Upper mantle P-wave speed variations beneath Ethiopia and the origin of the Afar hotspot

Abstract

The Afar hotspot has long been attributed to one or more thermal upwellings in the mantle, in particular starting thermal plumes characterized by a head that spreads laterally beneath the lithosphere, and a tail. New P-wave tomography images of the upper mantle beneath Ethiopia reveal an elongated low wave speed region that is deep (>400 km) and wide (>500 km). The location of the low wave speed anomaly aligns with the Afar Depression and Main Ethiopian Rift in the uppermost mantle, but the center of the anomaly shifts to the west with depth. The shape, depth extent, and location of the low wave speed anomaly is not consistent with a starting thermal plume presently beneath the hotspot. Instead, the anomaly suggests that the hotspot may be the surface manifestation of a broad mantle upwelling connected to the African Superplume in the lower mantle beneath southern Africa.