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

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.Les zones de cisaillement constituent des objets structuraux communs de la lithosphère. À grande échelle, elles sont le siège principal des déplacements entre plaques tectoniques, accommodant de grandes quantités de déformation. La compréhension de leur comportement mécanique dans le temps et l'espace est donc essentielle pour la connaissance générale de la dynamique de la lithosphère. La température joue un rôle majeur sur la loi de comportement rhéologique qui caractérise le domaine ductile (en profondeur), réduisant alors efficacement la résistance mécanique. Chaque roche possède en outre des propriétés mécaniques intrinsèques qui varient en fonction de sa composition minéralogique, de sa texture et de sa structure interne. Or, en l'absence de grandeurs directement mesurables en profondeur, la rhéologie de la lithosphère demeure sujette à diverses interprétations. Le comportement mécanique des zones de cisaillement est d'autant plus méconnu qu'elles sont le siège d'intenses changements de la nature des roches et de perturbations thermiques majeures. En particulier, l'énergie mécanique qui y est convertie en chaleur (shear heating) peut engendrer une étroite interrelation entre thermique et mécanique. Ce travail de thèse vise à contribuer à la connaissance générale de la rhéologie des zones de cisaillement lithosphérique. Une approche originale a été mise en place, se basant sur l'évolution thermique aux abords et au sein des zones de cisaillement. Sur la base de modèles numériques thermo-cinématiques 2-D et de développements analytiques, la variabilité de premier ordre de l'évolution et de la perturbation thermique est analysée et quantifiée au regard de l'influence des trois processus thermiques majeurs que sont la diffusion, l'advection et le shear heating. Les résultats sont confrontés aux signatures thermiques métamorphiques associées aux chevauchements intra-continentaux pour lesquels les influences des processus d'accrétion et d'érosion sont également examinées. Le cas du Main Central Thrust (Himalaya), associé à une inversion thermique métamorphique bien développée, est pris comme exemple de référence. Nos résultats quantitatifs mettent en avant le rôle crucial du shear heating, notamment de la variabilité de la résistance mécanique des zones de cisaillement. L'accent est mis sur l'importance des paramètres de fluage des roches. L'étude de zones de cisaillement centimétriques développées au sein de la granodiorite du Zillertal (fenêtre des Tauern, Alpes) à la faveur de faibles variations de la composition minéralogique révèle l'extrême sensibilité de la rhéologie des roches ignées représentatives de la croûte continentale. Les conséquences de cette variabilité intense à petite échelle sont finalement discutées au regard des rhéologies classiquement considérées dans les modèles qui s'intéressent aux processus qui régissent la dynamique de la lithosphère

Developing an inverted Barrovian sequence; insights from monazite petrochronology

In the Himalayan region of Sikkim, the well-developed inverted metamorphic sequence of the Main Central Thrust (MCT) zone is folded, thus exposing several transects through the structure that reached similar metamorphic grades at different times. In-situ LA-ICP-MS U–Th–Pb monazite ages, linked to pressure–temperature conditions via trace-element reaction fingerprints, allow key aspects of the evolution of the thrust zone to be understood for the first time. The ages show that peak metamorphic conditions were reached earliest in the structurally highest part of the inverted metamorphic sequence, in the Greater Himalayan Sequence (GHS) in the hanging wall of the MCT. Monazite in this unit grew over a prolonged period between ~37 and 16 Ma in the southerly leading-edge of the thrust zone and between ~37 and 14.5 Ma in the northern rear-edge of the thrust zone, at peak metamorphic conditions of ~790 ◦C and 10 kbar. Monazite ages in Lesser Himalayan Sequence (LHS) footwall rocks show that identical metamorphic conditions were reached ~4–6 Ma apart along the ~60 km separating samples along the MCT transport direction. Upper LHS footwall rocks reached peak metamorphic conditions of ~655 ◦C and 9 kbar between ~21 and 16 Ma in the more southerly-exposed transect and ~14.5–12 Ma in the northern transect. Similarly, lower LHS footwall rocks reached peak metamorphic conditions of ~580 ◦C and 8.5 kbar at ~16 Ma in the south, and 9–10 Ma in the north. In the southern transect, the timing of partial melting in the GHS hanging wall (~23–19.5 Ma) overlaps with the timing of prograde metamorphism (~21 Ma) in the LHS footwall, confirming that the hanging wall may have provided the heat necessary for the metamorphism of the footwall. Overall, the data provide robust evidence for progressively downwards-penetrating deformation and accretion of original LHS footwall material to the GHS hanging wall over a period of ~5 Ma. These processes appear to have occurred several times during the prolonged ductile evolution of the thrust. The preserved inverted metamorphic sequence therefore documents the formation of sequential ‘paleothrusts’ through time, cutting down from the original locus of MCT movement at the LHS–GHS protolith boundary and forming at successively lower pressure and temperature conditions. The petrochronologic methods applied here constrain a complex temporal and thermal deformation history, and demonstrate that inverted metamorphic sequences can preserve a rich record of the duration of progressive ductile thrusting

