Role of subaerial volcanic rocks and mantle plumes in creation of South Atlantic margins: implications for salt tectonics and source rocks

Seaward-dipping re¯ectors (SDRs) represent ¯ood basalts rapidly extruded during either rifting or initially subaerial sea-¯oor spreading. Evaporites can form on this basaltic proto-oceanic crust, as in the Afar Triangle today. Evidence for SDRs in South Atlantic deep-water regions comes from proximity to the uniquely large Paranaà ±Etendeka volcanic province onshore, the Tristan and Gough hot spots, drilled volcanic rocks, and seismic pro®les showing SDR provinces more than 100 km wide, as much as 7 km thick, and thousands of kilometers long. SDRs are clearest adjoining the Aptian salt basins. However, we speculate that SDRs are also present but seismically obscured below the salt basins. We argue that the conjugate Aptian salt basins are post-breakup, not pre-breakup; they were separated from the start by a mid-oceanic ridge; distal salt accumulated on proto-oceanic crust, not rift basins. This hypothesis is supported by: seismic stratigraphy and structure; magnetic anomalies; plate reconstructions; and hydrothermal potash evaporites. An important implication for exploration is that thick basalts, rather than rift-age source rocks, may underlie distal parts of the salt basins

The West African salt-bearing rifted margin—Regional structural variability and salt tectonics between Gabon and Namibe

Salt-bearing rifted margins comprise some of the most structurally complex and economically important sedimentary basin settings such as the South Atlantic and the Gulf of Mexico salt basins. They are also involved with some of the largest uncertainties regarding the crustal and syn-rift basin architecture and supra-salt tectonic evolution, as well as the link between rifted margin architecture with salt deposition and post-rift gravity-driven salt tectonics. We thus conduct a margin-scale study along nearly the entire West African salt basin, from South Gabon to Namibe, combining a vast data set of 2D and 3D seismic and well data with gravimetric and magnetic data to analyse its along-strike rift and salt tectonic structural variability. We construct regional structural and thickness maps of key salt and post-salt intervals to depict the history of individual margin segments and to investigate (1) how rifting and rifted margin architecture influences post-rift salt tectonics evolution, (2) how these vary through time and space and (3) what are the controls between their different salt tectonic styles. We show that rifting and rift structures controlled the salt basin geometry, thickness and base-salt relief in different ways for the different margin segments, and drastically influenced their post-rift salt tectonic evolution. Differences in post-salt sediment supply and continental uplift also had a role in their salt tectonic evolution. The results also have general implications to understand the interplay between rifted margin architecture with post-rift salt tectonics for salt-bearing rifted margins.publishedVersio

Seismic stratigraphy of the offshore basins of Argentina : characterization and modeling of the South Atlantic passive margin dynamics

El carácter multi-episódico del rifting observado en la evolución de las cuencas extensionales mesozoicas del sector costa afuera Argentino fue integrado en un marco regional del desmembramiento del supercontinente de Gondwana, con una revisión de la cronología del rifting para las principales cuencas mesozoicas en el sector austral de Sudamérica y África. El objetivo del estudio se focalizó en los eventos de rift pre-Cretácico que culminaron en el Cretácico temprano con la apertura del océano Atlántico Sur. En el sector costa afuera de Argentina, particularmente en la zona de las cuencas del Colorado/Salado, se identificaron a partir de interpretación sísmica tres eventos de rift independientes. Un primer evento de rift se asoció con la reactivación extensional, ocurrida durante el Triásico tardío a Jurásico temprano, de antiguos cabalgamientos de la faja plegada y corrida de Ventania-Cabo del Paleozoico superior. Este primer conjunto de fallas es cortado transversalmente por otro set de fallas, formadoras de los principales depocentros de la cuenca del Colorado, y posiblemente la adyacente cuenca de Salado. Esta segunda etapa de rifting (la principal en cuanto a formación de cuencas para la zona de estudio) está relacionada con un evento de rift de edad Jurásico temprano y medio, correlacionable con el rifting de Karoo. En el Cretácico temprano, un nuevo evento extensional orientado WNW-ESE es responsable de la formación de depocentros en la zona más externa de la plataforma, el emplazamiento de SDRs (Seaward Dipping Reflectors) y finalmente la apertura del Océano Atlántico Sur entre Sudamérica y África. Más al sur, en la cuenca de Malvinas, se reconocieron dos eventos de rift que fueron caracterizados sísmicamente, y para los que se obtuvieron dos edades absolutas mediante el método U-Pb en circón. El evento de rift más antiguo fue fechado Triásico superior, mientras que el segundo evento (el más importante en cuanto a formación de depocentros en la cuenca) fue datado Jurásico medio y correlacionado a la provincia magmática de Chon Aike. La caracterización sismo-estratigráfica de la evolución post-ruptura del margen Atlántico de Argentina (segmento Rawson-Colorado-Salado) se llevó a cabo con la interpretación de secciones sísmicas y preparación de mapas isópacos para las principales secuencias. Se identificaron tres etapas de evolución de la deriva continental. La primera etapa comienza con la ruptura continental en el Hauteriviano tardío a Barremiano y se extiende hasta el Cretácico superior. La subsidencia en esta etapa está condicionada por la subsidencia termal sobre los principales depocentros, que fueron conectándose progresivamente. Los altos de basamento entre cuencas son cubiertos recién en el Maastrichtiano-Daniano, con una transgresión de carácter regional que integra finalmente a las cuencas del margen en una sola plataforma continental abierta. Durante el Paleógeno, la subsidencia y el aporte sedimentario se concentran en el área norte (cuenca del Salado). El sector sur (cuenca de Rawson y sector sur de la cuenca de Colorado) actúa como un alto relativo, sin acomodación. Finalmente, el ciclo Neógeno se caracteriza por subsidencia cilíndrica continua en la zona de plataforma (posiblemente relacionado con procesos geodinámicos andinos) y la participación notable de las corrientes de contorno en la dinámica sedimentaria (reforzada luego de la apertura del pasaje de Drake en el Eoceno).The multi-stage character of rifting observed in the evolution of the Mesozoic extensional basins offshore Argentina was integrated into a more regional framework of Gondwana, with a review of the chronology of rifting of the main Mesozoic basins in southern South America and Africa. The focus of the study was placed on the pre-Cretaceous rifting events that ended with the Early Cretaceous opening of the South Atlantic Ocean. Three independent rifting events were identified in the Colorado/Salado basins area. A first rifting event was associated with the Late Triassic-Early Jurassic extensional reactivation of Late Paleozoic thrusts of the Ventania-Cape fold belt. This first set of faults is transected by a new set related to the main depocenters of the Colorado and the adjacent Salado basin. The second and main rifting stage is correlated with the Early-Middle Jurassic Karoo rifting. In the Early Cretaceous, WNW-ESE oriented extension produced a new rifting setting, emplacement of SDRs and finished with the South Atlantic breakup. Further South, in the Malvinas basin, two rifting events were recognized and seismically characterized, and the synrift units were dated using U-Pb in zircon. The oldest rifting event was dated Late Triassic, while the second and main rifting event was dated Middle Jurassic and correlated to the Chon Aike magmatic province. The post-breakup evolution of the Argentinean South Atlantic margin (Rawson- Colorado-Salado area) was also studied. Seismic stratigraphic characterization was carried out, with the preparation of chronostratigraphic charts and isopach maps. Three stages of drift evolution were identified. The first drift stage starts with the Late Hauterivian/Barremian breakup and lasts until the Top Cretaceous. Subsidence during this stage is conditioned by the thermal sagging over the main depocenters, which remained disconnected. With the Maastrichtian-Danian regional transgression, as the basement highs between basins are flooded, the margin becomes a single continental platform. The Paleogene drift stage two is characterized by subsidence and sedimentary input centered in the Salado area. The Rawson and southern Colorado area act as a high. Finally, during the Neogene, drift stage three is characterized by increased cylindrical subsidence (possibly related to Andean dynamic processes) and the remarkable interaction of contour currents (after the Drake Passage opening).Fil: Lovecchio, Juan Pablo. Universidad de Buenos Aires. Facultad de Ciencias Exactas y Naturales; Argentina

