The Influence of Mantle Structure on Dynamic Topography in Southern Africa

Abstract

Due to relatively high terrain and negligible active tectonics, the southern Africa region boasts over 30 independent estimates of dynamic topography. These published estimates display a wide variance due to both the variety of methods used in computation and a lack of constraints on the regional mantle structure. Here we show that a focus on regional mantle structure is important to generate models of lithospheric and mantle dynamics. Global average mantle properties are not representative of a particular region, and it is necessary to generate viscosity profiles specific to a region of interest. We develop a Bayesian inversion using dynamic geoid kernels, existing seismic tomography models, and Slepian functions to invert for a localized radial viscosity profile that best explains the geoid in southern Africa. With an understanding of viscosity uncertainty, we constrain dynamic topography in southern Africa to lie between 1,000 and 2,000 m. Additionally, we model vertical displacements from 112 Global Navigation Satellite System stations across our region to examine the long-term, long wavelength pattern of present-day vertical motion, suggesting that a mean of 1.5 mm/yr (1$\sigma$: 0.8–2.0 mm/yr) of vertical motion may be related to ongoing dynamic topography. Our study demonstrates the utility of dynamic geoid kernels in local nonlinear inversions of non-unique geophysical data. Furthermore, we present evidence that the mantle beneath southern Africa is generating significant dynamic support for and vertical displacement of the lithosphere in this region.