Shear band zones occur in various thicknesses which are controlled primarily by the number of bands within the zones as marked in Figure 1(a) and Figure 1(b). Figure 2 and the inset shows that individual bands are sequentially added. The bands generally have straight traces in sections which include the slip vector, while they are curvilinear and inosculating in sections at high angles to the slip vectors. Shear band zones are known as Riedel Shears with the opposite sense of shearing with reference to the experimental work of Riedel (1929). See 'Multiple Sets of Shear Band Zones.' It is also possible that the splays may be dominated by volumetric strain localization as shown under 'Splay Volumetric Bands.'

Figure 1. Shear bands in sandstone, (a) Single bands, two bands, and three or more bands in two sets. (b) A zone of shear bands with a complex internal architecture. The zone offsets an older single band by about half a cm as pointed out by the arrow. From Aydin (2006).

Figure 2. Shear band zones in the Entrada sandstone. (a) A thick shear band zone made up of numerous sub-parallel bands formed by sequential addition, (b) a zone which includes both sub-parallel bands as well as bands at an angle to the overall orientation of the zone. From Aydin and Johnson (1978).

Shear band zones remarkably similar to those observed in the field have also been produced in the laboratory (see Figure 3) by Muir et al. (2002). These zones formed by sequential addition of shear bands thereby supporting the mechanism proposed earlier based on the field data.

Figure 3. Photographs showing shear band zones made up of several discrete bands on radial cross-sections of two samples of a sandstone deformed to (a) 6% and (b) 9% axial strain in the laboratory. From Mair et al. (2000).