The continuous extrusion and expansion of the Tibetan Plateau toward the northeast have led to intense crustal shortening and left-lateral shearing in the Qilian Shan region. The Qilian−Haiyuan fault zone, a major block-boundary fault in the region, is the subject of considerable debate regarding the intensity of extrusion and thrusting, as well as the onset time and maximum displacement of left-lateral strike-slip motion. This study focuses on the Laohushan fault within the central-eastern segment of the Qilian−Haiyuan fault zone. Through geological mapping, stratigraphic correlation, and cosmogenic nuclide burial dating, we established a chronological framework for the Cenozoic strata. The provenance characteristics, such as paleocurrent directions and heavy mineral assemblages, suggest that the Pliocene strata on both sides originated from the same paleodrainage system. The conclusions suggest that the maximum strike-slip displacement of the Miocene strata is 15.2 ± 0.7 km. Based on the late Quaternary slip rate of 4.5 ± 1.0 mm/yr, the onset time of strike-slip activity is estimated to be 3.4 Ma. The base of the growth fold suggests that the onset of early compressional thrusting occurred earlier than 7.8 Ma. The Qilian−Haiyuan fault zone underwent two main evolutionary stages. During the middle Miocene (13−8 Ma), thrusting activities exhibited a west-to-east propagation pattern. During the Pliocene (5−2.5 Ma), it transitioned to predominantly strike-slip activity. The northeastward compressive expansion of the Tibetan Plateau may also be accommodated by a cyclic mechanism involving the alternating conversion of thrust and strike-slip faults, while simultaneously generating new thrust faults propagating forward.
Yu et al. (2026) studied this question.