Viscosity of the lower mantle as inferred from rotational data.

Until recently most estimates of the deep mantle viscosity were based on analyses of postglacial uplifts, and there were no serious estimates of the sensitivity of the derived viscosity solutions to variations of the input parameters, such as those associated with the surface loading. In this paper the viscosity of the lower mantle is arrived at by making comparisons of the observed secular motions of the earth's rotation axis with theoretical results from a layered viscoelastic, rotating earth, which has been subjected to glacial forcing. Our model, consisting of an elastic lithosphere, a two-layer, adiabatically stratified viscoelastic mantle, and an inviscid core, is essentially analytical, and this makes it economically feasible to use as an aid in conducting an extensive sensitivity analysis of the lower mantle viscosity from changes in the parameters connected with the deglaciaton phenomenon. Both sets of rotational data, polar wander and nontidal deceleration of the length of the day, have been employed to constrain the viscosity structure. On the basis of this type of investigation, the viscosity of the lower mantle is found invariably to be larger than that of the upper mantle and lies between 1-4 X 10²² P. -Authors