41 research outputs found
Devenir d'une lithosphère en subduction dans le manteau terrestre : Etude de sa déformation et des flux mantelliques associés
No full textIn this thesis, I provide a tighter constraint on the range of possible viscosity ratio between the slab and the surrounding mantle that mainly controls slab deformation and mantle flow associated. To quantify the viscosity contrast between the slab and surrounding mantle, I used the rheological dependence of the geometrical response of a viscous subducting slab, subjected to mantle flow induced by slab motion. By combining results from analogue experiments, semi-analytic solutions and 3D numerical methods, I was able to quantify the fate of a subducting slab in the mantle. The comparison of these model results, with geophysical data (mainly seismic tomography and earthquake distributions on a selection of subduction zones) indicated that the viscosity contrast between the slab and upper mantle is small and should not exceed 100. According to this weak slab prediction, the geometry of a subducting slab was observed to ultimately evolve into a "jellyfish" like shape. The weak slab prediction also provides a new explanation of the seismic anomalies observed in the deep mantle. Additionally, in order to support the observed slab penetration into the lower mantle, combined with the assumption of a weak slab, it was also possible to constrain the density contrast and the viscosity structure of the mantle, in particular, the ratio of upper to lower mantle viscosity.L'approche scientifique a consisté en la combinaison de trois différentes méthodes de modélisation (analogique, semi analytique et numérique) afin de contraindre la rhéologie qui régit la déformation de la lithosphère et les flux mantelliques associés au niveau des zones de subduction. Par l'étroite confrontation des prédictions (modèles) et les observations disponibles (données sismologiques pour l'essentiel), j'apporte de nouvelles contraintes sur le devenir d'une lithosphère en subduction dans le manteau, couplées avec une précision sur la structuration du manteau. Les résultats des modèles au regard des données géophysiques prédisent une lithosphère faiblement visqueuse (le rapport de viscosité entre la lithosphère et le manteau ne saurait excéder 100) qui atteint lors de sa descente dans le manteau la forme spécifique d'une méduse. Ce résultat majeur apporte une nouvelle interprétation sur l'origine des épanchements de lithosphère dans le manteau profond et une précision sur la stratification entre le manteau supérieur et le manteau inférieur faible pour permettre ainsi la pénétration de la lithosphère dans le manteau inférieur. Cette étude apporte de nouvelles données sur la compréhension de l'interaction entre le flux mantellique et la lithosphère
Distribution of porphyry copper deposits along the western Tethyan and Andean subduction zones: Insights from a paleotectonic approach
No full textInternational audienceAlong the western Tethyan and Andean subduction zones the distribution of Cretaceous and Cenozoic porphyry Cu deposits is not random and shows that they were emplaced in distinct regional clusters. To understand the appearance of these clusters within their geodynamical contexts and identify kinematic features which would favor the genesis of porphyry-type ore bodies, we use a paleotectonic approach. Two clusters in the Aegean-Balkan-Carpathian area, which were emplaced in upper Cretaceous and Oligo-Miocene, and two others in the Andes, which were emplaced in late Eocene and Miocene, are sufficiently well constrained to be studied in detail. It appears that they are associated with a specific polyphased kinematic context related to the convergence of tectonic plates. This context is characterized by: 1) a relatively fast convergence rate shortly followed by 2) a drastic decrease of this rate. From these observations, and assuming that the major part of plate convergence is accommodated along subduction zones, we propose a two-phase geodynamic model favoring emplacement of porphyry Cu deposits: 1) a high melt production in the mantle wedge, followed by 2) an extensional regime (or at least relaxation of the compressional stress) in the upper plate, promoting ascension of fertile magmas to the upper crust. Melt production at depth and the following extensional regime, which would be related to variations in convergence rate, are thus associated with variations in plate and trench velocities, themselves being controlled by both plate kinematics at the surface and slab dynamics in the upper mantle. In particular, along-strike folding behavior of the subducting slab may strongly influence trench velocity changes and the location of porphyry Cu deposits. Metallogenic data suggest that periods of slab retreat, which would favor mineralization processes during ~ 40 Myrs, would be separated by barren periods lasting ~ 10 to 20 Myrs, corresponding to shorter episodes of trench advance, as observed in laboratory experiments. These results confirm the control of the geodynamic context, and especially subduction dynamics, on the genesis of porphyry Cu deposits. This study also shows that the paleotectonic approach is a promising tool that could help identify geodynamic and tectonic criteria favoring the genesis of various ore deposits
