Replacing fireplace surround

I am planning to replace the Granite (http://tanbrown.org) fireplace surround in place now with 18" travertine Tiles (http://www.g682.net) cut to fit.. The dark color of the Granite (http://tanbrown.org) is just very unsightly.

My question is, since I will not be doing the stand (http://maplered.com) ard riser, legs and (http://maplered.com) header that exists now, will the thinset be strong enough to bond the 18" Tiles (http://www.g682.net) to the backerboard? I'll certainly support the Tiles (http://www.g682.net) while the thinset dries but I'm concerned they'll just pop off at some later point.