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= Pre-collisional Himalaya =
= Pre-collisional Himalaya =
[[Image:Hymalayas 86.15103E 31.99726N.jpg|thumb|Satellite Image of the Himalayas|339x339px]]
[[Image:Hymalayas 86.15103E 31.99726N.jpg|thumb|Satellite Image of the Himalayas|339x339px]]
'''Pre-collsional Himalaya''' is the original configuration of the Himalayan orogen before the Cenozoic orogeny that began with the Indo-Asian collision. Over the past 150 years, a long research history on the dynamic evolution of the Himalayan orogen has been conducted.The [[Geology of the Himalaya|geology of Himalaya]] is a type locality of a [[Convergent boundary|continental-continental collision]] scheme.<ref name=":0">{{Cite journal|last=Yin|first=An|date=2006|title=Cenozoic tectonic evolution of the Himalayan orogen as constrained by along-strike variation of structural geometry, exhumation history, and foreland sedimentation|journal=Earth-Science Reviews|volume=76|issue=1-2|page=1-131|doi=10.1016/j.earscirev.2005.05.004}}</ref> The reconstruction of the initial structural and stratigraphic configuration is highly controversial and major concerns are the arrangements of the different tectonostratigraphic zones in three dimensions. Several models have been postulated to explain the spatial and genetic relationships of the tectonostratigraphic zones prior to the Indo-Asian collision.
'''Pre-collsional Himalaya''' is the original arrangement of the Himalayan tectonostratigraphic zones before the Cenozoic orogeny that began with the Indo-Asian collision. Over the past 150 years, a long research history on the dynamic evolution of the Himalayan orogen has been conducted.The [[Geology of the Himalaya|geology of Himalaya]] is a type locality of a [[Convergent boundary|continental-continental collision]] scheme.<ref name=":0">{{Cite journal|last=Yin|first=An|date=2006|title=Cenozoic tectonic evolution of the Himalayan orogen as constrained by along-strike variation of structural geometry, exhumation history, and foreland sedimentation|journal=Earth-Science Reviews|volume=76|issue=1-2|page=1-131|doi=10.1016/j.earscirev.2005.05.004}}</ref> The reconstruction of the initial structural and stratigraphic configuration is highly controversial and major concerns are the arrangements of the different tectonostratigraphic zones in three dimensions. Several models have been postulated to explain the spatial and genetic relationships of the tectonostratigraphic zones prior to the Indo-Asian collision.


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Revision as of 18:29, 16 November 2016

Pre-collisional Himalaya

Satellite Image of the Himalayas

Pre-collsional Himalaya is the original arrangement of the Himalayan tectonostratigraphic zones before the Cenozoic orogeny that began with the Indo-Asian collision. Over the past 150 years, a long research history on the dynamic evolution of the Himalayan orogen has been conducted.The geology of Himalaya is a type locality of a continental-continental collision scheme.[1] The reconstruction of the initial structural and stratigraphic configuration is highly controversial and major concerns are the arrangements of the different tectonostratigraphic zones in three dimensions. Several models have been postulated to explain the spatial and genetic relationships of the tectonostratigraphic zones prior to the Indo-Asian collision.

Tectonostratigraphic Zones of Himalaya

Further information: Geology of the Himalaya and Geology of Nepal

The Himalaya orogeny is conventionally divided into four major tectonostratigraphic units.[2] From North to South, they are:

  • Tethyan Himalayan Sequence
  • Greater Himalayan Crystalline Complex
  • Lesser Himalayan Sequence
  • Sub-Himalayan Sequence

The Tethyan Himalayan sequence, Greater Himalayan crystalline complex and the Lesser Himalayan sequence are grouped together as the North Indian Sequence due to the overlapping age from Proterozoic to Phanerozoic.[3]

Tethyan Himalayan Sequence

The Tethyan Himalayan sequence is composed of mainly siliciclastic and carbonate sedimentary rocks from 1840 Ma to 40 Ma which is inter-bedded with volcanic rocks of Paleozoic and Mesozoic.[1] It is divided into several sub-units and it is a result of the Carboniferous- Jurassic rifting event that induce a change in the depositional settings. This event is associated with the separation of the Cimmerian Plate from Northern Indian and consequently the opening of Tethys Ocean.[3] The boundary between the several sub-units are poorly constraint[4] and the lower age bound of the whole sequence is generally considered to be Neoproterozoic in age.[5] The 1840 Ma was determined by rubidium-strontium dating of the Baragoan gneiss,[6] however some have allocated the gneiss into the Lesser Himalayan Sequence.[7]

