A TOMOGRAPHY STUDY OF THE CHYULU HILLS, KENYA

The Chyulu Hills, a Quatemary volcanic field on the eastern shoulder o f the Kenya rift in East Africa, have been the target of a multidiscip linary research project. In this contribution teleseismic travel-time residuals from a passive seismic experiment are imaged into P-velocity perturbations in the lithosphere underneath the Chyulu Hills. The ima ges reveal lateral velocity contrasts of about 5% with a prominent low -velocity zone which is located directly beneath the volcanic range do wn to 70 km depth. Due to the small size and amplitude of the anomaly this zone of decreased seismic velocity is interpreted as evidence for small magma chambers with elevated temperatures in the lower crust an d uppermost mantle. A major upwelling of the asthenosphere can be excl uded, because the seismic anomaly is too limited in size. However, the increased temperatures below the Chyulu Hills are interpreted as a si gn for the still active magmatic processes related to the rifting in E ast Africa.