ISOSTASY OF AFRICA

Bouguer gravity anomaly and topography data have been used to estimate the effective elastic thickness, T-e, of the African lithosphere. The highest values (T-e > 100 km) correlate with relatively old Archaean cratonic regions and the lowest (0 < T-e < 10) with younger late Palae ozoic fold-belts and Mesozoic/Tertiary rift systems. No simple relatio nship exists, however, between T-e and the age of the lithosphere at t he time of loading. Cratons (e.g., West Africa) and fold-belts (e.g., Atlas) of similar thermal age display both high and low T-e values. A good correlation exists between T-e and the present-day surface heat f low over Africa: regions of low T-e correlate with high heat flow wher eas high T-e regions have low heat flow. Thus, continental T-e depends more on the present-day geotherm and, hence composition, than on the cooling history of the lithosphere. The T-e and heat-flow data over th e West Africa craton, Kaapvaal-Zimbabwe craton, Damara fold-belt, Nige r basin, and the Central African Plateau, for example, can be explaine d by a thermal and mechanical model in which T-e is given approximatel y by the depth to the 400 degrees C isotherm. We have used the T-e str ucture of Africa to isolate that part of the gravity field which canno t be accounted for by flexure and must be caused by other processes. T he resulting anomalies, which have amplitudes of up to +/-25 mGal and wavelengths of about 2000 km, correlate with regional changes in topog raphy. There is a suggestion of a lineation of the long-wavelength gra vity field in the direction of absolute African plate motion which, if correct, implies that even beneath the slow moving plates there may b e some alignment of motions in the underlying mantle.