus can be reached on the strength of anisotropy in the inner core and its origin

complex seismic anisotropy at the border of a very low velocity province at the base of the earth s mantle 1 we constrain the anisotropy associated with a very low velocity province vlvp at the base of the earth s mantle using the sks and skks waves sampling the region our selected high quality data sets consist of 415 sks and 111 skks waveforms for 127 deep earthquakes recorded at distances between 90 and 150 by the seismic stations in three temporary broadband passcal seismic arrays the kaapvaal seismic array 1997 1999 the tanzania seismic array 1994 1995 and the ethiopia kenya seismic array 1999 2001 as well as the permanent stations in the global seismographic network these seismic data provide good sampling coverage for some portion of the vlvp and its surrounding areas our results show when the sks or skks phases sample the regions away from the border of the vlvp inside or outside the vlvp the apparent splitting parameters inferred from the sks phases are consistent with those inferred from the skks phases and their variations strongly correlate with seismic stations but not with the exit points at the core mantle boundary of these seismic phases however when the sks or skks phases sample near the border of the vlvp the apparent splitting parameters inferred from the sks phases and skks phases are different and their variations no longer correlate with seismic stations these features indicate that part of the shear wave splitting for the seismic data sampling the border of the vlvp has to originate from deep mantle most likely near the border of the vlvp we assume that the anisotropy in the shallow mantle beneath seismic stations has a horizontal hexagonal symmetry axis and infer the splitting parameters associated with the shallow anisotropy beneath the seismic stations using the sks and skks waveforms for the seismic data sampling the regions away from the border of the vlvp we then obtain the splitting parameters associated with the lowermost mantle anisotropy using the sks and skks waveforms corrected for the inferred shallow anisotropy beneath seismic stations assuming that the medium in the lowermost mantle also has a horizontal hexagonal symmetry axis our results reveal a complex anisotropy pattern near the border of the vlvp such a complex anisotropy pattern may be explained by lattice preferred orientation of anisotropic mantle aggregates aligned by a complex mantle flow near the vlvp margins the complex flow pattern near the vlvp margins may indicate strong interactions between the vlvp a compositional anomaly and the surrounding normal mantle and may provide an explanation to the concentration of some hot spots geographically near the borders of the vlvps in the lowermost mantle

seismic evidence for slab graveyards atop the core mantle boundary beneath the indian ocean geoid low abstract the indian ocean geoid low iogl that spans a vast areal extent south of the indian subcontinent is a spectacular feature on the earth whose origin still remains ambiguous in this study we investigate the seismic character of the lower mantle below this geoid low utilizing the travel time and amplitude residuals of high quality s and scs phases from 207 earthquakes recorded at 276 stations in the epicentral distance range of 36 90 for comparison we also perform a similar exercise for a region of geoid high in the vicinity results reveal large variations in the scs travel times indicating that the lowermost mantle beneath the iogl region is heterogeneous the scs s differential travel times are 3 s slower than those predicted by the iasp91 model primarily due to velocity increase in the lowermost mantle beneath the iogl region and 2 s higher than the iasp91 beneath the geoid high region due to velocity decrease in the lowermost mantle the largest negative residuals from manual method 7 72 s are concentrated below the iogl iterative matching of differential travel time residuals reveals that the maximum positive and negative residuals can be explained in terms of a reduction in shear velocity of 0 9 and an increase of 1 6 respectively in a 1000 km thick layer above the core mantle boundary further the scs s amplitude residuals beneath the iogl are positive implying high impedance contrast at the core mantle boundary owing to the presence of high velocity material we attribute these high velocities to the presence of dehydrated high density slab graveyards atop the core mantle boundary beneath the indian ocean release of water at the mid to upper mantle depths due to the dehydration of subducted slabs causing a reduction in density and velocity of the ambient mantle could be responsible for the geoid low

strong seismic scatterers near the core mantle boundary north of the pacific anomaly abstract tomographic images have shown that there are clear high velocity heterogeneities to the north of the pacific anomaly near the core mantle boundary cmb but the detailed structure and origin of these heterogeneities are poorly known in this study we analyze pkp precursors from earthquakes in the aleutian islands and kamchatka peninsula recorded by seismic arrays in antarctica and find that these heterogeneities extend 400 km above the cmb and are distributed between 30 and 45 n in latitude the scatterers show the largest p wave velocity perturbation of 1 0 1 2 in the center 160 180 e and 0 5 to the west and east 140 160 e 180 200 e scs s differential travel time residuals reveal similar features we suggest that these seismic scatterers are the remnants of ancient subducted slab material the lateral variations may be caused either by different slabs or by variations in slab composition resulting from their segregation process

