South Africa - Inkaba.org

Integrated Interpretation of the Deep Crustal Geophysical
Profile Across the Southern Karoo Basin and Beattie Magnetic
Anomaly, South Africa
A. Lindeque 1,2 , M.J. de Wit 2 , T. Ryberg 3
1. Currently at the Alfred Wegener Institute for Polar and Marine Research, Am Alten Hafen 26, 27568
Bremerhaven, Germany ansa.lindeque@awi.de
2. AEON - Africa Earth Observatory Network and Department of Geological Sciences, University of
Cape Town, Rondebosch 7701, South Africa
3. GeoForschungsZentrum Potsdam, Germany, trond@gfz-potsdam.de
ABSTRACT
Between 2004 and 2007, several geophysical experiments were conducted along transects on the southern margin
of Africa as part of the Inkaba yeAfrica initiative. One of these, the Agulhas-Karoo (AK) transect, crosses
prominent tectonic features such as the Paleozoic-Mesozoic Karoo Basin, Paleozoic Cape Fold Belt Front and
Beattie Magnetic Anomaly. The acquired geophysical data provide the opportunity to adjust the last published
crustal model of 1993, based on sparse gravity and magnetic data, with a one derived from the joint interpretation
of archive data (surface geology, aeromagnetic data, nearby deep boreholes, teleseismic receiver functions,
regional seismic reflection profiles), and the acquired high-resolution geophysical data (impedance spectroscopy
measurements on borehole samples, near vertical reflection data (NVR), shallow P- and S-wave velocity data, wide
angle refraction data and magnetotelluric data), at the centre 100 km of the western AK-transect. The integrated
model differentiates four components in the ~42 to 45 km thick crust: (1) the ~2 to 5 km thick folded Karoo
Supergroup with low-angle thrust faults rooted in a zone of local, resting paraconformably on (2), the ~2.5 to 12
km thick non-folded sandstone/shale Cape Supergroup (CSG) - represented by a distinctive band of horizontal
reflectors ending in a flat base, or seismic unconformity, separating it from (3), the ~13 to 21 km underlying midcrust
with a distinct north dipping seismic fabric, interpreted as ~1.2 Ga Mesoproterozoic Namaqua-Natal
Metamorphic Belt (NNMB). The NNMB continues below the CFB tectonic front, possibly to the continental
margin and contains the two-part BMA. The BMA is constrained to a ~10 to 12 km wide northern zone, and a ~5
to 7 km wide southern zone, both at ~7 to 8 km below surface, continuing for a depth of ~5 km. The BMA source
is interpreted to be a Namaqua-like massive to disseminated, deformed/metamorphosed strataform sulfide ore
body. A mid-crustal detachment interpreted as a thrust or shear plane, separates the mid-crust from component (4),
the highly reflective ~10 to 24 km thick lower crust, interpreted as older Palaeo-Proterozoic NNMB basement, and,
a ~2 to 5 km thick highly reflective layer, sub-parallel to a clear Moho, interpreted as mafic underplating. The
model presents evidence in support of a thin-skinned tectonic thrust model for the evolution of the flat-based CFB
and shows no significant fore-deep stratigraphic thickening in the Karoo Basin towards the CFB front. The tectonic
model suggests a Meso-Proterozoic collision orogen setting, overprinted by a mid-Phanerozoic thin-skinned fold
and thrust belt coupled to the far-field accretion/subduction margin to the south.
KEYWORDS: Seismic Reflection, crust, Beattie Magnetic Anomaly, Karoo Basin, Cape Fold Belt, South Africa,
Namaqua Natal Metamorphic Belt
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