DEFINITION OF TIBETAN PLATEAU

The Tibetan Plateau is the largest high

area of thickened continental crust on Earth, with an average

height of 16,000 feet (4,880 m) over 470,000 square miles

(1,220,000 km2). Bordered on the south by the Himalayan

Mountains, the Kunlun Mountains in the north, the Karakorum

on the west, and the Hengduan Shan on the east, Tibet is

the source of many of the largest rivers in Asia. The Yangtze,

Mekong, Indus, Salween, and Brahmaputra Rivers all rise in

Tibet and flow through Asia, forming the most important

source of water and navigation for huge regions.

Southern Tibet merges into the foothills of the northern

side of the main ranges of the Himalaya, but they are separated

from the mountains by the deeply incised river gorges of

the Indus, Sutlej, and Yarlung Zangbo (Brahmaputra) Rivers.

Central and northern Tibet consist of plains and steppes that

are about 3,000 feet (1,000 m) higher in the south than the

north. Eastern Tibet includes the Transverse Ranges (the

Hengduan Shan) that are dissected by major faults in the

river valleys of the northwest-southeast flowing Mekong, Salween,

and Yangtze Rivers.

Tibet has a high plateau climate, with large diurnal and

monthly temperature variations. The center of the plateau

has an average January temperature of 32°F (0°C) and an

average June temperature of 62°F (17°C). The southeastern

part of the plateau is affected by the Bay of Bengal summer

monsoons, whereas other parts of the plateau experience

severe storms in fall and winter months.

Geologically, the Tibetan Plateau is divided into four terranes,

including the Himalayan terrane in the south, and the

Lhasa terrane, the Qiangtang terrane, and Songban-Ganzi

composite terrane in the north. The Songban-Ganzi terrane

includes Triassic flysch and Carboniferous-Permian sedimentary

rocks, and a peridotite-gabbro-diabase sill complex that

may be an ophiolite, overlain by Triassic flysch. Another

fault-bounded section includes Paleozoic limestone and

marine clastics, probably deposited in an extensional basin.

South of the Jinsha suture, the Qiangtang terrane contains

Precambrian basement overlain by Early Paleozoic sediments

that are up to 12 miles (20 km) thick. Western parts of the

Qiangtang terrane contain Gondwanan tillites and Triassic-

Jurassic coastal swamp and shallow marine sedimentary

rocks. Late Jurassic–Early Cretaceous deformation uplifted

these rocks, before they were unconformably overlain by Cretaceous

strata.

The Lhasa terrane collided with the Qiangtang terrane

in the Late Jurassic and formed the Bangong suture, containing

flysch and ophiolitic slices, that now separates the two

terranes. It is a composite terrane containing various pieces

that rifted from Gondwana in the Late Permian. Southern

parts of the Lhasa terrane contain abundant Upper Cretaceous

to Paleocene granitic plutons and volcanics, as well as

Paleozoic carbonates and Triassic-Jurassic shallow marine

deposits. The center of the Lhasa terrane is similar to the

south but with fewer magmatic rocks, whereas the north

contains Upper Cretaceous shallow marine rocks that onlap

the Upper Jurassic–Cretaceous suture.

The Himalayan terrane collided with the Lhasa terrane

in the Middle Eocene forming the ophiolite-decorated

Yarlungzangbo suture. Precambrian metamorphic basement is

thrust over Sinian through Tertiary strata including Lower Paleozoic

carbonates and Devonian clastics, overlain unconformably

by Permo-Carboniferous carbonates. The Himalayan

terrane contains Lower Permian Gondwanan flora and probably

represents the northern passive margin of Mesozoic India,

with carbonates and clastics in the south, thickening to an allclastic

continental rise sequence in the north.