lowermost mantle anisotropy beneath the north pacific from differential s scs splitting summary seismic anisotropy is an important tool for studying the nature origin and dynamics of the lowermost mantle d we introduce differential s scs splitting as a tool for removing the effect of near source and near receiver anisotropy to estimate splitting accrued in the d region this is applicable to events recorded at epicentral distances between 60 and 85 near source anisotropy has often been ignored in previous studies of lowermost mantle anisotropy we apply differential s scs splitting to records from canadian national seismic network stations of western pacific earthquakes these sample the lowermost mantle beneath the north pacific the residual splitting in scs which we attribute to d shows lag times between 1 0 and 3 9 s given the near horizontal ray path of scs in d we interpret the recovered fast directions as the orientation of the fast shear wave in the plane defined by the vertical and transverse directions and observe a clearly non vti transverse isotropy with a vertical axis of symmetry style of anisotropy the largest population of results shows an approximately southeasterly dipping symmetry axis which we speculate might be explained by descending palaeoslab material being swept horizontally across the core mantle boundary towards an upwelling region beneath the central pacific non vti symmetry and the many possible contributions to d anisotropy from lower mantle minerals melt and subducted materials suggest that our understanding of the lowermost mantle could be greatly improved by trying to resolve a more general style of anisotropy

gypsum a joint tomographic model of mantle density and seismic wave speeds 1 gypsum is a 3 d model of mantle shear wave s speeds compressional wave p speeds and density the model is developed through simultaneous inversion of seismic body wave travel times p and s and geodynamic observations while using realistic mineral physics parameters linking wave speeds and density geodynamic observations include the global free air gravity field divergence of the tectonic plates dynamic topography of the free surface and the flow induced excess ellipticity of the core mantle boundary gypsum is built with the philosophy that heterogeneity that most closely resembles thermal variations is the simplest possible solution models of the density field from earth s free oscillations have provided great insight into the density configuration of the mantle but are limited to very long wavelength solutions alternatively scaling higher resolution seismic images to obtain density anomalies generates density fields that do not satisfy geodynamic observations the current study provides a 3 d density model for the mantle that directly satisfies geodynamic and seismic observations through a joint seismic geodynamic inversion process notable density field observations include high density piles at the base of superplume structures supporting the general results of past normal mode studies however we find that these features are more localized and have lower amplitude than past studies would suggest when we consider both fast and slow seismic anomalies in gypsum we find that p and s wave speeds are strongly correlated throughout the mantle however we find a low correlation of fast s wave zones in the deep mantle 1500 km depth with the corresponding p wave anomalies suggesting a systematic divergence from simplified thermal effects in ancient subducted slab anomalies the cratonic lithosphere and d regions are shown to have strong compositional signatures however we argue that temperature variations are the primary cause of p wave speed s wave speed and density anomalies throughout most of the mantle

a lower mantle s wave triplication and the shear velocity structure of d summary a lower mantle s wave triplication detected with short and longperiod wwssn and csn recordings indicates a substantial shear velocity discontinuity near 280 km above the core mantle boundary the triplication can be observed in rotated sh seismograms from intermediate and deep focus events throughout the distance range from 70 to 95 three distinct source region receiver array combinations that have been investigated in detail demonstrate consistent travel time and relative amplitude behaviour of the triplication with slight systematic shifts in the triplication indicating up to 40km variations in the depth of the discontinuity modelling of the observations with synthetic seismograms produced with the cagniard de hoop and reflectivity methods constrains the shear velocity increase to be 2 75 k 0 25 per cent comparable to upper mantle discontinuities short period observations indicate that the velocity increase may be a sharp first order discontinuity or may extend over a transition zone no more than 50 km thick the shear velocity gradient below the discontinuity within the d layer is not well constrained by the sh data but slightly positive or near zero velocity gradients are consistent with the long period amplitude ratios of scsh sh

