S-WAVE VELOCITY STRUCTURE OF THE CRUST AND UPPER MANTLE BENEATH KENYA IN COMPARISON TO TANZANIA AND ETHIOPIA: IMPLICATIONS FOR THE FORMATION OF THE EAST AFRICAN AND ETHIOPIAN PLATEAUS

New estimates of the S-wave velocity structure of the crust and upper mantle beneath the Kenya Rift and surrounding Kenya Highlands have been obtained by jointly inverting P wave receiver functions and Rayleigh wave phase and group velocities. The joint inversion was performed for seven broadband seismic stations, five that were part of the Kenya broadband seismic experiment and two belonging to the Global Seismic Network. 1-D S-wave velocity models obtained from the inversions have been compared to similar models for Tanzania and Ethiopia to identify differences in lithospheric structure beneath the East African and Ethiopian Plateaus. The S-wave velocity structure of the upper mantle to at least 100 to 150 km depth beneath the Kenya Highlands is similar to structure under the East African Plateau in Tanzania. It therefore appears that the plateau lithosphere under the Kenya Highlands has not been affected thermally to any greater extent than the plateau lithosphere beneath Tanzania. Under the Kenya Rift, however, the lithosphere has been substantially modified. S-wave velocities at depths of 80 to 150 km beneath the rift are 8 to 10% lower compared to under the Kenya Highlands, which may be a sufficiently large change to indicate the presence of partial melt. There is little similarity between the plateau lithosphere in East Africa and Ethiopia. The lithosphere under the Ethiopian Plateau is thin, extending to a depth of no more than about 80 to 90 km depth, compared to 100 to 150 km beneath Tanzania and the Kenya Highlands. The maximum S-wave velocity in the lithosphere is also very low, reaching only to 4.2 to 4.3 km/s, compared to 4.6 to 4.7 km/s beneath Tanzania and the Kenya Highlands. These differences indicate that the buoyant support for the plateau elevation in East Africa, including the Kenya Highlands, resides at deeper depths in the mantle than beneath the Ethiopian Plateau.