Deformation-induced Japan twinning in quartz during incipient mylonitization

Many crystalline rocks of the continental crust contain coarse-grained quartz as a main mineral (e.g., granitoids). Incipient deformation of coarse quartz, which likely controls the accumulation of bulk strain in heterogeneously deformed crustal rock volumes, commonly develops microshear zones (MSZs) of localized recrystallization. At mid-crustal conditions, where quartz deformation is mostly accomplished by subgrain rotation recrystallization, grains of MSZs can show an abrupt change in crystallographic orientation (large misorientation angle) with respect to the host quartz that is still not fully understood. We analyzed MSZs (20-200 mu m thick) from deformed coarse-grained (millimeter grain size) quartz veins in the Austroalpine Schobergruppe (Eastern Alps). Electron backscatter diffraction analysis reveals that the MSZs are characterized by a nearly 90 degrees misorientation angle between the c-axes of the host and new grains, which also share one {m} and one {1122} pole, compatible with Japan twinning. This abrupt switch of the c-axis orientation can promote geometrical softening and shear localization. So far, Japan twinning has been interpreted as a growth feature. We show that deformation-induced twinning in quartz, including Japan and Dauphine twinning, can play an important role in initiation of crystal-plastic deformation within the crust.