Seismic evidence for stratification in composition and anisotropic fabric within the thick lithosphere of Kalahari Craton

Abstract

[1] Based on joint consideration of S receiver functions and surface-wave anisotropy we present evidence for the existence of a thick and layered lithosphere beneath the Kalahari Craton. Our results show that frozen-in anisotropy and compositional changes can generate sharp Mid-Lithospheric Discontinuities (MLD) at depths of 85 and 150-200 km, respectively. We found that a 50 km thick anisotropic layer, containing 3% S wave anisotropy and with a fast-velocity axis different from that in the layer beneath, can account for the first MLD at about 85 km depth. Significant correlation between the depths of an apparent boundary separating the depleted and metasomatised lithosphere, as inferred from chemical tomography, and those of our second MLD led us to characterize it as a compositional boundary, most likely due to the modification of the cratonic mantle lithosphere by magma infiltration. The deepening of this boundary from 150 to 200 km is spatially correlated with the surficial expression of the Thabazimbi-Murchison Lineament (TML), implying that the TML isolates the lithosphere of the Limpopo terrane from that of the ancient Kaapvaal terrane. The

The seismic mid-lithosphere discontinuity

“…Additionally, our calculations ( Fig. 3) and those of Sodoudi et al (2013) do not image an MLD at such depths at BOSA. More generally, even assuming that the Till et al (2012) solidus applies to cratonic upper mantle worldwide, there is no obvious reason why melt, migrating by porous flow in peridotite, should accumulate to significant porosities at 80-100 km depth.…”

“…Such magnitudes are comparable to or larger than velocity reductions observed at the continental LAB Rychert et al, 2007;Sodoudi et al, 2013). Thicknesses of the velocity contrast are generally no more than 30 km to 40 km Lekić and Fischer, 2014;Wölbern et al, 2012).…”

“…1) (e.g. Abt et al, 2010;Chen, 2009;Ford et al, 2010;Foster et al, 2014;Heit et al, 2007;Kumar et al, 2013;Savage and Silver, 2008;Sodoudi et al, 2013;Wittlinger and Farra, 2007;Wölbern et al, 2012). In some areas this seismic velocity decrease appears to be continuous between tectonically Rychert et al, 2007), Australia , South America (Heit et al, 2007), Africa (Hansen et al, 2009;Wölbern et al, 2012) and India (Kumar et al, 2013).…”

Origins of cratonic mantle discontinuities: A view from petrology, geochemistry and thermodynamic models

“…The thinner "LAB" corresponds to a depth where the amplitude of non-thermal seismic variations beneath the Kaapvaal craton is sharply reduced, which has been interpreted as the base of the chemical boundary layer (Artemieva, 2009). In contrast, teleseismic tomography and S receiver functions have revealed a lithosphere thickness of 250-300 km (Zhang and Tanimoto, 1993;Sodoudi et al, 2013).…”

Thickness of the lithosphere beneath Turkey and surroundings from S-receiver functions

“…Reviews of the application of the S-receiver function technique in investigations of the lithosphere-asthenosphere boundary are found in Shearer (2009), Fischer et al (2010) and Rychert et al (2010). More recently, further studies of uppermantle discontinuities have been published by Levander and Miller (2012), Kumar et al (2012a, b), Hopper et al (2013), and Sodoudi et al (2013).…”