Fault (geology)

In geology, a fault is a planar fracture or discontinuity in a volume of rock, across which there has been significant displacement as a result of rock-mass movement. Large faults within the Earth's crust result from the action of plate tectonic forces, with the largest forming the boundaries between the plates, such as subduction zones or transform faults. Energy release associated with rapid movement on active faults is the cause of most earthquakes.

A fault plane is the plane that represents the fracture surface of a fault. A fault trace or fault line is a place where the fault can be seen or mapped on the surface. A fault trace is also the line commonly plotted on geologic maps to represent a fault.[1][2]

Since faults do not usually consist of a single, clean fracture, geologists use the term fault zone when referring to the zone of complex deformation associated with the fault plane.

Mechanisms of faulting

Falla normal Morro Solar Peru
Normal fault in La Herradura Formation, Morro Solar, Peru. The light layer of rock shows the displacement. A second normal fault is at the right.

Because of friction 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 called asperities. When a fault is locked stress builds up, and when it reaches a level that exceeds the strength threshold, the fault ruptures and the accumulated strain energy is released in part as seismic waves, forming an earthquake.

Strain occurs accumulatively or instantaneously, depending on the liquid state of the rock; the ductile lower crust and mantle accumulate deformation gradually via shearing, whereas the brittle upper crust reacts by fracture – instantaneous stress release – resulting in motion along the fault. A fault in ductile rocks can also release instantaneously when the strain rate is too great.

Slip, heave, throw

Fault in Seppap Gorge Morocco
A 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.

Slip is defined as the relative movement of geological features present on either side of a fault plane. A fault's sense of slip is defined as the relative motion of the rock on each side of the fault with respect to the other side.[3] In measuring the horizontal or vertical separation, the throw of the fault is the vertical component of the separation and the heave of the fault is the horizontal component, as in "Throw up and heave out".[4]

Microfault
Microfault showing a piercing point (the coin's diameter is 18 mm)

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; the direction and magnitude of heave and throw can be measured only by finding common intersection points on either side of the fault (called a piercing 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 foot wall

The two sides of a non-vertical fault are known as the hanging wall and footwall. The hanging wall occurs above the fault plane and the footwall occurs below it.[5] 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 above him.[6]

Fault types

Based on 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.
  • oblique-slip, combining strike and dip slip.

Strike-slip faults

Piqiang Fault, China detail
Satellite image of the Piqiang Fault, a northwest trending left-lateral strike-slip fault in the Taklamakan Desert south of the Tien Shan Mountains, China (40.3°N, 77.7°E)
Strike slip fault
Schematic illustration of the two strike-slip fault types.

In a strike-slip fault (also known as a wrench fault, tear fault or transcurrent fault),[7] 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 as sinistral faults. Those with right-lateral motion are also known as dextral faults.[8] 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 the transform fault, when it forms a plate boundary. This class is related to an offset in a spreading center, such as a mid-ocean ridge, or, less common, within continental lithosphere, such as the Dead Sea Transform in the Middle East or the Alpine Fault in New Zealand. Transform faults are also referred to as "conservative" plate boundaries, inasmuch as lithosphere is neither created nor destroyed.

Dip-slip faults

Normal faults - Arganda del Rey, Madrid, Spain
Normal faults in Spain, between which rock layers have slipped downwards (at photo's centre)

Dip-slip faults can be either normal ("extensional") or reverse.

In a normal fault, the hanging wall moves downward, relative to the footwall. A downthrown block between two normal faults dipping towards each other is a graben. An upthrown block between two normal faults dipping away from each other is a horst. Low-angle normal faults with regional tectonic significance may be designated detachment faults.

Nor rev
Cross-sectional illustration of normal and reverse dip-slip faults

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. The dip of a reverse fault is relatively steep, greater than 45°. The terminology of "normal" and "reverse" comes from coal-mining in England, where normal faults are the most common.[9]

A thrust fault has the same sense of motion as a reverse fault, but with the dip of the fault plane at less than 45°.[10][11] Thrust faults typically form ramps, flats and fault-bend (hanging wall and foot wall) folds.

Thrust with fault bend fold

Flat segments of thrust fault planes are known as flats, and inclined sections of the thrust are known as ramps. Typically, thrust faults move within formations by forming flats and climb up sections with ramps.

Fault-bend folds are formed by movement of the hanging wall over a non-planar fault surface and are found associated with both extensional and thrust faults.

