4:15 PM - 4:30 PM
[SGL23-04] Relationship between the Cenozoic movement of the Tanakura Fault Zone and the Miocene deformed conglomerate
Keywords:right-lateral movement, Tanakura Basin, Miocene, U-Pb age, FT age
The Cenozoic Tanakura Basin is developed in southern Fukushima Prefecture to northern Ibaraki Prefecture. The Tanakura Basin was formed by the movement of the Tanakura Fault Zone during the opening of the Sea of Japan, and rapid basin subsidence, basin filling, and uplift occurred in a short period around 17-15 Ma (Hosoi et al., 2020). However, there are various theories about the movement of the Tanakura Fault Zone that affected the development of the basin.
Otsuki (1975) examined the movement of the Tanakura Fault using deformed conglomerate recognized in the Neogene along the Tanakura Western Marginal Fault, but there is also a consideration that the deformed conglomerate was formed by the consolidation of weathered gravels (Tagiri et al., 1999). In this study, we examined the relationship between the movement of the Tanakura Western Marginal Fault and deformed conglomerate by using geological surveys around the fault, especially observation of fault outcrops and deformed conglomerate, and zircon U-Pb and FT dating.
As results, we reveal that the Tanakura Fault Zone underwent right-lateral movement after the Neogene deposition (after ~15 Ma) and that deformed conglomerate was formed because of this fault activity. The right-lateral movement of the Tanakura Western Marginal Fault is suggested by the shear fabric developed on the fault outcrop. The deformed gravels of the conglomerate were found to consist of granitic gravels with weakened solidity and arkose sands, but the gravels were apparently elongated along the P-plane of the Riedel shear plane developed by the right-lateral movement of the Tanakura West Margin Fault. We also found some gravels of the conglomerate which are thought to have originated from the Hitachi metamorphic rocks, which is located about 20 km southeast of the study area. It is suggested that the gravels may have been deposited and then transported to the study area by the right-lateral movement of the Tanakura Fault Zone, especially the Tanakura Eastern Marginal Fault. The right-lateral movement of the Tanakura Fault Zone occurred after the deposition of the Neogene conglomerate, and therefore after ~15 Ma.
Otsuki (1975) examined the movement of the Tanakura Fault using deformed conglomerate recognized in the Neogene along the Tanakura Western Marginal Fault, but there is also a consideration that the deformed conglomerate was formed by the consolidation of weathered gravels (Tagiri et al., 1999). In this study, we examined the relationship between the movement of the Tanakura Western Marginal Fault and deformed conglomerate by using geological surveys around the fault, especially observation of fault outcrops and deformed conglomerate, and zircon U-Pb and FT dating.
As results, we reveal that the Tanakura Fault Zone underwent right-lateral movement after the Neogene deposition (after ~15 Ma) and that deformed conglomerate was formed because of this fault activity. The right-lateral movement of the Tanakura Western Marginal Fault is suggested by the shear fabric developed on the fault outcrop. The deformed gravels of the conglomerate were found to consist of granitic gravels with weakened solidity and arkose sands, but the gravels were apparently elongated along the P-plane of the Riedel shear plane developed by the right-lateral movement of the Tanakura West Margin Fault. We also found some gravels of the conglomerate which are thought to have originated from the Hitachi metamorphic rocks, which is located about 20 km southeast of the study area. It is suggested that the gravels may have been deposited and then transported to the study area by the right-lateral movement of the Tanakura Fault Zone, especially the Tanakura Eastern Marginal Fault. The right-lateral movement of the Tanakura Fault Zone occurred after the deposition of the Neogene conglomerate, and therefore after ~15 Ma.