Types of Faults With Photos

Faults are fractures in the Earth's crust where there has been movement on either side of the fracture. Here, we explore the types of faults, each with its unique characteristics and geological implications, illustrated with photos to bring these concepts to life.

A fault is a fracture or zone of fractures between two blocks of rock, allowing the blocks to move relative to each other. This movement can occur rapidly, in the form of an earthquake, or slowly, in the form of creep.

Faults are primarily caused by tectonic forces such as compression, tension, and shear. There are several types of faults, categorized based on the type of movement and the direction of stress acting on the rocks.These geological faults are responsible for earthquakes, mountain building, and other geological processes that shape the Earth's surface over time.

Types of Faults and tectonic forces such as compression, tension, and shear.

Fault Terminology

Hanging Wall: The block of rock that lies above the fault plane.

Footwall: The block of rock that lies below the fault plane.

Dip: The angle at which the fault plane is inclined relative to the horizontal.

Strike: The direction of the fault line along the Earth's surface, typically measured as a compass bearing.

Here are the types of faults:

Normal Fault

Normal fault in Mosaic Canyon, Death Valley.

Movement: In a normal fault, the hanging wall (the block of rock above the fault plane) moves downward relative to the footwall (the block of rock below the fault plane).

Stress Type: Tensional (extensional) stress, which pulls the crust apart.

Appearance: The fault plane is often steep, and the surface above the hanging wall drops down relative to the footwall.

Common Location: Typically found in areas where the crust is being stretched, such as divergent plate boundaries (e.g., mid-ocean ridges or continental rift zones).

Examples: The Basin and Range Province in the western United States has numerous normal faults.

Reverse Faults

Reverse fault showing the hanging wall moving upward relative to the footwall.

Movement: In a reverse fault, the hanging wall moves upward relative to the footwall. A reverse fault has a steeper angle, typically greater than 30 degrees, often around 45 degrees or more.

Stress Type: Occurs due to compressional forces, which push the crust together.

Common Location: Reverse and thrust faults are typically found in areas of plate convergence, such as at subduction zones and continental collision zones (e.g., the Himalayas).

Example: The Himalayan Mountain Range, formed due to the collision of the Indian and Eurasian plates.

Thrust Fault

Example of a thrust fault with a low-angle dip, observed at Ketobe Knob, Utah.

Thrust Fault is a specific type of reverse fault characterized by a low angle of dip, usually less than 45 degrees, and often much flatter, sometimes even approaching horizontal. Over time, due to the low angle, older rocks can be pushed over younger rocks, which is a distinctive feature of thrust faults. It occurs in regions with very strong compressional forces. Example: The Rocky Mountains in North America, formed by thrust faulting.

Strike-Slip Faults

Satellite image of a part of the Piqiang Strike-Slip Fault that laterally partitions the Keping Shan Thrust Belt in the NW Tarim Basin, China. 70 kilometers.

Movement: In a strike-slip fault, the movement is primarily horizontal, with blocks sliding past each other laterally. There is little to no vertical movement.

Stress Type: Result from shearing forces that push blocks horizontally in opposite directions.

Common Location: Common at transform plate boundaries, where plates slide past each other.

Types:

Left-lateral (Sinistral): If you stand on one side of the fault and the other side moves to your left, it's left-lateral. Example: The Great Glen Fault in Scotland.
Right-lateral (Dextral): If the other side moves to your right, it's right-lateral. The San Andreas Fault is a famous example of a right-lateral strike-slip fault.

Transform Fault

The Dead Sea Transform Fault

Transform Fault is a specific type of strike-slip fault that occurs at plate boundaries where plates slide past each other. They are a type of plate boundary themselves, like the transform boundaries between tectonic plates

Common Location: Found at transform plate boundaries, especially along oceanic ridges.
Example: The Dead Sea Transform, which separates the Arabian Plate from the African Plate.

Oblique-Slip Fault

Oblique-Slip Fault: Fault that exhibits both horizontal (strike-slip) and vertical (dip-slip) displacement.

An oblique-slip fault combines elements of both strike-slip and dip-slip faults, meaning there is both horizontal (lateral) and vertical (up or down) movement along the fault plane.

Movement: These faults combine both vertical and horizontal movement.

