Structural segmentation and zonation and differential deformation across and along the Lomgmen thrust belt,West Sichuan,China

The NE-trending Longmen thrust belt defines the eastern margin of the Tibetan Plateau,and separates the Songpan-Ganzi fold belt to the west by the Maoxian-Wenchuan fault from the Sichuan Basin to the east by the Anxian-Dujiangyan(Guanxian) fault.The Longmen thrust belt comprises a serial of thrust sheets bounded by four major SE-directed thrusting faults,i.e.,from west to east the Qingchuan-Maoxian-Wenchuan fault,Beichuan-Yingxiu-Wulong fault,Anxian-Dujiangyan-Shuangshi fault and Guangyuan-Dayi-Ya'an(blind) fault.The deformation across the Lomgmen thrust belt changes from ductile to ductile-brittle to brittle from northwest to southeast,exhibiting a distinct structural zonation.According to the deformation style and the involvement of basement rocks,five structural zones are recognized,and each is bounded by the above four major faults.From west to east,they are:(1) arcuate structure and ductile deformation;(2) basement-involved thrusts and folds with cleavage;(3) concentric fold,imbricate thrusts and klippes;(4) moderate to gentle fold-thrust deformation;and(5) weakly deformed or undeformed foreland basin sequence.There are also structural segmentation along the Longmen thrust belt.The northern Longmen Mountains is characterized by Proterozoic Jiaoziding basement complex and Tangwangzhai syncline,and the middle and southern Longmen Mountains are symbolized respectively by the Proterozoic Peng-Guan and Baoxing basement complexes,both with klippes along the front of the complexes.Along the Longmen thrust belt,from northeast to southwest,folding and thrusting initiation and finalization tend to be young,the deformation is more brittle and extensive,and the Cenozoic activity is more intense.The structural zonation and segmentation across and along the Longmen thrust belt are mainly associated with the Indosinian amalgamation between North China Block,South China Block and Qiangtang Block,and the Cenozoic India-Asia collision.