ALBERT

Description: The South Atlantic Ocean evolved after rupture of the Sao FranciscoCongoRio de la PlataKalahari cratonic landmass and the Late Proterozoic fold belts. Break-up in the South Atlantic realm developed diachronously: rifting started in the south (Argentina) during the Jurassic and progressed towards the equatorial segment. The central portion was controlled by a rift-resistant cratonic nucleus (the Sao FranciscoCongo craton) and as a result underwent development of narrow basins; parts controlled by Neoproterozoic fold belts developed wide basins. The final break-up of western Gondwana and the onset of plate divergence were marked by thick wedges of seaward-dipping reflectors, located near the incipient ocean-ridge spreading centre that had already been formed by the time Aptian evaporites were deposited. Subsequently, a few episodes of intraplate tectonic and magmatic activity affected the Santos, Campos and Espirito Santo basins. Post-break up development of the offshore basins was affected by gravity gliding over the Aptian evaporites. Continental uplift may be invoked as the main cause of salt mobilization, generating prograding clastic wedges that thickened basin-wards and produced a loading effect on the salt basin. Coupled with onshore erosional unloading and the effects of the gravity gliding, this probably resulted in further flexural uplift of the continental margin.

Description: Seismic reflection and refraction profiles, and potential field data, complemented by crustal-scale gravity modelling, plate reconstructions and well cross-sections are used to study the evolution of the South Segment of the South Atlantic conjugate margins. Distinct along-margin structural and magmatic changes that are spatially related to a number of conjugate transfer systems are revealed. The northern province, between the Rio Grande Fracture Zone and the Salado Transfer Zone, is characterized by symmetrical seawards-dipping reflections (SDRs) and symmetrical continent–ocean transitional domain. The central province, between the Salado Transfer Zone and the conjugate Colorado–Hope transfer system, is characterized by along-strike tectonomagmatic asymmetry. The Tristan da Cunha plume, located on the central province of the South Segment, may have influenced the volume of magmatism but did not necessarily alter the process of rifted margin formation. Thus implying that, apart from voluminous magmatism, the extensional evolution of the central province of the South Segment may have much in common with ‘magma-poor’ margins.

Description: This work integrates the available geological information and geochronology data for the Cretaceous–Recent magmatism in the South Atlantic, represented by onshore and offshore magmatic events, including the oceanic islands along the transform faults and near the mid-ocean ridge. The analysis of the igneous rocks and their tectonic settings allows new insights into the evolution of the African and Brazilian continental margins during the South Atlantic opening. Following the abundant volcanism in the Early Cretaceous, the magmatic quiescence during the Aptian–Albian times is a common characteristic of almost all Brazilian and West African marginal basins. However, rocks ascribed to the Cabo Granite (104 Ma) are observed in NE Brazil. In West Africa, sparse Aptian–Albian ages are observed in a few coastal igneous centres. In the SE Brazilian margin, an east–west alkaline magmatic trend is observed from Poços de Caldas to Cabo Frio, comprising igneous intrusions dated from 87 to 64 Ma. Mafic dyke swarms trending NW also occur in the region extending from the Cabo Frio Province towards the Central Brazilian Craton. On the West African side, Early Cretaceous–Recent volcanism is observed in the Walvis Ridge (139 Ma), the St Helena Ridge (81 Ma) and the Cameroon Volcanic Line (Early Tertiary–Recent). Volcanic islands such as Ascencion (1.0–0.65 Ma), Tristão da Cunha (2.5–0.13 Ma) and the St Helena islands (12 Ma) most probably correspond to mantle plumes or hot spots presently located near the mid-Atlantic spreading centre. Within the South America platform and deep oceanic regions, the following volcanic islands are observed: the Rio Grande Rise (88–86 Ma), Abrolhos (54–44 Ma), the Vitória–Trindade Chain (no age), Trindade (2.8–1.2 Ma) and Fernando de Noronha (12–1.5 Ma). There are several volcanic features along the NW–SE-trending Cruzeiro do Sul Lineament from Cabo Frio to the Rio Grande Rise, but they have not been dated. The only known occurrence of serpentinized mantle rocks in the South Atlantic margin is associated with the Saint Peter and Saint Paul Rocks located along the São Paulo Fracture Zone. The Cameroon Volcanic Line in NW Africa is related to the magmatism that started in the Late Cretaceous and shows local manifestations up to the Present. The compilation of all available magmatic ages suggests an asymmetrical evolution between the African and South America platforms with more pre-break-up and post-break-up magmatism observed in the Brazilian margin. This is most likely to have resulted from the different geological processes operating during the South Atlantic Ocean opening, shifts in the spreading centre, and, possibly, the rising and waning of mantle plumes.Supplementary material: A complete table with radiometric dates that have been obtained by universities, government agencies and research groups is available at: www.geolsoc.org.uk/SUP18596

