Small Scale Variations in Seismic Anisotropy Near Kimberley, South Africa

We have exploited a new data set of broadband seismic waveforms near the region of Kimberley, South Africa, to place new constraints on seismic anisotropy in an area of extensive kimberlitic volcanism. The Kimberley region is significant in that it represents Archean cratonic mantle that has been extensively studied via petrologic, geochemical, and seismic means, and provides a unique opportunity to meld the results of a wide range of datasets. One of the key questions is the extent to which the mantle and crustal sections of the cratonic lithosphere correspond in their record of continental lithospheric evolution. Data for this study was recorded at the Kimberley Telemetered Array (KTA), one component of the larger Southern Africa Seismic Experiment. The KTA was installed near the large population of kimberlite pipes near Kimberley, South Africa, and is comprised of 31 stations that were deployed from January to May of 1999. Station spacing for the array ranges from 2 to 5 km with a total aperture of approximately 60 km by 40 km. For most stations, we determined well constrained shear wave splitting parameters from individual events. One event yielded a particularly striking example of the distinct variations in splitting observed across the array. This event exhibited clear, coherent radial energy for all stations across the array, while energy on the transverse component grades smoothly from strong to none over the full array length of only 60 km. Given the limited data set for the KTA, we applied the Wolfe and Silver [1998] stacking technique to place constraints on variations across the network. Shear wave splitting observations for the Kimberley array exhibit an average fast direction of roughly NE-SW, and splitting times that range from 0.15 s in the SE portion of the array to nearly 0.75 s in the NW portion of the array. The differences in seismic anisotropy beneath Kimberley are the first documented results in the world of such extreme small scale variations in shear wave splitting. Our results are most consistent with a model in which the anisotropy is limited to depths well within the mantle lithosphere interior, and is controlled by ancient (i.e., Archean) deformation events also evident at the surface. This model is also supported by a broad range of geochemical and petrologic evidence, which suggests that a significant change in lithospheric properties exists in the region.