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 go al in the Earth sciences. Deep mantle structure, as inferred from seis mic tomography or subduction history, has been shown to account well f or the observed surface gravity field and motions of tectonic plates(1 -3). But the origin of certain large-scale features, such as the anoma lous elevation of the southern and eastern African plateaux, has remai ned controversial. Whereas the average elevation of most cratons is be tween 400 and 500 m, the southern African plateau stands more than 1 k m above sea level, with the surrounding oceans possessing a residual b athymetry in excess of 500 m (ref. 4). Global seismic tomography studi es have persistently indicated the existence of a large-scale low-velo city anomaly beneath the African plate(5-10) and here we show that man tle flow induced by the density variations inferred from these velocit y anomalies can dynamically support the excess elevation of the Africa n 'superswell'. We also find that this upwelling mantle flow-which is most intense near the core-mantle boundary-constitutes a significant d riving force for tectonic plates in the region.