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-tract 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.