GNGTS 2016 - Atti del 35° Convegno Nazionale

226 GNGTS 2016 S essione 1.2 leading to the Mesozoic continental rifting (e.g. Marotta and Spalla, 2007; Marotta et al. , 2009; Spalla et al. , 2014), whereas on the basis of recent paleogeographic reconstructions it has also been interpreted as engaged by the neo-Variscan late-orogenic collapse (e.g. Spiess et al. , 2010; von Raumer et al. , 2013). In the northern Atlantic region for instance, a sequence of rift basins from Permian to Cretaceous has been described occurring before the opening of the ocean (e.g. Doré and Steward, 2002) making the rifting of the North Atlantic Ocean a long lasting process with several extensional events associated with a migration of eulerian poles as testified by the anticlockwise and successive clockwise rotation of superposed rift axes. Based on this idea, we test whether the lithospheric extension can lead the rifting of the Alpine Tethys by comparing numerical modelling of post-collisional extension and successive rifting and oceanization with Permian-Triassic to Jurassic natural data from the Alps and northern Apennines (Fig. 1). In particular, we focus our attention on the thermal state of the pre-rifting (Permian-Triassic in age) lithosphere in order to explore if the opening of the Alpine Tethys started on a stable continental lithosphere or rather developed on a thermally perturbed one. Results. We here discuss the results obtained for two subsequent numerical models that simulate the evolution of the European lithosphere from the late collision of the Variscan chain to the Jurassic opening of the Alpine Tethys. The first model accounts for the evolution of the crustal lithosphere after the Variscan subduction and collision (300 Ma) up to 220 Ma (Marotta et al. , 2009). The second model accounts for the rifting of the continental lithosphere from 220 Ma up to reach the crustal breakup and the formation of the oceanic crust (Marotta et al. , 2016). For both models different initial geodynamic configurations have been tested and we compare the results with natural data of Permian-Triassic metamorphic rocks and Jurassic gabbros and peridotites (Fig. 1), in order to evaluate which configuration best matches the observations. Natural data belong to different structural Alpine domains. Continental rocks are collected from Helvetic and Penninic domains (European paleomargin) and from Austroalpine and Southalpine domains (Adriatic paleomargin) and oceanic rocks are collected from Alpine and Apennine ophiolites (Fig. 1). The comparison is made in terms of contemporaneous agreement to lithology, pressure and temperature values, and ages. The differences between model predictions and natural P-T-age data are synthesized in Fig. 2, where the ages estimate for the rocks are shown using light grey bars for radiometric ages and dark grey for geologically determined ages. For the first model we compare the results of two different configurations. The first one is characterized by a purely gravitational evolution of the lithosphere in order to simulate a late- orogenic collapse. The second configuration instead, is characterized by a forced extension of the lithosphere of 2 cm/yr. With respect to the purely gravitational simulation, for which the fit between predictions and observations is obtained for few data only (Fig. 2), the forced extension simulation agrees well with all collected natural data (Fig. 2). The most peculiar character of the Permian–Triassic igneous activity is the widespread emplacement of gabbro stocks at the base of the crust and the occurrence of basaltic products in the volcanics. Therefore, we verify whether the P-T conditions predicted for the lithospheric and asthenospheric mantle by different configurations cross the solidus of peridotite. Although predictions from all configurations satisfy the thermal state for mantle partial melting, the latter is attained at 75 km depth for the purely gravitational configuration and at 50 km depth for the simulation with forced extension. Basaltic melt production is thus compatible with all the simulated tectonic settings but, to allow the partial melting of the continental crust, the thermal state must be similar to that suggested by simulation with forced extension. The final thermo-mechanical setting is very different between the two configurations. In the purely gravitational simulation both the crustal thickness and the lithospheric thermal state are similar to the initial conditions, while in the forced extension simulation a strong lithospheric thinning occurs together with a hot thermal state. The second model simulates the extension of the continental lithosphere up to reach the