Subduction systems are inherently three-dimensional, with significant along-strike variations in the timing and style of deformation and magmatism. Geodynamic models used to explain such variations and associated trench curvature typically invoke mantle flow or along-strike variations in the properties of the subducting lithosphere, ignoring the role of the overriding plate. Here we use analogue experiments to investigate the dynamics of diachronous backarc basin opening. In the models, horizontal tension increases in the upper plate due to progressive subduction of negatively buoyant oceanic lithosphere. The magmatic arc lithosphere eventually fails and backarc opening occurs via rollback of the forearc block. Arc failure initiates near the model edge and propagates diachronously along strike, producing an arcuate plate boundary. The experiments demonstrate that the trench rollback rate is limited by the propagation rate of arc failure. Conversely, slab rollback generates additional horizontal tension in the adjacent magmatic arc lithosphere, which drives along-strike propagation of arc failure. Feedback between the rates of trench rollback and arc failure propagation eventually leads to a steady state, which dictates the geometry of the backarc basin in map view. In the experiment reported here, the force and rate balance leads to a backarc basin with an ellipticity of ∼1.7, which matches the shape of the West Mariana basin (Pacific Ocean).