The rift-drift transition on magma-rich margins: insights into the evolution of seaward dipping reflectors

Magma-rich margins are characterised by thick sequences of seaward dipping reflectors (SDRs), which consist of basalts interbedded with variable amounts of clastic and volcaniclastic material. The processes responsible for the emplacement and tilting of SDRs remain uncertain, as does the composition of the crust that they overlie. In this thesis, novel insights into SDR emplacement are gained through analysing new seismic reflection data from the Orange Basin, offshore South Africa and Namibia. It is demonstrated that a series of abandoned sites of SDR emplacement are embedded within the Orange Basin SDRs. Analysis of their 3D structure indicates that these features record rift-jumps resulting from the interaction between separate magmatic segments. Abandoned sites of SDR emplacement record the syn-emplacement geometry of SDRs, providing new constraints on models of SDR formation. The stratal geometries observed were compared to predictions from numerical and conceptual models. This analysis indicates that the dips within SDRs result from magmatic-loading. It also suggests that SDRs are emplaced subaerially as the uppermost part of an entirely magmatic crust. Magnetic lineations were then correlated from within oceanic crust to within SDR-bearing magmatic crust, providing new insights into the lateral variability of spreading dynamics. The along-strike continuity of these lineations suggest that, during South Atlantic opening, magmatic spreading occurred along a continuous axial zone, part of which was submarine and part of which was subaerial. This organised spreading centre was not segmented by transfer zones active earlier in the rifting process. This provides an example of magmatic process overprinting structural segmentation present earlier in the rifting process. Finally, the processes accommodating asymmetric SDR emplacement were analysed. In contrast to previous studies, here it is demonstrated that three processes control asymmetry: asymmetric magmatic spreading; rift-jumps; and asymmetric continental stretching. The dominance of each of these processes varies both spatially and temporally.