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Late Cretaceous trench basin strata were deposited in the subduction zone that consumed Neo-Tethyan oceanic lithosphere along the southern margin of the proto-Tibetan Plateau. We conducted detrital zircon (DZ) U-Pb geochronology on six trench basin samples (n = 1716) collected near Denggar, Tibet (similar to 500 km west of Lhasa), to assess the provenance of these rocks and reconstruct Late Cretaceous sediment transport pathways. They contained DZ ages that point to a unique source around Lhasa city, north of the Late Cretaceous Gangdese magmatic arc. The modern Lhasa River catchment contains the requisite sources, and its main trunk transects the Gangdese magmatic arc, joining with the Yarlung River at a barbed junction at the India-Asia suture. We infer that the Lhasa River is an ancient feature that transported sediment to the subduction zone in Late Cretaceous time and persisted during India-Asia collision.