Faults may be reactivated at a later time with the movement in the opposite direction to the original movement (fault inversion). A normal fault may therefore become a reverse fault and vice versa.

Thrust faults form nappes and klippen 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.

Oblique-slip faults

Oblique slip fault
Oblique-slip fault

A fault which has a component of dip-slip and a component of strike-slip is termed an oblique-slip fault. Nearly all faults have some component of both dip-slip and strike-slip, so defining a fault as oblique requires both dip and strike components to be measurable and significant. Some oblique faults occur within transtensional and transpressional regimes, and others occur where the direction of extension or shortening changes during the deformation but the earlier formed faults remain active.

The hade angle is defined as the complement of the dip angle; it is the angle between the fault plane and a vertical plane that strikes parallel to the fault.

Listric fault

Rollover
Listric fault (red line)

Listric faults are similar to normal faults but the fault plane curves, the dip being steeper near the surface, then shallower with increased depth. The dip may flatten into a sub-horizontal décollement, resulting in horizontal slip on a horizontal plane. The illustration shows slumping of the hanging wall along a listric fault. Where the hanging wall is absent (such as on a cliff) the footwall may slump in a manner that creates multiple listric faults.

Ring fault

Ring faults, also known as caldera faults, are faults that occur within collapsed volcanic calderas[12] and the sites of bolide strikes, such as the Chesapeake Bay impact crater. Ring faults are result of a series of overlapping normal faults, forming a circular outline. Fractures created by ring faults may be filled by ring dikes[12].

Synthetic and antithetic faults

Synthetic and antithetic faults 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 by rollover anticlines (e.g. the Niger Delta Structural Style).

Fault rock

FaultGouge
Salmon-colored fault gouge and associated fault separates two different rock types on the left (dark gray) and right (light gray). From the Gobi of Mongolia.
CREIGHTON
Inactive fault from Sudbury to Sault 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 any mineralising fluids. Fault rocks are classified by their textures and the implied mechanism of deformation. A fault that passes through different levels of the lithosphere 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 of detachment faults and major thrust faults.

The main types of fault rock include:

  • Cataclasite – a fault rock which is cohesive with a poorly developed or absent planar fabric, or which is incohesive, characterised by generally angular clasts and rock fragments in a finer-grained matrix of similar composition.
    • Tectonic or Fault breccia – a medium- to coarse-grained cataclasite containing >30% visible fragments.
    • Fault gouge – an incohesive, clay-rich fine- to ultrafine-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 in sedimentary 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 rounded porphyroclasts and rock fragments of similar composition to minerals in the matrix
  • Pseudotachylite – ultrafine-grained glassy-looking material, usually black and flinty in appearance, occurring as thin planar veins, injection veins or as a matrix to pseudoconglomerates or breccias, which infills dilation fractures in the host rock.

Impacts on structures and people

In geotechnical engineering a fault often forms a discontinuity that may have a large influence on the mechanical behavior (strength, deformation, etc.) of soil and rock masses in, for example, tunnel, foundation, or slope construction.

The level of a fault's activity can be critical for (1) locating buildings, tanks, and pipelines and (2) assessing the seismic shaking and tsunami 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 the Holocene Epoch (the last 11,700 years) of the Earth's geological history.[13] Also, faults that have shown movement during the Holocene plus Pleistocene 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 studying soil features seen in shallow excavations and geomorphology seen in aerial photographs. Subsurface clues include shears and their relationships to carbonate nodules, eroded clay, and iron oxide mineralization, in the case of older soil, and lack of such signs in the case of younger soil. Radiocarbon dating of organic 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 past earthquakes over the past several hundred years, and develop rough projections of future fault activity.

See also

Notes

  1. ^ USGS & Fault Traces
  2. ^ USGS & Fault Lines.
  3. ^ SCEC & Education Module, p. 14.
  4. ^ "Faults: Introduction". University of California, Santa Cruz. Archived from the original on 2011-09-27. Retrieved 19 March 2010.
  5. ^ USGS & Hanging Wall.
  6. ^ Tingley & Pizarro 2000, p. 132
  7. ^ Allaby 2015.
  8. ^ Park 2004.
  9. ^ Peacock D.C.P.; Knipe R.J.; Sanderson D.J. (2000). "Glossary of normal faults". Journal of Structural Geology. 22 (3): 298. doi:10.1016/S0191-8141(00)80102-9.
  10. ^ "dip slip". Earthquake Glossary. USGS. Retrieved 13 December 2017.
  11. ^ "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. Retrieved 13 December 2017.
  12. ^ a b "Structural Geology Notebook - Caldera Faults". maps.unomaha.edu. Retrieved 2018-04-06.
  13. ^ Brodie et al. 2007

