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65-518: The Nevadan Orogeny began with the formation of a continental volcanic arc due to east dipping subduction of the Farallon Plate beneath the North American Plate. Continued subduction of oceanic crust transported multiple oceanic arc terranes to the western margin of North America where they were accreted onto the edge of the continent. During the accretion of the arc terranes onto North America,

130-485: 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

195-447: A consequence of ductile shear. An important group of microstructures observed in ductile shear zones are S-planes, C-planes and C' planes. The sense of shear shown by both S-C and S-C' structures matches that of the shear zone in which they are found. Other microstructures which can give sense of shear include: Transpression regimes are formed during oblique collision of tectonic plates and during non-orthogonal subduction . Typically

260-518: A focus for hydrothermal flow through orogenic belts . They may often show some form of retrograde metamorphism from a peak metamorphic assemblage and are commonly metasomatised . Shear zones can be only inches wide, or up to several kilometres wide. Often, due to their structural control and presence at the edges of tectonic blocks, shear zones are mappable units and form important discontinuities to separate terranes. As such, many large and long shear zones are named, identical to fault systems. When

325-415: 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

390-580: A mixture of oblique-slip thrust faults and strike-slip or transform faults are formed. Microstructural evidence of transpressional regimes can be rodding lineations , mylonites , augen-structured gneisses , mica fish and so on. A typical example of a transpression regime is the Alpine Fault zone of New Zealand , where the oblique subduction of the Pacific Plate under the Indo-Australian Plate

455-920: 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,

520-458: 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

585-532: 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

650-475: A sinusoidal set of foliations formed at a shallow angle to the main shear foliation, and which curve into the main shear foliation. Such rocks are known as L-S tectonites. If the rock mass begins to undergo large degrees of lateral movement, the strain ellipse lengthens into a cigar shaped volume. At this point shear foliations begin to break down into a rodding lineation or a stretch lineation. Such rocks are known as L-tectonites. Very distinctive textures form as

715-457: A south block up and west oblique sense of movement. Transtension regimes are oblique tensional environments. Oblique, normal geologic fault and detachment faults in rift zones are the typical structural manifestations of transtension conditions. Microstructural evidence of transtension includes rodding or stretching lineations , stretched porphyroblasts , mylonites, etc. Diagrams and definitions of shear ( Wayback Machine ), by University of

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780-663: 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,

845-659: 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

910-409: 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

975-406: Is converted to oblique strike-slip movement. Here, the orogenic belt attains a trapezoidal shape dominated by oblique splay faults , steeply-dipping recumbent nappes and fault-bend folds. The Alpine Schist of New Zealand is characterised by heavily crenulated and sheared phyllite . It is being pushed up at the rate of 8 to 10 mm per year, and the area is prone to large earthquakes with

1040-408: 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

1105-466: 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 . Rocks typical of shear zones include mylonite , cataclasite , S-tectonite and L-tectonite , pseudotachylite , certain breccias and highly foliated versions of

1170-455: 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

1235-497: 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

1300-481: 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:

1365-672: 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

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1430-596: 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

1495-428: 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

1560-495: The wall rocks . A shear zone is a tabular to sheetlike, planar or curviplanar zone composed of rocks that are more highly strained than rocks adjacent to the zone. Typically this is a type of fault , but it may be difficult to place a distinct fault plane into the shear zone. Shear zones may form zones of much more intense foliation , deformation , and folding . En echelon veins or fractures may be observed within shear zones. Many shear zones host ore deposits as they are

1625-526: 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

1690-668: The Central Belt and essentially consists of volcanic arc rocks along with small amounts of chert and argillite. The Shoo Fly complex is to the East of the Calaveras greenschist complex and is dominated by quartz sandstone with small amounts of limestone and phyllite. K-Ar dating of the Tuolumne River terrane indicates it is between 190 and 170 Ma in age. During this time there would have been significant amounts of folding and thrust faulting near

