upper mantle anisotropy beneath the geoscope stations seismic anisotropy has been widely studied this last decade particularly by measuring splitting of vertically propagating core shear waves the main interest in this technique is to characterize upper mantle flow beneath seismic stations on the other hand the major restriction in this method is that a single station gives a single anisotropy measurement alternative methods have been developed in order to avoid this restriction an accurate determination of upper mantle seismic anisotropy beneath a seismic station may allow one by doing anisotropy correction to characterize remote or deeper anisotropy the geoscope network is ideal for this purpose because it is composed of a large set about 26 of high quality broadband seismometers globally distributed and because some of these stations have run for more than 10 years and most of them for more than 5 years we selected about 100 events at each site generally of magnitude mb 6 0 and we performed systematic measurements of the splitting parameters fast polarization direction and delay time t on sks skks and pks phases splitting on oceanic islands has been difficult to observe owing to the low quality of the signal but also perhaps owing to complex upper mantle structures beneath the stations station kip kipapa hawaii in the pacific is the only oceanic geoscope station with a clear anisotropy we determined well constrained splitting parameters for 10 of the 17 continental stations that may be explained by a single anisotropic layer the poor correlation between fast polarization directions and the absolute plate motion together with the apparent incoherence between the plate velocities and the observed delay times suggest that a simple drag induced asthenospheric flow alone fails to explain most of the observations for some stations located on or near major lithospheric structures tam tamanrasset algeria for instance we observe a good correlation between fast polarization directions and regional structures at station scz santa cruz california we found clear variations of the splitting parameters as a function of the event backazimuth compatible with two layers of anisotropy three stations can canberra hyb hyderabad india and ssb saint sauveur badole france seem to be devoid of detectable anisotropy

a kinematic model for recrystallization and texture development in olivine polycrystals the interpretation of seismic anisotropy in the mantle requires a knowledge of the relationship between the lattice preferred orientation lpo of crystals and the convective flow field in order better to understand this link we present a model for the evolution of lpo in olivine aggregates that deform by both intracrystalline slip and dynamic recrystallization dynamic recrystallization depends on the dislocation density of the grains which is a function of the applied local stress grains with a large density of dislocations lower their bulk strain energy by nucleating strain free sub grains at a rate proportional to a dimensionless nucleation parameter grains with high energy are then invaded by grains with low energy by grain boundary migration at a rate proportional to a dimensionless grain boundary mobility m the value of is constrained by observed lpo patterns in experimentally deformed olivine aggregates and m is constrained by the temporal evolution of the strength of the lpo for m 125 75 and 3 the model predictions agree well with the experimental results numerical calculations of lpo using our model are significantly faster than those based on viscoplastic self consistent or equilibrium based theories making the model especially suitable for applications for complex convective flows

identifying global seismic anisotropy patterns by correlating shear wave splitting and surface wave data we compare a global compilation of shear wave splitting measurements with azimuthal seismic anisotropy parameters inferred from surface wave tomography the currently available splitting dataset is taken from a novel comprehensive collection of available publications that is updated interactively online the comparison between the two types of data is made by calculating predicted splitting parameters from the anisotropic tomography model comparing these predicted splitting parameters with the observed ones we find a considerable correlation between the two datasets at global scale this result is noteworthy since such correlation did not seem to exist in previous studies the spatial resolution associated with the two types of methods is rather different while surface waves have good vertical resolution and poor lateral resolution of several hundreds of kilometers sks splitting measurements have good lateral but poor vertical resolution the correlation can be understood in light of recent propositions that anisotropy seen by sks splitting constrains mostly the upper mantle and therefore a similar depth region as surface waves the correlation also confirms the generally good quality of the shear wave measurements as well as that of the anisotropic tomography model

two dimensional modelling of subduction zone anisotropy with application to southwestern japan summary we present a series of 2 d numerical models of viscous flow in the mantle wedge induced by a subducting lithospheric plate we use a kinematically defined slab geometry approximating the subduction of the philippine sea plate beneath eurasia through finite element modelling we explore the effects of different rheological and thermal constraints e g a low viscosity region in the wedge corner power law versus newtonian rheology the inclusion of thermal buoyancy forces and a temperature dependent viscosity law on the velocity and finite strain field in the mantle wedge from the numerical flow models we construct models of anisotropy in the wedge by calculating the evolution of the finite strain ellipse and combining its geometry with appropriate elastic constants for effective transversely isotropic mantle material we then predict shear wave splitting for stations located above the model domain using expressions derived from anisotropic perturbation theory and compare the predictions to 500 previously published shear wave splitting measurements from seventeen stations of the broad band f net array located in southwestern japan although the use of different model parameters can have a substantial effect on the character of the finite strain field the effect on the average predicted splitting parameters is small however the variations with backazimuth and ray parameter of individual splitting intensity measurements at a given station for different models are often different and rigorous analysis of details in the splitting patterns allows us to discriminate among different rheological models for flow in the mantle wedge the splitting observed in southwestern japan agrees well with the predictions of trench perpendicular flow in the mantle wedge along with b type olivine fabric dominating in a region from the wedge corner to about 125 km from the trench

shear wave splitting and subcontinental mantle deformation splitting observations are interpreted in terms of the strain induced lattice preferred orientation of mantle minerals especially olivine we consider three hypotheses concerning the origin of the continental anlsotropy 1 strain associated with absolute plate motion as in the oceanic upper mantle 2 crustal stress and 3 the past and present internal deformation of the subcontinental upper mantle by tectonic episodes it is found that the last hypothesis is the most successful namely that the most recent significant episode of internal deformation appears to be the best predictor of q for stable continental regions this is interpreted as fossil anisotropy whereas for presently active regions such as alaska the anisotropy reflects present day tectonic activity in the stable portion of north america there is a good correlation between delay time and lithospheric thickness this is consistent with the anisotropy being localized in the subcontinental lithosphere and suggests that intrinsic anisotropy is approximately constant the acceptance of this hypothesis has several implications for subcontinental mantle deformation first it argues for coherent deformation of the continental lithosphere crust and mantle during orogenies this implies that the anisotropic portion of the lithosphere was present since the deformational episode and rules out the addition of undeformed material to this layer by subsequent underplating or conductive growth of the thermal boundary layer one of the most important issues in the study of orogenies is the need to reconcile the formation of thickened lithosphere with the paradoxically high mantle temperatures often associated with orogenic episodes most efforts to date have focussed on modes of deformation whereby the cold lithospheric mantle is removed by convective instability or delamination and replaced by warm asthenosphere these models however are incompatible with the evidence for preserved coherent lithospheric deformation rather the deformed mantle appears to have been heated in place we suggest that the elevated mantle temperatures may be due to the strain heating accompanying the deformation

evidence for ancient lithospheric deformation in the east european craton based on mantle seismic anisotropy and crustal magnetics abstract we present new shear wave splitting measurements performed at 16 stations on the east european craton and discuss their implications in terms of upper mantle anisotropy for this geophysically poorly known region previous investigations of mantle anisotropy in central europe have shown fast directions aligning smoothly with the craton s margin and various suggestions have been proposed to explain their origin such as asthenospheric flow or lithospheric frozen in deformation here we aim at investigating the continuation of this shear wave splitting pattern further to the east into the east european craton for the craton the interpretation appears to be less ambiguous than for central europe since several arguments support lithospheric anisotropy in this region 1 the large scale coherence within either of the four constituting blocks and the significant variations between the blocks on a small scale 2 the weak correlation with absolute plate motion vectors and 3 the good correlation between anisotropy and crustal features for which we use magnetic field alignments as a proxy rather good correlation of these magnetic features with seismic fast orientations strongly supports the idea of vertically coherent deformation throughout upper mantle and crust the observed splitting orientations thus reflect the last tectonic events of each block frozen in into the lithosphere for hundreds of millions of years

mantle flow in subduction systems the subslab flow field and implications for mantle dynamics 1 the character of the mantle flow field in subduction zones remains poorly understood despite its importance for our understanding of subduction dynamics in particular little attention has been paid to mantle flow beneath subducting slabs in order to identify processes that make first order contributions to the global pattern of subslab mantle flow we have compiled shear wave splitting measurements from subduction zones worldwide from previously published studies and estimated average splitting parameters for the subslab region globally the subslab region is overwhelmingly dominated by trench parallel fast splitting directions we tested for relationships between splitting delay time a measure of the strength of anisotropy and parameters that are indicators of tectonic processes such as trench migration velocity convergence velocity and obliquity age of subducting lithosphere and slab dip curvature seismicity and thickness we used several different plate motion models to describe plate and trench motions and evaluated the differences among the models we find that only one parameter namely the trench migration velocity in a pacific hot spot reference frame appears to correlate well with the strength of the subslab splitting signal this finding supports a model in which the mantle beneath subducting slabs is dominated by three dimensional flow induced by trench migration we explore several implications of our model for various aspects of mantle dynamics including the choice of a suitable reference frame s for mantle flow the existence of a thin decoupling zone between slabs and the subslab mantle and consequences for mass transfer between the upper and lower mantle

interpreting spatial variations in anisotropy insights into the main ethiopian rift from sks waveform modelling seismic anisotropy is a common feature in the upper mantle and measuring shear wave splitting in core phases is a common approach in estimating its characteristics large lateral variations in estimated splitting parameters are observed over small spatial distances in many differing tectonic regions including areas of continental break up such as the main ethiopian rift mer we investigate the ability of shear wave splitting analysis to constrain spatial variations in anisotropy using a one way wave equation modelling scheme to generate bandlimited waveforms for a suite of models representing regions with rapidly changing anisotropy we show that shear wave splitting can identify lateral variation in anisotropy on the order of 20 50 km where a change in fast direction demarcates the transition in anisotropy in addition variation in the amount of splitting is complicated close to the transition and is sensitive to the vertical thickness of anisotropy we have used these modelling results to interpret shear wave splitting measurements for the mer the model that best fits the observations has a 100 km wide rift zone with a fast direction of 30 outside and 20 inside the rift the model has 9 per cent anisotropy close to the western margin with 7 per cent anisotropy elsewhere in all regions of the model we constrain the anisotropy to begin at a depth of 90 km the depth of anisotropy is consistent with geochemical estimates of the depth of melt initiation beneath the region also the elevated splitting beneath the western margin supports evidence of low velocities and highly conductive zones from seismic tomography and magneto tellurics suggesting melt is more focused along the western margin this study shows how observations of sks wave splitting from dense seismic networks can be used to map sharp lateral changes and constrain the depth of the anisotropy

