OLIGO-MIOCENE REORGANIZATION OF EASTERN HIMALAYAN RIVER SYSTEMS: NEW CONSTRAINTS FROM DETRITAL ZIRCON (U-TH)/PB PROVENANCE ANALYSIS OF THE BARAIL GROUP, NORTHERN INDO-BURMAN RANGES

The reorganization of continental-scale river systems is a fundamental control on the size and distribution of sedimentary basins along tectonically active plate boundaries. In the eastern Himalaya, river reorganization has been proposed to explain the evolution of modern drainages and foreland basin lithostratigraphy. Widespread application of detrital zircon (U-Th)/Pb geochronology by LA-ICP-MS identifies < 300 Ma Gangdese batholith and Tibeten-derived zircons in Miocene sedimentary deposits within the Siwalik and Indo-Burman basins. Presence of these Transhimalayan plutonic zircons imply the incorporation of the Yarlung river drainage system across the India-Eurasia continental suture by the Middle Miocene. This study presents new detrital zircon U-Pb geochronology for 7 samples of the late Oligocene Group, and one sample of Pleistocene conglomerate, in three proximal stratigraphic sections of the northern Indo-Burman Ranges. The percentage of < 300 Ma zircons increases from < 19% in marine deposits in the lower Barail to > 40% in terrestrial-coastal deposits in the Upper Barail. Detrital age populations in the Barail Group range from Ma to Ma and comprise distinct ca. 60, 80-100 Ma, 120 Ma, 180 Ma, 220 Ma and 270 Ma subpopulations. The 80-100 Ma population is dominant with an apparent local source in the Lohit Plutonic Complex, presently within the Lohit and Dibang river drainages. The Pleistocene sample has a wide range of ages < 300 Ma that may reflect recycling of younger (Miocene and Pliocene) deposits in the Naga Thrust belt. However, potential Gangdese pluton ages only constitute 2% of the total population. Zircon age populations are consistent with ancestral river drainage in the Northern and Eastern Transhimalayan plutons in and between the Bom. Application of Hf isotope analysis to selected zircons from this age range will discriminate between pluton sources. We interpret this provenance data as evidence for an Oligocene river system crossing the India-Eurasia suture zone at the easternmost end of the plate collision. Our results suggest that integration of the ancestral Yarlung river with the Himalayan foreland occurred after the Oligocene, consistent with recent provenance data from the Miocene Surma and Tipam Groups of the more distal Indo-Burman Ranges.