THE TECTONIC AND METAMORPHIC HISTORY OF THE PRE-CAMBRIAN ROCKS OF THE MBEYA REGION, SOUTH–WEST TANGANYIKA

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Abstract
The Mbeya region is situated at the branching of the western and eastern rift valleys. The Mbeya section of the western, or Rukwa, rift is bounded on its north-east flank by the Mbeya range, which is formed predominantly of quartzo-feldspathic cataclastic rocks. These were developed from the older Ubendian rocks of the region by axial-plane shear in a north-westerly trending fold-system provisionally correlated with the Ukingan orogeny. A north-westerly trending zone of lateral shear separates the Mbeya range from the Ubendian rocks of the Lupa area to its north-east. On the south-west flank of the western rift valley there is a further zone of mylonitic rocks which also trend north-westwards. Cataclastic rocks are thus widely developed and are predominant among the Pre-Cambrian rocks of the area. The north-westerly trend of the Pre-Cambrian rocks is closely followed by the younger structures of the western rift valley, but the Usangu scarp cuts across the older structures of the eastern rift valley.