Structural deformation in Kaladawan of North Altun orogenic belt, NW China: Constraints from magnetic and EBSD fabrics

Abstract

The North Altyn Tagh ductile shear zone is well exposed in the Kaladawan area of North Altun orogenic belt. The deformed rocks provide clues to the deformation conditions and the tectonic evolution of the North Altun orogen. This study investigated structural deformation in Kaladawan of North Altun orogenic belt using magnetic fabric and electronic backscattered diffraction (EBSD) quartz fabric in order to reveal the dynamic and kinematic characteristics of the North Altyn Tagh ductile shear zone. We conducted detailed field investigations and collected oriented samples to analyse magnetic and EBSD fabrics. The mean magnetic susceptibility (Km) value for the Kaladawan rocks is low, indicating that paramagnetic and diamagnetic minerals are the principal carriers of magnetic susceptibility. The corrected degree of anisotropy (Pj), magnetic lineation (L), magnetic foliation (F), shape parameter (T), and oblateness of the magnetic susceptibility ellipsoid (E) show that the deformation in the central part of a transect across Kaladawan is weak indicating elongated prolate strain, whereas the deformation in the north and south is strong indicating oblate strain. The anisotropic magnetic susceptibility (AMS) principal ellipsoidal axis K1 (magnetic lineation) trends mainly WNW–ESE and are oriented with variable magnetic foliation attitudes, whereas the AMS principal axis K3 (pole to magnetic foliation) trends NNE–SSW. Deformation under different conditions generated two types of quartz c‐axis fabric style: type A (point maximum) and type B (Y maximum plus crossed girdles). Fabric thermometry analyses suggest that the quartz fabrics formed at a medium temperature of 400–500°C. According to previous zircon U–Pb data from Kaladawan, deformation of the North Altyn Tagh ductile shear zone took place between 478.1 and 427.3 Ma. The ductile shear can be interpreted as a tectonic response to dextral oblique convergence between the Tarim and Central Altun blocks.