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. Slat) 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 witIx 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 witIx 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 witIx 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-kin 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 herore which the geoid becomes uncorrelated witIx 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 paleomagnetic studies.

S U M M A R YA model of lithospheric thickness and a recent compilation of M o h o depths are used t o compute the Earth's isostatic surface topography and associated gravity anomalies. T h e results are strongly influenced by the uncertainties in lithospheric depth and crustal density profiles. T h e preferred models explain most of the observed topography and are highly correlated with observed gravity anomalies for harmonic degrees larger than 10. Comparisons of our residual topography with geodynamical calculations of dynamic topography based on mantle circulation are rather poor.

We present observations of diffracted SV (SVd) for a path between the Fiji‐Tonga islands and the eastern coast of North America at distances greater than 110°. Observed features of S diffracted suggest that coupling between SVd and SHd can be ruled out as a first order effect for this path. Arrivals of SHd are late relative to IASP91 travel‐times by about 10 s, and those of SVd are late relative to SHd by 3 s, for most records. The slope of the log(SVd/SHd) spectral ratio is around 3Hz−1 in the range 0.06–0.15 Hz. A transversely isotropic low‐velocity layer in the lower‐most mantle with a thickness of 200–300 km may account for most of the observed properties of SVd.

Abstract. Although one-layer dynamic models of the Earth's mantle have successfully explained the geoid, they generate a surface dynamic topography that seems too large relative to geological observations. In this study, we hypothesize the possibility of partial advection of mantle equidensity surfaces by vertical motion induced by "driving" loads. These large-scale"flow-dependent" loads would greatly reduce the dynamic topography amplitude, while preserving a good fit to the observed geoid. Various physical processes related to nonequilibrium phase changes or to the existence of chemical heterogeneity in the mantle could justify a partial advection of the mean density. In this paper, we simply consider the flow-dependent loads as proportional to the vertical flow velocity. Two density mantle models a, re considered, one from subduction reconstruction [Ricard et al., 1993] and one from seismic tomography [Li and Romanowicz, 1995]. We show that a very moderate entrainment (a few kilometers) of the equidensity surfaces in the transition zone is sufficient to reduce dynamic topography amplitude by a factor of 2 or 3. The seismic velocity signal associated with this entrainment would be hidden by the signal of thermal origin. Using this new hypothesis, we compute sea level changes associated with epeirogeny for the Cretaceous, Paleocene, and Oligocene periods. The amplitude and phase of these changes are in fairly good agreement with geological hypsometric curves. Our results suggest that not only the thermodynamics, but also the kinetics of mineralogical phase changes in the transition zone are of crucial importance.

The main difficulties in anisotropic velocity analysis and inversion using surface seismic data are associated with the multiparameter nature of the problem and inherent trade-offs between the model parameters. For the most common anisotropic model, transverse isotropy with a vertical symmetry axis (VTI media), P-wave kinematic signatures are controlled by the vertical velocity V 0 and the anisotropic parameters and δ. However, only two combinations of these parameters-NMO velocity from a horizontal reflector V nmo (0) and the anellipticity coefficient η-can be determined from P-wave reflection traveltimes if the medium above the reflector is laterally homogeneous. While V nmo (0) and η are sufficient for time-domain imaging in VTI media, they cannot be used to resolve the vertical velocity and build velocity models needed for depth migration. Here, we demonstrate that P-wave reflection data can be inverted for all three relevant VTI parameters (V 0 , and δ) if the model contains nonhorizontal intermediate interfaces. Using anisotropic reflection tomography, we carry out parameter estimation for a two-layer medium with a curved intermediate interface and reconstruct the correct anisotropic depth model. To explain the success of this inversion procedure, we present an analytic study of reflection traveltimes for this model and show that the information about the vertical velocity and reflector depth was contained in the reflected rays which crossed the dipping intermediate interface. The results of this work are especially encouraging because the need for depth imaging (such as prestack depth migration) arises mostly in laterally heterogeneous media. Still, we restricted this study to a relatively simple model and constrained the inversion by assuming that one of the layers is isotropic. In general, although lateral heterogeneity does create a dependence of P-wave reflection traveltimes on the vertical velocity, there is no guarantee that for more complicated models all anisotropic parameters can be resolved in a unique fashion.