Global reflectivity- and discontinuity-structure of the mantle transition zone. Shown are 2-D contour-plots of the S-wave impedance contrasts (⁠|$\rm R_S$|⁠; cf. eq. 2; abscissa) against discontinuity depth (ordinate). Marginal histograms for each parameter are shown with solid black lines. For comparison, impedance contrast estimates from other studies are shown in black: PREM (Dziewoński & Anderson 1981), SF99 (Shearer & Flanagan 1999), CDW05 (Chambers et al. 2005a), LS06-1 and LS06-3 (results for first-order and finite-width discontinuities, respectively, from Lawrence & Shearer 2006). Coloured triangles represent impedance contrasts estimated from seismic models differing in basalt fraction (f) for an equilibrium assemblage (EA) and a mechanical mixture (MM) model, respectively. Thermal conditions are: |$\rm Z_{lit}=150~km, T_{lit}=1350^\circ C$|⁠.