The Structure of the Western Flank of the Gregory Rift. Part II. The Mantle

Slowness and delay time data for teleseismic P-wave arrivals recorded at the temporary seismic array station at Kaptagat on the western flank of the Gregory Rift are presented. The data are used to elucidate the shape and structure of the low velocity zone in the mantle shown from surface-wave dispersion data to underlie the East African Rift. The data suggest that a lateral variation in velocity associated with a steeply dipping upper surface for the zone is the principal cause of the large slowness anomalies observed at Kaptagat. This upper surface is mapped from the slowness data to show a broad ellipsoidal structure lying at some 150 km depth on which is superimposed at steep-sided structure connecting it to the crustal intrusion along the Rift axis. The zone appears to be attenuated not only westwards away from the Rift axis but also northwards to correspond to the dying out of the Rift in Northern Kenya.