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The '''Main Central Thrust ''' is a major fault where the [[Indian Plate]] has pushed under the [[Asian Plate]] along the [[Himalayas|Himalaya]]. The fault slopes down to the north and is exposed on the surface in a NW-SE direction (strike). It is a [[thrust fault]] that continues along 2200&nbsp;km of the Himalaya mountain belt.<ref name=":1">Upreti, B. N. "[http://www.sciencedirect.com/science/article/pii/S1367912099000474 An overview of the stratigraphy and tectonics of the Nepal Himalaya]" ''Journal of Asian Earth Sciences'' 17.5 (1999): 577-606.</ref>
The '''Main Central Thrust ''' is a major fault where the [[Indian Plate]] has pushed under the [[Asian Plate]] along the [[Himalayas|Himalaya]]. The fault slopes down to the north and is exposed on the surface in a NW-SE direction (strike). It is a [[thrust fault]] that continues along 2200&nbsp;km of the Himalaya mountain belt.<ref name=":1">Upreti, B. N. "[http://www.sciencedirect.com/science/article/pii/S1367912099000474 An overview of the stratigraphy and tectonics of the Nepal Himalaya]" ''Journal of Asian Earth Sciences'' 17.5 (1999): 577–606.</ref>


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.<ref name=":3">Helm, A., and A. Gansser. "Central Himalaya, Geological observations of the Swiss expedition 1936." ''Memoires de la Societe Helvetique des Sciences Naturelles'' 73.1,245 (1939).</ref> However, this definition is not perfect because of many difficulties and complications defining the Main Central Thrust.
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.<ref name=":3">Helm, A., and A. Gansser. "Central Himalaya, Geological observations of the Swiss expedition 1936." ''Memoires de la Societe Helvetique des Sciences Naturelles'' 73.1,245 (1939).</ref> 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,<ref name=":3" /><ref name=":6" /> metamorphic isograd,<ref name=":7" /> geochronology,<ref name=":8" /> geochemistry,<ref name=":9" /> and strain magnitude.<ref name=":10" /><ref name=":5" /> None of these is reliable if used independently. Furthermore, there 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.
Many geologists have researched the Main Central Thrust using various different criteria such as lithology,<ref name=":3" /><ref name=":6" /> metamorphic isograd,<ref name=":7" /> geochronology,<ref name=":8" /> geochemistry,<ref name=":9" /> and strain magnitude.<ref name=":10" /><ref name=":5" /> None of these are reliable if used independently. Furthermore, there 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.
[[File:2 2 himal tecto units.png|thumb|Figure 2. Simplified geological map of Himalaya.<ref>Pierre Dèzes 1999, "[http://doc.rero.ch/record/499 Tectonic and metamorphic Evolution of the Central Himalayan Domain in Southeast Zanskar (Kashmir, India)]". Mémoires de Géologie (Lausanne) No. 32, ISSN 1015-3578</ref> The Main Central Thrust is indicated by solid line and triangles.|599x599px]]
[[File:2 2 himal tecto units.png|thumb|Figure 2. Simplified geological map of Himalaya.<ref>Pierre Dèzes 1999, "[http://doc.rero.ch/record/499 Tectonic and metamorphic Evolution of the Central Himalayan Domain in Southeast Zanskar (Kashmir, India)]". Mémoires de Géologie (Lausanne) No. 32, ISSN 1015-3578</ref> The Main Central Thrust is indicated by solid line and triangles.|599x599px]]


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The Himalayan mountain belt was produced by the collision of the [[Indian Plate]] and the [[Asian Plate]]. It is structurally dominated by three north-dipping, fault-bound geological units stacked on each other. The major faults are [[South Tibetan Detachment]], the Main Central Thrust, the Main Boundary Thrust and the Main Frontal Thrust.<ref name=":3" /> These units (figure 3), from south to north, are:
The Himalayan mountain belt was produced by the collision of the [[Indian Plate]] and the [[Asian Plate]]. It is structurally dominated by three north-dipping, fault-bound geological units stacked on each other. The major faults are [[South Tibetan Detachment]], the Main Central Thrust, the Main Boundary Thrust and the Main Frontal Thrust.<ref name=":3" /> These units (figure 3), from south to north, are:


