THE LATE MIOCENE THROUGH PRESENT PALEOELEVATION HISTORY OF SOUTHWESTERN TIBET

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 similar to 9.2 to < 1 Ma. Based on delta O-18(cc) values from shell aragonite, we estimate that oxygen isotope ratios of Miocene - Pleistocene paleo-surface water these calculated values are lower than delta O-18(sw) values [-17.9 to -11.9% (VSMOW)] of modern water in the basin. The extremely low 8180Pyw values from fluvial mollusks and evaporatively elevated delta O-18(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 delta C-13(pm) values of well-preserved vascular plant material increase from -23.4 to -26.8 perm, 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 C-4 biomass in this high, arid watershed at - 7 Ma, and thus corresponds to the C-3 - C-4 transition observed in Neogene deposits of the northern Indian sub-continent.