The structure of the shallow crust beneath Olkaria geothermal field, Kenya, deduced from gravity studies

Abstract

The Olkaria region of the Kenya rift valley is a potentially important geothermal area due to the occurrence of young volcanic activity and surface geothermal manifestations which include hot springs, fumaroles and thermally altered ground. In order to determine subsurface structure nearly 255 new gravity measurements, together with existing regional gravity data, were used to propose a subsurface model. A few surface density measurements have been made within Olkaria and in adjoining areas, and these combined with density values inferred from the measured P-wave seismic velocities within Olkaria are used to define the density distribution within the shallow crust. A two-dimensional model to explain the local gravity anomalies indicates that an approximately horizontal three-layer volcanic sequence overlying the basement system is downfaulted on the western part of Olkaria, and is intruded by denser dyke-like bodies of rhyolitic composition along the north-south-trending fractures that occur mainly in the central part of the Olkaria. An increase in the thickness of volcanics in the western part of Olkaria is responsible for the observed negative Bouguer anomaly in that area, while vertical density contrasts between the dyke-like intrusions and the host rock generate model anomalies in good agreement with the observed positive Bouguer anomalies in the central part of Olkaria. The largest and most recent intrusion, which is probably still in a magmatic state, occurs along the Ololbutot fracture zone and is apparently the main heat source for the geothermal phenomena in Olkaria.