Architecture of the continental margin in the Gulf of Guinea as revealed by reprocessed deep-imaging seismic data

Abstract

Post-stack reprocessing of the PROBE seismic data set has resulted in a refined tectonic model of the continental margin between Cameroon and Southern Gabon. The margin is divided into the North Douala and Gabon Basins by the Kribi Fracture Zone, a 75 km-wide transform fault that trends NE-SW and intersects the coastline between 2-3 N. North of the Kribi Fracture Zone, oceanic crust extends essentially to the coastline and floors the narrow margin off Cameroon. Little or no offshore rift margin exists in the North Douala Basin. Reflection Moho beneath oceanic crust is relatively weak but continuous throughout the North Douala Basin, except under the aprons of the Cameroon Volcanic Line. Oceanic crustal thickness is extremely uniform in the North Douala Basin, averaging about 1.75 s in two-way travel time. A distinct NE-SW trending structural grain at both basement and reflection Moho levels occurs within the Kribi Fracture Zone. Much of the margin off Equatorial Guinea is influenced or controlled by fracture zone tectonics. South of the Kribi Fracture Zone, oceanic crust is offset about 350 km to the south-west, resulting in the broad rift margin off Gabon. Extension here has largely progressed without the complications (or simplifications) introduced by fracture zone tectonics. Rifting of the margin seems to involve brittle, upper continental crustal sheets overlying a series of ductile lower sheets or lenses that generally dip easterly to southeasterly. The ductile sheets may unroof toward outer highs, which mark the boundary between oceanic and continental lith-osphere. Strong reflection events are often associated with the ductile shear zones that define the lower sheets, but a single, continuous event representing reflection Moho cannot be delineated. We suggest the concept of a regional reflection Moho beneath rifted continental crust is invalid in this area. The tectonic and rheological models presented here may be useful in framing hydrocarbon exploration tactics and strategies along this passive margin. In a broader context, elucidation of margin architecture may be a cost-effective way to approach frontier passive margins in the next century.