The 17th International Conference on Thermochronology (Thermo2021) was held in Santa Fe, New Mexico, on September 12-17, 2021. This bi-annual conference series evolved via the coalescence of the International Workshops on Fission Track Thermochronology, held since 1978, and the European Workshops on Thermochronology. It has become the premier forum for thermochronology practitioners and users to discuss fundamental and methodological topics and opportunities related to their science and its future. Each conference is independently organized, and a Standing Committee consisting of past organizers and other community members helps to ensure their continuation into the future. Thermo2021 was greatly affected by the COVID-19 pandemic. Normally the meeting would have been expected to draw ~250 attendees, but travel restrictions limited in-person attendance to 86, plus 21 remote presenters. Nearly all in-person participants were from the US, and only four were international. Talks and posters were distributed among five themes: (U-Th)/He; fission track; other thermochronometers; frontiers in data handling, statistics, interpretation methods, and modeling; and integration and interpretation. Although COVID-19 presented many challenges, it also allowed the Organizing Committee to adapt creatively and transform adversity into opportunity. In particular, the smaller number of attendees permitted more talks by students and early-career scientists, both within the theme sessions and in the Charles & Nancy Naeser Early Career Session. Discussion time was prioritized: at a Tuesday evening “swap meet” for ideas, in 30-40-minute time slots within each theme session, and in Friday afternoon breakouts for the first four themes and another dedicated to early career and DEI issues. These were used to identify emergent ideas and concerns across a broad range of topics, from the theory and practice of the various thermochronometric techniques, to their interpretation through thermal history modeling and other methods, to anticipated trends in data dissemination and management, to the needs of the next generation of thermochronologists, particularly in the US. Each Friday breakout designated a scribe who recorded the discussion and distributed their notes. Each group then designated one or more writers to transform the notes into text for this White Paper. Notes or early write-up versions were provided to the international thermochronology community, and feedback solicited. In addition, cross-cutting themes that occurred across multiple breakout groups were identified and compiled. This White Paper is the outcome of these efforts. We hope that it will serve as a record for the meeting, and an overview of where the predominantly US-based component of the thermochronology community considers the current state of knowledge to be and where future efforts should be directed, for developing both the science and its human infrastructure.

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.

Abstract A new method for automatic counting of etched fission tracks in minerals is developed and recently published in Geochronology (see Nachtergaele and De Grave, 2021). Artificial intelligence techniques such as deep neural networks and computer vision were trained to detect fission surface semi-tracks on images. The deep neural networks can be used in an open source computer program for semi-automated fission track dating called “AI-Track-tive”. Our custom-trained deep neural networks use YOLOv3 object detection algorithm, which is currently one of the most powerful and fastest object recognition algorithms. Two Deep Neural Networks were trained for both apatite and mica using our training dataset with images from the available microscope. The developed program successfully finds most of the fission tracks in the microscope images, however, the user still needs to supervise the automatic counting. The presented deep neural networks have high precision for apatite (97%) and mica (98%). Recall values are lower for apatite (86%) than for mica (91%). These high values have been obtained on images using the same microscope that provided the training images. The application can be used online on the web page https://ai-track-tive.ugent.be or after download as an offline application for Windows. The online application can be used to analyse captured images and does not require installation or download. The offline application can be used for both live track recognition on live microscopy images and captured images of apatite or mica. AI-Track-tive is written in Python and can be downloaded on https://github.com/SimonNachtergaele.

