A Correlation Between the Southern North Falkland Basin and the Outeniqua Basin and its Implications on the Palaeogeographic Reconstruction of Gondwana

Abstract

Accurate palaeogeographic reconstructions are key to lowering risk during hydrocarbon exploration. However, the tectonic complexity of transform margins can make this challenging; the example considered in this study is the Falkland Plateau (FP). Despite decades of debate, the position of the plateau and the Falkland Islands (FI) within a Gondwana pre-break-up configuration remains contentious. Not only does this impact our understanding of the evolution of the surrounding sedimentary basins but it also influences the reconstruction of Southern Gondwana. Early reconstructions positioned the micro-continent between South America and South Africa, while later models argued for a palaeoposition offshore south-eastern South Africa. These later models favour either a rigid plateau or a ~180° rotated micro-continent interpretation. The scarcity of information regarding the deep structure of the basins around the FI hampers a direct comparison between the plateau and South Africa. This study provides new insights into the evolution of the FP by focussing on seismic reflection data coupled with gravity data from the Southern North Falkland Basin (SNFB). The integration of these data allows us to detail the geometry of the NW-SE striking reactivated thrusts bounding the half-grabens of the SNFB. These have shallow depth converted dips in the range of 20° - 40°, lengths of up to 150 km and up to 5 km throws. In the basins offshore the conjugate South African margin, faults show consistent shallowing of dip north-eastward from 60° across the Plettenburg Fault to 24° across the St. Croix Fault and a change in strike from WNW-ESE to NNW-SSE eastward thought to follow the crustal fabric developed during the Cape Orogeny. The architecture of the SNFB shows most similarities with the northernmost Algoa and Gamtoos Basins. We propose that the reactivated thrusts from the South African basins and the SNFB were originally along strike from each other, a statement supported by the aforementioned change in trend of the South African structural grain. This implies that prior to break-up the FI were situated offshore the south-eastern coast of South Africa but in a more southern position than previously suggested that requires a rotation of less than 100° during break-up. The proposed reconstruction impacts the surrounding basins, particularly the FP Basin by reducing the amount of extension expected to affect it and can influence future reconstructions of Southern Gondwana.

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