International audienceThe high elevation and deep incision of the Alps have traditionally been used as an argument for recent tectonic activity that has elevated the belt and increased erosion rates. Normal faulting and horizontal extension, however, dominate current tectonic activity, and isostatic compensation of thinning crust should lead not to increased but to decreased mean elevations. Here we test the idea that enhanced Quaternary erosion of the Alps and isostatic compensation of the mass removed can account for the distribution of present-day geodetically measured rates of vertical movement in the western Alps. Using geophysical relief and Kuhlemann's estimated average erosion rate for the Alps, we quantify the spatial distribution of erosion and the volume of eroded rock, respectively. From these, we obtain a map of rock eroded within a given time span. The calculated isostatic response of the Alpine lithosphere to erosional unloading for a variety of values of the flexural rigidity of the Alpine lithosphere reaches a maximum of [~]500 m since 1 Ma in the inner Swiss Alps, and vertical movement extends across the entire belt, including peri-Alpine basins. Assuming a steady erosion rate since 1 Ma, this rebound accounts for half of the measured vertical motion of 1.1 mm/yr in the southern Valais. Thus, a significant fraction ([~]50%) of the present-day vertical movement results from the isostatic response to enhanced erosion during Plio-Quaternary tim

S U M M A R YThe contrasted tectonics of the western/central Alps is examined using a synthesis of 389 reliable focal mechanisms. The present-day strain regime is mapped and interpolated for the entire Alpine belt based on a newly developed method of regionalization. The most striking feature is a continuous area of extension which closely follows the large-scale topographic crest line of the Alpine arc. Thrusting is observed locally, limited to areas near the border of the Alpine chain. A majority of earthquakes within the Alps and its forelands are in strikeslip mode. Stress inversion methods have been applied to homogenous subsets of focal plane mechanisms in order to map regional variations in stress orientation. The stress state is confirmed to be orogen-perpendicular both for σ 3 in the inner extensional zones and σ 1 in the outer transcurrent/transpressional zones. Extensional areas are well correlated with the part of the belt which presents the thickest crust, as shown by the comparison with the Bouguer anomaly and the average topography of the belt. In the northwestern Swiss Alps, extension is also correlated with currently uplifting zones. These observations and our strain/stress analyses support a geodynamic model for the western Alps in which the current activity is mostly a result of gravitational 'body' forces. Earthquakes do not provide any direct evidence for ongoing convergence in the Alpine system, but a relationship with ongoing activity of complex block rotations of the Apulian microplate cannot be ruled out.

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.

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

Internal parts of the Alps have undergone widespread extensional deformation in the course of their Neogene exhumation history. Palaeostress inversion methods are used to map the prevailing stress fields and their evolution through time. Here we present new data from 100 sites with a total of about 2000 faults/striae couples, covering a large portion of the inner north‐western Alps. Palaeostress tensors are mostly extensional, although one‐third of them are transcurrent. The dominant direction of minimum horizontal stress axes (σ3) is in an orogen‐parallel (N30° to N70°) orientation around the bend of the north‐west alpine arc. A comparison between this older (Neogene, post‐metamorphic) stress field with the current stress and strain field determined from seismotectonics and geodesy indicates a change in deformation mode from early orogen‐parallel extrusion to a late and ongoing orogen‐perpendicular spreading.