Jean Benkhelil

Deformational styles of the eastern Mediterranean Ridge and surroundings from combined swath mapping and seiemic reflection profiling

Abstract Recent swath mapping and seismic reflection profiling across the eastern(Levantine) branch of the Mediterranean Ridge (MR), in the eastern Mediterranean Sea, illustrate a strong variability of the deformational styles that characterize this precollisional accretionary prism. Along a north–south cross-section of the MR, a structural analysis, based on surface mapping, combined with vertical seismic reflection data, reveals two main structural domains. A southern Outer Domain consists of a series of three disconnected distinct fold belts. Folding affects a sedimentary cover which includes an approximately 2-km thick Pliocene and Quaternary wedge in this domain, resting on fairly thick Messinian evaporitic sequences that act as probable decollement layers. The MR Inner Domain includes three regions showing evidences of strong internal deformations and of numerous probable mud cones and mud flows, but no seismic evidence of significant underlying Messinian evaporites can be detected. The inner sub-regions are thrusting northwards over an area made of faulted, and locally uplifted, acoustic basement blocks that constitute the southernmost extension of the Crete continental margin, acting as a continental buttress for the MR. This innermost domain is itself structurally disconnected from the Crete continental margin by the en echelon Pliny troughs system that shows evidence of left lateral displacement. Altogether, the different structural and sedimentary cover patterns reveal a strong contrast between both MR Inner and Outer Domains, and important lateral variations within the Inner MR itself. Strike-slip faulting seams to characterizes both areas, and large-scale mud accumulations potentially exist in the northern one. Our results support a model of imbricated accretionary prisms, including, at least, two stages: (1) a pre-Messinian stage during which the Inner MR probably developed in response to northward subduction of the African lithosphere beneath southern Europe, and (2) a Messinian to post-Messinian period, during which the kinematics of the Aegean–Anatolian microplates and the presence of thick Messinian deposits became prevalent and which led to the creation of the Outer MR folded wedge piling against the previous one. In our interpretation, the occurrence of thick Messinian evaporites in this area induces important local and regional modifications of the structural pattern. Strike-slip activities reflect partitioning related to oblique subduction, and likely lateral escape of the sedimentary cover in the whole studied area.

Marine geologic evidence for a Levantine-Sinai plate, a new piece of the Mediterranean puzzle

Marine geophysical data recorded offshore Egypt illustrate the presence of an active fault belt, trending N145°E, that obliquely transects the eastern Nile deep-sea fan. This belt, more than 150 km long, consists of a series of linear transtensive faults, with an apparent right-lateral horizontal component. These fault zones bound thick-sediment-filled grabens where linear salt ridges and diapirs represent likely Messinian salt reactive response to regional transcurrent geodynamics. We infer that this tectonic belt might correspond to an offshore extension of the Gulf of Suez rift system. If our hypothesis is correct, this fault belt might represent the western boundary of a Levantine-Sinai microplate, locked between the major Arabia and Africa plates and the Anatolian-Aegean microplate.

The Guinea continental margin: an example of a structurally complex transform margin

Abstract The Guinean continental margin corresponds to a wide hinge zone which developed between the Jurassic Central Atlantic and the Cretaceous Equatorial Atlantic. The Guinea Marginal Plateau that characterizes the southern border of this margin is a wide triangular morphological feature bounded to the west by a typical rifted margin slope segment, while its southern edge corresponds to a narrow and morphologically complex continental slope probably resulting from intracontinental transcurrent motion during the Early Cretaceous. On the basis of seismic profiles recorded in two selected areas located along the southern Guinea margin, a seismic stratigraphy is established. At depth, the plateau is underlain by a thick Mesozoic and Cenozoic sedimentary wedge made of two main ensembles. A lower ensemble (or sequence 1) consists of Early Cretaceous clastics. An upper ensemble includes a series of four distinct sequences attributed to Albian to Tertiary deposits. Eastward, the sedimentary cover of the southern plateau edge is locally pierced by volcanic bodies of Paleocene age. Beneath the southern margin of the Guinea Plateau, the sedimentary sequences appear deformed by faults and folds resulting from, at least, two main tectonic episodes. A first event only affected sequence 1, which is cut by a set of extensional faults. A second tectonic phase including folding, reverse faulting and transcurrent faulting is responsible for the deformation of sequences 1 and 2. During this tectonic episode, former normal faults have been reactivated as reverse faults resulting in a structural inversion. A third tectonic phase was responsible for structures transverse to the general E-W trend of the southern Guinea margin and consisting of N-S-trending normal faults and associated volcanoes. The spatial distribution of the tectonic trends clearly substantiates a polyphase tectonic activity that can be related to plate motion changes during early stages of the continental separation. A first stage is characterized by a divergent rifting leading to N-S-trending synsedimentary normal faults. During the Cretaceous, the tectonic activity increased as a general transform motion occurred between the Guinea Plateau and its American conjugate margin (the Demerara Plateau). Local tensional stresses were responsible for a splay fault system active near the tip of the Guinea Plateau. In late Albian times, a slight adjustment of the Africa plate, with respect to the South American one, resulted in local squeezing of the whole sedimentary wedge. After this short compressive event, final continental separation occurred in an extensional regime characterized by a general collapse along two major scarps and volcanic activity.