External links

1857 Fort Tejon earthquake

The 1857 Fort Tejon earthquake occurred at about 8:20 a.m. (Pacific time) on January 9 in central and Southern California. One of the largest recorded earthquakes in the United States, with an estimated moment magnitude of 7.9, it ruptured the southern part of the San Andreas Fault for a length of about 225 miles (350 kilometers), between Parkfield and Wrightwood.

Though the shock was centered near Parkfield, the event is referred to as the Fort Tejon earthquake, because that was the location of the greatest damage. Fort Tejon is just north of the junction of the San Andreas and Garlock Faults, where the Tehachapi, San Emigdio, and Sierra Pelona Transverse Ranges come together.

The earthquake is the most recent large event to occur along that portion of the San Andreas Fault, and is estimated to have had a maximum perceived intensity of IX (Violent) on the Modified Mercalli scale (MM) near Fort Tejon in the Tehachapi Mountains, and along the San Andreas Fault from Mil Potrero (near Pine Mountain Club) in the San Emigdio Mountains to Lake Hughes in the Sierra Pelona Mountains. Accounts of the events' effects varied widely, including the time of the main shock as well as foreshocks that occurred at several locations earlier in the morning.

1931 Myitkyina earthquake

The 1931 Myitkyina earthquake, or also known as the 1931 Kamaing earthquake, occurred on January 28 at 02:35 local time (20:09 January 27 UTC). It was located in northern Burma, then part of British India. The magnitude of this earthquake was put at Mw 7.6. According to some sources the depth was 35 km, and according to a study of Phyo M. M. the depth was 5 to 30 km.The shock was very violent and lasted at least 30 seconds. The intensity reached MMI IX. There were numerous fissures and cracks. Sand blows were reported. The earthquake may have been caused by slip along the Sagaing Fault. The Sagaing Fault is a continental transform fault between the India Plate and the Sunda Plate. This earthquake is located along the northern Sagaing Fault. Sagaing Fault at 22° N is narrow, about 10 km wide. The part of Sagaing Fault between 25°30' and 26° is wider, with a shear zone about 70 km wide, and has four branches identified.

2009 Shizuoka earthquake

The 2009 Shizuoka earthquake occurred with a magnitude of 6.4 that hit Shizuoka Prefecture in the south of Honshū, Japan, on August 11 at 05:07 local time (August 10, 20:07 UTC).

Carbonate-hosted lead-zinc ore deposits

Carbonate-hosted lead-zinc ore deposits are important and highly valuable concentrations of lead and zinc sulfide ores hosted within carbonate (limestone, marl, dolomite) formations and which share a common genetic origin.

These ore bodies range from 0.5 million tonnes of contained ore, to 20 million tonnes or more, and have a grade of between 4% combined lead and zinc to over 14% combined lead and zinc. These ore bodies tend to be compact, fairly uniform plug-like or pipe-like replacements of their host carbonate sequences and as such can be extremely profitable mines.

This classification of ore deposits is also known as Mississippi Valley Type or MVT ore deposits, after a number of such deposits along the Mississippi River in the United States, where such ores were first recognised; these include the famed Southeast Missouri Lead District of southeastern Missouri, and deposits in northeast Iowa, southwest Wisconsin, and northwest Illinois.

Similarly Irish-type carbonate lead-zinc ores, exemplified by Lisheen Mine in County Tipperary, are formed in similar ways.

Cochabamba Fault Zone

The Cochabamba Fault Zone or Cochabamba Shear Zone is an east-southeast trending zone of sinistral strike-slip faults near the city of Cochabamba in the Bolivian Andes. The movements along Cochabamba Fault Zone are related to the bend in the Andes from running in a north-west direction to a north-south direction at this latitude. The compression of the crust at the Arica Elbow causes part of the thrust belt in the Bolivian Andes to acquire a lateral movement to escape from the compression taking place along the elbow axis.

Earth crust displacement

Earth crustal displacement or Earth crust displacement may refer to:

Plate tectonics, scientific theory which describes the large scale motions of Earth's crust (lithosphere).

