Constraining the timing of brittle deformation and faulting in the Toki granite, central Japan

Constraining of the timing of fault zone formation is of fundamental geotectonic importance to understand structural evolution and brittle fault processes. Here, we present authigenic illite K–Ar age data from brittle fault zones comprising two gouges within the Toki granite, central Japan. The gouge samples were collected at depths of 252.9 and 403.7 m from a shaft at the Mizunami Underground Research Laboratory, and were separated into five grain-size fractions (< 0.1, < 0.4, < 2, 2–6, 6–10 μm). K–Ar ages of clay fractions decrease with grain size, suggesting enrichment in finer fraction of more-recently grown authigenic illites. The K–Ar ages of the fractions range from 53.6 to 42.7 Ma (Paleogene–Early to Middle Eocene). The < 0.1 μm fractions yield ages of 42.7 ± 0.9, and 46.5 ± 1.0 Ma. This age range is consistent with the stability field of illite (i.e. 100–200 °C) and the main temperature field of brittle deformation (< 300 °C) within the cooling history of the Toki granite, supported by extensive thermochronological data of the host rock.