seismic anisotropy in the lowermost mantle near the perm anomaly the lower mantle is dominated by two large structures with anomalously low shear wave velocities known as large low shear velocity provinces llsvps several studies have documented evidence for strong seismic anisotropy at the base of the mantle near the edges of the african llsvp recent work has identified a smaller structure with similar low shear wave velocities beneath eurasia dubbed the perm anomaly here we probe lowermost mantle anisotropy near the perm anomaly using the differential splitting of sks and skks phases measured at stations in europe we find evidence for lowermost mantle anisotropy in the vicinity of the perm anomaly with geographic trends hinting at lateral variations in anisotropy across the boundaries of the perm anomaly as well as across a previously unsampled portion of the african llsvp border our observations suggest that deformation is concentrated at the boundaries of both the perm anomaly and the african llsvp

s to p conversions from mid mantle slow scatterers in slab regions observations of deep stagnated oceanic crust the fate of a subducted slab is a key ingredient in the context of plate tectonics yet it remains enigmatic especially in terms of its crustal component in this study our efforts are devoted to resolve slab related structures in the mid mantle below eastern indonesia the izu bonin region and the peru area by employing seismic array analysing techniques on high frequency waveform data from f net in japan and the alaska regional network and the usarray in north america a pronounced arrival after the direct p wave is observed in the recordings of four deep earthquakes depths greater than 400 km from three subduction systems including the philippines the izu bonin and the peru this later arrival displays a slightly lower slowness compared to the direct p wave and its back azimuth deviates somewhat from the great circle direction we explain it as an s to p conversion at a deep scatterer below the sources in the source region in total five scatterers are seen at depths ranging from 930 to 1500 km those scatterers appear to be characterised by an 7 km thick low velocity layer compared to the ambient mantle combined evidence from published mineral physical analysis suggests that past subducted oceanic crust possibly fragmented is most likely responsible for these thin layer compositional heterogeneities trapped in the mid mantle beneath the study regions our observations give a clue to the potential fate of subducted oceanic crust

mineralogical effects on the detectability of the postperovskite boundary the discovery of a phase transition in mg silicate perovskite pv to postperovskite ppv at lowermost mantle pressure temperature p t conditions may provide an explanation for the discontinuous increase in shear wave velocity found in some regions at a depth range of 200 to 400 km above the core mantle boundary hereafter the d discontinuity however recent studies on binary and ternary systems showed that reasonable contents of fe2 and al for pyrolite increase the thickness width of the mixed phase region of the pv ppv boundary 400 600 km to much larger than the d discontinuity 70 km these results challenge the assignment of the d discontinuity to the pv ppv boundary in pyrolite homogenized mantle composition furthermore the mineralogy and composition of rocks that can host a detectable pv ppv boundary are still unknown here we report in situ measurements of the depths and thicknesses of the pv ppv transition in multiphase systems san carlos olivine pyrolitic and midocean ridge basaltic compositions at the p t conditions of the lowermost mantle searching for candidate rocks with a sharp pv ppv discontinuity whereas the pyrolitic mantle may not have a seismologically detectable pv ppv transition due to the effect of al harzburgitic compositions have detectable transitions due to low al content in contrast al rich basaltic compositions may have a detectable pv ppv boundary due to their distinct mineralogy therefore the observation of the d discontinuity may be related to the pv ppv transition in the differentiated oceanic lithosphere materials transported to the lowermost mantle by subducting slabs

origin of crude oils from oilfields in the zagros fold belt southern iraq relation to organic matter input and paleoenvironmental conditions crude oil samples from cretaceous and tertiary reservoir sections in the zagros fold belt oil fields southern iraq were investigated using non biomarker and biomarker parameters the results of this study have been used to assess source of organic matter and the genetic link between oils and their potential source rocks in the basin the oils are characterized by high sulphur and trace metal ni v contents and relatively low api gravity values 17 4 22 7 api this indicates that these oils are heavy and generated from a marine source rock containing type ii s kerogen this is supported by their biomarker distributions of normal alkanes regular isoprenoids terpanes and steranes and the bulk carbon isotope compositions of their saturated and aromatic hydrocarbons the oils are characterized by low pr ph ratios 1 high values of the c35 homohopane index and c31 22r c30 hopane ratios relatively high c27 sterane concentrations and the predominance of c29 norhopane these biomarkers suggest that the oils were generated predominantly from a marine carbonate source rock deposited under reducing conditions and containing plankton algal and microorganisms source input the presence of gammacerane also suggests water column stratification during source rock deposition the biomarker characteristics of the oils are consistent with those of the middle jurassic sargelu carbonate as the effective source rock in the basin biomarker maturity data indicate that the oils were generated from early maturity source rocks