Structural evolution of the deepwater West Niger Delta passive margin

A detailed investigation of the structural evolution of the deepwater west Niger Delta was undertaken from the combination of industry 2D and 3D seismic reflection datasets. The study has been focused on three themes: crustal architecture, thrusting in oceanic crust and the role of multiple detachments in developing the structural style in the area. Detailed analysis and mapping of the basement structures, crustal thickness and distribution, identification and analysis of thrust-fault pattern and its relationships to detachment levels have provided a completely new understanding of the structural evolution of the deepwater west Niger Delta. The study shows that the area is underlain by oceanic crust that is characterised by a thickness of 5-7 km and by internal reflectivity consisting of both dipping and sub-horizontal reflectors. Inclined reflections can be traced up to the top of the crust where they offset it across a series of minor to major SW-NE striking basement thrusts in the SE of the study. The crust is thinnest around a major transform structure, the Chain Fracture Zone possibly related to the local geometry of the spreading fabrics with no significant variation the crustal thickness across the transform zone. Detachments are located within the 'Dahomey unit', and the transition between the Agbada and Akata Formations (Top Akata). Quantitative measurements of fault displacements show that the utilisation of different detachments results in contrasting styles of thrust propagation and fold growth. Two geographical zones are defined. In zone A, (NW sector of the study area), the stratigraphically shallowest Dahomey detachment is dominant and is associated with thrust truncated folds while in zone B, (SE sector of the study area) a stratigraphically lower detachment approximately at the Agbada-Akata Formation boundary is associated with thrust propagation folds

Structural evolution of the deepwater west Niger Delta passive margin

A detailed investigation of the structural evolution of the deepwater west Niger Delta was undertaken from the combination of industry 2D and 3D seismic reflection datasets. The study has been focused on three themes: crustal architecture, thrusting in oceanic crust and the role of multiple detachments in developing the structural style in the area. Detailed analysis and mapping of the basement structures, crustal thickness and distribution, identification and analysis of thrust-fault pattern and its relationships to detachment levels have provided a completely new understanding of the structural evolution of the deepwater west Niger Delta. The study shows that the area is underlain by oceanic crust that is characterised by a thickness of 5-7 km and by internal reflectivity consisting of both dipping and sub-horizontal reflectors. Inclined reflections can be traced up to the top of the crust where they offset it across a series of minor to major SW-NE striking basement thrusts in the SE of the study. The crust is thinnest around a major transform structure, the Chain Fracture Zone possibly related to the local geometry of the spreading fabrics with no significant variation the crustal thickness across the transform zone. Detachments are located within the 'Dahomey unit', and the transition between the Agbada and Akata Formations (Top Akata). Quantitative measurements of fault displacements show that the utilisation of different detachments results in contrasting styles of thrust propagation and fold growth. Two geographical zones are defined. In zone A, (NW sector of the study area), the stratigraphically shallowest Dahomey detachment is dominant and is associated with thrust truncated folds while in zone B, (SE sector of the study area) a stratigraphically lower detachment approximately at the Agbada-Akata Formation boundary is associated with thrust propagation folds.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Fig. S12: Oceanward rift migration during formation of Santos–Benguela ultra-wide rifted margin

Relationships between synrift unconformities, rotated fault blocks and the uplifted hinges in the Benguela Margin. This figure is equivalent to Figure 5g. Section location in Figures 1 and 3

Fig. S9: Oceanward rift migration during formation of Santos–Benguela ultra-wide rifted margin

Relationships between synrift unconformities, rotated fault blocks and uplifts in the distal margin of Santos Basin. This figure is equivalent to Figure 5d. Section location in Figures 1 and 3

Movie M2: Oceanward rift migration during formation of Santos–Benguela ultra-wide rifted margin

Detail of the model, showing only the evolution of the first 40 km