Although it is well appreciated that rheological properties control a number of geodynamic processes, inferring rheological properties in the deep mantle is challenging. Two methods have been used to infer the rheological properties of Earth’s mantle. One is to use the observed time-dependent deformation due to glacial isostatic adjustment (GIA) processes, and another is to analyze gravity-related observations in terms of the density distributions inside of the mantle (Hager 1984). GIA provides a robust estimate of mantle viscosity, but the GIA observations for the relative sea level (RSL) have little sensitivity to the viscosity of the mantle deeper than ~1200 km
The latter approach can be applied to Earth’s deep interior because density variation driving mantle flow can occur in the deep interior of the Earth and resultant gravity signals can be measured at the Earth’s surface. However, the estimation of density anomalies driving such a flow is difficult because the velocity-to-density conversion factor is not well constrained.
Here we examine two different data sets on time-dependent deformation, the observations on non-elastic deformation associated with Chandler wobble and tidal deformation to place new constraints on the rheological properties of the deep mantle. Deformation associated with Chandler wobble and tidal deformation occurs mostly in the deep mantle (Smith and Dahlen, 1981), and hence the analysis of these time-dependent deformations provides important constraints on the rheological properties of the deep mantle.
These two observations provide important constraints on the viscosity of the D” layer of the Earth’s mantle, the lowermost layer in the Earth’s mantle (~300 km thickness). That is, the effective viscosity in the bottom ~300 km layer is 10^19 ? 10^20 Pa s, and also the effective viscosity of the bottom part of the D” layer (~100 km thickness) is less than 10^18 Pa s. Such a viscosity structure of the D” layer would be a natural consequence of a steep temperature gradient in the D” layer. These estimates are also used to infer the temperature distribution within the D” layer and heat flow from the core to the mantle. Please see the following papers: