The crustal architecture of a magma-rich margin across the rift to drift transition: Insights from the Pelotas Basin, Uruguay

Magma-rich margins are often characterised by a proximal zone of attenuated continental crust thinning, followed basinwards by increasing volcanic involvement passing laterally into a zone of accumulation of basalts interbedded with volcaniclastic and clastic material. Such accumulations are identified as Seaward-Dipping reflections (SDRs), which occur prior to oceanic crust growth. The processes of emplacement of SDRs and the composition of the underlying crust remain under debate. This thesis provides new insights into SDR emplacement and geometry by analysing high-quality seismic reflection data from Pelotas Basin, offshore Brazil and Uruguay. The basin architecture is characterised by defining the crust types (i.e. continental, transitional and oceanic) based on their characteristics and stages of evolution of the margin. The current study found variations in the transitional domain that modify the geometry and accommodation space. Analysis of the 3D structure and geometry of SDRs provided new constraints to propose a model of SDRs formation. Progressive emplacement of SDRs was identified as occurring subaerially. SDR packages were observed to have accumulated obliquely to the spreading centre and over thinned continental crust for inner SDRs and magmatic crust for outer SDRs. The thickness distribution of SDRs is suggested to be governed by the amount of thermal subsidence and accommodation space created on the margin. The curved geometry of SDRs is likely to result from magmatic loading. Finally, the model proposed in this thesis is assessed in the regional context of the South Atlantic Margins. Comparison along the margin showed consistency of the interpreted crustal architecture and SDR distributions. Comparison with the conjugate margin of Namibia showed asymmetric emplacement of SDRs in volume and width. The factors identified to explain this asymmetry were rift jumps, crustal thinning distribution and the influence of the mantle plume and magma distribution.