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GEOREF RECORD

Structural variations of the crust in the Southwestern Cape, deduced from seismic receiver functions

Josh D. Harvey, Maarten J. de Wit, Jacek Stankiewicz and C. Moctar Doucoure
Structural variations of the crust in the Southwestern Cape, deduced from seismic receiver functions
South African Journal of Geology (September 2001) 104 (3): 231-242

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 approximately 46 km in the north, near Kenhard, to approximately 50 km, 300 km 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 150 km 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 40 km. Farther southward still, for approximately 50 km 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 45 km. 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 30 km. 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 approximately 4 km 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 approximately 8 and approximately 18 km 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 1000 km across southern South Africa. The intracrustal discontinuities are interpreted as the upper and lower bounds of a approximately 10 km thick block of altered Mesoproterozoic (Kibaran) maficultramafic 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 approximately 8 km to approximately 11 km before the magnetic signature disappears beneath the western branch of the Cape Fold Belt.


ISSN: 1012-0750
EISSN: 1996-8590
Coden: TGSAA3
Serial Title: South African Journal of Geology
Serial Volume: 104
Serial Issue: 3
Title: Structural variations of the crust in the Southwestern Cape, deduced from seismic receiver functions
Affiliation: University of Cape Town, Department of Geological Sciences, Rondebosch, South Africa
Pages: 231-242
Published: 200109
Text Language: English
Publisher: Bureau for Scientific Publications, Pretoria, South Africa
References: 53
Accession Number: 2002-064796
Categories: Solid-earth geophysics
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. sect., 2 tables, geol. sketch maps
S38°00'00" - S19°00'00", E10°00'00" - E40°00'00"
Country of Publication: South Africa
Secondary Affiliation: GeoRef, Copyright 2018, American Geosciences Institute.
Update Code: 200220
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