Dynamics of dikes versus cone sheets in volcanic systems

International audienceIgneous sheet intrusions of various shapes, such as dikes and cone sheets, coexist as parts of complex volcanic plumbing systems likely fed by common sources. How they form is fundamental regarding volcanic hazards, yet no dynamic model simulates and predicts satisfactorily their diversity. Here we present scaled laboratory experiments that reproduced dikes and cone sheets under controlled conditions. Our models show that their formation is governed by a dimensionless ratio (Π1), which describes the geometry of the magma source, and a dynamic dimensionless ratio (Π2), which compares the viscous stresses in the flowing magma to the host rock strength. Plotting our experiments against these two numbers results in a phase diagram evidencing a dike and a cone sheet field, separated by a sharp transition that fits a power law. This result shows that dikes and cone sheets correspond to distinct physical regimes of magma emplacement in the crust. For a given host rock strength, cone sheets preferentially form when the source is shallow, relative to its lateral extent, orwhen the magma influx velocity (or viscosity) is high. Conversely, dikes form when the source is deep compared to its size, or when magma influx rate (or viscosity) is low. Both dikes and cone sheets may form fromthe same source, the shift fromone regime to the other being then controlled by magma dynamics, i.e., different values of Π2. The extrapolated empirical dike-to-cone sheet transition is in good agreement with the occurrence of dikes and cone sheets in various natural volcanic settings

Laser-doppler Acoustic Probing of Granular Media with Varying Water Levels

International audienceLaboratory physical modelling and non-contacting ultrasonic techniques are frequently proposed to tackle theoretical and method- ological issues related to geophysical prospecting. We used an innovative experimental set-up to perform laser-Doppler acoustic probing of granular materials with varying water levels to target near-surface hydrogeological applications. The preliminary results presented here show a clear influence of the water level on both first arrival times and dispersion of guided waves, and significant differences in terms of amplitudes. They validate the use of such approach to benchmark recently developed methods for water saturation detection in hydrogeophysics

Early, far-field and diffuse tectonics records in the North Aquitaine Basin (France)

In Western Europe, the deformations related to the opening of the Bay of Biscay and the formation of the Pyrenean belt are well described in the southern part of the Aquitaine Basin, but little is known about the impact of these geodynamic events towards the Northern Aquitaine Platform. In this paper, we combine field observation with Unmanned Aerial Vehicle (UAV) imagery and calcite U-Pb geochronology to determine precisely the tectonic evolution in the Vendée Coastal domain. We evidence two main tectonic events: (1) At the transition between the Late Jurassic to Early Cretaceous, WNW-ESE striking normal faults formed horsts and grabens at the onset of the opening of the Bay of Biscay. The reactivation of Variscan faults during this tectonic event is consistent with oblique extension. This event triggered ascending fluid flows that mix with Basin fluids responsible for barite-pyrite-quartz mineralizations near the unconformity. (2) During the Late Cretaceous, fractures, wide-open folds, veins, and joints are consistent with the N-S shortening direction during the earliest stages of the Pyrenenan compression. In both cases, the Northern Aquitaine Platform records the early stages of the main regional tectonic events in a far-field position. In the northern Aquitaine Basin, as in many other places in Europe, the tectonic study of sedimentary platforms located far from the plate boundaries provides new constraints on the early diffuse deformation process that predate the main tectonic phases

Thrust wedges and fluid overpressures: Sandbox models involving pore fluids

International audienceThe well-known model for the critical taper of an accretionary wedge includes overpressure as a first-order parameter. Fluid overpressures reduce frictional resistance at the base of a wedge but they also act as body forces on all material particles of the wedge, in addition to that of gravity. By means of sandbox modeling, many workers have tried to verify the predictions of the critical taper model, but few of them have so far incorporated true fluid pressures. We have used scaled experiments, in which compressed air flows through sand packs, so as to model the deformation of overpressured wedges. A new apparatus provides for a horizontally varying fluid pressure, for example, a linear variation, as in the critical taper model. We have done three series of experiments, involving horizontal shortening of homogeneous or multilayered sand models for various gradients of fluid pressure. As predicted by the critical taper model, the apical angle of the resulting wedge depends on the overpressure gradient. In homogeneous sand at a high overpressure gradient, deformation becomes diffuse and looks ductile. In multilayered models, detachments form beneath layers of low permeability, so that thrusts propagate rapidly toward the undeformed foreland. The efficiency of a detachment and its ability to propagate depend not only on the fluid pressure but also on the permeability ratios between the various layers

