Structural Geology of a Central Segment of the Qilian Shan-Nan Shan Thrust Belt: Implications for the Magnitude of Cenozoic Shortening in the Northeastern Tibetan Plateau

Due to the lack of systematic geologic mapping across the Qilian Shan and Nan Shan region, the amount of crustal strain that accumulated in the northeastern margin of the Tibetan plateau during the Cenozoic Indo-Asian collision remains poorly understood. The aim of this study was to establish a structural framework of an ∼15,00 km2 region in the central Qilian Shan-Nan Shan thrust belt based on detailed geologic mapping. This study reveals the presence of two Cenozoic thrust systems in the region: (1) the Shule thrust system and (2) the Tuo Lai thrust system. The Shule thrust system consists of thinskinned, north-propagating thrust faults truncated by younger, thick-skinned, southpropagating thrusts and >5000 m-thick Permian to Upper Triassic folded strata. The Tuo Lai thrust system is composed five main thrust faults that duplicate a regional unconformity between Carboniferous strata above and Ordovician mélange below. Based on our geologic mapping, we have constructed and restored a balanced cross-section that suggests a minimum shortening during the Cenozoic of 18 km along the Shule thrust system, which corresponds to ∼45% crustal strain in this region. Interpretation of zircon U-Pb dates from a foliated granitoid yields an age of 911 ± 12 Ma, confirming previous work suggesting a phase of magmatism between 930 and 900 Ma in the Qilian Shan-Nan Shan region.