Crust-scale 3D model of the Western Bredasdorp Basin (Southern South Africa): data-based insights from combined isostatic and 3D gravity modelling

The southern South African continental margin documents a complex margin system that has undergone both continental rifting and transform processes in a manner that its present-day architecture and geodynamic evolution can only be better understood through the application of a multidisciplinary and multi-scale geo-modelling procedure. In this study, we focus on the proximal section of the larger Bredasdorp sub-basin (the westernmost of the five southern South African offshore Mesozoic sub-basins), which is hereto referred as the Western Bredasdorp Basin. Integration of 1200km of 2D seismic-reflection profiles, well-logs and cores yields a consistent 3D structural model of the Upper Jurassic-Cenozoic sedimentary megasequence comprising six stratigraphic layers that represent the syn-rift to post-rift successions with geometric information and lithology-depth-dependent properties (porosities and densities). We subsequently applied a combined approach based on Airy's isostatic concept and 3D gravity modelling to predict the depth to the crust-mantle boundary (Moho) as well as the density structure of the deep crust. The best-fit 3D model with the measured gravity field is only achievable by considering a heterogeneous deep crustal domain, consisting of an uppermost less dense prerift meta-sedimentary layer [=2600kgm(-3)] with a series of structural domains. To reproduce the observed density variations for the Upper Cenomanian-Cenozoic sequence, our model predicts a cumulative eroded thickness of ca. 800-1200m of Tertiary sediments, which may be related to the Late Miocene margin uplift. Analyses of the key features of the first crust-scale 3D model of the basin, ranging from thickness distribution pattern, Moho shallowing trend, sub-crustal thinning to shallow and deep crustal extensional regimes, suggest that basin initiation is typical of a mantle involvement deep-seated pull-apart setting that is associated with the development of the Agulhas-Falkland dextral shear zone, and that the system is not in isostatic equilibrium at present day due to a mass excess in the eastern domain of the basin that may be linked to a compensating rise of the asthenospheric mantle during crustal extension. Further corroborating the strike-slip setting is the variations of sedimentation rates through time. The estimated syn-rift sedimentation rates are three to four times higher than the post-rift sedimentation, thereby indicating that a rather fast and short-lived subsidence during the syn-rift phase is succeeded by a significantly poor passive margin development in the post-rift phase. Moreover, the derived lithospheric stretching factors [=1.5-1.75] for the main basin axis do not conform to the weak post-rift subsidence. This therefore suggests that a differential thinning of the crust and the mantle-lithosphere typical for strike-slip basins, rather than the classical uniform stretching model, may be applicable to the Western Bredasdorp Basin.