Mantle Rheology, Viscomagnetic Coupling At the Core‐Mantle Boundary and Differential Rotation of the Core Induced By Pleistocenic Deglaciation

Abstract

The influence of the viscomagnetic coupling at the core‐mantle boundary (CMB) on the differential rotation of the core and non‐tidal acceleration induced by the Pleistocenic deglaciation is investigated for stratified mantle models with steady‐state or transient rheologies. For a realistic modelling of the viscomagnetic coupling, the time‐dependent viscoelastic topography at the CMB induces a westward drift of the core with respect to the mantle that can be correlated to the zonal component of the secular drift of the geomagnetic field. Starting from a reference model based on the Maxwell rheology and a viscosity profile inferred from j2 data and non‐tidal acceleration, with 1021 Pa s and 5 × 1021 Pa s for upper and lower mantle respectively, we study first the effects of the hardening of the upper mantle in the transition zone between 420 and 670 km depth. This hardening is responsible for a 30 per cent reduction in the westward drift with respect to a uniform upper mantle. the impact of a viscosity decrease on the top of the lower mantle is considered next. the softening of the lower mantle beneath the 670 km discontinuity counteracts the effects of the transition zone, enhancing the differential rotation of the core. the D″ layer is also implemented in order to analyse the influence of the decoupling at the bottom of the mantle, responsible for faster relaxation of the topography. If lower mantle viscosity is increased beyond the threshold of 1022 Pas we obtain a differential rotation of the core in the opposite direction or an eastward drift. We also analyse the impact of a Burgers rheology for the lower mantle to simulate transient effects. For this rheology, steady‐state viscosities in the lower mantle higher than 1022 Pas, in agreement with estimates from long‐wavelength geoid anomalies and recent findings from true polar wander, allow a westward drift of the core. Non‐tidal acceleration is generally less affected by rheological variations than the differential rotation. Changes in the non‐tidal acceleration induced by the various rheological models are, on the other hand, comparable with the error bounds in the observed values. Copyright © 1994, Wiley Blackwell. All rights reserved