Body-Wave Tomographic Imaging of the Turkana Depression: Implications for Rift-Development and Plume-Lithosphere Interactions

The Turkana Depression, a topographically-subdued, broadly-rifted zone between the elevated East African and Ethiopian plateaus, disrupts the NS, fault-bounded rift basin morphology that characterizes most of the East African Rift. The unusual breadth of the Turkana Depression leaves unanswered questions about the initiation and evolution of rifting between the Main Ethiopian and Eastern rifts. Hypotheses explaining the unusually broad, low-lying area include superposed Mesozoic and Cenozoic rifting and a lack of mantle lithospheric thinning and dynamic support. To address these issues, in Kounoudis et al. (G3, 2021) we have carried out the first body-wave tomographic study of the Depressions upper mantle. Seismically-derived temperatures at 100 km depth exceed petrological estimates, suggesting the presence of mantle melt, although not as voluminous as the Main Ethiopian Rift, contributes to velocity anomalies. A NWSE-trending high wavespeed band in southern Ethiopia at <200 km depth is interpreted as refractory Proterozoic lithosphere which has likely influenced the localization of both Mesozoic and Cenozoic rifting. At <100 km depth below the central Depression, a single localized low wavespeed zone is lacking. Only in the northernmost Eastern Rift and southern Lake Turkana is there evidence for focused low wavespeeds resembling the Main Ethiopian Rift, that bifurcate below the Depression and broaden approaching southern Ethiopia further north. These low wavespeeds may be attributed to melt-intruded mantle lithosphere or ponded asthenospheric material below lithospheric thin-spots induced by the region's multiple rifting phases. Low wavespeeds persist to the mantle transition zone suggesting the Depression may not lack mantle dynamic support in comparison to the two plateaus. Kounoudis, R., Bastow, I.D., Ebinger, C.J., Ogden, C.S., Ayele, A., Bendick, R., Mariita, N., Kiangi, G., Wigham, G., Musila, M. & Kibret, B. (2021). Body-wave tomographic imaging of the Turkana Depression: Implications for rift development and plume-lithosphere interactions. G3, 22,doi:10.1029/2021GC009782.