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OCT Structure, COB Location and Magmatic Type of the SE Brazilian and S. Angolan Margins From Integrated Quantitative Analysis of Deep Seismic Reflection and Gravity Anomaly Data


Ocean-continent transition (OCT) structure, continent-ocean boundary (COB) location and magmatic type are of critical importance in evaluating petroleum systems in deep-water frontier oil and gas exploration. The OCT structure, COB location and magmatic type of the SE Brazilian and S Angolan rifted continental margins are much debated; exhumed and serpentinised mantle have been reported at these margins. Integrated quantitative analysis using deep seismic reflection data and gravity inversion have been used to determine OCT structure, COB location and magmatic type. Gravity anomaly inversion, incorporating a lithosphere thermal gravity anomaly correction, has been used to determine Moho depth, crustal basement thickness and continental lithosphere thinning. Residual Depth Anomaly (RDA) analysis has been used to investigate OCT bathymetric anomalies with respect to expected oceanic bathymetries and subsidence analysis has been used to determine the distribution of continental lithosphere thinning. These techniques have been validated on the Iberian margin for profiles IAM9 and ISE-01. In addition a joint inversion technique using deep seismic reflection and gravity anomaly data has been applied to the ION-GXT BS1-575 SE Brazil and ION-GXT CS1-2400 S Angola. The joint inversion method solves for coincident seismic and gravity Moho and calculates the lateral variations in crustal basement densities and velocities. Gravity inversion, RDA and subsidence analysis along the S Angolan ION-GXT CS1-2400 profile suggests that exhumed mantle, corresponding to a magma poor margin, is absent beneath the allochthonous salt. The thickness of earliest oceanic crust, derived from gravity and deep seismic reflection data is approximately 7km. The joint inversion predicts crustal basement densities and seismic velocities slightly less than expected for “normal” oceanic crust. Gravity inversion, RDA and subsidence analysis along the ION-GXT BS1-575 profile, crossing the SE Brazilian margin, predicts the COB to be located SE of the Florianopolis Ridge. Analysis shows no evidence for exhumed mantle on this margin profile. The joint inversion technique predicts normal oceanic basement seismic velocities and densities. Crustal basement thicknesses of between 10–15km are predicted beneath the Sao Paulo Plateau and Florianopolis Ridge, separated by a thin region of crustal basement interpreted as a regional transtensional structure.