雅鲁藏布江中下游流域地貌差异演化的岩屑磷灰石裂变径迹证据

以西藏米林派为分界点, 雅鲁藏布江中下游显示了截然不同的河流特性, 下游的雅鲁藏布大峡谷作为世界第一大峡谷, 围绕南迦巴瓦峰形成了马蹄形大拐弯, 与中游的宽谷河道显著不同. 分析了雅鲁藏布大峡谷上游米林河段和大峡谷下游地东河段的河床砂岩屑磷灰石裂变径迹(AFT)数据结果. 分析显示, 米林河段AFT年龄集中在10.7和26.8 Ma, 地东河段AFT显示年龄集中在2.5, 7.1和12.6 Ma. 由于河床砂岩屑是上游方向裂点向下到岩屑沉积点之间流域地质体经剥蚀和水流分选平均后的产物, 据此可以推定, 米林上游到加查河段流域的地质体和大峡谷下游地东以上到直白河段流域的地质体经历了不同的冷却历史. 从统计的结果上看, 米林以上到加查的中游河段地质体经历了两个不同阶段的冷却, 分别为10.7和26.8 Ma前后, 雅鲁藏布大峡谷所在流域地质体的冷却主要发生于2.5 Ma前后, 这一结果证实, 以南迦巴瓦峰为核心的东喜马拉雅构造结在2.5 Ma以来经历了快速和独特的构造地貌演化过程. 雅鲁藏布大峡谷与雅鲁藏布江中游河段的流域属于相同的大地构造单元, 即冈底斯岩带、雅鲁藏布缝合带和特提斯喜马拉雅带. 但是, 二者热史演化的迥然差异显示, 雅鲁藏布大峡谷河段流域地质体在2.5 Ma以来的冷却过程似乎不是区域构造作用的结果, 显示其可能与气候因素引起的强烈剥蚀作用密切相关.

AFT dating of detrital apatites from the Yarlung Zangbo Great Canyon: Implications for its distinct geomorphological evolution

The Yarlung Zangbo Great Canyon is distinct from upstream of the Yarlung Zangbo River, with a great U turn slicing the Namche Barwa Region. This paper reports apatite fission track (AFT) ages of the fluvial sands from Milin (ML-114) at upstream and Didong (DXL-20) at downstream of the Yarlung Zangbo Great Canyon. The AFT ages of ML-114 are concentrated at 10.7 and 26.8 Ma while those of DXL-20 at 2.5, 7.1 and 12.6 Ma. Considering that the fluvial sands are the mixture of the bedrocks in drainage catchments from the knickpoint at upstream to where the rock debris deposited after denudation and transportation, it can be concluded that the two drainage terrains, one from Milin to Jiacha of the upstream and another from Didong to Zhibai of the downstream, have different cooling ages. The obtained data indicate that the eastern Himalayan syntaxis surrounding the Namche Barwa-Gyala Peri has experienced rapid and unique tectonic geomorphological evolution since 2.5 Ma. Even though the drainage catchment of the Yarlung Zangbo Great Canyon and the middle reach of the Yarlung Zangbo River are considered to be the same tectonic unit consisting of the Gangdise belt, the Yarlung Zangbo suture and the Tethys Himalayan belt, the different geothermal histories between the upstream and downstream sections of the Yarlung Zangbo Great Canyon did not result from the regional neo-tectonic evolution, but from the rapid denudation caused by climatic factors.