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

Abstract

SUMMARY The deep structure of the West African continental margin between 5°S and 8°S was investigated using vertical reflection and wide‐angle reflection/refraction techniques, during the ZaïAngo 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.

South Atlantic continental margins of Africa: A comparison of the tectonic vs climate interplay on the evolution of equatorial west Africa and SW Africa margins

“…However, these typical rift-related structures are markedly absent in the middle and lower margin of equatorial western Africa margin (Moulin, 2003;Contrucci et al, 2004). Across this segment (Fig.2a), the continental crust rapidly thins down to less than 10 km over a distance of 50km.…”

“…The margin structure north of the Walvis Ridge has been investigated by several deep-seismic experiments since the early 90's and an increasing amount of data is being released and reprocessed Wannesson et al, 1991;Meyers and Rosendahl, 1991;Moulin et al, 2001;Dupré, 2003;Contrucci et al, 2004). The continental margin across the Angola segment comprises a 150 km-wide zone of extremely thinned continental crust (5 to 8 km thick), which is overlain by a 10-12 km sedimentary prism, and underlain by an anomalously high velocity zone, at the transition with moderately extended continental crust.…”

“…The continental margin across the Angola segment comprises a 150 km-wide zone of extremely thinned continental crust (5 to 8 km thick), which is overlain by a 10-12 km sedimentary prism, and underlain by an anomalously high velocity zone, at the transition with moderately extended continental crust. The origin of this anomalous interval could represent either an abnormal lower crust or mantle (Contrucci et al, 2004), or underplated material (Dupré, 2003). The narrow transition between extended continental crust and normal oceanic crust occurs a few kilometres landward of the Angola Escarpment (a prominent north-south bathymetric step at the slope-basin boundary).…”

Crustal breakup and continent-ocean transition at South Atlantic conjugate margins

“…Therefore, the high-amplitude and subhorizontal band of seismic reflections have been interpreted as the top of a high-density/high-velocity lower crustal body which is spatially related to a broad and prominent "edge effect" gravity anomaly high (Figure 2c). Similarly, an enigmatic high-velocity lower crustal body has also been documented farther south [Contrucci et al, 2004;Moulin et al, 2005]. Such a lower crustal body is not necessarily breakup related, but may be related to older tectonic episodes and, possibly, orogenies; e.g., inheritance from Transamazonian and Pan-African collision belts [e.g., Meyers et al, 1996a].…”

“…Since an anomalous high-velocity/density lower crust has been described along the northern Angola (Lower Congo Basin) [Contrucci et al, 2004] and southern Gabon margins [Wannesson et al, 1991;Blaich et al, 2010], the possibility exists that such lower crust is also present along the Kwanza Basin. However, with the available data and due to the nonunique interpretation of gravity anomalies, it is impossible to resolve this issue.…”