Tectono-metamorphic evolution of the Briançonnais zone (Modane-Aussois and Southern Vanoise units, Lyon Turin transect, Western Alps)

Abstract

In the central Western Alps, a combined structural, petrological and 40Ar–39Ar geochronological study of the Modane-Aussois and Southern Vanoise units yields important constraints on the timing of deformation and exhumation of the Briançonnais zone. These data help to decipher the respective roles of oceanic subduction, continental subduction and collision in the burial and exhumation of the main units through time. In the Modane-Aussois unit top to the NW thrusting (D1) was followed by top to the east shearing (D2) interpreted by some as normal faulting and by others as backthrusting. Pseudosection calculations imply that D1 deformation occurred at 1.0±0.1GPa and 350±30°C. Analysis of chlorite–phengite pairs yield P–T estimates between 0.15 and 0.65GPa and between 220 and 350°C for the D2 event. Phengites along the D1 schistosity (sample M80) yields an 40Ar–39Ar age of 37.12±0.39Ma, while D2 phengites yield ages of 35.42±0.38 (sample M173) and 31.60±0.33Ma (sample M196). It was not possible to test whether these ages are altered by excess argon or not. Our interpretation is that the D1/D2 transition occurred at ∼37Ma at the beginning of decompression, and that D2 lasted until at least ∼32Ma. Pseudosection calculation suggests that the Southern Vanoise unit was buried at 1.6±0.2GPa and 500–540°C. D1 deformation occurred during exhumation until 0.7–10.5GPa and 370±30°C. Published ages suggest that D1 deformation possibly started at ∼50Ma and lasted until ∼37Ma. D2 deformations started at P–T conditions close to that recorded in Modane-Aussois unit and lasted until 0.2±0.1GPa and 280±30°C at ∼28Ma. The gap of 0.6±0.3GPa and 150±130°C between peak metamorphic conditions in the two units was concealed by thrusting of the South Vanoise unit on top of the Modane-Aussois unit during D1 Deformation. Top to the east deformation (D2) affects both units and is interpreted as backthrusting.Based on these data, we propose a geodynamic reconstruction where the oceanic subduction of the Piedmont unit until ∼50Ma, is followed by its exhumation at the time of continental subduction of the continental Southern Vanoise unit until ∼45Ma. The Southern Vanoise is in turn underthrusted by the Modane-Aussois unit until ∼37Ma (D1). Between 37 and 31Ma the Modane-Aussois and Southern Vanoise units exhume together during backthrusting to the east (D2). This corresponds to the collision stage and to the activation of the Penninic Thrust. In the ∼50Ma to ∼31Ma time period the main thrusts propagated westward as the tectonic context switched from oceanic to continental subduction and finally to collision. During each stage, external units are buried while internal ones are exhumed.