Diachronous evolution of Himalayan drainage: comparing the western and eastern syntaxes through detrital zircon U-Pb geochronology of Middle Siwalik sandstones

Abstract Detrital zircon U–Pb ages from a Middle Siwalik Nagri Formation sandstone (~ 11–9 Ma) in the Kohala section, Hazara–Kashmir Syntaxis (western Himalaya, Pakistan), are used to assess sediment provenance and drainage connectivity in the western Himalayan foreland basin. The 98 concordant ages are dominated by Neoproterozoic–early Paleozoic grains (~ 1000–400 Ma; ~59%), with subordinate Mesoproterozoic (~ 20%) and Paleoproterozoic (~ 16%) populations. This pattern is typical of Higher Himalayan crystalline rocks and Tethyan Himalayan sedimentary successions. Zircons of Trans-Himalayan arc age (40–110 Ma) are sparse (two grains, ~ 2%); at this sample size they are unlikely to have formed more than ~ 7% of the zircon load. Multi-metric similarity analysis and multidimensional scaling group the Nagri Formation with coeval Middle Siwalik strata of India, Nepal and Sikkim–Bhutan. They separate it from Arunachal Pradesh and the Indo-Burman Ranges, where Trans-Himalayan zircons are common, and from the distal Indus Fan and Laxmi Basin. We interpret the Kohala depocentre as fed mainly by transverse rivers draining the Himalayan thrust belt, outside the catchment of the axial paleo-Indus. Compiled foreland records suggest that Trans-Himalayan detritus reached the proximal foreland at different times along strike. At Kohala, the scarcity of arc-derived zircons may reflect a drainage divide along the Higher Himalayan and Pir Panjal ranges during initiation of the Main Boundary Thrust (~ 11–9 Ma). The results suggest that efficient long-distance sediment routing to this part of the foreland developed only after substantial deformation and exhumation had already occurred in the orogen.