Rapid Along-strike Variation of Breakup Volcanism on the Pelotas Margin, Offshore SE Brazil, South Atlantic and its Control by Lithosphere Inheritance

The southern rifted margins of the South Atlantic are commonly regarded as some of the best examples of magma-rich margins with the Pelotas, Uruguay, Argentine and Namibia margins showing prominent Seaward Dipping Reflectors (SDRs). These volcanic SDRs are commonly interpreted as resulting from enhanced decompression melting during rifting and breakup from regionally elevated asthenosphere temperatures associated with the Parana-Etendeka mantle plume. We investigate the lateral variability of breakup volcanic addition along-strike of the Pelotas segment of the southern South Atlantic rifted margin offshore SE Brazil. Our analysis of regional seismic reflection profiles shows that magmatic addition on the Pelotas margin varies substantially along strike from extremely magma-rich to magma-normal within a distance of approximately 300 km.

In the north of the Pelotas margin, where SDRs are thickest, the Torres High shows SDRs up to  20 km thickness. In contrast, in the south of the Pelotas margin, the magmatic addition is normal and SDRs are very thin or absent. Further south of the Pelotas margin, offshore Uruguay and northern Argentina, margins are again magma-rich with SDRs thickness reaching 10 km or more.The very thick SDRs of the northern Pelotas margin lay offshore of the thick Serra Geral volcanics of similar Cretaceaous age found onshore in the Santa Catalina, Parana, Sao Paulo and northern Rio Grande do Sul states of SE Brazil. Further south, Serra Geral volcanics are absent in the cratonic southern Rio Grande do Sul, which is onshore of the southern Pelotas margin with thin or absent SDRs and normal magmatic addition. The abrupt decrease in rift and breakup decompression melting from north to south along the Pelotas margin, and its increase to the south on the Uruguay and northern Argentina margins is inconsistent with the simple Parana-Etendeka mantle plume model. The correlation of magma-normal breakup in the southern Pelotas margin with cratonic geology onshore implies a significant contribution of lithosphere inheritance to decompression melting during rifting and breakup to form the southern South Atlantic margins.

A relationship is observed between the amount of volcanic material and the two way travel time (TWTT) of first proximal volcanics in seismic sections.  First volcanics are observed at 1.25s TWTT for the highly magmatic Torres High profile while, in contrast, for the normally magmatic profiles in the south, first volcanics are observed at 4.2s TWTT or deeper. The observed inverse relationship between post-breakup accommodation space and SDR thickness is consistent with predictions of a simple isostatic model of continental lithosphere thinning and decompression melting during breakup. This relationship between TWTT of first volcanics in seismic sections and the magnitude of magmatic addition may provide an effective means of mapping the distribution of breakup magmatic volume for the southern South Atlantic margins and its correlation with onshore geological inheritance.