Lhasa terrane

The Lhasa terrane is a terrane, or fragment of crustal material, sutured to the Eurasian Plate during the Cretaceous that forms present-day southern Tibet. It takes its name from the city of Lhasa in the Tibet Autonomous Region, China. The northern part may have originated in the East African Orogeny, while the southern part appears to have once been part of Australia. The two parts joined, were later attached to Asia, and then were impacted by the collision of the Indian Plate that formed the Himalayas.

Lhasa terrane
Nam Tso
Portion of the terrane, Namtso lake to the north above Nyenchen Tanglha Mountains (white)
LocationTibet Autonomous Region, China
Coordinates30°N 91°E / 30°N 91°ECoordinates: 30°N 91°E / 30°N 91°E
GeologyTerrane

Location

2 2 himal tecto units
Tectonic units of the Himalaya. Green is the Indus-Yarlung suture zone. Red is the Transhimalaya. Lhasa to the east.
Bangong-Nujiang Suture Zone
Lhasa and Qiangtang Terranes and associated suture zones

The Lhasa terrane is separated from the Himalayas to the south by the Yarlung-Tsangpo suture, and from the Qiangtang terrane to the north by the Bangong-Nujiang suture.[1] The Lhasa terrane has a Precambrian crystalline basement overlaid with sedimentary strata from the Paleozoic (c. 541–252 Ma[a]) and Mesozoic (c.  252–66 Ma) and containing magmatic rocks from the Paleozoic to Cenozoic (66 Ma to the present). It is thought to be the last crustal block to accrete to the Eurasian plate before it collided with the Indian plate in the Cenozoic.[2]

Origins

The Lhasa terrane consisted of two blocks before the Mesozoic, the North Lhasa Block and the South Lhasa Block.[3] The two blocks have lithology and detrital zircon ages similar to the Qiangtang terrane and to Tethyan strata in the Himalaya, which suggests these areas were nearby in Gondwana. The detrital zircon ages differ somewhat between the North and South Lhasa terranes.[4] The South Lhasa terrane appears to have evolved as part of Australia in the late Precambrian and early Paleozoic. Isotopic analysis of detrital zircons of c. 1170 Ma from Paleozoic metasedimentary rocks in the Lhasa terrane shows identical values to detrital zircons of the same age from Western Australia. The detrital zircons probably came from southwest Australia's Albany-Fraser belt.[5]

The North Lhasa terrane may have been formed in part from the northern part of the East African Orogeny. Neoproterozoic oceanic crustal rocks are included in the crystalline basement of the North Lhasa terrane, which are probably from the Mozambique Ocean that formed when the Rodinia super-continent broke up. In the Late Cryogenian, around 650 Ma, the oceanic crustal basement of North Lhasa experienced HP metamorphism in the subduction zone associated with the closing of the Mozambique ocean. In the Early Paleozoic around 485 Ma it experienced MP metamorphism associated with the amalgamation of Eastern and Western Gondwana.[2]

In the Early Paleozoic the North and South Lhasa terranes and the Qiangtang terrane experienced magmatism that seems to have been the result of an Andean-type orogeny caused when the Proto-Tethys Ocean was subducted after Gondwana was finally amalgamated. In the Middle Paleozoic around 360 Ma the Lhasa and Qiangtang terranes again experienced magmatism, apparently due to the subduction of the Paleo-Tethys Ocean.[2]

Formation and evolution

Lhasa terrane approach to Qiangtang terrane
Lhasa terrane approach to Qiangtang terrane
Cross section depicting the tectonic evolution of the Bangong suture zone
More detailed view of the tectonic evolution of the Bangong suture zone

The Lhasa terrane was formed from the North and South Lhasa terranes, which were at first separated by the Paleo-Tethys Ocean, and were joined in a suture zone in the Late Paleozoic.[2] The Paleo-Tethys Ocean that separated the North and South Lhasa terranes closed, and around 260 Ma in the Late Permian an HP metamorphic belt formed between the two blocks. Around 220 Ma in the Triassic an MP metamorphic belt formed.[2]

