High and Dry: Central Tibetan Plateau During the Mid-Tertiary

Large mid-Tertiary sedimentary basins at 4.4 km elevation in the north-central Lhasa terrane archive the history of environments and paleoelevation in the central part of the Tibetan Plateau. In the Nyima basin, more than 3 km of strongly shortened fluvial, alluvial fan, and lacustrine sediment accumulated along the north flank of a major north-verging fold-thrust belt. The maximum age of the succession is 40 Ma, based on U-Pb ages of detrital zircons. Biotite 40Ar/39Ar ages from reworked tuffs in the lower third of the succession indicate deposition at 25 to 26 Ma. Provenance and paleocurrent data require sediment source areas to the south in the fold-thrust belt. d18O values from lacustrine marls and fossil shell material are strongly negative (-10 to -14); if unaltered by diagenesis these values suggest paleoelevations on the order of 4 km. Roughly 50 km to the north, in the Muggar Kangri Range, thick nonmarine strata record a wide variety of depositional environments, including eolian dune facies. Unlike the Nyima basin, the Muggar Kangri basin reflects longitudinal eastward flowing paleodrainage. Eolian dune facies yield generally eastward paleowind directions. Sediment source areas lay toward the north, in thrust-faulted ranges lying within the southern Qiangtang terrane. Regional correlations based on lithostratigraphy suggest that the upper part of the Muggar Kangri basin fill is roughly the same age as the Nyima basin fill. Taken together, the new geochronologic, oxygen isotopic, and sedimentary facies data indicate that (1) Nyima basin fill is younger than widely distributed Eocene-Oligocene strata in north-central Tibet, challenging models that predict a systematic northward younging in upper crustal shortening and basin development and (2) the central part of the Plateau was high (>4 km) and dry by mid-Tertiary time.