Brittle failures and vein formation in the evolution of the South Qiangtang accretionary complex in the Tibetan Plateau

The genesis of the subduction melange in the central Qiangtang terrane has been a long hot debate. However, little research has been conducted on the brittle failure within the accretionary wedge, which is very important to unveil the structural evolution of the melange. In this study, based on the recognition of multiple deformational phases, we analyse the characteristics and formation history of the vein system in the Gangma Co melange. Six groups of quartz veins are recognized. Foliation-parallel extension veins (G1 veins), shear veins (G2 veins) and foliation-perpendicular extension veins (G4 veins) are supposed to have formed during the subduction of oceanic crust, recording the repeated low-angle thrust-sense frictional sliding, tensile fracturing and stress changes generated by subduction-related earthquakes. Subsequent vertical extension veins (G5 veins) are suggested to be related to the exhumation of the underplated melange, while the horizontal extension veins (G6 veins) in the last phase represent the final horizontal thrusting. The temperature conditions for shear vein formation were examined by fluid inclusion analysis, ranging from 120 to 200 degrees C, coinciding with the temperature conditions of the slow earthquake region where episodic tremors and slow slip occur. This contribution supports that the Gangma Co melange represents an in situ subduction zone and that its internal vein system is a response to the tectonic evolution of the Longmu Co-Shuanghu Tethys Ocean.