Description: Integration of seismic, potential field, and borehole data from conjugate basins along the South Atlantic continental margin, particularly the northeastern Brazilian and northwestern African segments, indicates that the rift architecture is controlled by fracture zones that extend from the oceanic crust and penetrate through the continental crust, locally corresponding to Precambrian structures in cratonic regions. The fracture zones may divide the continental margin into several compartments with independent sedimentary depocentres, separate crustal domains along oceanic transforms, and affect the rift architecture by shearing. Oceanic transform zones may leak igneous rocks originated from the mantle. This work discusses conjugate sedimentary basins in the South Atlantic salt basins, particularly from Jacuipe to Sergipe-Alagoas on the Brazilian side, and from Gabon to Rio Muni on the African side. The following aspects are emphasized: (1) rift depocentres are controlled by border faults subparallel to the margin and by transverse faults that may continue as transform fractures in the oceanic crust; (2) the southernmost segment of the South Atlantic continental margins is characterized by Early Cretaceous volcanic rocks that underlie continental lacustrine Neocomian to Barremian syn-rift sediments; (3) the pre-rift sequences (Mesozoic and Palaeozoic sediments) that underlie the syn-rift depocentres in Gabon and Sergipe/Alagoas are mainly devoid of volcanics; (4) there is seismic evidence of magmatic underplating in the deeper portions of the continental crust, which are expressed by antiformal features locally aligned with transform fractures; (5) basement-involved extensional faults and volcanic activity along leaking transform faults are imaged along several conjugate segments of the margin, particularly along the equatorial margin (Romanche fracture zone); (6) in some segments of the divergent margin, the transition from outer rift blocks to oceanic crust is characterized by wedges of seaward-dipping reflectors with a possible origin associated with emplacement of oceanic ridges; (7) locally, the outermost rift blocks near the continental-oceanic crust boundary seem to be highly eroded by post-rift uplift caused by transform fault shearing or by magmatic underplating; (8) tectonomagmatic episodes climaxed in the Late Cretaceous/Early Tertiary in northeastern Brazil and extended to the Late Tertiary on the West African margin, forming large volcanic complexes along transverse lineaments that affect both oceanic and continental crust.

Description: This work discusses concepts related to the occurrence of salt along weakness planes, such as faults and fractures, which resemble igneous intrusions and may result in peculiar seismic features. We suggest that mechanisms for the formation of these structures basically involve the creation of extensional faults (commonly associated with crestal collapse grabens), which are rotated and migrated to structural flanks by domation, creating interesting seismic features here referred to as halokinetic rotating faults. At the time of their formation, some of these faults may be incipiently intruded by salt as a way of relieving sporadic intense internal overpressure episodes in the salt body, by regional compression, and/or by buoyancy effects compensating the density difference between salt and surrounding sediments. The relatively low overburden pressure at the crest of the diapir and the original high dip angles of these fault planes favor salt intrusions near the diapir apex. The process may occur in several cycles along the salt dome evolution, creating several generations of salt apophyses positioned in the diapir apex and flanks, resulting in different dips and areas of extension. These intrusions sometimes resemble the branches of Christmas tree structures, which are commonly formed by extrusive mechanisms. Although well and seismic data point to the occurrence of salt along fault planes, we recognize that salt is not a low-viscosity fluid, and the mechanisms to allow its penetration along fault planes remain unknown. Some of the possible mechanisms, which are commonly associated with a later phase of regional compression, are discussed in this work. The implications for petroleum exploration may have been overlooked in the recent exploration campaigns in the deep-water regions of the Brazilian margin. Halokinetic rotating faults, when partially filled with salt, are sometimes responsible for common pitfalls observed in seismic and well data interpretation. When fault planes present subhorizontal dips and high reflectivity, caused by the presence of salt, they have been mistakenly interpreted as flatspots, a well-known seismic hydrocarbon indicator. When drilled and proved to correspond to thin evaporite intervals in well data, these salt apophyses have also been misinterpreted as younger localized evaporitic events overlying the main salt body.