Page 179 - Microtectonics

168 6 · Dilatation Sites – Veins, Strain Shadows, Fringes and Boudins

Fig. 6.12. Effect of relative growth rate of crystals and vein opening
rate on the geometry of crystals in veins and on presence of inclu-
sion banding. Crystals grow anisotropically in a unitaxial vein, from
bottom to top. Their growth rate in the periodically opening crack
at the top depends on their orientation. In this section, slow and fast
growing crystals alternate. a Opening rate is slow, and both fast and
slow crystals can fill an opened crack before the next opening incre-
ment. Each renewed cracking and growing episode produces a con-
tinuous inclusion band in all crystals. b Fast growing crystals can
reach the contact after each cracking episode, but slow growing crys-
tals cannot; the result is the formation of discontinuous inclusion
bands. c Opening rate is so fast, that no crystals can reach the upper
boundary any more and blocky, euhedral crystals form. Inclusion
bands are completely lacking. (After Fisher and Brantley 1992)
6.2.3
Curved Fibres and Fibre Growth Direction

Fibres and in some cases elongate crystals are commonly
curved in veins. In some cases this curvature can be ex-
plained by deformation or grain boundary migration
(Williams and Urai 1989). However, in most cases, the
curved fibres are strain-free and contain undeformed
arrays of fluid inclusions or delicate banding visible in
cathodoluminescence (Sect. 10.2.1), which would be de-
stroyed or modified by grain boundary migration (Urai
et al. 1991; Bons and Jessell 1997). The presence of such
optically strain-free curved fibres has led to the inter-
pretation that fibres grow in a particular kinematic di-
rection, and that the growth direction changes when kin-
ematic axes rotate relatively to the rock volume in which
the veins develop (Ramsay 1980b); hence, the orienta-
Fig. 6.11. Aspects of microstructural development in extension
veins. a–c Development of discontinuous inclusion bands. If only tion of the fibres carries information on the opening vec-
the fastest growing crystals reach the boundary after crack open- tor of such dilating veins. For simple vein geometries,
ing, they will be the only ones that contain inclusion bands. d, e If this direction was even suggested to be parallel to the
opening rate is increased, no crystals can reach the boundary any extensional ISA of the flow (Figs. 6.5, 6.7; Durney and
more, euhedral crystal faces are formed, and no inclusion bands
are trapped in the grains. f If the fluid pressure decreases suffi- Ramsay 1973; Wickham 1973; Wilcox et al. 1973; Philip
ciently, the vein may collapse and the euhedral crystal faces can and Etchecopar 1978; Casey et al. 1983; Ramsay and Huber
indent the wall rock 1983; Ellis 1986). Such fibres or elongate crystals are said