Horizontal: Known as strike-slip, where blocks slide past each other side to side. This can be right-lateral or left-lateral.
Vertical: Known as dip-slip, where there's up or down movement, which could be normal (hanging wall moves down) or reverse/thrust (hanging wall moves up).

Stress Type: Result from a combination of tensional, compressional, and shearing forces.

Common Location: Oblique-slip faults are common in areas where the tectonic stress isn't purely compressional or tensional but has a significant shear component as well. Like: Transform boundaries where plates slide past each other but with some convergence or divergence.

An impressive view of a fault scarp formed by the South Leader Fault, resulting from the 2016 Kaikoura earthquake in New Zealand, which had a magnitude of 7.8. At this location, the earth moved vertically by approximately 3.5 meters. Through precise geological mapping, it's been determined that the movement along the fault was a combination of normal faulting with a left-lateral (sinistral) component. Normal oblique sinistral fault. Photo by: Kate Pedley.

There are two primary types of oblique-slip faults:

Right-Lateral Oblique-Slip: Combines right-lateral strike-slip with either normal or reverse dip-slip.
Left-Lateral Oblique-Slip: Combines left-lateral strike-slip with either normal or reverse dip-slip.

Examples: Many faults have oblique movement due to complex tectonic stresses; an example might be found along certain sections of the Alpine Fault in New Zealand.

Listric Fault

A geological model depicting a listric growth fault. The fault exhibits a concave upward shape, with newer sediments accumulating on the hanging wall side.

A listric fault is a type of normal fault characterized by a curved fault plane that is concave upwards. These faults begin at a steeper angle near the surface and gradually flatten with depth. They are commonly associated with gravitational sliding or extensional collapse in areas undergoing crustal stretching. The hanging wall slides downward along this curved plane. This fault geometry can cause rollover anticlines, which are curved folds in the hanging wall block.

Movement: Listric faults are curved faults, where the dip decreases with depth. The upper part of the fault is steeper, and it becomes flatter as it deepens.

Stress Type: Tensional stress, though the nature of curvature causes unique deformation.

Common Location: Often associated with extensional environments, such as rift zones or passive continental margins, but can also occur in areas of landslide activity.

Examples: Listric faults are commonly seen in extensional basins and are associated with the detachment of rock blocks. The Great Basin region in the western U.S. has several listric faults.

Growth Fault

Growth fault along the margin of a sedimentary basin.

Growth faults are typically normal faults where the fault movement occurs concurrently with the deposition of sediments. As the fault moves, more space is created on the hanging wall (the downthrown block) for sediments to accumulate compared to the footwall.

Movement: Growth faults are a special type of fault that develops in sedimentary basins where rapid deposition of sediment causes the underlying layers to fault and adjust.

Stress Type: Often related to the weight of sediments and gravity, though extensional forces may also be involved.

Common Location: Occurs in areas with large sedimentary accumulations, such as delta regions and continental margins. Often, growth faults have a listric shape, meaning they curve and flatten with depth, but this isn't a strict requirement.

Examples: The Niger Delta and the Gulf of Mexico are examples of regions where growth faults are common due to large amounts of sediment being deposited.

Graben and Horst

Graben and Horst Structures in Zanjan, Iran.
Photo: Mehdi Jahangiri and Reza Alipour

A graben is a down-dropped block of Earth's crust, bounded by parallel normal faults, forming a valley or trough. A horst is an elevated block of Earth's crust that has been uplifted or remained stationary while adjacent blocks have subsided, the counterpart to a graben.

Movement: Grabens are down-dropped blocks flanked by normal faults, while horsts are uplifted blocks.

Stress Type: Tensional stress causes crustal stretching.

Location: Graben and horst structures typically form in regions undergoing crustal extension, common in rift zones and extensional settings.

Normal faults occur when the crust is pulled apart, causing the hanging wall to move down relative to the footwall. Two parallel normal faults can create either a graben (land dropping) or a horst (land uplifted).

A model of geological strata (Artificial) in Crystal Palace Park, London, depicting normal faults with a horst (an uplifted block) in the center.

Examples: The Basin and Range Province in Nevada features alternating grabens and horsts. The East African Rift Valley is a graben, while the Sierra Nevada in California is a horst.

