EVIDENCE FOR AN OLIGOCENE YARLUNG-BRAHMAPUTRA CONNECTION AND CROSS SUTURE DRAINAGE FROM DETRITAL ZIRCON U-Pb AND εHf PROVENANCE ANALYSIS, NORTHERN INDO-BURMAN RANGES
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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 as an explanation for modern drainage patterns and abrupt changes in foreland basin lithostratigraphy. Widespread application of detrital zircon (U-Th)/Pb geochronology by LA-ICP-MS identifies < 300 Ma Transhimalayan zircons sourced from Tibetan plutonic rocks in Miocene sedimentary deposits within the Siwalik and Indo-Burman basins. The presence of these Transhimalayan plutonic zircons implies the integration of the Yarlung River drainage system across the India-Eurasia continental suture prior to the Middle Miocene. We report that new ages acquired from detrital zircons of late Oligocene deposits of the northern Indo-Burman Ranges (IBR) have a percentage of < 300 Ma from < 20% in marine deposits to > 30% in terrestrial-coastal deposits. However, potential Gangdese pluton ages only constitute 2% of the total population, whereas population of this source is abundant in Miocene deposits. Zircon age populations are consistent with ancestral river drainage in the northern and eastern Transhimalayan plutons of the Bomi-Chayu and Dianxi-Burma region. Utilization of single grain εHf isotope analysis of ages < 300 Ma discriminates plutonic source of zircons, which provides paleo-river reconfiguration of the eastern Himalayan River systems. εHf values are coupled with (U-Th)/Pb ages; our data shows clustered between -20 to 20 for ages < 300 Ma. Clusters of points are located in the Lohit Plutonic Complex, Bomi-Chayu, and Dianxi-Burma plutons. εHf and (U-Th)/Pb values of collected samples points to a shift in provenance in coastal-terrestrial deposits of the late Oligocene. We interpret this shift as an integration of the Yarlung River in the Miocene causing an expansion of a localized drainage area to continental-scale.