Thrust faults

Thrust faults are the result of compressional forces that exceed the natural strength of rocks and cause them to break and move. They can trigger earthquakes, create mountain ranges, and serve as natural traps for gas and oil deposits.

Thrust Fault Production

A mass of rock below the surface of the Earth usually cracks and fractures when it loses its resistance to an applied force. Rocks break when their ability to store energy is exceeded. When a rock shows some evidence of movement or displacement along the zone of breakage, a fault is created. Thrust faults are commonly the result of strong compressional (squeezing) forces acting on relatively brittle, older subsurface rock that has moved upward and over or on top of a mass of younger, adjacent rock. It is a particular kind of fault and one of many types that exist.

The zone of breakage between the once-united masses of rock is known as the fault plane. The motion of the rocks on either side of this plane and the plane itself is usually parallel to each other. The blocks of rock on both sides of a fault plane are known as walls, a term that comes from the days of the early prospectors, who were really the first field geologists. Because the presence of a fault marks a zone of weakness in the ground, either mineral-rich groundwater or hot fluid magmas will eventually find and follow this path of least resistance toward the surface and deposit ores, minerals, or gemstones. Prospectors would seek out faults, as they knew that a fault was likely to be the home of some valuable material. Once a fault was located, a mine shaft would be dug to follow the trace of the fault below ground.

The head wall, or hanging wall, was the wall above the miner's head; the footwall was the wall below his feet. Head walls and footwalls exist only in faults that are not vertical. In terms of the overall structure of the fault, the hanging wall is the block that occurs above the fault plane, and the footwall is the rock below the fault plane. In thrust faults, the head wall always moves relatively upward, and the footwall moves relatively downward. The term “relatively” is used because it is usually very difficult for a geologist to determine exactly which block has moved. For example, both blocks could have moved upward, but the hanging wall moved farther; both blocks could have dropped, but the footwall dropped farther; the hanging wall could have remained stationary while the footwall dropped; or the footwall could have remained stationary while the hanging wall moved upward.

Fault Orientation

The orientation of the fault relative to the Earth's surface is of great importance. It allows the fault to be located and mapped as a place to avoid during construction, especially if it is an active fault or one with the potential for continued movement. A fault's orientation, or strike, is measured by the trace of the fault plane as it would appear on a horizontal plane and is measured in degrees from the magnetic North Pole. The plane's angle of tilt, or dip, is measured from a horizontal position down to the fault plane. The dip direction is always perpendicular to the strike direction.

A fault can be straight in form and consist of one sharp, clean break, or it can have a highly irregular form and be composed of multiple breaks. Thrust faults of the latter type may be so closely spaced as to form a highly complex zone that may be hundreds of meters wide. Fault planes can also be curved, adding to the complexity. Geologists have located and mapped many small thrust faults at very shallow depths below the surface and have found some large thrust faults that extend down to a depth of 700 kilometers.

In general, the total displacement, or offset, in a rock along a thrust fault may be large or small, and horizontal, vertical, or oblique, depending on the strength of the compressional force and the rock type involved. Typical thrust faults have low dip angles in which lower strata are often pushed above higher strata. They often place older rocks above younger rocks. An important factor in determining this displacement is the angle of the fault plane. Steeply dipping planes will show a small vertical uplift on the order of a few meters or less; shallowly dipping planes may exhibit a long horizontal displacement extending for many kilometers. Displacements are also described in terms of their relative motion. A “dip-slip” occurs when the movement of the rock is parallel to the dip direction of the fault plane, a “strike-slip” indicates motion parallel to the strike, and an “oblique slip” occurs somewhere between the strike and dip.

Fault Varieties and Ages

Three types of faults involve movement of the hanging wall upward with respect to the footwall: Thrust faults are characterized by fault planes that dip at angles less than 30 degrees; overthrust faults are thrust faults with very large displacement; and reverse faults dip at angles greater than 45 degrees from horizontal. In large displacement thrust faults, the fault surface may be quite irregular; therefore, the fault plane may approach horizontal or even reverse direction. Overthrusts are very common in areas of intense folding where there is shortening, and usually thickening, of the crust. The offset of rock along the path of the fault is usually greatest near its middle and decreases at either end, or the thrust may terminate in strike-slip faults.

The age of rocks may be used to determine the relative age of faults. Since a fault cuts through a block of rock, the fault must be younger than the rock. Therefore, if a rock is found to be 100 million years old and is cut by a fault, then the fault must be younger than 100 million years.

Locating Thrust Faults

Structural geologists study faults to try to understand what was happening to the Earth's crust at the time of faulting and to determine the origin of the force that caused the rocks to break and move. To study a thrust fault, the geologist must first accurately locate and measure the fault in the field. It is sometimes difficult to determine where a fault exists, especially if it is very old. The geologist, acting as a detective, must rely on direct physical evidence that they can gather from above and below the surface.

When a fault intersects the surface, it usually forms a fault line. The existence of this line is commonly indicated by some noticeable feature. Photographs of the land taken from high-altitude aircraft reveal these features as offsets or disruptions in rows of planted crops or trees; sharp breaks in the channels of streams; unusual linear alignments of springs flowing at the base of a mountain or along a valley; raised sections of land, such as beach and river terraces; and fault scarps. A fault scarp is a recent, sharp break in the surface of the ground that has a straight and very steep slope. The height of a scarp is directly related to the amount of upward motion along a fault. Unless the fault is still active, however, it usually is not exposed at the surface but is buried beneath the cover of more recently deposited sediments. In this case, a geologist must rely on subsurface evidence.

