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

Cenozoic tectonics of the Western Approaches Channel basins and its control of local drainage systems

International audienceLe domaine des Approches occidentales de la Manche constitue une zone clé pour caractériser l'évolution post-rift des marges continentales NW européennes associées à la collision Afrique/Europe. Malgré les divers témoins des inversions cénozoïques jalonnant le pourtour de la Manche, la structuration et l'amplitude des mouvements demeurent néanmoins incertaines au sein de la partie méridionale française des Approches occidentales. Il en est de même sur le rôle de l'inversion de la mer du Nord dans la mise en place du Fleuve Manche qui drainait un bassin versant bien supérieur à la Manche actuelle durant les grandes régressions plio-quaternaires et alimentait les éventails sous-marins Celtique et Armoricain en bas de pente. La réalisation des campagnes de sismique-réflexion haute résolution GEOMOC et GEOBREST03 dont les résultats font l'objet de cet article permettent de répondre à ces questions en complétant la connaissance géologique de la Manche. Les nouvelles observations soulignent le diachronisme et le contraste de l'amplitude des mouvements du système de failles associées à l'inversion du bassin d'Iroise. Celle-ci se fait en deux épisodes: un épisode paroxysmal paléogène décomposé en deux phases, éocène (Yprésien probable) et oligocène, et un épisode néogène plus modéré réactivant partiellement les structures impliquées antérieurement. Les déformations se concentrent le long de l'accident nord Iroise (NIF) situé dans le prolongement de la faille Médio-Manche et entraîne localement des plissements de la couverture sédimentaire à l'aplomb des accidents profonds. L'inversion induit ainsi un soulèvement de près de 700 m du plateau médian situé au sud de l'accident nord Iroise. La cartographie isochrone des séquences sismiques identifiées démontre également le contrôle majeur des structures tectoniques sur la mise en place des dépôts néogènes. Le soulèvement de la partie orientale du bassin favorise ainsi la mise en place de vastes prismes progradants d'âge miocène supérieur, et contrôle le développement postérieur du réseau des paléo-vallées constituant l'extrémité occidentale du fleuve Manche. Ce réseau présente une géométrie en baïonnette marquée par de brutaux changements de directions variant de N040 à N070, cette dernière direction caractérisant la plus grande partie des failles néogènes associées au bassin d'Iroise. Les paléo-vallées se seraient développées lors d'une chute du niveau marin au-delà du rebord de plate-forme et la stratigraphie établie à travers cette étude amène à placer le début des incisions au Pliocène (Reurévien ou pré-Tiglien). La chute amplifiée par l'inversion du bassin d'Iroise serait suivie d'un basculement tardif de la plate-forme externe à l'instar des observations réalisées sur de nombreuses marges du pourtour nord atlantique

Extension as a major tectonic process of the evolution of Southern Ecuador compressional forearc

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High frequency sea level fluctuations recorded in the Black Sea since the LGM

