2. DEFORMATION PATTERNS AND TECTONIC REGIMES OF THE CÔTE D’IVOIRE-GHANA TRANSFORM MARGIN AS DEDUCED FROM LEG 159 RESULTS1Jean Benkhelil,2 Jean Mascle,3 and Caroline Huguen2 |
During the Early Cretaceous, the African and South American plates separated in their equatorial domain along wide transform zones made of a series of pull-apart basins including rifted and transform segments. The rifted Deep Ivorian Basin, and its southern border, the future Transform Margin studied during Ocean Drilling Program Leg 159, were areas of thick continental sedimentation from the latest Jurassic to Early Cretaceous. In Albian and early Cenomanian times, clastic sedimentation persisted in a progressively deepening marine environment. The newly created Côte d’Ivoire-Ghana Marginal Ridge (CIGMR) was eroded during part of the Cenomanian as a consequence of a tectonic uplift and subsequent emersion. A strong unconformity, recorded between the latest Albian and early Turonian at all Leg 159 sites, denotes a radical change in sedimentation, which became typically biogenic (shallow carbonates) during the Late Cretaceous. The Early Cretaceous to Albian thick clastic wedge, which constitutes the CIGMR, has recorded various tectonic deformations that reflect different states of stresses successively prevailing during the marginal ridge formation. Early deformations clearly relate to synsedimentary mass sliding, water-escape structures, presumably triggered by earthquakes, and to dewatering cleavage, itself a consequence of dewatering processes. Synsedimentary listric normal faults have recorded collapses, differential sedimentary loading, and compaction. In more lithified sediments, sets of small-scale normal faults created a pattern of horst and graben microstructures denoting a period of extensional regime prior to a general tilting of strata. A complex microfaulting pattern made of reverse and normal faults, associated with asymmetric microfolds and attributed to a parallel to bedding shear, is interpreted as the consequence of a progressive tectonic uplift of the CIGMR active during Albian times. The creation of the CIGMR thus appears to result from the combined effects of wrench tectonics and accompanying continental stretching and heating. On top of the marginal ridge, transpressive activity is well attested at Sites 960, 961, and 962, by shear faults showing vertical slickensides and associated crenulation and nascent cleavage. Below the Cenomanian unconformity, fractures are commonly filled with minerals such as calcite, kaolinite, quartz, pyrite, barite, and siderite, indicating that hydrothermal circulations mainly occurred during Albian to Cenomanian time span. The deformational history of the CIGMR may tentatively be summarized into four main stages: 1. An Early Cretaceous rifting stage, which created the rifted Deep Ivorian Basin, during which extensional synsedimentary deformations prevailed. 2. An Aptian–Albian syntransform stage during which a transcurrent activity along the rifted basin southern border lead to the formation of pull-apart basins. Incipient shear zones produced unstable conditions, and the extensional stresses were progressively vanishing to be progressively replaced by parallel to bedding shear, as a consequence of an early uplift of the CIGMR. 3. A late Albian stage that was contemporaneous with the development of flower structures and with the main uplift phase of the marginal ridge leading its emersion and subsequent erosion. 4. Turonian–Coniacian, in which shallow-water carbonates were deposited on top of the eroded ridge that started to subside progressively; subsidence persisted throughout the Cenozoic. |
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