statistical properties of seismic anisotropy predicted by upper mantle geodynamic models 1 we study how numerically predicted seismic anisotropy in the upper mantle is affected by several common assumptions about the rheology of the convecting mantle and deformation induced lattice preferred orientations lpo of minerals we also use these global circulation and texturing models to investigate what bias may be introduced by assumptions about the symmetry of the elastic tensor for anisotropic mineral assemblages maps of elasticity tensor statistics are computed to evaluate symmetry simplifications commonly employed in seismological and geodynamic models we show that most of the anisotropy predicted by our convection lpo models is captured by estimates based on a best fitting hexagonal symmetry tensor derived from the full elastic tensors for the computed olivine enstatite lpos however the commonly employed elliptical approximation does not hold in general the orientations of the best fitting hexagonal symmetry axes are generally very close to those predicted for finite strain axes correlations between hexagonal anisotropy parameters for p and s waves show simple bilinear relationships such relationships can reduce the number of free parameters for seismic inversions if this information is included a priori the match between our model predictions and observed patterns of anisotropy supports earlier more idealized studies that assumed laboratory derived mineral physics theories and seismic measurements of anisotropy could be applied to study mantle dynamics the match is evident both in agreement between predicted lpo at selected model sites and that measured in natural samples and in the global pattern of fast seismic wave propagation directions

upper mantle anisotropy beneath the african iris and geoscope stations summary upper mantle anisotropy beneath the african iris and geoscope stations is investigated through the measurements of splitting of teleseismic shear waves such as sks skks and pks phases seismic anisotropy data are interesting on their own as a measure of upper mantle active or frozen deformation beneath a given station but each station is of potential interest since it can be used to retrieve source side seismic anisotropy at remote sites if one is able to perform station side anisotropy correction we performed systematic investigations of teleseismic shear wave splitting at 15 stations from the iris and geoscope global seismic networks which are located on both the oceanic and the continental parts of the african plate anisotropy is generally well observed at continental stations the patterns we present generally show much more complexity than the results previously published from smaller data sets despite this complexity the splitting parameters generally appear in several places to contain a signature of the regional geodynamic setting rift structures archaean craton panafrican belt although a deeper source of anisotropy asthenospheric may be present at the oceanic stations anisotropy measurements are much more difficult to perform because the signal is generally of poor quality msey in the seychelles indian ocean is the exception and displays a clear correlation of the azimuth of the fast split shear wave with the trend of the absolute plate motion as defined by hotspot tracks

geochemical and mineralogical control of different genetic types of deep sea nodules from the pacific ocean deep sea nodules from the northeast pacific nodule belt and the southeast pacific sonne basin being formed in areas bordering the equatorial zone of high biological productivity accumulate by two basically different growth processes a early diagenetic growth by supply from pore water and b hydrogenetic growth by supply from near bottom sea water these growth processes lead to different genetic types of nodules early diagenetic type a hydrogenetic type b and mixed type ab a further type ac very rich in mn is being formed by increasing influence of early diagenesis these types can clearly be distinguished by their shapes surface textures mineral constituents of oxide fraction internal microstructures and geochemistry a genetical classification is being proposed on the basis of statistically computed interelement relationships todorokite very poor in fe is the main mn phase in the early diagenetic substance mno2 intimately intergrown with feooh xh2o is the main phase in the hydrogenetic substance consequently an important difference can be pointed out the metal supply for the growth of the early diagenetic nodules is based on an ionic solution of me2 e g mn2 ni2 cu2 zn2 whereas the supply for the hydrogenetic nodules is caused by transport of colloidal particles mobilization of mn2 and fractionation from fe is controlled by the amount of decomposing organic matter in the peneliquid layer of the sediments the main factor controlling the intensity of early diagenesis is the biological productivity in surface waters the crucial point of reversal at a mn fe ratio of about 5 obtained by hyperbolical regression of the analyses of nodules from the southeast pacific represents best concentrations in ni and cu mn fe quotients greater than 5 cause a decrease of ni and cu content nodules from the northeast pacific nodule belt generally contain higher concentrations in cu than nodules from the southeast pacific this can be explained by an additional supply of cu transported below ccd by siliceous plankton

a teleseismic shear wave splitting study to investigate mantle flow around south america and implications for plate driving forces summary closure of the pacific ocean basin by the convergence of its surrounding plates some of which have deep continental roots implies that there is net mass flux out of the mantle under the pacific here we report on a shear wave splitting study designed to test the prediction that there should be flow around its southern margin our results show no evidence for present day flow around the tip of southern south america instead the results suggest present day flow directions in the southern atlantic that parallel the south american absolute plate motion direction even under antarctica the results also provide evidence for absolute plate motion driven by the basal drag of ocean basin scale mantle flow and suggest that 200 km thick flow boundary layers exist under south america and antarctica and also demonstrate that mantle flow directions cannot be reliably inferred from present day plate morphology

a geodynamic model of mantle density heterogeneity using cenozoic and mesozoic plate motion reconstructions we derive a model of present day mantle density heterogeneity under the assumption that subducted slabs sink vertically into the mantle the thermal buoyancy of these slabs is estimated from the observed thermal subsidence cooling of oceanic lithosphere slab velocities in the upper mantle are computed from the local convergence rate we assume that slabs cross the upper lower mantle interface and continue sinking into the lower mantle with a reduced velocity for a velocity reduction factor between 2 and 5 our slab heterogeneity model is as correlated with current tomographic models as these models are correlated with each other we have also computed a synthetic geoid from our density model for a viscosity increase of about a factor of 40 from the upper to lower mantle our model predicts the first 8 spherical harmonic degrees of the geoid with statistical confidence larger than 95 and explains 84 of the observed geoid assuming that the model c21 and s21 terms are absent due to a long relaxation time for earth s rotational bulge otherwise 73 of the geoid variance is explained the viscosity increase is consistent with our velocity reduction factor for slabs entering the lower mantle since downwelling velocities are expected to scale roughly as the logarithm of viscosity loge 40 3 7 these results show that the history of plate tectonics can explain the main features of the present day structure of the mantle the dynamic topography induced by this heterogeneity structure consists mainly of about 1 km amplitude lows concentrated along the active continental margins of the pacific basin our model can also be used to predict the time variation of mantle heterogeneity and the gravity field we find that the age of the geoid defined as the time in the past before which the geoid becomes uncorrelated with the present geoid is about 50 m y our model for the history of the degree 2 geoid which is equivalent to the history of the inertia tensor should give us a tool to study the variations in earth s rotation pole indicated in paleomagiietic studies

upper mantle structure beneath the azores hotspot from finite frequency seismic tomography the azores archipelago is a classic expression of a hotspot that has interacted with a mid ocean ridge avariety of geochemical studies and geophysical observations point to the influence of some sort of mantle plume on the formation of the azores plateau and the adjacent mid atlantic ridge mar but the mantle structure beneath the azores has not heretofore been imaged tomographically at a sufficiently fine resolution to resolve a narrow plume we present a p wave velocity model of the mantle beneath the hotspot derived from teleseismic body waves recorded by six broadband seismic stations on the azores islands including the global seismographic network station cmla three dimensional travel time sensitivity kernels are used to account for the effects of wavefront healing and scattering of seismic waves an inversion of 228 p wave travel time delays reveal a lowvelocity anomaly in the shallow mantle less than 200 km deep along the azores archipelago and beneath the azores plateau centered approximately at 38 5 n 28 5 w the low velocity anomaly extends northeastward and downward to connect to a plume like column of low velocity material from 250 km to at least 400 km depth centered northeast of terceira these results are consistent with other geophysical and geochemical observations and provide seismic evidence for a model of plume ridge interaction in which the plume conduit is deflected to the southwest in the shallow mantle by asthenospheric flow and plate motion and contributes hot mantle material and excess melt to the mar south of the azores

foraminiferal ooze from the galapagos rift area hydrothermal impact and diagenetic mobilization of elements abstract chemical changes during diagenesis in foraminiferal oozes have been studied in five cores from the hydrothermally active spreading centre of the galapagos rift shifts of concentrations of elements such as mn co cu ni zn and ba are related to changes of eh potential in the range from 0 33 to 0 07 v mn co and cu are strongly mobilized in the reduction zone and reprecipitated as hydroxides near the sediment water interface the ree distribution patterns of these oxide rich sediments are very similar to those in the base metal poor diagenetically formed mn nodules during diagenesis several elements ree co sb ba and zn were redistributed between carbonate and non carbonate phases whereas others were not fe sc th and cr although the composition of these sediments may be attributed to diverse sources convincing evidence supporting a hydrothermal impact comes from the sb concentrations

constraints on upper mantle viscosity from the flow induced pressure gradient across the australian continental keel 1 the thickness of continental lithosphere varies considerably from tectonically active to cratonic regions where it can be as thick as 250 300 km embedded in the upper mantle like a ship when driven to move by a velocity imposed at the surface a continental keel is expected to induce a pressure gradient in the mantle we hypothesize that the viscosity of the asthenosphere or the shear coupling between lower lithosphere and asthenosphere should control this pressure effect and thus the resulting dynamic topography we perform three dimensional finite element calculations to examine the effects of forcing a continental keel by an imposed surface velocity with the australian region as a case study when the upper mantle is strong but still weaker than the lower mantle positive dynamic topography is created around the leading edge and negative dynamic topography is created around the trailing edge of the keel which is measurable by positive and negative geoid anomalies respectively for a weak upper mantle the effect is much reduced we analyze geoidal and gravity anomalies in the australian region by spatiospectral localization using slepian functions the method allows us to remove a best fit estimate of the geographically localized low spherical harmonic degree contributions regional geoid anomalies thus filtered are on the order of 10 m across the australian continent with a spatial pattern similar to that predicted by the models the comparison of modeled and observed geoid anomalies places constraints on mantle viscosity structure models with a two layer mantle cannot sufficiently constrain the ratio of viscosity between the upper and lower mantle the addition of a third weak upper mantle layer an asthenosphere amplifies the effects of keels our three layer models with lower mantle viscosity of 3 1022 pa s suggest that the upper mantle asthenosphere is 300 times weaker than the lower mantle while the transition zone 400 670 km depths has a viscosity varying between 1021 and 1022 pa s

