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57-776: The generally accepted definition of the Main Central Thrust is that it is a ductile shear zone along which the High-grade Great Himalayan Crystalline complex was placed above the low-grade to unmetamorphosed Lesser Himalayan Sequence. However, this definition is not perfect because of many difficulties and complications defining the Main Central Thrust. Many geologists have researched the Main Central Thrust using various different criteria such as lithology, metamorphic isograd, geochronology, geochemistry, and strain magnitude. None of these are reliable if used independently. Furthermore, there

114-475: A cobalt-60 interior acting as a radioactive heat source. This should take half a year to reach the oceanic Moho . Exploration can also be aided through computer simulations of the evolution of the mantle. In 2009, a supercomputer application provided new insight into the distribution of mineral deposits, especially isotopes of iron, from when the mantle developed 4.5 billion years ago. In 2023 JOIDES Resolution recovered cores of what appeared to be rock from

171-671: A broad zone which a few kilometers thick. This zone accommodates most of the ductile shear zones and brittle thrust faults between the lowermost part of the Greater Himalayan Crystalline complex and the uppermost part of the Lesser Himalayan Sequence. None of the above definitions are precise because the Main Central Thrust developed and changes its style not only vertically but also along its strike, and even through time. Also, its definition should not be limited to one thrust fault, but should be

228-468: A broader fault zone. To better understand the Main Central Thrust, more research should be done along its strike and through time. Shear zone In geology, a shear zone is a thin zone within the Earth's crust or upper mantle that has been strongly deformed, due to the walls of rock on either side of the zone slipping past each other. In the upper crust, where rock is brittle, the shear zone takes

285-433: A certain depth range - the so-called alternating zone , where brittle fracturing and plastic flow coexist. The main reason for this is found in the usually heteromineral composition of rocks, with different minerals showing different responses to applied stresses (for instance, under stress quartz reacts plastically long before feldspars do). Thus differences in lithology , grain size , and preexisting fabrics determine

342-466: A deeper discontinuity in colder regions and a shallower discontinuity in hotter regions. This discontinuity is generally linked to the transition from ringwoodite to bridgmanite and periclase . This is thermodynamically an endothermic reaction and creates a viscosity jump. Both characteristics cause this phase transition to play an important role in geodynamical models. There is another major phase transition predicted at 520 km (320 mi) for

399-414: A density of about 3.33 g/cm (0.120 lb/cu in) Upper mantle material that has come up onto the surface comprises about 55% olivine and 35% pyroxene, and 5 to 10% of calcium oxide and aluminum oxide . The upper mantle is dominantly peridotite , composed primarily of variable proportions of the minerals olivine, clinopyroxene , orthopyroxene , and an aluminous phase. The aluminous phase

456-414: A different rheological response. Yet other, purely physical factors, influence the changeover depth as well, including: In Scholz's model for a quartzo-feldspathic crust (with a geotherm taken from Southern California), the brittle–semibrittle transition starts at about 11 km depth with an ambient temperature of 300 °C. The underlying alternating zone then extends to roughly 16 km depth with

513-526: A temperature of about 360 °C. Below approximately 16 km depth, only ductile shear zones are found. The seismogenic zone , in which earthquakes nucleate, is tied to the brittle domain, the schizosphere . Below an intervening alternating zone, there is the plastosphere . In the seismogenic layer , which occurs below an upper stability transition related to an upper seismicity cutoff (situated usually at about 4–5 km depth), true cataclasites start to appear. The seismogenic layer then yields to

570-569: A wide depth-range, a great variety of different rock types with their characteristic structures are associated with shear zones. A shear zone is a zone of strong deformation (with a high strain rate ) surrounded by rocks with a lower state of finite strain . It is characterised by a length to width ratio of more than 5:1. Shear zones form a continuum of geological structures, ranging from brittle shear zones (or faults ) via brittle–ductile shear zones (or semibrittle shear zones ), ductile–brittle to ductile shear zones . In brittle shear zones,

627-513: A world record for total length for a vertical drilling string of 10,062 m (33,011 ft). The previous record was held by the U.S. vessel Glomar Challenger , which in 1978 drilled to 7,049.5 meters (23,130 feet) below sea level in the Mariana Trench . On 6 September 2012, Scientific deep-sea drilling vessel Chikyū set a new world record by drilling down and obtaining rock samples from deeper than 2,111 metres (6,926 ft) below

