So, this magically maintained desert region... it doesn't countour the existing land, but is as if a slice of the world were physically swapped...? Meaning that where the differing elevations of the two regions meet, there will be numerous sheer cliff-faces, etc.?

Or does the desert simply overlay the existing land countours and elevations? If the latter, the desert probably will be surrounded by a thin bit of hard rocky crags where the elevations don't properly line up. You might represent this with some kind of dark colored texture fade from the regions outside the desert and into the desert.

If the latter, I think Steel General pretty much has the right idea.