Page 195 - Microtectonics

184 6 · Dilatation Sites – Veins, Strain Shadows, Fringes and Boudins
Fig. 6.29.
Object centre path determined for
a fringe structure from Lourdes,
France. (After Köhn et al. 2001b;
Courtesy Daniel Köhn)

6.5 1973; Wickham 1973; Hedlund et al. 1994) assumes that 6.5
fibres are rigid. The second model assumes that fringes Non-Fibrous Strain Shadows and Strain Caps
deformed passively and homogeneously (Ramsay and
Huber 1983; Ellis 1986). In both models, the fibres are Massive (non-fibrous) strain shadows are elongate domains
subdivided into small segments that represent incremen- on both sides of a core object in which the fabric is different
tal deformation steps, and progressive deformation is from that in the rock matrix (Figs. 5.20, 6.30a, 6.31). The
restored by repeated multiplication of deformation ten- boundary between the strain shadow and the matrix may
sors derived from the fibre segments (fibre trajectory be sharp, as for a fringe, but is more commonly gradual
analysis) The results of such deformation path analyses (Figs. 6.1, 6.30a, 6.31, 7.35, 7.41, 7.42). Strain shadows are
have been used in studies of folding (Wickham and commonly enriched in soluble minerals such as quartz, car-
Anthony 1977; Beutner and Diegel 1985; Beutner et al. bonate and chlorite, whereas foliation-forming minerals
1988; Fisher and Anastasio 1994; Hedlund et al. 1994), such as micas or chain-silicates are under-represented. If a
foliation development (Fisher 1990) and tectonic evolu- foliation is present in the matrix, it is usually more weakly
tion of deformed terrains (Fisher and Bryne 1990; Clark developed or absent in the strain shadow (Figs. 6.31, 7.32,
et al. 1993; Kirkwood et al. 1995). Unfortunately, these 7.41). The shape of massive strain shadows can be used as a
studies are not generally applicable because in most cases, tool to determine shear sense as in the case of fringe struc-
individual fibres will not open parallel to ISA and the tures (Figs. 6.24, 7.35). Obviously, more care is needed than
behaviour of single fibres is not representative for the in the case of fringes, especially if the core object is angular.
entire fringe structure (Spencer 1991). In foliated rocks, strain shadows and fringe structures
Aerden (1996) and Köhn et al. (2000) suggested a new are commonly associated with strain caps, domains en-
method to analyse undeformed fringes where irregulari- riched in micas or insoluble minerals, where the main
ties on the surface of the central object are fitted to cer- foliation is strongly developed (Figs. 6.30a, 6.31, 7.11, 7.24,
tain fibres by sliding the central object along them in an 7.39). Strain caps occur at opposite sides of the core ob-
analysis; this defines an object-centre path that the cen- ject, in the quarters oblique to the strain shadow or fringe.
tral object must have taken along the fringe to produce Some strain fringes may develop their own strain caps if
its present shape (Fig. 6.29). Rotation of the fringe rela- deformation is advanced, when they may start to behave
tive to the core object can be separately determined. The as independent core objects (Fig. 6.29).
object-centre path does not depend on behaviour of in- The term strain shadow may suggest that strain in the
dividual fibres, is unique for each fringe, and can poten- ‘shadow’ of the core object is relatively low, but this is not
tially be used to determine finite strain and possibly the generally correct. In fact, strain shadows and strain caps
deformation path in the wall rock (Köhn et al. 2003). are structures representing complex partitioning of strain
A new type of development is the possibility to use and volume change around a core object, the exact na-
quartz-fringes to determine strain rate in the host rock ture of which is usually unknown.
for dating purposes using Rb-Sr geochronology. Müller Massive strain shadows can be formed by non-fibrous
et al. (2000) report successful Rb-Sr micro-sampling and infilling of a void at the surface of a core object as for
dating in fringes. Fringes clearly contain a lot of infor- strain fringes, by recrystallisation of a strain fringe, or by
mation on deformation conditions and history, and have redistribution of mineral phases in response to inhomo-
the potential to become a powerful tool in the collection geneous deformation around a core object. This may
of geological data. happen at low fluid pressure by pressure solution at the