TURNER, JONATHAN P, The University of Birmingham, UK; BRUCE R ROSENDAHL, Rosentiel School of Marine & Atmospheric Science, USA; DAVID A KIRKLAND,The University of Birmingham, UK

Abstract: Structural Architecture Of The Continental Margin Offshore Cameroon-Equatorial Guinea-Gabon, West Africa

Understanding of rifted continental margins has progressed rapidly with technological advances in seismic and deepwater/oceanic drilling, and with improved modelling of the structural, stratigraphic and thermal evolution of their sedimentary basins. However, in spite of the distinct segmentation of many rifted continental margins - often with strikingly contrasting geology between adjacent segments - the configuration of the deep-rooted structures controlling this segmentation is virtually unknown. Here, we report on a new structural interpretation and gravity modelling of the deep-imaging PROBE data. This dataset traverses the transition from continental to oceanic lithosphere between 5ºS and 3ºN, offshore West Africa and it represents one of the most extensive deep-imaging seismic collections on any rifted continental margin. The data show the margin divided by the Kribi Fracture Zone (KFZ -Figures 1 & 2), a 75km-wide transform fault zone trending NE. North of the KFZ, in the North Douala Basin, oceanic crust extends to the shoreline whereas south of the KFZ, in the Gabon Basin, a broad and prospective rift margin exists.This implies that in the North Douala basin almost all the rift geology must be located onshore and that potential offshore sources and targets lie entirely within the post-rift section. Offshore Equatorial Guinea, where the KFZ abuts the continental shelf, the setting is that of a continent-bounded transform fault, strongly influenced by fracture zone tectonics.

Detailed mapping of a high-quality commercial dataset offshore Equatorial Guinea allows us to identify links between the fracture zone tectonics and basin development. In particular, we highlight large-scale gravity-driven detachment of the basin fill above a weak, planar salty horizon.Albian gravitational detachment of the Equatorial Guinea margin took place immediately prior to oceanic spreading and it led to the emplacement of kilometres-wide blocks of shallow water limestones in a deep, basinal seting. The mechanism that triggered gravitational collapse of this part of the West African margin is contentious. However, within our study area, this style of gravity-driven deformation is confined to that segment where the KFZ abuts the continntal shelf. Consequently, we interpret it as a response to steep, basinward gradients at the abrupt join between continental and oceanic crust across the KFZ.
 
 

Figure 1. Structure architecture map of offshore West Africa between 5°S and 3°N , with location of deep-imaging PROBE data.

Figure 2. Structural architecture from interpretation of seismic transect (8) of continental margin, offshore West Africa.

AAPG Search and Discovery Article #90923@1999 International Conference and Exhibition, Birmingham, England