Tectonic evolution of the Cape and Karoo basins of South Africa
Introduction
The Karoo, a Khoikhoi (Hottentot) word meaning Thirstland, is the arid interior of South Africa, a broad plateau with the highest freeboard in the world. This plateau is supported by the Archean Kaapvaal craton and the surrounding Proterozoic basement, a suite of rigid basement blocks that controlled basin development throughout the Phanerozoic.
The Cape and Karoo basins represent two large-scale episodes of intermittent subsidence and sedimentation within the interior of Gondwana (Fig. 1, Fig. 2). The Early Ordovician to Early Carboniferous Cape basin (160 Ma) is believed to have developed by extensional processes (Cloetingh et al., 1992) in an episutural setting (Hälbich et al., 1993). However, located more than 1000 km from the paleo-Pacific plate, it was not a typical passive margin (e.g. Thamm and Johnson, 2006). The shallow marine and non-marine basin fill is notable for its great thicknesses of quartz sand accumulation and muddy deltaic deposits with a diverse marine invertebrate biota. After a 30 Ma hiatus, the Late Carboniferous to Jurassic Karoo basin (125 Ma) developed as a tabular cratonic cover. The Karoo is generally interpreted as a retro-arc foreland basin that developed behind an inferred magmatic arc and fold-thrust belt (e.g. Catuneanu et al., 1998, Johnson et al., 2006). But basement architecture, timing of the Cape orogeny, and stratigraphic relationships are atypical of a flexural foreland basin. An alternative explanation is that the long-wavelength component of subsidence resulted from lithospheric deflection due to mantle flow coupled to distant subduction (Pysklywec and Mitrovica, 1999). The terrestrial upper Karoo succession is acclaimed for its remarkable diversity of synapsid reptile fossils. Alex du Toit, in 1927, recognized the overarching importance of Karoo stratigraphy and paleontology in Gondwana-wide correlation.
There is a substantial amount of geophysical and geological evidence that allows us to elaborate on these earlier basin interpretations. We will address the large-scale evolution of the Cape and Karoo basins, and show that the styles of subsidence, the Cape orogeny, and late Karoo magmatism were controlled by first-order basement faults and the motions of the basement blocks between them. The sedimentary fill is made up of several unconformity-bounded megasequences that show that each basin episode consists of a three-stage evolution, comprising crustal uplift, fault-controlled subsidence, and long periods of regional subsidence during which faulting was subordinate. This interpretation suggests that the component of regional subsidence was accommodated by vertical stresses of a ductile mantle, and that the stratigraphy is an intimate surface monitor of this behaviour.
This paper focuses on the three-dimensional basement architecture and the stratigraphic response to tectonism. We have mapped the structural framework of the basement from published geological maps and total field magnetic and Bouguer gravity data (Stettler et al., 1999, Venter et al., 1999), and have derived depths to Moho from teleseismic data (Nguuri et al., 2001, James et al., 2003). Kaapvaal basement structure is partly imaged in 500 km of 16 s seismic (de Wit and Tinker, 2004). Deep reflection seismic line SAGS-03-92 Beaufort images the southern Namaqua basement between Beaufort West and Droogekloof. This line was recorded 96-fold with a record length of 16 s TWT, conducted with a vibrator source and a 50 m geophone spacing, giving a 6–60 Hz sweep of frequencies. Finally, Soekor drilled 19 exploration wells to a maximum depth of 5560 m (Winter and Venter, 1970), but with an enormous well spacing (up to 250 km).
Section snippets
Tectonic framework
Gondwana was assembled in the late Neoproterozoic and Cambrian by Pan-African – Brasiliano orogenic belts that sutured the cratons of West Gondwana, and by collision with East Gondwana along the Mozambique belt during the late-stage East-African – Ross orogeny (Grunow et al., 1996, Unrug, 1997). This assembly includes the continental lithosphere of the Falkland Plateau which abutted South Africa along the Agulhas–Falkland transform (Richards et al., 1996, Jacobs et al., 1999), and crystalline
Early Paleozoic Cape basin: large extensional subsidence
The Cape Supergroup represents 160 myr of Ordovician through Early Carboniferous subsidence. The basin fill is about 8 km thick. Exploration wells and reflection seismic show that these sediments, including the Msikaba Formation of the Transkei, onlap the Namaqua basement (Fig. 2, Fig. 7). Onland the basin measures about 200,000 km2 to the subcrop edge, and is known to extend at least 100 km offshore. In this section we briefly discuss the episutural Saldanian geology and how the Cape basin was
Permian early Karoo basin: large-scale epeirogeny
The Karoo Supergroup spans 125 myr of latest Carboniferous – Early Jurassic time. It measures nearly 600,000 km2, and has a maximum thickness of ∼5500 m above the Namaqua basement block. The lower Karoo is separated from the upper Karoo by the Permian–Triassic boundary. Subsidence of the Permian lower Karoo basin pre-dated the Cape orogeny. Seismic stratigraphy (Fig. 5), the relationship of cleavage fabrics to diagenetic mineral assemblages (de Swardt and Rowsell, 1974), and provenance studies (
Cape orogeny and late Karoo foreland basin
The Cape orogeny dates to the Triassic uppermost Beaufort and Stormberg groups (∼250–215 Ma). This event is linked to an eastward shift of the locus of subsidence, deposition of coarse alluvial fans, and the appearance of the Lystrosaurus assemblage after the end-Permian crisis. In the southern Karoo, Beaufort seismic reflections at the leading edge of the Cape fold belt have gentle northward dips, parallel to the roof thrust of a tectonic wedge (Fig. 5, Fig. 17), thus directly demonstrating
Discussion
The Cape and Karoo basins formed within the continental interior more than 1000 km from the plate edge. Basin formation was controlled by the strength and anisotropy of rigid blocks of Precambrian basement and the crustal faults between them (Fig. 4, Fig. 6). The distributions and amplitudes of several of the basin-filling events, and the minor contribution of contemporaneous upper crustal brittle failure, require accommodation by ductile mantle flow. Upper mantle rather than lower crustal flow
Summary and conclusions
The Karoo basin is commonly interpreted as a retro-arc foreland basin, despite there being no geophysical evidence for the nearby magmatic arc and the Cape fold belt dating to late Karoo time. Neither does the basin fill display the onlapping characteristics of a flexural foreland basin. Another particularly weak part of the interpretation is that the lithosphere is not laterally uniform.
Overwhelming mutually supportive evidence links the stratigraphy directly to the behaviour of rigid crustal
Acknowledgements
We remember with sadness Dairne Rowsell who died in 2007. Her diagenetic studies were of fundamental help to us.
This project has been an enormous undertaking, covering a long spell of basin evolution and many disciplines. We have checked as much of the data as we can, and greatly appreciate the help that we have received in doing so. In particular, we wish to thank Hugh Balkwill, Peter Booth, Dave Broad, Doug Cole, Bill Collins, Coenie de Beer, Erwin Ebner, Bob Ehrlich, Andrea Fildani, Miguel
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