Structural and metamorphic evolution of an ocean-continent transition (OCT) zone mélange deformed under HP conditions during Alpine subduction (Western Italian Alps).

We report on the structural architecture and metamorphic evolution of a mélange, developed originally in an ocean-continent transition (OCT) zone along the boundary between the continental crust of the Sesia-Lanzo (SLZ) and the oceanic Piemonte Zones (PZ) in the axial part of the Western Alps. All these units were deformed together under high-pressure conditions. The mélange consists of thin layers of calcschist, fine-grained gneiss, quartzite, minor metabasic rocks and serpentinite, and occurs all along the western margin of southern SLZ, extending from Santanel klippe to Lanzo Massif, over a distance of 50 km (Spalla et al., 1983; Battiston et al., 1984). Calcschist rocks range from phyllites to carbonatic schists and marbles; fine-grained gneisses of continental origin (very similar to those of SLZ) include phengitic white mica, chlorite, ± garnet ± albite and relict allanite. Thinly layered quartzites are white mica- and garnet-bearing. Metabasic rocks consist of metagabbros and metabasalts with minor mylonitic serpentinites. All these lithologies of the mélange unit and the rocks of SLZ and PZ together underwent four episodes of deformation, giving rise to a complex regional tectonostratigraphy. The earliest deformational structures are represented by up to ten meter-scale isoclinal rootless folds. The metamorphic mineral assemblages marking successive foliations indicate that all rock units in the mélange, SLZ and PZ (Spalla et al., 1983; Benciolini et al., 1984) experienced an early eclogite facies imprint, followed by re-equilibration under blueschist facies conditions, and that they were finally widely retrogressed under greenschist facies conditions during the last two deformational episodes (D3 and D4 structures). The strong synmetamorphic deformation of this mélange prevents an unequivocal interpretation of its origin; hence, we envisage two possible scenarios: i) the present day configuration of these thin, intermingled layers, including rootless refolded isoclinal folds, is entirely due to transposition that occurred in a mantle wedge at the early stages of deformation under eclogite facies conditions during active subduction; ii) a detrital origin of these alternating layers of terrigeneous and carbonaceous rocks corresponds to a primary sequence of an extensionally-thinned continental margin near an OCT that was reworked in the Alpine subduction system.