Deep structure of the West African continental margin (Congo, Zaire, Angola), between 5°S and 8°S, from reflection/refraction seismics and gravity data

The deep structure of the West African continental margin between 5 degreesS and 8 degreesS was investigated using vertical reflection and wide-angle reflection/refraction techniques, during the ZaiAngo project, a joint programme conducted in 2000 April by Ifremer and TotalFinaElf. To penetrate below the salt layer, a non-conventional, low-frequency seismic source was used in the 'single-bubble' mode, together with ocean bottom instruments (hydrophones and seismometers) and a 4.5 km long streamer that recorded multichannel seismic reflection (MCS). The data show that the continental crust thins abruptly over a lateral distance of less than 50 km, from 30 km thick below the continental platform (based on gravity data), to less than 4 km thick below the Lower Congo Basin that formed prior to the Aptian salt deposition. This subsalt sedimentary basin (180 km wide, 4 km thick, with velocities varying from 4.7 km s(-1) to 5.8 km s(-1) at the bottom) is located between the foot of the continental slope and the oceanic domain. It is underlain by crust of an intermediary or transitional type, between continental crust and what can be recognized as oceanic crust. In the transitional zone, a crustal upper layer is present below the pre-salt sedimentary basin, 3 to 7 km thick, with velocities increasing from 5.8 km s(-1) at the top to 6.8 km s(-1) at the bottom of the layer. This layer appears to thin regularly, from 6-7 km thick below the depocentre of the pre-salt sedimentary basin to 3-4 km thick below the western termination of the basin. Below this upper crustal layer, an anomalous velocity layer (7.2 to 7.8 km s(-1)), is documented, below the eastern side of the basin, where the crustal thinning is at a maximum. The origin of this layer is unknown. Several arguments, like rifting duration (between 15 Ma and 30 Ma) or the absence of seaward-dipping reflectors, precludes the hypothesis of underplated mantle material, but other hypotheses (such as serpentinized material or high-grade metamorphic crustal rocks or a mixture of mafic and ultramafic crustal rocks) are plausible. Near the ocean termination of the basin, the transitional zone is bounded to the west by a basement ridge that is clearly documented on two profiles ('7 + 11' and 14) having a dense ocean bottom seismometer/hydrophone (OBS/OBH) spacing. On these profiles, an anomalous velocity layer is present in the westernmost part of the transitional zone (below the basement ridge) and in the oceanic domain. This layer, absent on profile 3, may be related either to oceanization and slow seafloor spreading processes or to a consequence of the rifting process.