Structural features of the Southwest African continental margin according to results of lithosphere-scale 3D gravity and thermal modelling
Graphical abstract
Introduction
The Southwest African continental margin offshore the western coast of South Africa and Namibia (Fig. 1) is a passive volcanic margin (Blaich et al., 2009, Brown et al., 1995, Gladczenko et al., 1997, Gladczenko et al., 1998, Macdonald et al., 2003, O'Connor and Duncan, 1990, Talwani and Abreu, 2000). There, Early Cretaceous continental break-up resulted in the formation of oceanic lithosphere within the Atlantic Ocean between Africa and South America as well as in the formation of several sedimentary basins along the western African coast (e.g. Macdonald et al., 2003, Seranne and Anka, 2005, Stewart et al., 2000). Tectonically, the Southwest African continental margin encompasses the Orange, the Luderitz, the Walvis, and partially the Namibe basins (Fig. 2). All these basins have axes parallel to the present-day coastline of southwest Africa and contain thick sequences of Cenozoic to upper Lower Cretaceous post-breakup sediments, Upper Jurassic-lower Lower Cretaceous syn-breakup sediments and, possibly, Upper Carboniferous to Triassic-Lower Jurassic Karoo sediments (Clemson et al., 1999, Erlank et al., 1984) and sediments of older ages (Gladczenko et al., 1998). Consequently, the sedimentary archives in the area under consideration have recorded the main tectonic stages that occurred within the Southwest African continental margin. In addition, this segment of the West African continental margin is complicated by the presence of the NE–SW striking magmatic Walvis Ridge.
To analyse the present-day structure of the passive margin of South Africa and Namibia, a lithospheric-scale 3D model has been constructed. Offshore, the 3D structural model covers the marginal Cretaceous-Cenozoic Orange, Luderitz, Walvis and Namibe basins, the Walvis Ridge (Fig. 2). Onshore, the model includes two late-Proterozoic basins: Owambo (Etosha) and Nama (e.g. Clauer and Kröner, 1979, Miller, 1997).
Based on structural data, a 3D gravity modelling approach (IGMAS: Interactive Gravity and Magnetic Application System; Götze, 1978, Götze and Lahmeyer, 1988, Schmidt and Götze, 1998, Götze and Schmidt, 2010, Schmidt et al., 2011) has been applied to obtain a 3D density model of the entire Southwest African continental margin and adjacent continent and ocean to understand the regional configuration of the deep structure of the study area. Finally, the gravity-consistent 3D structural model has been used to evaluate the present-day conductive thermal field beneath the study area, using the software GeoModelling System (GMS) for 3D thermal analysis (Bayer et al., 1997, Scheck and Bayer, 1999). Consequently, the major results of this study are related to the 3D configuration of the regional-scale structural and thermal features of the Southwest African continental margin and adjacent areas according to 3D gravity and 3D thermal modelling.
Section snippets
Geological setting
Onshore, the Southwest African passive continental margin is bounded by the Precambrian Congo and Kalahari cratons with the superimposed Proterozoic-early Palaeozoic Kaoko and Damara orogenic belts (Miller, 1983, Passchier et al., 2002, Persits et al., 1997, Seth et al., 1998). This old structural base is complicated by the presence of two late-Proterozoic sedimentary basins, the Owambo (Etosha) Basin and the Nama Basin (Clauer and Kröner, 1979, Miller, 1997 Miller et al., 2010).
This passive
3D structural model
For the construction of the 3D structural model, the bathymetry and topography have been taken from the General Bathymetric Chart of the Oceans (GEBCO) Digital Atlas (IOC, IHO and BODC, 2003). Accordingly, sea water represents the uppermost layer of the 3D structural model.
The configuration of the sedimentary cover in the model is based on a set of structural depth maps derived from the interpretation of reflection seismic lines (Hartwig et al., 2010, Stewart et al., 2000). Four structural
3D gravity modelling
3D gravity modelling has been carried out by use of the Interactive Gravity and Magnetic Application System (IGMAS) which is a software package for 3D gravity and magnetic modelling (Götze, 1978, Götze and Lahmeyer, 1988, Götze and Schmidt, 2010, Schmidt and Götze, 1998, Schmidt et al., 2011). During the 3D gravity analysis, the geometrical approximation of the 3D structural model is done by a triangulation between predefined structural depth maps. In particular, the complex shape of the layers
3D gravity modelling
The modelled gravity response of the 3D structural/density model (Fig. 6b) demonstrates a good fit with the observed gravity data (Fig. 6a) in terms of large-scale structural features of the study area. In most places, the difference between the observed and the calculated gravity anomalies ranges from − 10 to + 10 mGal (Fig. 6c). Locally, some short-wavelength misfits between the observed and the calculated gravity anomalies reach higher values than ± 10 mGal. These misfits are confined to
Discussion
The results of 3D gravity modelling confirmed that the crystalline crust of the study area is not homogeneous. At the regional scale, the crystalline crust has been subdivided into four major layers, namely the middle and upper oceanic crust (oceanic layers 2 and 3A), the middle and upper continental crust, the high-density lower crustal layer (oceanic layer 3B beneath the ocean and the high-density lower crustal body along the continental margin) and the high-density zones within the
Conclusions
In summary it can be stated that the modelled gravity response of the 3D structural/density model demonstrates a good fit with the observed gravity data in terms of long wavelengths characteristics. In particular, the 3D structural/density model indicates the presence of a continuous high-density lower crustal layer along the continental margin as well as of the high-density zones within the continental crystalline crust of the study area. The high-density lower crustal layer is often
Acknowledgements
This work has been done in the framework of the Priority Program 1375 - SAMPLE (South Atlantic Margin Processes and Links with onshore Evolution), funded by the German Research Foundation (DFG). We would also like to acknowledge the support provided in frame of the INKABA yeAFRICA project. We are thankful to Hans-Jürgen Götze and Sabine Schmidt for permission to use the 3D gravity modelling software package (IGMAS) as well as for providing us support with this software. Zahie Anka's position at
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