Crustal Thickness and Continental Lithosphere Thinning on the N. Angolan Rifted Margins Predicted Using Gravity Inversion

Crustal thickness and continental lithosphere thinning have been determined for the N. Angolan Margin using gravity inversion; a margin in which seismic imaging of the ocean-continent transition is obscured by large thicknesses of allochthonous salt. The gravity inversion is carried out in the 3D spectral domain and includes a correction for the lithosphere thermal gravity anomaly generated by elevated geothermal gradients within stretched continental margin and adjacent ocean basin lithosphere. Sediment thicknesses used to determine the sediment gravity anomaly contribution are derived from the P7+11 and P3 seismic cross-sections of Contrucci et al. (2004). A compaction-controlled sediment density is assumed for non-salt lithologies. Moho depth predicted by the gravity inversion are in good agreement with the Contrucci et al. (2004) seismic estimates. Continental lithosphere thinning along the P3 and P7+11 profiles have been determined from crustal basement thicknesses predicted by gravity inversion and include a correction for volcanic addition generated by decompression melting during continental breakup. The sensitivity of Moho depth, crustal thickness and continental lithosphere thinning to sub-salt sediment thickness, breakup age and volcanic addition has been investigated. If the N. Angola margin is magma poor, gravity inversion predicts a region, for lines P3 and P7+11, up to 180 km wide of highly thinned pre-breakup continental crust between 15 and 6 km thick. If the N. Angolan margin has "normal" volcanic addition, then gravity inversion predicts the width of the highly thinned continental crust to be reduced to approximately 100km width. It is unclear whether the ocean-continent transition zone beneath the allochthonous salt is underlain by exhumed mantle.