I really do like the muted colour scheme you've used here. One easy way to get natural looking creases is just to get a white piece of paper fold it, unfold it and then scan it. Put it on it's own layer (either multiply or overlay blend mode - can't remember) and there you go. If you want to be a real perfectionist you can mesh warp the image so it fits the creases too but most of the time I don't think anyone would notice.
Welcome to the Guild btw - fascinating maps and subject matter.