The origin of the anticline founded on fluvial terraces at the eastern part of the Tokamachi Basin, Niigata Prefecture

Abstract

　 Many fluvial terraces formed by Shinano River can be seen in the Tokamachi Basin, which is located between Uonuma and Higashikubiki Hills. Previous researchers reported that some thrusts along the eastern and western margin of the basin cut and deformed those fluvial terraces, and have surveyed their distribution, activities, and structures. Earthquake Research Committee named these faults as western and eastern Tokamachi Fault Zone (TFZ) and revealed each last event based on the results of previous papers and contract works. However, the relationships with other faults around TFZ and their recurrence interval are still debatable. We found an anticline (hereinafter referred as “Hosoo-Nyoraiji anticline”) on the terrace surface by detailed geomorphological analyses around the eastern TFZ, as a part of the evaluation survey of active faults in 2015 conducted by Geological Survey of Japan. Hosoo-Nyoraiji anticline (HNA), which is likely to be symmetric fold, has about 1.5 km wavelength and uplifted the top terrace classified as Maibara group (formed in 140-300 ka) about 20-25 m high. Near Nakazaike village, some steps below of the top terrace classified as Hoonokizaka or Kaisaka (55 ka) groups have been uplifted progressively. In addition, geological survey of the Uonuma Formation covered with the terrace deposits revealed that the upper layer has deformed in line with the deformation of HNA. These results suggest that the anticline had deformed in a time period when the upper Uonuma Formation deposited. Wavelength of the anticline yields that the thickness of deformed layer is about 0.3 km, suggesting that HNA was formed by the slip on the detachment fault inside the Uonuma Formation. Yokokura et al. (Chishitsu News, 2008) also suggested that there is a detachment fault under HNA, and it is connected to the eastern TFZ. This suggests that the detachment fault was formed by horizontal compression along the NW-SE direction, possibly as a secondary fault.