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This thesis investigates a sequence of 17 cycles of coarse-grained syn-rift delta progradation in the Loreto basin, Baja California Sur. The Loreto basin is a Pliocene half-graben bounded by the 30 km long easterly dipping extensional Loreto fault, with 1.5 km throw. The Loreto basin uniquely provides a temporally constrained setting that couples preserved alluvial architecture with marine depositional geometries that enables these controls to be distinguished. The depositional architecture of each cycle is reconstructed by correlating 61 close-spaced (20 m to 320 m), detailed (1:5000) sections. Of the 17 progradational cycles, the lower 11 are particularly well exposed. Temporal constraints achieved through the dating of two sequence bounding tuff horizons, give an average duration for a single cycle of delta progradation to be 14±8.6 kyr., with most cycles being <10 kyr. This duration is too short to be explained by Pliocene eustatic variations (41 kyr). A consistency in measured palaeocurrents towards the NNE suggests that lobe switching was not a significant factor in controlling delta architecture. A facies framework is established that recognises both shoal-water and Gilbert-type delta deposition in the basin. The facies framework, coupled with reconstructed depositional architectures reveals that 7 of the 11 lower cycles, and at least one upper cycle (12) exhibit a palaeoseaward transition from shoal-water to Gilbert-type deltas. This nucleation of clinoforms with initially oblique, but increasingly sigmoidal geometries is characterised by a down-transport development of forests, which increase in height from 4 m to >26 m as they are traced basinward. Each cycle is capped by a shell bed that records drowning of the delta top. The geometry, and specifically the nucleation of clinoforms in single cycles of progradation requires a continued basinward increase in bathymetry. In one case this increase can be attributed to the topography of an underlying delta cycle. However for most of the cycles the increase in bathymetry can only be achieved if the rate of tectonically-controlled accommodation creation accelerates during their deposition.

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.