Small-Scale Variations in Seismic Anisotropy Near Kimberley, South Africa - fig. 1 Figure 1. Results from shear wave splitting and relative delay time analysis of SKS and PKP phases recorded at the Kimberley telemetered array. Proposed boundary of strong/weak anisotropic domain is delineated by dashed line. a) Stacked shear wave splitting results. Azimuth of black bars denote fast polarization direction; thin black bars denote 95% confidence limits for fast polarization directions; size of circles is scaled to splitting time. For figures 1b-1d, crosses (faster than average arrivals) and circles (slower than average arrivals) are scaled to magnitude of relative arrival time. Triangles denote relative arrival times of ± 0.025 s.
<br /><br />
Using broadband seismic data from the IRIS PASSCAL broadband telemetered array installed near Kimberley, South Africa, we place new constraints on seismic anisotropy in an area of extensive mantle modification within an Archean cratonic setting (Fouch et al., 2004). The array was installed as part of the multidisciplinary Kaapvaal Project, which extensively studied the region via petrologic, geochemical, and seismic means, and provided a unique opportunity to meld the results of a broad range of datasets to examine processes of cratonic evolution.
<br /><br />
We analyze the shear wave splitting of SKS phases recorded near Kimberley. These phases exhibited consistent fast polarization directions of ~NE SW (Figure 1a) and splitting times that ranged from 0.15 s in the SE to nearly 0.75 s in the NW regions of the array (Figures 1a, 2). Multi-channel cross correlation of relative arrival times of teleseismic SKS phases across the array revealed clear azimuthal variations and a relative arrival time range comparable to the shear wave splitting delay time range (Figures 1b, 1c). A like analysis of teleseismic PKPdf phases did not exhibit significant differences in relative arrival times across the array (Figure 1d). 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%, 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.
<br /><br />
We hypothesize that the observed seismic anisotropy is due to strain induced 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. Our results support the notion that seismic anisotropy beneath Archean continental regions is not created (or alternatively not preserved) during initial continental formation, but instead is generated during subsequent significant mantle deforming events.
<br /><br />
Fouch, M.J., P.G. Silver, D.R. Bell, and J.N. Lee, Small-scale variations in seismic anisotropy near Kimberley, South Africa, Geophys. J. Int., 157, 764-774, doi: 10.1111/j.1365-246X.2004.02234.x, 2004.
<br /><br />
Silver, P.G., S.S. Gao, K.H. Liu, and the Kaapvaal Seismic Group, Mantle deformation beneath southern Africa. Geophys. Res. Lett., 28, 2493 2496, 2001.
Small-Scale Variations in Seismic Anisotropy Near Kimberley, South Africa - fig. 1 Figure 1. Results from shear wave splitting and relative delay time analysis of SKS and PKP phases recorded at the Kimberley telemetered array. Proposed boundary of strong/weak anisotropic domain is delineated by dashed line. a) Stacked shear wave splitting results. Azimuth of black bars denote fast polarization direction; thin black bars denote 95% confidence limits for fast polarization directions; size of circles is scaled to splitting time. For figures 1b-1d, crosses (faster than average arrivals) and circles (slower than average arrivals) are scaled to magnitude of relative arrival time. Triangles denote relative arrival times of ± 0.025 s.
<br /><br />
Using broadband seismic data from the IRIS PASSCAL broadband telemetered array installed near Kimberley, South Africa, we place new constraints on seismic anisotropy in an area of extensive mantle modification within an Archean cratonic setting (Fouch et al., 2004). The array was installed as part of the multidisciplinary Kaapvaal Project, which extensively studied the region via petrologic, geochemical, and seismic means, and provided a unique opportunity to meld the results of a broad range of datasets to examine processes of cratonic evolution.
<br /><br />
We analyze the shear wave splitting of SKS phases recorded near Kimberley. These phases exhibited consistent fast polarization directions of ~NE SW (Figure 1a) and splitting times that ranged from 0.15 s in the SE to nearly 0.75 s in the NW regions of the array (Figures 1a, 2). Multi-channel cross correlation of relative arrival times of teleseismic SKS phases across the array revealed clear azimuthal variations and a relative arrival time range comparable to the shear wave splitting delay time range (Figures 1b, 1c). A like analysis of teleseismic PKPdf phases did not exhibit significant differences in relative arrival times across the array (Figure 1d). 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%, 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.
<br /><br />
We hypothesize that the observed seismic anisotropy is due to strain induced 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. Our results support the notion that seismic anisotropy beneath Archean continental regions is not created (or alternatively not preserved) during initial continental formation, but instead is generated during subsequent significant mantle deforming events.
<br /><br />
Fouch, M.J., P.G. Silver, D.R. Bell, and J.N. Lee, Small-scale variations in seismic anisotropy near Kimberley, South Africa, Geophys. J. Int., 157, 764-774, doi: 10.1111/j.1365-246X.2004.02234.x, 2004.
<br /><br />
Silver, P.G., S.S. Gao, K.H. Liu, and the Kaapvaal Seismic Group, Mantle deformation beneath southern Africa. Geophys. Res. Lett., 28, 2493 2496, 2001.
