Mechanisms for the generation of plate tectonics by two-phase grain-damage and pinning

Weakening and shear localization in the lithosphere are essential ingredients for understanding how and whether plate tectonics is generated from mantle convection on terrestrial planets. We present a new theoretical model for the mechanism of lithospheric weakening and shear-localization and hence plate generation through damage, grain evolution and Zener pinning in two-phase (polycrystalline) lithospheric rocks. Grain size evolves through the competition of coarsening, which drives grain growth, with damage, which drives grain reduction. However, in a two-phase medium the interface between phases induces Zener pinning, which impedes grain growth and facilitates damage. The size of the pinning surfaces is given by the roughness of the interface, and damage to the interface causes smaller pinning surfaces, which in turn drive down the grain-size, forcing the rheology into the grain-size-dependent diffusion creep regime. This process allows damage and rheological weakening to co-exist, which is normally considered impossible in single phase assemblages. Moreover pinning greatly inhibits grain-growth and shear-zone healing, which is much faster in single phase materials. Hence, the resulting shear-localization is rapid (less than 1 Myr), but the healing time for a dormant weak zone is very slow (greater than 100 Myrs); these effects therefore permit rapidly forming and long-lived plate boundaries. The model therefore provides a key ingredient and predictive theory for the generation of plate tectonics on Earth and other planets.