Nappe stack of Piemonte-Ligurian units south of Aosta Valley: New evidence from Urtier Valley (Western Alps)

New results of structural and lithostratigraphic investigations performed in the Urtier Valley, located south of Aosta Valley at the northern flank of the Gran Paradiso Massif, shed new light on the nappe stack architecture of the Western Alps. Two distinct Piemonte-Ligurian units were recognized in the area: (a) the blueschist facies Broillot unit, with a well-preserved lithostratigraphic succession that includes ophiolites and their metasedimentary cover and (2) the eclogitic Bardonney unit with clasts and/or slices of ophiolites embedded in a carbonate/quarzitic and/or metabasic matrix. Three major deformation stages have been recognized (D1, D2, and D3). D1 phase occurred during the peak metamorphic conditions (i.e., eclogite or blueschist facies). D2 and D3 phases are related to multistage exhumation processes, from early stacking to late refolding of former nappe contacts. Despite the intensity of deformation and metamorphism, the original lithostratigraphic succession of the Broillot unit allowed us to use a lithostratigraphic approach to reconstruct the structures in the study area. For the first time, differences in the nappe stack exposed south and north of the Aosta Valley are reported. In fact, contrary to what is observed in the north, the eclogitic units exposed in the Urtier Valley lay above and not below the blueschist units. A new model for the tectonic evolution, which ranges from stages of subduction to large-scale nappe refolding, is proposed. It is able to explain the difference in the architecture of the north and south of the Aosta Valley.