Tibet Geography(3)
  
Geology and Geography of the Lhasa River in Tibet

Geologic Map of the Lhasa River Area

Map produced by the Chinese Ministry of Geology and Mineral Resources
Original scale 1:1,000,000

The Lhasa or Kyi Chi river is located in the east central part of the Eocene block of Tibet , between the Tsangpo Suture to the south and the NW extension of the Ailao Shan Fault to the north. The area is characterized by subhorizontal Paleozoic and Mesozoic sedimentary rocks which have been intruded by significant quantities of Cretaceous granite, metamorphosed and deeply eroded to produce a rugged terrain (see photo below). Faults generally trend E-W. The average elevation is about 16,000 feet, typical of the Tibetan Plateau.

Samdainkangsang, a sacred mountain, elevation about 21,600'.
Photo by Phil Wegener Kantor

Carboniferous and Permian rocks are typically massive marble. They are exposed in the middle reaches of the river basin and represent an off shore environment prior to the initial subduction of Tethys oceanic crust. Mesozoic and early Cenozoic rocks were originally deposited as sandstones and shales in a coastal region during subduction of Tethys oceanic crust with younger rocks containing increasing amounts of volcanic material. Jurassic and Cretaceous rocks are widely exposed in the northern part of the river basin and have been intruded by moderate volumes of Cretaceous granite. Triassic rocks unconformably overlain by Eocene-Cretaceous volcaniclastic sediments dominate exposures in the southern part of the river basin, including the Yarlung Tsangpo valley. They have been intruded by large volumes of Eocene granitic rocks just north of the Tsangpo Suture.

Photo by Steve Swann

In its upper and middle reaches, the river bed is filled with glacial gravels and the few rapids present are due to constriction. The gravels form benches which are sometimes over a hundred feet thick feet thick. In its lower reaches, the river braids through sandbars.

For information about the river and the local culture, see