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Abstract: The Himalayan orogeny, a result of the collision of India and Eurasia, provides direct evidence of strain accommodation and large-scale rheological behavior of the continental lithosphere. Knowledge of the timing of the India–Eurasia collision is essential to understand the physical processes involved in collisional systems. Here we present a geochronological and multi-isotopic study on rocks from the upper crust of the Kohistan Paleo-Island Arc that formed in the equatorial part of the Neo-Tethys Ocean. In situ U–Pb geochronology and Hf isotopes in zircon, and whole-rock Nd and Sr isotopic data of plutonic rocks from the Kohistan-Ladakh Batholith, are used to construct a continuous record of the isotopic evolution of the source region of these granitoids that are related to both the subduction of the oceanic lithosphere and subsequent arc–continent collisions. We demonstrate that profound changes in the source region of these rocks correspond to collisional events. Our dataset constrains that the Kohistan–Ladakh Island Arc initially collided along the Indus suture zone with India at 50.2±1.5Ma, an age generally attributed to the final India–Eurasia collision for the entire Himalayan belt. In the western Himalaya, the final collision between the assembled India/Arc and Eurasia however, occurred ∼10Ma later at 40.4±1.3Ma along the so-called Shyok suture zone. We present evidence indicating that a similar dual collision scenario can be extended to the east and conclude that a final India/Arc–Eurasia collision at ∼40Ma integrates crucial aspects of the magmatic, tectonic, and sedimentary record of the whole Himalayan mountain belt. [Copyright &y& Elsevier]

Abstract: The Subhimalaya of NW India exposes a well developed marine to continental foreland sequence preserving sediments shed from the tectonically evolving Cenozoic Himalayan orogen. Detrital zircon U–Pb ages and their Hf isotopic compositions, of seven samples, were analyzed from the Subathu sub-basin foreland formations to constrain sediment provenance. The detrital zircon U–Pb age spectra yielded a similar pattern, for all foreland formations, with major peaks clustered at ~500–650Ma, 900–1000Ma and ~2500Ma, corresponding to (indistinguishable) Tethyan Himalaya and Higher Himalayan Crystalline sources and those at ~1800–1900Ma corresponding to lower a Lesser Himalayan arc source. Comparison of the foreland detrital zircon U–Pb age spectra with Himalayan lithotectonic source units, in conjunction with their Hf isotopes and previously determined sediment petrography and age constraints, suggest that the sediments to the flysch marine Subathu Formation (~65 to >41Ma) were sourced from a positive-relief area of the Tethyan Himalaya mixed with detritus from lower Lesser Himalaya and additional cratonic northern Indian Precambrian rocks. No Cretaceous to Eocene-aged detrital zircons, contributed by eroding Ladakh batholith north of the Indus Tsangpo suture zone, were recorded in the Palaeogene-aged foreland samples. The younger synorogenic continental Dagshai Formation (<30 to <22Ma) sediments were sourced from the very low-grade Tethyan Himalaya, whereas the alluvial-facies Kasauli Formation (<22 to 13Ma) began to receive sediments additionally from erosion of the medium-grade metamorphic rocks of the Higher Himalayan Crystalline and the lower Lesser Himalaya. The Lower Siwalik Formation sediments (~13–11.5Ma) were derived from the low-grade rocks of the Tethyan Himalaya, both the upper and lower Lesser Himalaya and additionally from the eroding high-grade migmatitic gneisses of the Higher Himalayan Crystalline. Thus, from the similar detrital zircon age spectra of the foreland formations, we infer that differences in the continental sediment detritus into the foreland basin reflect a straightforward sediment mixture of variable proportions from fixed Himalayan lithotectonic sources which progressively shifted towards the foreland with development of the orogen. [Copyright &y& Elsevier]