Geomorphological Evolution of Quaternary Alluvial Fans and Its Response to Tectonic Activity along the Fujiang River, Northeastern Longmen Mountains

Key words: 
• Longmen mountains /
• alluvial fan /
• geomorphological evolution /
• structural response /
• Fujiang River

Abstract: Fujiang river alluvial fan is located in the Wudu Basin, the North-Eastern part of Longmen mountains, which is a foreland basin formed near the eastern margin of Tibetan Plateau, and the alluvial fan covers an area of about 25 km². The alluvial fan has developed in a right-lateral strike-slip domain settings, it belongs to the Jiangyou Fault and Xiangshui-Rangshui Fault. Through the field survey, sedimentary sequence and cosmogenic nuclide burial ages data of alluvial fan, and combined with the previous research results and geological survey data, three alluvial fan's formation, distribution and tectonic activities along the Fujiang river are determined respectively. Early Pleistocene alluvial fan(1.84 Ma), Middle Pleistocene alluvial fan(0.54 Ma) and Holocene alluvial fan distributed near the northeast-southwest directions in foreland of Longmen mountains. In the middle Pleistocene, Jiangyou rupture is right-lateral strike-slip thrust fault, causing the early Pleistocene Fujiang river alluvial fan head right-lateral slip about 3.2 km, and the fan uplifted about 50 m. As Jiangyou rupture continued right-lateral strike-slip and thrust, the middle Pleistocene alluvial fan's head was right-lateral displaced about 0.8 km. During the Quaternary period, the early and middle Pleistocene alluvial fan total was right-lateral displaced about 4 km, 0.8 km, respectively. The middle Pleistocene alluvial fan uplifted about 50 m, The early Pleistocene alluvial fan total uplifted about 100 m. Sedimentary center of Fujiang river alluvial fan moved to the right downstream. Characterized by coarsening upward sedimentary sequence, gravel layer of alluvial fans asymmetrically distributed. The results show the Longmen mountains has been uplifted and the rupture right-lateral strike-slip and thrust, the Fujiang river alluvial fans expanded as a correspondence to the tectonic activities.