An ophiolite is a section of Earth's oceanic crust and the underlying upper mantle that has been uplifted and exposed, and often emplaced onto continental crustal rocks.
48-545: The Greek word ὄφις, ophis ( snake ) is found in the name of ophiolites, because of the superficial texture of some of them. Serpentinite especially evokes a snakeskin. (The suffix -lite is from the Greek lithos , meaning "stone".) Some ophiolites have a green color. The origin of these rocks, present in many mountainous massifs , remained uncertain until the advent of plate tectonic theory. Their great significance relates to their occurrence within mountain belts such as
96-490: A Nb depletion. These chemical signatures support the ophiolites having formed in a back-arc basin of a subduction zone. Ophiolite generation and subduction may also be explained, as suggested from evidence from the Coast Range ophiolite of California and Baja California, by a change in subduction location and polarity. Oceanic crust attached to a continental margin subducts beneath an island arc. Pre-ophiolitic ocean crust
144-413: A low occurrence of silica-rich minerals; those present have a high sodium and low potassium content. The temperature gradients of the metamorphosis of ophiolitic pillow lavas and dykes are similar to those found beneath ocean ridges today. Evidence from the metal-ore deposits present in and near ophiolites and from oxygen and hydrogen isotopes suggests that the passage of seawater through hot basalt in
192-945: A passive continental margin. They include the Coast Range ophiolite of California, the Josephine ophiolite of the Klamath Mountains (California, Oregon), and ophiolites in the southern Andes of South America. Despite their differences in mode of emplacement, both types of ophiolite are exclusively supra-subduction zone (SSZ) in origin. Based on mode of occurrences, the Neoproterozoic ophiolites appear to show characteristics of both mid-oceanic ridge basalt (MORB)-type and SSZ-type ophiolites and are classified from oldest to youngest into: (1) MORB intact ophiolites (MIO); (2) dismembered ophiolites (DO); and (3) arc-associated ophiolites (AAO) (El Bahariya, 2018). Collectively,
240-457: A range of trace elements as well (that is, chemical elements occurring in amounts of 1000 ppm or less). In particular, trace elements associated with subduction zone (island arc) volcanics tend to be high in ophiolites, whereas trace elements that are high in ocean ridge basalts but low in subduction zone volcanics are also low in ophiolites. Additionally, the crystallization order of feldspar and pyroxene (clino- and orthopyroxene) in
288-436: A section. The samples are analyzed to determine their detrital remanent magnetism (DRM), that is, the polarity of Earth's magnetic field at the time a stratum was deposited. For sedimentary rocks this is possible because, as they fall through the water column, very fine-grained magnetic minerals (< 17 μm ) behave like tiny compasses , orienting themselves with Earth's magnetic field . Upon burial, that orientation
336-531: A seismic study on an ophiolite complex ( Bay of Islands, Newfoundland ) in order to establish a comparison. The study concluded that oceanic and ophiolitic velocity structures were identical, pointing to the origin of ophiolite complexes as oceanic crust. The observations that follow support this conclusion. Rocks originating on the seafloor show chemical composition comparable to unaltered ophiolite layers, from primary composition elements such as silicon and titanium to trace elements. Seafloor and ophiolitic rocks share
384-683: A subduction zone, and contact with air. A hypothesis based on research conducted on the Bay of Islands complex in Newfoundland as well as the East Vardar complex in the Apuseni Mountains of Romania suggest that an irregular continental margin colliding with an island arc complex causes ophiolite generation in a back-arc basin and obduction due to compression. The continental margin, promontories and reentrants along its length,
432-652: A type of geosyncline called eugeosynclines were characterized by producing an "initial magmatism" that in some cases corresponded to ophiolitic magmatism. As plate tectonic theory prevailed in geology and geosyncline theory became outdated ophiolites were interpreted in the new framework. They were recognized as fragments of oceanic lithosphere , and dykes were viewed as the result of extensional tectonics at mid-ocean ridges . The plutonic rocks found in ophiolites were understood as remnants of former magma chambers. In 1973, Akiho Miyashiro revolutionized common conceptions of ophiolites and proposed an island arc origin for
480-416: Is a branch of geology concerned with the study of rock layers ( strata ) and layering (stratification). It is primarily used in the study of sedimentary and layered volcanic rocks . Stratigraphy has three related subfields: lithostratigraphy (lithologic stratigraphy), biostratigraphy (biologic stratigraphy), and chronostratigraphy (stratigraphy by age). Catholic priest Nicholas Steno established
528-431: Is also commonly used to delineate the nature and extent of hydrocarbon -bearing reservoir rocks, seals, and traps of petroleum geology . Chronostratigraphy is the branch of stratigraphy that places an absolute age, rather than a relative age on rock strata . The branch is concerned with deriving geochronological data for rock units, both directly and inferentially, so that a sequence of time-relative events that created
