ISOSTATIC DEFORMATIONS AND POLAR WANDER INDUCED BY REDISTRIBUTION OF MASS WITHIN THE EARTH

The effects at the surface of the Earth of time-dependent mantle mass anomalies are analyzed within the framework of a viscoelastic mantle with Maxwell rheology. The implications for the Earth's rotation are developed using the linearized Liouville equations valid for small polar displacements. Our approach is appropriate for a simplified modeling of subduction. The displacement of the Earth's axis of rotation, called true polar wander, is very sensitive to the viscosity profile of the mantle and to the nature of the 670-km seismic discontinuity. Phase change models generally yield a huge amount, of polar wander, except for large viscosity increases. For chemically stratified models, true polar wander is drastically reduced as a consequence of dynamic compensation of the mass anomalies at the upper-lower mantle interface. When the source is embedded in the upper mantle in the proximity of the chemical density jump, the polar wander stops after a finite time controlled by the slowest isostatic mode.