Heat flow measurements in East Africa

Abstract

All heat-flow measurements in East Africa until 1971 have been obtained from relatively deep rift valley lakes by oceanographic techniques. Twenty values from Lake Malawi range over more than an order of magnitude (Von Herzen and Vacquier, 1967), although the average is similar to that obtained from South African shield measurements, about 1.1 μcal.cm −2.sec −1 (H.F.U.). The values appear systematically distributed geographically over the lake area, with a possible correlation with localized tectonic activity. Except for one high value (3.5 H.F.U.), values from 12 locations in Lake Tanganyika range from low to normal (0.4–1.4 H.F.U.), with no apparent systematic geographical distribution (Degens et al., 1971). Similarly, some recent values from Lake Kivu have a range of 0.4–4.4 H.F.U., but most are above normal. The higher heat flow in Lake Kivu seems consistent with the recent volcanic activity in this region. The effects of the local environment, particularly high sedimentation rates and past changes in lake water temperature, have probably affected many of the measurements. An unfavorable combination of these effects at some stations may imply equilibrium values which are 2 to 3 times larger than those measured. These could make some values comparable to high values measured on oceanic rifts. However, other geophysical measurements, particularly gravity, imply a continental rather than oceanic crustal structure beneath the lakes, suggesting that the tectonic mechanisms responsible for the anomalous heat flux are different from those thought to be associated with oceanic rifts i.e., cooling of a spreading lithospheric slab.