Thermal and mechanical evolution of shear zones : analytical and numerical approach, and comparison with the field data

Les zones de cisaillement constituent des objets structuraux communs de la lithosphère. À grande échelle, elles sont le siège principal des déplacements entre plaques tectoniques, accommodant de grandes quantités de déformation. La compréhension de leur comportement mécanique dans le temps et l'espace est donc essentielle pour la connaissance générale de la dynamique de la lithosphère. La température joue un rôle majeur sur la loi de comportement rhéologique qui caractérise le domaine ductile (en profondeur), réduisant alors efficacement la résistance mécanique. Chaque roche possède en outre des propriétés mécaniques intrinsèques qui varient en fonction de sa composition minéralogique, de sa texture et de sa structure interne. Or, en l'absence de grandeurs directement mesurables en profondeur, la rhéologie de la lithosphère demeure sujette à diverses interprétations. Le comportement mécanique des zones de cisaillement est d'autant plus méconnu qu'elles sont le siège d'intenses changements de la nature des roches et de perturbations thermiques majeures. En particulier, l'énergie mécanique qui y est convertie en chaleur (shear heating) peut engendrer une étroite interrelation entre thermique et mécanique. Ce travail de thèse vise à contribuer à la connaissance générale de la rhéologie des zones de cisaillement lithosphérique. Une approche originale a été mise en place, se basant sur l'évolution thermique aux abords et au sein des zones de cisaillement. Sur la base de modèles numériques thermo-cinématiques 2-D et de développements analytiques, la variabilité de premier ordre de l'évolution et de la perturbation thermique est analysée et quantifiée au regard de l'influence des trois processus thermiques majeurs que sont la diffusion, l'advection et le shear heating. Les résultats sont confrontés aux signatures thermiques métamorphiques associées aux chevauchements intra-continentaux pour lesquels les influences des processus d'accrétion et d'érosion sont également examinées. Le cas du Main Central Thrust (Himalaya), associé à une inversion thermique métamorphique bien développée, est pris comme exemple de référence. Nos résultats quantitatifs mettent en avant le rôle crucial du shear heating, notamment de la variabilité de la résistance mécanique des zones de cisaillement. L'accent est mis sur l'importance des paramètres de fluage des roches. L'étude de zones de cisaillement centimétriques développées au sein de la granodiorite du Zillertal (fenêtre des Tauern, Alpes) à la faveur de faibles variations de la composition minéralogique révèle l'extrême sensibilité de la rhéologie des roches ignées représentatives de la croûte continentale. Les conséquences de cette variabilité intense à petite échelle sont finalement discutées au regard des rhéologies classiquement considérées dans les modèles qui s'intéressent aux processus qui régissent la dynamique de la lithosphère.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

50 ka of vegetation and climate history in Western Europe : pollen study and multi-proxy approach on the Bergsee lacustrine record (Black Forest, Germany)

Le Bergsee offre une séquence sédimentaire terrestre et continue sur l’histoire environnementale de l’Europe de l’Ouest entre 50 ka BP et aujourd’hui.Un enregistrement pollinique continu et séculaire permet la construction de l’évolution de la végétation et du climat sur la dernière période glaciaire.Outre l’opposition entre le Stade Isotopique Marin 3 et 2 (le second étant plus steppique), la succession de stades et d’interstades courts nord-atlantiques est reflétée par l’alternance de steppes (i.e. climat froid/sec) et de courts épisodes forestiers (i.e. réchauffements). Des phases glaciaires plus prononcées attestent de Stades de Heinrich en Europe de l’Ouest.Ces résultats sont validés par : 1) la confrontation avec des études européennes et 2) l’approche multi-proxy (chironomes, alkanes, géochimie) appliquée à des épisodes clés.Une comparaison avec les données archéologiques montre finalement le potentiel de contribution du contexte climato-environnemental du Bergsee à la compréhension des changements sociétaux du Paléolithique Supérieur.Bergsee Lake provides a terrestrial and continuous sediment record of environmental changes in Western Europe for the last 50 ka.A continuous pollen record established at secular resolution allow to reconstruct the vegetation and climate variability of the Last Glacial period.Contrasted climate/vegetation is recorded between Marine Isotope Stages 3 and 2 (more steppic for the second one), and the north-Atlantic stadial/interstadial succession is also reflected by alternating steppe (i.e. cold/dry climate) and short forested episodes (i.e. warming). Moreover, Heinrich Stadials are evidenced as pronounced glacial phases by the Bergsee record.These results are supported by 1) comparison with other European records and 2) the multi-proxy approach (chironomids, alkanes, sedimentary data) applied on key climatic periods.Finally, comparison with archaeological data highlights the great potential contribution of the Bergsee record to the understanding of society changes of the Late Palaeolithic

