Tectonics and volcanism of the southern Kenya Rift Valley and its influence on rift sedimentation

Abstract

Summary Volcanism in the northern half of the Kenya Rift Valley began at 30 Ma, and in the southern half it began at 15 Ma. In the southern rift tectonic development started with gentle downwarping followed by repeated flooding of the depression with lavas. A half-graben formed at 7 Ma by faulting on the W, succeeded by further eruptions in the rift floor. By 4 Ma a graben had formed with faulting of the flexed eastern margin of the depression. Succeeding periods of faulting and volcanism migrated inwards, creating step-fault platforms and a narrow inner graben, but in the Naivasha-Nakuru sector voluminous trachytic volcanism completely filled the rift depression at intervals between 6 and 2 Ma. The inner graben was cut by dense swarms of minor faults from 2 Ma; and in the last 0.5 Ma a series of trachytic caldera volcanoes was built axially in the inner graben. Large sedimentary basins are found in broad half-graben in the northern and southern splay-fault sectors, and are subdivided into local basins where volcanic damming has occurred. Minor basins formed on back-tilted step-fault platforms, and numerous smaller lakes existed in graben of the rift floor. In the central sector lake basins spread broadly over pyroclastic deposits between volcanic piles. Ramp and step-ramp structures locally divert drainage from the high rift flanks into rift basins. Hypersaline lakes with trona evaporites in the Natron-Magadi basin were caused by the arid climate, eruption of sodium carbonate-rich ash, and recirculation of alkaline groundwater by hot springs into closed structural basins.