A subset of faults formed by shearing of joints and joint zones in Jurassic Aztec Sandstone exposed at Valley of Fire State Park, Nevada, was investigated by Flodin (2003) and Flodin et al. (2005). The first two stations (stations 1 and 2) were located within a fault zone with 25 m left-lateral slip (Figure 1a and Figure 1b) and the third station (sample 3) from a fault zone with 160 m left-lateral slip (Figure 1c).

Figure 1. Internal architecture of faults formed by sheared joints and joint zones mechanism in the Aztec Sandstone exposed at Valley of Fire State Park, southeastern Nevada. The first two are from two stations (a and b) on a fault with 25 meters predominantly left-lateral slip and the other station (c) is from a left-lateral fault with 160 meters slip. All three zones contain slivers of deformed rocks with highly variable grain size distribution. From Flodin (2003).

Grain size distribution from samples collected from various components of the fault zones (fragmented rocks and fault rocks) as well as the adjacent host rocks were measured using laser particle size analysis, which has lower and upper detection limits of 0.04 and 2000 um, respectively. As shown by the plots in Figure 2 (a, b, and c), the grain size distributions are bimodel and the fault with the greater slip magnitude shows greater communition.

Figure 2. Grain size distribution plots of host rock, fragmented rock, and fault rock samples collected from stations 1, 2, and 3. See Figure 1 for the locations and the discussion text for the data trends. From Flodin (2003).

Host rock samples adjacent to the faults possess lower median grain sizes (48-139 µm) than host rock samples collected away from the faults. Three host rock samples (41, 42, 44) from station 3 show a trend for smaller median values and decreased sorting with proximity to the fault core (Figure 2c). There is a trend of reduction in median grain size from host rock (between 160 and 332 µm) to fragmented rock to fault rock (between 3 and 51 µm). In most cases, fault rock grain sizes span a much narrower range (between 0.06 and 20 µm) with respect to host rock samples (0.06 and 840 µm). The authors noted a lower limit of grain size reduction for fault rock samples irrespective of average shear strain.

Data from studies concerning fragmentation and grain size reduction associated with other faulting mechanisms, or faults with unspecified mechanisms are provided at other places in this Knowledgebase. In a study of deformed crystalline rocks cored from active faults related to the San Andreas system of southern California, Blenkinsop (1991) provided a conceptual model for grain size reduction associated with dilatant fractures or cataclastic faults. Please see the link, 'Grain Size Distribution of Cataclasis in Dynamic Faulting.'