Mid-to-late Cenozoic tectonostratigraphic archives of the Qingshuihe Basin, NW China: tracing northeastward stepwise growth of the Tibetan Plateau

The detailed sedimentary sequence and tectonic deformation characteristics of the Qingshuihe Basin and its surrounding areas in the northeastern margin of the Tibetan Plateau have provided significant insights into the stages and intensity of lateral orogenic plateau expansion during the Mid-to-late Cenozoic. Several key unconformities and a tectonically induced river terrace have been identified, shedding light on the timing, scale, and tectonic response of the Tibetan Plateau’s growth into northwestern China. The identified unconformities include the ~ 16.5 Ma parallel unconformity (U₁) between the Qingshuiying and Zhangenpu formations, the ~ 7.6 Ma slight-angular unconformity (U₂) between the Zhangenpu and Ganhegou formations, the ~ 2.77–1.04 Ma slight- to high-angular unconformity (U₃) between the Ganhegou and Yumen formations, and the ~ 0.135 Ma high-angular unconformity (U₄) between the Yumen and Salawusu formations. A ~ 5–4 ka Holocene river terrace of the Qingshuihe River has also been identified. These unconformities and the river terrace provide evidence for a stepwise northeastward expansion model of the Tibetan Plateau. The U₁ disconformity signifies the outward expansion of the orogenic plateau into its northeastern margin through uplift rather than shortening, possibly subsequent to the removal of the mantle lithosphere. The angular unconformities U₂, U₃, and U₄ indicate synchronous crustal shortening across the Tibetan region and its northeastern margin, with U₃ representing the most intense lateral expansion of the plateau. The Holocene river terrace represents the most recent tectonic event associated with the northeastward growth of the Tibetan Plateau in the study area, shaping modern landforms and topography.