MIOCENE CAPTURE OF THE YALU (YARLUNG) RIVER BY THE PALEO-SUBANSIRI RIVER: EVIDENCE FROM DETRITAL ZIRCON U-PB GEOCHRONOLOGY AND SEDIMENTOLOGY IN THE EASTERN HIMALAYA

The Yalu River is the largest drainage system in the Himalaya. Its current configuration may have existed prior to the Himalayan uplift (i.e., antecedent) or resulted from capture due to headward erosion of the Brahmaputra River. In order to examine these hypotheses, we conducted detrital zircon dating and sedimentological analyses of Tertiary strata from the eastern Himalayan foreland. Our studied Tertiary strata consist of the Miocene Dafla and Subansiri Formations, which are separated by a thrust fault, and the Pliocene to Quaternary Kimin Formation. We investigated two areas: Itanagar and Kimin, which are roughly 50 km apart. In the studied section, multistoried and massive sandstone beds prevalent in the Dafla and Subansiri Formations are overlain by coarse-grained sandstone and pebble conglomerates of the Kimin Formation. The Subansiri Formation preserves trough cross bedding, large sandbars, fine-grained overbank deposits, and south-flowing paleo-current indicators, all of which suggest the presence of a large south-flowing river at the time of its deposition. U-Pb detrital zircon dating reveals the presence of zircons derived from the Gangdese batholith of southern Tibet, north of the Indus-Tsangpo suture. In contrast, the concentration of the Gangdese zircons is much lower in the Kimin Formation. The dominance of locally derived clasts indicate that the Kimin deposits are primarily derived from erosion of local Himalayan basement rather than a transverse Himalayan river. We interpret Gangdese age zircon in the Kimin Formation in terms of recycling of the older Dafla and Subansiri Formations. Evidence for recycling is clearly indicated by the dramatic change of modern detrital zircon age distributions as the Subansiri river crosses the main boundary thrust and cut through foreland basin deposits in the eastern Himalaya. The combined sedimentologic and detrital zircon data from the Tertiary strata led us to propose that the Yalu River was linked in the Miocene with a large south-flowing river in our study area. The inferred river could be an earlier course of the Subansiri River, as its source region lies approximately 30 km from the main course of the Yalu River. The proposed river capture event may have been induced by tectonic forces (e.g., rift damming), climate change (e.g., intensification of the monsoon), or a glacial effect on topography.