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 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.

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.