4 Shear Fractures or Faults‌

Shear fractures or faults may be identified by the presence of offset marker horizons across the fracture (Figure 9a), which were continuous before the shearing motion. These offsets or displacements indicate relative movement of the fracture walls predominantly parallel to the fault planes. In layered rocks, faults cut and displace marker horizons as shown in the sketch. Shearing across fractures can also be identified by the presence of the so-called splay fractures as shown in Figure 9b. In this case, the motion indicated by half arrows occurred along the apparent dip direction of the discontinuity.

Figure 9 – a) Schematic field diagram illustrating that two marker beds (gray) are offset a few centimeters by small faults. The faults have no free ends meaning that they are vertically limited by interfaces between consecutive beds. b) Shearing of a series of discontinuities along the apparent dip direction marked by half arrows and the associated splays at the extensional quadrants above their free ends. Splay fractures can be used to infer shearing on the inclined fractures and to determine the direction of slip. Aztec Sandstone exposed at Valley of Fire State Park, Nevada, USA.

Faults have characteristic surface morphology, such as nearly planar surfaces with polished appearance (Figure 10a) and striations and grooves (Figure 10b), which are a series of lineaments on fault surfaces formed in the direction of slip. These elements are typically referred to as slickensides in structural geology.

Figure 10 – a) Fault zone with a well-developed planar slip surface offsetting flat beds by about 8 m. b) Grooves and striations on a polished slip surface. Entrada Sandstone near Goblin Valley, Utah, USA. From Aydin and Johnson (1978).

Splay fractures (Figure 9b) are also known as tail cracks, wing cracks and horsetail fractures. These features can be used to identify faults and the sense of shear across them. Splay fractures are oriented at an angle to faults. This angle is usually dihedral and is called splay angle or kink angle. Box 1 provides additional information about splay fractures associated with strike-slip faults and their properties. Interestingly, contrary to a wide-spread misconception of shear fractures as conduits for flow, it is primarily the splay fractures and related structures known as pull-aparts that aid fluid flow rather than the shear fractures/faults or slip surfaces themselves.

Generally, offsets of horizons such as the gray-shaded units in Figure 9a do not reveal the mechanism of faulting and the exact orientation of kinematics vectors (fault slip direction). This information can be obtained by investigating nearly planar fault surfaces with a polished appearance (Figure 10a) and/or with striations and grooves (Figure 10b). Of course, three-dimensional (3D) exposures of single faults and offset horizons (Figure 10a) allow the precise assessment of fault slip direction and magnitude. The small faults shown in Figure 9a have no free ends, rather they abut against interfaces and do not stop in the middle of a bed. This lack of free ends is relevant to fluid flow in two ways.

1. The vertical hydraulic connectivity provided by faults that lack free ends is limited to the beds they crosscut. In the case of Figure 9a, it means that the faults do not create direct hydraulic communication between the yellow bed where they stop and the bed directly above or below their tip location. Specifically, in Figure 9a:
   • the near-vertical fault in the center of the diagram provides communication between all three yellow layers;
   • the lower left fault provides communication between the lower two yellow layers; and,
   • the upper right fault provides communication between the upper two yellow layers.
2. The efficiency of fluid flow in settings where faults lack free ends depends not only on the permeability of the faults themselves but also on the geometry (continuity) and permeability of the interbed contacts (i.e., the horizontal lines between the yellow and grey beds in Figure 9a). For example, if the grey beds of Figure 9a are permeable, then flow in the near-vertical faults may enter those beds and continue horizontally along and/or vertically through the beds to eventually seep into:
   • the adjacent yellow beds; and/or,
   • into other faults which will in turn provide new pathways.