K-Ar dating of fossil seismogenic thrusts in the Shimanto accretionary complex, southwest Japan

K-Ar ages of clay-sized mineral grains are used to determine the timing of activity on fossil seismogenic faults within the Cretaceous-Paleogene Shimanto accretionary complex, southwest Japan. The samples were collected from the roof thrust of the Upper Cretaceous Okitsu Mélange, which is considered an ancient plate boundary décollement, and the Nobeoka Thrust, which divides the Eocene Kitagawa Group from the Eocene-Lower Oligocene Hyuga Group and is thought to be an ancient major splay fault cutting an accretionary complex. The K-Ar ages of both fault rocks decrease with decreasing 2M₁illite polytype component, indicating that they record a mixture of 1M_dand 2M₁illite polytypes. Calculated ages of the 2M₁illite polytype from two samples using illite dating analysis are 66.1 ± 8.1 Ma and 46.7 ± 8.2 Ma, respectively, which are similar to the depositional age of each host rock. In contrast, ages of the 1M_dillite polytype from two samples are calculated at 18.4 ± 1.2 Ma and 24.4 ± 1.4 Ma, respectively, and are significantly younger than the depositional ages. The diffusion of ⁴⁰Ar or formation of 1M_dillite polytype in the fault rocks was probably caused by frictional heating or high-temperature fluid migration that occurred when the faults were reactivated. The results indicate that the seismogenic faults have continued to slip as reactivated, upper plate faults long after accretion. Furthermore, the calculated ages of the 1M_dillite polytype are similar to the age obtained from another ancient plate boundary décollement exposed to the east of the study area (29-23 Ma; Tonai et al., 2016). Thus, these faults may record a period of early Miocene, active, upper plate fault slip across a wide area of the Shimanto accretionary complex.