13.3 Fractures, Joints, and Faults

When rocks break in response to stress, the resulting break is called a fracture. If rocks on one side of the break shift relative to rocks on the other side, then the fracture is a fault. If there is no movement of one side relative to the other, and if there are many other fractures with the same orientation, then the fractures are called joints. Joints with a common orientation make up a joint set (Figure 13.19).

Figure 13.19 Joint sets have broken these siltstone and shale beds into long rectangular planks. Source: Michael C. Rygel (2008) CC BY-SA 3.0 view source

Jointing

Most joints form when the overall stress regime is one of tension (pulling apart) rather than compression. The tension can be from a rock contracting, such as during the cooling of volcanic rock (Figure 13.9, upper left). It can also be from a body of rock expanding. Exfoliation joints, which make the rock appear to be flaking off in sheets (Figure 13.20), occur when a body of rock expands in response to reduced pressure, such as when overlying rocks have been removed by erosion.

Figure 13.20 Half Dome at Yosemite National Park is an exposed granite batholith that displays exfoliation joints, causing sheets of rock to break off. Source: HylgeriaK (2010) CC BY-SA 3.0 view source

Nevertheless, it is possible for joints to develop where the overall regime is one of compression. Joints can develop where rocks are being folded, because the hinge zone of the fold is under tension as it stretches to accommodate the bending (Figure 13.21).

Figure 13.21 Joints developed in the hinge zone of folded rocks. Source: Steven Earle (2015) CC BY 4.0 view source

Joints can also develop in a rock a rock under compression as a way to accommodate the change in shape (Figure 13.22).  The joints accommodate the larger compression stress  (larger red arrows) by allowing the rock to stretch in the up-down direction (along the green arrows).

Figure 13.22 Joints developing to accommodate the larger horizontal component of compression (large red arrows). Source: Steven Earle CC BY 4.0 view source


Faulting

A fault is a boundary between two bodies of rock along which there has been relative motion (e.g., Figure 13.23). Some large faults, like the San Andreas fault in California or the Tintina fault, extending from northern British Columbia through central Yukon and into Alaska, show evidence of hundreds of kilometres of motion. Other faults show only centimetres of movement. In order to estimate the amount of motion on a fault, it is necessary to find a feature that shows up on both sides of the fault, and has been offset by the fault. This could be the edge of a bed or dike as in Figure 13.23, or it could be a landscape feature, such as a fence or a stream.

Figure 13.23 View looking down on a fault (white dashed line) in intrusive rocks on Quadra Island, British Columbia. The pink dyke has been offset approximately 10 cm by the fault (length of the white arrow). Source: Steven Earle (2015) CC BY 4.0 view source

Types of Faults

Different kinds of faults develop under different stress conditions. We describe faults in terms of how the rocks on one side of the fault move relative to the other.

Dip-Slip Faults

Dip-slip faults are so named because the dominant motion involves moving up or down the dipping (tilting) fault plane. In dip-slip faults we identify rock above the fault as the hanging wall, (or headwall) and the rock beneath as the footwall. These terms were originally used by miners to describe the rocks above and below an ore body (Figure 13.24).

Figure 13.24 The hanging wall (or headwall) of a fault is the rock above the fault. The footwall is the rock below. These terms were originally used by miners to describe the rocks above and below an ore body. Source: Photo- Gold Hill Mine, Yukon Territory, by Eric A. Hegg (1898) Public Domain view source. Diagram- Karla Panchuk (2018) CC BY 4.0

Tension produces normal faults, in which the crust undergoes extension. This permits the hanging wall to slide down the footwall in response to gravity (Figure 13.25, left). Compression produces reverse faults, pushing the hanging wall up relative to the footwall. Reverse faults shorten and thicken the crust (Figure 13.25, right).

Strike-Slip Faults

Faults where the motion is mostly horizontal and along the “strike” or the length of the fault are called strike-slip faults (Figure 13.26 bottom). These happen where shear stress causes bodies of rock to slide sideways with respect to each other, as is the case along a transform boundary. If the far side moves to the right, as in Figures 13.23 and 13.26 (right), it is a right-handed, right-lateral,or dextral strike-slip fault. If the far side moves to the left it is a left-handed, left-lateral, or sinistral strike-slip fault.

Figure 13.25 Dip slip faults. Normal faults are caused by tension, while reverse faults happen during compression. Source: Karla Panchuk (2018) CC BY-SA 4.0, modifed after Woudloper (2010) CC BY-SA 3.0 view source
Figure 13.26 Strike-slip faults. Rocks on either side of the fault move parallel to the fault. In dextral strike-slip faults the far side moves to the right of the observer. In sinistral strike-slip faults the far side moves to the left of the observer. Source: Karla Panchuk (2018) CC BY 4.0

Different Tectonic Settings Have Distinct Types of Faults

Horst and Graben Structure

In areas that are characterized by extensional tectonics, and with many normal faults arranged side-by-side, some blocks may subside (settle downward) relative to neighbouring parts. This is typical in areas of continental rifting, such as the Great Rift Valley of East Africa or in parts of Iceland. In such situations, blocks that move down relative to the other blocks are graben, and elevated blocks with graben on either side are called horsts. There are many horsts and grabens in the Basin and Range area of the western United States, especially in Nevada. Part of the Fraser Valley region of British Columbia, in the area around Sumas Prairie, is a graben.

Figure 12.14  Depiction of graben and horst structures that form in extensional situations.  All of the faults are normal faults.  [SE]
Figure 13.27  Graben and horst structures form where extension is happening. All of the faults are normal faults. Source: Steven Earle (2015) CC BY 4.0 view source

Thrust Faults

Thrust faults are a type of reverse fault with a very low-angle fault plane. The fault planes of thrust faults typically slope at less than 30°. Thrust faults are relatively common in mountain belts that were created by continent-continent collisions. Some represent tens of kilometres of thrusting, where thick sheets of sedimentary rock have been pushed up and over other layers of rock (Figure 13.28).

Figure 12.15 Depiction a thrust fault.  Top: prior to faulting.  Bottom: after significant fault offset. [SE]
Figure 13.28 A thrust fault. Top: prior to faulting. Bottom: after significant fault offset. Source: Steven Earle (2015) CC BY 4.0 view source

There are numerous thrust faults in the Rocky Mountains, and a well-known example is the McConnell Thrust, along which a sequence of sedimentary rocks about 800 m thick has been pushed for about 40 km from west to east over underlying rock (Figure 13.29). The thrusted rocks range in age from Cambrian to Cretaceous, so in the area around Mt. Yamnuska Cambrian-aged rock (around 500 Ma) has been thrust over, and now lies on top of Cretaceous-aged rock (around 75 Ma) (Figure 13.30).

Figure 12.16  Depiction of the McConnell Thrust in the eastern part of the Rockies.  The rock within the faded area has been eroded. [SE]
Figure 13.29  The McConnell Thrust in the eastern part of the Rockies. The rock within the faded area has been eroded. Source: Steven Earle (2015) CC BY 4.0 view source
Figure 13.30 The McConnell Thrust at Mt. Yamnuska near Exshaw, Alberta. Cambrian limestones have been thrust over top of Cretaceous mudstone. Source: Steven Earle (2015) CC BY 4.0 view source

Exercise: Fault Types

 

Figure 13.31 A dip-slip fault. Source: Steven Earle (2015) CC BY 4.0 view source

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