In General Relativity, Faster-Than-Light travel is only prohibited locally. Its possible for space-time itself to expand and shrink faster than light. This is why distant-enough galaxies are receding from us at faster than the speed of light.

Mathematically, the structure of the proposed warp-drive is such that the spaceship is moving at << c locally, so no causality is violated. See the author's answer here:

http://www.icarusinterstellar.org/daydreaming-beyond-the-sol...