transform fault
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A transform fault or transform boundary, is a fault along a plate boundary where the
motion In physics, motion is the phenomenon in which an object changes its Position (geometry), position with respect to time. Motion is mathematically described in terms of Displacement (geometry), displacement, distance, velocity, acceleration, speed ...
is predominantly horizontal. It ends abruptly where it connects to another plate boundary, either another transform, a spreading ridge, or a
subduction zone Subduction is a geological process in which the oceanic lithosphere is recycled Recycling is the process of converting waste materials into new materials and objects. The Energy recycling, recovery of energy from waste materials is oft ...
. A transform fault is a special case of a ''
strike-slip fault In geology, a fault is a Fracture (geology), planar fracture or discontinuity in a volume of Rock (geology), rock across which there has been significant displacement as a result of rock-mass movements. Large faults within Earth's crust (geolo ...
'' that also forms a plate boundary. Most such faults are found in
oceanic crust Oceanic crust is the uppermost layer of the oceanic portion of the Plate tectonics, tectonic plates. It is composed of the upper oceanic crust, with pillow lavas and a dike (geology), dike complex, and the lower oceanic crust, composed of troct ...
, where they accommodate the lateral offset between segments of divergent boundaries, forming a
zigzag A zigzag is a pattern made up of small corners at variable angles, though constant within the zigzag, tracing a path between two parallel lines; it can be described as both jagged and fairly regular. In geometry, this pattern is described as a ...
pattern. This is a result of oblique seafloor spreading where the direction of motion is not perpendicular to the trend of the overall divergent boundary. A smaller number of such faults are found on land, although these are generally better-known, such as the
San Andreas Fault The San Andreas Fault is a continental transform fault that extends roughly through California. It forms the tectonics, tectonic boundary between the Pacific Plate and the North American Plate, and its motion is Fault (geology)#Strike-slip fau ...
and
North Anatolian Fault The North Anatolian Fault (NAF) ( tr, Kuzey Anadolu Fay Hattı) is an active right-lateral Fault (geology)#Strike-slip faults, strike-slip fault in northern Anatolia, and is the Transform fault, transform boundary between the Eurasian Plate and ...
.


Nomenclature

Transform boundaries are also known as conservative plate boundaries because they involve no addition or loss of
lithosphere A lithosphere () is the rigid, outermost rocky shell of a terrestrial planet or natural satellite. On Earth, it is composed of the crust (geology), crust and the portion of the upper mantle (geology), mantle that behaves elastically on time sca ...
at the Earth's surface.


Background

Geophysicist and geologist John Tuzo Wilson recognized that the offsets of oceanic ridges by faults do not follow the classical pattern of an offset fence or geological marker in Reid's rebound theory of faulting, from which the sense of slip is derived. The new class of faults, called transform faults, produce slip in the opposite direction from what one would surmise from the standard interpretation of an offset geological feature. Slip along transform faults does not increase the distance between the ridges it separates; the distance remains constant in
earthquakes An earthquake (also known as a quake, tremor or temblor) is the shaking of the surface of the Earth resulting from a sudden release of energy in the Earth's lithosphere that creates seismic waves. Earthquakes can range in intensity, from ...
because the ridges are spreading centers. This
hypothesis A hypothesis (plural hypotheses) is a proposed explanation for a phenomenon. For a hypothesis to be a scientific hypothesis, the scientific method requires that one can testable, test it. Scientists generally base scientific hypotheses on prev ...
was confirmed in a study of the fault plane solutions that showed the slip on transform faults points in the opposite direction than classical interpretation would suggest.Sykes, L.R. (1967). Mechanism of earthquakes and nature of faulting on the mid-oceanic ridges, Journal of Geophysical Research, 72, 5–27.


Difference between transform and transcurrent faults

Transform faults are closely related to transcurrent faults and are commonly confused. Both types of fault are strike-slip or side-to-side in movement; nevertheless, transform faults always end at a junction with another plate boundary, while transcurrent faults may die out without a junction with another fault. Finally, transform faults form a tectonic plate boundary, while transcurrent faults do not.