The Tibetan Plateau was formed from a number of continental terranes that rifted from northern Gondwana in the Paleozoic and Mesozoic, moved northward and accreted to southern Asia. The Lhasa terrane is the farthest south of these terranes.[3] The Lhasa terrane moved northward and collided with the Qiangtang terrane along the Banggongco-Nujiang Suture.[6][7] The collision began towards the end of the late Jurassic (c. 163–145 Ma), and collision activity continued until the early Late Cretaceous (c. 100–66) Ma . During this period the terrane may have been shortened by at least 180 kilometres (110 mi).[1] Strata from the Lower Jurassic in the Bangong suture between the Lhasa and Qiangtang terranes differ from the rocks in the Lhasa terrane and appear to have a unique source.[8]

The collision with the Qiangtang terrane caused a peripheral foreland basin to form in the north part of the Lhasa terrane, which persisted into the Early Cretaceous. In some parts of the foreland basin the north-dipping subduction of the Neotethyan oceanic crust below the Lhasa terrane caused volcanism. The Gangdese volcanic arc was formed as this subduction continued along the southern margin of the Lhasa terrane.[9] The Gangdese batholith intrudes the southern half of the Lhasa terrain.[10] There is evidence that by the end of the Cretaceous the Southern Tibet crust was approximately twice as thick as normal.[11]

Clastic sediments found in the terrane were deposited in shallow waters during the Early Cretaceous (c. 146–100 Ma.) In northern Lhasa these sediments formed in the foreland basin created during the Lhasa–Qiangtang collision. They are overlaid by marine limestone from the Aptian-Albian period, deposited in a shallow continental seaway. The Takena Formation developed in the Late Cretaceous in the foreland basin to the north of the Gangdese magmatic arc, and consists of marine limestone overlaid by fluvial red beds.[8] Outcropped folds in the Takena Formation between Lhasa and Yangbajain are upright or lean slightly to the north or south, and indicate 30% to 50% shortening in the Late Cretaceous before the Indian collision.[10]

India–Asia collision

India 71-0 Ma
India-Eurasia collision 70-0 Ma

Contact with India began along the Yarlung-Zangbo suture around 50 Ma during the Eocene, and the two continents continue to converge. Magmatism continued in the Gangdese arc until as late as 40 Ma.[10] There are competing hypotheses about the details of the tectonic processes during the collision between the Indian and Eurasian plates.[4] At one extreme, some consider that during the collision the Indian crust was underthrust beneath the southern Asian crust, or injected into this crust. At the other extreme, some consider that the convergence was mostly accommodated by shortening of the Asian crust.[12]

The results of seismic reflection profiling, reported in 1998, indicate that there may be a midcrustal partial-melt zone under the length of the Yangbajain-Damxung graben starting at a depth of 12 to 18 kilometres (7.5 to 11.2 mi). The reflection undulates, so the melt zone may have been tectonically deformed. North-dipping reflections deep in the crust below the Gangdese batholith at a depth of 40 to 60 kilometres (25 to 37 mi) may mark the downdip of the Yarlung-Zangbo suture, or may mark a more recent reverse fault. Taken together, the results indicate that the upper crust of the Lhasa terrane was moderately shortened by the collision, with melting in the middle crust. They neither support nor rule out underthrusting or fluid injection of the Indian continental crust below the Lhasa terrane.[12]

The Linzizong Formation is distributed widely along the Gangdese Belt. It was emplaced between 69 and 43 Ma near Lhasa and between 54 and 37 Ma in southwestern Tibet. It is slightly folded and slopes gently to the north. The formation is unconformably underlain by Cretaceous sedimentary sequences more than 3,000 metres (9,800 ft) thick, which are strongly folded.[13] The results of palaeomagnetic studies of the Linzizong Formation in the Linzhou Basin and the Takena Formation reported in 2009 indicate that there was little movement of the Lhasa terrane in the Cretaceous and Early Eocene. The measurements give a northward movement of the Lhasa terrane since then of 1,847 ± 763 kilometres (1,148 ± 474 mi). This implies that there was significant crustal shortening as the collision progressed.[14] The South Lhasa terrane experienced metamorphism and magmatism in the Early Cenozoic (55–45 Ma) and metamorphism in the Late Eocene (40–30 Ma), presumably due to the collision between the continents of India and Eurasia.[2]

