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

Abstract

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 ± Bt) assemblage is related to the regional foliation S 2 . PERPLE_X pseudosection modelling allowed to reconstruct a P-T path for a garnet-chloritoidbearing micaschist from the northern part of the Dora Maira Massif, which reached eclogite facies conditions at 18-20 kbar and 515-525°C (M 1 /S 1 event) and then was exhumed during increasing T (10-11 kbar and 555-565°C). The M 2 assemblage defining the S 2 regional foliation developed at P \ 7 kbar and T \ 575°C. According to the structural evolution, the tectonic coupling between Dora Maira and Piedmont Zone took place during exhumation along the subduction channel.

Review of metamorphic and kinematic data from Internal Crystalline Massifs (Western Alps): PTt paths and exhumation history

“…(3) As concerning the Internal Crystalline Massifs, the mineral chemistry of all the studied samples preserving the HP stage (M 1 assemblages) shows different degrees of re-equilibration and all the samples which are re-equilibrated at LP conditions (M 2 assemblages) always preserve some HP relic, mostly high-celadonitic Phe and Grt. The southern Monte Rosa and the eastern Gran Paradiso reached higher pressure during the D 1 /M 1 eclogite facies tectono metamorphic stage around 24-27 kbar and 550-600 • C (Le Bayon et al, 2006b;Gabudianu Radulescu et al, 2009;Gasco et al, , 2011a while the northern Dora-Maira was subducted to 20-21 kbar at 500-520 • C (Gasco et al, 2011b). The exhumation paths are different too and are characterized by isothermal to cooling decompression in the MR and GP, while the DM registered a decompressional heating.…”

“…7e) within the Combin calcschists, while it developed to a lesser extent in Susa Valley and is almost absent in the Gressoney study area (Gasco et al, , 2011aGasco and Gattiglio, 2011). The ECC in the Susa Valley consist of top to W extensional shear bands, while in the Orco section consist of top to SE extensional shear bands at the eastern border of the GP Massif (Gasco et al, 2009) and of top to NW extensional shear bands at the western border of the GP Massif (Rolland et al, 2000;Bucher et al, 2003).…”

“…Pseudosection modelling of the Alpine metamorphic assemblages of two different chemical systems allowed to reconstruct the Alpine PT paths for the Internal Crystalline Massifs in study areas (Gasco et al, , 2011a. The metamorphic evolution of the basement complex from the Internal Crystalline Massifs is marked by texturally distinct mineral assemblages, defining two main Alpine metamorphic stages (M 1 and M 2 ) developed under different PT conditions (Fig.…”

“…9. Alpine PT path reconstructed for the southern Monte Rosa massif (Gasco et al, 2011b), the eastern Gran Paradiso massif and northern Dora-Maira Massif (Gasco et al, 2011a). The depth is calculated on the base of an average crustal density of 2.95 kg/dm 3 .…”

“…This work has the aim to reconstruct the tectono-metamorphic evolution of the Internal Crystalline Massifs and their relationships with the overlying Piedmont Zone according to detailed geological mapping of three different key-areas reported in Gasco (2010), Gasco et al (2009), for GP, Gasco and Gattiglio (2011) for MR, and Gasco et al (2011a) for DM.…”