Constraints on Kunlun and Haiyuan left-lateral strike-slip fault evolution, northeastern margin of the Tibetan Plateau

Understanding the long-term role of major intracontinental strike-slip faults (>1000 km) in Tibet requires knowledge of slip histories on geologic timescales, including timing of fault initiation. We provide minimum bounds on the onset age of the Kunlun and Haiyuan left-lateral faults, two major structures within the northeastern margin of the Tibetan Plateau. We present new low-temperature thermochronometry ages and geologic observations to date periods of exhumation associated with transpressional deformation. Results suggest that Kunlun faulting initiated first during the early Miocene along the central fault segment followed by expansion along strike to the east and west. Thermal histories from ranges associated with the active Haiyuan fault record primarily late Cretaceous cooling and suggest that exhumation associated with Cenozoic motion is less than ~ 2-4 km. Between 15-6 Ma, structures between the Kunlun and Haiyuan fault become active, from which we infer simultaneous motion of both the Kunlun and Haiyuan faults that bound an ~250 km wide zone of associated transpressional shear. Thus, left-lateral strain appears to dominate the structural landscape of the interior northeastern plateau margin post-15 Ma. Miocene onset of Kunlun and Haiyuan faulting is also consistent with the age of initiation reported for other major Tibetan strike-slip faults (e.g., the Jiali, Karakorum, Xianshuihe, and Ganzi-Yushu faults). Strike-slip fault initiation in northeastern Tibet occurs earlier than most previous studies suggest, and evolution of this fault system spans more than 15 Ma. We propose that initiation of these faults, as well as rapid flow in the deep crust, may be a part of the evolution of a continental orogen during the later stages of long-term plate convergence rather than being tied to a specific event or abrupt change in orogen dynamics.