Abstract

The active East African Rift System forms a belt of anastomosing faults extending 4,000 km south-southwest from the junction of the Red Sea and Gulf of Aden to the Zambezi River. The presence of certain ancient features, including the Great Dyke of Rhodesia and an alignment of Bushveld-type igneous complexes, indicates that the system formerly extended as an infracrustal lineament another 1,500 km south-southwest down the eastern half of Africa to the Orange River. The rift system is entirely intracontinental, and formed in the ancient African Precambrian platform with whose 3-b.y. geological history it is intimately associated. The Cenozoic rift faults follow mobile belts moulded upon ancient shields and formed during at least seven major orogenic cycles affecting pre-Silurian “assemblages” comprising complex rock groups characterized by structural and metamorphic similarities but including units of greatly differing ages. An analysis of major structural features moulded on the Tanganyika Shield indicates that the belts may have originated at about 2.7 b.y. by dextral transcurrent movement between the ancient shields, but horizontal movement was subsequently impeded by cratonization and replaced by vertical displacement.

The nature of the rifted belts indicates control by mantle mechanisms and repeated reactivation, so the term perennial deep lineament is proposed. Comparison with other intra-continental rift systems and preliminary geophysical results in East Africa strongly suggest lithospheric states similar to those underlying the mid-ocean ridges, implying high heat flow restricted to the lineaments in a generally cool continental environment. Geological mapping proves, however, that no formation of new oceanic crust has taken place in spite of the great age of the lineaments. Profuse rift volcanism of continental type affects the northern part of the eastern rift lineament, but this restricted volcanic sector contrasts strongly with the major nonvolcanic portion of the rift system. The lack of spreading of the rifts may be due to the compression of the African Plate between the spreading Mid-Atlantic and Mid-Indian Ocean Ridges.

Long fault lineaments are features of the earth's crust, and a review shows that intra-continental rift valleys of East African type occur where such lineaments must cut through particularly resistant cratonized massifs, forming wide zones of fracturing and shearing with blastomylonites, flaser gneisses, and migmatites in the plastic infracrust. The mobile belts of the system may thus be compared with orogenies in which plastic deformation and migmatization at deep levels are succeeded by isostatic uplift, but the linear Cenozoic arches thus produced in East Africa are affected by typical rift valley faulting at the brittle surface levels and are thus genetically associated with infracrustal Precambrian structures and continued high heat flow while reflecting supracrustal mechanics. The lineaments do not always follow fold belts but may cut obliquely across their grain. Although the most superficial rift faulting is steeply normal and antithetic, observations suggest that the deeper faults are vertical, and it is proposed that subsurface plasticity and expansion due to heat flow is a causative factor in the sinking of strips of cool surface rock in a shield environment.

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