Natural resources group Rio Tinto has announced that its chief executive Sam Walsh and chief financial officer Chris Lynch have had their fixed-term retirement dates pushed back.Chairman Jan du Plessis said the board has agreed with both directors on new "long-term, open-ended commitments" and that the decision to extend the tenure was a "strong endorsement of their leadership".Walsh's initial contract was scheduled to end in December 2014 with a break clause at the end of this month, while Lynch's initial contract was due to finish in February 2017.Both will now move to a rolling contract with no end date and a 12-month notice period."Since their appointments early last year, Sam and Chris have led a transformation of the business and established a track record of delivering on their promises," du Plessis said."Rio Tinto has increased cash flows from operations, achieved significant operating cash cost improvements, reduced net debt and refocused capital expenditure on projects with the most compelling returns."Walsh and Lynch will transfer to their new contracts after Rio's annual general meetings in 2015, though their new terms will need shareholder approval. Rumeration for both will remain unchanged, the company said.