Breakup Magmatism in the South Atlantic: Mechanisms and Implications

Abstract

The Early Cretaceous opening of the South Atlantic Ocean was accompanied by extensive intrusive and extrusive magmatism collectively grouped in the South Atlantic Igneous Province (SAIP). The SAIP includes the onshore Paraná-Etendeka large igneous province (PELIP), offshore seaward-dipping reflectors sequences, the Rio Grande Rise and Walvis Ridge, and voluminous intrusive magmatism. Nonetheless, the nature of the processes that lead to continental breakup remain controversial, and the environmental impact of these events is not yet fully understood. In order to investigate the tectonomagmatic evolution of the South Atlantic, we present a new compilation of the various geochemical compositions and radiometric ages associated with the SAIP as well as an estimation of magma volumes constrained by extensive seismic, gravity, and magnetic datasets. The SAIP was formed during the Early Cretaceous, from the Valanginian to Albian (135–110 Ma), with the majority of the magmatic activity lasting c. 3.5 Myr interval between 135.5 and 132 Ma. Onshore magmatism is characterized by massive outpourings of tholeiitic basaltic flows formed by melting of hotter than normal mantle along with significant lithospheric thinning. Magma source compositions varied during magmatism with enriched “plume-like” components transitioning to more depleted “MORB-like” (Mid-Ocean-Ridge-Basalt) signatures. The total magma volume of the SAIP ranges between 10 × 10⁶ km³ and 16 × 10⁶ km³ with a striking asymmetry in volumes of the onshore and offshore volcanism along the conjugate margins. Offshore, ∼67 to 70 % of the total magma volume is located on the African side whereas 30 to 33 % of the total magma volume is located on the South American side. Onshore, ∼10 % of the total magma volume is located on the African side whereas 90 % of the total magma volume is located on the South American side. This asymmetry in volume distribution is attributed to different factors including variations in rifting mode, differences in initial lithospheric thickness, and structural inheritance. The geochemical data were analyzed and integrated to provide insights on magma generation and source compositions along with an updated geological model for the South Atlantic magmatism. The onset of the activity of the SAIP at the start of the Weissert Event suggests that the SAIP may have played a major role on the climatic and oceanographic changes during the mid-Valanginian.