Surpressions de fluide et décollements. Modélisations analogique et numérique

Thèse publiée dans la collection Mémoire de Géosciences-Rennes N° 113 ISBN : 2-914375-24-7Overpressures of fluid are very often called upon to explain the very weak frictions observed at the base of the overlapping tablecloths or the slips of sediments on very weak slopes. Analogical modeling had however never taken them into account. In this thesis, we develop a new technique which consists in injecting air in sand models in order to simulate these overpressures. Thanks to this method, we show how the fluids act on the deformation by the intermédiaire of the forces of current (gradient of pressure), by modifying l intensity and l orientation of the effective constraints. We carry out then experiments of compressive deformation, in which we generate levels of separation while exploiting the permeability of the sand used in the construction of the models. The efficacity of these separations depends then on the pressure of fluid applied and controls the style of the deformation. These analogical experiments are compared with a digital model of distinct elements coupled to a liquid phase, which we also developed during this thesis. The confrontation of the results resulting from the two methods appears rather convincing. We simulate then gravitating slips and show how overpressures of fluid can create listric normal faults by modifying the orientation of the constraints. These experiments are finally applied to the deformations of the delta of the Amazone. The correlation between the form of the slips, the distribution of the pressures and the presence d'une bed rock in probable generation of gas, enables us to specify the origine of overpressures in this delta. To finish, we place ourselves at the scale of the sand grain, and we use the numerical method to characterize the evolution of texture d'un granular medium subjected to a gradient of pressure of increasing fluid.Les surpressions de fluide sont très souvent invoquées pour expliquer les très faibles frottements observés à la base des nappes chevauchantes ou les glissements de sédiments sur de très faibles pentes. La modélisation analogique ne les avait cependant jamais prises en compte. Dans cette thèse, nous développons une nouvelle technique qui consiste à injecter de l'air dans des modèles de sable afin de simuler ces surpressions. Grâce à cette méthode, nous montrons comment les fluides agissent sur la déformation par l'intermédiaire des forces de courant (gradient de pression), en modifiant l'intensité et l'orientation des contraintes effectives. Nous effectuons ensuite des expériences de déformation compressive, dans lesquelles nous générons des niveaux de décollement en jouant sur la perméabilité du sable utilisé dans la construction des modèles. L'efficacité de ces décollements dépend alors de la pression de fluide appliquée et contrôle le style de la déformation. Ces expériences analogiques sont comparées avec un modèle numérique d'éléments distincts couplé à une phase fluide, que nous avons également développé au cours de cette thèse. La confrontation des résultats issus des deux méthodes apparaît assez convaincante. Nous simulons ensuite des glissements gravitaires et montrons comment les surpressions de fluide peuvent créer des failles normales listriques en modifiant l'orientation des contraintes. Ces expériences sont enfin appliquées aux déformations du delta de l'Amazone. La corrélation entre la forme des glissements, la distribution des pressions et la présence d'une roche mère en probable génération de gaz, nous permet de préciser l'origine des surpressions dans ce delta. Pour terminer, nous nous plaçons à l'échelle du grain de sable, et nous utilisons la méthode numérique pour caractériser l'évolution de la texture d'un milieu granulaire soumis à un gradient de pression de fluide croissant

Sandbox experiments on gravitational spreading and gliding in the presence of fluid overpressures

International audienceWhereas in previous analogue experiments on gravitational spreading and gliding, detachment occurred on a ductile layer, we have used a relatively new technique of injecting compressed air into sand packs so as to simulate the effects of fluid overpressures in sedimentary strata and to trigger slope instabilities. In our experiments, the governing equations yield scales for dimensions, stresses and fluid pressure. However, the more transitory phenomena of production and decrease of overpressure cannot be suitably scaled. By using layers of differing permeability, we are able to produce sharp detachments in models made of sand alone. The experiments involve gravity spreading or gravity gliding. In gravity spreading, propagation of the detachment and of extensional deformation depends on the fluid pressure. For medium values of fluid overpressure, normal faults are closely spaced, numerous and bound rotated blocks. They propagate progressively toward the back of the model. For the highest pressures, the deformation propagates very fast and faults bound non-rotated blocks, which slide on an efficient basal detachment. Fault dips are also controlled by fluid pressure and by frictional resistance at the base. To model gravitational gliding required an apparatus with a more complex system of air injection. We did a series of experiments using injection windows of various lengths and compared the results with predictions from a quasi-3D analytical model of sliding. In contrast with predictions for an infinite slope, sliding depends on (1) the fluid overpressure on the detachment, (2) the fluid overpressure in the body of the sliding sheet, and (3) the shape of the detachment surface. In particular, we show that frictional resistance at the lower edge is a primary control on the dynamics of gliding