1755-584: The K-Ar method and were determined to be between 169 and 209 Ma in age, which implies they were placed well before any deformation related to the Nevadan Orogeny would have occurred. As the age of these dikes are older than the deformation of the Nevadan Orogeny, it is evident that most of the deformation took place towards the western side of the Sierra Nevada, rather than in the eastern regions. The Central belt of

1820-552: The Klamath mountains involves multiple stages. The first stage of the formation of the Klamath mountains was arc magmatism on the western coast of North America which resulted in the formation of the Western Hayfork Terrane. Once the Western Hayfork Terrane was formed (and had subsequently stopped forming) the region was intruded by mafic dikes attributed to some form of extension at approximately 160 Ma. Once extension ceased in

1885-615: The Sierra Nevada consists of rocks from the Tuolumne River terrane which were accreted onto the western Margin of North America at an earlier time (>150 Ma) than the rocks of the Slate Creek terrane. In general there are two different zones in the Central belt, which are the Calaveras greenschist complex and the Shoo Fly complex . The Calaveras-greenschist complex is located in the western half of

1950-597: The Sierra Nevada while the Abrams and Salmon mica schists were thrust on top of the Stuart Fork Formation in the Klamath Mountains. Ophiolite 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. The Greek word ὄφις, ophis ( snake ) is found in the name of ophiolites, because of

2015-583: The Sierra Nevada: the Western belt, Central belt, and Eastern Belt. The Klamath Mountains are somewhat more complex in their overall structure than the Sierra Nevada. The eastern belt of the Sierra Nevada consists of the Northern Sierra Terrane. The Northern Sierra Terrane was formed from volcanism at the western edge of North America due to the subduction of an oceanic plate, which eventually resulted in

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2080-525: 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

2145-403: The accretion of the Tuolumne River and Slate Creek terranes to North America. This is analogous to the "Andean" style of orogenesis where subduction of an oceanic plate to approximately 110 km beneath the surface of Earth results in melting of the down-going slab and convecting asthenosphere. This melting may be assisted by the presence of water in what is known as flux melting . The melt from

2210-541: The accretionary ophiolite sequence in the Klamath Mountains appears to be the Josephine Ophoilite, which is dated to be about 155 to 150 Ma in age using both argon-argon (Ar-Ar) and lead-uranium (Pb-U) methods. Rather than being thrust on top of North America, the Josephine Ophiolite was accreted through a different process that involved being thrust underneath North America and then eventually being exhumed at

2275-438: The area, compression began again, resulting in the closure of a very small back arc basin produced by the extension and accreted the ophiolite sequences seen in the Klamath Mountains from the Nevadan Orogeny time (Josephine Ophiolite at 155 Ma). Continued convergence in the Klamath Mountains region would eventually lead to the emplacement of dikes and sills within the Josephine Ophiolite at approximately 153 Ma. The youngest of

2340-477: 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

2405-549: 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

2470-537: The collision zone for both the Tuolumne River terrane and the existing Northern Sierra Terrane. However, most of the deformation that would have been experienced in the collision was restricted to the Tuolumne River Terrane as minimal deformation is seen in the Eastern Belt. The rocks of the Western belt comprise dominantly sedimentary rocks including greywacke and mudstone that have undergone deformation. In

2535-401: 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

2600-420: The deformation. Shear zones which occur in more brittle rheological conditions (cooler, less confining pressure ) or at high rates of strain, tend to fail by brittle failure; breaking of minerals, which are ground up into a breccia with a milled texture. Shear zones which occur under brittle-ductile conditions can accommodate much deformation by enacting a series of mechanisms which rely less on fracture of

2665-503: 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

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2730-494: 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: Shear (geology) In 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

2795-497: 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

2860-410: 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

2925-481: 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

2990-429: The horizontal displacement of this faulting can be measured in the tens or hundreds of kilometers of length, the fault is referred to as a megashear. Megashears often indicate the edges of ancient tectonic plates. The mechanisms of shearing depend on the pressure and temperature of the rock and on the rate of shear which the rock is subjected to. The response of the rock to these conditions determines how it accommodates

