[S-CG47] Evolution and movement of the crustal surface and application of geo- and thermochronology

[SCG47-P01] Classification of developing stages of Japanese mountains based on the multi-timescale denudation rates

*Shigeru Sueoka¹, Tetsuya Komatsu¹, Yuki Hiura¹ (1.Japan Atomic Energy Agency)

Keywords:mountain development, denudation, sediment yields in catchments, in-situ terrestrial cosmogenic nuclides, low-temperature thermochronology

This study suggests a new classification of Japanese mountains based on the developing stages (Ohmori, 1978) identified from the multi-timescale (10¹-10⁷ yr) denudation data. The Japanese Islands are active island arcs, where mountains of varied sizes and morphologies are formed reflecting the variation in tectonics, geology, and climate. Mountain classifications are useful to organize their features and to understand the geoscientific implications. Japanese mountains were classified by their formation mechanisms (Kaizuka & Chinzei, 1986; Koaze, 2007) and morphologic features (Yamada, 1999; Yonekura et al., 2001). However, the formation mechanisms are not generally well-constrained and often consist of multiple factors. The morphologies provide clearer criteria for classifications, but the geoscientific implications are not simple. More recent attempts classified Japanese mountains based on Ohmori’s (1978) developing stages of mountains (Yasue et al., 2011; Asamori et al., 2012). They identified the developing stages from the ratio of uplift and denudation rates because the two rates will be in dynamic equilibrium as mountains develop. This approach has advantages of quantitative criteria and clear geoscientific implications. Nonetheless, there are still problems mainly on estimation of the uplift rates. Uplift rates are generally difficult to estimate in highly-denuded mountainous regions, where geomorphic markers are rarely developed/preserved. In addition, in slowly-denuded regions, uncertainties of the rates affect greatly on identification of the developing stages. This study attempts to identify the developing stages more accurately, by focusing on temporal variations in the denudation rates. Three datasets were used covering a different duration, i.e., sediment yields in catchments (<~10¹ yr), in-situ terrestrial cosmogenic nuclides (~10²-10⁴ yr), and low-temperature thermochronology (~10⁶-10⁷ yr). Details of the datasets are provided in Hiura et al. (submitting) and Sueoka & Tagami (2019). In the result, fourteen representative mountains were grouped into the following four; denudation rates are hereinafter referred as class A (1-10 mm/yr), class B (0.1-1 mm/yr), and class C (0.01-0.1 mm/yr). The first group has had denudation rates of class A in the last ~10⁶ yr. This group contains ~3,000-m-high mountains in the Japanese Alps, being at the “culminating stage” where uplift and denudation are in dynamic equilibrium (Yoshikawa, 1984). The second group increased denudation rates from class B to A in the last ~10⁶ yr. This group contains ~1,000 to ~2,000-m-high mountains in the Kinki region and back-arc of the NE Japan, being at the late “developing stage” due to the slower denudation rates than the first group. The third group has constant denudation rates of class C or B. This group consists of low-relief mountains with many low-relied erosion surfaces, e.g., the Kitakami, Abukuma, and Chugoku Mountains, being at the early “developing stage”. The fourth group decreased denudation rates from class A to B. This group contains the Hidaka Mountains which was actively uplifted in the middle Miocene and later inactivated, being at the “declining stage” in which uplift is aborted and elevations are reduced. We are planning to verify and enhance reliability of the classifications by 1) applying the approach to more mountains, 2) increasing denudation data in each mountain, and 3) obtaining denudation rates in ~10⁴-10⁵ yr using the recently-developed optical stimulated luminescence (OSL) thermochronology (Ogata & Sueoka, in press).



Acknowledgements:
This study was funded by the Ministry of Economy, Trade and Industry (METI), Japan as part of its R&D supporting program titled “Establishment of Advanced Technology for Evaluating the Long-term Geosphere Stability on Geological Disposal Project of Radioactive Waste (Fiscal Years 2018 and 2019)”.