Geophysical investigation of the subsurface through seismic refraction and reflection methods provides an efficient and non-invasive means towards addressing geologic problems across multiple scales. Both seismic techniques, in an active-source exploration setting, involve inducing acoustic waves into the subsurface and measuring their propagation velocities and amplitudes. These measurements have physically-based relationships with the properties of the underlying strata, thus allowing changes in the seismic measurements to be interpreted with respect to changes in the subsurface geology. Two applications of the seismic method are presented in this dissertation: (1) shallow seismic refraction acquisition and processing applied to the near-surface investigations of soil and regolith, which constitute the Critical Zone (CZ), beneath the upland hillslopes of the Valles Caldera, New Mexico; (2) interpretation of 2-D and 3-D marine seismic reflection data that image the upper 10-km of the crust beneath the Southern Guinea Plateau, offshore Guinea, West Africa. In both cases, the seismic data provide necessary constraints for the generation of accurate subsurface models that permit further geophysical modeling. The near-surface results, presented in Appendix A, provided a rich dataset of weathered thicknesses across hillslopes that supported an investigation of potential relationships between CZ geologic architecture and topographic attributes. Quantified relationships suggest that calibrated predictions based on the topography can provide first-order estimates of regolith thickness across upland landscapes. These results add to the ongoing CZ-science endeavor to understand proposed links between subsurface weathering processes and their surface expressions. In Appendix B, interpretations of high-resolution 3-D seismic data have illuminated deformational structures associated with Mesozoic rifting of the Southern Guinea Plateau. The interpretations were expanded onto regional 2-D seismic profiles, permitting a regional synthesis of the southern margin’s structural evolution. Additional tectonic subsidence and forward-gravity modeling highlight the influence of Jurassic rifting on the Southern Guinea Plateau prior to Early-Cretaceous rifting and separation, as well as crustal thickness estimates from the continental shelf out towards oceanic crust. Lastly, the Guinea-Demerara conjugate plateaus, and their associated deformations, were restored to 100 Ma, revealing an apparent upper-crustal asymmetry between the two margins. Appendix C presents two seismic-exploration methodologies based on 3-D seismic reflection data: (1) the calculation and interpretation of two co-rendered volumetric seismic attributes – most-positive curvature and semblance; (2) numerically modeling the tectonic subsidence of an entire 3-D seismic survey. Both techniques are used to address the inherent difficulty in interpreting the extent to which Jurassic rifting affected the Southern Guinea Plateau. Furthermore, the numerical model of subsidence provides a new exploration technique towards qualitatively and quantitatively assisting in the assessment of potential hydrocarbon-bearing basins.

The two end-member concept of mantle plume-driven versus far field stress-driven continental rifting anticipates high volumes of magma emplaced close to the rift-initiating plume, whereas relatively low magmatic volumes are predicted at large distances from the plume where the rifting is thought to be driven by far field stresses. We test this concept at the Guinea Plateau, which represents the last area of separation between Africa and South America, by investigating for rift-related volumes of magmatism using borehole, 3D seismic, and gravity data to run structural 3D inversions in two different data areas. Despite our interpretation of igneous rocks spanning large areas of continental shelf covered by the available seismic surveys, the calculated volumes in the Guinea Plateau barely match the magmatic volumes of other magma-poor margins and thus endorse the aforementioned concept. While the volcanic units on the shelf seem to be characterized more dominantly by horizontally deposited extrusive volcanic flows distributed over larger areas, numerous paleo-seamounts pierce complexly deformed pre and syn-rift sedimentary units on the slope. As non-uniqueness is an omnipresent issue when using potential field data to model geologic features, our method faced some challenges in the areas exhibiting complicated geology. In this situation less rigid constraints were applied in the modeling process. The misfit issues were successfully addressed by filtering the frequency content of the gravity data according to the depth of the investigated geology. In this work, we classify and compare our volume estimates for rift-related magmatism between the Guinea Fracture Zone (FZ) and the Saint Paul's FZ while presenting the refinements applied to our modeling technique.

