constraints on the correlation of p and s wave velocity heterogeneity in the mantle from p pp ppp and pkpab traveltimes we investigate the correlation of large scale p and s velocity heterogeneity in the mantle by determining how well 106 000 compressional p pp ppp and pkpab traveltimes can be explained by s wave velocity model s20rts scaled using a depth dependent factor and by a model in which the lateral p velocity variations are different we first assess the assumption that p wave traveltimes can be explained by a model in which lateral p velocity variations p are identical to s velocity variations s in model s20rts for a given depth we project s from s20rts into model s2p using a depth dependent scaling factor r defined as s r z x p we find by grid search that the highest reduction of data variance is obtained when r increases linearly from 1 25 at the surface to 3 0 at the core mantle boundary a comparison of s wave ss and p wave pp traveltimes for identical source receiver pairs also indicates that r increases with depth significantly higher variance reduction is not obtained when r is parametrized with an additional degree of freedom therefore the precise shape of r cannot be constrained by our data p and pp wave traveltime anomalies with respect to the scaled model s2p yield coherent geographic variations this indicates that there are large scale lateral p velocity variations in the mantle that are different from those in model s2p we estimate these variations by inverting p wave traveltime anomalies with respect to model s2p for a degree 12 model of p velocity heterogeneity this model p 12 s 2 p indicates where in the well sampled mantle regions we need to modify model s2p to further improve the fit to the traveltime data anomalies in p12 s 2 p exist throughout the mantle it is therefore not obvious that compositional heterogeneity is prominent in the lower 1000 km of the mantle only as suggested previously low p wave velocities in the upper mantle beneath oceans are the strongest anomalies in p12 s 2 p and explain better the delayed traveltimes of pp wave phases with oceanic surface refection points lower mantle anomalies include high and low p velocity structures beneath eastern asia and north america respectively the high p velocity anomaly in the lower mantle beneath the central pacific is consistent with the assertion made by other researchers that large scale lower mantle upwellings are not purely thermal in origin

the influence of thermochemical convection on the fixity of mantle plumes abstract a general feature of both isochemical and thermochemical studies of mantle convection is that horizontal plume velocities tend to be smaller than typical convective velocities however it is not clear which system leads to a greater fixity of mantle plumes we perform two and three dimensional numerical calculations and compare both thermochemical and isochemical cases with similar convective vigor to determine whether presence of a dense component in the mantle can lead to smaller ratios of horizontal plume velocity to surface velocity we investigate different viscosity and density contrasts between chemical components in the thermochemical calculations and we perform isochemical calculations with both free slip and no slip bottom boundary conditions we then compare both visually and quantitatively the results of the thermochemical and isochemical calculations to determine which leads to greater plume fixity we find that horizontal plume velocities for thermochemical calculations are similar to those from isochemical calculations with no slip bottom boundary conditions in addition we find that plumes tend to be more fixed for isochemical cases with free slip bottom boundary conditions for two dimensional calculations however in three dimensions we find that plume fixity is similar to that observed in thermochemical calculations

tomography of core mantle boundary and lowermost mantle coupled by geodynamics summary we propose an innovative approach to mapping cmb topography from seismic p wave traveltime inversions instead of treating mantle velocity and cmb topography as independent parameters as has been done so far we account for their coupling by mantle flow as formulated by forte peltier this approach rests on the assumption that p data are sufficiently sensitive to thermal heterogeneity and that compositional heterogeneity albeit important in localized regions of the mantle e g within the d region is not sufficiently strong to govern the pattern of mantle wide convection and hence the cmb topography the resulting tomographic maps of cmb topography are physically sound and they resolve the known discrepancy between images obtained from classic tomography on the basis of core reflected and core refracted seismic phases since the coefficients of mantle velocity structure are the only free parameters of the inversion this joint tomography geodynamics approach reduces the number of parameters nevertheless the corresponding mantle models fit the seismic data as well as the purely seismic ones

constraints on the lateral strength of slabs from three dimensional dynamic flow models if the viscosity of subducted lithosphere is described purely by temperature and pressure it should remain considerably more viscous than the surrounding material as it descends through the upper mantle many dynamic models of the long wavelength geoid associated with slabs require an increase in viscosity from the upper to the lower mantle but assume the perturbation to the lateral viscosity structure arising from the slabs themselves can be ignored previous studies indicate that strong and localized viscosity variations should have a dramatic influence on the geoid we present 3d finite element models of the regional geoid of the western pacific subduction zones slab buoyancies and viscosities are defined using the distribution of seismicity the geoid is very sensitive to the lateral strength of the slab very viscous slabs penetrating a low viscosity mantle generate significant 10 50 m long wavelength geoid lows opposite to the geoid high which is observed over slabs to obtain a geoid high comparable to that observed the lower mantle viscosity must be 60 200 times greater than the upper mantle viscosity and the slab must be in contact with the lower mantle these strict requirements suggest that slabs have been weakened and cannot act as stress guides from the deep mantle to the surface

the relationships between large scale variations in shear velocity density and compressional velocity in the earth s mantle a large data set of surface wave phase anomalies body wave travel times normal mode splitting functions and long period waveforms is used to investigate the scaling between shear velocity density and compressional velocity in the earth s mantle we introduce a methodology that allows construction of joint models with various levels of scaling complexity d ln d lnvs d lnvs d lnvp in order to detect seismological signatures of chemical heterogeneity we demonstrate that the data sets considered cannot be fit concurrently with a uniform or a positive and uniform throughout the mantle the variance reductions to p wave travel times and vp sensitive modes are up to 40 higher with our preferred model of anisotropic shear and compressional velocity than the recent anisotropic shear velocity model s362ani m which was constructed assuming a uniform throughout the mantle several features reported in earlier tomographic studies persist after the inclusion of new and larger data sets anticorrelation between bulk sound and shear velocities in the lowermost mantle as well as an increase in with depth in the lower mantle are largely independent of the regularization scheme when correlations between density and shear velocity variations are imposed in the lowermost mantle variance reductions of several spheroidal and toroidal modes deteriorate by as much as 40 recent measurements of the splitting of 0s2 in particular are largely incompatible with perfectly correlated shear velocity and density heterogeneity throughout the mantle a way to significantly improve the fits to various data sets is by allowing independent density perturbations in the lowermost mantle our preferred joint model consists of denser than average anomalies 1 peak to peak at the base of the mantle roughly coincident with the low velocity superplumes the relative variation of shear velocity density and compressional velocity in our study disfavors a purely thermal contribution to heterogeneity in the lowermost mantle with implications for the long term stability and evolution of superplumes

evidence for a sharp lateral variation of velocity at the core mantle boundary from multipathed pkpab rapid changes in differential travel times between the various pkp branches have been attributed to strong lateral velocity variations at the base of the mantle differential time pkpab df residuals showing jumps of up to 2 s for paths only 100 km apart have been reported such extreme changes in travel times for signals with comparable wavelengths suggest ray bifurcation with multipaths containing slow and fast contributions to pkpab here we report on some well documented normal and multipathed pkpab phases along with rapidly varying differential time residuals observed for fiji events with paths sampling the northern edge of the mid pacific large slow structure beneath hawaii systematic analyses of these data with two dimensional models suggest the presence of a ridge shaped structure at the core mantle boundary containing abrupt reductions of p wave velocity of up to 10 this narrow ultralow velocity zone ulvz is located at the boundary between a large scale low velocity and high velocity structure corroborating recent dynamic models which propose that ulvzs preferentially occur at the boundary between hot and cold mantle domains and could be related to plume activity at hawaii

