An integrated analysis of lithospheric structure across the East African plateau based on gravity anomalies and recent seismic studies

Abstract

Data from recent gravity surveys carried out by the University of Texas at El Paso (USA) and other institutions and existing data were combined and integrated with seismic results from the KRISP project and geologic data to analyze the regional lithospheric structure beneath the East African plateau. A series of gravity maps were constructed, and they show that the East African plateau is associated with a broad negative gravity anomaly which is about 1200 ± 100 km wide and which has an amplitude of -150 ± 20 mGal. Superimposed on this anomaly are narrower, steeper-sided negative anomalies about 200 km wide with amplitudes of -30 to -60 mGal. These anomalies are associated with the eastern and western arms of the East African rift. These gravity maps also show low gravity gradients in the Tanzanian craton, Kasai craton and Congo basin areas while there are high gravity gradients within the rifts and at the mobile belt-craton boundaries. With increased filter wavelength cutoff, the rift-related gravity lows broaden and eventually merge into one broad low whose apex is centered on Lake Victoria. Seismic refraction, teleseismic, mantle xenolith, petrochemical and geologic data were used as constraints during the construction of a series of gravity models which represent integrated models of lithospheric structure across the East African plateau. Three W-E profiles across the plateau and one N-S profile along the axis of the Eastern rift in Kenya and Tanzania were modeled. These models together with the filtered gravity maps demonstrate lithospheric thinning beneath the rifts and suggest the existence of a deep mantle anomaly centered beneath the East African plateau and the Tanzanian craton. The mantle anomaly is widely believed to be a plume and our models indicate that the diameter of its head is about 600 km. Two arms with diameters less than 250 km extend from the plume head and penetrate the lithosphere to shallow levels under the Eastern and Western rifts. In the Western rift and the southern part of the Eastern rift (Tanzania), these features are deeper than under Kenya. This study provides additional evidence that the long wavelength Bouguer gravity anomaly over the East African plateau can be explained by lithospheric heating and thinning. The model along the axis of the Eastern rift suggests that it is propagating southwards into central Tanzania which is also suggested by the age of initiation of volcanism.