We performed continuous ramped heating (CRH) on apatites from various tectonic settings and found two major types of 4He outgassing behavior. Apatites with good (U–Th)/He age reproducibility show simple and unimodal incremental gas-release curves that are similar to those predicted by volume diffusion, whereas samples exhibiting greater age dispersion have complex gas-release curves that feature He ‘spikes’ and secondary gas-release peaks deferred to higher temperatures. Age dispersion from the apatites with simple outgassing behavior can be explained by variability in their relative He retentivity observed on Arrhenius arrays—with similar activation energy but different diffusivities, which may be a result of fine-scale crystal imperfections. The observed high-temperature gas component and the resulting “too-old” AHe ages, combined with an attempt at age correction based on secondary peak gas-removal, seem to indicate the existence of sink-like crystal imperfections that can trap 4He both in nature and during laboratory heating. CRH analysis at different heating rates further suggests that the second gas-release peak occurs at varying temperatures, indicating that the sink is kinetically responsive, and if characterizable, may contain additional thermal-history information. These observations suggest that (1) CRH can be deployed as a routine screening tool for (U–Th)/He dating, (2) diffusion of 4He could be complicated by imperfections beyond radiation damage, and (3) if the proposed sinks exist and retain appreciable 4He, there are opportunities to explore additional thermal histories of natural samples.

A compilation of legacy and new low-temperature thermochronological data from the British-Irish Isles and their surrounding offshore shelves yielded c. 700 AFT ages and c. 180 AHe ages from 29 peer-reviewed papers, 27 Geotrack industry reports and several new unpublished studies from offshore Ireland. The compilation shows for the first time a regional age pattern, with older AFT ages in Scotland and Northern Ireland than in the rest of Ireland. This pattern is tentatively attributed to the influence of the Anton-Dohrn Transfer Zone (ADTZ) during an Early Cretaceous phase of plate-wide uplift that resulted in more exhumation to the SW of the transfer zone than to the NE. Caledonian faults might also create differential exhumation of the tectonic blocks between them, as is observed in the compilation of AFT data from northern Scotland and this could explain the dispersion in the timing of exhumation seen on the North Porcupine High, offshore Ireland. Finally, the Paleogene exhumation visible in the Central Irish Sea, and attributed in recent years to igneous underplating, has not been detected in the Malin Sea-Outer Hebrides, despite the area being underlain by a high-velocity body also interpreted as igneous underplating. In conclusion, a detailed analysis of a large dataset of low-temperature thermochronological has revealed the possible influence of major crustal structure on the Mesozoic exhumation of this part of the NE Atlantic Margin, with large-scale decoupling occurring at a transfer zone and medium-scale decoupling occurring along regional-scale faults. The dataset also shed some doubts on the generic nature of exhumation caused by igneous underplating which has been much discussed in recent years.

Zircon Raman dating is an emergent thermochronological method. It exploits the disruption of the zircon lattice due to α-disintegration of trace amounts of 238U, 235U, 232Th, and their daughters. The radiation damage broadens the Raman bands and shifts them to lower wavenumbers. The Raman bandwidths provide a sensitive measure for the accumulated lattice damage (Nasdala et al., 1995; Nasdala et al., 2001). The measured bandwidth and the effective uranium concentration define the Raman age (Härtel et al., 2021). Radiation damage anneals upon heating and the meaning of the Raman age depends on the zircon’s thermal history. The Raman age is a formation age if no annealing has taken place, or a reset age if all pre-existing damage has been annealed by a geological heating event. In the case of partial annealing, however, the Raman age is a mixed age with no obvious geological significance. Mixed ages are difficult to interpret and cannot be distinguished from reset ages using the standard procedure for annealing detection (Nasdala et al., 2001). On the other hand, inhomogeneous damage distributions due to actinide zoning within zircon grains present a problem for zircon Raman dating. Overlapping signals from more and less damaged zones lead to asymmetric Raman bands and overestimated bandwidths (Nasdala et al., 2005). We discuss Raman spectra of zircon from partially annealed samples and spectra with asymmetric bands. We introduce discrimination plots based on the 356, 439, and 1008 cm-1 bandwidths that provide a means for detecting and distinguishing asymmetry and partial annealing. We discuss examples of zircon Raman dating and present a measurement protocol.

