Geology of the southern Dora-Maira Massif: insights from a sector with mixed ophiolitic and continental rocks (Valmala tectonic unit, Western Alps)

Abstract

In the Valmala sector of the southern Dora Maira Massif (Western Alps), two different eclogiteand blueschist-facies units (i.e. the Rocca Solei and Dronero units, respectively), are separated by a shear zone (i.e. the Valmala Tectonic Unit), which peculiarly consists of mixed slices of ophiolitic and continental rocks. A detailed geological map at 1:10,000 scale allowed to point out that the tectonic slices within the Valmala Tectonic Unit consist of 'native' rock slices wrenched from the overlying Dronero Unit, and 'exotic' rocks likely sourced from other units of the Dora Maira and from a continental margin and an oceanic basin. On the contrary, rock slices sourced from the underlying Rocca Solei Unit are lacking. The overall tectonic stack results after an early subduction-related deformation phase (i.e. the D1), and the pervasive overprinting of two subsequent exhumation-related deformation phases (i.e. the D2 and D3). The Valmala Tectonic Unit is inferred to have played a role in decoupling the southern Dora Maira Massif during subduction, and/or in driving exhumation of the ultra-high pressure rocks occurring in the adjoining Brossasco-Isasca Unit.

Protracted Subduction of the European Hyperextended Margin Revealed by Rutile U‐Pb Geochronology Across the Dora‐Maira Massif (Western Alps)

“…These estimates imply a metamorphic gap of at least 1 GPa and 200°C between the hot eclogites of the coesite-bearing unit and the cold eclogites (Figure 3). No detailed P-T estimates exist for the OB, but Balestro et al (2020) described garnet-chloritoid assemblages, suggesting P-T conditions close to those of the "cold" eclogite units.…”

“…latter three corresponding to Units 1, 2, and 3 of Henry et al (1993). At the base of the Dronero-Sampeyre unit, the Ophiolitiferous Band (OB), or Valmala shear zone (Balestro et al, 2020;Henry et al, 1993) consists of mixed continental and oceanic rocks (including orthogneisses, calcschists, metabasalts, and serpentinites). Above the Dronero-Sampeyre unit, one or several slivers of mixed oceanic (Jurassic to Cretaceous calcschists) and continental (Triassic to upper Cretaceous) metasediments are intercalated below the Monviso metaophiolite complex (Balestro et al, 2011(Balestro et al, , 2015.…”

“…The BIU is the only unit where ultrahigh-pressure (UHP) metamorphism has been recognized, and peak P-T conditions of ∼3.5-4.0 GPa and 700°C-750°C (Figure 3) have been estimated from a variety of lithologies in the unit, including coesite-pyrope-bearing whiteschist (e.g., Campomenosi et al, 2021;Chopin, 1984;Ferrando et al, 2009;Hermann, 2003;Schertl et al, 1991;Sharp et al, 1993), mafic eclogite (Chopin et al, 1991; Malaroda (1972), Kerckhove and Monjuvent (1979), Henry (1990); Chopin et al (1991), Compagnoni et al (2012), Groppo et al (2019), and Balestro et al (2020). The location of the section is shown on the map.…”

Timing of exhumation of meta-ophiolite units in the Western Alps: New tectonic implications from 40Ar/39Ar white mica ages from Piedmont Zone (Susa Valley)

“…This implies that, at least in the northern DM, the D1-related P-T peak age should predate the Early Oliogocene age calculated in the southern DM (i.e. in the Brossasco-Isasca Unit; Moniè and Chopin, 1991;Gebauer et al, 1997;Itaya et al, 2017), wherein a complex tectonic stack of continental crust and ophiolitic units occurs (Henry et al, 1993;Groppo et al, 2019;Balestro et al, 2020).…”

Metasediments Covering Ophiolites in the HP Internal Belt of the Western Alps: Review of Tectono-Stratigraphic Successions and Constraints for the Alpine Evolution

“…The Monviso Complex is tectonically interposed between the continental Dora Maira crystalline Massif and the Queyras Schistes Lustrés Complex, to the East and West, respectively [61,62]. The Dora Maira Massif represents a slab of the paleo-European margin, which consists of various tectonic units, differing from each other for their lithostratigraphic features, Alpine metamorphic P-T peaks (from blueschistto eclogite-coesite facies), and structural position [82][83][84][85][86][87]. The Queyras Schistes Lustrés Complex, which is separated from the Monviso Complex along a major ductile fault [88,89], represents the external part of the Alpine accretionary wedge [90] (Figure 1B).…”