Basement-cover tectonostratigraphic relationships in the northern Dora-Maira Massif (Western Alps)

The internal sector of the Western Alpine axial belt consists of Paleozoic basement units and minor Mesozoic cover successions, which are more widespread in the frontal part of the accretionary wedge. The Europa-derived Internal Crystalline Massifs, in their present-day dome-shaped exposure, consist of pre-Variscan basements hosting Permian intrusive bodies, while remnants of the Mesozoic cover occur discontinuously only along the flanks of the domes. This is also the case of the Dora-Maira Massif (DMM), a stack of different tectonic units metamorphosed under different peak pressure-temperature (P-T) conditions during the Alpine metamorphic cycle. The Triassic to Jurassic cover successions of the DMM basement are detached in the southernmost sector (Balestro et al., 2022) and tectonically sliced along the western flank of the dome in the central sector. In the northern sector of the massif, the basement-cover relationships are more ambiguous, having been represented either as tectonic or as stratigraphic in different geological maps.

We investigated the tectonostratigraphic relationships in the northernmost sector of the DMM in the Susa Valley, by analysing two detailed lithostratigraphic and structural sections across the basement-cover interface. The basement consists of polycyclic garnet- and chloritoid-bearing micaschist with bodies of metabasite and monocyclic orthogneiss, whereas the cover succession mainly consists of Middle Triassic dolomitic marble with minor calcschist of supposed Jurassic age. Both the Paleozoic and Mesozoic rocks were deformed during four main Alpine deformation phases corresponding to the subduction-related D1 phase and the early (i.e., D2) to late (i.e., D3 and D4) exhumation-related phases.

The investigated basement-cover interface is currently represented by a tectonic mélange, varying in thickness from a few meters to tens of meters, mixing tectonic slices of different sizes and lithologies (i.e., monocyclic carbonate-bearing micaschist, vacuolar dolomitic marble and carbonate tectonic breccia, marble, quartzite and quartz-rich schist, gneiss and garnet- and chloritoid-bearing micaschist). The occurrence of this mélange highlights the tectonic nature of the basement-cover interface, which (i) involved lithologies sourced from the Paleozoic basement and the Mesozoic carbonate cover and (ii) is localized along relatively weak metasediments similar to those occurring in the adjoining Briançonnais successions, which are considered Late Permian to Early Triassic in age. The tectonic mélange is deformed by late exhumation-related D3 and D4 folds. Ongoing investigations are focused on better defining when (t) and under which metamorphic conditions (P-T) the mélange was formed.

 

 

Balestro G., Festa A., Cadoppi, P., Groppo, C. & Roà M. (2022) - Pre-Orogenic Tectonostratigraphic Evolution of the European Distal Margin-Alpine Tethys Transition Zone in High-Pressure Units of the Southwestern Alps. Geosciences 2022, 12, 358.