Structure, kinematics and ages of transpression during strain-partitioning in the Chongshan shear zone, western Yunnan, China

摘要

The Chongshan shear zone extends from the eastern Himalayan Syntaxis to the Lincang Granitic pluton in Yunnan Province, China. The structure and kinematics show that the shear zone comprises mainly of mylonitic gneiss-migmatite and schist with a dextral-dominated strike-slip motion in an N-S trending northern segment, and a sinistral strike-slip shear in NW-SE trending middle and southern segments. Both were developed under a bulk, regional-scale sinistral transpression. SHRIMP and LA-ICPMS U-Pb and ~(40)Ar/~(39)Ar dating reveal two Tertiary magmatic events along the zone, followed by younger sinistral strike-slip shear. The Eocene magmatic event (c. 55-38 Ma), followed by metamorphism at c. 36 Ma, happened before the strike-slip motion. The strike-slip shear along the zone began c. 32 Ma, which generated shear heating from 32 to 22 Ma. The ~(40)Ar/~(39)Ar ages of syn-kinematic micas, range from 19 to 14 Ma, and indicate that the strike-slip shear continued to this time with coeval transpressional exhumation and uplift of the metamorphic rocks along the Biluoxueshan-Chongshan chain. The Chongshan zone is thus a Cenozoic shear zone, which was contemporaneous with motion on the left-lateral Ailao Shan-Red River shear zone and the right-lateral Gaoligong shear zone, and should be important in accommodating the northwards movement of India during collision. During Oligocene to Miocene times, the continental block that was extruded between the Ailao Shan-Red River and Gaoligong shear zones was dismembered into at least two major fragments by the Chongshan shear zone.