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. 
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.
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 above 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.
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.
