Heat influx and exhumation of the Shimanto accretionary complex: Miocene fission track ages from the Kii Peninsula, southwest Japan

To determine how local geological events contributed to the evolution of accretionary complexes and eventual exposure of rocks with different structural levels, geochronological mapping was carried out using fission track (FT) analysis at the Kii Peninsula, southwest Japan. At this site, the original zonal structure of Cretaceous accretionary complexes parallel to the subduction zone is disturbed by the northward projection of the Shimanto accretionary complex. Twenty-six zircon FT ages were obtained from an area of similar to12 km in an east-west direction and similar to15 km in a north-south direction, and classified into three groups: (i) ages similar to15 Ma (range similar to10-20 Ma), which are distributed along the northwest-southeast valley; (ii) ages of similar to50 Ma in the northwest of the study area; and (iii) ages older than those in Groups 1 and 2. Based on results from eight zircon FT length distributions, the Miocene ages appear to be the result of spatial variations in heat influx and cooling after the regional exhumation of the area, as recorded by FT ages of similar to50 Ma.