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684-412: Is a conversion to a more dense mineral structure, the seismic velocity rises abruptly and creates a discontinuity. At the top of the transition zone, olivine undergoes isochemical phase transitions to wadsleyite and ringwoodite . Unlike nominally anhydrous olivine, these high-pressure olivine polymorphs have a large capacity to store water in their crystal structure. This has led to the hypothesis that

741-441: Is a very thick layer of rock inside the planet, which begins just beneath the crust (at about 10 km (6.2 mi) under the oceans and about 35 km (22 mi) under the continents) and ends at the top of the lower mantle at 670 km (420 mi). Temperatures range from approximately 500 K (227 °C; 440 °F) at the upper boundary with the crust to approximately 1,200 K (930 °C; 1,700 °F) at

798-466: Is an abrupt increase of P -wave and S -wave velocities at a depth of 220 km (140 mi) (Note that this is a different "Lehmann discontinuity" than the one between the Earth's inner and outer cores labeled in the image on the right.) The transition zone is located between the upper mantle and the lower mantle between a depth of 410 km (250 mi) and 670 km (420 mi). This

855-486: Is bound below by the Main Central Thrust. Neodymium isotope composition differs across the thrust. Nd composition changes mark the Main Central Thrust. For example, an average Nd Epsilon value of −21.5 has been reported in the Lesser Himalayan Sequence while an average Nd Epsilon value of −16 has been reported in the Greater Himalayan Sequence. By strain, the Main Central Thrust is defined as

912-467: Is generally less than 10 km (6.2 mi) thick. Continental crust is about 35 km (22 mi) thick, but the large crustal root under the Tibetan Plateau is approximately 70 km (43 mi) thick. The thickness of the upper mantle is about 640 km (400 mi). The entire mantle is about 2,900 km (1,800 mi) thick, which means the upper mantle is only about 20% of

969-415: Is plagioclase in the uppermost mantle, then spinel, and then garnet below about 100 kilometres (62 mi). Gradually through the upper mantle, pyroxenes become less stable and transform into majoritic garnet . Experiments on olivines and pyroxenes show that these minerals change the structure as pressure increases at greater depth, which explains why the density curves are not perfectly smooth. When there

1026-518: Is the most complex discontinuity and marks the boundary between the upper and lower mantle. It appears in PP precursors (a wave that reflects off the discontinuity once) only in certain regions but is always apparent in SS precursors. It is seen as single and double reflections in receiver functions for P to S conversions over a broad range of depths (640–720 km, or 397–447 mi). The Clapeyron slope predicts

1083-429: Is thought to occur as a result of the rearrangement of grains in olivine to form a denser crystal structure as a result of the increase in pressure with increasing depth. Below a depth of 670 km (420 mi), due to pressure changes, ringwoodite minerals change into two new denser phases, bridgmanite and periclase. This can be seen using body waves from earthquakes , which are converted, reflected, or refracted at

1140-510: Is uncertainty because of the differences along-strike in the active ages of the Main Central Thrust. It was not all formed at the same time. The Himalayan mountain belt was produced by the collision of the Indian plate and the Eurasian plate . It is structurally dominated by three north-dipping, fault-bound geological units stacked on each other. The major faults are South Tibetan Detachment ,

1197-629: The Earth's crust, sometimes extending into the upper mantle. They can be very long-lived features and commonly show evidence of several overprinting stages of activity. Material can be transported upwards or downwards in them, the most important one being water circulating dissolved ions . This can bring about metasomatism in the host rocks and even re-fertilise mantle material. Shear zones can host economically viable mineralizations , examples being important gold deposits in Precambrian terranes. Upper mantle The upper mantle of Earth

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1254-431: The Earth's surface and outer core and the ability of the crystalline rocks at high pressure and temperature to undergo slow, creeping, viscous-like deformation over millions of years, there is a convective material circulation in the mantle. Hot material upwells , while cooler (and heavier) material sinks downward. Downward motion of material occurs at convergent plate boundaries called subduction zones . Locations on

1311-644: The Japanese vessel Chikyū to drill up to 7,000 m (23,000 ft) below the seabed. On 27 April 2012, Chikyū drilled to a depth of 7,740 metres (25,390 ft) below sea level, setting a new world record for deep-sea drilling. This record has since been surpassed by the ill-fated Deepwater Horizon mobile offshore drilling unit, operating on the Tiber prospect in the Mississippi Canyon Field, United States Gulf of Mexico, when it achieved

