An important and novel mechanism for ductile failure of the lithosphere is identified here, which is intrinsic to the thermal-mechanical feedback in a temperature dependent plastic body with coupled elastic fields. Both a temperature-dependent power-law visco-elasto-plastic rheology and a temperature-dependent elasto-plastic rheology are employed to study in a self-consistent fashion the deformation of the lithosphere subject to extension by means of a two-dimensional, finite-element code. A structural perturbation initially localizes elasto-plastic deformation only in its immediate vicinity. However, after 800,000 years have elapsed the localized zone of deformation takes off in a 'crack-like' fashion and travels to the bottom of the lithosphere in about 50,000 years time. When the plate is severed, thermal runaway is caused by mechanical heating triggered by the rapid energy transfer of the globally stored elastic energy into localized plastic dissipation in the ductile fault. ¿ 1998 American Geophysical Union