The terrain
becomes an arid desert, whose bleak expanses are punctuated by
small green oases where a river exits the mountains. One of the
largest oasis is Dulan, where the land has been reclaimed from
the sands, and irrigation yields green fields. Lines of trees
have been planted as windbreaks, in an attempt to halt the
shifting dunes (right).
Seismic reflection work done for oil exploration and drilling
have revealed three major unconformities in the Tsaidam Basin.
These breaks in the rock record mark times where erosion
predominated and sediments were not deposited, often due to
uplift associated with mountain building. The oldest one of late
Paleocene-early Eocene age, formed at the same time that
thrusting was occurring on the Tibetan Plateau in the Fenghuo
Shan. The middle unconformity is the largest and probably
reflects when the Tsaidam Basin was over-ridden by faults along
its southern and northern margins.
The cliffs slightly farther along the highway
display spectacular columnar jointing. Fluted vertical rock walls
rise into a cloudless sky like a gigantic collection of organ
pipes (left). The rock is a pyroclastic tuff, material ejected
from a volcano that raced across the surrounding land buoyed by
hot gases. The resulting deposits weld together from the great
and slowly cool. Increasing the temperature of a material
produces a increase in volume - a property called thermal
expansion. Conversely, when something loses temperature its
volume decreases, and the material shrinks. As this happened to
the tuff, the thick carpet of pyroclastic rock did not cool
evenly; its lower parts were effectively insulated by the
overlying parts. Consequently the deposit shrank more at the top
than at the bottom, producing stresses within the flow. Tension
fractures formed at the top propagated downward, creating
beautiful polygonal columns.
We
cross more desert, following ancient silk route along the
southern edge of the Tsaidam Basin. The road winds like a black
asphalt snake across sandy expanses (above). Blasted peaks of
broken rock stick through the shimmering dunes. Further west, the
ground is flat and rocky, a desert pavement of small stones and
sand. The vegetation is sparse and wind-blown sand forms small
dunes in the lee of the bushes (right). The Tsaidam Basin was one
of the sources of the loess near Lanzhou.
The Burhan Budai Shan, or more familiarly Kunlun Mountains,
rise to the south and we catch occasional glimpses of snow on the
high summits on the horizon. The linear mountain front rises
abruptly from the gray stony desert, marking the position of the
strike-slip Kunlun Fault. This is a very active structure and
huge magnitude 7+ earthquakes occured near Dulan in 1937 and
1963. During the 1963 earthquake, the north side slid westward
relative to the south side by about 8 meters! The Kunlun Fault is
just one of the large strike-slip faults along the northern and
eastern margins of the Tibetan Plateau.
