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

Abstract 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 40 Ar/ 39 Ar 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 40 Ar/ 39 Ar 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.