the effect of plate motion history on the longevity of deep mantle heterogeneities abstract understanding the first order dynamical structure and evolution of earth s mantle is a fundamental goal in solid earth geophysics tomographic observations reveal a lower mantle characterised by higher than average shear wave speeds beneath asia and encircling the pacific consistent with cold slabs beneath regions of ancient subduction and lower than average shear wave speeds in broad regional areas beneath africa and the central pacific termed llsvps the llsvps are not well understood from a dynamical perspective and their origin and evolution remain enigmatic some numerical studies propose that the llsvp beneath africa is post pangean in origin formed as a result of return flow in the mantle due to circum pangean subduction countered by an older pacific llsvp suggested to have formed during the break up of rodinia this propounds that prior to the formation of pangea the lower mantle was dominated by a degree 1 convection pattern with a major upwelling centred close to the present day pacific llsvp and subduction concentrated mainly in the antipodal hemisphere in contrast palaeomagnetic observations which proffer a link between the reconstructed eruption sites of phanerozoic kimberlites and large igneous provinces with regions on the margins of the present day llsvps suggest that the anomalies may have remained stationary for at least the last 540 myr and further that the anomalies were largely insensitive to the formation and subsequent break up of pangea here we investigate the evolution and long term stability of llsvp like structures in earth s mantle by integrating plate tectonics and numerical models of global thermochemical mantle dynamics we explore the possibility that either one or both llsvps existed prior to the formation of pangea and improve upon previous studies by using a new true polar wander corrected global plate model to impose surface velocity boundary conditions for a time interval that spans the amalgamation and subsequent break up of the supercontinent we find that were only the pacific llsvp to exist prior to the formation of pangea the african llsvp would not have been created within the lifetime of the supercontinent we also find that were the mantle to be dominated by two antipodal llsvp like structures prior to the formation of pangea the structures would remain relatively unchanged to the present day and would be insensitive to the formation and break up of the supercontinent our results suggest that both the african and pacific llsvps have remained close to their present day positions for at least the past 410 myr

the composition and evolution of the continental crust rare earth element evidence from sedimentary rocks the composition of the present day upper crust inferred from the uniformity of sedimentary rock r e e rare earth element patterns is close to that of granodiorite a revised andesite model is used to obtain total crustal composition the lower crust is the composition remaining assuming that the upper crust one third of the total is derived from intracrustal partial melting the upper crustal r e e pattern has pronounced eu depletion eu eu 0 64 the lower crustal pattern has eu enrichment eu eu 1 17 and the total crust has no eu anomaly relative to chondritic abundances the eu depletion in the upper crust is attributed to retention of eu in plagioclase in the lower crust because plagioclase is not stable below 40 km 10 kbar the anomaly is intracrustal in origin the archaean upper crust has a different r e e pattern to that of the present day upper crust being lower in total r e e and la yb ratios and lacking an eu anomaly these data are used to infer the archaean upper crustal composition which resembles that of the present day total crust except that ni and cr contents are higher the archaean crustal composition can be modelled by a mixture of tholeiites and tonalite trondhjemites the latter have steep light r e e enriched heavy r e e depleted patterns consistent with equilibration with garnet and hence probable mantle derivation there is little reason to suppose that the archaean lower crust was different in composition from the upper crust except locally where partial melting episodes occurred the r e e evidence is consistent with isotopic and geological evidence for a low continental growth rate in the early archaean a massive increase to about 70 of the total crust between about 3000 and 2500 ma b p and a slow increase until the present day the change from archaean to post archaean r e e patterns in the upper crust is not isochronous but is reflected in the sedimentary rock r e e patterns at differing times in different continents on the basis of a model composition for the mantle 36 of the potassium 30 of uranium 15 of lanthanum and 3 of ytterbium are concentrated in the present continental crust this enrichment is related to ionic size and valency differences from common mantle cations e g mg fe pre 3 9 ga b p crusts were obliterated by meteorite bombardment no geochemical evidence exists for primordial anorthositic sialic or mafic crusts

deformation in the lowermost mantle from polycrystal plasticity to seismic anisotropy in the deep earth deformation occurs at many scales large scale convection produces subduction of slabs and upwelling of plumes in the mantle at the high temperature high pressure conditions strain is accommodated through crystal plasticity either by diffusion or the movement of dislocations slip causes crystal rotations and thus produces a characteristic pattern of crystal preferred orientation and corresponding anisotropy of physical properties at the macroscopic scale in this study we use polycrystal plasticity with experimentally derived deformation mechanisms for perovskite post perovskite and magnesiowuestite to predict texture development along streamlines in a 2d geodynamic convection model of the lowermost mantle strong preferred orientation develops during subduction and upwelling while during spreading along the core mantle boundary the orientation pattern is relatively stable from preferred crystal orientation and single crystal elastic properties bulk elastic properties can be calculated and compared with seismic observations post perovskite with predominant 001 slip and magnesiowuestite with 110 and 111 slip produce anisotropy patterns which are consistent with observed anisotropy i e fast s waves polarized parallel to the core mantle boundary and anti correlation between p and s wave anisotropies in contrast perovskite with dominant 001 slip and two post perovskite models with dominant slip on 010 and 100 produce anisotropy patterns which are inconsistent with seismic observations

mantle tomography and its relation to temperature and composition we propose a new method to constrain lateral variations of temperature and composition in the lower mantle from global tomographic models of shear and compressional wave speed we assume that the mantle consists of a mixture of perovskite and magnesio wustite in a first stage we directly invert vp and vs anomalies for variations of temperature and composition using the appropriate partial derivatives or sensitivities of velocities to temperature and composition however uncertainties in the tomographic models and in the sensitivities are such that variations in composition are completely unconstrained inferring deterministic distributions of temperature and composition being currently not possible we turn to a statistical approach which allows to infer several robust features comparison between synthetic and predicted ratios of the relative shear to compressional velocity anomalies indicates that the origin of seismic anomalies cannot be purely thermal but do not constrain the amplitude of the variations of temperature and composition we show that we can estimate these variations using histograms of the relative vp and vs anomalies at a given depth we computed histograms for a large variety of cases and found that at the bottom of the mantle variations in the volumic fraction of perovskite from 14 to 10 are essential to explain seismic tomography in the mid mantle anomalies of perovskite are not required but moderate variations up to 6 can explain the observed distributions equally well these trade offs between anomalies of temperature and composition cannot be resolved by relative velocity anomalies alone an accurate determination of temperature and composition requires the knowledge of density variations as well we show that anomalies of iron can then also be resolved 2003 elsevier b v all rights reserved

large scale structure at the core mantle boundary from diffracted waves at the base of the earth s mantle is a region called d which serves as a thermal and chemical boundary layer between the silicate mantle and the liquid iron outer core1 2 tomographic models of mantle compressional wave velocity vp have their worst resolution in d owing to limited ray sampling3 5 but have hinted at large scale lateral variations there here i use a new technique which has its greatest resolution within d to produce a map of the large scale vp variations within d the technique compares the arrival times of waves that have been refracted across and diffracted around the core mantle boundary cmb the diffracted waves travel a long way in d making them excellent probes of this layer and the differential technique removes many effects arising from ray paths outside of d ref 6 the new map provides an image of the continent sized variations at the cmb that is complementary to existing shear wave data7 8 and similarities between the map and projections of ancient subducted lithosphere9 verify previous suggestions10 of a strong coupling between surface plate tectonics and the base of the mantle

sediments at the top of earth s core unusual physical properties at the core mantle boundary have been inferred from seismic and geodetic observations in recent years we show how both types of observations can be explained by a layer of silicate sediments which accumulate at the top of the core as earth cools compaction of the sediments expels most of the liquid iron but leaves behind a small amount of core material which is entrained in mantle convection and may account for the isotopic signatures of core material in some hot spot plumes extraction of light elements from the liquid core also enhances the vigor of convection in the core and may increase the power available to the geodynamo

