Page 185 - Microtectonics

174 6 · Dilatation Sites – Veins, Strain Shadows, Fringes and Boudins
As for curved fibres in extension veins, this can be due to grade deformed metasedimentary rocks, especially in
tracking behaviour of the fibres when slip direction on a metaturbidites (Fig. 6.9; Ramsay and Huber 1983, 1987;
fault changes. As in all veins, fibres or elongate grains in Cox 1987; Gaviglio 1986; Fitches et al. 1986; Mawer 1987;
shear veins are not necessarily tracking; they may con- Mitra 1987; Tanner 1989, 1992a; Henderson et al. 1990;
tain fibres or elongate grains normal to the vein wall or Labaume et al. 1991; Cosgrove 1993; Jessell et al. 1994;
even equidimensional crystals. This may lead to errone- Fowler 1996; Fowler and Winsor 1997; Ohlmacher and
ous interpretation of such veins as extensional veins. Aydin 1997; Köhn and Passchier 2000; Passchier et al.
Wide continuous shear veins commonly show inter- 2002). They can be up to 30 cm thick and several hun-
nal striping or lamination formed by inclusion trails that dreds of metres long, are commonly composed of quartz
consist of isolated fragments of the wall rock, breccia and chlorite, and normally contain three structural ele-
zones or slickolites. The inclusion trails in such striped ments of different orientation: inclusion bands, inclusion
shear veins (Fig. 6.18, ×Video 6.18a,b,c) connect with steps trails, and boundaries of elongate crystals. Most veins
in the wall rock that may not be conspicuous, especially lack a clear median line. Inclusion bands mimic the shape
if outcrops or angles between steps and other wall rock of the wall rock while inclusion trails connect jogs in the
segments are small. In many cases, these inclusion trails inclusion bands and corresponding points in the wall
are anastomosing and form a linear fabric. In the field rocks that were originally joined. Inclusion trails are com-
such anastomosing trails are easily confused with fibres. monly parallel to jogs in the vein-wall rock contact that
Besides inclusion trails, less conspicuous inclusion bands may nucleate on older structures such as folds (de Roo
composed of solid and fluid inclusions are common in and Weber 1992). Inclusion trails are usually thin breccia
shear veins. These tend to be irregular in shape but par- zones of wall rock and vein material, or slickensides
allel to each other and mimic the shape of the wall rock (Stanley 1990; de Roo and Weber 1992). The planar and
between inclusion trails (Fig. 6.18b, ×Video 6.18b,c). linear striping in the veins is in some cases due to pre-
They form by crack-seal or pulsating growth of material ferred orientation of crystals or fibres, but more gener-
in the vein. Shear veins are commonly linked to exten- ally to parallel inclusion trails. In such cases, the veins
sion veins highly oblique to the shear vein. are composed of elongate or stubby quartz crystals that
Shear veins may occur in non-layered or fractured lie oblique to inclusion bands and trails, and to the edge
rock, but are quite common parallel to layering in sedi- of the veins (Fig. 6.18b, ×Video 6.18b,c; de Roo and We-
mentary rocks with a fine planar and/or linear striping ber 1992; Köhn and Passchier 2000). Since elongate grains
parallel or subparallel to the edge of the vein. Such bed- commonly nucleate on the vein boundary jogs, they tend
ding veins (de Roo and Weber 1992) are common in low- to be normal to the inclusion bands.
Fig. 6.18.
a Development of a shear vein
from a fault with jogs. After
small displacement, isolated
fibre packages lie along the fault,
typical of slickenfibres. After
larger displacement the fibre
packages overlap and become
separated by inclusion trails.
This produces the characteristic
banded nature of striped shear
veins and bedding veins. b Detail
of the internal structure of a
typical striped shear vein. In-
clusion bands, inclusion trails
and boundaries of elongate crys-
tals all have different orienta-
tion. Grain growth in this vein
is unitaxial and occurs at top
right, as shown by widening of
a decreasing number of crystals
in that direction. Inclusion bands
change shape when wall rock
fragments become detached.
Grain boundaries commonly
terminate or jog along inclusion
trails