Satellite image of a fault in theTaklamakan Desert. The two colorful ridges (at bottom left and top right) used to form a single continuous line, but have been split apart by movement along the fault.
Ingeology, afault is aplanar fracture or discontinuity in a volume ofrock across which there has been significant displacement as a result of rock-mass movements. Large faults withinEarth'scrust result from the action ofplate tectonic forces, with the largest forming the boundaries between the plates, such as the megathrust faults ofsubduction zones ortransform faults.[1] Energy release associated with rapid movement onactive faults is the cause of mostearthquakes. Faults may also displace slowly, byaseismic creep.[2]
Afault plane is theplane that represents the fracture surface of a fault. Afault trace orfault line is a place where the fault can be seen or mapped on the surface. A fault trace is also the line commonly plotted ongeological maps to represent a fault.[3][4]
Afault zone is a cluster of parallel faults.[5][6] However, the term is also used for the zone of crushed rock along a single fault.[7] Prolonged motion along closely spaced faults can blur the distinction, as the rock between the faults is converted to fault-bound lenses of rock and then progressively crushed.[8]
Due tofriction and the rigidity of the constituent rocks, the two sides of a fault cannot always glide or flow past each other easily, and so occasionally all movement stops. The regions of higher friction along a fault plane, where it becomes locked, are calledasperities.Stress builds up when a fault is locked, and when it reaches a level that exceeds thestrength threshold, the fault ruptures and the accumulatedstrain energy is released in part asseismic waves, forming anearthquake.[2]
Strain occurs accumulatively or instantaneously, depending on theliquid state of the rock; theductile lower crust andmantle accumulate deformation gradually viashearing, whereas the brittle upper crust reacts by fracture – instantaneous stress release – resulting in motion along the fault.[9] A fault in ductile rocks can also release instantaneously when the strain rate is too great.
Slip is defined as the relative movement of geological features present on either side of a fault plane. A fault'ssense of slip is defined as the relative motion of the rock on each side of the fault concerning the other side.[10] In measuring the horizontal or vertical separation, thethrow of the fault is the vertical component of the separation and theheave of the fault is the horizontal component, as in "Throw up and heave out".[11]The vector of slip can be qualitatively assessed by studying any drag folding of strata, which may be visible on either side of the fault.[12] Drag folding is a zone of folding close to a fault that likely arises from frictional resistance to movement on the fault.[13] The direction and magnitude of heave and throw can be measured only by finding common intersection points on either side of the fault (called apiercing point). In practice, it is usually only possible to find the slip direction of faults, and an approximation of the heave and throw vector.
The two sides of a non-vertical fault are known as thehanging wall andfootwall. The hanging wall occurs above the fault plane and the footwall occurs below it.[14] This terminology comes from mining: when working a tabularore body, the miner stood with the footwall under his feet and with the hanging wall above him.[15] These terms are important for distinguishing different dip-slip fault types: reverse faults and normal faults. In a reverse fault, the hanging wall displaces upward, while in a normal fault the hanging wall displaces downward. Distinguishing between these two fault types is important for determining the stress regime of the fault movement.
The problem of the hanging wall can lead to severe stresses androck bursts, for example atFrood Mine.[16]
Faults are mainly classified in terms of the angle that the fault plane makes with the Earth's surface, known as thedip, and the direction of slip along the fault plane.[17]Based on the direction of slip, faults can be categorized as:
strike-slip, where the offset is predominantly horizontal, parallel to the fault trace;
dip-slip, offset is predominantly vertical and/or perpendicular to the fault trace; or
In astrike-slip fault (also known as awrench fault,tear fault ortranscurrent fault),[18] the fault surface (plane) is usually near vertical, and the footwall moves laterally either left or right with very little vertical motion. Strike-slip faults with left-lateral motion are also known assinistral faults and those with right-lateral motion asdextral faults.[19] Each is defined by the direction of movement of the ground as would be seen by an observer on the opposite side of the fault.
A special class of strike-slip fault is thetransform fault when it forms aplate boundary. This class is related to an offset in aspreading center, such as amid-ocean ridge, or, less common, within continentallithosphere, such as theDead Sea Transform in theMiddle East or theAlpine Fault in New Zealand. Transform faults are also referred to as "conservative" plate boundaries since the lithosphere is neither created nor destroyed.
Verticalcross-sectional view, along a plane perpendicular to thefault plane, illustrating normal and reverse dip-slip faults
Dip-slip faults can be eithernormal ("extensional") orreverse. The terminology of "normal" and "reverse" comes fromcoal mining in England, where normal faults are the most common.[20]
With the passage of time, a regional reversal betweentensional andcompressionalstresses (or vice-versa) might occur, and faults may be reactivated with their relative block movement inverted in opposite directions to the original movement (fault inversion). In such a way, a normal fault may therefore become a reverse fault and vice versa.
