No abstract

The western Alps are an active collision belt whose current stress field is inhomogeneous [Müller et al., 1992]. We report new seismological data which significantly improve our knowledge of this stress field. About 1600 earthquakes which occurred in the western Alps during the last 10 years were precisely located, and 79 new focal solutions were computed. The analysis of this database shows that widespread extension affects all the internal zones of the belt. To better constrain the associated stress regime, six stress tensors were computed using the Gephart and Forsyth [1984] method. They show that the current tectonics of the western Alps are contrasted with close variation in the stress regime (transpression to the front of the belt contrasting with extension in the core of the belt). The extensional direction is radial to the arcuate geometry of the belt and bounded outboard by the former thrust of the internal zones onto the external zone, suggesting extensional reactivation of this inherited crustal discontinuity. Such widespread extension within the inner part of an actually ongoing collision belt cannot be explained by simple collision‐related tectonics. We propose that intrabelt buoyancy forces, such as those produced by a slab retreat or slab break‐off, interfere with the boundary forces driven by the ongoing Europe‐Africa convergence.

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.

Widespread brittle extension occurs in the internal zones of the southwestern Alps, forming a dense fault network, which overprints the compressional structures. The extension has been followed by transcurrent motions. In terms of paleostress this extension is radial to the belt close to the Crustal Penninic Front and multidirectional in the eastern part of the internal zones. The paleostress field of the strike‐slip phase is coherent with the extensional one and compatible with dextral shear along longitudinal faults. Globally, we propose that the internal zones of the southwestern Alps underwent a single Neogene transtensive tectonic regime. To strengthen our structural results, the current tectonics has been analyzed using seismotectonic tools. This approach allowed the recognition of several active faults. Moreover, the ongoing tectonics is coherent with the observed brittle deformation. The fault network controls the present‐day seismicity. The combined seismotectonic and structural approaches give coherency and continuity from the Neogene to ongoing extensional tectonics in the southwestern Alps. In the framework of the whole western Alps a synthesis of our results with neotectonic‐related data allowed us to provide an accurate map of the Neogene to present kinematics of the belt. Extension, which appears as a major feature of the internal zones, has been coeval with the propagation of thrusts in the outer zones during Neogene times. The geodynamic processes, which rule this major tectonic evolution of the belt, remain a matter of debate.