Average lithospheric structure of southern Africa

Abstract

We present a new 1-D earth model for the crust and upper mantle beneath the Archaean core of southern Africa. The crustal structure is constrained by published seismic refraction/reflection data and by the modelling of teleseismic receiver function data. The mantle structure is constrained by traveltime measurements for Pn and Sn waves, by waveform inversion of multimode surface waves, and by forward modelling of Pnl waveforms for regional earthquakes. The 42 km thick crust of southern Africa consists of four layers: a 2 km thick 5.05 km s-1 surface layer, a 5 km thick 6.08 km s-1 upper crust, a 20 km thick 6.30 km s-1 mid-crust and a 15 km thick 6.73 km s-1 lower crust. Below the crust there is an 80 km thick upper-mantle lid. VP and Vs are 8.09 and 4.62 km s-1, respectively, at the top of the lid, and the compressional- and shear-velocity gradients through the lid are 0.0008s-1 and 0.0013s-1, respectively. Below the lid there is a substantial shear-wave LVZ, with the shear-wave velocity dropping from 4.72 km s-1 in the lower part of the lid to 4.32 km s-1 at 250km depth. The shear-wave LVZ is required to fit both the VSn measurements and the regional surface waveforms. Forward modelling of Pnl suggests that no upper-mantle P-wave LVZ exists. There is an increase in the P-wave velocity gradient at 125km depth to 0.0015 s-1 and a second increase at 250 km depth to 0.0035 s-1. Velocities and densities in the seismic model of the lid agree closely with velocity and density estimates from the geochemical analysis of garnet periodotite nodules from kimberlites of the Kaapvaal Craton, implying that the estimates from kimberlites are representative of a wider region of the Archaean core of southern Africa-not just of the restricted region beneath which the kimberlites are found. A comparison of the southern African seismic model with seismic models for other shield regions where nodule data are not available shows that the nodule results may also be representative of the upper mantle in those regions.