Crustal structure of the southern margin of the African continent: Results from geophysical experiments

Abstract

[1] A number of geophysical onshore and offshore experiments were carried out along a profile across the southern margin of the African Plate in the framework of the Inkaba yeAfrica project. Refraction seismic experiments show that Moho depth decreases rapidly from over 40 km inland to around 30 km at the present coast before gently thinning out toward the Agulhas-Falkland Fracture Zone, which marks the transition zone between the continental and oceanic crust. In the region of the abruptly decreasing Moho depth, in the vicinity of the boundary between the Namaqua-Natal Mobile Belt and the Cape Fold Belt, lower crustal P-wave velocities up to 7.4 km/s are observed. This is interpreted as metabasic lithologies of Precambrian age in the Namaqua-Natal Mobile Belt, or mafic intrusions added to the base of the crust by younger magmatism. The velocity model for the upper crust has excellent resolution and is consistent with the known geological record. A joint interpretation of the velocity model with an electrical conductivity model, obtained from magnetotelluric studies, makes it possible to correlate a high-velocity anomaly north of the center of the Beattie magnetic anomaly with a highly resistive body.

Asymmetry of high-velocity lower crust on the South Atlantic rifted margins and implications for the interplay of magmatism and tectonics in continental breakup

“…1, line 8) across the sheared South African margin east of the Cape Peninsula in South Africa. This seismic profile across the FAFZ shows no evidence of magmatic features at the continent-ocean boundary, but there are small HVLC bodies well inland, which Stankiewicz et al (2008) attributed to igneous crust formed during the 180 Ma Karoo event, but may also be interpreted as high-density metamorphic rocks (garnet amphibolite, mafic granulite) in the continental basement (as in Norway; see Gernigon et al, 2006).…”

“…With respect to these basement features, the onshore parts of seismic lines 4 and 5 lie within the Damara Belt, whereas lines 6 and 7 cross the Gariep Belt at the coast and extend into the Namaqua-Natal Mobile Belt to the east. The seismic traverse across the southern margin of Africa (Stankiewicz et al, 2008) crosses the Cape Fold Belt and extends into the Namaqua-Natal Mobile Belt to the north (line 8 in Fig. 1).…”

“…1, line 7) and the seismic velocity profile derived by Hirsch et al (2009) shows a well-developed body of HVLC below SDRs. Finally, it is worth mentioning for reference that Stankiewicz et al (2008) published a seismic velocity profile ( Fig. 1, line 8) across the sheared South African margin east of the Cape Peninsula in South Africa.…”

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

“…Namaqua-Natal metamorphic terranes (NNMB): R = Richtersveld; G = Gordonia; B= Bushmanland of the Namaqua province; N = Natal province. Black = Green and Durrheim (1990), Mahanyele et al (2004), Hirsch et al (2008); Lines on the South coast: light grey line = onland seismic refraction line of Stankiewicz et al (2007;2008) and the longer onshore-offshore seismic refraction line (Stankiewicz et al 2008;Parsiegla et al 2007;; black line perpendicular to CFB front = IyA-The regions with complex reflectivity patterns cross-cut by the high conductivity anomalies in the mid-crust (Figures 7 and 9) could also be interpreted as ore bodies, since ore minerals such as Pyrrhotite and other iron sulphides have a high conductivity. Impedance spectroscopy on samples from boreholes SA-1/66 and KW-1/67, proved high conductivity in the Wh/PA Formations, associated with pyrite and chalcopyrite (Branch et al, 2007).…”

Reflection/Refraction Seismology

“…This routine was originally developed for mid-and deep crustal seismic refraction tomography (e.g., Zelt et al, 2004;Stankiewicz et al, 2008) but has been utilized for shallow subsurface imaging by appropriately adjusting model parameters (Deen and and Gohl, 2002). As required for a linearized inversion, an initial model has to be implemented first.…”