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Four mud volcanoes of several kilometres diameter named Amon, Osiris, Isis, and North Alex and located above gas chimneys on the Central Nile Deep Sea Fan, were investigated for the first time with the submersible Nautile. One of the objectives was to characterize the seafloor morphology and the seepage activity across the mud volcanoes. The seepage activity was dominated by emissions of methane and heavier hydrocarbons associated with a major thermal contribution. The most active parts of the mud volcanoes were highly gas-saturated (methane concentrations in the water and in the sediments, respectively, of several hundreds of nmol/L and several mmol/L of wet sediment) and associated with significantly high thermal gradients (at 10 m below the seafloor, the recorded temperatures reached more than 40 °C). Patches of highly reduced blackish sediments, mats of sulphide-oxidizing bacteria, and precipitates of authigenic carbonate were detected, indicative of anaerobic methane consumption. The chemosynthetic fauna was, however, not very abundant, inhibited most likely by the high and vigorous fluxes, and was associated mainly with carbonate-crust-covered seafloor encountered on the southwestern flank of Amon. Mud expulsions are not very common at present and were found limited to the most active emission centres of two mud volcanoes, where slow extrusion of mud occurs. Each of the mud volcanoes is fed principally by a main narrow channel located below the most elevated areas, most commonly in the centres of the structures. The distribution, shape, and seafloor morphology of the mud volcanoes and associated seeps over the Central Nile Deep Sea Fan are clearly tectonically controlled. © 2007 Elsevier Ltd. All rights reserved.

Albania, situated at the boundary between the Dinaric and the Hellenic branchs of the Dinaro-Hellenic fold belt, has experienced a multiphase geodynamic evolution. The internal zones show a Mid-Jurassic episode of deformation characterized by ophiolite obduction, followed by development of a fold-and-thrust belt in the external zones during the Cenozoic. More recently, Albania has experienced a tensional regime. We present apatite and zircon fission-track (AFT and ZFT) measurements for 22 samples, and seven measurements of track-length distributions to elucidate the thermal evolution. AFT ages vary from 10.8 ± 0.7 Ma to 50.5 ± 5.7 Ma. The oldest ages (Eocene) occur in the western Albanides, corresponding to Eocene emplacement of the internal zones over the external ones. Neogene ages in the eastern Albanides suggest rapid exhumation, which we relate to an extensional regime. The ZFT ages show that the internal Albanides did not reach temperatures > 200 °C during the Cenozoic.

Géomorphologie : Relief, Processus, Environnement, v. 2, p. 91-104, 2005; International audience; Since 80 My, the eastern Mediterranean basin has been subjected to a complex evolution, controlled by the ongoing convergence between African and Eurasian plates (fig. 1). One of the consequences of this specific kinematics is the formation, within the Ionian and Levantine basins, of the Mediterranean Ridge, an atypical tectono-sedimentary accretionary wedge 1500 km long and 200-250 km wide. From the Ionian to the Levantine basins, the Mediterranean Ridge appears as a southward arcuate high, lying at water depths ranging from 1400 m in its central area, to 3500 m, bounded to the north by the Hellenic trench system and to the south by a series of discontinuous abyssal plains. Within the western part of this system (Ionian basin), C. Truffert et al. (1993) identified three different morphostructural provinces: the outer front, the central domain, and the northern area, in contact with a flat inner domain. This classification was completed by E. Chaumillon (1995) who identified laterally a western (or Ionian) province, a central (or Libyan) narrow domain, and an eastern (or Levantine) branch. Although this prism has already been partly investigated using reflection and refraction seismics (

