Metamorphism
The unit shows traces of high-grade metamorphism in the form of eclogite and blueschist relicts. Most of it, esecially the external and intermediate zones, is in the greenschist facies though. This greenschist metamorphic grade is seen as a late (Meso-Alpine) overprint, most researchers think the whole Sesia zone or at least part of it has been in eclogite or blueschist conditions during Paleogene subduction. Because clear evidence for high pressure metamorphism is restricted to the internal zone, it is not clear whether the other two zones have also subducted to great depth.
Read more about this topic: Sesia Zone