Sediment loading at continental margins has been interpreted as a very inefficient mechanism for initiating subduction. We have re-investigated this problem using a time-dependent, non-linear ductile failure analysis in which the effects of power-law plastic strain-hardening are assessed. The results follow classical flexural rigidity solutions until wholesale lithosphere failure occurs. Sediment loading causes two shear zones to develop from the bottom of the lithosphere upwards. For strong strain-hardening, shear zones nucleate earlier but the plate may never break. For small hardening, the instability grows much faster and the plate can break under the sediment load with a limiting yield stress lower than 0.4 GPa. When the ductile faults growing from the top and the bottom of the plate merge in the center, the flexurally supported gravitational energy is released; the lithosphere becomes unstable and shears off super-exponentially within a few million years. ¿ 2000 American Geophysical Union