Deep‐penetrating MCS images of the South Gabon Basin: implications for rift tectonics and post‐breakup salt remobilization

Abstract

Abstract Rifted margin architecture along part of the southern Gabonese margin is interpreted from four deep‐penetration, multichannel seismic reflection (MCS) profiles. A series of synthetically faulted crustal blocks are identified, separated by dominantly seaward‐dipping fault zones formed during Cretaceous rifting between Africa and South America. Extensional strain ratios are ≅ 1.5. These faults appear either to transect the entire crustal section or are interrupted by discontinuous zones of midcrustal reflections which may represent detachments.Outer acoustic basement highs are situated just seaward of the continental 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 continental 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 °165 km south‐west of the edge of the continental shelf. The salt forms an° 1.5‐km‐thick horizon overlying the outer highs, and it may be autochthonous there, suggesting salt was deposited contemporaneously with emplacement of proto‐oceanic crust.Differential subsidence and tilting between continental rift‐blocks during post‐rift margin subsidence has resulted in a sympathetic terrace‐ramp geometry in overlying Aptian salt. Salt terraces form above tops of crustal blocks, where salt tends to rise vertically, creating pillows and diapirs. Ramps connecting terraces tend to form above seaward‐facing fault zones; salt flowage there has been both lateral and vertical, creating triangular diapirs along the footwalls of growth faults. Most of these growth‐faults sole within the salt base, but a few continue into the interpreted synrift succession.