Geophysical experiments and models of the Kenya Rift before 1989

Abstract

Geophysical data acquired in the last 50 years have led to increasingly complex models of the deep structure and origin of the Kenya Rift since Bullard argued that the broad negative isostatic anomalies he observed could be best explained in terms of compression. All subsequent models invoked tension. Detailed gravity surveys showed that the rift axis was associated with an intermittent narrow positive anomaly and a variety of models involving an upwarp of the asthenosphere and intrusion of the crust have been proposed and refined in the light of seismic refraction, teleseismic and local earthquake data. All the models using seismic and gravity data show that the crustal structure beneath the rift is different from that beneath the flanks. The 1968 refraction profile along the axis in the north showed that the crust is only 20 km thick and underlain by mantle with the anomalously low velocity of 7.5 km/s. This velocity was also observed beneath the more detailed 1985 profile in the south where the thickness was over 34 km. These complex data along with those from studies of heat flow, electrical conductivity, uplift, and gravity point to the need for high resolution refraction experiments to obtain the crustal thickness and velocity-depth structure within and outside the rift and teleseismic studies to further understand the deeper structure and processes beneath the rift.