High pressure metamorphism in the nappes of the Valle dell'Orco traverse (Western Alps collisional belt) *

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Abstract
A detailed field and petrological study of rocks from nappes cut by the Valle dell'Orco (Italian Western Alps), in particular the Sesia–Lanzo composite unit, has revealed geological and metamorphic histories which started in pre‐alpine times and lasted up to the alpine subduction‐collisional processes. During these processes the nappes sustained an early high P–low T stage and a later low P greenschist facies stage, but followed partly distinctive P–T–time trajectories. This paper discusses the kinematic evolution and the thermal history of the alpine belt from the early subduction/underthrust to the later exhumation stage. The metamorphic crystallization is often governed by incomplete and/or local equilibrium, and the pervasive syn‐metamorphic deformation and the composition of the syn‐metamorphic fluid phase (if present) have exerted an effective local control on reaction kinetics.