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The geology and tectonics of central Bhutan Journal Item

9 Lithotectonic mapping, metamorphic observations and U-Pb zircon ages underpin a substantial revision 10 of central Bhutan geology – notably a more extensive and continuous outcrop of the Tethyan 11 Sedimentary Series (TSS) than previously mapped. Metamorphic grade in the TSS increases downward 12 towards a basal north-vergent tectonic contact with the underlying Greater Himalayan Series (GHS), 13 interpreted as a southward continuation of the South Tibetan Detachment (STD). Miocene (~17 – 20 14 Ma) leucogranite sheets are associated with the STD in this region but appear to diminish southwards. 15 Two leucogranite dykes that cross-cut TSS structures yield ages of 17.8±0.2 and 17.9±0.5 Ma. A 500±4 16 Ma (U-Pb zircon) metamorphosed ash bed in the Pele La Group within the psammite-dominated lower 17 TSS yields the first direct isotopic age for the TSS in the eastern Himalaya, confirming existing age 18 constraints from detrital zircon and fossil studies. A continuation of the Paro metasedimentary unit 19 underlying the GHS was mapped near Wangdue Phodrang. Our observations, notably the exposure of a 20 wholly ductile STD so far south and the significance of large nappe-like structures in the TSS, prompt a 21 major revision to the geological map of the Bhutan Himalaya and require a reassessment of tectonic 22 interpretations of the Bhutan Himalaya. 23