NEW YORK (TheStreet) -- The Yahoo! (YHOO) board of directors has unananimously approved a deal to purchase Tumblr for $1.1 billion, according to The Wall Street Journal and the Web site AllThingsD.com.There was no immediate word about whether the Tumblr board had approved the all-cash deal but an announcement by Yahoo! was expected on Monday. As part of the deal, Tumblr CEO David Karp would stay with Yahoo! for a least four years and would continue to run Tumblr with a certain amount of autonomy. Yahoo! CEO Marissa Mayer was reported to be interested in Tumblr, a blogging and social media website, because it would bring a younger audience to Yahoo!. But some vocal Tumblr users have said they would not continue with that service if it became part of Yahoo! Tumblr's valuation had been put at $800 million. Among its investors are Spark Capital, Union Square Ventures, Sequoia Capital, Greylock Partners, Insight Venture Partners and the Chernin Group.