PERSISTENT FAULT CONTROLLED BASIN FORMATION SINCE THE PROTEROZOIC ALONG THE WESTERN BRANCH OF THE EAST-AFRICAN RIFT

The Western Branch of the East African Rift System is outlined by elon gate sedimentary basins, frequently occupied by Cenozoic rift lakes. T he role of the inheritance of the leading rift faults from pre-existin g basement structures has often been invoked. Recent studies in wester n Tanzania confirm the extent of the northwest orientated Palaeoproter ozoic Ubende Belt contribution to the Phanerozoic Rift. Attention is d rawn here on the occurrence of different Meso-and Neoproterozoic sedim entary basins that developed along the ductile shear belt as a result of repeated sinistral wrench fault reactivation. These basins partly o verlap each other and typically bear shallow and weakly evolved sedime nts. North of the Ubende Belt, the Mesoproterozoic Kibara Belt is infe rred to have originated as a basin controlled by the complex terminati on of the Ubende wrench fault. Phanerozoic rift basins also develop al ong the northwest orientated Ubende Belt structure. They display the s ame elongate shape as the Proterozoic basins. In Late Palaeozoic-Early Mesozoic the Karoo rift basins formed from a dextral lateral shear re activation of the inherited Proterozoic shear faults. During the first phase of development the Lake Tanganyika Basin is believed to bear th e same characteristics as all previous basins along the Ubende Shear B elt, mainly controlled by strike-slip movements along pre-existing she ar faults. The present Lake Tanganyika Basin is subdivided in two sub- basins, separated by the transverse Mahali Shoal, which is an active s tructure located on the Ubende Shear. The deep lake basin mainly devel oped outside the Ubende Belt. The northern sub-basin appears to be str ucturally controlled by the reactivation of the Mesoproterozoic sinist ral wrench fault termination of the Ubende shear faults. Structural co ntrol of the Palaeoproterozoic basement is however unclear for the sou thern sub-basin of Lake Tanganyika: this part of the rift segment is f lanked by Palaeoproterozoic basement which has not been affected by th e Ubende Shear. (C) 1998 Elsevier Science Limited.