Strata

Sedimentary strata from the Palaeozoic are mainly Carboniferous sandstone, metasandstone, shale and phyllite, and lesser Ordovician, Silurian and Permian limestone. Precambrian strata are rarely exposed. Rocks from the Triassic include inter-bedded limestone and basaltic volcanic units, most common along the terrane's southern margin. In the northern terrane the Jurassic strata are deepwater sandstone and shale, often with ophiolitic assemblages. In the southern terrane the Jurassic strata are marine limestone and mudstone. Strata from the Lower Cretaceous are clastic mudstone, sandstone and local conglomerate units. The Lower Cretaceous clastic units are overlaid by a shallow marine limestone from the Aptian-Albian period, exposed in many places, which in some places holds Cenomanian fossils. The strata from the Upper Cretaceous are successions of arkosic fluvial sandstone and mudstone.[9]

References

  1. ^ Ma – Millions of years ago
  1. ^ a b Ozacar 2015.
  2. ^ a b c d e f Zhang et al. 2014, p. 170–171.
  3. ^ a b Wan 2010, p. 139.
  4. ^ a b Leier et al. 2007, p. 361.
  5. ^ Di et al. 2011.
  6. ^ Wan 2010, p. 210.
  7. ^ Metcalfe 1994, pp. 97–111.
  8. ^ a b Leier 2005.
  9. ^ a b Leier et al. 2007, p. 363.
  10. ^ a b c Alsdorf, BrownNelson & Makovsky 1998, p. 502.
  11. ^ Leier et al. 2007, p. 363–364.
  12. ^ a b Alsdorf, BrownNelson & Makovsky 1998, p. 501.
  13. ^ Liebke et al. 2010, p. 1200.
  14. ^ Liebke et al. 2010, p. 1199.

Sources

  • Alsdorf, Douglas; Brown, Larry; Nelson, K. Douglas; Makovsky, Yizhaq; Klemperer, Simon; Zhao, Wenjin (August 1998). "Crustal deformation of the Lhasa terrane, Tibet plateau from Project INDEPTH deep seismic reflection profiles". Tectonics. 17 (4): 501–519. Bibcode:1998Tecto..17..501A. doi:10.1029/98tc01315. Retrieved 2015-02-19.
  • Di, Cheng Zhu; Zhi, Dan Zhao; Niu, Yaoling; Dilek, Yildirim; Mo, Xuan-Xue (2011-03-13). "Lhasa terrane in southern Tibet came from Australia". Geology. Geological Society of America. 39 (8): 727–730. doi:10.1130/g31895.1. Retrieved 2015-02-18.
  • Leier, Andrew (2005). "The Cretaceous Evolution of the Lhasa Terrane, Southern Tibet". The University of Arizona. hdl:10150/193796. Cite journal requires |journal= (help)
  • Leier, Andrew L.; Kapp, Paul; Gehrels, George E.; DeCelles, Peter G. (2007). "Detrital zircon geochronology of Carboniferous–Cretaceous strata in the Lhasa terrane, Southern Tibet" (PDF). Basin Research. 19 (3): 361–378. doi:10.1111/j.1365-2117.2007.00330.x. Retrieved 2015-02-19.
  • Liebke, Ursina; Appel, Erwin; Ding, Lin; Neumann, Udo; Antolin, Borja; Xu, Qiang (2010). "Position of the Lhasa terrane prior to India–Asia collision derived from palaeomagnetic inclinations of 53 Ma old dykes of the Linzhou Basin: constraints on the age of collision and post-collisional shortening within the Tibetan Plateau". Geophysical Journal International. 182 (3): 1199–1215. doi:10.1111/j.1365-246x.2010.04698.x. Retrieved 2015-02-19.
  • Metcalfe, I (1994). "Late Paleozoic and Mesozoic paleogeography of eastern Pangea and Thethys". In Embry, Ashton F.; Beauchamp, Benoit; Glass, Donald J. (eds.). Pangea: Global Environments and Resources. Calgary, Alberta, Canada: Canadian Society of Petroleum Geologists. ISBN 978-0-920230-57-2.
  • Ozacar, Arda (2015). "Paleotectonic Evolution of Tibet". Retrieved 2015-02-18.
  • Wan, Tianfeng (2010). The Tectonics of China: Data, Maps and Evolution. Berlin: Springer. ISBN 978-3-642-11866-1.
  • Zhang, Z.M.; Dong, X.; Santosh, M.; Zhao, G.C (January 2014). "Metamorphism and tectonic evolution of the Lhasa terrane, Central Tibet". Gondwana Research. 25 (1): 170–189. doi:10.1016/j.gr.2012.08.024.

