Joint inversion of receiver functions and surface wave dispersion to resolve the depth dependent shear wave velocity structure of the Botswana crust

Abstract

I jointly inverted receiver functions with surface wave dispersion data to resolve the one-dimensional shear wave velocity structure below the seismic stations of the Botswana Seismic Network. This was done to improve our understanding of the Precambrian crust of Botswana. A joint inversion allows for better constraints on the velocity structure than either data set alone could and was done using a trans-dimensional Bayesian approach. The one- dimensional models showed four main features. Many models showed evidence for a low velocity sedimentary layer overlying the crystalline basement. I also often found a well resolved discontinuity at around 11 kilometers depth. I have interpreted this discontinuity as the Conrad discontinuity marking the transition from a felsic upper crust to an intermediate middle crust. At a depth of approximately 35 kilometers most models show a discontinuity where the shear wave velocity jumps to a higher value of between the 4.2 km/s and 4.5 km/s. This high velocity layer is interpreted as the consequence of mafic underplating during one of the magmatic events that affected southern Africa. I have interpreted the Moho where the shear wave velocity exceeds 4.2 km/s. In Botswana, the crustal thickness varies quite strongly between 33 and 52 kilometers. The thin crust in north- northeast Botswana is probably related to recent rifting in the Okavango Rift Zone and the thick crust near the boundary between the Zimbabwe craton and the Magondi belt is probably related to intrusion of the Okavango dyke swarm.