The isostatic compensation of East Africa

Abstract
By means of the methods of Lewis & Dorman (1970), the isostatic response has been computed for an area 800 km square of East Africa, surrounding the Kenya rift valley. By applying linear programming to the inversion of these data, it can be shown that local compensation models involving only negative density contrasts fit the response data at confidence levels of up to 80%. Comparison of the gravity anomalies of the compensation with the results of seismic array measurements made by Long & Backhouse (1976) shows that the East African plateau is compensated differently from the highland region around the rift valley known as the Kenya dome. The dome is supported by a region of the mantle with low density, low seismic P velocity, and high electrical conductivity, almost certainly a zone of partial melting. The linear programming technique can be used to show that, to satisfy the data, the top of the anomalous zone must be no deeper than 55 km. If the density anomaly is entirely due to melting, the average melt concentration in the uppermost 50 km of the mantle must exceed 6%.