6.3 Shear Band Zones‌

The photographs in Figure 16a and Figure 16b show zones of shear deformation bands, often referred to as zones of shear bands. The slip on the small zone in Figure 16a is about 5 mm, whereas it is about 3 to 4 m across the zone in Figure 16b. The exposed portion of the scale in Figure 16b is about 75 cm. Similar to the growth of compaction band zones illustrated earlier, shear band zones also grow by sequential addition of individual shear bands, one adjacent to the other.

Figure 16 – Shear band zones.a) A small zone with a few shear bands offsetting an older band for about 0.5 cm. b) A larger shear band zone with a normal sense of offset of about 3.5 m. The exposed portion of the scale in Figure 16b is about 75 cm. From Aydin and Johnson (1978).

The plot in Figure 17 shows how the permeability of shear band zones (simply referred to as deformation bands) may depend on the number of bands as well as on the permeability of individual bands (Shipton et al., 2002). Overall, the value of permeability across shear band zones decreases as the number of bands increases.

Figure 17 – A plot showing how the number of bands and band permeability (in millidarcy, mD) may affect the permeability of shear band zones. From Shipton and others (2002).

More realistic values of a deformation band zone permeability can be estimated by using actual single band permeability. The compilation of data shown in Table 1 includes porosity and permeability of compaction bands in various orientations to the cross-bedding and shear bands. Permeability was obtained by use of a field or lab mini-permeameter.

Table 1 – Permeability (in millidarcy) and porosity of deformation bands and their host rocks. Compiled by Deng and others (2012) from the sources listed in the first column.