Subsidence evolution of the conjugate passive continental margins of southwestern Africa and eastern Argentina
Aachen (2016) [Dissertation / PhD Thesis]
Page(s): 1 Online Ressource (VI, 130 Blätter) : Diagramme, Karten
Even though the present-day structure of the South Atlantic passive continental margins has been thoroughly investigated, uncertainties remain, particularly regarding the subsidence evolution during the post-rift phase and its causative processes. This thesis therefore focuses on the subsidence history of the passive continental margins within the Southern Segment of the South Atlantic Ocean. The subsidence history is inferred from restored paleobathymetries along the passive margins using their present-day structural configuration as well as their thermal field.In a first part of this thesis a 1D backward modelling approach is used to reconstruct the paleobathymetries offshore SW Africa. Therefore, individual subsidence components are separated according to the load induced by the sediments and the thermal cooling of the lithosphere. Starting from the present-day configuration, sedimentary units are successively “backstripped” to quantify the amount of load induced subsidence. The amount of thermal subsidence is calculated assuming uniform stretching of the lithosphere. Considering both subsidence components allows reconstructing paleobathymetries of the SW African margin in order to infer its subsidence history. The results obtained show a general subsidence of the margin with intermittent phases of seafloor uplift, as indicated by elevations above sea level in some parts of the research area. Summing up, the amount of elevations above sea level (i.e., seafloor uplift) during the post-rift phase yields about 1200 m. Although this quantification cannot fully identify the causative processes behind the vertical movements, their timing and magnitude give evidence for a mantle mechanism causing seafloor uplift rather than ridge-push related to spreading of the South Atlantic Ocean.Improving the understanding of the subsidence evolution of the South Atlantic passive margins requires also a subsidence analysis along the conjugate margin offshore SE South America. Hence the Colorado Basin along the Argentine margin is investigated. To accomplish this, the present-day structural configuration of the Colorado Basin is used and the same subsidence analysis used to analyse the SW African margin is applied. The results of the backward modelling approach show continuous subsidence of the Colorado Basin since the breakup of Gondwana. Therefore, they are in contrast with the subsidence history of the sedimentary basin on the conjugate margin, where the general subsidence pattern was interrupted by multiple phases of seafloor uplift. In terms of causative processes responsible for the subsidence history, a tectonic force alone, such as ridge-push due to seafloor spreading, is unlikely to cause the seafloor uplift along the SW African margin, since the effects of ridge-push should have the same impact on both margins. This further supports the hypothesis of a mantle mechanism rather than ridge-push as cause for the seafloor uplift along the SW African margin.Finally, to assess how far the obtained seafloor uplift is influenced by the assumptions of the 1D backward modelling and in order to also take the initial syn-rift subsidence into account, 3D forward modelling is performed for the same research area along the SW African margin as in the first part of this work. The 3D forward modelling approach used in this study has the particular advantage of coupling the thermal and mechanical subsidence components. Hence, the interaction between thermal relaxation of the lithosphere and the flexural isostatic response due to sedimentary loading, as well as the interplay of both is considered. The results also indicate elevations above sea level with a similar magnitude, as shown in the backward modelling approach and only minor temporal variations. Following the interpretation of the forward modelling results, it appears that the elevations above sea level are strongly related to the syn-rift phase. Taking the limitations of the approach used to reconstruct the syn-rift phase into account leads to the conclusion that the elevation above sea level might be the result of missing subsidence during the syn-rift phase. This missing subsidence component might be related to unconsidered lower crustal flow while calculating the syn-rift subsidence. According to the previously described approaches, a successful analysis of the subsidence history of the South Atlantic passive continental margins based on present-day observations is possible. After summarizing the results of the three parts of this work and integrating them into a comprehensive discussion of the possible causative processes behind the evolution of the South Atlantic passive continental margins, it is evident that a mantle mechanism affected the SW African margin causing phases of uplift.
Dressel, Kai Ingo
Kukla, Peter A.