Greater Himalayan Crystalline Complex

Generally, the greater Himalayan crystalline complex is a belt of high-grade metamorphic rock that is along the east-trending axis of the Himalayan range[8], with deformed and undeformed early- middle Miocene leucogranites throughout the complex.[9] In Nepal, it is bounded by the Main Central Thrust below and the overlying South Tibetan Detachment and Tethyan Himalayan sequence. Estimation of the age from 1800 Ma to 480 Ma is made although these ages are poorly constrained.[10] The metamorphic grade of the complex increases up section in the lower part; and starting from the middle to the upper part, the metamorphic grade decreases up section.[11] In addition, inverted metamorphism appears at Central Nepal.[12]

Lesser Himalayan Sequence

The lesser Himalayan sequence is characterized by non-fossil bound low-grade meta-sedimentary rocks,[2] metavolcanic rocks and augen gneiss. It has a age range of 1870 Ma to 520 Ma.[5] In North West of India, the Neoproterozoic strata are overlain comformbly with Cambrian strata,[13] while in Pakistan, Cambrian or Carboniferous sequences of the Tibetan Himalaya Sequence is overlying the Mesoproterozic strata of the Lower Himalayan Sequence.[8]

Sub-Himalayan Sequence

This is a Cenozoic Sequence in the Main Frontal Thrust and Main Boundary Thrust. The sequence is mainly composed of Siwalik strata form the Neogene, occupying the footwall of the Main Boundary Thrust and Paleogene to early Miocene strata in both the hanging wall and the footwall of the same thrust.[14] This sequence is mainly separated into two sub-units characterized by the difference in depositional environment and age. The older strata is Paleocene to Eocene of age while the younger strata has an age range of Miocene to Pliocene. They are deposited in an marine environment and a terrestrial environment respectively. An unconformity exists between the two sub-units. While the lower bound age of the unconformity is upper Eocene, the upper bound age is poorly constraint and it is believed to possibly be between lower Oligocene and lower Miocene .[15] Documented exceptions of the unconformity include Bengal Basin, eastern Shillong plateau and the Kirthar Mountains.[1]

Concepts

Models on the reconstruction of Pre-tectonic Himalaya (curly arrows represents the direction of sedimentation), modified from Yin (2006), Myrow (2003), DeCelles (2000)

The initial configuration of pre-tectonic Himalaya can be expressed in the four following models:[1]

  • Passive continental margin model
  • Crystalline Axis model
  • Accreted Terrane model
  • Carboniferous-extension model

Passive Continental Margin Model

Background

This model is a single margin model where the North Indian sequence was deposited on Northern India on a continental margin that was north facing. The units in the North Indian sequence represents separate facies, the proximal and distal part of a continental margin.[16][17]

Predictions

This model predicts that all three units namely the Lesser Himalayan, Greater Himalayan and Tethyan Himalayan must have a nearly identical depositional age, depositional setting and are composed of similar sources. Some basic lines of evidence in support of this model include detrital zircon ages, paleocurrent data and paleontological data.[18] Firstly, the detrital zircon data for the Lesser Himalaya and Tethyan Himalaya sequence yield similar age spectra when similar aged samples were used. Furthermore, when samarium-neodymium dating of the entire North Indian sequence was made, significant overlap of Nd isotopic signatures between the different tectonostratigraphic zones indicates a sharing of similar sources.[19] [20]Secondly, the paleocurrent data orientated from south-south west to north-north is common to both Lesser Himalaya and Tethyan Himalaya, more specifically in the Tal Group and Kunzam La Formation respectively.[21] Moreover, the lithology of the two sequences connotes a fluvial depositional setting and the lithofacies of the rocks strongly supports the idea that the Lesser Himalaya and Tethyan Himalaya are proximal and distal part of a continental margin setting.[18] Finally, both sequences contain the same Early Cambrian equatorial trilobite species and thus reinforces the likelihood of the passive continental margin model.[22]

Additionally, it is proposed that the protolith of the Greater Himalaya may have been sedimentary in nature and correlate with the Lesser and Tethyan Himalayan sequences.[9] Although precise matching of the ancient stratigraphy of the Greater Himalaya with the other zones are not possible, the Greater Himalayan sequence share correlative strata with Neoproterozoic to Cambrian age rocks in Tethyan Himalaya. Similar transition form siliciclastic rocks to carbonates occurs in both sequences in strata of similar ages.[23] In spite of the metamorphic grade of Greater Himalaya, the protolith lithology is nevertheless similar to the other zones and possibly share the same depositional setting.

