ScS Reverberations Map Widespread Subducted Slab Stagnation in the Mantle Transition Zone

Earth's mantle transition zone (MTZ) is a region of substantial transformations in the structure and composition of rocks. The mineralogical phase changes, observed seismically as semi‐discontinuous features, act as a semi‐permeable barrier to thermochemical flux between the upper and lower mantle. They partly control global mantle circulation patterns. However, understanding regional‐scale convection processes relies on high‐quality seismic measurements with improvements in observational approaches in terms of coverage and resolution. Here, we perform a comprehensive analysis of the MTZ discontinuities using new large global data sets of ScS reverberations. The data sets are compiled and processed using data‐driven and machine learning methodologies, highlighting the prospective capacity of automated approaches for rapid data analysis. Our maps reveal significant topography variations in the 410 and 660 km discontinuity depths, and hence MTZ thickness, and provide complementary spatial geometry to existing MTZ maps. These results are consistent with first order correlation of MTZ thickness with seismic wavespeed, and further indicate additional smaller scale lateral variation in topography across all regions of data coverage. We integrate our observations with mineralogical modeling to derive new thermal and compositional models of the MTZ. Our models display a correlation between locations of recent subduction and basalt accumulation, and between hotspots and regions of high temperature. Both temperature and composition are dominated by short‐length scale patterns in their variability, indicating ubiquitous and persistent heterogeneities within the MTZ. Together, these observations evidence widespread and long‐term stagnation of material at and within the MTZ.