Is the mantle transition zone uniform beneath Precambrian shields?

In the present study, we examine the mantle transition zone (MTZ) structure below the prominent Precambrian shields on the Earth to understand its thermal and compositional properties and depth distribution of MTZ discontinuities. For this study, we used data from Canadian, Brazilian, Baltic, African and Australian Shields. We migrate the receiver functions to the depth domain using 3D tomographic models to examine the topography of MTZ discontinuities that define the upper and lower boundaries of the MTZ. The depth migration results were obtained using two different 3D tomographic velocity models, LLNL_G3D_JPS and GyPSuM. The analysis shows that both models yield similar results. The findings indicate a thinner than usual MTZ beneath all the Precambrian shields with an average thickness of ∼238 ± 8 km. Notably, the present study reveals the upper boundary of the MTZ (410 km discontinuity) displays distinctive topography; in contrast, the lower boundary of the MTZ (660 km discontinuity) is situated at shallower depths. Therefore, the main factor causing the variation in MTZ thickness is the shallowing of the 660 km discontinuity, indicating that the post-spinel transition occurred at higher temperatures with a negative Clapeyron slope, supporting the theory of whole-mantle convection. This suggests that mantle plumes have some appreciable impact on the MTZ beneath Precambrian shields, which are limited to the base of the MTZ. Alternatively, the global mantle warming explanation could be invoked, as it best explains the similar characteristics of the 660 km discontinuity beneath the Precambrian shields. However, this phenomenon needs to be tested through numerical modelling.