The Effects of Mantle Flow and Plate Rheology on the Lithospheric Stress Field

The relationship between surface deformation and mantle-lithosphere coupling depends strongly on the viscosity structure of both the lithosphere and convecting mantle. Lateral variations in the thickness and viscosity of the lithosphere affect both the flow pattern and shear stress magnitude at the base of deeply penetrating continental roots. We examine how lateral variations in mantle shear influence the deformation pattern in a simplified elastic lithosphere. We use CitcomS to compute the instantaneous mantle flow driven by both tomographically-inferred density heterogeneity (mantle) and surface plate motions (plate), and the corresponding basal shear tractions, for both layered and laterally-varying viscosity structures. We use ABAQUS to solve for the response of an elastic lithosphere to the applied net horizontal shear tractions (mantle + plate) for a 3-D spherical shell. While the viscosity structure of the lithosphere has only a minor influence on the orientations of the net basal tractions acting on the lithosphere, the magnitudes of these tractions vary strongly as a function of the lithospheric viscosity structure. As a result, the presence of thick cratonic roots enhances coupling at the base of the lithosphere leading to higher traction magnitudes in and around these regions, which affects patterns of elastic stresses within the lithosphere. Interestingly, the presence of thick viscous roots also leads to lower traction magnitudes in regions adjacent to some of the roots. The magnitude of the elastic stress field produced by basal tractions reflects both the gradients and absolute value of the tractions, so peaks in the elastic stress field magnitude often occur both within and adjacent to thick lithospheric roots. In general, changes in the magnitude of elastic stresses are smaller than those in the net horizontal tractions. In addition to the effects of laterally varying viscosity, we also examine the roles of plate boundary strength and magnitude scaling of the plate-driven basal tractions in determining regional traction and elastic stress patterns. Although an improvement to previous models, our results so far do not account for variable lithospheric rheology between the base of the lithosphere and the base of the elastic lithosphere. As the rheological structure of the lithosphere varies significantly as a function of tectonic province and age, future global flow models with regional mesh refinement should help to determine how sharp variations in the strength of lithosphere influence the relationship between large-scale mantle flow and surface deformation.