The zircon (U-Th)/He (ZHe) system with a typical closure temperature of ~160-200°C*1, but lower for higher radiation damaged grains*2, offers the potential for evaluating thermal histories in the uppermost ~10 km of the crust. ZHe thermochronometry has been applied to different geological settings in order to estimate tectonics, uplift and denudation, basin evolution, etc.*3, which can also contribute to evaluating long-term tectonic stabilities for the geologic disposal project. So far, the effectivity of ZHe thermochronology has been verified, however improved age standards for the method are required. To date, the method has conventionally employed zircon fission-track age standards such as the Fish Canyon Tuff (FCT) zircon*4. ZHe grain ages are sometimes over-dispersed owing to factors such as zoning of parent nuclei, radiation damage, grain size and He-bearing inclusions*2,5. Considerable parent isotope zonation was reported in some FCT crystals*6, inviting a search for alternative potential ZHe standards*7,8,9. These works reported robust ZHe data with little age dispersion because of homogeneous U-Th distribution in zircon megacrysts, making them possible reference material candidates. However, a practical issue remains because ZHe analyses of unknown samples are carried out grain-by-grain as opposed to analyzing large pieces of a single grain. We have attempted to assess suitable zircon samples as ZHe age standards by using rapid cooling rock samples of relatively young (<100 Ma) age. This is because such rock samples are expected to empirically exhibit simple thermal histories and little radiation damage. Therefore, age dispersion caused by radiation damage can be relatively small. In order to reassess previous data obtained by Tagami et al. (2003)*10, ZHe analyses of the Pliocene Utaosa rhyolite (TRG-04 and -07) and the Miocene Buluk Tuff have been carried out. In addition, OD-3 zircon*11, a zircon U-Pb age standard, was also analyzed. In this presentation, preliminary ZHe age data from these samples will be presented and compared to evaluate their suitability as ZHe reference materials e.g., FCT. References 1: Reiners et al. (2004), Geochim. Cosmochim. Acta, 68, p. 1857–1887 2: Guenthner et al. (2013), Am. J. Sci., 313, p. 145–198 3: Ault et al. (2019), Tectonics, 38, p. 3705–3739 4: Gleadow et al. (2015), Earth Planet Sci. Lett., 424, p. 95–108 5: Danišík et al. (2017), Sci. Adv., 3, p. 1–9 6: Dobson et al. (2008), Geochim. Cosmochim. Acta, 72, p4745–4755 7: Li et al. (2017), Geostand. Geoanal. Res., 41, p. 359–365 8: Yu et al. (2020), Geostand. Geoanal. Res., 44, p. 763–783 9: Kirkland et al. (2020), Geochim. Cosmochim. Acta, 274, p. 1–19 10:Tagami et al. (2003), Earth Planet Sci. Lett., 207, p. 57–67 11:Iwano et al. (2013), Isl. Arc, 22, p. 382–394 Acknowledgements This study was supported by the Ministry of Economy, Trade, and Industry (METI) of Japan.

Optically stimulated luminescence (OSL) thermochronometry is a tool for constraining cooling histories in low-temperature domains (several tens of degree Celsius) during the past 104–105 years [1][2][3]. This method is currently applied only to rapidly denuded regions (~5mm/yr when a general geothermal gradient in is assumed to be ~0.03℃/m) because luminescence signals in slowly denuded regions saturate before the rocks are exhumated to the surface. However, cooling histories in slowly denuded regions may be constrained if unsaturated samples are obtained from deep boreholes. In addition, using deep borehole core enable to compare the results between samples at multiple depths, which is useful to isolate the denudation history from other events, such as faulting or hydrothermal activity. We applied multi-OSL-thermochronometry [2] to the deep borehole core drilled at the Rokko Mountains, Japan, where slow denudation rates (0.1-1.0 mm/yr) are expected from previous studies [4][5][6]. We used the Kabutoyama core collected by National Research Institute for Earth Science and Disaster Resilience [5][7]. The total length of Kabutoyama core is 1,313 m and we collected the samples at 408, 642, 818 and 1048 m for OSL-thermochronometry. Our results showed that useful thermal information can be extracted from the infrared stimulated luminescence signals of samples collected at depths ≥408 m. We found that the sample temperatures remained around the present ambient temperature at each depth for the last 0.1 Myr, indicating that the Rokko Mountains is topographically stable, which was consistent with previous findings. Thus, the thermal denudation history of slowly denuded regions may be constrained by multi-OSL-thermochronometry using samples from deep borehole cores. However, the denudation rates in the Rokko Mountains were too low and could not be determined by this method. Further research is required to quantify the denudation rate. 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 2019-2021)”. References: [1] Herman et al. (2010). Earth and Planetary Science Letters, 297, 183-189; [2] King et al. (2016). Quaternary Geochronology, 33, 76-87; [3] Herman and King (2018). Elements, 14, 33-38; [4] Huzita (1968). The Quaternary Research, 7, 248-260; [5] Sueoka et al. (2010). Journal of Geography, 119, 84-101; [6] Matsuhi et al. (2014). Transactions, Japanese Geomorphological Union, 35, 165-185; [7] Yamada et al. (2012). Technical Note of the National Research Institute for Earth Science and Disaster Prevention, 371, 27p.

