DETRITAL ZIRCON PROVENANCE ANALYSIS OF RONGMAWA FORMATION STRATA ALONG THE INDIA-ASIA SUTURE IN SOUTHERN TIBET SUGGEST THAT THE LHASA RIVER IS AN ANTECEDENT TRANS-ARC RIVER THAT TRANSPORTED SEDIMENT FROM THE CENTRAL LHASA TERRANE TO THE SUBDUCTION ZONE IN LATE CRETACEOUS TIME

During Late Cretaceous time, Rongmawa Formation trench basin strata were deposited in the subduction zone that consumed Neo-Tethyan oceanic lithosphere along the southern margin of the Andean-style proto-Tibetan Plateau. We conducted detrital zircon (DZ) U-Pb geochronology on six trench basin samples (n = 1,716) collected near Dênggar, Tibet (~500 km west of Lhasa city) to assess the provenance of these rocks and reconstruct Late Cretaceous sediment transport pathways. Provenance analysis consisted of qualitative comparisons, multi-dimensional scaling, and non-negative matrix factorization to extract source spectra. All trench basin samples contain DZ ages that point to a unique source around Lhasa city, north of the main body of the Late Cretaceous Gangdese magmatic arc. This signal is only rarely observed in forearc strata, suggesting that the transport pathway to the trench largely bypassed the forearc basin. 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. At this location, forearc assemblages are not preserved, which could either reflect pre-Late Cretaceous subduction erosion or later modification during India-Asia collision. We interpret 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. The Ancestral Lhasa River was likely analogous to the modern Fuji River, which crosses magmatic arc highlands in southern Japan and feeds the Nankai Trough by connecting to a submarine canyon and a continuous, >400-km-long, trench-parallel submarine channel.