Tectono-metamorphic evolution of the Simano-Adula nappe boundary, Central Alps, Switzerland

his paper presents a regional study undertaken in the Penninic zone of the Central Alps in southern Switzerland. The studied region is situated on the boundary of the lower Penninic Simano nappe and Adula nappe. The region is characterized by three major phases of deformation designated D1, D2 and D3, and Post-D3 sequentially. During phase D1, the Adula nappe was thrust northwards onto the Simano nappe. Phase D2 represents the main phase of Alpine metamorphism and deformation under upper amphibolite facies conditions. During this event the nappe boundary was isoclinally folded. The structures due to the deformation phase D2 represent the dominant structures in the entire region. Phase D3 corresponds to the backfolding of the Alpine nappe pile. This deformation phase is only locally preserved in the study region. Post-D3 deformation is a brittle deformation of the Alpine nappes under greenschist facies conditions. P-T calculations were carried out using different calibrations of the GARB (garnet- biotite) thermometer and the GASP (garnet–aluminosilicate–quartz–plagioclase) barometer. The average P-T con- ditions obtained for the main metamorphic event D2 are of about 7 kbar and 625 °C. Metapelites of the Adula nappe complex appear to record high-pressure conditions that they once were subjected to in the Alpine subduction zone during Early Tertiary. The difference of pressure conditions was 2 to 4 kbar relative to the Simano nappe in which metapelites seem to record only the Barrovian-type metamorphism of the Central Alps.