Rapid Transition From High-Pressure Metamorphism to Exhumation Driven by Orogen-Parallel Extensional Shearing (Adula Unit, Eastern Central Alps)

In the Central Alps, the Adula unit exposes high- and ultra-high-pressure (HP-UHP) metamorphic rocks, providing key insights into the subduction and exhumation dynamics of the former distal European margin. We investigate the San Bernardino Shear Zone (SBSZ), a top-to-the-east extensional shear zone developed within orthogneiss and paragneiss at the top of the Adula unit, containing eclogite boudins. Using detailed structural analysis, petrography, thermodynamic modeling, and 40Ar/39Ar geochronology, we constrain the Pressure-Temperature-deformation-time (P-T-d-t) path of these high-pressure (HP) rocks. Phengite in eclogites yields minimum age of peak metamorphism of 38–39 Ma, making them the northernmost Alpine HP record in the Adula unit. Eclogite facies conditions reached ∼2.0–2.35 GPa and 580°–655°C. Progressive deformation in the eclogite host rocks led to the overprinting of earlier NW-directed fabrics by top-to-the-east shearing, recorded in synkinematic mica and associated with resetting of 40Ar/39Ar ages to 36–29 Ma. This temporal-spatial pattern indicates a short interval between subduction, nappe stacking, and extensional exhumation. Our results demonstrate that the SBSZ played a key role in the exhumation of the eastern Lepontine Dome, supporting a model of distributed orogen-parallel extension. This mechanism, partially coeval with amphibolite-facies metamorphism and crustal anatexis, suggests that HP units were exhumed during orogen-perpendicular shortening and orogen-parallel extension.