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Fault (geology)

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(Redirected fromGeologic fault)
"Fault line" redirects here. For other uses, seeFault line (disambiguation).
"Shatter belt" redirects here. For the concept in geopolitics, seeShatter belt (geopolitics).

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,afaultis aplanar fractureor discontinuity in a volume ofrockacross which there has been significant displacement as a result of rock-mass movements. Large faults withinEarth'scrustresult from the action ofplate tectonicforces, with the largest forming the boundaries between the plates, such as the megathrust faults ofsubduction zonesortransform faults.[1]Energy release associated with rapid movement onactive faultsis the cause of mostearthquakes.Faults may also displace slowly, byaseismic creep.[2]

Afault planeis theplanethat represents the fracture surface of a fault. Afault traceorfault lineis a place where the fault can be seen or mapped on the surface. A fault trace is also the line commonly plotted ongeologic mapsto represent a fault.[3][4]

Afault zoneis 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]

Mechanisms of faulting

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See also:Fault mechanics

Due tofrictionand 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.Stressbuilds up when a fault is locked, and when it reaches a level that exceeds thestrengththreshold, the fault ruptures and the accumulatedstrain energyis released in part asseismic waves,forming anearthquake.[2]

Strain occurs accumulatively or instantaneously, depending on theliquid stateof the rock; theductilelower crust andmantleaccumulate 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, heave, throw

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Slipis defined as the relative movement of geological features present on either side of a fault plane. A fault'ssense of slipis 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, thethrowof the fault is the vertical component of the separation and theheaveof 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.

Hanging wall and footwall

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Hanging & footwall

The two sides of a non-vertical fault are known as thehanging wallandfootwall.The hanging wall occurs above the fault plane and the footwall occurs below it.[14]This terminology comes from mining: when working a tabularorebody, 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.

Fault types

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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.[16] 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
  • oblique-slip,combining strike-slip and dip-slip.

Strike-slip faults

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Schematic illustration of the two strike-slip fault types, as seen from above
Further information:Strike-slip tectonics

In astrike-slip fault(also known as awrench fault,tear faultortranscurrent fault),[17]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 assinistralfaults and those with right-lateral motion asdextralfaults.[18]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 faultwhen it forms aplateboundary. This class is related to an offset in aspreading center,such as amid-ocean ridge,or, less common, within continentallithosphere,such as theDead Sea Transformin theMiddle Eastor theAlpine Faultin New Zealand. Transform faults are also referred to as "conservative" plate boundaries since the lithosphere is neither created nor destroyed.

Dip-slip faults

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Vertical cross-sectional view, along a plane perpendicular to thefault plane,illustrating normal and reverse dip-slip faults

Dip-slip faultscan be eithernormal( "extensional") orreverse.The terminology of "normal" and "reverse" comes fromcoal miningin England, where normal faults are the most common.[19]

With the passage of time, a regional reversal betweentensionalandcompressionalstresses(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.

Normal faults

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In a normal fault, the hanging wall moves downward, relative to the footwall. Thedipof most normal faults is at least 60 degrees but some normal faults dip at less than 45 degrees.[20]

Basin and range topography
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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.

Listric faults
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Normal faults can evolve intolistric faults,with their plane dip being steeper near the surface, then shallower with increased depth, 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.

  • A diagram of a listric fault.
    A diagram of a listric fault.
  • Listric faults in a cliff wall.
    Listric faults in a cliff wall.
Detachment faults
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The fault panes of listric faults can further flatten and evolve into a horizontal or near-horizontal plane, where slip progresses horizontally along adecollement.Extensionaldecollements can grow to great dimensions and formdetachment faults,which are low-angle normal faults with regionaltectonicsignificance.

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 fil in the gap. If faults form,imbrication fansordomino faultingmay form.

  • Listric fault (red line), with a resulting rollover fold
    Listric fault (red line), with a resulting rollover fold
  • A diagram showing how an imbrication fan forms
    A diagram showing how an imbrication fan forms

Reverse faults

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Reverse fault

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.

Thrust faults
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Thrust fault with a fault-bend fold

Athrust faulthas the same sense of motion as a reverse fault, but with the dip of the fault plane at less than 45°.[21][22]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 aflatand a section where the thrust fault cut upward through the stratigraphic sequence is known as aramp.[23]Typically, thrust faults movewithinformations 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 formnappesandklippenin the large thrust belts.Subductionzones are a special class of thrusts that form the largest faults on Earth and give rise to the largest earthquakes.

Oblique-slip faults

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Oblique-slip fault

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 withintranstensionalandtranspressionalregimes, and others occur where the direction of extension or shortening changes during the deformation but the earlier formed faults remain active.

Thehadeangle is defined as thecomplementof the dip angle; it is the angle between the fault plane and a vertical plane that strikes parallel to the fault.

Ring fault

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Ring faults,also known ascaldera faults,are faults that occur within collapsed volcaniccalderas[24]and the sites ofbolidestrikes, 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.[24]

Synthetic and antithetic faults

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Syntheticandantitheticare 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 DeltaStructural Style).

