2011 Volume 120 Issue 6 Pages 1003-1012
The applicability of low-temperature thermochronology has been extended considerably over the past decade and the successful application of (U-Th)/He (He) thermochronometry is one of the most noteworthy advances. In this study, a zircon He analysis is used to identify the denudational history and pattern of the Akaishi Range, which has been uplifted since the late Pliocene. Zircon He grain ages from nine samples range from 21.5 to 3.0 Ma, while the ages are systematically younger to the east. These ages are interpreted to reflect the uplifting of the Akaishi Range because the youngest ages are consistent with the age at which uplifting was initiated according to the depositional ages of gravel. The decreasing ages to the east can be explained by subsequent denudation of the uplifted Akaishi Range, assuming a westerly tilting uplift of the region west of the Itoigawa-Shizuoka Tectonic Line (ISTL). Although denudation cannot be identified exactly because of a lack of precise estimates of the paleo-geothermal gradient of the study area, it is certain that the entire area between the Median Tectonic Line and ISTL has been denuded by a few kilometers since the onset of the range uplift. This implies that the topography of the Akaishi Range reflects post-uplift factors, e.g., spatial distribution of bedrock uplift rates and various denudation processes, rather than inherited geometry from the pre-uplift topography.
