Deep Crustal Profile Across the Southern Karoo Basin and Beattie Magnetic Anomaly, South Africa: An Integrated Interpretation With Tectonic Implications

Abstract

Two geophysical onshore-offshore lines on the southern margin of Africa form the Agulhas-Karoo Geophysical Transect (AKGT) and cross prominent geological features such as the Karoo Basin, Cape Fold Belt (CFB) and the Beattie Magnetic Anomaly (BMA). Geophysical data acquired along this AKGTransect between 2004 and 2007 within the Inkaba yeAfrica (lyA) framework, provide the platform for constructing a deep crustal section (IyA-200501) for the centre 100 km of the western AKGT-transect in order to resolve these features at depth. We present a detailed deep crustal model constructed from the joint interpretation of: i. archive data comprising surface geology, aeromagnetic data, nearby deep boreholes, teleseismic receiver functions and regional seismic reflection profiles, and ii. line coincident newly acquired high-resolution geophysical data consisting of near vertical seismic reflection data, shallow P-and S-wave velocity data, wide-angle refraction data, high resolution magnetotelluric data and impedance spectroscopy measurements on borehole samples. Our model differentiates four components in the up to 45 km thick crust: 1. a ~2 to 5 km thick folded Karoo Supergroup, disrupted by low-angle thrust faults rooted in a zone of local décollements in the lower Ecca Group and resting paraconformably on 2. a continuous undeformed sub-horizontal ~1.5 to 10 km thick wedge of the Cape Supergroup (CSG). This CSG wedge stretches from the Escarpment in the north to the tectonic front of the CFB in the south, and rests on an unconformity that dips about three degrees to the south. The angular unconformity is interpreted as an erosional peneplain that separates the CSG wedge from component 3. the ~13 to 21 km mid-crust basement below. The mid-crust contains a distinct north-dipping seismic fabric, here interpreted as 1.4 to 1.0 Ga Mesoproterozoic Namaqua-Natal Metamorphic Belt (NNMB) crust. A south-dipping mid-crustal detachment, interpreted as a ductile thrust zone, separates the mid-crust from component 4. a highly reflective ~10 to 24 km thick lower crust. The latter is interpreted as an older Palaeoproterozoic section of the NNMB (or even Archean cratonic basement), and bounded by a ~2 to 5 km thick, highly reflective bottom layer below that lies subparallel to a clear Moho. This bottom layer is interpreted as a mafic underplate, metasomatic reaction zone, or lower-crust to mantle transition zone. Collectively the seismic reflection and wide-angle refraction data support an interpretation that the NNMB mid-crustal layer contains the BMA source, possibly connected to two zones of strong reflectivity: a ~10 to 12 km wide northern zone and a ~5 to 7 km wide southern zone, both about 5 km thick and 7 to 8 km below surface. We interpret the BMA source to be at least in part, a Namaqua-like massive to disseminated, deformed/metamorphosed stratiform sulphide-magnetite ore body with metasomatic overprint.

Palinspastic restoration of an exhumed deepwater system: A workflow to improve paleogeographic reconstructions

“…Geomechanically weak mudstone horizons that facilitate detachment of overlying units are inferred in other thrust systems, for example, the Moine Thrust Zone (Watkins et al, 2014) and the offshore Orange Basin (De Vera et al, 2010;Butler and Paton, 2010;Dalton et al, in press). Lindeque et al (2012) used subsurface data to infer that the Prince Albert and Whitehill Formations are detachment horizons (Fig. 3).…”

“…The two-armed Cape Fold Belt is broadly east-west trending in southern South Africa, but changes orientation to a north-south trend in the west (Johnston et al, 2000, Lindeque et al, 2012, Fagereng et al, 2014 (Fig.1). The southern arm of the belt is characterised by north verging folds that transition northwards into upright, open fold structures and grade into horizontal strata further north (Lock 1980;De Beer, 1990;de Wit and Ransome, 1992;Hälbich, 1993;Johnston et al, 2000;Paton et al, 2006;Fagereng, 2014;Fagereng and Byrnes, 2015).…”

“…The southern arm of the belt is characterised by north verging folds that transition northwards into upright, open fold structures and grade into horizontal strata further north (Lock 1980;De Beer, 1990;de Wit and Ransome, 1992;Hälbich, 1993;Johnston et al, 2000;Paton et al, 2006;Fagereng, 2014;Fagereng and Byrnes, 2015). The fold belt is thought to be an example of both thin-skinned and thick-skinned thrusting ; a domain of shallowly dipping thrusting under the Karoo Basin coalesces at depth with steep, basement-involved faults beneath the Swartberg Range along a common mega-detachment (Lindeque et al, 2012). Metamorphic conditions in the fold belt reached lower greenschist facies during deformation (Tinker et al, 2008;Hansma et al 2015).…”

Refining the chronostratigraphy of the Karoo Basin, South Africa: magnetostratigraphic constraints support an early Permian age for the Ecca Group

“…The Karoo Basin (~700,000 km 2 ) has been classically interpreted to be a retro-arc foreland basin that developed behind an inferred magmatic arc and fold-thrust belt (Catuneanu et al, 1998;Johnson et al, 1996;Lindeque et al, 2011;Milani and De Wit, 2008). In this interpretation, the basin formed ~1000 km inland from the southern Panthalassan margin of Gondwana and to the north and east of the two branches of the Cape Fold Belt (Fig.…”

South Africa's coalfields — A 2014 perspective

“…Recent interpretations of the basin evolution and tectonic setting of the MKB range from: a retro-arc foreland basin (Catuneanu et al, 1998;Catuneanu et al, 2002;Veevers, 2004;Johnson et al, 2006); a transtensional foreland system created by subsidence and tilting in a strike-slip regime ); a thin-skinned fold belt that developed from collisional tectonics and distant subduction to the south (Lindeque et al, 2011), to a transient hypothetical mantle plume related model (Turner 1999). A full review of these individual models is beyond the scope of this paper and the interested reader is referred to the references above as well as Bordy et al (2005).…”