The Ultraviolet Laser Ablation Microprobe (UVLAMP) method of releasing helium from samples is an excellent, but under-utilized, tool in the diverse toolkit of gas extraction approaches available to researchers working with the (U-Th-Sm)/He thermochronology method. So far, most applications have involved some form of Laser Ablation (U-Th-Sm)/He dating (LAHe) or combined LAHe and Laser Ablation U-Th/Pb double dating (LADD) (e.g. 1, 2, 3, 4, 5, 6, 7). Other applications using UVLAMP have focused on 2D-mapping of helium distributions within zircon crystals (8) and stepwise Laser Ablation Depth Profiling (LADP) of induced helium diffusional loss profiles in apatite and zircon (9, 10). Based on the latter examples the stepwise helium LADP method would appear to be an excellent method to study the intricacies associated with a variety of aspects of the (U-Th-Sm)/He dating method and the interpretation and modeling of its results. Given that it creates high resolution helium profiles from the crystal margin to its core without the need to heat the sample to release the gas. Thus, it avoids issues of within-experiment radiation damage annealing, diffusional flattening of helium zonation, and/or the sudden release of helium from fluid and/or melt inclusions that can be associated with approaches using step heating of samples to acquire similar information about the helium distribution within a sample. In this contribution we focus on the results of high spatial resolution helium LADP experiments in a variety of accessory minerals (apatite, zircon, monazite, and titanite). The experiments are intended to a) empirically determine the alpha ejection distance and how those results compare to the distance for each mineral derived from SRIM calculations (11) and b) image natural helium distribution profiles from rim to core in zircons to produce data that are equivalent to those produced by 4He/3He thermochronology (12) experiments, but without the need to proton irradiate the sample. Initial LADP results on Durango apatite yielded an alpha ejection distance that is within error of the theoretical value, while results from several larger (>5 mm) zircon crystals did not yield profiles consistent with the presence of a straightforward alpha ejection zone. The helium depth profile results from the zircons were suggestive of either natural diffusional loss profiles, showing evidence of U-Th zoning, or a combination thereof. 1 Boyce et al. GCA 70, 2006; 2 Vermeesch et al. GCA 79, 2012; 3 Tripathy-Lang et al. JGR-ES 118, 2013; 4 Evans et al. JAAS 30, 2015; 5 Horne et al. GCA 178, 2016; 6 Horne et al. CG 506, 2019; 7 Pickering et al. CG 548, 2020; 8 Danisik et al. Sci Adv 3, 2017; 9 Van Soest et al. GCA 75, 2011; 10 Anderson et al. GCA 274, 2020; 11 Ziegler and Biersack, 1985; 12 Shuster and Farley EPSL 217, 2004.

Most thermochronological studies aimed at constraining exhumation rates rely on bedrock datasets. Often, they involve the analysis of samples collected along an elevation profile in terrains with high relief. However, there are several limitations to this approach, most importantly access to an appropriately steep traverse and sufficient relief to overcome uncertainties, and to have a broad enough range in closure ages as a function of elevation. Detrital thermochronology offers an alternative approach which can mitigate these challenges through coordinated dating of modern river sediments using multiple thermochronologic methods. Modern detrital sediments from active catchments provide an excellent source of material, typically rich in rock-forming and accessory minerals. Detrital thermochronologic data for material in sedimentary basins has been used widely to infer exhumation histories of sedimentary source terrains, reconstruct paleorelief, and evaluate spatial and temporal variations in erosion rates; however, there have been comparably fewer studies that apply this technique to evaluate regional exhumation patterns using detrital samples from active catchments. Based on the approach presented in Gallagher and Parra, (2020), we are exploring the capability of detrital thermochronologic data to infer regional exhumation patterns in the southeastern Sierra Nevada, CA. Here the uplift history remains debated and the potential mechanism of uplift has yet to be thoroughly constrained. Many catchments along the eastern side of the Sierra Nevada exhibit advantageous characteristics for detrital thermochronologic studies, including steep topography and high relief (that make it more difficult to sample bedrock), limited lithologic variability (which minimizes point-source biasing), relatively simple geologic structure, and relatively easy access to detrital sampling localities. Additionally, the dominant source of the southeastern Sierra Nevada catchments, the igneous units of the Sierra Nevada batholith, include abundant rock-forming minerals for 40Ar/39Ar thermochronology (hornblende, biotite, and sometimes muscovite) as well as abundant accessory minerals for (U-Th)/Pb geochronology(zircon), (U-Th)/Pb thermochronology (apatite), and (U-Th)/He thermochronology (zircon and apatite). Collectively, detrital thermochronological data from these minerals can elucidate much of the post-crystallization thermal history of the eastern flank of the Sierra Nevada. Preliminary results of this technique demonstrate the potential of this cost- and labor-efficient approach for exhumation history studies.

