Page 181 - Microtectonics

170 6 · Dilatation Sites – Veins, Strain Shadows, Fringes and Boudins

Fig. 6.14. Effects of vein-wall smoothness and opening rate of veins on crystal geometry in unitaxial veins, following Urai et al. (1991) and
Oliver and Bons (2001) for anisotropic crystal growth. True fibres only form if opening is by small increments (bottom left); in this case, they
are also tracking if the growth surface is rough. In case of larger opening rate, elongate crystals form due to growth competition, which can
be partly tracking if the growth surface is rough. If opening rate exceeds growth rate, blocky euhedral crystals form that are non-tracking.
Ghost fibres may track the opening direction in all cases
surface have a small amplitude below a critical value, longer period of growth before reaching the wall rock in
growth competition proceeds for a longer time (Urai et al. one opening step, and grains will expand by growth com-
1991; Hilgers et al. 2001). In this case, crystals have a petition to few elongate grains (Bons 2001; Hilgers et al.