Tectonics of the Kenya Rift Valley

Abstract

The more important contributions of previous work to an understanding of the structure and evolution of the Kenyarift valley are reviewed, and the various hypotheses of origin of the Kenya and other rift valleys are critical examined. The distribution and present elevation of cyclic erosion surfaces of late Cenozoicage is illustrated and it is concluded that three principal phases of uplift of early Miocene, late , Miocene-early Pliocene and late Pliocene-early Pleistocene ages occurred during the formation of the rift valley. The distribution of the Neogenevolcanic formations is illustrated and their sequence is established, partly from field relationships and partly with the aid of radioisotopic age determinations, which are listed. The tectonics o! the structural provdnces of the rift zone are described, and !or each an attempt is madeto trace the sequence of volcanic and tectonic events. Theoretical aspects of faulting in arched uplifts are considered together with the role of pre-existing structures upon the developmentof the rift valley. A summary of the available gravity measurements makes it possible to deduce that crustal thinning has occurred beneath the rift valley. An outline of the structure and probable development of the whole Afro-Arabian rift system is given, and it is concluded that there is a genetic connexion between the formation of the Red Sea and Gulf of Aden by processes akin to sea-floor spreading and the formation of the eastern rift system of Africa. The limitations of applying plate tectonics to eastern Africa are analysed. In conclusion a summary of the structural features and evolution of the Kenya rift'valley is presented, and a mechanism for its formation by crustal dilatation and dyke injection is proposed.