Thermal structure of the Kebara Formation and its proximal areas in the western Kii Peninsula, SW Japan: Application of the carbonaceous material Raman geothermometry

Abstract

Thermal structure of the Kebara Formation and its proximal areas in the western Kii Peninsula was examined by Raman spectra of carbonaceous material (RSCM) geothermometer for pelitic rocks. A mean temperature of 313 ± 5 °C is obtained for the Kebara Formation, which is comparable with that of the neighboring unit of the Mikabu belt (319 ± 5 °C). The Sanbagawa belt of the relevant area, within a few kilometers to the north of the Kebara Formation, shows a mean temperature of 285 ± 7 °C, which is slightly but evidently lower value than those of the above two units. Peak temperatures estimated from the Chichibu belt and Shimanto belt located to the south of the Kebara Formation are 289 ± 13 °C and 216 ± 24 °C, respectively. Published geochronological data of the Kebara Formation are slightly older than those of the Sanbagawa belt in the Kii Peninsula and are similar to those of the Mikabu belt in the relevant area. These two data sets (geothermometry and geochronology) suggest that the Kebara Formation is possibly correlated with the Mikabu belt, but is not coherent to the southern margin of the Sanbagawa belt in the western Kii Peninsula, in tectonic contact with each other.