Longmen Shan


Dujiangyan Diversion Project - J.H. Wittke

Dujiangyan

Construction of the Dujiangyan irrigation project was begun by Li Bing, the king of western Sichuan, in 256 BCE; his son, Er Lang, completed the project. Workers using only hand tools cut a trunk canal, called the "Mouth of the Precious Jar," through a towering mountain to feed an extensive system of canals on the plains. A diversion dam was constructed upstream (right), at the "Fish Mouth," that splits the Minjiang into "Outer" Flow and "Inner" Flow that is directed to the irrigation system. Another longitudinal weir, located just above the Mouth of the Precious Jar, controlsthe amount of water fed into the system. Flood waters top this lower weir and about 70% of the total flow returns to the Outer Flow of the Minjiang. The Dujiangyan system has continued in uninterrupted use since its initial construction.



Dujiangyan Stamps

In 1991, China issued these stamps showing the Dujiangyan irrigation project. The leftmost stamp shows the same area as the photograph above, but from a slightly different perspective.

Valley of the Minjiang - J.H. Wittke

Minjiang Valley

We drive up the deep valley of the Minjiang into the Longmen Shan (left). The Longmen Shan are a northeast-southwest-trending orogen, a nappe belt separating the Songban-Ganzi terrane of the Tibetan Plateau from the South China plate, on which Chengdu is located. Nappes are large folded sheets of rocks that have been moved along low-angle thrust faults. The rocks of the Longmen Shan were deposited upon the southern margin of the Eurasian Plate in a continental shelf environment, reminiscent of the present-day eastern coast of the United States. These include Silurian to Devonian metasedimentary rocks, late Paleozoic platform carbonates, and Triassic continental rocks, including coal seams. The lowlands of the South China Plate to the southeast are covered by alluvial sediments carried by the Minjiang and other rivers. These were deposited in the Chengdu Trough, a foreland basin produced when the weight of the Longmen Shan produced a depressed region in front of the mountains. NASA's on-line geomorphology book describes the geology of the northwestern Sichuan Basin, which shows dramatic folding and thrusting.


Tectonics of the eastern margin of the Tibetan Plateau (after Royden et al. 1997)

 

The Longmen Shan have experienced repeated deformations during their history as the series of microplates that make up the Tibetan Plateau collided with the southern margin of Eurasia. Each collision reactivated previous structures, causing movement to reoccur along previous faults and zones of weakness, pushing the mountains southward. The latest deformational event is Himalayan, associated with the impact of the Indian subcontinent and Eurasia.

However, the area does not have high seismic activity; for now, orogenesis has stopped in the Longmen Shan. Recent Global Positioning Satellite (GPS) studies have revealed that the triangular block carrying the Longmen Shan is no longer moving with respect to the South China Block. Instead, deformation is concentrated in the crust, which is moving independently from the underlying lithospheric mantle, along major strike-slip faults north and south of the range (left). The extension of the Kunlun Fault to the north and the Xianshuihe Fault to the south are very active. These fault are slipping at rates of about 1 centimeter per year and have produced six major (magnitude 7+) earthquakes in this century. Rupture of the Longmenshan Thrust fault caused the catastrophic Wenchuan earthquake of 2008 thast killed over 68,000 people. The main earthquake was followed for many month by over 100 aftershocks, some with magnitudes of over 6. These were generated as the region adjusted to the new set of stresses produce by the main earthquake.


Landslide in the Longmen Shan - Anne Barton Wittke

 

The road improves slightly, with stretches of intact asphalt appearing at increasingly greater intervals. However, landslides have covered the roadway in places, forming a rough veneer of mud and rocks. Recent studfies have revealed that landslides are the dominant erosional mechanism in the northwestern Himalayas; the results probably also apply to the Longmen Shan. Shovel-wielding road crews remove the smaller boulders and dirt, while a bulldozer pushes the largest blocks from the roadway into the raging torrent below.


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