Structural style and geochronology of ductile shear zones in the western north Qinling orogenic belt, Central China: Implications for Paleozoic orogeny in the Central China orogeny

摘要

The Qinling Orogen, which was formed by long-lived subduction and collision between the North and South China blocks, can be divided into the South Qinling orogenic belt (SQOB) and the North Qinling orogenic belt (NQOB) by the Shangdan suture zone. Orogen-scale shear zones developed on the boundaries within the NQOB can be used to enhance our understanding of collisional orogeny that occurred during the Paleozoic in the Qinling Orogen. Previous studies have identified several ductile shear zones on boundaries between different lithotectonic units in the eastern NQOB. However, ductile shear zones related to orogenic processes in the western NQOB have attracted relatively little attention. Herein, we present structural and chronological data for the ductile shear zones in the Tianshui region, western NQOB. Three sub-parallel ductile shear zones (DSZ-1, DSZ-2, and DSZ-3) are recognised in the southern Qinling Complex or as boundaries between the Qinling Complex and adjacent lithotectonic units in the Huamiao area, Tianshui City. Microstructural and fabric analyses indicate that these shear zones have kinematics of transpressive sinistral strike-slip. Zircon U-Pb and Ar-40-Ar-39 dating data suggest that these ductile shear zones experienced a prolonged period of deformation from 420 to 370 Ma during exhumation, which was related to oblique convergence between the SQOB and the NQOB. Combining with the synchronous Xinyang-Yuanlong dextral shear zone in the northern Qinling Complex, we suggest that the transpressive sinistral shearing and dextral transpressive ductile shearing on both sides of the Qinling Complex subjected it to orogen-parallel oblique-slip extrusion during the middle Paleozoic. Our results, together with previously published data in the eastern NQOB, indicate the occurrence of orogen-scale strike-slip shearing along the whole NQOB during the Paleozoic orogeny.