Héritages glaciels sur les côtes du massif Armoricain, France

La présence de blocs erratiques a fréquemment été observée sur les fonds de la Manche et sur les côtes du massif Armoricain. Leur répartition et leur nature pétrographique permet de leur attribuer une origine glacielle, avec des déplacements locaux, régionaux ou lointains (basaltes) par des glaces de rivière, des glaces marines ou encore des icebergs. Les dérives sont commandées par les vents dominants assistés par les courants de marées. Un épisode littoral glaciel a pu être reconnu dans les formations littorales contemporaines d'une brève transgression lors de la fin du dernier interglaciaire; il est associé à un faciès sédimentaire d'estran comparable à ce qui peut être observé actuellement en milieux à couverture glacielle saisonnière. Cette dynamique littorale a pu exister à maintes reprises au cours du Quaternaire sur les côtes de la Manche. Elle pourrait peut-être expliquer une part du façonnement des plates-formes littorales qui bordent cette région.Occurrence of erratic boulders on the floor of the English Channel and along the shorelines of the Massif Armoricain is a common feature. The distribution and the lithology of these erratics suggest ice rafting as the main transport mechanism and dispersion from local, regional and far away (basalt) sources areas. They were transported either by river ice, sea ice or icebergs. Drifting direction was controlled by prevailing winds assisted by tidal currents. One episode of shore rafting has been identified in shore formations and associated to a short transgression close to the end of the last interglacial. The observed relict sedimentary faciès is comparable to modern shore environments influenced by seasonal ice processes. Sea ice action on shore dynamics is believed to have taken place repeatedly during the Pleistocene along the coast of the English Channel. It could partly be responsible for the shaping of the shore platforms in the area

Le prélude au Quaternaire : les modalités d'entrée en glaciation (65 Ma - 2,2 Ma)

Le Cénozoïque enregistre une englaciation croissante par étapes, d’abord dans l’hémisphère sud puis, depuis le début du Néogène, dans l’hémisphère nord. Cette évolution est en relation surtout avec le rifting et la tectonique des plaques qui ouvrent ou ferment des détroits. La création de reliefs de collision ou de reliefs de décharge isostatique par érosion crustale ou superficielle, après le Crétacé, favorise l’englaciation. L’extension du plancher océanique et le stockage croissant de glace amènent une baisse eustatique importante. Les modifications des circulations océaniques et atmosphériques, que les reliefs entraînent, accentuent le refroidissement par le biais de la consommation du CO2 atmosphérique. La Terre passe d’un régime d’englaciation unipolaire à un régime bipolaire à partir du Néogène, d’une circulation océanique zonale à la circulation thermohaline que nous connaissons aujourd’hui. À partir de 3 Ma, en relation avec la configuration particulière des océans et des continents, le régime glaciaire est mondial, contrôlé préférentiellement par l’intensité de l’insolation, avec également un retour de la convergence intertropicale en position équatoriale. L’augmentation de l’aridité a influencé la biodiversité et la production de poussières. À la veille du Pléistocène, l’ère glaciaire est largement commencée : l’Antarctique passe par un maximum d’englacement au Miocène alors qu’il faut attendre le Pliocène pour voir de vraies calottes s’installer dans l’hémisphère nord.The Cainozoic era records a step-like glacierization, first on the southern hemisphere and since the onset of Neogene on the northern hemisphere. This evolution is linked to the rifting and to plate tectonic that opened or closed several straits. Since the Cretaceous, the creation of collisional or isostatic deloading reliefs thanks to underplating or superficial erosion favoured the glacierization. Sea floor spreading and ice storage induced an important sea-level drop. Modifications of the marine and atmospheric circulations induced by the new-born reliefs intensified the cooling trend. The Earth shifted from an unipolar glaciation to a bipolar one from the Neogene, from a zonal oceanic circulation to the present thermohaline circulation throughout CO2 consumption. From 3 Ma, in relation with the land masses and oceans configuration, the glacierization is global with a strong control by insolation strength, and an equatorial positioning of the Intertropical convergence zone (ITCZ). The increasing aridity influenced the biodiversity and dust production at least since the Oligocene. At the onset of the Pleistocene, the glacierization is mostly completed: the Antarctic ice maximum occurred during the Miocene, although true ice sheets only developed from the Pliocene in the northern hemispher

Enlargement of the active rift during glaciations

During the last glaciation, an ice sheet covered Iceland approximately 1000 m thick. A reconstruction of the ice flow lines shows that the ice sheet was partly drained through fast-flowing streams. The major drainage routes correlate with locations of geothermal anomalies, suggesting that ice stream activity was favoured by water produced in regions of high geothermal heat flux. A widening of active rift zone was also deduced revealing a coupling between deep and surface processes

