The Deepwater Thrust Belt of Cape Three Points, Ghana: An Unexpected Transpressional System Offshore West Africa

Gabor Tari, Marek Kaminski, Jim Molnar, Dave Valasek, and Gary Walters
Vanco, Houston, TX

The fossil marginal ridge of Côte d'Ivoire and Ghana is a prominent structural and topographic feature on the transform margin of equatorial West Africa. The acquisition and interpretation of some 3,000 kms of modern 2D reflection seismic data over the deep-water Cape Three Points area of Ghana resulted in some unexpected structural observations. Most importantly, the new seismic data clearly image a large landward-verging thrust system beneath the marginal ridge. Interestingly, the seismic reflectors associated with the individual thrust imbrications within this “nappe” were attributed to prograding sediments by previous interpretations. However, the internal geometry of the allochthonous nappe system is identical to those observed at the leading edge of classical folded belts. A recently acquired 1,500 square km 3D survey confirmed the initial structural interpretation and revealed additional details about the geometry of the folded belt. Additional evidence for regional-scale compressional deformation is provided by a series of inverted syn-rift half-grabens and a small, but very well-developed “foredeep basin” which formed due to the load of the incoming folded belt. Whereas the numerous anticlines are undoubtedly the result of repeated transpressional deformation along the transform margin, it is the dominating compressional component which is responsible for the presence of a number of plays identical to those found in folded belts. The folding of the Cenomanian foredeep sequence occurred during the Senonian. This intra-Senonian compression might be the same event which have been recorded in many other regions of Equatorial Africa, such as Gabon, Equatorial Guinea, Cameroon and Nigeria.