ORIGIN OF THE ZHADA BASIN, SW TIBET: A TECTONICALLY DAMMED PALEO-RIVER VALLEY

The Zhada Basin is located just north of the South Tibetan Detachment System in southwestern Tibet, and sits at between 3500 and 5000 m. The basin is unique, being a large, low-elevation (relative to the rest of Tibet), Mio-Pliocene basin in a position occupied elsewhere in the Himalayan arc by high mountains. The fluvial, lacustrine, alluvial fan and eolian deposits of the Zhada Basin contain a rich tectonic, climatic and faunal record. However, the origins of this basin are not well understood. New sedimentological evidence shows that the basin was occupied by a low-gradient, northwest-flowing paleo-Sutlej River; periodically dammed to form shallow lakes. At the northwestern end of the Zhada Basin, paleocurrent indicators record a drainage reversal, from northwest-flowing in the lower Zhada Formation to southeast-flowing rivers at the top. Northwest oriented paleocurrent indicators show that, unlike the modern which is diverted southward, the paleo-Sutlej River was unobstructed by the Qusum Metamorphic Core Complex at the northwestern edge of the basin. Across the same stratigraphic intervals in the central Zhada Basin, paleocurrent indicators display a shift from axial to transverse drainage. At both locations, changes in paleocurrent indicator orientation are accompanied by changes in source terrane. Zircons dating to < 60 Ma dominate detrital zircon populations from the lower Zhada Formation. Sandstones from these levels are rich in low-grade metasedimentary and volcanic lithic grains. Medium- to high-grade metamorphic grains and a detrital zircon signal consistent with a Greater Himalayan Sequence source dominate detritus for sandstones from the top of the Zhada Formation. These observations are consistent with a scenario in which uplift of the Qusum MCC dammed the paleo-Sutlej River, producing shallow lakes in the middle Zhada Formation, followed at the top of the formation by transverse alluvial fan progradation from the basin flanks.