Abstract
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 com positional layering, these grains must first be categorized as either α-type or δ-type porphyroclasts; thereafter they are extremely valuable indicators. Mica fish, which fall under the heading of α-type porphyroclasts, also allow the “fish-flash” outcrop method to be used, provided they are sufficiently large and numerous. Other sense-of-shear criteria require more care in their interpretation. Broken a...
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