Influence of continental roots and asthenosphere on plate‐mantle coupling

TLDR

Mantle flow shear tractions at the lithosphere base generate plate‑driving forces and lithospheric stresses. The study examines how sub‑lithospheric viscosity variations affect mantle‑induced shear tractions. The authors compare shear tractions from a mantle flow model with laterally varying lithosphere and asthenosphere viscosity to those from a layered‑viscosity model. Lateral viscosity variations leave traction direction unchanged, but thick continental roots amplify traction magnitudes 2–5×, whereas a low‑viscosity asthenosphere modestly reduces magnitudes and only matters if >100 km thick; thus, strong coupling beneath thick lithosphere can enhance plate‑driving forces, surface deformation, and lithospheric stresses, while a low‑viscosity asthenosphere does not decouple the lithosphere.

Abstract

The shear tractions that mantle flow exerts on the base of Earth's lithosphere contribute to plate‐driving forces and lithospheric stresses. We investigate the sensitivity of these tractions to sub‐lithospheric viscosity variations by comparing shear tractions computed from a mantle flow model featuring laterally‐varying lithosphere and asthenosphere viscosity with those from a model with layered viscosity. Lateral viscosity variations generally do not change the direction of shear tractions, but deeply penetrating continental roots increase traction magnitudes by a factor of 2–5 compared to 100 km thick lithosphere. A low‐viscosity asthenosphere decreases traction magnitudes by a smaller amount, and is important only if >100 km thick. Increased plate‐mantle coupling beneath thick continental lithosphere may increase plate‐driving forces, surface deformation, and mantle‐derived lithospheric stresses in these regions. By contrast, a low‐viscosity asthenosphere does not decouple the lithosphere from mantle flow, highlighting the geological importance of mantle tractions on the lithosphere.