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.
The greenschist metamorphic grade is seen as a late (Meso-Alpine) overlay; most researchers believe that parts of the Sesia zone or at least some of it were in eclogite or blueschist conditions during Paleogene subduction. The uncertainty regarding whether other zones have descended to great depths due to limited evidence of high-pressure metamorphism.