new inferences of mantle viscosity from joint inversion of long wavelength mantle convection and post glacial rebound data we perform joint inversions for mantle viscosity of geophysical observables associated with both mantle convection and glacial isostatic adjustment gia our data include coefficients of the non hydrostatic geoid up to degree 8 and decay times associated with post glacial relative sea level rsl variations in hudson bay and fennoscandia we find that both data sets may be reconciled using a single profile of mantle viscosity which is characterized by a significant increase with depth across this region this result weakens previous arguments that the mantle rheology has significant transient effects the viscosity profiles we obtain are found to reconcile the observed free air gravity anomaly over hudson bay

a waveform migration for the investigation of p wave structure at the top of d beneath northern siberia we present the first waveform migration for the lower mantle focusing on the lowermost 400 km of the mantle beneath northern siberia we migrate vertical component p wave seismograms recorded in western europe for earthquakes located in the kuril islands region we focus on the time window between the direct p wave and the pcp wave reflected at the core mantle boundary and we investigate the origin of pcp precursors the data selected for their good signal to noise ratio are deconvolved by the source time function determined by the beam of the p wave we observe the signature of a finite size scattering object located around 75 n 85 e at an apparent depth of 2650 km with respect to the iasp91 model its scale length is probably of the order of 100 km this object is characterized by an anisotropic scattering diagram and may be related to the presence of an ancient subducted plate near the core mantle boundary

polar low le cygne satellite observations and numerical simulations a polar low pl which occurred in october 1993 over the norwegian sea is investigated from an observational and a numerical point of view this pl has several unique features it developed early in the season it lasted for about 3 days and its trajectory was such that it passed over weather stations so that conventional observations of the low are available the conditions of the formation development and decay of the pl are investigated using a double approach satellite data from several instruments are used together to document the mesoscale structure of the low and two versions of a limited area model are run to investigate the dynamics of the low numerical model fields are compared to quantities derived from tiros n operational vertical sounder the special sensor microwave imager and satellite radar altimeter data in spite of a better spatial resolution of the models humidity and surface wind speeds are less organized in the simulations than in satellite retrievals the number of vertical levels especially for the lowest layers of the atmosphere appears to be an essential component for a good simulation of the trajectory of the low there is however good overall agreement between modelled and satellite derived fields and the good quality of the simulations allows inferences to be made regarding the essential physical and dynamical processes taking place during the formation and development of the pl we find that the pl was the result of favourable flow conditions at the surface in the form of a shallow arctic front established south of the ice edge together with an upper level potential vorticity anomaly setting the stage for a positive interaction later on the strong surface sensible and latent heat fluxes contributed to the extensive vertical development this study demonstrates the usefulness of the approach adopted here which relies not only on simulations but also on observations to get a very complete description of such disturbances copyright 2004 royal meteorological society

double beam imaging mapping lower mantle heterogeneities using combinations of source and receiver arrays we present a new technique for imaging spatially distributed heterogeneities using information from combinations of source and receiver arrays the method is based on the single scattering assumption and is closely related to the double beam method of kruger et al 1993 1995 1996 in that it exploits amplitude delay time slowness and azimuth information in two arrays simultaneously a crucial step in the method is the application of static time corrections for a chosen reference phase here pcp one type of image is obtained as a spatial likelihood distribution for prescribed source and receiver array slowness and azimuth values and delay times with respect to a reference phase this allows the determination of the spatial origin of coherent phases in the p coda in a complementary approach we perform a double beam stack migration according to the theoretical slowness and azimuth values for candidate scatterers distributed over a three dimensional 3 d grid the scattering strength is expressed as the resulting beam power beam amplitude or semblance in a time window determined from the theoretical delay time with respect to the reference phase while likelihood mapping provides an image reflecting the probability of a region to explain observed kinematic phase properties by single scattering double beam migration provides information about the scattering strength the method is applied to nuclear explosion sources in eastern kazakhstan recorded at the yellowknife array in northern canada we identify several scattering volumes within the lower mantle below the arctic producing individual anomalous lower mantle phases in the p coda these anomalies range in depth from the core mantle boundary cmb up to 500 km into the lowermost mantle and are mainly located under the eurasian side of the arctic the joint interpretation of these results with the results of previous studies suggests a connection of these anomalies to the cenozoic and mesozoic subduction of the pacific plate and the kula plate the determination of the nature of such 3 d anomalies necessitates further array studies with better azimuthal coverage

deformation in the lowermost mantle beneath australia from observations and models of seismic anisotropy observations of seismic anisotropy near the core mantle boundary may yield constraints on patterns of lowermost mantle flow we examine seismic anisotropy in the lowermost mantle beneath australia bounded by the african and pacific large low shear velocity provinces we combined measurements of differential splitting of sks skks and s scs phases sampling our study region over a range of azimuths using data from ten long running seismic stations observations reveal complex and laterally heterogeneous anisotropy in the lowermost mantle we identified two sub regions for which we have robust measurements of d associated splitting for a range of ray propagation directions and applied a forward modeling strategy to understand which anisotropic scenarios are consistent with the observations we tested a variety of elastic tensors and orientations including single crystal elasticity of lowermost mantle minerals bridgmanite post perovskite and ferropericlase tensors based on texture modeling in post perovskite aggregates elasticity predicted from deformation experiments on polycrystalline mgo aggregates and tensors that approximate the shape preferred orientation of partial melt we find that post perovskite scenarios are more consistently able to reproduce the observations beneath new zealand the observations suggest a nearly horizontal 100 axis orientation with an azimuth that agrees well with the horizontal flow direction predicted by previous mantle flow models our modeling results further suggest dominant slip on the 010 plane in post perovskite aggregates provide a good fit to the data but the solution is non unique our results have implications for the mechanisms of deformation and anisotropy in the lowermost mantle and for the patterns of mantle flow

probing two low velocity regions with pkp b caustic amplitudes and scattering summary we probe low velocity regions of the lower mantle with pkp waves using south pacific earthquakes recorded at stations from the temporary ethiopia afar geoscientific lithospheric experiment array in ethiopia and the german regional seismic network grsn germany the ethiopian stations show strong scattering of pkpab waves which arrive as precursors to pkpdf they also show anomalously large amplitude diffractions from the pkp b caustic in contrast the events recorded at the grsn show scattering of pkpab as precursors to pkpdf but more normal amplitudes in the pkp b caustic diffractions to understand the cause for the observed amplitudes we test a large set of 1 d models with anomalies in p and s wave velocities and density in the lowermost mantle and outer core structures that can cause the observed large pkp b caustic amplitudes are low velocity regions just above the core mantle boundary of the order of 50 km thick for the south pacific to ethiopia path with velocity reductions of around 5 per cent for p and 25 per cent for the s wave velocity alternatively a 30 km thick lamella starting at 50 km above the core mantle boundary with 10 per cent p wave velocity reduction and 25 per cent s wave velocity reduction also produces a good fit to the data we find that the low velocity regions act as waveguides enhancing the amplitude of the pkp b caustic diffracted waves and causing superposition of pkp b diffractions with pkpdf the low velocity regions cannot be smooth layers but must show a certain degree of small scale heterogeneity to produce the high frequency scattering observed in the data the path to germany needs small scatterers in the lowermost mantle but the low velocity region is less strong compared with the south pacific ethiopia path a plausible explanation for these low velocity regions is the presence of melt inclusions perhaps entrained in small scale convective systems that act as high frequency wavefield scatterers in regions where these inclusions or convective systems are abundant they appear as a large scale low velocity zone to lower frequency waves these observations support ideas that ultra low velocity zones at the base of the mantle may be heterogeneous regions of melt rich material