Evidence of Thrust Fault Motion

The subsurface location of a thrust fault is not always easy to find. The field geologist must rely on direct or indirect evidence that is not usually visible on the surface. Direct subsurface evidence of the existence of a thrust fault can be obtained from the examination of rocks within mine shafts, highway tunnels, or excavation sites dug for building foundations. Similarly, “roadcuts” (highway excavations that run through mountains and valleys) and natural outcroppings of rock at the surface may indicate the presence of a thrust fault. In these situations, a geologist would look for any evidence that suggests that massive blocks of rock have moved relative to one another.

When huge blocks of rock are broken and continue to rub against one another, certain physical features are produced as a result of the friction between these moving blocks. Sure indicators of faulting are slickensides, which are recognized as highly polished and finely scratched, or striated, surfaces of rock along the fault plane. The direction of the striations is parallel to the direction of the last movement along the fault. Depending on the amount of friction generated between these blocks, their inward-facing surfaces may be crushed into a fine, soft, claylike powder known as “gouge” or may become fault breccias, rocks consisting of small, angular fragments. Microbreccias are formed when the crushed fragments along the fault plane are microscopic. Mylonites are a special type of solid microbreccia; they have a streaked appearance in the direction of motion. Pseudotachylites are a kind of microbreccia that does not appear streaked but exists as a thin glassy film because of the melting of rock from frictional heat. Other evidence of thrust fault motion would be the overlapping or repetition of the same rock units (like overlapping shingles on a roof) and an abrupt termination of a rock unit along its trend. One or more of these faulting criteria may be present, or evidence may be completely missing.

Drilling for Evidence

If there are no accessible outcrops or underground viewpoints, the field geologist must turn to the expensive direct method of evidence-gathering: drilling. Drilling into the ground with specialized drilling rigs allows access to the subsurface. The examination of small broken rock samples brought up by a diamond drill bit or solid “rock cores” brought up by hollow drill bits is a direct means of studying rocks that do not exist at the surface. These samples can be compared with those taken from well-known nearby regions, and any disruption or missing units of rock may indicate the presence of a fault. If a subsurface geologic map exists, the geologist can predict which rocks exist below the drill site and use this information in conjunction with the collected rock samples.

If the evidence from drilling does not match the regional geology, then one or more faults may have been at work, shuffling the sequence of rocks below. Sometimes large blocks of rock get caught between faults when they move and become bent, folded, or twisted, creating a highly complex pattern that is not easy to understand using drill core data alone; the uncertainty of what lies below ground increases.

Evidence from Geologic Maps

Indirect evidence of thrust faulting can be had from a geologic map of an area. Geologic maps show the distribution, thickness, age, and orientation of the various types of rock that would be seen at the surface of the Earth were all the soil covering removed. Any older rock formation that sits on top of a younger rock formation was most likely moved to its location by thrust faulting. Rock formations that appear out of place with the overall sequence of rocks probably suffered a similar fate.

Several notable large thrust faults exist in the United States. They can be seen in the Rocky Mountain region as a series of sharp, parallel ridges similar to the teeth on a saw—hence the term “sawtooth,” as in the peaks of the Sawtooth Mountain Range in Montana. In the same state, a low-lying slab of rock known as the Lewis Overthrust shows a horizontal displacement of about 24 kilometers with a fault plane that dips at an angle of less than 3 degrees. Thrust faults occur in most mountain ranges on the Earth, such as the Appalachian Mountains, the European Alps, and the Himalayas.

A 2020 PNAS study revealed that Himalayan blind thrusts had propagated over 37 km south into the Ganga foreland basin, an area long assumed to be tectonically stable. This finding showed that modern thrust systems actively deform the landscape far beyond the visible mountain fronts.

Engineering and Economic Applications

A fault is a zone of weakness in a rock; therefore, it may continue to move over a long period. Information about the rate of rock movement is very valuable, especially when the motion along a thrust fault (or any fault) is rapid enough to trigger the release of stored energy within a rock, causing an earthquake. Accurate mapping of thrust faults is also important, since the potential for future earthquakes must be carefully evaluated, especially in highly urbanized areas. A complete study of thrust faults is not easy, however, because there are many variables to consider: the fault's exact location and total horizontal extent, the orientation and strength of the rocks relative to the fault, the direction and amount of movement, and the fault's previous earthquake history. These factors are critical to decisions of where to construct nuclear power plants, dams, housing projects, and cities.

Thrust faults have great economic potential, since they have the ability to act as “traps,” or reservoirs, for deposits of migrating oil and natural gas. In such a case, one impervious rock type is brought into fault contact with a petroleum-bearing rock. The impermeable rock now acts as a barrier to any further upward fluid migration and allows oil to accumulate beneath it. Similarly, in mineral exploration, large thrust faults have been known to harbor exploitable quantities of radioactive and other rare minerals, needed for use in industry and medicine, that were either deposited by igneous activity or precipitated by circulating, mineral-rich groundwater. Thrust faults also serve as natural underground pipelines, allowing circulating groundwater easier access to the surface of areas that otherwise might have been deserts.

Principal Terms

dip: the angle between a fault plane and a horizontal surface

fault: a fracture or zone of breakage in a mass of rock that shows evidence of displacement or offset

footwall: the block of rock that lies directly below the plane of a fault

head wall: the block of rock that lies directly above the plane of a fault; it is also known as a hanging wall

reverse fault: the same thing as a thrust fault, except that its fault plane dips at more than 45 degrees below the horizontal

scarp: a steep cliff or slope created by rapid movement along a fault

slip: amount of offset or displacement across the plane of the fault, relative to either the dip or the strike

strike: the orientation of a fault plane on the surface of the ground measured relative to north

Bibliography

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