Influences of lower-mantle properties on the formation of asthenosphere in oceanic upper mantle

Asthenosphere is a venerable concept based on geological intuition of Reginald Daly nearly 100 years ago. There have been various explanations for the existence of the asthenosphere. The concept of a plume-fed asthenosphere has been around for a few years due to the ideas put forth by Yamamoto et al. Using a two-dimensional Cartesian code based on finite-volume method, we have investigated the influences of lower-mantle physical properties on the formation of a low-viscosity zone in the oceanic upper mantle in regions close to a large mantle upwelling. The rheological law is Newtonian and depends on both temperature and depth. An extended-Boussinesq model is assumed for the energetics and the olivine to spinel, the spinel to perovskite and perovskite to post-perovskite (ppv) phase transitions are considered. We have compared the differences in the behavior of hot upwellings passing through the transition zone in the mid-mantle for a variety of models, starting with constant physical properties in the lower-mantle and culminating with complex models which have the post-perovskite phase transition and depth-dependent coefficient of thermal expansion and thermal conductivity. We found that the formation of the asthenosphere in the upper mantle in the vicinity of large upwellings is facilitated in models where both depth-dependent thermal expansivity and conductivity are included. Models with constant thermal expansivity and thermal conductivity do not produce a hot low-viscosity zone, resembling the asthenosphere. We have also studied the influences of a cylindrical model and found similar results as the Cartesian model with the important difference that upper-mantle temperatures were much cooler than the Cartesian model by about 600 to 700 K. Our findings argue for the potentially important role played by lower-mantle material properties on the development of a plume-fed asthenosphere in the oceanic upper mantle.