stability metastability and elastic properties of a dense silica polymorph seifertite 1 dense silica polymorphs with sixfold coordinated si have been found in snc and lunar meteorites and may be important minerals for silica rich components in the lower mantle however the stable crystal structure in the lower mantle and properties of dense silica remain controversial under stable heating and quasi hydrostatic stress conditions we found that the cacl2 type undergoes a phase transition to the pbo2type seifertite at 130 140 gpa and 2500 k our data suggest that this phase transition occurs at a greater depth than the perovskite postperovskite transition in the lowermost mantle the molar volume measured at 1 bar is the smallest among the reported silica polymorphs therefore having the highest calculated density and in excellent agreement with recent first principles calculations the greater molar volume of seifertite found in the shergottite meteorite and previous experiments supports a metastable synthesis of the phase outside its stability field our data combined with the hugoniots of silica polymorphs also rule out the possibility of the formation of seifertite in the meteorite within its stability field we found very little change in bulk sound speed across the cacl2 type seifertite transition if shear wave velocity decreases at the transition to seifertite as suggested by some computational studies this silica transition may provide an alternative explanation for the discontinuities with a shear wave velocity decrease found at depths greater than the d discontinuity

cross correlation weighted migration towards high resolution mapping of mantle heterogeneities summary we present a new type of born migration scheme to obtain a high resolution image of smallscale scatterers in the mantle seismic migration is a technique that projects scattered energy back to its origin by shifting and stacking seismograms using theoretical traveltimes migration methods in global seismology because of the limited distribution of sources and receivers tend to suffer from considerable smearing owing to isochronal artefacts this type of artefacts is caused by migration grids that have the set of theoretical traveltimes from a given source to all receivers similar to that for a true scattering point previously the information of slowness and backazimuth was explicitly explored to reduce such migration artefacts here we propose a simpler and more computationally efficient method but with similar effects in which the coherency of shifted seismograms measured by cross correlation is exploited to suppress isochronal artefacts the new migration scheme makes it possible to compute a large number of migrated images efficiently and thus evaluate the robustness of observed scattering objects by statistical approaches such as bootstrap resampling

transient d discontinuity revealed by seismic migration 1 the d region beneath north central asia is investigated using a weighted migration based on the generalised radon transform we image complex structure including a d discontinuity with variable topography and velocity contrast we also present observations of an area where the d reflector stops using synthetic data sets generated for distributions of point scatterers we show that this is not due to deficiencies in the data coverage but requires that the d discontinuity is not a continuous surface the termination of the discontinuity appears to restrict it to a region high seismic velocity this could be interpreted as confirmatory evidence for the existence and behaviour of the post perovskite phase transformation however compositional variations may also play a role

sks and spdks sensitivity to two dimensional ultralow velocity zones 1 seismic wave propagation through two dimensional core mantle boundary cmb ultralow velocity zones ulvzs is modeled using a global pseudospectral algorithm seismograms are synthesized for several types of ulvz models to investigate the effect of these structures on sks and spdks phases one dimensional models and two dimensional models with different quasi 1 d velocity structures on the source and receiver sides of the cmb provide a baseline for comparison with other 2 d models models with finite length ulvzs are used to test the sensitivity of the spdks travel time and waveform to different portions of the p diffracted wave path this test shows that spdks waves are only sensitive to ulvzs with lengths 100 km our results give three tools for identifying and characterizing 2 d ulvz structures first dual spdks pulses indicate exposure to at least two separate cmb velocity structures either different source and receiver side cmb velocities or different adjacent velocity regions for which pdiff inception occurs outside of and propagates into a ulvz second high amplitude sks precursors indicate a very strong i e thick and or large velocity perturbations ulvz hence the absence of sks precursors in most previous ulvz studies indicates that very strong sharp ulvzs are not common third mean spdks delays relative to prem constrain the minimum ulvz strength and length combinations required to produce a given travel time delay

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

the role of the heating mode of the mantle in intermittent reorganization of the plate velocity field summary the geological record indicates that stages of relatively steady plate motion have been punctuated by comparatively brief periods in which plate velocities have reorganized the distribution of buoyancy sources in the mantle has generally been regarded as evolving too slowly to explain these rapid transitions in plate velocity we investigate the feedback between mantle convection and plate velocity using 2 d and 3 d mantle convection models that incorporate mobile dynamic plates we focus on the influence of internal heating in the mantle and consider the effect of mantle viscosity stratification and different plate geometries on the plate velocity time dependence as either the rayleigh number or the internal heating rate is increased to magnitudes approaching mantle values the record of the plate motion from our calculations becomes characterized by intermittent changes in direction this behaviour is a result of the influence of plates on heat loss from the inherently unsteady internally heated convecting system plate motion instills a pattern of organization on the underlying convection that reflects the plate geometry and results in the formation of sheet like downwelling structures at convergent plate boundaries in both 2 d and 3 d calculations in contrast upwellings in 3 d models are not sheet like the role of the sheet like downwellings is critical in the observed episodic reorganization of the plate velocities warm material below the plates is entrained by plate motion into regions enveloping the downwelling sheets during periods of fairly steady plate motion buoyancy associated with the build up of heat around the downwelling sheets leads to the creation of an unstable convection pattern this build up of heat is dramatic in calculations with mantle like internal heating rates and resists continued long term plate motion towards mature downwellings when there are limitations on the degree of freedom of the direction of plate movement such as in 2 d models these effects become even more pronounced accordingly the effect of plates on mean global thermal quantities is more dramatic in 2 d calculations than it is in 3 d calculations nevertheless 3 d calculations incorporating plates of different sizes do exhibit rapid reorganizations in their convection patterns as the pull of young slablike features supersedes the pull of mature downwelling sheets we compare the timing and frequency of the reorganization events in our calculations with the general characteristics of plate motions determined from plate reconstruction studies

