Magnetic fabric of rock in the source area of the Yellow River and its geological significance

Abstract：The source area of the Yellow River located in northeast Tibetan Plateau is composed mainly of Triassic sedimentary strata. From north to south, a series of nappe structures and some early nearly-vertical ductile shear zones are developed. This paper presents the magnetic fabric of Bayankala Group sedimentary rocks in the Yellow River source area. It is shown that the rock magnetic susceptibility anisotropy（P value）and the percentage of anisotropy of magnetic susceptibility（H value）are not large either, suggesting overall weak ductile deformation in the area and strong ductile deformation in some parts. The magnetic fabric of rock is characterized by development of magnetic foliation, underdevelopment of magnetic lineation and flattened magnetic susceptibility ellipsoid, implying that rock endured flattening deformation in the compressive stress. The major principal stress is in NNE-SSW (near SN) derection, followed by NE-SW direction. Based on an analysis of rock magnetic fabric, the authors found that there are two ductile shear zones in the source area of the Yellow River. The ductile shear zones and the current distribution of lakes have certain coupling relationship. The northern ductile shear zone along the present Yellow River valley dominates the distribution of Zhalinghu, Elinghu and "Four Sister Lakes " in Maduo. The southern ductile shear zone along Gangnagema Co－Yeniugou belt controls the distribution of remnant lakes.