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.

THE THERMO-MECHANICAL EVOLUTION OF THE SUBDUCTION-COLLISION SYSTEMS / A. Regorda ; tutor: A.M. Marotta, M.I. Spalla, J.-M. Lardeaux; coordinatore: E. Erba. DIPARTIMENTO DI SCIENZE DELLA TERRA "ARDITO DESIO", 2017 Apr 05. 29. ciclo, Anno Accademico 2016. [10.13130/regorda-alessandro_phd2017-04-05].

THE THERMO-MECHANICAL EVOLUTION OF THE SUBDUCTION-COLLISION SYSTEMS

A. Regorda
2017

Abstract

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.
5-apr-2017
La finalité de ce travail est de développer un modèle thermomécanique 2D pour analyser en détails les effets de la dissipation visqueuse et de l'hydratation du coin de manteau sur l’état thermique et la dynamique dans les zones de subduction. L’état thermique et la dynamique résultant des modèles prenant en compte la dissipation visqueuse et/ou l'hydratation du manteau sont comparés aux modèles le les prenant pas en compte (Marotta and Spalla, 2007), afin d’analyser leurs effets sur la viscosité et sur la vitesse de déformation. Notre nouveau modèle démontre l’activation de la convection du manteau à courte longueur d’onde en fonction de l'hydratation et de la serpentinisation du coin de manteau. Il en résulte un recyclage des croûtes continentales et océaniques subduites. En outre, les effets de la vitesse de subduction sur l’ampleur de la région hydratée ont été analysés. Les évolutions des conditions P-T des marqueurs de crustaux et l'état thermique enregistré dans les différentes portions du complexe de subduction sont utilisés pour avoir une meilleure compréhension de la distribution et de l'évolution, dans le temps et dans l'espace, de conditions métamorphiques caractérisées par des rapports P/T contrastés. Une fois ces modèles établis, les évolutions P-T prédites par les modèles sont comparées aux données métamorphiques naturelles observées dans la chaine varisque, plus particulièrement dans les Alpes et le Massif Central français. Afin de prendre en compte l’exhumation de croûte subduite jusqu’aux niveaux les plus superficiels, le modèle prends en compte le rôle de l'atmosphère et donc des mécanisme d’érosion et de sédimentation. Cette condition induit une limite supérieure libre qui permet à la croûte subduite d’ arriver à la surface. L'analyse des trajets simulés est compatible avec des scénarios impliquant deux cycles de subduction et de collision dans l'évolution orogénique de la chaîne varisque. L'hypothèse de deux subduction successives est en accord avec les modèles géodynamiques proposés par de nombreux auteurs (e.g., Matte 2001; Guillot et al., 2009; Lardeaux 2014). Sur la base de ces résultats, un modèle caractérisé par deux subductions opposées a été mis en œuvre à fin de mieux rendre compte de l'évolution thermo-mécanique de l'orogenèse varisque.
Settore GEO/10 - Geofisica della Terra Solida
Settore GEO/03 - Geologia Strutturale
Subduction; Thermo-mechanical modelling; Metamorphism; Variscan chain
MAROTTA, ANNA MARIA
ERBA, ELISABETTA
Doctoral Thesis
THE THERMO-MECHANICAL EVOLUTION OF THE SUBDUCTION-COLLISION SYSTEMS / A. Regorda ; tutor: A.M. Marotta, M.I. Spalla, J.-M. Lardeaux; coordinatore: E. Erba. DIPARTIMENTO DI SCIENZE DELLA TERRA "ARDITO DESIO", 2017 Apr 05. 29. ciclo, Anno Accademico 2016. [10.13130/regorda-alessandro_phd2017-04-05].
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/2434/481243
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