The Ethiopia Afar Geoscientific Lithospheric Experiment (EAGLE): Probing the transition from continental rifting to incipient seafloor spreading

The Miocene-Holocene East African Rift in Ethiopia is unique worldwide because it subaerially exposes the transition between continental rifting and seafloor spreading within a young continental flood basalt province. As such, it is an ideal study locale for continental breakup processes and hotspot tectonism. Here, we review the results of a recent multidisciplinary, multi-institutional effort to understand geological processes in the region: the Ethiopia Afar Geoscientific Lithospheric Experiment (EAGLE). In 2001-2003, dense broadband seismological networks probed the structure of the upper mantle, while controlled-source wide-angle profiles illuminated both along-axis and across-rift crustal structure of the Main Ethiopian Rift. These seismic experiments, complemented by gravity and magnetotelluric surveys, provide important constraints on variations in rift structure, deformation mechanisms, and melt distribution prior to breakup. Quaternary magmatic zones at the surface within the rift are underlain by high-velocity, dense gabbroic intrusions that accommodate extension without marked crustal thinning. A magnetotelluric study illuminated partial melt in the Ethiopian crust, consistent with an overarching hypothesis of magma-assisted rifting. Mantle tomographic images reveal an similar to 500-km-wide low-velocity zone at >= 75 km depth in the upper mantle that extends from close to the eastern edge of the Main Ethiopian Rift westward beneath the uplifted and flood basalt-capped NW Ethiopian Plateau. The low-velocity zone does not interact simply with the Miocene-Holocene (rifting-related) base of lithosphere topography, but it provides an abundant source of partially molten material that assists extension of the seismically and volcanically active Main Ethiopian Rift to the present day.