Generating plate tectonics from the grain to global scale

The generation of plate tectonics on Earth relies on complex mechanisms for shear-localization, as well as for the retention and reactivation of weak zones in the cold ductile lithosphere. Pervasive mylonitization, wherein zones of high deformation coincide with extensive mineral grain-size reduction, is an important clue to this process.  In that regard,  the grain-damage model of lithospheric weakening provides a physical framework for both mylonitization and plate generation, and accounts for the competition between grain-reduction by deformation and damage, and healing by grain growth. Zener pinning at the evolving interface between mineral components, such as olivine and pyroxene, plays a key role in helping drive grains to small mylonitic sizes during deformation, and then retards their growth once deformation ceases.  The combined effects of damage and pinning, however, rely on the efficiency of  inter-grain mixing between phases (e.g., olivine and pyroxene) and grain dispersal.  Grain-scale mixing can be represented as a complex anisotropic, stress-driven chemical diffusion model.  This model predicts mylonitization, self-weakening and shear-localization in mixing zones, while neighboring unmixed zones remain strong.  This shows that  even with uniform stress and temperature at a given layer in the lithosphere, two states of deformation  can emerge and coexist in a hysteretic state, just as weak plate boundaries and strong plates coexist on Earth.