Subduction signature in the Internal Ligurian units (Northern Apennine, Italy): Evidence from P–T metamorphic peak estimate

The Internal Ligurian units (Northern Apennine) represent deformed and metamorphosed fragments of the oceanic lithosphere of the Ligure–Piemontese oceanic basin. Different tectonic models have been proposed for the geodynamic setting in which the deformation and metamorphism have been acquired. However, the lack of updated, clear, thermo-barometric data has made it hard to unambiguously discriminate between these different proposed models. In this article, we provide evidence for the deformation of the Palombini Shale, i.e., pelagic deposits belonging to the Internal Ligurian units, under P and T peak conditions of 230–300°C. and 0.6–0.9 GPa, respectively. These data indicate that the Internal Ligurian units were affected by a blueschist facies metamorphism achieved during the underplating within the accretionary wedge developed during the Late Cretaceous—Early Tertiary Alpine subduction. These data support the hypothesis of a unique, pre-Oligocene orogenic system for the Alpine belt and the westernmost sector of the Northern Apennines belt.