Abstract The main objective of this book is to provide a global overview of divergent margins based on geological and geophysical interpretation of sedimentary basins along the South, Central and North Atlantic conjugate margins, from plate tectonics and crustal scales to a more detailed description of stratigraphical and structural elements that are responsible for petroleum plays. These themes are complemented by geodynamic concepts based on physical and numerical models, and by comparisons with present-day embryonic margins, which are succinctly discussed in some papers. Supplementary material: Three plate animations of the Atlantic Ocean are available at www.geolsoc.org.uk/SUP18620 .

Abstract This chapter presents a synthesis of the pre-break-up plate tectonics of western Gondwana and the pre- and syn-rift magmatism in the SW South Atlantic margin (Brazil, Uruguay and Argentina) and the conjugate African continental margin (Angola, Namibia and South Africa). An updated database of the geographical locations of the primary available radiometric ages is also included in this work. A systematic analysis of the K–Ar and Ar/Ar ages from outcrops and boreholes shows a marked Mid to Late Jurassic peak in the southernmost segment of the South Atlantic, related to the emplacement of the Karoo volcanics in South Africa and in Argentina (including the Falkland Islands), and an important Early Cretaceous peak with age distributions that are related to the Gondwana break-up and formation of rift basins along the incipient continental margins. In both the southern Brazilian and Argentinian margins, as well as in the conjugate Namibian and South African margins, several igneous centres and basaltic lava flows are suggestive of the influence of mantle plumes in the Early Cretaceous, which were heralded by mafic dyke swarms in Argentina, Brazil, South Africa and Namibia. Supplementary material: A complete table with Radiometric ages of Jurassic–Early Cretaceous magmatism in the southern portion of West Gondwana is available at: www.geolsoc.org.uk/SUP18589

Abstract Conservational models, like simple shear, pure shear or polyphase models that exclude exchanges between the lower continental crust and upper mantle, are usually proposed to explain the lithospheric stretching and consequent crustal thinning of passive continental margins. These models need large amounts of horizontal movement, and have, therefore, important implications for plate kinematic reconstructions and intraplate deformation. In this paper we propose to show these implications in the Central Segment of the South Atlantic Ocean. In the Angola–Brazilian system, these models imply about 240 km of horizontal movement. This movement can be compensated by two end-member mechanisms: (1) an intraplate deformation located in Africa; and (2) an intraplate deformation located in South America. We detail for each solution the strong geological and geodynamical implications, and discuss the consequences for the genesis of passive continental margins.

Abstract Understanding the genesis of the very peculiar 600 km-wide Santos Basin–São Paulo Plateau system and its narrow conjugate Namibe Margin is a kinematic and structural problem. Several hypotheses have been proposed in order to explain the genesis of this system that imply the same amount of horizontal movement. We investigate the consequences of the horizontal movement in the Santos Basin, based in plate kinematic reconstructions. The kinematic history of this system that we present here, based on the interpretation of seismic profiles and kinematic constraints, has the following consequences: (1) there is no evidence of a ridge jump sensu stricto but, rather, a southwards propagation in the Central Segment of the South Atlantic that starts in the northern part, between the NE Brazilian and Gabonese margins; (2) the Namibe margin evolved as a transform passive margin; (3) the opening direction of the Santos Basin–São Paulo Plateau system is oblique to the general opening motions of the South American and African plates; and (4) this opening is younger (6 Ma) than those of the other basins of the Central Segment of the South Atlantic.

