2D numerical model of an ocean/continent subduction system: examples from the Variscan crust

Mechanisms that favor the exhumation of subducted crustal material, both continental and oceanic, have been explored by mean of several models and 2D numerical studies. Petrological and numerical models (e.g. Ernst and Liou, 2008; Roda et al., 2010 and refs. therein) reveal that the dehydration process of the oceanic slab, with a consequent hydration of the mantle wedge, have a primary role for developing a convective dynamics in the area between the slab and the upper plate, since the beginning of the subduction. The geodynamics of a convergent ocean/continent margin, evolving from subduction to continental collision, was analyzed by means of a 2D finite element thermo-mechanical model, in which the physics of the crust-mantle system is described by the equations for continuity, conservation of momentum and conservation of energy. A viscous behavior for the whole system is assumed, with both density and viscosity depending on temperature and composition. Different values of convergence velocities, 3, 5 and 8 cm/yr, have been used, as representative of slow, medium and fast subduction systems, respectively. Our analysis is particularly focused on the effects of viscous heating and mantle hydration on the dynamics in the wedge area. The results support that these mechanisms, differently from our reference model without hydration and viscous heating (Marotta and Spalla, 2007), induce the development of short wavelength convective cells in the wedge area, that favor the exhumation of buried crustal material since the early stages of the subduction. Model predictions, in terms of pressure, temperature, lithology and time, will be compared with structural, petrological and age natural data from the European Variscan crust to check and interactively improve 2D numerical models of the explored ocean/continent subduction system

Effects of mantle hydration and viscous heating on the dynamics of mantle wedge in a subduction system: differences and similarities of 2D model predictions with examples from the Variscan crust

Mechanisms that favor the exhumation of subducted crustal material, both continental and oceanic, have been explored by mean of several models and 2D numerical studies. Petrological and numerical models (e.g. Ernst and Liou, 2008; Roda et al., 2010; Regorda et al., 2013 and refs. therein) reveal that the dehydration process of the oceanic slab, with a consequent hydration of the mantle wedge, have a primary role for developing a convective dynamics in the area between the slab and the upper plate, since the beginning of the subduction. The geodynamics of a convergent ocean/continent margin, evolving from subduction to continental collision, was analyzed by means of a 2D finite element thermo-mechanical model, in which the physics of the crust-mantle system is described by the equations for continuity, conservation of momentum and conservation of energy. A viscous behavior for the whole system is assumed, with both density and viscosity depending on temperature and composition. Different values of convergence velocities, 3, 5 and 8 cm/yr, have been used, as representative of slow, medium and fast subduction systems, respectively. Our analysis is particularly focused on the effects of viscous heating and mantle hydration on the dynamics in the wedge area. The results support that these mechanisms, differently from our reference model without hydration and viscous heating (Marotta and Spalla, 2007), induce the development of short wavelength convective cells in the wedge area, that favor the exhumation of buried crustal material since the early stages of the subduction. Model predictions, in terms of pressure, temperature, lithology and time, will be compared with structural, petrological and age natural data from the European Variscan crust to check and interactively improve 2D numerical models of the explored ocean/continent subduction system

Deciphering orogeny: a metamorphic perspective. Examples from European Alpine and Variscan belts. Part I: Alpine metamorphism in the western Alps. A review,

International audienceIn this paper we review and discuss, in a synthetic historical way, the main results obtained on Alpine metamorphism in the western Alps. First, we describe the finite metamorphic architecture of the western Alps and discuss its relationships with subduction and collision processes. Second, we portray the progressive metamorphic evolution through time and space with the presentation of 5 metamorphic maps corresponding to critical orogenic periods, namely 85-65 Ma, 60-50 Ma, 48-40 Ma, 38-33 Ma and 30-20 Ma. We underline the lack of temporal data on high-pressure/low-temperature metamorphic rocks as well as the severe uncertainties on the sizes of rock units that have recorded the same metamorphic history (i.e. coherent P-T-t/deformation trajectories). We discuss the role of subduction-driven metamorphism in ocean-derived protoliths and the conflicting models that account for the diachrony of continental subductions in the western Alps

Tectonic significance of P-T-t paths in metamorphic rocks: examples from Alpine and Variscan orogenic belts

The recognition of several metamorphic re-equilibration steps in a single metamorphic complex has been fruitfully app!ied to mobile or collisional belts for tracing their tectonic history. Case studies from the Alps and other orogenic belts are here reviewed. All these applications of metamorphic studies share a common strategy in selecting the samples to be considered the ones representative of the petrologic and structural history: a mesostructural evolutionary sequence is first determined, that guides the individuation of microstructural sites representing relics or advancing steps of metamorphic re-equilibration. The basic physical principles allowing this policy to be followed are mentioned and some applications of importance to the Alpine case are resumed. Construction of' P-T paths can acquire a geotectonic significance and suggest the geodynamic environment inducing the metamorphism, when established across nappe piles and confronted with current thermal models

From granulites to eclogites in the Sesia zone (Italian Western Alps): a record of the opening and closure of the Piedmont ocean

The Sesia zone (Italian Western Alps) offers one of the best preserved examples of pre-Alpine basement reactivated, under eclogite facies conditions, during the Alpine orogenesis. A detailed mineralogical study of eclogitized acid and basic granulites, and related amphibolites, is presented. In these rare weak to undeformed rocks microstructural investigations allow three main metamorphic stages to be distinguished. The inferred P-T path is consistent with an uplift of continental crust produced by crustal thinning prior to the subduction of the continental rocks. In the light of the available geochronological constraints we propose to relate the pre-Alpine granulite and post-granulite retrograde evolution to the Permo-Jurassic extensional regime. The complex granulite-eclogite transition is thus regarded as a record of the opening and of the closure of the Piedmont ocean. -from Author

Finite Strain Pattern in Andriamena unit (North-Central Madagascar): Evidence for Late Neoproterozoic-Cambrian Thrusting during Continental Convergence

Precambrian Research, v. 123, n. 2-4, p. 135-157, 2003. http://dx.doi.org/10.1016/S0301-9268(03)00065-2International audienc

The Southeast France basin during Late Cretaceous times: The spatiotemporal link between Pyrenean collision and Alpine subduction,

International audienceWe present and discuss the Late Cretaceous evolution of the Southeast France Basin (SEFB) owing to the Pyrenean and Alpine belts. The available geological data (isopachs maps, boreholes and field data) were integrated in 3D GeoModeller software to build a 3D model of the geometry of the Cenomanian to Campanian sedimentary series of the Late Cretaceous period. Maps, 3D block diagrams and cross-sections extracted from the 3D model reveal a significant eastward marine regression during the Late Cretaceous with an average velocity of 0.5 to 1 cm per year. According to the location of the Late Cretaceous depocenters, two sub-basins are recognized in the SEFB and correspond to "en-échelon" synclines filled by syn-buckling sediments. These events are related to the sub-meridian "Pyrenean-Provence" crustal shortening. During Campanian time, the deepening and the tilting of the SEFB are interpreted as a consequence of the subduction of the Alpine Tethys. The Late Cretaceous SEFB is the prolongation on the European foreland of the Alpine subduction trench

Contrasted modes of amphibole development in coronitic metagabbros: a TEM investigation

International audienc