Depth migration of seismic reflection profiles across the Mediterranean Ridge accretionary complex between the African and Eurasian blocks illustrates profound variations in the geometry and internal structure along strike. Structural interpretations of four cross sections, together with bathymetric and acoustic surface information and drilling data, are used to volumetrically balance the amount of subduction versus accretion with time. Results suggest the existence of three distinct scenarios, with a jump in décollement in the west, intense backthrusting in the central part between Libya and Crete, and transcurrent tectonism in the east. The onset of accretion coincides with exhumation of thrust sheets (∼19 Ma), followed by rapid sediment accretion with thick, evaporite-bearing incoming successions facilitating outward growth of the wedge. The minimum rate of accretion (20–25% of the total sediment supply) is observed in the central portion where the ridge suffers maximum deformation. Here the indenting leading edge of the African Plate apparently forces the sediment into subduction, or local underplating. In contrast, an estimated 40–60% of the available sedimentary input was accreted in the western domain where collision is less accentuated. The results support the hypothesis that highly destructive forearc collisional events, like slab break off and exhumation of thrust sheets, can be followed by periods of accretion and continuous growth of accretionary wedges.

In the eastern Mediterranean, numerous argilo-kinetic manifestations, commonly named 'mud volcanoes', have been identified and studied in some detail during the last twenty years using several techniques. The Medinaut survey (December 1998) has provided new insights into this phenomenon through twenty deep dives performed with the submersible Nautile within two areas particularly active in terms of mud expulsions: the central Mediterranean ridge south of Crete, and the Anaximander mountains area, south of Turkey. The morphology, petrography, microstructure and biostratigraphy of different clasts sampled in the two areas helped to better understand the mud volcano formation and subsequent evolution. Paleoenvironmental conditions, ranging from deep marine deposition in distal deep-sea-fan environment during the Miocene (Mediterranean ridge) to continental margin conditions (Anaximander mountains), have been evaluated for the composition and age of the various rock types. Macroscopic and microscopic structural observations have revealed a close control of clast shapes by sets of early microfractures (calcite veins, joints) created before the clast expulsion to the seabed surface. Surface features, such as erosion marks, alteration aureoles or calcitic coatings, have also been described on most samples. Finally, erosion traces, alteration aureoles and calcitic coating observed for the first time are interpreted as the result of a complex cold seep context related to mud expulsion and associated fluid venting. © 2001 Ifremer/CNRS/IRD/Éditions scientifiques et médicales Elsevier SAS.

Methane seepage at mud volcanoes around the world continues to be of broad international interest for a number of reasons. Among these, seafloor methane is a potentially important contributor to global warming. In addition, dewatering and degassing of sediments and the formation of fluid migration pathways is important not only to sedimentologists, but also to hydrocarbon reservoir engineers and hydrologists.

During May 1990 and January-February 1991, an extensive geophysical data set was collected over the Côte d'Ivoire-Ghana continental margin, located along the equatorial coast of West Africa. The Ghana margin is a transform continental margin running subparallel to the Romanche Fracture Zone and its associated marginal ridge - the Côte d'Ivoire-Ghana Ridge. From this data set, an explosive refraction line running approximately 150 km, ENE-WSW between 3°55'N, 3°21'W and 4°23'N, 2°4'W, has been modelled together with wide-angle airgun profiles, and seismic reflection and gravity data. This study is centred on the Côte d'Ioivre Basin located just to the north of the Côte d'Ivoire-Ghana Ridge, where bathymetric data suggest that a component of normal rifting occurred, rather than the transform motion observed along the majority of the equatorial West African margin. Traveltime and amplitude modelling of the ocean-bottom seismometer data shows that the continental Moho beneath the margin rises in an oceanward direction, from approximately 24 km below sea level to approximately 17 km. In the centre of the line where the crust thins most rapidly, there exists a region of anomalously high velocity at the base of the crust, reaching some 8 km in thickness. This higher-velocity region is thought to represent an area of localized underplating related to rifting. Modelling of marine gravity data, collected coincident with the seismic line, has been used to test the best-fitting seismic model. This modelling has shown that the observed free-air anomaly is dominated by the effects of crustal thickness, and that a region of higher density is required at the base of the crust to fit the observed data. This higher-density region is consistent in size and location with the high velocities required to fit the seismic data. (Résumé d'auteur)