High times on the Tibetan Plateau: Paleoelevation of the Thakkhola graben,Nepal

East-west extension in the Tibetan Plateau is generally assumed to have res ulted from gravitational collapse following thickening and uplift. On the b asis of this assumption, several studies have dated east-west extensional s tructures to determine when the plateau attained its current high elevation . However, independent estimates of elevation are needed to determine wheth er extension occurred before, during, or after the plateau achieved its cur rent elevation. Because the isotopic composition of meteoric water decrease s with increasing elevation, significant change in local elevation througho ut the Thakkhola graben depositional history should be recorded by change i n delta(18)O values of fluvial and lacustrine carbonates. The delta(18)O va lues of -16% to -23 parts per thousand of Thakkhola graben carbonates refle ct meteoric water values similar to modern values and suggest that the sout hern Tibetan Plateau attained its current elevation prior to east-west exte nsion. Initiation of Thakkhola graben extension is constrained between 10 a nd 11 Ma, based on magnetostratigraphy of the older Tetang Formation. The d elta(13)C values of soil carbonates suggest an age younger than 8 Ma for th e base of the Thakkhola Formation.