MULTIPROXY PALEOALTIMETRY OF THE LATE OLIGOCENE-PLIOCENE OIYUG BASIN, SOUTHERN TIBET

The stable isotope compositions of carbonate and organic samples from the Oiyug basin in southern Tibet allows for model calculations of the Oligocene to Pliocene paleoelevation of the south central Tibetan Plateau. We measured the oxygen isotope composition of pedogenic and lacustrine calcite, dolomite, and siderite, and the hydrogen isotope composition of n-alkanes from plant waxes to reconstruct the delta O-18 and delta D values of Oiyug basin paleometeoric water. Calculated water isotope values from Oiyug basin carbonate and organic samples, respectively, are in close agreement, suggesting the preservation of an unaltered paleometeoric water isotopic signal in these archives. Late Oligocene-middle Miocene paleoelevation estimates from groundwater/pedogenic calcite and lacustrine dolomite indicate basin elevations of 4.1 km + 1.2/-1.6 km. Plant-wax n-alkanes delta D and lacustrine-siderite delta O-18 compositions of middle Miocene (similar to 15 Ma) samples indicate paleoelevations of 5.1 km + 1.3/-1.9 km. This estimated elevation is similar to the 5.4 km paleoelevation estimate based on fossil-floral physiognomy from the same stratigraphic interval. Calculated late Miocene-Pliocene paleoelevation estimates derived from the delta O-18 composition of lacustrine marls and carbonate/siderite concretions, as well as the delta D from plant wax n-alkanes indicate a mean elevation of 5.5 km + 1.4/-2.0 km at similar to 5 Ma. Although calculated mean paleoelevations for the Oiyug basin all fall within the errors associated with the model calculations, the close agreement of the different paleoelevation proxies provides an additional degree of confidence in the fidelity of the calculated paleoelevations. Calculated paleoelevations indicate a possible increase in Oiyug basin elevations of similar to 1.4 km between the early and late Miocene. Given the modern Oiyug basin elevation of similar to 4.3 km, study results allow for a possible > 1 km decrease in elevation since the early Pliocene. These findings, in conjunction with other Tibetan paleoaltimetry studies, are consistent with tectonic models supporting high elevations of the Tibetan Plateau since the initiation of India-Asia collision during the Eocene, and subsequent late Cenozoic extensional collapse.