Abstract:

Since the Miocene, strong thrust uplift has occurred in the Longmen Shan nappe tectonic belt, resulting in the formation of a steep topographic gradient zone.In the foreland region, ancient alluvial fans composed of gravels have been widely developed, primarily including the Mingshan-Qionglai (MQ)and Danling-Simeng (DS)gravel layers.These layers, deposited by the ancient Qingyi River, are ideal for exploring the river’s evolution and the paleoenvironmental changes in this region.We conducted field investigation in the Longmen Shan Foreland region and performed cosmogenic nuclide ²⁶Al/¹⁰Be dating analysis on the MQ and DS gravel layers.The analysis indicates that the burial age of the MQ gravel layer is 0.608,9±0.018,7 Ma, and the burial age of the DS gravel layer is 0.218,0±0.094,1 Ma.Therefore, we conclude that the MQ gravel layer formed in the late Early Pleistocene to early Middle Pleistocene ( Q_1^3- Q_2^1), and the DS gravel layer formed in the Middle Pleistocene ( Q_2^2).Combing previous research results, the two gravel layers correspond to the δ¹⁸O curve of deep-sea ice core and the sequence curve of Chinese loess, indicating they formed during a warm interglacial period.Based on statistical analysis of gravel composition, size, orientation, and paleoflow directions in the region, three major diversion events have been identified in the Qingyi River system.In the early stage, the Qingyi River flowed from Ya’an in a north-northeast direction towards Jianshan.With the uplift of the Xiongpo anticline and the continued uplift of the Mingshan Qionglai terraces, the Qingyi River, which initially flowed from the southwest-north to the northeast, was forced to cut through the Xiongpo anticline and divert southeast at Caoba.

Key words: cosmogenic nuclide, Mingshan Qionglai gravel bed, Danling-Simeng gravel bed, the Qingyi river