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

This study reviews and synthesizes the present knowledge on the Sesia-Dent Blanche nappes, the highest tectonic elements in the Western Alps (Switzerland and Italy), which comprise pieces of pre-Alpine basement and Mesozoic cover. All of the available data are integrated in a crustal-scale kinematic model with the aim to reconstruct the Alpine tectono-metamorphic evolution of the Sesia-Dent Blanche nappes. Although major uncertainties remain in the pre-Alpine geometry, the basement and cover sequences of the Sesia-Dent Blanche nappes are seen as part of a thinned continental crust derived from the Adriatic margin. The earliest stages of the Alpine evolution are interpreted as recording late Cretaceous subduction of the Adria-derived Sesia-Dent Blanche nappes below the South-Alpine domain. During this subduction, several sheets of crustal material were stacked and separated by shear zones that rework remnants of their Mesozoic cover. The recently described Roisan-Cignana Shear Zone of the Dent Blanche Tectonic System represents such a shear zone, indicating that the Sesia-Dent Blanche nappes represent a stack of several individual nappes. During the subsequent subduction of the Piemonte-Liguria Ocean large-scale folding of the nappe stack (including the Roisan-Cignana Shear Zone) took place under greenschist facies conditions, which indicates partial exhumation of the Dent Blanche Tectonic System. The entrance of the Briançonnais micro-continent within the subduction zone led to a drastic change in the deformation pattern of the Alpine belt, with rapid exhumation of the eclogite-facies ophiolite-bearing units and thrust propagation towards the foreland. Slab breakoff probably was responsible for allowing partial melting in the mantle and Oligocene intrusions into the most internal parts of the Sesia-Dent Blanche nappes. Finally, indentation of the Adriatic plate into the orogenic wedge resulted in the formation of the Vanzone back-fold, which marks the end of the pervasive ductile deformation within the Sesia-Dent Blanche nappes during the earliest Miocene