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Timing of East African Rift development in southern Ethiopia: Implication for mantle plume activity and evolution of topography

¹ Centre de Recherches Pétrographiques et Géochimiques, CRPG, CNRS-UPR 2300, BP 20, 54501 Vandoeuvre-Lès-Nancy Cedex, France
² Addis Ababa University, Geology and Geophysics Department, P.O. Box 1176, Addis Ababa, Ethiopia

Accurate determination of rifting chronology and associated uplift is crucial to understanding the evolution of the East African Rift System (EARS) and for identifying the significance of mantle plumes during continental breakup. This investigation of rift-related cooling along a major fault scarp in southern Ethiopia, using (U-Th)/He thermochronometry, shows that rifting started not before 20 Ma. Therefore, there is an absence of significant rift activity synchronous with the earliest volcanics of the EARS, which are Eocene in age. In contrast, this initial magmatic episode, which preceded the main flood basalts and rifting events by 15–20 Ma, is attributed to convective instabilities above the rising Afar mantle plume. A detailed spatial and temporal quantification of uplift and denudation along this rift shoulder shows that rift development in southern Ethiopia has been continuous since initiation in the Miocene. This direct evidence of denudation is inconsistent with the hypothesis that massive Plio-Pleistocene rifting and associated uplift occurred in this part of the EARS and could have triggered recent aridification. To the contrary, our study rather supports a major contribution of plume-related doming for creation of topographical barriers in the Ethiopian province.

Key Words: rift • East Africa • Ethiopia • mantle plume • (U-Th)/He • thermochronology

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