Structural and petrological analyses of the Frido Unit (southern Italy): New insights into the early tectonic evolution of the southern Apennines–Calabrian Arc system

Abstract This study provides new data on the deformation and metamorphic evolution of the Jurassic to Upper Oligocene Frido Unit, an Ocean Continent Transition (OCT) Unit belonging to the Ligurian Accretionary Complex (LAC), by means of the integration of structural analysis, petrological investigations and a revision of the stratigraphical setting. The Frido Unit, representing the main metamorphic component of the Lower–Middle Miocene LAC in southern Italy, is characterized by a multistage tectonic evolution including: (i) two progressive deformation phases involving the development, under relatively high-pressure metamorphic conditions, of an early cleavage associated with isoclinal folds within the framework of a dominant SE-ward tectonic transport; (ii) a third deformation stage involving the growth of lower-pressure Na-amphibole along extensional shear surfaces (therefore probably marking the onset of tectonic exhumation); and (iii) two very low-temperature deformation phases characterized by the development of kink folds and associated thrusts, probably related to the late tectonic emplacement of the accretionary wedge onto the outer sectors of the Apennine domain (with a prevailing tectonic transport first toward the NE and then toward the NW). In order to clarify the metamorphic evolution of the Frido Unit, petrologic analyses were focused on its metasedimentary pelitic succession. Here, mineral parageneses including carpholite (well-documented for the first time in this study) and potassic white mica yielded peak pressure of ~ 1.2–1.4 GPa and temperature around 350 °C, thus indicating a high pressure/very low temperature metamorphism and a P–T–t path characterized by a rapid exhumation without any greenschist-facies overprint. In comparison with similar units cropping out in northern Calabria, Tuscany and Corsica, the Frido Unit experienced one of the coldest burial-exhumation histories.