Three representative basins in the Western rift system of East Africa are bordered along one side by high-angle normal faults with 2- to 5-km throws (border faults). In plan view ∼100-km-long systems of linear border faults form curvilinear border fault segments bounding the East Kivu, West Kivu, and Rusizi basins. The opposite sides of these asymmetric basins are bounded by lower relief faulted monoclines or en echelon ramps. The largely unfaulted rift flanks have been uplifted 2 km above the 1.3-km-high East African plateau, with uplift narrowing basins during Quaternary time. Maximum estimates of ∼E-W crustal extension within basins are less than 25% (< 16 km), and planar border faults may penetrate the crust. The East Kivu and West Kivu basins are linked across the rift valley by a horst that serves as a hinge for subsidence in both basins. The westward tilted East Kivu and eastward tilted Rusizi border fault segments are linked along the rift by oblique-slip transfer faults that also accommodate along-axis differences in elevation. Upper Miocene-Recent eruptive volcanic centers within the comparatively high-strain interbasinal region (accommodation zone) generally coincide with the tips of border fault segments and transfer faults. The orientations of Miocene-Recent dip-slip and oblique-slip faults show little correlation with Precambrian shear zones or foliations in metamorphic basement. Differences between the East Kivu, West Kivu, and Rusizi basins in the age of initial faulting, subsidence, and age/composition of volcanic products suggest that border fault segments developed diachronously and propagated along the length of the rift. This along-axis border fault propagation and the crosscutting geometry of transfer faults contribute to the segmentation of the Western rift valley.
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