Page 194 - Microtectonics

6.4 · Fringes and the Deformation Path 183
6.4 genetic and tectonic strain to some extent. Even so, the 6.4
Fringes and the Deformation Path obtained finite strain is a minimum value, since fringes
may not have formed continuously during the entire
Fringe structures are informative and may be used deformation history.
to estimate sense of shear and finite strain (Sect. 9.2; Many fibrous veins and fringes have curving fibres
Durney and Ramsay 1973; Reks and Gray 1982, 1983; with a geometry that is more complex than that in
Ramsay and Huber 1983; Beutner and Diegel 1985; Gray Figs. 6.17 and 6.24. Such fibres are interpreted to form
and Willman 1991; Kirkwood et al. 1995; Kanagawa 1996; by a deformation history with changing flow parameters.
Köhn et al. 2003). If fibres in a fringe are straight, the The irregular curves in fibres of Figs. 6.3 and 6.28, for
long axis of the fringe is thought to represent the X-di- example, can be explained as changes in the orientation
rection of finite strain. If the total length of both fringes of ISA with respect to the developing fringe and the fo-
and the rigid inclusion is divided by the length of the liation in the wall rock, provided the fibres are displace-
inclusion, a minimum value for the principal stretch in ment-controlled. In fact, if fibres in fringes are tracking
the X-direction is obtained. With this method, only ex- they trace the displacement path (Box 2.1) of a particle
tension (stretch > 1) can be measured, not shortening. on the surface of the rigid core object with respect to the
It is therefore only possible to determine a strain value wall rock. The shape of fibres in a fringe therefore con-
in the case of plane strain and known volume change, tains information on the deformation path in the wall
or if stretch in the Y- and Z-directions are known by rock (Sect. 2.2; Durney and Ramsay 1973; Elliott 1973;
other means. Framboidal pyrites are assumed to have Wickham 1973; Gray and Durney 1979b; Casey et al. 1983;
grown during early diagenesis, and fringes around such Ramsay and Huber 1983; Beutner and Diegel 1985; Ellis
pyrites can therefore monitor more than just the tec- 1986; Gray and Willman 1991; Hedlund et al. 1994; Aerden
tonic strain (Durney and Ramsay 1973; Ramsay and 1996). Two models have been proposed to reconstruct
Huber 1983). Euhedral pyrite in mineralised zones may the deformation path from fibre shape in fringes, both
have grown after diagenesis. If both types of pyrite are of which assume that fibres track the extensional ISA;
present in a rock, it should be possible to separate dia- the most commonly used model (Durney and Ramsay

Fig. 6.28. Quartz fringe adjacent to a spherical pyrite framboid in chert. All fibres show a sharp bend, which is probably due to a change in the
instantaneous extension direction during progressive deformation. Leonora, Yilgarn Craton, Australia. Width of view 10 mm. CPL