The Indian plate rifted from Gondwana and started its

rapid (3.2–3.5 inches per year, 80–90 mm/yr) northward

movement about 120 million years ago. Subduction of the

Indian plate beneath Eurasia until about 70 million years ago

formed the Cretaceous Kangdese batholith belt containing

diorite, granodiorite, and granite. Collision of India with

Eurasia at 50–30 million years ago formed the Lhagoi-

Khangari of biotite and alkali granite and the 20–10-millionyear-

old Himalayan belt of tourmaline-muscovite granites.

Tertiary faulting in Tibet is accompanied by volcanism,

and the plateau is presently undergoing east-west extension

with the formation of north-south graben associated with hot

springs and probably deep magmatism. Seismic reflection

profiling has detected some regions with unusual characteristics

beneath some of these graben, interpreted by some seismologists

as regions of melt or partially molten crust.

Much research has been focused on the timing of the

uplift of the Tibetan Plateau and modeling the role this uplift

has had on global climate. The plateau strongly affects atmospheric

circulation, and many models suggest that the uplift

may contribute to global cooling and the growth of large continental

ice sheets in latest Tertiary and Quaternary times. In

addition to immediate changes to airflow patterns around the

high plateau, the uplift of large amounts of carbonate platform

and silicate rocks expose them to erosion. The weathering

of these rocks causes them to react with atmospheric

carbon dioxide, which combines these ions to produce bicarbonate

ions such as CaCO3, drawing down the atmospheric

carbon dioxide levels and contributing to global cooling.

The best estimate of the time of collision between India

and Asia is between 54 million and 49 million years ago.

Since then convergence between India and Asia has continued,

but at a slower rate of 1.6–2.0 inches per year (40–50

mm/yr), and this convergence has resulted in intense folding,

thrusting, shortening, and uplift of the Tibetan Plateau. Timing

the uplift to specific altitudes is difficult, and considerable

debate has centered on how much younger than 50 million

years ago the plateau reached its current height of 16,404 feet

(5 km). Most geologists would now agree that this height was

attained by 13.5 million years ago, and that any additional

height increase is unlikely since the strength of the rocks at

depth has been exceeded, and the currently active east-west

extensional faults are accommodating any additional height

increase by allowing to crust to flow laterally.

When the plateau reached significant heights it began to

deflect regional airflow currents that in turn deflect the jet

streams, causing them to meander and change course. Global

weather patterns were strongly changed. In particular, the

cold polar jet stream is now at times deflected southward

over North America, northwest Europe, and other places

where ice sheets have developed. The uplift increased aridity

in central Asia by blocking moist airflow across the plateau,

leading to higher summer and cooler winter temperatures.

The uplift also intensified the Indian Ocean monsoon,

because the height of the plateau intensifies temperature-driven

atmospheric flow as higher and lower pressure systems

develop over the plateau during winter and summer. This has

increased the amount of rainfall along the front of the

Himalaya Mountains, where some of the world’s heaviest

rainfalls have been reported, as the Indian monsoons are

forced over the high plateau. The cooler temperatures on the

plateau led to the growth of glaciers, which in turn reflect

back more sunlight, further adding to the cooling effect.

Paleoclimate records show that the Indian Ocean monsoon

underwent strong intensification 7–8 million years ago,

in agreement with some estimates of the time of uplift, but

younger than other estimates. The effects of the uplift would

be different if the uplift occurred rapidly in the Late

Pliocene–Pleistocene (as suggested by analysis of geomorphology,

paleokarst, and mammal fauna), or if the uplift occurred

gradually since the Eocene (based on lake sediment analysis).

Most geologists accept analysis of data that suggests that

uplift began about 25 million years ago, with the plateau

reaching its current height by 14 million or 15 million years

ago. These estimates are based on the timing of the start of

extensional deformation that accommodated the exceptional

height of the plateau, sedimentological records, and on uplift

histories based on geothermometry and fission track data.

See also ATMOSPHERE; FLOOD; HIMALAYA MOUNTAINS;

SUPERCONTINENT CYCLES.

tidal wave See TSUNAMI.

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