When rocks break in response to stress, the result is fracturing. 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.
Most joints form when the overall stress regime is one of tension (pulling apart) rather than compression. This can happen when a body of rock is expanding because of reduced pressure, such as when overlying rocks have been removed by erosion (Figure 13.11). It can also happen if the rock is contracting, such as during the cooling of volcanic rock (Figure 13.5a).
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 in tension as it stretches to accommodate the bending (Figure 13.12).
If there is differential stress on a rock, joints can also develop at angles to the direction of compression (Figure 13.13). You could think of these joints as accommodating the compression (red arrows) by allowing the rock to stretch in the up-down direction (along the green arrows).
A fault is boundary between two bodies of rock along which there has been relative motion (Figure 13.14). Some large faults, like the San Andreas Fault in California or the Tintina Fault, which extends from northern B.C. through central Yukon and into Alaska, show evidence of hundreds of kilometres of motion. Other faults show only centimetres. In order to estimate the amount of motion on a fault, we need 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 13.14, or it could be a landscape feature, such as a fence or a stream.
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 (Figure 13.15).
Dip-slip faults are so named because a large part of the 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, and the rock beneath as the footwall. If the fault develops in a situation of compression, then it will be a reverse fault because the compression causes the hanging wall to be pushed up relative to the footwall (Figure 13.15, upper left). This results in a shortening of the Earth’s crust. If the fault develops in a situation of tension, then it will be a normal fault, because the extension allows the hanging wall to slide down relative to the footwall in response to gravity (Figure 13.15, upper right).
Faults where the motion is mostly horizontal and along the “strike” or the length of the fault are called strike-slip faults (Figure 13.15 bottom). The happen where the bodies of rock are sliding 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 13.14 and 13.15, it is a right lateral or dextral strike-slip fault. If the far side moves to the left it is a left lateral or sinistral strike-slip fault.
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, it is not uncommon for a part of the upper crust to subside (settle downward) with respect to neighbouring parts. This is typical along areas of continental rifting, such as the Great Rift Valley of East Africa or in parts of Iceland. In such situations a down-dropped block is known as a graben, while an adjacent block that doesn’t subside is called a horst (Figure 13.16). 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 B.C., in the area around Sumas Prairie is a graben.
A special type of reverse fault, with a very low-angle fault plane, is known as a thrust fault. Thrust faults are relatively common in areas where fold-belt mountains have been created during continent-continent collision. Some represent tens of kilometres of thrusting, where thick sheets of sedimentary rock have been pushed up and over other layers of rock (Figure 13.17).
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 (Figure 13.18). 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 13.19).
Exercise 13.2 Types of Faults
Below are four faults that formed in different tectonic settings. For each fault indicate whether it is normal or reversed, and whether it was formed under compression or tension.