Post-Collisional Evolution in the Southern Variscan Belt: Insights From Numerical Modeling of the Valpelline Series (Western Alps)

The post-collisional evolution of orogenic belts is commonly marked by high-temperature metamorphism and widespread magmatism, yet the processes linking collision to post-collisional dynamics remain difficult to reconstruct due to limited exposure of deep-crustal levels and uncertainties in the determination of pressure-temperature conditions and geochronology. The southern Variscan belt provides a rare opportunity to investigate these processes through the exposure of deep-crustal sections, as shown in the Alps, where early Devonian-Carboniferous Variscan subduction and collision were followed by a Permian post-collisional exhumation. In the Valpelline Series (western Alps), Permian deep-crustal exhumation has been constrained by integrating U-Pb zircon geochronology with pressure-temperature estimates. Building on these constraints, we explore the post-collisional evolution using 2D numerical modeling of a convergent-divergent tectonic system and compare predicted pressure-temperature-time paths with those inferred from the Valpelline Series. The results show a strong agreement between the metamorphic evolution recorded in the rocks and the trajectories predicted by the models during post-collisional stages. The transition from convergence to extension involved the reactivation of the inherited structures, their progressive migration from the lower to the upper plate, and the development of shallow sedimentary basins, consistent with regional Alpine observations. The best fit is obtained in models that invoke a rapid shift from convergence to forced extension at plate margins near the Permo-Carboniferous boundary, rather than a prolonged gravitational collapse of the southern Variscan belt. These findings indicate that early Permian basin formation and deep-crustal exhumation were primarily controlled by plate-boundary forces rather than long-lasting gravitational processes.