[S-GL23] Geologic structure and tectonic history of East Asia and Japanese Islands

[SGL23-P05] Strike-slip faulting stress accompanying map-scale folds in the lower Miocene Kurami Group, central Shizuoka

*Noriaki Abe¹, Katsushi Sato¹ (1. Division of Earth and Planetary Sciences, Graduate School of Science, Kyoto University)

Keywords:Kurami Group, Kanto Syntaxis, Izu collision zone, fault-slip analysis, paleostress, Miocene

Kanto Syntaxis is a map-scale bending structure of the Median Tectonic Line and surrounding beltlike geologic structures in central Japan. The syntaxis is considered to form related to the collision between the Honshu arc and the Izu-Bonin-Mariana arc. Therefore, elucidating the evolution process of the syntaxis will give constraints about the plate configuration and the subducting direction. The western side of the syntaxis is divided into some blocks by map-scale N–S trending sinistral strike-slip faults (e.g. Akaishi Tectonic Zone). In the southern end of the Akaishi Mountains, lower–middle Miocene basins are distributed intermittently. Among them, the Futamata, Ohira, and Ieda groups locate on the western side and are considered to be taken in strike-slip duplex through the formation and evolution of the Akaishi Tectonic Zone (Kano et al., 1993). On the other hand, the Kurami Group located on the eastern side suffered more small strain than the western groups, and the deformation style of the Kurami Group has not been discussed enough. Therefore, it is important to elucidate the deformation and stress history in the Kurami Group for understanding the spatiotemporal strain distribution at the early time of Kanto Syntaxis formation. This study measured and analyzed outcrop-scale faults in the Kurami Group and estimated paleostress history.
The Kurami Group is characterized by map-scale folds with NE–SW trending axes. Whereas, the Saigo Group covering the Kurami Group unconformably shows south to southeastward monoclinal structure and is not folded (Shiba et al., 2020). It indicates that the folding in the Kurami Group had occurred before the Saigo Group deposited (around 16 Ma). In the Kurami Group, 229 outcrop-scale fault data were measured and analyzed. In addition, this study applied tilt correction of the faults to determine the temporal relationship between detected stresses and folding. As a result, it was revealed that strike-slip faulting stress with NW–SE σ₁ axis and large stress ratio (Stress α) had acted during folding, and strike-slip faulting stress with ENE–WSW σ₁ axis and small stress ratio (Stress β) had exerted after folding. The outcrop-scale faults consistent with Stress α are often accompanied by quartz veins of en echelon array, suggesting that the faults were formed under strike-slip faulting stress.
The directions of fold axes in the Kurami Group are consistent with the σ_Hmin direction of Stress α. If the Kurami Group folded under strike-slip faulting stress, this region would be transpressional around 16 Ma. The Kurami Group is distributed at the extension of map-scale N–S trending sinistral strike-slip faults, characterizing the western side of Kanto Syntaxis. The Kurami Group may have folded related to the map-scale strike-slip fault activities.

References
Kano, K. et al. (1993) Mem. Geol. Soc. Japan, 42, 203–223.
Shiba, M. et al. (2020) Earth Sci. (Chikyu Kagaku), 74, 137–155.