Seismic Evidence for an Ultralow Velocity Zone Beneath the Cape Verde Hotspot

Prajna Paramita Das , Heng-Yi Su, Jonathan Wolf , Barbara Romanowicz

Mega ultralow velocity zones (mega-ULVZs), thin patches of strongly reduced seismic velocity with large horizontal extent just above the core-mantle boundary (CMB), are increasingly found beneath deep mantle plumes, suggesting a link to hotspot volcanism. The Cape Verde hotspot is thought to overlie a deep plume, but whether a ULVZ exists at its base has remained unknown. We present core-diffracted shear-wave (Sdiff) waveforms whose postcursory arrivals are not present in synthetics computed from 3D tomographic mantle models. Instead, the data are best explained, to first order, by a quasi-cylindrical ULVZ near (9°N, 20°W) with ~800 km lateral extent, ~40 km height, and a ~20% shear-velocity reduction. These results suggest the presence of a mega-ULVZ at the root of the Cape Verde plume and strengthen evidence that such structures are common features of deep mantle upwellings.