Tectonic evolution of the Brianconnais units along a transect (ECORS-CROP) through the Italian-French Western Alps

Based on new structural data from an area in the Italian-French Western Alps. situated between the Petit Saint Bernard pass and the Gran Paradiso massif, the large-scale geometry of the tectonic features is established for the tectonic units derived from the Brianconnais paleogeographic domain. Based on this, and other new data on the metamorphic evolution and geochronology of the area. a consistent model for the tectonic evolution for the Brianconnais domain is proposed. A nappe stack consisting of, from bottom to top, Zone Houillere unit, Ruitor unit, Internal unit and Piemont-Ligurian oceanic unit, was affected by three deformation phases. The third phase of deformation (35-31 Ma), referred to as post-nappe folding, is responsible for the formation of two large-scale folds: the Ruitor and Valsavaranche mega-folds. An overturned nappe stack, characterized by foreland-dipping foliations, is found between the axial planes of these two mega-folds. No evidence was found for back-thrusting, postulated by many previous authors. The individual thrusts between the tectonic units were active during a second phase of deformation (43-35 Ma), characterized by top-NW to -NNW shearing. This D2 is related to nappe stacking and accompanied by substantial exhumation. Thrusting becomes relatively younger towards the external parts of the study area. We emphasize that extension played no significant role in the exhumation of the high-pressure units of the Italian-French Alps. The contacts between the tectonic units were already active during the first phase of deformation (50-43 Ma), which is related to subduction and peak pressures.