Seismic discontinuities in the mantle transition zone and at the top of the lower mantle beneath eastern China and Korea: Influence of the stagnant Pacific slab

Abstract

We have analyzed broadband data to identify and determine mantle discontinuities in the mantle transition zone (MTZ) beneath eastern China, where the Pacific slab is stagnant. The depths of the 410 and 660km discontinuities are generally shallower and deeper, respectively, than the global averages in and near the Pacific slab beneath eastern China. The MTZ is thicker in the slab than the global average. This observation is consistent with the thermally controlled olivine to wadsleyite transformation for the 410km discontinuity and the post-spinel transformation for the 660km discontinuity. Other discontinuities appear from 690 to 750km in the lower mantle part of the Pacific slab. Recent mineralogical experiments indicate that the most plausible interpretation of these deep discontinuities is that they represent the ilmenite to perovskite transformation in a cold environment, such as that in the Pacific slab.