Two subduction events in a polycyclic basement: Alpine and pre-Alpine high-pressure metamorphism in the Suretta nappe, Swiss Eastern Alps

This study is essentially based on coupling macrostructures, microstructures and metamorphic petrology in polymetamorphic mafic rocks from the Swiss Eastern Alps (Suretta nappe, Penninic domain). Petrographic criteria are used in conjunction with structural analysis and microprobe work to define crystallization/deformation relationships and to establish a relative but precise sequence of tectonometamorphic events. A first eclogite facies overprint and related exhumation occurred before emplacement of late Palaeozoic intrusives. During the Alpine cycle, the Suretta nappe was part of the thinned European continental margin. The Tertiary burial due to subduction and collision is responsible for D1 ductile thrusting and blueschist facies metamorphism. Late deformation phases, related to exhumation, are responsible for the development of extensional structures under greenschist facies conditions. Quantitative metamorphic petrology based on Gibbs free energy minimization (DOMINO by de Capitani) gives a constraint on the P-T conditions during the polymetamorphic and polycyclic evolution. The first high-P metamorphic event related to pre-Alpine structures occurred at c. 700 degrees C and at least 2.0 GPa. These conditions are compatible with pre-Alpine high-P re-equilibration already described in several Alpine units. The Alpine high-P metamorphism occurred under blueschist facies conditions at c. 400-450 degrees C and 1.0 GPa. Similar high-P, low-T conditions have already been described in the Mesozoic and Permian rock types. The two high-P events are clearly related to two different geothermal regimes and geodynamic environments.