Page 173 - Microtectonics

162 6 · Dilatation Sites – Veins, Strain Shadows, Fringes and Boudins

Box 6.1 Veins as a source of information on bulk deformation
The geometry of vein patterns is determined by the stress field in up of jogs in the bands. If fibres, elongate crystals or inclusion
a deforming rock and fluid pressure, but its interpretation is be- patterns are not straight, it is important to establish whether veins
yond the scope of this book. On a microscopic scale, fibres or elon- are unitaxial, antitaxial or syntaxial before interpreting the mo-
gate crystals can be used to determine the relative motion of vein tion history of vein walls. It is also important to establish if mo-
walls. As discussed above and shown in Fig. 6.15 crystals growing tion is in the plane of the thin section, or moves out of it; in the
in a vein will only track the opening direction if the vein has suit- latter case, more sections or even a full 3D study are necessary.
ably rough growth surfaces and will open in small increments. Once vein wall motion direction and history has been established,
Real fibres of a normally non-fibrous mineral such as calcite or a simple interpretation in terms of regional tectonics is only pos-
quartz are most reliable, since they can only form by tracking. sible for shear veins. Extension veins may themselves rotate with
Elongate crystals are more difficult to interpret, but if they are respect to bulk kinematic axes in non-coaxial flow, and even if
oblique to a grain wall, or have identical bends, they must be track- the relative motion of the wall rocks can be established, this car-
ing to some extent; elongate crystals at right angle to a vein sur- ries only indirect information on the far-field flow. In such cases,
face, however, are not indicative of straight opening; they can form microstructural information must be combined with field evi-
by tracking or non-tracking growth. In veins with elongate or even dence to reconstruct bulk deformation patterns. Fringe structures
blocky crystals, inclusion trails and ghost fibres are the most reli- are even more difficult to interpret, since except for coaxial flow
able tools to establish opening direction. If they are not present, histories, fringes will always rotate with respect to the kinematic
inclusion bands may indicate movement direction by the lining- frame and the central object as outlined in Sect. 6.3.

fracture (Fig. 6.8), fluid pressure may actually fall strain shadows contain much and detailed information
during crack opening because of the increasing volume on the deformation path, and are therefore some of the
in the jog or strain shadow. Material deposited in a vein most useful structures to reconstruct tectonic events in
or strain shadow can be transported towards the dila- thin section (e.g. Durney and Ramsay 1973; Ramsay and
tation site from outside in an open system along frac- Huber 1983; Etchecopar and Malavieille 1987; Hilgers and
ture networks, through the pore space, or along grain Urai 2002).
boundaries. This process is commonly referred to as In microtectonic analysis, veins and strain shadows
advection and may cause changes in the chemical and have a number of important applications:
isotope composition of a vein and its wall rock. Fluid
may even move along veins in dislocation-fashion, with- 1. Veins indicate the transport of material by fluids over
out permanent opening of large segments (Cosgrove some distance. Pressure solution may have been im-
1993; Oliver and Bons 2001) or be pumped in and out of portant, and the possibility of volume change must
a fracture network by occasional breaking and healing be considered.
of mineral “seals”, or by jog-opening (Vrolijk 1987; Cos- 2. Veins can be used to unravel polyphase deformation
grove 1993; Ohlmacher and Aydin 1997; Oliver and Bons by crosscutting relations, (Wallis 1992a); they can also
2001). Material deposited in veins and strain shadows give information on the earthquake cycle (Davison
can also be derived from the surrounding wall rock in 1995) and hydrocarbon migration (Parnell et al. 2000).
a closed system, e.g. by dissolution and precipitation of 3. The shape of veins and strain shadows and of the crys-
quartz or calcite. In this case, material can be transported tals in these structures can be used to determine
in a circulating fluid, or in a stationary fluid by diffusion shear sense and, in some cases, other deformation
(Oliver and Bons 2001). It has been suggested that in some parameters.
veins, pressure of the growing crystals on the wall rock 4. The composition of veins, strain shadows and fluid
in the case of super saturation may also open a crack inclusions in them allow assessment of the chemical
(“force of crystallisation”: Means and Li 2001; Wiltschko composition of fluids that accompany deformation
and Morse 2001). (Sect. 10.5).
Veins usually form in consolidated rocks, but there 5. Some veins and strain shadows can be dated (Sect. 9.8).
are indications that they may also form in unconsolidated
sediment (Fisher and Bryne 1990; Orange et al. 1993; Although field observations and thin sections are the
Vannucchi 2001). Criteria proposed to recognise such a main source of information on the development of veins
setting includes deformation of the wall rock during vein and strain shadows, it is now also possible to use two
growth, deformation of the wall rock by the tips of types of experimental modelling for the study of these
euhedral crystals in the vein and clouds of randomly ori- structures; analogue experiments (Means and Li 2001;
ented inclusions. Hilgers et al. 1997; Bons and Jessell 1997; Köhn et al. 2003;
In most cases, crack opening and filling is no unique Chap. 11) and numerical modelling (Bons 2001; Köhn
event, but occurs repeatedly or continuously during a et al. 2000, 2001a,b; Hilgers et al. 2001; ×Videos 6.23,
deformation phase. In such cases, the resulting veins or 6.27).