SEISMIC-GEODYNAMIC CONSTRAINTS ON 3-DIMENSIONAL STRUCTURE, VERTICAL FLOW, AND HEAT-TRANSFER IN THE MANTLE

Joint inversions of seismic and geodynamic data are carried out in whi ch we simultaneously constrain global-scale seismic heterogeneity in t he mantle as well as the amplitude of vertical mantle flow across the 670 km seismic discontinuity. These inversions reveal the existence of a family of three-dimensional (3-D) mantle models that satisfy the da ta while at the same time yielding predictions of layered mantle flow. The new 3-D mantle models we obtain demonstrate that the buoyancy for ces due to the undulations of the 670 km phase-change boundary strongl y inhibit the vertical flow between the upper and lower mantle. The st rong stabilizing effect of the 670 km topography also has an important impact on the predicted dynamic topography of the Earth's solid surfa ce and on the surface gravity anomalies. The new 3-D models that predi ct strongly or partially layered mantle flow provide essentially ident ical fits to the global seismic data as previous models that have, unt il now, predicted only whole-mantle flow. The convective vertical tran sport of heat across the mantle predicted on the basis of the new 3-D models shows that the heat flow is a minimum at 1000 km depth. This su ggests the presence at this depth of a globally defined horizon across which the pattern of lateral heterogeneity changes rapidly.