haskell 1935 revisited the haskell 1935 value of 1021 pa s for mantle viscosity is a classic and enduring constraint on the rheology of the earth s interior we revisit this inference using spherically symmetric self gravitating maxwell viscoelastic earth models our inference is based on both forward and inverse analyses of decay times associated with uplift at two sites considered by haskell angerman river sweden and oslo norway rather than the raw relative sea level rsl data at these sites we demonstrate that predictions of the decay time associated with the angerman river data are insensitive to variations in both the late pleistocene ice load history and the lithospheric thickness of the earth model the predictions at oslo are sensitive to both these inputs and hence decay times at this site provide a remarkably robust constraint on mantle viscosity we derive a constraint on the average viscosity of the mantle of 0 65 1 10 1021 pa s where the average resolved by the data encompasses a region which extends from the base of the lithosphere to a depth of near 1400 km this indicates that many previous analyses which have invoked the haskell value of 1021 pa s as a constraint on the average upper mantle i e above 670 km depth viscosity alone have misinterpreted the resolving power of the inference furthermore our analysis indicates that a number of apparently contradictory inferences of viscosity based on fennoscandian data satisfy the new rigorous interpretation of the haskell constraint finally we demonstrate how the ambiguity in the upper mantle lower mantle viscosity contrast associated with the haskell average may be reduced by invoking decay time constraints estimated from rsl curves in hudson bay a preliminary inversion of decay times at angerman river and richmond gulf in hudson bay suggests a contrast of approximately an order of magnitude between the average viscosities of the upper mantle and the top 1000 km of the lower mantle however a conclusive analysis in this regard must await the determination of consistent decay time estimates for the hudson bay region

comparison of azimuthal seismic anisotropy from surface waves and finite strain from global mantle circulation models summary we present global models of strain accumulation in mantle flow to compare the predicted finite strain ellipsoid fse orientations with observed seismic anisotropy the geographic focus is on oceanic and young continental regions where we expect our models to agree best with azimuthal anisotropy from surface waves finite strain derived models and alignment with the largest fse axes lead to better model fits than the hypothesis of alignment of fast propagation orientation with absolute plate motions our modelling approach is simplified in that we are using a linear viscosity for flow and assume a simple relationship between strain and anisotropy however results are encouraging and suggest that similar models can be used to assess the validity of assumptions inherent in the modelling of mantle convection and lithospheric deformation our results substantiate the hypothesis that seismic anisotropy can be used as an indicator for mantle flow circulation derived models can contribute to the establishment of a quantitative functional relationship between the two

detection of upper mantle flow associated with the african superplume a continental scale low seismic velocity anomaly in the mid to lower mantle beneath africa is a robust feature of global tomographic models assuming the low velocities are associated with warm less dense material the african seismic anomaly has been ascribed to a long lived thermal upwelling from the lower mantle such a large scale upwelling should also affect the regional horizontal flow field in the upper mantle to test this model we compare seismic anisotropy inferred from shear wave splitting measurements with instantaneous flow calculations that incorporate mantle density structure inferred from seismic tomography we calculate splitting parameters at 13 ocean island stations surrounding africa splitting measurements from island stations are ideal for interpreting anisotropy induced by asthenospheric flow because they lack a thick overlying lithosphere that may also contribute to the observed anisotropy we tested for a possible lithospheric contribution by comparing the splitting measurements with the fossil spreading directions we find that although the fossil lithospheric fabric closely matches the observed fast polarization directions at stations 500 km from a ridge axis they are a poor fit to the data at stations located farther off axis thus we conclude that far from a ridge axis the observed anisotropy is dominated by asthenospheric flow to test for an active component of mantle upwelling we considered several models with varying assumptions about the velocity at the base of the asthenosphere that it is 1 stationary below plates moving in the no net rotation nnr or hotspot reference frames 2 driven by plate motions at the earth s surface or 3 driven by a combination of plate motion and mantle density heterogeneity inferred from either seismic tomography or the history of subduction we find that the best fitting flow field is generated by plate motions and density heterogeneity associated with large scale upwelling originating in the lower mantle beneath southern africa and is manifest as a radial pattern of flow at the base of the asthenosphere this model provides a significantly better fit to the observed anisotropy than a model in which mantle flow is driven through a passive response to subduction the resulting sub asthenospheric flow field is estimated to have velocities of 0 3 cm year and an asthenospheric viscosity of 3 1019 pa s is found to be most consistent with the regional anisotropy geoid height and dynamic topography

the many surface expressions of mantle dynamics plate tectonic theory suggests that present day topography can be explained by the repeated interactions between the tectonic plates moving along earth s surface however mounting evidence indicates that a significant proportion of earth s topography results from the viscous stresses created by flow within the underlying mantle rather than by the moving plates this dynamic topography is transient varying as mantle flow changes and is characterized by small amplitudes and long wavelengths it is therefore often hidden by or confused with the more obvious topographic anomalies resulting from horizontal tectonic movements however dynamic topography can influence surface processes and thus enter the geological record it has for example played a role in the establishment of amazon drainage patterns in turn surface processes such as the erosion of topographical anomalies could affect mantle flow this emerging view of dynamic topography suggests that the concept of plate tectonics as the driver of surface deformation needs to be extended to include the vertical coupling between the mantle and the surface unravelling this coupling back in time with the help of models and the geological record can potentially provide unprecedented insights into past mantle dynamics

the unique dynamics of the pacific hemisphere mantle and its signature on seismic anisotropy abstract the portion of earth s mantle lying below the hemispherical region occupied by the pacific ocean basin is characterized by a unique style of convection the unique dynamics below the pacific hemisphere are revealed by analyzing the mantle flow predicted on the basis of high resolution seismic images of mantle structure in the pacific hemisphere we find a dome like upwelling originating in the lower mantle which actively transports material into the upper mantle a unique aspect of this flow found only in the pacific hemisphere is a demi tour or u turn of the circulation which occurs in the shallow mantle west of the east pacific ridge this demi tour connects the westward movement of the pacific plate to the dome like upwelling below the central pacific ocean and it also provides an explanation for the asymmetric distribution of seismic anisotropy on either side of the ridge axis in a global analysis of the correlation between mantle flow and seismic anisotropy we find the strongest agreement occurs in the pacific hemisphere mantle

viscosity structure beneath northeast iceland the dynamics of crustal rifting in iceland depend strongly on the lower crustal rheology which controls the intensity of upper crustal stress concentration and scale time of heat diffusion from the underlying mantle plume while magnetotelluric surveys suggest the presence of a pervasive hot and highly ductile lowermost crust with possibly high fraction of partial melt observations of low seismic attenuation and strong shear wave transmission suggest a much cooler lower crust and upper mantle since viscosity is also sensitive to the degree of partial melt present viscosity estimates for these regions could shed light on the factors responsible for these observations in this study we utilize horizontal and vertical displacement vectors determined in gps campaigns in northeast iceland since 1986 these are modeled in terms of steady state tectonic loading plus postseismic postdiking relaxation following the 1975 1984 krafla rifting episode as first proposed by foulger and others with the elastic part of the model fixed by external constraints these data have a high sensitivity to the viscosity structure beneath iceland lower crust and upper mantle viscosities of about 3 1019 pa s and 3 1018 pa s respectively yield the closest agreement with the data our lower crustal viscosity estimate is consistent with the low attenuation and low subsolidus temperature for the lower crust inferred in recent studies inversions for fissure opening during the krafla rifting episode yield about 7 m of opening centered on the krafla rift as is observed allowing for contemporaneous deep rifting on vertical faults along the askja segment partially accounts for the observed increase in separation across the rift during 1987 1992 but does not account for large displacements in the southeastern part of the network or the large relative subsidence around the askja rift during 1987 1990 recent deep normal faulting beneath the askja rift and further south might explain all of these remaining features

a new velocity field for africa from combined gps and doris space geodetic solutions contribution to the definition of the african reference frame afref 1 we analyzed 16 years of gps and 17 years of doppler orbitography and radiopositioning integrated by satellite doris data at continuously operating geodetic sites in africa and surroundings to describe the present day kinematics of the nubian and somalian plates and constrain relative motions across the east african rift the resulting velocity field describes horizontal and vertical motion at 133 gps sites and 9 doris sites horizontal velocities at sites located on stable nubia fit a single plate model with a weighted root mean square residual of 0 6 mm yr maximum residual 1 mm yr an upper bound for plate wide motions and for regional scale deformation in the seismically active southern africa and cameroon volcanic line we confirm significant southward motion 1 5 mm yr in morocco with respect to nubia consistent with earlier findings we propose an updated angular velocity for the divergence between nubia and somalia which provides the kinematic boundary conditions to rifting in east africa we update a plate motion model for the east african rift and revise the counterclockwise rotation of the victoria plate and clockwise rotation of the rovuma plate with respect to nubia vertical velocities range from 2 to 2 mm yr close to their uncertainties with no clear geographic pattern this study provides the first continent wide position velocity solution for africa expressed in international terrestrial reference frame itrf2008 a contribution to the upcoming african reference frame afref except for a few regions the african continent remains largely under sampled by continuous space geodetic data efforts are needed to augment the geodetic infrastructure and openly share existing data sets so that the objectives of afref can be fully reached

effect of lateral viscosity variations in the top 300 km on the geoid and dynamic topography summary we have investigated the impact of lateral viscosity variations lvv in the top 300 km of the mantle on the long wavelength gravitational response of the earth in contrast to the previous studies we demonstrate that the lvv may play a crucial role if a model with imposed plate velocities and partial layering is considered assuming lvv in the top mantle associated with the continental roots and the oceanic asthenosphere we are able to explain a significant portion of the geoid 85 per cent and of the free air gravity data 55 per cent without invoking a complex radial viscosity profile or assuming a lithosphere with an unrealistically low uniform viscosity the best fit to the data is found for models in which mass anomalies located at the boundary between upper and lower mantles reduce the mass flux across the 660 km discontinuity by a factor of 3 these models are characterized by a low average value of viscosity in the asthenosphere below the oceans 10 19 pa s and a large viscosity contrast 100 between the young suboceanic mantle and the deep continental roots the viscosity in the deep upper mantle is found to be around 3 10 20 pa s while a value higher by at least two orders of magnitude is typical for the lower mantle the predicted dynamic surface topography is influenced by the surface plate velocities and its amplitude amounts to a few hundred metres besides investigating the viscosity structures that reflect the different thicknesses of the lithosphere in continental and oceanic regions we have searched for a large scale pattern of lateral viscosity variation in the top 300 km that yields the best fit to the geoid the pattern obtained by this inversion shows a good correlation with the ocean continent distribution and 90 per cent of the geoid and 67 per cent of the free air gravity can be explained

