AGES OF CENOZOIC STRATA DEPOSITED IN THE TUOTUOHE BASIN AND IMPLICATIONS FOR THE EARLY OLIGOCENE PALEOELEVATION OF THE NORTHERN TIBETAN PLATEAU

Deciphering the uplift history of northern Tibet is critical for understanding both the geodynamic processes that lead to plateau formation as well as the regional climatic impacts that the growth of the plateau has had on eastern Asia. Despite its importance, this history has remained elusive for at least two reasons. First, absolute age control is sparse within sedimentary strata in northern Tibet, and second, complex spatial patterns of stable isotopes in precipitation complicate their interpretation as paleoelevation proxies. We present new constraints on the ages of Cenozoic strata deposited in the Tuotuohe Basin within northern Tibet. These constraints are derived from a new magnetostratigraphic study that is pinned to the geologic timescale through fossil assemblages and maximum depositional ages from U-Pb geochronology of detrital zircons. From these data we construct a new chronology for basin deposition. The new chronology refines the age of the Tuotuohe Fm to ~37-33 Ma, the Yaxicuo Fm to 33-23.6 Ma and the Wudaoliang Fm to 23.6-19.7 Ma. In addition, existence temperatures for fossil fauna found in the Tuotuohe Basin are determined from existence temperature of extant species. Ostracod assemblages and stoneworts from the Oligocene (~29 Ma) Yaxicuo Fm yield paleotemperature estimates of ~11-15.5 °C in the Tuotuohe Basin at the time of deposition. Presently, the mean annual temperature in this region is -4 ℃. After accounting for a -5.5 ℃ temperature change from the Oligocene to the present due to global cooling, the paleotemperatures derived from the fossil fauna require the Tibetan Plateau to have cooled ~9.5-14 ℃ since the Oligocene. Given a current mean elevation of 4700 m, this implies an Oligocene paleoelevation of the Tibetan Plateau of 2300-3100 m assuming a vertical lapse rate of 5.8℃/1000 m. Integration of this paleoelevation estimate with those published for basins in central Tibet, we find two major uplift events across central and northern Tibet. The first occurred from Oligocene to early Miocene time and is coincident with observed upper crustal shortening. The second occurred from the middle Miocene to present in the absence of upper crustal shortening and may reflect convective removal of lithospheric mantle and/or lower crustal flow beneath the central Tibetan Plateau.