new insights into the p and s wave velocity structure of the d discontinuity beneath the cocos plate summary broad band p and s waves from earthquakes in south america recorded at californian network stations are analysed to image lateral variations of the d discontinuity beneath the cocos plate we apply two array processing methods to the data set a simplified migration method to the p wave data set and a double array method to both the p and s wave data sets allowing us to compare results from the two methods the double array method images a dipping reflector at a depth range from 2650 to 2700 km in the southern part of the study area we observe a step like topography of 100 km to a shallower reflector at about 2600 km depth to the north as well as evidence for a second deeper reflector at a depth range from 2700 to 2750 km in the north results from the simplified migration agree well with those from the double array method similarly locating a large step in reflector depth in a similar location about 2650 km depth in the south and about 2550 km in the north as well as the additional deeper reflector at the depth of about 2750 km in the north waveform modelling of the reflected waves from both methods suggests a positive velocity contrast for s waves but a negative velocity contrast for p waves for the upper reflector in agreement with predictions from mineral physical calculations for a post perovskite phase transition the data also show some evidence for the existence of another deeper reflector that could indicate a double intersection of the geotherm with the post perovskite stability field that is the back transformation of post perovskite to perovskite close to the core mantle boundary

investigating d structure beneath the north atlantic abstract data from the grsn and spiced array are used to investigate lower mantle structure beneath the north atlantic ocean just south of nova scotia we study s waves in an attempt to determine whether a d discontinuity can be observed in this region the reflected phase sds cannot be observed unambiguously so we cross correlate the traces with synthetic seismograms generated using a number of different 1d earth models that incorporate positive and negative velocity discontinuities and gradients as well as thin high velocity lamellae the cross correlation shows that a discontinuity with an s wave velocity increase of magnitude 1 3 exists which exhibits significant topography between 86 and 286 km above the cmb the study area is located on the edge of a region of anomalously high seismic velocity identified by tomographic modelling which fits with our observation of a high velocity discontinuity

in situ observations of phase transition between perovskite and cairo 3 type phase in mgsio 3 and pyrolitic mantle composition abstract in situ observations of the perovskite cairo3 phase transition in mgsio3 and in pyrolitic compositions were carried out using a laser heated diamond anvil cell interfaced with a synchrotron radiation source for pure mgsio3 the phase boundary between the orthorhombic mg perovskite and cairo3 type phases in the temperature range of 1300 3100 k was determined to be p gpa 130 3 0 0070 0 0030 t 2500 k using platinum as a pressure calibrant we confirmed that the cairo3 type phase remained stable up to pressures of at least 156 gpa and temperatures of 2600 k the consistency of our results with previous theoretical calculations leads us to conclude that the 2700 km seismic discontinuity at the bottom of the lower mantle can be attributed to a phase transition to the cairo3 type phase the phase change from an orthorhombic mg perovskite to a cairo3 type bearing assemblage in a pyrolitic mantle composition was also observed at p 125 gpa which corresponds to the same mantle depth as the seismic discontinuity the phase boundary between the orthorhombic mg perovskite and cairo3 type bearing assemblage was determined to be p gpa 124 4 0 008 0 005 t 2500 k using gold as a pressure calibrant this transition boundary indicates that the temperature at a depth of 2700 km is about 2600 k and the adiabatic temperature gradient in the lower mantle is estimated to be 0 31 k km the partition coefficients and the effect of some elements on the phase equilibrium between the orthorhombic mgsio3 perovskite and cairo3 type mgsio3 were estimated from ab initio calculations our experimental and theoretical results indicate that the d layer consists of a cairo3 type bearing assemblage which is likely to have significant effect on the chemical and thermal evolution of the earth s mantle

d beneath the arctic from inversion of shear waveforms 1 the structure of the d region beneath the arctic has not previously been studied in detail using waveform inversion we find that the average s wave velocity in d beneath the arctic is about 0 04 km s higher than prem which is consistent with the existence of post perovskite ppv in d it is difficult to strongly constrain the fine structure of s velocity within d due to the small number of stations at epicentral distances 90 but by weighting those stations heavily in the inversion we show that the data suggest the existence of high s velocity in the upper half of d and low s velocity in the lower half consistent with the possibility of a double crossing ppv pv reverse phase transition within d we conduct a computational experiment to show that resolution of the velocity structure within d could be significantly improved by temporary installation of a portable array of seismographs in northern canada which would greatly increase the number of stations in the range 70 90

seismic evidence links the subducted mongol okhotsk slab to deformation in d near the northeastern margin of the perm anomaly abstract numerical simulations have indicated that the large low shear velocity provinces llsvps dynamically interact with subducted slabs in d and slabs in turn play an important role in influencing the shape and location of the llsvps in this study we find evidence for the existence of the d discontinuity near the northeastern margin of the perm anomaly a smaller llsvp beneath eurasia by analyzing the scd the lower mantle triplication phases generated from two deep events occurring in the northwestern pacific subduction zone and recorded at stations in europe and turkey the d discontinuity in the north is characterized with a 2 velocity increase occurring at a height of 300 km above the core mantle boundary cmb whereas the discontinuity in the south is characterized with a 3 velocity increase occurring at a height of 260 km above the cmb the d discontinuity is most likely attributed to a phase transition from perovskite to post perovskite in conjunction with the presence of relic slab materials likely related to the subducted mongol okhotsk slab our observations along with previous seismic anisotropic detections provide a clue to dynamically link the subducted slab to deformation taking place near the edges of the llsvp

seismic imaging of the laterally varying d region beneath the cocos plate summary we use an axisymmetric spherical earth finite difference algorithm to model sh wave propagation through cross sections of laterally varying lower mantle models beneath the cocos plate derived from recent data analyses synthetic seismograms with dominant periods as short as 4 s are computed for several models 1 a d reflector 264 km above the core mantle boundary with laterally varying s wave velocity increases of 0 9 2 6 per cent based on localized structures from a 1 d double array stacking method 2 an undulating d reflector with large topography and uniform velocity increase obtained using a 3 d migration method and 3 cross sections through the 3 d mantle s wave velocity tomography model txbw we apply double array stacking to assess model predictions of data of the models explored the s wave tomography model txbw displays the best overall agreement with data the undulating reflector produces a double scd arrival that may be useful in future studies for distinguishing between d volumetric heterogeneity and d discontinuity topography synthetics for the laterally varying models show waveform variability not observed in 1 d model predictions it is challenging to predict 3 d structure based on localized 1 d models when lateral structural variations are on the order of a few wavelengths of the energy used particularly for the grazing geometry of our data iterative approaches of computing synthetic seismograms and adjusting model characteristics by considering path integral effects are necessary to accurately model fine scale d structure