#The Lesser Himalayan Sequence, which is mainly composed of low-grade Proterozoic metasediments to unmetamorphosed rocks, fringed by the Main Boundary Thrust and the Main Central Thrust
# the Lesser Himalayan Sequence, which is mainly composed of low-grade Proterozoic metasediments to unmetamorphosed rocks, fringed by the Main Boundary Thrust and the Main Central Thrust;
#The Greater Himalayan Crystalline complex, which is mainly composed by high-grade [[gneiss]] and [[migmatite]], fringed below by the Main Central Thrust and the South Tibetan Detachment
# the Greater Himalayan Crystalline complex, which is mainly composed by high-grade [[gneiss]] and [[migmatite]], fringed below by the Main Central Thrust and the South Tibetan Detachment; and
#The Tethyan Himalayan Sequence, mainly composed by [[Proterozoic]] to [[Eocene]] sediments. deformed in an [[Paleogene]] fold-thrust belt, fringed below by the South Tibetan Detachment.<ref name=":0">Webb, A. Alexander G. "[http://geology.lsu.edu/files/Alex%20Webb%20Files/item61473.pdf Preliminary balanced palinspastic reconstruction of Cenozoic deformation across the Himachal Himalaya (northwestern India)]."''Geosphere'' 9.3 (2013): 572-587.</ref>
# the Tethyan Himalayan Sequence, mainly composed by [[Proterozoic]] to [[Eocene]] sediments, deformed in an [[Paleogene]] fold-thrust belt, fringed below by the South Tibetan Detachment.<ref name=":0">Webb, A. Alexander G. "[http://geology.lsu.edu/files/Alex%20Webb%20Files/item61473.pdf Preliminary balanced palinspastic reconstruction of Cenozoic deformation across the Himachal Himalaya (northwestern India)]."''Geosphere'' 9.3 (2013): 572–587.</ref>


[[File:The Main Central Thrust.png|thumb|306px|Figure 3. Cross section showing the Main Central Thrust.<ref>Ali, Jason R., et al. "Paleomagnetic data support Early Permian age for the Abor Volcanics in the lower Siang Valley, NE India: significance for Gondwana-related break-up models." [[Thrust|Journal of Asian Earth Sciences]] 50 (2012): 105-115.</ref> MBT: the Main Boundary Thrust; MCT: the Main Central Thrust; STDT: the South Tibetan Detachment Fault. YTS: YalungTsangpo Suture. Modified after Ali, Jason R., et al., 2012.]]
[[File:The Main Central Thrust.png|thumb|306px|Figure 3. Cross section showing the Main Central Thrust.<ref>Ali, Jason R., et al. "Paleomagnetic data support Early Permian age for the Abor Volcanics in the lower Siang Valley, NE India: significance for Gondwana-related break-up models." [[Thrust|Journal of Asian Earth Sciences]] 50 (2012): 105–115.</ref> MBT: the Main Boundary Thrust; MCT: the Main Central Thrust; STDT: the South Tibetan Detachment Fault. YTS: YalungTsangpo Suture. Modified after Ali, Jason R., et al., 2012.]]