In a normal fault, the hanging wall moves downward, relative to the footwall. Thedip of most normal faults is at least 60 degrees but some normal faults dip at less than 45 degrees.[21]
Diagram illustrating the structural relationship between grabens and horsts.
A downthrown block between two normal faults dipping towards each other is agraben. A block stranded between two grabens, and therefore two normal faults dipping away from each other, is ahorst. A sequence of grabens and horsts on the surface of the Earth produces a characteristicbasin and range topography.
A listric fault is a type of normal fault that has a concave-upward shape with the upper section near Earth's surface being steeper, becoming more horizontal with increased depth. Normal faults can evolve into listric faults with the fault plane curving into the Earth. They can also form where the hanging wall is absent (such as on a cliff), where the footwall may slump in a manner that creates multiple listric faults.
Cross-section diagram of a listric fault
Cross-section diagram of multiple listric faults in a cliff wall
The fault panes of listric faults can further flatten and evolve into a horizontal or near-horizontal plane, where slip progresses horizontally along adecollement.Extensional decollements can grow to great dimensions and formdetachment faults, which are low-angle normal faults with regionaltectonic significance.
Due to the curvature of the fault plane, the horizontal extensional displacement on a listric fault implies a geometric "gap" between the hanging and footwalls of the fault forms when the slip motion occurs. To accommodate into the geometric gap, and depending on itsrheology, the hanging wall might fold and slide downwards into the gap and producerollover folding, or break into further faults and blocks which fill in the gap. If faults form,imbrication fans ordomino faulting may form.
Cross-section diagram of a listric fault (red line), with a resulting rollover fold
Cross-section diagram showing how an imbrication fan forms
A reverse fault is the opposite of a normal fault—the hanging wall moves up relative to the footwall. Reverse faults indicate compressive shortening of the crust.
Cross-section diagram of a thrust fault with a fault-bend fold
Athrust fault has the same sense of motion as a reverse fault, but with the dip of the fault plane at less than 45°.[22][23] Thrust faults typically form ramps, flats and fault-bend (hanging wall and footwall) folds.
A section of a hanging wall or foot wall where a thrust fault formed along a relatively weak bedding plane is known as aflat and a section where the thrust fault cut upward through the stratigraphic sequence is known as aramp.[24] Typically, thrust faults movewithin formations by forming flats and climbing up sections with ramps. This results in the hanging wall flat (or a portion thereof) lying atop the foot wall ramp as shown in the fault-bend fold diagram.
Thrust faults formnappes andklippen in the large thrust belts.Subduction zones are a special class of thrusts that form the largest faults on Earth and give rise to the largest earthquakes.
A fault which has a component of dip-slip and a component of strike-slip is termed anoblique-slip fault. Nearly all faults have some component of both dip-slip and strike-slip; hence, defining a fault as oblique requires both dip and strike components to be measurable and significant. Some oblique faults occur withintranstensional andtranspressional regimes, and others occur where the direction of extension or shortening changes during the deformation but the earlier formed faults remain active.
Thehade angle is defined as thecomplement of the dip angle; it is the angle between the fault plane and a vertical plane that strikes parallel to the fault.
Ring faults, also known ascaldera faults, are faults that occur within collapsed volcaniccalderas[25] and the sites ofbolide strikes, such as theChesapeake Bay impact crater. Ring faults are the result of a series of overlapping normal faults, forming a circular outline. Fractures created by ring faults may be filled byring dikes.[25]
Synthetic andantithetic are terms used to describe minor faults associated with a major fault. Synthetic faults dip in the same direction as the major fault while the antithetic faults dip in the opposite direction. These faults may be accompanied byrollover anticlines (e.g. theNiger Delta Structural Style).
Structure of a fault[26]Salmon-coloredfault gouge and associated fault separates two different rock types on the left (dark gray) and right (light gray). From theGobi ofMongolia.Inactive fault fromSudbury toSault Ste. Marie, Northern Ontario, Canada
All faults have a measurable thickness, made up of deformed rock characteristic of the level in the crust where the faulting happened, of the rock types affected by the fault and of the presence and nature of anymineralising fluids. Fault rocks are classified by theirtextures and the implied mechanism of deformation. A fault that passes through different levels of thelithosphere will have many different types of fault rock developed along its surface. Continued dip-slip displacement tends to juxtapose fault rocks characteristic of different crustal levels, with varying degrees of overprinting. This effect is particularly clear in the case ofdetachment faults and majorthrust faults.