joint seismic tomography for bulk sound and shear wave speed in the earth s mantle high quality p and s travel times are now available from careful reprocessing of data reported to international agencies a restricted data set has been extracted for which comparable ray coverage is achieved for p and s and used for a joint inversion to produce a three dimensional model for shear and bulk sound velocities represented in terms of 2 2 cells and 18 layers in depth through the mantle about 106 times for each of p and s are combined to produce 312 549 summary rays for each wave type linearizing about the ak135 reference model 583 200 coupled tomographic equations are solved using an iterative partitioned scheme clear high resolution images are obtained for both bulk sound speed and shear wavespeed the bulk and shear moduli have differing sensitivity to temperature and mineral composition and so the images of the two velocity distributions help to constrain the nature of the processes which produce the variations different heterogeneity regimes can be recognised in the upper mantle the transition zone most of the lower mantle and the lowermost mantle in the upper mantle many features can be explained by thermal effects but in some orogenic zones e g western north america the opposite sense of the bulk sound and shear wave speed variation requires compositional effects or volatiles to outweigh any thermal effects in the lower mantle pronounced narrow structures which may represent remnant subduction are most marked in shear the level of large scale variations in bulk sound speed compared to shear diminishes with depth in the lower mantle reaching a minimum near 2000 km below this depth the variability of both wave speeds increases near the core mantle boundary the variations of the two wave speeds show little concordance suggesting the presence of widespread chemical heterogeneity

effects of low viscosity post perovskite on thermo chemical mantle convection in a 3 d spherical shell 1 numerical simulations of thermo chemical multi phase mantle convection in a 3 d spherical shell are performed to determine how a low viscosity of post perovskite affects dynamics and structures in the deep mantle low viscosity post perovskite weakens the deepest part of slabs allowing them to more effectively spread over the core mantle boundary cmb and it also results in a greater volume of basalt segregating both of which increase the size of dense chemical piles the horizontal lengthscale of regions of pooled slab material and the steepness of piles edges in composition and phase consistent with the existence of steep sharp sided edges found in seismic analyses cmb heat flux is strongly enhanced in regions of low viscosity post perovskite consistent with a theoretical prediction and both cmb and surface heat flux are increased on average by a low viscosity of post perovskite which could have important implications for the evolution of earth s core and mantle

testing the tracer ratio method for modeling active compositional fields in mantle convection simulations 1 tracer methods are attractive for modeling compositional fields because they offer the potential of zero numerical diffusion composition is typically taken to be proportional to the absolute local concentration of tracers but an increasingly popular method is to have dense and regular tracers with composition being equal to the local fraction of dense tracers this paper tests this ratio method using established benchmarks and by comparing the performance of the two tracer methods and grid based methods for simulating the long term evolution of a convecting mantle with a thick dense stable layer for this scenario the ratio method is found to have several advantages giving sharp stable long term layering with no tracer settling minimal statistical noise and low entrainment even with only 5 tracers per cell the method is equally applicable to finite volume and finite element treatments of the underlying flow entrainment in grid based advection methods is heavily dependent on resolution and numerical details and is reduced 1 order of magnitude by the filter proposed by a lenardic numerical determination of physically correct entrainment rates remains a challenging problem comparing tracer and grid based methods the spatial pattern of the thermal and chemical fields appear to be converging on the finest grids however the estimated entrainment differs significantly

three dimensional structure at the base of the mantle beneath the central pacific forward modeling of differential travel times of phases sensitive to lowermost mantle beneath the central pacific reveals lateral heterogeneity that is higher in amplitude than predicted by tomographic models a broad zone of low s velocity 4 percent with respect to standard models which may correspond to the base of a thermal plume narrows and is deflected as it extends to about 1000 kilometers above the core mantle boundary to the east of this zone a localized region of fast s velocity 5 percent suggests strong heterogeneity or anisotropy related to the presence of high pressure and temperature assemblages which may or may not involve core material its presence could also explain the observation of precursors to core reflected phases in this region

tomographic inversion of s sks times for shear velocity heterogeneity in d degree 12 and hybrid models differential travel times between s or diffracted s and sks were measured to study the global distribution of shear wave velocity heterogeneity in the lowermost 250 km of the mantle the d region commencing with 3000 s sks times with variable qualities we minimize uneven path coverage by thinning redundantly sampled regions e g the fiji tonga to north america corridor and collect additional data for sparsely sampled areas especially in the southern hemisphere about 1500 paths were retained distributed to reconcile both high density and homogeneous sampling we compare 1 spherical harmonic and 2 equal area block parameterizations of d shear velocity heterogeneity for identical minimum resolving lengths and mean model errors we show that the two parameterizations result in indistinguishable patterns of heterogeneity and power distribution for resolution down to a block size of 4 5 4 5 and maximum spherical harmonic degree of 40 we demonstrate synthetically that a high degree l inversion followed by a lower degree ls spherical harmonic synthesis effectively circumvents model contamination from expansion truncation effects tomographic inversion of this data set yields a global distribution of d heterogeneity robust up to degree 12 in our preferred model l 40 ls 12 rms 1 surface hotspots and estimated lower mantle plume roots are located in or at edges of low velocity regions vs 2 for both the atlantic indian low velocity corridor as well as the low velocities beneath the central pacific although the above parameterizations entail no intrinsic difference in resolvability higher resolution is regionally achieved by removing the derived degree 12 model from the data then inverting the corrected residuals using 4 5 4 5 blocks this hybrid method prevents power loss at intermediate to long wavelengths from otherwise severe damping and recovers smaller scale structure where constraints are better than the global average such as beneath the pacific and eurasia a synthetic recovery test yields resolvability maps that reflect both the path geometry and the quality of data which help to identify robust features in both the degree 12 and the hybrid d models

core mantle boundary topography and its relation to the viscosity structure of the lowermost mantle abstract two large areas of anomalously low seismic velocities are visible in all tomographic models of the lowermost mantle depending on the density structure of these large low shear velocity provinces llsvps the core mantle boundary cmb will deform upwards or downwards due to isostatic and dynamic topography the latter being sensitive to the viscosity structure of the lowermost mantle heterogeneities in the viscosity structure although difficult to constrain might be especially important if the llsvps are thermochemical piles with elevated intrinsic viscosity as suggested by mineral physics based on numerical models we identify a short wavelength about 80 120 km wide up to a few km deep topographic depression that forms around the pile edges if the pile is more viscous than the surrounding mantle the depression forms when a wedge of thermal boundary layer material becomes compressed against the viscous pile and is enhanced by relative uplift of the cmb beneath the pile by plumes rising above it the depth and asymmetry of the depression constrain the magnitude of the viscosity contrast between pile and the surrounding mantle furthermore periodic plume initiation and pile collapse at the pile margin systematically modify the characteristic depression with a maximum in asymmetry and depth at the time of plume initiation core reflected waves or scattered energy may be used to detect this topographic signature of stiff thermochemical piles at the base of the mantle

