Complete Ar resetting in an ultracataclasite by reactivation of a fossil seismogenic fault along the subducting plate interface in the Mugi Mlange of the Shimanto accretionary complex, southwest Japan

Tonai, Satoshi*; Ito, Shun*; Hashimoto, Yoshitaka*; Tamura, Hajimu; Tomioka, Naotaka*

We used the K-Ar ages of clay-sized mineral grains to investigate the timing of activity on the fossil seismogenic Minami-Awa Fault. The K-Ar ages from matrix shale of the mlange range from 85 to 48 Ma and decrease with decreasing amount of detrital mica. In contrast, the K-Ar ages of an ultracataclasite within the fault core are significantly younger, ranging from 29 to 23 Ma, and are unrelated to grain size. This indicates that Ar diffused completely from the ultracataclasite between 29 and 23 Ma. The diffusion of Ar in the ultracataclasite was probably caused by frictional heating or high-temperature fluid migration that occurred when the fault was reactivated. The results indicate that seismogenic faults that separate tectonic mlange from coherent strata in accretionary complex may slip, not only during accretion, but also long after accretion.