Fringe joints occur at the borders of joints as extensions of twisted and rotated hackle marks from their initially planar parent joints (Pollard and Aydin, 1988). They diverge gradually usually by a small angle of 5-25° from the planar faces of the parent joints (Figure 1), but larger angles with sharp divergence are also observed (Figure 2). In cross section view fringe joints appear to be echelon indicating mode I-III propagation. Fringe joints described in the geological literature only occur in relatively small areas with respect to the whole planar parent joint surfaces. However, if the fringe joints are extended well beyond the parent joints, then the relationship between the parent joints and the new system of joints isn't readily apparent. Perhaps that is why the word 'fringe' has not been used for these latter cases.

Figure 1. A zone of fringe joints making a small angle to the planar main joint surface in Aztec Sandstone across from the Mouse's Thank at the Valley of Fire State Park, Nevada. The flat surface on the top right of the image represents another joint surface. Students attending a field trip are for scale.

Figure 2. Joint surface morphology (top) showing several elliptical rib marks indicating mode I-II propagation. Here the emphasis is on a series of sharp fringe joints (bottom) indicating a breakdown of the parent joint surface in mode I-III propagation. From Aydin et al. (2006).