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46-573: The Valdivia Fracture Zone ( VFZ ) is a transform fault zone off the coast of southern Chile which runs between the continental slope near Valdivia in Chile and the Panov Seamount crossing the Chile Ridge . This tectonics article is a stub . You can help Misplaced Pages by expanding it . Transform fault Most such faults are found in oceanic crust , where they accommodate

92-496: A monoclinic crystal structure). This distinction is important in the classification of pyroxene peridotites since clinopyroxene melts more easily than orthopyroxene or olivine. The most common orthopyroxene is enstatite , Mg 2 Si 2 O 6 , in which iron substitutes for some of the magnesium. The most important clinopyroxene is diopside , CaMgSi 2 O 6 , again with some substitution of iron for magnesium ( hedenbergite , FeCaSi 2 O 6 ). Ultramafic rock in which

138-409: 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. 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 in rock at the surface or deep in

184-461: A low-cost, safe and permanent method of capturing and storing atmospheric CO 2 as part of climate change -related greenhouse gas sequestration . It was already known that peridotite reacts with CO 2 to form a solid carbonate -like limestone or marble mineral; and this process can be sped up a million times or more by simple drilling and hydraulic fracturing to allow injection of the CO 2 into

230-424: A process in which the pyroxenes and olivines are converted to green serpentine . This hydration reaction involves considerable increase in volume with concurrent deformation of the original textures. Serpentinites are mechanically weak and so flow readily within the earth. Distinctive plant communities grow in soils developed on serpentinite, because of the unusual composition of the underlying rock. One mineral in

276-467: A smaller section is also present in the Tasman District in the island's northwest. Other examples include: Peridotite Peridotite ( US : / ˈ p ɛ r ɪ d oʊ ˌ t aɪ t , p ə ˈ r ɪ d ə -/ PERR -ih-doh-tyte, pə- RID -ə- ) is a dense, coarse-grained igneous rock consisting mostly of the silicate minerals olivine and pyroxene . Peridotite is ultramafic , as

322-418: 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. [REDACTED] [REDACTED] 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,

368-439: A variety of size scales. Layered peridotites may form the base layers of layered intrusions. These are characterized by cumulate textures , characterized by a fabric of coarse (>5mm) interlocking euhedral (well-formed) crystals in a groundmass of finer crystals formed from liquid magma trapped in the cumulate. Many show poikilitic texture in which crystallization of this liquid has produced crystals that overgrow and enclose

414-448: Is a magnesium orthosilicate containing some iron with the variable formula (Mg,Fe) 2 SiO 4 . The magnesium-rich olivine of peridotites is typically olive-green in color. Pyroxenes are chain silicates having the variable formula (Ca,Na,Fe ,Mg)(Cr,Al,Fe ,Mg,Mn,Ti,V)Si 2 O 6 comprising a large group of different minerals. These are divided into orthopyroxenes (with an orthorhombic crystal structure) and clinopyroxenes (with

460-421: Is an older term for an ophiolite emplaced in a mountain belt during a continent-continent plate collision. Peridotites also occur as fragments ( xenoliths ) carried up by magmas from the mantle. Among the rocks that commonly include peridotite xenoliths are basalt and kimberlite . Although kimberlite is a variant of peridotite, kimberlite is also considered as brecciated volcanic material as well, which

506-507: Is being created to change that length. [REDACTED] [REDACTED] 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. [REDACTED] [REDACTED] The most prominent examples of

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552-488: Is bright green with some specks of black, although most hand samples tend to be darker green. Peridotitic outcrops typically range from earthy bright yellow to dark green; this is because olivine is easily weathered to iddingsite . While green and yellow are the most common colors, peridotitic rocks may exhibit a wide range of colors including blue, brown, and red. Coarse-grained igneous rocks in which mafic minerals (minerals rich in magnesium and iron ) make up over 90% of

598-435: Is constantly created through the upwelling of new basaltic magma . 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

644-403: Is converted to the higher-pressure mineral wadsleyite . Oceanic plates consist of up to about 100 km of peridotite covered by a thin crust. The crust, commonly about 6 km thick, consists of basalt, gabbro, and minor sediments. The peridotite below the ocean crust, "abyssal peridotite," is found on the walls of rifts in the deep sea floor. Oceanic plates are usually subducted back into

