  We report the direct imaging of a novel modulated flux striped domain phase
in a nearly twin-free YBCO crystal. These domains arise from instabilities in
the vortex structure within a narrow region of tilted magnetic fields at small
angles from the in-plane direction. By comparing the experimental and
theoretically derived vortex phase diagrams we infer that the stripe domains
emerge from a first order phase transition of the vortex structure. The size of
domains containing vortices of certain orientations is controlled by the
balance between the vortex stray field energy and the positive energy of the
domain boundaries. Our results confirm the existence of the kinked vortex chain
phase in an anisotropic high temperature superconductor and reveal a sharp
transition in the state of this phase resulting in regular vortex domains.