The Guinea Plateau is a shallow-marine, flat-lying bathymetric province situated along the equatorial West African margin, offshore Republic of Guinea. The Guinea Plateau and the conjugate Demerara Plateau hold particular geologic significance, as they represent the final point of separation between the African and South American continents during Gondwana break-up. Recent interpretation of both 2-D and 3-D seismic surveys along the Guinean margin have illuminated subsurface features related to Early Cretaceous crustal extension. Seismic structural investigations on these datasets suggest that the majority of extension is accommodated along large-scale listric normal faults located on a relatively narrow (<50 km) continental slope (up to ~39% extension). Minimal faulting reveals that little upper-crustal extension has occurred on the Guinea Plateau. Additionally, multiple 2-D seismic profiles image the transition from continental crust on the plateau and slope, to oceanic crust in the deeper marine basin. This continent-ocean boundary is the most representative boundary when testing the accuracy of plate reconstructions. Mapping of both the continent-ocean boundary and fracture zones across the equatorial Atlantic suggests that the Demerara Plateau and the South American plate are too far south in previous pre-rift reconstructions. A revised model introduces 20 km of Early Cretaceous NNW-oriented contraction across the Amazon Basin; an area of relative weakness where both geologic and geophysical data support such accommodation. Sea-surface flow models, which used this revised reconstruction and interpreted paleo-bathymetric data, predict upwelling throughout the newly formed equatorial seaway, and later along the West African margin during periods of regional organic-rich black shale deposition. With reduced decomposition of organic matter strongly correlated to upwelling, being able to predict these zones is of particular significance to petroleum companies, who have recently started exploring both the equatorial South American and West African coastlines.

Les marges transformantes, formées par le coulissage des plaques lors des premières étapes de l’ouverture océanique, ont été peu étudiées relativement aux marges divergentes. La marge de Côte d’Ivoire-Ghana fait figure d’exemple-type, et l’absence d’inventaire exhaustif ainsi que le peu d’étude de marges transformantes conjuguées entraîne une méconnaissance de la véritable diversité de ce type de marge. Deux approches ont été abordées dans ce mémoire. La première a consisté à les inventorier dans le monde à partir d’une méthodologie simple. Elles ont ensuite été comparées (physiographie et structure). La deuxième s’est concentrée sur l’étude tectono-sédimentaire des marges transformantes conjuguées des plateaux de Demerara et de Guinée. L’inventaire des marges transformantes a montré que les marges transformantes représentent 30% des marges passives. Leur distribution au sein de domaines océaniques ouverts de façon très oblique ou à l’intersection entre grands domaines reflète des conditions d’initiation spécifiques. 30% des marges transformantes se trouvent notamment en bordure d’un plateau marginal ; un type de bassin à l’histoire double nouvellement défini, qui semble associé à un fort amincissement crustal. Par ailleurs, on montre que les marges transformantes présentent des architectures diverses et sont souvent segmentées. L’étude des plateaux marginaux de Demerara et Guinée a permis de montrer qu’elles se mettent en place au niveau d’un domaine pré-aminci, sans doute guidées par des structures préexistantes. La nature du soubassement des plateaux est discutée : éventails sédimentaires ou volcaniques type SDR. La formation et l’évolution des marges transformantes en bordure des deux plateaux a été accompagnée de déformations intenses à l’approche des frontière océan-continents ainsi que de plusieurs épisodes de soulèvement et subsidence successifs de la bordure transformante, dont les origines et mécanismes sont discutés. / Transform margins form by transform motion between plates during early stages of oceanic spreading. They have been poorly studied in comparison with divergent margins, at least for the last 15 years. The Côte d’Ivoire-Ghana margin represents the best known example of a transform margin and is often considered as a model. However, the lack of a global complete inventory of such margins may lead to underestimate their diversity. Two approaches have been developed in this work. The first one consisted in a compiling world transform margins using a systematic and simple methodology. Those margins have then been compared (physiography, structure). In a second approach we analysed the tectono-sedimentary evolution of the Demerara and Guinea conjugated transform-derived plateaus. The worldwide review of transform margins shows that those represent 30% of the existing passive margins. Their distribution inside very oblique oceanic domains, and at the intersection between oceanic domains of contrasted ages, reflects specific initiation conditions. 30% of those transform margins locate on the edge of marginal plateaus —newly defined type of basins— that seem to be associated with important crustal thinning. Furthermore, this inventory shows that transform margins display a great variety of architectures. The Demerara and Guinea marginal plateaus are bounded by transform margins that occur on an already thinned crust, probably along a preexisting structure. The nature of these plateaus basement is discussed: sedimentary or volcanic SDRs fan-shaped units? During the Equatorial Atlantic opening, the formation and evolution of Demerara and Guinea transform margins were accompanied by intense deformations close to the continent-ocean boundary and by several successive uplift and subsidence phases along the plateaus borders. Their origin and mechanisms are discussed.