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Are Along-Strike Alternations of Symmetrical and Non-Symmetrical South Atlantic Conjugate Margins Controlled by Volcanic vs. Non-Volcanic Rifting Process?

Abstract

The mirror hypothesis widely applied to South Atlantic exploration proposes that large oil and gas fields on one margin can be found by moving along the opening flow lines of oceanic fracture zones to the conjugate margin in South America or Africa. The mirror hypothesis would only be correct if the rifting process is symmetrical and evenly divides the hydrocarbon-rich basin into two equal halves. If the rifting process is asymmetrical it is less likely that the same hydrocarbon-rich basin would be divided equally on both conjugate margins. In order to identify areas of symmetrical rifting assumed to form by pure shear vs. areas of asymmetrical rifting assumed to form by simple shear rifting in the area from the Equatorial Atlantic to the Falkland Islands, we have constructed 10, trans-oceanic isostatically-corrected basement cross sections extending from the South American to African continents. These sections which include the locations of the continent-ocean boundary (COB) known from previous studies and previously published regional seismic profiles show that asymmetrical margins characterize the equatorial Atlantic (latitude 5.0° S) to the Florianopolis fracture zone. In this northern zone of non-volcanic passive margins, the broad, low relief rift zone inferred to be the lower plate margin formed by a lower temperature simple shear process alternates at scales of 1,000 km (or less) with the narrow, high standing upper plate margin. Within this region, distribution of oil field concentrations follow the alternating patterns and have higher frequency on the interpreted lower plate margin segments. South of the Florianopolis fracture zone to the Falkland Islands, the volcanic passive margin becomes symmetrical with the width of the rifted zone roughly equidistant on both conjugate margins. We propose that this area reflects a pure shear rifting process involving a higher temperature and more ductile deformed crust with early rift basins equally divided on both conjugate margins. Analysis of the distribution of oil productive oil and gas provinces supports an alternating rift architecture and where either pure shear or simple shear processes occurred.