Miocene lateral extrusion in the inner western Alps revealed by dynamic fault analysis

Abstract

From early Miocene to the present‐day the core parts of the western European Alps experienced brittle extensional deformations, mostly in a strike‐parallel direction. Here we present new data constraining the brittle deformation of the Vanoise area (French Alps) and a synthesis of 312 paleostress tensors in the whole arc of the internal western Alps. The data show a continuous change in the direction of extension, from N065° (Simplon area), to N‐S (Vanoise area) and to NNW‐SSE (Briançon area). The abundance of orogen‐perpendicular σ3 axes increases from the north to the south. In the Briançonnais area, an extensional reactivation of the Basal Penninic Thrust seems to be the origin of the E‐W to NE‐SW oriented σ3. In light of these new data and the regional paleostress synthesis, we propose a predominant orogen‐parallel extension in the internal zone as a whole. This orogen‐parallel extension is related to the indentation/rotation of the Apulian microplate and to the opening of the Ligurian Sea during the lower‐middle Miocene. The locally observed orogen‐perpendicular extension is interpreted as an effect of the exhumation of the Internal Crystalline massifs, the uplift of the External Crystalline massifs and/or the present‐day geodynamics (postorogenic gravitational collapse). Some transcurrent tectonics, older than the extension in the Valais area and younger than the extension farther south is observed in the entire inner western Alps; strike‐slip movements are correlated with the Apulian rotation and local permutation of stress axes.