Seminar

Cretaceous–Paleogene strata exposed across the Himalayan orogen preserve a record of the early and pre-collisional stages of India-Asia continental convergence. Evidence for the initial arrival and deposition of Asian detritus on the Indian plate has proven a robust means to track the timing of India–Asia collision, and such constraints from across the orogen are imperative in understanding the origin and mechanics of Himalayan tectonics. Within the Lesser Himalaya and frontal thrust system of northwest India, discontinuous exposures of Cretaceous and Paleocene–Early Eocene rocks of the Singtali and Subathu Formations, respectively, provide invaluable insight into these initial stages, including the onset of collision. We generated new detrital zircon U-Pb ages and Hf isotopic data from these rocks in order to distinguish between Indian plate, Asian plate, and/or Kohistan–Ladahk Island arc sources and track major provenance shifts. Compared to zircon U-Pb results from the Singtali Formation, results from the Subathu Formation record (1) a significant increase in the relative abundance of Cambrian–Proterozoic grains, (2) a significant decrease in the abundance of ~1.6–1.8 Ga grains of Indian craton affinity and, (3) a distinct population of Permian and Late Cretaceous¬–Paleocene grains. All Cretaceous grains from the Singtali Formation yielded an enriched Hf isotopic signature, indicating a single unique Indian source for these grains which cluster in U-Pb-Hf space. Zircon Hf isotopic signatures from grains <320 Ma in the Subathu Formation show a significant increase in source diversity which include grains of Asian affinity—most likely indicating that collision commenced by the time of Subathu deposition. These results reveal a major shift in provenance from a southern Indian craton source during the Cretaceous to a northern Himalayan source at the time of Subathu deposition. Based on the biostragraphic and U-Pb constrained maximum depositional age of the Subathu Formation, terminal India¬¬–Asia collision must have occurred in northwest India by 50 Ma.