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

Abstract

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.

Structural Evolution and Metasomatism of Subducted Metaophiolites in the Northwestern Alps

“…The dominant structure in the study area is represented by a pervasive S 2 foliation (Figure ) marked by eclogite‐facies mineral assemblages partly retrogressed under blueschist‐ to greenschists‐facies conditions. The S 2 foliation is developed at the regional scale, as documented in nearby areas (e.g., Ellero & Loprieno, ; Le Bayon & Ballèvre, ). In the metaophiolites of the Vallon des Eaux Rousses, the S 2 foliation mostly trends E‐W (see Figures a and c) and is clearly recognizable in serpentinites, calcschists, and in more foliated metagabbros.…”

“…Since a few N‐S trending fold axes attributable to D 2 folds were found in the southern part of the study area (i.e., close to the Gran Paradiso massif), a regional‐scale reorientation of D 2 structures from a N‐S trend in the south, toward a E‐W trend to the north, can be envisaged. E‐W trending structures have been widely recognized also in nearby areas (e.g., Beltrando et al, ; Ellero & Loprieno, ; Le Bayon & Ballèvre, ). To the north, in the Mt.…”

“…According to Beltrando et al (, ), the Africa‐related units and metaophiolites record various Alpine deformation phases, the most recent of which is characterized by E‐W trending schistosity and shear zones, these latter being correlated to the exhumation of the Gran Paradiso massif (e.g., Brouwer et al, ; Elter, ; Freeman et al, ; Le Bayon & Ballèvre, ). In the same area, other investigators (e.g., Ellero & Loprieno, ) have recently distinguished, from the top downward, (1) the Africa‐related Tour Ponton klippe (e.g., Nervo & Polino, ) and “Acque Rosse” (i.e., Eaux Rousses) slice consisting of albite gneisses and garnet‐bearing micaschists with bodies of eclogite‐facies mafic rocks including the hectometric metagabbro of the Pointe Noire; (2) the underlying blueschist‐facies ophiolitic Bardoney unit (made of serpentinite and metagabbro blocks embedded in a carbonate/quartz‐rich or metabasic matrix) and Broillot unit (consisting of metabasites, rare serpentinites, and quartzites that pass upward to impure marbles and calcschists); and (3) the Pene Blanche carbonatic slice, corresponding to the “Faisceau de Cogne,” including a sequence of Lower Triassic metaquartzite, dolomitic marble, Liassic marble, and Liassic calcschists and marbles.…”

Hydrochemical and Isotopic Applications in the Western Aosta Valley (Italy) for Sustainable Groundwater Management

“…The Gran Paradiso Unit is one of the Internal Crystalline Massifs belonging to the Briançonnais Nappes system. This nappes system consists of several tectonic units of basement rocks and sedimentary cover [15,16] and is characterized by different metamorphic evolutions within a range of greenschist to blueschist facies [17].…”

Superposed Sedimentary and Tectonic Block-In-Matrix Fabrics in a Subducted Serpentinite Mélange (High-Pressure Zermatt Saas Ophiolite, Western Alps)

“…The study area is located in the high Champorcher Valley (NW Alps), in the Italian side of the Graian Alps, to the south of the Aosta-Ranzola fault system ( Figure 1). In this area, several units of both continental and oceanic origin are tectonically juxtaposed [95][96][97][98][99]. Metaophiolites mainly consist of serpentinites with metarodingitic dykes, metamafic rocks (mostly metagabbros), mafic and metaophicarbonate breccias, and a metasedimentary cover dominated by flysch-type calcschists [40,41,74,100].…”