Seismic Evidence for Accumulated Oceanic Crust Above the 660-km Discontinuity Beneath Southern Africa - fig. 1

Seismic Evidence for Accumulated Oceanic Crust Above the 660-km Discontinuity Beneath Southern Africa - fig. 1 Figure 1. (a) Locations of broadband seismic stations (triangles) and the profile of the receiver function stacks (line). Circles with a radius of 2° outline two of the overlapping patches used in stacking. (b) Waveforms of the stacked receiver functions along the profile in (a) and their 95% confidence limits determined by bootstrapping. Arrow marks the arrival of the negative-polarity phase. An nth root (n=2) stacking process is used to enhance coherent phases and suppress random noise. (c) A comparison of the receiver functions stacked along Pds moveout curves (solid lines) and reverberation moveout curves (dotted lines). Every other trace in (b) is shown for legibility. (d) Waveforms of two linearly stacked receiver functions from the two patches outlined in (a) and their 95% confidence limits. The bottom trace corresponds to the patch near the center of the array. The scale of the vertical axis is relative to the amplitude of the P wave on the vertical component. The top trace has been shifted upwards by 0.05.

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.

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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