the thermal signature of subducted lithospheric slabs at the core mantle boundary abstract we study the thermal structure around a cold deformable lithospheric slab as it sinks to the core mantle boundary and migrates along it we present analytical results for the steady thermal structure established by a steady but spatially varying motion the analysis gives a time like criterion for the thermal signature of a cold slab to persist by the time that the slab moves along the core mantle boundary the model is used to assess the feasibility of a purely thermal origin for some of the observed seismic reflectors near the core mantle boundary calculations of the time like criterion show that the dynamical conditions in our model namely the velocity and the thickness of the descending slab are hard to reconcile with observations of subduction and seismic features seismic reflections and refractions from anomalously fast regions above the core mantle boundary could be explained as thermal slabs if the thickness of the slab at subduction was larger than 200 km or somewhat less if the slab did not split at the core mantle boundary a simple thermal model also predicts from mineral physics a certain correlation between s and p wave velocity anomalies which is not observed however a purely thermal origin cannot be ruled out if the slab is buckling this process could be in agreement with the observations the amplitude of the seismic anomalies the vertical extent of high gradient zones and the p versus s comparisons chemical heterogeneities and phase transformations remain alternative or complementary explanations

investigation of laterally heterogeneous shear velocity structure in d beneath eurasia seismic waves are used to explore large scale radial structure and lateral heterogeneity of shear velocity in the d region the chemical and thermal boundary layer at the base of the mantle we analyze long period horizontally polarized sh shear waveforms from earthquakes in the northwest pacific recorded in europe and the middle east for evidence of proposed laterally varying shear velocity stratification in d beneath eurasia specifically our data sample the lower mantle extending from beneath the arctic ocean near novaya zemlya ussr to under northern india using a reflectivity technique to model the sh waveforms we determine a locally stratified shear velocity model sgle that provides a good average travel time and waveform fit to over 80 of the data sensitive to d structure sgle is characterized by a rapid 2 75 increase in shear velocity approximately 290 km above the core mantle boundary similar to previously proposed models for this region unlike for other regions analyzed using similar waveform modeling techniques the percentage of data inconsistent with this average radial model is fairly substantial indicating that laterally heterogeneous d velocity structure is important in this region some waveforms lack the reflected arrival expected for a stratified model while others show evidence of two arrivals both possibly originating within d while our data are insufficient for direct inversion of the three dimensional structure we utilize several simple parameterizations of the waveforms to gain some understanding of the region both the relative amplitude and differential travel time variations of the waveforms indicate that d beneath eurasia is characterized by 1 broad regions in which the structure has large scale stratification giving coherent long period reflections with superimposed velocity variations that scatter shorter wavelength waves and 2 smaller regions which are sufficiently heterogeneous to scatter even the long period energy the scale length of the lateral heterogeneity in d that produces these variations is inferred to be somewhat less than 500 km which contrasts with weaker lateral heterogeneity that is observed in the central region of the lower mantle where heterogeneity scale lengths of approximately 500 1000 km appear to dominate these observations are consistent with a model of the lower mantle in which distributed heterogeneity is concentrated in d generating a locally stratified layer in some regions the effects of a diffuse or topographically variable d discontinuity must be further explored using two and three dimensional synthetic calculations before inversions for the detailed structure can be undertaken

the geoid constraint in global geodynamics viscosity structure mantle heterogeneity models and boundary conditions summary the earth s non hydrostatic gravity field or geoid provides a first order constraint on mantle density structure and dynamics geodynamic models for the geoid have proliferated since the advent of seismic mapping of mantle heterogeneity structure tomography because the geoid offers perhaps the best measured independent constraint on mantle density heterogeneity however dynamic geoid models involve a number of questionable physical assumptions and uncertainties whose effects need to be evaluated before geodynamic inferences based upon the geoid can be considered sound troubling issues include the appropriate surface boundary conditions free slip no slip plates and parametrization of radial viscosity variations how many layers can be resolved in addition to lateral viscosity variations possible chemical layering of the mantle phase transitions etc there are also uncertainties in the density heterogeneity models used as input to dynamic geoid models most of which are derived from seismic tomography and require weakly constrained empirical conversion factors to go from seismic velocity variations to density variations here we address several of the most straightforward problems inherent in geoid modelling namely the issues of viscosity structure resolution uncertainties in appropriate boundary conditions and differences among mantle heterogeneity models a robust feature of all models is a lower mantle viscosity at least a factor of 30 greater than that of the upper mantle but there is little resolution with regard to finer details such as lithospheric or uppermost mantle low viscosity zone viscosity ironically free slip boundary conditions result in the best fits to the geoid in all cases but all boundary conditions exhibit predictable trade offs with the uppermost mantle viscosity models with a single viscosity layer representing the lower mantle yield similar dynamic topography estimates of the order of 700 1000 m in amplitude regardless of the finer details of upper mantle viscosity structure boundary conditions or input heterogeneity models comparing mantle heterogeneity models based on two independent seismological determinations harvard and berkeley models and on the history of subduction we find that these models are virtually indistinguishable regarding inferences of mantle viscosity structure and amplitude of dynamic topography and in terms of the effects of different boundary conditions uncertainties concerning which type of boundary condition is appropriate are much more important than which mantle heterogeneity model is chosen given other uncertainties in modelling the geoid particularly the strong effects due to lateral viscosity variations for intermediate 10 000 km wavelengths we conclude that the class of dynamic geoid models explored so far cannot reliably elucidate the details of upper mantle viscosity structure

convection in three dimensions with surface plates generation of toroidal flow this work presents numerical calculations of mantle convection that incorporate some of the basic observational constraints imposed by plate tectonics the model is three dimensional and includes surface plates it allows plate velocity to change dynamically according to the forces which result from convection we show that plates are an effective means of introducing a toroidal component into the flow field after initial transients the plate motion is nearly parallel to transform faults and in the direction that tends to minimizes the toroidal flow field the toroidal field decays with depth from its value at the surface the poloidal field is relatively constant throughout the layer but falls off slightly at the top and bottom boundaries layered viscosity increasing with depth causes the toroidal field to decay more rapidly effectively confining it to the upper low viscosity layer the effect of viscosity layering on the poloidal field is relatively small which we attribute to its generation by temperature variations distributed throughout the system the generation of toroidal flow by surface plates would seem to account for the observed nearly equal energy of toroidal and poloidal fields of plate motions on the earth a low viscosity region in the upper mantle will cause the toroidal flow to decay significantly before reaching the lower mantle the resulting concentration of toroidal flow in the upper mantle may result in more thorough mixing there and account for some of the geochemical and isotopic differences proposed to exist between the upper and lower mantles

