Post-nappe brittle tectonics and kinematic evolution of the north-western Alps: an integrated approach

Data from remote sensing, structural geology and thermochronology provide the basis for this integrated reconstruction of the Oligocene to Present kinematic evolution of the north-western Alpine nappe stack. Two brittle tectonic phases post-date the Cretaceous–Eocene ductile deformation. A NW–SE extension developed in the Oligocene (D1) along three main conjugate fault systems arranged in orthorhombic symmetry (N-, NW- and SE-dipping). Cooling rate contour maps, from published apatite and zircon fission-track ages and Rb/Sr biotite ages, highlight the differential exhumation of large fault-bounded blocks during this phase, whilst synkinematic hydrothermal veins and calc-alkaline dykes (29–32 Ma) help to constrain its age. From the Miocene onwards, a general rearrangement of the strain pattern led to SW-directed lateral extrusion (D2) of the Pennine-Graian Alps block, bounded by a network of seismogenic shear zones, the most important being the Ospizio Sottile, Simplon, Rhone and Chamonix faults. The internal deformation of the Pennine-Graian Alps block is characterised by an overall more or less homogeneous NE–SW extension. The approach undertaken, integrating remote sensing, structural analysis on different scales, and thermochronology (with the cooling rate map representation), is therefore effective in reconstructing the late-orogenic extensional tectonic evolution of metamorphic nappe stacks.