2007 GSA Denver Annual Meeting (28–31 October 2007)

Paper No. 9
Presentation Time: 3:50 PM

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


SAYLOR, Joel1, DECELLES, Peter G.2 and GEHRELS, George E.1, (1)Department of Geosciences, University of Arizona, Tucson, AZ 85721, (2)Geosciences, University of Arizona, Gould-Simpson Building #77, 1040 E 4th St, Tucson, AZ 85721, jsaylor@email.arizona.edu

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.