Multidisciplinary Approach to Investigate Tectonic Mélanges at the Alpine Subduction Interface (Western Alps)

At the contact between the metaophiolites of the Piemonte Zone (PZ) and the Austroalpine continental crust, a tectonic mélange composed of thin slivers of calcschists, quartzites, metabasites, serpentinites, and fine-grained gneisses (including metapelites and metagranitoids) is exposed along the entire western margin of the Sesia-Lanzo Zone (SLZ), stretching from the Santanel klippe to the Lanzo Massif (Spalla et al., 1983; Battiston et al., 1984). Both the SLZ and PZ units have undergone four distinct deformation phases, resulting in a complex framework. Successive metamorphic mineral assemblages associated with different foliations, observed from the central to the southern segments of the SLZ, reveal varying metamorphic conditions across this zone of tectonic mixing. The origin of this intricate rock assemblage could be attributed to tectonic transposition processes occurring within the mantle wedge, or alternatively, it may represent the reworked remnants of a previously extended and thinned continental margin. The structural setting at the boundary between the PZ and the southern SLZ suggests that, during Alpine convergence, slices of oceanic lithosphere—each preserving distinct thermal and structural histories from their burial and exhumation paths—were tectonically intermixed with units of the Adria continental margin. This type of tectonic mélange, comprising interlayered fragments of oceanic and continental crust along with variably serpentinized peridotites, is consistent with the products expected to form within a hydrated mantle wedge during ablative subduction, as shown by 2D numerical models of ocean–continent subduction systems (Roda et al., 2020).