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

• Published in: Geochemistry, geophysics, geosystems : G3 Vol. 23; no. 8
• Main Authors: Vaughan‐Hammon, Joshua D., Candioti, Lorenzo G., Duretz, Thibault, Schmalholz, Stefan M.
• Format: Journal Article
• Language: English
• Published: Washington John Wiley & Sons, Inc 01.08.2022; AGU and the Geochemical Society; Wiley
• Subjects: Convergence; Distribution; Earth Sciences; Earth surface; Evolution; Exhumation; Geological processes; Greenschist facies; Laboratory experimentation; Laboratory experiments; Mathematical models; Metamorphic facies; Metamorphic rocks; Metamorphism; Modelling; Mountains; numerical modeling; Numerical models; Ocean basins; Passive margins; Petrography; Physics; Plate convergence; Plate tectonics; Sciences of the Universe; Serpentinite; Shear zone; Subduction; Tectonics; Western Alps; Alps; Western Alps; Western Alps; numerical modeling; metamorphic facies
• ISSN: 1525-2027; 1525-2027
• DOI: 10.1029/2021GC009898

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 (mélange), (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. Plain Language Summary Evidence for deep geological processes (>c. 50 km) can be found in places throughout the Earth's surface where tectonic plates have converged and crustal rocks have been buried and subsequently exhumed back to the surface. Collision zones, such as those found in the western European Alps, provide a record of plate convergence and the associated burial‐exhumation history. The pattern of mineral changes due to pressure and temperature conditions (so‐called metamorphic facies) provides clues for the burial and exhumation history of rocks and, hence, for the geodynamic evolution of mountain belts. In the Western Alps, rocks with the highest metamorphic grade, indicating the most extreme burial and exhumation history, are closest to the observed collision front. This study presents results of two‐dimensional computer simulations, which are based on fundamental laws of physics, laboratory experiments, and natural observations. These simulations predict the evolution of the large‐scale metamorphic facies architecture of the Western Alps during continuous plate convergence. Key Points Local upper‐plate extension and serpentinites, a proxy for a weak subduction interface, enable syn‐convergent exhumation Syn‐convergent exhumation generates a continuous metamorphic facies gradient without tectonic mixing (mélange) Modeled P‐T paths, minor erosion, crustal structure, and metamorphic facies distribution agree with Western Alps