8.5: Faults

Figure \(\PageIndex{1}\): Common terms used for normal faults. Normal faults form when the hanging wall moves down relative to the footwall.

Faults are the places in the crust where brittle deformation occurs as two blocks of rocks move relative to one another. This breakage happens when stress is placed on cold rock such that it breaks instead of bends. These ruptures often occur at plate boundaries but can also occur in plate interiors as well.

The surface where the slip occurs is called the fault plane. The fault scarp is the offset of the surface produced where the fault breaks through to the surface (see figure above).

Normal and reverse faults, discussed in more detail below, display vertical, also known as dip-slip, motion. Dip-slip motion consists of relative up-and-down movement along a dipping fault between two blocks, the hanging wall, and footwall. In a dip-slip system, the footwall is below the fault plane and the hanging wall is above the fault plane. A good way to remember this is to imagine standing on the fault plane; the hanging wall would be hanging above you and your feet would be on the footwall.

A joint or fracture is a plane of brittle deformation in the rock created by the movement that is not offset. Joints can result from many processes, such as cooling, depressurizing, or folding.

Normal Faults

Normal faults move by a vertical motion where the hanging-wall moves downward relative to the footwall along the dip of the fault. Normal faults are created by tensional forces in the crust. Normal faults and tensional forces commonly occur at divergent plate boundaries, where the crust is being stretched by tensional stresses.

Figure \(\PageIndex{1}\): Example of a normal fault in an outcrop of the Pennsylvanian Honaker Trail Formation near Moab, Utah. Notice the thin gray bed on the left side of the fault is higher than it is on the right side of the fault. This is because the fault block on the right (hanging wall) has slid down.

Figure \(\PageIndex{1}\): Faulting that occurs in the crust under tensional stress.

Grabens, horsts, and half-grabens are blocks of crust or rock bounded by normal faults. Grabens drop down relative to adjacent blocks and create valleys. Horsts are left over and become areas of higher topography. Where occurring together, horsts and grabens create a symmetrical pattern of valleys surrounded by normal faults on both sides and mountains. Half-grabens are a one-sided version of a horst and graben, where blocks are tilted by a normal fault on one side, creating an asymmetrical valley-mountain arrangement. The mountain-valleys of the Basin and Range Province of Western Utah and Nevada consist of a series of full and half-grabens from the Salt Lake Valley to the Sierra Nevada Mountains.

Reverse Faults

Figure \(\PageIndex{1}\): Simplified block diagram of a reverse fault.

In reverse faults, compressional forces cause the hanging wall to move up relative to the footwall. A thrust fault is a reverse fault where the fault plane has a low dip angle of less than 45°. Thrust faults carry older rocks on top of younger rocks as you can see in the diagram below where the older gray rock appears on top of the younger white rock.

Figure \(\PageIndex{1}\): Terminology of thrust faults (low-angle reverse faults). A klippe is the remnant of the hangingwall (aka nappe), where the surrounding material has been eroded away. A window is where part of the hangingwall has been eroded away to expose the footwall (autochthon). Note the symbol shows flags on the overlying thrust plate.

Figure \(\PageIndex{1}\): Ketobe Knob in the San Rafael Swell of Utah displays an example of a thrust fault. (CC BY-NC-SA 4.0 International; Ron Schott via Flickr

Strike-slip Faults

Strike-slip faults have side-to-side motion. Strike-slip faults are most commonly associated with transform plate boundaries. In pure strike-slip motion, fault blocks on either side of the fault do not move up or down relative to each other, rather move laterally, or side to side.

Bends along strike-slip faults create areas of compression or tension between the sliding blocks. Releasing bends are created by tensional stresses which create features like normal faults and basins, such as the Salton Sea in California. Restraining bends are created by compressional stresses which create features like reverse faults causing mountain building, such as the San Gabriel Mountains in California.

Figure \(\PageIndex{1}\): Structures created by bends in strike-slip faults. Depending on the relative movement in relation to the bend in the fault, these structures can create basins or mountains. On the left is a restraining bend where compression causes uplift. On the right is a releasing bend where tension causes blocks to drop down.

An example of a strike-slip fault is the San Andreas Fault, which denotes a transform boundary between the North American and Pacific plates.