Geology of the southern Gran Paradiso Massif and Lower Piedmont Zone contact area (middle Ala Valley, Western Alps, Italy)

Abstract

This work presents the structural evolution of a poorly studied key-area in the middle Ala Valley, Western Alps, where two tectono-metamorphic units are exposed. A geological map at the 1:10.000 scale, integrated with meso-and microstructural analysis, has been realised. We investigated the contact area between Gran Paradiso Massif in the footwall and Lower Piedmont Zone in the hanging wall. Both tectono-metamorphic units, with a different paleogeographic affinity, preserve similar polyphasic deformation histories, defined by four deformation phases. The Dp phase, strongly transposing previous structural relicts, is marked by a high-pressure assemblage associated with the Sp foliation. Dp controls the lithological boundary attitude. A mylonitic zone, developed during the Dp, showing kinematic indicators pointing to a top-to-the N-NW sense of shear, is responsible for the juxtaposition of the two units. Dp structural elements are deformed by Dp+1 and Dp+2 subsequent phases. A greenschist-facies overprinting was observed during the Dp+1 phase.

Structural, lithostratigraphic and thermal features of a Permian lower crust from the Western Italian Alps (Valpelline Series, Valle d’Aosta)

“…The Western Alps consist of three main structural domains (Bousquet et al 2008): (i) the southern external domain, corresponding to the Adria-derived domains, i.e., the Southern Alps; (ii) the external domain, representing the European foreland (Helvetic domain) and (iii) the axial zone between the Penninic Front to the north and the Insubric Line to the south (Schmid et al 1989;Handy et al 2010). The axial zone includes remnants of the subducted oceanic lithosphere of the Piedmont-Ligurian Ocean (i.e., Piedmont Zone; Pognante, 1979;Dal Piaz, 1999;Zanoni et al 2016;Luoni et al 2020;Caso et al 2021) and the continental crust of Adria (Giuntoli & Engi, 2016;Austroalpine Domain) and European pertinence (Penninic Domain). The DBTS (Manzotti et al 2014;Manzotti & Ballèvre, 2017) is a continental klippe of the Austroalpine domain, which is divided into the Dent-Blanche s.s. to the northwest and the Mont-Mary Nappe (Fig.…”

The Memory of a Fault Gouge: An Example from the Simplon Fault Zone (Central Alps)

“…The Alpine Orogeny was and still is the result of convergence between the European and African Plate [43][44][45]. In the western Alps the convergence movement caused, since the Cretaceous, the first subsidence of the Piedmont-Ligurian Ocean [46,47], interposed between the two continental plates, and later (Cenozoic) orogenic collision [48]. The latter caused crustal thickening during continental collision and, in particular, by the subduction of the European passive margin in the Early Eocene.…”

“…The latter caused crustal thickening during continental collision and, in particular, by the subduction of the European passive margin in the Early Eocene. A new post-collisional stage of extension, heralded by normal faults and conjugate strike slip faults, began after the crustal thickening and allowed the formation of tectonic windows that expose the lower crust made of Penninic or European units [42][43][44][45][46][47][48][49][50]. In the Central Alps, the SFZ is related to this last stage post-collisional extension [25,26,[31][32][33][34][35][36][37][38][39].…”

Quantitative combined multiscale structural and minero-chemical analysis to unravel the tectono-metamorphic evolution of cordierite- migmatite gneiss from the Valpelline Unit (Dent-Blanche Nappe, Western Italian Alps, Valle d’Aosta)

“…In the Valpelline Unit, previous authors described cordierite-bearing migmatites (Gardien et al, 1994), but without clearly putting them in a framework of a detailed multiscale structural evolution. Geological mapping and outcrop-scale studies for the description of geometrical overprinting relationships are fundamental starting tools in structural geology (e.g., Spalla et al, 2005;Caso et al, 2021;Petroccia et al, 2020Petroccia et al, , 2022, but they need to be integrated with quantitative microstructural and chemical data to be fully understood (Spalla & Zucali, 2004;Fazio et al, 2009;Corti et al, 2019;Roda et al, 2021;Zucali et al, 2021). Quantitative parameters such as grainsize distribution, mineral shape factors and orientations, measured using ArcGis "ready-to-run" toolboxes, on the main rock-forming mineral phases, could be combined with quantitative minerochemical data (i.e., calibrated X-ray maps and point mineral analyses).…”