The U.S. debt is denominated in dollars. If the U.S. Treasury cannot redeem the debt by using tax revenue or by issuing more debt then holders of U.S. debt will simply sell their securities to the Federal Reserve. Moreover, if investors don't want to lend to the U.S. government for some bizarre reason the Fed will be happy to step into the breech. The result in both cases would be quantitative easing on a scale never seen before. This would be incredibly bullish for U.S. and world stock markets, at least in its early stages.