Significant surface uplift of southeastern Tibet during the late Middle Eocene–late Eocene: Evidence from vegetation turnover in the Gonjo Basin and regional geological records

When and how the southeastern Tibetan Plateau attained high elevation remains debated, yet resolving this question is fundamental to understanding the Cenozoic growth of the Tibetan Plateau. Here, we present a new palynological record spanning the late Middle Eocene (~42.3–41.5 Ma) from the Gonjo Basin and integrate it with an early Middle Eocene (~47.8–>44 Ma) pollen record from the same basin to reconstruct vegetation evolution across an important interval of plateau development. The integrated record documents a pronounced vegetation turnover from assemblages rich in broad-leaved taxa (e.g., Caryapollenites , Celtispollenites , and Liquidambarpollenites ) and ferns to conifer-rich assemblages (e.g., Pinuspollenites and Inaperturopollenites ) with the appearance of Piceapollis and more xerophytic taxa ( Ephedripites ), showing that the turnover was established by ~42.3 Ma. This ecological transition occurred during a period of relatively stable global climate, suggesting that global cooling alone cannot explain this shift. We infer that regionally significant surface uplift generated cooler, drier environments through elevation-dependent cooling and enhanced rain-shadow effects. Two exploratory pollen-based paleoelevation approaches yield estimates consistent with the moderate-to-high elevations inferred from vegetation turnover, providing a consistency check rather than precise paleoelevation constraints. Together with published paleoaltimetric, thermochronological, and structural evidence, the vegetation transition supports substantial topographic growth in southeastern Tibet during the late Middle Eocene–late Eocene. Such early uplift may have enhanced regional silicate weathering and influenced long-term Cenozoic seawater geochemical evolution and carbon cycling.