Development of the Median Tectonic Line-related shear zone, southwest Japan: An analysis of strain localization processes

The development of a mylonite zone along the Median Tectonic Line (MTL), southwest Japan, which developed in the Cretaceous granitic rocks, was examined to understand strain localization during cooling. The mylonites are E-W trending, north-dipping, and represent the hanging wall of the MTL. The mylonite zone includes both protomylonite and mylonite, which were derived from two distinct protoliths: tonalite (protomylonite) and granite (mylonite) and occur at distances of c. 0–300 m and 300–700 m from the MTL, respectively. We used quartz microstructures and crystallographic preferred orientations (CPOs) and two-feldspar thermometry to infer the spatiotemporal development of deformation conditions in the MTL mylonite zone. The mylonitic granite is sub-divided into southern and northern mylonite zones, which exhibit A- and B-type quartz microstructure formed by subgrain rotation and grain boundary migration recrystallization, respectively. In both the mylonitic granite and protomylonitic tonalite, quartz c-axis CPOs primarily display a moderate-temperature Y-maximum pattern, as well as a less common low-temperature type-I crossed-girdle pattern. In the northern zone, quartz deformation initially occurred at moderate temperatures of 435 °C–490 °C, forming mylonitic granite with B-type quartz microstructure. As temperature decreased to 355 °C–435 °C, the strain localized into the southern zone to form A-type quartz microstructure. The protomylonitic tonalite formed under similar temperature conditions. At temperatures below 355 °C, ductile deformation ceased in the majority of the granite and tonalite, with only a narrow zone of ultramylonite up to 50 m wide, deforming adjacent to the MTL at conditions approximating the brittle–ductile transition in quartz (c. 300 °C). Strain localization did not occur within a single shear zone, but in several deforming strands that formed in distinct protoliths, one of which developed into the narrow ultramylonite zone reworked as the brittle MTL, likely controlled by cooling.