Magnetic fabric evolution in the northwestern Sichuan basin, China and its strain characteristics

An investigation of magnetic fabrics has been carried out in northwestern Sichuan from the front zone in the northern segment of the Longmen fold-thrust belt to the margin of the Sichuan basin. Between Jiangyou and Guangyuan, 173 oriented specimens were obtained from 18 well sites along four sampling routes perpendicular to the strike of the Longmen belt. All the specimens were taken from Jurassic and Cretaceous sandstones and silty mudstones. Integrated analysis indicates that the magnetic fabrics in northwestern Sichuan reflect Cenozoic deformation. There are three types of magnetic fabrics in the study area: sedimentary magnetic fabric, initial deformation magnetic fabric and pencil-shaped magnetic fabric. Except the sites showing sedimentary magnetic fabrics, the preferred orientation of K1 at all the specimen sites is NE-SW, which indicates that the maximum principal compressional stress in the study area is oriented in a NW-SE direction, which shows that the stress mainly came from the Longmen fold-thrust belt. Along four sampling routes perpendicular to the strike of the Longmen belt, the sedimentary magnetic fabric changes to initial deformation magnetic fabric and finally to pencil-shaped magnetic fabric from the Sichuan basin to the front of the Longmen Mountains. This suggests that the strain in the Sichuan basin is feeble and becomes strong progressively near the orogen, and that since the Jurassic and Cretaceous, the tectonic deformation in the study area mainly took place at the edge of the orogen or has not been transferred to the Sichuan basin.