1. M. Gurnis is at the Seismological Laboratory, California Institute of Technology, Pasadena, CA 91125, USA.. R. D. Müller is in the Department of Geology and Geophysics, University of Sydney, Sydney, NSW 2006, Australia.. L. Moresi is at the Australian Geodynamics Cooperative Research Centre, Commonwealth Scientific and Industrial Research Organization Exploration and Mining, Nedlands, WA 6009 Australia.