constraints on core mantle boundary topography from normal mode splitting 1 the frequencies of earth s normal modes are split by rotation ellipticity and internal structure of the earth thus models of mantle heterogeneity and discontinuity topography generate splitting that may be tested against observations we insert maps of core mantle boundary cmb topography which are derived via either a purely seismic or a joint tomographic geodynamic inversion of body waves data on top of tomographic model s20rts we then calculate synthetic splitting functions for normal modes that have been shown to be sensitive to cmb topography and compare these to observed normal mode splitting data the cmb topography maps obtained via geodynamically constrained tomography fit normal mode data better than purely seismic maps in particular when the geodynamic constraint also accounts for the presence of post perovskite in the d 00 region we test the significance of the reduction in misfit using the concept of observability which suggests that normal modes are able to observe the difference between the different cmb topography maps in addition we find that the statistical significance assessed by checking what fraction of 1000 randomly generated cmb models achieve a comparatively good fit as the desired model is higher than 92 for degree 2 and 98 for all degrees in summary we have identified a model of cmb topography that fits body wave data and improves at least to some extent the fit to normal mode data and is coherent with the large scale pattern of deep mantle heterogeneity expected on the basis of convection modeling

archean and proterozoic crustal evolution evidence from crustal seismology seismic velocity models for archean and proterozoic provinces throughout the world are analyzed the thickness of the crust in archean provinces is generally found to be about 35 km except at collisional boundaries whereas proterozoic crust has a significantly greater thickness of about 45 km and has a substantially thicker high velocity 7 0 km s layer at the base we consider two models that may explain these differences the first model attributes the difference to a change in the composition of the upper mantle the higher temperatures in the archean mantle led to the eruption of komatiitic lavas resulting in an ultradepleted lithosphere unable to produce significant volumes of basaltic melt proterozoic crust developed above fertile mantle and subsequent partial melting resulted in basaltic underplating and crustal inflation in the second model convection in the hot archean mantle is considered to have been too turbulent to sustain stable long lived subduction zones by the proterozoic the mantle had cooled sufficiently for substantial island and continental arcs to be constructed and the high velocity basal layer was formed by basaltic underplating

petrogenesis of neoarchean metavolcanic rocks in changyukou northwestern hebei implications for the transition stage from a compressional to an extensional regime for the north china craton abstract the 2 5 ga metavolcanic rocks in changyukou northwestern hebei can be classified into three groups based on major and trace elements high mg basalts tholeiitic basalts and the calc alkaline series basaltic andesites andesites and dacites rhyolites both high mg basalts and tholeiitic basalts have negative anomalies of nb zr ti and heavy rare earth elements hree as well as enrichments of sr k pb ba and light rare earth elements lree and show typical subduction zone affinities the petrogenesis of high mg basalts can be ascribed to high degree partial melting of an enriched mantle source in the spinel stability field that was previously enriched in large ion lithophile elements lile and lree by slab derived hydrous fluids melts supercritical fluids as well as the subsequent magma mixing processes of different sources at different source depths with little or no influence of polybaric fractional crystallization the flat hree of tholeiitic basalts indicates they may also originate from the spinel stability field but from obviously shallower depths than the source of high mg basalts they may form at a later stage of the subduction process when rapid slab rollback leads to extension and seafloor spreading in the upper plate we obtain the compositions of the archean lower crust of the north china craton based on the archean wutai jining section by compiling the average tonalite trondhjemite granodiorite ttg components average mafic granulite components and average sedimentary rock components the modeling results show that the generation of high al basalts basaltic andesites and andesites can be attributed to assimilation by high mg basalts primary basalts of relatively high al 2 o 3 thickened lower crust and the subsequent crystallization of prevailing mafic mineral phases while al 2 o 3 rich plagioclase crystallization is suppressed under high pressure and nearly water saturated conditions dacites and rhyolites may be the result of further fractional crystallization of basaltic andesites high al basalts and andesites mixing of magmas at various stages along the fractionation course of basaltic andesites high al basalts toward rhyolites promotes the trend of the calc alkaline series to reconcile the 2 55 to 2 5 ga ttgs derived from overthickened crust the 2 51 to 2 50 ga calc alkaline volcanic rocks derived from thickened crust tholeiitic basalts representing low pressure and an extensional tectonic setting 2493 ma leucosyenogranites derived from overthickened crust 2437 ma biotite monzogranites derived from slightly thinner crust than leucosyenogranites but still thickened as well as the clockwise hybrid itd and ibc p t paths of the hp granulites and widespread extension and rifting setting within the ncc from 2300 ma we propose a model of an evolving subduction process among them the composition of the 2 5 ga changyukou volcanic rocks and potassic granites as well as the clockwise hybrid itd and ibc p t paths of the hp granulites may reveal that the tectonic setting in northwest hebei was in a transition stage from a subduction related compressional regime to an extensional regime related to plate rollback

further constraints on the african superplume structure it is well established that there is a large scale low velocity structure in the lowermost mantle beneath africa extending from the southeastern atlantic ocean to the southwestern indian ocean with a volume greater than 10 billion km 3 7000 km long 1000 km across and 1200 km high earth planet sci lett 206 2003 119 this low velocity structure is often called the african superplume various studies also require sharp boundaries for the plume however as for its height and shear velocity reduction there has been some controversy especially concerning the velocities at the core mantle boundary cmb here we present an assortment of phases involving s diff sks s and s c s with both vertical and horizontal paths sampling a 2d corridor through the structure travel time and waveform modeling of these seismic phases argues for a model with shear velocity reduction of approximately 3 within the superplume which is basically a 200 km thick layer low velocity layer beneath the southern atlantic ocean and a 1200 km high structure beneath south africa and against a model of a substantially reduced low velocity layer up to 10 300 km beneath the superplume we also analyzed p diff and the differential times of p c p p and compared them with sdiff and s c s s observations along the same great circle paths the p velocity is not very anomalous at most 0 5 much smaller than 1 as expected from a thermal anomaly with 3 lower s velocity geophys res lett 27 2000 421 thus again arguing for a chemical origin which was suggested from the modeling of african superplume sharp sides science 296 2002 1850

core mantle boundary topography from short period pcp pkp and pkkp data abstract we use a total of 839 369 pcp pkpab pkpbc pkpdf pkkpab and pkkpbc residual travel times from bull seism soc am 88 1998 722 grouped in 29 837 summary rays to constrain lateral variation in the depth to the core mantle boundary cmb we assumed a homogeneous outer core and the data were corrected for mantle structure and inner core anisotropy inversions of separate data sets yield amplitude variations of up to 5 km for pcp pkpab pkpbc and pkkp and 13 km for pkpdf this is larger than the cmb undulations inferred in geodetic studies and moreover the pcp results are not readily consistent with the inferences from pkp and pkkp although the source receiver ambiguity for the core refracted phases can explain some of it this discrepancy suggest that the travel time residuals cannot be explained by topography alone the wavespeed perturbations in the tomographic model used for the mantle corrections might be too small to fully account for the trade off between volumetric heterogeneity and cmb topography in a second experiment we therefore re applied corrections for mantle structure outside a basal 290 km thick layer and inverted all data jointly for both cmb topography and volumetric heterogeneity within this layer the resultant cmb model can explain pcp pkp and pkkp residuals and has approximately 0 2 km excess core ellipticity which is in good agreement with inferences from free core nutation observations joint inversion yields a peak to peak amplitude of cmb topography of about 3 km and the inversion yields velocity variations of 5 in the basal layer the latter suggests a strong trade off between topography and volumetric heterogeneity but uncertainty analyses suggest that the variation in core radius can be resolved the spherical averages of all inverted topographic models suggest that the data are best fit if the actual cmb radius is 1 5 km less than in the earth reference model used i e the average outer core radius would be 3478 km

