Nobody really knows because the US has never defaulted before.
This isn't quite true. On June 5, 1933, the US government committed an immense default. From the CRS:
In 1933, the gold standard was ended for the United States. Despite the creation of the Fed, a wave of bank runs resulted in massive bank failures over the period 1930-1933. The Fed failed to provide sufficient liquidity to enable the banks to meet their customers’ demands for cash.
This failure was due in part—and possibly largely—to the gold standard. For the Fed to generate enough cash to meet the public’s changed demand for it, it would have had to create much more money and to lower interest rates. Lower interest rates, however, would have sped up the export of gold from the country as investors looked abroad for higher returns. Creating more paper money, moreover, would have created doubts about the ability of the United States to remain on gold. The greater these doubts, the greater the incentive to export gold, reducing gold reserves, and making it harder to maintain the dollar at its legal gold value. Hence, to keep the economy from collapsing, the Fed needed a policy of expansion. To stay on the gold standard, it needed one of contraction. Until 1933, it largely went with the latter.
With the inauguration of Franklin Roosevelt, the government’s policy changed. In a series of executive orders, legislative actions, and court decisions, the United States was taken off the gold standard. Convertibility into gold was suspended. Private holdings of gold were nationalized. A new parity with gold was established amounting to a devaluation of approximately 40%. This parity was only important for international transactions, however. Because Americans could not hold gold, their dollars were not convertible. The gold value of the dollar was largely meaningless. With no convertibility, the result was a quasi-gold standard.
Shortly after taking office, President Roosevelt closed the banks in order to stop the bank runs and the export of gold from the country. His order also prohibited the banks from paying out gold or dealing in foreign exchange. He did this based on the Trading With the Enemy Act of 1917, which gave him broad powers over banking and currency. Although the 1917 act appeared to confer these powers only in wartime, President Roosevelt acted on the basis of a “national emergency” and summoned Congress to a special session to prepare legislation to confer the powers he wanted to deal with the situation.
Three days later, Congress passed the Emergency Banking Act, which amended the 1917 act to include national emergencies, retroactively approved the President’s actions of the previous three days, and granted him power to regulate or prohibit the payment of gold. President Roosevelt promptly used these powers to continue the prohibition on gold transactions, even for banks that reopened. By executive order, on April 5, 1933, the “hoarding” of gold was forbidden. Gold had to be turned in to the government at the official price of $20.67 per troy ounce. Essentially, the country’s gold was nationalized.
This action was endorsed by Congress in a joint resolution. The resolution called for a suspension of the gold standard and abrogated gold clauses. The Thomas Amendment to the Agricultural Adjustment Act of 1933 granted authority to the President to alter the gold content of the dollar, with power to reduce it to 50% of its previous value.
In addition, the amendment gave the President power to authorize the issuance of up to $3 billion in U.S. notes, and the power to compel the Fed to issue money to finance up to $3 billion in government borrowing. It also set out new conditions for the issuance of more silver certificates.
Under the authority of the Thomas Amendment, the market price of gold was allowed to increase to $35 by January 1934. At that time, the Gold Reserve Act was passed, and the President thereby empowered to fix the new value of the dollar at not less than 60% of its previous value. The Gold Reserve Act also gave legislative force to the nationalization of gold. Under its terms, title to all bullion and coin was vested in the U.S. government, gold coin was withdrawn from circulation, and the Treasury Secretary was given control of all trading in gold. Private holdings of gold were outlawed (except for numismatic and various industrial/artistic uses).
In June 1934, the Congress passed the Silver Purchase Act. The act called for at least one-fourth of the United States’ monetary stocks to be held in silver, so long as the government did not have to pay more for the silver than its official monetary value. The silver could be coined or issued as silver certificates. Silver certificates were exchangeable for silver coin. Because the market value of silver was below its monetary value, this law provided for the issuance of a limited quantity of another form of what amounted to fiat money.
The government’s abrogation of gold clauses in contracts was upheld by the Supreme Court in February 1935. Thus, the government could discharge all its interest and principal due in paper money. Because the dollar had depreciated due to official policy, it meant that the outlawing of gold clauses effectively reduced the amounts the government paid on its debts relative to what it would have paid in gold.
http://www.fas.org/sgp/crs/misc/R41887.pdf (PDF)