15.6: Geologic Structures: Faults

Geologic Structures: Faults

As rocks undergo brittle deformation Links to an external site., they may produce cracks in the rocks. If no appreciable displacement has occurred along these cracks, they are called joints Links to an external site.. If appreciable displacement does occur, they are referred to as faults Links to an external site..

Dip-Slip Faults

We will first examine dip-slip faults Links to an external site., in which movement along the fault is vertical, either up or down. The two masses of rock that are cut by a fault are termed the fault blocks Links to an external site.. The type of fault is determined by the direction that the fault blocks have moved. Fault block movement is described based on the movement of the hanging wall Links to an external site.: the fault block located above the fault plane (Figure 15.14). The other fault block, located beneath the fault plane, is called the foot wall Links to an external site. (Figure 15.14). These terms come from the idea that if a miner were climbing along the fault plane, they would hang their lantern above their head, along the hanging wall. Alternately, you can draw a stick figure straight up and down across the fault plane. Its head will be on the hanging wall and its feet will be on the foot wall.

A cross-section of a dip-slip fault. The head of the stick figure is on the hanging wall (mauve) and the feet of the stick figure are on the foot wall (blue). — **Figure 15.14:** A cross-section of a dip-slip fault. (CC-BY-SA 3.0, Randa Harris Links to an external site.; modified by Chloe Branciforte)

When extensional stresses are applied to the fault blocks, the hanging wall will move down, creating what is called a normal fault Links to an external site.. An easy way to remember this is the phrase “It’s normal to fall down”. Extensional stresses cause the crust to become stretched (elongated) and thinned out (Figure 15.15 and Table 15.4). These faults are common, but not exclusive to, rifts and divergent boundaries. Check out this USGS normal fault video Links to an external site. to visualize the motion further.

Block diagram of a normal fault, with the hanging wall block moving down due to extensional stresses. — **Figure 15.15:** A normal fault. (Public Domain, USGS Links to an external site.; modified by Chloe Branciforte)

Tensional forces acting over a region can produce normal faults that result in landforms known as horst and grabens. The graben Links to an external site. is the crustal block that down drops, and is surrounded by two horsts Links to an external site., the relatively uplifted crustal blocks (Figure 15.16). This terrain is typical of the Basin and Range of the western United States Links to an external site.. Check out this USGS horst and graben video Links to an external site. to visualize the motion further.

Block diagram of a normal fault, with the down block (graben) and up block (horst). — **Figure 15.16:** An area that has been stretched by tensional forces, resulting in numerous normal faults and horst and graben landforms. (Public Domain, Gregors Links to an external site.)

When compressional forces are applied to the fault blocks, the hanging wall will move up, creating a reverse fault Links to an external site. (Figure 15.17 and Table 15.4). This causes the crust to shorten in the area. A special type of reverse fault is a thrust fault Links to an external site., a low angle reverse fault (dip angle of less than 45°) that has a much thinner hanging wall. Check out this USGS thrust fault video Links to an external site. to visualize the motion of a reverse fault further. These faults are common at, but not exclusive to, convergent boundaries.

Block diagram of a reverse fault, with the hanging wall block moving up due to compressional stresses. — **Figure 15.17:** A reverse fault. (Public Domain, USGS Links to an external site.; modified by Chloe Branciforte)

Strike-Slip Faults

In a strike-slip fault Links to an external site., horizontal motion occurs, in the direction of strike (hence the name), with blocks on opposite sides of a fault sliding past each other due to shear forces (Figure 15.18 and Table 15.4). Movement is therefore either to the left or to the right.

Block diagram of a strike-slip fault, with fault blocks horizontally due to shear stresses. — **Figure 15.18:** A left-lateral strike-slip fault. (Public Domain, USGS Links to an external site.; modified by Chloe Branciforte)

The San Andreas Fault here in California is a classic example of a right-lateral strike-slip fault (Figure 15.19). To determine the relative sense of motion, an observer would stand along one side of the fault, looking across at the opposite fault block. If that fault block appears to have moved right, it is right-lateral; if it has moved left, it is left-lateral. Check out this USGS strike-slip fault video Links to an external site. to visualize the motion further.

Left, map of faults in California. Bold numbers show the average time between big earthquakes, determined at paleoseismic sites (triangles). Thick red lines show the extent of historic ruptures. Right, aerial image of the San Andreas Fault at the Carrizo Plain. — **Figure 15.19:** Left, map of faults in California. Right, aerial image of the San Andreas Fault at the Carrizo Plain. (Left, Public Domain; Karen Scharer Links to an external site./USGS Links to an external site.; Right, CC-BY 4.0, Ikluft Links to an external site.; modified by Chloe Branciforte)

**Table 15.4:** The fault types. (CC-BY 4.0, Chloe Branciforte)
Fault Type	Stress	Direction of Block Movement	Resulting Strain	Associated Plate Boundary
Normal	Tensional (extensional)	Vertical; Hanging Wall (HW) moves down	Stretching and thinning	Divergent and rifting
Reverse	Compressional	Vertical; HW moves up	Shortening and thickening	Convergent
Strike-slip	Shear	Horizontal; left or right	Tearing	Transform