Abstract: In the deep-incised valleys along the middle reaches of the Dadu River, field investigation led to the discovery of several types of sediments, which include rockfalls, landslides and paleo-dammed lake sediments. These sediments have preserved the activity records of the Lengqizhen fault, which is the southernmost segment of the Longmen Mountain fault zone. Two strike-slip events were determined on the basis of high resolution SPOT imagery interpretation, field observations, and stimulated luminescence dating (OSL). The early event occurred at 18Ma and the later at 11Ma. During the early deformation event, right slip faulting of the Lengqizhen fault cut into a brecciated sand and a suite of massive blocks. This deformation caused a damming event in the Dadu River and formed the Dewei paleo-dammed lake. The dammed lake lasted for 8ka and eventually broke about 11ka ago. The position of the damming event occurred in the main valley, from Jiajun Township to Dewei Township, along the V-shaped stream of the Dadu River. The late deformation event of the fault cut into the lacustrine deposits. This study has a great significance for understanding the history, activities and the full sequence of the Longmen Mountain fault zone.