core mantle boundary topography as a possible constraint on lower mantle chemistry and dynamics article i nfo the origin of large low shear wave velocity provinces llsvps in the lowermost mantle beneath the central pacific and africa is not well constrained we explore numerical convection calculations for two proposed hypotheses for these anomalies namely thermal upwellings e g plume clusters and large intrinsically dense piles of mantle material e g thermochemical piles each of which uniquely affects the topography on earth s core mantle boundary cmb the thermochemical pile models predict a relatively flat but elevated cmb beneath piles presumed llsvps with strong upwarping along llsvp margins the plume cluster models predict cmb upwarping beneath upwellings that are less geographically organized both models display cmb depressions beneath subduction related downwelling while each of the two models produces a unique characteristic style of cmb topography we find that seismic models will require shorter length scales than are currently being employed in order to distinguish between the end member dynamic models presented here

normal mode and free air gravity constraints on lateral variations in velocity and density of earth s mantle with the use of a large collection of free oscillation data and additional constraints imposed by the free air gravity anomaly lateral variations in shear velocity compressional velocity and density within the mantle dynamic topography on the free surface and topography on the 660 km discontinuity and the core mantle boundary were determined the velocity models are consistent with existing models based on travel time and waveform inversions in the lowermost mantle near the core mantle boundary denser than average material is found beneath regions of upwellings centered on the pacific ocean and africa that are characterized by slow shear velocities these anomalies suggest the existence of compositional heterogeneity near the core mantle boundary

secular changes in tonalite trondhjemite granodiorite composition as markers of the progressive cooling of earth archean tonalite trondhjemite granodiorite associations ttg are classically thought to generate through partial melting of hydrous metabasalts however the chemical composition of the least differentiated ttg parental magmas evolved from 4 0 to 2 5 ga during this interval the mg as well as the ni and cr contents increased which is interpreted as reflecting increased interactions between felsic melts generated by metabasalt melting and mantle peridotite similarly cao na2o and sr also increased over time thus reflecting an increase in the abundance of plagioclase in the melt residue the presence or absence of residual plagioclase is interpreted in terms of melting depth the demonstrated interaction between ttg parental magmas and the mantle rules out their genesis by fusion of previously underplated metabasalt and favors the melting of subducted slab material at 4 0 ga earth s geothermal gradient was sufficiently high to allow slab melting at shallow depths where plagioclase was stable consequently due to the small thickness of the overlying mantle wedge felsic magmas interacted little with the mantle at 2 5 ga however owing to lower geothermal gradients the melting depth was greater and plagioclase became no longer stable in the thick mantle wedge overlying the subducted slab as a result felsic magmas reacted strongly with the mantle peridotite the changes of ttg composition during archean time can be thus interpreted as reflecting the progressive cooling of earth

simultaneous inversion for 3 d variations in shear and bulk velocity in the mantle until recently most of the seismic tomographic modeling has been addressing the question of lateral heterogeneity either in p or s wave velocities the s wave velocity variations are larger and hence provide stronger signal on long period waveforms the direct p travel times being the first arrivals on the other hand are most frequently reported in the international seismological centre isc bulletins in mineral physics experiments the variation in bulk velocity is more often measured to better understand the differences between vp and vp patterns and better link the results from mineral physics to those of seismic tomography we formulate the inverse problem in terms of relative perturbations in the shear velocity vs 12 and bulk sound velocity v k 12 we use a large data set which consists of waveforms waveform derived travel times and travel times from the isc bulletins the earthquakes are relocated using corrections for lateral heterogeneity the events which cannot be reliably determined are discarded the model is defined as spherical harmonics to degree 12 horizontally and as chebyshev polynomials to order 13 radially for both shear and bulk sound velocity the inversion is performed under smoothness constraint the resolution tests and bootstrapping analysis indicate that the model is well recovered particularly at long wavelength the results indicate a much larger variability of shear than bulk sound velocity the model explains observations well the most intriguing result obtained in this study is that the variations in shear velocity and bulk sound velocity are negatively correlated in the lowermost mantle the explanation is not very clear from the mineral physics point of view it is not unlikely that this could be explained in terms of thermal variation even though we are unwilling to rule out the possibility of large wavelength compositional variations

an ichthyornithiform bird from the campanian of canada the geographic range of the ichthyornithiform birds is extended to the canadian province of manitoba the associated fauna from the pembina member of the vermilion river formation of southern manitoba closely resembles that from the sharon springs member of the pierre shale formation in kansas south dakota and wyoming

linking lowermost mantle structure core mantle boundary heat flux and mantle plume formation abstract the dynamics of earth s lowermost mantle exert significant control on the formation of mantle plumes and the core mantle boundary cmb heat flux however it is not clear if and how the variation of cmb heat flux and mantle plume activity are related here we perform geodynamic model experiments that show how temporal variations in cmb heat flux and pulses of mantle plumes are related to morphologic changes of the thermochemical piles of large scale compositional heterogeneities in earth s lowermost mantle represented by the large low shear velocity provinces llsvps we find good correlation between the morphologic changes of the thermochemical piles and the time variation of cmb heat flux the morphology of the thermochemical piles is significantly altered during the initiation and ascent of strong mantle plumes and the changes in pile morphology cause variations in the local and the total cmb heat flux our modeling results indicate that plume induced episodic variations of cmb heat flux link geomagnetic superchrons to pulses of surface volcanism although the relative timing of these two phenomena remains problematic we also find that the density distribution in thermochemical piles is heterogeneous and that the piles are denser on average than the surrounding mantle when both thermal and chemical effects are included

three dimensional structures and dynamics in the deep mantle effects of post perovskite phase change and deep mantle layering 1 three dimensional numerical simulations of thermo chemical mantle convection with multiple phase transitions olivine spinel perovskite post perovskite are used to investigate the morphology of compositional and post perovskite ppv boundary structures above the core mantle boundary cmb compositionally dense subducted morb partially segregates above the cmb forming ridge like dense piles morphologically similar to structures in previous spherical or cartesian studies in which a dense layer was inserted a priori an anti correlation is observed between regions where a thick post perovskite layer occurs and regions containing piles of dense material as in our previous two dimensional study lateral variations in the thickness or occurrence of a ppv layer cause very strong seismic shear wave velocity anomalies which may dominate thermal or composition related seismic heterogeneity in the depth range where the variable ppv boundary occurs and also generate sharp sided structures as observed seismologically both compositional and ppv effects may be necessary to simultaneously explain large scale heterogeneity and the global distribution of the discontinuity atop d

geoid and topographic swells over temperature dependent thermal plumes in spherical axisymmetric geometry geoid and topographic profiles over constant viscosity thermal plumes in spherical axisymmetric geometry at moderate rayleigh numbers are compared with results from previous studies in other geometries plumes in spherical axisymmetric models produce larger geoid anomalies than plumes in either cartesian or cylindrical axisymmetric geometries however the topographic anomalies over the plumes are smaller than the corresponding anomalies in a cylindrical axisymmetric geometry but larger than those in a cartesian geometry for plumes in a temperature dependent viscosity fluid the geoid and topographic profiles are reduced by a factor of two when compared to a constant viscosity fluid with the same background viscosity profile in this case the radially averaged temperature profile around the plume creates a low viscosity layer beneath the lithosphere

the effects of boundary topography on convection in earth s core summary we present the first investigation that explores the effects of an isolated topographic ridge on thermal convection in a planetary core like geometry and using core like fluid properties i e using a liquid metal like low prandtl number fluid the model s mean azimuthal flow resonates with the ridge and results in the excitation of a stationary topographic rossby wave this wave generates recirculating regions that remain fixed to the mantle reference frame associated with these regions is a strong longitudinally dependent heat flow along the inner core boundary this effect may control the location of melting and solidification on the inner core boundary theoretical considerations and the results of our simulations suggest that the wavenumber of the resonant wave lr scales as ro 1 2 where ro is the rossby number this scaling indicates that small scale flow structures wavenumber in the core can be excited by a topographic feature on the core mantle boundary the effects of strong magnetic diffusion in the core must then be invoked to generate a stationary magnetic signature that is comparable to the scale of observed geomagnetic structures

