青藏高原-四川盆地过渡带第四纪冰川作用及其对新构造运动的响应

Substantial investigation has been done on the glaical landforms and sediments in the Tibetan Plateau by numerous researchers over the past tens of years. In addition to the achievement in the understanding of glacial sequences, extent, and distribution features, a preliminary, significant conclusion has been achieved that two uplift events, Kunhuang and Gonghe Movements, played an important role in the Quaternary glacial history on the Tibetan Plateau, and some of the mountains along the eastern marginal zone of the Tibetan Plateau were glaciated during the last glacial period only after they had been elevated by the Gonghe Movement for a significant magnitude. However, this standpoint, significant to the understanding of the glacial history of the Quaternary glaciation on the Tibetan Plateau, was raised mainly based on the evidence from field investigations, including weathering degree of tills and morphostratigraphic correlation. Absolute dating of glacial landforms and sediments are required for verifying this opinion. Widespread, identifiable glacial landforms and sediments are present in the Chapingshan, Xiaoxiangling, and Luojishan Mountains in the transitional zone from the Tibetan Plateau to the Sichuan Basin. Tectonically, the three mountains are surrounded by active faults including Wenchun-Maoxian, Yingxiu-Beichun, Anninghe, Daliangshan, and Zemuhe Fault systems. Furthermore, a few steps of fluvial terraces are well-preserved along each of the rivers which flow down the faults. In this study, we will examine the response of the paleoglaciers in the above mountains to neo-tectonics, especially the Gonghe Movement, by systematically defining the chronology of glacial landforms and sediments in the mountains, with the combination of investigating the associated fluvial terraces and active faults.This research will help further the understanding of the response of paleoglaciers in the Tibetan Plateau to climate change and tectonics.

历经数代人几十年的调查研究，现已基本查清青藏高原第四纪冰川的序列、规模及分布特征，同时也初步确认高原的两次构造抬升事件- - -昆黄运动和共和运动对高原第四纪冰川发育的启动与规模演变起着至关重要的作用。其中，共和运动使高原东缘部分山地抬升到雪线以上，在末次冰期时首次发育了冰川。然而，这个有关高原第四纪冰川发育与演变的认识主要是源于冰川地貌地层学研究，亟待更进一步的年代学工作来证实与支持。夹持于青藏高原-四川盆地过渡区活动断裂带间的茶坪山、小相岭和螺髻山保留有形态清晰的冰川遗迹，且周边的河谷中发育有多级河流阶地。本项目拟运用宇宙成因核素和光释光等测年技术对茶坪山、小相岭和螺髻山的古冰川遗迹进行系统定年，并结合相邻河流阶地等方面的研究来查验上述山地第四纪冰川发育对新构造运动特别是共和运动的响应情况。本项目研究对进一步认识青藏高原第四纪冰川发育的构造-气候耦合效应具有重要理论意义。

自晚新生代以来，青藏高原经历了大幅度隆升，但隆升幅度在高原的不同部位是不均匀的，这种不均匀性的抬升一直持续到现在。这造就了一个现象，在同一气候带，不同山地进入冰冻圈的时间不同，最终历经第四纪冰川作用的次数和冰川发育规模也不尽相同。因此，借助不同山地的冰川作用次数及其规模，则可以了解这些山地第四纪冰川作用对构造抬升的响应，以及冰川作用发生时的山体大致高度。. 本项目对青藏高原东缘的螺髻山、小相岭及茶坪山第四纪冰川遗迹进行了系统考察，并选取有代表性的冰川序列运用宇宙成因核素和光释光技术进行测年。结果显示（1）茶坪山主峰附近的冰碛垄形成于2.2万年前左右，当时的冰川平衡线高度约为4060 m，大约在1.7万年前这里的冰川基本消失；（2）小相岭东侧水电站附近谷地中的两套冰碛垄形成于2.1万年前和1.65万年前左右，两次冰川前进时的冰川平衡线高度分别为3950 m和4050 m；（3）螺髻山经历了两次主要冰川前进，分别发生在6.3万年前和2.1万年前左右，当时的冰川平衡线高度分别为3900 m和4000 m，冰川在1.7万年前左右消失于主峰附近。另外，对鲜水河断裂带中段错断地貌和牙着库河流阶地的研究表明，自57万年以来，本区的构造抬升速率为2-2.5 mm/a。. 综合上述研究结果及前人的研究成果发现，青藏高原东北缘（比如太白山、达理加山）、东缘（比如螺髻山、小相岭、茶坪山）、东南缘（比如点苍山）的大多数山地均只在末次冰期时才历经第四纪冰川作用。这可能意味着这些山地在倒数第二次冰期时（氧同位素6阶段）尚未达到当时的雪线高度，只是在末次冰期到来之前才隆升到末次冰期雪线高度之上。结合我们通过河流阶地和活动断层得到的山地抬升速率（2-2.5 mm/a）以及对青藏高原东缘末次冰期和倒数第二次冰期冰川平衡线高度的估算，认为螺髻山、小相岭和茶坪山从倒数第二次冰期至末次冰期之间抬升了约200 m左右。这200 m的抬升幅度可能与青藏高原在第四纪晚期的构造运动即“共和运动”有关。