Upper Mantle Seismic Structure Beneath Southern Africa: Constraints on the Buoyancy Supporting the African Superswell

Abstract

We present new one-dimensional SH-wave velocity models of the upper mantle beneath the Kalahari craton in southern Africa obtained from waveform inversion of regional seismograms from an Mw = 5.9 earthquake located near Lake Tanganyika recorded on broadband seismic stations deployed during the 1997-1999 Southern African Seismic Experiment. The velocity in the lithosphere beneath the Kalahari craton is similar to that of other shields, and there is little evidence for a significant low velocity zone beneath the lithosphere. The lower part of the lithosphere, from 110 to 220 km depth, is slightly slower than beneath other shields, possibly due to higher temperatures or a decrease in Mg number (Mg#). If the slower velocities are caused by a thermal anomaly, then slightly less than half of the unusually high elevation of the Kalahari craton can be explained by shallow buoyancy from a hot lithosphere. However, a decrease in the Mg# of the lower lithosphere would increase the density and counteract the buoyancy effect of the higher temperatures. We obtain a thickness of 250 ± 30 km for the mantle transition zone, which is similar to the global average, but the velocity gradient between the 410 and 660 km discontinuities is less steep than in global models, such as PREM and IASP91. We also obtain velocity jumps of between 0.16 ± 0.1 and 0.21 ± 0.1 km/s across the 410 km discontinuity. Our results suggest that there may be a thermal or chemical anomaly in the mantle transition zone, or alternatively that the shear wave velocity structure of the transition zone in global reference models needs to be refined. Overall, our seismic models provide little support for an upper mantle source of buoyancy for the unusually high elevation of the Kalahari craton, and hence the southern African portion of the African Superswell.

Upper Mantle P and S Wave Velocity Structure of the Kalahari Craton and Surrounding Proterozoic Terranes, Southern Africa

“…A large number of seismic studies have investigated crust and mantle structure in southern Africa over the past few decades using body wave tomography (e.g., Fouch et al, 2004;Youssof et al, 2015;Yu et al, 2016), surface wave tomography (e.g., Chevrot & Zhao, 2007;Priestley et al, 2008;Fishwick, 2010;Adams & Nyblade, 2011), receiver functions (e.g., Fadel et al, 2018;Hansen et al, 2009;Kgaswane et al, 2009;Nguuri et al, 2001), and waveform modeling (e.g., Brandt et al, 2011;Moorkamp et al, 2019;Zhao et al, 1999). Four relevant body wave tomography studies are briefly summarized.…”

Contrasting Mesozoic evolution across the boundary between on and off craton regions of the South African plateau inferred from apatite fission track and (U‐Th‐Sm)/He thermochronology

“…The presence of a large mantle plume, or plumes, beneath the African continent [e.g., Nyblade and Robinson, 1994;Lithgow-Bertelloni and Silver, 1998;Brandt et al, 2012;Fishwick and Bastow, 2011] has often been cited as a mechanism to drive regional uplift and support the elevated African topography over long timescales [Gurnis et al, 2000;Roberts and White, 2010;Braun et al, 2014;Moucha and Forte, 2011]. However, the predicted spatial pattern, timing, and amount of "dynamic" uplift and subsidence for Africa are still unresolved [Moucha and Forte, 2011;Braun et al, 2014;Flament et al, 2014], and isolating the mantle "dynamic" uplift contribution from that of isostatically compensated topography remains challenging [e.g., Molnar et al, 2015;Colli et al, 2016;Artemieva and Vinnik, 2016].…”

Geochemical and geophysical constrains on the dynamic topography of the Southern African Plateau

“…[] (260–280 km) and on 1‐D SH velocity modeling of SASE data by Brandt et al . [] (220 km). However, our results are considerably deeper than the 180–200 km estimate from the surface wave tomographic model of Fishwick [].…”

“…Subsequent seismic tomography models showed an extensive low velocity anomaly, interpreted as a thermal anomaly, rising from the core‐mantle boundary through the whole mantle beneath the SE Atlantic [ Romanowicz and Gung , ]. Recent work using the Southern African Seismic Array (SASE) data is interpreted as not providing any support for an explanation for the high topography of the Kalahari Craton (Kaapvaal and Zimbabwe Cratons taken together) restricted to the upper mantle only [ Brandt et al ., ], although quantitative constraints on the amount of dynamic topography are not provided.…”

“…However, in contrast to the approach of Brandt et al . [] and of others that are driven primarily by consideration of only seismological data, we perform a joint thermochemical inversion of the geoid anomaly, Rayleigh wave dispersion data, magnetotelluric (MT) data and surface heat flow data, using a self‐adaptive Markov Chain Monte Carlo algorithm, within a thermodynamically and internally consistent framework [ Afonso et al ., , , ]. Our inversions take advantage of the wealth and quality of information that exists on the crustal structure, the chemistry and thermal structure of the lithospheric mantle, and the likely lithospheric thickness in southern Africa.…”