Tectonics and geochronology of the northern margin of the Zhongba terrane, Southern Tibet: implications for the closing processes of the western Neo-Tethys

Abstract:

The Zhongba terrane represents a special tectonic unit sitting between the northern and the southern ophiolitic sub-belt in the western segment of the Yarlung Zangbo Suture Zone (YZSZ), which separates the Indian and the Eurasian continental plates. The central-east section of the Zhongba terrane can be divided into fault-fold belt, fold belt and fault belt. The fault-fold belt, composed of northward-dipping thrust faults and asymmetric folds in the Carboniferous and late Devonian strata, mainly occurs at the northern margin of the Zhongba terrane. Four stages of deformation (D₁-D₄) can be recognized in this belt. Stage D₁ is recorded by the rolling folds in the Devonian strata in which the residual S₀ can be observed in asymmetric small folds. Quartz c-axis fabrics show that the dominant slip system of quartz is basal <a> under transpression and low temperature (~ 400℃). In the stage D₂, many small folds with axial plane inclined at high angle to the north were formed. The highly inclined C foliation in mylonitic limestones suggests that this belt represents the strain concentration zone during the northward subduction of the Neo-Tethyan lithosphere beneath the Gangdese arc belt. Coaxial progressive deformation is demonstrated by strongly shortened layers and abundant cleavages that are parallel to the axial plane of pre-existing folds in the stage D₃. Mylonitic foliation is replaced by the cleavages plane (S₂) associated with southward thrusts. In the stage D₄, some northward thrusts and relevant cleavage with medium angle replaced early folds. Zircon U–Pb ages found in nearby quartz diorites at Gangdese arc belt yielded 91.8 ± 1.3 Ma. Since these quartz diorites are compositionally similar to island-arc magma from active continental margins, we infer that the northward subduction of the north sub-belt of the YZSZ started in the upper Cretaceous. Muscovite separates from representative tectonites within the strain concentration zone and the southward thrust belts yielded ⁴⁰Ar/³⁹Ar age of 71.6 ±1.1 Ma and 56.5±2.5 Ma, respectively. The former represents the collisional time between the Zhongba terrane and the Gangdese arc, while the latter indicates the final closing time of the Neo-Tethys ocean.