Anisotropy Of Magnetic Susceptibility Study Of The Jurassic Rocks In The Eastern Qiangtang Block And Its Tectonic Implications

Anisotropy of magnetic susceptibility(AMS)technology,with its rapid and nondestructive measurement of rock fabric,can accurately measure the content of magnetic minerals in sediments,and analyze the advantages of its structural composition and combination relationship,has been applied to the study of structural evolution,provenance evolution and paleo-modern environment by more and more scholars.The Jurassic strata in the eastern Qiangtang block are widely exposed,with continuous deposition and clear occurrence,which provides a good condition for this study.Based on the study of AMS in the eastern part of the qiangtang block and the comprehensive analysis of previous research results and geological data,the following conclusions or understandings are obtained:(1)The main magnetic minerals of the Jurassic Wangbu Formation,Dongdaqiao Formation and Xiaosuoka Formation are hematite and magnetite,and the Wangbu Formation may also contain a small amount of pyrrhotite.In the rock magnetic experiments,the meausurement of thermomagnetic curves of magnetic susceptibility versus temperature and thermal demagnetization of a three-component IRM revealed the unblocking temperatures of hematite and magnetite,indicating that the main magnetic minerals of the Jurassic Wangbu Formation,Dongdaqiao Formation and Xiaosuoka Formation are hematite and magnetite.(2)The AMS of Wangbu formation of Lower Jurassic is a kind of structurally generated magnetic fabric related to weak deformation,and its maximum principal compressive stress direction is perpendicular to the maximum axis K1 of the susceptibility ellipsoid.However,the maximum axis K1 of the susceptibility ellipsoid has an obvious NW-SE preferred direction.Therefore,it is inferred that the formation of Wangbu formation may be affected by NE-SW direction structural stress after its formation,so that the primary sedimentary magnetic fabric is destroyed,magnetic lineation is developed,and the magnetic fabric related to structural genesis is formed.(3)The AMS of the Middle Jurassic Dongdaqiao Formation and the Upper Jurassic Xiaosuoka Formation implies that the magnetic foliation is more developed than the magnetic lineation,the anisotropy degree is very low,and the minimum axis K3 of magnetic susceptibility ellipsoid is near vertical to the beds.These findings indicate the primary sedimentary fabric for the Middle Jurassic Dongdaqiao Formation and the Upper Jurassic Xiaosuoka Formation.The magnetic fabric of the Middle Jurassic Dongdaqiao formation indicates that the paleocurrent direction is SE(138.3°),while the magnetic fabric of the Upper Jurassic Xiaosuoka formation indicates that the paleocurrent direction is NNW(328.3°).The obvious change of the paleocurrent direction between the Dongdaqiao Formation and the Xiaosoka Formation suggests that the uplifting process in the southern part of the Qamdo area was earlier than the north,and the provenance of the sediment had also undergone a corresponding transformation.