Detachment Fault

Detachment fault in Natural Bridge Canyon, Death Valley

A detachment fault, also called a low-angle normal fault or décollement, has a shallow dip, often nearly horizontal. Unlike typical normal faults with steeper dips, detachment faults can dip as low as 10 degrees or less.

Movement: These faults occur at low angles, separating rock units in extensional tectonic environments.

Stress Type: Tensional stress leads to horizontal stretching of the crust.

Location: Common in regions of significant crustal thinning, such as rift zones.

Examples: The Basin and Range Province in the western U.S. features large detachment faults.

Blind Fault

Northridge earthquake in 1994 (California) occurred on a blind thrust fault

A blind fault does not extend to the Earth's surface, remaining hidden from direct observation. It terminates beneath the surface, making it more difficult to detect. Blind faults terminate before reaching the surface, with no visible scarp or direct evidence, making detection harder. They can be thrust, reverse, or normal faults where movement is absorbed by folding or deformation in overlying rock layers.

Movement: A fault that remains below the surface and is not visible.

Stress Type: Compressional, tensional, or shear stress depending on the fault type.

Common Location: Occurs in various tectonic settings, especially in thrust or reverse fault zones.

Examples: Blind thrust faults are common in seismically active areas like the Los Angeles Basin, Northridge earthquake in 1994 (California) occurred on a blind thrust fault.

Rotational Faults

Rotational fault

Rotational fault is a type of fault where the blocks on either side of the fault plane rotate relative to each other around an axis perpendicular to the fault plane causing complex deformation patterns in the surrounding rock.

Movement: The movement involves rotation of one or both blocks around an axis that is perpendicular to the fault plane. This can result in one block tilting or pivoting relative to another.

Stress Type: Shear or Oblique Stress: Rotational faults often result from complex shear or oblique stresses where both lateral and vertical forces are applied to the faulted area.

Types of Rotation:

Hinge Fault: Here, one end of the fault block might remain stationary while the other end swings or rotates. This can sometimes be visualized as a door hinge where the fault line acts as the hinge.
Scissor Fault: In this scenario, the fault rotates around a central point, causing one side to move up and the other to move down, or vice versa, much like scissors opening or closing.

Location: Occurs in both extensional and compressional tectonic regimes.

Examples: The Transverse Ranges in California show block rotation due to the San Andreas fault system. In the Basin and Range Province, there are examples of rotational normal faulting with tilted blocks.

Scissor Fault

A scissor fault, also known as a pivot fault is a type of fault where the displacement along the fault plane changes direction along its length.

A scissor fault operates somewhat like a pair of scissors. One side of the fault moves up relative to the other, but this vertical movement reverses direction along the strike of the fault. This means if one end of the fault has the hanging wall moving up, the other end will have the hanging wall moving down.

Movement: A fault where one end of the fault experiences more vertical movement than the other. The motion along the fault is like the opening or closing of a pair of scissors.

Stress Type: Can vary, but often occurs in extensional or compressional regimes.

Common Location: Common in regions experiencing differential movement or tilting of blocks.

Real-world examples can be less straightforward to identify due to their complex nature, but they might be found in areas with intricate fault interactions or where faults pass through varying lithologies or structural domains.

Synthetic and Antithetic Faults

Synthetic and Antithetic are terms are used to describe smaller faults that either run parallel (synthetic) or in the opposite direction (antithetic) to a larger main fault in extensional settings.

Synthetic Faults:

Synthetic faults are minor faults that have the same slip direction as the main fault. They dip in the same direction as the primary fault and displace the rock in a manner that is consistent with the major fault movement.
They have the same dip direction as the main fault but are generally shorter and smaller.

Antithetic Faults:

Antithetic faults are secondary faults that dip in the opposite direction to the main fault and have a sense of movement that appears to counteract that of the main fault. However, they are related to and often caused by the same stress regime that created the primary fault.

Characteristics:

These fault systems can lead to complex fault blocks where blocks of rock are bound by a combination of synthetic and antithetic faults.

They are common in graben and rift zones, where large extensional stresses produce a combination of fault orientations.

