Dating faulted terrace surfaces with thin aeolian loess cover by using terrestrial Be-10 depth profiles: an attempt along the Nobi active fault system, central Japan

High-definition digital elevation models (DEMs) from airborne light detection and ranging (LiDAR) are very powerful tools in detecting unknown tectonic-geomorphic features and quantifying cumulative slip from repeated faulting events. The faulted geomorphic features, however, need to be somehow dated to convert the slip to long-term slip rate, and this task still remains as a challenging part of many tectonic geomorphic and paleoseismic studies. The dating is particularly difficult in mountainous and densely vegetated regions, where we are most benefitted from LiDAR DEMs but most often confront challenges in finding datable organic materials in high-energy gravelly deposits. Here we attempted to date left-laterally faulted fluvial terrace surfaces discovered along the Nobi active fault system (NAFS) in the Etsumi Mountains, central Japan, by using a terrestrial cosmogenic nuclide Be-10. In this region, terrace surfaces are covered with thin aeolian loess deposits of <1 m thick, with generally thinner loess cover on younger and lower surfaces. We employed the depth-profile method to simultaneously determine the age and inherited nuclide concentration, incorporating the effect of loess deposition after terrace abandonment. Exploratory pits for depth profiling were excavated at two sites along the NAFS; the Nukumi-Shiratani site on the low terrace surface along the Nukumi fault and the Nogo site on the middle terrace surface along the Neodani fault. Our results show the terrace abandonment ages that are consistent with the generally accepted terrace-formation and incision history modulated by global climate changes (MIS 2 and MIS 4 for the low and middle terrace surfaces, respectively) and also with crypto tephras identified in the loess deposits. In turn, the long-term left-lateral slip rate for the Nukumi fault was first determined whereas that for the Neodani fault proved to be substantially larger than estimates from previous studies.