Predicting paleoelevation of Tibet and the Himalaya from δ18O vs. altitude gradients in meteoric water across the Nepal Himalaya

The δ18O value of meteoric water varies with elevation, providing a means to reconstruct paleoelevation if the δ18O values of paleowater are known. In this study, we determined the δ18O values of water (δ18Omw) from small tributaries along the Seti River and Kali Gandaki in the Nepal Himalaya. We found that δ18Omw values decrease with increasing altitude for both transects. δ18Omw vs. altitude along the Kali Gandaki in west-central Nepal fit a second order polynomial curve, consistent with increasing depletion of 18O with increasing elevation, as predicted by a Rayleigh-type fractionation process. This modern δ18Omw vs. altitude relationship can be used to constrain paleoelevation from the δ18O values of Miocene–Pliocene carbonate (δ18Oc) deposited in the Thakkhola graben in the southern Tibetan Plateau. Paleoelevations of 3800±480 m to 5900±350 are predicted for the older Tetang Formation and 4500±430 m to 6300±330 m for the younger Thakkhola Formation. These paleoelevation estimates suggest that by ∼11 Ma the southern Tibetan Plateau was at a similar elevation to modern.