Crustal structure of the East African Rift zone

A review of seismological data on the crustal structure of the East African Rift zone is presented. The only refraction line is that along the Gregory Rift, which indicates a 7.5 km/sec refractor which is presumed to be the Moho. The bulk of data is provided by surface-wave dispersion studies. Some preliminary measurements of crustal and sub-Moho velocities using the University of Durham array at Kaptagat in Kenya are included.

There is now a growing body of evidence that the crust is generally of shield type over the whole rift zone. The exception is along the axis of the Gregory Rift, where a low-velocity Moho and some crustal modification is apparent. This is presumably the result of magma intrusions and suggests some crustal separation along this section of the rift. Sub-Moho velocities are probably normal outside the rifts themselves, though anomalously low upper-mantle velocities are to be associated with rifting. There is firm evidence for thinning of the lithosphere along the eastern branch of the rift. A cross-section of the Gregory Rift which is consistent with the current data is presented.