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Notes: The structure of the central Kaapvaal Craton of southern Africa has been investigated by deep seismic sounding, using mine tremors as energy sources. Seismometers were deployed at approximately 10km intervals on two profiles stretching between major mine tremor source regions. Mine tremors are rich in shear energy enabling joint interpretation of P- and S-waves and produce substantial energy at frequencies as low as 1 Hz. Record sections are presented for both P- and S-waves, and the traveltimes and amplitudes interpreted using 2-D ray-tracing techniques. Synthetic seismograms computed for a 1-D velocity model by the reflectivity method compare well with the observed data. A generalized seismic model of the Kaapvaal Craton has the following features: supracrustal strata 0–10 km thick; upper crystalline basement with P-wave velocities of 6.0–6.2 km s−1; the boundary between upper and lower crust at a depth of 14–18 km; a lower crust with a relatively uniform seismic velocity in the range 6.4–6.7 km s−1; and the crust/mantle transition over 1–3 km with the Moho at a depth of about 35 km. The lower crust is found to be seismically attenuating and has a Poisson's ratio of about 0.28. It is also known to be electrically conductive. These observations are in accord with the presence of hydrated mantle rock at the base of the crust.The velocity–depth model of the Kaapvaal Craton is similar to models derived for other Archaean cratons. The Proterozoic provinces adjacent to the Kaapvaal craton are significantly thicker, and have an intermediate- to high-velocity layer developed at the base of the crust. This is interpreted to indicate a change in the process of crustal growth, with basaltic underplating becoming more important since the Archaean. This change is attributed to a change in the composition of the upper mantle. The higher temperatures in the Archaean mantle led to the eruption of komatiitic lavas, resulting in an ultradepleted mantle unable to produce significant volumes of basaltic melt. Proterozoic crust developed above fertile mantle, and subsequent partial melting resulted in basaltic underplating and crustal inflation.

Notes: Abstract Some seismic refraction observations undertaken during the IGY are reported here together with a summary of other refraction studies carried out within the Transkei Basin, the Mozambique Ridge and the South African continental shelf area. A 2.5 km section of Cretaceous and younger rocks is associated with profiles observed on the continental shelf; directly below this group are rocks with velocities in the range 4.0–5.5 km s-1, probably representatives of the Karroo and Cape supergroups. The basement material velocity variations were from 5.3 to 6.5 with an average of 5.9 km s-1, and is correlated with granite or Malmesbury Formation plus granite. This crustal structure is similar to that found on the eastern continental shelf of southern South America. The profiles in the Transkei Basin show a thick layer of sediment with velocity range 1.50 to 3.50 km s-1, underlain by a refracting layer in which the average velocity is 4.5 km s-1. The velocity of 6.6 km s-1 obtained for the oceanic layer is similar to the velocities of the crustal layer measured in the Argentine Basin. The mantle velocity (8.1 km s-1) is consistent with the average mantle velocity for the Indian Ocean but significantly lower than the Pacific Ocean average of 8.20 km s-1. The depth to Moho is about 12.0 km and the crustal section is typical oceanic. A plate tectonic model of the early opening of the South Atlantic is used to describe the evolution of the Transkei Basin. On the Mozambique Ridge the thin sediments (0.7 km) are underlain by rocks with velocities averaging 5.6 km s-1. This is more than 1.0 km s-1 faster than the velocity for layer 2 from the Transkei Basin and the Agulhas Plateau, indicating rocks of a younger age or of a different type. Moreover the crustal section of the Ridge has a thickness in excess of 22 km and is in isostatic equilibrium when compared with the adjacent Transkei Basin and Agulhas Plateau. DSDP site 249, situated on the Ridge, penetrated basalt at a depth of 0.4 km. Whether this is continental or oceanic basalt is not known; when this site 249 basalt was compared to the cored basalts of the adjacent Mozambique Basin, inconclusive results were obtained. The essential constitution of the Mozambique Ridge remains an enigma, but solution of this problem is vital for the proper understanding of the Mesozoic history of this oceanic region.