The influence of pre-existing structures on the evolution of the southern Kenya Rift Valley — evidence from seismic and gravity studies

Abstract

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 uplift and faulting, the influence of pre-existing lithospheric structural controls is poorly understood. The interpretation of a 430-km-long seismic refraction and gravity line across the southern part of the Kenya Rift shows that the rift is developed across a transition zone, thought to represent the sheared Proterozoic boundary between the Archaean Nyanza Craton and the mobile Mozambique Belt. This zone of weakness has been exploited by the recent thermal rifting event. The Moho is at a depth of 33 km beneath the Archaean craton in the western part of the profile, and 40 km beneath the Mozambique Belt in the east. A few kilometres of localised crustal thinning has developed across the transition from thin to thick crust. At the surface, brittle faulting has formed an asymmetric rift basin 3.6 km deep, filled with low-velocity volcanic rocks. Basement velocities show a transition across the same area from low velocities (6.0 km s−1) in the Archaean, to high velocities (6.35 km s−1) in the Proterozoic. Mid-crustal layers show no deformation that can be attributed to the rifting event. Poorly constrained upper mantle velocities of 7.8 km s−1 beneath the southern rift confirm the continuation of the axial low-velocity zone imaged in previous seismic experiments. This is interpreted as the effect of small degrees of partial melt caused by elevated mantle temperatures. Gravity modelling suggests a contribution to the Bouguer anomaly from below the Moho, invoking the need for deep density contrasts. The regional gravity gradient necessary to model the Bouguer anomaly is used as supporting evidence for mantle-plume type circulation beneath the uplifted East African Plateau to the west of the Kenya Rift.