Gangdese Batholith


Nyetang Big Buddha - J.H. Wittke

 

Kyi Chu River

The road out of Lhasa follows the Kyi Chu (Lhasa River), which is in flood with the monsoonal rains. About 20 km south of Lhasa, we make one of our infrequent geology stops along the Kyi Chu. The highly deformed rocks of the Lower Cretaceous Takena Formation are inaccessible due to a large flooded patch, but the outcrop is highlighted by the immense carved and painted Nyetang Buddha. Lacking a witness to attest to the truth of his teachings upon his enlightenment, the Buddha touched one hand to the ground, calling the earth to be his witness. It seems an appropriate gesture for all geologists also.


Skin Boat on the Kyi Chu - J.H. Wittke

 

The wind has built enormous climbing dunes along the river, sand dunes plastered against the valley walls. Near Qüqü, where the Kyi Chu joins the Yarlong-Tsangpo (Brahmaputra River), a group of people are crossing the flooded river in a skin boat. They appear as a small black smudge on the vast gray surface of the waters.


Lhasa & Tsangpo

Tsangpo River Gorge

North is toward the upper right of this Space Shuttle photograph. The braided course of the Yarlung-Tsangpo cuts from the upper left to the lower right, with the sinuous Yamdrok Tso (lake) to the left. The Kyi Chu joins the Tsangpo in the middle of the photograph, near the town of Qüqü. Lhasa is located near where the Kyi Chu makes a sharp bend toward the top of the image. The Tsangpo cuts through the Gangdese Batholith in a deep gorge, which is marked by a dark triangular shadow to the right of a few white clouds above the top arm of Yamdrok Tso.

West of Qüqü, we enter the narrow gorge of the Yarlong-Tsangpo (right). The river has cut through the Gangdese batholith, a great mass of coarsely crystalline rocks that represent the roots of a great volcanic chain that once towered along the southern margin of the Eurasian continent. The granite was formed above a late Cretaceous-Paleocene subduction zone, the place where ocean floor plunged beneath Asia as the Indian subcontinent moved rapidly northward. The water in the minerals carried down in the subducting oceanic plate was forced out as the rocks descended deep into the Earth, flushing into the overlying parts of the mantle. This added water decreased the melting point of the rocks in this portion of the mantle, causing them to melt. Some of the resulting magma erupted to form a lofty volcanic arc, like the present-day Andes Mountains of South America, but the larger part of it remained within the crust, forming great overlapping chambers. The magma cooled, crystallized and froze to form the Gangdese Batholith.


Tsangpo Gorge - J.H. Wittke

The walls of the gorge are steep and, in places, waterfalls cascade down the sides. The plutonic rocks exposed here are mostly granodiorites, a slightly darker version of granite and contain lustrous needles of black amphiboles and biotite set in a groundmass of the lighter minerals quartz and feldspar. In places there are remnants of the rocks that hosted the plutons, forming enclaves of metamorphosed sediments, termed roof pendants. The road is carved along the side of the canyon, blasted through the massive igneous rocks. In places it has been covered by hastily-removed landslide debris. Where the road is especially rutted, we climb out to march along by foot. At one point the liquid mud is ankle-deep and people lift their pant legs to avoid soaking them in the brown fluid.

The Himalayans are being rapidly uplifted, at least by geological standards. Geologists have documented the rate of uplift by studying the variations in isotopic ages with elevation in a pluton near Qüqü. Isotopic ages are locked in when a rock cools below a specific temperature, called the "blocking temperature." As uplift occurs, the upper portions of a pluton reach the blocking temperature earlier than lower portions do. Thus, by dating rocks at different elevations, one can determine how quickly the pluton was uplifted. The Qüqü, the pluton was uplifted at about 0.07 mm per year between 42 and 21 Ma. This rate suddenly jumped to over 2 mm per year in the Miocene (between 20 and 18 Ma). At 2 mm per year, a mass of rock would be raised 2 km over the course of a million years.



Free Tibet!