Post-salt magmatism in the Campos Basin, offshore SE Brazil: style, distribution, and relationship to salt tectonics

Abstract

Substantial magmatism occurred during the development of the marginal basins of the central and southern South Atlantic. Situated on the Brazilian side of the central segment, the Campos and Santos basin represent a transitional margin, located between the magma-rich margin in the north and the magma-poor margin in the south. In addition to magmatism associated with the initiation of continental rifting, the southeast Brazilian basins experienced three main magmatic events; one during the early post-rift (i.e., sag stage) and two during the late post-rift (i.e., passive margin stage), both occurring after deposition of a thick, late Aptian salt layer. The products of post-salt, intrusive and extrusive magmatism in the southern Campos Basin are imaged in seismic reflection data and have been directly penetrated by boreholes, yet they remain poorly understood in terms of their style, detailed geometry, and distribution. Furthermore, the temporal, spatial and possibly genetic relationships between these post-salt igneous products and salt-tectonics are largely unknown. Documenting these possible salt-magma interactions is important to understanding the tectono-stratigraphic evolution of Campos Basin and other salt-bearing passive margin basins subject to magmatic activity. We use 2D and 3D seismic reflection and borehole data from the southeastern Campos Basin, offshore Brazil to map and characterize post-salt igneous sills and volcanoes, and to understand the interaction between salt tectonics and igneous products. We identified 99 sills emplaced within uppermost Aptian-to-Maastrichtian strata, 83 emplaced above the salt and 16 intra-salt, and 15 volcanoes within Upper Cretaceous-to-lower Paleogene strata, associated with a >310 km² lava field. We also identify four vent-like structures at two stratigraphic levels, sub-salt to Paleogene and sub-salt to Upper Cretaceous. Salt and overburden structures define a domain of extensional deformation on a base-salt basement high, and a domain of contractional deformation in the adjacent basement low. The occurrence of volcanoes, lava flows, and hydrothermal vents at various stratigraphic levels indicates that Cretaceous-to-Paleogene igneous activity was incremental rather than synchronous. The relationship between igneous intrusions and salt tectonics is complex, i.e., in areas where they are common, sills appear to have locally restricted salt flow and the growth of large salt structures, whereas in other cases, salt structures and overlying faults appear to have partially acted as pathways to intruding magma, allowing it to ascend to relatively shallow depths. In contrast, extrusive igneous products exclusively developed after the main phase of salt movement. However, salt distribution influenced the location of volcanoes, given the latter developed where salt is welded or very thin. Salt flow and thickness also controlled the distribution of lava flow products, which accumulated in structural lows between large salt tectonic structures. Our work indicates a complex relationship between salt tectonics and magmatic emplacement that impact the post-rift evolution of Campos Basin.