Magnetic fabric investigation in the northwestern Sichuan Basin and its regional inference

An investigation of the anisotropy of magnetic susceptibility (AMS) in 39 sites has been carried out in the northwestern Sichuan Basin which is adjacent to the northeastern Tibetan plateau, discussing the deformation pattern of the basin and the orogenic process of the corresponding orogens. Stepwise demagnetization of three orthogonal isothermal remanent magnetization suggests that hematite is the main magnetic carrier mineral. Combined with the low value of magnetic susceptibility, it is considered that the AMS is some combination of hematite and paramagnetic matrix minerals. Different types of magnetic fabrics from sedimentary to pencil structure magnetic fabric and sharps of susceptibility ellipsoids from oblate to prolate were observed. The existence of sedimentary magnetic fabric and the accordance of magnetic lineation and bedding strike indicate the magnetic fabrics along the front of Longmen Shan belt were the result of tectonic deformation. Thermochronology researches in the orogens and our apatite fission track analysis in the northwestern Sichuan Basin suggest that magnetic fabric mainly reflects the Cenozoic deformation. The magnetic lineation which is consistent with the fault and fold strikes in the Longmen Shan was induced by Mesozoic and Cenozoic deformation, and the latter is the primary one. Besides the typical weak deformation magnetic fabrics, atypical magnetic fabrics characterized by a magnetic lineation that is oblique to the bedding strike are shown in 53% sites along the front of Micang Shan. It is proposed that such a type of atypical magnetic fabric is the result of two competing sub-fabrics in the weakly deformed tectonic superposition region.