The peculiar style of Archean continental tectonics: Insights from multi-layered seismic anisotropy beneath southern Africa
Authors: Lebedev Sergei , Ravenna Matteo , Adam Joanne M.-C. 
Published: March 2020

&lt;p&gt;Anisotropy beneath the Kalahari Craton in southern Africa has been a subject of a long-standing controversy: does the shear-wave splitting measured on it&amp;#8212;small in amplitude but following a smoothly varying, regional fast-azimuth pattern&amp;#8212;indicate dominant anisotropy within the lithosphere or, instead, within the underlying asthenosphere? Here we show that the thick lithosphere of cratons can contain multiple anisotropic layers with different rock fabric within each, recording different episodes of deformation at different times in the ancient past. We invert very broadband measurements of surface-wave phase velocities for the layering of anisotropy from the upper crust down to the asthenosphere (up to 350 km depth) beneath different cratonic blocks within the Kalahari Craton. Our Bayesian inversion yields both the most likely values and the uncertainties of S-velocity isotropic averages and the azimuthal and radial anisotropy at different depths.&lt;/p&gt;&lt;p&gt;We detect four main layers of azimuthal anisotropy. In the upper crust, fast-propagation directions across the region are aligned N-S, perpendicular to the direction of extension, as evidenced by the earthquake source mechanisms. The upper-crustal anisotropy can thus be accounted by aligned micro-cracks, opened by the regional tectonic stress. In the asthenosphere (350 km depth), fast-propagation directions are also uniform across the region and aligned NNE-WSW, parallel to the absolute plate motion of Africa. This indicates that athenospheric anisotropy reflects the shear associated with the plate motion. In the lower lithosphere, anisotropic fabric is oriented differently in every cratonic sub-block. This anisotropy is likely to pre-date the assembly of the Kalahari Craton. Finally, in the lower crust and upper mantle down to ~80 km, the fabric is oriented uniformly E-W.&lt;/p&gt;&lt;p&gt;The regionally uniform anisotropic fabric in the upper lithosphere and the contrast of this uniformity with the lateral variability shown by the lower lithosphere suggest a previously unknown style of tectonics, likely to be unique to the Archean-Paleoproterozoic times. Following the formation of the cratons&amp;#8217; thick continental crust, the high radiogenic heat production within it resulted in peculiar geotherms (as modelled previously), with particularly hot lower crust and uppermost mantle. Ductile flow within this mechanically weak layer, driven by regional stresses, could account for the observed anisotropy; the geological record confirms the occurrence of significant, late-Archean, E-W extension. The mechanically stronger deep lithosphere, by contrast, appears to have remained largely undeformed, preserving pre-existing fabric.&amp;#160;&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;&lt;p&gt;References&lt;/p&gt;&lt;p&gt;Ravenna, M., S. Lebedev, J. Fullea, J. M.-C. Adam. Shear-wave velocity structure of southern Africa's lithosphere: Variations in the thickness and composition of cratons and their effect on topography. Geochem. Geophys. Geosyst., 19, 1499&amp;#8211;1518, https://doi.org/10.1029/2017GC007399, 2018.&lt;/p&gt;&lt;p&gt;Ravenna, M., S. Lebedev. Bayesian inversion of surface-wave data for radial and azimuthal shear-wave anisotropy, with applications to central Mongolia and west-central Italy. Geophys. J. Int., 213, 278-300, DOI:10.1093/gji/ggx497, 2018.&lt;/p&gt;&lt;p&gt;Adam, J. M.-C., S. Lebedev. Azimuthal anisotropy beneath southern Africa, from very-broadband, surface-wave dispersion measurements. Geophys. J. Int., 191, 155&amp;#8211;174, 2012.&lt;/p&gt;