Multiproxy paleoaltimetry of the Late Oligocene-Pliocene Oiyug Basin, southern Tibet

Abstract

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 δ¹⁸O and δD 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 δD and lacustrine-siderite δ¹⁸O compositions of middle Miocene (∼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 δ¹⁸O composition of lacustrine marls and carbonate/siderite concretions, as well as the δD from plant wax n-alkanes indicate a mean elevation of 5.5 km +1.4/−2.0 km at ∼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 ∼1.4 km between the early and late Miocene. Given the modern Oiyug basin elevation of ∼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.