Evolution of Rifting in Africa

Abstract

THE recent report by Girdler et al.¹ on the rift system of East Africa is of great interest because it presents a crustal model, based on new gravity observations, which suggests an intrusion of low velocity mantle material rising to the base of the sialic crust which underlies the eastern and western rift valleys of East Africa between 3° N and 9° S over an east–west width of about 1,000 km. The crustal model proposed is thus in a general way comparable with the postulated to underlie the worldwide mid-ocean ridge² and the Upper Rhinegraben³. Studies of seismicity, age of faulting, age of volcanoes and short and long wavelength gravity anomalies are interpreted by the authors as indicating attenuation of the African crustal plate and the early stages of its breakup. The structural behaviour of East Africa is compared with the conveyor belt mechanism of sea floor spreading from mid-ocean ridges as recently reviewed by Vine⁴ and rates of movement away from the rift axis of 0.5–2.0 cm yr⁻¹ are said to be indicated.