Abstract

A variety of host-fabric elements (HE) cut by cross-cutting elements (CE) in rocks defines flanking structures (FS) on mesoscopic and microscopic scales. There has been renewed interest in studying and classifying the FS for their morphologies, useful as shear sense indicators and geneses. Existing non-genetic morphologic parameters for the FS are reviewed, and two new classification schemes are presented. One of these is based on the nature of the CE and whether HE penetrates it. The other scheme takes account of all the potential combinations of drag/no drag and slip/no slip of the HE. Deciphering the shear sense of the rock body from FS is complicated because the angular relationship between the CE and the primary shear planes might be opposite to what is found between S- and C- ductile shear fabrics. Further, single CEs can curve and several similar FS occur in reverse forms. As with mineral fish, the shape asymmetries of microscopic CEs indicate the shear sense. Conjugate FS (with non-parallel CEs) with interfering perturbation fields around the CEs are more reliable shear-sense indicators than FS with single CE. During low but increasing bulk strains, FS may evolve from one type to another, e.g. from a- to s-type. At high strain, FS can resemble intrafolial or sheath fold. Whether the drag is normal or reverse depends fundamentally on the initial angle between the HE and the CE and the relative magnitudes of throw and vertical separation.

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