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JGE 1986 - Volume 34


May

Determination of Movement Sense in Mylonites
Carol Simpson, Virginia Polytechnic Institute and State University

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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