A study of the Kenya rift using delay-time tomography analysis and gravity modelling

Abstract

A combined study of 2-D delay-time tomography analysis and gravity modelling was carried out for a profile across the Kenyan rift at about 1°S. The results of the P-wave delay-time tomography analysis along the profile, carried out in 1985 within the framework of KRisp81 (Kenya Rift International Seismic Project), reveals a low-velocity zone (LVZ) down to depths of at least 200 km beneath the rift, slightly dipping to the west. To the east of the rift a second LVZ can be observed restricted to the upper mantle. Crustal “stripping” was applied to look in more detail at the delays related to the sub-Moho structure. The resulting model shows an even more pronounced low-velocity zone in the upper mantle, with a velocity contrast of up to 10%. The resulting seismic image was then used to construct a gravity model fitting not only the observed Bouguer gravity anomaly but also the seismic image in shape and size. To match the amplitudes of the Bouguer anomaly, as well as the observed seismic structure, anomalously large values for the ratio of velocity contrast to density contrast are required. To explain this, the occurrence of partial melt under the rift is proposed. The amount of melt is estimated to be about 5%. The low velocity zone east of the rift, centered at about 60–100 km depth, is consistent with petrological studies.