Flexural unloading and uplift along the Cote d'Ivoire-Ghana Transform Margin, equatorial Atlantic

摘要

Recent Ocean Drilling Program sampling of the Cote d'Ivoire-Ghana margin of West Africa provides for the first time the opportunity to study the development of a marginal ridge that formed along a sheared passive margin adjacent to the continent-ocean transition after the end of intracontinental wrenching. We model its evolution using a two dimensional flexural backstripping technique. The model is constrained by existing seismic refraction and reflection data on the crustal structure and stratigraphy and paleobathymetric evidence from the cores. Following rifting at 120 Ma (Aptian), intracontinental wrenching continued until approx 105 Ma (mid Albian), when South America and Africa separated along this transform. While the possible presence of thicker crust under the Marginal Ridge compared to the Deep Ivorian Basin may explain some of the ridge's topography, the entire uplift can be readily modeled as a flexural response to unloading along a shallow-dipping (approx 25 deg) fault at the time of continental separation. Forward subsidence modeling of the Marginal Ridge suggests that an effective elastic thickness of 2.5 km at 105 Ma is most appropriate to match the observed structure. Conduction of heat from the oceanic plate across the continent-ocean transition drove temporary uplift of at least 1200 m peaking just before 89 Ma, when sedimentary data show deposition within the photic zone (0-50 m). This study shows that significant, transient thermal uplift can be found in sheared continental passive margin settings even where magmatism is insignificant.