Résumé
The seismic sections of the post-rift sequence on the Gabon continental margin have been correlated with borehole-controlled chronologies to analyse the evolution of subsidence and tectonic activity from the Albian to the Present. The subsidence pattern has been approximated by backstripping wells. Backstripping results suggest a punctuated pattern of subsidence with a sudden decrease in the Coniacian-Santonian, and brief periods of increased subsidence in the Maastrichtian, the late Eocene and the late Miocene. Tectonics of the post-rift sequence is dominated by gravity-driven salt tectonics. Reconstruction of the activity of individual listric faults and of the decollement on two reference sections indicate that salt tectonic activity was strongly heterogeneous across and along the margin, as well as discontinuous with time. Data indicate reactivation of pre-salt transfer faults, normal to the margin, which were not previously reported. Activity on these transfer-faults may record regional tectonics affecting the African lithospheric plate. Temporal correlation of the rates of tectonic subsidence, salt tectonic activity, and basement fault activity, suggests an interaction of local (e.g. sedimentary load) and regional (e.g. thermal subsidence, far-field stress...) controls on the evolution of the margin. The post-rift evolution of the S. Gabon margin features an initial period of subsidence associated with intense salt tectonic activity, triggered by tilting of the margin. Subsidence and salt tectonics stopped in the late Coniacian-Santonian. This correlates with the "Santonian event" recorded throughout Africa, and with a drop in S. Atlantic spreading rate. The late Eocene and late Miocene accelerations of subsidence are associated with renewal of salt tectonics, the later driven by increases of sedimentation rate (the Zaire deep-sea fan). The late Maastrichtian and late Miocene pulses correlate with period of basement tectonic reactivation, contemporaneous with tectonic activity recorded in continental Africa. This study shows that, although subdued compared to gravity-driven salt tectonics, regional (platescale) tectonics expressed by reactivation of pre-salt basement faults, may be recorded in the sedimentation of the continental passive margins. This has important bearings on modelling the thermal evolution and the petroleum systems on continental "passive" margins.