L’origine de la sismicité intermédiaire est discutée dans la communauté scientifique depuis des décennies. L’occurrence des séismes sous des conditions élevées en pression et température ne peut pas être expliquée par les modèles classiques qui reposent sur le comportement fragile des matériaux. Les observations géophysiques combinées aux calculs thermodynamiques montrent que dans les zones de subduction, une partie des tremblements de terre du plan Wadati-Benioff supérieur peut être corrélée avec l’éclogitisation des schistes bleus de la croûte océanique. Ces interprétations, basées sur la géophysique et la modélisation thermodynamique, sont supportées par la présence des pseudotachylites dans les roches de haute pression sur le terrain. La motivation de cette thèse est d’établir un lien entre les réactions métamorphiques pendant l’éclogitisation des schistes bleus/granulites et la formation des séismes intermédiaires en utilisant des méthodes expérimentales (D-DIA et Griggs). Les résultats expérimentaux sur tous les échantillons testés démontrent que la densification pendant les réactions d’éclogitisation joue un rôle clé dans l’activation des mécanismes fragiles, mis en lumière par l’enregistrement d’émissions acoustiques pendant les expériences. Pour les expériences réalisées sur les échantillons de schistes bleus, la transition ductile-fragile est associée à la dégradation thermique du glaucophane pendant la réaction lawsonite- éclogite. Ce résultat est en accord avec la relocalisation et la fréquence de la sismicité intermédiaire dans la partie supérieure du plan Wadati-Benioff. Les résultats préliminaires sur la déformation des granulites montrent que les évènements fragiles enregistrés à haute pression et haute température sont associés à la dégradation thermique des plagioclases, qui induit par une importante réduction de volume. Ces résultats sont également en accord avec les observations géophysiques et géologiques. La distribution des émissions acoustiques enregistrées pendant les expériences révèlent des lois de Gutenberg-Richter similaires à celles observées pour les tremblements de terre naturels. Grace aux corrélations entre nos résultats expérimentaux et les connaissances sur les séismes intermédiaires naturels, nous concluons que les mécanismes expliquant la sismicité pendant nos expériences peuvent expliquer la formation des séismes intermédiaires naturels. Nous proposons ainsi que le processus de densification pendant les réactions d’éclogitisation, qui est associé à une réduction de la taille des grains, est un mécanisme d’affaiblissement commun qui peut expliquer la sismicité intermédiaire. / The origin of intermediate-depth seismicity has been debated for decades, because its formation at elevated depths cannot be explained by classic rock fracture and friction theory. Geophysical and field observations demonstrate that intermediate-depth earthquakes occur in hydrous rocks of the subducting oceanic crust as well as in nominally anhydrous rocks of the lower continental crust. n subduction zones the eclogitization of blueschist correlates very well with the relocation of intermediate-depth earthquakes forming the upper Wadati-Benioff plane and in continent-continent collision zones seismicity is recorded in areas of partially eclogitized granulites. The aim of this thesis is to experimentally investigate the potential of metamorphic reactions during eclogitization on the formation of intermediate-depth earthquakes in the oceanic and the continental crust. For the deformation experiments (D-DIA and Griggs) natural samples, were selected to study the eclogitization of blueschist and granulite in the laboratory. Experimental results on all investigated samples demonstrate that densification during eclogite-facies reaction plays a key role to enable faulting accompanied by acoustic emissions in the laboratory. In blueschist samples the breakdown of glaucophane during the transition from lawsonite-blueschist to lawsonite-eclogite acts as trigger for brittle failure. This is in good agreement with the relocation and frequency of natural intermediate-depth earthquakes of the upper Wadati-Benioff-plane. The preliminary results on granulite deformation show that the breakdown of plagioclase, accompanied by a pronounced volume decrease, weakens the sample to enable brittle failure at high-pressure/high-temperature conditions. Also these results match very well with geophysical and field observations. Gutenberg-Richter distribution of the acoustic emissions reveal that the events recorded during deformation follow the same physical law as earthquakes in nature. Together with the good correlation to natural observation we conclude that natural seismic events are caused by the same underlying mechanisms as our small lab-earthquakes. Based on the experimental results on various natural rock samples we propose densification in conjunction with grain-size reduction during eclogitization as a common weakening mechanism for intermediate-depth earthquakes in lower crustal rocks.

Antarctic ice shelves control the ice flux from the continent to the ocean. As such, they play a major role in the stability of the ice sheet and its potential contribution to sea level rise, especially in the context of global change. Below some of these ice shelves, marine ice can be found which is a product of the Deep Thermohaline Circulation. Due to its specific genetic process, marine ice has intrinsic physical (grain size, ice fabric, bubble content, ) and chemical (impurities, water stable isotopes) properties, that differ from those of 'meteoric ice' formed on the continent through snow metamorphism or 'sea ice' resulting from sea water freezing at the ocean-atmosphere surface. Until now however, the effect of these specific properties on marine ice rheology is still very poorly understood.<p><p>The principal objective being to include realistic mechanical parameters for marine ice in ice shelf flow models, uniaxial compression experiments have been performed on various types of marine ice samples. Technical developments are an important component of this thesis has they were necessary to equip the laboratory with the appropriate tools (pneumatic rig, automatic ice fabric data handling).<p><p>Results from experimental compression on isotropic marine ice show that it represents the higher boundary for meteoric ice viscosity throughout the whole temperature range, thereby validating Cuffey and Paterson's relationship with an enhancement factor equals to 1.<p><p>Marine ice is however often quite anisotropic, showing elongated crystals and wide single maximum fabric, that should impact its mechanical properties. Experiments on pre-oriented marine ice samples have therefore been carried out combining the study of epsilon_{oct} vs. tau_{oct} with a thorough analysis of microstructural data 'before' and 'after' the experiment. <p><p>Depending on the orientation of the sample in the applied stress field and on the intensity of the latter, anisotropic marine ice can be harder or softer than its isotropic counterpart, with n=4 often observed in Glen's flow law. Associating the experimental geometrical settings to potential natural equivalent, results suggest that anisotropic marine ice would strengthen ice shelf flow in most areas (for a same given temperature), apart from suturing areas between individual ice streams as they merge to form the ice shelf, where it could become weaker than meteoric ice in certain circumstances.<p><p>Finally, preliminary sensitivity studies, using a simple ice shelf model with our experimental parameters of Glen's flow law have allowed us to discuss the potential impact of rift location, rift size and thermal regime in the ice shelf behavior. / Doctorat en Sciences / info:eu-repo/semantics/nonPublished