PARTIAL ADVECTION OF EQUIDENSITY SURFACES - A SOLUTION FOR THE DYNAMIC TOPOGRAPHY PROBLEM

Although one-layer dynamic models of the Earth's mantle have successfu lly explained the geoid, they generate a surface dynamic topography th at seems too large relative to geological observations. In this study, we hypothesize the possibility of partial advection of mantle equiden sity surfaces by vertical motion induced by ''driving'' loads. These l arge-scale ''flow-dependent'' loads would greatly reduce the dynamic t opography amplitude., while preserving a good fit to the observed geoi d. Various physical processes related to nonequilibrium phase changes or to the existence of chemical heterogeneity in the mantle could just ify a partial advection of the mean density. In this paper, we simply consider the flow-dependent loads as proportional to the vertical flow velocity. Two density mantle models a;re considered, one from subduct ion reconstruction [Ricard et al., 1993] and one from seismic tomograp hy [Li and Romanowicz, 1995]. We show that a very moderate entrainment (a few kilometers) of the equidensity surfaces in the transition zone is sufficient to reduce dynamic topography amplitude by a factor of 2 or 3. The seismic velocity signal associated with this entrainment wo uld be hidden by the signal of thermal origin. Using this new hypothes is, we compute sea level changes associated with epeirogeny for the Cr etaceous, Paleocene, and Oligocene periods. The amplitude and phase of these changes are in fairly good agreement with geological hypsometri c curves. Our results suggest that not only the thermodynamics, but al so the kinetics of mineralogical phase changes in the transition zone are of crucial importance.