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

Abstract

ABSTRACT The Qinling Orogenic Belt was formed by multi-stage convergence and subsequent collision between the North China Block and South China Block during the Palaeozoic to Late Triassic. Following the collision between the South Qinling Belt and the South China Block in the Late Triassic, the whole Qinling Orogenic Belt evolved into an intracontinental orogeny. However, the exact termination time of the Late Mesozoic intracontinental contractional deformation and the tectonic transformation from compression to extension remains unclear. The extensional shear zone developed in the West Qinling Orogenic Belt provides a crucial clue to revealing the tectonic transformation and uplift and exhumation of the orogenic belt. This study focuses on kinematics, deformation temperature, and geochronology of the shear zone in the West Qinling Orogenic Belt through detailed field structural analysis and laboratory analysis. The Taibai shear zone (TBSZ), located in the West Qinling Orogenic Belt, cuts across the Qinling Orogenic Belt along the NE–NEE direction and separates the Taibai pluton in the east from the Baoji pluton in the west. The NE–NEE-striking TBSZ is an extensional shear zone showing a top-to-the-NW sinistral shear sense. The TBSZ was formed under the high greenschist–low amphibolite facies conditions (300–550°C) in the middle-upper crust level (8–15 km). Zircon U–Pb dating and biotite and muscovite 40Ar/39Ar dating indicate that the TBSZ occurred at 120–113 Ma. The extensional TBSZ and normal faults on its hanging wall indicate the tectonic transformation and the extensional collapse of the Mesozoic intracontinental orogenic belt during the late Early Cretaceous. The Cretaceous extensional collapse of the Qinling Orogenic Belt also led to the rapid uplift and exhumation of the Taibai pluton and the North Qinling Belt in the east.