The tectonometamorphic evolution of the Sesia–Dent Blanche nappes (internal Western Alps): review and synthesis

International audienceThis study reviews and synthesizes the presentknowledge on the Sesia–Dent Blanche nappes, the highesttectonic elements in the Western Alps (Switzerland andItaly), which comprise pieces of pre-Alpine basement andMesozoic cover. All of the available data are integrated in acrustal-scale kinematic model with the aim to reconstructthe Alpine tectono-metamorphic evolution of the Sesia–Dent Blanche nappes. Although major uncertainties remainin the pre-Alpine geometry, the basement and coversequences of the Sesia–Dent Blanche nappes are seen aspart of a thinned continental crust derived from the Adriaticmargin. The earliest stages of the Alpine evolution areinterpreted as recording late Cretaceous subduction of theAdria-derived Sesia–Dent Blanche nappes below theSouth-Alpine domain. During this subduction, severalsheets of crustal material were stacked and separated byshear zones that rework remnants of their Mesozoic cover.The recently described Roisan-Cignana Shear Zone of theDent Blanche Tectonic System represents such a shearzone, indicating that the Sesia–Dent Blanche nappes representa stack of several individual nappes. During thesubsequent subduction of the Piemonte–Liguria Oceanlarge-scale folding of the nappe stack (including the Roisan-Cignana Shear Zone) took place under greenschistfacies conditions, which indicates partial exhumation of theDent Blanche Tectonic System. The entrance of the Brianc¸onnais micro-continent within the subduction zone ledto a drastic change in the deformation pattern of the Alpinebelt, with rapid exhumation of the eclogite-facies ophiolitebearingunits and thrust propagation towards the foreland.Slab breakoff probably was responsible for allowing partialmelting in the mantle and Oligocene intrusions into themost internal parts of the Sesia–Dent Blanche nappes.Finally, indentation of the Adriatic plate into the orogenicwedge resulted in the formation of the Vanzone back-fold,which marks the end of the pervasive ductile deformationwithin the Sesia–Dent Blanche nappes during the earliestMiocene