Fig. 1

Oligocene and Neogene structures and metamorphism of the Central and Western Alps, modified after Frey et al. (1999), Steck et al. (2001) and Oberhänsli (2004) showing the location of the structural map of the central Lepontine Alps (Fig. 2) and the cross-section A–A’ (see also Fig. 8). The zones of the Late Cretaceous and Eocene high-pressure metamorphism are not represented. It is suggested that the anchizone-greenschist facies limit, corresponding to a temperature of about 300 °C represents an important rheological boundary in the quartz-rich granitic European crust that controls the position of the frontal NW verging and en echelon Belledonne, Mont Blanc and Aar F2 basement folds in the zone of NW–SE directed Alpine compression between the European and Adriatic plates. The greenschist-amphibolite facies boundary which delimits the Lepontine dome is defined by the reaction: albite An 0–3 + epidote + hornblende = oligoclas An >17 + hornblende (Wenk and Keller 1969; Steck 2008). The Adamello gabbros, diorites and granites (Ad), 32–29 Ma Bergell tonalites and granites (Be), 31–30 Ma Biella pluton (Bi) and Traversella diorite (Tr) are Oligocene mantle-derived intrusions located along the Insubric line, the limit between the European and Adriatic plates (Beccaluva et al. 1983; Reusser 1987; Romer et al. 1996; Berger et al. 2012; Kapferer et al. 2012). The dextral Rhône-Simplon fault continues to the west in the Chamonix zone and after new observations by Daniel Egli (personnel communication) also along the Penninic front to the east of the Mont Blanc massif. (1) Oligocene–Miocene greenschist facies metamorphism, (2) Oligocene–Miocene amphibolite facies metamorphism, (3) Mantle derived dioritic magmatism