1368-465: The Main Central Thrust follows the kyanite isograd. Under this criterion, crystals of kyanite appear upward of several meters from the lithologic change. By the difference in U-Pb detrital zircon ages, 1.87–2.60 Ga zircons have been reported from the Lesser Himalayan Sequence which is bound above by the Main Central Thrust, and 0.8–1.0 Ga zircons have been reported from the Greater Himalayan Sequence which

1425-665: The Main Central Thrust, the Main Boundary Thrust and the Main Frontal Thrust. These units (figure 1), from south to north, are: Knowledge of the kinematics of the Himalayan fault system is not as ideal as it has long been debated. To help understand the structural position the Main Central Thrust and role it played in the tectonic evolution of Himalaya, there are three general kinematic models: extrusion model, channel flow model, tectonic wedging model. for

1482-399: The Main Central Thrust, the following definitions of the Main Central Thrust have been made based on various criteria: By lithologic criteria, the Main Central Thrust is defined as the boundary between quartzite and phyllite , from the Lesser Himalayan Sequence; and the orthogneiss biotite -rich schist , which belongs to the Greater Himalayan Crystalline complex. By metamorphic isograd,

1539-478: The alternating zone at 11 km depth. Yet big earthquakes can rupture both up to the surface and well into the alternating zone, sometimes even into the plastosphere. The deformations in shear zones are responsible for the development of characteristic fabrics and mineral assemblages reflecting the reigning pressure – temperature (pT) conditions, flow type, movement sense, and deformation history. Shear zones are therefore very important structures for unravelling

1596-417: The base of the transition zone, ringwoodite decomposes into bridgmanite (formerly called magnesium silicate perovskite), and ferropericlase . Garnet also becomes unstable at or slightly below the base of the transition zone. Kimberlites explode from the earth's interior and sometimes carry rock fragments. Some of these xenolithic fragments are diamonds that can only come from the higher pressures below

1653-429: The boundary with the lower mantle. Upper mantle material that has come up onto the surface comprises about 55% olivine , 35% pyroxene , and 5 to 10% of calcium oxide and aluminum oxide minerals such as plagioclase , spinel , or garnet , depending upon depth. The density profile through Earth is determined by the velocity of seismic waves. Density increases progressively in each layer, largely due to compression of

1710-557: The boundary, and predicted from mineral physics , as the phase changes are temperature and density-dependent and hence depth-dependent. A single peak is seen in all seismological data at 410 km (250 mi), which is predicted by the single transition from α- to β- Mg 2 SiO 4 (olivine to wadsleyite ). From the Clapeyron slope this discontinuity is expected to be shallower in cold regions, such as subducting slabs, and deeper in warmer regions, such as mantle plumes . This

1767-474: The core-mantle boundary. The highest temperature of the upper mantle is 1,200 K (930 °C; 1,700 °F). Although the high temperature far exceeds the melting points of the mantle rocks at the surface, the mantle is almost exclusively solid. The enormous lithostatic pressure exerted on the mantle prevents melting because the temperature at which melting begins (the solidus ) increases with pressure. Pressure increases as depth increases since

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1824-423: The crust. The rocks that come with this are ultramafic nodules and peridotite. The composition seems to be very similar to the crust. One difference is that rocks and minerals of the mantle tend to have more magnesium and less silicon and aluminum than the crust. The first four most abundant elements in the upper mantle are oxygen, magnesium, silicon, and iron. Exploration of the mantle is generally conducted at

1881-450: The deformation is concentrated in a narrow fracture surface separating the wall rocks, whereas in a ductile shear zone the deformation is spread out through a wider zone, the deformation state varying continuously from wall to wall. Between these end-members, there are intermediate types of brittle–ductile (semibrittle) and ductile–brittle shear zones that can combine these geometric features in different proportions. This continuum found in

1938-437: The direction of movement. With the aid of offset markers such as displaced layering and dykes , or the deflection (bending) of layering/foliation into a shear zone, one can additionally determine the sense of shear. En echelon tension gash arrays (or extensional veins), characteristic of ductile-brittle shear zones, and sheath folds can also be valuable macroscopic shear-sense indicators. Microscopic indicators consist of

