Offscraping and shallow ophiolite accretion in the Ligurian Accretionary Wedge (Tuscan Apennines): role of seafloor structural inheritance

The Morello Tectonic Unit in the Tuscan Apennines (Italy) represents the result of tectono-sedimentary deformation, occurring in the frontal part of the non-metamorphosed Ligurian Accretionary Wedge, which consists of ophiolitic slices imbricated with a sedimentary succession containing ophiolite-bearing sedimentary mélanges. Geological mapping, structural and stratigraphic observations, allow us to document that (i) the ophiolite-bearing sedimentary mélanges formed by gravitational reworking of material sourced from intrabasinal structural highs facing the oceanic basin environment from Jurassic to lower Eocene, before the middle Eocene accretion stage, and (ii) the ophiolitic tectonic slices represent the scraping off at shallow structural levels of part of these crustal oceanic highs sequence. The final internal architecture of the shallow frontal portion of the Ligurian Accretionary Wedge does not differ from those observed in metamorphosed orogenic belts and exhumed accretionary complexes throughout the world (e.g., blueschist and eclogite units of Western Alps). This suggests that to the classical model of subduction and metamorphism followed by accretion and mélange formation (i.e., underplating), a model in which frontal accretion and ophiolite mélange formation at a shallow level are followed by underthrusting and subduction can be added, providing additional constraints to a better reconstruction of orogenic belts and accretionary complexes evolution.