Petro-structural geology of the Northern Adula Nappe Complex (Central Penninic Alps, Switzerland)

The Adula Nappe Complex is the largest fragment of Penninic continental crust in the Central Alps, where it deformed alongside the Valais Ocean ophiolites and metasediments. This study presents an integrated structural and petrological analysis of its northern tip, where pre-Alpine eclogites occur within phengite-bearing schists. Field investigations identified five Alpine ductile deformation phases in the phengite-bearing schists. D1 developed under prograde conditions, progressing to epidote-amphibolite facies during D2, amphibolite facies during D3, greenschist facies during D4, while D5 involved no new mineral growth. Metabasics record five deformation episodes, with pre-Alpine D1, D2, D2m, and D3a with prograde D1 and D2-D3a developing under eclogite facies followed by Alpine D3b under amphibolite facies. The surrounding Valais ophiolites and metasediments experienced three greenschist-facies deformation phases (D1, D2 and D3). An interpretative geological map at 1:20,000 scale illustrates the sequence of deformation, foliation patterns, and the complex polydeformed, polymetamorphosed lithostratigraphy of the area.