Extracting Exhumation and Relief History From Thermochronological Data: Can we Really Quantify Relief Changes Using age-Elevation Profiles?

We aim to establish what independent constraints on both the exhumation and relief history of mountain belts can be extracted from thermochronological data, in particular from age-elevation profiles. Age-elevation relationships (AER) have been widely used to provide information about orogen exhumation but the analyses of AER generally remain qualitative and limited to the identification of breaks or changes in slope. These methods potentially miss crucial information contained in the data, especially concerning the evolution of the topography and its effects on the thermochronometric record. Although numerical models are now used to address these issues, most of these are 1D simulations and do not capture the 2D effects of topography on the underlying isotherms. To better address the effect of relief on cooling histories, we use a three dimensional model that enables predictions of thermal histories and thermochronological ages from input exhumation and relief histories. We combine this model with an inversion method based on the neighbourhood algorithm to extract independent information on exhumation and change of relief. We start by using synthetic data to test the method, and then apply it to interpret a set of thermochronological dataset (zircon/apatite fission-track and (U-Th)/He) collected along an age-elevation profile in the French western Alps. Our results on synthetic data suggest that multiple thermochronometers are required to discriminate between different exhumation and relief history scenarios. However, relief development is difficult to quantify using these datasets and we suggest that these thermochronometers may not be the most suitable for recording recent relief changes during their cooling. We then apply these methods to the western Alps data (Ecrins- Pelvoux Massif) and we find a relatively well constrained scenario with varying exhumation rates for the last ~25-30 Ma. We suggest that relief increase, potentially related to climate change, has been important in the last stages of the history of this massif although this increase can not be quantitavely constrained with our data. Using new thermochronometers (for instance 4He/3He on apatite), as well as improving the ways relief changes are included in numerical models, may be powerful tools for extracting relief history and thus better constraining the impact of climate change on orogen development.