Abstract

Receiver functions were generated from seismic traces recorded by seismographs in the southwestern Cape to estimate the crustal thickness and other crustal features in this area. The Moho here is found at variable depths. Crustal thicknesses below the Karoo basin gradually increases from ~46km in the north, near Kenhard, to ~50km, 300km farther south near the northern margin of the Southern Cape Conductive Belt that is buried beneath the southern Karoo basin. From there southward for about 150km beneath the frontal sector of the eastern branch of the Cape Fold Belt (as far as the Kango Fault), the crust thins to less than 40km. Farther southward still, for ~ 50km beneath the central sector of the eastern branch of the Cape Fold Belt to the extension of the Worcester Fault, the crust thickens to about 45km. Then towards the south-coast, beneath the southernmost sector of the eastern branch of the Cape Fold Belt, the crust thins rapidly to less than 30km. This thinning is likely associated with Jurassic-Cretaceous crustal stretching recorded in the stratigraphy of extensional basins of South Africa and its continental margin. Small scale variations in crustal thicknesses of ~4km around each recording station reflect variability of the Moho on a small scale which, beneath the Namaqua-Natal Mobile Belt is possibly related to the tectonic accretion history of this Kibaran-age belt; and beneath the Cape Fold Belt due to differential crustal extension and thinning in the Mesozoic.

Beneath the frontal sector of the Cape Fold Belt, sharp crustal discontinuities at ~8 and ~18km depth are detected at four seismic stations. These stations are all located above the Southern Cape Conductive Belt (SCCB) and its associated regional east-west trending positive magnetic anomaly (the Beattie Anomaly; the largest of its kind in Africa) that can be traced for almost 1000km across southern South Africa. The intracrustal discontinuities are interpreted as the upper and lower bounds of a ~10km thick block of altered Mesoproterozoic (Kibaran) mafic-ultramafic rocks that are assumed to be the source for the Beattie Anomaly and the electrically conductive SCCB. Towards the west, the upper contact of the source of the Beattie Anomaly/SCCB deepens from ~8km to ~11km before the magnetic signature disappears beneath the western branch of the Cape Fold Belt.

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