Arc-arc collision plays an important role in the formation and evolution of continents (e.g., Yamamoto et al., 2009; Tamura et al., 2010). The Izu collision zone central Japan, an active collision zone between the Honshu Arc and the Izu-Bonin Arc since the middle Miocene (Matsuda, 1978; Amano, 1991; Kano, 2002; Hirata et al., 2010), provides an excellent setting for reconstructing the earliest stages of continent formation. Multi-system geo-thermochronometry was applied to different domains of the Izu collision zone, together with some previously published data, in order to reveal mountain formation processes, i.e., vertical crustal movements. For this study nine granitic samples yielded zircon U–Pb ages of 10.2–5.8 Ma (n = 2), apatite (U–Th)/He ages of 42.8–2.6 Ma (n = 7), and apatite fission-track (AFT) ages of 44.1–3.0 Ma (n = 9). Thermal history inversion modelling based on the AFT data using HeFTy ver. 1.9.3 (Ketcham, 2005), suggests rapid cooling events confined to the study region at ~5 Ma and ~1 Ma. The Kanto Mountains are thought to be uplifted domally in association with collision of the Tanzawa Block at ~5 Ma. But this uplift may have slowed down following migration of the plate boundary and late Pliocene termination of the Tanzawa collision. The Minobu Mountains and possibly adjacent mountains may have been uplifted by collision of the Izu Block at ~1 Ma. Mountain formation in the Izu collision zone was mainly controlled by collisions of the Tanzawa and Izu Blocks and motional change of the Philippine Sea plate at ~3 Ma (Takahashi, 2006). Earlier collisions of the Kushigatayama Block at ~13 Ma and Misaka Block at ~10 Ma appear to have had little effect on mountain formation. Together with ~90° clockwise rotation of the Kanto Mountains at 12-6 Ma (Takahashi & Saito, 1997), these observations suggest that horizontal deformation predominated during the earlier stage of arc-arc collision, whereas vertical movements due to buoyancy resulting from crustal shortening and thickening developed at a later stage. References: Amano, K., 1991, Modern Geol., 15, 315-329; Hirata, D. et al., 2010, J. Geogr., 119, 1125-1160; Kano, K., 2002, Bull. EQ Res. Inst. Univ. Tokyo, 77, 231-248; Ketcham, R.A., 2005, Rev. Min. Geochem., 58, 275-314; Matsuda, T., 1978, J. Phys. Earth, 56, S409-S421; Takahashi, M., 2006, J. Geogr., 115, 116-123; Takahashi, M. & Saito, K., 1997, Isl. Arc, 6, 168-182; Tamura et al., 2010, J. Petrol., 51, 823, doi:10.1093/petrology/egq002; Yamamoto, S. et al., 2009, Gond. Res., 15, 443-453.

For quantitative understanding of thermal features of fossil fluid activity derived from the Philippine Sea slab, we applied fluid-inclusion and thermochronometric analyses to hydrothermal veins and their host rocks outcropping in the Hongu area in southwestern Japan. Although hydrothermal events at ~150{degree sign}C and ~200{degree sign}C were identified by fluid-inclusion analyses of quartz veins, no thermal anomaly was found associated with the veins’ host rocks. Cooling ages showed no variation as a function of distance from the veins. Using zircon, we determined U-Pb ages of 77.3-66.9 Ma in the youngest population, fission-track pooled ages of 34.1-24.0 Ma, and (U-Th)/He single-grain ages of 23.6-8.7 Ma. Apatite yielded pooled fission-track ages of 12.0-9.0 Ma. All these ages can be explained by fluid flow that occurred either (1) before ~10 Ma at a depth where ambient temperature is higher than closure temperature of the apatite fission-track system (90{degree sign}C-120{degree sign}C, equivalent to ~3-km depth) or (2) after ~10 Ma but of such duration that is too short to have annealed fission tracks in apatite, which process requires ~10 yr at ~150{degree sign}C or as short as a few months at ~200{degree sign}C. Apatite fission-track ages of ~10 Ma might reflect regional mountain uplift and exhumation related to rapid subduction of the Philippine Sea slab and associated with clockwise rotation of the Southwest Japan Arc.