directions of seismic anisotropy in laboratory models of mantle plumes 1 a recent expansion in global seismic anisotropy data provides important new insights about the style of mantle convection interpretations of these geophysical measurements rely on complex relationships between mineral physics seismology and mantle dynamics we report on 3 d laboratory experiments using finite strain markers evolving in time dependent viscous flow fields to quantify the range in expected anisotropy patterns within buoyant plumes surfacing in a variety of tectonic settings a surprising result is that laboratory proxies for the olivine fast axis overwhelmingly align tangential to radial outflow in plumes well before reaching the surface these remarkably robust and ancient anisotropy patterns evolve differently in stagnant translational and divergent plate tectonic settings and are essentially orthogonal to patterns typically referenced when prospecting for plume signals in seismic data results suggest a fundamental change in the mineral physics seismology circulation relationship used in accepting or rejecting a plume model

formation of clay minerals during early diagenesis of a calcareous ooze abstract deep sea sediments from a basin in the southeast pacific are described in which strong carbonate dissolution and strong diagenetic migration of metals can be observed clay minerals of smectite type are formed in large quantities during the early diagenesis of this sediment in a first stage as an x ray amorphous phase which later crystallizes to smectite rb sc na and k become enriched during the process of clay mineral formation probably due to uptake of these elements from sea water to interlayer positions of smectite the unsolved question is what sort of mechanism connects the calcite dissolution with the formation of smectite the source of the al necessary for the formation of smectite is also unknown

mantle flow under the western united states from shear wave splitting we show that sks splitting in the westernmost united states polarization of the fastest shear waves and splitting times including their back azimuthal dependence can be explained by a geodynamic model that includes a continuum mechanics description of plate motions and underlying asthenospheric circulation models that include a counterflow at depths of 300 km are preferred which may indicate a far field effect of the farallon slab anomaly sinking underneath the central continental united states this finding is broadly consistent with earlier suggestions and we demonstrate that a mechanically coupled system though with a strong viscosity contrast with depth is consistent with the data we explore the depth dependence of predicted anisotropy by means of computing seismogram synthetics and comparing synthetic splits with observations some patterns in the data including null observations are matched well linked models of geodynamic flow and mineral alignment in the mantle provide a means to test the relationship between strain and the saturation of texturing lower fabric saturation strains than for global models are preferred by the data which may reflect the relatively active tectonic setting and thin asthenosphere of the study region in general our results show that seismic anisotropy when interpreted jointly with mineral physics theories may be used to quantitatively constrain the spatial character of flow and the degree of force coupling at depth

current plate velocities relative to the hotspots incorporating the nuvel 1 global plate motion model nuvel 1 is a new global model of current relative plate velocities demets et al 1990 which differ significantly from those of prior models here we incorporate nuvel 1 into hs2 nuvel1 a new global model of plate velocities relative to the hotspots hs2 nuvel1 was determined from the hotspot data and errors used by minster and jordan 1978 to determine am1 2 which is their model of plate velocities relative to the hotspots am1 2 is consistent with minster and jordan s relative plate velocity model rm2 here we compare hs2 nuvel1 with am1 2 and examine how their differences relate to differences between nuvel 1 and rm2 hs2 nuvel1 plate velocities relative to the hotspots are mainly similar to those of am1 2 minor differences between the two models include the following 1 in hs2 nuvel1 the speed of the partly continental apparently non subducting indian plate is greater than that of the purely oceanic subducting nazca plate 2 in places the direction of motion of the african antarctic arabian australian caribbean cocos eurasian north american and south american plates differs between models by more than 10 3 in places the speed of the australian caribbean cocos indian and nazca plates differs between models by more than 8 mm yr although 27 of the 30 rm2 euler vectors differ with 95 confidence from those of nuvel 1 only the am1 2 arabia hotspot and india hotspot euler vectors differ with 95 confidence from those of hs2 nuvel1 thus substituting nuvel 1 for rm2 in the inversion for plate velocities relative to the hotspots changes few euler vectors significantly presumably because the uncertainty in the velocity of a plate relative to the hotspots is much greater than the uncertainty in its velocity relative to other plates

sub slab anisotropy beneath the sumatra and circum pacific subduction zones from source side shear wave splitting observations understanding the dynamics of subduction is critical to our overall understanding of plate tectonics and the solid earth system observations of seismic anisotropy can yield constraints on deformation patterns in the mantle surrounding subducting slabs providing a tool for studying subduction dynamics while many observations of seismic anisotropy have been made in subduction systems our understanding of the mantle beneath subducting slabs remains tenuous due to the difficulty of constraining anisotropy in the sub slab region recently the source side shear wave splitting technique has been refined and applied to several subduction systems worldwide making accurate and direct measurements of sub slab anisotropy feasible and offering unprecedented spatial and depth coverage in the sub slab mantle here we present source side shear wave splitting measurements for the central america alaska aleutians sumatra ryukyu and izu bonin japan kurile subduction systems we find that measured fast splitting directions in these regions generally fall into two broad categories aligning either with the strike of the trench or with the motion of the subducting slab relative to the overriding plate trench parallel fast splitting directions dominate beneath the izu bonin japan and southern kurile slabs and part of the sumatra system while fast directions that parallel the motion of the downgoing plate dominate in the ryukyu central america northern kurile western sumatra and alaska aleutian regions we find that plate motion parallel fast splitting directions in the sub slab mantle are more common than previously thought we observe a correlation between fast direction and age of the subducting lithosphere older lithosphere 95 ma is associated with trench parallel splitting while younger lithosphere 95 ma is associated with plate motion parallel fast splitting directions finally we observe source side splitting for deep earthquakes transition zone depths beneath japan and sumatra suggesting the presence of anisotropy at midmantle depths beneath these regions

joint seismic geodynamic mineral physical modelling of african geodynamics a reconciliation of deep mantle convection with surface geophysical constraints recent progress in seismic tomography provides the first complete 3 d images of the combined thermal and chemical anomalies that characterise the unique deep mantle structure below the african continent we present a tomography based model of mantle convection that provides an excellent match to fundamental surface geodynamic constraints on internal density heterogeneity that includes both compositional and thermal contributions where the latter are constrained by mineral physics the application of this thermochemical convection model to the problem of african mantle dynamics yields a reconciliation of both surface gravity and topography anomalies to deep seated mantle flow under the african plate over a wider range of wavelengths than has been possible to date on the basis of these results we predict flow in the african asthenosphere characterised by a clear pattern of focussed upwellings below the major centres of late cenozoic volcanism including the kenya domes hoggar massif cameroon volcanicline cape verde and canary islands the flow predictions also reveal a deep seated large scale active hot upwelling below the western margin of africa under the cape verde islands that extends down to the core mantle boundary the scale and dynamical intensity of this west african superplume is comparable to the south african superplume that has long been assumed to dominate the large scale flow dynamics in the deep mantle under africa we evaluate the plausibility of the predicted asthenospheric flow patterns through a comparison with seismic azimuthal anisotropy derived from independent analyses of african shear wave splitting data

understanding cratonic flood basalts the origin of continental flood basalts has been debated for decades these eruptions often produce millions of cubic kilometers of basalt on timescales of only a million years although flood basalts are found in a variety of settings no locale is more puzzling than cratonic areas such as southern africa or the siberian craton where strong thick lithosphere is breached by these large basaltic outpourings conventionally flood basalts have been interpreted as melting events produced by one of two processes 1 elevated temperatures associated with mantle plumes and or 2 adiabatic decompression melting associated with lithospheric thinning in southern africa however there are severe problems with both of these mechanisms first the rifting circumstances of several wellknown basaltic outpourings clearly reflect lithospheric control rather than the influence of a deep seated plume specifically rift timing and magmatism are correlated with stress perturbations to the lithosphere associated with the formation of collisional rifts second the substantial lithospheric thinning required for adiabatic decompression melting is inconsistent with xenolith evidence for the continued survival of thick lithosphere beneath flood basalt domains as an alternative to these models we propose a new two stage model that interprets cratonic flood basalts not as melting events but as short duration drainage events that tap previously created sublithospheric reservoirs of molten basalt formed over a longer time scale reservoir creation existence stage i requires long term e g 1 ma supersolidus conditions in the sublithospheric mantle that could be maintained by an elevated equilibrium geotherm appropriate for the archean a slow thermal perturbation e g thermal blanketing or large scale mantle upwelling or a subduction related increase in volatile content the drainage event stage ii occurs in response to an abrupt stress change in the lithosphere which leads to the initiation and propagation of lithospheric dikes such a model accounts for the short eruption time of flood basalts the evidence for lithospheric control of the eruptions and the continued survival of cratonic lithosphere following these magmatic events the most notable consequence of this model is that it implies the existence of large reservoirs of magma comparable to the eruption volume of flood basalts that have been and are still likely present beneath stable continental lithopshere

comparison of sir b sar wave image spectra with wave model predictions implications on the sar modulation transfer function during the spaceborne imaging radar sir b mission in october 1984 the onboard synthetic aperture radar sar acquired data in the vicinity of hurricane josephine ocean wave spectra computed from the sar imagery are compared with ocean data gathering project odgp model forecasts because the apparent propagation direction of the dominant wave system rotated over 90 degrees along the pass the data set affords a unique opportunity to observe the same wave system as imaged by different sar mechanisms it is shown that for the sir b geometry the wave numbers and propagation directions extracted from the wave spectra are not strongly dependent on the exact nature of the sar imaging mechanism however the estimation of ocean significant wave height swh requires accurate knowledge of such mechanisms the spatial variation of swh as estimated from sar derived wave spectra and odgp model spectra compare most favorably when the phase of the hydrodynamic portion of the sar modulation transfer function is approximately 90 degrees past the long wave crest

