K-Ar age of mica clay minerals in an ultracataclasite of a fossil seismogenic fault in the Mugi Mélange, Shimanto accretionary complex, southwest Japan

Abstract:

We used the K–Ar ages of clay-sized mineral grains to investigate the timing of activity on the fossil seismogenic Minami-Awa Fault, which separates coherent strata of the Shimanto accretionary complex to its north from tectonic mélange to south. The K–Ar ages from the matrix shale of the mélange range from 85 to 48 Ma and decrease with decreasing grain size, indicating that they record a mixture of authigenic illite and 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 ⁴⁰Ar diffused completely from the ultracataclasite between 29 and 23 Ma, which postdates the formation of authigenic illite by about 20 Myr. The diffusion of ⁴⁰Ar 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 mélange from coherent strata in accretionary complexes may slip, not only during accretion, but also long after accretion.