Page 160 - Microtectonics

5.6 · Microscopic Shear Sense Indicators in Mylonite 149

Fig. 5.36. Truncated sillimanite fish in ultramylonite. In the matrix fine-grained ilmenite outlines an oblique foliation inclined to the mylonitic
foliation. The inclusion in the sillimanite is apatite. Dextral shear sense. Monte Mary, Italy. PPL. (Photograph courtesy Giorgio Pennacchioni)

porphyroclast lies between two large rigid objects, its
asymmetry may reflect the relative movement of these
two objects rather than the bulk shear sense in the shear
zone. Therefore, mantled clasts are most reliable as shear
sense indicators where they occur isolated in a fine-
grained mylonitic matrix, e.g. in ultramylonite.
Sigmoids have the same shape and orientation as
σ-type mantled clasts and, although their development
is not completely understood, their shape can be used to
determine shear sense in the same way (Fig. 5.20).
The geometry of porphyroclasts with pressure shad-
ows and fringes is formed in a different way as mantled
porphyroclasts, and both structures should not be con-
fused. Mantled clasts occur exclusively in mylonites, while
strain shadows can also be found in low strain rocks. Their
use as shear sense indicators is explained in Chap. 6. Fig. 5.37. Schematic diagram showing the geometry of normal min-
All mineral fish are good shear sense indicators, ir- eral fish and truncated fish
respective of their mechanisms of development. The
inclination of the long axis of fish with respect to the can be used as a shear sense indicator as in the case of
foliation and a commonly developed monoclinic shape mantled clasts. An exception are mica fish of group 6
symmetry, with one curved and one planar side, can (Fig. 5.28), which are localised on shear bands, and
be used as shear sense indicators (Figs. 5.20, 5.28, 5.30, truncated sillimanite fish (Fig. 5.37; Type 2 of Manckte-
5.31). If trains of small fragments trail away from the low et al. 2002) which have a different geometry from
tips of the fish, these will show stair stepping which other types.