690-434: Is present in peridotite mostly as a consequence of alteration by hydrous fluids. Although peridotites are classified by their content of olivine, pyroxenes, and hornblende, a number of other mineral families are characteristically present in peridotites and may make up a significant fraction of their composition. For example, chromite is sometimes present in amounts of up to 50%. (A chromite composition above 50% reclassifies

736-994: Is the San Andreas Fault on the Pacific coast of the United States. The San Andreas Fault links the East Pacific Rise 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 , making it a ridge-to-transform-style fault. The formation of the San Andreas Fault system occurred fairly recently during

782-435: 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

828-455: Is why it is referred to as a source of peridotite xenoliths. Peridotite xenoliths contain osmium and other elements whose stable isotope ratios provide clues on the formation and evolution of the Earth's mantle. Such xenoliths originate from depths of up to nearly 200 kilometers (120 mi) or more. The volcanic equivalent of peridotites are komatiites , which were mostly erupted early in

874-705: The Oligocene 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 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,

920-430: The Earth's history and are rare in rocks younger than Archean in age. Small pieces of peridotite have been found in lunar breccias. The rocks of the peridotite family are uncommon at the surface and are highly unstable, because olivine reacts quickly with water at typical temperatures of the upper crust and at the Earth's surface. Many, if not most, surface outcrops have been at least partly altered to serpentinite ,

966-420: The Earth's subsurface. Transform faults specifically accommodate lateral strain 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 zones . Transform faults are commonly found linking segments of divergent boundaries ( mid-oceanic ridges or spreading centres). These mid-oceanic ridges are where new seafloor

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1012-461: The Earth's surface. 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

1058-695: The San Andreas Continental Transform-Fault system was created. In New Zealand , the South Island 's Alpine Fault is a transform fault for much of its length. This has resulted in the folded land of the Southland Syncline being split into an eastern and western section several hundred kilometres apart. The majority of the syncline is found in Southland and The Catlins in the island's southeast, but

1104-456: 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

1150-719: The deformation associated with their tectonic mode of emplacement. Peridotites have two primary modes of origin: as mantle rocks formed during the accretion and differentiation of the Earth, or as cumulate rocks formed by precipitation of olivine ± pyroxenes from basaltic or ultramafic magmas. These magmas are ultimately derived from the upper mantle by partial melting of mantle peridotites. Mantle peridotites are sampled as ophiolites in collisional mountain ranges, as xenoliths in basalt or kimberlite, or as abyssal peridotites (sampled from ocean floor). These rocks represent either fertile mantle (lherzolite) or partially depleted mantle (harzburgite, dunite). Alpine peridotites may be either of

1196-401: The depth of formation of the peridotite. For example, the aluminium in lhertzolite is present as plagioclase at depths shallower than about 20 kilometers (12 mi), while it is present as spinel between 20 km and 60 kilometers (37 mi) and as garnet below 60 km. Peridotite is the dominant rock of the Earth's mantle above a depth of about 400 km; below that depth, olivine

1242-442: 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 and gabbro 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

1288-528: The fraction of pyroxenes exceeds 60% are classified as pyroxenites rather than peridotites. Pyroxenes are typically dark in color. Hornblende is an amphibole , a group of minerals resembling pyroxenes but with a double chain structure incorporating water. Hornblende itself has a highly variable composition, ranging from tschermakite ( Ca 2 (Mg,Fe) 3 Al 2 Si 6 Al 2 O 22 (OH) 2 ) to pargasite ( NaCa 2 (Mg,Fe) 4 AlSi 6 Al 2 O 22 (OH) 2 ) with many other variations in composition. It

1334-525: The lateral offset between segments of divergent boundaries , forming a zigzag pattern. This results from 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 and North Anatolian Fault . Transform boundaries are also known as conservative plate boundaries because they involve no addition or loss of lithosphere at

1380-606: The mantle in subduction zones . However, pieces can be emplaced into or overthrust on continental crust by a process called obduction , rather than carried down into the mantle. The emplacement may occur during orogenies , as during collisions of one continent with another or with an island arc . The pieces of oceanic plates emplaced within continental crust are referred to as ophiolites . Typical ophiolites consist mostly of peridotite plus associated rocks such as gabbro , pillow basalt , diabase sill-and-dike complexes, and red chert. Alpine peridotite or orogenic peridotite massif

1426-846: The mid-oceanic ridge transform zones are in the Atlantic Ocean between South America and Africa . 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 , which meets up with San Andreas Fault to the North. Transform faults are not limited to oceanic crust and spreading centers; many of them are on continental margins . The best example