Fast and partitioned postglacial rebound of southwestern Iceland

International audienceLocated both on the Mid-Atlantic Ridge and above a mantle plume, Iceland is subject to horizontal and vertical motions. Many studies described these deformations in terms of rifting episodes that have combined both extensional tectonics and magmatism. However, few studies have described the glacio-isostatic response induced by the retreat of the Weichselian ice cap. The melting of this ice cap induced a postglacial rebound for the whole of Iceland that may be controlled by the geodynamic setting and the rheological layering of the lithosphere. This study is devoted to (1) understanding the Holocene rebound on the southwestern coast and (2) estimating the asthenosphere viscosity and depth beneath Iceland. Two stages of holocene evolution were determined by means of GPS profiles, morphological observations, and data compilation. The first stage corresponds to a vertical uplift of 67.5 to 157.5 m. It started at 10,000 years BP and ended at 8500 years BP implying uplift rates between 4.5 and 10.5 cm/a. It was a quick isostatic response to the fast ice retreat. The second stage had vertical motion of tens of meters with a probable tectonic origin and started at 8500 years BP. The uplift rate is 1 to 2 orders of magnitude slower than the one during the first stage. Uplift partitioning during the first stage was controlled by the thermal state of the lithosphere, the highest geothermal flux inducing the maximum uplift rates. The relaxation time for uplift provides a viscosity estimate of 5.4­5.8 — 1019 Pa s for the asthenosphere. This value is similar to those determined for glacial areas in different continental contexts. However, the flexural wavelength indicates a shallower asthenosphere than that occurring in continental domains. Therefore this study highlights a coupling between the thermal structure of the Icelandic asthenosphere and the glacial rebound

Periglacial morphogenesis in the Paris Basin: insight from geophysical survey and consequences for the fate of soil pollution.

International audienceGeophysical survey by of the Pierrelaye-Bessancourt area revealed conductive polygon patterns of 20-30 m diameter detected between 0.5 and 1.7 m depth. The patterns are formed by greenish glauconite and carbonated sand hollows where clay-rich pedological horizons bend downward, forming narrow tongs extending up to 2-3 m depth. Such structures were interpreted as a buried polygonal ice-wedge network. Geometrical relationships between the lithological units allowed the identification of successive landscape events and a landscape chronology. The sequence started during the Saalian glaciation with (1) development of patterned grounds by thermokarstic cryoturbation; (2) consecutive deflation/erosion during post-permafrost aridity; (3) loess and eolian sand deposits; (4) weathering of the former deposits with development of pedogenic horizons during the Eemian interglacial; (5) recurrent cryoturbation and thermal cracking leading to infolding of the pedogenic horizons during the Pleniglacial optimum (Weichselian); (5) finally erosion that levelled the periglacial microreliefs, leading to the modern landscape. In this agricultural area, urban waste water has been spread and has led to high levels of metal pollution in the surface horizons of the soils. The polygonal cryogenic structures have major impacts on soil hydrology and dispersion/distribution of heavy metals toward the geological substrate

The Late Palaeozoic glaciation subsurface record, Chaco Basin (Bolivia)

Late Palaeozoic glaciation is the longest of the Phanerozoic era. It is recorded in numerous Gondwanian basins, some having a high petroleum potential like the Chaco Basin. In this basin, the quality of the available seismic, well and outcrop data permits to characterise the Late Palaeozoic glacial record. Palaeovalleys >500 m deep and ~7 km wide have here been analysed. Focusing on the glaciogenic Carboniferous deposits, the seismic data with well-ties and their outcrop analogues provide new sedimentological insights. The palaeovalley infill is imaged as a chaotic seismic facies overlain by an aggrading-prograding prism, interpreted as tillites covered by a fluvio-deltaic system respectively. Tillites form both under the ice and during rapid ice recession whereas fluvio-deltaic systems can only originate from a stable ice margin and last until the ice sheets withdraw inland. These two depositional modes are repeated several times generating the progressive burial of the Carboniferous palaeovalleys. This succession of erosions and fills records major glacial stages containing a series of glacial and interglacial phases from the Late Devonian to the Early Permian. Depicting the Late Palaeozoic glacial history of the Chaco Basin seems crucial for the localisation of potential good reservoirs

The quaternary of Britanny: Guidebook of the excursion in Britanny 12-15 September 1997

