Sinistral strike-slip shearing along the Jiali shear zone around the Eastern Himalaya syntaxis region: Evidences for oligocene eastward limited translation of Tibet

Abstract

In and around the Eastern Himalaya Syntaxis (EHS), the metamorphic complex and its structural boundaries are key tectonics to investigate the kinematics and thermal evolution of the continental collision. We provide new structural, kinematic and geochronological data along the northeastern boundary of the EHS, namely the Jiali strike-slip shear zone. Structures and various kinematic indicators record sinistral strike-slip shearing in the zone. Based on the microstructural features, crystallographic preferred orientation patterns of quartz and their slip systems, coupled with Raman Spectroscopy of Carbonaceous Materials (RSCM) geothermometry, the ductile sinistral shearing fabrics formed at upper greenschist and amphibolite facies (>500 °C). Two hornblende, one muscovite and five biotite separates from the mylonites in the shear zone were dated by the 40Ar/39Ar method. A hornblende plateau age of 23.7 Ma is interpreted as representing or being close to the time of sinistral movement. Muscovite and biotite 40Ar/39Ar ages from 22.9 to 16.9 Ma represent cooling ages following sinistral shearing. It is concluded on the basis of these data that the Jiali sinistral strike-slip shear zone was active in latest Oligocene. This sinistral movement on the Jiali shear zone is considered to link with the Ailao Shan-Red River shear zone, and form a unique, continuous structural boundary around the EHS. The Jiali-Ailao Shan-Red River sinistral shear zone and Gaoligong dextral shear zone formed a huge conjugate strike-slip shear pair around the EHS to accommodate the N–S shortening corresponding to northward indentation of the folded Indian continent. The conjugate strike-slip shear systems also contribute to transporting Tibet southeastward in a limited scale.