Evidence from Gravity Anomalies for Thinning of the Lithosphere beneath the Rift Valley in Kenya

Abstract

A detailed gravity survey of part of the Gregory Rift Valley in Kenya has shown a positive Bouguer anomaly over the rift floor between 0.25° N and 1.25° S. The anomaly is between 40 and 80 km wide and has an amplitude of 300‐600 g.u. (30‐60 mgal). Attempts to account for the anomaly with shallow mass‐distributions lead to geologically un‐reasonable models. The geophysical and geological data are best satisfied if the anomaly is due to a dense intrusion. The required body must be about 20 km wide, and its upper surface must be less than 3 km below the land surface. Other evidence in favour of this interpretation is presented, and it is shown that the models developed here are consistent with the crustal models of the area deduced from long‐wavelength gravity anomalies by other workers. An intrusion of the magnitude inferred in this paper represents extreme thinning of the lithosphere below the rift valley. Copyright © 1970, Wiley Blackwell. All rights reserved