Fault rock

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Structure of a fault[25]
Salmon-coloredfault gougeand associated fault separates two different rock types on the left (dark gray) and right (light gray). From theGobiofMongolia.
Inactive fault fromSudburytoSault 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 theirtexturesand the implied mechanism of deformation. A fault that passes through different levels of thelithospherewill 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 faultsand 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 angularclastsand rock fragments in a finer-grainedmatrixof 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 insedimentarysequences 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 roundedporphyroclastsand rock fragments of similar composition tomineralsin the matrix
  • Pseudotachylyte– ultrafine-grained glassy-looking material, usually black andflintyin appearance, occurring as thin planarveins,injection veins or as a matrix topseudoconglomeratesorbreccias,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.[26]

Impacts on structures and people

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Ingeotechnical engineering,a fault often forms adiscontinuitythat may have a large influence on the mechanical behavior (strength, deformation, etc.) ofsoiland rock masses in, for example,tunnel,foundation,orslopeconstruction.

The level of a fault's activity can be critical for (1) locating buildings, tanks, and pipelines and (2) assessing theseismicshaking andtsunamihazard 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 theHoloceneEpoch (the last 11,700 years) of the Earth's geological history.[27]Also, faults that have shown movement during the Holocene plusPleistoceneEpochs (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 studyingsoilfeatures seen in shallow excavations andgeomorphologyseen in aerial photographs. Subsurface clues include shears and their relationships tocarbonatenodules,erodedclay, andironoxidemineralization, in the case of older soil, and lack of such signs in the case of younger soil.Radiocarbon datingoforganicmaterial buried next to or over a fault shear is often critical in distinguishing active from inactive faults. From such relationships,paleoseismologistscan estimate the sizes of pastearthquakesover the past several hundred years, and develop rough projections of future fault activity.

Faults and ore deposits

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Many ore deposits lie on or are associated with faults. This is because the fractured rock associated with fault zones allow for magma ascent[28]or the circulation of mineral-bearing fluids. Intersections of near-vertical faults are often locations of significant ore deposits.[29]

An example of a fault hosting valuableporphyry copper depositsis northern Chile'sDomeyko Faultwith deposits atChuquicamata,Collahuasi,El Abra,El Salvador,La EscondidaandPotrerillos.[30]Further south in ChileLos BroncesandEl Tenienteporphyry copper deposit lie each at the intersection of two fault systems.[29]

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.[31]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.[31]

Groundwater

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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.[32]Fault zones act asaquifersand also assist groundwater transport.

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  • Microfault showing a piercing point (the coin's diameter is 18 mm (0.71 in))
    Microfault showing apiercing point(the coin's diameter is 18 mm (0.71 in))
  • A normal fault in Morocco. 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.
    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.
  • Fault breccia from Spain
    Faultbrecciafrom Spain

