Rift structure in southern Ethiopia

Abstract

The Main Ethiopian Rift, extending southward from the Afar triangle, is not continuous with the Gregory Rift in Kenya, but attenuates in southern Ethiopia. Here the Stefanie and Lake Rudolf rifts dominate and connect directly with the Kenya rift system. These rifts are terminated northward by northeast-striking normal fault systems and by diminishing throws on the boundary faults. In particular, the Gofa basin-and-range province interrupts the Stefanie Rift, and the entire system loses its expression north of an east-northeast fault set on strike with the southern boundary of the Afar depression. Estimates of fault movements are based on displacement of the sub-planar, pre-Tertiary surface eroded on the basement gneisses. The Main Ethiopian and Stefanie rifts are relatively symmetrical and lie at the crests of basement arches; gneisses are exposed at 2750 m a.s.l. west of Lake Chamo, and structural relief is at least 2 km. The other rifts are chiefly half-gräben; they and the north-trending systems are composed dominantly of westerly-tilted blocks with fault throws reaching at least 3 km. Thick alkali basalt and trachyte—pantellerite effusions vary in age from Mid-Eocene to Mid-Miocene. Mafic and salic successions are in part contemporaneous and predate major rifting, which occurred between Late Miocene and Mid-Pliocene time, with diminished movements to the present. Rifting is evident over a width in excess of 300 km in southern Ethiopia. Although the strikes of individual fault sets can be locally related to basement foliation trends, the gross alignments of the major rift valleys must stem from deeper-seated control.