Southeastern Atlantic and southwestern Indian Ocean: reconstruction of sedimentary and tectonic development since the Cretaceous - AISTEK-I
Agulhas-Karoo Geoscience Transect
Figure 1: The Agulhas-Karoo Geoscience Transect consists of two seismic refraction/reflection onshore-offshore profiles crossing the sheared southern African continental margin. APA=Agulhas Passage, AFFZ=Agulhas-Falkland Fracture Zone, CFB=Cape Fold Belt, KP=Karoo province
Objectives:
Within the German-South African co-operation project Inkaba yeAfrica which aims to understand Southern Africa from core to space, the Agulhas-Karoo Geoscience Transect represents an important component dealing with questions on crustal scale. Some of the issues being addressed are:
- What is the crustal nature of the Agulhas Plateau? When and how was it formed?
- What is the structure of the crust across the sheared margin? Where does the transition from continental to oceanic crust occur?
- Which geodynamic processes were involved in the formation of this margin?
- What is the present state of the Agulhas-Falkland-Fracture-Zone? Did it experience any phases of reactivation?
How did the Outeniqua Basin form? What kind of basement floors the basin?
Data acquisition and processing
Figure 2: Photo of the RV Sonne in Cape Town
Seismic reflection/refraction data were collected along two profiles (Figure 1) during RV Sonne cruise SO-182 (04.04.2005-18.05.2005, Figure 2). Profile AWI-20050100 has a length of 247 nm (20 ocean- bottom seismometers) and profile AWI-20050200 is 358 nm long (27 ocean-bottom seismometers). Both profiles have landward extensions with 48 Refteks each. These two combined land-sea seismic refraction/reflection profiles are part of the Agulhas-Karoo Geoscience Transect.
Figure 3: top) example of seismic refraction data collected at OBS station 20 with identified phases; middle) comparison between modelled (lines) and measured arrivals (bars); bottom) rays through the velocity-depth model (Figure 3). From Parsiegla et al. (2007).
Our seismic refraction data were processed (time correction, offset correction etc.) and arrivals were picked. Figure 3 below shows an example of the data measured at ocean-bottom seismometer (OBS) station 20 on the western offshore profile AWI-20050100. Velocity-depth models of the profiles AWI-20050100 (Parsiegla et al., 2007; Figure 4), AWI-20050200 (Parsiegla et al., 2008; Figure 5) and both combined land-sea profiles (Parsiegla et al., in prep., Stankiewicz, submitted) were created using a raytracing technique.
Results
The Agulhas-Karoo Geoscience Transect led to an unprecedented deep crustal view across the southern African continental margin and enhanced process understanding of the dynamic processes involved in the formation of the margin and the Agulhas Plateau Large Igneous Province. Figures 4 and 5 showing the velocity-depth structure across the southern African continental margin represent just a little part of the outcome of this successful project. The results are published in Parsiegla (2007; 2008; in prep.) and Stankiewicz (submitted). See these for full detail.
Figure 5: Velocity-depth structure along profile AWI-20050200 by Parsiegla et al. (2008). As on the sub-parallel profile AWI-20050100 the continent-ocean transition is located at the Agulhas-Falkland Fracture Zone. The Agulhas Plateau consists of overthickened oceanic crust with an up to 10 km thick lower crustal body. Parsiegla et al. (2008) discuss its formation as a Large Igneous Province.
Figure 4: Velocity-depth model of profile AWI-20050100 (Parsiegla et al., 2007). It shows the velocity structure across the margin with the continent-ocean transition zone centred at the Agulhas-Falkland Fracture Zone. AFFZ=Agulhas-Falkland Fracture Zone, DMR=Diaz Marginal Ridge
Publications
Parsiegla, N., Gohl, K., Uenzelmann-Neben, G. (2007), Deep crustal structure of the sheared South African continental margin: first results of the Agulhas-Karoo Geoscience Transect. S. Afr. J. Geol., 110, 393-406.
Parsiegla, N., Gohl, K., Uenzelmann-Neben, G. (2008), The Agulhas Plateau: Structure and Evolution of a Large Igneous Province, Geophys. J. Int., 174, 336-350, doi: 310.1111.j.1365-1246X.2008.03808.x.
Parsiegla, N., Stankiewicz, J., Gohl, K., Ryberg, T., Uenzelmann-Neben, G., The southern African continental margin – dynamic processes of a transform margin, in prep.
J. Stankiewicz, N. Parsiegla, T. Ryberg, K. Gohl, U. Weckmann, R. Trumbull & M. Weber, Crustal Structure of the Southern Margin of the African Continent: Results from Geophysical Experiments, submitted
Acknowledgements:
This project is funded by the Bundesministerium fuer Bildung, Forschung und Technologie under contract No 03G0182A 'AISTEK-I'.