2023 Volume 129 Issue 1 Pages 223-238
The lower Miocene strata in the San’in region were presumed to be a graben-fill deposit based on their distribution and the extensional setting. However, previous research has presented little evidence for grabens; only a few syndepositional faults have been found, and the regional stress has been called into question. To collect more basic data that can improve our understanding of the origin of the Miocene basins, we present geological data for the Tajima Mihonoura area, eastern San’in region, Southwest Japan, including the structure of the base of the lower Miocene Yoka Formation, the zircon U‒Pb age of the formation, and the paleostress during its deposition. There is talus or basal breccia at the contact between the basement and the Yoka Formation, and no faults were found along the contact. These observations indicate that the Yoka Formation unconformably overlies the basement. Although the thickness of the lower Miocene strata was used to estimate the shape of the grabens in the San’in region, we show that the thickness in the study area was dependent on the paleogeography during the early Miocene. Zircon from a felsic pumice tuff from the Yoka Formation was dated, yielding a weighted mean 238U‒206Pb age of 19.6±0.15 Ma (2σ) from the grains with concordant Miocene ages. This mean age is consistent with other chronological constraints on the formation obtained in the Tango Peninsula and the Tajima-Myokensan area, which form its eastern and southern margins, respectively. Paleostress inversion from the orientation of dikes related to the Yoka Formation suggests that the study area was subject to NE-SW extension with a low stress ratio. This stress probably represents the regional stress in the eastern San’in region, given the similarity of paleostresses reported recently from other areas of Hokutan.