Thermal history models are interpretive tools that incorporate data from chronometers and implement the published kinetics in the context of independent constraints on a sample’s known geologic history, in order to explore specific gaps in geologic knowledge. Despite their central role in the interpretation of thermochronologic datasets, our community has no standards for what characterizes a “robust” thermal history model result, how a model result’s rigor can and should be demonstrated, and how to communicate the key layers of interpretation produce a preferred thermal (and geologic) history. As a result, and through no fault of any one study or modeling program, published models are a patchwork of modeling philosophies, assumptions, and auxiliary hypotheses that are rarely sufficiently explored—to the frustration of authors, reviewers, and readers. This patchwork can give rise to conflicting conclusions and generate apparent controversies that distract from the geologic questions at hand. Therefore, our community needs to both embrace a diversity of modeling approaches and collectively discuss and set broad expectations for what constitutes thermal history modeling best practices. Here, we argue that the fundamental characteristic of any robust thermal history model result is that it is accompanied by a clear articulation of the “why”—e.g., the reason(s) that a model produces a distinctive history, be it the power of a geologic constraint, a grain’s age, a spatial relationship between samples, the choice of kinetic model, etc. We demonstrate this approach using (U-Th)/He data from basement rocks in the Front Range, CO, which when modeled require a distinctive Neoproterozoic thermal history: heating to 235-280°C after ca. 650 Ma and then cooling to <60°C during Paleozoic time. We demonstrate “why” through a suite of models that add, modify, and remove geologic constraints and data from the preferred model. We find that a heating event is required to produce the observed zircon He age-[eU] trend because (1) there is no more than ~600 My of radiation damage accumulated in the zircon crystals, (2) the geologic record places the samples at the surface prior to 650 Ma, and (3) published Ar ages require that these rocks were colder than ~250˚C for most of the last 1.5 Gy. By identifying these key factors, our sensitivity test facilitates comparisons to other studies and directs further discussion to how confident we are in the parts of our data and model set-up that produce this distinctive result. More broadly, this exercise demonstrates one of the challenges of deep-time thermochronology: the potential to accumulate multiple auxiliary assumptions that control the model result in ways that are not obvious, even to the experienced model user, without deliberate exploration of alternative solutions—further underscoring the need for more open discussion of this topic.

Apatite – Ca5(PO4)3(OH,F,Cl) – is a common accessory mineral in many terrestrial rocks and has a low (~75˚C) closure temperature for He retention[1]. As such the (U-Th-Sm)/He thermochronometer has become one of the most used dating techniques to constrain low temperature cooling histories of terrestrial samples. Apatite has been documented in many meteoritic, lunar, and martian samples, and is a particularly common accessory phase in many martian lithologies[2]. In meteoritic samples, (U-Th-Sm)/He dating of apatites provides some methodological challenges in that grains are frequently anhedral making it complicated to do a FT correction[2]. Results are commonly interpreted to represent the timing of shock metamorphism related to impacts onto the (planetary) surface which often correlates with cosmic-ray exposure ages[2]. NWA 7034, and its pairings, represents a piece of martian regolith that documents events on the martian surface that span approximately 4 Ga[3]. NWA 7034 is known to contain upwards of 4 wt.% apatite as detrital mineral fragments and as accessory phases in many lithic and impact-derived clasts[4]. These apatites have been previously analyzed geochronologically via U-Pb apatite and isotopically via δD, δ37Cl, and H2O using microbeam techniques[5,6]. At present, (U-Th)/He dating of NWA 7034 and its pairings has been restricted to bulk rock analyses[4,7,8,9]. This whole-rock approach relies on assumptions that the He, and its parent isotopes, are evenly distributed throughout the sample and that resetting of the WR-He thermochronometer is universal throughout the sample in response to the thermal pulse associated with the impact. These studies have yielded widely dispersed datasets that range from 50 Ma to 200 Ma, putting some of those assumptions into question[4,7,8,9]. In this contribution, we utilize the laser ablation double-dating (LADD) technique on individual apatites present within several polished slabs of NWA 7034. This targeted approach allows us to acquire (U-Th-Sm)/He ages, U-Pb ages, and trace element information within a petrological framework. The (U-Th-Sm)/He ages will provide additional constraints on the low temperature history of NWA7034 and help test the veracity of WR-He retention ages. References: [1] Zeitler et al. 1987 GCA, [2] Min 2005 Rev. Mineral Geochem; [3] Cassata et al. 2018, SciAdv; [4] Agee et al. 2013 Sci; [5] Hu et al. 2019 MAPS; [6] Davidson et al. 2020 EPSL; [7] Cartwright et al. 2014 EPSL; [8] Lindsay et al. 2021 MAPS; [9] Stephenson et al. 2017 MAPS

