Comparison of the Tanganyika, Malawi, Rukwa and Turkana Rift zones from analyses of seismic reflection data

Abstract

Northwest-southeast extension has opened the East African Rift System along two main branches, the Western and Eastern Branches. Rift zones along the Western Branch are marked by narrow lakes floored by thick piles of fluvial clastic and ‘pelagic’ sediment. Magmatism is restricted to a few small areas in the ‘arches’ between the lakes. In contrast, rift zones along the Eastern Branch are largely filled with volcanic and volcaniclastic materials and magmatism is generally perceived to be an integral part of the rifting process. In an attempt to sort out the significance and meaning of these and other differences, we have compared multifold seismic data from three Western Branch rift zones (Tanganyika, Rukwa and Malawi) and one Eastern Branch zone (Turkana). The Tanganyika and Malawi Rift Zones are composed of half-graben basins linked in complex ways by accommodation zones which generally trend oblique to the rift axes, and sometimes oblique to the extension direction. Half-grabens alternate basinal polarities where the rift crosses Proterozoic dislocation zones. Complex fault geometries are associated with some accommodation zones; elsewhere faults are almost exclusively planar. Sedimentary fill reaches at least 4–5 km and the section is mostly Cenozoic in age. Patches of Permo-Triassic sedimentary rocks are believed to occur within both rift zones. The Rukwa Rift is a pull-apart zone that connects the northern (Livingstone) basin of Lake Malawi to the Kalemie Basin in central Lake Tanganyika. The entire pull-apart system may be a series of down-to-the-east half-grabens. An accommodation zone develops along a short stretch of the Rukwa Rift, but no full polarity reversal occurs. The break-away faults of the Livingstone, Rukwa and Kalemie basins are essentially coincident with the Proterozoic Rukwa dislocation zone, which sub-parallels the inferred extension direction. Fault geometries in the Rukwa Rift are markedly listric, especially in the pre-Cenozoic section. Sedimentary fill ranges in age from pre-Karroo through Cenozoic and locally exceeds 10 km in thickness. The Turkana Rift is composed of short, linear, NNE-trending normal fault segments that are offset in a left-lateral sense by numerous, NW-SE trending transfer faults, linking facing border fault segments together. The overall trend of the rift zone is oblique to the opening direction, like the Tanganyika and Malawi cases, but the border fault segments are sub-perpendicular. Fault geometries are highly variable, but flower structures associated with transfer faults predominate. Igneous activity is ubiquitous and appears to be localized along the transfer faults. Basin fill reaches 4–5 km in thickness and is dominated by fluvial clastic, volcaniclastic and volcanic materials. The structural differences within the Tanganyika-Rukwa-Malawi system stem mainly from the modifying effects of pre-rift anistropies on strain expressions. Fundamentally, this system is a NW-SE trending series of single-polarity pull-apart basins. At the two ends of the pull-apart zone, the rift is deflected into more N-S trending basins which have a high tendency to alternate polarities along strike. This explanation does not account for the differences in fault forms between the Tanganyika-Malawi (planar) and Rukwa (listric) Rifts. For the time being, we presume these differences arise from systematic differences between Tanganyika-Malawi and Rukwa in the age ranges of the fill and/or the maximum depths of seismic imaging. Rifting in Turkana is profoundly different than in the Tanganyika-Rukwa-Malawi sub-branch and seems to involve a softer, more ductile, more organized style of extension which may be closer to the ideal case. In a thermal sense, rifting has progressed further in Turkana than along the Western Branch zones. This does not preclude original, fundamental difference in the thermal states of two branches.