Abstract The first comprehensive geological and geophysical surveys of the Brazilian continental margin during the 1970s recognized the crust in the SE Brazilian basins as ‘anomalous’ but models for the opening of the South Atlantic proposed at that time invoked a very narrow continent–ocean transition. Nevertheless, such studies established the presence of a thick sedimentary prism, including an extensive salt layer under the São Paulo Plateau. The earliest reconstructions for the South Atlantic invoked a seaward shift of the spreading axis to account for the asymmetric widths of the salt layer between the Brazilian margin and its conjugate in offshore Africa. Although our understanding of continent–ocean transition has progressed since then, direct seismic imaging at crustal scale has only been possible recently through long offset (10 km), deep recording (18 s), pre-stack depth migrated (PSDM) to 40 km, seismic-reflection data. These data allow us to generally image the Moho from under thick continental crust (>30 km) to thin oceanic crust ( c. 5 km). Although the nature of the transitional crust is still contested, these seismic data allow for constraints on various models for continent–ocean transition. Future integrated studies utilizing PSDM and refraction-seismic data will further refine these models.

Abstract New seismic and well data in the deep-water basins of Campos, Santos, South Kwanza and Benguela, supported by plate reconstructions, help answer fundamental questions on the rifting history of the central South Atlantic, specifically on the amount and effect of fault-related deformation, and on when and where sea-floor spreading started. The Paraná mantle plume played a fundamental role – dynamically raising the plate, prolonging continental rifting by heat-softening the crust and, after break-up, delaying the onset of marine conditions. Previous discrepancies in extension and subsidence have been solved, and the location and age of the continent–ocean boundary can now be determined. Rifting involved approximately 450 km of homogeneous pure shear, equivalent to a β factor (lithosphere stretching factor) of 4.5. Break-up occurred at 123 Ma (Barremian–Aptian boundary), 7–8 Ma later than the southern South Atlantic but 6 Ma before widespread salt deposition. The mid-Atlantic ridge was initially subaerial, marked by a volcanic high. Sea-floor spreading was at a rate of 24 mm year −1 , similar to syn-rift deformation prior to break-up. Transcontinental strike-slip shear zones are not evident but a major NW–SE lithospheric lineament associated with a failed triple junction arm had a major influence on the magmatic history, both prior to and after break-up. Supplementary material: A4 versions of the seismic sections shown in Figures 6 & 7 are available at http://www.geolsoc.org.uk/SUP18563 .

Abstract The Almada Basin, located in the southern Bahia State segment of NE Brazil, shares a similar sedimentation history and stress regime with the other eastern Brazilian basins. But when considering the composition of the transitional crust, a remarkably different behaviour is observed between the Almada Basin and the other eastern Brazilian basins. The architectural elements of the basin such as the reflection Moho discontinuity, the oceanic and continental basements, and the sedimentary section, as well as the tectonic style, are discussed in this study using gravity, seismic, regional geology and well-drilling results. The Almada Basin is part of a continental rift system that developed during the Early Cretaceous, heralding the Gondwana break-up and subsequently evolving into a passive divergent margin. Deep seismic profiles show the progressive thinning of the continental crust but there is no clear evidence of mantle exhumation. The Almada Basin is the conjugate margin of the South Gabon Basin in West Africa. Although both basins display similar stratigraphic record and geological evolution, the rifting mechanism resulted in a considerable asymmetric break-up. The Almada Basin is floored by the Itabunas–Salvador–Curaçá Belt of the São Francisco Craton and is characterized by Palaeoproterozoic granulites in a high-angle east-dipping thrust fault regime. However, the South Gabon Basin lies on the Neoproterozoic West Congolian Belt that is made up of low- to medium-grade metamorphic rocks in a low-angle thrust tectonic regime. The Precambrian basement fabric of both the Almada and South Gabon basins is marked by lithospheric-scale discontinuities that controlled the implantation of the rift zone and the extension style during the Mesozoic. The strong basement structural inheritance can be recognized in both the geological and geophysical records. The key architectural elements of a volcanic margin, such as large igneous provinces, seaward-dipping reflectors and the basinal syn-rift magmatism, are not recognized in the Almada Basin. Although the South Atlantic margin is mostly volcanic in the southernmost segment, the presence of these two non-volcanic segments – the southern Bahia State and the southern Gabon – are important as a record of the differences in the geotectonic process that governed the formation of the South Atlantic divergent margins.

