The large-scale deformational structure of Archean mantle beneath the Kaapvaal Craton from seismic anisotropy
Abstract
The Kaapvaal/Zimbabwe craton of southern Africa provides one of the best locales for studying the structural properties of the Archean mantle, given the prevalence of Kimberlite-derived mantle nodules, detailed geochronology of the crust and mantle, and the structural information obtained from the recent Southern African Seismic Experiment. Based on data from both the craton-wide, 81-station Kaapvaal Seismic Array (KSA), and small-aperture (60km), 31-station Kaapvaal Telemetered Array (KTA) near Kimberley, South Africa, the anisotropic properties of southern Africa have been carefully studied through measurements of shear-wave splitting. The observed anisotropy appears to be dominated by deformational events that occurred in the Archean mantle and thus provides a valuable tool for studying the processes governing the formation and subsequent deformation of this ancient craton. A single, large-scale feature dominates the spatial pattern of anisotropy across the craton. It is a spatially continuous, roughly NE/SW-trending 1000km-long arc, running from the Limpopo belt, southern Zimbabwe and northern Kaapvaal cratons in the northeast to the western Kaapvaal in the southwest. Along its length, fast polarization directions from shear wave splitting analyses are oriented approximately parallel to the trend of the arc. Based on crust/mantle geochronology, the arc is associated with 2.9Ga (and later Neo-Archean) deformational events. This feature wraps around the 3.0Ga and older Meso-Archean eastern Kaapvaal Shield to the south and east, whose mantle, in contrast, is seismically isotropic. The crustal boundary between these two domains corresponds to the Colesberg Lineament (CL) in the southwest and Thabazimbi-Murchison Lineament (TML) in the northeast. These major lineaments must also represent primary mantle deformation boundaries, since they mark a clear change from anisotropic to isotropic mantle. In the case of the CL, shear wave splitting results from the KTA show that the anisotropic/isotropic boundary is very sharp, producing a systematic change in splitting delay time from 0.15 s to 0.75 s over only about 60km, and must be located within the mantle lithosphere. The close correspondence between an Archean age boundary and a mantle anisotropic boundary suggests that distinct continental tectonic processes were operating during these two periods. Deformation occurring in the Neo-Archean (2.9Ga and later) generated significant mantle fabric in what appears to be a convergent-margin setting, consistent with the strong anisotropy commonly observed in convergent margins elsewhere in the world. Tectonic activity in the Meso-Archean (older than 3.0Ga) eastern Kaapvaal Shield occurred by processes that failed to produce a coherent mantle fabric. We hypothesize that the eastern Kaapvaal, representing the Earth's earliest era of continental history, formed and evolved in a non-convergent-margin setting that may reflect a pre-plate-tectonic period in Earth's history.
- Publication:
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EGS - AGU - EUG Joint Assembly
- Pub Date:
- April 2003
- Bibcode:
- 2003EAEJA....12850S