The central Andean Precordillera, between 30-31°S, has experienced active faulting and deformation from the early Miocene to present driven by a flat-slab segment of the down-going Pacific Plate. Basic models for fault propagation, in this region, involve progressive eastward stepping of deformation; however, out-of-sequence faulting has been postulated. Furthermore, deformation appears to have started earlier in the northern part of this region and later in the southern part. We use apatite (U-Th-Sm)/He (AHe) low-temperature thermochronology to quantify timing of fault exhumation and fault growth patterns to test hypotheses about out-of-sequence thrusting and the southward propagation of deformation in the region. Nine vertical transects were collected in the eastern-most part of the Precordillera. Preliminary AHe data indicate complete and partial age resetting in middle to late Miocene sedimentary units, that were deposited, buried, and subsequently exhumed. AHe ages range between 30 - 2Ma and trend younger to the south, supporting previous suggestions of north to south deformation migration. Additionally, we use cosmogenic radionuclides (CRN) to assess modern erosion rates across the landscape. Initial erosion rates range from 22 – 1330 m/my, with generally lower rates in the north and higher rates to the south. Ongoing analysis and modeling of both thermochronologic and CRN data will help to constrain the recent exhumation and erosion history in the central Andean Precordillera and determine if combining these two techniques can be used to identify out-of-sequence faulting and changes in spatial patterns of tectonically driven deformation. 

We describe new cosmogenic Be-10 and C-14 exposure age dating on previously glaciated bedrock samples from Lyell Canyon as constraints to model the glacier’s rate and timing of thinning and retreat after the Last Glacial Maximum (LGM). Close analysis of deglaciation following the LGM (22-12 ka) can offer insight into how glacier retreat proceeds in a warming climate. The extent and age of the LGM glaciation in Yosemite National Park, California are relatively well-constrained. Our new exposure ages from Yosemite can quantify the change of the glaciation after the LGM. This is important because the rate and timing of glacier retreat after the LGM allows us to learn about the LGM-Holocene climate transition. We collected 16 granodiorite bedrock samples from the Lyell Canyon walls in three vertical transects: at the end, in the middle, and near the head of Lyell Canyon. Sample elevations range from 2781m to 3388m. The samples are being processed for cosmogenic Be-10 and C-14 concentrations (for the lower and higher elevations in the transects, respectively). Together with previously acquired Be-10 exposure ages from glacial polished bedrock and boulders at the canyon floor, our vertical transects will help to define the relationship between glacier retreat and thinning along the valley. The combination of different nuclide measurements has the potential to reveal whether the glacier melted rapidly or went through multiple thinning and thickening cycles. We created several simple forward models of cosmogenic Be-10 and C-14 exposure ages on the valley wall for different glacier thinning patterns: (i) rapid thinning, (ii) thinning and thickening cycles during the melting, (iii) thickening first, followed by thinning, and (iv) breaking an upper small cirque glacier from the main glacier during the thinning. After we have obtained all our data, we will compare the exposure age data to our modeled scenarios, as well as local paleoclimate records, to quantify the glacier’s geometry and mass balance during the climate warming period. Understanding the timing, rates, and patterns of LGM retreat and thinning constitute a useful test case that aids mountain glacier melting predictions and water budget planning under contemporary climate change in analogous environments.

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.