SOSEM, South Santos Seismic and modelling experiment: analyzing rift-plume interaction during break-up - Preliminary results.

What are the factors that control the generation and emplacement of magma during the rifting and breakup of continents? The Southeastern margin of Brazil along the South Atlantic Ocean offers an unprecedented opportunity to analyze this question. Here, the Tristan mantle plume appears to have exerted a significant influence on the magmatic processes associated with rifting. Yet, the influence of the plume on magmatism was spatially variable and heterogeneous along the margin. The basins south of the Rio Grande Fracture Zone (RGFZ) show clear evidence of magma-rich rifting, characterised by seaward-dipping reflectors and lower crustal magmatic intrusions emplaced during rifting. However, to the north of the RGFZ, the Santos and Campos Basins, generally lack the typical features of magma-rich margins. This asymmetric distribution of magmatism around the original plume head, differs from the classical view of plume-rift interaction which assumes that volcanism should be symmetrically distributed with respect to the plume head, as observed in the North Atlantic¹.

To unravel the geological controls on the spatio-temporal distribution of magmatism during rifting, we carried out a wide-angle seismic experiment across the transitional zone between the Santos and Pelotas basins in November 2025. This area has been well-imaged with deep commercial MCS imaging (e.g. [2]). However, information on the nature of the crust is currently lacking and questions persist on the compositional nature of the São Paulo plateau, which has been interpreted as either extended and potentially intruded continental crust (e.g. [3], [4]) or as an oceanic plateau⁵.

During cruise MSM141 on board the R/V Maria S. Merian we acquired three wide-angle lines overlapping with pre-existing ION-GXT multichannel seismic lines 150 and 140 across the margin and 220 across the RGFZ. In total, 126 stations were deployed at ~8.5 km spacing. Simultaneously, 29 onshore stations were deployed along a ~200-km-long transect aligned with line 150. These three-component broadband stations were spaced 5-10 km apart and operated continuously at 250 Hz for up to 42 days. During shooting, an airgun array with a total volume of 64 L (4,160 in3) was used as the seismic source. The seismic experiment aims to reveal how magmatism changed with distance from the RGFZ, and the crustal nature of the Abimael Ridge and of the São Paulo Plateau. Our specific goals are to understand the 3D kinematic history of the area, and the role of the preexisting lithospheric structure and the RGFZ in controlling the spatio-temporal distribution of magmatism. The project has been funded by DFG and Petrobras and will include seismic tomography of the wide angle data and numerical modelling of the opening of this area of the South Atlantic.

References

Morgan, J. P. et al. (2020). PNAS, 117(45), 27877-27883. doi:10.1073/pnas.2012246117

McDermott et al. (2019). EPSL, 521, 14-24. doi:10.1016/j.epsl.2019.05.049

Evain et al., 2015. JGR, v. 120, p. 5401–5431.

Araujo et al. (2022). Geol. Soc. Lon. Spec. Publ., 524(1). doi:10.1144/SP524-2021-123

Karner et al. 2021, in Marcio R. Mello, Pinar O. Yilmaz, and Barry J. Katz, eds., AAPG Memoir 124, p.215–256.