3055-429: The initiation of shearing, a penetrative planar foliation is first formed within the rock mass. This manifests as realignment of textural features, growth and realignment of micas and growth of new minerals. The incipient shear foliation typically forms normal to the direction of principal shortening, and is diagnostic of the direction of shortening. In symmetric shortening, objects flatten on this shear foliation much

3120-581: 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

3185-500: 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

3250-412: 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

3315-628: 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

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3380-465: 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

3445-528: The phyllite in the Stuart Fork Formation is from the older Abrams and Salmon mica schists being thrust on top of the Stuart Fork rocks during the end of the Nevadan Orogeny. The Sierra Nevada and the Klamath Mountains were the result of continental magmatic arc and then oceanic arc accretion during the Nevadan Orogeny between 155 and 145 Ma. At nearly the same time the Eastern Belt of the Sierra Nevada

3510-412: The rock and occur within the minerals and the mineral lattices themselves. Shear zones accommodate compressive stress by movement on foliation planes. Shearing at ductile conditions may occur by fracturing of minerals and growth of sub-grain boundaries, as well as by lattice glide . This occurs particularly on platy minerals, especially micas. Mylonites are essentially ductile shear zones. During

3575-407: The same way that a round ball of treacle flattens with gravity. Within asymmetric shear zones, the behavior of an object undergoing shortening is analogous to the ball of treacle being smeared as it flattens, generally into an ellipse. Within shear zones with pronounced displacements a shear foliation may form at a shallow angle to the gross plane of the shear zone. This foliation ideally manifests as

3640-572: 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

3705-413: The sediment and crustal material between North American and the incoming arc terrane were thrust onto the continent forming ophiolite sequences that are preserved in both the Klamath Mountains and the Sierra Nevada. These mountain ranges are located in northern California-southern Oregon, and central California respectively. The accretion of arc terranes resulted in the generation of three distinct belts in

3770-495: The slab then rises up through the asthenosphere and through the crust to create large batholiths and volcanism. Although deformation in the western and central regions of the Sierra Nevada is widespread, deformation from the Nevadan Orogeny in the Eastern Belt is somewhat limited. It was determined that the deformation was minimal in the Eastern Belt by looking at dikes that had intruded the rocks which appeared to be mostly undeformed. These mostly undeformed dikes were dated using

3835-515: The southern part of the Western Belt the rocks have undergone folding as the main type of deformation. The Western Belt is generally separated from the Central Belt by the Melones fault zone which also distinguishes between the metamorphic rocks of the Western and Central Belts of the Sierra Nevada. The Western Belt rocks are interpreted to be a part of the Slate Creek terrane, which was accreted onto

3900-455: 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

3965-453: The surface. In the Klamath Mountains it has also been observed that there are two other plutons of rock that were accreted during the Nevadan Orogeny, the Abrams and Salmon mica schists of the Stuart Fork Formation. Using the potassium-argon (K-Ar) method of isotopic dating on phyllite, the age of metamorphism in the Stuart Fork Formation was determined to be about 148 Ma. The metamorphism related to

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4030-483: 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

4095-465: The western margin of North America at approximately 150 Ma. The age of these rocks was dated using potassium-argon dating (K-Ar). At the western foothills of the Sierra Nevada there are numerous dikes that have intruded the rocks that range in age from 148 to 155 Ma. These dikes are proposed to have been formed when the North American plate underwent a change in motion direction so that subduction

4160-591: Was forming, the Western Hayfork Terrane of the Klamath Mountains was being constructed. As the Nevadan Orogeny progressed, the Tuolumne River Terrane was accreted to the Sierra Nevada at approximately the same time as the formation of the Josephine Ophiolite in the Klamath Mountains (150-155 Ma). During the last stages of orogenesis, the sedimentary rocks of the Western Belt were accreted to

4225-415: Was no longer occurring in a northeast direction but in the southeast direction. The shear sense along the dikes is a sinistral shear sense which indicates later southeast subduction of the oceanic plate. The Klamath Mountains tell a similar story to the Sierra Nevada in that they are the product of multiple different accretionary events of island arc terranes. The current proposed model for the formation of

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