Seismological studies of the interface between downgoing slabs of subducted lithosphere and the overriding mantle provide constraints against which models of mantle and slab temperature and bulk composition can be tested. These studies, employing waves reflected and converted at the slab/mantle interface, suggest that a velocity contrast of 5-10% occurs over a distance of 10-20 km to depths of at least 350 km. We have investigated whether such sharp contrasts can result from thermal structure...