AMS investigation in the Pingluoba and Qiongxi anticlines, Sichuan, China: Implications for deformation mechanism of the Qiongxi structure

Abstract

Stratigraphic attitude is one of the most important field data for structural geology research. However, it is difficult to measure directly sometimes, especially in weak deformed sediments with low dip angle. Pingluoba anticline, mainly consisting of medium-high dip angle strata, is located on the Longmen Shan fold-and-thrust belt front. Qiongxi anticline is located in the southwestern Sichuan Basin, and it is mainly composed of sub-horizontal to low dip angle strata. An investigation of the anisotropy of magnetic susceptibility (AMS) in 78 sites has been carried out in these two folds. Stepwise demagnetization of three orthogonal isothermal remanent magnetizations suggests that hematite is the main magnetic carrier mineral in the Qiongxi anticline, meanwhile, hematite and magnetite are both existing in the Pingluoba anticline. It reveals three types of magnetic fabrics: sedimentary magnetic fabric, initial deformation magnetic fabric and pencil structure magnetic fabric, which indicate weak deformation. Throughout AMS theoretical analysis and example test in the Pingluoba anticline, magnetic fabric is a suitable technique to confirm the stratigraphic attitude in weak compressional deformed sediments. For the sedimentary magnetic fabric, initial deformation magnetic fabric and one atypical initial deformation magnetic fabric developed in the weak deformed tectonic superposition region, dip direction of bedding plane = pitch direction of K 3 +(or −) 180°, and dip angle of bedding plane = 90°-pitch angle of K 3 . 38 stratigraphic attitudes with low dip angle in the Qiongxi anticline have been rectified by magnetic fabric results. Qiongxi structure is a weak tectonic superposition deformation anticline. The deformation mechanism of Qiongxi structure may contain two stages: NW–SE compression stage started from the latest Cretaceous–early Cenozoic and nearly E–W shortening stage in the Late Cenozoic. • AMS is reasonable to confirm the sedimentary stratigraphic attitude. • Qiongxi structure is a weak tectonic superposition deformation anticline. • Qiongxi structure was formed as two compressional stages.