The Maira-Sampeyre and Val Grana Allochthons (south Western Alps): review and new data on the tectonometamorphic evolution of the Briançonnais distal margin

Abstract

Here we describe the structure, the high-pressure, low-temperature (HP-LT) metamorphism and tectonic evolution of the Briançonnais distal margin units from the south Western Alps. The studied area extends southwest of the Dora-Maira (U)HP basement units and east-southeast of the classical Briançonnais nappes. A new structural map accompanied by geological profiles shows the thrusting of the oceanic nappes (Monviso and Queyras units) onto the distal Briançonnais units (D1 and D2 late Eocene deformation phases) under blueschist-facies conditions. Subsequent deformation during the Early Oligocene (D3 deformation phase) took place under greenschist-facies conditions and was associated with back-folding and -thrusting in the units overlying the Dora-Maira massif and with exhumation related to normal reactivation of former thrusts within the latter massif. Two large cover units, detached from their former distal Briançonnais basement, are redefined as the Maira-Sampeyre and Val Grana Allochthons (shortly: Maira-Grana Allochthons = MGA) including, (i) the Val Maira-Sampeyre unit involving Lower and Middle Triassic formations, seemingly detached from the Dora-Maira units during the subduction process, and (ii) the Val Grana unit with Middle-Upper Triassic and Early-Middle Jurassic formations, which was probably detached from the Maira-Sampeyre unit and correlates with the “Prepiemonte units” known from the Ligurian Alps to the Swiss Prealps. Three major shear zones involving tectonic mélanges of oceanic and continental rocks at the base of the Val Grana, Maira-Sampeyre and Dronero units testify to an early phase of exhumation within the subduction channel in front of the Adria plate. We present a new metamorphic map based on published and new petrological data, including new thermometric data obtained by Raman spectroscopy of carbonaceous material (RSCM). The TRSCM values range from ~ 400 °C to > 500 °C, going from the most external Val Grana unit and overlying Queyras schists to the uppermost Dora-Maira unit. During the Late Triassic, the width of the Briançonnais s.l. domain can be restored at ~ 100 km, whereas it reached ~ 150 km after the Jurassic rifting. A significant, second rifting event affected the Briançonnais domain during the Late Cretaceous-Paleocene, forming the Longet-Alpet chaotic breccias, which deserve further investigations.

A journey towards the forbidden zone: a new, cold, UHP unit in the Dora-Maira Massif (Western Alps)

“…The Dora-Maira, Gran Paradiso, and Monte Rosa Massifs in the Western Alps constitute the Internal Crystalline Massifs and belong to the distal part of the Briançonnais palaeomargin (Schmid et al 2004;Handy et al 2010;Ballèvre et al 2020;Michard et al 2022;Manzotti and Ballèvre 2022). They are made of slices of Palaeozoic basement and minor remnants of dismembered Mesozoic covers.…”

“…The internal geometry of the southern Dora-Maira Massif is relatively well known (Fig. 1a): several tectonic units have been identified on the basis of lithological, structural and petrological data and they are separated by thrust sheets and slivers of Mesozoic meta-sediments and ophiolites (e.g., Chopin 1984;Chopin et al 1991;Kienast et al 1991;Compagnoni et al 1995;Michard et al 1995;Chopin and Schertl 1999;Compagnoni and Hirajima 2001;Rubatto and Hermann 2001;Compagnoni and Rolfo 2003 and refs therein;Compagnoni et al 2012;Groppo et al 2019;Bonnet et al 2022;Michard et al 2022). The Alpine metamorphism reached by these units varies from garnet blueschist (Dronero Unit), to quartz eclogite (Rocca Solei) to coesite eclogite facies (Brossasco-Isasca) conditions (Fig.…”

“…Finally, we cannot a priori exclude on a petrological basis the hypothesis that the Chasteiran Unit is monocyclic and made of Mesozoic sediments. Mesozoic continental sediments in the Alps are by far dominated by meta-limestones and meta-marls, i.e., calcite-bearing schists (e.g., Michard et al 2022). Calciumpoor or calcium-absent lithologies derived from pelitic schists developed during the anoxic events, mainly of mid-Cretaceous age.…”

Assessing the importance of H2O content in the tectono‐metamorphic evolution of shear zones: A case study from the Dora‐Maira Massif (Western Alps)

“…The Dora-Maira Massif (Figure 1a), one of the Internal Crystalline Massifs, belonging to the distal part of the Briançonnais paleomargin (Ballèvre et al, 2020;Gasco et al, 2013;Sandrone et al, 1993;Schmid et al, 2017), is described as a nappe-stack made by three major units (Argand, 1934;Chopin et al, 1991;Michard, 1967;Michard et al, 1993). Moving upward, they are: (i) the Sanfront-Pinerolo Unit (SPU; Avigad et al, 2003); (ii) the Dora-Maira Basement Complex (Michard et al, 1993) and (iii) the Dronero Unit (DU; Michard et al, 2022). After structural mapping, several authors (e.g., Chopin et al, 1991;Compagnoni et al, 2012;Henry et al, 1993;Manzotti et al, 2022;Michard et al, 2022;Vialon, 1966) divided the Dora-Maira Basement Complex into minor (sub-)units (Table S1).…”

“…Moving upward, they are: (i) the Sanfront-Pinerolo Unit (SPU; Avigad et al, 2003); (ii) the Dora-Maira Basement Complex (Michard et al, 1993) and (iii) the Dronero Unit (DU; Michard et al, 2022). After structural mapping, several authors (e.g., Chopin et al, 1991;Compagnoni et al, 2012;Henry et al, 1993;Manzotti et al, 2022;Michard et al, 2022;Vialon, 1966) divided the Dora-Maira Basement Complex into minor (sub-)units (Table S1). From the structurally lower to the higher, around the investigated area they are (Figure 1b; see Michard et al, 2022 for review): (i) the Brossasco-Isasca Unit (BIU); (ii) the Rocca Solei Unit (RSU); (iii) the Ricordone Unit (RU).…”