Ductile fracture of fine-grained plagioclase in the brittle–plastic transition regime: implication for earthquake source nucleation

Crush zones along strain-localized zones of plastic deformation were found in a small natural brittle–plastic shear zone in the Abukuma Mountains, NE Japan. The optical microstructures indicate that crush zones are developed where plastic deformation is strong. Scanning electron microscopy (SEM) revealed the presence of microvoids which connect together to form intergranular fractures in fine-grained plagioclase layers in strain-localized zones with crush zones. This suggests that the microvoids coalesce in the fine-grained plagioclase layers allowed ductile fracture to form crush zones during plastic deformation. Transmission electron microscopy (TEM) revealed that the dislocation densities of all grains in fine-grained plagioclase layers in strain-localized zones with crush zones are high, while evidence for grain boundary migration and dynamic recrystallization is found away from crush zones. During plastic deformation of plagioclase in the brittle–plastic transition regime, strong plastic deformation with insufficient recrystallization may have resulted in the nucleation of microvoids which link together to form intergranular fractures. This process may be an important mechanism during earthquake source nucleation.