R.V. Dingle, 1983. "Slump Structure S on the Outer Continent AlMargin of Southwestern Africa", Seismic Expression of Structural Styles: A Picture and Work Atlas. Volume 1–The Layered Earth, Volume 2–Tectonics Of Extensional Provinces, & Volume 3–Tectonics Of Compressional Provinces, A. W. Bally
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Over 260,000 sq km (96,525 sq m) of the outer continental margin of southwest Africa have been affected by Neogene slump structures. They occur along the whole of the margin between the eastern Walvis Ridge and the southern tip of the African continent (about 200S to 370S) North of about 300S, four discrete slump zones have been recognized, but to the south, the margin has been cut by several (at least four) coalescing features which form an almost continuous zone over 850 km (527 mi) long. Three seismic profiles and interpretative sections are shown to illustrate the geometry and associated structures of these slumps (Figure 1) - two from the large feature (Chamais Slump) immediately north of 300S (sections A and B) off southern Namibia, and one from the complex zone southwest of Cape Town (section C). Further details of the geological setting and overall geometry of these and similar large allochthonous structures on the nearby southeast continental margin can be found in Emery et al (1975), Dingle (1977, 1979), Bolli et al (1978), Summerhayes, Bornhold, and Embley (1979), and Dingle, Seisser, and Newton (1983). Figure 2 diagrammatically shows the main morphological features that have been used by various workers to identify large slumped masses from seismic profiles.
In the following account of the seismic profiles A-C, vertical scales are shown in seconds of traveltime (two-way time), which in the interpretive sections have been converted to water depths at 1,500 m/sec (4,921 ft/sec) (meters on the left, fathoms on the right). In the discussion, thicknesses of strata are quoted in meters, based on a nominal P-wave velocity of 2,000 m/sec (6,562 ft/sec) in the sediment. On the interpretive sections, thick lines indicate glide planes, and opposed arrows show directions of relative movement along some of the larger glide planes. Slope inclinations are calculated from the horizontal, and quoted as a ratio (for example, 1 in 10 =1:10).