Melting in the crust and upper mantle beneath the Kenya Rift: evidence from Geomagnetic Deep Sounding experiments

Abstract

A Geomagnetic Deep Sounding experiment in and around the Kenya Rift Valley has shown that telluric currents are concentrated by 3 regions with high electrical conductivity. Two of the anomalies are related to the rift structure, and the combination of different lines of geophysical evidence strongly suggests that the high conductivity is due to the presence of molten material in the rocks of the lower crust and upper mantle. The shallower zone where melt is present is located directly beneath the floor of the rift valley. Its upper surface is no deeper than 20 km; it may be as shallow as 5 km. The electrical conductivity is compatible with an average melt concentration of 5-10%, but could also be explained by discrete magma chambers where the concentration was much higher. The deeper conductor is located to the E of the rift valley beneath the Aberdare Mountains, and possibly extends beneath Mount Kenya. The Geomagnetic Deep Sounding data suggest a depth of 100 km to the high conductivity body, which appears to correspond to the core (the region with the highest melt concentration) of a zone of more diffuse melting in the mantle, that is responsible for seismic and regional gravity anomalies, and which supports a part of the topographic elevation of the Kenya dome