Abstract The discovery of the Jubilee Oil Field in 2007 in the Tano Basin of Ghana is a primary example of the potential for Late Cretaceous deep-water stratigraphic traps in basins previously overlooked by the exploration industry. The Tano Basin forms the eastern extension of the Deep Ivorian Basin, which resulted from Aptian–Albian trans-tension associated with the opening of the Atlantic between the St Paul and Romanche fracture zones and the subsequent post-rift subsidence. The basin was the focus for deposition of a thick Upper Cretaceous, deep-water clastic sequence, which, in combination with a modest Tertiary section, provided sufficient thickness to mature a Cretaceous source rock in the central part of the Tano Basin. This well-defined reservoir and charge fairway forms the play, which, when draped over a large plunging nose (the South Tano high), resulted in the formation of a stratigraphic–structural combination trapping geometry that is characteristic of the Jubilee Field and subsequent discoveries in this play.

Abstract Tectonic reconstructions made across the southern South Atlantic Ocean indicate a diversity of rift and drift basin characteristics on the conjugate margins that define them as different stratigraphic and structural entities. In terms of petroleum systems, the basins are not as unlike as some characteristics suggest. Given the lack of significant hydrocarbon discoveries to date south of the Walvis Ridge, doubts have been cast on the presence in this area of the prolific Lower Cretaceous lacustrine and marine source rock systems, which are well known in the Greater Campos Basin and offshore Angola. Oils and condensates from the basins south and north of the Walvis Ridge exhibit geochemical similarities suggesting that comparable source rock systems are present in both areas. The condensate geochemical analysis results from the Kudu Field in Namibia are compared with oils from marine and lacustrine sources in Brazil, indicating that the Kudu condensates are derived from at least two different source rocks. These results suggest that the underexplored basins offshore Namibia contain thermally mature Lower Cretaceous lacustrine and marine source rocks, offering a new frontier for petroleum exploration in Africa's southern South Atlantic.

Abstract The Nova Scotia and Morocco margins formed within a transition between volcanic margins to the south and non-volcanic margins to the north. We present deep seismic profiles to document the nature of this transition. Profiles on the Nova Scotia margin show two major transitions. The first transition represents a sharp reduction in syn-rift volcanism coincident with major changes in the East Coast Magnetic Anomaly and with the southern limit of the Slope Diapiric Province. The second transition represents a further restriction in syn- and post-rift volcanism leading to exposure of serpentinized mantle or the creation of highly tectonized oceanic crust. This transition is marked by highly extended and faulted continental crustal blocks. Revised plate reconstructions show similar transitions along the Moroccan margin. The southern transition occurs at a major change in the West Africa Coast Magnetic Anomaly and the southern limit of the Morocco Salt Basin. The second transition occurs at a major basement high (Tafelney Plateau), which is considered a high relief accommodation zone and contains highly extended faulted crustal blocks similar to those in a conjugate position off central Nova Scotia. This transition marks a major change in rifting asymmetry and separates the margins into two fundamentally distinct segments.

Abstract Regional gravity data and a three-dimensional (3D) inversion approach were used to examine variations in crustal thickness and to extend seismic-based interpretations along the Scotian margin, Atlantic Canada. Constraints on Moho, crust and sedimentary layers were based on seismic refraction models and profiles of deep multichannel seismic data. A subsurface 3D density anomaly distribution was developed, using initial constraints from bathymetric data and seismic basement depth estimates, and inverting for crustal and subcrustal geometries. Predictions from the model include regional maps of Moho structure, crustal thickness and stretching factor. Depth slices allow comparison with seismic interpretations along three transects. The model predictions of variable width of crustal extension and northward thinning of oceanic crust agree with the seismic profiles. The density anomaly model shows significantly greater thinning and numerous block faulting beneath the northern margin and more uniform thinning to the SW. The fairly sharp transition between the two regions may be explained by a transfer zone separating different rifting regimes.