Surpression de fluide et décollements (modélisations analogique et numérique)

Les surpressions de fluide sont très souvent invoquées pour expliquer les très faibles frottements observés à la base des nappes chevauchantes ou les glissements de sédiments sur de très faibles pentes. Dans cette thèse, nous développons une méthode analogique qui permet d’introduire ces effets dans des modèles de sable. Nous avons également écrit un code numérique d'éléments distincts couplé avec un fluide. Nous utilisons ces deux méthodes pour montrer comment les fluides agissent sur les déformations par l'intermédiaire des forces de courant qui modifient l'état des contraintes (intensité et orientation). Nous simulons ensuite la formation de prismes d'accrétion soumis à des surpressions de fluide variées. La confrontations des résultats analogiques et numériques est assez convaincante et en accord avec les modèles théoriques. Nous appliquons enfin nos expériences aux déformations gravitaires rencontrées dans le delta de l'Amazone. La corrélation entre la forme des glissements, la distribution des pressions et la présence d'une roche mère en probable génération de gaz, nous permet de préciser l'origine des surpressions dans ce delta.RENNES1-BU Sciences Philo (352382102) / SudocSudocFranceF

The Coulomb critical taper theory applied to gravitational instabilities

International audienceThe critical Coulomb wedge theory has been widely applied to the structural evolution of accretionary wedges and fold and thrust belts, but it also predicts the spreading of a wedge under gravity. This solution may be applied to the formation of gravitational spreading and gliding along passive margins, where elevated porefluid pressure is common. Following the initial hypotheses of the theory, we provide an alternative expression of the exact solution for a noncohesive wedge, better suited to slope instabilities. Our formulation allows a direct calculation of the fluid pressure required for the system to deform and predicts two kinds of gravitational deformation: shallow slumping and/or deep gravitational spreading rooting on the basal detachment. To verify the predictions of the model, we performed scaled experiments with pore pressure under conditions close to the critical taper hypotheses. The good agreement between our experimental results and the theory confirms the applicability of the critical taper model to the formation of gravitational structures in sedimentary wedges subjected to pore fluid pressure

Modelling the emplacement of cone sheets and dykes in volcanic systems

International audienc

Horizontal and vertical fluid flows as a key control of ore deposition at the basement/cover unconformity: insight from drone imagery of the Vendée coast, France

International audienceBasement/cover interfaces are important transfer zones for hydrothermal fluids responsible for ore deposition, such as U and Pb-Zn deposits. Unconformities are peculiarly mixing zone where basement-derived fluids encounter sedimentary- and/or meteoric-derived fluids; leading to precipitation of these ores. Fluids are channelized by permeability contrast, i.e. impermeable barriers, until being trapped in porous units, i.e. intrinsic porosity and/or secondary porosity (dissolution and karstification process). In this configuration fracturing channelize the fluid flow by breaking impermeable barriers allowing external fluids to enter and react with the rocks (precipitation and/or dissolution). In this way, structural studies are crucial to highlight the fracture network and the potential of geological units to be good reservoirs.In France, many occurrences of sediment-hosted deposits are known in Mesozoic basins (i.e. Aquitaine and Paris Basin) especially above the Variscan basement (Morvan district, SW Massif Central district, Poitou High district). The Vendée coast deposit (South Armorican Massif, France) is known for two Pb-Zn(-Ag) occurrences located in Liassic sediments overlying the Variscan basement. Previous works show that, during the Upper Jurassic extensional event (NNE-SSW horizontal stretching), the ore deposition results from the mixing of two different fluids: (1) low temperature brines following a horizontal path from evaporite to basin borders within Liassic sediments along the unconformity, (2) a high temperature and low salinity fluid rising up through the basement from several kilometres depth by a probable vertical pathway.However, the permeability architecture leading to such mixing remains poorly constrained. The Vendée ore deposits present favourable outcrop conditions to study the structural control of the fluid plumbing system along the basement/cover unconformity. Structural studies assisted by drone imagery coupled with the characterization of the alteration-mineralization pattern show that:(1) Horizontal path for basin brines is controlled by the impermeable barrier of the Toarcien layer overlying Liassic hosting karsts.(2) Vertical path of basement-derived fluids is enhanced by new faults and inherited fractures, respectively generated and reopened by the Jurassic extension.(3) Relative abundance of faults and veins in the Liassic sedimentary cover and the basement is consistent with a mechanical decoupling in a context of fluid overpressure