A Narrow, Mid-Mantle Plume below Southern Africa

A Narrow, Mid-Mantle Plume below Southern Africa a) The travel time delay ΔT and b) multi-pathing differential values ΔLR indicate the boundaries of the superdome.
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Current tomographic models of the Earth display perturbations to a radial stratified reference model. However, if these are chemically dense structures with low Rayleigh numbers, they can develop enormous relief, perhaps with boundaries closer to vertical than radial. Here, we develop a new tool for processing array data based on such a decomposition referred to as a multi-path detector which can be used to distinguish between horizontal structure (in-plane multi-pathing) vs. vertical (out-of-plane multi-pathing) directly from processing array waveforms. We demonstrate the usefulness of this approach by processing samples of both P and S data from the Kaapvaal Array in South Africa. The result displays a narrow plume-like feature emitting from the top of the large African low-velocity structure in the lower mantle. A detailed SKS wavefield is assembled for a segment along the structure's southern edge by combining multiple events recorded by a seismic array in the Kaapvaal region of southern Africa. With a new processing technique that emphases multi-pathing, we locate a relatively jagged, sloping wall 1000 km high with low velocities near it's basal edge. Forward modeling indicates that the plume's diameter is less than 150 km and consistent with an iso-chemical, low-viscosity plume conduit.
</p><p>References
</p><p>Sun, D., D. Helmberger, and M. Gurnis (2010), A narrow, mid-mantle plume below southern Africa, Geophys. Res. Lett., 37, L09302, doi:10.1029/2009GL042339.
</p><p>Acknowledgements: The waveform data were obtained from IRIS. This work was supported by NSF grant MCG.00021-NSF.CSEDIFINE.</p>

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