Syn-convergence flow inside and at the margin of orogenic plateaus: Lithospheric-scale experimental approach

International audienceThis study investigates three-dimensional flow modes of orogenic plateaus by means of physical modeling. Experiments consist of shortening two contiguous lithospheres of contrasting strength, one being a weak plateau-type lithosphere and the other a strong craton-type lithosphere. The lateral boundaries are either totally confined or allow escape toward a lateral foreland on one side. Two synconvergence flow regimes are distinguished, which are governed by the balance between the gravity potential and the strength of the plateau crust and the resistance of its lateral foreland. The first regime implies transversal (orogen-normal) injection of plateau lower crust into the collision zone as a result of confinement of the plateau by an increasingly stiffer lateral boundary. As a precursor mechanism to channel flow, transversal injection responds to downward thickening of the plateau crust that is forcedly extruded into the orogenic wedge. The second regime is that of collapse-driven lateral escape of the plateau. This regime is established after a threshold is attained in the interplate coupling in the collision zone, which allows the gravity potential of the plateau to overcome the resistance of its lateral boundary. Under the collapse-driven escape regime (orogen parallel), such as that governing Tibet during the last 13 Ma, most of the convergence in the plateau and the top and rear of the collisional wedge is transformed into lateral flow and extension

Gravity-driven sliding and associated deformations along complex submarine slopes: a laboratory modeling approach based on constraints observed offshore Martinique Island (Lesser Antilles)

Submarine gravity-driven sliding of sediments are common processes in the vicinity of volcanic islands. In the Lesser Antilles arc, the Montagne Pelée volcano on Martinique Island underwent several flank-collapse events during its long-term eruptive history, resulting in debris avalanches. When the debris avalanches entered into the seawater, they were emplaced over the unstable slope of the volcano, triggering a seafloor sediment failure and massive landslides downslope. Using a laboratory modeling approach, we simulated the gravity-driven sliding of a sand layer lying above a silicone layer. The experiments were performed using various slope geometries (slope lengths and number of slope breaks separating the slopes with different angles), under both dry and aqueous conditions, and while varying the amount of additional sand inputs upslope. The resulting deformations were characterized in each experiment in order to compare the obtained structures with those shown by the seismic lines offshore to the west of Martinique Island. During all the experiments, a compressional frontal deformation zone made of several reverse faults formed downslope, often near the slope breaks. Downslope, a portion of the sediments was mostly displaced and poorly deformed in a damping zone, while an extensional deformation zone formed upslope. The displacements of the surficial markers were measured through time to characterize the sliding dynamics. Our study demonstrates that the slope geometry and additional sand inputs primarily favor and increase the sliding deformation, whereas the hydrostatic pressure plays a secondary catalytic role over time. These results provide new constraints on the driving factors and their consequences on gravity-driven sliding in terms of deformations and runout distance over time. This may have a significant impact on the associated hazard assessment related to offshore infrastructures, in a region known for its seismic and volcanic risks

Transfert et préservation des signaux sédimentaires, un nouveau dispositif source-to-sink expérimental à Géosciences Rennes

National audienceIl reste difficile d'anticiper la réponse des paysages à la suite d'un évènement intense comme une tempête ou un glissement de terrain, ou d'une perturbation plus long-terme comme le changement climatique actuel. En effet, les interactions entre les processus en jeu ainsi que les échelles de temps considérées rendent difficiles les observations sur le terrain. La physique de ces processus n'est pas toujours bien connue et malgré les développements récents, il reste difficile d'appréhender le transfert et le dépôt de sédiments multi-tailles à l'échelle d'un bassin versant. L'approche expérimentale permet alors de réduire les échelles de temps et d'espace en laissant la physique du transport émerger dans un cadre connu et contrôlé. Je présente ici un dispositif développé au laboratoire de modélisation de Géosciences Rennes pour simuler le transfert et le dépôt de sédiments en charge de fond dans un réseau de drainage imposé. Le bassin versant est imprimé en 3D à partir d'une topographie numérique et il est couplé à une zone de dépôt où les sédiments s'accumulent pour construire un cône alluvial (échelle typique x10-100 km). Les expériences se concentrent donc sur le transfert et le dépôt des sédiments, en s'affranchissant de la mise en place du réseau de drainage qui peut être complexe et peu reproductible. Les sédiments et l'eau sont apportés dans le bassin versant par des points sources, dont le nombre et la location sont modulables. Ce dispositif permet de simuler la réponse d'un paysage à un changement (rapide ou lent, ponctuel ou transitoire) du flux d'eau, de sédiments ou du calibre des grains injectés. Le suivi topographique, photographique et les coupes réalisées dans les dépôts en fin d'expérience permettent de suivre la propagation et la modulation d'un signal, ainsi que son enregistrement géométrique et granulométrique dans les dépôts. Ce dispositif offre une vision source-to-sink de la propagation de signaux vue à la fois par les flux et la distribution de taille des sédiments. La facilité de mise en oeuvre et la relative rapidité des expériences permet d'envisager une approche expérimentale statistique

