Southern African continental margin: Dynamic processes of a transform margin

Abstract

[1] Dynamic processes at sheared margins associated with the formation of sedimentary basins and marginal ridges are poorly understood. The southern African margin provides an excellent opportunity to investigate the deep crustal structure of a transform margin and to characterize processes acting at these margins by studying the Agulhas-Falkland Fracture Zone, the Outeniqua Basin, and the Diaz Marginal Ridge. To do this, we present the results of the combined seismic land-sea experiments of the AgulhasKaroo Geoscience Transect. Detailed velocity-depth models show crustal thicknesses varying from 42 km beneath the Cape Fold Belt to 28 km beneath the shelf. The Agulhas-Falkland Fracture Zone is embedded in a 50 km wide transitional zone between continental and oceanic crust. The oceanic crust farther south exhibits relatively low average crustal velocities (6.0 km/s), which can possibly be attributed to transform-ridge intersection processes and the thermal effects of the adjacent continental crust during its formation. Crustal stretching factors derived from the velocity-depth models imply that extension in the Outeniqua Basin acted on regional as well as more local scales. We highlight evidence for two episodes of crustal stretching. The first, with a stretching factor b of 1.6, is interpreted to have influenced the entire Outeniqua Basin. The stresses possibly originated from the beginning breakup between Africa and Antarctica (169-155 Ma). The second episode can be associated with a transtensional component of the shear motion along the Agulhas-Falkland Transform from 136 Ma. This episode caused additional crustal stretching with b = 1.3 and is established to only have affected the southern parts of the basin. Crustal velocities directly beneath the Outeniqua Basin are consistent with the interpretation of Cape Supergroup rocks underlying most parts of the basin and the Diaz Marginal Ridge. We propose that the formation of this ridge can be either attributed to a transpressional episode along the Agulhas-Falkland Transform or, more likely, to thermal uplift accompanying the passage of a spreading ridge to the south.

Transform continental margins – Part 2: A worldwide review

“…Ben-Avraham et al, 1997;Parsiegla et al, 2009), Côte d'Ivoire-Ghana (e.g. Basile et al, 1998) and Exmouth (e.g.…”

Growth and Dispersal of a Southeast African Large Igneous Province

“…However, the concept of a potential common evolutionary history of both features has not been taken into account. Gohl and Uenzelmann-Neben (2001) and Parsiegla et al (2008;2009) found geophysical evidence for the AP to be regarded an oceanic Large Igneous Province (LIP), consisting of over-thickened oceanic crust formed at 110 to 95 Ma. Small continental fragments, which may explain the few dredged rock samples of felsic composition (Tucholke et al, 1981, Ben-Avraham et al, 1995, cannot be entirely excluded.…”

“…The data analyses provide new evidence for the structure and composition of the MOZR and are compared with geophysical models of the AP. (Smith and Sandwell, 1997) Gohl and Uenzelmann-Neben (2001) and Parsiegla et al (2008) (Hartnady and le Roex, 1985;Martin, 1987 Overthickened oceanic crust observed from deep crustal data For OBS system description, airgun sources as well as seismic refraction and MCS acquisition and processing parameters of the data collected during the SO-182 cruise, we refer to Schlüter and Uenzelmann-Neben (2008) and Parsiegla et al (2008;2009). Of the ten OBS systems deployed along the 240 km long profile AWI-20050300, eight systems recorded good-quality and useful data.…”

“…We collected deep crustal seismic data, using oceanbottom seismometers (OBS), and multichannel seismic (MCS) reflection data from the southern MOZR as part of a larger research program during RV Sonne cruise SO-182 (Schlüter and Uenzelmann-Neben, 2008;Parsiegla et al, 2008;2009) (profiles and OBS stations in Figure 1). The data analyses provide new evidence for the structure and composition of the MOZR and are compared with geophysical models of the AP.…”

Crust‐scale 3D model of the Western Bredasdorp Basin (Southern South Africa): data‐based insights from combined isostatic and 3D gravity modelling

“…The density for the seawater is set to 1030 kg m À3 and the assigned mean bulk densities (as derived from the well data constrained porosity-depth compaction curves; Figs 8-10) for the modelled sedimentary units are given in Table 1. 6.0 to 7.0 km s À1 ) of Parsiegla et al (2009) based on their onshore-offshore Agulhas-Karoo deep seismic transects through the Pletmos and Gamtoos sub-basins both of which are adjacent to the study area in the far eastern end of the larger Outeniqua Basin (Fig. With this approach, we accept slight deviations from these mean values in the domains of shallow basement highs (in the NW, SW and NE; Figs 3, 12 and 13) with very thin overburden thickness.…”