Brittle deformation in the inner NW Alps: from early orogenparallel extrusion to late orogen-perpendicular collapse

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 northwestern Alps. Palaeostress tensors are mostly extensional, although one-third of them are transcurrent. The dominant direction of minimum horizontal stress axes (sigma3) is in an orogen-parallel (N30degrees to N70degrees) 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.