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. Samples were collected from three regional faults that separate hanging wall coherent rocks from footwall subduction mélange: the Goshikinohama Fault that caps the Yokonami mélange, the roof thrust of the Okitsu mélange, and the Nobeoka Thrust that caps the Hyuga mélange. The K-Ar ages of fault rocks decrease with decreasing 2M₁ illite polytype component, indicating a mixture of 1M_d and 2M₁ illite polytypes. Based on illite dating analysis, the extrapolated ages of the pure 2M₁ illite polytype from the Goshikinohama Fault, the roof thrust of the Okitsu mélange, and the Nobeoka Thrust are 79.3 ± 5.0, 66.1 ± 8.1, and 46.7 ± 8.2 Ma, respectively, similar to the depositional age of each host rock. Lower intercepts of regression lines, which correspond to samples containing 100% authigenic illite, are calculated as 50.7 ± 1.4, 18.4 ± 1.2, and 24.4 ± 1.4 Ma, respectively. These ages are significantly younger than both the depositional ages and the timing of accretion. These results indicate that authigenic illite associated with fault slip is not related to underthrusting along the subduction interface but rather formed during out-of-sequence thrusting in the upper plate. Early Miocene slip along faults of the northern Shimanto belt is coincident with major tectonic events along the convergent margin, including collision with elements of the Izu-Bonin volcanic arc-backarc system, and opening of the Japan Sea.