Early Cretaceous Thrust and Nappe Tectonics in North Qilian Shan, Northern Tibetan Plateau: Evidence from Field Mapping, Geochronology, and Deep Structural Analysis

The North Qilian Shan fold and thrust belt, located at the northern Tibetan Plateau and southern margin of the Hexi Corridor, is a key tectonic unit to decode the formation and expansion of the plateau. Previous studies emphasize the Cenozoic deformation due to the far-field response to the Indo-Asian collision, but the Mesozoic deformations are poorly constrained in this area. We conducted detailed field mapping, structural analysis, geochronology, and structural interpretation of deep seismic reflectional profiling and magnetotelluric (MT) sounding, to address the superposed results of the Mesozoic and Cenozoic deformation. The results recognized the North Qilian thrust and nappe system (NQTS), the root and the frontal belt are the North Qilian thrust (NQT), and the Yumu Shan klippe (YK), respectively. The middle belt is located between the NQT and the YK. Monzonitic granite zircon U-Pb dating from the middle belt yields an age of ca. 415 Ma, which is similar to south NQT. The thrusting displacement is estimated at ca. 48 km by structural interpretation of deep profiles. The timing is constrained in the early stage of the Early Cretaceous by the formation of simultaneous growth strata. We suggest that the NQTS has resulted from the far-field effect of the Lhasa-Qiangtang collision, and the Yumu Shan is uplifted by the superposed Cenozoic deformation. image