Deep seismic structure of the continental margin in the Gulf of Guinea: a summary report

Abstract

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 zone that trends northeast-southwest and intersects the coastline between 2 and 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 northern Douala Basin. Reflection Moho beneath oceanic crust is relatively weak but continuous throughout the northern Douala Basin. Oceanic crustal thickness is extremely uniform in the North Douala Basin, averaging c. 1.75 s in two-way travel time. A distinct northeast-southwest 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 c. 350 km to the southwest, resulting in a broad rift margin off Gabon that is unaffected by major-offset fracture zones. This margin seems to have evolved by a form of imperfect pure shear, involving brittle, upper continental crustal sheets overlying a series of ductile lower sheets that generally dip easterly to southeasterly. The ductile sheets may unroof at outer highs, which mark the boundary between oceanic and continental lithosphere. Dominant reflection events are often associated with the ductile shear zones that define the lower sheets, but a regionally coherent event representing reflection Moho cannot be delineated. The concept of reflection Moho beneath rifted continental crust may be invalid. The tectonic and rheological models presented here can be used to frame 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.