Abstract We use seismic, field, core, borehole and vitrinite-reflectance data to constrain the development of the Newark Rift Basin, one of the largest and most thoroughly studied basins of the eastern North American rift system that formed during the break-up of Pangaea. These data provide critical information about the geometry of the preserved synrift section and the magnitude of post-rift erosion. We incorporate this information into a new structural restoration of the basin. Our work shows that the Newark Basin was initially narrow (<25 km) and markedly asymmetric; synrift strata show significant thickening towards the basin-bounding faults. Subsequently, the basin became wider (perhaps >100 km wide), deeper (up to 10 km) and less asymmetric; synrift strata exhibit subtle thickening towards the basin-bounding fault system. Several intrabasin faults dissected the Newark Basin after synrift deposition, and the basin fill was tilted ( c. 10°NW) and folded. Erosion (up to 6 km) accompanied the intrabasin faulting, NW tilting and folding, significantly reducing the basin size. Our work suggests that the eastern North American rift system is characterized by a very broad zone of upper-crustal extension in which a few, wide, deep, long-lived, fault-bounded basins (like the Newark Basin) accommodated much of the extension.

Abstract During the complex evolution and transition of the United States (US) Central Atlantic from a rift system to a passive margin, five post-rift sedimentary depocentres developed along its approximately 1850 km length. Varying in size, shape and thickness of sediment fill, these depocentres are separated by interbasin arches and regions lacking major post-rift sedimentary depocentres. From 1976 until 1984, a single phase of exploratory drilling was carried out in three of these depocentres. Located primarily on the continental shelf, the tested play types resulted in a single, modest natural gas discovery. The drilling clarified the risks of various petroleum system elements and processes in the areas and plays tested. During 2010, a new resource inventory covering the area was completed by a team of Bureau of Ocean Energy Management geologists and engineers. The inventory incorporated and applied modern exploration concepts and key new learnings from NE-adjacent offshore Nova Scotia, conjugate NW Africa and the African transform margin. Nine new conceptual plays and a single proven high-risk play have been identified and their resources inventoried.

Abstract The Moroccan salt basin appears to be very unique in the sense that it is not sedimentary loading but tectonic inversion which appears to drive the latest stages of salt-related deformation in the central part of the basin. The gravity potential to maintain salt tectonics is provided by the differential uplift of the Atlas Mountains, located adjacent and striking almost perpendicular to the margin. Along the offshore part of the Moroccan salt basin there are many play types, most of them related to the Triassic syn-rift salt. Toe-thrust anticlines at the basinwards edge of the salt basin form very large structures. Traps associated with salt tongues and diapirs define a more ‘classical’ salt-flank play. Numerous salt tongues, sheets and canopy complexes provide for a ‘Gulf of Mexico-style’ subsalt play. Despite the numerous untested play types, there have been only four deep-water exploration wells drilled in the entire Moroccan salt basin, none of them having subsalt penetrations.

