Figure 7 - uploaded by Michel Guiraud
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Sketch showing the deformation related to décollement zones induced by a slump mass causing shearing by loading observed in the Leg 159 samples. The décollement surface (C plane) is parallel to the bedding and may pass to reverse faults or terminate in the axial plane of asymmetric fold (S planes). The normal faults (C′ planes) and boudinage result from the extensional strain, which affects the trailing edge of the structure.
Source publication
Regional tectonics and detailed examination of cores recovered during Leg 159 of the Ocean Drilling Program show that four types of deformation occurred during early extension, marginal uplift and shearing of the Cote d'Ivoire-Ghana Marginal Ridge (CIGMR): (1) extensional deformation related to the opening up of pull-apart basins; (2) synsedimentar...
Citations
... The geological history of Côte d'Ivoire sedimentary basin is related to the opening and expansion of the South Atlantic, was formed following the intracratonic divergence of Benue rift, during Lower Cretaceous (Digbehi, 1987;Aka, 1991;Chierici, 1996;Benkhelil et al, 1998;Pickett and Allerton, 1998;Marcano et al, 1998). The sedimentary evolution of the basin of Côte d'Ivoire proceeded according to four principal phases described by Digbehi (1987) then by Chierici (1996). ...
We studied the argillaceous fractions of three wells carried out in Samo area, located in the South-Eastern part of Côte d'Ivoire sedimentary basin. Preliminary works whose results are presented in this paper related to the mineralogical studies of clays of the levels crossed by these wells. They precede the study aiming at establishing the palynostratigraphy as well as the variation of the depositional environments in connection with the eustatic episodes which occurred in this sector of the Northern Gulf of Guinea The results indicate that analyzed samples are mainly made up of quartz and kaolinite with a small proportion of illite. In the upper part sequence identified like the Continental terminal, the deposits are primarily made up of quartz and kaolinite. In bituminous gray clays of the lower section, the samples are dominated by quartz and illite. These results lead to distinguish two lithostratigraphical sequences: the variegated clays of Continental terminal Mio-Pliocene age are provided by Quaternary lateritic continental detritus; the Lower sequence of bituminous clays is mainly marine origin.
... Hibbard and Karig, 1987;Agar, 1988;George, 1990), oceanic transform margins (e.g. Benkhelil et al., 1998), thrusting of poorly lithified sediments (e.g. Nigro and Renda, 2004), together with sediment slump sheets (Figs. ...
Sheath folds are developed in a broad spectrum of geological environments in which material flow occurs, including gravity-driven surficial deformation in ignimbrites, unconsolidated sediments and salt, together with deeper level ductile shear zones in metamorphic rocks. This study represents the first geometric comparison of sheath folds in these different settings across a wide range of scales. Elliptical closures defining eye-folds represent (y–z) cross sections through highly-curvilinear sheath folds. Our analysis of the published literature, coupled with field observations, reveals remarkably similar ellipticities (Ryz) for sheath folds in metamorphic shear zones (Ryz 4.23), salt (Ryz 4.29), sediment slumps (Ryz 4.34), glaciotectonites (Ryz 4.48), and ignimbrites (Ryz 4.34). Nested eye-folds across this range of materials (N = 1800) reveal distinct and consistent differences in ellipticity from the outer- (Ryz) to the inner-most (Ry′z′) elliptical “rings” of individual sheath folds. The variation in ratios from outer to inner rings (R′ = Ryz/Ry′z′) in gravity-driven surficial flows typically displays a relative increase in ellipticity to define cats-eye-folds (R′ < 1) similar to those observed during simple and general shear in metamorphic rocks. We show that sheath folds develop across a range of scales within these different environments, and display elliptical ratios (Ryz) that are remarkably constant (R2 > 0.99) across 9 orders of magnitude (sheath y axes range from ∼0.1 mm to >75 km). Our findings lead us to conclude that the geometric properties of sheath folds are scale invariant and primarily controlled by the type and amount of strain, with R′ also reflecting the rheological significance of layering associated with original buckle fold mechanisms. The scaling pattern of sheath folds reflects the length scales of the precursor buckle folds (and width of deformation zones) across a broad range of materials and environments. With continued deformation, the layering marking the original folds may become increasingly passive to define sheath folds. These empirical relationships suggest sheath folding is a fundamental mode of viscous response across a broad spectrum of materials, strain rates and scales encompassing a variety of deformation settings.
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.
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.
