The Western Alps' active tectonics is characterized by ongoing widespread extension in the highest parts of the belt and transpressive/compressive tectonics along its borders. We examine these contrasting tectonic regimes using a multidisciplinary approach including seismotectonics, numerical modeling, GPS, morphotectonics, fieldwork, and brittle deformation analysis. Extension appears to be the dominant process in the present-day tectonic activity in the Western Alps, affecting its internal areas all along the arc. Shortening, in contrast, is limited to small areas located along at the outer borders of the chain. Strike-slip is observed throughout the Alpine realm and in the foreland. The stress-orientation pattern is radial for r3 in the inner, extensional zones, and for r1 in the outer, transcurrent/tranpressional ones. Extensional areas can be correlated with the parts of the belt with the thickest crust. Quantification of seismic strain in tectonically homogeneous areas shows that only 10-20% of the geodesy-documented deformation can be explained by the Alpine seismicity. We propose that, Alpine active tectonics are ruled by isostasy/buoyancy forces rather than the ongoing shortening along the Alpine Europe/Adria collision zone. This interpretation is corroborated by numerical modeling. The Neogene extensional structures in the Alps formed under increasingly brittle conditions. A synthesis of paleostress tensors for the internal parts of the West-Alpine Arc documents major orogen-parallel extension with a continuous change in r3 directions from ENE-WSW in the Simplon area, to N-S in the Vanoise area and to NNW-SSE in the Briançon area. Minor orogen-perpendicular extension increases from N to S. This second signal correlates with the present-day geodynamics as revealed by focal-plane mechanisms analysis. The orogen-parallel extension could be related to the opening of the Ligurian Sea during the Early-Middle Miocene and to compression/ rotation of the Adriatic indenter inducing lateral extrusion.

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International audienceFrom 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 aboundance 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 orogenparallel 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 (post-orogenic gravitational collapse). Some transcurrent tectonics, older than the extension in the Valais area, and younger than the extension further South is observed in the entire inner Western Alps; strike-slip movements are correlated with the Apulian rotation and local permutation of stress axes