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Marine geophysical studies have revealed that crustal accretion processes at mid-ocean ridges are largely dependent on the relative contribution of magmatism and tectonism to plate separation. At high levels of magmatic accretion, the ridge is associated with large amounts of surficial volcanism and the formation of closely spaced, continuous abyssal hills that parallel the ridge axis. In contrast, when magmatic accretion is low, tectonic extension prevails and crustal structure is markedly different. In this latter scenario, long-lived detachment faulting forms oceanic core complexes (OCCs) that expose lower crustal rocks and mantle material at the seafloor. Whilst many of the general features of this ‘magma-poor’ terrain have been identified, there is still widespread debate as to the exact conditions that necessitate and regulate OCC formation. This study presents the results of a deep-towed and shipboard geophysical survey of the Mid-Atlantic Ridge between 13 – 14°N. This area is an ideal place to test and develop models for OCC structure and formation as the ridge axis exhibits multiple core complexes at various evolutionary stages. The data presented here include: deep-towed TOBI sidescan sonar and tri-axial magnetometer data, shipboard gravity and bathymetric data, and detailed seabed sampling results. High resolution sidescan sonar data reveal that active core complexes in this region are associated with a neovolcanic hiatus within the axial valley. In contrast, a recently terminated near-axis OCC is associated with widespread neovolcanism, thus confirming that OCC formation is regulated by variations in melt supply to the ridge axis. Forward modelling of gravity data shows that beneath the domal sections of OCC footwalls (from which serpentinised peridotites were ubiquitously sampled), a low density zone (LDZ) exists. This area is interpreted as comprising predominantly gabbroic material that has been captured by each detachment fault at depth beneath the ridge axis (thus explaining the gap in neovolcanism at the surface). Furthermore, magnetic data suggest that OCC footwalls are highly heterogeneous, reflecting significant compositional and thickness variations of the LDZ. Analyses of sidescan sonar imagery, combined with models of gravity and magnetic anomalies, reveal significant across- and along-axis asymmetry in the region. In general, the quadrants of the survey area within the inside-corners of the non-transform offset at 13°38’N are associated with widespread OCC formation, elevated tectonic strain (Tε = 25 – 30%), thin crust and faster spreading (20 – 40%) compared with conjugate ridge flanks; these areas are characterised by crust that is typically ~0.5 – 1.0 km thicker than OCC-forming areas, and tectonic strain (Tε = 10 – 15%) is partitioned across numerous, small faults. On the basis of these results, a model is presented for the structure of the 13 – 14°N region, and for the life cycle of oceanic core complexes. These models have implications for our current understanding of the magmato-tectonic conditions within which OCCs are expected to operate, and, in a broader sense, for the long wavelength processes that govern crustal accretion at low levels of magmatism.

Gas hydrates are natural ice-like solids formed from the combination of gas and water molecules. Hydrates could be considered a potential geohazard in the marine environment due to their possible inuence on seaoor stability in environments where social infrastructure such as telecommunication/IT cables is present. If hydrates dissociate there will be a reduction in sediment shear strength and an increased susceptibility to induced soft sediment deformation. This research aims to analyse the impact of hydrate dissociation coupled with seismic ground motion on the stability of submarine slopes as described by the Factor of Safety within the sediment column. A simple one-dimensional numerical model is developed that simulates heat flow and hydrate dissociation within the sediments. The dissociation model considers many competing factors such as seawater depth, increasing seabed temperatures, the natural geothermal gradient, the thermal properties of sediments, the quantity of gas hydrate and the rate of heat owing into and out of the hydrate during dissociation. The model also takes into account the changing salinity of the surrounding pore space fluids as the hydrate forms or dissociates. Estimates of pressure change during dissociation are derived from first principles based on conservation laws and the ideal gas law. The model of gas hydrate dissociation is used to inform estimates of shear strength applied to a slope stability model of continental slope sediments under earthquake loading. A sand-rich sequence is modeled. Although, during dissociation, pore-water pressure within the hydrate/sediment mix was found to increase by approximately 10%, this resulted in a reduction of effective stress to almost zero within the shallower sediments, causing slope instability in the depth range 3-7 mbsf in water depths of 600m and an increase in the ground wave acceleration by an average of 0:01m². The greatest reduction in the factor of safety (0:08) and the greatest increase in acceleration (0:016753m²) was shown to occur 3mbsf. It was determined that the dissociation of hydrate without drainage increases the likelihood of slope failure. This in turn leads to instability in the sediments on slopes as shallow as 1.32 deg in the model. These model results are compatible with previous studies of the shallow AFEN slide, NW Scotland.

