A Geophysical Study of Lake Kivu, East Africa *

Summary The geomagnetic field over Lake Kivu, an East African rift valley lake, is generally characterized by gentle gradients, and small, isolated anomalies of 50‐100 gammas amplitude. Lineated anomalies do not occur except in association with Cenozoic volcanics. In the Northern Basin, which is probably Pliocene in age, the well‐bedded sediments average about 300 m in thickness, exceeding 500 m in places. Elsewhere in the lake, the sediment thickness averages only about 100 m. Sonobuoy refraction studies show that the basement underlying the sedimentary column is composed of material having a compressional wave velocity of 5.3–5.4 km s −1 , probably representative of sialic plutonic or metamorphic rock. Gravity measurements show that the western branch of the East African rift is approximately the surface trace of a transition in deep structure across Africa. Bouguer gravity anomalies in the immediate vicinity of Lake Kivu show a strong negative correlation with elevation. The Bouguer anomaly versus elevation relationship, however, is rather different from that for the Gregory Rift or from East Africa in general, thus implying differences in structure. The geophysical data in Lake Kivu is consistent with the idea that it is at a stage of volcanic and hydrothermal activity following graben formation. Evidence for sea‐floor spreading is not observed, but this tectonic process may be in a nascent stage of development.