Well, it is kind of hard to say because Ant is built on some particular architecture. I can safely say, however, that they probably use standard object oriented development methodologies to keep Ant stable and flexible. May I ask why you need to know the architecture upon which Ant is built?
There are many well-known design patterns use in ant - for example, tasks are a variation of the Command pattern. Don't know wether you could give the architecture a single name. I also wouldn't say that MVC describes a whole architecture - it's an architectural pattern which could simply be combined with many others (for example, it doesn't say anything about persistence).
The soul is dyed the color of its thoughts. Think only on those things that are in line with your principles and can bear the light of day. The content of your character is your choice. Day by day, what you do is who you become. Your integrity is your destiny - it is the light that guides your way. - Heraclitus