Tectonics: Retro‐foreland basin development in response to Proto‐Tethyan Ocean closure, NE Tibet Plateau
The compositions and ages of the sediments within retro or foreland basins that are formed and preserved adjacent to collisional orogens can reflect the nature of colliding tectonic elements. The Yaoshuiquan and Huabaoshan formations in the South Qilian belt on the NE Tibetan Plateau represent a fluvial succession of sediments deposited during latest Ordovician to Late Silurian time within a retro‐foreland basin setting in response to arc‐continent collision. Lower parts of sections through both formations are dominated by mafic to felsic volcanic detritus with youngest detrital zircon populations of ~446 Ma. Stratigraphically higher beds contain abundant metamorphic and felsic igneous detritus with youngest detrital zircon populations of ~440 Ma. Paleocurrent indicators point to sediment derivation from a Cambro‐Ordovician arc‐ophiolite complex containing mixed 530‐480 Ma oceanic‐crust together with contributions from a 479‐450 Ma continental‐arc early in development of the basin. The Cambrian arc‐accretionary system and Central Qilian block united to form the basement of a continental arc at ~450 Ma and both then contributed sediments to the Lianhuashan‐Huabaoshan basin. After the Hualong complex accreted to the north a broad Andean‐type margin developed along the southern margin of the Central Qilian block from 450‐440 Ma. These processes generated a wider basin that received detritus from both the south and the north. Consumption of the Proto‐Tethyan Ocean ended with collision between the Qaidam and Hualong blocks which led to mass wasting of detritus from the Andean‐type igneous rocks and both blocks with the basin from 440‐420 Ma.