This research demonstrates the utility of repeat acoustic and photographic surveys for monitoring biotopes and hard-substratum communities in temperate waters. Kappa analysis of results derived from time-lapse backscatter surveys conducted at Church Bay, Rathlin Island (1999,2008 and 2009) indicate that the spatial distribution of biotopes is highly similar between each biotope map, possibly due to the stability of bedforms shaped by the local tidal regime. Maximum change is represented by seasonal and annual changes in the growth of Zostera marina. Changes induced by kelp harvesting are not detected in backscatter data. Low-cost photo-mosaics derived from stills and video imagery of fixed quadrats (1 OO-x-l 00 cm) are compared. Although video-collection is quicker, more species and less-conspicuous taxa are identified from stills imagery. Data extraction using point-intercepts proves efficient and generates data sensitive to 10% community change (N=16), but detects fewer species than the visual estimation measure. Frequency-of-occurrence extraction measures significantly over-estimate benthic cover. Seasonal growth, recruitment and mortality of Caryophyllia smithii, Hymeniacidon perleve and Plocamium cartilagineum are estimated to nearest 0.04 ern? month-l using digitisation. The presence of algal canopies reduce the accuracy of sessile-invertebrate cover estimates in spring and summer. Exploratory analysis of results from photoquadrat surveys conducted at Damicornis Bay, Rathlin Island (2009 and 2010), identifies six distinct sub-communities on a boulder slope. Community variability obscures detection of change. However, analysis of the most dominant sub-community indicates that boulder-slope community shifted, possibly due to local erosion of sediment. Optimal balance between precision, efficiency and species-detection ability for photoquadrats (25-x-25 cm) is 50 points-per-image. Functional groups retain structural relevance of communities, while improving precision, sensitivity and efficiency of data extraction and community-representation measures relative to full-species compositions. Continued monitoring of hard-substratum communities using high-resolution photoquadrats and fixed-photomosaics is recommended.

Fluid inclusions offer the only available samples of uncontaminated sub-seafloor fluids. To date, microthermometry of such fluid inclusions trapped in ocean crust rocks has revealed that fluids of a wide range of salinities exist in both modem and ancient hydrothermal systems. LAICPMS analyses of fluid inclusion chemistry are reported here.

Many aquatic environments have soft, muddy substrates, but this important property has largely been ignored in process-based models of turbidity currents. Previous turbidity current studies have focussed on flows over hard or non-cohesive, movable substrates. However, the flume experiments conducted in this study show that soft mud beds interact differently with a passing flow. Flow-bed interaction caused deformation and erosion of the beds and changed the flow structure of the turbidity currents. Subsequently, these changes might impact the geometry of the resulting turbidite deposits. Five different interaction types bet: ween turbidity currents and soft substrates were defined: 1) no interaction, 2) interfacial waves, 3) mixing and erosion, 4) severe mixing and erosion, and 5) leading wave formation. The now geometry and velocity, turbulence and concentration profiles of flows with no flow-bed interaction resembled those of flows over a hard substrate. But with increasing intensity of the flow-bed interaction, the friction at the flow-bed interface and erosion depth increased, tl1e geometry of tl1e front of the flow changed from blunt to pointed, and leading waves increased in size. This resulted in a decrease in the flow velocity, an increase in the height of the maximum velocity, an increase in near-bed turbulence, and an increase in flow concentration. These changes were strongest in the head of the flows, where the highest intensity of flow-bed interaction occurred. To study spatio-temporal changes in the now structure of turbidity currents, a turbidity current model was created using image analysis on videos taken during the experiments. Results of the model indicated a trend in the head velocity of turbidity currents overriding a soft bed, with an initial decrease in flow velocity followed by an increase until a constant flow velocity was reached. The experimental results were summarised in flow-bed interaction phase diagrams which showed a decrease in flow-bed interaction with increasing yield strength of the bed and an increase in flow-bed interaction with an increase in bed shear stress. For the flow-bed concentration ratios used in this study, the bed shear stress had the largest influence on the flow-bed interaction. Fieldwork was conducted in the Gres de PeYra-Cava (France) which focussed on the lower bed boundary characteristics of turbidite deposits with an underlying mud bed. Flat, non-eroded bed boundaries were inferred to result from flows with no interaction with the bed, while wavy, non-eroded boundaries were believed to be an indication for interfacial wave interaction. Flame and loading structures were interpreted as products of mixing and erosion and sediment injections were related to leading waves, although no direct evidence for this was found in the field. Predictions of turbidite deposit geometry and composition based on the results of this study show a large variation in deposit thickness, run-out length and composition which is of great importance to the hydrocarbon industry.