finite frequency effects on global s diffracted traveltimes summary many seismic observations have shown that strong heterogeneities exist in the bottom few hundreds kilometres of the mantle among different seismic phases this region that is the d layer can be most globally sampled by diffracted waves along the core mantle boundary here we assess the amplitude and distribution of s wave velocity variations in the d layer of an existing tomographic model we compare observed shdiff traveltime anomalies to synthetic ones obtained using 1 the coupled spectral element method csem which is our reference exact method 2 non linear asymptotic coupling theory nact and 3 1 d ray theory synthetic waveforms are calculated down to 0 057 hz with a corner frequency at 0 026 hz in the first part of this paper we compare the traveltime anomaly predictions from the three different methods the anomalies from csem and nact are obtained by taking cross correlations of the 3 d and 1 d synthetic waveforms both nact and standard ray theory which are used in other recent tomographic models suffer from biases in traveltime predictions for vertically varying structure near the core mantle boundary nact suffers from saturation of traveltimes due to the portion in the kernel calculation that is based on the reference 1 d model while ray theory suffers from wave front healing effects in the vertical plane exacerbated in the presence of thin low velocity layers in the second part we compare observed traveltime anomalies and predictions from csem the data consists of 506 sdiff traveltime anomalies from 15 events obtained form global seismograph network records the tomographic model does a good job at predicting traveltimes of sdiff phases especially when the path mostly samples fast s velocity regions at the base of the mantle such as beneath india china north america and northern pacific the underprediction of the positive observed traveltime anomalies seems to occur in regions where the paths sample close to the border of the large low shear velocity provinces such as beneath southern pacific south of australia philippine sea plate and northwestern pacific ocean inspection of the observed seismograms at higher frequencies indicate that 1 the gradient of structure is underestimated and 2 there are post cursors to the sdiff which bias the lower frequency traveltime measurements and which are not taken into account by the tomographic inversion because of the approximate theoretical framework

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

is there a first order discontinuity in the lowermost mantle abstract in 1983 lay and helmberger geophys j r astron soc 75 1983 799 837 reported the detection of a precursor to the seismic phase scs they attributed this precursor to a sharp seismic discontinuity located several hundred kilometers above the core mantle boundary such a lowermost mantle discontinuity implies the existence of a sharp phase change or a chemical boundary precursors to scs and less frequently pcp have since been observed in numerous locations but are not a global phenomenon frequently pcp precursors are weak or absent when scs precursors are observed in the same location and vice versa there can be significant variations in the amplitude and arrival time of the precursor relative to the main phase the presence or absence of these precursors has led to speculations about the nature of the lowermost mantle here we demonstrate that scs or pcp precursors may be produced by gradients in seismic wave speed associated with large scale lowermost mantle heterogeneity rather than a phase or chemical boundary with substantial topography such gradients require lateral variations in temperature and close to the core mantle boundary composition

geophysically consistent values of the perovskite to post perovskite transition clapeyron slope the double crossing hypothesis posits that post perovskite bearing rock in earth s d layer exists as a layer above the core mantle boundary bounded above and below by intersections between a curved thermal boundary layer geotherm and a relatively steep phase boundary increasing seismic evidence for the existence of pairs of discontinuities predicted to occur at the top and bottom of this layer motivates an examination of the consistency of this model with mineral physics constraints for the clapeyron slope of this phase transition using independent constraints for a lower bound on temperature in earth s deep mantle and the temperature of earth s inner core boundary we show that a post perovskite double crossing is inconsistent with plausible core temperatures for a clapeyron slope less than about 7 mpa k with the higher range of experimental values yielding better agreement with recent estimates of the melting temperature of earth s core

multiple scales in mantle convection abstract the multiple scales structure of mantle convection remains largely unknown a 3d model of convection is used to investigate the effect of a viscosity step on the multiple scales pattern of an infinite prandtl number convection for rayleigh numbers ra from 10 5 to 1 75 10 7 the planform of the large scale convection is polygonal and the ascending plumes are quite steady even for a rayleigh number of 1 75 10 7 when the rayleigh number exceeds a value of 3 10 5 the upper thermal boundary layer becomes unstable small scale instabilities appear away from the plume and they are advected by the large scale flow the pattern of these small scale flows evolves with the rayleigh number they form circular like waves centered on the axis of the plume for ra 10 6 an increase of the rayleigh number above ra 5 10 6 leads to the generation of cold drops inside the circular waves finally for ra 1 75 10 7 the circular waves disappear at some distance from the plume axis then the small scale flow consists of cold drops as they are advected by the large scale flow the cold drops mature inside cold radial gutters this superimposition of small scale instabilities on the large scale flow clearly results from the viscosity jump the implications of these models appear more straightforward in the case of venus than of the earth due to the absence of moving plates in the model the existence of small scale convection around the plumes could explain the presence of volcanoes in the plains around the equatorial highlands of venus

dynamic feedback between a continentlike raft and thermal convection seismic observations of the mantle which include long wavelength structure a k 1 dependence of heterogeneity on harmonic k and a heterogeneous upper boundary layer and supercontinent kinematics may be explained by the dynamic interaction between a continent like raft and thermal convection we have formulated finite element models of convection with rafts simulating continental plates in a cylindrical geometry the azimuthal interconnectivity of this geometry is vital to resolve the two way dynamics between rafts and convection computations show that 1 raft motion is periodic 2 long wavelength thermal structure is significant within both thermal boundary layers and the fluid interior and 3 the large scale thermal structure with a wavelength longer than the width of raft is responsible for raft motion these three results which are observed for a range of rayleigh numbers internal heating rates and raft sizes are a direct consequence of the dynamic interaction between the raft and convection the physical processes for a model with a rayleigh number of 10 5 are representative when the raft is stationary due to the less efficient heat transfer through the raft and instabilities from the bottom boundary layer heat accumulates beneath the raft and results in long wavelength thermal anomalies the long wavelength thermal anomalies enhance raft motion accompanying the enhanced raft movement the long wavelength thermal anomalies diminish and the raft velocity decreases or the raft comes to rest since convection models without rafts generate less long wavelength heterogeneity compared to the models with rafts or continental plates we suspect that continental plates may play a crucial role in mantle dynamics interestingly raft motion with a period of about 10 transit times is usually significant 10 transit times is about 600 m y if scaled to the earth this is close to the observed 300 500 m y period of supercontinent aggregation and dispersal

mantle transition zone structure along a profile in the sw pacific thermal and compositional variations summary events from the south pacific recorded in the tien shan region are studied with a migration method to measure discontinuity depth along a profile between the mariana and molucca sea subduction zones deflections of the upper mantle discontinuities within and between the subduction zones are investigated using pp precursors filtered to periods of 3 to 10 s we find p wave reflections near the 410 520 and 660 km discontinuities the 410 km discontinuity is elevated in the subduction region and near the predicted depth 410 km in the average velocity region in some cases we find negative polarities of the reflection from the 410 km discontinuity which may indicate water induced melting above the olivine to wadsleyite transition the 520 km discontinuity is shallow between the two subduction zones and deepens towards the mariana backarc region this discontinuity topography may be due to detection of both the wadsleyite to ringwoodite transition in a depleted downwelling elevated region and the formation of calcium perovskite in a more fertile upwelling depressed region the 660 km discontinuity is split in the halmahera slab with one reflection each near 600 and 700 km depth consistent with a cold sinking slab and detection of phase transitions for both garnet and olivine to perovskite respectively in the region between the subduction areas we find a downward deflection of the 660 km discontinuity with short length scale 100 300 km undulations that may reflect compositional variation

