THE THERMO-MECHANICAL EVOLUTION OF THE SUBDUCTION-COLLISION SYSTEMS

The aim of this work was to develop a 2D thermo-mechanical model to analyse in detail the effects of the shear heating and mantle wedge hydration on the thermal state and dynamics of an ocean/continent subduction system. The thermal setting and dynamics that result from models with shear heating and/or mantle hydration are directly compared to a model that does not account for either (Marotta and Spalla, 2007) to analyse their effects on both the strain rate and the viscosity. The new model show the activation of short-wavelength mantle convection related to the hydration and the serpentinisation of the mantle wedge, with the consequent recycling of oceanic and continental subducted material. The effects of the subduction velocities on the size of the hydrated area are also analysed, and predictions of the pressure-temperature evolutions of crustal markers and the thermal field, which affect different portions of subduction systems, are used to infer the thermal regimes that affect the models. Similarly, the model can help to understand extensively both the distribution and the evolution, in time and space, of metamorphic conditions characterised by contrasting P/T ratios in subduction systems. In a second phase, P-T predicted by the model has been compared with natural P max -T estimates related to the Variscan metamorphism, from both the present domains of the Alps and from the French Central Massif. However, the model did not allow to compare simulated P-T paths with successive metamorphic stages recorded and preserved by the rocks during their metamorphic evolution, because of the lack of exhumation of subducted material up to the shallowest portion of the crust. Then, the model has been implemented by the introduction of the atmosphere and erosion-sedimentation mechanism, to verify that a free upper boundary could allow the rising of material up to the upper continental crust. The analysis of the simulated paths suggests the possibility to have two cycles of subduction and collision involved in the evolution of the Variscan belt. The hypothesis of two successive subduction systems is in agreement with geodynamics models proposed by many authors (e.g. Matte, 2001; Guillot et al., 2009; Lardeaux, 2014). On these results, a model characterised by two opposite subduction systems has been developed, to verify that it could represent a better evolutionary system for the reconstruction of the Variscan orogeny. Lastly, a comparison between the new model and P-T data of Variscan metamorphism has been performed.