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The late Miocene through present paleoelevation history of southwestern Tibet

J. E. Saylor^*,, J. Quade^*, D. L. Dettman^*, P. G. DeCelles^*, P. A. Kapp^* and L. Ding^**

^* Department of Geosciences, University of Arizona, Tucson, Arizona 85721, USA, (520) 621-2672
^** Institute of Tibetan Plateau Research, Chinese Academy of Sciences, P.O. Box 2871, Beijing, China 100029

Corresponding author's present address: Department of Geological Sciences, Jackson School of Geosciences, The University of Texas at Austin, Austin, Texas 78712-0254; jsaylor{at}mail.utexas.edu

Recent research using stable isotopes of carbon and oxygen from carbonates and fossil teeth seems to support both a pre- and post-mid-Miocene uplift of the southern Tibetan Plateau. We examined this issue by analysis of well-preserved fossil mollusks and plant remains from the Zhada Basin in southwestern Tibet, which ranges in age from 9.2 to <1 Ma. Based on ¹⁸O_cc values from shell aragonite, we estimate that oxygen isotope ratios of Miocene –Pleistocene paleo-surface water (¹⁸O_psw) in Zhada Basin ranged from –24.5 to –2.2 (VSMOW). The lowest of these calculated values are lower than ¹⁸O_sw values [–17.9 to –11.9 (VSMOW)] of modern water in the basin. The extremely low ¹⁸O_psw values from fluvial mollusks and evaporatively elevated ¹⁸O_psw values from lacustrine mollusks, show that the peaks surrounding the Zhada Basin were at elevations at least as high as, and possibly up to 1.5 km higher than today, and that conditions have been arid since at least 9 Ma. A decrease in elevation since the Miocene is not specifically predicted by any existing mechanical models for the development of the Tibetan Plateau.

Paleoenvironmental modeling and physical evidence shows that the climate in Zhada Basin was cold and arid, indistinguishable from modern. The ¹³C_pm values of well-preserved vascular plant material increase from –23.4 to –26.8 permil at the base of the Zhada Formation to as high as –8.4 permil above 250 to 300 m. This shift denotes the expansion of C₄ biomass in this high, arid watershed at 7 Ma, and thus corresponds to the C₃ –C₄ transition observed in Neogene deposits of the northern Indian sub-continent.

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