AAPG Annual Convention and Exhibition

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De-Constructing the Gravity Field Through Time: Implications for the Angola-Brazil Conjugate Margin


2D gravity modelling has long been used as a tool for validating and discriminating between geological models of crustal composition and geometry. In this presentation, we present an approach where the post rift sedimentary series is stripped off and the isostatic response of the pre-drift Geology is used to model the crustal configuration at the end of continental rifting. The method consists of: 1- Modelling the gravity field across the continental margin to constrain architecture and densities in the crust and uppermost mantle 2- Removing the salt and post salt series 3- De-compacting the pre-salt sediment layer 4- Calculating the gravity effect of the remaining section and considering what vertical movements are required to bring to bring the model to isostatic equilibrium 5- Comparing the result with other geological evidence including bio-stratigraphy and limits on depositional environments Using examples from the offshore Angolan and Brazilian continental margins, we demonstrate how this approach helps broaden our understanding of the relationship between rifting and salt deposition in the Aptian. As the model is constructed back to the end of the synrift phase, we expect the crustal geometry to be identical to the present day architecture. The model is then adjusted isostatically on a broad scale to account for the effect of the sediment load. Local isostatic imbalances remain in the resultant model; these can be related to: – Lateral strength of the lithosphere – Short term plate tectonic stresses – The creation of an accommodation space at the rift margin which we hypothesise was the site for syn-rift salt deposition that flowed later during Albian time after the tilting of the whole continental margin. This approach provides additional insights into: 1- The nature of the Aptian salt basins 2- Timing and distribution of the deformation along the continental margin 3- The timing and the mechanism of rifting.