CENOZOIC PALEOALTIMETRY AND PALEOHYPSOMETRY OF THE HIMALAYAS AND TIBET PLATEAU

The paleoaltimetry and paleohypsometry of the Himalayas and Tibet are examined using the parcel-based model of Rowley et al. (2001- EPSL 188, 253-268) that combines atmospheric thermodynamics of water vapor condensation with isotopic fractionation of oxygen and hydrogen isotopes to predict the relationship between elevation and isotopic composition in orographically forced precipitation systems for latitudes equatorward of about 40°. Oxygen isotopes derived from lacustrine and paleosol carbonates in the Thakkhola and Gyirong basins of the Himalaya indicate that the Late Miocene paleohypsometry of Himalayas is indistinguishable from the present. Approximately 15 Ma paleosol carbonates from the Ouyug Basin in southern Tibet, immediately north of the Indus-Yarlung Tsangpo suture, also contain highly depleted oxygen, implying paleo-elevations in excess of 5 km. The hypsometric mean elevation of the Ouyug basin today is about 5 km and our paleohypsometric estimate is indistinguishable from the Present. Thus it appears that southern Tibet had also achieved its current elevation by 15 Ma. Farther north, in northern Tibet, samples of lacustrine limestones from the Eocene Fenghuoshan Group imply that the hypsometric mean elevation of the drainage basins feeding the Eocene Fenghuoshan lakes were about 2 km high and that the Fenghuoshan lakes were situated at elevations below 1 km. The present elevation of the Fenghuoshan is above 4.8 km implying in excess of about 4 km of surface uplift since the Eocene of this part of Tibet. Results from both Miocene and Eocene paleosol and lacustrine carbonates derived from recently collected samples from the Lunpola Basin in central Tibet will be presented. These data provide for the first time indications of the spatial and temporal patterns of Himalayan and Tibetan surface uplift. High elevations existed in southern Tibet and the Himalayas well before the hypothesized 8 Ma uplift nominally correlated with onset of east-west extension. Uplift of northern Tibet post-dates the Fenghuoshan sediments, but the timing is not yet resolved.