Title

Tomographic and Depth Phasing Imaging of Mantle Structure beneath the Southern Africa Seismic Array

Abstract

The Southern Africa Seismic Experiment was designed specifically to image the deep structure of cratonic roots. It is part of a multidisciplinary research project by Carnegie Institution, MIT, southern African academic institutions and industry collaborators to study the structure, composition, and evolution of the cratons and adjacent Proterozoic belts of southern Africa. An array of fifty five portable broadband REFTEK/STS-2 seismic stations was deployed from April, 1997 to July, 1999 along a NNE-SSW transect about 1800 km long by 600 km wide in southern Africa. Approximately half the station were redeployed to new sites in April/May, 1998 for a total of eighty two stations. The PASSCAL telemetered array of 30 REFTEK/STS2 stations was deployed in the region of Kimberley, South Africa, for a period of about 6 months early in 1999. Teleseismic P-wave and S-wave delay times from the broadband array have been analyzed to obtain tomographic images of mantle structure beneath southern Africa to depths in excess of 1000 km. Crustal time delay corrections have been applied to the data based on receiver function and surface wave inversions. Mantle velocities beneath the craton are significantly higher than those beneath the adjacent Proterozoic belts, with shear velocity constrasts disproportionately large. Mantle velocities vary across the craton itself at about the 1 percent level, with the highest velocities found beneath the southern part of the Kaapvaal craton and in the region of the Zimbabwe craton of NE Botswana and SW Zimbabwe. Clear evidence for deep cratonic root structures is observed to at least 200--250 km depth, with the most prominent root structures typically underlying diamondiferous regions. Very preliminary results from depth phasing images of P-S conversions across the PASSCAL broadband telemetered array near Kimberley, indicate a possible velocity reversal at a depth of about 300 km. Tomographic images for greater depths in the mantle indicate a prominent low velocity feature in the mantle below about 700 km, in the general region beneath the Bushveld province and north. The Bushveld province within the Kaapvaal craton is marked by relatively lower velocities in the mantle root. While the velocity contrast with adjacent undisturbed craton is comparatively small, it does correlate also with slightly greater crustal delay times (Nguuri et al., this meeting) and with a zone of null SKS splitting (Gao et al., this meeting). The seismic results coupled with evidence of younger Re/Os model ages of mantle nodules suggests possible cratonic disruption during formation of the Bushveld.

Meeting Name

AGU Fall Meeting (1999: Dec. 1, San Francisco, CA)

Department(s)

Geosciences and Geological and Petroleum Engineering

Document Type

Article - Conference proceedings

Document Version

Citation

File Type

text

Language(s)

English

Rights

© 1999 American Geophysical Union (AGU), All rights reserved.

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