Dipping Low-Velocity Layer in the Mid-Lower Mantle: Evidence for Geochemical Heterogeneity
Satoshi Kaneshima,
1*
George Helffrich
2
Data from western United States short-period seismic networks
reveal a conversion from an S to a P wave within
a low seismic velocity layer (greater than or equal to the 4 percent
velocity difference compared to the surrounding mantle) in the
mid-lower mantle (1400 to 1600 kilometers deep) east of the Mariana
and Izu-Bonin subduction zones. The low-velocity layer (about 8 kilometers thick) dips 30° to 40° southward and is at least 500 kilometers by 300 kilometers. Its steep dip, large velocity contrast,
and sharpness imply a chemical rather than a thermal origin. Ancient oceanic crust subducted into the lower mantle is a plausible candidate for the low-velocity layer because of its broad thin extent.
1 Department of Earth and Planetary Sciences, Faculty
of Science, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo
152, Japan.
2 Department of Geology, University of Bristol,
Queens Road, Bristol, UK.
*
To whom correspondence should be addressed.