on the likelihood of post perovskite near the core mantle boundary a statistical interpretation of seismic observations abstract recent experimental studies indicate that perovskite the dominant lower mantle mineral undergoes a phase change to post perovskite at high pressures however it has been unclear whether this transition occurs within the earth s mantle due to uncertainties in both the thermochemical state of the lowermost mantle and the pressure temperature conditions of the phase boundary in this study we compare the relative fit to global seismic data of mantle models which do and do not contain post perovskite following a statistical approach our data comprise more than 10 000 pdiff and sdiff travel times global in coverage from which we extract the global distributions of dln vs and dln vp near the core mantle boundary cmb these distributions are sensitive to the underlying lateral variations in mineralogy and temperature even after seismic uncertainties are taken into account and are ideally suited for investigating the likelihood of the presence of post perovskite a post perovskite bearing cmb region provides a significantly closer fit to the seismic data than a post perovskite free cmb region on both a global and regional scale these results complement previous local seismic reflection studies which have shown a consistency between seismic observations and the physical properties of post perovskite inside the deep earth

shale gas accumulation potential of the upper cretaceous qingshankou formation in the southeast songliao basin ne china abstract the purpose of this paper is to provide both quantitative and qualitative visual analyses of the nanometer scale pore systems of immature and early shales as well as to discuss the biogenic shale gas accumulation potential of the upper cretaceous section of the songliao basin to achieve these goals mineralogical compositions were determined using transmitted and reflected light petrography x ray diffractometry and scanning electron microscopy sem while the nanostructure morphology and pore size distributions psds were quantified using field emission scanning electron microscopy fe sem and low pressure nitrogen gas adsorption lp n2ga the results of these analyses indicate that nanometer scale pores are well developed in the immature and low maturity shale and that these shales contain many types of reservoir pores the mudstone layer of the qingshankou formation k2qn contains a high permeability characteristic and good rock fracturing conditions while it is also thick 9 m in thickness and rich in fine organic matter overall analysis of the entire formation using source rock and reservoir evaluations indicate that the first member of the qingshankou formation k2qn1 has a greater shale gas accumulation potential than the second and third members of the qingshankou formation k2qn2 3

high resolution imaging of lowermost mantle structure under the cocos plate 1 broadband seismic shear waves are analyzed to investigate the fine velocity structure in the lowermost mantle beneath the cocos plate a region where previous studies have indicated the presence of a shear velocity increase about 200 300 km above the core mantle boundary data from 14 south american earthquakes recorded at california broadband networks provide dense ray path sampling of the lowermost mantle in an approximately 700 km long north south corridor roughly 150 km wide application of a simplified seismic migration method that uses a homogeneous background velocity model suggests topography of the previously imaged positive impedance jump varying in depth from north to south by as much as 150 km with a weakly reflecting transition zone in between the migration approach enables examination of small scale spatial variations and out of plane scattering effects topography of the discontinuity may account for observed variations in the amplitude of reflected arrivals or there may be lateral variations in the velocity contrast across the boundary lateral variations of the shear velocity structure within the d layer may produce some apparent topography in the discontinuity image but any such volumetric structure is not yet well enough determined to incorporate in the migration a localized negative impedance contrast reflector or scatterer is imaged at depths about 100 km below the positive reflector in the northern portion of the study area several scenarios can explain these results including 1 a slab that reaches the lowermost mantle 2 the birth of an upwelling beneath a recumbent slab or 3 chemical layering in this region

postperovskite phase transition and its geophysical implications 1 the stability of mg fe sio3 perovskite in the deep lower mantle has long been uncertain recently a phase transition from perovskite to postperovskite was discovered through a significant change in the x ray diffraction pattern at high pressure and high temperature conditions corresponding to the core mantle boundary region this phase transition was also confirmed by first principles calculations these suggest that mg fe sio3 postperovskite is the predominant mineral in the lowermost mantle called the d layer and phase transition from perovskite to postperovskite has significant geophysical implications for its nature and dynamics the postperovskite phase may account for the large seismic anomalies observed in the d region such as the d discontinuity s wave anisotropy and anticorrelation between the anomalies in s wave and bulk sound velocities in addition this phase transition is a strongly exothermic reaction it destabilizes the thermal boundary layer at the base of the mantle and should promote the formation of high temperature upwelling plumes

mantle avalanches and the dynamic topography of continents abstract the dynamic topography of continents associated with mantle avalanche events are discussed our flow simulations involve two dimensional axisymmetric multi phase mantle models which incorporate compressibility and a realistic factor of 40 increase in viscosity with depth the surface kinematic boundary conditions involve a mix of free and rigid i e no slip constraints with the latter intended to mimic the influence of a spherical continental cap we find that avalanche events support regional vertical deflections of up to 2 5 km the horizontal length scale of the dynamic topography is 2500 km and non negligible deflections can persist on the order of 100 200 ma we also find that convergence rates in regions surrounding the continents are variable but can reach rates of the order of 3 cm a finally we speculate that an avalanche event may have been responsible for the development of the large scale intracontinental basin system in the western interior of north america from the early devonian to late carboniferous 1

small scatterers in the lower mantle observed at german broadband arrays seismograms of earthquakes from the south pacific recorded at a german broadband array and network show precursors to pkpdf these precursors mainly originate from off path scattering of pkpab or a nearby pkpbc to p for receiver side scattering or from scattering of p to pkpab or pkpbc on the pkpdf path for source side scattering standard array processing techniques based on plane wave approximations such as vespagram or frequency wavenumber analysis are inadequate for investigating these precursors since scattered waves cannot be approximated as plane waves for arrays and networks larger than 300 300 km for short period waves we therefore develop a migration method to estimate the location of scatterers in the mantle at the core mantle boundary and at the top of the outer core with our method we are able to find isolated scatterers at the source side and the receiver side although the depth of the scatterer is not well constrained however from looking at the first possible arrival time of precursors at different depth and the region where scattering can take place scattering volume we believe that the location of the scatterers is in the lowermost mantle since we have detected scatterers in regions where ultralow velocity zones have been discovered recently we think that the precursor energy possibly originates from scattering at partial melt at the base of the mantle comparing results from broadband and band pass filtered data the detection of small scale structure of the ultralow velocity zones becomes possible

