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Metamorphic facies evolution and distribution in the Western Alps predicted by petrological-thermomechanical models
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  • Joshua David Vaughan Hammon,
  • Lorenzo Giuseppe Candioti,
  • Thibault Duretz,
  • Stefan Markus Schmalholz
Joshua David Vaughan Hammon
University of Lausanne

Corresponding Author:joshua.vaughan-hammon@unil.ch

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Lorenzo Giuseppe Candioti
UNIL Lausanne
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Thibault Duretz
Universite de Rennes 1
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Stefan Markus Schmalholz
University of Lausanne
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The evolution and distribution of metamorphic rocks throughout the western European Alps is indicative of subduction-related metamorphism. The present-day distribution of metamorphic rocks in the Western Alps exhibits a regional trend, with an internal high-pressure domain and decreasing grade towards the foreland. However, the processes by which high-grade continental rocks are formed and exhumed, as well as the evolution of the metamorphic architecture remains unclear. Here, we present a two-dimensional petrological-thermomechanical model to investigate the evolution and distribution of metamorphic facies within an orogenic wedge formed by subduction and continental collision. The model simulates an entire geodynamic cycle of extension, with passive margin formation and mantle exhumation, followed by thermal equilibration without applied far-field deformation, convergence, with subduction initiation, basin closure and collision. After thermal equilibration, we consider ad-hoc the serpentinization of the exhumed mantle. Models developing a weak subduction interface, due to 6 km serpentinite thickness, display a laterally varying peak metamorphic facies distribution, with the highest grade rocks within the core of the orogeny, agreeing with distributions in the Western Alps. In contrast, models with a stronger subduction interface (3 km serpentinite thickness) develop an orogenic wedge with a vertical metamorphic gradient. The metamorphic distribution is calculated using the peak P and T values of 10’000 numerical markers during their modelled P-T trajectories. The models indicate, during overall convergence, local extensional tectonics between the exhuming material and overriding plate, whereby the upper-plate hanging-wall is unroofed, moving with a normal sense of shear relative to the exhuming high-pressure rocks.