Magnetic Fabric And Finite Strain Investigation Of Fault-related Folds In The Southern Longmenshan

The structural deformation of the Longmenshan fold-and-thrust belt in Cenozoic time is a direct reflection of the crustal shorting in the eastern margin of the Tibetan plateau, its characteristic of Cenozoic deformation is important in understanding the uplift mechanism and evolutionary process of the Tibetan plateau. Fault-related folds are common structures in fold-and-thrust belt, research on their strain features will facilitate the apprehension of the growth and evolution of fold-and-thrust belts, and meanwhile, magnetic fabric can be used to analyze the strain feature of deformed rocks, thus playing a very important role in tectonic evolution studies. This paper chooses the southern Longmenshan fold-and-thrust belt as its study area, and selects a cross section in the middle and northern part of the Qiongxi fault-bend fold respectively, and a total of48sampling sites of magnetic fabric are designed in certain intervals on different parts, so as to conduct an analysis on magnetic fabric and strain features of the Qiongxi fault-bend fold and on basis of which discuss the Cenozoic deformation features and regional stress field change in the southern Longmenshan.Magnetic mineralogy analysis shows that hematite is the main magnetic carrier in the layer of upper Cretaceous of the Qiongxi area. Analysis of magnetic fabrics in the Qiongxi fault-bend fold reveals three types of magnetic fabrics:sedimentary magnetic fabric, initial deformation magnetic fabric and pencil structure magnetic fabric. According to the analysis of magnetic fabric feature on different parts of the fold, we find that strain on the forelimb is relatively higher than that on the backlimb and on the core of the fold, and strain on the middle part is somehow stronger than other parts of the fault-bend fold. To clarify the similarities and differences between a fault-bend fold and a fault-propagation fold, this thesis carries out a comparison of the research results of magnetic fabrics and strain in the Qiongxi fault-bend fold with that of the published Feixianguan fault-propagation fold and Yanjinggou fault-propagation fold. The result suggest that for both a fault-bend fold and a fault-propagation fold, the magnetic fabric types on the backlimb of the fold are mainly initial deformation and sedimentary magnetic fabrics while pencil structure magnetic fabrics developed on the forelimb of the fold, indicating that strain on forelimb are both relatively stronger than on the backlimb. Meanwhile, on fault-propagation folds the trishear zone is the strain focus zone, but no such strain focus zone is found on fault-bend folds.Present studies indicate that deformation during Cenozoic time was obviously stronger than that of late Triassic in the southern Longmenshan, analyzing the seismic section of the Qiongxi fault-bend fold, we find that the fold involved rocks of Paleogene time, so we suggest that magnetic fabric data of the fold reflect deformation during the Cenozoic time in the southern Longmenshan. This paper makes a statistics of the correlation parameters obtained from magnetic fabric test of the48sampling sites in the Qiongxi fault-bend fold and draws Flinn and Pj-T diagrams of magnetic fabric data, analyzing the distribution of the sampling sites in these two diagrams, we find that all the sampling sites show oblate magnetic ellipsoid, indicating that strain in this fold is weak, and we don’t find relatively stronger deformation. Combine with the published magnetic fabric research achievements of Tianquan-Leshan section in the southern Longmenshan, this thesis suggests that magnetic data from the front of the southern Longmenshan show weak strain of rocks, and thus reflects a feature of weak deformation in Cenozoic time in front of the southern Longmenshan.To discuss the deformation regime of the Qiongxi fault-bend fold, we makes a stereographic projection of magnetic lineations after bedding correction in the fold, with the exception of which showing as sedimentary magnetic fabrics. The result shows that the orientation of the maximal susceptibility axils cluster at nearly north-south direction (N10°E), reflecting the general N-S tensile strain direction of the fold. Combine with the published result on the deformation time of Qiongxi fault system, we suggest that the Qiongxi fault-bend fold is a product of nearly E-W crustal shortening since late Pliocene-early Pleistocene, and meanwhile indicates that the related regional maximal stress direction being east-west. Carrying out a comparison between the N-S extended Qiongxi fault-bend fold with the published results of magnetic fabrics and stress field of NE-SW extended structures in our study area, we suggest that magnetic fabric results from the southern Longmenshan reflect that a change of stress field in the southern Longmenshan occurred during the Cenozoic time, with the orientation of the maximal stress changing from NW-SE during early Cenozoic to E-W since late Pliocene-early Pleistocene to present day.