pkp precursors implications for global scatterers precursors to the core phase pkp are generated by scattering of seismic energy from heterogeneities in the mantle here we examine a large global data set of pkp precursors in individual seismograms and array data to better understand scattering locations the precursor amplitudes from individual seismograms are analyzed with respect to the inner core phase pkikp and mantle phase pp we find and correct for a hemispherical asymmetry in the precursor pkikp amplitudes resulting from inner core structure using ray tracing we locate scatterers in our array data and use these to infer scattering locations in the individual data the scattering strength displays regional variation however we find no relationship with long scale core mantle boundary velocity structure scattering is observed in all regions of data coverage as are paths with no precursors this indicates that scattering occurs from various small scale heterogeneities including but not limited to ultralow velocity zones or partial melt and slabs

a new method for the computation of global viscoelastic post seismic deformation in a realistic earth model ii horizontal displacement summary post seismic deformation caused by the 2004 sumatra andaman earthquake m 9 3 has been observed by space geodetic techniques such as the global positioning system gps and the gravity recovery and climate experiment grace to estimate such deformation with a spatial scale exceeding 100 km we can use theories of global post seismic deformation in which the earth is treated as a self gravitating viscoelastic sphere previous theories have imposed limitations on employed earth models such as neglecting compressibility and time variation of self gravitation or simplifying the radial profile of the viscosity this is because the normal mode method on which these theories are based cannot evaluate a denumerably infinite set of eigenmodes that will appear when those limitations are removed in the practical computation to bypass this difficulty in our previous paper we showed that the numerical inverse laplace integration with a rectangular path enabled us to obtain the vertical deformation without using those approximations in this paper we apply the same method to the horizontal deformation for a complete set of the fault mechanisms strike slip dip slip and vertical and horizontal tensile next we reveal error sources in our approach the largest error arises in different selections of the integration path this error is less than 1 per cent in the lower degree deformations n 10 and 0 1 0 5 per cent in the higher degree ones 100 n 1000 relative to the result obtained with an independent method finally we elucidate differences in far field deformations obtained for the earth model employed in this study and those used in the previous studies for a large dip slip event mw 8 3 the result shows that 1 differences of 6 50 per cent are seen in the coseismic offset and 2 horizontal rate differences of 1 5 mm yr 1 5 25 per cent of the rates are observed at the epicentral distance of 100 km during the first thirty years after the event these differences are detectable with gps therefore for theory to meet observational accuracy effects of compressibility and stratification on global post seismic deformation should be considered

modeling the ratios of skks and sks amplitudes with ultra low velocity zones at the core mantle boundary 1 between 105 115 degrees the sks waveform is complicated by the formation of spdks a wave that has segments of p diffraction along the core mantle boundary while previous studies have primarily focused on the move out of spdks from sks we analyze the concomitant reduction of the sks amplitude long period skks sks amplitude ratios present a coherent global pattern skks sks is relatively large in north and south american recordings of deep tonga fiji earthquakes but prem like in european recordings of earthquakes in south america and north american recordings of earthquakes in indonesia modeling of skks sks indicate that ultra low velocity zones ulvzs layers at the base of the mantle with a thickness of about 10 20 km and a shear velocity reduction between 20 30 are required to explain high skks sks ratios and the early move out of spdks

thermochemical structures beneath africa and the pacific ocean large low velocity seismic anomalies have been detected in the earth s lower mantle beneath africa and the pacific ocean that are not easily explained by temperature variations alone the african anomaly has been interpreted to be a northwest southeast trending structure with a sharp edged linear ridge like morphology the pacific anomaly on the other hand appears to be more rounded in shape mantle models with heterogeneous composition have related these structures to dense thermochemical piles or superplumes it has not been shown however that such models can lead to thermochemical structures that satisfy the geometrical constraints as inferred from seismological observations here we present numerical models of thermochemical convection in a three dimensional spherical geometry using plate velocities inferred for the past 119 million years we show that earth s subduction history can lead to thermochemical structures similar in shape to the observed large lower mantle velocity anomalies we find that subduction history tends to focus dense material into a ridge like pile beneath africa and a relatively more rounded pile under the pacific ocean consistent with seismic observations

