GNGTS 2016 - Atti del 35° Convegno Nazionale

228 GNGTS 2016 S essione 1.2 thermal state: for a cold and strong lithosphere, the thinning is very rapid (4.4 Ma) with respect to a hot and weak lithosphere (15.4 Ma). Similarly, the occurrence of the crustal breakup is shorter for a cold lithosphere (7.4 Ma) than for a hot lithosphere (approximately 31.4 Ma). For both the chosen initial thermal configurations of the lithosphere, the exhumation of the serpentinized mantle starts before the oceanic spreading and the mantle partial melting, making the model compatible with a magma-poor rifting, as suggested for the Alpine case (e.g., Manatschal et al. , 2015). In the hot configuration the continental crust thickness sensibly decreases during the extension from 30 km to approximately 5 km close to the OCTZ. In the cold model instead, the crustal thickness decreases from 30 km to approximately 20 km. The comparison between the natural data and the model predictions shows a good agreement with all of the oceanic data for both hot and cold configurations. Taking into account that a hyperextended system has been proposed for the Alpine Tethys rifting (e.g. Manatschal et al. , 2015) and a time span of approximately 30-40 Ma is considered between the first extensional structures related to the rifting (200 Ma, Mohn et al. , 2012) and the oceanic gabbros emplacement (170-160 Ma, see review in Marotta et al. , 2009, 2016), a rifting developed on thermally perturbed lithosphere better agrees the natural data available in ophiolites. Discussion and conclusion. The comparison between Permian-Triassic to Jurassic natural data from theAlps and the northernApennines and two subsequent numerical models simulating the evolution of the lithosphere from the late collision of the Variscan chain to the Jurassic opening of the Alpine Tethys suggests that: i) a forced extension of the lithosphere results in a thermal state that better agrees the Permian-Triassic high temperature event(s) than a solely Fig. 3 – Schematic geodynamic cartoon illustrating some stages of the proposed transition from late collisional slab breakoff after the Variscan subduction to Jurassic ocean opening. Legend: pink = Variscan continental crust (dark = European; light = Adriatic); orange = lithospheric mantle; light green = asthenosphere; yellow = area of mantle partial melting generating oceanic gabbros; dark green = Permian-Jurassic sediments.