Imaging the Crustal Structures of Southernmost Africa Using Wide Angle Seismics

One of the projects within the framework of Inkaba ye Africa, a research initiative between German and South African geoscience research communities, is the Agulhas-Karoo transect. This 800 km north-south off-onshore transect runs from the offshore Agulhas Plateau onto the South African coast, through the Cape Fold Belt, Beattie Magnetic Anomaly, the Karoo Basin, the Great Escarpment and into the Kaapvaal Craton. Among the number of geophysical measurements taken along the transect are two wide-angle seismic lines collected in April and May 2005. The lines run roughly parallel to each other approximately 200 km apart, starting at Mossel Bay and St. Francis, respectively, and running about 200 km north to Fraserburg and Graaf Reinet. At each line approximately 50 receivers were used to record data from 13 shots. First (P-wave) arrivals were manually picked on the available traces, and tomographic inversion was done using these travel times. The ray coverage allowed to create the P-wave velocity model to depths of up to 30km. The overall quality of data is very good, and most of the shallow features agree with previous studies and expectations, such as low velocities beneath the Mesozoic/Cenozoic inliers in the Cape Fold Belt. We also observe a high velocity anomaly that could relate to the Beattie Magnetic Anomaly.