Grain damage, phase mixing and plate-boundary formation

Abstract

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 size 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, which likely depends on grain size itself. Here we present a new model for inter-grain mixing and damage and the onset of rapid mixing. The model considers the competition between the formation of new grains behind a receding interphase triple junction (e.g., olivine growing into a boundary between two pyroxene grains) and their severance or spalling during progressive deformation and damage. The newly formed grains of one phase are then transported along the opposing phase's grain-boundaries and the two phases become dispersed at the grain-scale in a growing mixed layer. The small intermixed grains also affect the grain evolution of the surrounding host grains by Zener pinning, and hence influence the rheology and growth of the mixed layer. As the grains in the mixed layer shrink, subsequently spalled new grains are also smaller, causing a feedback that leads to more rapid mixing and shear localization in the mixed layer. The early stages of mixing can be compared to laboratory experiments, but the transition to a steady-state localized mixed layer, as a proxy for a mylonitic zone, can take a few million years at mid lithospheric conditions. Moreover, a transition in mixing efficiency occurs as grain size reduces, which can induce hysteretic behavior, wherein strong, slowly deforming regions can co-exist with weak rapidly deforming zones, analogous to plate tectonic states with large strong plates and narrow weak boundaries.