Main panel: bending moment variation with age. The plot shows predicted age-saturation moment relationships for various strength models, as well as various published data. Filled circles shows observed moment estimates from Levitt & Sandwell (1995) (blue). Black crosses show bending moments from the forward models of (Garcia et al. 2019) extracted at the point of maximum curvature; these points are within 5–10 per cent of the saturation moment for the ‘weak’ strength model investigated (shown with solid black line). Red circles show the bending moment at the first zero crossing, redigitized from the paper of Hunter & Watts (2016), using the variable elastic thickness results (A2). As discussed in Section 6, we developed a family of models by varying the friction coefficient and the LTP strength subject to the neutral plane constraint (30 km) for 100 Myr lithosphere, with zero net axial force. These models are used to define weak, intermediate and strong model regions. The solid blue line shows the member of this family of models that minimizes the misfit (RMS) for the data set of Levitt & Sandwell (1995); the dashed blue line shows the strength model inferred in the original study. The dashed black line shows the strength model proposed by Chapple & Forsyth (1979), chosen to fit data for ‘old’ subducting plates. The top panel shows the variation in neutral plane depth as a function of age.