Development History of Landslide-Related Sagging Geomorphology in Orogenic Belts: Examples in Central Japan

Small-scale geomorphic features of sagging have been known to occur in mountainous areas in Japan by the analyses of high resolution map images made from the LiDAR data. Development histories of the sagging landforms were analyzed by the GIS analyses, field mapping, machine and hand auger boring, AMS C-14 dating, and tephra geochronology in the Mt. Kanmuriyama, Tsuenomine and Nogo-Hakusan areas. Sediments accumulated in the ridge-top depression in the Mt. Kanmuriyama area yield wood fragments with the ages of 1,234-1060, 6,191-5,996 and 7,931-7,731 cal BP, and also include the K-Ah tephra of ca 7.3 ka. These ages indicate that the depression started to form at the beginning of warm and humid climate after the last glacial period. On the other hand, the double ridges in the Mt. Tsuenomine area formed much earlier before the deposition of the Kikai-Tozurahara tephra about 95 ka. The GIS analyses of sagging in the Mt. Nogo-Hakusan area indicate that most of them are distributed on the flat or gentle surface just above the knick line (erosion front); it means that the knick lines retreat through the repetitive formation and destruction of the sagging landforms.