A three-dimensional (3D) thermal–kinematic modelling approach based on finite-element techniques is used to study lower-crustal viscosity at transform margins during the continent–ocean transform development stage and after the ridge has passed by. Nine modelling scenarios combining different equilibrium surface heat flows and lower-crustal rheologies are studied. Modelling results indicate that substantial parts of the lower crust at transform margins have the potential to flow at geologically appreciable strain rates, which can lead to uplift/subsidence, as well as lateral variations, in upper- and lower-crustal thicknesses and Moho depth. These low-viscosity zones (i.e. parts of the lower crust with effective viscosities of less than 10¹⁸ Pa s) make up distinct ductility distributions that vary in space and time during margin evolution. Three basic ductility patterns and related thermal processes can be identified: reduced lower-crustal viscosities originating at the continental rift and the continent–ocean boundary (COB), respectively; reduced lower-crustal viscosities along the transform caused by the migrating ridge; and the background distribution of lower-crustal ductility resulting from the equilibrium temperature field. Superposition of all three ductility patterns and the complex interaction of the underlying perturbations of the temperature field result in distinct differences in the potential of lower-crustal flow both in space (parallel and perpendicular to the transform) and with time. Thus, modelling results provide templates for understanding lower-crustal flow at transform margins in general and await further studies comparing model predictions with actual field observations.