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 Many people are intimidated by the magnetic method, but they are quite willing to attack gravity interpretation. This is usually a result of the inclination/declination issues with magnetics, and the fact that most people view a gravity map as a structural map. Magnetic maps are not structural maps; they are a contouredrepresentation of magnetization changes in the geology. This is harder for people to “visualize” in terms of real rocks and structure. Gravity interpretation seems easier because a gravity high usually correlates with a structural high (except in the case of salt), but simple criteria such as that can cause problems if the interpreter is not careful On the other hand, some aspects of the magnetic method are quite straightforward. One is the direct mathematical relationship between anomaly wavelength and source depth. Another important relationship is that basement lithology changes typically have amplitudes of hundreds of nanotesla (nT), but structural changes usually cause anomalies of only tens of nT (see Gibson's magnetic susceptibility versus structure illustration later in this volume). It is also important to note that, depending on magnetic inclination, a fault and a lithology change can have distinctly different magnetic signatures. One might cause a symmetrical anomaly while the other causes an asymmetrical anomaly. This underscores the importance of preliminary models prior to undertaking any gravity or magnetic interpretation.

Abstract The vector nature of the Earth's magnetic field dictates that interpreters must take care to understand pitfalls related to the orientation of the field (i.e., magnetic inclination and declination), and the relationship of the magnetic field to a region's geology. The case history presented here demonstrates one such pitfall. Present models for the formation of the Grenada Basin vary from north-south extension to northeast-southwest extension to east-west extension. Gridded magnetic anomalies over the basin provide a picture of the Earth's field that contributes to this spectrum of possible extensional origins. The Grenada Basin is a back-arc basin located near the eastern edge of the Caribbean Plate. The basin is bounded on the east and west by the roughly north-south-trending active Lesser Antilles and remnant Aves Ridge Island Arcs, respectively. Although this physiography, as well as gravity data, supports formation by near east-west extension, magnetic anomalies over the basin exhibit predominantly east-west trends. The crust of the Grenada Basin and of other back-arc basins forms similarly to the crusts of ocean basins. If the observed magnetic anomalies over the basin are produced by sea-floor spreading, then the orientation of extension may be complex. Extension in most back-arc basins is roughly normal to their trenches and subduction zones, but some basins appear to exhibit oblique extension. A careful interpretation of magnetic profiles reveals low-amplitude magnetic anomaly trends, oriented subparallel to the island arc, over the southern part of the Grenada Basin, which supports a model for basin development by near east-west extension.