constraints on small scale heterogeneity in the lowermost mantle from observations of near podal pcp precursors abstract volumetric heterogeneities on large 1000 km and intermediate scales 100 km in the lowermost mantle have been established with seismological approaches however there are controversies regarding the level of heterogeneity in the lowermost mantle at small scales a few kilometers to tens of kilometers with lower bound estimates ranging from 0 1 to a few percent we take advantage of the small amplitude pcp waves at near podal distances 0 12 to constrain the level of small scale heterogeneity within 250 km above the cmb first we compute short period synthetic seismograms with a finite difference code for a series of volumetric heterogeneity models in the lowermost mantle and find that pcp is not identifiable if the small scale heterogeneity in the lowermost mantle is above 2 5 we then use a functional form appropriate for coda decay to suppress p coda contamination by comparing the corrected envelope of pcp and its precursors with synthetic seismograms we find that perturbations of small scale 8 km heterogeneity in the lowermost mantle is 0 2 0 5 beneath regions of the china myanmar border area okhotsk sea and south america whereas strong perturbations 1 0 are found beneath central america in the regions studied we find that this particular type of small scale heterogeneity in the lowermost mantle is weak yet there are some regions requiring heterogeneity up to 1 0 where scattering is stronger such as under central america more chemically complex mineral assemblages may be present at the core mantle boundary

chemical trends in ocean islands explained by plume slab interaction earth s surface shows many features of which the genesis can be understood only through their connection with processes in earth s deep interior recent studies indicate that spatial geochemical patterns at oceanic islands correspond to structures in the lowermost mantle inferred from seismic tomographic models this suggests that hot buoyant upwellings can carry chemical heterogeneities from the deep lower mantle toward the surface providing a window to the composition of the lowermost mantle the exact nature of this link between surface and deep earth remains debated and poorly understood using computational models we show that subducted slabs interacting with dense thermochemical piles can trigger the ascent of hot plumes that inherit chemical gradients present in the lowermost mantle we identify two key factors controlling this process i if slabs induce strong lower mantle flow toward the edges of these piles where plumes rise the pile facing side of the plume preferentially samples material originating from the pile and bilaterally asymmetric chemical zoning develops ii the composition of the melt produced reflects this bilateral zoning if the overlying plate moves roughly perpendicular to the chemical gradient in the plume conduit our results explain some of the observed geochemical trends of oceanic islands and provide insights into how these trends may originate

seismic evidence for the depression of the d discontinuity beneath the caribbean implication for slab heating from the earth s core the lowermost 100 300 km of the earth s mantle commonly regarded as the thermal boundary layer tbl of mantle circulation is characterized by its complex physical properties beneath the caribbean this so called d layer features relatively high velocities and abrupt impedance increase at the top designated as the d discontinuity these seismic characteristics have been attributed to the accumulation of ancient subducted slab material and the phase transition in the major lower mantle mineral of pervoskite geodynamic models predict that the blanketing cold slabs may trap enough heat from core to be buoyantly destabilized and eventually broken apart and entrained into the bottom of the convection cell here we explore the d structure with unprecedented resolution through modeling traveltimes amplitudes and waveform shapes from the usarray we find an east to west asymmetrical undulation of the d discontinuity with a v shaped depression of 70 160 km over a lateral distance of 600 km beneath northern south america the shear velocity perturbations vary in the same trend showing the most pronounced reduction of 3 4 below the thinnest d layer in close proximity to an intermittently undetected discontinuity the strong correlation between the d topography and velocity variations indicates the phase transition boundary has been perturbed or even disrupted by the large lateral temperature gradient of slab material which has been reheated from the core over extended periods of time

