Widespread active seepage activity on the Nile Deep Sea Fan (offshore Egypt) revealed by high-definition geophysical imagery

Fluid escape structures on the Nile Deep Sea Fan were investigated during the MEDIFLUX MIMES expedition in 2004. Mud volcanoes, pockmarks and authigenic carbonate structures were surveyed for the first time with a high-resolution deep-towed 75 kHz sidescan sonar and a 2–8 kHz Chirp sediment echosounder. In combination with existing multibeam bathymetry and detailed seafloor in situ geological observations, these new data allowed detailed seep analyses. About 60 gas flares were detected acoustically in the water column from the sidescan sonar raw data atwater depths from 770 to 1700 m. These gas flares coincide at the seabed with 1) the centres of the mud volcanoes where mud is also extruded, 2) the borders of the mud volcanoes where the emitted gases contribute to the precipitation of authigenic carbonates, and 3) to the edges of broad sheets of authigenic carbonates. Subsurface sediments are commonly disturbed by ascending fluids throughout the delta, with an abundance of seep-related carbonate structures on the seafloor. The feeder channels below mud volcanoes, similar to the gas conduits below the widespread carbonate crust structures and pockmarks, are relatively narrow and, for the vast majority of them, do not exceed a few metres in diameter. The seeps on the Nile Deep Sea Fan clearly follow lineations on the seafloor that ee can relate to faults

Phanerozoic geological evolution of the Equatorial Atlantic domain

The Phanerozoic geological evolution of the Equatorial Atlantic domain has been controlled since the end of Early Cretaceous by the Romanche and Saint Paul transform faults. These faults did not follow the PanAfrican shear zones, but were surimposed on Palaeozoic basins. From Neocomian to Barremian, the Central Atlantic rift propagated southward in Cassiporé and Marajó basins, and the South Atlantic rift propagated northward in Potiguar and Benue basins. During Aptian times, the Equatorial Atlantic transform domain appeared as a transfer zone between the northward propagating tip of South Atlantic and the Central Atlantic. Between the transform faults, oceanic accretion started during Late Aptian in small divergent segments, from south to north: Benin-Mundaú, deep Ivorian basin-Barreirinhas, Liberia-Cassiporé. From Late Aptian to Late Albian, the Togo-Ghana-Ceará basins appeared along the Romanche transform fault, and Côte dÍvoire-Parà-Maranhão basins along Saint Paul transform fault. They were rapidly subsiding in intra-continental settings. During Late Cretaceous, these basins became active transform continental margins, and passive margins since Santonian times. In the same time, the continental edge uplifted leading either to important erosion on the shelf or to marginal ridges parallel to the transform faults in deeper settings

Multi-disciplinary investigation of fluid seepage on an unstable margin: The case of the Central Nile deep sea fan

We report on a multidisciplinary study of cold seeps explored in the Central Nile deep-sea fan of the Egyptian margin. Our approach combines in situ seafloor observation, geophysics, sedimentological data, measurement of bottom-water methane anomalies, pore-water and sediment geochemistry, and 230Th/U dating of authigenic carbonates. Two areas were investigated, which correspond to different sedimentary provinces. The lower slope, at ∼ 2100 m water depth, indicates deformation of sediments by gravitational processes, exhibiting slope-parallel elongated ridges and seafloor depressions. In contrast, the middle slope, at ∼ 1650 m water depth, exhibits a series of debris-flow deposits not remobilized by post-depositional gravity processes. Significant differences exist between fluid-escape structures from the two studied areas. At the lower slope, methane anomalies were detected in bottom-waters above the depressions, whereas the adjacent ridges show a frequent coverage of fractured carbonate pavements associated with chemosynthetic vent communities. Carbonate U/Th age dates (∼ 8 kyr BP), pore-water sulphate and solid phase sediment data suggest that seepage activity at those carbonate ridges has decreased over the recent past. In contrast, large (∼ 1 km2) carbonate-paved areas were discovered in the middle slope, with U/Th isotope evidence for ongoing carbonate precipitation during the Late Holocene (since ∼ 5 kyr BP at least). Our results suggest that fluid venting is closely related to sediment deformation in the Central Nile margin. It is proposed that slope instability leads to focused fluid flow in the lower slope and exposure of ‘fossil’ carbonate ridges, whereas pervasive diffuse flow prevails at the unfailed middle slope

