Earth Science & Engineering, Imperial College London
Doi
Abstract
The southern South Atlantic has often been considered a classic example of continental break-up in the presence of a starting mantle plume, evidenced from the Paraná-Etendeka continental flood basalts, the Rio Grande Rise & Walvis Ridge, and wide-spread presence of seaward dipping reflectors & high-velocity lower-crustal bodies along the conjugate margins. However, inconsistencies remain, including a stark contrast of non-volcanic and volcanic passive margins north and south of the aseismic ridges, and evidence from seaward dipping reflector distributions suggesting segmentation influenced magmatism. Within this project, different methods for the formation of the volcanic passive margins in the southern South Atlantic have been explored. To test the main influence on magmatism during break-up, observational constraints of post and syn-rift magmatism originate from 38 wide-angle and multichannel seismic profiles. These measurements show that at 450km south of the Walvis Ridge, oceanic crust has a thickness of 11.7 km, thinning to 6.1km at a distance of 2300km along-strike. Overall, oceanic crustal thickness decreases linearly along-strike with little indication of segmentation. Active mantle upwelling was also tested using the relationship between melt volumes and seismic velocity from syn-rift magmatism, with little evidence for this process. Using a numerical model of continental rifting capable of producing melt thickness and major element oxide compositions through time, the observational constraints from post and syn-rift magmatism required a hot layer present decreasing in temperature relative to the asthenosphere from 250°C in the north to 50°C at a distance of 2300km south along-strike. By reconstructing the lithosphere thickness at break-up, it was suggested that the sublithospheric topography had a strong influence on magmatism throughout the region, including on the asymmetry in magmatism along the margins. Overall however, modelling results suggest that temperature, and in turn possibly a plume, was the primary control of magmatism during the opening of the southern South Atlantic.Open Acces