50 000 ans d'histoire de la végétation et du climat en Europe occidentale : étude pollinique et approche multi-proxy sur la séquence sédimentaire du Bergsee (Forêt Noire, Allemagne)

Bergsee Lake provides a terrestrial and continuous sediment record of environmental changes in Western Europe for the last 50 ka.A continuous pollen record established at secular resolution allow to reconstruct the vegetation and climate variability of the Last Glacial period.Contrasted climate/vegetation is recorded between Marine Isotope Stages 3 and 2 (more steppic for the second one), and the north-Atlantic stadial/interstadial succession is also reflected by alternating steppe (i.e. cold/dry climate) and short forested episodes (i.e. warming). Moreover, Heinrich Stadials are evidenced as pronounced glacial phases by the Bergsee record.These results are supported by 1) comparison with other European records and 2) the multi-proxy approach (chironomids, alkanes, sedimentary data) applied on key climatic periods.Finally, comparison with archaeological data highlights the great potential contribution of the Bergsee record to the understanding of society changes of the Late Palaeolithic.Le Bergsee offre une séquence sédimentaire terrestre et continue sur l’histoire environnementale de l’Europe de l’Ouest entre 50 ka BP et aujourd’hui.Un enregistrement pollinique continu et séculaire permet la construction de l’évolution de la végétation et du climat sur la dernière période glaciaire.Outre l’opposition entre le Stade Isotopique Marin 3 et 2 (le second étant plus steppique), la succession de stades et d’interstades courts nord-atlantiques est reflétée par l’alternance de steppes (i.e. climat froid/sec) et de courts épisodes forestiers (i.e. réchauffements). Des phases glaciaires plus prononcées attestent de Stades de Heinrich en Europe de l’Ouest.Ces résultats sont validés par : 1) la confrontation avec des études européennes et 2) l’approche multi-proxy (chironomes, alkanes, géochimie) appliquée à des épisodes clés.Une comparaison avec les données archéologiques montre finalement le potentiel de contribution du contexte climato-environnemental du Bergsee à la compréhension des changements sociétaux du Paléolithique Supérieur

Syn-deformational inverted metamorphism: Insights from 2D Thermo-kinematic numerical models

International audienceInverted metamorphism is characterized by the stacking of high-grade metamorphic unit structurally above unit presenting a lower metamorphic grade. Some main thrusts in major orogens are characterized by such an inverted metamorphism. Several cases have been already described (e.g. Variscan belt, Himalayas, Caledonian belt, Canadian cordillera), but the most popular and the most studied corresponds to the case associated to Main Central Thrust (MCT) in the Himalayan belt. The models that propose to explain how inverted metamorphism can occur are multiple, but they are still debated. Moreover, another issue related to the preservation of a thermal inversion is still not well solved. In this study, we propose to address the problem by using a 2D-thermo-kinematical model. The velocity field (including isostasy compensation) is imposed in the whole model in order to simulate a crustal scale thrust. At each time step, we solve the heat diffusion equation on the grid. The temperatures are then advected by markers following the velocity field. We voluntary decided to simplify the model in order to control each parameter and test their influence on the possible inversion of the geotherms. We thus realized a parametric study in order to quantify the impact of the initial conditions (thrust angle and convergence velocity) and the thermal properties of the rocks on the thermal evolution around a major compressive shear zone. Our results show that heat capacity, density and heat production of rocks have negligible effect on the geotherms inversion. However initial conditions, thermal conductivity and shear heating seem to be the most important parameters controlling the inverted metamorphism. Since the shear heating depends on the convergence velocity and the viscosity of the material in the thrust, we provide here the possible range of values of velocity and viscosity allowing the inversion and the preservation of inverted geotherm during time

