Is the mantle transition zone uniform beneath Precambrian shields?

To understand the ubiquitous nature of the Precambrian shield, we investigated the structure of the mantle transition zone (MTZ) beneath the Canadian, Brazilian, Baltic, African, and Australian Shields. Receiver Functions were computed from data collected from various stations sampling these regions, and the topography of the MTZ boundaries was mapped using depth-migrated RF images generated with two 3D tomographic velocity models, LLNL_G3D_JPS and GyPSuM. The depth-migrated images from both models reveal a thinner-than-usual MTZ beneath all the Precambrian shields, with an average thickness of approximately 238 ± 8 km. The upper boundary of the MTZ (the 410 km discontinuity) shows distinct topography, while the lower boundary (the 660 km discontinuity) is found at shallower depths. This suggests that the main cause of MTZ thickness variation is the shallowing of the 660 km discontinuity, pointing to a post-spinel transition occurring at higher temperatures with a negative Clapeyron slope, supporting the theory of whole-mantle convection. These results suggest that mantle plumes have appreciable influence the MTZ beneath Precambrian shields, extending to its base, which also gains support (/may also be accounted by) from the global mantle warming observations at the 660 km discontinuity. Further numerical modeling/ simulation studies are needed to test this hypotheses.