LATE CENOZOIC CRUSTAL SHORTENING ACROSS THE MIDDLE PART OF THE TIBETAN PLATEAU-HIMALAYAN OROGENIC BELT AND ITS TECTONIC ORIGIN

In contrast with western and eastern parts of the Tibetan plateau, the middle part of the Tibetan plateau contains rugged landforms, characterized by a series of NWW-trending water divides alternating with drainage basins, along which the Tongtianhe, the Lancang and the Nu rivers flow to the southeast. Based on field observations, the topographic features of these water divides and drainage basins are commonly controlled by the NW-SE folding, specifically, the stronger the folding, the steeper of the slop. The consistency between the deformational and topographic features indicates that the rugged landforms of the middle part of the plateau resulted from the Cenozoic NE-SW folding. This zone of crustal shortening can be traced northward to the northeastern end of the Altyn Tagh fault through the northern margin of the Tibetan plateau-Himalayan (TH) orogenic belt, where late Cenozoic crustal shortening mainly takes place along the western end of the Haiyuan fault, the Qaibeiyuan thrust fault, the central part of the Qaidam basin and the middle segment of the East Kunlun belt, and southward to the Himalayan thrust belt, where the crustal shortening mainly occurred along the southern boundary fault of the Gangdese magmatic belt, the North Himalayan gneiss dome system and the middle-east Himalayan thrust fault that is overlain by the Mt. Everest. This zone of NE-SW crustal shortening across the widest part of the TH orogenic belt is 1500 km in the NE-SW direction, 650 km wide in the south and 100 km wide in the north. It lies between two large Cenozoic strike-slip fault systems, the Altyn Tagh and the Karakoram faults to the west and the Haiyuan, the East Kunlun, the Xianshuihe, the Jiali and the Red River-Ailao Shan fault to the east. Thus, the northward motion induced by the convergence of India and Eurasia occurring within the western and eastern parts of the plateau that lie within these two fault systems is interpreted to have largely taken up by the strike-slip motion. In contrast the middle part of the TH orogenic belt was shortened in NE-SW direction to accommodating the northward motion.