Kinematics of the Southern Rhodope Core Complex (North Greece)

The Southern Rhodope Core Complex is a wide metamorphic dome exhumed in the northern Aegean as a result of large-scale extension from mid-Eocene to mid-Miocene times. Its roughly triangular shape is bordered on the SW by the Jurassic and Cretaceous metamorphic units of the Serbo-Macedonian in the Chalkidiki peninsula and on the N by the eclogite bearing gneisses of the Sideroneron massif. The main foliation of metamorphic rocks is flat lying up to 100 km core complex width. Most rocks display a stretching lineation trending NEâ SW. The Kerdylion detachment zone located at the SW controlled the exhumation of the core complex from middle Eocene to mid-Oligocene. From late Oligocene to mid-Miocene exhumation is located inside the dome and is accompanied by the emplacement of the synkinematic plutons of Vrondou and Symvolon. Since late Miocene times, extensional basin sediments are deposited on top of the exhumed metamorphic and plutonic rocks and controlled by steep normal faults and flat-ramp-type structures. Evidence from Thassos Island is used to illustrate the sequence of deformation from stacking by thrusting of the metamorphic pile to ductile extension and finally to development of extensional Plio-Pleistocene sedimentary basin. Paleomagnetic data indicate that the core complex exhumation is controlled by a 30� dextral rotation of the Chalkidiki block. Extensional displacements are restored using a pole of rotation deduced from the curvature of stretching lineation trends at core complex scale. It is argued that the Rhodope Core Complex has recorded at least 120 km of extension in the North Aegean, since the last 40 My

Les marges continentales transformantes ouest-africaines. Côte d’Ivoire, Ghana, Guinée. Campagne EQUANAUTE 3 juin - 2 juillet 1992

Nineteen scientific dives have been performed on board the submarine "Nautile" during the EQUANAUTE cruise (june 3 to july 2 - 1993) along three main areas off western Equatorial Africa ; fourteen dives concern the deepest parts of the Côte d'Ivoire and Ghana transform margin ; three dives were made on a wide volcanoe along the southern Guinean margin ; two dives were devoted to a deep submarine relief along the extinct Romanche fracture zone. This report gives a brief review of direct in situ observation as well a synthetic geological cross section for each dive.  Dix-neuf plongées scientifiques ont été effectuées, à bord du submersible "Nautile" porté par le N/O "Nadir", au cours de la campagne ÉQUANAUTE du 3 juin au 2 juillet 1993, au long de trois régions caractéristiques de la façade africaine équatoriale de l'Atlantique : quatorze plongées ont concerné la marge profonde de Côte d'Ivoire - Ghana, trois plongées la marge méridionale de la Guinée, deux plongées ont été réalisées sur un relief profond jalonnant le tracé de la zone de fracture de la Romanche. Cet ouvrage présente de manière synthétique les principales observations et mesures provenant de cette campagne

1975-1995, vingt ans de recherches sur les marges continentales « transformantes »