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

on the emergence of plate tectonics at present young oceanic lithosphere is positively buoyant and it does not become negatively buoyant until it is older than about 20 m y if in the past the mantle was hotter the oceanic crust would have been thicker and the lithosphere9s age of neutral buoyancy greater on the other hand the hotter mantle would have had a lower viscosity and convected faster so the average age of oceanic plates at subduction would have been less than the present 100 m y at some time in the past average oceanic lithosphere would have only just become negatively buoyant as it reached a subduction zone it is estimated here that this condition occurred when the mantle was only about 50 c hotter than at present and as recently as 0 9 1 4 b y ago transformation of basaltic crust to eclogite might have enhanced the vigor of the plates but possibly only modestly and intermittently in earlier times plate tectonics could have operated but more slowly so that it could not by itself have accomplished the necessary rate of heat removal from the earth a different tectonic mode must also have operated perhaps subcrustal delamination or dripping plate tectonics would have gradually taken over from the earlier mode plumes are a omplementary mode of mantle convection driven by a lower thermal boundary layer and so they could have operated in conjunction with both regimes of the upper boundary layer

the archean proterozoic transition evidence from the geochemistry of metasedimentary rocks of guyana and montana abstract chemical and isotopic compositions of ancient sedimentary rocks have in some cases provided evidence of first order changes yin the composition and extent of continental crust from the archean to the proterozoic we reversed the typical sampling pattern comparing metapelites and carbonates from proterozoic greenstone belts with those from an archean continental margin sequence and found the reverse of the typical age composition relations metapelites and volcaniclastic metagreywackes from the early proterozoic greenstone belts of northern guyana resemble those of many archean greenstone belts in ree trace and major element compositions mean ratios la sm 5 0 la th 5 1 eu eu 0 80 th u 3 4 median k 2 o na 2 o 0 8 schists from an archean continental marginal sequence in montana have compositions like many post archean metapelites derived from continental sources mean ratios la sm 6 8 la th 2 7 eu eu 0 67 th u 5 2 median k 2 o na 2 o 1 7 other archean continental basin sediments also resemble their post archean counterparts implying that upper continental crusts of these eras may not have differed significantly in these respects however temporal change may have occurred in upper crustal contents of ni and cr many archean clastic metasedimentary rocks of both continental and greenstone belt provenance have contents of these compatible elements higher than in their post archean equivalents dolomitic limestone from a greenstone belt of northern guyana has a very low 87 sr 86 sr ratio of 0 7007 such low values may reflect the local influence of mantle derived volcanic rocks during carbonate deposition diagenesis or metamorphism and they may not be reliable indicators of the evolution of sr in seawater archean dolomitic marbles from the continental marginal sequence of montana have 87 sr 86 sr ratios of 0 7065 and higher significantly more radiogenic than many accepted values for carbonates of archean greenstone belts

geochemistry and zircon u pb geochronology of paleoproterozoic arc related granitoid in the northwestern yangtze block and its geological implications abstract the early history of the yangtze block has not been well constrained yet due to the scarce outcrops of archean to paleoproterozoic rocks in this study we report an integrated study of zircon u pb age major and trace element data and sr nd hf isotope compositions for gray gneisses from the houhe complex in the northwestern part of the yangtze block zircon u pb dating yields a weighted mean 207 pb 206 pb age of 2081 9 ma for a gneiss this age is interpreted as the formation age of the houhe complex and thus the houhe complex represents the oldest rocks found in the western part of the yangtze block the gray gneisses from the houhe complex range in sio 2 contents from 58 63 to 68 59 and na 2 o from 3 88 to 5 28 and have relatively high fe 2 o 3 contents of 2 86 6 69 al 2 o 3 of 16 01 18 88 and mgo of 0 97 2 65 these rocks show low sr 149 390 ppm cr 9 07 45 1 ppm and ni 4 97 21 3 ppm contents but high y 12 9 32 7 ppm and yb 0 95 2 25 ppm they are characterized by a relative enrichment in liles and lrees but a depletion in hfses these features are similar to those of calc alkaline granitoids suggesting that their formation might be related to a subduction related process or remelting of preexisted arc rocks compiled age spectra of archean to paleoproterozoic zircon grains reveal that the western and eastern parts of the yangtze block have similar archean and early paleoproterozoic age ranges indicative of the occurrence of an old continental nucleus in both parts on the other hand there are large amounts of ca 2050 2400 ma zircon grains revealed in the western part of the yangtze block implying that there might be a microcontinent with an active type continental margin during the paleoproterozoic times in the western part of the yangtze block because of the poor outcrops of archean to paleoproterozoic rocks in the yangtze block it is no possible to establish the exact nature of such an active margin yet the occurrence of ca 2 0 ga khondalitic and metasedimentary rocks has been documented in the eastern part of the yangtze block implying that there might be a passive type continental margin in the eastern part of the yangtze block at ca 2 0 ga the collision between the western microcontinent and the eastern part may have resulted in the final arc continental collision and the amalgamation of the yangtze block

a numerical study of thermal and chemical structures at the core mantle boundary abstract the core mantle boundary cmb represents the lower boundary layer of the actively convecting earth s mantle and is structurally very complex thermal and chemical structures such as thermal plumes and thermochemical piles have been considered to explain the complexities both affect the dynamics of the earth s mantle and its temporal evolution but also the surface plates are an essential aspect of mantle convection that strongly influence the dynamics of the interior we use numerical thermochemical models of mantle convection to study the structure and dynamics of the lowermost mantle our approach allows for the investigation of plumes and piles in combination with plate like surface motion and deep subduction the models show that the presence of a dense cmb layer generally reduces the mobility of the surface plates but that during plate evolution a variety of plume classes occur leaving a complex structure at the cmb the cmb topography shows large elevated areas with sharp edges and a flat or slightly dented top for piles with plumes atop also plume clusters can cause large elevated areas with sharp edges but a flat top with a few smaller peaks smaller scale patterns often close to the edges of the large structures cause smaller peaks in cmb topography and can be explained by either line plumes or wind driven thermals the latter often arise when heat is trapped beneath subducted slabs that spread over the cmb

the 2 65 ga a type granite in the northeastern yangtze craton petrogenesis and geological implications abstract the presence of old residual continental crustal rocks in cratons is crucial for investigation of their early evolution due to the poor exposure of archean rocks the early evolution of the yangtze craton has not been well constrained yet in this study a combined study of zircon u pb and hf o isotopes as well as whole rock major and trace elements and nd isotopes have been conducted for a granite pluton at huji town in the northern part of the yangtze craton u pb dating for magmatic zircons yielded a weighted mean age of 2656 6 ma revealing another archean outcrop in the yangtze craton besides the kongling terrane the granite samples show relatively evaluated k 2 o na 2 o cao tfeo mgo ratios and zr nb ce ga and y contents sharing the same features as the a type granite they have pronounced eu sr and ba negative anomalies on the primitive mantle normalized trace element spidergram implying the residual of plagioclase in the magma sources their high zr contents of 440 529 ppm correspond to initial magma temperatures of 930 962 c the zircons have negative hf t values of 2 7 to 0 1 and hf model ages of ca 3 2 ga respectively the 18 o values of the zircons range between 4 94 and 5 72 and give an average of 5 26 0 14 suggesting no contribution of supracrustal rock into the host magma therefore the hf model ages indicate the generation of the huji granite by reworking of ca 3 2 ga juvenile crustal rocks combined with previous studies the finding of the neoarchean huji granites with ca 3 2 ga hf model ages suggests the widespread of archean basement in the yangtze craton the occurrence of the ca 2 65 ga huji granites likely records the thermally stable lithosphere and thus heralds the craton stabilization of the eastern part of the yangtze block