International audienceThis paper presents a 3D geometric interpretation of very high resolution seismic Chirp profiles acquired on the Romanian shelf during ASSEMBLAGE European Project. The results provide a solid record of the Black Sea Last Glacial Maximum (LGM) water level fluctuations. This pseudo-3D seismic interpretation shows that the Black Sea lacustrine shelf deposits form a significant basinward-prograding wedge system. On top of these prograding sequences is a set of sand dunes that delineates a wave cut-terrace like feature around the isobath − 100 m. Landward of this dune field are small depressions containing barkhan-like bodies. The upper part of the last prograding sequence is incised by anastomosed channels which end in the Danube (Viteaz) canyon which are also built on the lacustrine prograding wedge. Ten stratigraphic units were distinguished and correlated with analyses of cores retrieved from this area. The results demonstrate that the first eight sequences represent lacustrine prograding wedges, the ninth sequence is the dune system itself and the tenth is a marine mud drape which covers the entire continental shelf. The lacustrine prograding wedges represent a lowstand deposit characterised by forced regression-like reflectors mapped from the pseudo-3D seismic data. Their hinge point corresponds to the wave erosion surface mapped around − 100 m isobath on the multibeam mosaic. Dated cores give age control on this lowstand period, which lasted from 11 to 8.5 kyr 14C BP as implied by: (1) the continuously dry climatic conditions in the region inferred by high percentages of herbs and steppe elements determined from ASSEMBLAGE cores and, (2) the formation of dunes between 10 and 8.5 kyr 14C BP on the desiccated north-western Black Sea shelf at − 100 m and (3) all of these covered by a marine mud drape confirming that the dune system is no longer active. The buried, anastomosed fluvial channels that suddenly disappear below 90 m depth, and a unique wave-cut terrace between 95 and 100 mbsl on the outer shelf are also consistent with a major lowstand base-level at around − 100 m water depth. Preservation of sand dunes and the occurrence of small, buried incised valleys mark a rapid transgression within less than a century during which ravinement processes related to the water level rise had no time to significantly erode the seafloor

From orogenic collapse to rifting: A case study of the northern Porcupine Basin, offshore Ireland

International audienceOffshore Ireland, the North Atlantic opening is generally interpreted as successive, 10-to-15-Myr-long, rifting events during the Mesozoic. However, their interaction is poorly documented in terms of structural inheritance and fault reactivation. From extensive seismic and well data in the northern Porcupine Basin, we show that extension actually evolved over an abnormal period of ca. 220 Myrs with overlapping tectonic stages that, all together, reflect the Irish Atlantic Margin construction from the Palaeozoic. During the Carboniferous, extension initiated throughout the Variscan and Caledonian thickened crusts and controlled the deposition of generally slightly-tilted, continental clastics. Two subsequent rifting events, during the Triassic-Lower Jurassic and Upper-Jurassic-Lower Cretaceous, controlled shallow-to-deep marine sedi-mentation in separated sub-basins that progressively widen and connect once extension localised along bounding faults. We propose that extensional tectonics, first controlled by a general orogenic collapse reflecting an early stage of stretching, is then followed by several rifting periods that ultimately evolved toward the hyper-thinning of the margin and shaped an aborted rift propagator. This evolution is due to pre-existing orogenic-related structures and boundary conditions variations. Such a continuum of deformation, although pulsed, implies varying structural interactions of crustal scale that are often recorded in syn-tectonic sediments

Successive Rifting Events in Marginal Basins: The Example of the Coral Sea Region (Papua New Guinea)

International audienceReactivation of extensional structures is commonly inferred during rift evolution. In that context, we present original seismic interpretation to explore the geometry and interactions of three successive rifting events in the Coral Sea region, Papua New Guinea. The first event (R 1), poorly documented, occurred during the Triassic along an older N-S Permian structural fabric. During the Jurassic, extensional faults were reactivated through a second extensional episode (R 2), which formed small (~10/20 km) basins bounded by N-S, NE-SW, and E-W listric faults. Extension prolonged during the Lower Cretaceous with seafloor spreading in the Owen Stanley Oceanic Basin, now incorporated in the Papuan fold and thrust belts. A third Late Cretaceous extensional phase (R 3) gently reactivated some of the faults with very limited landward tilt in most basins and deformation located along the present continent-ocean transition. Seafloor spreading in the Coral Sea followed from Danian to Ypresian. This extensional system is sealed by unequally preserved Eocene strata that mark the onset of postrift thermal subsidence prior to the margin inversion from Oligocene onward. This overall evolution suggests various extensional systems that are geographically and temporarily defined the one another. The early rifting of the crust is controlled by preexisting continental features resulting in the local Pangaea breakup. In contrast, the Coral Sea propagator cuts through the rifted margin and is controlled by a subduction complex in accordance with the Tasman Sea opening. This evolution underlines the interactions existing between two extension modes in agreement with variations of the regional geodynamical setting around Australia

