Karel Theunissen

The Palaeoproterozoic Ubendian shear belt in Tanzania: geochronology and structure

The Ubendian belt is a linear, NW-SE trending orogenic belt in western Tanzania. It is part of a larger Palaeoproterozoic orogen, developed around the west and south-western margin of the Archaean Tanzanian craton. The Ubendian Belt has experienced several periods of reactivation since the Palaeoproterozoic, acting as a zone of displacement during successive orogenic and rift-forming events. The Ubendian Belt is characterized by an early deformation and granulite-facies metamorphism, isotopically dated at 2100–2025 Ma, and marked by an E-W to ESE-WNW trending foliation. This phase also affected the adjacent Usagaran (and Bangweulu?) Belt and is interpreted as a product of collisional orogeny along the SW margin of the Tanzanian, and possibly Congo, cratons. A second phase of deformation, apparently restricted to the Ubendian Belt, is characterized by large, NW-SE trending, dextral shear zones. This phase is responsible for the creation of the eight crustal blocks developed throughout the belt and overprints much of the earlier deformation fabric. This second deformation phase is terminated by late- to post-kinematic calc-alkaline granitic batholiths (ca 1860 Ma). A phase of tectonic reactivation occurred locally at ca 1725 Ma. Notable by its absence, is any evidence of Kibaran isotopic ages in the Ubendian Belt. Major Kibaran orogenic belts occur to the north (Burundi) and south (Zambia) of the Ubendian Belt, although they are apparently not isotopically recorded within it. A third phase of deformation is characterized by Neoproterozoic (ca 750 Ma) reactivations of the Ubendian shear zones. This deformation is marked by the development of sinistral, brittle-ductile shear zones, displaying retrograde metamorphic mineral assemblages, and intruded by alkaline plutons. These late shear zones were the preferential locus for the brittle rift faults of the western branch of the East African Rift.

Persistent fault controlled basin formation since the Proterozoic along the Western Branch of the East African Rift

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 belived 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 Palaeoproterozoic 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.