Rheology of the Upper Mantle: A Synthesis

TLDR

Rheological properties of the Earth's upper mantle are key to lithosphere and asthenosphere dynamics, but the dominant flow mechanisms remain poorly resolved. Synthesis of laboratory, geophysical, and geological data shows that hot shallow mantle flows by dislocation creep, cold shallow or deep mantle by diffusion creep, and stress‑induced grain‑size reduction weakens the mantle near the transition, localizing deformation and mechanically decoupling it.

Abstract

Rheological properties of the upper mantle of the Earth play an important role in the dynamics of the lithosphere and asthenosphere. However, such fundamental issues as the dominant mechanisms of flow have not been well resolved. A synthesis of laboratory studies and geophysical and geological observations shows that transitions between diffusion and dislocation creep likely occur in the Earth's upper mantle. The hot and shallow upper mantle flows by dislocation creep, whereas cold and shallow or deep upper mantle may flow by diffusion creep. When the stress increases, grain size is reduced and the upper mantle near the transition between these two regimes is weakened. Consequently, deformation is localized and the upper mantle is decoupled mechanically near these depths.