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#1732783093365576-409: Is attached to the subducting oceanic crust, which dips away from it underneath the island arc complex. As subduction takes place, the buoyant continent and island arc complex converge, initially colliding with the promontories. However, oceanic crust is still at the surface between the promontories, not having been subducted beneath the island arc yet. The subducting oceanic crust is thought to split from
624-534: Is due to physical contrasts in rock type ( lithology ). This variation can occur vertically as layering (bedding), or laterally, and reflects changes in environments of deposition (known as facies change). These variations provide a lithostratigraphy or lithologic stratigraphy of the rock unit. Key concepts in stratigraphy involve understanding how certain geometric relationships between rock layers arise and what these geometries imply about their original depositional environment. The basic concept in stratigraphy, called
672-407: Is generated by a back-arc basin. The collision of the continent and island arc initiates a new subduction zone at the back-arc basin, dipping in the opposite direction as the first. The created ophiolite becomes the tip of the new subduction's forearc and is uplifted (over the accretionary wedge ) by detachment and compression. Verification of the two above hypotheses requires further research, as do
720-411: Is preserved. For volcanic rocks, magnetic minerals, which form in the melt, orient themselves with the ambient magnetic field, and are fixed in place upon crystallization of the lava. Oriented paleomagnetic core samples are collected in the field; mudstones , siltstones , and very fine-grained sandstones are the preferred lithologies because the magnetic grains are finer and more likely to orient with
768-454: Is that the thick gabbro layer of ophiolites calls for large magma chambers beneath mid-ocean ridges. However, seismic sounding of mid-ocean ridges has revealed only a few magma chambers beneath ridges, and these are quite thin. A few deep drill holes into oceanic crust have intercepted gabbro, but it is not layered like ophiolite gabbro. The circulation of hydrothermal fluids through young oceanic crust causes serpentinization , alteration of
816-506: The Alps and the Himalayas , where they document the existence of former ocean basins that have now been consumed by subduction . This insight was one of the founding pillars of plate tectonics , and ophiolites have always played a central role in plate tectonic theory and the interpretation of ancient mountain belts. The stratigraphic -like sequence observed in ophiolites corresponds to
864-493: The Integrated Ocean Drilling Program and other research cruises have shown that in situ ocean crust can be quite variable in thickness and composition, and that in places sheeted dikes sit directly on peridotite tectonite , with no intervening gabbros . Ophiolites have been identified in most of the world's orogenic belts . However, two components of ophiolite formation are under debate:
912-696: The closure of the Tethys Ocean . Ophiolites in Archean and Paleoproterozoic domains are rare. Most ophiolites can be divided into one of two groups: Tethyan and Cordilleran. Tethyan ophiolites are characteristic of those that occur in the eastern Mediterranean sea area, e.g. Troodos in Cyprus, and in the Middle East, such as Semail in Oman, which consist of relatively complete rock series corresponding to
960-603: The geosyncline concept. He held that Alpine ophiolites were "submarine effusions issuing along thrust faults into the active flank of an asymmetrically shortening geosyncline". The apparent lack of ophiolites in the Peruvian Andes , Steinmann theorized, was either due to the Andes being preceded by a shallow geosyncline or representing just the margin of a geosyncline. Thus, Cordilleran-type and Alpine-type mountains were to be different in this regard. In Hans Stille 's models
1008-431: The law of superposition , states: in an undeformed stratigraphic sequence, the oldest strata occur at the base of the sequence. Chemostratigraphy studies the changes in the relative proportions of trace elements and isotopes within and between lithologic units. Carbon and oxygen isotope ratios vary with time, and researchers can use those to map subtle changes that occurred in the paleoenvironment. This has led to
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#17327830933651056-427: The lithosphere -forming processes at mid-oceanic ridges . From top to bottom, the layers in the sequence are: A Geological Society of America Penrose Conference on ophiolites in 1972 defined the term "ophiolite" to include all of the layers listed above, including the sediment layer formed independently of the rest of the ophiolite. This definition has been challenged recently because new studies of oceanic crust by
1104-564: The natural remanent magnetization (NRM) to reveal the DRM. Following statistical analysis, the results are used to generate a local magnetostratigraphic column that can then be compared against the Global Magnetic Polarity Time Scale. This technique is used to date sequences that generally lack fossils or interbedded igneous rocks. The continuous nature of the sampling means that it is also a powerful technique for
1152-536: The "Steinmann Trinity": the mixture of serpentine , diabase - spilite and chert . The recognition of the Steinmann Trinity served years later to build up the theory around seafloor spreading and plate tectonics . A key observation by Steinmann was that ophiolites were associated to sedimentary rocks reflecting former deep sea environments. Steinmann himself interpreted ophiolites (the Trinity) using
