SEISMIC EVIDENCE FOR SMALL-SCALE DYNAMICS IN THE LOWERMOST MANTLE AT THE ROOT OF THE HAWAIIAN HOTSPOT

The hot thermal boundary layer produced by heat transport from the Ear th's core to the base of the mantle is thought to contain strong horiz ontal shear flows and to nucleate instabilities in which hot material rises into the convecting mantle as thermal plumes(1-3). A recent stud y(4,5) proposes that the Hawaiian plume is deflected by mantle convect ion and, in the lowermost mantle, is located to the southeast of its s urface manifestation. Here we present seismic data that densely sample , with core-reflected shear waves, a region beneath the central Pacifi c Ocean which includes the predicted location of the deflected root of the Hawaiian hotspot. Our mapping of the structure in this region of the lowermost mantle reveals strong lateral gradients in shear-wave ve locity and anisotropic shear-wave polarization direction over distance s of only several hundred kilometres. We interpret these gradients as being indicative of small-scale dynamical structure in the thermal bou ndary layer, where vertical flow into the Hawaiian plume at its root i s accompanied by horizontal flow towards the plume.