Apple’s press release on the matter reads:
Apple today announced that Bob Mansfield, Apple’s senior vice president of Hardware Engineering, will retire and the role will be transitioned to Dan Riccio, Apple’s vice president of iPad Hardware Engineering, over several months. The entire hardware engineering team will continue to report to Mansfield until his departure.
“Bob has been an instrumental part of our executive team, leading the hardware engineering organization and overseeing the team that has delivered dozens of breakthrough products over the years,” said Tim Cook, Apple’s CEO. “We are very sad to have him leave and hope he enjoys every day of his retirement.”
“Dan has been one of Bob’s key lieutenants for a very long time and is very well respected within Apple and by the industry,” added Cook. “Our hardware engineering team is the best engineering team on earth and will not miss a beat during the transition.”