Apatite (U-Th)/He (AHe) thermochronology depends on accurate knowledge of how diffusion occurs. This involves measurement of core diffusion kinetics as well as understanding the behavior of migrating He atoms. Drawing from previous studies as well as data obtained via continuous ramped heating (CRH), we assess several processes that need to be integrated into a single model for He diffusion in apatite. CRH analyses conducted at different heating rates show a kinetic response for both the “normal” lower-temperature and the higher-temperature release peaks, with peaks shifting to lower temperatures at lower heating rates. Where we do see a rollover in Arrhenius trends it also shows a kinetic response, being deferred to higher temperatures at higher heating rates, though many samples with unimodal release peaks do not show a significant rollover; fluorapatites seem to show more prominent rollover. For samples showing multiple release peaks, we find that their Arrhenius data often transition from one lower-temperature trend to another at higher temperatures that has about the same slope and thus activation energy. This looks very much like MDD behavior in K-feldspar, and MDD domain analysis fits the observed data very well, even if mechanisms involving discrete domain sizes are implausible. This interesting and unexplained result must speak to the nature of what is happening during analysis of samples having trapped He. To explore our data, we coded a simple diffusion model in which single He atoms are free to jump within a grid, but can also arrive at grid nodes designated as reversible sinks, escape from which depends on an exponentially temperature- dependent probability. The model includes radiogenic He production over geological thermal histories followed by laboratory CRH outgassing. When conditioned using D values observed for AHe, the model accurately predicts parameters such as closure temperature and fractional loss. When traps are introduced, the model simulates the essential nature of the dual-peak CRH results we see. Three important results emerge from this model. (1) Few sinks need be present. (2) Trapping occurs twice during diffusion, first in nature and then again during laboratory outgassing, meaning that the ratio of the gas amounts beneath each CRH peak overestimates the geological trapping. (2) Trapping in nature is very dependent on the sample’s thermal history: it is smallest for ancient rapid cooling and largest for samples that reside in the PRZ (allowing more radiogenic production to find traps before diffusion ceases). This model raises the possibility that complex CRH data record extended thermal-history information. If CRH and 4He/3He analysis were combined the 3He lab outgassing would record the sample’s trapping dynamics, and the 4He outgassing would reflect that plus a segment of the sample’s thermal history, which could be extracted using the 3He observations.

