Evidence of Geologically Controlled Seismic Anisotropy beneath Southern Africa
Based on shear-wave splitting measurements at 8 sites within the southern Kaapvaal craton, Vinnik et al. (1996, 1998) suggested that the observed NE-SW fast polarization directions of 7 of the 8 were caused by the absolute plate motion (APM) of the African plate. However, new measurements at 70 sites using data from the 1997-99 Southern African Seismic Experiment (SASE), completely spanning the craton along a NE-SW transect, strongly support the notation that in old cratonic areas, anisotropy is primarily caused by structural fabric frozen in the mantle by Archean deformational events. This is best illustrated in the northeastern Kaapvaal craton and Limpopo belt, where there is the best geologic control. The available splitting directions for this region are essentially parallel to the large-scale ENE trending structures found there and significantly different from the APM direction, as well as the NE-SW splitting directions found in the southern craton. Moving northward across the boundary from the Limpopo belt to Zimbabwe craton, the fast polarization directions abruptly change by about 45⁰ from ENE to NNE, again suggesting geologic control. The one region where ancient deformation provides an unsatisfactory explanation is in the younger Proterozoic and Phanerozoic belts to the southwest of the craton. The fast polarizations are roughly oriented NE-SW, while the dominant geologic fabric is at a high angle to this direction. In this area there may be a contribution from mantle flow.
S. S. Gao et al., "Evidence of Geologically Controlled Seismic Anisotropy beneath Southern Africa," Eos, Transactions American Geophysical Union, American Geophysical Union (AGU), Dec 1999.
AGU Fall Meeting (1999: Dec. 1, San Francisco, CA)
Geosciences and Geological and Petroleum Engineering
Article - Conference proceedings
© 1999 American Geophysical Union (AGU), All rights reserved.
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