Volcanism in the Afar Rift sustained by decompression melting with minimal plume influence

Continental breakup is caused by some combination of heating and stretching(1,2). The Afar Rift system in Africa is an example of active continental rifting, where a mantle plume probably weakened the lithosphere through thermal erosion and magma infiltration. However, the location and degree of plume influence today are debated(2,3). Here we use seismic S-to-P receiver functions to image the mantle structure beneath Afar. We identify the transition between the lithosphere and underlying asthenosphere at about 75 km depth beneath the flanks of the continental rift. However, this boundary is absent beneath the rift itself and we instead observe a strong increase in seismic velocities with depth, at about 75 km. We use geodynamic modelling to show that the velocity increase at this depth is best explained by decompression melting of the mantle in the absence of a strong thermal plume. So, although the absence of mantle lithosphere beneath the rift implies a plume may have once been active, we conclude that the influence of a thermal plume directly beneath Afar today is minimal.