[S-CG61] Dynamics in mobile belts

[SCG61-18] Formation of the Kinki triangle zone, SW Japan

*Hiroshi Sato^1,2 (1.Earthquake Prediction Research Center, Earthquake Research Institute, The University of Tokyo, 2.Center for Integrated Research and Education of Natural Hazards, Shizuoka University)

Keywords:Kinki Triangle zone, Opening of the sea of Japan, Inversion tectonics, deformation of overriding plates

The Kinki triangle zone (KTZ) is marked by dense distribution of reverse active faults. The formation of such reverse faults is investigated using deep seismic profiling, earthquake tomography and geology. The common idea on the formation of the reverse active faults is due to strong horizontal EW trending compressional stress. However following this idea, some paradoxical problem remains. The crustal thickness revealed by seismic tomography (Matsubara et al., 2017) in KTZ shows slightly tinner than that of surrounding area. According to the deep seismic profiling, the dip angle of the active faults in the seismogenic zone are moderate and obviously higher than the initial dip angle of thrust faults. Structure of Japanese islands was strongly controlled by the rifting and drifting processes from Asiatic continent. In the final phase of the opening of the Sea of Japan, SW Honshu bent associated with the collision of Izu-Ogasawa arc. The inner zone of the eastern SW Honshu deformed forming mega-kink structures (Kano et al., 1990). The Median Tectonic Line (MTL), which divides inner and outer Japan, acts as weak surface and allowed the deformation of inner zone as a single beam. The bend in the SW Japan formed by the collision of Izu-Ogasawara arc is recognized by the trend of MTL, it starts from the eastern end of the KTZ. By the northward bend of inner zone of SW Japan, extensional deformation occurred southern part of the KTZ, which produced NS-trending normal faults (Figure 1). By the EW horizontal compressional stress in Quartenary, reactivation of NS-trending normal faults as reverse fault produced the moderate angle reverse faults. Extended crustal thickness in the Miocene, has been back up due to Quaternary shortening of KTZ. The increase of EW-horizontal compressional stress is manifested by the increase of slip rates of NS-trending reverse active faults, such as the Uemachi fault. The change in stress state probably derived from the change in motion of PHS from NNW to WNW. As additional factor to increase the horizontal stress is due to the compression from the contact of upper surface of PHS. The deep seismic profiling revealed that beneath the Yoro Mountains the upper surface PHS located in the lower crust. WNW-motion of the PHS pushes the NNW-trending arch beneath the Yoro Mountains and increases the horizontal compression of the KTZ. The rheology of the overriding plate, such as jelly sandwich structure, makes the complicate process to transfer the additional stress in jelly layer (lower crust) to the upper crust. To solve this problem research on the rheology of overriding plate will be required.
Reference: Kano, K. et al. (1990) Tectonophys., 176, 333-3. Matsubara, M. et al. (2017) Tectonophys., 710-711, 97-107.