As mentioned under fault zone properties, the geostatistical distribution of fault rock thickness shows large variations. These properties are directly related to the mechanism of faulting, slip magnitude, lithology and fault zone architecture. Let's consider two mechanisms to illustrate this point. Figure 1 includes a series of idealized joint zone geometries and potential fault rock distributions for faults developed from shearing of these initial joint zones in sandstone following Flodin and Aydin (2004).

Figure 1. Conceptual model for faults formed by shearing of joint zones with various configurations and the resulting damage zone and fault rock distributions. From Flodin and Aydin (2004).

Figure 2 includes actual field data along a relatively large fault zone with 83 meters maximum slip formed by this mechanism in Aztec Sandstone cropping out over Valley of Fire State Park, Nevada. The maps and the measurements illustrate the fault geometry (a) and the distributions of fault rock along the fault zone architecture for a certain portion of the zone (b, c). The diagonal lines in (c) show the local thickness of fault rock with a scale bar of 20 m on the right hand side of the figure.

Figure 2. Slip distribution (b) along a relatively large fault in sandstone cropping out over Valley of Fire State Park with the corresponding fault rock thickness (c) and damage zone architecture (a). From Flodin (2000).

The plots in Figure 3 show linear (a) and log-log (b) plots of fault rock thickness from different faults with increasing slip magnitudes formed by the sheared-joint mechanism in the same rock type. The data suggest that the fault rock thickness shows a large variation for faults even formed by the same mechanism in a relatively homogeneous parent rock (Flodin, 2000). Furthermore, the fault rock along a given fault zone is highly heterogeneous (Flodin et al., 2005).

Figure 3. Linear (a) and log-log (b) plots of fault rock distribution for a number of faults with various maximum slip in sandstone, Valley of Fire State Park, Nevada. Although there is an increasing trend of thickness with increasing slip, there is a significant spread in this data set. From Flodin (2000).

Faults formed by shale smearing is another distinctive faulting mechanism which was covered in more detail somewhere else in this Knowledgebase. Here we note that the shale fault rock thickness in this type of faulting mechanism has a decreasing trend as slip increases (Figure 4) and eventually the shale smearing diminishes at some slip value (Younes and Aydin, 1997; 1998). Shale smear thickness is reported to depend on the original thickness of the shale beds, the magnitude of the slip, the ductility contrast between the shale and nonshale lithologies, and the rate of deformation. In the presence of multiple shale units, the percentage of the sum of all shale bed thicknesses is divided by the fault throw. Shale Gouge Ratio (SGR) is commonly used to measure shale fault rock thickness, which is defined as the percentage of shale or clay in the slip interval and is calculated at a point on a fault surface (Yielding et al, 1997; Yielding, 2002).

Figure 4. Shale smear thickness (normalized by the original formation thicknesses) decreases as a function of fault offset for two faults in the Gulf of Suez, Egypt. The original undisturbed thicknesses of the shale units were 35 and 50 meters. Data from Younes and Aydin 1997; 1998.

Shear band faults have cores in which parent sandstones have been deformed cataclastically. This topic has been described under 'Shear Bands.'