The deep seismic structure of the Ethiopia/Afar hotspot and the African superplume

The Ethiopia/Afar hotspot has been frequently explained as an upper mantle continuation of the African superplume, with anomalous material in the lower mantle under southern Africa, rising through the transition zone beneath eastern Africa. However, the significantly larger amplitude low velocity anomaly in the upper mantle beneath Ethiopia/Afar, compared to the anomalies beneath neighboring regions, has led to questions about whether or not alongstrike differences in the seismic structure beneath eastern Africa and western Arabia are consistent with the superplume interpretation. Here we present a new P-wave model of the hotspot's deep structure and use it to evaluate the superplume model. At shallow (< ~400 km) depths, the slowest velocities are centered beneath the Main Ethiopian Rift, and we attribute these low velocities to decompression melting beneath young, thin lithosphere. At deeper depths, the low velocity structure trends to the northeast, and the locus of the low velocity anomaly is found beneath Afar. The northeast-trending structure with depth is best modeled by northeastward flow of warm superplume material beneath eastern Africa. The combined effects of shallow decompression melting and northeastward flow of superplume material explain why upper mantle velocities beneath Ethiopia/Afar are significantly slower than those beneath neighboring East Africa and western Arabia. The superplume interpretation can thus explain the deep seismic structure of the hotspot if the effects of both decompression melting and mantle flow are considered.