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The tectonic evolution of the NW alpine front and foreland basin is reviewed in the light of new structural and chrono-stratigraphical data. Seismic-reflection profiles from the Jura fold thrust belt and Molasse basin, surface-geology and thrust-system considerations lead to a complete cross-section of the NW Alpine front including the Helvetic domain. Restoration of this section places individual Cenozoic formations in their approximate palaeogeographic position. ‘Geohistory’ plots are constructed for five profiles along a SE-NW transect. Thrust front, onlap and forebulge advanced at high rates of 10–20 km/Ma −1 at the onset of foreland basin formation in the late Eocene/ early Oligocene (40–30 Ma). In these early stages, the foreland basin is an underfilled flexural trough with about 100 km width, less than 600 m water depth at the deepest point and less than 200 m of total accumulated sediments. From 30 to 22 Ma, thrust front and ‘pinch-out’ migrate at a decreased rate of about 5 km/Ma −1 northwestward. The basin width remains constant at around 100 km; an increased total subsidence ( c. 2.7 km) is compensated by sedimentation. At around 22 Ma, the thrust front seems to come to a halt southeast of Lausanne, whereas a strong subsidence trend prevails. After the Serravallian ( c. 12 Ma) the Alpine thrust front jumps by about 100 km northwestward from a position southeast of Lausanne to the external Jura leading to thrust related uplift, deformation and concommitant erosion of the entire basin fill. No new flexural foreland basin in response to the modified thrust- and load-geometry has yet been developed. The present-day Molasse basin is only a small remnant of a much larger foreland basin in a very advanced stage of its evolution.

Twinning of the e-plane is the dominant crystal -plastic deformation mechanism in calcite deformed below about 400 8C. Calcite in a twin domain has a different crystallographic orientation from the host calcite grain. So-called thin twins appear as thin black lines when viewed parallel to the twin plane at 200-320 £ magnification under a petrographic microscope. Thick twins viewed in the same way have a microscopically visible width of twinned material between black lines. Calcite e-twin width and morphology has been correlated with temperature of deformation in naturally deformed coarse-grained calcite. In this paper, we present a compilation and analysis of data from limestones of the frontal Alps (France and Switzerland) and the Appalachian Valley and Ridge and Plateau provinces (eastern United States) to document this temperature dependence. Mean calcite twin width correlates directly with temperature of deformation such that thin twins dominate below 170 8C and thick twins dominate above 200 8C. Above 250 8C dynamic recrystallization is an important deformation mechanism in calcite. Mean twin intensity (twin planes/mm) correlates negatively with temperature, and a cross plot of twin intensity with twin width can yield information about both strain and temperature of deformation. These relationships provide a deformation geothermometer for rocks that might otherwise yield little or no paleotemperature data.

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.