1995-439: The fault system of Himalaya shown in shown in figure 2. Although the general definition of the Main Central Thrust has been given, it is not enough due to the complication and difficulties in defining the Main Central Thrust. For long, many researchers have defined the Main Central Thrust by different criteria, including by lithology that differs between the hanging wall and the footwall , by metamorphic grade changes from

2052-406: The following structures: The width of individual shear zones stretches from the grain scale to the kilometer scale. Crustal-scale shear zones (megashears) can become 10 km wide and consequently show very large displacements from tens to hundreds of kilometers. Brittle shear zones (faults) usually widen with depth and with an increase in displacements. Because shear zones are characterised by

2109-448: The form of a fracture called a fault . In the lower crust and mantle, the extreme conditions of pressure and temperature make the rock ductile . That is, the rock is capable of slowly deforming without fracture, like hot metal being worked by a blacksmith. Here the shear zone is a wider zone, in which the ductile rock has slowly flowed to accommodate the relative motion of the rock walls on either side. Because shear zones are found across

2166-421: The hanging wall to the footwall, by the different Uranium-Lead (U-Pb) detrital zircon ages, by the different Neodymium isotope compositions, by different strain, etc. Some of these criteria have also been combined. However, none of these criteria are reliable if they are used by themselves. Meanwhile, these criteria are not all be satisfied together. The dominant problems are: Despite the difficulties in defining

2223-454: The history of a specific terrane . Starting at the Earth's surface, the following rock types are usually encountered in a shear zone: Both fault gouge and cataclasites are due to abrasive wear on brittle, seismogenic faults. Mylonites start to occur with the onset of semibrittle behaviour in the alternating zone characterised by adhesive wear . Pseudotachylites can still be encountered here. By passing into greenschist facies conditions,

2280-557: The localisation of strain, some form of strain softening must occur, in order for the affected host material to deform more plastically. The softening can be brought about by the following phenomena: Furthermore, for a material to become more ductile (quasi-plastic) and undergo continuous deformation (flow) without fracturing, the following deformation mechanisms (on a grain scale) have to be taken into account: Due to their deep penetration, shear zones are found in all metamorphic facies . Brittle shear zones are more or less ubiquitous in

2337-451: The mantle is defined by a sudden increase in the speed of seismic waves, which Andrija Mohorovičić first noted in 1909; this boundary is now referred to as the Mohorovičić discontinuity or "Moho." The Moho defines the base of the crust and varies from 10 km (6.2 mi) to 70 km (43 mi) below the surface of the Earth. Oceanic crust is thinner than continental crust and

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2394-611: The material beneath has to support the weight of all the material above it. The entire mantle is thought to deform like a fluid on long timescales, with permanent plastic deformation. The highest pressure of the upper mantle is 24.0 GPa (237,000 atm) compared to the bottom of the mantle, which is 136 GPa (1,340,000 atm). Estimates for the viscosity of the upper mantle range between 10 and 10 Pa·s , depending on depth, temperature, composition, state of stress, and numerous other factors. The upper mantle can only flow very slowly. However, when large forces are applied to

2451-481: The pseudotachylites disappear and only different types of mylonites persist. Striped gneisses are high-grade mylonites and occur at the very bottom of ductile shear zones. The sense of shear in a shear zone ( dextral , sinistral , reverse or normal) can be deduced by macroscopic structures and by a plethora of microtectonic indicators. The main macroscopic indicators are striations ( slickensides ), slickenfibers , and stretching– or mineral lineations. They indicate

2508-408: The rock at increased depths. Abrupt changes in density occur where the material composition changes. The upper mantle begins just beneath the crust and ends at the top of the lower mantle. The upper mantle causes the tectonic plates to move. Crust and mantle are distinguished by composition, while the lithosphere and asthenosphere are defined by a change in mechanical properties. The top of

2565-413: The seabed rather than on land because of the oceanic crust's relative thinness as compared to the significantly thicker continental crust. The first attempt at mantle exploration, known as Project Mohole , was abandoned in 1966 after repeated failures and cost overruns. The deepest penetration was approximately 180 m (590 ft). In 2005 an oceanic borehole reached 1,416 metres (4,646 ft) below

2622-747: The seafloor from the ocean drilling vessel JOIDES Resolution . On 5 March 2007, a team of scientists on board the RRS James Cook embarked on a voyage to an area of the Atlantic seafloor where the mantle lies exposed without any crust covering, midway between the Cape Verde Islands and the Caribbean Sea . The exposed site lies approximately 3 kilometres (1.9 mi) beneath the ocean surface and covers thousands of square kilometers. The Chikyu Hakken mission attempted to use

