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

A 'trefoil' joint in granite of the Bohemian Massif, in the Variscan belt o f 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. Frac ture 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. (C) 2001 Elsevier Science Ltd. All rights reserved.