Seismic imaging the D″ region beneath the Central Atlantic

The lowermost mantle beneath the central Atlantic is bounded by the western edge of African Large Low Shear Wave Province (LLSVP) and a high-velocity region beneath the eastern Americas, which is attributed to the accumulated subducted slab material at the core-mantle boundary (CMB). Data recorded at broadband seismic arrays in western Europe and Morocco allow us to investigate the detailed structure beneath the Central Atlantic, which provides an important constraint on how remnant slab material and LLSVP interact and reshape the D″ layer. Here, we exploit lower mantle triplicated S waveform data from deep earthquakes beneath South America. Our modelling results show that the D″ discontinuity is constantly observed from north to south beneath the Central Atlantic but at varied depths and with varied shear velocities. In the lowermost mantle beneath east of Caribbean, our results indicate a sharp 2.1% shear velocity increase at 340 km above the CMB, representing a possible bridgmanite dominant normal mantle D″ layer. Whereas, the models of northern and southern portions of Central Atlantic indicate a gradual velocity increase of 4.2% across 200 km starting from 360 km above the CMB downwards. We also find a distinct low velocity zone on top of the velocity jump, which can be interpreted as an upwelling produced by the dynamic response of ancient slab material reaching the CMB.