subducted slabs and the geoid constraints on mantle rheology and flow geoid anomalies are primarily the result of the density contrasts driving mantle convection and plate motion the total geoid anomaly resulting from a given density contrast in a convecting earth is affected by the mass anomalies associated with the flow induced deformation of the upper surface and internal compositional boundaries as well as by the density contrast itself these boundary deformations and hence the total gravity field depend on the radial distribution of effective viscosity if the internal density contrasts can be estimated as is the case for subducted slabs useful constraints can be placed on the depth and on the variation of viscosity with depth of the convecting system the degree 4 9 components of the observed long wavelength geoid are highly correlated with those predicted by a density model for seismically active subducted slabs the positive sign of the correlation requires that the effective viscosity increases with depth by a factor of 30 or more the amplitude of the correlation cannot be explained by the density contrasts associated with just the seismically active parts of subducted slabs however the amplitude can be explained if the density contrasts associated with subduction extend into the lower mantle or if old lithosphere is piled up at the base of the upper mantle beneath subduction zones to a thickness in excess of 350 km over horizontal distances of thousands of kilometers mantlewide convection in a mantle that has a viscosity increasing with depth provides a simple explanation of the long wavelength geoid anomalies over subduction zones

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

lower mantle heterogeneity beneath eurasia imaged by parametric migration of shear waves abstract lower mantle structure beneath central eurasia is examined using a recently developed technique for migrating parametric representations of shear waves from teleseismic events the migration method of lay and young lay t young c j 1996 imaging scattering structures in the lower mantle by migration of long period shear waves j geophys res 101 20 023 20 040 assumes isotropic scattering from discrete heterogeneities to account for s wave coda arrivals and provides resolution at scale lengths of about 500 km the migration is applied to 15 s period shear wave data from 21 western pacific earthquakes recorded in europe and the middle east that exhibit extra arrivals between s and scs on the transverse components migrations are performed using prem as well as model sgle gaherty j b lay t 1992 investigation of laterally heterogeneous shear velocity structure in d beneath eurasia j geophys res 97 417 435 which has a discontinuity at 2605 km 286 km above the core mantle boundary the migration images for the most coherent coda arrival scd resemble those expected for a lower mantle discontinuity model and simulations are performed for models with discontinuities at various depths quantitative correlation of the images for the data and synthetic migrations show the optimal average depth of the discontinuity to be 2605 km migrations of subsets of the data suggest that the discontinuity depth varies from 2605 km in the northern end of the study area to 2620 km in the southern end an additional intermediate arrival scd2 is observed in a limited portion of the dataset and migrations indicate this phase is generated by localized discrete scatterers located at depths of 2750 to 2770 km under north central eurasia these results are generally consistent with previous forward modeling studies but the migration approach allows lateral variations to be modeled more systematically the heterogeneous structure near the base of the mantle appears to be a manifestation of a complex boundary layer

seismic evidence for small scale heterogeneity throughout the earth s mantle details in the amplitude structure of short period precursors to the seismic core phase pkp can be used to constrain the depth extent of mantle scattering the simplest model consistent with the observations invokes small 8 km weak r m s velocity perturbations of 1 random heterogeneities uniformly distributed throughout the mantle the data do not support previously proposed models that place an increase in the concentration of scattering sites near the base of the mantle and instead place an upper limit on the amplitude of any short wavelength topography at the core mantle boundary

investigating the lowermost mantle using migrations of long period s scs data summary the lowermost mantle is investigated using a data set of high quality waveforms and a phase stripping technique which removes the s and scs phases in order to isolate more subtle arrivals two migration methods were applied to locate scattering bodies in the lower mantle the first a simple backprojection is useful for identifying regions of strong scattering the second migration scheme uses weights based on the generalized radon transform we present results for a region of lower mantle beneath alaska and western canada and test the influence of experimental geometry on the migration results by migrating synthetic data sets generated for various distributions of point scatterers we show that although the exact geometry of scattering bodies is poorly constrained the majority of the features in the data can be recreated with a few simple structures the results show the presence of a d discontinuity which can be simulated using a flat scattering body 250 km above the core mantle boundary near the alaska canada border we also present evidence for positive velocity anomalies in the bottom 100 km of the mantle beneath the d reflector and near the aleutians the association of these scattering structures with a region of high velocity is consistent with a cause related to lower mantle heterogeneity introduced by a subducting slab

p and s wave reflections from anomalies in the lowermost mantle summary seismic p and s waves recorded at the grf array in germany are used to study the inhomogeneous structure of the boundary layer d at the base of the mantle the use of the seismic array allows the detection of small scale anomalies in the lowermost mantle the lateral resolution attainable is about 100 to 400 km i e more than 10 times better than with tomographic methods the analysis of 13 years of grf broad band array data yields 255 events with high signal to noise ratio which are used to map the lowermost mantle 74 of these events show anomalous p waves pdp which arrive 3 6 s after the direct p wave and have a slowness 0 7 0 8 s smaller than the slowness of direct p other events close to the events with pdp do not have such an anomalous phase using slowness traveltime amplitude and waveform information it is demonstrated that pdp is caused by an anomalous lower mantle velocity structure below the turning point of the p wave if the pp phase i e a p wave first reflected at the free surface near the source is used together with the p phase distinct and well sepaarted p velocity anomalies can be determined under the nansen basin the kara sea and northern siberia the areas of the bounce points of pdp in the lower mantle have a lateral extension of about 100 by 200 km but this is not the size of the anomaly since the resolution of p waves at 1 hz in this depth is 130 km by 260 km fresnel zone the accuracy to which the depth of the reflector 2612 km under the nansen basin and 2605 km under northern siberia respectively can be determined for 1 d models is 10 to 20 km the velocity contrast at the lower mantle discontinuity is about 3 per cent 1 to 1 5 per cent areas which have velocity fluctuations smaller than about 1 per cent can not be detected as anomalous areas 2 d models of the anomalies reveal the range of adequate models and possible trade offs if the lateral extension of the anomaly is about 7 the reflector has to be 40 km deeper than in the 1 d model if the dip of the reflector in the lowermost mantle is only about 1 5 it is difficult to resolve if the reflector is tilted towards the source or towards the receiver for the anomaly under the nansen basin deviations from the great circle path are observed for pdp indicating 3 d effects below the three anomalies the core mantle boundary cmb can be located by pcp with a depth in agreement with standard earth models under northern siberia has a region where p and s velocity anomalies coincide but also a region where only an s velocity anomaly but no p velocity anomaly is observed this results in changes of the poisson ratio from 5 9 per cent to 4 8 per cent 1 5 per cent across the discontinuity in the lowermost mantle for the regions studied the analysis of the s waves s sds and scs reveals two s velocity anomalies in the lowermost mantle under northern siberia the first anomaly coincides with the p velocity anomaly under northern siberia and can be explained by a 1 d model with a reflector depth of 2610 km 15 km and a velocity contrast of 2 3 per cent the second s velocity anomaly is in an area where no p velocity anomaly can be detected the corresponding 1 d s velocity model has a reflector depth of 2575 km 15 km and an s velocity contrast of 2 6 per cent the smallest structures that can be resolved with the s waves in this depth are about twice as large as for the p waves i e 230 km by 460 km fresnel zone the joint analysis of p and s waves therefore shows a region with a p velocity anomaly together with weak indications for an s velocity anomaly nansen basin and a second region with a p but no s anomaly kara sea the lowermost mantle