From longitudinal slab curvature to slab rheology
No full textInternational audienceThe curvature of a subducting lithosphere is chiefly controlled by the viscosity ratio between the slab and the surrounding mantle. On the basis of a semi-analytical flow model, we explore the rheological dependence of the geometrical response of a viscous slab subjected to toroidal mantle flow. Mantle flow is excited by slab retreat at a prescribed mean velocity and is iteratively solved for by using a stream function approach, in turn providing the stresses that bend the slab. Comparison between model predictions and geophysical observations of slab curvature gives an average slab-to-mantle viscosity ratio of 45
Devenir d'une lithosphère en subduction dans le manteau terrestre (étude de sa déformation et des flux mantelliques associés)
No full textDans une zone de subduction, différents acteurs principaux sont mis en jeu : la lithosphère en subduction, les lithosphères chevauchante et adjacentes et le manteau environnant. Leur interaction provoque une dynamique aux niveaux des zones de subduction qui se caractérise principalement par la sismicité, le volcanisme arrière arc, la migration du slab, la déformation de la plaque supérieure et enfin des flux mantelliques. Plus particulièrement, ces phénomènes soulignent l'existence d'une dynamique profonde associée à l'interaction entre la lithosphère en subduction et le manteau environnant. Les différentes étapes de l évolution d'une lithosphère en subduction dans le manteau terrestre correspondent à des événements géologiques qui seront enregistrés et potentiellement préservés. De plus, les données sismologiques (tomographie sismique, anisotropie sismique), provenant des zones de subduction, nous renseignent sur la diversité des géométries des panneaux plongeants en profondeur et des flux mantelliques. Cependant, la géologie de surface et la tomographie sismique ne nous permettent pas de comprendre l évolution temporelle du comportement de la lithosphère dans le manteau et la dynamique associée. De plus, malgré un effort permanent sur la compréhension de la dynamique des zones de subduction, la viscosité de la lithosphère est encore mal connue: à présent, le rapport de viscosité entre la lithosphère en subduction et le manteau avoisinant utilisé atteint des gammes très larges, allant de 1 à l'infini, ce qui est insatisfaisant et ce qui laisse une grande liberté d'interprétation sur la dynamique interne de la Terre. Les travaux que j'ai conduits sur cette problématique durant la thèse permettent désormais de remédier à cette lacune. Mon approche scientifique a consisté en la combinaison de trois différentes méthodes de modélisation (analogique, semi analytique et numérique) afin de contraindre la rhéologie qui régit la déformation de la lithosphère et les flux mantelliques associés. Par l'étroite confrontation des prédictions (modèles) et les observations disponibles (données sismologiques pour l'essentiel), j'apporte de nouvelles contraintes sur le devenir d'une lithosphère en subduction dans le manteau, couplées avec une précision sur la structuration du manteau. Les résultats des modèles au regard des données géophysiques prédisent une lithosphère faiblement visqueuse (le rapport de viscosité entre la lithosphère et le manteau ne saurait excéder 100) qui atteint lors de sa descente dans le manteau la forme spécifique d'une méduse. Ce résultat majeur apporte une nouvelle interprétation sur l'origine des épanchements de lithosphère dans le manteau profond et une précision sur la stratification entre le manteau supérieur et le manteau inférieur faible pour permettre ainsi la pénétration de la lithosphère dans le manteau inférieur. Cette étude apporte de nouvelles données sur la compréhension de l interaction entre le flux mantellique et la lithosphère.A subduction zone is composed of several components: the subducting lithosphere, the surrounding mantle and the overriding and adjacent lithospheres. The interaction of theses components is responsible for the complex dynamics of subduction zones which are characterised by seismicity, back-arc volcanism, trench retreat, deformation of the overriding plate and the flow within the surrounding mantle. Such surface phenomena highlight deep dynamics associated with the interaction between the subducting lithosphere and the surrounding mantle. The various phases in the evolution of a subduction system correspond to geological events that are recorded and, to some degree, preserved in the geological record. Seismological data (seismic tomography, seismic anisotropy) at different subduction