THE EAST-AFRICAN RIFT SYSTEM IN THE LIGHT OF KRISP-90

On the basis of a test experiment in 1985 (KRISP 85) an integrated sei smic-refraction/teleseismic survey (KRISP 90) was undertaken to study the deep structure beneath the Kenya rift down to depths of 100-150 km . This paper summarizes the highlights of KRISP 90 as reported in this volume and discusses their broad implications as well as the structur e of the Kenya rift in the general framework of other continental rift s. Major scientific goals of this phase of KRISP were to reveal the de tailed crustal and upper mantle structure under the Kenya rift, to stu dy the relationship between mantle updoming and the development of sed imentary basins and other shallow structures within the rift, to under stand the role of the Kenya rift within the Afro-Arabian rift system a nd within a global perspective, and to elucidate fundamental questions such as the mode and mechanism of continental rifting. The KRISP resu lts clearly demonstrate that the Kenya rift is associated with sharply defined lithospheric thinning and very low upper mantle velocities do wn to depths of over 150 km. In the south-central portion of the rift, the lithospheric mantle has been thinned much more than the crust. To the north, high-velocity layers detected in the upper mantle appear t o require the presence of anistropy in the form of the alignment of ol ivine crystals. Major axial variations in structure were also discover ed, which correlate very well with variations in the amount of extensi on, the physiographic width of the rift valley, the regional topograph y, and the regional gravity anomalies. Similar relationships are parti cularly well documented in the Rio Grande rift. To the extent that tru ly comparable data sets are available, the Kenya rift shares many feat ures with other rift zones. For example, crustal structure under the K enya, Rio Grande, and Baikal rifts and the Rhine Graben is generally s ymmetrically centered on the rift valleys. However, the Kenya rift is distinctive, but not unique, in terms of the amount of volcanism. This volcanic activity would suggest large-scale modification of the crust by magmatism.