Strain and kinematic vorticity analysis: An indicator for sinistral transpressional strain-partitioning along the Lancangjiang shear zone, western Yunnan, China

This work deals with the preliminary relationship between strain path and strain partitioning pattern in a sinistral transpressional zone, Lancangjiang shear zone, located to the southeast of Tibet. Various ductile rocks provide an opportunity to investigate quantitative finite strain (R (s)), kinematic vorticity values (W (m) ), and proportions of simple and pure shear components. The mean kinematic vorticity values (W (m) ) were evaluated based on three methods, such as R (s)-theta, prophyroclast hyperbolic distribution method (PHD), and polar Mohr diagram construction (PMD). W (m) for mylonitic metasediments of the low-grade schist belt ranged from 0.19 to 0.54. Protomylonites, mylonites, and ultramylonites recorded differential W (m) values, 0.19-0.73, 0.11-0.8, and 0.84-0.95 (even 0.99) respectively in the high-grade gneiss belt. Based on classical strain analysis and these kinematic vorticity evaluations, the shear zone is classified as a preliminarily bulk monoclinic symmetry. The pattern of vorticity path suggests that a previously undocumented transpressional setting and strain partitioning pattern in the southeast region of Tibet Plateau, characterized by steep-narrow domain of simple-shearing (Tertiary strike-slip shearing) deformation and wider domain of mostly pure-shearing (Tertiary contraction) to accommodate intracontinental deformation.