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

an olivine fabric database an overview of upper mantle fabrics and seismic anisotropy we present a unique database of 110 olivine petrofabrics and their calculated seismic properties the samples come from a variety of the upper mantle geodynamic environments ophiolites subduction zones and kimberlites with a wide range of micro structures a phenomenological relationship is established between p and s wave seismic anisotropy and the degree of crystal alignment fabric strength seismic anisotropy increases rapidly at low fabric strength before reaching a near saturation level of 15 to 20 for p waves and 10 to 15 for s waves despite a large variation in the symmetry of fabric patterns the average seismic properties of the different fabric micro structural and geodynamic settings have similar anisotropies in both magnitude and symmetry hence it would seem possible to determine some measure of fabric strength from seismic anisotropy if the dimensions of the anisotropic region are known but not geodynamic environment or details of the petrofabric pattern a simple pattern of seismic anisotropy characterises the average sample of the database which has the following features the polarisation plane of the fastest s wave is parallel or sub parallel to the foliation plane the maximum shear wave splitting is parallel to the y structural direction in foliation plane and normal to the lineation the maximum of the p wave velocity is parallel to the high concentration of 100 axes which is sub parallel to the lineation the 100 orientation distribution has the greatest influence on the p wave seismic anisotropy the 100 and 001 orientation distributions have the greatest influence on the symmetry of s wave anisotropy although the magnitude of anisotropy is influenced by the distribution of all three principal axes although the database only contains olivine petrofabrics this statistical study clearly shows that seismic anisotropy can be used to deduce the orientation of the structural frame in the upper mantle 1998 elsevier science b v all rights reserved

a sharp cratonic lithosphere asthenosphere boundary beneath the american midwest and its relation to mantle flow abstract beneath the american midwest s to p sp converted wave imaging and multi mode surface wave tomography identify a north trending transition in seismic structure at 150 250 km depth to the east of this american midwest transition amt the lithosphere asthenosphere boundary lab is imaged as a 1 2 sp sv amplitude arrival at 200 240 km depth consistent with the depth of negative shear velocity and azimuthal anisotropy gradients imaged by surface wave tomography to the west of the amt sp conversions are much shallower at 150 190 km depth and are much weaker 0 7 or absent azimuthal anisotropy constrained by surface wave tomography also changes across the amt with stronger anisotropy to the east of the transition beneath the thicker lithospheric root we suggest that the seismic changes across the amt can be explained by considering the effects of asthenospheric flow beneath the leading edge of the thick lithospheric root the mantle flow is dominantly driven by the drift of the north america plate locally higher flow velocities are expected where the asthenosphere is forced to flow beneath the thicker root this mantle underflow could create a sharper seismic lab east of the amt via two effects first the local increase in flow velocities could steepen the thermal gradient at the base of the lithosphere and hence the isotropic velocity contrast second the increased strain rate along edge of the lithosphere could enhance the magnitude of azimuthal anisotropy our results suggest that seismically detectable lab sharpness variations could be used to constrain geographic variations in coupling between plates and mantle convection a secondary result is the image of a mid lithospheric discontinuity arrival at 80 110 km depth that is found primarily to the east of the amt this arrival is interpreted as produced by a layer of low velocity metasomatic minerals that have accumulated since the 1 8 ga creation of the lithosphere

crustal radial anisotropy beneath cameroon from ambient noise tomography abstract to increase the understanding of crustal deformation and crustal flow patterns due to tectonic processes in cameroon we study the lateral variability of the crustal isotropic velocity and radial anisotropy estimated using ambient noise tomography ant rayleigh and love wave noise correlation functions ncfs were retrieved from the cross correlation of seismic ambient noise data recorded in cameroon and phase velocities at periods of 8 to 30 s were measured to perform surface wave tomography joint inversion of rayleigh and love wave data for isotropic velocity models could not fit the observed dispersions simultaneously we attribute the love rayleigh discrepancy to the presence of radial anisotropy in the crust and estimated its magnitude our 3 d radial anisotropic model reveals the spatial variation of strong to weak positive vsh vsv and negative vsv vsh radial anisotropy in the crust we observe negative radial anisotropy in the upper crust that is associated mainly with the location of a previously reported mantle plume the anisotropy could be attributed to the vertical alignment of fossil microcracks or metamorphic foliations due to the upwelling of plume material a strong positive radial anisotropy is centered at the location of an inferred boundary between the congo craton and the oubanguides belt that might be related to the preferred orientation of crustal anisotropic minerals associated with shearing in this fault zone the middle crust is characterized by a widespread negative radial anisotropy that is likely caused by the flow induced alignment of anisotropic minerals that crystallized during magma intrusion the magnitude of the radial anisotropy varies systematically from predominantly negative in the middle crust to positive in the lower crust the imaged patterns of the isotropic velocity and radial anisotropy are consistent with previous studies and agree with regional tectonics

length scales patterns and origin of azimuthal seismic anisotropy in the upper mantle as mapped by rayleigh waves summary we measure the degree of consistency between published models of azimuthal seismic anisotropy from surface waves focusing on rayleigh wave phase velocity models some models agree up to wavelengths of 2000 km albeit at small values of linear correlation coefficients others are however not well correlated at all also with regard to isotropic structure this points to differences in the underlying data sets and inversion strategies particularly the relative damping of mapped isotropic versus anisotropic anomalies yet there is more agreement between published models than commonly held encouraging further analysis employing a generalized spherical harmonic representation we analyse power spectra of orientational 2 anisotropic heterogeneity from seismology we find that the anisotropic component of some models is characterized by stronger short wavelength power than the associated isotropic structure this spectral signal is consistent with predictions from new geodynamic models based on olivine texturing in mantle flow the flow models are also successful in predicting some of the seismologically mapped patterns we substantiate earlier findings that flow computations significantly outperform models of fast azimuths based on absolute plate velocities moreover further evidence for the importance of active upwellings and downwellings as inferred from seismic tomography is presented deterministic estimates of expected anisotropic structure based on mantle flow computations such as ours can help guide future seismologic inversions particularly in oceanic plate regions we propose to consider such a priori information when addressing open questions about the averaging properties and resolution of surface and body wave based estimates of anisotropy

structure beneath the alboran from geodynamic flow models and seismic anisotropy 1 upper mantle heterogeneity beneath the alboran sea western mediterranean as inferred from seismology has been associated with a range of subduction and lithospheric delamination scenarios however better constraints on the deep dynamics of the region are needed to determine the cause and consequence of complex surface tectonics here we use an improved set of shear wave splitting observations and a suite of mantle flow models to test a range of suggested structures we find that the observed seismic anisotropy is best reproduced by mantle flow models that include a continuous deeply extending slab beneath the alboran which elongates along the iberian margin from granada to gibraltar and curves southward toward the high atlas other models with detached slabs slabs with spatial gaps or drip like features produce results inconsistent with the splitting observations sw directed shear flow when combined with sublithospheric deflection in response to a dense sinker generates nnw splitting orientations most similar to the patterns observed along gibraltar slab viscosities of 250 times that of the upper mantle are preferred because they provide a balance between the poloidal flow induced by any sinker and toroidal flow induced by stiff slabs the best match to anisotropy across the atlas is a model with a stiff continental keel in northwestern africa which deflects flow northward our results show that quantitative predictions of seismic anisotropy are useful in distinguishing the spatial and depth extent of regional density structures which may otherwise be ambiguous

global miocene tectonics and the modern world abstract an amazing congruence of seemingly unrelated diverse global events began in the middle and upper miocene and established our modern world two global orogenic belts were active mostly in the middle and upper miocene while backarc basins formed along the eastern margin of asia coincident with these events global temperatures cooled in both the ocean and atmosphere desertification occurred from central asia into and across most of northern africa and also in australia and in southern south america coincident with the expansion of the antarctic ice cap at 14 ma there was initial widespread deep sea erosion and changes in patterns of deep sea sedimentation muddy pelagic sedimentation increased six fold in the north and central atlantic and pacific oceans and global changes in circulation lead to more diatomites in the pacific and fewer in the atlantic by the end of the miocene most of the mediterranean sea had evaporated broadly coincident with these events many old large river systems were destroyed and new ones formed as much of the world s landscape changed collectively these global on shore tectonic and ocean atmospheric events provide the foundation for our modern world a mixture of new and rejuvenated orogenic belts and their far field effects distant epiorogenic uplift rain shadow deserts large alluvial aprons and distant deltas as inherited gondwanan landscapes persisted remote from plate boundaries thus at the end of the miocene much of the world s landscape except for that changed by pleistocene continental glaciation would be recognizable to us today we argue that all of these events had the same ultimate common cause an internal earth engine that drove plate motions in two broad ways first the opening and closing of seven key gateways to deep water oceanic currents radically altered global heat transfer and changed a lingering greenhouse to an icehouse world secondly these events were in part coincident with renewed heat flow in the african and pacific superplumes that energized global plate motions in the middle and upper miocene we hope this global synthesis will stimulate more research on the many global events of the miocene to understand better both our modern world and earlier global orogenies

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

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

slab stagnation and detachment under northeast china abstract results of tomography models around the japanese islands show the existence of a gap between the horizontally lying stagnant slab extending under northeastern china and the fast seismic velocity anomaly in the lower mantle a simple conversion from the fast velocity anomaly to the low temperature anomaly shows a similar feature this feature appears to be inconsistent with the results of numerical simulations on the interaction between the slab and phase transitions with temperature dependent viscosity such numerical models predict a continuous slab throughout the mantle i extend previous analyses of the tomography model and model calculations to infer the origins of the gap beneath northeastern china results of numerical simulations that take the geologic history of the subduction zone into account suggest two possible origins for the gap 1 the opening of the japan sea led to a breaking off of the otherwise continuous subducting slab or 2 the western edge of the stagnant slab is the previous subducted ridge which was the plate boundary between the extinct izanagi and the pacific plates origin 2 suggesting the present horizontally lying slab has accumulated since the ridge subduction is preferable for explaining the present length of the horizontally lying slab in the upper mantle numerical models of origin 1 predict a stagnant slab in the upper mantle that is too short and a narrow or non existent gap preferred models require rather stronger flow resistance of the 660 km phase change than expected from current estimates of the phase transition property future detailed estimates of the amount of the subducted izanagi plate and the present stagnant slab would be useful to constrain models a systematic along arc variation of the slab morphology from the northeast japan to kurile arcs is also recognized and its understanding may constrain the 3d mantle flow there

diagenetic mobilization of manganese in peru basin sediments abstract the mn contents and sedimentation rates of two cores from different areas of the peru basin have been determined the southern core is associated with mn nodules of conventional slow accretion rates mm 106 yr while the northern one accompanies nodules with very high growth rates 102 mm 106 yr the depletion of mn observed within the top 40 cm of the cores is interpreted as resulting from a diagenetic remobilization of mn 2 due to the oxidative characteristics of the sedimentary column in both cores the calculated flux of mn provided by the sediment is higher than the total mn recovered in the overlying nodules this indicates that the mn content of the peru basin nodules can be supplied by diagenetic processes from the sediment not only for the slowly growing nodules but also for the extremely fast growing concretions found in the north of the basin

a global geoid model with imposed plate velocities and partial layering most inversions of the long wavelength geoid in conjunction with the seismic tomographic information have so far been carried out under the assumption of either purely whole mantle or perfectly layered circulation moreover modeling the lithosphere as a spherical shell with a uniform low viscosity was found to yield the best fit to the observed geoid we have tested whether a good prediction of the geoid can also be achieved by including two constraints a semipermeable behavior of the 660 km discontinuity and surface plate velocities equal to the observed ones the mass transfer between upper and lower mantle has been changed by imposing a surface density anomaly at a depth of 660 km which is proportional to the mass anomaly needed to achieve perfectly layered circulation on the top of the mantle we assume a stiff lithosphere that moves with a velocity corresponding to the observed plate motion the viscosity only varies with depth considering a simple three layer viscosity structure and changing the permeability of the 660 km interface we have obtained a satisfactory variance reduction of the geoid data 75 for degrees 2 12 the best fit to the geoid is obtained if the mass transfer across the 660 km boundary is reduced to one third in comparison with the purely whole mantle model the best fitting viscosity profile is characterized by a clearly defined asthenosphere and a viscosity increase by at least 2 orders of magnitude in the lower mantle the amplitudes of dynamic topography predicted by our model are remarkably small 100 m thus fully compatible with the observation

