Tectono-metamorphic evolution of the Briançonnais Units along the southwestern edge of the Alps: Constraints from the Marguareis Massif (Western Ligurian Alps)

The Briançonnais Units are regarded as the witnesses of the thinned European continental crust progressively involved in collisional processes starting from middle Eocene age (i.e. Vanossi, 1986; Michard et al., 2004). In the southwestern Alps a SW-verging stack composed of Helminthoid Flysch located in between Briançonnais Units (Sanità et al., 2020) crops out along the southwest sector of the Marguareis Massif (Western Ligurian Alps). The topmost Marguareis Unit (Briançonnais Domain) shows a Meso-Cenozoic sedimentary succession and recorded a deformation history dealing with the overprinting of fold systems (D1-D4 events), the latter developed at different structural levels, and later faults (Sanità et al., 2020). Combining micro-scale structural observations and chlorite-phengite multiequilibrium thermobarometry on middle Eocene metapelites we reconstruct the Pressure-Temperature-time-deformation path of the Marguareis Unit. Microstructural analysis highlights that only the D1 and D2 events are characterized by metamorphic re-crystallization. D1 event is represented by S1 slaty cleavage marked by preferred orientations of Chl + Phg + Qtz + Alb + K-Fsp + Epd ±Cal whereas the D2 event is testified by a crenulation cleavage outlined by Phg + Chl + Qtz + Alb + K-Fesp ± Cal. D3 and D4 events are associated with stylolitic surfaces without metamorphic re-crystallization suggesting their development at shallower crustal levels. We applied the Chl-Qz-wt, Phg-Qz-wt and Chl-Phg-Qz-wt methods on D1-and D2-related Chl-Phg pairs. The thermobarometric estimations show that the D1-related Chl-Phg couples registered 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 event at 0.85-0.7 GPa and 230-300°C. 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. Thus, the reconstructed P-T path strongly indicates that the the Marguareis Unit recorded HP-LT metamorphic conditions in a continental subduction setting. The latter was already documented in others sectors of the Alpine belt.