On the meaning of peak temperature profiles in inverted metamorphic sequences

International audienceInverted metamorphic sequences (IMS) are common features of main thrust systems on Earth. They exhibit an upwards continuous increase in peak temperature conditions and thereby constitute evidence of the close relationship between the thermal field evolution and tectonic processes. Heat advection and shear heating are known to allow the formation of such metamorphic signatures. Heat diffusion also plays an important role in temperature distribution on both sides of the thrust. Other advection processes such as erosion or accretion may also cause a local peak temperature inversion. Each one of these processes therefore affects the thermal field around the thrust. However, despite the crucial importance of all these processes for the interpretation of the inverted peak temperature signatures, their respective influences have never been quantified and compared all together. To address this issue, we propose an innovative coupled approach. (i) We use two-dimensional numerical models that simulate various thrust systems, allowing for a wide diversity of setups. To illustrate this study, we focus on intracontinental thrust systems for which all processes listed are likely to play a key role in the thermal evolution. We perform a parametric study including kinematic settings (i.e. convergence, erosion and accretion), thermal properties, mechanical strength and heat sources. (ii) Dimensionless numbers based on parameters are used to quantify the relative contributions of each process to the thermal budget evolution. Hence, the three thermal processes (i.e. heat diffusion, heat advection and shear heating) are compared with each other via three dimensionless combinations of the Peclet and Brinkman numbers: RDif, RAdv and RPro, respectively. Erosion and accretion are compared separately, based on a fourth dimensionless number Rea. (iii) We analytically examine the inverted peak temperature recorded along profiles that are perpendicular to the thrust zone defined in our numerical experiments. Each peak temperature profile presenting an inversion can then be characterized by a function of approximation involving six meaningful parameters: the location μFF and width σFF of the maximum peak temperature inversion, the characteristic peak temperature Tcte and gradient GLB beneath the inversion zone, and the inversion-related contrasts in the peak temperature ΔT and gradient ΔG. This coupled approach, linking numerical modelling and analytical treatment, allows to quantitatively interpret IMS in terms of the processes involved. The application of our method to intracontinental thrust systems demonstrates that shear heating and erosion support significant inversions, but that the relative contributions of each process have meaningful consequences. Our results reveal that competition between shear heating and heat diffusion on the one hand, and between erosion and accretion on the other hand have a high impact. In particular, the variability in the rock's mechanical strength strongly influences the features of peak temperature inversions. Consequently, none of these processes can be ignored. Our results highlight the major importance of the rheology of rocks in the thermal evolution of shear zones. Finally, our methodology is not only restricted to the analysis of numerical data but also constitutes a way of broad interest to analyse peak temperature signatures around any shear zone

What we can learn from peak temperatures profiles in inverted metamorphic sequences

International audienceInverted metamorphic sequences correspond to the stacking of structural units through which the metamorphicpeak temperatures progressively increase upwards. Such thermal profiles, already studied for years, are characteristicof lithospheric-scale thrust zones. Nevertheless, the processes allowing their formation still remaincontentious. Several processes can, indeed, lead to peak temperatures inversion: heat advection, shear heating, indepthaccretion and/or erosion (allowing the exhumation of the overthrusting block). Furthermore, heat diffusionalso has an important effect on temperatures distribution on both sides of the thrust. Each one of these processesdistinctly impacts on the metamorphic thermal field in the vicinity of the thrust zone. However, their respectiveinfluences were never clearly analyzed and compared despite their crucial importance for the interpretation of theinverted peak temperatures signatures.Here, we thus propose to address this shortcoming by using two-dimensional numerical models simulatingintra-continental thrusts systems. To do so, we combine a parametric numerical study and the “analytical characterization”of the computed inverted peak temperatures recorded, in our models, along profiles perpendicularto the thrust zone. The parametric combinations, including kinematic setting (i.e. convergence, erosion andaccretion), thermal properties, mechanical strength and heat sources, control the processes into play during thethrust activity whose relative importances can be quantified. When the resulting peak temperatures profiles presenta noticeable inversion, they are converted into a function of approximation characterized by six parameters. Thesesix outputs then constitute the keys to quantitatively decipher the inversions features, not only in terms of spatialextent and intensity over time, but also by characterizing the peak temperatures trends on either sides of thedomain of inversion. This numerical and analytical coupled approach then allows to give the significance of peaktemperatures profiles in relation with the different processes into play.Our results allow to quantify the influence of each process (i.e. heat diffusion, heat advection, shear heating,erosion and accretion) on the different features of inverted peak temperatures signatures. They show that noneof them can be considered alone. Finally, the function of approximation used in this study, allowing to efficientlyfit a discrete dataset to a continuous signal, can also be applied to natural peak temperatures estimations followingthe same way. We thus propose to illustrate this on the example of the inverted metamorphic sequence associatedto the Main Central Thrust zone in the Himalayas

Inverted metamorphism and feedback between temperature and non-Newtonian viscosity in compressive shear zones : A 2D thermo-kinematic study.

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Time to reconcile thermal inversion models around thrusts

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