seismic anisotropy of oceanic upper mantle shear wave splitting methodologies and observations we develop methodologies to obtain accurate measurements of shear wave splitting and apply these techniques to examine the pattern of oceanic upper mantle anisotropy to obtain high quality estimates of receiver splitting at island stations we devise a stacking method that finds the optimum splitting parameters and 95 error region for teleseismic shear wave phases from a suite of earthquake events to obtain additional measurements in oceanic regions we develop techniques to measure splitting parameters and errors for ss phases that sample upper mantle anisotropy at their bounce points however we find the data are often of low resolution and anomalous characteristics are sometimes found that make splitting difficult to interpret ten second splitting of ss is observed across the banjo seismic array but we cannot unambiguously attribute this signal to mantle anisotropy at the bounce the receiver splitting methods are used to assess the adequacy of a two anisotropic layer model for the pacific region with fast polarization azimuth in the lithosphere oriented in the fossil spreading direction and in the asthenosphere oriented in the absolute plate motion direction this model has been proposed to explain surface wave data in the pacific ocean but our splitting results demonstrate that oceanic anisotropy patterns are more heterogeneous than would be predicted while the island splitting measurements could reflect the influence of individual hotspot upwellings hotspot effects do not appear to be universally dominant we propose that splitting observations alternatively indicate broad scale differences in the underlying character of oceanic upper mantle anisotropy associated with coherant patters of lithospheric structure and asthenospheric flow in particular splitting surface wave models and regional studies all support a model where lithospheric anisotropy throughout the south pacific has been erased or reoriented toward the absolute plate motion direction whereas more limited observations in the north pacific indicate that the fossil lithospheric signature appears to be preserved

p and s wave velocity and vp vs in the wake of the yellowstone hot spot 1 seismic vp vs and vp vs structure is imaged across the yellowstone hot spot swell including the hot spot track where magmatism occurred at the eastern snake river plain 6 10 m y b p data are teleseismic p and s travel time delays that have been corrected for the well understood upper mantle anisotropy and crustal structure amplitude variations in the imaged structures are 6 2 11 2 and 8 for vp vs and vp vs respectively the dominant structure is a zone which extends beneath the snake river plain to a depth of 190 km that is high in vp vs and low in vp and vs the physical state of the upper mantle is inferred by assuming isostasy using the volume of melt segregated from the mantle that is inferred from estimates of magma addition to the crust and using relations that scale changes in temperature partial melt fraction and composition to density specifically we infer that the low velocity mantle beneath the snake river plain is partially molten up to 1 0 and the high velocity yellowstone swell mantle away from the snake river plain is 80 k cooler and 5 depleted in basaltic component the imaged large seismic velocity variations occur under near isothermal conditions

upper mantle seismic anisotropy as a constraint for mantle flow and continental dynamics of the north american plate abstract the alignment of intrinsically anisotropic olivine crystals under convection is typically invoked as the cause of the bulk of seismic anisotropy inferred from shear wave splitting sws this provides a means of constraining the interplay between continental dynamics and the deep mantle in particular for densely instrumented regions such as north america after usarray there a comparison of fast orientations from sws with absolute plate motions apm suggests that anisotropy is mainly controlled by plate motions however large regional misfits and the limited realism of the apm model motivate us to further explore sws based anisotropy if sws is estimated from olivine alignment in mantle circulation instead plate driven flow alone produces anisotropy that has large misfits with sws the addition of large scale mantle density anomalies and lateral viscosity variations significantly improves models although a strong continental craton is essential varying its geometry does however not improve the plate scale misfit moreover models based on higher resolution tomography degrade the fit indicating issues with the flow model assumptions and or a missing contributions to anisotropy we thus compute a lithospheric complement to achieve a best fit joint representation of asthenospheric and frozen in lithospheric anisotropy the complement shows coherent structure and regional correlation with independently imaged crustal and upper mantle anisotropy dense sws measurements therefore provide information on depth dependent anisotropy with implications for tectonics but much remains to be understood about continental anisotropy and its origin

continental interior and edge breakup at convergent margins induced by subduction direction reversal a numerical modeling study applied to the south china sea margin the dynamics of continental breakup at convergent margins have been described as the results of back arc opening caused by slab rollback or drag force induced by subduction direction reversal although the rollback hypothesis has been intensively studied our understanding of the consequence of subduction direction reversal remains limited using thermo mechanical modeling based on constraints from the south china sea scs region we investigate how subduction direction reversal controls the breakup of convergent margins the numerical results show that two distinct breakup modes namely continental interior and edge breakup edge refer to continent above the plate boundary interface may develop depending on the maturity of the convergent margin and the age of the oceanic lithosphere for a slab age of 15 45 ma increasing the duration of subduction promotes the continental interior breakup mode where a large block of the continental material is separated from the overriding plate in contrast the continental edge breakup mode develops when the subduction is a short duration event and in this mode a wide zone of less continuous continental fragments and tearing of the subducted slab occur these two modes are consistent with the interior relic late mesozoic arc and edge relic fore arc rifting characteristics in the western and eastern scs margin suggesting that variation in the northwest directed subduction duration of the proto scs might be a reason for the differential breakup locus along the strike of the scs margin besides a two segment trench associated with the northwest directed subduction is implied in the present day scs region

mantle flow and melting beneath oceanic ridge ridge ridge triple junctions abstract plate boundary geometry likely has an important influence on crustal production at mid ocean ridges many studies have explored the effects of geometrical features such as transform offsets and oblique ridge segments on mantle flow and melting this study investigates how triple junction tj geometry may influence mantle dynamics an earlier study georgen j e lin j 2002 three dimensional passive flow and temperature structure beneath oceanic ridge ridge ridge triple junctions earth planet sci lett 204 115 132 suggested that the effects of a ridge ridge ridge configuration are most pronounced under the branch with the slowest spreading rate thus we create a three dimensional finite element variable viscosity model that focuses on the slowest diverging ridge of a triple junction with geometry similar to the rodrigues tj this spreading axis may be considered to be analogous to the southwest indian ridge within 100 km of the tj temperatures at depths within the partial melting zone and crustal thickness are predicted to increase by 40 c and 1 km respectively we also investigate the effects of differential motion of the tj with respect to the underlying mantle by imposing bottom model boundary conditions replicating a absolute plate motion and b a three dimensional solution for plate driven and density driven asthenospheric flow in the african region neither of these basal boundary conditions significantly affects the model solutions suggesting that the system is dominated by the divergence of the surface places finally we explore how varying spreading rate magnitudes affects tj geodynamics when ridge divergence rates are all relatively slow i e with plate kinematics similar to the azores tj significant along axis increases in mantle temperature and crustal thickness are calculated at depths within the partial melting zone temperatures are predicted to increase by 150 c similar to the excess temperatures associated with mantle plumes likewise crustal thickness is calculated to increase by approximately 6 km over the 200 km of ridge closest to the tj these results could imply that some component of the excess volcanism observed in geologic settings such as the terceira rift may be attributed to the effects of tj geometry although the important influence of features like nearby hotspots e g the azores hotspot cannot be evaluated without additional numerical modeling

seismic tomography surface uplift and the breakup of gondwanaland integrating mantle convection backwards in time mantle density heterogeneities imaged using seismic tomography contain information about time dependent mantle flow and mantle structures that existed in the past we model the history of mantle flow using a tomographic image of the mantle beneath southern africa as an initial condition while reversing the direction of flow and analytically incorporating cooling plates as a boundary condition if the resulting backwards integrated model for structures is used as a starting point for a forwards convection model today s mantle can be adequately reconstructed if we do not integrate backwards more than than about 50 75 ma flow can also be reliably reversed through the mesozoic but only if instability of the lower boundary layer can be suppressed our model predicts that the large seismically slow and presumably hot structure beneath southern africa produced 500 700 m of dynamic topography throughout the cenozoic since 30 ma uplift has moved from eastern to southern africa where uplift rates are 10 m myr consistent with observations during the mesozoic the modeled topographic high is situated near gondwanaland rifting raising the possibility that this buoyant structure may have been involved with this breakup

seismic anisotropy beneath stable continental interiors a robust knowledge of seismic anisotropy beneath the continents is essential to our understanding of plate tectonic theory as anisotropy provides a unique constraint on the character of past and present deformation in the lithosphere and sublithospheric mantle this review paper summarizes the range of techniques currently available to image seismic anisotropy with passive source seismic data and addresses current issues surrounding the observation and interpretation of continental seismic anisotropy to this end we present case studies for four regions where seismic anisotropy has been extensively investigated in recent years eastern north america the canadian shield australia and southern africa based on this full suite of results we infer that stable continental regions generally contain seismic anisotropy that is clearly located within both the lithosphere and the sublithospheric mantle usually to depths of at least 200 km and perhaps more an implication of these results is that tectonic plates are at most only partially coupled to the underlying mantle the results from these case studies also demonstrate that while remarkable progress in seismic anisotropy imaging has been achieved in recent years it is clear that much more work will be required to adequately understand the origin of continental seismic anisotropy we suggest that a more robust characterization of anisotropic parameters can only be achieved by integrating complementary seismic datasets and by incorporating constraints from key related datasets from mineral physics magnetotellurics gravity and geodesy

mantle anisotropy beneath the earth s mid ocean ridges observations of seismic anisotropy at oceanic spreading centres offer insights into mid ocean ridge processes and the formation of new plates here remote observations of seismic anisotropy beneath mid ocean ridges are made using measurements of source side shear wave splitting over 100 high quality measurements are made using earthquakes that occur near mid ocean ridges and transform faults but are observed at teleseismic distances in general for off axis ridge events the polarisation of fast shear waves phi prime prime is approximately parallel to the spreading direction nearer the ridge lesssim 50 km phi prime prime becomes more scattered and is often ridge parallel delay times delta t tend to increase from 1 s near the ridge axis to sim 3 s further away slow spreading regions gakkel and southwest indian ridges show smaller amounts of splitting than faster spreading centres at transform zones the pattern is more complex coverage beneath the east pacific rise is especially good and we observe a systematic increase in delay times in s wave splitting measurements compared to previous sks splitting observations made at ocean bottom seismometers one compatible explanation is the presence of horizontally aligned connected layers of melt at depth with an inclusion density which increases near the top of the layer this is also compatible with other observations of the lab discontinuity and surface wave derived measurements of radial anisotropy

