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

Abstract

Abstract 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 conditions. The different rocks and metamorphic evolutions and the abundance of serpentinites in the tectonic mixture suggest a possible 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 evolutions with the results of a 2-D numerical model of ocean-continent subduction with mantle wedge serpentinization. The predictions 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 degree of mixing estimated from field data is consistent with that predicted by the numerical simulation. Finally, the present-day location of the RCT, which marks the boundary between the orogenic wedge (Penninic and Austroalpine domains) and the southern hinterland (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 natural data and the model results confirms the interpretation of the RCT as a subduction-related mélange that occurred during exhumation within a serpentinized mantle wedge.