Title

Seismic Structure and Tectonics of Southern Africa -- Progress Report

Abstract

The Southern African Seismic Experiment (SASE), designed to sample the crust and mantle beneath southern Africa, consists of nearly 80 recording sites and spans a 2000km northeast-southwest section that extends from Capetown to Zimbabwe. The experiment is part of a multidisciplinary research project by the Carnegie Institution of Washington, MIT, and several southern African institutions. Results to date suggest that upper mantle structure is closely related to geologic structures at the surface. Body and surface-wave tomography reveal a high velocity mantle root beneath the Kaapvaal craton that extends to at least 200km depth. Shear wave splitting in SKS is observed at most of about 80 sites, indicative of deformation-induced anisotropy within the upper mantle. The orientation of the fast polarization directions Φ, show significant lateral variations that closely follow the orientation of the ancient surface geologic deformation: NE-SW in the southern Kaapvaal, EW within the Limpopo mobile belt, in addition to NE-SW to NNE-SSW directions in the Zimbabwe craton. This appears to represent Archean mantle deformation. Finally,the splitting delay times are small, suggesting weak anisotropy. Indeed, calculated seismic anisotropy based on a suite of mantle nodules from southern Africa are compatible with this observation, as well as the apparently stronger transverse isotropy inferred from horizontally propagating waves, as obtained by the MIT group. One of the intriguing features is the seismic structure beneath the 2.05 Ga Bushveld Complex, the world's largest layered igneous intrusion. At the latitude of this intrusion, 25⁰, there is a roughly E-W zone of low velocity mantle, weak mantle anisotropy and a maximum in crustal thickness. The spatial correspondence suggests a causal relationship, namely that ancient pervasive igneous activity of the Bushveld has altered mantle structure and added crustal material. Concerning deeper structure, non-linear stacking of about 1000 receiver functions finds clear P-to-S conversions from transition zone discontinuities. The 410km discontinuity is estimated to be about 400 km towards the south, gradually increasing to about 420 km at the northern end of the profile. The 660 km discontinuity shows greater lateral variation: 650km in the southern, 630km in the middle, and 680km in the northern part of the network. The corresponding variation in transition zone thickness may be related to strong three-dimensional heterogeneity in the lower mantle. Travel time residuals from SKS phases which are primarily controlled by the structure in the lower mantle, show delays in the center of the array that are as much as 4 s slow relative to the northern and southern ends of the profile. Since the slow arrivals correspond spatially to the region where the transition zone is thinnest, we suggest that there is thermal control of discontinuity depth by lateral heterogeneity at the top of the lower mantle.

Meeting Name

AGU Fall Meeting (1998: 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

© 1998 American Geophysical Union (AGU), All rights reserved.

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