Asymmetrical Conjugate Margins of the South Atlantic: Effects of Variable Basement Architecture and Magmatic Additions on the Continental Rifting Process and Petroleum Systems

Abstract

The conjugate margins of Uruguay/Southern Brazil and Namibia exhibit similar, asymmetrical zones of thinned continental and igneous crust related to the opening of the South Atlantic. Previous workers have suggested simple- or pure-shear mechanisms for observed crustal and magmatic asymmetries on conjugate margins. We use ~23,000 km of deep-record, depth-migrated 2D seismic data from both conjugate margins to assess the boundaries of thinned continental crust versus the volumes of magmatic additions in order to test pure vs. simple-shear crustal thinning mechanisms. Pre-existing structural grains and basement architecture are a primary control on the basinward extent of thinned continental crust and magmatic additions on conjugate margins. In the syn-rift stages, rift-centered magmatism resulted in the emplacement of igneous material related to west-to-east track of the Tristan de Cunha mantle plume. Orogenic belts are located at the passive margins of Uruguay/Southern Brazil and Namibia and their degree of crustal thinning was influenced by the margin's strike-parallel thinning versus strike-perpendicular orientation. Wide zones (~250km) of thinned continental curst are found at orogenic belts orthogonal to the rift axis, where rift-parallel belts average much narrower zones (~80km). This effect of pre-existing structural grain resulted in a zone of thinned continental crust totaling 117, 500 km2 on the South American margin and 150,500 km2 on its West African, conjugate margin. Volcanic margins exhibit zones of seaward-dipping reflectors (SDRs) produced by intense magmatism accompanying continental breakup. These >200 km wide zones of SDRs and underlying igneous crust reach thicknesses up to 22 km and show distinctive seismic facies in their proximal and distal domains. South of these conjugate volcanic plateaus, we have mapped zones of SDR's on the South American and West African conjugate margins that total 81,100 km2 and 110,800 km2, respectively. The Kudu-1 discovery and wells with oil/gas shows give evidence of a functioning petroleum system on the Namibian margin and within a strongly volcanic margin. Although no discoveries have been made on the Uruguay/Southern Brazilian margins, positive hydrocarbon indicators include 6 km of sediment overburden, gas chimneys, bottom-simulating reflectors, and oil seeps.

AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017