Transform, or sheared, continental margins are generated at first as consequences of large-scale intra-continental strike-slip displacements, then as the results of successive shear motions between progressively thinning continental crusts and, finally, active motions along oceanic lithosphere. They, later on, give birth to oceanic fracture zones before to become seismically inactive and thus passive continental margin segments. In this paper we review how the concept of transform margin has progressively merged, mainly in France, during the seventies. We then show how a full program of sea-going researches, devoted to the progressive in situ analysis of one of the major transform margin, the Côte d’Ivoire/Ghana margin, has been built and managed during twelve years, between 1983 and 1995. Since that time, no specific systematic complementary studies were conducted on transform margins; only recent promising oil industry discoveries in such specific setting have reactivated new scientific interests on the study of transform margin structures and of their evolution.1975-1995, vingt ans de recherches sur les marges continentales « transformantes » Résumé. Les marges continentales dites transformantes résultent des effets d'un grand mouvement de coulissage lithosphérique, d'abord intracontinental, puis mettant successivement en contact actif des lithosphères continentales amincies et enfin océaniques ; finalement ce mouvement donne naissance à d'importantes zones de fracture océaniques, alors que le segment de marge créé devient, quant à lui, passif après avoir été pendant un grande partie de son évolution tectoniquement et sismiquement actif. Cet article relate les différentes phases de la genèse, en France, au cours des années 1970, du concept de « marge transformante » ; il présente aussi la séquence des campagnes à la mer qui, pendant une douzaine d'années (entre 1983 et 1995), se sont succédé afin d'analyser la marge continentale au large de la Côte d'Ivoire et du Ghana, considérée comme un segment typique de marge transformante et d'en proposer un modèle d'évolution. Curieusement pendant presque vingt ans pratiquement aucun programme spécifique n'a été consacré à ces objets jusqu'à un regain d'intérêt récent lié à des découvertes industrielles prometteuses

Un canyon sous-marin revisité : le trou sans fond de Côte d'Ivoire

The " Trou sans Fond " canyon belongs to the specific category of " gouf " that represents the best expressed submarine canyons. The " Trou sans Fond " is particulary characterized by the following : — A dual head system where abundant clastics are entrapped. These clastics are provided by a strong W-E trending longshore drift, supplemented since 1950 by the lagoonal Ebrié-Comoe river system input (Vridi channel). The western head, directly connected to the Vridi channel, appears mainly experimented to dominant erosional processes. — The presence of two beds : — the greater one has a wide regular section bordered by lateral levees, and appears chiefly shaped by sedimentary overflow and aggradational processes. The minor bed expressed by a narrow, sinuous, « U » shaped valley, is deeply entrenched in the major bed by turbulent erosion. The creation of its " pseudo-meanders " is likely relaed to : (1) competency and load of gravitational flows scouring the bed ; and (2) mass movements (slidings) attacking the bluffs. — The superposition of two distinct canyons. The present day canyon is excavated within Neogene sedimentary strata that have themselves infilled a previous Oligocene canyon bisecting the palaeotransform margin. The " Trou sans Fond " canyon seems thus to be the curtailed progressively westwards shifted system of an oligocene major ancestor.Le Trou sans fond appartient à la catégorie des canyons sous-marins la mieux individualisée, celle des « goufs ». Les caractères suivants le distinguent : — Le dédoublement de sa tête qui est un bassin de réception alimenté par un abondant matériel sédimentaire fourni par la puissante dérive littorale et, depuis 1950, par les décharges du Système Lagune Ebrié-Comoé. La tête occidentale en raison de sa position à la sortie du Canal de Vridi, est la plus soumise à l'érosion (ravinement, approfondissement, recul). — L'étagement des lits. Le lit majeur est large, profilé en berceau entre des rives supérieures, réhaussé de levées, façonné par le débordement et l'accumulation sédimentaires des flux de gravité. Le lit mineur est étroit, profilé en auge, profondément encaissé entre les rives inférieures, sinueux et entretenu par l'érosion turbulente. La formation des «pseudo-méandres » du lit mineur est conditionnée par (1) la force vive et la charge des flux gravitaires qui le creusent ; et (2) les mouvements de masse (glissements) qui dégradent les rives élevées. — L'emboîtement des cours. Le cours actuel est le résultat de la réexcavation persistante opérée dans les séries sédimentaires néogènes qui fossilisent un paléocours (oligocène) creusé transversalement à la zone de fractures mise en place lors de l'ouverture de l'Atlantique équatorial. Le Trou sans Fond est ainsi la version en réduction et décalée vers l'ouest de son devancier.Vanney Jean-René, Mascle Jean. Un canyon sous-marin revisité : le trou sans fond de Côte d'Ivoire . In: Annales de Géographie, t. 101, n°563, 1992. pp. 43-67