the lowermost mantle beneath northern asia ii evidence for lower mantle anisotropy summary we have analysed prediffracted s waves with turning points beneath northern siberia in a study of anisotropy in the lowermost mantle deep focus earthquakes beneath the marianas izu bonin and the sea of japan recorded at stations in western europe are used a correction for upper mantle anisotropy is applied to the data comparisons of the data with synthetic data for models with and without a high velocity d layer suggest that there is a velocity discontinuity at the top of the d region and that the style of anisotropy is transversely isotropic in this region time separations between s waves on the radial and transverse component show a weak trend where the separation increases with epicentral distance a normalization of this separation with the travel distance within d 300 km thick in this region suggests that the anisotropy is uniformly distributed within this layer and has an average value of 0 5 per cent a combination of different studies which investigate the structure of the lowermost mantle beneath europe and northern siberia reveals a complicated picture tomographic models from this area and evidence of d anisotropy lower mantle scatterers reflections from a d discontinuity and ultralow velocity zones suggest two distinct regions one exhibits high velocities d anisotropy a d discontinuity and no evidence of scatterers or ultra low velocity zones these features are likely associated with the palaeosubduction of the izanagi plate well into the lowermost mantle the other region has a lower overall velocity and shows evidence of scatterers and ultra lowvelocity zones perhaps suggesting the presence of partial melt these results suggest dramatic lateral variations in the nature of the lowermost mantle beneath northern asia over a length scale of roughly 30 degrees

d discontinuity structure beneath the north atlantic from scd observations we analyzed transverse and radial component recordings from the 2010 m6 3 southern spain earthquake depth 620 km recorded on 370 broadband stations in north america we grouped these seismograms into subarrays and applied fourth root vespa processing vespagram analysis in order to enhance low amplitude arrivals these vespagrams show clear scd arrivals which indicate the existence of the d discontinuity beneath the north atlantic ocean 45 60 n 45 55 w these observations are best fit with a 2 4 velocity increase at the top of the d discontinuity at a height above the core mantle boundary of 304 14 km we do not observe scd arrivals at the eastern end of our study region which is consistent with the presence of the easternmost edge of the ancient farallon plate

lowermost mantle anisotropy beneath the northwestern pacific evidence from pcs scs sks and skks phases 1 in this study we utilize data from broadband seismic stations of the japanese f net network to investigate anisotropic structure in the lowermost mantle beneath the northwestern pacific the comparison of shear wave splitting from phases that have similar paths in the upper mantle but different paths in the lowermost mantle such as pcs scs or sks skks can yield constraints on anisotropy in the d region we measured splitting for sks skks pcs and scs phases at f net stations and compared the measured splitting parameters to previously published estimates of upper mantle anisotropy beneath the region we observed many examples of discrepant sks skks splitting and splitting of scs pcs sks and skks phases that does not agree well with the known upper mantle anisotropy beneath individual stations the most likely explanation for these discrepancies is a contribution to splitting from anisotropy in the lowermost mantle in particular for sks skks phases that sample the lowermost mantle just to the south and east of the kamchatka peninsula we observed generally n s fast directions with delay times between 0 5 and 1 5 s these data suggest the presence of a fairly large coherent region of deformation in the lowermost mantle beneath the northwestern pacific our preferred model for these observations is that solid state flow at the base of the mantle induces a lattice preferred orientation of lowermost mantle minerals generating a seismically anisotropic fabric

the trouble with seeing double 1 the recently discovered phase transition for mg fe sio3 from perovskite to post perovskite structure for pressure temperature conditions in the lowermost mantle provides a plausible mechanism to explain a several percent shear velocity increase detected at the top of the d region in many regions the phase transition has a large positive clapeyron slope and given the likely presence of a strong temperature increase just above the core mantle boundary d conditions may reintersect the phase boundary resulting in a thin layer of perovskite below a lens of post perovskite separated by a few percent velocity decrease detection of such a velocity decrease is shown to be much harder than detection of the shallower velocity increase due to fundamental properties of seismic waves purported observations in support of a double discontinuity are re evaluated based on waveform modeling waveform stacking procedures are required for reliable detection of the lower half of any double crossing of the phase boundary

possible evidence for a double crossing phase transition in d beneath central america from inversion of seismic waveforms 1 we invert seismic body wave waveform data for the vertical dependence of isotropic shear velocity in the d layer beneath central america using the transverse components of relatively long period broadband waveforms 20 200 s as data our models suggest that the s velocity increase in d may be localized in the zone from 100 200 km above the core mantle boundary cmb while the s velocity does not significantly deviate from prem in the zone from 0 100 km above the cmb this suggests the possible existence of a double crossing a reverse phase transition from post perovskite back to perovskite at a depth of 100 km above the cmb resolution tests indicate that our methods and data can resolve the vertical velocity profile within d and that the lower half of d is especially well resolved

