Late Mesozoic tectonic transformation of the Qinling Orogenic Belt, central China: Evidence from the Taibai ductile shear zone

The Taibai ductile shear zone (TDSZ), located in the critical tectonic position, is of great significance for revealing the intercontinental tectonic evolution of the QOB during the Late Mesozoic. The NEE-trending TDSZ is an oblique extensional shear zone with top-to-the-NW sinistral shear sense kinematics. The shear zone underwent steady-state pure-shear-dominated ductile deformation with kinematic vorticity values of 0.26-0.72. Mineral deformation characteristics, two-feldspar thermometers, and Kruhl thermometer suggest the TDSZ was formed in the middle-upper crust at the deformation temperature of 400-500 ℃. The shear deformation occurred under the differential stresses of 25.29-36.58 MPa with strain rates of 10^-14-10^-10s^-1, indicating slow ductile deformation. Zircon U-Pb ages and biotite and muscovite ⁴⁰Ar-³⁹Ar ages for samples suggest that the ductile deformation occurred at <i>c.</i> 120 Ma and TDSZ uplifted during 120-113 Ma. In combination with regional data, we suggest the QOB underwent the tectonic regime transformation from compression to extension during the Late Mesozoic. The TDSZ may record the westward extrusion of the QOB. The Baoji pluton and adjacent area were extruded to the northwest along the TDSZ and the Taibai pluton uplifted rapidly to the upper crust under the NE-SW compression.