Tectono-metamorphic evolution of the Briançonnais Units along the southwesternmost sectors of the Alps: Insights from the Marguareis Massif (Western Ligurian Alps)

n the Marguareis Massif, the SW-verging stack is composed of Helminthoid Flysch located in between Briançonnais Units (European continental margin, Vanossi et al., 1986). The latter are represented by the topmost Marguareis Unit and the bottommost Cabanaira Unit both involved in collisional processes into the Alpine wedge starting from middle Eocene age (i.e. Vanossi, 1986). Here, the Briançonnais Unit show a Meso-Cenozoic sedimentary succession and recorded, during middle Eocene-late Eocene, a pre-coupling deformation history dealing with the overprinting of fold systems confined to each unit. These are developed at different structural levels. After the coupling of the units (late Eocene-Early Oligocene) the whole tectonic pile shared the same deformation history represented by flat-lying fold system and subsequent faults. Combining micro-scale structural observations, Chlorite-Phengite multi-equilibrium thermo-barometry and Illite Crystallinity index (IC) on middle Eocene metapelites we reconstruct the tectono-metamorphic history of the Briançonnais Units. Microstructural analysis highlights that only the D1 MU and D2 MU and D1 CU events, in the Marguareis and Cabanaira Units respectively, are characterized by metamorphic re-crystallization. The subsequent deformation, indeed, developed at shallower structural levels. D1 MU event produced S1 MU slaty cleavage marked by preferred orientations of Chl + Phg + Qtz + Alb + K-Fsp + Epd ±Cal whereas the D2 MU event is testified by a S2 MU crenulation cleavage outlined by Phg + Chl + Qtz + Alb + K-Fesp ± Cal. The D1 CU event is represented by a S1 CU slaty cleavage marked by very small grains of Chl + Phg + Qtz + Alb ± Cal. Thermobarometric estimations indicate for the D1 MU-related Chl-Phg couples a Pressure-peak conditions of 1.00-0.90 Gpa and 280-330°C. The retrograde path of the Marguareis Unit is well constrained by the Chl-Phg pairs re-crystallized during the D2 MU event at 0.85-0.7 GPa and 230-300°C. For the Cabanaira Unit the IC was measured on the smallest phyllosilicates along the S1 CU. The latter suggests that the Cabanaira Unit is characterized by an epizonal metamorphic imprint. So, our results show that the prograde path is not recorded in the Marguareis Unit and the Chl-Phg pairs grown along the S1 and S2 foliations depict its retrograde trajectory. The exhumation already started during the D1 and it continues during the D2 up to the shallower structural levels into the Alpine wedge. The Cabanaira Unit seems recorded a different metamorphic history at lower pressure and temperature into the Alpine wedge. So, petrological and structural data indicate that the Marguareis Units recorded HP-LT metamorphic imprint whereas the Cabanaira Unit recorded a epizone-related metamorphic history but both in a continental subduction setting.