zones show a large variety of slab geometries and mantle ow around convergent plate boundaries. Whilst seismic tomography does not allow us to understand the temporal evolution of the slab behaviour, surface geology does provide some insight into the time evolution, even though it carries little information regarding the dynamics of the slab and the surrounding mantle during subduction. Despite the vast amount of research conducted by the geodynamics community to understand the dynamics of subduction zones, the viscosity of the subducting lithosphere is not well constrained. At present, the viscosity ratio between the slab and surrounding mantle used in models of subduction zones ranges from one to infinity. Clearly this is unsatisfactory and such a wide range of viscosity contrasts leaves the dynamics of the Earth open to numerous interpretations. In this thesis, we aim to rectify this unresolved issue by providing a tighter constraint on the range of possible viscosity contrasts between the slab and the surrounding mantle. To quantify the viscosity contrast between the slab and surrounding mantle, I used the rheological dependence of the geometrical response of a viscous subducting slab, subjected to mantle ow induced by slab motion. By combining results from analogue experiments, semi-analytic solutions and 3D numerical methods, I was able to quantify the fate of a subducting slab in the mantle. The comparison of these model results, with geophysical data (mainly seismic tomography and earthquake distributions on a selection of subduction zones) indicated that the viscosity contrast between the slab and upper mantle is small and should not exceed 100. According to this weak slab prediction, the geometry of a subducting slab was observed to ultimately evolve into a jellyfish" like shape. The weak slab prediction also provides a new explanation of the seismic anomalies observed in the deep mantle. Additionally, in order to support the observed slab penetration into the lower mantle, combined with the assumption of a weak slab, it was also possible to constrain the density contrast and the viscosity structure of the mantle, in particular, the ratio of upper to lower mantle viscosity.RENNES1-BU Sciences Philo (352382102) / SudocSudocFranceF
Transformer les savoirs tacites des geosciences en pa-trimoine collectif : stratégies et perspectives
No full textCette étude explore la capture et la valorisation des savoirs tacites au sein d’organisations scientifiques, en prenant le BRGM comme cas d’étude. Face à l’enjeu stratégique de la gestion des connaissances, nous examinons la transformation des savoirs individuels en patrimoine collectif via une approche méthodologique en trois volets : cadre théo-rique, utilisation d’outils d’IA pour recenser et transcrire ces savoirs, et proposition d’une architecture CBR avec un agent IA ("beregem") pour résoudre des problématiques en géosciences. Cette recherche contribue à la gestion des savoirs tacites scientifiques par des solutions basées sur l’IA
Influence of surrounding plates on 3D subduction dynamics
No full textInternational audienceOur 3D modelling study shows that the presence of lithospheric plates around a subducting plate has a significant influence on subduction dynamics, in particular on trench retreat rate, slab dip, and lateral shortening of the subducting plate. Neighbouring plates prevent unrealistic plate behaviour with no need for complex rheologies. Because, at the Earth's surface, plates form a continuous shell, they should not be neglected
A knowledge-driven modeling formalism for automatic structural interpretation
No full textInternational audienceBuilding structural models of geological entities is generally addressed as an interpolation problem that requires human experts to interpret input data and use knowledge. Although experts can effectively interpret, their interpretations can be subjective and occasionally prone to error. This is largely due to under-sampling of data, requiring experts to make choices in the selection and preparation of these data and knowledge, and selection and configuration of modeling algorithms. Modeling algorithms also do not reflect the complex expert interpretation process, as they incorporate only a portion of the knowledge typically held by experts and have limited ability to directly interact with experts during the interpretation process itself. This makes it challenging to build geologically complex models and systematically identify and address inconsistencies in a model. Part of the solution to these issues is the formalization of the interpretation process, which incorporates more knowledge and better reflects expert decision-making. In this paper we develop and prototype such a formalization. A prototype algorithm and tool are presented and applied to simple folding structures, and the results are favorably compared to existing approaches. This comparison highlights the potential of the proposed approach to reduce the need for expert involvement and increase the range of knowledge utilize
