Explanatory notes to the map : metamorphic structure of the Alps transition from the Western to the Central Alps

The northern-western Alps, located between two major tectonic structures, the Simplon and the Aosta-Ranzolla faults, represent a "transition” zone where all paleogeographic domains involved within the alpine orogenic wedge are present and clearly distinguishable on a map (e.g. BIGI et al., 1990; SCHMID et al., 2004). The structural style and metamorphic record in the area linking the West and South-West Lepontine to the Western Alps has particular characteristics, which warrant this separate chapter. This concerns the Lepontine zone from just East of Valle d’Ossola to the western limit set by the continental Bernhard nappe system in the North-West, the ocean- derived Piedmont-Ligurian zone and its prolongation to the west (Préalps), as well continental units issued either from the Adriatic continent domain (Sesia and Dent-Blanche massifs) or from the European margin (Mt Blanc massif). All units will be described after that from east to west following Figure 1, while structural relationship between different units is described in details in SCHMID et al. (2004).