The anatomy of a large submarine slump on a sheared continental margin (SE Africa)

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Abstract
The morphology and structure of a large submarine slump (Agulhas Slump) on the sheared continental margin off SE Africa are described from bathymetric and continuous seismic reflection records. It is a composite feature consisting of proximal and distal allochthonous sediment masses separated by a large glide plane scar. The locations of the various structural elements of the slump are related to underlying features: in the head region these are controlled by large-scale Cretaceous and Palaeogene depositional features, and in the toe region by older basement ridges. In the western part of the slump, the basement ridges have dammed the slump over continental crust, whereas in the east, allochthonous material has spread into the oceanic Transkei Basin. Characteristics related to a structural setting on a sheared continental margin are emphasized and discussed. The Agulhas Slump is probably the largest slumped mass so far recognized from modem oceans (750 km long, 106 km wide, with a volume of over 20,000 km 3 ) and is post-Pliocene in age. A seismic triggering mechanism is tentatively proposed: the slump lies on two major fault zones whose extensions are known to be seismically active (the Cape Fold Belt and the Agulhas marginal fracture zone).