The mantle transition zone as seen by global Pds phases: No clear evidence for a thin transition zone beneath hotspots

Abstract

We present a new global study of the transition zone from Pds converted waves at the 410‐ and 660‐km discontinuities. Our observations extend previous global Pds studies with a larger data set, especially in oceanic regions where we have been able to measure Pds travel times, sampling the mantle transition zone (MTZ) beneath 26 hotspot locations. We find significant lateral variations of the MTZ thickness. Both the maximum variations (±35–40 km) and the long‐wavelength pattern are in overall agreement with previous SS precursors studies. The MTZ is generally thick beneath subduction zones, where the observed MTZ variations are consistent with thermal anomalies ranging between −100°K and −300°K. In Central and North America, we observe an NW–SE pattern of thick MTZ, which can be associated with the fossil Farallon subduction. We do not find clear evidence for a thin MTZ beneath hotspots. However, the 410‐km discontinuity remains generally deepened after correcting our Pds travel times for the 3D heterogeneities located above the MTZ, and its topography variations can be explained by thermal anomalies between +100°K and +300°K. The depth of the 660‐km discontinuity may be less temperature sensitive in hot regions of the mantle, which is consistent with the effect of a phase transition from majorite garnet to perovskite at a depth of 660 km.