Fault (geology) From Wikipedia, the free encyclopedia Jump to: navigation, search "Fault line" redirects here. For other uses, see Fault line (disambiguation). "Faults" redirects here. For other uses, see Fault. This article is missing citations or needs footnotes. Please help add inline citations to guard against copyright violations and factual inaccuracies. (August 2007) In geology, a fault or fault line is a planar fracture in rock in which the rock on one side of the fracture has moved with respect to the rock on the other side. Large faults within the Earth's crust are the result of differential or shear motion and active fault zones are the causal locations of most earthquakes. Earthquakes are caused by energy release during rapid slippage along a fault. A fault that runs along the boundary between two tectonic plates is called a transform fault. Since faults do not usually consist of a single, clean fracture, the term fault zone is used when referring to the zone of complex deformation that is associated with the fault plane. The two sides of a non-vertical fault are called the hanging wall and footwall. By definition, the hanging wall occurs above the fault and the footwall occurs below the fault. This terminology comes from mining. When working a tabular ore body the miner stood with the footwall under his feet and with the hanging wall hanging above him. Fault in shales near Adelaide, Australia Contents [hide] • • • 1 Mechanics o 1.1 Microfracturing and AMR theory 2 Slip, heave, throw 3 Fault types dividing the Allegheny Plateau and the true Appalachian Mountains in Pennsylvania. United States.2 Strike-slip faults 3. Because of friction and the rigidity of the rock. [edit] Microfracturing and AMR theory Microfracturing. is controlled by the relative motion of rocks on either side of the fault surface. the rocks cannot simply glide or flow past each other.• • • • 3. A fault in ductile rocks can also release instantaneously when the strain rate is too great. where small-scale failures. The energy released by instantaneous strain release is the cause of earthquakes. a common phenomenon along transform boundaries. the accumulated potential energy is released as strain.3 Oblique-slip faults 4 Fault rock 5 See also 6 References o o o 7 External links [edit] Mechanics The Junction fault. The creation and behaviour of faults. perhaps on areas the size of a dinner plate . Strain is both accumulative and instantaneous depending on the rheology of the rock. or microseismicity. which is focused into a plane along which relative motion is accommodated — the fault. stress builds up in rocks and when it reaches a level that exceeds the strain threshold.1 Dip-slip faults 3. in both an individual small fault and within the greater fault zones which define the tectonic plates. the ductile lower crust and mantle accumulates deformation gradually via shearing whereas the brittle upper crust reacts by fracture. or instantaneous stress release to cause motion along the fault. is sometimes thought of as a symptom caused by rocks under strain. Rather. release stress under high strain conditions. after a large earthquake. It is only when sufficient microfractures link up into a large slip surface that a large seismic event or earthquake can occur. The sense of slip[clarification needed] defines the type of fault. accelerating moment release (AMR). In practise it is usually only possible to find the slip direction of faults. i. [edit] Slip. According to this theory. and an approximation of the heave and throw vector. and is a displacement vector. heave.e. The vector of slip can be qualitatively assessed by studying the fault bend folding. The warping of the rock layers on the right is likely due to drag folding. the drag folding of strata on either side of the fault. the majority of the stress is released and the frequency of microfracturing is exponentially lower. The left side moves down while the right side moves up. A related theory. Slip is defined as the relative movement of geological features present on either side of a fault plane. the direction and magnitude of heave and throw can be measured only by finding common intersection points on either side of the fault. [edit] Fault types . throw A fault in Bédarieux. This is distinct from the throw of the fault. and may be a promising tool for earthquake prediction on the scale of days to years. France.or a small area. Heave is the measured horizontal[clarification needed] offset of the fault. Similar behaviour is observed in the tremors preceding volcanic eruptions. hypothesizes that the seismicity rate accelerates in a well-behaved way prior to large earthquakes. which is the vertical offset. This is being increasingly used to predict rock failures within mines and applications are being attempted for the portions of faults within brittle rheological conditions. The hanging wall moves downward. A downthrown block between two normal faults dipping towards each other is called a graben. the fault is known as a transcurrent or strike-slip fault. Low-angle normal faults with regional tectonic significance may be designated detachment faults. Faults may be reactivated at a later time with the movement in the opposite direction to the original movement (fault inversion). Typically thrust faults move within formations by forming flats. Cross-sectional illustration of normal and reverse dip-slip faults. Flat segments of thrust fault planes are known as flats. which may have been altered by local or regional folding or tilting. but with the dip of the fault plane at less than 45°.Faults can be categorized into three groups based on the sense of slip. A normal fault may therefore become a reverse fault and vice versa. Where the slip is approximately horizontal. [edit] Dip-slip faults Dip-slip faults can be again classified into the types "reverse" and "normal". not the present-day orientation. The fault plane is the plane that represents the fracture surface of a fault. An upthrown block between two normal faults dipping away from each other is called a horst. and inclined sections of the thrust are known as ramps. Thrust faults typically form ramps. A thrust fault has the same sense of motion as a reverse fault. A fault where the relative movement (or slip) on the fault plane is approximately vertical is known as a dipslip fault. and climb up section with ramps. greater than 45°. relative to the footwall. flats and fault-bend (hanging wall and foot wall) folds. Fault-bend folds are formed by movement of the hangingwall over a non-planar fault surface and are found associated with both extensional and thrust faults. Reverse faults are indicative of shortening of the crust. A reverse fault is the opposite of a normal fault — the hanging wall moves up relative to the footwall. Alternatively such a fault can be called an extensional fault. [edit] Strike-slip faults . An oblique-slip fault has non-zero components of both strike and dip slip. Thrust faults are responsible for forming nappes and klippen in the large thrust belts. it is the orientation of the net dip and sense of slip of the fault which must be considered. A normal fault occurs when the crust is extended. For all naming distinctions. The dip of a reverse fault is relatively steep. Transform faults are often referred to as transform plate boundaries. . Strike-slip faults with left-lateral motion are also known as sinistral faults. the San Andreas Fault.One of the United States's most famous. which is a plate tectonics feature related to spreading centers such as mid-ocean ridges. it caused the massive 1906 San Francisco Earthquake. a right-lateral strike-slip fault. The fault surface is usually near vertical and the footwall moves either left or right or laterally with very little vertical motion. A special class of strike-slip faults is the transform fault. Schematic illustration of the two strike-slip fault types. [edit] Oblique-slip faults Fault along the southern boundary of Makhtesh Ramon. Those with right-lateral motion are also known as dextral faults. Negev. southern Israel. made up of deformed rock that is characteristic of the level in the crust where the faulting happened.A fault which has a component of dip-slip and a component of strike-slip is termed an oblique-slip fault. Nearly all faults will have some component of both dip-slip and strike-slip. A fault that passes through different levels of the lithosphere will have many different types of fault rock developed along its surface. [edit] Fault rock All faults have a measurable thickness. Look into the Cambrian shield of Northern Ontario. The main types of fault rock are: . Continued dip-slip displacement will tend to juxtapose fault rocks characteristic of different crustal levels. the rock types affected by the fault and the presence and nature of any mineralising fluids. it is the angle between the fault plane and a vertical plane that strikes parallel to the fault. Some oblique faults occur within transtensional and transpressional regimes. others occur where the direction of extension or shortening changes during the deformation but the earlier formed faults remain active. Marie. so defining a fault as oblique requires both dip and strike components to be measurable and significant. This effect is particularly clear in the case of detachment faults and major thrust faults. with varying degrees of overprinting. Canada. Fault rocks are classified by their textures and the implied mechanism of deformation. Inactive fault from Sudbury to Sault Ste. The hade angle is defined as the complement of the dip angle. Hess. Stephen J. (1996). ISBN 0-47152621-5. Proposed classification of fault rocks Diagrams explaining thrust fault geometries [edit] External links .• • • • • • Cataclasite . Fault Gouge ..A fault rock which is cohesive with a poorly developed or absent planar fabric. Reynolds. usually black and flinty in appearance. Davis. "The Internal Processes: Types of Faults".An incohesive.to ultrafine-grained cataclasite. pp. injection veins or as a matrix to pseudoconglomerates or breccias. and commonly containing rounded porphyroclasts and rock fragments of similar composition to minerals in the matrix Tectonic or Fault Breccia . Clay smear clay-rich fault gouge formed in sedimentary sequences containing clay-rich layers which are strongly deformed and sheared into the fault gouge. Darrel (2000).to coarse-grained cataclasite containing >30% visible fragments. Mylonite . occurring as thin planar veins.Ultrafine-grained vitreous-looking material. characterised by generally angular clasts and rock fragments in a finer-grained matrix of similar composition. George H. Tom L. pp. Rock clasts may be present Pseudotachylite . which may possess a planar fabric and containing <30% visible fragments.A fault rock which is cohesive and characterized by a well developed planar fabric resulting from tectonic reduction of grain size. NJ: Prentice Hall. John Wiley & Sons. clay-rich fine. [edit] See also Wikimedia Commons has media related to: Faults • • • • • • • Allegheny Plateau Fault mechanics Mitigation of seismic motion Seismic hazard Shear (geology) Transform fault Striation (geology) [edit] References • • • • McKnight. Structural Geology of Rocks and Regions. which infills dilation fractures in the host rock. or which is incohesive. New York. 372–424. ISBN 0-13-020263-0.. 416–7. Upper Saddle River. "Folds". Physical Geography: A Landscape Appreciation.A medium. Central California LANDSAT image of the San Andreas Fault in southern California Retrieved from "http://en.• • • Fault Motion Animations An aerial view of the San Andreas fault in the Carrizo Plain.org/wiki/Fault_(geology)" .wikipedia.