Abstract:
The deep basins, uplifted flanks, and volcanoes of the Western and Kenya rift
systems have developed along the western and eastern margins of the 1300 km-wide East
African plateau. Structural patterns deduced from field, Landsat, and geophysical studies
in the Western rift reveal a series of asymmetric basins bounded by approximately 100 km-long
segments of the border fault system. These basins are linked by oblique-slip and
strike-slip faults cross-cutting the rift valley. Faults bounding the Kenya and Western rift
valleys delineate two north-south-trending, 40-75 km wide zones of crustal extension, and
little or no crustal thinning has occurred beneath the uplifted flanks or the central plateau.
In the Western rift, volcanism in Late Miocene time began prior to or concurrent with
basinal subsidence, followed by rift flank uplift. Individual extensional basins developed
diachronously, and basinal propagation may give rise to the along-axis segmentation of the
rift valley. The coherence between gravity and topography data indicates that the
mechanical lithosphere beneath the two rift valleys has been weakened relative to the central
plateau and adjacent cratonic regions. Gravity and topography data at wavelengths
corresponding to the overcompensated East African plateau can be explained by density
variations within the upper mantle that are dynamically maintained.