L'objectif de ce travail est d'évaluer les rôles respectifs des facteurs environnementaux et de la tectonique synsédimentaire en milieu de plate-forme propice à l'accumulation de matière organique. La zone atelier est le Jurassique supérieur du Boulonnais et ses dépôts terrigènes (grès et marnes), accumulés dans un contexte tectonique extensif, à une époque où les variations du niveau marin pouvaient être importantes. Nous montrons que les bancs calcaires et les récifs à huîtres que nous avons étudiés dans ces formations sont d'origine diagénétique très précoce et qu'ils se sont formés à l'interface eau-sédiment ou juste au-dessous. Ces carbonates sont induits par des réactions chimiques bactériennes rendues possibles par des circulations de fluides de long des failles synsédimentaires. Nos résultats montrent également que les conditions de préservation de la matière organique sont soumises à de nombreux facteurs mais nous insistons ici sur le rôle du fer qui, par sa présence, peut empêcher les réactions de sulfuration entre les ions sulfures et les molécules organiques. De ce fait, cette protection naturelle par sulfuration est empêchée et la matière organique est plus intensément reminéralisée. Les sources de fer sont multiples en milieu de plate-forme mais nos travaux suggèrent que les smectites puissent véhiculer du fer alors qu'elles sont souvent interprétées comme des vecteurs de matière organique. Enfin, nous montrons que des gradients de distribution de la smectite peuvent être très marqués sur de courtes distances, malgré la flottabilité de ce minéral. Cette information est utile pour les reconstitutions fondées sur les argiles. / The goal of this study was to evaluate the respective roles of environmental factors and synsedimentary tectonic in a platform environment favorable to the accumulation of organic matter. The study area is the Upper Jurassic of the Boulonnais (Northern France) and the terrigenous deposits (sandstone and marl) accumulated in an extensive tectonic context, at a time when the sea level variations could be noticeable. We show that the limestone beds and oyster reefs that we studied are of earliest diagenetic origin and that they were formed at the water-sediment interface or just beneath. These carbonates are induced by bacterial chemical reactions enabled by fluid circulation along synsedimentary faults. Our results also show that the conditions of preservation of organic matter is subject to many factors but we focus here on the role of iron which, by its presence, can prevent sulfuration reaction between sulfide ions and organic molecules. Thus, this natural protection by sulfuration is prevented and the organic material is more intensely mineralized. The sources of iron are multiple in a platform environment but our work suggests that smectites can convey iron while they are often considered to be organic-matter vectors. Finally, we show that smectite distribution gradients can be very marked over short distances in spite of the buoyancy of this mineral. This information is useful for clay-based reconstructions.

