Formation and evolution of a subduction-related mélange: The example of the Rocca Canavese Thrust Sheets (Western Alps)

In the Sesia-Lanzo Zone, Western Alps, the Rocca Canavese Thrust Sheets (RCT) subunit is characterized by a mixture of mantle- and crust-derived lithologies, such as metapelites, metagranitoids, metabasics, and serpentinized mantle slices with sizes ranging from meters to hundreds of meters. Structural and metamorphic history suggests that the RCT rocks experienced a complex evolution. In particular, two different peak conditions were obtained for the metabasics, representing different tectono-metamorphic units (TMUs), namely, D1a under eclogite facies conditions and D1b under lawsonite- blueschist-facies conditions. The two TMUs were coupled during the syn-D2 exhumation stage under epidote-blueschist-facies condi- tions. The different rocks and metamorphic evolutions and the abundance of serpenti- nites in the tectonic mixture suggest a possi- ble subduction-related mélange origin for the RCT. To verify whether a subduction-related mélange can record tectono-metamorphic histories similar to that inferred for the RCT, we compare the pressure-temperature evo- lutions with the results of a 2-D numerical model of ocean-continent subduction with mantle wedge serpentinization. The predic- tions of the numerical model fully reproduce the two peak conditions (D1a and D1b) and the successive exhumation history of the two TMUs within the subduction wedge. The de- gree of mixing estimated from field data is consistent with that predicted by the numeri- cal simulation. Finally, the present-day loca- tion of the RCT, which marks the boundary between the orogenic wedge (Penninic and Austroalpine domains) and the southern hin- terland (Southalpine domain) of the Alpine chain, is reproduced by the model at the end of the exhumation in the subduction wedge. Therefore, the comparison between natu- ral data and the model results confirms the interpretation of the RCT as a subduction- related mélange that occurred during exhu- mation within a serpentinized mantle wedge.