Seismic structure across the Kenya Rift Valley: Data analysis and geodynamic implications.

During the 1990 Kenya Rift International Seismic Project (KRISP 90) a 450 km E-W crustal refraction profile was undertaken across the Kenya Rift, a late Tertiary to Recent extensional feature associated with extensive volcanic activity. The P-wave data have been analysed using 2-D ray-tracing, finite difference and reflectivity dynamic modelling. A simultaneous velocity and travel time inversion has been applied to the forward model to test its uniqueness and resolution. The analyses show an asymmetric sedimentary basin which is thickest against the rift's major western boundary fault. The crustal velocities vary from 6.2 km/s in the Archaean craton to the west of the rift to about 6.0 km/s in the Proterozoic orogenic belt along the remainder of the profile. The crustal thickness outside the rift varies from 38 3 km adjacent to the rift's western margin and 34 2 km to the east. Beneath the rift itself the thickness is only 30 2 km. The upper mantle velocity is generally about 8.0 km/s except beneath the rift where it is consistently low at 7.6 - 7.8 km/s. This anomalously low velocity suggests a 5 - 6% partial melt. The combined seismic and gravity model supports the contention that convective processes in the mantle are dynamically supporting the uplifted East African Plateau. Kinematic and dynamic modelling of the S-wave field show that upper crustal phases have been recorded only outside the rift. Mid and lower crustal S arrivals do not seem to have been attenuated underneath the rift axis, precluding an extensive hot regime at lower crustal depth. A reflected phase is observed from an interface within the mantle beneath the western flank of the rift. Detailed analyses of this phase confirm the presence of a high velocity layer (8.4 km/s) below 60 km: compositional anomalies as well as crystal orientation have been suggested as an explanation for the observed velocity structure. This evidence may delimit the lateral extent of the upper mantle low velocity zone underneath the graben itself. A model of extension via simple shear in the upper crust and pure shear in the lower crust and upper mantle is suggested. The presence of a small diapir under the Kenya Rift, radiating from a 'weak' plume seated under the East African Plateau, is envisaged; the diapir appears to have spread asymmetrically towards the Proterozoic lithosphere to the east of the rift.