Magnetic fabrics in Jingzhuba-Shiwo section of foreland fold belt of the South Dabashan：Constraints on its tectonic evolution
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摘要: 通过野外构造观测、岩石磁学与磁组构综合分析，本文研究了南大巴山前陆褶皱带荆竹坝—石窝剖面的叠加构造特征及其形成演化.从北东向南西，剖面构造变形总体呈减弱趋势，褶皱轴面总体倾向北东，大尺度褶皱枢纽均以小角度向北西倾伏.古应力分析显示最大主压应力为北东—南西向，反映以南大巴山的推覆为主.剖面J3之前的采样层位主要表现为变形组构，而J3-K1的采样层位则表现为初始弱变形组构.磁线理呈NW-SE向的优势方位，与剖面主构造线基本平行，主要反映来自南大巴山的推覆挤压.剖面发育特殊磁组构：①磁面理与地层面斜交，主要与褶皱作用中的平行层简单剪切相关；②磁线理均不同程度斜交于地层走向，指示构造叠加背景.沿剖面北东向南西区段Kmin的倾伏角随构造变形强度减弱而增大，据此相关性可将Kmin的倾伏角作为判别弱变形沉积岩变形强度的标志.本文认为，在晚侏罗世以南大巴山的推覆为主而米仓山短轴背斜与川东褶皱带挤压次之的联合作用使南大巴山前陆褶皱带具有构造叠加特征，之后的早白垩世仍主要表现为南大巴山的推覆，而其他两个方向的挤压较之前相对较弱.该结果也反映了秦岭J3-K1陆内造山作用及燕山期雪峰陆内构造变形的影响，为探索陆内构造与陆内造山的大陆动力学提供了佐证.Abstract: A detailed investigation of the structural geometry and magnetic fabrics together with rock magnetism has been carried out along Jingzhuba-Shiwo section located in the foreland fold belt of the South Dabashan, which aims to shed light on the superimposed deformation and tectonic evolution. As a whole, the intensity of deformation decreases from the northeast to the southwest, and the axial planes dip northeastward and the hinges of large-scale folds plunge northwestward at small angle. Moreover, the maximum principal stress in NE-SW direction indicates that the compression of the South Dabashan is dominant as per palaeostress analysis. The sampling horizons before the Late Jurassic are mainly characterized with magnetic fabrics in deformed rocks, whereas the sampling horizons ranging from the Late Jurassic to the Early Cretaceous are characterized with incipient magnetic fabrics in weakly deformed rocks. The magnetic lineations are dominantly oriented NW-SE and remain almost parallel to the main tectonic structures of the section, suggesting that AMS can be ascribed to tectonic processes. Also there are two types of unusual magnetic fabrics revealed. The first one merely occurring in the northeast section is characterized by an oblique magnetic foliation with respect to bedding, which is related with parallel-bedding simple shear during fold growth. The other one caused by tectonic superimposition is characterized by an oblique magnetic lineation with respect to bedding strike. The plunges of Kmin increase with decreasing deformation intensity from northeast to southwest along the section, which can be taken as a gauge to distinguish the deformation intensity of weakly-deformed sedimentary rocks according to the relativity. Consequently it can be concluded that the superimposed tectonic deformation of the section was formed by the combined compression which arose from the South Dabashan primarily and the Micangshan and the eastern Sichuan fold belt secondly during the Late Jurassic. Afterwards the compression from the South Dabashan was still dominant, the compression from the other two directions, however, became much weaker in the Early Cretaceous than in the Late Jurassic. Furthermore, the effects resulting from Qinling intracontinental orogeny during J3-K1 and Xuefeng intracontinental deformation could be reflected by the results mentioned above, which can also provide evidences for exploration of continental dynamics.
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