Modeling of an eroding thrust belt indicates that immediately after thrusting the lithosphere is about twice as strong as unthickened (cratonic) lithosphere, but after 10 to 20 m.y. it becomes weaker than cratonic lithosphere, reaching a minimum of 40% to 70% of cratonic strength 30 to 100 m.y. after thrusting. This may explain why rifting of thrust belts commonly occurs several tens of million years after thrusting ceases. Relative weakening results from (1) elevation of the geotherm caused by burial of radioisotope-enriched surface rocks and (2) replacement of strong mantle rocks by weak crustal rocks in the lithospheric column. Low denudation rates enhance weakening by allowing radioisotope-enriched rocks to remain buried longer. Our modeling indicates that (1) magmatism after thrusting is favored by high concentrations of heat-producing elements in the lower plate and by slow denudation rates and (2) factors that affect denudation rate, such as climate and the development of external drainage, can significantly influence evolution of a thrust belt.