=== Kinematic models ===
=== Kinematic models ===
[[File:Kinematic models.png|thumb|Figure 4. Kinematics models of Himalayan orogen (modified from Webb et al, 2011) showing three models of how the Greater Himalayan Crystalline complex was placed over the Lesser Himalayan Sequence. LHS: the Lesser Himalayan Sequence; GHC: the Greater Himalayan Crystalline complex; THS: the Tethyan Himalayan Sequence; MCT: the Main Central Thrust; STD: the South Tibetan Detachment; ITS: Indus-Tsangpo suture zone.|621x621px]]
[[File:Kinematic models.png|thumb|Figure 4. Kinematics models of Himalayan orogen (modified from Webb et al, 2011) showing three models of how the Greater Himalayan Crystalline complex was placed over the Lesser Himalayan Sequence. LHS: the Lesser Himalayan Sequence; GHC: the Greater Himalayan Crystalline complex; THS: the Tethyan Himalayan Sequence; MCT: the Main Central Thrust; STD: the South Tibetan Detachment; ITS: Indus-Tsangpo suture zone.|621x621px]]
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,<ref>Burchfiel, B. C., and L. H. Royden. "North-south extension within the convergent Himalayan region." ''Geology'' 13.10 (1985): 679-682.</ref> channel flow model,<ref>Beaumont, C., et al. "Himalayan tectonics explained by extrusion of a low-viscosity crustal channel coupled to focused surface denudation." ''Nature''414.6865 (2001): 738-742.</ref> tectonic wedging model.<ref>Webb, A. Alexander G., et al. "The leading edge of the Greater Himalayan Crystalline complex revealed in the NW Indian Himalaya: Implications for the evolution of the Himalayan orogen." ''Geology'' 35.10 (2007): 955-958.</ref><ref>Webb, A. Alexander G. "Preliminary balanced palinspastic reconstruction of Cenozoic deformation across the Himachal Himalaya (northwestern India)." ''Geosphere'' 9.3 (2013): 572-587.</ref> for the fault system of Himalaya shown in shown in figure 4.
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,<ref>Burchfiel, B. C., and L. H. Royden. "North-south extension within the convergent Himalayan region." ''Geology'' 13.10 (1985): 679–682.</ref> channel flow model,<ref>Beaumont, C., et al. "Himalayan tectonics explained by extrusion of a low-viscosity crustal channel coupled to focused surface denudation." ''Nature''414.6865 (2001): 738–742.</ref> tectonic wedging model.<ref>Webb, A. Alexander G., et al. "The leading edge of the Greater Himalayan Crystalline complex revealed in the NW Indian Himalaya: Implications for the evolution of the Himalayan orogen." ''Geology'' 35.10 (2007): 955–958.</ref><ref>Webb, A. Alexander G. "Preliminary balanced palinspastic reconstruction of Cenozoic deformation across the Himachal Himalaya (northwestern India)." ''Geosphere'' 9.3 (2013): 572–587.</ref> for the fault system of Himalaya shown in shown in figure 4.


== Various definitions of the Main Central Thrust ==
== Various definitions of the Main Central Thrust ==
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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.
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 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.<ref name=":5">Searle, Michael P., et al. "Defining the Himalayan main central thrust in Nepal."''Journal of the Geological Society'' 165.2 (2008): 523-534.</ref> Meanwhile, these criteria are not all be satisfied together.<ref name=":4">Yin, An. "Cenozoic tectonic evolution of the Himalayan orogen as constrained by along-strike variation of structural geometry, exhumation history, and foreland sedimentation." ''Earth-Science Reviews'' 76.1 (2006): 1-131.</ref> The dominant problems are:
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 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.<ref name=":5">Searle, Michael P., et al. "Defining the Himalayan main central thrust in Nepal."''Journal of the Geological Society'' 165.2 (2008): 523–534.</ref> Meanwhile, these criteria are not all be satisfied together.<ref name=":4">Yin, An. "Cenozoic tectonic evolution of the Himalayan orogen as constrained by along-strike variation of structural geometry, exhumation history, and foreland sedimentation." ''Earth-Science Reviews'' 76.1 (2006): 1–131.</ref> The dominant problems are:


Lithology and stratigraphy have not been completely investigated and understood;
* lithology and stratigraphy have not been completely investigated and understood;
metamorphic grades across the Main Central Thrust shear zone are continuously changing, thus any one particular [[isograd]] is not reliable for determining the location of the Main Central Thrust;
* metamorphic grades across the Main Central Thrust shear zone are continuously changing, thus any one particular [[isograd]] is not reliable for determining the location of the Main Central Thrust;
strain magnitude cannot be determined as most of the fabrics of the Main Central Thrust that resulted from shearing, have disappeared because of strong heating and deformation;
* strain magnitude cannot be determined as most of the fabrics of the Main Central Thrust that resulted from shearing, have disappeared because of strong heating and deformation; and that
and some geologists do not believe that the whole shear surface was active at the same time because they think that the Main Central Thrust ductile shear zone is caused by finite strain deformation.<ref name=":4" /><ref>Mark Harrison, T., et al. "A late Miocene-Pliocene origin for the central Himalayan inverted metamorphism." ''Earth and Planetary Science Letters'' 146.1 (1997): E1-E7.</ref>
* some geologists do not believe that the whole shear surface was active at the same time because they think that the Main Central Thrust ductile shear zone is caused by finite strain deformation.<ref name=":4" /><ref>Mark Harrison, T., et al. "A late Miocene-Pliocene origin for the central Himalayan inverted metamorphism." ''Earth and Planetary Science Letters'' 146.1 (1997): E1–E7.</ref>