Problems

Opposition of the model include the lack of thick and well develop strata, younger than Precambrian of age in the Lesser Himalaya, while very well preserved in Tethyan Himalaya.[3] This model also fail to explain the relationship of Greater Himalaya and Lesser Himalaya, while the latter is structurally higher, the former has thrust over the latter over the entirety of the main central thrust in Nepal.[10] In addition, contrasting results in the εNd values revealing a more negative value in Lesser Himalaya compared to both the Greater Himalaya and Tethyan Himalaya suggest different sources amongst the sequences. The latter two have values that resemble the Arabian Shield and Eastern Antarctica which conflict with the Indian Shield source that composes Lesser Himalaya.[24] [25]

Tectonic Evolution of the Passive Continental Margin Model

Crystalline Axis Model

Background

This model shows that the Lesser and Tethyan Himalaya were deposited in distinct basins that are separated by the Greater Himalaya complex.[26]

Problems

Results of the zircon ages and possible protolith lithologies and their corresponding first-order similarities between the tectonicstratigraphic zones have generally discredited this model.[18] To begin with, the Greater Himalaya yields younger detrital zircon ages than that of Lesser and Tethyan Himalaya, which makes it very unlikely for Greater Himalaya to be a topographic high that separates two depositional basins.[27][28] Furthermore, all the evidences that supports the passive margin model are also problematic for this model since the strong paleontological, lithological and sedimentological relationships between Lesser and Tethyan Himalaya basically rejects the connotation that they were once separated. Missing suture zone rocks in the Main Central Thrust also proves difficult to explain win this model.[18]

Accreted Terrane Model

Background

In this model, Lesser and Greater Himalaya were developed in separate areas during Precambrian to Cambrian and soon after in late Cambrian to Early Ordovician Greater Himalaya accreted as an exotic terrane into the Northern India margin and came into contact with Lesser Himalaya. Tethyan Himalaya was later deposited on top of Greater Himalaya as an overlying sequence.[10]

Predictions

This reconstruction predicts that the Greater Himalaya thrust over Lesser Himalayan rocks during early Paleozoic as such it is able to more successfully explain the age relationship between the strata across the main central thrust.[1] Other models would require greater slip along the main central thrust during the Cenozoic orogeny to achieve the present stratigraphical arrangement. Deformation produced by Paleozoic tectonics may have been overprinted by the Cenozoic reactivation of the main central thrust which result in the lack of old suture zone rocks. Moreover, aligned with the model, sediments in northern India have experienced a transition for turbidites to syn-collision sediments from Cambrian to Ordovician.[29] Additional evidences with regards to isotopic signatures and detrital zircon age may also increase the credibility of this model.[24] Further investigation is required to support more predictions of this model since metamorphism of Greater Himalaya strata in early Paleozoic and Cambrian to Ordovician syn-tectonic sediments in the Tethyan Himalaya are lacking.[10]

Problems

This model fails to reconcile with detrital zircon age, and paleontological evidence with respect to the similarities between Tethyan and Lesser Himalaya. Similar to the crystalline axis model, this model faces the same challenges with respect to the evidences that support the passive margin model. Trilobite fauna, paleocurrent and lithological similarities would be unlikely if the tectonostratigraphic zones were formed in separate terranes.[18] Earlier predictions[10] that suggest a thrust/ shortening event in Tethyan Himalaya in the early Paleozoic and the simultaneous formation of granitic intrusions under the sequence was also undermined by the opposing rifting isotopic signatures in the granites.[6]

Carboniferous-extension Model

Background

This model illustrates that in Carboniferous, due to possible rifting,[3] the Lesser and Greater Himalaya were separated by a north-dipping normal fault. In this reconstruction, the former is the footwall and the latter is on the hanging wall.

Predictions

This explains,the apparent missing lower Paleozoic strata in Lesser Himalaya due to footwall uplift and erosion, and provides a possible solution to the age relationship across the main central thrust by the reactivation of this ancient normal fault.[1]

Problems

The missing strata in Lesser Himalaya could be a result of glaciation in late Carboniferous. [30] Similar to the passive margin model, this model also implies that all the zones within the North Indian sequence share the same source, however the εNd value and detrital zircon ages are currently controversial and may not favor this interpretation.[24] [31]

Evolution of the Carboniferous-extension model, modified from Yin (2006)

