Constraints from 40Ar/39Ar geochronology on the timing of Alpine shear zones in the Mont Blanc-Aiguilles Rouges region of the European Alps

The Mont Blanc and Aiguilles Rouges massifs, which form part of the external crystalline massifs of the western European Alps, record a young and fast exhumation history, as established by an extensive low-temperature thermochronology and geochronology data set. Various kinematic and dynamic models for Oligocene to Neogene deformation and exhumation of the Aiguilles Rouges-Mont Blanc system have been proposed. However, the timing of deformation along major shear zones in and around the massifs, which is crucial to these models, is still controversial. Our new 40Ar/39Ar data from key deformation zones in the Mont Blanc area show that NW directed thrusting lasted from Oligocene to mid-Miocene times in the Mont Blanc massif before a NW-ward jump of the deformation front to below the Aiguilles Rouges massif, which led to updoming of the Aiguilles Rouges and Mont Blanc massifs. This deactivated the main NW verging shear zones and caused back-folding and back thrusting east of the Mont Blanc, as well as upward extrusion of the Aiguilles Rouges massif. Subsequently, there was a switch to more coaxial shortening between the Aiguilles Rouges and Mont Blanc massifs associated with strain partitioning and related dextral strike-slip deformation on the eastern side of the Mont Blanc massif.