Massive Rock Avalanche Deposits at the Glaciated Tanbosawa Basin, East Face of Mount Tateyama (Oyama), Northern Japanese Alps, Central Japan

A thick mass of and unsorted debris is located in a valley-head cirque in the Tateyama area of the Northern Japanese Alps. The debris shows a characteristic deformation structure with jigsaw cracks, dike-like injections of fines, and shearing. Therefore it is judged to be composed of massive rock avalanche materials that are not of glacial or fluvial origin. Based on the cosmogenic nuclide exposure ages of granitic boulders, the debris is covered by late Pleistocene (18 to 12 ka) glacial, fluvioglacial, or debris flow deposits. The debris suggests that a massive rock slope failure occurred prior to glacial advance in the cirque during MIS2. Further investigations are required at other areas of the glaciated high mountains in central Japan.