Pergamon Journal of African Earth Sciences, Vol. 26, No.3, pp. 347-361, 1998 c 1998 Elsevier Science Ltd All rights reserved. Printed in Great Britain PI/:S0899-5362(98)00020-7 0899-5362198 $19.00 + 0.00 Persistent fault controlled basin formation since the Proterozoic along the Western Branch of the East African Rift J. KLERKX, K. THEUNISSEN, and D. DELVAUX Department of Geology and Mineralogy, Royal Museum of Central Africa, B-3080 Tervuren, Belgium Abstract- The Western Branch of the East African Rift System is outlined by elongate sedimentary basins, frequently occupied by Cenozoic rift lakes. The role of the inheritance of the leading rift faults from pre-existing basement structures has often been invoked. Recent studies in western Tanzania confirm the extent of the northwest orientated Palaeoproterozoic Ubende Belt contribution to the Phanerozoic Rift. Attention is drawn here on the occurrence of different Meso- and Neoproterozoic sedimentary basins that developed along the ductile shear belt as a result of repeated sinistral wrench fault reactivation. These basins partly overlap each other and typically bear shallow and weakly evolved sediments. North of the Ubende Belt, the Mesoproterozoic Kibara Belt is inferred to have originated as a basin controlled by the complex termination of the Ubende wrench fault. Phanerozoic rift basins also develop along the northwest orientated Ubende Belt structure. They display the same elongate shape as the Proterozoic basins. In Late Palaeozoic-Early Mesozoic the Karoo rift basins formed from a dextral lateral shear reactivation of the inherited Proterozoic shear faults. During the first phase of development the Lake Tanganyika Basin is believed to bear the same characteristics as all previous basins along the Ubende Shear Belt, mainly controlled by strike-slip movements along pre-existing shear faults. The present Lake Tanganyika Basin is subdivided in two sub-basins, separated by the transverse Mahali Shoal, which is an active structure located on the Ubende Shear. The deep lake basin mainly developed outside the Ubende Belt. The northern sub-basin appears to be structurally controlled by the reactivation of the Mesoproterozoic sinistral wrench fault termination of the Ubende shear faults. Structural control of the Palaeo proterozoic basement is however unclear for the southern sub-basin of Lake Tanganyika: this part of the rift segment is flanked by Palaeoproterozoic basement which has not been affected by the Ubende Shear. © 1998 Elsevier Science Limited. Resume-La branche occidentale du rift est-africain est caracterisee par une serie de bassins sedimentaires. souvent occupes par des lacs de rift. L'influence des structures pre-existantes du socle comme heritage des structures principales du rift a ete souvent invoquee. Des etudes recentes en Tanzanie occidentale confirment I'importance de la contribution des structures paleoproterozoiques orientees NW-SE de la chaine ubendienne aux structures phanerozoiques du rift. L'attention est attires ici sur la presence de bassins sedimentaires d'age meso- a neoproterozoique, qui se sont developpes Ie long de la zone de cisaillement ductile, suite a des reactivations decrochantes senestres repetees, Ces bassins se superposent partiellement et contiennent typiquement des sediments peu evotues, d'eau peu profonde. Au nord de la chaine ubendienne, la chaine rnesoproterozoique Kibarienne est consideree comme issue d'un bassin controle par la termination complexe du svsterne decrochant ubendien. Les bassins phanerozoiques se developpent aussi Ie long de la structure NW-SE de la chaine ubendienne. lis ont la merne forme allonqee que les bassins proterozoiques. Journal of African Earth Sciences 347