=== Definitions based on various criteria ===
=== Definitions based on various criteria ===
Despite the difficulties in defining the Main Central Thrust, the following definitions of the Main Central Thrust have been made based on various criteria:
Despite the difficulties in defining 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.<ref name=":3" /><ref name=":6">Daniel, C. G., et al. "Exhumation of the Main Central Thrust from lower crustal depths, eastern Bhutan Himalaya." ''Journal of Metamorphic Geology'' 21.4 (2003): 317-334.</ref>
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.<ref name=":3" /><ref name=":6">Daniel, C. G., et al. "Exhumation of the Main Central Thrust from lower crustal depths, eastern Bhutan Himalaya." ''Journal of Metamorphic Geology'' 21.4 (2003): 317–334.</ref>


By metamorphic isograd, the Main Central Thrust follows the [[kyanite]] isograd. Under this criterion, crystals of kyanite appear upward of several meters from the lithologic change.<ref name=":7">Le Fort, Patrick. "Himalayas: the collided range. Present knowledge of the continental arc." ''Am. J. Sci'' 275.1 (1975): 1-44.</ref>
By metamorphic isograd, the Main Central Thrust follows the [[kyanite]] isograd. Under this criterion, crystals of kyanite appear upward of several meters from the lithologic change.<ref name=":7">Le Fort, Patrick. "Himalayas: the collided range. Present knowledge of the continental arc." ''Am. J. Sci'' 275.1 (1975): 1–44.</ref>


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 is bound below by the Main Central Thrust.<ref name=":8">Parrish, Randall R., and V. Hodges. "Isotopic constraints on the age and provenance of the Lesser and Greater Himalayan sequences, Nepalese Himalaya." ''Geological Society of America Bulletin'' 108.7 (1996): 904-911.</ref>
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 is bound below by the Main Central Thrust.<ref name=":8">Parrish, Randall R., and V. Hodges. "Isotopic constraints on the age and provenance of the Lesser and Greater Himalayan sequences, Nepalese Himalaya." ''Geological Society of America Bulletin'' 108.7 (1996): 904–911.</ref>


[[Samarium-neodymium dating|Neodymium isotope]] composition differs across the thrust. [[Neodymium|Nd]] composition changes mark the Main Central Thrust. For example, an average Nd [[Samarium-neodymium dating#Epsilon notation|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.<ref name=":9">Robinson, Delores M., et al. "The kinematic evolution of the Nepalese Himalaya interpreted from Nd isotopes." ''Earth and Planetary Science Letters'' 192.4 (2001): 507-521.</ref>
[[Samarium-neodymium dating|Neodymium isotope]] composition differs across the thrust. [[Neodymium|Nd]] composition changes mark the Main Central Thrust. For example, an average Nd [[Samarium-neodymium dating#Epsilon notation|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.<ref name=":9">Robinson, Delores M., et al. "The kinematic evolution of the Nepalese Himalaya interpreted from Nd isotopes." ''Earth and Planetary Science Letters'' 192.4 (2001): 507–521.</ref>


By strain, the Main Central Thrust is defined as 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.<ref name=":10">Searle, M. P., et al. "The structural geometry, metamorphic and magmatic evolution of the Everest massif, High Himalaya of Nepal–South Tibet." ''Journal of the Geological Society'' 160.3 (2003): 345-366.</ref><ref name=":5" />
By strain, the Main Central Thrust is defined as 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.<ref name=":10">Searle, M. P., et al. "The structural geometry, metamorphic and magmatic evolution of the Everest massif, High Himalaya of Nepal–South Tibet." ''Journal of the Geological Society'' 160.3 (2003): 345–366.</ref><ref name=":5" />