L’Europe occidentale enregistre d’importants changements paléogéographiques entre la fin du Mésozoïque et le Paléogène, en réponse à des évènements géodynamiques en limites de plaques. Les déformations affectant le NW de la plaque européenne ont déjà bien été identifiées dans plusieurs bassins, mais elles sont en revanche mal contraintes en Manche. L’objectif de cette thèse est ainsi de reconstruire les évolutions sédimentaires et structurales de la Manche orientale au cours du Paléogène et de les replacer dans le contexte géodynamique du domaine Ouest-Européen à travers l’étude du Bassin de Dieppe-Hampshire, dont la stratigraphie et le calendrier des déformations ne sont pas précisément connus. Ces travaux s’appuient sur l’apport de nouvelles données de sismique réflexion très haute résolution et de nouveaux carottages. L’analyse du remplissage sédimentaire a permis d’établir le cadre stratigraphique, de proposer un découpage séquentiel avec l’identification de quatre cycles de 2ème ordre entre le Thanétien et le Bartonien, et de cartographier la partie méridionale du bassin. La corrélation de ces cycles avec les bassins cénozoïques proches a permis de mettre en évidence cinq phases de déformation ayant affecté le Bassin Anglo-Parisien. Grâce à une analyse structurale fine, nous avons également pu détailler le style et la cinématique de la déformation en Manche orientale, proposer un calendrier de cette déformation, et enfin établir le rôle joué par l’héritage structural régional sur l’évolution paléogène du domaine. / Major paleogeographic changes are recorded within Western Europe between the end of Cretaceous and Paleogene times, in response of tectonic events along European plate boundaries. The deformation affecting the NW part of the European plate are already well identified in the Paris Basin, the Belgian Basin and in Southern England, however it is poorly constrained within the English Channel. The objective of this thesis is thus to reconstruct sedimentary and structural evolutions of the Eastern English Channel during Paleogene times, and to replace these evolutions within the geodynamic context of the Western European domain. This thesis has been focalized on the Dieppe-Hampshire Basin, for which the stratigraphy and the timing of the deformation are not precisely known. This work is based on the contribution of new very high resolution seismic reflection data and of new coring samples. The analysis of the sedimentary filling allowed to establish the stratigraphic framework, to propose a sequence stratigraphic division with the identification of four 2nd order cycles between the Thanetian and the Bartonian, and to map the southern part of the basin. The correlation of these cycles with surrounding Cenozoic basins led to evidence five deformation phases which have affected the Anglo-Paris Basin. Thanks to a careful structural analysis, we also detailed the style and the kinematics of the deformation in the Eastern English Channel, we proposed a deformation schedule and we established the role played by the regional structural inheritance on the Paleogene evolution of the domain.

L’objectif de ce travail est d’étudier la sédimentologie des dépôts organiques dans un bassin hémipélagique (séries mésozoïques du Bassin du S-E de la France) afin de comprendre les processus de transport et de préservation de la matière organique (MO), ainsi que sa distribution dans les différents cortèges sédimentaires. Les résultats démontrent qu’il n’y a pas de relation entre le cortège organique et la granulométrie de la fraction terrigène ou la minéralogie de la fraction argileuse. Des turbidites sont présentes dans un niveau organique particulier, mais elles n’ont pas acheminé de MO terrestre dans le bassin ; au contraire, elles ont eu un rôle négatif sur la préservation du contenu organique. De la MO terrestre est observée dans certains niveaux organiques, mais sans lien avec des dépôts gravitaires, signifiant donc que la MO terrestre peut être transportée vers le domaine marin distal sans nécessairement impliquer des processus gravitaires. Il n’y a pas de lien évident entre la nature de la MO et les cortèges de dépôts dans les séquences stratigraphiques de 3e ordre. L’apport majeur de ce travail concerne l’approche multi-facteurs de la sédimentologie de la MO en domaine marin. Pour une productivité primaire marine constamment faible à normale, il a été mis en avant le rôle de 3 facteurs clés pour la formation de dépôts organiques en bassin hémipélagique : le taux de sédimentation, la nature intrinsèque de la MO, et les conditions d’oxygénation du bassin. Les niveaux organiques les plus riches en MO sont les événements anoxiques globaux, démontrant que l’anoxie est le facteur permettant les plus fortes accumulations organiques dans