Seismic Evidence for Accumulated Oceanic Crust Above the 660-km Discontinuity Beneath Southern Africa - fig. 2

Seismic Evidence for Accumulated Oceanic Crust Above the 660-km Discontinuity Beneath Southern Africa - fig. 2 Figure 2. (a) Two possible scenarios may cause a negative-polarity P-to-S conversion near a 590-km depth: a uniform reduction in velocity at the base of the upper mantle (dashed line) and a velocity reduction near 590 km followed by a greater than normal velocity gradient (dotted line). The reference shear velocity structure (solid line) is a modified iasp91 model with a smaller velocity contrast at the 660-km discontinuity to provide a better fit to the observed P660s amplitude. (b) The top traces are the synthetic receiver functions for the three velocity models in (a). Line styles match those in (a). Arrow marks the converted phase from the velocity reduction near 590 km depth. The waveform of a linearly stacked receiver function from the center patch in Figure 1a and its 95% confidence limit are shown for comparison. The top traces are shifted upwards by 0.05.
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High-pressure assemblages of subducted oceanic crust are denser than the normal upper mantle but less dense than the uppermost lower mantle (Ringwood, 1991; Hirose et al., 1999). Thus, subducted oceanic crust may accumulate at the base of the upper mantle. Direct observational evidence for this hypothesis, however, remains elusive. We present an analysis of a negative-polarity shear wave converted from a compressional wave at a seismic discontinuity near 570 - 600 km depth beneath southern Africa. The negative polarity of the converted phase indicates a ~2.2 ± 0.2% S-velocity decrease with depth at the seismic discontinuity. This velocity reduction is associated, however, with a low-velocity contrast at the 660-km discontinuity. The exsolution of Ca-perovskite in former oceanic crust at depths greater than 600 km and the associated small volume fraction of ringwoodite are plausible explanations for the apparent paradox between the negative velocity discontinuity and the low velocity contrast at the 660-km discontinuity.
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Shen, Y., and J. Blum, Seismic evidence for accumulated oceanic crust above the 660-km discontinuity beneath southern Africa, Geophys. Res. Lett., 30, 1925, doi:10.1029/2003GL017991, 2003.

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