HR: 11:35h
AN: S31H-06    [PDF]
TI: Seismic Evidence for Accumulated Oceanic Crust Above the 660-km Discontinuity Beneath Southern Africa
AU: * Shen, Y
EM: yshen@gso.uri.edu
AF: University of Rhode Island, Graduate School of Oceanography, Narragansett, RI 02882 United States
AU: Blum, J
EM: jblum@igpp.ucsd.edu
AF: Scripps Insititution of Oceanography, UC San Diego, La Jolla, CA 92093 United States
AB: High-pressure assemblages of subducted oceanic crust are denser than the normal upper mantle but less dense than the uppermost lower mantle. 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 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 $\pm$ 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. Accumulated oceanic crust at the base of the upper mantle provides a plausible explanation for the apparent paradox between the negative velocity discontinuity and the low velocity contrast at the 660-km discontinuity. Measurements of the elastic constants supplemented by estimates based on systematics and first principles theoretical predictions suggest that subducted oceanic crust has a shear wave velocity ~2% lower than that of pyrolite at 530 - 630 km depth. From 600 km depth to the base of the upper mantle, Ca-perovskite exsolution in former oceanic crust could increase the shear wave velocity by 1-2 percent. This scenario is in general agreement with an anomalously large P-wave velocity gradient over a depth range of ~80 km around 660 km depth inferred from the travel times and waveforms of direct P waves beneath southern Africa.
DE: 1030 Geochemical cycles (0330)
DE: 7218 Lithosphere and upper mantle
DE: 8124 Earth's interior--composition and state (old 8105)
DE: 9305 Africa
SC: Seismology [S]
MN: 2003 Fall Meeting