Lateral variations of the mantle density and fluctuation of the core-mantle boundary

Abstract

An inverse method to determine lateral density differences in the mantle, as well as fluctuation of the core-mantle boundary, has been developed using long-wavelength geopotential anomalies and the surface velocity field. The mantle is treated dynamically as a self-gravitating, incompressible Newtonian viscous fluid shell with spherically symmetric viscosity, but with a lateral inhomogeneous density. For any one density distribution a mantle flow is calculated. Among possible density distributions that all satisfy observational constraints, one is selected in which the associated velocity field has the least viscous dissipation. The same type of calculation has been carried out for three radially different viscous models. Results show that the level of lateral density variations decreases with depth. A mantle structure, in which the lower mantle viscosity (10 24 P) is 100 times higher than the upper mantle's is unacceptable.