core mantle boundary structure investigated using sks and skks polarization anomalies summary occasionally sks and skks waveforms in the same seismogram are affected differently by anisotropy one source of this discrepancy may be structures in d in this study we examine the discrepancy in order to determine where it arises in the propagation path and what it is due to we find that d is the most likely source of the signal the relatively minor differences in differential shear wave splitting in sks and skks limit large scale azimuthal anisotropy in d to less than 2 per cent though it may be locally stronger the most effective way to develop the splitting differences is through polarization differences between sks and skks imposed at the cmb we examine how effective relief on and lateral gradients in cmb structure are at generating polarization anomalies and conclude that topography generates them best ramps and ridges are more effective generators of polarization anomalies than hills laterally extensive 500 1000 km sloping cmb topography greater than 15 can develop the observed polarization anomalies in the data the topography required exceeds constraints from other sources so is unlikely to be the major factor explaining the anomalies we produce a global map of sks skks exit points from the core where anomalous polarization behaviour is found in relation to the velocity structure known to exist in d the anomalies appear to be restricted to faster than average areas of the cmb suggesting a contribution from anisotropy related to the post perovskite phase transition

mgsio3 postperovskite at d conditions the postperovskite transition in mgsio3 at conditions similar to those expected at the d discontinuity of earth s lower mantle offers a paradigm for interpreting the properties of this region despite consistent experimental and theoretical predictions of this phase transformation the complexity of the d region raises questions about its geophysical significance here we report the thermoelastic properties of cmcm postperovskite at appropriate conditions and evidences of its presence in the lowermost mantle these are i the jumps in shear and longitudinal velocities similar to those observed in certain places of the d discontinuity and ii the anticorrelation between shear and bulk velocity anomalies as detected within the d region in addition the increase in shear modulus across the phase transition provides a possible explanation for the reported discrepancy between the calculated shear modulus of postperovskite free aggregates and the seismological counterpart in the lowermost mantle

lowermost mantle flow at the eastern edge of the african large low shear velocity province abstract observations of seismic anisotropy in the lowermost mantle are plentiful but their interpretation in terms of mantle flow remains challenging here we interrogate the anisotropic structure of the lowermost mantle beneath the afar region just outside the edge of the african large low shear velocity province using a combination of shear wave splitting techniques applied to phases propagating at five distinct raypath orientations we then model the resulting data set by testing various candidate mechanisms for anisotropy the observations are best fit by a model that invokes the lattice preferred orientation lpo of post perovskite with the 100 crystallographic axis oriented either nearly vertically or highly obliquely to the horizontal plane plausible corresponding mantle flow scenarios involve a significant vertical flow component which suggests that the african large low shear velocity province edge may deflect ambient mantle flow upwards or may be associated with a sheet like upwelling

anisotropy as cause for polarity reversals of d reflections recordings of seismic events that sample the deep mantle can test different hypotheses of mantle processes and composition seismic reflections from structures in the d region the bottom 200 400 km of the earth s mantle can provide information on the velocity contrasts in this region by studying the waveforms and polarities of the d reflections in p and s waves we can potentially distinguish between different explanations for the observed structures such as phase transitions mineral texture or thermal anomalies here we use source receiver combinations that contain reflections from d in two different regions that are both characterised by fast seismic velocities in tomographic models beneath the caribbean a positive s velocity contrast but negative p wave velocity contrast across the d reflector has been reported previously consistent with a model of a phase change in mgsio3 in the second fast velocity region eurasia we detect positive p and s wave velocity contrasts in two orthogonal paths crossing in the lowermost mantle indicating a different scenario for d a path that crosses this region in 45 to the other two great circle paths shows evidence for a negative p velocity contrast one explanation to reconcile observations in both regions is a phase transition from perovskite to post perovskite with a fraction of 12 preferred crystal alignment in the post perovskite phase depending on the travel direction of the waves with respect to the flow direction in the lower mantle positive or negative velocity jumps can be expected other anisotropic models are considered but cannot fully explain the range of observations we find in the data c 2011 elsevier b v all rights reserved

