Gravity and seismic measurements in Kenya.

This thesis is mainly concerned with the use of gravity data to study the structure of the crust and upper mantle beneath the Kenya Rift Valley. Following a review of previous geophysical studies of the Rift there are descriptions of some methods used in producing a gravity map of Kenya, including an account of the "minimum curvature" interpolation method. The techniques of Dorman and Lewis (1970) are used (a) to show that Kenya's topography is to a large extent isostatically compensated and (b) to compute the gravity effect of the compensation. The latter is explained in terms of the model of the upper mantle structure proposed by Long and Backhouse (1976). It is also used in place of the conventional "regional" in the interpretation of crustal gravity anomalies within the Rift. A seismic experiment carried out in 1975 is described and the data interpreted. This is used to control the interpretation of the gravity data. The results of this study support the idea of some crustal thinning below the Rift but not of a continuous wedge of mantle derived material intruding the crust to high levels. The seismic data imply that normal basement exists below the volcanic rocks of the Rift floor at 0.50 N; after the effects of the volcanic and sediments have been removed from the gravity data there is a residual "axial high" which is best explained by a wide tabular body, within the basement, which could be similar in form to the elongated layered intrusions seen in ancient rocks (e.g. Rhodesian Great Dyke). It is suggested that below the Rift floor, apart from this intrusion there is normal crustal layering, perhaps intruded by feeders or small magma chambers which do not produce measurable gravity anomalies