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1472-424: The nodules preserve isotope ratios of osmium and other elements that record processes that occurred when Earth was formed, and so they are of special interest to paleogeologists because they provide clues to the early composition of Earth's mantle and the complexities of the processes that occurred. The word peridotite comes from the gemstone peridot , which consists of pale green olivine. Classic peridotite

1518-451: The ophiolite association and representing the uppermost mantle below ocean basins, or masses of subcontinental mantle emplaced along thrust faults in mountain belts. Layered peridotites are igneous sediments and form by mechanical accumulation of dense olivine crystals. They form from mantle-derived magmas, such as those of basalt composition. Peridotites associated with Alaskan-type ultramafic complexes are cumulates that probably formed in

1564-407: The original cumulus crystals (called chadrocrysts ). Another texture is a well-annealed texture of equal sized anhedral crystals with straight grain boundaries intersecting at 120°. This may result when slow cooling allowed recrystallization to minimize surface energy. Cataclastic texture, showing irregular fractures and deformation twinning of olivine grains, is common in peridotites because of

1610-505: The other continent. 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

1656-422: The relative proportions of pyroxenes , chromite , plagioclase , and amphibole . Peridotite is the dominant rock of the upper part of Earth's mantle . The compositions of peridotite nodules found in certain basalts are of special interest along with diamond pipes ( kimberlite ), because they provide samples of Earth's mantle brought up from depths ranging from about 30 km to 200 km or more. Some of

1702-471: The rock as a peridotitic chromitite .) Other common accessory minerals include spinel , garnet , biotite , or magnetite . A peridotite containing significant amounts of one of these minerals may have its classification refined accordingly; for example, if a lhertzolite contains up to 5% spinel, it is a spinel-bearing lhertzolite , while for amounts up to 50%, it would be classified as a spinel lhertzolite . The accessory minerals can be useful for estimating

1748-402: The rock contains less than 45% silica . It is high in magnesium (Mg ), reflecting the high proportions of magnesium-rich olivine, with appreciable iron . Peridotite is derived from Earth's mantle , either as solid blocks and fragments, or as crystals accumulated from magmas that formed in the mantle. The compositions of peridotites from these layered igneous complexes vary widely, reflecting

1794-598: The root zones of volcanoes. Cumulate peridotites are also formed in komatiite lava flows. Komatiites are high temperature partial melts of peridotite characterized by a high degree of partial melting deep below the surface. Eclogite , a rock similar to basalt in composition, is composed primarily of omphacite (sodic clinopyroxene) and pyrope -rich garnet. Eclogite is associated with peridotite in some xenolith occurrences; it also occurs with peridotite in rocks metamorphosed at high pressures during processes related to subduction. Peridotite may potentially be used in

1840-444: 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,

1886-428: The serpentine group, chrysotile , is a type of asbestos. Most peridotite is green in color due to its high olivine content. However, peridotites can range in color from greenish-gray to nearly black to pale yellowish-green. Peridotite weathers to form a distinctive brown crust in subaerial exposures and to a deep orange color in submarine exposures. Peridotites can take on a massive form or may be in layers on

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1932-634: 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 because the ridges are spreading centers. This hypothesis 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. 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

1978-793: The subsurface peridotite formation. Peridotite is named for the gemstone peridot , a glassy green gem originally mined on St. John's Island in the Red Sea and now mined on the San Carlos Apache Indian Reservation in Arizona. Peridotite that has been hydrated at low temperatures is the protolith for serpentinite , which may include chrysotile asbestos (a form of serpentine) and talc . Layered intrusions with cumulate peridotite are typically associated with sulfide or chromite ores. Sulfides associated with peridotites form nickel ores and platinoid metals; most of

2024-408: 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

2070-440: The total volume of these four mineral families in the rock. Peridotites are further classified as follows: Mantle peridotite is highly enriched in magnesium, with a typical magnesium number of 89. In other words, of the total content of iron plus magnesium, 89 mol% is magnesium. This is reflected in the composition of the mafic minerals making up the peridotite. Olivine is the essential mineral found in all peridotites. It

2116-431: The volume of the rock are classified as ultramafic rocks . Such rocks typically contain less than 45% silica. Ultramafic rocks are further classified by their relative proportions of olivine , orthopyroxene , clinopyroxene , and hornblende , which are the most abundant families of mafic minerals in most ultramafic rocks. Peridotite is then defined as coarse-grained ultramafic rock in which olivine makes up 40% or more of

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