龙门山冲断带分段-分带性构造格局及其差异变形特征

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

龙门山冲断带地处青藏高原东缘,介于松潘-甘孜褶皱带与四川盆地之间,由一系列走向北东、倾向北西的逆冲断层及其所夹的岩片组成。分段-分带性构造格局是龙门山冲断带的基本特点和重要特色。龙门山自西向东发育有4条主干断裂:茂县-汶川断裂、北川-映秀-五龙断裂、安县-都江堰(原称"灌县")-双石断裂和广元-大邑-雅安(隐伏)断裂。由这些主干断裂分隔,龙门山冲断带沿倾向表现出明显的分带性变形特征,以其典型构造特征自西向东称为弧形韧性变形带、基底冲断-同劈理褶皱带、同心褶皱-叠瓦冲断带、前陆扩展变形带和前陆坳陷带,它们自西向东显示层次渐浅、韧性递减、脆性渐强的变形特点。龙门山冲断带沿走向以北川-安县与卧龙-怀远一线为界可三分为北段、中段和南段,北段以出露轿子顶基底杂岩和唐王寨向斜及其前缘叠瓦冲断系为主要特征,中段以出露彭灌基底杂岩及其前缘发育飞来峰为典型特征,南段以出露五龙、宝兴基底杂岩及其前缘发育飞来峰为典型特征。自北东向南西,龙门山冲断带沿走向具有构造起始和定型时期渐晚、脆性渐强、前陆卷入变形的程度渐强、新生代隆升渐快和活动性增强的变化趋势。

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.