High-relief glacial valleys shape the modern topography of the Southern Patagonian Andes, but their formation remains poorly understood. Two Miocene plutonic complexes in the Andean retroarc, the Fitz Roy (49°S) and Torres del Paine (51°S) massifs, were emplaced between 16.9–16.4 Ma and 12.6–12.4 Ma, respectively. Subduction of oceanic ridge segments initiated ca. 16 Ma at 54°S, leading to northward opening of a slab window with associated mantle upwelling. The onset of major glaciations caused drastic topographic changes since ca. 7 Ma. To constrain the respective contributions of tectonic-mantle dynamics and fluvio-glacial erosion to rock exhumation and landscape evolution, we perform inverse thermal modeling of a new dataset of zircon and apatite (U-Th)/He from the two massifs, complemented by apatite 4He/3He data for Torres del Paine. Our results show rapid rock exhumation recorded only in the Fitz Roy massif between 10 and 8 Ma, which we ascribe to local mantle upwelling forcing surface uplift and intensified erosion around 49°S. Both massifs record a pulse of rock exhumation between 7 and 4 Ma, which we interpret as enhanced erosion during the beginning of Patagonian glaciations. After a period of erosional and tectonic quiescence in the Pliocene, increased rock exhumation since 3-2 Ma is interpreted as the result of alpine glacial valley carving promoted by reinforced glacial-interglacial cycles. This study highlights that glacial erosion was the main driver to rock exhumation in the Patagonian retroarc since 7 Ma, but that mantle upwelling might be a driving force to rock exhumation as well.

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.