Lhasa (prefecture-level city)

Lhasa is a prefecture-level city, one of the main administrative divisions of the Tibet Autonomous Region of China. It covers an area of of rugged and sparsely populated terrain. Its urban center is Lhasa, with around 300,000 residents, which mostly corresponds with the administrative Chengguan District, while its suburbs extend into Doilungdêqên District and Dagzê District. The consolidated prefecture-level city contains an additional five, mostly rural, counties.
The city boundaries roughly correspond 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 terrain 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. In the future, the Sichuan–Tibet railway currently under construction will expect to start operations in 2030. Two large dams on the Lhasa River deliver hydroelectric power, as do many smaller dams and the. 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.

History

Lhasa is the capital and largest city in Tibet. Founded in the 5th century AD and largely closed to foreigners until the early 1980s, its name means "God's Home" or "City of the Gods" and is still very much a pilgrimage destination among Tibetans. After several conflicts of the Mongol invasions and the Qing rule of Tibet, the British led by Francis Younghusband led the expeditions of Tibet. In 1912, the Qing collapse led to Tibet becoming independent on 4 April 1912 with Thubten Gyatso as "the Great Thirteenth" for Tibet's national independence from Qing rule.
The annexation of Tibet by the People's Republic of China took place as Tenzin Gyatso fled to McLeod Ganj, India in 1959 at the end of the Tibetan uprising.

Geography

Location

Lhasa lies in south-central Tibet, to the north of the Himalayas.
The prefecture-level city is from east to west and from north to south. It covers an area of.
It is bordered by Nagqu City to the north, Nyingchi City to the east, Shannan/Lhoka City to the south and Xigazê City to the west.
The prefecture-level city roughly corresponds to the basin of the Lhasa River, which is the center of Tibet politically, economically and culturally. Chengguan District is also the center of Tibet in terms of transport, communications, education and religion, as well as being the most developed part of Tibet and a major tourist destination with sights such as the Potala Palace, Jokhang and Ramoche Temple.

Lhasa River basin

Lhasa prefecture-level city roughly corresponds to the basin of the Lhasa River, a major tributary of the Yarlung Tsangpo. Exceptions are the north of Damxung County, which crosses the watershed of the Nyenchen Tanglha Mountains and includes part of the Namtso lake, and Nyêmo County, which covers the basin of the Nimu Maqu River, a direct tributary of the Yarlung Tsangpo.
The river basin is separated from the Yarlung Tsangpo valley to the south by the Goikarla Rigyu range.
The largest tributary of the Lhasa River, the Reting Tsangpo, originates in the Chenthangula Mountains in Nagqu Prefecture at an elevation of about, and flows southwest into Lhasa past Reting Monastery.
The Lhasa River drains an area of, and is the largest tributary of the middle section of the Yarlung Tsangpo.
The average altitude of the basin is around.
The basin has complex geology and is tectonically active. Earthquakes are common.
Annual runoff is.
Water quality is good, with little discharge of sewage and minimal chemical pesticides and fertilizers.
The Lhasa River forms where three smaller rivers converge.
These are the Phak Chu, the Phongdolha Chu which flows from Damxung County and the Reting Tsangpo, which rises beyond the Reting Monastery.
The highest tributary rises at around on the southern slope of the Nyenchen Tanglha Mountains.
In its upper reaches, the river flows southeast through a deep valley.
Lower down the river valley is flatter and changes its direction to the southwest, The river expands to a width of.
Major tributaries in the lower reaches include the Pengbo River and the Duilong River.
At its mouth, the Lhasa Valley is about wide.
The bulk of the water is supplied by the summer monsoon rains, which fall from July to September.
There are floods in the summer from July to September, with about 17% of the annual runoff flowing in September.
In winter the river has low water, and sometimes freezes.
Total flow is about, with average flow about.
The total hydropower potential of the river basin is 2,560,000 kW.
Zhikong Hydro Power Station in Maizhokunggar County delivers 100 MW.
The Pangduo Hydro Power Station in Lhünzhub County has total installed capacity of 160 MW.

Geology

The former Lhasa prefecture lies in the Lhasa terrane, to which it gives its name. This is thought to be the last crustal block to accrete to the Eurasian plate before the collision with the Indian plate in the Cenozoic.
The 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.
The Lhasa terrane consisted of two blocks before the Mesozoic, the North Lhasa Block and the South Lhasa Block.
These blocks were joined in the Late Paleozoic.
The Lhasa terrane moved northward and collided with the Qiangtang terrane along the Bangong suture.
The collision began towards the end of the late Jurassic, and collision activity continued until the early Late Cretaceous.
During this period the terrane may have been shortened by at least.
The collision caused a peripheral foreland basin to form in the north part of the Lhasa terrane.
In some parts of the foreland basin the north-dipping subduction of the Neotethyan oceanic crust below the Lhasa terrane caused volcanism.
The Gangdese batholith was formed as this subduction continued along the southern margin of the Lhasa terrane.
The Gangdese intrudes the southern half of the Lhasa terrain.
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.
There was significant crustal shortening as the collision progressed.
The South Lhasa terrane experienced metamorphism and magmatism in the Early Cenozoic and metamorphism in the Late Eocene, presumably due to the collision between the continents of India and Eurasia.
Rocks in this region include sedimentary rocks from the Paleozoic and Mesozoic into which granite has intruded during the Cretaceous.
The rocks have metamorphosed and are deeply eroded and faulted.
The rocks exposed in the Reting Tsangpo canyon range in age from 400 Ma to 50 Ma.
The result of faulting has been to often juxtapose relatively recent rocks with much older rocks.
Some parts of the ocean floor were pushed up onto the Tibetan Plateau and formed marble or slate.
Sea fossils from 400 Ma are found in the river's canyons, and houses are roofed with slate.
The Yangbajing Basin lies between the Nyainquentanglha Range to the northwest and the Yarlu-Zangbo suture to the south.
The Yangbajain Geothermal Field is in the central part of a half-graben fault-depression basin caused by the foremontane fault zone of the Nyainqentanglha Mountains.
The SE-dipping detachment fault began to form about 8 Ma.
The geothermal reservoir is basically a Quaternary basin underlaid by a large granite batholith.
The basin has been filled with glacial deposits from the north and alluvial-pluvial sediments from the south.
Fluid flows horizontally into the reservoir through the faults around the basin.
Chemical analysis of the thermal fluid indicate that there is shallow-seated magmatic activity not far below the geothermal field.
During the ice ages of the last two million years, the Tibetan plateau and the Himalayas have been covered by the expanded polar ice cap several times. As the ice moved it eroded the rock, filling the river canyons with gravel. In some sections the rivers have cut through the gravel and flow swiftly over bedrock, and in some areas large boulders have fallen into the rivers and formed rapids.

Climate

The Lhasa valley is roughly the same latitude as the southern United States, but at an altitude of or more it is cooler.
The central river valleys of Tibet are warm in summer, and even in the coldest months of winter the temperature is above freezing on sunny days.
The climate is semi-arid monsoon, with a low average temperature of.
Average annual precipitation is, with 85% falling in the June–September period.
Typically, there are 3,000 hours of sunshine each year.
It is cooler in the northern regions, warmer in the south. Annual figures:
Studies of temperature and precipitation data from 1979 to 2005 indicate that higher temperatures are leading to longer snow-free periods at the lower elevations.
However, at higher levels the amount of precipitation has increased, so despite warming the snow-free period is shorter.