External links

2019 in paleobotany

This article records new taxa of fossil plants that are scheduled to be described during the year 2019, as well as other significant discoveries and events related to paleobotany that are scheduled to occur in the year 2019.

2019 in paleontology

Paleontology or palaeontology is the study of prehistoric life forms on Earth through the examination of plant and animal fossils. This includes the study of body fossils, tracks (ichnites), burrows, cast-off parts, fossilised feces (coprolites), palynomorphs and chemical residues. Because humans have encountered fossils for millennia, paleontology has a long history both before and after becoming formalized as a science. This article records significant discoveries and events related to paleontology that occurred or were published in the year 2019.

Bangong suture

The Bangong suture zone is approximately 1200 km long and trends in an east–west orientation, and a key location in the central Tibet conjugate fault zone. Located in central Tibet between the Lhasa (southern block) and Qiangtang (northern block) terranes, it is a discontinuous belt of ophiolites and mélange that is 10–20 km wide, up to 50 km wide in places. The northern part of the fault zone consists of northeast striking sinistral strike-slip faults while the southern part consists of northwest striking right lateral strike-slip faults. These conjugate faults to the north and south of the Bangong intersect with each other along the Bangong-Nujiang suture zone.

Cimmeria (continent)

Cimmeria was an ancient continent, or, rather, a string of microcontinents or terranes, that rifted from Gondwana in the Southern Hemisphere and was accreted to Eurasia in the Northern Hemisphere. It consisted of parts of what is today Turkey, Iran, Afghanistan, Tibet, Shan–Thai, and Malay Peninsula. Cimmeria rifted from the Gondwanan shores of the Paleo-Tethys Ocean during the Carboniferous-earliest Permian and as the Neo-Tethys Ocean opened behind it, during the Permian, the Paleo-Tethys closed in front of it. Cimmeria rifted off Gondwana from east to west, from Australia to the eastern Mediterranean.

It stretched across several latitudes and spanned a wide range of climatic zones.

Continental arc

A continental arc is a type of volcanic arc occurring as an "arc-shape" topographic high region along a continental margin. The continental arc is formed at an active continental margin where two tectonic plates meet, and where one plate has continental crust and the other oceanic crust along the line of plate convergence, and a subduction zone develops. The magmatism and petrogenesis of continental crust are complicated: in essence, continental arcs reflect a mixture of oceanic crust materials, mantle wedge and continental crust materials.

Gangdese batholith

The Gangdese batholith or Gangdese volcanic arc is a major geological structure in the south of the Lhasa terrane in Tibet, to the north of the Himalayas. The batholith formed around 100 million years ago, and was volcanically active for about 20 million years. It reactivated around 65 million years ago as the Indian plate approached Eurasia, and was active for another 20 million years.

Geology of China

The geology of China (or the geological structure of the People's Republic of China) consists of three Precambrian cratons surrounded by a number of orogenic belts. The modern tectonic environment is dominated by the continued collision of India with the rest of Asia starting 40–50 million years ago. This has formed the Himalaya and continues to deform most of China. China has vast mineral reserves, a significant earthquake risk in its Western regions and rare isolated active volcanoes throughout the country.Many geological concepts were discovered very early in China's history. However, it was not until the adoption of European natural science in the late 19th century that geology became a science in China.

High pressure metamorphic terranes along the Bangong-Nujiang Suture Zone

High pressure terranes along the ~1200 km long east-west trending Bangong-Nujiang suture zone (BNS) on the Tibetan Plateau have been extensively mapped and studied. Understanding the geodynamic processes in which these terranes are created is key to understanding the development and subsequent deformation of the BNS and Eurasian deformation as a whole.

