DEEP-PENETRATING MCS IMAGES OF THE SOUTH GABON BASIN - IMPLICATIONS FOR RIFT TECTONICS AND POSTBREAKUP SALT REMOBILIZATION

Rifted margin architecture along part of the southern Gabonese margin is interpreted from four deep-penetration, multichannel seismic reflec tion (MCS) profiles. A series of synthetically faulted crustal blocks are identified, separated by dominantly seaward-dipping fault zones fo rmed during Cretaceous rifting between Africa and South America. Exten sional strain ratios are approximate to 1.5. These faults appear eithe r to transect the entire crustal section or are interrupted by discont inuous zones of midcrustal reflections which may represent detachments . Outer acoustic basement highs are situated just seaward of the conti nental slope. On the combined basis of seismic geometry, an associated positive magnetic anomaly and an increase in free-air gravity, these outer highs are interpreted to mark faulted transitions from rifted co ntinental crust to 'proto-oceanic crust', presumably composed of mafic volcanic rocks and possibly slivers of attenuated continental crustal blocks. The outer edge of Aptian salt lies approximate to 165 km sout h-west of the edge of the continental shelf. The salt forms an approxi mate to 1.5-km-thick horizon overlying the outer highs, and it may be autochthonous there, suggesting salt was deposited contemporaneously w ith emplacement of proto-oceanic crust. Differential subsidence and ti lting between continental rift-blocks during post-rift margin subsiden ce has resulted in a sympathetic terrace-ramp geometry in overlying Ap tian salt. Salt terraces form above tops of crustal blocks, where salt tends to rise vertically, creating pillows and diapirs. Ramps connect ing terraces tend to form above seaward-facing fault zones; salt flowa ge there has been both lateral and vertical, creating triangular diapi rs along the footwalls of growth faults. Most of these growth-faults s ole within the salt base, but a few continue into the interpreted synr ift succession.