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The Roles of Magmatism and Loading in the Formation of Seaward Dipping Reflectors: Insights From Abandoned Volcanic Segments

Abstract

Deepwater rifted margins are major hydrocarbon exploration targets, however our understanding of their formation is still developing. On volcanic rifted margins, the rift-drift transition was accompanied by the emplacement of large volumes of flood basalt. In seismic reflection data these basalts, alongside intercalated sediments, are imaged as Seaward Dipping Reflectors (SDRs). SDRs are important to the hydrocarbon industry for several reasons: i) they may contain reservoir sands, ii) they overlie conventional syn-rift systems and hence may act as a seal and iii) their emplacement will affect the thermal history of the post-rift. However, the emplacement and seaward tilting of these flood basalts remains contentious, with numerous models proposing that they form via either: 1) landward dipping normal faulting, or 2) loading due to the axial emplacement of lava flows. These models are based on observations from widely spaced seismic lines over ancient margins, or observations from the Ethiopian rift system. Here we use a densely spaced seismic grid from the Orange Basin, offshore SW Africa, to map the SDRs at a higher resolution than has been previously shown. This allows us to differentiate the roles of faulting, magmatism and loading in SDR emplacement. Strong reflectors and seismic facies variations were used to divide the SDRs into several sub-units. Along strike, the earliest SDR packages transition into a series of confined volcanic depocentres. These contain up to 3 km of volcanic material and thin both landwards and seawards. Each segment is underlain by a feeder dyke complex which cross-cuts syn-rift stratigraphy. Stratal thickening is not fault-controlled and the underlying reflectors are warped downwards. This indicates that subsidence of up to 3 km was caused by loading from the extrusive and intrusive igneous bodies. This magnitude of subsidence is sufficient to produce the SDRs observed on many margins worldwide. It is therefore likely that during breakup in a volcanic environment, strain is accommodated through magmatism and associated loading, and not through faulting. Through gaining an understanding of SDR evolution, our results can inform the basin analysis and modelling required for hydrocarbon exploration on volcanic rifted margins.