Abstract
Gravelly deposits around mountain ranges are important clues to elucidate the uplifting process of the ranges. Our lithological analysis of 3139 alluvial-fan gravels from the Karuma Gravel Bed (3-4 ka) at the eastern foot of the Northern Japan Alps (the Ushirotateyama Range) clarifies that they are derived mostly from the lower unit of caldera fills and the flooring granite exposed on the higher slopes of the range. Major provnances of older two gravel beds (ca. 0.8 Ma and 1.7-2.2 Ma) are the upper unit of the caldera fills and the Pre-Neogene basement rocks on the caldera rim slopes, respectively. The caldera fills, called the Jiigatake-Shirasawatengu caldera complex (2.3-1.6 Ma), tilted eastward up to 70°- 80° by 0.6 Ma (Harayama et al., 2003). The first change of gravel source in around 1.5-1.0 Ma thus seems to have been due to the beginning of the tilting and uplifting of the caldera and the second in around 0.7-0.1 Ma also to the subsequent erosion of the caldera fills.
