Subduction characteristics of the ordovician erlangping back-arc basin in the east qinling mountains, china: implications for the tectonic evolution of the northern margin of the proto-tethys ocean

A relatively complete trench-arc-basin system developed on the northern margin of the Proto-Tethys Ocean, and its subduction mode is key in reconstructing the tectonic evolution of the northern Proto-Tethyan margin. The Erlangping Group in the East Qinling Mountains is thought to be the product of subduction in the back-arc basin. Analysis of the metamorphism and deformation recorded by the group may elucidate the subduction mode of the back-arc basin. In this study, the tectonites in the southern and northern parts of Erlangping Group are taken as the research object, and their dynamic parameters such as geometry, kinematics and vorticity are determined by field observation and indoor tectonic deformation analysis. Deformed rocks in the northern part of the Erlangping Group are mainly feldspathic schists, primary mylonite, and mylonite, and the deformed rocks in the southern part of the Erlangping Group are mainly schists. The geometric characteristics of Erlangping group show that the foliation (30 degrees-60 degrees angle 42 degrees-70 degrees) and dip lineation (5 degrees-62 degrees angle 33 degrees-51 degrees) in the north of Erlangping group incline to NE, while the foliation (174 degrees-234 degrees angle 50 degrees-80 degrees) and dip lineation (195 degrees-230 degrees angle 45 degrees-65 degrees) in the South incline to SW; the kinematic characteristics show that both sides of Erlangping Group move under Qinling Group and Kuanping Group, respectively. Kinematic vorticity calculation shows that the vorticity values of the northern and southern parts of Erlangping Group are 0.79 and 0.78, respectively, both of which are more than 0.75, indicating that the shear type is dominated by simple shear and supplemented by pure shear. The average values of kinematic vorticity on the northern and southern parts of Erlangping Group are 0.78 and 0.79, respectively, both more than 0.75, indicating that subduction was dominated by simple shear, with minor pure shear deformation. The subduction deformation temperature range of Erlangping Group is estimated to be 380 similar to 500 degrees C by the degree of quartz dynamic recrystallization. There is a positive correlation between the differential stress value sigma (southern parts of Erlangping Group: 17.84-46.50; northern parts of Erlangping Group: 17.87-71.66) and strain rate epsilon (southern parts of Erlangping Group: 2.09 x 10(-14) - 2.51 x 10(-13); northern parts of Erlangping Group: 2.08 x 10(-14) - 8.23 x 10(-13)) during subduction, while the grain size of dynamically recrystallized quartz is negatively correlated with strain rate. The specific performance is that in the northern part of Erlangping Group, from the Waxuezi-Qiaoduan shear zone to the Damiao-Dongjie shear zone, with the increase of the particle size of dynamically recrystallized quartz particles, the values of differential stress and strain rate gradually decrease, and have the same performance in the southern part of Erlangping Group. Through the above structural deformation analysis, combined with the previous research results on the material composition, sedimentary formation, magmatism, metamorphism and deformation of the Erlangping back-arc basin, it is considered that the Erlangping back-arc basin has the characteristics of bidirectional subduction during the late Palaeozoic subduction.*