The structural pattern of the eastern Himalayan syntaxis in Namjagbarwa consists of two series of structures with different styles. One series compiles the earlier ductile contractional and lateral-slip deformation system, formed by nearly north-south shortening within the syntaxis, left-lateral and right-lateral slipping along its western and eastern boundaries respectively. They were possibly produced by the indentation of the Indian continent into Asian continent after India-Asia collision. The peak deformation-metamorphic ages in these structures are 62-60 Ma, ～23 Ma and ～13 Ma. The other series is composed of ductile-brittle normal faults distributing concentrically and dipping toward the outsides of Namjagbarwa Peak. They were probably the collapse structures caused by rapid uplift in a later time and the beginning ages for the normal faulting are about 7.3-6.3 Ma.
ZHANG Jinjiang, Jl Jianqing, ZHONG Dalai, DING Lin & HE Shundong1. Department of Geology, Peking University, Beijing 100871, China
The eastern Himalayan syntaxis in Namjagbarwa is a high-grade metamorphic terrain formed by theIndia-Eurasia collision and northward indentation of the Indian continent into Asia. Right- and left-lateral slip zoneswere formed by the indentation on the eastern and western boundaries of the syntaxis respectively. TheDongjug-Mainling fault zone is the main shear zone on the western boundary. This fault zone is a left-lateral slip beltwith a large component of thrusting. The kinematics of the fault is consistent with the shortening within the syntaxis, andthe slipping history along it represents the indenting process of the syntaxis. The Ar-Ar chronological study shows thatthe age of the early deformation in the Dongjug-Mainling fault zone ranges from 62 to 59 Ma. This evidences that theIndia-Eurasia collision occurred in the early Paleocene in the eastern Himalayan syntaxis.
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(Rs),kinematic vorticity values(Wm),and proportions of simple and pure shear components.The mean kinematic vorticity values(Wm) were evaluated based on three methods,such as Rs-θ,prophyroclast hyperbolic distribution method(PHD),and polar Mohr diagram construction(PMD).Wm for mylonitic metasediments of the low-grade schist belt ranged from 0.19 to 0.54.Protomylonites,mylonites,and ultramylonites recorded differential Wm 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 accommo-date intracontinental deformation.
