A model for rift development in Eastern Africa

Abstract

A tectonic survey has been completed in Eastern Africa, using remote sensing as well as structural analysis in the field. New data have been collected in the eastern and western branches of the East African Rift system. The geomorphology suggests that the main deformation occurred in the two principal branches of the East African Rift system, but compilations of earthquake data show that recent movements are widespread over a large area in Eastern Africa. Many ancient structures have been reactivated during Neogene times, especially large Precambrian lineaments which are evident on satellite imagery. The Tanganyika–Rukwa–Malawi lineament, along its segment between Lakes Tanganyika and Malawi, now serves as an intracontinental transform zone, with folds and transcurrent faults. The Aswa lineament is, at the present time, only partly reactivated between Lake Mobutu and the Gregory Rift, and also along its southeastern continuation to the Indian Ocean. In many areas, tension gashes or striations on the slickensides have given the local palaeostress orientation. It appears to be changing with time, but the most typical orientation indicates horizontal extension striking NW–SE or WNW–ESE. At the earliest stage of rifting compression was horizontal, then it later became vertical. Compilation of earthquake foci from publications, confirms these results. All these observations together with data concerning the geometry of the deformation, as well as the mechanisms of the Cenozoic intracontinental deformation, implies a near NW–SE movement of the Somalian block, relative to the African continent. A dynamic tectonic model for the rift opening and propagation of the main fracturing across Africa is proposed. It suggests that opening is more important in the north than in the south. Four stages can be identified in the evolution of the East African Rift. The pre-rift stage corresponds to a dense fracturing, characterized by strike-slip faults, while a shallow but wide depression formed, and open tension gashes gave way to tholeiitic volcanism. The initial rifting stage is recognized by oblique-slip faults, bounding tilted blocks, but the uplift of the rift shoulders is not very important. The following ‘typical’ rift formation stage is associated with normal faults bordering the main tilted blocks, while subsidence of the rift floor and uplift of the shoulders are important. The advanced rifting stage, shown by important magmatic intrusions along the rift axis, corresponds to the initial formation of oceanic crust. These stages correspond to different present day aspects of various rift segments in Eastern Africa.