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-θ, 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. Supported by National Natural Science Foundation of China (Grant Nos. 40802050, 49802020, 40172074) and China Postdoctoral Science Foundation (Grant No. 20070440065)