Differential occurrence of rapid and slow mass movements on segmented hillslopes and its implication in late Quaternary paleohydrology in Northeastern Japan

Tephrochronology-assisted studies of the development of hillslopes, which are segmented by a convex break of slope in northeastern Japan, revealed that lower slope segments began to develop in the early Holocene by consumption of upper segments which mostly developed in the late Pleistocene. Detailed distribution surveys of present-day rapid regolith slides, which were induced by heavy rain in the hills composed of semiconsolidated sandstone and siltstone of the Neogene, showed remarkable concentration of slide heads on lower segments where regolith was distributed not so unequally. Radiocarbon dating suggested that time frequency of regolith slides on lower segments is several times higher than on upper segments in the same hills. On the other hand, in the hills where considerably thicker regolith such as saprolite was maintained on their upper portion, upper segments are also extensively affected by rain-induced regolith slides. Time frequency of regolith slide occurrence on headmost walls in upper segments with thick saprolite is almost as high as that on lower segments of the hills composed of semiconsolidated sedimentary rocks. The spatially and temporally differential occurrence of regolith slides in combination with slow mass movements of creep-type allowed a hypothesis which explains the morphological development of hillslopes in a framework of climatic and hydrogeomorphic change in the late Quaternary. The hypothesis implies that such information of hillslope morphology will be useful in the reconstruction of paleohydrologic environment and its change.