Page 177 - Microtectonics

166 6 · Dilatation Sites – Veins, Strain Shadows, Fringes and Boudins

Fig. 6.8. Transitions between extension and shear veins. Extension
veins open normal to the vein wall, shear veins at a low angle or
parallel to the vein wall with opening of jogs
Bryne 1990). This could be due to partial filling of a crack
related to different growth rate of crystals with different
orientation; slowly growing crystals may not be able to
fill a crack completely until the next opening cycle. Such
interrupted inclusion bands, and the presence of wall
rock fragments along inclusion bands are evidence for
activity of the crack-seal mechanism (Fig. 6.12).
Aggregates of solid or fluid inclusions may also occur
in surfaces oblique to the vein-wall contact, usually con-
necting jogs in the inclusion bands, and corresponding
points in the vein-wall contact. These inclusions define
inclusion trails (Fig. 6.10; Ramsay and Huber 1983; de
Roo and Weber 1992; Köhn and Passchier 2000). While
inclusion bands are parallel to the vein wall, inclusion
trails are thought to track the opening trajectory of a
vein (Ramsay and Huber 1983; Köhn and Passchier 2000).
Fig. 6.7. Development of the five types of veins with tracking fibres Isolated elongate fluid inclusions or fibres such as mica
described in the text. A change in the relative motion of the wall grains may also lie oblique to the edge of a vein and to
rocks (arrows) can cause curvature of the growing fibres in the
veins. Notice that the sense of curvature of the fibres depends on inclusion bands or trails (Fig. 6.10). None of the inclu-
the type of vein, and that ataxial veins develop straight fibres which sions is necessarily parallel to fibres or elongate grains
indicate a mean displacement direction of the vein wall rocks that build up the bulk of the vein (Figs. 6.4, 6.9), since these