[S-CG44] Evolution and movement of the crustal surface and application of geo- and thermochronology

[SCG44-P02] Fission-track thermochronology and zircon U-Pb geochronology in the Tateyama area of the Hida Range, central Japan: Preliminary results

*Shigeru Sueoka¹, Hideki Iwano^2,3, Tohru Danhara², Takafumi Hirata³, Melanie Kranz-Bartz^4,5, Georgina E. King⁵, Takahiro Tagami⁶ (1.Tono Geoscience Center, Japan Atomic Energy Agency, 2.Kyoto Fission-Track Co., Ltd., 3.Geochemical Research Center, Graduate School of Science, The University of Tokyo, 4.Institute of Geology, Mineralogy, and Geophysics, Ruhr-University Bochum, 5.Institute of Earth Surface Dynamics, University of Lausanne, 6.Division of Earth and Planetary Sciences, Graduate School of Science, Kyoto University)

Keywords:mountain building, Hida Range, fission-track thermochronometry, U-Pb geochronometry

The Hida Mountains are known to expose very young Miocene to Quaternary granitoids, including the world's youngest Kurobegawa Granite, mainly in the Kurobe and Takidani areas (Harayama 1992; Sano+ 2002; Ito+ 2013, 2021). Recent application of Al-in-Hornblende geobarometry (Mutch+ 2016) to the young granitoids in the Kurobe area revealed that rapid exhumation, ranging from ca. 7 to 14 mm/yr, led to the exposure of these granitoids at the surface after ~0.8 Ma (Sueoka+ in rev.). The rapid and localized uplift was caused by the concentration of deformation due to the regional E-W compression on a high geothermal gradient zone along the Kurobe area (Sueoka+ in rev.). The next questions to be addressed in order to validate and develop this model are: 1) to what extent does this concentration of deformation extend across the Hida Mountains, and 2) how does the concentration of deformation correspond to the extent of exposure of young granite? The Tateyama area is located ~5-10 km southwest of the Kurobe area and is one of the steepest areas in the Hida Mountains, with several ~3000 m peaks. The Tateyama Caldera, a Quaternary volcano, is located to the west of the Tateyama area, but other major peaks in the area are non-volcanic mountains composed of Jurassic Funatsu Tonalites. In the Tateyama area, there are no known young granitoids reported by previous studies. In this study, FT thermochronometry and zircon U-Pb geochronometry were conducted on the samples used for OSL/ESR thermochronometry by Bartz+ (2024) to estimate the uplift and denudation history and to confirm the presence of young granitoids. Zircon U-Pb dates were obtained for four samples, collected in the most easterly part of the area, all of which were dated to Jurassic age or older. The three ages are ~200.6-193.8 Ma whilst one age is ~265.4 Ma, which are consistent with previously reported zircon U-Pb ages of Funatsu tonalites in other areas (Takehara & Horie 2019; Yamada+ 2021). Namely, the present measurements did not confirm the distribution of young granitoids in the Tateyama area as in the Kurobe area. On the other hand, the FT dates of zircon and apatite were young, ~30.4-9.1 Ma and ~9.3-1.8 Ma, respectively. The closure temperature of the apatite FT system is ~100°C (e.g., Ketcham+ 2007), yielding a preliminary estimation of exhumation rates of ~0.6-2.8 mm/yr calculated using age2exhume which assumes a steady-state geothermal structure (van der Beek & Schildgen 2023). These exhumation rates are among the highest in the world's mobile belts (e.g., Montgomery & Brandon 2002; Herman+ 2013), although somewhat lower than those in the Kurobe area. In GNSS observations in the Hida Mountains over a period of ~10 years, vertical displacement rate projected onto an E-W direction appears to have reached a maximum near the Kurobe area and decreased westward from the Tateyama area (Nishimura+ 2013). This is consistent with the million-year-scale exhumation pattern in the Kurobe and Tateyama areas and suggests that the uplift pattern as observed by GNSS may have continued over the late Quaternary. In addition, based on the seismic wave velocity structure, the low-velocity zone beneath the Hida Mountains is most prominent beneath the Kurobe area, but also extends to the Tateyama area including the Tateyama caldera (Matsubara+ 2000). Based on the above, it is possible to interpret that the concentration of deformation in the high-temperature zone peaking in the Kurobe area extends to the Tateyama area, but the rate of uplift and denudation is inferior to that in the Kurobe area and did not result in the exposure of young granitoids. Harayama+ (2003) tried to explain the rapid uplift in the Kurobe area by assuming an east-dipping thrust on the west side of the Kurobe area, but since the Tateyama area, which is the foot wall side of the fault, was also estimated to have rapid exhumation rates. It is more reasonable to explain the uplift of Kurobe and Tateyama areas by concentration of deformation along the Kurobe area (Sueoka+ in rev.). The deformation was concentrated in the zone of high geothermal gradient along the Kurobe area, and as a result the central part of the Hida Mountains, including the Kurobe and Tateyama areas, was deformed into something like an anticlinal fold, resulting in the exposure of young granitoids only along the Kurobe area.

Acknowledgments: This study was partly funded by the Ministry of Economy, Trade and Industry, Japan, as part of its R&D supporting program titled “Establishment of Technology for Comprehensive Evaluation of the Long-term Geosphere Stability on Geological Disposal Project of Radioactive Waste” (grant no. JPJ007597; fiscal year 2024).