Page 184 - Microtectonics

6.2 · Veins 173
Fig. 6.17.
Schematic diagram of the growth
of tension gashes (from left to
right) in a dextral shear zone in
the case of syntaxial and anti-
taxial fibre growth. Ornamenta-
tion indicates growth zones of
the fibres in the gashes for each
stage of deformation. At extreme
right, growth zones have been
omitted to show the geometry
of fibres in the tension gashes

that foliation curvature due to a strain gradient has a of the growth process. Antitaxial grains have the same
similar geometry as the tension gashes, but with the low- sense of curvature as the external shape of the tension
strain (outer) parts of the foliation in the direction of gash; syntaxial grains have the opposite sense of curva-
the instantaneous extension axis (Fig. 6.16a). ture (Fig. 6.17). More complex internal structures are also
In three dimensions, many tension gashes as described possible (Smith 2005).
above consist of isolated lenses. In some cases however,
echelon tension gashes are laterally grading into planar veins 6.2.5
(Fig. 6.16b; Nicholson and Pollard 1985; Nicholson and Shear Veins, Slickenfibres and Bedding Veins
Ejiofor 1987; Nicholson 1991; Tanner 1992b). In many cases
it can be shown that echelon tension gashes lie at the tip If veins form by opening at a small angle to the vein wall,
of planar veins, especially where they cut a lithological e.g. along a fault, their internal structure deviates in some
contact. Feather veins may occur between the planar and aspects from that of tension gashes which open highly
en-echelon sections, and are interpreted to form by propa- oblique to the vein wall. Such shear veins are easily rec-
gation of a straight vein trough tension gashes (Fig. 6.16b). ognisable if markers are present but in many cases dis-
If flow in a shear zone is by simple shear, the instanta- placement on them is significant, and markers are miss-
neous shortening direction will lie at 45° to the shear zone ing. In such cases, they can be recognised by the lensoid
boundary (Fig. 2.6) and the tips of the tension gashes shape of the vein segments, commonly separated by faults
will lie in this direction (Fig. 6.16a, ×Video 6.16). If flow filled with microbreccia or cataclasite. Many shear veins
is between pure and simple shear (0 < W < 1; Sect. 2.5), can be recognised because they contain fibres or elon-
k
the angle between the tips and the shear zone boundary gate crystals that have grown at a small angle to the vein
will either be smaller or larger than 45°. Such structures wall: The fibres may consist of carbonate (Labaume et al.
are known as shortening- and stretching shear zones re- 1991; Cosgrove 1993) quartz, mica or even minerals such
spectively (Sect. 5.6.3). as sillimanite (Argles and Platt 1999) or tourmaline. These
If fibres or elongate grains are present in curved ten- structures are also known as slickenfibres in slicken-
sion gash veins, they may also have a complex shape that fibre veins. (Figs. 6.8, 6.9; Sect. 5.2.2).
can be used to determine shear sense (Fig. 6.17). The Slickenfibre veins commonly contain curved fibres paral-
pattern of curvature of the grains depends on the nature lel to the plane of the slickenside (Ramsay and Huber 1983).