晚新生代岷江下蚀速率及其对青藏高原东缘山脉隆升机制和形成时限的定量约束

Late Cenozoic Minjiang Incision Rate and Its Constraint on the Uplift of the Eastern Margin of the Tibetan Plateau

青藏高原东缘具有青藏高原地貌、龙门山高山地貌和山前冲积平原三个一级地貌单元 ,本文以岷江作为切入点 ,研究了该地区河流下蚀速率与山脉的隆升作用之间的相互关系。在建立岷江阶地序列的基础上 ,利用阶地高程和热释光年代学测年资料分别定量计算了岷江在川西高原、龙门山和成都盆地的下蚀速率 ,结果表明岷江各河段的下蚀速率明显不同 ,分别为 1.0 7～ 1.6 1mm / a、1.81m m/ a和 0 .5 9mm / a;在龙门山地区岷江的下蚀速率最高 ,约为川西高原地区的 1.5倍 ,约为成都平原地区的 3倍 ;而同一河段不同时期岷江的下蚀速率基本是连续的 ,具有很好的线性关系 ,可作为该河段整个河谷的下蚀速率。基于龙门山的表面隆升速率 (0 .3～ 0 .4 mm / a) ,在约束局部侵蚀基准面和气候变化对阶地形成的控制作用的基础上 ,本文建立了青藏高原东缘岷江下蚀速率与龙门山表面隆升速率之间的线性关系 ,结果表明河流下蚀速率约为山脉表面隆升速率的 5倍。根据龙门山表面在隆升速率和下切速率等方面均大于川西高原 ,并结合龙门山活动构造以走滑作用为主 ,笔者认为青藏高原东缘的边缘山脉以剥蚀隆升为主 ,兼有构造隆升作用。最后 ,根据岷江最大切割深度所需的时间 (3.4 8Ma)和成都盆地最古老的岷江冲积扇大邑砾岩的时间 (3.6

The large-scale morphology of the eastern margin of the Tibetan plateau can be divided into three zones, the Tibetan plateau, Longmen Mnt. and Sichuan basin. To understand the relationship between uplift rate and Minjiang incision rate in the region, we calculated the incision rate of the Minjiang River using terrace and its TL ages, and the results indicate that the Minjiang incision rate is 1.07~1.61 mm/a in the Tibetan Plateau, 1.81 mm/a in the Longmen Mnt., and 0.59 mm/a in the Sichuan Basin. Based on these key data and surface uplift rate of the Longmen Mnt.(0.3~0.4 mm/a, we established the late Cenozoic coupling relationship between incision rate and surface uplift rate, and incision rate is five times the surface uplift rate in the Longmen Mnt. According to the time span (3.48 Ma) of forming the maximum depth of dissection and the age (3.6 Ma) of oldest Minjiang Fan deposit in the Chengdu Basin, we inferred that the uplift of the Longmen Mnt. began from 3.6 Ma, and the mountain building models of the Longmen Mnt. had been constrained to both erosion uplift and tectonic uplift.