Abstract
Six magnetic variometers were installed at sites along a 300 km east-west line, crossing the eastern branch of the East African Rift Valley near the Equator. The instruments were operated from March to May 1971, during which time four magnetic disturbances were recorded. Transfer functions have been computed for each station, and are presented both as real and imaginary induction arrows and in an alternative ‘maximum and minimum’ form. They show that a relatively shallow concentration of induced current flows along the axis of the rift valley. However, the anomaly is not symmetric, and the phase of the response shows a complex behaviour on the east side of the Rift. The computer program of Jones and Pascoe has been used to compute the response of two-dimensional conductivity models. The type of model that fits the experimental data involves a strip of high conductivity material at a depth of no more than 20 km beneath the floor of the rift with, in addition, a thick slab (100 km) of material of more moderate conductivity at a depth of 50 km beneath the eastern flanks of the rift. The lateral extent of this slab was not precisely defined by the 1971 experiment, but it may well be several hundred kilometres. The two regions of high conductivity are interpreted as zones of partial melting in the upper mantle. Their positions coincide with the regions of most recent (late Quaternary) volcanism.
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