ce type de bassin sédimentaire. / The goal of this work is to study the sedimentology of organic deposits in a hemipelagic basin (Mesozoic deposits of the S-E France Basin) in order to understand the processes responsible for transport and preservation of organic matter (OM), as well as its distribution in various sedimentary settings. The results demonstrate that there is no relationship between the organic content and the grain size of the terrigenous fraction or the mineralogy of the clay fraction. Turbidites are present in a particular organic level, but they did not convey terrestrial OM in the basin; on the contrary, they had a negative role upon the preservation of the organic content. Terrestrial OM is observed in some organic levels, but not related to gravity deposits, meaning that terrestrial OM can be transported to the distal marine domain without necessarily involving gravity processes. There is no clear link between the nature of the OM and the place of the deposits in the 3rd order stratigraphic sequences. The major contribution of the present work concerns the multi-factor approach to the sedimentology of OM in the marine domain. For a constantly low to normal marine primary productivity, it was highlighted the role of 3 key factors for the formation of organic deposits in the hemipelagic basin: the sedimentation rate, the intrinsic nature of OM, and the basin oxygenation conditions. The organic levels the richest in OM are the global anoxic events, which show that anoxia is the factor allowing the strongest organic accumulations in this type of sedimentary basin.

Le bassin de Cariaco représente un emplacement idéal pour l’étude de la préservation de la matière organique (MO) en particulier la sulfuration naturelle, car il a présenté des conditions euxiniques récurrentes au cours de son histoire. Pour contraindre les processus de préservation de la MO, nous avons étudié les sédiments varvés du bassin de Cariaco sur une période de 130 ka. Nous avons mis au point un protocole basé sur des hydrolyses séquentielles permettant d’isoler des kérogènes libres d'artefacts. Ce protocole a permis de libérer d’importantes quantités de carbohydrates représentant jusqu'à 3% du carbone organique total. Ces carbohydrates d’origine algo-bactérienne présentent une diminution en profondeur lié à des processus de dégradation in situ et/ou à l’incorporation dans la matrice macromoléculaire. L’étude des marqueurs moléculaires présents dans les extraits organiques corrobore l’origine principalement autochtone de la MO. Cependant la contribution des végétaux terrestres a été mise en évidence dans les échantillons correspondant aux périodes glaciaires. Le processus de sulfuration naturelle a un rôle important dans la préservation de la MO à Cariaco. La sulfuration intramoléculaire est indiquée par la présence des thiophènes et thiolanes à l’état libre. La désulfuration au nickel de Raney et la pyrolyse des kérogènes révèlent également une sulfuration intermoléculaire. La sulfuration ne présente pas une tendance générale à l’augmentation dans l’intervalle étudié. Le processus se révèle sensible aux variations climatiques, et facteurs tels que les conditions redox du milieu et la disponibilité et réactivité des précurseurs moléculaires. / The Cariaco basin provide an ideal location to study the organic matter (OM) preservation processes in particular the natural sulfurization, since it has presented recurrent euxinic conditions along its history. To better understand the OM preservation processes, we have studied the varved sediments of Cariaco in a time interval of 130 ky. In this study, we have employed an artifact free method based on stepwise hydrolysis for the isolation of kerogen. This method permitted to obtain important amounts of monomeric carbohydrates, accounting for up to 3% of the total organic carbon. Those algal/bacterial-derived carbohydrates present a decrease with depth related to in situ degradation and/or incorporation to the macromolecular matrix. The investigation of the organic extracted biomarkers corroborate the authochtonous origin of the OM. Nevertheless, an important contribution of terrestrial OM was observed in the samples from glacials periods. The natural sulfurization process have an important role in the preservation of OM in Cariaco basin. The intramolecular sulfurization was revealed by the presence of free alkylthiophenes and thiolanes. Raney nickel desulfurization and kerogen pyrolysis also revealed intermolecular sulfurization. The natural sulfurization does not present a global increase in the time interval studied. This process respond to variations in climatic conditions, redox conditions in the water column and sediments and also to the reactivity and availability of the molecular precursors.