Permian-Triassic high thermal regime in the Alps: result of Late Variscan collapse or continental rifting? Validation by numerical modeling.

The Permian-Triassic igneous activity, associated with regional scale deformation developed under high-temperature/low-pressure (HT-LP) metamorphic conditions and widely recorded in the pre-Alpine crust of the European Alps, can result from late orogenic collapse of a collisional belt or from lithospheric thinning leading to a continental rifting process. In order to reduce this ambiguity, we use a two-dimensional finite element model to give new insights on the sequence of mechanisms operating during active ocean-continent convergence, followed by continental collision and pure gravitational evolution and on the regional geodynamic interpretation of the Paleozoic-Mesozoic evolution of the Alpine area. The modeling predictions are compared with the PT climax conditions of Variscan and Permian-Triassic metamorphism affecting the continental crust of the Helvetic to Southalpine domains. The good agreement between model predictions and natural data realized for the early to Neovariscan evolution indicates that during Paleozoic, the pre-Alpine crust of the Alps was part of an active ocean-continent convergence margin and an intracontinental suture zone. Furthermore, modeling results support the interpretation envisaging a Permian-Triassic lithospheric extension as responsible for the HT metamorphism and related intense igneous activity. This evolution was precursor of the Mesozoic oceanization, during which the tectonic units, coupled and accreted during the Variscan subduction and collision, were separated to form the two passive European and Adriatic continental margins.