PRIMARY RESEARCH OF ACTIVITY SEGMENTATION OF LONGMENSHAN FAULT ZONE SINCE LATE-QUATERNARY

The Longmenshan Fault zone,trending in NE,lies between the Songpan-Ganzi orogenic belt and the Yangzi block.It consists of four major faults,such as the Houshan Fault,and related thrusting sheets.It is a nappe structure characterized by forward spreading.It formed during the Indo-China movement and was active many times since then.Since the Quaternary,this fault zone has been very active,and exhibits differential activity on its various sections.In this work,we make an integrated analysis of this fault zone based on data of geomorphology,geology,Bouguer gravity anomalies,and earthquakes.The results show that the near NS-striking Leidong Fault,located along Beichuan-Anxian,is the boundary between the middle and northeastern sections of the Longmenshan Fault zone.The two sections have high distinctive activity.The middle section is very active since late Pleistocene time with frequent medium-and small-sized earthquakes,while the northeastern section has weak activity since Quaternary with occasional small earthquakes.Due to the compression and uplift of the Tibetan Plateau and lateral sliding of blocks,the Sichuan-Qinghai block moves towards SEE,resulting in an active boundary of the block.This boundary includes the Mingshan uplift and the middle and southwestern sections of the Longmenshan Fault zone,both of which are truncated by the Mingshan uplift.And the northeastern section of the Longmenshan Fault zone has been abandoned.