Geoscience Reference
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consistent should you infer a defi nite, non-coaxial shear
sense.
In zones of strong non-coaxial (simple) shear strain, one shear
sense dominates most, if not all, kinematic indicators. Some rocks
may have been subjected to more than one episode of shearing,
however, commonly under different conditions (pressure,
temperature, fl uid, etc.). Later shear fabrics and kinematic
indicators may have partially overprinted earlier ones, so it is
important to note which features you have used to deduce which
sense of shear, as well as their orientations. Figure 8.20 provides
some examples of common ductile shear sense indicators.
8
(
(
(
)
(
Figure 8.20
Ductile kinematic indicators, all indicating dextral ('top-to-the-right') shear sense. (a) Asymmetric
tails on feldspar porphyroclasts in a mylonite, northwest India. (b) Composite sketch depicting various features
used for determining shear sense (width
c
. 40 cm). (c) S-C fabric (shear band cleavage) in a mica schist,
Switzerland. Camera case near the base is 25 cm across. (d) Asymmetric pressure shadows on a boudin in a
gneiss, northwest India. (a, c and d: Tom W. Argles, The Open University, UK.)
