1. 中国科学院地质与地物理研究所岩石圈构造演化开放研究实验室 北京 100029; 2. 国土资源部成都地质矿产研究所 成都 610082; 3. Department of Earth Atmospheric and Planetary Sciences M. I. T. Cambridge 02139 USA; 4. 北京大学地质学系 北京 100871
GEOLOGIC AND GEOMORPHIC ORIGINS OF THE EAST HIMALAYAN GAP
Wang Erchie1, Zhou Yong1, Chen Zhiliang2, Clark B. Burchfiel3, Ji Jianqing4
1. Institute of Geology and Geophysics, Chinese Academy of Science, Beijing 100029; 2. Chen Zhiliang Chengdu Institute of Geology and Mineral Resources, Chengdu 610082; 3. Department of Earth Atmospheric and Planetary Sciences M. I. T. Cambridge 02139 USA; 4. Department of Geology, Peking University, Beijing 100871
Two transects, mapped across the eastern boundary between the Tethys Himalayan sedimentary sequence and the High Himalayan metamorphic sequence in Milin area of southeastern Tibet, demonstrate that a roughly W-dipping normal fault, namely Milin, separates these two sequences. Triassic rocks in the hanging wall consist of schist and quartzite, dipping to SW with an angle of ～60 degree. They contain mylonitic fabrics and show evidence for normal slip. Those rocks in the footwall consist of augen gneiss, with dipping direction varying from NW to SW. The sense of the rotation of plagioclase augens indicates the hanging wall going downward. The contrasting difference between their metamorphic degree indicates that amount of normal displacement along the Milin fault is very large. In this region the average elevation of the Himalayas is only about 4500 m, much lower than that of any other part of the Himalayas, namely here the East Himalayan gap. Its formation can be interpreted to have resulted from the normal faulting along the Milin fault. To the south, the Milin fault joins with the eastern end of the South Tibet detachment system (STDS), suggesting that it may have constituted the eastern flank fault of STDS. With the highest peak of 7756 m (Namche Barwa), the core of the eastern Himalayan syntaxis is bounded by the Milin fault on the west, indicating that its rapid uplift may have also accommodated by the normal faulting, which occurred in the Miocene. Occurrence of the normal faulting across the eastern Himalayan gap indicates that the Himalayas also underwent E-W elongation while the N-S shortening occurred in the Miocene.