EVOLUTION OF THE EAST AFRICAN RIFT: DRIP MELTING, LITHOSPHERIC THINNING AND MAFIC VOLCANISM

The East African Rift System is one of the longest continuous tectonic features exposed at the Earth’s surface. It encompasses two major areas of uplift and extension (the Ethiopian and Kenya domes) and persists for ~3000 km from the Red Sea to Mozambique. Topographic and volcanic features of the EARS are broadly understood to be related to asthenospheric upwelling. An integrated along-axis interpretation remains elusive because spatial geochemical patterns documented in Oligocene (Pik et al. 1999) and Quaternary (Rooney et al. 2012) Ethiopian volcanics vanish in the Turkana depression and do not reappear in the Kenya and Western Rifts. Recent geophysical results (Hansen et al. 2012) support geochemical models that indicate the African superplume is active across this entire region, sparking renewed interest in a corresponding geochemical synthesis. We explore the importance of small-scale convection, density-driven lithospheric removal and associated “mantle-drip” melting (Elkins-Tanton 2007; Holbig & Grove 2008) to the volcanic and topographic features of the EARS. Primitive mafic lavas from both Afar and Turkana that have been interpreted as defining the composition of upwelling asthenosphere have signature petrographic and geochemical features associated with drip melting, suggesting that they represent melts of locally delaminated lithosphere rather than exclusively deep mantle plumes. The resulting mass balance of source components and the fate of continental lithosphere must be revised. We focus also on a period of HIMU-like volcanism ~22-26 Ma that is present in the eastern (Nelson et al. 2014) and northwestern (Kieffer et al. 2004; Meshasha & Shinjo 2008) Ethiopian plateau and in Turkana (Furman et al. 2006). Key mafic lavas erupted in these episodes exhibit geochemical evidence for drip melting that suggests they correspond to a period of widespread lithospheric removal subsequent to plume head volcanism. Hf-Os isotopic results requiring a contribution from ancient subduction-modified mantle are presumably related to the Pan-African orogeny. We suggest that this slab-derived material is responsible for the HIMU-like signature. Post-Miocene plateau uplift in Ethiopia and Kenya may reflect this event as buoyant asthenospheric material replaced dense lowermost lithosphere.