Page 159 - Microtectonics

148 5 · Shear Zones
Fig. 5.34.
EBS-image of a tourmaline fish
from Lambari, Brazil, showing
zoning in the centre and new
growth of tourmaline at the tips
of the fish

Fig. 5.35. SEM image of quartz fish in marble from Thasos. Width of view 0.8 mm. (Courtesy M. Bestmann)

5.6.8 Equidimensional naked or Θ-type clasts cannot be used
Porphyroclast Systems as Shear Sense Indicators as shear sense indicators. σ -type-, σ -type-, δ-type- and
a
b
complex mantled clasts can be used as shear sense indica-
Summarising our present understanding of the devel- tors using their internal asymmetry, and the stair-stepping
opment of porphyroclasts and related structures, the fol- of the wings. Stair stepping is a useful shear sense indi-
lowing applies. If cohesion between clast and matrix is cator; the wings step up in the direction of movement of
perfect, naked or mantled clasts and sigmoidal systems the upper block (Figs. 5.21, 5.22–5.25). However, care
form; if cohesion is reduced, mineral fish form, or strain should be taken in inhomogeneous mylonite with many
shadows if voids can form alongside the clast (Fig. 5.20). large porphyroclasts or low strain lenses; if a mantled