core mantle boundary topography estimated from numerical simulations of instantaneous mantle flow core mantle boundary cmb topography is estimated from numerical simulations of instantaneous mantle flow in a three dimensional spherical shell geometry density anomaly models of the earth s mantle are derived from geodynamic models constrained principally by seismic tomography results to account for the small 1 5 km cmb topographic relief inferred from recent seismological results lateral viscosity variations in the mantle compositionally dense piles in the deep mantle and a low viscosity d layer are all required for the numerical models low viscosity subducting slabs in the lower mantle serve to reduce the topographic amplitude below high velocity regions i e the circum pacific belt the existence of dense piles may cause the local topographic depressions even below low velocity regions i e the south pacific and atlantic africa regions

thermochemical structures within a spherical mantle superplumes or piles 1 heterogeneous compositional mantle models are frequently invoked to explain some observations obtained from geochemistry and seismology in particular two regions in the earth s lower mantle under the pacific and africa are often interpreted as being either piles or superplumes of dense material we perform numerical modeling of thermochemical convection in a three dimensional spherical geometry to investigate whether the geometry can focus a dense chemical component into a small number of isolated rounded piles or superplumes of material we study the effect of temperature and compositionally dependent viscosity and we find two different modes of dense layer deformation temperature dependent rheology leads to a low viscosity dense layer which is passively swept aside by downwellings into linear piles that are spread throughout the entire lower mantle the addition of compositionally dependent rheology in which the dense layer is more viscous results in active deformation of the dense material forming large isolated superplumes our results indicate that piles and superplumes are separate features which in general do not occur together and in order for isolated rounded superplumes to form an intrinsic compositional viscosity increase is required for the dense material

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

can we resolve 3d density heterogeneity in the lower mantle we reexamine the possibility of resolving 3d density structure in the mantle by performing test inversions using a dataset of well constrained degree 2 splitting coefficients we show that degree 2 in vs and vp can be resolved and confirm that dlnvs dlnvp 2 5 at depths 2000km indicating a chemical component to large scale heterogeneity models with positively correlated with vs in the deepest mantle fit the data at least as well as those in which density and velocities are anti correlated however some rough bounds on dln dlnvs can be inferred models obtained from geodynamic considerations are acceptable in contrast to models with large negative 0 6 dln dlnvs throughout the lower mantle the most robust feature is a marked increase in dln dlnvs at the top of the lower mantle reaching a maximum around 1500 km

a strong lateral shear velocity gradient and anisotropy heterogeneity in the lowermost mantle beneath the southern pacific 1 velocity heterogeneity and seismic anisotropy in the d region beneath the southern pacific ocean is investigated using broadband shear waves from southwest pacific and south american earthquakes recorded by permanent and temporary stations in antarctica australia south america and on islands in the south pacific shear velocity perturbations vs are inferred from the differential travel times between hand picked horizontally polarized sh direct or diffracted s arrivals and vertically polarized sv sks arrivals derived patterns in vs roughly agree with global shear velocity models but reveal a stronger shear velocity lateral gradient north of approximately 52 s where vs transitions from approximately 0 5 to 1 0 relative to radially averaged reference models over less than 600 km along the base of d waveform analyses provide an even stronger constraint on the transitional region where there is a change in waveform behavior occurring over length scales less than 300 km differential travel times between skks and sks calculated by cross correlating sks with hilbert transformed skks support large vs amplitudes and help to constrain the location of the transitional region anisotropy in d ks is inferred from handpicked differential travel times between the sv and sh components of direct or diffracted s and display a slight spatial trend where the sh component precedes the sv in the north and east of the study area but arrives after sv in the center and southwest of it however there is high ks variability in the center of the study region there is no apparent correlation between vs and ks abrupt changes in the character of velocity and anisotropy in this region of d may be related to chemical heterogeneity at the boundary of the pacific superswell as well as small scale convection in the deep mantle

did the character of subduction change at the end of the archean constraints from convergent margin granitoids large ion lithophile and high field strength element distributions in juvenile upper continental crust are controlled chiefly by the abundance of tonalite trondhjemite granodiorite ttg in the archean shifting to a combination of ttg calc alkaline granitoid and graywacke control thereafter geochemical differences between ttg and high silica adakites do not require production of most ttg magmas in descending slabs changes in the ratio of ttg to calc alkaline granitoids after 2 5 ga indicate that archean subduction zones must have differed from younger subduction zones in two very important ways 1 a deep mafic crust served as a ttg magma source either as thickened crust or in descending slabs and 2 they did not give rise to significant volumes of calc alkaline magma thickened mafic crust in the late archean may have resulted from plate jams in subduction zones caused by thicker oceanic crust and oceanic plateaus produced during late archean mantle thermal events

mapping the lower mantle determination of lateral heterogeneity in p velocity up to degree and order 6 the data from the international seismological centre bulletins for the years 1964 1979 are used to derive a three dimensional model of lateral variations of the p velocity in the lower mantle unlike previous studies in which such perturbations were represented by a blocklike parameterization we seek the solution in the form r k 0k l 0l m 0l k r kalm cos m kblm sin m plm cos some 500 000 travel time residuals for teleseismic distances from 5000 earthquakes are used in an iterative procedure to derive the coefficients a and b the maximum k and l used are 4 and 6 respectively in each iteration in addition to solving for the three dimensional structure we also relocate all the earthquakes perturb the average travel time curve and determine station corrections for over 1000 stations particular attention is given to the problem of weighting the individual observations in order to avoid as much as possible the bias due to the uneven distribution of sources and receivers the resulting model shows a high level of perturbations just below the 670 km discontinuity and just above the core mantle boundary where the maximum perturbations reach 1 1 5 of the average velocity even for this highly smoothed model at a depth of 2000 km the rms perturbations are some 3 to 4 times lower than at either of the two boundaries the model predicts well the large scale pattern of observed travel time residuals for various source regions except for the distinct effects of the subduction zones the most striking large scale three dimensional feature of the model is a ring of high velocities circumscribing the pacific basin within a depth range from 1000 km to the core mantle boundary if the proportionality between perturbations in the velocity and temperature is assumed the derived pattern of anomalies could be consistent with a two boundary layer model of convection in the lower mantle some caution however should be exercised in interpreting the results near the 670 km discontinuity where the resolving power of our data set is limited by the lack of adequate control of the upper mantle structure at the same time a significant global increase in the level of perturbations in the lowermost mantle is established beyond question the results of dziewonski et al 1977 on the correlation between the equivalent geoid and the observed geoid for the angular order numbers 2 and 3 is confirmed but there is no significant correlation for degrees from 4 to 6 these calculations are made for the rigid earth richards and hager this issue show that introduction of finite viscosity can change the sign of a geoid anomaly in addition recent studies of lateral variations in the shear velocity reveal the presence of large scale anomalies in the upper mantle thus while the inferences made here with respect to the origin of the gravest orders of the geopotential field are not conclusive they point the way in which results from seismology can be used to address some of the basic questions of geodynamics

