The deep-seated equivalents of brittle faults are ductile-shear zones and mylonite zones but
these often do not contain any unequivocal offset marker layers for the determination of
relative movement sense. Mylonite samples can be used to determine sense of shear provided they
are observed on planes cut parallel to the mineral elongation lineation and perpendicular to
the mylonitic compositional banding.
Shear-band foliations in single sets, and S-C mylonites are the most reliable shear-sense
indicators. Where porphyroclasts have dynamically recrystallized tails that are asymmetric with
respect to the compositional layering, these grains must first be categorized as either σ-type or δ–type porphyroclasts; thereafter they are extremely valuable
indicators. Mica fish which fall under the heading of σ–type porphyroclasts also
allow the "fish-flast" outcrop method to be used, provided they are sufficiently
large and numerous.
Other sense-of-shear criteria require more care in their interpretation. Broken and displaced
grains may show offset that is either sympathetic or antithetic to the overall shear sense,
depending upon (a) the initial orientation of the fractures to the flow plane, and (b) how much
rotation the fragmented grains have undergone. Grain-shape and lattice preferred orientation
paterns may only reflect the latest increment of deformation history, but are extremely
reliable movement-sense indicators for this latest phase. Microfolds are potentially useful
provided that they fold the mylonitic layering due to minor local perturbations in the flow
regime. As many as possible of the above criteria should be used and a test made for internal
consistency. Where all available evidence is in agreement, the sense of movement can be
predicted with a high degree of confidence.
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