Thermal and structural evolution of the external Western Alps: Insights from (U–Th–Sm)/He thermochronology and RSCM thermometry in the Aiguilles Rouges/Mont Blanc massifs

Abstract

In the Western Alps, the External Crystalline Massifs (ECM) are key places to investigate the kinematics and thermal structure of a collisional crustal wedge, as their paleo-brittle/ductile transition is now exhumed at the surface. New (U–Th–Sm)/He data on zircon and new Raman Spectroscopy on Carbonaceous Material (RSCM) data from the Aiguilles Rouges and the Mont Blanc massifs, coupled to HeFTy thermal modeling, constrain the thermal evolution and exhumation of the massifs. In the cover of the Aiguilles Rouges massif, we found that the maximal temperature was about 320°C (+/−25°C), close to the maximal temperature reached in the cover of the Mont Blanc massif (~350°C+/−25°C). We show that, after a fast heating period, the thermal peak lasted 10–15Myrs in the Mont Blanc massif, and probably 5–10Myrs in the Aiguilles Rouges massif. This thermal peak is synchronous with crustal shortening documented in the basement. (U–Th–Sm)/He data and thermal modeling point toward a coeval cooling of both massifs, like other ECM, at around 18Ma +/−1Ma. This cooling was related to an exhumation due to the initiation of frontal crustal ramps below the ECM, quite synchronously along the Western Alps arc.