MECHANISMS OF INHERITANCE OF RIFT FAULTING IN THE WESTERN BRANCH OF THE EAST-AFRICAN RIFT, TANZANIA

The western branch of the East African Rift system is commonly cited a s a result of Phanerozoic reactivation of the Paleoproterozoic Ubendia n belt in western Tanzania. Geological evidence is provided to show th at prominent mechanical anisotropies successively appeared during Prot erozoic evolution of the Precambrian basement and that their different reactivation behavior contributed to the Phanerozoic rift pattern. Th e Ubende belt (1950-1850 Ma) is a NW oriented, amphibolite facies duct ile lateral shear belt in which older (2100-2025 Ma) and complex granu lite facies terranes are included along trend. Retrograde multiphase s inistral strike-slip mylonites developed along the NW oriented ductile shear belt. They reflect persistent Proterozoic wrench fault reactiva tion of the latter. Shallow level sedimentary basins upon and along th e ductile shear belt display deformational structures attributable to the Proterozoic wrench fault reactivation. Neoproterozoic sinistral tr anspression produced the final geometrical pattern of the wrench fault zone, which appears as an elongate and NW trending positive flower st ructure, locally enhanced by late Proterozoic contraction. Phanerozoic rifting is demonstrated by others to occur in three distinct episodes , during which the complex rift segment formed upon the multiphase Pro terozoic wrench fault zone. The evaluation of the relationship between multiphase rift and multiphase prerift fabrics is reconsidered. The P roterozoic prerift fabrics correspond with a dextral transpressional a nd ductile deformational pattern, which became selectively reactivated by sinistral transpressional ductile- brittle mylonites. Proterozoic mylonites constitute shallow level mechanical anisotropies and define the general trend of the rift faults. According to the position of the se mylonites in the center or in the external parts of their NW orient ed Neoproterozoic transpression, they reactivate as complex and multip hase rift faults or as normal and recent faults, respectively. The Pal eoproterozoic NW oriented and ductile lateral shear belt constitutes t he deep level mechanical anisotropy. Its reactivation in Phanerozoic s tress fields is likely dextral oblique transtension, considered as a l eading mechanism of the pluriphase and NW oriented deep rift basins.