3D subduction dynamics: A first-order parameter of the transition from copper- to gold-rich deposits in the eastern Mediterranean region
No full textInternational audienceThe natural variability of geometry and dynamics of subduction zones leads to a variety of mantle and crustal processes that may influence the genesis of ore deposits in the overriding plate. These complex interactions cannot be fully represented by two-dimensional (2D) models but require that the spatial and temporal evolution of ore deposits be examined in a detailed 3D tectonic framework. We compare and discuss the geodynamic and metallogenic evolution of the eastern Mediterranean subduction zone since the late Cretaceous by integrating a newly compiled metallogenic database with a recent kinematic reconstruction model. The resulting paleotec-tonic maps identify (1) a late Cretaceous Cu-rich metallogenic period with large deposits formed above a linear and stable subduction zone that produced large amounts of potentially fertile magmas in a narrow magmatic arc and (2) a late Eocene-Miocene Pb-Zn-followed by Au-rich metallogenic period with generally smaller deposits spread over a wide back-arc basin that was opened due to subduction retreat and lateral slab tearing. Supported by high-resolution numerical modeling, the proposed dynamic arc and back-arc metallogenic models emphasize (1) the influence of 3D slab dynamics and associated asthenospheric flow on ore distribution in space and time and (2) the importance of exhuming metamorphic domes in extensional back-arc setting to focus metal-bearing fluid circulations. These models differ from collision and post-collision models and may be valid on the western and eastern terminations of the whole Tethyan orogenic belt (i.e. Mediterranean and southeast Asia) where fast 3D subduction dynamics has influenced the geodynamic and metallogenic evolution of the overriding plate
Projet BD GTH DOM : Vers une plateforme d'information sur l'exploration géothermique de haute température en contexte volcanique dans les îles d'Outre-mer - Rapport final
No full textSpatial and temporal distribution of Cu-Au-Mo ore deposits along the western Tethyan convergent margin: a link with the 3D subduction dynamics
No full textInternational audienceAlong the western Tethyan convergent margin, where Tertiary subduction history is well constrained, porphyry, epithermal and skarn ore deposits show a variable evolution of their spatial distribution. Using different and complementary database on European and Middle East ore deposits, three metallogenic episodes have been highlighted: (1) a late Cretaceous - Paleocene phase characterized by a copper mineralization within the Balkan chain and in the Kaçkar mountains (eastern Turkey), (2) an Eocene phase with a few copper ore deposits in eastern Turkey and small Caucasia and (3) an Oligocene - Neogene phase with a more southern distribution along the margin and mainly constituted by epithermal Au systems in the west (Carpathians, Rhodope, Aegean and western Turkey) and by porphyry copper deposits in the east (Zagros). Using paleogeographic tools, it turned out that, in the eastern Mediterranean area, the late Cretaceous - Paleocene and Oligocene - Neogenemetallogenic episodes arecoeval with a significant decrease of the Africa - Eurasia convergence rate, from about 1.5 to 0.4 cm/yr. Indeed, compressional tectonics promote the storage of large volumes of metal-rich magma and the development of an extensive MASH (melting, assimilation, storage and homogenization) zone. When this convergence rate decreases, a stress relaxation occurs in the overriding crust, inducing the ascent of a sufficient flux of this fertile magma and allowing the formation of numerous mineralized systems within the upper crust. The Au-rich Oligocene - Neogenemetallogenic episode in the eastern Mediterranean region is also correlated with an increase of mantle-derived and/or subduction-modified lithospheric mantle components in magmas. This feature may be a consequence of the emplacement of hot asthenosphere at shallow depth related to (1) the development of a wide back-arc region due to slab retreat such as in the Aegean domain and (2) a slab tear and/or a lithospheric delamination, suspected notably in the Carpathians and western Turkey where alkaline to shoshoniticvolcanism occurs. As the behavior of the slab and asthenosphere below the upper plate seems to play a key-role in controlling the distribution of ore deposits, it is worth studying the dynamics of the 3D mantle flow related toslab retreat. Thus, 3D numerical models of subduction dynamics with realistic rheologies have been developed. Around the slab edges, the poloidal (i.e. in a vertical plane) and toroidal (i.e. in a horizontal plane) components of the mantle flow in subduction zone appear to depend on the slab rollback to plate velocity ratio
