Abstract
The first <i>in situ</i> Hf and U-Pb isotope analyses of detrital zircons from Upper Miocene sandstone in the Inner-Burma Tertiary Basin, together with data of igneous zircons from the eastern Transhimalayan batholith from southern Tibet to Burma, enable us to study the sedimentary source to sink relation and river evolution around the eastern Himalayas. Among 47 out of 62 dated zircons from the Miocene sandstone that exhibit Cretaceous and Paleogene <sup>206</sup>Pb/<sup>238</sup>U ages, 24 grains have positive ε<sub>Hf</sub>(T) isotope values up to +16. Whilst zircons of such ages are common in the Transhimalayan plutons, those showing high ε<sub>Hf</sub>(T) values have been observed so far only in the Gangdese batholith. Our results, therefore, support the notion that by Late Miocene time the Yarlu Tsangpo, which flows past the Gangdese batholith in southeastern Tibet, drained into the Irrawaddy River. We attribute this river routing to not only regional topographic control but also the dextral movement of the Jiali-Gaoligong-Sagaing fault system that appears most active during the Middle Miocene. Subsequent reorganization of these mountain rivers was affiliated with headward erosion of the Brahmaputra River that eventually cut across the Namche Barwa Syntaxis and captured the Yarlu Tsangpo drainage to form the modern eastern Himalayan river system.
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