Mechanics

Faults in general are focused areas of deformation or strain, which are the response of built-up stresses in the form of compression, tension, or
shear stress Shear stress, often denoted by (Greek alphabet, Greek: tau), is the component of stress (physics), stress coplanar with a material cross section. It arises from the shear force, the component of force vector parallel (geometry), parallel to the ...
in rock at the surface or deep in the Earth's subsurface. Transform faults specifically accommodate
lateral strain In continuum mechanics, lateral strain, also known as transverse strain, is defined as the ratio of the change in diameter of a circular bar of a material to its diameter due to deformation in the longitudinal direction. It occurs when under the act ...
by transferring displacement between mid-ocean ridges or subduction zones. They also act as the plane of weakness, which may result in splitting in
rift zone A rift zone is a feature of some volcanoes, especially shield volcanoes, in which a set of linear cracks (or rifts) develops in a volcanic edifice, typically forming into two or three well-defined regions along the flanks of the vent. Believed t ...
s.


Transform faults and divergent boundaries

Transform faults are commonly found linking segments of divergent boundaries (
mid-oceanic ridge A mid-ocean ridge (MOR) is a seafloor mountain system formed by plate tectonics. It typically has a depth of about and rises about above the deepest portion of an ocean basin. This feature is where seafloor spreading takes place along a Diverge ...
s or spreading centres). These mid-oceanic ridges are where new seafloor is constantly created through the
upwelling Upwelling is an physical oceanography, oceanographic phenomenon that involves wind-driven motion of dense, cooler, and usually nutrient-rich water from deep water towards the ocean surface. It replaces the warmer and usually nutrient-depleted ...
of new
basaltic Basalt (; ) is an aphanite, aphanitic (fine-grained) extrusive igneous rock formed from the rapid cooling of low-viscosity lava rich in magnesium and iron (mafic lava) exposed at or very near the planetary surface, surface of a terrestrial ...
magma Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma is found beneath the surface of the Earth, and evidence of magmatism has also been discovered on other terrestrial planets and some natural sa ...
. With new seafloor being pushed and pulled out, the older seafloor slowly slides away from the mid-oceanic ridges toward the continents. Although separated only by tens of kilometers, this separation between segments of the ridges causes portions of the seafloor to push past each other in opposing directions. This lateral movement of seafloors past each other is where transform faults are currently active. Transform faults move differently from a strike-slip fault at the mid-oceanic ridge. Instead of the ridges moving away from each other, as they do in other strike-slip faults, transform-fault ridges remain in the same, fixed locations, and the new ocean seafloor created at the ridges is pushed away from the ridge. Evidence of this motion can be found in paleomagnetic striping on the seafloor. A paper written by geophysicist Taras Gerya theorizes that the creation of the transform faults between the ridges of the mid-oceanic ridge is attributed to rotated and stretched sections of the mid-oceanic ridge. This occurs over a long period of time with the spreading center or ridge slowly deforming from a straight line to a curved line. Finally, fracturing along these planes forms transform faults. As this takes place, the fault changes from a normal fault with extensional stress to a strike-slip fault with lateral stress. In the study done by Bonatti and Crane,
peridotite Peridotite ( ) is a dense, coarse-grained igneous rock consisting mostly of the silicate minerals olivine and pyroxene. Peridotite is ultramafic, as the rock contains less than 45% silica. It is high in magnesium (Mg2+), reflecting the high prop ...
and
gabbro Gabbro () is a phaneritic (coarse-grained), mafic intrusion, intrusive igneous rock formed from the slow cooling of magnesium-rich and iron-rich magma into a crystallinity, holocrystalline mass deep beneath the Earth's surface. Slow-cooling, coa ...
rocks were discovered in the edges of the transform ridges. These rocks are created deep inside the Earth's mantle and then rapidly exhumed to the surface. This evidence helps to prove that new seafloor is being created at the mid-oceanic ridges and further supports the theory of plate tectonics. Active transform faults are between two tectonic structures or faults. Fracture zones represent the previously active transform-fault lines, which have since passed the active transform zone and are being pushed toward the continents. These elevated ridges on the ocean floor can be traced for hundreds of miles and in some cases even from one continent across an ocean to the other continent.


