Landslide-induced terminal moraine-like landforms on the east side of Mount Shiroumadake, Northern Japanese Alps

Previous studies have interpreted landforms east of Mount Shiroumadake (2932 m asl), in the Northern Japanese Alps, as lateral and terminal moraines and outwash terraces of glacial origin. However, recent studies in other areas of the Northern Japanese Alps have interpreted similar landforms as characteristic landslide bodies. We re-examined the origin and age of moraine-like landforms on the eastern side of Mount Shiroumadake through field geological surveys and landform analysis. We show that terminal moraine-like landforms in our study area consist of beds of brecciated or crushed gravel with locally derived crackle clasts and are unrepresentative of the variety of bedrock expose in the plausible source areas of Pleistocene glaciers. We interpret these deposits as induced by mass rock creep and landsliding at the terminus of a steep ridge or a steep valley side slope. Similarly, terrace surfaces formerly-interpreted as of outwash or fluvial origin consist of gravel beds with many fractured rock clasts of landslide origin. Although the timing of the landslides is not well constrained, they may have occurred in the early to middle Holocene. The highly deformed rocks of the study area, combined with the humid climate with much snowfall and rainfall, have left the landscape with an inherent predisposition for landsliding. Paleoearthquakes on nearby active faults in the piedmont area of Mount Shiroumadake may have provided a direct trigger for landsliding.