SEISMIC STRUCTURE OF THE UPPERMOST MANTLE BENEATH THE KENYA RIFT

A major goal of the Kenya Rift International Seismic Project (KRISP) 1 990 experiment was the determination of deep lithospheric structure. I n the refraction/wide-angle reflection part of the KRISP effort, the e xperiment was designed to obtain arrivals to distances in excess of 40 0 km. Phases from interfaces within the mantle were recorded from many shotpoints, and by design, the best data were obtained along the axia l profile. Reflected arrivals from two thin (< 10 km), high-velocity l ayers were observed along this profile and a refracted arrival was obs erved from the upper high-velocity layer. These mantle phases were obs erved on record sections from four axial profile shotpoints so overlap ping and reversed coverage was obtained. Both high-velocity layers are deepest beneath Lake Turkana and become more shallow southward as the apex of the Kenya dome is approached. The first layer has a velocity of 8.05-8.15 km/s, is at a depth of about 45 km beneath Lake Turkana, and is observed at depths of about 40 km to the south before it disapp ears near the base of the crust. The deeper layer has velocities rangi ng from 7.7 to 7.8 km/s in the south to about 8.3 km/s in the north, h as a similar dip as the upper one, and is found at depths of 60-65 km. Mantle arrivals outside the rift valley appear to correlate with this layer. The large amounts of extrusive volcanics associated with the r ift suggest compositional anomalies as an explanation for the observed velocity structure. However, the effects of the large heat anomaly as sociated with the rift indicate that composition alone cannot explain the high-velocity layers observed. These layers require some anisotrop y probably due to the preferred orientation of olivine crystals. The s eismic model is consistent with hot mantle material rising beneath the Kenya dome in the southern Kenya rift and north-dipping shearing alon g the rift axis near the base of the lithosphere beneath the northern Kenya rift. This implies lithosphere thickening towards the north and is consistent with a thermal thinning of the lithosphere from below in the south changing to thinning of the lithosphere due to stretching i n the north.