Abstract Deposition of the Abenaki Formation carbonate complex began during the Middle Jurassic, shortly after the interpreted onset of western Atlantic sea-floor spreading, and continued through Late Jurassic time, resulting in thick carbonate deposits along the Jurassic shelf edge. This carbonate complex was the exploration target of 16 wells drilled from 1970 to 1989. Although there were no discoveries in the carbonate objectives, oil was discovered in Cretaceous siliciclastics in structural drape closures overlying the carbonate margin. The Province of Nova Scotia decided to undertake a technical study to further the interest in the carbonate bank objective while the shallow oil play was being developed. Subsequently, in 1989, two studies were undertaken to investigate a porosity play in the Jurassic Baccaro Member of the Abenaki Formation because significant porosity had been encountered in a number of the dry holes that targeted the carbonate margin, particularly the Demascota G-32 well. The first study modelled porosity in several wells utilizing one-dimensional (1D) and wedge models, and amplitude v. offset (AVO) models. The Demascota G-32 well was used to seismically identify and model several types of potential hydrocarbon-bearing prospects along the bank. Modelling indicated that zones of 11–14% porosity with a minimum thickness of 10 m could be detected. However, a thickness of 50 m was required to resolve the top and base of the porosity. The AVO model for the Demascota well demonstrated that an increase in amplitude with offset (AVO effect) in the presence of gas and fracturing (which dramatically decreases Poisson’s ratio) should be detectable and may be useful in identifying prospects in this type of play. The assumption of a fractured reservoir was reasonable due to the number of seismically identified faults at the front of the carbonate bank and the description of fractures in conventional cores taken in the Baccaro Member. The second study involved acquiring a pair of dip and strike seismic lines adjacent to the Demascota well and reprocessing them to preserve relative amplitudes while providing a high-resolution, stacked section, especially at the Baccaro Member level. Interpretation of these lines was undertaken in the AVO domain. An increase in amplitude with offset was observed up-dip of the Demascota G-32 well that indicated a fractured, gas-filled porous zone could exist up-dip of this well. Following these studies, 3D seismic data was acquired over the shallower Panuke Oil Field and similar studies were conducted on that dataset in 1997. As a result, Encana drilled a structural high containing bright amplitude reflectors and encountered over 100 m of net gas pay in vuggy and cavernous limestones and dolomites in 1999. This validated the earlier studies, confirming that these reservoirs could be seen on seismic data and proved that economic gas reservoirs exist in the carbonate bank. These results should encourage exploration along the extensive, sparsely drilled Late Jurassic carbonate margins bounding offshore eastern North America and its conjugates in the Central Atlantic Region.

Abstract Continental break-up mechanisms vary systematically between slow- and fast-extension systems. Slow-extension break-up has been established from studies of the Central Atlantic, European and Adria margins. This study focuses on the intermediate and fast cases from Gabon and East India, and draws from the interpretation of reflection seismic, gravimetric and magnetic data. Interpretation indicates continental break-up via continental mantle unroofing in all systems, with modifications produced by magmatism in faster-extension systems. Break-up of the intermediate-extension Gabon system involves partial upper continental crustal decoupling from continental mantle; whereas, in the fast East Coast India system, decoupled and lower-crustal regimes underwent upwarping in ‘soggy’ zones in the footwalls of major normal faults. Usually, upper-crustal break-up is affected by pre-existing anisotropies, which form systems of constraining ‘rails’ for extending continental crust. This modifies the local stress regimes. They regain a regional character as the function of constraining rails vanishes during progressive unroofing of the upper mantle. Different regions attain different amounts of upper-crustal stretching prior to the break-up. The break-up location is then controlled by the upper-crustal energy balance principle of ‘wound linkage’, by which the minimum physical work is performed for linking upper-crustal ‘wounds’, leading to successful upper-crustal break-up. Supplementary material: Supplementary information and figures on the modelling of the mechanisms and architecture is available at http://www.geolsoc.org.uk/SUP18525 .

Abstract Hyperextended, magma-poor margins are characterized by a wide continent–ocean transition and anomalously small fractions of magmatism during mantle exhumation prior to oceanic spreading. Here, I bring together several aspects of their rift to drift transition and give a coherent picture of their evolution from platform to deep-sea environments. I focus mainly on the West Iberia Margin (WIM)–Newfoundland (NF) conjugates in the North Atlantic Ocean. The architectural evolution of these margins is characterized by upper-crustal faulting and lower-crustal deformation that are tightly coupled, resulting in fault displacement that is accompanied by underlying, equal and coeval crustal thinning. Lower crust deforms first ductilely, but then progressively switches to brittle due to enhanced conductive cooling at very slow extension velocities (< c. 6 mm a −1 half-rate). The switch from ductile to progressively brittle lower crust is accompanied by the emergence of a dominant basinwards fault dip and oceanwards younging of fault activity. It is shown that these processes, acting in concert: (1) reconcile the horizontal extension on faults with crustal thinning without the need of lower-crustal flow; (2) explain, within one common Andersonian framework (faults active at 65°–30°), the change in fault geometry from planar to listric to detachment-like with increasing extension; and (3) generate the tectonic asymmetry observed between conjugate pairs. This work also discusses a high-resolution seismic section of the WIM showing that the ‘detachment-like’ fault S is truncated prior to the peridotite ridge where mantle exhumation first takes place. This suggests that serpentinized mantle rises due to its own buoyancy, separating and pulling the thinned crustal blocks apart. Once the crust has been separated, further mantle exhumation takes place by magma-poor extension of the underlying mantle. I show with numerical models that either a small reduction of mantle potential temperature ( c. 25–50 °C), a mantle depletion of more than 10% or very slow half-extension velocities ( c. 6 mm a −1 ) are required to reproduce the small amount of magmatism inferred. Available observations support either a very slow extension velocity or a smaller than normal mantle temperature; however, estimation errors may be large. Ultimately, unravelling which of these factors most contribute to magma-poor mantle exhumation will provide an improved understanding of the mantle lateral homogeneity and the three-dimensional nature of the rifting to drifting process.