Types

In his work on transform-fault systems, geologist Tuzo Wilson said that transform faults must be connected to other faults or tectonic-plate boundaries on both ends; because of that requirement, transform faults can grow in length, keep a constant length, or decrease in length. These length changes are dependent on which type of fault or tectonic structure connect with the transform fault. Wilson described six types of transform faults: ''Growing length:'' In situations where a transform fault links a spreading center and the upper block of a subduction zone or where two upper blocks of subduction zones are linked, the transform fault itself will grow in length. ''Constant length:'' In other cases, transform faults will remain at a constant length. This steadiness can be attributed to many different causes. In the case of ridge-to-ridge transforms, the constancy is caused by the continuous growth by both ridges outward, canceling any change in length. The opposite occurs when a ridge linked to a subducting plate, where all the lithosphere (new seafloor) being created by the ridge is subducted, or swallowed up, by the subduction zone. Finally, when two upper subduction plates are linked there is no change in length. This is due to the plates moving parallel with each other and no new lithosphere is being created to change that length. Decreasing length faults: In rare cases, transform faults can shrink in length. These occur when two descending subduction plates are linked by a transform fault. In time as the plates are subducted, the transform fault will decrease in length until the transform fault disappears completely, leaving only two subduction zones facing in opposite directions.


Examples

The most prominent examples of the mid-oceanic ridge transform zones are in the
Atlantic Ocean The Atlantic Ocean is the second-largest of the world's five oceans, with an area of about . It covers approximately 20% of Earth's surface and about 29% of its water surface area. It is known to separate the " Old World" of Africa ...
between
South America South America is a continent entirely in the Western Hemisphere and mostly in the Southern Hemisphere, with a relatively small portion in the Northern Hemisphere at the northern tip of the continent. It can also be described as the souther ...
and
Africa Africa is the world's second-largest and second-most populous continent, after Asia in both cases. At about 30.3 million km2 (11.7 million square miles) including adjacent islands, it covers 6% of Earth's total surface area ...
. Known as the St. Paul, Romanche, Chain, and Ascension fracture zones, these areas have deep, easily identifiable transform faults and ridges. Other locations include: the East Pacific Ridge located in the South Eastern
Pacific Ocean The Pacific Ocean is the largest and deepest of Earth's five oceanic divisions. It extends from the Arctic Ocean in the north to the Southern Ocean (or, depending on definition, to Antarctica) in the south, and is bounded by the continen ...
, which meets up with
San Andreas Fault The San Andreas Fault is a continental transform fault that extends roughly through California. It forms the tectonics, tectonic boundary between the Pacific Plate and the North American Plate, and its motion is Fault (geology)#Strike-slip fau ...
to the North. Transform faults are not limited to oceanic crust and spreading centers; many of them are on
continental margin A continental margin is the outer edge of continental crust abutting oceanic crust under coastal waters. It is one of the three major zones of the ocean floor, the other two being oceanic basin, deep-ocean basins and mid-ocean ridges. The conti ...
s. The best example is the
San Andreas Fault The San Andreas Fault is a continental transform fault that extends roughly through California. It forms the tectonics, tectonic boundary between the Pacific Plate and the North American Plate, and its motion is Fault (geology)#Strike-slip fau ...
on the Pacific coast of the United States. The San Andreas Fault links the
East Pacific Rise The East Pacific Rise is a mid-ocean rise (termed an oceanic rise and not a mid-ocean ridge due to its higher rate of spreading that results in less elevation increase and more regular terrain), a divergent boundary, divergent tectonic plate bou ...
off the West coast of Mexico (Gulf of California) to the Mendocino Triple Junction (Part of the Juan de Fuca plate) off the coast of the
Northwestern United States The Northwestern United States, also known as the American Northwest or simply the Northwest, is an informal geographic region of the United States. The region consistently includes the states of Oregon, Washington (state), Washington, Idaho, ...
, making it a ridge-to-transform-style fault. The formation of the San Andreas Fault system occurred fairly recently during the
