Mechanisms of strain localization and nucleation of earthquake faulting by grain-scale processes at the middle crustal level

The mechanism of strain localization is the key to our understanding of the transition from steady-state to unstable flow, and therefore of earthquake faulting in the middle crust. In this study, biotite grains in mylonitic gneisses along the Jinzhou detachment fault zone, Liaodong peninsula, northeast China, acted as a preexisting weak phase that had important influences on deformation of mid-crustal rocks. High phase strength contrasts between biotite and other mineral phases resulted in stress concentrations at the tips of biotite grains and induced semi-brittle deformation of neighboring quartz and feldspar grains. As a consequence, the biotite grains became interconnected to form zones of weakness, while basal plane slip and grain boundary sliding operated in biotite grains and fine-grained biotite-feldspar-quartz aggregates, respectively. The zones filled with biotite grains and fine-grained quartz-feldspar aggregates continued to propagate and coalesce during the deformation. These processes led to transition from load-bearing (i.e., coarse plagioclase grains) framework to interconnected weak phase (i.e., biotite grains and finegrained feldspar aggregates) domination, that further led to the formation of initial strain localization zones (SLZs). With the propagation and linkage of the SLZs, high stress concentrations at the tips of the SLZs led to nucleation of rupture along the SLZs. As a consequence, there occurred an abrupt increase in strain rates that resulted in transition from stable to unstable slip within the SLZs. The processes were accompanied by occurrence of mid-crustal earthquake faulting and formation of pseudotachylites along the SLZs.