The term splay joint refers to a type of joint formed by the shearing of an existing discontinuity. Splay joints (Figure 1) occur in the extensional quadrants with respect to the sense of shear across a fracture and are oriented at an angle to the sheared fracture called splay angle or kink angle. Splay joints are also referred to as tail cracks, wing cracks, and horse tail fractures. Horse tail fractures are used for multiple splay joints spaced closely at or near their initiation and fan out as the splays propagate away (Figure 2). In simple configurations, splay joints usually occur at the extensional quadrants of the two ends of sheared discontinuity. Figure 3 shows splay joints in granitic rock at a higher angle than those of the previous examples. Examples of splays forming at right-angle to the sheared fractures are also presented in this Knowledgebase. For additional images of splay joints and their kink angles, please see the links under 'Related Concepts.'

Figure 1. Photo (rotated by 90 degrees CCW for conserving space) of a vertical cliff face showing splay joints associated with a series of joints sheared in a normal sense in sandstone. The offset across each segment is on the order of a few millimeters.

Figure 2. Splay joints with a horse tail geometry are associated with a series of short right-lateral strike-slip faults (map view) in sandstone, Valley of Fire State Park, Nevada. The slip across the sheared fractures is about 2 centimeters. From de Joussineau and Aydin (2007).

Figure 3. Splay joints, opening fractures inclined at the sheared fracture at high-angle on a glaciated nearly horizontal surface in granodiorite, Donner Pass, Sierra Nevada. White half-arrow head on upper block shows sense of shearing and scale bar on the lower block is 10 cm long. See Aydin et al. (2002) for additional photographs and maps from the same area.

Splay joints may have a significant impact on the fluid flow properties of rocks as indicated by hydrothermal precipitations in them. They are then called 'veins' (Figure 4). A little bit of shearing across a sheared discontinuity may produce splays that reach adjacent fractures, thus significantly enhancing the connectivity of sub-parallel fractures. In mature fault zones, the associated damage zones are intensively broken up by splay joints and sheared splay joints. The density, the maximum length, and the kink or splay angle of these fractures in a damage zone will determine the fluid flow properties of the fault zones as well as their strength. For more discussion on this topic, please refer to sections Fracture Frequency in Fault Damage Zone, Scaling between Fault Length and Maximum Splay Length, and Splay (Kink) Angle.

Figure 4. A splay vein associated with a sheared vein in granodiorite. Note that the splay vein narrows down away from the sheared fracture and is located slightly behind the apparent tip of the sheared fracture. From Segall and Pollard (1983).

Splay fractures with exceptionally large size may also occur. Examples of these are shown in the map in Figure 5, which are interpreted to be associated with some strike-slip faults on the surface of Venus (Koenig and Aydin, 1998). There, the maximum length of the splay fractures appears to be on the order of several hundred kilometers.

Figure 5. Interpreted splay fractures (blue lines) associated with strike-slip faults (red lines) on the surface of Venus using the Magellan data. The maximum length of the splays is on the range of several hundred kilometers. Black lines on the map represent ridges which are not the focus of this presentation. From Koenig and Aydin (1998).

There is a sister structure closely related to a splay joint, called a joint branch structure (Figure 6). Splay joints are considered to be associated with primary shear fractures. On the other hand, a joint branch is due to the bifurcation of a primary fracture into two or more branches with similar kinematics. The latter is referred to as a dynamic fracturing. Crack branching is generally associated with high fracture propagation velocity.

Figure 6. Crack branching produced experimentally from notches on the top surface of glass plates subjected to tension. From Lawn and Wilshaw (1975) (credited to J. E. Field (1971) by the authors).