The Cenozoic magmatism of East Africa: Part II – Rifting of the mobile belt

Abstract

Despite the foundational role East Africa has played in the advancement of our understanding of continental rifting, there remains substantial ambiguity as to how magmatism has evolved during progressive rift development. This contribution is a comparative study that explores the temporal development of magmatism in both rifts, and within the interposing Turkana Depression. Notwithstanding the independent evolution of the Main Ethiopian Rift and Kenya Rift prior to their Quaternary linkage, magmatic events within them show remarkable parallelism. Following an initial pulse of basaltic magmatism ca. 20 Ma, more evolved compositions (flood phonolites in Kenya, rhyolites in Ethiopia) dominated the landscape until ca. 12 Ma. From ca. 12 Ma to 9 Ma, a renewed phase of widespread basaltic volcanism (Mid-Miocene Resurgence Phase) impacted the entire region from the Afar margin to Kenya, though activity in the south slightly predates equivalent basaltic events to the north. Following this widespread basaltic event, silicic magmatism again dominated the now nascent rifts until a renewed phase of basaltic activity commenced ca. 4 to 1.6 Ma termed the ‘Stratoid Phase’. Following the termination of the Stratoid Phase, the modern expression of volcanism in both rifts has been dominated by central silicic volcanoes. Magmatic activity within the Turkana Depression broadly parallels that in both rifts – basaltic events are evident at ca. 20 Ma, 12 Ma, and 4 Ma, however the silicic interludes that are evident within the rifts, are generally absent. Moreover, from 4 Ma to present, there is a progressive decrease in magmatic activity from an initial fissural basalt, to a series of large shield volcanoes, to modern belts of cinder cones. The results of this synthesis suggest widespread pulsed basaltic events precede silicic flare-ups throughout the East African Rift System, and the lack of such flare-ups in Turkana may relate to the Mesozoic rifting that modified the continental lithosphere in that region.