slabs in the lower mantle and their modulation of plume formation numerical mantle convection models indicate that subducting slabs can reach the core mantle boundary cmb for a wide range of assumed material properties and plate tectonic histories an increase in lower mantle viscosity a phase transition at 660 km depth depth dependent thermal expansivity and depth dependent thermal diffusivity do not preclude model slabs from reaching the cmb we find that ancient slabs could be associated with lateral temperature anomalies 500 c cooler than ambient mantle plausible increases of thermal conductivity with depth will not cause slabs to diffuse away regional spherical models with actual plate evolutionary models show that slabs are unlikely to be continuous from the upper mantle to the cmb even for radially simple mantle structures the observation from tomography showing only a few continuous slab like features from the surface to the cmb may be a result of complex plate kinematics not mantle layering there are important consequences of deeply penetrating slabs our models show that plumes preferentially develop on the edge of slabs in areas on the cmb free of slabs plume formation and eruption are expected to be frequent while the basal thermal boundary layer would be thin however in areas beneath slabs the basal thermal boundary layer would be thicker and plume formation infrequent beneath slabs a substantial amount of hot mantle can be trapped over long periods of time leading to mega plume formation we predict that patches of low seismic velocity may be found beneath large scale high seismic velocity structures at the core mantle boundary we find that the location buoyancy and geochemistry of mega plumes will differ from those plumes forming at the edge of slabs various geophysical and geochemical implications of this finding are discussed

constraints from seismic anisotropy on the nature of the lowermost mantle the d layer lies at the bottom of the earth s rocky mantle and separates it from the liquid metal alloy core this region extending from the core mantle boundary to a few hundred kilometres above fig 1 is geodynamically analogous to the more easily studied lithosphere at the top of the mantle the structure of d may reflect the style of lower mantle convection the nature of core mantle interaction and perhaps even the fate of subducting lithosphere1 observations of lithospheric seismic anisotropy have provided valuable insight into the nature of the upper mantle boundary layer but discussion of lower mantle seismic anisotropy has been somewhat contentious2 5 here we present evidence from seismic waves that have traversed the lowermost mantle beneath the caribbean region for a zone of seismic anisotropy below the d discontinuity which in this region lies 250 km above the core mantle boundary the anisotropy is most probably due to horizontal layering or aligned inclusions of a material with differing shear wave velocity if d is a graveyard for subducted lithosphere a plausible explanation of the anisotropy may be the contrast between cold lithospheric mantle and material that formerly constituted the oceanic crust which may have lower shear wave velocity owing to the presence of melt

seismic velocity structures and detailed features of the d discontinuity near the core mantle boundary beneath eastern eurasia abstract shear and compressional velocity structures and features of the d discontinuity near the core mantle boundary cmb beneath eastern eurasia are studied based on forward waveform modeling and differential travel time analysis of scsh sh sscs ss pcp p and ppcp pp phases high quality differential travel time analysis reveals two high velocity patches spanning a width of 3000 km and separated by a 300 km wide normal velocity gap the maximum shear and compressional velocity perturbations reach 3 8 and 2 7 respectively seismic data indicate that the d discontinuity emerges in the high velocity patches and disappears in the normal velocity gap furthermore the features of the d discontinuity although constrained by rather sparse seismic data vary from diffusive discontinuities in the eastern patch to sharp discontinuities in the western patch the d discontinuities occur at about 270 km above the cmb in the eastern patch with a shear velocity jump of 3 0 5 0 and a transitional depth of at least 100 km and at 250 km above the cmb in the western patch with an abrupt shear velocity jump of 5 0 a diffusive p wave discontinuity is also observed in the eastern patch with the similar features of the s wave discontinuity the inferred features of the d discontinuity can be explained by the post perovskite transformation in the lower mantle the observations that the d discontinuities occur only in the high velocity patches may be due to the effect of the positive clapeyron slope of the post perovskite transition and the varying transitional depths of the d discontinuity in the high velocity regions may be caused by the compositional effects on the post perovskite transition in the presence of strong variations of composition within the high velocity anomalies the high velocity patches geographically correlate with the subduction of the pacific and indian plates in the past 30 60 ma and the normal velocity gap falls into the subduction gap between the pacific and indian plates these geographical correlations raise the possibility that the existence of the d discontinuity is intimately related to the presence of the past subduction material at the cmb and varying features of the discontinuity further suggest existence of strong variations of composition within the subduction materials at the cmb