Dipping low-velocity layer in the mid-lower mantle: Evidence for geochemical heterogeneity

Data from western United States short-period seismic networks reveal a conv ersion from an S to a P wave within a low seismic velocity layer (greater t han or equal to the 4 percent velocity difference compared to the surroundi ng mantle) in the mid-lower mantle (1400 to 1600 kilometers deep) east of t he Mariana and Izu-Bonin subduction zones. The low-velocity layer (about 8 kilometers thick) dips 30 degrees to 40 degrees southward and is at least 5 00 kilometers by 300 kilometers. Its steep dip, large velocity contrast, an d 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.