The mantle under Africa: An overview of global and regional tomography models, lithosphere thickness models and their effect on present-day and past dynamic topography

Abstract

The African plate is affected less by subduction than other plates and, as a consequence its surface experienced less subduction-related deformation while its mantle contains fewer subducted slabs. This region therefore appears suitable to study the etfect of large-scale mantle density anomalies and flow on both present-day dynamic topography and past surface uplift and subsidence events. Dynamic topography quantifies the mantle component to Earth's surface topography and as such influences the formation of basins and natural resources. It is computed here from a mantle flow model with density variations infelTed from seismic tomography and radial mantle viscosity variations. In order to assess the robustness of tomography features, we have compiled here a total of 18 models (13 whole-mantle and 5 upper mantle). We have also computed a weighted mean tomography model that gives a fit between dynamic topography and observation-based \"residual topography\" substantially better than any individual model. To better understand discrepancies between dynamic and residual topography we compute their con-elations and ratios (I) regionally in caps of 30 degrees of arc and (2) spectrally as a function of spherical harmonic degree up until degree 31. We compare the observed con-elation and ratio of geoid and residual topography with the values \"expected\" from a geodynamic model. Correlations between residual and dynamic topography are typically high in a region centred on northeastern Africa. Ratios tend to be higher in the oceans than the continent, indicating substantial lateral viscosity variations in the asthenosphere. We compute models of past dynamic topography by backward advection of density anomalies. Combining these with models of African plate motion, we compute upl ift and subsidence of points moving with the plate. However, when interpreting these results it needs to be carefully considered at what depth the density anomalies that cause these uplift and subsidence are located. Results can only be considered if those density anomalies are not advected in or out of thennal boundary layers, such that the neglect of diffusion backward in time does not introduce a substantial error. To study the effects of including a deformable lithosphere with a pressure-and temperature-dependent rheology and a free surface, we apply pressures and velocities trom the mantle flow computations at the base of a model lithosphere. Results indicate little deformation in the African basins, such that the treatment with a pure mantle flow code appears valid there.