extended boussinesq thermal chemical convection with moving heat sources and variable viscosity we have studied with an aspect ratio four box the thermal chemical convective evolution with strongly temperature and depth dependent viscosity and moving heat sources within the extended boussinesq framework in which both adiabatic and viscous heating are included and a depth dependent thermal expansivity is assumed in the equation of state our focus is to show how this type of mantle evolution with an averaged ra of 0 106 may develop with a linear chemical stratification and a uniformly hot mantle as an initial condition the effects of extended boussinesq and depth dependent thermal expansivity are to prevent the effective destruction of the chemical heterogeneities our results show that this initial condition would after the age of the earth lead to a lava lamp mode consisting of a thick chemically stratified and intensely internally heated layer with a thickness of around a quarter of the whole mantle thickness however in this isolated internally convecting layer exceedingly high temperatures greater than 4500 k would be reached in the deep mantle plumes can be launched from the top of this thick denser layer this lava lamp stage would give way to the formation of denser hill like structures at the core mantle boundary then upwellings with deep lower mantle origins can be induced by the interaction of the downwellings with the d layer our simulations show the possibility for some long range mass transfer interaction between these widely separated chemical hills promoted by the fast horizontal flow induced by the sinking currents along the low viscosity zone due to temperature dependent rheology at the core mantle boundary

small scale ultralow velocity zone structure imaged by scp 1 short period teleseismic scp phases from events in the tonga fiji region recorded at the short period small aperture warramunga array in australia are used to study structure at the core mantle boundary cmb we examine the data for evidence of precursors and postcursors to scp such additional arrivals are generated by the interaction of scp with structure at the reflection and conversion point at or near the cmb we show precursors and postcursors to scp indicating an ultralow velocity zone ulvz at the cmb in the region between tonga fiji and australia one dimensional forward modeling of the short period data leads to models of the ulvz with thickness varying between 5 and 20 km velocity reductions of 0 10 and 10 30 for p velocity and s velocity respectively and 0 50 density increase many events however show simple waveforms consistent with a locally simple cmb models for proximal events can vary greatly indicating very complex three dimensional structure with scale lengths of a few tens of kilometers these results point to complex chemical and physical processes in the lowermost mantle

the interaction between the post perovskite phase change and a thermo chemical boundary layer near the core mantle boundary numerical simulations of thermo chemical mantle convection with the newly discovered post perovskite ppv phase change are used to characterize its dynamical consequences for the dq region which has frequently been interpreted as being thermo chemical in origin a strong lateral variation in the appearance of the ppv region is observed with high cmb temperatures leading to bdouble crossingq of the phase boundary in average or cold regions or no crossing in hot regions lower cmb temperatures result in a single crossing everywhere but with highly variable depth the strongly exothermic nature of the post perovskite phase change induces an anti correlation between regions containing a thick ppv layer and piles of chemically dense material formed by the segregation of the crustal component from subducted slabs the ppv phase change destabilizes chemically dense material allowing subducted slabs to easily penetrate into a dense layer forming isolated patches of the post perovskite phase whereas the lateral spectrum of thermal anomalies in the deep mantle is dominated by long wavelengths the lateral spectrum of composition is relatively flat and is the dominant contributor to seismic velocity variations at all wavelengths except the very longest ones d 2005 elsevier b v all rights reserved

numerical models of a dense layer at the base of the mantle and implications for the geodynamics of d to investigate the dynamics of the mantle s d layer we explore numerical models of mantle convection which feature a dense basal boundary layer we use a double diffusive finite element convection scheme and vary thermal and chemical rayleigh numbers and properties including viscosity thermal conductivity and internal heating for isoviscous models with heating only from below the thermal rayleigh number ra is set at either 106 or 107 the negative chemical buoyancy of a dense layer produces a stable boundary layer when the ratio of chemical to thermal buoyancy the buoyancy number b is around 1 for b 0 5 the layer is unstable while b 0 6 may produce a layer which remains stable for long periods depending on other factors such as rayleigh number layer thickness and thermal diffusivity of the layer at high enough ra small scale convection can occur within the layer using ra 107 we look at changes in layer thickness from 100 to 300 km at two values of b 0 6 and 1 convection within the layer takes place most easily when the layer has a greater initial thickness for b 0 6 the layer is not always continuous along the lower boundary so convection within the layer does not occur unless the initial thickness is fairly high 300 km increasing the thermal diffusivity of the layer to simulate enrichment in metals enhances heat conduction across the layer and can suppress internal convection within d enhanced conduction also leads to higher plume temperatures including internal heating mainly increases the overall heat flow through the mantle leading to higher surface heat flux finally we examine models with temperature dependent viscosity and pressure dependent viscosity using rayleigh numbers in the range of 107 convection within the dense layer is enhanced by the relative reduction in viscosity our models exhibit only a modest decrease in viscosity an order of magnitude but illustrate how a dense low viscosity layer interacts with cold viscous downwellings

flat to steep transition in subduction style we use a thermal history of the mantle to calculate the evolution of oceanic crustal thickness over earth history and use residual depth anomalies from the present day pacific to find the crustal thickness range 9 2 11 6 km where the subduction style changes from mainly flat to mainly steep we find that steep subduction was well developed by 2 5 ga which coincides with a major change in sedimentary rare earth element patterns over 50 of all oceanic crust subducted steeply by 2 0 1 6 ga the same interval over which the average thickness of continental plates declined rapidly because producing thick plates requires many episodes of flat subduction our model can explain why there are no known thick plates

influence of thermochemical piles on topography at earth s core mantle boundary abstract numerous seismic studies reveal the presence of two large low velocity anomalies beneath africa and the central pacific efforts to characterize these anomalies have yielded a variety of interpretations over the years both isochemical and thermochemical previous interpretations have included large isochemical superplumes clusters of smaller thermal plumes and doming thermochemical superplumes a conceptual mantle model that is presently growing favor involves long lived thermochemical piles in anticipation that nutation studies will provide better topographic constraints on the core mantle boundary cmb in the future we examine the effects of thermochemical piles at this boundary in this study we perform numerical modeling of thermochemical and isochemical convection as a function of convective vigor and temperature dependent viscosity to predict the topography at earth s cmb our results show that in isochemical convection downwellings always lead to negative depressed cmb topography while upwellings cause positive elevated topography consistent with previous studies however in thermochemical convection cmb topography and its relationship to downwellings and thermochemical piles are significantly affected by temperature dependent viscosity for isoviscous or weakly temperature dependent viscosity the piles cause negative cmb topography while positive topography is often below cold downwellings however for realistic more strongly temperature dependent viscosity the most negative cmb topography is below downwellings while the topography below the piles is often slightly more positive and or flat similar to upwellings in isochemical models we also show that although thermochemical piles are intrinsically more dense the large thermal buoyancy associated with them leads to overall relative buoyancy that is on par with slabs consequently thermochemical models lead to an overall reduction in magnitude of cmb topography with respect to isochemical models

global mantle shear velocity model developed using nonlinear asymptotic coupling theory we present a three dimensional shear velocity model of the whole mantle developed using s h waveform data the model is expressed horizontally in terms of spherical harmonics up to degree 12 and vertically in terms of legendre polynomials up to degrees 5 and 7 in the upper and lower mantle respectively what distinguishes this model from other tomographic models published to date is 1 the theoretical normal mode based wave propagation approach where we include across branch mode coupling terms in order to model the body wave sensitivity to structure along the path more accurately 2 the wave packet weighting scheme which allows to balance contributions from high amplitude and low amplitude phases increasing the resolution in some parts of the mantle we also relax the constraints on the moho depth which is allowed to vary in the inversion thus absorbing some uncertainties in crustal structure the resulting model is generally in good agreement with other recent global mantle s velocity models and with some regional models the rms profile with depth has more power than other models in the upper mantle lower mantle transition region and the zone of increased power and low degree structure near the base of the mantle is confined to the last 500 km in depth this model provides a particularly good fit to the non hydrostatic geoid through harmonic degree 12 79 variance reduction as well as good fits to observed splitting functions of s velocity sensitive mantle modes indicating that both large scale and small scale features are really well constrained