Structure and development of the Changliangshan ductile shear zone, North Tibet: implications for the initial closure of the Paleo-Tethys Ocean in the central Qiangtang region

Abstract

The Changliangshan ductile shear zone extends ~80 km in an east–west direction in central Qiangtang, Tibetan Plateau. Structural and kinematic analyses show that the shear zone comprises mainly protomylonite, mylonite, and phyllite with dextral-dominated strike–slip motion in the northern branch of the shear zone and sinistral strike–slip shear in the southern branch. Concordant U–Pb ages on the metamorphic rims of zircons from mylonite indicate that deformation along the shear zone started at 249.1 ± 6.7 Ma during the initial collision between the North and South Qiangtang terranes. The 40Ar/39Ar ages of sericite (221–215 Ma), combined with available kinematic and structural data, indicate that the metamorphic rocks of the shear zone experienced transpressional exhumation and uplift during the Late Triassic. In addition, the E–W-trending Changliangshan ductile shear zone marks the contact between Yangtze-type (warm-water) and Gondwana-type (cold-water) deposits. The initial NE–SW-directed collision gave rise to shearing along the Changliangshan ductile shear zone under lower- to middle-greenschist facies conditions during the Early–Middle Triassic. Subsequently, the Yangtze-type sediments passively pierced the Gondwana-type deposits as a wedge-shaped unit along the shear zone. Furthermore, the data reveal that the Changliangshan shear zone formed during oblique convergence between the North and South Qiangtang terranes during initial closure of the Paleo-Tethys.