Fine structure of the lowermost crust beneath the Kaapvaal craton and its implications for crustal formation and evolution

High quality data from a dense seismic array covering an area of approximately 60×40 km2 are used to obtain tight quantitative estimates of the fine-scale velocity and density structure of the lowermost crust and the crust–mantle boundary (Moho) beneath the Kaapvaal craton in the vicinity of Kimberley, South Africa. Results based on a modified receiver function waveform analysis of Moho conversions and crustal reverberation phases show that the crust beneath the array is thin (35.4 km) with an average Poisson’s ratio of 0.254. The minimum S-wave velocity contrast across the Moho is 17.3% while the contrast in density is 15.4%. The density contrast across the Moho is particularly diagnostic. For an assumed uppermost mantle density beneath Kimberley of 3.3 gm/cc as determined from mantle xenoliths, the density of the lowermost crust is 2.86 gm/cc, indicating rocks of felsic to intermediate composition. Analysis of waveform broadening of the crustal reverberation phases relative to that of the direct P-wave shows the thickness of the Moho transition zone to be less than 0.5 km and the maximum variation in crustal thickness over the region of the array to be less than 1 km. The flat and almost perfectly sharp Moho, together with the absence of a mafic lower crust, suggests large-scale crustal reworking in the period between crustal formation and the time of cratonic stabilization.