CRUSTAL STRUCTURE OF THE SOUTHEASTERN FLANK OF THE KENYA RIFT DEDUCEDFROM WIDE-ANGLE P-WAVE DATA

The 420-km-long northwest-southeast KRISP (Kenya Rift International Se ismic Project) flank line F, part of the KRISP 94 experiment, extends from Athi River (30 km southeast of Nairobi) to the Indian Ocean near Mombasa, Kenya. Line F crosses the Chyulu Hills area, a young Quaterna ry volcanic field, surrounded by the basement of the Mozambique belt. The basement is at the surface almost along the entire profile with tw o exceptions, one in the Chyulu Hills area due to the presence of volc anic pyroclastics, the other next to the Indian Ocean where sediments reach a thickness of about 8 km. Below the basement the crust can be d ivided into three layers. The upper layer extends to about 10 km depth with P-wave velocities of roughly 6.25-6.4 km/s. The mid-crustal laye r reaches a depth of about 20 km where P-wave velocities range from 6. 55 km/s to 6.7 km/s. The lower crust in the area of the Chyulu Hills i s unexpectedly thick (>20 km) and generates strong refracted phases wi th velocities of about 7.0 km/s. Crustal thickness is about 40 km but thickens in the area of the Chyulu Hills and thins towards the Indian Ocean to 22 km. P-n-phases, refracted waves travelling through the upp ermost mantle, are identified on four record sections and give good co ntrol over the upper mantle velocities which are slightly reduced from 8.1-8.2 km/s to 7.9 km/s underneath the Chyulu Hills area, The upper and lower crust in the region of the Chyulu Hills are significantly he terogeneous, producing strong signal-induced noise which masks seconda ry arrivals in the corresponding distance range. Underneath the Chyulu Hills PMP-reflections are hard to identify indicating that the crust- mantle boundary is a transition zone, rather than a first-order discon tinuity.