I don't think that the geophysics of our modern world would necessarily apply to Tolkien's world. As has been noted elsewhere on the Downs, he was not a physicist. Tolkien's world seems real in comparison to our world's mythology and folklore, but not when set along side science as we now know it. Which is fantastic, I think, since a lot of the magic is lost when fantasy writers try to overexplain phenomena.
I think it's highly likely that the Valar, or whatever force caused the Sinking of Beleriand, were not subject to the usual physical laws, and that it would be possible for them to only destroy Beleriand without affecting the rest of Middle-Earth. A possible explanation could be fault lines like the San Andreas fault, if any explanation really needs to be given.
The most important thing in the tale is that at the end the ocean comes in to wash clean the land, and the importance of what happened in the rest of the world at that time is minor at best. With the whole cleansing idea (remember bad guys hate water), I find it interesting that part of Taur-nu-Fuin was still left standing. It would have been cool if Fingolfin's cairn was left above the water! (*sigh*, so many what-ifs).
Another neat thread that hasn't been mentioned is
What Happened to Beleriand?