It's called a pen name.
Stephan King writes under three I believe. (I could be wrong but I know he uses more than one writer name). He has also written dramas (green mile) and fantasy (Talisman/dark tower) as well as western (gunslinger). And yeah, I realize Stephan King is a pen name too.
All of which were madly successful.
Nora Roberts is another example who writes under at least two different names and has been successful at both.
There are so many others. But no, it doesn't pigeon hole a writer into being forced to write a certain thing. Now that doesn't guarantee they'll be successful in other arenas (Dr. Seuss was a good example) but it certainly won't stop them from writing in other genres unless they let it.
Stephen King isn't a pen name, it's real name. And the problem with this example is that his pen name, Richard Bachman, is also one that everyone knows. In fact, whenever a Bachman book comes out now, it's a bit fun to read because you know it's going to be an artistic journey into the same writer writing the story from a different perspective, so to speak. It's a bit like Joe Hill (his son). Yes Joe Hill is his pen name, and I call him Joe Hill, but it doesn't mean people don't know who he is.
I wish this was true, btw. One of the things I've always liked about writers is that even famous ones, unlike other celebrities, aren't necessarily out there. I'm a private person. I don't like random people knowing things about my life or my opinions or anything that isn't relevant to my writing. I actually think of the current situations in terms of, "Oh great. I guess I'll have to actually have a blog one day. No idea how that'll work because my life is boring and I'm certainly not going to be telling strangers about it anyway." (Have I ever mentioned I'm not a fan of blogs in general, either? I kinda hate the things...well 99% of them). I will never tweet, period. I've managed to get this far without a Facebook page and would love it if I could manage to never have one.
I think even if you do have a pen name, though, you're still expected to be "out there" so to speak. The only decent thing about the whole situation is that I really don't think people go into a bookstore and say, "Oh I'm not going to buy this book because the author doesn't have a blog." At best, it might give some people a personal sense of knowing you (am I the only person in the world who doesn't necessarily want to "know" some of my idols? I sorta like idolizing them, honestly) and encourage them to buy more of your books in the future or build up a fan base. At worst, you might act like Anne Rice and have a bunch of people turned off by the crazy stuff you say online and lose all desire to ready anything you write.
I also think there's a tendency to overpersonalize and reveal too much information in blogs, though, and that always makes me nervous to see. Partly it's usually boring, but you never know who's out there reading. Going back to Stephen King, I remember a few years back when he had a crazy guy show up in his kitchen with a "bomb" (luckily it wasn't a real one, he just thought it was). And Stephen King isn't exactly the most exposed person out there. There are some really crazy people out there, and if I don't have control over who sees the things I write, I want to darn sure have control over what
information they see.
I'm half asleep and rambly, so pardon any lack of coherence that may be present.