A reappraisal of epeirogenic flexure axes in southern Africa

Major drainage divides in southern Africa are interpreted to reflect lines of epeirogenic flexuring of the subcontinent associated with the formation of co-related basins. The Great Escarpment, which separates coastal and inl and drainage systems, marks the locus of the Escarpment axis. It was initia ted by Early Cretaceous rift flank uplift associated with the break-up Gond wana. Geophysical studies suggest that subsequent erosion, coupled with sed imentation on the continental shelf, would have resulted in progressive inl and migration of this flexure. The divide between the Orange-Vaal River sys tem and the Limpopo and Molopo-Nossib-Auob drainage basins is designated th e Etosha-Griqualand-Transvaal (EGT) axis. Upper Cretaceous flexuring along this axis disrupted old drainage lines, and initiated deposition of the Kal ahari formation. The end-Cretaceous Ovamboland-Kalahari-Zimbabwe (OKZ) axis forms the watershed between the Zambezi and Limpopo Rivers in Zimbabwe, an d separates the latter river system from fossil endoreic drainage lines in the Kalahari, which originally emptied into the Makgadigadi Pans system. In the south of Botswana, this axis is defined by the Kalahari Schwelle, whic h separates the fossil Kalahari drainages from the Molopo-Nossib River syst em. Processes responsible for initiating the EGT and OKZ flexures are poorl y understood. However, the inferred ages of both these two axes and the Esc arpment axis correspond with episodes of alkaline volcanism in southern Afr ica. This argues for a link between continental flexuring and volcanic acti vity. Major Pliocene uplift occurred along a line intermediate between the Great Escarpment and the present coastline in the east of the country (the Ciskei-Swaziland axis). More subdued Plio-Pleistocene flexuring along a sou thwest-northeast axis (designated the Bushmanland-Harts axis) traversing th e interior of South Africa was responsible for the formation of major pans ('floors') in Bushmanland and the Orange Free State. There are a number of subordinate lines of uplift (the Khomas, Otavi, and Zoutpansberg axes) whic h are parallel to the Bushmanland-Harts axis. They are presumably related t o the same stress field, and thus probably of similar age. These latter axe s are all sub-parallel to active faults in northern Botswana which are inte rpreted to reflect southwestwards migration of the east African rift system , following lines of structural weakness. Sequential uplift along the axes which have been identified provides a framework for interpreting the evolut ion of drainages and erosion surfaces on the sub-continent.