The main types of fault rock include:
Cataclasite – a fault rock which is cohesive with a poorly developed or absent planarfabric, or which is incohesive, characterised by generally angularclasts and rock fragments in a finer-grainedmatrix of similar composition.
Tectonic orfault breccia – a medium- to coarse-grained cataclasite containing >30% visible fragments.
Fault gouge – an incohesive,clay-rich fine- toultrafine-grained cataclasite, which may possess a planar fabric and containing <30% visible fragments. Rock clasts may be present
Clay smear – clay-rich fault gouge formed insedimentary sequences containing clay-rich layers which are strongly deformed and sheared into the fault gouge.
Mylonite – a fault rock which is cohesive and characterized by a well-developed planar fabric resulting from tectonic reduction of grain size, and commonly containing roundedporphyroclasts and rock fragments of similar composition tominerals in the matrix
Pseudotachylyte – ultrafine-grained glassy-looking material, usually black andflinty in appearance, occurring as thin planarveins, injection veins or as a matrix topseudoconglomerates orbreccias, which infills dilation fractures in the host rock. Pseudotachylyte likely only forms as the result of seismic slip rates and can act as a fault rate indicator on inactive faults.[27]
The level of a fault's activity can be critical for (1) locating buildings, tanks, and pipelines and (2) assessing theseismic shaking andtsunami hazard to infrastructure and people in the vicinity. In California, for example, new building construction has been prohibited directly on or near faults that have moved within theHolocene Epoch (the last 11,700 years) of the Earth's geological history.[28] Also, faults that have shown movement during the Holocene plusPleistocene Epochs (the last 2.6 million years) may receive consideration, especially for critical structures such as power plants, dams, hospitals, and schools. Geologists assess a fault's age by studyingsoil features seen in shallow excavations andgeomorphology seen in aerial photographs. Subsurface clues include shears and their relationships tocarbonatenodules,eroded clay, andironoxide mineralization, in the case of older soil, and lack of such signs in the case of younger soil.Radiocarbon dating oforganic material buried next to or over a fault shear is often critical in distinguishing active from inactive faults. From such relationships,paleoseismologists can estimate the sizes of pastearthquakes over the past several hundred years, and develop rough projections of future fault activity.
Many ore deposits lie on or are associated with faults. This is because the fractured rock associated with fault zones allow for magma ascent[29] or the circulation of mineral-bearing fluids. Intersections of near-vertical faults are often locations of significant ore deposits.[30]
Faults may not always act as conduits to surface. It has been proposed that deep-seated "misoriented" faults may instead be zones where magmas forming porphyry copper stagnate achieving the right time for—and type of—igneous differentiation.[32] At a given time differentiated magmas would burst violently out of the fault-traps and head to shallower places in the crust where porphyry copper deposits would be formed.[32]
As faults are zones of weakness, they facilitate the interaction of water with the surrounding rock and enhance chemicalweathering. The enhanced chemical weathering increases the size of the weathered zone and hence creates more space forgroundwater.[33] Fault zones act asaquifers and also assist groundwater transport.
Microfault showing apiercing point (the coin's diameter is 18 mm (0.71 in))
A normal fault inMorocco. The fault plane is the steeply leftward-dipping line in the centre of the photo, which is the plane along which the rock layers to the left have slipped downwards, relative to the layers to the right of the fault.
^Lutgens, Frederick K.; Tarbuck, E.J.; Tasa, D. (illustrator) (2012).Essentials of geology (11th ed.). Boston: Prentice Hall. p. 32.ISBN978-0321714725.
^Fillmore, Robert (2010).Geological evolution of the Colorado Plateau of eastern Utah and western Colorado, including the San Juan River, Natural Bridges, Canyonlands, Arches, and the Book Cliffs. Salt Lake City: University of Utah Press. p. 337.ISBN9781607810049.
^Childs, Conrad; Manzocchi, Tom; Walsh, John J.; Bonson, Christopher G.; Nicol, Andrew; Schöpfer, Martin P.J. (February 2009). "A geometric model of fault zone and fault rock thickness variations".Journal of Structural Geology.31 (2):117–127.Bibcode:2009JSG....31..117C.doi:10.1016/j.jsg.2008.08.009.
^Fossen, Haakon (2016).Structural geology (2nd ed.). Cambridge, UK: Cambridge University Press. pp. 117, 178.ISBN9781107057647.
Davis, George H.; Reynolds, Stephen J. (1996)."Folds".Structural Geology of Rocks and Regions (2nd ed.). John Wiley & Sons. pp. 372–424.ISBN0-471-52621-5.
Hart, E.W.; Bryant, W.A. (1997). Fault rupture hazard in California: Alquist-Priolo earthquake fault zoning act with index to earthquake fault zone maps (Report). Special Publication 42. California Division of Mines and Geology.
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