Asymmetric Conjugate Volcanic Margins of the South Atlantic: A Result of Variable Seaward Dipping Reflector Domain Accretion.

Abstract

The conjugate margins of Uruguay/Southern Brazil and Namibia have comparative asymmetric zones of thinned continental crust and distributions of volcanic material related to the opening of the South Atlantic. In this study we use a combined total of >23,000 km of deep-record, depth-migrated 2D seismic data to assess the true limits of the continental, seaward dipping reflectors (SDRs) and oceanic crustal domains within the study area. The seismic data were analyzed via a combination of a consistent seismic facies within the interpreted volcanic features established in previous works and new observations from this study. These data present volcanic margins that characteristically possess zones of SDR/igneous crust related to the intense magmatism present during continental breakup. These conjugate zones of SDRs and their underlying igneous crustal basement are bound by the limit of continental crust and the limit of oceanic crust. Subaerially-emplaced and tabular basalt flows have rotated downward 20° in the direction of the spreading ridge and creating packages of SDRs up to ~22 km thick near the limit of continental crust. The SDR packages are wedge-shaped and thin abruptly basinward towards the limit of continental crust to transition to a normal, 6-8-km thick oceanic crust. Our model to explain an approximately 30% greater volume of SDRs/igneous crust on the trailing Namibian margin than on the leading Brazilian margin during the syn- and post rift phases invokes a combination of diverging tectonic plates and a north-westerly plate motion with respect to a fixed mantle position of the mantle plume. This plate and ridge geometry resulted in the eruption of voluminous magmatic material on both conjugate margins form a “magmatic ridge” prior to the nucleation of the mid-oceanic ridge. Our proposed model for volcanic margin asymmetry in the South Atlantic does not require simple shear mechanism to produce the observed asymmetrical volcanic material distribution observed both in my data and in seismic refraction studies by previous workers. Understanding the processes and conditions that produce these asymmetries is critical in performing accurate tectonic plate reconstructions and understanding volcanic passive margin evolution.

AAPG Datapages/Search and Discovery Article #90332 © 2018 AAPG International Conference and Exhibition, Cape Town, South Africa, November 4-11, 2018