H. Walford and N. White

Bullard Laboratories, University of Cambridge, UK

ABSTRACT: Sedimentation, Vertical Motions and Mantle Convection on the African Continent

Africa differs from other continents in that many of its important geological features are thought to be dynamically supported by mantle convection (e.g. Holmes, 1944; Hartley & Allen, 1994; Nyblade & Robinson, 1994; Burke, 1996; Ebinger & Sleep, 1998). The pattern of convection is inferred from long wavelength free-air gravity anomalies. Lithospheric variations (e.g. changes in crustal thickness) generate short wavelength (<1000 km) gravity anomalies, while the longer-wavelength variations reflect sub-lithospheric processes. Long wavelength gravity anomalies coincide with features such as the Afar "hotspot", the anomalously high topography of Southern Africa and the Congo Basin (Democratic Republic of Congo). Models of dynamic topography (elevation supported by mantle convection) predict vertical motions of up to 1 km or more on the continents (e.g. Grand et al., 1997). If this is the case, dynamic topography should be identifiable in the geology and geomorphology, and be distinct from vertical motions induced by other mechanisms, such as rifting. By looking for relationships between the distribution of sediment and the pattern of convection, the contribution of mantle convection to drainage development and continental topography can be quantitatively assessed.

Africa is unusual in that it is almost entirely surrounded by passive continental margins. Large thicknesses of sediment (up to ~10 km) have accumulated on the continental shelf at the sites of major deltas and have remained largely undisturbed throughout the Cenozoic. The offshore stratigraphy should provide an indirect record of the pattern of vertical motions onshore, as variations in sediment flux to the shelf reflect topographic changes in the catchment area. The most important factors governing the sediment load of a river appear to be basin area and topography (Milliman & Syvitski, 1992). Factors such as variations in climate are an order of magnitude less important. Hence, major changes in palaeo-flux can be attributed to changes in the average elevation of the basin and/or variations in the size of the catchment area, due to capture or loss of significant tributaries. Half of Africa is drained by just ten rivers, so by analysing the sediment flux to a few major deltas the drainage history of much of the continent can be reconstructed.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado