Changes of crack velocities at the transition from the parent joint through the en échelon fringe to a secondary mirror plane

A ‘trefoil’ joint in granite of the Bohemian Massif, in the Variscan belt of Europe, displays intriguing surface morphologies. It consists of ‘primary’ and ‘secondary’ mirrors as well as ‘primary’ and ‘secondary’ fringes, an arrangement not previously described. These fractographic features are seen to record two cycles of increase and decrease in fracture velocities. Fracture velocities increased along the propagating mirrors in association with the increase of released energy. However, beyond a certain velocity energy dissipated along the new multi-surfaces at the fringes, and this involved reduction in fracture velocities.