Revealing the Complexities of the Gabon Southern Margin Through Gravity, Magnetics, and Seismic Data Integration

Abstract

A new multi-client broadband 3D survey located off the SW coast of Gabon has revealed clues about the structural behaviour of this margin. Marine seismic, gravity and magnetic data were acquired concurrently, and interpreted using integrated 3D modelling. The rifting of the South Atlantic margin resulted in the development of a deep salt basin, prior to the onset of oceanic crust further to the west. This extensional zone is accommodated by low-angle normal faults evidenced in the seismic data. It is 40km wide in the south towards the Congo border and narrows northward where it terminates against a detached continental ribbon. It is interpreted that this early phase of extension led to mantle exhumation in the south and transiting northwards to hyper-extended continental crust near the continental ribbon. Higher magnetic susceptibilities are modelled in the SE compared to lower ones to the NW. The marked magnetic contrast between these two areas is further correlated to South Atlantic spreading patterns evident in the global public domain gravity and magnetic datasets. The 3D gravity model required the emplacement of zones of higher density lower crustal structures, which correlated with the changes in seismic character. In the NW, the continental ribbon rests on low-angle detachment faults converging towards the Moho interface, with trajectories connecting both flanks of the ribbon at depth. We interpret that the embrittlement of the lower crust and subsequent coupling with the upper crust favoured these regional detachments along the Moho interface. These features can be interpreted as release detachments, overlapping and interlinked through complex relay ramps drawing an upper-plate model. The lateral transition in structural style from low angle brittle faults to deep rooted detachment faults occurs at the boundary between the magnetic susceptibility areas. In this setting, the relatively low density of the continental ribbon was surrounded by denser material from the lower crust, resulting in uplift by buoyancy during the Aptian-Albian. The seismic interpretation of the crest of the continental ribbon shows a clear network of drainage features formed by sub-aerial exposure, which is proof of local uplift. Processes on a relatively small scale, such as drainage patterns, are put in the context of the tectonic changes observed at the crustal scale, via a quantitative 3D integrated model of the study area, tied in turn to regional gravity and magnetic grids.

AAPG Datapages/Search and Discovery Article #90332 © 2018 AAPG International Conference and Exhibition, Cape Town, South Africa, November 4-11, 2018