Silurian Sedimentation in the South Qilian Belt: Arc‐Continent Collision‐related Deposition in the NE Tibet Plateau?

The South Qilian belt mainly comprises an early Paleozoic arc‐ophiolite complex, accretionary prism, microcontinental block, and foreland basin. These elements represent accretion‐collision during Cambrian to Silurian time in response to closure of the Proto‐Tethyan Ocean in the NE of the present‐day Tibet Plateau. Closure of the Proto‐Tethyan Ocean between the Central Qilian block and the Oulongbuluke block and the associated collision took place from NE to SW in a zipper‐like style. Sediment would have been dispersed longitudinally SW‐ward with a progressive facies migration from marginal alluvial sediments toward slope deep‐water and deep‐sea turbidites. This migration path indicates an ocean basin that shrank toward the SW. The Balonggongga'er Formation in the western South Qilian belt represents the fill of a latest Ordovician‐Silurian remnant ocean basin that separated the Oulongbuluke block from the Central Qilian block, and records Silurian closure of the Proto‐Tethyan Ocean and subduction beneath the Central Qilian block. However, alluvial deposits in the Lajishan area were accumulated in a retro‐foreland basin, indicating that continent‐continent collision in the eastern South Qilian belt occurred at c. 450–440 Ma. These results demonstrate that the Proto‐Tethyan Ocean closed diachronously during early Paleozoic time.