Two-phase kinematic evolution of the Qilian Shan, northern Tibetan Plateau: Initial Eocene-Oligocene deformation that accelerated in the mid-Miocene

The Cenozoic growth of the Tibetan Plateau and the distribution of deformation across it are a consequence of India -Asia collision and continued convergence, which have implications for studies of continental tectonics. The spatio-temporal development of Cenozoic deformation along the northern margin of the plateau is an important issue that can be better understood by testing various models of plateau growth. The northern Tibetan Plateau is bounded by the Cenozoic Qilian Shan thrust belt and the Haiyuan leftslip fault. We conducted geologic mapping, field observations, electron spin resonance (ESR) dating, and apatite (U-Th)/He (AHe) and apatite fission -track (AFT) analysis in the Qilian Shan thrust belt to improve our understanding of the timing of brittle faulting and range exhumation in the northern Tibetan Plateau. We document the first direct age constraints for Oligocene deformation within the central Qilian Shan via ESR dating, which correlates with AHe-AFT cooling ages in adjacent ranges. We demonstrate that the Qilian Shan thrust belt experienced a two-phase growth history, including Eocene-Oligocene fault -related uplift shortly after the India -Asia convergence, and midMiocene regional overprinting deformation that reactivated the proximal thrust faults. This deformational pattern suggests that the Qilian Shan thrust belt has experienced out of -sequence development since the Eocene- Oligocene and has persisted as the stationary northeastern boundary of the HimalayanTibetan Orogen throughout the Cenozoic. The Paleozoic Qilian suture systems acted as a pre-existing weakness and played a decisive role in controlling the lithospheric rheology, which therefore impacted the timing, pattern, and strain distribution of Cenozoic deformation across the northern Tibetan Plateau.