GuidebookDonnées générale sur la géologie de la Bretagne: socle, évolution paléogène, néogène et quaternaire (loess, niveaux marins). Descriptions des coupes depuis la baie de St Brieuc, le Finsitère et le Morbihan. Sites archéologiques de Piégu, Barnenez, Menez Drégan

Trente Ans de l'Evolution Spatiotemporelle du Glissement De "Bou Halla" à l'Aide de la Photogrammétrie Numérique et du MNT

International audienceLa caractérisation géométrique et cinématique, à différentes échelles d'un glissement de terrain, est une étape indispensable à la compréhension de la dynamique d'un tel phénomène. La télédétection spatiale est un outil particulièrement bien adapté pour cette caractérisation puisqu'elle permet d'obtenir une vision synoptique de l'édifice, à différentes échelles spatiales (du centimètre à la dizaine de mètres) et temporelles (quelques heures à plusieurs années), en s'affranchissant des problèmes d'accès. Jusqu'à ce jour peu de travaux ont mis à profit les avantages de la photogrammétrie numérique dans le domaine des risques géomorphologiques et des mouvements de terrains en particulier. La tâche principale restant pour l'instant est le développement des techniques et des matériels. Plusieurs géologues ont défini selon cette méthode les vecteurs de déplacements d'un certain nombre de repères naturels identifiés sur deux clichés aériens. Ceci a permis aussi d'établir des cartes de déformations superficielles et de mesurer le déplacement d'éléments remarquables de la surface topographique sur des sites instables (Casson et al., 2005 ; Van Ash et al., 2006 ; et Méric et al., 2007 ...). L'originalité de notre travail est basée sur la construction automatique des MNTs à partir des couples stéréoscopique, et l'orthorectification des photoaériennes permettant de projeter chaque image dans un même référentiel, afin de poursuivre l'activité spatiotemporelle d'un mouvement de terrain

Preliminary results of a paleoseismological analysis along the Sahel fault (Algeria): New evidence for historical seismic events

International audienceThe ∼60 km-long Sahel ridge west of Algiers (Tell Atlas, north Algeria) is considered as an ENE-WSW fault-propagation fold running along the Mediterranean coast and associated with a north-west dipping thrust. Its proximity with Algiers makes this structure a potential source of destructive earthquakes that could hit the capital city, as occurred in 1365 AD and 1716 AD. The first paleoseismologic investigation on the Sahel ridge was conducted in order to detect paleo-ruptures related to active faulting and to date them. From the first investigations in the area, a first trench was excavated across bending-moment normal faults induced by flexural slip folding in the hanging wall of the Sahel anticline thrust ramp. Paleoseismological analyses recognize eight rupture events affecting colluvial deposits. 14C dating indicates that these events are very young, six of them being younger than 778 AD. The first sedimentary record indicates two ruptures before 1211 AD, i.e. older than the first historical earthquake documented in the region. Three events have age ranges compatible with the 1365, 1673 and 1716 Algiers earthquakes, whereas three other ones depict very recent ages, i.e. younger than 1700 AD. Potential of these secondary extrados faults for determining paleoseismic events and thrust behaviour is discussed

Neogene and Pleistocene geodynamics: the paleoseismic evolution of Armorica (Western France)

The evolution of the passive Armorican margin (Western France) during the Neogene and Quaternary was analyzed using field data. The morphology of the margin attests to a late Hercynian shaping, further deformation during the Mesozoic mid-Atlantic opening, during the Alpine Orogeny, and ultimately, a Late Cenozoic uplift, mostly related to an onshore isostatic accommodation in response to erosion and limited tectonic activity. A very limited strike–slip dynamic, with very low seismicity, accommodated the Neogene–Pleistocene N170 strains around the rigid Armorican terrane. The South Armorican domain and English Channel floor include shear zones that adjusted the Alpine convergence, facilitating its transpressive slip to the west. The Permo-Triassic N150 faults were reactivated during the inversion phases that began after the Bartonian under the distal control of the Alpine convergence and the decrease in the Atlantic spreading rate after 34 Ma. The Armorican marine platforms were stable after the late Eocene and slightly subsident, experiencing pulsed episodes of transient lithospheric doming during the Neogene and Quaternary. Co-seismic activity onshore without surface rupture was recorded around ∼5.3 Ma, ∼3.7 Ma, ∼2.4–1.2 Ma, and ∼400–250 ka, in tandem with an inland exhumation driven by isostatic adjustment due to an intensification of periglacial erosion at the onset of the early interstadials or by agriculture. Low-magnitude and ubiquitous shallow seismic activities seem to be related today to an isostatic uplifted old brittle–ductile transition due to the accumulation of shearing strain