Soutenance de thèse de Sylvia Duprat-Oualid (Géosciences Rennes)


Le vendredi 12 décembre 2014 à 14:30, en salle de conf de l'OSUR (RDC bât.14B, campus de Beaulieu, Université de Rennes 1), Sylvia Duprat-Oualid soutient sa thèse intitulée

Le vendredi 12 décembre 2014 à 14:30, en salle de conf de l'OSUR (RDC bât.14B, campus de Beaulieu, Université de Rennes 1), Sylvia Duprat-Oualid soutient sa thèse intitulée

Évolution thermique et mécanique des zones de cisaillement : approche analytique et numérique et confrontation aux données de terrain

Jury :
Jean BRAUN - Professeur, Université Joseph Fourier, Grenoble (Rapporteur)
Bernhard GRASEMANN - Professeur, Université de Vienne, Autriche (Rapporteur)
Denis GAPAIS - Directeur de Recherche CNRS, Université de Rennes 1 (Examinateur)
Muriel GERBAULT - Chargée de recherche IRD, GET, Toulouse (Examinatrice)
Loïc LABROUSSE - Professeur, Université Pierre et Marie Curie (UPMC), Paris VI (Examinateur)
Pavel PITRA - Maître de Conférence, Université de Rennes 1 (Directeur)
Philippe YAMATO - Maître de Conférence, Université de Rennes 1 (co-Directeur)

Résumé :
Shear zones are common structural features in the lithosphere and occur at various scales (from microscopic to lithospheric). At the lithospheric scale, they concentrate most of the relative movements between tectonic plates, and therefore, accommodate a high amount of strain. Consequently, the understanding of both their spatial and temporal mechanical behaviour is crucial for the general knowledge of the lithosphe dynamics.
Rheology of rocks, which define their mechanical behaviour, is controlled by physical laws that predict how they deform under some stresses. Temperature plays a major role in the creep-dislocation behaviour, which characterizes the ductile domain (in depth), decreasing efficiently the rock strength. Furthermore, each rock has intrinsic mechanical properties, which depend on its mineralogical composition, texture and internal structures. However, due to the lack of data directly measurable deeper than a few kilometres, the lithosphere rheology, and in particular the continental lithosphere remains subject to drastically different interpretations. The mechanical behaviour of major shear zones is not fully understood, as they are the location of intense changes of both the rock internal nature and major thermal perturbations. Especially, the mechanical energy, converted into heat (shear heating) causes a close interaction between thermal ad mechanical evolutions.
This thesis aims to better understand the rheological state of lithospheric scale shear zones. For this purpose, we used an original approach, based on the temperature field evolution around and within such shear zones. From 2D numerical thermo-kinematic models and analytical developments, the first order variability of thermal evolution and perturbation is anal- ysed and quantified with respect to the impact of three major thermal processes, defined as diffusion, advection and shear heating. Results are compared to metamorphic thermal signatures associated to intra-continental thrust zones for which the influence of both accretion and erosion was also investigated. The case of the Main Central Thrust (MCT) in the Himalayas, whose the inverse metamorphic thermal zonation has been extensively studied, was chosen as the main natural analogue.
Our quantitative results highlight the crucial role of shear heating, and more particularly of mechanical strength variability within shear zones. We thus emphasise on the importance of rock creep parameters. The study of centimetre-scale shear zones, which developed within the granodiorite of the Zillertal nappe (Tauern window, Tyrol, Alps) thanks to little local variations of the mineralogical composition, reveals the extreme sensitivity of igneous rocks rheology, representative of the continental crust. The consequences of such an intense variability, revealed at small scale are finally discussed with regard to rheologies usually considered in models that focus on processes controlling lithosphere dynamics.

Key-words: shear zones; rheology of the lithosphere; shear heating; thermal processes; strain localization; numerical modeling; analytical development.

Contact : Sylvia Duprat-Oualid (Géosciences Rennes)