“…The recent rapid growth of global geophysical data bases has been accompanied by a heightened interest in the implications and origins of the lateral heterogeneity in the earth's mantle which they have revealed. In particular, global variations of heat flow [Pollack and Chapman, 1977;Chapman and Pollack, 1980;Sclater et al, 1980], bathymetry [Parsons and Sclater, 1977], gravity [Gaposhkin, 1979;Lerch et al, 1979;Marsh et al, 1984], chemistry of igneous extrusions [DePaolo and Wasserburg, 1976;O'Nions et al, 1977;Allegre et al, 1983], seismic surface wave [Toksoz and Anderson, 1966;Knopoff, 1972;Woodhouse and Dziewonski, 1984] and body wave [Julian and Senqupta, 1973;Dziewonski et al, 1977;Dziewonski, 1984] velocities, and frequency shifts of fundamental spheroidal modes of elastic gravitational free oscillation [Masters et al, 1982], all provide loose constraints on possible models of lateral heterogeneity. As these data continue to be refined, it may ultimately be possible to use them to directly constrain models of mantle flow on the basis of the characteristic lateral heterogeneity implied by the different flow models.…”