Erosion and tectonics in the East African Rift System

Abstract

Summary From consideration of the Karroo and Jurassic structures within the Rift Zone it is suggested that major folding that took place in later Jurassic time raised locally to great height the resistant pre-Karroo complex, out of which, in successive cycles, the high-level residual plateaux have been carved. In general, throughout the Rift Zone the surviving larger tracts of Karroo and Jurassic sediments still occupy low-lying areas representing the original major synclines, but in some cases trough-faulting that succeeded the folding carried down into the elevated areas strips of sediments which were subsequently eroded to form ancestral " rifts "; within these " rifts " much of the Pleistocene rifting took place and thus formed narrow lake basins within an ancient topography. The pattern of the ancient residuals on the main watersheds of the Rift Zone indicates that these watersheds were original features on the Miocene peneplain, inherited from the great framework of the Jurassic folds. Until the rifting of about the Middle Pleistocene carried down the floor of the Nyasa and Tanganyika troughs to form the existing lakes, the history and form of these troughs differed in no essential respect from that of the Karroo troughs of the Luangwa Valley and the middle and lower Zambezi. These troughs had been developed in their present form by a prolonged sequence of planation, continental uplift, and erosion that intervened between the fracturing of the post-Karroo period and that of the Pleistocene. The Pleistocene rifting accordingly took place along existing troughs of ancient origin of which the form was accentuated by the erosion of the Miocene and Pliocene cycles. Similarly, many of the Pleistocene rift fractures of Tanganyika Territory followed ancient fault-line scarps and troughs. The major rifts of the northern part of the Rift System, including the Abyssinia Rift, also show a pronounced pre-Pleistocene phase of ancient origin preceding the Pleistocene rifting, and in two localities there is evidence, too, of movements of Lower Miocene age