1200-523: The above observations, there are inconsistencies in the theory of ophiolites as oceanic crust, which suggests that newly generated ocean crust follows the full Wilson cycle before emplacement as an ophiolite. This requires ophiolites to be much older than the orogenies on which they lie, and therefore old and cold. However, radiometric and stratigraphic dating has found ophiolites to have undergone emplacement when young and hot: most are less than 50 million years old. Ophiolites therefore cannot have followed
1248-615: The ambient field during deposition. If the ancient magnetic field were oriented similar to today's field ( North Magnetic Pole near the North Rotational Pole ), the strata would retain a normal polarity. If the data indicate that the North Magnetic Pole were near the South Rotational Pole , the strata would exhibit reversed polarity. Results of the individual samples are analyzed by removing
1296-477: The classic ophiolite assemblage and which have been emplaced onto a passive continental margin more or less intact (Tethys is the name given to the ancient sea that once separated Europe and Africa). Cordilleran ophiolites are characteristic of those that occur in the mountain belts of western North America (the " Cordillera " or backbone of the continent). These ophiolites sit on subduction zone accretionary complexes (subduction complexes) and have no association with
1344-556: The classic ophiolite occurrences thought of as being related to seafloor spreading (Troodos in Cyprus , Semail in Oman ) were found to be "SSZ" ophiolites, formed by rapid extension of fore-arc crust during subduction initiation. A fore-arc setting for most ophiolites also solves the otherwise-perplexing problem of how oceanic lithosphere can be emplaced on top of continental crust. It appears that continental accretion sediments, if carried by
1392-401: The continental margin to aid subduction. In the event that the rate of trench retreat is greater than that of the island arc complex's progression, trench rollback will take place, and by consequence, extension of the overriding plate will occur to allow the island arc complex to match the trench retreat's speed. The extension, a back-arc basin, generates oceanic crust: ophiolites. Finally, when
1440-502: The downgoing plate into a subduction zone, will jam it up and cause subduction to cease, resulting in the rebound of the accretionary prism with fore-arc lithosphere (ophiolite) on top of it. Ophiolites with compositions comparable with hotspot -type eruptive settings or normal mid-oceanic ridge basalt are rare, and those examples are generally strongly dismembered in subduction zone accretionary complexes. Ophiolites are common in orogenic belts of Mesozoic age, like those formed by
1488-660: The famous Troodos Ophiolite in Cyprus , arguing that numerous lavas and dykes in the ophiolite had calc-alkaline chemistries . Examples of ophiolites that have been influential in the study of these rocks bodies are: Serpentinite Too Many Requests If you report this error to the Wikimedia System Administrators, please include the details below. Request from 172.68.168.226 via cp1108 cp1108, Varnish XID 221994988 Upstream caches: cp1108 int Error: 429, Too Many Requests at Thu, 28 Nov 2024 08:38:13 GMT Stratigraphy Stratigraphy
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1536-493: The first, he used ophiolite for serpentinite rocks found in large-scale breccias called mélanges . In the second publication, he expanded the definition to encompass a variety of igneous rocks as well such as gabbro , diabase , ultramafic and volcanic rocks. Ophiolites thus became a name for a well-known association of rocks occurring in the Alps and Apennines of Italy. Following work in these two mountains systems, Gustav Steinmann defined what later became known as
1584-410: The full Wilson cycle and are considered atypical ocean crust. There is yet no consensus on the mechanics of emplacement, the process by which oceanic crust is uplifted onto continental margins despite the relatively low density of the latter. All emplacement procedures share the same steps nonetheless: subduction initiation, thrusting of the ophiolite over a continental margin or an overriding plate at
1632-481: The gabbros is reversed, and ophiolites also appear to have a multi-phase magmatic complexity on par with subduction zones. Indeed, there is increasing evidence that most ophiolites are generated when subduction begins and thus represent fragments of fore-arc lithosphere. This led to introduction of the term "supra-subduction zone" (SSZ) ophiolite in the 1980s to acknowledge that some ophiolites are more closely related to island arcs than ocean ridges. Consequently, some of
1680-512: The gap may be due to removal by erosion, in which case it may be called a stratigraphic vacuity. It is called a hiatus because deposition was on hold for a period of time. A physical gap may represent both a period of non-deposition and a period of erosion. A geologic fault may cause the appearance of a hiatus. Magnetostratigraphy is a chronostratigraphic technique used to date sedimentary and volcanic sequences. The method works by collecting oriented samples at measured intervals throughout
1728-591: The investigated ophiolites of the Central Eastern Desert (CED) fall into both MORB/back-arc basin basalt (BABB) ophiolites and SSZ ophiolites. They are spatially and temporally unrelated, and thus, it seems likely that the two types are not petrogenetically related. Ophiolites occur in different geological settings, and they represent change of the tectonic setting of the ophiolites from MORB to SSZ with time. The term ophiolite originated from publications of Alexandre Brongniart in 1813 and 1821. In
