Tsaidam Basin


Dulan - J.H. Wittke

 

The Tsaidam (Qaidam) Basin is located between the Qilian Shan and the Kunlun Shan at the northern margin of the Tibetan Plateau. The basin is filled with young Cenozoic sediments, averaging 4-5 kilometers in thickness. During the deformation that produced the high Tibetan Plateau to the south and mountains to the north, it remained stable, becoming a basin as the mountains rose around it. NASA's on-line geomorphology book describes the western end of the Tsaidam Basin, which is defined by the enormous Altyn Tagh strike-slip fault.

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 (left).


Tsaidam Basin - J.H. Wittke

 

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.


Columnar Joints in Welded Tuff - J.H. Wittke

 

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.


Kunlun Mountains - J.H. Wittke

 

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.


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