SOME REMARKS ON THE STRUCTURE AND GEODYNAMICS OF THE KENYA RIFT

Since 1985 the KRISP working group has recorded a number of seismic profiles and carried out array experiments in the Kenya Rift valley. Long range profiling has shown that across the Rift at latitude 0.5-degrees, the crust thins from 40 km beneath the western flank to 30 km beneath the graben proper, and thickens again to 35 km beneath the eastern flank. The crustal thickness beneath the graben decreases northwards from 35 km beneath the culmination of the Kenya dome to 20 km under Lake Turkana, suggesting a change to a highly extended terrain in the northern Kenya Rift. Furthermore, the anomalous low mantle velocities of 7.5-7.6 km/s suggest that the upper mantle immediately beneath the Rift may contain reservoirs of magmas, which were generated at greater depth. The anomalous mantle under the graben proper, as seen by the tomographic imaging, is characterized by a large, steep walled low velocity zone with a velocity decrease of at least 10% in the central part between 65 and 100 km depth which reaches down to more than 150 km. A comparison of the shape and size of the observed Bouguer anomaly with the LVZ suggests that the upper mantle beneath the Rift probably includes some 5% partial melt, concentrated in the areas with lowest velocities. These results provide new constraints for geodynamic models of the evolution of the Kenya Rift.