A seismic investigation of the lithosphere of the Gregory rift

for profiles running southeast over the flank of the dome and across the rift. Interpretations for the flank show a sharply increasing thickening of the anomalous zone towards the rift, with a secondary thickening near or under Mt. Kilimanjaro. The rift profile shows that the anomalous zone penetrates the crust to within about 20 km of the surface. A depth of 120 km is deduced for the base of the anomalous zone, but this may be in error due to systematic error in the baseline of station delays. To circumvent the significant errors associated with the assumption of horizontal layering, a three-dimensional ray tracing technique is devised. Flat bottomed models are derived assuming uniform anomalous zone velocities of 7.5 and 7.0 km/sec. The 7.0 km/sec model shows a thinner and shallower anomalous zone, but the overall shapes of these models are in good agreement with the preliminary models. Deficiences in the ray tracing technique are discussed and it is shown that the parameters characterising the three-dimensional models are not well controlled. Suggestions are made for improving the technique. The models are all consistent with the theory that upward perturbation of the 1ithosphere-asthenosphere boundary, giving rise to magmatic activity, thinning of the lithosphere and domal uplift, is the primary cause of rifting.