Mantle transition zone (MTZ) thickness. (a) Map of mean MTZ thickness (⁠|$\rm \Delta Z_{MTZ}$|⁠), which is compared to the global model of (Tian et al. 2020, b). Green triangles and lines indicate locations of hotspots (Anderson & Schramm 2005) and contours of slabs (Hayes et al. 2018). (c) Histograms of MTZ thickness sorted by tectonic setting. Continental (Q, P, S) and oceanic regions (A, B, C) follow the global tectonic regionalization ‘GTR1’ (Jordan 1981,cf. text for details). Additional results are shown for hotspots (HS; see map in a), subduction zones with a slab in the upper mantle (SZ1) or in the MTZ (SZ2; see map in a), and normal mantle (NM) away from hotspots and slabs.