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Investigation of the crust in the Southern Karoo using the seismic
reflection technique
L. Loots 1,2 , R.J. Durrheim 2,3 , T. Ryberg 4
1. Council for Geoscience, South Africa, lloots@geoscience.org.za
2. University of Witwatersrand, South Africa
3. CSIR, South Africa, rdurrhei@csir.co.za
4. GeoForschungsZentrum Potsdam, Germany, trond@gfz-potsdam.de
ABSTRACT
A deep seismic reflection survey was conducted in September 1992 as part of the National Geophysics
Programme. The data were re-processed at GFZ using ProMax TM . The 100.5 km profile (running from Beaufort
West in the north to Klaarstroom in the south along the N12 national road) presents a high quality seismic image of
the crust down to the Moho. The profile traverses the southern Karoo Basin and the northern part of the Cape Fold
Belt (CFB). The upper crust is characterized by strong reflectors that dip gently to the south up to where the CFB
starts. The reflections are produced by the bedded strata of the Karoo and Cape Supergroups. The Karoo and Cape
Supergroups are separated by an unconformity, which has a very strong seismic signal. The upper and the lower
crust are also separated by an unconformity which can be traced from the north of the profile to where the Cape
Fold Belt starts. The lower crust is not very reflective, but is likely part of the Bushmanland sub-province of the
Namaqua-Natal Mobile Belt. It is also here that the most probably source for the Beattie magnetic anomaly (BMA)
is hosted. A ~10 km wide, highly reflective zone coinciding with the location of the BMA occurs at a depth of 7 –
15 km. The seismic fabric of the lower crust exhibits both northerly and southerly dips. A strong continuous
seismic signal can be seen at 20 – 25 km from the north of the profile at a depth of ~ 37 km. This is interpreted to
be the boundary between the crust and the mantle. All this preliminary results are in accord with other published
results.
KEYWORDS: Seismic reflection, crust, Beattie magnetic anomaly, Moho.
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