DETRITAL ZIRCON GEOCHRONOLOGY OF TERTIARY AND MODERN SEDIMENTS IN THE EASTERN HIMALAYA: IMPLICATIONS FOR LARGE-SCALE DRAINAGE REORGANIZATION

The evolution of major drainage systems in the eastern Himalaya and Tibetan Plateau has been affected by deformation related to the Indo-Asian collision. The occurrence of river-reversal and river-capture events has been suggested for many of the large rivers on the basis of the unusual geometric arrangement of drainage networks. In contrast to models which identify drainages as passive stain markers, the river-capture processes make different predictions about the relationship between drainage-network evolution and strain distribution and strain history. In order to test the hypothesis of river capture for the Yalu Tsangpo-Brahmaputra River, we have examined the detrital zircon geochronology of Late Cenozoic Himalayan foreland sediments. If the modern shape of the Yalu Tsangpo-Brahmaputra drainage system was created by capture of the eastward-flowing Yalu Tsangpo by headward erosion of the southward flowing Brahmaputra drainage, then this should be recorded by a sudden influx of sediment sourced from behind the Himalayan range into the foreland, characterized by a large number of zircons derived from the Gangdese batholith north of the Indus-Tsangpo suture. Preliminary detrital zircon U-Pb age data obtained from four samples of sandstones in the upper Miocene Dafla Formation and the upper-Pliocene-lower Pleistocene Kimin Formation in the eastern (Arunachal) Himalayan foreland of NE India, as well as modern Yalu Tsangpo sand near Lhasa, is consistent with a river capture event in the late Tertiary. This interpretation is based on the absence of a large spike in zircon-age distributions at ~50 Ma in the Dafla Formation and the appearance of this spike in the Kimin Formation. These detrital zircons are the signature of the Gangdese batholith. Other possible explanations include changes in channel width of the Brahmaputra River in the Himalayan foreland during the late Tertiary, sequential eastward capturing of southward flowing rivers in the southern Himalayan range by the westward flowing Brahmaputra River, or northward migration of the Brahmaputra River due to northward tilting of the Shillong Plateau.