THE INFLUENCE OF PREEXISTING STRUCTURES ON THE EVOLUTION OF THE SOUTHERN KENYA RIFT-VALLEY - EVIDENCE FROM SEISMIC AND GRAVITY STUDIES

The Kenya Rift is an active continental rift that has developed since the Late Oligocene. Although a thermal origin for the rifting episode is indicated by the scale of volcanism and its relative timing with up lift and faulting, the influence of pre-existing lithospheric structur al controls is poorly understood. The interpretation of a 430-km-long seismic refraction and gravity line across the southern part of the Ke nya Rift shows that the rift is developed across a transition zone, th ought to represent the sheared Proterozoic boundary between the Archae an Nyanza Craton and the mobile Mozambique Belt. This zone of weakness has been exploited by the recent thermal rifting event. The Moho is a t a depth of 33 km beneath the Archaean craton in the western part of the profile, anti 40 km beneath the Mozambique Belt in the east. A few kilometres of localised crustal thinning has developed across the, tr ansition from thin to thick crust. At the surface, brittle faulting ha s formed an asymmetric rift basin 3.6 km deep, filled with low-velocit y volcanic rocks. Basement velocities show a transition across the sam e area from low velocities (6.0 km s(-1)) in the Archaean, to high vel ocities (6.35 km s(-1)) in the Proterozoic. Mid-crustal layers show no deformation that can be attributed to the rifting event. Poorly const rained upper mantle velocities of 7.8 km s(-1) beneath the southern ri ft confirm the continuation of the axial low-velocity zone imaged in p revious seismic experiments. This is interpreted as the effect of smal l degrees of partial melt caused by elevated mantle temperatures. Grav ity modelling suggests a contribution to the Bouguer anomaly from belo w the Moho, invoking the need for deep density contrasts. The regional gravity gradient necessary to model the Bouguer anomaly is used as su pporting evidence for mantle-plume type circulation beneath the uplift ed East African Plateau to the west of the Kenya Rift.