Road cut near Arches National Park Entrance, Grand County, Utah, showing Moab fault with synthetic and antithetic faults. by: Enry Horas Sihombing

Common Location: They can occur in various tectonic settings, including extensional (like rifts), compressional (like fold and thrust belts), or strike-slip environments. The presence of antithetic and synthetic faults can significantly complicate the geological structure of an area, leading to complex patterns of uplift, subsidence, or lateral movement.

Examples:

The East African Rift contains many synthetic and antithetic normal faults within its overall extensional regime.
Rhine Graben in central Europe is also a region with a network of synthetic and antithetic faults.

Riedel Shear Faults

Riedel Shear Faults are a specific type of fracture or shear structure that forms within shear zones, They are typically found in strike-slip fault zones, but can also occur in normal and reverse faults. They are named after the German geologist Walter Riedel, who studied these structures.

Common Location: Riedel shears form during the early stages of shear zone development as a rock mass begins to deform under shear stress. They are often observed in experiments with clay or sand models before a through-going fault develops.

Types of Riedel Shears:

R Shears (R-Shears): These are synthetic to the overall shear direction, meaning they have the same sense of movement as the main shear zone but are oriented at a low angle (typically around 15 degrees) to the direction of the main shear. They are the most common type and form first.
R' Shears (R'-Shears or Antithetic Shears): These are antithetic, meaning they move in the opposite sense relative to the main shear zone. They form at a high angle (approximately 75 degrees) to the main shear direction.
P Shears: These are also synthetic but form at an angle to R shears, often symmetrical to R shears with respect to the maximum principal stress direction. They are less common than R shears but help in accommodating the shear deformation.

Y Shears: These are parallel to the main shear zone and represent the eventual through-going fault that might develop as deformation progresses.

In the field, Riedel shears can be observed in outcrops where strike-slip faulting has occurred. They might appear as smaller fractures or slickensides within a larger fault zone.

Conjugate Faults

Conjugate normal faults in marble of Noonday Dolomite, Death Valley National Park, California.

Conjugate faults are two sets of faults that form under the same stress regime and intersect at an angle, typically around 60 degrees in ideal conditions. However, in real geological settings, this angle can vary due to anisotropic rock properties, pre-existing structures, or variations in the stress field.

Movement: Can be strike-slip, normal, or reverse, with each set having opposite senses of movement relative to each other (e.g., one dextral, one sinistral for strike-slip).

Types of Movement:

Strike-Slip Conjugate Faults: Here, the movement is horizontal. If one fault exhibits right-lateral (dextral) movement, the other will show left-lateral (sinistral) movement, or vice versa.
Normal Conjugate Faults: Occur in extensional environments where one block moves downward relative to the other, creating structures like grabens and horsts.
Reverse (or Thrust) Conjugate Faults: Found in compressional environments where one block is pushed up over another.

Examples:

The East African Rift System: Exhibits normal conjugate faults due to the extensional forces pulling the African continent apart.
The Alpine Fault System in New Zealand: Shows examples of strike-slip conjugate faults within a broader context of oblique continental collision.

Duplexes Faults

A duplex is a structural arrangement where a series of thrust faults create a stacked sequence of fault-bound rock slices, known as horses. These horses are displaced along the faults, leading to a complex internal geometry where rock units are repeated vertically.

Duplexes typically form in compressional tectonic regimes where layers of rock are being pushed over each other, leading to the creation of thrust faults. As these faults propagate, they can stack up, creating a duplex structure.

Formation:

Duplexes form in regions undergoing compression, often within fold-thrust belts. The process involves:

Initial Thrusting: A thrust fault forms due to compressional forces.
Branching: As the thrust propagates, it might encounter layers or mechanical boundaries that cause it to branch, forming new faults below the initial one.
Sequential Development: New faults form sequentially at the base (floor thrust), lifting and moving older faults and their associated horses upward.

Examples:

Moine Thrust Zone, Scotland: One of the classic areas where duplex structures were first recognized and studied.
Canadian Rockies: Numerous duplex structures are part of the thrust sheets that have been pushed eastward during the formation of the mountain range.
Sub-Andean Zone: In regions like Bolivia, duplexes are common in the eastward-verging thrust faults associated with the Andean orogeny.

Conclusion

Faults are critical for understanding the dynamic nature of Earth's crust, providing insights into past and present tectonic activities, and are essential for assessing geological hazards like earthquakes.