The missing piece of the South Atlantic jigsaw: when continental break-up ignores crustal heterogeneity
Van der Spuy, D
- Publisher: Geological Society
Crustal heterogeneity is considered to play a critical role in the position of continental break-up, yet this can only be demonstrated when a fully constrained pre-break-up configuration of both conjugate margins is achievable. Limitations in our understanding of the pre-break-up crustal structure in the offshore region of many margins preclude this. In the southern South Atlantic, which is an archetypal conjugate margin, this can be achieved because of the high confidence in plate reconstruction. Prior to addressing the role of crustal heterogeneity, two questions have to be addressed: first, what is the location of the regionally extensive Gondwanan Orogeny that remains enigmatic in the Orange Basin, offshore South Africa; and, second, although it has been established that the Argentinian Colorado rift basin has an east–west trend perpendicular to the Orange Basin and Atlantic spreading, where is the western continuation of this east–west trend? We present here a revised structural model for the southern South Atlantic by identifying the South African fold belt offshore. The fold belt trend changes from north–south to east–west offshore and correlates directly with the restored Colorado Basin. The Colorado–Orange rifts form a tripartite system with the Namibian Gariep Belt, which we call the Garies Triple Junction. All three rift branches were active during the break-up of Gondwana, but during the Atlantic rift phase the Colorado Basin failed while the other two branches continued to rift, defining the present day location of the South Atlantic. In addressing these two outstanding questions, this study challenges the premise that crustal heterogeneity controls the position of continental break-up because seafloor spreading demonstrably cross-cuts the pre-existing crustal heterogeneity. Furthermore, we highlight the importance of differentiating between early rift evolution and subsequent rifting that occurs immediately prior to seafloor spreading.