Small-scale variations in seismic anisotropy near Kimberley, South Africa

Abstract

SUMMARY We have exploited a new data set of broad-band seismic waveforms recorded near the region of Kimberley, South Africa to place new constraints on seismic anisotropy in an area of extensive mantle modification within an Archaean cratonic setting. The Kimberley region is significant in that it represents Archaean cratonic mantle that has been studied extensively via petrologic, geochemical and seismic means, and provides a unique opportunity to meld the results of a broad range of data sets to examine processes of crust and mantle deformation. Shear wave splitting observations using SKS phases recorded by a dense broad-band seismic array in the Kimberley region exhibit consistent fast polarization directions of ∼NE‐SW and splitting times that range from 0.15 s in the SE to nearly 0.75 s in the NW regions of the array. Multichannel cross-correlation of relative arrival times of teleseismic SKS phases across the array reveals clear azimuthal variations and a relative arrival-time range comparable to the shear wave splitting delay-time range. A like analysis of teleseismic PKPdf phases does not exhibit significant differences in relative arrival times across the array. The combined relative arrival time and splitting analyses indicate that a significant change in the strength of anisotropic structure is required across the region. Our results are most consistent with a model in which variations in seismic anisotropy near the Kimberley region are constrained to the lithosphere with an average anisotropic strength of ∼1.8 per cent, extend to depths no greater than 150 km and are primarily controlled by a significant change in the strength of anisotropy within the lateral bounds of the seismic array. This inferred structural boundary is near the location of the ca 2.9 Gyr old Colesberg magnetic lineament, a prominent aeromagnetic feature interpreted as the primary suture zone joining the eastern Meso-Archaean and western Neo-Archaean blocks of the Kaapvaal craton. The proposed boundary is also located within the southern exposure of the ca 2.7 Gyr old Ventersdorp volcanic-sedimentary sequence, a key continental rifting feature in the region. We hypothesize that the observed seismic anisotropy is the result of straininduced lattice-preferred orientation of olivine caused by the amalgamation and deformation of the eastern and western blocks of the Kaapvaal craton. This collisional process mechanically weakened the region to generate the observed mantle fabric, which was exploited by significant rifting events that may have subsequently enhanced the regional fabric. This model is also consistent with SKS splitting results for the whole of southern Africa, which suggest either weak anisotropic or isotropic mantle structure in the eastern Kaapvaal and coherent mantle anisotropy across much of the western Kaapvaal. Our results support the notion that seismic anisotropy beneath Archaean continental regions is not created (or, alternatively, not preserved) during initial continental formation, but is instead generated during subsequent significant mantle-deforming events.