Structural and petrographic map of the Sassa gabbro complex (Dent Blanche nappe, Austroalpine tectonic system, Western Alps, Italy)

The Sassa gabbro complex outcrops in the upper Valpelline and is part of the Permian gabbros of the Collon – Matterhorn group, set in the Dent Blanche nappe of the Western Austroalpine tectonic system. This 1:2500-scale map was constructed through the synergic use of meso- and microscopic analysis of fabrics to identify the relationships of the superposed foliations with the growth of equilibrium mineral assemblages. Throughout the foliation trajectories, a mosaic of rock volumes, in which strain is differently partitioned, is highlighted on the map for all the successive deformation stages. Therefore, this mosaic shows deformation gradients spanning from unstrained or weakly strained domains, where the primary magmatic features are still well preserved, to domains in which the Alpine fabrics are pervasive and obliterate the previous structures. The pre-Alpine history is characterized by the polyphasic emplacement of the Sassa gabbro and by a coronitic growth of successive mineral assemblages during crustal thinning-related exhumation. The Alpine history is characterized by multistage heterogeneous deformation consisting of a first D1 stage developed in blueschist facies conditions followed by D2 and D3 stages developed in greenschist facies conditions. In summary, by applying this analytical method, the geologic traces of the pre-Alpine lithospheric thinning and Alpine subduction histories have been separated.