The São Francisco Craton (SFC) and its marginal Araçuaí and Brasília orogens exhibit a significant diversity in their lithospheric architecture. These orogens were shaped during the Neoproterozoic–Cambrian amalgamation of West Gondwana. The rigid cratonic lithosphere of the SFC and the relatively weak lithosphere of the Araçuaí Orogen were disrupted during the Cretaceous opening of the South Atlantic Ocean, whereas the Brasília Orogen remained in the continental hinterland. In earlier research, the thermal effects of the Phanerozoic reactivations in the shallow crust of the Araçuaí Orogen have been revealed by low-temperature thermochronology, mainly by apatite fission track (AFT) analysis. However, analyses from the continental interior are scarce. We present new AFT data from forty-three samples from the Brasília Orogen, the SFC and the Araçuaí Orogen, far from the passive margin of the Atlantic coast (~150 to 800 km). Three main periods of basement exhumation were identified: (i) Paleozoic, recorded both by samples from the SFC and Brasília Orogen; (ii) Early Cretaceous to Cenomanian, recorded by samples from the Araçuaí Orogen; and (iii) Late Cretaceous to Paleocene, inferred in samples from all domains. We compare the differential exhumation pattern of the different geotectonic provinces with their lithospheric strengths. We suggest that the SFC likely concentrated the Meso-Cenozoic reactivations in narrow weak zones while the Araçuaí Orogen displayed a far-reaching Meso-Cenozoic deformation. The Brasília Orogen seems to be an example of a stronger orogenic lithosphere, inhibiting reworking, confirmed by our new AFT data. Understanding the role of the lithosphere rigidity may be decisive to comprehend the processes of differential denudation and the tectonic–morphological evolution over Phanerozoic events.

The “Laramide-style” uplifts of North America—characterized by blocks of Proterozoic-Archean basement that were exhumed along reverse faults within the Cordilleran foreland basin—are widely interpreted to be a result of flat-slab subduction of oceanic lithosphere beneath the continent. Despite this general consensus, the causal mechanisms of basement-cored uplifts remain unclear. Assessment of the hotly debated geodynamic models that relate flat-slab subduction and upper crustal deformation hinge on the availability of accurate estimates for the timing of exhumation of Laramide uplifts. A major problem with current models is that they do not incorporate timing constraints from the Laramide region of central Montana. This region represents the northernmost extent of Laramide deformation and timing constraints are critically needed to enhance our understanding of how stress is transmitted inboard during flat-slab subduction. We present the first low-temperature thermochronological ages from the Little Belt Mountains (LBM) of central Montana, which is the northernmost Laramide-style uplift with exposed basement gneisses. Apatite fission track ages ranging from ca. 90-70 Ma suggest that the core of the LBM was exhumed through the 120-60°C apatite partial annealing zone in the Late Cretaceous. These ages corroborate recent low-T thermochronology and sedimentological work that propose an earlier than previously recognized onset of Laramide deformation in southwestern Montana and eastern Idaho (>80 Ma), but additional data are needed to reduce the uncertainty between ages and ascertain the exhumation history of the LBM. To this end, we will integrate new low-temperature thermochronology (<150°C) and associated thermal history models in order to constrain the Cretaceous tectonic evolution of the LBM and the extent of Laramide deformation in the western USA.

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.

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.

We report a new series of step-etch experiments to reveal the influence of microscopy technique on track selection bias. Two different aliquots of induced tracks in Durango apatite were etched for 10-15-20-25-30s and for 20-25-30s in 5.5M HNO3 at 21°C. Three different track selection criteria were applied after the initial etch step of the etching procedure: (1) all tracks with reasonable measurability under transmitted and reflected light switch with 100x objective and 2.5X optovar magnification; (2) ”fully etched” tracks under transmitted light with 100x objective; and (3) ”fully etched” tracks under reflected light with 100x objective. Approach 1 was applied to both aliquots and the approaches 2 and 3 to the latter aliquot. Comparing the mean track lengths, approaches 2 and 3 result in similar values over all experiments, while approach 1 provides a ~0.4 μm higher in the first aliquot and ~0.3 μm lower in the second aliquot than approach 2 and 3 due to higher and lower variations of effective etch times. Comparing the c-axis angles, in the 0-30° range approach 3 provides a severely reduced fraction of tracks due to their weak appearance under reflected light. Furthermore, approach 2 provides ~%14 lower track densities comparing with approaches 1 and 3. We recommend using both transmitted and reflected light during entire track selection and measurement procedures. We are working to develop a new 2+-step etching procedure, where tracks are located after 10s of etching but measured after a second 10s etch step, resulting in better-controlled etching times while reducing the bias associated with analyst choice. Furthermore, this two-step etching procedure can be iterated for more etch-steps, by identifying newly-appeared tracks after each etch step and etching them for 10s more, to increase the number of measured tracks while maintaining consistent selection criteria.

