Syn-rift regional subsidence across the West African continental margin: the role of lower plate ductile extension

Abstract New ostracode data from the West African margin indicate that the Outer Basin Sediment Wedge (also termed the ‘pre-salt wedge’ and the ‘pre-salt sag basin’) is Neocomian to Aptian in age and is contemporaneous with syn-rift deposits developed inboard of the Atlantic hinge zone. Despite the fact that the Outer Basin Sediment Wedge is clearly a syn-rift deposit, it does not exhibit any of the diagnostic characteristics of brittle deformation, such as the existence of normal faults and the faulting and rotation of crustal blocks. Such features are common between the Atlantic and Eastern hinges for the early stages of rifting between West Africa and Brazil, which occurred as a series of extensional phases commencing in the Berriasian and culminating in the Late Aptian. To reconcile the concomitant development of fault-controlled subsidence between the hinges and across the Atlantic hinge zone and sag-basin development seaward of the Atlantic hinge zone requires that: (1) extension seaward of the Atlantic hinge is the result of strain-partitioning between a relatively non-deforming upper crust (i.e. the upper plate) and a ductile-deforming lower crust and lithospheric mantle (i.e. the lower plate) during the second and third rift phases, while (2) between the hinges, early brittle deformation (normal faulting) progresses to ductile deformation in the third rift phase. During the third rift phase, lower plate ductile deformation across the entire region generated regional subsidence both seaward of the Atlantic hinge and between the hinges with little attendant brittle deformation. This extension style produced, directly or indirectly, a sequence of crucial events across the West African margin: (1) the development of the pre-Chela unconformity as lake level dropped in the Early Aptian, exposing the prograding deltas of the Argilles Vertes Formation; (2) the regional development of the Chela unconformity and transgressive lag deposits of the Chela Formation in the Mid-Aptian; (3) the development of regionally extensive, shallow-water, restricted marine conditions across the entire margin (between West Africa and Brazil) immediately prior to evaporite precipitation; and (4) the development of significant post-rift accommodation (deposition of the Late Cretaceous, Paleogene and Neogene formations) in the same region previously characterized by minor syn-rift faulting, repeated dessication cycles (allowing the precipitation of thick evaporites) and negligible erosional truncation of earlier syn-rift units. Previous workers have suggested that the Loeme evaporites were formed as part of the rapid, early post-rift phase of basin subsidence as the region became inundated by sea water across the Walvis Ridge. In this model, it is difficult to develop the restrictive environments required to deposit the thick (>1 km) evaporites of the Loeme Formation (and the equivalent Ezanga and Ibura evaporites of Gabon and Brazil, respectively) across the entire West African-Brazilian rift system. The existence of shallow-water environments across the entire region is not consistent with water depths determined from the relief of clinoform foresets existing immediately prior to evaporite deposition thus requiring tectonic uplift of the deep-water regions. These evaporites, therefore, appear to be part of the late-stage syn-rift sediment package and the break-up unconformity, if it exists, separates the Loeme evaporites below from the overlying Albian carbonates. A direct consequence of ductile extension is one of increased heat input accompanying the rift stage in those areas dominated by syn-rift sag-basin development. The distribution and amplitude of the heat pulse is governed by the geometry of the mid-crustal weak zone and the distribution and amplitude of the lower plate extension. Seaward of the Atlantic hinge zone, the maximum heat flow is predicted to be in excess of 200 mW m −2 , whereas between the hinge zones, the heat flow is significantly less and ranges between 20mW/m 2 and 100 mW/m 2 . Because sediment temperature is a function of thermal conductivity and thickness of sediment overburden, the viability of syn-rift sources and prospectivity of the deep-water West African margin will, to a large degree, depend on the delicate interplay between the cooling of the extended lithosphere and subsequent burial of source rocks as a function of time.