Influences on continental margin development: a case study from the Santos Basin, South-eastern Brazil

Abstract

South Atlantic margins display significant variation in structural style along their length. An understanding of factors that influence this variation in structural style is a key component in any understanding of the evolution of the South Atlantic. The Santos basin is an obliquely rifted basin located offshore SE Brazil. Structures in the basin are thought to have been influenced by the reactivation of basement structures and/or fabrics during the onset of the Gondwana breakup. The onshore region adjacent to the Santos Basin offers the chance to test whether basement structures have played a significant role in influencing the initiation and development of syn-rift faults, dykes and fractures.
Field and remote sensing studies of the onshore region and on the conjugate margin in Africa show the strong geometric and temporal link between major 135Ma dyke swarms and the development of faults and fractures. 3 structural domains are defined, and correlated with lineament domains from the African margin. Faults, dykes and fractures within the lineament domains in SE Brazil share consistent orientations as large scale morphotectonic lineaments. Brittle structures show orientations and kinematics that are consistent with regional E-W oriented transtension. Little evidence of basement influence can be seen at outcrop scale, although it is speculated that larger scale structures may be reactivated.
Faults in the offshore basin share similar geometries to brittle structures onshore. As rifting progressed, and dyke intrusion ceased, NNE-SSW trending structures to the north of the basin ceased to be active, while N-S trending extensional faults continued to deform. N-S trending faults and fractures are also observed to cut dykes in the onshore region. The importance of understanding onshore geology when interpreting offshore basins is highlighted in this study, as is the need to integrate diverse datasets when trying to understand the complex influences on margin development.