Transfert et préservation des signaux sédimentaires, un nouveau dispositif source-to-sink expérimental à Géosciences Rennes

National audienceIl reste difficile d'anticiper la réponse des paysages à la suite d'un évènement intense comme une tempête ou un glissement de terrain, ou d'une perturbation plus long-terme comme le changement climatique actuel. En effet, les interactions entre les processus en jeu ainsi que les échelles de temps considérées rendent difficiles les observations sur le terrain. La physique de ces processus n'est pas toujours bien connue et malgré les développements récents, il reste difficile d'appréhender le transfert et le dépôt de sédiments multi-tailles à l'échelle d'un bassin versant. L'approche expérimentale permet alors de réduire les échelles de temps et d'espace en laissant la physique du transport émerger dans un cadre connu et contrôlé. Je présente ici un dispositif développé au laboratoire de modélisation de Géosciences Rennes pour simuler le transfert et le dépôt de sédiments en charge de fond dans un réseau de drainage imposé. Le bassin versant est imprimé en 3D à partir d'une topographie numérique et il est couplé à une zone de dépôt où les sédiments s'accumulent pour construire un cône alluvial (échelle typique x10-100 km). Les expériences se concentrent donc sur le transfert et le dépôt des sédiments, en s'affranchissant de la mise en place du réseau de drainage qui peut être complexe et peu reproductible. Les sédiments et l'eau sont apportés dans le bassin versant par des points sources, dont le nombre et la location sont modulables. Ce dispositif permet de simuler la réponse d'un paysage à un changement (rapide ou lent, ponctuel ou transitoire) du flux d'eau, de sédiments ou du calibre des grains injectés. Le suivi topographique, photographique et les coupes réalisées dans les dépôts en fin d'expérience permettent de suivre la propagation et la modulation d'un signal, ainsi que son enregistrement géométrique et granulométrique dans les dépôts. Ce dispositif offre une vision source-to-sink de la propagation de signaux vue à la fois par les flux et la distribution de taille des sédiments. La facilité de mise en oeuvre et la relative rapidité des expériences permet d'envisager une approche expérimentale statistique

Modélisation analogique de brèches sédimentaires dans un talusd’éboulis avec ou sans extension—comparaison avec un casnaturel

International audienceSedimentary breccias deposited on a talus slope during extension at the front of a normalfault or at the base of a slope are related to the creation of substantial topography. The architectureof talus slopes resulting from exhumation during extension is different from that of rock cliffs withoutdeformation. In this work, we run several analogue models, with or without extension, in order toanalyse the external geometry and internal organization of sedimentary breccias and their evolutionin time and space. This temporal organization of the sand layers in the experiments shows that normalunroofing sequences are characteristic of progressive exhumation and continuous deformation duringextension, whereas reverse unroofing is related to cliff destruction without vertical movement. Inthe experiment with wet layers of sand, the blocks that resulted from the aggregates of sand grains aregenerally deposited farther away than single sand grains and are located at the base of the slope; thisis one of the main characteristics of rockfalls. The analogue results are compared to natural examplesLes brèches sédimentaires déposées sur un talus d’éboulis pendant l’extension au frontd’une faille normale ou à la base d’un versant abrupt sont liées à la création d’une topographie importante.L’architecture des talus d’éboulis résultant de l’exhumation pendant l’extension est différentede celle des falaises rocheuses sans déformation. Dans ce travail, nous avons réalisé plusieursmodèles analogues, avec ou sans extension, afin d’analyser la géométrie externe et l’organisation internedes brèches sédimentaires et leur évolution dans le temps et l’espace. L’organisation temporelledes couches de sable dans les expériences montre que la séquence de dépôt normale est caractéristiqued’une exhumation progressive et d’une déformation continue pendant l’extension, alors qu’uneséquence de dépôt inverse est liée à la destruction de la falaise sans mouvement vertical. Dans les expériences réalisées avec des couches de sable humide, les blocs résultant des agrégats de grains desable sont généralement déposés plus loin que les grains de sable individuels et sont situés à la base dela pente; c’est l’une des principales caractéristiques des éboulements. Les résultats des modélisationssont comparés à des exemples naturels

Supercritical river terraces generated by hydraulic and geomorphic interactions

International audienceThe alternating cycle of valley widening through lateral erosion ("strath planation") and valley narrowing through vertical incision into bedrock ("strath terrace abandonment") due to variations in sediment supply (Qs) relative to river transport capacity (Qsc) is a common feature in many mountainous environments, yet our understanding of the mechanics of the processes that drive this landscape change remains poorly quantified. Here, we use an experimental and numerical study to identify the geomorphic and hydraulic controls driving the response of mixed bedrock-alluvial rivers to variable sediment supply, water discharge, and tectonic tilting. The experimental channels exhibit a multistage response of channel narrowing, accompanied by stripping of the alluvial cover in a downstream-migrating incision wave, followed by destabilization of the bed and development of a single vertical step in the bed profile ("knickpoint") when the hydraulic conditions are supercritical. In our experiments, headward erosion by knickpoints is the most efficient process of strath terrace abandonment, contributing the majority of the total vertical incision in a short period of time. We show experimentally that knickpoint development under supercritical flow conditions drives the rapid response of fluvial systems to upstream perturbations in Qs/Qsc despite no base-level fall. This has implications for the understanding of distributions of strath terrace ages, the inference of base-level variations from knickpoint propagation, and how landscapes respond to climatic or tectonic perturbations

