The Pre-Alpine Basement of the Lower Austro-Alpine Nappes in the Bernina Massif (Grisons, Switzerland; Valtellina, Italy)

Abstract

Pre-Alpine basement rocks in the Bernina Massif occur in the cores of large recumbent anticlines which define Eo-Alpine nappe structures. Polyphase Alpine overprinting led to a strong recrystallization of the lower parts of the nappe pile. Basement rocks with a pre-Alpine tectonic and metamorphic overprinting were the country rocks of late Variscan intrusive rocks, which show exclusively Alpine overprints. Namurian intrusives (326 Ma) display a transition from calc-alkaline to alkaline igneous activity. Uplift and erosion led to pre-Permian exhumation of calcalkaline and alkaline intrusive rocks. In the lowermost Permian (285 Ma), rhyolitic extrusive activity was followed by continental clastic sedimentation marking the onset of the Alpine sedimentation cycle. Incipient rifting is indicated by basic igneous activity which preceded the opening of the oceanic Piemonte basin and the formation of a passive continental margin in Jurassic time.