Lithostratigraphic, geophysical and morpho-tectonic studies of the South Cameroon shelf

Abstract A geological and structural sketch of the southern Cameroon shelf was drawn on the basis of a high-resolution seismic survey, dredgings and Kullenberg core sampling of the outcropping rocks at sea bed. Palynological and mineralogical data (X-ray diffractometry), supported by occasional foraminiferal evidence, revealed four main lithostratigraphic sets resting on a Precambrian basement: Upper Cretaceous (Albian to Senonian), Paleocene–Lower Eocene, Miocene–Pliocene and Pleistocene. Three successive deformational types characterise the structure of this margin: (1) N–S trending fractures parallel to the onshore Precambrian structural directions formed during the rifting period and crosscut by N60°E trending cross-faults induced by the transform movement of the South Atlantic; (2) compressive structures governed by local stresses during the Eocene transcurrent reactivation along the N60°E transform faults excluding a salt-controlled deformation; (3) a set of approximately N–S faults affecting the unfolded sequence of the Miocene–Pliocene or the Quaternary and indicative of distensive conditions. The geodynamic evolution is placed in the general framework of the Central and South-Atlantic rifting and passive margin history. Interestingly, the age of the compressive deformation varies from Late Albian to Mid-Eocene depending on the regional plate motion events which are generated along the narrow fracture zones extending as far as or even into the continental domain.

Occurrence of marine mid-Cretaceous sediments along the Guinean slope (Equamarge II cruise): their significance for the evolution of the central Atlantic African margin

Abstract During the Equamarge II cruise (1988), several coring attempts made along the slope edge of the southern Guinean Plateau led to the recovery of middle Cretaceous sediments in two cores. One contained a carbonate block, dated as middle Albian (H. rischi Zone); the micropaleontological and sedimentological data suggest an open marine environment and paleodepths of approximately 150–200 m. Another core included a dark shale level corresponding to the global anoxic event “CTBE” (Cenomanian/Turonian Boundary Event, W. archaeocretacea Zone). As shown by seismic reflection profiles, these mid-Cretaceous beds lie very close to a major unconformity, which distinctly divides the sedimentary cover into two units (the lower one folded and faulted, the upper one undisturbed). Geodynamic and paleoenvironmental implications of these new data better constrain models of evolution for this crucial area of the Atlantic realm. The compressive and extensional events which affected the Equatorial Atlantic during the progressive separation of Africa from South America are thus more accurately dated. These results also strengthen the hypothesis of an open seaway between the Central and South proto-Atlantic as early as the middle Albian.

Synlithification deformation processes of the Cretaceous sediments of the Ivory Coast‐Ghana transform margin: A way to detail the margin history

The Ivory Coast-Ghana margin is a typical transform margin along which large-scale tectonism has been well documented. The Cretaceous sediments of the Ivory Coast-Ghana transform margin were subjected to numerous tectonic, gravitational, and hydrothermal forces during lithification. In this paper, we infer the order of occurrence of deformation regimes from characteristic deformations observed in samples of Ocean Drilling Program (ODP) Leg 159. Three main types of minor scale deformation are identified and characterized in terms of geometry, rheology, and kinematics: (1) extensional structures such as normal faults and irregular veins with angular, indented walls that are widely recorded at all ODP sites; (2) compressional structures that are confined to the marginal ridge of the Ivory Coast-Ghana transform margin and include reverse faults, folds, and vertical crenulation cleavage and (3) various types of shear zone at low angles with respect to bedding, which are continuous throughout the Cretaceous succession of the ridge. Structural analysis combined with thin section observation shows that most of these deformations occurred in water-rich sediments. The extensional, minor-scale structures are related to the Early Cretaceous rifting stage of the margin as characterized at regional scale by half-grabens and tilted blocks. The compressional structures observed in the samples are attributed to Turonian-Santonian positive inversion tectonics and related to the occurrence of large-scale flower structures and inverted extensional blocks. In contrast, the shear zones at low angles with respect to bedding observed throughout the Cretaceous series are linked to local gravity-induced tectonics.

Submersible observations of Cretaceous deformations along the Cote d'Ivoire-Ghana transform margin

Abstract The Côte 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 Côte 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.

Thermicité et déformation de la marge continentale dans le Sud de la Tasmanie (Australie): résultats préliminaires d'une analyse par traces de fission et d'une étude microstructurale

Abstract We present and discuss a few fission track data, and microstructural observations, from rock samples dredged along the western and southwestern continental margin of Tasmania. The results allow assessing the thermal and tectonic regimes that were active prior to and during the margin creation. The different ages, as provided by fission tracks, and deformational styles, as evidenced from microstructures, are then tentatively correlated with the two main rifting episodes, in Late Jurassic–Cretaceous times and Eocene–Oligocene respectively, deduced from kinematical reconstructions, that have led to the present- day southern margin of Tasmania. To cite this article: M. Selo et al., C. R. Geoscience 334 (2002) 59–66