MULTIPLE EPISODES OF RIFTING IN CENTRAL AND EAST-AFRICA - A REEVALUATION OF GRAVITY-DATA

A compilation of new and existing gravity data, as well as geophysical and geological data, is used to assess the cumulative effects of mult iple rifting episodes on crustal and upper mantle density structures b eneath the Uganda - Kenya - Ethiopia - Sudan border region. This compi lation includes new gravity and geological data collected in 1990 in s outh-western Ethiopia. Variations in the trends and amplitudes of Boug uer gravity anomalies reveal three overlapping rift systems: Mesozoic, Paleogene and Miocene-Recent. Each of these rift systems is a number of 40-100 km long sedimentary basins, and each system is approximately 1000 km long. The Bouguer anomaly patterns indicate that the Ethiopia n and East African plateaux and corresponding gravity anomalies are di screte tectonic features. Models of structural and gravity profiles of two basins (Omo and Chew Bahir basins) suggest that pre-Oligocene (Cr etaceous?) strata underlie 3 km or more of Neogene- Recent strata with in the northern Kenya rift, and that more than 2 km of Neogene - Recen t strata underlie parts of the southern Main Ethiopian rift. The super position of perhaps three rifting episodes in the Lake Turkana (Omo) r egion has led to 90% crustal thinning (beta approximate to 2).