PROVENANCE OF CRETACEOUS–PALEOGENE STRATA OF NORTHWEST INDIA: DETRITAL ZIRCON GEOCHRONOLOGIC AND HF ISOTOPIC INSIGHTS INTO THE TIMING OF INDIA–ASIA COLLISION

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 the Cretaceous Singtali and Paleocene–Eocene Subathu formations provide invaluable insight into the onset of collision and initiation of Himalayan orogenesis. We generated new detrital zircon U-Pb ages and Hf isotopic data from these rocks in order to identify major provenance shifts associated with transitions between detrital input from the Indian plate, from accretionary arc terranes (e.g., Kohistan-Ladahk), and from the Asian plate to the north. 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. These differences reveal a major shift in provenance from a southern Indian cratonic source during the Cretaceous to a northern Himalayan source at the time of Subathu deposition. Preliminary zircon Hf isotopic data from grains <320 Ma in the Subathu Formation include more evolved grains of Asian affinity, indicating that India­­–Asia collision commenced by the time of Subathu deposition during the Eocene, in agreement with previous studies from the western and central Himalaya.