Lower-mantle seismic discontinuities and the thermal morphology of subducted slabs

The nature of the lowermost few 100 km of the mantle holds insights into the scale of convection of the Earth's mantle. Here we investigate the fine structure of this region using seismic migration of shear waves reflecting from velocity discontinuities near the core-mantle boundary (CMB), recorded by a dense network of European seismic stations. Two main features are found. One structure exhibits large-scale topography and can be explained by a sharp increase in seismic velocity 206-316 km above the CMB. The second structure lies 55-85 km above the CMB and marks a sharp reduction in velocity. To explain the origin of these discontinuities, we appeal to numerical simulations of three-dimensional (3D) mantle convection, incorporating rigidly moving plates. The plates organize the convective flow to allow for the development of large convection cells and cool sheet-like downwellings at a convergent plate boundary. The results suggest that, in places, observed seismic complexity in D discontinuity structure may be due to the thermal gradients resulting from the accumulation of cool subducted plate material at the CMB. (C) 2004 Elsevier B.V. All rights reserved.