Faults formed by shearing of pre-existing joints (Figure 1) or veins take advantage of weak mechanical discontinuities provided by joints and veins. Considering that rocks have lower resistance to opening mode failure, joints (or veins) are one of the earliest formed structure types in Earth's crust and consequently they are often prone to later shearing. Thus, the sheared joint-based deformation mechanism is very common in Earth's upper crust.

Figure 1. (a) A set of sheared joints with apparent offsets on the order of a few millimeters in sandstone, Valley of Fire State Park, NV. Photograph by Shang Deng. (b) A set of sheared vertical set of joints with splays at the tips (a few pointed out by arrows) in Navajo Sandstone, Zion National Park, Utah.

Shearing of joints, or any plane of weakness for that matter, is commonly associated with splay fracturing. This subject has been presented under 'Splay Joints.' This mechanism results in an assemblage of a sheared joint and a splay joint (or vein). Please follow the link under 'Splay Joints' for the details of this phenomenon. In some cases, shearing of an initial discontinuity such as a joint or a vein, produces splay pressure solution seams as well as splay joints. This is discussed separately under 'Assemblages of Joints/Veins and Pressure Solution Seams.'

If splay fracturing associated with shearing of a set of joints (Figure 2a) results in a set of splay joints, then the system comprises a sheared joint set and a splay joint set with predominantly opening displacement discontinuity (Figure 2b). Figure 3 shows an example for such a system in thinly bedded siltstone cropping out at Chilean Patagonia (Gonzales and Aydin, 2008). Here the splay angle is nearly 45 degrees.

Figure 2. An initial set of joints (a) subsequently sheared producing a set of predominantly opening-mode splays (b).

Figure 3. A sheared joint set (nearly up-and-down the image) and diagonal splay fractures in siltstone, Chilean Patagonia. From Gonzales and Aydin (2008).

Figure 4 and Figure 5 show fracture networks similar to that in Figure 3 but with somewhat different intersection angles. Additional examples of such fracture networks can be found in multiple joint sets with non-orthogonal configurations which require shearing of the older sets. The readers should be aware that the traditional use of the term 'joints' in this context is because the sheared set initially formed in opening mode or the offsets are so small that a casual observer often neglects them. However, the implication of such discriminating observation is quite significant.

Figure 4. One dominant systematic set of joints which were subsequently sheared producing a highly dense group of splay joints in granodioritic rocks, Donner Pass, Nevada. There is another set of splays in near E-W orientation in the upper left part of the map. Note that some splays occur at high-angle to the sheared joints between narrowly spaced sheared joints. From Aydin (2002).

Figure 5. A sheared systematic set of joints trending from right to left and their splays at low-angle to the sheared set, Arches National Park, Utah. This superposition has been interpreted as being caused by the shift in axis of folding. From Dyer (1979).

The geometry of a sheared joint-splay joint assemblage is controlled by the spacing and degree of saturation of the initial joint set. In addition, splay, or kink, angles and their variations are critical. Figure 6 shows two different intersection angles between the splay joints and the sheared systematic joints in the Moab member of the Entrada Sandstone exposed at Arches National Park, Utah. The intersection angles vary from 0 to 90 degrees. Please see the links for 'Growth of Faults Based on Initial Plane of Weakness' as well as several related case studies.

Figure 6. Idealized diagrams showing high- and low-angle intersections between sheared joints and their splays. Simplified from Dyer (1983).