Broadband seismograms collected from USArray and other networks available from the DMC have been used to map topography of phase boundaries in earth’s upper mantle. The relative timing of reflections from earth’s surface and boundaries within earth can be measured with high-resolution stacks of precursors to the seismic phase SS, if the data are of sufficiently high quality. The discontinuities usually found near depths of 410 km and 660 km are both deeper in the down-dip direction of subduction zones, which is inconsistent with cold material at 410-km depth. Several mechanisms invoking chemical heterogeneity within the mantle transition zone might explain this feature. In some regions, there are multiple reflections from the discontinuities, consistent with partial melt near 410-km depth and/or additional phase changes near 660-km depth. Thus, the origin of upper mantle heterogeneity has both chemical and thermal contributions, and is associated with deeply rooted tectonic processes.


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