Multiscale structural analysis in the subducted continental crust of the internal Sesia-Lanzo Zone (Monte Mucrone, Western Alps)

Francesco Delleani, M. Iole Spalla, Daniele Castelli, and Guido Gosso

Intrusives of Permian age and their high-grade country rocks, subducted and exhumed in Alpine time, have been studied with petro-structural high resolution mapping. In this subduction environment of the Sesia-Lanzo Zone of the internal Western Alps, focus was oriented on interaction mechanisms between deformation and progress of metamorphic transformations, in an area offering a mosaic of different tectonometamorphic evolutionary steps. Such apparently incoherent distribution of structural imprints, representing various discrete states of the tectonic sequence heterogeneously frozen in adjacent spaces, has been previously recomposed into a coherent progression of deformation events by means of foliation trajectory mapping. This work guaranteed a petrographic analysis of the mineralogical support of sequentially ordered planar fabrics. Six deformation episodes (D1 to D6) were recognised, as superposed folds, foliations and ductile shear zones systems, evidently related to the Alpine tectonic history. Petrologic estimates on time-related microstructures well manifest that eclogite facies metamorphic conditions assisted D1 and D2 deformation stages, the blueschist-facies re-equilibration was contemporaneous with D3 and the greenschist-facies re-equilibration with D4. Recognition of numerous assemblages and textures of pre-Alpine protoliths supported validity of the individuation of the earliest, more cryptic tectonic environment of the widespread eclogitisation that predates D2. The coherent correlation between structural and metamorphic mineral growth histories on a wide area served as a base for further refinement and revealed that full granular scale diffusion of newly-forming parageneses is generally related to attainment of a threshold of intensity in the development of the new planar fabrics, as similarly established in another subducted-exhumed metamorphic complex of the internal Central Alps.