A new subdivision of the central Sesia Zone (Aosta Valley, Italy)

International audienceThe Sesia Zone in the Western Alps is a continental terrane probably derived from the NW-Adriatic margin andpolydeformed at HP conditions during Alpine convergence.Subdivisions of the Sesia Zone classically have been based on the dominant lithotypes: Eclogitic MicaschistComplex, Seconda Zona Diorito-Kinzigitica, and Gneiss Minuti Complex. However, recent work (Regis et al.,2014) on what was considered a single internal unit has revealed that it comprises two or more tectonic slices thatexperienced substantially different PTDt-evolutions.Therefore, detailed regional petrographic and structural mapping (1:3k to 1:10k) was undertaken and combinedwith extensive sampling for petrochronological analysis. Results allow us to propose a first tectonic scheme forthe Sesia Zone between the Aosta Valley and Val d’Ayas.A set of field criteria was developed and applied, aiming to recognize and delimit the first order tectonic units inthis complex structural and metamorphic context. The approach rests on three criteria used in the field: (1) Discontinuouslyvisible metasedimentary trails (mostly carbonates) considered to be monocyclic (Permo-Mesozoicprotoliths); (2) mappable high-strain zones; and (3) visible differences in the metamorphic imprint. None of thesekey features used are sufficient by themselves, but in combination they allow us to propose a new map that delimitsmain units.We propose an Internal Complex with three eclogitic sheets, each 0.5–3 km thick. Dominant lithotypes includemicaschists associated with mafic rocks and minor orthogneiss. The main foliation is of HP, dipping moderatelyNW. Each of these sheets is bounded by (most likely monometamorphic) sediments,