Shear zones

Shear zones are microscopic to regional scale domains across which displacement has occurred. Brittle, brittle-ductile, or ductile deformation processes occur in shear zones at shallow, intermediate, or deep levels in the crust , respectively.

In brittle shear zones (equivalent to fault zones), displacement occurs on discrete fracture surfaces. In brittle-ductile shear zones, all or portions of the zone may undergo both ductile and brittle deformation. Displacement across brittle-ductile shear zones can be accommodated by oblique, en échelon stepping extensional veins (tension fractures) and/or shear fractures in addition to through-going shears parallel to zone boundaries. Extensional veins may be subsequently deformed into sigmoidal shapes (S shapes) and cut by younger veins. The sense of stepping of veins and their sigmoidal shape is used to determine the sense of displacement. Folds within transcurrent brittle-ductile shear zones (i.e., where displacement across steeply dipping shear zones is sub-horizontal) are en échelon stepping and doubly plunging. Fold axial surfaces initiated at approximately 45° to zone boundaries progressively rotate towards parallelism with the shear zone in areas of greatest deformation commonly overlying a crustal-scale structure.

Layers offset by ductile shear zones are thinned and progressively bent into parallelism with the zone. Grain size reduction in shear zones produces rocks called mylonites. The word mylonite is derived from the Greek word mulon or mulos for mill and the suffix –ite, for product of. Despite the origin of the word mylonite, only harder minerals such as feldspar are fractured and mechanically ground. Minerals such as quartz deform in a plastic manner instead, and are smeared out to form quartz ribbons. The term ultramylonite is used where grain size reduction has been extreme. The precursor rock type may be difficult to distinguish, e.g., ultramylonitic granite, felsic volcanic, and metasedimentary rock may all appear almost identical. Where the sense of offset of markers is not apparent, as is often the case in regional-scale ductile shear zones, the sense of displacement may be determined from observation of sections perpendicular to the foliation and parallel to the displacement direction (given by the orientation of mineral elongation lineations). This often requires microscopic, thin section examination. Shear criteria include:

• The relative orientations of flattening (S) and shear (C) foliations. S develops at about 45° to C, and is bent towards parallelism with C with increasing strain.
• Synthetic (C' or ecc1) shears at approximately 15–30° to C displace the shear foliation in mylonitic rocks with the same sense as the bulk shear sense. Antithetic shears (ecc2) with the opposite sense of displacement form at a large angle to the foliation.
• The asymmetry of foliations and strain (pressure) shadows around hard mineral grains or competent clasts. Low strain areas caused by ductile flow around rigid objects are sites where minerals (e.g., quartz, calcite, amphiboles, and biotite) crystallize. Mineral growths in strain shadows and surrounding foliations may be deformed when rigid objects rotate during shearing.
• Synthetic or antithetic slip on mineral cleavages. Slip and separation may occur in minerals such as pyroxene with pronounced mineral cleavage planes. Antithetic domino or bookshelf style slip occurs on planes inclined against the sense of shear at a large angle to the shear foliation. Cleavages inclined at small angles towards the sense of shear show synthetic slip.
• Mica fish (fish-shaped mica crystals whose extremities are asymmetrically bent into shear planes).
• X-ray, laser, or optical measurements of crystallographic axes provide shear criteria in quartz-rich mylonites. Folds develop within ductile shear zones where layers are inclined to shear zone boundaries or are adjacent to irregularities that perturb ductile flow. Folds in ductile shear zones generally initiate with axes at a large angle to the displacement direction and are overturned in a sense consistent with the sense of shear. Care must, however, be taken in using fold asymmetry to establish the sense of shear. Folds in overturned layers at high strain within shear zones, and folds formed by the back-rotation of layers between two shear zones, may be overturned in a sense opposite to the sense of shear displacement. Fold axes progressively rotate towards the shear direction with progressive deformation. Folds with curved axes and sheath-like folds may form at high strains, producing complex, refolded folds within a single deformation event.

See also Faults and fractures; Plate tectonics