PERSISTENT HIGH TOPOGRAPHY IN THE HIMALAYAN-TIBETAN OROGEN

The paleoelevation history of the Tibetan Plateau remains a contentious topic in tectonics. Ggeodynamic models and stable isotopic, as well as geological evidence, support the early Cenozoic emergence of high topography following the collision of the Indian and Eurasian plates. However, recent General Circulation Model (GCM) simulations of paleoprecipitation isotopes as well as paleobotanical records of climate challenge previous interpretations of elevated early Cenozoic topography.

Here, we employ an orographic precipitation isotope model (OPI) to investigate the spatial controls of precipitation isotopes during orographic lifting across more than 1000 km of the Tibetan Plateau. These data are coupled with analyses of stable hydrogen isotopes in organic molecular biomarkers to quantify molecular fractionation across the region. Utilizing the OPI model alongside modern organic molecular biomarker data, we assess Eocene to recent plant biomarker δ2H records of paleoprecipitation isotopes across the Tibetan Plateau.

Our study reveals that after accounting for long-term climate variations and their effects on the relationship between water isotopes and orographic lifting, areas north of the present-day Himalayan crest are characterized by greater isotopic fractionation than modern since at least the Eocene. Indeed, prior to the Miocene epoch, reconstructed paleoprecipitation isotopes indicate at least a 20% higher isotopic fractionation than that observed in today's topography. While these findings do not directly constrain the paleoelevation of basins now situated north of the Himalayan crest, they provide compelling evidence for orographic lifting comparable to or exceeding modern barriers and persistent high topography in the Himalaya-Tibetan orogen.