Lhasa (prefecture-level city)

Lhasa is a prefecture-level city, formerly a prefecture until 7 January 1960, one of the main administrative divisions of the Tibet Autonomous Region of China. It covers an area of 29,274 square kilometres (11,303 sq mi) of rugged and sparsely populated terrain.

The consolidated prefecture-level city is divided into five mostly rural counties and three partially urban districts Chengguan District, Doilungdêqên District, and Dagzê District, which contain the main urban area of Lhasa.

The prefecture-level city roughly corresponds to the basin of the Lhasa River, a major tributary of the Yarlung Tsangpo River. It lies on the Lhasa terrane, the last unit of crust to accrete to the Eurasian plate before the continent of India collided with Asia about 50 million years ago and pushed up the Himalayas. The terrane is high, contains a complex pattern of faults and is tectonically active. The temperature is generally warm in summer and rises above freezing on sunny days in winter. Most of the rain falls in summer. The upland areas and northern grasslands are used for grazing yaks, sheep and goats, while the river valleys support agriculture with crops such as barley, wheat and vegetables. Wildlife is not abundant, but includes the rare snow leopard and black-necked crane. Mining has caused some environmental problems.

The 2000 census gave a total population of 474,490, of whom 387,124 were ethnic Tibetans. The Han Chinese population at the time was mainly concentrated in urban areas. The prefecture-level city is traversed by two major highways and by the Qinghai–Tibet Railway, which terminates in the city of Lhasa. Two large dams on the Lhasa River deliver hydroelectric power, as do many smaller dams and the Yangbajain Geothermal Field. The population is well-served by primary schools and basic medical facilities, although more advanced facilities are lacking. Tibetan Buddhism and monastic life have been dominant aspects of the local culture since the 7th century. Most of the monasteries were destroyed during the Cultural Revolution, but since then many have been restored and serve as tourist attractions.

List of tectonic plates

This is a list of tectonic plates on the Earth's surface. Tectonic plates are pieces of Earth's crust and uppermost mantle, together referred to as the lithosphere. The plates are around 100 km (62 mi) thick and consist of two principal types of material: oceanic crust (also called sima from silicon and magnesium) and continental crust (sial from silicon and aluminium). The composition of the two types of crust differs markedly, with mafic basaltic rocks dominating oceanic crust, while continental crust consists principally of lower-density felsic granitic rocks.

Maizhokunggar County

Maizhokunggar County or Meldro Gungkar County is a county of Lhasa and east of the main center of Chengguan, Tibet Autonomous Region. It has an area of 5,492 square kilometres (2,120 sq mi) with an average elevation of over 4,000 metres (13,000 ft). Most of the people are ethnic Tibetan and are engaged in agriculture or herding. Mining is a major source of tax revenue, but has created environmental problems. The county has various tourist attractions including hot springs and the Drigung Monastery.

Outline of plate tectonics

This is a list of articles related to plate tectonics and tectonic plates.

Provenance (geology)

Provenance in geology, is the reconstruction of the origin of sediments. The Earth is a dynamic planet, and all rocks are subject to transition between the three main rock types: sedimentary, metamorphic, and igneous rocks (the rock cycle). Rocks exposed to the surface are sooner or later broken down into sediments. Sediments are expected to be able to provide evidence of the erosional history of their parent source rocks. The purpose of provenance study is to restore the tectonic, paleo-geographic and paleo-climatic history.

In the modern geological lexicon, "sediment provenance" specifically refers to the application of compositional analyses to determine the origin of sediments. This is often used in conjunction with the study of exhumation histories, interpretation of drainage networks and their evolution, and forward-modelling of paleo-earth systems. In combination, these help to characterise the "source to sink" journey of clastic sediments from hinterland to sedimentary basin.

Terrane

A terrane in geology, in full a tectonostratigraphic terrane, is a fragment of crustal material formed on, or broken off from, one tectonic plate and accreted or "sutured" to crust lying on another plate. The crustal block or fragment preserves its own distinctive geologic history, which is different from that of the surrounding areas—hence the term "exotic" terrane. The suture zone between a terrane and the crust it attaches to is usually identifiable as a fault.

Older usage of terrane simply described a series of related rock formations or an area having a preponderance of a particular rock or rock groups.

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