Dynamic topography, plate driving forces and the African superswell

Discovering the connection between processes observed to occur at the surface of the Earth and its internal dynamics remains an essential goal in the Earth sciences. Deep mantle structure, as inferred from seismic tomography or subduction history, has been shown to account well for the observed surface gravity field and motions of tectonic plates(1-3). But the origin of certain large-scale features, such as the anomalous elevation of the southern and eastern African plateaux, has remained controversial. Whereas the average elevation of most cratons is between 400 and 500 m, the southern African plateau stands more than 1 km above sea level, with the surrounding oceans possessing a residual bathymetry in excess of 500 m (ref. 4). Global seismic tomography studies have persistently indicated the existence of a large-scale low-velocity anomaly beneath the African plate(5-10) and here we show that mantle flow induced by the density variations inferred from these velocity anomalies can dynamically support the excess elevation of the African 'superswell'. We also find that this upwelling mantle flow-which is most intense near the core-mantle boundary-constitutes a significant driving force for tectonic plates in the region.