Slip surfaces refer to the smoothed, lineated, and grooved surfaces, with the linear marks called slickensides (Figure 1 and Figure 2). They are associated with frictional sliding and form either on the original wall rock or within the core.

Figure 1. Fault surface, from Key lms Quarry, Caryville, TN.

Figure 2. Photograph of a zone of deformation bands with slip surface on the left margin that offsets Entrada Sandstone beds by about 7 m. From Aydin and Johnson (1978).

In deformation band based faults, the slip surfaces normally develop on the boundary of the fault rock as shown in Figure 2. Since slip surfaces may have some opening along their lengths, they act as conduit to fluid flow. As a result of diagenetic processes, these openings are commonly filled by precipitations (Figure 3).

Figure 3. Photomicrograph, in plain polarized light, of partially filled slip band within a fault core. (A) filled with calcite and (B) filled with broken quartz grains and iron-rich mineral matrix. From Ahmadov (2006).

Figure 4 shows red-stained slip surfaces in the outcrop view as evidence of paleofluid flow activities. The filled slip surfaces are then called slip bands (Ahmadov, 2006; Ahmadov et al, 2007) due to their tabular appearance. After being partially or fully filled, their ability to carry fluid decreases. For detailed discussion of the permeability of individual slip bands and upscaled permeability of this type of faults, please see the links at the bottom of this page. For example, Ahmadov (2006) documented the geometry of slip surface and slip bands within Aztec Sandstone at Valley of Fire State Park, Nevada. The slip surfaces and slip bands are the most continuous elements within the fault zones. They are nearly planar in map view, with lengths ranging from several centimeters to several tens of meters and widths ranging from 0.1 to 1 mm. Slip surfaces and bands form normally as single features, but in some cases individual slip bands may merge into a zone (Figure 5). The density of the slip bands varies dramatically along a single fault, but can exceed 10 per 10 cm in both fault-parallel and fault-normal directions for well-developed zones.

Figure 4. Outcrop photo showing cross section view of stained slip surfaces in a fault core in Aztec sandstone at Valley of Fire State Park, Nevada.

Figure 5. Photograph of the sample from the fault zone in Aztec Sandstone. Sample is 20 cm long. From Ahmadov (2006).