[S-CG54] Evolution and movement of the crustal surface and application of geo- and thermochronology

[SCG54-P04] Exhumation history of the central Akaishi Range based on zircon (U-Th)/He thermochronology (preliminary report)

*Shigeru Sueoka¹, Barry Paul Kohn², Takahiro Tagami³ (1.Japan Atomic Energy Agency, 2.School of Geography, Earth and Atmospheric Sciences, University of Melbourne, 3.Division of Earth Sciences, Graduate School of Science, Kyoto University)

Keywords:(U-Th)/He thermochronology, Akaishi Range, exhumation, arc-arc junction

The Akaishi Range, located in the southern Japanese Alps, formed at the junction of the NE and SW Japan arcs. It also forms part of the South Fossa Magna region, which is situated in an active collision zone between the Izu-Bonin Arc and Honshu Island. Elucidating the history and uplift mechanism of the Akaishi Range contributes to an improved understanding of tectonic processes at the junction of island arcs. To date, the uplift history of the Akaishi Range has been constrained mainly by the depositional ages of gravels sourced from the Range into adjacent basins. Gravel deposition in the middle reaches of the Fuji river to the east commenced in the late Pliocene (Kano, 2002, in the Ina basin to the west in the early Pleistocene (Moriyama & Mitsuno, 1989; Matsushima, 1995), and in the Kakegawa region to the south in the middle Pleistocene (Muto, 1985). These observations suggest that uplift of the Akaishi Range began in the northeast in the late Pliocene and then extended to the west and south through the Quaternary. The uplift mechanism has mainly been attributed to two factors: thrusting of the Itoigawa-Shizuoka Tectonic Line fault zone (ISTL-FZ) along the eastern margin of the Akaishi Range (e.g., Ikeda, 1990) and collision of the Izu-Bonin arc from the southeast (e.g., Kano, 2002). Sueoka et al. (2017) reported low-temperature thermochronometry data and thermo-kinematic modeling (Braun et al., 2012) across the northern Akaishi Range to elucidate the uplift mechanism. Based on those results, it was proposed that tilting to the west accompanied uplift of the northern part and commenced in the late Pliocene as a result of thrusting of ISTL-FZ. However, the southern part may have been uplifted by an alternative mechanism because the Akaishi Range is not bounded by active faults in that area. Furthermore, topographic cross-sections across the northern part indicate asymmetric profiles with peaks shifted to the eastern side (i.e., adjacent to the ISTL-FZ), whereas peaks in the southern part were displaced further to the western side. This study reports preliminary zircon (U-Th)/He thermochronometry data from across the central Akaishi Range. The weighted mean ages obtained range between ~19 to 2 Ma and generally young to the east indicating that, similar to the northern part, the central part was uplifted and tilted to the west as a result of thrusting on the ISTL-FZ. Using the age2exhume code (van der Beek and Schildgen, 2023) and accounting for the dip of ISTL-FZ, exhumation rates for the central part are tentatively computed to be 0.7-3.2 mm/yr. Future work will aim at acquiring further thermochronometric data from the central and southern parts and performing further thermo-kinematic modeling towards obtaining a more comprehensive understanding of the uplift mechanism of the Akaishi Range.

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 (JPJ007597) (Fiscal Year 2022)”. Sample collection was helped by Dr. Masakazu Niwa, Dr. Kunimi Yamada, Mr. Kazuo Kobori, and Mr. Ayumu Makuuchi at Japan Atomic Energy Agency. Mineral separation was performed by Dr. Tohru Danhara and Dr. Hideki Iwano at Kyoto Fission-Track Co., Ltd.