EOCENE-MIOCENE PALEOALTIMETRY OF THE TIBETAN PLATEAU

The stable-isotopic composition of lacustrine and pedogenic carbonates from the Hoh Xil, Lunpola, and Oiyug basins of Tibet allow model estimates of the Eocene-Miocene paleoaltimetry of the Tibetan Plateau. Lacustrine carbonates from the Middle Eocene Fenghuoshan Group of the Hoh Xil basin in northern Tibet have δ¹⁸O values ranging from –11.7‰ to –10.26‰ (PDB). Model results using these values indicate the hypsometric mean elevation of the drainage basins feeding Hoh Xil lakes ≤ 2 km. In the Lunpola basin of central Tibet, Eocene-Oligocene Niubao Fm. lacustrine limestones and marls have δ¹⁸O values of –2.9‰ to –13.8‰ (PDB), while pedogenic carbonates from the same unit have average δ¹⁸O compositions of –17.1‰ ±1.7‰ (PDB). Model results using these compositions yield predicted paleoelevations for the basin of 4.0-4.6 km. The oxygen isotopic composition of lacustrine marls and micritic limestones from the Miocene Dingqing Formation from the Lunpola basin range from –0.4‰ to –14.6‰ (PDB). Model results using these compositions yield predicted Miocene paleoelevations of ~4.2 km. Pedogenic carbonates from the Rigonla Formation in the Oiyug Basin of southern Tibet have δ ¹⁸O values -20.2‰ which indicates surface elevations ~5.3 km prior to 15 Ma.

Collectively, these results indicate that the southern and central Tibetan plateau achieved elevations similar to those that presently exist by ≥15 Ma, and for the Lunpola region of central Tibet at > 34 Ma. To date, the data from the Fenghuoshan Group are the only indications of relatively lower elevations (≤ 2 km) for any part of the present-day Tibetan Plateau since the Middle Eocene and suggests that the northern boundary of the plateau during the Eocene was situated between the Lunpola and Hoh Xil basins. This suggests that uplift of the Tibetan plateau has progressed northward through time and that the northern part of the plateau has experienced uplift in excess of 2.7 km since ~36 Ma. Exactly when uplift of the northern plateau occurred, however, has yet to be resolved.