Variations in crustal thickness and uppermost mantle structure across the Kaapvaal Craton from Pn and Sn arrivals and receiver functions : research article

lIgPnl/Ig and lIgSnl/Ig wavespeeds in the uppermost mantle of the Kaapvaal Craton are high and uniform with average values of 8.35 and 4.81 km s -1 , respectively, indicating the presence of highly depleted magnesium-rich peridotite. The lIgPnl/Ig times also suggest azimuthal anisotropy with maximum wavespeeds of 8.40 km slsupg-1l/supg at an azimuth of about 15 o . A simple method of estimating crustal thicknesses below the stations of the Kaapvaal broad-band network using lIgPnl/Ig times provided results that have been compared with those from receiver functions. Receiver functions indicate that the mean thickness of the crust below the Witwatersrand Basin is about 42 km. The average crustal thicknesses determined from lIgPnl/Ig times for 19 centrally-located stations on each of the northern and southern regions of the craton away from the Witwatersrand Basin were 50.52 + 0.88 km and 38.07 + 0.85 km, respectively. In contrast, the corresponding average thicknesses determined from receiver functions were 43.58 + 0.57km and 37.58 + 0.70 km, respectively. A proposed explanation of the systematically lower values of crustal thicknesses for receiver functions in the northern part of the Kaapvaal Craton that was affected by the Bushveld magmatism at 2.05 Gyr, is that variations in composition and metamorphic grade in an underplated, mafic lower crust can result in a petrological crust-mantle boundary that yields weak P-to-SV conversions.