== Prospect ==
== Prospect ==
None of the above definitions is precise because the Main Central Thrust developed and changes its style not only vertically but also along its strike, and even through time.<ref name=":5" /><ref name=":4" /> Also, its definition should not be limited to one thrust fault, but should be a broader fault zone.<ref name=":5" /> To better understand the Main Central Thrust, more research should be done along its strike and through time.<ref name=":4" />
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.<ref name=":5" /><ref name=":4" /> Also, its definition should not be limited to one thrust fault, but should be a broader fault zone.<ref name=":5" /> To better understand the Main Central Thrust, more research should be done along its strike and through time.<ref name=":4" />


== See also ==
== See also ==

Revision as of 23:23, 25 January 2015

The Main Central Thrust is a major fault where the Indian Plate has pushed under the Asian Plate along the Himalaya. The fault slopes down to the north and is exposed on the surface in a NW-SE direction (strike). It is a thrust fault that continues along 2200 km of the Himalaya mountain belt.[1]

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.[2] 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,[2][3] metamorphic isograd,[4] geochronology,[5] geochemistry,[6] and strain magnitude.[7][8] None of these are reliable if used independently. Furthermore, there 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.

Figure 2. Simplified geological map of Himalaya.[9] The Main Central Thrust is indicated by solid line and triangles.

Geologic background

The Himalayan mountain belt was produced by the collision of the Indian Plate and the Asian Plate. It is structurally dominated by three north-dipping, fault-bound geological units stacked on each other. The major faults are South Tibetan Detachment, the Main Central Thrust, the Main Boundary Thrust and the Main Frontal Thrust.[2] These units (figure 3), from south to north, are:

  1. the Lesser Himalayan Sequence, which is mainly composed of low-grade Proterozoic metasediments to unmetamorphosed rocks, fringed by the Main Boundary Thrust and the Main Central Thrust;
  2. the Greater Himalayan Crystalline complex, which is mainly composed by high-grade gneiss and migmatite, fringed below by the Main Central Thrust and the South Tibetan Detachment; and
  3. the Tethyan Himalayan Sequence, mainly composed by Proterozoic to Eocene sediments, deformed in an Paleogene fold-thrust belt, fringed below by the South Tibetan Detachment.[10]
File:The Main Central Thrust.png
Figure 3. Cross section showing the Main Central Thrust.[11] MBT: the Main Boundary Thrust; MCT: the Main Central Thrust; STDT: the South Tibetan Detachment Fault. YTS: YalungTsangpo Suture. Modified after Ali, Jason R., et al., 2012.

Kinematic models

Figure 4. Kinematics models of Himalayan orogen (modified from Webb et al, 2011) showing three models of how the Greater Himalayan Crystalline complex was placed over the Lesser Himalayan Sequence. LHS: the Lesser Himalayan Sequence; GHC: the Greater Himalayan Crystalline complex; THS: the Tethyan Himalayan Sequence; MCT: the Main Central Thrust; STD: the South Tibetan Detachment; ITS: Indus-Tsangpo suture zone.

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,[12] channel flow model,[13] tectonic wedging model.[14][15] for the fault system of Himalaya shown in shown in figure 4.

Various definitions of the Main Central Thrust

Difficulties in understanding

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 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.[8] Meanwhile, these criteria are not all be satisfied together.[16] The dominant problems are:

  • lithology and stratigraphy have not been completely investigated and understood;
  • metamorphic grades across the Main Central Thrust shear zone are continuously changing, thus any one particular isograd is not reliable for determining the location of the Main Central Thrust;
  • strain magnitude cannot be determined as most of the fabrics of the Main Central Thrust that resulted from shearing, have disappeared because of strong heating and deformation; and that
  • some geologists do not believe that the whole shear surface was active at the same time because they think that the Main Central Thrust ductile shear zone is caused by finite strain deformation.[16][17]

Definitions based on various criteria

Despite the difficulties in defining 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.[2][3]

