3D reconstruction of fabric domains in the eclogitised continental crust of the Mt. Mucrone area, Sesia-Lanzo Zone, Western Alps

In metamorphic basements, 3D geological modelling, based on multiscale correlation of superposed structural and metamorphic imprints, can be an excellent tool for evaluating rock volumes that recorded different fabric evolution during the tectonometamorphic history in lithospheric slices, reworked within convergent systems. During each deformation stage, strain partitioning generates rock volumes recording different degree of fabric evolution: a quantitative 3D representation of domains characterized by homogeneous degree of fabric evolution (DFE) is inferred for the HP-LT metamorphic rocks of Mt. Mucrone, consisting of a continental rock sequence polydeformed during Alpine subduction. Polyphase Alpine heterogeneous deformation allowed the preservation of pre-Alpine magmatic features and HT metamorphic relicts in widely eclogitised metagranitoids and metapelites, respectively. Multiscale structural analysis reveals seven groups of superposed Alpine structures developed under eclogite facies (D1 to D3) and successively under blueschist facies (D4) during subduction, to greenschist facies (D5 to D7) throughout the continental collision. D2 structures are dominant at regional scale and are characterized by isoclinal folds associated with a pervasive foliation. In this contribution, we synthesises multiscale data, defining syn-D2 structural and metamorphic heterogeneities, on maps of fabric evolution and metamorphic reaction progress. The contouring of such domains is based on the estimation of volumes occupied by different degree of grain-scale reorganization during D2 (planar fabric: 0-20%; 21-60%; 61-100%). The individuation of the domains with homogeneous degree of reaction progress (DRP) is determined based on the modal amount of D2 mineral assemblages (0-20%; 21-60%; and 61-100%). These domains are georeferenced and stored in a geo-database set on a geographic information system (GIS). The fabric maps linked to digital elevation model, eight interpretative cross-sections, and orientation data of the structural elements are used to constrain volumes with Geomodeller software. The model allows defining the size, shape, and spatial relationships of rock volumes showing homogeneous DFE and DRP. The results indicate that D2 mylonitic and tectonitic fabric domains covered ca. 85% of the modelled rock volume and their relationships with DRP show that the dominant tectonometamorphic imprint is characterized by the highest DFE. The results also demonstrated that this approach is a powerful tool to unravel the variation in size of rock volumes, homogeneous in structural and metamorphic evolution, which shared a common subduction-collision-exhumation history.