Abstract

The Western rift, the western branch of the East African rift system, is bordered by high-angle normal fault systems bounding one side of spoon-shaped basins. Depth-to-detachment estimates of 20-30 km, the rollover geometry of asymmetric basins, and seismicity throughout the depth range 0-30 km suggest that planar border faults along one side of rift basins penetrate the crust. Along the length of the rift, ∼100-km-long en echelon border-fault segments are linked by obliqueslip transfer faults, ramps, and monoclines within comparatively high-strain accommodation zones. These transfer faults accommodate significant along-axis variations in the elevation of basins and uplifted rift flanks but do not appear to extend outside the 40- to 70-km-wide rift basins across the uplifted flanks. Kinematic constraints from several basins indicate a regional east-west extension direction that is consistent with existing seismicity data. Estimates of crustal extension based on fault geometries interpreted from seismic reflection data and field observations within 13 basins are less than 15% (<10 km). Volcanism began at ∼12 Ma in the north and at ∼7 Ma in the south prior to or concurrent with the initial development of Western rift sedimentary basins. During Pleistocene time, asymmetric basins have narrowed through progressive hanging-wall collapse and with uplift of the rift flanks. These geometric and temporal patterns of development are similar to those of the more magmatically active Kenya rift, where volcanic activity began ∼11 m.y. prior to initial volcanism in the Western rift. Both the poor correlation between Miocene-Recent border and transfer faults and pre-Miocene shear zones, and the repetitive basin geometries indicate that the along-axis segmentation generally is not inherited. Western rift border-fault segments propagated to the north and south to link originally isolated basins, and this along-axis propagation contributes to the segmentation of the Western rift valley.

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