The Sesia Nappe is a Salassic tectonic unit located between the Canavese Fault to the east and the Upper Penninic units to the west. It is composed of three units of crystalline basement characterized by different lithological associations: the Second Dioritic-Kinzigitic Zone, The Gneiss Minuti Unit and the Eclogitic Micaschist Unit.

Couverture monométamorphique de la nappe de Sesia.

The Second Dioritic-Kinzigitic Zone (2DK) of the Sesia Nappe consists of kilometer-scale slices of micaschists, characterized by well-preserved pre-Alpine amphibolite to granulite-facies mineral assemblages as well as minor amphibolites and marbles, locally overprinted by the early Alpine high-pressure metamorphism.

The Eclogitic Micaschist Unit of the Sesia Nappe comprises mostly paragneisses with minor mafic rocks, orthogneisses and calcitic marbles. Eclogitic mineral assemblages are well preserved, rarely overprinted by the subsequent greenschist-facies metamorphism.

The Gneiss Minuti Unit of the Sesia Nappe consists mainly of orthogneisses, derived from granitoid of probable Permian age and interbedded with more mafic layers. High-pressure metamorphism of this unit was largely obliterated by the later greenschist-facies metamorphism.