MODELING THE 2-D SEISMIC VELOCITY STRUCTURE ACROSS THE KENYA RIFT

A 460-km-long seismic refraction/wide-angle reflection profile across the East African rift in Kenya has been interpreted using a travel-time inversion method to calculate a two-dimensional crustal and uppermost mantle seismic velocity model. The derived model is consistent with the crustal structure determined by independent interpretation of axial (along the rift) and flank (near the eastern end of the cross profile) data sets. The velocity model indicates that the Kenya rift at this location (near the Equator) is a relatively narrow (about 100 km wide) feature from surface expression (fault-bounded basins) to upper-mantle depths. A 5-km-deep, sediment- and volcanic-filled basin is present beneath the rift valley. Seismic velocities in the underlying crust are slightly higher directly beneath the rift valley than in the adjacent terranes. Additionally, the crust thins by about 8 km (to a thickness of about 30 km) in a 100-km-wide zone beneath the rift valley and anomalously low upper-mantle seismic velocity (Pn approximate to 7.6 km/s) is present only beneath the thinned crust and extends depths of greater than 120 km.