1776-499: The oceanic lithosphere is entirely subducted, the island arc complex's extensional regime becomes compressional. The hot, positively buoyant ocean crust from the extension will not subduct, instead obducting onto the island arc as an ophiolite. As compression persists, the ophiolite is emplaced onto the continental margin. Based on Sr and Nd isotope analyses, ophiolites have a similar composition to mid-ocean-ridge basalts, but typically have slightly elevated large ion lithophile elements and
1824-411: The origin of the sequence and the mechanism for ophiolite emplacement. Emplacement is the process of the sequence's uplift over lower density continental crust. Several studies support the conclusion that ophiolites formed as oceanic lithosphere . Seismic velocity structure studies have provided most of the current knowledge of the oceanic crust's composition. For this reason, researchers carried out
1872-628: The other hypotheses available in current literature on the subject. Scientists have drilled only about 1.5 km into the 6- to 7-kilometer-thick oceanic crust, so scientific understanding of oceanic crust comes largely from comparing ophiolite structure to seismic soundings of in situ oceanic crust. Oceanic crust generally has a layered velocity structure that implies a layered rock series similar to that listed above. But in detail there are problems, with many ophiolites exhibiting thinner accumulations of igneous rock than are inferred for oceanic crust. Another problem relating to oceanic crust and ophiolites
1920-465: The peridotites and alteration of minerals in the gabbros and basalts to lower temperature assemblages. For example, plagioclase , pyroxenes , and olivine in the sheeted dikes and lavas will alter to albite , chlorite , and serpentine , respectively. Often, ore bodies such as iron -rich sulfide deposits are found above highly altered epidosites ( epidote - quartz rocks) that are evidence of relict black smokers , which continue to operate within
1968-437: The rock layers. Strata from widespread locations containing the same fossil fauna and flora are said to be correlatable in time. Biologic stratigraphy was based on William Smith's principle of faunal succession , which predated, and was one of the first and most powerful lines of evidence for, biological evolution . It provides strong evidence for the formation ( speciation ) and extinction of species . The geologic time scale
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2016-484: The rocks formation can be derived. The ultimate aim of chronostratigraphy is to place dates on the sequence of deposition of all rocks within a geological region, and then to every region, and by extension to provide an entire geologic record of the Earth. A gap or missing strata in the geological record of an area is called a stratigraphic hiatus. This may be the result of a halt in the deposition of sediment. Alternatively,
2064-571: The seafloor spreading centers of ocean ridges today. Thus, there is reason to believe that ophiolites are indeed oceanic mantle and crust; however, certain problems arise when looking closer. Beyond issues of layer thicknesses mentioned above, a problem arises concerning compositional differences of silica (SiO 2 ) and titania (TiO 2 ). Ophiolite basalt contents place them in the domain of subduction zones (~55% silica, <1% TiO 2 ), whereas mid-ocean ridge basalts typically have ~50% silica and 1.5–2.5% TiO 2 . These chemical differences extend to
2112-405: The significance of strata or rock layering and the importance of fossil markers for correlating strata; he created the first geologic map of England. Other influential applications of stratigraphy in the early 19th century were by Georges Cuvier and Alexandre Brongniart , who studied the geology of the region around Paris. Variation in rock units, most obviously displayed as visible layering,
2160-406: The specialized field of isotopic stratigraphy. Cyclostratigraphy documents the often cyclic changes in the relative proportions of minerals (particularly carbonates ), grain size, thickness of sediment layers ( varves ) and fossil diversity with time, related to seasonal or longer term changes in palaeoclimates . Biostratigraphy or paleontologic stratigraphy is based on fossil evidence in
2208-429: The theoretical basis for stratigraphy when he introduced the law of superposition , the principle of original horizontality and the principle of lateral continuity in a 1669 work on the fossilization of organic remains in layers of sediment. The first practical large-scale application of stratigraphy was by William Smith in the 1790s and early 19th century. Known as the "Father of English geology", Smith recognized
2256-481: The vicinity of ridges dissolved and carried elements that precipitated as sulfides when the heated seawater came into contact with cold seawater. The same phenomenon occurs near oceanic ridges in a formation known as hydrothermal vents . The final line of evidence supporting the origin of ophiolites as seafloor is the region of formation of the sediments over the pillow lavas: they were deposited in water over 2 km deep, far removed from land-sourced sediments. Despite
2304-529: Was developed during the 19th century, based on the evidence of biologic stratigraphy and faunal succession. This timescale remained a relative scale until the development of radiometric dating , which was based on an absolute time framework, leading to the development of chronostratigraphy. One important development is the Vail curve , which attempts to define a global historical sea-level curve according to inferences from worldwide stratigraphic patterns. Stratigraphy
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