A Comparison between the Older (Karroo) Rifts and the Younger (Cenozoic) Rifts of Eastern Africa

Abstract

The older rifts or troughs are confined to southern Africa and extend northwards into Zambia and southern Tanzania. They were primarily infilled with Karroo sediments, with a final capping¹ of basaltic volcanics, remarkably similar in sequence to that seen in the main Karroo basins of South Africa and Botswana, despite their mutual isolation. Subsidence by downwarping was contemporaneous with deposition, although, faulting was prominent particularly in late or post-Karroo times. The trends of the troughs are often SW-NE and side trends NW-SE, suggesting a stress system at right angles to this direction; nevertheless, there is a common alignment with older Precambrian ‘grains’. They were excavated and partly refilled by Cretaceous deposits and their present relief is the result of erosional exhumation. Volcanism that can be regarded as directly associated with the rifts is rare. The rifts were already ‘dead’ before the separation of Africa as a continent and, indeed, comparable rifts in west Africa and S. America have been transected by continental separation. The courses of the Zambesi and its tributaries, the Luangwa and Kafue, illustrate not only the typical diversity of trends of the troughs, but also the effects of later exhumation: indeed, in places, as at the Kariba gorge, a block of underlying basement has been reached and transected. The extent of possible marginal uplifts to the troughs cannot now be judged because of later events of uplift and erosion, but it is notable that the Zambesi has maintained its course to the sea despite continued uplift of the eastern side of Africa.