For many weeks , the UN Security Council was deadlocked over the search for a successor before finally settling on Kurt Waldheim to succeed U Thant as secretary-general on December 21 , 1971-Waldheim 's 53rd birthday-and just ten days before U Thant 's second term was to end .
For many weeks , the UN Security Council was deadlocked over the search for a successor before finally settling on Kurt Waldheim to succeed U Thant as Secretary-General on December 21 , 1971 - Waldheim 's 53rd birthday - and just ten days before U Thant 's second term was to have ended .