2679-553: The seafloor off the Shimokita Peninsula of Japan in the northwest Pacific Ocean. A novel method of exploring the uppermost few hundred kilometers of the Earth was proposed in 2005, consisting of a small, dense, heat-generating probe that melts its way down through the crust and mantle while its position and progress are tracked by acoustic signals generated in the rocks. The probe consists of an outer sphere of tungsten about 1 metre (3 ft 3 in) in diameter with

2736-418: The structural geometries of shear zones reflects the different deformation mechanisms reigning in the crust, i.e. the changeover from brittle (fracturing) at or near the surface to ductile (flow) deformation with increasing depth. By passing through the brittle–semibrittle transition the ductile response to deformation is starting to set in. This transition is not tied to a specific depth, but rather occurs over

2793-403: The surface that lie over plumes are predicted to have high elevation (because of the buoyancy of the hotter, less-dense plume beneath) and to exhibit hot spot volcanism . The seismic data is not sufficient to determine the composition of the mantle. Observations of rocks exposed on the surface and other evidence reveal that the upper mantle is mafic minerals olivine and pyroxene, and it has

2850-797: The thrust type is the Moine Thrust in northwestern Scotland . An example for the subduction zone setting is the Japan Median Tectonic Line . Detachment fault related shear zones can be found in southeastern California, e.g. the Whipple Mountain Detachment Fault . An example of a huge anastomosing shear-zone is the Borborema Shear Zone in Brazil . The importance of shear zones lies in the fact that they are major zones of weakness in

2907-472: The total mantle thickness. The boundary between the upper and lower mantle is a 670 km (420 mi) discontinuity. Earthquakes at shallow depths result from strike-slip faulting ; however, below about 50 km (31 mi), the hot, high-pressure conditions inhibit further seismicity. The mantle is viscous and incapable of faulting . However, in subduction zones , earthquakes are observed down to 670 km (420 mi). The Lehmann discontinuity

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2964-403: The transition of olivine (β to γ) and garnet in the pyrolite mantle. This one has only sporadically been observed in seismological data. Other non-global phase transitions have been suggested at a range of depths. Temperatures range from approximately 500 K (227 °C; 440 °F) at the upper boundary with the crust to approximately 4,200 K (3,930 °C; 7,100 °F) at

3021-510: The transition zone may host a large quantity of water. In Earth's interior, olivine occurs in the upper mantle at depths less than 410 kilometres (250 mi), and ringwoodite is inferred within the transition zone from about 520 to 670 kilometres (320 to 420 mi) depth. Seismic activity discontinuities at about 410 kilometres (250 mi), 520 kilometres (320 mi), and 670 kilometres (420 mi) depth have been attributed to phase changes involving olivine and its polymorphs . At

3078-1142: The underlying dominant sense of movement of the terrane at that time. Some good examples of shear zones of the strike-slip type are the South Armorican Shear Zone and the North Armorican Shear Zone in Brittany , the North Anatolian Fault Zone in Turkey , and the Dead Sea Fault in Israel . Shear zones of the transform type are the San Andreas Fault in California , and the Alpine Fault in New Zealand . A shear zone of

3135-426: The upper crust. Ductile shear zones start at greenschist facies conditions and are therefore restricted to metamorphic terranes. Shear zones can occur in the following geotectonic settings: Shear zones are dependent neither on rock type nor on geological age. Most often they are not isolated in their occurrence, but commonly form fractal -scaled, linked up, anastomosing networks which reflect in their arrangement

3192-474: The upper mantle after drilling only a few hundred meters into the Atlantis Massif . The borehole reached a maximum depth of 1,268 meters and recovered 886 meters of rock samples consisting of primarily peridotite . There is debate over the extent to which the samples represent the upper mantle with some arguing the effects of seawater on the samples situates them as examples of deep lower crust. However,

3249-420: The uppermost mantle, it can become weaker, and this effect is thought to be important in allowing the formation of tectonic plate boundaries. Although there is a tendency to larger viscosity at greater depth, this relation is far from linear and shows layers with dramatically decreased viscosity, in particular in the upper mantle and at the boundary with the core. Because of the temperature difference between

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