Model Comparison

Predictions[1][10][18][26]
Models Same source Older Greater Himalaya Early Paleozoic tectonics Current age relationships along the Main Central Thrust Rifting in Carboniferous
Passive Continental Margin Model ✔ × × × ×
Crystalline Axis Model × ✔ × × ×
Accreted Terrane Model × × ✔ ✔ ×
Carboniferous- extension Model ✔ × × ✔ ✔

ee Also

References

  1. ^ a b c d e f g Yin, An (2006). "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 (1–2): 1-131. doi:10.1016/j.earscirev.2005.05.004.
  2. ^ a b Heim, Arnold; Gansser, Augusto (1939). Central Himalaya Geological Observations of Swiss. pp. 1–246.
  3. ^ a b c d Brookfield, M.E (1993). "The Himalayan passive margin from Precambrian to Cretaceous times". Sedimentary Geology. 84 (1–4): 1-35.
  4. ^ Yin, An; Harrison, T. Mark (2000). "Geologic Evolution of the Himalayan- Tibetan Orogen". Annual Review of Earth and Planetary Sciences. 28: 211-280. doi:10.1146/annurev.earth.28.1.211.
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  7. ^ Pandey, A.K.; Virdi, N.S. (2003). "Microstructural and fluid inclusion constraints on the evolution of Jakhri thrust zone in the Satluj valley of NW Himalaya". Current science. 84 (10): 1355-1364.
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  11. ^ Hubbard, Mary S.; Harrison, T. Mark (1989). "40Ar/39Ar age constraints on deformation and metamorphism in the main central thrust zone and Tibetan slab, eastern Nepal Himalaya". Tectonics. 8 (4).
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  19. ^ Whittington, Alan; Foster, Gavin; Harris, Nigel; Vance, Derek; Ayres, Michael (1999-07-01). "Lithostratigraphic correlations in the western Himalaya—An isotopic approach". Geology. 27 (7): 585–588. doi:10.1130/0091-7613(1999)0272.3.CO;2. ISSN 0091-7613.
  20. ^ Ahmad, T.; Harris, N.; Bickle, M.; Chapman, H.; Bunbury, J.; Prince, C. "Isotopic constraints on the structural relationships between the Lesser Himalayan Series and the High Himalayan Crystalline Series, Garhwal Himalaya". Geological Society of America Bulletin. 112 (3): 467–477. doi:10.1130/0016-7606(2000)112<467:icotsr>2.0.co;2.
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  22. ^ Hughes, Nigel C.; Jell, Peter A. (1999-02-01). "Biostratigraphy and biogeography of Himalayan Cambrian trilobites". Geological Society of America Special Papers. 328: 109–116. doi:10.1130/0-8137-2328-0.109. ISSN 0072-1077.
  23. ^ Garzanti, Eduardo; Casnedi, Raffaele; Jadoul, Flavio (1986-07-01). "Sedimentary evidence of a Cambro-Ordovician orogenic event in the northwestern Himalaya". Sedimentary Geology. 48 (3): 237–265. doi:10.1016/0037-0738(86)90032-1.
  24. ^ a b c Spencer, Christopher J.; Harris, Ron A.; Sachan, Himanshu Kumar; Saxena, Anubhooti (2011-05-25). "Depositional provenance of the Greater Himalayan Sequence, Garhwal Himalaya, India: Implications for tectonic setting". Journal of Asian Earth Sciences. 41 (3): 344–354. doi:10.1016/j.jseaes.2011.02.001.
  25. ^ Imayama, Takeshi; Arita, Kazunori (2008-04-28). "Nd isotopic data reveal the material and tectonic nature of the Main Central Thrust zone in Nepal Himalaya". Tectonophysics. Asia out of Tethys: Geochronologic, Tectonic and Sedimentary Records. 451 (1–4): 265–281. doi:10.1016/j.tecto.2007.11.051.
  26. ^ a b Saxena, M.N. (1971). "The Crystalline Axis of the Himalaya: The Indian Shield and Continental Drift". Tectonophysics. 12: 433-447.
  27. ^ Parrish, Randall R.; Hodges, V. (1996-07-01). "Isotopic constraints on the age and provenance of the Lesser and Greater Himalayan sequences, Nepalese Himalaya". Geological Society of America Bulletin. 108 (7): 904–911. doi:10.1130/0016-7606(1996)1082.3.CO;2. ISSN 0016-7606.
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  29. ^ Pogue, Kevin R.; Hylland, Michael D.; Yeats, Robert S.; Khattak, Wali Ullah; Hussain, Ahmad (1999-02-01). "Stratigraphic and structural framework of Himalayan foothills, northern Pakistan". Geological Society of America Special Papers. 328: 257–274. doi:10.1130/0-8137-2328-0.257. ISSN 0072-1077.
  30. ^ Vannay, Jean-Claude; Steck, Albrecht (1995-04-01). "Tectonic evolution of the High Himalaya in Upper Lahul (NW Himalaya, India)". Tectonics. 14 (2): 253–263. doi:10.1029/94TC02455. ISSN 1944-9194.
  31. ^ Yoshida, Masaru; Upreti, Bishal N. (2006-11-01). "Neoproterozoic India within East Gondwana: Constraints from recent geochronologic data from Himalaya". Gondwana Research. 10 (3–4): 349–356. doi:10.1016/j.gr.2006.04.011.

Further Readings

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