See also

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References

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  1. ^Lutgens, Frederick K.; Tarbuck, E.J.; Tasa, D. (illustrator) (2012).Essentials of geology(11th ed.). Boston: Prentice Hall. p. 32.ISBN978-0321714725.
  2. ^abOhnaka, M. (2013).The Physics of Rock Failure and Earthquakes.Cambridge University Press.ISBN978-1-107-35533-0.
  3. ^USGS,Earthquake Glossary – fault trace,retrieved10 April2015
  4. ^USGS, Robert Tristram (30 April 2003),Where are the Fault Lines in the United States East of the Rocky Mountains?,archived fromthe originalon 18 November 2009,retrieved6 March2010
  5. ^| “Fault zone.”Merriam-Webster Dictionary, Merriam-Webster. Retrieved 8 Oct. 2020.
  6. ^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.
  7. ^Caine, Jonathan Saul; Evans, James P.; Forster, Craig B. (1 November 1996). "Fault zone architecture and permeability structure".Geology.24(11): 1025–1028.Bibcode:1996Geo....24.1025S.doi:10.1130/0091-7613(1996)024<1025:FZAAPS>2.3.CO;2.
  8. ^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.
  9. ^Fossen, Haakon (2016).Structural geology(Second ed.). Cambridge, United Kingdom. pp. 117, 178.ISBN9781107057647.{{cite book}}:CS1 maint: location missing publisher (link)
  10. ^SCEC & Education Module,p. 14.
  11. ^"Faults: Introduction".University of California, Santa Cruz.Archived fromthe originalon 27 September 2011.Retrieved19 March2010.
  12. ^Choi, Pom-yong; Lee, Seung Ryeol; Choi, Hyen -Il; Hwang, Jae-ha; Kwon, Seok-ki; Ko, In-sae; An, Gi-o (June 2002). "Movement history of the Andong Fault System: Geometric and tectonic approaches".Geosciences Journal.6(2): 91–102.Bibcode:2002GescJ...6...91C.doi:10.1007/BF03028280.S2CID206832817.
  13. ^Fossen 2016,p. 479.
  14. ^USGS,Hanging wall Foot wall,archived fromthe originalon 8 May 2009,retrieved2 April2010
  15. ^Tingley, J.V.; Pizarro, K.A. (2000),Traveling America's loneliest road: a geologic and natural history tour,Nevada Bureau of Mines and Geology Special Publication, vol. 26, Nevada Bureau of Mines and Geology, p. 132,ISBN978-1-888035-05-6,retrieved2 April2010
  16. ^"What is a fault and what are the different types?".USGS: Science for a Changing World.Retrieved13 October2021.
  17. ^Allaby, Michael, ed. (2015)."Strike-Slip Fault".A Dictionary of Geology and Earth Sciences(4th ed.). Oxford University Press.ISBN978-0-19-965306-5.
  18. ^Park, R.G. (2004).Foundation of Structural Geology(3 ed.). Routledge. p. 11.ISBN978-0-7487-5802-9.
  19. ^Peacock, D. C. P.; Knipe, R. J.; Sanderson, D. J. (2000). "Glossary of normal faults".Journal of Structural Geology.22(3): 298.Bibcode:2000JSG....22..291P.doi:10.1016/S0191-8141(00)80102-9.
  20. ^Oskin, Michael E. (3 June 2019)."Normal Faults".LibreTexts.Retrieved6 April2022.
  21. ^"dip slip".Earthquake Glossary.USGS.Archivedfrom the original on 23 November 2017.Retrieved13 December2017.
  22. ^"How are reverse faults different than thrust faults? In what way are they similar?".UCSB Science Line.University of California, Santa Barbara.13 February 2012.Archivedfrom the original on 27 October 2017.Retrieved13 December2017.
  23. ^Park, R.G. (2004).Foundation of Structural Geology(3 ed.). Routledge. p. 15.ISBN978-0-7487-5802-9.
  24. ^ab"Structural Geology Notebook – Caldera Faults".maps.unomaha.edu.Archivedfrom the original on 19 November 2018.Retrieved6 April2018.
  25. ^Jin-Hyuck, Choi; Paul, Edwards; Kyoungtae, Ko; Kim, Young-Seog (January 2016)."Definition and classification of fault damage zones: A review and a new methodological approach".Earth-Science Reviews.152:70-87.Bibcode:2016ESRv..152...70C.doi:10.1016/j.earscirev.2015.11.006.
  26. ^Rowe, Christie; Griffith, Ashley (2015)."Do faults preserve a record of seismic slip: A second opinion".Journal of Structural Geology.78:1–26.Bibcode:2015JSG....78....1R.doi:10.1016/j.jsg.2015.06.006.
  27. ^Brodie, Kate; Fettes, Douglas; Harte, Ben; Schmid, Rolf (29 January 2007),Structural terms including fault rock terms,International Union of Geological Sciences
  28. ^Troll, V R; Mattsson, T; Upton, B G J; Emeleus, C H; Donaldson, C H; Meyer, R; Weis, F; Dahrén, B; Heimdal, T H (9 October 2020)."Fault-Controlled Magma Ascent Recorded in the Central Series of the Rum Layered Intrusion, NW Scotland".Journal of Petrology.61(10).doi:10.1093/petrology/egaa093.hdl:10023/23208.ISSN0022-3530.
  29. ^abPiquer Romo, José Meulen; Yáñez, Gonzálo; Rivera, Orlando; Cooke, David (2019)."Long-lived crustal damage zones associated with fault intersections in the high Andes of Central Chile".Andean Geology.46(2): 223–239.doi:10.5027/andgeoV46n2-3108.Archivedfrom the original on 8 August 2019.Retrieved9 June2019.
  30. ^Robb, Laurence (2007).Introduction to Ore-Forming Processes(4th ed.).Malden, MA,United States:Blackwell ScienceLtd. p. 104.ISBN978-0-632-06378-9.
  31. ^abPiquer, José; Sanchez-Alfaro, Pablo; Pérez-Flores, Pamela (2021)."A new model for the optimal structural context for giant porphyry copper deposit formation".Geology.49(5): 597–601.Bibcode:2021Geo....49..597P.doi:10.1130/G48287.1.S2CID234008062.
  32. ^Pradhan, Rudra Mohan; Singh, Anand; Ojha, Arun Kumar; Biswal, Tapas Kumar (12 July 2022)."Structural controls on bedrock weathering in crystalline basement terranes and its implications on groundwater resources".Scientific Reports.12(1): 11815.Bibcode:2022NatSR..1211815P.doi:10.1038/s41598-022-15889-x.ISSN2045-2322.PMC9276672.PMID35821387.

Other reading

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  • 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). Vol. Special Publication 42. California Division of Mines and Geology.
  • Marquis, John; Hafner, Katrin; Hauksson, Egill,"The Properties of Fault Slip",Investigating Earthquakes through Regional Seismicity,Southern California Earthquake Center, archived fromthe originalon 25 June 2010,retrieved19 March2010
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The WikibookHistorical Geologyhas a page on the topic of:Faults
Wikimedia Commons has media related toFaults.
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