Fault (geology), fracture in Earth's crust where one side moves with respect to the other side.

Supercontinent cycle, the quasi-periodic aggregation and dispersal of Earth's continental crust.

Cataclysmic pole shift hypothesis, where the axis of rotation of a planet may have shifted or the crust may have shifted dramatically.

Fault breccia

Fault breccia ( or ; Italian for "breach"), or tectonic breccia, is a breccia (a rock type consisting of angular clasts) that was formed by tectonic forces.

Fault breccia is a tectonite formed by localized zone of brittle deformation (a fault zone) in a rock.

Fault gouge

Fault gouge is a tectonite (a rock formed by tectonic forces) with a very small grain size. Fault gouge has no cohesion and it is normally an unconsolidated rock type, unless cementation took place at a later stage. A fault gouge forms in the same way as fault breccia, the latter also having larger clasts. In comparison to fault breccia, which is another incohesive fault rock, fault gouge has less visible fragments (less than 30% visible fragments regarding fault gouges, and more than 30% regarding fault breccia).Gouge-filled faults can be weak planes in rock masses. If compressive stresses are enough these can cause compressive yielding or eventually rock fracture.

Fracture mechanics

Fracture mechanics is the field of mechanics concerned with the study of the propagation of cracks in materials. It uses methods of analytical solid mechanics to calculate the driving force on a crack and those of experimental solid mechanics to characterize the material's resistance to fracture.

In modern materials science, fracture mechanics is an important tool used to improve the performance of mechanical components. It applies the physics of stress and strain behavior of materials, in particular the theories of elasticity and plasticity, to the microscopic crystallographic defects found in real materials in order to predict the macroscopic mechanical behavior of those bodies. Fractography is widely used with fracture mechanics to understand the causes of failures and also verify the theoretical failure predictions with real life failures. The prediction of crack growth is at the heart of the damage tolerance mechanical design discipline.

There are three ways of applying a force to enable a crack to propagate:

Mode I fracture – Opening mode (a tensile stress normal to the plane of the crack),

Mode II fracture – Sliding mode (a shear stress acting parallel to the plane of the crack and perpendicular to the crack front), and

Mode III fracture – Tearing mode (a shear stress acting parallel to the plane of the crack and parallel to the crack front).

Haast Schist

The Haast Schist which contains both the Alpine and Otago Schist is a metamorphic unit in the South Island of New Zealand. It extends from Central Otago, along the eastern side of the Alpine Fault to Cook Strait. There are also isolated outcrops of the Haast Schist within the central North Island. The schists were named after Haast Pass on the West Coast. The Haast Schist can be divided geographically from north to south into the Kaimanawa, Terawhiti, Marlborough, Alpine, Otago and Chatham schist.

Hakatai Shale

The Hakatai Shale is a Mesoproterozoic rock formation that outcrops in the Grand Canyon, Coconino County, Arizona. It consists of colorful strata that exhibit colors that vary from purple to red to brilliant orange on outcrop. The colors are the result of the oxidation of iron-bearing minerals in the Hakatai Shale. It consists of lower and middle members that consist of bright-red, slope-forming, highly fractured, argillaceous mudstones and shale and an upper member composed of purple and red, cliff-forming, medium-grained sandstone. Its thickness, which apparently increases eastwards, varies form 137 to 300 m (449 to 984 ft). In general, the Hakatai Shale and associated strata of the Unkar Group rocks dip northeast (10°-30°) toward normal faults that dip 60° or more toward the southwest. This can be seen at the Palisades fault in the eastern part of the main Unkar Group outcrop area (below East Rim). In addition, thick, prominent, and dark-colored basaltic sills and dikes cut across the purple to red to brilliant orange strata of the Hakatai Shale.The bright orange-red slopes of the Hakatai Shale contrasts sharply against the grayish outcrops of the Bass Formation. The outcrop of the Hakatai Shale also contrasts greatly with the steep cliffs formed by Shinumo Quartzite as seen at the base of Isis Temple. In the central Grand Canyon north of Grand Canyon Village and viewed from the south at the South Rim, the bright orange-red unit can be seen below the Isis Temple and Cheops Pyramid landforms at the intersection of Bright Angel Canyon and Granite Gorge; the Bright Angel Trail from the South Rim traverses through the geographic region to the north, the North Kaibab Trail in Bright Angel Canyon.The Hakatai Shale is part of a conformable sequence of sedimentary strata that comprise the Unkar Group. The Unkar Group is about 1,600 to 2,200 m (5,200 to 7,200 ft) thick and composed, in ascending order, of the Bass Formation, Hakatai Shale, Shinumo Quartzite, Dox Formation, and Cardenas Basalt. In ascending order, the Unkar Group is overlain by the Nankoweap Formation, about 113 to 150 m (371 to 492 ft) thick; the Chuar Group, about 1,900 m (6,200 ft) thick; and the Sixtymile Formation, about 60 m (200 ft) thick. The Grand Canyon Supergroup, of which the Unkar Group is the lowermost part, overlies deeply eroded granites, gneisses, pegmatites, and schists that comprise Vishnu Basement Rocks.

