Monazite fission-track presents itself as a novel, low-temperature thermochronometer with annealing studies placing its closure temperature between ~45 and 25 °C. Previously, monazite has been unsuitable for fission-track dating due to high abundance of gadolinium and insufficient investigation of the etching protocol. Gadolinium causes self-shielding via thermal neutron capture and substantial associated nuclear heating during irradiation which prevented robust monazite fission-track dating using the traditional external detector method. Further, early etching studies were found to be extremely corrosive to monazite grains. However, developments in LA-ICP-MS fission-track analysis allow for measurement of 238U and improvements in monazite fission-track etching protocols mean that dating monazite through the fission-track method is now viable. In this study, we present monazite fission-track data from an elevation profile (2260 m, 2000 m, 1600 m, and 1200 m) from the Catalina metamorphic core complex (Catalina MCC), in southern AZ, USA. We follow the etching protocol described in Jones et al. (2019), etching the monazites in 6 M HCl for 90 minutes at 90 °C. We measure the 238U concentration via LA-ICP-MS and compare the dates to other multi-method thermochronology from the same rocks. Traditional low-temperature thermochronology (apatite and zircon fission-track, apatite and zircon (U-Th-Sm)/He) from the Catalina MCC reveals cooling at 25-20 Ma and 18-10 Ma. Preliminary monazite fission-track analysis yields a date of 6.1 ± 0.4 Ma, far younger than all the traditional thermochronometric data, in-line its far lower closure temperature. The 6 Ma monazite fission-track date is consistent with the youngest phase of hematite (U-Th)/He dates observed in the nearby Rincon metamorphic core complex and suggest that these dates correspond to the latest phase of exhumation in response to Basin and Range extension and/or climate enhanced erosion. These preliminary results show that monazite fission-track can reveal shallow crustal processes and contribute to constraining thermal histories below ~60 oC, which are traditionally difficult to resolve.

The principal structural elements of the Himalayan arc can be traced more or less continuously for nearly 2500 km. It is therefore understandable that along-strike variations in structure and denudation have not received the same attention as equivalent arc-normal trends. However, it is now clear that arc segmentation can be controlled by lateral variations in the geometry of the Main Himalayan Thrust (MHT). The Bhutan Himalaya has a distinctive physiography and hosts nominal modern seismicity despite experiencing long-term strain accommodation comparable to the wider arc. This enigmatic section of the orogen presents an opportunity to test the case for local arc segmentation through applied tectonic geomorphology. By integrating low temperature thermochronology, cosmogenic radionuclide methods and quantitative geomorphometry, this study documents the spatial and temporal variability of denudation to infer partitioning of deformation across crustal structures. Laboratory methods include apatite fission track (AFT), and (U-Th)/He dating of zircon from in situ bedrock, synorogenic sediments and modern detrital samples. Additionally, 10Be concentrations from detrital quartz samples adds to a nation-wide compilation of previously published data. Results suggest prominent along- and across-strike variation in deformation within Bhutan. High normalised channel steepness and hillslope characterise a prominent east-west trending zone of elevated millennial-scale erosion rates. Here termed the Naka Zone, this geomorphic region is coincident with the estimated rupture extent of an early 18th Century great earthquake and terminates to the east in the vicinity of the Kuru Chu reentrant in the Main Central Thrust (MCT). Greater Himalaya basement rocks west of the Sakteng Klippe show a phase of rapid, monotonic cooling, the timing of which is largely latitude-dependent, consistent with exhumation above a mid-crustal ramp on the MHT ~100 km from the front followed by horizontal translation above the AFT partial annealing zone. This framework explains the decoupling of AFT ages in sampled catchments from millennial-scale erosion rates. Small catchments that straddle the MCT show bimodal distributions in single grain AFT ages, suggestive of activity on the MCT during the late Miocene. Further, central ages show a marked decrease towards Arunachel Pradesh, suggesting that in far eastern Bhutan the mid-crustal ramp extends towards the foreland, possibly invoking a lateral ramp. Synorogenic detrital thermochronometers are unreset and thus provide information on source area bedrock cooling and provenance. ZHe and AFT age distributions in the Siwaliks are bimodal. Comparisons with large modern drainage systems links a young age peak (Mio-Pliocene) to the Greater Himalaya and a dominant older age peak (Mid Miocene) to the Lesser Himalaya and points to persistent elevated topography in the range front east of Kuru Chu.

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.

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.