长江源区新生代地堑的构造特征与形成机制

Tectonic characteristics and formation mechanism of Cenozoic grabens in the source region of the Yangtze River, China.

长江源地区近南北走向的地堑构造是该区现今最为显著的构造现象,也是青藏高原迄今发现的同类构造发育的最北部地区。长江源地区地堑构造包括温泉、常错、当拉错纳玛和沱沱河4个规模较大的近南北向地堑。断层年代学和断陷盆地沉积作用研究结果表明,研究区伸展变形至少自中新世末—上新世初就已开始,对长江源地区现今的地貌格局、水系型式具有显著的制约作用。长江源区主要水系是沿南北走向的地堑构造和正断层侵蚀发育而成的。青藏高原新生代伸展地堑构造是高原经历早期地壳强烈缩短变形之后,在深部动力学机制作用下快速隆升的产物。

Field mapping reveals that the nearly N-S-trending grabens are the present most prominent structural features in the source region of the Yangtze River, which are also the northernmost grabens ever found on the Qinghai-Tibet Plateau. There are four major grabens distributed in the region, namely the Wenquan graben, Changcuo graben, Danglacuonama graben and Tuotuo River graben. Fault chronology and sedimentation of the downfaulted basins indicate that extensional deformation in the study region was initiated in the late Miocene to early Pliocene at least, which has had a marked controlling effect on the present geomorphologic framework and drainage pattem in the region. The main drainage system in the source region of Yangtze River formed and developed by riverhead erosion along the N-S-trending grabens and normal faults. The Cenozoic extensional grabens on the Qinghai-Tibet Plateau are the product of rapid uplift of the plateau caused by the deep dynamic mechanism after earlier strong crustal shortening.