Magnetic Fabrics and Tectonic Implications of the Higher Himalayan Rocks in Nyalam, South Tibet

Abstract

AbstractWe gives a detailed description of the magnetic fabrics of the rocks from the Nyalam region, south Tibet in order to fathom the deformational phases and processes in the Himalayan orogenic belt. The characteristics of the magnetic fabrics indicate that the anisotropy (P) of magnetic susceptibility in the rocks is highest in the schist 1km north of Zham Town, with a maximum value of 1.72, while lowest in the rocks 1km south of Kangshanqiao, with a minimum value of 1.13. The percentage anisotropies (H) of magnetic susceptibility range between 11.82% and 45.07%, indicating that the rocks in these places were once subjected to severe thrusting and plastic deformation during the early stages of the collision between the Indian plate and the Eurasian plate. The Nyalam and Rouqiecun Group Complexes may be considered as a giant ductile shear zone. The magnetic lineations (L) vary between 1.02 and 1.09, and magnetic foliations (F) between 1.06 and 1.60. The magnetic lineations are apparently better developed than the magnetic foliations. The shape factors of magnetic ellipsoids (T) range between 0.19 and 0.76, and the Eccentricity (E) of the magnetic ellipsoids between 1.04 and 1.49. The magnetic ellipsoids tend to take the shape of a flattened ellipsoid. From the region 1km north of Zham Town to Rouqiecun, the main axis (D3) of mean minimum magnetic ellipsoids is oriented S‐N (with the exception of the MA9 1 km north of Zham Town), and main axial dip angle (I3) > 51ο, even up to 72.6ο. The results of research have demonstrated that the combination of NS‐directed compressional stress and NWW‐trending structural plane resulted in the strike‐slipping shear of dextral ductile oblique thrusting, rapid uplifting of mountain masses and high topographic reliefs on the Higher Himalayan crystalline basement. The locally developed elongated magnetic ellipsoids indicate that the combination of gravitational potential energy of the mountain masses and northward compressional force caused by the Indian plate led to nearly NS‐trending extension and collapse and the formation of a dextral ductile shear zone similar to tilted structures.