MESOZOIC AND EARLY TERTIARY RIFT TECTONICS IN EAST-AFRICA

A complex history of crustal extension occurred in east and central Africa during the Mesozoic and early Tertiary. Beginning in the Late Jurassic, this resulted in a large system of rifts, the Central African rift system, that spanned from central Sudan to southern Kenya. Late Jurassic rifting is best documented in the White and Blue Nile rifts of the Sudan, and records east-west extension in half-graben that were connected by large-scale shear zones and pull-apart basins. Early Cretaceous rifting re-activated Jurassic basins and spread to the large South Sudan rifts and Anza rift in Kenya. By the Late Cretaceous, the extension direction shifted to the NE-SW, and the presently observed large-scale rift geometry was established. In the early Tertiary, some Mesozoic basins were again reactivated, while other regions underwent wrench faulting and basin inversion. The large number of basins preserved in the Central African rift system can be used to construct an evolutionary model of continental rift tectonics. Early phases of extension at low strains produced alternating half-graben/accommodation zone geometries similar to those observed in most young and active continental rifts. At higher strains, some border faults were abandoned so that through-going, simpler active fault systems could evolve. This is interpreted as representing a switch from complex, oppositely dipping detachment structures, with strike dimensions of 50-150 km, to regional detachment structures that continue for hundreds of kilometers parallel to the rift. This change in the type of detachment was accompanied by a shift in the position of the subsidence away from the breakaway to a position focused further within the regional upper plate. Non-rotational, high angle, normal faulting dominates in the development of these late basin geometries. Deciphering similar rift basin histories from passive continental margins may, in many cases, exceed the limits of available reflection seismic data. East-west striking shear zones, including the Umm Hani and South Sudan shears, served as basin-terminating Structures in the Late Jurassic rift system. These shear zones were later reactivated during the Cretaceous and early Tertiary, controlling the positions of some accommodation zones and "jogs" (apparent offsets) in rift trends. The South Sudan shear may have continued to play a role in Neogene extensional tectonics, as it connects "jogs" in both the Abu Gabra/Anza rift (Jurassic? to Early Tertiary) and Turkana/Chew Bahir rift (Neogene to Recent).