Structural and geochronological constraints on a Late Mesozoic extension event in the West Qinling Orogenic Belt, China

The Qinling Orogenic Belt (QOB) has been well documented that it was formed by multiple steps of convergence and subsequent collision between the North China Block (NCB) and South China Block (SCB) during the Paleozoic and Late Triassic. Following the collision in the Late Triassic, the whole QOB evolved into an intracontinental orogeny. Large-scale N-S compressional and thrusting deformation tectonics along the previous major boundary faults were developed. Meanwhile, several extension-related tectonics were also developed in the QOB during or slightly later than the compression. However, the expiry date of the Late Mesozoic intracontinental compression orogeny and the tectonic transformation of the QOB remains unclear.

The extensional shear zone developed in the West QOB provides a crucial clue for revealing the orogenic belt's tectonic transformation and uplift and exhumation process. This paper focuses on kinematics, geochronology, and deformation temperature of the Taibai shear zone in the west QOB (TBSZ) to understand the intracontinental orogenic evolutionary process of the QOB.

The Taibai shear zone was developed at the northwestern margin of the Taibai pluton. It cuts across the QOB and separates the Taibai pluton in the east from the Baoji pluton in the west. The NE–NEE-striking and NW–NNW-dipping TBSZ is an extensional shear zone showing a top-to-the-NW sinistral shear sense. Mineral deformation characteristics and two-feldspar geothermometers constrain that the TBSZ was formed under the high greenschist–low amphibolite facies conditions (300-550 ℃) in the middle-upper crust (8-15 km). Zircon U-Pb dating and biotites and muscovites ⁴⁰Ar-³⁹Ar dating analysis suggest that the TBSZ was formed and uplifted rapidly during 120-113 Ma.

Combined with the regional geological data, the TBSZ records the extensional collapse of the Late Mesozoic intracontinental orogenic belt during the 120–113 Ma. The TBSZ also led to the rapid uplift and exhumation of the Taibai pluton and the North Qinling Belt in the east.