盆-山转换带构造变形特征及成因机制--以四川盆地东南部桑木场背斜为例

Characteristics and genetic mechanism of tectonic deformation in basin-mountain transitional zone:A case study of Sangmuchang Anticline in southeastern Sichuan Basin

为了剖析湘鄂西“隔槽式”褶皱带前缘与川东南“隔挡式”褶皱带的叠置关系,探讨四川盆地东南缘盆山转换带构造变形特征及成因机制,基于现有的地震、测井、DEM数字高程及相关地质资料,以断层相关褶皱理论为指导,对切过桑木场背斜的多条二维地震剖面进行了精细的解释,分析了其地质结构特征;利用平衡剖面技术建立了其运动学模型,分析了其构造演化特征。研究认为,桑木场背斜存在分层滑脱变形的变形机制,在垂向上具有相互叠置的结构特征,盖层的变形较为一致,以沿滑脱面的顺层剪切变形为主,背斜的整体构造格局受到盆地基底形态与齐岳山断裂的共同影响。齐岳山断裂是由数条倾向南东的基底逆冲断层组合而成的隐伏断裂系统,在其控制下形成的深部基底构造楔及叠瓦构造相互叠置,共同控制了背斜的变形。进一步分析表明,背斜的变形始于燕山期中期,其构造演化经历了多期构造运动的叠加作用。在雪峰山西推应力的作用下,湘鄂西“隔槽式”褶皱带由东向西的递进构造变形控制了深部构造楔及叠瓦构造的活动,进而控制了背斜的主体构造形态。位于盆地内部的川东南“隔挡式”褶皱带对背盆山转换带构造变形特征的影响主要体现在背斜前翼,造成了背斜前翼宽缓,后翼陡窄的构造差异。

The Sangmuchang Anticline,sitting at a crucial location where the front of“trough-like”fold belt in western Hubei and Hunan Provinces superimposing the“barrier-style”fold belt in southeastern Sichuan Basin,was chosen to be studied to gain insight into the superimposition relationship between the belts as well as the tectonic deformation characteristics and genetic mechanisms of this mountain-basin transitional zone from Xuefeng Mountains to the Sichuan Basin.Available seismic and logging data together with DEM digital elevation and other related geological information were combined with fault-related fold theories to perform a fine interpretation of multiple 2D seismic sections crossing the anticline for architectural characterization and to establish a kinematics model with the balanced section restoration technique for tectonic evolution analysis.The study has concluded that the deformation of the anticline involves layered decollement with a mutual structural superposition in vertical direction,and that with some consistently deformed cap-rocks and bedding shear deformation dominating along the decollement surface,the overall structure of the anticline is under the joint influence of basement occurrence and Qiyueshan Fault.The fault is a concealed system formed by the combination of several basement thrusting faults striking near south-east,and the deep basement wedge and imbricated structures are thereby in superimposition,jointly controlling the anticline deformation.Further analysis shows that the anticline deformation initiated since the middle of Yanshan period,and its tectonic evolution has experienced the superposition of multiple tectonic movements.Under the westward thrusting of Xuefeng Mountains,the east-to-west progressive tectonic deformation of the west of Hunan and Hubei“trough-like”fold belt controls the activity of the deep tectonic wedges and the imbricated structures,which in turn control the main tectonic form of the anticline.The influence of the“partition-style”fold belt in the interior of the southeastern Sichuan Basin on the tectonic deformation of the dorsal basin-mountain transitional zone is mainly reflected in the anterior flank of the dorsal slope,resulting in the broad gentle fore wing and the steep narrow back wing of the anticline.