龙门山冲断带北段前锋带新生代构造变形

The Cenozoic Structural Deformation in the Northern Longmenshan Front Belt

龙门山北段前锋构造的地震剖面解释和前缘盆地内沉积地层的磁组构研究表明前锋构造中发育两期构造挤压作用,即整体强烈的晚三叠世变形和由北向南逐渐减弱的弱新生代构造变形。受这两期构造挤压作用的控制,龙门山北段前锋构造中发育上、下两套构造层,地表构造为晚三叠世时期形成,而深部隐伏构造则形成于新生代。北部的矿山梁和天井山构造几何学上表现为一个双重构造,浅层是一个晚三叠世形成的断层转折褶皱;深层是新生代形成的多个逆冲岩片叠置所构成的隐伏堆垛背斜;南部的青林口和中坝构造主体表现为叠瓦状逆冲,前锋构造是断层转折褶皱和断层传播褶皱。新生代构造冲断位移量以及造成早期构造抬升由北向南逐渐减小,反映新生代变形强度由北向南的减弱。磁组构研究表明新生代变形从龙门山冲断带边缘到盆地内部,磁组构从铅笔状磁组构到初始变形磁组构并逐渐过渡到沉积磁组构。由南向北磁组构由初始变形磁组构转变为铅笔状磁组构,说明应变越来越强,从而进一步证明了龙门山前锋新生代构造的弱变形作用和变形强度的北强南弱分布特征。

Seismic interpretation, field investigations and magnetic fabrics study reveal a week Cenozoic tectonic compressive deformation occurred in the northern Longmenshan front belt, which changed the older structures formed during the Late Triassic. So the front belt of the Northern Longmenshan exhibits a double-layer structural style characterized by shallow thrust structures of early deformation and deep blind ones of Cenozoic. The Kuangshanliang and Tianjingshan present a duplex which comprises of a shallow fault-bend fold of late Triassic and a deep blind stacking anticline imbricated by several thrust sheets of Cenozoic. And Qinglinkou and Zhongba shows a shallow imbricate structure of late Triassic and a blind fault-bend fold or fault-propagation fold which may be created or altered in the Cenozoic. The Cenozoic deformation intensity becomes much weaker from the north to south and results in the smaller displacement and structural uplift of the thrust folds. Magnetic fabrics study in the Jurassic and Cretaceous also illustrate the degressive distribution of the week Cenozoic deformation of the Northern Longmenshan front belt from the north to south, where the sedimentary magnetic fabric changes to initial deformation magnetic fabric and finally to pencil-shaped magnetic fabric from the Sichuan basin to thrust belt and initial deformation magnetic fabrics to pencil-shaped magnetic fabrics from the south to north along the northern frontier Longmenshan belt.