研究業績

Determining the depositional ages of the Early–Middle Miocene forearc basin deposits in southwestern Japan is essential for understanding the Neogene deformation and stress history of the forearc region. To constrain the depositional age of the Tanabe Group, an Early–Middle Miocene forearc basin sedimentary sequence, we analyzed the U–Pb ages of detrital zircon separated from eight sedimentary rock samples, and obtained 221 concordant ages from 330 zircon grains. The age of the youngest grain is 19.4 ± 0.6 Ma, which constrains an upper limit for the depositional age. However, the youngest grain is much older than a foraminiferal age (16.3–15.1 Ma) for the deposition of the group. The absence of zircon ages corresponding to the widespread mid-Miocene felsic volcanic activities on Kii Peninsula at ~15 Ma suggests that the deposition of the Tanabe Group ended before these volcanic events. The discrepancy between the youngest zircon age and depositional age can be attributed to volcanic quiescence in the provenance area during the deposition of the Tanabe Group. We also suggest that the E–W tensional stress recorded by synsedimentary clastic dikes was exerted before ca. 15 Ma in the southern part of Kii Peninsula.