Mechanisms of inheritance of rift faulting in the western branch of the East African Rift, Tanzania

Abstract

The western branch of the East African Rift system is commonly cited as a result of Phanerozoic reactivation of the Paleoproterozoic Ubendian belt in western Tanzania. Geological evidence is provided to show that prominent mechanical anisotropies successively appeared during Proterozoic evolution of the Precambrian basement and that their different reactivation behavior contributed to the Phanerozoic rift pattern. The Ubende belt (1950–1850 Ma) is a NW oriented, amphibolite facies ductile lateral shear belt in which older (2100–2025 Ma) and complex granulite facies terranes are included along trend. Retrograde multiphase sinistral strike‐slip mylonites developed along the NW oriented ductile shear belt. They reflect persistent Proterozoic wrench fault reactivation of the latter. Shallow level sedimentary basins upon and along the ductile shear belt display deformational structures attributable to the Proterozoic wrench fault reactivation. Neoproterozoic sinistral transpression produced the final geometrical pattern of the wrench fault zone, which appears as an elongate and NW trending positive flower structure, 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 Proterozoic wrench fault zone. The evaluation of the relationship between multiphase rift and multiphase prerift fabrics is reconsidered. The Proterozoic prerift fabrics correspond with a dextral transpressional and 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 these mylonites in the center or in the external parts of their NW oriented Neoproterozoic transpression, they reactivate as complex and multiphase rift faults or as normal and recent faults, respectively. The Paleoproterozoic NW oriented and ductile lateral shear belt constitutes the deep level mechanical anisotropy. Its reactivation in Phanerozoic stress fields is likely dextral oblique transtension, considered as a leading mechanism of the pluriphase and NW oriented deep rift basins.

Rheological heterogeneity, mechanical anisotropy and deformation of the continental lithosphere

“…Many examples are found in the literature: the North Atlantic rift in eastern North America (e.g., Wilson, 1966) and in Morocco (Piqué and Laville, 1996), the Rio Grande rift (Olsen et al, 1987), the Northeast China rift (e.g., Ma and Wu, 1987), the Baikal rift (e.g., Delvaux et al, 1995), the East African rift (Ring, 1994;Theunissen et al, 1996), the West African rift (e.g., Fairhead and Binks, 1991), the Cape Graben (Burke, 1976), and the Eastern Brazilian rift (Chang et al, 1992). Recent shear wave splitting surveys in the Kenya and the Baikal rifts (Gao et al, 1997) show that at the rift margin, where large-scale modification of the lithospheric mantle by upwelling asthenospheric mantle is unlikely, fast shear waves are polarized parallel to the trend of the Mozambique and Mongolian orogenic belts respectively (Fig.…”

Lithogeochemistry, geochronology and geodynamic setting of the Lupa Terrane, Tanzania: Implications for the extent of the Archean Tanzanian Craton

“…The proposed temporally 624 distinct deformation events are only those that are readily distinguished in the field and it is expected 625 that Paleoproterozoic structures have been reactivated during tectonism that has continued to the 626 present day (Theunissen et al, 1996). 627…”

Palaeoproterozoic to Neoproterozoic growth and evolution of the eastern Congo Craton: Its role in the Rodinia puzzle

“…1). The belt has been subdivided into various elongated blocks, which are bound by NW-SE oriented faults or shear zones, and which have contrasting lithologies and structural features (McConnell, 1972;Daly, 1988;Daly et al, 1989;Lenoir et al, 1994;Theunissen et al, 1996). The structural trends within these blocks generally follow a NW-SE trend, and developed under amphibolite facies conditions (Sklyarov et al, 1998).…”