The transition from Variscan collision to continental break-up in the Alps: advice from the comparison between natural data and numerical model predictions

Records of Variscan structural and metamorphic imprints in the Alps indicate that before Pangaea fragmentation, the continental lithosphere was thermally and mechanically per- turbed during Variscan subduction and collision. A diffuse igneous activity associated with high-temperature (HT) metamorphism, accounting for a Permian–Triassic high thermal regime, is peculiar to the Alpine area and has been interpreted as induced either by late-orogenic collapse or by lithospheric extension and thinning leading to continental rifting. Intra-continental basins hosting the Permian volcanic products have been interpreted as developed either in a late- collisional strike-slip or in a continental rifting setting. Two-dimensional finite element models have been used to shed light on the transition between late Variscan orogenic evolution and litho- spheric thinning that, since Permian – Triassic time, announced the Tethys opening. Comparison of model predictions with a broad set of natural metamorphic, structural, sedimentary and igneous data suggests that the late collisional gravitational evolution does not provide a thermo-mechanical outline able to justify mantle partial melting, accounted by emplacement of huge gabbro bodies and regional-scale high-temperature metamorphism during Permian–Triassic time. An active exten- sion is required to obtain model predictions comparable with natural data inferred from the volumes of the Alpine basement poorly reactivated during Mesozoic–Tertiary convergence.