the influence of plate boundary motion on planform in viscously stratified mantle convection models 1 a number of studies examining the influence of plates on mantle convection have concluded that planform and thermal structure are strongly influenced by plate geometry however studies that have modeled evolving plate geometries over periods greater than a mantle transit time indicate that mantle planform may not correlate with plate geometry to assess the influence of plate boundary motion on mantle convection we investigate convection in a plane layer system featuring four polygon shaped plates new to this work plate boundaries are moved at specified velocities that are consistent with the velocities associated with the convection driven flow in the system interior plate velocities are time dependent and use a force balance method to ensure that plate motion neither drives nor resists the convection the influence of the plate boundary motion on convection is compared in models featuring viscosity profiles that increase by factors of 30 90 300 and 1000 across the lower mantle the effective rayleigh numbers of these systems are held at a nearly constant value we find that convection planform is sensitive to both divergent and convergent plate boundary motion for a system featuring as much as a 90 fold contrast in viscosity between the upper and lower mantle however as the viscosity stratification is increased the response of the convection planform to the motion of divergent plate boundaries diminishes in contrast we find planform and specifically plume positions respond to the motion of convergent plate boundaries even when the lower mantle viscosity is 1000 times greater than the upper mantle viscosity

radial 1 d seismic structures in the deep mantle in mantle convection simulations with self consistently calculated mineralogy 1 numerical thermo chemical mantle convection simulations in a spherical annulus geometry with self consistently calculated mineralogy and mineral physics are used to predict detailed deep mantle seismic structures particularly local radial profiles of shear wave velocity vs and bulk sound velocity vb the predicted structures are compared to seismological observations and used to guide the interpretation of seismic observations and to test the model the mantle composition is described as a mixture of morb mid oceanic ridge basalt and harzburgitic end members in the na2o cao feo mgo al2o3 sio2system to assess the influence of chemical variability four different sets of end member compositions are evaluated results confirm that the post perovskite ppv phase causes anti correlated s wave and bulk sound velocities in the deep mantle due to ppv being fast in vs but slow in bulk sound velocity local 1 d seismic profiles display great lateral variability and often have multiple discontinuities in the deep mantle due to morb layers in folded slabs with a positive vs anomaly and negative bulk sound anomaly or the perovskite ppv phase transition the ppv transition is not visible inside piles of segregated morb because of the high temperature and small velocity contrast of ppv in morb piles of segregated morb are seismically slow in both vs and bulk sound despite being expected to be fast in vs because they are hotter than the surrounding material anelasticity has a significant influence on profiles of vs only in the lower thermal boundary layer which corresponds to below 2600 to 2800 km depth depending on region where temperatures are higher than the extrapolated adiabat these results indicate the importance of using a joint geodynamical mineralogical approach to predict and aid in the interpretation of deep mantle seismic structure because interpretations based on seismology and mineral physics alone may be misleading and do not capture the strong lateral variability in 1 d structure obtained here for example multiple reflections arising from folded slabs and the precise balance between thermal and compositional influences on seismic structure

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

the influence of tectonic plates on mantle convection patterns temperature and heat flow summary the dynamic coupling between plate motion and mantle convection is investigated in a suite of cartesian models by systematically varying aspect ratios and plate geometries the aim of the study presented here is to determine to what extent plates affect mantle flow patterns temperature and surface heat flux to this end we compare numerical convection models with free slip boundary conditions to models that incorporate between one and six plates where the geometries of the plates remain fixed while the plate velocities evolve dynamically with the flow we also vary the widths of the plates and the computational domain in order to determine what constraint these parameters place on the mean temperature heat flux and plate velocity of mantle convection models we have investigated the influence of plates for three whole mantle convection cases that differ in their heating modes internally heated and basally heated and rheologies isoviscous and depth dependent viscosity we present a systematic investigation of over 30 models that exhibit increasingly complex behaviour in order to understand highly time dependent systems using the insight gained from simpler models in models with aspect ratios from 0 5 to 12 we find that for the same heating mode variations in temperature can be as much as 40 per cent when comparing calculations with unit width plates to models incorporating plates with widths equal to five times the model depth mean surface heat flux may decrease by 60 per cent over the same range of plate widths we also find that internally heated mantle convection models incorporating plates exhibit novel behaviour that we believe has not been described previously in mantle convection studies specifically in internally heated models plate motion is characterized by episodic reversals in direction driven by changes in the mantle circulation from clockwise to counterclockwise and vice versa these flow reversals occur in internally heated convection and are caused by a build up of heat in the interiors of wide convection cells close to mantle downwellings we find that flow reversals occur rapidly and are present in both single plate and multiple plate models that include internal heating this behaviour offers a possible explanation for why the pacific plate suddenly changed its direction some 43 ma

geoid anomalies in a dynamic earth in order to obtain a dynamically consistent relationship between the geoid and the earth s response to internal buoyancy forces we have calculated potential and surface deformation love numbers for internal loading these quantities depend on the depth and harmonic degree of loading they can be integrated as green functions to obtain the dynamic response due to an arbitrary distribution of internal density contrasts spherically symmetric self gravitating flow models are constructed for a variety of radial newtonian viscosity variations and flow configurations including both whole mantle and layered convection we demonstrate that boundary deformation due to internal loading reaches its equilibrium value on the same time scale as postglacial rebound much less than the time scale for significant change in the convective flow pattern by calculating relaxation times for a series of spherically symmetric viscous earth models for uniform mantle viscosity the geoid signature due to boundary deformations is larger than that due to internal loads resulting in net negative geoid anomalies for positive density contrasts geoid anomalies from intermediate wavelength density contrasts are amplified by up to an order of magnitude geoid anomalies are primarily the result of density contrasts in the interior of convecting layers density contrasts near layer boundaries are almost completely compensated layered mantle convection results in smaller geoid anomalies than mantle wide flow for a given density contrast viscosity stratification leads to more complicated spectral signatures because of the sensitivity of the dynamic response functions to model parameters forward models for the geoid can be used to combine several sources of geophysical data e g subducted slab locations seismic velocity anomalies surface topography to constrain better the structure and viscosity of the mantle

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