Reconstructing paleo-drainage in southern Tibet prior to the India-Asia collision: Provenance constraints from the Shexing Formation

The configuration of drainage networks along the southern Asian margin prior to the India-Asia collision is critical for understanding long-term landscape evolution, yet remains poorly constrained. Here, we integrate sedimentological, petrographic, and detrital zircon UPb geochronological data from the Shexing Formation, deposited in the backarc basin of the Gangdese arc, to reconstruct the pre-collisional drainage system of southern Tibet. Sedimentological evidence indicates predominantly fluvial deposition with localized shallow marine influence. Maximum depositional ages derived from detrital zircon UPb data constrain basin fill to ∼119–84 Ma. Quantitative provenance analysis reveals that the majority of sediment was sourced from the north-central Lhasa terrane (contributing 41–99 %), with only minor contributions from the Gangdese arc. This provenance pattern, when integrated with the published tectonic subsidence history, supports a backarc basin model that was likely triggered by the rollback of the Neo-Tethyan oceanic lithosphere. Combining paleocurrent measurements, spatial correlation with coeval strata in the Xigaze forearc and trench basins, and detrital zircon age signatures, we propose that during the deposition of the Shexing Formation, multiple transverse rivers, rather than a single axial system, cut through the Gangdese arc along the southern Asian margin. The inferred paleogeographic configuration may have resembled that of modern Taiwan, where sub-parallel rivers sourced in the Central Mountain Range traverse sedimentary plains and incise through the Coastal Range before discharging into the ocean. Continued uplift of the Gangdese arc subsequently reorganized regional drainage, shifting from a southward flow to an axial configuration sometime between the Late Cretaceous and the Miocene.