The Anchimetamorphic Tectonic Mélange of Tempa Roccarossa (Southern Apennines, Italy): Insights on the Kinematic and Thermobaric Evolution of the Upper Miocene‐Pliocene Orogenic Wedge

Studies of tectonic mélanges provide constraints on the evolution of active plate margins. However, resolving the pressure-temperature trajectories of these deformed rocks, which are exhumed from low-temperature conditions, can be challenging. We analyzed a Late Miocene-Early Pliocene tectonic mélange formed in a shear zone in the southern Apennines (Basilicata, Italy), located in the hanging wall of a regional thrust, to provide estimates of temperature, pressure and strain. The mélange comprises slates with a fine-grained phyllosilicate matrix embedding larger porphyroclasts with relict S0 bedding. Electronic microscope analysis revealed a disjunctive cleavage (S1), partially to fully transposed by a top-to-the-E/SE crenulation cleavage (S2) marked by white mica and chlorite. A late weak cleavage (S3) is not accompanied by newly formed minerals. Kinematic vorticity analysis indicates a range of 20%–35% coaxial strain, whereas 3D strain analysis of deformed clasts suggests oblate strain. X-Ray Powder Diffraction analysis of grains <2 μm indicates anchimetamorphic conditions between 200 and 250°C, with temperatures increasing by 50°C toward the thrust contact. Multi-equilibrium modeling of coarser S1-S2 grains ranges from 300 to 380°C, independent of their position in the shear zone. We attribute the low-temperature range of finer grains to Apennine anchimetamorphism, whereas grains >2 μm are likely detrital and record higher pressure-temperature conditions. Assuming a regional paleogeothermal gradient of 20°C/km, we estimate a maximum burial depth of about 12 km and a pressure of 0.32 GPa. This approach can be applied to similar contexts worldwide, providing a tool for regional tectonic reconstruction and process-oriented studies.