Map of the strain and reaction-rate partitioning in the Mombarone-Mt. Mucrone area (Sesia Lanzo Zone, Italian Western Alps)

The Mt. Mucrone – Mt. Mars – Mombarone area includes several Permian intrusive bodies and metasediments preserving granulitic to amphibolitic pre-Alpine metamorphism widely re-equilibrated under eclogite-facies conditions during the Cretaceous. They are now part of the Eclogitic Micaschists Complex of the Sesia-Lanzo Zone in the Alpine HP metamorphic belt of the Western Alps. During the field structural analysis, seven groups of ductile structures were recognized as having evolved as follows: the D1 – D3 fabrics took place under eclogite-, D4 under blueschist- and D5 to D7 under greenschist-facies conditions, respectively. Combined structural and petrologic studies generated maps that correlate the degree of fabric evolution and metamorphic transformation at map scale and allow to conclude that, over a critical threshold of fabric reworking, the accomplishment of metamorphic transformations increase proportionally with the degree of fabric evolution. Such map-scale counterpart of outcrop-size deformation zones makes strain-partitioning a critical factor controlling reaction progress.