CRUSTAL HETEROGENEITY AND BASEMENT INFLUENCE ON THE DEVELOPMENT OF THE KENYA RIFT, EAST-AFRICA

Structural and metamorphic studies within the exposed Precambrian basement in Kenya indicate that a number of heterogeneities exist within the lithosphere beneath the line of the Kenya Rift. Of these, the most important is the mechanical and thermal contrast between thick, cold, and rigid Archean lithosphere and thinner anisotropic Proterozoic mobile belt crust/lithosphere. Structural, geophysical, and heat flow data indicate that during a late Proterozoic collisional event the margin of the Tanzanian Archean craton was reworked, overthrust, and effectively buried by tectonically emplaced ''Mozambique Belt'' rocks. Its position now lies some 100 km east ard northeast of the exposed outcrop. These variations in crust/lithosphere type may be directly correlated with the morphotectonic and structural framework of the Kenya Rift and in particular are reflected in the spatial patterns of Cenozoic volcanism within the Gregory Rift zone. Along the craton margin and within the mobile belt a series of late Proterozoic continental-scale NW-SE and N-S trending ductile/brittle shear zones are fundamental. The reactivation of these shear zones under varying stress field conditions is presented as a model to account for the patterns of rift subsidence, including the location and geometry of graben structures, and the intrusion of magmas since early Miocene times.