GNGTS 2019 - Atti del 38° Convegno Nazionale

GNGTS 2019 S essione 1.4 237 2. a post-collisional phase (phase 2), which lasts 10 Myr (from 373.5 to 363.5 Ma) and is controlled by sole gravitational forces; 3. a second opposite active oceanic subduction (phase 3) that lasts 26.5 Myr (from 363.5 to 337 Ma), until the second continental collision, with a prescribed velocity of 5 cm/yr of plate O2; 4. a final post-collisional phase (phase 4) that lasts 42 Myr (from 337 to 295 Ma) and, as phase 2, is controlled by sole gravitational forces. The time span covered by the four phases covers the same time span of one cycle model (now on “models SS”) after Regorda et al. (2017), which is characterised by two tectonic phases: 1. an initial oceanic subduction (phase 1) lasting 51.5 Myr (from 425 to 373.5 Ma), with a prescribed velocity of 5 cm/yr; 2. a post-collisional phase (phase 2) lasting 78.5 Myr (from 425 to 295 Ma). P-T conditions inferred from Variscan metamorphic rocks of the Alps and the FMC have been compared with those predicted for different lithospheric markers by the different models of double subductions. For the comparison we used P max -T Pmax estimates because they are the most representative to investigate the interaction between two active oceanic subductions. The comparison with natural data shows a different agreement for rocks from the Alps and from the French Massif Central . Metamorphic conditions recorded by the rocks with high P/T ratios from theAlps show a good agreement with P-T predicted in both hot and cold subductions, being characterised by both different metamorphic gradients and different estimated ages (Fig. 2). Data with high P/T ratios from the Alps show a general improvement in their agreement with phases 3 and 4 of model DS.2.5 with respect to phase 2 of model SS.5, fitting with subducted markers related to the second subduction (Fig. 2). Their precise radiometric-measured ages make them more significant than data from the FMC, suggesting that a polycyclic scenario is more appropriate for the geodynamic reconstruction of the Variscan orogeny. In general, the fitting improvement between predictions of model DS.2.5 and data from the Alps with an estimated age compatible with the beginning of phase 2 of model SS.5 and phases 3 and 4 of model DS.2.5 is related to the activation of the second subduction that produces a lower thermal state, more compatible with data characterised by intermediate-to-high and high P/T ratios. Fig. 1 - Setup, boundary conditions, initial thermal configuration and acronyms of the numerical models. The distances are not to scale. UP stands for upper plate; LP stands for lower plate.