ON THE ORIGIN OF THE LONGITUDINAL VALLEY BETWEENHONGWEN VILLAGE AND DIANNAN VILLAGE, NW.YUNNAN, AND THE DISCUSSION ON THERIVER CAPTURE OF JINSHAJIANG
Xu Zhong-lu Li Xing-jian (Chengdu Geological College)
The .longitudinal valley between Hongwen Village, NW Yunnan, near Shigu, at the first curve of Jinshajiang and Diannan village trends from NNE to SSW and stretches more than 53 Km. Professor P. Misch and Professor Ben Mei-ngo et al. believed that it is the valley of the old Jinshajiang, and was the result of river capture. The old Jinshajiang, after passing through Shigu, flowed southward along this longitudinal valley into Lancangjiang or Yuanjiang. The authors after two field studies in this district express their diffrent opinion as follows. The longitudinal valley when look at a distance is a single trough. But, actually it is a compound of different landform units, including seven depressions or sink holes, subsequent valley and fault basin, which stretch in the same direction. There wasn't any river deposits and river landforms. In addition, the lacustrine deposits in Jian-chuan Basin and the wide valley of Jiuhe, and the alluvial deposits of river along Jinloghe, the main Quaternary bed in this valley, are pluvial deposits. Its composition is rigorously controlled by the rocks exposed in the coombes, such as the pluvial gravels consisting of mica-schist and the mica fragments originated from mica-schist's gravels and the lamprophyre. These rocks, located at the base and the outside of coombes of Bailashao, came from the basal conglomerates of Upper Tertiary and exposed in these coombes. But Professor Ken Mei-ngo described that these pluvial gravels and fine sand beds consisting of mica fragments were alluvial deposits transported by the old Jinshajiang from the areas of schist rock of the upper river. In the longitudinal valley, the terraces which only present in the south of the Guanyinmiao may be divised into two to four classes, they consist of lacustrine, alluvial and pluvial deposits, all of which belong to the Middle Pleistocene or even later. We believe that those terraces are one of the phenomena of the newer and strong uplift tectonic movement in this region. The development of the different landform types in the longitudinal valley has been controlled by the great thrust fault zone between Hongwen Village and Hejiang Village that strikes NNE-SSW. This fault zone stretches about 100 Km., and the fault plane dips to the east. The Upper Tertiary and the underlying beds, even the Qua-ternary rock beds that near the fault zone had been faulted. This indicates that the fault is quite young and active. The authors think that the abrupt change of the direction of Jinshajiang at Shigu is a natural result of its adaptation to local geological conditions during the process of development.