Page 153 - Microtectonics

142 5 · Shear Zones
Experiments by ten Grotenhuis et al. (2002), Manck- lised or cataclastically torn-off grains along the upper
telow et al. (2002) and Ceriani et al. (2003) have shown and lower parts (Fig. 5.32), possibly accompanied by dis-
that once an elongate lozenge shape of mineral fish has solution an/or diffusive mass transfer (Fig. 5.28b) and
developed, and if the fish have poor bonding with the by slip on (001) in a synthetic sense, that could explain
matrix at least along their long sides, they may obtain a the complementary rounded parts of the lens shapes
stable position in any non-coaxial flow, even in simple (Fig. 5.28b). The shape of group 2 fish is thought to evolve
shear (Figs. B.5.1, B.5.5). The strong preferred orientation from group 1 by drag along zones of concentrated shear,
of mineral fish with respect to the mylonitic foliation can localised along the upper and lower contacts (Fig. 5.28b),
be explained in this way. Also, low bonding or a soft man- comparable to the development of σ-type mantled clasts
tle implies low differential stresses in the rim of the por- (Sect. 5.6.7.2). The shape of mica fish of group 3 can eas-
phyroclast and therefore reduced recrystallisation and ily be attained by slip on (001), starting from a position
little tendency to form a mantle (Kenkmann 2000). parallel to the foliation (Fig. 5.28b). Mica fish of group 4
Of all mineral fish, white mica fish are most common. are thought to have formed by antithetic slip on (001)
Their initial shape is probably due to a combination of from grains with an original high angle between inter-
the factors mentioned above. Figures 5.28 and 5.29 show nal cleavage and foliation (Fig. 5.28b). Alternatively they
a number of white mica fish shapes that were recognised could have developed from fish of groups 1 or 2 by fur-
by ten Grotenhuis et al. (2003). Each group is considered ther removal of the upper and lower parts. Group 5 mica
to result from a specific combination of mechanisms fish are explained as originated from short thick micas
(Fig. 5.28b). Micas of groups 1 and 2 may have attained in a similar way as group 4 ones, but with additional
their apparently stable inclined position by back rota- modification by removal along C'-type shear bands
tion from an original position approximately parallel to (Sect. 5.6.3), at a small synthetic angle with the foliation
the foliation to a new stable position. The typical lens (Fig. 5.28b). Finally group 6 micas may result from drag
shape can be explained by removal of small recrystal- folding along C'-type shear bands (Fig. 5.28b).

Fig. 5.28.
a The main types of white mica
fish recognised in thin section.
b Inferred development of the
different types of mica fish (af-
ter ten Grotenhuis et al. 2003)