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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. |
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