a simple global model of plate dynamics and mantle convection cooling and thickening of lithospheric plates with age and subduction result in large scale horizontal density contrasts tending to drive plate motions and mantle flow we quantify the driving forces associated with these density contrasts to determine if they can drive the observed plate motions first two dimensional models are computed to evaluate the effects of assumed rheologies and boundary conditions we are unable to obtain platelike behavior in viscous models with traction free boundary conditions the piecewise uniform velocities distinctive of plate motion can be imposed as boundary conditions and the dynamic consistency of the models evaluated by determining if the net force on each vanishes if the lithosphere has a newtonian viscous rheology the net force on any plate is a strong function of the effective grid spacing used leading to ambiguities in interpretation incorporating a rigid plastic lithosphere which fails at a critical yield stress into the otherwise viscous model removes these ambiguities the model is extended to the actual three dimensional spherical plate geometry the observed velocities of rigid plastic plates are matched to the solution of the viscous stokes equation at the lithosphere asthenosphere boundary body forces from the seismically observed slabs from the thickening of the lithosphere obtained from the actual lithospheric ages and from the differences in structure between continents and oceans are included interior density contrasts such as those resulting from upwellings from a hot bottom boundary layer are assumed to occur on a scale small compared to plate dimensions and are not included the driving forces from the density contrasts within the plates are calculated and compared to resisting forces resulting from viscous drag computed from the three dimensional global return flow and resistance to deformation at converging boundaries the rms residual torque is 30 of the driving torque the density contrasts within the plates themselves can reasonably account for plate motions body forces from convection in the interior may provide only a small net force on the plates at converging boundaries the lithosphere has a yield stress of 100 bars drag at the base of the plates is 5 bars and resists plate motion the net driving forces from subducting slabs and collisional resistance are localized and approximately balance driving forces from lithospheric thickening are distributed over the areas of the plates as is viscous drag the approximate balance of these two forces predicts plate velocities uncorrelated with plate area as observed the model represents a specific case of boundary layer convection the dynamical results are consistent with either upper mantle or mantle wide convection

sediment supply to the orange sedimentary system over the last 150 my an evaluation from sedimentation denudation balance the south african plateau is bordered by passive margin basins preserving the terrigeneous sediment produced during onshore erosion as such these basins potentially provide a record of the variation in onshore elevation and relief over time here we bring new constraints on the uplift and erosion of the southern african plateau over the last 150 ma from the perspective of the stratigraphic architecture of these basins we review published data to quantify the terrigeneous supply eroded in the drainage area and preserved in the basins the novel aspect of our approach is the integration of the evolution of the whole domain in sedimentation i e not only the platform as well as the onshore eroding region along the south african and namibian atlantic margins we determined the long term signal of sedimentary supply at the 10 ma scale from 3d mass balance calculations comparing sedimentary volumes deduced from offshore isopach maps with a particular attention to associated uncertainties on one hand and denudation volumes deduced from thermochronology data on the other we show that in this case the volumes of denudation and of sediments preserved in the basin are consistent both in total and incremental amount the latter is therefore a reliable estimation of the denudation in the associated continental domain subject to certain assumptions such as the drainage area over time the long term signal of sedimentary supply of the south african and namibian atlantic margins shows that the major part of the denudation occurred during the post rift lower cretaceous and the upper cretaceous and subsequently decreased this is not the expected behavior for the simple relaxation of topography acquired during rifting that would show a progressive decrease of the sediment supply over time this suggests a significant rejuvenation of relief during the upper cretaceous 50 ma after the rifting event if erosion is directly linked to elevation or relief variation this implies 2 generations the first before break up i e before 130 ma presumably during rifting and the second during the upper cretaceous

asymmetric mantle dynamics in the melt region of the east pacific rise abstract the mantle electromagnetic and tomography melt experiment found a surprising degree of asymmetry in the mantle beneath the fast spreading southern east pacific rise melt seismic team science 280 1998 1215 1218 forsyth et al science 280 1998 1235 1238 toomey et al science 280 1998 1224 1227 wolfe and solomon science 280 1998 1230 1232 scheirer et al science 280 1998 1221 1224 evans et al science 286 1999 752 756 pressure release melting of the upwelling mantle produces magma that migrates to the surface to form a layer of new crust at the spreading center about 6 km thick canales et al science 280 1998 1218 1221 seismic and electromagnetic measurements demonstrated that the distribution of this melt in the mantle is asymmetric forsyth et al science 280 1998 1235 1238 toomey et al science 280 1998 1224 1227 evans et al science 286 1999 752 756 at depths of several tens of kilometers melt is more abundant beneath the pacific plate to the west of the axis than beneath the nazca plate to the east melt investigators attributed the asymmetry in melt and geophysical properties to several possible factors asymmetric flow passively driven by coupling to the faster moving pacific plate interactions between the spreading center and hotspots of the south pacific an off axis center of dynamic upwelling and or anomalous melting of an embedded compositional heterogeneity melt seismic team science 280 1998 1215 1218 forsyth et al science 280 1998 1235 1238 toomey et al science 280 1998 1224 1227 wolfe and solomon science 280 1998 1230 1232 evans et al science 286 1999 752 756 here we demonstrate that passive flow driven by asymmetric plate motion alone is not a sufficient explanation of the anomalies asthenospheric flow from hotspots in the pacific superswell region back to the migrating ridge axis in conjunction with the asymmetric plate motion can create many of the observed anomalies

electrical lithosphere beneath the kaapvaal craton southern africa 1 a regional scale magnetotelluric mt experiment across the southern african kaapvaal craton and surrounding terranes called the southern african magnetotelluric experiment samtex has revealed complex structure in the lithospheric mantle large variations in maximum resistivity at depths to 200 250 km relate directly to age and tectonic provenance of surface structures within the central portions of the kaapvaal craton are regions of resistive lithosphere about 230 km thick in agreement with estimates from xenolith thermobarometry and seismic surface wave tomography but thinner than inferred from seismic body wave tomography the mt data are unable to discriminate between a completely dry or slightly damp a few hundred parts per million of water structure within the transitional region at the base of the lithosphere however the structure of the uppermost 150 km of lithosphere is consistent with enhanced but still low conductivities reported for hydrous olivine and orthopyroxene at levels of water reported for kaapvaal xenoliths the electrical lithosphere around the kimberley and premier diamond mines is thinner than the maximum craton thickness found between kimberley and johannesburg pretoria the mantle beneath the bushveld complex is highly conducting at depths around 60 km possible explanations for these high conductivities include graphite or sulphide and or iron metals associated with the bushveld magmatic event we suggest that one of these conductive phases most likely melt related sulphides could electrically connect iron rich garnets in a garnet rich eclogitic composition associated with a relict subduction slab

evolving viscous anisotropy in the upper mantle and its geodynamic implications asthenospheric shear causes some minerals particularly olivine to develop anisotropic textures that can be detected seismically in laboratory experiments these textures are also associated with

sediments and micronodules in the northern and central peru basin the mineralogy and composition of sediments and manganese micronodules from the northern and central sectors of the peru basin are discussed because of the proximity of the basin to the carbonate compensation depth c c d surface sediments vary between calcareous oozes and siliceous muds besides biogenic components clay minerals are important by far the most abundant clay mineral is smectite which is thought to be of diagenetic origin on a carbonate free basis the surface sediments are rather uniform in composition throughout the basin and are similar in composition to those of the equatorial pacific sedimentation rates for the uppermost core sections are in the range of 3 to 5 mm 1000 yr

using probabilistic seismic tomography to test mantle velocity density relationships we use a neighborhood algorithm to explore the fit to long period seismic data of a wide variety of long wavelength mantle models this approach to the global tomographic inverse problem yields probability distributions for seismic velocities density and related properties as functions of depth such distributions can be robust even when individual models are not and allow us to test several assumptions about the earth that have long been enforced a priori in inversions in particular we are able to test the paradigm ofdeep mantle heterogeneity that is dominantly thermal in origin producing velocity and density perturbations that are well correlated and have relative amplitudes given by nlnb nlnvs 6 0 5 our distributions show that such relationships are unlikely and even though the results are consistent with recent best fitting models from damped seismic inversions they demonstrate that many specific properties ofsuch models are not robust the data clearly f density perturbations that are poorly or negatively correlated with velocity heterogeneity and have amplitudes several times larger yielding nlnb nlnvs s 1 0 than damped inversions allow these characteristics are most pronounced in the upper mantle transition zone and the base ofthe lower mantle suggesting layered convection the negative density velocity correlations f at these depths imply dominantly chemical heterogeneity while the likelihood ofrelatively high amplitude density variations suggests that variable iron content is an important component ofthis heterogeneity these results which we show to be consistent with independent gravity constraints represent a profound change in the interpretation of seismic constraints in addition the distributions show that even though best fitting density models from recent inversions or our sampling are consistent with the data most specific properties of such models are not robust this implies that it is more appropriate to use seismic model distributions rather than individual models to make geodynamic and geochemical inferences

how mantle slabs drive plate tectonics the gravitational pull of subducted slabs is thought to drive the motions of earth s tectonic plates but the coupling between slabs and plates is not well established if a slab is mechanically attached to a subducting plate it can exert a direct pull on the plate alternatively a detached slab may drive a plate by exciting flow in the mantle that exerts a shear traction on the base of the plate from the geologic history of subduction we estimated the relative importance of pull versus suction for the present day plates observed plate motions are best predicted if slabs in the upper mantle are attached to plates and generate slab pull forces that account for about half of the total driving force on plates slabs in the lower mantle are supported by viscous mantle forces and drive plates through slab suction

seismic anisotropy in the upper mantle 2 predictions for current plate boundary flow models 1 the anisotropic seismic structure due to flow induced mineral alignment is investigated for a series of models designed to simulate deformation in the upper mantle within a few hundred kilometers of a plate boundary the orientation distributions of olivine enstatite aggregates evolve along streamlines of each flow model based on each grains plastic response to the local stress strain field the effective elastic tensor for these textured aggregates provides predictions of p wave anisotropy and shear wave splitting throughout the model space p and s travel time delay patterns and fast shear wave polarization angles are found to vary significantly with incidence angle for a given model comparison of predicted fast p direction for our method versus a finite strain based estimate shows that agreement is acceptable for much of the model space but notable differences occur in regions up to several tens of kilometers in size two dimensional models of spreading center flow are presented for slow and fast rates and for several cases in which the ridge migrates over the deeper mantle the effect of flow in the third dimension is addressed in a few calculations for one comparison of flow in the mantle wedge at a subduction zone the introduction of trench parallel flow causes significant changes in the predicted patterns of p wave anisotropy magnitude more than orientation and sks splitting