Abstract Two different types of ‘transitional lithosphere’ have been documented along magma-poor rifted margins. One consists of apparently subcontinental mantle that has been exhumed, brittlely deformed, and serpentinized during late stages of rifting. A second is thinned (<10 km) continental crust, which in some cases is known to have been supported near sea level at least early in the rift history and thus is interpreted to reflect depth-dependent extension. In both cases, it is typically assumed that oceanic crust forms at the time that the brittle continental crust is breached or soon thereafter, and thus that transitional lithosphere has relatively limited width. Here three representative cases of transitional lithosphere are examined: one in the Newfoundland–Iberia rift and one at Goban Spur (both exhumed mantle), and one off the Angola–Congo margin (thin continental crust flanked seaward by apparently exhumed lower continental crust±exhumed mantle). Considering the geological and geophysical evidence, it appears that depth-dependent extension (riftward flow of weak lower continental crust and/or upper mantle) may be a common phenomenon on magma-poor margins and that this can result in a much broader zone of transitional lithosphere than has hitherto been assumed. Transitional lithosphere in this wide zone may consist of subcontinental mantle, lower continental crust or some combination thereof, depending on the strength profile of the pre-rift continental lithosphere. Transitional lithosphere ceases to be emplaced (i.e. ‘final break-up’ occurs) only when emplacement of heat and melt from the rising asthenosphere becomes dominant over lateral flow of the weak lower lithosphere. This model implies a two-stage break-up: first, the rupture of the brittle continental crust; and, second, the eventual separation of the ductile subcontinental lithosphere which is coincident with emplacement of normal oceanic crust. Well defined magnetic anomalies can form in transitional lithosphere that consists of highly serpentinized, exhumed mantle, and such anomalies therefore are not diagnostic of oceanic crust. Where present, the anomalies can be helpful in interpreting and dating the rifting history.

Abstract The segmented East Indian continental margin developed after the Early Cretaceous break-up from Antarctica. Its continental crust terminates abruptly without considerable thinning along the Coromondal strike-slip segment and thins considerably before it terminates in the orthogonal rifting segments. The segments have an exhumed continental mantle corridor oceanwards of them. This, proto-oceanic crust, corridor varies in width from segment to segment, indicating a relationship with varying break-up-controlling tectonics of the adjacent margin segments. The top of the proto-oceanic crust is imaged by a higher reflectivity zone, while its base does not have any distinct signature. A contorted system of reflectors represents its internal structure. Its gravity signature is a longer-wavelength anomaly with peak values up to 30 mGal less negative than surrounding values. Its magnetic signature is represented by a positive anomaly with peak values of 0–56 nT. Wide proto-oceanic segments are adjacent to margin segments that are preceded by the orthogonally rifting Cauvery, Krishna–Godavari and Mahanadi rift zones. A narrow proto-oceanic segment is adjacent to the margin segment initiated by the dextral Coromondal transfer zone. A combination of seismic interpretation and gravity/magnetic forward modelling indicates that proto-oceanic crust is most probably composed of lower crust slivers and unroofed hydrated upper mantle, being formed between the late rifting and the organized sea-floor spreading.