Structural features of the Southwest African continental margin according to results of lithosphere-scale 3D gravity and thermal modelling

doi:10.1016/j.tecto.2013.04.014

Tectonophysics

Volume 604, 24 September 2013, Pages 104-121

https://doi.org/10.1016/j.tecto.2013.04.014 Get rights and content

Highlights

•
3D structural model of the Southwest African continental margin has been derived.
•
3D gravity modelling: the configuration of the crystalline crust has been resolved.
•
3D density model: presence of a high-density lower crustal layer along the margin.
•
3D thermal model predicts temperature distribution beneath the margin.

Abstract

To understand the structure of the Southwest African continental margin, a lithosphere-scale 3D structural model has been developed, covering the marginal Cretaceous-Cenozoic Orange, Luderitz, Walvis and Namibe basins, the easternmost Walvis Ridge offshore. Onshore, the model includes two late-Proterozoic Owambo (Etosha) and Nama basins. This 3D model integrates published thickness maps (sediment isopach maps), shallow seismic and well data as well as published deep seismic information and has been additionally constrained by 3D gravity and thermal modelling.

Using 3D gravity modelling, the first order configuration of the crystalline crust has been resolved with respect to the location of the continent–ocean boundary. The distribution of a high-density lower crustal layer indicates a continuous body extending below the Cretaceous-Cenozoic depocentres and aligned parallel to the coast line. In addition, high-density zones within the continental crystalline crust had to be included in the model to fit observed and calculated gravity. The obtained Moho topography correlates with the major tectonic units of this continental margin.

The results of the 3D thermal modelling indicate that there is a clear relationship between the location of thickened sediments and areas with increased temperatures within the upper 10 km of the 3D model. This indicates that the low thermal conductivity of the sediments increases heat storage within the areas covered by thick sediments. Within the deeper crust, the main feature of the temperature distribution is the transition across the continental margin from the relatively cold oceanic part to the warm continental one. This regional pattern is controlled by the thickness of the crystalline continental crust, which is characterized by an increased radiogenic heat production. At a depth of 80–90 km, the temperature becomes higher beneath the oceanic domain than beneath the continent, reflecting the configuration of the lower thermal boundary which is represented by an isothermal lithosphere–asthenosphere boundary.

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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Structural features of the Southwest African continental margin according to results of lithosphere-scale 3D gravity and thermal modelling

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Geological setting

3D structural model

3D gravity modelling

3D gravity modelling

Discussion

Conclusions

Acknowledgements

Earth and Planetary Science Letters

Earth and Planetary Science Letters

Tectonophysics

Lithos

Marine and Petroleum Geology

Marine Geology

Tectonophysics

Marine and Petroleum Geology

Marine and Petroleum Geology

Marine and Petroleum Geology

Earth and Planetary Science Letters

Sedimentary Basins of the World

Tectonophysics

Journal of African Earth Sciences

Tectonophysics

Physics and Chemistry of the Earth

Tectonophysics

Journal of African Earth Sciences

Precambrian Research

Tectonophysics

The DNSC08GRA global marine gravity field from double retracked satellite altimetry

Journal of Geodesy

Deep structure of the Namibia continental margin as derived from integrated geophysical studies

Journal of Geophysical Research

Modeling of the 3D thermal field in the Northeast German Basin

Geologische Rundschau

Crustal architecture and deep structure of the Namibian passive continental margin around Walvis Ridge from wide-angle seismic data. EGU General Assembly, memory stick

Geophysical Research Abstracts

Anomaly map: Regional Free Air or Bouguer anomaly grid data derived from the EGM2008 model

Crustal-scale architecture and segmentation of the Argentine margin and its conjugate off South Africa

Geophysical Journal International

Offshore Mesozoic basins

Sequence stratigraphy in offshore South African divergent basins: an atlas on exploration for Cretaceous lowstand traps by SOEKOR (Pty) Ltd

American Association of Petroleum Geologists, Studies in Geology

Morphotectonic evolution of the South Atlantic margins of Africa and South America

Conduction of Heat in Solids

Thermal properties: thermal conductivity and specific heat of minerals and rocks

Seismic velocity structure and composition of the continental crust: a global view

Journal of Geophysical Research

Continental material in the shallow oceanic mantle—how does it get there?

Geology

The Namib Rift: a rift system of possible Karoo age, offshore Namibia

The Oil and Gas Habitats of the South Atlantic

The Geological Society, London, Special Publications

Deep structure of the West African continental margin (Congo, Zaire, Angola), between 5oS and 8oS, from reflection/refraction seismics and gravity data

Geophysical Journal International

A model for flood basalt volcanism

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Gravity modeling of the ocean–continent transition along the South Atlantic margins

Journal of Geophysical Research

The SW African volcanic rifted margin and the initiation of the Walvis Ridge, South Atlantic

Marine Geophysical Researches

Geochemistry and petrogenesis of the Etendeka volcanic rocks from SWA/Namibia

Geological Society of South Africa Special Publications

Tectonic and Thermal Evolution of South Atlantic Marginal Basins

Crustal structure across the Colorado Basin, offshore Argentina

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Theoretical analysis of regional groundwater flow: 2. Effect of water-table configuration and subsurface permeability variation

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Post Palaeozoic succession and structure of the south-western African continental margin

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South Atlantic volcanic margins

Journal of the Geological Society