Metamorphic Facies Distribution in the Western Alps Predicted by Petrological-Thermomechanical Models of Syn-Convergent Exhumation

The distribution of metamorphic rocks throughout the western European Alps indicates subduction-related metamorphism. However, processes by which high-grade metamorphic rocks exhume remain disputed. Here, we present two-dimensional petrological-thermomechanical numerical models to investigate the metamorphic facies evolution during orogenesis. We model closure of an oceanic basin with exhumed mantle bounded by passive margins. To ensure thermomechanical feasibility, we model also this basin configuration. Before convergence, we replace the uppermost portions of exhumed mantle with serpentinite. Location and orientation of subduction are not predefined and subduction initiates self-consistently during convergence. A weak subduction interface develops if serpentinite is initially thick enough (here 6 km) and can distribute along the interface. Syn-convergent exhumation of (ultra)high-pressure rocks occurs with minor erosion, enabled by local upper-plate extension and a crustal-scale normal-sense shear zone. We calculate metamorphic facies evolutions with peak P and T values of 10,000 markers. Results show (a) peak P and T values agreeing with estimates from natural rocks, (b) exhumed, structurally coherent regions with identical metamorphic facies, indicating absence of significant mixing (melange), (c) facies distributions corresponding to that of the Western Alps, which is from eclogite to blueschist to greenschist facies when going from internal to external domains, and (d) exhumation velocities larger than burial velocities. Models with stronger subduction interface (3-km serpentinite thickness) develop an orogenic wedge with vertical metamorphic gradient and minor exhumation. Syn-convergent exhumation is feasible for the Western Alps and calculating metamorphic facies distribution is useful when testing the applicability of models to natural orogens.

Automated summary

In this study, numerical models were used to investigate the metamorphic facies evolution during orogenesis in the western European Alps. The results showed that syn-convergent exhumation of high-grade metamorphic rocks is feasible, and the calculated metamorphic facies distribution agrees with observations in the Western Alps.