Helmberger, Don V. and Ni, Sidao (2005) Seismic Modeling Constraints on the South African Super Plume. In: Earth's Deep Mantle: Structure, Composition, and Evolution. Geophysical Monograph Series. No.160. American Geophysical Union , Washington, DC, pp. 63-81. ISBN 0-87590-425-4 http://resolver.caltech.edu/CaltechAUTHORS:20110818-084011694
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Tomographic studies of the structure of the lower mantle beneath South Africa reveal large-scale low velocities above the core-mantle boundary. Predicted SKS delay patterns (up to 3 s) for some of these models fit observations (Kaapvaal Array data) quite well except for magnitude level, explaining less than one-half the observed anomaly. Moreover, the sharpness in travel-time offsets and waveform complications require that nearly vertical walls separate the anomalous structure from the normal preliminary reference Earth model (PREM) mantle. We present numerous record sections along with 2D and 3D synthetics displaying multipathing of arrivals (S_(d') SKS, SKKS, S, and ScS), based on a large-scale 3D structure. This kidney-shaped structure has one apex beneath the Indian Ocean (Kerguelen) and the other extending beneath the Mid-Atlantic (Cape Verde). The structure is about 1200 km wide beneath South Africa and extends upward to at least 1000 km through the lower mantle, similar to Grand's model but with an average uniform velocity decrease of about 3% relative to PREM. We have not found any evidences for ultra-low-velocity zones (ULVZ) beneath the main structure but ample evidence at some locations near the edges. We also analyzed Pd and the differentials between PcP travel times and P travel times (PcP-P) along the same great circle paths from the same events. The P-velocity is not very anomalous, perhaps -0.5%. The sharpness of the lateral boundaries (walls) and the large contrast in P and S velocities can be used in arguments for a thermochemical origin.
|Item Type:||Book Section|
|Additional Information:||© 2005 American Geophysical Union. We thank Lianxing Wen and Ed Garnero for their excellent reviews, Evelina Cui for her efforts in producing this manuscript, and the editors of this book, in particular, Rob van der Hilst and production coordinator Virginia Marcum. This research was supported by NSF Grant EAR-0229885. Contribution Number 9116 of the Division of Geological and Planetary Sciences, California Institute of Technology.|
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|Deposited By:||Ruth Sustaita|
|Deposited On:||18 Aug 2011 16:42|
|Last Modified:||18 Aug 2011 16:42|
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