Tectonic Evolution of the Back-Arc Basins in Southern Tibet: Insights from Detrital Zircon U–Pb–Hf Isotopic Constraints

The Late Mesozoic marks an important period in the tectonic evolution of southern Tibet. We examine the Late Mesozoic sedimentary sequences of the back-arc basins in southern Tibet using over 1000 detrital zircon U–Pb ages with in-situ Lu–Hf isotopic data, to fingerprint sediment provenance and reconstruct basin evolution. Our new geochronological results establish a refined chronostratigraphic framework and constrain the maximum depositional ages from ca. 157 to 84 Ma. Specifically, the Late Jurassic–Early Cretaceous successions (Quesangwenquan to Takena Formations; dated at 157–105 Ma) record multiple major zircon age peaks between 151 and 571 Ma with enriched Hf isotopic compositions (-24.56 to -0.13), indicating derivation mainly from the central Lhasa sub-terrane. In contrast, the Late Cretaceous Shexing Formation (ca. 84 Ma) exhibits a prominent age peak at ca. 125 Ma and predominantly positive ɛ Hf (t) values (+ 0.37 to +14), indicating a provenance shift toward the south Lhasa sub-terrane. This provenance turnover documents a major tectonic transition from an extensional back-arc basin to a compressional retro-arc foreland basin. These results provide key constraints on the paleotectonic architecture of the back-arc basins and highlight the Neo-Tethys Ocean northward subduction and the Lhasa–Qiangtang collision in driving changes in sedimentary provenance and paleogeography.