Inversion of thermochronological age–elevation profiles to extract independent estimates of denudation and relief history — II: Application to the French Western Alps

Thermochronologic data collected along age–elevation profiles are commonly interpreted as recording temporally variant but spatially constant exhumation rates. However, thermochronologic age–elevation relationships are known to be perturbed by topographic effects and potential changes in relief, which are neglected in the inherently 1-D interpretation commonly applied. Such data thus potentially record both the denudation and relief history of the sampled region but extracting this information is challenging. In a companion paper, we develop a methodology for rigorously interpreting thermochronologic age–elevation profiles in terms of exhumation rates and relief development through time, and to independently quantify the resolution of these constraints. Here we test this approach using a thermochronological dataset consisting of apatite and zircon fission-track and (U–Th)/He data, collected at La Meije Peak in the Pelvoux-Ecrins massif (French Western Alps). Our data and models suggest a three-phase exhumation history in the Pelvoux–Ecrins massif, including a pulse of rapid exhumation at ∼ 6–5.5 Ma, preceded and followed by more moderate rates of denudation in the order of 0.3–0.4 km Myr− 1. This rapid exhumation event appears to occur coevally in other external crystalline massifs in the Alps but is not detected by qualitative inspection of the age–elevation relationships. Both our synthetic results and inversion of the La Meije data strongly suggest that apatite fission-track age–elevation relationships alone cannot resolve both denudation and relief histories independently and that multiple thermochronometers are required. Combining apatite fission-track and (U–Th)/He ages and, particularly, including fission-track-length data greatly improves the resolution of the inferred exhumation histories. Although denudation rates through time and the timing of rate changes are generally well resolved, our data have insufficient resolution to satisfactorily constrain the relief history. Synthetic results reported in the companion paper suggest that the reason for this limitation is that relief increase through valley carving has been insufficient with respect to the regional denudation rates to be unambiguously extracted from the data.