In February l994, the Kenya Rift International Seismic Project carried out two wide-angle reﬂection and refraction seismic proﬁles between Lake Victoria and Mombasa across southern Kenya. Our investigation of the data has revealed evidence for the presence of two upper mantle reﬂectors beneath southwestem Kenya, sometimes at short range, from seven shotpoints. Two-dimensional forward modelling of these reﬂectors using a pre-existing two-dimensional velocity—depth model for the crust [Birt, C.S., Maguire, P.H.K., Khan, M.A., Thybo, H., Keller, G.R., Patel, J., l997. The inﬂuence of pre-existing structures on the evolution of the Southern Kenya Rift Valley — evidence from seismic and gravity studies.
Tectonophysics 278, 211—242], has shown them to lie at depths of approximately 51 and 63 km. The upper reﬂector, denoted d1, shallows by about 5-10 km in the area beneath Lake Magadi, situated in the rift itself. Correlations for the deeper reﬂector, denoted d2, are sparse and more dificult to determine, so it was not possible to deﬁne any shallowing corresponding to the surface expression of the rift. Only limited control exists over the upper mantle velocities used in the modelling. Immediately beneath the Moho we use a value of Pn calculated from the crustal model, and constraints from previous refraction, teleseismic and gravity studies, to determine the velocity at depth. At the d2 reﬂector a reasonable velocity contrast was introduced to produce a reﬂector for modelling purposes. Beneath the d1 reﬂector the velocity decreases to the average value over 3 km. Beneath the rift the velocity also rises across d1 and again, decreases to the average value over the next 3 km. At the d2 reﬂector a similar model is used. This model accounts for the presence of the mantle reﬂectors seen in the data by using layers of thin higher velocity in a lower background velocity. Due to the uncertainty in the velocities the absolute position of both dl and d; could vary, but the relative upwelling beneath the rift is reasonably well constrained and data from four different shotpoints which indicate the shallowing show good agreement. A signiﬁcant result of this study is that the continuity of the d, reﬂector indicates that the sub-Moho lithosphere has not been substantially disrupted by mantle upwelling, even though probably thinned and stretched.
Keywords: rifting; upper mantle; Kenya; wide-angle reﬂections; tectonics; P-wave velocities