Kern Canyon Fault

The Kern Canyon Fault (Late-Quaternary Active Kern Canyon Fault) is a dextral strike-slip fault (horizontal) that runs roughly around 150 km (93 mi) beside the Kern Canyon River through the mountainous area of the Southern Sierra Nevada Batholith. The fault was a reverse fault in the Early Cretaceous era during the primal stages of the Farallon Plate subduction beneath the North American Continental Plate and fully transitioned into a strike-slip shear zone during the Late Cretaceous.Professor Robert W. Webb of the University of Chicago was the first to research the fault in 1936; He found a lava flow (Pliocene age) that inducted the northern end of the fault line where the Little Kern and Kern River coincided. Without any evidence of deformation upon the hardened lava and without any evidence found previously when investigating the fault line, Webb deemed the fault an inactive site.In 2007, Professor Elisabeth Nadin (University of Alaska Fairbanks) discovered that while mapping the faults within the Southern Sierra Nevada, there had been several accounts of activity along the Kern Canyon Fault Line well into the Quaternary Era. Her research continued into 2010, which explicitly entailed the lines of evidence that overturn the proposition that the fault line was inactive for more than 3.5 million years.

Lewis Range

The Lewis Range is a mountain range located in the Rocky Mountains of northern Montana, United States and extreme southern Alberta, Canada. It was formed as a result of the Lewis Overthrust, a geologic thrust fault resulted in the overlying of younger Cretaceous rocks by older Proterozoic rocks. The range is located within Waterton Lakes National Park in Alberta, Canada and Glacier National Park and the Bob Marshall Wilderness Complex in Montana, United States. The highest peak is Mount Cleveland at 10,466 ft (3,190 m).

Oxia Palus quadrangle

The Oxia Palus quadrangle is one of a series of 30 quadrangle maps of Mars used by the United States Geological Survey (USGS) Astrogeology Research Program. The Oxia Palus quadrangle is also referred to as MC-11 (Mars Chart-11).The quadrangle covers the region of 0° to 45° west longitude and 0° to 30° north latitude on Mars. This quadrangle contains parts of many regions: Chryse Planitia, Arabia Terra, Xanthe Terra, Margaritifer Terra, Meridiani Planum and Oxia Planum.

Mars Pathfinder landed in the Oxia Palus quadrangle at 19.13°N 33.22°W / 19.13; -33.22, on July 4, 1997. Crater names in Oxia Palus are a Who's Who for famous scientists. Besides Galilei and da Vinci, some of the people who discovered the atom and radiation are honored there: Curie, Becquerel, and Rutherford.Mawrth Vallis was strongly considered as a landing site for NASA's Curiosity Mars rover, the Mars Science Laboratory. It made it to at least the top two sites for NASA's EXoMars 2020 Rover mission. The exact location proposed for this landing is 22.16 N and 342.05 E.The Mawrth Vallis region is well studied with more than 40 papers published in peer-reviewed publications. Near the Mawrth channel is a 200 meter high plateau with many exposed layers. Spectral studies have detected clay minerals that present as a sequence of layers.

Clay minerals were probably deposited in the Early to Middle Noachian period. Later weathering exposed a variety of minerals such as kaolin, alunite, and jarosite. Later, volcanic material covered the region. This volcanic material would have protected any possible organic materials from radiation.Another site in the Oxia Palus quadrangle has been picked for the EXoMars 2020 landing is at 18.14 N and 335.76 E. This site is of interest because of a long-duration aqueous system including a delta, possible biosignatures, and a variety of clays.This quadrangle contains abundant evidence for past water in such forms as river valleys, lakes, springs, and chaos areas where water flowed out of the ground. A variety of clay minerals have been found in Oxia Palus. Clay is formed in water, and it is good for preserving microscopic evidence of ancient life. Recently, scientists have found strong evidence for a lake located in the Oxia Palus quadrangle that received drainage from Shalbatana Vallis. The study, carried out with HiRISE images, indicates that water formed a 30-mile-long canyon that opened up into a valley, deposited sediment, and created a delta. This delta and others around the basin imply the existence of a large, long-lived lake. Of special interest is evidence that the lake formed after the warm, wet period was thought to have ended. So, lakes may have been around much longer than previously thought.

