Lithostratigraphic setting and P-T metamorphic evolution for the Dora Maira Massif along the Piedmont Zone boundary (middle Susa Valley, NW Alps)

A new structural and petrological study, associated with a detailed geological mapping, allowed to better understand the tectonic relationships between the Dora Maira Massif and the Piedmont Zone in the middle Susa Valley. In the study area, a thick sequence of calcschists, previously attributed to the stratigraphic cover of the Dora Maira, is interpreted as belonging to the Piedmont Zone. Four deformation phases were identified: D-1 represents the eclogite facies stage, D-2 developed under greenschist facies conditions, transposed the early foliation and was responsible for the development of the regional schistosity S-2. D-3 is characterized by close to open folds with N dipping axial surfaces and finally, D-4 developed macro-scale folds with E-dipping axial planes. Structural analyses allow to infer a relative timing for the tectonic contact between the two nappes which were coupled after the eclogite facies metamorphism but before the development of the S-2 foliation under greenschist facies conditions. Petrographic investigation into metapelite samples permitted to identify two main metamorphic assemblages within the Dora Maira polymetamorphic basement: M-1 (Phe + Pg + Cld + Grt + ChlI + Qtz + Rt) assemblage defines the S-1 relict foliation, while M-2 (Ms + Pg + Ab + ChlII + Qtz + Ilm +/- A Bt) assemblage is related to the regional foliation S-2. PERPLE_X pseudosection modelling allowed to reconstruct a P-T path for a garnet-chloritoid-bearing micaschist from the northern part of the Dora Maira Massif, which reached eclogite facies conditions at 18-20 kbar and 515-525A degrees C (M-1/S-1 event) and then was exhumed during increasing T (10-11 kbar and 555-565A degrees C). The M-2 assemblage defining the S-2 regional foliation developed at P < 7 kbar and T < 575A degrees C. According to the structural evolution, the tectonic coupling between Dora Maira and Piedmont Zone took place during exhumation along the subduction channel.