Gilbert Bellaiche

The Nile deep sea fan: preliminary results from a swath bathymetry survey

We present and discuss a set of data, mainly swath bathymetry, backscatter images and a few seismic data, that have been recently recorded over large areas of the Nile deep-sea fan between water depths of 1200 and 3000 m. These data demonstrate the presence of at least three distinct morphostructural provinces where interacting sedimentary, tectonic, and salt tectonic processes control the present day morphology and the recent evolution of the major Mediterranean deep-sea fan. The role of the ongoing collision presently occurring between the continental Eratosthenes Seamount and the Cyprus Arc may have been, and still is, a determining parameter for the development of the Nile deep-sea fan.

The Ebro and the Rhone deep-sea fans: First comparative study☆

Abstract Bathymetric and continuous-seismic profiles enable us to define the main morphological and structural outlines of the Ebro and Rhone deep-sea fans. Despite great differences of morphology and volume, these two fans are characterized by a similar structure with a 500 milliseconds thick series of superposed lenticular acoustic units lying above more homogeneous sediments. These units are tentatively attributed to channel-infilling, associated with overbank deposits shifting through time. Nevertheless, some of them may represent large sedimentary slides, especially off the Ebro river.

Le cône sous-marin du Nil et son réseau de chenaux profonds : nouveaux résultats (campagne Fanil)

The meandrous leveed channels of the Nile Cone show clear evidence of avulsions. Their sedimentary architecture is founded on numerous stacked lens-shaped acoustic units. In the areas of the distal fan, lobe deposits are apparent from multichannel imagery. Huge debris flow deposits, sometimes associated with pockmarks, are recognized. Mud volcanoes and gas seeping are closely associated with faulting. In the East, a very long north-trending channel, originating from the Egyptian coast, merges with a network of channels, very probably originating from the Levantine coasts. Both networks outlet in the sedimentary basin located south of Cyprus.

Interpretation de quelques profils de sismique refraction en mer Ligure

Abstract Reversed marine seismic refraction profiles, run south of Imperia (Italy) with sonobuoys, permitted us to enlarge our investigation area from the Ligurian shores to the abyssal plain and to provide new data about the structure of the continental margin-abyssal plain transition zone. The main results are: (1) the absence in the abyssal plain of the Cretaceous series observed on land and on the continental slope; (2) the extension and the wedging out of the uppermost series of the Messinian evaporites on the continental slope while halite is only present in the deepest area of the Ligurian Sea basin; (3) the presence of the infrasalt series on the continental slope; (4) a change in the nature of the basement (6.07–6.41 km/s) which passes from 3 to 9 km deep between Monaco and the abyssal plain. A cross-section through the margin with the refraction data superimposed on a seismic reflection profile is presented.

Age et nature des formations volcaniques prismées observées et prélevées dans le canyon des Moines (Sud-Ouest Corse) pendant la campagne Cyaligure

Abstract During the “Cyaligure” submersible cruise (1977), investigations in the “Canyon des Moines” (Corsican margin) led us to observe and to sample a prismatic volcanic cliff at a depth of 1000–1200 m. The radiometric data ( 40 K− 40 Ar and 39 Ar− 40 Ar on bulk rock and 40 K7minus; 40 Ar on separated plagioclases) are interpreted to give the most likely age of the volcanism as between 18.5 and 14.7 Ma. The mineralogical and geochemical analyses of this lava show that its parentage is clearly different from that of an alkaline or a shoshonitic series. This lava, of late Burdigalian-Langhian age, may be considered as evidence of an oceanic crust or of an orogenic magmatism of Sardinian type synchronous with the end of the accretion period in the Ligurian Basin.

Detailed morphology, sedimentary structure and evolution of the continental margin of the western Provençal basin (south of France) since the late Miocene

Abstract Seabeam bathymetry and high-resolution seismic profiles allow the study of different sedimentary units existing off western Provence and to establish their continuity throughout the continental margin from the shelf to the rise. The morphology of the slope is greatly controlled by gravity processes leading to phenomena such as canyon piracy or channel avulsion. This morphology quite accurately expresses the configuration of the old underlying sedimentary and tectonic structures. The sedimentary structure of this domain is the result of the complex arrangement of lobes and ridges that originated from different continental sources and were deposited during different epochs of the Plio-Quaternary. Knowledge of the old hydrographic networks draining Provence during the different Quaternary climatic stages allows palaeogeographic reconstructions to be proposed linking these sedimentary units to their continental sources.

The Jarrafa Trough (Pelagian Sea): Structural evolution and tectonic significance☆

Abstract The structural and sedimentary characteristics of the Jarrafa Trough (Pelagian Sea) are presented. Buried ridges attributed to reefs of Miocene age, transgressive Pliocene sedimentary layers and progradational Quaternary series are well observed on the edges of the trough. A distensive regime occurred along WNW-ESE and ENE-WSW tectonic axes throughout the Miocene to the Present. Intrusive bodies related to salt diapirism are recognized. The Jarrafa Trough is interpreted as an element of the Tertiary intracontinental belt of grabens running from Europe to Africa.

Sedimentation and structure of the Izu-Ogasawara (Bonin) Trench off Tokyo: new lights on the results of a diving campaign with the bathyscape “Archimede”

Abstract Direct observations from submersible prove that in the explored areas, the Izu-Ogasawara Trench is devoid of any significant sediment cover. The effects of the tensional forces affecting the oceanic crust have been observed in the outer wall of this trench, especially near its foot between 8200 and 8500 m depth. They lead to a staircase morphology (with rock exposures) corresponding probably with vertical faulting parallel to the trend of the trench axis. The inner wall, at about 5000 m depth, shows large rock exposures that have been sampled. These rocks are volcanic lavas of the tholeiitic series (icelandites). The staircase morphology observed in this part of the overriding plate seems to prove the occurrence of distensive forces induced probably by the underthrusting of the oceanic crust. No indication of compressive stress has been registered in the whole investigated area.

Structure of the French Riviera submarine canyons: Evidence of a polygenetic history from a submersible study (“Cyaligure” campaign)

Abstract The results of the dives made, during the “Cyaligure” campaign with the submersible “Cyana”, in the lower part of the Provencal canyons (Pampelonne and Saint-Tropez canyons) at between 1300 and 2160 m water depth, complete the observations made during the previous surveys and establish the very poor thickness or even the absence, on this margin, of the Miocene sedimentary series — probably owing to erosion during the Messinian (Late Miocene) desiccation period. They also establish the importance, especially during the Quaternary, of sedimentary in-filling processes, diagenetic (lithification) and cutting phenomena that lead us to consider these canyons (like probably many others in the world) as complex polygenetic structures developing throughout their first cutting epoch, here inferred to the Messinian up to the present time.

Off-shore seismological experiments south of Provence in relation to “Glomar Challenger” deep sea drillings (JOIDES-DSDP, Leg 13)

Abstract The evaporitic layer located at 35 km south of Toulon, according to compressional-wave velocity measured in samples from JOIDES-DSDP Leg 13 drillings in the Mediterranean Sea, could be constituted by gypsum or anhydrite, halite having been deposited in the most central parts of the basin. These deductions are in favour of the supposed “bulls eye” structure for evaporitic layers in the Mediterranean Sea.