The Natal rift (South Mozambique, Africa): tectonic inheritance and magmatism
Résumé
The break-up of the Gondwana supercontinent started during the Early Jurassic and lead to the formation of the Mozambique passive margins, as Africa and Antarctica separated during the mid-Jurassic period. Albeit plate kinematics during the oceanic spreading stage are relatively well constrained, the initial fit of Africa and Antarctica, their earliest relative movements and the architecture of distinct margin segments from Mozambique to South Africa in relation to those from Antarctica remain sparsely known. While margins architecture has been recently evidenced along the central Mozambique, the spatial extent of rifting further south remains poorly documented.
Based on detailed observations related to seismic reflection profiles and oil company wells, findings show the existence of a magma-rich segment with large volume of seaward dipping reflectors in the South Mozambique (i.e. the Natal segment). Such segment is defined by a rapid transition from continental to oceanic domains that differs from classical divergent margins. Such particular architecture may be related to a difference in lithosphere rheology (i.e. Archean vs. Neoproterozoic) that favored extension and upwelling from a deep thermomechanical anomaly, called Karoo superplume. In parallel, the birth of the Natal ocean occurred at ca. 165 - 160 Ma and bring new constrains on Gondwana breakup. Here, we inferred a simplified conceptual geodynamic model for the whole area, from the initiation of extension to the seafloor spreading. We summarize this evolution in three main steps. Stage T1 represents the first extensional event inducing crustal thinning during the Permo-Trias before the breakup of the Gondwana. It is characterized by an E-W extension trend responsible for the formation of large N-S fault-controlled basins. Stage T2 is marked by the onset of a plume activity at about 180 Ma. Characterized by large N-S trending basins and by seawards dipping reflectors infilling, deformation related to T2 is consistent with a NW-SE trending extension. Finally, Stage T3 corresponds to the continuation of the rift with a stress field rotation ranging from NW-SE to N-S, suggesting that Antarctica moving in a SSE direction with respect to Africa after 156 Ma. The entire region is then going through an episode of uplift related to mantle dynamics.