Polygenetic mélanges in the Ligurian accretionary wedge (Northern Apennines, Italy)

Interpretations on the origin of the External Ligurian Units in the Northern Apennines (Italy) range from a typical unmetamorphosed broken formation (chaotic rock unit without exotic blocks) to an undifferentiated chaotic complex, produced by offscraping–tectonic imbrication during the evolution of the Ligurian accretionary wedge (LAW). We report here on the internal structure of the late Cretaceous–late Oligocene LAW, which consists of polygenetic chaotic bodies of different scales formed by the superposition in time and space of tectonic, diapiric and sedimentary processes. A Broken Formation is the oldest unit in the LAW, showing bedding-parallel boudinage structures developed due to layer-parallel extension at the toe of the LAW-front during the late Cretaceous–middle Eocene. This Broken Formation experienced an overprint of tectonic, diapiric and sedimentary processes as a result of the late Oligocene continental collision. Contractional deformation produced a structurally ordered block-in-matrix fabric through mixing of both native and exotic blocks, forming a Tectonic Mélange. Concentration of overpressurized fluids along thrust faults triggered the upward rise of shaly material, producing a Diapiric Mélange, which provided the source material for the downslope emplacement of the youngest, late Oligocene Sedimentary Mélange. Sedimentary Mélange units unconformably cover the thrust faults, constraining the timing of the youngest episode of contractional deformation in the LAW. The interplay and superposition of tectonic, diapiric and sedimentary processes plays a significant role in the dynamic equilibrium of accretionary wedges. Our findings provide useful criteria to differentiate between different types of polygenetic mélanges in ancient and modern accretionary wedges.