Control of crustal strength, tectonic inheritance, and stretching/ shortening rates on crustal deformation and basin reactivation: insights from laboratory models

International audienceGeological settings characterized by multiple coeval tectonic regimes provide a unique opportunity to understand complex interactions among different geodynamic processes. However, they remain comparatively less studied from an experimental point of view than areas with more simple patterns of deformation resulting from primary plate–boundary interactions. Here, we carried out analog experiments involving simultaneous shortening and orthogonal extension under different rheological conditions, including the effect of crustal inheritance. We performed brittle experiments and brittle–ductile experiments to simulate cases of “strong” and “weak” crusts, respectively. We present two types of experiments: (i) one-stage experiments with either shortening only or synchronous orthogonal shortening and stretching and (ii) two-stage experiments with a first stage of stretching and a second stage with either shortening only or synchronous orthogonal shortening and stretching. In our models, deformation is accommodated by a combination of normal, thrust, and strike-slip faults with structure location depending on boundary conditions and crustal inheritance. For brittle models, we show that the three types of structures can develop at the same time for intermediate ratios of stretching (extension) over shortening rates (1.40). For larger ratios and for the same amount of stretching, deformation is accommodated by normal faults at edges and in the center of the model as well as by conjugate strike-slip faults at the edges of the model. For brittle–ductile models, we always observe strike-slip faults that crosscut the entire model. They are associated with shortening-orthogonal thrust faults for models with low Ve/Vs and no initial extensional stage or stretching-orthogonal normal faults for models with high Ve/Vs and an initial extensional stage. Whatever the crustal strength, the past deformation history, and the stretching / shortening ratio, both normal and thrust faults remain with similar orientations, i.e., stretching-orthogonal and shortening-orthogonal, respectively. Instead, strike-slip faults exhibit orientations with respect to the shortening direction that vary between ∼0 and ∼65∘. Strike-slip faults parallel to the shortening direction develop in previously extended portions of models with a brittle–ductile crust, while strike-slip faults with a high angle form at the boundaries of the brittle model, their orientation being to some extent influenced by pre-existing or newly forming graben in the center of the model. We also show that extensional structures formed during a first stage of deformation are never inverted under orthogonal shortening but can be reactivated as normal or strike-slip faults depending on Ve/Vs. Our experiments reproduce V-shaped conjugate strike-slip systems and normal faulting during compression similar to structures observed in the Tibetan Plateau, the eastern Alps, western Anatolia, and the Central Asia orogen. Models with two-stage deformation show variable extensional to strike-slip reactivation of former extensional basins during basin-parallel shortening, which resembles synorogenic foreland transtensional reactivations documented in the Baikal and Golfo de San Jorge basins

Indentation as an extrusion mechanism of lower crustal rocks: Insight from analogue and numerical modelling, application to the Eastern Bohemian Massif

International audienceRecent petrological, structural and geochronological studies of the eastern margin of the Bohemian Massif (Czech Republic) suggest a conceptual geodynamical model to explain exhumation of lower crustal (20 kbar, 800 degrees C) felsic rocks. The model involves indentation of a weak orogenic lower crust by an adjacent rigid mantle lithosphere, resulting in crustal-scale buckling of the weak orogenic lower/middle crust interface followed by extrusion of a ductile nappe over the rigid promontory. The hypothesis has been investigated using both analogue and numerical models. Analogue experiments using a three layer sand-silicone setup were carried out in Rennes laboratory (France). Results show that the most important features of the conceptual model can be reproduced: extrusion of lowermost silicone over the indenter and flow of horizontal viscous channel underneath a rigid lid above the actively progressing promontory. Furthermore, experimental results show that a plateau develops above the channelling lower crust. Two sets of sandbox-scale numerical simulations were performed. The first set of experiments is designed to study the influence of viscosity stratification within the crust on the extrusion process. A second set of experiments were performed in order to quantify the influence of the viscosity and the geometry of the indentor. Non-dimensional scaling laws were derived to predict the maximum extrusion rates associated with the indentation mechanism. Such laws enable the computation vertical extrusion rates that are in good agreement with natural exhumation rates inferred from petrological data. Finally, we discuss the potential positive feedback of Rayleigh Taylor instability on vertical extrusion for the case of Eastern Bohemian Massi

Consequences of tectonic advection on relief dynamics

International audienc