By metamorphic isograd, the Main Central Thrust follows the kyanite isograd. Under this criterion, crystals of kyanite appear upward of several meters from the lithologic change.[4]

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 is bound below by the Main Central Thrust.[5]

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.[6]

By strain, the Main Central Thrust is defined as 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.[7][8]

Prospect

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.[8][16] Also, its definition should not be limited to one thrust fault, but should be a broader fault zone.[8] To better understand the Main Central Thrust, more research should be done along its strike and through time.[16]

See also

South Tibetan Detachment

References

  1. ^ Upreti, B. N. "An overview of the stratigraphy and tectonics of the Nepal Himalaya" Journal of Asian Earth Sciences 17.5 (1999): 577–606.
  2. ^ a b c d Helm, A., and A. Gansser. "Central Himalaya, Geological observations of the Swiss expedition 1936." Memoires de la Societe Helvetique des Sciences Naturelles 73.1,245 (1939).
  3. ^ a b Daniel, C. G., et al. "Exhumation of the Main Central Thrust from lower crustal depths, eastern Bhutan Himalaya." Journal of Metamorphic Geology 21.4 (2003): 317–334.
  4. ^ a b Le Fort, Patrick. "Himalayas: the collided range. Present knowledge of the continental arc." Am. J. Sci 275.1 (1975): 1–44.
  5. ^ a b Parrish, Randall R., and V. Hodges. "Isotopic constraints on the age and provenance of the Lesser and Greater Himalayan sequences, Nepalese Himalaya." Geological Society of America Bulletin 108.7 (1996): 904–911.
  6. ^ a b Robinson, Delores M., et al. "The kinematic evolution of the Nepalese Himalaya interpreted from Nd isotopes." Earth and Planetary Science Letters 192.4 (2001): 507–521.
  7. ^ a b Searle, M. P., et al. "The structural geometry, metamorphic and magmatic evolution of the Everest massif, High Himalaya of Nepal–South Tibet." Journal of the Geological Society 160.3 (2003): 345–366.
  8. ^ a b c d e Searle, Michael P., et al. "Defining the Himalayan main central thrust in Nepal."Journal of the Geological Society 165.2 (2008): 523–534.
  9. ^ Pierre Dèzes 1999, "Tectonic and metamorphic Evolution of the Central Himalayan Domain in Southeast Zanskar (Kashmir, India)". Mémoires de Géologie (Lausanne) No. 32, ISSN 1015-3578
  10. ^ Webb, A. Alexander G. "Preliminary balanced palinspastic reconstruction of Cenozoic deformation across the Himachal Himalaya (northwestern India)."Geosphere 9.3 (2013): 572–587.
  11. ^ Ali, Jason R., et al. "Paleomagnetic data support Early Permian age for the Abor Volcanics in the lower Siang Valley, NE India: significance for Gondwana-related break-up models." Journal of Asian Earth Sciences 50 (2012): 105–115.
  12. ^ Burchfiel, B. C., and L. H. Royden. "North-south extension within the convergent Himalayan region." Geology 13.10 (1985): 679–682.
  13. ^ Beaumont, C., et al. "Himalayan tectonics explained by extrusion of a low-viscosity crustal channel coupled to focused surface denudation." Nature414.6865 (2001): 738–742.
  14. ^ Webb, A. Alexander G., et al. "The leading edge of the Greater Himalayan Crystalline complex revealed in the NW Indian Himalaya: Implications for the evolution of the Himalayan orogen." Geology 35.10 (2007): 955–958.
  15. ^ Webb, A. Alexander G. "Preliminary balanced palinspastic reconstruction of Cenozoic deformation across the Himachal Himalaya (northwestern India)." Geosphere 9.3 (2013): 572–587.
  16. ^ a b c d Yin, An. "Cenozoic tectonic evolution of the Himalayan orogen as constrained by along-strike variation of structural geometry, exhumation history, and foreland sedimentation." Earth-Science Reviews 76.1 (2006): 1–131.
  17. ^ Mark Harrison, T., et al. "A late Miocene-Pliocene origin for the central Himalayan inverted metamorphism." Earth and Planetary Science Letters 146.1 (1997): E1–E7.
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