Page 162 - Microtectonics

5.6 · Microscopic Shear Sense Indicators in Mylonite 151
5.6.12 by fracturing and precipitation of material from solu-
Potential Shear Sense Markers tion (Chap. 6). Such microveins have been studied in
deformed pseudotachylyte (Passchier 1986b) and meta-
A number of structures in mylonites are potential shear chert (Wallis 1992a).
sense markers, but they are either more cumbersome to use Veins can shorten, extend, or shorten and extend in
than the ones mentioned above, or the results are not con- sequence depending on vein orientation in the flow, and
sistent. This is probably mainly because their development on flow parameters such as vorticity and volume change.
is as yet incompletely understood. In time, these structures If sufficient viscosity contrast exists between veins and
may also be useful, but we advise not to rely exclusively matrix, veins can suffer boudinage on extension, or they
on one of them at present. The most common ones are: can fold on shortening. Figure 5.39 shows the distribu-
tion of domains of shortened (s), extended (e), first short-
Deformed Veins ened-then-extended (se) and first extended-then-short-
ened (es) material line categories for different types of
If a large number of deformed veins of different orien- progressive deformation (see also Sect. 2.7; Fig. 2.8). Since
tation are present in a deformed rock, they can be used veins do not start folding or enjoying boudinage at the on-
to obtain detailed information on the deformation set of deformation, and may unfold when being stretched,
process (Talbot 1970; Hutton 1982; Passchier 1990a; the boundaries of material line categories do not corre-
Fig. 5.39). This applies to veins in outcrop, but also to spond exactly to domains of folding (f), boudinage (b),
microveins that can be studied in thin section, formed folding-then-boudinage (fb) and boudinage-then-

Fig. 5.39.
Distribution of domains of de-
formed lines in space for four
categories of progressive defor-
mation. Note how the distribu-
tion of the domains is depend-
ent on the type of progressive
deformation; in non-coaxial
progressive deformation the
distribution has monoclinic
symmetry and can be used to
determine sense of shear. The
largest field of first shortened-
then-extended lines (se) indi-
cates the dominant rotation
direction of lines, and therefore
sense of shear (dextral). Only in
the case of polyphase deforma-
tion are domains of first exten-
sion-then-shortening possible