Christophe Basile

The Ivory Coast-Ghana transform margin: A marginal ridge structure deduced from seismic data

Abstract The Ivory Coast-Ghana (ICG) marginal ridge is a prominent feature of the ICG transform margin and includes a fossil ridge partially buried by a thick, undeformed sedimentary cover. The fossil ICG ridge is 130 km long and 25 km wide, and towers over the adjacent rifted basin (deep Ivorian basin, DIB) and the oceanic crust by 1.3 km and more than 4 km, respectively. It formed in three successive stages. 1. (1) During the rifting of the DIB, both vertical and horizontal motions between the DIB and the South American plate varied along the plate boundary. This relative motion occurred in an accommodation zone that tilted the northern slope of the ICG ridge along en-echelon, mainly strike-slip, faults. 2. (2) After the rifting of the DIB, the relative motion remained constant along the transform plate boundary. At this time strike-slip deformation was localized into a narrow and highly deformed belt that truncated the accommodation zone. 3. (3) Finally, the transform motion occurred between the DIB and an occanic plate. Thermal exchanges between the two adjacent plates induced thermal uplift of the ICG ridge that amplified previous tectonic tilting.

Crustal structure and ocean-continent transition at marginal ridge: the Cote d'Ivoire-Ghana marginal ridge

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.

The Côte d’Ivoire–Ghana transform margin: sedimentary and tectonic structure from multichannel seismic data

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.

Mapping of a segment of the Romanche Fracture Zone: A morphostructural analysis of a major transform fault of the equatorial Atlantic Ocean

One square nautical degree within the active part of the Romanche Fracture Zone was surveyed at about long 17°W during the 1988 Equamarge II cruise of the R/V Jean Charcot to the equatorial Atlantic. SeaBeam mapping and seismic-reflection profiles of the trench and adjacent northern and southern transverse ridges emphasize the contrast between the apparent simplicity of the major morphostructural lineaments and the complexity of the second-order features. The vertical displacement associated with the main strike-slip movement of major oceanic transform faults is clearly illustrated by positive topographic anomalies of the transverse ridges. The bathymetric map and samples collected confirm that the flat-topped northern transverse ridge actually emerged at a time when peridesertic climate shaped basaltic basement into ventifact pebbles coated with desert varnish.

Structure of oceanic crust adjacent to a transform margin segment: the Côte d’Ivoire–Ghana transform margin

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.

Fission track study: heating and denudation of marginal ridge of the Ivory Coast–Ghana transform margin

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.

Statistical classification of log response as an indicator of facies variation during changes in sea level: Integrated Ocean Drilling Program Expedition 313

In this study, a novel application of a statistical approach is utilized for analysis of downhole logging data from Miocene-aged siliciclastic shelf sediments on the New Jersey Margin (eastern USA). A multivariate iterative nonhierarchical cluster analysis (INCA) of spectral gamma-ray logs from Integrated Ocean Drilling Program (IODP) Expedition 313 enables lithology within this siliciclastic succession to be inferred and, through comparison with the 1311 m of recovered core, a continuous assessment of depositional sequences is constructed. Significant changes in INCA clusters corroborate most key stratigraphic surfaces interpreted from the core, and this result has particular value for surface recognition in intervals of poor core recovery. This analysis contributes to the evaluation of sequence stratigraphic models of large-scale clinoform complexes that predict depositional environments, sediment composition, and stratal geometries in response to sea-level changes. The novel approach of combining statistical analysis with detailed lithostratigraphic and seismic reflection data sets will be of interest to any scientists working with downhole logs, especially spectral gamma-ray data, and also provides a reference for the strengths and weaknesses of multicomponent analysis applied to continental margin lithofacies. The method presented here is appropriate for evaluating successions elsewhere and also has value for hydrocarbon exploration where sequence stratigraphy is a fundamental tool.

Marges continentales transformantes

We present and discuss three main stages that successively control the sedimentary, tectonic and crustal evolution along continental margin segments which result chiefly from transform rifting between two parting continents. During an intracontinental stage, the future margin area is submitted to specific thermal and wrench tectonic activities and to a dominantly clastic sedimentation. In a second stage, an active transform contact between continental crust and a newly created oceanic crust, induces a sharp transition between ocean and continent crusts, as well as the creation of a very steep and linear slope. Finally, the margin is submitted to a more standard cooling subsidence and accompanying sedimentation.

Fresh-water and salt-water distribution in passive margin sediments: Insights from Integrated Ocean Drilling Program Expedition 313 on the New Jersey Margin

On the New Jersey shelf (offshore North America), the presence of pore water fresher than seawater is known from a series of boreholes completed during the 1970s and 1980s. To account for this fresh water, a fi rst hypothesis involves possible present-day active dynamic connections with onshore aquifers, while a second involves meteoritic and/or sub- ice-sheet waters during periods of lowered sea level. Expedition 313 drilled three boreholes on the middle shelf, offering a unique opportunity for the internal structure of the siliciclastic system to be accessed, at scales ranging from the depositional matrix to the continental margin. This enables the stratigraphic architecture to be correlated with the spatial distribution and salinity of saturating fl uids. Expedition 313 revealed both very low salinities (<3 g/L) at depths exceeding 400 m below the seafl oor and evidence for a multilayered reservoir organization, with freshand/ or brackish-water intervals alternating vertically with salty intervals. In this study we present a revised distribution of the salinity beneath the middle shelf. Our observations suggest that the processes controlling salinity are strongly infl uenced by lithology, porosity, and permeability. Saltier pore waters generally occur in coarse-grained intervals and fresher pore waters occur in fi ne-grained intervals. The transition from fresher to saltier intervals is often marked by cemented horizons that probably act as permeability barriers. In the lowermost parts of two holes, the salinity varies independently of lithology, suggesting different mechanisms and/or sources of salinity. We present an interpretation of the sedimentary facies distribution, derived from core, logs, and seismic profi le analyses, that is used to discuss the marginscale two-dimensional reservoir geometry and permeability distribution. These proposed geometries are of primary importance when considering the possible pathways and emplacement mechanisms for the fresh and salty water below the New Jersey shelf.

Late Jurassic sedimentation and deformation in the west Iberia continental margin: insights from FMS data, ODP Leg 173

In the Iberia Abyssal Plain, Ocean Drilling Program (ODP) Site 1065 drilled the top of a continental tilted block, penetrating a Middle to Late Jurassic probable pre-rift sedimentary unit. Borehole images of this unit were obtained by the Formation MicroScanner (FMS) logging tool, thus allowing the identification and 3D measurement of two block tilting phases. The last one is an eastward 15° tilt, probably related to the main Early Cretaceous rifting phase, and consistent with the tilted block image on seismic lines. The first tilt, previously unknown, is Late Jurassic in age. The block tilting is 15° to the southeast, and is not associated with any noticeable change in the depositional environment. Both tilting episodes induced sedimentary instabilities, leading to listric faults and slumps. FMS images also showed that the sediments originated from the north (Galicia Bank) during Late Jurassic times.