Spatial and temporal distribution of glacial erosion as recorded by apatite (U-Th)/He and 4He/3He thermochronology.

Abstract

<p>Constraining the impact of Quaternary glaciations on landscape dynamics is required to better understand the interaction between tectonics, climate, and erosion. Over the years, low-temperature thermochronology such as apatite (U-Th)/He (AHe) has been used to quantify glacial erosion in different climatic and tectonic settings. However, in some contexts, AHe records lack temporal resolution because of limited exhumation due to glacial incision and/or low geothermal gradients. In addition, significant spatial variability in erosion can affect the quality of thermal-kinematic inversions when combining spatially distributed AHe data. This effect may be significant in glacial settings where a switch from a fluvial to a glacial landscape induced a significant change in the spatial distribution of erosion.</p> <p>However, the <sup>4</sup>He/<sup>3</sup>He thermochronology can extract lower-temperature and higher-resolution thermal histories from an AHe dataset. The method uses the spatial distribution of natural <sup>4</sup>He in an apatite crystal, which reflects the rate of cooling through the AHe partial retention zone. It has been successfully applied to track glacial incision and relief-development histories that would have been untraceable with conventional AHe thermochronology. Consequently, thermochronology data can now provide more detailed and localized thermal history. While <sup>4</sup>He/<sup>3</sup>He thermochronology has been successfully used in settings where background exhumation rates are moderate, the sensitivity of the technique remains untested in settings with notably low exhumation-rates, such as at passive margins.</p> <p>Here, we couple a glacial landscape-evolution model (iSOSIA) with a new version of a thermo-kinematic model (PecubeGUI), incorporating radiation-damage effects on helium diffusion, to explore the ability of apatite (U-Th)/He and <sup>4</sup>He/<sup>3</sup>He thermochronometers to record glacial incision. To do so, we model a range of synthetic glacial scenarios in different tectonic, climatic, and thermal settings.  Our landscape-evolution models include glacial, fluvial and hillslope erosion, as well as sediment transport. We assess model predictions of thermochronologic parameters, including age-elevation relationships and <sup>4</sup>He/<sup>3</sup>He spectra, and their evolution when switching from a steady-state fluvial to a glacial topography. This modelling exercise aims to provide a guide for sampling strategies and interpretations for both conventional apatite (U-Th)/He and <sup>4</sup>He/<sup>3</sup>He thermochronology when working in glacial settings, considering their particular tectonic and climatic context.</p>