The thermochronological method was applied to metamorphic rocks distributed in eastern Nepal to elucidate the denudation process of the upper-crust of the continental collision zone. New results of systematic fission-track (FT) age dating and FT length measurements of zircon and apatite were utilized in the thermochronological inverse analysis to reconstruct the time-temperature (t-T) paths in the temperature range of 60–350°C. Eight t-T paths obtained along the across-strike section in eastern Nepal showed that the cooling process of the metamorphic rocks are characterized by 1) gradual cooling (<30°C/Myr) followed by rapid cooling (~150°C/Myr) and subsequent gradual cooling (gradual-rapid-gradual cooling: GRG cooling), 2) northward-younging of the timing of the rapid cooling since ca. 9 Ma. The observed FT ages and t-T paths were then compared with the FT ages and t-T paths obtained by forward calculations using 3-D thermokinematic models to test the following four tectono-thermal models which have been proposed in the Central and Eastern Himalayas: 1) The denudation of the upper-crust is associated with the movement of the plate boundary fault (Main Himalayan Thrust: MHT) showing flat geometry (the Flat MHT model) and 2) flat-ramp-flat geometry (the Flat-Ramp-Flat MHT model), 3) the denudation of the upper-crust is mainly controlled by the focused uplift associated with the growth of the Lesser Himalayan Duplex (the Duplex 01-03 model) or 4) slip of the splay fault of the MHT (the Splay Fault model). As a result, only the Flat-Ramp-Flat MHT model reconstructed similar t-T paths and age distribution patterns obtained from eastern Nepal. This suggests that the observed FT ages and t-T paths reflect a denudation process driven by the movement of the MHT showing a flat-ramp-flat geometry. The GRG cooling and the northward-younging of the timing of rapid cooling indicate that the flat-ramp-flat geometry of the MHT was established by ca. 9 Ma and has been stable thereafter. The result of the thermokinematic inverse analysis also indicates that the denudation rate and its spatial distribution have been stable since ca. 9 Ma.

Exhumation in continental and oceanic arc settings is linked to tectonic and climatic forces that result in some of the highest topography and erosion rates in the world. Regional exhumation studies of oceanic arcs are sparse due to poor exposure, accessibility, complex inherited structures, and thermal overprinting due to magmatism, reburial, and metamorphism. In contrast, the Aleutian arc has an unusually thick crust, exceptional subaerial exposure of plutons, and limited regional thermal overprinting, providing the ideal conditions for a systematic thermochronology study that investigates the mechanisms that lead to arc exhumation. By analyzing multiple chronometers (apatite and zircon (U-Th)/He, zircon U-Pb) with different temperature sensitivities within the same plutonic sample, we can constrain uplift and erosion rates over ~40 million years of Aleutian arc history. Here, we present preliminary apatite and zircon (U-Th)/He ages from 23 plutonic samples from the islands of Unalaska, Umnak, Atka, Ilak, and Amatignak. These data are part of a larger study where we will analyze a total of 78 samples collected from 11 plutons along >1400 km of the Central Aleutian Islands. Ultimately, this dataset will provide a regional framework to quantitatively assess the various proposed mechanisms for Aleutian arc exhumation, such as; 10° Pacific plate rotation, subduction of the Kula ridge, and/or pluton emplacement, each of which has predictable and testable geographic age trends. The samples analyzed are from plutons that crystallized in the Oligocene to Miocene based on zircon U-Pb dating. Both zircon and apatite (U-Th)/He ages also show an Oligocene to Miocene spread, with a majority of the ages from both chronometers pooling at 7-13 Ma and no particular geographic trend. These preliminary results suggest that a significant exhumation event occurred in the late Miocene, as has been observed in other circum-Pacific arcs. These preliminary data may support the hypothesis that the late Miocene pulse of high plutonism in the Aleutian Islands led to arc exhumation. However, additional samples need to be analyzed to provide an arc-scale view of exhumation timing, trends, and rates of the Central Aleutian arc in order to test possible uplift and erosion scenarios.