GNGTS 2019 - Atti del 38° Convegno Nazionale

236 GNGTS 2019 S essione 1.4 HOW MANY SUBDUCTIONS IN THE VARISCAN OROGENY? INSIGHTS FROM NUMERICAL MODELS A. Regorda 1 , J.-M. Lardeaux 2 , M. Roda 1 , A.M. Marotta 1 , M.I. Spalla 1 1 Università degli Studi di Milano, Dipartimento di Scienze della Terra ‘A. Desio’, Milano, Italy 2 Université Nice Sophia-Antipolis, UMR Geoazur, Valbonne, France The Variscan belt is the result of the Pangea accretion that most marks the European continental lithosphere from Iberian Peninsula to Poland (von Raumer et al. , 2003; Lardeaux et al. , 2014) and the debate on the number of oceans and subduction systems that have been active during the orogen formation is still open (Faure et al. , 1997; Franke et al. , 2017). Two scenarios concerning the geodynamic evolution of the Variscan orogeny have been proposed: 1. Monocyclic scenario: this type of geodynamic reconstruction assumes a single long- lasting south-dipping subduction of a large oceanic domain. For some authors Armorica remained more or less closed to Gondwana during its northward drift, from Ordovician to Devonian times, in agreement with the lack of biostratigraphic and paleomagnetic data that suggest a short-lived narrow oceanic domain, smaller than 500-1000 km (Matte, 2001; Faure et al. , 2009; Lardeaux, 2014). For the Variscan crust of Alps 2D numerical modelling generated predictions showing a good fitting with the natural petrological and geochronological data from Helvetic, Penninic, Austroalpine and Southalpine continental crust (Marotta and Spalla, 2007; Spalla et al. , 2014; Roda et al. , 2019); 2. Polycyclic scenario: this geodynamic scenario envisages two main oceanic basins opened by the successive northward drifting of two Armorican microcontinent and closed by opposite subductions (Lardeaux, 2014; Lardeaux et al. , 2014; Franke et al. , 2017), as suggested by the occurrence of HP/UHP metamorphism (approximately at 400 and 360 Ma) on both sides of the Variscan belt. The northern oceanic basin is identified as the Saxothuringian ocean, while the southern basin can be identified as the Medio-European (Lardeaux, 2014; Lardeaux et al. , 2014) or Galicia-Moldanubian (Franke et al. , 2017) ocean. The width and the duration of the Medio-European oceanic domain are debated, due to discrepancies between metamorphic and paleo-geographic data. However, the duration of the southern ocean is testified by the records of low temperature (LT) and high to ultrahigh pressure (HP/UHP) metamorphism produced under a low-thermal regime that last at least 30 Myr, which implies the subduction of a significant amount of oceanic lithosphere. For the French Massif Central (FMC) many authors (e.g., Faure et al. , 2009; Lardeaux, 2014) proposed a Silurian north-dipping subduction of Medio- European ocean and the northern margin of Gondwana underneath a magmatic arc developed on continental crust of either the southern margin of Armorica or an unknown and lost microcontinent followed by a late Devonian south-dipping subduction of the Saxothuringian ocean. The evolution inferred from the pre-Alpine basement of the External Crystalline Massifs of the Western Alps has been interpreted as compatible with the one proposed for the FMC (Guillot et al. , 2009). Numerical models characterised by two opposite verging ocean/continent subduction systems at short distance have been developed for the first time and here proposed to verify which scenario better fit with the Variscan P-T evolutions from the French Massif Central and the Alps. The proposed models of two opposite subductions (now on “models DS”) simulate the thermo-mechanical evolution of an ocean/continent/ocean/continent subduction complex during four tectonic phases over a period of 130 Myr (Fig. 1): 1. a first active oceanic subduction (phase 1) that lasts 51.5 Myr (from 425 to 373.5 Ma), until the continental collision, and characterised by three different velocities of plate subduction O1: 1, 2.5 and 5 cm/yr;