关键词: 龙门山断裂带, 岷江阶地, 晚第四纪, 断裂滑动速率

Abstract: We estimate the slip rates of three faults in the central segment of Longmenshan Fault zone, namely, the Maowen-Wenchuan Fault, the Beichuan-Yingxiu Fault and the Jiangyou-Guanxian Fault, based on the measurement and dating of deformation of Late Quaternary terraces along the Minjiang River and its tributaries. The three-level Late Quaternary terraces, T₁, T₂ and T₃, are well developed along the Dujiangyan-Wenchuan reach of the Minjiang River. The cross profiles show that T₃ and T₁ have undergone a complete development process. The thermoluminescence ages of the upper deposits on T₁, T₂ and T₃ are 9～13ka BP, 19～30(ka BP) and 51～58ka BP, respectively. Therefore, we can constrain the ages of T₁, T₂ and T₃ surfaces to 10ka BP, 20ka BP and 50ka BP, respectively. The synchronous three-level terraces also exist in the valleys of the Baishahe River and other tributaries of the Minjiang River. These river terraces are laterally and vertically dislocated by the Longmenshan Fault zone. According to the dating and vertical displacements of river terraces, the Late Quaternary reverse slip rates of the Maowen-Wenchuan Fault, the Beichuan-Yingxiu Fault and the Jiangyou-Guanxian Fault are estimated to be 0.5mm/a, 0.6～0.3mm/a, 0.2mm/a, respectively. According to the lateral displacements of river terraces, the Late Quaternary dextral strike-slip rate of the Maowen-Wenchuan Fault and the Beichuan-Yingxiu Fault averages 1mm/a each. The late Quaternary faulting of the Longmenshan Fault zone is characterized with intermittency. Three faulting stages can be identified. The first one is between 50ka BP to 20ka BP, the second one between 20ka BP to 10ka BP, and the latest starts at 10ka BP. The existence of thick alluvial deposits in the present channel indicates that the uplift of Longmenshan Mountains is more complex than previously estimated.

Key words: Longmenshan Fault zone, Minjiang river terrace, late Quaternary, fault slip rate