5 Deformation Bands
This section briefly introduces structures collectively called deformation bands, which represent localized deformations grouped into narrow bands (Rudnicki and Rice, 1975; Aydin et al., 2006). The image in Figure 11a shows one compaction band (the pen is for scale) and two shear bands, which can be identified by the offsets of the older bands, in the Aztec Sandstone at Valley of Fire State Park, Nevada, USA.
Figure 11 – a) A compaction band (marked by a pen and blue arrows) that is cut and offset by two shear bands (marked by red half arrows), showing that the compaction band is older. Aztec Sandstone, Valley of Fire State Park. b) A low-angle thrust fault (marked by shovel) in poorly consolidated terrace deposits near Arcata, northern California, USA. Based on the sense of fault motion, subhorizontal bands must have been dilated. Additional information about structures known as dilation bands is provided by du Bernard and others (2002).
Often, different types of deformation bands crosscut the same rock volumes (Figure 11a). In Figure 11a, converging blue arrows represent the constriction direction which is normal to the compaction band, whereas red half arrows mark upward-to-left direction of shearing across two subparallel shear bands. The compaction band is older than the shear bands. Generally, compaction bands are thicker than shear bands by as much as an order of magnitude. The annotated image in Figure 11b shows a shear band in the diagonal orientation with the thrust motion marked by red half arrows in young, poorly-consolidated, terrace deposits in Northern California, USA (Cashman and Cashman, 2000). The bands in the subhorizontal orientation are dilation bands (du Bernard et al., 2002) with the opening direction marked by diverging solid blue full arrows as inferred from the sense of shearing on the shear band. Mollema and Antonellini (1996) present similar configurations of compaction bands.
The striking differences in the geometry and orientation of the bands reported in Figure 11a and Figure 11b are strictly related to their formation mechanics. The two factors prominent in the formation of deformation bands are stress state (Figure 12a) and material properties (Figure 12b). The semi-parabolic curve in the p-q (mean stress p vs. shear stress q) diagram of Figure 12a represents the failure envelope limiting the admissible stress states and controlling the nature (arrow labeled “m”) and orientation (labeled “n”) of the failure structure. The notion is that the loading paths and incremental plastic deformation control the geometry and nature of the failure structures. Material properties of the granular medium (porosity, grain size, and degree of cementation and strength of grain contacts) control the micromechanics of deformation (Figure 12b) involving pore enlargement (dilation) or pore collapse (compaction), grain-to-grain sliding without appreciable volume change (isochoric shear) and grain fracture (cataclastic deformation with compaction). More information on this topic is provided by Aydin and others (2006) and Schultz (2019).
Figure 12 – a) A diagram, commonly referred to as p–q diagram, showing the failure envelope and range of mechanical behaviors as a function of mean stress p and shear stress q. b) A ternary diagram showing domains of micro-mechanisms with isochoric shear, dilation and compaction (Dippenaar, 2022, based on Aydin et al., 2006).