The structure of speke Gulf, Tanzania, and its relation to the East African rift system

Abstract

A gravity survey has been made of the region between Lake Victoria and the Gregory Rift, in an attempt to establish the relationship of Speke Gulf to the East African Rift System. Regional and residual Bouguer anomaly maps show the main feature of interest to be the negative residual Bouguer anomaly in line with, and over Speke Gulf. The anomaly is trough shaped (width 50 km) and has an amplitude of −200 g.u., extending for nearly 200 km. Two explanations for the negative anomaly are plausible: 1. (1) an elongate, low-density granitic intrusion 2. (2) Speke Gulf occupies part of a much eroded Precambrian graben, within which a thicker segment of the earth's crust is preserved. At present, the “graben” model seems to offer a better correlation with the geology and is preferred. Either model implies the presence of fundamental lines of weakness in the crust in this region, and the seismicity along these lines indicates incipient faulting, probably due to stresses arising from north—south variations in the extension across the Gregory Rift. It is possible that a future rift may be developing at Speke Gulf, paralleling the Kavirondo Rift Valley.