Magnetic fabric and finite strain analysis of the Qiongxi fault-bend fold in the southern Longmen mountain

The characteristics of Cenozoic deformation in the southern Longmen Mountain fold-thrust belt, which is located to the west of Sichuan basin, is important in recognizing the deformation regime of the eastern margin of the Tibetan Plateau. Magnetic fabric is a sensitive strain indicator particularly useful in weak deformed sedimentary areas. In this paper, a magnetic fabric investigation was carried out into the Qiongxi fault-bend fold, which is located to the south of Longmen Mountain fold-thrust belt on the basis of 48 sampling sites scattered in different parts so as to analyze the finite strain of the bending fold and discuss the regional deformation regime. The results show that hematite is the main magnetic carrier mineral in the upper Cretaceous of Qiongxi area, and the fold suffered weak strain; in addition, there exist three types of magnetic fabrics in the study area, and most of the sampling sites show sedimentary and initial deformation magnetic fabrics, with pencil structure magnetic fabrics only existent on the forelimb of the middle part of the fold. These phenomena indicate that the strain on the forelimb is relatively higher, and the strain on the layer of the middle part is somewhat stronger than that of other parts of the fold-bend fold. In addition, the stereonet map of magnetic lineation shows that the orientation of the maximal susceptibility axial cluster is in nearly SN direction (N10°E) , which indicates that the deformation of the bending fold of Qiongxi fault is related to the late Cenozoic EW-trending crustal shortening in the southern Longmen Mountain, and this may imply a change of the direction of the maximal stress in the southern Longmen Mountain in late Cenozoic.