A seismic refraction investigation of the Archaean Kaapvaal Craton, South Africa, using mine tremors as the energy source

Abstract

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. Copyright © 1992, Wiley Blackwell. All rights reserved