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Complementary data on previously surveyed areas have been obtained and more than 3000 km of continuous seismic profiles along the eastern Hellenic continental margin, between eastern Crete and Rhodes, have been made. Among the important preliminary observations, we note: the relatively great thickness, due to probable coarse rapid fill, of the margin sedimentary cover, which is also intensively affected by extensional faulting. The presence of a well-defined sedimentary basin (lying between the Strabo- and Pliny Trench), where evaporitic strata exist but where the acoustic facies also suggest coarse fill, is probably derived from surrounding topographical highs exposed to erosion during the Messinian. Finally we observe a progressive and perplexing change of the Strabo Trench structural pattern from west to east, which indicates a more complex tectonic regime than we expected. We speculate that at the southern margin of the Rhodes Basin, northward thrusting may now be occurring.

Abstract:  Crustal structure of the Co^te d’Ivoire–Ghana marginal ridge and its transition with oceanic lithosphere are deduced from multichannel seismic reflection, wide-angle seismic, and gravity data. The CIGMR is cut into rotated blocks and displays a crustal structure quite similar to that of the nearby northern Ivorian extensional basin. These results strongly support that the CIGMR represents an uplifted fragment of continental crust. Transition with the oceanic crust appears sharp; continental crustal thinning occurs over less than 5 km. We did not find evidence for underplating and/or contamination as anticipated from such a sharp contact between continental and oceanic crust.

Abstract:  The Cte d’Ivoire–Ghana marginal ridge, mainly formed by thick detrital sediments of Lower Cretaceous age, was surveyed by deep dives with the submersible Nautile (Equanaute survey). Samples retrieved during the dives, combined with observations of video and photos records, allowed us to document a compressional event that occurred along the continental margin off Cte d’Ivoire and Ghana, during Early Cretaceous times. Remarkable fracturing, folding, and cleavage characteristics in the sedimentary wedge are best explained by large-scale strike-slip transform activity between the two parting African and South American continents in Cretaceous times.

Abstract:  We present multi channel seismic data recorded at the transition between the Ivorian (rifted) basin and the Côte d’Ivoire–Ghana marginal ridge (formed in a Cretaceous transform margin). The ridge is made of sedimentary sequences continuous with the synrift sediments of the Ivorian basin. Clinoformal structures suggest synrift progradational sedimentation originating from the Brazilian craton, which was located to the south during the Cretaceous. Subsequent to rifting, southward migration of the transform motion isolated the ridge from the Brazilian shelf. In the western part of the marginal ridge, crustal half-grabens are buried by postrifting progradational sedimentation, suggesting important posttransform subsidence.

Abstract:  Six sandstone blocks sampled during dives along the southern slope of the Ivory Coast–Ghana continental margin have been studied using fission tracks in apatite and zircon. Measurements demonstrate that the rocks were heated above 120°C but below 390°C and cooled quickly. The ages of cooling recorded by the apatite crystals are 90 Ma in the western part of the margin, and 80–70 Ma in the central and eastern part. Heating is interpreted by the heat liberation due to the friction along the active transform fault and by the vicinity of an oceanic spreading center, which slipped along the margin. Cooling is interpreted by two stages of denudation due to minor faults and landslides produced by the increasing of the bathymetric step between the continental margin and the oceanic crust.

Abstract:  The structure of the oceanic crust adjacent to the Côte d’Ivoire–Ghana transform margin is deduced from multichannel seismic reflection and seismic wide-angle data, showing crustal heterogeneities within oceanic basement; the oceanic crust adjacent to the transform margin is half as thick as standard Atlantic oceanic crust. Refraction data indicate a gradual velocity transition towards typical mantle velocities. Such an abnormal oceanic crustal structure appears quite similar to crustal structures known along transform faults. This crustal thinning may be related to thermal effects of the nearby continental crust, on the oceanic accretion processes. We did not find geophysical evidence for oceanic crust contamination by continental lithosphere.

Abstract:  During the Equanaute cruise (June 1992), 14 submersible deep dives were conducted along the steep southern slope of the Cte d’Ivoire–Ghana marginal ridge. In situ diving observations and sample analysis were done to specify the clastic lithology of a sedimentary sequence about 2200 m thick. Most of the rocks collected consist of dark shales, greenish lenticular- to wavy-bedded siltstones and large-scale trough cross-bedded sandstones. Lithofacies analysis favors a deltaic origin for this sedimentary synrift succession, which is probably of Early Cretaceous age.

Abstract:  Fourteen deep dives were conducted along the southern slope of the Côte d’Ivoire–Ghana marginal ridge. Detrital rock samples reveal Early Cretaceous synrift sediments. In situ observations and microtectonic analysis identify the nature and the timing of three successive episodes of deformation. Clay layers are affected by early diagenetic deformational structures such as slump microfolds and normal microfaults. Most of the clastic deposits display a second deformational episode. These structures result from a layer-parallel extension and are related to extensional synrift tectonics. The last stage of deformation is identified by right lateral wrench folds associated with brittle joints and slaty cleavage. It is attributed to the syntransform evolution of the Côte d’Ivoire–Ghana marginal ridge.

Abstract:  The Mediterranean Ridge (eastern Mediterranean) is a large accretionary complex that results from the Africa–Europe–Aegean plates convergence. Multichannel seismic data, combined with previous results showed that the ridge comprises distinct major structural domains facing different forelands: (1) An outer domain is bounded to the south by the ridge toe. Underneath the Ionian and Levantine outer Ridge, Messinian evaporites act as a major decollement level. (2) An axial, or crestal, ridge domain with mud diapiric and mud volcano activity is bounded to the north by backthrust. (3) A less tectonized inner Ridge domain, possibly a series of former forearc basins, abuts the Hellenic Trench. The ridge displays strong along-strike variations. These variations can be interpreted as consequences of an ongoing collision against the Libyan continental promontory.