In October 2015, Oxia Planum, a plain located near 18.275°N 335.368°E / 18.275; 335.368, was reported to be the preferred landing location for the ExoMars rover. An erosion-resistant layer on top of clay units may have preserved evidence of life.

Pothole (geology)

Potholes are frequently encountered during mining operations in the Bushveld Igneous Complex in South Africa. Two orebodies, the Upper Group 2 (UG2) and the Merensky Reef, host about 70% of the world's platinum group metals (PGM), and pose major extraction problems for the mining industry in their faults, dykes, joints, domes, iron-rich ultramafic pegmatoids, rolls and dunite pipes. The greatest mining problems, though, are presented by potholes.

The massive intrusion of molten magma, predating the nearby Vredefort impact by at least 30 million years, led to partial to complete melting of the cumulus floor already in place. Flow and turbulence, high temperatures and chemical reactions, sculpted and potholed the surface of the floor in a process similar to the erosion caused by running water. With the end of the outpouring, when emplacement ceased, cooling of the magma started, leaving the potholes filled, and creating a fault surface at the interface of the pothole and the filling magma. The final phase was when crystallisation of chromitite was followed by that of pyroxene and plagioclase, while the contents of the potholes, also consisting of UG2 or Merensky Reef, but having followed a different cooling profile, showed little or no crystallisation of chromitite.

The problems posed by potholes to mining operations are rooted in the fracturing and fragmentation of the material in potholes and their surroundings. These lead to ground instabilities, especially in the hanging wall, and serious safety hazards. Mining is generally abandoned when more than about 40% of the raise line is potholed. A crew for developing new raise lines is usually on standby for such cases, incurring considerable development and labour costs.

In horizontal section potholes are roughly circular to elliptical and vary in diameter from 20 m to more than 1 km. In vertical section their shape is generally dish-like and may be quite asymmetric.

Shear (geology)

Shear is the response of a rock to deformation usually by compressive stress and forms particular textures. Shear can be homogeneous or non-homogeneous, and may be pure shear or simple shear. Study of geological shear is related to the study of structural geology, rock microstructure or rock texture and fault mechanics.

The process of shearing occurs within brittle, brittle-ductile, and ductile rocks. Within purely brittle rocks, compressive stress results in fracturing and simple faulting.

Splay

Splay may refer to:

Splay, a verb meaning slant, slope or spread outwards

Splay (physiology), the difference between urine threshold and saturation

Splay (Japanese band), a J-pop band from Osaka

Splay Networks, a Sweden-headquartered group of multi-channel networks for Sweden, Finland, Norway, Denmark, and Germany

In architecture

chamfer, a beveled edge connecting two surfaces

talus (fortification), a sloping face at the base of a fortified wall

Splay (plastics), off-colored streaking that occurs in injection molded plastics

Splay tree, a type of search tree

Splay fault, geology

Splay leg, a condition in birds and poultry

Splay (album), a 1996 album by Shiner

Splay (Jim Black album), a 2002 album by Jim Black's AlasNoAxis

West Napa Fault

The West Napa Fault is a 57 km (35 mi) long geologic fault in Napa County, in the North Bay region of the San Francisco Bay Area in northern California. It is believed to be the northern extension of the Calaveras Fault in the East Bay region.

It has been mapped as a Late Pleistocene-Holocene active fault, and is considered to be predominantly a right lateral strike-slip fault. The fault was discovered in 1976 by Gene Boudreau, a ground water drilling specialist from Sebastopol.

Overviews
History of geology
Сomposition and structure
Historical geology
Motion
Water
Geophysics
Applications
Occupations
Underlying theory
Measurement conventions
Large-Scale Tectonics
Fracturing
Faulting
Foliation and Lineation
Folding
Boudinage
Kinematic Analysis
Shear zone
Overview
Subfields
Physical
phenomena

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