Oligocene The Oligocene ( ) is a geologic epoch (geology), epoch of the Paleogene Geologic time scale, Period and extends from about 33.9 million to 23 million years before the present ( to ). As with other older geologic periods, the rock beds that define ...
Period between 34 million and 24 million years ago. During this period, the Farallon plate, followed by the Pacific plate, collided into the
North American plate The North American Plate is a Plate tectonics, tectonic plate covering most of North America, Cuba, the Bahamas, extreme northeastern Asia, and parts of Iceland and Azores, the Azores. With an area of , it is the Earth's second largest tectonic p ...
. The collision led to the subduction of the Farallon plate underneath the North American plate. Once the spreading center separating the Pacific and the Farallon plates was subducted beneath the North American plate, the San Andreas Continental Transform-Fault system was created. In
New Zealand New Zealand ( mi, Aotearoa ) is an island country in the southwestern Pacific Ocean. It consists of two main landmasses—the North Island () and the South Island ()—and over 700 List of islands of New Zealand, smaller islands. It is the ...
, the
South Island The South Island, also officially named , is the larger of the two major islands of New Zealand in surface area, the other being the smaller but more populous North Island. It is bordered to the north by Cook Strait, to the west by the Tasman S ...
's
alpine fault The Alpine Fault is a geological fault that runs almost the entire length of New Zealand's South Island The South Island, also officially named , is the larger of the two major islands of New Zealand in surface area, the other being the smal ...
is a transform fault for much of its length. This has resulted in the folded land of the
Southland Syncline The Southland Syncline is a major geological structure located in the Southland, New Zealand, Southland Region of New Zealand, New Zealand's South Island. The syncline folds the Mesozoic Greywacke, greywackes of the Stratigraphy of New Zealand, Mu ...
being split into an eastern and western section several hundred kilometres apart. The majority of the syncline is found in Southland and
The Catlins The Catlins (sometimes referred to as The Catlins Coast) comprises an area in the southeastern corner of the South Island of New Zealand. The area lies between Balclutha, New Zealand, Balclutha and Invercargill, straddling the boundary between ...
in the island's southeast, but a smaller section is also present in the
Tasman District Tasman District () is a districts of New Zealand, local government district in the northwest of the South Island of New Zealand. It borders the Canterbury, New Zealand, Canterbury Region, West Coast, New Zealand, West Coast Region, Marlborough R ...
in the island's northwest. Other examples include: *
Middle East The Middle East ( ar, الشرق الأوسط, ISO 233: ) is a geopolitical region commonly encompassing Arabian Peninsula, Arabia (including the Arabian Peninsula and Bahrain), Anatolia, Asia Minor (Asian part of Turkey except Hatay Pro ...
's Dead Sea Transform fault *
Pakistan Pakistan ( ur, ), officially the Islamic Republic of Pakistan ( ur, , label=none), is a country in South Asia. It is the world's List of countries and dependencies by population, fifth-most populous country, with a population of almost 24 ...
's Chaman Fault *
Turkey Turkey ( tr, Türkiye ), officially the Republic of Türkiye ( tr, Türkiye Cumhuriyeti, links=no ), is a transcontinental country located mainly on the Anatolia, Anatolian Peninsula in Western Asia, with a East Thrace, small portion on th ...
's
North Anatolian Fault The North Anatolian Fault (NAF) ( tr, Kuzey Anadolu Fay Hattı) is an active right-lateral Fault (geology)#Strike-slip faults, strike-slip fault in northern Anatolia, and is the Transform fault, transform boundary between the Eurasian Plate and ...
*
North America North America is a continent in the Northern Hemisphere and almost entirely within the Western Hemisphere. It is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South America and the Car ...
's Queen Charlotte Fault *
Myanmar Myanmar, ; UK pronunciations: US pronunciations incl. . Note: Wikipedia's IPA conventions require indicating /r/ even in British English although only some British English speakers pronounce r at the end of syllables. As John C. Wells, Joh ...
's Sagaing Fault


See also

* * * * * *


References

* International Tectonic Dictionary – AAPG Memoir 7, 1967 * The Encyclopedia of Structural Geology and Plate Tectonics – Ed. by Carl K. Seyfert, 1987 {{physical oceanography, expanded=other Structural geology Plate tectonics Faults (geology) Strike-slip earthquakes