The geological evolution of northwestern Africa and its continental margins is investigated in the light of nine Meso-Cenozoic paleogeological maps, which integrate original minimal extent of sedimentary deposits beyond their present-day erosional limits. Mapping is based on a compilation of published original data on the stratigraphy and depositional environments of sediments, structures, magmatism, and low-temperature thermochonology, as well as on the interpretation of industrial seismic and borehole data.We show that rifting of the equatorial domain propagated eastward from the Central Atlantic between the Valanginian (ca. 140 Ma) and the Aptian (ca. 112 Ma) as an en echelon strike-slip and rift system connected to an inland rift network. This network defines a six-microplate synrift kinematic model for the African continental domain. We document persistent, long-wavelength eroding marginal upwarps that supplied clastic sediments to the offshore margin basins and a large intracratonic basin. The latter acted as a transient sediment reservoir because the products of its erosion were transferred both to the Tethys (to the north) and the Atlantic Ocean. This paired marginal upwarpintra cratonic basin source-to-sink system was perturbed by the growth of the late Paleogene Hoggar hotspot swell that fragmented the intracratonic basins into five residual depocenters. By linking the evolution of the continental margins to that of their African hinterland, this study bears important implications for the interplay of long-wavelength deformation and sediment transfers over paired shield-continental margin systems.
We investigated the variability of the first-order crustal structure and early post-rift stratigraphy of six segments of the African Equatorial Atlantic margin using sub-surface data (seismic and wells). Extension obliquity of the segments varies from 0° for the West Ivory Coast and Ghana transform segments to 30° for the Togo-Benin oblique segment and 75° for the East Ivory Coast normally divergent segment. The Sierra Leone and Liberia segments underwent probably deformation during both the early Jurassic rifting of the Central Atlantic and the early Cretaceous rifting of the Equatorial Atlantic with contrasted divergence obliquities. For segments that underwent a single rifting, we show that, the higher the obliquity, the wider the crustal thinning domain. This has a major influence on the first-order geometry of all the post-rift horizons, including the present-day slope: the lower the obliquity, the larger the differential subsidence across the margin and the steeper the present-day slopes of post-rift horizons. This also has a major influence on the flexural isostatic response of the lithosphere to thermal-and erosion/sedimentation-driven (un)loads during the early post-rift. Narrow (transform) segments underwent higher flexural (and/or thermal) uplifts in the proximal domain than wider divergent segments. Along the same margin, divergent segments therefore may preserve early post-rift deposits in their proximal domains, whereas they are not preserved on nearby transform segments.
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