Effects of hydrothermal activity and weathering in the active fault area: formation of large landslide and landslide dam lake, Lake Nakatsuna, Nagano, Japan

Lake Nakatsuna, the central lake of the Nishina Three Lakes located in a narrow intermontane valley in the northern Japanese Alps, is thought to have formed by large landslides damming the paleo-Himekawa River at approximately 30 ka B.P. The mechanism of these large landslides in relation to geological and geomorphological attributes is less understood for the Nakatsuna Landslide. This paper focuses on the geomorphological evolution of Lake Nakatsuna in relation to the lithofacies of landslide blocks in tectonically active, hydrothermally altered, and weathered areas. Geomorphological and geological mapping, characterization of weathering grades and alteration intensity through field observation, petrographic observation, clay mineral analysis by XRD and major bulk geochemistry by XRF methods were adopted to establish the trends and intensity of source rock weathering and its relation to landslide mechanisms. Three paleo-landslide blocks have been identified from geomorphological field observations that may have formed the Lake Nakatsuna landslide dam. Petrographic observation reveals that approximately 30% of the mineral grains are slightly to moderately altered to form clay minerals (chlorite, kaolinite, illite, etc.) derived from hydrothermal alteration and weathering of rocks. CIA values range between 58 and 94 and reveal slight to strong hydrothermal activity. Along with the concentrations of abnormal geomorphic features such as steep slopes, isolated hills and depressions, and extremely loose sediments containing small and large blocks that have broken like a jigsaw puzzle in the hillslope and upper reach, it was determined that there was a large landslide that dammed the paleoriver and formed Lake Nakatsuna.