Geophysical models of the Kenya rift

Abstract

Geophysical models of the Kenya rift before 1990, based on gravity and seismic data, showed that the crustal structure beneath the rift is different’ from that beneath the flanks. The early pendulum gravity measurements of Bullard (1936) revealed that the East African plateau is in isostatic equilibrium and that the adjoining Western and Eastern rifts are associated with broad negative anomalies. Subsequent work focussed on the presence within the broad negative of a narrow axial positive anomaly first identified, but not analysed by McCall (19671, in the vicinity of Menengai. This feature was mapped in some detail along the rift’s length by Searle (1970) between the equator and 1.25”s where it was found to be continuous but variable in amplitude. He explained the positive anomaly in terms of a dense intrusion about 20 km wide, with its top within about 3 km of the surface. Fairhead (1976) reinterpreted one of Searle’s profiles through Nairobi and Suswa using a different regional, which reduced the amplitude of the positive anomaly by half. His favoured model included a dense dyke injection zone 10 km wide intruding a 2 km thickness of basaltic lavas filling the rift floor below 2 km of less dense phonolite lavas. Baker and Wohlenberg (1971) reinterpreted one of Searle’s profiles just south of Menengai to obtain a much-quoted model for the crust and