Evolutionary dynamics of rifted basins: new thoughts about the formation of long-lived passive margins

International audienceThe polyphased propagation of the deformation, generally suspected in rift systems, remains often poorly defined in terms of timing and fault reactivation and the role of transversal discontinuities in the crust is often poorly understood during the overall extensional history of a basin. Herein, we present some new concepts about the evolution of two well-separated rift propagators, the marginal basin of the Coral Sea (offshore Papua New Guinea), and the Porcupine Basin (offshore Ireland). Both basins are parts of long-lived rift systems that formed across former orogenic sutures in respect of local plate tectonics frameworks. On one side, the Coral Sea opened through the Australian Craton and the Tasmanides Orogen and its propagation ahead of the Tasman Sea was broadly controlled by the subduction of the Pacific Ocean. On the other side, the Porcupine Basin cut through the Variscides and Caledonides fold and thrust belts as a response of the North Atlantic Rift system. Despite these two different settings, our observation shows two common extensional modes for both systems. The first highlights the role of the generalised orogenic collapse in the initiation of the extension, by reactivating orogenic faults over several tens of millions of years. The second, called "real rifting", articulates within overprinted 10-to-20-Myrs-long extensional megacycles, each geographically and temporarily well defined and composed of diffused and then localised faulting events. Such a scenario implies to consider a multiphased extension, which provides variously-tilted and -oriented fault-blocks filled-in by several sedimentary sequences showing a vertical stack of syn- and post-rift unconformities. This geological architecture directly undersigns the dynamics of the continental crust in which transverse orogenic features have a dominant role onto the basin propagation by either activating or turning off the faults independently of the general plate tectonic force

Dynamics of dikes versus cone sheets in volcanic systems

International audienceSwarms of hundreds to thousands of igneous sheet intrusions represent the main magma pathways through the Earth’s brittle crust. Igneous 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, but yet no dynamic model simulates and predicts satisfactorily their diversity. Here we present scaled laboratory experiments that reproduced dike and cone sheet intrusion geometries under controlled conditions (Galland et al., 2014). The model rock is fine-grained, cohesive Coulomb crystalline silica flour. The model magma is a molten vegetable oil injected at constant flow rate. After the experiments, the oil solidifies and the intrusion is excavated to observe its shape

Toward a unified dynamic model for dykes and cone sheets in volcanic systems

International audienceIgneous sheet intrusions, such as dykes and cone sheets, represent various geometries of magma channels throughthe crust. In many volcanoes, they coexist as parts of complex plumbing systems and are likely fed by commonsources. How they form is fundamental regarding volcanic hazards, but yet no dynamic model simulates and predictssatisfactorily the diversity of sheet intrusions observed in volcanic systems. Here we present scaled laboratoryexperiments that reproduced dyke and cone sheet intrusion geometries under controlled conditions. Combined to aparametric study, a dimensional analysis shows that two dimensionless numbers 1 and 2 govern the formationof these intrusions. 1 is geometrical and describes the geometry of the magma source; 2 is dynamical and comparesthe local viscous stresses in the flowing magma to the host-rock strength. Plotting our experiments againstthese two numbers results in a phase diagram evidencing a dyke and a cone-sheet field, separated by a sharp transitionthat fits a power law. This result shows that dykes and cone sheets correspond to two distinct physical regimesof magma emplacement in the Earth’s crust. Cone sheets preferentially form when their source is shallow relativeto their size, when the magma influx (or viscosity) is large, or when the host rock is weak. In addition, both dykesand cone sheets may form from the same source, the shift from one regime to the other being then controlled bymagma dynamics, i.e. different values of 2. We compare our phase diagram to geological data and show that theextrapolated empirical dyke-to-cone sheet transition predicts the occurrence of dykes and cone sheets in variousnatural volcanic settings. This study thus provides a unified dynamic model of sheet intrusions emplacement andcaptures fundamental mechanisms of magma transport in the Earth’s crust