Beyond the Axial Zone: Unveiling the Regional Mylonitic Complex in the Variscan Sardinian Basement

Sardinia (Italy) represents one of the most comprehensive cross-sections of the Variscan orogen, characterised by its structurally and metamorphically complex axial zone, which still presents significant unresolved questions. This study integrates newly acquired field data with an extensive re-evaluation of previously published geological datasets from 1984 to 2022. Through detailed structural and petrographic analyses, the research identifies two major geological complexes: an older and higher-grade Old Gneiss Complex and a New Gneiss Complex that contain remnants of the Old Gneiss Complex embedded into it. This refined framework redefines the axial zone of Sardinia as a medium-temperature Regional Mylonitic Complex developed during the activity of the intracontinental East Variscan Shear Zone. Moreover, the data collected and the results obtained have enabled a reinterpretation of the Posada Valley Shear Zone. Traditionally regarded as part of the Posada Asinara Shear Zone (which itself was thought to represent the transitional boundary between the Internal Nappe Zone and the Axial Zone), the Posada Valley Sear Zone is now understood as a localised shear band, varying in width from 10 to 70 meters and marked by transitions from mylonite I S-C to mylonite II S-C over a 5 km extent. This shear zone developed during the late Carboniferous, between the activity of the EVSZ and the emplacement of Late Variscan granites. So, the study demonstrates that the Condensed Isogrades Zone, rather than the Posada Valley Shear Zone, represents the proper transition between the Internal Nappe Zone and the Axial Zone. This zone is distinguished by pronounced metamorphic gradients and a structural alignment consistent with the NE–SW D2 tectonic event observed across both the Old Gneiss Complex and the Internal Nappe Zone lithologies. The analysis reveals an anticlockwise rotation of shortening directions during the late Carboniferous, challenging previously accepted models of Variscan metamorphic zonation increasing north-eastward. These results align Sardinia’s evolution with similar tectonic features in Morocco, Corsica, the Maures Massif, and the Argentera Massif, providing new insights into the geodynamic processes governing the Gondwana-Laurussia collision during the late Carboniferous to early Permian.