Deep crustal structure of the sheared South African continental margin: first results of the Agulhas-Karoo Geoscience Transect

Abstract

The southern margin of South Africa developed as a consequence of shear motion between the African and South American plates along the Agulhas-Falkland Transform Fault during the Early Cretaceous break-up of Gondwana. The Agulhas-Karoo Geoscience Transect crosses this continental margin, and includes two combined offshore-onshore seismic reflection/refraction profiles. We present results from the western offshore profile. Using ocean-bottom seismometers and a dense airgun shot pattern, a detailed image of the velocity -depth structure of the margin from the Agulhas Passage to the Agulhas Bank was derived. Modelling reveals crustal thicknesses of between 7 km and 30 km along the profile. The upper crust has P-wave velocities of between 5.6 and 6.6 km/s and the lower crust has velocities that lie between 6.4 and 7.1 km/s. Uppermost mantle velocities range from 7.8 to 8.0 km/s. The 52 km wide continent-ocean-transition zone, where the Moho rises steeply, occurs at the Agulhas-Falkland Fracture Zone.Beneath the Southern Outeniqua Basin and the Diaz Marginal Ridge, a zone of relatively low velocities (~5 km/s) with a thickness of up to 3 km, can be discerned in the upper crust. We interpret this zone as an old basin filled with pre-break-up metasediments, which may be related to the Cape Fold Belt. We suggest that the origin of the Diaz Marginal Ridge is bound up with the tectonic history of this basin as it exhibits a similar velocity structure. Almost no stratified sediment cover exists in the Agulhas Passage because of strong erosion due to ocean currents. The velocity structure and seismic reflection results indicate the presence of alternating layers of volcanic flows and sediments, with a mean velocity of about 4 km/s. We suggest these volcanic flows were an accompaniment of possible re-activation events of parts of the Agulhas-Falkland Fracture Zone. Tectonic motion seems to be sub vertical instead of strike-slip along the re-activated part of the fracture zone. Therefore, a relation to the uplift of the South African crust may be possible.

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

“…Our interpreted extension of the NNMB to the continental shelf and up to the Agulhas FalklandFracture Zone (AFZZ) as illustrated in Figure 11, is further supported by the seismic wide-angle refraction work and subsequent tomography crustal models of Parsiegla et al, (2007; 2008). Their data indicate that the mid-and lower crust beneath the Cape-/Karoo Basin have similar seismic properties up to the continental boundary, thereby implying the nature of the crystalline basement beneath the CFB, and beyond, is likely Mesoproterozoic NNMB.…”

“…Eaton, 2005). Alternatively, the strong reflective layer could be interpreted as a decoupling-or transition zone between the lower crust and uppermost mantle, with a vertical extent of a few kilometres (Oueity and Clowes, 2010 Figure 2), as well as results from the amphibian wide-angle refraction experiment along the entire western line of the AKGTransect (Figure 1; Parsiegla et al, 2007;). …”

“…upper-, middle-, lower crust and Moho depth) against crustal divisions recorded at the nearest teleseismic receiver function station (SA05 in Harvey et al, 2001, ~25 km east of IyA-200501, Figure 2), and in the large scale regional Agulhas-Karoo onshore-offshore refraction crustal model (Parsiegla et al, 2007;Stankiewicz et al, 2007;2008).…”

“…Under the auspices of the Inkaba yeAfrica research initiative (de Wit and Horsfield, 2006; www.inkaba.org) the Agulhas-Karoo Geophysical Transect (AKGT) was designed to investigate these, and other features through deep crustal geophysical data, with the aim of resolving the tectonic evolution of the southern continental margin of Africa further (e.g. Lindeque et al, 2006;Parsiegla et al, 2007;Stankiewicz et al, 2007;2008;Weckmann et al, 2007a;2007b).…”

Comparison of electrical conductivity structures and 2D magnetic modelling along two profiles crossing the Beattie Magnetic Anomaly, South Africa

“…In this study we report on a comparison of both profiles (MT1 and MT4) and explore if a common source of the BMA and SCCB is supported by the electrical conductivity results. In addition to the MT data, a 100 km long seismic near-vertical reflection line was carried out along the western profile (MT1), and wide angle reflection/refraction measurements as part of an on-shore/off-shore experiment (Parsiegla et al, 2007) were conducted along both transects.…”