full waveform tomography of the upper mantle in the south atlantic region imaging a westward fluxing shallow asthenosphere abstract a prominent feature of the south atlantic region is its strongly asymmetric residual bathymetry across the ocean basin it has been suggested that the residual bathymetry is dynamic in nature arising from the large slow velocity seismic anomaly located in the lower mantle beneath the african plate unfortunately the pattern of mantle heterogeneity particularly in the upper mantle is not well known owing to the sparsity of seismic stations and the existence of large aseismic regions on the african and south american plates here we present a new seismic tomographic study of the south atlantic upper mantle our model is based on a full seismic waveform inversion of 4000 high quality seismograms for isotropic 3 d seismic structure using a powerful adjoint methodology capable of extracting maximum information from each seismogram the theory requires simulation of seismic wave propagation in 3 d heterogeneous earth models computed with a spectral element method where the differences between observed and synthetic seismograms are quantified using phase misfits obtained through a time frequency transform the model images a continuous channel of pronounced slow seismic velocity in the shallow sublithospheric mantle between 150 and 300 km depth that branches in between the cratonic roots under the african and south american continents at greater depth below 300 350 km the slow anomalies are less pronounced and a change in the convective planform is indicated by isolated round shaped patches in an overall faster mantle it is possible that the depthwise change of the convective planform from vertical to horizontal advection of hot buoyant material in a low viscosity asthenosphere can reconcile the anomalous residual bathymetry in the region and we present a simple fluid dynamic model of pressure driven flow to assess the feasibility of this scenario

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

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

layered mantle convection a model for geoid and topography the long wavelength geoid and topography are dynamic effects of a convecting mantle the long wavelength geoid of the earth is controlled by density variations in the mantle and has been explained by circulation models involving whole mantle flow however the relationship of long wavelength topography to mantle circulation has been a puzzling problem in geodynamics we show that the dynamic topography is mainly due to density variations in the upper mantle even after the effects of lithospheric cooling and crustal thickness variation are taken into account layered mantle convection with a shallow origin for surface dynamic topography is consistent with the spectrum small amplitude and pattern of the topography layered mantle convection with a barrier about 250 km deeper than the 670 km phase boundary provides a self consistent geodynamic model for the amplitude and pattern of both the long wavelength geoid and surface topography

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

azimuthal seismic anisotropy constrains net rotation of the lithosphere 1 net rotation nr of the lithosphere is found in hot spot reference frames important for tectonics and plume models but difficult to constrain using mantle flow and crystallographic texture modeling i show that nrs lead to mantle shearing which is recorded in azimuthal seismic anisotropy the nr amplitude in some hot spot models is so large that it degrades the model fit to anisotropy significantly smaller nrs such as predicted by flow models with stiff continental keels are consistent with seismology however

global plate motion frames toward a unified model 1 plate tectonics constitutes our primary framework for understanding how the earth works over geological timescales high resolution mapping of relative plate motions based on marine geophysical data has followed the discovery of geomagnetic reversals mid ocean ridges transform faults and seafloor spreading cementing the plate tectonic paradigm however so called absolute plate motions describing how the fragments of the outer shell of the earth have moved relative to a reference system such as the earth s mantle are still poorly understood accurate absolute plate motion models are essential surface boundary conditions for mantle convection models as well as for understanding past ocean circulation and climate as continent ocean distributions change with time a fundamental problem with deciphering absolute plate motions is that the earth s rotation axis and the averaged magnetic dipole axis are not necessarily fixed to the mantle reference system absolute plate motion models based on volcanic hot spot tracks are largely confined to the last 130 ma and ideally would require knowledge about the motions within the convecting mantle in contrast models based on paleomagnetic data reflect plate motion relative to the magnetic dipole axis for most of earth s history but cannot provide paleolongitudes because of the axial symmetry of the earth s magnetic dipole field we analyze four different reference frames paleomagnetic african fixed hot spot african moving hot spot and global moving hot spot discuss their uncertainties and develop a unifying approach for connecting a hot spot track system and a paleomagnetic absolute plate reference system into a hybrid model for the time period from the assembly of pangea 320 ma to the present for the last 100 ma we use a moving hot spot reference frame that takes mantle convection into account and we connect this to a pre 100 ma global paleomagnetic frame adjusted 5 in longitude to smooth the reference frame transition using plate driving force arguments and the mapping of reconstructed large igneous provinces to core mantle boundary topography we argue that continental paleolongitudes can be constrained with reasonable confidence

numerical modeling of mantle flow beneath madagascar to constrain upper mantle rheology beneath continental regions over the past few decades azimuthal seismic anisotropy measurements have been widely used proxy to study past and present day deformation of the lithosphere and to characterize convection in the mantle beneath continental regions distinguishing between shallow and deep sources of anisotropy remains difficult due to poor depth constraints of measurements and a lack of regional scale geodynamic modeling here we constrain the sources of seismic anisotropy beneath madagascar where a complex pattern cannot be explained by a single process such as absolute plate motion global mantle flow or geology we test the hypotheses that either edge driven convection edc or mantle flow derived from mantle wind interactions with lithospheric topography is the dominant source of anisotropy beneath madagascar we therefore simulate two sets of mantle convection models using regional scale 3 d computational modeling we then calculate lattice preferred orientation that develops along pathlines of the mantle flow models and use them to calculate synthetic splitting parameters comparison of predicted with observed seismic anisotropy shows a good fit in northern and southern madagascar for the edc model but the mantle wind case only fits well in northern madagascar this result suggests the dominant control of the measured anisotropy may be from edc but the role of localized fossil anisotropy in narrow shear zones cannot be ruled out in southern madagascar our results suggest that the asthenosphere beneath northern and southern madagascar is dominated by dislocation creep dislocation creep rheology may be dominant in the upper asthenosphere beneath other regions of continental lithosphere

seismic anisotropy of northeastern algeria from shear wave splitting analysis abstract there are few studies of internal deformation under northern africa here we present such a study we analyze teleseismic shear wave splitting for northeast algeria to improve our knowledge of lithospheric and asthenospheric deformation mechanisms in this region we study waveform data generated by tens of teleseismic events recorded at five recently installed broadband bb stations in algeria these stations cover an area 2 across extending from the tellian geological units in the north to the saharan atlas units in the south analysis of sks wave splitting results insignificant spatial variations in fast polarization orientation over a scale length of at most 100 km the seismic anisotropy shows three clear spatial patterns a general ene wsw orientation is observed under the stations in the north this polarization orientation follows the direction of the tell atlas mountain chain which is perpendicular to the convergence direction between africa and eurasia delay times vary significantly across the region between 0 6 and 2 0 s at several stations there is an indication of a wnw ese polarization orientation which is apparently related to a later geodynamic evolutionary phase in this region a third pattern of seismic anisotropy emerges in the south with an orientation of roughly n s we discuss these observations in light of geodynamic models and present day geodetic motion

seismic anisotropy beneath the continents probing the depths of geology seismic anisotropy beneath continents is analyzed from shear wave splitting recorded at more than 300 continental seismic stations anisotropy is found to be a ubiquitous property that is due to mantle deformation from past and present orogenic activity the observed coherence with crustal deformation implies that the mantle plays a major if not dominant role in orogenies no evidence is found for a continental asthenospheric decoupling zone suggesting that continents are coupled to general mantle circulation

the mantle flow field beneath western north america although motions at the surface of tectonic plates are well determined the accompanying horizontal mantle flow is not we have combined observations of surface deformation and upper mantle seismic anisotropy to estimate this flow field for western north america we find that the mantle velocity is 5 5 1 5 centimeters per year due east in a hot spot reference frame nearly opposite to the direction of north american plate motion west southwest the flow is only weakly coupled to the motion of the surface plate producing a small drag force this flow field is probably due to heterogeneity in mantle density associated with the former farallon oceanic plate beneath north america

constraining the extent of crust mantle coupling in central asia using gps geologic and shear wave splitting data abstract we have obtained constraints on mechanical crust mantle coupling for tibet and yunnan indo china by comparing the observed surface deformation field inferred from gps and quaternary fault slip rate data with the mantle deformation field inferred from several sks shear wave splitting data sets we first determined whether the anisotropy is dominantly asthenospheric or lithospheric by testing simple models of both types against the observed values of the fast polarization direction which is assumed to be parallel to the horizontal projection of the direction of maximum shear for asthenospheric flow we solved for a best fitting uniform sub asthenospheric velocity model for eurasia the fit however was not satisfactory rms misfit 20 solving for separate uniform flow fields in each region improves the fit 15 for tibet 11 for yunnan although the resulting flow fields are inconsistent with several geophysical and geological constraints and thus considered unlikely we then considered lithospheric models for tibet vertically coherent deformation i e maximum shear direction from surface deformation is parallel to yields an improved match 11 for left lateral shear both the goodness of fit and the dominance of left lateral surface faulting in tibet argue for a lithospheric source of anisotropy the misfit for yunnan is large for either right 53 or left lateral 49 shear which argues for complete crust mantle decoupling in yunnan we show that the fast polarization directions throughout both the tibet and yunnan region can be fit by a single lithospheric dynamic model in which there is strong coupling between crust and mantle beneath tibet but a complete decoupling between crust and mantle beneath yunnan crust this dynamic model predicts left lateral maximum shear directions within the mantle that align with fast polarization directions in both regions 9 these maximum shear directions within the mantle align with the left lateral maximum shear directions in the crustal deformation field in tibet but are not in yunnan our results have the following implications first the coherence between crust and mantle deformation in tibet implies strong crust mantle mechanical coupling since this is the only way that crustal buoyancy forces required to account for the surface deformation field can be transmitted into the mantle this behavior is consistent with a uniform strength lithosphere or strong crust but not with a substantially weaker crust for example it is inconsistent with the popular jelly sandwich rheology and thus precludes behavior such as large scale lower crustal flow in tibet the crust mantle coherence is also incompatible with mantle delamination second crust mantle decoupling within the yunnan lithosphere argues that mantle deformation there is controlled only by boundary conditions crustal buoyancy forces are not transmitted into the mantle beneath yunnan moreover the dynamic model for the mantle shows that the yunnan crust is moving south with respect to the mantle at rates as high as 30 mm yr third there is a fundamental rheological lithospheric transition between tibet and yunnan that may provide a key to understanding this significant orogen